Abstract:
A dispersible granular formulation has been developed which contains oxyfluorfen herbicide. This formulation provides excellent suspensibility characteristics, and reduces post-emergent vapor phytotoxicity.

Description:
This application is a continuation of application Ser. No. 667,136 filed Mar. 11, 1991 (abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to novel dispersible granule formulations of the herbicide oxyfluorfen and processes for their preparation. 
     Water dispersible granular pesticide formulations are known. These formulations are desirable because they avoid the use of potentially toxic solvents and permit the use of easily-disposable paper containers or water soluble containers. Potential exposure of pesticide applicators and the general public to the pesticide or solvent is thereby reduced. 
     Typical dispersible granular pesticide formulations are described for example in GB 1,433,882, EP 0,252,896 and U.S. Pat. No. 3,920,442. GB 1,433,882 describes a process for preparing dispersible granules by blending premilled, water-insoluble active ingredient, dispersing agents, disintegrating agents and wetting agents in an aqueous suspension. The aqueous mix is extruded to form granules which are then dried to yield the final product. U.S. Pat. No. 3,920,442 describes water dispersible pesticide aggregates containing 5 to 95% by weight of pesticide. The aggregates are prepared by contacting the finely divided solid ingredients in a fluidized bed with a fine spray of water or a solution of the binder-dispersant followed by drying. 
     In attempts at making pesticide granules, it has often been found that granules or agglomerates prepared from the formulated wettable powders of the art using well-known agglomerating techniques and using water as the agglomerating agent, are not easily dispersible in water. On the other hand, agglomerates which are readily water-dispersible are often not sufficiently resistant to attrition and form a fine dust fraction upon handling and shipping. If conventional binders are added to make the granules strong, then they are not dispersible in water. Techniques such as tabletting, extrusion and rolling which involve high-pressure compaction of moistened mixtures containing finely divided pesticides, diluents, binder and dispersant, as described in U.S. Pat. No. 3,617,246, lead to dense pellets, tablets, plates, and rods which are subsequently dried and crushed. These latter techniques have also been used to form granules containing up to 50% of active pesticide, but the resulting granules are not rapidly or completely water dispersible and are not suitable for use in preparing sprayable suspensions. 
     Low-melting solids such as oxyfluorfen present an especially difficult problem in the preparation of a dispersible granule formulation. The low-melting solid, herein defined as melting below 100° C., tends to melt or become sticky during or subsequent to the grinding process which is a necessary step in preparing dispersible granules. EP 0,252,896 describes a possible solution to this problem which requires micro-encapsulation of low-melting pesticides prior to granulation. However, microencapsulation involves additional processing steps and adds to the cost of the overall formulation. 
     In the case of the herbicide oxyfluorfen, it has not heretofore been possible to produce a dispersible granular product which combines the features of a high active ingredient content, good suspensibility and dispersion properties as well as resistance to attrition thereby avoiding the formation of a dust. 
     Conventional oxyfluorfen formulations also suffer from the loss of herbicide vapor from the site of application which is undesirable because of reduction of herbicide in the soil and possible vapor injury to the growing crops in or near the location to which the herbicide is applied. 
     Oxyfluorfen is known to cause injury to soybeans by volatization under field conditions. Grabowski and Hopen, Weed Science 1985, Vol. 33, pg. 306-309 show that emulsifiable concentrate, wettable powder and soil applied granular formulations of oxyfluorfen cause vapor injury to plants under greenhouse conditions. 
     SUMMARY OF THE INVENTION 
     This invention relates to dispersible granule formulations of oxyfluorfen herbicide and processes for preparing these formulations including pan granulation or extrusion of a pre-wet mixture comprising finely-ground active ingredient, surfactant and optionally wetting agent, dispersing agent and carrier. The dispersible granules produced by these processes provide good suspensibility and also significantly reduce post-emergent oxyfluorfen vapor injury to plants as compared to emulsifiable concentrate and wettable powder formulations without reducing pre-emergent herbicide activity. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to this invention, there is provided a composition, in granular form, comprising oxyfluorfen and optionally one or more surfactants which significantly reduces vapor phase injury to growing plants. 
     By granular form, we mean granules substantially all of which have a mean particle size of at least 0.1 mm., which is a particle size much larger than the mean particle size of a powder, the mean particle size of which is measured in microns. 
     Oxyfluorfen is 2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene. Dispersible granular, also known as dry flowable, formulations are designated in agricultural literature as DG, DF or WG. The percentage active ingredient in the formulation is sometimes indicated, as for example, oxyfluorfen 80 DG or oxyfluorfen 60 DF. 
     A dispersible granular herbicide composition designed for dispersion in a liquid carrier should ideally have a high content of active material, should be readily dispersible in the carrier and should then form a dispersion which is as stable as possible, requiring the minimum of subsequent agitation for homogeneity. The liquid carrier will, of course, for convenience normally be water. We have now devised a way to make granules containing from about 10% to about 90% oxyfluorfen which readily break down when they are stirred into a liquid carrier to give a stable dispersion of the active ingredient. 
     This invention is a dispersible granule comprising oxyfluorfen having a suspensibility of at least about 70%. It has been found that this dispersible granule formulation causes significantly less vapor injury to growing plants compared to emulsifiable concentrate and wettable powder formulations of oxyfluorfen without reducing preemergent herbicide activity. 
     In this specification &#34;significantly less vapor injury&#34; means at least a 30% reduction in injury by the dispersible granules of this invention compared to conventional formulations in an assay equivalent to the vapor bioassay described in Example 12. 
     In addition to the active ingredient, oxyfluorfen, the mixture to be formed into dispersible granules will contain one or more surfactants and, optionally, flow aids, dispersants, disintegrants, wetting agents and defoaming agents. 
     The suspensibility test is determined in accordance with a variation of the procedure of the Collaborative International Pesticides Analytical Council (CIPAC) Handbook, Vol. 1, Ed. G. R. Raw (1970), Method Number MT 15.1. Standard hard water (342 ppm as calcium carbonate) was prepared according to CIPAC method MT 18.1.4., also known as Army Hard Water. The suspensibility test is further described in Example 1. 
     The term &#34;surfactant&#34; is used in the broad sense to include materials which may be referred to as emulsifying agents, dispersing agents and wetting agents, and the surfactant component may comprise one or more surfactants selected from the anionic, cationic and nonionic types. 
     Examples of surfactants of the anionic type include soaps, salts of aliphatic monoesters of sulfuric acid such as sodium lauryl sulfate, salts of sulfonated aromatic compounds, for example sodium dodecylbenzene sulfonate, sodium or ammonium lignosulfonate or butylnaphthalene sulfonate, and a mixture of the sodium salts of diisopropyl- and triiso-propylnaphthalene sulfonates. Suitable agents of the non-ionic type include, for example, the condensation products of ethylene oxide with fatty alcohols such as oleoyl alcohol or cetyl alcohol, or with alkyl phenols such as octyl phenol, nonyl phenol and octyl cresol. Other non-ionic agents are the partial esters with ethylene oxide and the lecithins. 
     Preferably the surfactant component will comprise at least one wetting agent such as those selected from alkyl naphthalene sulfonates, alkylaryl polyoxyethylene ammonium sulfonates, sulfosuccinates and nonionics such as tridecyl alcohol ethoxylate; or at least one dispersing agent such as those selected from the group of naphthalene sulfonates, lignosulfonates and polyacrylates. 
     Typically the total surfactant component will comprise from 0.1 to 25% and preferably from 1 to 15% by weight of the dry weight of the composition. 
     In the context of this specification, a dispersing agent is a surfactant which facilitates the dispersion of the pesticide particles when the product is added to a liquid, for example water. The dispersing agents used are preferably water-soluble ones. Phosphate esters, polycarbonate/acrylates and calcium lignosulfonates produced granules with poor suspensibility. Examples of dispersants preferred for the dispersible granule formulations of this invention include: Polyfon® H, Polyfon O, Reax® 88B, Morwet® D-425, Reax 45A, Polyfon T, Polyfon F, Lignosol™ XD-65, Reax 45L, Reax 85A, Reax 910, Polyfon OD, PC-825, described in Table 1. Most preferred dispersants are: Reax® 85A, Polyfon® H and Morwet® D425. 
     Examples of surfactants preferred as wetting agents for the dispersible granule formulations of this invention include Morwet® B, Morwet EFW, Sellogen® DFL, Morwet IP, Igepon® AC-78, Igepon T-77, Aerosol OT-B, and Triton® XN-45S, described in Table 1. Most preferred wetting agents are Morwet B and Triton XN-45S. 
     All surfactants act as dispersing agents in some degree, and also in some degree as wetting agents; most surface-active agents are however, more efficient in one capacity than the other. The worker of ordinary skill in the formulation art can select a surfactant most suitable for the purpose in view. 
     Small particles of a low-melting solid such as oxyfluorfen often tend to stick together thereby causing flow problems in processing the material. Flow enchancing agents also called flow aids such as clays or silica particles may be used to minimize these problems. Flow aids preferred for the oxyfluorfen dispersible granular include HiSil® 233, Wessalon® 50S, Cab-O-Sil® M-5, Wessalon S, Barden® Clay, and Microcel® E. Most preferred is HiSil 233, a silicate flow aid. The flow aid content of the dispersible granule may vary from 0 to 90% and preferably from 1 to 8%. 
     Addition of a silicon containing antifoaming agent is desirable to aid in the processing and use of oxyfluorfen dispersible granules. Defoaming agents may be used in amounts of 0.1% to 5%; a preferred range is about 0.2% to 1.0%. The preferred defoaming agent is Mazu DF-1300. 
     Disintegrants, which are water soluble organic compounds such as starch or sugar or water soluble inorganic salts such as sodium acetate or sodium bicarbonate, are sometimes used in dispersible granule formulations. See GB 1,433,882 for example. We have found that certain disintegrants have different effects on the oxyfluorfen dispersible granules of this invention; formulations containing sodium bicarbonate were found to disperse less effectively. On the other hand, Primojel® disintegrant, a sodium starch glycolate provided a dispersible granule with good physical properties and biological activity. 
     This invention comprises a dispersible granule containing by weight percent: oxyfluorfen, 10-90%; dispersent 0-25%; wetting agent 0-25%; disintegrant 0-5%; flow aid 0-90%; and defoaming agent 0-5%. 
     The most preferred composition of this invention comprises by weight percent: oxyfluorfen 80.4%; dispersant, Reax 85A, 17.5%; flow aid, HiSil 233, 1.6%; defoaming agent, Mazu DF-1300, 0.5%; and water, less than 1%. 
     Examples of dispersant, wetting agents, flow aids and defoaming agents useful in this invention are shown in Table I. 
     
                       TABLE 1______________________________________   Producer     Chemical Type______________________________________DispersantsPolyfon ® H     Westvaco Chemicals                    Aliphatic and aromaticPolyfon F P.O. Box 70848 sulfonated ligninPolyfon O Charleston Hts., SCPolyfon OD     29415-0848Reax ® 88BReax 45AReax 45LReax 85AReax 910Reax 80CReax 83AReax 100MPC-876A                  Ammonium                    lignosulfonateLignosol ®     Reed Lignin, Inc.                    Sodium lignosulfonateXD-65     81 Holly Hill Lane     Greenwich, CT 06830Orzan ® A     ITT Rayonier Inc.                    Ammonium     18000 Pacific  lignosulfonate     Highway S.     Suite 900     Seattle, WA 98188Wetting AgentsMorwet ® B     DeSoto, Inc.   Sodium n-butyl     2001 N. Grove  naphthalene     Fort Worth, TX sulfonate     76113Morwet EFW               Napthalene sulfonateMorwet D425              Naphthalene sulfonateMorwet IP                Sodium diisopropyl                    naphthalene sulfonateSellogen ®     Henkel Corp.   Alkyl naphthaleneDFL       Emery Group    sulfonate     11 501 Northlake     Drive     Cincinnati, OH 45249Igepon ®     GAF Corp.      Sodium cocylAC-78     140 W. 51st. St.                    isethionate     New York, NY 10020Igepon T-77              Sodium methyl oleoyl                    taurateAerosol ®     American Cyanamid                    Sodium dioctyl sulfoOT-B      One Cyanamid Plaza                    succinate     Wayne, NJ 07470Triton ®     Union Carbide Co.                    Ammonium alkyl/arylXN-45S    Industrial Chem. Div.                    polyoxyethylene sulfate     39 Old Ridgebury Rd.     Danbury, CT 06817Flow AidsHiSil ® 233     PPG Industries Silica     One Gateway Center     Pittsburgh, PA 15222Wessalon ® 505     Degussa Corp.  Silica     Rt. 46, Hollister Rd.     Teterboro, NJ 07608Wessalon S               SilicaCab-O-Sil ®     Cabot Corp.    SilicaM-5       Boston, MA 02110Barden ® Clay     J. M. Huber Corp.                    Clay     Rt. #4     Macon, GA 30201Microcel ® E     Johns-Manville Silicate     P.O. Box 5108     Denver, CO 80217DefoamingAgentsMazu ®     Mazer Chemicals                    Silicone and SilicaDF 1300   3938 Poreti Drive     Gurnee, IL 60031______________________________________ 
    
     The dispersible granules of this invention are prepared by milling oxyfluorfen, optionally with flow aid to a particle size of less than 20 microns preferably less than 10 microns and more preferably less than 5 microns; then adding a dispersant, water and optionally a wetting agent and a defoaming agent and mixing until a paste is obtained; agglomerating the paste; and drying the granules thus produced. 
     Agglomeration or granulation may be accomplished by any operable means such as wet granulation, tabletting, pan agglomeration, fluid bed agglomeration or extrusion. Extrusion is the preferred method. 
     The preferred amount of water to be added to the mixture to be agglomerated is 18 to 25 parts per 100 parts by weight. 
     After extrusion, the dispersible granules are dried. For storage stability, it is preferred to reduce the residual water to at least 2% and preferably below 1% by weight. Drying temperatures above about 100° C. are injurious to the product. The preferred drying temperature is less than about 60° C. and more preferably less than about 50° C. Drying may be accomplished by any suitable drying means preferably one which supplies inert gas at a controlled temperature. A two stage fluid bed dryer is preferred. 
     A preferred process for producing the oxyfluorfen dispersible granules of this invention comprises: 
     (a) milling a mixture of oxyfluorfen, and flow aid to a particle size between 3 and 15 microns; 
     (b) adding a surfactant, and 18-25% water (based on the total weight of ingredients) to the milled mixture of step (a) and mixing until a homogeneous, extrudable paste is obtained; 
     (c) extruding the paste obtained in step (b); 
     (d) drying the extruded granules at a temperature of less than about 60° C. to a moisture content of less than about 2% by weight. 
    
    
     The following examples describe the preparation of the oxyfluorfen dispersible granules of this invention and are intended only to illustrate the invention and not intended to limit the scope of the invention which is defined by the claims. 
     EXAMPLE 1 
     Preparation of Oxyfluorfen 80 DG by Extrusion 
     Technical oxyfluorfen (97% purity by weight) was melted by heating at 100° C. for 24 hours. The molten material was then poured into aluminum foil lined trays to a depth of about one inch. After cooling at room temperature for 24 hours, the solid was broken up and milled in a coffee mill. 
     The coarse milled oxyfluorfen was blended with HiSil 233 flow aid in a Marion mixer (manufactured for Rapid Machinery Co., Marion, Iowa by Texas Div. Tranter Inc. Old Burk Road, Wichita Falls, Tex.) in the following ratio of ingredients in parts by weight: oxyfluorfen 80.4; HiSil 1.6 and mixed for 20 minutes. 
     The above blend was milled in a Bantam micropulverizer, (manufactured by Mikropul, 10 Chantaur Road, Summit, N.J. 07901) fitted with a 0.042 inch screen and automatic feeder. The mean particle size of micropulverized product was about 40 microns. 
     The micropulverized product was then air milled in an 8&#34; horizontal (pancake) jet mill (manufactured by Fluid Energy Processing and Equipment Co., 153 Penn Energy, Hatfield, Pa. 19440). An Accu-Rate feeder (made by Accu-Rate, 746 E. Milwaukee Street, White Water, Wis. 53190) was fitted with a 2&#34; screw feed nozzle positioned to deliver micropulverized material to the air mill. High pressure nitrogen was used for both feeding and grinding to maintain an oxygen concentration below 10%. The ground material was collected in the air bag. The air mill was operated at a nitrogen feed pressure of 65 psig, a nitrogen grind pressure of 60 psig, and an accurate-rate setting of 100. 
     Under these conditions the milling rate was 8-10 lbs./hour. The mean particle size was about 5 microns. 
     The above blend (82.0 parts by weight), Reax 85A dispersant (17.5 parts by weight), Mazu DF 1300 antifoam (0.5 parts by weight) and water (25 parts by weight) were mixed in a Kitchen Aid mixer (made by Hobart, Kitchen Aid Div., Troy, Ohio) for approximately 2 to 3 minutes. 
     The mixture was transferred to a KAR 130 extruder (made by Tsu Tsui Rikagaku Kikai Co., Ltd., Japan) fitted with a 1.0 mm screen. After extrusion, the extrudate was air dried to a moisture content of 1 to 2% by weight at a temperature of less than 40° C. 
     Suspensibility (variation of CIPAC method) 
     Two grams of oxyfluorfen DG was placed in a glass-stoppered graduated cylinder containing 248 ml of Army Hard Water at 25° C. The cylinder was inverted 30 times over a period of 90 seconds and allowed to stand for 30 minutes. The bottom 25 ml was separated, evaporated and dried for 16 hours at 60° C.; and the resulting residue was used to calculate the percent suspensibility by the following formula. ##EQU1## 
     EXAMPLES 2-9 
     A number of formulations of oxyfluorfen DG were prepared following the general procedure of Example 1 using variations of dispersants, wetting agents, disintegrants, defoaming agents and flow aids. The components and results are shown in Table 2 below. 
     
                                           TABLE 2__________________________________________________________________________ %       Dispersant                Disintegrant                        Wetting Agent                                 Mazu Defoam                                          Flow aid                                                  PercentExample # Oxyfluorfen         (%)    (%)     (%)      (%)      (%)     Suspensibility__________________________________________________________________________1     80.4    Reax 85A                --      --       0.5      HiSil 233                                                  90         17.5                             1.62     80.4    Reax 85A                --      Triton XN 455                                 0.5      HiSil 233                                                  90         15.5           2.0               1.63     80.4    Morwet D425                --      --       0.5      HiSil 233                                                  86         17.5                             1.64     77.9    Reax 85A                Primogel                        --       0.5      HiSil 233                                                  84         18.0   2.0                       1.65     77.9    Reax 85A                NaHCO.sub.3                        --       0.5      HiSil 233                                                  71         18.0   2.0                       1.66     77.9    Reax 85A                --      --       0.5      --      95         21.67     77.9    PC-876 H                --      --       0.5      HiSil 233                                                  95         20.0                             1.68     83.4    Reax 85A                --      --       --       HiSil 233                                                  89         15.0                             1.69     21.0*   Reax 85A                --      --       --       HiSil 233,                                                  856         20.0                             Barden Clay                                          5710    77.9    Reax 85A                --      --       0.5      HiSil 233                                                  83         20.0                             1.6__________________________________________________________________________ *75% active ingredient 
    
     EXAMPLE 10 
     Preparation of Oxyfluorfen DG by Granulation 
     Oxyfluorfen (77.9 parts by weight) and HiSil 233 (1.6 parts by weight) were milled to a particle size of approximately 5 microns. Reax 85A (20.0 parts by weight) and Mazu DF 1300 (0.5 parts by weight) were added to the oxyfluorfen and HiSil and mixed in a Hobart blender until the mixture was well blended. 
     Water was added slowly to the powder while mixing. When the mixture began to granulate, addition of water was stopped and stirring was continued until granulation was complete. 
     The resulting granules were dried under vacuum and screened to remove fines and oversized particles. 
     EXAMPLE 11 
     Pre-Emergent Herbicide Test Procedure 
     The following test procedure was employed to assess the herbicidal activity of the formulations of the invention. Seeds of selected plants were planted in flats or pots. The test compound was sprayed directly onto the soil surface immediately after planting. The flats or pots were then placed in a greenhouse and watered by subirrigation and overhead. Subsequently, only overhead watering was used. 
     Test species employed are listed in Tables 3 and 4. 
     Each formulation to be evaluated was suspended in water and sprayed over the flats or pots using a carrier volume equivalent to 25 gallons per acre at the rate of application in grams per hectare (g/Ha) specified in the tables. About two or three weeks after application of the test formulation, the state of growth of the plants was observed. Each species was evaluated on a scale of 0 to 100 in which 0 equals no control and 100 equals total control of the species. 
     Tables 3 and 4 show the pre-emergent herbicide activity of the oxyfluorfen dispersible granule formulations of this invention compared with oxyfluorfen emulsifiable concentrate (1.6 E), a commercial formulation of Rohm and Haas Company, Philadelphia, Pa. 19105. 
     
                       TABLE 3______________________________________Pre-emergent Herbicide Test; 300 g/Hectare Active Ingredient        Average Dicot                     Average MonocotFormulation  % Control    % Control______________________________________Oxyfluorfen DG        87           100Example 1Oxyfluorfen DG        87           100Example 2Oxyfluorfen DG        91           100Example 3Oxyfluorfen  94           100EmulsifiableConcentrate (1.6 E)Untreated    0            0______________________________________Species used in the above test are listed belowDicots in Test           Monocots in Test______________________________________Bidens   Bidens pilosa   Barnyardgrass,Morningglory    Ipomoea lacunosa                    Echinochloa crus-galliNightshade    Solanum nigrum  Foxtail Setaria viridisPigweed  Amaranthus retreflexusSmartweed    Polygonum lapathifoliumVelvetleaf    Abutilon theophrasti______________________________________ 
    
     
                       TABLE 4______________________________________Pre-emergent Herbicide Test; 300 g/Hectare Active Ingredient        Average Dicot                     Average MonocotFormulation  % Control    % Control______________________________________Oxyfluorfen DG        88           99Example 2Oxyfluorfen DG        91           98Example 4Oxyfluorfen DG        93           100Example 5Oxyfluorfen  86           100EmulsifiableConcentrate (1.6 E)Untreated    0            0______________________________________Species used in the above test are listed below.Dicots                   Monocots______________________________________Bidens    Bidens pilosa  Barnyardgrass,Morningglory     Ipomoea lacunosa                    Echinochloa crus-galliNightshade     Solanum nigrum Foxtail Setaria viridisVelvetleaf     Abutilon theophrasti______________________________________ 
    
     EXAMPLE 12 
     Vapor Bioassay 
     The vapor assay was designed to detect herbicidal injury that can occur only from vaporized herbicides. Unsprayed plants were exposed to potential vapors within an enclosed system as follows. 
     Typical greenhouse soil (50:50 of soil to Redi Earth®) was added to an eight inch diameter circular pot six inches tall to within one inch of the top and then covered with a 2:1 soil and sand mix. The pot was sprayed with a herbicide in the conventional manner. The sprayed pot was immediately placed in the greenhouse and the soil wetted from the bottom up using a water-filled saucer. After wetting the soil, a 2 inch pot of velvetleaf was placed in a small petri dish in the center of the pot. 
     After placing the untreated plants on the treated soil, a one gallon clear plastic bag was inverted over the eight inch pot forming a tightly sealed tent over the entire system. The pot/tent system was left in the greenhouse for four days at which time injury evaluations were made using a scale of 0 (no injuries) to 100 (complete kill). 
     Tables 5 and 6 show the vapor injury caused by dispersible granule formulations of this invention in comparison to the oxyfluorfen emulsifiable concentrate (1.6 E) and oxyflourfen wettable powder (25 WP) commercial formulations of Rohm and Haas Company, Philadelphia, Pa 19105. 
     
                       TABLE 5______________________________________Oxyfluorfen DG Vapor Test300 g/Hectare Active IngredientVelvetleaf4 Days after ApplicationFormulation          % Injury______________________________________Oxyfluorfen DG       30Example 1Oxyfluorfen DG       25Example 2Oxyfluorfen DG       35Example 3Oxyfluorfen          100Emulsifiable Concentrate (1.6 E)Untreated            0______________________________________ 
    
     
                       TABLE 6______________________________________Oxyfluorfen DG Vapor Test300 g/Hectare Active IngredientVelvetleaf4 Days after ApplicationFormulation          % Injury______________________________________Oxyfluorfen DG       40Example 2Oxyfluorfen DG       45Example 4Oxyfluorfen DG       50Example 5Oxyfluorfen          90Emulsifiable Concentrate (1.6 E)Untreated            0Oxyfluorfen 25 WP    80Oxyfluorfen          100Emulsifiable Concentrate (1.6 E)Untreated            0______________________________________ 
    
     Although the invention has been described in regard to its preferred embodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having ordinary skill in this art may be made without departing from the scope of the invention which is defined in the claims.