longer uncommon for aerospace, oil-and-gas, industrial and some electronics
applications to require materials systems that withstand temperatures of 300°C
or greater. Other applications have to contend with extreme cold, even down to
cryogenic temperatures in the neighborhood of 4K.
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
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
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.
example, it would be unwise to pick a structural adhesive whose Tg is 100°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°C and
service temperatures up to 315°C. Other
options, such as a newly developed bismaleimide adhesive from Master Bond (see
sidebar), has a Tg of 300°C and predicted service temperatures in excess of
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.
differences between the measured Tg and maximum service temperature values
highlight the fact that the two are not the same thing. Plenty of successful
real world applications have employed adhesives whose operating temperatures
exceed Tg for short periods of time or by small margins that will not cause
mechanical properties to degrade enough to matter.
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
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.
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
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°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.
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°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.
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
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
resistant adhesive products are also available with added functional attributes-including
low-outgassing behavior, thermal conductivity, electrical conductivity, biocompatibility
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.
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.
information, visit: www.masterbond.com