Environmental Engineering Reference
In-Depth Information
is currently manufactured by NatureWorks LLC (United States) and Hycail
(Netherlands), Mitsui Chemicals, and Toyota (Japan).
4.6.2.4 Poly(Hydroxyalkanoates)
Unlikebio-basedPE,PET,andPLA,thepoly(hydroxyalkanoates) (PHA)are
bioplastics synthesized by bacteria. It was the first bacterial polymer to be
harvested commercially. PHAs are deposited within the bacterial cells of
many species as a lipoic material (Burdon, 1946). It is also unusual in that
PHAs though hydrophobic still rapidly biodegrade in the environment. All
bacterial polymers are not necessarily biodegradable (Steinbüchel, 2005);
PHA's biodegradability is attributed to its saturated polyester chemical
structure.
Bacteriagrownunderspecificconditionsoflowlimitingnutrients(N,P,and
S) generate PHAs as storage materials in high yield. A variety of substrates
can support the growth of PHA-producing bacteria. These include milk
whey (Nath et al., 2008), molasses (sugarcane or beet) (Solaiman et al.,
2006), and glycerol (Ibrahim and Steinbüchel, 2009). Over 250 bacterial
species produce PHAs, and some of the best species (
Cupriavidus necator
)
can accumulate the polymer up to 80% of its dry cell weight. In addition
to naturally occurring species, genetically manipulated recombinant strains
(mainly of
E.coli
modified to increase PHA yield and bioreactor expression)
have been used for PHA production (Keshavarz and Roy, 2010). The
schematic in
Figure 4.16
illustrates the process for a specific pilot plant
in a sugarcane factory in Brazil (Nonato et al., 2001) set up to produce
poly(hydroxybutyrate) [PHB], the first type of PHA to be discovered. This
example is particularly interesting because of the sustainability aspects of
using bagasse to produce energy to run the operation and the near
closed-loop recycling of the waste in the plant.
PHAs are manufactured by Telles (United States), Biomer (Germany),
Mitsubishi Gas and Kaneka (Japan), PHB Industrial S/A (Brazil), and
Metabolix (United States).
The chemical structures of PHB and its copolymer PHBV are shown below,
and the polymer has an average molecular weight of
Mn
~ 500,000 to a
HB monomer. Because of high stereoregularity, PHB polymer has a high
degree of crystallinity (~80%) and a high Tm of 179°C. It is a plastic that
matches the properties of popular thermoplastics such as polystyrene or
