Environmental Engineering Reference
In-Depth Information
mutants in various parts of the biosynthetic pathway through genetic engineering, a series of
polymers with different rheological properties can be produced by fermentation [9, 10].
2.2. Plant Proteins
Several potentially useful plant proteins are available as byproducts of biorefining. For
example, corn zein (the alcohol soluble fraction of corn gluten) is becoming increasingly
available as a result of expansion in the production of bioethanols. Protein based adhesives
appear promising because many proteins in nature are used for adhesion, for example, marine
mussels use proteins to adhere to surfaces.
Widespread cultivation of soybeans in the United States has encouraged a great deal of
research into the development of biopolymers derived from soy protein. Soy proteins are
complex macromolecules with many sites available for interaction with plasticizers and other
copolymeric constituents, enabling soy protein to be converted to soy protein plastic through
extrusion with a plasticizer or cross-linking agent. Although the mechanical properties of soy
protein plastic can be controlled and optimized by adjusting the molding temperature and
pressure and initial moisture content, applications are limited because of its low strength and
great tendency to absorb moisture. The most effective method is to blend soy protein plastic
with biodegradable polymers to form soy protein-based biodegradable plastics. Currently, the
biodegradable plastics being used to blend soy plastic include polyester amide,
polycaprolactone, Biomax®, and poly(tetramethylene adipate-co-terephthalate) [11, 12].
While these approaches have been effective, other promising options are found in the use of
soy proteins in structural composites, addressed below.
Proteins can be synthesized using recombinant DNA technologies. Because of this there
is great interest within the polymer science community in exploiting proteins to make
specialized supramolecular structures. The primary protein sequence of amino acids
(residues) dictates its molecular conformation and resulting supramolecular structure. If this
structure formation can be controlled novel and interesting materials will result. It is unclear
however, if such materials will have widespread utility as commodity plastics.
2.3. Plant Oil-based Polymers
Soy bean oil and other plant oils have already proven useful as plastic materials when
converted using chemical techniques. Their great utility lies in the fact that the much of the
oil present in plants is unsaturated (i.e., the oils contain reactive carbon double bond) and
contain ester linkages. These chemical functionalities allow a range of polymerization
chemistries to be practiced leading to a wide variety of plastic resins. From a bioengineering
point of view, genetic engineering techniques may be used to manipulate and control the
distribution of the type of oil present in the plant. DuPont has developed a soybean that
contains in excess of 80 percent oleic within the fatty acid distribution. In 2002, 75 percent of
U.S.-soybean acres were planted with biotech soybeans-up from 68 percent in 2001,
according to statistics released by the USDA [13]. The widespread acceptance of transgenic
crops likely means that plant-based oils of defined character will become increasingly
available in the future. Once again, the conversion of these specialty plant oils to plastics is
expected to be more economical than conversion to biodiesel.
Natural oils are comprised of triglycerides and are abundant in many areas of the world.
Triglycerides consist of three fatty acids (i.e., carbon numbers from 22 down to 14 and double
bonds down from 3 to 0) covalently linked to a central glycerol.
