Vacuum diffusion pump oil is the core working medium of vacuum diffusion pumps. As a key consumable for achieving high and ultra-high vacuum environments, its performance directly determines the ultimate vacuum degree, pumping speed and service life of the pump. This type of special oil is mainly composed of organosilicon, with extremely low vapor pressure, excellent thermal stability and chemical inertness. Different from ordinary industrial lubricating oil, it needs to adapt to the cyclic working conditions of high-temperature evaporation, high-speed jet and condensation reflux in the vacuum system. With the improvement of downstream high-end fields' requirements for vacuum degree, its molecular structure optimization and preparation process upgrading have become the core direction of industrial development.
The core characteristics of vacuum diffusion pump oil stem from its precisely designed molecular structure. Mainstream products take phenylmethylsiloxane as the core skeleton. The introduction of phenyl groups significantly improves the thermal stability and radiation resistance of the oil, while methyl groups ensure its fluidity and chemical inertness. The synergy of the two endows the oil with wide temperature adaptability and low oil return rate. High-quality products need to meet three core indicators: the ultimate vacuum degree can reach 10⁻⁹~10⁻¹¹ Torr, the saturated vapor pressure at 25℃ is lower than 3×10⁻¹⁰ Torr, and it can still maintain chemical stability under high-temperature 300℃ working conditions, without decomposition or harmful volatiles, avoiding pollution of the vacuum system and processed workpieces.
At present, the mainstream preparation process of vacuum diffusion pump oil is mainly polycondensation reaction. Using high-purity siloxane monomers (such as methylphenyldichlorosilane) as raw materials, polycondensation reaction occurs under the action of composite catalysts, and the finished product is obtained through purification, removal of low-boiling substances, precision filtration and other processes. The traditional process has two pain points: first, insufficient monomer purity leads to high product vapor pressure and excessive oil return rate; second, poor controllability of polycondensation reaction, wide molecular weight distribution, affecting the stability of pumping speed, making it difficult to adapt to ultra-high vacuum scenarios.
Process upgrades focus on two major directions: precise control and purity improvement. At the raw material end, high-purity monomers with purity ≥99% are used to remove impurities and light fractions, reducing the vapor pressure of the oil from the source; in the reaction process, organotin and amine composite catalysts are selected to precisely control the temperature and time of the polycondensation reaction, narrow the molecular weight distribution, and improve the uniformity of product performance. At the same time, the low-boiling substance removal process is optimized, using vacuum distillation combined with membrane separation technology to completely remove light component impurities, further improving the ultimate vacuum degree of the product. In addition, the application of solvent-free production process reduces the emission of volatile organic compounds, conforms to the requirements of green chemical industry, and promotes the upgrading of products from general-purpose to high-end special type, adapting to harsh scenarios such as nuclear industry and ultra-high vacuum research.