Over the past 30 years, significant technology gains in polymer engineering have greatly expanded the applications suited to adhesive bonding with epoxy resins. Advanced bonding methods that incorporate epoxy resin technology are among the most reliable and cost-effective fastening options in highly demanding industries such as aerospace, automotive, marine, construction, and electronics. In many applications, epoxy resins are finding use in welding, brazing, and soldering operations.
High-performance epoxies combine substantial mechanical strength with dimensional stability, resistance to harsh chemicals, and user-friendly handling characteristics at a reasonable cost. These advanced structural adhesives can be formulated to bond steel, aluminum, and other non-ferrous metals, in addition to many thermosets and thermoplastics, fiber-reinforced composites, ceramics, concrete, brick, glass, wood, and foam structures. Compared to other commercial adhesives, epoxies feature several important advantages: they fill gaps, resist water and chemicals, and achieve high strength and durability within timely cure schedules.
Of all the available adhesive technologies, epoxies also feature the highest tensile shear strengths — 6,000 psi and greater — with fiber-reinforced compounds offering shear strengths in excess of 10,000 psi. Bonds are generally rigid, but can be made more pliant with flexibilizers if necessary. Service temperatures range from below -60 °F to higher than 500 °F. Further, these structural adhesives cure with minimal shrinkage and without creating volatile compounds. When used to join dissimilar metals, the bondline functions as a barrier against galvanic corrosion. Another key benefit of epoxy adhesives is that they offer robust electrical insulation, and are therefore suited for applications involving electrical and electronic assemblies.
Most epoxy adhesive systems consist of a base resin, hardener, accelerator, flexibilizers, fillers, diluents, and additives. The base resin significantly influences both thermal stability and chemical resistance. Depending on the selected hardener, an epoxy adhesive may cure quickly (in just 40 seconds) or slowly (over 48 hours), either at room temperature or elevated temperatures in the 150 to 400 °F range.
Bond strength, mechanical strength, flexibility, heat and chemical resistance, electrical and thermal conductivity, and many other properties all may be adjusted according to the type and amount of various chemical components added to the epoxy formulation.
Epoxy adhesives are usually produced as one- or two-component formulas, and as either ambient or heat-cured liquids, pastes, or films. Films are available as supported or unsupported tapes, with the principal supporting elements made of glass, cloth, graphite fibers, or nylon film.
Substantial technology development has occurred over the last several years with regard to improving the performance characteristics of epoxy adhesives. Many research and development projects were in response to the aerospace industry’s need for lighter-weight, more fuel-efficient components. These technology advances resulted in improved toughness and peel strength, increased high-temperature service capabilities, and greater resistance to water and other chemicals — including fuels and lubricants — at elevated temperatures. Further, the shelf life of one-component pastes and films has been extended without requiring low-temperature (40 °F) storage. Cure times have been shortened, while superior bonding properties have been maintained, and in some cases, improved upon.
Gains achieved with regard to increased peel strength are particularly noteworthy. While structural adhesives have traditionally featured high tensile shear strength, they have also exhibited low peel strength, a marked disadvantage for many bonding operations. Today, this deficiency is being corrected with more sophisticated chemistry and formulation technology, including modification with liquid elastomers. Furthermore, epoxy-based adhesives with significantly improved peel strength continue to feature the high tensile shear strengths typical of these materials.