Drying is one of those processes that come up repeatedly in any discussion about adhesives but isn’t always explained clearly. This can lead to confusion over terminology, which leads to poor understanding of application practices, which will affect how well our project turns out (if we do things like not let our epoxy resin fully cure before moving onto the next step). So let’s dig into some background information, and then we’ll be able to troubleshoot any problems.
What is drying?
Drying is the process by which water evaporates from an adhesive, whether it’s wet paint on a wall or two pieces of cured resin stuck together with an epoxy adhesive. Drying happens because the adhesive’s solvents have evaporated.
What is curing?
Curing is a chemical reaction caused by a resin that causes it to harden into a solid. Without this, it would remain a liquid forever. Eventually drying out, though, but not having any real structural stability until completely dry. Curing happens because of the nature of epoxy resins and other resins used in adhesives, which undergo an exothermic (heat-producing) chemical reaction when the mixture is complete. This means they start cooling at this point due to their inner energy production, so if you do slow experiments like submerging an epoxy resin in water, you can see it start to solidify on its own after some time due to the chemical reaction.
What does humidity have to do with it?
Humidity can be difficult, so this section will break it into something more understandable. An excellent way to understand humidity is thinking about steam in terms of molecules rather than water vapor since most adhesive chemists are concerned with humidity. Steam comprises highly tiny water particles called “molecules,” which are atomically small but can easily be visible in large numbers if you have enough of them together in an enclosed area (think clouds). Another analogy is that air or gas itself is mostly space. Imagine lots of “air space” between each molecule instead of full solid objects everywhere, where a few pieces of matter are stuck together by the forces holding them together, but not so tightly that it would be as rigid as a solid.
Imagine that these water molecules in the air are bouncing around everywhere, which they always are even though they’re invisible most of the time. Sometimes one will bounce right into an adhesive molecule and stick there for a while. Eventually, the steam molecule will be thrown off again by another randomly moving steam molecule, or maybe simply because it was floating around aimlessly. It collided with something else (such as a piece of epoxy resin).
But this is important. Though some humidity molecules will come from water evaporating from our adhesive resins, other moisture molecules can form when these humidity molecules hit one another and make a new larger humidity particle.
How does this affect the adhesive?
This is important to understand. Let’s say our adhesive is wet with water. But it hasn’t started the curing process yet (no heat production). Then you set it out somewhere with high humidity, so more of these water vapor molecules are floating around -a lot of them- which means more chances for them to hit our adhesive and stick. This increases their chance of sticking overall, which increases the chance that the resin will cure faster (due to more water molecules sticking) and become more rigid (because of the chemical reaction).
But curing isn’t affected by humidity. It’s only increased because more moisture molecules are hitting our adhesive. So if you do stick our wet adhesive somewhere with high humidity, its chances for curing will be increased. But if you stick it somewhere with low humidity, then any added chance of curing would also be offset by a lower overall amount of water vapor stuck to your adhesive due to less humidity in this environment.
In summary, there is no significant difference between drying vs. curing. The main difference is whether your adhesive is exposed to moisture condensed from the air (drying) versus formed from other humidity molecules stuck together (curing). In both cases, adding more moisture may increase the chance for the resin to cure on its own. The humidity happens to be easier to understand if you think about it in terms of steam molecules rather than tiny invisible particles.