Adhesives have increasingly been taking over the bonding function from mechanical fasteners, and in doing so bonding wider ranges and types of substrates as well as bonding of dissimilar materials. Metals are no exception, and using adhesives to bond them either to the same or dissimilar substrates is becoming an increasingly popular and effective method of replacing welding.
Shifting from welding metal to adhesive bonding has been especially challenging because of metal’s non-porous nature and smooth surface, but three types of adhesives (epoxy, polyurethane and cyanoacrylate) are now bonding metal to metal as well as to dissimilar substrates like glass, wood, and fabric, according to their abilities to meet the specific needs of different metal bond projects.
However, many different products falling within these ranges of adhesive products not all of which are designed for metal bonding or the substrates to which it will be bonded, choosing the right one for a specific metal application needs to be based on the suitability of individual products within the range to fit the needs of each project. This information is vital, particularly when it comes to tensile strength, reactions to environmental conditions, curing times, the window for correction during application, and how easy they are to use. All are factors which can differ considerably with different adhesives.
Epoxy: For bonding metal to metal, two part epoxies made up of a separate hardener and resin, which are mixed in equal proportions, offer multiple advantages such as high tensile strength, and flexibility in the materials that can be bonded with metal. They also stand up to various temperatures, don’t conduct electricity, are waterproof, hard, and can be drilled and sanded after they are fully cured.
Some epoxies will involve more difficult and time-taking applications. Mixing the two parts before application, the surface preparation (without contamination) required by some; and the need for application on both substrates, all involve time. Initial drying can take from 5 to 30 minutes, depending on the epoxy used, and the full cure period (anywhere from 4 to 24 hours) can cause long delays. Clamping is usually recommended in the first drying period.
Polyurethane: While polyurethane adhesive metal bonds are not as strong as the epoxies and require the correct amount of heat or moisture (usually in the environment) to cure well, the expansion of coverage and gap filling abilities, are strong benefits . Polyurethanes are suitable for securing metal to other substrates, and can be used outdoors because of their high resistance to water and UV light. Polyurethane adhesives are single part adhesives, usually easy to apply and spread (with different ones requiring application to one or both substrates). There is open time for correction of mistakes before the adhesive starts to harden (around 10 minutes) provided the adhesive chosen does not have a strong grab; leaks from joints can be cleaned with a damp cloth, and when fully hardened the adhesive can be filed, chiseled or sanded.
Cyanoacrylates: When it comes to super glues, high strength versions of these polymers can also bond both metal to metal as well as to other materials in simple repairs and small tasks. This adhesive requires water in the environment to form its strong chemical chains, but its water resistance is too low for use outdoors. Applying it involves care. Using the liquid version can involve spillage, and application be difficult, while the gel version is easier to handle.