Rubber Testing

Butyl rubber - This material generally had the least endurance in fatigue tests, but it may be adequate for some cardiovascular applications. Advantages include less sensitivity to stress concentrators than Pellethane, a very low permeability to fluids, a moderate creep resistance and widespread availability at low cost. Disadvantages include a relatively low fatigue resistance compared to the elastomers specifically designed for these applications. The rubber tested was not designed for medical applications and had standard rubber additives and modifiers that were cytotoxic unless the material was extracted after manufacture. 

The residual decrease in thickness of a rubber test piece, expressed as a percentage of the original thickness, after the test piece has been subjected to a compression stress under specified conditions of time of loading, time of recovery and temperature. BS 903 PartA6. Determination of compression set. 

Decrease in the amplitude of an oscillation or wave motion with time. Damping in rubber testing results from hysteresis. 

A type of test for determining abrasion resistance. Fixed knives, under constant load, scrape across a rotating rubber test piece. The resulting loss in weight is a measure of the abrasion resistance. 

The elongation remaining after a rubber test piece has been stretched for a given time and allowed to rest for a given time it is expressed as a percentage of the original distance between the bench marks. The term permanent set (correctly, permanent set in tension) is still popularly used to denote tension set, see standard BS903-A5. 

Statistical principles should be applied to the planning and analysis of all test programmes. Details of commonly needed techniques are given in a series of ISO standards as well as in standard text books. An alternative is BS 903 Part 2, Guide to the application of statistics to rubber testing , which includes the basics of design of experiments and has a bibliography [3]. 

BS 903-2, Physical testing of rubber. Guide to the application of statistics to rubber testing, 1997. 

Standard method of test for accelerated ozone cracking of vulcanized rubber [Test method D-1149-64 (reapproved 1970)], pp. 554-560. In 1972 Annual Book of ASTM Standards. Part 28. Rubber Carbon Black Gaskets. Philadelphia American Society for Testing and Materials, 1972. 

ISO 1658 Natural Rubber - Test Recipes and Evaluation of Vulcanisation Characteristics, International Oiganization for Standardization, Geneva,... 

In rubber testing the elastic structure is not damaged and the original shape is restored, since the deforming forces are lower than the recovery forces which are exerted by the elastic nature of rubber. In this respect rubber test methods differ from those applied to metals, bitumens, waxes, greases and ceramics where measurements are made of permanent deformation. 

International Rubber Test Method for Rubber Property International Hardness D1415... 

It is perhaps less easy to excuse the lack of a chapter on non-destructive testing. The reason is a mixture of the fact that the major NDT techniques are, in the main only applied to a few particular rubber products and the realisation that to properly describe all methods would require a book, not a chapter. It is, however, worth remembering that it is not only ultrasonics, radiography, holography and so on which are non-destructive. A number of the more traditional rubber tests, for example electrical properties, many dynamic tests, hardness and dimensional measures leave you with the product intact. There are text books which deal with NDT techniques generally and. a comprehensive review of NDT of polymers by Gross in Handbook of Polymer Testing3. 

A number of subjects common to all areas of physical testing have been addressed in chapter 2. These include discussion of the reasons for testing, the trends in test development, the use of statistics and quality control of laboratories. Whilst these matters are not unique to rubber testing, it is most important that they are fully appreciated in the context of our particular test procedures and class of materials. 

Until the 1980s most people remained unaware of how reproducible their rubber test methods were. Then, interlaboratory test programmes revealed the true scale of the problem. Long established test methods were found to have far poorer reproducibility than previously realised, in some cases to the... 

Earlier editions of this book had a complete chapter devoted to this subject which opened by commenting that it was tempting to claim that it was the most important chapter. The reasoning was that, whatever property we measure, whatever test method we use we end up with results and the question "What do the figures really mean " Results are useless unless we know their significance significance means statistics. However, at that time it was very unusual for statistical methods to be applied to rubber testing. 

For many reasons, not least the influence of the quality movement and the widespread availability of personal computers, statistical methods are now much more widely appreciated and more frequently applied to the results of rubber tests. Also, a practical reason for not now needing a statistics chapter is the existence of the comprehensive British Standard Application of Statistics to Rubber Testing1 which at the time of writing is being considered for adoption as an ISO standard. It contains references to standards on statistical methods and also has a small bibliography. 

The most suitable physical properties are likely to depend on the particular material, with plastics test methods being used for the harder elastomers (where the title elastomer may not even seem appropriate) and rubber methods for the less hard and more elastic materials. Where thermoplastic elastomers are to compete with conventional rubbers then clearly rubber test methods will be expected. On the other hand, where they are being compared to normal thermoplastics it would seem reasonable to use appropriate plastics test methods. 

Some of the conditions used in rubber test methods may need modifying for application to thermoplastic elastomers because of their intrinsic thermoplastic nature. If the temperatures generally used in ageing and compression set tests on thermosetting rubbers were applied to thermoplastic materials they could appear to perform extremely badly. Whether this was significant would depend on the service temperature. Data sheets need to be checked as those for thermoplastic elastomers may have used much lower temperatures that would be found for conventional rubbers, and it is only too easy to get a misleading impression of performance. 

ASTM D6600, 2000. Standard practice for evaluating test sensitivity for rubber test methods. 

CEN operates through a series of technical committees in a similar manner to ISO but there scopes do not necessarily coincide. At present there is no CEN committee for rubber although a considerable number of rubber products are covered by particular CEN product committees. It becomes clear that the relationship between CEN, ISO and national standards can cause difficulties. The people developing standards in a particular CEN product committee may not be the same as those at ISO TC 45 and a European country could have difficulty aligning its national standards with both those of CEN and ISO. Fortunately, as regards test methods CEN make considerable effort to adopt the ISO procedures and there are mechanisms for revisions to proceed simultaneously in the two bodies. Because there is no CEN committee for rubber there have been no problems with rubber test methods. 

ISO 2393, 1994. Rubber test mixes - Preparation, mixing and vulcanisation - Equipment and procedures. 

BS 903-A64, 1995. Method for the preparation, mixing and vulcanization of rubber test mixes. 

It is generally assumed that humidity is not important in most rubber tests and, hence, conditioning in an atmosphere with control of temperature only is usually specified. However, control of humidity is considered necessary in certain cases, for example testing latex rubber and electrical tests. In many instances the 16h minimum conditioning period will not be sufficient for equilibrium to have been reached, especially with relatively thick test pieces. Hence, all that this conditioning can hope to achieve is to bring test pieces having similar dimensions into more nearly comparable conditions than they would otherwise be. To reach complete moisture equilibrium would in many cases take several days and for thicker test pieces probably weeks. 

In ISO, general directions for achieving both elevated and sub-normal temperatures for rubber testing is now included in ISO 23529. This advice is useful in that it lists the various types of chamber construction and heat transfer media which may be used and specifies a number of general... 

ISO 1658, 1994. Natural Rubber. Test recipes and evaluation of vulcanisation characteristics. 

The most common dimensional measurements relate to the size of test pieces because this information is required for virtually all physical test methods. There is also sometimes need to measure dimensions of components of the apparatus, such as the thickness of spacers in compression set tests. Other aspects of dimensional measurement that are relevant to rubber testing include extensometry, surface roughness, dimensional stability and dispersion. 

In rubber testing, the surface finish of metals is of importance, for example on mould surfaces and compression set plates. There are a number of standards in the ISO Geometric Product Specification series but the most relevant is ISO 428729 which covers terms, definitions and surface texture parameters relating to the profile method of measuring surface finish. There are apparently over 1000 different parameters to characterize surface finish30 but only a few are generally encountered. The most commonly found is Ra (previously called CLA) which is the mean deviation of the surface profile above and below the center line, followed by Rz, a measure of the peak to valley height. For example, the arithmetic mean deviation (Ra) of the compression plates for compression set tests must be better than 0.2 m. 

There is a British standard19 giving guidance on the application of rubber testing to finite element analysis. Several of the models for stress strain behaviour are appraised and advice given on selection. The point is made that the models considered treat the rubber as a perfectly elastic material,... 

It is usual in rubber testing to calculate tensile stresses, including that at break, on the initial cross-sectional area of the test piece. Strictly, the stress should be the force per unit area of the actual deformed section but this is rather more difficult to calculate and in any case, it is the force that a given piece of rubber will withstand which is of interest. The stress calculated on initial cross-section is sometimes called nominal stress. ... 

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