Abstract:
The invention provides novel mixtures and methods for significantly lowering the temperature at which chloroacetamides begin to form solid precipitant, without significantly diluting the chloroacetamides.

Description:
FIELD OF THE INVENTION 
     The present invention relates to novel mixtures and methods having the effect of lowering the precipitation point of chloroacetamides. 
     BACKGROUND OF THE INVENTION 
     Chloroacetamides are known herbicides. More particularly, they are plant growth inhibitor herbicides which primarily inhibit growth of roots and shoots of seedlings. Examples of chloroacetamide herbicides are alachlor, metolachlor, acetochlor, metazachlor, diethatyl, propachlor and thiophenamines. An example of a known thiophenamine plant growth inhibitor herbicide is dimethenamid, whose chemical name is 2-chloro-N-(2,4-dimethyl-3-thienyl)-N-(2-methoxy-1-methylethyl)-acetamide. Processes for its production, herbicidal compositions containing it and its use as a herbicide are described in U.S. Pat. No. 4,666,502, the contents of which are incorporated herein by reference. Dimethenamid consists of 4 stereoisomers as diastereomeric mixtures (1S, aRS (known as S-dimethenamid) and 1R, aRS (known as R-dimethenamid)) and as a racemic mixture (1RS, aRS). References herein to chloroacetamides, including dimethenamid, refer to their various forms, including their various stereoisomers, unless stated otherwise. 
     One commercial dimethenamid product is available under the registered trademark FRONTIER® (BASF AG, Germany), with either G.0 lb/gal. or 7.5 lb/gal. dimethenamid, along with other inert ingredients, such as petroleum distillates, xylene or xylene range aromatic solvents. 
     While the use of, chloroacetamides, including dimethenamid, as growth inhibitor herbicides is known in the art, one drawback to their commercial use is their precipitation point--the temperature, at about standard atmospheric pressure, at which liquid chloroactamides begin to solidify to form a solid precipitant. The racemic mixture of dimethenamid has a precipitation point of about 20° C. -22° C. As a result of this property, these commercial products tend to precipitate from liquid formulations at temperatures which are common in commercial use of herbicides. 
     For example, the FRONTIER® product comprising 7.5 pounds of dimethenamid per gallon, tends to form a solid precipitant at temperatures of 12-13° C. and below. The temperatures experienced by these formulations during normal distribution and field application commonly drop to temperatures well below 12-13° C., thus resulting in formation of dimethenamid precipitant. This is problematic to commercial users because, among other things, precipitation inhibits the users&#39; ability to uniformly apply the herbicide to crops. Thus, commercial users typically must heat the dimethenamid products prior to use, which can be costly and time consuming. Alternatively, manufactures of dimethenamid products are required to rotate stock of dimethenamid in heated storage with unused dimethenamid at the commercial users&#39; facilities that has been exposed to temperatures below 120°-13° C. 
     It is well known that the temperature at which a dissolved liquid freezes, or precipitates, can be lowered by decreasing the mole fraction of the solute in solution of the liquid solvent. The extent to which the precipitation temperature is affected is generally directly proportional to the extent to which the mole fraction of the solvent has been decreased. 
     If a solution is an &#34;ideal&#34; solution, the extent to which the precipitation temperature decreases by addition of a solute is not affected by the composition of the solvent or solute, and a curve made by plotting precipitation temperature versus concentration will not vary when different compounds are used to dilute the liquid. The term &#34;ideal solution&#34; refers to a solution in which little or no specific molecular interaction occurs between its components. An &#34;ideal solution&#34; conforms with Raoult&#39;s law. 
     Thus, one theoretical alternative approach to avoiding the need to heat chloroactetamide herbicides prior to use is to significantly lower the mole fraction of (i.e., dilute) the herbicide in solution. One preferred diluent known as gamma butyrolactone can be so used to lower the melting point of dimethenamid to minus twenty degrees celsius, but in order to do so, the dimethenamid in the solution must be diluted to twenty mole percent (forty five percent by weight). 
     However, by significantly diluting the herbicide, its effectiveness is reduced. Furthermore, significant dilution of the herbicide results in a significant increase in the amount of total product required to achieve the desired herbicidal result. This not only results in greater cost to the user based on the amount of product purchased, but also increases significantly the costs of shipping, handling and applying the product. 
     Extensive experimentation was conducted in attempting to lower the precipitation temperature of the chloroacetamide herbicide, dimethenamid, by combining it in solution with various substances, and lowering the temperature of the solutions incrementally while observing them for solid precipitant formation. The dimethenamid precipitation temperature for each solution was determined as the temperature, at about standard atmospheric pressure, at which the solutions yielded at least a trace of solid dimethenamid precipitant. The term &#34;trace&#34; is used herein to mean an amount of solid precipitant that can be detected visibly without the aid of magnification, but which cannot be measured quantitatively without the aid of magnification. If the amount of solid precipitant can be measured visibly without the aid of magnification, then it is considered to be more than a trace. 
     It is understood that most substances form ideal, or nearly ideal, solutions with dimethenamid, and therefore, that the melting point of dimethenamid is not depressed substantially without significant dilution of the dimethenamid. Although some compounds have a minor effect on precipitation temperature, the deviation from ideality with these substances is not significant enough to be useful, and the substances are not acceptable in commercial herbicide formulations. 
     Accordingly, no method of inhibiting solid precipitant growth in chloroacetamide solutions at conventional shipping, storage and application temperatures, other than unacceptable dilution, is currently available. Therefore, commercial users of chloroacetamide herbicides, such as dimethenamid, have been unable to use such liquid products, substantially free of solid precipitant, if such products have been shipped or stored at temperatures substantially below 12°-13° C., without having to heat the product to melt, or re-dissolve, the solid precipitant therein. Because known diluents can depress the precipitation point only by substantially diluting the herbicide, the users&#39; only alternatives in those conditions have been to either use such products containing solid precipitant therein or to employ the costly and time consuming step of heating the products to melt, or re-dissolve, the solid precipitant. 
     SUMMARY OF THE INVENTION 
     It has been found surprisingly that the temperature at which the chloroacetamide herbicide, dimethenamid, forms a solid precipitant can be lowered significantly with significantly less dilution of the dimethenamid than has heretofore been available. The invention provides chloroacetamide compositions having improved low temperature stability and methods for lowering the precipitation point of chloroacetamides. The chloroacetamide precipitation temperature is lowered by combining chloroacetamides with chemical compounds of the following formula: ##STR1## 
     Wherein R 1  is either chlorine or methoxy, and R 2  is, optionally, hydrogen, halogen, or a lower alkyl, a lower alkyl ether, or a lower alkyl halide. 
     According to one preferred embodiment of the invention, a herbicidal mixture comprises a herbicidally effective amount of dimethenamid and 3,6-dichloro-2-methoxybenzoic acid, known as dicamba acid, wherein the molar concentration of the dicamba acid is from 30% to 50% of the total molar concentration of the dimethenamid and dicamba acid. The mixture can also be diluted with known inert ingredients, such as gamma butyrolactone, petroleum distillates, xylene or xylene range aromatic solvents, to adjust the concentration of herbicidal components thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing which forms a portion of the original disclosure of the invention: 
     FIG. 1 is a graph depicting precipitation point of dimethenamid at various molar concentrations in combination with dicamba and with gamma butyrolactone. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description, preferred embodiments of the invention are described to enable practice of the invention. Although specific terms are used to describe and illustrate the preferred embodiments, such terms are not intended as limitations on practice of the invention. Moreover, although the invention is described with reference to the preferred embodiments, numerous variations and modifications of the invention will be apparent to those of skill in the art upon consideration of the foregoing and the following detailed description. 
     The compositions of the invention include a herbicidally effective amount of a chloroacetamide, such as alachlor, metolachlor, acetochlor, metazachlor, diethatyl, propachlor or thiophenamines such as dimethenamid, combined with a precipitation point lowering agent. These are prepared in a ratio of from about 1:1 chloroacetamide to precipitation point lowering agent, on a mole/mole basis, up to about 2.5:1, with a preferred ratio being about a 3:2 ratio. 
     In accord with the invention, the precipitation point lowering agents are compounds having the following chemical formula: ##STR2## 
     wherein R 1  is either chlorine or methoxy, and R 2  is, optionally, hydrogen, halogen, a lower alkyl, a lower alkyl ether, or a lower alkyl halide. Examples of such precipitation point lowering compounds include dicamba acid (3,6-dichloro-2-methoxybenzoic acid)and 2,6-dichlorobenzoic acid. Dicamba is a known plant growth regulator herbicide, which is commonly used in post-emergence herbicidal control of broad-leaf weeds in monocot crops. One commercially available dicamba product is known as BANVEL® (BASF AG, Germany), which contains 4.0 lb/gal. of dicamba acid in inert diluents. 
     Although dicamba acid and other benzoic acids are known herbicidal plant growth regulators, they have not heretofore been combined with chloroacetamides, such as dimethenamid, in accordance with the ratios of the present invention and have not achieved the precipitation point lowering effects of the present invention. 
     Mixtures of these compounds with the chloroacetamide herbicide dimethenamid within the parameters of the above ratios exhibit surprisingly low precipitation temperatures, enabling the temperatures of such solutions to be lowered to below -20° C. before a trace of solid precipitant is observed. Also, when temperatures of these solutions were lowered to a point where precipitation occurred, the amount of solid precipitant formed was much less, and formed much slower, than when dimethenamid is combined with other known diluents. 
     Infrared spectra for mixtures of dimethenamid and dicamba reveal a shift in the carbonyl bands of both dimethenamid and dicamba, as compared with infrared spectra of unmixed dimethenamid and dicamba. Furthermore, both substances, when in stoichiometric excess of the other, exhibited both shifted and non-shifted carbonyl bands, suggesting some chemical bonding in relation to the carbonyl components of the two substances. However, testing of mixtures of dimethenamid and dicamba with thin layer chromatography showed that the two substances are easily separated thereby. This demonstrates that no covalent bond is formed between dimethenamid and dicamba, and that the association between the two substances is fairly weak and dynamic. 
     Interestingly, in determining precipitation temperatures for solutions containing varying amounts of dicamba and of 2,6-dichlorobenzoic acid mixed with dimethenamid, it was learned that dicamba and 2,6-dichlorobenzoic acid are soluble in dimethenamid up to molar concentrations about equal to the molar concentration of dimethenamid. When the molar concentration of dicamba or 2,6-dichlorobenzoic acid exceeds that of dimethenamid, the dicamba or 2,6-dichlorobenzoic acid precipitates and requires significant heating to return to the solution. 
     To determine whether the anomalous precipitation temperature depression in dimethenamid is attributable to the benzoic acid structure of these compounds, solubilities of other structurally similar benzoic acids in dimethenamid were measured. Surprisingly, structurally similar benzoic acids, such as 3,5-dicamba acid, are much less soluble in dimethenamid than are dicamba and 2,6-dichlorobenzoic acid, suggesting a strong structural specificity in the interaction between dimethenamid and both dicamba and 2,6-dichlorobenzoic acid. It is believed that this structural specificity is found in the location of the chlorine and methoxy groups adjacent the acid group in both dicamba acid and 2,6-dichlorobenzoic acid, and that the electron affinity of these groups enhances the interaction of the acid group of those molecules with the carbonyl components of dimethenamid. 
     The mixtures and formulations described herein can be prepared by a manner known per se, in particular by stirring compounds and the other usual formula adjuvants into the dimethenamid while stirring and optionally while heating. In a preferred embodiment, the dimethenamid is heated to about 115° F. before adding dicamba thereto. Also, the concentration of the components can be varied by combining the mixtures, using methods known per se, in particular by stirring the compounds with known diluents. 
     As used herein, the term diluents means any liquid or solid agriculturally acceptable material which may be added to the components to provide a more easily or improved applicable form, or to achieve a usable or desired strength of activity. Examples are gamma butyrolactone, petroleum distillates, xylene, or xylene range aromatic solvents. On preferred embodiment of the present invention comprises about 5 pounds per gallon dimethenamid and about 1 pound per gallon dicamba with commercially known diluents such as petroleum distillates, xylene or xylene range aromatic solvents. At this concentration, the dicamba has the desired effect of lowering the precipitation temperature of dimethenamid, and the mixture has a desirable viscosity to facilitate application by commercial users. 
     The formulations of the present invention can also include other ingredients or adjuvants commonly employed in the art, including penetrants, surfactants, crop oils, drift control agents, defoaming agents, preservatives, wetting agents, adherents, antimicrobial agents, and the like, including mixtures thereof, as are also well known in the art and disclosed, for example, in the aforementioned U.S. Pat. No. 4,666,502. 
     Herbicidally acceptable additives can be added to the mixtures, using methods known per se, in particular by stirring, including other compounds having similar or complementary herbicidal activity or compounds having antidotal, fungicidal or insecticidal activity. Particular formulations, to be applied in spraying form, can contain surfactants such as wetting and dispersing agents, for example, an ethoxylated alkylphenol or an ethoxylated fatty alcohol. Also, compatibility enhancing agents, such as emulsifiers, can be used to improve compatibility of the formulations when combined by an end user, for example, with products containing water. For example, in one embodiment, the formulations of the present invention are combined with a blend of nonionic/anionic surfactants, and a phosphate ester to emulsify the formulations of the present invention in water. Moreover, the mixtures and formulations described herein can be used in various herbicidal applications as are known, per se, in the art, and as are described in the above-mentioned U.S. Pat. No. 4,666,502. 
     The following examples set forth the dimethenamid crystallization point lowering effects of several combinations of the present invention. 
     EXAMPLES 
     Solutions were prepared at room temperature and about standard atmospheric pressure. The solutions were the cooled to -20° C. The precipitation temperatures for the solutions of dimethenamid with dicamba were recorded as the temperature below which at least a trace of solid dimethenamid formed in the solution. The results are set forth in Table 1 below: 
     
                       TABLE 1______________________________________                                 Precipi-  Dimethenamid Dimethenamid Dicamba Dicamba tation  Mole Weight Mole Weight Tempera-  Fraction Percent Fraction Percent ture ° C.______________________________________0.938    95%         0.062     5%     19  0.878 90% 0.122 10% 17  0.820 85% 0.180 15% 16  0.762 80% 0.238 20% 12  0.706 75% 0.294 25% 9 (trace)  0.652 70% 0.348 30% -20  0.598 65% 0.402 35% -20  0.546 60% 0.454 40% -20  0.495 55% 0.505 45% -20  0.445 50% 0.555 50%  81*  0.396 45% 0.604 55%  89*  0.348 40% 0.652 60%  94*  0.301 35% 0.699 65%  96*  0.256 30% 0.744 70% 100*  0.211 25% 0.789 75% 102*  0.167 20% 0.833 80% 104*  0.124 15% 0.876 85% 108*  0.082 10% 0.918 90% 110*  0.040  5% 0.960 95% 112*______________________________________ *The solid formations at dicamba concentrations of 0.505 mole fraction, and below, were dimethenamid precipitant. Above that concentration, dicamba precipitant formed, which required significant heating to dissolv back into solution. 
    
     As shown in Table 1 above, when dicamba is present in concentrations of greater than about 0.30 mole percent (i.e., greater than 25% weight), the dimethenamid precipitation temperature is depressed significantly. 
     For comparison, the precipitation temperature of dimethenamid was measured in similar fashion in solutions with varying concentrations of gamma butyrolactone (&#34;Gamma Blo&#34;), a known diluent. The dimethenamid precipitation temperatures, in those solutions are set forth in Table 2 below: 
     
                       TABLE 2______________________________________                Gamma-   Gamma-  Precipi-  Dimethenamid Dimethenamid Blo Blo tation  Mole Weight Mole Weight Tempera-  Fraction Percent Fraction Percent ture ° C.______________________________________0.856    95%         0.144     5%     18  0.737 90% 0.263 10% 18  0.639 85% 0.361 15% 18  0.555 80% 0.445 20% 17  0.484 75% 0.516 25% 15  0.421 70% 0.579 30% 13  0.367 65% 0.633 35%  9  0.319 60% 0.681 40%  7  0.276 55% 0.724 45%  4  0.238 50% 0.762 50% -7  0.203 45% 0.797 55% -20  0.172 40% 0.828 60% -20  0.144 35% 0.856 65% -20______________________________________ 
    
     As shown in Table 2 above, a significantly greater amount of gamma butyrolactone is required to lower the precipitation temperature of dimethenamid, as compared with the amount of dicamba required to achieve a similar dimethenamid precipitation temperature. The amount of dicamba necessary to achieve a dimethenamid precipitation temperature of -20° C. is about 30% w--equivalent to a mole fraction of about 0.35. By comparison, the amount of gamma butyrolactone necessary to achieve a precipitation temperature of -20° C. is about 55% w--equivalent to a mole fraction of about 0.8. 
     The difference in precipitation temperature depression achieved with dicamba as the precipitation temperature lowering agent, in comparison to the normal depression caused by dilution of dimethenamid, is more easily seen in the graph shown in FIG. 1. In FIG. 1, the curve 1 indicates the precipitation temperature observed in solutions of dimethenamid and dicamba. As seen in FIG. 1, the precipitation temperature of dimethenamid is depressed significantly by dicamba beginning at the point where the dimethenamid mole fraction is approximately 0.70 and the dicamba mole fraction is approximately 0.30. 
     Line A--A indicates the approximate point at which dicamba begins to precipitate and requires significant heating to return the dicamba to the solution. The curve 2 in FIG. 1 illustrates depression of the dimethenamid precipitation temperature by dilution with gamma butyrolactone. As FIG. 1 illustrates, in order to depress the dimethenamid precipitation temperature significantly with gamma butyrolactone, the mole fraction of dimethenamid must be diluted significantly more than with dicamba. For instance, with dicamba, the mole fraction of dimethenamid at which dimethenamid has a precipitation temperature of 10° C. is approximately 0.70, whereas, with gamma butyrolactone, the mole fraction of dimethenamid at which the dimethenamid has a precipitation temperature of 10° C. is approximately 0.37. Similarly, with dicamba as the precipitation temperature lowering agent, dimethenamid has a precipitation temperature of -20° C. with a dimethenamid mole fraction of approximately 0.65. With gamma butyrolactone as a diluent, in order to lower the dimethenamid precipitation temperature to -20° C., the dimethenamid mole fraction must be lowered to approximately 0.20. 
     Tests of a similar protocol were conducted using combinations of dimethenamid with 2,6-dichlorobenzoic acid. The results of these tests showed a similar precipitation point suppression as was exhibited with combinations of dimethenamid with dicamba acid. The results are set forth in Table 3 below: 
     
                       TABLE 3______________________________________                2,6-Di-  2,6-Di-                                Precipi-  Dimethenamid Dimethenamid Chloro- Chloro- tation  Mole Weight Mole Weight Tempera-  Fraction Percent Fraction Percent ture ° C.______________________________________0.929    95%         0.071     5%     18  0.862 90% 0.138 10%  16  0.797 85% 0.203 15%  15  0.735 80% 0.265 20% -20  0.675 75% 0.325 25% -20  0.618 70% 0.382 30% -20  0.563 65% 0.437 35% -20  0.510 60% 0.490 40% -20  0.458 55% 0.542 45%   49**  0.409 50% 0.591 50%   71**  0.362 45% 0.638 55%   73**  0.316 40% 0.684 60%   91**______________________________________ **As with the precipitation temperature determination relating to dicamba beginning at the point where the mole fraction of 2,6dichlorobenzoic acid exceeds the mole fraction of dimethenamid, the 2,6dichlorobenzoic acid precipitates and requires significant heating to dissolve in the dimethenamid. Also, as with dicamba, infrared spectra for mixtures of dimethenamid and 2,6dichlorobenzoic acid reveal a shift in the carbonyl bands of both  # substances, further suggesting some chemical bonding in relation to the carbonyl components of the two substances. 
    
     To demonstrate the commercial utility of the precipitation temperature suppression provided by this invention, formulations of dimethenamid and dicamba acid were prepared at dimethenamid:dicamba weight ratios of 2:1 and 3:1, which correspond to mole ratios of 1.6:1 and 2.4:1, respectively. With gamma butyrolactone, a known diluent, samples of the formulations were diluted so that the concentration of total active ingredients (i.e., dimethenamid and dicamba) were 8 pounds per gallon, 7 pounds per gallon and 6 pounds per gallon. 
     The diluted samples, and samples of the undiluted 2:1 and 3:1 weight ratio dimethenamid to dicamba mixtures, were then cooled in 10° C. increments, seeded with solid dimethenamid and solid dicamba after each cooling increment, and observed for solid precipitant growth. At -20° C., none of the samples exhibited precipitation, even after seeding. At -30° C., four days after seeding, the undiluted samples and the 3 to 1 mixture that had been diluted to 8 pounds per gallon began to show slight traces of solid precipitant. The mixtures were then cooled to -40° C. and again seeded with solid dimethenamid and solid dicamba. Three days after seeding, the mixtures comprising 2 to 1 weight ratio of dimethenamid to dicamba showed only traces of solid dimethenamid. The mixtures comprising 3 to 1 weight ratio of dimethenamid to dicamba showed more significant solid precipitant growth. 
     The solutions were then heated in 1° C. increments up to a temperature of 0° C. to observe temperatures at which only a trace of the solid precipitant remained and at which all solid precipitant returned to the liquid solution. The results are shown in Table 4 below: 
     
                       TABLE 4______________________________________                  Only       No  Weight Ratio Concentration Trace Solids  Dimethenamid After Solids Remaining  to Dicamba Dilution Remaining ° C. ° C.______________________________________2:1      No Dilution   N/A*       N/A*  2:1 8 lbs/gal. N/A** -9  2:1 7 lbs/gal. N/A** -11  2:1 6 lbs/gal. N/A** -11  3:1 No Dilution -39 N/A***  3:1 8 lbs/gal. -10 -3  3:1 7 lbs/gal. -15 -8  3:1 6 lbs/gal. -16 -12______________________________________ *Only trace solid precipitant present at beginning of warmup (-39° C.) and also at end of warmup (0° C.). **Only trace solid precipitant present at beginning of warmup (-39.degree C.). ***Trace solid precipitant remained at end of warmup (0° C.). 
    
     The invention has been described in considerable detail with reference to its preferred embodiments. However, numerous variations and modifications can be made within the spirit and scope of the invention without departing from the invention as described in the foregoing specification and defined in the appended claims.