It wasn’t that long ago that if you took a stroll down the adhesives aisle of your local hardware store, choosing the right glue for the job was a simple task. Your choices were limited to a five-minute epoxy and a small bottle of "super glue." Anecdotal evidence indicated that failure was as common as success, likely forever tainting the thought of adhesives in your mind. Today, the selection of adhesive types can be dizzying in a single store, but there’s still no handy guide to help you choose the right tool for the job. Hopefully, this article can shed some light on different types of adhesives, and help you find the right one for any job.
Adhesives aren’t just the last resort for fixing broken things. It has been scientifically proven that under normal conditions, a small quantity of adhesive will provide more strength than any mechanical fastener at a fraction of the cost. When bonding metals, structural adhesives can provide strengths comparable to welding without any of the distortion or skill requirements. As an added benefit over welding, adhesives can almost always be used to join different materials, such as plastics, rubber gaskets and different metals, while accommodating differing rates of thermal expansion and insulating against galvanic corrosion.
For example, a 316 stainless steel joint that might cost four dollars in material and labour to bolt together-could be replaced with approximately 15 cents of adhesive, while doubling the potential joint strength. So assuming that adhesives can be used almost anywhere, why aren’t they more common in OEM manufacture and repairs? The lack of a "universal" adhesive, leading to bad experiences is probably the biggest culprit. As a result, identifying the different families of adhesives and what they do best is paramount.
Without a universal adhesive, it may appear that selecting an adhesive can quickly fall into an intimidating tangle of names and details. While some of the names can seem complex, there are only seven major chemistries (or families) to choose from. Each of these chemistries has a niche in which it excels and, once that niche is identified, the selection process becomes much simpler.
Here’s a brief introduction to the various families of adhesive. These descriptions should give you a fairly accurate idea of an appropriate adhesive technology for your application:
Epoxies: Likely to be the most familiar adhesive to work with, this two-part adhesive was originally developed in the 1920s. It’s still the strongest adhesive system, and the resin/hardener curing system gives it the best chemical resistance and an almost unlimited ability to cure thick profiles.
Cyanoacrylates: Also called "instant adhesive" or " super glue," this product was discovered in the 1960s. Like epoxies, it has an incredible strength, but cures by reaction with the ambient moisture in the air. This limits the bond line to very close fitting parts, but also makes it suitable for bonding almost any two surfaces.
Acrylics: These adhesives are the most modern of the three structural types. While almost as strong in pure strength as epoxies and Cyanoacrylates, it represents a significant increase in toughness, able to survive repeated impacts that might shatter a more rigid adhesive. Testing has shown that joints bonded with acrylic adhesive will result in distorted steel or failed fibreglass before the adhesive will fail.
Hot melts: These are the oldest of all adhesive technologies, dating back to the first time man stuck things together with a lump of warm tar. Fortunately, technology has improved since then and hot melts are currently the most common adhesive used (by volume) due to the low cost and rapid curing (cooling). Although significantly weaker than epoxies or Cyanoacrylates, hot-melt adhesives can still develop 300 pounds per square inch of surface area!
Elastomers: This group of adhesives is used more for their elastic properties rather than their strength. There are four different groups of elastomers:
Silicones: A high-temperature elastomer, able to produce reasonably high strengths;
Urethanes: A high-strength elastomer, but this adhesive is falling out of favour in many industries because it contains potentially harmful isocyanates;
Modified silanes: Despite its name, this elastomer is completely unrelated to silicone and is a non-toxic and paint able version of a urethane; and
Flextec: A very recently developed elastomer, it’s similar in performance to a modified silane, but cures significantly faster.
Anaerobics: These adhesives were initially used for bonding machine threads, as they cure in the absence of air and the presence of metal. Since their discovery in the 1950s, they’re used as a simple, but effective way of securing two metal surfaces, as well as sealing the gap between them.
Solvent-based cements: These adhesives cover a wide spectrum, but are all some type of volatile material (i.e. rubber) suspended in a solvent. Once applied, the solvent evaporates and the volatile material bonds the two surfaces. Many fast-acting solvents are now banned in the workplace, making water the most popular solvent. However, the slow evaporation rate of water makes this an unattractive option for many processes.
The overall strength of a joint is directly proportional to the bonded area. While it’s true that changing the width of the joint (perpendicular to the line of force) results in a linear increase in bond strength, this doesn’t hold true for increasing the depth of the bonded area. The load distribution across the depth of the joint prevents any large gains in bond strength beyond a 1:1 ratio.
In addition to the surface area, the forces on a joint will greatly affect the strength. Adhesives have been proven to be very strong in tension, compression and shear, but can lose 90 percent of their strength on flexible joints where they can be peeled apart. In these cases, an elastomer is more appropriate, but these joints should be avoided whenever possible.
In order to gain acceptable strength, a common misconception is that adhesives require thorough cleaning and roughing prior to application. While roughening will generally increase the bond strength due to the mechanical locking of the adhesive, any surface finish produced by machining or casting will be acceptable for bonding.
Of greater importance to bonding are surface treatments and coatings to adhesive performance. Since these barriers are typically inert, their unresponsiveness can make them unsuitable for bonding. However, since the actual adhesives act as a barrier agent in most cases, it may be possible to eliminate the additional (and usually expensive) coating.
Cleaning of the surfaces prior to bonding is another process that creates much confusion. As a general rule, if contamination is significant enough that the adhesive will bond to it rather than the surface-then it should be cleaned. If an adhesive stops working in a production situation, there’s a very good chance that contamination is an issue.
Since the clamping force is generally low (usually a few pounds per square foot), the clamping time is usually a prime concern in an adhesive application. Generally, the window of assembling and clamping is determined by the cycle time of other parts of the process. As such, the handling time should be one of the prime considerations when selecting an adhesive.
As mentioned earlier, adhesive families usually have multiple curing mechanisms, such as moisture, heat, UV light or the presence of a separate activator. By selecting an adhesive from within the same family with a different curing mechanism, adhesives can adapt to any manufacturing situation with little or no loss in performance.
The bottom line
Adhesives may seem overly complex to someone with no previous experience with them. However, many leading companies are turning to adhesives as their preferred method of attaching. High strength, versatility and lower costs are all common benefits associated with taking a new look at this technology. Understanding the different adhesive chemistries, as well as the role of surface preparation and clamping, ensures that the adhesive selection process will result in the right tool for the job.
In some complex cases, choosing the adhesive that would provide the best performance, while integrating with the current manufacturing process, can be difficult. In these cases, turn to an adhesive supplier to help you select the best adhesive for your application.
This is an edited article provided by Henkel Loctite Canada. Martin Woollard, P.Eng., is an applications engineer at Henkel Loctite Canada. For more information, visit www.henkelna.com or www.loctite.com.