Preimpregnates

Small quantities of higher molecular weight resin in powder form are also manufactured. They are used in solution or emulsion form as binders for glass-fibre preforms and also for the manufacture of preimpregnated cloths. 

Process in whicli continuous strands of roving or roving tape are wound, at a specified pitch and tension, onto the outside surface of a mandrel. Roving is saturated with liquid resin or is preimpregnated with partially cured resin. Application of heat may be required to promote polymerization. 

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. 

EMS hybrid yarns or Schappe s preimpregnated yarns are a combination of reinforcing fibres (glass, aramid or carbon) and polyamide 12. 

A fourth, but rather unusual, impregnation alternative involves the use of preimpregnated reinforcement (prepregs). The reason for its limited use is the high cost of prepregs, whereas the reason for it ever having been considered is that improved composite properties may be achieved. 

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. 

Figure 13.10 The uneven surface of a preimpregnated thermoplastic sheet. The rough surface is oriented relative to the fiber longitudinal axis as shown...

In fiber placement, a moving head is mounted to a multidegree of fieedom carriage. The fiber placement head is designed to cut and place each tow individually. The head may also include a controlled heat source and compaction roller for in situ consolidation. Tow placement with in situ consolidation has made it possible to place preimpregnated tape along nongeodesic paths, manufacture parts with concave sections, and to locate ply dropoffs precisely. 

The most widely used allyls are the monomers and prepolymers of diallyl phlhalate and diallyl isophlhalate. These are readily converted into thermoset molding compounds and into preimpregnated glass cloths and papers. 

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 PEEK resin is marketed as neat or filled pellets for injection molding, as powder for coatings, or as preimpregnated fiber sheet and tapes. Applications include parts that are exposed to high temperature, radiation, or aggressive chemical environments. Aerospace and military uses are prominent. At present, polyamideimide (PAI) resin and poly(arylene sulfides) are the main competitors for applications requiring service temperatures of 280°C. At lower temperatures, polyethersulfones, amorphous nylons, and polyetherimides (PEI) can be considered. 

Autoclave molding has been particularly adapted for the cure of laminates of preimpregnated plies of continuous fibers (prepregs). A lay-up of... 

Figure 9.11 Lay-up of preimpregnated plies placed between a heated plaque and the bleeder cloth.

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. 

In the preimpregnation process, the dye liquor initially contains only the dye as a water-insoluble dispersion. After the temperature is increased and, if necessary, sodium hydroxide solution and salt are added, the pigment partially deposits on the fiber. After the reducing agent is added, vatting occurs rapidly and the dye is absorbed by the fiber. As a result of partial deposition of the dye in the nonsubstantive form, the levelness of dyeing is improved. Prepigmentation of piece... 

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