Lubricant Additives

52 Lubricant Additives: Chemistry and Applications

This synthetic route is still used today in the manufacturing of ZDDP. The P 2 S 5 , a fl ammable solid
produced from the high-temperature reaction between elemental sulfur and phosphorus, is provided
to the ZDDP manufacture in sealed aluminum bins containing 500–7200 lb P 2 S 5. The P 2 S 5 is hop-
pered into the reactor containing alcohol under a blanket of nitrogen. This is due to the ignitability
of both the alcohol and the P 2 S 5 when exposed to air. The hydrogen sulfi de by-product, a highly
toxic gas, is either converted to sodium sulfi de solution in a caustic scrubber or thermally oxidized
to sulfur dioxide. The heat of reaction and rate of hydrogen sulfi de evolution are controlled by the
addition rate of the P 2 S 3 as well as the fl ow rate of the cooling water. The acid is then neutralized
by zinc oxide; the reaction temperature is controlled by the addition rate of reaction depending on
whether it is an acid to oxide or oxide to acid, addition scheme. Enough zinc oxide is used to neu-
tralize the acid to a pH range, which will give a product suitably stable to thermal degradation and
hydrogen sulfi de evolution. The water formed from the reaction and the residual alcohol is vacuum-
distilled. Any unreacted zinc oxide is then fi ltered, requiring a fi ltration system capable of removing
particles as small as 0.1–0.8 μm. A larger molar excess of zinc oxide is often necessary to obtain the
pH required for stability. The various manufactures have done much work to reduce the amount of
zinc oxide used to obtain product stability (such as the addition of low-molecular-weight alcohols or
carboxylic acids to lower the amount of residual sediment in the product before fi ltration) [2]. The
fi ltered liquid product, with or without additional petroleum oil, is then provided to the customer in
drums or in bulk.

2.3 CHEMICAL AND PHYSICAL NATURE

ZDDP is an organometallic compound having four sulfur atoms coordinated to the zinc atom,
which is in a tetrahedral, sp^3 hybridized state. A Raman spectrum of ZDDP shows a strong P–S
symmetric stretching band near 540 cm–1 and the absence of a strong Raman band near 660 cm–1,
indicating a symmetrical sulfur–zinc coordination arrangement as in the following structure

RO OR

RO OR

P P

S

S S

S

Zn (2.3)

versus structure 2.4

RO OR

RO OR

PP

S

SS

S

Zn

(2.4)

often given in the literature. The strong IR band at 600 cm–1 pointing to P=S stretching would be
more consistent with PS 2 antisymmetrical stretching in light of the Raman spectrum [3]. The neu-
tral ZDDP molecules as represented in structure 3.1 actually exist as monomer, dimer, trimer, or
oligomer depending on the state of the ZDDP, crystalline or liquid, the concentration of ZDDP in
solvent, and the presence of additional compounds. The proposed structure for a tetramer in the case
of a neutral zinc diisobutyldithiophosphate in hexane as determined by dynamic light scattering is
shown in the following structure [4]:

RO

RO RO RO

RO

P

S

S S S S

S S

P P

S

S

S

S

S

S

P P

P

S

P

P

S

Zn Zn Zn Zn

S

RO OR RO OR RO OR

OR OR OR

OR

OR

(2.5)