Rambling Thoughts – Temperature, Humidity, and Adhesives

I’m going to go really far out on a limb and touch on a subject I have a little armchair knowledge on. I am not and should not be treated as an authority, really, but I always thought it was an interesting subject because I had never really stopped and looked at what “relative humidity,” the most often discussed form, actually was. For anyone interested in a more detailed explanation, check out the ideal gas law. That’s the mathematical model of what’s going on at the chemical level.

My interest stemmed from working with adhesives. In particular, I was trying to describe and identify factors in the variation of curing time. I’m of the opinion that manufacturing and industry largely thrive on consistency and predictability. Variation creates inefficiency and acts as a barrier to automation, which like it or not, is our best ticket to getting humans away from the damaging and dehumanizing nature of repetitive work.

While I will not bother to identify the specific product in question, it is a two part silicone product with a relatively inert base resin and a hardener with a rather intense ammonia quality to it. It’s extraordinarily thick. Somewhere in the 60k-90k centipoise (cP) range. Your average ketchup is in the 50k-70k range. It’s also quite heavy, about one and a half times the weight of water.

It was observed that cure times varied significant between summer and winter in our region. I live in “the South” where summers tend to be hot and humid and the winters are typically cool and dry. Actual trials in controlled conditions showed that both temperature and relative humidity had an impact on cure time. Not a huge surprise, many chemical reactions proceed more quickly with more available heat energy to speed it along, and the product was stated to have a “moisture cure” component to it as well. It seem reasonable to assume that more available water vapor means faster curing.

The typical working time in the summer could be 1-3 minutes, with about 1 1/2 being typical. In the winter It was usually 2-4 with 2 1/2-3 being normal. I realize a minute doesn’t sound like a lot, but if you’re using the product in an application where it can be seen, it can be the difference between a nice smooth finish and the Rocky Mountains meets Grand Canyon.

Now, it’s not realistic to expect to control the actual climate. Evidence indicates that as a species we’re not even committed to preserving it, much less controlling it. You can control for these things by adjusting the working conditions, but you need to know which adjustment to make and how far to turn the dial.

When I tried to understand “relative humidity,” thought, I quickly learned that temperature is an important factor. That method of humidity doesn’t tell you how much actual water vapor exists in the air, but rather how much it’s holding relative to its current maximum.

My favorite analogy has always been a glass of water. Mathematically we’re usually talking about a volume of one cubic meter, so let’s say that cubic meter of gas is an empty glass that can contain water. If I have a 20oz glass, and it contains 10oz of water, we would say that it’s half full. For this example, it’s at 50% relative humidity. The glass contains 50% of the maximum amount of water that it can hold.

What makes this unique is that as heat increases, the “air” in our cubic meter gets thinner, creating more room for other things like water vapor. In my example, this means that we have a larger glass. We’ll say the new warmer glass as a 30oz capacity. If I pour my 10oz of water into it, the actual quantity of water hasn’t changed, but the relative humidity has. The warm glass is only 33% full. We have a relative humidity of 33%.

This meant that a cold winter day with high humidity (something we don’t tend to get much of locally) still has less available water vapor than a hot summer day with low humidity. The question it left me with was what impact did the actual humidity, the literal quantity of water vapor, have on the curing of the adhesive?

Unfortunately I never gathered sufficient data to answer this question. My understanding of the mathematical model I needed to test it was critically flawed, which meant my actual humidity humidity numbers weren’t usable. Not exactly a satisfying ending to the story, but I’ve always appreciated what I learned in the process. Maybe you will too, I hope.

Y’all take care. Watch out for that humidity.


Hey, it’s Blaugust time! The goal is to simply promote and stimulate the blogging community by encouraging people of all skill levels and backgrounds to post. The official post can be found here and it’s never too late to start.

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