It's no
longer uncommon for aerospace, oil-and-gas, industrial and some electronics
applications to require materials systems that withstand temperatures of 300 degrees C
or greater. Other applications have to contend with extreme cold, even down to
cryogenic temperatures in the neighborhood of 4K.
Picking
the right adhesive product for extreme temperature applications may seem
straightforward. After all, just about every adhesive supplier publishes
temperature resistance values on their data sheets. Relying on that data,
engineers will sometimes address temperature issues by simply selecting an adhesive
rated for temperatures beyond their application's expected operating
temperature.
Unfortunately,
good design practice is not that simple. Temperature resistance values on data
sheets are notoriously inconsistent, in part because suppliers test adhesives
so differently--with some suppliers taking a far more conservative approach to
reporting temperature data than others. To
select the right temperature resistant adhesive for a given application, you
will have to dig a bit deeper than a line or two on a data sheet.
Glass Transition Temperature
As
polymeric materials, adhesives share a generalized thermo-mechanical response
to temperature extremes. As the temperature rises past a certain point, the
adhesive will begin to soften and lose some tensile strength. The adhesive will
also experience a rise in its coefficient of thermal expansion (CTE). The point
at which very high temperatures start to pose a problem differs with the
individual adhesive and the application requirements. But the adhesive's glass
transition temperature (Tg) provides a window on where that point lies.
Not to be
confused with a melting point, Tg is the temperature at which thermosetting
amorphous polymers- including most temperature resistant adhesives-change from
a rigid "glassy" state to a more pliable "rubbery" state. This intrinsic
thermal property serves as a good indication of an adhesive product's ability to
stand up to an application's temperature requirements.
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For example, it would be unwise to pick a structural adhesive whose Tg is 100 degrees C less than the application's continuous use temperature. Even in less obvious cases, picking an adhesive with too low a Tg can exacerbate more subtle failure mechanisms such as creep and thermal stresses from CTE mismatch.
With a few important exceptions, adhesives with the best heat resistance tend to have high Tg values, making them rigid across their operating temperature range. Epoxies used to be unmatched in this regard, with some grades offering Tg of 230 degrees C and service temperatures up to 315 degrees C. Other options, such as a newly developed bismaleimide adhesive from Master Bond (see sidebar), has a Tg of 300 degrees C and predicted service temperatures in excess of 340 degrees C.
The exception to this relationship between heat resistance and a high Tg involves some silicones and B-stage epoxies. The unique nature of silicones' molecular backbone and B-stage epoxies' flexible cure state allows these adhesives to combine relatively low Tg and decent heat resistance-though not as good as the best epoxies or bismaleimide.
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So while Tg is not a shorthand for continuous use temperature, it does serve as an indicator of good design practice in high-temperature applications. By selecting an adhesive whose Tg is above the expected service temperature, you can reduce the risk of inadequate mechanical properties or thermal stresses.
Cold Cases
At very cold temperatures, Tg does not provide the same clear window into adhesive performance as it does in high temperature applications. As temperatures dip farther and farther below the Tg, adhesives become increasingly brittle and subject to low failure stresses. That reasoning would in theory seem to favor flexible adhesives with low Tg values for the coldest applications.
Yet in practice, the opposite is often true. Epoxy adhesives in particular do not experience a significant loss of properties even at cryogenic temperatures--meaning they work best in a rigid state that extends from their Tg into far colder territory.
How Limited Understanding of Adhesives Can Limit Choices Exaggerating maximum use temperature as a safety measure usually results in the selection of adhesives with higher glass transition temperatures (Tg). That's fine if you actually need all of that added temperature resistance. However, if you don't, you may end up with an adhesive that requires more difficult curing and handling methods. For example, you may find yourself moving from a product with simple room-temperature cure to a product that requires an oven cure. What's more, adhesives with a higher Tg tend to be more rigid materials, which provide less "give" in thermal cycling applications. By opting for more temperature resistance than you actually need, you can unwittingly sacrifice a bit of thermal cycling capabilities. One often neglected factor in determining an appropriate margin of safety is the duration of exposure to elevated temperatures. Many adhesives can withstand 300 degrees C for a few seconds. Increase that exposure to hours, days or months, and the list of suitable adhesive products shrinks to a few epoxies and bismaleimides with high Tg. The message here is not to ignore safety margins, but to determine them realistically and with an eye to the length of exposure. |
As an example, Master Bond makes one- and two-component epoxy adhesives that provide structural bonds that can handle temperature extremes from cryogenic 4K up to 205 degrees C. Other high temperature epoxies likewise exhibit excellent performance at cold temperatures above cryogenic levels. This ability to work in environments that mix extreme heat and cold is particularly important in aerospace applications.
Adhesives with a low Tg that function fine in a purely cryogenic environment tend to exhibit large CTEs as they heat up-with all the thermal stress problems that large CTEs can imply. High Tg adhesives tend to have smaller, manageable CTEs across their entire operating temperature range, and may be more suitable for these mixed extreme environments.
Managing Trade-offs
The good news about temperature resistant adhesives is that they offer a balance of properties that requires little in the way of design trade-offs. These epoxies, silicones and bismaleimides offer all the physical and mechanical properties required to address a range of structural bonding, encapsulation and sealing applications.
Temperature resistant adhesive products are also available with added functional attributes-including low-outgassing behavior, thermal conductivity, electrical conductivity, biocompatibility and more.
Of course, there is no free lunch in the engineering world, and the price of enhanced temperature resistance comes in the form of more difficult and costly curing and mixing regimens. With epoxies, the grades with the very best temperature resistance are two-component systems requiring both mixing and an oven cure, possibly with fixturing. One-component systems needing an oven cure are next best in the temperature resistance department. Systems requiring room temperature cure, while easiest to use, trade off some temperature resistance.
Temperature resistant products also require careful attention to the manufacturer's cure recommendations. While that advice applies to most adhesive applications, it's all the more crucial with temperature resistant products because the Tg can be lowered by improper curing. Simply put, optimal properties and an optimal cure go hand in hand.
Robert Michaels is vice president of technical sales for Master Bond.
For more information, visit: www.masterbond.com