Liquid now, Hard later
Most of the traditional Thermoset Unsaturated Polyesters, Thermoset Unsaturated Vinylesters, and Epoxies arrive at the fabrication shop as liquids in buckets, drums, totes, or tankers. They require liquid hardeners to be mixed in at specific ratios in order to transform into a solid physical state. The esters use peroxide catalysts in the 1% to 3% range, and the epoxies use a specifically formulated part B designed in tandem with the part A manufacturer. The esters also can come prepromoted or unpromoted, which basically is another part of the chemistry package typically containing a cobalt-based mixture that promotes the curing process. Where this is added depends upon several variables, and can also affect the shelflife of the product.
Nontraditionally the Epoxies and esters can come into the fabrication shop as prepregs. These are resin and glass mixtures that are formable to shape but have all of the chemicals and glass mixed in, and are activated by heat (ovens). This of course requires that they be transported and stored in refrigerated containers. There are several tradoffs for prepregs including cost, delivery, process control, shelflife, etc.
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