Abstract:
The invention relates to a process for reducing the monomeric aziridine content in a polyaziridine forming reaction mixture by adding to the polyaziridine forming reaction mixture an excess of a carbodiimide scavenger wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine, and to a product obtained by this process and to a coating composition containing the product obtained by the process.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a process for reducing monomeric aziridines in a polyaziridine reaction by adding carbodiimides as a scavenger, the products obtained by that process and coating compositions prepared from these products. 
     Polyfunctional aziridines have been shown to be useful as crosslinking agents in various types of waterborne and solventborne coating systems such as carboxylated acrylics, vinyl-acetate, carboxylated urethanes such as polyurethane dispersions (PUDs), styrene acrylics or mixtures thereof. 
     One important subclass of commercially available polyfunctional aziridines includes reaction products of ethylene imine (EI, aziridine) or propylene imine (PI, methyl aziridine) with trimethylol propane triacrylate (described for example in U.S. Pat. No. 2,596,299 to Bastian). Other commercially important polyfunctional aziridines can be prepared from ethylene imine or propylene imine and pentaerythritol triacrylate. Other polyfunctional aziridines are based on alkoxylated polyols. 
     Another method of preparing polyfunctional aziridines includes the transesterification of methyl (1-aziridinyl)propionates with polyols catalyzed with tertiary amines (as described in DE 2334656 to Miksovsky), whereby the methyl (1-aziridinyl)propionates are prepared from monomeric aziridines. 
     Ethylene imine and propylene imine are volatile low molecular weight toxic compounds which are undesired residuals in the processes to form polyfunctional aziridines. The residual monomeric aziridine compound has to be removed sometimes tediously by elaborate distillation methods or lengthy aging processes. This is especially true, if the reaction temperature is low, which is sometimes necessary to avoid discoloration or viscosity problems. Sometimes the residue can exceed 1000 ppm in crude reaction mixtures. It is very desirable to reduce this amount for a commercial product below 500 ppm and even more preferred to reduce it below 100 ppm or ultimately below 50 ppm. 
     To drive the Michael-type addition reaction of the aziridine and the acrylate to completion, it is possible to use an excess of aziridine. A disadvantage of this approach is the amount of aziridine to be removed from the reaction product under vacuum. With a simple distillation column and a vacuum of about 50 mm Hg it is usually not possible to remove the aziridine level to below 300 ppm on a commercial time scale, for example in a day. To achieve an aziridine level below that, it is necessary to use refined equipment, like an expensive falling-film or wiping-film evaporator, or longer distillation times which is economically unfavorable. Another method to drive the Michael addition uses an excess of acrylate. The residual aziridine levels can thereby be reduced to less than 50 ppm, which sometimes require considerable aging times up to several months, which again is economically unfavorable. 
     It is an object of the present invention to provide an easy process for making polyfunctional aziridines without advanced and expensive distillation or purification steps. It is another object of the invention to produce polyfunctional polyaziridines that are rheologically stable and are low in color. It is another object of the invention to develop a process that results in low monomeric aziridine without incurring elaborate vacuum steps or long batch or aging times 
     The present invention uses a scavenger. The reactions between electrophiles and aziridines have been described in detail in the literature. Copending application Ser. No. 10/137,805 describes an isocyanate as a scavenger. One example of a suitable electrophile is a carbodiimide. 
     The reaction between carbodiimides and aziridines is known. Baeg et al. disclose a regiospecific palladium-catalyzed cycloaddition of N-substituted aziridines and carbodiimides in the J. Org. Chem. (1992), 57(1), 157-62. 
     None of the above disclosures however describes the present invention. 
     SUMMARY OF THE INVENTION 
     The invention relates to a process for reducing monomeric aziridines in a polyaziridine forming reaction mixture by adding to the polyaziridine forming reaction mixture an excess of a carbodiimide scavenger, wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine. 
     The invention also relates to the product obtained by this process and a coating composition containing this product. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various aziridines and substituted aziridines can be used to form polyfunctional aziridines. The suitable aziridines are well known in the art and generally correspond to the formula 
                        
 
where R 1 , R 2 , R 3 , and R 4  independently represent hydrogen; alkyl with up to about 20 carbon atoms, preferably methyl, ethyl, or propyl; aryl, preferably phenyl; alkaryl, preferably tolyl or xylyl; or aralkyl, preferably benzyl or phenethyl.
 
     The groups R 1 -R 4  may represent substituted radicals wherein the substituents include cyano, halo, amino, hydroxy, alkoxy, carbalkoxy, and nitrile. Suitable examples of substituted groups R 1 , R 2 , R 3 , and R 4  thus include cyanoalkyl, haloalkyl, aminoalkyl, hydroxyalkyl alkoxyalkyl, carbalkoxyalkyl, and similar substituted derivatives of aryl, alkaryl and aralkyl groups. 
     Specific examples of suitable aziridines include ethylenimine (aziridine), 1,2-propylenimine (2-methyl aziridine), 2-ethyl aziridine, 1,2-dodecylenimine (2-decyl aziridine), 1,1-dimethyl ethylenimine (2,2-dimethyl aziridine), phenyl ethylenimine (2-phenyl aziridine), tolyl ethylenimine (2-(4-methylphenyl) aziridine), benzyl ethylenimine (2-phenylmethyl aziridine), 1,2-diphenyl ethylenimine (2,3-diphenyl aziridine), hydroxyethyl ethylenimine (2-(2-hydroxyethyl) aziridine), aminoethyl ethylenimine (2-(2-aminoethyl) aziridine), 3-chloropropyl ethylenimine (2-(3-chloropropyl) aziridine), p-chlorophenyl ethylenimine (2-(4-chlorophenyl) aziridine), methoxyethyl ethylenimine (2-(2-methoxy-ethyl) aziridine), dodecyl aziridinyl formate (dodecyl 1-aziridinyl carboxylate), carbethoxyethyl ethylenimine (2-(2-carbethoxyethyl) aziridine). 
     Because of their availability and because they have been found to be among the most effective, the preferred aziridines are ethylenimine, 1,2-propylenimine and 2-ethylaziridine. 
     The suitable aziridines are usually reacted with acrylates. 
     Preferred acrylates are polyacrylates having a functionality f≧2, which can be synthesized e.g. by an esterification reaction between a polyol and acrylic acid. However, other methods are also possible to synthesize those polyfunctional acrylates. Examples of polyols used in this kind of polyacrylate synthesis include neopentyl glycol, 2,2′-bis(p-hydroxy-phenyl)propane (bis-phenol A), bis(p-hydroxyphenyl)methane (bis-phenol F), glycerol, trimethylolpropane, pentaerythritol and others. It is also possible to use diols commonly used in polyester synthesis. Examples of these diols include ethylene and propylene glycol, butandiol, hexanediol and others. 
     It is also possible but less preferred to react the monomeric aziridine with monoacylates e.g. esters of acrylic and methacrylic acid and subsequently perform an optionally base catalyzed transesterification reaction. 
     The reaction temperature in the Michael-addition between the acrylate and the aziridine is above the melting point of the components and below 100° C. Reactions at room temperature will usually work well. It is preferred to react between 0 and 60° C., more preferred between 25 and 50° C. Theoretically higher temperatures can be applied, but are not preferred. It is also possible to run the reaction under pressure with or without elevated temperatures, if suitable equipment is available, which is less preferred. Typically the monomeric aziridine is added to the acrylate over a period of time to control the exothermic reaction. The reaction is then performed in the above mentioned temperature range. Typically the reaction time is less than 24 hours, but will vary with batch size. It is also possible to add the acrylate to the monomeric aziridine, however this is less preferred. In either case, at the end of the reaction the monomeric aziridine level is usually less than 1000 ppm, for example in the range between 100 ppm and 1000 ppm. In cases where the monomeric aziridine level is significantly higher it is possible to apply a weak vacuum (around 50 mm Hg) and use a distillation column to reduce the monomeric aziridine level into the above mentioned range. However, to further reduce the monomeric aziridine content sophisticated vacuum equipment or very long application times are necessary. 
     To reduce the level of monomeric aziridine to below 50 ppm according to one embodiment of the invention an excess of a suitable carbodiimide scavenger is added to the reaction mixture, wherein the excess is based on the equivalent ratio of scavenger to monomeric aziridine. The invention uses an equivalent ratio of scavenger to residual monomeric aziridine of 1:1 to 15:1, preferably 1:1 to 14:1, more preferably 1.7:1 to 3:1. In one embodiment of the invention the ratio is as low as 1:1 to 1.01:1. If the residual amount in the polyaziridine forming reaction mixture is between 200 and 1000 ppm, usually 0.01 to 2% by wt of the scavenger, based on the reaction mixture is sufficient. Preferred amounts of scavenger are between 0.05 and 1.5% by wt., more preferred between 0.1 and 1.0% by wt., and most preferred between 0.15 and 0.6% by wt., based on the weight of the reaction mixture. 
     The monomeric aziridine-scavenger-reaction can be performed at ambient temperature. Since the reaction is exothermic a means for constantly cooling the reaction can be optionally employed. 
     The scavenger is generally added slowly into the reactor containing the polyaziridine forming reaction mixture. The reactor is usually equipped with a condenser, a stirring means and a temperature measuring device. If the acrylate in this reaction is a polyacrylate then the reaction is finished; however, if the acrylate is a monoacrylate a transesterification reaction can be performed afterwards. In this less preferred case it is possible to perform the Michael-reaction first, then the transesterification reaction and finally the scavenging reaction. 
     Scavengers are compounds that react fast with the NH-functionality of aziridines. Several classes of compounds fit this description. In the context of the invention it is also desirable that the resulting mixtures of the polyaziridine and the reaction product of the aziridine and the scavenger are useful for coatings applications in that they have appropriate properties including color stability and rheological stability. Additionally, suitable scavengers in the context of the present invention do not release acidic by-products. Examples of suitable scavengers include carbodiimides. 
     Suitable carbodiimides include symmetric and unsymmetric carbodiimides represented by formula
 
R′—N═C═N—R″
 
wherein
         R′ and R″ independently represent alkyl and cycloalkyl with up to about 20 carbon atoms, aryl, alkaryl, or aralkyl.
 
Preferred residues R′ and R″ include methyl, ethyl, propyl, cyclohexyl, phenyl, tolyl, xylyl, benzyl or phenethyl. Examples include N,N-dicyclohexylcarbodiimide (DCC), 1,3-diisopropylcarbodiimide, 1,3-di-tertbutylcarbodiimide, 1,3-di-sec-butylcarbodiimide, N,N′-di-o-tolylcarbodiimide, bis-(2,6-diisopropylphenyl)-carbodiimide, or 1-ethyl-3-(3dimethylaminopropyl)-carbodiimide.
       

     It is of course also possible to add mixtures of scavengers (e.g. different carbodiimides). Some of the scavengers can also be mixtures of different stereo- and regioisomers. 
     It can be useful to add a stabilizer to the reaction mixture. It is preferred to use tertiary amines, more preferred the use of aliphatic tertiary amines and especially preferred is the use of tetramethylethylene diamine (TMEDA). Other additives e.g. antioxidants, rheology modifier, light stabilizer among others can be added as necessary for the final application, if they are not nucleophilic or acidic in nature. 
     It is possible to add the stabilizer to the reaction mixture before, during or after the reaction. 
     The scavenger can be added with or preferred without solvent or solvent mixtures, however preferred solvents are non-nucleophilic and non-acidic in nature. In general, it is preferred to add the scavenger or the mixture of scavengers neat without solvent. 
     Catalysts that enhance the reactivity of the scavenger are usually not necessary. Preferred catalysts are non-nucleophilic and non-acidic in nature. 
     The products prepared by the present invention can be used in all known polyaziridine applications, including use as cross-linkers, in adhesive applications, in coating compositions and in inks and printing compositions, in the field of photography, thermal and electrostatic imaging, fiber and fabric treatment and other uses. 
     The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified. 
     EXAMPLES 
     Production of Polyaziridine (I) 
     4800 lbs trimethylolpropane triacrylate were charged to a reactor and 17 lbs 4-methoxyphenol were added as an inhibitor against free radical polymerization. 2 lbs triethylamine were added to the reactor to maintain basic conditions. 
     An excess of 10% molar excess of propylene imine (2-methyl-aziridine) was then added slowly over 10-12 hours, while keeping reaction temperature at 35-40° C. 
     After all of the propylene imine was added, the temperature was maintained around 37.5° C. for about 9 hours. Some propylene imine was removed by distillation at around 50 mm Hg for 7 hours. 
     The polyaziridine product contained between 200 ppm and 300 ppm of residual propylene imine. 
     The examples below were compared by mixing at room temperature 0.5 grams of the scavenger (such as N,N-dicyclohexylcarbodiimide (DCC) in example 1) with 100 grams of polyaziridine I. The resulting products were clear, slightly viscous liquids. 
     This dramatically lowered the PI content from about 214 ppm to less than 50 ppm. The modified polyaziridine was unaffected in terms of physical properties and application properties and provided a high viscosity and high color stability. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                   
                   
                 Initial 
                 Final 
                   
               
               
                   
                   
                 Scavenger 
                 PI 
                 PI 
               
               
                   
                   
                 Weight 
                 (ppm) 
                 (ppm) 
               
               
                 Example 
                 Scavenger 
                 Grams 
                 LC-MS 
                 LC-MS 
                 Appearance 
               
               
                   
               
               
                 1 
                 DCC 
                 0.50 
                 214 
                 30 
                 clear 
               
               
                 Comp 1 
                 Benzoyl 
                 0.50 
                 268 
                  4 
                 hazy 
               
               
                   
                 chloride 
               
               
                 Comp 2 
                 Phthalic 
                 0.50 
                 268 
                 n.d. 
                 did not 
               
               
                   
                 anyhyride 
                   
                   
                   
                 dissolve 
               
               
                   
               
             
          
           
               
                   
                   
                 Unmodified 
                   
                 DCC-modified 
                   
               
               
                   
                 Storage at 
                 polyaziridine (I) 
                   
                 polyaziridine (I) (EX. 1) 
               
             
          
           
               
                   
                 50° C. 
                 Color 
                 Viscosity 
                 Color 
                 Viscosity 
               
               
                   
                   
               
               
                   
                 0 weeks 
                  61 
                  434 
                  78 
                 530 
               
               
                   
                 1 week 
                 102 
                  892 
                  94 
                 534 
               
               
                   
                 2 weeks 
                 115 
                 1170 
                 104 
                 583 
               
               
                   
                 3 weeks 
                 140 
                 1740 
                 120 
                 587 
               
               
                   
                   
               
             
          
         
       
     
     The examples also show that not all possible scavengers result in resins that are commercially usable, but that the scavengers according to the invention have excellent results. 
     Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.