May 1 2003
PHB and its copolymers with polyhydroxyvalerate (PHV) are melt-processable semi-crystalline thermoplastics produced by biological fermentation from renewable carbohydrate feedstocks.
These biodegradable materials have been described as "the first example of a true thermoplastic from biotechnology." Although relatively stable under everyday conditions, these biopolymers degrade gradually in the body and when in landfill sites or composted. The HB monomer unit is a typical constituent of human blood.
These biopolymers have somewhat limited chemical resistance as they dissolve in chlorinated solvents and are attacked by alkalis and acids. They are optically active polymers with a chiral site in each molecular repeat unit, all of which are in the D- (or R) configuration.
PHB homopolymer is a stiff and somewhat brittle polymer of high crystallinity, with mechanical properties similar to those of polystyrene, though it is more temperature resistant and less brittle.
In addition, the degradation rate of PHB homopolymer is relatively high at its normal melt processing temperature. Therefore, copolymers are the preferred materials for general purposes. It is believed that the potential application of homopolymer is in the medical/biological fields.
Background
Polyhydroxybutyrate/polyhydroxyvalerate (PHB/PHV) copolymers are used in preference to polyhydroxybutyrate (PHB) homopolymers for general purposes (e.g. moulding containers) in order to obtain a better balance of stiffness and toughness.
Polyhydroxybutyrate/polyhydroxyvalerate (PHB/PHV) copolymers contents of 5 - 20% give a useful range of properties broadly similar to those of the polyolefins (the polyethylenes and polypropylene). They melt at lower temperatures than the homopolymer, giving a useful improvement in melt-processability. Their other properties are similar to those of polyhydroxybutyrate .
Polyhydroxybutyrate/polyhydroxyvalerate (PHB/PHV) copolymers are being used for biodegradeable containers (of which shampoo bottles are the most high-profile example) and other articles difficult to recycle e.g. disposable razors or medically contaminated articles.
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PHB/PHV copolymers are being used for disposable razors and other biodegradeable containers and that are diffcult to recycle. Image Credit: ShutterStock/Stockforlife
Key Properties
The key properties of Polyhydroxybutyrate/polyhydroxyvalerate (PHB/PHV) copolymers are tabulated below.
Table 1. Key properties
| Chemical Resistance |
| Acids – dilute |
Fair |
| Alcohols |
Fair |
| Alkalis |
Poor |
| Greases and Oils |
Good |
| Electrical Properties |
| Dielectric constant @ 1 MHz |
3.0 |
| Volume resistivity ( Ohm.cm ) |
1016 |
| Mechanical Properties |
| Elongation at break ( % ) |
15 |
| Izod impact strength ( J.m-1 ) |
100 |
| Tensile modulus ( GPa ) |
0.9 |
| Tensile strength ( MPa ) |
28 |
| Physical Properties |
| Density ( g.cm-3 ) |
1.25 |
| Resistance to Ultra-violet |
Fair |
| Thermal Properties |
| Specific heat ( kJ.K-1.kg-1 ) |
1.4 |
| Thermal conductivity ( W.m-1.K-1 ) |
0.15 @ 23 |
Properties of Polyhydroxybutyrate/Polyhydroxyvalerate 8% - Biopolymer Film
| Property |
|
Value |
| Extension to break - Longitudinal |
% |
7-15 |
| Extension to break - Transverse |
% |
5-10 |
| Permeability to Oxygen @ 25 °C |
x10-13 cm3.cm.cm-2.s-1.Pa-1 |
0.1-0.2 |
| Permeability to Water @ 35 °C |
x10-13 cm3.cm.cm-2.s-1.Pa-1 |
1000-2000 |
| Tensile modulus - Longitudinal |
GPa |
0.6-1.0 |
| Tensile modulus - Transverse |
GPa |
0.8-1.2 |
| Tensile strength - Longitudinal |
MPa |
25-30 |
| Tensile strength - Transverse |
MPa |
25-30 |
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