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.

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