Page 206 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 7.6 General Properties and Applications of Thermoplastics

against degradation caused by oxidation, particularly at elevated temperatures,
is achieved by adding antioxidants to the polymer. Various coatings are another
means of protecting polymers.
° Fillers used in plastics are generally wood flour (fine sawdust), silica flour (fine
silica powder), clay, powdered mica, talc, calcium carbonate, and short fibers of
cellulose, glass, or asbestos. Because of their low cost, fillers are important in re-
ducing the overall cost of polymers. Depending on their type, fillers also may im-
prove the strength, hardness, toughness, abrasion resistance, dimensional
stability, or stiffness of plastics. These properties are greatest at specific percent-
ages of different types of polymer-filler combinations. As with reinforced plas-
tics (Section 9.2), a filler’s effectiveness depends on the nature of the bond
between the filler material and the polymer chains.
° The wide variety of colors available in plastics is obtained by adding colorants-
either organic (dyes) or inorganic (pigments). The selection of a colorant de-
pends on the service temperature and the expected amount of exposure to light.
Pigments are dispersed particles; they generally have greater resistance than do
dyes to temperature and light.
° If the temperature is sufficiently high, most polymers will ignite and burn; flame
colors are usually yellow or blue. The flammability (ability to support combus-
tion) of polymers varies considerably, depending on their composition (especially
on their chlorine and fluorine content). The flammability of polymers can be re-
duced either by making them from less flammable raw materials or by adding
flame retardants, such as compounds of chlorine, bromine, and phosphorus.
Cross-linking also reduces polymer flammability.
The following list gives several common polymers with different burning
characteristics:

l. Plastics that do not burn: flnorocarbons (e.g., Teflon)
2. Plastics that do burn but are self-extinguishing: carbonate, nylon,
1/inyl chloride
3. Plastics that burn and are not self-extinguishing: acetal, acrylic,
acrylonitrile-batadiene-styrene, cellulose, polyester propylene, styrene.
' Lubricants may be added to polymers to reduce friction during their subse-
quent processing into useful products and to prevent parts from sticking to the
molds. Typical lubricants are linseed oil, mineral oil, and waxes (natural and
synthetic); metallic soaps, such as calcium stearate and zinc stearate also are
used. Lubrication also is important in preventing thin polymer films from
sticking to each other.

7.6 General Properties and Applications
of Thermoplastics

The general characteristics and typical applications of major thermoplastics, partic-
ularly as they related to the manufacturing and service life of plastic products and
components, are outlined in this section. General recommendations for various plas-
tics applications are given in Table 7.3, and Table 7.4 lists some of the more com-
mon trade names for thermoplastics.
Acetals (from acetic and alcohol) have good strength, good stiffness, and good
resistance to creep, abrasion, moisture, heat, and chemicals. Typical applications
include mechanical parts and components requiring high performance over a long