All About Epoxy Resin: Pro&Cons, Curing Agents

8th January 2024

What is epoxy resin?

Epoxy resin, also known as polyepoxides, is a type of thermosetting polymer containing two or more epoxide groups. Epoxide group is a ring composed of carbon-carbon-oxygen atoms, exhibiting high reactivity that enables epoxy resins to undergo cross-linking reactions with various types of curing agents and form three-dimensional polymeric networks. Coatings  formulated with epoxy resins as film-forming materials are known as epoxy coatings.

Epoxy resin is the condensation product of epichlorohydrin with bisphenol A or polyols. Due to the chemical reactivity of the epoxy group, it can be opened by various compounds containing active hydrogen, leading to cross-linking and the formation of a network structure. Therefore, epoxy resin is a thermosetting resin.

Pros of epoxy coating

To impart excellent performance to epoxy resin paint, it is necessary to react epoxy resin with a curing agent or fatty acid, forming a cross-linked, network structure of large molecules. The advantages of epoxy resin paint are as follows:

1.Strong Adhesion. Epoxy paint films exhibit excellent adhesion to various substrates such as metals (steel, aluminum, etc.), ceramics, glass, concrete. This strong adhesion is attributed to the presence of numerous epoxy groups and ether bonds in epoxy resin paint, allowing for effective bonding with substrates. Additionally, epoxy resins have a low volume shrinkage rate (approximately 2%) during curing, compared to that of unsaturated polymers (11%). High shrinkage rates can lead to internal stresses that may compromise adhesion.

2. Excellent Chemical Resistance. Epoxy resin is characterized by the presence of hydrocarbon and ether linkages without ester bonds, making it exceptionally resistant to alkali. Featuring excellent resistance to alkali and strong adhesion, epoxy resin is widely used as anti-corrosive primers, including cathodic electrodeposition primers in automotive applications.

Furthermore, epoxy coating form a robust three-dimensional network structure after curing. It can exhibit resistance to oil and other immersions, which make epoxy coating widely used in applications such as oil tanks, oil tankers, and the interior walls of aircraft integral fuel tanks.

Cons of epoxy coating

  1. Poor resistance to light. Epoxy resin contains aromatic ether bonds which make its paint film susceptible to degradation and chain scission after exposure to sunlight.
  2. Limited low-temperature curability. Epoxy resin typically requires curing temperatures above 10°C, and the reaction is slow and challenging at temperatures below 10°C. This poses inconvenience for cold-season construction of large objects such as ships, bridges, ports, and oil tanks.

 Is epoxy resin toxic?

Epoxy resin is considered a practically non-toxic product. However, the curing agents for epoxy resin and the solvents added during use typically exhibit toxicity. Modified amine curing agents generally have lower toxicity. Other types of curing agents also possess some level of toxicity but are generally classified as low-toxicity substances.

Curing agents of epoxy resin

  1. Aliphatic Polyamine Curing Agent: Aliphatic polyamines can cure at room temperature with fast curing speed and low viscosity. They are used to formulate solvent-free or high solids coatings that cure at room temperature.
  2. Fatty Amine Adduct Curing Agent: This curing agent is formed by reacting aliphatic polyamines with a small amount of epoxy resin. Coatings using this amine adduct are less prone to moisture absorption, exhibit minimal whitening, have a mild odor, and can be used directly without the need for pre-maturation.
  3. Amidoamine Curing Agent: Amidoamine is synthesized by condensing plant oil fatty acids (or tall oil) with polyamines, resulting in a compound containing amide and amine groups.Chemical structure of amidoamine
  4. Amino Polyamide Resin Curing Agent: Amino polyamide resin is not a simple compound but a viscous resin containing free amino groups. It has excellent properties and is widely used as a curing agent. It is produced by heating the polymerization of unsaturated fatty acids to form a dimer acid, which is then condensed with polyamines. So far, amino polyamide resin curing agent is the most widely used curing agent in epoxy coatings.
  5. Cycloaliphatic Amine Curing Agent: Cycloaliphatic amine curing agents are characterized by their light color, excellent color retention, and low viscosity. However, they exhibit slow reactivity, often necessitating their combination with other curing agents, the addition of accelerators, formation of adducts, or requiring heat for curing.