Effect of process variable on production of thermoplastic vulcanizates based on SEBS/PP/LLDPE via reactive extrusion

Properties of thermoplastic vulcanizates (TPVs) depend on their microstructure. Processing conditions including shear rate, processing temperature and shearing time can affect the final microstructure of TPVs. Morphological study of resultant TPVs was carried out using a scanning electron microscope. The object of this study is to investigate the effect of temperature (210–230 °C, 170–190 °C, 150–170 °C and 140–160 °C) and speed of production process on the properties of TPV based on poly(styrene-b-(ethylene-co-butylene)-b styrene) triblock copolymer/polypropylene (PP)/linear low-density polyethylene prepared via reactive extrusion. Melt flow index (MFI) of the prepared samples was sharply decreased (from 24.4 to 2.63 g/10 min at 190 °C, 10 kg) with decreasing temperature (from 210–230 °C to 170–190 °C). However, hardness of the prepared TPVs was increased to a limiting value (80 shore A) with decreasing the process temperature. Compression set sharply decreased (from 70 to 49% at 120 °C, 70 h) and it has reached to a limiting value (46%). The process temperature, however, is not significantly affected on the tensile strength and tensile strength at 20% extension of the TPV. The elongation has the lowest value in 170–190 °C. Result of elongation at break after aging was improved (from 12.2 to 3.3%) with decreasing process temperature (from 140–160 °C to 210–230 °C). The lowest value of tensile strength (4.1%) was obtained in 170–190 °C after aging. Due to the increase in speeds of the twin screw extruder (180–250 rpm, in 140–160 °C) surface quality was suitable and smooth. Also TPV samples were prepared by either one-step or two-step reactors using the extruder system and the results were compared. In the one-step method, a pronounced higher value in elongation at break (897%) and MFI (6.8 g/10 min at 190 °C, 10 kg) were observed. It was found that the results of heat aging were improved using the two-step process. The two-step process led to higher tensile stress at 20% extension (from 2.7 to 3.3 MPa), too. In addition, a comprehensive experimental study was carried out to achieve the optimal process conditions for the production of TPVs using the two-step process. In one method (method A), the curing agent was added in the first step of the production. Other components such as PP, antioxidant, which may interfere with the peroxide curing were mixed in the second step of the process. In another method (method B), thermoplastic elastomer was produced in the first step, and then curing agent was added in the second step of the productions. The TPV then was prepared in the second step of the process. Generally, the results show that the properties of the prepared TPV with the method B are superior to the method A.