What are the factors and methods affecting the curing of coatings?
Coating curing is the process of coating being applied to the coated object and forming a dry coating film (including hard and soft film) by various means.
The factors affecting the curing speed are mainly the type of coating, coating thickness, curing method, curing conditions, curing equipment and specific curing protocols, etc.
(1) Coating type
Under the same curing conditions, the type of paint varies greatly in curing speed. Generally speaking, volatile paint curing fast, oil-based paint curing slow, polymeric paint is very different, polymeric paint in the photosensitive paint curing fastest, while other polymeric paint is more between volatile paint and oil-based paint, when using mechanized assembly line for finishing, volatile paint, acid-curing amino alkyd paint is more commonly used.
(2) Coating thickness
In the finishing process, the coating is basically not formed at once, usually using the method of multiple thin coats (such as oil-based lacquer is generally applied once about 35μm, nitrocellulose lacquer about 15μm, etc.). Under the same curing conditions, the thin coating in curing, the internal stress is small, the formation of less coating defects; while the coating is too thick, the internal stress is larger, easy to produce wrinkles and other defects, while due to the evaporation of solvent, the coating shrinkage, resulting in uneven gloss, internal non-curing, etc.. Practice has proved that, in addition to polyester paint, the physical properties of the paint film formed by multiple coats are better compared with the same thickness of paint film formed by one coat.
(3) Curing conditions
1- Curing temperature Curing temperature plays a decisive influence on the curing speed of most paint coatings. When the curing temperature is too low, the solvent evaporation and chemical reaction is sluggish, and the coating is difficult to be cured, raising the temperature can accelerate the solvent evaporation and water evaporation, accelerate the oxidation reaction and thermochemical reaction of the coating, and the curing speed of the coating is accelerated, but the temperature cannot be raised indefinitely, because the temperature and the curing speed are not proportional to each other, when the curing temperature is too high, the curing speed is not significantly increased, but the paint film will be yellowed or Not only that, the temperature in the process of curing the coating also has an impact on the substrate, the substrate is heated, causing changes in moisture content, the substrate shrinkage and deformation, and even warping, cracking, volatile paint coating, curing temperature exceeds 60 ℃, the solvent evaporates fiercely, the surface layer quickly dry solid, the internal solvent vapor reaches the surface layer is easy to produce bubbles, so, when using artificial curing methods, the surface temperature is generally not exceed 60℃.
Paint curing, by having the following three commonly used methods
I. Natural curing.
Under natural conditions, the use of air convection to make solvent evaporation, oxidation polymerization or reaction with curing agent to form a film, suitable for volatile coatings, air-drying coatings and curing agent curing coatings and other self-drying coatings, drying quality is greatly affected by environmental conditions.
1、Solvent evaporation curing
It is the solvent evaporates through the surface of the coating, leaving the solids of the coating and being attached to the surface of the object to be coated, forming a dry solid coating film.
2、Air oxidation curing
It is the use of oxygen in the air to make the coating dry film, the oxygen in the air and the coating cross-linking reaction to form a dry coating film.
3、Thermal reaction or chemical reaction curing
This kind of coating is heated or chemically cross-linked under the action of catalyst (including assimilator), and the various film-forming components in the coating fuse with each other and cross-link to form a three-dimensional mesh structure of the coating film.
Second, traditional heating curing.
Drying according to the drying temperature can be divided into low temperature drying (below 100°C, mainly for drying the surface coating of self-drying coatings or materials with poor heat resistance). Medium-temperature drying (100-150°C, mainly for condensation polymerization reaction curing of coatings into films). High temperature drying (higher than 150°C, mainly used for powder coating, electrophoretic coating, etc.)
1、Hot air convection plus radiation combination curing
Generally first radiation and then convection, using the advantages of radiation heating fast, so that the workpiece heated, and then use hot air convection insulation to ensure the drying quality.
2、Hot air convection curing
Hot air convection heating uniform, high temperature control, suitable for high quality coating, not affected by the shape of the workpiece and the complexity of the structure, but the slow heating rate, low thermal efficiency, equipment is huge, the coating is easy to blister, wrinkle, high dust requirements. The heat sources used are steam, electricity, diesel, gas, liquefied gas and natural gas, etc.
Melt curing coating generally refers to the solid powder type of coating products.
1、Ultraviolet light (UV) radiation curing
It is a kind of chemical formula (paint, ink and adhesive) with the help of energy irradiation to realize the process of conversion from liquid to solid state.
2、Near infrared curing (short-wave infrared)
Near-infrared technology enables powder coatings to work and cure quickly within seconds.
3、Infrared (long-wave, medium-wave) radiation curing
Usually use infrared, far infrared radiation to the object after direct absorption into heat energy, so that the coating curing.
4、Infrared catalytic thermal reaction curing
Is the use of the coating itself to absorb infrared energy into heat energy to make the coating film curing a method.
Microwave refers to the electromagnetic wave with the frequency of 0.3~300GHz. The material will produce physical phenomena such as heating and melting under the action of microwave, and chemical reaction will also occur.
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