Leveling agent is a commonly used coating additive, which can promote the formation of a flat, smooth, and uniform coating film during the drying and film-forming process. A type of substance that can effectively reduce the surface tension of coating fluid and improve its leveling and uniformity. It can improve the permeability of the coating fluid, reduce the possibility of spots and marks during brushing, increase coverage, and make the film formation uniform and natural. Mainly surfactants, organic solvents, etc.
The quality of a leveling agent depends on its ability to provide uniform surface tension and is not directly related to the level of surface tension. The migration orientation of the leveling agent follows the “minimum energy principle” in physics, that is, it flows spontaneously from places with high energy to places with low energy. When the paint is applied to the surface of the substrate, two interfaces are formed, namely the “substrate-paint film interface” and the “paint film-air interface”.
There are many types of leveling agents, and the types of leveling agents used in different coatings are also different. High boiling point solvents or butyl cellulose can be used in solvent-based finishes. In water-based coating agents, surfactants or polyacrylic acid, carboxymethyl cellulose, etc
Leveling agents can promote the paint to form a flat, smooth and uniform coating film during the drying and film-forming process. Whether the coating film can achieve flat and smooth characteristics is called leveling. Leveling agents can be divided into surface state control leveling agents (short wave leveling) and flow and leveling accelerators (long wave leveling).
Classification
Leveling agents are broadly divided into two categories. One works by adjusting the viscosity of the paint film and the leveling time. Most of these leveling agents are high-boiling organic solvents or their mixtures, such as isophorone, diacetone alcohol, Solvesso150, etc.; the other It works by adjusting the surface properties of the paint film. Most of the leveling agents that people generally refer to refer to this type of leveling agent. This type of leveling agent migrates to the surface of the paint film through limited compatibility, affecting surface properties such as the interfacial tension of the paint film, so that the paint film can achieve good leveling. According to different chemical structures, there are three main categories of this type of leveling agent: acrylic, silicone, and fluorocarbon.
Silicone type
Commonly used silicone oils include polydimethylsiloxane and polymethylphenylsiloxane. Polydimethylsiloxane is used in coatings and inks. Although polymethylphenylsiloxane has good compatibility, it cannot control the surface state, so it is basically not used in leveling agents and is mostly used for high-temperature resistance.
Surface condition control leveling agent (short wave leveling)
It is usually a silicone leveling agent. The common structure is polyether-modified polydimethylsiloxane, which can reduce the surface tension of the paint and prevent shrinkage, ripples, Bernal vortex, blooming, orange peel, etc. Disadvantages. But it usually has effects such as foam stabilization and poor recoating. The function diagram is shown in the figure. Related products include BYK-333, BYK-3760, Tego Glide 450, etc.
Poor short-wave leveling means that there are vortex-like traces formed by Bernal vortices on the surface of the paint film. Good short-wave leveling does not mean that the paint film is flat, but that the appearance is uniform.
Bernal vortex
Polydimethylsiloxane
Although it has good surface state control capabilities, it has many shortcomings. such as poor compatibility, which will affect the gloss of the coating film, and shrinkage holes and interlayer adhesion problems often occur. Polydimethyl siloxanes vary in molecular weight and have different compatibility and uses. Organically modified polydimethyl loxane has obvious advantages over silicone oil. lt not only retains the advantages of silicone but also uses modified products to overcome its shortcomings and exert many special functional effects key to the performance and use of modified siloxane is the molecular weight and type of siloxane, the type of modified compound and its position in the molecule there are many ways to modify it.
Polyether polyester-modified organosilanes
Organopolysiloxanes belonging to the comb structure. The n+m is about 50-250 and the molecular weight is controlled between 1000-150000. Its compatibility is dependent on the polyether and polyester to adjust, the longer the chain the better the compatibility. Polyether modification is the most common in this category, usually using ethylene oxide and propylene oxide. With the increase of ethoxyl content, its compatibility with water also improves, so it is also possible to synthesize water-soluble silicone-based leveling agents. Ethylene oxide and propylene oxide can be used alone or in combination to control hydrophilicity and lipophilicity. If both ethoxy and propoxy are contained, water- and oil-soluble silicone leveling agents are made, such as the 1080 and 1090 series products of Moenen Chemical.
The higher the molecular weight, the better the surface control ability, the better the slip and anti-adhesion, e.g. 1070, 1071 series products of Monon Chemical.
There are two ways to link the modified polyester or polyether with silicone: one is silicone-oxygen bonding; the other is a silicone-carbon chain, generally speaking, the former is not as good as the latter regarding thermal stability and water resistance.
Siloxane modified with polyether, polyester, and resin compatibility has been greatly improved, reduce surface tension, the ability to control surface flow, slip enhancement, anti-shrinkage, and anti-adhesion effects are also very good, and individual products and interlayer adhesion problems. Especially polyether-modified polysiloxane, thermal stability is not good, easy to stabilize foam. Be sure to pay attention to the negative effects of these products when applying.
Alkyl modified organosilane
As mentioned earlier, polyether-modified polysiloxanes have some shortcomings; alkyl-modified polysiloxanes have precisely these advantages.
This series of polysiloxanes also belongs to the comb structure. The molecular weight of these products is relatively small, around 10,000, and n+m is about 30-50. The purpose of alkyl modification is mainly to improve thermal stability, compatibility unsteady foaming, and even anti-foaming function. However, with the growth of the modified alkyl chain, its ability to reduce surface tension decreases. The relationship between alkyl chain length and surface tension is shown in Table 1-1.
Table 1-1 Relationship between alkyl chain length and surface tension
| modified alkyl chain | ——CH3 | ——CH2——CH3 | ——(CH2)9 CH3 | |
| Surface tension/(mN/m) | 20.6 | 26.2 | 31.4 | |
Generally, the carbon chain is controlled between C1-C14, so the molecular weight is not too large.
The three modification methods of polydimethylsiloxane are introduced above. The modification methods are different, the dosage and structure of the modifier are different, and the performance of the products is also different. The leveling agents produced by the three different modification methods and their heat resistance are also quite different. Moneng Chemical’s 1060 silicone-modified polysiloxane has good heat resistance and can be used within the temperature range of 200-220°C.
End group modified silicone
Terminal-modified silicone that imparts excellent slippery properties. In order to impart good slipperiness to the coating film, Moneng Chemical has launched some terminal-modified silicones.
Reactive leveling agent
In radiation-cured coatings and ink systems, there are defects such as poor wetting of the substrate, insufficient smoothness, easy scratching, and poor leveling properties. In response to these problems, Moneng Chemical Company provides a series of reactive organically modified polysiloxane acrylic leveling agents, including Moneng Chemical’s 1073, 1074 and other products. The former has good compatibility and poor slippery properties. The latter has poor compatibility and good slipperiness.
It can be seen from the structural formula that the modified organic compound is acrylate, which is used to adjust its fluidity and compatibility, and its slipperiness is determined by silicone. The double bonds of the acrylic group can participate in the free radical polymerization reaction, and together with the resin form a coating film firmly anchored on the surface of the coating film.
Acrylic type
product description
Resin-type surface flow control agents are mostly linear resin polymers, mainly including acrylic resin, urea-formaldehyde resin and melamine-formaldehyde resin. In general systems, the compatibility of these resins is limited. They will accumulate on the surface to form a new resin film, which will balance the surface tension of the coating. However, they will not reduce the surface tension, so they will not affect the surface tension. The flow of paint is often called a flow accelerator. This type of leveling agent contains acrylic resin as the main component.
Acrylic leveling agents can not only promote the flow and leveling of the coating film, but also will not affect the interlayer adhesion of the coating film, and also have a defoaming effect.
Flow and leveling accelerator (long wave leveling)
It is usually a non-silicon leveling agent – acrylic polymer leveling agent. This type of leveling agent can balance the difference in surface tension of the paint film and obtain a truly flat, mirror-like paint film surface. The local surface tension identity is adjusted by orienting within the coating through compatibility with the system. The long-wave leveling agent has too good compatibility, which is not conducive to the formation of interfaces on the surface and cannot provide leveling; the compatibility is too poor, uneven dispersion, and shrinkage-like defects, resulting in haze and loss of gloss in the paint film; suitable phase Capacitive, the leveling agent forms a new interface film on the surface to play a leveling role, as shown in the figure below. Therefore, acrylate polymer leveling agents with medium molecular weight (4000＜Mn＜20000) are often used for long-wave leveling.
The driving force for long-wave leveling to help paint film leveling is the surface tension of the paint film. The time it takes for the paint film to be completely flat is inversely proportional to the surface tension. That is to say, the higher the surface tension, the better the leveling effect. Therefore, acrylic leveling agents can provide better flatness and good long-wave leveling effects.
Leveling principle
The compatibility of acrylic leveling agents is an important indicator of its ability to control the surface condition of the coating film. The compatibility is too good, and it dissolves in the coating film, and will not form a new interface on the surface of the coating film, and cannot provide leveling effect; the compatibility is too poor, and it is impossible to evenly distribute on the surface of the coating film, and will aggregate with each other. , prone to shrinkage-like defects. It will reduce the gloss of the coating film and produce undesirable side effects such as haze. Only with ideal controlled compatibility will a new interface layer be formed on the surface of the coating film to play a leveling role.
Product Categories
The controlled nature of acrylate leveling agents is achieved by changing molecular weight and polarity. Homopolymer compatibility is not as good as copolymer, such as homopolymerized acrylic acid is usually poorly compatible with epoxy, polyester, polyurethane, and other coatings used in the resin, if it is physically mixed, the formation of the surface state of the non-glossy film, so acrylic homopolymer is not very suitable for leveling agent. Ideal leveling agents mostly copolymer can be terpolymer, can also be modified copolymer, only copolymer can change the polarity of the polymer through different monomers and glass flower temperature.
Usually, the number average molecular weight of the acrylate leveling agent is controlled between 6000-20000, the molecular weight distribution is narrow, the glass bloom temperature is controlled below -20℃, the surface tension is below 25-26mN/m. Such compatibility-limited acrylic copolymers are considered good leveling agents.
Acrylate leveling agents can be homopolymers or copolymers; they can be linear structures or with branched chains; they can be random copolymers or block copolymers.
(1)Fluorine-modified acrylic leveling agent This type of leveling agent is widely used, fluorine-modified acrylic so that the advantages and disadvantages of fluorine and acrylic complement each other, so that this type of leveling agent tends to be more perfect. The advantages and disadvantages of acrylic and fluorine leveling agent see table 1-1
Advantages and Disadvantages of Acrylic and Fluorine Leveling Agents
| Leveling agent | advantage | shortcoming |
| Acrylic leveling agent | Strong leveling and defoaming capabilities, without affecting inter-layer adhesion | The base material has poor wettability and cannot eliminate shrinkage cavities. |
| Fluorine leveling agent | Good substrate wettability and strong anti-shrinkage ability | Stable foam, poor adhesion between layers, unable to be recoated, expensive |
Through the modified leveling agent, it has better surface control ability, unstable foaming, can be recoated, and has good anti-crater and substrate wetting capabilities. Representative products include 1154 from Moneng Chemical.
Application of acrylic leveling agents. Pure acrylic leveling agents have little effect on surface tension, so they are often used as flow and leveling aids, especially for iron printing coatings and coil coatings, to eliminate roller marks. is beneficial. There are also wood paints that can be applied to help eliminate brush marks.
When applying, special attention should be paid to the compatibility with the paint. Generally speaking, those with large molecular weights have poor compatibility, but have good flow and leveling properties; those with small molecular weights have good compatibility, but have poor flow and leveling properties.
Acrylic leveling agents can not only be used in oil-based paints and water-based paints, but are also commonly used leveling agents in powder coatings. Such products include 1153 from Mo Neng Chemical.
Main Effect
paint is decoration and protection. If there are flow and leveling defects, it will not only affect the appearance, but also damage the protective function. For example, the formation of shrinkage cavities will cause insufficient thickness of the paint film, and the formation of pinholes will lead to discontinuity of the paint film, which will reduce the protective properties of the paint film. During the construction and film-forming process of paint, some physical and chemical changes will occur. These changes and the properties of the paint itself will significantly affect the flow and leveling of the paint.
After the coating is applied, new interfaces will appear. Generally, they are the liquid/solid interface between the coating and the substrate and the liquid/air interface between the coating and the air. If the interfacial tension of the liquid/solid interface between the paint and the substrate is higher than the critical surface tension of the substrate, the paint will not be able to spread on the substrate, and leveling defects such as fish eyes and shrinkage holes will naturally occur.
of the paint film will cause temperature, density and surface tension differences between the surface and the interior of the paint film. These differences will in turn lead to turbulent motion inside the paint film, forming the so-called Benard vortex. Benard vortices can cause orange peel; in systems containing more than one pigment, if there is a certain difference in the mobility of the pigment particles, Benard vortices are also likely to cause floating colors and blooming, and vertical surface construction can cause silk streaks.
During the drying process of the paint film, some insoluble colloidal particles are sometimes produced. The production of insoluble colloidal particles will lead to the formation of a surface tension gradient, which often leads to the generation of shrinkage holes in the paint film. For example, in a cross-linked curing system, the formula contains more than one resin. During the drying process of the paint film, as the solvent evaporates, the less soluble resin may form insoluble colloidal particles. In addition, in formulas containing surfactants, if the surfactant is incompatible with the system, or as the solvent evaporates during the drying process, its concentration changes, resulting in a change in solubility and the formation of incompatible droplets. A difference in surface tension will also occur. These may cause shrinkage cavities.
During the construction and film-forming process of the coating, if there are external pollutants, it may also cause leveling defects such as shrinkage cavities and fish eyes. These pollutants are usually oil, dust, paint mist, water vapor, etc. from the air, construction tools and substrates.
The properties of the paint itself, such as application viscosity, drying time, etc., will also have a significant impact on the final leveling of the paint film. Too high an application viscosity and too short a drying time will often produce a poorly leveled surface.
Therefore, it is necessary to add a leveling agent, adjust some changes during the construction and film-forming process of the paint, and adjust the properties of the paint to help the paint obtain a good leveling.
V.Application
For a certain formula system, the appropriate leveling agent type should be selected based on the properties of the formula and the performance expected to be achieved by the leveling agent.
1. Solvent-based coating system
Acrylic levelers are often used in primer and midcoat formulations. If degassing and substrate wettability are required, choose
Choose medium or high molecular-weight acrylic, leveling agents. In primers, if stronger substrate wetting is required, consider using silicone leveling agents and modified acrylic leveling agents that can significantly reduce surface tension (such as fluorine-modified acrylic leveling agents and phosphate esters). Modified acrylic leveling agent), if silicone leveling agents and fluorine-modified acrylic leveling agents have side effects such as foam stabilization and affecting interlayer adhesion, phosphate-modified acrylic leveling agents should be used.
In topcoat and clearcoat formulations, the requirements for the appearance of the paint film are relatively high. Generally, low molecular weight acrylic leveling agents can be used, which will achieve good leveling properties and will not easily produce haze in the paint film. In cross-linked curing systems, the use of acrylic leveling agents containing reactive functional groups often achieves better leveling properties while improving the physical and chemical properties of the paint film. If the paint film needs to have better fluidity or needs to be slippery and scratch-resistant, a silicone leveling agent is necessary. In this case, it is best to use a silicone leveling agent and an acrylic leveling agent in combination.
It should be noted that when constructing on vertical surfaces, silicone leveling agents can effectively reduce the tendency of the coating to sag while providing leveling properties. In addition, in the formulation of metallic flash paint, silicone leveling agents should be used with caution, because they may cause uneven arrangement of flake aluminum pigments and uneven color of the paint film.
2. Powder coating system
The powder coating leveling process is divided into two stages. The first stage is the melting of the powder particles, and the second stage is the melting of the powder particles and their flow into a smooth paint film. Powder coatings do not contain solvents and do not produce surface tension gradients during the film formation process. Leveling is more related to the wetting of the substrate.
Acrylic leveling agents are often used in powder coatings. If the leveling agent is in liquid form, it generally needs to be made into a masterbatch before it can be used. There are also acrylic leveling agents made into powders, which are specially used for powder coatings. These products adsorb liquid acrylic leveling agents on silica powder. Some low-end leveling agents use calcium carbonate to adsorb.
If the powder coating requires slipperiness and scratch resistance, a silicone leveling agent must be used. There are already silicone leveling agents made into powder that are specially used for powder coatings. When using silicone leveling agents, care should be taken to avoid the formation of shrinkage cavities.
3. Water-based coating system
Water-based coating systems are divided into water-soluble systems and latex systems.
In water-soluble systems, it is necessary to strongly reduce the surface tension of the system. The most commonly used are silicone leveling agents and fluorocarbon compounds.
Physical leveling agents perform the same function as they would in solvent-based coating systems. Of course, if a truly smooth surface is required, acrylic leveling agents for water-based systems are necessary.
For latex systems, the film-forming mechanism is completely different, and the viscosity does not change with the volatilization of the solvent. The use of leveling agents in the formula may improve the substrate wettability of the paint. Acrylic leveling agents can improve the flatness of the paint film, but the main flow properties of the paint are more controlled and adjusted by adding rheology control agents.