Curing agents

Reactive compounds in thermosetting matrices of composite materials are monomers or oligomers characterized by a functionality that ranges from a minimum of two up to six or more, in order to produce a crosslinked network. The generic term of resin is usually referred either to the monomer present in the larger amount either to the mix containing the main monomer and all other reactive components, i.e. curing agents, catalysts and initiators. The same base monomer can thus be used to produce crosslinked polymers characterized by properties strongly different depending on the curing agents. In this section, the curing agents involved in and strictly necessary to the curing reactions, are only considered. In very basic terms, curing agents are divided in two classes depending on the role they play in the curing reactions. In step reactions curing agents participate to form the crosslinked network being present in a significant amount and are often referred as hardeners. In radical chain reactions the initiator, producing the first radical promoting the initiation step, is present in a very limited amount in the reactive mixture (a few percent by weight). The discussion will be essentially limited to epoxy resin and partly to polyester and vinylester resins.