We all know that temperature can affect the cure (and the pot life too) of two-part adhesives, like epoxies and urethane’s. But what exactly does that mean? Is there a “rule of thumb” that can be used to predict the affect? Yes, indeed there is! We call it the “10 Degree Rule”.
10 Degree Rule
Between roughly 30 degree F and 120 degrees F, there is a rough rule of thumb that can be used to predict the cure behavior of these adhesives. For every 10 degree increase in temperature (we’re talking Celsius here—more on Fahrenheit later..), the reaction rate will double, meaning the cure speed will also double. Meaning it will be cut in half. But pot life (or working time, aka “gel time”) will also change by a factor of 2—- or ½, depending on your perspective. Now, with Fahrenheit, the 10 degree rule actually becomes the 18 degree rule. Clear as mud, right?! Well, let’s walk through a few examples, it will become much easier to understand….
First, why the difference between 10 degree C and 18 degree F? Blame the metric system (or blame the U.S. for not being on the metric system!), but the reality is the world does indeed use two different temperature measurement systems. The easiest rule to remember is the 10 degree rule—but bear in mind that it actually translates to 18 degrees if using the Fahrenheit scale.
Now, on to some examples:
- Let’s say our adhesive at 25 degrees C (or 78 degrees F) has a gel time of 10 minutes, and a cure time of 60 minutes. What is the gel time and cure time if the temperature is increased to 35 C (or 95 F)? The temperature difference is 10 C, so the cure speed will be twice as fast, meaning the cure time is now only 30 minutes. And the 10 minute gel time will be reduced to 5 minutes.
- Same as before, but now the temperature is 45 C (113 F). The cure speed is again double what it was in the previous example, so now it’s going to set off like a rocket and cure in just 15 minutes! But you’d better work fast, because this puppy is now going to gel in the container in 2-1/2 minutes.
- Now let’s go the other way—colder temperatures. Using our same adhesive, what if the temperature drops to 15 C (or 59 F)?. Now the cure will be twice as slow, and take twice as long, so the new cure time is 2 x 60 = 120 minutes. However, you’ll have lots of time to work with it, because your 10 minute gel time will now be 20 minutes.
Knowing this relationship, there are some “tips and tricks” than can be utilized. Let’s say you’re working on a cold day, but still need a fast cure. Well, if there is some way that you can heat up the ambient area by just 10 C (or 18 F)—you will cure in half the time. You might use a heat gun, you might use a tented-in area—- it may be as simple as doing the job in full sunshine rather than a shaded area. The same concept might apply in Houston on a sunny day in August—if you can shade your job and reduce the temperature by 10 C you get more working time. Other tricks: use an Igloo cooler to keep your containers of Part A and Part B cooler (or warmer) than your jobsite until you need to mix them. Your installer will appreciate the adjustment in working time.
So this rule of thumb is fairly accurate with most 2-part adhesives like epoxy and urethane; and it’s even somewhat predictive for other chemistries like polyesters and acrylates. Once you get to extreme temperatures, though (over 120 F, or below freezing) all bets are off— the thermodynamics of the reactions change dramatically, and our rule of thumb is not so good anymore. But who’s working with glues when it’s 20 below or 140 F anyhow?!