Polyhydroxybutyrate-valerate copolymers

Another important family of biobased polyesters is the polyhydroxyalkanoates (PHAs) the most common members of this family are polyhydroxybutyrate (PHB) and its copolymer, polyhydroxybutyrate-valerate (PHBV). 

After many years of research initiated in the 1970s, ICI introduced polyhydroxybutyrate-valerate copolymers in 1990. Because of their particularly interesting manufaeturing technology, these materials are dealt with in depth separately in the next section. 

Table 31.3 Some selected properties of polyhydroxybutyrate-valerate copolymers (Biopol—Zeneca)...

If the homopolymer decomposes at the fabrication temperature another approach is to make a copolymer that can be melt processed at a lower temperature. For example, polyhydroxybutyrate decomposes at the processing temperature (190°C), whereas the copolymer with valeric acid can be processed at 160°C without decomposition. These aliphatic polyesters are biodegradable and most importantly, the decomposition products are not toxic, hence their use in medical applications (e.g., sutures). 

Metabolix s PHBV (polyhydroxybutyrate valerate) was initially developed by ICI. PHBV and related copolymers are made in a pilot plant using different bacteria to create compositions with up to 70% crystallinity. Elongation can be manipulated from 5% to 100%, and melting points range between 135 and 185 °C (275-365 °F). 

Polyhydroxybutyrate-valerate copolymer Biodegradable polyester used in degradable plastic... 

PHA constitute a whole class of microbially synthesized polyesters [33], of which polyhydroxybutyrate (PHB) and the copolymer with valerate (PHBV) have attracted much attention as possible replacements of oil-based thermoplastics. To serve as a useful injection moldable plastic with reasonable ductility, PHA... 

A more recent biodegradable polymer is polyhydroxybutyrate-valerate copolymer (PHBV) from ICI Americas Inc.. These copolymers are produced through biochanical means. In fact, these natural thermoplastics are derived from bacteria. They are fully degradable in many microbial environments. Other biodegradable polymers include Konjac, a water-soluble polysaccharide produced by FMC Chitin, another polysaccharide that is insoluble in water and Chitosan, which is soluble in water. 

A more recent biodegradable polymer is polyhydroxybutyrate-valerate copolymer (PHBV). These copolymers differ from many of the typical plastic materials in that they are produced through biochemical means. It is produced commercially by ICI using the bacteria Alcaligenes eutrophus, which is fed a carbohydrate. The bacteria produce polyesters, which are harvested at the end of the process. When the bacteria are fed glucose, the pure polyhydroxybutyrate polymer is formed, while a mixed feed of glucose and propionic acid will produce the copolymers. Different grades are commercially available that vary in the amount of hydroxyvalerate units and the presence of plasticizers. The pure hy-... 

Aliphatic polyesters based on monomers other than a-hydroxyalkanoic acids have also been developed and evaluated as drug delivery matrices. These include the polyhydroxybutyrate and polyhydroxy valerate homo- and copolymers developed by Imperial Chemical Industries (ICI) from a fermentation process and the polycaprolactones extensively studied by Pitt and Schindler (14,15). The homopolymers in these series of aliphatic polyesters are hydrophobic and crystalline in structure. Because of these properties, these polyesters normally have long degradation times in vivo of 1-2 years. However, the use of copolymers and in the case of polycaprolactone even polymer blends have led to materials with useful degradation times as a result of changes in the crystallinity and hydrophobicity of these polymers. An even larger family of polymers based upon hydroxyaliphatic acids has recently been prepared by bacteria fermentation processes, and it is anticipated that some of these materials may be evaluated for drug delivery as soon as they become commercially available. 

The two most common PHAs are polyhydroxybutyrate (PHB) and a copolymer of polyhy-droxybutyrate and polyhydroxy valerate (PHBV). PHAs can be used as films, fibers, and coatings for hot beverage cups made of paper. 

Ahmed, T., Margal, H., Lawless, M., Wanandy, N. S., Chiu, A., Foster, L. J. R., Polyhydroxybutyrate and its copolymer with polyhydroxy-valerate as biomaterials Influence on progression of stem cell cycle. Binmocromoip.cuip.s. 10, 2707-2715 (2010), DOl 10.1021/ bml007579. 

In the presence of limited nutrients, bacteria can be induced to make polyhydroxybutyrates and valerates, which are processed into a copolymer known as BiopoP. BiopoP has properties similar to polypropylene, but it is biodegradable and obtained from nonpetroleum sources. 

Polyhydroxybutyrate (PHB) is a polyhdroxyalkmioate polymer with pl tic-like properties that is synthesized by some soil bacteria (e.g. Alcaligenes eutrophus). The genes for the pathway have been cloned and inserted into plants with successful production of PHB. Additional research is required to optimize the properties the butyrate-valerate copolymer has superior functionality but it remains more difficult to express this material in plants compared to the pure polymer (75). Improved expression levels and localization would also assist in separations and in the economic cost to produce this renewable biodegradable plastic. 

The simplest PHA is polyhydroxybutyrate (PHB), whose formula is shown in Fig. 22.1. This polymer and its copolymer with polyhydroxy valerate seem to be, at present, the only PHAs relevant for practical applications. 

One of the early truly biodegradable polymers was polyhydroxybu-tyrate/valerate (PHBV). This is a member of the polyester family which is produced by certain types of bacteria when they have a diet which is carbon-rich but poor in some essential nutrient. Under these conditions, they produce polyhydroxybutyrate (PHB) as a food store to be called upon when carbon sources are less available. With manipulation of the diet, the bacteria can be induced to form a copolymer, PHBV, which has more useful properties than PHB. 

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