Page 94 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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Encyclopedia of Physical Science and Technology EN012K-587 July 26, 2001 10:35

464 Plastics Engineering

amorphous plastics offering a broad working-temperature 7. Melamine formaldehyde: The hardest of all plas-
range. Very low water absorption and so excellent dimen- tics, this material is excaellent for food contact applica-
sional stability. Very amenable to blending with other plas- tions and is fire resistant. Requires higher pressure than
tics to produce alloys. phenolics to mold but its color properties are superior to
17. Polyphenylene sulfide: Good strength characteris- those of the polyesters.
tics from low to high temperatures. Inert to most chemicals 8. Phenolics: General purpose, cost-effective resins.
and inherently flame retardant. One of the first synthetic plastics; developed by Leo H.
18. Polypropylene: Part of the same generic group as Baekeland in the early 1900s. Color limited to black or
polyethylene. Low-density, tough plastic (above room brown. Easily molded into complex shapes with good di-
temperature) and very easy to mold. Outstanding fatigue mensional stability. Good creep resistance and very hard
resistance and stress-crack resistance. surface.
19. Polysulfone: A clear, rigid, tough plastic capable 9. Polyesters: Unlimited colors and also available
◦
of continuous use at 160 C. Excellent chemical resistance transparent.Easytomoldbutshrinkageishigh.Verypopu-
but notch-sensitive and expensive. Polyether sulfone is lar resin for use with glass fibers; product is often referred
one of the most commercially successful members of the to as fiberglass. Also form basis for the popular (easily
polysulfone family. molded) bulk molding compound (BMC) and sheet mold-
20. Polyurethane:Versatilematerialavailableinawide ing compound (SMC).
range of forms. Very tough, with high abrasion and chem- 10. Polyimides: These materials have a low dielectric
ical resistance. Popular foaming material (rigid or flexible constant and a high dielectric strength. Their chemical
foams). Susceptible to ultraviolet attack. Also available as resistance and creep resistance are both excellent. Also
a thermoset. available as a thermoplastic.
21. Polyvinyl chloride: Wide range of formulations 11. Polyurethane: Wide range of formulations. The
from hard–rigid to soft–flexible material. Hard and tough greater the degree of cross-linking, the greater the
with excellent electrical insulation properties. Good hardness. Excellent toughness, good traction properties,
outdoor weatherability and resistance to moisture and and resistance to wear make it ideal for skate wheels and
chemicals. shoe heels. Also available as a thermoplastic.
12. Silicones: Can be available as elastomeric or rigid
B. Thermosets materials. They are very inert and can be used over a
broad temperature range. Popular as gaskets, sealing rings,
1. Alkyd: Easy, fast-molding material with no volatile and O-rings. They have good weatherability and excellent
by-products. Excellent electrical insulation with good heat electrical properties.
resistance and low moisture absorption. Developed in the 13. Urea formaldehyde: Can be available in a range
1930s; one of the earliest applications was for automotive of colors. Good surface hardness and electrical properties
distributor caps. but relatively poor heat resistance.
2. Allyl: Excellent all-round electrical properties even
under long-term heat and moisture. Also good resistance
C. General Behavior Characteristics
to chemicals even at high temperatures.
of Engineering Plastics
3. Aminos: Urea and melamine formaldehyde are the
most common forms. Very easily colored. Urea is less exp- The successful design of plastic articles requires a broader
ensive and molds more easily but melamine has higher range of knowledge of materials behavior than would be
hardness and better chip resistance and is more resistant the case for designing a metal product. This is because
to temperature and chemicals. factors such as time, temperature, environment, etc. are
4. Bismaleimide (BMI): Excellent high-temperature variables that must be considered in almost all cases for
performance but, like most thermosetting materials, it plastics, whereas these are only relevant in extreme cases
lacks toughness. for metals. The simplest illustration of this is the design
5. Cyanates: Tough materials with excellent adhesive of a product to be subjected to a static load at room tem-
peel strength. Low dielectric loss and moisture absorption. perature. If the product is made from metal and it is able
Good dimensional stability. to withstand the applied load in the short term, one can
6. Epoxies: Excellent combination of mechanical have reasonable confidence that there will be no prob-
strength, electrical resistance, and adhesion to most lems in the long term. However, with plastics the modu-
materials. Low mold shrinkage, and some formulations lus (related to the stiffness) of the material will decrease
cure without heat or pressure. Most grades are relatively with time, so the ability to withstand the applied load in
brittle. the short term should not lead one to assume that it will