Laminates high-pressure reinforced

Table 5.5 High-pressure reinforced TS laminates using different resins...

The most commonly used reinforcement for high pressure decorative and industrial laminates is paper (qv). The strong substrate layers, or filler, are kraft paper. Kraft is a brown paper made from a sulfate pulp process (8). It consists of both short cellulose fibers from hardwoods and long fibers from conifers. The long fibers impart most of the wet strength required for resin saturation processes. 

The quantity of resin appHed to the reinforcing ply to achieve a state of full densification varies inversely with the laminating pressure. Therefore, high pressure laminates pressed at about 7 MPa (1000 psi) need only about 25—30% phenoHc resin in kraft paper, whereas low pressure (1 MPa = 145 psi) laminates need 50—60% resin in the reinforcing ply if all voids are to be filled in the final product. 

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, which is then heated. This process is favored over the high-pressure process because of the use of a simpler, lower cost mold and production of strain-free products. 

Overlays for plywood deserves mentioning. Overlays for plywood may be anything that conceivably can stick to the panel and have end use utility. Overlays include polyester or phenolic impregnated paper (either medium or high density types), fiberglass-reinforced plastic, fabric, high pressure laminates, aluminum, lead, polyurethane insulation and pebbles. The uses for plywood are extensive and marriage with other overlay materials expand these uses tremendously. 

Kraft paper is widely used as a reinforcement. When saturated with phenolics, it is made into a common printed wiring board. When combined with melamine, it becomes a decorative high-pressure laminate used in furniture, countertops, and wall panels. Paper reinforcement is inexpensive and easy to machine, drill, and pxmch. It imparts good electrical properties, but it is sensitive to moisture and cannot withstand high temperatures. 

Thermosetting resins can be combined with reinforcing fiber directly in a molding press by pouring a liquid resin onto a fiber mat or fabric or by combining the resins and reinforcement in a treater. The molding press process is nsed to produce continuous low-pressure laminates of finite thickness. The treater process is used to produce high-pressure laminates in sheet form. 

Phenolic, melamine, and silicone varnishes are used to impregnate reinforcement in high-pressure laminates. Polyester varnishes are nsed in continuous low-pressure laminates. The first group is also used in insnlating varnishes for electrical coils and other equipment. 

High performance composites may be laminates wherein veils of carbon fiber ate treated with an epoxy resin, stacked up to the desired final product thickness, and then laminated together under heat and pressure (see Composite materials Carbon and graphite fibers). Simply mixing together carbon or glass fibers and polymeric resins to form a reinforced plastic leads to a composite material, but this is not a laminate if not constmcted from discrete phes. 

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