**4.1 Thermoplastic elastomers based on Styrene Block Copolymers (SBC)**

Commercial available TPE-S materials are generally a compound of a styrene block copolymer, commonly poly(styrene-b-ethylene/butylene-b-styrene) (SEBS) or poly(styrene-b-butadiene-b-styrene), and a thermoplastic polymer, mostly polypropylene (PP). Additionally, plasticizer, mineral fillers and other components are used to achieve the demanded properties. In Fig. 6 (left) force - temperature curves and the corresponding relaxation spectra of two different types of SBC - compounds are represented. Up to 110 °C, both materials behave almost identical, but at higher temperatures the force of the SEBS/PE compound drops down to zero close to 120 °C, whereas the force of the SEBS/PP compound decreases more or less slightly until the base line is approached at about 165 °C. In the relaxation spectrum of both materials a significant peak at about 100 °C is observable which corresponds to the glass transition temperature of the styrene hard phase of the SEBS. At higher temperature (120 °C or 160 °C) an additional peak appears which is caused by the melting of the thermoplastic component, i.e. polyethylene or polypropylene, respectively. From these measurements it becomes clearly obvious; the upper service temperature range of SBC compounds is limited by the glass transition of the polystyrene hard phase. An increased upper service temperature limit may result from the existence of a co-continuous phase of a thermoplastic component having a higher melting temperature. In case of polyethylene as the thermoplastic component, an improvement up to 120 °C can be achieved, whereas by use of polypropylene higher temperature, up to a maximum value of 160 °C, is possible.

Characterization of Thermoplastic Elastomers

a. Commercial grades based on EPDM/PP

by Means of Temperature Scanning Stress Relaxation Measurements 357

In Figure 6 (right) a series of stress - temperature - curves and relaxation spectra is also represented, which were obtained from SEBS/PPE blends, containing increasing proportion of PPE. Due to increasing content of PPE, the glass transition of the SEBS hard phase is shifting to higher temperatures and thus makes the hard phase more heat resistant. Furthermore, the level of the stress - temperature - curves is also increasing, caused by the reinforcing effect of the hard domains. As can be seen from the AFM phase images shown in Figure 7, the size of the hard domains (dark) is increasing, after PPE was added to the system. Obviously, the hard domains of the SEBS and also of the SEBS/PPE blends act as physical network junctions and additionally as filler particles. In this case, no co-continuous phase of the thermoplastic component exists, as in case of the SEBS/PP blends. The stress - temperature curves and the relaxation spectra reveal the failure of the samples occurred slightly above the glass transition temperature of the hard phase. If combining the SEBS/PPE system with an additional thermoplastic blend component, e.g. polypropylene or polyamide 12, the thermal mechanical behaviour of the material can be improved on further (Barbe et al., 2005).

**4.2 Thermoplastic polyolefin blends (TPO) and dynamic vulcanizates (TPV)** 

Most of the commercial available thermoplastic vulcanizates (TPV) are produced from polyolefin blends, in particular EPDM/PP, by the process of dynamic vulcanization. TPV exhibit several advantages over simple thermoplastic polyolefin blends (TPO). Due to selective crosslinking of the EPDM rubber phase, almost all material properties are improved. Especially compression set, creep, stress relaxation and swell behaviour are highly important for automotive applications, e.g. all kinds of sealing systems. As shown in Figure 8 for different types of commercial TPO and TPV, dynamic vulcanization has strong impact on relaxation behaviour (Reid et al., 2004). Whereas the stress - temperature - curve of a simple TPO blend exhibits a strong decrease of stress with increasing temperature, the decrease of stress of a TPV material of comparable hardness is significantly lower. As it becomes also obvious from Figure 8, the differences between TPO and TPV depend on hardness; the lower the hardness, the bigger the differences, and vice versa. Only one significant peak is observable in the relaxation spectra of the TPV, which is assigned to the melting of the PP matrix.

Sample Shore - A σo T10 T50 T90 TSSR - Index

SEBS/PP (100/60) 69 1.098 63.7 107.5 159.0 0.655 SEBS/PE (100/60) 70 1.106 61.9 106.2 120.5 0.796

SEBS/PPE (100/0) 22 0.183 80.6 111.1 111.8 0.922 SEBS/PPE (100/20) 28 0.269 90.5 128.1 145.7 0.844 SEBS/PPE (100/40) 43 0.460 69.9 125.7 151.3 0.765 SEBS/PPE (100/60) 64 0.929 54.0 112.6 150.7 0.659

In case of low hardness TPO a smaller peak appears at about 40 to 60 °C, which might be related to the -relaxation process of the PP phase. At higher temperatures, when the PP ma-

Table 1. Results of the TSSR tests as represented in Fig. 6.

MPa °C °C °C RI

However, the latter values have to be considered as theoretical maxima. For obvious reasons, the upper limits of the service temperature have to be significantly lower than those maximum values. Because the melting temperature of polypropylene is considerably higher than of polyethylene, PP is favoured as thermoplastic component for commercial TPE-S materials. Normally, it is not possible to detect the glass transition temperature of the PS hard phase of commercial SBC compounds by means of traditional DSC and DMA measurements, because of the complex compound composition and the limited sensitivity of the instruments. In contrast, TSSR measurements are very sensitive with respect to relaxation processes of the hard phase and therefore more suitable, to characterize and improve those materials.

Fig. 6. *Left:* Force - temperature curves and relaxation spectra obtained from SEBS/PE and SEBS/PP blends. *Right:* Stress - temperature - curves and relaxation spectra obtained from SEBS/PPE blends with increasing proportion of PPE.

Fig. 7. AFM phase images of pure SEBS (left) and SEBS/PPE blend (right) with blend ratio of 100/20.

An alternative route to improve the heat resistance of TPE-S materials, i.e. TPE based on SBC, exists by blending the SBC with poly(p-phenylene ether) (PPE). PPE and PS are known to be thermodynamically miscible over the entire composition range, i.e., they form a blend with only one glass transition temperature (Tucker et al., 1988). Thus, by blending with PPE the glass transition of the SBC hard phase may be increased up to 150 °C (Barbe et al., 2005).

However, the latter values have to be considered as theoretical maxima. For obvious reasons, the upper limits of the service temperature have to be significantly lower than those maximum values. Because the melting temperature of polypropylene is considerably higher than of polyethylene, PP is favoured as thermoplastic component for commercial TPE-S materials. Normally, it is not possible to detect the glass transition temperature of the PS hard phase of commercial SBC compounds by means of traditional DSC and DMA measurements, because of the complex compound composition and the limited sensitivity of the instruments. In contrast, TSSR measurements are very sensitive with respect to relaxation processes of the hard

Fig. 6. *Left:* Force - temperature curves and relaxation spectra obtained from SEBS/PE and SEBS/PP blends. *Right:* Stress - temperature - curves and relaxation spectra obtained from

Fig. 7. AFM phase images of pure SEBS (left) and SEBS/PPE blend (right) with blend ratio of

An alternative route to improve the heat resistance of TPE-S materials, i.e. TPE based on SBC, exists by blending the SBC with poly(p-phenylene ether) (PPE). PPE and PS are known to be thermodynamically miscible over the entire composition range, i.e., they form a blend with only one glass transition temperature (Tucker et al., 1988). Thus, by blending with PPE the glass transition of the SBC hard phase may be increased up to 150 °C (Barbe et al., 2005).

SEBS/PPE blends with increasing proportion of PPE.

100/20.

phase and therefore more suitable, to characterize and improve those materials.

In Figure 6 (right) a series of stress - temperature - curves and relaxation spectra is also represented, which were obtained from SEBS/PPE blends, containing increasing proportion of PPE. Due to increasing content of PPE, the glass transition of the SEBS hard phase is shifting to higher temperatures and thus makes the hard phase more heat resistant. Furthermore, the level of the stress - temperature - curves is also increasing, caused by the reinforcing effect of the hard domains. As can be seen from the AFM phase images shown in Figure 7, the size of the hard domains (dark) is increasing, after PPE was added to the system. Obviously, the hard domains of the SEBS and also of the SEBS/PPE blends act as physical network junctions and additionally as filler particles. In this case, no co-continuous phase of the thermoplastic component exists, as in case of the SEBS/PP blends. The stress - temperature curves and the relaxation spectra reveal the failure of the samples occurred slightly above the glass transition temperature of the hard phase. If combining the SEBS/PPE system with an additional thermoplastic blend component, e.g. polypropylene or polyamide 12, the thermal mechanical behaviour of the material can be improved on further (Barbe et al., 2005).
