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A day after painting a closet, I set a piece of wood on the wall of the closet that was painted a day ago as well. Within minutes, both of their layers of paint fused together (please correct me if there is a better term than 'fused').

I'm assuming that the paint approaches a state of being a solid, but the process is never stopping, (as I've heard:) similar to how concrete is much stronger thousands of years after it is made rather than a couple of years.

So I understand that it started off as a liquid with a high viscosity, and liquids with high viscosity's turn into a solid when spread thin, but why does that even happen. As well, was I right when I explained that it is constantly 'drying', explaining why it was able to fuse with the other layer of the same paint even a day later?

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Ullmann’s Encyclopedia of Industrial Chemistry lists numerous components of commercial paints:

  • dyes, pigments, extenders
  • organic solvents, water, coalescing agents
  • binders
  • plasticizers
  • paint additives (levelling agents, film-formation promoters, wetting agents, dispersants, antisettling agents, antifoaming agents, catalysts, antifloating and antiflooding agents, antiskinning agents, matting agents, neutralizing agents, thickening agents, preservatives, corrosion inhibitors)

The application method, drying and hardening behaviour of a paint is essentially determined by the binder. Binders are macromolecular products. The low molecular mass binders include alkyd resins, phenolic resins, polyisocyanates, and epoxy resins. These binders must be chemically hardened after application to produce high molecular mass cross-linked macromolecules in order to form acceptable films. The higher molcular mass binders include cellulose nitrate, polyacrylate, and vinyl chloride copolymers, which are readily suitable for film formation.

As the paint dries, a firmly bonded film is formed. Drying and film formation take place physically and chemically. Physical drying involves evaporation of the solvents (organic solvents and/or water). This is the main process for paints with high molecular mass polymer binders. Chemically drying paints contain binder components with a relatively low molecular mass that react together on drying to form cross-linked macromolecules. This process can occur by polymerization, polyaddition, or polycondensation.

In practice, physical drying and the various mechanisms of chemical drying can proceed concurrently or consecutively, depending on the composition of the binder system.

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