Good film leveling requires a long enough time to keep the viscosity at the lowest level, and the film is fully leveled, but this often leads to sag problems.
In the past, the pigment flocculation method was used to obtain thixotropy of the paint, but the degree of flocculation is large, and the hiding power, gloss, fluidity, and leveling of the paint are deteriorated, and the pigment is usually controlled to be in a slight flocculation state. Pigment flocculation has now been replaced by the use of rheology additives.
Rheological additives can achieve a proper balance between sagging and leveling, that is, the viscosity of the coating temporarily decreases during construction, and the viscosity gradually increases slowly during the viscosity recovery period, so that the coating film has time to level off. After leveling, the viscosity of the paint film is very high, which prevents sagging. Effective rheological additives can properly control the viscosity recovery speed, make the paint film leveling well, and effectively control sagging.
When spraying solvent-based coatings, as long as the solvent volatilization rate is controlled and the Spray Gun is used correctly, sufficient leveling can be achieved without sagging. Thermal spraying is beneficial for controlling sagging. When the paint mist particles hit the cold surface of the substrate, the temperature drops, the viscosity increases, and the sagging is reduced. The use of carbon dioxide spraying under supercritical conditions is particularly helpful in controlling sagging, because the paint mist almost completely flashes off the carbon dioxide on the way, so the viscosity increases greatly. High-speed electrostatic rotary cup can be applied with higher viscosity paint, so it also helps to control sagging.
The solvents used in brush and stick coatings evaporate slowly, the coatings need to be thixotropic, and rheological additives need to be added. Latex paints are thixotropic and sag less than solvent-based paints.
(1) Sag of high solid content coating
There are three reasons for the slow evaporation of solvents in high solids coatings:
①Because of the high surface tension of the paint, the diameter of the paint mist is larger than that of the conventional paint, the surface area to volume ratio is smaller, and the solvent volatilization is less. However, the operator can obtain the same atomization by adjusting the spraying equipment and conditions .
②The resin molecules of the high-solid coatings are the lowest and the concentration is high, so the ratio of the number of solvent molecules to the number of resin molecules is lower than that of conventional coatings, which reduces the volatilization speed of the solvent.
③The Tg of high-solid coatings changes faster with the concentration. After the solvent volatilizes slightly, it reaches the stage where the volatilization speed of the solvent is controlled and diffused, so that the volatilization speed is greatly reduced.
High-solid coatings are prone to sag due to slow solvent volatilization, not only that, but also prone to drying tunnel sag (also known as thermal sag). Because the viscosity of the high-solid paint enters the hot drying tunnel, the decrease in viscosity is larger than that of conventional coatings, and the wet film is fine before entering the drying tunnel, and sagging occurs in the drying tunnel. The partition of the drying tunnel can control the thermal sagging to a certain extent, the low temperature zone makes the solvent volatilize more time, and some crosslinking may also occur, and the wet film reaches a higher solid content and molecular maximum before entering the high temperature zone.
Adjusting solvent composition is not sufficient to control sag in many high solids coatings and requires the use of thixotropic agents such as fine-grained silica, bentonite clay or polyamide gels treated with quaternary compounds.
A small degree of sag does not appear on the white paint film, but it will affect the orientation of the metal flakes in the paint film, which is very obvious on the paint film of metallic paint. Therefore, the sag problem of high solid automotive metallic paint is particularly special. serious. Sio2 cannot be used in this coating to obtain thixotropy, because the scattering of Sio2 affects the flop of the paint film, and acrylic microgels are usually used to form swollen colloidal particles to obtain thixotropy, the refractive index of the microgel Very close to cross-linked acrylic polymers, no interference with flop, and microgels also improve film strength.
(2) Sagging of water-diluted paint
The solvent for water-thinning resin coatings is a mixture of water-organic solvents. Water-ethylene glycol butyrate is used as a water-diluted paint as a solvent. Under a certain RH (relative humidity), the relative volatilization speeds of water and ethylene glycol butyrate are equal, so that the composition of the residual solution remains unchanged. This RH is called critical relative humidity (CRH). The water-diluted resin is soluble in organic solvents but not in water. The concentration of ethylene glycol butyrate in the resin particles is high. The concentration of ethylene glycol butyrate in the continuous water phase is actually low, and the proportion of water is high, while the volatilization is the water phase. Therefore, the CRH of a paint containing 10.6% ethylene glycol butyrate is 65%, because there is more water to be volatilized; the CRH of the same proportion of water and ethylene glycol butyrate mixture without resin is 80%.
When the water and solvent evaporate, the water evaporates quickly, and the organic solvent is enriched, which makes the viscosity of the paint drop and sag.
