Лако-красочные материалы — производство

Hydrophobic solvents decrease the MFFT most efficiently and increase the hardness of the coating due to improvements in film formation. They also penetrate into the shell and swell the polymer, increasing the viscosity dramatically. Coalescing solvents distribute throughout the three phases, water, polymer shell and polymer core, in differing volumes. Optimum film formation takes place when the coalescent is present in the shell of the polymer.

The most stable systems are obtained with the minimum volume of hydrophobic coalescent, but this volume should be sufficient to retain good film formation under adverse conditions. The hydrophobic coalescent should be coupled with the water soluble type and pre-blended with water to prevent problems such as polymer “shock” during addition. An extremely efficient hydrophobic coalescent is the ester alcohol, Texanol from Eastman, best utilised where stoving facilities are available since there is a tendency for it to be retained in the film. Being insoluble, Texanol is preferentially absorbed by the polymer particles. This not only provides better coalescence at a lower level, but also prevents rapid release from the surface, thereby aiding more complete film formation. Texanol’s ability to produce a continuous, fully coalesced, film often provides an improved barrier when resistance to the elements is required.

Clear coatings coalesced with diethylene glycol monobutyl ether demonstrate improved flow and wetting characteristics, especially on plastics. The use of coalescents such as butyl diglycol acetate should be avoided, as they are susceptible to alkaline hydrolysis.

Recent developments have seen the release of a coalescing solvent that is classified as a non-VOC for paint applications (B. Pt. 250°C). This could well be the answer to a formulator’s dream, since many acrylic polymer latices with high Tg require levels of coalescent beyond the compliant limits. A listing of some coalescing solvents is included in Table 7-8.