Environmental Engineering Reference
In-Depth Information
important ones. Direct esterification is the simplest route, but the unavoidable degradation of
starch chains diminishes mechanical strength of the end product. Alternatively, starch can be
covalently linked to other materials during a polymerization process; starch-poly(vinyl
acetate) materials, for example, have been prepared via in situ polymerization of vinyl acetate
in the presence of starch with a ferrous ammonium sulfate-hydrogen peroxide redox initiator
system. Other methods include melt blending of starch with synthetic polymers, such as
poly(ethyleneco-vinyl acetate) and polyethylene with anhydride functionality [3].
Current work in this area is directed toward the modification of TPS by reactive blending
with polymers containing functional groups, which are intended to bond with starch hydroxyl
groups during the blending procedure by means of a polymer analog (trans-esterification)
reaction. Poly(vinyl acetate) and poly(vinyl acetate-co-butyl acrylate) are of special interest
because of their potentials to diminish moisture sensitivity and the glass transition
temperature of resulting blends. Preliminary experiments have shown successful reactive
blending, increases in thermal stability, and decreases in swelling due to water, although work
remains to be done to improve the processability of the blends [3].
In addition, mathematical and computational efforts involving models such as the lattice-
fluid hydrogen-bonding model are assisting experimental work by facilitating the prediction
of mechanical and volumetric properties of starch-based polymers and water [3].
Transgenic plants again offer the hope of improving polymer properties. Transgenic
plants have been studied to understand the biosynthesis of starch [5, 6]. Manipulation of these
biosynthetic pathways provides a means for affecting the distribution of amylose and
amylopectin, potentially providing exquisite control over material properties within a single
crop without the need to blend different varietals.
Several other polysaccharides are valuable bioresources from a materials point of view
[7]. Naturally this includes cellulose, but also includes pectins. Pectins are used in the food
industries as coatings and additives and can be extracted from apple pomace and citrus peels;
pectin is partially methylated poly-รก-1,4-D-galacturonic acid. Konjac (a copolymer of
mannose and glucose with a ratio of about 1.6:1) is derived from plant tubers and can be
formed into films. Alignates are also film formers that are derived from the cell wall of brown
seaweed; structurally alignate is poly(1,4 uronic acid). Guar gum and gum Arabic are two
widely recognized materials from the family of plant gums that consist of hydrateable
polysaccharides; these gums find application as binders, adhesives, flocculants, emulsifiers,
and even lubricants in the food, papermaking, and petroleum industries.
Polysacharides are also available from animal sources; the primary example is chitin.
Chitin is the second only to cellulose in its natural abundance in biomass being found in the
exoskeletons of crustacean and insects. Because chitin and the related chitosan are available
from shellfish waste, they are inexpensive in their raw forms and have been widely studied
[8]. Applications include absorbants used in wastewater treatment, films used as membranes,
beads for metal chelation, and coatings for improved seed germination. To date, most
manipulation of the raw materials to final products has involved chemical methods with
relatively little emphasis on bioengineering techniques. Given the abundance of chitin, better
more benign processing techniques are warranted.
From the perspective of bioengineering, bacterial polysaccharides are the most interesting
as they lend themselves to the host of metabolic engineering techniques. For example,
xanthan gum is a high molecular weight branched polysaccharide extracted from
Xanthomonas campestris that is used as a viscosity modifier in drilling fluids. By introducing