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The Leveling Principle of Leveling Agent

Nov 16, 2020

The leveling principle of silicone leveling agent: surface tension is the main driving force for the leveling of the coating film, but poor surface tension is the main source of surface defects of the coating film. Increase the surface tension of the coating film to achieve a balance of surface tension, which is very beneficial to the flow and leveling of the coating film, but it cannot be done. Therefore, people have to reduce the surface tension of the coating film to eliminate the difference in surface tension, control the surface state of the coating film, and eliminate surface defects.
Leveling principle
Surface tension is the main driving force for coating film leveling, but poor surface tension is the main source of coating film surface defects. Increase the surface tension of the coating film to achieve a balance of surface tension, which is very beneficial to the flow and leveling of the coating film, but it cannot be done. Therefore, people have to reduce the surface tension of the coating film to eliminate the difference in surface tension, control the surface state of the coating film, and eliminate surface defects. For example, eliminate the Benal vortex to prevent blooming and orange peel. Eliminate the difference in surface tension gradient on the surface of the coating film to prevent shrinkage and ripples. Reduce the interfacial tension between the coating and the substrate, enhance the spread of the coating, improve the adhesion, and reduce or eliminate the shrinkage caused by the substrate. Silicone oil and organically modified polysiloxane are a kind of surface condition control agent that has been used earlier and widely used in the coating industry, and can basically eliminate surface defects caused by poor surface tension.
Product Category
Silicone oil
Commonly used silicone oils are polydimethylsilane and polymethylphenylsiloxane. Polydimethylsiloxane is used in coatings and inks. Although polymethylphenylsiloxane has good compatibility, it does not have the ability to control the surface state, so it is basically not used in leveling agents and is mostly used for high temperature resistance.

Polydimethylsiloxane
Although it has a good surface condition control ability, it has many shortcomings. The compatibility is not good, which will affect the gloss of the coating film, and often appear shrinkage, interlayer adhesion problems, etc. The molecular weight of polydimethylsiloxane is different, and its compatibility and use are also different. Compared with silicone oil, organically modified polydimethylsiloxane has obvious advantages. It not only retains the advantages of siloxane, but also uses modified compounds to overcome its shortcomings and exert many special functional effects. The key to the performance and use of modified siloxane is the molecular weight, type, type of modified compound and position in the molecule of siloxane. There are many ways to modify it.

Polyether polyester modified organosiloxane
It is an organopolysiloxane with a comb structure. n+m is about 50-250, and the molecular weight is controlled between 1000-150000. The compatibility is adjusted by polyether and polyester. The longer the chain, the better the compatibility. Among these types of polyethers, the most modified ones are usually ethylene oxide and propylene oxide. As the content of ethoxy groups increases, its compatibility with water also increases, so it is possible to synthesize water-soluble siloxane leveling agents. Ethylene oxide and propylene oxide can be used alone or in a mixture to control hydrophilicity and lipophilicity. If it contains both ethoxy group and propoxy group, it is made into a water-oil dual-use siloxane leveling agent, such as: MONENG-1080, 1090 series products.

The larger the molecular weight, the stronger the ability to control the surface state, and the better the slip and anti-blocking properties, such as: MONENG-1070, 1071 series products.

There are two ways to link modified polyester or polyether with siloxane: one is silicon-oxygen bond; the other is silicon-carbon chain. Generally speaking, the thermal stability and water resistance of the former are not as good as the latter.

The compatibility of polyether and polyester modified siloxane with resin has been greatly improved, the surface tension is reduced, the ability to control surface flow, slippage, anti-cratering, and anti-blocking effects are also very good. , Some products still have problems with adhesion between layers. Especially polyether modified polysiloxanes have poor thermal stability and are easy to stabilize foam. Be sure to pay attention to the negative effects of these products when applying.

Alkyl modified organosiloxane
As mentioned earlier, polyether-modified polysiloxanes have some shortcomings; alkyl-modified polysiloxanes have just these advantages.

This series of polysiloxane products also belong to the comb structure. The molecular weight of this type of product is relatively small, about 10,000, and n+m is about 30-50. The purpose of modification with alkyl groups is mainly to improve thermal stability, compatibility and unstable foam, and even have defoaming function. But as the modified alkyl chain grows, its ability to reduce surface tension also decreases. The relationship between alkyl chain length and surface tension is shown in 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 described above. The modification methods are different, the amount and structure of the modifier are different, and the performance of the product is also different. The leveling agent produced by the three different modification methods, Its heat resistance is also completely different. MONENG-1060 silicone modified polysiloxane has good heat resistance and can be used in the temperature range of 200-220℃.

End group modified silicone
End-modified silicones with excellent slip properties In order to impart good slip properties to the coating film, Moneng Chemical has introduced some end-modified silicones.

Reactive leveling agent
In the coating and ink system of radiation curing, there are defects such as poor wetting of the substrate, insufficient slip, easy scratching, and poor leveling. In response to these problems, Moneng Chemical Co., Ltd. provides a series of reactive organically modified polysiloxane acrylic leveling agents, including MONENG-1073, 1074 and other products. The former has good compatibility and poor slippage. The latter has poor compatibility and good smoothness.

It can be seen from the structural formula that the modified organic substance is acrylate, which is used to adjust its fluidity and compatibility. Its smoothness is determined by siloxane. The double bond of the acrylic group can participate in the polymerization reaction of free radicals, and together with the resin form a coating film firmly anchored on the surface of the coating film.

Collapse edit this paragraph application
It can be seen from the above that the modification methods of silicone leveling agents are different, the modified materials are different, the structure is different, the molecular weight is different, and the application is also different. So pay attention to the following points when applying.

Choose leveling agent according to the problem
Silicon leveling agents have no "universal type" that can solve any problems. If you choose improperly, it will bring negative effects. If you want to eliminate the difference in surface tension, it is best to choose a polyether-modified polysiloxane that reduces the surface tension and does not choose an alkyl-modified polysiloxane. Requires high temperature baking. Can not choose polyether modified, to choose alkyl modified or polyester modified. Long-term slipperiness is required. Additives cannot be selected. Instead, reactive types must be selected. It is required to improve the interface relationship and reduce the interfacial tension. It is best to use a low molecular weight polyether modified substrate wetting agent.

Adhesion between layers
Although this type of leveling agent has been modified to improve compatibility and adhesion problems, some still have problems. For leveling agents with poor mobility, it is easy to apply a second layer after the film is completely dry. Something went wrong. In addition, overheated baking destroys the structure of the leveling agent, which is prone to adhesion problems. Choose a silicone leveling agent that does not contain active groups and has good mobility at the end of the modifier.

In addition, to improve adhesion, you can also choose a leveling agent with a small molecular weight. Because short-chain siloxane is not easy to form a continuous film at the air/liquid interface, it increases the contact area between the topcoat and the primer and improves the layer. Between adhesion.

Stabilization
Almost reduce the surface tension, the stronger the leveling agent, the stronger the foam stabilization, especially the polyether modified polysiloxane, it is easy to be oriented at the air/liquid interface, and the air in the coating is leveled The agent "wrapped" and formed bubbles. Pay special attention to the application, you can choose unstable foam or low foam.

Thermal stability
When using in heating and solid-type coatings, you must pay attention to the heat resistance range of the leveling agent, and the baking temperature must be within the heat resistance range of the leveling agent, otherwise the leveling agent will decompose during overheating and baking, resulting in coating The film has negative effects such as shrinkage, difficulty in recoating, and decreased gloss. For the primer or secondary primer, it is best to use an alkyl-modified polysiloxane that does not reduce the surface tension. The top coat can be a polyester-modified polysiloxane that can be recoated.

Dosage and adding method
The dosage depends on the need. The total recommended dosage is 0.01%-1.0% of the paint amount. How much is appropriate? It depends on the composition of the formula, the solubility of the solvent, the compatibility of the resin, and the interaction between the additives. Depending on factors such as relationship. The effect of insufficient dosage is not obvious, and excessive dosage will cause fine wrinkles and even shrinkage. So the dosage must be moderate.

In order to eliminate brush marks and roll marks, it is best not to use leveling agents that reduce surface tension. Acrylic leveling agents or alkyl-modified leveling agents can be used. Keeping the surface tension of the coating as much as possible is helpful for leveling. Add The amount of 0.3-0.5% can also be used with solvent-based leveling agents.

Other leveling agents
There are also fluorocarbons and high-boiling solvents. Fluorocarbon leveling agent has the highest surface activity and can reduce the surface tension of the coating to 16-18mN/m, so it has the strongest surface state control ability. In a nutshell, fluorocarbon leveling agents have all the functions of silicone leveling agents. Compared with silicone leveling agents, the ability to reduce surface tension is stronger and the effect of surface state control is better. But it also has its negative effects, like interlayer adhesion, and foam stabilization is more difficult to overcome. Its price is also very expensive, which affects its application.

Solvent-based leveling agents are mostly mixtures of high-boiling point solvents, and their main mechanism of action is to slow down the volatilization rate of solvents, reduce coating viscosity, and improve leveling agents. But it is against the requirements of environmental protection, anti-sagging control, and quick-drying, so people don't use it much.

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