Degradation of pre-aged polymers exposed to simulated recycling:: Properties and thermal stability

Accelerated thermal and photo-aging of four homopolymers, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and high-impact polystyrene (HIPS), was performed and the impact of subsequent reprocessing conditions on their properties studied. Polymer samples oven-aged at 100 degrees C for varying periods of time or UV irradiated in a Weather-o-meter (WOM) at lambda = 340 nm were reprocessed in a Brabender plasticorder at 190 degrees C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical properties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI, tensile impact strength). LDPE and HIPS were found to be more susceptible to thermo-oxidation than HDPE and PP, whereas HDPE and PP were affected to a greater extent by UV exposure; the crucial role here is being played by the stabilization of the studied resins. In HDPE the scission and crosslinking reactions competed both in thermo-and photo-degradation. In the case of LDPE, scission prevailed over branching during thermo-oxidation, whereas photo-oxidation of the same sample led predominantly to crosslinking. Abrupt deterioration of the LDPE rheological properties after one week of thermal exposure was suppressed by re-stabilization. The scission reaction was also predominant for PP during thermo-oxidation, and it took place even faster during UV exposure. In the case of HIPS a slight photo-degradation of PS matrix is accompanied by simultaneous crosslinking of the polybutadiene component. (c) 2005 Elsevier Ltd. All rights reserved.