Film-forming additives are important components of architectural coatings, which can improve the production process, improve product performance, enhance the performance of coatings, reduce environmental pollution, and develop new coatings with special features.
Film-forming additives, also known as coalescents and agglomerates, are usually high-boiling solvents that act as a “temporary” plasticizer to lower the glass transition temperature (Tg) of the polymer, and once the particle deformation and film formation process is complete, the film-forming additives will evaporate from the coating film, thereby restoring the polymer Tg value to its initial value. Typically, most film-forming aids evaporate 1-2 hours later than water at room temperature.
Therefore, film forming aids should consist of solvents with slower volatility. A big prerequisite for a film-forming additive is that the water evaporates during the drying process while the film-forming additive remains in the coating and it evaporates from the coating on its own afterwards. Usually applied to the coating latex have a low glass transition temperature (Tg). Such as VAE emulsions in a 3 ℃ or so, therefore. In most of the temperature higher than 5 ℃ conditions. These emulsions can be normal film formation, and the addition of film-forming additives, to accelerate the film dry solid played a role. Figure 1 shows the effect of glycol as a film-forming additive on the drying time of acrylic coatings. With the increase of glycol doping, the drying time of the coating film is also reduced.
In practical applications, the use of a single rheology modifier often fails to achieve the best balance of leveling and sag resistance. Usually two or more rheology modifiers are used together to achieve Better coordination between leveling, sag resistance, splash resistance and thickening effect. In addition, because most of the rheology additives are water-soluble, too much dosage will have side effects on the water resistance of the coating film. Therefore, the dosage should just meet the requirements of leveling and anti-sagging, and it is not suitable for multiple use.
Cellulose ethers and hydrophobically modified cellulose ether rheology additives generally need to be dispersed in water by shearing force before use, and it is best to use them in a pre-gel state. Alkali swellable acrylate copolymers and hydrophobically modified alkali swellable acrylate copolymers are anionic and micronized polymer wax are very sensitive to pH. Their application effects are related to the pH of the system and are effective in the range of pH 7.5-10.
When using, it can be diluted with several times of water and then added to the system slowly under stirring, but it should be noted that the adding speed should not be too fast and the pH value of the system should not be too high, otherwise it will cause uneven thickening or even agglomeration. After mixing, the pH value of the system is increased to 8~9. The hydrophobically modified polyurethane is non-ionic, and its application effect is not affected by the pH value. It can be slowly added to the system under stirring, but the speed is not Too fast, otherwise it will cause flocculation. Table 8-5 lists some typical waterborne coating rheology modifiers.
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