Introduction of leveling agent
Leveling agent is a kind of common coating additive, which can promote the coating to form a flat, smooth and even film in the process of drying and film formation. There are many kinds of leveling agents, and the types of leveling agents used in different coatings are different.
Overview of leveling agent
After coating construction, there is a flow and drying process, and then gradually form a flat, smooth and even film. Whether the coating can achieve the smooth and smooth characteristics is called leveling. Shrinkage cavity is one of the characteristic defects of coating during leveling and film forming. In the actual construction process, due to the poor leveling, brush marks occur during brush coating, roll marks occur during roll coating, orange peel occurs during spraying, shrinkage, pinhole, sagging and other phenomena occur during the drying process, all of which are called poor leveling, which reduces the decorative and protective functions of the coating. There are many factors that affect the leveling of coatings, such as the volatile gradient and solubility of solvents, the surface tension of coatings, the wet film thickness and surface tension gradient, the rheology of coatings, construction technology and environment, etc., among which the most important factors are the surface tension of coatings, the surface tension gradient produced by wet film during the film forming process and the surface tension homogenization ability of wet film surface. In order to improve the leveling property of the coating, it is necessary to adjust the formulation and add appropriate additives, so that the coating has the ability of proper surface tension and reducing the gradient of surface tension.
Leveling agent mechanism
The common defects of dry film formation are shrinkage, orange peel, brush mark, roll mark, sagging, etc. Shrinkage cavity refers to the irregular, bowl like small depression formed on the film, which makes the film lose flatness. It usually centers on a drop or a small piece of impurities, forming a ring-shaped edge around. From the point of view of leveling, it is a kind of special "point type" flow unevenness, which is produced on the surface of the coating film. Its shape can be divided into plane type, crater type, point type, exposed bottom type, bubble type, etc.
Commonly used anti shrinkage hole leveling agent
Solvent leveling agents are mainly high boiling point solvent mixtures. Solvent based coatings can improve the leveling only by increasing the solvent to reduce the viscosity, which will reduce the solid content of the coating and lead to such disadvantages as sagging; or to maintain the solvent content, only adding high boiling point solvent to adjust the evaporation rate to improve the leveling, and the drying time will be correspondingly prolonged. So the two schemes are not ideal. Only by adding high boiling point solvent mixture, it can show various increasing characteristics (volatilization index, distillation curve, dissolving capacity). Solvent leveling agent is mainly composed of various mixed solvents with high boiling point, which has good solubility and is also a good wetting agent for pigments. It is very effective to use this kind of additives in the curing coatings at room temperature because the solvents volatilize too fast, the viscosity of the coatings increases too fast, which hinders the flow and causes the brush marks, and the solvent volatilizes, which causes the shrinkage caused by the poor solubility of the base materials, or the boiling marks and bubbles in the baking coatings. In addition, the use of high boiling point leveling agent to adjust the volatilization rate can also overcome the problem of whitening. The long-chain resins with limited compatibility are commonly used, such as polyacrylic acid, cellulose acetate, etc. Their low surface tension can reduce the surface tension between the coating and the substrate, improve the wettability of the coating on the substrate, remove the gas molecules adsorbed on the solid surface, prevent the gas molecules adsorbed from being removed too late and form pits, shrinkage holes, orange peel and other defects on the surface of the cured film; in addition, they are not completely miscible with the resin, and can be quickly transferred to the surface A single molecular layer is formed on the surface to ensure the uniformity of the surface tension and increase the anti shrinkage effect, so as to improve the smoothness and flatness of the coating surface. Polyacrylate leveling agent can be divided into pure polyacrylate, modified polyacrylate (or combined with silicone), acrylic acid-base resin, etc. the compatibility between pure polyacrylate leveling agent and common epoxy resin, polyester resin or polyurethane resin for coating is very poor, which will form a foggy film when applied. In order to improve its compatibility, copolymers with good miscibility are usually used. The long-chain silicone resins with limited compatibility are commonly used as polydimethylsiloxane, polymethylphenylsiloxane, organic modified polysiloxane, etc. This kind of material can improve the wettability of the substrate and control the surface flow to improve the leveling effect. When the solvent volatilizes, the silicone resin forms a single molecular layer on the surface of the film to improve the luster of the film. The modified polysiloxane can be divided into polyether modified silicone, polyester modified silicone and reactive silicone. The introduction of organic groups helps to improve the compatibility of polysiloxane and coating resin. Even if the concentration is increased, there will be no incompatibility and side effects. The modified polysiloxane can reduce the interfacial tension between coating and substrate, improve the wettability of substrate, improve the adhesion and prevent Flower, orange peel, reduce shrinkage, pinhole and other film surface diseases. Fluorinated surfactants are mainly polyfluorinated polyolefins. They have good compatibility and surface activity for many resins and solvents. They are helpful to improve wettability, dispersion and leveling. They can also adjust the rate of solvent evaporation in solvent based paints.
Why leveling agent should be used in paint formulation
The main function of coating is decoration and protection. If there are flow and leveling defects, it will not only affect the appearance, but also damage the protection function. If the formation of shrinkage cavity results in the insufficient thickness of the film, and the formation of pinhole will lead to the discontinuity of the film, these will reduce the protection of the film. In the process of coating construction and film formation, there will be some physical and chemical changes. These changes and the properties of the coating itself will significantly affect the flow and leveling of the coating. After coating construction, there will be new interfaces, generally the liquid / solid interface between coating and substrate and the liquid / gas interface between coating and air. If the interfacial tension between the coating and the substrate is higher than the critical surface tension of the substrate, the coating will not spread on the substrate, and will naturally produce such leveling defects as fisheye and shrinkage cavity. The volatilization of solvent in the drying process of paint film will lead to the difference of temperature, density and surface tension between the surface and the interior of paint film. These differences will lead to the turbulent movement of the interior of paint film and form the so-called Benard vortex. Benard vortex will lead to orange peel; in systems containing more than one pigment, if there is a certain difference in the movement of pigment particles, Benard vortex is likely to lead to floating color and blooming, and the construction of vertical plane will lead to silk grain. In the drying process of the paint film, some insoluble colloidal particles sometimes appear, which will lead to the formation of surface tension gradient and the formation of shrinkage cavity in the paint film. For example, in the crosslinking curing system, the formula contains more than one resin. During the drying process of the paint film, with the evaporation of the solvent, the resin with poor solubility may form insoluble colloidal particles. In addition, in the formula containing surfactant, if the surfactant is not compatible with the system, or with the evaporation of the solvent in the drying process, its concentration changes, resulting in the change of solubility, forming incompatible droplets, will also form the surface tension difference. All of these may cause shrinkage cavity. In the process of coating construction and film-forming, if there are external pollutants, it may also lead to shrinkage cavity, fish eye and other leveling defects. These pollutants are usually oil, dust, paint mist and water vapor from air, construction tools and base materials. The properties of the coating itself, such as construction viscosity, drying time, etc., will also have a significant impact on the final leveling of the film. Too high construction viscosity and too short drying time will usually produce poor leveling surface. Therefore, it is necessary to add leveling agent to help the coating to achieve a good leveling by adjusting some changes and coating properties during the construction and film-forming process.
Types of leveling agents
Leveling agents can be roughly divided into two categories. One is to adjust the film viscosity and leveling time. Most of these leveling agents are high boiling organic solvents or mixtures, such as isophorone, diacetone alcohol, solveso150, etc. The other is to adjust the surface properties of the paint film to play a role. Generally speaking, the leveling agent mostly refers to this kind of leveling agent. This kind of leveling agent migrates to the surface of the paint film through limited compatibility, which affects the surface properties such as the interfacial tension of the paint film, and makes the paint film obtain good leveling. According to the different chemical structure, there are three kinds of leveling agents: acrylic acid, organosilicon and fluorocarbon. ● acrylic leveling agent
Acrylic leveling agent includes pure acrylic leveling agent and modified acrylic leveling agent. Pure acrylic leveling agent includes traditional non reactive acrylic leveling agent and new reactive acrylic leveling agent with tube energy group. This is a kind of acrylic homopolymer or copolymer with different molecular weight. This kind of leveling agent can only slightly reduce the surface tension of the paint, but it can balance the difference of the surface tension of the paint film and obtain a truly flat, mirror like surface of the paint film. If the molecular weight is high enough, this kind of leveling agent also has the function of degassing and defoaming. The disadvantages of traditional non reactive acrylic leveling agent are high molecular weight products, which may produce fog shadow in the paint film, low molecular weight products, and may reduce the surface hardness of the paint film. The acrylic leveling agent with reactive functional groups can solve this problem well. It can provide good leveling property without fogging, reducing the surface hardness and sometimes improving the surface hardness. The main varieties of modified acrylic leveling agent are fluorine modified acrylic leveling agent and phosphate modified acrylic leveling agent. Different from the pure acrylic leveling agent, the modified acrylic leveling agent can significantly reduce the surface tension of the coating, which will have leveling property and good substrate wettability. ● silicone leveling agent
Organosilicon leveling agent has two remarkable characteristics. First, it can significantly reduce the surface tension of the paint, improve the wetting ability of the paint substrate and the fluidity of the paint film, and eliminate Benard vortex to prevent blooming. The ability to reduce surface tension depends on its chemical structure. Another remarkable characteristic is that it can improve the smoothness, anti hanging and anti adhesion of the coating. The disadvantage of this leveling agent is that there is a tendency to stabilize foam and affect adhesion between layers. Some of them also pollute the construction environment, such as oven drying. At present, there are three main types of its structure: polydimethylsiloxane, polydimethylsiloxane and organic modified polysiloxane. The organic modified polysiloxane is the most important one. Pure polydimethylsiloxane is rarely used because of its poor compatibility with coating system. Fluorocarbon leveling agent
Fluorocarbon leveling agent is characterized by high efficiency, but expensive. It is usually used when acrylic leveling agent and organic silicon leveling agent are difficult to play. However, there is a tendency to stabilize foam and affect adhesion between layers.
Application of leveling agent
For a certain formulation system, the appropriate type of leveling agent should be selected according to the nature of the formulation and the performance expected to be achieved by the leveling agent. 1. In the formulation of primer and intermediate coat, acrylic leveling agent is usually used in solvent paint system. If degassing property and substrate wettability are needed, medium molecular weight or high molecular weight acrylic leveling agent should be selected. In the primer, if a stronger substrate wettability is required, silicone leveling agent and modified acrylic leveling agent (such as fluorine modified acrylic leveling agent and phosphoric acid) which can significantly reduce the surface tension can be considered