Adding a leveling agent will affect the appearance, interlayer adhesion and recoatability of the coating film. Conventional leveling agents include acrylic copolymers, silicones and fluorocarbon surfactants. Homopolymers and copolymers with methacrylate as monomers can usually be used as leveling agents, which can play a leveling role by controlling the degree of incompatibility between them and the base resin. Choosing different monomers, controlling the average molecular weight and modifying with different functional groups can all change the leveling effect. Silicone leveling agents have low surface tension and extremely high surface activity. This is because silicones can accumulate a large number of methyl groups on the surface. The new generation of silicone leveling agents are usually polyether modified, polyester modified or alkyl modified. In addition, the purpose of adjusting the performance of the leveling agent can be achieved by adjusting the molecular weight and silicon content. Fluorocarbon leveling agents are the most effective in reducing surface tension among all leveling agents, but they have the disadvantage of easy foaming when recoating.
The pigments and fillers are directly dispersed in the neutralized water-based polyester resin base material, the pH value is adjusted to 810-814, and the viscosity is adjusted to 0112 Pa·s with deionized water. We compared four acrylic leveling agents: A is an unmodified acrylic resin; B is a hydroxyl-containing acrylic resin; C is a fluorine-modified acrylic resin; D is a fluorine-modified hydroxyl-containing acrylic resin. Evaluate the effect of leveling agent on the leveling and anti-crater of the coating. The effect of the leveling agent's molecular weight, polarity (represented by hydroxyl content), fluorine modification, curing temperature and leveling agent dosage (012%, 014%, 016%, based on the total amount of coating) on the performance of the coating film was investigated. . The evaluation method is: apply the intermediate paint on a glass plate, flash dry at room temperature for 10 minutes, then bake at 150°C for 20 minutes, and finally over-bake at 190°C for 20 minutes (the over-baking condition is for the metal primer ). The effect of the leveling agent is evaluated by examining the gloss and defects of the coating film. The test result is expressed as 0～5, 0 is the best.
Results and discussion test results are shown in Figures 1 to 3 respectively. The ordinate indicates the degree of defects of the coating film from 0 to 5. The abscissa of each small chart represents three different hydroxyl content from 0 to 3. The four columns from left to right indicate different fluorine content, the leftmost column is 0 fluorine content, and the rightmost column is the highest fluorine content. Each row in each chart represents a specific molecular weight, the top row is low molecular weight, about 3000-5000, the middle row is medium molecular weight, about 15,000 to 20,000, and the bottom row is high molecular weight, about 50,000 to 100,000.
A. Class leveling agent is a low molecular weight polymer without any modification, and its leveling effect and anti-fisheye effect are poor. There is obvious migration when baking at higher temperature, which leads to stickiness and oil slick on the surface of the coating film. When the molecular weight of the leveling agent increases, the leveling effect is significantly improved, and migration is effectively inhibited. The high molecular weight leveling agent has high lipophilicity. When the amount is too high, it will cause the gloss to decrease and the fish eyes to increase. This is due to the incompatibility of the leveling agent with the base resin. Type A leveling agent will reduce the resistance to stone impact, especially when its dosage is large and the effect is more obvious in the state of over-baking. Type B leveling agent improves the polarity of the polymer. When the polarity is low, the leveling is very good. The leveling agent with high hydroxyl content has almost no leveling property. This is because the intra-molecular and intermolecular forces exceed other combined forces, making the leveling agent unable to move to the surface of the coating film. If the molecular weight of the high-polarity leveling agent is increased, the coating film will lose gloss.
B. When the molecular weight of the leveling agent is from medium to high, it has a great influence on the adhesion of recoating. The low-molecular-weight type B leveling agent slightly affects the stone-strike resistance of the coating film in the case of high dosage and over-baking.
C. The leveling agent has excellent leveling properties, but the degree of migration is high. If the amount is large and baked at high temperature, the coating film may become sticky. The fluorine-modified leveling agent is very efficient, and only a small amount of it needs to be added to have a significant effect. However, this kind of additives has poor interlayer adhesion, stone impact resistance and poor cross-cutting adhesion, and is not suitable for multilayer coatings.
D. Class leveling agent is much more efficient than non-fluorine modified leveling agent. It only migrates when low cross-linkable polar groups, high fluorine content, and baking at high temperatures. In a wide range of molecular weight and wide range of polar group content, the effect of this type of leveling agent is excellent. Only high molecular weight, high fluorine content and low hydroxyl content will cause the coating to lose gloss. As far as the recoatability is concerned, only when the fluorine content is high and the hydroxyl content is low will it have an effect.
The molecular weight of the leveling agent has the same effect on the leveling and recoatability of the coating film. An increase in the hydroxyl content will improve the adhesion of the coating, but it will have a negative impact on the leveling. Increasing the fluorine content can improve the leveling of the coating film, but it will reduce the adhesion. The combination of suitable fluorine content and hydroxyl content can achieve excellent leveling without reducing the adhesion of the coating film.
nylon leveling agent