Color stabilizers for zinc dithiophosphates

A small amount of hydrogen sulfide, an olefin, a mercaptan, an epoxide, a phosphite or a phosphine are added to a zinc dialkyldithiophosphate salt to decrease the rate of color generation upon storage and/or exposure to heat.

FIELD OF THE INVENTION 
The field of this invention relates to a composition comprising a lubricant 
oil additive useful as an inhibitor of engine wear, lubricant oxidation 
and engine part corrosion. More particularly, this invention relates to a 
composition of a lubricating oil additive comprising an oil-soluble zinc 
dialkyldithiophosphate wherein addition of small amounts of a compound 
selected from the group consisting of olefins, mercaptans, epoxides, 
phosphites, and phosphines decreases the rate of oxidation of the zinc 
dialkyldithiophosphate under conditions of heat and/or prolonged storage. 
The decreased rate of oxidation decreases the rate of color generation or 
color formation. 
It is an object of this invention to provide a lubricant composition 
comprising a wear inhibitor which has increased oxidative stability as an 
additive in a lubricating oil composition and good color stabilization in 
storage at elevated temperatures. 
It is an object of this invention to provide a lubricant composition 
comprising a wear inhibitor mixture which has improved oxidative stability 
in a lubricating oil composition wherein the wear inhibitor comprises an 
oil-soluble zinc dialkyldithiophosphate. 
It is an object of this invention to provide a process for the preparation 
of a wear inhibitor mixture which demonstrates improved color stability at 
elevated temperatures. 
It is an object of this invention to provide a process for preparation of a 
lubricating oil composition which comprises an oil-soluble zinc 
dialkyldithiophosphate and demonstrates improved color stability at 
elevated temperatures. 
These and other objects will become apparent from the description given 
hereafter. 
BACKGROUND OF THE INVENTION 
Oil-soluble zinc dithiophosphates are used as compounding agents or 
additives in lubricating oils to inhibit engine wear, to inhibit oxidation 
of the lubrication oil, and to inhibit engine part corrosion. It is highly 
desirable for such oil-soluble zinc dialkyldithiophosphates to provide the 
aforesaid properties to the lubricating oil without affecting the 
appearance characteristics of the formulated lubricant composition even 
though the performance characteristics of the lubricant composition may 
not be affected noticeably. 
As is well known, oil-soluble zinc dialkyldithiophosphates are 
conventionally prepared by reacting monohydric alcohols or phenols with 
phosphorus pentasulfide, usually in a mole ratio of 4:1, alcohol to 
phosphorus pentasulfide, in the presence of a diluent oil at a temperature 
within the range of from about 70.degree. C. to about 135.degree. C. to 
form partial esters of dithiophosphoric acid, a green odoriferous liquid. 
The so-obtained partial esters are then neutralized with zinc oxide at a 
temperature within the range of from about 70.degree. C. to about 
135.degree. C. to form a zinc dialkyldithiophosphate, typically a light 
yellow liquid. 
It has been found that, upon storage, the zinc dialkyldithiophosphate 
prepared as described often will acquire a dark objectionable color, often 
of a brownish hue. The dark color is objectionable as it affects the 
marketing of the zinc dialkyldithiophosphates. The dark product has been 
considered as inferior to product of a light color. 
It has long been known that the metal salts of partial esters of 
dithiophosphoric acids can be decolorized by incorporation of a small 
amount of a trialkylolamine whose alkylol groups contain 1 to 4 carbon 
atoms each. U.S. Pat. No. 2,983,742 teaches that excellent results have 
been obtained with triethanolamine but other amines of the class disclosed 
can be used. Amounts can be used of from 0.25 to 1.0 percent by weight. 
U.S. Pat. No. 3,361,668 teaches and claims a lubricating composition 
containing light-colored metal phosphorodithioates wherein the said 
phosphorodithioate is prepared by reacting phosphorus pentasulfide with a 
mixture of from about 95% to 99.95% by weight of a monohydroxy alcohol or 
phenol having from 1 to 30 carbon atoms and from about 0.05% to 5.0% by 
weight of an alkyl amine, cycloalkylamine or heterocyclic amine having up 
to about 20 carbon atoms. In an alternative procedure, the 
phosphorodithioic acid diester is first prepared and a small amount of a 
suitable amine is then added. 
In the prior art, it has been postulated that the cause of the dark color 
in metal salts of dithiophosphate esters is the presence of traces of 
heavy metals, principally iron, in the phosphorus pentasulfides. U.S. Pat. 
No. 2,983,742, Boba, et al, considers that addition of a trialkylolamine 
forms complexes with traces of the heavy metal compounds in the 
dithiophosphate esster salt that are more stable than the uncomplexed 
metal compounds. However, Boba, et al, U.S. Pat. No. 2,983,742 indicate 
this postulate is somewhat negatived by the fact that other agents capable 
of forming complexes with heavy metal compounds do not produce equivalent 
results. Wiese, U.S. Pat. No. 3,361,668, teaches that commercially 
prepared phosphorodithioic acid diesters are sufficiently strong acids to 
be corrosive to metals and the use of special corrosion-resistant 
equipment is generally required in their manufacture. Wiese U.S. Pat. No. 
3,361,668 teaches that the corrosiveness of the phosphorodithioic acid 
esters upon mild steel is shown to be reduced in the presence of an 
amine-reacted product of a phosphorodithioic acid ester. Wiese U.S. Pat. 
No. 3,361,668 accordingly teaches that the amine-reacted product of a 
phosphorodithioic acid is less reactive to iron compounds and retains its 
light color as prepared. 
It has now been found unexpectedly that acid salts comprising zinc 
dialkyldithiophosphates substantially retain their original light color 
despite exposure to elevated temperatures for prolonged periods of time 
upon the addition of color stabilizers in an amount not greater than 1.0 
(wt)% of the zinc dialkyldithiophosphate metal salt in concentrations of 
from 0.0001 to 0.5 mole of stabilizer per mole of zinc 
dialkyldithiophosphate metal salt. A preferred range is 0.002 to 0.05 mole 
of stabilizer per mole of zinc dialkyldithiophosphate. The color 
stabilizers are added after the dialkyldithiophosphoric acid diester is 
neutralized with zinc oxide to prepare the zinc dialkyldithiophosphate. 
Some of the color stabilizers are preferably added to the dialkyl 
dithiophosphoric acid diester after it is neutralized with zinc oxide. The 
color stabilizers comprise hydrogen sulfide, olefins, mercaptans, 
epoxides, phosphites, and phosphones. 
Although the mechanism has not been definitely established by which the 
color stabilizers cause the zinc dialkyldithiophosphate to retain a light 
color, it is believed that the addition of the aforementioned color 
stabilizers decreases the rate of oxidation of the zinc 
dialkyldithiophosphate under conditions of heat and/or prolonged storage; 
nor has the darkening of the zinc dialkyldithiophosphate been definitely 
established as having been caused by the presence of oxygen and consequent 
oxidation of the dithiophosphate ester. It is also considered that traces 
of other compounds may be present which react with zinc 
dialkyldithiophosphate to cause a darkening of the ester. 
SUMMARY OF THE INVENTION 
A small amount of hydrogen sulfide, an olefin, a mercaptan, an epoxide, a 
phosphite or a phosphine is added to a zinc dialkyldithiophosphate salt to 
decrease the rate of color formation or color generation upon storage 
and/or exposure to heat. 
DETAIL OF THE INVENTION 
O,O'-phosphorodithioic acid diesters are manufactured by reacting about 4 
equivalents of an organic monohydroxy compound, e.g., an alcohol or a 
phenol with one mole of phosphorus pentasulfide at temperatures of from 
about 70.degree. C. to about 135.degree. C. with the evolution of hydrogen 
sulfide as illustrated by the following equation: 
##STR1## 
The oil-soluble metal phosphorodithioate diester salts are usually 
manufactured by neutralizing the phosphorodithioic acid diester with a 
basically reacting metal compound, preferably the oxide, frequently with a 
small amount of water added, at temperatures of from about 50.degree. C. 
to about 150.degree. C. 
A phosphorodithioic acid diester is usually assumed to have the structure 
shown in the above equation wherein the thiol group attached to the 
phosphorus atom accounts for its relatively strong acidic character. In 
addition, unless the alcohols used are anhydrous, traces of water which 
are present in the reaction mixture react with the phosphorus pentasulfide 
to cause the formation of small amounts of monoesters of phosphorodithioic 
acid or phosphoromonothioic acid and even some unesterified phosphorus and 
sulfur-containing acids. 
Suitable monohydroxy organic compounds useful in the preparation of the 
improved O,O'-diesters of phosphorodithioic acid include alcohols, 
phenols, and alkyl phenols, including their substituted derivatives, e.g., 
nitro-, haloalkoxy-, hydroxy-, carboxy-, etc. Suitable alcohols include, 
for example, methanol, ethanol, n-propanol, isopropanol, n-butanol, 
2-butanol, 2-methyl-propanol, n-pentanol, 2-pentanol, 3-pentanol, 
2-methylbutanol, 3-methyl-2-pentanol, n-hexanol, 2-hexanol, 3-hexanol, 
2-methyl-2-pentanol, 4-methyl-3-pentanol, cyclohexanol, 
chlorocyclohexanol, methylcyclohexanol, heptanol, 2-ethylhexanol, 
n-octanol, 2,2-dimethyloctanol, nonanol, dodecanol, octadecanol, 
eicosanol, etc. The phenols suitable for the purposes of this invention 
include alkyl phenols and substituted phenols, e.g., phenol, chlorophenol, 
bromophenol, nitrophenol, methoxyphenol, cresol, propylphenol, 
butylphenol, amylphenol, heptylphenol, octylphenol, nonylphenol, 
octadecylphenol, etc. Ordinarily, the monohydroxy organic compounds 
suitable for purposes of this invention may have from 1 to about 30 carbon 
atoms. Mixtures of monohydroxy organic compounds can also be used without 
penalty to produce oil-soluble metal salts. Alcohols most frequently used 
are isobutanol, isoamyl, alcohol, isooctanol, 2-propanol, 
4-methyl-2-pentanol, capryl alcohol, and nonylphenol. 
The color stabilizers suitable for use in this invention for preparing the 
improved zinc dialkyldithiophosphate include olefins, mercaptans, 
epoxides, phosphites, and phosphines. Olefinic compounds useful as color 
stabilizers can be selected from a large variety of compounds containing 
olefinic double bonds such as alpha olefins of 2 to 100 carbon atoms, 
cyclic olefins such as cyclohexene, dicyclopentadiene, terpenenes, and 
olefinic compounds with functional groups such as allyl and oleyl alcohol, 
acrylic acid, and methacrylic acid and their esters, oleic acid, styrene, 
vinylethers, vinylesters, and vnylpyrrolidone. Mercaptans can be selected 
from primary, secondary, or tertiary mercaptans of 4 to 40 carbon atoms; 
H.sub.2 S can also be used as a color stabilizer. Epoxides are selected 
from compounds with 2 to 100 carbon atoms, such as epoxybutane or 
epoxidized soybean oil. Trialkyl or triarylphosphites can be selected from 
compounds of the formula (RO).sub.3 P where R is an alkyl group or a 
phenyl or substituted phenyl group of from 1 to 30 carbon atoms. 
Trialkylphosphines have the general formula R.sub.3 P where R is an alkyl 
group of from 1 to 20 carbon atoms. 
The proposed color stabilizers are usually added after the dialkyl 
dithiophosphoric acid is neutralized with zinc oxide. Some of the 
stabilizers such as phosphites, phosphines, or mercaptans can be added 
before neutralization with zinc oxide is completed. 
The color stabilizing additive can be added in concentrations ranging from 
0.0001 to 0.5 mole of stabilizer per mole of zinc dialkyldithiophosphate 
(ZnDTP). A preferred range is 0.002 to 0.05 mole of stabilizer per mole of 
zinc dialkyldithiophosphate (ZnDTP).