Sendtime：2019-05-06 source：Makersoon Powder Coatings
Coating extinction principle and its application
A semi-gloss or matte powder coating is sometimes required depending on the application. In order to reduce the gloss of the coating film, conventional matting agents include ultrafine silica, talc, aluminum stearate, calcium stearate, and low molecular weight thermoplastic resins. To get the matting effect, you can use the following methods:
a. The powder coatings with different curing speeds are mixed by the dry mixing method, and the gloss of the obtained coating film is remarkably lowered to play a matting effect. Due to the rapid curing of the highly reactive powder coating, the slower curing of the continuous phase of the resin and the reaction are hindered, and the final cured coating loses its luster. Although this method is effective, it increases the manufacturing process and costs.
b. Add a thermoplastic resin. A cellulose powder is added to the epoxy powder coating, and a fine powdery low molecular weight polyethylene or a polypropylene resin is added to the polyester powder coating to obtain a matte powder coating. The gloss of the coating film is affected by the manufacturing conditions. Since the addition of the thermoplastic resin affects the storage stability of the powder coating, the amount of addition is limited to 2 to 3% of the amount of the resin.
c. Use an extraordinary curing agent with matting effect. d. Add a filler with matting effect. The addition of fillers such as ultrafine silica and talc causes a significant decrease in gloss. Extraordinarily the particle size of the filler has a great influence on the gloss of the coating film. The larger the particle size, the lower the gloss. It is important to note that increasing the amount of filler will result in a decrease in the flatness and mechanical strength of the film. In the epoxy powder coating, the effect of calcium carbonate particle size on the gloss of the coating film is shown in the table. The addition of ultrafine silica and kaolin to acrylic, polyester powder coatings gives matt coatings. In addition to this, a matte coating can be obtained by adding a wax such as polyethylene paraffin or hydrogenated castor oil.
The principle of coating extinction and its application In recent years, China's coatings industry is in the ascendant, and the total output of coatings has also leapt to the forefront of the world, forming an industrial group related to this, actively promoting the technological revolution and development in the field of traditional coating. During this period, high-gloss bright paints were favored by consumers with their bright colors and bright colors, and they have been in the paint market for quite a long time. However, with the continuous improvement of people's living standards, on the one hand, consumers feel that high-gloss bright paints are more reflective after being formed, which is harmful to people's eyes; on the other hand, consumers' aesthetic concepts are more and more inclined to leisure. Stylish and personal. Coupled with the booming demand in China's automotive and home appliance industry. These factors have led to a sharp increase in the demand for low-gloss coatings with a soft appearance. At the same time, how to produce coatings with matting properties has become a problem that coating designers must consider.
Factors Affecting Gloss and Gloss 1. Measurement of Gloss and Gloss Light is reflected on the surface of an object, and the ability of the surface of the object to reflect light is called gloss. The gloss of the surface of different objects is different. The measure of the ability of the surface of the object to reflect light is called gloss. Gloss is expressed as a percentage. The higher the gloss of the surface of the object, the stronger its ability to reflect light and the higher the brightness. The size of the gloss was measured by a photoelectric gloss meter. According to the degree of gloss, the paint can be divided into bright, matt, matt and so on. Classification of paint gloss (as an example of a 60° reflection gloss (ie, a light incident angle of 60°)): matt paint: gloss < 10%; matt paint: gloss 15 to 60%; gloss paint: Gloss > 60%.
2. Factors affecting gloss 1 Roughness of the surface of the coating The surface gloss of the object is closely related to the roughness of the surface of the object. When light hits the surface of the object, part of it is absorbed by the object, part of it will be reflected and scattered, and some will be refracted. The smaller the roughness of the surface of the object, the more light is reflected and the higher the gloss. On the contrary, if the surface of the object is uneven, the amount of light scattered is increased, resulting in a decrease in gloss. The roughness (h) of the surface of a bright object that human vision can perceive, using the knowledge of micro-surface theory, can be calculated according to the formula: h = λ/cosα, where the wavelength of the human light is entered; α is the angle of incidence. For example, when the angle of incidence of the person is 60°, h=1.1 μm can be calculated, and when the surface roughness h of the object is larger than 1.1 μm, the unevenness is exhibited and the gloss is lowered.
2 Coating film formation process After the coating material is applied to the surface of the object, it is solidified into a film by solvent evaporation (for solvent-based coatings). The formation of the coating film is critical to the surface roughness and gloss of the coating. In the wet film stage, the evaporation rate of the solvent is controlled by the diffusion of the solvent on the surface of the coating film. When the evaporation rate of each component of the solvent is not much different, it is possible to obtain a high-gloss surface; conversely, when the components of the solvent are wet When the volatilization rate of the film stage is different, it tends to form a curl or even precipitate out of the polymer molecules, and becomes a particle or agglomerate of a different size, and the surface of the coating film is uneven. In the dry film stage, the evaporation rate of the solvent is mainly controlled by the diffusion of the solvent in the entire coating, and also affects the roughness of the surface of the coating film. In addition, during the formation of the coating film, as the solvent evaporates, the coating film becomes thinner and shrinks, and some suspended heavy particles in the coating are rearranged on the surface of the coating film, causing unevenness of the surface of the coating film.
3 Particle size and distribution of pigments and fines The particle size and particle size distribution of pigments and fillers in coatings are one of the important factors affecting the gloss of coatings. It has been found in research that a high gloss coating film can be obtained when the diameter of the pigment particles is less than 0.3 μm. The reason is that the pigment particles dispersed in the coating are made into a coating film of a certain thickness, and after drying, only the uppermost pigment particles locally protrude, and the surface roughness of the coating film caused by the pigment ions having a particle diameter of less than 0.3 μm Will not exceed 0.1μm. When the average particle diameter of the pigment is between 3 and 5 μm, a coating film having a good matting effect can be obtained.
In addition to the above three factors can affect the gloss of the surface of the coating film, the volume concentration of the pigment (PVC), the dispersibility of the pigment, and the surface structure and surface reflection characteristics of the coating film also affect the gloss of the surface of the coating film. Among them, as the PVC of the pigment increases, the gloss of the surface of the coating film first decreases, and a minimum value occurs at the limit volume concentration (CPVC) of the pigment, and then the gloss increases as the PVC increases. When the type and amount of the pigment are determined, the better the dispersion, the higher the gloss of the surface of the coating film.
Second, the extinction problem of the paint extinction is to use a certain method to reduce the gloss of the surface of the coating film.
1. The principle of extinction combined with the mechanism of the gloss of the surface of the coating film and the factors affecting the gloss, it is considered that extinction is to use various means to destroy the smoothness of the coating film, increase the surface micro-roughness of the coating film, and reduce the surface light of the coating film. Reflection. Can be divided into physical extinction and chemical extinction two ways. The principle of physical extinction is: adding a matting agent to make the coating surface uneven during the film formation process, increasing the scattering of light and reducing reflection. Chemical matting is achieved by introducing into the coating a structure or group such as a polypropylene grafted species that absorbs light to achieve low gloss.
2. Extinction method 1 Matting agent In the paint industry today, the method of adding a matting agent is widely used. There are mainly the following categories:
(1) Metal soap Metal soap is a matting agent commonly used in the early days. It is mainly metal stearates such as aluminum stearate, zinc stearate, calcium stearate, magnesium stearate and the like. Among them, aluminum stearate is the most widely used. The extinction principle of metal soap is based on its incompatibility with the composition of the coating. It is suspended in the coating with very fine particles. When it is formed into a film, it is distributed on the surface of the coating film to cause micro-roughness on the surface of the coating film. The reflection of light on the surface of the film achieves the purpose of extinction.
(2) Wax wax is a kind of matting agent which is widely used earlier, and it belongs to organic suspension type matting agent. After the coating is completed, the wax in the coating film is deposited with the evaporation of the solvent, and the fine crystal is suspended on the surface of the coating film to form a rough surface of scattered light to serve as a matting effect. Wax as a matting agent is characterized by its ease of use and imparts a good hand and water resistance, damp heat resistance and stain resistance. However, after the wax layer is formed on the surface of the coating film, the volatilization of the solvent and the infiltration of oxygen are prevented, which affects the drying and overcoating of the coating film. The future development trend is to use synthetic polymer wax and silica together to obtain the best extinction effect.
(3) Functional fine pigments, such as diatomaceous earth, kaolin, fumed silica, etc., are all functional fine materials used as matting agents. They are inorganic filled matting agents. When the coating film is dried, their fine particles form a micro-rough surface on the surface of the coating film, reducing the reflection of light to obtain a matte appearance. The matting effect of such matting agents is subject to many factors. Taking silica as an example, when it is used as a matting agent, its pore volume, average particle size and particle size distribution, dry film thickness and whether the particle surface is treated or not, etc., will affect its matting effect. Experiments have shown that larger pore volume, uniform particle size distribution and silica with matching particle size and dry film thickness have better extinction properties.
In addition to the above three types of commonly used matting agents, some dry oils, such as: tung oil can also be used as a matting agent in paints. It mainly utilizes the high reactivity of the conjugated double bond in tung oil to make the surface of the coating film have different oxidative crosslinking speeds to cause irregularities on the surface of the coating film to achieve matting effect.
In order to obtain low gloss, the matting agent plays an irreplaceable role. However, in the course of its use, there are still many defects waiting for improvement. For example, the selection and use conditions of the matting agent are more demanding; the price of the matting agent is high, and most of the matting agents used in China need to be imported from abroad, which makes the low-gloss coating The price has also risen; the domestically produced matting agent contains many cyclic quinones, so the resulting coating film is easy to yellow and has poor baking resistance. How to produce a matting agent with good performance and moderate price is still one of the problems that coating workers need to solve.
3. Matting resin resin is an indispensable component of coatings. Therefore, some people synthesize matting resin, and use it to form a film with other resins to obtain low gloss. The use of matting agents can be avoided and coating costs can be reduced. There are already corresponding products listed in foreign countries. The domestic Qi Jixian and Jiang Qibin and others have also studied this. They have synthesized acrylic matting resins, which are used in powder coatings. The effect is quite good, and the coating film is easy to yellow. , defects such as poor baking resistance. Its mechanism: First, the use of the curing agent in the matte resin and the composition of the coating composition and the curing temperature of the other resin, resulting in successive curing, resulting in uneven shrinkage on the surface of the coating film, thereby destroying the smoothness of the surface of the coating film, resulting in matting . G9010 matting resin uses this mechanism to obtain matting. The second is to increase the difference in surface tension between the two resins, resulting in uneven shrinkage of the coating film to produce micro-roughness. The matting resin synthesized by Jiang Qibin has the reason to produce a matting effect. The third is to introduce a poorly compatible monomer into the synthetic resin. The film formation of the coating causes the synthetic resin to be analyzed from the middle of the coating film, thereby increasing the micro-roughness of the surface of the coating film and obtaining low gloss.
Third, it is expected that low-gloss coatings will attract more and more attention in the future coating market, which makes the extinction problem more and more important. Modified matting agents with excellent synthetic properties (such as silica treated with polymer wax surface) and synthetic matting resins will undoubtedly become the focus of research.