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, and prevent shrinkage and ripples. Reduce the interfacial tension between the coating and the substrate, enhance the spreadability 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.
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.
Although it has a good surface condition control ability, it has many shortcomings. The compatibility is not good, it will affect the gloss of the coating film, and there will often be shrinkage and interlayer adhesion problems. 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 overcomes its shortcomings with modified compounds and exerts many special functional effects. The key to the performance and application 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-like 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. Therefore, 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 and propoxy groups, it is made into a water-oil dual-purpose silicone leveling agent, such as: MONENG-1080, 1090.
The larger the molecular weight, the stronger the ability to control the surface state, and the better the slip and anti-blocking properties. For example: MONENG-1070, 1071 series products.
There are two ways to link the modified polyester or polyether with siloxane: one is a silicon-oxygen bond; the other is a 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 exactly 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.
End group modified silicone
End-modified silicones with excellent smoothness In order to impart good smoothness to the coating film, Moneng Chemical has introduced some terminal-modified silicones.
Reactive leveling agent
In the coating and ink system of radiation fixation, there are defects such as poor wetting of the substrate, insufficient slippage, easy scratching, and poor leveling. In response to these problems, Moneng Chemical Company provides a series of reactive organically modified polysiloxane acrylic leveling agents, such as 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 form a coating film together with the resin to be firmly anchored on the surface of the coating film.