Page 213 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 7 Polymers: Structure, General Properties, and Applications
One property of elastomers is their hysteresis loss in stretching or compression
(Fig. 7.14). The clockwise loop indicates energy loss, whereby mechanical energy is
converted into heat. This property is desirable for absorbing vibrational energy
(damping) and sound deadening.

Natural Rubber. The base for natural rubber is latex, a milk-like sap obtained
from the inner bark of a tropical tree. Natural rubber has good resistance to abra-
sion and fatigue, and characteristic high friction, but low resistance, to oil, heat,
ozone, and sunlight. Typical applications are tires, seals, shoe heels, couplings, and
engine mounts.

Synthetic Rubbers. Examples of synthetic rubbers are butyl, styrene butadiene,
polybutadiene, and ethylene propylene. Compared to natural rubber, they have bet-
ter resistance to heat, gasoline, and chemicals, and they have a higher range of use-
ful temperatures. Synthetic rubbers that are resistant to oil are neoprene, nitrile,
urethane, and silicone. Typical applications of synthetic rubbers are tires, shock ab-
sorbers, seals, and belts.

Silicones. Silicones (see also Section 7.7) have the highest useful temperature range
of elastomers (up to 315°C), but other properties (such as strength and resistance to
wear and oils) generally are inferior to those in other elastomers. Typical applications
of silicones are seals, gaskets, thermal insulation, high-temperature electrical switch-
es, and electronic apparatus.

Polyurethane. This elastomer has very good overall properties of high strength,
stiffness, and hardness, and it has exceptional resistance to abrasion, cutting, and
tearing. Typical applications are seals, gaskets, cushioning, diaphragms for the rub-
ber forming of sheet metals (Section 16.8), and auto body parts.

SUMMARY

° Polymers are a major class of materials and possess a very wide range of mechan-
ical, physical, chemical, and optical properties. Compared to metals, polymers
generally are characterized by a lower density, strength, elastic modulus, thermal
and electrical conductivity, and cost; by a higher strength-to-weight ratio, higher
resistance to corrosion, higher thermal expansion, and wider choice of colors and
transparencies; and by a greater ease of manufacture into complex shapes.
° Plastics are composed of polymer molecules and various additives. The smallest
repetitive unit in a polymer chain is called a mer. Monomers are linked by poly-
merization processes (condensation and addition) to form larger molecules. The
glass-transition temperature separates the region of brittle behavior in polymers
from that of ductile behavior.
° The properties of polymers depend on their molecular weight, structure (linear,
branched, cross-linked, or network), degrees of polymerization and crystallinity,
and on additives present in their formulation. Additives have such functions as
improving strength, flame retardation, lubrication, imparting flexibility and color,
and providing stability against ultraviolet radiation and oxygen. Polymer structures
can be modified by several means to impart a wide range of desirable properties.
° Two major classes of polymers are thermoplastics and thermosets. Thermoplastics
become soft and easy to form at elevated temperatures; they return to their origi-
nal properties when cooled. Their mechanical behavior can be characterized by