By Kent Pitcher
Q. I understand that hot melts don't offer sufficient strength for most structural purposes. Is there an alternative that offers the speed advantages of hot melts *and* the structural strength of PVA?
Also, what are the upper "in use" temperature limits I should observe for adhesives?
A. The ideal alternative to traditional hot melts when structural strength is required are reactive polyurethane hot melts. These "new" hot melts cure in two stages; the first is a mechanical bond created as the adhesive cools, the second and final stage occurs over the next 48 hours as the adhesive cures chemically. The final bond is "thermoset" meaning it won't re-flow with heat. It is also extremely strong and waterproof. This type of adhesive is available in a variety of sizes including 300 gm cartridges and larger bulk sizes. Our company handles a product called HoBond.
Your other inquiry is also topic for good discussion. Some polyvinyl glues (PVA's) can give you heat resistance up in the 200 degrees F range, but not much higher. Most of them will begin to soften at around 165 degrees F, and run the risk of opening up a glue line . Liquid polyurethanes are not much better, having heat resistance up to about 190-200 degrees F. Urea resins are a good option, as they give you a thermoset bond and will not soften or reflow with heat.
The reactive polyurethane hot melts mentioned ealier will also provide very high heat resistance.