PHB (Polyhydroxybutyrate)

Quick Overview

PHB is the simplest member of the PHA family, produced through bacterial fermentation. Fully biodegradable in soil, marine, and composting environments, PHB offers true biological sustainability despite processing challenges from brittleness.

Overview

Polyhydroxybutyrate (PHB) is the simplest and most abundant member of the polyhydroxyalkanoate (PHA) family, representing nature’s own solution to polymer production. Unlike synthetic plastics manufactured through chemical processes, PHB is produced entirely by bacteria as an intracellular energy storage compound—essentially bacterial fat. This biological origin makes PHB one of the most environmentally sustainable polymers available, fully biodegradable in virtually all natural environments while being produced from renewable feedstocks through fermentation.

PHB’s story began in 1926 when French microbiologist Maurice Lemoigne discovered bacterial inclusions that turned out to be polyester granules. For decades, PHB remained a scientific curiosity until oil crises and environmental concerns sparked interest in renewable, biodegradable alternatives to petroleum plastics. Today, PHB and its copolymers represent the forefront of truly sustainable polymer science—materials that nature knows how to make and break down.

While PHB offers exceptional environmental credentials, it faces commercial challenges from brittleness, narrow processing windows, and higher costs compared to both conventional plastics and other bioplastics like PLA. However, ongoing research into copolymers, processing additives, and novel production methods continues to address these limitations while preserving PHB’s unique environmental advantages.

Production and Biosynthesis

Bacterial Fermentation Process:

PHB production represents true biological manufacturing:

1. Bacterial Selection: Specific bacteria naturally produce PHB, including:

   • Cupriavidus necator (formerly Ralstonia eutropha) - most studied PHB producer
   • Alcaligenes latus
   • Recombinant E. coli engineered for PHB production
   • Various cyanobacteria and purple bacteria

2. Fermentation: Bacteria are cultivated under nutrient-limited conditions (typically nitrogen or phosphorus deficiency) that trigger PHB accumulation as energy reserves:

   • Carbon source (glucose, sugars, oils, methane, CO2) is provided in excess
   • PHB accumulates as granules inside bacterial cells (up to 80-90% of cell dry weight)
   • Fermentation takes 24-96 hours depending on bacterial strain and conditions

3. Harvesting: Bacterial cells are lysed (broken open) using:

   • Chemical methods (surfactants, solvents)
   • Enzymatic digestion
   • Mechanical disruption

4. Purification: PHB granules are separated from cell debris and purified

5. Processing: Purified PHB is dried and pelletized for manufacturing

Feedstock Flexibility: One of PHB’s advantages is the diversity of carbon sources bacteria can convert:

• First-generation: Glucose, sucrose from corn or sugarcane
• Second-generation: Glycerol from biodiesel production, molasses from sugar refining
• Third-generation: Methane from landfills or agriculture, industrial off-gases (CO2, CO)
• Fourth-generation: Direct CO2 fixation using cyanobacteria or engineered bacteria
• Waste streams: Used cooking oil, food processing waste, whey from dairy

Chemical Structure and Properties

Molecular Structure: PHB is a linear polyester with repeating 3-hydroxybutyrate units:

• Molecular weight: typically 200,000-600,000 Da
• Highly crystalline structure (60-80% crystallinity)
• Isotactic stereoregular configuration
• Similar backbone to polypropylene but with ester linkages

Physical Properties:

• Melting point: 170-180°C
• Glass transition temperature: 5-10°C
• Density: 1.25 g/cm³
• Highly crystalline and rigid

Mechanical Properties:

• Tensile strength: 25-40 MPa (comparable to polypropylene)
• Elongation at break: 3-8% (very brittle - major limitation)
• Young’s modulus: 3-4 GPa (stiff and rigid)
• High crystallinity causes brittleness and limited flexibility

Thermal Properties:

• Narrow processing window: melting point (170-180°C) close to degradation temperature (190-200°C)
• Thermal degradation during processing limits molecular weight
• Requires careful temperature control during manufacturing

Biodegradability: PHB exhibits exceptional biodegradability across all environments:

• Industrial composting: 60-90 days (complete degradation)
• Soil: 6-12 months
• Marine water: 6-24 months (certified marine biodegradable)
• Freshwater: 8-18 months
• Anaerobic digestion: Degrades producing biogas

Biodegradation occurs through:

• Enzymatic hydrolysis by extracellular depolymerases
• Microbial consumption of degradation products
• Complete mineralization to CO2, water, and biomass

Applications and Markets

Packaging:

• Rigid containers and bottles
• Blister packaging for pharmaceuticals
• Cosmetic packaging
• Food containers for dry goods
• Single-use items (cups, lids, straws)

Agricultural Applications:

• Mulch films for specialized crops
• Biodegradable plant pots
• Seed coatings
• Controlled-release fertilizer encapsulation

Marine and Aquatic: PHB’s marine biodegradability makes it valuable for:

• Fishing gear and nets that biodegrade if lost
• Marine coatings
• Aquaculture equipment
• Personal care products (microbeads, exfoliants)
• Packaging for marine-based industries

Medical and Pharmaceutical:

• Sutures and wound closure devices
• Bone fixation screws and pins
• Controlled drug delivery microspheres
• Tissue engineering scaffolds
• Cardiovascular patches

Consumer Products:

• Disposable razors and toothbrushes
• Cosmetic applicators
• Single-use hygiene products
• Rigid components requiring biodegradability

Advantages and Environmental Benefits

Complete Biodegradability: PHB biodegrades in virtually all natural environments without requiring specific industrial conditions—a key advantage over PLA and many other bioplastics.

Marine Biodegradability: Unlike most bioplastics, PHB is certified marine biodegradable, addressing ocean plastic pollution concerns.

True Biological Origin: PHB is produced by living organisms through natural metabolic processes, not chemical synthesis, making it inherently renewable and sustainable.

Carbon Sequestration: During biomass feedstock growth (plants, algae), CO2 is captured from the atmosphere. PHB production from CO2-utilizing bacteria can be carbon-negative.

No Fossil Fuel Dependence: PHB can be produced entirely from renewable resources without petroleum inputs.

Biocompatibility: Excellent biocompatibility enables medical applications, with PHB approved for certain medical devices.

Challenges and Limitations

Brittleness: Pure PHB’s low elongation at break (3-8%) makes it unsuitable for applications requiring flexibility or impact resistance. Solutions include:

• Copolymerization with hydroxyvalerate (PHBV) or hydroxyhexanoate (PHBH)
• Blending with plasticizers or other polymers
• Processing modifications to reduce crystallinity

Processing Challenges:

• Narrow window between melting and degradation temperatures
• Thermal degradation reduces molecular weight during processing
• Requires specialized processing conditions and equipment modifications
• Limited shelf life due to continued crystallization during storage

Cost: PHB production is expensive:

• Fermentation costs (media, energy, time)
• Downstream processing (cell lysis, purification, drying)
• Lower production volumes maintain high prices
• Typical cost: $3.00-6.00/kg compared to $1.00-1.50/kg for conventional plastics

PHB costs 2-3 times more than PLA and is comparable to or higher than other PHAs.

Limited Production Capacity: Global PHB production is relatively small (estimated 20,000-40,000 tonnes annually), limiting commercial availability and maintaining high costs.

Property Variability: Batch-to-batch variations in PHB properties (molecular weight, crystallinity) can occur due to fermentation inconsistencies.

Recent Innovations and Future Outlook

Copolymer Development: Research focuses on PHB copolymers with improved properties:

• PHBV (3-15% hydroxyvalerate): Increased flexibility and impact resistance
• PHBH (10-25% hydroxyhexanoate): Rubber-like elasticity
• P34HB: Enhanced toughness and elongation

Advanced Production Methods:

• Continuous fermentation systems reducing batch-to-batch variability
• Engineered bacteria with higher PHB yields (90%+ of cell weight)
• Photosynthetic bacteria producing PHB directly from CO2 and sunlight
• Consolidated bioprocessing eliminating separate enzyme production

Cost Reduction Strategies:

• Using waste feedstocks (food waste, industrial off-gases) reduces raw material costs
• Improved downstream processing reduces purification costs
• Scaling production to 100,000+ tonne capacity could reduce costs 40-60%

Blends and Composites:

• PHB/cellulose nanocomposites improving mechanical properties
• PHB/PLA blends balancing cost and performance
• PHB-based wood-plastic composites for durable applications

Novel Applications:

• 3D printing filaments with controlled degradation
• Smart packaging with time-release properties
• Biomedical devices leveraging tunable degradation rates
• Marine biodegradable products addressing ocean pollution

Market Growth: The PHB market is projected to grow significantly:

• Current capacity: ~30,000-50,000 tonnes annually
• Projected growth to 200,000-300,000 tonnes by 2030
• Growth drivers: Marine biodegradability requirements, medical applications, premium sustainable packaging

Regulatory Support: Increasing regulations addressing marine plastic pollution and single-use plastics favor PHB’s unique marine biodegradability.

PHB represents polymer production as nature intended—biological synthesis, renewable feedstocks, and complete biodegradability in all environments. While processing challenges and costs currently limit widespread adoption, ongoing innovations in bacterial engineering, copolymer formulations, and production scaling promise to unlock PHB’s potential as a truly sustainable alternative to conventional plastics. As the world seeks materials that work within natural systems rather than against them, PHB’s biological origins and environmental compatibility position it as a critical component of future sustainable materials portfolios.