Study on the Degradation and Synergistic /antagonistic Antioxidizing Mechanism of Phenolic /aminic Antioxidants and Their Combinations

Oxidation stability of lubricating oil is critical for the smooth operation of industrial equipment such as turbines.Sludge formation is a major issue in turbine operation.Currently,the turbine oil industry employs antioxidants,especially,the combination of aminic and phenolic antioxidants,as the key additives to reduce sludge formation.Aromatic amines,primarily PANA,are considered as the most effective antioxidants in terms of oxidation induction time(OIT)measured by the RPVOT test.However,PANA is also a sludge former.Phenolic antioxidants are normally used in combination with the aminic antioxidants.It is widely believed that there exists a synergistic effect between the phenolic and aminic antioxidants.Regarding the interaction mechanism between the two types of antioxidants,it is generally accepted that phenolics act as a hydrogen donor to help regenerate the consumed aminic antioxidants.This mechanism can only explain some of the phenomena observed in field practice,but there are many other questions remaining unanswered including the antagonistic effect between PANA and phenolics.In this study,we used HPLC,FTIR and LC-MS to monitor the degradation products of the oil systems that were formulated with various aminic and phenolic antioxidants.For the APANA alone system,we identified the formation of dimers and trimers of APANA.For the PANA alone system,we identified the formation of dimers and tetramers.The dimers,trimers,and tetramers of PANA have very limited solubility.These findings therefore help explain why PANA is a sludge former.The oligomerizations(dimerization,trimerization,etc.)regenerate the N-H functionality,thus explaining why the PANA as well as other amines are very effective antioxidants.For the PANA/phenolic system,the oligomers of PANA were greatly reduced.Instead,we identified species that were formed by the recombination of the PANA radicals,or the PANA oligomeric radicals with the phenolic radicals.This,to our knowledge,has not been reported so far.The recombination of aminyl radicals and the phenolic radicals decreases the capability of regenerating the N-H functionality,which helps explain the antagonistic effect observed in the PANA/phenolic system.It also suggests that the phenolic antioxidants are very important in suppressing the sludge formation in the turbine oil system because the recombination products of PANA and phenolics have much better solubility than the PANA oligomers.