Analysis of rubber and rubber-like polymers

Analysis

of

Rubber-like

Rubber

and

Polymers

Fourth Edition

M.J.R. Loadman

KLUWER ACADEMIC PUBLISHERS DORDRECHT / BOSTON / LONDON

Library of Congress Cataloging-in-Publication Data

ISBN O 412 81970 8 Published by Kluwer Academic Publishers, P.O. Box 17, 3300 AA Dordrecht, The Netherlands. Sold and distributed in North, Central and South America by Kluwer Academic Publishers, 101 Philip Drive, Norwell, MA 02061, U.S.A. In all other countries, sold and distributed by Kluwer Academic Publishers P.O. Box 322, 3300 AH Dordrecht, The Netherlands.

Printed on acid-free paper

All Rights Reserved © 1998 Kluwer Academic Publishers No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. Printed in The UK

Preface

The first edition of this book (1958) described an analytical situation which had existed for a number of years for maintaining quality control on vulcanizates of natural rubber although the situation had recently been disturbed by the introduction of a range of synthetic rubbers which required identification and quantitative estimation. For the former purpose 'wet' chemistry, based on various imperfectly understood organic reactions, was pressed into service. Alongside this was the first introduction of instrumental analysis, using the infrared spectra of either the polymers or, more usually, their pyrolytic products to 'fingerprint7 the material. The identification of a range of organic accelerators, antioxidants and their derivatives which had been introduced during the 1920s and 30s was, in the first edition, dealt with by a combination of column chromatography and infrared spectroscopy or by paper chromatography. Quantitative procedures were, however, still classical in the tradition of gravimetric or volumetric assays with an initially weighed sample yielding, after chemical manipulation, a carefully precipitated, dried and weighed end product, or a solution of known composition whose weight or titre, as a percentage of the initial sample, quantified the function being determined. The second edition of this work (1968) consolidated the newer techniques which had been introduced in the first without adding to them although, in other applications of analytical chemistry, instrumental analysis had already brought about a transformation in laboratory practice. In 1983 the third edition was published and gave full credit to modern instrumentation in all spheres of the analysis of rubber and rubber-like polymers, describing techniques and illustrating applications where equipment still at the 'research stage' could add to the strength of the analysts' armoury of the future. Nevertheless, the financial stric-

tures confronting modern 'instrumental7 laboratories were appreciated so, within each area of analysis, there was a variety of techniques presented, from the I)UITI test', costing essentially nothing, to those using instrumentation costing many tens of thousands of pounds. In this, the fourth edition, the structure of the previous edition has been maintained and expanded in that each chapter provides a complete package of information on a particular topic as viewed by an enquirer or analyst rather than discussing the range of uses of a particular instrument or technique. After covering a range of topics, the book continues by showing how specific primary analytical data can be intercorrelated and how this can then be expressed in the technological language of compound or product 'formulation7. Finally, the validity of any conclusions drawn from the analytical data is discussed in terms of its statistical significance so that a reasoned interpretation may be made of the final information package. The impact of 'health and safety' oriented legislation has taken its toll of many of the older chemical methods of analysis. Not only are the chemicals used now considered potentially hazardous, but it is also important to note that many of the older methods present in the literature of the last century have not been fully validated against the thousands of new substances which may, today, be found in a commercial rubber product and which may interfere with a colorimetric or spot test which would have been perfectly satisfactory in earlier times. Many new or extended instrumental techniques have, however, replaced those which have been eliminated, and, at the same time, the opportunity has been taken to invite my colleagues in the Materials Characterization Group of the Tun Abdul Razak Research Centre to comment on, rewrite, or expand any areas which they believed to be deficient. Because these experts operate under areas of instrumental expertise and the book is structured under topics of interest to the rubber analyst or technologist, individual contributions are scattered throughout the text and I can only claim to have attempted to produce a coherent whole! To my staff, in alphabetical order, I give my thanks: Bob Crafts (elemental analysis and statistics), Paul Cudby (microscopical techniques), Jim Gleeson (GC and TLC), Colin Hull (NMR, thermal methods and carbon black), Kevin Jackson (spectroscopic and thermal methods), Chris Lewan (LC and GPC), and Sue Stephens (GC and TLC). To others of my staff whose contributions were indirect in that they freed those listed above to make their direct contributions I also offer my thanks. Acknowledgement is also due to the Board of the Tun Abdul Razak Research Centre (TARRC) for permission to undertake this project and for the facilities made available to my staff and me. MJRL (1998)

Acknowledgements

In a book of this nature it is inevitable that a wide range of publications be consulted to afford as balanced a picture as possible of the current position in the analysis of rubber and rubber-like polymers. From these publications many tables and figures have been culled to illustrate relevant points throughout the text and it is with much gratitude that I and the publisher thank the copyright holders for permission to use their data. The very number of these necessitates only the briefest of comments but this brevity in no way reduces the sincerity of our appreciation to: The British Standards Institution for Figure 6.3, taken from BS 7164: Part 24: 1966 and the American Society for Testing and Materials, together with A. Krishen (1974) for Figures 7.10, 7.11 and 7.12. Full copies of these documents may be obtained from 389 Chiswick High Road, London W4 4AL and 100 Barr Harbour Drive, West Conshohocken, PA19428, USA respectively. The National Institute of Standards and Technology, Technology Administration, US Department of Commerce, for permission to reprint Table 11.7. John Wiley & Sons, Inc. with Evans, Higgins, Lee and Watson (1960) /. Appl. Polym. ScL for Figure 5.2; with Gelling, Loadman and Sidek (1979) /. Polym. ScL Polym. Chem. Edn. for Figures 7.16, 7.17 and 7.18; with Kim and Mendelkern (1972) /. Polym. ScL Part A2 for Figure 7.20; with Lee and Singleton (1979) /. Appl. Polym. ScL for Figure 7.21 and with Billmeyer (1971) Textbook of Polymer Science, 2nd Edn for Figure 8.2 The Managing Editor of Rubber Chemistry and Technology with Swarin and Wims (1974) for Figures 12.2, 12.3 and 12.4 as well as Tables 12.6 and 12.7; Sircar and Lamond (1978) for Figure 12.5; Brazier and Nickel (1975) for Table 12.5 and Pautrat et al (1976) for Figure 11.4.

The Editor of The European Journal for the following tables which appeared in Rubber /.: White (1967) for Table 4.2 and Lamond and Gillingham (1970) for Tables 11.5 and 11.6. The American Chemical Society with Carman (1973) Macromolecules for Figure 8.14 and Krishen (1972) Anal. Chem. for Figure 7.9. The Editor of Materials World with Davies and Kam (1967) /. IRI for Table 11.3, Ney and Heath (1968) /. IRI for Figures 7.7 and 7.8, McSweeney (1970) /. IRI for Figure 3.4, Davey et al. (1978) Plast. and Rubb. Mat. and Applic. for Figure 6.4 and Charsley and Dunn (1981) Plast. and Rubber Process Applic. for Figure 11.5. MCM Publishing for allowing Figures 11.2 and 11.3 to be taken from Maurer (197Oa), Rubber Age, Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam for allowing Figure 7.23 to be reprinted from Thermochim. Acta (1980), 39, 593 (Goh) and Addison Wesley Longman Ltd for permission to reproduce Davies and Goldsmith's table of '%age of Student's t distribution' from 'Statistical Methods in Research and Production (0-852-45087-X) as Figure 14.1. Figure 10.2 was supplied by the Parr Instrument Company and is published with its permission, Figures 6.5 and 6.6 are published with the permission of Dionex (UK) Ltd whilst Figure 7.13 was provided by, and is published with the permission of, the Perkin Elmer Corporation. Finally I thank the Director of the Rubber Research Institute of Malaysia for permission to use the data shown in Tables 6.2 (Davey (1989) /. Nat Rubber Res.) and 14.2 and the Board of the Tun Abdul Razak Research Centre (TARRC), through the Director of Research, for permission to refer to unpublished work carried out within the Research Centre over many years and for Figure 7.2 taken from the house publication, NR Technol. (G.M.C. Higgins and M.J.R. Loadman, 1970). Work carried out under the earlier name of the Research Centre - the Malaysian Rubber Producers' Research Association (MRPRA) - is credited to that name.

Contents

Preface ............................................................................

xii

Acknowledgements .........................................................

xiv

1.

2.

3.

Introduction .............................................................

1

The Nature of Materials .......................................................

1

The Historical Perspective ..................................................

4

Scope of the Book ...............................................................

14

The Analytical Problem .......................................................

16

Compositional Categories ...................................................

19

References ..........................................................................

22

Sampling and Sample Preparation ........................

25

Analysis of Average Composition .......................................

25

Homogenization of Sample .................................................

27

Analysis of Localized Composition .....................................

28

Size of Test Portion .............................................................

29

Sample Preparation ............................................................

29

Extraction ................................................................

31

Preliminary Remarks ...........................................................

31

Nature of the Extraction Process ........................................

32

Standard Apparatus for Determination of Extract Level .............................................................................

37

Choice of Solvent ................................................................

38

Time of Extraction ...............................................................

40

Rapid Extraction ..................................................................

41

Microwave Extraction ..........................................................

42

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v

vi

4.

5.

6.

Contents Micro Scale Extraction ........................................................

43

Multiple Extractions .............................................................

44

Specific Extractions .............................................................

45

Supercritical Fluid Extraction ...............................................

46

Latex ....................................................................................

47

Thermal Extraction ..............................................................

48

Adsorption/Extraction ..........................................................

49

References ..........................................................................

52

Analysis of Extracts ...............................................

54

Identifications with no Separation .......................................

55

Identification with Separation ..............................................

65

Identification after Separation .............................................

77

References ..........................................................................

79

Solution Methods ....................................................

81

Theoretical Considerations .................................................

82

Practical Considerations .....................................................

88

References ..........................................................................

93

Quantitative Elemental Analysis ...........................

95

Carbon and Hydrogen .........................................................

95

Nitrogen ...............................................................................

96

Oxygen ................................................................................

98

Chlorine and Bromine .........................................................

100

Fluorine ................................................................................

103

Silicon ..................................................................................

105

Phosphorus .........................................................................

106

Sulphur ................................................................................

109

Ion Chromatography (IC) ....................................................

123

References ..........................................................................

126

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Contents 7.

8.

9.

vii

Instrumental Polymer Analysis ............................. 129 Introduction ..........................................................................

129

Infrared Spectroscopy (IR) ..................................................

129

Nuclear Magnetic Resonance Spectroscopy (NMR) ..........

143

Pyrolysis-Gas Chromatography (PGC) ..............................

148

Derivative Thermogravimetry (DTG) ...................................

154

Differential Scanning Calorimetry (DSC) ............................

163

Scanning Electron Microscopy (SEM) ................................

168

References ..........................................................................

171

Polymer Characterization ...................................... 174 Molar Mass ..........................................................................

174

Microstructure ......................................................................

193

Metathesis ...........................................................................

201

Latex Particle Sizing ............................................................

202

References ..........................................................................

205

Blend Morphological Analysis ............................... 208 Light Microscopy (LM) .........................................................

208

Scanning Electron Microscopy (SEM) ................................

209

Transmission Electron Microscopy (TEM) ..........................

211

SEM Based Scanning Transmission Electron Microscopy (S(T)EM) ...................................................

212

TEM Based Scanning Transmission Electron Microscopy (STEM) .....................................................

214

Microtomy and Associated Techniques ..............................

215

Freeze Fracture ...................................................................

226

Chemical Staining ...............................................................

226

Chemical Etching ................................................................

229

Case Study ..........................................................................

231

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viii

Contents Swollen Vulcanized Elastomer Network Observation ........

238

References ..........................................................................

242

10. Inorganic Fillers and Trace Metal Analysis .......... 243 Ashing ..................................................................................

243

Bulk Filler Analysis ..............................................................

251

Trace Metals ........................................................................

252

Analysis of Prepared Solutions ...........................................

252

Total Sample Elemental Analysis .......................................

256

References ..........................................................................

263

11. Carbon Black ........................................................... 265 Obtaining Free Carbon Black from the Rubber Matrix .......

265

Types of Carbon Black ........................................................

267

Analysis of Carbon Black Particles and Aggregates ..........

270

Analysis of Carbon Black Type ...........................................

270

Surface Area Measurements ..............................................

274

Black Type by Thermogravimetry .......................................

279

Carbon Black Dispersion in Vulcanizates ...........................

284

Other Techniques Used to Examine Carbon Black ............

285

References ..........................................................................

287

12. Formulation Derivation and Calculation ............... 290 Polymer Content ..................................................................

290

Formulation Derivation ........................................................

303

Formulation Calculation ......................................................

309

References ..........................................................................

310

13. Blooms and Visually Similar Phenomena ............ 312 True Blooms ........................................................................

312

Modified Blooms ..................................................................

314

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Contents

ix

Pseudo Blooms ...................................................................

314

Surface Contamination ........................................................

315

Hazing of Transparent Rubbers ..........................................

315

Staining/Discoloration ..........................................................

315

Pre-Analytical Check-List ....................................................

317

Analytical Methods ..............................................................

319

Removal of Bloom Prior to Analysis ...................................

320

References ..........................................................................

323

14. Validity of Results ................................................... 325 Introduction ..........................................................................

325

Meaningful Information from Imprecise Data ......................

328

Traceability ..........................................................................

341

Validation of Analytical Methods .........................................

343

References ..........................................................................

346

Appendices .................................................................... 347 Appendix A Table of Official National and International Standards ...............................................

347

Appendix B Elastomers: Nomenclature, Description and Properties .............................................................

352

Appendix C Intercorrelation of Analytical Techniques .......

359

Author Index .................................................................. 369 Index ............................................................................... 361

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Introduction

I

THE NATURE OF MATERIALS Most readers will have their own idea of the materials they would expect to find in a book such as this but it is not easy to define them in a way which makes it immediately apparent whether or not any one is, or is not, included. The difficulty has been in part avoided by giving an appendix of the materials which have been considered in drawing up the methods and schemes in this book but, even so, some attempt must be made at definition, or at least at description. Natural rubber derived from the tree Hevea brasiliensis is the prototype of a wide range of materials which have a high extensibility combined with an ability to recover from extension. It is usual to refer to these materials as highly elastic, and to group their properties as high elasticity. These properties have been found to be dependent on a certain type of molecular structure, and rubber-like materials have physical properties similar to natural rubber because they, too, have the same molecular pattern in their structure. The essential features of this structure are the ability of certain atoms to unite forming long, very flexible, chains coupled with the existence of a range of molecular attractions between the chains which modifies the degree of flexibility. If the chains are perfectly uniform and symmetrical, the molecular attraction between them will reduce flexibility and even lead to crystal formation. If they are completely irregular then the material will possess little strength and will break at a comparatively low extension. For dimensional stability over long periods of time it is further necessary that the molecular chains should be linked together by occasional crosslinks to form a three-dimensional network. All the materials in the appendix conform in structure to the first two of the above requirements but those which also conform to the last condition, that of possessing a crosslinked three-dimensional structure, are those referred to as rubbers whereas

the remainder are rubber-like. In general one may say that the rubberlike materials are the flexible plastics or thermoplastics. Rubber and rubber-like materials are therefore materials composed of long-chain molecules, which show high elasticity and it is this property which led to the generic term elastomer being coined by Fisher in 1939. The term polymer has not yet been introduced into this discussion; if we consider the long molecular chain of plastic sulphur, the sulphur atoms themselves form the simple unit from which it is built, but most other long-chain molecules are formed by the repetition of a rather more complex unit consisting of several atoms, which will constitute the backbone, to which other atoms or atomic groupings are appended. When this unit is repeated to build up a long molecular chain, the unit is defined as the monomer and the polymer can be represented as (monomer)n. In this instance, the polymer should more correctly be referred to as a homopolymer. Details of methods whereby polymers can be built from monomers are outside the scope of this book but it should be realized at this early stage that many important synthetic rubbers are copolymers derived from two or more monomers being mixed before polymerization. In these situations, the polymer chain does not necessarily contain a regular and uniform alternating sequence of the monomer units nor are they necessarily distributed randomly along the length of the chain. Indeed the extent to which monomers exist in 'blocks', and the length of the blocks, plays an important part in determining the properties of that particular copolymer. The use of only one monomer naturally leads to a greater regularity in the polymer chain but even a relatively simple monomer such as 1,3butadiene can give rise to polymeric irregularity due to combination in the 1,2 instead of the 1,4 position and due to the possibility of the spatial configuration around the central double bond of each unit of the polymerized material being either trans or cis. (It will be appreciated that polymerization of butadiene involves a loss of unsaturation since CH2=CH-CH=CH2 becomes, in the polymer, (-CH2-CH=CHCH2—)n with the monomer units connecting end to end during the 1,4polymerization process). This method of polymerization is the addition method and the resulting polymers are addition polymers. An alternative approach to the formation of a long molecular chain is the condensation of two or more types of molecular units (monomers) each possessing two, and no more than two, functional groups accompanied by the elimination of some simple molecule such as hydrogen chloride or water. It is by this method that the alkyd resins used in paint technology are made from dibasic acids and glycols with the elimination of water. There is, however, a limit to the size of the molecule that can be made in this way and this limit is below the size

where useful rubber-like properties are developed. In order to achieve sufficient polymerization a further stage is added whereby the medium length molecules obtained from the condensation process are subsequently linked together by other di-functional compounds such as the highly reactive diisocyanates. This process is the basis of some important synthetic rubber-like materials such as highly elastic lacquers and foams. Unlike addition polymers, polycondensates must, from their chemical nature, be completely regular although not all the medium length molecular chains will be the same length. So far, only the formation of chain molecules has been discussed but rubbers possess a crosslinked three-dimensional structure which results from chemical 'joins' or 'links' from one chain to another. These crosslinks must be sufficiently frequent to prevent chains from sliding over each other but not frequent enough to destroy their essential flexibility; in practice about 1% of monomer units take part in the crosslinking process. Convenient crosslinking agents are chemically dissimilar from the chains they link and, as is well known, sulphur is the most common. The term curing was used by Charles Goodyear in the USA to describe the process of heating natural rubber with sulphur to 'cure' it of its propensity to turn brittle on cooling and sticky when hot whilst, in the UK, the term vulcanization was preferred. Today, whilst curing and vulcanization are used synonymously for the sulphur crosslinking of elastomers, the term curing is also applied to all other forms of crosslinking where three-dimensional networks are built up from polymer chains without the use of sulphur. Given a suitable choice of solvent and temperature, most high polymers (that is, polymers with very long polymer chain lengths) will dissolve, but ease of solution decreases with increasing molecular size and crosslinking effectively gives a molecule whose molecular size is the same as its physical size. At this extreme, dissolution of the polymer in solvent cannot occur but mixing of solvent molecules with the network of the polymer is possible and this is the mechanism of solvent swelling. The swollen polymer still retains the crosslinked structure of its unswollen state but is rendered softer, more flexible and weaker because the strength and rigidity of the network structure are no longer enhanced by the attraction of the polymer chains for each other. The controlled mixing of a polymer with a suitable solvent can be used to advantage in modifying the properties of some rubber-like materials and this is the basis of the conversion of rigid polyvinylchloride to a flexible plastic. Pure polyvinylchloride is a hard, horn-like material which can be mixed with a solvent such as dibutylphthalate to give the familiar plastic material which is sufficiently soft and flexible for garments to be made from it. In this application the solvent is referred to as a plasticizer and it functions by separating the molecular chains

Table 1.1 Classification of thermoplastic materials Type

Soft component

Hard component

Structure

Plasticizer

Polyvinyl chloride or Ethyiene-vinyl acetate copolymer

Swollen polymer

Rigid polyvinyl plastics Polyurethanes

Polyether glycol or Methylene (diphenyl Polyester glycol (A) isocyanate) (B)

Polyether esters

Polyether glycol (A)

(AB)2

Polyolefin (A)

Polystyrene (B)

ABA or ABA A

'Soft' elastomer i.e. EP rubber or NR

Polypropylene or Polyethylene

physical blend

Styrene block/star copolymers Thermoplastic elastomers

1,4-Benzene dicarboxylic acid (terephthalic acid) (B)

(AB)2

one from another. Popular technical usage historically restricted the term plasticizer to solvents used with synthetic materials such as polyvinylchloride and nitrile rubbers and used the terms softener or extender, depending upon the level present, for materials performing a similar function in natural and general-purpose rubbers. More recently this distinction has become blurred. Following from this discussion, rubber-like materials, or thermoplastics, can conveniently be divided into five types as illustrated in Table 1.1. THE HISTORICAL PERSPECTIVE It would be inappropriate in a book of this nature to consider the historical perspective of these materials in any depth but, having identified the categories into which the various materials fall, a knowledge of their historical development should be both of interest to, and advantageous for, any analyst working in this area. NATURAL RUBBER

The history of natural rubber over the last three thousand years is a fascinating story and in many areas it is confused in detail where, even today, the truth is sometimes obscure. It also involved many dozens of famous scientists who, space decrees, must forfeit mention here.

The oldest rubber known was reputed to have been found in 1924, in Germany, fossilized in lignite deposits some 60 million years old (Schidrowitz and Dawson, 1952), and this could be the same material described by Auleytner (1953) which was again found in Germany and dated to the Eocene period, some 30 million years ago. An attempt by the editor to trace this material in 1994 at its last known location QagielIonian University, Cracow) met with failure. There seems to be only one other reference to natural rubber appearing in the 'old world' and this is to the Ethiopians making play-balls and other rubber objects which then spread to ancient Egypt. Herodotus attributed their origins to the Lydians. With these exceptions, the early history of rubber is solely a story of the 'new world', centred round the equatorial regions of South America and Mexico. The earliest records which refer to natural rubber in the Americas are Aztec picture writings dating from the 6th Century AD which show that rubber was used as a material for paying tributes and was also associated with devil-worship. In the Mayan city of Chichen Itza excavations have uncovered many sacrificial items (including human remains), rubber figurines and torches with rubber cores which were burnt to generate thick black smoke, possibly to suggest rain clouds homeopathic witchcraft! There is no doubt that sport was a fundamental part of the preColumbian Americas and it seems that one game which spread through the whole region was an early version of handball or basketball. The same game was played as far south as Paraguay and north into what is now Arizona. In 1993 Stuart described a rubber ball found in El Manati which was over three thousand years old. Although the ball game appears to have covered a vast area, the same is not true for the artefacts manufactured by the natives of the Amazon or Peruvian regions, possibly because these had more practical and/or religious significance. One example was the use of rubber for the manufacture of shoes. The Amazonian native was concerned with protecting his feet and did this by a straight over-dipping process, with his feet as the mould, to produce a perfectly fitting pair of galoshes. The earliest western references to rubber inevitably involve Christopher Columbus but the honour for the first certain reference to rubber in print belongs to Pietro Martire d'Anghiera (1530) who talked of 'gummi optima', and described how it was obtained as a white juice from certain trees which dried to a transparent material whose properties were improved by fumigation. For a few years the literature flowed. Captain Gonzalo Fernandez de Oviedo y Valdes (1535) gave a detailed description of the ball games played in the Greater Antilles whilst Antonio de Herrera Tordesillas (1601) described how Cortez had watched such a game at the court of

Montezuma. In 1615 Torquemada documented the first applications other than play-balls. He described how his soldiers were taught by natives to waterproof their clothing by dipping it in the milky juice from the rubber tree and he also described the making of footwear, bottles and a variety of hollow goods by the process of dipping over clay formers then breaking out the latter. However, neither the reports nor the rubber products which came out of the Americas stimulated more than a passing interest in Europe. The latter were just regarded as curiosities and there was no appreciation of the commercial landslide which was to come. From 1615 to 1736 rubber-related literature was minimal but from the latter date the start of the western rubber industry can be dated. This was due to the activities of two Frenchmen, Charles de Ia Condamine and Francois Fresneau. La Condamine was born at the turn of the 17th/18th Century and was a soldier, social climber, dilettante, and poet but he was also a friend of Voltaire and had interests in chemistry, astronomy and botany. When the Paris Academy of Science organised two expeditions to determine the exact shape of the Earth, he seemed a reasonable choice to lead one which was sent to Ecuador (or Esmeraldis). Soon after his arrival in Quito, in 1736, he sent a package of rubber to the Academy with a long memoir describing many aspects of its origins and production. These included the words 'Heve' as the name of the tree from which the milk or 'latex' flowed and the name given to the material by the Maninas Indians: 'cahuchu' or 'caoutchouc'. He later described the smoking procedure by which the natives made the rubber stable and the wide range of goods which were produced, including the following. 'They [the natives on the banks of the Amazon] make bottles of it in the shape of a pear, to the neck of which they attach a fluted piece of wood. By pressing them, the liquid they contain is made to flow out through the flutes and, by this means, they become real syringes/ From this the Portuguese called the tree 'pao de Xiringa' (syringe wood) and the rubber tappers or harvesters 'Seringueiros'. The present name for the tree which is universally accepted as producing the best rubber is 'Hevea braziliensis' and this is the source of all modern plantation rubber. It was not, however, the tree which produced much of the rubber spoken of in pre-Columbian times. The tree which Ia Condamine called 'Heve' was 'Castilloa elastica', but he did not realise that the one he described a decade later, the 'pao de Xiringa' or Seringa tree, was different. In 1775, Fusee-Aublet identified and named 'Hevea guyahensis' as the rubber-producing tree of the Guianas and it was left to Willdenhow in 1811 (Dean, 1987), Director of the Berlin Botanical Gardens, to classify the Seringa as 'Hevea braziliensis'. Meanwhile, Persoon (1807) had proposed the name 'Siphonia elastica'

and the matter was only laid to rest by Muller (1865-6) who suppressed the classification 'Siphonia' in favour of 'Hevea braziliensis' some 50 years later. 'Latex', the word used by Ia Condamine to describe the juice of the tree, was derived from the Spanish word for milk and remains in use to this day. The name 'rubber' was coined in 1770 by the scientist Joseph Priestley when he found some in a shop of artists' materials, being sold to erase pencil marks by rubbing them out. The full name of 'indiarubber', intended to reflect the perceived source of the material, soon became shortened. La Condamine's word 'caoutchouc' is generally taken to be based on the Indian 'caa ochu' - 'the tree that weeps' - but in view of the early religious significance of rubber it is interesting to note that in a dictionary of Kechuan language of the ancient Incas, Holguin (1608) translated 'cauchu' as 'he who casts the evil eye' whilst other writers have also noted the connection between the word and things magical. It has also been related to a native word for blood, and this could complete the circle to the weeping (bleeding) tree. Regardless of which is correct (and both could be), these are the likely origins of the current German and French words, 'kautschuk' and 'caoutchouc'. One final observation about the confusion of words: the reader of older books about rubber production in Amazonia will see rubber described as 'fine Para' or the like. This is named after the port of Para, close to the mouth of the Amazon river. However, the whole state, a substantial part of Brazil, is also called Para whilst the town is also known as Belem. Before returning to France, Ia Condamine met Fresneau who was a trained engineer and amateur botanist. Fresneau became infected with Ia Condamine's enthusiasm for rubber and was the first European person to consider it as a potential industrial material. When Ia Condamine returned to France, Fresneau remained in Guiana, detailing all aspects of rubber production, treatment and usage and forwarding his reports to his friend for publication. In 1751 Ia Condamine presented a paper by Fresneau to the French Academy (eventually published in 1755) which described many of the latter's findings and this can truly be called the first scientific paper on rubber. Fresneau deserves one further mention. After he returned to France in 1749 he continued to devote his life to research into rubber and, according to his biographer and descendant, the Comte de Chasseloup Laubat (1942), he eventually concluded that turpentine was the ideal solvent from which to prepare rubber solutions which could be used to emulate latex in the manufacture of articles in Europe, the latex itself being too unstable to ship to Europe. This enabled rubber to become an industrial raw material and justified Fresneau's title as 'the father of the rubber industry'.

For the next fifty years after the work of Fresneau, progress was slow but then, between 1820 and 1839, there was a resurgence of interest as, in the UK, Hancock invented his machine to convert lumps of solid rubber into a useable homogenous gum, a process he called 'pickling7 to confuse his competitors, Macintosh developed his three-layer waterproof fabric and, in North America, Chaffee invented his rubber mill and calender, the designs of which are basically the same as those in use today. Chaffee also founded the Roxburgh Rubber Co., the first American rubber company. In 1839 Goodyear discovered that heating a mix of rubber, white lead and sulphur resulted in a highly elastic material which was rubber 'cured' of its problems. It no longer went brittle in the cold and soft in the heat - nor did it seem to putrefy so easily. Thus the process of heating rubber with sulphur became known as the curing process. In the UK, Hancock acquired of some of Goodyear's cured rubber and, identifying sulphur as the 'magic' ingredient, developed a curing process which he patented ten weeks before Goodyear. The name vulcanization was coined by a friend of Hancock's - a Mr Brockedon and fell into popular use. In 1857 Thomas Hancock published his classic guide to the UK rubber industry and his illustrations give some idea of the breadth of uses to which rubber was being put. Not many are missing from a list of today since they include airproof products, hoses and tyres, nautical, domestic and travel equipment as well as a range of seals, washers and medical devices. As early as 1791 the idea of transplanting the South American rubber tree to more convenient (and politically more acceptable) locations was proposed by James Anderson but it was not until the 187Os that Sir Joseph Dalton Hooker brought the concept to fulfilment. Around 1870 Sir Clements Markham was feeling very pleased with his new knighthood, bestowed on him by Queen Victoria for having 're-located' the cinchona (quinine) tree to India, and was looking for new ideas. The idea of repeating the process with the Hevea tree seemed appealing and, through his contacts with the Cabinet, the Consul in Para was asked to obtain some Hevea seeds. In 1873 the first (2000) seeds came to England but only 12 germinated and these died either at Kew or in India. Hooker then suggested that a planter he knew be commissioned to collect some seeds. Thus Henry Wickham arrived on the scene. The story of how Henry Wickham brought his famous Hevea seeds out of South America to Kew Gardens and thence to Ceylon (Sri Lanka) and on to Malay(si)a has been told many times - mostly by Wickham himself with more and more added refinements until his death in 1928 but even the earlier versions seem to owe more to poetic licence than

fact if one judges by his wife's diaries and other contemporaneous reports (Wolf and Wolf, 1936; Dean, 1987). Interestingly, whilst Brazil continues to revile the name of Wickham for carrying out what was later to be called 'an exploit hardly defensible in international law', the country glorifies the names of Francisco Inocentcio de Souza Coutinho, who smuggled seeds of many spices from Cayenne to Para in 1797, and Francisco de MeIo Palheta who had been able to charm the wife of the French Governor into providing him with, amongst other forbidden fruit, seeds of that flavour of delight coffee - in 1727. There is, however, no doubt that some 70 000 seeds arrived from Brazil as a result of Wickham's exploits and that just 2397 germinated. In August 1876, 1919 of these were sent to Ceylon and 90% survived the journey to Colombo, arriving in September of that year. It was then discovered that no-one had arranged for the freight charges to be paid and only after furious correspondence the matter was finally settled. History does not relate how many survived but by 1880 there were only 320 of the original stock remaining in the plantation at Heneratgoda. It does relate that 100 were also sent to Singapore, again with no arrangement for freight charges to be paid, and that these all died. The importance of tracing these seeds and seedlings lies in the fact that in 1876 Markham also arranged for Robert Cross to travel to Brazil to 'back up' Wickham by shipping further Hevea stock to England. These were shipped mainly as seedlings and in the spring of 1887 it was recorded that only 26 had survived. By the end of 1877, Kew had distributed over 3000 seedlings, much more than their primary stock, so there must have been considerable propagation from cuttings and, within this set, a further 100 were sent to Ceylon - of which 22 were forwarded to Singapore. The planters noted that these were quite different from other Heveas they had seen and this led Henry Ridley, the Director of the Singapore Botanical Gardens and the man who, more than any other, could claim to have got the Malaysian rubber industry off (or into) the ground, to suggest that these were 'cross' plants and that 'it was from these 22 plants ... that three quarters of the cultivated plants of Hevea braziliensis have sprung'. The question remains: who should be called 'the father of the plantation rubber industry'? It must be appreciated that the story of natural rubber is not only that of Hevea braziliensis although, in the industrialized world, most other sources were of only passing importance. By far the most important in the closing years of the nineteenth century and first decade of the twentieth was the Congo vine. Before Stanley's epic three year journey from Zanzibar to the mouth of the Congo in 1877, the centre of Africa was a blank on any map.

However, the fact that he had made the journey, and the stories he had to tell, opened up the possibility of commercial exploitation of the Congo basin. Stanley first tried to interest the British but they had other things on their minds so he turned to Leopold II, King of the Belgians, who was quick to realise the potential profits to be made. Amongst the raw materials available for exploitation was rubber from the Congo vine and when it was explained to the natives that the Europeans wanted it and would pay for it, they could hardly believe their luck. However, it was not long before the proverb 'lootoji loo Ie iwa' (rubber is death) came into being. In 1887, 30 tons came down the Congo but by 1908 the total had reached 50 000 tons. Without the wild rubber of America and Africa the world of 1914 would have been a very different place. By 1914 the world's output of plantation rubber had equalled that of wild rubber and by 1918, plantation rubber was the only natural way forward. The story of wild rubber was essentially over. THE SYNTHETIC RUBBERS

The first phase in the search for a synthetic rubber was the fundamental scientific research in which natural rubber was broken down so that its structure could be determined, followed by the recombination of the monomer unit, or other low molecular weight materials with a similar chemical structure which could be obtained from commercially sensible sources, to give an elastomeric material. C.G. Williams decomposed rubber by pyrolysis as early as 1860 and identified 'spirit7, 'oil' and 'tar' - the 'spirit' or volatile substance he named isoprene and correctly gave its elemental composition as C5H8. In 1879 Bouchardat reported the recombination of isoprene to a rubbery material as did Wallach (1887) in Germany and Tilden (1892) in the UK, the last having correctly proposed the structure of isoprene as 2-methyl1,3-butadiene ten years earlier (1882) and having written (1884) of the possible industrial significance of polymerizing isoprene if it could be obtained from a more convenient source. Tilden used turpentine as the source of his isoprene and there is an interesting footnote to history in that a small container labelled 'Sir William Tilden's Rubber' recently came to light at Birmingham University. This was analysed using proton and 13C NMR spectroscopy at the laboratories of the Tun Abdul Razak Research Centre by C. D. Hull (1995) and unambiguously identified as poly-(2,3-dimethylbutadiene), not polyisoprene. This is difficult to reconcile with the information which Tilden gave in his presentation to the Birmingham Philosophical Society on May 18th 1892 but it may be that a number of experiments were set up and that this one, assumed to be with isoprene, actually contained 2,3-dimethy!butadiene. The extra interest here is that, although initial polymerization studies

were carried out using isoprene as 'feedstock', Kondakoff polymerized 2,3-dimethylbutadiene in 1900 to produce 'methyl rubber'. This became the first commercial rubber when it was produced by Hoffmann and Coutelle, working for Bayer, in 1909. The timing was propitious with the Great War on the horizon. Hoffmann deserves more than a passing mention because he was also involved in the invention of new accelerators and antidegradents which were essential to impart reasonable performance to the methyl rubber he was manufacturing. In 1912 the first synthetic car tyres were made of this elastomer for Professor Duisberg and these were followed with a set for Kaiser Wilhelm II. One, at least, of these is still in existence and was displayed at an exhibition 'Rubber, The Fascinating Material' which toured Europe during 1995-6. When the author tapped one of these tyres it was as hard as rock! Germany was obtaining natural rubber from America before that country entered the Great War but, from 1916, its problems became acute and production of 'methyl rubber' was recommenced with some 2.5 thousand tons being manufactured by the war's end. Russia was also active during this period with Lebedev polymerising 1,4-butadiene in 1910 and Ostromislensky taking out a patent on the synthesis of PVC and PVBr in 1912. In 1913 Ostromislensky published a book detailing a vast range of procedures for synthesizing different feedstocks. However, neither the Russian nor the American synthetic rubber industries were under the same pressures as Germany and, with the price of natural rubber low, there was little incentive for other than academic research. At this point mention should be made of the UK firm, Strange and Graham Ltd of London, which patented (Mathews and Strange, 1910) the use of sodium as the first chemical polymerization catalyst. It should not be imagined that the procedures used to polymerize the various dienes were similar to those in use today; there were many routes to polymerization affording nominally the same materials with, generally, very poor and unpredictable properties. They were also time consuming, reaction times being measured in weeks or even months! The situation changed drastically in 1922 when the Stephenson Reduction Plan, which cut production from the British controlled plantations to force up the price of the natural material, was introduced. Over the next three years there was a tenfold price rise followed by a catastrophic fall as producers outside the control of Britain flooded the market. It was this uncertainty which was a major catalyst for the next phase in the development of the synthetics. One of the first of these materials was far removed from the work of the preceding years in that it was prepared, by accident, by J.C. Patrick in the early 20s (although not patented until 1932) and was an ethylene

polysulphide - the first of the 'Thiokols' which are still in use as sealants today. Working independently in Switzerland, Baer (1926) produced a similar material on which IG Farbenindustrie based its Perdurens. In the States the thiokol rubbers were referred to as GR-P rubber. IG (which now included Bayer) resumed its research in 1925 and came on stream with Buna (polybutadiene rubber, BR) as well as two copolymers synthesized by mixing two different monomers together before the polymerization stage - Buna S (styrene butadiene copolymer, SBR or GR-S in America) and Buna-N (butadiene acrylonitrile copolymer, NBR or GR-A). These had reached laboratory production by 1930 but then there was a further hiatus as the bottom dropped out of the natural rubber market yet again. In 1933, when Hitler came to power, work restarted with a vengeance. One important feature of these new polymerizations was that they took place in an aqueous emulsion and were very much more efficient than the earlier gas phase reactions. Unfortunately, there seems to be no record as to whether the emulsion process was serendipity or was intended to mimic the biosynthesis of natural rubber. Given the political situation in Europe during this period it is, perhaps, ironic that IG and the Standard Oil Co. of New Jersey formed a joint study group with carefully designated areas of co-operation and privacy. At that time IG was making acetylene, its primary feedstock for elastomer synthesis, from calcium carbide (its private field) but in about 1930 it changed to natural gas and Standard was then entitled to an interest. Thus Standard held the US patents to all the Buna rubbers, a crucial factor in the development of the American synthetic rubber industry as the Second World War developed. A further valuable material to come out of the IG/Standard agreement was butyl rubber. Originally synthesized by IG as polyisobutylene it had no olefinic groups remaining after polymerization and therefore could not be vulcanized. Standard added a little butadiene and produced a vulcanizable product with a low level of residual unsaturation. At that time all of America's rubber development programme was privately funded and, when the Second World War started, indigenous American synthetic rubber production was in its infancy. In 1941 it was producing less than 1% of the country's consumption and of this some 227 metric tons was SBR. The first government-owned plant came on stream in mid 1942 and by 1945 the year's production exceeded 830 000 metric tons. Thus is the incentive of war and the availability of blank cheques! There was one other major elastomer which made its appearance during this period and that was polychloroprene. This originated in the academic work of Father Julius Nieuwland (1922) into the dimerization

of acetylene to form vinyl acetylene and when Du Pont de Nemours became aware of this work its significance was appreciated. The addition of hydrogen chloride across the acetylenic bond would produce 2-chloro-l,3-butadiene, a substance analogous to isoprene except that the side chain methyl group had been replaced by a chlorine atom. This was prepared by Carothers et al. (1931) and called chloroprene. It polymerized to give polychloroprene although this is often called by Du Font's trade name, initially Duprene and later Neoprene. Although being last in this 'between-the-wars' part of the history of the synthetics, it was the first real commercial synthetic rubber. Over the same period Russia also synthesized a polychloroprene - Sovprene. The American contribution to synthetic rubber production during the war paid for a vast amount of fundamental research as well as production technology but, when the war finished in 1945, the cycle of cheap natural rubber leading to diminished research completed another circle. However, in the early 1950s Ziegler and Natta revolutionized the industry with their new catalysts which enabled high cis 1,4-polybutadiene to be synthesized whilst novel organo-metallic catalysts also led to the synthesis of epichlorohydrin and propylene oxide. The third phase of production techniques had arrived. All of the elastomers mentioned so far have been either homopolymers, that is one monomer polymerized, or random copolymers but, when some structure is fed into this randomness, quite different properties can be obtained and this is the principle behind many of today's thermoplastic elastomers. In these materials there are soft 'rubbery7 regions to provide extensibility coupled with 'glassy' regions which serve as physical network junctions at their operating temperatures but become thermoplastic and thus mouldable (or remouldable) when they are heated (see Table 1.1). Their nomenclature gives an indication of their structure, thus polyisoprene, 'tipped' at both ends with polystyrene, is designated SIS. These have been available now for some 25 years and are taking an ever-increasing share of the elastomer market, recent figures suggesting about 20% of the non-tyre market. Other materials with similar properties are alloys of plastics and elastomers such as natural rubber and polypropylene. As with the synthetic rubbers, the range of these materials is vast and they have a number of books devoted solely to them. The interested reader is referred to, inter alia, publications by Legge, Holden and Schroeder (1987) or De and Bhowmick (1990). Although quite different from the classic concept of a vulcanized or crosslinked elastomer their requirements for analysis are similar to those of conventional vulcanizates and their particular differences will be highlighted throughout the analytical test procedures where relevant.

Of the many elastomers not covered by this historical introduction there is one class which must be mentioned as it is unique in containing no carbon - it is thus not even an organic material. This is the class of silicone rubbers which were introduced in 1944 (Hyde, 1944). SCOPE OF THE BOOK There are four comprehensive sources of analytical methods for rubber written in English: the publications of the International Organization for Standardization (ISO), British Standards Institution (BS), Comite Europeen de Normalisation (CEN) and the American Society for Testing and Materials (ASTM). Each provides standard methods for performing a range of analyses, the details given being precise and comprehensive, covering everything from the design of suitable apparatus and the quality of reagents to the manipulative details for each step. For many estimations alternative methods are given. As BS Standards are revised they are generally double referenced with both BS and ISO references and it should also be noted that where a BS and CEN Standard co-exist, the former must be withdrawn if there is conflict between the two. It is not proposed that this book should supersede the published works of the standardizing bodies but rather that it should supplement them and for reference a list of current ISO, BS, ASTM and DIN (Deutsches Institut Fur Normung. e.V.) documents relevant to the analysis of rubber and rubber-like materials is given in Appendix A. There are two ways in which supplementation is needed and should be useful. In the first place, the standard methods often give no indication of when they should be used or why one method is preferable to another. In the second place, there is no attempt to incorporate the discrete methods into an analytical scheme designed for this or that purpose. When an analyst is asked to investigate a faulty product, or to advise on suitable procedure for factory control, he or she needs a conspectus of available methods together with information illustrating their use, range and limitations. In short there is a need for a critical assessment of analytical practice in the field of the material in question, and it is for such a person that the present work is intended. In the following chapters an attempt is made to assess critically the tools and practice of analysis applied in the field of rubber and rubber-like materials. Although the major concern of this book is the identification and estimation of the components of the complex material of a manufactured product, this includes, of necessity, certain aspects of raw rubber analysis. Published standard methods are not in general repeated here and only where a method is not in a British or International Standards publication are procedural details fully set out.

At the time of publication of the first edition of this book, books devoted solely or even principally to the analysis of rubber and rubberlike materials had been rare although most textbooks on rubber chemistry and, more recently, on high polymers, devote some space to the topic. The first textbook of analysis was either that of Ditmar or that of Pontio, since both were published in 1909. Die Analyse Des Kautschuks der Guttapercha Balata und ihrer Zusatze is the title of Ditmar's work. It contains much discussion on the theory of the constitution of rubber, the preparation of chemical derivatives such as brominated rubber, and the alleged structures of these. Pontio's Analyse du Caoutchouc et de Ia Gutta-Percha is altogether lighter but nevertheless contains the essential processes for the examination of rubber from various botanical sources as well as alternative methods for analysing vulcanized rubber. The first work in English seems to have been that of Caspari (1914) which, in spite of its title, India-Rubber Laboratory Practice, was concerned almost exclusively with analysis. Of course, analysis had been dealt with extensively in Weber's much earlier book The Chemistry of Rubber (1902), and the popularity of this work may have accounted for the lack of a book specifically on the subject. Tuttle followed in 1922 with the first American book, The Analysis of Rubber, and after this there was a gap until the Ministry of Supply published, at first for limited circulation only, its Users' Memorandum No U.9, Identification and Estimation of Natural and Synthetic Rubbers, in 1944, and a revised edition in 1946. This was actually a pamphlet rather than a book, and the first real textbook of analysis dealing with synthetic as well as natural rubber was that of H. E. Frey, Methoden zur Chemischen Analyse von Gummimischungen published by Springer in 1953. Roff in 1956 dealt extensively with analytical matters in his reference book Fibres, Plastics and Rubbers which has the advantage of giving the salient features in a concise form and setting them out in relation to other high polymers covering a greater range of properties than are dealt with here. The journals Analytical Chemistry and Rubber Chemistry and Technology have published critical reviews in the field, such as Analysis, Composition and Structure of Rubber and Rubber Products (Tyler, 1967). Full textbooks of methods and critical discussions have also been published in the United States by Try on and Horowitz (1963), Tyler and Try on (1963) and the very extensive study in three volumes edited by Kline (1959, 1962). In England, the publication by Haslam and Willis (1965) entitled Identification and Analysis of Plastics, now in its second edition with Squirrell as co-author (1972), includes many data on rubbers as well as plastics. The two atlases, Infra Red Analysis of Polymers, Resins and Additives Volumes I and II by Hummel and Scholl (1969, 1973), revised in three volumes. Atlas of Polymer and Plastics Analysis (Hummel, 1981a, b;

Scholl, 1981), provide many thousands of reference spectra as well as much practical analytical advice. The publication of new books in this field has been limited; for instance the Handbook of Analysis of Synthetic Polymers and Plastics, by Urbanski et al. appeared in 1977 but it is a reprint of a Polish publication of 1972. More recently the trend has been towards producing books of conference papers, which lack specificity and tend to be a mixture of literature surveys, promotional literature and speculative research, or volumes such as Applied Polymer Analysis and Characterization, VoI II, 1991, edited by Mitchell, which describe a wide range of technical advances but leave one searching for their applicability in the 'real', rather than 'research', world. Although the first edition of this work could claim priority in its field this was not the case for the second, third, or this, the fourth edition. However, the justification for the second and third editions still holds true; no other work seems to deal with the problems of the general analyst or technologist, nor do other books discuss the significance of each individual analysis in the total concept of the vulcanizate formulation, the relevance of state of cure or of blooming, or the analysis of degraded materials to provide data on the reason for, or mechanism of, degradation. The opportunity has been taken to continue to expand details of modern instrumental techniques but it remains a fact that many rubber industry laboratories and factories will not have these facilities and thus some pre-instrumental methods are still covered providing as wide a range as possible for each type of analysis. The increasing pressures of 'Health and Safety' legislation, however, inevitably mean that a number of useful experiments have had to be deleted. The practising technologist, or rubber-chemist, who provides an analytical service will soon find that as well as analysing vulcanizates, he or she will be asked to study thermoplastics, compounds prior to vulcanization, raw rubbers and possibly latex. At each stage throughout this edition variations in experimental technique which will broaden the scope of the analytical procedure are described and discussed. THE ANALYTICAL PROBLEM A rubber vulcanizate, or rubber-like product, can be considered to consist of five major classes of materials: 1. 2. 3. 4. 5.

polymers plasticizers/oils solid fillers ancillary chemicals and their residues adventitious materials

and herein lies the paradox since this is the breakdown which is often required but it is the very breakdown which the analyst cannot directly obtain. Polymers contain extractable materials which appear in (3) whilst carbon black can contain up to 1% sulphur as well as 5% other components. Polymers may be used as dust-free carriers for curatives whilst inorganic powders can be used to carry organic curatives such as peroxides. Some inorganics, such as whiting, decompose during thermal analysis and none of these is classically 'pure'. Oils and plasticizers may be metered accurately into a mix but suffer a degree of loss due to leakage in the mixer whilst many protective additives are complex mixtures, components of which may react differently during cure and ageing. A formulation analysis therefore usually consists of a set of analytical data followed by inspired interpretation. The more information there is, the more closely will the derived formulation reflect the true composition. The qualitative and quantitative separation and identification of any, or all, of these chemicals can be a complex and time-consuming process and it is thus important to consider the purpose for which an analysis is required, what degree of accuracy is needed, and which of a variety of available methods, if any, will enable it to be achieved. It is worth remembering that Parkinson's Law applies as much to the analytical laboratory as elsewhere, and here it may be stated: 'Whenever new equipment, techniques or automation are introduced, demand will increase to fully occupy the equipment available'. Nothing is as cost effective as a sceptical approach to the question of the need for a particular analysis. Analysis of rubbers or rubber-like materials in a commercial consultancy tend to fall in one or more of the following categories: 1. complete analysis of a competitor's product; 2. partial analysis, e.g. fillers only, or nature and percentage of polymer, under similar circumstances; 3. reasonably complete analysis of representative samples purchased to a defined specification; 4. specific analysis, e.g. type and level of antioxidant (problems often linked to environmental or toxicological concerns); 5. analysis as a means of checking product behaviour, e.g. pH of aqueous extract of a gasket intended for use in contact with metal; 6. analysis of deteriorated or faulty products to determine, if possible, the cause; 7. analysis to detect factory errors. The reasons for desiring to know the exact make-up of a competing product may or may not be regarded as an ethical problem but this has but little bearing on the analytical problem. One point which should

always be borne in mind is that the cost of identifying and quantifying every component in a product is exceedingly high in both work hours and range of equipment required. A selective approach to the depth of analysis coupled with an input from an experienced rubber technologist will generally provide the most cost-effective route to a formulation equivalent to or better than the one being investigated. Sometimes it is an interest in the cost of materials which prompts analysis, and then differentiation between various antioxidants or stabilizers would probably be unnecessary since the cost difference, if one were substituted for another, would be insignificant relative to small bulk ingredient changes. In general, analysis for costing purposes only requires the identification and estimation of the polymer and bulk fillers. The routine examination of a certain percentage of products, purchased to a specification which lays down their composition, is rarely carried out nowadays in areas of general rubber goods or engineering products although it is common in areas of medical or pharmaceutical products. Many organizations regard a specification as laying down the performance required, leaving the manufacturer to achieve this in his or her own way. This largely abolishes the need for extensive analysis and substitutes the easier and cheaper methods of physical testing, before and after accelerated ageing if this is required. This can, however, cause problems for the analyst if he or she is asked by a user to comment on the reasons for a particular product's failure to meet its required performance specification. With no knowledge of the polymer, fillers or other chemicals present, a complete analysis will be necessary in order to see whether or not it would be expected to meet the specification, even if correctly mixed and cured, before considering possible errors in manufacture. It might also cause the organization problems if it is multi-sourcing components and nominally identical products in one application have different compositions. On the other hand, certain contracts contain a clause requiring disclosure of the materials of manufacture and some of the reasons for this are not sufficiently appreciated. Where the rubber or rubber-like material is in contact with complex materials such as explosives, living tissue, food or medical supplies, the manufacturer cannot be expected to foresee all possible effects of the chemicals incorporated into the finished article which he or she supplies. Even the user may not have sufficient knowledge of which ingredients are, or are not, acceptable. Disclosure by the manufacturer allows consideration of the materials by third parties with a wider field of knowledge but disclosure without the possibility that subsequent departures from the disclosed formula will be detected offers no safeguard. Where health or safety is at stake, analyses may be necessary on every batch but disclosure considerably

lightens the analyst's task, since the analysis can be designed around the known formula with the omission of many steps that would be essential were the product unknown. Care must be taken, however, that in designing one particular analytical sequence it does not become so specific that it excludes the observation of extra-specification materials, i.e. whilst designing an analytical protocol to make sure that one particular antioxidant is present, the protocol must be broad enough to confirm that others are absent. Further reasons for the analysis of rubber and rubber-like materials are those of examining deteriorated or faulty products and here it should be remembered that there is often a long chain between custom compounder, component manufacturer, trade component user (one or more) and final retail product purchaser. At any stage of the manufacturing or assembling processes the rubber component may be rejected or the product may be returned after use or misuse. In any event it will be necessary to carry out an investigation to a greater or lesser extent and, almost inevitably, in the early stages of such an investigation the faulty article will be examined by analysis. It may be, for example, that a bloom has developed in the warehouse; the analyst is consulted on the nature of the bloom and once this is unambiguously known the problem is usually more than half solved. The improvement in physical testing of materials has led manufacturers increasingly to depend upon physical properties as a criterion of correct manufacture, but advances in instrumental techniques over recent years should make each factory manager consider whether any particular one could be of use to him or her in his or her search for quality. Typically, a vulcanized product can be analysed non-destructively for sulphur content in under two minutes whilst an 'oil', polymer, black, inorganic filler analysis can be obtained on a few milligrams of sample in under ten minutes. Perhaps more importantly still, batches of uncured compound can be checked and adjusted if necessary before cure, thus preventing wastage and reducing product variability. Even if absolute identifications are not carried out, compositional profiles and accepted deviations can be defined and mixtures 'flagged' if they fall outside permitted ranges. COMPOSITIONAL CATEGORIES POLYMERS

The elastomeric phase of a rubber product is just one of the categories which has been defined and even this expands beyond just polymer identification when one realizes that several polymers could have been blended together to optimize a particular property, or to cheapen a

compound without damaging its properties sufficiently to put it out of specification. Historically there is no one reference which introduces the concept of rubber blends to the manufacturing industry but it probably occurred within days of the first synthetic elastomers being prepared. The precise ways in which the various polymers intermix if they are blends, or their structure if they are copolymers (block or random), can also critically affect the final performance of the product. One must also consider the level of polymer in the product, the blend ratio if more than one polymer is present, whether the polymers are vulcanized or not - thermoplastic or thermoset - and the morphology of the system. PLASTICIZERS AND OILS

The level of complexity of these materials is close to that of the polymers with a wide range of materials being documented, each of which may be uniquely selected to impart specific properties to a particular elastomeric product and, as with elastomers, blends are often used to improve further or refine product properties. These materials have different solubility properties in different solvents and the wrong choice can lead to incorrect raw data from which erroneous conclusions will be drawn. SOLID FILLERS

In addition to blending and plasticizing the polymers, it is frequently desirable to incorporate powders into the materials to increase their bulk, alter their density, reduce their resilience, cheapen their cost, or to modify some special property. This practice certainly extends back to the beginning of the nineteenth century and probably back to the Aztecs. The powders are incorporated before crosslinking and are dispersed in the polymer, which provides the continuous matrix. The bulk filler may consist of a single material or may be a mixture of several components and an error in determining the total filler loading can arise from the nature of the fillers themselves. Thus, precipitated calcium carbonate may contain up to 5% of stearic acid and, since calcium stearate is soluble, the material remaining will differ from that originally added to the polymer by amounts up to 5.5%. As a further complication, 'rubber grade' stearic acid is only some 40% stearic, 57% palmitic and 2% myristic acid so an appropriate analysis must be used or the limitations of the chosen one realised. Some clays used in rubber and PVC compounds contain added organic materials, 2-3% of which may be extractable, leading to analytical figures which differ from those the compounder would claim. In describing the analysis of fillers, a distinction is made between

carbon black and inorganic fillers as the identification of the former requires completely different techniques and there are different factors which are important. ANCILLARY CHEMICALS AND THEIR RESIDUES

In the case of rubber vulcanizates, the formulation complexity does not end with the major 1^uIk' components because the final crosslinking stage is rarely carried out by sulphur alone. Accelerators are added to both speed up and 'fine tune' the chemistry of the rubber-sulphur reaction, zinc oxide to 'activate' the accelerator, and some fatty acid, usually 'stearic' acid, to assist in the activation. These materials first appeared at the dawn of the synthetic era with the dithiocarbamates being invented by Bruni (1919), mercaptobenzthiazole (MBT) by Bruni and Romani (1921), diphenylguanidine (DPG) by Weiss (1922) and mercaptobenzthiazole disulphide (MBTS) by Sebrell and Boord (1923). These earliest materials are still the materials of choice in many applications today. It will be appreciated that both rubber vulcanizates and rubber-like materials, natural or synthetic, are organic in nature, and age in the presence of air. This ageing is partially counteracted or deferred (but never prevented) by small amounts of stabilizing agents which may be present in natural materials or added during the manufacture of synthetic materials. Even so, more of these materials are usually added when mixing the polymer with the other ingredients. Rubbers with some degree of unsaturation are stabilized with antioxidants or antiozonants whilst, with PVC, a metal oxide may be added to protect against loss of hydrogen chloride. For over a century wax has been appreciated as an inert coating which will prevent oxygen coming in contact with a substrate of rubber (Schidrowitz and Dawson, 1952) but, even today, this may be wiped off a product as being unsightly, thus negating its whole purpose. Aminebased antioxidants were first used at the turn of the century but it took until the 1950s for the non-staining phenolic antioxidants to make their presence felt. Most of today's protective materials are developments of these two categories and the developments continue as ever greater service demands are placed on modern elastomers. ADVENTITIOUS 3VtATERIALS

This category would normally include dirt contamination, present in either the polymer or compounding ingredients, together with protein and other insoluble non-rubbers from natural rubber, or catalyst residues from synthetic polymers. The analysis of any of these could be

significant in terms of both problem solving and polymer identification. Many of these adventitious materials are the subject of environmental or health and safety-related controls, examples being nitrosamines derived from dithiocarbamate curatives or the trace metals covered by regulations such as EN71.3. It should also be appreciated that adventitious materials can be generated during the manufacturing process, thus thiurams will form dithiocarbamates and these, in turn, will lead to N-nitrosamines, the levels of which are restricted in a range of products. A product made from rubber, or a rubber-like material, can thus be considered to be a mechanical mixture of polymer(s), plasticizer(s) or extending oil(s) and inert powders (or fillers as they are usually called) together with a number of other ingredients which may be regarded as being dissolved or suspended in the polymer. The temperature at which the mixing and vulcanization steps are carried out, coupled with the presence of a range of reactive species, cause changes in the composition of these 'other ingredients' so that they frequently no longer exist in the form in which they were added to the vulcanizate and it will therefore be necessary to identify the products derived from them to deduce their original presence. No discussion of rubber analysis is complete without intelligent anticipation of the errors expected, and their significance in the interpretation of the results. In some areas of chemical analysis it is quite possible, and reasonable, to quote percentages to two places of decimals, with equivalent implied precision for those components present at much lower levels. In the field of rubber analysis these levels of accuracy are neither sought nor, usually, attained and typically one would expect an accuracy of no better than 1-2% of the measured value. The meaning of the term 'accuracy' is discussed at length in the final chapter of this book but it should be borne in mind at this stage that there is little to be gained by analysing components to a much greater accuracy than that with which they were added to the mix, whilst arguably the accuracy need only be sufficient to indicate technologically significant variations from the norm. It should also be remembered that virtually none of the materials used in the rubber industry could be considered 'pure' as one would normally define the term and thus, however accurate the analysis itself is, it will not enable a more accurate estimation of the actual added material to be made. REFERENCES d'Anghiera, P.M. (1530) De Orbe Nouo, Compluti (now Alcala) folio xxxv. Auleytner, J. (1953) Bulletin de I'Academie Polonaise des Sciences, 1, 5, 189.

Baer, J. (1926) BP 279,406. Bouchardat, G. (1879) Compte rend. 89, 1117. Bruni, G. (1919) DRP 380774. Bruni, G. and Romani, E. (1921) Ind. Rubb. J. 62, 18. Carothers, W.H., Williams, L, Collins, A.M. and Kirby, J.E . (1931) /. Amer. Chem. Soc. 53, 4203. Caspari, W. A. (1914) India-Rubber Laboratory Practice, Macmillan, London, de Chasseloup Laubart, F. (1942) Francois Fresneau Pere de Caoutchouc, Paris. Ia Condamine, C. M. (1755) Sur une Resine elastique nouvellement decouverte par M. Fresneau, in Histoire et Memoires de I'Academic pour I'annee 1751, 319 Paris. De, S.K. and Bhowmick, A.K. (eds) (1990) Thermoplastic Elastomers from RubberPlastic Blends, Ellis Horwood, London. Dean, W. (1987) Brazil and the Struggle for Rubber, Cambridge University Press, Cambridge. Ditmar, R. (1909) Die Analyse des Kautschuks der Guttapercha Balata und ihrer Zusatze, Hartleben, Vienna and Leipzig. Fisher, H.L. (1939) Ind. Eng. Chem. 31, 941. Frey, H.E. (1953) Methoden zur Chemischen Analyse von Gummimischungen, Springer Verlag, Berlin. Fusee-Aublet, J.B.C. (1755) Histoire des Plantes de Ia Guiane Frangaise, London and Paris, 2, 871. Hancock, T. (1857) The Origin and Progress of the CAOUTCHOUC or India-rubber Manufacture in England, London. Haslam, J. and Willis, H.A. (1965) Identification and Analysis of Plastics, Iliffe, London. Haslam, J., Willis, H.A. and Squirrell, D.C.M. (1972) Identification and Analysis of Plastics, 2nd edn., Iliffe, London, de Herrera Tordesillas, A. (1601) Historia General de los Hechos de los Castillanos, 1, 231, Madrid. Hoffman, F. and Coutelle, C. (1909) GP 260690. Holguin, D.G. (1608) Vocabulario de Ia Lengua General de todo el Peru llamada Lengua Quichua, o del Inca, Ciudad de los Reyes (Lima). HuU, C.D. (1995) TARRC internal report reference D576. Hummel, D.O. (198Ia) Atlas of Polymer and Plastics Analysis, Volume I, Polymers, Structures and Spectra, Carl Hanser Verlag, Munich. Hummel, D.O. (198Ib) Atlas of Polymer and Plastics Analysis, Volume II, Plastics, Fibres, Rubbers, Resins, Carl Hanser Verlag, Munich. Hummel, D.O. and Scholl. F.K. (1969) Infra Red Analysis of Polymers, Resins and Additives, an Atlas: Volume I, Plastics, Elastomers, Fibres and Resins, Carl Hanser Verlag, Munich. Hummel, D.O. and Scholl, F.K. (1973) Infra Red Analysis of Polymers, Resins and Additives, an Atlas: Volume II, Additives and Processing Aids, Carl Hanser Verlag, Munich. Hyde, J.F. (1944) BP 561136/561226. Kline, G.M. (1959) Analytical Chemistry of Polymers I, Interscience, New York. Idem. II and III (1962). Kondakoff, I, (1900) /. Prakt. Chem. 62, 172. Lebedev, S.V. (1910) /. Russ. Phys. Chem. Soc. 42, 949. Legge, N.R., Holden, G. and Schroeder, H.E. (eds) (1987) Thermoplastic Elastomers. A Comprehensive Review, Hanser Publishers, Munich.

Mathews, F.E. and Strange, E.H. (1910) EP 24790. Ministry of Supply (1944) Identification and Estimation of Natural and Synthetic Rubbers, Users' Memorandum U.9, London, and (1946) Users' Memorandum U.9A, London. Mitchell, J. (ed.) (1991) Applied Polymer Analysis and Characterization, Vol. 2, Hanser, Munich. Miiller (1865-6) Linnoea, Vol. xxxiv. Nieuwland, J.A. (1922) Science 56, 486. Ostromislensky, I. (1912) GP 264123. Ostromislensky, I. (1913) Caoutchouc and its Analogues, Moscow, de Oviedo y Valdes, G.F. (1535) Historia natural y general de las Indias, Seville. Patrick, J.C. (1932) USP 1,890,191. Persoon, C.H. (1807) Synopsis Planarium sive Encheiridicum, Paris, 2, 588. Pontio, M. (1909) Analyse du Caoutchouc et de Ia Gutta-Percha, Gauthier-Villars, Paris. Roff, WJ. (1956) Fibres, Plastics and Rubbers, Butterworth, London. Schidrowitz, P. and Dawson, T.R. (eds) (1952) History of the Rubber Industry, Heffer and Sons, Cambridge. Scholl, F.K. (1981) Atlas of Polymer and Plastics Analysis, Volume III, Additives and Processing Aids, Carl Hanser Verlag, Munich. Sebrell, L.B. and Boord, C.E. (1923) Am. Soc. 45, 2390. Stuart, G.E. (1993) National Geographic, November 1993, 101. Tilden, W.A. (1882) Chem. News 46, 120. Tilden, W.A. (1884) /. Chem. Soc. 47, 411. Tilden, W.A. (1892) Chem. News 65, 265. Torquemada, J. (1615) Monarchia Indiana 2, 664, Seville. Try on, M. and Horowitz, E. (1963) Methods for the analysis of rubber and related products, in Handbook of Analytical Chemistry, Meites, L. (ed.), McGraw-Hill, New York. Turtle, J.B. (1922) The Analysis of Rubber, Chemical Catalog Co., New York. Tyler, W.P (1967) Rubber Chem. Technol 40, 238. Tyler, W.P. and Tryon, M. (1963) in Standard Methods of Chemical Analysis, 6th edn, Welcher, FJ. (ed.), 2B, 43, Van Nostrand, Princeton. Urbanski, J., Czerwinski, N., Janicka, K., Majewska, F. and Zowall, H. (1977) Handbook of Analysis of Synthetic Polymers and Plastics, Halsted Press, New York. Wallach, O. (1887) Annalen 238, 88. Weber, C.O. (1902) The Chemistry of Rubber, Griffin, London. Weiss, M.L. (1922) USP 1411231. Wolf, H. and Wolf, R. (1936) Rubber, Covici Friede, New York.

Sampling

and sample

preparation

r\ tL

It is essential that the material actually analysed either be representative of the material available, or be that most appropriate for solving the particular problem presented. International and British Standards distinguish between the sample (that which one is given to analyse) and the test portion (that which one separates from the sample and uses entire for a given investigation). Using this differentiation, our discussion centres upon the choice of a test portion and its subsequent treatment so that the maximum relevant information is obtained. Procedures for the sampling of both natural and synthetic latices are fully detailed in ISO 123-1985 whilst for raw rubber ISO 1795-1992 should be consulted. It is not intended to cover these here as they are extremely detailed and specific. Our more immediate concern is to indicate the problems confronting an analyst when examining a compounded, thermoplastic or thermoset elastomer. In mixing and processing rubber and rubber-like polymers, powders are added and form a disperse phase in a matrix or continuous phase of polymer. The degree of dispersion may vary considerably both over short distances and long distances. The analyst is usually given a sample on which an analysis is needed and before taking the test portion required for a given determination he or she ensures that the test portion is appropriate for that particular analysis. ANALYSIS OF AVERAGE COMPOSITION

In those cases where one or more aspects of the overall formulation is or are to be determined, it is necessary that the test portion is taken from a large enough volume to ensure that inhomogeneities arising either from mixing or from peculiarities of the particular manufacturing process can be averaged out by homogenization prior to the relevant

analysis. The following examples, though not exhaustive, are illustrative of the general principles to be observed. THIN CALENDERED SHEETS OR PROOFINGS

Economic considerations will usually prevent sampling from the centre of the length of a roll but the extreme ends should be avoided and samples taken near to both ends rather than at one end only. The samples should be from the entire width, preferably cut diagonally. Whether separate test portions are cut from a sample from each end or, alternatively, whether the samples from the two ends are blended and homogenized before taking the test portion, will depend on the nature of and reason for the analysis. DIPPED GOODS Many rubber products such as catheters, condoms and gloves are made by a dipping process and, particularly in the last instance, care should be taken in the choice of a piece for analysis. There is no doubt that cure residues and protective agents can, on occasion, become concentrated in the fingertips of gloves and, unless there is a specific reason for a different course of action, samples should be taken from the centre of the palm or the equivalent region on the back of the glove. All dipped products should be sampled with the possibility of dipping variability in mind. SMALL MOULDED ARTICLES

The quantity of material required for a particular set of analyses will largely determine the number of mouldings required. A sufficient number of mouldings to allow all necessary analyses to be carried out should be homogenized together before taking the test portion(s). LARGE MANUFACTURED ARTICLES

In this case the nature of, and reason for, the analysis will influence profoundly the procedure to be followed. Composite articles such as tyres must be sectioned and dismantled, the various components being separated and handled separately. Homogenization of the separated components will usually but not necessarily be carried out. RUBBERIZED FABRICS

Thick rubberized fabrics may sometimes be separated by cutting with a

razor blade, but in cases where this is not feasible it is often possible to separate rubber from fabric after swelling the rubber with vapours of a suitable solvent such as chloroform or dichloromethane. The rubber is freed of solvent by evaporation in air or vacuum at room temperature, and then homogenized. If rubber cannot be separated from the fabric, then the material must be analysed as a whole, after cutting into small pieces (ISO, to pass a 2mm sieve; ASTM, 1.5mm square). HOMOGENIZATION OF SAMPLE

Two methods are available for rubbers and rubber-like polymers: (i) the sample may be finely divided by cutting or grinding and the cuttings well mixed before taking the test portion, or (ii) the mixing may be carried out by passing through the tightly closed nip of a roll mill, the cutting of the test portion being delayed until after sheeting the sample. In some cases a piece of the homogenized sheet can itself form the test portion without the necessity for finely dividing by cutting. In all cases it is essential to ensure that any extraneous foreign matter is excluded from the sample prior to homogenization. International, British and American standardizing bodies prefer comminution of the material by passing through the cold, tightly closed rolls of a two-roll rubber-mill. If this machinery is available it is undoubtedly the best way to prepare the material for most analytical procedures and anyone regularly analysing rubber-like samples would be well advised to install one. The rolls need not be machine driven as perfectly satisfactory results can be obtained with a long handle on each roll and human effort to turn them. Failing this, a rotating rasp may be used but is not favoured. Rasping causes a considerable local temperature rise which can lead to chemical reactions such as 'maturing' processes, and reaction of any residual sulphur, whilst oxidation occurs with consequent increase in extractable material. Also, it is unsuitable for unvulcanized rubber and the rubber-like plastics. The obvious alternative, grinding or buffing, is not acceptable since the powder obtained will be oxidized and contaminated with material from the grinding wheel. Cutting with scissors or knife (razor blade) is laborious but is essential if a mill is not available. The International Standard ISO 4661 Part II, 1987, allows cutting and specifies that material 'shall be comminuted to pass a 1.7mm aperture sieve'. The ASTM Standard on rubber products, ASTM D 297-1993, also allows cutting but requires the sample to be rubbed or passed through a 14mesh sieve (this sieve has an opening of 1.4mm). Both specifications require the sheeting, if this is the method of preparation used, to be to 0.5 mm or less in thickness.

ANALYSIS OF LOCALIZED COMPOSITION

There are many occasions when homogenization of a sample destroys the very features which are important in a particular investigation. The situations where homogenization is inappropriate are too varied for a comprehensive discussion to be presented; nevertheless, the following examples are illustrative and highlight the need for closely defining the analytical problem and designing both the sampling and the analysis procedures appropriately. VULCANIZATION STATE OF THICK ARTICLES

An example where homogenization might not be desirable is the determination of the 'free' sulphur content of a truck tyre. Assuming that this is required because inadequate vulcanization is suspected, it would be reasonable to take the test portion only from the inner face of the tread rubber rather than from a homogenized cross-section of the tread. Similarly with large blocks of rubber for mounting engines or for bridge bearings, where the state of cure may well vary with the distance from the surface, free sulphur determinations carried out on test portions taken separately from the centre and outer parts of the block would be more informative than those carried out on a test portion taken from a homogenized cross-section of the block. ANALYSIS OF BLOOMS (SEE CHAPTER 13)

Where blooms form on the surface of a rubber mix or rubber article, it is clearly inappropriate to homogenize the bulk material prior to identification of the bloom. BOND FAILURE PROBLEMS

Bonds between rubber and metal are sensitive to the state of cure of the rubber. It is, however, the state of cure in the immediate vicinity of the metal which is important, and so the test portion must be taken from this area rather than from a homogenized cross-section. INHOMOGENEITY AND POOR DISPERSION

These can cause a variety of problems, such as variable physical properties, article-to-article variation, unevenness of colour etc. Such problems can be investigated by the reverse of the normal procedure. By cutting down on the size of the test portion, and with no homogenization stage, an idea may be obtained of the degree of inhomogeneity existing,

provided that the analytical technique employed is sufficiently sensitive to cope with the small sample size. PHASE MORPHOLOGY WITHIN A BLEND (SEE CHAPTER 9)

In this area, an awareness of the artefacts which are inevitably introduced during the manufacturing process is a prerequisite to selecting an appropriate volume to sample. For example, injection moulded test plaques are subject to high levels of flow orientation but these are at a minimum level at a point roughly a quarter of the way up the plaque directly opposite the tab, so blocks for sectioning should be removed from this region if the bulk morphology is the major concern but from other well defined areas if it is the orientation, or flow effect, which is being studied. Likewise, in many commercial products, edge, surface and bulk orientation effects are likely to be present and may make the selection of a genuinely artefact-free volume difficult. In a series of similar samples, once a sampling point has been established, it should be adhered to for the whole series. Finally, to judge how representative a thin section is of the bulk it is often more appropriate to check the entire length of one section, where the regions will be separated from another by lmm or more, than to check one section against the next where the separation between the two will only be 150 nm or so. SIZE OF TEST PORTION The size of the test portion must be chosen with several factors borne in mind. 1. It must be sufficiently large to allow the carrying out of all the analytical techniques which might be required. 2. It must be sufficiently large to give adequate sensitivity for each technique being employed. 3. It must be sufficiently large to average out any irrelevant inhomogeneities. 4. It must be within reasonable limits such that handling during subsequent analysis is not an insuperable problem. 5. It must be sufficiently small that reagent volumes and apparatus are not unpractically large. 6. It must be sufficiently small that relevant inhomogeneities of adventitious contaminants are not swamped out. SAMPLE PREPARATION Although International and other standards organizations define the material actually being analysed as the test portion, the general analyst,

and indeed the scientist, in the English-speaking world, uses the word sample to refer to the material he is actually analysing. Indeed this is implicit both in the heading of this section, and in the heading of the corresponding parts of International Standards, which also use the word sample. In conformity with common convention, therefore, the word sample is used from this point onwards throughout the book to refer to that piece of material which the analyst is actually using. The sample may be one section of the test portion, taken to carry out one of a series of interrelated analyses, or it may be a discrete micro-portion on which one specific analysis will be carried out, and which will not necessarily be representative either of the bulk material or of another micro-sample. Having selected a test portion which is most appropriate for a given analytical problem, the analyst must then decide on the most appropriate procedure for preparation of the sample to be analysed. The exception to this general nomenclature is in terms of microscopical analysis in which specimen is often used to denote that portion of the sample that has been prepared or is undergoing preparation (often by a lengthy procedure) for examination. In general, sample preparation techniques other than the initial homogenization procedure are specific to the analytical technique being employed. An exception to this is solvent extraction, partial or exhaustive, which is considered in some detail in the next chapter. Other preparation techniques range in complexity from cutting into thin strips with scissors, through hot pressing, microtoming, or ashing, to sophisticated total or selective degradative procedures used primarily for infrared or nuclear magnetic resonance spectroscopic investigation. These preparation techniques are considered during the discussions of the particular analytical technique, in subsequent chapters.

Extraction

O

PRELIMINARY REMARKS Although the concept of extraction is thoroughly understood by most analysts, its applications to the analysis of rubbers and rubber-like materials are diverse and complicated. As a first step it is advisable to differentiate between extraction, solution and dissolution. Extraction is here defined as the procedure for removing organic additives from the polymer/black/inorganic components without simultaneously removing significant amounts of the polymeric phase, whilst solution and dissolution involve the removal of polymer from the remaining components. It must, however, be borne in mind that most polymers, even in the uncompounded state, contain non-rubbers which will be extracted by these techniques, and in any quantitative extraction due correction must be made for them. In general those extracted organic materials are of low molar mass, but they may include polymeric plasticizers, factice and mineral rubber, more realistically considered as plasticizers than polymers. Extractions do not necessarily require solvents: useful information may be provided by a thermal extraction whilst extraction using a solvent may be carried out in the cold, or heated, for periods of time ranging from seconds to days, and be either quantitative, qualitative, or selective depending upon the exact nature of the experiment. It would be realistic to say that in the vast majority of cases the purpose of an 'extraction' is to use an appropriate solvent to provide essentially complete separation of the extractable materials from the bulk matrix so that each can be examined without interference from the other; for this reason the classic theory of extraction merits detailed consideration. The choice of an 'appropriate solvent' is a potential difficulty. Until a completely extracted sample is available identification of an unknown polymer may not be possible but, paradoxically, until the polymer has been identified, one does not know the correct solvent to

use for extraction. This difficulty is more apparent than real since the analyst will usually obtain some information from the appearance of the sample, its use, colour or smell. If a wrong solvent is mistakenly used for extraction, the fault will be detected and remedial action taken. All the solvents commonly employed for hot extractions and certainly those recommended in these pages, will usually extract all extractable material; where they fail is in extracting some polymer as well as the non-polymeric material. In such a case, the polymer is available for identification but the extract will be too great and may mislead the analyst as to its composition and nature. As soon as the polymer is identified, however, the analyst will realize his or her mistake and will take steps to correct it if information on its amount or composition is required. Natural rubber, being the oldest of the class of materials we are considering, serves as the prototype for extraction procedures. Henriques (1892) introduced extraction with alcoholic 'potash7 to remove factice and an abstract by Weber in 1894 records the extraction of 'asphaltum' by cold nitrobenzene. Henriques also used carbon disulphide in 1894 to extract vulcanized rubber, and Holde, about the same time, used ether-alcohol mixtures. Acetone seems to have been used at the turn of the century and Weber (1902) argues in favour of its use, while Caspari (1914) records it as the standard extractant. Acetone is very suitable for the extraction of natural rubber but is not the best solvent for use with all synthetic rubbers. It is not suitable for extracting unvulcanized synthetic rubbers or thermoplastic materials such as polyvinylchloride; these remarks are amplified later in this chapter. NATURE OF THE EXTRACTION PROCESS

The extraction of soluble substances from a rubber by a solvent utilizing a continuous extraction process as described later in this chapter is a diffusion controlled process. In the case of a vulcanized rubber, the substance on which most extractions are likely to be carried out, the rubber acts as a semi-permeable membrane. Some time after the start of the extraction, the rubber is swollen to its maximum extent by imbibition of the extracting liquid which forms a relatively concentrated solution, inside the rubber, of the substances to be extracted. The solvent outside the rubber is continually renewed so the concentration outside the rubber is virtually zero and diffusion of the soluble substances follows the direction of the concentration gradient. The rubber acts as a semi-permeable membrane by reason of its crosslinked nature, giving a mesh the size of which limits absolutely the size of the molecules which can diffuse out. Since the

process is taking place in a (relatively) non-ionizing solvent with largely neutral molecules and the mesh itself is non-ionizing, many of the considerations normally important with semi-permeable membranes can be ignored. Instead of a single mesh size there will be, of course, a size-range depending on the distribution of the crosslinks forming the structure, and thermal agitation will lead to variation in the space through which a molecule could move. However, the occurrence of the maximum space given by the fully extended chains forming the sides of any mesh will have a finite probability and therefore only molecules corresponding to this fully stretched mesh size, or smaller, will be extractable. For a soft vulcanized natural rubber this mesh will have sides of about 20 nm so that molecules whose smallest dimension, considered as a radius, exceeds 10 nm will be inextractable. The more the rubber is swollen by solvent, the more rapidly will extraction occur but the absolute limit to the size of molecules which can be extracted will not be affected. When the molecular size is near the limit of extraction, the effect of swelling on the rate of extraction can make all the difference between extraction in a few hours and extraction necessitating weeks. This may be illustrated by reference to bitumen. The higher molar mass portion of this can be extracted from vulcanized natural rubber in a few hours if chloroform is used but even after extraction for many hours with acetone, a constant weight will not be achieved. The fundamental law in the study of extraction processes as examples of diffusion phenomena is Pick's first law (Eq. 3.1). If dmg of the substance diffuses in time At across an area A under a concentration gradient dc/dx, then m=-DA.dc/dx.dt

(3.1)

where D is the diffusion coefficient, which is defined by Pick's law. The elimination of m from Pick's first law gives the general differential equation of diffusion sometimes known as Pick's second law (Eq. 3.2): dc/dt = Dd2c/dx2

(3.2)

The various solutions to this which can be obtained after the imposition of certain boundary conditions are discussed by Barrer (1941). The case with which we are concerned may be visualized as diffusion from a thin membrane of, say, 0.5mm thickness, the concentration of soluble material in the membrane being given by the ratio of the extractable solids to the solvent imbibed, and the concentration falling discontinuously to zero at the interface between membrane and liquid. This is, of course, an idealized approximation, but as the solvent surrounding the sample is continuously agitated by the arrival of freshly condensed solvent and is also completely drained at frequent intervals, it suffices

as a model. For this model, the amount of extractable material (w) left in the membrane at any time t is given by: m = 0.0405C0.£ l/(2n + l)2 exp[-(3.95 x 103)(2n + l)2 Dt] (3.3) n=0

In this equation C0 is the initial concentration of extractable material in the volume of the membrane, and provided consistent units are used for C0 and ra (grams or moles) the diffusion coefficient (D) is given in cm2 s-1. The magnitude of D is a function of molecular size and some idea of its variation can be obtained from Figure 3.1, which gives D plotted against log (molar mass) for an aqueous system and is derived from data given by Alexander and Johnson (1949). It will be observed that the diffusion coefficient for oxygen in water is about 2 x ICT5 (at 180C) whereas values given in Table 3.1 for nitrogen, which has about the same molecular size as oxygen, show that for normal vulcanized rubbers it is of the order of 2 x 1(T7. Any increase in the value of D due to the higher temperature and to the presence of the solvent needs to be set against its decrease with increasing molar mass, but with the smaller molecules the presence of solvent would bring D up to the same order as exhibited by molecules of similar size in water. It is clear from Figure 3.1 that, in the absence of the restraining influence of a semi-permeable membrane, the diffusion coefficient decreases relatively slowly with increasing molar mass but once the diffusing molecule approaches the size of the membrane mesh, it will cause D to drop rapidly to zero. It seems probable that, for the resins and plasticizers normally extracted from rubbers, D is about HT6 - 1(T7. We are now able to discuss the question of completeness of extraction. If we return to Eq. 3.3 it will be seen that it converges very rapidly indeed and a reasonable approximation can be obtained by expanding for two terms only and rearranging to give Eq. 3.4. m/0.0405C0 = exp( - 3.95. 1O3Df) +1/9 exp( - 35.55.103 Dt)

(3.4)

Table 3.1 Diffusion coefficients for nitrogen in polymers Polymer Vulcanized polychloroprene Butadiene copolymers: Acrylonitrile Methacrylate Styrene

Temperature, 0C

D, Cm2S-1

27.1

1.9 x 10~7

17 20 20

0.66 x 1(T7 3.4 x 10~7 2.4 x 10~7

Diffusion coefficient, D x IO5

Nitric acid

Oxygen

Sodium chloride Oxalic acid

Logio (molar weight) Figure 3.1 Coefficient of diffusion of water as a function of molecular size.

Initially, when f = 0, ra/0.0405C0 = 1.111, whereas, if the summation had been carried to infinity, this expression would be equal to 1.234 since, by definition and for the thickness of membrane taken, ra/C0/ = 0.05. Figure 3.2 shows a plot on double logarithmic scale of ra/0.0405C0 obtained from the right-hand side of Eq. 3.4 against log t for a time scale of 10-105 seconds (about 28 hours) and for two values of D. Whilst it is true that an infinite time is required to complete the extractions, the amount unextracted after even a short time can be below that detectable by the analytical operation involved. When D = 10~7, the unextracted material is reduced to 1% of its initial value in 3 hours and to 0.1% in 5 hours after which the amount remaining becomes too small to have any analytical meaning. The diffusion equation with the awkward summation of exponentials in its integrated form has been avoided by experimentalists in discussing the effect of extraction time and other variables. In addition, it is rare in analytical practice to be extracting from a polymer a single molecular species of definite molar mass. In the past, extraction was always from raw or vulcanized natural rubber where the mixture of extractable substances certainly defied any attempt to ascribe a definite, even if average, value of D, because the range of molar masses of the non-rubber constituents increases smoothly from that of quebrachitol

Log,o[0.0405CoJ

Logio (time in seconds) Figure 3.2 Influence of diffusion coefficient on extraction time. and those of the fatty acids to molar masses of tens of thousands. However, the shapes of the curves in Figure 3.2 indicate that definite extract levels should be obtained if the polymer is a true membrane, i.e. if it is either crosslinked or, if free from crosslinks, free of polymer material of low molar mass that might be soluble in the extracting solvent. Failure to extract to constant weight is most likely to be a chemical phenomenon due to slow oxidative scission giving a constant supply of material of low molar mass derived from the polymer. The simple application of the diffusion equation assumes the concentration to fall to zero at the surface of the rubber. This is not true since each of the pieces of equipment illustrated in Figure 3.3 has a finite time between siphoning and, during each cycle, there is a build-up in the concentration of extracted materials in the solution surrounding the rubber sample. It will also be appreciated that the rubber is always wetted by the extracting liquid and a layer of this remains even after siphoning has removed the bulk of it. The effect of this will be to decrease the concentration gradient thus depressing the value of the diffusion coefficient. The diffusion theory expounded above gives a reasonable physical picture of the extraction process and, when applied quantitatively, gives values for the parameters of the equation used which are of the right order. The corollary, that extraction can never be complete, is not of analytical significance for the amount remaining unextracted when D is of the order of 1(T6 can be reduced below the limits of analytical sensitivity within reasonable periods of extraction. This is no longer so

when high molar mass polymers are to be extracted from another crosslinked polymer. Polymethylmethacrylate of high molar mass (intrinsic viscosity 6.6) was extracted with acetone from a natural rubber vulcanizate only to the extent of 14.6% of the amount there, after 27 days (Cooper and Smith, 1962). Their data do not allow calculation of the diffusion coefficient but an approximate treatment suggests that it is smaller than 10~10. STANDARD APPARATUS FOR DETERMINATION OF EXTRACT LEVEL The apparatus used for extraction should preferably be of all-glass construction and two forms recommended in the International Standards Organization document ISO 1407:1992 are illustrated in Figure 3.3(a) and (b) which, in addition, allows a form of extraction apparatus in which a coiled metal condenser is inserted into the neck of a conical flask, which is closed by a metal disc through which the condenser tube passes, and is integral with it. This form of apparatus, usually known as the Underwriters, is illustrated in ASTMD 297-93 and is shown here as Figure 3.3(c). The objections to it are that when several are connected in series the tubing tends to prevent the closing plate sitting squarely on the flask; flask irregularities have a similar effect, both resulting in a loss of solvent, a loss aggravated by the fact that the

Figure 3.3 Four basic types of extraction apparatus suitable for the extraction of rubber and rubber-like polymers.

condenser is necessarily tightly coiled and its effective surface area small. In addition, a condenser cut from block tin is expensive and its cheaper equivalent, dipped or plated brass, has been known to contain pin-holes in the plating, flaws identified as the cause of polymerization of the extractant (acetone). Figure 3.3(d) illustrates the routine extraction apparatus used in the author's laboratory. A 150cm3 round-bottomed flask is used since this is preferred for convenient heating on a bank of heating mantles, and enables the solvent to be removed under reduced pressure at low temperature. The Soxhlet cup, although having a capacity of only 8 cm3, will hold the weight of sample usually extracted (3 g) and has considerable advantages in terms of the length of time required for extraction for reasons discussed later. Raw rubbers, unvulcanized compounded rubbers and some thermoplastic materials often become tacky during extraction and tend to coalesce, thus invalidating the quantitative extraction data. This can be overcome with sheeted samples by placing them between lens tissue or nylon filter cloth prior to extraction whilst, for cut-up samples BS 1673: Part 11-1954 (now withdrawn) suggests the use of silver sand to dilute the polymer. In all cases the anti-coalescing materials should be extracted before use. A valuable technique with thermoplastics and compounded rubbers is to prepare a thin film using a hot laboratory press. Temperatures up to 18O0C may be required for some thermoplasts but it is a simple matter to press for a few seconds and obtain a sheet 0.2-0.5 mm thick. Similarly a compound can be lightly cured by holding it at approximately 15O0C for 1-2 minutes and the sheet will then have a much reduced tendency to flow during the extraction although the extract will then contain cure residues, together with the original curatives. If the edges of the sheet are discarded there is no evidence for degradation of the polymer in this time scale. CHOICE OF SOLVENT For the extraction of natural rubber, whether vulcanized or raw, acetone is usually specified as it fulfils most of the criteria for a good extractant. These are that the polymer should be swollen slightly by the solvent but should not be soluble in it; it is convenient that it should boil at a temperature well below that of any extracted material so that it can be removed easily from the extract without loss of any extracted liquid or heat damage to any solids, and, in addition, the solvent should be inert to any possible ingredient of the extract and not objectionable by virtue of excessive toxicity, inflammability or odour (although it must be remembered that all solvents are toxic to some extent). It is advanta-

Table 3.2 Solvents for the extraction of rubbers and rubber-like polymers Elastomers NR synthetic polyolefins: (i.e. BR/SBR/IR etc) UR CR and NBR PVC Thermoplastic block copolymers (ie SIS) NR/PE/PP types EPR/PE/PP types

Raw/compounded

Vulcanized

acetone methanol 2-propanol acetone 2-propanol acetone butanone light petroleum (60-80) methanol 2-propanol diethyl ether methanol methanol

acetone methanol butanone acetone butanone acetone butanone light petroleum (60-80) methanol 2-propanol -

methanol methanol

geous if the solvent used is cheap, as the reuse of recovered solvent carries with it a certain element of risk and should be avoided. Table 3.2 indicates solvents which have been found generally acceptable in the extraction of common elastomers, but attention is also drawn to Table 3.3 which gives a much broader picture of the resistance Table 3.3 Resistance of rubbers to various liquids Rubber

Natural cis- P l S B R NR cis BR E P R etc. C R N B R AU/EU O T MQ etc. FPM/CFM C S M Acrylates

Aliphatic Aromatic Halogenated Ketones Alcohols Animal Water & veg. oils P P P P P P G E E E E P-G E F E

P P P P P P-F F F F P-F E F E

P P P P P P P P F G F G P P

Ratings: E = excellent, G = good, F = fair, P = poor. For polymer types see Appendix B.

G G P-G E G G G-P E G F P - G G G - E E E G - E G F-G P-G E G P P E P F G F G P E E F - G P - FG G G G G E F F G G F F P E E E P G G G P P E E

of a range of rubbers to various solvent types, and may help in the selection of an unusual solvent for a specific application. Acetone used for extraction should be free from its polymers and from water and this means that for all accurate work, if it is taken other than from a newly opened container which has been stored in the dark, it should be redistilled before use. Methanol is also a useful solvent for the qualitative extraction of natural rubber, raw or vulcanized, for subsequent chromatographic examination of the extract. Whilst additives are generally extracted quantitatively, the extract appears cleaner than when obtained with acetone. Extraction of polar synthetic rubbers may be with ether or with light petroleum of specified boiling range. If the latter is preferred it should be noted that the Soxhlet-type extraction apparatus sometimes gives trouble due to the lower boiling components of the solvent creating vapour locks in the siphon tube. The remedy is to use a Soxhlet with an external syphon (see Figure 3.3(d)). Ether, being of constant boiling point, is free from this trouble provided that excessive heat is not applied to the flask, preventing the condensate from running back. Methanol has been found to be an acceptable solvent for selectively extracting plasticizers from block copolymers such as SBS or SIS, and those based on a polyolefin/polypropylene blend, whilst it also affords a good separation of the polymeric plasticizers used with acrylonitrile rubbers and chloroprenes (Williamson, 1957). Robertson and Rowley (1960) recommend the carbon tetrachloride-methanol azeotrope for the removal of polymeric plasticizers from PVC but this is unlikely to find favour today because of the toxicity of carbon tetrachloride. Because of the vast range of solvents suggested by different authors it is imperative that any quantitative results, or specifications, define fully the solvent system used and the extraction process. TIME OF EXTRACTION Rubber extractions with acetone or the other solvents listed are usually carried out overnight, with ASTMD297-93 and ISO 1407:1992 tending to agree on some 300 cycles through the extraction cup, although ISO allows as few as 160. If the sample has been properly comminuted the longer times required by these standards are probably unnecessary although, when new apparatus is used, the extraction rate should be checked, as also should its behaviour with any unusual solvents before leaving extractions overnight. A simple rubber compound containing only a minimum of added materials is usually quantitatively extracted in periods less than eight hours but this gives the necessary margin of safety to allow for the presence of unusual substances.

RAPID EXTRACTION

The apparatus already illustrated in Figure 3.3(d), using the low-volume Soxhlet extractor, enables relatively rapid extractions to be carried out. The cycle time is of the order of one minute (with 3g rubber) and thus after three hours the minimum number of ISO passes has been exceeded and, in the author's experience, essentially quantitative extraction of general purpose vulcanizates or compounds etc. has occurred. The particular advantage of reducing the extraction time from eight to three hours is that an extraction with subsequent examination of the extract and residue can be carried out within one working day. An even more rapid method has been proposed by Kress (1956), and is referenced in ISO 1407. The procedure is as follows: Sheet out Ig of the vulcanizate with the mill nip set as tightly as possible. Make several passes with a single unfolded sheet until the sample is as coherent as possible. Where this is not possible place the milled crumbs into a filter paper envelope and for uncured stock sandwich between filter paper. Cut, with scissors, a test portion of between 80 and HOmg and drop the weighed test piece into the boiling solvent and continue rapid boiling for 30 minutes. (Kress uses 20cm of solvent in a 250cm conical flask with a condenser but states that a beaker of solvent on a hot-plate and covered with a watch glass is equally satisfactory.) After extraction, press the test piece between 'folds of absorbent paper towels' to remove excess solvent and dry at 105-11O0C for 10 minutes. Kress advocates the use of a mixture of methyl ethyl ketone and ethanol, 75:25 by volume, and weighing the test portion before and after in order to determine the extract quantitatively. This mixed solvent is chosen empirically to give results in line with the standard ASTM acetone extraction, MEK itself giving too high a figure. However, we are not here concerned with Kress's suggestion that his method should replace the existing quantitative procedure but only that it provides a convenient means of obtaining quickly an extracted sample for qualitative examination and for this purpose the use of MEK without ethanol is probably preferable. A completely different philosophy was adopted by Higgins (1978) who used a high-speed macerator which generated ultrasonic pulses to extract quantitatively 2-5 g samples of raw rubbers in a matter of minutes. Samples of SMR 5, 20, 50 and 5-LV rubbers, together with an experimental set of oil-extended NRs (OENR) containing up to 25% oil were examined by the rapid method and ASTM D297. The results illustrated that the method allows a rapid determination of the extract level with extraction times of less than 5 minutes.

Subsequent unpublished work has shown that this technique is equally valid for the analysis of compounded or vulcanized rubber and thus has a particular significance in that it allows the cold extraction of unvulcanized samples permitting the identification of the added curatives themselves, rather than their decomposition products, particularly important for the identification of thiazole and thiuram accelerators. It is, of course, quite possible to use a 'cold Soxhlet' extractor, in which the hot solvent vapours by-pass the sample holder and only after condensing do they flow over the material being extracted, but this is extremely time consuming and cumbersome whilst the extracted substances are still contained within the boiling extractant for the period of the experiment. MICROWAVE EXTRACTION

Accelerated extraction using microwave heating is a relatively recent development, the first commercial ovens only appearing some 10-15 years ago. The main attraction of this type of apparatus is its ability to achieve extraction temperatures above the normal boiling point of the extraction solvent, without using high pressure containment vessels. Microwave heating achieves super-heated conditions because the container is not heated directly by the microwaves. The microwaves are usually at a frequency corresponding to rotational energy bands of -CH or -O-H, that is around 4000 cm"1 or 2450 MHz. Although the vessels used to contain the samples are often made from PTFE, or similar inert polymers, any residual -C-H linkages present in non fully fluorinated polymers are rigidly fixed and incapable or absorbing the microwave radiation therefore the solvent is heated from the inside outwards, rather than from the outside in, as is the more usual situation. This mode of heating also promotes superheating since it reduces the potential for nucleation of bubbles by asperities on the surface of the container. When boiling does eventually occur, the temperature of the solvent does not return to its 'normal' boiling temperature, but it remains at an elevated one which is characteristic of the particular solvent being used. This higher temperature enables extraction to be carried out in significantly shorter times and the extraction times for microwave extraction are of the same order as those for micro scale extraction. It should, however, be noted that these elevated temperatures will tend to increase the possibility of further cure-related chemical reactions or the decomposition of labile additives during the period of the extraction. This technique has been used extensively in extracting additives from plastics (Freitag and John, 1989; Neilson, 1991) but less so for elastomers. The additives for which extraction was demonstrated by these

authors were all either actual rubber processing chemicals, or closely related substances but the difficulty of using the data from these papers as a guide to the suitability of microwave extraction for rubber products relates directly to the difficulty of producing very small particles from rubbery materials without inducing chemical changes to title analytes, a difficulty which is also relevant to supercritical fluid extraction as mentioned below. Since the extraction process is diffusion-limited, the larger particles inevitably require longer extraction times which may reduce the cost effectiveness of the extraction process. MICRO SCALE EXTRACTION The first published account describing applications and a procedure for carrying out extractions of rubber on the micro scale is that of Wyatt (1941). An illustration of his apparatus was included in the second edition of this book (Wake, 1969) but it appears obsolete today since the practising analyst would tend to use standard 'micro glassware' not available in 1941. The size of the test portion was some 20 mg and by strict definition, therefore, the method should be considered a semimicro one as the sample is over 10 mg in weight. Nevertheless the extract will normally be between 1 and 5mg thus the use of the word 'micro' could be acceptable. Wyatt's results indicate the rapidity of extraction using this apparatus, 2 hours being sufficient for an extraction with acetone as compared with 7 hours for the conventional modern macro method. The shorter time equates well with the time required using the small extraction apparatus illustrated in Figure 3.3(d) when 3 hours is generally adequate for a 3g sample. Wyatt also gives data for chloroform and alcoholic 'potash' extractions, and again illustrates that the more rapidly obtained results have recoveries comparable with those of the slower macro method. The test portion size of 20 mg is also a point which requires consideration. If a piece of material of adequate weight is available (ISO 1407 requires 3-5 g) then there is little point in carrying out a micro extraction unless there is a specific reason as discussed in Chapter 2. In this case the problem of obtaining a representative sample does not arise as one is specifically looking for differences on the micro scale. Should a micro extraction be required of a 5g sample such that it is representative of the whole, then it should be appreciated that the 20 mg constitutes only some 1/250 part of this, and it is advisable to carry out a two-part homogenization by taking a 0.1-0.2 g sample from the initially homogenized 5g and carrying out a further homogenization before taking the 20 mg samples. The statistical rationale behind this argument and the validity of micro sampling techniques are given in Chapter 14.

MULTIPLE EXTRACTIONS

In rubber technology some use is made of factice and mineral rubber with both natural and general-purpose synthetic rubbers whilst polyesters are increasingly used as plasticizers for PVC and oil-resisting synthetic rubbers. Factice, also known as rubber substitute, exists in two broad classes distinguished by colour and known as 'light sub/ and 'dark sub/. Actually, the colour difference signifies a chemical difference of some importance to the analyst. Brown, or dark sub., is formed by reacting together a mixture of vegetable oils and sulphur and is a polymeric material of moderate molar mass (about 7000) (Stamberger and Knight, 1928), whereas white, or light sub., is formed from the same oils by reaction with sulphur chloride (S2Cl2) and the molecule, probably of similar molar mass, contains chlorine as well as sulphur. Factice is not extractable from vulcanized rubber by acetone although its presence usually leads to a slight increase in the acetone extract due to the presence of small quantities of free oils; neither does a simple change of solvent, to chloroform for example, enable this to be done, so recourse has to be made to a degradative extraction with alcoholic potash after the rubber has undergone an initial extraction with acetone. Mineral rubber is the 'trade name7 given to asphaltic hydrocarbons used as 'extenders' or cheap filling material or processing aids for rubbers when high-grade mechanical properties are not required. Asphalt, whether derived from native asphalt or obtained as a distillery residue from some petroleum sources, consists of a mixture of oils, resins, and asphaltenes the latter two being of medium high molar mass, and it is part of these which resist extraction with acetone. If asphalt is suspected, the procedure to adopt is to follow the acetone extraction with one of chloroform until a colourless liquid is obtained from the extraction cup since no other materials in common commercial use in rubber vulcanizates are insoluble in acetone but soluble in chloroform. However, it does not suffice to quote the chloroform extract so obtained as the mineral rubber content, since part of the mineral rubber will have been extracted by acetone and this part will depend on the source of the mineral rubber and probably also on the temperature of vulcanization of the compound. In matching a specification for a compound to the analytical figures this must be allowed for. Rather similar to the factice problem in natural rubber is that of polymeric plasticizers in the oil-resistant synthetic rubbers. Typical materials are polypropylene adipate and polypropylene sebacate. The molar mass is not very high and, as with condensation polymers generally, the molar mass distribution is rather broad. This causes difficulty in finding a solvent which will extract ordinary monomeric plasticizers

without extracting any of the polymeric plasticizer and, in fact, this cannot be done. Ether, which is normally used, definitely takes out some of the polymeric plasticizer. Difficulty is also experienced at the other end of the molar mass distribution in finding a solvent which will completely remove the high molar mass material without removing the base polymer. Acetone will successfully remove all the polymeric plasticizer but will also remove some of the rubber, particularly that of the butadiene-acrylonitrile type. As already mentioned, Williamson (1957) has shown that methanol can be used successfully in certain cases for the analysis of polymeric plasticizers. SPECIFIC EXTRACTIONS Multiple extractions are, by definition, specific since the first must leave something behind for the subsequent ones. There is, however, no need for subsequent extractions to be carried out if the first one extracts the substance to be analysed either qualitatively, reproducibly or quantitatively depending on the purpose of the exercise and this is the concept behind a specific extraction. There are two fundamental reasons for carrying out such an analysis. The first is to extract an analyte cleanly whilst leaving behind possible contaminants or interfering substances whilst the second is to extract an article in a way which mimics a process it might experience during its service life so that any substances being leached or extracted can be investigated. An example of the former is the aqueous acid extraction and subsequent estimation of hydroxylamine from raw or vulcanized natural rubber (Davey and Loadman, 1988) whilst the latter is often related to health and safety considerations where a water-based extraction medium may be used to emulate a physiological activity. Examples of this are the procedure of Blosczyk (1992) for extracting MBT and ZMBT from rubber products with water, although Edwards (1994) claimed that the insolubility of ZMBT was such that it was not extracted unless the extractant had been acidified prior to the extraction being carried out, and the analysis for volatile N-nitrosamines in baby feeding bottle teats and soothers by the German (BGA) method of 1984 (soon to be replaced by CEN Standard in response to the EU directive 93/11/EEC). It should be noted that the American (FDA) procedure for nitrosamine analysis uses dichloromethane as extractant on the grounds that this is exhaustive and gives a maximum level of nitrosamines which, potentially, could be bio-available. Recently there has been a growing interest in the levels of bio-available nitrosamines in a wide range of products such as the BGA

Table 3.4 Chemical compositions of artificial saliva and artificial sweat Artificial saliva sodium bicarbonate sodium chloride potassium carbonate distilled water

Artificial sweat 4.2 g 0.5 g 0.2 g 3 100Om

potassium chloride sodium chloride sodium sulphate ammonium chloride !active acid (90%) urea distilled water

0.3 g 4.5 g 0.3 g 0.4g 3.Og 0.2 g 3 100Om

Recommendation 21 'special category' products, condoms, gloves and catheters. Only in some cases are analytical procedures described and this must cause concern for the analyst who is confronted with a product for which no documented or recognized procedure exists. For instance, when analysing latex gloves or condoms, consideration should be given to using artificial perspiration instead of artificial saliva. As Table 3.4 illustrates, these are appreciably different chemically although whether the difference is significant in terms of extracting bio-available nitrosamines and nitrosatable amines appears not to have been documented. Some representative Standards are DIN 53160, which measures the resistance to saliva and perspiration of coloured toys, and the European Standard, EN71-3, which is concerned with the bio-available toxic elements in children's toys. Here the extractant is hydrochloric acid of pH 1.0-1.5, being intended to simulate the potential for dissolution of ingested materials in the stomach. Regulations concerning rubber in contact with food are typified by the FDA Code of Federal Regulations, title 21 which defines, inter alia, extraction limits with water for aqueous based foods and n-hexane for fatty ones whilst the BGA Recommendation 21 identifies four other categories as well as the 'special category' and sets limits for each category when extracted with water, 10% aqueous ethanol and 3% aqueous acetic acid. SUPERCRITICAL FLUID EXTRACTION Supercritical fluid extraction can be considered an extension of both the specific and sequential extraction procedures described above in that selective extraction may be achieved by varying the temperature, density (pressure), flow and time of the extracting liquefied gas. By progressively altering one of the parameters, typically the pressure and hence density of the extractant, a controlled extraction can be achieved which mirrors the use of solvent gradients in HPLC analysis and,

indeed, SFE is often directly coupled to HPLC to exploit this 'precolumn' selectivity (King, 1989). Nevertheless two major disadvantages remain in applying this technique to the analysis of rubbers or rubber-like materials. Firstly, it is experimentally complex and the validation of any result is a timeconsuming exercise for which there is no short cut whilst secondly, and perhaps of more pragmatic importance, it is necessary for a solid test portion to be prepared as a finely divided powder, with individual particles in the size range 10-50 |im. Whilst this is possible if one uses a macerator followed by cryogenic grinding and sieving it may well be difficult to justify the time and cost involved in the operation. It must also be remembered that the minor extractable components of the rubber might have undergone at the least a quantitative change during the preparation of the powder. The reader who wishes to consider this alternative approach to 'solvent' extraction is referred to excellent reviews by Gere and Derrico (1994). LATEX

ISO documentation provides three standards concerned with the conversion of the liquid latex to solid rubber. For our purposes two of them - ISO 124-1992 and ISO 498-1992 - can be considered the same in that they both afford total solids in the form of a thin sheet of dry rubber. The difference is that whereas ISO 124 is solely concerned with obtaining a total solids value, and dries a layer of latex at 7O0C (16 h) or 10O0C (2h), ISO 498 is specifically written to prepare a smooth thin film of rubber and thus dries at 350C to constant weight, a procedure taking several days. The third document (ISO 126-1995) describes the determination of the dry rubber content by coagulation with acetic acid, separation from the latex liquids, and subsequent drying at 700C. It is obvious that this sample will be quite different from the other two in that most of the non-rubbers will have been separated from the rubber itself, and it should not therefore be used for further general analysis if this could be significant. The dried latex may be extracted by any appropriate solvent in the same way as a raw rubber or vulcanizate, but its physical state can be used to advantage in preparing a suitable sample for extraction. Chin et al. (1975) used a rotary evaporator to spread a thin film of NR latex (LA-SPP) over the sides of a 150cm3 round-bottomed flask which was then simultaneously coagulated and extracted by the addition of a methanol-acetic acid mixture. After a few minutes of rotating the solution in contact with the latex film, effectively quantitative extraction of pentachlorophenol was achieved in the form of a solution amenable to direct analysis.

A similar procedure was adopted by Edwards (1981) who prepared a latex (LATZ) film as described above and used pure methanol to extract tetramethyl thiuram disulphide (TMTD) by rotating the flask containing the solvent and latex film for 1 hour before analysis of the extract by liquid chromatography. It should be commented that no TMTD (Mz /=1

f>,-Mi 3

Z=I

Mn =

; X

Mw =

Z=I

; X

Mz =

. X

^n1

I>/M/

2>;M/

Z=I

Z=I

Z=I

2

where HI is the number of molecules of molar mass Mi. The Z-average molar mass is especially sensitive to high molar mass components in the polymer. The difference between number- and weight-average molar masses is important. The weight-average value is always greater than the number-average one except in an ideal system of uniform molar mass (mono-disperse system) when the two are equal. This leads to the ratio Mw/Mn being used_to describe the dispersity, or spread, of molar masses in a sample. Mw is more sensitive to high molar mass components whilst the converse is true for Mn. It is a simple matter to illustrate the difference by considering two polymers, one in which all the molecules have a molar mass of 100 000, and the other in which all the

molecules are of exactly 10000 molar mass. In the former case Mn = Mw = 100000 whilst in the latter, Mn = Mw = 10000. If equal weights of each are mixed, the resulting measured values would be Mn = 18 200

Mo; = 55 000

but if equal numbers of molecules of each are taken,

Mn = 55 000

Mw = 92 000

In subsequent sections dealing with methods of measurement, the type of molar mass so measured will be indicated. There are many methods for determining some form of average molar mass of a polymer but relatively few for studying the distribution of that molar mass. Both these properties are important in helping to understand variations in processing such as mastication (Baijol, 1972), cold flow characteristics (Purdon and Mati, 1966) and adhesive properties (Koldunovich et al. 1968). A number of the more relevant and successful ones will be discussed in some detail. END GROUP ANALYSIS (Mn)

With certain linear polymers it is possible to estimate the number of end groups by chemical analysis and so derive the number-average molar mass. The total number of chain ends is twice the number of polymer molecules, but if each polymer molecule contains one group of a particular type at the end of its chain then the number of that end group equates with the number of molecules. It is obviously of fundamental importance that the particular group which is the subject of the analysis is confined only to the end of each chain and also, with branched polymers, that each branch is not terminated with the group of interest. (Obviously this situation would provide data on 'branching' if each branch was so tipped, and a 'true' value of the molar mass was obtained by a different method.) Given these criteria it will be apparent that there are innumerable methods of analysis and it must be up to the analyst to find the most suitable one for any particular end group. Techniques to be considered include chemical methods, used by Ogg et al. as early as 1945 to estimate terminal hydroxyl groups, infrared spectroscopy, first used by Pfarm et al. a year later, and also nuclear magnetic resonance, pyrolysis-gas chromatography and mass spectrometry. It may also be advantageous to react the functional group chemically prior to analysis. Heacock (1963) estimated carboxyl groups in the presence of carbonyl groups by reacting the former with sulphur tetrafluoride to obtain the thionyl halides which were then quantitatively measured by infrared spectroscopy, whilst Edwards and Loadman

(1976) determined the molar mass of hydroxyl tipped polystyrenes by reaction with hexamethyldisilazane and trimethylchlorsilane in pyridine solution followed by measurement of the trimethylsilyl end group relative to a quantitatively added standard using NMR spectroscopy. The replacement of one proton by nine greatly enhances the sensitivity and usefulness of the technique and Mn values up to 80000 were measured. MEASUREMENT OF COLLIGATIVE PROPERTY (Mn)

A colligative property is one which depends primarily upon the number of molecules in the system and not upon their nature. Perhaps one of the earliest observed was the relationship between the depression of freezing point and the concentration of the freezing solution (cryoscopy) enshrined as Blagden's Law some two hundred years ago. Others include the elevation of boiling point (ebulliometry), the reduction of osmotic pressure (membrane osmometry, MO) and the reduction of vapour pressure (vapour pressure osmometry, VPO). Under ideal conditions a general equation defines the calculation of Mn: X/c = K/Mn+ be

(8.1)

where X is the colligative property, c is the concentration of the solution, and K and b are 'constants' which differ for each technique and for each piece of equipment. In practice a series of solutions of varying concentrations is prepared and the colligative property measured for each. A graph is then plotted, as illustrated in Figure 8.1, of X/c against c, the number-average molar mass being the reciprocal of the intercept of the plot, after extrapolation to infinite dilution (C = O), with the abscissa. This method requires a knowledge of the constant K which is normally obtained by calibration against standards of known molar masses. In rare instances the solutions do not behave ideally and the plot obtained shows a distinct curvature due to the concentration dependence of the colligative property becoming significant. In these cases a plot of ^/(X/c) vs. c usually provides an acceptably straight line for reliable extrapolation to zero concentration. Cryoscopy and ebulliometry Cryoscopy and ebulliometry can be considered together briefly as they do not feature largely in polymer analysis although a number of publications, Newitt and Kokle (1966) (cryoscopy) and Ezrin (1968) (ebulliometry), have appeared. These indicate that the techniques are valid up

Figure 8.1 Diagrammatic illustration of the determination of Mn from colligative property measurements.

to molar masses of some 30000, although there are problems with supercooling and frothing respectively. Commercial instrumentation does not, however, appear to be available, probably due to the much greater ease of operation of the vapour pressure osmometer. Vapour pressure osmometry The technique of indirectly measuring the lowering of vapour pressure of a solvent due to the presence of a dissolved material was first proposed by Pasternack et al. (1962). It is based on measurement of the temperature difference between droplets of pure solvent and of polymer solution maintained in an isothermal atmosphere saturated with the solvent vapour. The temperature difference results from the different rates of solvent evaporation from and condensation on to the two droplets. As with the two previous methods, the colligative property, a temperature difference, is measured electronically and the value of K is obtained by calibration against appropriate standards of known molar masses. In the 1970s and 80s there appeared a number of papers which questioned the validity of an absolute constant in the determination by VPO of a molar mass and these illustrated how the 'constant7 K varies with molar mass; see Bersted (1973), Brzezinski et al. (1973), Morris (1977), and Marx-Figini and Figini (1980). Edwards (1977) tabulated K values for a range of molar mass standards as shown in Table 8.1. However, examination of the literature quoted shows that all these studies were carried out using a Hewlett Packard 302B instrument.

Table 8.1 Relationship between Mn and K (solvent, toluene at 4O 0 C) (Hewlett Packard 302B) Substance 8-Hydroxyquinoline Hexachlorobenzene Polystyrene standard Polystyrene standard Polystyrene standard Polystyrene standard

Mn

K

145 285 970 2300 8500 17500

10670 11600 12300 13300 14000 14950

When Edwards (1981) repeated this work with a Corona-Wescan 232A vapour pressure osmometer he found that K was indeed a constant, an observation also made by Burge (1979). It would seem, therefore, that each analyst must calibrate his or her particular instrument for the full molar mass range over which it is likely to be used, since subtle design and manufacturing differences are obviously important. Membrane osmometry Membrane osmometry is the last of the techniques to be considered which relies upon a colligative property of a polymer. The success of this technique depends upon a membrane, between the polymer solution and solvent, being permeable to the latter but completely impermeable to the polymer molecules in solution. In practice this means that whereas VPO finds application in the 500-50 000 Mn range, MO is restricted to polymers having no components with molar masses less than 15000; for unfractionated polymers this means an effective minimum Mn of 50 000. The principle of operation is simple. A solution of the sample is placed above the membrane below which is the pure solvent connected to a reservoir by a flexible tube. The height of the solvent reservoir is automatically adjusted to keep a bubble, introduced into a capillary tube below the membrane, in line with an optical detector, and thus equalize the rate of migration of solvent molecules from both sides of the membrane. The colligative property is then directly measured as h (the height difference between the membrane and solvent meniscus). In practice the displacement is converted to a voltage and plotted on a chart recorder or monitor to allow the observation of slow drifts due to the diffusion of any relatively low molar mass components which might be present. Extrapolation to zero time provides a realistic value for Mn

although Elias (1968) reviewed theoretical treatments of this problem and showed that even such extrapolation fails to provide a 'true' figure. Unlike the previously described techniques, membrane osmometry requires no calibration as the constant K = RT where R is the gas constant and T the absolute temperature. VISCOMETRY (Mn OR Mw)

The use of a viscometer to measure the viscosity (T/) of a solution of a polymer, followed by the calculation of intrinsic viscosity ([TJ]) and hence its molar mass, has been a standard procedure for many years, with one of the earliest reports being that of Staudinger and Heuer (1930). Many analysts will have used a viscometer since schooldays and thus the apparatus requires little description, although ISO 3105 may be consulted for the design of various accepted glass capillary viscometers, the type generally used in measuring polymer solution viscosity. The principle of operation is extremely simple in that the time (t) taken for a solution of known concentration to flow between two marks on a capillary tube is compared with the time taken by the solvent (t0), and the ratio is a measure of the viscosity of that solution. Full practical details were described in the British Standard BS 5858-1980 which is now withdrawn. Successive dilutions afford a range of concentrations (c) and times (t) from which the intrinsic viscosity [rj] may be calculated. The viscosity of the solution (solvent) = rj (rj0) from which r]sp (the specific viscosity) may be obtained: risp =

n - no no

t-t0 to

(8.2)

- —

and hence [^], using either the Huggins equation (1942)

-£--[,!+KH M%

V^^

(14-2)

In practice, since 100% sampling is not carried out, one does not have a value for the true population mean and an estimate of this must be made from the few analyses actually carried out. Under these conditions a more realistic estimate of the sample standard deviation (s) is obtained by the equation:

-^W