Because of their ability to adhere to a wide variety of materials, their high strength, their resistance to chemicals and environments, and their ability to resist creep under sustained load, epoxies are the most widely used structural adhesive. They are available in one component, heat curing and two component, room temperature curing systems. Unmodified epoxies cure to hard, brittle solids. Most adhesive formulations include modifiers to increase flexibility or toughness of the cured adhesive. This results in bond lines that are able to resist more peel and cleavage stress as well as impact.
One component systems typically cure at temperatures from 250 to 350oF (120 to 175oF). Cold storage is required to provide sufficient shelf life. They provide rigid but tough bond lines and have excellent adhesion to metals. Chemical and environmental resistance is excellent. Most formulations have a paste consistency and can be applied by trowel or extruded as beads. They easily fill gaps and provide excellent sealing properties particularly against harsh chemicals. They are often used as alternatives to welding and rivets. Some formulations can tolerate processing oil on the substrate and still obtain satisfactory bond strength.
One component heat curing film adhesives are typically based on epoxy resin formulated with curatives and modifiers. They are very high performance adhesives providing high strength, high fatigue resistance, and high temperature resistance. These curing film adhesives require cold storage and have limited shelf-life after warming to room temperature. They are especially suited for bonding and laminating large areas. Epoxy film adhesives find most of their applications in the aerospace industry for assembly of components such as aircraft panels and helicopter rotor blades. To obtain optimal performance and durability, aluminum substrates are usually chemically treated.
Two component epoxy adhesives are found in all market segments. The worklife (time adhesive can be processed and bonded after mixing) can vary from a few minutes to several hours. Assemblies must be fixtured until the adhesive has cured sufficiently to have enough strength for handling and additional processing. Final cure and ultimate strength is obtained over hours to weeks depending on formulation. High ambient temperature accelerates the rate of cure and shortens the work life. Low ambient temperature slows the rate of cure and extends the time before assemblies can be further processed. In general, adhesives that cure faster have lower final strength than those that cure more slowly. The major advantage of two component epoxy adhesives is that they are suitable for bonding nearly all substrates - metal, plastic, glass and ceramic, wood and wood products, and many types of rubber. In general, they have high resistance to physical and chemical influences and in addition they have high long-term stability because they only have a limited tendency to undergo creep. Depending on the type, they can withstand continuous temperatures from 200oF (95°C) up to 390oF (200°C). Cured adhesives are typically hard and rigid and range from brittle to tough depending on formulation.