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 bond line 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.
Of equal significance are recent improvements in thermal stability for one- and two-component structural adhesives. Advanced epoxy adhesive systems now perform satisfactorily at temperatures of 500 °F and higher. Key to this development is the production of new heat-resistant epoxy resins based on novel chemical structures. These advanced resins can be cured over a wide temperature range with specially designed curing agents that yield impressive bond strengths.
Two-component epoxies are usually cured at ambient or moderately elevated temperatures in the 75 to 200 °F range and tend to achieve somewhat lower strengths and more limited service temperature capabilities compared to one component formulas. Single-component paste and film epoxy adhesives are often used in the aerospace and transportation industries because they offer the highest shear strengths, service temperature capabilities, and ease of processing. These epoxies require elevated temperature cures, frequently in the 250 to 400 °F range.
Beyond punishing temperatures, epoxies are also finding use in highly corrosive environments, such as those in the chemical processing industries. Specialized two-component liquid and paste adhesives now feature superior resistance to strong mineral acids, bases, and organic solvents after only ambient temperature cures.
Today’s advanced epoxy resins often function as thermal and electrical insulators as well. In certain electronics applications, it is now required that adhesives conduct electricity, heat, or both. Electrically and thermally conductive adhesives have been developed in response to this demand. Electrically conductive epoxies contain metallic fillers — such as silver, copper, and nickel — in finely divided powder form. For less stringent requirements, graphite fillers are acceptable. Thermal conductivity is achieved using either specialized metals or inorganic fillers, including alumina. Both one- and two-component conductive adhesive systems are available with either ambient or elevated temperature cures. Advanced adhesive systems are also being utilized in NASA-compliant low-outgassing and fiber-optic applications.
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