Epoxy resin will fall under two principal categories - glycidyl or non-glycidyl. Glycidyl resins come under the subcategories of glycidyl-ether, glycidyl-ester, and glycidyl-amine. The non-glycidyl variety will either be aliphatic or cyclo-aliphatic. Glycidyl-ether epoxies are the most commonly used, particularly those of the bisphenol and novolac variety.
Bisphenol Epoxy Resins
Bisphenol-A diglycidyl ether (DGEBA) is a commonly used variety of commercial epoxy resin. It is produced as a result of the reaction of bisphenol-A on contact with epichlorohydrin in the presence of a basic catalyst. This variety of epoxy resin has the lowest molecular weight.
Aliphatic Epoxy Resins
These types of epoxy resins are produced either as a result of double bond epoxidation (involving cycloaliphatic epoxides and epoxidized vegetable oils) or reaction with epichlorohydrin (glycidyl ethers and esters). Cycloaliphatic epoxides feature one or more aliphatic ring in the molecule containing the oxirane ring. They have a defined aliphatic structure, high oxirane content, and absence of chlorine. This makes for low levels of viscosity, good weather resistance, low dielectric constants, and high Tg.
Novolac Epoxy Resins
These types of resins are produced as a result of contact and reaction between phenol and methanol (formaldehyde). The reaction of epichlorohydrin and novolacs results in the formation of novolacs with glycidl residues including epoxyphenol novolac (EPN) or epoxycresol novolac (ECN). Such epoxies do not contain solvents or volatile organic compounds. Being free of VOCs, they are extremely safe to use and respirators aren’t required. They have a relatively high level of adhesive strength, making for good levels of coverage. Protection and durability are also assured in the application of these resins.
Halogenated Epoxy Resins
These epoxy resins are admixed for special properties. This involves the application and mixture of brominated and fluorinated varieties. Brominated bisphenol A is the preferred option for flame resistance and electrical applications. However, the commercial production and use of such resins is limited due to the associated cost and low Tg.
Epoxy Resin Diluents
The creation of epoxy diluents involves the glycidylation of aliphatic alcohols or polyols. The materials produced as a result of such processes may be either monofunctional (e.g. dodecanol glycidyl ether), difunctional (butanediol diglycidyl ether), or higher functionality (e.g. trimethylolpropane triglycidyl ether).
Glycidylamine Epoxy Resins
These epoxy resins have relatively high levels of functionality and are formed as a result of the contact reaction between aromatic amines and epichlorohydrin. Industrial grades include triglycidyl-p-aminophenol (functionality 3) and N,N,N′,N′-tetraglycidyl-bis-(4-aminophenyl)-methane (functionality 4). They have a low-medium viscosity at room temperature, making them easier to process than EPN or ECN varieties.