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Winter 2005 - Article 2

From The Lab
The Drying Phases of a Floor Finish

One of the many things we take for granted with a floor finish is that after application it will dry to an even, glossy, durable film. What many people don't realize is that the drying process of a coating has distinct phases. Understanding each of these phases will assist you in making better decisions of when to recoat when applying multiple coats of floor finish. We'll go ahead and explore each phase to bring more understanding to this fascinating process.

Floor finishes are composed of several components and when applied to a floor, form a continuous film at room temperature. These components fall into two major groups: non-volatile material (NVM) and volatiles. The NVM consists of polymer, resin, wax and sometimes urethanes - basically, anything left on the floor after it has dried. The volatiles include the water, co-solvents and anything else that evaporates in the drying process.

When you observe a dry finish, it is the polymer that is
responsible for the lion's share of what you see. Polymers under an electron microscope look like balls or spheres. The trick is to get all of these uneven, separated particles to form a flat, level, hard, glossy surface.

There are three distinct phases involved in the drying of a finish: set to touch, dry to touch and dry through. These three phases can actually be measured in the laboratory with special equipment.

Phase 1 (set to touch) begins as soon as the finish is applied to the floor. The volatiles immediately work their way up to the liquid-air interface (where the wet surface meets the air). While this is happening, the NVM starts to concentrate and come together. This goes on until most of the volatile matter has left the finish and a film begins to form at the surface. If you were to touch it at this point, you would feel the film (set to touch), but it would move as you moved your finger around.

Phase 2 (dry to touch) occurs as the polymer particles start to come in contact with each other. As this happens, they begin to deform in order to fill any voids among the particles. This deformation occurs more easily in soft polymers than it does with harder ones. If a finish is recoated at this point (too soon), it would not be surprising if blushing, streaking or other recoat phenomena occurred since there are still volatiles trapped in the finish.

Phase 3 (dry through) starts with the initial formation of a
continuous film. As the drying process continues to move down the film, small amounts of water and volatiles left behind continue to work their way out of the film along tiny channels remaining between the NVM (polymer) particles. The film is nearly 100% dried and from now on any water or solvent vapors escape through the polymer film via a process called diffusion. It is at this stage that the particles become homogenous and gain their mechanical properties and recoating can begin.

One final note; drying and curing are different phenomenon and curing takes place after drying. Depending on the finish, curing may take from two hours to 48 hours.