(1) Leveling of paint film
The wet paint film obtained by most application methods of coatings is rough and needs to be leveled. The leveling of the paint film is not good, brush marks will appear when brushing, orange peel will appear when spraying, and rolling marks will appear when rolling. Because spraying is important in industry, orange peel is important. The ridges formed by the paint mist particles on the surface of the paint film are somewhat like orange peels. These ridges are larger than the spray point and are called orange peels. Paints containing highly volatile solvents are most prone to orange peel when sprayed. Coatings often use mixed solvents with fast and slow volatilization.
The combination of fast and slow volatilizing solvents is often used for spraying baking paint, which has two effects: ① Before the paint mist reaches the workpiece during spraying, most of the fast volatilizing solvents have volatilized, increasing the viscosity of the coating and reducing the tendency of the wet paint film. The solvent that evaporates slowly makes the wet paint film have enough time to level off, and the air mixed in the wet paint film can also escape, and the possibility of popping holes during baking is minimized. ② The volatility of the solvent is very low, the wet paint film can maintain low viscosity for a long time, the paint can be fully leveled by the surface tension, and a smooth paint film can be obtained, but the paint film on the convex part of the rough substrate surface is too thin. Because the surface tension increases with the decrease of temperature, the surface tension of the solvent is usually lower than that of the resin, so when the solvent volatilizes, the surface temperature of the paint film decreases and the concentration increases, which will increase the surface tension. It is volatile, and the viscosity in the center is still low, which is driven by the surface tension gradient of the paint. The paint with low surface tension covers the surface of the substrate with high surface tension to achieve a uniform paint film, but the paint film is uneven with the unevenness of the substrate surface. , that is, not smooth. With a combination of fast and slow volatile solvents, a paint film with reasonable flatness and no thin spots can be obtained.
The change of surface tension during solvent evaporation affects leveling, because the higher the surface tension, the easier the leveling of the paint film. Water-diluted coatings contain fast-volatile solvents (such as iso-T alcohols). Because the surface tension of water is large and that of iso-T alcohols is small, iso-T alcohols volatilize faster than water, which increases the relative proportion of water in the mixed solvent. The surface tension of the system gradually increases, which can promote leveling. When containing slow volatile solvents (such as ethylene glycol monohexane), the relative proportion of water will gradually decrease due to solvent volatilization, and the surface tension will also decrease accordingly, which is unfavorable for leveling.
The leveling of the actual paint film is a dynamic process. In addition to the surface tension, the smaller the surface roughness of the wet paint film, the lower the viscosity of the paint, and the thicker the wet paint film, the faster the leveling, but the paint with fast leveling is usually Sagging is prone to occur. Drooping is the downward flow of the wet paint film under the action of its own gravity, that is, the downward flow of the wet paint film.
(2) Retraction
涂料施工时的机械作用力能把表面张力高的涂料涂覆于表面张力较低的底材上,但涂料并不能润湿底材表面,湿漆膜就倾向于收缩成球状,而溶剂挥发造成湿漆膜黏度增大,在被拉成球之前,黏度已高到使流动基本停止,这样就油和有机分子的表面张力通常很低,在有油污的钢材上涂漆会造成漆膜回缩。赤手接触底漆膜,然后再涂表面张力较高的面漆,指印留下的油污就会将面漆的湿膜推开,面漆漆膜表面出现手的图案,这称为透印。大多数塑料的表面张力通常很低,在塑料上涂漆就更容易出现回缩。在含有硅油或氟碳表面活性剂的漆膜上重新涂漆时,表面活性剂的疏水端(即低表面活性剂部分)排列在漆膜的表面上,也使新的湿漆膜产生回缩。
为消除漆膜回缩,得到厚度均匀一致的漆膜,涂料涂布前通常需要对工件进行预处理。金属等材料的表面张力大,需要清除其表面的有机污物(主要是各种油污,包括工件加工后涂的防锈油),这称为除油。塑料等材料本身的表面张力小,需要在它们的表面上引进极性大的基团(如通基、殘基等),以增加其表面张力。
常用金属(钢铁、铝、锌等)的表面实际上覆盖的是它们的氧化物,这些自然形成的氧化物的性质并不均匀一致,需要进行化学处理,使金属表面的性质均匀一致,如通常对钢铁表面进行磷化,生成均匀的磷化膜。
总之,为提高涂料对底材的润湿性,涂装时底材表面通常需要预处理,使底材表面的表面张力高、性质基本一致,然后再涂布涂料,既可避免回缩问题,又能增大附着力。
高固体分涂料比常规涂料的表面张力高。因为要达到高固体分,就需要降低树脂的分子量,需要增加树脂中极性官能团的数量,如通基和殘基,这类官能团具有高极性,会产生更高的表面张力。为降低黏度,就要用氢键受体型溶剂,而不用要登类溶剂。氢键受体型溶剂为醇、配、酯、酮、遷基等强极性化合物,能产生更高的表面张力。溶剂的表面张力也较高,所以高固体分涂料因表面张力高,更易回缩,通常需要加表面活性剂以降低表面张力。
(3)缩孔
Shrinkage cavities are produced by small particles or droplets of low surface tension. These low surface tension substances exist on the substrate, in the coating, or fly down on the freshly coated wet film, generating a surface tension gradient. The low surface tension part flows to try to cover the surrounding high surface tension part, while the solvent The volatilization of volatilization increases the viscosity, hinders the flow, and finally forms concave shrinkage cavities at the low surface tension, and the appearance is small round, like a crater. Impurity particles can generally be seen in the center of the shrinkage cavity.
When spraying a flat panel, the paint film is thickest on the edge, and thinner slightly away from the edge, which is the thick edge phenomenon. Because on the side, the air flow is the largest and the solvent volatilizes the fastest, resulting in an increase in the resin concentration on the side and a large temperature drop, which increases the surface tension on the side and causes adjacent low surface tension coatings to flow over.
When spraying, the spatter in the air falls on the wet film. If the surface tension of the spatter is lower than that of the wet film, it will cause shrinkage; if it is higher than that of the wet film, it will cause the adjacent wet film with low surface tension to flow over and produce orange peel.
The higher the decorative requirements of the paint film, the higher the cleanliness required for the painting site. Modern painting workshops are usually arranged according to the required cleanliness level to ensure the required cleanliness. However, there are always some debris particles in most factories, and the actual coating environment conditions are not good. To avoid the formation of shrinkage cavities, the following measures can be taken: ①Increase the thickness of the wet paint film. The wet paint film is covered by dust particles and Nako dirt with low surface tension. If the paint film is thick, there will be no obvious shrinkage cavities; ②Increase the viscosity of wet paint film. If the viscosity of the wet paint film is increased enough to prevent the paint from flowing, even if there is a possibility of shrinkage cavities in the paint, shrinkage cavities will not occur, so adding a thixotropic agent can increase the viscosity of the wet paint film and prevent shrinkage cavities. ③ Low surface tension coatings are less likely to form shrinkage cavities, because there are fewer sundries with lower surface tensions than such coatings. Alkyd coatings have very low surface tension. The surface tension of polyester coatings is higher than that of acrylic coatings, and the shrinkage problem is greater. ④ Coating formula should be reasonable. It should not appear that the mutual solubility of the various components in the coating is not good, and some resins are precipitated due to poor solubility of the solvent due to solvent volatilization during the drying process.
During the coating storage, a small amount of macromolecules in the resin react to produce insoluble particles, and the poorly soluble resin precipitates insoluble small particles during the drying process, which may cause shrinkage. The surface active substance is incompatible with the coating, and a small amount of liquid droplets are precipitated. The concentration of the surfactant during drying exceeds its solubility, producing immiscible droplets. If excessive silicone oil is added to the paint or the viscosity is too high, shrinkage cavities will easily occur, so when designing the formula, it is necessary to reduce the shrinkage cavities as much as possible.
During the curtain coating construction, the paint curtain must be complete. If a particle or droplet with a lower surface tension than the paint curtain falls on the paint curtain, the flow driven by the surface tension difference creates a hole in the curtain. Curtain coating requires the use of low surface tension coatings.
The adhesion of the paint film to the substrate is mainly intermolecular force. In the extremely small distance (about 0.5nm), the intermolecular force can play a role, and the coating cannot have good adhesion to the substrate without good wetting. In the actual coating process, on the one hand, the surface tension of the substrate should be increased; on the other hand, if the surface tension of the coating is too high, the surface tension of the coating should be reduced so that the coating can fully wet the surface of the substrate and avoid shrinkage. and shrinkage cavity. High solids coatings always have higher surface tension and are more likely to develop shrinkage and craters than conventional coatings. Powder coatings are prone to shrinkage and shrinkage, and leveling agents are often required. The leveling agent can reduce the surface tension of the coating to below the surface tension of most debris that can cause shrinkage, and at the same time can promote the leveling of the coating, so it is also called anti-shrinkage crater leveling agent.
(4) leveling agent
The main function of coatings is decoration and protection. If the coating film is uneven or has irregular shrinkage cavities, it will not only fail to achieve the decorative effect, but also reduce or damage its protective function.
Traditional oil-based paints have less surface tension and usually have fewer film defects. Polyurethane coatings, epoxy-amine coatings, powder coatings, water-diluted coatings, high-solid coatings, etc. have high surface tension, and the disadvantages caused by surface tension need to be considered, and the leveling of these coatings needs to be improved to overcome shrinkage cavities. The measures taken from the aspect of coatings are: firstly, it is necessary to optimize the formula, manufacturing, process, etc. of the coatings, and secondly, use anti-crater leveling agent.
The function of anti-crater leveling agent: make the solvent have a uniform volatilization speed; reduce the viscosity of the coating and prolong the leveling time; form a very thin monomolecular layer on the surface of the coating film to provide uniform surface tension; reduce the surface tension of the coating, both Make the coating and the substrate have good wettability, and make no difference in surface tension between the coating and the substance that causes shrinkage.
There are two types of anti-crater leveling agents: high-boiling point mixed solvents and limited miscible resins, both of which have different mechanisms of action.
The first type is mixed with high-boiling point solvents such as aromatics, ketones, and esters. It has strong dissolving power and always maintains the dissolution of film-forming substances, and the paint is improved by reducing the viscosity of the paint by the solvent. Improve the leveling of the film, but reduce the solid content of the coating, resulting in sagging and prolonging the drying time.
In normal temperature drying coatings, because the solvent evaporates quickly, it is very effective to use a mixed solvent with a high boiling point and strong solubility as a leveling agent. Both F-1 nitro paint moisture-proof agent and F-2 perchlorethylene paint moisture-proof agent have this function. Solvent leveling agent not only has a leveling effect, but also a good wetting agent for pigments.
The second category is resins with limited miscibility, including cellulose acetate, polyacrylates, and silicone resins. This kind of substance can move to the surface of the wet paint film. Because of the small miscibility, it is enriched on the surface of the paint film, thus forming a "shell" on the wet film, hindering the volatilization of the solvent, prolonging the leveling time, and weakening the paint. The flow of the surface layer of the film, thus weakening the "trace" of flow, makes the paint film smoother. This type of leveling aid inhibits the volatilization of the solvent to prolong the flow time of the wet film, and also aggravates the sagging, and the limited compatibility affects the mechanical properties and gloss of the paint film, so the minimum dosage is usually used.
The higher the butyryl content of cellulose acetate, the better the leveling effect. It is used in polyurethane coatings and powder coatings.
The polyacrylate used as an anti-crater leveling agent is a copolymer with an average molecular weight of 6000-20000 and a viscosity of 4-12Pa·s at 65°C. If the viscosity is higher, the coating film will have almost no leveling. Conversely, if the viscosity is too low, polyacrylate will migrate to the surface of the paint film, causing changes in the physical properties of the paint film. The Tg of polyacrylate is generally below -20°C to ensure that the resin has a certain fluidity at room temperature, and the surface tension is very small, in the range of (2.5~2.6)x10-5N/cm. The amount of this type of leveling agent: epoxy resin is 0.5%~1.0% (mass) of film-forming substances, polyester resin is about 1.0%, acrylic resin is 1.0%~1.5%, which can be applied to powder coatings, solvent-based coatings and in water-based paints. With the increase of the amount of polyacrylate leveling agent, the surface tension reaches a minimum value, and then remains basically constant, and the amount of leveling agent is too large, which often produces fog on the surface of the paint film, reducing the recoatability of the paint film. Therefore, it should not be excessive.
Silicone oil and silicone modified resin are a kind of leveling agent used earlier and widely in the coating industry. Silicone oils include polymethylsiloxane and polymethylphenylsiloxane. The compatibility between dimethylpolysiloxane and coating resin is poor, and the compatibility can be increased through phenyl modification, but the effect is limited, so polymethylsiloxane modified with organic resin is often used.
According to the type of organic modification group, it can be divided into polyether modification and polyester modification. The larger m is, the better the affinity with the resin is. Use ,rz to control leveling. The compatibility of this leveling agent with the resin has a certain limit, the purpose is to enable it to migrate to the surface of the coating film and reduce the surface tension. Polyether-modified silicone is widely used, but it degrades in the presence of high temperature and water, and polyester-modified silicone leveling agent needs to be used at this time. In order to evenly mix the leveling aid into the powder coating, it is often loaded on a carrier such as silica or made into a masterbatch with a resin.
