Page 309 - Carrahers_Polymer_Chemistry,_Eighth_Edition

272 Carraher’s Polymer Chemistry

Laminar materials are produced by a variety of techniques. Coextrusion blow molding produces
a number of common food containers that consist of multilayers such as layers consisting of poly-
propylene/adhesive/poly(vinyl alcohol)/adhesive/adhesive/polypropylene.
Sandwich composites are combinations where a central core(s) is surrounded generally by stron-
ger outer layers. Sandwich composites are present in the modern ski and as high-temperature stable
materials used in the space program. Some cores are very light acting something like fi ller with
respect to high strength, with the strength provided by the outer panels. Simple corrugated card-
board is an example of a honeycomb core sandwich structure except that the outer paper-intense
layers are not particularly strong. Even in the case of similar polyethylene and polypropylene cor-
rugated structures, the outer layers are not appreciatively stronger than the inner layer. In these cases
the combination acts to give a light weight somewhat strong combination, but they are not truly
composites but simply exploit a common construction.

8.7.3 LAMINATING
Laminating is a simple binding together of different layers of materials. The binding materials are
often thermosetting plastics and resins. The materials to be bound together can be paper, cloth,

wood, or fibrous glass. These are often referred to as the reinforcing materials. Typically, sheets,
impregnated by a binding material, are stacked between highly polished metal plates, subjected to
high pressure and heat in a hydraulic press producing a bonded product, which may be subsequently

treated, depending on its final use. The end product may be flat, rod-shaped, tubular, rounded, or

some other formed shape.
Reinforced plastics differ from high pressure laminates in that little or no pressure is employed.
For instance, in making formed shapes, impregnated reinforcing material is cut to a desired shape,
the various layers are added to a mold, and the molding is completed by heating the mold. This pro-
cess is favored over the high pressure process because of the use of a simpler, lower-cost mold and
the production of strain-free products.

8.7.4 PARTICULATE
Particulate composites consist of the reinforcing materials being dispersed throughout the resin.
Unlike fibrous composites, the reinforcing material in the particulate composites is more bulky

and is not fibrous in nature. These particulates can be of many shapes and relative sizes. One of the
employed shapes is that of a flat rounded rectangular “disk-like” shape where the disk-like particles

can form overlapping layers, thus reinforcing one another (Figure 8.7). Particle board is an exam-
ple of particulate composites. At times, there is little to differentiate between fillers and reinforcing

agents. The emphasis on reinforcing agents is one where increased strength results from the pres-
ence of the reinforcing agent, whereas fi llers fulfill the role of simply increasing the bulk of the

material. Further, fillers are generally smaller than reinforcing agents.
8.8 METAL-MATRIX COMPOSITES
There were driving forces for the development of composites in the 1950s and 1960s. These included
the need for materials that offered greater strength/density ratios. Another driving force was the
promise that the marriage between two materials would produce materials with especially high-
strength and elastic moduli. There was also the need for new materials to meet the demands of the
aerospace industries that were rapidly developing. Applications for these materials rapidly spilled
over from the aerospace and military into the general public with advancements made in one sector
fueling advances in another sector. Many of the fi rst advanced materials were of the clad or lami-
nate type of composite. Here we will not be concerned with clad or laminate type of composites but
rather those where the two phases are mixed together.

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