51

2

Zinc Dithiophosphates

Randolf A. McDonald

CONTENTS

2.1 Introduction ............................................................................................................................. 51
2.2 Synthesis and Manufacture ..................................................................................................... 51
2.3 Chemical and Physical Nature ................................................................................................ 52
2.4 Thermal and Hydrolytic Stability ...........................................................................................53
2.5 Oxidation Inhibition ............................................................................................................... 56
2.6 Antiwear and Extreme-Pressure Film Formation ................................................................... 58
2.7 Applications ............................................................................................................................ 59
References ........................................................................................................................................ 61

2.1 INTRODUCTION

Zinc dialkyldithiophosphates (ZDDPs) have been used for more than 50 years in the lubricant
industry as low-cost, multifunctional additives in engine oils, transmission fl uids, hydraulic fl uids,
gear oils, greases, and other lubricant applications. The power of this particular compound is in
its ability to simultaneously function as an excellent antiwear agent, a mild extreme-pressure (EP)
agent, and an effective oxidation and corrosion inhibitor, all at a very low cost in comparison with
the alternate chemistries available in the market. This is why it is still manufactured on a large
scale by companies such as the ExxonMobil Corporation, Chevron Corporation, Ethyl Corporation,
Lubrizol Corporation, and others. To date, as much as 300 million lb of ZDDP is still manufac-
tured annually in the industrialized West.

2.2 SYNTHESIS AND MANUFACTURE

ZDDP was fi rst patented on December 5, 1944, by Herbert C. Freuler of the Union Oil Company of
California in Los Angeles [1]. The multifunctionality of ZDDP was immediately noticed as Freuler
indicated a noticeable increase in both the oxidation and the corrosion resistance of the lubricants
tested with the novel compound at a 0.1–1.0% treatment level. The initial synthesis Freuler carried
out involved the reaction of 4 mol of the intermediate dialkyldithiophosphate acid and 1 mol of
hydrogen sulfi de

4 ROH + P 2 S 5 2(RO) 2 P SH + H 2 S

S

→ (2.1)

followed by neutralization of the acid with 1 mol of zinc oxide

2(RO)^2 P SH + ZnO^ → 2(RO)^2 P SZnS P(OR)^2 + H^2 O

S S S

(2.2)