Study on the Formation Mechanism of Oil Sludge in Polyol Esters in Presence of High-Temperature Antioxidant N-Phenyl-α-naphthylamine

Compared with traditional lubricants, polyol ester lubricants exhibit superior oxidative stability and have been widely applied in extreme operating conditions such as aviation engines. However, under high-temperature conditions, polyol esters are still susceptible to oxidation and therefore require the addition of antioxidants. N-phenyl-α-naphthylamine is an excellent high-temperature antioxidant used in polyol ester. However, a notable issue is that oil sludge may form when this antioxidant is used at high temperatures. Excessive sludge can lead to a series of problems such as oil circuit blockage, more severe mechanical wear, and poor heat dissipation performance. In this work, oil sludge formation from N-phenyl-α-naphthylamine was simulated via high-temperature oxidation experiments in a polyol ester base oil. The formed sludge was then characterized by various advanced techniques, such as FT-IR, GPC, TGA, MALDI-TOF MS, and XPS. The results showed that the oil sludge was mainly composed of derivatives of polyol esters and N-phenyl-α-naphthylamine, along with some metal components. Further analysis showed that polymerization reactions between antioxidant molecules are the key factors leading to sludge generation, and polycyclic aromatic compounds formed by polymerization are responsible for inducing sludge generation in polyol esters.