Biodegradable biopolymer PHB: production, properties and copolymers for sustainable plastics

Overview

The podcast discusses polyhydroxybutyrate (PHB), a biodegradable polymer produced by bacteria, and how its chemistry and production influence its potential as a sustainable alternative to conventional plastics.

• PHB is a thermoplastic biodegradable polymer from the polyhydroxyalcanoate family.
• Its isotactic, crystalline structure makes it stiff and brittle, with processing limited by degradation near its melting point.
• Copolymerisation with polyhydroxyvalerate (PHV) improves processability and reduces crystallinity.
• Applications span drinks cups, utensils, packaging, and some medical devices, with ongoing research into production from yeasts and plants.

Introduction

The podcast explores PHB, a biodegradable polymer produced naturally by bacteria, highlighting why it matters for waste management and sustainable materials. It emphasizes the context of non-biodegradable plastics, landfill waste, and oceanic plastic accumulation, setting the stage for a green chemistry discussion about PHB as an alternative.

PHB: origin and structure

PHB, a member of the polyhydroxyalkanoate family, is synthesized by bacteria as an energy reserve. It was first discovered in Bacillus megaterium in the 1920s by Maurice Lemoigne. In bacterial cells, PHB forms discrete granules typically 100 to 800 nanometers in diameter. The polymer often accounts for a substantial fraction of cell dry mass, up to 30% under normal conditions and potentially up to 80% with carbon excess and nitrogen limitation. Its highly regular, isotactic structure causes side groups to align in the same direction, promoting tight packing into crystals that render PHB brittle and stiff. The polymer is thermoplastic and insoluble in water, though it degrades slowly in acids and bases and dissolves in chlorinated solvents. Its relatively high regularity contributes to processing challenges, including degradation at temperatures not much higher than the melting point around 175°C.

Production and processing challenges

PHB production can be carbon-source flexible, with food-derived carbohydrates such as glucose, sucrose, molasses, starch and ethanol serving as substrates. However, high production costs and limited mechanical performance have restricted large-scale use in consumer goods. Processing PHB molten state is difficult due to rapid thermal degradation near its melting temperature, complicating manufacturing and shaping of end products.

PHB PHV copolymers: improving processability

To address brittleness and processing limitations, PHB can be copolymerised with polyhydroxyvalerate (PHV). PHB PHV copolymers become less crystalline and less brittle, while maintaining biodegradability. This copolymerization can lower the melting temperature to around 75°C, enabling easier processing and broader manufacturing compatibility. The degradation time depends on both composition and environment; in sewage works with lots of bacteria the material can degrade in months, while in marine environments it may take years, and on a shelf the polymer can persist for decades. The late 1980s saw commercialization under the Biopol brand, and recent research has explored producing PHB and its copolymers from non-bacterial sources such as yeasts and genetically modified plants like rapeseed and cress.

Applications and future prospects

PHB and PHV PHB copolymers have found uses in disposable cups, utensils, packaging, and medical devices such as sutures, wound dressings, and bone plates and screws. Their biocompatibility and biodegradability drive interest in sustained drug delivery applications, where the drug is dispersed in the polymer and released as it degrades. The ongoing trend toward rising petroleum costs and advancing biotechnology suggests that biopolymers like PHB and PHV copolymers may become more prominent in homes, workplaces, and compost environments as processing methods improve and production costs fall.

Conclusion

PHB represents a compelling eco-friendly polymer option owing to its natural production, biodegradability, and potential for modification via copolymerization. Overcoming production costs and refining processing techniques will be key to broader adoption and real-world impact.