1. Coating curing mechanism
During the process of changing the coating from liquid to solid, there are physical changes such as volatilization of solvents, and chemical changes such as cross-linking reactions between molecules of coating components. The mechanism of paint film formation of commonly used coatings for wood products can generally be divided into three types: solvent volatilization curing film formation, oxidation curing film formation and polymerization curing film formation.
(1) Solvent volatile type
The curing mechanism of this kind of coating is mainly the physical process of solvent volatilization. With the continuous volatilization of solvent, the viscosity of the coating gradually increases, from a flowing state to a non-flowing state. After the solvent volatilizes, the curing process of the coating ends and a solid paint film is formed. Coatings include shellac paint, nitro paint, perchlorethylene paint and volatile acrylic paint. Under natural conditions, the curing speed of this kind of paint is faster than that of oil-based paint.
5 to 10 times.
Factors that affect the curing speed of such coatings include the type of solvent and its mixing ratio, the relationship between film-forming substances and solvents, etc.
Under other conditions being the same, the volatilization state of the solvent is related to its saturated vapor pressure. The higher the saturated vapor pressure, the lower the boiling point and the faster the volatilization speed. Therefore, the coating containing more low boiling point solvents cures faster. However, in production, the content of low boiling point solvents should not be too much, otherwise the solvent will evaporate too fast, resulting in whitening of the paint film, poor leveling, and defects such as pinholes and wrinkled skin after drying. However, if there are too many medium and high boiling point solvents, the coating will cure slowly. Usually low boiling point and high boiling point mixed solvents are used.
The curing speed of volatile coatings is not only determined by the vapor pressure of the solvent, but also by the relationship between the solvent and the film-forming substance. Some film-forming substances have a kind of occlusion for the solvent, that is, the release of the film-forming substance to the solvent is poor. When the same solvent volatilizes from the film-forming substance solution with good release property, it will be volatilized at the same speed within a certain period of time from the beginning to the end, while in the film-forming substance solution with poor release property, the speed will drop to the later stage. Very low, it takes a long time to evaporate part of the residual solvent. It is easier for a solvent with the same boiling point to volatilize from nitro lacquer than from ethylene peroxide paint, and the speed when it reaches surface dryness is the same, but the time to reach the actual dryness is very large.
(2) Oxidative curing
The curing mechanism of this type of coating is the result of the combined action of the oxidative polymerization reaction between the polymers of the film-forming substance and the physical process of solvent volatilization, but the polymerization reaction is the main one. Commonly used such coatings include: grease paint (clear oil, blended paint), ester glue paint, calcium grease paint, one-component polyurethane paint, alkyd paint, oil-based phenolic resin paint and other oil-based paints containing a large amount of vegetable oil. During the curing process of the coating, the unsaturated fatty acids in the oil molecules absorb oxygen in the air, and a series of complex polymerization reactions occur, which increases the molecular weight of the film-forming substance, changes from small molecules to large molecules, and finally becomes a network Polymer paint film with body shape structure. Since the rate of oxidative polymerization is slower than the volatilization rate of the solvent, the drying rate of this type of paint mainly depends on the rate of oxidative polymerization.
(3) Polymerization curing
The curing mechanism of this kind of coating is to promote various types of polymerization or condensation crosslinking chemical reactions of film-forming substances through the action of oxygen, catalysts, ultraviolet rays, electron beams, etc. to form a network-shaped polymer paint film. Commonly used such coatings include unsaturated polyester resin, modified acrylic resin, polyurethane paint, photosensitive paint, acid-cured amino alkyd paint, etc.
For example, the curing of acid-cured amino alkyd paint is to use the ether bond in the amino resin and the hydroxyl group in the alkyd resin to undergo a crosslinking reaction at room temperature under the catalysis of an acid to cure. The curing of unsaturated polyester paint is the result of the addition polymerization reaction between unsaturated polyesters initiated by peroxide initiators. The curing of photosensitive paint is to irradiate with ultraviolet light to make the photosensitizer decompose free radicals, and trigger the polymerization reaction of photosensitive resin and reactive diluent to form a film.
2. Factors affecting curing speed
Factors affecting the curing speed include coating type, coating thickness, curing method, curing conditions, curing equipment and specific curing procedures.
(1) Coating type
Under the same curing conditions, different types of coatings have great differences in curing speed, while the same type of coatings have little difference. Generally speaking, volatile paints cure quickly, oil paints cure slowly, and the situation of polymer paints is very different. Photosensitive paints in polymeric paints cure quickly, while other polymeric paints are mostly between volatile paints and oily paints. When using mechanized assembly lines for finishing, volatile paints and acid-cured amino alkyd paints are more commonly used.
(2) Coating thickness
In the finishing process, the coating is basically not formed at one time, usually by the method of multiple thin coatings (such as oil paint with a thickness of about 35 μm, and nitro paint with a thickness of 15 μm).
left and right, etc.). Under the same curing conditions, when the thin coating is cured, the internal stress is small and the coating defects formed are few; when the coating is too thick, the internal stress is large, and wrinkles and other defects are prone to occur. , Coating shrinkage leads to uneven gloss, internal non-curing, etc. Practice has proved that, except for polyester paint, the paint film formed by multiple coatings of other paints has better physical properties than the paint film of the same thickness formed by one coating.
(3) Curing conditions
1. Curing temperature
The curing temperature has a decisive influence on the curing speed of most paint coatings. When the curing temperature is too low, the solvent volatilizes and the chemical reaction is slow, making it difficult to cure the coating. Increasing the temperature can accelerate solvent volatilization and water evaporation, accelerate coating oxidation reaction and thermochemical reaction, and accelerate coating curing speed. However, the temperature cannot be increased without limit, because the temperature is not directly proportional to the curing speed. When the curing temperature is too high, the curing speed will not be significantly increased, but it will make the paint film yellow or darken. Not only that, but the temperature also affects the substrate during the curing process of the coating. When the substrate is heated, the moisture content changes, the substrate shrinks and deforms, and even warps and cracks. For coatings of volatile lacquers, the curing temperature exceeds
At 60 °C, the solvent evaporates violently, the surface layer dries quickly, and bubbles are easily formed when the internal solvent vapor reaches the surface layer. Therefore, when the artificial curing method is used, the surface temperature should generally not exceed 60 °C.
2. Air humidity
The air humidity should be moderate. When the humidity is too high, the evaporation rate of the water in the coating will decrease, and the evaporation rate of the solvent will slow down, thus slowing down the curing rate of the coating. Most of the coatings should be kept at a relative humidity of 45% to 60%.
Curing in air is most suitable. If the air in the dry curing place is too humid, not only will the curing process be slow, but also the formed paint film will be hazy and other defects will appear. Relative humidity has little effect on the curing speed of volatile paints, but has a great relationship with the film quality. Especially when the temperature is low and the relative humidity is high, the coating is prone to "whitening".
For oil-based paints, when the relative humidity exceeds 70%, the effect on the curing speed of the coating is more significant than that of temperature.
During the curing process of the coating, the internal stress generated is related to the relative humidity.
3. Ventilation conditions
Corresponding ventilation measures should be taken when the coating is cured, so that the surface of the coating has suitable air circulation, and the solvent vapor can be discharged in time. Increasing the air circulation can reduce the curing time, improve the curing efficiency, and ensure the curing quality.
(1) Airflow velocity
Air circulation is beneficial to the natural curing of the paint film. Because in a closed environment with a high concentration of solvent vapor, the paint film cures slowly, or even does not dry. Ventilation is conducive to the volatilization of solvents in the paint film and the removal of solvent vapors, and can ensure the safety of natural curing places.
The forced circulation air drying room is to circulate the hot air by the ventilator, and its drying and curing effect depends on the air flow speed to a large extent. The greater the air flow velocity, the better the heat transfer effect. Hot air drying usually adopts low air velocity, generally
0.5 ~ 5.0m/s; temperature is 30 ~ 150 ℃.
(2) Air flow direction The direction of air flow is also very important. The direction of air flow and coating has two forms: parallel and vertical. According to the test, vertical air supply hot air drying is better than parallel air supply.
(4) External conditions
For coatings that are polymerized and cured to form a film, the curing of the coating is a complex chemical reaction process. The curing speed is related to the nature of the resin contained, the amount of curing agent and catalyst added, and external conditions such as temperature, infrared rays, ultraviolet rays, electron beams, etc., can often accelerate the progress of this reaction. The size of the effect of external conditions depends on the degree to which the external conditions are compatible with the properties of the coating. For example, photocurable coatings can be cured into a film in just a few seconds under the irradiation of strong ultraviolet light. If infrared or other heating methods are used, it is difficult to cure, or even not cured. Another example is electron beam curing coatings, whose coatings are cured faster than photosensitive varnishes under the irradiation of electron beams emitted by electron accelerators. Other coatings do not respond as strongly to electron beams. Therefore, the coating curing method should be reasonably selected according to the nature of the coating used.
