**5. Conclusion**

366 Thermoplastic Elastomers

of the curve in Figure 15 (right), the slope of the stress - curve increases immediately, after the non-isothermal test period has started. It becomes also obvious, that the slope of the non-isothermal stress - curve is partially reduced by the influence of the ongoing stress relaxation.

Fig. 15. Entire stress - curve (left) of a TSSR - test and zoomed part (right) of the curve, as

Fig. 16. Temperature coefficient (left) and apparent crosslink density (right), with and with-

In order to determine the entropic part entropy of the experimentally obtained temperature coefficient , it is necessary to eliminate the contribution of stress relaxation by means of Eq. (13). Thus, an apparent value of crosslink density of the sample can be calculated from Eq. (8) when 0 is replaced by the entropic part entropy of the temperature coefficient. To demonstrate the influence of the correction, the experimentally obtained values of temperature coefficient are plotted as a function of isothermal relaxation period, with and without correction for comparison, on the left side of Figure 16. On the right hand side of Figure 16 the corresponding values of apparent crosslink density are presented, as calculated from Eq. (8) without and with correction by Eq. (13). Without correction, the values of temperature coefficient and crosslink density vary over a wide range, starting from negative values and ap-

indicated by the rectangle.

out correction, of a commercial TPV.

The aim of this paper is to describe the opportunities of a new test method, especially developed to characterize the stress relaxation behaviour and thermoelastic properties of TPE. In contrast to conventional stress relaxation measurements the new TSSR test method is less time consuming and requires only a minimum on manual effort. Generally, three phenomena have to be considered if a stretched rubber sample is annealed under the conditions of non-isothermal TSSR tests. Stress relaxation or decrease of stress, immediately occurs after the strain has been applied to the sample. Because relaxation time constants strongly decrease with increasing temperature, stress relaxation is accelerated when temperature is scanned during a TSSR test. An opposite effect results from entropy elastic behaviour of the rubber sample. Due to increasing temperature a stretched rubber sample tends to contract and therefore the stress increases if the strain is kept constant. Furthermore, stress relaxation and entropy elastic behaviour will be superimposed by a slight increase of sample length, caused by thermal expansion. It is shown that thermal expansion of the sample is negligible, if the strain is sufficiently high. Basic equations for evaluation have been developed, taking into account the specific conditions of TSSR tests.

The versatility of TSSR measurements has been demonstrated at several examples of commercial TPE and model compounds. The relaxation spectra of commercially available TPE based on SBC exhibit two significant peaks, which can be identified as glass transition temperature of the polystyrene end blocks and the melt temperature of an additional blend component, e.g. polypropylene. Blends of SBC and PPE were investigated to improve the heat resistance of the material. It has been shown that PPE and the polystyrene end blocks of SBC form a mixed phase with elevated glass transition temperature. The corresponding shift of glass transition temperature of the hard phase could be clearly identified from TSSR relaxation spectra. Thus, TSSR measurements are a suitable tool to determine the stress relaxation properties of such complex systems.

Results obtained from investigations of thermoplastic polyolefin blends (TPO) and dynamic vulcanizates (TPV) based on EPDM/PP and EPDM/HDPE, demonstrate the versatile opportunities of TSSR measurements to characterize stress relaxation behaviour and crosslink density. Comparison of commercial TPO and TPV of different hardness clearly show that the relaxation behaviour of the material is significantly improved by crosslinking of the rubber phase. It is also seen, the impact on stress relaxation is more pronounced for materials of lower hardness.

A model system of peroxide cured TPV based on EPDM/PP was investigated to determine the crosslink density of the rubber phase. By varying the amount of curatives the crosslink density of the samples has been altered within certain limits. These samples were subjected

Characterization of Thermoplastic Elastomers

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to TSSR measurements and from the initial slope of the TSSR stress temperature curves the crosslink densities of the samples were determined, by considering the entropy effect. Additionally, TPV based on EPDM/HDPE which was designed for hard/soft - combinations with HDPE and UHMW-PE. Due to better compatibility of EPDM and HDPE, the phase morphology and also the properties differ from EPDM/PP based TPV. From the results of TSSR measurements the differences with respect to rubber elasticity and heat resistance become clearly obvious.

Although TSSR tests are relatively fast and easy to perform, an accelerated test procedure has been developed for rapid determination of crosslink density of TPV. Based on a theoretical approach a basic equation has been developed to separate the phenomenon of stress relaxation from the initial part of the experimentally observable stress - temperature curve. Thus, reliable values of crosslink density can be obtained even at strongly reduced test duration.
