Environmental Engineering Reference
In-Depth Information
desirable starting feedstock. DuPont's high oleic soybean is a good example of this type of
manipulation; the soybean has been genetically engineered to produce a more desirable
distribution of soy oils-in this case, one particularly rich in oleic oil.
2. State of the Science
2.1. Thermoplastic Starch (TPS)
Starch, an energy storage material found abundantly in cereals and tubers, was one of the
first natural polymers studied for the production of biodegradable materials and is widely
used in the food, paper, and textile industries. It is composed of amylose and the branched
polymer amylopectin, both are polysaccharides of alpha-D-glucopyranosyl units linked by (1-
4) and (1-6) linkages [1].
Starch was first used in biodegradable materials in the 1970s as a filler in synthetic
polymers such as polyethylene or, in its gelatinized form, as a component of blends with
water- soluble or water-dispersible polymers. These were best described as bio-
disintegratable rather than biodegradable; data showed that only the surface starch was
decomposed, leaving behind recalcitrant polyethylene fragments. Because products made
from these resins did not meet the criteria of biodegradability for defined disposal systems
like composting, further applications were soon sought [2].
More recently, starch has been used as the primary component in thermoplastic
compositions. Although starch is not itself a thermoplastic material, at moderately high
temperatures (90-180°C), under pressure and shear stress, starch granules melt and flow to
give an amorphous material called TPS, which can be processed just like a thermoplastic
synthetic polymer. Conventional plastic processing techniques such as injection molding and
extrusion can then be applied successfully. The ability to mold TPS, and the advantages of
starch in terms of low cost and high availability from renewable resources, have made it a
highly attractive resource for the development of biodegradable polymers. Unfortunately,
however, the use of regular TPS has been limited by its brittleness, by degradation under
conditions of normal use, and by hydrophilicity [1, 2)] The latter is especially problematic
because water is a plasticizer of starch, with the result that the performance of regular TPS is
unstable at sufficiently great relative humidity [3].
To address these problems, a number of modifications of TPS have been attempted to
improve its material properties. Surface modifications form one class of approaches; in these,
the superficial hydroxyl groups of the material are derivatized, especially to reduce
hydrophilicity, without changing the bulk composition and characteristics of the TPS. This
type of approach has also been used with cellulose fibers, considered below, to improve their
compatibilities with polymer matrices when they are used as natural reinforcements in
composites [1].
The use of biodegradable plasticizers has also been investigated, with the goals of
increasing durability as well as diminishing brittleness. Polyols such as glycerol, which is
often used with biodegradable polymers, effectively reduce degradation of thermoplastic
starch with increasing glycerol content corresponding to diminishing starch degradation under
conditions of normal use [4].
The starch structure can also be modified directly, particularly to increase durability and
plasticity. Among the many possibilities of modification, esterification is one of the most
