ANALYTICAL STRATEGIES FOR THE QUALITY ASSESSMENT OF RECYCLED HIGH-IMPACT POLYSTYRENE
Francisco Javier Vilaplana Domingo

Departamento de Máquinas y Motores Térmicos; Instituto de Tecnología de Materiales
Escuela Técnica Superior de Ingeniería del Diseño; Universidad Politécnica de Valencia
Camino de Vera, s/n; 46022 Valencia (Spain)

ABSTRACT

Polymers are subjected to physical and chemical changes during their processing, service life, and further recovery, and they may also interact with impurities that can alter their composition. These changes substantially modify the stabilisation mechanisms and mechanical properties of recycled polymers. The assessment of the quality properties of recycled polymers is therefore crucial to guarantee the performance of recyclates in further applications. Three key quality properties have been defined for this quality analysis: degree of mixing (composition), degree of degradation, and presence of low molecular weight compounds (degradation products, contaminants, additives). Furthermore, detailed knowledge about how the different stages of the polymer life cycle affect the degree of degradation of polymeric materials is important when discussing their further waste recovery possibilities and the performance of recycled plastics. 
A dual-pronged experimental approach employing multiple processing and thermo-oxidation has been proposed to model the life cycle of recycled high-impact polystyrene (HIPS) used in packaging applications, and electrical and electronic equipment (E&E). Both reprocessing and thermo-oxidative degradation are responsible for coexistent physical and chemical effects (chain scission, crosslinking, apparition of oxidative moieties, polymeric chain rearrangements, and probably physical ageing) on the microstructure and morphology of polybutadiene (PB) and polystyrene (PS) phases; these  effects ultimately influence the long-term stability, and the rheological and mechanical behaviour of HIPS. The PB phase has proved to be the initiation point of HIPS degradation throughout the life cycle. Thermo-oxidation seems to have more severe effects on HIPS properties; therefore, it can be concluded that previous service life may be the part of the life cycle with the greatest influence on the recycling possibilities and performance of HIPS recyclates in second-market applications.
The results from the life cycle degradation simulation were compared with those obtained from real samples from a large-scale mechanical recycling plant. A combination of different analytical strategies (thermal analysis, vibrational spectroscopy, and chromatographic analysis) is necessary to obtain a detailed understanding of the quality of recycled HIPS as defined by the three key quality properties. Special attention has been devoted to the determination of brominated flame retardants in recycled HIPS from electrical and electronic equipment using chromatographic techniques, due to the legislative and environmental implications of these additives, both in terms of waste management and emissions into the environment.

Keywords: Recycling; high-impact polystyrene; degradation; quality analysis; thermal analysis; vibrational spectroscopy; chromatography; thermo-oxidation; reprocessing; polybutadiene microstructure; advanced solvent extraction; brominated flame retardants; waste electrical and electronic equipment (WEEE).