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Lube-Tech


The effect of the Sulphur content in base oils was investigated in a thin-film oxidation study, using High Pressure Differential Scanning Calorimetry (HP-DSC) [4]. The oxidation induction time (OIT) of HP-DSC is determined at 35 bar O2, 200 °C and with 3.0-3.3 mg sample size, Figure 2.


Antioxidant selection Primary Antioxidants


The primary antioxidants, BHT and Diphenyl amine (Figure 1) were dosed at 0.1% each. This corresponds to a fairly modest treat rate, suitable for generating OIT results under reasonably mild conditions.


Secondary Antioxidants A selection of six different, commercially readily available, additives, known or expected to display (also) a secondary antioxidant effect is shown in Figure 3.


Figure 4. Oxidation Induction time, OIT.


Figure 3. Secondary antioxidants, a) Dithiocarbamate (methylene-bis-(N,N’-dibutyl) thiocarbamate); b) an active Sulphur carrier, dialkylpentasulfide; c) an inactive Sulphur carrier (di-tert-dodecyl trisulfide); d) a Sulphur- containing Phenolic antioxidant; e) a Calcium Phenate-type detergent additive; f) an organic Phosphite EP additive.


Initially, Nybase 150 was doped with a sulfide model compound to correspond to the Sulphur levels of a typical Group I base oils. A low dose and high dose Nybase 150 was prepared, 700 ppm Sulphur, and 0.3 wt% (3000 ppm) Sulphur, respectively. The results outlined in Figure 5 contains several important leads. The initial base oil Sulphur shapes the overall oxidation stability behaviour. A lower total base oil Sulphur gives a shorter OIT. The Nybase at 300 ppm has an OIT of 3.74 min, and is followed by the SN 150 base oil A with 700 ppm Sulphur and an OIT of 6.36 min. A Nybase with 700 ppm Sulphur, whereof 400 ppm in the form of added benzothiophene, clocks in at an OIT of 4.23 minutes. So evidently benzothiophenic Sulphur is not the key to improved antioxidation in this chemical system [5]. At an OIT of 20.10 minutes, Nybase with 700 ppm Sulphur doped up with sulfidic species displays about five times (5x) higher oxidation stability. Finally, at a proposed “saturation level”, the Nybase with 3000 ppm overall Sulphur yields an OIT of 45.39 minutes, with some peak broadening.


LUBE MAGAZINE NO.148 DECEMBER 2018 35 Results


Response to primary antioxidants The HPDSC-Oxidation Induction Time (OIT) measures how long time the inhibited oil can withstand severe oxidation at the chosen elevated temperature (isothermal measurement) and oxygen pressure. When the inhibitor is depleted, the oil is subjected to rapid oxidation and a strong exothermic reaction is noted (Figure 4) and the onset time can thus be determined.


PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE


No.119 page 4


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