Compression and tension set

Increasing the crosslink density of the elastomer dispersion results in improvements of the strength and tension set of the blend. The difference between the earlier commercial grades of partially cured EPDM/PP blends (TPR, Uniroyal) and the more recent commercial grades of completely cured EPDM/PP blend is in the improved elastomeric properties, viz. reduced compression and tension set and improved flexural fatigue. More important, the chemical resistance and resistance to oil swelhng is improved. Typical properties of commercial dynamically vulcanized EPDM/PP blend (Santoprene ) are shown in Table 15.13. 

The network structure of linear and branched TPUs is obviously different. The branched thermoplastics are capable of forming allophanate and/or possibly biuret crosslinks under suitable conditions. These conditions are partly met by the processing temperatures but usually a subsequent hot-air cure is necessary to achieve the optimum set properties. Inevitably, therefore, the branched TPUs have considerably lower compression and tension set properties than the truly all-linear thermoplastic TPUs, and in this respect approximate to the castable polyurethanes. 

TPEs a good elastomeric recovery and the properties of thermoplastic polymers. Their preparation, structure, and properties were carefully analyzed by Coran and Patel [38]. Their morphology has many common characteristics with the thermoplastic elastomeric polyolefins and involves a highly vulcanized elastomeric phase uniformly distributed in a melt-processable matrix Rader [5] compares it to a raisin pie . The vulcanization of the elastomer takes place when in a molten plastic. The crosslinking improves several TPE properties behavior with respect to temperature, resistance to swelling in fluids, compression and tension set, creep and stress relaxation. 

The obvious ways to measure the temperature at which the ability to recover from a deformation is lost are by the loss tangent from dynamic tests or by compression or tension set measurements. Dynamic analyzers arc an excellent way of characterizing low-temperature characteristics, stiffness as well as viscous loss, but they are a relatively modern invention and expensive. It is, how ever. a little. surprising that rebound resilience tests have not been commonly used. Set is quite often used, with compression set being favored over tension set. A particular form of recovery test developed and standardized for measurement of low-temperature behavior is the so-called temperature retraction test. This consists of stretching a dumbbell test piece, placing it in a bath at -70 C. and allowing it to retract as the temperature is raised- in a sense a variation on tension set. 

ISO 2285 2001 Rubber, vulcanized or thermoplastic - Determination of tension set under constant elongation, and of tension set, elongation and creep under constant tensile load ISO 7616 1986 Cellular plastics, rigid - Determination of compressive creep under specified load and temperature conditions... 

Data can be obtained from tests in uniaxial tension, uniaxial compression, equibiaxial tension, pure shear and simple shear. Relevant test methods are described in subsequent sections. In principle, the coefficients for a model can be obtained from a single test, for example uniaxial tension. However, the coefficients are not fully independent and more than one set of values can be found to describe the tension stress strain curve. A difficulty will arise if these coefficients are applied to another mode of deformation, for example shear or compression, because the different sets of values may not be equivalent in these cases. To obtain more robust coefficients it is necessary to carry out tests using more than one geometry and to combine the data to optimize the coefficients. 

The present ISO standard for creep is ISO 80131 which specifies procedures for measurements in compression and shear. In earlier standards, creep and stress relaxation were covered in the same documents and creep in tension was included. One reason for the separation was that stress relaxation became more important for seal performance, whereas creep remained a more minority interest. Measurements in tension were dropped on the basis that engineering components are not generally stressed in this manner. However, it is worth noting that, if a general indication of creep performance is required, the strains in tension can be relatively large and only quite simple apparatus is necessary. Such a simple method is included in the ISO standard for tension set described in Section 3.2. The British equivalent, BS903 Part A152 is identical to ISO 8013. 

Set tests are made in either tension or compression and for their prime use, quality control, the choice of mode can be made according to the convenience of the test piece available. If intended to simulate service conditions, e.g. indentation of flooring, the most relevant mode of deformation would be used. Tests can be carried out in which the test piece is subjected to either constant stress or constant strain but, as the latter is by far the most widely used, the illustration of set measurement given in Figure 11.3 is based on constant strain in the compression mode. 

The most straightforward way to measure the effect of low temperatures on recovery is by means of a compression set or tension set test. Tests in compression are favoured and a method has been standardised internationally. The procedure is essentially the same as set measurements at normal or elevated temperatures and has been discussed in Chapter 10, Section 3.1. As the recovery of the rubber becomes more sluggish with reduction of temperature the dynamic loss tangent becomes larger and the resilience lower (see Chapter 9), and these parameters are sensitive measures of the effects of low temperatures. Procedures have not been standardized, but rebound resilience tests are inherently simple and quite commonly carried out as a function of temperature. It is found that resilience becomes a minimum when the rubber is in its most leathery state and rises again as the rubber becomes hard and brittle. 

In engineering design. Yuung s modulus is used for tension and compression and the rigidity modulus lor shear, as in lorsion springs. According to Hooke s Law, Ihe stress set up within an elastic body is proportional 10 the strain lo which the body is subjected by the applied load. 

When considering the bending of a beam and attempting to extract a modulus value one must make several assumptions, the most important being that the modulus in tension is the same as in compression, and is independent of strain (at least for the range of strain involved). The simple Bernoulli-Euler theory is usually used to interpret the data. When performing resonance tests it is particularly useful to find a set of resonances and compare the measured frequency ratios with the theoretical ones given in the previous chapter. 

Gas saturated solutions possess lower viscosity (e.g. in the region of viscosity of water). The surface tension between gas and liquid phase is also lowered. By the expansion of such gas saturated solution over a nozzle or other expansion device the compressed medium is set free, due to its high vapour pressure. Therefore the solution is cooled. At the same time high... 

We note that the elasticity discussed above is only for planar bilayers under compression or tension and does not extend to the bending of bilayers. On the contrary, our analysis has shown that the bending of a bilayer is favoured down to the critical packing radius (assuming that the lipids can freely rearrange by lateral movement and/or flip-flop), and that bending elasticity sets in only for radii smaller than this critical value. The elasticity of a fluid bilayer is therefore seen to be profoundly different from that of a classical elastic plate or shell. 

When a thin rod of cross section A and length I, initially at equilibrium, is stretched or compressed by application of a force / (Fig. 1.5.3) it is conventional to introduce a stress a = fj A that induces a strain s = dl/1 and to set cr > 0, dl > 0 when the rod is under tension and a <0, dl <0 under compresion. Note that in either case work is performed on the rod. In the elastic limit when the work element is carried out reversibly. 

Figure 1.33. Test for the pristine state of the water-decane interface. The Interfaclal tension is measured during compression and expansion cycles (a) meticulously purified decane, (b) commercial decane sample. (Redrawn from R. Miller, P. Joos and V.B. Falnerman, Adv. Colloid Interface Set 49 (1994) 249.)...

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