Chemistry Reference
In-Depth Information
centrifugation followed by press filtration and subsequent drying of the final bio-
plastic
[21]
. Bacteria used for this purpose are sometimes genetically engineered
to metabolize different feed stocks as well as to increase conversion efficiency of
feedstock into polymer. Important variables in choosing bacteria strains include
high density growth, high polymer content, rapid growth rate as well as the ability
to produce controllable structures
[21]
. The most widely studied bioplastics pro-
duced from this process are polyhydroxylalkanoates (PHAs).
PHAs are the most widely studied types of polyesters that are produced by
bacteria
[22]
. The most common type of PHA is poly(hydroxybutyrate) (PHB)
and poly(hydroxyvalerate) (PHV). PHAs are classified in terms of short chain
length (PHAscl) possessing thermoplastic properties and medium chain length
(PHAmcl) with elastomeric properties
[22]
. PHAs have the potential to be a com-
petitive bioplastic due to the range of possible monomers that can be polymerized
by the microorganisms. The range of material substrates allows the possibility of
tailoring the mechanical properties for a particular application. Types of PHAs
that are produced commercially include poly-(R)-3-hydroxybutyrate (PHB),
copolymer of (R)-3-hydroxybutyrate (3HB) and (R)-hydroxyvalerate (3HV)
termed PHBV, copolymer of (R)-3-hydroxybutyrate (3HB) and (R)-hydroxyhex-
anoate (3HHx) termed PHBHHx
[21]
.
13.4
Biopolymer Blends and Biocomposites
Many other uses of biopolymers exist than are discussed in the previous sections.
A biopolymer can be blended with (1) another biopolymer, (2) a biodegradable
synthetic polymer, or (3) a non-degradable synthetic polymer. Also biopolymers
can be combined with different reinforcing materials such as mineral particles or
natural fibers to create a bio-polymer matrix composite. In fact, natural rubbers
are usually compounded with various inorganic fillers, antioxidants, pigments, to
name a few, to make it more useful.
13.4.1
Biopolymer Blends
As mentioned in Chapter 5, blending is a useful strategy to modify the materials
properties for specific applications. Biopolymers are no exception. Biopolymers are
often blended with one another to improve total degradation time as well as
mechanical properties. For instance, TPS is more susceptible to microbial action
and is more ductile compared to PLA, and a blend leads to intermediate properties.
Bioplastics are often blended with synthetic polymers in order to reduce the
cost of the material. Along with the reduction of cost, if the bioplastic chosen is
biodegradable, the overall material may be considered biodegradable depending
on the particle size of the remaining synthetic plastic once biodegradation has
been completed.
