Anhydride curing of epoxy coating

In the previous post, we discussed the concept of epoxy coating and the reaction between an amine hardener and an epoxy resin. You can read this post here.

In this post, we will discuss the reactions between epoxy resins and other types of hardeners such as phenol, anhydride, and thiol.

Epoxy resin with anhydride hardener

Acid anhydride curing agents are diacid molecules from which water molecules have been eliminated. This removal of water is a reversible reaction, and the anhydride curing agent can change their molecular structure if they come in contact with moisture.

The following image shows the typical structure of an anhydride curing agent molecule.

Figure: Transformation of diacid molecule to anhydride molecule due to water elimination

This reaction differs from the one between amine and epoxy because here different parts of the epoxy resin react with the hardener. The amine reaction is only between the epoxide group and the amine reaction. Let us go through the main steps for the epoxy-anhydride reaction.

Reaction mechanism

The reaction proceeds as described in the following steps:

  • In this scheme of reactions, the O atom in the anhydride reacts with the epoxide group as well as the hydroxyl group in the resin but not in the same step.
  • In the beginning, the oxide ring in anhydride opens up. The O atom in anhydride takes the H atom from the OH group in the epoxy resin backbone chain. This O atom combines with one of the C atoms from the anhydride. Here, the epoxide ring is not affected.
  • Next, the combined anhydride-epoxy molecule reacts with another epoxy resin molecule. Here, the OH group in the anhydride loses the H atom. The remaining O atom breaks the epoxide ring and connects one of the C atoms in the epoxide ring. Please note that all the epoxide rings do not open up at the same time in the same reaction. Every epoxide ring requires an OH group to open up.
  • The tertiary molecules thus formed then react with each other or other unreacted epoxide groups. Here one epoxide group from one molecule reacts with the OH group from another molecule to form a crosslinking network.
  • This reaction continues until all the epoxy resins are crosslinked with ether (C-O-C-) groups in the backbone chain.
Figure: Reaction scheme of epoxy and anhydride reactions to form epoxy coating

Such a complex reaction mechanism results in a long pot life of the coating as the molecules take time to react with each other in multiple ways.

Features of the reaction mechanism

  • Requires heat for curing
  • Needs catalysts and a small amount of moisture for reaction initiation
  • There is no definite end for the cure cycle. The curing is deemed completed when the curing cycle is stopped. This does not mean that all the molecules have reacted completely.
  • Anhydrides can react with water before mixing and curing and lose their reactivity to epoxy resin.

Advantages of anhydride-cured epoxy

  • High chemical resistance – due to the low number of reactive sites and higher crosslinking
  • High Tg and thermal resistance – as the greater crosslinking and interlinking make it difficult for the polymer chains to become flexible
  • High dielectric strength – due to less moisture penetration and a greater amount of reacted molecules
  • Low toxicity – as no other chemical is used for reaction and the original chemicals react with each other till the end of the cure cycle
  • Low cure shrinkage and exotherm – as the high amount of crosslinking makes the coating stiffer and resistant to shrinking of polymer chains.

Wrap up

The properties of the coating are related to how the polymer chains are formed during the reaction. hence, to remember the properties, it is crucial to relate them to the reaction mechanism and the resulting morphology for the coating chains.

In this article, we dealt with the epoxy-anhydride reactions. In the next article, we will talk about the thiol and phenol curing agents and their reactions with epoxy.