Polychloroprene mixtures and the use thereof
This invention relates to sulphur-modified solid chloroprene rubber mixtures having excellent storage stability, good processibility and high tensile strength, and the invention also relates to the use thereof. 
     It has been possible, using mixtures of pre-cross-linked polychloroprene (gel polymer) and benzene-soluble polychloroprene (sol polymer) to reduce the swelling after extrusion of polychloroprene rubbers and to improve the dimensional accuracy, stability and surface smoothness of extruded articles and calandered films. A particularly advantageous process for the production of such mixtures is described for example, in DE-AS No. 1,720,107. 
     However, the unsatisfactory storage stability of this polymer mixture and the reduced mechanical properties of the vulcanisates, such as the tensile strength are noticeable disadvantages. 
     However, if the sol and gel polymer is produced in the presence of sulphur as comonomer, then, after peptization of the latices and after working up the mixture to produce solid rubber, a product is produced which surprisingly has excellent storage stability as a polymer mixture, good processing properties, such as rapid rolled sheet formation, a low energy consumption during production as well as a good injecting ability during extrusion of the rubber mixture, a high scorch reliability and very good mechanical properties of the vulcanisates. This combination of properties cannot be achieved by mixing sol and gel polymers, as they are described in, for example DE-AS No. 1,720,107 or by the combination of sol-gel-chloroprenepolymers, in which only one of the components is a sulphur-containing chloroprene polymer. 
     Therefore, the present invention provides solid polychloroprene mixtures of sol and gel polymer, characterised in that the sol component consists of a copolymer of chloroprene and from 0.75 to 0.32% by weight of sulphur, based on the total quantity of unsaturated compounds of the sol polychloroprene, and up to 10% by weight of the chloroprene may be replaced by other comonomers, and the gel component consists of a copolymer of chloroprene, from 0.4 to 1.0% by weight of sulphur, based on the total quantity of unsaturated compounds of the gel polychloroprene and from 7.5×10 -3  to 3.0×10 -2  mols, based on 100 g of the total quantity of monomer of the gel polychloroprene, of a compound corresponding to the general formula: ##STR2## wherein R 1  and R 2  each represents a hydrogen atom or a C 1  -C 4  alkyl groups, and X represents a C 2  -C 10  alkylene group, wherein up to 10% by weight of the chloroprene may be replaced by other comonomers. 
     1-Chloroprene, styrene, 2,3-dichlorobutadiene and acrylonitrile are particularly included as other comonomers of the sol polymer and of the gel polymer. 
     The solid polychloroprene mixtures are produced by mixing the components which are present at latex, and optionally by adding substances which improve the storage stability of the polymer, such as tetra-ethylthiuramdisulphide (TETD), following which coagulation takes place. 
     The mixtures preferably contain the sol and gel components in a ratio of from 9:1 to 4:6. 
     A preferred diester is ethylene glycol dimethacrylate. After peptization, the gel polymer is to have a swelling index in toluene of from 10 to 90, preferably from 20 to 60. 
     The polymerisation of the two components may take place continuously or discontinuously, a continuous production method being preferred. 
     Polymerisation and peptization of the sol component takes place according to, for example, DE-OS No. 1,807,298 or DE-OS No. 2,755,074. 
     Polymerisation of the sulphur-containing gel component takes place according to, for example, DE-OS No. 3,120,992 or DE-OS No. 3,123,905. The polymerisation is stopped and the polymer peptized according to, for example DE-OS No. 2,755,074. The term &#34;peptization&#34; is understood in the present context as designating the cleavage of the polymer chain at its sulphur segments. The peptization agent, TETD, which is frequently used may be combined with nucleophilic acting substances, such as amines or dithiocarbamate (DE-OS No. 2,018,736; DE-AS No. 1,230,204). The latex which is obtained after polymerisation is usually peptized at a temperature of from 30° to 70° C. The peptization rate depends on, among other factors, the quantity of peptization agent, on the type and quantity of the nucleophilic substance, on the temperature and on the pH of the latex. The polymer may be decomposed in the latex or on the solid crude polymer, for example according to freeze coagulation of the latex. 
     Depending on the peptization conditions, the sulphur bridges in the polychloroprene network are dissolved, as a result of which the network density which is visible in an increasing swelling index, is reduced. 
     The latex mixture may be worked up, for example, by freeze coagulation as described in DE-OS No. 2,645,921. 
     The polychloroprene mixtures according to the present invention are particularly suitable for the production of rubber articles which may be subjected to a high mechanical strain, such as V-belts and conveyor belts, and these may be produced by extrusion, by calandering and by stamping. 
    
    
     EXAMPLE 1 
     Production of sol polymer latex I 
     970 g of chloroprene and 30 g of 2,3-dichlorobutadiene are emulsified in 1500 g of water to which are added 55 g of disproportionated resin acid (solids content 70%), 5 g of sodium salt of a naphthalenesulphonic acid-formaldehyde condensation product, 5 g of sodium hydroxide, 3 g of anhydrous sodium pyrophosphate, 5 g of triisopropanolamine and 12 g of sulphur dispersion (50%). 
     10 g of potassium persulphate and 0.2 g of sodium-β-anthraquinone sulphonate, dissolved in 490 g of water are prepared as catalyst solution. 
     The emulsion is flushed with nitrogen, heated to 50° C. and polymerisation started by adding the catalyst solution. During polymerisation, more catalyst solution is metered in such that the temperature of the mixture does not exceed 50° C. After a monomer conversion of 65%, polymerisation is stopped with 1 g of phenothazine and the excess monomer is separated under reduced pressure by steam distillation. The latex is mixed with 6.5 g of TETD (22% by weight emulsion) and is peptized for 3 hours at 40° C. 
     EXAMPLE 2 
     Production of sol polymer latex II 
     The process is carried out as described in Example 1, except that the 2,3-dichlorobutadiene is replaced by chloroprene. 
     EXAMPLE 3 
     Production of sol polymer latex III (Comparative Example) 
     1000 g of chloroprene are emulsified together with 2.5 g of n-dodecylmercaptan in 1300 g of water, to which are added 55 g of disproportionated resin acid (solids content 70%), 7 g of the sodium salt of a naphthalene-sulphonic acid-formaldehyde condensation product and 7 g of sodium hydroxide. 
     32 ml of a 3% aqueous formamidine sulphinic acid solution are allowed to continuously run in as catalyst solution. The emulsion is flushed with nitrogen, heated to 40° C. and polymerisation started by adding the catalyst solution. During polymerisation, more catalyst solution is metered in such that the temperature of the mixture does not exceed 45° C. After a monomer conversion of 65%, polymerisation is stopped with 1 g of phenothiazine and the excess monomer is separated under reduced pressure by steam distillation. 
     EXAMPLE 4 
     Production of sulphur-containing gel polymer latex I 
     970 g of chloroprene and 30 g of ethylene glycol dimethacrylate are emulsified in 1500 g of water, to which are added 55 g of disproportionated resin acid (solids content 70%), 6 g of the sodium salt of a naphthalenesulphonic acid-formaldehyde condensation product, 3 g of sodium hydroxide, 5 g of anhydrous sodium pyrophosphate, 5 g of triisopropanolamine and from 10 to 18 g of a sulphur dispersion (50%). 
     10 g of potassium persulphate and 0.2 g of sodium-β-anthraquinone sulphate, dissolved in 490 g of water are prepared as the catalyst solution. 
     Polymerisation is carried out as described in Example 1 up to a monomer conversion of 78%, and the latex which is degassed and stopped with 7 g of tetraethylthiuramdisulphide is peptized with 4.2 g of dibutyldithiocarbamate, dissolved in water, for 3 hours at 40° C. 
     EXAMPLE 5 
     Production of sulphur-containing gel polymer latex II 
     The process is carried out as described in Example 4, except that 960 g of chloroprene and 40 g of ethylene glycol dimethacrylate are used. 
     EXAMPLE 6 
     Production of gel polymer latex III (Comparative Example) 
     950 g of 2-chloroprene are emulsified together with 50 g of ethylene glycol dimethacrylate and 3 g of n-dodecylmercaptan in 1300 g of water, to which are added 60 g of disproportionated resin acid (solids content 70%), 4 g of sodium salt of a naphthalenesulphonic acid-formaldehyde condensation product and 4 g of sodium hydroxide. 
     Polymerisation is carried out in analogous manner to Example 3 up to a conversion of 78%. 
     Compilation of the polymer mixtures, numbers in parts by weight (solid based on solid). Except for Examples 7 and 8, all the latex mixtures are mixed with 1.5% by weight of TETD, based on polymer solids, before processing. 
     
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Example                                                                   
       7.sup.1                                                            
         8.sup.1                                                          
           9.sup.1                                                        
             10                                                           
               11                                                         
                 12                                                       
                   13                                                     
                     14                                                   
                       15                                                 
                         16                                               
                           17                                             
                             18                                           
                               19                                         
                                 20                                       
                                   21                                     
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Sol latex I                                                               
           80        80                                                   
                       90                                                 
                         80                                               
                           70                                             
                             60                                           
                               80                                         
                                 80                                       
                                   80                                     
Sol latex II 80                                                           
               50                                                         
                 80                                                       
                   50                                                     
Sol latex III                                                             
       80                                                                 
         50                                                               
Gel latex I  20.sup.3                                                     
               50.sup.3                                                   
                     20.sup.3                                             
                       10.sup.2                                           
                         20.sup.2                                         
                           30.sup.2                                       
                             40.sup.2                                     
                               20.sup.2                                   
                                 20.sup.4                                 
                                   20.sup.5                               
Gel latex II     20                                                       
                   50                                                     
Gel latex III                                                             
       20                                                                 
         50                                                               
           20                                                             
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 .sup.1 = Comparative Example                                             
 .sup.2 = Contains 0.9 parts of sulphur                                   
 .sup.3 = Contains 0.8 parts of sulphur                                   
 .sup.4 = Contains 0.7 parts of sulphur                                   
 .sup.5 = Contains 0.5 parts of sulphur                                   
 
    
     EXAMPLE 22 
     Storage stability and mastication 
     Storage stability: 
     After the latex mixtures have been processed into rubber, the Mooney viscosity of the samples is determined according to DIN 53 523; ML 4 (100° C.). In order to determine the viscosity stability, the samples are stored for 3 days at 70° C. and the viscosity is re-determined. The more stable the products, the lower the viscosity difference (ΔML) between the two measurements. 
     Mastication test 
     800 g of polymer are squeezed twice through a 0.4 mm roller gap on a rolling mill (200×390 mm) at 40° C. roller temperature and with a friction of 1:1.2. 200 g thereof are then weighed twice and masticated over a period of from 0 to 8 minutes with the same roller gap. The end value which is achieved, in addition to the viscosity difference is used for the assessment. 
     
         ______________________________________                                    
Polymer from Example                                                      
               10     11     12   13   7.sup.1                            
                                            8.sup.1                       
______________________________________                                    
ML-4 (ME)      47     52     50   48   52   50                            
ML-4 (ME 3 days 70° C.)                                            
               44     49     49   48   62   62                            
ΔML      -3     -3     -1   ±0                                   
                                       +10  +12                           
Decomposition by                                                          
mastication                                                               
(ME)           33     36     36   32   44   43                            
Decomposition in %                                                        
               30     31     28   33   15   14                            
______________________________________                                    
 .sup.1 Comparative Example                                               
 
    
     Compared to conventional types of gel, the gels according to the present invention exhibit a high mastication effect which is very advantageous for the processing behaviour, and they only exhibit a small viscosity change during the heat storage, which is equivalent to a high storage stability. 
     EXAMPLE 23 
     Processing and vulcanisate properties 
     Scorch time (MS 5 ) min. at 120° C.: 
     A test mixture produced according to 150 specifications 2475-1975 E is vulcanised in a shearing disc plastometer according to Mooney (DIN 53 523) at 120° C. The time span from the start of measuring up to the plasticity increase by 5 Mooney units over the plasticity minimum is specified (in min.) as the prevulcanisation time (scorch). 
     Vulcanisation time at 150° C.: 
     A test mixture produced according to ISO specifications 2475-1975 E is vulcanised in a shearing vulcameter according to DIN 53 529 at 150° C. The reaction time t R  in minutes which results from the difference of the times at 80% (t 80 ) and 10% (t 10 ) conversion with respect to the cross-linking density which may be achieved, is recorded. 
     Vulcanisation strength: 
     A polymer-carbon black mixture according to ISO standard 2475 1975 E is vulcanised at 150° C. in three stages (20, 40, 60 min.). The strength of the corresponding samples is determined according to DIN 53 455. In each case, the average value of the three stages is stated. 
     
         ______________________________________                                    
Polymer from Example                                                      
               10     11     12   13   7    8.sup.1                       
______________________________________                                    
MS.sub.5 (min) 21     20     22   25   8    8                             
Reaction time (min)                                                       
               8.2    9.0    8.4  9.8  17.5 16.0                          
Vulcanisation  19.6   19.4   19.5 19.1 18.5 16.3                          
strength (MPa)                                                            
______________________________________                                    
 .sup.1 Comparative Example                                               
 
    
     As may be seen, the polymer mixtures according to the present invention (numbers 10 to 13) have clearly longer scorch times, i.e., a higher processing reliability, shorter reaction times and a higher vulcanisation strength, even in the case of high gel quantities (numbers 11 and 13) compared to the Comparative product (number 8). 
     EXAMPLE 24 
     Extrusion behaviour of the rubber mixture 
     
         ______________________________________                                    
Test mixture                                                              
______________________________________                                    
Polymer            100    parts by weight                                 
Carbon black N 990 25     &#34;                                               
Carbon black N 539 25     &#34;                                               
Aromatic mineral oil                                                      
                   15     &#34;                                               
α-naphthylamine                                                     
                   2      &#34;                                               
Stearic acid       1      &#34;                                               
Ozone protective agent                                                    
                   1      &#34;                                               
Magnesium oxide    4      &#34;                                               
______________________________________                                    
 
    
     According to this formulation, a total mixture amounting to 1.2 kg is mixed in a 1 liter kneader at 30° C. and at 30 r.p.m. 300 g of this test mixture are injected to form a Garvey profile according to ASTM-D 2230-78 as an 8 mm wide and a 2 mm thick strip by means of a Brabender laboratory extruder (10-D screw) at a temperature of from 70° (cylinder) to 90° C. (injection head), and the swelling after extrusion (in %) and the extrusion velocity (cm/sec) is determined therefrom. 
     500 g of the test mixture are allowed to revolve as a sheet on a rolling mill (320×200 mm) at 30° C., 20 r.p.m. and with a roller gap of 0.9 mm. Two strips having dimensions of 5×50 cm are cut out of the sheet and are left on a small talcum-covered support. After 24 hours, the length is measured and the average from both samples is stated as the shrinkage (in %). 
     
         ______________________________________                                    
Polymer from Example                                                      
                  10        12     7.sup.1                                
______________________________________                                    
Swelling after extrusion (%)                                              
                  45.0      45.0   46.5                                   
Extrusion velocity (cm/sec)                                               
                  2.20      2.24   2.10                                   
Shrinkage (%)     42        40     48                                     
______________________________________                                    
 .sup.1 Comparative Example                                               
 
    
     EXAMPLE 25 
     Comparison of polymer mixtures having sulphur-containing and sulphur-free gel parts 
     The processing and vulcanisate properties are determined according to Example 23. 
     
         ______________________________________                                    
Polymer from Example 9.sup.1                                              
                            14                                            
______________________________________                                    
ML-4 (ME)            60     60                                            
MS.sub.5 (min)       24     36                                            
Reaction time (min)  12.2   10.4                                          
Vulcanisation strength                                                    
                     18.6   19.4                                          
(MPa)                                                                     
Hardness (Shore A)   59     63                                            
Elasticity (%)       45     48                                            
______________________________________                                    
 .sup.1 Comparative Example                                               
 
    
     Compared to Example 9 in which only one polymer component contains sulphur, Example 14 according to the present invention exhibits a clearly higher scorch reliability, a faster reaction time and more favourable vulcanisate values. 
     EXAMPLE 26 
     Influence of the gel proportion and of the sulphur content of the gel component on the raw material- and vulcanisate properties of the polymer mixtures 
     The storage stability and the processing and vulcanisate properties are determined, as described in Examples 22 and 23, the test mixtures being produced according to ISO 2475-1975 E without thiourea accelerators. 
     
         ______________________________________                                    
Polymer from                                                              
Example     15     16     17   18   19   20   21                          
______________________________________                                    
ML-4 (ME)   37     37     36   36   35   37   38                          
ML-4 (3 days                                                              
            36     36     36   36   34   37   39                          
70° C.) (ME)                                                       
MS.sub.5 (min)                                                            
            45     42     42   40   42   39   39                          
Vulcanisate 19.3   19.2   19.4 19.6 19.2 19.6 20.1                        
strength (MPa)                                                            
Hardness (Shore A)                                                        
            65     65     65   65   65   64   64                          
Elasticity (%)                                                            
            48     48     48   48   49   49   49                          
______________________________________                                    
 
    
     As may be seen, the storage stability and the processing behaviour is independent of the gel content or of the sulphur content of the gel component. In contrast to the hitherto conventional decrease in the vulcanisate strength with an increasing gel content in the polymer mixture, in this case, the strength is independent thereof.