The basic building blocks of an epoxy resins are Epichlorhydrin and Bisphenol -A continued polymerization reactions create higher molecular weight solid epoxy resins.
2CH2 – CH – CH2 + HO – R – C – R – OH à CH2 – CH – CH2 – O – R – C – R – O – CH2 – CH – CH2
O CH3 O CH3 O
EPICHLORHYDRIN BISPHENOL – A – EPOXY RESIN ( R = Aromatic group)
Why epoxy ?
To effectively protect reinforcing steel against corrosion a coating must provide a continuous film that will:
- Resist penetration by salt ions,
- Resist the action of osmosis,
- Adhere to and expand/contract with the steel substrate,
- Resist breakdown from weathering and exposure,
- Be flexible and durable enough for handling,
- Strong resistance to oxygen & chloride &
- Highly insulating with very low conductivity & high dielectric resistance.
Fusion-bonded epoxy coating satisfies all of these requirements.
It is a thermoset material, meaning that once it is cured, the coating will not tend to soften with higher temperatures. It achieves its beneficial properties as a result of a heat catalyzed chemical reaction.
Epoxy coating starts out as a dry powder. The powder is produced by combining organic epoxy resins with appropriate curing agents, fillers, pigments, flow control agents. When heated, the powder melts and its constituents react to form complex cross-linked polymers.
Epoxy coatings are environmentally friendly materials. Unlike many paints, the fusion-bonded epoxy coatings used for steel reinforcement do not contain appreciable solvents or other environmentally hazardous substances. Systems used to apply the coating are very efficient, resulting in little material loss to the atmosphere and little waste disposal
How Epoxy Coating Protects
Fusion-bonded epoxy coating principally protects against corrosion by serving as a barrier that isolates the steel from the oxygen, moisture, & chloride ions that are needed to cause corrosion.
Epoxy coating also has a high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion.
In addition to serving as a circuit breaker, the coating protects in way that is less obvious: coating reduces the size and number of potential cathodic sites, which limits the rate of corrosion reaction that could occur.
In order for macrocell corrosion to take place, a large area of steel surface is needed to serve as the cathode where oxygen reduction can occur.
Fusion bonded epoxy coating process
The application of fusion-bonded epoxy to reinforcing steel is straightforward and uncomplicated: clean the steel, heat it to the proper temperature, apply the powdered-epoxy coating material, allow the coating to cure, and inspect. However, the details are important and must be understood and implemented to assure a quality coating that will extend the working life of a structure in a corrosive environment. .