Prepregs

Prepregs are used to mold different products. Different forms of reinforcements are used (nonwoven mat, woven fabric, braided, preform, roving, etc.). The TS catalyzed compounded resin is impregnated into the continuous reinforcement and partially cured to a tack-fi-ee state in the B-stage (Chapter 3). The impregnated liquid resin provides for precise placement of reinforcement. 

The reinforcement can be predesigned to meet performance shape requirements. The molder uses the prepreg in a compression mold or other molding process that will allow the required temperature (low to high) and pressure (low to high) conditions to be met, based on how the resin was compounded. With proper storage condition of temperature [at least about 21°C (70°F)], their shelf life can be controlled lasting at least 6 months. 

Techniques for locating and orienting them onto a molding surface, in accordance with the RP design pattern, are adapted to the tack and drape characteristics of the prepreg. The woven fabrics make possible 

Prepregs have advantages over resin wet lay-up fabrication systems (Chapter 5). The wet lay-up places the reinforcement in a mold cavity followed with literally pouring resin over the reinforcement. Resin or a gel coating can be applied to the mold surface prior to placement of the reinforcement to provide an improved molded part surface. The main advantages of prepregs over wet lay-up fabrication are  

Reinforcement Carbon/epoxy Aramid/ epoxy Glass/epoxy  

Fiber reinforced prepregs containing a MMBS modified thermoset resin are particularly useful in the fabrication of electrical and office equipment enclosures (24). 

Electrical and office equipment enclosures, such as computer cases, copier cases and telecommunications equipment, are conventionally prepared from thermoplastic resins such as PC, ABS, and poly(propylene). These materials have the advantageous properties of toughness, flexibility and the ability to meet the UL specifications by including fire retardant additives. 

However, the thermoplastics have the disadvantages of not being stiff due to their low modulus and a lack to flow into detailed molds due to their inherently high melt viscosity. Because of the relatively 

These disadvantages can be overcome by using thermoset resins instead of thermoplastics. However, conventional thermoset resins, e.g., epoxies and phenolics, are not tough enough for these applications resulting in cracking due to their relatively low impact strength. 

Instead, a prepreg that is particularly suitable for the fabrication of electrical and office equipment has been developed. This prepreg consists of a thermoset, preferably a phenol formaldehyde resin, that is formulated with a MMBS resin. The MMBS resin is added to the phenolic novolak preferably at the stage of the condensation reaction of the phenolic resin. 

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Blends have also been prepared by dissolving DMPPO in a monomer and then polymerizing the monomer. An example is an epoxy—DMPPO blend prepared by curing a solution of DMPPO in Epon 828 at 85°C with an alurninum—tetramethylguariidine catalyst. Some copolymer formation is observed. The solutions can be appHed to glass cloth before curing to produce prepregs for composites in appHcations such as printed circuit boards (67). 

Table 3. Typical Epoxy Resin Prepreg Formulation...

The reaction product of 4,4 -bismaleimidodiphenylmethane and 4,4 -diaminophenylmethane, known as Kerimide 601 [9063-71-2] is prepolymerized to such an extent that the resulting prepolymer is soluble in aprotic solvents such as /V-methy1pyrro1idinone [872-50-4] dimethylformamide [68-12-2] and the like, and therefore can be processed via solution techniques to prepreg. Kerim ide 601 is mainly used in glass fabric laminates for electrical appHcations and became the industry standard for polyimide-based printed circuit boards (32). 

The Michael addition reaction of amines and thiols with bismaleimides or functionalized monomaleimides is a versatile tool ia the synthesis of chain-extended maleimide-terroinated prepolymers. These prepolymers generally are soluble ia organic solvents from which they can be processed to prepreg and molded to high quaUty, void-free laminates. 

Bismaleimide Resins via EI E Reaction. The copolymerization of a BMI with o,o -diallylbisphenol A [1745-89-7] (DABA) is a resia coacept that has beea widely accepted by the iadustry because BMI—DABA bleads are tacky soHds at room temperature and therefore provide all the desired properties ia prepregs, such as drape and tack, similar to epoxies. Crystalline BMI can easily be blended with DABA, which is a high viscosity fluid at room temperature. Upon heating BMI—DABA blends copolymerize via complex ENE and Diels-Alder reactions as outlined ia Eigure 8. 

The resin system V-378A, mentioned eadier, is a bismaleimide system that has been modified with divinylbenzene to achieve drape and tack in prepreg form. Divinylbenzene-modifted BMI is appreciated because of its outstanding hot—wet environmental resistance and epoxylike cure (18). 

Commercial BMI Resins. Since the late 1960s, bismaleimide resins have been commercially available. The first company in the marketplace was Rhc ne Poulenc Chimie, Erance. Several others, Technochemie, Germany CIBA GEIGY, Switzerland DSM, The Netherlands, BASE, West Germany Mitsubishi Gas Chemicals and MitsuiToatsu, both of Japan Reichhold and Polyorganics from the United States and Shell Chemical Company, United States (through the acquisition of Technochemie by Deutsche Shell AG), followed. Some of them lost interest in this speciaUty market sector. Table 13 indicates the BMI resins available at the end of 1992. No reference is made to prepreg systems available from various sources. A complete Hst of available products and information on any particular material should be obtained from the suppHers. 

PEI resins reinforced with up to 40 wt % fiber glass are available. The American Cyanamid Co. uses an Ultem-type resin with carbon fibers to form their Cypac 1000 series prepregs (31). The moisture content of the resin must be less than 0.05% prior to melt processing to minimise thermal degradation. 

LARC-CPI has a T of 222°C and melts at 350°C (37). The high melting temperature iUustrates one disadvantage of the high temperature thermoplastics ia order to process or melt coasoUdate the prepreg, it must be processed at temperatures greater than 360°C. 

Another method that has great potential for the preparation of advanced prepregs and has been explored extensively requites fine powders. The reinforcing fibers are coated with fine particles of the resin and, when heated, the resin flows over the fiber. This method requites finely divided particles either in aqueous solution, in an inert volatile solvent, or as high dielectric material that can be charged and coated by electrostatic attraction to the fiber. Synthetic methods that make fine particles, similar to that described for PEEK (23), are needed. 

Fig. 9. The effect of voids due to poor wetting on adhesive strength, (a) The zippering effect of voids aligned in the plane of shear, (b) Macro-voids in the resin formed during the manufacture of a carbon fiber reinforced prepregs. (c) Micro-voids caused by axial crenulations along carbon fiber surfaces.

Boron-Epoxy Prepreg Tape (Courtesy of General Dynamics)... 

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