Composite MaterialsKrishan K. Chawla

Chapter 5. Polymer Matrix Composites

5.1 Fabrication of PMCsHand lay-up – Glass fibers are laid onto a mold by hand and the resin is sprayed on or brushed on. Frequently, resin and fibers are sprayed together onto the mold.

Filament winding – another very versatile technique in which continuous tow or roving is passed through a resin impregnation bath and wound over a rotating or stationary mandrel. - a roving consists of thousands of individual filaments

Pultrusion – aligned fiber composites in the form of section

Bag molding process - for making large parts. Vacuum or pressure bags containing fibers in predetermined orientations in a partially cured matrix are used in an autoclave for densification and curing of resin.s

5.2 Structure and properties of PMCs

A traverse section of continuous glass fiber in an unsaturated polyester matrix

The layer structure of an injection-molded composite consisting of short glass fibers in a semicrystalline polyethylene terephthalate(PET)

Essentially, it is a three-layer structure of Different fiber orientations :More fibers are parallel to the mold fill directionIn the two surface layers, S , than in the transverse Direction in the central layer, C.

- Laminates of polymer matrix composites made by the stacking of appropriately oriented plies also result In composites with highly anisotropic characteristics.

-The epoxy resins commonly employed as matrices in the composites meant for use in the aerospace industry are fairly impervious to the range of fluids commonly encountered, for example, jet fuel, lubricants. - There are, however, two fundamental effects that must be taken into account when designing components made of PMCs, namely, temperature and humidity. The combined effect of these two, that is, hygrothermal effects, can result in a considerable degradation in the mechanical characteristics of the PMCs.

-Degradation owing to ultraviolet radiation is another important environmental effect. Ultraviolet radiation breaks the covalent bonds in organic polymers.

-Fracture in PMCs, as in other composites, is associated with the characteristics of the three entities : fibers, matrix, and interface. fiber/matrix debonding, fiber pullout, fiber fracture, crazing and fracture of the matrix are the energy- absorbing phenomena that can contribute to the failure process of the composite.

5.3 Interface in PMCs-Hull gives a good description of the glass fiber/polyester resin interface. The glass surface has randomly distributed groups of various oxides; SiO2, Fe2O3, Al2O3, absorb water as hydroxyl groups, -M-OH where M can be Si, Fe, and Al and as molecular water attached to the (-M-OH) groups. - If the glass surface stays in contact with water for too long a time, hygroscopic elements can get dissolved and a porous surface consisting of nonhydrated oxides is left.-The coupling agents create a chemical bridge between the glass surface and the resin matrix.

-The chemical bridge theory goes as follows: R-SiX3 X: hydrolyzable groups bonded to Si.(ex. –OC2H5) R: resin-compatible group

5.4 Applications

-Glass fiber reinforced polymers are used in a wide variety of industries: - from sporting goods to civil construction to aerospace. - Tanks and vessels in the chemical process industry.