Study On The Synthesis Of Alkyl Phenothiazines And Their Anti-Oxidant Activities In Lubricants

Less emissions and better fuel economy as well as longer oil drain intervals are driving modern lubricants to improve their anti-oxidation activity at higher temperature,which makes some antioxidants such as phenothiazines being more and more studied abroad,while the similar research is seldom reported domesticly.This paper is going to study the relationship between the alkyl phenothiazines' performance and their structures for the purpose of developing high efficient antioxidants.7 ring-substituted phenothiazines,3-methyl- phenothiazine,3,7-di(methyl) -phenothiazine,3-tert-butyl-phenothiazine,3,7-di(tert-butyl)-phenothiazine, 3,7-di(1,1,3,3-tetra-methyl)-phenothiazine,3-(α-methyl-benzyl)-phenothiazine and 3,7-di(α-methyl-benzyl)-phenothiazine were synthesized by sulfuration of corresponding alkyl diphenylamine in the presence of an iodine catalyst.The effects of sulfuration conditions and substituents were investigated.Higher temperature and longer reaction time lead to promote conversion,but decrease the objective product's yield.The sulfurtion rate decreased with the increase of the alkyl groups on the ring.The optimized conditions were determined by orthogonal method,the yield of 3,7-di-(1,1,3,3-tetra methyl)- phenothiazine was enhanced to 91.7%.The by-product was 3-(1,1,3,3-tetra-methyl)- phenothiazine which was formed later in the reaction by thermal split.7 N-substituted phenothiazines,N-ethyl-phenothiazine,N-n-butyl-phenothiazine, N-iso-butyl-phenothiazine,N-n-octyl-phenothiazine,N-iso-octyl-phenothiazine, N-(ethyl-phenyl)-phenothiazine were obtained by alkylation of phenothiazine with corresponding alkyl halides or thio-alkanols.The effects of steric hinderance on alkylation rate were dominant,bulkier the alkylating agent,lower the conversion.The anti-oxidant activities of ring-substituted diphenylamines,ring-substituted phenothiazines and N-substituted phenothiazines were evaluated by RPVOT,PDSC and General Oxidation Test.The results revealed that the ring-substituted phenothiazines exhibited better performances on extending induction time and inhibiting viscosity increase than the ring-substituted diphenylamines,while increasing of deposits.Except for N-(ethyl-sulfur-phenyl)-phenothiazine,all other N-substituted phenothiazines were observed with very short inducing oxidation time.Alkyl substituents introducing on the ring or N atom of phenothiazine are not obviously influence the inhibition of viscosity increase and reduction of deposits.It showed a tendency that the longer alkyl group leads to more deposits. The oxidation process of 3,7-di-(1,1,3,3-tetra methyl-butyl)-phenothiazine(OO-PTZ) was followed by HPLC-MS,the results revealed the intermediate oxidation product was a more potent antioxidant than OO-PTZ itself.The intermediate's structure was confirmed by NMR as 1,10'-bis-(1,1,3,3-tetramethyl-butyl)-phenothiazine).The synergism of OO-PTZ with 4,4'-di(1,1,3,3,-tetramethyl )-diphenylamine(OO-DPA) are evaluated by General Oxidation Test.The results showed that OO-PTZ/OO-DPA mixture exhibited a significant synergistic effect on reducing deposits,owing to the decrease of OO-DPA's depleting rate analysed by HPLC-MS.Alkyl phenothiazine/diphenylamine mixtures were synthesized by partial sulfuration of industrial alkyl diphenylamine Irganox L 57.The antioxidant mixtures were evaluated in turbine oils,air compressor oils,and gasoline engine oils as well as heavy duty diesel engine oils,certain antioxidant mixtures exhibited satisfying performances in these oils.