Investigation of zinc dialkyldithiophosphates by ion exchange method

Abstract

UDC 665.7.038.5:543.544.6 Along with metal-containing or ashless detergent/dispersant additives used in lube oil formulations, zinc dialkyldithiophosphates are frequently used; these additives have antioxidant, anticorrosion, and antiwear properties. The schemes described in the literature for investigating the composition of additives [1-3] necessarily include a stage in which metal-containing additives are hydrolyzed, so that the salts are converted to the corresponding acids. The hydrolysis is usually performed by treating the additive with 5 N hydrochloric acid, either hot [i] or at room temperature [3], after which the organic products from the hydrolysis are extracted with diethyl ether. We had shown previously [4, 5] that it is desirable to replace the acid hydrolysis of metal-containing detergent additives by ion exchange on the sulfonated polystyrene cation exchange resin KU-2-8 in the H form. In benzene/alcohol medium (7:3), additives such as calcium and barium sulfonates, alkylphenolates, and alkylsalicylates are converted quantitatively by ion exchange to the corresponding acids without any loss and without any side reactions. In view of this background, it appeared advisable to investigate the behavior of zinc dialkyldithiophosphates under the conditions of ion exchange. A further point of interest is that the conversion of zinc dialkyldithiophosphates to dialkyldithiophosphoric acids will open up the way to obtain additional information on the composition of dithiophosphate additives [6]. In the present work, we used individual zinc dialkyidithiophosphates that were synthesized* by reactions of the corresponding acids with zinc dust in a dry nitrogen atmosphere, followed by recrystallizatio n from isopropanol to a constant melting point. The dialkyldithiophosphoric acids were synthesized* by interaction of phosphorus pentasulfide with the appropriate alcohols in accordance with a procedure given in [7], followed by purification of the materials through the sodium salts by a procedure given in [8] and vacuum distillation. The melting points of the zinc dialkyldithiophosphates coincided with those reported in the literature, and the zinc and phosphorus contents and the acid n~nbers coincided with the calculated values. The ion exchange of the zinc dithiophosphates was performed on KU-2-8 cation exchange resin in the H form, which was prepared by a procedure given in [9].