Waterborne coating construction technology and operation skills
Text / Yang Wenliang
The water-based paint coating process is basically the same as the solvent-based paint, but the equipment selection materials and process parameters must be selected according to the characteristics of the water-based paint. Its typical process is:
Pretreatment [Upline→Pre-degumming→Degreasing→Washing 1→Washing 2→(Deionized Water Washing 1)→(Table Adjusting)→(Phosphating)→Washing 3→Washing 4→(Deionized Water Washing 2)→Artificial Water Blowing→ Drying→→Strong cooling→painting→leveling→preheating period→drying→cooling (strong cooling)→offline
1, pre-processing
Since water-based paint uses water as a diluent, the surface tension of water is 72.6dyn/cm (dynes/cm), which is 2.5 times larger than that of common organic solvents, and is 2.5 times that of common organic solvents for coatings (see table below), so it is difficult to wet. The surface of the steel drum is prone to shrinkage, so on the one hand, by adding a surfactant to the aqueous coating production formula to reduce the surface tension and improve the wetting property, on the other hand, when the workpiece is pretreated, the surface of the workpiece is treated. Higher quality requirements.
Surface tension of organic solvents commonly used in coatings
name Surface Tension
Dyn/cm
name
Surface Tension
Dyn/cm
Ethyl acetate
26.29
Butanol
27.18
Propyl acetate
26.60
acetone
26.26
Butyl acetate
27.55
Amyl acetate
27.66
O-xylene
32.51
Ethanol
24.05
Meta-xylene
31.23
Ethyl propionate
26.72
Paraxylene
30.69
Methyl acetate
27.95
Butyl acetate
26.55
Toluene
30.90
2. Preheating before spraying and strong cooling after drying
The latent heat of evaporation of water is 2547.22kJ/kg, while the latent heat of evaporation of common solvents: 343.2kJ/kg for xylene, 590.2kJ/kg for butanol, and 343～444kJ/kg for ketones, which are much lower than water. Therefore, spraying the same thickness of the coating film on the workpiece to lower the temperature of the water-based paint is larger. Therefore, it is better to strongly cool the steel drum after drying, and generally use the method of cold air blowing to accelerate the evaporation of water vapor.
Due to the latent heat of the water-based paint, the heat absorption ability and the surface tension are large. For steel drums, when the requirements for monochrome spraying or decoration are generally not high, usually when the ambient temperature of the spray shop reaches about 30 °C, there is no preheating before spraying. However, for two-color or multi-color spraying, or when the decorative requirements are high, it is necessary to preheat the steel drum before spraying. Especially in the northern part of China, the cold season is even more so. Generally, the preheating temperature of the barrel before spraying is not lower than 70 °C.
3, spray paint
The general spray gun used for spraying water-based paint is the same as the solvent-based paint, but the requirements for environmental conditions (mainly temperature and humidity) are different from those of solvent-based paint. When the water-based paint is sprayed in the spray booth, the water becomes 1244 times the volume expansion of the water vapor, which can instantly increase the humidity in the spray booth. When the humidity reaches 90% or more, the coating film applied to the object will flow down.
Therefore, it is important to control the humidity of the spray booth when spraying water-based paint. The temperature should also be kept within the range of no sag and runny. It is generally considered that the temperature is 15-30 ° C and the relative humidity is 60-80%. If it is lower than 15 ° C, film formation will be affected. Since water is more corrosive than solvent, the spray booth, especially the circulating water treatment system, is preferably made of stainless steel. The air supply in the paint booth must have an appropriate temperature and humidity control device.
4, preheating in the early stage of drying
The preheating period is the leveling of the paint film after painting and the pre-evaporation process of water. The purpose is to prevent the wet paint film containing a large amount of moisture (the water content of the water-based paint is about 63%) suddenly entering a higher temperature (> 100 ° C) drying oven. The specific parameters of the preheating time and temperature should be determined according to the performance of the water-based paint used, the quality requirements of the steel drum coating and the temperature required by the drying furnace. Under normal circumstances, the temperature is set at about 80 °C.
The length of the warm-up period is mainly affected by the chain speed. The preheating period is short, the moisture in the coating is not volatilized in time, and defects such as pinholes and scorpions appear after baking at high temperature; the preheating period is long, the surface of the paint film is too dry, and defects such as orange peel are easily generated. It is necessary to choose the warm-up period reasonably, otherwise it is prone to quality problems such as sag and air bubbles.
5, drying
Similar to the design of the spray booth, the inner wall of the drying room is also made of stainless steel. Since the latent heat of evaporation of water is large, it is 5 to 7 times that of a general solvent, and the heat capacity of the drying chamber is moderately increased. At the same time, the evaporation of water is slower than that of the organic solvent, so that the drying time of the water-based paint is long, and the temperature of the coating film and the coated object should not be too fast. When the water-based paint adopts the high-pressure airless spray process, the water-based paint coating film needs to pass the preheating period before it can be raised to the process temperature due to the high water content of the paint.
6, transfer paint system
Solvent-based paint transfer system can be made of carbon steel, and water-based paint will corrode and rust due to water. Therefore, the paint system of water-based paint must use stainless steel and should be chemically passivated. There is also a need for a highly thixotropic supply hose for aqueous coatings and a pump with sufficient processing capacity.
7, spraying equipment
It should be specially noted that when the electrostatic spraying process is adopted, since the water-based paint has a smaller electric resistance than the solvent-based paint, if a conventional electrostatic printing method causes electric leakage, a special electrostatic spraying device is required. Waterborne coatings are usually applied by direct (internal) charging and indirect (external) charging.
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