Preimpregnated material

Special requirement for Class A finish on preimpregnated material such as SMC is to being used on some of DaimierChysler s 2004 coupe models. It is in response to the reliance by today s vehicles on electronic systems that communicate via satellite and electromagnetic waves. Modern cars need several antennae, and for styling and design reasons these are, where possible, hidden beneath a body panel section. However, an all-metal body needs external antennae since electromagnetic waves cannot penetrate sheet metal. 

In larger-volume production, preimpregnated sheet compounds are used. The fabricator forms the sheet compound to shape and initiates the cure. These compounds are known as prepregs (preimpregnated materials), SMCs (sheet molding compounds), and BMC (bulk molding compounds). There are many variations within each group (Chapter 4). 

R Dyksterhouse, JA Dyksterhouse. Production of Improved Preimpregnated Material Comprising a Particulate Thermoplastic Polymer Suitable for Use in the Formation of a Substantially Void-Free Fiber Reinforced Composite Article. U.S. Patent 5128198, July 7, 1992. 

The prepreg (preimpregnated materials) is then delivered to the lamination press. Here the required number of prepreg sheets is stacked between polished steel plates. The plates are further stacked together to form a pack that is then placed into the laminating press s platens. Each pack can consist of 10 laminations, and the press may have 24 platens, making a press load of up to 240 laminations per cycle. The press is closed, and the laminates are consolidated and polymerized (cured) at temperatures from 121 to 204°C, pressures from 200 to 3000 psi, and times from 3 to 120 min. Molded laminates are then trimmed to final size. Some laminates must be postcured by additional heating in ovens. 

In addition to wet systems, where reinforcements are resin-coated during the process, more convenient prepregs are available that can be used for many applications. Suppliers assist in the selection of preimpregnated materials, which are available as unidirectional, woven, and nonwoven tape, tow, mat, and so forth. 

Preimpregnated materials usually are a compound of a reinforcement and a hot melt or solvent system. Prepreg also includes wet systems without solvent using TS polyester. They are stored for use at a latter time either in-house or to ship to a fabricator. The plastic is partially cured, B-stage, ready-to-mold material in web form that may have a substrate of glass fiber mat, fabric, roving, paper, cotton cloth, and so forth. With proper temperature storage conditions, their shelf life can be controlled to last at least 6 months. 

Composite patches may also be used to repair metallic structures, and recent studies have indicated the potential of such repairs for naval and offshore structures (e.g., Bowditch et al. 2004 Turton et al. 2005). These may be applied, using infusion or preimpregnated materials, or adhesively bonded, and can be applied to repair fatigue cracks, corrosion damage, or for reinforcement purposes. Advantages over metallic repair are ... 

Applications. Boron fibers are used as unidirectional reinforcement for epoxy composites in the form of preimpregnated tape. The material is used extensively, mostly in fixed and rotary wing military aircrafts for horizontal and vertical stabilizers, mdders, longerons, wing doublers, and rotors. They are also used in sporting goods. Another application is as reinforcement for metal matrix composites, in the form of an array of fibers pressed between metal foils, the metal being aluminum in most applications. 

Cylinders may be wound with a wet winding process while maintaining high fiber volume fractions typically seen in parts made from preimpregnated (B staged) materials. 

The prepreg winding technique offers better control of fiber volume fraction, but at a cost. Material costs are 1.5 to 2 times higher, and there are additional costs associated with storing the preimpregnated (thermosetting matrix) tows. Preimpregnated tows are used almost exclusively for thermoplastic matrix materials, where there are no shelf-life restrictions. 

Engineering thermoplastics have also been used in preimpregnated constructions. The thermoplastic is thoroughly dispersed as a continuous phase in glass, other resins, carbon fibers (qv), or other reinforcement. Articles can be produced from these constructions using thermoforming techniques. For example, the aerospace industry uses polyetheretherketone (PEEK) in woven carbon-fiber tapes (26). Experimental uses of other composite constructions have been reported (27) (see also COMPOSITE MATERIALS, POLYMER-MATRIX). 

The use of an emulsion of PDMS particles is attractive because its low viscosity makes it suitable for preimpregnation of fibers to produce composite materials. A possible drawback is the emulsion stability which has to be controlled in order to avoid particle coalescence. 

Depending on the resin preparation, the material in or around a mold can be cured with or without heat, and commonly without pressure. Curing needs include room temperature conditions, heat sources, vacuum bags, pressure bags, autoclaves, etc. An alternative is to use preimpregnated, B-stage TS polyester or sheet molding compound (SMC), but in this case heat is applied with low pressure via a impermeable sheet over the material. This process can produce compact... 

The simple and nondestructive Barcol hardness test has the added advantage that it can be conducted in situ on the factory floor (see Section 6). The other techniques are more costly, require a greater degree of operator training, and are not practical for in situ factory use. However, they are suitable for checking incoming material as preimpregnates or laminates. 

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