Decomposition of ethylenically unsaturated carboxylic esters
This invention relates to a process for the decomposition of ethylenically unsaturated carboxylic esters, in particular acrylate and methacrylate esters. It also relates to latexes treated by the process. 
     Methyl, ethyl, butyl and higher acrylates and methacrylates are useful monomers which can be polymerised to form plastics especially thermoplastics materials having a wide range of uses. In particular these materials find extensive applications in coating compositions, e.g. paints, varnishes and stains and in wall covering materials. The utility of acrylate and methacrylate polymers is limited to some extent because they are generally contaminated with unreacted acrylate and methacrylate monomers which, particularly in the case of ethyl and butyl acrylates, can impart an unpleasant smell to the polymer. The unreacted monomer may be present in amounts up to 1000 or 2000 ppm in some polymers and is considered quite malodorous when present in amounts above 50 ppm. Contamination with unreacted monomer is particularly a problem when the polymer is used in paints and wall coverings when the smell is especially noticeable. This problem has for some considerable time concerned manufacturers of paints and wall coverings but, to date, no fully satisfactory solution has been found. Proposed methods for removing unreacted monomers from acrylate and methacrylate polymers have suggested using chemical treatments including those disclosed in U.S. Pat. No. 4,766,173 (treatment with cysteine and lysine), European Published Patent Application 0273651A (treatment with a scavenger monomer such as vinyl acetate) and German Offenlegenschrift 3635367. 
     We have now found that unreacted acrylate and methacrylate monomers can be removed from polymers by treatment with suitable enzymes. 
     Accordingly the present invention provides a method of removing residual amounts of an ethylenically unsaturated carboxylic acid ester monomer from a dispersion in a continuous liquid phase of particles of a polymer which contains residues derived from the ester monomer, which method comprises contacting the dispersion with a hydrolytic enzyme capable of decomposing the ester monomer in the presence of the stabilized dispersed polymer, under conditions suitable for the enzyme to decompose the ester. 
     Further according to the present invention we provide a latex comprising a polymerised ethylenically unsaturated carboxylic ester which contains at least 0.001% by weight (based on the weight of the latex) of a hydrolytic enzyme capable of decomposing the ester monomer. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     In the accompanying Figures, FIGS. 1 to 3 are graphs showing the reduction of acrylate monomer levels occurring on treatment of polymer dispersions with enzymes according to the invention. 
    
    
     The process of the invention is especially suitable for the decomposition of unreacted acrylate and methacrylate monomers from acrylate and methacrylate polymers. Typically, the corresponding monomers are alkyl acrylate and/or methacrylate esters and usually the alkyl groups are C 1  to C 10  alkyl groups including, for example, ethyl, butyl and `iso-octyl` (2-ethylhexyl) alkyl groups, together with pigments fillers and additives such as are commonly used in paint formulations. The process is also especially useful when the unreacted monomer is present in only trace amounts, say less than 2000 ppm. It can be used to treat polymers which are intended for any purpose but has particular advantages when used to treat polymers for use in wall coverings and coatings compositions e.g. paints, varnishes and stains. Treatment of the polymer with the enzyme can be carried out at any stage during its production or the treatment can be applied to the final products, e.g. the paint containing the polymer together with pigments fillers and additives such as are commonly used in paint formulations. 
     In the process of the invention any hydrolytic enzyme may be used which is capable of causing the decomposition of the ester monomer, especially an acrylate or methacrylate ester, to the corresponding acid. Particularly suitable classes of enzymes include esterases and lipases although amidases and proteases can be used. An especially suitable individual amidase enzyme for use in the process of the invention is that disclosed in our European Published Patent Application 0272025A which can be induced in strains of Methylophilus methylotrophus (formerly named Pseudomonas methylotropha), described in UK specification 1370892. Examples of suitable strains of Methylophilus methylotrophus include those deposited at the National Collections of Industrial and Marine Bacteria Ltd., 23 St. Machar Drive, Aberdeen, AB2 1RY. Scotland as strains NCIB 10508 to 10515 and 10592 to 10596 (particularly NCIB 10515). 
     Suitably the process of the invention is carried out at a temperature in the range 10° to 70° C., but not usually below ambient temperature, typically up to 50° C. preferably 30° to 50° C. and at a pH in the range 3 to 10, preferably 5 to 9. Preferably the enzyme is added to the medium containing the ester in an amount in the range 0.001 to 1% by weight (based on the weight of the medium). 
     Suitably the latex of the invention is an aqueous latex. Aqueous latexes suitable for coating compositions such as paints comprise homopolymers or more usually copolymers or ethylenically unsaturated carboxylic esters. In the copolymers other co-monomers include styrene, vinyl toluene, ethylene, propylene, butene-1 or copolymerised unsaturated carboxylic acids such as acrylic acid. 
     Further examples of suitable enzymes are listed below in Tables 1, 2 and 3. Table 1 lists enzymes which are generally suitable for use in the process of the invention to obtain marked reductions in levels of ethylenically unsaturated carboxylic esters. Table 2 lists preferred enzymes and Table 3 lists particularly prefered enzymes which for instance have been found to be capable of reducing the level of ester monomers present in their polymers to below 10 ppm. The lists are of examples only and are not exhaustive of the enzymes which may be used in the process of the invention. 
     In the lists the following abbreviations are used to indicate the different types of enzymes: 
     L: lipase or esterase 
     CE: acetylcholinesterase 
     P: protease 
     A: amidase 
     A(N): nitrilase 
     PL: phospholipase 
     AP: alkaline phosphatase 
     
                       TABLE 1                                                     
______________________________________                                    
Enzyme            Source         Type                                     
______________________________________                                    
 1. Aspergillus niger lipase                                              
                      Biocatalysts Ltd                                    
                                     L                                    
 2. Chromobacterium viscoseem                                             
                      Biocatalysts Ltd                                    
                                     L                                    
    lipase                                                                
 3. Candida cylindracea lipase                                            
                      Biocatalysts Ltd                                    
                                     L                                    
 4. Mucor miehei lipase                                                   
                      Biocatalysts Ltd                                    
                                     L                                    
 5. Pseudomonas fluorescens                                               
                      Biocatalysts Ltd                                    
                                     L                                    
    lipase                                                                
 6. Rhizopus niveus lipase                                                
                      Biocatalysts Ltd                                    
                                     L                                    
 7. Pancreatic lipase Biocatalysts Ltd                                    
                                     L                                    
 8. Hog pancreas lipase                                                   
                      Fluka Chemicals Ltd                                 
                                     L                                    
 9. Porcine pancreatic lipase                                             
                      Sigma Chemical L                                    
                      Company                                             
10. Psuedomonas fluorescens                                               
                      Sigma Chemical L                                    
    lipase            Company                                             
11. Porcine liver esterase                                                
                      Sigma Chemical L                                    
                      Company                                             
12. &#34;LIPOLASE&#34; lipase Novo-Nordisk A/S                                    
                                     L                                    
13. Acetylcholinesterase type                                             
                      Sigma Chemical CE                                   
    VI-S              Company                                             
14. Subtilisin Carlsberg (type                                            
                      Sigma Chemical P                                    
    VIII)             Company                                             
15. Thermolysin (Bacillus                                                 
                      Diawa Kasei kk P                                    
    thermoproteolyticus)                                                  
16. &#34;ALCALASE&#34; type DX                                                    
                      Novo-Nordisk A/S                                    
                                     P                                    
    (Bacillus licheniformis)                                              
17. &#34;ESPERASE&#34; (Bacillus sp)                                              
                      Novo-Nordisk A/S                                    
                                     P                                    
18. Amidase (Methylophilus                                                
                      ICI plc        A                                    
    methylotrophus)                                                       
19. NOVO Sp 361 (Nitrilase)                                               
                      Novo-Nordisk A/S                                    
                                     A(N)                                 
20. NOVOZYM&#34; 244      Novo-Nordisk A/S                                    
                                     PL                                   
    Phospholipase A2 (bovine                                              
    pancreas)                                                             
21. Alkaline Phosphatase                                                  
                      Sigma Chemical AP                                   
                      Company                                             
22. Candida Cylindracea LIP F5                                            
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
23. Mucor Miehei LIP F7                                                   
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
24. Pseudomonas Fluorescens                                               
                      Enzymatix Ltd, L                                    
    LIP B1            Cambridge, UK                                       
25. Porcine pancreatic                                                    
                      Enzymatix Ltd, L                                    
    Extract           Cambridge, UK                                       
26. Rhizopus Sp LIP F1                                                    
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
27. Rhizopus Sp LIP F3                                                    
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
28. Rhizopus Sp LIP F4                                                    
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
29. Rhizopus delemar LIP F2                                               
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
30. Candida Sp LIP F6 Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
31. Mucor Sp LIP F8   Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
32. Aspergillus Sp LIP F9                                                 
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
33. Aspergillus Usamii LIP F10                                            
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
34. Penicillium SP LIP F11                                                
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
35. Penicillium SP LIP F12                                                
                      Enzymatix Ltd, L                                    
                      Cambridge, UK                                       
36. Humicola lanuginosa LIP                                               
                      Enzymatix Ltd, L                                    
    F13               Cambridge, UK                                       
37. Geotrichum candidum LIP                                               
                      Enzymatix Ltd, L                                    
    F14               Cambridge, UK                                       
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
Enzyme            Source          Type                                    
______________________________________                                    
 4. Mucor miehei lipase                                                   
                      Biocatalysts Ltd                                    
                                      L                                   
 5. Pseudomonas fluorescens                                               
                      Biocatalysts Ltd                                    
                                      L                                   
    lipase                                                                
 3. Candida cylindracea lipase                                            
                      Biocatalysts Ltd                                    
                                      L                                   
 6. Rhizopus niveus lipase                                                
                      Biocatalysts Ltd                                    
                                      L                                   
 7. Pancreatic lipase Biocatalysts Ltd                                    
                                      L                                   
11. Porcine liver esterase                                                
                      Sigma Chemical  L                                   
                      Company                                             
12. &#34;LIPOLASE&#34; lipase Novo-Nordisk A/S                                    
                                      L                                   
14. Subtilisin Carlsberg (type                                            
                      Sigma Chemical  P                                   
    VIII)             Company                                             
18. Amidase (Methylophilus                                                
                      ICI plc         A                                   
    methylotrophus)                                                       
22. Candida Cylindracea LIP F5                                            
                      Enzymatix Ltd,  L                                   
                      Cambridge, UK                                       
20. NOVOZYM&#34; 244      Novo-Nordisk A/S                                    
                                      PL                                  
    Phospholipase A2 (bovine                                              
    pancreas)                                                             
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Enzyme           Source           Type                                    
______________________________________                                    
 3. Candida cylindracea lipase                                            
                     Biocatalysts Ltd L                                   
 5. Pseudomonas fluorescens                                               
                     Biocatalysts Ltd L                                   
    lipase                                                                
11. Porcine liver esterase                                                
                     Sigma Chemical Company                               
                                      L                                   
12. &#34;LIPOLASE&#34; lipase                                                     
                     Novo-Nordisk A/S L                                   
______________________________________                                    
 
    
     Other enzymes useful in the process which may be mentioned include lipases derived from Rhizopus species such as delemar, javanacius, Japonicus and oryzae; Mucor species such as avanicus;  Streptomyces species; Pseudomonas species such as gladioli and cepacia; Humicola lanuginosa; Candida lipolytica and various Penicillium species. In referring to enzymes as being obtained from particular sources, we mean that the specified sources represent the original genotype of the enzyme. Of course, using nucleic acid cloning techniques, enzymes can be expressed in species different from the source of the original genotype and enzymes expressed in a genotypically alien species are included as enzymes of the original genotypical source. For example, the commercially available enzyme `Lipolase` from Novo-Nordisk A/S is stated to be genotypically lipase from Humicola languinosa but commercially manufactured using an Aspergillus expression vector for cloned DNA encoding for the enzyme. 
     The invention is illustrated by the following Examples: 
     EXAMPLE 1 
     Samples of acrylic latex containing 1000 ppm butyl acrylate or more were treated with pig liver esterase at 0.01% (w/w) or 0.1% (w/w) Pseudomonas fluorescens lipase. If necessary the pH of the latex was adjusted by the addition of 1.0M NaOH. The samples containing enzyme were mixed thoroughly and incubated at room temperature for 10 days. At intervals during the incubation, aliquots were removed and the levels of butyl acrylate and acrylic acid were determined by analysis. 
     FIG. 1 shows typical treatment profiles with the two enzymes for an acrylic latex at pH 6.0. The Figure shows graphs of butyl acrylate concentration (ppm) against incubation time (days) for the two enzymes. Controls performed at the same time showed no change in butyl acrylate concentrations over 10 days and no appearance of acrylic acid. 
     EXAMPLE 2 
     Samples of acrylic latex containing in excess of 1000 ppm butyl acrylate were adjusted to various pH values by the addition of 1.0M NaOH. Pig liver esterase was added to a final concentration of 0.01% (w/w) with thorough mixing. The latices were then incubated at room temperature and sampled at various time intervals for the determination of butyl acrylate and acrylic acid concentrations. Table 4 shows the effect of pH on the initial rate of butyl acrylate hydrolysis in acrylic latex by pig liver esterase compared to controls lacking enzyme. 
     
                       TABLE 4                                                     
______________________________________                                    
EFFECT OF pH ON THE HYDROLYSIS OF BUTYL                                   
ACRYLATE IN ACRYLATE LATEX BY PIG LIVER                                   
ESTERASE                                                                  
         Initial rate of butyl acrylate                                   
pH       hydrolysis (ppm/day)                                             
______________________________________                                    
5.4      210                                                              
6.0      590                                                              
7.0      780                                                              
8.0      920                                                              
______________________________________                                    
 
    
     EXAMPLE 3 
     Samples of commercial white matt emulsion paint containing between 1100-1700 ppm butyl acrylate were treated with the following enzymes (dose rate in % (w/w) in brackets): pig liver esterase (0.01) Candida cylindracea lipase (0.1), Novo Lipolase (0.1), Methylophilus methylotrophus amidase (0.1) and Subtilisin carlsberg (0.1). Incubations were carried out at room temperature except for Novo Lipolase and Subtilisin carlsberg which were at 30° C. After 3 days, samples were removed from the treated paint and analysed for butyl acrylate and acrylic acid concentration. Table 5 shows the reductions in butyl acrylate concentration achieved after 3 days treatment with enzyme. The most active enzyme reduced the butyl acrylate concentration to below 10 ppm in the paint. 
     
                       TABLE 5                                                     
______________________________________                                    
REDUCTIONS IN BUTYL ACRYLATE CONCENTRATIONS                               
IN EMULSION PAINT BY TREATMENT WITH VARIOUS                               
ENZYMES                                                                   
                    Amount of Butyl                                       
                    Acrylate Removed                                      
Enzyme              after 3 days (ppm)                                    
______________________________________                                    
Pig liver esterase  1700                                                  
Candida cylindracea lipase                                                
                    1040                                                  
Novo lipolase       830                                                   
Methylophilus methylotrophus amidase                                      
                    370                                                   
Subtilisin carlsberg                                                      
                    258                                                   
Control (enzyme)     0                                                    
______________________________________                                    
 
    
     EXAMPLE 4 
     Aliquots of the white matt emulsion paint used in Example 3 were treated with Methylphilus methylotrophus amidase at 0.1% (w/w). After thorough mixing, samples were either incubated at room temperature or 50° C. At various time periods, aliquote were removed from the quantitation of butyl acrylate and acrylic acid levels. The initial rates of butyl acrylate hydrolysis observed were 129 ppm/day at room temperature and 364 ppm/day at 50° C. Thus a threefold increase in activity was seen at the higher temperature. 
     EXAMPLES 5 TO 7 
     De-odourisation of a latex: 
     Various amounts (as specified in Table 6) of the lipase derived from the yeast Candida cylindracea supplied by Sigma Chemical Company Limited of Poole, Dorset, England were added and stirred into a latex consisting of 50 wt % water and 50 wt % of a copolymer of butyl acrylate and methyl methacrylate. The latex contained 560 ppm of residual unpolymerised butyl acrylate monomer. The latex were maintained at 30° C. for 10 days during which time de-odourisation occurred by removal of a major proportion of the malodourous butyl acrylate monomer. After 10 days, 200 wt % of acetone was added to the latex to enable acrylate monomer remaining to be determined by high pressure liquid chromatography. The results obtained are shown in Table 6. The latex was found to have no odour of butyl acrylate when sniffed by an observer having a normal sense of smell. 
     
                       TABLE 6                                                     
______________________________________                                    
                     Amount Butyl Acrylate                                
          *Wt % lipase                                                    
                     remaining at 10 days                                 
Example   added      ppm                                                  
______________________________________                                    
5         0.1        19                                                   
6         0.05       31                                                   
7         0.025      33                                                   
______________________________________                                    
 *wt % is based on the weight of the latex.                               
 
    
     EXAMPLES 8 TO 10 
     Effectiveness with various esters: 
     0.5 wt % (based on the weight of the sample) of the lipase used in Examples 5 to 7 was added to each of three 100 g of samples of de-ionised water containing 1000 ppm of one of three ethylenically unsaturated carboxylic ester monomers as specified in Table 7. The latex was maintained at 30° C. and shaken for 10 days. Then 200 g acetone was added to each sample to enable the amount of ester monomer remaining to be determined by gas/liquid phase chromatography. The results obtained are shown in Table 7. In all cases the amount of ester monomer remaining was below 100 ppm which was the lower limit of the sensitivity of the gas/liquid phase chromatography technique. The samples were found to have no odour of the esters which indicates that the concentration of ester was below 50 ppm. 
     
                       TABLE 7                                                     
______________________________________                                    
                        Amount remaining                                  
Example   Ester Monomer ppm                                               
______________________________________                                    
8         butyl acrylate                                                  
                        &lt;100                                              
9         ethyl acrylate                                                  
                        &lt;100                                              
10        methyl methacrylate                                             
                        &lt;100                                              
______________________________________                                    
 
    
     EXAMPLES 11 TO 13 
     Effectiveness of supported lipase: 
     The procedure of Examples 8 to 10 was repeated except that the lipase used was obtained from the fungus Mucor miehei, was supported on a polystyrene support and was supplied by Novo Enzymes Limited of Fareham, Surrey, England under the trade name &#34;LIPOZYME&#34;. The results obtained are shown in Table 9. 
     
                       TABLE 9                                                     
______________________________________                                    
                        Amount of Ester                                   
                        remaining after                                   
Example   Ester Monomer 10 days ppm                                       
______________________________________                                    
11        butyl acrylate                                                  
                        &lt;100                                              
12        ethyl acrylate                                                  
                        &lt;100                                              
13        methyl methacrylate                                             
                        &lt;100                                              
______________________________________                                    
 
    
     In all cases, the samples showed no odour of the ester indicating that the concentration of ester was below 50 ppm. 
     EXAMPLE 14 
     Activity at higher temperature: 
     2.5 g of the supported lipase used in Examples 10 to 13 were added to 50 g of de-ionised water containing 1000 ppm of butyl acrylate. The water was maintained at 45° C. and stirred for two hours. Then 150 g acetone was added to the water (to enable the amount of butyl acrylate remaining to be determined by high pressure liquid chromatography) and the supported lipase was removed by filtration. The chromatography was performed and the concentration of butyl acrylate was found to be less than 500 ppm. The water was also found to have lost its odour of butyl acrylate. 
     The recovered supported lipase was re-used four times as above and was found not to have lost any of its activity. 
     EXAMPLE 15 
     Samples of a latex of a copolymer of methyl methacrylate and butylacrylate having a total solids content of about 52% of a type commercially used in and having a residual level of butyl acrylate monomer of 340 ppm were treated with the stated amounts (w/w) of following enzymes: 
     
         ______________________________________                                    
Methylophilus methylotrophua amidase (ICI)                                
                           0.625%                                         
Candida cylindricae lipase (Biocatalysts)                                 
                           1.25%                                          
Mucor miehei lipase (Biocatalysts)                                        
                           2.5%                                           
Lipolase (Novo)            2.5%                                           
Pig liver esterase (Sigma) 0.625%                                         
Pseudomonas fluorescens lipase (Biocatalysts)                             
                           2.5%                                           
______________________________________                                    
 
    
     The latex samples containing the enzyme were incubated at 20° C. At suitable intervals, the levels of butyl monomer were determined by sampling, extraction and HPLC analysis. The results are set out in FIG. 2 which is a graph of measured butyl acrylate monomer lever against incubation time. 
     EXAMPLE 16 
     Samples of a latex of a copolymer of styrene, butyl acrylate and methacrylic acid and including a small proportion of α-methyl styrene and having a total solids content of about 40% commercially used in and having a residual level of butyl acrylate monomer of 790 ppm were treated with the stated amounts (w/w) of the following enzymes: 
     
         ______________________________________                                    
Candida cylindricae lipase (Biocatalysts)                                 
                           1.25%                                          
Lipolase (Nova)            2.5%                                           
Mucor miehei lipase (Biocatalysts)                                        
                           2.5%                                           
Pseudomonas fluorescens lipase (Biocatalysts)                             
                           2.5%                                           
______________________________________                                    
 
    
     The latex samples containing the enzyme were incubated at 20° C. At suitable intervals, the levels of buty acrylate monomer were determined by sampling, extraction and HPLC analysis. The results are set out in FIG. 3 which is a graph of measured butyl acrylate monomer level against incubation time.