Epoxy resins can be cured by several means. The most common curing agents are Lewis bases, such as amines and amides; Lewis acids, such as boron trifluoride; and other materials including phenols, organic acids, and anhydrides. Alkyds have been developed since 1926 for use in paints and protective coatings and GE had much experience with them. An alkyd is the reaction product of a polyhydroxy alcohol and a polybasic acid or anhydride, the same way that polyesters are formed. In the late 1940s, these materials were widely available from various alcohols and organic acids at reasonable costs. The epoxy curing reaction with organic acids depends on the carboxyl groups on the acids. When equivalent molar ratios of hydroxyl and carboxyl groups are reacted, most of these groups are used up in the esterification reaction, which produces the alkyd or polyester resin, and such materials have little reactivity with epoxy resins. When there is an excess of the acid, there will be unused carboxyl groups left over that may then be used to react with the epoxy group to cure the system with little or no volatile by-products. These alkyds are called high-acid-number resins. By choosing a medium-length carbon chain organic acid, such as adipic acid with six carbons, to react with glycerol, a three-carbon alcohol with three hydroxyl groups—glycerol adipate resin—is formed. When five moles of adipic acid are reacted with four moles of glycerol, the resulting high-acid-number polyester will have an average molecular weight of about 800. Using this resin as a hardener or coreactant with an epoxy resin will impart a degree of toughness, without brittleness, to the epoxy formulation. GE used variations of this chemistry in making their Micapal 1 epoxy-bonded-mica high-voltage ground insulation system. GE produced this system into the 1990s.