Challenges and opportunities of solvent-based additive extraction methods for plastic recycling.

Additives are ubiquitously used in plastics to improve their functionality. However, they are not always desirable in their 'second life' and are a major bottleneck for chemical recycling. Although research on extraction techniques for efficient removal of additives is increasing, it resembles much like uncharted territory due to the broad variety of additives, plastics and removal techniques. Today solvent-based additive extraction techniques, solid-liquid extraction and dissolution-precipitation, are considered to be the most promising techniques to remove additives. This review focuses on the assessment of these techniques by making a link between literature and physicochemical principles such as diffusion and Hansen solubility theory. From a technical point of view, dissolution-precipitation is preferred to remove a broad spectrum of additives because diffusion limitations affect the solid-liquid extraction recoveries. Novel techniques such as accelerated solvent extraction (ASE) are promising for finding the balance between these two processes. Because of limited studies on the economic and environmental feasibility of extraction methods, this review also includes a basic economic and environmental assessment of two extreme cases for the extraction of additives. According to this assessment, the feasibility of additives removal depends strongly on the type of additive and plastic and also on the extraction conditions. In the best-case scenario at least 70% of solvent recovery is required to extract plasticizers from polyvinyl chloride (PVC) via dissolution-precipitation with tetrahydrofuran (THF), while solid-liquid extraction of phenolic antioxidants and a fatty acid amide slip agents from polypropylene (PP) with dichloromethane (DCM) can be economically viable even without intensive solvent recovery.