Page 212 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 7.9 Elastomers (Rubbers) l9l

The long-term performance of biodegradable plastics (both during their useful
life cycle as products and in landfills) has not been assessed fully. There also is con-
cern that the emphasis on biodegradability will divert attention from the issue of the
recyclability of plastics and the efforts for conservation of materials and energy. A
major consideration is the fact that the cost of today’s biodegradable polymers is
substantially higher than that of synthetic polymers. Consequently, a mixture of
agricultural waste-such as hulls from corn, wheat, rice, and soy (as the major
component)-and biodegradable polymers (as the minor component) is an attrac-
tive alternative.

Recycling of Plastics. Much effort continues to be expended globally on the col-
lecting and recycling of used plastic products. Thermoplastics are recycled by remelt-
ing them and then reforming them into other products. They carry recycling symbols,
in the shape of a triangle outlined by three clockwise arrows and having a number in
the middle. These numbers correspond to the following plastics:
1-PETE (polyethylene)
2-HDPE (high-density polyethylene)
3-V (vinyl)
4-LDPE (low-density polyethylene)
5-PP (polypropylene)
6-PS (polystyrene)
7-Other
Recycled plastics increasingly are being used for a variety of products. For
example, a recycled polyester (filled with glass fibers and minerals) is used for the
engine cover for an F-series Ford pickup truck, as it has the appropriate stiffness,
chemical resistance, and shape retention up to 18O°C.

1.9 Elastomers (Rubbers)

Elastomers consist of a large family of amorphous polymers having a low glass-tran-
sition temperature. They have a characteristic ability to undergo large elastic defor-
mations without rupture; also, they are soft and have a low elastic modulus. The
term elastomer is derived from the words elastic and mer.
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The structure of elastomer molecules is highly kinked (tightly twisted or 3 0°
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curled). They stretch, but then return to their original shape after the load is re-
moved (Fig. 7.14). They can also be cross-linked, the best example of this being the Q0
elevated-temperature vulcanization of rubber with sulfur, discovered by Charles
Goodyear in 1839 and named for Vulcan, the Roman god of fire. Once the elas- Elongation
tomer is cross-linked, it cannot be reshaped (for example, an automobile tire, which
is one giant molecule, cannot be softened and reshaped). FIGURE 7.I4 Typical load-
The terms elastomer and rubber often are used interchangeably. Generally, an elongation curve for rubbers.
elastomer is defined as being capable of recovering substantially in shape and size The clockwise loop, indicat-
after the load has been removed. A rubber is defined as being capable of recovering ing the loading and the
from large deformations quickly. unloading paths, displays the
hysteresis loss. Hysteresis
The hardness of elastomers, which is measured with a durometer (Section 2.6),
gives rubbers the capacity to
increases with the cross-linking of the molecular chains. As with plastics, a variety
dissipate energy, damp vibra-
of additives can be blended into elastomers to impart specific properties. Elastomers
tion, and absorb shock
have a wide range of applications in high-friction and nonskid surfaces, protection
loading, as is necessary in
against corrosion and abrasion, electrical insulation, and shock and vibration insu- automobile tires and in
lation. Examples include tires, hoses, weatherstripping, footwear, linings, gaskets, vibration dampers placed
seals, printing rolls, and flooring. under machinery.