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Rosanna Briscoe, Property Management Representative, of Kelly Moore Paints. For more information, you can reach her at [email protected], or
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Sometimes, the painter will think that this is just a visual effect in the paint and declare the job done. The problem is that the film is not continuous and will allow water to penetrate, causing premature peeling and flaking in the ensuing months. The reason for this phenomenon is that most water-borne house paints are emulsion paints, where the resin particles are separated from each other in the paint can by the water. This enables them to move around easily and gives the mixture a low viscosity so the paint can be transferred from the can to the wood, metal or other substrate by spray, brush or roller. When the paint has been applied to the surface, the water evaporates, allowing the resin particles to come closer together and “reach out and touch each other.” This “touching” process requires a certain amount of energy to accomplish. The energy can come from the sun or from a heated house, but if this is not available, the resin particle essentially doesn't have the energy to blend with its adjoining resin particles.
The glass-transition temperature Resin molecules are shaped like long strands of spaghetti, (or worms, or snakes) that give the paint film strength by wrapping themselves around each other and overlapping, creating friction that makes it difficult to pull them apart once they have intermingled. There is a certain temperature, at which the paint film has enough energy to do this, and it is called the TG (T-sub-G) or “glass transition temperature,” At temperatures below the TG, the film will crack because the resin molecules haven't coalesced or “interpenetrated.” Above the TG, the paint film will flow out and the resin molecules will possess enough energy to penetrate adjoining resin particles, causing greater adhesion and cohesion in the film and creating a continuous blanket to keep the water out.
Paint-formulating companies can vary this TG by the addition of solvents and additives, but their quality-control test usually is done in a 40° F refrigerator, where the sample is left to dry overnight. If the paint film is continuous with no cracking, it is deemed satisfactory to put in the can for sale. The reason the manufacturer puts 50° F on the paint-can label is to provide a safety margin of 10 degrees. Often in cooler weather, the surface temperature of the house or garage is less than 40° F even if the surrounding air is 50° F or greater. The cold night air will cool the house and garage down to 35° F, let’s say, and due to the heat capacity of the house, it will warm up to 40° or 50° the next day. Of course, the house will be heated to 70° F or 80° F inside, so during the daytime its walls will warm up to 40° F quickly, being heated from inside and out.
The unheated garage, meanwhile, will stay cold longer, inside and out. This means that the garage wall might be painted with the temperature at 35° F, which is below the 40° F TG of the paint film. As a result, cracks will appear in the paint film due to low coalescence. The paint film on the inhabited part of the house will look great, however, leaving the painter to theorize that maybe some contamination on the surface of the garage walls was to blame. Remedy: Take the wall’s temperature in cold weather. The surface temperature should be taken with an infrared thermometer to make sure the film will coalesce properly.
Painting when the temperature and weather conditions are right is always a smart move.