Recycling high impact polystyrene: Material properties and reprocessing in a circular economy business model

Abstract

An account is presented of successfully implemented Extended Producer Responsibility (EPR) with recycling to high value products, to help overcome barriers and generate confidence in moving towards Circular Economy (CE) business models. A template for organisations proposing to recycle a thermoplastic is provided by describing appropriate tests and considerations in implementing the recovery and re-use of high impact polystyrene (HIPS) based on a practical, industry case. Simulating the repeated 100% closed loop reprocessing of production scrap, original HIPS has been injection molded to produce tensile and impact test pieces, reground and reprocessed eight times. Assessing the present results together with literature, repeated recycling of production scrap is possible without the need for a remedial compounding step, or changes to processing parameters. Integral to the EPR model is recovery of plastic from returned end-of-life (EoL) products, and in relation to the in-use environment, studies of the effects of UV exposure on virgin material show that significant property degradation can occur. However, with indoor use, as in the present case, this is not seen. Within the CE business model it is desirable to use blends of recovered HIPS originating from different original resin manufacturers. Feeding dry blend regrind directly to the molding machine proved satisfactory, avoiding the need for a compounding/pelletizing step. As an outcome of this study, products are being successfully manufactured from 100% HIPS recovered from EoL products in an environmentally and economically positive CE plan.

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“…The main changes in the mechanical properties of HIPS upon UV exposure were observed for strain-at-break, which decreased from around 20 % to a range of 7-9% for samples UV-aged between 6 h to 10 h. Cross-linking increases the rigidity of the PB domains and eliminates their toughening effectiveness, which results in the crack propagation seen along the interface between the PB domains and the PS. The resulting reduction in impact performance is accompanied by dramatic reductions in elongation at break, with these properties approaching those of pure PS [31]. It seems that loss-in-flexibility plateaued at a UV exposure time of around 6 h to 10 h. This effect may be related to the saturation of the crosslinking density of the PB phase, where, usually, the first main chemical changes occur.…”

“…When oxygen is present formation of unstable alkyl and peroxyl radicals and photo-unstable intermediary products occurs. These products produce further radicals with UV irradiation, resulting in cross-linking and chain scission together with the formation of lower molecular weight products [25,[27][28][29][30][31]. In Figure 2, a comparison of the carbonyl region in the FT-IR spectrum is made for the different time steps of UV ageing.…”

“…It seems that loss-in-flexibility plateaued at a UV exposure time of around 6 h to 10 h. This effect may be related to the saturation of the crosslinking density of the PB phase, where, usually, the first main chemical changes occur. However, it can be expected that upon additional UV exposure, additional loss of mechanical properties would occur due to chain scission of the PS phase [31]. In Figure 7b stress-strain diagrams of recycled HIPS containing various concentrations of ADP are presented.…”