Page 63 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.3

cessed by injection molding, with the remainder used in extruded sheet and rod. Their low
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coefficient of friction make acetal resins good for bearings.
2.2.2 Biodegradable Polymers
Disposal of solid waste is a challenging problem. The United States consumes over 53 bil-
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lion lb of polymers a year for a variety of applications. When the life cycle of these poly-
meric parts is completed, they may end up in a landfill. Plastics are often selected for
applications based on their stability to degradation; however, this means degradation will
be very slow, adding to the solid waste problem. Methods to reduce the amount of solid
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waste include either recycling or biodegradation. Considerable work has been done to
recycle plastics, both in the manufacturing and consumer area. Biodegradable materials
offer another way to reduce the solid waste problem. Most waste is disposed of by burial
in a landfill. Under these conditions, oxygen is depleted, and biodegradation must proceed
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without the presence of oxygen. An alternative is aerobic composting. In selecting a
polymer that will undergo biodegradation, it is important to ascertain the method of dis-
posal. Will the polymer be degraded in the presence of oxygen and water, and what will be
the pH level? Biodegradation can be separated into two types: chemical and microbial
degradation. Chemical degradation includes degradation by oxidation, photodegradation,
thermal degradation, and hydrolysis. Microbial degradation can include both fungi and
bacteria. The susceptibility of a polymer to biodegradation depends on the structure of the
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backbone. For example, polymers with hydrolyzable backbones can be attacked by acids
or bases, breaking down the molecular weight. They are therefore more likely to be de-
graded. Polymers that fit into this category include most natural-based polymers, such as
polysaccharides, and synthetic materials, such as polyurethanes, polyamides, polyesters,
and polyethers. Polymers that contain only carbon groups in the backbone are more resis-
tant to biodegradation.
Photodegradation can be accomplished by using polymers that are unstable to light
sources or by the used of additives that undergo photodegration. Copolymers of divinyl
ketone with styrene, ethylene, or polypropylene (Eco Atlantic) are examples of materials
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that are susceptible to photodegradation. The addition of a UV absorbing material will
also act to enhance photodegradation of a polymer. An example is the addition of iron
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dithiocarbamate. The degradation must be controlled to ensure that the polymer does not
degrade prematurely.
Many polymers described elsewhere in this book can be considered for biodegradable
applications. Polyvinyl alcohol has been considered in applications requiring biodegrada-
tion because of its water solubility. However, the actual degradation of the polymer chain
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may be slow. Polyvinyl alcohol is a semicrystalline polymer synthesized from polyvinyl
acetate. The properties are governed by the molecular weight and by the amount of hydrol-
ysis. Water soluble polyvinyl alcohol has a degree of hydrolysis near 88 percent. Water in-
soluble polymers are formed if the degree of hydrolysis is less than 85 percent. 24
Cellulose-based polymers are some of the more widely available naturally based poly-
mers. They can therefore be used in applications requiring biodegradation. For example,
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regenerated cellulose is used in packaging applications. A biodegradable grade of cellu-
lose acetate is available from Rhone-Poulenc (Bioceta and Biocellat), where an additive
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acts to enhance the biodegradation. This material finds application in blister packaging,
transparent window envelopes, and other packaging applications.
Starch-based products are also available for applications requiring biodegradability.
The starch is often blended with polymers for better properties. For example, polyethylene
films containing between 5 and 10 percent cornstarch have been used in biodegradable ap-
plications. Blends of starch with vinyl alcohol are produced by Fertec (Italy) and used in

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