Studies of the Evaluation of Solid Phase Dispersants in Polymer Blends

Abstract

The present study is concerned with the problem of recycling mixtures of thermoplastic polymers. Low-density polyethylene (LDPE) and polystyrene (PS) were chosen for this study because of the mechanical properties of these blends. To improve performance, block and graft copolymers of ethylene-styrene as solid phase dispersants (SPDs) were added to blends containing equal proportions of LDPE and PS. Since this composition represents the poorest balance of properties in this system, it has been found that the addition of SPDs generally increases yield strength, impact strength, and elongation at break of the blends. The desired SPDs were prepared by four major procedures: 1. Grafting of styrene onto processed LDPE, in which the preformed hydroperoxide acts as a free radical initiator for the graft copolymerisation. 2. By grafting of styrene into LDPE in the presence of free radical generators such as benzoyl peroxide (BPO) and cumene hydroperoxide (CHP). 3. By mechano-chemical synthesis, involving polymer-monomer interaction in the RAPRA Torque Rheometer in the absence and presence of BPO and CHP. 4. By polymer-polymer block interchange in the Torque Rheometer in the absence and presence of the initiators and also in the Brabender (Plasticorder) without initiator. In order to measure yields of block and graft copolymers, the homopolymer (PS) was first extracted by toluene or methyl-ethyl-ketone (MEK) at 40-50°C and then the amount of the copolymer in the remaining fractions was determined by IR-spectroscopic method. The highest yields of copolymers were obtained by the mechano-chemical method (polymer-polymer interaction) at 170°C. Under these conditions, favourable reactions and unfavourable degradation reactions appear to be balanced. Crystallinity, dynamic mechanical properties, morphology and stress-strain behaviour of the blends in the absence and presence of the corresponding SPDs were examined before and after thermal and ultraviolet (UV) irradiation.