The factors affecting the curing speed are mainly the type of coating, coating thickness, curing method, curing conditions, curing equipment and specific curing procedures. (1) Coating type
Under the curing conditions, the type of coating is different, and the curing speed is very different. In general, volatile paints cure quickly and oil paints cure slowly. The situation of polymeric paints is very different. Among the polymer paints, the photosensitive paint cures the fastest, while other polymer paints are mostly between the volatile paint and the oil paint. When using the mechanical assembly line for painting, the volatile paint and the acid-cured amino alkyd paint are more commonly used.
(2) Coating thickness
During the finishing process, the coating is not formed at one time, usually by multi-pass thin coating method ( For example, oil paint is usually applied at a time of about 35 μm, and nitrocellulose is about 15 μm. Under the same curing conditions, the thin coating has less internal stress and less coating defects when it is cured. When the coating is too thick, the internal stress is large, which is prone to wrinkles and other defects, and at the same time due to solvent evaporation. The coating shrinks, resulting in uneven gloss, internal curing, and the like. It has been proved that, besides polyester paint, the paint film formed by multiple paints of other paints has better physical properties than the paint film of the same thickness formed by one finish.
(3) Curing conditions
1 – Curing temperature The curing temperature has a decisive influence on the curing speed of most coatings. When the curing temperature is too low, the solvent volatilizes and the chemical reaction is sluggish, and the coating is difficult to cure. Increasing the temperature can accelerate the evaporation of the solvent and evaporation of water, accelerate the oxidation reaction of the coating and the thermochemical reaction, and accelerate the curing speed of the coating. However, the temperature cannot be increased indefinitely because the temperature is not proportional to the curing speed. When the curing temperature is too high, the curing speed is not significantly improved, but the paint film is yellowed or the color is darkened. Not only that, the temperature also affects the substrate during the curing of the coating. The substrate is heated, causing a change in moisture content, and the substrate undergoes shrinkage deformation and even warpage and cracking. In the volatile paint coating, when the curing temperature exceeds 60 ° C, the solvent is volatilized fiercely, the surface layer is quickly dried, and the internal solvent vapor is likely to generate bubbles when it reaches the surface layer. Therefore, when the artificial curing method is employed, the surface temperature generally does not exceed 60 °C.
2-Air humidity The air humidity should be appropriate. If the temperature is too large, the evaporation rate of the water in the coating is lowered, and the solvent volatilization rate is slowed, thereby slowing down the curing speed of the coating. Most coatings are most suitable for curing in air with a relative humidity of 45% to 60%. If the air in the dry curing place is humid, not only the curing process is slow, but also the formed film is unclear and other defects occur. When the relative humidity is high, the coating is prone to ‘whitening’.
Due to the oily paint, when the relative humidity exceeds 70%, the effect on the curing speed of the coating is significantly greater than the temperature.
The internal stress generated by the coating during curing is related to its relative humidity. Table 5-1 lists the shrinkage stresses produced by various coatings after curing at different relative humidity.
3-Ventilation conditions The coating should be cured when it is cured to allow proper air circulation on the surface of the coating. Disposal of solvent vapor in time and increased air circulation can reduce curing time. Improve curing efficiency and ensure curing quality.
a. Airflow speed. Air circulation is beneficial to the curing of the paint film itself. In a closed environment with a high solvent vapor concentration, the paint film cures slowly or even does not dry. Ventilation facilitates the volatilization of the solvent of the paint film and the elimination of solvent vapors, and ensures the safety of the natural curing place.
Forced air drying chamber is caused by ventilation to circulate hot air, and its drying and solidifying effect depends largely on the air flow speed. The higher the air speed, the better the heat transfer effect. The better the heat transfer effect, the hot air drying usually uses a low air flow rate, generally 0.5 to 5 m / s, and the temperature is 30 to 50 ° C.
b. Airflow direction. The direction of air flow is also very important. The direction of the airflow and the direction of the coating are parallel and vertical. According to the experiment, the hot air drying of the vertical air supply is superior to the parallel air supply.
4-External conditions are used to polymerize and cure film-forming coatings whose coating cures into a complex chemical reaction process. The curing rate is related to the nature of the resin contained, the amount of curing agent and the amount of catalyst added. External conditions, such as temperature, infrared, ultraviolet, and electron rays, can often accelerate the reaction. The magnitude of the external conditions depends on the extent to which the external conditions are compatible with the properties of the coating. When the photocurable coating is irradiated with ultraviolet light, it takes only a few seconds to form a film. If infrared or other heating methods are used, it is difficult to cure or even cure. Another example is an electron beam curing coating, in which the coating is cured faster than the photosensitive varnish under the electron beam emitted by the electron accelerator, while other coatings do not react strongly to the electron beam. Therefore, the coating curing method should be reasonably selected according to the nature of the coating used.
(provided by Shenyang Building Materials Network) <
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