Method of preparing sulfur-modified polychloroprene
BACKGROUND OF THE INVENTION 
     The present invention is directed to an improved method of preparing a sulfur-modified polychloroprene latex. 
     Sulfur-modified chloroprene polymers are well known. They are obtained by polymerization of chloroprene, possibly with another unsaturated monomer, in the presence of elemental sulfur. The polymer obtained has a structure which is strongly cross-linked via polysulfide bonds and is difficult to work. In order to obtain a product of increased plasticity, the polymer is partially degraded by means of suitable agents which produce a breaking of some of the polysulfide bonds. This operation is referred to as peptization. Suitable plasticizing agents are described in general in U.S. Pat. No. 2,234,215. 
     Methods of peptization in alkaline aqueous emulsion have been described, for instance in French Pat. Nos. 1,376,314; 1,392,446; 1,393,099; 1,398,602; 2,076,922 and 2,089,811, as well as in U.S. Pat. No. 3,318,832. In accordance with these methods, the peptization is carried out after the polymerization by treating the polymer latex with an alkylthiuram disulfide, combined or not with a xanthogen disulfide or a dialkyldithiocarbamate. The main disadvantage of these processes is the slowness of the peptization (requiring several hours at 40° C.). The reaction proceeds during the treatment of the latex for the elimination of the residual monomer as well as during the phase of isolation of the rubber. Furthermore, the plasticity of the rubbers obtained varies in time, which represents a serious drawback for their use and their conversion into manufactured products. 
     It is also known, in accordance with a different technique, to increase the plasticity of the sulfur-modified chloroprene polymers by means of modifying agents or chain-transfer agents, such as xanthogen disulfides, iodoform or alkyl mercaptans, which are present during the polymerization and which act as molecular weight regulators. These methods are described, for example, in French Pat. No. 1,457,004 and U.S. Pat. No. 3,397,173. The modified copolymers obtained can still be peptized subsequently. 
     Finally, French Pat. No. 2,008,562 and its corresponding U.S. Pat. No. 3,808,183 describe a method of preparing polychloroprene modified with sulfur and xanthogen or iodoform in which the polymer latex is peptized instantaneously by means of a thiol. 
     It is, therefore, an object of the present invention to provide a method of preparing, in a single step, sulfur-modified chloroprene polymers without auxiliary modifying agents, in which the polymerization and the peptization are carried out directly during the polymerization and not after completion of the polymerization, as in accordance with the known methods. 
     It is also an object of the present invention to provide an improved method for preparing sulfur-modified polychloroprene latices. 
     Other objects will be apparent to those skilled in the art from the present description. 
     GENERAL STATEMENT OF THE INVENTION 
     The method of the invention is conducted by copolymerization of chloroprene with sulfur, the chloroprene optimally being replaced up to 20% of its weight by another copolymerizable vinyl monomer, in alkaline aqueous emulsion, in the presence of a free-radical initiator, said method employing in the reaction mixture, one or more organic polysulfides selected from among the class consisting of: 
     (a)-the disulfides of di- and tetra-alkylthiurams in which the alkyl group contains from about 3 to 5 carbon atoms; 
     (b)-benzothiazyl disulfide; 
     (c)-dibenzyl polysulfides in which the number of sulfur atoms is equal to or greater than 3; 
     (d)-2,4,5-trichlorophenyl trisulfide; and 
     (e)-benzanilide disulfide. The said polysulfide is introduced before the begninning of the polymerization in proportions of between about 0.1 and 2 parts to 100 parts (by weight) of monomer charged. 
     The polysulfides which can be employed belong to the general class of plasticizing compounds described in U.S. Pat. No. 2,234,215. These compounds do not all have the same activity. It is known that some of them act in the reaction medium as chain-transfer agents, while others such as the disulfides of tetraethyl and tetramethyl thiurams, have a polymerization-retarding or inhibiting effect. By the present invention it has been found that the polysulfides employed in accordance with the invention have no polymerization retarding effect and have a rapid and complete peptizing action so that the polymers obtained can no longer be peptized subsequent to their preparation and do not undergo any change in viscosity during storage. 
     In combination with the said compounds it is possible, in accordance with one specific embodiment of the method of the invention to use a disulfide or tetrasulfide of tetramethyl or tetraethyl thiuram in an amount of less than 0.25 part per hundred parts by weight of monomer. The polymerization is carred out in alkaline aqueous emulsion in known manner in the presence of free-radical producers, such as hydrogen peroxide, the organic peroxides, cumyl peroxide, dibenzoyl peroxide, alkaline-metal or ammonium ferricyanides and, preferably alkaline-metal or ammonium persulfates. 
     As emulsifying agents there can be employed any of the usual compounds or mixture of usual compounds, such as water-soluble salts of acids derived from crude dismutated hydrogenated or partially polymerized rosins derived from pine oleoresin, wood, or tall oil. The emulsion generally contains a certain amount of dispersing and stabilizing agents, such as the fatty alcohol sulfates, alkyl sulfates and sulfonates, the alkaline salts of alkyl aryl sulfonic acids and, more particularly, the alkaline salts of methylene-bis-naphthalene sulfonic acid. Small amounts of a conventional chain transfer agent can also be added. 
     Up to 20% of the weight of the chloroprene monomer can be replaced by another copolymerizable monomer. Among the monomers which are copolymerizable with chloroprene are the vinyl aromatic compounds, such as styrene, the vinyl toluenes, and vinyl naphthalenes, the acrylic and methacrylic acids, as well as their ester and nitrile derivatives, such as ethyl acrylate, methyl methacrylate and acrylonitrile, the aliphatic conjugated diolefines, such as 1,3-butadiene, isoprene, 2,3-dichloro-1, 3-butadiene and 2,3-dimethyl-1, 3-butadiene, and the vinyl ethers and ketones, such as methyl vinyl ether, vinyl acetate and methyl vinyl ketone. 
     The concentration of the monomer or monomers in the emulsion is generally between about 30 and 60% by weight, referred to the total weight of the emulsion. 
     The amount of sulfur may vary from about 0.1 to 0.6% by weight, referred to the chloroprene monomer or the mixture of monomers. 
     The polysulfides which can be employed in the method of the invention are introduced into the reaction medium at the same time as the other reagents, but before the start of the polymerization. 
     The polymerization temperature may vary from about 10° C. to 80° C., but it is preferably between about 40° C. and 60° C. 
     The pH of the emulsion during the polymerization is greater than about 10 and preferably between about 11 and 13. 
     The final conversion content does not present any particular limit and it may vary from about 50 to 90%, and, preferably, from about 75% to 85%. 
     The polymerization can be stopped at any time by addition of a conventional polymerization-inhibiting agent. 
     The residual monomer is eliminated by flash distillation and the resulting rubber is isolated from the resulting latex by any known means. 
     The process of the invention has an appreciable economic advantage due to the fact that it is carried out in a single step without increasing the duration of the polymerization. Furthermore, the amount of peptizing agent necessary can be decreased to one-third of its value as compared with the conventional two-step processes of the prior art. 
     The chloroprene polymers obtained have a Mooney viscosity of between about 20 and 100 and, preferably, between about 35 and 70. This value is stable and does not change during the storage of the latex or the treatment of the rubber. The value desired can be fixed by the man skilled in the art by means of a few preliminary tests as a function of the amount of sulfur, the nature and concentration of peptizing agent, as well as the polymerization conditions. The polychloroprenes can be vulcanized with the vulcanization systems customarily employed for sulfur-modified polychloroprenes, without addition of accelerators derived from thiourea, and in the same manner as the polychloroprenes peptized in conventional manner by thiurams. The vulcanized products have excellent mechanical properties. They are superior to the polychloroprenes peptized or modified in accordance with the prior art with respect to the values of compression-set and tear-strength. Therefore, they are particularly suitable for the manufacture of technical parts such as joints and damping members. 
     SPECIFIC DESCRIPTION OF THE INVENTION 
     In order to disclose more clearly the nature of the present invention, the following examples illustrating the invention are given. It should be understood, however, that this is done solely by way of example and is intended neither to delineate the scope of the invention nor limit the ambit of the appended claims. In the examples which follow, and throughout the specification, the quantities of material are expressed in terms of parts by weight, unless otherwise specified. 
    
    
     EXAMPLE 1 
     A sulfur-modified polychloroprene latex is prepared by means of an aqueous emulsion system of the following composition: 
     
         ______________________________________                                    
                        Parts                                             
______________________________________                                    
chloroprene               100                                             
resin acids: dismutated rosin                                             
                          4                                               
sulfur                    0.20                                            
tetrabutylthiuram disulfide                                               
                          1                                               
water                     100                                             
sodium hydroxide (100%)   0.64                                            
sodium salt of methylene-bis-alkyl-                                       
                          0.70                                            
naphthalene sulfonic acids                                                
Catalyst solution to be added to the emulsion system:                     
sodium persulfate         1                                               
water                     99                                              
______________________________________                                    
 
    
     The polymerization is effected at 50° C. in a nitrogen atmosphere at a pH of 12.9 and the catalyst solution is added continuously at a rate so as to obtain a uniform rate of polymerization (the total amount introduced is about 5 parts by weight of the weight of the emulsion system). After 4 hours and 15 minutes the conversion of the monomer is 80%. The polymerization is stopped by adding 0.01 part of p-tertiarybutyl catechol and 0.01 part of phenothiazine in the form of an emulsion. The latex is then treated with steam to eliminate the residual monomer and then cooled. After isolation of the polymer by coagulation on a cold drum, the measurement of the Mooney viscosity (ML 1+4 at 100° C.) gives a value of 50. 
     COMPARATIVE TEST A 
     A test is carried out under the same operating conditions as in the above example, but without adding thiuram during the polymerization. After the termination of the polymerization, the polymer, isolated on a sample of latex, has a viscosity of 170. One part of tetraethylthiuram disulfide is then introduced into the latex; the latex is stirred for four hours at 45° C. and treated by the customary method. The Mooney viscosity of the polymer obtained is 155 and the material is very unstable upon storage. 
     COMPARATIVE TEST B 
     (Peptization process of the prior art) 
     The test is again carried out but this time without thiuram and the amount of sulfur is increased to 0.6 parts. After the termination of the polymerization, 1.6 part of tetraethylthiuram disulfide is added. The latex is stirred for 4 hours at 45° C. and treated in accordance with the customary method. The Mooney viscosity of the polymer obtained is 50. 
     COMPARATIVE TEST C 
     (Modified non-peptized polymer in accordance with the prior art) 
     The test is again carried out replacing the tetrabutylthiuram disulfide by 0.4 part of diisopropyl xanthogen disulfide. The amount of sulfur is 0.6 part. The polymer isolated after the termination of the polymerization has a Mooney viscosity of 50. 
     MECHANICAL PROPERTIES OF THE FOREGOING POLYMERS 
     The polychloroprene obtained in accordance with each of Example 1 and Comparative Tests B and C mixed with the following components: 
     
         ______________________________________                                    
                       Parts by weight                                    
______________________________________                                    
polychloroprene          100                                              
magnesium oxide          4                                                
zinc oxide               5                                                
stearic acid             0.5                                              
phenyl-β-naphthylamine                                               
                         2                                                
SRF black                75                                               
aromatic extending oil (DUTREX 729 FC of                                  
                         15                                               
Shell Oil)                                                                
______________________________________                                    
 
    
     Vulcanization is carried out at 153° C., for 40 minutes. The properties of the vulcanized products are entered in Table I below: 
     
                       TABLE I                                                     
______________________________________                                    
Compression Set - ASTM standard 395-61 - Method B - after 96              
hours at 100° C. - Tear strength - ASTM Standard D 624-73 -        
Angular Test Piece C                                                      
                            Tear                                          
Polychloroprene                                                           
             Compression Set %                                            
                            strength in kg/cm                             
______________________________________                                    
Example I    54             63                                            
Comparative Test B                                                        
             63             54                                            
Comparative Test C                                                        
             64             44                                            
______________________________________                                    
 
    
     EXAMPLES 2 to 9 
     A polychloroprene is prepared in the same manner and using the emulsifying system described in Example 1, above. The nature and quantity of the thiuram or of a mixture of thiurams in accordance with the invention is varied. The amount of sulfur, the polymerization temperature and the degree of conversion of the monomer are also varied (the polymerization time is 4 to 5 hours). These changes and the viscosity values measured on the polymer obtained are set forth in Table II, below. In Table II the following abbreviations are employed: 
     TETD--tetraethylthiuram disulfide 
     TITD--tetraisopropylthiuram disulfide 
     TBTD--tetrabutylthiuram disulfide 
     DPTD--dipentamethylene thiuram disulfide 
     DPTT--dipentamethylene thiuram tetrasulfide 
     The results obtained and set forth in Table II show that the method of the invention makes it possible to obtain stable Mooney viscosity values within the range of about 30 to 65. Example E in Table II, given by way of comparison, shows that the thiuram tetrasulfide used does not operate as peptizing agent in the method of the invention. 
     
                                           TABLE II                                
__________________________________________________________________________
Sulfur-parts                                                              
           Thiuram     Operating conditions                               
                                   Mooney viscosity                       
Examples                                                                  
     by Weight  parts by Wt.                                              
                       T (°C.)                                     
                           Conversion %                                   
                                   ML 1 + 4 - 100° C.              
__________________________________________________________________________
2    0.1   TBTD 2      40  60      60                                     
3    0.6   TITD 0.6    40  80      47                                     
4    0.2   (TBTD                                                          
                0.8    50  70      66                                     
           (TETD                                                          
                0.2                                                       
5    0.6   (TBTD                                                          
                0.1    45  70      39                                     
           (TETD                                                          
                0.1                                                       
6    0.2   (TBTD                                                          
                1.3    30  80      62                                     
           (TETD                                                          
                0.2                                                       
7    0.2   DPTD 0.5    40  80      75                                     
8    0.2   DPTD 0.8    40  80      57                                     
9    0.2   DPTD 1      40  80      30                                     
E    0.2   DPTT 1      50  80      170                                    
__________________________________________________________________________
 
    
     EXAMPLES 10 to 13 
     The thiuram sulfide employed in the foregoing examples is replaced by a different polysulfide. 0.2 part of sulfur is used. The polymerization is carried out at 50° C. and is terminated when the conversion of the monomer into polymer reaches 80%. The total time of the polymerization varies from 4.5 to 5 hours. The results are entered in Table III, below. Examples, F, G, H are given by way of comparison. 
     
                       TABLE III                                                   
______________________________________                                    
 Examples       10    F      11  12  G    H    13                         
______________________________________                                    
Polysulfide (parts by weight)                                             
Benzothiazyl disulfide                                                    
                1                                                         
Mercaptobenzothiazol  1                                                   
Benzanilide disulfide        0.5                                          
Benzyl polysulfide               1.1                                      
Benzyl disulfide                     1.1                                  
Benzoyl disulfide                         1                               
2,4,5-trichlorophenyl-                                                    
trisulfide                                     1                          
Mooney viscosity                                                          
(ML 1 + 4 - 100° C.)                                               
                43    170    41  65  150  180  52                         
______________________________________                                    
 
    
     The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but is recognized that various modifications are possible within the scope of the invention claimed.