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

a. Commercial grades based on EPDM/PP

352 Thermoplastic Elastomers

nal information about the composition, morphology and structure of the sample can be de-

0 50 100 150 200

Fig. 3. Theoretical stress - temperature - curves, calculated by use of Eq. (5) and Eq. (7) with

High molecular weight poly(styrene-b-ethylene/butylene-b-styrene) (SEBS) with a polystyrene (PS) content of 33%, a molar mass of the PS-blocks of 29000 g/mol and a total molar mass of *M*w = 174000 g/mol were used as the basis for the compounds prepared. In SBC/ polyolefin blends a standard isotactic polypropylene and, alternatively a standard high density polyethylene were used as the polyolefin component of the compounds. In SBC/PPE blends high molecular weight poly(p-phenylene ether) (PPE) with Tg = 215°C and molar mass of Mw = 38900 g/mol was used as the modifier. Additionally, high purity medicinal paraffin oil was used as the extender oil for all compounds and a small amount of stabilizer

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

was added to protect the polymers against degradation during the mixing process.

are described in earlier papers (Vennemann et al., 2004) and (Barbe et al., 2005).

SBC/polyolefin compounds were produced using a twin-screw extruder (L/D: 32/1, 25 mm diameter; Berstoff GmbH). SBC/PPE compounds were produced by means of a single-screw extruder (Göttfert GmbH, L/D: 15/1). In all cases the ingredients were mixed together prior feeding to the extruder, having a barrel temperature of 260°C. Test plates of 2mm thickness of all compounds were produced in a pneumatic injection moulding press. Further details

temperature T / °C

duced from the entire relaxation spectrum.

Eq. (5) Eq. (7)

0

ν = 100 mol/m3, 0 = 1.5 and = 3 . 10-4 K-1 .

**3.1 Materials and preparation of the samples** 

0.1

0.2

stress

**3. Experimental** 

σ / MPa

0.3

0.4

Several commercial grades of thermoplastic vulcanizates based on EPDM/PP covering a wide range of hardness were obtained from Solvay Engineered Polymers (TX/USA) and tested as received. The novel TPV-AP materials were produced via a dynamic vulcanization process using a new curative system and DVA process developed by Solvay Engineered Polymers (Reid et al., 2004). The new cure system results in a material with non-hygroscopic behaviour, white colour, and low odour. Properties of TPV-AP are compared to two other commercially available TPV materials. TPV-HS is a commercial TPV based upon EPDM and PP where the elastomer is crosslinked with a hydrosilation process. TPV-PH is also a commercial TPV based upon EPDM and PP where the elastomer is crosslinked with a phenolic resin curing process. The samples of both TPV-HS and TPV-PH were not produced by Solvay Engineered Polymers, but commercial grades, produced by other suppliers.

b. Model compounds of peroxide cured TPV based on EPDM/PP

Commercial available EPDM rubber and isotactic polypropylene homopolymer (PP) were used as the basis for the dynamic vulcanizates (TPV). The EPDM contains 50 wt % ethylene and 4 wt % ethylidene norbornene (ENB). It has a Mooney viscosity ML(1+4) at 125 °C, of 36. The melt flow rate of the polypropylene, measured at 230 °C and 2.16 kg is 12 g/10 min. The crosslink system consists of di(tert-butylperoxyisopropyl)benzene (abbrev.: DTBPIB) as peroxide and trimethylolpropane trimethacrylate (abbrev.: TRIM) as co-agent. The peroxide and co-agent are supplied commercially on a silica carrier, with active agent content of 40 wt % and 70 wt %, respectively. The TPV samples are designated as TPV1 to TPV6, whereas the total amount of curatives (DTBPIB and TRIM) is increasing from 1 phr to 6 phr in steps of 1 phr. The volume fraction of polypropylene was PP = 0.23 in all compounds. An uncured compound of identical EPDM/PP ratio was also produced and tested as reference sample. All samples were produced in a two-step mixing process using a Haake Rheocord 600 laboratory internal mixer (Thermo Electron Corporation, Karlsruhe). Further details of the production process are published elsewhere (Vennemann, 2006).

c. Model compounds of phenolic cured TPV based on EPDM/HDPE

Two different commercially available EPDM rubber and two different grades of high density polyethylene (HDPE) were used as the basis for the dynamic vulcanizates, in this study. Crosslinking of the EPDM in all compounds was performed with a phenolic resin cure system, consisting of stannous chloride (SnCl2 2 H2O), zinc oxide (ZnO) and SMD 31214. The latter is a commercially available solution of paraffinic mineral oil and 30 wt % of phenolic resin SP 1045. Further details of the composition and preparation of the compounds are published elsewhere (Vennemann, 2009).

#### **3.2 TSSR instrument and test procedure**

The temperature scanning stress relaxation tests were performed by use of a commercial available TSSR instrument obtained from Brabender GmbH (Duisburg, Germany). The TSSR instrument (Fig. 4) consists of an electrical heating chamber where the sample, a S2 testing rod, is placed between two clamps. The clamps are connected to a linear drive unit to apply

Characterization of Thermoplastic Elastomers

temperatures T10, T50 and T90.

**4. Results and discussion** 

by Means of Temperature Scanning Stress Relaxation Measurements 355

0

(9)

*F F dT*

90 0

90

*T*

0

*T T*

*T*

Fig. 5. Normalized force as a function of temperature and determination of characteristic

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.

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

*RI*

a certain uniaxial extension to the sample. A high quality signal amplifier in combination with a high resolution AD-converter is used to detect and digitize the analogue signals of the high-resolution force transducer and the thermocouple. In order to detect the current temperature the thermocouple is placed near the centre of the sample. All signals are transferred to a personal computer. A special software program is used for treatment and evaluation of the data as well as for the control of the test procedure.

Fig. 4. TSSR instrument and test procedure

The test procedure starts with placing the sample in the electrical heated test chamber, which is controlled at the initial temperature T0 of 23 °C. After the initial strain of 0 = 50% is applied, the isothermal relaxation period starts, whereas the temperature remains constant at 23 °C within +/- 0.1 °C. During this time most of the short time relaxation processes occur and the sample reaches a quasi equilibrium state. Then the sample is heated linearly at a constant rate of β = 2 K/min, until the stress relaxation has been fully completed or rupture of the sample has occurred.

From the obtained force – temperature curve certain characteristic quantities such as T10, T50, T90 and the TSSR index RI can be calculated. The temperature Tx stands for the temperature at which the force ratio F/F0 has decreased about x% with respect to the initial force F0. The TSSR index RI is a measure of the rubber like behaviour of the material and is calculated from the area below the normalized force – temperature curve, as represented in Figure 5 and given by Eq. (9). Additionally, the temperature coefficient and the relaxation spectrum H(T) are calculated from the initial slope and the derivative of the stress–temperature curve, respectively, as described in chapter 2, in more detail.

Fig. 5. Normalized force as a function of temperature and determination of characteristic temperatures T10, T50 and T90.
