Making The Transition: Coalescing Aids For Latex Paint
To understand the function of coalescing aids, it is necessary to examine the film formation process of a latex paint (see Figure 1). The emulsified polymer that will ultimately form the paint film exists as tiny droplets of solid material dispersed in water. On a porous substrate, some water and - depending on solubility - some coalescing aid will migrate into the substrate. As the amount of water decreases during the drying process, the latex particles are forced closer and closer together. When the repulsive forces on the droplet surface can no longer keep the particles separated, the emulsion collapses, and the system solidifies.
The ability of a polymer to form a film is a temperature-related phenomenon. Each polymer composition has a characteristic temperature below which it can no longer form a film. This is called the minimum film-formation temperature (MFFT) of the polymer. This property is normally measured using a MFFT bar apparatus according to ASTM Method D 2354. Polymers with low MFFT values form films easily, but the films have undesirable properties for paint applications such as low durability or high surface tack. The polymers generally used in paint formation have MFFT values in the range of 15-45˚C. These emulsions form durable, nontacky films when dried above the MFFT; however, in the hands of a painter, latex paint may have to be applied at temperatures approaching 0˚C. To form a good film at temperatures below the MFFT of the base emulsion, it is necessary to add a coalescing aid to the system.