Do adhesives melt at high temperature?
…depends on the type of adhesive is the quick answer to that one…
Adhesive users are often under the false impression that all adhesives melt at high temperature. While thermoplastic adhesives do melt or reflow at high temperature, thermoset adhesives do not. Examples of thermoplastic adhesives include hotmelt adhesives such as used on cardboard boxes or the little sticks used in craft glue guns. These products melt at high temperature and freeze at room temperature. Just like water you can melt and freeze repeatedly.
Permabond adhesives are thermoset and although they may weaken and soften at high temperatures, they don’t actually turn liquid again. Thermoset adhesives are made by polymers joining together with chemical bonds producing a crosslinked polymer structure. Examples of thermoset adhesives include epoxy adhesives, anaerobic adhesives, cyanoacrylate adhesives, UV curable adhesives, polyurethane adhesives.
So an application which is going to be subjected to high service temperatures during the assembly process or during use (such as paint bake oven, solder reflow, autoclave or oven sterilization), with a thermoset adhesive is not going to end up with a pool of molten adhesive under the joint thankfully!
Applying high temperatures to joints made with thermoset adhesives can be an easy way of weakening them to get them undone again. Generally temperatures above 200°C significantly weaken organic-chemistry based adhesives (although new developments of higher temperature resistant adhesives may mean having to crank up that oven!). Take care when handling hot parts and undo them whilst they’re still hot otherwise adhesive strength returns on cooling.