Adhesives beat the heat-and the cold too

Engineers have to be judges of temperature when they select materials for the job. Both the high and low end of this temperature range can be a challenge, yet there are adhesive families that rise to that challenge when operating temperatures reach extreme levels. Epoxies may come to mind because they are without equal at functioning across the temperature spectrum. But there are other adhesive families that tolerate temperature extremes, including some silicones and a newly developed bismaleimide adhesive that offers even better high temperature resistance than the best epoxy. Get to know the glass transition temperature As polymeric materials, adhesives share a generalized thermo-mechanical response to temperature extremes. The "certain point" at which 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. A look at cold cases At cold temperatures, Tg does not provide the same clear window into adhesive performance as it does in high temperature applications. As temperatures dip farther below the Tg, adhesives become increasingly brittle and subject to low failure stresses. That reasoning would seem to favour flexible adhesives with low Tg values for the coldest applications. Yet in practice, the opposite is often true. Epoxy adhesives 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 colder territory.