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.

What is epoxy coating?

The importance of epoxy coatings for corrosion protection cannot be overstated. The global market for epoxy coatings is predicted to grow at a CAGR of 5.32% to surpass USD 44.61 billion by 2028. It is no wonder that a quick search of the keywords ‘epoxy coating’ brings up more than 88 million results.

This article will go through the definition and main reactions that form the epoxy coating.

What is epoxy coating? Definition and working principle

Epoxy coating is a layer of protective organic polymer coated on a metallic or non-metallic substrate. It is the chemical product formed after a reaction between two components—resin and hardener. It consists of additional components that contribute towards specific properties.

The resin is a polymer that contains the functional group epoxide which is a triangular ring of one oxygen and two carbon atoms.

Figure: Epoxide ring

This epoxide ring may then be connected to a varying number of hydrogen atoms or other C-H groups in a long polymer chain. The backbone chain may be made up of esters, acrylics, ethyl or methyl groups, etc.

The key to recognising an epoxy coating is the epoxide ring in the resin chemical structure.

The hardener may vary depending on the curing properties required for the application. Here’s a quick list of the various hardeners:

  • Amines – aliphatic, aromatic, and cycloaliphatic
  • Anhydrides
  • Phenols
  • Thiols

The reactivity of the hardeners increases in the following order:

Phenols < Anhydrides < Aromatic amines < Cycloaliphatic amines < Aliphatic amines < Thiols

The following images show the typical chemical reaction for an epoxide resin with an amine hardener. The epoxy reaction occurs in three steps:

  1. The epoxide ring in the resin reacts with the amine group in the hardener to form a secondary amine. In this step, one H atom from amine goes and forms OH with the O in the epoxide group leading to the opening up of the ring. The N gets connected with the C in the open epoxide molecule.
  2. Next, the formed secondary amine once again reacts with the epoxide group. Here, the remaining H from the secondary amine breaks the epoxide ring. H goes to the O from the epoxide group. The C from the backbone molecule connects with the N from the amine group. This reaction forms the tertiary amine.
  3. The last step is the etherification of the tertiary amines. The tertiary amine reacts with one more epoxide ring. this step is different from the previous steps because the N in the amine group does not participate in this reaction. The Hydroxyl OH groups are the active sites. The H from the OH group of the tertiary amine breaks the epoxide ring and reacts with the O to form OH. Thus, it shifts from OH in tertiary amine to OH in the epoxide group. The remaining O in the tertiary amine now forms a crosslinking bond with the C of the backbone CH2 group of the epoxide ring. This is the final epoxy coating molecule.
Figure: Reaction scheme of the epoxy resin and amine hardener to form epoxy coating.

Wrap up

Epoxy coating is a popular coating. However, its reaction mechanism is often misunderstood or not understood at all. This series of articles will cover the principle, reactions, and failures of epoxy coatings.

This article dealt with the amine + epoxy reaction. In the next part of the article, we will deal with the reaction mechanisms of other hardeners used to form epoxy coatings.