Who hasn’t heard of epoxy? Epoxies have been used since the 1940s to manufacture a wide variety of products and, in the last three decades, have become a primary tool for repairing building materials, especially wood and concrete. While some companies tout their epoxy products as close to miracle cures that can mend broken metal or revive deteriorated wood, there are also architectural conservators who believe that epoxies do more harm than good. To make things worse, there are many products on the market that are used similarly to—and often confused with—epoxies. All this can leave old-house owners scratching their heads. Is there a practical, middle ground where epoxies can be used to save exterior woodwork successfully? And just what is this epoxy stuff? A look at the nature and growing applications of this remarkable chemical repair technology can help answer these questions.
Epoxy belongs to a broad family of thermosetting compounds—in simple terms, plastics that cure by heat reaction. Widely used epoxies like adhesives and casting materials are commonly made up of two liquids: an epoxy resin and a curing agent called a hardener. When the resin and hardener are mixed together, a chemical reaction takes place and the resin transforms into a solid mass. During the reaction, single molecules (monomers) of the epoxy resin and the curing agent combine to form chains of molecules (polymers). As the mixture cures, the chains grow longer, the solution gels, and then it hardens.
Epoxy materials are very adaptable. The chemists who design and formulate epoxies can give them a wide range of pre-cure and post-cure characteristics. It all depends on which epoxy monomers, curing agents, solvents, and fillers they add. The pre-cure mix can be thin as water to penetrate porous materials, or thick and viscous to stay put on vertical surfaces. The cured epoxy can be nearly as hard and brittle as glass, or almost as soft and flexible as rubber bands. Even the rate of cure can be adjusted to meet very specific needs. An epoxy that takes hours to gel can penetrate deeply into porous wood, while products that set up in minutes can be good for adhesives and floor coatings where the area must be put back into service as soon as possible. Many epoxy systems also contain additives, such as plasticizers to make them more flexible, organic solvents to make them more spreadable, and fillers, such as sand, to add bulk and reduce costs. Mixing in pigments adds color.
Since epoxies bond exceptionally well to a wide range of materials, they make excellent adhesives. Good bonding also means that epoxies are useful for making composite materials. For example, we can reinforce a repair by layering epoxy and fabric so that the resulting composite bonds to its surroundings, or by mixing the resin with powdery fillers or chopped fibers to make a paste that fills voids.