The interaction between sulphur antioxidants and iron in mineral oils

Abstract

Mechanisms of antioxidant action of zinc dialkvldithio­ phosphate (ZnDRP) and several structurally related compounds( (basic zinc dialkyldithiophosphate (b-ZnDRP), dialkylthio­ phosphoryl disulphide (DRDS), dialkyldithiophosphoric acid (DRDPA), dialkylthiophosphoric acid (DRTPA) ) were studied in both the presence and absence of a soluble iron catalyst (iron (III) stearate (FeST) ). Oxygen absorption studies in two hydrocarbon substrates, white mineral oil and decalin, show that each of the above thiophosphoryl compounds is an effective inhibitor of oxidation at 130°C in the absence of FeST. In the presence of FeST, however, their effectiveness is severely reduced and a higher concentration of antioxidant is required to provide stabilisation against oxidation. The initial presence of hydroperoxide is shohl1 to have a marked effect on the antioxidant activity of each of the above thiophosphoryl compounds in both the presence and absence of FeST. All the above compounds are shown to be capable of decomposing cumene hydroperoxide (CHP) in both the presence and absence of FeST at 110°C. The long term stabilisation against hydrocarbon oxidation provided by the above thio­ phosphoryl compounds in the presence of added CHP may be attributed to ionic hydroperoxide decomposition, promoted by sulphur containing acids formed as a result of the oxidation of the parent additive molecule by CHP. Although reaction with FeST may lead to the removal of some of the sulphur acids, hydroperoxide decomposition is shown to still be occurring in the presence of soluble iron. Of the above thiophosphoryl compounds, only DRDPA gives iron (III) dialkyldithiophosphate (FeDRP) on reaction with FeST. In the absence of CHP, FeDRP was shown to be an inhibitor of hydrocarbon oxidation. In the presence of CHP, however, FeDRP becomes inactive due to free radical decomposition of the hydroperoxide by the iron complex. FeDRP is shown to be an effective hydroperoxide decomposer even when present in small amounts, the mechanism of decomposition depending on the initial CHP:FeDRP ratio. The initial homolytic decomposition of hydroperoxide is caused by the FeDRP itself, but the subsequent ionic reactions are associated with the oxidation product(s) of the iron complex.