The majority of post-consumer plastic waste is not recycled. Impediments to the recycling of commodity polymers include separation, impurities and degradation of the macromolecular structures, all of which can negatively affect the properties of recycled materials. An attractive alternative is to transform polymers back into monomers and purify them for repolymerization — a form of chemical recycling we term chemical recycling to monomer (CRM). Material recycled in this way exhibits no loss in properties, creating an ideal, circular polymer economy. This Review presents our vision for realizing a circular polymer economy based on CRM. We examine the energetics of polymerization and other challenges in developing practical and scalable CRM processes. We briefly review attempts to achieve CRM with commodity polymers, including through polyolefin thermolysis and nylon 6 ring-closing depolymerization, and closely examine the recent flourishing of CRM with new-to-the-world polymers. The benefits of heterocycle ring-opening polymerization are discussed in terms of synthetic control and kinetically accessible polymer-backbone functionality. Common chemical and structural characteristics of CRM-compatible ring-opening-polymerization monomers are identified, and the properties, benefits and liabilities of these recyclable polymers are discussed. We conclude with our perspective on the ideals and opportunities for the field. Unrecycled post-consumer plastic waste is an enormous, growing problem. Chemical recycling to monomer (CRM) delivers recycled material without degradation in properties. This Review assesses the viability of commercial polymer CRM, the flourishing of CRM with new polymers and opportunities for the field.

Chemical recycling to monomer for an ideal, circular polymer economy

Recycling of bioplastics, their blends and biocomposites: A review

Degradation of polyolefins during melt processing

An investigation of the mechanisms of degradation of a branched polymer in extrusion was performed using starch as substrate. Starch has the advantage that the distribution of degree of polymerization of individual branches can be readily obtained using a debranching enzyme and also that it does not undergo any reaction except scission during extrusion, thereby aiding mechanistic interpretation. Various starches, containing a range of the highly branched amylopectin component and the much less branched amylose component, were extruded in the presence of water and glycerol as plasticizers with extruder barrel temperatures ranging from 50 °C at the hopper zone through to 140 °C near the die exit. Analysis by size-exclusion chromatography of both whole and debranched samples subject to various levels of extrusion showed that the extrusion degradation process involved preferential cleaving of larger molecules, while causing the size distribution to narrow and converge toward a maximum stable size. This is ana...

Mechanism of Degradation of Starch, a Highly Branched Polymer, during Extrusion

Oxo-biodegradable carbon backbone polymers – Oxidative degradation of polyethylene under accelerated test conditions

The response of a HDPE, a L-LDPE and a PP homopolymer to processing conditions and processing stabilizer formulations in the 220°C to 280°C temperature range was investigated. The technique of multiple extrusion was used. Melt flow and melt flow ratio were measured after the first, third and fifth extrusion pass. In the case of the HDPE and the L-LDPE, degradation by cross-linking was the predominant reaction and the molecular weight distribution showed a significant broadening compared to the virgin samples. The PP homopolymer degraded by chain scission. By adding a multifunctional hindered phenol and a hydrolysis resistant phosphite, all polymers could be efficiently protected against degradation on processing. For the HDPE and the L-LDPE, increased amounts of phosphite provided increased processing stability at constant phenol concentration. A 4:1 ratio of phosphite to phenol exhibited the best performance. The most efficient ratio for protecting melt flow of the PP homopolymer was found when a two to one ratio of phosphite to phenol was added.

Processing stabilization of polyolefins

Recycled mixed plastics from domestic and commercial waste can be stabilised against thermal oxidative degradation by adding a combination of a sterically hindered phenol with a phosphite or phosphonite.

Process for stabilising recycled mixed plastics

A process for stabilising olefin polymers having a mean particle diameter of from 1 to 5 mm that have been polymerised on carrier catalysts, which comprises treating the polyolefins with a stabiliser mixture prepared by heating to from 120 to 260°C a mixture containing a) at least one carrier and/or b) at least one stabiliser that melts at from 20 to 120°C and c) at least one stabiliser that melts at from 120 to 260°C, and then cooling said mixture until solidified, components b) and c) being different.

Process for stabilising polyolefins

Recycled plastic mixtures of domestic and industrial waste can be stabilised against thermo-oxidative degradation by addition of a combination of a sterically hindered phenol with a phosphite or phosphonite.

Jean-Roch Pauquet

Papers

Degradation of polyolefins during melt processing

Processing stabilization of polyolefins

Process for stabilising recycled mixed plastics

Process for stabilising polyolefins

Process for stabilising recycled plastic mixtures