Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Bob Goss

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.ismithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder. A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-139-5 (hardback) 978-1-84735-138-8 (softback) 978-1-84735-140-1 (ebook)

Typeset by Integra Software Services Pvt. Ltd. Printed and bound by Lightning Source Inc.

C

ontents

Preface ................................................................................................................. vii 1.

Introduction to Adhesives.............................................................................. 1 1.1

Cyanoacrylates .................................................................................... 1 1.1.1

1.2

UV-curing Adhesives ............................................................................ 9 1.2.1

The Curing Process ................................................................. 9

1.2.2

Health and Safety with UV ................................................... 13

1.2.3

The Curing Equipment .......................................................... 13

1.2.4

Curing Adhesive Tack-free .................................................... 14

1.2.5

Types of UV Adhesives .......................................................... 16

1.2.6

Benefits of UV Adhesives ....................................................... 17

1.3

Two-part Acrylics .............................................................................. 17

1.4

Epoxies .............................................................................................. 19 1.4.1

1.5

1.6

Advantages and Disadvantages of Epoxies ............................ 21

Flexible Adhesive Sealants ................................................................. 21 1.5.1

Silicone Adhesive Sealants ..................................................... 22

1.5.2

Polyurethane Adhesive Sealants ............................................ 23

1.5.3

Modified Silane Adhesive Sealants ........................................ 24

Hot Melt Adhesives ........................................................................... 25 1.6.1

2.

Types of Cyanoacrylate ........................................................... 3

Reactive Hot Melts ............................................................... 25

Engineering Thermoplastics......................................................................... 27 2.1

Introduction ...................................................................................... 27

2.2

Amorphous Thermoplastics ............................................................... 27

2.3

Semi-crystalline Polymers .................................................................. 29

2.4

Adhesive Performance on Thermoplastics .......................................... 31

i

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts 2.4.1

ABS ....................................................................................... 32

2.4.2

LCP....................................................................................... 32

2.4.3

Polyamide ............................................................................. 33

2.4.4

PBT ....................................................................................... 34

2.4.5

PC ......................................................................................... 36

2.4.6

PEEK .................................................................................... 37

2.4.7

PES and PUS ......................................................................... 38

2.4.8

PE ......................................................................................... 39

2.4.9

PET ....................................................................................... 40

2.4.10 PMMA.................................................................................. 40 2.4.11 POM ..................................................................................... 41 2.4.12 PPO ...................................................................................... 42 2.4.13 PPS........................................................................................ 43 2.4.14 PP ......................................................................................... 43 2.4.15 PS.......................................................................................... 45 2.4.16 PTFE ..................................................................................... 46 2.4.17 PVC ...................................................................................... 47 2.5 3.

Engineering Thermoset Plastics ................................................................... 51 3.1

Introduction ...................................................................................... 51

3.2

Adhesive Performance on Thermoset Plastics..................................... 53

3.3 4.

ii

General Comments on Adhesive Bonding of Thermoplastics ............. 47

3.2.1

Diallyl Phthalate (DAP) ......................................................... 54

3.2.2

Epoxies ................................................................................. 55

3.2.3

Phenolics ............................................................................... 55

3.2.4

Polyester (Thermoset) ........................................................... 56

3.2.5

Polyurethanes........................................................................ 57

3.2.6

Polyimides............................................................................. 58

General Comments on Adhesive Bonding of Thermoset Plastics ........ 59

Elastomers and Thermoplastic Elastomers (TPE)......................................... 61 4.1

Introduction ...................................................................................... 61

4.2

Adhesive Performance on Elastomers................................................. 61 4.2.1

Butyl Rubber......................................................................... 62

4.2.2

Copolyester TPE ................................................................... 63

Contents 4.2.3

Ethylene Acrylic (EEA) Rubber ............................................. 64

4.2.4

Ethylene Propylene Diene Monomer Rubber (EPDM) .......... 64

4.2.5

Ethylene-Vinyl Acetate Co-polymer (EVA) ............................ 65

4.2.6

Fluorosilicone Rubber ........................................................... 66

4.2.7

Natural Rubber ..................................................................... 66

4.2.8

Nitrile Rubber ....................................................................... 67

4.2.9

Neoprene Rubber .................................................................. 68

4.2.10 Polyisoprene.......................................................................... 68 4.2.11 Polyolefin Elastomers ............................................................ 68 4.2.12 Silicone Rubber ..................................................................... 70 4.2.13 Styrene-Butadiene Rubber (SBR) ........................................... 70 4.2.14 Styrenic TPE ......................................................................... 70 4.2.15 Thermoplastic Vulcanisates (TPV)......................................... 72 4.3 5.

6.

General Comments on Bonding of Elastomers ................................... 72

Joint Design................................................................................................. 75 5.1

Introduction ...................................................................................... 75

5.2

Lap Joint ........................................................................................... 75 5.2.1

Joint Width versus Joint Overlap .......................................... 76

5.2.2

Optimising Joints to Minimise Stress .................................... 78

5.3

Double Lap Joint (Tongue and Groove) ............................................. 80

5.4

Cylindrical Joints ............................................................................... 81 5.4.1

Design Details ....................................................................... 82

5.4.2

Cross Holes........................................................................... 84

5.4.3

Blind Holes ........................................................................... 85

5.5

Butt Joint ........................................................................................... 86

5.6

Bond Line Thickness .......................................................................... 88

5.7

Thermal Effects ................................................................................. 89

5.8

Selecting the Viscosity of the Adhesive ............................................... 89

5.9

Surface Preparation ........................................................................... 91

Bonding of Low-energy Plastics and Rubbers .............................................. 93 6.1

Surface Wetting.................................................................................. 93

6.2

Measuring Surface Energy ................................................................. 97

6.3

Surface Treatments ............................................................................ 97 iii

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

6.4 7.

6.3.1

Abrasion ............................................................................... 98

6.3.2

Corona Discharge ................................................................. 98

6.3.3

Plasma Treatment.................................................................. 98

6.3.4

Flame Treatment ................................................................... 98

6.3.5

Use of Primers ....................................................................... 99

Two-part Acrylics .............................................................................. 99

Selecting the Adhesive ............................................................................... 101 7.1

Introduction .................................................................................... 101

7.2

Factors for Consideration ................................................................ 102 7.2.1

Selection of Materials .......................................................... 102

7.2.2

Adhesive Performance ......................................................... 103

7.2.3

Durability and Long Term Performance and Temperature Resistance....................................................... 104

7.2.4

Surface Preparation ............................................................. 104

7.2.5

Ease of Application ............................................................. 105

7.2.6

Joint Design ........................................................................ 105

7.2.7

Viscosity.............................................................................. 106

7.2.8

Cure Speed .......................................................................... 107

7.2.9

Gap-filling Capability .......................................................... 107

7.2.10 Sealing Capability ............................................................... 107 7.2.11 Health and Safety (H&S) .................................................... 108 7.2.12 Approvals ........................................................................... 111 7.2.13 Recycling Adhesives ............................................................ 111 7.3 8.

Dispensing Adhesives in Production .......................................................... 115 8.1

iv

A Summary for Adhesive Selection .................................................. 112

Basic Principles ................................................................................ 115 8.1.1

Single- or Two-part Adhesive .............................................. 115

8.1.2

Viscosity (see Section 5.8) ................................................... 116

8.1.3

Cycle Time .......................................................................... 118

8.1.4

Cure Method ...................................................................... 118

8.1.5

Dispense Quantity ............................................................... 118

8.1.6

Open Time .......................................................................... 120

8.1.7

Health and Safety ................................................................ 120

Contents 8.1.8 8.2

8.3 9.

Cost .................................................................................... 121

Dispensing Systems .......................................................................... 121 8.2.1

Manual Units ...................................................................... 122

8.2.2

Semi-automatic Dispensers .................................................. 123

8.2.3

Syringe Dispensing .............................................................. 123

8.2.4

Pressure Pot Dispensing ...................................................... 123

Automatic Systems .......................................................................... 125

Durability and Environmental Testing ....................................................... 127 9.1

Introduction .................................................................................... 127 9.1.1

Surface Finish and Surface Preparation ............................... 127

9.1.2

Joint Design ........................................................................ 128

9.1.3

Substrate Bonded ................................................................ 129

9.2

Effect of Humidity and Water Absorption ....................................... 130

9.3

Durability of Cyanoacrylates ........................................................... 130 9.3.1

9.4

Cyanoacrylates for Medical Applications ............................ 132

Durability of UV-curing Adhesives................................................... 132 9.4.1

UV Adhesives for Medical Applications .............................. 133

9.5

Durability of Two-Part Acrylics ....................................................... 135

9.6

Durability of Epoxies ....................................................................... 135

9.7

Environmental Testing ..................................................................... 135

10. Troubleshooting ........................................................................................ 139 10.1 ‘No Glue’ – Inspecting for the Presence of Adhesive ........................ 139 10.1.1 ‘No Glue’ – Verifying the Adhesive Has Been Dispensed ..... 140 10.1.2 ‘No Glue’ – Air Bubbles and Voids...................................... 141 10.1.3 ‘No Glue’– Destructive and Non-destructive Methods ........ 142 10.1.4 ‘No Glue’ – Other Factors ................................................... 142 10.2 ‘No Cure’ ........................................................................................ 143 10.2.1 ‘No Cure’ – Odour .............................................................. 143 10.2.2 ‘No Cure’ – Factors Inhibiting Cure .................................... 143 10.2.3 ‘No Cure’ – Disturbing Partially Cured Adhesive ................ 144 10.2.4 ‘No Cure’ – Differential Scanning Calorimetry (DSC) ......... 144 10.2.5 ‘No Cure’ – Adhesive Curing Problems ............................... 145

v

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts 10.2.6 Blooming of Cyanoacrylates ............................................... 145 10.3 ‘No Stick’ ........................................................................................ 148 10.3.1 Theories of Adhesion .......................................................... 148 10.3.2 Cohesive ............................................................................. 149 10.3.3 Adhesive Failure .................................................................. 150 10.3.4 Substrate Failure ................................................................. 151 10.4 No Performance .............................................................................. 152 10.4.1 Surface Analysis .................................................................. 153 10.4.2 Defining the Failure Mode .................................................. 153 Abbreviations .................................................................................................... 155 Author Index..................................................................................................... 157 Subject Index..................................................................................................... 159

vi

P

reface

Plastics are an integral part of everyday life. There are huge numbers of manufacturing processes that use plastics either for the complete build or for sub-components within the assembly. Even components nominally composed entirely of metals may well include a polymeric material to coat, seal or adhesively bond the various components. There are countless different grades available and new trade names are introduced every year. Plastic manufacturers are able to formulate grades to meet the performance needs for almost any application. Not only is there a limitless number of plastics available but also there is a vast number of adhesives available. You only have to type the word ‘adhesives’ into an internet search engine and you will receive a cascade of data on adhesives of all types. This guide is written to help designers of plastic engineering components select an adhesive that may be suitable for their intended application. It is not intended for the packaging industry or for the bonding of composite materials and deliberately no mention is given to mechanical clips, ultrasonic welding or other plastic joining methods as these would be outside the scope of this guide. The adhesives discussed in this guide are aimed primarily at ‘small part’ bonding – parts typically (but not exclusively) smaller than an A4 pad. There are chapters on the bonding of ‘difficult’ plastics, joint design and dispensing systems but an adhesive cannot be selected for an application solely on the information given in this guide as it is not possible to provide environmental data for every combination of adhesive, plastic grade and joint configuration and so it is always recommended that adhesive manufacturers are consulted and trials are conducted. The guide discusses 30 of the most commonly used generic families of both thermoplastics and thermoset plastics and also includes a number of commonly used rubbers and elastomers. The final chapter discusses methods of troubleshooting possible reasons for an adhesive failure. Bob Goss

vii

1

Introduction to Adhesives

Choosing the best adhesive grade for a production application can be an exacting design task. This is especially true when joining dissimilar materials and when bonding certain engineering plastics. Adhesives can provide the optimum – indeed often the only – assembly method. However, it is all too often that the adhesive is not fully considered at the design stage. This can result in much time and trouble for engineers to get the prototypes into production. Four main types of adhesives are discussed in this book: •

Cyanoacrylates,

•

UV-curing acrylics,

•

Two-part acrylics, and

•

Epoxies.

There are of course many other types of adhesives suitable for bonding plastics and mention of some of these is given in Section 1.5.

1.1 Cyanoacrylates Cyanoacrylates are one of the most widely used adhesives for bonding small plastic parts. In liquid form cyanoacrylates or ‘super glues’ are stored in high-density polyethylene bottles to minimise the ingress of moisture into the liquid. These adhesives are generally available as colourless liquids and grades vary in viscosity range from 3 milliPascal seconds (mPa-s) (i.e., a thin liquid) through to a thixotropic gel for application to vertical surfaces or for highly porous materials. Cyanoacrylate technology has been available since the early 1960s and is extensively used by both industry and consumer alike. Considerable advancements in this technology have been made over the years and new grades and versions are not uncommon in the ‘New Product’ announcements in the Technical Press. 1

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Monomer

Acidic Stabiliser

Moisture

Figure 1.1 Curing of cyanoacrylates Cyanoacrylate adhesives cure readily on most surfaces where there are traces of moisture present. The moisture neutralises the stabiliser to initiate curing within a few seconds. In Figure 1.1 above, the large spheres represent the adhesive monomer and the smaller spheres represent the acidic stabiliser with the dark spheres representing the surface moisture. As the cyanoacrylate comes into contact with the surface moisture, the acidic stabiliser is neutralised and chains of adhesive molecules build up on the surfaces and inter-weave to bind the surfaces together and polymerise the adhesive. The cure speed of a cyanoacrylate, if left open on a surface (as on the left-hand side of Figure 1.2), will be relatively slow (several hours) because there is insufficient moisture (although the cyanoacrylate will cure at the surface interface). When the adhesive is between two close fitting surfaces (right-hand side of Figure 1.2), there is moisture on both surfaces and the cyanoacrylate will cure rapidly.

FAST CURE

SLOW CURE

Water Vapour

Monomer

Surface Moisture

Acidic Stabiliser

Monomer

Figure 1.2 Closing the joint The two major factors affecting cure speed are the percentage relative humidity and the gap. The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower relative humidities, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) results in a faster cure. High relative humidity can be detrimental as the cyanoacrylate sometimes cures so fast 2

Introduction to Adhesives that the adhesive polymerises before it has properly adhered to the surface and the resulting bond is poor. 100 90

% Strength

80 70 60 50 40 30 20 10

0

5 sec

30 sec

1 min 10 mins

1 hr

4 hrs 12–24 hrs

Figure 1.3 Cure speed of a cyanoacrylate The gap between parts should ideally be less than 0.1 mm and the thinner the gap the faster the cure; generally thin gaps will produce the strongest joints. Some grades of cyanoacrylate will fill gaps up to 0.5 mm and the ultraviolet (UV) curing grades are capable of curing through gaps up to 5 or 6 mm. Figure 1.3 above shows how the strength of a cyanoacrylate develops with time. Generally, cyanoacrylates will gain handling strength within the first minute or so but they continue to cure over the next 24 hours and can in some circumstances increase in strength two-fold in this time [1]. Data sheets for cyanoacrylates will quote ‘fixture speed’ or ‘handling strength’ and this will vary between different substrates for the same grade of cyanoacrylate adhesive. In a production situation the time to gain fixture strength is often a key factor in the cycle time and the ‘fixture strength’ will depend on the actual application but is generally regarded as the time when parts can be picked up gently without the adhesive joint failing. The ISO test for fixture strength is 0.1 N/mm2. The cyanoacrylate should not be disturbed during the critical time whilst it is polymerising, as the adhesive may never subsequently gain its full strength.

1.1.1 Types of Cyanoacrylate There are many different types of cyanoacrylate and so when designers are contemplating the use of an adhesive for specific project, they have to consider which type is most 3

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts appropriate – especially when it comes to production considerations and performance criteria. The following types of cyanoacrylate will be discussed in this section: •

Ethyl,

•

Methyl,

•

Alkoxy ethyl,

•

Surface insensitive,

•

Toughened,

•

Thermally resistant,

•

Flexible, and

•

UV-curing grades.

1.1.1.1 Ethyl Cyanoacrylates The ethyl cyanoacrylates are probably the most common of all the standard cyanoacrylates and the most widely used. The ethyl cyanoacrylates are best suited for bonding most plastics and elastomers to themselves and have excellent adhesion to polycarbonate, acrylonitrile butadiene styrene, poly(vinyl chloride) (PVC) and butyl rubber amongst many.

1.1.1.2 Methyl Cyanoacrylates With a smaller molecule size the methyl cyanoacrylates have better affinity to metals and can sometimes offer better resistance to solvents. The methyl grades of cyanoacrylate would often be specified for bonding plastic to metal or rubber to metal applications. The basic monomer is a very thin (low viscosity) liquid typically around 3–5 mPa-s. These methyl cyanoacrylate adhesives can be used to ‘wick-in’ to pre-assembled items such as for locking small adjustment screws or for increasing the strength of two press-fitted components.

1.1.1.3 Alkoxy Ethyl Cyanoacrylates Ethyl and methyl cyanoacrylates have a sharp odour and are eye and nasal irritants, especially in enclosed spaces or where large volumes of cyanoacrylate are present.

4

Introduction to Adhesives This high volatility of cyanoacrylates can lead to the formation of a white ‘bloom’ adjacent to the bond line (see Section 10.2.6). Alkoxy ethyl cyanoacrylates with a high molecular weight are available that virtually eliminate the odour and minimise the blooming. However, these high-molecular-weight products are slower curing and do not always offer the same adhesion performance as the standard ethyl grades. In some applications a slower curing product is useful as it can give the operator more time to align component parts during the assembly process.

1.1.1.4 Surface Insensitive Cyanoacrylates (For Acidic and Porous Substrates) Some substrates, for example paper, cork, cardboard, leather, dichromated metals and some fabrics, can be slightly acidic and this surface acidity will result in a very slow cure or in some circumstances inhibit curing completely. For these acidic substrates, a surface insensitive grade of cyanoacrylate must be used to ensure proper cure. The surface insensitive grades include special additives that can convert surface contamination into an activating species towards cyanoacrylates and then allow the adhesive to continue curing in the usual way. Such ‘surface insensitive’ cyanoacrylates are good general-purpose bonders – making them particularly suited to applications where the condition of the substrate is not well defined. This additive is now used in some consumer grades of ‘super glue’ because it reduces the dependence on surface moisture to cure and ultimately leads to good bond strength on many substrates.

1.1.1.5 Toughened Cyanoacrylates Cyanoacrylates can be toughened by the addition of rubber particles. The standard methyl or ethyl cyanoacrylates can have poor impact strength and low peel strength but modified ethyl cyanoacrylates were developed in the mid 1980s that show superior peel strength and often high humidity resistance. The principle of toughening the adhesive is that the rubber particles minimise the propagation of cracks as the adhesive is subjected to high peel or cleavage loads. Figure 1.4 shows a high peel load situation. The diagram on the left shows a crack propagating through the adhesive bond line on a standard ethyl cyanoacrylate. On the righthand side, the adhesive is toughened by the addition of rubber particles and so the crack only gets as far as the rubber particle and the stress is then dissipated and the crack has to reform only to meet another ‘crack arrester’. The resulting adhesive bond line is therefore considerably more resistant to peel and cleavage loads than

5

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts a conventional cyanoacrylate. However, the toughened cyanoacrylates are slower curing than standard grades.

Non Toughened

Toughened

Figure 1.4 Toughened cyanoacrylates offer better resistance to peel loads due to the presence of rubber particles in the adhesive matrix

Figure 1.5 Using a toughened cyanoacrylate to bond the rubber grip on to the handlebar of a trials motorcycle The rubber toughened cyanoacrylates are particularly well suited for rubber-to-metal bonding applications. In Figure 1.5 below, a toughened cyanoacrylate is used to bond the rubber grip onto the handlebars of a motorcycle. In this application, the benefit of using the rubber-toughened grade is that it is slower curing than a standard ethyl grade as, with a long engagement length and close fitting parts such as this, a standard cyanoacrylate would be too fast and it would not be possible to assemble the parts.

1.1.1.6 Thermally Resistant Cyanoacrylates The typical maximum operating temperature of a standard ethyl cyanoacrylate is 85 °C to 100 °C and the bond strength can fall rapidly after 100 °C (Figure 1.6). 6

Introduction to Adhesives

% RT Strength

100 75 50 25 0

0

150

50 100 Temperature, °C

Figure 1.6 Hot strength of a standard ethyl cyanoacrylate – measured on steel lap shears Allyl based cyanoacrylates were developed to improve the hot strength but these products required a secondary heat cure and parts to be clamped during the heat cycle to allow the allyl group to fully crosslink. Other cyanoacrylates with additives (such as phthalic anhydrides) have been formulated and these products do provide improved long-term strength at 120 °C.

% Initial Strength, measured at RT

For the cyanoacrylate with the high-temperature additives, the data are presented slightly differently as the adhesive gains hot strength as it is aged at temperature [1]. However, it can be seen (Figure 1.7) that there is still a 50% improvement in the performance. 30 °C

100

100 °C

76 121 °C

60 26

0

600

1000

1600

2000

Hours

Figure 1.7 Heat ageing strength curve for a cyanoacrylate with high-temperature additives

1.1.1.7 Flexible Cyanoacrylates Flexible cyanoacrylates were introduced in 2003 and are based on a modified ethyl monomer with considerably improved flexibility over the standard product. These 7

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts cyanoacrylates were developed for the loudspeaker industry (Figure 1.8), where a degree of flexibility in the coil to suspension bonded joint is desirable. The flexible grades of cyanoacrylate are also surface insensitive as they are used for the bonding of leather goods and fabrics and other similar applications where a flexible adhesive joint is required together with the ability to bond to these acidic substrates. Flexible cyanoacrylates have a slightly lower high-temperature resistance than standard ethyl grade products (maximum 75 °C).

Figure 1.8 A flexible adhesive is used to bond the suspension to the coil in a loudspeaker

1.1.1.8 UV-curing Cyanoacrylates One of the more recent introductions to cyanoacrylate technology are UV-curable grades. These are modified ethyl-based products with added photoinitiators absorbing UV light in the UVA region (around 365 nm). Light-curing cyanoacrylates cure almost immediately to a hard, tack-free finish in less than 3 seconds when exposed to highintensity UV light. Whereas most cyanoacrylates require a thin bond line (5000 mW/cm2) and, in general, the higher the intensity, the faster the cure. As a general rule, the amount of energy required to cure a UV acrylic adhesive is 1500 mJ/cm2. So if the intensity of the UV light (at the bond line) is 100 mW/cm2 then the cure time should be 15 seconds. 11

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 1.13 The intensity of UV light reduces as the distance and thickness of substrate increase Many of the original UV products (developed for glass bonding) cured at a wavelength of 365 nm (UVA) and this was ideal for glass bonding as the UVA wavelength is transmitted by most grades of glass. Many plastics, however (e.g., polycarbonate), can act as a UV filter (especially to UVA and to shorter wavelengths) and thus the UV adhesives with UVA photoinitiators would not cure under polycarbonate (see Figure 1.14). The wide use of polycarbonate and flexible PVC in the medical industry resulted in the development of a range of adhesives which were not only considerably more flexible but also cured at the visible end of the UV spectrum (UVV) 390 – 470 nm, thus overcoming the problem of absorbance at shorter wavelengths. Whilst these products are described as ‘visible light curing’ adhesives, it should be noted that an investment in UV equipment is still required for rapid curing.

420 (nm)

420 (nm)

420 (nm)

420 (nm)

365 (nm)

365 (nm)

365 (nm)

365 (nm)

220 (nm)

220 (nm)

Glass

Clear Plastic

Figure 1.14 Glass will filter out all wavelengths 120 °C for around 15 minutes.

1.2.6 Benefits of UV Adhesives UV adhesives offer the following advantages to the user: •

Speed of cure – usually less than 30 seconds thus allowing increased productivity,

•

Cure on demand – the adhesive only cures when exposed to light thus allowing time for the alignment of component parts,

•

Optical clarity – most of the UV adhesives are clear and this is ideal for aesthetic or optical applications,

•

Single part – there is no need to mix adhesive or apply primer, and

•

Solvent free – UV adhesives are 100% solids and contain no hazardous solvents.

1.3 Two-part Acrylics Two-part acrylics are available either as two-part resins or as a resin for Part A with Part B as a liquid activator. The two-part resin products are formulated with Part A and Part B of similar viscosities and designed to be dispensed as bead-on-bead or mix-in-the-nozzle. Two-part acrylics (also referred to as methyl methacrylates or methylmethacrylates (MMA)) can offer benefits over cyanoacrylates with improved gap fill and typically 17

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts improved durability on metals. Many different grades are available and some are more suited for metals than plastics although they do not give good performance on elastomers, fluoropolymers or polyolefins. There is also a risk that they may stress crack certain thermoplastics (especially the liquid activators) and care is required to ensure that the solvent-based liquid activator is applied to the least susceptible surface. The MMA products are differentiated from other two-part adhesive systems like epoxies or polyurethanes by the fact that the cure is relatively insensitive to the mix ratio. They are comparatively inexpensive and although earlier versions were brittle when cured, the more recent developments are significantly tougher and more flexible generally give improved peel strength over cyanoacrylates. The mix-in-the-nozzle grades can cure in less than 60 seconds although more typically the cure time is 5–15 minutes to handling strength and 12–24 hours to full strength. Some grades of MMA give outstanding performance on unprepared or oily steel with fixture times of about 10 minutes. They will also withstand a paint bake cycle (and benefit from the heat cure) and so are used in some sheet metal industries to replace rivets. Some toughened acrylic grades can be difficult to dispense due to the ‘stringiness’ of the additives in the adhesive matrix and so difficult to manage on small component parts. They often have a high odour (although low-odour versions are available) and some acrylics are flammable. The MMA are usually toughened with rubber dissolved in the monomer (Figure 1.22) and it is these rubber particles that provide the high peel strength and toughness. MMA have proved to be extremely useful in the bonding of larger plastic components such as bonding automotive bumpers, fibreglass components and assembling boats. A more recent development in MMA technology are products that will bond polyolefin plastics successfully (see Section 6.4).

Non Toughened

Toughened

Figure 1.22 MMA are toughened with rubber particles 18

Introduction to Adhesives

1.4 Epoxies Epoxies are thermoset products normally prepared by mixing a resin and a hardener or curing agent. The mixing can be achieved by mixing the correct quantities by hand on a small pallet (Figure 1.23) or (more usually for production applications) by dispensing directly from the original containers via a helix nozzle (Figure 1.24). Precise measuring of the resin and hardener is essential to ensure the epoxy reaction is completed and thus the correct properties of the final adhesive are achieved. This critical mix ratio is perhaps a disadvantage of epoxies over other two-part adhesive systems. The helix nozzle ensures good mixing, provided there are sufficient mixing segments in the nozzle (Figure 1.25).

Figure 1.23 Mixing epoxy on a pallet

Figure 1.24 Mixing epoxy in a helix nozzle 19

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 1.25 An epoxy nozzle

Epoxies exotherm during the cure process and so when a large amount of mixed epoxy is dispensed onto a component part, the temperature will increase thus accelerating the cure time. Correspondingly if only small quantities of epoxy are dispensed, the cure speed may well be longer than quoted on the data sheet as there is insufficient mass of adhesive present to help drive the cure along. Epoxies can be heat cured and as a good rule of thumb the cure time will be halved for every 10 °C rise in temperature. Single-part epoxies are available (where the resin and hardener are pre-mixed) but these usually require refrigerated storage and a heat cure. Epoxies are extremely versatile adhesives due to the large number of combinations of epoxy resins and hardeners that are available, each of which can give a different molecular structure in the resulting cured polymer [3]. Additives include fillers, rubber tougheners, plasticisers and other modifiers to customise adhesives for specific applications. A range of electrically conductive (silver-filled) epoxies are available and these are becoming popular in electronics bonding and sealing applications. An electrically conductive adhesive will never have the same performance in terms 20

Introduction to Adhesives of conductivity or durability as a soldered joint (where a true inter-metallic layer is achieved) but they can offer high resistance to humidity and continuous service temperatures up to 140 °C [4]. Epoxies generally have excellent adhesion to metals, ceramics and glass, although on most amorphous thermoplastics epoxies will usually be outperformed by MMA, UV adhesives or cyanoacrylates. Epoxies will bond well to thermoset plastics and are widely used for bonding sheet moulding compound door and body panels in the transportation industries. Epoxies do not adhere well to elastomers, fluoropolymers or polyolefin plastics. They will fill very large gaps and have excellent thermal and environmental resistance and so are widely used for the potting and encapsulation of electronic components and printed circuit boards. Epoxies can be modified with various additives, fillers, plasticisers, rubber tougheners and other polymers and so there are a huge number of epoxy adhesives available and numerous specialist grades.

1.4.1 Advantages and Disadvantages of Epoxies Advantages include the high bond strength, adhesion to a wide range of materials and excellent solvent and water resistance. Epoxies will fill large gaps, which is a distinct advantage over cyanoacrylates and reactive acrylics. Epoxies are widely used and generally lower cost than UV or cyanoacrylates. They can often be the most durable and toughest of the adhesive technologies. Epoxies, however, are not always the best adhesive for bonding small engineering plastic components as the dispensing of the two-part versions can be problematic (see Section 8.1) and they are relatively slow to cure compared to most adhesive technologies (although the cure speed can be accelerated by heat). Some hardeners can be toxic and have a high odour and so this can cause health and safety issues in the workplace or at the dispensing station. Very-low-viscosity versions are generally not possible because the base resins often have a high molecular weight.

1.5 Flexible Adhesive Sealants Somewhere in the whole catalogue of adhesive families, the product is classified as a ‘sealant’ and not necessarily as an ‘adhesive’. The application itself will tend to define the difference as much as the generic adhesive type. Very often, in order for a sealant 21

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts to successfully fulfil its function, it must offer a good degree of adhesion against the surface it is sealing otherwise the media will permeate along the joint line between the sealant and the substrate (Figure 1.26). One possible method of distinguishing sealants and adhesives is to compare the typical cohesive strength of the products with flexibility (% extension). Figure 1.27 shows how some of the most common adhesives and sealants for the bonding of plastic materials might be classified. This graph is not entirely correct as there will always be exceptions to the categories but it does show that there is no distinct definition between an adhesive and a sealant.

A Figure 1.26 If the sealant does not adhere, a leak path is possible (arrow A)

Epoxies

Bonding UV Acrylics

CA

Elastic Bonding MMA

Elastic Sealing

PU Adhesives

elt

tm Ho

Cohesive Strength [N/mm2]

High performance bonding

PU, MS Elastic Bonding Silicones PU, MS Elastic Sealing

So lv Ba ent sed

Butyl tapes

Elongation [%]

Figure 1.27 Adhesives versus sealants CA = Cyanoacrylate adhesives, MS = modified silane adhesives, PU = polyurethane adhesives, MMA = methylmethacrylate adhesives, UV = ultraviolet cure adhesives

1.5.1 Silicone Adhesive Sealants Single-component silicone adhesives are usually RTV based and so cure slowly in the presence of atmospheric moisture. The RTV will release a by-product during the cure 22

Introduction to Adhesives cycle and the most common of these is acetic acid. These ‘acetoxy’ curing silicones will skin over in about 15–20 minutes and achieve a full depth of cure of 4–6 mm over 24–48 hours depending on the % RH at the time of cure (Figure 1.28).

Depth of Cure, mm

5 4 3 2 1 0 0

6

12

18

24

Cure Time, hours

Figure 1.28 Cure speed curve for an RTV acetoxy silicone (50% RH, 23 °C)

The acetic acid by-product can be corrosive, particularly to copper terminals on printed circuit boards and other electrical items. For this reason other RTV silicone sealants have been developed with oxime- or alkoxy-based by-products. These can be slightly slower to cure than the acetoxy-based products but often the adhesion to plastics is considerably improved. Two-component systems can also be formulated with no by-products and which cure very quickly. One of the main benefits of silicone technology is its outstanding temperature resistance (typically >250 °C) but a disadvantage is that the products cannot be over-painted after cure. In many industrial premises where there is a paint shop, RTV silicones are prohibited due to the contamination from the by-products.

1.5.2 Polyurethane Adhesive Sealants Polyurethane adhesives for plastics are available primarily in both single-component and two-component forms. They are very tough, flexible products with excellent peel strength and are also extremely versatile for the bonding of larger plastic components. Many grades of polyurethanes are certainly adhesives and not just sealants and most automotive windscreens are now bonded in with a polyurethane-based adhesive. The excellent adhesion, durability and good flexibility are key properties. Note that most 23

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts automotive manufacturers will include a black strip at the periphery of the windshield and this strip is not only to improve the aesthetics of the bond where the black polyurethane might have extruded outside the joint but it also serves as a UV protection to the adhesive as polyurethanes generally have poor resistance to ultraviolet light. Single-part polyurethanes rely on an atmospheric moisture cure and will release isocyanates during the cure cycle. Cure times are relatively slow with a skin-over time of about 20–30 minutes and complete cure in 3–7 days depending on the grade. The isocyanates can lead to health and safety concerns in the workplace as they would be classified as irritants and in some cases toxic. Two-part polyurethanes still release isocyanates but the cure time is considerably improved. Two-component polyurethanes are the most common type used for plastics bonding, although primers are sometimes required to improve adhesion.

1.5.3 Modified Silane Adhesive Sealants Modified silanes (MS) are odourless, non-corrosive and do not contain isocyanates, solvents, silicones or PVC. They are normally available as single-component adhesive sealants (although two-part systems are also available) and require atmospheric moisture during the cure cycle. When fully cured they are a flexible elastomeric product with excellent adhesion properties to a wide range of materials and are compatible with paint systems. MS products demonstrate good UV resistance and can therefore be used for interior and exterior applications. They do not usually have the cohesive strength of a polyurethane or the temperature resistance of a silicone but high- and low-modulus versions are available. Modified silane products bond well to most metals, glass and ceramics and depending on the grade will bond to PVC, polyamide and most polyesters [5]. Trials are recommended and roughening the surface will generally result in an increase in adhesion. Modified silane products are used for seam sealing in a wide variety of industries including the elastic bonding of metals and plastics used in side panelling and roof-skin bonding in vehicle and caravan manufacture; elastic interior and/or exterior seam and joint sealing in vehicle body construction, railway carriage construction, the air conditioning and ventilation industry, the electrical industry and for shower doors. The slow cure cycle of MS adhesive sealants has been addressed by some manufacturers and two-part (mix-in-the-nozzle) versions are increasingly being made available. The cure time on these products is typically less than 15 minutes.

24

Introduction to Adhesives

1.6 Hot Melt Adhesives Hot melts were introduced into the market many years ago and are widely used in the packaging, labelling, woodworking and bookbinding industries but are also used for bonding plastics. Hot melts are 100% solid single-component adhesives that are applied in a molten state and solidify by cooling. They do not contain any carrier material (e.g., water or solvent) and so can offer extremely fast setting times and excellent gap-filling capabilities [6]. Hot melts are available in a variety of forms including tapes, films, pellets, cylinders, cubes and blocks. For small part bonding, they are usually dispensed in ‘stick’ form via a heated hand-gun or in bulk from semiautomatic or automatic application systems. The two most important characteristics of hot melts are their open time and green strength. The open time is the time that the adhesive remains fluid enough after dispensing the adhesive before it will no longer wet the mating surface and thus create an effective bond. The green strength is a measure of the rate of cure and is often also referred to as the handling strength. These two factors are inexorably related as the longer the adhesive is left ‘open’ on a surface, the thicker it gets and so its ability to wet the surface reduces, thus leading to a drop in bond strength. Hot melts are easy-to-use, medium-strength adhesives and can be made rigid or flexible. They are thermoplastic materials and so typically have a maximum operating temperature of 90 °C but higher temperature grades are available (see Section 1.6.1). Hot melts are often used where the bond area is large as they can be difficult to dispense onto small components where the width of the bond is less than 5 mm, as the rheology of these products is such that they have a tendency to ‘string’ during the dispensing cycle. On larger parts, however, where there is a bigger bonded area and more scope for accommodating less precise dispensing criteria, they invariably show faster curing times than cyanoacrylates and certainly considerably improved gap-filling capabilities. One concern for hot melts is the relatively high application temperature (160–180 °C) as this can damage heat sensitive components and incur health and safety issues.

1.6.1 Reactive Hot Melts Reactive hot melts are single-component adhesives that are applied in the molten form (like conventional hot-melt adhesives) but have an additional secondary cure with atmospheric moisture to give a crosslinked polymer. Reactive hot melts will give 25

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts almost instant fixture strength as the hot melt solidifies and over the next few days will increase in strength as the secondary cure urethane element is completed. The reactive hot melts bond well to many substrates, and have good high-temperature performance and considerably improved cold flexibility over conventional hot melts. They also have excellent water and vapour resistance and improved solvent resistance and are used for major structural applications such as the assembly of large panels in insulated garage doors and for the lamination of PVC sections in window construction.

References 1.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2000, p.264 and p.273.

2.

D.E. Packham, Handbook of Adhesion, Longman Scientific & Technical, Harlow, UK, 1992, 369.

3.

D.J. Dunn, Engineering and Structural Adhesives, Rapra Review Report No.169, Rapra Technology Ltd, Shrewsbury, UK, 2004, p.6.

4.

Henkel Technical Data Sheet Loctite 3880, 2003.

5.

Henkel Technical Information Sheet Terostat MS 939, 2006, March.

6.

D.J. Dunn, Adhesives and Sealants – Technology, Applications and Markets, Rapra Market Report, Rapra Technology Ltd, Shrewsbury UK, 2003, p.34.

26

2

Engineering Thermoplastics

2.1 Introduction The annual consumption of engineering plastics continues to grow [1] and the commodity plastics (polyethylene, polypropylene, poly(vinyl chloride) (PVC) and polystyrene) make up approximately 75% of the total consumption. These commodity plastics are widely used in the packaging industry and in the building/ construction industries (water pipes, vessels, guttering, road cones). Engineering plastics is a term loosely used to describe thermoplastics and thermoset plastics which offer a range of engineering properties (stiffness, strength, toughness and heat resistance) and are used in sectors such as automotive, household appliances and medical devices, etc., and this adhesive guide focuses on these engineering materials as this is where most detailed design effort for the manufacture of small parts is concentrated. The automotive sector is perhaps the largest consumer of engineering and high performance plastics but other sectors (medical device, electrical and electronics, etc.,) are also considerable users. In the automotive sector, engineering polymers such as polybutylene terephthalate (PBT) are replacing polyamide in some car electrical components because of their improved dimensional stability. Also, with the ever increasing temperature requirements in under-bonnet applications, liquid crystal polymer (LCP) and polyphenylene sulfide (PPS) have also been replacing polyamide [1]. There are four main polymer classes (thermoplastics, thermosets, elastomers and thermoplastic elastomers) but thermoplastics fall into two distinct classes as amorphous thermoplastics and semi-crystalline thermoplastics (Figure 2.1).

2.2 Amorphous Thermoplastics Plastics that can be repeatedly solidified from the melt state without significant crystallisation are described as amorphous thermoplastics. Amorphous thermoplastics 27

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts have long macromolecule chains which are highly entangled with large side chains (Figure 2.2).

Polymer Classes

Thermoplastics

Amorphous

Thermoplastic Elastomers

Thermosets

SemiCrystalline

Elastomers

Figure 2.1 Polymer classes Amorphous thermoplastics are frequently used in applications where clarity is important and in applications where they may be thermo-processed (e.g., ultrasonic welding). They can be moulded to a high-dimensional accuracy and stability, and have a good appearance and mechanical properties (depending on the grade). At elevated temperatures or at high stress levels, the polymer chains can uncoil and thus alter the properties of the polymer. Generally, amorphous thermoplastics are less chemically resistant than semi-crystalline thermoplastics and can be subject to stress cracking and can swell or dissolve in some solvents.

Figure 2.2 Amorphous thermoplastic

28

Engineering Thermoplastics Some typical examples of amorphous thermoplastics are acrylonitrile-butadiene-styrene (ABS), polymethylmethacrylate (PMMA), polystyrene (PS) and polycarbonate (PC). Applications for amorphous thermoplastics vary from car bumpers to Lego® bricks, and from motorcycle helmets to ski boots.

2.3 Semi-crystalline Polymers Semi-crystalline polymers are plastics that contain areas of both crystalline molecular structure and amorphous regions as well (Figure 2.3). The degree of crystallisation depends on the chemical structure (generic type), the rate of cooling and the thermal history in the post-moulded state. Very rapid cooling in thin sections can result in insignificant levels of crystallisation and this is used to great effect in the production of transparent film (e.g., polyethylene (PE), polyethylene terephthalate and polypropylene (PP)). Increased crystallisation increases and improves hardness, modulus, strength, abrasion resistance, shrinkage, density and opacity. Semi-crystalline thermoplastics are generally used in applications where high fatigue resistance or repeated cyclic loading is involved (bearings, linkages, etc.). They are often reinforced with glass fibres and this considerably improves the high-temperature stiffness and the strength. However, the shrinkage can be variable and difficult to predict. Semi-crystalline thermoplastics are noted for very good electrical properties as well as the ability to withstand both high heat and severe chemical environments. Examples of semi-crystalline thermoplastics include polyamide, polyarylamide, LCP and PBT [1].

Figure 2.3 Semi-crystalline plastic 29

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts The amorphous thermoplastics are generally easier to adhesively bond than the semicrystalline thermoplastics, although some nylons will bond quite well (Table 2.1).

Table 2.1 Some examples of amorphous and semi-crystalline thermoplastics Amorphous Semi-crystalline Polyphenylene oxide (PPO) (Noryl) Polyetheretherketone (PEEK) PC Polytetrafluoroethylene (PTFE) Polyethersulfone (PES) PBT PS Polyamide (PA) Polysulfone (PUS) PPS Polyamide (amorphous) (Nylon) Polyethylene terephthalate (PET) PMMA, Acrylic Polyoxymethylene (POM) (Acetal) PVC PP ABS PE Ionomers LCP The glass transition temperature (Tg) is an important property of thermoplastics and is the temperature at which the plastic will change in its mechanical properties (depending on the degree of crystallinity) due to the onset of the breakdown of the physical bonds within its structure (see Figure 2.4). The glass transition

10 102 103 104

B

10−2

10−1

1

A

10−3

Relaxation Modulus (MPa)

Glass transition region

50

100 Tg

150

200 Tm

Temperature °C

Figure 2.4 Influence of temperature on relaxation modulus for thermoplastics ((A) amorphous, and (B) semi-crystalline) 30

Engineering Thermoplastics temperature is not a sharp temperature point but the average value of the transition range. Thermoplastics are elastic and flexible above a glass transition temperature Tg, specific for each one, and in some cases a low Tg is useful and in other applications a high Tg is required. A polyethylene ice box container has a very low Tg as it is essential that it remains flexible at temperatures below −20 °C. A blood filter must retain all of its structural integrity at temperatures >37 °C and therefore polycarbonate is often the selected plastic as it has a high Tg (amongst other properties).

2.4 Adhesive Performance on Thermoplastics In this section, a guide is given to indicate the performance of several different adhesives for a selection of thermoplastics. The bulk of this information was taken from ‘The Loctite Design Guide for Bonding Plastics’ [2] issued by Henkel Ltd and for more detailed information the reader should refer to this guide. An adhesive cannot be selected solely on the basis of bond strength information as other factors such as cure speed, environmental resistance and dispensing method will all influence the final choice. The values given for the shear strength are given to provide a general idea of how each of the nine adhesives performed on the particular material. Different performances will be achieved depending on the fillers, lubricants, colourants and anti-static additives. The nine different adhesives selected for these charts were as follows: •

Three cyanoacrylates (standard ethyl, toughened grade and ethyl + primer),

•

Two acrylics (a standard methylmethacrylate (MMA) and a polyolefin bonder),

•

Two epoxies (a 5-minute epoxy and a longer cure grade),

•

One room-temperature-vulcanising silicone (alkoxy cure), and

•

One ultraviolet (UV) acrylic (cure at 420 nm).

A brief description of each adhesive is given, together with a few general comments on the overall performance of the adhesives. The alkoxy silicone often showed a strength of 1.4 N/mm2 for many of the plastics tested but in this case the adhesive has failed cohesively. A silicone adhesive is often used where sealing might be the main priority of the adhesive rather than structural strength.

31

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

2.4.1 ABS ABS is a generic name for a family of amorphous thermoplastics produced by combining three monomers: acrylonitrile, butadiene and styrene. Different ratios of these monomers provide variations in strength, stiffness, impact resistance and surface appearance and so there are many different versions of ABS, each with their own particular properties. Many adhesives are stronger than the substrate itself when tested in tensile shear (Table 2.2) and ABS would typically be regarded as a relatively easy-to-bond material. However, ABS can sometimes be stress cracked by some adhesives.

2.4.2 LCP Liquid crystal polymers (also called aromatic copolyesters) have outstanding mechanical properties at both ambient and extreme temperatures. They are also highly resistant to many chemicals and so are used for many automotive under-bonnet and aerospace applications. A number of filled and reinforced grades are available and this material can be processed with very short cycle times.

Table 2.2 Adhesive shear strengths (ABS) Adhesive type Description Shear strength (N/mm2) Standard ethyl >24.1* Toughened >16.6* Cyanoacrylate Ethyl + primer >23.1* MMA 11.7 Two-part acrylic Polyolefin bonder 13.8 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 12.4 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >24.1* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

32

Engineering Thermoplastics

Table 2.3 Adhesive shear strengths (LCP) Adhesive type Description Shear strength (N/mm2) Standard ethyl 5.2 Cyanoacrylate Toughened 3.5 Ethyl + primer 2.8 MMA 3.8 Two-part acrylic Polyolefin bonder 3.1 ‘5-minute’ epoxy 4.1 Epoxy Standard epoxy 6.9 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure 4.5 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

LCP is flame resistant and so is often used in the electrical industry for stator insulation and connectors, etc. Surface roughening of LCP typically improves adhesion and this plastic can be bonded with most adhesives with epoxies generally giving the highest tensile shear strengths (Table 2.3).

2.4.3 Polyamide Polyamide Nylon was one of the earliest plastics and has been commercially available since the late 1930s. The standard grades are PA6, PA66 and PA12 but there area also many speciality grades available including lubricated, plasticised, flame retardant and, perhaps most common of all, glass filled. Polyamide is tough, inexpensive and has good resistance to heat and chemicals but polyamides can sometimes have poor dimensional stability due to water absorption. The main advantage of PA6 over PA66 is that it is easier to process and produces lower mould shrinkage but PA66 has better low-temperature toughness and good fatigue resistance.

33

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts PA12 has lower water absorption and improved UV resistance but has a lower temperature resistance than the other polyamide grades. Polyamides are used in a wide range of applications including under-bonnet applications such as the air intake manifold, and the air and cooling system peripherals. It is also used to manufacture electrical components such as switches, connectors and contactors. Applications are as diverse as castor wheels for furniture to aerosol valves. Polyamides can usually be bonded quite easily (Table 2.4) but long-term durability can be an issue in some applications due to water absorption. The alkoxy silicone product shows good strength and durability on polyamide and the result of 1.7 N/mm2 is a measure of the cohesive strength of the silicone rather than the adhesive strength to the base material.

2.4.4 PBT The most notable properties of PBT are its chemical resistance and mechanical properties. It is also noted for its high stiffness and strength, low water absorption and high dimensional stability.

Table 2.4 Adhesive shear strengths (polyamide) Adhesive type Description Shear strength (N/mm2) Standard ethyl >25.0* Toughened 16.9 Cyanoacrylate Ethyl + primer 11.0 MMA 6.6 Two-part acrylic Polyolefin bonder 3.8 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 5.5 Silicone Alkoxy silicone 1.7 UV acrylic Visible light cure 1.4 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

34

Engineering Thermoplastics There are a number of different grades and types of PBT, including glass reinforced grades. It can be used for single-use medical devices but not for devices that might be subjected to repeated autoclaving as it does not have sufficient temperature resistance. PBT polymer blends combine the properties of partially crystalline PBT with those of amorphous thermoplastics such as polycarbonate or ABS. The amorphous partner improves the impact strength and warpage behaviour, while the PBT ensures the chemical resistance of the blend. PBT is used in the electrical lighting industry for its electrical insulation properties, dimensional stability and colour stability. It is also used in this industry for printedcircuit-board connectors and sockets, junction boxes and housings for switches and capacitors. PBT would typically be used for car interior or exterior components rather than underbonnet applications, including ash trays, windscreen wiper arms and door handles, or where a high-quality, weather-resistant surface is a key requirement. In these trials [2], (Table 2.5) the cyanoacrylate with a primer showed excellent results but bonding PBT with engineering adhesives is often variable as it can depend on the

Adhesive type

Table 2.5 Adhesive shear strengths (PBT) Description Shear strength (N/mm2) Standard ethyl 1.7

Cyanoacrylate

Toughened Ethyl + primer

0.7 >21.9*

MMA

2.4

Polyolefin bonder

7.6

‘5-minute’ epoxy

3.8

Standard epoxy

4.8

Silicone

Alkoxy silicone

1.4

UV acrylic

Visible light cure

1.4

Two-part acrylic Epoxy

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

35

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts particular grade and in some applications a surface pre-treatment may be necessary. Roughening of the surface generally shows an improvement in bond strength.

2.4.5 PC There is a wide range of polycarbonate resins and compounds available and they are often blended with ABS for greater versatility. Polycarbonate is often selected for its strength and clarity and its ability to be moulded into almost any shape and so is used as a replacement for glass for safety goggles, lenses, electrical switch panels and business machine housings. PC/ABS is used in the automotive sector for instrument panels and loudspeaker chassis and grilles. Polycarbonate is also widely used in the medical industry for items such as blood centrifuge bowls, safety syringes and intravenous connectors as it is generally biocompatible. Polycarbonate is usually easy to bond with many adhesives giving higher strengths than the substrate material. However, some adhesives will stress crack polycarbonate. The use of cyanoacrylate primer showed a statistical decrease in strength in these trials [2], but substrate failure was achieved with both a standard cyanoacrylate and a UV acrylic (Table 2.6). The epoxy also showed excellent strengths on this material.

Table 2.6 Adhesive shear strengths (polycarbonate) Adhesive type Description Shear strength (N/mm2) Standard ethyl >26.6* Toughened 5.2 Cyanoacrylate Ethyl + primer 13.8 MMA 7.8 Two-part acrylic Polyolefin bonder 5.9 ‘5-minute’ epoxy 6.2 Epoxy Standard epoxy 18.3 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >25.5* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

36

Engineering Thermoplastics

2.4.6 PEEK PEEK is a high-performance thermoplastic which is well suited for high-temperature environments. It is often one of the highest-priced engineering plastics but does offer a unique combination of properties. The high-temperature performance (continuous service temperature up to 200 °C) and excellent mechanical and electrical properties make it one of the leading contenders as a replacement for metal parts. It also has outstanding radiation resistance (for medical applications) and very good sunlight and weathering resistance. PEEK can be processed on all conventional processing technologies and so this material is often used in demanding and niche high-temperature engineering plastic applications. It has been used in a wide variety of applications including chemical pumps, coffee machines and microchips. PEEK is also easy to machine and so is often used in industry for the production of high-precision plastic components in small batches. PEEK is difficult to bond with engineering adhesives due to its low surface energy and usually requires some form of surface treatment (e.g., plasma) to give a good bond (Table 2.7).

Table 2.7 Adhesive shear strengths (PEEK) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.7 Cyanoacrylate Toughened 1.0 Ethyl + primer 1.7 MMA 2.1 Two-part acrylic Polyolefin bonder 2.1 ‘5-minute’ epoxy 1.7 Epoxy Standard epoxy 3.5 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 3.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

37

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

2.4.7 PES and PUS PES is a high-temperature amorphous thermoplastic and is used in applications where high stiffness and high continuous operating temperatures are a pre-requisite. PES can, however, be degraded by UV light and so has a low resistance to weathering. PES can be sterilised repeatedly (autoclaved) and so is used for some medical devices including instrument trays and infusion equipment. The major disadvantage to adhesively joining PES (and polysulfone) is that they are extremely prone to stress cracking by uncured adhesives and so any excess uncured adhesive should be either cured or removed from the surface immediately. The UV acrylic gave excellent results on the grade of polyethersulfone tested [2] (Table 2.8) but good results were also achieved with the specialist ‘polyolefin bonder’, a 10:1 two-part acrylic.

Table 2.8 Adhesive shear strengths (PES) Adhesive type Description Shear strength (N/mm2) (PES) Standard ethyl 11.0 Cyanoacrylate Toughened 4.5 Ethyl + primer 1.0 MMA 6.9 Two-part acrylic Polyolefin bonder 13.8 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 4.5 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure 21.0 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

38

Engineering Thermoplastics

2.4.8 PE Polyethylene is a lightweight, semi-crystalline thermoplastic and is available in the following major physical grades: •

Low-density polyethylene,

•

Medium-density polyethylene,

•

High-density polyethylene, and

•

Ultra-high-molecular-weight polyethylene.

Polyethylene is not generally regarded as a high-performance engineering plastic but it is extremely versatile and inexpensive and so is one of the most popular of all plastics. It offers good or excellent resistance to many chemicals and can be processed by all conventional methods including rotational moulding. Polyethylene has a low surface energy (31 mN/m) and so will generally require surface preparation prior to bonding. The two-part acrylic ‘polyolefin bonder’ showed good strengths in these trials [2] (Table 2.9).

Table 2.9 Adhesive shear strengths (polyethylene) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.0 Cyanoacrylate Toughened 9.7* ‘5-minute’ epoxy 1.4 Epoxy Standard epoxy 1.0 Silicone Alkoxy silicone 22.1* Toughened 3.8 Cyanoacrylate Ethyl + primer >12.1* MMA 2.4 Two-part acrylic Polyolefin bonder 3.1 ‘5-minute’ epoxy 2.1 Epoxy Standard epoxy 3.1 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 7.9 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

2.4.10 PMMA PMMA is the most common member of the acrylic family and widely known as the ICI trade name of ‘Perspex’. Acrylic has excellent transparency and no appreciable yellowing under sunlight. They are therefore used in the construction industry and in the electrical industry for lighting systems and lenses.

40

Engineering Thermoplastics

Table 2.11 Adhesive shear strengths (PMMA) Adhesive type Description Shear strength (N/mm2) Standard ethyl >27.2* Toughened 4.1 Cyanoacrylate Ethyl + primer 1.7 MMA 6.6 Two-part acrylic Polyolefin bonder 12.1 ‘5-minute’ epoxy 2.1 Epoxy Standard epoxy 6.9 Silicone Alkoxy silicone 0.1 UV acrylic Visible light cure 12.1 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

Acrylics are not usually recommended for high-temperature applications but they do offer good chemical resistance. Acrylics can be bonded with most adhesives and in these trials [2] cyanoacrylates (without primer) and UV-curing acrylics gave best results (Table 2.11). The silicone adhesive did not show good strengths.

2.4.11 POM POM is widely known as ‘acetal’ and a well-known trade name is ‘Delrin’, manufactured by DuPont. This is a popular engineering plastic to replace parts that might otherwise be manufactured from metals. It is generally easy to machine and therefore makes for a good engineering plastic for small batch production or prototype applications. Acetal is a highly crystalline polymer with good abrasion resistance, low water absorption and favourable frictional and wear properties. It exhibits high physical strength as well as excellent creep and impact resistance. Applications include bearings, gears and conveyor belt links but also chemical pumps and, due to its resilience and recovery, it is also used for clips and snap-fit connections. 41

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.12 Adhesive shear strengths (acetal) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.4 Cyanoacrylate Toughened 0.7 Ethyl + primer 11.7 MMA 1.4 Two-part acrylic Polyolefin bonder 2.4 ‘5-minute’ epoxy 1.7 Epoxy Standard epoxy 2.1 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 1.7 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

Like PEEK, this material is also difficult to bond (Table 2.12) with engineering adhesives and usually requires some form of surface treatment to give a good bond.

2.4.12 PPO PPO is an engineering thermoplastic known for its excellent radiation resistance, oxidation resistance, thermal stability and electrical properties. Typical applications include television cabinets, car spoilers and laptop computer outer shells. PPO has one of the lowest water absorption rates of any of the engineering thermoplastics and has excellent flame retardance, electrical properties and impact strength. One of the best-known trade names for this material is Noryl (GE Plastics) and this is not only widely used in the automotive industry, but also in the building and construction sector and for switchgear, etc. PPO is relatively easy to bond but can be stress cracked by uncured cyanoacrylates, UV acrylics and solvent-based activators and primers. Best results were achieved in these trials [2] with a standard cyanoacrylate and the use of primer only produced a marginal increase in strength (Table 2.13). All the other adhesives tested gave medium to lower strengths (including the toughened cyanoacrylate).

42

Engineering Thermoplastics

Table 2.13 Adhesive shear strengths (PPO) Adhesive type Description Shear strength (N/mm2) Standard ethyl 11.0 Cyanoacrylate Toughened 3.5 Ethyl + primer 12.1 MMA 2.1 Two-part acrylic Polyolefin bonder 9.3 ‘5-minute’ epoxy 1.4 Epoxy Standard epoxy 5.9 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 6.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

2.4.13 PPS PPS is a crystalline polymer with a relatively high melt temperature (285 °C). It can sometimes be difficult to process but it does offer an excellent combination of longterm thermal stability and superior chemical resistance below 200 °C. Its inherent flame resistance and good mechanical properties make this material a popular choice for high-quality moulded parts that are uniform and reproducible. PPS is widely used in the automotive industry in fuel systems, coolant systems, brake systems and many other under-bonnet applications. Outside of the automotive industry it is used for various components including pumps, impellors and valves, motor relays and some thermostat components. Although shear strengths achieved in these trials [2] were relatively low, roughening the PPS generally gives a significant increase in bond strengths (Table 2.14).

2.4.14 PP Polypropylene is one of the lightest engineering thermoplastics (SG = 0.90) and it has excellent moisture resistance. One of the major disadvantages of polypropylene is its poor impact strength at low temperatures. It does, however, offer excellent fatigue resistance and it is widely used for luggage, packaging, toys and storage battery cases.

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 2.14 Adhesive shear strengths (PPS) Adhesive type Description Shear strength (N/mm2) Standard ethyl 1.0 Cyanoacrylate Toughened 0.7 Ethyl + primer 2.8 MMA 2.1 Two-part acrylic Polyolefin bonder 4.1 ‘5-minute’ epoxy 1.0 Epoxy Standard epoxy 5.5 Silicone Alkoxy silicone 0.7 UV acrylic Visible light cure 3.8 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

Table 2.15 Adhesive shear strengths (polypropylene) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.3 Cyanoacrylate Toughened 0.3 Ethyl + primer >13.5* MMA 19.7* MMA 16.2 Two-part acrylic Polyolefin bonder 10.0 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 9.3 Silicone Alkoxy silicone 1.0 UV acrylic Visible light cure >17.6* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information

thermoplastic applications, especially if the gaps are relatively large or if the adhesive is to be used as a potting or encapsulation material as well as an adhesive. Epoxies will not bond to the non-polar plastics (polyethylene, polypropylene or the fluoropolymers but they do show good adhesion to ABS, polyamide, polycarbonate and other similar widely used materials. As a ‘rule of thumb’, the faster-curing (5-minute) epoxies often show lower strengths on plastics than the longer curing grades. Two-part acrylics (or MMA) bond exceptionally well to many thermoplastics and these products are often used for the assembly of point-of-sale plastic assemblies where ease of application, strength and aesthetics are important. Cyanoacrylates are probably the most versatile adhesives for the bonding of thermoplastics [3] as they are single-part, fast-curing adhesives with good strength and can offer excellent durability if the joint design and plastic grades are chosen carefully at the onset (see Section 7.2.2). Cyanoacrylates can be toughened and are particularly suitable for bonding rubber to plastics if over-moulding is not possible. Cyanoacrylates are typically suitable for temperatures up to 100 °C, although some speciality cyanoacrylates are available for higher operating temperatures. One of the major benefits of cyanoacrylates is that they can be used (with a primer) to bond the non-polar plastics. More details about these primers are given in Section 6.2

48

Engineering Thermoplastics and, although this primer makes the adhesive system two-part, it can be a lower cost method for production than an investment in other surface treatments such as plasma or corona treatment. Cyanoacrylates are best suited where the gaps are small (21.7* Toughened >13.5* Cyanoacrylate Ethyl + primer 1.0 MMA 5.2 Two-part acrylic Polyolefin bonder 4.1 ‘5-minute’ epoxy 3.1 Epoxy Standard epoxy 12.0 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 2.4 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset

54

Engineering Thermoset Plastics

3.2.2 Epoxies Epoxy resins are usually supplied as one-part frozen pre-mixes or two-part systems. There are numerous formulations; each has its own particular characteristic and can vary in viscosity from a thin liquid to a gel consistency. Epoxies are used as adhesives, coatings and binding resins and have excellent abrasion resistance and chemical resistance. Due to the vast array of fillers and different types of epoxy resin, properties vary substantially. Epoxies are generally strong, and also heat, chemical and abrasion resistant. Many printed circuit boards are fibreglass-filled epoxy. Other applications range from marine coatings and encapsulations to adhesives and floorings. The Araldite® series of epoxy resins, manufactured by Huntsman, are perhaps the most well-known, typically two-part systems and these epoxy adhesives would generally be expected to bond well to epoxy thermoset plastics. Cyanoacrylates showed good results in these trials [3], but would not be expected to offer the same durability as epoxy-based adhesives. Note that there are so many different grades of epoxy materials that it is impossible to give specific data (Table 3.2) is intended as a guideline and tests would always be necessary to confirm adhesion performance.

Table 3.2 Adhesive shear strengths (epoxy resins) Adhesive type Description Shear strength (N/mm2) Standard ethyl 13.1 Cyanoacrylate Toughened 17.1 Ethyl + primer 1.7 MMA 11.7 Two-part acrylic Polyolefin bonder 12.1 ‘5-minute’ epoxy 17.9 Epoxy Standard epoxy 20.6 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 10.3 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of epoxy, fillers, surface finish, etc. Recycling information = thermoset

3.2.3 Phenolics Phenolics are one of the earliest bonding agents and are typically heat cured to form a highly crystalline thermosetting polymer. Phenolics have moderate strength compared 55

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts to other plastics but have higher hardness and greater rigidity then most thermoset and thermoplastics. They are made up from a reaction between phenol and formaldehyde so are often referred to as phenol formaldehyde or melamine formaldehyde. Typical applications include handles, electrical terminals and switches. There are many different variants and many have excellent flame resistance and so are used in many public sector applications (mass transit, marine, etc). Epoxy-based and two-part acrylic engineering adhesives generally bond well to phenolics and roughening the surface was found to be beneficial for the UV-curing acrylics (Table 3.3).

3.2.4 Polyester (Thermoset) ‘Fibreglass’ (an unsaturated polyester resin) is one of the best known commercial thermosetting resins for composites and is widely used for large structural applications such as boat hulls and aerospace applications. Polyesters are known for their excellent electrical properties and are widely used in home electrical appliances that require high temperature stability. Specialty grades available include flame retardant, glass filled and magnetisable ferrite filled grades.

Table 3.3 Adhesive shear strengths (phenolics) Adhesive type Description Shear strength (N/mm2) Standard ethyl 4.1 Cyanoacrylate Toughened 11.0 Ethyl + primer 1.0 MMA 9.0 Two-part acrylic Polyolefin bonder 13.1 ‘5-minute’ epoxy 7.6 Epoxy Standard epoxy 17.6 Silicone Alkoxy silicone 1.7 UV acrylic Visible light cure 7.6 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

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Engineering Thermoset Plastics

Table 3.4 Adhesive shear strengths (thermoset polyesters) Adhesive type Description Shear strength (N/mm2) Standard ethyl >9.3* Toughened >9.3* Cyanoacrylate Ethyl + primer 2.4 MMA 7.2 Two-part acrylic Polyolefin bonder 5.5 ‘5-minute’ epoxy >8.6* Epoxy Standard epoxy 5.9 Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure 4.1 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

Most engineering adhesives will bond readily to thermoset polyesters – often achieving substrate failure on test pieces (Table 3.4). Roughening the surface is usually beneficial, especially with the faster-curing cyanoacrylate adhesives. With the exception of the UV-curing acrylic, good strengths were achieved on polyester with all the different adhesives tested [3]. The alkoxy silicone showed a strength of 1.4 N/mm2 but in this case the adhesive has failed cohesively. A silicone adhesive is often used where sealing might be the main priority of the adhesive rather than structural strength.

3.2.5 Polyurethanes Polyurethanes can have the physical structure of a solid casting, a flexible elastomer or soft or rigid foams. It can be either a thermoplastic or more usually it is a thermoset material. The isocyanates in polyurethanes often result in health and safety implications during cure or processing and so must be carefully controlled. Polyurethane plastics can sometimes be difficult to bond and some surface treatment may be necessary. With the wide variety of grades and end-products available, it is 57

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts difficult to give specific data. The data given in Table 3.5 is for guidelines only. Some polyurethanes can be stress cracked by cyanoacrylate adhesives and by some solventbased activators or primers.

Table 3.5 Adhesive shear strengths (polyurethanes) Adhesive type Description Shear strength (N/mm2) Standard ethyl 2.4 Cyanoacrylate Toughened 1.4 Ethyl + primer 9.7 MMA 4.8 Two-part acrylic Polyolefin bonder 4.8 ‘5-minute’ epoxy 2.8 Epoxy Standard epoxy 5.2 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 7.9 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoset plastic

3.2.6 Polyimides Available as both thermoplastic and thermoset resins, polyimides (PI) are a family of some of the most heat- and fire-resistant polymers known. Polyimide is said to possess a greater resistance to heat than any other unfilled organic material. Unlike most plastics, PI are available in laminates and shapes, moulded parts and stock shapes. Polyimide parts are fabricated by techniques ranging from powder-metallurgy methods to conventional injection, transfer and compression moulding and extrusion. In general, mouldings and laminates are based on thermoset resins, although some are made from thermoplastic grades. Laminates are based on continuous reinforcements where moulding resins contain chopped (short) fibre reinforcements. ‘Kapton®’ is one of the best-known trade names. Polyimides can be bonded with cyanoacrylates (Table 3.6) but the rigidity of these adhesives often results in a high stress concentration at the periphery of the joint and this induces premature failure of the material (see Section 10.3.4).

58

Engineering Thermoset Plastics

Table 3.6 Adhesive shear strength (polyimides) Adhesive type Description Shear strength (N/mm2) Standard ethyl >5.5* Toughened >5.5* Cyanoacrylate Ethyl + primer >4.5* MMA 6.6 Two-part acrylic Polyolefin bonder 3.5 ‘5-minute’ epoxy 6.6 Epoxy Standard epoxy >11.7* Silicone Alkoxy silicone 1.4 UV acrylic Visible light cure >5.5* Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of plastic, fillers, surface finish, etc. Recycling information = thermoplastic

3.3 General Comments on Adhesive Bonding of Thermoset Plastics Structural adhesives for thermoset plastics are more often than not epoxies, two-part acrylics and sometimes cyanoacrylates [4]. UV-cure adhesives are not widely used as many thermoset plastics are not clear. Epoxies will bond to most thermoset plastics but as mentioned in Section 1.4 there are many different epoxy adhesive grades available and some epoxies are far more suited for bonding metals than plastics. Two-part acrylics (or MMA) bond well to many thermoset plastics and these products are often used for the assembly of larger thermoset plastic components. They generally cure faster than epoxies and the mix ratio of two-part MMA acrylics is not as critical as it can be for epoxy adhesives. However, many MMA have a high odour and have a tendency to string and many grades are flammable. Standard ethyl-based cyanoacrylates will bond thermoset plastics but the modified toughened grades are often specified for thermosets due to their improved impact resistance.

59

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Cyanoacrylates are best suited where the gaps are small (0.8* Toughened >0.7* Cyanoacrylate Ethyl + primer >1.0* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.6 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information [3] = rubber

4.2.2 Copolyester TPE Copolyester TPE is a high-cost elastomer but has above-average performance. Plasticisers are not used when forming copolyesters. This makes them purer than most TPE, which consequently makes them especially well suited for medical and food applications. Typical applications include fuel tanks, drive belts and cables. Like most elastomers, copolyesters can be bonded with cyanoacrylates and in these trials [2] no improvement was gained by using the primer (Table 4.2).

Table 4.2 Adhesive shear strength (copolyester TPE) Adhesive type Description Shear strength (N/mm2) Standard ethyl 10.8 Cyanoacrylate Toughened 3.5 Ethyl + primer 10.4 Two-part acrylic MMA 2.4 Silicone Alkoxy silicone 1.2 UV acrylic Visible light cure 8.4 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.3 Ethylene Acrylic (EEA) Rubber EEA rubbers have better heat resistance and low-temperature flexibility than polyacrylate rubbers and they also offer excellent resistance to water. This, coupled with its resistance to UV and ozone, gives it excellent weathering resistance. EEA rubbers offer poor resistance to non-mineral oil brake fluid, esters and ketones. They do, however, offer excellent resistance to diesel fuel, kerosene, ethylene glycol and water. Table 4.3 gives typical shear strengths for this material.

4.2.4 Ethylene Propylene Diene Monomer Rubber (EPDM) EPDM is known for its superior resistance to ozone and oxidation as well as its low cost. Typical engineering applications include seals, hoses, belts, cable covers and weather-strips. EPDM can be difficult to bond (Table 4.4) and many cyanoacrylates will not show particularly good adhesion to EPDM. However, some speciality grades of cyanoacrylate will show good adhesion to EPDM without the use of primer. The silicones, UV acrylics and two-part acrylics all show relatively poor performance on EPDM.

Table 4.3 Adhesive shear strength (EEA rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >2.3* Toughened 0.9 Cyanoacrylate Ethyl + primer >2.9* Two-part acrylic MMA 1.2 Silicone Alkoxy silicone 0.8 UV acrylic Visible light cure 1.3 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.4 Adhesive shear strength (EPDM) Adhesive type Description Shear strength (N/mm2) Special ethyl >4.7* Toughened 1.6 Cyanoacrylate Ethyl + primer >4.0* Two-part acrylic MMA 0.9 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

4.2.5 Ethylene-Vinyl Acetate Co-polymer (EVA) EVA is thermoplastic and it is available in various forms depending on the level of co-polymer in the vinyl acetate. This affects the elasticity of the material and also the performance of adhesives. EVA is used for disposable gloves, hoses, tubes and anaesthesia face masks. Table 4.5 gives some typical adhesive shear strengths for this versatile material.

Table 4.5 Adhesive shear strength (EVA) Adhesive type Description Shear strength (N/mm2) Standard ethyl 3.8 Cyanoacrylate Toughened 3.6 Ethyl + primer >5.7* Two-part acrylic MMA 1.5 Silicone Alkoxy silicone 0.07 UV acrylic Visible light cure 4.6 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.6 Fluorosilicone Rubber Fluorosilicones are renowned for their fuel resistance and utility in extreme-temperature service environments. The siloxane backbone results in a polymer with excellent UV, ozone and thermal resistance. The maximum recommended service temperature is in excess of 200 °C (392 °F) for most grades with brittle points as low as −65 °C. Fluorosilicones are used for ‘O’ rings and for wire and cable insulation. Whilst some adhesive shear strengths are given in Table 4.6 fluorosilicones can often be recommended for high-temperature applications and so if the operating temperature is >100 °C, it is unlikely that cyanoacrylates could be used as suitable adhesives.

4.2.7 Natural Rubber Natural rubber is created by processing the latex of a plant, Hevea brasiliensis, which is indigenous to the Amazon valley and is the only known plant to produce highmolecular-weight linear polymer. The latex is ‘tapped’ from the tree then collected and treated with a stabiliser and brought to a processing centre. There are many different types and grades of natural rubber varying with colour, cleanliness and uniformity of appearance. As with most elastomers, natural rubber can be readily bonded with cyanoacrylates although in these trials [2] the adhesion achieved with the toughened cyanoacrylates was relatively low (Table 4.7).

Table 4.6 Adhesive shear strengths (fluorosilicone rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >1.4* Toughened 0.5 Cyanoacrylate Ethyl + primer >1.7* Two-part acrylic MMA 0.6 Silicone Alkoxy silicone 1.6 UV acrylic Visible light cure 0.8 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Elastomers and Thermoplastic Elastomers (TPE)

Table 4.7 Adhesive shear strengths (natural rubber) Adhesive type

Description Standard ethyl

Shear strength (N/mm2) >2.1*

Toughened

Cyanoacrylate

Ethyl + primer

0.9 >1.8*

Two-part acrylic

MMA

0.3

Silicone

Alkoxy silicone

0.3

UV acrylic

Visible light cure

1.6

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

4.2.8 Nitrile Rubber Nitrile rubber is one of the most widely used elastomers and is known for its superior high- and low-temperature performance and its exceptional oil, petrol and solvent resistance. Typical applications include hoses, shoes and flooring. Nitrile rubber is generally easy to bond with cyanoacrylates (Table 4.8) and the use of a primer is not normally necessary. Of the other adhesives tested, the two-part acrylics and the UV acrylics showed promising adhesion.

Table 4.8 Adhesive shear strengths (nitrile rubber) Adhesive type

Description Standard ethyl

Cyanoacrylate

Toughened Ethyl + primer

Shear strength (N/mm2) >2.0* 1.8 >2.0*

Two-part acrylic

MMA

1.7

Silicone

Alkoxy silicone

0.9

UV acrylic

Visible light cure

1.6

*

Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

4.2.9 Neoprene Rubber Neoprene rubber is a thermoset elastomer used for window seals, hoses, cable insulation and for gaskets. There are various forms of neoprene available but they will all bond well with cyanoacrylates and a primer is not usually required (Table 4.9).

4.2.10 Polyisoprene Polyisoprene is a thermoset elastomer and has high tensile properties and its main advantage over natural rubber is that it is generally easier to process. It is used for the manufacture of rubber bands, baby milk bottle teats, sporting goods and engine mounts. Cyanoacrylates show good adhesion to polyisoprene and primers are not usually required (Table 4.10).

4.2.11 Polyolefin Elastomers This TPE is used for handles on power and hand tools but also as grips on everything from pens to golf clubs. Manufacturers use two-shot mouldings to form these grips but sometimes adhesives are required for prototype parts or for supplementing

Table 4.9 Adhesive shear strength (neoprene rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl >2.1* Toughened >1.8* Cyanoacrylate Ethyl + primer >1.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.3 UV acrylic Visible light cure 1.4 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = thermoset rubber

68

Elastomers and Thermoplastic Elastomers (TPE)

Table 4.10 Adhesive shear strength (polyisoprene) Adhesive type Description Shear strength (N/mm2) Standard ethyl >1.7* Toughened >1.7* Cyanoacrylate Ethyl + primer >2.0* Two-part acrylic MMA 0.3 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

mechanical fits or small production runs where the tooling for a two-shot moulding is prohibitively expensive. Thermoplastic elastomers can be quite difficult to adhesively bond (Table 4.11) and invariably a primer or some other form of surface treatment (see Section 6.3) is necessary.

Table 4.11 Adhesive shear strengths (polyolefin elastomers) Adhesive type Description Shear strength (N/mm2) Standard ethyl 2.7 Cyanoacrylate Toughened 1.9 Ethyl + primer >3.4* Two-part acrylic MMA 1.1 Silicone Alkoxy silicone 1.3 UV acrylic Visible light cure 1.7 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Table 4.12 Adhesive shear strengths (silicone rubber) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.5 Cyanoacrylate Toughened 1.8* Toughened >1.3* Cyanoacrylate Ethyl + primer >1.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.8 * Notes: Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber •

Styrene-butadiene-styrene block co-polymers

•

Styrene-isoprene-styrene block co-polymers

•

Styrene-ethylene-butylene-styrene block co-polymers [4]

From an adhesive viewpoint, these materials will often require a primer or some other kind of surface preparation for an effective bond (Table 4.14). However, there are many different blends and additives for styrenic TPE and so trials are recommended to verify the optimum adhesive/primer combination.

Table 4.14 Adhesive shear strengths (styrenic TPE) Adhesive type Description Shear strength (N/mm2) Standard ethyl >3.0* Toughened 2.5 Cyanoacrylate Ethyl + primer >2.8* Two-part acrylic MMA 0.4 Silicone Alkoxy silicone 0.9 UV acrylic Visible light cure 1.2 Notes: * Substrate failure All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts Again cyanoacrylates are amongst the first choice adhesives for these materials and would be suitable for many applications up to 80 °C operating temperatures.

4.2.15 Thermoplastic Vulcanisates (TPV) One of the most widely used TPV is sold under the trade name of Santoprene™ and this material has a low surface energy, thus making it difficult to bond with engineering adhesives. Whilst cyanoacrylates with a surface primer give good results (Table 4.15), the failure modes are invariably at the adhesive-Santoprene™ interface. On some grades of Santoprene™ (e.g., 103-50), adhesive shear strengths up to 8 N/mm2 can be achieved with an ethyl-based cyanoacrylate and primer but with other grades (e.g., Santoprene™ 201-55), only 1.5 N/mm2 is achieved.

4.3 General Comments on Bonding of Elastomers Polychloroprene, nitrile, natural rubber, styrene butadiene rubber and butyl rubber can all be readily bonded with cyanoacrylates. EPDM and fluoroelastomers (such as Viton) can also be bonded, although only with specific grades of cyanoacrylate. The silicone rubbers and thermoplastic elastomers will usually require a primer but will also bond with cyanoacrylates. Cyanoacrylates are relatively rigid adhesives and so may not be suitable in some applications where a high degree of flexibility is required. The toughened cyanoacrylates do show improved flexibility but are a little slower curing and sometimes

Table 4.15 Adhesive shear strengths (TPV) Adhesive type Description Shear strength (N/mm2) Standard ethyl 0.5 Cyanoacrylate Toughened 0.6 Ethyl + primer 1.5 Two-part acrylic MMA 0.3 Silicone Alkoxy silicone 0.4 UV acrylic Visible light cure 0.8 Notes: All shear strengths are given as guidelines only and may vary considerably depending on grade of rubber, fillers, surface finish, etc. Recycling information = rubber

72

Elastomers and Thermoplastic Elastomers (TPE) show slightly lower shear strengths than a standard ethyl grade. However, where an elastomer is to be bonded to a metal, then a rubber-toughened cyanoacrylate might well prove the best adhesive. Note that cyanoacrylates are best suited for non-structural applications and are best suited for temperatures up to 100 °C. Modified flexible cyanoacrylates (see Section 1.1.1.7) also show improved flexibility (Figure 4.1) but the glass transition temperature of these adhesives is lower and so the hot strength and heat aging properties of these products is limited to around 70 °C.

Figure 4.1 Nitrile rubber bonded with a flexible cyanoacrylate

If, therefore, a high degree of flexibility is required at higher temperatures then a two-part acrylic- or silicone-based product may be more suitable.

References 1.

Handbook of Rubber Bonding, Ed., B. Crowther, Rapra Technology Ltd, Shrewsbury, UK, 2000.

2.

The Loctite Guide to Bonding Rubber and Elastomers, Volume 2, Henkel Ltd, Hatfield, UK, 2006.

3.

V. Goodship, Introduction to Plastics Recycling, 2nd Edition, Rapra Technology Ltd, Shrewsbury, UK, 2007, 52.

4.

P. Dufton, Thermoplastic Elastomers Industry Analysis Report, Rapra Technology Ltd, Shrewsbury, UK, 2001, 8 to p.8.

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5

Joint Design

5.1 Introduction This chapter discusses the overall concepts for designing adhesively bonded joints and some guidelines for some of the more common joint designs are discussed. The specific types of joints that will be considered include: •

Lap joint,

•

Double lap joint (tongue and groove),

•

Cylindrical joint, and

•

Butt joint.

5.2 Lap Joint This is the most standard adhesive joint and the one widely used as the standard for testing the performance of adhesives. When a lap joint (Figure 5.1) is loaded as shown in the direction of the arrows, the adhesive in the joint is subjected to primarily shear loads but also with an element of tensile loading, especially if the substrates show some flexibility (Figure 5.2).

Figure 5.1 Lap shear joint

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Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.2 Tensile shear loading (exaggerated to show effect)

5.2.1 Joint Width versus Joint Overlap In a standard lap shear joint, if the overlap length is increased, the strength of the joint does not necessarily increase linearly for the same standard conditions (i.e., same adhesive, same substrate, same surface preparation, and so on). In Figure 5.3 below the bond area has been increased from 1000 mm2 to 2000 mm2 but the strength will only increase by a small percentage. The reason for this can be seen in the stress distribution curves in Figure 5.4. Similarly, in Figure 5.5, both these joints have the same bonded area (1000 mm2) but the joint on the right will be the stronger joint due to the reduced stress concentrations at the end of the joint. From the shear stress distribution curve in Figure 5.4 we can see that the highest stress is at the ends of the bond and by increasing the joint overlap there is no significant change in the strength of the bond.

40 mm 25 mm

80 mm 25 mm

25 mm × 80 mm

Figure 5.3 Extending the overlap on a lap shear joint 76

Joint Design

2

3

Failure load/N

1

2

3

1

Overlapping length/mm

Figure 5.4 The joint strength does not increase linearly with joint overlap due to the stress concentrations at the ends of the joint

25 mm

40 mm 50 mm 20 mm

25 mm × 40 mm

50 mm × 20 mm

Figure 5.5 Which joint is the strongest?

This is because the joint starts to break at the stress peak at the end of the overlap where the adhesion or cohesive strength of the adhesive is exceeded. By increasing the width of the joint, the shear stress distribution is not changed and so the failure load of lap joints increases in the same proportion as the joint width increases (Figure 5.6). 77

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Bond Strength

Increasing Width

Increasing Overlap

Bond Area

Figure 5.6 Increasing joint width will increase the bond strength

Shear forces can cause mechanical deformations in the substrates and the appearance of peak stress points; this is of particular concern when the components being assembled have a thin cross section and when the materials have a low modulus of elasticity. Failure is most likely to occur at the ends where maximum stress is present. The ‘nominal shear strength’ of adhesives (as indicated by manufacturers) is measured under controlled experimental conditions. Lower shear strengths must be expected to take into account the specific characteristics of an application. If long-lasting forces of varying nature acting on the bonded joints appear under actual operating conditions, the value used for calculations should be less than the nominal shear strength of the adhesive. When the surfaces of the components being bonded are poorly fitting, the contact between such surfaces must be good and ensured by the assembly technique. When clamping pressure is removed after the adhesive is cured, elastic recovery of the components may introduce pre-service stresses in the assembly; these stresses may be in addition to the forces experienced under operating conditions and should be allowed for.

5.2.2 Optimising Joints to Minimise Stress A fillet of adhesive is almost invariably formed outside the joint (Figure 5.7) and this can be beneficial in that it reduces the stress concentrations at the end of the joint. For this reason the excess adhesive should only be removed for aesthetic purposes.

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Joint Design

Figure 5.7 A small fillet of adhesive outside the joint can reduce the stress concentrations

The lap shear joint has been the subject of much academic study, particularly with metal substrates but due to the wide variety of plastics available there seems to be little published data when it comes to bonding plastics. In some plastic engineering applications the load on the joint is very low, perhaps only the weight of the bonded parts, and so it will not be necessary to pay too much attention to joint design and the shape most easily fabricated can be selected. For higher load joints the joint design will require more careful consideration and testing prior to manufacture. A few simple suggestions for improving the stress distribution across a lap shear joint are shown in Figure 5.8 but the application and manufacturing process will define the options available. Rigid adhesives (e.g., standard ethyl cyanoacrylates) are used for bonding assemblies when the joints can be designed to be subjected only to shear and normal static forces. If the application is subjected to peel stresses or impact loads, a toughened adhesive (e.g., epoxy, two-part acrylic or toughened cyanoacrylate) would be more suitable.

Figure 5.8 Some options for a lap shear joint [1] 79

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

5.3 Double Lap Joint (Tongue and Groove) A tongue and groove (or double lap shear) joint (Figure 5.9) can be an ideal joint design for many applications, especially for bonding a lid onto a box. The adhesive can be dispensed into the groove, thus ensuring positive location of the liquid adhesive and when the joint is closed and the adhesive cured, the adhesive is essentially under shear loading and so will have high strengths. Note, however, even with this design which would appear to allow symmetrical loading, under high load the substrates may distort and thus subject the adhesive to a cleavage/peel load [2]. The wall thickness of the substrate will define whether there is sufficient width to include a double lap shear joint and if the thickness is less than 3 mm, it may well be difficult to achieve the tolerances required. One of the issues that can occur with a tongue and groove joint is allowing for where the adhesive will flow to when the joint is closed. In Figure 5.9, when the joint is closed, excess adhesive has nowhere to flow to and may therefore spill out and impair the aesthetics of the assembly.

Figure 5.9 Tongue and groove joint (with no allowance for excess adhesive) 80

Joint Design

Figure 5.10 A modified tongue and an offset ensures the adhesive finds the easiest path to the larger gap and thus minimises excess adhesive outside the joint

In Figure 5.10, the design has been modified slightly to allow for the adhesive to flow into a slightly larger gap, thus improving the aesthetics of the joint.

5.4 Cylindrical Joints A common application for adhesives is where co-axial (cylindrical parts) require to be joined. Figure 5.11 shows a typical application where the black polypropylene spigot is to be bonded and sealed into the red elastomeric (thermoplastic elastomers) housing. In this case, both these plastics are low-surface-energy materials and so will require either a surface primer or some form of surface treatment to increase the surface wetting (see Chapter 6). In this application a medium viscosity ethyl cyanoacrylate with a primer was selected. The length to diameter (L/D) ratio and the diametral clearances are both important factors in terms of the final strength of the bond. In this application the L/D ratio is 1.2 and so ideally suited for an adhesive. 81

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.11 Joining dissimilar plastics (cylindrical joint) Thin bond lines where the diametral clearance is less than 0.1 mm are usually best for most industrial applications. Figure 5.12 shows a much higher L/D ratio and in this application a cyanoacrylate might not be the most appropriate adhesive as it may cure before the parts can be fully assembled. For long engagement lengths such as these and where the outer (female) substrate is opaque (and a ultraviolet (UV) adhesive cannot be used) then the adhesive can be wicked into the joint after assembly [3]. Note the small chamfer in Figure 5.12 to allow for this capillary action. In this case a diametral clearance of 0.04 mm has been selected as the low viscosity cyanoacrylates have limited gap-filling capabilities. If the gaps were larger (0.2 mm) then a two-part acrylic or epoxy may be the most appropriate adhesive. If the outer (female) substrate had been transparent in Figure 5.12, it may have been possible to use an ultraviolet-curing adhesive. The benefit of UV adhesives is that the product will only cure ‘on demand’ and so there will be plenty of time to rotate the component parts to ensure full joint coverage. Where the assembly process is horizontal, the chamfer is not always necessary but the male part may require to be rotated to ensure full joint coverage.

5.4.1 Design Details In many cylindrical bonding applications involving plastics, it is common practice to dispense the adhesive onto the inner (male) substrate and then close the joint with a rotating action. This in principle is fine but invariably excess adhesive is applied and the excess will be forced out of the joint as shown in Figure 5.13.

82

Joint Design

d

D

D – d = 0.04 mm

L

Figure 5.12 Tube bonding with long engagement lengths

Figure 5.13 Applying adhesive to male component 83

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.14 Applying adhesive to the female component

Perhaps the easiest option is to control the dispensing carefully to minimise excess adhesive (see Section 8.1.5). However, one option to overcome excess adhesive being pushed outside the joint is to apply the adhesive to the inside of the female component (Figure 5.14). In this case (Figure 5.14) the adhesive tends to get pushed along the component and will end up on the inside. This may in its turn cause issues with the possibility that the adhesive may block or interfere with other fluids passing through the connector. A third option for cylindrical component parts is to design in a small recess so that the adhesive has somewhere to escape to when the joint is closed (Figure 5.15).

5.4.2 Cross Holes Another method of applying adhesive to cylindrical parts is to introduce the adhesive via a cross hole (Figure 5.16), although this does involve additional detailed mould design.

84

Joint Design

Recess

Figure 5.15 Parts designed with a recess to minimise excess adhesive

Figure 5.16 Dispensing the adhesive via cross holes

5.4.3 Blind Holes Where a ‘blind’ hole exists if the adhesive is applied to the male part and the joint closed, it is likely that the air trapped in the base of the hole will push all the adhesive out of the hole and thus starve the joint of adhesive (Figure 5.17). The best practice is to apply the adhesive into the base of the hole and then assemble the parts (Figure 5.18). Note, however, that in small-diameter holes it is entirely possible that ‘hydraulic locking’ can occur and so a small bleed hole to allow the air to escape may also be necessary.

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Figure 5.17 Air trapped in the base of a blind hole will force the adhesive out of the joint

5.5 Butt Joint The butt joint can be in various configurations (Figure 5.19) and this is almost certainly the easiest and lowest-cost joint. The success of this joint will depend on the materials bonded, the loads on the joint and the thicknesses of the substrates involved. If the adherends are rigid and a moment or offset load is applied to this joint then the adhesive will be subjected to quite a severe cleavage or peel load (Figure 5.20) and as such the butt joint is generally regarded as a poor joint design. In some applications the butt joint may be the only possible method of assembling the component parts and, if aesthetics are important, the small fillet of excess adhesive outside the joint (which adds to the strength of the joint) may also be undesirable. One example of this in the plastics industry is the assembly of shelving where acrylic or polycarbonate sheets are bonded together to form ‘points of sale’ display equipment. 86

Joint Design

Figure 5.18 For blind holes, apply the adhesive to the base of the hole

Figure 5.19 Various forms of a butt joint 87

Practical Guide to Adhesive Bonding of Small Engineering Plastic and Rubber Parts

Figure 5.20 An offset tensile load or a moment can create high cleavage or peel loads in the joint line

Small chamfer

Figure 5.21 A small chamfer will increase the strength of the joint

In these cases a small internal chamfer can help to improve the integrity and aesthetics of the joint and it allows for some tolerance on the dispensed quantity of adhesive (Figure 5.21) as it can be quite difficult to dispense the exact quantity of adhesive so that it stops exactly at the edge of the joint. The chamfer creates a small gap to allow for the adhesive flow. In these applications a variety of adhesives are used including UV cure, cyanoacrylates and two-part adhesives (epoxies or acrylics) and the adhesive is selected for its clarity, cost, speed of cure and ease of use.

5.6 Bond Line Thickness The gap between the parts and therefore the thickness of the adhesive film has an important bearing on the characteristics of the joint [4]. A thick bond line (>0.25 mm) will generally be a weakening feature for cyanoacrylates as the mechanical strength of the cured cyanoacrylate film is likely to be less than the plastic or other substrate 88

Joint Design bonded. For most applications where cyanoacrylates are involved, thin films (