Adhesives and Metal: A Bond That Lasts (and Saves Money!)
Based on an original article by Loctite, "Bonding Metals with Adhesives" By Patrick J. Courtney, Engineering Project Manager Loctite Corporation
When most people think of adhesives, they envision small plastic bottles that occupy space in a kitchen drawer to be used to repair a broken coffee cup or to re-attach a chipped part back onto a holiday decoration. For those of us in industry, adhesives go far beyond simple repair jobs. Adhesives are fast replacing metal fasteners, welding, soldering and brazing in many application where attaching two parts together is part of the process.
When we talk about adhesives, we're referring to a group of chemicals that are classified into two large groups: structural adhesives and machinery adhesives. Structural adhesives are used in situations where the adhesive itself primarily supports the load, whereas machinery adhesives "augment the seal or holding force of a mechanically joined assembly."
Structural Adhesives vs. Thermal Mechanical Fastening
Assembling equipment and devices with metal substrates requires varied bonding methods, including thermal bonding (welding, soldering, brazing), mechanical fastening and adhesive bonding.
Thermal joining, while delivering a strong and lasting bond, can cause aesthetic problems (distortion, coloration) because of uneven heating and cooling of the parts. When problems arise with thermally bonded parts, repairs are both difficult and costly.
Mechanical fasteners provide a strong and fast bonding solution for both similar and dissimilar substrates. However, they require holes to be drilled into the materials to be bonded, which can result in weakened substrates. In addition, this type of bonding tends to concentrate stress in a small area rather than distributing it over a larger area, which can lead to premature failure. While suitable for stationary substrates, this bond is susceptible to flex or vibration stress damage. Aesthetically, they can detract due to their being visible in the product.
Adhesives give manufacturers many unique advantages. First of all, adhesives, whether bonding metal to metal or other substrates, evenly distribute stress over a wide area and are hidden from view since they are applied within the joint. Second, adhesives resist damage that can be inflicted by flex or vibration. In addition, the bond they provide doubles as a sealant, warding off corrosion. Another distinct advantage adhesives provide is that they allow easy joining of irregular-shaped and/or dissimilar substrates. They are "one-size-fits-al" and are "easily automated." However, adhesives may require long curing times and may also include unique surface preparation prior to application. Disassembly is also an issue, given the nature and the strength of the bond.
Structural Adhesive Options for Metal Bonding
Metals are the preferred substrate in demanding environments because of higher heat and chemical resistance when compared to other materials. Due to the wide differences in coefficients of thermal expansion (CTE) between metal substrates and adhesives, selected adhesives must be able to meet the demands of repeated heating and cooling. Commonly used adhesives for bonding metal substrates are: