Action mechanism of leveling agent and summary of using skills
Leveling agent is a commonly used coating additives, it can promote the formation of coating in the dry film forming a smooth, smooth, uniform coating. Many types of leveling agent, leveling agent used in different types of paint are not the same.
Flowing agent overview
After the paint construction, there is a flow and drying film forming process, and then gradually form a smooth, smooth, uniform coating. Coating can achieve the smooth features, known as leveling. Shrinkage is the coating in the leveling and film forming process produced one of the defects. In the actual construction process, due to poor leveling, brush marks appear when brushing, roller rolling roll marks occur when orange peel spray, accompanied by shrinkage in the drying process, pinholes, sagging phenomenon, Both are called poor leveling, the occurrence of these phenomena reduces the decoration and protection of coatings.
Many factors affect the leveling of the paint, the solvent volatility gradient and dissolution properties, the coating surface tension, wet film thickness and surface tension gradient, coating rheology, construction technology and the environment, the most important factor is the paint surface Tension, the surface tension gradient generated by the wet film during film formation and the surface tension uniformity of the wet film surface. Improve the leveling of the coating need to consider adjusting the formulation and adding appropriate additives to make the coating have the appropriate surface tension and ability to reduce the surface tension gradient.
Flowing agent mechanism
Film drying film defects are common shrinkage, orange peel, brush marks, rolling marks, sagging and so on.
Shrinkage refers to the coating formed on the irregular, like bowl-like small depression, the coating lost flatness, often with a drop or a small piece of impurities as the center, around the formation of a circular edge. From a leveling point of view, it is a special kind of "point-like" flow unevenness, resulting from the surface of the coating film. Its shape can be divided into planar, crater, point, Type and so on.
Common anti-shrinkage leveling agent
Solvent-based leveling agents are mainly high boiling point solvent mixtures. Solvent-based coatings can only improve the leveling by increasing the solvent to reduce the viscosity, reduce the solid content of the coating and lead to sagging and other defects; or to maintain the solvent content, and only add the high-boiling solvent to adjust the volatilization rate to improve leveling, drying The time is also extended accordingly. Therefore, the two programs are not ideal. Only adding high boiling point solvent mixture, showing a variety of incremental properties (volatility index, distillation curve, the ability to dissolve) is more ideal. The main components of the solvent leveling agent is a variety of high boiling point mixed solvent, has good solubility, but also a good pigment wetting agent. Curing paint at room temperature due to solvent evaporation is too fast, the paint viscosity is too fast to prevent flow caused by brush marks, solvent evaporation lead to deterioration of the solubility of the base shrinkage, or in the baking paint boiling marks, foaming Such disadvantages ills using such additives is very effective. In addition the use of high boiling point leveling agent to adjust the volatilization rate, but also overcome the blemishes.
Restrictions on the compatibility of long-chain resins commonly used are polyacrylic acid, vinegar cellulose and so on. Their low surface tension, can reduce the surface tension between the coating and the substrate to improve the wettability of the coating to improve the substrate, remove the gas molecules adsorbed on the solid surface to prevent the adsorption of gas molecules too late In addition, they are not completely mixed with the resin, and can migrate to the surface to form a monolayer rapidly, so as to ensure the uniform surface tension on the surface and increase the resistance of the surface Shrinkage effect, thereby improving the smooth surface of the coating film. Polyacrylate leveling agent can be divided into pure polyacrylate, modified polyacrylate (or with silicone), acrylic alkali resin, etc., pure polyacrylate leveling agent and ordinary epoxy resin, polyester Resin or polyurethane coatings with poor compatibility of the resin, the application will form a misty coating. In order to improve their compatibility, usually with better compatibility of the copolymer.
Polydimethylsiloxanes, polymethylphenylsiloxanes, organo-modified polysiloxanes and the like are commonly used for long-chain silicone resins with limited compatibility. These substances can improve the wettability of the substrate and control the flow of the surface, play a role in improving the leveling effect, when the solvent is volatile, the silicone resin coating film surface to form a monolayer, to improve the gloss of the coating. Modified polysiloxane can be divided into polyether modified silicone, polyester modified silicone, reactive silicone, the introduction of organic groups can help improve the compatibility of silicone and coating resin, even if Concentration increases will not produce incompatibility and side effects, modified polysiloxane can reduce the coating and the substrate interfacial tension, improve the wettability of the substrate, improve adhesion, prevent hair loss, orange peel, reduce shrinkage Hole, needle eye and other coating surface sick.
Fluorosurfactants, whose main components are polyfluorinated polyolefins, also have good compatibility and surface activity with many resins and solvents, helping to improve wetting, dispersibility and leveling, as well as Adjust solvent evaporation rate in solvent-based paint.
Why use leveling agent in the paint formula
The main function of paint is to decorate and protect, if flow and leveling defects, not only affect the appearance, but also detrimental to protection. As the formation of shrinkage caused by the film thickness is not enough, the formation of pinholes will lead to the film of non-continuity, which will reduce the protective film.
Paint in the construction and film forming process, there will be some physical and chemical changes, these changes and the nature of the paint itself, will significantly affect the paint flow and leveling.
After the paint is applied, a new interface appears, typically the liquid / solid interface between the paint and the substrate and the liquid / air interface between the paint and the air. If the interfacial tension of the liquid / solid interface between the coating and the substrate is above the critical surface tension of the substrate, the coating will not spread on the substrate and will naturally result in flow defects such as fisheyes and shrinkage cavities.
Volatilization of the solvent during the drying of the paint film results in a difference in temperature, density and surface tension between the paint film surface and the interior, which in turn causes turbulent motion inside the paint film to form a so-called Benard vortex. The Benard vortex can cause orange peel; in systems with more than one pigment, Benard vortices are likely to cause floating and blossoming if the motility of the pigment particles is somewhat different, and vertical construction can lead to serrations.
During the drying of the paint film, some insoluble rubber particles are sometimes produced. The formation of insoluble rubber particles leads to the formation of a surface tension gradient, which often leads to shrinkage in the paint film. For example, in a cross-linked curable system, the formulation contains more than one resin. As the solvent evaporates during drying of the paint film, poorly soluble resins can form insoluble micelles. In addition, in surfactant-containing formulations, if the surfactant is incompatible with the system or changes its concentration as the solvent volatilizes during drying resulting in changes in solubility, incompatible droplets are formed, Surface tension differences also form. These may cause shrinkage holes.
Paint in the construction and film-forming process, if there are external pollutants, may also lead to shrinkage, fisheye and other flow defects. These contaminants are usually oil, dust, paint mist, water vapor and the like from air, construction tools and substrates.
The nature of the paint itself, such as construction viscosity, drying time, etc., can also have a significant effect on the final leveling of the paint film. Too high construction viscosity and too short a drying time usually result in poor leveling surfaces.
Therefore, it is necessary to add a leveling agent to help achieve a good leveling of the coating by making some changes in the coating during construction and film formation and adjusting the coating properties.
The type of leveling agent
Leveling agents can be broadly divided into two broad categories.
One is by adjusting the film viscosity and leveling time to play a role, such leveling agents are mostly high boiling point organic solvents or mixtures thereof, such as isophorone, diacetone alcohol, Solvesso150 and so on.
The other is by adjusting the surface properties of the paint film to play a role, most people call the leveling agent mostly refers to this type of leveling agent. Such leveling agents migrate to the surface of the paint film with limited compatibility and affect the surface properties such as the interfacial tension of the paint film so as to obtain a good leveling of the paint film. According to the different chemical structure, there are three main types of these leveling agents: acrylics, silicones and fluorocarbons.
Acrylic leveling agent
Acrylic leveling agents include pure acrylic leveling agents and modified acrylic leveling agents.
Pure acrylic leveling agents include conventional non-reactive acrylic leveling agents and new energetic acrylic leveling agents. These are a class of homopolymers or copolymers of acrylic acid of varying molecular weights. These leveling agents only slightly reduce the surface tension of the coating, but balance the difference in surface tension of the coating to obtain a truly smooth mirror-like surface. Such leveling agents also have the effect of degassing and defoaming if the molecular weight is high enough. The disadvantage of the traditional non-reactive acrylic leveling agent is that high molecular weight products may produce haze, low molecular weight products in the paint film, and may reduce the surface hardness of the paint film. Acrylic acid leveling agents containing reactive functional groups are a good solution to this paradox, providing good leveling without fogging nor reducing the surface hardness and sometimes increasing the surface hardness.
The main varieties of modified acrylic leveling agent for fluorine-modified acrylic leveling agent and phosphate-modified acrylic leveling agent. In contrast to pure acrylic leveling agents, modified acrylic leveling agents can significantly reduce the surface tension of the coating and thus provide good substrate wetting while leveling.
● Silicone leveling agent
Silicone leveling agents have two salient features. The first is that it can significantly reduce the surface tension of the coating, improve the substrate coating ability of the coating and the fluidity of the coating, and eliminate the Benard vortex to prevent the flowering. The ability to reduce surface tension depends on its chemical structure. Another notable feature is the ability to improve the smoothness, hang-off and blocking resistance of the coating. Disadvantages of these leveling agents are the presence of a stable foam, a tendency to affect the adhesion between the layers, and some also contaminate the construction environment, such as ovens. The structure of the current three main categories: polydimethylsiloxane, polymethyl alkyl siloxane, organically modified polysiloxane to organically modified polysiloxane most important, pure polydimethylsiloxane Alkanes are rarely used due to their poor compatibility with coating systems.
● Fluorocarbon leveling agent
Fluorocarbon-based leveling agents are characterized by their high efficiency but high cost, and are generally used when an acrylate leveling agent and a silicone leveling agent are hard to work with, however, there is also a tendency to stabilize the foam and affect interlayer adhesion.
Application of leveling agent
For a given formulation system, the choice of a suitable leveling agent should be based on the nature of the formulation and the desired performance of the leveling agent.
1, solvent coating system
Acrylic leveling agents are commonly used in primer and midcoat formulations. If degassing and substrate wetting are desired, medium or high molecular weight acrylic leveling agents should be selected. In the primer, if more substrate wetting is required, consider the use of silicone leveling agents and modified acrylic leveling agents that significantly reduce the surface tension (such as fluorine-modified acrylic leveling agents and phosphoric acid esters Acrylic acid leveling agent), if the silicone leveling agent and fluorine-modified acrylic leveling agent appears to foam, affect the side effects such as adhesion layer should be used phosphate-modified acrylic leveling agent.
In the topcoat and clear paint formulations, the appearance of the paint film is relatively high, the general choice of low molecular weight acrylic leveling agent, so that will get a good leveling, the film is not easy to produce haze. In the cross-linked curing system, the selection of reactive functional groups containing acrylic leveling agent often get better leveling, while improving the physical and chemical properties of the film. Silicone leveling agents are required if the film needs to have better fluidity or requires slip and scratch resistance, in which case it is best to use a silicone leveling agent in combination with an acrylic leveling agent.
It should be noted that the silicone leveling agent provides leveling performance while vertical construction, while effectively reducing the tendency of the coating to sag. In addition, metal flashing paint formulations, should be used with caution silicone leveling agent, because it may lead to the uneven arrangement of flaky aluminum pigment film appears uneven color.
2, powder coating system
The flow of powder coating process is divided into two stages. The first phase is the melting of the powder particles, the second phase of the powder particles melt and flow into a smooth film. Powder coatings do not contain solvents and do not create surface tension gradients during film formation. More leveling is associated with substrate wetting.
Acrylic acid leveling agent often used in powder coatings. If the leveling agent is liquid, it is generally necessary to prepare masterbatch before use. Also made of powder acrylic leveling agent, designed for powder coatings, such products is the liquid acrylic acid leveling agent adsorbed on the silica powder, some low-grade leveling agent with calcium carbonate adsorption.
If powder coatings require slip and anti-seize properties, silicone leveling agents, as well as powder-based silicone leveling agents that are specifically used for powder coatings, are used. Use silicone leveling agent should pay attention to avoid shrinkage.
3, water-based paint system
Water-based paint system is divided into water-soluble system and latex system.
In water-soluble systems, the need to strongly reduce the surface tension of the system, the most commonly used silicone leveling agent and fluorocarbon
Leveling agents act as they are used in solvent-borne coating systems. Of course, if a truly flat surface is required, an acrylic leveling agent for aqueous systems is necessary.
As for the latex system, the film forming mechanism is completely different, the viscosity does not change with the volatilization of the solvent. The leveling agent used in the formulation may increase the substrate wettability of the coating. Acrylic acid leveling agent can improve the smoothness of the paint film. However, the main flowability of the coating is controlled and adjusted more by the addition of a rheology control agent .