| Ester oil as lubricant base stock is of particular significance, due to its good tribological properties, biodegradability and renewability. Unfortunately, ester oils are usually susceptive to chemical reactions with oxygen and metal under elevated temperature, thereby losing lubricity. Therefore, it is urgent to develop antioxidants so as to prolong their service life in tribological application. Present antioxidants for ester oil are always obtained through improving the structures of common antioxidants and synergistic effect of different antioxidants whose thermal stability and antioxidant efficiency need to be improved. This thesis adopts enzymatic polymerization, organic synthesis and peroxide treatment to synthesize several antioxidants with multifunctional groups for ester oil, with the hope to increase their molecular weight and the number of antioxidant groups thereby improving their antioxidant performance at high temperature. The antioxidant properties and antioxidant mechanism of the as-synthesized antioxidants are also dealt with.(1) Common phenolic antioxidants are a type of widely used antioxidants with low toxicity and low sludge production, but their thermal stability needs to be improved.Therefore, polymerization, one of the effective ways to improve the thermal stability of compounds, is adopted to synthesize poly(p-methoxyphenol)(denoted as PMOP) from p-methoxyphenol(denoted as MOP) through a one-step polymerization in the presence of hydrogen peroxide as the oxidizing agent and horse radish peroxidase as the catalyst. Findings indicate that the as-synthesized PMOP exhibits a number-average molecular weight of about800 and has good dispersion stability in the tested ester oils. The introduction of PMOP(0.5wt%) increases the oxidation induction time(denoted as OIT) of di-iso-octyl sebacate(denoted as DIOS) from 48 min to 1050 min in rotary oxygen bomb test(denoted as ROBT)at 150℃, which indicates that PMOP as the antioxidant of ester oil like DIOS is much superior to the tested commercial antioxidants. Moreover, PMOP shows the maximum endothermic peak at 360 ℃, showing good thermal stability. Furthermore, PMOP is able to increase the ester oils’ OIT in pressurized differential scanning calorimetry(denoted as PDSC)at 210 ℃. It is supposed that the phenolic hydroxyl group in MOP also takes part in the polymerization, and hence the as-synthesized PMOP has a greater antioxidant capacity than MOP. Namely, PMOP with the structure similar to that of hindered phenols is the majorcontributor to the improved antioxidant capacity.(2) Despite of its good thermal stability at high temperature, PMOP cannot effectively prolong the OIT of ester oils(differing from amine type antioxidants). Thus phenylamine was introduced in polymerization to improve the antioxidant efficiency of PMOP.Poly(p-methoxyphenol-phenylamine)(denoted as P(MOP-PA)) was prepared by the copolymerization of MOP and phenylamine in the presence of sodium dodecyl benzene sulfonate as the surfactant and horse radish peroxidase as the catalyst in liquid-phase system.It was found that the as-synthesized P(MOP-PA) exhibits a number-average molecular weight of about 612 and has good dispersion stability in DIOS and petrochemical diester(coded as Priolube 3959). The introduction of P(MOP-PA)(0.5 wt%) increases the OIT of DIOS from48 min to 1616 min, which indicates that P(MOP-PA) has stronger antioxidant ability than PMOP. This could be attributed to the secondary amine groups generated in polymerization process.(3) Alkyl phenothiazine and alkyl diphenylamine, two kinds of commercial antioxidants in ester oils at high temperature, can effectively inhibit the oxidation of ester oils when used either alone or synergistically. With respect to this fact, N-substituted phenyl-2-amino-10H-phenothiazine was prepared from substituted aniline and2-chlorophenothiazine by one-step synthesis in argon atmosphere in the presence of Pd-catalyst and aprotic solvents. The antioxidation behavior of the as-synthesized product was evaluated by rotary oxygen bomb test(denoted as ROBT) and pressurized differential scanning calorimetry(denoted as PDSC); and its structure was characterized by Fourier transform infrared spectrometry(FT-IR), high resolution mass spectra(HRMS) and nuclear magnetic resonance spectroscopy(NMR). Results show that the as-synthesized product is a kind of intramolecular synergistic antioxidants with structure similar to the structure of phenothiazine and diphenylamine monomers. Since alkyl or alkoxy groups are introduced into the structure of the product to improve the dissolvability and ability of controlling sludge in ester oils, the as-synthesized product exhibits better thermal stability than phenothiazine and diphenylamine, and it can effectively prolong the OIT of the tested ester oils at high temperature.(4) N,N’,N’’-triphenyl-1,3,5-benzenetriamine and a series of its substitution derivatives were prepared from phloroglucinol and substituted anilines in methylbenzene solvents in argon atmosphere. The antioxidation behavior of the as-synthesized product and its substitution derivatives as antioxidants in several ester oils was evaluated, while theoreticalcalculations were conducted to examine their antioxidation mechanism. It was found that each molecule of the products consists of structures similar to the structure of diphenylamine monomer; namely, each molecule of the products contains three secondary amino groups.Compared with diphenylamine, 1,3,5-tris(phenylamino) benzene exhibits better antioxidation ability at 150 ℃ and 210 ℃. The experimental data and the results of Gaussian calculation prove that the substitution groups of the products not only influence the dissolvability and ability of controlling sludge in ester oils, but also affect their antioxidation behavior. This is because the substituents result in changes in the molecular structure and electronic effect of the as-synthesized products thereby causing change in their antioxidation behavior.(5) The excellent synergy between amine and phenolic antioxidants has been widely reported, but it is hard to achieve stronger antioxidant effects at high temperatures by simply mixing the two types of antioxidants. Therefore, a new synergistic antioxidant was obtained by the reaction of di-tert-butyl peroxide with diphenylamine and 2,6-di-tert-butylphenol at140 ℃ with decane solvents in argon atmosphere. The composition and structure of the as-synthesized antioxidant was analyzed by FT-IR, gas chromatograph mass spectrometry and X-ray single crystal diffraction; and its antioxidant behavior was tested by TG, RBOT and PDSC. Results show that the product is a mixture of2,6-di-tert-butyl-4-(diphenylamino)phenol and diphenylamine. The product exhibits better antioxidation behavior than raw material diphenylamine and 2,6-di-tert-butylphenol in DIOS(estimated by oil sludge production and OIT). Besides, the product exhibits better antioxidation behavior at elevated temperature in various base oils, which is closely dependent on its action mechanism. |
PDF Full Text Request