Within TWI a core competency is being developed in the design and selection of green solvents as replacements for conventional VOCs and as novel media in their own right. The Polymers section within the Materials Group is now half way through an EU-funded project developing novel solvents for the separation of pure polymers from mixed waste. The Polysolve process uses highly selective yet cost-effective and biodegradable solvents to specifically extract polystyrene and polycarbonate from mixed waste streams, resulting in recycled polymers comparable in purity and performance to virgin material. This greatly elevates the utility and value of the recycled plastic as well as reducing both the levels of non-biodegradable waste going to landfill and the pressure on non-sustainable oil-derived feedstocks for polymer manufacturing.
The key to the design and selection of the right solvent from a huge range of possibilities lies in matching the properties of the solvent as closely as possible to the requirements of the process. To achieve this, TWI has access to unique solvent selection software, which is able to rapidly and accurately screen large numbers of solvents against multiple predetermined criteria. For the Polysolve project the software was able to screen over 3,500 solvents for the capability to selectively dissolve the two polymer types to meet the specific purity and performance criteria defined. The solubility parameters and properties of the target polymers and their additives were determined and candidate sets of solvents were produced. Each candidate set was then refined by further rounds of screening based on decreasing vital process criteria. The screening process is able to simultaneously assess as varied requirements as solubility, boiling point, viscosity, toxicity and cost, facilitating the selection of a solvent that is genuinely optimized around the specific needs of the process.
Successful and efficient dissolution of each polymer was achieved using environmentally benign solvents of low toxicity. The project will now focus on up-scaling this process to a pilot plant level with a capacity of processing over 50kg per batch over the next few months.
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