Designed to Degrade: Tailoring Polyesters for Circularity

A powerful toolbox is needed to turn the linear plastic economy into circular. Development of materials designed for mechanical recycling, chemical recycling, and/or biodegradation in targeted end-of-life environment are all necessary puzzle pieces in this process. Polyesters, with reversible ester bonds, are already forerunners in plastic circularity: poly(ethylene terephthalate) (PET) is the most recycled plastic material suitable for mechanical and chemical recycling, while common aliphatic polyesters are biodegradable under favorable conditions, such as industrial compost. However, this circular design needs to be further tailored for different end-of-life options to enable chemical recycling under greener conditions and/or rapid enough biodegradation even under less favorable environmental conditions. Here, we discuss molecular design of the polyester chain targeting enhancement of circularity by incorporation of more easily hydrolyzable ester bonds, additional dynamic bonds, or degradation catalyzing functional groups as part of the polyester chain. The utilization of polyester circularity to design replacement materials for current volume plastics is also reviewed as well as embedment of green catalysts, such as enzymes in biodegradable polyester matrices to facilitate the degradation process.

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Recent progress on biodegradable polylactic acid based blends and their biocomposites: A comprehensive review

Being less dependent on non-renewable resources as well as protecting the environment from waste streams have become two critical primers for a global movement toward replacing conventional plastics with renewable and biodegradable polymers. Despite all these efforts, only a few biodegradable polymers have paved their way successfully into the market. Polylactic acid is one of these biodegradable polymers that has been investigated thoroughly by researchers as well as manufactured on a large industrial scale. It is synthesized from lactic acid obtained mainly from the biological fermentation of carbohydrates, which makes this material a renewable polymer. Besides its renewability, it benefits from some attractive mechanical performances including high strength and stiffness, though brittleness is a major drawback of this biopolymer. Accordingly, the development of blends and biocomposites based on polylactic acid with highly flexible biodegradable polymers, specifically poly(butylene adipate co terephthalate) has been the objective of many investigations recently. This paper focuses on the blends and biocomposites based on these two biopolymers, specifically their mechanical, rheological, and biodegradation, the main characteristics that are crucial for being considered as a biodegradable substitution for conventional non-biodegradable polymers.

A Review of Biodegradable Plastics: Chemistry, Applications, Properties, and Future Research Needs

Environmental concerns over waste plastics' effect on the environment are leading to the creation of biodegradable plastics. Biodegradable plastics may serve as a promising approach to manage the issue of environmental accumulation of plastic waste in the ocean and soil. Biodegradable plastics are the type of polymers that can be degraded by microorganisms into small molecules (e.g., H2O, CO2, and CH4). However, there are misconceptions surrounding biodegradable plastics. For example, the term "biodegradable" on product labeling can be misconstrued by the public to imply that the product will degrade under any environmental conditions. Such misleading information leads to consumer encouragement of excessive consumption of certain goods and increased littering of products labeled as "biodegradable". This review not only provides a comprehensive overview of the state-of-the-art biodegradable plastics but also clarifies the definitions and various terms associated with biodegradable plastics, including oxo-degradable plastics, enzyme-mediated plastics, and biodegradation agents. Analytical techniques and standard test methods to evaluate the biodegradability of polymeric materials in alignment with international standards are summarized. The review summarizes the properties and industrial applications of previously developed biodegradable plastics and then discusses how biomass-derived monomers can create new types of biodegradable polymers by utilizing their unique chemical properties from oxygen-containing functional groups. The terminology and methodologies covered in the paper provide a perspective on directions for the design of new biodegradable polymers that possess not only advanced performance for practical applications but also environmental benefits.

Assessing labelled carbon assimilation from poly butylene adipate-co-terephthalate (PBAT) monomers during thermophilic anaerobic digestion

PBAT (poly butylene adipate-co-terephthalate) is a widely used biodegradable plastic, but the knowledge about its metabolization in anaerobic environments is very limited. In this study, the anaerobic digester sludge from a municipal wastewater treatment plant was used as inoculum to investigate the biodegradability of PBAT monomers in thermophilic conditions. The research employs a combination of ¹³C-labelled monomers and proteogenomics to track the labelled carbon and identify the microorganisms involved. A total of 122 labelled peptides of interest were identified for adipic acid (AA) and 1,4-butanedio (BD). Through the time-dependent isotopic enrichment and isotopic profile distributions, Bacteroides, Ichthyobacterium, and Methanosarcina were proven to be directly involved in the metabolization of at least one monomer. This study provides a first insight into the identity and genomic potential of microorganisms responsible for biodegradability of PBAT monomers during anaerobic digestion under thermophilic conditions.

The Use of Branching Agents in the Synthesis of PBAT

Biodegradable polyesters represent an advanced alternative to polyolefin plastics in various applications. Polybutylene adipate terephthalate (PBAT) can compete with polyolefins in terms of their mechanical characteristics and melt processing conditions. The properties of PBAT depend on the molecular weight, dispersity, and architecture of the copolymer. Long-chain branching (LCB) of the PBAT backbone is an efficient method for the improvement of the copolymer characteristics. In the present work, we studied branching agents (BAs) 1–7 of different structures in the two-stage polycondensation of 1,4-butanediol, dimethyl terephthalate, and adipic acid and investigated the composition and melt rheology of the copolymers. According to the results of the research, 1,1,1-tris(hydroxymethyl)ethane 2 and 3-hydroxy-2-(hydroxymethyl)-2-methylpropanoic acid 5 outperformed glycerol 1 as BAs in terms of shear thinning behavior and viscoelasticity.

Experimental Determination of Molecular Weight-Dependent Miscibility of PBAT/PLA Blends

Blends of poly(butylene adipate-co-terephthalate) (PBAT) and polylactide (PLA) have attracted the attention of academia and industry as a sustainable material. Unfortunately, this combination results in problems related to poor miscibility on the molecular level. This study mainly aims to determine the influence of molecular weights on the miscibility of PBAT/PLA blends. First, polymers with various molecular weights were obtained by the hydrolysis of PBAT and methanolysis of PLA. Second, the two components were solution-blended with different molecular weights and weight ratios. Third, each blend was heated to the molten state and subsequently stored at room temperature. Finally, the samples were tested using DSC and SEM. The thermal analysis indicated that the difference in glass transition temperature between both components decreased from about 91 °C to 57 °C and 0 °C, as the number-average molecular weights (M_n) decreased from 52/127 to 9.4/9 and 6.3/6.6 kg/mol. Moreover, the morphology changed from phase-separated with dispersed large particles gradually to uniform and homogeneous. This experimental work validated the trends predicted in the previous study, namely that PBAT/PLA blends changed the state from immiscible to partially miscible to fully miscible with decreasing M_n values. Moreover, we discussed the influencing factors such as weight ratio, temperature, and molecular structure on the miscibility. Based on the results, this work contributes to developing partially miscible and compatible blends without additives.