All silicones suitable for paints and coatings are based on polydimethylsiloxane with a relatively simple structure. Their basic chain consists of several Si-O units (siloxane units), each unit
Silicon atom with two methyl groups. One way to modify this structure is to change the chain length. As the chain length increases, the viscosity of these silicone oils increases, so their viscosity is a useful measure of molecular weight. For some commercial products, the viscosity value is even given as part of the product name. The relatively high molecular weight also means reduced solubility and poor compatibility in the coating system.
In low molecular weight compounds, not only linear molecules but also cyclic structures are possible. In both cases (linear and cyclic), the typical silicone effect is to reduce the surface tension caused by the dimethyl structure.
Low molecular weight polydimethylsiloxane (number of dimethyl x <60) and cyclic silicone (x = 3 to 6) can reduce surface tension and improve fluidity. Their viscosity is between 1 and 50 mPas. Cyclic products (and linear products with x < 6) are volatile, so their use is not without problems: they evaporate from the paint film and may then volatilize on the surface, for example.
In the oven, it condenses into a high molecular weight product, which then contaminates other coatings and substrates and causes shrinkage. High molecular weight products (60 <x <100; viscosity 50-100 mPas) will also improve the slipperiness of the coating system. Polydimethylsiloxanes of x~1200 (viscosity~60,000 mPas) have an appropriate balance between compatibility and may not be compatible with ordinary adhesive systems as defoamers. Higher molecular weight products (x> 1400, viscosity> 100,000 mPas) are ultimately so incompatible and insoluble in coating systems that can cause cratering, so they can only be used for special effects (hammer finishes).
Polymethylalkylsiloxane
Not only can the performance range of the polysiloxane be changed by changing the chain length, but also by replacing (partially) the methyl group of the siloxane unit with other side chains.
Polysiloxanes with methylalkyl instead of dimethyl also exhibit the typical characteristics of silicones, but the surface tension of these compounds increases significantly with increasing chain number. The length of the alkyl side chain.
Organically modified polysiloxane
For coating applications, a more important factor is that some methyl groups can be replaced with chemically completely different groups. Then the general term used is "organic" modification? Polysiloxane. Organic modification is an elegant method to control the compatibility of silicone products and coating systems. The introduced side chain is improved. Therefore, the compatibility of the polysiloxane-based chain and the influence as described above are secondary. Polysiloxanes are the most commonly used polyether modified chains. This type of modification offers a variety of possibilities, and it is possible to synthesize silicone additives with a very specific range of properties to suit various applications.
The dimethyl structure is the cause of the low surface tension of the silicone additive, so it is possible to obtain some specific surface tension values for the modification of polyether (x:y) by changing the ratio. As mentioned above, the remaining dimethyl groups can also be replaced with methylalkyl groups. As a result, the surface tension further increased, and the decrease in the surface tension of the coating due to additives proved to be correspondingly smaller.
In addition, the structure of the polyether chain itself can vary. Polyethers are usually composed of ethylene oxide units (EO) and propylene oxide units (PO). Polyethylene oxide is very hydrophilic (polar), while polypropylene oxide is hydrophobic (non-polar). Therefore, the polarity of the entire silicone additive can be controlled by the EO/PO ratio. A high proportion of EO even increases the polarity, and the additives become more compatible in polar coating systems, even if they are water soluble. However, there is also a tendency for foam to stabilize. On the other hand, a high PO ratio reduces water solubility and increases defoaming performance. In addition to the EO/PO ratio and the number of polyether chains, other important factors are whether the side chains are randomly distributed along the silicone-based chain (comb structure) or in a block structure, or whether the polysiloxane chain position is modified only at the end.
The last detail is the type of connection between the polyether side chain and the basic skeleton of the polysiloxane. It can be seen from the structural formula above that the polyether chains are not directly connected but through (short) alkyl chains; such molecules are resistant to hydrolysis. If the connection is direct, hydrolysis usually easily leads to polyether chain breakage and condensation.
Polysiloxane to provide a higher molecular weight structure (compatibility loss, the risk of shrinkage).
Other modifications
Although modification with polyether chains is the most common type of modification, other possibilities are also conceivable and realized in practice. Polyester and aralkyl are interesting as modifying elements. Silicone additives modified in this way can also reduce the surface tension of the coating system. Compared with polyether modified products, these additives, however, have much higher heat resistance. Polyether modified polysiloxanes thermally decompose at temperatures above about 150°C, while polyester and aralkyl modified products are stable at temperatures up to 200°C and higher. If possible, the degradation of additives should be avoided because it may cause coating problems and interlayer adhesion problems.
Hangzhou Silva New Material Technology Co., Ltd. was established in 2014. It is a young but fast-growing company, specializing in the research, development, production and sales of silicone chemicals. Silway specializes in modified silicone and formulation technology, and the product line covers agricultural silicone adjuvant , silicone waterproofing agents, cosmetic silicones, silicone adjuvant , modified silicones for die casting, etc. https://www.silways.com/