Tougher and tougher films with catalysts

We mentioned catalysts when we talked about polyurethane varnishes or those acrylic. They are not only used with these types of varnishes, they are also part of the reaction with other varnishes. With some materials they are only used to improve certain qualities and not as a condition for drying. Catalysts are not like a solvent but are part of the reaction, forming together with the base component the varnish film (polyester varnishes are an exception, but about that another time). That's why I think their role deserves a more detailed explanation.

I'll start by asking you not to be scared, because I'm going to talk a bit like in chemistry class. I know the word "chemistry" alone has already made you cringe, but please get over your phobia. I'll try to humanize it a little. Like I said, catalysts are part of the film formation process. The mixture between the two components A and B is so designed that the chemical reaction between them results in a totally new product - the varnish film - with different properties. If the mixing ratio is not respected, another product with different qualities results.

Take the example polyurethane varnishes. Between the components there is a chemical reaction called cross-linking. It's like people from 2 different groups shaking hands and getting caught up in a game. The game is calculated in such a way that each person binds to 2 other people in the other group. Now think of each person as having 4 hands (that's pretty much how reticulation happens) and linking with 4 people. Can you imagine the result? How well will each person be caught in the game and how hard will it be for this mass of people to be attacked from the outside? This explains the much higher hardness of a catalyzed film and its superior properties.

There is always a mixing ratio between the 2 components. If it is not respected, problems can occur. Going back to the above model, if the number of people is not respected there will be loose hands or even people left out, either on one side or the other. These loose hands can always get caught by something else and this can damage the game. When the number of free hands is small, even if they get caught on something else they cannot influence a table as large as the total game. If, however, free hands come to dominate, they can lead the game in directions that cannot be predicted.

That's pretty much what happens with the mixing ratio. At small variations the influence is also small. When the mix differs considerably from the recommendations, the excess part starts to dominate and often you cannot predict the outcome. Basically, excess lacquer means a soft or non-drying film, and excess catalyst means a more brittle, friable film. But no one can say exactly what other side effects may occur.

There are products that can be used both with and without catalysts. This is the case for some water-soluble materials, but also for nitrocellulose (combined nitro) or acrylics. In this case, the products also dry without catalyst, sometimes over a longer period of time. By adding catalyst, the degree of cross-linking is achieved which gives the film superior strength.

It should also be known that hardeners (as catalysts are also called) are moisture-sensitive substitutes. Continuing the point above, they play with water for fun, so when they find it they react immediately and turn into a solid, white and very brittle substance. It also reacts with moisture in the air, that in the form of vapour, which is why catalyst canisters should not be left open.

Calculate well how you will use the catalyst because even if the canister is kept closed, after a while it will still harden. The moisture in the air, which entered the can when it was opened, will slowly activate the catalyst and make it react. To avoid this happening to the catalyst in the original canister, the packaging is done in a nitrogen medium.

In the case of water soluble products catalyzed, a certain way of mixing must be respected to avoid problems. First add the dilution water to the base product, mix very well, then add the catalyst in a thin thread under mixing. This prevents the catalyst from coming into direct contact with the water and turning into small solid particles that can clog the pump or spray gun.

I really want this article to help you better understand some of the rules when finishing wood and I hope it wasn't too academic. What I want is for us to end up making the most beautiful furniture together and not to remind you of chemistry classes. 🙂