The structure of leveling agents for coatings generally consists of a chain segment compatible with the system and a chain segment incompatible with the system and with a surface tension lower than the system. In the process of film formation, due to the separation and migration of incompatible chain segments with low surface tension, the additives will spontaneously migrate to the surface of the coating system, forming a single molecule film, so as to provide uniform surface tension for the system, and eliminate all kinds of film surface defects caused by surface tension gradient.
Silicone leveling agent is an important category of coating leveling agent, in general, low molecular weight (< 5 000) of polydimethylsiloxane can be used as leveling agent, but because of its poor compatibility, the use of more restrictions, leveling effect is not good enough, can not be applied to high grade system. Is widely used in coating of organic silicon flow ping agent is all sorts of modified silicone oil, according to the type of the modification, there are mainly polyether modified silicone oil, modified polyester modified silicone oil, phenyl silicone oil, alkyl modified silicone oil, phenyl alkyl modified silicone oil and alkyl - polyether modified silicone oil, including polyether modified silicone oil due to its structure can be adjusted in large scale, Performance can cover a variety of different requirements, can be applied to a variety of systems, is currently the main varieties of organic silicon leveling agent.
The basic structure of polyether modified silicone is shown in Formula (1).
The values of M and N determine the content of siloxane chain segments in the structure. The smaller the m/ N value, the higher the corresponding silicon content. The values of x and Y determine the relative molecular weight and hydrophilicity of the polyether chain segment (x represents the number of propylene oxide units, which is a hydrophobic unit; y represents the number of ethylene oxide units, which is a hydrophilic unit; when x > y, the polyether chain segment is mainly hydrophobic; when x < y, the polyether chain segment is mainly hydrophilic). The values of M, N, X and y determine the properties of the corresponding polyether modified silicone oil and their uses. If leveling agent is needed to have a good feel and shrinkage ability, usually need higher silicon content and longer siloxane chain length, can be achieved by increasing the M, N value; When the leveling agent is required to have good defoaming ability, high silicon content and propylene oxide content are required, and the value of M, N and X can be increased and the value of Y can be reduced to meet the requirements. When the need to improve the safety of the use of additives, reduce the risk of film defects, the need for relatively low silicon content, can be increased by m, reduce the value of N to meet the requirements; When it needs to be used in water-based system, due to the need for a certain water solubility, it needs not too high silicon content and a high content of ethylene oxide in the hydrophilic chain segment, which can be achieved by increasing the values of M and Y. When the flow ping agent in the high temperature system has to be used (such as) of the lacquer that bake, due to the low decomposition temperature of polyether, easy in use process decomposition, causing some paint film on the surface of the ills, such as painting, shrinkage cavity, then need to choose contain high temperature resistant groups (such as alkyl modified silicone oil, phenyl modified silicone oil and polyester modified silicone oil, etc.) of modified silicone oil, relatively speaking, Alkyl-modified silicone oil can not only provide leveling effect, but also has a good defoaming effect, but its compatibility is not good, usually not suitable for varnish. The phenyl silicone oil and polyester modified silicone oil have good compatibility, suitable for high temperature varnish system, while providing excellent compatibility and high temperature resistance. With the development of technology, there have been some leveling agents combining silicone and acrylate recently. This kind of leveling agent combines the advantages of both at the same time, with excellent early leveling and late leveling effect. According to the different chemical structure, it can be divided into two types of acrylate modified silicone leveling agent (silicone as the main chain, acrylate as the side chain) and silicone modified acrylic ester, and silicone modified acrylic ester leveling agent in addition to providing leveling effect, but also can provide pollution resistance.