Page 242 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 9.2 The Structure of Reinforced Plastics

catalytic crackers in petroleum refining) also can be used as precursors. Pyrolysis is
the process of inducing chemical changes by heat-for instance, by burning a length
of yarn and causing the material to carbonize and become black in color. With PAN,
the fibers are partially cross-linked at a moderate temperature (in order to prevent
melting during subsequent processing steps) and are elongated simultaneously. At
this point, the fibers are carburized: They are exposed to elevated temperatures to
expel the hydrogen (dehydrogenation) and the nitrogen (denitrogenation) from the
PAN. The temperatures for carbonizing range up to about 1500°C; for graphitizing
up to 3000°C. PAN fibers, which have an oriented long-chain molecules, are white
and have a density of 1170 kg/ms.

Conductive Graphite Fibers. These fibers are produced to make it possible to
enhance the electrical and thermal conductivity of reinforced plastic components.
The fibers are coated with a metal (usually nickel) using a continuous electroplating
process; the coating is typically 0.5-/.tm thick on a 7-um-diameter graphite fiber
core. Available in a chopped or a continuous form, the conductive fibers are
incorporated directly into injection-molded plastic parts. Applications include elec-
tromagnetic and radio-frequency shielding and lightning-strike protection.

Ceramic Fibers. One family of ceramic fibers is Nextel, a trade name. Nextels are
oval in cross section and consist of alumina, silica, and boric oxide. Typical mechan-
ical properties are given in Table 9.2. These fibers have low elongation, low thermal
conductiity, and good chemical resistance, in addition to being suitable for high-
temperature applications.

Polymer Fibers. Polymer fibers may be made of nylon, rayon, acrylics, or aramids;
the most common are aramid fibers. Aramids (Section 7.6), such as Kevlar, are
among the toughest fibers and have very high specific strength (Fig. 9.3). Aramids
can undergo some plastic deformation prior to fracture and, hence, have higher
toughness than brittle fibers. However, aramids absorb moisture (hygroscopic), thus
degrading their properties and complicating their application.
Another high-performance polyethylene fiber is Spectra (a trade name); it has
an ultra-high molecular weight and high molecular-chain orientation. Spectra, a
bright white polyethylene, has better abrasion resistance and flexural-fatigue resist-
ance than aramid fibers at a similar cost. In addition, because of its lower density
(970 kg/m3), it has a higher specific strength and specific stiffness than aramid
fibers (see Table 9.2). However, a low melting point and poor adhesion characteris-
tics as compared to other polymers are its major limitations to applications.
Polymer fibers are made by two processes: melt spinning and dry spinning (see
Section 19.2.2). Melt spinning involves extruding a liquid polymer through small
holes in a die (spinnerets). The fibers are then cooled before being gathered and
wound onto bobbins. The fibers may be stretched to further orient and strengthen
the polymer. In dry spinning, the polymer is dissolved in a liquid solution to form a
partially oriented liquid-crystal form. As the polymer passes through the spinnerette,
it is oriented further, and at this point, the fibers are washed, dried, and wound.
Aramids are oriented in solution and are oriented fully when they pass through the
spinnerette and therefore, do not need to be drawn any further.

Boron Fibers. These fibers consist of boron deposited (by chemical vapor-deposition
techniques) onto tungsten fibers (Fig. 9.4b). Boron also can be deposited onto carbon
fibers. Boron fibers have desirable properties, such as high strength and stiffness, both
in tension and in compression, and resistance to high temperatures. However, because
of the high density of tungsten, they are heavy and also are expensive.