Page 204 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 7.3 Thermoplastics |83

Water Absorption. An important characteristic of some polymers, such as nylons, is
their ability to absorb water (hygroscopic). Water acts as a plasticizing agent: It makes
the polymer more plastic (see Section 7.5). In a sense, it lubricates the chains in the
amorphous region. With increasing moisture absorption, the glass-transition tempera-
ture, the yield stress, and the elastic modulus of the polymer typically are lowered
severely. Dimensional changes also occur, especially in a humid environment.

Thermal and Electrical Properties. Compared to metals, plastics generally are
characterized by low thermal and electrical conductivity, low specific gravity (rang-
ing from 0.90 to 2.2), and a high coefficient of thermal expansion (about an order
of magnitude higher; see Tables 3.1 and 3.2.) Because most polymers have low elec-
trical conductivity, they can be used for insulators and as packaging material for
electronic components.
The electrical conductivity of some polymers can be increased by doping (intro-
ducing impurities, such as metal powders, salts, and iodides, into the polymer).
Discovered in the late 1970s, electrically conducting polymers include polyethylene
oxide, polyacetylene, polyaniline, polypyrrole, and polythiophene. The electrical
conductivity of polymers increases with moisture absorption; their electronic proper-
ties also can be changed by irradiation. Applications for conducting polymers include
adhesives, microelectronic devices, rechargeable batteries, capacitors, catalysts, fuel
cells, fuel-level sensors, deicer panels, radar dishes, antistatic coatings, and thermoac-
tuating motors (used in linear-motion applications such as for power antennae, sun
roofs, and power windows).
Thermally conducting polymers also are being developed for applications
requiring dimensional stability and heat transfer (such as heat sinks), as well as for
reducing cycle times in molding and processing of thermoplastics. These polymers
are typically thermoplastics (such as polypropylene, polycarbonate, and nylon) and
are embedded with nonmetallic thermally conducting particles; their conductivity
can be as much as 100 times that of conventional plastics.

Shape-memory Polymers. Recent investigations have shown that polymers also
can behave in a manner similar to the shape-memory alloys described in Section 6.13.
The polymers can be stretched or compressed to very large strains, and then, when
subjected to heat, light, or a chemical environment, they recover their shape. The
potential applications for these polymers are similar to those for shape-memory met-
als, such as in opening blocked arteries, as well as probing neurons in the brain and
improving the toughness of spines.

EXAMPLE 7.2 Use of Electrically Conducting Polymers in Rechargeable Batteries

One of the earliest applications of conducting poly- LiyC6 is oxidized, emitting free electrons and lithium
mers was in rechargeable batteries. Modern lithium ions. The electrons drive external electronics, and the
rechargeable batteries use lithium or an oxide of lithi- Lil" ions are stored in the polymer. When the cathode is
um as the cathode and lithium carbide (LiyC6) as depleted, the battery must be recharged to restore the
the anode, separated by a conducting polymer layer. cathode. During charging, Li+ is transferred through
Lithium is used because it is the lightest of all metals the polymer electrolytes to the cathode. Lithium-ion
and has a high electrochemical potential, so that its batteries have good capacity, can generate up to 4.5 Y
energy per volume is highest. and can be placed in series to obtain higher voltages.
The polymer, usually polyethylene oxide (PEO), Battery cells are now being developed in which both
with a dissolved lithium salt, is placed between the electrodes are made of conducting polymers; one has
cathode and anode. During discharge of the battery, been constructed with a capacity of 3.5 V