Too much hardener in the lake. What can happen?

I'm going to turn again a discussion I had with a collaborator into an article. The discussion was about the consequences of not respecting the mixing ratio for polyurethane varnish used in furniture finishing. As the principle is the same for most A+B products, be they industrial or DIY products, I think that presenting the consequences is useful for everyone.

The A+B principle is used for several wood products (and not only): varnishes, adhesives, glues. Basically, it is a matter of 2 components which, when mixed, react to form a third product, totally different in properties from the first two. In the case of the polyurethane products mentioned above, the reaction between components A and B leads to the formation of the varnish film.

Being a chemical reaction, the ratio between the 2 parts is well studied and determined, so that when they are mixed, they react, the molecules bind to each other and turn into a polymer with very good surface properties - the varnish film.

But what happens if the report is not respected? One side will remain free, unreacted and this can influence the properties of the resulting film. The same is true for adhesive or putty.

Now, don't think that adding 52-55% instead of 50% will be a big tragedy. As they know that their products are not used in pharmacies, all manufacturers provide a safety margin so that if there is no high-performance measuring equipment, the products can still be used. Problems become noticeable after 15-20%.

And what can happen to the lake? If there is too much hardener in the mix, the film can become brittle. In contact with water, it instantly hardens, forming a brittle solid. Excess hardener left unreacted can come into contact with moisture in the air. Thus, as the spent catalyst is embedded in the varnish film, it can also become brittle.

If, on the contrary, the varnish is in excess, the unresponsive part remains soft and will transfer this property to the varnish film in which it is trapped. The film becomes spongy, soft, without mechanical and chemical resistance, i.e. without precisely those properties for which a polyurethane varnish is chosen.

What happens to the polyurethane varnish if the catalyst (component B) is not added at all? It will remain soft and sticky. If the catalyst is applied on top, the whole film will crack. All you can do is remove the layer of varnish with thinner and start again from the beginning, mixing correctly. There are varnishes, however, that even if you don't add catalyst, they still harden. These are products where the catalyst percentage is low, 10-20% and where the catalyst is added to increase certain properties of the film (mechanical strength, chemical resistance) and shorten the drying time. The products will dry even if the hardener is not added, but in a longer time.

Before using A+B products, carefully read the instructions on the data sheets in the case of industrial products or on the packaging in the case of DIY products and, above all, follow them. They are not there for no reason.