Process and composition for providing reduced discoloration of pyrithiones

Aqueous pyrithiones and a process and composition for providing reduced discoloration of these pyrithiones using an alkali metal or alkaline earth metal salt of 1-hydroxyethane-1,1-diphosphonic acid.

FIELD OF THE INVENTION 
This invention relates generally to aqueous pyrithiones and, more 
specifically, to a process and composition for providing reduced 
discoloration of these pyrithiones in the presence of ferric ion. 
DESCRIPTION OF THE PRIOR ART 
Sodium pyrithione [also called the sodium salt of 
1-hydroxy-2-pyridinethione, sodium pyridine-2-thiol-N-oxide, or 
2-pyridinethiol-1-oxide, Na salt] is typically employed as a biocide and 
preservative in fuctional fluids, such as metalworking fluids, lubricants, 
cosmetics and toiletries. 
Likewise, zinc pyrithione [also known as zinc pyridine-2-thiol-N-oxide or 
bis[1-hydroxy-2(H)pyridinethionato]-zinc] is an excellent biocide. It has 
been employed as a broad-spectrum anti-microbial agent and preservative in 
metalworking fluids, plastics, and cosmetics. Its principal uses are as an 
anti-dandruff agent in hair products or as a preservative in various 
cosmetics and toiletries. 
Since the aesthetics of metalworking fluids, cosmetics and toiletries 
normally require certain desirable colors, and the formulators of such 
products go to great lengths to achieve specific color effects, any 
ingredient which causes the functional fluid to vary much from white or 
colorless may take the colorant formulators' task very difficult. 
In the presence of ferric ion, pyrithione-containing compositions tend to 
turn blue even though the ferric ion is present in mere trace amounts. 
This blue discoloration is undesirable for aesthetic reasons, as discussed 
above. 
In addition to the aesthetics problems, the blue coloration problem 
associated with the presence of ferric ion causes a funtioning problem in 
the pyrithione-containing compositions. This problem results from the fact 
that the pyrithione tends to form a blue precipitate in the presence of 
ferric ion. The precipitate reduces the amount of available pyrithione 
available throughout the composition, thereby diminishing the biocidal 
protection thereof. 
Various additives for pyrithione-containing functional fluids have been 
suggested in the past in an attempt to solve the problems associated with 
discoloration thereof. These additives include EDTA, nitrilotriacetic acid 
and ethylenediamine tetra(methylene)-phosphonic acid. However, none of 
these additives are as effective in avoiding or reducing the blue 
coloration problem associated with ferric ion as might be desired and 
several exhibit high toxicity. 
In view of the above, new approaches to avoiding or reducing the blue 
coloration problem associated with ferric ion in pyrithione-containing 
compositions including those with functional fluids, during manufacture 
and storage thereof, would be highly desired from a commercial standpoint. 
SUMMARY OF THE INVENTION 
The present invention is directed to a process for preventing or reducing 
discoloration of aqueous sodium or zinc pyrithione by treating the 
pyrithione with an effective amount of a selected agent to prevent or 
reduce the discoloration. 
Thus, in one aspect, the present invention relates to a process for 
reducing or inhibiting the presence of a blue coloration caused by the 
presence of ferric ion in an aqueous sodium or zinc pyrithione which 
comprises adding thereto an alkali metal or alkaline earth metal salt of 
1-hydroxyethane-1,1-diphosphonic acid (also referred to herein as "HEDP"). 
In yet another aspect, the present invention relates to a composition free 
of blue coloration otherwise caused by the presence of ferric ion therein 
comprising an aqueous sodium or zinc pyrithione and an alkali metal or 
alkaline earth metal salt of 1-hydroxyethane-1,1-diphosphonic acid. 
DETAILED DESCRIPTION OF THE INVENTION 
The sodium pyrithione employed in the process and composition of the 
present invention is a well-known commercial product and is commonly made 
by reacting 2-chloropyridine-N-oxide with NaSH and NaOH. See U.S. Pat. No. 
3,159,640, which issued to McClure on Dec. 1, 1964, incorporated herein by 
reference. 
Zinc pyrithione may be made by reacting 1-hydroxy-2-pyridinethione or a 
soluble salt thereof with a zinc salt (e.g., ZnSO.sub.4) to form a zinc 
pyrithione precipitate. See U.S. Pat. No. 2,809,971, which issued to 
Bernstein and Losee on Oct. 15, 1957, incorporated herein by reference. 
A wide variety of alkali metal and alkaline earth metal salts of HEDP are 
useful within the scope of the present invention such as, for example, the 
sodium, potassium, calcium and magnesium salts of HEDP. Of these, sodium 
and potassium are preferred. The most preferred salt is potassium based 
upon its ease of preparation by reacting HEDP with KOH (while cooling the 
reaction mixture) within a pH range of between about 11 and about 13, more 
preferably between about 11 and about 12. Below a pH of about 11, 
precipitation of the HEDP-K.sub.4 and/or the pyrithione is frequently 
encountered, whereas a pH of above about 13 tends tpo cause pyrithione 
stability problems upon aging of the composition. 
In contrast to the ease of preparation of the potassium salt, the sodium 
salt of HEDP generally requires maintaining the temperature at an elevated 
level of as high as 90.degree. C. or higher and a pH of between about 10.5 
and about 13, preferably between about 11.5 and about 12. Lower 
temperatures can result in the heavy precipitation of the hydrated 
trisodium salt of HEDP during the preparation. 
Although in its broadest aspect, the present invention encompasses 
compositions containing only aqueous pyrithione and the above-specified 
metal salt of HEDP, the invention is advantageously employed with 
conpositions additionally containing a functional fluid, such as a 
metalworking fluid. When such a fluid is present, high levels of ferric 
ion are often encountered. For example, a level of ferric ion of 150 ppm 
or higher is not uncommon in commercial metalworking fluids. By 
incorporating an effective amount of the metal salt of HEDP into the 
composition, the blue coloration attributable to the presence of ferric 
ion bound with pyrithione is suitably reduced, eliminated, or avoided. 
The amount of the above-specified metal salt 1f HEDP incorporated into the 
compositions of the present invention can vary over a wide range. If the 
preferred HEDP-K.sub.4 salt is used, the amount of HEDP-K.sub.4 is 
desirably between about 33 and about 75 weight percent based on the total 
weight of HEDP-K.sub.4 and pyrithione in the composition. The upper limit 
on HEDP-K.sub.4 in this range of ratios provides an adequate amount of 
HEDP-K.sub.4 if the total amount of ferric ion in the composition is no 
greater than about 150 ppm. If larger quantities of ferric ion are 
expected to be encountered, the amount of HEDP-K.sub.4 is increased 
accordingly. 
Normally, the preferred composition is supplied as an aqueous concentrate 
containing a functional fluid, pyrithione, and HEDP-K.sub.4. In the 
aqueous concentrate, sufficient pyrithione is provided such that the 
"working" functional fluid will contain a biocidally effective amount 
thereof. In order to satisfy this requirement, the concentrate for a 
metalworking fluid, for example, preferably contains between about 450 ppm 
and about 5000 ppm of pyrithione, thereby providing at least about 45 ppm 
in the "working" fluid based upon a dilution rate of the concentrate to 
the "working" fluid of between about 1:10 and about 1:100. Other 
functional fluids, such as cosmetics, are often formulated directly 
(without the need for a concentrate) and can contain up to 5000 ppm, or 
more, for the pyrithione salt. If the composition is pre-determined to 
have a high ferric ion content, the HEDP-K.sub.4 level can be adjusted 
accordingly to a higher level as is required to achieve the objective of 
reduced blue coloration. Alternatively, if periodic influxes of ferric ion 
are expected to be encountered during use of the composition, such as when 
using a metalworking fluid that picks up iron during use, periodic or 
continuous additions of HEDP-K.sub.4 to the fluid can suitablly be made to 
compensate for the discoloration and loss of microbial effectiveness that 
would otherwise occur. If significant amounts of calcium or magnesium ions 
are expected to be present or in the working functional fluid (e.g., 
caused by the use of hard water to dilute the concentrate), a chelator 
such as ehtylenediamine tetraacetic acid (EDTA) is suitably utilized in an 
amount of up to 500 ppm or higher as needed to chelate the calcium or 
magnesium ions. 
Without wishing to be bound by any particular theory, the efficacy of the 
HEDP-K.sub.4 in preventing or reducing blue coloration in the compositions 
of the present invention is believed by the present inventors to be 
attributable to the superior ferric ion binding capability of the 
above-specified metal salts of HEDP, as compared to the ferric ion binding 
ability of the pyrithione in the composition. More specifically, since the 
blue coloration is believed by the instant inventors to be caused by 
ferric ion bound to pyrithione, blue color elimination or prevention is 
believed to be effected in accordance with this invention by virtue of the 
superior ferric ion-binding capability of HEDP-K.sub.4 in competition with 
the pyrithione present in the composition. 
Indeed, some compositions within the scope of the present invention exhibit 
an initial bluish coloration upon addition of ferric ion. Upon standing 
for a few minutes, the blue color disappears indicating the effectiveness 
of ferric ion binding by HEDP-K.sub.4 upon equilibration. 
The term "discoloration" as employed herein with respect to 
pyrithione-containing compositions may mean any unacceptable gray, blue, 
black, purple or color other than the natural color or desired artificial 
color of the formulation. It is noted that the natural color of a sodium 
pyrithione itself is a clear yellow. One way of quantitatively measuring 
for discoloration in zinc pyrithione is by measuring the Hunter color 
parameters and calculating a whiteness value from them. Another method is 
to visually inspect the composition for any signs of off-whiteness, as 
compared to the desired or white color.

The following examples are intended to illustrate, but in no way limit the 
scope of, the present invention. 
EXAMPLE 1 
Efficacy of HEDP-K.sub.4 in Eliminating Blue Coloration Caused by The 
Presence of Ferric Ion in Pyrithione-Containing Synthetic Metalworking 
Fluids 
In order to test the effectiveness of HEDP-K.sub.4 in eliminating or 
reducing blue color in a pyrithione-containing synthetic metalworking 
fluid, the following experiments were conducted. 
HEDP-K.sub.4 was prepared as follows: 
A 60 weight percent aqueous HEDP acid solution was added to a reaction 
vessel in an amount of 40.2 weight percent based on the weight of this 
acid plus KOH to be added. The reaction vessel was adequately cooled with 
an ice bath. Fifty weight percent aqueous KOH in an amount of 59.8 percent 
by weight based upon the total weight of HEDP plus KOH was added slowly to 
the reaction vessel containing the HEDP slowly until the pH of 
neutralization was 12.0. 
Various formulations of either the tetrasodium or tetrapotassium salt of 
HEDP, Sodium Omadine.RTM. (40 weight percent aqueous sodium pyrithione), 
and, optionally, the tetrasodium salt of EDTA were prepared. The specific 
formulations (A through F) are described in TABLE I below. 
Formulations A through E were then added to a synthetic metalworking fluid 
in aliquots sufficient to provide a final amount of each active component 
as specified in TABLE I. The synthetic metalworking fluid composition was 
as follows: 
______________________________________ 
COMPOSITION OF 
SYNTHETIC METALWORKING FLUID (pH.about.9) 
Composition 
Amount in 
Synthetic Metalworking Fluid Ingredient 
Weight Percent 
______________________________________ 
Mazer RI #4 (an amine-based corrosion 
6.75 
inhibitor, a product of Mazer Chemical) 
TP 2098 (a carboxylic acid derivative, a product 
2.25 
of American Hoescht) 
Triethanolamine (99 percent active; 1 percent 
15.00 
H.sub.2 O) 
Water 65.00 
Caprylic acid (a product of Emery 
10.00 
Chemical Co.) 
Pluronic L-101 (a surfactant, a product of 
0.50 
Rohm & Haas) 
Poly-Solv .RTM. EB (an ethylene glycol monobutyl 
0.50 
ether, a product of Olin Corporation) 
100.00 
______________________________________ 
The various formulations were as follows: 
TABLE I 
__________________________________________________________________________ 
Amount of Active 
Final pH 
Component of Each 
of Synthetic 
Component Formulation Added 
Metalworking 
Formulation 
Component Weight 
pH of to Synthetic 
Fluid Plus 
No. Composition Percent 
Formulation 
Metalworking Fluid 
Formulation 
__________________________________________________________________________ 
A HEDP--K.sub.4 
60.0 11.9 828 ppm 9.2 
(24 percent active) 
Sodium OMADINE .RTM. 
40.0 920 ppm 
(40 percent active) 
B HEDP--K.sub.4 
60.0 11.0 966 ppm 9.2 
(24 percent active) 
Sodium OMADINE .RTM. 
35.0 920 ppm 
(40 percent active) 
EDTA--Na.sub.4 
5.0 329 ppm 
C HEDP--K.sub.4 
60.0 11.5 946 ppm 9.2 
(24 percent active) 
Sodium OMADINE .RTM. 
34.3 920 ppm 
(40 percent active) 
EDTA--Na.sub.4 
5.7 382 ppm 
(100 percent active) 
D HEDP--Na.sub.4 
60.0 10.2* 1008 ppm 9.2 
(28 percent active) 
Sodium OMADINE .RTM. 
40.0 920 ppm 
E HEDP--Na.sub.4 
60.0 10.7 1008 ppm 9.2 
(28 percent active) 
Sodium OMADINE .RTM. 
35.0 920 ppm 
EDTA--Na.sub.4 
5.0 328 ppm 
__________________________________________________________________________ 
Comparison F 
A commercial product containing sodium pyrithione and containing a 
nitriloacetic acid ferric ion chelator to prevent blue color formation, 
added to the synthetic metalworking fluid in an amount sufficient to 
provide 920 ppm of sodium pyrithione. 
*pH of Formulation D was 10.7 when testing at 100 ppm Fe.sup.+++ in 
synthetic metalworking fluid. 
In a first set of experiments, ferric ion (added as FeCl.sub.3.6H.sub.2 O) 
in an amount of 50, 100, or 150 ppm, was added to an aliquot of the 
synthetic metalworking fluid. One of the formulations was then added and 
visual color changes and the presence of precipitates were noted as given 
in TABLE II. 
TABLE II 
______________________________________ 
Effectiveness of Formulations A-E and Comparison 
Formulation F in Reducing Blue Coloration 
in a Synthetic Metalworking Fluid 
Containing Various Levels of Ferric Iron 
Formulation 
Fe.sup.+++ Visual 
No. ppm Observation 
______________________________________ 
B 50 Initially Lt Purple, 
turns to Clear Yellow 
in 10 seconds 
C 50 Initially Blue, Clear 
Yellow in 15 Sec after 
16 Hr same 
D 50 Initially Lt. Purple, 
turns to Clear Yellow 
in 10 Seconds 
E 50 Initially Lt. Purple, 
turns to Yellow in 
10 Seconds 
Comparison 
50 Initially Dk. Blue, 
F turns to Lt. Gray in 
2 min, then Yellow/Gray 
after one hour, then to 
Clear Yellow after 16 hr 
A 100 Initially Dk. Purple, 
turns to Clear Yellow 
in 30 Seconds 
B 100 Initially Dk. Purple, 
turns to Clear Yellow 
in 25 Seconds 
C 100 Initially Dk. Purple, 
turns to Clear Yellow 
in 30 Seconds 
D 100 Initially Lt. Purple, 
turns to Clear Yellow 
in 25-30 Seconds 
Comparison 
100 Dk. Purple turns to 
F Purple/Gray after 4 hr, 
then to clear Dk. Yellow 
standing overnight 
noticeable precipitate 
A 150 Initially Dk. Purple, 
turns to Clear Yellow 
in 1 min 
C 150 Initially Dk. Purple, 
turns to Clear Yellow 
in 1 min 
D 150 Initially Clear Purple, 
turns to Yellow in 
30 Seconds 
E 150 Initially Clear Purple, 
turns to Yellow in 
30 Seconds 
Comparison 
150 Initially Dk. Purple, 
F turns to Purple/Gray 
After 4 hr, then to 
Clear Dk. Yellow after 
standing overnight 
noticeable precipitate. 
______________________________________ 
On the basis of the visual observation tests given in TABLE II, it is 
readily apparent that Formulations A through E generally provide superior 
blue color reduction as compared to Comparison Formulation F at each of 
the three ferric ion levels (50 ppm, 100 ppm, and 150 ppm) in the 
synthetic metalworking fluid tested. In addition, none of the Formulations 
A through F formed a precipitate in the synthetic metalworking fluid upon 
standing over one night, whereas the synthetic metalworking fluids 
containing 100 ppm and the one containing 150 ppm of ferric ion plus 
Comparison Formulation F each formed noticeable precipitate upon standing 
overnight. 
A second set of experiments was run using the same formulations, synthetic 
metalworking fluid, and ferric ion levels as described above. However, 
calcium ion (Ca.sup.++) was added to simulate a hard water-containing 
metalworking fluid. Calcium ion was present in the synthetic metalworking 
fluid at the levels described in TABLE III below, and the visual 
observations for these fluids are given in TABLE III. 
TABLE III 
______________________________________ 
Effectiveness of Formulations A Through E 
And Comparison Formulation F in Reducing 
Blue Coloration in a Synthetic Metalworking Fluid 
Containing Various Levels of Iron and Calcium 
Formulation 
Fe.sup.+++ 
Ca.sup.++ 
No. ppm ppm Observation 
______________________________________ 
A 50 216 Initially Dk. Purple, 
turns to Clear Yellow 
in 30 Sec 
C 50 216 Initially Clear Blue, 
turns to Yellow in 
20 Sec/16 Hr 
same 
Comparison 
50 216 Dk. Purple after 16 Hr 
A 100 216 Initially Dk. Purple, 
turns to Yellow/Gray 
in 1.5 min 
C 100 216 Initially Dk. Purple, 
turns to Clear Yellow 
in 40 seconds 
Comparison 
100 216 Dk. Purple after 2 days 
F Noticeable Precipitation 
A 150 216 Initially Dk. Purple, 
Purple after 2 days, turns 
to clear yellow on standing 
Slight Precipitation 
C 150 216 Initially Dk. Purple, 
Purple after 2 days, turns 
clear yellow on standing 
Slight Precipitation 
Comparison 
150 216 Initially Dk. Purple 
F turns Purple in 2 days to 
clear, Dk. amber on standing 
Heavy Precipitate 
C 50 36 Initially clear blue, 
turns to Yellow in 
20 Sec/16 Hr 
Comparison 
50 36 Initially Dk. Purple, 
F turns to Lt. Purple 
after 2 min, then to 
Gray/Yellow after 16 hr 
______________________________________ 
The results as given in TABLE III show the superior performance of 
Formulations A and C, as compared to Comparison Formulation F at 100 ppm 
and 150 ppm ferric ion and 216 ppm calcium ion based upon visual 
observations over a two-day period. At 100 ppm Fe.sup.+++ and 216 ppm 
Ca.sup.++, Formulation C provided a quicker clearing of the synthetic 
metalworking fluid to its natural yellow color than did Formulation A, 
apparently due to the calcium ion chelating ability of the EDTA present in 
Formualtion C (but not in Formualation A). 
EXAMPLE 2 
Efficacy of HEDP-Na.sub.4 in Eliminating Blue Coloration Caused by The 
Presence of Ferric Ion in Pyrithione-Containing Synthetic Metalworking 
Fluids 
As a further example, the tetrasodium salt of HEDP was prepared using the 
set-up and procedure described in EXAMPLE 1, but by reacting NaOH with 
HEDP at a 90.degree. C. reaction temperature to give a final pH of 
neutralization of about 11.5. 
Formulations analogous to those described in EXAMPLE 1 (but replacing the 
HEDP-K.sub.4 with identical amounts of HEDP-Na.sub.4) were prepared and 
added to aliquots of the synthetic metalworking fluid described in EXAMPLE 
1. Ferric ion was also added to the aliquots of the synthetic metalworking 
fluid in an amount of 50 ppm, 100 ppm, or 150 ppm. 
Performance of the HEDP-Na.sub.4 were comparable to that provided in 
EXAMPLE 1 by HEDP-K.sub.4. All test fluids cleared to a clear yellow 
natural coloration in less than one minute.