Method to produce pesticide suspension concentrates

Details Method to produce pesticide suspension concentrates Method to produce pesticide suspension concentrates

2000-05-31

2003-04-01

Clardy, S. Mark (Department: 1616)

Plant protecting and regulating compositions

Plant growth regulating compositions

Organic active compound containing

C504S352000, C504S363000, C514S937000, C424S405000, C023S29300R, C023S29500G

Reexamination Certificate

active

06541426

ABSTRACT:

The present invention relates to a method to produce suspension concentrates of pesticides, pharmaceuticals, biocides, and similar materials, using a melt emulsification process. The method is particularly applicable to pesticides which have a low water solubility. The particle size of the material in the concentrate can be controlled to give particles less than 5 microns (&mgr;), preferably less than 1&mgr;.
The efficacy of pesticides is often related to the size of the pesticide particle. Typically, the smaller the particle the greater the efficacy due to factors such as increased release rate and wider and more uniform coverage upon application. For this reason, there is a need to prepare pesticide formulations in which the pesticide has a small particle size, preferably less than 5&mgr;. Small particles are typically prepared by milling larger particles using any one or more conventional milling techniques such as, for example, air milling, hammer milling, crushing (jaw, gyratory cone, roller, impact), impact milling (stationary plates), tumble milling with grinding media (balls, rods), roller milling (feeding through a small gap), pin milling, jet air milling (spiral, opposed, fluidized). Unfortunately, pesticides which melt at temperatures below about 120° C. are often difficult to mill to small particle size using conventional milling techniques. This is because the extended milling times needed to reach such small size often heats up the pesticide. This, in turn, leads to particle agglomeration and/or fouling of the milling apparatus due to melting of the pesticide. To avoid such problems the milling apparatus often requires cryogenic or refrigerated cooling.
Small particle size formulations of pesticides which are amorphous materials may be prepared by melting the pesticide and then emulsifying it in an aqueous medium. However, for pesticides which are crystalline, it is very difficult to achieve a stable crystalline product by this route and at the same time maintain pesticide particle size in the micron to sub-micron range because of heat of crystallization and unfavorable crystallization kinetics.
British Patent 1 518 568 describes both batch and continuous methods of suspension production using a melt emulsification technique wherein melted pesticide is introduced slowly, with vigorous stirring, into an aqueous phase kept at a uniform temperature. The described process results in particles which vary in size from 1 to 100&mgr;, with a considerable portion greater than 10&mgr;. U.S. Pat. No. 5,539,021 describes the preparation of high internal phase ratio emulsions and latexes using a high shear mixer to prepare small particle size latexes and emulsions. This method requires hydrophobic liquids which are emulsified at very high ratios in a water phase and subsequently diluted for further use. In the case of solid resins, the resin is first dissolved in a solvent, then emulsified, then the solvent is removed to get a solid. This method does not generally work well for highly crystalline solids because large crystals are frequently formed. Thus, there is still a need for a process which will allow production of suspension concentrates of low melting but crystalline materials with small particle size.
We have discovered specific conditions under which a continuous melt emulsification/crystallization method is used to form a suspension concentrate of a crystalline, preferably low melting, solid in which the final particle size is controlled even into the sub-micron size range. The method comprises the steps of:
a) combining a stream comprising melted solid with a stream comprising a solvent; wherein:
1) the melting point of the solid is from 40 to 180° C.;
2) the temperature of the stream comprising melted solid is 5° C. or more above the melting point of the solid;
3) the solid has a solubility of less than 1%, by weight, in the solvent at 25° C.;
4) one or both of the streams further comprises a surfactant or dispersant or both; and
b) mixing the combined streams in a confined chamber under high shear conditions wherein the melted solid first emulsifies forming particles 5&mgr; or less in size which then cool to below their melting point and solidify before the particles leave the chamber.
This method will produce a suspension concentrate formulation comprised of particles of the solid which are less than 5&mgr;, often less than 1&mgr; in size in the solvent. The small particle size ensures that the biological activity of the solid approaches that of a solvent-based emulsifiable concentrate of the solid, but when water is used as the solvent stream, with the added benefit of eliminating organic solvents. Small particle size also ensures high suspensibility of the solid in the solution concentrate, typically greater than 90%. The particles comprising the resulting composition have a unique morphology as a result of the crystallization process that occurs during the emulsification/crystallization under high shear conditions. In practice, when each particle crystallizes it often, but not always, forms a single crystal which will reflect the basic crystal structure of the solid itself. For example, the particle shape of the product from Example 1 below is that of flat plates. This morphology is far different from what would be obtained through conventional milling processes wherein the solid is crushed.
The method of this invention may be applied to any solid with a melting point from 40 to 180° C. which is crystalline. The term “crystalline” means a material which when melted and then cooled to a temperature below its melting point rapidly crystallizes (solidifies) through a process of nucleation and accretion. Although particularly applicable to pesticides, the method is also applicable to pharmaceuticals, biocides, dyes, other organic chemicals, and mixtures thereof. Preferably the melting point of the solid is from 40 to 120° C., more preferably from 50 to 110° C., even more preferably from 60 to 100° C.
If the solubility of the solid in the solvent is more than 1%, by weight, the stability of the resulting suspension may be poor. Poor stability is usually caused by “Ostwald ripening”, a process in which small crystals present in a suspension gradually dissolve while large crystals grow larger or form agglomerates. To avoid this problem, the solubility of the solid in the solvent must be less than 1%, by weight at 25° C.; preferably less than 0.5%; more preferably less than 0.1%.
Solvent composition is not critical. However, the solid must have a solubility in the solvent of less than 1%, by weight at 25° C. and be capable of being emulsified in the solvent. The solvent may be water or an organic solvent such as, for example; oils, alcohols, ethers, ketones, alkanes, cycloalkanes, aromatic compounds, pyridines and other aromatic nitrogen containing compounds, or mixtures thereof. Water is the most preferred solvent because of its ability to form an emulsion of the melted solid as well as a stable suspension concentrate.
When the solid is a pesticide, the solvent stream may contain a second pesticide which may be the same as but, preferably, different than the solid. The second pesticide should be compatible with the solid pesticide in terms of use rates and locus to be treated. Such pesticides may be selected from herbicides, insecticides, fungicides, acaricides, and the like. When the solvent is water, preferably the second pesticide is water soluble, most preferably, a water soluble salt. Examples of such pesticides include sodium acifluorfen, salts of glyphosate, and the like.
The stream comprising the melted solid may further comprise a solvent in which the solid is soluble. This may aid in transporting the solid to the confined chamber. However, care must be taken that the solvent does not interfere with the emulsification process that occurs in the chamber. Preferably no solvent is used in the melted solid stream.
Either the stream comprising the melted solid, the stream comprising the solvent, or both must contain one or more surfactants, dispersants, or both

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