TESTS ON UNVULCANIZED RUBBERS

A number of international standards have been published which give specific test mix recipes and evaluation procedures for particular polymers. These are recorded in references 2-11 and similar British and ASTM methods also exist. These standards refer to the general test methods and standard methods of preparation which are discussed below but include additional detail relevant to the polymer in question. They also include very limited testing on the vulcanisates. The main purpose of these standards is to provide a basis for comparison and evaluation of particular polymers as regards their processing and vulcanisation characteristics their scope does not extend to the general physical properties of vulcanisates. 

Mixing and moulding have been considered under the general heading of preparation of test pieces in Chapter 4. Where compounded but unvulcanised rubbers are to be tested, the same standard mixing procedures will be relevant together with further details relevant to particular polymers, as referred to in Section 1 above. 

Additionally, general preparation procedures have been standardised for the tests discussed in this chapter. ISO 179512 gives simple instructions for taking a sample from selected bales or, where the rubber is in the form of chips or powder, taking the sample from the package. The sample is specified to be between 350 and 1500 g depending on the tests to be carried out and a note mentions that a surface layer my need to be removed if talc or a release agent is present. 

By weighing the first and last passes, the mill homogenization procedure is also use to measure volatile matter content. Samples are then allocated to such tests as are required in accordance with ISO methods. In particular, it is specified that volatile matter is measured according to the oven method of ISO 24813. Apparently, we have two measures of volatile matter Mooney viscosity and plasticity retention index are measured as discussed later in the chapter. Although the title and the procedure indicate that the method is intended for raw rubber, it is also specified that vulcanization characteristics are measured. 

For synthetic rubbers, the mill homogenization is generally omitted, although it is specified as an alternative where the appropriate evaluation procedure requires it before measuring Mooney viscosity. Different conditions are given for specific polymers. The mill method of ISO 248 for determining volatile matter is specified but the oven method may be substituted if the material sticks to the mill rolls. Regardless of which volatile matter method is used, the mill procedure is required to dry samples for any chemical tests needed - unless this is not possible. To the uninitiated at least, this is not a model of clarity. Vulcanization characteristics are determined for synthetic rubbers, but not plasticity retention index. 

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Rheometer measurements measure cure and cure rate characteristics of the rubber. The component manufacturer performs this test on unvulcanized rubber. The rheometer measures the viscosity of the rubber as a function of time at a constant temperature. As time increases, the degree of cure or cross-linking increases and thus the viscosity increases. 

Several tests may be performed on unvulcanized rubber or on standard-shaped test specimens to measure the properties of a rubber compoundJ These include the following ... 

ASTM D6048, Standard practice for stress relaxation testing of raw rubber, unvulcanized rubber compounds and thermoplastic elastomers. New ASTM standard prepared in Stress Relaxation Task Group to D11.12, Subcommittee on Processability. 

Dynamic mechanical measurements for elastomers that cover wide ranges of frequency and temperature are rather scarce. Payne and Scott [12] carried out extensive measurements of /a and /x" for unvulcanized natural mbber as a function of test frequency (Figure 1.8). He showed that the experimental relations at different temperatures could be superposed to yield master curves, as shown in Figure 1.9, using the WLF frequency-temperature equivalence, Equation 1.11. The same shift factors, log Ox. were used for both experimental quantities, /x and /x". Successful superposition in both cases confirms that the dependence of the viscoelastic properties of rubber on frequency and temperature arises from changes in the rate of Brownian motion of molecular segments with temperature. 

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