Abstract:
Poly(tri-n-butyltin) methacrylate, an anti-fouling compound, is prepared bysterifying a cross-linked polymethacrylic acid with tributyltin oxide in the presence of a poly solvent. The poly solvent, such as acetone, dioxane or methyl-ethyl ketone, causes particle size reduction due to hydraulic shattering. The in-reaction particle size reduction eliminates the need for mechanical post grinding in the current process.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to preparation of anti-fouling compounds, and more particularly to an improved method for batch production of crosslinked poly(tri-n-butyltin)methacrylate. 
     Crosslinked poly(tri-n-butyltin)methacrylate, an organometallic polymer, hereafter referred to as OMP-5, is used as an anti-fouling agent to prevent the growth of marine organisms on the hulls of ships. The current process for preparing OMP-5 calls for esterification of crosslinked polymethacrylic acid with tributyltin oxide (TBTO) in the presence of toluene. The toluene permits azeotropic distillation of water in an effort to drive the esterification reaction between the crosslinked polymethacrylic acid and the TBTO. The resultant product has an enlarged particle size which must be mechanically ground down to a size which is more useful as a pigment in antifouling coatings. Generally, the grinding step is performed after the esterification, however, the order may sometimes be reversed. 
     In either case, the mechanical grinding step and the associated handling constitute a significant portion of the cost of producing OMP-5. Also, substantial material losses are often incurred in the grinding operation. Furthermore, the grinding operation generates respirable dusts which could be hazardous to the health of personnel exposed thereto. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to produce OMP-5 in a particle size small enough to be useful as a pigment in a coating. 
     It is a further object of this invention to produce OMP-5 in a useful particle size without the need for grinding. 
     The above and other objects are realized in a process which esterifies tributyltin oxide with crosslinked polymethacrylic acid in the presence of a polar solvent such as acetone. The end product has a particle size which is sufficiently small for use as a pigment without grinding. 
     Other objects, advantages, and novel features of the invention will become apparent from the detailed description which follows the accompanying drawing wherein: 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The FIGURE shows a schematic representation of a system for producing OMP-5 by the process according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawing there is shown generally a system for production of OMP-5 by a process according to the present invention. In the preferred technique of practicing the present invention the equipment is cleaned and dried, a vacuum is drawn on a glass-lined steel reactor 10, and a solvent such as acetone is drawn into reactor 10. The vacuum is then relieved with nitrogen and crosslinked polymethacrylic acid (e.g., Rohm and Haas&#39; Amberlite IRC 50) is added. As the polymethacrylic acid is introduced agitation is applied to the mixture by means of an agitator or stirrer 12. 
     When the introduction of polymethacrylic acid is complete a vacuum is again drawn on reactor 10 and tributyltin oxide (TBTO) is then drawn into reactor 10. The vacuum is again relieved with nitrogen and a very slight nitrogen purge is maintained while the reactor 10 and contents are heated to reflux. The actual reaction temperature will depend on such parameters as type and quantity of the materials used, etc. 
     The esterification reaction is continued for a sufficient period of time to produce the desired quantity of OMP-5. The reactor 10 and contents are then cooled. The OMP-5 solids are drawn into a filter 14 made of a suitable material such as stainless steel. The filtrate from filter 14 is retained in a vacuum filtration receiver 16 for disposal or recovery. 
     Reactor 10 is then filled with acetone and the OMP-5 filtered product. The mixture is stirred by stirrer 12. The solids are then isolated and slurried with acetone again. The acetone washes are retained for recovery or disposal in receiver 16. The filtered product is then vacuum dried. The particle size of the OMP-5 end product is generally less than 45 μm. For comparison, the particle size of OMP-5 prepared by the standard process using toluene is generally in the range of 300 to 500 μm before grinding. 
     The reaction has also been run with dioxane and methyl ethyl ketone (MEK) and significant particle size reduction was realized. The most significant reduction was obtained using dioxane. Table I shows a comparison of the use of various solvents in the process as well as the influence of certain reaction variables. 
     
                                           TABLE I__________________________________________________________________________INFLUENCE OF REACTION VARIABLES ON OMP 5PREPARATION IN VARIOUS SOLVENTSIRC 50Reaction Parameters OMP 5PercentTBTO/     Percent        Percent                         Percent                              ParticleH.sub.2 Oacid.sup.1     solids.sup.2          Solvent   yield.sup.3                         Sn.sup.4                              size, um.sup.5__________________________________________________________________________11.2 1.18 18   Toluene   70   25.2 300-50052.  1.06 18   Toluene   80   28.1 300-50052.  1.06 40   Toluene   80        300-5001.4  1.12 37   Dioxane   75.sup.6                         28.1 100%&lt;454.   1.14 18   Dioxane   70.sup.6                         28.4 100%&lt;4552.  1.06 18   Dioxane   76.sup.6  15%&lt;45&lt;0.5 1.08 33   Acetone             60%&lt;451.4  1.12 18   Acetone   88        95%&lt;4552.  1.06 18   Acetone   88        50%&lt;4552.  1.06 60   Acetone             300-5001.2  1.16 18   MEK                 80%&lt;451.3  0.73 17   acetonitrile                    96        29%&lt;451.3  0.73 17   tetrahydrofuran                    92        33%&lt;451.3  0.73 17   n-butyl alcohol                    90        33%&lt;451.3  0.73 17   1,2-dimethoxyethane                    90        38%&lt;45__________________________________________________________________________ .sup.1 Molar ratio of Bu.sub.3 Sn moiety to acid moiety. .sup.2 IRC50 plus TBTO/total reaction volume. .sup.3 Assuming 100% conversion of acid to tributyltin ester. .sup.4 Values adjusted based on percent recovery of Sn in NBS standard. .sup.5 As measured by microscopic analysis of approximately 200 particles .sup.6 Values a minimum due to loss of fines during filtration. 
    
     Three variables appear to have an influence on particle size and tin level desired: (1) the initial water content of the polymethacrylic acid, (2) the TBTO/acid ratio, and (3) the solids content of the reaction mixture. The preparations listed in Table I were all performed in the same manner using identical lots of starting materials. 
     Particle size reduction in the preparation of OMP-5 by the process according to the present invention is believed to be caused by hydraulic shattering. Hydraulic shattering of crosslinked polymers is thought to occur when a sufficiently crosslinked polymer is exposed to a solvent that causes extensive swelling of a lightly crosslinked polymer or that dissolves an uncrosslinked polymer. Under these conditions the solubility parameter of the polymer and solvent are equal or very nearly equal. This implies a fairly complex situation for shattering of OMP-5 during its preparation. 
     It has been determined that acetone, MEK, and dioxane do not cause shattering of either the crosslinked polymethacrylic acid or the OMP-5 end product. Therefore, the shattering must occur at some intermediate stage of the esterification process. During the conversion of the crosslinked polymethacrylic acid to OMP-5 the solubility parameter of the polymer decreases. 
     The solvent system present during the reaction consists of three components: solvent, water and TBTO. The amount of TBTO decreases and the amount of water increases during the reaction. The net result is that the solubility parameter of the solvent increases during the reaction. Therefore, shattering must occur at that stage of the reaction when the degree of conversion to OMP-5 brings the solubility parameter of the polymer to the same level as that of the solvent at that particular stage of conversion. 
     Thus, a three-factor interaction is present with the three-component solvent system described above. The optimum conditions for shattering can be determined empirically with very little difficulty. 
     Some of the many advantages and new features of the subject invention should now be apparent in view of the foregoing description. A process for the preparation of crosslinked poly(tri-n-butyltin) methacrylate (OMP-5), an antifouling compound, has been described which eliminates the necessity of grinding the end product in order to obtain a particle size which is useful as a pigment in a coating. 
     Numerous modifications and variations of the subject invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.