Depending on the application, sometimes semi gloss or matte powder coatings are required. In order to reduce the glossiness of the coating, commonly used matting agents include ultrafine silica, talc powder, aluminum stearate, calcium stearate, low molecular weight thermoplastic resin, etc. To achieve a cutout effect, the following methods can be used:

One kind. If powder coatings with different curing speeds are mixed using dry mixing method, the gloss of the resulting coating will be significantly reduced and a matting effect will be produced. Due to the fast curing speed of highly active powder coatings, it hinders the leveling and reaction of the resin that slowly solidifies continuously, resulting in a loss of gloss in the final cured coating. Although this method is effective, it increases manufacturing processes and costs.

Add thermoplastic resin to the bay. Adding cellulose acetate to epoxy powder coating and fine powder low molecular weight polyethylene and polypropylene resin to polyester powder coating can result in matte powder coating. The glossiness of the coating is affected by manufacturing conditions. Due to the fact that the addition of thermoplastic resin can affect the storage stability of powder coatings, the amount added is limited to 2-3% of the resin content.

C. Use a special curing agent with extinction effect. d. Add fillers with extinction effect. Adding fillers such as ultrafine silica and talc powder can significantly reduce glossiness. Especially the particle size of the filler has a significant impact on the glossiness of the coating. The larger the particle size, the lower the gloss. It should be noted that increasing the amount of filler will reduce the flatness and mechanical strength of the coating. The influence of calcium carbonate particle size on the glossiness of epoxy powder coatings is shown in the table. Adding ultrafine silica and kaolin to acrylic and polyester powder coatings can obtain matte coatings. In addition, adding waxes such as polyethylene paraffin and hydrogenated castor oil can also obtain matte coatings.

In recent years, the extinction principle and application of coatings have been thriving in China's coatings industry, and the total production of coatings has also jumped to the forefront of the world, forming related industrial clusters and actively promoting technological revolution and development. The development of traditional coatings field. During this period, high gloss coatings were highly favored by consumers for their bright and vibrant colors, and have long dominated the paint market. However, with the continuous improvement of people's living standards, on the one hand, consumers feel that high gloss paint has more reflection after film formation, which is harmful to human eyes; On the other hand, consumers' aesthetic concepts are increasingly inclined towards leisure, fashion, and personalization. In addition, the demand for automobiles and home appliances in China is strong. These factors have led to a sharp increase in demand for low gloss coatings with a soft appearance. At the same time, how to produce coatings with extinction performance has become a problem that coating designers must consider.

1. Factors affecting glossiness and glossiness

1. Glossiness and measurement of glossiness: When light is projected onto the surface of an object, it undergoes reflection, and the reflective ability of the object's surface is called glossiness. The glossiness of different object surfaces varies. The measure of the ability of an object's surface to reflect light is called glossiness. Glossiness is expressed as a percentage. The higher the glossiness of an object's surface, the stronger its ability to reflect light and the higher its brightness. Glossiness is measured by a photoelectric gloss meter. According to the level of glossiness, coatings can be divided into glossy, matte, and matte. Classification of paint gloss (taking 60 ° reflective gloss (i.e. light incident angle of 60 °) as an example): Matte paint: gloss<10%; Matte paint: glossiness 15-60%; Lacquer: Glossiness>60%.

2. Factors affecting gloss

① The roughness of the coating surface is closely related to the glossiness of the object surface. When light shines on the surface of an object, some will be absorbed by the object, some will be reflected and scattered, and some will be refracted. The smaller the surface roughness of an object, the more light it reflects and the higher its glossiness. On the contrary, if the surface of an object is uneven, scattered light will increase and gloss will decrease. The surface roughness (h) of bright objects that can be perceived by human vision can be calculated using the knowledge of micro surface theory according to the following formula: h = λ/cosα， The input is the wavelength of light emitted by the object; α is the incident angle. For example, when the shooting angle is

At 60 °, h=1.1 μ m can be calculated. When the surface roughness h of an object is greater than 1.1 μ m, unevenness will appear, reducing glossiness.

② The film-forming process of coating film: After the coating is applied to the surface of the object, it solidifies into a film through the evaporation of the solvent (for solvent based coatings). The formation process of the coating film is very important for the surface roughness and glossiness of the coating film. 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 rates of the solvent components are not significantly different, a high gloss surface can be obtained; On the contrary, when the components of the solvent are moist, the evaporation rate during the film stage is different, which can cause polymer molecules to tend to form curls or even precipitate, forming particles or clusters of different sizes, resulting in uneven coating surfaces. In the dry film stage, the evaporation rate of the solvent is mainly controlled by the diffusion of the solvent throughout the coating, which also affects the roughness of the coating surface. In addition, during the formation of the coating, as the solvent evaporates, the coating will become thinner and shrink, and some suspended heavy particles in the coating will rearrange on the surface of the coating, causing unevenness in the coating. The surface of the coating film.

③ The particle size and distribution of pigments and fine materials. The particle size and particle size distribution of pigment (filler) materials in coatings are important factors affecting the glossiness of coating films. It has been found in research that high gloss coatings can only be obtained when the diameter of pigment particles is less than 0.3 μ m. The reason is that after the pigment particles dispersed in the coating are made into a certain thickness of coating and dried, only the topmost pigment particle part protrudes, and the particle size is less than 0.3 μ m and will not exceed 0.1 μ m. When the average particle size of the pigment is between 3-5 μ m, a coating with good extinction effect can be obtained.

In addition to the above three factors that affect the glossiness of the coating surface, the volume concentration of pigment (PVC), the dispersibility of pigment, and the surface structure and surface reflection characteristics of the coating also affect the glossiness of the coating. Coating surface. Among them, with the increase of pigment PVC, the surface glossiness of the coating first decreases, and a minimum value appears at the maximum volume concentration (CPVC) of the pigment. Then, with the increase of PVC, the glossiness also increases When the type and amount of pigment are determined, the better the dispersibility, the higher the glossiness of the coating surface.

2、 The problem of paint extinction is to use certain methods to reduce the glossiness of the coating surface.

1. The extinction principle combines the mechanism of coating surface gloss and the factors that affect gloss. People believe that extinction is the use of various methods to disrupt the smoothness of the coating, increase the micro roughness of the coating surface, and reduce the exposure of the coating surface. Reflection. It can be divided into two types: physical extinction and chemical extinction. The principle of physical extinction is to add an extinction agent during the film formation process to make the coating surface uneven, increase light scattering, and reduce reflection. Chemical extinction is achieved by introducing structures or functional groups that can absorb light, such as polypropylene grafted materials, into the coating to achieve low glossiness.

2. Extinction method: In today's coating industry, people commonly use the method of adding extinction agents. There are mainly the following categories:

(1) Metal soap is an early commonly used matting agent. Mainly some metal stearates, such as aluminum stearate, zinc stearate, calcium stearate, magnesium stearate, etc. Among them, aluminum stearate has the highest dosage. The extinction principle of metal soap is based on its incompatibility with coating components. It is suspended in the coating with extremely fine particles and distributed on the surface of the coating during the film formation process, causing micro roughness on the surface of the coating and reducing the coating. The reflection of light on the surface of a thin film achieves the purpose of extinction.

(2) Wax is an early and widely used matting agent, belonging to the organic suspension type matting agent. After painting, wax me

The coating film will precipitate as the solvent evaporates and suspend in the form of small crystals on the surface of the coating film, forming a rough surface that scatters light and plays a role in extinction. The characteristic of wax as a matting agent is its ease of use, which can give the coating a good hand feel, water resistance, moisture resistance, and stain resistance. However, the formation of a wax layer on the surface of the coating film can also prevent the evaporation of solvents and the penetration of oxygen, thereby affecting the drying and recoating of the coating film. The future development trend is to combine synthetic polymer wax with silica to achieve the best extinction effect.

(3) Functional fine materials such as diatomaceous earth, kaolin, and gas-phase silica are specialized functional fine materials used as matting agents, belonging to inorganic filling type matting agents. When the coating is dry, their tiny particles will form a slightly rough surface on the coating surface, reducing light reflection and achieving a dull appearance. The extinction effect of this type of matting agent is constrained by multiple factors. Take silicon dioxide as an example. When used as a matting agent, the pore volume, average particle size and particle size distribution, dry film thickness, and whether the particle surface has been treated all affect its matting effect. Experiments have shown that silica with larger pore volume, uniform particle size distribution, and particle size matching dry film thickness has better extinction performance.

In addition to the three commonly used matting agents mentioned above, some dry oils such as tung oil can also be used as matting agents in coatings. It mainly utilizes the high reactivity of the conjugated double bonds in tung oil to give the bottom surface of the coating different oxidation crosslinking rates, resulting in unevenness on the surface of the coating and achieving extinction effect.

In order to achieve low gloss, matting agents play an irreplaceable role. However, there are still many defects that need to be improved during use. For example, the selection and usage conditions of matting agents are quite strict; The price of matting agents is relatively high, and most of the matting agents used domestically need to be imported from abroad, which has led to a corresponding increase in the price of low gloss coatings; Most domestic matting agents contain cyclic substances, which can cause yellowing of the coating and poor baking resistance. How to produce high-performance and affordable matting agents is still one of the problems that coating workers need to solve.

3. Matte resin is an essential component of paint, so some people have synthesized matte resin and used it to form a film with other resins to achieve low gloss. It can avoid the use of matting agents and reduce coating costs. Corresponding products have already been launched on the market abroad, and Gu Jixian and Jiang Qibin in China have also conducted research on this and synthesized acrylic extinction resin for powder coatings, which has a very good effect and eliminates yellowing of the coating film, Defects such as poor baking resistance. The mechanism is to use the functional groups in the extinction resin and the curing agent in the coating composition to cure at different temperatures, resulting in sequential curing and uneven surface shrinkage of the coating, thereby damaging the flatness of the coating. The thin film surface causes extinction. G9010 extinction resin uses this mechanism to achieve extinction effect. The second is to increase the surface tension difference between the two resins, causing uneven shrinkage of the coating and producing micro roughness. The extinction resin synthesized by Jiang Qibin produces extinction effect precisely because of this. The third is the introduction of monomers with poor compatibility in synthetic resins. The formation of the coating causes these monomers to cause the synthetic resin to analyze from the middle of the coating, thereby increasing the micro roughness of the coating surface and achieving low glossiness.

3、 It is expected that low gloss coatings in the future paint market will receive increasing attention, making the issue of extinction even more important. Modified extinction agents with excellent synthetic properties (such as silica surface treated with polymer wax) and synthetic extinction resins will undoubtedly become the focus of research.