GTBL042-20 GTBL042-Callister-v3 September 21, 2007 19:48
Summary • 797Rubber materials present some disposal and recycling challenges. When vul-
canized, they are thermoset materials, which makes chemical recycling difficult. In
addition, they may also contain a variety of fillers. The major source of rubber scrap
in the United States is discarded automobile tires, which are highly nonbiodegrad-
able. Scrap tires have been utilized as a fuel for some industrial applications (i.e.,
cement plants), but yield dirty emissions. Recycled rubber tires that have been split
and reshaped are used in a variety of applications such as automotive bumper guards,
mud flaps, door mats, and conveyor rollers; and, of course, used tires may also be
recapped. In addition, rubber tires may be ground into small chunks that are then
recombined into the desired shape using some type of adhesive; the resulting mate-
rial may be used in a number of nondemanding applications such as place mats and
rubber toys.
The most viable recyclable alternatives to the traditional rubber materials are
the thermoplastic elastomers (Section 13.16). Being thermoplastic in nature, they are
not chemically crosslinked and, thus, are easily reshaped. Furthermore, production
energy requirements are lower than for the thermoset rubbers since a vulcanization
step is not required in their manufacture.Composite Materials
Composites are inherently difficult to recycle because they are multiphase in nature.
The two or more phases/materials that constitute the composite are normally in-
termixed on a very fine scale and trying to separate them complicates the recycling
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 tech-
niques 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.SUMMARY
Economic Considerations
The economics of engineering is very important in product design and manufactur-
ing. To minimize product cost, materials engineers must take into account component
design, what materials are used, and manufacturing processes. Other significant eco-
nomic factors include fringe benefits, labor, insurance, profit, etc.Environmental and Societal Considerations
Environmental and societal impacts of production are becoming significant en-
gineering issues. In this regard, the material cradle-to-grave life cycle is an
important consideration; this cycle consists of extraction, synthesis/processing, prod-
uct design/manufacture, application, and disposal stages. Materials, energy, and