Callister - An introduction - 8th edition

22.5 Recycling Issues in Materials Science and Engineering • 881
process. However, some techniques have been developed, with modest success, for
recycling polymer-matrix composites. Recycling technologies will differ only slightly
for thermoset-matrix and thermoplastic-matrix composite materials.
The first step in recycling both thermoset- and thermoplastic-matrix composites
is shredding/grinding, wherein the components are reduced in size to relatively small
particles. In some instances, these ground particles are used as filler materials that
are blended with a polymer (and perhaps other fillers) before fabrication (usually
using some type of molding technique) into postconsumer products. Other recycling
processes allow for separating of the fibers and/or matrix materials.With some techniques
the matrix is volatilized; with others it is recovered as a monomer. Of course,
the recovered fibers have short lengths as a result of the shredding/grinding process.
In addition, fibers will experience a reduction of mechanical strength, the degree
of which will depend on the specific recovery process as well as fiber type.
MATERIALS OF IMPORTANCE
Biodegradable and Biorenewable Polymers/Plastics
Most of the polymers manufactured today are
synthetic and petroleum based. These synthetic
materials (e.g., polyethylene and polystyrene)
are extremely stable and resistant to
degradation, particularly in moist environments.
In the 1970s and 1980s it was feared that the large
volume of plastic waste being generated would
contribute to the filling of all available landfill
capacity. Thus, the resistance to degradation of
polymers was viewed as a liability rather than as
an asset. The introduction of biodegradable polymers
was perceived as a means to eliminate some
of this landfill waste, and the response of the polymer
industry was to start developing biodegradable
materials.
Biodegradable polymers are those that degrade
naturally in the environment, normally by microbial
action. With regard to degradation mechanism, microbes
sever polymer chain bonds, which leads to a
decrease in molecular size; these smaller molecules
may then be ingested by microbes, in a process that
is similar to the composting of plants. Of course, natural
polymers such as wool, cotton, and wood are
biodegradable inasmuch as microbes can readily digest
these materials.
The first generation of these degradable materials
was based on common polymers such as
polyethylene. Compounds were added to make
these materials decompose in sunlight (i.e., photodegrade),
to oxidize by reacting with oxygen in
the air, and/or to degrade biologically. Unfortunately,
this first generation did not measure up to
expectations.They degraded slowly if at all, and the
anticipated reduction in landfill waste was not
realized. These initial disappointments gave
degradable polymers a bad reputation that hindered
their development. By way of response, the
polymer industry instituted standards that accurately
measure degradation rate as well as characterize
the mode of degradation.These developments
led to a renewed interest in biodegradable polymers.
Development of the current generation of
biodegradable polymers is frequently directed to
niche applications that take advantage of their
short lifetimes. For example, biodegradable leaf
and yard waste bags can be used to contain compostable
matter, which eliminates the need to debag
the material.
Another important application of biodegradable
plastics is as mulch films for farming (Figure
22.3). In colder regions of the world, covering
seedbeds with plastic sheets can extend the growing
season so as to increase crop yields and, in addition,
reduce costs.The plastic sheets absorb heat,
raise the ground temperature, and increase moisture
retention. Traditionally, black polyethylene
(nonbiodegradable) sheets were used. However,
at the end of the growing season these sheets had
to be manually gathered from the field and disposed
of because they did not decompose/biodegrade.
More recently, biodegradable plastics have been
developed for use as mulch films. After the crops
have been harvested, these films are simply
plowed into and enrich the soil as they decompose.