Adhesives that stand up to harsh chemical environments

Of all the factors to consider when selecting adhesives, chemical exposure too often gets short shrift. A quick peek at a few data sheets or a chemical-resistance chart may be all the work that goes into evaluating adhesives for use in specific chemical environments.

This kind of half-hearted effort simply won’t cut it if you want to make sure the adhesives, sealants, and encapsulants you use will withstand the chemicals found in so many industrial, medical, automotive, and aerospace applications.

All polymers, including adhesives, are potentially vulnerable to chemical attack that can degrade physical properties. The challenge is knowing exactly how an adhesive will react, given all the variables that govern the effects of chemical exposures.

It starts with the interaction of substrates and adhesives with specific chemical agents. The challenge here is there are literally thousands of possible combinations. The type of exposure is also a factor, and it can range from a momentary splash to continuous immersion. Finally, chemical resistance can vary substantially under different mechanical and thermal loads.

Getting a handle on how an adhesive will perform in service, given all these variables, can be difficult and time consuming. Here are some basic guidelines for selecting adhesives that will hold fast against any chemical onslaught.

Understand chemical interactions
The first thing to understand is that no single adhesive is best for every chemical environment. But some adhesive families have broad resistance to many types of chemicals. Epoxies clearly lead the pack in this regard, while polyurethanes, silicones, UV curables, and polysulfides resist a more limited range of chemicals.

Within each adhesive family, thermal and chemical resistance often go hand in hand. Cross-linked adhesives, like the ones mentioned above, tend to have the best chemical resistance below their glass-transition temperatures (T_g). So grades with higher T_g can often beat the heat and withstand more chemicals.

Generalities, however, only get you so far in the adhesive selection process. Keep in mind that adhesive chemistries can vary substantially even within a family. Individual adhesive grades can have different additives and curing reactions that affect their ability to withstand chemicals.

Consider epoxies. As a family, they are the most chemically resistant adhesives, encapsulants, and coatings. But individual epoxy formulations do differ in their specific chemical-resistance traits. The nearby table, which lists the relative resistance of epoxy coatings to a lineup of industrial chemicals, solvents, and fuels, shows how different epoxies react. For example, assuming all epoxies resist ethyl alcohol, because some grades do, can be a big mistake. So it’s always important to consider how individual grades resist specific chemical exposures, and this strategy applies to every adhesive.

Understand exposure variables
Figuring out which adhesives withstand which chemicals is only half the battle. Engineers also need to understand the type of exposure. On a basic level, exposures should be characterized by the intensity of contact with a chemical agent. Low-intensity exposure is best thought of as a splash. Higher-intensity exposure would involve intermittent or continuous immersion. And remember, exposure can involve gases, not just solids or liquids.

Chemical exposure should also be considered in the context of the application’s thermal and mechanical loads. Many adhesives incrementally lose chemical resistance at elevated temperatures — especially above T_g. High stresses also exacerbate adverse effects a chemical agent has on adhesive or cohesive strength. Adhesive and chemical combinations that perform well under one set of loading conditions won’t necessarily make the grade in others.

All these variables may sound straightforward, but mischaracterizing the type of exposure is a surprisingly common mistake with potentially serious ramifications. Understating the exposure intensity or severity of the loads can result in adhesives that don’t perform as well as expected, or perhaps even fail.

Usually, prudent design engineers tend to overstate exposure or loads. That strategy, while safe and appropriate up to a point, can limit the number of suitable adhesives for a given application. Why? Because for any potentially harmful chemical agent, there are many more adhesives that can resist splashes, low temperatures, and low stresses than can resist continuous immersion, high temperatures, and high stresses.

The consequences of overengineering for chemical resistance are twofold. One is that designers could end up trading off desirable adhesive properties for a level of chemical resistance the application doesn’t really require. The other is that adhesives with the best chemical resistance are apt to have more-difficult mixing and curing regimens, potentially bumping up assembly costs unnecessarily.

Understand testing
Because it’s crucial to get the details of chemical exposure right, it is a good idea to test bonding, sealing, and encapsulation applications that could be subject to harmful chemical interactions. There are dozens of ASTM and industry-specific tests that attempt to capture these interactions. General Motors alone has more than 30 adhesives specifications, many of which contain conditions related to chemical or moisture exposure.

Whichever test is most accepted in your particular industry, keep in mind that testing at best only approximates real-world conditions. For instance, ASTM D896, one of the most widely cited standards for adhesive chemical resistance, makes no distinction between chemical adsorption in the bulk adhesive or penetration at the adhesive-substrate interface. Yet this difference is a key factor in an adhesive’s chemical-degradation behavior.

Most tests also require shorter-term exposures than products experience in actual applications. Some military tests immerse adhesives in fuels and hydraulic fluid for just one week, and to high-humidity conditions for 30 days. Real-world exposures can obviously last much longer.

Savvy engineers don’t put much credence in short-term test data. Experts at Master Bond, Hackensack, N. J., for instance, rely instead on a unique database of long-term-exposure information. Some of this chemical exposure data comes from immersion tests that have lasted as long as 10 years. And the data cover many combinations of adhesives and chemicals — including many organic and inorganic acids, alcohols, chlorinated compounds, hydrocarbons, and solvents.

A related issue involves accelerated testing, which exposes adhesives to exaggerated loads for short time periods in an effort to predict long-term service life. Many times, these test regimens elevate temperatures to the point they introduce thermal effects a product would never experience in the field. Because actual service temperature is such a critical part of any adhesive’s true chemical resistance, accelerated testing can actually point engineers in the wrong direction. For example, epoxies that might be an excellent choice in a room-temperature chemical environment can be made to fail at accelerated-testing temperatures.

None of these warnings should be construed as an argument against testing. Just bear in mind that test conditions often deviate substantially from real-world conditions and time scales.

An effective adhesives-selection strategy, then, requires engineers to evaluate specific chemical-and-adhesive combinations in the context of exposure and loading that are as close as possible to the expected service conditions. That involves a lot more effort than picking adhesives from a chemical resistance chart, but it’s a small price to pay for some confidence that an adhesive is right for the job.

© 2011 Penton Media, Inc.