Antioxidative Properties of Methyltin <i>S</i>-2-Ethylhexyl Thioglycolates for Mineral Oils

Abstract

Methyltin S-tris- and dimethyltin S-bis(2-ethylhexyl thioglycolate) (I and II) were prepared from methane stannoic acid and dimethyltin oxide, respectively, and their antioxidative activities for mineral oils have been evaluated. Synergism between I or II and 2, 6-di-tert-butyl-p-cresol (BHT) or phenothiazine (PA) was also examined. Two methods were adopted for this purpose. One was the measurement of time required for the peroxide value to reach 10 (oxidative induction period) as dry air was bubbled under atmospheric pressure through 50g of base oil (liquid paraffin) with 1mmol/kg of additive concentration at 170°C or 180°C, and the other was the measurement of time required for a difinite amount of oxygen to be taken up by 50ml of base oil (150# neutral oil, additive concentration 0.05-1.0wt%) in a rotary bomb in the presence of 5g of water and 3m of copper wire (RBOT method).By the former method, I and II showed much more excellent antioxidative activity than the standard BHT and PA and a remarkable synergism with BHT (Table 1). These properties were also revealed even at low additive concentration levels (0.2-1mmol/kg) (Table 2). However, only very low activity and no remarkable synergism with BHT and PA were observed by the RBOT method (Table 3). This phenomenon was considered to be due to the hydrolysis of I and II to some inert substances such as methane stannoic acid and dimethyltin oxide which might undergo undesirable reactions with BHT or PA to diminish their antioxidative activities.Organotin compounds have been widely recognized as excellent stabilizers for polyvinyl chloride (PVC) involving several different functions9) for stabilization. Meanwhile, it might be suggested that dialkyltin dilaurates or dialkyltin bisalkylthioglycolates behave also as radical scavengers in the stabilization of PVC9). From the comparison of the antioxidative activities of tetraphenyltin, PA, and BHT2), organotin compounds which contain only alkyl or aryl-Sn bonds seemed to exhibit lower activities than PA and BHT. Therefore, it was considered that the more excellent activities of I and II than those of PA and BHT observed in this work stemmed from the existence of mercaptoester groups which were bonded with an S-Sn linkage. I and II were also found to behave as peroxide decomposer (Fig. 1) in this work. Zinc dicyclohexyl dithiophosphate (DTPZn) and dilauryl thiodipropionate (TPL) which are generally recognized as peroxide decomposers showed only low activities when they were used without other kinds of antioxidants under the conditions adopted in this work (Table 1). From the consideration of these facts, the excellent activities of I and II were assumed to be based on the effects of two processes, radical scavenging and hydroperoxide decomposing, taking place simultaneously in the antioxidative process of mineral oils.