29/03/2024
Selection and Use of Coalescing Agents by J H Bieleman.
Introduction Polymer dispersions, used as binders in dispersion paints, consist of particles finely distributed in water. The layers of surfactant and/or colloid, present on the surface of the particles, act as stabilizers, preventing coagulation and provide stability in the dispersed stage. The particles have diameters of about 30 to 500 nm, or even higher in case of course, opaque dispersions. The film formation process, after application, involves evaporation of the solvent and formation of a continuous polymer film by purely physical means.
The film formation of polymer dispersions can be divided into 3 steps:
1. Concentration
2. Compaction
3. Coalescence
After application of the polymer dispersion or the dispersion paint, containing polymer dispersion, a volume reduction occurs, as the result of evaporation of water and solvent as well as absorption by appropriate substrates. The decrease in volume forces the particles to move closer together and finally the repulsive forces -as a result of adsorbed stabilizers- is overcome and there is a direct contact between the particles. The total solids content of the film at this stage is typically around 70 to 80%. After the stabilizing system has collapsed, film formation is irreversible. Next, the particles are forced to deform, mainly by capillary forces, until ultimately there is a homogeneous, coherent film (coalescence; fig 1). The total solids content at this stage is typically around 90% and slowly increasing, approaching 100%.
However, in order to accomplish full coalescence, deformation resistance forces, associated with the polymer hardness (Tg: glass-transition temperature), have to be overcome. Hard particles are unable to take part in coalescence. For any polymer dispersion there is a typical limiting temperature above which the particles are so soft enabling capillary forces to overcome deformation resistance and allow full coalescence. This temperature is termed the minimum film-formation temperature: MFFT and can be determined by a MFFT bar (temperature gradient plate).
