Functional Properties and Morphology of Recycled Post-consumer WEEE Thermoplastic Blends
This work concerns a waste electrical and electronic equipment blend of recycled plastics (WEEEBR), reprocessed from a low density thermoplastics batch (600 kg) by removal of non-thermoplastic contamination (1.2 weight % (wt%)). Some well-distributed inorganic domains (mainly 5-20 µm) containing Ca, Na, Mg, Fe or Ba still remained in the WEEEBR after melt-filtration.
The WEEEBR consisted of high impact polystyrene (HIPS, 42 wt%), acrylonitrile-butadiene-styrene copolymer (ABS, 38 wt%), polypropylene (PP, 10 wt%) and other thermoplastics
(10 wt%). Although PP was a minor component in the blend, the phase separation of the polymers and the shear yielding observed in the fracture surfaces indicated the existence of partial co-continuous structures at appropriate mixing times, viscosity ratios and temperatures. Co-continuity of WEEEBR was suggested as a transitory morphological state, which was only observed by twin screw extrusion under certain processing conditions (around 60 rpm,
200 oC). These processing conditions resulted in a yield point and an elongation at break (εb) at about 5 %. Increasing the screw rotation rate by only 30 rpm or increasing the barrel temperature by only 20 oC resulted in a WEEEBR material without a yield point and about half the εb-value.
The processing conditions in single screw extrusion or injection moulding were not observed to yield a co-continuous PP phase. Furthermore, injection moulding resulted in a layered structure with a small variation in composition in the layered structure. In addition, single screw extruded WEEEBR contained a significant amount of voids (50-300 µm), reducing the load-bearing cross-sectional area and probably giving rise to stress concentrations.
The WEEEBR was already chemically degraded in the as-received state, which was indicated by a smaller than expected exotherm associated with the lower activation energy and the antioxidants were mainly de-activated (consumed). An increase in stiffness and a significant decrease in ductility were observed after reprocessing, which could to some extent be attributed to physical ageing caused by the cooling conditions used.
The ductility of WEEEBR was significantly improved by compatibilisation, which was expected to promote the shear yielding deformation mechanism, particularly around defects and inclusions. Among four studied compatibilisers, the addition of only 2.5 wt% SEBS was found to increase the εb-values of uncompatibilised WEEEBR more than 5 times.