Epoxy Paints

Epoxy paints consist of two components that react with each other to form a hard, inert coating. Part A typically consists of an epoxy resin with pigments & extenders and Part B is the epoxy-curing agent, often called the hardener. Epoxy paints are typically referred to as anticorrosive barrier coatings. The excellent adhesion of epoxies coatings is due to the strong polar bonds it forms with the surfaces when it comes in contact and cures on a surface. The cross-linking reaction of epoxies for the most part is independent of the surrounding environment except temperature.

The chemical reaction between Bisphenol-A and Epichlorohydrin reacts to form two glycidyl groups to the ends of the Bisphenol-A creating a diglycidyl ether of Bisphenol-A (called DGEBA). This resin is the backbone to most anticorrosive coating systems. The glycidyl groups on both ends of the Bisphenol-A resin produced are also referred to as an oxirane or epoxy group. The size of the resulting polymer depends upon the ratio of Epichlorohydrin to Bisphenol-A. Typically the higher the molecular weight the higher the viscosity of the resin produced. These resins are thermosetting polymers and are used as high performance anticorrosive coatings and potting and encapsulating materials. These resins have excellent electrical properties, low shrinkage, good adhesion to many metals and resistance to moisture, thermal and mechanical shock.

Novolac epoxy resins are different from Bisphenol-A epoxy resins. Typically called Bisphenol-F epoxides. They are glycidyl ethers of phenolic-novolac resins. To produce this resin, phenols are reacted in excess with formaldehyde in presence of acidic catalyst to produce phenolic-novolac epoxy resin. Novolac-epoxy resins are synthesized by reacting phenolic novolac resin with epichlorohydrin in presence of sodium hydroxide as a catalyst. Novolac epoxy resins contain multiple epoxide groups. The number of epoxide groups per molecule depends upon the number of phenolic hydroxyl groups in the starting phenolic novolac resin, the extent to which they reacted. The multiple epoxide groups make these resins achieve a high cross-link density resulting in excellent temperature, chemical and solvent resistance when formulated into paint.

Curing anticorrosive paints are typically done by reacting the epoxy resin with an amine catalyst. The amine curing agent selection plays a major role in determining many of the properties of the final cured epoxy paint. These properties include chemical resistance, pot life, dry time, penetration and wetting ability. Curing agents come in many different chemical types, generally based upon amines or amides. Some of the common amines and amides available for use include:

Aliphatic and cycloaliphatic amines and polyamines. Amines are typically ammonia with one or more hydrogen atoms replaced by organic groups.

• Amides and polyamides. Amides are typically ammonia with a hydrogen atom replaced by a carbon/oxygen and organic group.
• Amine based curing agents are considered to more durable and chemical resistant than amide based curing agents but most have a tendency to 'blush' in moist conditions. Most curing agents will react with water molecules rather than the epoxy base, resulting in a waxy film known as amine blush. This will affect the overall performance of the coating.

Amine curing agents have a molecular structure that typically consists of four hydrogen reactive sites. The hydrogen's react with the epoxy ring on the ends of the epoxy resin. The result is a new carbon-hydrogen bond using the hydrogen from the curing agent to free the epoxy group's hydrogen to react with the group's oxygen to form an OH (hydroxyl) pendant. This hydroxyl group contributes to the epoxy's outstanding adhesion to many substrates. The aromatic ring unit that the hydroxyls attach to provides the epoxies excellent thermal and corrosion properties. The four hydrogen sites cause excellent cross-linking when reacting that give these material superior chemical and thermal stability.