Method of treating synthetic polymer latex of styrene-butadiene or polyisoprene to make solid polymer derived therefrom re-dispersible to a latex
This application is a divisional application of my co-pending application Ser. No. 390,246 filed Aug. 21, 1973, now U.S. Pat. 3,904,567. 
    
    
     The present invention relates to the preservation of synthetic rubber latex in the form of re-dispersible solids or low moisture content pastes. 
     Synthetic rubber in the form of its latex is an important material particularly in the production of soft and foamed rubber goods, the manufacture of which is frequently performed in places far removed from the source of latex. The present invention is directed in part to the provision of latices in solid form which are particularly convenient for transport and for the preparation of for example foam materials on site. 
     Further the solid latices are believed to be useful in the manufacture of so called &#34;flexible plasters&#34;. 
     The present Invention provides a method of making a solid or pasty synthetic rubber which is dispersible in water, which method comprises providing a latex of the synthetic rubber in admixture with a re-dispersing agent, and removing water from the latex. The invention also includes synthetic rubber in solid or paste form which contains a re-dispersing agent and is dispersible in water. 
     Whilst it is believed that the invention is, in general, applicable to a wide range of synthetic rubbers, it has been found that it is particularly applicable to styrene-butadiene rubber (SBR) and polyisoprene rubber. Because of the slightly differing surface properties of the latex particles of these rubbers, there are some differences in the compounds found to be completely effective in these two types of synthetic latices. Among SBR latices those prepared by emulsion polymerisation (e.g. Intex 100) are preferred and among polyisoprene latices those prepared by solution polymerisation (e.g. Cariflex IR 700) are preferred. 
     While it will normally be preferred to use the minimum quantity of re-dispersing agent necessary to confer adequate re-dispersibility on the solid rubber, there is no reason in principle why greater amounts of re-dispersing agent should not be used. The maximum concentration of redispersing agent is thus not critical, and will generally be that at which other properties of the rubber start to significantly deteriorate. 
     STYRENE BUTADIENE RUBBER 
     By this term, we mean elastomeric copolymers of styrene and butadiene, which copolymers may optionally contain minor proportions of other monomers copolymerised therewith. 
     Preferred re-dispersing agents for styrene-butadiene rubber latices are urea, polyhydric alcohols and mono- and di-saccharides. Generally, these re-dispersing agents are used in amounts of from 0.05 to 10% by weight on the weight of the dry rubber. Amounts below 0.1 phr may not be sufficient to achieve complete re-dispersibility of the solid or pasty rubber. Little added advantage is gained by the use of amounts above 5.0 phr. 
     Preferred polyhydric alcohols having at least 3 hydroxyl groups per molecule include glycerol, mannitol, sorbitol, quebrachitol and inositol. Pentaerythritol is less effective. Preferred saccharides include d (+) glucose, fructose, d (+) galactose, 1 (+) arabinose, d (-) arabinose, sucrose, d (-) mannose and lactose. 
     In general, where mixtures are used there is no need to add supplementary agents over and above the fatty acid soap level normally used in the production of SBR latices. Attention is drawn to the publication entitled &#34;Description of Synthetic Rubbers and Latices&#34; published by the International Institute of Synthetic Rubber Producers Inc., 45 Rockefeller Plaza, New York, N.Y.10020, 1968 - Jan - edition, wherein it is noted that four types of emulsifying systems are used in the production of SBR latex. These are fatty acid, rosin-acid, a mixture of a fatty acid and a rosin-acid and salt acid. The experiments made so far have been using a SBR latex which contained a fatty acid soap emulsifying system. The results have indicated that the further addition of a rosin-acid soap to the SBR latex used in the experiments, interferes with the re-dispersibility of the rubber. This is illustrated in Example No. 7. It is accordingly preferred in the present invention to prepare the redispersible rubber of the invention from a latex sustantially free from rosin-acids. 
     POLYISOPRENE RUBBER 
     By this term we mean an elastomeric polymer of polyisoprene, either cis- or trans-polyisoprene or a mixture of the two, either a homopolymer or a co-polymer of a major proportion of polyisoprene with a minor proportion of another co-polymerisable monomer. 
     Preferred re-dispersing agents for polyisoprene rubber latices are either saponin alone, or a mixture of 
     a. at least one compound selected from glycosides, polyhydric alcohols, urea, and mono- and di-saccharides, with 
     b. at least one alkali metal or ammonium salt of an organic acid having a C 7  to C 30  aliphatic carbon chain. 
     Generally, the redispersing agent is used in a amount from 2 to 20 phr. Saponin is very effective alone in concentrations of the order of 10 phr. Where mixtures are used, each component is preferably present in an amount of at least 1 phr., and the mixtures in an amount of at least 3 phr. 
     Examples of suitable glycosides are the saponin and digitonin. Preferred polyhydric alcohols include glycerol, mannitol, sorbitol, quebrachitol and inositol. Preferred saccharides include d (+) glucose, fructose, d (+) galactose, 1 (+) arabinose, d (-) arabinose, sucrose, d (-) mannose, and lactose. 
     Among compounds of group b) above, there may be mentioned potassium palmitate; potassium oleate; the ammonium, sodium and potassium salts of capric, caprylic, lauric, stearic, ricinoleic and linoleic acids either by themselves or in various combinations as they would occur in natural vegetable oil soaps, e.g. coconut oil soap and castor oil soap; and the agent sold by ICI under the Trade Mark Vulcastab LS, believed to be sodium oleyl para-anisidine sulphonate. 
     The method used to dry the treated latex is not critical but in commercial operation the normal spray drying techniques are satisfactory. The specific examples use a laboratory method of drying the rubber as thin films on glass plates. The general technique used throughout the examples, unless otherwise specified, is as follows. 
     The redispersing agent either in aqueous solution or dispersion, typically at about 10% by weight, or as the free material, is added to portions of the synthetic rubber latex and the mixture is thoroughly stirred. The mixture is spread onto glass plates and dried in air at 29°C (ambient temperature) typically for from 48 to 54 hours, producing films about 2.5 mm. thick. The typical moisture content is from 0.8 to 2.0%. A typical average figure is 1.8%. The dried paste is removed from the glass plate with a spatula. The rubber is then redispersed in a minimum amount of water by high speed stirring in a commercial liquidiser for ca. 30 seconds. The redispersibility can be quantatively assessed by determining the quantity retained by sieving through fine muslin. 
     In the Examples, Examples 1 to 7 relate to SBR latex and Examples 8 to 10 to polyisoprene latex. 
     The SBR latex used in the examples is a commercial type sold under the trade name Intex 100. The polyisoprene latex is a commercial cis-1,4-polyisoprene latex sold under the trade name Cariflex IR 700. 
    
    
     EXAMPLE 1 
     Samples of styrene-butadiene latex were treated with (a) 1.5% by weight glycerol as a 10% aqueous solution and (b) 2% by weight commercial brown sugar, both percentages being by weight on the weight of the dry rubber. The reconstituted latices had the following properties: 
     
         Treatment  Dry  Total                                                     
                     pH   Mechanical                                      
                                Brook-                                    
           rubber                                                         
                solids    stability                                       
                                field                                     
           content                                                        
                % wt.     time (sec)                                      
                                viscosity                                 
           (% wt.)              (cps)                                     
__________________________________________________________________________
SBR control                                                               
(Intex 100 or 105)                                                        
           66.14                                                          
                68.72                                                     
                     9.60 2000  765                                       
Re-dispersed SBR                                                          
treated with 1.5%                                                         
wt. glycerol                                                              
           65.10                                                          
                68.82                                                     
                     10.00                                                
                          2000  745                                       
Re-dispersed SBR                                                          
treated with 2%                                                           
wt. commercial                                                            
brown sugar                                                               
           63.44                                                          
                65.59                                                     
                     9.90 2000  420                                       
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     Electron micrographs of these re-constituted latices show no observable differences between the reconstituted and control. 
     The latices were compounded using the following formulations: 
     
                             Parts by weight dry                                   
______________________________________                                    
Latex                 100.0                                               
Zinc oxide            5.0                                                 
Zinc diethyldithiocarbamate                                               
                      1.0                                                 
Zinc mercaptobenzothiazole                                                
                      1.0                                                 
Antioxidant           1.0                                                 
Sulphur               2.5                                                 
Foaming agent (potassium oleate)                                          
                      1.0                                                 
Trimene base          1.0                                                 
Sodium silicofluoride 5.0                                                 
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     Foams made from unmatured latex and no filler was added. Vulcanisation was carried out at 100°C for 30 minutes and drying for 16 hours at 70°C. The examples were tested according to the technique described in the appropriate British Standards Specification. The results are given below. 
     
         __________________________________________________________________________
                        Redispersed                                       
                                 Redispersed                              
                        SBR treated                                       
                                 SBR treated                              
                  Control                                                 
                        with 1.5% wt.                                     
                                 with 2% wt.                              
                  SBR   glycerol commercial                               
                                 brown sugar                              
__________________________________________________________________________
Density, D (gm cm.sup.-.sup.3)                                            
                  0.114 0.105    0.107                                    
Volume shrinkage %                                                        
                  14.2  15.5     12.7                                     
Compression modulus, C (gm cm.sup.-.sup.2)                                
 Single flex      55.9  44.3     49.1                                     
 Initial          55.8  42.9     49.1                                     
 Fatigued         54.8  41.4     47.6                                     
 Aged 22 hr/70°C                                                   
                  67.0  56.6     n.d.                                     
Fatigue loss %    1.8   3.5      2.9                                      
Age hardening %   20.0  32.0     n.d.                                     
C/D.sup.2.5.sup.&#39; 12.6gle flex                                            
                        12.5     13.0                                     
    Initial       12.6  12.1     13.0                                     
    Fatigued      12.4  11.7     12.7                                     
    Aged 22 hr/70°C                                                
                  15.1  16.0     n.d                                      
Compression set                                                           
 Static 72 hr/20°C                                                 
                  2.2   3.4      1.5                                      
 Static 22 hr/70°C                                                 
                  7.4   7.6      5.2                                      
 Dynamic          1.1   1.5      1.1                                      
Tensile strength kg cm.sup.-.sup.2                                        
                  0.70  0.64     0.59                                     
Elongation at break %                                                     
                  205   170      153                                      
__________________________________________________________________________
 n.d. = not determined.                                                   
 
    
     Using the same technique further samples were prepared and the data thus obtained indicate that the addition of as much as 5% by weight glycerol or the commercial grade of brown sugar to SBR latex do not adversely affect the properties of the raw latex. 
     EXAMPLE 2 
     This Example illustrates the effect of adding urea, at various concentrations, on the redispersibility of SBR latex. 
     
         ______________________________________                                    
Concen-                         Coagulum                                  
tration  Appearance on addition of water                                  
                                content                                   
phr      to dry rubber          % wt.                                     
______________________________________                                    
Control      Film was coherent      100                                   
0.1          Film was partially coherent.                                 
                                    54                                    
             Milkiness noted after 20 minutes.                            
             Partially redispersible.                                     
0.3          Film was completely non-coherent;                            
                                    4                                     
             immediate milkiness on addition of                           
             water.                                                       
0.7          Film was completely non-coherent;                            
                                    0                                     
             immediate milkiness on addition of                           
             water.                                                       
1.0          Completely redispersible.                                    
                                    0                                     
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     As in Example 1 a sample of styrene-butadiene latex was treated with 3% by wt. of urea. The reconstituted latex had the following properties: 
     
                         Total             Mechanical                              
Treatment       Solids   pH       stability                               
                % wt.             time (sec)                              
______________________________________                                    
SBR control (Intex 100)                                                   
                68.87    9.82     2000                                    
Redispersed SBR treated                                                   
                67.90    9.55     2000                                    
with 3% urea                                                              
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     The latices were compounded to the formulation described in Example 1 and the properties of the foam rubber are shown below. 
     
         ______________________________________                                    
                         Redis-                                           
                         persed SBR                                       
                  Control                                                 
                         treated with                                     
                  SBR    3% wt. urea.                                     
______________________________________                                    
Density, D (gm. cm.sup.-.sup.3)                                           
                    0.125    0.17                                         
Volume shrinkage %  12.9     14.9                                         
Compression modulus, C (gm. cm.sup.-.sup.2)                               
 Single flex        79.6     64.7                                         
 Initial            80.3     66.2                                         
 Fatigued           77.7     64.3                                         
Fatigue loss %      3.3      2.9                                          
C/D.sup.2.5 Single flex                                                   
                    14.5     13.7                                         
    Initial         14.6     14.1                                         
    Fatigued        14.1     13.6                                         
Compression set                                                           
 Static 72 hr/20°C                                                 
                    2.2      3.0                                          
 Dynamic            1.1      1.1                                          
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     EXAMPLE 3 
     This example illustrates the effect of adding glucose, at various concentrations, on the redispersibility of SBR latex. 
     
         ______________________________________                                    
Concen-                         Coagulum                                  
tration  Appearance on addition of water                                  
                                content                                   
phr      to dry rubber          % wt.                                     
______________________________________                                    
Control      Film was coherent      100                                   
0.2          Films were completely non-                                   
                                    0                                     
0.4          coherent, a very milky 0                                     
             dispersion of latex                                          
0.8          particles.             0                                     
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     Similar results were obtained with the following mono-saccharides: fructose, d-mannose, d-galactose, 1-arabinose and d-arabinose. 
     EXAMPLE 4 
     This Example illustrates the effect of adding sucrose, at various concentrations, on the redispersibility of SBR latex. 
     
         ______________________________________                                    
Concen-                         Coagulum                                  
tration  Appearance on addition of water                                  
                                content                                   
phr      to dry rubber          % wt.                                     
______________________________________                                    
Control      Film was coherent      100                                   
0.2          Film was partially coherent.                                 
                                    10                                    
             Milky dispersion of latex                                    
             particles. Partial re-                                       
             dispersibility of the rubber                                 
             was evident.                                                 
0.4          Film was completely non-coherent.                            
                                    0                                     
             Milky dispersion of latex particles.                         
0.8          Complete redispersibility.                                   
                                    0                                     
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     Similar results were obtained with lactose. 
     EXAMPLE 5 
     This Example illustrates the effect of adding Glycerol, at various concentrations, on the redispersibility of SBR Latex. 
     
         ______________________________________                                    
Concen-                         Coagulum                                  
tration  Appearance on addition of water                                  
                                content                                   
phr      to dry rubber          % wt.                                     
______________________________________                                    
0            Film was coherent      100                                   
0.2          Films were completely non-                                   
                                    0                                     
0.4          coherent. Immediate milkiness                                
                                    0                                     
0.8          on addition of water.  0                                     
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     The effectiveness of glycerol in redispersing SBR is evident from the above results. Glycerol mono-oleate, mono-ricinoleate and mono-stearate were also experimented at concentrations between 3 - 10 phr, but the effects on redispersibility were not similar to that of glycerol. Large flocs of rubber particles were observed and re-dispersion was incomplete even at such high concentrations. Glycerol diacetate was also found to be ineffective even at concentrations of 2 - 10 phr. 
     EXAMPLE 6 
     This Example illustrates the effect of adding Mannitol, at various concentrations, on the redispersibility of SBR Latex. 
     
         ______________________________________                                    
Concen-                         Coagulum                                  
tration  Appearance on addition of water                                  
                                content                                   
phr      on dry rubber          % wt.                                     
______________________________________                                    
0            Film was coherent      100                                   
0.2          Film was mainly non-coherent.                                
                                    2                                     
             Immediate milkiness on addition of                           
             water. Almost complete re-dispersibility.                    
0.4          Films were completely non-coherent.                          
                                    0                                     
             Immediate milkiness on addition of                           
0.8          water. Complete re-dispersibility.                           
                                    0                                     
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     Similar results were obtained with sorbitol, meso-inositol and quebrachitol. 
     EXAMPLE 7 
     This Example illustrates how certain emulsifiers present in commercial SBR Latex can interfere with the redispersion of latices according to the invention. 
     SBR latex (under the trade name of Intex 100 which contains 7.5% potassium oleate) was treated with 1 phr glucose and divided into seven equal portions. To each portion was added 0-5 phr of a commercial rosin-acid soap. The latices were separately dried on glass plates as described above. Water was added to the dry films and coagulum contents were determined. Results are shown in the following table. 
     
         ______________________________________                                    
Addition of potassium                                                     
              Coagulum content                                            
                           Re-dispersibility                              
rosin-acid soap                                                           
              % wt.        %                                              
______________________________________                                    
0             0            100                                            
0.5           0            100                                            
1.0           0            100                                            
2.0           7             93                                            
3.0           24            76                                            
4.0           100           0                                             
5.0           100           0                                             
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     It is clear that the addition of 2 phr of potassium rosin-acid soap to a treated SBR latex interferes with the redispersibility of the rubber. Potassium palmitate interferes, to a lesser extent, with the redispersibility of the rubber as illustrated in the following table. 
     
         ______________________________________                                    
Addition of potassium                                                     
              Coagulum content                                            
                           Redispersibility                               
palmitate (phr)                                                           
              % wt.        %                                              
______________________________________                                    
0             0            100                                            
1             0            100                                            
2             0            100                                            
3             12            88                                            
4             12            88                                            
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     EXAMPLES 8 to 10 
     Using the method described above the effectiveness of various redispersing agents on Cariflex IR 700 latex. 
     The supplier of Cariflex IR 700 has stated that a potassium soap of rosin-acid was used in the production of this latex. Rosin-acid is sometimes described as a &#34;monocarboxylic acid of an alkylated hydrophenanthrene nuclei&#34;. The type used in Cariflex IR 700 is the abietic acid type. Approximately 2 percent of this soap is present in Cariflex IR 700. This figure has been confirmed by analysis. 
     
                                           TABLE                                   
__________________________________________________________________________
Component (a)                                                             
         Component (b)                                                    
                 Appearance on addition                                   
                               Coagulum                                   
                 of water to dry rubber                                   
                               Content wt.%                               
__________________________________________________________________________
Control rubber                                                            
           Nil   Film was coherent                                        
                               100                                        
  Nil                                                                     
Example 8. Effect of saponin                                              
10 phr saponin                                                            
           Nil   Film was completely non-                                 
                                0                                         
                 coherent. Milky disper-                                  
                 sion of latex particles.                                 
 8 phr saponin                                                            
         5 phr Vulca-                                                     
                     &#34;          0                                         
         stab LS.                                                         
 0 phr saponin                                                            
         5 phr Vulca-                                                     
                 Film was coherent.                                       
                               100                                        
         stab.                                                            
Example 9. Effect of mono-saccharide                                      
 2 phr glucose                                                            
         5 phr potass-                                                    
                 Film was completely non-                                 
                                0                                         
         ium oleate                                                       
                 coherent. Milky dispersion                               
                 of latex particles.                                      
  Nil    5 phr potass-                                                    
                 Film coherent 100                                        
         ium oleate                                                       
 2 phr glucose                                                            
           Nil      &#34;          100                                        
Example 10. Effect of polyhydric Alcohol                                  
 2 phr glycerol                                                           
         5 phr potass-                                                    
                 Film was completely non-                                 
                                0                                         
         ium oleate                                                       
                 coherent. Milky disper-                                  
                 sion of latex particles.                                 
 2 phr glycerol                                                           
           Nil   Film coherent.                                           
                               100                                        
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     What I claim is: