Page 301 - Carrahers_Polymer_Chemistry,_Eighth_Edition

264 Carraher’s Polymer Chemistry

As with many polymers, the limits of strength are due to the presence of voids. For glass fi bers,
these voids generally occur on the surface, thus care is taken to protect these surfaces through sur-
face treatments with methacrylatochromic chloride, vinyl trichlorosilanes, and other silanes. These
surface agents chemically react with the fiber surface acting to repel and protect the surface from

harmful agents such as moisture.

A number of kinds of carbon-intense fibers are used, the most common being carbon and graph-

ite fibers and carbon black. As in the case of fibrous glass, surface voids are present. Carbon-intense

fibers are often surface-treated with agents such as low molecular weight epoxy resins. Such surface

treatments also aim at increasing the fi ber–matrix adhesion.
Two general varieties of aromatic nylons are often employed. A less-stiff variety is employed
when some flexibility is important, while a stiffer variety is used for applications where greater

strength is required. While good adhesion with the resin is often desired, poor adhesion is some-
times an advantage such as in the construction of body armor where “delamination” is a useful
mode for absorbing an impact.
As we understand materials better, we are able to utilize them for additional applications. It is

known that “elongational” flow through orifices can result in the stretching and reorientation of

polymer chains giving a stronger fiber in the direction of pull. Some polymers become entangled
and the flow gives additional orientation. Finally, polymer solutions may be stable at rest, but under

high rates of extrusion they may be removed from solution, forming a gel phase. These observations

have allowed the production of a number of new polyolefi n fibers, including ultrahigh-modulus

polyethylene fibers that have low density but relatively high tensile strength with an elongation at
break over two times greater than glass and aromatic nylon fi bers.
Both thermoset and thermoplastic resin systems are employed in the construction of composites
(Table 8.3). The most common thermoset resins are polyimides, unsaturated polyesters, epoxys,
phenol-formaldehydes, and amino-formaldehydes. A wide variety of thermoplastic resins have been
developed.
8.5 LONG-FIBER COMPOSITES—APPLICATIONS

Many of the applications for composite materials involve their (relative) light weight, resistance to
weathering and chemicals, and their ability to be easily fabricated and machined. Bulk applications
employ composites that are relatively inexpensive. Combinations of rigorous specifications, low vol-

ume, specific machining and fabrication specifications, and comparative price to alternative materi-

als and solutions allow more expensive specialized composites to be developed and utilized.
Applications are increasing. Following is a brief description of some of these. One of the larg-
est and oldest applications of composites is the construction of water-going vessels from rowboats,
sailboats, racing boats, and motor craft to large seagoing ships. The use of fresh water and salt
water resistant composites allowed the boating industry to grow and today includes a range from
individually operated backyard construction to the use of large boatyards producing craft on an

assembly line. Most of these craft are composed of fiberglass and fi berglass/carbon-combination
composites.
Compositions are also important in the construction of objects to both propel material into and
material to exist in outer space. Because of the large amount of fuel required to propel spacecraft
into outer space, weight reduction, offered by composites, is essential. The polymeric nature of com-
posites also makes it an ideal material to resist degradation caused by the vacuum of outer space.
Many biomaterials are composites. Bone and skin are relatively light compared to metals.
Composite structures can approach the densities of bone and skin and offer the necessary inertness
and strength to act as body-part substitutes.
Power-assisted arms have been made by placing hot-form strips of closed-cell PE foam over the
cast of an arm. Grooves are cut into these strips before application and carbon/resin are added to the
grooves. The resulting product is strong, light, and the cushioned PE strips soften the attachment

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