Page 308 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Composites and Fillers 271

M M
M M
M M M
M M M
M M
M M
M M M M
M M M
M M M
M M M
M M M

FIGURE 8.6 Silicate sheets impregnated with caprolactam monomer, M, which are eventually
polymerized.

8.7.1 PROCESSING OF FIBER-REINFORCED COMPOSITES

These exists a wide variety of particular operations but briefly they can be described in terms of fi la-
ment winding, preimpregnation of the fiber with the partially cured resin, and pultrusion. Pultrusion

is used to produce rods, tubes, and beams employing continuous fibers that have a constant cross-

sectional shape. The fiber (as a continuous fiber bundle, weave or tow) is impregnated with a ther-

mosetting resin and pulled through a die that shapes and establishes the fiber to resin ratio. This

stock is then pulled through a curing die that can machine or cut, producing the final shape such as
filled and hollow tubes and sheets.

The term used for continuous fi ber reinforcement preimpregnation with a polymer resin that
is only partially cured is “prepreg.” Prepreg material is generally delivered to the customer in the
form of a tape. The customer then molds and forms the tape material into the desired shape fi nally
curing the material without having to add any additional resin. Preparation of the prepreg can be

carried out using a calendaring process. Briefl y, fiber from many spools are sandwiched and pressed
between sheets of heated resin with the resin heated to allow impregnation but not so high as to be
very fl uid.
Thus, the fiber is impregnated in the partially cured resin. Depending upon the assembly the fi ber

is usually unidirectional, but can be made so that the fibers are bidirectional or are in some other

combination. The process of fitting the prepreg into, generally onto, the mold is called “lay-up.”
In general, a number of layers of prepreg are used. The lay-up may be done by hand, called hand
lay-up, or done automatically, or by some combination of automatic and hand lay-up. As expected,
hand lay-up is more costly but is needed where one-of-a-kind products are produced.
In fi lament winding, the fiber is wound to form a desired pattern, usually but not necessarily

hollow and cylindrical. The fiber is passed through the resin and then spun onto a mandrel. After

the desired number of layers of fiber is added, it is cured. Prepregs can be filament wound. With the

advent of new machinery, complex shapes and designs of the filament can be readily wound.
8.7.2 STRUCTURAL COMPOSITES
Structural composites can be combinations of homogeneous and composite materials. Laminar
composites are composed of two-dimensional sheets that generally have a preferred high-strength
direction. The layers are stacked so that the preferred high-strength directions are different, gen-
erally at right angles to one another. The composition is held together by a resin. This resin can be
applied as simply an adhesive to the various surfaces of the individual sheets or the sheet can be
soaked in the resin before laying the sheets together. In either case, the bonding is usually of a phys-

ical type. Plywood is an example of a laminar composite. Laminar fibrous glass sheets are included
as part of the modern ski construction. These fibrous glass sheets are fi ber-reinforced composites

used together as laminar composites.

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