PHA comprising medium-chain monomers (6 or more carbon length) are more elastomeric and may contain unsaturated carbon bonds. They are more conducive for coating and film materials, and offer greater possibilities for chemical modifications. The short-chain and medium-chain PHA arise from different biosynthetic routes and therefore are made by different microorganisms. The composition of culture medium (particularly carbon substrate), influences the microbial polymer (e.g. range of polymers formed, molecular weight, crystallinity) which in turn determines the physical properties (e.g. mechanical and tensile strength). Poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) is a copolymer comprised of short-chain monomers of 4-5 carbon length).
The properties of PHA polymers range from stiff, highly crystalline materials like PHB to rubbery elastomers like polyhydroxyoctonate (PHO) to other PHAs , which are completely amorphous, tacky substances.
Biopol is a copolymer where hydroxybutyrate and hydroxyvalerate units are incorporated randomly along the chain. Adding controlled amounts of organic acid may vary the hydroxyvalerate content.
There are two causes of variation in grades. (i) the level of hydroxyvalerate units and (ii) the presence of plastizer. The hydroxyvalerate copolymer has a Tmin the range of 173-180 degree centi. and Tg about 5 degree centi. The hydroxyvalerate content is about 5-12%. The ethyl side chain of the valerate unit will reduce chain packing and lower crystalline melting point, modulus and tensile strength and at the same time increase flexibility, impact strength and ductility. Higher the hydroxyvalerate content, lower is the crystallization rate. All commercial grades contains nucleating agent to facilitate crystallization and shorten processing cycles during the moulding operations. The use of plasticizers has similar effect as that of increasing the hydroxyvalerate content. It depresses Tg and therefore improves impact strength and ductility at lower temperatures. These polymers have good resistance to oils but are hydrolysed by acids and bases. Because of the enhanced crystallinity due to the presence of nucleating agents, the biodegradation commences at the surface of the polymer.
Processing of Bioplastics -
Presence of nucleating agents (which facilitate crystallization) or the use of plasticiser shortens the processing cycles during the moulding operations. There are two main points about processing of PHBV bioplastics -
(i) The limited thermal stability of the polymer and so it degrades rapidly above 195 degree centi. (ii) The need to optimise conditions to allow a maximum crystallization rate (which reduces cycle times). The maximum rate of crystallization is reported to be at about 55-60 degree centi. which is significantly closer to Tg than the Tm. Processing temperatures should not exceed 180 degree centi. and duration of time when the material is in melt state should be kept minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding the blow-pin and the mould should be at about 60 degree centi. to optimise crystallisation rates. Similarly injection moulds are recommended at 55-65 degree centi.
The low-hydroxyvalerate, unplasticised grades are most critical to process, requiring the higher processing temperatures. Conditions are slightly less critical with the higher hydroxyvalerate containing and plasticised grades.
In addition to producing PHAs in dry powder form for melt processing, Metabolix is also developing PHA latexes. These materials have unique film forming properties, which are finding application in higher performance applications as well as in more traditional commodity uses. Metabolix company supplies PHA samples to companies under research and development agreements.
The properties of PHA polymers range from stiff, highly crystalline materials like PHB to rubbery elastomers like polyhydroxyoctonate (PHO) to other PHAs , which are completely amorphous, tacky substances.
Biopol is a copolymer where hydroxybutyrate and hydroxyvalerate units are incorporated randomly along the chain. Adding controlled amounts of organic acid may vary the hydroxyvalerate content.
There are two causes of variation in grades. (i) the level of hydroxyvalerate units and (ii) the presence of plastizer. The hydroxyvalerate copolymer has a Tmin the range of 173-180 degree centi. and Tg about 5 degree centi. The hydroxyvalerate content is about 5-12%. The ethyl side chain of the valerate unit will reduce chain packing and lower crystalline melting point, modulus and tensile strength and at the same time increase flexibility, impact strength and ductility. Higher the hydroxyvalerate content, lower is the crystallization rate. All commercial grades contains nucleating agent to facilitate crystallization and shorten processing cycles during the moulding operations. The use of plasticizers has similar effect as that of increasing the hydroxyvalerate content. It depresses Tg and therefore improves impact strength and ductility at lower temperatures. These polymers have good resistance to oils but are hydrolysed by acids and bases. Because of the enhanced crystallinity due to the presence of nucleating agents, the biodegradation commences at the surface of the polymer.
Processing of Bioplastics -
Presence of nucleating agents (which facilitate crystallization) or the use of plasticiser shortens the processing cycles during the moulding operations. There are two main points about processing of PHBV bioplastics -
(i) The limited thermal stability of the polymer and so it degrades rapidly above 195 degree centi. (ii) The need to optimise conditions to allow a maximum crystallization rate (which reduces cycle times). The maximum rate of crystallization is reported to be at about 55-60 degree centi. which is significantly closer to Tg than the Tm. Processing temperatures should not exceed 180 degree centi. and duration of time when the material is in melt state should be kept minimum. At the end of a run the processing equipment should be purged with polyethylene. When blow moulding the blow-pin and the mould should be at about 60 degree centi. to optimise crystallisation rates. Similarly injection moulds are recommended at 55-65 degree centi.
The low-hydroxyvalerate, unplasticised grades are most critical to process, requiring the higher processing temperatures. Conditions are slightly less critical with the higher hydroxyvalerate containing and plasticised grades.
In addition to producing PHAs in dry powder form for melt processing, Metabolix is also developing PHA latexes. These materials have unique film forming properties, which are finding application in higher performance applications as well as in more traditional commodity uses. Metabolix company supplies PHA samples to companies under research and development agreements.
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