Epoxy resins can be cured with a variety of compounds called curing agents which are also known as curatives, hardeners, or converters. One of themost widely used classes/types of curing agents for epoxy resins is anhydrides. These acid anhydride curing agents are derived from the elimination of water from diacids as shown below. This is a reversible reaction. Therefore, anhydride curing agents must be adequately protected from moisture since they will revert to the diacid.
If diacid is present, it will increase the reactivity with epoxy resin and may affect cured properties. As with amine cured epoxy resins, anhydride cured epoxy resins produce a heteropolymer consisting of epoxy molecules linked together through the reactive sites of the anhydride curing agent. The majority of anhydrides used for curing epoxy resin matrices are liquids but solid dianhydrides also find limited use as curing agents in fiber reinforced composite applications. The reactivity of most liquid anhydride curing agents with epoxy resins is very slow without the addition of a catalyst, even at high temperature. Different from amine curing agents, the reaction mechanism of anhydrides with epoxies is complex and subject to many competing reactions. Some of the variables affecting the reaction mechanism include, the gel time and temperature, post-cure time and temperature, type and concentration of accelerator, hydroxyl concentration present, ratio of anhydride to epoxy (A/E ratio), amount of free acid, and type of epoxy resin. While there are many possible reactions with epoxy resins, the 3 major reactions are shown below using phthalic anhydride as an example.
Secondary alcohols from the epoxy backbone or other alcohols (aliphatic) react with the anhydride forming a monoester.
The corresponding carboxylic acid can then react with an epoxy group forming a diester.