wyowindworks

Just for clarification:

Epoxy resins (bonding or laminating) cure by poly-addition. The monomer molecules (resin) create molecular links with the hardener molecules to create cross-linked polymer chains/networks. The monomers can only link together through a hardener molecule. If you add too much or not enough hardener then the you end up with molecules that do not link together......a.k.a. lower than desired cross-link density. The extra unlinked molecules end-up creating spaces between the polymers network resulting in a pooly organized molecular lattice. This lowers the Tg (glass transition temp), HDT (heat deflection temperature), strength, and stiffness. The same things happens when you add a solvent to reduce the viscosity of an epoxy resin. If the solvent molecules do not evaporate out of the mix then they will just inhibit the molecular linking of the epoxy. With epoxies you want to accurately mix it according to the specified ratio to get the maximum properties out of the resin. You do not want to alter the mixing ratio to alter the pot life. Instead, you want to use a different hardener to alter the working time. Weighing the resin mix on an accurate scale (.01 grams) is the best way to accurately mix the resin. The mix ratio by weight is typically different than the mix ratio by volume due the differing densities between the resin and the hardener.

Polyester resin work completely differently. In this scenario the monomers in the resin directly link with other monomers. The catalyst (MEKP) is a polymerization initiator. Add more MEKP, within reason, and the reaction happens faster. Add less and the reaction is slower.

Both resins create heat as a bi-product of the polymerization. Both resins will react faster when heated. Lower the temperature and the pot life increases. Increase the temperature and the pot life shortens. In large masses of mixed resins the heat generated by the polymerization will cause the resin to cure faster. The faster is cures the more heat is developed. This cycle can drive the polymerization at a rate that cause the resin to become very hot, cure very quickly, and result in a poor performing final product (low link density, pooly developed molecular lattice, high shrinkage). Spreading the resin out in a larger diameter cup/pot/tray increases the surface area of the resin so it can dissipate the heat. This will extend the pot life.