1. Sag theory (paint sag equation)
The flow mark caused by the wet coating film driven by gravity is called sag. The sag model is shown in State 8-l3. The tension (ie shear stress X) caused by the thin layer of x bit area in the coating film is:
Obviously, Newtonian coatings have poor sag resistance, and the sagging becomes more serious as the film thickness increases. But in fact, the coatings are all pseudoplastic, and the temperature decreases and the concentration increases due to the volatilization of the solvent during the drying process. .cause the viscosity to rise; at the same time, it also makes the coating film thinner. All these factors can inhibit the sag. However, the viscosity, as the sag resistance, is always the main factor that should be considered in the formulation design, usually adding a small amount of rheological additives to the In Newtonian coatings, make the viscosity at low shear rate reach tens to hundreds of Pa·s or give the coating a yield value that resists sagging.
The effect of hand viscosity on leveling and sagging is opposite. Changing the viscosity can only adjust the leveling and sagging to an appropriate level. The effective solution to the contradiction between leveling and sagging can only be given to the system by using thixotropy. In order to obtain better leveling and sag resistance.
High-solid coatings, because the solid content is as high as 70% (mass), the solvent volatilization is less [about 5% (mass)] during the spraying process, and the ordinary solvent-based coatings, the construction solid content is about 30% (mass), and the solvent volatilization during the coating process The loss is as high as about 5o% (mass). Obviously, the high solid content coating does not have the viscosity increase caused by solvent volatilization and the thinning of the coating film. The best way is to form a primary anti-sagging yield value.
2. Hanging prevention measures
From the sagging equation, it can be seen that the main factors that cause sagging are the thickness of the coating film and the viscosity of the coating. Among them, the viscosity of the paint is also changed by temperature, solvent volatilization and denaturation of the paint. The root causes and countermeasures are described below.
(1) The reason why the sprayed wet film is too thick (related equipment: Wet Film Applicator)
①Repeated spraying for many times;
②The distance between the Spray Gun and the workpiece is too close;
③A spray is too thick, such as the spray inspection moves too slowly or stops.
Solution
①During wet-on-wet construction, there should be sufficient drying time between two coats of spraying, and the flash-drying time for thin coatings should be 3-5 minutes. As the number of sprays and the total thickness of wet film increase, the drying time should be extended to 5-8 minutes min; (related instrument: drying time Tester)
② Ensure that the Spray Gun and the workpiece are vertical and the distance is constant;
③The moving speed of the spray inspection should be uniform, avoid stopping, and avoid shooting and returning to the workpiece
(2) The viscosity of the paint is too low. Strictly control the dilution ratio and dilute to the specified construction viscosity according to the paint variety. (Related instruments: viscometer)
(3) The solvent volatilizes too slowly. For example, if ① the thinner is not properly selected;
(4) Spraying on the smooth old paint film The paint has poor wettability to the surface, and it is easy to cause field fall, so the old paint film should be polished and roughened in advance. (Related instruments: adhesion Tester)
