A numerical study on the thermal expansion coefficients of fiber

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(orthorhombic) of polyethylene has the dimensions of 0.741, 0.494, and 0.255 nm
(Chawla, 1998). There are four carbon and eight hydrogen atoms per unit cell. Its
strength and modulus are slightly lower than those of aramid fibers but on a per-unitweight
basis, specific property values are about 30% to 40% higher than those of
aramid. As is true of most organic fibers, both polyethylene and aramid fibers must
be limited to low-temperature (lower than 150°C) applications. High-modulus
polyethylene fibers are hard to bond with any polymeric matrix. Some kind of surface
treatment must be given to the polyethylene fiber to bond with resins such as
epoxy and (PMMA). Photomicrographs of polyethylene fibers are given in Figure
2.9.
Figure 2.9 Photomicrographs of polyethylene fibers (Wypych, 2000).
2.2.2.5.3 Aramid Fibers
Aramid fiber is a generic term for a class of synthetic organic fibers called
aromatic polyamide fibers. The U.S. Federal Trade Commission gives a good
definition of an aramid fiber as “a manufactured fiber in which the fiber-forming
substance is a long-chain synthetic polyamide in which at least 85% of the amide
linkages are attached directly to two aromatic rings”. The basic chemical structure of
aramid fibers consists of oriented para-substituted aromatic units, which make them
rigid rodlike polymers (Figure 2.10). The rigid rodlike structure results in a high
glass transition temperature and poor solubility, which makes fabrication of these
polymers, by conventional techniques, difficult.