Zinc Dithiophosphates 53
Under overbased condition, when the ratio of dialkyldithiophosphate acid to zinc oxide is less than
2:1, a basic zinc salt
(RO) 2 PS Zn 4 O
6S
(2.6)will be synthesized along with the neutral salt. The basic salt is a tetrahedron of zinc atoms
surrounding a central oxygen atom with (RO) 2 PS 2 ligands along each edge of the tetrahedron.
Crystallographic analysis of pure basic zinc salts has established the near equivalency of P–S–Zn
bonds. Raman spectra have also shown symmetrical P–S stretching, supporting a symmetrical
sulfur–zinc coordination arrangement for the basic ZDDPs [3]. In the presence of water, as one
would encounter during commercial ZDDP manufacture, the basic zinc salt will be in equilibrium
with the basic zinc double salt as seen in the following reaction:
(RO) 2 PS Zn 4 OZH 2 O n 2 OH
6S
+
23R = alkyl, phenyl, or alkylphenyl(RO) 2 PSS
(2.7)The stoichiometric excess of zinc oxide used in commercial ZDDP manufacture gives rise to a
mixture of basic zinc salt (or zinc double salt) and neutral salt, the ratio depending on the amount
of excess zinc oxide used and the molecular weight of the alkyl groups involved, where short alkyl
groups tend to promote the formation
3 Zn ZnO
2+R = alkyl, phenyl, or alkylphenyl(RO) Zn^4 O
2 PSS
(RO) 2 PSS
(2.8)but performance differences seen between the two salts with respect to wear would imply that a
more complex situation may exist [5]. As reported in the literature, basic ZDDP salts spontaneously
decompose in solution into neutral complexes and zinc oxide when the temperature is increased [6].
Pure ZDDPs, with alkyl groups of four carbons or less, are solid at ambient temperatures (with
the exception of sec-butyl, which is a semisolid at room temperature) and tend to have limited
or no solubility in petroleum base stocks. ZDDPs with aryl or alkyl groups with more than fi ve
carbons are liquid at ambient temperature. To utilize the less-expensive and more readily available
low-molecular-weight alcohols and yet to produce oil-soluble products, commercial manufactures
use m ixtures of high- (i.e., more than four ca rbons) and low-molecula r-weight alcohols to obtain a
statistical distribution of products favoring lesser amounts of pure low-molecular-weight ZDDPs.
Other methods have also been developed to increase the amount of lower- molecular-weight alco-
hols in ZDDPs. These include the addition of ammonium carboxylates to inhibit precipitation [7]
and the use of alkyl succinimides as solubilizing-complexing agents [8].
2.4 THERMAL AND HYDROLYTIC STABILITY
The study of the thermal degradation of ZDDP is important in that much of the tribologi-
cal characteristics of ZDDP can be explained by the effects of its decomposition products. The
thermal decomposition of ZDDP in mineral oil has been found to be extremely complex. ZDDP in
oil, upon heating to degradation, will give off volatile compounds such as olefi n, alkyl disulfi de,