Mechanical and chemical recycling of solid plastic waste.

Present estimates suggest that of the 359 million tons of plastics produced annually worldwide1, 150–200 million tons accumulate in landfill or in the natural environment2. Poly(ethylene terephthalate) (PET) is the most abundant polyester plastic, with almost 70 million tons manufactured annually worldwide for use in textiles and packaging3. The main recycling process for PET, via thermomechanical means, results in a loss of mechanical properties4. Consequently, de novo synthesis is preferred and PET waste continues to accumulate. With a high ratio of aromatic terephthalate units—which reduce chain mobility—PET is a polyester that is extremely difficult to hydrolyse5. Several PET hydrolase enzymes have been reported, but show limited productivity6,7. Here we describe an improved PET hydrolase that ultimately achieves, over 10 hours, a minimum of 90 per cent PET depolymerization into monomers, with a productivity of 16.7 grams of terephthalate per litre per hour (200 grams per kilogram of PET suspension, with an enzyme concentration of 3 milligrams per gram of PET). This highly efficient, optimized enzyme outperforms all PET hydrolases reported so far, including an enzyme8,9 from the bacterium Ideonella sakaiensis strain 201-F6 (even assisted by a secondary enzyme10) and related improved variants11–14 that have attracted recent interest. We also show that biologically recycled PET exhibiting the same properties as petrochemical PET can be produced from enzymatically depolymerized PET waste, before being processed into bottles, thereby contributing towards the concept of a circular PET economy. Computer-aided engineering produces improvements to an enzyme that breaks down poly(ethylene terephthalate) (PET) into its constituent monomers, which are used to synthesize PET of near-petrochemical grade that can be further processed into bottles.

An engineered PET depolymerase to break down and recycle plastic bottles

Increasing the stream of recycled plastic necessitates an approach beyond the traditional recycling via melting and re-extrusion. Various chemical recycling processes have great potential to enhance recycling rates. In this Review, a summary of the various chemical recycling routes and assessment via life-cycle analysis is complemented by an extensive list of processes developed by companies active in chemical recycling. We show that each of the currently available processes is applicable for specific plastic waste streams. Thus, only a combination of different technologies can address the plastic waste problem. Research should focus on more realistic, more contaminated and mixed waste streams, while collection and sorting infrastructure will need to be improved, that is, by stricter regulation. This Review aims to inspire both science and innovation for the production of higher value and quality products from plastic recycling suitable for reuse or valorization to create the necessary economic and environmental push for a circular economy.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201915651

Beyond Mechanical Recycling: Giving New Life to Plastic Waste

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

The current global plastics economy is highly linear, with the exceptional performance and low carbon footprint of polymeric materials at odds with dramatic increases in plastic waste. Transitioning to a circular economy that retains plastic in its highest value condition is essential to reduce environmental impacts, promoting reduction, reuse, and recycling. Mechanical recycling is an essential tool in an environmentally and economically sustainable economy of plastics, but current mechanical recycling processes are limited by cost, degradation of mechanical properties, and inconsistent quality products. This review covers the current methods and challenges for the mechanical recycling of the five main packaging plastics: poly(ethylene terephthalate), polyethylene, polypropylene, polystyrene, and poly(vinyl chloride) through the lens of a circular economy. Their reprocessing induced degradation mechanisms are introduced and strategies to improve their recycling are discussed. Additionally, this review briefly examines approaches to improve polymer blending in mixed plastic waste streams and applications of lower quality recyclate.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/marc.202000415

Mechanical Recycling of Packaging Plastics: A Review.

On the role of flame retardants in mechanical recycling of solid plastic waste.

Within this research the effect of injection molding temperature on polypropylene (PP)/poly(ethylene terephthalate) (PET) blends and microfibrillar composites was investigated. Injection molding blends (IMBs) and microfibrillar composites (MFCs) of PP/PET have been prepared in a weight ratio 70/30. The samples were processed at three different injection molding temperatures (Tim) (210, 230, 280 °C) and subjected to extensive characterization. The observations from the fracture surfaces of MFCs showed that PET fibers can be achieved by three step processing. The results indicated that Tim has a big influence on morphology of IMBs and MFCs. With increasing the Tim, distinctive variations in particle and fiber diameters were noticed. The differences in mechanical performances were obtained by flexural and impact tests. Establishing relationships between the processing parameters, properties, and morphology of composites is of key importance for the valorization of MFC polymers.

https://www.mdpi.com/2073-4360/8/10/355/pdf

The Effect of Injection Molding Temperature on the Morphology and Mechanical Properties of PP/PET Blends and Microfibrillar Composites.

Nanocomposites have attracted a great deal of interest during recent years. Much research has been conducted towards the incorporation of clay particles in a polypropylene matrix. However, the effect of extrusion reprocessing on the material properties has not been studied in depth. In this study, composites of polypropylene (PP) reinforced with organic modified montmorillonite (MMT) (4 wt%) and coupling agent were subjected to 15 extrusion cycles. The materials were characterized by melt flow index (MFI), plate-plate rheometry, tensile testing and impact measurements, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed an improved intercalation of the MMT clay in the first few extrusions, thus improving the mechanical properties. Increasing the processing steps furthermore lead to a decrease in elastic modulus and impact strength, which was attributed to a reduction in matrix-filler interaction, most probably caused by organoclay degradation as shown by thermogravimetric analysis (TGA). The results of this study provide a useful insight in either the recycling of PP/MMT waste streams or the use of clay particles as a strengthening filler agent in the recycling process of different polymer waste streams.

https://www.mdpi.com/2073-4360/6/12/2912/pdf

The Effect of Multiple Extrusions on the Properties of Montmorillonite Filled Polypropylene

https://biblio.ugent.be/publication/8678961/file/8678962.pdf

Laurens Delva

Papers

Mechanical and chemical recycling of solid plastic waste.

On the role of flame retardants in mechanical recycling of solid plastic waste.

The Effect of Injection Molding Temperature on the Morphology and Mechanical Properties of PP/PET Blends and Microfibrillar Composites.

The Effect of Multiple Extrusions on the Properties of Montmorillonite Filled Polypropylene

A recycler’s perspective on the implications of REACH and food contact material (FCM) regulations for the mechanical recycling of FCM plastics