The load-carrying properties of diester disulphides

Abstract A range of diester disulphides was synthesised and their extreme-pressure and antiwear properties assessed using the Four-Ball machine. The diester disulphides had better antiwear properties, and poorer extreme-pressure properties than the corresponding unsubstituted disulphides. Dialkoxy substituted disulphides had similar extreme-pressure and antiwear properties to unsubstituted disulphides. Maximum antiwear protection at low additive concentration, followed by subsequent increase in wear with increasing concentration, was obtained for diester disulphides [-S(-CH 2 ) n COOC 2 H 5 ] 2 where n = 1, 2 and 3, but not when n = 5, 10. Explanations for these results are given and these are supported by heats of adsorption and electron probe microanalysis studies. It is suggested that the prime requirement for improving the antiwear properties of organo-sulphur compounds is the presence of strongly adsorbing groups at both ends of the molecule. These results also highlight the fact that the load-carrying properties of additives for one application can be improved only at the expense of its possible use in other fields.