What Does Epoxy Coating Mean?
Epoxies are resins that contain the reactive CH(O)CH2 function (called an epoxide or oxirane ring), where the oxygen atom is found in a three-membered cyclic ring arrangement so that it is positioned outside the carbon backbone.
Simple epoxies are synthesized through reaction of epichlorohydrin with the compound, bisphenol-A. The product is usually called the diglycidyl ether of bisphenol-A (DGEBA) or bisphenol-A diglycidyl ether (BADGE). The small-ring geometry of the epoxide functional group at the end of the monomer chain structures makes it ideally suited to chemical addition reactions that lead to full-chain saturation in resin chemistry, which is better known as polymer curing. Cured polymers produce materials of outstanding durability, making them suitable for very demanding applications. Amines are the usual choice for the curing reaction with epoxies, and form the component often referred to as the hardener. Other hardener compounds may include amides or acid anhydrides.
Through the wide variety of chemical chain lengths and chemical structural components available in both the epoxide and the hardener components, epoxy synthesis can be tailored to achieve chemical structures that fulfill the performance requirements of any particular application.
Epoxy chemistry built in this way from structurally more complex analogues creates a host of major industrial applications such as coatings – particularly protective and powder coatings, floor coatings, adhesives, fiber-reinforced polymers and encapsulation materials for the electrical industry. All of these applications demand exceptional durability. Epoxies occupy more than one segment of the coatings sector and may be manufactured into liquid coatings or powder coatings. Anti-corrosion applications of epoxies are found in both forms.
The epoxy segment is one of many protective coatings chemistries. The global market for epoxy coatings has been valued at US$ 32 billion by Mordor Intelligence and will grow by nearly a quarter between 2023 and 2028. Building and construction are the main drivers for this growth, along with the shift to water-based epoxies, with Asia being the largest demand driver.
Types of Epoxy Coating
Novolac resins are phenol-formaldehyde resins and novolac epoxies are closely similar except that that the phenoxy group is etherified with a glycidyl molecule that places the epoxy function in a side chain, thereby allowing a more extensive crosslinking. The high aromatic ring content and high crosslink density conferred by crossing an epoxy with a novolac resin furnishes chemical structures of outstanding durability that are ideal for protective applications. These are used in areas where the risk of corrosion or chemical attack is very high, including tank linings where solvents and corrosive or aggressive chemicals may be stored.
Epoxy powder coatings
Epoxy powder coatings are coatings that are applied as a dry powder and then heat cured. Some are pure epoxy powder coatings, some are epoxy hybrids and some are epoxy polyester coatings which offer superior performance characteristics. Epoxy powder coatings provide excellent corrosion resistance to metals.
Fusion-bonded epoxies (FBEs) are epoxy coatings that are produced by high-temperature fusion (melting) of the epoxy resin with its corresponding hardener in a fluid bed. (At ambient temperatures, the reaction between the resin and hardener is unable to take place.) The resulting fused coating is a thermoset coating which after cooling, does not lose its form at higher temperatures. It bonds to metal surfaces at high temperatures, forming an excellent protective layer.
Coal tar epoxies
Coal tar epoxies incorporate coal tar to create comparatively rudimentary waterproofing coatings. The hydrophobic hydrocarbon content of coal tar forms a basic protection against water, however the presence of polycyclic aromatic hydrocarbons (PAHs) is considered a health hazard on the grounds of carcinogenicity, mutagenicity and reprotoxicity and so these are banned in developed countries.
What are epoxy coatings used for?
Protection in harsh environments
Novolac epoxy coatings are used in areas where there the risk of corrosion or chemical attack is very high, including tank linings where solvents and corrosive or aggressive chemicals may be stored.
Protection in mildly corrosive environments
In heavy industry, epoxy coatings are used for the protection of steel and concrete where they are subject to mild chemical attack. This includes the dilute presence of corrosive compounds in water, the presence of organic acids – which are much weaker than mineral acids – or acid mists.
Protection in salt/salt-water environments
Marine applications of epoxy coatings are largely the underwater surfaces of ships and boats where they protect against the corrosive effects of salt water. The main areas are hulls, cargo and ballast tanks and decks where the threat of corrosion is repetitive. In the sea-going environment, salt carried by the wind poses an additional corrosive threat to ships.
Protection of pipelines and pipe fittings
Fusion-bonded epoxies (FBEs) are applied mainly to steel pipes, piping connections, valves, fittings, welded areas and so on.
Marine and underwater applications
Epoxy powder coatings are used in corrosive environments and especially in propellers, boat trailers and other marine components.
Coal tar epoxies are used for sewers, marine docks, ship hulls, pipelines interiors and exteriors, marine barge exteriors, and for metal protection in most chemical and saltwater environments
Protection of concrete and use in floor coatings
Floor coatings are another major application for epoxies insofar as they protect concrete floors in industrial interiors. The resistance qualities that epoxies offer to mild chemicals (e.g. oils, greases and cleaning chemicals found in the working environment) and their superior resistance to abrasion in heavy pedestrian traffic and medium transportation traffic areas render them ideal for warehouses and logistics centers.
Other applications of epoxy coatings
Epoxy coatings are also used for the priming automobile chassis and based on different characteristics, they are used in the electronics industry for the protection of electronic components.
Epoxies for domestic applications
Domestic applications of epoxies favor their use as resinous adhesives rather than coatings. The same type of two-component chemistry is used for the formation of high-strength adhesives and superglues that have the advantages of being tough and resistant to chemicals. They form bonds of extreme durability.
One application where epoxy coatings are used domestically is for flooring in garages, basements and patios. The two components of the coating – resin and hardener – are mixed together to crosslink and set when used in concrete floor repairs. They have the advantages of being durable, resistance to weather, chemicals, impact while remaining easy to clean and maintain.