Rubber Analysis

Characterisation, Failure Diagnosis and Reverse Engineering

Martin J. Forrest

• Publisher: De Gruyter
• Year: 2019
• Print ISBN: 9783110640274
• eBook ISBN: 9783110640441
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Industrial & Technical Chemistry
• Index: Technology & Engineering

Rubber analysis plays a vital part in ensuring that manufactured products are fit for purpose. This comprehensive, application-based book with up-to-date referencing covers all important applications and subject area associated with the analysis of rubber compounds and rubber products. Includes characterization of rubber polymers, rubber fumes, identification of extractables and leachables, as well as reverse engineering on compounded products.

1 Introduction

This book follows on from a review of rubber analysis published by the author as a Rapra Review Report in 2001 [1]. In the intervening 17 years, there have been number of important developments in the field. Some of these were associated with the development and commercialisation of new analytical techniques and others led to the appearance of improved versions of established techniques, both of which have increased the speed, efficiency and capability available to the rubber analyst. Other developments were associated with new testing methodologies created to help protect the environment and to safeguard human health. Alongside these advances and developments, there has also been a significant amount of new research and investigative work carried out and published. Taking everything into account, it was, therefore, considered to be timely and important to reflect and represent all of the these changes in a new, and very much enlarged edition of the book.

The analysis of rubber products to demonstrate compliance with regulations is one area that has grown significantly since 2001. In addition to the many regulations that existed prior to 2001, new regulations have been published, such as those concerning the presence of polyaromatic hydrocarbons in extender oils [2] and a new Chinese standard for the use of rubber products for food-contact applications [3]. As well as these specific, rubber-related regulations, major new European Union regulations have been published which have had a significant and wider impact on industry, such as the Registration, Evaluation, Authorisation and restriction of Chemicals [4] and the Restriction of Hazardous Substances [5]. Regulatory bodies, such as the US Food and Drug Administration, have put more emphasis since 2001 on identifying and quantifying the substances that could migrate from the rubber products used in medical devices, and in drug storage, dispensing and manufacturing systems.

It is for these reasons that dedicated sections that deal specifically with the analysis work that has been undertaken in these subject areas have been included in this book, as well as other sections that address additional specific areas of interest, such as the characterisation of rubber process fume, studying the curing behaviour of rubbers, and the identification of blooms and contaminants in rubber products.

To ensure that the major ‘themes’ of rubber analysis work are fully represented, there are also extensive sections in the book that deal with the identification, quantification and characterisation of the principal constituents in a rubber compound (i.e., polymer, plasticiser/oil and filler), and of the wide range of additives that can be used in rubber compounds (e.g., cure system and antidegradant system species). To provide a thorough background to all the investigative and themed sections, there is also a major section which describes the extensive range of analytical techniques available to rubber analysts when they are in pursuit of their various and diverse goals.

The overall objective of this book, therefore, is to present an introduction and overview of rubber analysis and the analytical methods and techniques used to achieve a wide range of objectives, including:

• – Quality-control work
• – Failure investigations
• – Deformulation studies
• – Characterisation work (e.g., curing behaviour)
• – Work on regulatory compliance (e.g., for food-contact applications)
• – Work to address health and safety or environmental concerns (e.g., characterisation of rubber fume)

To set the scene for a book on rubber analysis, it is useful to provide a brief introduction to rubber technology and, hence, provide the background to the complexity of the challenges that the rubber analyst often encounters. This is the intended function of the next six paragraphs.

To obtain useful products that perform under various demanding conditions, a rubber compound can be ‘tailor-made’ by selecting from a broad range of polymers and additives. With rubbers, the possible compositional permutations are made more numerous by an extensive array of plasticisers, fillers, process aids, antidegradants and cure systems. The technology of rubbers is, therefore, a mature one, and allows ‘fine tuning’ of a compound to fit a number of seemingly conflicting design criteria and product requirements. Hence, rubber compounds and products present analysts with one of their most difficult, but satisfying, challenges.

Natural and synthetic rubber compounds are used in a highly diverse range of rubber products for many applications and are manufactured throughout the world for various sectors of industry and end users. The examples of the products that can be manufactured from rubber compounds vary from bridge bearings to rubber seals, gaskets, adhesives and elastic cord (Table 1.1). The majority of these products are manufactured using traditional thermoset rubbers (i.e., vulcanised rubbers), but there are also a number of thermoplastic rubbers available in which the crosslinks are physical as a result of crystalline domains, or those that have a glass transition temperature above ambient. Of the sectors where rubber is used, the automotive industry is of particular importance in that tyre-related products, account for ~60% of the synthetic rubber and ~75% of the natural rubber used worldwide [6].

Table 1.1: Examples of commercial rubber products.

Generic class of product	Examples of commercial products
Tyres	Passenger car, truck tyres, racing, cycle, off-road tyres, inner tubes, curing bladders
Conveyor/transmission belting	Steel cord conveyor belting, repair material for conveyor belting, scrapers, mining conveyors, V-belts, flat belts, synchronous belts
Industrial hose	Water hose, high-pressure hose, welding hose, hydraulic hose, spiral hose, offshore hose, oil hose, chemical hoses
Automotive products	Hoses and gaskets, anti-vibration mounts, timing belts, window and door seals, transmission and engine components, wiper blades
General mouldings/ sheeting	Seals and gaskets, floor and roller coverings, sheeting, protection linings, microcellular products, rubberised fabric, wire and cable jackets, pump impellors, rail mounts, bridge bearings
Medical/pharma products	Surgeons’ gloves, medical tubing, gaskets, catheters, dialysis products, implants, contraceptives, baby soothers, tubing and valves, masks and respirators
Clothing	Footwear, protective suits, household gloves, industrial gloves, footwear/boot heels and soling, cellular rubber soles, diving suits, coated fabrics, sports footwear and clothing
Food contact products	Transportation belts and hoses, gloves, pipe and machinery seals and gaskets, plate heat exchanger gaskets, seals/gaskets for cans and bottles, tank linings
Miscellaneous products	Adhesives, rubberised asphalt, high-vacuum and radiation components, carpet backing, latex thread, sealants and caulking, toys

Table 1.1 provides an indication of the diverse range of rubber products that are manufactured throughout the world today. Both the composition of a rubber compound and the manufacturing processes that are used to produce it will depend upon the individual product that is being manufactured.

As mentioned above, rubber products have complex compositions and the company or individual who designs a rubber formulation for a specific product has a large number of ingredients to choose from [7]. Although there are general guidelines for designing rubber compounds, the final formulation depends upon the knowledge and expertise of each rubber compounder, i.e., there are no ‘standard’ rubber compounds.

One of the reasons that the composition of a rubber compound can vary enormously is because there are many types of base polymer (>20), each having a unique chemical structure which has a direct influence on its inherent chemical and physical properties, processing behaviour and the applications and products in which it can be used [8]. It is also the case that for each generic type of rubbery polymer (e.g., ethylene propylene diene monomer rubber), there are a significant number of different grades available, each of which may differ in a number of aspects (e.g., molecular weight or comonomer type). For the ethylene propylene class of rubbers, for example, over 100 different grades are available.

In addition to the base polymer, rubbers are formulated with a range of additives (typically between 5 and 20) to achieve a compound that has the desired processing and service properties [9]. There are also many cases where two or more base polymers are mixed and blended together in order to achieve the desired properties (e.g., some of the compounds used in the manufacture of tyres). Also, within each class of ingredient (e.g., antioxidant) there is a wide range of different chemical compounds available on the market, each having different structures and properties, from a number of manufacturers. Within another additive group, the carbon blacks, there are in excess of 30 possible products. The composition of rubber compounds can also alter during manufacturing because some of the ingredients are volatile at typical processing temperatures and others (e.g., cure accelerators and antioxidants) generate breakdown and reaction products [10–12], all of which makes accurate quantification of the original concentration extremely difficult to derive. It is also common industry practice to use more than one ingredient from a particular class in a formulation to achieve the desired properties (e.g., two or more accelerators). Certain rubber compounds may also contain reactive ingredients (e.g., pre-vulcanisation inhibitors) at a relatively low (<0.5%) level, which can make detection difficult.

The rubber analyst, therefore, has to bear in mind all these complexities to ensure an effective analytical strategy is devised and, also consider the possible interferences and difficulties that may arise and influence the data. The usual result is that when, for example, reverse engineering work is carried out on a rubber compound, a number of wet chemistry, elemental, spectroscopic, chromatographic and thermal techniques have to be used in an integrated and structured approach in order to ensure a successful result. It is still the case, though, that even with the greatest care, and in the most advantageous situations, >90% of a complex rubber formulation can be completely elucidated by analytical means alone. Some compound development work will still be required in order to fill knowledge gaps and find a satisfactory match for the processing, curing, physical properties and in-service performance of the target sample.

To assist the practicing analyst, the emphasis throughout the book is of an empirical nature, with the author drawing on his 40 years of technical experience within the polymer ind...

Table of contents

• Cover
• Title Page
• Copyright
• Contents
• Preface
• 1 Introduction
• 2 Analytical techniques
• 3 Principal components and bulk composition
• 4 Additives
• 5 Reverse engineering and product deformulation
• 6 Curing and cure state studies
• 7 Characterisation of rubber process fume
• 8 Compliance with food-contact regulations
• 9 Extractable and leachable studies on rubber products
• 10 Analysis of surface blooms and contaminants
• 11 Analysis of rubber latices
• 12 Conclusions
• Appendix 1: Standard nomenclature system for rubbers
• Abbreviations
• Index