How to evaluate the role of paint defoamers for solvent based systems?

The evaluation of defoamer cannot be based on the final judgment of the effect of the freshly formulated paint after construction. The paint should be done after rapid storage for 1 month (60 ℃ oven), and then take out the construction, still have the effect of defoaming and construction does not produce defects in the paint film, and just formulated the same effect, then to show that this defoamer effective, or natural storage for six months or a year to test the surface effect of defoaming and construction.

The stability of defoamer in the system is important. In solvent-based wood coatings may produce defects due to improper selection of defoamer, such as in glossy varnish due to poor compatibility of defoamer and lead to a decline in gloss, glossy varnish in the film fogging and thus affect the transparency, micro-fine ptfe powder some produce shrinkage phenomenon.

Generally speaking, foam is a coarse dispersion of gas in liquid, which belongs to gas-liquid non-homogeneous system. The bulk density is close to the gas and not close to the liquid gas-liquid dispersion. Gas-liquid dispersions are divided into “bubble dispersions” with more liquid and less gas, and “foam” with more gas and less liquid.　　What is a foam? A foam can be defined as a stable gas in a liquid medium. Foam is a gas in the liquid dispersion system, the gas becomes many bubbles separated by the continuous phase of the liquid, the gas is the dispersed phase, the liquid is the dispersion medium.

Foam is a thermodynamically unstable system, which cannot be stable. The thermodynamic instability of foam is due to the fact that the total surface area of the liquid in the system is greatly reduced after the bubble is broken, and thus the energy of the system (free energy) is much reduced.

The Marangoni effect prevents the discharge of the bubble film and restores the bubble film thickness. The bubble discharges gas into the air and the bubble bursts. Factors affecting this process are the apparent viscosity and consistency of the bubbles which affect the diffusion of defoamer particles on the bubble surface film.