What Is PHB? Its Properties, Production, and Applications

Polyhydroxybutyrate, or PHB, is a bioplastic derived from natural sources. Unlike traditional plastics made from petroleum, PHB is made by microorganisms. This polymer represents a shift towards materials that are both bio-derived and biodegradable.

Understanding PHB: Composition and Classification

Polyhydroxybutyrate is a naturally occurring polymer in the bioplastics category. It belongs to the polyhydroxyalkanoates (PHAs) class of biopolymers, which are polyesters synthesized by microorganisms from renewable resources.

PHB’s chemical structure is a polyester formed from repeating monomer units of 3-hydroxybutyrate. This linkage creates a material with properties that can mimic conventional plastics.

Within the PHA family, PHB is the most common member, specifically referred to as poly-3-hydroxybutyrate (P3HB). Different microorganisms can produce a variety of PHAs by incorporating other similar monomers, which results in polymers with slightly different properties.

Microorganisms accumulate this biopolymer to store carbon and energy when food is scarce. This natural function is what scientists have harnessed for its production.

The Natural Production of PHB

PHB production is a biological process where microorganisms create it as an energy and carbon reserve. This is triggered by physiological stress, such as when essential nutrients are limited but a carbon source is plentiful, causing the bacteria to accumulate PHB.

Synthesis occurs inside microbial cells, where the polymer is stored as granules that can comprise a large portion of the cell’s dry weight. A well-known bacterium used for PHB production is Cupriavidus necator (formerly Alcaligenes eutrophus).

For commercial production, this process is replicated in large fermentation tanks. Microorganisms are cultivated in a controlled environment, first receiving all nutrients needed to multiply. Once a sufficient population is reached, the nutrient balance is altered to induce the stress that promotes PHB accumulation.

The biosynthesis pathway starts with two acetyl-CoA molecules from a carbon source like glucose. Enzymatic reactions convert these into the hydroxybutyryl-CoA monomer, which is then polymerized into PHB. The bacteria are then harvested, and the polymer is extracted and purified.

Distinctive Properties of PHB

A primary characteristic of PHB is its biodegradability. In environments like soil or compost, microorganisms break down PHB into carbon dioxide and water. This contrasts with petroleum-based plastics that persist for centuries.

Another property is biocompatibility, meaning it is not harmful to living tissues. This allows its use in medical applications without adverse reactions and is a direct result of its natural origin.

PHB is a thermoplastic with a melting point of approximately 175°C. It can be melted and molded into various shapes using conventional plastic manufacturing equipment.

Physically, PHB is a stiff and brittle material with a tensile strength of around 40 MPa, comparable to polypropylene. It is insoluble in water, resistant to hydrolytic degradation and ultraviolet radiation, but has poor resistance to acids and bases.

Applications of PHB

In packaging, PHB is used for biodegradable films, containers, and single-use items like cutlery. These are an alternative to traditional plastics, especially for food packaging that can contaminate recycling streams.

The medical field is another area for PHB applications. Its biocompatibility allows for manufacturing devices that the body can safely absorb over time. Examples include surgical sutures, bone plates, tissue engineering scaffolds, and drug delivery systems.

In agriculture, PHB is used for applications like mulch films that biodegrade in the soil, eliminating removal and disposal. It is also used in systems for the controlled release of fertilizers and pesticides to reduce environmental impact.

Blending PHB with other polymers creates copolymers with enhanced properties, such as improved flexibility, broadening its potential uses. As production becomes more efficient, PHB may become a more common material in consumer and industrial products.