UV Coatings: Basics, Recent Developments and New Applications

UV Coatings Basics, Recent Developments Applications Reinhold Schwalm Publisher: Elsevier Science (December 21, 20...

Author: Reinhold Schwalm

169 downloads 2258 Views 16MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

UV Coatings Basics,

Recent

Developments

Applications

Reinhold Schwalm

Publisher: Elsevier Science (December 21, 2006) Language: English ISBN-10: 0444529799 ISBN-13: 978-0444529794

and

New

Table of Contents

Preface, Pages v-vii Chapter 1 - Introduction to Coatings Technology, Pages 1-18 Chapter 2 - The UV Curing Process, Pages 19-61 Chapter 3 - Network Formation and Characterization, Pages 62-93 Chapter 4 - Raw Materials, Pages 94-139 Chapter 5 - Formulations, Pages 140-159 Chapter 6 - Structure-Property Relationships, Pages 160-178 Chapter 7 - Tackling the Drawbacks of UV Systems, Pages 179-194 Chapter 8 - Classical Applications, Pages 195-205 Chapter 9 - Recent Developments, Pages 206-251 Chapter 10 - New Applications, Pages 252-290 Chapter 11 - Health, Safety and Environment, Pages 291-302 Subject index, Pages 303-310

Preface Curing a car coating as fast as lightning – dream or becoming reality soon? Few flash lights are sufficient to completely convert falling drops of liquid UV-curable materials into hard balls before they reach the table top. This experiment has been recorded for a 3sat TV documentation and is depicted in the different stages in Figure P1. UVcurable formulations respond so fast to a complete phase change from liquid to solid because the radiation employed forms an active species as initiator for a catalytic curing reaction. The predominantly used radiation out of the electromagnetic spectrum is in the UV (wavelength range: 200 to 400 nm). Such liquid formulations applied to a substrate are termed UV-curable coatings and after curing upon UV exposure solid UV-cured coating are generated. The traditional applications of UV coatings are on thermo-sensitive substrates, like wood and paper. Since UV curing is a very eco-efficient and low-energy curing method, the growth rates of UV-curable coatings are in the range of 10% per year. The typical UV coatings containing multifunctional oligomers are solvent free (100% solids), thus helping industry to reduce significantly emissions into the atmosphere and hence protect the environment. Recently, the automotive industry has discovered that UV-cured coatings are very scratch resistant, which stimulated very extensive work into the development of UV coatings for automotive applications. This book reviews in a comprehensive fashion the basics of the technology, describing the decision process from the application requirements over the choice of the appropriate chemistry, the raw materials (resins, diluents, photoinitiators) and formulations employed, to the curing process including network formation as well as the equipment necessary. Not only the advantages but also the drawbacks of this fast emerging technology are discussed together with proposed technical solutions to tackle these disadvantages. The main part will focus extensively on recent developments, like UV powder coatings, dual cure systems, curing under inert conditions or UV plasma curing and new applications, like DVD bonding, ink-jet printing and various automotive applications. These new systems are extending the application areas considerably, for example to the curing of three-dimensional substrates. These progressions will enable applications in the automotive industry, at first on trim parts, then for coating repair and finally in OEM car body coating. The last chapter addresses health, safety and environmental (HSE) aspects, which are of considerable importance not only to the manufacturers and operators, but also to the end users. Furthermore, manufactures, suppliers and end users have to respond to current and emerging chemical regulations, like REACH, VOC- and Food-Contact legislation. The ecological and economical benefits of the UV-curing technology are demonstrated on some examples using the tool of eco-efficiency analysis. v

vi

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . P1. Flash light curing of a falling “UV-curable drop”.

The televised experiment shows impressively the potential of UV-curable coatings. Not only the very fast curing speed, but also the low curing temperature, as well as the solvent free composition of the formulations are the highlights of this technology. UV curing has the potential to replace thermal hardening as the curing technology of the future.

PREFACE

vii

The author appreciates the contributions of several colleagues working in the field of UV technology for providing text, figures, discussions and/or corrections of the manuscript and thanks especially the experts in specific fields for their help in the design and revisal of the corresponding chapters: Rainer Königer (GE Plastics) Heinz-Hilmar Bankowsky (BASF AG) Nick Gruber (BASF AG) Oscar Lafuente (BASF AG) Erich Beck (BASF AG) Rolf Müller (IST-Metz) Bernward Röttgers (Fusion Corp.) Nazire Dogan (AKZO Nobel Coatings) Thomas Fey (DuPont Performance Coatings) Chapter 2: Christian Decker (Mulhouse) and Richard W. Stowe (Fusion Corp.) Chapter 3: Georg Meichsner (FH Esslingen) Chapter 5: Wolfgang Schrof, Klaus Menzel (BASF AG) Chapter 10: Michael Kutschera (BASF AG), Andreas Poppe (BASF Coatings), Bradley Richards (BASF Corp.)

C HAPTER 1

Introduction to Coatings Technology Coatings are found almost anywhere in daily life, the most prominent examples are architectural wall coatings and automotive paints. They are applied in order to provide: • decorative appearance, and/or • protective barrier. The main functions of a coating are thus on the one hand to ensure the desired appearance (colour, gloss) and on the other hand the necessary protection, against corrosion, stone chipping, scratches, abrasion or chemical attack, like red wine, coffee or mustard on furniture coatings or acid rain, tree resins or bird excrements on automotive coatings, as shown in Figure 1.1.

F IG . 1.1. General function of a coating (e.g., automotive). 1

2


Whereas the do-it-yourself architectural coatings are almost all water-based, the vast majority of industrially used coatings, applied in factories on various substrates, like vehicles, furniture, metal cans, paperboards, etc., still contain solvents. The coatings and application spectrum discussed in this book are predominantly based on the industrial coatings sector, which had a share of about 40% of the whole worldwide coatings market (60% architectural).

1.1 COATING MARKETS AND MARKET PROSPECTS The market prospects of future coating technologies in the industrial paint sector are reflecting the environmental concerns about the use of solvents, and hence governed by VOC (volatile organic carbon) regulations. According to these regulations, the market share of solvent-based coatings is declining significantly and the share of alternative, environmentally friendly systems, especially water-based, powder, and radiation curable (UV/electronbeam) coatings is steadily increasing, as depicted in the chart in Figure 1.2 (Paulus, reported at RadTech Conference, Barcelona 2005).1 Further information about market developments in the specific sectors of the coatings industry is available by numerous market research institutes.2 As can be seen from the technology split in Figure 1.2, the predictions of the total amount of solvent-based systems for 2015 are only slightly lower than the amounts in 2003, however, a considerable part of the classical solvent-based systems containing from 50–70% solvents are shifted to “higher solids” systems with up to 80% solids. The reluctance to switch away from solvent-based systems is often related to the

F IG . 1.2. Market development of industrial coatings by technology and region.

INTRODUCTION TO COATINGS TECHNOLOGY

3

excellent properties of such coatings, as well as the ease of handling and the high comfort factor gained over the years of working with solvent-based coatings. A comparison of the most suitable future coating technologies reveals that all alternatives to the classical solvent-borne coatings have specific advantages and drawbacks. “High solids” systems are closest to conventional solvent-based coatings and hence most easily adopted by manufacturers of solvent-borne coatings. However, they still consist of up to 30% solvents and have to be replaced in the long run. Water-based systems are well developed, however, they still lack performance when directly exposed to the environment, mainly due to their sensitivity to humidity, which is a consequence of the use of water compatible groups for solubilizing or dispersing the systems in water. Furthermore drying of water-based systems requires more energy and specially designed drying units. The most environmentally friendly coatings are powder and radiation curable (UV/EB) systems, which are based on 100% solid or liquid formulations. The drawbacks of these coating systems are related to their performance. Due to the interference of melting and film formation with the cross-linking reaction, powder coatings often exhibit an orange peel structure. Radiation curable systems struggle with oxygen inhibition reactions of the radical induced polymerisation, mainly at the surface. Furthermore, UV light absorbing components that are present in the formulation like pigments, additives or UV absorbers can cause through-cure issues. The science and technology of organic coatings, including the chemistry, processing, applications, properties and performance, is described in detail in a number of comprehensive books.3

TABLE 1.1. Comparison of advantages and drawbacks of future coating technologies Coating

Advantages

Drawbacks

High solids

Excellent properties Ease of handling Users are familiar with solvent-based coatings

Still solvent containing Long curing time

Waterborne

Low VOC Wide range of chemistries, properties, application techniques

Weak chemical resistance Difficult (cleanup) to dry Foaming

Powder

100% solids Environmentally nearly ideal

Narrow process window Orange peel structure Expensive Long curing time

UV/EB

100% liquid Low energy consumption Low emission Low capital investment Low space consumption Marginal substrate heating

Higher raw material costs Difficult surface cure (due to oxygen inhibition) Difficult cure of pigmented coatings No cure in shadow areas

4


1.2

UV COATING MARKETS AND MARKET PROSPECTS

Compared with the total resins market for industrial coatings and inks the share of radiation curable products is still small (5 µm) Topics to eliminate weaknesses • Improving adhesion to metal, plastics • Minimizing skin irritation caused by some reactive diluents • Reducing odor (of the formulations) • Reducing extractables of cured coatings • Improving photoinitiators (cost, migration, volatility) • Direct food contact packaging approval While the advantages and good performance characteristics of this technology are very obvious, the reasons for the limited penetration into large volume coating applications must lie in some substantial disadvantages. Major reasons are the limited availability of three-dimensional curing equipment, the very limited use of UV cured coatings in exterior applications, due to the existing paradigm that UV curing would not be possible in the presence of UV exterior durability stabilizers, and higher material costs compared to conventional coatings.

12


One of the major reasons for the almost exclusive application of UV curing in twodimensional curing systems is the fact that the radiant power of the lamp decreases with the square of the distance. Thus, it is difficult to control the effective energy (radiant power arriving at the surface/curing time) necessary to cure the coating sufficiently at every point of a three dimensional substrate. Major advances in the design and radiometric control of three-dimensional curing equipment have already been achieved, but still have to be improved. The development of UV curable coatings for exterior applications had been disregarded in the past, since such coatings have to be stabilized with UV absorbers and radical scavengers (HALS types) in order to provide enough long-term stability. This was due to the preconception, that the UV induced radical polymerization could not be possible in the presence of UV absorbers and radical scavengers. Since this prejudice has been disproved, the whole field of exterior applications opened to UV coatings, which had seemed closed to UV curing forever. Especially the high scratch resistance obtainable with UV cured coatings, as proven in parquet flooring, has attracted the attention of automotive companies, which are looking for coatings which can withstand the typical scratches resulting in car wash units. When thinking about curing of such complex geometries as available in car bodies (doors, hoods) it is getting obvious that solutions have to be found to cure areas within three-dimensional shapes, which are in the shadow of the exposure source. Dual cure coatings for example are developed in order to cure also shadow areas of complex threedimensional objects. They may use as a second functionality, besides the UV curing chemistry, the complete range of available thermal curing chemistries, like isocyanates (in combination with hydroxyl-functional compounds, known as 2 component PU systems) or carbamate groups (curable with melamine chemistry, known as 1 C coatings). The problem of curing in shadow areas has also been tackled from the equipment side, which resulted in the development of UV curing in a plasma chamber or under inert conditions. Further improvements have to be achieved in order to overcome the oxygen inhibition effect, which leaves a tacky surface, unless very intense radiation or other measures are employed. This effect is caused by the high reactivity of oxygen with radical species and the formation of an unreactive peroxy radical, which does not continue the curing chain reaction. Therefore, the cross-linking reaction and the formation of a solid network are retarded until all oxygen is consumed. Since the use of high energy density radiation is undesirable several alternative measures will be discussed. The discussion of tackling the drawbacks of UV technology, the evaluation of structure property relationships, relating mainly to mechanical and scratch resistant properties, constitutes the basis for understanding the advantages of using this technology for exterior, especially for automotive and industrial applications. The economical and ecological benefits of UV curable coatings often appear when the question arises “which coating system should be chosen in order to coat a specific surface area” and different coating alternatives may be considered. This is preferably done by comparing the whole process of coating a substrate from cradle to grave with the ecoefficiency method.


13

1.5 ECO-EFFICIENCY The method of eco-efficiency11 has been developed by BASF and Roland Berger&Partners starting in 1996 as a strategic instrument in order to identify products combining the optimum in the desired application with good environmental performance in conjunction with the lowest costs. It compares economical and ecological advantages and drawbacks across various solutions based on the view of the end-user, for example, for the application of “painting a square meter of furniture”. With this method it is possible to study the complete life-cycle of a product, starting from crude oil as a resource for the raw materials all the way to recycling after use. Besides the ecological factors, like the pollution of the environment or the hazard potential caused by the products, also the economic picture, for example as a cost consideration, is evaluated and the economical and ecological advantages and disadvantages are compared. This list is very valuable in order to be used to identify the decisive influence factor in order to design alternative scenarios and eventually develop more suitable products. The environmental impact is evaluated by five main criteria, the consumption of energy (25% weighting), the consumption of raw materials (25%), the emissions (20%), the toxicity potential (20%) and the risk potential for misuse and hazard (10%). Each of these categories covers a large number of individual criteria, for example the emissions are evaluated as air emissions (50%), water emissions (35%) and soil or wastes (15%), whereof the air emissions are classified by the greenhouse warming potential (50%), the ozone depletion potential (20%), the photochemical ozone creation potential (20%) and the acidification potential. In parallel the economic data are compiled in the form of costs for all alternatives. From the materials and energy flow chart, weak points, driving costs and potentials for cost reductions can be identified. These data are plotted as a two-dimensional graph, known as an eco-efficiency portfolio. This portfolio classifies the different alternatives based on the assumptions put into the evaluation of the data. Several consequences can be derived from this portfolio: products with high degree of eco-efficiency can be sold accordingly in order to improve market share, products with lower eco-efficiency can be optimized depending on the necessity of improvement on the ecological or economical side, or products with low eco-efficiency may be abandoned or substituted by an alternative. For the example of the analysis of several production lines running with different coating technologies to coat wooden door fronts,12 the resulting portfolio is shown in Figure 1.9. For the door front analysis different coating alternatives are compared, an UV formulation based on low-viscous polyether acrylates, an aqueous varnish, a two component polyurethane, an acid curing varnish and a nitro cellulose varnish. From the evaluation of the economical (cost) and ecological data, the UV roller coating shows the best results in the environmentally relevant points, due to lowest raw material and low energy consumption, as well as high productivity and lowest production costs. Further eco-efficiency examples are discussed in Chapter 10. Although the UV technology is already known as an environmentally friendly technology, the eco-efficiency analysis can support the decision of companies to switch over to UV technology, if it is shown that the cost situation is also favorable for an UV solution.

14


F IG . 1.9. Eco-efficiency portfolio for coating of 1000 wooden door fronts.

F IG . 1.10. Driving forces of UV technology.

1.6 DRIVING FORCES OF UV TECHNOLOGY From the discussion of the advantages and drawbacks, as well as the eco-efficiency of the UV technology the driving forces for future developments of this environmentally friendly technology become evident (Figure 1.10). In general the main driving forces are listed below: Performance, exemplarily mentioned are: • High surface quality • Chemical and mechanical resistance • Gloss, scratch and abrasion resistance


15

Economy, exemplarily mentioned are: • Energy and material saving process • Cold cure, no additional heating Ecology, exemplarily mentioned are: • Nearly no VOC’s • Very low emissions after curing • Very low extractables after curing These driving forces result in double digit growth rates in classical applications. Due to the high curing speed of UV polymerization other eco compatible coating systems, besides the classical 100% solids UV coatings, such as water-based, powder and dual cure systems, are also being modified in such a way to be curable by UV polymerization. And ultimately, general new exterior applications and especially automotive usage will contribute to further vital growth. Innovative concepts are introduced which will further trigger the use of UV coatings in three-dimensional curing. UV curing under inert (nitrogen, carbon dioxide) atmosphere has been described long ago; however, the influence on the scratch resistance, especially with test methods relevant to automotive top coats, has been investigated in detail only recently. Furthermore, since radiation curing has mainly been used for two-dimensional industrial substrates, the equipment, conditions and influencing factors of three-dimensional UV curing still has to be developed. The investigation of several influencing factors has triggered the development of 3D UV exposure equipment, which resulted in the establishment of the Larolux® curing process in a carbon dioxide filled pool with conventional tanning lamps, which makes the UV curing process easily accessible to everybody, particularly for craftsmen, like joiners. Another very innovative concept for 3D curing even in shadow areas is realized in the UV plasma processing, which can be visualized by the picture of placing the 3D object into the “enlarged lamp”, as described in Chapter 9. In the whole coatings industry, the introduction of nanomaterials improved the performance of coatings considerably. For instance, the introduction of a thermal curable 2C polyurethane clear coat by PPG (Ceramiclear® ) improved the scratch resistance of 2C PU coatings dramatically. Thus, also a lot of work to incorporate nanoparticles into UV curable systems has been done and will as well be described in Chapter 9.

1.7

CURRENT R&D TOPICS AND TRENDS IN UV TECHNOLOGY

The following spot list (according to Paulus1 and Bankowsky4 ) covers some of the hot topics worked on within raw materials and coating companies. Wood coatings: • Products with less extractables • Products with high scratch resistance • Spray coating of 3 D objects

16


Graphic arts: • Inks and overprint varnishes (OPV’s) with approval for food packaging • Inks and adhesives for DVD’s • Ink jet Plastic coatings: • Adhesion promoters • Adhesives for DVD and flat panel displays Metal coatings: • Products with good adhesion • Products for coil coatings • Weather resistant coatings for 1. Automotive OEM 2. Automotive refinish Most of these new targets require new products, like UV curable waterborne systems or dual cure systems, relying on UV cure and a second independent curing mechanism, for example thermal or oxidative curing. A surely incomplete selection shows what future trends have been identified so far and what developments can be expected in Europe. Trend at raw material suppliers: • Become a solution provider to customer’s problems • Work on segment and customer specific innovations • Active participation in panels to follow the changes in legislation related to health, safety, working hygiene and environment • Globalization of services and standards Trend at coatings manufacturers: • Short lead-time to meet new legislative regulations (VOC, Preparation Directive, etc.) • Reduced numbers of suppliers and raw materials • Transfer formulation development towards the raw material supplier • Provide package of services and problem solutions • Even smaller companies must become global Trend at end-users: • Short lead-time to meet new legislative regulations (VOC, Preparation Directive, etc.) • Reduced numbers of suppliers • Transfer of development activities towards the coating manufacturer

1.8 PERSPECTIVES In order to provide a basis for the understanding of the technology of the new developments and applications, a brief description of the basic process technology, the raw materials and formulation chemistries involved, the network structures obtained, as well as structure– property relationships are given. A more detailed insight into the chemistry, processes,


17

F IG . 1.11. Perspectives of UV curing technology to catch a bigger share of the coatings market.

equipment, formulations and mainly classical applications of radiation curable coatings is given in a number of excellent reviews and books already published.13 The technical perspectives of the UV curing technology leading to the expected increase in market share are drafted in Figure 1.11. The application field will be widened significantly by extending the employment from the classical wood and paper substrates to metal, plastics, leather, minerals etc., from almost exclusively interior and industrial use to exterior and handcraft applications. The development of novel equipment and processes facilitates the curing of three-dimensional shapes. The progression of advanced materials and formulations enable the through-cure of pigmented coatings, the creation of flexible and elastic coatings as well as change the whole coatings process from liquid application to prefabricated shape-able coating films, an approach that may pave the way to completely substitute painting of objects by foil coating, for example wallpapering a car as a vision.

REFERENCES 1. Paulus, W., Status of UV/EB in Europe, RadTech Europe 2005, Barcelona (paper available at the RadTech web site: http://www.radtech-europe.com/download/paperpaulusrteconference2005.pdf). 2. (a) Information Research Limited (IRL), a division of Business Research Group, UK, www.informationsresearch.co.uk; (b) SKEIST Incorp., Whippany, NJ, www.skeistinc.com; (c) Lovett, P.D. & Company, Business planning and market research, www.pdlovett.com/coatingsresearch.html; (d) Kansai Research Institute (KRI, JP); Stanfort Research Institute (SRI, USA). 3. Comprehensive standard works about organic coatings: Wicks Jr., Z.W., Jones, F.N. and Pappas, S.P., “Organic Coatings, Science and Technology”, Vol. 1 and 2. John Wiley & Sons, New York, 1994. Mischke, P., Groteclaes, M. and Brok, T., “European Coatings Handbook”. Vincentz, 2000. Goldschmidt, A. and

18

4. 5. 6. 7. 8. 9. 10. 11.

12. 13.

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS Streiberger, H.J., “BASF Handbook on Basics Coatings Technology”. Vincentz, 2003. Streitberger, H.J. and Kreis, W. (Eds.), “Automotive Paints and Coatings”, 2nd edn. Wiley–VCH, 2006. Bankowsky, H.H., Radiation curing in Europe, RadTech USA, May 2004, Charlotte, Conference Proceedings. 2004, on CD. Zhao, E., Radcure markets: trends across regions, RadTech Asia 2005, Proceedings. 2005, pp. 420–426. Luo, V., RadTech Asia 2005, Proceedings. 2005, pp. 24–25. Ukachi, T., Otaka, T. and Shinohara, N., RadTech Asia 2005, Proceedings. 2005, pp. 26–33. Kiatkamjornwong, S., Phattanarudee, S. and Jiratumnikul, N., RadTech Asia 2005, Proceedings. 2005, pp. 34–39. (a) Shi, W., Jin, Y. and Jin, Y., RadTech Asia 2005, Proceedings. 2005, pp. 40–47. (b) Shi, W. and Jin, Y., RadTech 2006, Conference Procedings, Chicago, IL. 2006, on CD. Come-back of EBC at the RadTech ’05, Coating, 12/2005, pp. 518–520. (a) Overview about the method is available at the web page: http://corporate.basf.com/en/sustainability/ oekoef/fizienz/veroeffentlichungen.htm?id=K*4JH7ixmbcp.l2; (b) Seufert, W., Umwelt 4/5, 3 (2001); and (c) Chem. Daily (2002). Biehler, M., Menzel, K. and Saling, P., UV spectrum, Fusion UV, Winter 2003, www.fusionuv.com/news_ events/spectrum_newsletter. Comprehensive standard works about UV curing: Pappas, S.P. (Ed.), “UV-Curing: Science and Technology”. Technology Marketing Corp., Stanford, 1978. Holman, R. (Ed.), “UV& EB Curing Formulations for Printing Inks, Coatings & Paints”. SITA Technology, London, 1984. Allen, N.S. (Ed.), “Photopolymerisation and Photoimaging Science and Technology”, Elsevier Applied Science, New York, 1989. Fouassier, J.P. and Rabek, J.F. (Eds.), “Radiation curing in Polymer Science and Technology”. Vol. I-IV. Elsevier Applied Science, 1993. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring – Fundamentals and Applications”. Hanser Publishers, New York, 1995. Garett, P.G., “Strahlenhärtung”. Hrsg. U. Zorl, Vincentz, Hannover, 1996. “Chemistry and Technology of UV and EB Formulations for Coatings, Inks and Paints”, In Wiley/SITA Series in Surface Coatings Technology, Vol. I-VI, (Oldring, P.K.T. ed.), John Wiley and Sons, New York, 1991–1996. Mehnert, R., Pincus, A., Janorsky, I., Stowe, R. and Berejka, A., “Chemistry and Technology of UV and EB Formulations for Coatings, Inks, and Paints: UV and EB Curing Technology and Equipment”. John Wiley and Sons, New York, 1998. Davidson, R.S., “Exploring the Science, Technology and Applications of UV and EB-Curing”. SITA Technology Limited, 1999.

C HAPTER 2

The UV Curing Process The UV curing process is predominantly determined by the desired application of the coating. The intended end-product governs the substrate to be coated. This may be an abrasion resistant clear coat for ready-to-install parquet or an overprint varnish for paper cards, a coloured base coat and a clear coat for plastic automotive parts or metal coils, as well as a flexible protective coat for window frames. The function of the coating, for instance the colouration of the part, the protection against corrosion, scratching, chemical attack or against weathering deterioration, determines the type and property requirements of the coating as well as the thickness required. The targeted properties, like high gloss appearance, abrasion resistance, colour effects, hardness, flexibility, resistance against chemicals or scratches, have to be provided by the chemical formulation, consisting of base resins, diluents, photoinitiators and various additives. Furthermore, an appropriate selection of the components has to be done in order to enable an effective curing process; for instance, in coatings containing pigments or UV light stabilizers, the spectral absorbance of the photoinitiator has to be adjusted to a spectral region where the pigments or UV absorbers are fairly transparent. This fine tuning is necessary to match the characteristics of the lamp system with the chemistry of the coating to provide an economic curing process. Besides the physical properties of the cured material to be obtained, the economics of the coating process is the most important variable which decides over the type of coating used. Thus, in order to calculate the total costs of a coating process, not only materials costs but the whole process design and the equipment set-up have to be considered in order to compare different coating processes with each other. UV curable coatings are always in competition with thermally curable systems of the classical solvent-type, water-based or powder coatings. Some economic factors of UV curing have been discussed by Stowe,1 with cost examples for ink, coating and adhesive applications in comparison with thermal hardening, if applicable. UV curing in general offers a number of advantages over competitive coatings, while some can be related to costs, others relate to performance, environmentally compliance or processes not achievable with other methods. However, no general comparison of process economics can be made; it has to be done rather in a case to case study. Thus, the UV curing process relies crucially on an efficient cogging of the required application properties with the chemistry chosen to fulfill the performance requirements as 19

20


F IG . 2.1. Interaction of UV process design parameters.

well as the UV curing equipment applied to provide a fast and complete cure in order to meet the economical and ecological aspects of coating technology (Figure 2.1). UV curing in its basics is a fast, room temperature curing process indicating low energy consumption and requiring little space for the equipment.

2.1 APPLICATION (PROPERTY REQUIREMENTS) From the application point of view, UV curable coatings are mainly used in such industrial applications where thermal curing is hardly possible, like curing of coatings on temperature sensitive substrates, like wood, paper and plastics, and in imaging applications, where only selected areas should be polymerized, like in polymer printing plates and photoresists. Specific applications of photocurable coatings are clear coats for parquet, furniture, vinyl flooring, on plastic substrates (skies, boards), compact discs, headlight lenses, overprint varnishes (posters, high gloss packaging), adhesives, protective coatings for optical fibres, electronic parts. Applications of photocurable coatings on metals (automotive, coil coating) and exterior uses are just emerging. These applications cover a large range of properties. The function of the coating and the desired end product properties determine the chemistry to be used in order to fulfill the application requirements. For instance, for overprint varnishes (OVP) on book covers, art prints, post cards, photos, etc. used to protect the printed image and increase the appearance by a high gloss finish, the OVP layer thickness is in the range of 8–10 µm, cured at a speed of about 60–80 m/min. Such a clear coat of moderate thickness can be composed of standard acrylate resins and diluents; merely the photoinitiator system has to be selected to comply with the high curing speed. The curing

THE UV CURING PROCESS

21

F IG . 2.2. Interaction of UV process parameters.

can be performed with a standard mercury lamp set-up. UV coatings on wood, for example, fibre boards, plywood or veneer, have different functions. UV primer may have to stabilize the wooden support and ensure adhesion. This layer is also very thin and will be cured with standard equipment. If a wooden décor, however, is printed on top of the primer or printed on a foil and laminated onto the particle board, this décor layer will contain pigments and therefore the photoinitiator has to be chosen to absorb at longer wavelength in order to match with a transparent area of the pigment. In this case, a lamp system should be chosen, which also has significant emissions in the longer wavelength range. The same applies if a clear coat is used for exterior applications, for instance a clear coat for polycarbonate headlamps. Here a UV light absorber has to be used, which also absorbs at least in the UV-B range. UV printing inks are used for example in the offset, flexographic and gravure printing process at a layer thickness in the range of up to 2 µm. Since these inks are highly pigmented, even at the relatively low thickness, the through-cure is often difficult to achieve. These few examples have been selected to demonstrate, that the required properties of the coating determine the chemistry to be used, and that photochemistry and the exposure equipment then have to be adjusted to achieve the target properties in an efficient cure process (Figure 2.2).

2.2 CHEMISTRY (PHOTOCHEMICAL PROCESS) This section describes briefly the principles of the chemistry and kinetics involved in the UV curing process.

22 2.2.1


Photoinduced Curing Chemistry

Photoinduced curing can be realized as in the preparation of conventional linear polymers by a step like process, as used in polyaddition and polycondensation reactions or by a chain process occurring in polymerization reactions (Figure 2.3). The photoinduced polyaddition technology has been for a long time the workhorse of photoresist technology,2 for example, the crosslinking of resins was achieved by photoinduced dimerization of cinnamates. This photodimerization is an example of a direct photoreaction where every step of polymer built-up is initiated by an absorbed photon, thus every single reaction step is dependent on the quantum yield of the photoreaction (generally very much smaller than 1). On the contrary, in polymerization reactions induced by light only the initiating step is dependent on the photoreaction (Φ < 1). The photopolymerization reaction then is a chain reaction, where one produced initiator radical can add up to several thousand monomer units, thus the overall quantum yield of the total reaction is much bigger than 1. Whereas the photoinduced radical polymerization is now the mainstream technology, the photoinduced ionic curing reactions are not so well explored and developed, mainly due to the lack of easily available photoinitiators. In recent years, considerable progress has been made in the development of new cationic photoinitiators,3,4 however, there are only a few anionic type photoinitiators described.5 The basic principles of curing and network formation are similar in radical and cationic induced curing. The cationic curing has its main advantages in the oxygen insensitive curing and in the good adhesion mainly to metals achieved with the cationic curable epoxy systems. The cationic curing will be described briefly in the section of the raw materials. The main focus will be placed on the photoinduced radical polymerization. The UV curing technology is based on the photoinitiated rapid transformation of a reactive liquid formulation into a solid coating film. The initiating species may be a cation, an anion or a radical. The vast majority of UV curable coatings are based on radical producing photoinitiators. The main components of such formulations based on radical polymeriza-

F IG . 2.3. Possibilities of photoinduced curing.


23

tions are: • Reactive resins containing a plurality of polymerizable double bonds, which govern mainly the desired properties of the final coating; • Copolymerizable, monomeric diluents, which are responsible for the reduction or adjustment of the viscosity of the formulation, a function taken by the solvent in conventional formulations; • Photoinitiators or a photoinitiating system containing photoinitiator and photosensibilizer or coinitiators; and, if necessary, other coating additives, like surface active additives, slip additives, fillers, pigments, light stabilizers, etc. The chemistry involved in the radical initiated UV induced crosslinking can be divided into the three steps, initiation, propagation and termination. Although the UV energy applied in photocuring may cleave C–C and C–H bonds, the commonly used monomers do not produce sufficient amounts of initiating species, which is due to low absorbance and poor cleavage efficiency. Thus, a special photoinitiator is usually applied, which is excited and ultimately yields via intersystem crossing, accompanied by various deactivation reactions, the formation of a radical species, which can initiate radical polymerization. The following polymerization reaction follows almost exactly the rules of conventional radical polymerization. Thus, only the initiation step is different to thermal initiated radical polymerization. The basic principles of photoinitiation, photopolymerization and photocuring are described in detail in a book edited by Fouassier.6 The light absorption and the following processes are commonly pictured in a Jablonski diagram (Figure 2.4). The process starts with the absorption of a photon by the photoinitiator molecule, which results in excitation of an electron into higher singlet states. From

F IG . 2.4. Jablonsky-type diagram for photoinduced radical photoinitiation.

24


F IG . 2.5. Photoinitiator types.

these excited states, various processes can follow. First, deactivation can proceed by radiationless internal conversion and evolution of heat back to the ground state or by emission of fluorescence. Second, by intersystem crossing (ISC) an electron spin inversion leads to the excited triplet state. The photochemical processes which lead to the desired active species (e.g., free radicals) often take place from the excited triplet state, where the molecule posses two unpaired electrons, rather than from the singlet state. The formation of the reactive species, namely free radicals, competes with further deactivation processes, like monomer quenching, oxygen quenching and phosphorescence. The direct oxygen quenching of the photoinitiator excited states is not very likely in the case of the extremely shortlived triplet states of α-cleavable type photoinitiators, but much more pronounced in the hydrogen abstraction type owing to the relatively long-lived triplet states.7 From the triplet state two main reactions can lead to initiating species, the intramolecular scission of an α-bond, or the intermolecular abstraction of a hydrogen atom. The intramolecular scission is the most effective process in the formation of radicals, since the hydrogen abstraction is a bimolecular type reaction, which is diffusion controlled and may be accompanied by several deactivation reactions. The quantum yield of initiation, representing the number of growing chains per photon absorbed reflects the importance of the processes leading to initiation over all the indicated processes of deactivation. The efficiency of the photoinitiation is a function of different quantum yields, since several side reactions can occur in every step. Thus, the overall yield of initiation is a complex function of different quantum yields, represented exemplarily in Figure 2.4. Two


25

F IG . 2.6. Propagation and transfer.

F IG . 2.7. Termination reaction.

examples of photoinitators, an alpha type scission initiator and a hydrogen abstraction type photoinitiator are shown in Figure 2.5. The chemical structures of photoinitiators are discussed in more detail in the raw materials section (Chapter 4, photoinitiators). Furthermore, photoinitiator chemistry, the formation of the excited states, and the reactivity of radical and cationic photoinitiators are very well described in the literature (Fouassier, ref. 8, Chapters 3 and 4). Propagation (Figure 2.6) is the key step to very efficient curing, since it is a chain reaction where for instance one produced radical can add more than 1000 monomer units within a fraction of a second. The steps after the initiation are very similar to the normal radical polymerization of monofunctional monomers, which are widely used to synthesize thermoplastic polymers, like polyethylenes, polypropylene or polystyrenes. The main difference in coating systems is the use of multifunctional monomers or oligomers, which

26


leads to the formation of networks. In the propagation reaction transfer reactions also often play a significant role, where the growing radical chain does not add to another monomer unit, but abstracts hydrogen radical from a neighbouring R–H group. The remaining Rradical can then start another growing chain, thus leading to the termination of the growing polymer chain, but not to the termination of the chain reaction. The reaction of the radicals with oxygen does not play a significant role in the polymerizations of linear polymers, since they are normally conducted under inert conditions. However, the curing of coatings is normally performed under atmospheric conditions, thus, the oxygen interference plays a major role. The termination reactions are also manifold (Figure 2.7). Besides the termination with an initiator radical, several other termination reactions play a role, especially the recombination of growing radical species or elimination reaction of the chain end.

2.2.2

Kinetics

indexKinetics Conventional UV curing lines are operating at curing speeds of 5 m/min to 100 m/min, corresponding to a residence time of roughly a fraction of a second under a typical lamp setup. Thus, the polymerization kinetics has to be very fast. Why are these reactions so fast, while typical radical polymerizations of linear homopolymers take hours? The kinetics of the photopolymerization of multifunctional monomers is in principle similar to the classical radical polymerization of monomers. For those, the kinetics has been well investigated and for a vast number of classical monomers used in the synthesis of linear homopolymerization, kinetic constants have been determined.9 However, the kinetics and mechanism of the curing of multifunctional monomers is much more complex due to different basic conditions. For instance, the number of initiating radicals produced per second is much higher in photopolymerizing systems, having pronounced effects on the rate of polymerization and termination. The mobility of the active species changes dramatically while the medium turns from a liquid over to a viscoelastic rubber, and, finally, to a glassy solid, and the formation of microgels at an early stage has considerable influences on the structural composition, the local heat evolution, volume shrinkage and/or radical trapping reactions. Therefore it is not advisable to use published kinetic constants of monomers, despite multifunctional monomers being structurally similar, in order to estimate the photopolymerization kinetics of coatings. Rather than taking data from literature compiled under various conditions, it is better to compare the different systems under the conditions to be used, in the case of UV curable coatings, for example in a thin film and irradiation at room temperature. A lot of kinetic studies have been done by Decker and coworkers.10 They have followed the conversion of the reactive groups, for example the acrylate double bonds, for a number of resins and reactive diluents used in UV curing with real-time IR spectroscopy (RT-IR) (see 2.2.5.1), and determined the kinetic constants under UV curing process conditions. In the polymer growth reaction (−MA∗ is a radical species), kp =kAA

P−MA∗ + MA → P−MA −MA∗ ,

(2.2.1)


27

the rate of polymerization (rp ) is calculated as follows: rp = kp /(2kt )0.5 [M0 ]ri0.5 ,

(2.2.2)

where kp is the rate constant of polymerization, kt is the rate constant of termination, [M0 ] is the monomer concentration, and ri is the initiation rate. In general, the high propagation rate constants of acrylates (kp ≈ 104 l mol−1 s−1 ) together with a relatively slow termination process are the reason why the curing of acrylate based UV curable coatings proceeds so fast.11 The photopolymerization reactions proceed within some seconds, and from the rate constants Decker has calculated the average time for the addition of one monomer unit to the growing polymer chain being about 20 µs. The propagation reaction drops slightly as the polymer network is formed, whereas the termination reaction drops rapidly due to reaction diffusion control of the termination step. Besides C. Decker (University of Mulhouse, France), kinetic studies of UV curable systems have been a major research topic in the research group of C. Bowman (University of Colorado, US). Some of the most important publications are dealing with the reaction diffusion during the photopolymerization, the volume relaxation and mechanistic studies of Iniferter (initiation and transfer) photopolymerizations.12 Recent studies are also dealing with modelling, for instance the effects of chain length on the termination kinetics in crosslinking photopolymerization.13 In this work a model has been developed which allows the prediction of multivinyl free-radical photopolymerization kinetics, incorporating termination reactions dependent on the chain lengths (at low conversions) and diffusion controlled termination (at high conversions). The rate of initiation (ri ) is defined by the quantum yield of initiation (Φi ) and the irradiance absorbed (Ia ): ri = Φi Ia , and is directly related to the photoinitiator concentration [PI]: ri = Φi I0 1 − e−2.3[PI]ε A/V ,

(2.2.3)

(2.2.4)

where I0 is the incident irradiance, ε is the PI extinction coefficient, l is the sample thickness, A is the exposed area and V is the exposure volume. Addition rate constants of some of the most important photoinitiator radicals to acrylate and methacrylate monomers have been compiled recently14 and evaluated as a function of the polymerization medium. The benzoyl radical, produced in alpha type photoinitiators adds with a relatively low rate constant to the (meth)acrylate monomers (in the order of 105 M−1 s−1 ), whereas the second radicals formed in the cleavage reaction (aliphatic ketyl radicals) add with a much higher rate constant in the order of about 107 M−1 s−1 to the monomers. The kinetic chain length (kcl) in crosslinking reactions have to be shown as considerably high in spite of the high rate of initiation, due to the high number of radicals produced during intense illumination and to the short exposure. The kinetic chain length can be expressed as: kcl = rp /ri .

(2.2.5)

28


In an example presented in the paper of Decker,10 the kinetic chain length of a formulation based on Lucirin® TPO as photoinitiator, has been determined to be a value of 7700 mol per radical. This high value surprises to some extent, since the high exposure energies used in UV illumination should increase the rate of initiation (2.2.3), which would consequently, decrease kcl (2.2.5). The long kinetic chain length observed in UV curable acrylate based coatings, however, is considerably attributed to the high monomer concentrations used (in a typical formulation in the order of [M0 ] = 3 mol/l). In the Lucirin based formulation 1 wt% of photoinitiator was used, thus, each photoinitiator molecule is surrounded by about 100 acrylate double bonds. After a short exposure time, about 1% of the initiator is destroyed (forming two radicals), thus resulting in a situation where each radical produced is surrounded by 5000 acrylate bonds. Finally about 10,000 double bonds were found to be polymerized by every produced radical. 2.2.2.1 Influence of the irradiance The effect of irradiance (mW/cm2 ) (often misleading termed light intensity (I0 )) on the polymerization kinetics has been described in several studies.15 The increase of irradiance has been shown not only to increase the rate of polymerization, but also the conversion of double bonds, thus the extent of cure is much higher. This effect has been explained by the increase of the sample temperature at higher irradiance, due to higher heat production of the lamp itself as well as to higher heat of polymerization produced within the short polymerization time, leading to higher mobility and thus higher conversion. 2.2.2.2 Copolymerization The curing kinetics is also dependent on the type of reactive groups used. The polymerization rate of acrylates is for instance considerably higher than for methacrylates, since the α-methyl group stabilizes the radical. Data found in the Polymer Handbook16 for the thermal initiated polymerization roughly give a difference of a factor of 5 in the polymerization rate at room temperature. However note that the termination reactions are also faster with acrylates: Methyl methacrylate

kp = 310 l/(mol s) (25 ◦ C)

kt = 17 × 10−6 l/(mol s)

Methyl acrylate

kp = 1580 l/(mol s) (25 ◦ C)

kt = 55 × 10−6 l/(mol s)

Similar results, obtained by photocalorimetric measurements,17 have been published for the photoinitiated polymerizations of acrylates and methacrylates (I = 5 mW/cm2 , 4 wt% PI):

TetrahydrofurfurylTrimethylolpropane tri-

Acrylate kp (l/min)

Methacrylate kp (l/min)

1.94 6.41

0.54 1.55

In homopolymerizations, these differences result in a higher polymerization rate of acrylates compared to methacrylates. If different monomer types are used, like acrylates and

29


methacrylates (for example, when using methacrylates as reactive diluents), the rules of the classical copolymerization apply. In a simple copolymerization model, it is assumed, that the reactivity of an active center only depends upon the monomer unit in the chain where the active center is located. The kinetic scheme is shown in Figure 2.8. These reactivity ratios define the relative reactivities of the growing radical center towards the comonomers. Reactivity ratios of comonomers have been determined for the classical copolymerization and several values can be extracted from the Polymer Handbook. Examples are presented in the tabular form below: Monomer A

rA

Monomer B

rB

Acrylic acid Acrylic acid butyl ester Acrylic acid butyl ester Acrylic acid ethylhexylester Acrylic acid hydroxybutylester Acrylic acid methylester

0.42 0.15 3.07 0.26 0.73 0.35

Acrylic acid, methyl ester Styrene Vinyl acetate Styrene Styrene Methylmethacrylate

0.98 0.8 0.06 0.94 0.72 1.8

The reactivity ratios of the methyl esters of acrylates and methacrylates in a copolymerization clearly show, that the acrylate radical as well as the methylacrylate radical react faster with the methacrylate monomer than with the acrylate monomer, because of the formation of the most stabilized (methyl acrylate) radical. Thus, in a copolymerization, every formed radical reacts faster with methacrylates than with an acrylate monomer. However, at least the further addition of another methacrylate proceeds with the slower polymerization rate of methacrylates, thus often leading to an overall reduced rate. Furthermore, this behaviour leads to polymer structures, which should be more block like rather than statistical.

F IG . 2.8. Propagation possibilities in copolymerization and reactivity ratios of different monomer units.

30


F IG . 2.9. Comparison of acrylate to methacrylate conversion in photoinitiated homopolymerization of HEA and HEMA, respectively.

In Figure 2.9 the polymerization behaviour of acrylates and methacrylates in air and inert atmosphere (CO2 ) is shown.18 The homopolymerization of Hydroxyethyl methacrylate (HEMA) did not occur at all in the presence of air in a 10 µm thick film at low irradiance (30 mW/cm2 ), and only about 10% of the Hydroxyethyl acrylate (HEA) is polymerized under air. This is due to the oxygen inhibition reaction, discussed in more detail in Chapter 7. The great differences in reactivity of acrylates and methacrylates can be revealed when exposed under inert conditions, for example, under carbon dioxide. These monofunctional acrylates or methacrylates are generally used as reactive diluents in oligomeric acrylate resins. The kinetics of the reaction of urethane acrylate resins in the presence of 30% of the HEA and HEMA, respectively, is shown in Figure 2.10. In the inert atmosphere, again, the differences can be shown clearly. The formulation containing the acrylate (HEA) diluent polymerizes readily within 1.5 s to a conversion plateau of about 95%. When the methacrylate (HEMA) is used as a reactive diluent, the polymerization is significantly retarded and is similar to the polymerization behaviour of the pure HEMA. After most of the HEMA reactive diluent is consumed, the polymerization rate increases significantly. This example demonstrates that the curing behaviour is dependent on the type of the functional groups used. In radical based UV curing, the number of different functional groups used is limited. Still, unsaturated polyesters, based on maleic acid as the unsaturated group, rely on the use of styrene as a diluent. Due to toxicological reasons, the use of styrene is rapidly decreasing. Styrene has been substituted by acrylates, even in systems based on unsaturated polyesters. Since the vast majority of resins are acrylate functionalized, the preferred reactive diluents are also acrylates (tri- or dipropyleneglycol diacrylate, hexandiol diacrylate, trimethylolpropane triacrylate). Since these low molecular weight acrylates are eye and skin irritating, higher molecular weight acrylates as well as other functionalities have been


31

F IG . 2.10. UV curing of UA acrylate resin-acrylate diluent (HEA) and UA acrylate resin-methacrylate (HEMA) diluent.

tested. The methacrylates are less irritating, however, the kinetics of the copolymerization with the acrylate resins has to be considered. There are also other functional groups besides acrylates and methacrylates employed in UV curing systems. Very effective in the reduction of the formulation viscosity are vinyl ether (VE) monomers,19 which at the same content are more effective in viscosity reduction of, for example, an epoxy acrylate resin than TPGDA (tripropylene glycol diacrylate). The kinetics of the radical-induced copolymerization of divinyl ethers with acrylate resins has been evaluated by Decker.20 In acrylate/vinylether systems the copolymerization behaviour is much different than in acrylate/acrylate or acrylate/methacrylate systems, since the vinyl ethers do not homopolymerize radically. Here, the reactivity ratio rA = kAA /kAB was found in the order of RA = 1.8, indicating, that an acrylate radical is about twice as reactive towards an acrylate double bond than towards a vinyl ether bond.21 Depending on the monomer feed ratio, unreacted vinyl ether bonds may be present in the coating, since no homopolymerization occurs. In order to get close to 100% conversion of the vinyl ether groups, the vinyl ether content should not exceed about 10% of the resin composition with acrylates as copolymerization partners and about 50% in unsaturated polyesters (ref. 19). Vinyl ethers also undergo a radical copolymerization with monomers having electron poor double bonds, like maleimides and unsaturated esters (e.g., maleates or fumarates).22 Such monomers also do not homopolymerize either, but polymerize radically in an alternating fashion with high efficiency if the stoichiometric mixture based on the double bond content was kept.23 The polymerization of the VE–maleimide or maleate systems proceeds through a donor– acceptor mechanism, which is responsible for a less oxygen inhibited polymerization behaviour,24 but still present as shown by Dias.25 The charge transfer complex furthermore can directly be photolyzed to an exciplex (VE-MA)∗ , which initiates the polymerization

32


without the use of a photoinitiator. The polymerization, however, proceeds much slower than in the presence of an additional photoinitiator since the absorbance of the donor– acceptor complex is very low. The absorbance of the donor–acceptor complex is higher with systems based on vinyl ether–maleimide monomers, resulting in a more efficient copolymerization in such photoinitiator free systems.26 This is mainly due to the higher absorbance of the maleimide monomer as well as the strong hydrogen abstractability of the excited maleimide, which is considerably higher than benzophenone. The maleimides thus can act simultaneously as a photoinitiator and as a (co)monomer. If the curing is performed under inert atmosphere (nitrogen) and readily abstractable hydrogens are present, like aliphatic maleimide structures with a methylene group next to the imide-nitrogen and ideally with even better abstractable hydrogens, such systems can be competitive to conventional hydrogen abstraction type systems.

2.2.3

Frontal Polymerization

Because of the limited penetration of UV light into a coating, UV polymerization is mainly used in thin film (up to 200 µm) applications. The term frontal polymerization27 refers to a polymerization process, where the UV light is absorbed by the photoinitiators at the surface layers of UV curable coatings and upon bleaching the photopolymerization front moves into the deeper layers of the coating. Up to 2 mm thick samples have been cured by such frontal polymerizations in a system containing 5 wt% triarylacylphosphine oxides as strongly absorbing photoinitiators and polyurethane acrylates as resins.28 Further systems described therein are based on the use of N-substituted maleimides as photobleachable components in photoinitiator-free coatings, and interpenetrating networks (IPN) based on the mixture of radically (acrylates) and cationically (epoxides) polymerizing systems. These polymerizations can be initiated simultaneously or consecutively, whereby the polymerizing front velocity is directly controlled by the bleaching rate of the corresponding photoinitators. Photoinitiated frontal polymerisation can for example be used for the repair of holes in many materials or the curing of composites.29

2.2.4

Heat of Polymerization

For a polymerization to occur, the (Gibbs) free energy difference between the monomer and the polymer must be negative G = H − T S. For a polymerization process, the gain in enthalpy (H ) caused by a conversion of a π to a sigma bond is a highly exothermic reaction (–34 to –160 kJ/mol) and dominates the loss of translational entropy, when monomers are bound to chains. The entropy loss is in the range of –100 J/mol, thus at normal temperatures, the enthalpy term will be the dominating and favouring the polymerization.


33

Thus, the curing process of acrylates and methacrylates is exothermic and the change in enthalpy for acrylates and methacrylates is typically in the range of minus 50–90 kJ/mol. In general, the most important factors for the enthalpy gains are the resonance stabilization of the monomer double bonds and the steric strains in the polymer formed. Thus, the better the resonance stabilization of the monomer, the less exothermic the reaction (for example, α-methyl styrene exhibits only a low heat of polymerization of 35 kJ/mol), on the other hand, tetrafluorethylene produces a polymer with little steric strain, thus exhibiting a heat of polymerization of 155 kJ/mol.30 In acrylate based photopolymerizing coatings this exothermic reaction can increase the sample temperature cured at room temperature up to 90 ◦ C within a fraction of a second. The rate at which the temperature rises was found to correlate well with the rate of polymerization.31 Recording the maximum temperatures reached upon curing of a sample of a urethane diacrylate under different light intensities: • 10 mW/cm2 : 40 ◦ C at final conversion of 80% • 40 mW/cm2 : 65 ◦ C at final conversion of 90% • 80 mW/cm2 : 90 ◦ C at final conversion of 95% shows, that the final conversion is dependent on the maximum temperature reached for a UV cured sample. At the same energy (energy = irradiance × exposure time, in the past often expressed as dose = light intensity × exposure time), the final conversion increases with increasing light intensity, since the heat of polymerization is released in a much shorter time, thus resulting in the higher sample temperature. The temperature profiles have been recorded by Decker with IR spectroscopy, using a temperature sensitive IR band of poly (propylene) substrates at 842 cm−1 .32

2.2.5 Evaluation of Cure Extent 2.2.5.1 Real-time infrared (RT-IR) spectroscopy As mentioned before, the cure conversion can be measured with infrared spectroscopy, either classically by measuring the IR spectrum before and after cure, or with a method developed by Decker, the real-time FTIR,33 following the course of acrylate double bond conversion during UV polymerization (Figure 2.11). With this method, the temporal reaction conversion could be followed in the millisecond range by the decreasing absorption of the acryl ester double bonds at 810, 1410 or 1636 cm−1 , while simultaneously curing with UV light. Important information on the polymerization rate, content of the unreacted double bonds, inhibition by oxygen, and so on can be gained with this method. The decreasing absorption for example of the 1410 cm−1 band can be directly converted to conversion of the acrylate bands as a function of exposure time, as shown in Figure 2.12. The rate of polymerization (Rp ) can easily be obtained from the slope of the conversion versus exposure time curve and the initial monomer concentration Rp = [M0 ]

dx . dt

(2.2.6)

34


F IG . 2.11. Set-up of real-time Fourier transform-infrared spectroscopy (RT-FTIR).

F IG . 2.12. Transfer of RT-IR spectra to acrylate double bond conversion.

Furthermore, the final conversion can be extracted from the conversion–time curve and the residual remaining unreacted double bonds calculated. The IR measurement gives a good correlation with the total double bond conversion of the coating layer, but no information about the cure extent throughout the film thickness can be deduced. Thus, another technique has been developed, which allows the measurement of acrylate double bonds throughout the film thickness, Confocal Raman Microscopy. 2.2.5.2 Confocal Raman Microscopy Confocal Raman Microscopy combines the chemical information from vibrational spectroscopy with the spatial resolution of confocal microscopy.34 The spatial resolution of this technique is in the range of 1 µm3 . This allows a mapping of the surface as well as a depth profiling of, for example, the remaining acrylate double bonds. The set-up of the Raman microscope is shown in Figure 2.13.


35

F IG . 2.13. Confocal Raman spectroscopy set-up.

For example, Raman microscope LABRAM (Dilor company) can be equipped with a He– Ne laser (632.8 nm, 10 mW power at the surface). The Confocal Raman Spectroscopy has been used for evaluating the oxygen inhibition effect (see Figure 7.2, Chapter 7), the light attenuation effect described by Lambert–Beer’s law, the cure extent within pigmented coatings, or the effects of additives on the curing conversion (UV absorbers, HALS radical scavengers, etc.).35 Raman spectra mapping of a coating into the deeper layers with one micron steps is shown exemplarily in Figure 2.14. The indicated peaks are attributed to acrylate double bond vibrations. As a function of the depth of the cured coating the remaining double bonds are diminishing with cure depth. Thus, real-time IR spectroscopy and confocal Raman microscopy are two complementary non-destructive techniques useful for the characterization of UV curable and cured coatings. Whereas the RT-IR gives information about the conversion development during curing and the residual double bonds over the whole film thickness, the confocal Raman spectroscopy is able to give a detailed picture about the spatial distribution of residual double bonds within the coating depth as well as a surface mapping. Thus, IR and Confocal Raman Spectroscopy are complementary techniques applied to gain information about conversion, residual monomer content, oxygen inhibition effects and so on (Figure 2.15). 2.2.5.3 In-line monitoring by near-infrared (NIR) spectroscopy The in-line monitoring in process control of various reactions including the radical polymerization with nearinfrared spectroscopy has gained widespread use.36 Since the extinction coefficients in the near-infrared are relatively low, the NIR spectroscopy measurements are usually performed with samples having layer thicknesses in the mm range. Due to calibration of the NIR spec-

36


F IG . 2.14. Depth profile of a UV cured polyether acrylate by confocal Raman microscopy.

F IG . 2.15. Characterization of cure extent by IR and confocal Raman.

troscopy with either FTIR spectroscopy or specific properties of the coating, like hardness, it has been shown that NIR spectroscopy can be used as an effective tool for monitoring the conversion of acrylate double bonds directly in the running UV curing line.37 2.2.5.4 Photo DSC The differential scanning calorimetry (DSC) is the most widely used technique to study photocuring reactions (Figure 2.16).38 It is very well suited for the determination of kinetic parameters, like enthalpy, degree of conversion, rate constants, Arrhenius parameters, etc., however, since the DSC has relatively long response times (2 s),


37

F IG . 2.16. Differential photocalorimetry set-up.

the monitoring of fast UV polymerizations is less accurate, unless low irradiance exposures are chosen.

2.3 EQUIPMENT (FOR UV CURING PROCESS) 2.3.1 Coating Application Processes The application of the UV coating to the substrate is usually done in automated processes. There are several application processes in operation, which are very well described in the literature39 : 1. 2. 3. 4.

Roll coaters and curtain coaters, used for flat-panel production; Airless or conventional spray guns, used for three-dimensional or shaped objects; Vacuum coaters; Electrostatic application.

The typical UV formulations are in a viscosity range (380 nm) and power in the UV region. This leads to difficulties in the curing of some materials, especially in the cationic curing, since the standard cationic photoinitiators all have absorption bands below 300 nm. The technical feasibility of LED’s has been evaluated by comparing the properties of a urethane acrylate adhesives cured with an LED light vs. a multiple wavelength mercury arc

48


F IG . 2.22. Reflector geometries.

lamps, showing that similar properties can be obtained. Curing studies with epoxies, on the other hand, illustrate that curing with LED systems is limited by the absorption of common cationic photoinitiator systems. Current LEDs are commercially available at wavelengths of 400 nm or higher. In order to match the spectral emission of the LED light, the cationic photoinitiators may have to be modified.51 UV LED’s are, however, still in the development stage and have not yet developed high power or shorter wavelength sources. 2.3.3.2 Reflectors Since a UV lamp emits in all directions, a reflector is used to direct the light emitted in the “wrong direction” onto the surface of the coating. Well designed reflectors collect and project about 75% of the radiant energy from the bulb to the surface, whereas parabolic reflectors collect no more than 55% (Figure 2.22). The material of the reflector is mainly aluminium. Aluminium, however, also reflects IR, which may lead to unwanted heat up of the surface. Therefore, reflectors have been developed, which also reflect UV radiation very well, but not visible and IR radiation. Furthermore, the geometry of the reflector is responsible for the light distribution at the surface, whether homogeneously distributed or well focused.

2.3.4

Exposure Parameter Measurement

Why measure the exposure parameters? When a UV cured coating has been developed to exhibit satisfactory performance characteristics with a specific equipment setup, the operator often does not know what UV exposure conditions are required to obtain this cure. Furthermore, it is difficult to define the necessary conditions for repeating the process later or even at a different curing line. For practical reasons it is important to measure and compare the total energy density applied on the coating for the setup of an exposure unit. However, since a lot of other


49

equipment specific parameters may not be the same, this may not lead to exactly the same curing results, assessed by curing extent and coating performance. Reasons for such deviations may be that the wavelength spectrum of the lamp, which in conjunction with the absorbance characteristics of the coating determines the rate of polymerization, can be different. Second, the configuration of the unit determines the distance to the coating and thus the heat distribution at the coating surface. Furthermore it is important how the energy is measured, since the tools are registering an integral spectral distribution. Thus, the identification of the key exposure parameters, which have the most significant effect on the performance of the final coating is most important. These parameters are the total energy, irradiance profile, the spectral distribution and the temperature of the coating by infrared energy. Infrared (IR) energy is primarily emitted by the quartz envelope of the UV source, but also absorbance of UV and visible light and any exotherm of the curing reaction contribute to the temperature profile of the coating. Thus, it is important for process design to measure these key parameters needed for the UV curing process. For process control a reduced measurement package may be sufficient. How to measure process relevance? The challenge of measuring the UV energy is not the energy measurement per se, but the correlation of the measured values with the performance of the UV curing reaction, since the measured energy only represents the energy density applied at the surface. The interaction with the photoactive material and the curing process occurring is not reflected. Thus, as mentioned before, the radiometric measurement must be relevant to the process, thus, for example, if the photoinitiator absorption is in the range below 300 nm, the measurement of the radiant power in the UVA range of 315 to 400 nm would not be very relevant. In Figure 2.23 the situation is elucidated and for the purpose of explanation divided into 3 stages, which are related to the optimization of the effective energy arriving at the surface, the interaction of this energy with the photoactive components in the coating and the control and monitoring of the optimized process. The design of the process is related to an excellent correlation of the desired cure result with the employed UV energy. The difficulty related to the control and monitoring of the process is due to the fact, that the cure extent can not be monitored easily, but instead the radiometric measurements are measuring only the energy density arriving at the surface. Thus, it is assumed that the same energy density arriving at the surface results in the same cure extent. Therefore, in the design of the process the “process window” 52 also has to be defined, in order to determine which variations in the energy density still give a satisfactory curing result and at which extent of deviations adjustments have to be done. In the process design, the choice of the appropriate UV lamp type is dependant on the matching of the photoinitiator (PI) absorption and lamp spectrum. After the choice of the proper lamp type, the effective energy arriving at the surface can be optimized by several factors, like reflector design, irradiance, or peak irradiance. These factors have to be correlated with the efficiency of curing. Thus, the photoinitiator type and content, the absorption characteristics of interfering additives, like pigments or UV absorbers, the cure speed and other factors have to be correlated with the exposure parameters and optimized to result in the desired coating properties.

50


F IG . 2.23. Phases of process design, optimization, characterization, and monitoring/control.

F IG . 2.24. Spectral absorption of an aliphatic UA resin and a α-hydroxy-ketone (HK) photoinitiator.

If the key parameters are identified, the “process window” can be determined and the control and monitoring of the curing can then be done by monitoring only few key parameters. The complexity of energy measurement and significance to process design and control can be exemplified by the examples given in Figures 2.24 and 2.25.


51

The example shown in Figure 2.24 corresponds to a common clear coat formulation, where only the absorption of the resins and the photoinitiators plays a major role. In this case, the photoinitiator absorption is in the range of 220–280 nm, the UV-C range, which is defined from 200 to 280 nm. So, the most meaningful measurements to the process will be the energy density applied in the UV-C range. However, the measurement of the total energy in this range may not be sufficient, since the distribution of the different wavelengths within the spectrum may be considerably different with two lamps. As shown in the spectral distribution of the different bulbs (Figure 2.21), the radiant power applied below 300 nm is much different with the standard mercury compared to the gallium added bulb. Thus, even if the total energy in the UV-C is the same the curing result will not be the same; for example, in the case of the standard bulb, 1000 mJ/cm2 is applied within 4 s (at 250 mW/cm2 radiant power) and with the gallium bulb 1000 mJ/cm2 is applied within 20 s (at 50 mW/cm2 ). It is therefore also important at what radiant power the energy was delivered. The PowerPuck® , for example, measures the total energy and the peak radiant power per transition band defined. This presents another difficulty, namely that the wavelength range is defined different by the radiometer types and manufacturers (UV-C with the PowerPuck® is measured between 250 and 260 nm). In Figure 2.25 the absorption spectra are plotted of compounds contained in a pigmented coating formulation, representing a more complicated situation concerning the absorption behaviour of the photoinitiators. There are two photoinitators employed, one type (acylphosphine oxide, e.g., TPO) absorbing at long wavelength outside the absorption spectrum of the pigment, responsible for through cure, and one photoinitiator absorbing in the short wavelength region responsible for surface cure. In principle, the TPO type photoinitiator could also be used for surface cure, however, it is the more expensive type. Thus, in this case the wavelength ranges in the UV-A (315–400 nm) and UV-C play a role and

F IG . 2.25. Overlap of lamps spectrum (Hg–Fe) with absorbance curves of a pigmented UV formulation.

52


are the most meaningful for the process, however, the exact impact of the measured data to the process can not be deduced. The overlay of the output spectrum of the ferrum doped mercury lamp with the absorbance curves of the coating components illustrates the importance of matching the spectral output of the lamp with the absorption characteristics of the photoinitiators (Figure 2.25). For a process control, the measurement of the total energies in these ranges can be kept constant by increasing the exposure time (or decreasing belt speed) in order to maintain the curing result. If the radiant power of the lamp decreases for any reason, the curing result will be satisfactory only as long as the radiant power is high enough for the specific optimized curing process. Thus, for process control it is important to track energy density and peak radiant power, in order to correctly run the process and have the right measures at hand to respond, if the curing results are no longer satisfactory. These examples represent only a simplification of designing and monitoring the UV curing process. A more detailed description of radiometric methods for UV process design and process monitoring has been given recently by Stowe 53 as well as an insight into three-dimensional curing design processes.54 A short description of main aspects is given in the section below. How to measure the exposure parameters? With the use of an appropriate radiometric process control, the UV curing process can be quantified sufficiently. Ideally, after the coating has been cured satisfactory, the process will be repeated under the same conditions by exposing a dynamic radiometer instead of the coating. Since there is not the radiometer to be used for all setups on the market, the question arises which type of instrument will be suited for the specific process. There are radiometers available, which are positioned at a specific place in an exposure unit and can perform static measurements, mainly used to monitor the stability of a lamp system. They measure irradiance and spectral distribution, but not energy, since they are not coupled to the exposure time. Radiometers recording the data of the whole exposure process are moving under the lamps. Such dynamic measurements record the total energy, irradiance profile and spectral distribution. Most UV radiometers do not measure IR irradiance. The usual method of determining the effect of IR is the measurement of the surface temperature. The heat produced by the lamp may be a benefit or a nuisance, but it is an inseparable factor in the curing process. Currently, a non-contacting optical thermometer55 is recommended for surface temperature measurement. Radiometers give the total UV energy density in a given wavelength range, but since the energy is the time-integral of irradiance, information about neither irradiance nor time can be extracted from it. To illustrate this statement, for example, consider a total energy in the UV-A range of 1000 mJ/cm2 , the energy can be applied by an irradiance of 250 mW/cm2 with a mercury lamp within 4 s, or by daylight exposure in the sun at an irradiance of about 2.5 mW/cm2 within 400 s. In both cases, the total energy is the same, but the final properties of the coating exposed to the different conditions would differ very much. Consequently, data on energy alone is less important to design, but more useful in monitoring or control, where the exposure unit is kept constant and only the variation or decay of the energy during operation is monitored. Thus, information about the peak of irradiance or of the entire exposure profile is important to the design. Figure 2.26 illustrates several irradiance profiles, all of which exhibit approximately the same energy content, but distinctly different profiles – radiant peak power


53

F IG . 2.26. Different exposure profiles with the same total energy.

as a function of time. The fact that these exposures produce different physical properties in most UV-curable materials is the reason that profile information is needed in the process design stage. The exposure profile is characteristic of any lamp design. It is not possible to increase the peak-to-energy ratio of a lamp – this is determined by its physical design.56 For process design, it may be necessary to evaluate a significant number of parameters and variables in order to optimize a process and assure a wide operating “process window”. Process optimization means matching the lamp system and the required exposure variable to the coating and its application, and matching the UV curable chemistry to the performance required. In order to evaluate the effects on the physical properties of the final cured product, correlation with exposure variables is essential. As mentioned, these variables can be expressed in terms of irradiance profile, spectral distribution, total energy, and infrared energy (or temperature). Multi-band radiometers, mapping radiometers (to evaluate profile), and spectrophotometers can record information on a significant number of these parameters. In addition to facilitating the optimization process, these measurements are used to determine the operating limits for production process control. Once designed and optimized, monitoring the process in production may be limited to monitoring only a few key parameters; namely those which, when out of pre-determined limits, affect the result. From process design, these critical parameters were identified. Relatively inexpensive, simple tools and methods can be used in production monitoring. These may be on-line monitors, dosimeter strips, single band radiometers, and the like. Ultimately, these measurements must be related to target properties to be achieved and must be correlated with measurements of the optimized process from the process design parameters. Selecting a method of measurement or a particular radiometer should be based on the specific process and the identification of the key variables which have the greatest effect on the process. For further information see also ref. 57. A wide variety of radiometric instruments (EIT, Inc., International light) is available for measuring the radiant characteristics of industrial and laboratory UV lamps. Relating these characteristics to the performance of a UV-cured product depends on how well the selected parameters match the critical factors of the cure process. Effective radiometry must reflect the understanding of the interactions of the lamp characteristic with the chemistry needed

54


for the specific application related to the development of the physical properties of the final coating. The first step of consideration of an UV equipment setup is related to the process design, the optimization of the UV exposure conditions based on the performance requirements of coating. This is done by varying the formulation ingredients and/or UV exposure, and by evaluation of the performance of the resulting cured coating. In this respect, the optical characteristics of the film, the spectral absorption mainly determined by the photoinitiator type and concentration, the film thickness, other absorbing additives, like pigments, UV absorbers, etc., in conjunction with the spectral characteristic of the used UV lamp play a major role. These parameters determine what exposure is most effective in obtaining the desired physical properties of the cured coating. To characterize this design information together with the limits where the process still works, is important to achieve an efficient process with an adequate process window. Radiometric measurements are useful in quantifying the successful exposure parameters, so the process can be reliably repeated. 2.3.4.1 Radiometric devices There are a lot of instruments and devices available for radiometric measurements, the selection of the proper instrument is dependant on the compatibility with the process equipment (e.g., UV line, on-line monitoring or 3D exposure) and the relevant exposure parameters. Radiometers measure irradiance (W/cm2 ) over a uniquely defined wavelength band range. Most of them are narrow band radiometers, differing in construction, filters, detectors and mode of operation. Therefore, different radiometers give different results when measuring broad-band sources, due to the different responsivity or wavelength sensitivity. Even if comparing data from instruments of the same type, the actual UV energy may be different if the type of source is different. The predominant number of commercial radiometers measures in the UV-A wavelength range (320–390 nm), mainly due to the fact, that most applied chemistries (photoinitiators) respond in this wavelength region. The most important parts to know about the radiometer are: 1. Responsivity (wavelength band or range) (EIT UVC (240–260 nm), EIT UVA (320– 390 nm), International Light, IL 390B (250–400 nm)), 2. Dynamic range (W/cm2 ), and 3. Capacity for recording energy (J/cm2 ), and if it is a sampling type instrument: 4. Sampling rate (slow sample rates may reduce accuracy at high speeds; very fast sampling rates can cause errors under lamps that pulse with line (mains) frequency), and if peak irradiances are measured; 5. Threshold (if exposures are in the low irradiance level, for example, as it applies in 3D exposures, where the irradiance may be in the order of less than 100 mW/cm2 , a radiometer with a 50 mW/cm2 threshold would be subject to significant errors). The responsivity is the degree of response of a radiometric detector to the different wavelength applied. Due to the design and filters used, the responsivity of every instrument


55

F IG . 2.27. Selected examples of responsivity and mercury lamp UV emission (R.W. Stowe).

is different, and therefore leads to different collected data despite exposure to the same lamp (Figure 2.27). The responsivity data are given by the equipment manufacturers. For high energy applications, almost all radiometers can be used, however, for energy measurements below 100 mW/cm2 , only few instruments are available (e.g., UVRAD from EIT; low power version of PowerPuck). A widely used radiometer is for instance the EIT PowerPuck® . The UV Power Puck also has the distinct advantage of being the only radiometer able to monitor the peak irradiance in W/cm2 in each bandwidth. This allows the operator to determine not only the total energy density, but also how that energy density is delivered; i.e., what irradiance at what wavelength. The PowerPuck® measures four separate UV transmission band ranges (UV-V: 445– 395 nm; UV-A: 390–320 nm; UV-B: 320–280 nm; UV-C: 260–250 nm) simultaneously and indicates the total energy density (J/cm2 ), as well as the peak irradiance (W/cm2 ) of each band range. Dosimeters measure the accumulated energy (J/cm2 ) over the whole exposure time, also at a uniquely defined wavelength range. Since this instrument type also includes the time of exposure, it is commonly used (e.g., RADCHECK UV EB DOSIMETER). Spectroradiometers measure spectral irradiance at a very narrow bandwidth, thus they are used when the effect of the irradiance in a specific wavelength on the curing behaviour has to be evaluated. Radiachromic dosimeters are tabs or strips that measure the time integrated energy by changing colour. They exhibit normally broad band response. The values of colour change can be correlated with a radiometer and then these strips can be used as an effective tool for process monitoring. One example are UV FastChek® strips,58 composed of 5 separate color changing zones, which indicate the applied level of energy by a simple visual inspection, measuring energies from 0 to 1500 mJ/cm2 . They are very versatile and can especially

56


F IG . 2.28. Typical custom engineered UV system (Fusion UV Systems, Inc.).

be placed on positions where the use of a radiometer is difficult (e.g., three-dimensional objects). Correlating parameters measured with different radiometers? In the development of a coating it happens very often, that the optimization of the formulation and the curing conditions were done with laboratory equipment and afterwards the process has to be transferred to production lines with different lamps and equipment. Often, the type of the radiometer also has to be changed, for instance when the device may not be attached to 3D substrates easily. Thus, it is a very difficult task to correlate different radiometers (may have different responsivities) with different lamps (may have different spectral emissions). Some correlations can be done with different radiometers when the results are correlated with the exact same lamps and spectral distribution.

2.3.5

UV Coating Line Examples

The setup of a typical 2D curing UV line is shown schematically in Figure 2.28, consisting of a conveyor belt for the transport of the substrate, a coating unit for roller or curtain coating, the lamp system, consisting of one or multiple lamp arrays, an exhaust integrated in the lamps system to remove ozone and other volatiles generated during the exposure and often cooling units for the lamp and eventually the substrate. There are several case studies about operating UV lines, available by the US EPA (Environmental Protection Agency) in cooperation with the Midwest Research Institute, on UV curable wood coatings, covering different companies (e.g., Columbia Forest Products, Chatham, VA, Lane, Altavista, VA; Loewenstein, Pompano Beach, FL), different products (e.g., panels, office furniture, cabinets, seatings), and different UV cured layers (e.g., UV


57

sealer, UV topcoat, UV filler).59 Some of the lines and their characteristics are introduced briefly. One example of existing UV coating lines is at Columbia Forest Products, where a wide variety of hardwood plywood panels are produced, with a clearcoat on one or both sides. The coating process includes a UV-cured sealer and a UV-cured topcoat. The panels are either multi-ply or medium density fibreboard (MDF). The UV coating line consists of two sanders, two roll coaters, and two UV exposure units, connected by conveyor belts. The panels first pass through a multi-head sander that also cleans the panels for a smoother coating application. The panels then pass through the roll coater where the sealer is applied. Afterwards, the coating is cured by UV lamps. The number of lamps and cure time vary depending on the product, however cure time is only a few seconds. In the second half of the line the top coat is applied by a roll coater and cured by two UV lamps. There are several advantages of using the UV-cured coating system instead of traditional solventborne nitrocellulose coatings. The short curing time reduces the amount of space required in the facility to house the UV line. The UV equipment also provides a highly automated coating process and requires a smaller labour force than hand spraying traditional coatings. Coors Can Manufacturing Plant (Golden, CO), the largest single aluminium can producer in the world, producing approximately 4 billion cans per year, converted to UV-cured coatings in 1975 to increase can printing speeds and reduce energy consumption. Printing speeds are approximately 1600–1800 cans per minute, and cure time is 0.7 s. The UV ovens require less floor space and less start-up time than thermal ovens meeting the same throughput requirements. Coors estimated that VOC emissions resulting from UV-curable coatings was 1.6 tons (1 t) per billion cans cured, whereas thermally cured cans resulted in 28.5 tons (26 t) of VOC’s per billion cans cured. Sutherland Golf Inc. (Barberton, OH) manufactures golf balls, typically coated with a waterborne primer and a urethane clearcoat. The company chose an UV clearcoat instead because it provides a glossy, tough coating but remains more elastomeric. Hussey Seating, the world leader in spectator seating, switched already 1994 from a polyurethane finishing line to an automated UV-cured system. VOC emissions decreased from 199 tons/year to 99 kg/year, even though production increased from 9000 units/week to greater than 14,000 units/week. Switching to the new system averted costs for constructing a new storage area ($200,000) and reduced labour costs by two-thirds. Since UV-curable coatings are 100% solids, coating costs decreased by 17% on a per-unit basis. The UV coatings proved more durable than the polyurethane coatings when exposed to sunlight, heavy use, and water. Paintbox Ltd., located in Banbury, UK, is a contract coater and decorator of plastic components. Paintbox coats parts for customers in the automotive, domestic appliance, and electronics industries. They used to coat their parts with solvent-based products and cured the parts in thermal curing ovens. However, due to customer concerns, they switched to UV technology, which offers superior scratch and scuff resistance while improving aesthetics. By switching to UV-curable coatings, Paintbox improved their overall rejection rates. This improvement can be attributed to reduced contamination from dust particles on the coated parts. UV curing dries much quicker, which reduces the possibility of contamination. IKEA has converted many of its wood coating lines to UV coatings. The resulting conversion helped IKEA reduce the cost per-square-meter finish surface for flat production.

58


F IG . 2.29. Typical UV printing press set-up (IST-Metz, GmbH).

The UV coating system resulted in a more environmentally friendly system that improved quality and productivity. UV technology enabled IKEA to produce products with a better quality finish, as well as better scratch resistance and resistance to liquids, such as water and coffee. IKEA’s line productivity increased due to UV’s faster drying and curing times. The UV system also provided a safer work environment (i.e., no solvent in the air) with reduced cleaning time. What are the equipment costs/benefits? While the investment required in UV application equipment is steep, the potential benefits are tremendous. Costs to outfit a UV coating line typically range from $50,000 at the very low end to $1 million, depending on the sophistication of the system. It is unusual for a manufacturer to recoup that investment in a relatively short time from production benefits. Typical coating lines are schematically shown in Figures 2.28 and 2.29.

2.3.6

Equipment Suppliers

An overview about UV equipment supplier companies, mainly in the US, is given at the GlobalSpec website.60


59

Main suppliers are listed below. UV lines and lamps: • • • •

Fusion UV Systems, Inc. (www.fusionuv.com) Hönle (www.hoenle.com) IST Metz (www.ist-uv.de) Nordson (www.nordson.com/Businesses/UV) Lamps:

• Primarc (www.primarc.com) • Heraeus amba (www.heraeusamba.com/uv_curing_lamps.html) • Heraeus Noblelight Excimer lamps (www.heraeus-noblelight.com/HNG/eng/uvfb/uvfb_ Home.nsf/$frameset/start) Instrument suppliers: • • • • • •

Solatell Instruments, 4D Controls Ltd., Cornwall, UK (www.solatell.com) EIT Instruments, Sterling, VA (www.eitinc.com) International Light, Newburyport, MA (www.intl-light.com) UV/EB Labels; UV Process Supply, Inc. Chicago, IL (www.uvprocess.com) Rad Check Dosimeter; UV Process Supply, Chicago, IL (www.uvprocess.com) Far West Technology, Inc., Goleta, CA (www.fwt.com) REFERENCES

1. Stowe, R.W., Some economic factors of UV curing, RadTech (1994), Conference Proceedings, North America, Orlando, FL. 1994, pp. 353–359. 2. “Materials for Microlithography”, In ACS Symposium Series, Vol. 266 (L.F. Thompson, C.G. Willson and J.M.J. Frechet, eds.). 1984. 3. (a) Crivello, J.V. and Ahn, J., J. Polym. Sci. Polym. Chem. Ed. 41 (16), 2556–2569 (2003); (b) Dietliker, K. and Crivello, J.V., In “Photoinitiators for Free Radical, Cationic and Anionic Photopolymerization”, 2nd edn, Vol. 3, In Chemistry and Technology of UV and EB Formulation for Coatings, Inks and Paints (G. Bradley, ed.). John Wiley and Sons/SITA Technology, London, 1998, chapter IV; (c) http://www.rpi.edu/dept/chem/faculty/crivello/crivello.html. 4. Irgacure 250 – Ciba Specialty Chemicals. 5. http://www.cibasc.com/first_commercial_photolatent_base_catalyst_for_uv__a_clearcoat_applications. pdf. 6. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring, Fundamentals and Applications”. Hanser Publishers, Munich–Vienna–New York, 1995. 7. Allen, N. (Ed.), “Photopolymerization and Photoimaging Science and Technology”. Elsevier Applied Science, London, 1989, Chapter 1, pp. 40–44. 8. Fouassier, J.P., “Photoinitiation, Photopolymerization and Photocuring, Fundamentals and Applications”. Hanser Publishers, Munich–Vienna–New York, 1995, Chapters 3 and 4. 9. Brandrup, J. and Immergut, E.H., Polymer Handbook, 2nd edn, Wiley–Interscience, 1975, II-45. 10. Decker, C., Macromol. Rapid Commun. 23 (18), 1067–1093 (2002). 11. Decker, C. and Moussa, K., Eur. Polym. J. 26, 393 (1990). 12. (a) Anseth, K.S., Wang, C.M. and Bowman, C.N., Macromolecules 27, 650 (1994); (b) Anseth, K.S., Bowman, C.N. and Peppas, N.A., J. Polym. Sci. A Polym. Chem. 32 (1), 139–147 (1994); (c) Anseth, K.S., Kline, L.M., Walker, T.A., Anderson, K.J. and Bowman, C.N., Macromolecules 28, 2491 (1995); (d) Kannurpatti, A.R., Lu, S., Bunker, G. and Bowman, C.N., Macromolecules 29, 7310 (1996).

60


13. Lovestead, T.M., Berchtold, K.A. and Bowman, C.N., Macromol. Theor. Simulat. 11, 729–738 (2002). 14. Lavevée, J., Allonas, X., Jradi, S. and Fouassier, J.P., Macromolecules 39, 1872–1879 (2006). 15. (a) Klosterboer, J.G., Adv. Polym. Sci. 84, 1 (1984); (b) Decker, C., Elzaouk, B. and Decker, D., Macromol. Sci. Pure Appl. Chem. A33, 173 (1996); (c) Decker, C., Polym. Int. 45, 133–141 (1998). 16. Brandrup, J. and Immergut, E.H., “Polymer Handbook”, 2nd edn. John Wiley & Sons, New York, 1975. 17. Abadie, M.J.M. and Voytekunas, V.Y., Eurasian ChemTech J. 6, 67–77 (2004). 18. Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 101–111 (2003). 19. Schwalm, R., Binder, H., Funhoff, D., Lokai, M., Schrof, W. and Weiguny, S., RadTech Europe 1999, Conference Proceedings. 1991, pp. 103–109. 20. Decker, C. and Decker, D., J. Macromol. Sci. A34, 4 (1997). 21. Decker, C. and Decker, D., J. Macromol. Sci. Pure Appl. Chem. A34 (4), 605–625 (1997). 22. Decker, C. and Decker, D., Polymer 38 (9), 2229–2237 (1997). 23. Noren, G.K., Tortello, A.J. and Vanderberg, J.T., Proceedings RadTech Conference USA, Northbrook, IL, Vol. 2. 1990, p. 201. 24. Jönsson, S., Sundell, P.E., Ericsson, J., Hultgren, J., Sheng, D., Hoyle, C.E., Clark, C.C., Miller, C. and Owens, G., RadTech 96, Conference Proceedings. 1996, pp. 377–392. 25. Dias, A.A., Jansen, J.F.G.A. and vanDijck, M., RadTech Europe 1999, Conference Proceedings. 1999, pp. 473–482. 26. (a) Hoyle, C., Proceedings RadTech Conference USA, Northbrook, IL, Vol. 1. 1990, p. 194; (b) Decker, C., Morel, F., Jönnson, S., Clark, S.C. and Hoyle, C., Polym. Mater. Sci. Eng. 75, 198 (1996). 27. Rytov, B.L., Ivanov, V.B. and Anisimov, V.M., Polymer 37, 5695 (1996). 28. Decker, C., Polym. Int. 45, 133–141 (1998). 29. Pojman, J.A., RadTech 2006, Conference Proceedings, Chicago, IL. 2006, on CD. 30. Allen, G. and Bevington, J.C. (Eds.), “Chain Polymerization”, Part I. Vol. 3. Comprehensive Polymer Science, Pergamon Press, 1989. 31. Decker, C. and Decker, D., F-Morel, In “Photopolymerization Fundamentals and Applications” (A.B. Scranton, C.N. Bowman and R.W. Pfeiffer, eds.), In ACS Symposium Series, Vol. 673. Am. Chem. Soc., Washington, 1997, p. 63. 32. Decker, C., Macromol. Rapid Commun. 23 (18), 1067–1093 (2002). 33. (a) Decker, C. and Moussa, K., Macromol. Chem. 189, 2381 (1988); (b) Decker, C. and Moussa, K., J. Coat. Technol. 62, 55–61 (1990). 34. Williams, K.P.J., Pitt, G.D., Batchelder, D.N. and Kip, B.J., Appl. Spectrosc. 48, 232–235 (1995). 35. Schrof, W., Beck, E., Königer, R., Meisenburg, U., Menzel, K., Reich, W. and Schwalm, R., Characterization of radiation curable coatings by confocal Raman microscopy, RadTech 1998, North America, Conference Proceedings. 1998, pp. 363–374. 36. Siesler, H.W., Ozaki, Y., Kawata, S. and Heise, H.M. (Eds.), “Near-Infrared Spectroscopy: Principles, Instruments, Applications”. Wiley–VCH, Weinheim, 2002. 37. Scherzer, T., Mehnert, R., Volland, A. and Lucht, H., Process and quality control during UV curing of acrylate coatings using near-infrared reflection spectroscopy, Proceedings RadTech Asia 2005, Shanghai, China. 2005, pp. 574–582. 38. Abadie, M.J.M., Eur. Coat. J. 11, 788–795 (1991). 39. Wicks Jr., Z.W., Jones, F.N. and Pappas, P. (Eds.), “Organic Coatings – Science and Technology”, In Applications, Properties, and Performance, Vol. 2. John Wiley & Sons, 1994, Chapter XXII, pp. 65–82. 40. Raith, Th., Bischof, M., Degner, M. and Gemmler, E., 3-D UV technology for OEM coatings, RadTech Report, November/December 2001. 2001, pp. 26–29. 41. Mills, P., Robotic UV curing: A cost-effective way to cure large, 3D parts, RadTech 2006, Conference Proceedings. Chicago, IL, 2006 on CD. 42. http://www.radtech.org/RadTechGlossary.pdf. 43. Laser induced polymerization: Decker, C. In “Materials for Microlithography” (L.F. Thompson, C.G. Willson and J.M. Frechet, eds.), In ACS Symposium Series, Vol. 266. 1984, Chapter 9, pp. 207–223. 44. http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/beers1.htm. 45. Thommes, G.A. and Webers, V.J., J. Imaging Sci. 29, 112–116 (1985). 46. Seao, N., RadTech Asia 1991, Osaka, Proceedings. 1991, pp. 478–484.


61

47. Schaeffer, W., Jönsson, S. and Amin, M.R., Greater efficiency in UV curing through the use of high peak energy sources, Proceedings, RadTech Europe, 1995. 48. Stowe, R.W., Aspects of radiometry and process verification for 3D UV processing, RadTech Europe 2003. 2003, pp. 821–828. 49. (a) see Chapter 2, pp. 41–65, in Roffey, C.G., “Photopolymerization of Surface Coatings”. Wiley Interscience, 1982; (b) Calvert, J.G. and Pitts, J.N., “Photochemistry”. Wiley, New York, 1966. 50. Stowe, R.W., Equipment for UV curing, International Coatings Expo (ICE) and Technology Conference 2000, Chicago, IL, USA, October 18–20. 2000, pp. 1–17. 51. Dake, K., Montgomery, E., Koo, Y.C. and Hubert, M., LED curing versus conventional UV curing systems: Property comparisons of acrylates and epoxies, RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 52. Stowe, R.W., “Relationship of the Significant Elements of UV Lamps to the UV Curing – Process Window. Paint Line Design and Implementation”. Society of Manufacturing Engineers, Nashville, TN, USA, December 1998. 53. Stowe, R.W., J. Coat. Technol. March (2003), www.radtech-europe.com/download/stowemarch.pdf. 54. Stowe, R.W., Verifying UV exposure in 3D curing systems, RadTech North America, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. 55. Raytek, Inc.; Santa Cruz, CA (www.raytek.com). 56. Stowe, R.W., Practical aspects of irradiance and energy in UV curing, RadTech 1998. Proceedings, North America, Chicago, IL. 1998, pp. 640–645. 57. (a) Stowe, R.W., Practical relationships between UV lamps and the UV curing process window, Proceedings, RadTech North America, 1994; (b) Stowe, R.W., Practical aspects of irradiance and energy density in UV curing, Proceedings, RadTech North America, 2000. 58. www.uvprocess.com/products/Curecon/UVmon/FastCheck.asp. 59. http://www.epa.gov/ttn/atw/wood/low/ultravio.html. 60. http://process-equipment.globalspec.com/LearnMore/Manufacturing_Process_Equipment/Industrial_ Assembly/UV_Curing_Systems.

C HAPTER 3

Network Formation and Characterization In the year 2000 about 180 mln. tons of polymers were sold, whereof about 30 mln. tons are coatings (lacquers), glues and dispersions. According to their functions and structures these polymers can be classified into different categories, for instance, natural (proteins, polysaccharides, rubber) and synthetic polymers (thermoplastics, elastomers, duromers). The main group of synthetic polymers belongs to the class of thermoplastics and fibres, which are linear, uncrosslinked polymers, which can be processed into different shapes by thermal treatment. Besides other classifications, one classification of the synthetic polymer types can be done by the increasing degree of crosslinking, whereof the elastomers are slightly or moderately crosslinked, occurring very often as rubber-like flexible materials, and duromers, which are highly crosslinked, available for instance as hard coating materials on kitchen worktops, on cars or on industrially produced parquet floorings (Figure 3.1). This type of classification however reflects only the structural composition, whereas the names of the different classes (thermoplastic, elastomer, duromers) reflect the mechanical behavior of the polymer classes at the normal temperatures of use. Thermoplastics appear at room temperature in a glassy state, but become plastic upon heating at temperatures

F IG . 3.1. Classification of polymer subgroups. 62

NETWORK FORMATION AND CHARACTERIZATION

63

above the so-called glass transition temperature, the elastomers are used at temperatures above their glass transition and therefore are elastic at room temperature, and, finally, the duromers are very durable and exhibit extraordinary mechanical properties. The mechanical properties of materials are those properties that involve a reaction to an applied stress or load, and determine the range of usefulness of a material and establish the product life that can be expected. They are often used to help classify and identify materials. The most common properties considered are hardness, tensile strength, ductility, impact resistance, and fracture toughness. These properties are dependent on the physical state of the materials, polymers or coatings in particular. The mechanical properties of a material are not constants and often change as a function of temperature, rate of loading, and other conditions. For illustration, H2 O can appear in the physical state of a solid (snow), in the liquid state (water) or in the gaseous phase (steam), depending on the surrounding temperature. As it is with water, the mechanical properties of polymers are also very temperature dependent, thus they appear as hard or soft materials. Solid polymers are either in a glassy (amorphous) or crystalline state. In a melt polymers are liquid. In between, in the transition between solid and liquid a more or less broad transition state exists, where the polymers become softer with increasing temperature, called the glass transition region. So, the most important question is, what are the requirements put on a material (or coating) and how do they behave at the application temperature and conditions. For example, an automotive topcoat should withstand mechanical attack during the lifetime of several impacts, like scratching during car wash (and ideally also scratches resulting from vandalism) or gravel chipping, as well as chemical attack of acid rain, spilled gasoline, tree resin or bird droppings, and this within a huge temperature and climate range, from hot and humid conditions in Florida to cold and dry conditions in Siberia or Canada. So, the coatings have to be designed to fulfill a bunch of properties, some of them are diametrically opposed, like hardness and flexibility, and the test methods have to be designed to match the reality as close as possible (and enable a fast screening under time lapse conditions). The scientific classification of the material states according to their flow properties is similar to the classification according to their uses. Very common test method for the characterization of material properties are stress-strain experiments, where a specimen is subjected to a stress (force/area) and the resulting strain (elongation, compression, bending, torque or shear strain) is measured. In the simplest approximation (which is almost always good enough) the relation between stress and strain is taken to be linear (at low deformations). In this linear region, the relationship is defined by Hooke’s law where the ratio of stress to strain is a constant. The slope of the line in this region where stress is proportional to strain is called the modulus of elasticity or Young’s modulus. The modulus of elasticity (E) defines the properties of a material as it undergoes stress, deforms, and then returns to its original shape after the stress is removed. It is a measure of the stiffness of the material. Depending on the way the material is being stretched, bent, or otherwise distorted, further moduli can be derived, like the torsion modulus (T ) or shear modulus (G). In the temperature dependent curves of the moduli different regions can be identified, at “low” temperatures and high moduli the glass region, where the cooperative mobility of the chain segments is frozen and the material reacts on stress like an elastic solid (the material is hard). Then the glass transition region where the molecular mobility increases. This re-

64


F IG . 3.2. Modulus (shear G) as a function of temperature for different polymer classes.

gion is defined by a glass transition temperature which, however, is not a thermodynamic defined “sharp” temperature like the melting temperature, but rather a more or less broad region. The definition of the glass transition temperature is dependent on the determination method used (see below). After the glass transition region often a rubber-elastic plateau appears, and at higher temperatures a further drop in the modulus leads into the viscous flow region. As shown exemplarily for the shear (G)-modulus in Figure 3.2, the modulus of the elastomers drops significantly below room temperature. The uncrosslinked elastomer precursors become very soft at temperatures well below room temperature and pass directly into viscous flow, whereas slightly crosslinked (typical) elastomers keep a relatively low, but nearly constant rubber-elastic plateau at temperatures around and above room temperature. This plateau formation is due to the slight crosslinking, which is necessary for the function as elastomers, since the elastic deformation of the polymer backbone is only reversible if crosslinks prevent the sliding and relaxation of the polymer chains, as happens in the uncrosslinked elastomer precursor. Entanglements of polymer chains are a special form of (temporary) crosslinks. The temperature region at which the drastic drop occurs is called the glass transition temperature, since the material drops from a glassy into a soft or rubber-like state. The thermoplastics usually exhibit a high modulus up to well above room temperature, until they reach their glass transition temperature, where the modulus drops steeply and steadily. In the highly crosslinked duromers the modulus drop is shifted further to higher temperatures and the rubber-elastic plateau is on a much higher level than obtained with the elastomers. It will be shown later that the crosslink density can be derived from the hight of the rubber-elasticity plateau. However, with higher crosslink density the drop of the modulus is getting less pronounced and disappears completely in a ideal duromers (since Tg is approaching infinity for x = 1).


65

F IG . 3.3. Network structures.

Thus, the main influencing factors on the classification and on the mechanical properties of polymers are the physical states of the polymers at the temperature of use (normally room temperature), as well as on the niveau of the modulus beyond the glass transition temperature. Thus, the polymer classes range from (at room temperature) soft uncrosslinked elastomers, which upon slight crosslinking remain soft, but posses restoring forces, over (at room temperature) hard, but above its glass transition temperature formable thermoplastics, to the highly crosslinked duromers. On the one hand these highly crosslinked duromers can be considered similar to thermoplastics with extended usable temperature range, and on the other hand they exhibit extraordinary properties with respect to solubility, swelling, chemical and abrasion resistance, etc., as will be discussed further in the book. Most of the polymer networks (Figure 3.3) belong to the imperfect network type, thus, they are inhomogeneous, contain cycles (or loops), dangling ends and entanglements of molecular chains between crosslinks. However, as shown later, highly crosslinked systems with short chains between crosslinks are approaching the ideal network type, which should behave perfectly energy-elastic (a deformation is completely restored) and do not show plastic flow. As can be seen in parts of a cobweb, nature almost builds perfect networks.

3.1 FILM FORMATION Preliminary to network formation is film formation, which in case of UV curable coatings is not disturbed by drying processes, as in the case of solvent- or water-borne coatings. The first step in the film formation is the wetting of the substrate and the spreading of the droplets or the liquid applied. This process can be measured by the determination of the wetting angle α (>90◦ for a drop sitting on the surface), which should approach zero for ideal spreading. In order to obtain a good spreading the surface tension of the paint should be equal or lower than that of the surface.1 After wetting the droplets or the liquid flows to

66


form a smooth, continuous film. The driving force behind the flow is the surface tension, surface tension gradients and gravity, facilitated by low viscosity. Although certain terms may be disregarded depending on the situation.2 On horizontal surfaces gravity helps for ideal film formation, whereas on vertical surfaces and edges sagging and runaway from edges occurs. The sagging and edge flow can be reduced or prevented by controlling the viscosity and introducing thixotropic behaviour. The transition of a liquid lacquer to a solid coating passes through various intermediate stages, which can be described by examining the changes in viscosity lg η = k Mn NV,

(3.1.1)

where NV is the solids content. Thus, the viscosity is dependent on the molecular weight and the solids content. However, this relationship is linear only at low enough concentrations where individual polymer molecules and their associated solvent molecules are isolated from each other. For concentrated solutions of polymers and resins as used in the coatings field the relationship can be complex, however, viscosity generally increases with increasing molecular weight. Among the factors controlling the viscosity are the molecular weight, the molecular weight distribution, the resin structure and the concentration, which determine the Tg of the resin solution.4 As resin solution viscosity increases, for example due to molecular weight increase by crosslinking, at some concentration the Tg of the solution becomes the dominating factor, and above that concentration the viscosity increases rapidly. Since the application viscosity depends on the application method (e.g., ∼100 mPa s for spray or 3000 mPa s for roller coating), in solvent based systems the higher molecular weights of the resins can be compensated by the solvent content. In 100% liquid UV coatings the molecular weights of the resins have to be in the lower molecular weight range. The resin is then diluted with reactive diluents to reach the application viscosity range. Finally, upon curing the molecular weight and the viscosity increase into the area of desired properties, like mechanical strength and chemical resistance (Figure 3.4). Thus, during the coating process (of a UV-curable system) the physical state of the coating changes dramatically from a liquid to a solid state, with two major processes involved, film formation and buildup of a network during the curing reaction.

3.2

NETWORK FORMATION

General principles of polymer network formation, structures and properties are well described.5 The network formation during the curing reaction can be described by two major transitions, the gelation, which involves the transformation of a liquid into a gel-like state and in the case of duromers, the vitrification, the transition of the rubber state into a solid glassy state. This change in the physical properties is caused by a molecular weight increase during the curing reaction. In the build-up of molecular weight, there is a big difference between chain growth polymerization reactions and stepwise polycondensation or polyaddition reactions. Starting from low molecular weight monomers, the polymerization yields


67

F IG . 3.4. Viscosity changes from monomer to UV cured coating.

F IG . 3.5. Molecular weight built-up at low conversions for polymerization (chain process) and polyaddition/-condensation (step process).

already high molecular weight chains at rather low conversions, whereas the step growth polyaddition/-condensation does not builds up high molecular weights until very high conversions (Figure 3.5). The classical polymerization process of monofunctional monomers normally leads to linear polymers. Some side reactions, like abstraction of hydrogen atoms from a polymer chain and subsequent growth from this reactive site can lead to branching or even forma-

68


F IG . 3.6. Network formation during radical induced UV polymerization.

tion of some crosslinks by dimerization reactions. Intended crosslinking reactions are used widely in low crosslinked systems of natural rubber, elastomers or polyacrylic acids, like in tires or superabsorbent polymers, and in highly crosslinked coatings. However, to obtain high crosslink densities (XLD) in polycondensation or polyaddition type reactions the network formation is usually not done by the reaction of multifunctional monomers, but rather by reaction of preformed multifunctional resins, like in two-component polyurethanes by the use of polyacrylate resins containing hydroxyl functions, with multifunctional isocyanates. In this case, the network formation can already proceed at much lower conversions. This benefit, however, has to be traded-off by the higher viscosity of the resins, which must be dissolved in solvents. The network build-up of radically initiated polymerizations of multifunctional monomers has been illustrated by Klosterboer22 (Figure 3.6). It is characterized by the fact that the gel points in radically polymerization reactions occur at a very early stage and therefore microgels are formed and with increasing conversion of reactive groups the number of microgels increases until at the final stage the microgels grow together and only few unreacted monomer units or dangling double bonds exist. Depending on the chemistry used different types of polymer networks can be generated (Figure 3.7). By using the same type of (multi)functional groups, like (meth)acrylates, epoxides, polyenes or vinyl ethers, homopolymer networks are obtained. Applying copolymerizable different functional groups, for example, radically polymerizable acrylates and polyenes, acrylates and vinyl ethers or cationically polymerizable epoxides and vinyl ethers, copolymer networks result. If the networks are formed independently, like in the case where in a coating a radically polymerized acrylate network and a cationically polymerized network are generated together, interpenetrating networks are generated. The structure of such interpenetrating networks is symbolized in Figure 3.8. The interpenetrating networks are characterized by the interpenetration of molecular chains, where no chemical links exist between the different networks. The networks cannot be separated unless chemical bonds are broken.


69

F IG . 3.7. Network types formed with different chemistries.

F IG . 3.8. Display detail of an interpenetrating network structure.

The reaction possibilities of a chain radical and the formation of networks using multifunctional monomers is depicted in Figure 3.9. There are several possibilities of the addition of an initiator or a growing chain radical to the double bonds. If one chain is added and the thus formed radical abstracts a hydrogen atom, the molecular chain is terminated (however, the kinetic chain propagation can go on with the corresponding radical produced!). The remaining double bond, however may join in another chain building process (Xi,k ). This nomenclature according to Macosko and Miller indicates the functionality of the structural unit with (i) and the maximum functionality of the net point with (k) (e.g.,

70


F IG . 3.9. Possibility of chains connected to difunctional monomers.

F IG . 3.10. Soluble fraction and fraction of crosslinked diacrylate (HDDA) with thre- and four-connected polymer chains.

HDDA = X1,4 to X4,4 ). If the second double bond reacts in the same way (addition and hydrogen transfer), only two sites are connected together resulting in linear growth, but no crosslinking. The same is the case, if a growing chain adds to one double bond and this radical starts another chain, but the remaining second double bond is trapped in the network and does not react. A real network formation is only achieved with the reactions X3,4 and X4,4 . With increasing conversion during a polymerization reaction, the amount of the structures X3,4 and X4,4 increases (Figure 3.10). Thus, a difunctional acrylate containing two double bonds functions as a tetrafunctional branching point in the ideal case. The amount of the network formed can be determined by extraction methods. After exceeding the gel point, which is characterized by the transition of a hyperbranched into a crosslinked molecule, the coating consists of gel, which is insoluble in all solvents, and a soluble fraction. With increasing conversion the soluble fraction decreases. This has been calculated by Meichsner et al.6


71

F IG . 3.11. Insight into a network structure.

Crosslinked polymers contain per chain at least two bridges to another chain. If the crosslink density is not too high, these networks can be swollen by a solvent to a so-called gel. The transition of a soluble polymer to a gel is often so sharp, that it is entitled as the “gelpoint”. Gel formation occurs if the functionality of the monomers exceeds two (a monoacrylate has a functionality of 2, a di-acrylate of 4). A closer insight into a network (Figure 3.11) reveals the existance of junction points to form a network structure, the network density determined by the molecular weight between X-links, but also inhomogeneities caused by dangling ends (radical termination by hydrogen abstraction), residual unsaturations and long living radicals. Since the networks consist of “infinite” molecular weight molecules (Mw approaches infinity, whereas Mn stays finite), which are virtually unsoluble, the characterization of crosslinked polymers can not be done by molecular weight determinations, but rather by crosslink density and molecular weight between crosslinks. The nomenclature is often inconsistent. The crosslink density is expressed by ν (mol/g; ν 0) or by the degree of crosslinking (Xc ; 0 Xc 1). The degree of crosslinking can be related to the molecular weight between crosslinks Xc = 1/Mc , which is also defined by the number of moles of elastically effective network chains per cubic centimeter of film (also abbreviated as νe ). Mc can be calculated by the following equation7 : Mc =

M f −2

(3.2.1)

where f is the functionality of the molecule. The subtraction of 2 from the functionality is due to the fact that a functionality of 2 contributes only to linear molecular weight build-up, but not to branching or crosslinking. If several components are used, the molecular weight M is calculated by the medium molecular weight M0 M0 =

n 1 M1 + n 2 M2 + · · · + n i Mi n1 + n2 + · · · + ni

(3.2.2)

72


and f0 =

n1 f1 + n2 f2 + · · · + ni fi n1 + n2 + · · · + ni

(3.2.3)

M0 f0 − 2

(3.2.4)

resulting in Mc =

and in the case the conversion of the functional groups (e.g., double bonds) is considered: Mc =

M0 pf0 − 2

(3.2.5)

where p is the fraction of converted groups (p 1). To give some examples about the values of Mc and Xc , the crosslinking of an oligomer of the molecular weight 1000 g/mol, endcapped with two acrylic groups, in combination with the equivalent molecular weight of a monofunctional acrylate (10 mol of ethyl acrylate: 100 g/mol) can be calculated to: Mc = (1 × 1000 + 10 × 100)/11 (1 × 4 + 10 × 2)/11 − 2 = 181/0.18 = 1010, thus resulting in a crosslink density of Xc = 1 × 10−3 . If the same oligomer is reacted with hexanediol diacrylate instead of ethyl acrylate with approximately the same molecular weight (4 molecules of 226 g/mol) Mc = (1 × 1000 + 4 × 226)/5 (1 × 4 + 4 × 4)/5 − 2 = 380/2 = 190, then the resulting (higher) crosslink density is Xc = 5.26 × 10−3 . Some further values, all calculated for a (theoretical) double bond (DB) conversion of 100%, starting with a long-chain diacrylate with molecular weight of 1000 g/mol, over a trifunctional oligomer and the pure hexandiol diacrylate as a highly crosslinked system to the even higher crosslinked dendrimers are given in Table 3.1. It should be noted, that it will be difficult in practice to drive the high functional systems to complete conversion. The calculation of the network quantities can be done according to the theories developed by Macosco and Miller.8 The prerequisites of this theory are: • Equal reactivity of all functional groups; • The reactivity of the groups being independent from each other; and • Cyclizations are not considered. The network characterizing values can be experimentally determined by dynamical mechanical analysis, based on the theory of rubber elasticity.9 The crosslink density (Xc ) is: Xc = E /3ρRT ,

(3.2.6)

73


TABLE 3.1. Theoretical calculation of crosslink density of different functional molecules Xc (×10−3 )

Molecule

Molecular weight

Functionality

Mc

Difunctional oligomer Trifunctional Oligomer Hexanediol diacrylate Dendrimer∗ of Generation 1 Generation 2 Generation 3

1000 750

4 (2 DB) 6 (3 DB)

500 188

2.0 5.3

226

4 (2 DB)

113

8.8

74 80 83

13.5 12.5 12.0

1040 2416 5168

16 (8 DB) 32 (16 DB) 64 (32 DB)

∗ Pentaerythritol as a core and esterification with dimethylolpropane carboxylic acid (DMPA).

where E is the E-modulus in the rubber elastic region, R is the gas constant, ρ is the density and T is the absolute temperature. With this equation, the network densities can be calculated from the E-modulus data in the rubber elasticity region. However, it has been shown by Meichsner et al.6 that equation (3.2.6) holds not true for highly crosslinked networks. They found considerable deviations from the linear behavior and derived an exponential correlation between E-modulus and crosslink density (Xc ). This deviating behavior may be attributed to the short molecular weights between crosslinks, where the theory of rubber elasticity, which reflects the entropic elastic behavior of the chain segments between the X-links may not be valid anymore and instead energy elastic deformations may become dominant, which can be described by deformations of valence bonds or bond angles. They found the following correlation for modulus and X-link density: ln E = mXc + b, where m is the slope and b is the axis intercept; or expressed as: E = b emXc .

(3.2.7)

3.3 MONITORING FILM FORMATION AND CURING Numerous methods exist which are able to describe film formation and detect the chemical changes occurring during curing. Among the methods are infrared and Raman spectroscopy, differential scanning calorimetry, differential thermal analysis, measurement of dielectric constants or dielectric loss (see ref. 1). Recently, two novel methods have been developed to monitor the film formation and curing, exemplified with UV curable coatings.10 Real-time dynamic mechanical analysis at low frequencies, as well as ultrasonic frequencies have been developed to monitor the physical changes occurring, like gelation and vitrification, and have been combined with near-infrared analysis to relate them to the chemical conversion of reactive bonds caused by the polymerization reaction.

74


Low-frequency dynamical mechanical analysis is able to monitor gelation, while the high-frequency ultrasonic reflection technique has been used to characterize the vitrification. In the investigated UV-cured coatings, it could be shown, that the cure of the coating is frozen due to vitrification at a high, but not complete conversion. The increase of the shear modulus during cure of almost two orders of magnitude (to about 1 GPa, which is typical for glassy materials) has been related to the cure conversion, which stops at about 80% conversion in the coating studied. The ultrasonic method has also been used to study drying and film formation of waterborne coatings.11

3.4 NETWORK CHARACTERIZATION Networks elude most of the chemical characterization methods used for linear polymers, like molecular weight determination, end group analysis, etc., because of their insolubility. The insolubility of coatings or of parts of the coatings occurs after a certain degree of crosslinking, defined by the gel point, has been exceeded. The gel point, which is defined as the conversion degree where the weight average of the molecular weight approaches infinity, can occur at very low conversions (below 10%), as has been shown before in the case of radically polymerizing acrylate systems. Thus, the characterization of networks is mainly restricted to the analysis of physical and mechanical properties and their response to the impact of chemicals, like solvents, or physical impacts, like particles, causing scratches or abrasions, on the coating surface. Some of the characteristic properties and responses to impacts are listed in Table 3.2. The characterization of coatings can be roughly classified TABLE 3.2. Characterization of coatings Physical

Chemical

Network characterization Viscoelastic properties Modulus Elongation Transition temperatures Glass transition Brittle–ductile transition Hardness and flexibility Crosslink density Elastically effective chain length Curing gradient

Network characterization Extractables

Response to impacts Abrasion resistance Adhesion Scratch resistance Stone chipping

Response to impacts Solvent resistance Swelling behavior Chemical resistance Household chemicals Acid rain and other automotive chemicals Exterior durability Photooxidation Hydrolysis

75


F IG . 3.12. Network structures of low- and high-crosslink density and effect of stress application on network structure.

in the physical and the chemical characterization, and subdivided into the characterization of the network structure and in the response of the network to different impacts, be it the resistance to chemicals (solvent, water, household chemicals, like mustard, coffee, etc.), or to physical attack, like stone chipping or scratching. Most of the properties and their test methods for coatings are described very well in the literature, for example, in “BASF Handbook on Basics of Coating Technology, Chapter 3.2”,12 or in “Organic Coatings, Science and Technology, Vol. 2, chapter 24”.13 Some of the test methods often used for the characterization of the networks are described briefly below. 3.4.1 Test Methods for Analyzing Physical Properties The physical properties of the coatings will be dependent on the network structures (Figure 3.12) obtained as a function of chemical composition and crosslink density. 3.4.1.1 Viscoelastic properties The viscoelastic properties of coatings are considerably dependent on temperature, rate of deformation and time or frequency, and often measured in stress-strain analysis (tensile or shear stress) as a function of temperature (Figure 3.13).14 The behavior of the material upon the applied stress in the initial, essentially straight line, is characterized by the proportionality constant, upon tensile stress by the elasticity (E )- and upon shear stress by the torsion (G )-modulus, σ = Eε, where σ is the tensile stress and ε is the elongation in relation to initial length (l/ l); τ = Gγ , where τ is the shear stress and γ is the distortion.

(3.4.1)

76


F IG . 3.13. Determination of mechanical properties (typical diagram for viscoelastic polymers).

The thus obtained moduli can be converted into each other by the following equation: G=

E , 2(1 + μ)

where μ is the Poisson ratio, the quotient of the relative transverse and longitudinal contraction, typical values are in the region of 0 to 0.5 (glassy polymers: 0.33). Thus, the value of the elastic modulus is generally about two to three times the shear modulus. At the end of the stress-strain curves in Figure 3.13, the sample breaks. This point defines the elongation-at break (percents of elongation) and the tensile-at-break (or tensile strength). The area under the curve represents the work-to-break. Viscoelastic materials are often characterized by the fact that at an intermediate strain less stress has to be applied to further elongate the sample. The maximum stress at that point is called the yield point. Whereas ideal elastic deformation is (over a wide range) almost independent of temperature, viscous deformation is time (frequency) and temperature dependent, thus viscoelastic deformation is very dependent on the time and temperature over which the stress is kept. At fast rates of stress application or at low temperatures, elastic response dominates, however resulting in significantly lower elongations-at-break, whereas at low rates of applied stress and high temperatures the viscous response (plastic flow) will be proportionally greater. Therefore, the same viscoelastic response curves can be obtained at low temperature or high stress rates or on the other hand at high temperatures or low stress rates. Thus, it is possible to calculate the data, for example, of high rate of stress application from low temperature measurements or vice versa with the method of time-temperature superpositioning.15 This relationship is relevant to all mechanical responses of a coating upon


77

F IG . 3.14. Elastic (E ) and loss (E ) modulus as a function of temperature.

application of stresses as a function of temperature and time interval of applied stress. To illustrate this further lets consider an automotive top coat and its response to scratches. During hand wash of a car at normal conditions (25 ◦ C) small gravel particles on the surface of a top coat will be pushed into the surface with low shear rates. The coating will respond mainly elastic and probably no scratches will emerge, whereas in automatic car wash units the brushes exert high impacts on the gravel particles. This high impact causes a brittle response of the coating and fracture type scratches will remain. Similar behaviour can be expected if stones are catapulted onto a coating surface. The coating may behave elastic and/or plastic in summer (high temperatures) with no visible damage, but brittle in winter (low temperatures) with significant scratches produced. Stress-strain analysis can also be done dynamically. Figure 3.14 shows the typical diagrams obtained in dynamic mechanical analysis (DMA) measurements as a function of temperature. The dynamic mechanical analysis has the advantage over linear stress-strain curves (as shown in Figure 3.13) that the elastic and plastic components of the modulus can be separated, however, neither tensile-at-break nor elongation-at-break can be determined, since the coating must stay intact during the measurement. The stress is usually applied in an oscillating way and the phase angle difference between applied stress and resultant strain and amplitude of deformation is measured and calculated into the storage modulus (E = σ0 cos δ/ε0 ) and the loss modulus (E = σ0 sin δ/ε0 ). In the low temperature regions the network is in a glassy state, where only local molecular motions (vibrations or rotations) can take place which may give rise to so called β-transitions. With increasing temperature, the main transition region (also called α-transition) defined by the glass transition temperature is reached. Above the glass transition temperature the E -(storage) modulus, which measures the recoverable part of the energy imparted by the applied stress,

78


F IG . 3.15. Temperature dependence of elasticity modulus.

drops considerably (note that the scale is logarithmic) by several orders of magnitude, thus indicating the transition of a glassy into a rubber-elastic state. The glass transition temperature is not as sharp as a melting point (it is a second order “kinetic” transition in contrast to a first order thermodynamic transition of Tm ), instead it appears as a transition region. Hence, the value of Tg is dependent on the cooling rate of the sample as well as the fundamental chemical/molecular composition, the molecular weight, and junction point density. The most important influencing factors on the glass transition temperature are given in terms of structure–property relationships in Chapter 6. The loss modulus, which is a measure of the viscous response, reflects that a part of the imparted stress is dissipated as heat in the form of viscous flow. The maximum of the tangent δ (= E /E ) is used to define the Tg . This value, however corresponds to a temperature, where the E-modulus already has dropped by more than one order of magnitude. Therefore, often instead of the tangent δ, , is used as the T . The maximum of the E curve the maximum of the loss modulus, Emax g corresponds more to the onset of the decay of the E -modulus and therefore to the beginning of the glass transition region. Thus, in coatings where a broad glass transition region appears, the differentiation if a coating is in the glassy or already in a soft state, is better when using the Emax value16 as the indicator for the glass transition temperature (Tg ). According to equation (3.2.6) the crosslink density can be calculated from the E-modulus in the rubber-elastic region. This can be shown by the E-modulus curves of Figure 3.15. We have seen from Figure 3.13 that the stress strain analysis gives information about the modulus, the elongation at yield (where viscous flow starts) and the elongation at break. Wu17 has studied the modulus and elongation data of linear polymers (e.g., polymethyl methacrylate) and superimposed the curves (Figure 3.16) as a function of temperature. He has also shown that the modulus drops above the glass transition temperature. The elongation is very low at low temperatures and there is no yield point, that means that the


79

F IG . 3.16. Temperature dependence of mechanical properties, elongation and modulus in a linear polymer (e.g., PMMA).

polymer breaks by brittle failure. At around 50 ◦ C (still below Tg ) greater elongations show up and a yield point appeared. At somewhat higher temperatures the elongation of break of this polymer starts to increase rapidly still at a temperature below Tg . Wu defined the temperature at the intercept of the elongation at break and the elongation at yield curves as the brittle–ductile transition temperature (Tb ). Below this temperature the polymer is brittle, between Tb and Tg it is hard and ductile and above Tg it is getting soft. Theses transitions have been determined for linear polymers, in Chapter 5 it will be discussed how Tb is influenced by crosslinking. The homogeneity of polymer networks may also be evaluated by DMA measurements. It has been shown for the polycondensation of polyol-melamine (1 K) thermal curing systems, that the E exhibits a relatively sharp drop at a baking temperature of 110 ◦ C, and the transition curve shifts parallel to higher temperatures up to a baking temperature of 130 ◦ C. Beginning at 130 ◦ C and higher baking temperature the onset and Tg remain constant, but the transition region broadens, which can be explained in this case with the beginning selfcondensation of the melamines, resulting in inhomogeneous network structures.18 Thus, the evaluation of the broadness of the glass transition region may also be used to determine the homogeneity of network structures. A complete phase diagram of curing (phase transformations from liquid to glass) has been provided in the case of thermal curing systems with the TTT (transition, time, temperature)-diagram analysis (Gilham,19 Figure 3.17). In these (isothermal cure schedule) diagrams the vitrification line marks the time necessary for the Tg to coincide with the cure temperature. However, the same Tg can be obtained by isochronal cure schedules (not designed to be depicted in the TTT diagram). It has been found that samples cured isothermally or isochronically, despite having the same Tg , exhibited different shear strength. Thus, the choice of the cure schedule, in the case of radiation cured systems, the irradiance, exposure time, cure temperature, may greatly influence the ultimate mechanical properties.20 A time-temperature-energy (TTE) diagram for e-beam cured coatings has

80


F IG . 3.17. What happens during cure? Temperature–time–transition (TTT) diagram: A map of thermal curing (from liquid over rubber to glass).

been proposed.21 For UV cured coatings the influence of the irradiance on the Tg has been shown by DMA analysis of the UV induced homopolymerization of hexanediol diacrylate (HDDA). The Tg (tangent delta) increases from −23 to 62 ◦ C as a result of increasing irradiance.22 Furthermore, the conversion limitation due to vitrification of room temperature UV cured systems will be discussed in Chapter 6 (see Figure 6.3). By increasing the cure temperature an increase in conversion and Tg can also be obtained. To get a full phase diagram about UV curable systems an irradiance(time)-temperature diagram should be generated, since conversion limitations of room temperature cured UV systems due to vitrification can be overcome by increasing irradiance (leads to temperature increase due to polymerization heat) or increasing cure temperature. 3.4.1.2 Hardness and flexibility What is hardness? Hardness can be defined as the resistance of a material to localized deformations, caused, for example, from indentation, scratching, cutting or bending. In metals, ceramics and most thermoplastics and coatings, the deformation considered is plastic or fracture deformation of the surface. For elastomers and some polymers, hardness is defined as the resistance to elastic deformation of the surface. The lack of a fundamental definition indicates that hardness is not a basic property of a material, but rather a composite one with contributions from the yield strength, true tensile strength, modulus, and others factors. In practice, hardness is measured in terms of the size of an impression made on a specimen by an indenter of a specified shape when a specified force is applied for a specified time, the indent being measured after the force has been removed. There are three principal standard methods for expressing the relationship between hardness and the size of the impression, these being Brinell, Rockwell, and Vickers. For practical and calibration reasons, each of these methods is divided into a range


81

of scales, defined by a combination of applied load and indenter morphology, to cover the range of hardness. Recently, with the introduction of instrumented indentation hardness, it has become possible to measure the indent under the applied force. In the coating industry very common for measuring the hardness of coatings are pendulum, pencil, and indentation hardness. The pendulum method evaluates hardness by measuring the damping time of an oscillating pendulum. These tests are performed according to ASTM D 4366 and DIN EN ISO 1522 or DIN 53157 (König). The pendulum rests with two steel balls on the coating surface. When it is kicked off, the viscoelastic behavior of the coating determines the oscillation time. Hard coatings exhibit long oscillation times. The pendulum hardness tester may be equipped with “König” or “Persoz” pendulum: König: 200 g, ball weight, 5 mm ball diameter; deflection start at 6 ◦ and end at 3 ◦ with a period of oscillations of 1.4 s (reference: damping on glass 250 s); Persoz: 500 g ball weight, 8 mm ball diameter; deflection start at 12◦ and end at 4 ◦ with a period of oscillations of 1 s (reference: damping on glass 430 s). In the literature values are mostly expressed as König or Persoz hardness in seconds, however, sometimes also as number of oscillations, which may be recalculated into seconds (one oscillation with König corresponds to 1.4 s). The rapid and inexpensive Pencil hardness (ASTM D 3363) test relies on the comparison of film hardness with the hardness of calibrated drawing leads (hardness scale from soft to hard: 6B-5B-4B-3B-2B-B-HB-FH-2H-3H-4H-5H-6H). A coating is placed on a horizontal surface and the pencil is held against the film at an angle of about 45 ◦ and pushed away from the operator in a stroke (about 7 mm). The pencil hardness given is the hardness of the pencil which does not cut into the film. The indentation hardness (ASTM D 1474) is the quotient of the applied load and the surface area of the impression that is present underneath at a given load. The indentation can be done with a point or ball or in the case of nano-indentation with tips of different shapes (Fischerscope, Berkovich, Cube corner, see Figures 3.19 and 3.21). Instrumented indentation hardness23 provides the ability to measure the indenter penetration h under the applied force F throughout the testing cycle and is therefore capable of measuring both the plastic and elastic deformation of the material under test (see, for example, Figures 3.21 and 6.16). Flexibility is tested in a mandrel bend test (ASTM D 522), where the coated panels are bent over a mandrel and the resistance to cracking is determined or in a test where a ball type mandrel is forwarded into the backside of a coated metal plate and the penetration depth without cracking of the coating is expressed in mm (Erichsen cupping; ISO 1520; DIN 53156). 3.4.1.3 Crosslink density The crosslink density can be experimentally determined by measuring the modulus in the rubber elastic state as described before (3.2.6) by Zosel.9 Furthermore, the crosslink density can be determined by swelling experiments. The swelling can be measured by gravimetric methods (weight before and after immersed in a (good) solvent for a given period), or by measuring the change in height as the sample swells.

82


F IG . 3.18. Laboratory scratch testing apparatuses.

According to Flory’s theory the crosslink density can be calculated from the volume ratio of the swollen and unswollen network.24 3.4.1.4 Test methods for abrasion resistance The most common test method for determination of abrasion resistance is the Taber Abraser method (Figure 3.18). It consists of measuring the weight loss occurring when the coated (wooden) substrate is subjected to rotating abrasive wheels with defined load. This method is chosen in order to simulate the abrasion occurring on wooden flooring during walking. The tests are standardized by many methods (e.g., DIN EN 438, ASTM D 4060-90, DIN 53754, DIN 53799). The most common used wheels are Calibrase resilient wheels composed of rubber and aluminum oxide abrasive particles, whereof the CS-10 is relatively soft and causes mild abrasion, whereas CS-17 is harder and causes harsh abrasion (an overview about the wheels available is given in ref. 25). Another method for differentiating very abrasion resistant coatings is the use of sand paper. Paper of the size no. 180 is extremely abrasive and can also be employed to assess the coating sandability. The resistance to abrasion can be expressed in several ways, the most common are the weight loss (in mg) per number of cycles or the number of cycles (per unit of film thickness) needed to wear through. Since the abrasive paper gets clogged easily, the DIN EN 438 requires the changing of the paper after 500 cycles or after getting clogged. Based on this, the reproducibility is rather low and therefore in Scandinavia the method known as falling sand has been developed (SIS 923509). In this test a fixed amount of corund particles of


83

specific size fall into the path of a leather covered wheel that rotates in contact with the coated substrate to be examined. The abrasion resistance is given in weight loss per cycles. The falling sand test is a simple method of testing abrasion. Standardized sand is released and guided through a tube onto the test specimen. The volume of sand required to obtain total erosion of a known coating thickness indicates the abrasion resistance (ASTM D 968, DIN 1164). Since the mechanism involved in abrasion and the rates of stress application vary so widely one single test will be comprehensive to predict the wear life of different coatings for different applications. Therefore, for every application the best simulation test has to be chosen and the mechanical properties of the coatings to be optimized to perform well in the application and the simulation test. 3.4.1.5 Test methods for scratch resistance A multitude of test methods exist to simulate the scratching conditions to which a coating may be subjected to, for example, the scratches occurring during car wash or on parquet flooring due to high heel shoes. Among them are the multi scratch tests, like the laboratory car wash simulation unit of Amtec–Kistler26,27 or Elcometer 1730, modified Crockmeter28,29 tests, Rota–Hub30 or ScotchBrite® scratching tests (Figure 3.18). The abrasive tests, like Taber test,31,32 or falling sand tests, are rarely used in automotive clear coat testing, but in clear coats for parquet, detecting the abrasive materials loss. The single scratch tests for indentation only (Fischerscope), scratch (CSM-Nanoscratch) or combined indenter and scratch (Hysitron Triboscope) method are used to characterize the material properties. Furthermore many other more or less standardized tests exist. Most of these tests have a drawback in common: they typically damage the surface more severely than typical real life cycles in order to generate optical visible and distinguishable damage in short time scales. The extent of the damages is dependent on the forces, velocities, and geometries applied as well as on the test conditions, like humidity and temperature. In all tests, some sort of abrasive bodies (sandpaper, particles, brushes) are moved with specified normal force several times across the sample surface. Classification of scratch resistance is then done by visual inspection, optical gloss (Figure 3.19), color change (greying) or abrasive material loss detection. For automotive coatings, nearly every automotive company uses different tests or test parameters which sense different but not completely clarified aspects of scratch resistance. Since it simulates the car wash reality very closely, the laboratory car wash unit AMTEC-Kistler is used very often as a standard test method.33 In the AMTEC test, usually two types of coatings exhibit good scratch resistance, highly crosslinked and rubber-type materials, which is due to the high restoring forces caused by the “rubber-like” elastic behavior. Since the rubber-type materials, however, are susceptible to swelling when contacted with typical “automotive” chemicals, which may interact with the coating surface, like gasoline, they exhibit poor chemical resistance. In order to distinguish between hard and the rubber-type coatings, often a second scratch test method reflecting the hardness of the surface, like the Crockmeter test or methods which reflect the crosslink density, like dynamical mechanical analysis (DMA), are applied in addition to the AMTEC test. The DMA method, which evaluates the viscoelastic properties of the material has also been used to correlate crosslink density with scratch resistance.34

84


F IG . 3.19. Principle of gloss and haze measurement.

In order to simulate the scratching and simultaneously measure the mechanical response parameters of coatings occurring during indentation or scratching, a variety of single contact measurement systems has been developed. A review of different test methods for generating scratches and evaluating the scratch resistance of the coating has been published recently.35 A widely used instrument to evaluate hardness and mechanical parameters is the Fischerscope, which measures the surface mechanics of coatings by vertical indentation on the surface with a diamond tip under defined forces, whereas the nano-scratcher (e.g., CSM Nano-Scratch Tester) evaluates the slip properties by lateral scratching with defined tip geometries and forces. The newest generation of scratch test equipment is able to perform both, indentation vertical to the surface, lateral scratch set-ups as well as imaging of the resulting damage.36 Kutschera et al.37 for instance describe the single contact testing using a Hysitron TriboIndenter® system mounted on a Digital Instruments Nanoscope Dimension 3100 AFM, a Berkovich type indenter tip with sharp edges (pyramidal diamond 142.3◦ opening angle) to perform the indentation of different coating surfaces with load, hold and unload cycles (Figures 3.20 and 3.21). This method allows the determination of the mechanical response as a function of elastic, plastic and fracture response ratio. The correlation of the detailed mechanical responses with scratch data can therefore help to optimize the coating properties in order to develop scratch resistant systems (see chapter 10 for scratch resistant UV cured automotive top coats). 3.4.1.6 Internal stress and shrinkage38 ent reasons:

The build-up of internal stress can have differ-

• Temperature stress, caused by different thermal expansion coefficients of coating and substrate, resulting in a different volume contraction; • Polymerization shrinkage in a gel or glassy state, resulting in a stress due to insufficient relaxation; • Volume expansion or contraction due to humidity or solvent/water evaporation in a gelled or glassy state.


85

F IG . 3.20. AFM tool with nanoindenter set-up.

F IG . 3.21. Sketch of the nanoindentation measurement principle (top) and analysis scheme (elastic, plastic, fracture according to Shen et al.).

86


F IG . 3.22. Shrinkage as a function of functionality per volume.

Although UV curing proceeds normally at room temperature, the heating of the coating may be substantial, since the curing reaction proceeds very fast and therefore considerable heat is released which can heat up the coating by more than 50 ◦ C. Thus, the Tg of the final coating will rise above room temperature and if the expansion coefficients of the coating and the substrate are different, during cooling, and even more if the adhesion of the coating to the substrate is high, internal stress will develop. The thermal stress can be described by the following equation39 : σ =

E (αf − αs ) dT ,

from T1 to T2 ,

(3.4.2)

where αf and αs are the expansion coefficients of the film and substrate, respectively. In radically polymerizable acrylate coatings the acrylate double bonds are converted during cure to covalent bound single bonds. Due to this coupling reaction, the total volume changes from the van der Waals distance of the acrylates to the shorter covalent bonds formed, thus, resulting in a shrinkage due to volume contraction. According to the literature, there is a correlation between the internal stress and the gap of the temperature of measurement to the glass transition temperature of the coating.40 As shown in Figure 3.22, the cure of highly crosslinked UV materials can result in shrinkages of up to 35%. Although shrinkage occurs during the whole curing process, internal stress will only develop after gelation has occurred. However, since the gel point of radically cured UV systems occurs at a very early stage, the shrinkage of the material can result in large internal stresses during the cure after gelation. These developed internal stresses can lead to deformation or cupping, as well as to delamination or adhesion failure.


87

F IG . 3.23. Determination of internal stress by the cantilever-beam method.

The internal stress can be measured by optical41 or mechanical methods, whereof the cantilever beam method42 has been used for the shrinkage and stress determination in model UV-curable formulations.43 The latter method and the equations for the determination of the internal stress σi are shown in Figure 3.23. The internal stress developed in the model UV coating based on the difunctional hexanediol diacrylate (HDDA) and the monofunctional trimethylolpropan-monoformal-acrylate (TMPFMA) as a function of the HDDA content and the temperature is shown in Figure 3.24. In this example it is shown that considerable stress is built-up due to shrinkage occurring during polymerization. The internal stress decreases with increasing temperature, since relaxations are occurring. The internal stress increased exponential with increasing glass transition temperature of the coatings (Figure 3.25), corresponding to a higher content of the difunctional resin (HDDA), similar to the curves reported by Zosel.44 The glass transition temperatures plotted are based on Tg measured with DSC (differential scanning calorimetry) and with DMA (dynamic mechanical analysis), using the maximum of the E loss modulus. In Figure 3.26, the internal stress of the same HDDA/TMPFMA (0% HDDA to 100% HDDA) coatings is plotted as a function of the shrinkage. From this figure it is obvious that the coating with no HDDA crosslinker develops no stress despite a shrinkage of about 10% (obtained by the density method: shrinkage = 100% × (density cured film − density liquid)/density cured film). The glass transition temperature of this linear homopolymer is below 20 ◦ C, thus below the measurement temperature. A considerable internal stress develops not until shrinkage occurs at Tg ’s above room temperature. Figure 3.27 shows the internal stress as a function of humidity and temperature. Firstly, the internal stress is higher the lower the temperature is, secondly, it looks

88


F IG . 3.24. Internal stress as a function of temperature in coatings based on HDDA and TMPFMA.

F IG . 3.25. Internal stress as a function of Tg in coatings based on HDDA (%) and TMPFMA (Tg by Emax and DSC).

like water penetrating into the film has a softening effect and hence reduces internal stress. A crosslinked polymer that, upon exposure to light, exhibits stress and strain relaxation without changes in materials properties has been reported recently by the Bow-


89

F IG . 3.26. Internal stress as a function of shrinkage in coatings based on HDDA (%) and TMPFMA (shrinkage by cantilever beam method).

F IG . 3.27. Internal stress as a function of humidity in coatings based on HDDA (60%) and TMPFMA (40%).

man group.45 The approach is based on addition-fragmentation chain transfer reactions, similar to the RAFT mechanism in controlled radical polymerization,46 with allyl sulfides as addition-fragmentation agents. The crosslinked polymer is build-up with the thiol-ENE chemistry containing a tetrafunctional thiol (pentaerythritol tetra-

90


(3-mercapto-propionate), triethylenglycol divinylether, the ring opening monomer 2methyl-7-methylene-1,5-dithiacyclooctane (MDTO47 ) and 1,6-hexanedithiol. The MDTO was introduced into the backbone as the reversible cleavage allyl sulfide functionality. The stress relaxation during exposure has been shown. This effect is not due to photodegradation, since the moduli of the samples are the same before and after irradiation. 3.4.1.7 Measurement of the curing and hardness gradient within a coating The radiation attenuation within the coating layer (according to Beer’s law) causes a gradient of energy density throughout the coating layer resulting in a decrease of conversion of functional groups from the surface to the bottom layer. Such a gradient can be measured by Confocal Raman Microscopy (see Chapter 2, Figure 3.13).48 As a result of this gradient the cure conversion and thus, the crosslink density decreases from the surface to the bottom, if negative effects of oxygen inhibition at the surface are eliminated (e.g., by inert curing). Added pigments or UV absorbers can enhance such a gradient formation. With decreasing X-link density also the hardness should decrease. Since the hardness gradient can not be followed directly by Confocal Raman microscopy and furthermore added pigments may cause complications in the Raman determination, Meichsner and coworkers evaluated the hardness gradient by micro- (Fischerscope® H100) and nano-indentation (Hysitron two-dimensional triboscope® ) methods.49 The micro-indentation hardness can only be measured at film thicknesses above 40 µm, because at lower thicknesses the hardness measurement is influenced by the hardness of the substrate. Thus the gradient was determined by evaluating the difference of the hardness of a normally exposed sample (under inert conditions) to a sample, where the exposure was done from the backside through quartz glass (transparent to UV light). This exposure setup should introduce a gradient from the quartz interface to the surface, with higher X-link density at the interface and decreasing hardness in the direction of the surface. The measurement of the hardness here was also done at the surface, but the surface layer in this setup corresponds to the bottom layer of a normal exposure. This setup showed that the hardness of the films with thicknesses higher than 40 µm exhibited a gradient in hardness. Whereas the hardness stayed constant in the case of the normally exposed sample, as expected, the hardness decreased with increasing film thickness in the quartz setup. At a thickness of 105 µm the hardness corresponding to the bottom layer was about two-third and at 160 µm about half of the hardness of the surface. With nano-indentation the hardness can be measured at very thin films and this allows the measurement of a gradient directly of a cross-section of the exposed film layer. The hardness and elasticity modulus measurements corroborated that a hardness gradient exists. In both cases the absolute values decrease to about 40% at the bottom of a 200 µm thick film.

3.4.2

Chemical characterization

The chemical characterization of networks is pretty difficult since they are not soluble. Thus, test methods evaluate the extractables or the swelling behaviour. The choice of the


91

F IG . 3.28. Chemical resistance test methods.

extraction or swelling solvent is usually made by durability needs of the coating. Extraction tests are done in order to characterize the sol fraction of the network. They are often done with good solvents for the employed raw materials (e.g., methylene chloride, tetrahydrofurane, N-methyl-pyrollidone). The extractable can then be characterized by standard analytical methods (NMR, IR, SEC, . . .). Standard tests with a specific solvent, like methylethyl-ketone (MEK) are used in double rub tests for the characterization of the network quality. The test is done with a tissue soaked with MEK, rubbing over the coating (one double rub is a stroke forth and back). An uncrosslinked thermoplastic rubs away within few double rubs, whereas with increasing crosslinking the number of rubs increases. Highly crosslinked coatings withstand 200 double rubs (where the test is usually stopped) and more. 3.4.2.1 Chemical resistance The chemical resistance of the coating surface is usually tested according to occurring or anticipated stresses. Automotive coatings, for example, are subjected to a variety of chemicals, like acid rain, bird excrements, tree resins, gasoline and so on, which may act upon the surface at various climate conditions, for example, at temperatures as high as 80 ◦ C. These chemicals should not leave any marks or damage at the coating surface. Therefore, the chemical resistance test for automotive applications determines the highest temperature where no visible marks of applied droplets after treatment are occurring (with the gradient oven method). Applied chemicals are sulfuric acid (simulation of acid rain (1 or 10% concentration)), pancreatine (as a model for bird excrements), tree resin, caustic soda, water, and gasoline. For wood coatings typical household chemicals, like coffee, mustard, red wine or ballpen ink, are applied and tested at room temperature (Figure 3.28).

92


REFERENCES 1. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, Hannover, 2003, Chapter 3, p. 323. 2. (a) Myers, T.G., Surface tension driven thin film flows, In “The first European Symposium on The Mechanics of Thin Film Coatings” (P.H. Gaskell, M.D. Savage and J.L. Summers, eds.). World Scientific, 1996, pp. 259– 268; (b) Orchard, S.E., Appl. Sci. Res. A 11, 451–464 (1962); (c) Smith, N.P.D., Orchard, S.E. and RhindTutt, A.J., J. Oil Col. Chem. Assoc. 44, 618 (1961); (d) Overdiep, W.S., Prog. Org. Coat. 14, 159–175 (1986); (e) Wilson, S.K., IMA J. Appl. Math. 50 (2), 149–166 (1993). 3. Mercurio, A. and Lewis, S., J. Paint Technol. 47 (607), 37 (1975). 4. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), “Organic Coatings, Science and Technology”, Vol. II. John Wiley & Sons, New York, 1994, pp. 15–21. 5. Stepto, R.F.T. (Ed.), “Polymer Networks, Principles of their Formation, Structure and Properties”. Blackie Academic&Professional, London, 1998. 6. Meichsner, G., Deuter, F., Groß, Th., Beck, E. and Menzel, K., Farbe&Lack 103 (8), 45–50 (1997). 7. Berger, J. and Huntgens, F., Angew. Makromol. Chem. 76/77, 109 (1979). 8. (a) Macosko, C.W. and Miller, D.R., Macromolecules 9, 199 (1976); (b) Miller, D.R. and Macosko, C.W., Macromolecules 9, 206 (1976). 9. Zosel, A., In “Lack und Polymerfilme” (U. Zorl, ed.). Vincentz Verlag, Hannover, 1996. 10. Steeman, P.A.M., Dias, A.A., Wienke, D., Alig, I. and Lellinger, D., Recent developments in monitoring film-formation and cure of coatings, In “Coatings Science International (CoSi 2005), Book of Abstracts”. 2005, pp. 24–29. 11. Alig, I. and Lellinger, D., Chem. Innov. 2, 12–18 (2000). 12. Goldschmidt, A. and Streitberger, H.-J., “BASF Handbook on Basics of Coating Technology”. Vincentz Network, Hannover, 2003. 13. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P., “Organic Coatings, Science and Technology”, Vol. 2. Wiley, 1994, p. 105. 14. Schnecko, H., Makromol. Chem. 76/77, 1 (1979). 15. Hill, L.W., “Mechanical Properties of Coatings”. Federation of Societies for Coating Technology, Blue Bell, PA, 1987. 16. Frey, Th., Große-Brinkhaus, K.-H. and Röckrath, U., Progr. Org. Coat. 27, 59–66 (1996). 17. Wu, S.J., J. Appl. Polym. Sci. 20, 327 (1976). 18. (a) ref. 16; (b) Hill, L.W. and Kozlowski, K., J. Coat. Technol. 59 (571), 63 (1987). 19. (a) Enns, J.B. and Gillham, J.K., J. Appl. Polym. Sci. 28, 2831 (1983); (b) Wisanrakkit, G., Gillham, J.K. and Enns, J.B., J. Appl. Polym. Sci. 41, 1895 (1990). 20. Zumbrunn, M.A., Wilkes, G.L. and Ward, Th.C., In “Radiation Curing in Polymer Science and Technology, Vol. III, Polymerization Mechanism” (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, Chapter 4, pp. 101–151. 21. Kim, H.-C. and Wilkes, G.L., J. Radiat. Cur. 16 (1), 8 (1989). 22. Klosterboer, J.G., Adv. Polym. Sci. 84, 1 (1988). 23. http://www.npl.co.uk/force/guidance/hardness/references.html. 24. http://campoly.com/notes/005.pdf (Cambridge Polymer Group, Boston, MA). 25. http://www.labomat.com/english/Taber.html. 26. http://www.amteckistler.de. 27. Meier-Westhues, U., Klimmasch, Th. and Tillack, J., Eur. Coat. J. 9, 258 (2002). 28. (a) American Association of Textile Chemists and Colorists, AATCC 8, 116 & 165; (b) ASTM D6279–98 (Method for rub abrasion mar resistance of high gloss coatings). 29. Trumbo, D.L., Rudelich, J.C. and Mote, B.E., In “Perspectives on New Crops and New Uses” (J. Janick, ed.). ASHS Press, Alexandria, VA, 1999, pp. 267–271. 30. Used, for example, in “UV curable clearcoats” by Jung, T. and Valet, A., RadTech Report, November/December, 2001, pp. 30–35. 31. Gregorovich, B. and Hazan, I., Progr. Org. Coat. 24, 131 (1994). 32. ASTM Committee in Annual Book of ASTM Standards, American Society for Testing and Materials, West Conshohocken, 1998, p. 6.


93

33. ISO/FDIS 20566:200, Paints and varnishes-determination of scratch resistance of a coating system using laboratory car wash (also German Standard DIN 55665). 34. AIF project 12567: http://www.fpl.uni-stuttgart.de/Aktuelles/AiF12567.pdf. 35. Courter, J.L. and Kamenetzky, E.A., Eur. Coat. J. 7, 100 (1999). 36. (a) Schwalm, R., Beck, E. and Pfau, A., Eur. Coat. J. 1, 107 (2003); (b) Meichsner, G., Burk, T., Zhang, H., Larbig, H., Sander, R. and Kutschera, M., Farbe&Lack 111 (7), 27–31 (2005). 37. Kutschera, M., Sander, R., Herrmann, P., Weckenmann, U. and Poppe, A., http://www.coatings-science. chem.tue.nl/2005/speakers/Poppe.htm. 38. Meichsner, G., Mezger, T. and Schröder, J., “Lackeigenschaften Messen und Steuern – Fließeigenschaften, Grenzflächen, Kolloide”. Vincentz Verlag, Hannover, 2003. 39. Perera, D. and van den Eynde, D., J. Coat. Technol. 53 (677), 39 (1981). 40. (a) Perera, D. and van der Eynde, D., Progr. Org. Coat. 28, 21 (1996); (b) Pang, Z. and Ling, X., Beijing Huiagong Daxue Xuebao, Ziran Kexueban 22, 22 (1995). 41. Askadskii, A., “Physical Properties of Polymers”. Overseas Publishers Association, Amsterdam, 1996. 42. Corcoran, E., J. Paint Technol. 41 (538), 635 (1969). 43. Maday, M., Diploma Thesis, FH Esslingen, 1998. 44. Zosel, A., In “Lackeigenschaften Messen und Steuern – Fließeigenschaften, Grenzflächen, Kolloide” (G. Meichsner, T. Mezger and J. Schröder, eds.). Vincentz Verlag, Hannover, 2003. 45. Scott, T.F., Schneider, A.D., Cook, W.D. and Bowman, C.N., “Photoinduced Plasticity in Crosslinked Polymers”, Vol. 308. Science, June 2005, pp. 1615–1617. 46. Meijs, G.F., Rizzardo, E. and Tang, S.H., Macromolecules 21, 3122 (1988). 47. Evans, R.A. and Rizzardo, E., J. Polym. Sci. A 39, 202 (2001). 48. Schrof, W., Farbe&Lack 103 (7), 22 (1997). 49. Meichsner, G., Burk, T., Zhang, H., Larbig, H., Sander, R. and Kutschera, M., Monitoring the cure gradient of UV-cured coatings by microindentation hardness and atomic force microscopy, 8th Nürnberg Congress, Creative Advances in Coating Technology, Congress Proceedings, Vol. 2. Vincentz Network, Hannover, 2005, pp. 173–180.

C HAPTER 4

Raw Materials The raw materials described in this section are classified into the categories of radical polymerization systems, cationic polymerization systems and radical polyaddition systems (Thiol-ENE). The raw materials used for the formulation of UV coatings consist of low molecular weight resins, typically in the molecular weight range of 300–5000 g/mol. The main resins types are radically polymerizable unsaturated polyesters, acrylate terminated molecules, like polyepoxides, polyesters, polyethers and polyurethanes as well as epoxides and vinylethers. Epoxide terminated oligomers and vinyl ethers are cured cationically. An overview of the main resin types used in UV curable systems is given in Figure 4.1. By far the most applied resins are the radical polymerization type acrylates and unsaturated polyesters, the thiol-ene systems are often discussed because of their insensitivity against oxygen inhibition, but yet only used in few niche applications.1 The cationic polymerization

Radical Acrylates Epoxy acrylates Polyester acrylates Urethane acrylates Polyether acrylates Acrylated polyacrylates Acrylated oils

Cationic Epoxides Bisphenol Adiglycidyl-ether 3,4-epoxycyclohexylmethyl-3 ,4 epoxycyclohexane carboxylate (ECC)

Photoinitiator free Donor–acceptor systems Maleimide-acceptor N-ethyl-MI N-phenyl-MI Vinyl ether-donor Triethylenglycol di-vinylether Allyl ether-donor

Unsaturated Polyesters

Vinyl ethers VE oligomers (RadTech’98 US, Conf. Proc. p. 53)

Self initiating β-keto-Michael addition oligomers

Poly-ENE-Thiol systems Isoprene–styreneblock copolymers F IG . 4.1. Resin types for UV curing systems. 94

RAW MATERIALS

95

systems are also used only in a few metal coating applications, whereas the photoinitiatorfree systems are just in the developmental stage. Since most of these resins are often too high in viscosity, they are diluted with reactive diluents to adjust the application viscosity. Mono- or multifunctional acrylates are mainly used as reactive diluents, whereas methacrylates, or non-acrylate monomers, like styrene, vinyl pyrrolidone, divinyl ethers and few others are less frequently applied. Vinyl ethers and monoepoxides are used as reactive diluents in cationically curable coatings. The photoinitiators are responsible for the effectiveness of the curing reaction. The vast majority of used photoinitiators are forming radicals, which add to unsaturated double bonds, whereas the cationic photoinitiators upon exposure form a Broensted (H+ ) or Lewis acid, which initiates the cationic polymerization of epoxides or vinyl ethers. Photoinitiatorfree systems rely on the direct formation of radicals upon exposure, for example, with electron beams, where σ -bonds are cleaved and radicals formed. The donor–acceptor systems form radicals from the excited complex by rearrangements. Such systems therefore do not need an external photoinitiator. Besides these essential raw materials of a UV curable formulation, additives, like surfactants, defoamers, leveling agents, flow regulators, flexibilizers, pigments (mainly in printing inks), UV stabilizers (UV absorbers and HALS type radical scavengers), or fillers (clay, calcium carbonate, silica) as well as nanoparticles, which are transparent and may provide higher scratch resistance, are used according to requirements.

4.1 RADICAL POLYMERIZATION SYSTEMS 4.1.1 Standard Resins 4.1.1.1 Unsaturated polyesters The unsaturated polyesters were the first class of resins used in UV curing, now their overwhelming use is in fibre reinforced composites and only a small fraction is used in UV curable laminating adhesives and wood fillers. Such unsaturated polyesters (UPE), derived by the condensation reaction of maleic or fumaric acid with various diols, dissolved in styrene, were the earliest used UV curable resins. Because of the toxicity of styrene, these systems are not used extensively any more. Multifunctional acrylates, like TPGDA or TMPTA, have been used instead of styrene as a reactive diluent in UPE resins for adhesives and ink applications. Recently, powder resins based on unsaturated polyesters have been introduced, obtained by mixtures of UPE with vinyl ether polyurethane crosslinkers2 or mixtures of UPE with allyl ether polyesters.3 4.1.1.2 Standard acrylate terminated oligomers The acrylate resins now dominate the market. The schematic structure of the main acrylate terminated resin classes is shown in Figure 4.2. From the resin classes the epoxy acrylates are the biggest on the market, prepared by the reaction of epoxides, e.g., Bisphenol-A diglycidylether, with acrylic acid (Figure 4.3). These Bisphenol-A type acrylated epoxides are the dominant products on the market and account for about 70% of all epoxy acrylates used. Phenol-formaldehyde resin based (Novolac)-glycidylethers acrylates are used as a speciality in solder film resists, because

96


F IG . 4.2. Schematic chemical structure of main acrylate resin types for acrylate based radically curing UV coatings.

F IG . 4.3. Synthetic scheme of aromatic epoxy and urethane acrylates.

RAW MATERIALS

97

TABLE 4.1. Synthesis of an epoxy acrylate4 Using a vessel equipped with an agitator, a reflux condenser, an inert gas inlet and a charging port, there was charged 660 g Epon 828. Epon 828 is a diglycidyl ether of Bisphenol-A, having an epoxy equivalent weight of about 185–192 g and an epoxide value of 0.50–0.54. Through the gas inlet was introduced a nitrogen flow which was maintained throughout the resin preparation cycle. While under agitation, 238 g of glacial acrylic acid was added and stirred until epoxy resin dissolved. Upon dissolution of the acrylic acid, 1.3 g of triethyl amine and 0.1 g of hydroquinone were added. Under continued agitation, heat was raised to 95–100 ◦ C and held at this temperature until an acid number of 5–10 was obtained (∼10 h). Temperature was reduced to 65–70 ◦ C and 600 g hydroxypropyl acrylate was added and the mixture was stirred until uniform. The final product was a clear solution of prepolymer in monomer at a 60/40-weight ratio.

of their high temperature resistance. The epoxy precursors are to a lesser extent modified with fatty acids or amines. Similar products to the fatty acid modified epoxies can also be obtained by the reaction of epoxidized soybean oil with acrylic acid. Due to the high viscosity of especially the Bisphenol-A type epoxy acrylates, they are delivered usually diluted with reactive diluents, like hydroxypropyl acrylate, dipropylene glycol diacrylate, tripropylene glycol diacrylate or hexanediol diacrylate. Besides these aromatic epoxide precursors, more expensive aliphatic epoxy resins play only a minor role in the market. The epoxy acrylates are distinguished by a high reactivity and the cured coatings exhibit good chemical stability. Main uses are paper coatings and inks as well as wood coatings. An example for the synthesis of such a resin is given in Table 4.1. Urethane acrylates are simple addition products of multifunctional isocyanates, like toluene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate or their condensation products, e.g., isocyanurates, biurets, allophanates, with polyols and hydroxyalkyl acrylates, for instance, hydroxyethyl acrylate, hydroxybutyl acrylate or pentaerythritol triacrylate. Since the addition reaction proceeds very well, the coatings or ink formulating companies produce a large portion of the urethane acrylates captively. However, also a large variety of different urethane acrylate resins are available by the raw materials suppliers. The applications are mainly on plastics, with the dominant application on PVC floor coverings, wooden parquet, screen inks and optical fibres. These applications require good optical properties and non-yellowing behaviour, thus more than 80% of the used urethane acrylates are based on aliphatic isocyanates. Urethane acrylates with low functionality exhibit a high flexibility and are often based on flexible polyester or polyether diols, which are reacted with bifunctional isocyanates and endcapped with hydroxyalkyl acrylates (Figure 4.3). A typical synthesis procedure of a urethane acrylate is given in Table 4.2. Since the viscosity of the urethane acrylates is relatively high, they are often diluted with reactive thinners like TPGDA or HDDA. However, if the flexibility of the coatings should be increased, rather than using flexible diols, monofunctional diluents, like ethylhexyl acrylate, 2-(2-ethoxyethoxy) ethyl acrylate or trimethylolpropane-formal-monoacrylate are also used.

98


TABLE 4.2. Synthesis example of an aromatic urethane acrylate5 A polycaprolactone (98 g, produced as described in US Patent 3,169,945), having an average molecular weight of 530, and 52 g of 2-hydroxypropyl acrylate were placed in a 500 ml flask that was equipped with a stirrer, thermocouple and two dropping funnels. After the addition of one drop of dibutyl tin dilaurate the mixture was heated to 70 ◦ C in an oil bath. The solution was stirred while 69.6 g of an 80/20 mixture of 2,4- and 2,6-tolylene diisocyanates was added in a drop wise manner over a period of 2 h. The mixture was stirred for an additional 1.5 h and then 0.002 g of 4-methoxyphenol was added as a stabilizer. This product was a clear liquid. A solution containing 73 wt% of the acrylate capped polycaprolactone and 27 wt% of 2-butoxyethyl acrylate had a Brookfield viscosity of 2270 cps at 23 ◦ C.

F IG . 4.4. Toolbox to tune the properties of urethane acrylates.

The higher functional urethane acrylates are often used to obtain hard, scratch and chemical resistant coatings. Examples of such multifunctional type of urethane acrylate resins are given in Figure 4.3. Besides the good mechanical properties, these aliphatic type urethane acrylates exhibit good weatherability and do not yellow upon exposure to exterior conditions. Thus, they are the preferred class of resins for exterior applications. The structure of the urethane acrylates can be designed to the required properties by choosing the right balance of hard phase and soft phase, by tuning the setscrews molecular weight, glass transition temperature and crosslink density (Figure 4.4). The compilation of the desired properties of urethane acrylates, however, reveals that the individual measures are often diametrically opposed and that a compromise always has to be made in order to adjust the most desired properties. Examples of available aliphatic urethane acrylate resins by raw materials suppliers are listed in Table 4.3.

99

RAW MATERIALS

TABLE 4.3. Available flexible and hard urethane acrylates BASF AG

Bayer AG

Cognis

Cytec Surf

Sartomer/CV

Rahn

Laromer® UA

Desmolux® UA

Photomer®

Ebecryl®

Sartomer® (SR) Craynor® (CN)

Genomer®

CN966H90 CN965 CN9001 CN936

4188/EHA 4215 4269/1122

CN922 CN925 952B75 CN 975

4302 4590/PPTTA

Flexible 19T 9028V 9030V 9033V

VP LS 2220 VP LS 2258 VP LS 2220 XP LS 2413 XP LS 2430 XP LS 2989

6008 6210 6891

401 8307 8402 8406

Hard LR 8987 9029V 9048V 9050V

VP LS 2265 VP LS 2308

6623 6613

1290 8305 264

F IG . 4.5. Conventional and non-NCO route to urethane acrylates (UA).

Recently, novel routes to urethane acrylates have been published which do not rely on the reaction of hydroxyalkyl acrylates with isocyanates, but make use of the well known reaction of amines with cyclic ethylene or propylene carbonate to form hydroxyethyl- or hydroxypropyl urethanes, which are then acrylated by classical transesterification reactions6 or by enzymatic esterification7 (Figure 4.5). The use of this route has the clear advantage

100


F IG . 4.6. Examples of the synthesis of polyester and polyether acrylates.

TABLE 4.4. Synthesis example of a polyester acrylate8 A mixture of pentaerythritol (78.0 g), stabilized methacrylic acid (344.0 g) (molar ratio of alcohol:acid = 1:8), concentrated sulfuric acid (7.75 g) and cupric oxide (0.5 g), dry toluene (300 ml) was stirred under reflux. The water formed during the reaction was continuously removed using a Dean and Stark trap. After 24 h the required quantity of water (32.0 ml) had been removed. Some charring and polymer formation was evident. The organic layer was washed with water, dried (magnesium sulphate) and the solvent removed in vacuum to afford viscous brown oil. Upon triturating with ethanol, a white crystalline solid (163 g) was obtained, which, after recrystallisation from the same solvent, had an mp. 66–67 ◦ C (70% yield).

that the portfolio of available products is not restricted any more to the limited number of available isocyanates, but rather on the multitude of available amines. Polyester and polyether acrylates are synthesized by esterification of polyester/ether polyols with acrylic acid (Figure 4.6). Examples of polyesterols are the condensation products of adipic acid with diethylene glycol, 1,6-hexane diol or trimethylol propane or their ethoxylated or propoxylated derivatives. Higher functional resins can be obtained by the use of triols, like trimethylolpropane or glycerol, or tetrols, like pentaerythritol, in the reaction with for example adipic acid and acrylic acid. The molecular weights of such resins are typically in the range of 500–2000 g/mol. There is a large variety of polyester acrylates available on the market. These resins are mainly used in wood coatings and paper coatings, and to a lesser extend in inks. The polyester acrylates used in wood coatings are mainly applied in top and undercoats.

RAW MATERIALS

101

F IG . 4.7. Amine modification of acrylates.

Polyetherols often used are ethoxylated or propoxylated glycerol or trimethylol propane. Such polyether acrylates represent a class of resins of low viscosity, which can be used as sole resins as well as reactive diluents. Due to their higher molecular weight the skin irritation levels are lowered significantly compared to conventional diluents. Applying the substance on rabbit eyes and evaluating the redness often determine the irritancy. The irritancy is expressed with a Draize value (Draize rating: 0–2: slightly irritant; 2–5: irritant; 6–8: severe irritant). Rabbits are used because they are inexpensive, have large eyes, and are easy to handle. However, the transferability to humans is disputed, since the rabbit is considered as an inappropriate and inaccurate model for human ocular damage.9 The Draize values of such ethoxylated and/or propoxylated reactive diluents are in the range of 0–2. The amine modification of acrylate resins is usually done in order to incorporate the amine synergist, which is used mainly in combination with hydrogen abstraction type photoinitiators, into the polymer backbone, and therefore increases reactivity, and reduce odour and extractables. The amines are introduced into the resins by a Michael Addition type reaction of primary or secondary amines with acrylates (Figure 4.7).10 The amine content used is normally rather low (in the 2–10% range). This type of resins is mainly used in graphic and wood applications, where high reaction speeds are required and yellowing does not play a significant role. Acrylated polyacrylates are obtained preferably by the polymer analogous modification of functional polyacrylates containing pendant hydroxyl, epoxide, acid, or anhydride groups. These groups are subsequently reacted with complementary functional group containing unsaturated monomers by esterification or addition reactions, as pointed out in Figure 4.8. The acrylated acrylic prepolymers offer good exterior durability, low colour and good chemical resistance.

102


F IG . 4.8. Synthesis of acrylated polyacrylates by transesterification (left) and acrylic acid addition (right).

Aromatic epoxy acrylates – Based on Bisphenol-A diglycidether – High viscosity – High reactivity – Diluted with reactive thinners – Good chemical stability – Good cost/performance ratio – Tendency to yellowing

Aliphatic urethane acrylates – Pronounced toughness and flexibility – Non-yellowing – High weathering resistance – High viscosity – High price

Polyester acrylates – Good overall performance (allrounders) – Large variety of hard or flexible resin – Low residual monomers – Low odour – Good compatibility (with other resins)

Polyether acrylates – Low viscosity – Applicable as resins or reactive diluent – High reactivity

Amine modified acrylates – Coinitiators – High reactivity – Low odour, low migratables

Acrylated polyacrylates – Large variety of polyacrylatols available – High exteriour stability

F IG . 4.9. Principle property spectrum of standard type acrylate resins.

Since the requirement spectrum of a coating is so manifold, in most of the cases one resin type cannot fulfill all the demands. This is the reason why so many different resin types exist. The principle property spectrum of the different standard resin types is given in Figure 4.9.

RAW MATERIALS

103

F IG . 4.10. Specialty resins: Example of a dendritic acrylate.

4.1.2 Specialty Resins 4.1.2.1 Dendritic and hyperbranched resins Dendritic polymers have gained attractiveness because of their unique structure as well as unique properties compared to their linear or branched homologues. They are classified into the two categories of dendrimers (a typical structure is shown in Figure 4.10), characterized by a perfect symmetrical globular shape and monodisperse molecular weight distribution, and hyperbranched polymers, which are less perfect in their structure and shape, exhibiting a polydispersity, but are easier to prepare at a lower cost level. Hyperbranched polyacrylates exhibit a considerable advantage against conventional acrylates as they offer low viscosity at relatively high molecular weights and high functionalities. Comparisons of the properties of linear and hyperbranched polymers have been published recently in general reviews.11 Low-viscosity hyperbranched polymers have been developed for radiation curing applications.12 A new product has been described to be based on acrylated hyperbranched polyester/polyether polyol blend, which is hard and flexible, with hardness and scratch resistance as good as dipentaerythritolhexaacrylate (DPHA), but lower viscosity and better flexibility than DPHA. Hyperbranched polyacrylates described earlier are based on the polycondensation of dimethylolpropane carboxylic acid onto a polyol core (trimethylolpropane) and about 50% acrylation of the third generation polyester with about 24 hydroxyl groups.13 Dendritic polyols are commercially available under the trade name Boltorn® by Perstorp Corp. The series number H20, H30, H40 indicates the generation of the dendritic build up (second, third, fourth) showing an average hydroxyl number of 16, 32, and 64, respectively. Hyperbranched urethane acrylates have also been prepared on the basis of similar hyperbranched polyester structures, but functionalization with the monoadduct of isophoronediisocyanate with (alkoxylated) hydroxyalkyl (meth)acrylates.14

104


Boltorn® -type hyperbranched aliphatic polyesters have further been used for the design of waterborne polyurethane acrylate dispersions.15 The described hyperbranched acrylates are synthesized in at least two-steps by first producing the hydroxyl or carboxyl terminated hyperbranched molecule, which is then functionalized with unsaturated acrylate groups. A one-step synthetic process has been reported by the use of Diels–Alder reaction of polyfunctional sorbic and polyfunctional acrylates, where for example the condensation products of sorbic acid with trimethylolpropane and dipropylene glycol-diacrylate are reacted in a Diels–Alder reaction.16 The acrylates have to have at least one function less than the sorbates in order to yield dendritic polymers under reproducible conditions. The viscosities are low compared to linear polyester acrylates (in the range of 1–10 Pa for molecular weight in the range of 1000–3000 g/mol). They exhibit good UV reactivity and low UV energies are sufficient for curing. Dendritic acrylate oligomers of a different structure type have been prepared by the Michael-type reaction of ethylenediamine with trimethylolpropane-triacrylate in a molar range of 1 to 5 under mild conditions.17 In order to use acrylated Boltorn type polyesters for powder applications, the end groups have been modified with semi crystalline (octadecyl) groups, in order to introduce a crystalline melting point at 50–60 ◦ C.18 4.1.2.2 Photoinitiator-free vinyl ether/maleimide systems A limited number of UVcurable systems containing a combination of electron donor and electron acceptor monomers, which are capable of being cured without an additional photoinitiator have been recently developed by Jönsson.19 The best studied and best performing systems are based on a N-substituted maleimide as the acceptor monomer and a vinyl ether donor monomer. The reactivity of such systems without any photoinitiator was found to be in the order of the acrylate resins containing a photoinitiator.20 The maleimide has an absorption in the 300 nm wavelength range, whereas the vinyl ether does not absorb above 250 nm. The mechanism proposed is based on two types of initiation, the intermolecular excimer formation to generate diradical species and the hydrogen abstraction type from the triplet state of the maleimide (Figure 4.11). The formed maleimide radical as well as the hydrogen donor radical can add to another maleimide or to vinyl ether. Since the vinyl ether does not homopolymerize, the structure of the resulting network is composed of alternating MI and VE units, as well as homopolymer MI sequences and excess MI with minor isolated VE units. The aliphatic maleimides exhibit the highest reactivity, especially those containing readily abstractable hydrogens, like propoxylated or ethoxylated derivatives; however, the toxicity of the specific maleimides has to be evaluated carefully. Other monomers used in the photoinitiator-free polymerization of donor and acceptor molecules are given in Table 4.5.20 4.1.2.3 Miscellaneous resin types Self-initiating UV-curable resins of the structure shown in Figure 4.12 have been introduced recently.21 They are obtained by the Michaeladdition-type reaction of beta keto esters to diacrylates. This chemistry is very versatile to the design of new UV-curable resins that do not need an additional photoinitiator. This is, because upon UV exposure these resins undergo alpha cleavage next to the ketone moiety

RAW MATERIALS

105

F IG . 4.11. Donor–acceptor-type photoinitiator-free initiation mechanism.

TABLE 4.5. Donor–acceptor combinations Vinyl ethers

Maleimides

Triethyleneglycol divinyl ether 4-Hydroxybutyl-vinyl ether

Triethyleneglycol bismaleimide (Ciba) C36-alkyl bismaleimide (Q-bond, Quantum) 2-Ethylcarbonate ethylmaleimide

to split off an acetyl radical and generate another radical on the polymer backbone. Both radicals can then polymerize the acrylate groups. Another new raw material type of resins is based on metal containing monomers (Figure 4.12). Such polymers, like crosslinked zinc diacrylate22 promote adhesion to metal and glass substrates and may provide reversible crosslinking at the ionic site in order to be used for UV hotmelt adhesives or self-healing films. 4.1.2.4 Silicon based oligomers Silicon based oligomeric (meth)acrylates are used in so-called hybrid polymers based on sol–gel reactions, where an inorganic network is obtained via the siloxane condensation and an organic network via the UV or thermal polymerisation of reactive groups like acrylates or methacrylates. Such systems can also be classified as dual cure systems. Oligomers of this type can be synthesized by the reaction of functional (trialkoxy)-siloxanes (–OH, –NH2 , epoxy) with functional (meth)acrylates. UV-curable hybrids, like ORMOCER® (organic modified ceramic) systems,23 as described in Chapter 9, can, for example, be based on the commercially available 3-methacryloxypropyl-trimethoxy silane24 as part of the system (Figure 4.12).

106


F IG . 4.12. Specialty resins: Self-initiating, metal ion bridged and silicone acrylates.

4.1.2.5 Polyenes The photocrosslinking of thermoplastic elastomers combines the properties of thermoplastics and elastomers.25 Polymers used are based on block copolymers building a hard phase (e.g., polystyrene, glassy domain) and an elastomeric phase (e.g., polybutadiene), which can be crosslinked via the unsaturation in the elastic phase. Such crosslinking reactions have been described for styrene-butadiene26 and acrylonitrilebutadiene (ABA).27 The crosslinking reaction can be greatly enhanced by the addition of mulitfunctional acrylate monomers or multifunctional thiols. The reaction leads to rapid insolubilization of the thermoplastics and to the improvement of other properties, like higher adhesion failure temperatures and moderate hardening of the elastomeric phase, however, the elastic character was retained. Such systems can advantageously be used in patterning applications, like flexography and photolithography. 4.1.2.6 Dual cure resins Dual curing (UV initiated and thermal) can be performed by using blends of the classical resins, like acrylate terminated oligomers and polyisocyanates/polyol or melamine–formaldehyde/polyol combinations. According to this concept a large variety of viable chemistries is around. Discussed here briefly are resins, which contain two functional groups in “one” molecule, where one group is accessible to UV and the other to thermal curing. Belonging to this class are the described epoxyacrylates, which contain the unsaturated acrylate group and a secondary hydroxyl group, which can for instance be reacted with isocyanates or melamine resins. Complementary to such hydroxy acrylates are isocyanato acrylates, containing the acrylate and an isocyanate group in the same molecule. Such compounds can be prepared by the understochiometric reaction of hydroxyalkyl acrylates with polyisocyanates, like the trimers (isocyanurate, biuret) of hexamethylene diisocyanate or isophorone diisocyanate. Since such molecules suffer from relatively high viscosity, novel molecules based on allo-

RAW MATERIALS

107

F IG . 4.13. Specialty resins: Dual-cure resins based on isocyanato acrylates.

F IG . 4.14. Composition scheme of PU (PUD) and dual cure dispersions.

phanates, synthesized by the allophanatization of hexamethylene-diisocyanate and hydroxyethyl acrylate as the alcohol component have been commercialized.28 Different synthetic routes have obtained similar structures.29 The intramolecular instead of intermolecular hydrogen bridging, as explained in Figure 4.13, is responsible for the reduced viscosity of this type of isocyanato acrylates. 4.1.2.7 Water-based systems The composition scheme of polyurethane acrylate and dual curable dispersions is shown schematically in Figure 4.14. The polyurethane acrylate

108


dispersions typically comprise polyisocyanates reacted with diols (and/or small amounts of polyols), hydroxyalkyl acrylates and compounds containing acid groups, like carboxylic or sulfonic acids, as well as a functional groups which reacts with the isocyanate, e.g., amino or hydroxyl groups. Such UV-curable polyurethane dispersions are very well known in the field (see ref. 30 and references therein). Dual curable dispersions furthermore contain blocking groups for the isocyanates and functional groups that are able to react with the (finally) released isocyanates. 4.1.2.8 Polymer definition According to the new EU regulation REACH, new chemical species have to be registered (see also Chapter 11) and subjected to a bunch of testing procedures. However, there are exemptions to the registration obligation, and one exemption refers to polymers according to the OECD polymer definition, which just have to be notified and require only a reduced test package. This exemption will foster the future development of resins to fulfill the polymer criteria. In order to be considered as polymers according to the OECD definition the following four prerequisites on a substance are required. Polymer means a substance consisting of: a) Molecules characterized by the sequence of one or more types of monomer units; b) A simple weight majority of molecules containing at least three monomer units that are covalently bound to at least one other monomer unit or other reactant; c) Less than a simple weight majority of molecules of the same molecular weight; and d) Molecules distributed over a range of molecular weights; wherein differences in the molecular weights are primarily attributable to differences in the number of monomer units. There are also several exclusions from the exemption (e.g., cationic polymers, degradable or unstable polymers, polymers containing more than 2% of a monomer not listed in EINECS). Furthermore, several other constraints have to be considered, for example, if a multifunctional starter molecule, like glycerol is used in an ethoxylation reaction, the sequence of monomers has to be 3 mol of oxirane attached to one alcohol group of the glycerol, thus for statistical reasons (if not otherwise evidenced), a polymer is only formed if more than 6 mol of oxirane is applied per mol glycerol, see, for example, ref. 31. 4.1.2.9 Stabilizer/inhibitors The stabilization of the acrylate resins during synthesis and storage against undesired polymerization has been described in several publications;32 most of the occurring reactions are also involved in the degradation and autooxidation of polymers. The stabilization issue can be separated into the stages of the synthetic process and long-term stabilization, as well as differentiated by the conditions of handling under aerobic or inert atmosphere. In all many different radicals may be involved in the reaction scheme, as depicted in Figure 4.15.33 As indicated, these different radical types need and enable various paths of possible stabilization measures. The formation of an initial radical may be triggered by heat, shear force induced by pumping, by catalyst residues in redox reactions or by exposure to light. Such primarily generated carbon-centered radicals form in the presence of oxygen (right side of Figure 4.15) in a very fast reaction a hydroperoxide radical (R–OO• ). The further addition of a monomer unit is very slow, thus a very effective way of stabilization of the monomers against unwanted polymerization in the presence of

RAW MATERIALS

109

F IG . 4.15. Types of radicals produced during processing or storage and stabilization possibilities.

oxygen is the scavenging of the hydroperoxide radical by phenolic antioxidants. The thus formed (hindered) phenolic radicals do not initiate polymerization. If the hydroperoxide radicals are not captured by the phenolic antioxidants they abstract further hydrogen and form hydroperoxides (R–OOH) and regenerate alkyl radicals. The hydroperoxides can also produce further radicals by decomposition to the very reactive alkoxy and hydroxy radicals under the impact of UV light, heat or catalytic residues (left side of Figure 4.15). Thus, in order to prevent the decomposition of hydroperoxides to the detrimental radical, often hydroperoxide decomposers are added, which destroy the hydroperoxide to form harmless products. Very effective hydroperoxide decomposers are organophosphorus compounds, like phosphites and phosphonites.34 Under inert conditions the primarily generated C-radical or the reaction products of these radicals with monomers must be scavenged effectively. There are only few effective C-radical scavengers mentioned in the literature. 4.1.2.9.1 Inhibition under aerobic conditions The inhibition of the undesirable polymerization of acrylates during synthesis or storage in the presence of air or reduced air content (of about 6% air in nitrogen) is mainly due to the formation of less reactive peroxide radicals, which are then completely deactivated by inhibitors reacting with oxygen centered radicals. The mechanism of the inhibition under oxygen conditions is explained in Figure 4.16. The most commonly used aerobic inhibitors belong to the chemical class of phenols, like monomethylether of hydroquinone (MEHQ), hydroxyanisol or 2,6-di-tertbutyl-kresol. There are a numerous number of phenolic antioxidants available, either partly or fully substituted with steric bulky substituents in the 2, 4, and/or 6 position. These

110


F IG . 4.16. Mechanism of inhibition of polymerization under oxygen conditions (MEHQ inhibitor).

F IG . 4.17. Inhibition mechanism of phenolic antioxidants and side reactions.

phenolic antioxidants inhibit the polymerization effectively, however, reaction products formed may also contribute to discolouration (Figure 4.17). Whereas the phenols are mostly applied as long-term stabilizers during storage, MEHQ is also used in the synthesis, for example, the esterification with acrylic acid, because of its high volatility, which contributes to the stabilization of the vapour phase. The deactivation

RAW MATERIALS

111

is mainly based on the scavenging of the low reactivity peroxy radicals. In the absence of oxygen MEHQ does not inhibit the polymerization at all. A very effective natural occurring antioxidant is Vitamin E, which has also been used as stabilizer in synthesis of acrylates35 and has been evaluated as a model for improving the action of synthetic phenolic antioxidants,36 showing, that the introduction of an alkoxy group in any of the 2, 4, or 6 positions should have a superior stabilizing power. Other molecules described to scavenge both; alkyl and alkoxy radicals under low oxygen concentrations are benzofuranone derivatives.37 Hydroperoxide decomposers are often used during synthesis, in order to decompose hydroperoxides formed during the course of the reaction. An example is represented by the following equation: P–[O–Ar]3 + ROOH → O=P–[O–Ar]3 + ROH.

(4.1.1)

Since these compounds are susceptible to hydrolysis, in practise mainly substituted hydrolytically stable phosphites or phosphonites are used. These stabilizers are especially useful if polyetherols are used in the esterification reaction with acrylic acid, since the ethers are very susceptible to the formation of hydroperoxides. Further effective hydroperoxide decomposers frequently used are copper salts, thioesters or metal complexes of thiocarbamates.38 4.1.2.9.2 Inhibition under anaerobic (inert) conditions The mechanism associated with inhibition of the polymerization under anaerobic or inert conditions is related to two reactions occurring, the addition of the inhibitor molecule to a primarily formed radical, and second, the stabilization of the unpaired electron. The technically most used inhibitor for the stabilization under inert conditions is phenothiazin.39 Another class of very effective C-radical scavengers are the nitroxyl radicals, for example the 2,2,6,6-tetramethyl piperidine-N-oxyl (TEMPO), which are very reactive towards carbon centered radicals, but not towards the unpaired electron at an oxygen. Despite the fact that phenothiazin or N-oxyles are very effective inhibitors, their widespread use is hampered by two disadvantages. The first being related to their effectiveness, since in the desired photopolymerisation, the inhibitor has to be overrun by a higher concentration of photochemically produced radicals, in order to start the crosslinking reaction. And the second is due to the discoloration associated with the use of these compounds.

4.1.3 Reactive Diluents As reactive diluents monomers and oligomeric acrylates or vinyl ethers are used in order to adjust the application viscosity (Figures 4.18 and 4.19). Despite the fact that monomers like styrene, N-vinyl pyrrolidone40 and monofunctional esters of acrylic acid, belong to the best diluents, their use is decreasing due to their high volatility, strong odour, skin irritation and flammability. Since their mono-functionality provides a higher molecular weight between crosslink’s or lower crosslink density, which results in a better flexibility, monomers with

112

Methacrylates Hydroxypropyl-MA Isobornyl-MA Dicyclopentenyl-oxy-ethyl-MA Hydroxyethyl-MA

Cationic: Mono-epoxides, Vinyl ethers, allyl ethers, oxetanes F IG . 4.18. Reactive diluent types for UV-curing systems.

Others Styrene N-Vinyl caprolactam N-Vinyl pyrrolidone N-Vinyl formamid Acrylamidomorpholine Silanes Vinyl ethers Divinylether of Tripropylenglycol Divinylether of cyclohexanedimethanol Vinylether capped urethanes


Acrylates Monofunctional acrylates Isobornyl acrylate (IBOA) Trimethylolpropane-formal-mono-acrylate Phenoxyethyl acrylate (POEA) Difunctional acrylates Tripropylene glycol diacrylate (TPGDA) Dipropylene glycol diacrylate (DPGDA) Hexandiol diacrylate (HDDA) Neopentylglycol diacrylate (NPGDA) Multifunctional acrylates Trimethylolpropane triacrylate (TMPTA) (Ethoxylated/propoxylated) TMPTA Propoxylated glycerole triacrylate Pentaerythritol triacrylate (PETIA) Pentaerythritol tetraacrylate (PT4 A) Dipentaerythritol penta/hexa acrylate

RAW MATERIALS

113

F IG . 4.19. Structure of main acrylate reactive diluents.

lower volatility and odour, like isobornyl acrylate or trimethylol propane formal monoacrylate, have been developed. Multifunctional monomers are available in a broad range since they are produced by esterification reactions of acrylic acid with polyols; the main representatives are tripropylene glycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA), propoxylated glycerol triacrylate (GPTA, like OTA480), and hexanediol diacrylate (HDDA). Most of the others have only a small market share. Higher functional acrylates such as pentaerythritol tetraacrylate (PTA) or dipentaerythritol hexaacrylate (DPHA) are also available, but only used as minor ingredients since they increase dramatically the crosslink density resulting rapidly in hard but brittle films. Such resins and diluents are commercially available from Cytec, BASF, Akcros, Cray Valley/Sartomer and Henkel. Reactive diluents used in cationic polymerizing systems are mainly mono- or difunctional epoxides and vinyl ethers. The idealized structure of these compounds however does not represent exactly the real composition of such reactive diluents, since some side reactions occur during the esterification with acrylic acid, mainly Michael-type additions of the acrylic acid as well as Michael addition of the various alcohols on the double bonds of the formed multifunctional acrylates (Figure 4.20). The vinyl ethers have been evaluated as reactive diluents in radical polymerizable acrylate and unsaturated polyester41 systems, as well as in cationic polymerizable epoxy coatings.42 It has been shown, that the advantages of vinyl ethers compared to acrylates are their high diluting power, which is often better than acrylates (e.g., tripropylene glycol diacrylate

114


F IG . 4.20. Structure of trimethylolpropane triacrylate (TMPTA) and addition by-products formed during synthesis.

(TPGDA)), and their lower toxicity (no labelling, no irritancy). The performance of the systems evaluated, triethylene glycol divinylether (DVE-3) compared to TPGDA as a reactive diluent, revealed similar mechanical properties, but higher reactivity and scratch resistance. The disadvantages are due to the kinetics, since the vinyl ethers do not homopolymerize radically, but rather react in an alternating polymerization, the content should be no higher than 50-mol%. Furthermore, it has been shown by Decker43 that in such formulations upon radical photoinitiation the acrylate radical is twice as reactive towards the acrylate double bond than towards the vinyl ether double bond. Thus, in order to be fully converted, and therefore avoid unreacted residual vinyl ether groups in the coating, which are prone to acidic hydrolysis, resulting in the formation of acetaldehyde, the content of the vinyl ether reactive diluents should not exceed about 10% in acrylate systems, and the range of 10– 20% in epoxy systems, however, in unsaturated polyesters the vinyl ether content can be up to 50%.

4.1.4

Radical Photoinitiators

Photoinitiators are molecules that absorb photons upon irradiation with light and form reactive species from their excited state, which initiate consecutive reactions (see Figure 2.4). The initiating species may be radicals, cations or anions. Radical photoinitiators, which represent more than 90% of commercially used initiators, are available in large number from companies like Ciba Specialties (trade names Irgacure® and Darocure® ), Lamberti (Esacure® ), BASF (Lucirin® ), and many others. Available in smaller number are cationic photoinitiators, mainly sulfonium salts, from Union Carbide (Cyracure® ), Degussa (Degacure® ) as well as iodonium salts from General Electric and iron complexes from Ciba. Examples of commercially available photoinitiators are listed in reviews published by Crivello,44 Fouassier,45 and Davidson.46

RAW MATERIALS

115

F IG . 4.21. Examples of “alpha-cleavage type” and non-cleavable “electron transfer-hydrogen abstraction type” photoinitiators.

4.1.4.1 Standard radical type photoinitiators Almost all radical photoinitiators contain the benzoyl(phenyl–CO–) structure element. The two most important classes are the αcleavable (Norrish type I) and the non-cleavable (“hydrogen abstraction” type II) photoinitiators (Figure 4.21 and Table 4.6). A photoinitiator for radical polymerization should exhibit a bunch of properties, from which the following are the most important: a) High absorption at the exposure wavelength and high molar extinction coefficient; b) High quantum yield of formation of initiating species; c) High reactivity of the radical to the monomer. The α-cleavage-type photoinitiators are very versatile, exhibiting higher efficiency compared to hydrogen abstraction types due to the unimolecular cleavage reaction and consequently they are the most widely used. The quantum yields of dissociation of the cleavable photoinitiators are for example 0.3 (2-benzyl-2-dimethylamino-1-[4-(4-morpholinyl) phenyl]-1-butanone), 0.7 (2,4,6trimethyl-benzoyl)-diphenyl phosphine oxide), 0.8 (1-hydroxy-cyclohexyl-phenyl-ketone), 2-hydroxy-2-methyl-1-phenyl-1-propanone) or 0.95 (α, α-dimethoxy-α-phenylacetophenone). The addition rate constants (M−1 s−1 ) of the formed radicals to monomers (acrylates and methacrylates) are shown in Figure 4.22.

116


TABLE 4.6. Most important photoinitiator classes, product examples and main uses Chemical class

Examples

Physical state

Main use

Benzophenones

Benzophenone (Genocure® BP)

Powder

General: low cost; in combination with amine synergists

α-Hydroxy ketones (α-HK)

1-Hydroxy-cyclohexylphenyl-ketone (Irgacure® 184)

Powder

General surface cure; non-yellowing

2-Hydroxy-1-[4(2-hydroxyethoxy) phenyl]-2-methyl-1propanone (Irgacure® 2959)

Powder

Clear coats

2-Hydroxy-2-methyl1-phenyl-1-propanone (Darocure® 1173)

Liquid

Benzil-dialkylketal

α, α-Dimethoxy-αphenylacetophenone

Powder

Wood, PCB, printing plates

(BDK)

(Irgacure® 651)

α-Amino ketones

2-Benzyl-2dimethylamino-1[4-(4-morpholinyl) phenyl]-1-butanone (Irgacure® 369)

Powder

Electronics (PCB), inks

Phenyl glyoxylates (PG)

Methyl-benzoylformate (Genocure® MBF)

Liquid

Labelling free

Thioxanthones (ITX)

Isopropyl-thioxanthone (Genocure® ITX)

Powder

Inks

Acylphosphine oxides (APO)

2,4,6-Trimethylbenzoyl)-diphenyl phosphine oxide (Lucirin® TPO) Phenyl-bis-(2,4,6trimethylbenzoyl) phosphine oxide (Irgacure® 819)

Powder TPO-L (liquid)

General through cure, inks (pigmented)

Powder, mix: liquid

Exterior coatings (UV-absorber containing)

RAW MATERIALS

117

F IG . 4.22. Dissociation quantum yields of photoinitiators and addition rate constants of the corresponding radicals to monomers.

The benzoyl radical formed in almost all cleavable type photoinitiators adds at a low rate to the acrylate or methacrylate monomers. The aliphatic ketyl or phosphinoyl radicals react at a two orders of magnitude higher rate with (meth)acrylate monomers.47 To increase reactivity amine-containing coinitiators (synergists) are used which have two effects. First, the C–H group adjacent to the nitrogen is a good hydrogen atom donor and the thus formed radical can initiate the polymerization and/or second, the radical can scavenge oxygen very effectively and the initially formed hydroperoxide also abstracts a hydrogen from another amine moiety, forming again the reactive amine radical, which can initiate polymerization or add oxygen in another cycle until all oxygen is consumed (Figure 4.23). The photoinitiators are typically differentiated by their absorption profiles (Figure 4.24), characterized by the absorption wavelength range and the molar absorption strength defined by the molar absorption coefficient. Typical loadings of photoinitiators are in the range of 1–5%, often 2–3% are reasonable. Such high concentrations are often only necessary in order to overcome oxygen inhibition effects, since under inert curing conditions loadings of 0.5–1% are enough. The loading should not be too high in order to avoid filter effects that may prevent light penetration to the bottom layers resulting in through cure problems. The photoinitiators also have to be selected in order to match with the output spectrum of the UV light source. The medium pressure mercury lamps are the standard lamps used since they provide high power and emission lines where most of the commercially available photoinitiators absorb (see Figure 4.25).

118


F IG . 4.23. Amine synergist action (hydrogen donor; initiation and oxygen scavenging).

F IG . 4.24. Absorption spectra of different photoinitiators in acetonitrile at a concentration of c = 0.01 g/l (Lucirin® TPO, c = 0.025 g/l).

4.1.4.2 Low yellowing photoinitiators The yellowing during photoinitiation is a general problem to which several factors contribute, the photoinitiators, the resins, stabilizers, exposure conditions and so on. The effect of the various influencing factors on the yellowing behaviour will be discussed in detail in Chapter 7. The structures of the photoinitiator as well as the photoproducts, however, also contribute to yellowing. Especially the formation of semiquinoid or aromatic bi-ketonic structures contributes to yellowing, as, for example, in the case of benzildimethylketal (BDK).48 Thus, one strategy in the design of non-yellowing photoinitiators was the design of systems that generate non-absorbing

RAW MATERIALS

119

F IG . 4.25. Chemical structure of aromatic and cycloaliphatic epoxides, standard sulfonium salt and novel-type of cationic photoinitiator (arylthianthrenium salt).

(non-yellowing) photoproducts. Photoinitiators whose benzoyl moiety is linked to aliphatic residues are following this design rule, as it is the case with hydroxyalkylacetophenones (α-HK). 1-Hydroxy-cyclohexyl-phenyl-ketone is therefore one of the best non-yellowing type photoinitiator. Another approach is based on the photobleaching, represented by the class of acyl phosphine oxides, which are yellowish per se, but turn into colourless products after photocuring. 4.1.4.3 Photoinitiators for pigmented, UV-stabilized coatings and through cure The longer wavelength (>300 nm) absorbing photoinitiators of the acyl phosphine oxide type (Lucirin® TPO or Irgacure® 819) have been developed for the use in pigmented or UV absorber containing films as well as to provide good through cure in thicker coatings. The normal loading of this type of photoinitiators is in the range of 0.5–1%. They are often used in combination with hydroxyketone type photoinitiators that provide the surface cure while phosphine oxides provide the through cure. 4.1.4.4 Low emission photoinitiators Novel photoinitiators for low odour and low migration have been developed by Ciba, especially for graphics applications,49 based on a difunctional hydroxyketone, a special substituted aminoketone and a difunctional phenyl glyoxylate.50 Polymer bound photoinitiators are described in an overview by Carlini.51 Special types, based on the coupling of hydroxy benzophenone or hydroxy-thioxanthone to a polytetrahydrofurane diole52 have been introduced recently under the trade names Omnipol BP and Omnipol TX.

120


4.2

CATIONICALLY-CURABLE SYSTEMS

Interest and research activity in cationic UV curing has increased due to the insensitivity to oxygen inhibition. Until now this technology has not yet found the widespread use in many industrial applications compared to radical systems. Decorative and protective coatings, printing inks and adhesives are just a few examples of applications in which photoinitiated cationic polymerizations have experienced the most commercial growth.53 As with radically polymerizing UV coatings, the cationic curing is also a very rapid polymerization consuming little energy without the need for an inert atmosphere, hence providing important economic incentives. The cationic polymerization process is shown schematically on the example of epoxide polymerization (4.2.1). A proton or a Lewis acid activates the oxygen atom in order to facilitate a nucleophilic attack of epoxide oxygen onto the (positivated) carbon next to the activated oxygen. Upon ring opening a secondary hydroxide is formed and the chain reaction continues at the next activated oxirane ring.

(4.2.1) The thermal, mechanical, chemical resistance, low shrinkage and adhesion characteristics of the network polymers that are formed are excellent. However, their limited use up to now may be due to other problems of the cationic polymerization system than oxygen inhibition; for example, moisture can terminate the chain reaction. The industrial impact of cationic UV curing is predicted to increase markedly in the future as this technology undergoes further maturation.54 4.2.1

Cationically Curable Resins and Monomers

In general, two main groups of monomers are used in cationic polymerization, the first being ethylenic monomers containing groups that stabilize the carbocation by resonance, like aromatic rings (styrene, α-methyl styrene), a double bond (butadienes, isoprenes) or an ether group (vinyl ethers, oxetanes), and the second being heterocyclic monomers containing at least one heteroatom (O, S or P, like epoxides or caprolactones), for which the propagation reaction proceeds via onium ions. A large number of cationic resins and monomers have been evaluated.55 By far the main resin types used in radiation curable cationic coatings are based on epoxides, for example, the bis-glycidyl ether of Bisphenol-A or the cycloaliphatic 3,4-epoxy-cyclohexylmethyl3 ,4 -epoxy-cyclohexane-carboxylate (Cyracure UVR 6110 from Dow) and a large number of other cycloaliphatic epoxides,56 followed by vinyl ether terminated resins (Figure 4.25).

RAW MATERIALS

121

As reactive diluents epoxides (1,6-hexanediol-diglycidylether, HDDGE) or vinyl ethers (triethyleneglycol-divinyl ether, TEGDVE) are used. The cationic curing of epoxies together with vinylether57 or oxetane groups58 containing monomers has been evaluated. Triethylene glycol divinyl ether was shown to be more effective as reactive diluent in cationic curing epoxy systems (Cyracure UVR 6110) than HDDGE. With optimum TEGDVE concentrations in the range of 10–20% the viscosity could be decreased to the range of 50–100 mPa s, cure rate and conversion increased as well as solvent resistance. The use of 3-ethyl-3-hydroxymethyl oxetane (EHMO) also reduced the viscosity down to 150–100 mPa s at concentrations of 10–25%. Here also the cure rate and the solvent resistance are increased, as shown with typical formulations for clear coats and inks. Novel hyperbranched polymers, based on dipentaerythritol as the core molecule and 4,4 -bis-(4-hydroxyphenyl) valeric acid as AB2 monomer and functionalized with hydroxy, epoxy or oxetane groups have been evaluated in cationic UV curing59 of epoxy and oxetane systems as additives. They were incorporated into the network either by copolymerization or via chain transfer involving the hydroxy group. They showed an increase of toughness and modulus and better thermal stability, expressed as increase of weight residue above 450 ◦ C in TGA experiments.

4.2.2 Cationic Photoinitiators The most widely employed cationic photoinitiators are compounds belonging to two classes of onium salts, the diaryliodonium and triarylsulfonium types. Less important are diazonium salts and organometallic complexes, like ferrocenium salts (Irgacure® 261). Fouassier has reviewed the chemistry, excited state processes and reactivity of a large number of such initiators.60 The iodonium and sulfonium compounds are very efficient photoinitiators, producing Broensted acids as initiating species, however, they exhibit some disadvantages. Their spectral sensitivity is predominantly in the short wavelength UV, their toxicity, especially of the iodonium salts, has to be considered, and in general their solubility, in particular of the cheaper sulfonium salts, in coating formulations is rather low. Furthermore, these diaryl- or triaryl-onium salts release undesirable benzene during the photoreaction, which prompted regulation of these types in food packaging.61 New developments have been undertaken to tackle the problems described. A new photoinitiator recently introduced by Ciba is based on long alkyl chain substituted diaryl iodonium salts, thus avoiding the release of benzene (Ciba Irgacure® 250). Recent work published by Crivello62 makes use of the class of Dialkylphenacylsulfonium salts (DPS, see Figure 4.26), which can be easily synthesized in high yields by simple, straightforward reactions. The photoreactivity is comparable to the reactivity of the iodonium and sulfonium salts (Figure 4.26). Due to this new, easy synthetic route, it has been feasible to optimize the structure in order to produce highly photosensitive photoinitiators that are both very soluble in a wide variety of polar and non-polar monomers, thus, are suitable for the polymerization of all kinds of available cationic monomers. The DPS salts prepared by this method are stable, colourless, light-sensitive compounds. Since the crystalline DPS salts do not undergo thermal decomposition at temperatures up to their melting

122


F IG . 4.26. Comparison of the curing conversion of different cationic photoinitiators.

points, they can also be used as latent thermal initiators for the cationic polymerization of reactive monomers and oligomers. Another approach in order to avoid the release of benzene was published by Lamberti S.p.A.63 with the development of cyclic sulfonium salts of the Arylthianthrenium type (Figure 4.25). They are characterized by a fast cure and absence of toxic by-products during the photoreaction. Due to the good performance of the monothianthrenium salts and their good solubility in the formulations, a series of 5-alkoxyphenyl-thianthrenium derivatives particularly suitable for food packaging applications has been developed.

4.2.3

Photosensitization of Cationic Photopolymerisation

Diaryliodonium, triarylsulfonium and dialkylphenacylsulfonium salt cationic photoinitiators typically possess major absorption bands in the short wavelength region (65

2.0 4 >65

3.2 36 >65

2.4 20 >65

absorbing acylphosphine oxides (either Irgacure® 819 or Lucirin® TPO can be used) enables the throughcure. The coating is exposed to two ferrum added lamps (80 W/cm) at curing speed up to 10 m/min. Starting formulations for further optimization according to the specific needs are given for UV offset, UV flexo and UV screen inks, recommended for paper, board and film applications (e.g., BASF brochure: Make up your print products7 ) in Tables 5.8–5.10. The formulations contain either yellow, red, blue and/or black pigments, usually a grinding resin, like Laromer® LR 9013, polyester or epoxy acrylate binder resins, a photoinitiator combination containing standard photoinitiators facilitating surface cure and an acylphosphine oxide type photoinitiator (e.g., Lucirin® TPO-L), responsible for through cure, and optionally levelling agents and defoamers.

5.2.2

Formulation for Overprint Varnishes (OPV)

Overprint varnishes are applied to impart high gloss and protection to a printed surface, like post cards, catalogue covers, cosmetic cartons, etc.

147

FORMULATIONS

TABLE 5.10. UV screen ink UV screen ink components Pigment Heliogen® Blue D 7092 Special black 250 TiO2 Kronos® 2063S Resin Laromer® LR 9013 Laromer® LR 9019 Laromer® UA19T Laromer® LR 8986 Dipropyleneglycol diacrylate

Cyan (wt%)

Black (wt%)

White (wt%)

5 – –

1 4 –

– – 34

5 – 54 7 15

5 – 54 7 15

– 37 – 12

Photoinitiator Lucirin® TPO-L Irgacure® 369 Irgacure® 907 Darocure® 1173

1.5 1 0.5 2

1.5 1 0.5 2

4 – – 3

Co-initiator Laromer® LR 8956

6

6

–

2 1 – 100

2 1 – 100

3 6 1 100

Additives Aerosil® 200 CAB 551-001 (20% in DPGDA) Byk® 164 Viscosity (Pa s, 23 ◦ C) Yield (Pa, 23 ◦ C) Reactivity (m/min; 120 W/cm)

4 – –

3.7 – –

3.5 15 60

The basic performance requirements on overprint varnishes are characterized by: • • • •

High reactivity; High gloss; Excellent wetting of the ink; Low price.

These overprint varnishes are applied with a thickness of about 6–10 µm (g/m2 ) typically in a coating unit as part of a printing press or in a separate step on a coating machine. A typical formulation is based on a high content of multifunctional crosslinkers (for high reactivity), oligomeric resins (epoxy-, polyester- or amine-modified polyether acrylates) and high photoinitiator amounts (Table 5.11). Thus, a general formula for overprint varnishes contains about 30–70% of highly functional reactive diluent, 10–20% resins and photoinitiator amounts up to 8% and often also amine-synergist, in order to obtain high cure speeds.

148


TABLE 5.11. General composition of an overprint varnish formulation Component

(%)

Function

Oligomer acrylate resin

10–20

Film formation Basic performance properties

Reactive diluent

20–30

Film formation Viscosity adjustment

Trifunctional X-linkers

30–70

Coating properties

Photoinitiator

3–8

Initiation of curing; speed

Photosynergists

2–5

Cure speed increase

Additives

0–3

Surfactants Wetting, levelling agent, slip, . . .

TABLE 5.12. Comparison of standard OPV formulation and OPV for inert exposure Component

OPV (%)

Type

Standard

Inert

Oligomer resin

52

52

Epoxy acrylate (high reactivity and chemical resistance), Laromer® LR 8986

Reactive diluent

35

35

Viscosity adjustment, tripropyleneglycol diacrylate (TPGDA)

Photoinitiator

8 –

– 1

Benzophenone Lucirin® TPO

Additives

0.4

0.4

Surfactant and slip additives

In order to reduce the photoinitiator content, for example for formulations, which are designed for packages with indirect food contact (where only very small amounts of migratables are tolerated), curing can be done under inert conditions. The same cure speed can be achieved with formulations containing only 1% photoinitiator compared to 8% when curing under air (Table 5.12). Furthermore, due to the reduced photoinitiator contents, migratables and odour are reduced with such formulations cured under inert conditions.

5.3 SPECIALTY FORMULATIONS (FOR GLASS, POLYCARBONATE, METAL) In the following three examples of specialty formulations applicable as starting formulations for coating glass, plastics or metal surfaces are given.

149

FORMULATIONS

TABLE 5.13. Scratch resistant topcoat for glass Component

(%)

Type

Dipentaerythrolpenta-acrylate Propoxylated neopentylglycol-diacrylate Urethane acrylate Photoinitiator Photoinitiator

64.2 16.4 10 7 2

Sartomer® SR 399 Sartomer® SR 9003 Sartomer CN 965 Benzophenone Darocure® 1173

TABLE 5.14. Flexible starting formulation for UV coating of polycarbonate Component

(%)

Type

Aliphatic epoxyacrylate Trimethylolpropane formal-monoacrylate

60 10

Sartomer® CN 132 Sartomer® SR 531 or Laromer® LR 8887

1,6-Hexanediol diacrylate (HDDA) Photoinitiator Photoinitiator

25 2 3

Irgacure® 184 Darocure® 1173

TABLE 5.15. Formulation for cationic metal coatings Component

(%)

Function

Epoxy resin Inert flexibilizer Photoinitiator

60–70 20–30 1–3

Dow UVR-6160 Polycaprolactone triol Sulfonium salt (e.g., Dow UVI-6990) or Iodonium salt (e.g., Irgacure® 250)

Pigments or dyes Additives

1–2 0–3

Surfactants

Coatings suggested for high scratch resistance on glass (ref. 6) are based on high functional pentaerythrol pentaacrylate (Table 5.13). The reported thickness was about 100 µm and no basecoat was used. The starting formulation proposed for coating plastic polycarbonate (ref. 6) is much more flexible than the formulations described before. It is pointed out, that the use of a flexible basecoat improves abrasion resistance, as well as adhesion and flexibility. Since cationic coating systems based on epoxy resins exhibit little shrinkage, the adhesion of such coatings on metal surfaces is generally better than that of acrylate based formulations. Most of the cationic coatings rely on epoxy resins, flexibilizers and sulfonium or iodonium salts as photoinitiators (Table 5.15). 5.4 FORMULATION SCREENING FOR NEW APPLICATIONS Frequently asked questions are about how to start the formulation screening for a new application. For the traditional applications, like wood, plastic coatings and graphic arts

150


the formulation examples discussed before can be used as a starting point. Some general considerations for the design and selection of raw materials for formulations are given in Figure 5.2. For interior applications, the selection of the raw materials is governed by the demands of the application segment, the required properties and the application method. There are no overall general restrictions. On the other hand, exterior applications and UV curing of pigmented systems require a careful selection of raw materials as well as of the exposure equipment. Since pigments absorb more or less intensely in the UV, the photoinitiators and the lamp have to be tuned to allow enough through-cure. Thus, usually a combination of photoinitiators, a standard hydroxyalkyl ketone (HAK) for surface cure and an Acylphosphine oxide (APO) type for through-cure, is selected. The exposure lamp also should exhibit considerable output at longer wavelength, as do gallium or ferrum added mercury lamps (see Figure 2.21). For exterior applications the selection of all components is critical. The resins have to be “weather-resistant”, particularly oxidative and hydrolytically stable. Hence, the only appropriate resin types are aliphatic urethane acrylates (UA) or acrylated polyacrylates (APA), to be used in combination with hydrocarbon type acrylate diluents (e.g., hexanediol diacrylate, decanediol diacrylate). In order to stabilize the coating and the substrate against degradation and discolouration, the coatings have to contain UV absorbers and radical scavengers, usually of the hindered amine light stabilizer (HALS) type. Since these compounds, similar to pigments, also absorb in the UV, the photoinitiator package is also composed of a (non-yellowing) surface cure type (e.g., Irgacure® 184) and an acylphosphine oxide type photoinitiator. The exposure lamp is preferably of the ferrum added mercury type. The main performance requirements in wood coatings are good adhesion (flexible waterbased primers or reactive acrylates, like isocyanato acrylates) and especially for parquet coatings high abrasion resistance (corundum fillers). The graphic applications often ask for high reactivity (high functional oligomers or amine modified resins) and high gloss. Can and coil coating as well as plastics demand excellent adhesion (low shrinkage, reactive primers) and good flexibility (low functional higher molecular weight resins and/or monofunctional reactive diluents). High scratch resistance can be obtained with high crosslink densities and/or added nanoparticles. High hardness can be achieved with high Tg coatings (high crosslink density or rigid components) and high flexibility by low crosslink density (high molecular weight between crosslinks) and flexible chains (polyether, polysiloxanes). Last but not least the desired application method determines the choice of the raw materials. Powder applications require a glass transition temperature of the powder above room temperature, preferably above 40 ◦ C, and melting temperatures below 120 ◦ C. Thus, high Tg resins and/or crystalline resins/diluents have to be used in combination with low volatile, solid photoinitiators. In liquid systems, the application viscosity decreases from roller over curtain and vacuum coating to spray coatings. The viscosity adjustment can be done preferably with reactive diluents, however, where low molecular weight monomers are undesirable or prohibited (porous substrates, spraying), solvents or waterbased emulsions or dispersions may be used.

FORMULATIONS

F IG . 5.2. Considerations for selection of starting formulations.

151

152 UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS

F IG . 5.3. Schematic structures of flexible and highly crosslinked urethane acrylate resins and different functional diluents for formulation screening with high-throughput methods.

153

FORMULATIONS

TABLE 5.16. HTS set-up for screening urethane acrylates Varied components

Components kept fixed in all formulations

Binder

Reactive thinner

Reactive thinner (wt%)

Photoinitiator

Additives

UV protection

UA 1 ↓ UA 26

TMPMFA HDDA TMPTA

0, 10, 30, 50 0, 10, 30, 50 0, 10, 30, 50

3.5% Irgacure® 184 0.5% Lucirin® TPO

1% defoamer 0.3% levelling

1.3% UV absorber 0.7% HALS

5.5

INCREASING EFFICIENCY IN FORMULATION SCREENING: HIGH-THROUGHPUT SCREENING (HTS)

In order to illustrate the method of high-throughput screening (HTS), the course of the development of weathering and scratch resistant aliphatic urethane acrylate resins is given in the following example.8 The screening was based on the formulation of aliphatic urethane acrylate resins together with reactive diluents of varying functionality. The aliphatic urethane acrylate resins used were synthesized by the reaction of a low viscous isocyanato-acrylate (ICA: Laromer® 9000 (BASF AG)) with commercially available diols and hydroxyalkyl acrylates. Molecular weight and acrylate functionality of the urethane acrylates was varied systematically. Twenty-six different aliphatic urethane acrylates were synthesized and formulated with the 3 reactive thinners as shown in Figure 5.3. The combination of 26 UA resins with the 3 diluents in 4 different concentrations resulted in 260 formulations. As fixed components 4 wt% photoinitiator (mixture of Irgacure® 184, Lucirin® TPO), 1 wt% defoamer, 0.3% levelling additive and 1.3 wt% UV absorber and 0.7 wt% HALS (UV protection) were added to the formulations (Table 5.16). The workflow of the high-throughput formulation testing is elucidated in Figure 5.4. This tool setup can be used for formulation screening with already existing raw materials, however, a parallel synthesis robot can also be applied to synthesize tailor-made new components (resins) in a very efficient way before the formulation step. A modular coatings robot (Figure 5.6) was used to dispense the raw materials and stir the formulations, then to draw down the coating films and finally to cure them using UV light. Various semiautomated application tests yielded materials properties like hardness, elasticity, scratch and chemical resistance as well as UV conversion. Sophisticated data management supported the design of experiments, configures the robot, captures the recipes, process parameters and measurement results in a database and offers data visualization. This allows selection of promising formulations and to harvesting of structure property correlations of coating properties, process parameters and chemistries applied. In subsequent steps more focused libraries can be generated. The resins can be synthesized in a classical manner or with a HTS reactor with automated dosing and parallel setup as shown in Figure 5.5. The formulations were coated with the HTS equipment shown in Figure 5.6 onto glass substrates by doctor blading (slit width 200 µm) with a wet paint thickness of 150 µm. After 15 min flash-off at room temperature the coatings were subjected to a 20 min treatment at 100 ◦ C. Subsequently, UV curing was performed at 60–80 ◦ C with 5 J/cm2 under

154


F IG . 5.4. Scheme of high throughput formulation testing.

F IG . 5.5. Parallel reactors for synthesis.

inert conditions in a CO2 bath in order to eliminate oxygen inhibition. All the properties were measured with the HTS equipment (Figure 5.7). The mechanical properties of the cured films, like hardness and elasticity, were determined by Fischerscope® measurements

FORMULATIONS

F IG . 5.6. HTS parallel coating equipment.

F IG . 5.7. HTS combinatorial application testing.

155

156


F IG . 5.8. Determination of hardness and elasticity (fischerscope® ).

F IG . 5.9. Fischerscope measurement (hardness versus elastic work fraction) of highly X-linked UA formulations from HTS.

FORMULATIONS

157

F IG . 5.10. Fischerscope measurement (hardness versus elastic work fraction) of UA formulations from HTS of all 260 formulations.

(Figure 5.8). Scratch resistance was evaluated by a sand scrub test. UV conversion was determined by confocal Raman microscopy at the surface (0 µm) and 15 µm layer depth. Chemical resistance was evaluated for H2 SO4 , NaOH, Pancreatin, tree resin and H2 O at two different temperatures each. At first glance, one might be overwhelmed by the flood of data generated from the screening. With visualization tools, however, interesting correlations can be revealed. A plot of hardness vs. elasticity, as shown in Figure 5.9, for a subset of highly crosslinked urethane acrylate resins (see Figure 5.3, top right) as a function of the used hydroxyacrylate building block on one hand and the type (i.e., functionality) and concentration of the reactive thinner on the other hand. The picture becomes even more complex (Figure 5.10), if additionally urethane acrylate resins are incorporated which contain flexibilizing diols in the center of the resin molecule (see Figure 5.3, top left). From the data in Figure 5.10 molecular resin structures can be identified which result in coatings of high elastic work (>70% of total) at low hardness (highly flexible systems) or at high hardness (highly crosslinked coatings). The evaluation of the scratch testing showed high scratch resistance for the coatings which exhibited high elastic work, either the flexible or the highly crosslinked. The chemical resistance data (Figure 5.11) were plotted in the same type of graph (hardness versus elastic work) and the differentiation of good and bad chemical resistance is shown by the shape (filled circle, empty circle and plus (+), respectively). The results demonstrate that the best chemical resistance is in the upper right corner of every plot, thus produced by the coatings with high crosslink density. The highly crosslinked urethane acrylate coatings identified by the HTS screening have then been subjected to intensive classical scratch resistant tests. According to a model published by Jones et al.9 , there are three different ways

158


F IG . 5.11. Chemical resistance results of UA formulations from HTS (the formulations are plotted the same as in diagram in Figure 5.10).

TABLE 5.17. “Three response, two mechanism model” for evaluation of scratch resistance Responses to marring stress

Mechanisms of marring

Mechanisms of healing

Fracture Plastic deformation Elastic deformation

Fracture Plastic deformation

Viscoelastic creep

coating surface can respond to marring stress, addressing two different mechanisms of marring, and one mechanism of healing. In this model, fracture and plastic deformation lead to observable marring, while elastic deformation does not. Instead, elastically deformed mars recover their original dimensions almost instantaneously. Thus, the three responses in this model lead to only two marring mechanisms (Table 5.17). According to the model described above and the results of the high-throughput screening, the urethane acrylates which combine a high ratio of elastic work (We /Wtot ) and high universal hardness (Hu ) in Fischerscope testing should exhibit a very interesting combination of hardness and elasticity.10 After further optimization of the respective formulations these products have been tested as automotive clear coats, especially on plastic parts against the conventional two pack (2C PU) formulations. The results are shown in Table 5.18. Such highly crosslinked coatings based on aliphatic urethane acrylates, identified by the high-throughput screening, showed excellent chemical resistance and extreme scratch resistance.

159

FORMULATIONS

TABLE 5.18. Applications test for automotive clear coats High functional aliphatic urethane acrylate (UV) based on Laromer® 9000 3.5% Irgacure® 184/0, 5% Lucirin® TPO, UV curing Larolux® Scratch resistance Initial gloss at 20◦ AMTEC-Kistler 10 cycles at 20◦ (%) DIN 55668 10 cycles after reflow 2 h 80 ◦ C at 20◦ (%) 40 cycles at 20◦ (%) Chemical resistance Gradient oven Base coat brilliant silver

H2 SO4 (◦ C) NaOH (◦ C) Pancreatin (◦ C) Tree resin (◦ C) H2 O (◦ C)

UV

89 85 87 83

2C PU scratch resistant (bench mark) 85 74 87 48

75 57 69 75 75

44 69 71 69 75

Such HTS methods are now often used for the screening of formulations. Further examples describing the development of UV refinish primers and clear coats have been published recently.11

REFERENCES 1. Wicks Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), “Organic Coatings, Science and Technology, Vol. 2, Applications, Properties, and Performance”. John Wiley, 1994, Chapters XXXIII and XXXIV, pp. 273–311. 2. Holman, R. (Ed.), “UV&EB Curing Formulations for Printing Inks, Coatings and Paints”. SITA Technology, London, 1984. 3. Müller, B. and Poth, U., “Lackformulierung und Lackrezeptur: Das Lehrbuch für Ausbildung und Praxis”. Vincentz Verlag, 2003, Chapter 2. 4. Bankowsky, H.H., Paint Coat. Ind. June, 44–48 (1997). 5. Bankowsky, H.H., Enenkel, P., Beck, E., Lokai, M. and Sass, K., The formulation and testing of radiation curing binders for abrasion-resistant parquet varnishes, RadTech, Baltimore, MD, End User Workbook. 2000, pp. 210–222. 6. Silberzan, I., Magny, B., Elie, K., Roche, S. and Loubert, J.L., An insight in scratch resistance of UV cured coatings, RadTech Europe, Conference Proceedings, Vol. I. 2003, pp. 171–177. 7. www.basf.com/pigment and www.basf.com/resin. 8. Heischkel, Y., Schwalm, R., Kutschera, M., Schrof, W., Koltzenburg, S., Beck, E., Larbig, H., Menzel, K. and Gruber, N., RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 297–303. 9. Jones, N., Shen, W., Smith, S.M., Huang, Z. and Ryntz, R.A., Progr. Org. Coat. 34, 119 (1998). 10. Schwalm, R., Polym. Paint Colour J. 189 (4421), 18 (1999). 11. Bach, H., Gambino, C., Galeza, L., Dvorchak, M.J., Fäcke, T., Detrembleur, C., Ehlers, M., Mundstock, H., Schmitz, J. and Weikard, J., UV refinish primer and clearcoat, RadTech Europe 2003, Conference Proceedings. 2003, pp. 731–738.

C HAPTER 6

Structure–Property Relationships We have seen in Chapter 3, that the properties of networks and thus of the coatings depend strongly on the physical state of the materials, being at the application temperature in a glassy state, in a rubber-like soft region or even in the transition between glass and rubber. In this transition region, the modulus is heavily influenced by temperature and may drop by several orders of magnitude. This transition is defined by the glass transition temperature (Tg ). Since almost all crosslinked coatings are amorphous polymer networks, the glass transition temperature has the most prominent influence on the mechanical properties. The Tg describes the deflection from linearity of the increase or decrease of volume of a polymer during heating, from a glassy to a soft state or during cooling from the soft to a glassy state. This behaviour is not characterized by a sharp discontinuity of the increase of the specific volume with temperature, as it is with a melting transition, but just as a discontinuity identified by a change in the rate of increase of the specific volume with temperature. Thus, the transition of a polymer into an amorphous glass is more a kinetic than a thermodynamic transition. The glass transition temperature increases with increasing molecular weight, since the mobility of the chain segments decreases. In linear polymers the glass transition temperature reaches a plateau at a polymerization degree above about 100–600, depending on the molecular structure. Consequently it does not further increase with increasing molecular weight. Below the Tg , the polymer behaves like an immobile glassy material, whereas above the glass transition temperature the chains are able to move to a certain extent, and the material behaves like a rubber. The absolute value of the glass transition temperature depends on the structure of the polymer and can vary from Tg ’s well below room temperature, like in linear polysiloxanes to above 300 ◦ C in polyimides, polybenzimidazole and others. Crosslinking restricts the mobility of chain segments, if the distance between the crosslinks is shorter than the segment length required for mobility. It has been described, that the mobility of about 30–50 chain-links is necessary to observe a glass transition. Elastomers (rubbers) are used above their glass transition and may exhibit elongations beyond 500%, and in addition to that, they are able to restore the original shape, due to the few crosslinks. Highly crosslinked polymers, like duromers, which are used below their Tg , often exhibit elongations less than 10%, and are therefore not really extendable or formable. Structure–property relationships of highly crosslinked UV curable duromers have been evaluated for many different systems. A review of characterization methods and under160

STRUCTURE–PROPERTY RELATIONSHIPS

161

standing of the relationships between exposure conditions, chemical structure and network properties are given by Zumbrunn.1 The properties of radiation curable materials can be tailored to the application needs. Some fundamental influencing factors on the main properties, like mechanical behaviour, are discussed in the next section. The main influences of the acrylate (resin) functionalities and resin types on the polymerization process and general properties are exemplarily shown in Figures 6.1 and 6.2. Increasing the monomer (diluent or resin) functionality leads to higher cure speed, higher Tg , higher crosslink density, higher hardness and higher chemical and scratch resistance, but lower flexibility and lower conversion, causing a higher content of uncured residual unsaturated groups. Generalized structure–property relationships of the typical resins used in radiation curable acrylate systems are given in Figure 6.2. The performance criteria shown

F IG . 6.1. Influence of resin/diluent functionality on the polymerization process and some properties.

F IG . 6.2. Generalized properties of typical resins of the different UV curable acrylate resin classes (spider diagram: 0, worst; 10, best).

162


in the diagram are rated relative to a defined scale, ranging from poor (=0) to excellent (=10). The (aromatic) epoxy acrylates provide high hardness, reactivity and chemical resistance, whereas urethane acrylates are known for their high flexibility, toughness and abrasion resistance. The polyester acrylates have a well balanced property profile, making them typical all-rounder, and the polyether acrylates are very good reactive diluents due to their low viscosity but also usable as pure resin materials. It will be discussed in this chapter how these properties are influenced by chemical composition, cure extent and crosslink density.

6.1

EFFECT OF CROSSLINKING ON MECHANICAL PROPERTIES

Mechanical properties relevant for coatings are hardness, stiffness, toughness, elasticity and many others. All these properties are not only material properties, but also depend on the molecular interactions, morphological structures (amorphous, crystalline) as well as on the processing conditions. The basic concepts of the physical behaviour of coating films have been adopted from the concepts developed by the plastics and rubber industries, for example, stress-strain analysis as an effective tool to characterize mechanical film properties, as discussed in Chapter 3. A brief and good introduction into determination and methods of mechanical properties of coatings is given in Vol. 2 of “Organic Coatings”.2 The fact that coating films are viscoelastic materials is fundamental for the understanding of coating properties. Upon deformation they respond with elastic restoring and/or viscous flow. The ideal elastic behaviour (illustrated by the deformation of a spring) is described by Hook’s law. The elongation of the material is proportional to the stress applied, and if the stress is released, the material returns to its original shape. On the other hand, an ideal viscous material will show elongation by viscous flow upon stress applied, and remain at this elongated stage even after the stress is released. The coating films are viscoelastic materials, thus the behaviour can be regarded as intermediate between full recovery and viscous flow. Thermoplastics generally do not recover their initial state after deformation, since the viscous flow results in permanent deformation. In crosslinked films, the stress on the crosslinks produces restoring forces to partly or completely reverse viscous flow. The yield point in the stress strain diagram (Figure 3.13) corresponds to the point where the material begins to have permanent (unrecoverable) deformation. The stress-strain analysis can be done in a static way, where strain is applied to elongate the sample and trace the stress relaxation or in a dynamic oscillating strain application at a specific frequency, where the resulting stress development and phase angle difference is measured. From these experiments the storage modulus (E ), which is a measure of the restorable energy imparted by the applied strain, the loss modulus (E ), which reflects the loss of the applied strain due to viscous flow of the sample, and the ratio of loss modulus to storage modulus (E /E ), called tan δ, can be determined. The drop in the modulus of E is caused by transition of the material from a glass into a rubber-like material. This transition is characterized by the glass transition temperature, which can be determined by the maximum of the tangent δ, which corresponds more to the mean value of the glass transition region or by the maximum of the loss modulus (E ), which corresponds more to the beginning of the glass transition region. In a sharp drop of


163

the modulus (E ) these values do not differ significantly, but in highly crosslinked heterogeneous networks, the Tg region can be relatively broad and in these cases the values will differ significantly. Especially in the cases where the glass transition region is within or near to the use temperature it is very important to know if the network (or coating) is or behaves like it is in the physical state of a glass or a rubber. Since the mechanical properties of crosslinked coatings are strongly influenced by the glass transition temperature,3 the structural influences on the Tg are discussed. If the Tg of the coating is below the application temperature, the coatings often exhibit some of the following properties: • • • • •

Soft and flexible, Difficult to sand (rubber-like), Exhibiting poor blockability (tacky), Low barrier properties (against chemicals, oxygen, water), High water uptake and swelling. If Tg is above the application temperature, the coating can often be characterized as:

• • • • •

Hard, to some extent brittle, Exhibiting low water uptake and swelling, High barrier properties against chemicals and water, High scratch resistance, and High chemical resistance.

Physically, Tg can be explained as the lowest temperature where segments of polymer molecules start moving relative to neighbouring segments and start occupying a larger volume. The glass transition is not a sharp point, but rather a transition range depending on the thermal history of the sample. With fast heating rates the Tg appears at higher temperatures and if the material was cooled down rapidly, the Tg appears to be lower than with samples that are cooled down more slowly. What are the main structural influencing factors on the glass transition temperature? Most relevant factors are those that influence the thermal energy necessary for the maintenance of the rotation of chain segments. These are the chain flexibility, the molecular constitution (sterical effects), the molecular weight, as well as branches and crosslinks. The relative flexibility of the polymer chains will determine the magnitude of the glass transition temperature. It is a measure for the ability of the chain segments to rotate around the bonds. Therefore a rigid chain needs high thermal energy in order for the chain segments to rotate, thus is has a high Tg , whereas a flexible chain can maintain rotations already at low temperatures, thus exhibiting low glass transition temperatures. Since the mobility of chain segments is also dependent on the molecular weight, values for Tg are often given as the glass transition temperature at infinite molecular weight. Tg increases with increasing molecular weight, but the Tg versus molecular weight curve approaches a constant value (Tg∞ ). The Tg∞ can vary over a wide temperature range (Table 6.1), as shown for different linear polymers. Besides the chemical constitution of the backbone, including the steric hindrance effects on rotation, which is reflected by Tg∞ , the glass transition is influenced by the molecular

164


TABLE 6.1. Tg values of linear polymers at infinite molecular weight Polymer

Tg∞ ( ◦ C)

Poly(dimethyl siloxane) Polypropylene oxide Polybutyl acrylate Polyethyl acrylate Polymethyl acrylate Polycyclohexyl acrylate Polymethyl methacrylate Polyvinyl carbazol

−123 −65 −54 −24 +10 +19 +105 +150

weight (Mn ), and by the crosslink density (X) (see Chapter 3 for definition), as expressed by the simple equation (6.1.1)4 : Tg = Tg∞ − [K/Mn ] + [Kx X],

(6.1.1)

where Tg∞ is the limiting glass temperature of the uncrosslinked polymer at infinite molecular weight, K is a constant; the term [K/Mn ] characterizes the decrease in Tg due to the remaining end groups of the uncrosslinked polymer chains, since the end groups require a higher volume than the chain segments and therefore contribute to an easier rotation. Kx is a constant characterizing the increase of Tg due to the presence of crosslinks and X is the crosslink density. Equation (6.1.1) holds true only for linear polymers with low crosslink densities, since it assumes that the reference glass transition temperature of the linear polymer does not change with the degree of cure (end group conversion). A better fit of data with higher crosslinked systems was derived with an equation published by Stutz,5 taking into account that the reference glass temperature is not a constant, but dependent on the cure conversion Tg = Tg∞ − K1 (1 − p) 1 + K2 X/(1 − X) ,

(6.1.2)

where Tg∞ is the true backbone Tg without endgroups or crosslinks, K1 characterizes the influence of endgroups, thus reflecting the degree of cure (p), and K2 is another constant accounting for the influence of crosslinks. Thus, the Tg of the linear polymer is lowered by endgroups if the degree of cure is not 100%, and increased due to the crosslinking reaction. This equation also explains how Tg changes during cure. As the cure conversion increases the term lowering Tg gets smaller and with increasing crosslink density the last term increases exponentially.

6.1.1

Cure Conversion and Glass Transition Temperature

The degree of cure and the dependence on the glass transition temperatures of different resins has been evaluated,6 and will be described in the following.


165

F IG . 6.3. Cure conversion as a function of cure temperature for three different Laromer® resins.

In order to verify the degree of cure of the resins, the conversion of the acrylic double bonds as a function of cure time has been followed for resins of different molecular structure. According to equation (6.1.2), Tg is dependent on the conversion of the functional groups. The conversion of the acrylate groups was determined by real-time infrared spectroscopy as described by Decker.7 It is shown in Figure 6.3 (top left) that the conversions of the three coatings are relatively high (>60%), however, never close to completion. The limitation of acrylate conversion is mainly due to vitrification of the system, analogous to thermosetting systems published by Gillham.8 According to equation (6.1.2) Tg increases with increasing curing conversion. It has been known from thermoset coatings that the curing reaction stops when Tg approaches or exceeds the curing temperature by about 20 ◦ C. In order to evaluate if this observation also holds true for radiation curable systems three UV curable coatings of different resin structure were chosen. When UV cured at room temperature the obtained Tg ’s of the coatings based on the three resin types were −3 ◦ C (LR 8907), 51 ◦ C (LR 8861) and 75 ◦ C (LR 8713), respectively. The corresponding acrylate conversions were 90, 62 and 59%. The measurement of the acrylate conversion as a function of cure temperature showed, that already nearly complete conversion is obtained in the case of the resin where the backbone Tg is below RT (LR 8907), while almost no change occurs with increasing cure temperature (Figure 6.3, right top). The situation changes with experimental product LR 8861. The cure conversion of the room temperature curing experiment is restricted to about 70% conversion (Tg of 51 ◦ C

166


F IG . 6.4. Increase of Tg as a function of cure temperature for a Laromer® resin.

reached), however is steadily increased with increasing cure temperature up to 90% final conversion (Figure 6.3, right middle). The LR 8713 resin (Tg of 75 ◦ C at RT cure) clearly shows that the cure reaction stops when the increasing Tg (during curing) approaches or slightly exceeds cure temperature, demonstrated by the large plateau up to ∼80 ◦ C cure temperature. Not until the cure temperature was increased above 80 ◦ C the conversion increased again (Figure 6.3, right bottom). Thus, the higher curing temperatures increased conversion, and also resulted in a Tg increase, as shown in Figure 6.4. According to the demonstrated limitation of cure conversion and Tg one could expect that the final Tg ’s of the room temperature cured resins should be limited to upper values of about 30–40 ◦ C. Nevertheless Tg ’s of 51 and 75 ◦ C were obtained at room temperature cure. However, this can be explained by the experiments of Decker,9 who recorded the temperature profiles of photocuring at room temperature. These measurements revealed that a temperature increase (T ) of up to 70 ◦ C might occur during curing, due to the evolved heat of polymerization. This temperature increase within the coating film can explain the obtained Tg ’s in the range up to 75 ◦ C in the case of room temperature curing. The influencing factors on Tg have been discussed in such detail, since it is of paramount importance to know the physical state of the coating when discussing the mechanical behaviour.

6.1.2

Hardness and Flexibility as a Function of Tg

For a series of UV curable resins of different types (epoxy, polyether, polyester and urethane acrylates of Laromers) the hardness (Figure 6.5) and flexibility (Figure 6.6, as determined by the Erichsen cupping method) have been determined as a function of the glass transition temperature. The basic mechanical properties of this series are summarized in Table 6.2.


167

F IG . 6.5. Hardness as a function of Tg of UV cured Laromer® resins.

F IG . 6.6. Flexibility as a function of Tg of UV cured Laromer® resins.

The hardness measurements of the coatings can be considered as single temperature modulus determinations and were done with an indentation (Vickers) and a pendulum test. The Erichsen cupping represents a reverse impact test, where a ball is pressed into the coated substrate from the backside until the coating breaks. This test reflects the flexibility of the coating and is measured as the extent of elongation in mm. The hardness and flexibility data of the various acrylate resins were plotted against the glass transition temperature. From the data in Figure 6.5 it is very obvious that there is a significant increase in hardness for coatings with Tg ’s above room temperature, which

Laromer

Type

Conv.h (%)

Tg i ( ◦ C)

4.78 2.64 5.49

59 60 93

(−10) 54 75 (−8) 24

234 176 145 615 143

4.27 5.68 6.90 1.63 6.99

70 72 58 90 61

19 14 34 −3 27

1.5 4.6 4.7 2

275 164 91 161

3.60 6.10 10.99 6.21

29 82 81 78

66 (−10) 19 (−9) 26 (−15) 25

12 172

8 4.0

1100 285

0.91 3.5

60 75

26 65

157 225

5 6.8

121 793

8.26 1.26

66 57

56 57

Func.c

PDd (s)

Vickerse (N/mm2 )

Erichsenf (mm)

HDDA (20) BuAc –

2 2 2

186 144 31

194 193 8.8

1 1.1 4.1

209 379 182

– – – DPGDA (25)

2.5 3.5 3.5 2.5 3.5

42 45 116 29 97

11.9 16.7 95 3 89

6.5 4.5 2.7 6.8 2.6

– – – –

3 3 3 3

113 52 73 55

161 19.6 47.5 22.9

TPGDA (35) HDDA (30)

2 2.5

43 164

50% 40%

3.5 2.2

154 192

1/Mc ×10−3

a All acrylates used are Laromer® types (BASF AG) and were pure or with the specified diluent containing 4 wt% of the photoinitiator Irgacure® 500. Coating thickness was 50 µm. The films were exposed with 1280 mJ/cm2 . b Diluent: HDDA, hexanediol diacrylate; PO 33F, oligoetheracrylate; D(T)PGDA, di(tri)propyleneglycol diacrylate. c Func., functionality. d PD, pendulum hardness. e Vickers, vickers hardness. f Erichsen cupping. g M , elastically effective chain length. c h Conv., conversion of acrylates. i T , glass transition temperature (a second T in brackets). g g


Epoxy acrylates EA 81 Aromatic LR 8713 Aromatic LR 8765 Aliphatic Polyester acrylates PE 55F Aliphatic LR 8799 Aliphatic LR 8800 Aliphatic LR 8907 Aliphatic LR 8912 Aliphatic Polyether acrylates PO 33F PO 84F LR 8894 PO 83F Urethane acrylates UA 19T Aliphatic LR 8987 Aliphatic Water based systems LR 8895 LR 8949

Mc g (g/mol)

Diluentb (%)

168

TABLE 6.2. Mechanical data of various radiation cured acrylatesa


169

appears with both hardness measurement methods. The data for flexibility show nearly the opposite behaviour. There is a drop of flexibility when Tg rises above room temperature. The step is not as pronounced as in the hardness diagram, probably due to the existence of a second Tg in several coatings below room temperature, which indicates phase separations. However, there are two exceptions with the water-based systems, which exhibit extraordinarily high flexibility at high Tg ’s.

6.1.3 Influence of Mc (Molecular Weight Between Crosslinks) The principle influence of the molecular weight between crosslinks (Mc ) on the flexibility has been shown schematically already in Figure 3.12. The mechanical data of the waterbased systems as listed in Table 6.2 exhibit high hardness in agreement with the high Tg ’s, however, considerably more flexibility than expected according to a pure Tg consideration (Figure 6.6). Therefore the flexibility as a function of Mc has been evaluated. In the case of the investigated resins there was no exact linear correlation between flexibility and Mc as shown in Figure 6.7, but a clear trend that the flexibility decreases with decreasing molecular weight (1/Mc ). There are some coatings, which deviate from the trend line, but this deviation behaviour could be explained by the specific structures of these resins (see ref. 6). Generally, the water-based coatings had a high Tg , which is responsible for the high hardness, and a relatively high molecular weight between crosslinks, which is responsible for the high flexibility. The explanation for the high flexibility of the chain segments can be derived from investigations of the mechanical behaviour of linear polymers. The mechanical analysis has been done for linear polymers to a much greater extent than for coating networks. In such polymers (e.g., polymethyl-methacrylate (PMMA) investigated by Wu,10 Figure 6.8), it has been detected, that a considerable change of the mechanical properties occurs at the glass transition temperature, however, a considerable

F IG . 6.7. Flexibility versus 1/Mc (molecular weight between crosslinks).

170


Transition temperatures of homopolymers Polymer Tg (◦ C) Tb (◦ C) Polystyrene 100 90 PMMA 105 45 PVC 80 10 Bisphenol-A–PC 150 −200

Tg –Tb (◦ C) 10 60 70 350

F IG . 6.8. Transition temperatures of linear polymers.

increase of the elongation behaviour (elongation at yield as well as elongation at break) occurs already well below the glass transition temperature. This elongation behaviour is responsible for the observed mechanical performance of the polymer being ductile (or flexible in a coating sense) in a region below Tg . The lower boundary of this region has been defined by Wu by another transition temperature, the brittle–ductile transition (Tb ), which can be detected from stress-strain analysis as a function of temperature. Wu has defined this Tb transition temperature at the intercept of the elongation at break and the elongation at yield point. Thus, in the range between Tb and Tg , the polymer material exhibits a ductile behaviour. This transition (Tb ) is due to starting chain segment mobility (sometimes also described as second order β-transition) and renders the deep-drawing of plastics possible well below Tg . The mechanical properties of UV cured coatings can be well explained by modulus and elongation curves as shown in Figure 6.9. According to the mechanical property values listed in Table 6.2, the polyester acrylate (LR 8907) appears soft (3 N/mm2 ) and flexible (6.8 mm Erichsen), which can be correlated with the low Tg (−3 ◦ C), which dominates the mechanical properties. In the case of the water-based LR 8949, the Tg is above room temperature (57 ◦ C), but due to a lower crosslink density (see high Mc value) a Tb transition exists below room temperatures, which is responsible for the relatively high flexibility (6.8 mm Erichsen) at high hardness (225 N/mm2 ). With increasing crosslinking, the mobility of segments is reduced and the difference of Tb and Tg decreases, until in highly crossliked materials only one transition, the glass transition exists (schematically shown in Figure 6.10). In these cases, the Tg virtually marks the transition temperature below which the coating is brittle and above which it is flexible. An example is the resin LR 8987 with a Tg of 65 ◦ C and a reduced, but moderate flexibility of 4.0 mm (probably due to the broad Tg region) and even more pronounced in the case of the resin LR 8713 with a Tg of 75 ◦ C and a elongation of only 1.0 mm (Erichsen).


171

F IG . 6.9. Superimposing modulus and elongation curves of three UV cured resins with different composition.

F IG . 6.10. Effect of crosslinking on brittle-ductile transition (Tb ).

Although the mechanical behavior can be explained by the physical state of the coatings and the position of the transition temperatures relative to the application temperatures, it has to be considered that the glass transition temperature and therefore the properties is influenced by the velocity of the impact in reality as well as the velocity of the measurements performed for the determination of the values (see Chapter 3.4.1). The obtained Tg distinguishes between hard and soft coatings. If Tg is above room temperature, the coating is hard and turns soft at temperatures above Tg , which is reflected in the modulus curves. The flexibility can be determined by elongation measurements. Linear polymers often have brittle–ductile transitions below room temperature, they are tough (flexible) at room temperature. With increasing crosslink density the elastically effective chain length between crosslink decreases and the Tb transition increases until at high crosslink densities only one transition, the glass transition temperature, exists. Since

172


most of the radiation cured coatings are highly crosslinked only Tg determines the mechanical properties and that’s the reason why most of the coatings are either hard/brittle or soft/flexible. Thus, in order to get hard and flexible coatings, the crosslink density should be decreased to allow a Tb -transition below room temperature and the glass transition has to be increased above RT by means of chemical composition and crosslinking during photopolymerization. Low crosslink densities are difficult to realize in 100% coatings without using large amounts of monofunctional reactive diluents, since viscosity also increases with molecular weight. Therefore, water-based systems, where the viscosity is independent of molecular weight of the resin, or systems applicable by melting, like hot melt or powder coatings, are option towards hard and flexible coatings.11 6.2 EFFECT OF CROSSLINKING ON COATING PROPERTIES In the preceding section some structure–property relationships have been discussed influencing the mechanical properties of UV cured coatings. In daily life however, the application properties of the coatings, like abrasion-, scratch- or chemical resistance, are the crucial key factors. For linear polymers a lot of data exist characterizing the structural influences on mechanical properties, however much less data exist for crosslinked coatings. And even less data are available for correlating property–property relationships, like the influences of mechanical properties on scratch resistance, pendulum hardness, elasticity or Taber abrasion (Figure 6.11). The reason why such correlations hardly exist is based on the fact, that many of these application properties, like scratch resistance, are not a defined material property. The scratch resistance of a material is dependent on the scratch test, which means on the scratching material (hardness, shape, etc.) as well as on the impact (velocity, mass) affecting the coating. Thus, depending on the scratch test different material properties will be required to obtain optimal scratch resistance, for instance, in a car wash test or an abrasive Taber test. Several scratch test methods used in the evaluation of coatings have been described in Chapter 3 (see Figure 3.18).

F IG . 6.11. From structure–property to property–property relationships.


173

In the following we will discuss the effects of X-linking on some performance data, like hardness, flexibility and scratch resistance of a series of model coatings based on aliphatic urethane acrylate chemistry. The scratch resistance was evaluated in a ScotchBrite® test, where a ScotchBrite® fleece is moved under a specific load (750 g) with 50 double strokes over the surface and the gloss difference of the coating measured before and after scratching. Urethane acrylates have been identified as the most promising candidates of UV curable raw materials for outdoor applications.12 The resins used to obtain valuable structure– property relationships are based on a bifunctional resin containing a linear diol to introduce flexibility (UA1, flex) and a three-functional aliphatic urethane resin (UA2, hard). These resins were formulated with different functional monomers (monofunctional to hexafunctional) to vary the crosslink density (Figure 6.12 and Table 6.3). The crosslink density has been calculated from the molecular composition, the functionality and the conversion with the DryAdd program of OxMat13 : the crosslink density of the gel is the cycle rank divided by the weight of the gel. Cycle rank is the number of “cuts” required to reduce the network structure to a tree. Some basic properties as a function of crosslink density are shown in Figure 6.13. In the diagram the performance values are given as relative values. The 100% values are defined for the scratch resistance, given as the gloss retention after scratching (Gloss 100%), the flexibility (Erichsen 10 mm), the hardness (Pendulum 200 s), the elongation (Elongation at break 100%) and the chemical resistance as grades ranging from 0 to 5 (0 refers to no visible mark after chemical treatment). For the bifunctional, flexible resin in combination with reactive diluents, the relative values show, that at the lowest crosslink density, the flexibility and elongation at break are at a high level, the scratch resistance starts at about 60% gloss retention, but approaches almost 100% at high X-link densities. The hardness starts at a very low level, but increases to about 45%, the chemical resistance does not change very much on a low level (Fig-

F IG . 6.12. Structure of urethane acrylates and reactive diluents used for the determination of basic coating properties as a function of crosslink density.

174


ure 6.13). Using the tri-functional, hard urethane acrylate resin as a basis, the diagram (Figure 6.13) clearly shows the pronounced influence of crosslink density on the increase of scratch resistance (gloss retention), pendulum hardness and chemical resistance. The gloss retention is increased from about 80% up to about 97% at crosslink densities around TABLE 6.3. Crosslink (X) density of UA resins (70%) in combination with 30% reactive diluents of different functionality Resin

Diluent

X-link density (×10−3 mol/cm3 )

UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex) UA 1 (flex)

TMPMFA (1) DDDA (2) HDDA (2) TMPTA (3) PTA (4) DPHA (6)

0.31 0.88 1 1.88 2.4 2.22

UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard) UA 2 (hard)

TMPMFA (1) DDDA (2) HDDA (2) TMPTA (3) PTA (4) DPHA (6)

0.9 1.7 2 2.7 2.4 2.6

F IG . 6.13. Correlation of coating performance with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).


175

F IG . 6.14. Correlation of scratch resistance with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).

F IG . 6.15. Correlation of E-modulus with crosslink density of a bifunctional and a trifunctional resin, diluted with 30% diluents (of functionalities 1–6).

2.5×10−3 (corresponds to Mc < 330 g/mol). The trade-off by increasing crosslink density is in flexibility (Erichsen cupping), which decreases significantly with increasing crosslink density. Depending on the application a compromise between high scratch resistance and enough flexibility has to be found. The scratch resistance, tested in the ScotchBrite® test (see Figure 3.18), is depicted in more detail in Figure 6.14 as a function of the crosslink density of the two different resin type series.14 It is shown that a high scratch resistance in the test can be obtained with both concepts if the crosslink density is high enough. This scratch resistance behaviour, however, is not due to the same “hardness” of the surface, since the E-moduli of the two series are significantly different (Figure 6.15). While the moduli of both coating series

176


F IG . 6.16. Elastic work (W elast ) and plastic displacement (hplast ) as a function of crosslink density from nano-indentation measurements.

F IG . 6.17. Hardness vs. contact depth during nano-indentation of a UA coating exposed under air and nitrogen atmosphere.

are increasing with increasing X-link density, the absolute values of the hard UA1 series are higher by at least a factor of three. Thus, the high restoring forces induced by high crosslink density cause high elastic response in hard as well as in soft coatings, resulting in high gloss retention after scratching. In order to more directly simulate the mechanical stress induced by scratching15 these samples were also investigated with a nano-scratcher/indenter system.16 This system is equipped with two orthogonal actuator/sensor systems that are designed to apply a welldefined load (p) perpendicular to the surface for indentation and parallel to the surface for scratching and to sense the penetration depth at the same time. During indentation load displacement curves are generated (see insert in Figure 6.17). From these curves apart from the reduced modulus of elasticity and the hardness,17 several other parameters can be


177

derived: hmax The maximal indentation depth under load, consisting of the elastic deformation and the plastic deformation including viscous flow; helas The elastic part of the deformation or the elastic recovery; hplast The remaining plastic deformation when the load is fully removed; Welast The work which during the deformation is transferred to potential energy and which is finally recovered during release; Wplast The work that is lost in the plastic and viscous processes during deformation. Altogether this set of parameters allows an assessment of the “hardness-flexibility balance” of the respective material. In Figure 6.17 the elastic work and the plastic displacement as a function of crosslink density are illustrated. With increasing X-link density, the material reacts less plastic, which is also reflected in an increasing share of recovered elastic work (W elast ). The conclusions of the indentation measurements can be summarized in the way that increased crosslink density increases the E-modulus, hardness and scratch resistance of the material – as expected. At the same time the share of plastic deformation is reduced and the “shape” recovery is enhanced, by the reactive interlocking of the chains. This interpretation is – on first glance – clearly not in line with the standard notion, that crosslinking hinders reflow. However the latter classical picture hints at reflow kinetics driven by surface tension, whereas here the crosslinking seems to induce a – so to speak – thermodynamic restoring force by interlocking strained polymer chains, which then try to release the strain by recovering their original positions. It has to be noticed, however, that besides the structural influences, in UV curing systems the exposure atmosphere, due to the oxygen inhibition effect, has a considerable effect on the curing result, especially when scratching is addressed, since the scratching happens only at the uppermost surface layer. This inhibition effect is most pronounced at the surface, probably leading to a reduced crosslink density. At least a difference in hardness has been found in hardness measurements with nanoindentation showing a higher hardness at the surface of a coating exposed under nitrogen compared to air curing (Figure 6.17).

REFERENCES 1. Zumbrunn, M.A., Wilkes, G.L. and Ward, T.C., The characterization of the dynamical mechanical and dielectric properties of UV- and EB-cured coatings, In “Radiation Curing in Polymer Science and Technology”, Vol. III (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, 1993, pp. 101–152. 2. Wicks, Jr., Z.W., Jones, F.N. and Pappas, S.P. (Eds.), Chapter XXIV: Mechanical properties, In “Organic Coatings, Science and Technology, Vol. 2: Applications, Properties, and Performance.” John Wiley & Sons, New York, 1993, pp. 105–131, and main references: e.g., (a) Hill, L.W., J. Coat. Technol. 64 (808), 29, (1992); (b) Hill, L.W., “Mechanical Properties of Coatings”. Federation of Societies for Coating Technology, Blue Bell, PA, 1987; (c) Hill, L.W., Prog. Org. Coat. 5, 277 (1977). 3. Zosel, A., Farbe&Lack 83, 804 (1977). 4. Fox, T.G. and Loshaek, S., J. Polym. Sci. 15, 371 (1955). 5. Stutz, H., Illers, K.-H. and Mertens, J., J. Polym. Sci., Part B: Polym. Phys. 28, 1483 (1990). 6. Schwalm, R., Polym. Paint Colour J. Okt., 18–22 (1999). 7. Decker, C. and Moussa, K., J. Coat. Technol. 62 (786), 56 (1990).

178 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

UV COATINGS – BASICS, RECENT DEVELOPMENTS AND NEW APPLICATIONS Simon, S.L. and Gillham, J.K., J. Appl. Polym. Sci. 53, 709–727 (1994). Decker, C., Elzaouk, B. and Decker, D., Preprints RadTech Europe 1995, p. 115. Wu, S.J., J. Appl. Polym. Sci. 20, 327 (1992). Schwalm, R., Häußling, L., Reich, W., Beck, E., Enenckel, P. and Menzel, K., Progr. Org. Coat. 32 (1–4), 191–196 (1997). Königer, R., Beck, E. and Menzel, K., Farbe&Lack 4, 233 (1999). DryAdd Simulation software (Intelligensys), http://www.intelligensys.co.uk/sim/dryadd.htm. Gruber, N., presented at API Conference Ulm 2004. (a) Shen, W., Ji, C., Jones, F.N., Everson, M.P. and Ryntz, R.A., Surf. Coat. Intl. 79, 253 (1996); (b) Oliver, W.C. and Phar, G.M., J. Mater. Res. 7, 1564 (1992). Schwalm, R., Beck, E. and Pfau, A., Eur. Coat. J. 1–2 (03), 39–46 (2003). Klinke, E., Kordisch, M., Kunz, G. and Eisenbach, C.D., Farbe&Lack 4, 54–60 (2002).

C HAPTER 7

Tackling the Drawbacks of UV Systems The most important intrinsic drawbacks of the radically induced radiation curing technology together with proposed and practiced solutions to overcome these disadvantages are summarized in this chapter. Two of these drawbacks are related to the radical intermediates itself, which firstly hamper the curing reaction at the surface (oxygen inhibition) and secondly contribute to yellowing (initial yellowing), until they are completely decayed. The third main drawback is associated with the lack of radical formation in areas to which the light does not penetrate (shadow areas) and therefore no curing occurs.

7.1 OXYGEN INHIBITION The reaction of oxygen with the different species formed during the photopolymerisation and the effects on the network formation throughout the film thickness of a coating are

F IG . 7.1. Scheme of the oxygen inhibition reactions. 179

180


sketched in Figure 7.1. The termination of the reaction by hydroperoxides, the incomplete network formation at the surface and the remaining acrylate double bonds are commonly called the “problem of oxygen inhibition”. In the photoinduced radical polymerization of multifunctional monomers, the efficiency of crosslinking is due to the efficiency of the propagation step. In the presence of air, the oxygen diradical reacts much faster with the photoinitiator or propagating radical to form a relatively stable peroxy-radical,1 which does not initiate the acrylate polymerization, but rather acts as an inhibitor. This inhibition results in an induction period of the polymerization until all oxygen is consumed.2 Therefore, in thin films the complete polymerization is retarded and in thicker films the acrylate conversions at the air–coatings interface are very low, resulting in tacky surfaces. This detrimental action of oxygen inhibition is shown schematically in Figure 7.1, depicting the lower acrylate consumption especially at the interface to air. The extension of the inhibited layer thickness is dependent on the oxygen diffusion into the coating. The penetration or diffusivity of oxygen in the coating layer itself is dependent on several factors, for instance the type and polarity of the materials used, as well as the viscosity, which is a major influencing factor. This oxygen penetration into the film can be derived from an approximate solution of Fick’s diffusion equation (7.1.1). 1/2 d = 6D(t) ,

(7.1.1)

with d = distance (cm), D = oxygen diffusivity (cm2 /s) and t = exposure time. The oxygen diffusivity in water-like liquids (viscosity 1 mPa s) is on the order of 10−5 cm2 /s, and in typical UV resins with increasing viscosity from 10−6 to 10−8 cm2 /s. Thus in typical UV polymerizations times ranging from 0.5 to 5 s, the oxygen molecules can penetrate distances of 0.1–10 µm. This theoretical estimate3 can be confirmed, as shown in Figures 2.14 and 7.2, where a polyether acrylate has been photopolymerized and the remaining concentration of double bonds has been determined by confocal Raman microscopy4 as a function of layer thickness. During curing of the unmodified resin,

F IG . 7.2. Double bond conversion as a function of depth into the films with a neat polyether acrylate resin, with amine modified resin (oxygen consumption) and with wax additive (oxygen barrier).

TACKLING THE DRAWBACKS OF UV SYSTEMS

181

a polyether acrylate, up to a layer depth of about 10 µm the double bond conversion remains very low, until in deeper layers the conversion increases significantly. By modifying the resin with amines or by applying a wax additive, the conversion is already high at the surface and almost as high as in the bulk regions. It has been shown,5,6 that the detrimental effects of oxygen inhibition are less pronounced when the diffusion of atmospheric oxygen into the liquid coating film is decreased by: • decreased oxygen concentration in the atmosphere, • increased formulation viscosity or • decreased sample temperature; and the cure speed increased by • high photoinitiator concentrations and/or efficient photoinitiators, • highly reactive formulations or • high light irradiance. Several studies have shown that this inhibitory effect of oxygen in the photopolymerisation of acrylate based formulations depend on the type and concentration of the photoinitiators selected, the formulation reactivity and the irradiance.7,8 The Bowman group has presented a detailed study into the impact of oxygen on photopolymerisation kinetics recently.9 They found that the inhibition rate constant is ∼106 times greater than the propagation rate constant. They also investigated the effect of dissolved oxygen on the mechanical properties of the film and found, that it has a negligible effect on glass transition and modulus, since the concentration of oxygen terminated short-chain species is very low compared to the crosslinked polymer chains. There are several methods known to reduce the effect of oxygen inhibition.

7.1.1 Physical Methods 1. High irradiance and/or high energy density 2. Inerting the exposure atmosphere 3. Physical barriers, like wax or protective films 7.1.1.1 High irradiance and high energy density The most applied method for overcoming oxygen inhibition is the use of high irradiance and high energy density in order to produce a high concentration of radicals, which quench the oxygen effectively and finally result in a high curing speed and a tack-free surface. The disadvantage associated with this method is the multiple overexposure of the coating compared with the energy density needed to cure the bulk of the coating. The effect of high irradiance on the double bond conversion of a urethane acrylate formulation is shown in Figure 7.3. The influence of the irradiance on the conversion of a urethane acrylate formulation under air shows, that the total conversion improves significantly when the irradiance is increased from 15 to 90 mW/cm2 . A comparison of the effect of irradiance of curing under inert conditions (carbon dioxide) with air is given in Figure 9.41.

182


F IG . 7.3. Conversion as a function of irradiance.

7.1.1.2 Inerting the exposure atmosphere The detrimental effects of atmospheric oxygen can be successfully overcome by inerting the exposure atmosphere.10 It does not play any significant role, which type of inerting medium will be applied, either nitrogen, argon, carbon dioxide or other inert gases. The comparison of nitrogen and carbon dioxide, as well as the different influence factors, like oxygen content, sample temperature, monomer viscosity, film thickness, type and concentration of photoinitiator, monomer reactivity, light irradiance have been evaluated, with special emphasis on carbon dioxide as the inert atmosphere are described in more detail in Chapter 9.2.1 (see refs. 5,6). Carbon dioxide has the advantages over nitrogen that it is: • easily available and cheaper than nitrogen, • heavier than air and can therefore be maintained in a container without much loss. In recent years, the use of inert gas was extended to several applications, especially in foil coating and in the printing sector.11 The motivation behind this was to achieve faster curing, to reduce the photoinitiator content, to reduce the number of lamps and to improve the quality. The cost for producing the inert atmosphere in belt systems is offset by savings in the plants and for the photoinitiators, so that this scenario may be worthwhile even independently of the improvement in quality.12 The influencing factors on the coating performance by working under carbon dioxide atmosphere and application examples are discussed in Chapter 9. 7.1.1.3 Physical barriers Simple but not widely usable possibilities for preventing the admission of oxygen include the use of floating waxes, as used in the 1960s for UV curable styrene/UP resin systems in combination with relatively low-power 30 W/cm lamps.13 The effect is demonstrated in Figure 7.2.14 Furthermore, inerting can be achieved by covering the coated substrate with a transparent protective foil,15 an evident example is realized in the UV curing of adhesive through a transparent foil.


183

7.1.2 Chemical Methods 1. 2. 3. 4. 5. 6. 7.

Amine synergists High photoinitiator (PI) concentrations and PI type Acrylate monomer structure High reactivity Formulation viscosity Oxygen scavenging by a dye sensitizer Other additives

Hoyle has recently published an overview covering several of these methods to reduce the detrimental effects of oxygen inhibition on photocuring.16 He also demonstrated the effect of various additives on the polymerization exotherms by polymerizing acrylates together with additives in a photo-DSC (differential scanning calorimetry) in air with and without the additive in comparison to nitrogen atmosphere. 7.1.2.1 Amine synergists A long and well-known method for overcoming the oxygen inhibition is the addition of amines to the formulation, either as additives or chemically bound to acrylates via Michael addition.17 The proposed mechanism of the amine-cosynergist has been discussed in Chapter 4 (Figure 4.23). The main effect is the oxygen scavenging reaction of the amine, based on the good hydrogen atom donor properties of the C–H-group adjacent to the nitrogen. The once formed C-centered radical, produced by hydrogen abstraction of a photoinitiator or propagating acrylate radical, either can scavenge an oxygen molecule to form a peroxy radical, which itself can further abstract a hydrogen from another amine, or initiate the polymerization directly. Despite the excellent oxygen scavenging properties of the amines, they exhibit some distinct disadvantages, like yellowing of the coating, poor weatherability, plasticizing effect, which limit their general usage. 7.1.2.2 High photoinitiator concentrations Since oxygen reacts readily with photoinitiator or propagating radicals, a high concentration of radicals in the system consumes oxygen and prevents diffusion into deeper layers. The effect of the photoinitiator concentration on the polymerization kinetics is given in Figure 9.41 as a comparison of the effect of curing under air and inert atmosphere. Furthermore, of course, the quantum yield of the radical formation is dependent on the photoinitiator type (see Chapter 4.1.4). 7.1.2.3 Acrylate monomer structure and viscosity Effective monomer structures in order to reduce the effect of oxygen inhibition are based on considerations to provide either labile hydrogen atoms as discussed in the case of amines, or to increase viscosity. As discussed in Chapter 2, the type and structure of the resins and diluents have to be chosen by the application requirements, however, if the requirements allow the use of ethylene or propylene glycol or their thioether analogues to be used, the oxygen inhibition effect is significantly reduced.18 The proposed mechanism is similar to the mechanism demonstrated in Figure 4.23, however with the substitution of the –N–CH– group by the –O–CH– counterpart.

184


7.1.2.4 High reactivity The reactivity of the monomers or oligomers governs the cure speed and therefore the reaction time during which oxygen can penetrate into the sample. However, photopolymerization will not occur if the diffusion of oxygen is very high (e.g., low viscosity), even if the reactivity of the resin is very high, until the concentration of the oxygen dissolved in the sample has dropped by two orders of magnitude,19 and the monomer can compete successfully with the oxygen for the addition to the initiating radicals. 7.1.2.5 Formulation viscosity The rate of oxygen diffusion into the un-cured liquid film is significantly determined by the viscosity of the UV-curable formulation.20 The effect of the viscosity has been demonstrated by comparing the polymerization rates of films of an urethane resin (Laromer LR 8987) as a function of temperature, which changes dramatically the viscosity. The film thickness of 5 µm has been chosen in a range where the oxygen inhibition is most pronounced. An increase in the temperature causes a decrease in formulation viscosity. At −19 ◦ C, where the viscosity is rather high the polymerization rate (per [M]) is on the order of 1, and drops continuously to 0 by increasing the curing temperature over 6, 25, 50 ◦ C to 80 ◦ C. This behaviour is plotted in Figure 9.42 in comparison to the curing under carbon dioxide. The behaviour under air is due to decreased viscosity and therefore enhanced oxygen diffusion, leading to reduced polymerization rates. Similar results were obtained by photo-DSC evaluations.21 7.1.2.6 Oxygen scavenging by a dye The conversion of dissolved oxygen into singlet oxygen in the presence of a dye sensitizer and the scavenging of the singlet oxygen by 1,3-diphenylisobenzo-furan to generate 1,2-dibenzoyl-benzene, which can work as a photoinitiator has been described.22 However, associated with the use of this dye, the coating is slightly coloured, and therefore this approach has not been used extensively, probably due to the limited application range for slightly coloured coatings. Recently, a novel system of singlet oxygen generator (zinc 2,9,16,23-tetra-tert-butyl29H ,31H -phthalocyanine (Zn-ttp)) and singlet oxygen scavenger (dimethylanthracene (DMA)), has been published.23 The combination of Zn-ttp/DMA and pre-illumination can effectively consume the molecular oxygen dissolved in the system. As a result, the inhibition period was significantly reduced and the rate of polymerization increased dramatically.

7.1.3

Conclusions

As discussed in this chapter, there are several possibilities to overcome the detrimental effects of oxygen inhibition, especially at the surface of an UV-cured coating. The most convenient way for working in an air atmosphere is to compensate the oxygen inhibition reaction by high irradiance and energy density, by selection of efficient photoinitiators and use of high photoinitiator concentrations, by selection of highly reactive formulations or by incorporation of additives, like amines or waxes. Each of these methods has its own advantages and disadvantages. The very simple, yet, more expensive way is the inerting of the curing atmosphere. There are exposure units available where nitrogen is cycled around the exposure set-up.


185

F IG . 7.4. Possibilities to cure in shadow areas.

Well known, however, only recently further developed is the inerting with carbon dioxide. The influencing factors on the curing conversion in the presence of carbon dioxide as the inerting medium are discussed in Chapter 9. Since carbon dioxide is heavier than air, a new process called Larolux® has been introduced, where the exposure can be performed in a carbon dioxide pool with very low-irradiance lamps. This process, as well as the curing under vacuum conditions (UV plasma curing), which can favourably be used for threedimensional curing, will also be described in more detail in the Chapter 9.

7.2 SHADOW AREAS “The brighter the light, the deeper the shadow” and “every cloud has a silver lining”. These worldly wisdoms also hold true for UV curing. The curing of relatively complex threedimensional objects is non-trivial. Such objects may appear for instance in car bodies or car doors, exhibiting hollow regions, the inside of which can hardly be reached by UV radiation and will therefore remain uncured. To resolve this issue, several approaches have been published: • Light tunnels with lamps illuminating the object from all sides • Placing lamps on robots This approach has been pursued in several evaluations (see Figure 2.17). • Curing under inert conditions using reflective walls The curing under inert conditions using reflective (aluminium) walls will be described in Chapter 9 (Larolux® process).

186


• Dual cure systems: • UV and thermal, • UV and oxidative drying. Dual cure systems combining UV-radiation with thermal curing have recently been developed.24–26 They contain two types of functional groups: usually UV curable acrylate double bonds, and thermally curable groups, for example, polyol and isocyanate or melamine-amino functionalities. The basic principle of these chemistries is depicted in Figure 7.4. There are several possibilities to design such systems. The simplest approach of just combining the individual molecules of multifunctional acrylates, polyols and polyisocyanates for example, results in the formation of interpenetrating networks, which are often inhomogeneous. This is due to a phase separation of the independently formed relatively unpolar polyacrylate network and the relatively polar polyurethane network. Therefore molecules have been designed which combine the different functionalities in one molecule, like acrylate double bonds and isocyanates and on the other hand hydroxyl groups and acrylate double bonds. The newly developed chemistries are discussed in more detail in Chapter 9. The dual curing approach can also be transferred to water-based systems.27 Furthermore, dual curing can also be obtained with one type of chemistry, but different initiator types, namely a photoinduced radical initiator and a thermal activatable peroxide. With these initiators pure acrylate based coating systems can be used for dual curing. • UV plasma curing As shown in Fig. 7.4, the UV plasma curing is as well as the Larolux® process a novel innovative concept of “inert” curing, which is based on the UV curing of (preferably threedimensional) parts in an evacuated plasma chamber. The process is described in more detail in Chapter 9. 7.3 INITIAL PHOTOYELLOWING28 Certain effects that are unique to radiation curing hamper the development of new UV coating applications. One such effect is initial photoyellowing that develops in the coating directly after exposure to ultraviolet (UV) or electron beam (EB) radiation (Figure 7.5), which has to be distinguished from the yellowing occurring (with almost all coatings) during aging and weathering. This initial photoyellowing is at least in part reversible and will, therefore bleach to some extent in the first few hours after the exposure. This variability in the colour of the coating makes exact colour matching very difficult until several hours after the radiation curing step, which, in turn, makes an inline quality control difficult. To expand the use of radiation curing into new applications this effect needs to be better understood. There are several influencing factors, which have been evaluated (ref. 28). The initial yellowing resulting from the curing reaction diminishes in the hours after exposure, the rate depending on the storage conditions (Figure 7.6). The bleaching rate is faster the higher the storage temperature. The yellowing development is not exclusively dependent on the presence of a photoinitiator, but rather due to the retarded decay of the formed radicals.


187

F IG . 7.5. Different stages of photoyellowing (due to photocuring and photoaging, respectively).

F IG . 7.6. Initial photoyellowness decay as a function of storage conditions.

The effect of the photoinitiator on initial photoyellowing is shown in Figure 7.7, where the same resin is exposed to e-beam radiation in the absence of a photoinitiator, and to UV in the presence of an α-hydroxy alkyl acetophenone photoinitiator. The absolute values of yellowing (b∗ ) are a function of the e-beam dose (7 mrad, 6.8; 3 mrad, 4.0; 15 mrad, 11) or the photoinitiator concentration. Attempts at measuring the initial photoyellowing with various concentrations of photoinitiator were realized. As expected, the higher the concentration, the more coloured the film becomes up to a certain limit, after which the initial yellowing plateaus or even starts to decline again. A similar dependence of the photoinitiator concentration on the reactivity of UV-printing inks has also been observed.29 This effect is not well understood, however, it is possible that at high photoinitiator concentrations and therefore high radical concentrations termination reactions (radical combination) could remove radicals from the system or shorten the average chain length of the polymer. This effect in turn would yield less long-lived radicals in the polymer matrix. Therefore,

188


F IG . 7.7. Comparing initial photoyellowness and yellowness decay of e-beam exposed with UV exposed resin.

F IG . 7.8. Photoproducts of the decomposition of irgacure® 184 photoinitiator.

it is not the presence of photoinitiators per se which is responsible for the initial yellowing, but rather, the initial photoyellowing seems to be an inherent effect of the radical concentration. The GC-MS spectrum of a cured solution of Irgacure® 184 in methanol showed the presence of a variety of scission products (Figure 7.8): benzene, cyclohexanol, cyclohexanone, benzaldehyde, 2-hydroxycyclohexanone, 1,1-dimethoxy-cyclohexane, methyl benzoate, 2-hydroxy-1-phenylethanone, Irgacure® 184, 2-diphenylethanone, benzoic acid, benzil,


189

F IG . 7.9. Initial photoyellowing due to photoinitiator type in UA resin laromer® LR8987 under nitrogen.

o-benzoylbenzoin. It is conceivable that many of these by-products could lead to yellowing upon undergoing secondary reactions such as oxidation or condensation. However, none of the fragments found can directly account for the initial photoyellowing phenomenon. Similar experiments were performed for other α-hydroxy alkyl acetophenones: Darocur® 1173 and Irgacure® 2959 were found to behave very similarly to Irgacure® 184. Consequently, all the α-hydroxy alkyl acetophenone photoinitiators, which have been investigated, also showed similar initial photoyellowing values. Acylphosphine oxides were introduced in radiation curable systems more than a decade ago by BASF and more recently by Ciba. 2,4,6-Trimethylbenzoyldiphenylphosphine oxide (Lucirin® TPO) and 2,4,6-trimethylbenzoylethoxyphenylphosphine oxide (Lucirin® TPO-L) are commonly used for pigmented systems or systems with high film thickness. Despite being yellow, these photoinitiators display photobleaching upon irradiation due to the destruction of the visible-light chromophore group of TPO. As the absorption of the photoinitiator bleaches, UV or visible light may penetrate deeper and deeper into the film. The initial photoyellowing of these photoinitiators is similar to the initial photoyellowing of α-hydroxy alkyl acetophenones (see Figure 7.9). However, the initial discoloration presented a slightly red coloration instead of the typical yellow coloration. Furthermore, combinations of Lucirin® TPO with other Photoinitiators may display synergistic effects and therefore show less initial photoyellowing than would be expected by simply adding the contributions from each photoinitiator. In order to avoid the formation of the yellow-coloured benzil, two other classes of photoinitiators have been tested. These are photoinitiators that undergo a primary process of hydrogen atom abstraction from the environment (or an intramolecular H-abstraction) and onium salt photoinitiators, which are primarily used for cationic polymerisation, but may also be used to initiate radical reactions. Benzophenone presents the disadvantage of developing significant discoloration when exposed to sunlight. Moreover, the yellowing during ageing becomes even worse if amines are used as co-initiator. Fortunately, some resins may also function as H-donors, so that the use of amines may be avoided. Despite the poor long-term performance of benzophenone yellowing, it shows only a very slight initial photoyellowing (see Figure 7.9). No benzil was found in GC-MS

190


analysis of a cured solution of benzophenone in propan-2-ol, which further confirmed that no α-cleavages took place. It has been reported30 that the initial photoyellowing of isopropyl-thioxanthone (ITX) with N-methyldiethanolamine as co-initiator is due to photoproducts from ITX. Unfortunately, ITX already exhibits a yellow coloration and shows an even stronger initial photoyellowing. For these reasons thioxanthones are usually only used in pigmented systems such as screen inks.31 Similar to benzil dimethyl ketal (BDK), the initial photoyellowing for ITX decreases in the presence of amines when N,N-dimethylethanolamine is added as co-initiator. Phenylglyoxylates have, unfortunately, only been the subject of a limited number of studies probably because of its non-trivial photochemistry. It has been claimed32 that these molecules undergo an intramolecular Habstraction followed by a fragmentation similar to a Norrish type II. It may, however, be feasible that this structure could yield a Norrish type I reaction. Initial photoyellowing results were excellent with this initiator. However, the initial photoyellowing of this photoinitiator depends on the conditions under which the curing takes place. The initial photoyellowing was very slight with a urethane acrylate under air. However, the initial photoyellowing was significant under inert conditions. Furthermore, the real time-IR spectra showed that the polymerisation realized under inert conditions achieved a high conversion of 71%, whereas, the reaction under air only reached a conversion of 44% (in thin films (film thickness ca. 10 µm)). The presence of benzaldehyde found in the GC-MS spectrum of a photolyzed solution of methyl phenylglyoxylate (Nuvopol® PI 3000) suggests that (apart from H-abstraction) a reaction type Norrish I may also take place. Segurola et al.33 drew similar conclusions by evaluating the photoyellowing and discoloration of the photoinitiator types, summarizing that 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184) exhibited very little discoloration and gave the best results of all photoinitiators studied and from the near UV absorbing photoinitiators Lucirin TPO (acyl phosphine oxides) caused lower yellowing compared to α-aminoalkyl ketone photoinitiators due to photobleaching. Amines are usually used to improve the reactivity of a system or as hydrogen donors. However, as systems containing amines present a strong discoloration when exposed to high temperatures it is advantageous to avoid their use in formulations. However, amines can also have a positive influence on the initial discoloration; as mentioned above, BDK shows a lower initial photoyellowing in the presence of amines, as does Nuvopol® PI 3000. It has been claimed34 that the reduction of the initial photoyellowing in the presence of amines with BDK could be explained by the photoreduction of the carbonyl chromophore to carbinol compounds through intermolecular hydrogen abstraction. However, in the case of most Norrish Type I photoinitiators the presence of amines leads to significantly stronger initial photoyellowing. Triarylsulfonium salts are commonly used as cationic photoinitiators. Cyracure® UVI6990 is a mixture of sulfonium salts. In the unexposed form it already exhibits a slight red colour. The acid obtained with this photoinitiator after curing leads to a very violent reaction with some vinylethers. However, it is suitable for initiating a polymerisation with epoxides. As in the case of iodonium salts urethane acrylates could be polymerised radically too. After UV curing a film of the aliphatic epoxide Basoset® 162 and a film of Laromer® LR 8987 with Cyracure UVI-6990 as photoinitiator, the films showed no increased initial photoyellowness. However, a slight red colour was observed. This initial


191

discoloration may be a result of (i) coloured by-products which are formed from radicals or (ii) side reactions of the super acid HPF6 with the radiation curable resin (e.g., condensation reactions or rearrangements). Dektar and co-workers have already studied the photoproducts resulting from the photolysis of triphenylsulfonium salts by GC-MS.35 Cyracure UVI-6990 is expected to yield additional photolysis products as a result of being a mixture and because of its more complex structure. The initial photoyellowing may depend on the quantity of photoinitiator which is photolyzed and thus on the quantity of light received by the sample. The more the sample is irradiated, the more yellow the film is. This observation is true for low irradiation doses, however, at higher doses this simple correlation no longer holds – in fact it reverses. It is feasible that at very high irradiation doses, the coloured photoproducts, which are themselves sensitive to UV-light, are further photolyzed. Also at higher radical concentrations, radical combination reactions become more favoured, thereby removing reactive species from the system. This is also reflected in the dependence of the photoyellowing on the photoinitiator content (Figure 7.10). The photoyellowing goes through a maximum, indicating that with higher photoinitiator contents more radicals are produced, however, under these conditions more radical termination reactions also become more likely. The binder seems to play an important role in the initial photoyellowing. Four classes of radiation curable resins have been tested: (i) urethane acrylates, UAs (aliphatic polyurethane acrylates are the most suitable for industrial outdoor applications), (ii) oligoether acrylates, POs, (and amine modified oligoether acrylates, POAs), (iii) epoxy acrylates, EAs, and (iv) polyester acrylates, PEs. Using Irgacure® 184 as photoinitiator, different behaviours were observed between the different binders, which suggest that the initial photoyellowing depends on both the photoinitiator and the matrix it is in. The best initial photoyellowing result was obtained with the aliphatic urethane acrylate Laromer® UA19T (see Figure 7.11). Tripropylene glycol diacrylate (TPGDA) and diethylene glycol diacrylate (DEGDA) gave better results, however, TPGDA and DEGDA are very rarely used as sole binders in a formulation.

F IG . 7.10. Influence of the content of the photoinitiator on the initial photoyellowing.

192


F IG . 7.11. Initial photoyellowing of different (Laromer® ) acrylate resins types exposed under air and nitrogen atmosphere.

In most resin classes a slight trend is detectable favouring either air or inert conditions for curing, e.g., aromatic epoxy acrylates (EAs) show less initial photoyellowing under nitrogen, while UV monomers show less initial yellowing under air. These differences are generally considered to be too small to be significant for normal applications. Furthermore, the data also show that amine modified polyether acrylates (POAs) exhibit a stronger initial discoloration than the polyether acrylates (POs). UAs and POs yielded the best results. Investigations employing Lucirin TPO as the photoinitiator gave very similar results to the results obtained with Irgacure® 184. Further experiments investigating the reactivity, absorption and viscosity of different radiation curable resins showed that there was no direct correlation between these properties and the observed initial photoyellowing. It is worth noting that in many cases the extent of initial photoyellowing does not correlate with long-term yellowing from artificial weathering or ageing experiments. On the contrary, the systems known for their low yellowing in ageing experiments (urethane acrylates) seem to be particularly prone to initial photoyellowing.

7.3.1

Conclusions

Thus, both the photoinitiator and the binder play an important role for the initial photoyellowing of radiation cured coatings. Except for BDK, photoinitiators undergoing photoscissions exhibit very similar initial photoyellowing results – qualitatively and quantitatively – presumably because they all have the benzoyl radical in common. With the exception of ITX, which exhibits a very strong discoloration after UV exposure, photoinitiators that undergo H-abstraction present low levels of initial photoyellowing. However, it is these photoinitiators that present yellowing upon ageing, especially, benzophenone and its derivatives. Cationic onium photoinitiators show only a relatively slight initial photoyellowing;


193

F IG . 7.12. Scheme for explanation of initial photoyellowness in radical initiated coatings.

their low solubility as well as the reddish colouring of the cured films make these cationic photoinitiators suitable for only a small number of applications. The complete absence of photoinitiators in the case of electron beam curing does not solve the problem of the initial photoyellowing effect either. On the contrary, electron beam curing has led to relatively strong initial photo-yellowing effects. Binders show varying trends in their initial photoyellowing depending on their chemical composition or resin class. Stabilisers, amines and the atmosphere during curing influence the strength of the initial photoyellowing to some extent and may allow incremental modifications of the initial photoyellowing. However, these differences in initial photoyellowing are minor in comparison with the contribution from the combination of resin and photoinitiator used in a formulation. The chemical nature of the polymer network as well as it’s density seems to be strong influencing factors regarding initial photoyellowing. Both electron beam and thermal curing also show strong initial yellowing which suggests that the yellowing is inherent to these highly crosslinked polymers. It is conceivable that the coloration is due to radicals or highly reactive species that are trapped in the polymer matrix and therefore have a significant lifetime of several hours to weeks36 (Figure 7.12). Any modification of the conditions that increases the mobility (i.e., delay vitrification) of these reactive centres (temperature, solvents) will reduce initial photoyellowing. Furthermore, reducing the number of radical species also reduces the observed photoyellowing (photoinitiator, energy density, stabilizers). Unfortunately, the phenomenon of initial photoyellowing is very complex and there is no universal solution. Therefore, curing conditions and raw materials need to be carefully examined for each application to reduce initial photoyellowing to a minimum.

REFERENCES 1. Decker, C., “Handbook of Polymer Science and Technology”, Vol. 3. 1989, p. 541. 2. Kloosterboer, J.G., Adv. Polym. Sci. 84, 1–61 (1988). 3. Krongauz, V.V. and Chawla, C.P., Photopolymerization in compact and digital versatile disks manufacturing: Peculiarities of oxygen effects, RadTech Report September/October 2001. pp. 34–46.

194 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14.

15. 16. 17.

18. 19. 20. 21. 22. 23.

24. 25. 26. 27. 28. 29. 30. 31.

32.

33. 34. 35. 36.


Schrof, W., Beck, E., Königer, R., Reich, W. and Schwalm, R., Progr. Org. Coat. 35, 197–204 (1999). Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 92–100 (2003). Studer, K., Decker, C., Beck, E. and Schwalm, R., Progr. Org. Coat. 48, 101–111 (2003). Wight, F.R. and Nunez, I.M., J. Radiat. Curing 16 (1), 3 (1989). Müller, U. and Vallejos, C., Angew. Makromol. Chem. 206, 171 (1993). O’Brien, A. and Bowman, C., The impact of oxygen on photopolymerization kinetics, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. (a) Müller, R., Proceedings of RadTech Europe Conference, Basel, 2001, p. 149; (b) Henke, T., Proceedings of RadTech Europe Conference, Basel, 2001, p. 145; (c) Beck, E., Proceedings of RadTech Europe Conference, Basel, 2001, p. 643. Menzel, K., Bankowsky, H.-H., Enenkel, P. and Lokai, M., RadTech Europe 1999, Conference Proceedings. 1999, pp. 165–170. Jung, J., RadTech Europe 1999, Conference Proceedings. 1999, p. 41. Bolon, D.A. and Webb, K.K., J. Appl. Polym. Sci. 22 (9), 2543 (1978). (a) Schrof, W., Beck, E., Königer, R., Meisenburg, U., Menzel, K., Reich, W. and Schwalm, R., RadTech North America, 1998. pp. 363–374; (b) Schrof, W., Häußling, L., Schwalm, R., Reich, W., Menzel, K., Königer, R. and Beck, E., RadTech Europe, 1997, Conference Proceedings. 1997, pp. 535–547. Davidson, S., “Exploring the Science, Technology and Applications of UV and EB Curing”. SITA Technology Limited, London, 1999, p. 249. Hoyle, C., An overview of oxygen inhibition in photocuring, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. 2004, on CD. Davidson, R.S., The role of amines in UV-curing, In “Radiation Curing in Polymer Science and Technology: Polymerization Mechanism”, Vol III (J.P. Fouassier and J.F. Rabek, eds.). Elsevier Applied Science, London, 1993, p. 153. Pappas, S.P. (Ed.), “UV Curing: Science and Technology”, Vol. II. 1985, Technology Marketing Corporation, p. 125. Decker, C. and Jenkins, A.D., Macromolecules 18, 1241 (1985). Kunz, M., Strobel, R. and Gysau, D., Proceedings of RadTech North America Conference, 1996, p. 278. Abadie, M.J.M. and Voytekunas, V.Y., Eurasian ChemTech J. 6, 67–77 (2004). (a) Decker, C., Faure, J., Fizet, M. and Rychla, L., Photogr. Sci. Eng. 23 (3), 137 (1979); (b) Decker, C., Makromol. Chem. 180, 2027 (1979). Gou, L. and Scranton, A., A photochemical method to eliminate oxygen inhibition in photocured systems, RadTech North America, eI5, UV&EB Technology, Charlotte, NC, May 2004, Technical Conference Proceedings. (2004), on CD. Königer, R., Farbe&Lack 105, 233 (1999). Maag, K., Lenhard, W. and Löffles, H., Progr. Org. Coat. 40, 93 (2000). Fischer, W. and Weikard, J., Farbe&Lack 107, 120 (2001). Decker, C., Masson, F. and Schwalm, R., Macromol. Mater. Eng. 288, 17–28 (2003). Königer, R. and Studer, K., Eur. Coat. J. 1–2, 26–58 (2001). Seng, H.-P., Coatings 5, 199 (2000). Hult, A. and Ranby, B., Polym. Degr. Stab. 8, 89 (1984). Crivello, J.V. and Dietliker, K., “Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation”, Volume III, 2nd edn. Wiley, 1998, p. 107. Crivello, J.V. and Dietliker, K., “Chemistry and Technology of UV & EB Formulation for Coatings, Inks & Paints, Photoinitiators for Free Radical Cationic and Anionic Photopolymerisation”, Volume III, 2nd edn. Wiley, 1998, p. 186; Encinas, M.V., Lissi, E.A., Zanocco, A., Stewart, L. and Scaiano, J.C., Can. J. Chem. 62, 386 (1984); and Huyser, E.S. and Neckers, D.C., J. Org. Chem. 29, 276 (1964). Segurola, J., Allen, N.S., Edge, M., McMahon, A. and Wilson, S., Polym. Degrad. Stab. 64, 39–48 (1999). Herlihy, S., Proceedings of RadTech Europe 1999 Berlin, November 8–10. 1999, p. 489. Dektar, J.L. and Hacker, N.P., J. Chem. Soc., Chem. Commun. 1591 (1987). (a) Atherton, N.M., Melville, H.W. and Whiffen, D.H., Trans. Faraday Soc. 54, 1300 (1958); (b) Selli, E., Trends Photochem. & Photobiol. 4, 55 (1997).

C HAPTER 8

Classical Applications The traditional applications of UV-curable systems are in the market sectors where temperature sensitive substrates are coated, like wood, paper and plastics as well as in imaging applications, like electronics and printing plates (Figure 8.1).1 The worldwide consumption of UV curable acrylate formulations in the year 2000 was approximately 140,000 tons. The dominating market sector worldwide is the graphic arts, with a share of about 50% in NAFTA, 42% in Europe and 35% in Japan, the wood sector being the second largest with 50% in Europe and about 14% each in NAFTA and Japan (Figure 8.2).

8.1 ON WOODEN SURFACES The wood finishing industry today deals with solid wood constructions, composition boards, like particle-boards or hard-boards, as well as with moulded polymers or foams,

F IG . 8.1. Classical applications of radiation curable coatings. 195

196


F IG . 8.2. The World market for UV-curable acrylate resins in the traditional applications (wood and graphic arts).

F IG . 8.3. Wood coatings market in Europe by chemistries.

decorated with wood imitating finishes. The main segments of wood processing are the direct processing of solid wooden products, like solid wood furniture, window/door frames or construction components, articles made of wooden pieces, like (veneer) coated fibre boards or chipboards, and wooden floorings, like parquet. The main markets for furniture production are in Europe, America and China. The market for wood coatings in Europe in the year 2000 was about 460,000 tons, polyurethane coatings accounting for about 34%, followed by nitrocellulose based (16%) and acid catalyzed coatings (15%) (Figure 8.3).

CLASSICAL APPLICATIONS

197

Wood processing in Europe is mainly done in Italy (33%), Germany (19%) and Spain (18%). In Europe UV-curable coatings have a considerable market share of about 12%.2,3 The main applications of UV coatings in the wood sector are on furniture and on flooring. The furniture wood coatings sector is dominated by large furniture manufacturers consuming large quantities of coatings for mass production, like IKEA, and to a lesser extent by artisans producing smaller production runs or pieces of furniture. Examples are residential furniture, like drawers, chairs, Tables, kitchen cabinets, bathroom vanities, hotel furniture, office furniture, doors, or parquet hardwood flooring. The coating of wooden flat panels with UV-curable materials started at least 30 years ago, mainly with filler sealer applications using unsaturated polyester-styrene coatings, few basecoat formulations and several topcoat applications,4 based on unsaturated polyesters and, due to higher speed and better performance, acrylate based coatings. Nowadays, a large variety of several different types of acrylate coatings are available, the major classes being:5 • 100% liquids for flat panel coating with roller application, and coating mouldings or shaped panels; • Water-based UV coatings for flat panels with curtain coating and coating mouldings or shaped panels with spray applications; • UV powder coats. Flat panels are coated mainly with 100% liquid UV systems. Complete solutions for UV staining, UV sealers and UV topcoats are available as well as the adequate equipment, like curtain and roller coaters and exposure systems. Large scale production is realized with parquet finishes, flat panel wood with open or closed grain, either mat or high gloss, and flat doors. For coating of shaped panels with 100% solids, sprayable formulations have to be used, which either contains appropriate amounts of monomers or low viscous oligomers, which can be applied with hot spraying systems (“unisprayer”, CEFLA). Industrially manufactured parquet is one of the domains of UV curable coatings, since they can be easily roller coated and cured at high speed. They exhibit high-abrasion and scratch resistance. Formerly used coatings based on unsaturated polyesters diluted with styrene have been replaced by acrylic type systems. As already discussed in the formulations section (Chapter 5.1), such parquet coatings are usually composed of: • a primer layer, responsible for the adhesion, • filler/sealer, and/or • several undercoats, which may contain abrasive resistant hard particles (corundum), responsible for abrasion resistance (Figure 8.4), and • a top coat layer, responsible for scratch and chemical resistance and the optical appearance, the entire layer thickness being up to 100 to 150 µm. Water-based UV coatings are often used as primers in order to enhance adhesion of the base and/or top coat, as well as for top coats with matt finishes, since the traditional 100% liquid UV coatings are more difficult to matt6 due to marginal shrinkage compared to solvent-borne and water-based coatings (Figure 8.5).

198


F IG . 8.4. Abrasion resistance of parquet flooring coatings containing filler and unfilled.

F IG . 8.5. Matting process.

Unsaturated polyesters/styrene or epoxy acrylate resins are often used in filler/sealers. Furthermore, clay, talcum or calcium carbonate are commonly used as fillers in these systems. Typically, antibubbling and leveling additives are also included in the formulation to yield coatings with a layer thickness of up to 50 µm. The parquet top coatings are predominantly 100% liquid acrylic systems based on polyester or urethane acrylates applied at thicknesses of around 50 µm. As alternatives to the 100% liquids, UV powder coats are heavily discussed as a viable coating alternative on MDF (medium density fibre boards) products, since they melt at low temperatures. When heated with IR and exposed to UV relatively short curing times (1– 2 min) prevent excessive heating stress on the substrate. This is important to avoid flaking problems due to heating up the MDF board and helps reduce the required cooling capacity needed. The chemistry of UV powder coatings will be discussed in Chapter 9. The coverage of wooden boards (chipboards, fiberboards) with foils, consisting of impregnated paper, which is printed with wood pattern and sealed with clear coats, is exem-

CLASSICAL APPLICATIONS

199

plified in Chapter 10. The clear coats used are often UV curable systems because of high production speed and good chemical and scratch resistance of the final coating. The challenges of industrial wood finishing beyond environmental concerns, to also address performance issues. For instance wear resistance of floorings, including new test methods to predict wear resistance, similarly, increased durability for external joinery and once again methods to predict durability. Increase of wear resistance by application of nanoparticles is an area of extensive research, since the nanoparticles introduce higher strength into the coating without negatively influencing transparency and gloss. Nanoparticles have also been used to introduce antibacterial properties into wood coatings,7 thus enabling the use of wooden articles in hygienic applications, like hospitals. The unsatisfactory durability of exterior coatings is not related to the topcoat performance but mainly attributed to poor adhesion. Therefore, the improvement of adhesion of the coating to wooden surfaces is a major issue. Beside the use of waterborne primers and low surface tension coatings by plasma treatment, crosslinking of the coating with the wood by reaction with isocyanato-acrylates has been discussed.8

8.2

IN GRAPHIC ARTS INDUSTRY

UV-curable systems are used in graphic arts in various applications, in imaging, as inks, as well as overprint coatings/varnishes. This market is by far the largest of all UV coatings applications (see also Figure 1.4).

8.2.1 Imaging Applications Photocuring is one possibility out of a wide range of photochemical induced reactions applied in imaging technology, such as typical copying processes (electrophotography, diazotyping), in graphic arts applications (diazo systems, colloid-dichromate emulsions, photopolymer printing plates) and in photoresists (photosolubilizing diazoquinone systems, photoacid catalyzed systems, azide crosslinking, photodimerization, photopolymerization). The process relies on applying a coating on a substrate, exposing through the image mask and subsequent removal (development) of the unexposed (negative) or exposed (positive) areas (Figure 8.6). Photocurable materials are only used in negative acting systems, where they induce a crosslinking reaction in the exposed areas and the unexposed parts can be dissolved (developed) in the solvent, which has been used in the coating formulation. In the negative process swelling during development is often observed, which may reduce the resolution capabilities. In positive imaging usually the solubility is changed by exposure from a hydrophobic to an alkaline soluble state, for example, blocked phenolic resins are deblocked to restore the alkaline soluble phenol resin. The imaging process is similar for photoresists and printing plates. The required resolution is much lower in printing plate applications (several µm pattern size) than in photoresists (from 1000 tons/year and CRM’s (>1 ton/year) have to be registered between 2007 and 2010. Their evaluation period extends to 2012. Substances produced or imported >100 tons/year have to be registered between 2007 and 2013, the evaluation done by 2016. The registration period for substances >1 ton/year ends 2018, when all of the existing substances >1 ton/year have to be registered. Up to now substances subject to authorization are limited to CMR, PBT, vPvB and endocrine disrupters, for which the producing or importing companies have to apply for an authorization for the foreseen use. If the risk is adequately controlled the EU

HEALTH, SAFETY AND ENVIRONMENT

299

F IG . 11.3. A uniform system for registration, evaluation and authorization of chemicals in the EU (REACH).

commission does grant an authorization, or if the socio-economic benefits outweigh the risks the commission may grant authorization with a time-limit. The impact of the REACH process on the production chain, as examined by Bias, considers the registration requirements of all chemicals used in the production chain (process chemicals for production of raw materials, e.g., solvents; raw materials, preparations, e.g., formulations from lacquer components, process chemicals, e.g., for surface treatment, etc.) compared with low or moderate level requirements put on the registration of imported final articles. It is therefore more attractive to import articles or finished goods instead of importing or producing within the EU individual substances or preparations. According to internal studies by companies about 20–100% of the formulations used in plastic additives, paints, electronics and cosmetics have to be re-examined and re-qualified due to higher costs. For some substances in the 10–100 tons/year range production may be stopped entirely. A rough estimate of the testing and registration costs was given (Figure 11.4). However, considering pros and cons of the REACH system, it may be a burden as well as an opportunity in respect to the changes in markets and competition profiles, alternative product developments, customer relationships and comprehension of uses, product developments and markets. The direct effects of REACH on the UV coatings market, and especially on the availability of the numerous raw materials are not yet conceivable. The trends, however, are foreseeable: Since UV-curable resins are low molecular weight oligomers, the sustainability of existing resins and the development of new types will shift to polymeric resins, which are exempt from registration. The reactive diluents portfolio will be cut down to a few commodities. The use of additives, like photoinitiators, synergists or other functional additives, is in the range of 10–100 tons (for about 10%), the range of 1–10 tons (for the

300


F IG . 11.4. Cost estimation of testing and registration (REACH).

other 90%). Thus, the number of available additives will most likely decrease significantly and/or the raw materials prices will increase. This scenario will create high cost burdens for reformulations. EuPia has assessed the impact on the European printing industry.11

11.6 UV COATED MATERIALS FOR FOOD CONTACT A new regulation of the EU (EC 1935/2004) on materials and articles intended to come into contact with food entered into force in November 2004. The general requirements of this regulation are laid down in Article 3, stating that materials coming into contact with food shall be manufactured such that under normal and foreseeable conditions of use, they do not transfer their constituents to food in quantities, which could endanger human health or bring about an unacceptable change in the composition of food or deterioration in the organoleptic characteristics thereof. In Article 5 specific measures for groups of materials and articles are defined, but no positive list of allowed substances with specific migration limits for printing inks exist. Therefore the compliance of UV inks can only be assessed on the basis of the overall migration limit (60 mg/kg food), the level of benzophenone as photoinitiator (0.6 mg/kg food) and the sum of acrylic acid esters (6 mg/kg food). Migration tests with UV printed materials have been performed and the results reported,12 showing that the migration levels in UV systems regarding the photoinitiators and acrylates are not as bad as often expected. The migration data of a large number of investigated photoinitiators was below the 10 ppb level (proposed for unknown substances in the “super directive draft”) and the level of acrylic acid was also below the proposed sum for acrylates of 6 mg/kg food.

HEALTH, SAFETY AND ENVIRONMENT

301

11.7 CONCEPT OF BEST AVAILABLE TECHNIQUE (BAT) The Directive on the Integrated Pollution Prevention and Control (IPPC 96/61/EG), adopted by the EU gives a list of industrial activities with a significant contribution to environmental pollution, for which an information exchange on the Best Available Technology (BAT) was organized. One of these activities is the field of surface treatment,13 which includes coatings. A concept of integrated technique assessment14 has been developed, where some fundamental aspects for the determination of the BAT have to be considered: • • • • • • •

Use of low-waste technology, Use of less hazardous substances, Consumption and efficient use of raw materials and energy, Recovery and re-use of raw materials, Technological and scientific advances, Nature and amount of emissions, Cost benefit relationship.

In a Case Study presented in this paper the application of different refinish primers for vehicle coating has been evaluated. Among them were, 1K UV cure, 2K PU thermal cure, Epoxy thermal cure, 2K PU infrared cure, Epoxy infrared cure, and 1K UV cure using aerosol cans. Considering the total costs, energy, global warming potentials, photochemical oxidant creation, health effects, risk, and resources, the 1K UV curing coating for vehicle refinish applications has been identified as the best alternative.

REFERENCES 1. (a) Jansen, I., The “Sevilla Process”: A driver for environmental performance in industry, RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 149–152; (b) http://eippcb.jrs.es. 2. RadTech Europe, http://www.radtech-europe.com/orghse.html. 3. (a) Raulfs, F.W., Classification and labelling of UV/EB acrylates, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 37–42; (b) http://www.cefic.be/Templates/shwAssocDetails.asp?NID=473&HID= 26&ID=39. 4. “UV protocol” download at the website: www.bgdp.de/e/pages/health-safety/UV-technology.htm or available at the institutions noted in the text. 5. http://www.radtech-europe.com/download/rte05signinguvprotokol.pdf. 6. (a) Knoop, R., UV protocol – requirements related to occupational health and safety stipulated consistently in Europe, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 113–118; (b) Riester, M., Küter, B. and Mayer, A., Safe use of UV/EB Technology in the Coating and Printing industry, RadTech Europe 2005, Conference Proceedings, Vol. I. 2005, pp. 27–29. 7. Nuijten, L. and Mayer, A., REACH – Exposure Scenarios (Categories versus UVITECH data), RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 105–110. 8. Roux, M.L., The impact of the European VOC Directive on the coating process of the furniture industry, RadTech Europe 2003, Conference Proceedings, Vol. I. 2003, pp. 459–465. 9. Drexler, H.J. and Sell, J., Eur. Coat. J. 04, 2–8 (2002). 10. Bias, W.-R., REACH – Impact and workability affecting the production chain, RadTech Europe 05, Conference Proceedings, Vol. II. 2005, pp. 87–102. 11. www.eupia.org/doc/easnet.dll/GetDoc?APPL=2&DAT_IM=0201C7&TYPE=PDF.

302


12. Rüter, M., Benz, H. and Piringer, O., UV Printing Inks in food contact material – Migration and set-off problems, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 139–146. 13. (a) Jansen, I., The “Sevilla Process” – Best available techniques in surface treatment using organic solvents, In “Integrated Scenario Analysis and Decision Support for the Modern Factory” (J. Geldermann, H. Schollenberger, I. Hubert and O. Rentz, eds.). French–German Institute for Environmental Research (DFIU/IFARE), Karlsruhe, 2004, pp. 127–136; (b) Schollenberger, H., Treitz, M., Geldermann, J. and Rentz, O., “Integrated environmental protection: Best Available Coating Techniques, International Coater congress: The Power of Surfaces.” Vincentz Network, Hannover, 2004, pp. 329–338. 14. Treitz, M., Schollenberger, H., Schrader, B., Geldermann, J. and Rentz, O., Multi-criteria decision support for integrated technique assessment, RadTech Europe 2005, Conference Proceedings, Vol. II. 2005, pp. 155– 160.

Subject Index

Automotive applications, 252 Automotive clear coats, 159

3D-UV curing pilot unit, 245 α-cleavage-type photoinitiators, 24, 115 Abrasion resistance, 224 of ORMOCER® coatings, 225 of parquet flooring coatings, 198 Absorbance, 42 Absorption spectra of different photoinitiators, 118 Accelerated weathering tests on UV resins, 134 Acid number, 127 Acrylate reactive diluents, 113 Acrylate resin type: effect on photoyellowing, 192 Acrylate terminated oligomers, 95 Acrylated polyacrylates, 101 Acrylates, 28 Adhesion, 211 Adhesion primer, 142 Adhesives, 281 Advantages and drawbacks of UV coatings, 11 of water-based UV coatings, 213 Advantages of UV coatings, 11 ecological, 11 economical, 11 performance, 11 Advantages of UV coatings for automotive clear coats, 256 Advantages of UV-curable powders, 215 Advantages of water-based coatings, 209–211 Aluminum can, 57 Amine modification, 118 of acrylate resins, 101 synergist action, 118 Amine synergist action, 118 Amine synergists, 183 Amtec–Kistler test, 83, 271 Application determines substrate and properties, 20 Application fields of UV-curable formulations, 140 Application spectrum of UV-curable coatings, 253 Application viscosity, 66

Best available technique (BAT), 301 Blue ray disk with UV protective coating, 285 Brittle–ductile transition temperature, 79 C-radical scavengers, 111 Calculation of crosslink density, 73 Cantilever-beam method, 87 Car wash field test, 269 Carbamato acrylates, 221 Cationic curing of epoxies, 121 Cationic metal coatings, 149 Cationic photoinitiators, 119, 121 avoiding the release of benzene, 121 Cationically curable systems, 120 Ceramiclear, 224 Chain type radical polymerization, 67 Challenges for exterior durable UV coatings, 229 Characterization of coatings, 74 Characterization of raw materials, 126 Chemical characterization, 90 Chemical methods to reduce oxygen inhibition, 183 Chemical resistance for automotive applications, 91 for wood applications, 91 Chemistry being determined by required properties, 21 Chemistry of typical automotive top coats, 255 Chromatographic characterization, 130 Classification of polymers duromers, 62 elastomers, 62 thermoplastics, 62 Classical UV applications, 195 in graphic arts, 199 on wooden surfaces, 195 Classification of the results of nanoscratch experiments, 274 Coating additives, 125 303

304


Coating application processes equipment advances, 37 roll and curtain coating, 37 spray coating, 37 UV robots, 37 Coating line examples aluminum cans, 57 hardwood panels, 57 plastic components, 57 printing press, 58 Coating markets, 2 Coil coating process, 279 Coil coatings, 278 Combinatorial application testing, 155 Comparison of UV coatings to standard 1K and 2K clears, 268 Components of UV formulations, 22 Composition of a water-based adhesion primer, 142 Composition of wood coatings, 141 Compositions of UV curable coatings, 9 Confocal Raman Microscopy, 34 Considerations for selection of starting formulations, 151 Conversion limitation by Tg , 165 Conversion limitation due to vitrification, 80 Copolymerization, 28 Correlation of coating performance with crosslink density, 174 E-modulus with crosslink density, 175 scratch resistance with crosslink density, 175 Crockmeter, 83 Crosslink density: correlation with modulus, 73 Crosslink density, 68, 71, 72, 78, 81, 173 Crosslinking reaction of AcResin® adhesives, 282 CSM-Nanoscratch, 83 Cure conversion in the presence of HALS radical scavenger, 230 of UV absorber, 230 Cure extent, 33, 36 Curing and hardness gradient, 90 Curing under inert conditions, 185 Curing under nitrogen and carbon dioxide, 238 Current R&D topics, 15 E value, 134 Deep UV photoresists, 202 Degree of crosslinking, 71 Dendritic acrylate oligomers, 104 Dendritic resins, 103 Depth profile of acrylate conversion, 36 Differential scanning calorimetry (DSC), 130 Differential thermal analysis (DTA), 130 Discolouration, 110

Dissociation quantum yields of photoinitiators, 117 Donor–acceptor mechanism, 31 Donor–acceptor systems, 104, 105 Dosimeters, 55 Double bond conversion, 34 Drawbacks of UV coatings, 11, 179 Driving forces of UV technology, 14 Dual cure coatings, 218 adhesion improvement, 221 Dual cure dispersions, 108, 222 Dual cure powder coating hybrids, 221 Dual cure resins, 106 Dual cure systems, 186 Dual cure water-based coatings, 221 Durability tests, 253 Duromers, 62 DVD bonding, 283 Dynamic mechanical analysis, 77 Dynamic viscosity, 126 e-beam technology, 7, 8 E-modulus, 78 Easy matting, 210 Eco-efficiency, 13 analysis, 258 of automotive primer/scaler, 258 of wooden door fronts, 13 portfolio, 263 Economic factors of UV curing, 19 Effect of crosslinking on coating properties, 172 Effect of crosslinking on mechanical properties, 162 Effective energy, 40, 43 Efficiency of photoinitiation, 24 Elastic response, 268, 271–274 Elastic response pattern, 273 Elasticity modulus, 75 Elasticity modulus as function of crosslink density, 78 Elastomers, 62 Electrical input power, 39 Electromagnetic spectrum, 7 Elongation at break, 76 Elongation at yield, 76 Energies as a function of wavelenght, 8 Energy density, 39, 41 Energy of a photon, 42 Energy output of a mercury bulb, 40 ENE’s: structures and reactivities, 124 Environmental benefit of UV-cured coatings, 260 Environmental impact assessment matrix, 296 Enzymatic esterification, 99

SUBJECT INDEX Epoxy acrylates, 95–97 synthesis, 97 Equipment, 37 Equipment suppliers, 58 Evaluation of scratch resistance, 158 Excimer lamps, 47 Explanation of initial photoyellowness, 193 Exposure parameter measurement, 48 Exposure process, 38 Exposure profile, 52 Exterior applications, 135 Exterior durable UV coatings: challenges, 229 Falling sand test, 83 Fe-added mercury lamp spectrum, 47 Fe-additive, 47 Field test car wash, 273 Field test scratch performance, 270, 273, 274 Film coating instead of painting, 286 Film coating technology, 286 Film formation, 65 Fischerscope® , 83 Fischerscope® measurements, 154 Flash light curing, vi Flexibility, 81 Flexibility as a function of Tg , 167 Flexographic printing plate, 200 Foil coating process, 288 Foil coating technology, 287 Food contact, 300 Formulation robot, 155 Formulation screening for new applications, 149 Formulations, 140 Formulations for graphic inks, 144, 145 overprint varnish, 147 pigmented UV ink, 145 UV flexo ink, 146 UV offset ink, 145 UV screen ink, 148 Formulations for wood coatings, 141 Fracture response, 268, 271, 272, 274 Frontal polymerization, 32 Function of UV absorbers, 227 Functions of a coating, 1 Furniture foil coatings, 286 Furniture wood coatings, 197 Future coating technologies, 3 advantages, 3 drawbacks, 3 Ga-added mercury lamp spectrum, 47 Ga-additive, 47 Gelpoint, 71 Glass region, 63

305

Glass transition region, 63 Glass transition temperature (Tg ), 64, 160, 162 determines mechanical properties, 171 influence of crosslink density, 164 influence of cure conversion, 164 influence on flexibility, 167 influence on hardness, 167 structural influencing factors, 163 Gloss and haze measurement, 84 Graphic application requirements, 132 Graphic arts industry, 199 Gravure printing plates, 200 Guidelines for classification and labelling of UV acrylates eurymeric acrylates, 292 stenomeric acrylates, 292 Guides for the safe use of UV technologies, 291 H-abstraction type photoinitiators, 24, 115 HALS radical scavengers, 227, 230, 231 Hardness, 80 Hardness and flexibility as a function of Tg , 166 Hardness as a function of Tg , 167 Hardness at the surface as a function of UV exposure atmosphere, 176 Hardness-flexibility balance, 177 Hardwood panels, 57 Hazard classes, 292 Health, safety and environmental (HSE) aspects, 291 Heat of polymerization, 32 High performance liquid chromatography (HPLC), 130 High-throughput screening (HTS), 153, 154 Homogeneity of polymer networks, 79 Hybrid-Cure systems, 218 Hydrogen abstraction type photoinitiators type II, 115 Hydroperoxide decomposers, 109, 111 Hyperbranched resins, 103 Illumination, 41 Imaging applications, 199, 202 Indentation hardness, 81 Industrial parquet coating, 142 Inert atmosphere, 182 Influence of curing conditions, 240 Influence of Mc (molecular weight between crosslinks) on flexibility, 169 Influence of resin/diluent functionality on polymerization process and properties, 161 Influence of the oxygen content on curing reaction, 240

306


Infrared spectroscopy, 128 Inhibition under aerobic conditions, 109 Inhibition under anaerobic (inert) conditions, 111 Initial photoyellowing, 186 decay of yellowing, 187, 188 effect of photoinitiator content, 191 effect of photoinitiator type, 189 effect of resin type, 192 Initial yellowing, 179 Inks, 202 Interaction of UV process parameters, 20, 21 Internal stress, 84, 87 as a function of shrinkage, 89 as a function of Tg , 88 as a function of temperature, 88 Interpenetrating networks, 68 Iodine value, 127 Irradiance, 39, 40 Isocyanato acrylates, 106, 219 Jablonsky-type diagram, 23 Key exposure parameters, 49 Kinematic viscosity, 126 Kinetic chain length, 27 Kinetics of the photopolymerization, 26 Labelling guide, 291 Labelling recommendations in case of incomplete data, 294 Laboratory scratch tests, 271 Lambert–Beer law, 42 Larolux® , 239 process, 185 UV inert curing, 244 Layer composition of parquet flooring coating, 142 LED based light sources, 47 Letterpress printing plates, 200 Light stabilizers, 227 Lithographic printing plates, 201 Low emission photoinitiators, 119 Low yellowing photoinitiators, 118 Lower extractables, 209 Macosko–Miller theory, 69, 72 Maleimides, 31 Mar resistance, 267 Market development of industrial coatings, 2 Market UV acrylate resins, 196 Market of UV coatings, 4, 5 Mass spectrometry, 129

Matching the spectral output of the lamp with the absorption characteristics of the photoinitiators, 52 Matt formulations, 143 Matting process, 198 Measurement of exposure parameters, 52 Measures to decrease detrimental oxygen effect, 181 Mechanical properties of coatings, 162 Mechanical properties of materials, 63 Mechanical properties of UV cured coatings brittle–ductile transition (Tb ), 170 effect of crosslinking on Tb , 170 effect of Tg (hard or soft), 171 measures to obtain hard and flexible coatings, 172 Mechanical response, 271 Mechanism of monomer inhibition and stabilization possibilities, 109 Mercury based lamps, 45 Metal containing monomers, 105 Methacrylates, 28 Microgel formation, 68 Microwave powered mercury lamps, 45 Modulus as a function of temperature, 64 Modulus of elasticity, 63 Molecular weight between crosslinks, 71 Molecular weight build-up during curing, 66 Monitoring film formation and curing, 73 Monitoring of optimized UV process, 50 Monomers, 111 Multifunctional monomers, 113 Nano-indentation, 176 Nano-Scratch Tester, 84 Nano-scratching, 176 Nanocoatings, 223 Nanocomposites, 223 Nanoindentation measurements correlation to laboratory tests, 272 correlation with field tests, 274 principle, 85 Nanoparticles, 226 Near-infrared (NIR) spectroscopy, 35 Network characterization, 74 Network formation, 62, 66, 68 Network formation by chain connections, 70 Network structures, 65, 71 Network types, 69 New applications for radiation curable coatings, 252 Norrish type I photoinitiators, 115 Novel routes to urethane acrylates, 99 Nuclear magnetic resonance (NMR), 129

SUBJECT INDEX OH number, 128 One-component dual cure coatings, 221, 222 Onium salts, 121 Optical density, 43 ORMOCER® coatings, 223 ORMOCER® systems, 105 Overcoming the detrimental effects of oxygen inhibition, 184 Overprint varnishes (OPV), 146 application, 204 for inert exposure, 148 formulations, 148, 204 performance requirements, 204 Oxetane groups, 121 Oxygen diffusion, 184 Oxygen diffusion scheme, 179 Oxygen inhibition, 179 influence of viscosity on polymerization rate, 184, 243 Oxygen inhibition as function of layer depth, 180 Oxygen inhibition reaction, 30 Oxygen penetration, 180 Oxygen scavenging by a dye, 184 Painted film, 287 Parquet coatings, 197 Parquet flooring UV coatings: scratch resistance, 225 Particle size measurement, 131 Peak irradiance, 43, 44 Pendulum hardness, 81 Perception of scratches, 268 Performance requirements for automotive coatings, 252 Perspectives of UV curing technology, 17 Phases of process design, 50 Phenolic antioxidants, 109 Photo DSC, 36 Photo latent base, 265 Photochemical yellowing, 133 Photoinduced curing chemistry, 22 Photoinitiation, 23 Photoinitiator classes, 116 Photoinitiator content: effect on photoyellowing, 191 Photoinitiator-free polymerization, 104 Photoinitiators, 115 hydrogen abstraction type II, 115 Norrish type I, 115 Photoinitiators for, 119 cationic curing, 119, 121 pigmented coatings, 119

307

radical curing, 114 through cure, 119 UV-stabilized coating, 119 Photoinitiators used in combination with light stabilizer package (UV absorber/HALS), 232 Photopolymer printing plates, 199 Photopolymerization kinetic chain length, 27 propagation, 25 rate of initiation, 27 termination, 25 Photoresists, 200, 202 Photosensitization of cationic photopolymerisation, 122 Photoyellowing, 186 Physical methods to reduce oxygen inhibition, 181 Pigmented UV coating formulation absorption spectra, 51 Pigmented UV coatings, 246 Pigmented UV ink, 145 Pigmented UV-curable coatings, 280 Plastic applications in automotive, 276 Plastic components, 57 Plastic response, 268, 271, 272, 274 Polycarbonate headlamp lenses, 258 Polyenes, 106 Polyester acrylates, 100 property spectrum, 161 synthesis, 100 Polyester and polyether acrylates, 100 Polyether acrylates, 101 Polymer bound photoinitiators, 119 Polymer definition, 108 Polymerization behaviour of acrylates and methacrylates, 30 Polymerization rates, 28 acrylates, 28 methacrylates, 28 Polymerization shrinkage, 84 Polyurethane acrylate dispersions, 108 Printed circuit boards (PCB), 202 Printing ink formulation, 144 Printing inks, 145 Printing plates, 200 Printing press, 58 Printing techniques, 201 Process control, 49 Process design, 53 Process window, 49, 50 Processing of UV powders, 215 Propagation, 25 Properties of UV curable acrylate resins fundamental influencing factors, 161

308


generalized property spectrum, 161 Property spectrum of standard acrylate resins, 102, 161 Property–property relationships, 172 Radiachromic dosimeters, 55 Radiant flux, 42 Radiant power, 39, 40 Radiation attenuation throughout coating thickness, 43 Radiation cured coil coatings, 279 Radical initiated UV induced crosslinking, 23 photoinitiation, 23 propagation, 24 termination, 24 Radical photoinitiators, 114 Radical polymerization systems, 95 Radical scavengers, 227, 228 Radical types occurring during processing/storage, 109 Radiometers, 52, 54 Radiometric devices, 54 Radiometric methods, 52 Rate of initiation, 27 Rate of polymerization, 33 Raw material suppliers, 135 Raw materials for exterior applications, 131 Raw materials for UV coatings, 94 REACH (Registration, Evaluation and Authorisation of Chemicals), 297, 298 impact on production chain, 298 motivation, 298 registration obligation and timeline, 298 Reactive diluent types, 112 Reactive diluents, 111 Reactivity ratios of comonomers, 29 Real-time infrared (RT-IR) spectroscopy, 26, 33 Recovery, 271, 272 Recovery behaviour, 274 Reduction of oxygen inhibition, 243 by chemical methods, 183 by physical methods, 181 Reflectors, 48 Registration, evaluation and authorization of chemicals (REACH), 297, 298 Relationship of scratch and acid etch resistance, 275 Resin types for UV curing systems, 94 Responsivity, 54 Risk assessment matrix, 296 Risk assessment of UV curing, 295 Rota–Hub, 83 Rubber-elastic plateau, 64

Safe application of inks, 291 Safe application of UV-curable systems, 295 Safe handling of acrylates, 292 Scratch laboratory test methods, 267 Scratch resistance, 175, 224, 271, 275 coating response to impacts, 77 correlation of field tests with nanoindentation measurements, 274 requirements coatings have to fulfill, 274 Scratch resistance after Jacksonville testing, 275 Scratch resistance of automotive coatings correlation with mechanical response, 271 Scratch resistance of different clear coats, 275 Scratch resistance of weather-resistant UV coatings, 244 field tests, 270 Scratch resistant coatings for automotive applications, 266 Scratch resistant top coat formulations, 144 Scratch resistant UV coatings, 258 Scratch resistant UV coatings for DVD/Blue ray, 284 Scratch tests, 267 Selection criteria for raw materials for exterior applications, 135 Selection of raw materials, 131 exterior applications, 131, 135 graphic applications, 132 wood applications, 131 Selection of resin types for outdoor use, 132 Self-initiating UV-curable resins, 104 Shadow areas, 179, 185, 249 Shrinkage, 84, 86 Shrinkage as a function of functionality, 86 Side reactions contributing to discolouration, 110 Side reactions during esterification, 113 Silicon based oligomers, 105 Single scratch tests, 83 Sol–gel technology, 223 Solvent-borne UV coatings, 206 Spectroradiometers, 55 Spray application, 206 Spray coating, 37 Stabilizer/inhibitors, 108 Standard mercury lamp, 46 spectrum, 47 Stenomeric acrylates, 293 Step type polyaddition/condensation, 67 Stress-strain analysis, 77, 162 Stress-strain curves, 76 Stress-strain experiments, 63 Structural adhesives, 283

SUBJECT INDEX Structure–property relationships, 160, 172 Structures of typical thiols, 125 Suitability of UV coatings for automotive applications, 254 Superfluid chromatography (SFC), 130 Surface tension, 65, 66 Taber Abraser method, 82 Tangent δ, 78 Temperature dependence of mechanical properties, 79 Termination in radical polymerization reactions, 25, 26 Terminology, 39 Terminology related to emission of light, 38, 39 electrical input power, 38 energy density, 39, 41 irradiance, 39, 40 radiant power, 39, 40 Terms and SI units of light emission, 41 Test methods for abrasion resistance, 82 coatings, 75 scratch resistance, 83 AMTEC-Kistler, 83 Crockmeter, 83 Indentation, 83 Rota–Hub, 83 Thermal analysis, 130 Thermal stress, 86 Thermal yellowing, 133 Thermogravimetric analysis (TGA), 131 Thermoplastics, 62 Thiol-ENE systems, 123 general mechanism, 123 insensitivity to oxygen inhibition, 123 Thiols structures, 125 Time-temperature superpositioning, 76 Topcoat for glass, 149 Topcoat formulations, 143 Torsion modulus, 75 Traditional applications of UV-curable systems, 195 Transesterification, 99 Transmittance, 42 Trends in UV technology, 15 at coatings manufacturers, 16 at end-users, 16 at raw material suppliers, 16 TTT (time, temperature, transition)-diagram, 79 Two component dual cure coatings, 219 Unsaturated esters, 31

309

Unsaturated polyesters, 95 Urethane acrylates, 96–100, 135, 173, 256, 277 enzymatic esterification, 99 exterior applications, 98, 132–135 flexible, 99 hard, 99 novel routes to, 99 property spectrum, 161 property tuning, 98 synthesis, 98 transesterification, 99 UV absorbers, 227, 228 UV spectra, 229 UV clear coat, 287 UV clearcoat developments for automotive OEM coatings, 220 UV coated materials for food contact, 300 UV coating line examples, 56 UV coating markets, 4, 5 regional split, 4 sectorial split, 4 trends across the regions, 5, 6 UV coating of polycarbonate, 149 UV coatings application examples, 8 from traditional to new applications, 9 UV equipment setup, 54 UV flexo ink, 146 UV inkjet, 284 UV inks for flexographic printing, 203 for lithographic printing, 203 for screen-printing, 203 UV lamps, 45 UV line equipment, 44 UV offset ink, 145 UV plasma cure® , 248, 249 UV plasma curing, 186, 246, 247 UV powder coating formulations, 216 UV powder coating process, 216 UV powder coatings, 213, 258 advantages, 215 applications, 214 chemistry, 216 processing, 215 why UV powder?, 214 UV powder coatings on MDF, 198 UV protocol, 294 UV Protocol of safe handling of coatings, 291 UV refinish primer, 258, 265 UV robot, 37 UV screen ink, 147 UV spectra of photoinitiators, 128

310


UV systems for dental applications, 285 UV technology, 7 UV wood coatings 100% liquid coatings, 197 UV powder coatings, 197 water-based UV coatings, 197 UV-A clearcoats, 265 UV-A curable coating, 266 UV-curable adhesives for optical disks and displays, 283 UV-curable clear coats, 256 UV-curable coatings, 275 for car refinish, 264 for hard top coats on plastic, 277 key for film technology, 286–289 on automotive plastics, 254 UV-curable dispersions, 208 UV-curable emulsions, 208 UV-curable functional groups, 31 UV-curable inks, 202, 203 UV-curable ink-jet inks, 284 UV-curable nanocoatings, 223 UV-curable nanocomposites, 226 UV-curable ORMOCER® coatings, 224 UV-curable polyurethane dispersions (PUD), 222 UV-curable primer/sealer, 254, 258 UV-curable refinish primers, 258 UV-curable water-based systems, 207 UV-cured parts in automotive applications, 257 UV-curing adhesives, 281 UV-curing in the presence of UV absorbers and radical scavengers, 227, 230 UV-curing of highly flexible coatings, 277 UV-curing of pigmented systems, 246 UV-curing pilot line for automotive bodies, 257 UV-curing process, 10, 19 application, 20 chemistry, 21 equipment, 37 UV-curing under inert atmosphere, 238 advantages of carbon dioxide, 239 comparison nitrogen versus carbon dioxide, 238

influence of curing conditions, 240–243 influence of oxygen content, 240 influence of viscosity, 243 UV-dual cure processing for automotive top coat application, 276 UVITECH Project, 291, 295 Vinyl ether reactive diluents, 114 Vinyl ether (VE) monomers, 31 Vinyl ether/maleimide systems, 104 Viscoelastic materials, 162 Viscoelastic properties, 75 Viscosity, 66, 126, 243 Viscosity changes, 67 Vitrification, 165 VOC regulation, 296 Water-based systems, 107 Water-based UV coating, 206 types of (soluble, emulsion, dispersion), 208, 209 why water-based UV?, 206 Water-soluble UV-curable resins, 208 Weathering performance of UV coatings, 234 aging followed with IR spectroscopy, 234 Florida tests of UV clear coats, 238 of water-based UV coatings, 234–237 Weathering resistance of UV-cured acrylics, 227 Weathering resistance of water-based coatings, 234 Weathering stability of coatings, 232 Weathering stability of polyurethane dispersion, 236 Weathering stability of UV-cured clear coats, 238 Wood application requirements, 131 Wood coatings market in Europe, 196 Wooden door fronts: eco-efficiency, 13 Years in Florida, 238 Yellowing, 186 Yellowness decay, 188

UV Coatings: Basics, Recent Developments and New Applications

Particle image velocimetry: new developments and recent applications

Recent developments and applications of invariant integrals

Nanomaterials Chemistry: Recent Developments and New Directions

Nanomaterials Chemistry: Recent Developments and New Directions

Coffee: Recent Developments

Flavour Science: Recent Developments

Tree Transgenesis: Recent Developments

New developments in formal languages and applications

New Developments and Applications in Sensing Technology

Tree transgenesis: recent developments

Coffee: Recent Developments

Nonstandard Analysis. Recent Developments

Handbook of Time Series Analysis: Recent Theoretical Developments and Applications

Recent Developments and Innovative Applications in Computational Mechanics

Handbook of Time Series Analysis: Recent Theoretical Developments and Applications

Magnetism: Basics and Applications

New Developments and New Applications in Animal Cell Technology

Recent Developments of Electrical Drives

Recent developments in vector optimization

Recent Developments in Vector Optimization

Recent Synthetic Developments in Polyquinane

Recent developments in nonlinear analysis

Recent Developments of Electrical Drives

Recent developments of electrical drives

Recent Developments in Nonlinear Analysis

Robot Motion and Control: Recent Developments

Recent Developments in Alcoholism: Alcoholism and Women

Computer Science Today.. Recent Trends and Developments

Artificial Intelligence Today.. Recent Trends and Developments

Recent developments in fractals and related fields

UV Coatings: Basics, Recent Developments and New Applications

Particle image velocimetry: new developments and recent applications

Recent developments and applications of invariant integrals

Nanomaterials Chemistry: Recent Developments and New Directions

Nanomaterials Chemistry: Recent Developments and New Directions

Coffee: Recent Developments

Flavour Science: Recent Developments

Tree Transgenesis: Recent Developments

New developments in formal languages and applications

New Developments and Applications in Sensing Technology

Tree transgenesis: recent developments

Coffee: Recent Developments

Nonstandard Analysis. Recent Developments

Handbook of Time Series Analysis: Recent Theoretical Developments and Applications

Recent Developments and Innovative Applications in Computational Mechanics

Handbook of Time Series Analysis: Recent Theoretical Developments and Applications

Magnetism: Basics and Applications

New Developments and New Applications in Animal Cell Technology

Recent Developments of Electrical Drives

Recent developments in vector optimization

Recent Developments in Vector Optimization

Recent Synthetic Developments in Polyquinane

Recent developments in nonlinear analysis

Recent Developments of Electrical Drives

Recent developments of electrical drives

Recent Developments in Nonlinear Analysis

Robot Motion and Control: Recent Developments

Recent Developments in Alcoholism: Alcoholism and Women

Computer Science Today.. Recent Trends and Developments

Artificial Intelligence Today.. Recent Trends and Developments

Recent developments in fractals and related fields

Recommend Documents