Epoxy resins and epoxies are reactive adhesives named for the reactive part of the molecule responsible for the curing chemistry. Epoxies are the cured version of epoxy resin.
An epoxide or oxirane ring is a three-membered ring containing two carbons and one oxygen. It’s highly strained and prone to ring-opening reactions with amines, hydroxyl groups, and thiols. Epoxy resins commonly contain aromatic di-epoxide, bisphenol-A diglycidyl ether. A second component, typically a hardener, is mixed to start the curing. The most common hardeners are diamines.
Epoxy resins are typically sticky. Before curing, they adhere well and coat several substrates. An advantage of use is that the joints for bonding can be repositioned until the resin sets. The cure time varies, however, with formulation from seconds to days. Many epoxies do not attain full strength until after 24 hours.
Cured epoxy resins are typically hard, crystalline materials that offer strong bonds for various materials, whether porous or non-porous.
Epoxy adhesives find use in many high-performance applications due to their unmatched mechanical strength. Formulators often use additives to make epoxies more resilient because these adhesives can crack under stress.
Two-part epoxy resins are by far the most common. Two tubes, a double-barreled syringe, or a sachet with two compartments are a few of the ways epoxies are sold. Most require mixing equal volumes of resin and hardener to start the cure. The mixed resin is applied to the surfaces to be joined and clamped while the resin is allowed to cure. One-part epoxy resins are used industrially, usually using heat to initiate a reaction.
Regardless of whether a one or two-part epoxy resin, the crosslinking that occurs during curing means the cured epoxy does not dissolve easily in any solvent. The window to clean or repair a sloppy application with a solvent is short. Acetone is recommended for many uncured epoxies. Waiting for the resin to cure before trying to clean up can be a mistake. Once cured, the resin is hard and difficult to remove mechanically.
Epoxides are reactive chemical species and do pose risks. For example, epoxy resins and hardeners are skin irritants. Avoid contact with the resin and hardener, or with epoxy resins that are mixed but not yet cured. Prolonged contact can lead to skin sensitization, with some people also developing asthma-like conditions.
Anyone showing a potential sensitization should discontinue using or working with epoxy resins.
As with all adhesives, carefully follow the directions when using epoxies, work in well-ventilated areas, and use the appropriate personal protective equipment.
The cure chemistry is exothermic. This means the temperature can rise sufficiently to burn skin and melt plastics. This is a problem in large batches, thick applications of epoxy, or assemblies where the bond is thermally insulated. Applying the epoxy in thin layers typically allows sufficient cooling to avoid a sudden temperature rise. Cured epoxy resin usually presents no health risks.
Did you know?
Aside from its bonding capabilities, epoxy resin is also used as a coating. It hardens when applied, creating a protective layer on surfaces.
The benefits include:
- Protecting wood from rotting
- Preventing corrosion and rust from forming on metal surfaces
- Providing a waterproof barrier for applications, such as for use around windows or doors, or in boat restoration
- Ensuring excellent insulating properties, including for electronics and other applications