Compression set Tests

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

In addition to normal compression set test conditions, usually 22 hours at 70 °C in a hot air oven, pharmaceutical elastomeric closures may be subjected to compression set conditions simulating steam sterilization cycles in an autoclave for 30 minutes at 121 °C. Also, sterilizing cycles employing ETO, radiation, or dry heat are used. Comparison data between formulations are used to develop compression set values that will identify potentially acceptable compounds under these conditions. 

It should be noted that the compression set test is often conducted on wet foam or in a humid atmosphere. This implies that the numbers will be different, and indeed this is the case. More often than not, polyurethane chemists work in wet or humid environments, and a procedure that uses this option is advisable. 

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. 

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. 

A previous edition of the dimensions standard also had a method intended for compression set test pieces which was similar except that the force on the foot was 850 30 mN and the contact members were either domed surfaces of 12.5 mm radius or a spherical contact of 6.35 mm diameter and a raised platform of 9.5 mm diameter. This use of curved surfaces for compression set is based on the fact that after compression, particularly with non-lubricated test pieces, the rubber may well have concave surfaces. This does not happen if the test piece is lubricated as is now the usual practice and, hence, the curved surfaces were eliminated. However, if concave test pieces are encountered it may well be better to resort to the old method... 

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. 

ASTM D429 also has another direct tension procedure as Method D, which is for test pieces bonded after vulcanization (normally the bond is formed during curing of the rubber). Curiously, this differs from method A in that a compression set test piece is specified, but which one is not stated, and the speed is 0.83 mm/s. 

Figure 9.2 illustrates the sample sizes and basic test equipment for the compression set test. 

Constant-Deflection-Compression-Set Test ASTM D 3574 -Test D. This test consists in deflecting the flexible foam specimen under specified conditions of time and temperature and noting the effect on the thickness of the specimen. Ordinarily the specimen is deflected to 50%, 75%, or 90% of its thickness. The entire assembly is then placed in a mechanically convected air oven at 70°C (158°F) and 5% RH for 22 hours. Following this exposure the specimen is then removed from the apparatus and the recovered thickness measured. The constant-deflection compression set, expressed as a percentage of the original thickness, is then calculated. 

In principle any of the tests could be used to study crystallization of rubbers by conditioning the test pieces for much longer times than normal, but in practice the favored method is change in hardness, one reason being that unvulcanized materials can be tested. Because crystallization is more rapid in the strained state, a particular type of compression set test has also been standardized for rubbers. 

Compression stress relaxation is described in ISO 3. 84 (BS903, Part A42). The principal procedure uses Type A or Type B compression set test pieces compressed by 25% against lubricated surfaces, but ring test pieces are specified for tests conducted under fluids. 

The determination of set in tensile strain is much less commonly specified than that in compression, although in principle it is a particularly straightforward procedure a strip, dumbbell or ring lest piece of known reference length is. stretched to a given extension, exposed in this condition to a combination of temperature and time, and then released for a specified period before measurement of the reference length. One clear advantage for the test, as say a measure of the state of cure, is that test pieces can be cut from even the thinnest of latex films, whereas the minimum ply thickness of a laminated compression set test piece is 2 mm. 

Compression set tests are usually run at elevated temperatures to simulate conditions or aging effects. Common test conditions are 70 hr at 70°C or 100°C, although heat-resistant materials such as fluoroelastomers may be tested for longer periods of time at temperatures up to 200°C or more. If the end product is expected to perform at low temperatures, e.g., below 0°C, compression set is measured at the expected service temperature. 

Compression set, which is a measure of the recovery of rubber after release from compressive forces, is a combination of creep and stress relaxation and, since tests are conducted at elevated temperatures, the chemical processes of stress relaxation are likely to predominate. Compression set test conditions have been specified by seal users, and are considered by some engineers to be an excellent indicator of sealing efficiency. Compression tests are used in service specifications as a measure of cross-linking. 

Figures 5 and 6 compare the compression set of vulcanized Parel, natural rubber, and neoprene elastomers at 100 and 150 C., respectively. At 100 C., the compression set of Parel elastomer is within experimental error of that of natural rubber neoprene is clearly superior. When the compression set test was run for a long time at 150 C. (Figure 7), Parel elastomer was the best of these three rubbers. Natural rubber had 100% compression set...

One-half sized ASTM dumbbells of rubber ( <85 mils thick) were aged in a forced draft air oven. Their tensile properties were measured on a Model LACC Scott Tester, at 20 inches/min. Compression set tests were made on buttons 0.5 inch thick and 0.75 inch in diameter. They were compressed 25%, according to ASTM D-395 method B. 

More commonly, compression-set tests are used on rubber-type materials. Rubber-seal type materials are used in compression because the test apparatus is simple and inexpensive to construct. Despite this popularity, stress-relaxation testing gives a more useful insight into the suitability of a material for seals [5]. 

The best compression set is obtained with ST type of polysulfide and with a zinc peroxide-cure system. Optimum results are obtained when the vulcanization temperature is 155°C and a post-cure of 24 h at lOO C is used. Compression sets tested for 22 h at 70°C when cured for 30 min at the following temperatures are 52% at 142°C, 36% at 147.4°C, and 24% at 154°C. These results may be improved after post-curing for 24 h at 100°C to 23%, 15%, and 14%, respectively. The maximum recommended operating temperature for Thiokol ST in compression is 70°C and Thiokol FA is not suitable for applications requiring even fair compression set at elevated temperatures. The maximum recommended operating temperature for polysulfides in parts not under compression is 100°C for continuous service. The temperature of vulcanization should not exceed 163°C. 

It is now possible by incorporating acid acceptors, such as certain metal oxides, to prevent this bloom occurring. Normally less than 0 5% of a suitable metal oxide is found completely effective. However, such additives do not by themselves remove the need for a post-cure where compression set and heat stability properties are of paramount concern. In these cases a post-cure temperature should be selected at least 20°C above the expected service temperature or compression set test temperature. 

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