From the Desk of Dr. Walter Brenner

STRUCTURAL ADHESIVES CHALLENGE
MECHANICAL FASTENERS

by Walter Brenner, Technical Director, Master Bond Inc.

Adhesives are becoming a formidable competitor to mechanical fasteners in structural applications. The reason: recent improvements in peel strength, flexibility, and resistance to moisture, temperature, and chemicals.

The potential advantages of adhesives over mechanical fasteners have long been recognized; however, industrial acceptance of adhesives for structural applications has been slow. The main reason for this lack of use is that adequate information about the characteristics of adhesives was not available until recently. Development studies by the aerospace industry greatly improved the technical database on structural adhesives, and many industrial designers are now convinced that the adhesives can provide bonds that are both cost effective and durable.

Structural adhesive bonds have replaced both mechanical fasteners and assembly techniques such as soldering and welding in many industrial applications. The advantages of structural adhesive bonding over other joining techniques include:

• Cost savings, including lower labor costs.
• Weight reduction.
• Elimination of stress point concentrations by even distribution of stress over the entire bonded surface, plus improved load bearing capacity.
• Protective sealing against contamination by liquids or gases.
• Bonding of dissimilar materials. Often the adhesive bond line acts as an insulator against galvanic corrosion in metal assemblies.
• Improved fatigue resistance, and resistance to shock, vibration, and thermal cycling.
• Enhanced structural appearance because protrusions, punctures, and attachments are eliminated.
• Increased tolerances on machined parts because some structural adhesives can fill gaps left when parts mate poorly.

Few available adhesive materials meet the stringent requirements of structural bonding, such as durability, high strength, and dimensional stability over a wide range of environmental conditions. The main classes of adhesives meeting structural requirements are epoxies, cyanoacrylates, reactive acrylics, and polyurethanes. Also, recently developed materials that appear to be especially suited to high temperature use include polyimides and some silicones.

Epoxies, the Workhorses

Epoxies are by far the most widely used of the structural adhesives. In these one and two-part adhesives, base resins, curing agents, fillers, and various additives can be formulated to provide high strength bonds to steels, ceramics, glass, wood, thermoplastics, fiber-reinforced composites, rubber, and plastic foam..

High tensile strength—up to 10,000 psi for reinforced epoxies—makes epoxy bonds especially resistant to impact, vibration, and shock. The service temperature range of these adhesives extends from about -70°F to more than 450°F. Recent improvements in the high temperature resistance of epoxies have enhanced their advantages for metal-to-metal bonds. A few epoxies now retain their strength at temperatures exceeding 500°F.

Epoxy bonds are usually rigid and exhibit poor peel strength. However, some new formulas have greatly improved peel strength and allow more flexible bonds. Flexible epoxies maintain the desirable characteristics of other epoxies during cure; namely, they exhibit little shrinkage and do not emit volatiles.

A severe problem with earlier epoxies was their lack of resistance—especially at high temperatures—to moisture, fuel, hydraulic fluids, and other chemicals. Newer epoxies have much better resistance to chemicals over a wide temperature range. Other advantages of epoxy adhesives include low to moderate cost, excellent gap filling, electrical and thermal insulating properties, and ability to protect against galvanic corrosion.

Epoxies come in liquids, pastes, and films. They can be cured with or without heat. However, most epoxies require a heat cure. Two-component liquid and paste types are used extensively in aerospace, construction, and electrical applications. One-part pastes and films are often used in automobiles. One component adhesives provide more shear strength and wider temperature resistance than other types of epoxies.

For optimal bond quality, all substrate surfaces should be cleaned and dried before epoxy is applied. Some substrates need special pretreatment (see table, Substrate Surface Pretreatments). Primers usually are not required, but they can improve corrosion resistance when substrates will not be bonded immediately after cleaning. Pressures required to ensure wetting of surfaces range from mere contact for two-component liquids to above 50 psi of direct pressure for adhesive films.

Cyanoacrylates for Quick Cures

Cyanoacrylates are high strength, rapid curing, single-component adhesives that allow fast bonding at room temperature. They are known as "instant" glues because they fixture within seconds of application. They bond a range of surfaces including metals, elastomers, plastics, glass, and ceramics.

These reactive adhesives do not emit volatiles during cure. The cured adhesive is an excellent electrical insulator, and major industrial applications include electrical assembly such as wire tacking, and coil and loudspeaker bonding. Cyanoacrylates bond best on slightly alkaline surfaces. Minimal surface preparation is required for these adhesives, though dry, clean, and non-oily surfaces are preferred.