Several approaches are found in the literature together with a large number of patents describing the preparation of prepregs. It is very desirable that prepregs are produced using simplified manufacturing processes. The prepreg must be pliable, easy to handle, be shelf stable and meet the end use performance requirements for which it was designed. Resin systems should have low viscosities to facilitate impregnation of the reinforcement and air removal during the process. The prepreg should be tack free; the resin must not drain or migrate and cure at a specific temperature within a specified time. Most common resin materials used in the preparation of prepregs are epoxy intermediates.
However, epoxy resins are limited by the type of substrates to which they can be successfully applied and their ability to conform to irregular surfaces. This paper describes a unique process for producing prepregs from relatively low viscosity unsaturated polyester resins. These same resins can be formulated to increase viscosity within minutes or hours after initial impregnation of the reinforcement. Final viscosities may be from a few thousand centipoises to several million and are attainable without altering the reactive monomer or primary backbone of the polymer. Stability and curing properties will be presented together with potential applications.
Industrial processes using reinforced thermosetting resins in open mold operations are facing an increase in environmental regulations that require a better control to minimized organic emissions. During the curing process of thermosetting resins such as unsaturated or vinyl esters large amounts monomer diluents flash off from surface evaporation. Styrene monomer is the most common diluent in these types of resins. The U.S. Environmental Protection Agency (EPA)1 classifies styrene as both a hazardous air pollutant (HAP) and a volatile organic compound (VOC). Styrene is a monomer with the lowest cost and most value due to its reactivity and salvating power. It reduces the viscosity of resins effectively, simplifying spray up, injection molding and hand lay up operations. Its reactivity with the most common unsaturated monomers such as fumaric, maleic and acrylates makes it very effective in building resin hardness. The crosslinked network formed provides enhanced mechanical properties compared to other monomers.
Reduction of styrene in thermosetting resins has been a major task in the composites industry. Lowering the amounts of styrene increases the viscosity and changes the Rheology of the resin. These problems in turn, affect the resin processability such as fiber wet out and curing. To minimize these difficulties, resin producers have taken different approaches. The most common approach to lower the styrene content is to decrease the molecular weight of the polymer composition. Low molecular weight polymers require less monomer to obtain the appropriate workable viscosities. However, it is more difficult for resins with lower molecular weight to achieve similar mechanical properties as their high molecular weight counter parts.
Styrene vapor suppression is another approach to lower the styrene emissions. Suppression of organic emissions is most commonly accomplished by adding paraffin waxes. During the curing process, the wax spreads over the surface of the resin and prevents the styrene from migrating through the laminate, minimizing emissions. Waxes do not adversely affect the resin viscosity and curing properties. Mechanical properties of the cured parts remain the same. However, one problem with adding waxes is the poor interlaminar adhesion between plies.
