Page 240 - Manufacturing Engineering and Technology - Kalpakjian, Serope : Schmid, Steven R.

Section 9.2 The Structure of Reinforced Plastics 2

40

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FIGURE 9.3 Specific tensile strength (tensile-strength~to-density ratio) and specific tensile
modulus (modulus-of-elasticity-to-density ratio) for various fibers used in reinforced plastics.
Note the wide range of specific strengths and stiffnesses available.

practical fiber content is 65 %; higher percentages generally result in poorer structural
properties.
When more than one type of fiber is used in a reinforced plastic, the composite
is called a hybrid. Hybrids generally have even better properties than single-fiber
composites, but they are more costly. Glass or carbon fiber-reinforced hybrid plastics
have been developed for high-temperature applications up to about 300°C.
Generally, however, these fibers are brittle, abrasive, lack toughness, and can degrade
chemically when exposed to the environment. Also, the properties of fibers can vary
significantly with the quality of the material and the method of processing.

9.2.l Reinforcing Fibers
Glass, carbon, ceramics, aramids, and boron are the most common reinforcing fibers
for polymer-matrix composites (Table 9.2).
Glass Fibers. Glass fibers are the most widely used and the least expensive of all
fibers. The composite material is called glass-fiber reinforced plastic (GFRP) and may
contain between 30 and 60% glass fibers by volume. Fibers are made by drawing
molten glass through small openings in a platinum die (see Section 18.3.4). The
molten glass is then elongated mechanically, cooled, and wound on a roll. A protec-
tive coating or sizing may be applied to facilitate their passage through the machin-
ery. The glass fibers are treated with silane (a silicon hydride) for improved wetting
and bonding between the fiber and the matrix.
The following are the principal types of glass fibers:

° E-type: a calcium aluminoborosilicate glass, the type most commonly used.
° S-type: a magnesia aluminosilicate glass, offering higher strength and stiffness,
but at a higher cost.
° E-CR-type: a high-performance glass fiber, offering higher resistance to elevated
temperatures and acid corrosion than does the E glass.