Introduction
Plastic pollution has become a serious environmental issue around the world with conventional plastics taking hundreds of years to decompose in landfills and the natural environment. This has led researchers to actively pursue the development of biodegradable and compostable plastics that can degrade within a reasonable timeframe without leaving behind toxic residues. Some of the key areas of research include developing plastics from renewable biomass sources, engineering microbial degradation, optimizing conditions for compostability, and improving mechanical and barrier properties to match conventional plastics. This paper aims to discuss the current state of research on biodegradable plastics by reviewing relevant literature.
Biomass-Derived Polymers
Plant-based polymers like poly-lactic acid (PLA), polyhydroxyalkanoates (PHAs), and polybutylene succinate (PBS) are among the most prominently researched biomass-derived biodegradable plastics (Laycock et al., 2017). PLA is derived from corn starch or sugarcane and has good mechanical strength and processability. Its hydrophobicity and slow degradation rate limit its applicability. Genetic engineering approaches are being used to develop new strains of bacteria that can produce a wider range of PHAs at lower costs (Chen, 2009). PBS is synthesized from petrochemical based succinic acid and 1,4-butanediol but efforts are ongoing to produce these monomers from renewable resources. Researchers are also exploring the potential of cellulose, chitosan, starch, lipids and proteins from agricultural/forestry wastes to develop novel bioplastics (Shen et al., 2010). Blends and composites of different biopolymers are another strategy to improve material properties.
Microbial Degradation
For a plastic to be truly biodegradable, microorganisms like bacteria and fungi must be able to metabolize and mineralize it into CO2, water, and biomass. A detailed understanding of the microbial enzymes and pathways involved in depolymerization is crucial. Several studies have identified various bacteria, actinomycetes and fungi capable of degrading bioplastics (Tokiwa et al., 2009). Some key enzymes reported are cutinase for PLA, lipases and esterases for aliphatic polyesters. Metagenomic studies are providing insights into the catabolic gene clusters in plastic-degrading microbial communities (Yu and Chen, 2018). Researchers are also engineering new microbial strains with enhanced depolymerase activity and exploring synergies between different microbes for complete mineralization (Zumstein et al., 2018). Standardization of biodegradation test methods is still needed to ensure consistency and reproducibility of results.
Composting Conditions
Controlling environmental parameters like temperature, moisture level, aeration, carbon/nitrogen ratio etc. is critical for optimized biodegradation during composting. Studies have characterized the effect of these process variables on the breakdown of different bioplastics. For example, satisfactory degradation of PLA and PBS was achieved at 58°C in compost while cellulose-based plastics required higher temperatures of 50-70°C (Iovino et al., 2008). Moisture levels of 40-60% were found suitable. Incorporating bulking agents in compost helps maintain porosity while nitrogen sources accelerate microbial activity. Continuous aeration is essential to provide oxygen and mix the degrading plastic. Standard protocols have been developed specifying conditions and durations for aerobic compostability certification (EN 13432, ASTM D6400). Scale-up trials are required to validate lab findings at industrial compost facilities.
Applications and Commercialization
Research efforts are leading to increased commercialization of bioplastics. For example, products made from PLA include food packaging films, disposable cutlery, 3D printing filaments etc. Many restaurants and coffee shops use PLA-based cups, containers and straws. Car manufacturers like Ford are incorporating bioplastics in interior parts. PHAs find use in surgical sutures and drug delivery. Compost bags meeting EN 13432 standards are also available. High production costs remain a bottleneck compared to fossil-fuel based plastics. Development of integrated biorefinery systems leveraging agricultural and municipal organic wastes can help lower costs and improve economic viability through value-added co-products. Successful implementation will also require development of separate collection, sorting and composting infrastructure to divert biodegradable plastics from landfills to industrial composting facilities.
Conclusion
To summarize, research on biodegradable plastics is an actively growing field with the goal of developing truly sustainable and end-of-life compatible alternatives to conventional plastics. Promising progress has been made in utilizing renewable biomass, understanding microbial mechanisms, optimizing composting protocols and scaling up production. Challenges still remain in making bioplastics competitive on cost while meeting performance standards. Continued multidisciplinary collaborations between material scientists, chemists, microbiologists and engineers hold the key to further advancements. Standardization and development of supporting infrastructure will also be required to realize the full potential of bioplastics in transitioning to a circular bioeconomy with reduced environmental impacts.
Andrew Stroud