Callister - An introduction - 8th edition

15.19 Advanced Polymeric Materials • 603
or a chemical reaction. Rather, these materials contain polymer tackifying resins;
during detachment of the two bonding surfaces, small fibrils form that are attached
to the surfaces and tend to hold them together. Polymers used for pressure-sensitive
adhesives include the acrylics, styrenic block copolymers (Section 15.19), and
natural rubber.
Films
Polymeric materials have found widespread use in the form of thin films. Films having
thicknesses between 0.025 and 0.125 mm (0.001 and 0.005 in.) are fabricated
and used extensively as bags for packaging food products and other merchandise,
as textile products, and in a host of other uses. Important characteristics of the materials
produced and used as films include low density, a high degree of flexibility,
high tensile and tear strengths, resistance to attack by moisture and other chemicals,
and low permeability to some gases, especially water vapor (Section 14.14).
Some of the polymers that meet these criteria and are manufactured in film form
are polyethylene, polypropylene, cellophane, and cellulose acetate.
foam
Foams
Foams are plastic materials that contain a relatively high volume percentage of small
pores and trapped gas bubbles. Both thermoplastic and thermosetting materials are
used as foams; these include polyurethane, rubber, polystyrene, and poly(vinyl chloride).
Foams are commonly used as cushions in automobiles and furniture as well
as in packaging and thermal insulation. The foaming process is often carried out by
incorporating into the batch of material a blowing agent that, upon heating, decomposes
with the liberation of a gas. Gas bubbles are generated throughout the nowfluid
mass, which remain in the solid upon cooling and give rise to a spongelike
structure. The same effect is produced by dissolving an inert gas into a molten polymer
under high pressure. When the pressure is rapidly reduced, the gas comes out
of solution and forms bubbles and pores that remain in the solid as it cools.
15.19 ADVANCED POLYMERIC MATERIALS
A number of new polymers having unique and desirable combinations of properties
have been developed over the past several years; many have found niches in
new technologies and/or have satisfactorily replaced other materials. Some of these
include ultra-high-molecular-weight polyethylene, liquid crystal polymers, and thermoplastic
elastomers. Each of these will now be discussed.
ultra-high-molecularweight
polyethylene
Ultra-High-Molecular-Weight Polyethylene
Ultra-high-molecular-weight polyethylene (UHMWPE) is a linear polyethylene that has
an extremely high molecular weight. Its typical M is approximately 4 10 6 w
g/mol,
which is an order of magnitude greater than that of high-density polyethylene. In
fiber form, UHMWPE is highly aligned and has the trade name Spectra. Some of
the extraordinary characteristics of this material are as follows:
1. An extremely high impact resistance
2. Outstanding resistance to wear and abrasion
3. A very low coefficient of friction
4. A self-lubricating and nonstick surface
5. Very good chemical resistance to normally encountered solvents