Cheng Jianqiang, Zhang Yong, Chen Congzong, Zuo Juanjuan (Marine Chemical Research Institute, State Key Lab of Marine Coatings, Qingdao, Shandong 266071, China)
Abstract : The flooding defect occurring at the process of film drying and curing has been analyzed via internal factors and external factors. Meanwhile, some prevention treatments have been given accordingly.
Coatings are marine, petroleum, One of the most widely used anti-corrosion and functional materials in the fields of chemical industry, machinery, aerospace, traffic engineering, etc., can protect and decorate various parts of the coated object, improve the quality of the coated object, and extend the The service life has been widely used and developed in various industries.
The coating is a “semi-finished product” of the coating film. It can show its protection, decoration and other special functions only after it is applied to the surface of the coating to form a coating film. However, some coatings may suffer from whitening and dulling, dull spots, or white haze during drying or after film formation. The color of the coating film is paler than the original. This phenomenon is called whitening of the coating film, also called whitening and whitening. Whitening affects the surface decoration effect and scratch resistance of the coating film. Although sometimes it is not easy to detect visually, the phenomenon of lameness or poor adhesion between layers may occur in the future.
The causes of whitening of the coating film are very complicated. This article will infer the whitening mechanism according to the characteristics of the whitening phenomenon, focusing on the analysis of the internal causes of the coating components that cause the whitening of the coating film and the external causes such as the external environment. The preventive measures and improvement methods for whitening of the paint film are put forward.
1 Mechanism of whitening phenomenon of coating film
During the drying process or after curing to form a film for a few days, a few months or a longer period of time, there may be whitening phenomenon. Coatings are generally composed of film-forming substances, solvents, pigments, auxiliaries and other components. After forming a coating film, it is a relatively stable equilibrium system. With changes in the external environment such as time and temperature, relative humidity, this balance may be broken, and the single-phase continuous structure of the film-forming substance in the coating film will change. These irreversible changes are mainly manifested in the local formation of micro air sacs, micro fluid sacs, amorphous micro crystals, etc., and the appearance of the coating film shows a whitening phenomenon.
The whitening of the coating film may also be caused by the formation of incompatible compounds after the moisture in the air reacts with the coating. The water cannot be completely dissolved in the volatile matter and then volatilizes, but forms a white milky body with the film-forming substance. After that, the water gradually evaporates. At this time, if the milky body can be dissolved by the residual solvent, the white coating film layer disappears; otherwise, the continuous phase of the coating film is destroyed, causing the resin to precipitate and whitening.
2 Analysis of the causes of whitening of the coating film
The whitening phenomenon that occurs due to water absorption and uneven dispersion of pigments and fillers in the 4 major components of the coating is called internal cause, which is caused by external environmental factors during the coating construction process. The phenomenon of whitening is called external cause.
2.1The relative molecular mass distribution of the internal
cause of the film whitening of the coating film has a great effect on the whitening of the coating film. Resin molecules and solvent molecules differ not only in size but also in molecular movement speed. This will cause the solvent molecules to penetrate into the resin quickly, but the resin molecules diffuse into the solvent very slowly. The resin undergoes swelling and then The process of dissolution. The closer the polarities of the resin and the solvent are, the easier it is to dissolve. The solubility of the same resin in the same solvent decreases as the relative molecular mass increases. The greater the relative molecular mass of the resin, the worse the compatibility with the solvent. When the relative molecular weight is the same, when a resin with a wide relative molecular mass distribution and a large relative molecular mass is selected as the film-forming material, some resins may appear whitening.
Water infiltration in coatings can also cause whitening. The necessary condition for water to penetrate into the resin or solvent is that its molecular structure contains hydroxyl groups (-OH) -type hydrophilic groups. Theoretically, the more hydrophilic groups contained in the resin or solvent, the more water vapor may enter the coating. . Coatings containing low relative molecular weight alcohol are more prone to whitening. Because low relative molecular weight alcohols such as methanol and ethanol can be miscible with water at any ratio, the solubility of n-butanol and isopropanol in water can also reach 8%. about. In addition, low-molecular-weight ketones (such as acetone) are easily soluble in water because they contain hydrophilic polar carbonyl groups and non-polar methyl groups, so they are easily soluble in water. As the number of atoms increases, the solubility will continue to decrease, and the relative molecular mass will become insoluble in water after it reaches a certain level. Carboxylic acids, aromatics, esters, and alcohol ethers have similar rules. The hydrophilicity of nitrocellulose containing hydrophilic groups increases with decreasing nitrogen content, so the whitening of nitro paints is generally more severe than other coatings. Controlling the use of hydrophilic solvents in coating formulation design can effectively reduce the occurrence of whitening of the coating film.
Water vapor entering the coating can cause whitening of the coating, but it does not mean that no moisture can be mixed in the coating. Virtually all solvents will contain a small amount of water, but as long as the amount of water intrusion is within the tolerance of the coating system, whitening can be avoided. From the dissolution mechanism, it is a question of solubility parameters after water and solvent are mixed. Tables 1 and 2 list the solubility parameters of commonly used polymers and organic solvents in the coatings industry. The solubility parameter of water is 42.8. Polymer polymerization The solubility parameters of the substances are far lower than the solubility parameters of water. After water enters the coating system, because its solubility parameters are much higher than the resins and solvents in Tables 1 and 2, the solubility parameters of the entire system increase rapidly. When it increases beyond a certain amount of resin solubility parameters, the resin will If it is insoluble and precipitates from the solvent, it will appear as whitening of the coating film.
The powder has two effects on the whitening of the coating film. When the powder contains a certain amount of water, because the refractive index of the water and the film-forming resin are different, the coating film will appear dull and dull in the appearance and optical effect after drying. When the amount of powder exceeds the threshold, When the volume concentration of pigment and filler (CPVC), or uneven dispersion, poor dispersion stability, and the formation of local powder content exceeding CPVC, some powder particles will be surrounded by air without being wetted by the resin. Because the difference between the refractive index of air and the refractive index of the base resin is greater, it causes the phenomenon of unevenness in the hiding power of the coating film, which is reflected in the visual effect of the coating film being white.
Excellent wetting and dispersing agents can significantly improve the whitening behavior of paint films. After adding talc powder to the coating, on the one hand, the resin and talc powder are physically intertwined or formed into a chemical bond under the action of a wetting and dispersing agent; on the other hand, the resin and talc powder are intercalated with each other. The coating film showed a uniform and ordered layered structure at different thicknesses. The more uniform the powder is dispersed, the better the dispersion stability, and the stronger the whitening resistance of the coating film.
2.2 The external cause of whitening of the coating film
The decrease in the surface temperature of the coating during coating is an important cause of the phenomenon of whitening. The volatilization of the diluent is an endothermic reaction. If the diluent volatilizes too quickly during the coating process, the surface temperature of the coated object can be reduced by 15-20 ° C. Under the same relative humidity conditions, the more heat is taken away from the substrate, the more the surface temperature of the coating is lowered, the more water vapor is condensed. When the substrate surface temperature drops below the air dew point temperature, the coating film enters the whitening stage.
The faster the evaporation rate of the thinner in the coating, the greater the temperature drop. The most fundamental factor determining the volatilization rate is the vapor pressure of the solvent at this temperature, followed by the relative molecular mass of the solvent. Table 3 lists the volatilization rates of commonly used solvents. Among them, methyl acetate and acetone are solvents with fast volatilization rates.
The material and coating process of the coated object also have a great effect on the whitening of the coating film surface. The better the thermal conductivity and the larger the heat capacity, the better the constant temperature effect of the coating, and the slower the surface temperature drops. When spraying a coated object with a convex shape, the solvent easily diffuses around the corners, taking away more heat and causing the surface temperature to decrease, and the coating film becomes white. On the contrary, the solvent evaporates slowly in the concave coating, and the coating film is not easy to whiten. . Excessive airflow and air pressure during spraying, excessive solvent addition, and excessive wet film thickness can easily increase the air velocity on the coating surface, evaporate too quickly, and cause the surface of the coated object to cool down excessively, which in turn can cause whitening of the coating film.
Chen Xiaowen and others examined the factors that affect the whitening of PU wood paints, and focused on laboratory simulation of the coating coating curing environment. It was found that temperature and humidity had the greatest effect on the whitening phenomenon. Further research found that the phenomenon of whitening of the coating film was not the humidity Caused by condensation on the coating film. Under the same external conditions, the larger the thickness of the coating film, the more severe the whitening of the coating film; the longer the drying time of the coating film, the lighter the whitening phenomenon.
In addition, when spraying or brushing, there is a lot of water on the tool. When the water is mixed into the paint during construction, the paint surface will appear white.
3 Prevention of whitening phenomenon
In summary, the whitening phenomenon of the coating film is caused by very complicated reasons. There are also various preventive methods. In order to effectively prevent such phenomena, the following methods can be comprehensively considered.
Organic solvents are reasonably selected with reference to the solubility parameters of commonly used resins and solvents. While ensuring that the resin has good compatibility in the solvent to prevent the resin from precipitating from the coating, try to avoid organic solvents with strong hydrophilicity. Organic solvents with high boiling points and slow volatility can be selected, such as butyl acetate and cyclohexanone in Table 3. However, it should be used with caution when solvents with boiling points higher than 200 ℃, because it will cause slow drying and easy construction. Sickness, coating film becomes soft and sticky.
During formulation design, the CPVC of the system is reasonably controlled. There are phenomena of dry powder aggregation and wet powder flocculation in paint to varying degrees. Select a suitable wetting and dispersing agent to improve the uniformity and dispersion stability of the powder in the coating and prevent the aggregation of the powder in the coating.
If you encounter high temperature and high humidity weather, if necessary, you can dehumidify the construction site and pre-bake the coating to increase the surface temperature of the coating, so that the surface temperature of the coating after the solvent has evaporated Keep above the dew point of air humidity. The pre-baking temperature should be set according to the temperature and relative humidity during construction. Generally, the surface of the coated object can be raised by about 10 ℃ to effectively prevent the occurrence of whitening.
After coating is completed, the coated object should be heated quickly if conditions permit. The phenomenon of whitening of the coating film often occurs when the solvent is about to evaporate. At this time, the solvent content is the smallest, the water vapor infiltration is the largest, the surface temperature of the coating is minimized, and the solubility parameter is most likely to lose balance. If the temperature is raised to about 60 ℃ immediately after coating, it can effectively prevent the entry of water vapor, and then prevent the occurrence of whitening.
4 Whitening treatment
When the whitening phenomenon occurs, you should use gauze to dip a small amount of strong solvent in time, and gently wipe on the whitening surface, but be careful not to rub it repeatedly and repeatedly, otherwise it will affect the appearance due to the dissolution of the resin. You can also spray a thin layer of strong solvent on the paint film that is not yet dry to dissolve the precipitated resin, so that the whitening coating film will gradually fade. For surfaces that require more than one coat of the same color and model, there is no need to pay attention to the whitening of the coating film in the previous coats. Dilute the last coat with a high boiling point true solvent and apply it thinly on the surface. Can effectively avoid whitening of the coating film.
Based on the mechanism of the whitening phenomenon of the coating film, combined with the analysis of the internal and external factors that cause the whitening phenomenon, it pointed out the measures and methods that should be taken to avoid the whitening phenomenon during the coating production, the optimization of the coating formula, The design provides some reference for the quality control and quality assurance of the coating process.