Page 210 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 7.7 General Properties and Applications of Thermosetting Plastics

grains together in grinding wheels; and electrical components (such as wiring devices,
connectors, and insulators).
Polyesters (thermosetting polyesters; see also Section 7.6) have good mechani-
cal, chemical, and electrical properties. They generally are reinforced with glass (or
other) fibers and also are available as casting resins. Typical applications include
boats, luggage, chairs, automotive bodies, swimming pools, and materials for
impregnating cloth and paper.
Polyimides possess good mechanical, physical, and electrical properties at
elevated temperatures; they also have good creep resistance, low friction, and low
wear characteristics. Polyimides have the nonmelting characteristic of a thermoset,
but the structure of a thermoplastic. Typical applications include pump components
(bearings, seals, valve seats, retainer rings, and piston rings), electrical connectors
for high-temperature use, aerospace parts, high-strength impact-resistant structures,
sports equipment, and safety vests.
Silicones have properties that depend on composition. Generally, they weather
well, possess excellent electrical properties over a wide range of humidity and temper-
ature, and resist chemicals and heat (see also Section 7.9). Typical applications include
electrical components requiring strength at elevated temperatures, oven gaskets, heat
seals, and waterproof materials.

Health Hazards. There is increasing concern that some of the chemicals used in
polymers may present health hazards, especially in products such as polycarbonate
water containers and baby bottles, and also medical devices, sports safety equip-
ment, and eating utensils. The chemical that is of particular concern is bisphenol A,
or BPA, a widely produced and used chemical. Numerous worldwide investigations
are being conducted to determine whether there is any link to human diseases,
specifically heart disease and diabetes.

EXAMPLE 7.3 Materials for a Refrigerator Door Liner
In the selection of candidate materials for a refrigera- on material properties and behavior over a
tor door liner (where eggs, butter, salad dressings, and period of time, compatibility with other com-
small bottles are stored) the following factors should ponents in the door, and cost of materials and
be considered: manufacturing.

l. Mechanical requirements: strength, toughness An extensive study, considering all of the
(to withstand impacts, door slamming, and factors involved, identified two candidate materials
racking), stiffness, resilience, and resistance to
for door liners: ABS (acrylonitrile-butadiene-styrene)
scratching and wear at operating temperatures. and HIPS (high-impact polystyrene). Gne aspect of
2. Physical requirements: dimensional stability the study involved the effect of vegetable oils, such
and electrical insulation. as from salad dressing stored in the door shelf, on
3. Chemical requirements: resistance to staining, the strength of those plastics. Experiments showed
odor, chemical reactions with food and bever- that the presence of vegetable oils significantly
ages, and cleaning fluids. reduced the load-bearing capacity of HIPS. It was
found that HIPS becomes brittle in the presence of
4. Appearance: color, stability of color, surface
oils (solvent-stress cracking), Whereas ABS is not
finish, texture, and feel.
affected to any significant extent.
5. Manufacturing properties: methods of manu-
facturing and assembly, effects of processing