Biopolymers - Polymer Science and Engineering - page 522

with other biopolymers increases biodegradability. Focus in research has been
placed on engineering TPS for packaging applications.

13.3.1.3 Cellulose
Cellulose can be harvested from many plant sources as it is the major part of the
cell wall. Also cellulose can be produced by fermentation of certain bacteria.
Fermentation occurs in a bioreactor and is the process of microorganisms meta-
bolically breaking down organic substances in the absence of oxygen, anaerobic
metabolism. Important parameters controlled during the process are temperature,
pH, and sugar content from the feedstock material. Fermentation can also be car-
ried out with genetically engineered microorganisms (GEMs). GEMs can be
designed to break down specific substrate materials during fermentation, as well
as surviving the harsh conditions [6] . Cellulose produced by bacteria can be
obtained as a continuous film by cultivating the bacteria in a glucose solution [5] .
Cellulose is also secreted by marine chordates such as the sea squirt [5] .
Cellulose from plant sources can be extracted by chemical solubilisation type
processes such as pulping, which results in a refined structure. The cellulose
structure is disrupted and the cellulose remaining in solution is then precipitated.
This method is used to prepare regenerated cellulose. Regenerated cellulose is dif-
ferent from native cellulose because extensive degradation takes place during the
process of dissolution and the final product is usually less crystalline [5] .
Other products derived from cellulose include cellulose nitrate, cellulose ester,
and cellulose acetate. Work has been conducted on grafting cellulose as copolymer,
as well as the potential to cross-link cellulose. There is interest in cellulose chemis-
try and polymerization due to the abundance of cellulose as a renewable resource.

13.3.2 Polymerization of Bio-monomers
Bioplastics produced from bio-monomers use well-established polymerization
techniques. In general, such bio-monomers are molecules from renewable
resources that can be polymerized [6] . Specific examples of bioplastics produced
in this way are given below.
Polyesters, polyamides, and polyurethanes can be manufactured from triglycer-
ides, or commonly known as fatty acids, in vegetable oils [17] . The vegetable oils
are collected from the oil-containing seeds such as flax, canola, and soybeans.
These products are then subsequently crushed to release their fatty acids. These
vegetable oils can be polymerized using techniques such as free radical, cationic,
olefin metathesis, and condensation polymerization [17] . The resulting polymer
ranges from ductile linear thermoplastics to rigid cross-linked thermosets as well
as soft to hard flexible rubbers [17] . However, purification of the monomers prior
to polymerization is always challenging. There is controversy regarding utilization
of food such as vegetable oils for synthetic materials.
Polylactic acid (PLA) is a thermoplastic polyester synthesized from the bio-
intermediate,

lactic acid (2-hydroxypropionic acid), produced from microbial