It is June again and with the coming of summer it is time to make adjustments to our installation process. The summer season brings warmer temperatures and higher humidity, which means faster curing times for our urethane adhesives. Is this a good thing? Faster cure times means faster safe-drive-away times, right? Yes it does, but it also means a shorter working time during the application and setting of the glass.
I remember how I learned the hard way about “tack-free time,” “skin-over time” or working time. I was doing a DW763, a General Motors full-size van back when I was a rookie. It was 90°F and 80 percent humidity and I applied the urethane before I realized I forgot to prep the glass. I quickly cleaned and primed the glass. I allowed it to dry. Just before I was going to set the glass, I checked the urethane bead and found it to be already tack free (the urethane was no longer pasty). The amount of time I had spent cleaning, priming and waiting for the dry time to pass was a short 11 minutes. The product’s printed “tack-free time” was written on the technical data sheet as 30 minutes. I should have had plenty of time to set the glass, right?
What I didn’t notice until too late was the small print at the bottom of the page: “Tack-free time 30 minutes after application at 72°F (22°C) and 50 percent relative humidity.” Of course, at a higher temperature (and higher humidity), tack-free time was considerably shorter.
This was a lesson I did not soon forget. Trying to scrape up two and a half tubes of not fully cured urethane on a hot sticky day never will be my idea of a good time.
When temperature and humidity values rise, working time decreases, sometimes dramatically. Remember that as soon as the urethane is opened, the product begins to cure. A technician should not dawdle when applying, paddling and setting the glass. In fact, any procedure that can be done before the urethane tube is opened should be completed. At the very least the glass should be clean, primed and ready to go. Then apply, paddle and set the glass without delay. Remember that warm air has the ability to hold more moisture, thus your curing times and working times are shortened in relation to the increase in temperature and humidity.
By the way, here is a little hint if you use the aluminum tubes of urethane. If you run into the situation I described above and you have to remove a partially cured bead of urethane from the pinchweld, use the other end of an emptied tube as a scooping tool. Push the tube along the pinchweld and scoop up the bead. If you oval the tube slightly, it makes for a smooth surface to re-apply the new bead. It works slick with little to no mess. Stay cool out in the field this summer.