From the Desk of Dr. Walter Brenner

EPOXIES PACE GROWTH
OF HIGH-PERFORMANCE
BONDING TECHNOLOGY

by Walter Brenner, Technical Director, Master Bond Inc.

Adhesive bonding is finding increasing use. Here is a basic discussion of one of the most prominent bonding agents.

Adhesive bonding, especially when it incorporates epoxy resins, is becoming more and more accepted as a cost-effective production method in the aerospace, automotive, marine, construction, mechanical and electrical/electronic industries. Epoxy resins are a technically and economically attractive alternative to mechanical fasteners as well as for welding, brazing, and soldering operations.

Epoxy adhesives have an unmatched record and an extensive 30-year history of bonding experiences. They combine high mechanical strength properties with dimensional stability, chemical resistance and good handling characteristics at moderate cost. These workhorses of the adhesives world can be formulated to bond to steels, aluminum and other non-ferrous metals, most thermosetting and many thermoplastic materials, fiber reinforced composites, concretes, bricks, ceramics, glasses, woods and foamed structures. When compared to other commercial adhesives, epoxies provide many important advantages: they fill gaps, resist water and chemicals and offer durability with convenient cure schedules.

The A,B,Cs of Epoxy Adhesives

Generally, epoxy adhesive systems are made up of a base resin, a hardener, an accelerator, flexibilizers, fillers, diluents and additives.

The base resin greatly affects thermal stability and chemical resistance. Depending on the selected hardener, an epoxy adhesive can cure quickly (40 sec) or slowly (48 hrs) at room and/or elevated temperatures (150 to 400°F). The bond strength, mechanical strength properties, flexibility, heat and chemical resistance, electrical and thermal conductivity, etc. can be varied by the choice of type and amount of various components to best meet specific application requirements. The list of additives for upgrading the properties of an epoxy adhesive in a given end category is steadily increasing.

Epoxies show the highest tensile shear strengths of all adhesives—up to 6000 psi and more—with fiber-reinforced compounds featuring shear strengths in excess of 10,000 psi. Bonds are generally rigid, but may be made more flexible if necessary. Service temperatures range from below -67°F to over 500°F. Resistance to moisture, fuels and many other chemicals is of a very high order. Epoxies generally offer exceptionally good gap-filling capability and cure with minimal shrinkage without the evolution of volatiles. When used to bond dissimilar metals, the bond line functions as a barrier against galvanic corrosion. Since they offer good electrical insulation characteristics, they are valuable in many electrical and electronic assemblies.

Epoxy adhesives are usually produced as one- or two-component, ambient or heat curing liquids, pastes or films. The films are supported or unsupported tapes, the principal supports being glass cloth, graphite fibers or nylon film.

Two-component liquid and paste epoxy adhesives are widely employed in the construction, marine, electrical/electronic and various mechanical industries as well as for repair and maintenance in the vast consumer markets.

One-component paste and film adhesives are used in the aerospace and transportation industries because they offer the highest shear strengths and the greatest service temperature capabilities. They require elevated temperature cures, frequently carried out in the 250- 400°F range. Two-component epoxy adhesives are usually cured at ambient or moderately elevated temperatures e.g., 75-200°F. Some tend to exhibit somewhat lower strengths and more limited service temperature capabilities compared to one-component pastes and films.

Great Leaps Forward

Much progress has been made in the last few years in improving performance characteristics of epoxy adhesives. These advances relate to improving toughness and peel strength, increasing their high-temperature service capabilities and upgrading their resistance to water and other chemicals, including fuels and lubricants, at elevated temperatures. Also, for one-component pastes and films, shelf life without the need for low-temperature (40°F) storage has been substantially extended and cure schedules shortened while bonding properties are retained and sometimes advanced. Much of this development work was stimulated by the needs of aerospace industries for lighter weight, more fuel-efficient aerospace components.

The gains achieved in upgrading peel strength and heat resistant characteristics of epoxy adhesives are particularly impressive. Epoxy adhesives have traditionally exhibited high-tensile shear strengths but have shown low peel strength, a marked disadvantage for many bonding operations. This deficiency is being overcome by more sophisticated chemistry and formulation technology including modification with liquid elastomers. Furthermore, epoxy-based adhesives with greatly improved peel strength retain the desirably high tensile shear strengths typical of these materials.

Equally significant are the improvements in thermal stability for one and two component epoxy-based adhesives. Today's advanced epoxy adhesive systems perform quite satisfactorily at temperatures at and above 500°F. The key to this remarkable development has been the production of new heat-resistant epoxy resins based on novel chemical structures. These advanced epoxy resins can be cured over a wide temperature range with specially designed curing agents and give impressive bond strengths.

Many advances have been made towards developing epoxy-based adhesives capable of performing in highly corrosive environments (like those found in chemical process industries). Two-component liquid and paste adhesives have been recently commercialized with excellent resistance to strong mineral acids, bases and organic solvents after only ambient temperature cures. One such adhesive even resists 98% sulphuric acid, 30% phosphoric acid, 30% ammonia, 100% ethanol and strong organic solvents including cellosolve and butyl acetate.