Page 16 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

1.2 CHAPTER 1

Materials are often classified as either metals, ceramics, or polymers. Polymers differ
from the other materials in a variety of ways but generally exhibit lower densities, thermal
conductivities, and moduli. Table 1.1 compares the properties of polymers to some repre-
sentative ceramic and metallic materials. The lower densities of polymeric materials offer
an advantage in applications where lighter weight is desired. The use of additives allows
the compounder to develop a host of materials for specific application. For example, the
addition of conducting fillers generates materials from insulating to conducting. As a re-
sult, polymers may find application in EMI shielding and antistatic protection.
Polymeric materials are used in a vast array of products. In the automotive area, they
are used for interior parts and in under-the-hood applications. Packaging applications are a
large area for thermoplastics, from carbonated beverage bottles to plastic wrap. Applica-
tion requirements vary widely but, luckily, plastic materials can be synthesized to meet
these varied service conditions. It remains the job of the part designer to select from the ar-
ray of thermoplastic materials available to meet the required demands.

1.2 POLYMER STRUCTURE AND SYNTHESIS

A polymer is prepared by stringing together a low molecular weight species (monomer;
e.g., ethylene) into an extremely long chain (polymer; in the case of ethylene, the polymer
is polyethylene) much as one would string together a series of bead to make a necklace
(see Fig. 1.1). The chemical characteristics of the starting low molecular weight species
will determine the properties of the final polymer. When two low different molecular

TABLE 1.1 Properties of Selected Materials 48
Thermal Electrical
Specific conductivity, resistivity, Modulus,
2
Material gravity Joule cm/(°C cm s) µΩ cm MPa
Aluminum 2.7 2.2 2.9 70,000
Brass 8.5 1.2 6.2 110,000
Copper 8.9 4.0 1.7 110,000
Steel (1040) 7.85 0.48 17.1 205,000
Al O 3.8 0.29 >10 14 350,000
2 3
Concrete 2.4 0.01 – 14,000
Borosilicate glass 2.4 0.01 >10 17 70,000
5
MgO 3.6 – 10 (2000° F) 205,000
14
Polyethylene (H.D.) 0.96 0.0052 10 –10 18 350–1,250
Polystyrene 1.05 0.0008 10 18 2,800
Polymethyl methacry- 1.2 0.002 10 16 3,500
late
Nylon 1.15 0.0025 10 14 2,800

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