When it comes to silicone oil, its solubility, heat resistance, and low-temperature resistance are the key factors determining whether it can be put to use.
Solubility: The "social circle" of oils
Can silicone oils "get along" with each other?
The answer is yes. Different viscosities of dimethyl silicone oil can be mixed in any proportion to form a uniform mixture with an intermediate viscosity. Similarly, dimethyl silicone oil and hydrogen-containing silicone oil can also be mixed in any proportion.
However, some combinations have "distances". For example, dimethyl silicone oil and methyl phenyl silicone oil can only be miscible when the viscosity of dimethyl silicone oil is lower than 50 mm2/s and the phenyl mole fraction of methyl phenyl silicone oil does not exceed 10%. While different phenyl-containing methyl phenyl silicone oils do not mix at all.
With organic solvents' "affection"
The solubility of silicone oil in organic solvents mainly depends on its type and viscosity.
Methyl silicone oil is particularly easy to dissolve in non-polar solvents, and its solubility decreases as the viscosity increases. Its solubility is poorer in polar solvents.
While methyl phenyl silicone oil or silicone oils with higher alkyl substituents have better solubility, the higher the phenyl content, the better the solubility.
Heat resistance: The "tolerance" of temperature
In an air environment below 150°C, silicone oil has extremely high chemical stability and is suitable for long-term operation. However, once the temperature rises above 200°C, methyl will start to undergo an oxygen reaction, resulting in the formation of cross-linked structures of methyl siloxane, a significant increase in viscosity, and ultimately leading to gelation. Therefore, the safest temperature for use in the air should be limited to 170-180°C. Long-term use at temperatures above 200°C will not only increase viscosity but also produce irritating low-molecular silanol and methanol with a pungent smell. When the temperature further rises to above 450°C, it will start to burn, ultimately generating silica residue.
However, under conditions of reduced contact with air, such as in a closed system, the decomposition temperature can be raised to 200°C. In a vacuum environment or an inert gas atmosphere, silicone oil can withstand temperatures up to 300°C without decomposition. It is worth noting that in an inert gas, methyl phenyl silicone oil undergoes depolymerization rather than cross-linking when heated, resulting in a decrease in viscosity.
The heat resistance of methyl phenyl silicone oil is better than that of dimethyl silicone oil. High-molecular-weight methyl phenyl silicone oil can operate stably at 250°C in the air for several hours and can withstand short-term 450°C temperatures. In an inert gas, methyl phenyl silicone oil can even be used at 400°C.
Low-temperature resistance: "Hard core" at low temperatures
The pour point or freezing point of dimethyl silicone oil increases with the increase in viscosity. By replacing the methyl group with a small amount of phenyl group, the pour point of dimethyl silicone oil can be reduced. This is because the introduction of phenyl group disrupts the symmetry of the polysiloxane molecule. When the phenyl mole fraction is approximately 5%, the pour point reaches the lowest. As the phenyl content further increases, the pour point will rise again.