STUDY OF RHEOLOGICAL BEHAVIOR OF REPROCESSING HIGH IMPACT POLYSTYRENE

The effect of reprocessing High Impact Polystyrene (HIPS) has been studied in this paper. To simulate recycled HIPS, we reprocessed virgin HIPS through 5 cycles. The HIPS has been rheologically characterized after the various cycles of reprocessing in order to evaluate their corresponding properties and correlate them with the number of cycles undergone. Our results show that viscosity decrease, as the number of reprocessing cycles increases. 1. INTRODUCCTION High Impact Polystyrene is a thermoplastic formed in two phases: a styrene phase and a butadiene phase. Among the diverse applications of this material is its use in the manufacture of packaging materials. This type of product can be characterized as having a short life cycle, and consequently a huge quantity of domestic waste is generated, which must be dealt with to reduce environmental impact. The problems which arise in the recovery of polymer materials are basically the variations that occur in their properties, due to either thermic degradation or the presence of impurities. Many studies have been carried out on the degradation of polymers. Su (3) analyzed the influence of the reprocessing cycles on the mechanical properties of polyamide 6 (PA6). Su's work is relevant to our work because he carried out a study of mechanical and rheological properties, although it was with a different material. Su's work is interesting, but we do not consider that reprocessing the material 16 times is very useful, because other studies show that the loss of property occurs in the first 5 reprocessed. (2,12) Other authors, including Santana (12) in their work in 2002 and Soriano (3) have carried out similar studies on HIPS. Santana investigated the themo-mechanical properties of post-consumer HIPS (from disposable cups) through five consecutive injection moulding steps to simulating the recycling cycles. Santana states that tensile strength, modulus of elasticity and elongation at break properties of HIPS were slightly reduced, which indicate an effect of decreasing of molecular weight. Soriano analyzes influence of the number of processing cycles on the microstructure and macroscopic properties on a HIPS in coextruded sheet, maintaining a constant composition of 70 wt% of virgin HIPS and 30 wt% of recycled HIPS. With respect to the rheological properties, Soriano shows that the viscosity of the material subjected to one processing cycle is higher than of the virgin material, especially at low shear rate, but as shear rate increases, all, curves tent to concur to a similar value and this difference in viscosities becomes very small. After the second and up to the sixth processing cycle, the viscosity decreases. Soriano calculates the value of n (the model constant which symbolizes the pseudoplastic behavior slope of the material) obtaining a value of 0.31 for virgin material and 0.35 for the sixth processing cycle. This data is calculated from apparent viscosity (ηap) as Soriano used only one die (L/D 30mm) instead of the three recommended by ISO 11443. On the other hand, Balard (10) analyzed the compatibility of polycarbonate (PC) with acrylonitrile-butadiene-styrene (ABS) using waste material from the electrical sector,