Mechanical and thermal properties of recycled WEEE plastic blends

Doctoral thesis, 2016

Electronic waste is the fastest growing waste stream today, and the recycling of the plastics from waste electrical and electronic equipment (WEEE) has attracted great attention recently for environmental reasons and to comply with the European Union’s (EUs) WEEE Directive. The plastics fraction in WEEE is between 20 and 35 weight % (wt%). The WEEE plastics contain up to 15 different types which makes it difficult and costly to separate the plastics from each other, which is how plastics material recycling mainly is done today. In this work the opposite approach has been taken and the possibility to do a plastics blend of all the WEEE plastics has been investigated. This has been done in means of characterizing different WEEE plastic waste streams regarding the mechanical and thermal properties and enhance the mechanical properties of the recycled material with the addition of compatibilizers and/or gamma irradiation. The WEEE plastics study was based on a 600 kg batch of blended post-consumer recycled WEEE plastics (WEEEBR). This low-density, brominated flame retardant free blend was melt-filtered to remove contaminants, mostly thermosets such as rubbers (1.2 wt%). The composition analysis showed that the WEEEBR consisted of three main thermoplastics constituents: high impact polystyrene (PS/HIPS, 42 wt%), followed by acrylonitrile-butadiene-styrene copolymer (ABS, 38 wt%), and lastly polypropylene (PP, 10 wt%). The remaining 10 wt% were other thermoplastics, thermosets and contaminants such as wood and paper. Antimony leaching from an ABS computer casing showed that sodium hydrogen tartrate in dimethyl sulfoxide (DMSO) worked as a leaching medium with almost 50 % leaching efficiency. The hypothesis that gamma irradiation of ABS should enhance the mechanical properties by creating free radicals and making crosslinks in the plastics was not confirmed. Instead the plastics became brittle and degraded with lower mechanical properties compared with non-irradiated ABS. The melt flow rate (MFR) of gamma irradiated WEEEBR showed a decrease in viscosity of up to 100 kGy (indicating chain scissoring of the polymer chains) and then an increase in viscosity of up to 600 kGy (indicating crosslinking of the polymer chains). The WEEEBR went from being brittle to becoming a ductile material by adding only a small amount (2.5 wt%) of a styrene-b(ethylene-co-butylene)-b-styrene copolymer (SEBS) containing compatibilizer. A considerable increase in the impact strength was seen, from 2.1 kJ/m2 to 3.8 kJ/m2 with 5 wt% compatibilizer. Based on the achieved results, WEEEBR and similar blends can potentially be used as a replacement for virgin plastics when they have been melt-blended, melt-filtrated and a suitable compatibilizer has been added.