The purpose of spraying plastic products is to make the coating form a solid and continuous coating on the surface of the object to be coated, so that it can play its role of plutonium, protection and special functions.
Surface crusting of paints:
The surface of oxidized polymeric paints, alkyd resins and primers with more pigments or ammonia-based alkyd resins can be easily crusted in the storage process. Generally by adding carbonic acid and oxime compounds as an effective anti-crusting agent, each as long as the addition of less than 1% of the amount can be.
Coating gel:
The viscosity of the paint will become larger and larger during storage, and this phenomenon is called viscosity increase or hysteresis thickening. If further development, the paint will lose fluidity and form a gel state. For oil-based paints, the amount of drying agent should be reduced and oxidation polymerization prevention agent should be added. However, it should be noted that this will affect the drying performance of the coating, and the reduction must be limited. For the coating of amino alkyd resin, alcoholic polar solvent with hydroxide root should be added, which has significant effect on crusting.
Coating hardening bonding:
After crusting on the surface of the coating forms a gel, if further development often appears to harden and produce bonding phenomenon. Usually, a low acid value color carrier should be chosen, and fatty acid esters or amines can be added to prevent hardening bonding. It is worth noting that alkyd resin does not have hardening bond for high acid value color carrier, but will have hardening bond for low acid value color carrier.
Pigment Deposition Separation:
Usually, the pigment particles are suspended and dispersed in the polymer solution of the coating. Due to the different specific gravity of the solution and the pigment particles, the pigment often precipitates and is in a sub-stable state, resulting in uneven color or surface gloss difference of the coating film due to insufficient stirring during the coating process, as well as blockage of the coating in the pipeline transportation. In order to prevent precipitation and separation, suitable surfactants should be selected to maintain a certain viscosity of the dispersant, and the composition of the solvent should be reasonably determined. If the coating has produced sedimentation and separation, good dispersing apparatus and dispersion method should be used and stirred sufficiently.
Sagging or hanging of the coating film:
When the coating film on the vertical surface hangs, it will produce columnar or wavy stripes, the long columnar stripes are called dripping and curtain-like stripes are called hanging. It is directly related to the type and amount of pigment in the coating and the presence or absence of additives, and has a greater impact on it. Generally, coatings with good fluidity, slow curing speed and solvent evaporation rate are prone to dripping. In addition, because coatings containing pigments tend to show thixotropy, coatings with mechanical mixing are prone to dripping. The viscosity of the paint has a certain influence on dripping, usually the higher the viscosity of the paint, the less likely it is to drip. However, the same viscosity of the paint has different dripping properties, which is mainly influenced by the thixotropy of the paint and the volatility of the solvent. In order to prevent paint dripping and hanging, consider using solvents with better volatility, spraying the coating as thin as possible, properly adjusting the dilution ratio of thinner, properly reducing the viscosity of the paint, and reasonably controlling the distance and running speed of the spray gun.
Coating whitening:
During or after the drying process, the surface of the coating film produces a white atomization phenomenon called whitening. The main reason for whitening is that when the solvent in the coating evaporates sharply, despite the high ambient temperature around, the temperature of the coating liquid drops below the dew point, and the water vapor in the air condenses into water into the material liquid, making the polymer in the coating either precipitate, or due to the evaporation of condensed water, the coating film is filled with gas, and after drying, a tiny white cloudy mark appears on the film surface.
Under the condition of high temperature and humidity, if the bright paint containing a lot of fast evaporating solvent is used for painting, the evaporation of the solvent will suddenly take away a lot of heat, resulting in the temperature of the surface of the coating liquid dropping, so that the moisture in the air condenses and mixes into it, causing the polymer in the paint to precipitate and produce whitening. In addition, if a weakly soluble thinner is used to dilute the paint, the solvent components with strong solvency will evaporate quickly, which will increase the proportion of non-solvent components in the paint, causing the dissolved polymer to precipitate and produce whitening. It can be seen that the whitening mainly depends on the relative humidity, the temperature difference between the paint and the surrounding environment, the solvent and polymer type and other factors. If the paint is painted under the condition of relative humidity of 80% or more, it is very easy to produce whitening. In this case, 20%-30% high boiling point solvent should be added to the bright paint thinner, i.e. slow drying thinner, and the painted products should be preheated for a certain time before spraying.
It is worth noting that if excessive use of slow-drying thinner, it is easy to produce poor drying, and after the use of thinner with weak solvency, in the drying process, there will be polymer into analysis. In addition, the water content in the compressed air and the amount of low boiling point solvent should be reduced as much as possible.
Coating Chalking:
The phenomenon that the surface of the coating film decomposes in the outdoor environment and becomes powdery and loses its luster is called chalking. In this regard, when spraying white paint, you can add the right amount of zinc oxide or antimony and barium sulfate. You can also add the right amount of UV absorbers as additives.
Surface color bleeding and frosting:
The phenomenon that the base coating or base color leaks out from the surface coating film and makes the surface coating film discolored is called surface bleeding. In addition, after using the pigment that can easily cause surface bleeding, the pigment often precipitates out from the coating, and this phenomenon is called frosting.
The reason of surface bleeding and frosting is mainly due to the coloring substance contained in the bottom coating or bottom color is dissolved by the solvent of the surface coating, usually the inorganic pigment will not produce surface bleeding, but the organic pigment such as red or maroon color precipitation pigment is easy to cause surface bleeding. Therefore, the solubility of organic pigments should be improved. In order to prevent surface bleeding and frosting, one is to try not to use pigments that cause bleeding in the bottom coating; the second is to wait for the bottom coating to be completely cured before painting the surface layer; the third is for the bottom coating or bottom color that is easy to produce surface bleeding, first paint the surface containing alcohol-soluble polyvinyl butyral or aluminum powder to cover the bleeding surface, and then paint the surface layer.
Surface color floating and blooming:
When surface coating, in order to get a variety of colors, the paint is mostly composed of more than two kinds of pigments mixed. Due to the different lubricity, particle size, specific gravity and cohesiveness of various pigments, differences in settling speed will occur when coating, resulting in a different color between the surface layer and the bottom layer when the coating film is formed, a phenomenon called floating color or surface color separation. For example, if a paint containing cyanine and yellow lead is used for coating, a green film should be formed, but due to the continuous precipitation of yellow lead, the yellow tone is lost and the film only shows a strong cyan tone. In addition, if there is no difference between the color of the surface layer and the bottom layer of the coating film, but the color difference or the so-called color unevenness occurs locally in the coating film, it is called blooming or color surfacing.
Surface color floating and flowering:
When surface coating, in order to get a variety of colors, the paint is mostly composed of more than two kinds of pigments mixed. Due to the different lubricity, particle size, specific gravity and cohesiveness of various pigments, differences in settling speed will occur when coating, resulting in a different color between the surface layer and the bottom layer when the coating film is formed, a phenomenon called floating color or surface color separation. For example, if a paint containing cyanine and yellow lead is used for coating, a green film should be formed, but due to the continuous precipitation of yellow lead, the yellow tone is lost and the film only shows a strong cyan tone. In addition, if there is no difference between the color of the surface layer and the bottom layer of the coating film, but the color difference or the so-called color unevenness occurs locally in the coating film, it is called blooming or color surfacing.
Surface color floating and flowering:
Surface color floating and flowering mostly occurs in the gray, blue and green department, which is closely related to the dispersion stability of the pigment and solvent evaporation process of vortex, convection phenomenon. Along with the convection phenomenon of solvent evaporation of the coating liquid, the pigment with fine particles can flow easily, and the pigment with larger particles can flow difficult. For this reason, in the convection process, the fine and small relative density pigments move to the surface layer, and the coarse and large relative density pigments move to the bottom layer, thus presenting the phenomenon of color separation. Usually, high viscosity of coating liquid is not easy to flow and the convection is small; low viscosity of coating liquid, convection phenomenon is more serious and color separation is easily produced. In addition, in the convection process, the flowing pigment particles clash with each other, and the unstable particles at the interface produce agglomeration and become large particles, which are distributed in the bottom layer of the coating film.
Since the causes of surface color floating and flowering are quite complicated, there is no effective measure in preventing this phenomenon. At present, the main is a reasonable choice of pigments, color spreaders and solvents, especially the deployment of moderate additives. Research shows that the addition of soybean emulsifier or lead naphthenate, etc., can prevent flowering. Adding lubricants such as ricinoleic acid can reduce the agglomeration of titanium dioxide, increase the agglomeration of cyanine, and keep the balance of sinking, thus preventing the color from floating out. Adding the right amount of silicone oil can also prevent surface blooming, which is due to the surface viscoelasticity produced by silicone oil adhering to the surface of the coating solution, mechanically inhibiting the convection effect of the vortex within the coating solution. In addition, the paint should be fully stirred before use, carefully filtered, and even more stirred after adding thinner, the oil on the surface of the product must be washed away; for the metallic texture of the paint can not be painted too thick.
Surface discoloration and fading:
There are three main reasons for film discoloration after painting: First, the color of the coating film migrates to other colors, such as the yellow coating film containing yellow lead becomes black after contact with hydrogen sulfide; second, the color pigment of the coating film becomes lighter and loses its original color, such as the coating film containing organic red pigment loses its red color and becomes white after sunlight exposure; third, the white or light color and transparent coating film becomes yellow due to the effect of sunlight, artificial light or heat or brown. Therefore, discoloration and fading are mainly caused by paint factors and environmental factors or a mixture of two factors. The discoloration phenomenon is usually more obvious when the temperature is high. Dust or soot adhesion, mold or the influence of the substrate can also cause discoloration. The organic pigment has poor light resistance, and its coating film will fade easily after being exposed to light. Inorganic colored pigments have poor acid and alkali resistance, and their coating film will be easily discolored by acid and alkali erosion or contact with acid or alkaline gas. With unsaturated phenolic or oxyindene resin as the joint material, the coating film of lead white, zinc barium white and titanium dioxide as the pigment is easy to turn yellow. When too much desiccant is added, it is also easy to cause discoloration or yellowing. In order to prevent the discoloration of the coating film, the pigment should be selected and combined reasonably. The surface coating film should be kept clean after the product is painted.