Selective dissolution and precipitation for mechanical recycling of post-use plastics
Creators: name: Ng, Wenfa
Publisher: name: figshare
Year: 2014
Subjects: subject: Environmental Science
subject: Organic Chemistry
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Environmental Chemistry

description: Plastics is ubiquitous in almost all aspects of modern life – but characteristics such as good durability and low mass-to-volume ratio that afford its use in myriad applications also present significant challenges to its end-of-life disposal. In particular, poor biodegradability, production from non-renewable resources, and, more important, the propensity of emitting hazardous substances during incineration meant that plastics use incurs a significant environmental footprint. Although various policy initiatives (ranging from reuse, reduce and recycle) have been promulgated for tackling the problem, the relatively low take-up of such initiatives around the world meant that significant challenges remain in solving the plastics disposal problem. This essay aims to provide readers with a snapshot of the environmental challenges surrounding plastic use and the various methods available for recycling post-consumer plastic waste – with special emphasis on the selective dissolution and reprecipitation (SDP) approach. More important, problems such as deterioration of polymers’ properties due to leaching of additives and stabilizers during recycling would also be delineated. Given an expansive literature, discussion of the various plastic recycling methods (such as mechanical, chemical/feedstock, and thermal) would inherently not be exhaustive. Nevertheless, the conceptual basis, advantages and disadvantages of each class of methods would be highlighted. While recovered polymers are usually employed in lower value applications relative to the plastics’ original use (known as “down-cycling”), the essay would also discuss recent trends and proof-of-concept methods in “up-cycling” plastics waste for use as adsorbents in wastewater treatment, self-healing thermal polymers, or as carbon nanotubes in supercapacitor applications. Finally, substantial coverage would be dedicated to explaining the working principles and performance parameters (e.g., choice of solvents and anti-solvents, temperature gradient etc.) governing the application of SDP in mechanical recycling of waste polymers. Briefly, SDP separates individual polymer types from a mixture by utilizing the differential dissolution temperatures of different polymers in a particular solvent. Subsequent addition of an anti-solvent to the polymer-solvent mixture induces the precipitation and recovery of the dissolved polymer as granular materials – a form-factor well-suited for a variety of applications and polymer fabrication techniques. Collectively, besides highlighting the environmental sustainability challenges surrounding plastic use, the current exposition should also be useful as a general introduction to various plastic recycling methods. Discussion of up-cycling approaches further illuminates an important trend in environmental research: i.e., treading away from accepting redeployment of recycled materials for original or low-value use to developing innovative approaches for converting waste into high performance materials. Such an approach, if successful, would provide a sizeable market for recycled plastics – thereby, helping anchor recycling into the fabric of life by providing an economic incentive for an activity that hitherto relies on voluntary participation.