Callister - An introduction - 8th edition

882 • Chapter 22 / Economic, Environmental, and Societal Issues in Materials Science
Figure 22.3 Photograph showing biodegradable plastic mulch
films that have been laid out on farmland that is being cultivated.
(Photograph courtesy of Dubois Agrinovation.)
Other potential opportunities for this group
of materials exist in the fast-food industry. For example,
if all of the plates, cups, packaging, and so
on were based on biodegradable materials, they
could be commingled with food waste and then
composted in large-scale urban operations. Not
only would these measures reduce the amount of
material entering landfills, but if the polymers
were derived from renewable materials, a reduction
in greenhouse gas emissions would result.
In order to reduce our dependence on petroleum
and the emissions of greenhouse gases, there
has been a major effort to develop biodegradable
polymers that are also biorenewable—based on
plant-derived materials (biomass 2 ). These new
materials must be cost-competitive with existing
polymers and capable of being processed using
conventional techniques (extrusion, injection
molding, etc.).
Over the past 30 or so years a number of
biorenewable polymers have been synthesized
that have properties comparable to the petroleumderived
materials; some are biodegradable, and
others are not. Perhaps the best known of these
bioderived polymers is poly(l-lactic acid) (abbreviated
PLA), which has the following repeat unit
structure:
C
CH 3
O
C
Commercially, PLA is derived from lactic acid;
however, the raw materials for its manufacture are
starch-rich renewable products such as corn, sugar
beets, and wheat. Mechanically, the modulus of
elasticity and tensile strength of PLA are comparable
to poly(ethylene terephthalate), and copolymerization
with other biodegradable polymers
[e.g., poly(glycolic acid) (PGA)] promotes property
alterations so as to allow the use of conventional
manufacturing processes such as injection
molding, extrusion, blow molding, and fiber forming.
Other of its properties make PLA desirable
O
n
2
Biomass refers to biological material such as the stems, leaves, and seeds of plants that
can be used as fuel or as an industrial feed stock.