Introduction to Materials Chemistry by Allcock H. R

Author:Allcock, H. R.
Language: eng
Format: epub
Publisher: John Wiley & Sons, Inc.
Published: 2011-08-27T04:00:00+00:00

Which block type forms the matrix and which forms the coronas? This depends on the relative lengths of the two blocks. The spherical micellar structure tends to appear when one block length (call it block A) is appreciably longer than the other. As the block length of A becomes shorter, the morphology may change progressively to cylindrical, to gyroidal, and then for nearly equal block lengths, to laminar. If the length of the other block (B) is now increased, the reverse sequence of phases will be formed—gyroidal, cyclindrical, to spherical, with block B now constituting the matrix phase. These phase components have dimensions that are in the nanometer range, because they depend on the 10–100 nm lengths of the blocks.

9. Laminates

Laminates are composites in which layers of two or more different and incompatible materials are bonded together to yield a material that is stronger than the individual components. Two types of laminates are of particular interest. First, laminates that consist of two outer layers of a glass cemented by a central layer of an impact-resistant adhesive polymer are used in automobile windshields and related applications. The glass provides the strength and transparency, while the thin central layer holds the broken structure together if an impact occurs. Poly(vinyl butyrate) is often used as the central, impact-absorbing layer.

A second type of laminate consists of layers of a reinforcing woven cloth, typically of glass fiber or carbon fiber, infused with a thermosetting monomer (Figure 9.10). In a laminated assembly of this type, multiple layers of the cloth are infused one by one with the liquid monomer. The assembly is then compressed and heated to crosslink the polymer, thus trapping the reinforcing agent within the polymer matrix. The advantage of a woven cloth or mat of fibers over chopped fibers is that the cloth or mat can provide reinforcement in two dimensions over a large area. Carbon fiber–epoxy composites of this type are now widely used in aircraft construction. Glass fiber–epoxy laminates are the basis of boat manufacture and printed circuit boards. Glass fiber– or nylon fiber/rubber laminates are used in automobile tires. The ultimate strength of laminates of this type depends on the thermal stability of the fiber and the adhesion between the fibers and the resin. Failure of a composite is usually the result of a breakdown of the adhesive bond between the components.

Figure 9.10. Construction of a typical laminated composite.

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