Marc: In printed circuit board fabrication, fabricators place sheets of prepreg between each pair of glass cores. They stack these layers into books. As they heat and press the books, the sheet melts into an epoxy glue. The thickness of the prepreg sheet determines the spacing between the core layers. The prepreg cures to a hard epoxy layer with the same dielectric as the core layer.
Now getting to your point, resins to be used in PCB fab are reacted with materials to improve they cross linking and latency properties. Latency is the ability of a resin to be partially cured and stable at room temperature. That�s why sometimes they�re called a "B-Stage Resin," which is an intermediate stage in curing a thermoset resin.
The simplest epoxy used in PCB fab is difunctional. Difunctionals have relatively low tg, moisture, and chemical resistance. Multifunctional (and tetrafunctional) resins have improved tg, moisture, and chemical resistance. Broadly, Glass transition temperature, tg (�C):