Plastic is a broad name given to different polymers with high molecular weight, which can be degraded by various processes. However, considering their abundance in the environment and their specificity in attacking plastics, biodegradation of plastics by microorganisms and enzymes seems to be the most effective process. When plastics are used as substrates for microorganisms, evaluation of their biodegradability should not only be based on their chemical structure, but also on their physical properties (melting point, glass transition temperature, crystallinity, storage modulus etc.). In this review, microbial and enzymatic biodegradation of plastics and some factors that affect their biodegradability are discussed.

Poly(lactide) (PLA) has been developed and made commercially available in recent years. One of the major tasks to be taken before the widespread application of PLA is the fundamental understanding of its biodegradation mechanisms. This paper provides a short overview on the biodegradability and biodegradation of PLA. Emphasis is focused mainly on microbial and enzymatic degradation. Most of the PLA-degrading microorganisms phylogenetically belong to the family of Pseudonocardiaceae and related genera such as Amycolatopsis, Lentzea, Kibdelosporangium, Streptoalloteichus, and Saccharothrix. Several proteinous materials such as silk fibroin, elastin, gelatin, and some peptides and amino acids were found to stimulate the production of enzymes from PLA-degrading microorganisms. In addition to proteinase K from Tritirachium album, subtilisin, a microbial serine protease and some mammalian serine proteases such as alpha-chymotrypsin, trypsin, and elastase could also degrade PLA.

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Microbial polyhydroxyalkanoates (PHAs), one of the largest groups of thermoplastic polyesters are receiving much attention as biodegradable substitutes for non-degradable plastics. Poly(D-3-hydroxybutyrate) (PHB) is the most ubiquitous and most intensively studied PHA. Microorganisms degrading these polyesters are widely distributed in various environments. Although various PHB-degrading microorganisms and PHB depolymerases have been studied and characterized, there are still many groups of microorganisms and enzymes with varying properties awaiting various applications. Distributions of PHB-degrading microorganisms, factors affecting the biodegradability of PHB, and microbial and enzymatic degradation of PHB are discussed in this review. We also propose an application of a new isolated, thermophilic PHB-degrading microorganism, Streptomyces strain MG, for producing pure monomers of PHA and useful chemicals, including D-3-hydroxycarboxylic acids such as D-3-hydroxybutyric acid, by enzymatic degradation of PHB.

In this study, the use of cotton fiber (CF) as a filler in poly(butylene succinate) (PBS) and the effect of silane treatment on the mechanical properties, thermal stability, and biodegradability of PBS/CF composites are investigated. The results showed that the tensile strength of PBS was improved (15%–78%) with the incorporation of CF (10–40 wt%) and was further increased (25%–118%) when CF was treated with a silane coupling agent. Scanning electron microscopy (SEM) observation of the fracture surfaces of PBS/CF composites showed that there was slight improvement in fiber-matrix compatibility. Thermogravimetric (TG) analysis showed that the thermal stability of the composites was lower than that of neat PBS and decreased with increasing filler loading. The biobased carbon content of the composites increased with increasing CF content. The incorporation of CF (with and without silane treatment) in PBS significantly increased the biodegradation rate of the composites