Epoxy coatings have some general characteristics. The most important being:
- Good water resistance
- Good adhesion to the substrate
- Good chemical resistance
- Very good alkali resistance
- Great resistance to mechanical damage
- High durability
- Temperature resistant up to 120oC (somewhat lower/ higher for certain systems)
- Certain systems officially approved for potable water tanks and in contact with food
- High solids content/low VOC possible
- Poor resistance to UV rays: chalks in sunlight
- Application and curing depends on the temperature (normally above +10oC, cold climate versions down to -5oC)
- It may be difficult to overcoat cured epoxy
- They are two-component products and therefore require good mixing and may give increased wastage
- Moderate resistance to acids
- Can cause allergy (eczema)
- Require knowledge to be used correctly
Epoxy coatings have good chemical resistance, particularly to alkalis. They have good adhesion both to steel and concrete and good water resistance. Epoxy can be modified using phenol, coal tar and hydrocarbon resin to give special properties,
e.g. better chemical resistance, better penetration, better water resistance etc. One drawback with many epoxy coatings is that they contain large quantities of solvent. However, types have now been developed with high solids content (mastic products) with excellent "all round" properties. There are also a solvent-free epoxy coatings which are used for drinking water tanks. Water-borne epoxy coatings are increasingly being used today because they give a better working environment. Chemical resistance, however, is slightly reduced.
Before painting, the two components must be mixed in the correct ratio. The curing process is a chemical reaction between the base and curing agent, so application and curing are temperature dependent. It is important to apply the paint before the chemical reaction has proceeded too long after mixing. When the pot life (usage time) has elapsed, the paint becomes dry and hard and impossible to apply.
Areas of use
Different types of epoxies have different areas of use:
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- Epoxy mastics. Very versatile coatings: Industry, ships and offshore. Under and above water (Topcoat required when exposed to UV-light). On most substrates due to good penetration and adhesion properties.
- Pure epoxy. Chemical tanks, potable water tanks and as an all-round coating system on ships and industrial plants. Require blast cleaning to minimum Sa 2½.
- Phenolic epoxy. Chemical tanks. Even better properties than pure epoxy. Require blast cleaning to minimum Sa 2½.
- Coal tar epoxy. Ships. Under water, particularly water ballast tanks.
- Solvent free epoxy. Drinking water tanks and where environmental restrictions are decisive.
- Water-borne epoxy. Industry where conditions can be controlled (humidity and temperature)