Process for the cross-linking of halogenated polymers
This invention relates to a process for the cross-linking of halogenated polymers such as chlorinated polyethylene by means of bis-(2,5-dimercapto-1,3,4-thiadiazole), hereinafter referred to as BDTD, or bis-(2,5-dimercapto-1,3,4-thiadiazole) derivatives such as the monobenzoyl derivative and basic substances, in particular certain amidamines. 
     It is known from German Offenlegungsschriften Nos. 2 754 060 and 2 845 125 to cross-link saturated, halogenated polymers by heating them in the presence of 2,5-dimercapto-1,3,4-thiadiazole and a basic material. The basic material used is in particular a combination of an organic base and an inorganic base, the organic base used being an amine having a boiling point above circa 110° C. and a pK-value below circa 4.5. Dicyclohexylamine and a reaction product of aniline and butyraldehyde are mentioned as preferred amines for this purpose. 
     These amines are difficult to handle as they are powerfully corrosive organic liquids and are difficult to incorporate in rubber mixtures. 
     Further, when these amines are used for the cross-linking of chlorinated polyethylene, the vulcanizate properties are found to be unsatisfactory and the moulds used for the production of the articles are found to be badly soiled and the articles are difficult to remove from the mould. 
     The present invention relates to a process for the cross-linking of halogenated polymers such as chlorinated polyethylene with bis-(2,5-dimercapto-1,3,4-thiadiazole) or derivatives thereof in the presence of a basic material. The basic material is preferably an amidamine corresponding to the following formula 
     
         R--CO--NH--[CH.sub.2 --CH.sub.2 --NH].sub.n --CO--R.sub.1  (I) 
    
     wherein 
     R stands for saturated or unsaturated hydrocarbon group having 6 to 22 carbon atoms, 
     R 1  stands for alkyl, aryl, cycloalkyl, aralkyl or R--COCH 2  -CH 2  and 
     n stands for an integer from 1 to 10. 
     The alkyl group may be straight chained or branched and may in particular have 1 to 18 carbon atoms. The aryl group preferably has 6 to 12 carbon atoms and may carry substituents such as halogen, C 1  -C 4  -alkyl, nitro, cyano or C 1  -C 4  -alkoxy; the cycloalkyl group preferably has 5 to 8 carbon atoms; the aralkyl in particular has 7 to 12 carbon atoms and its aryl moiety may carry the same substituents as the above-mentioned aryl group. 
     Preferred compounds of formula I correspond to formula II 
     
         R.sub.2 --CO--NH--[CH.sub.2 --CH.sub.2 --NH].sub.m --CO--R.sub.3(II) 
    
     wherein 
     R 2  and R 3  may be identical or different and stand for straight chained, saturated alkyl with 8 to 18 carbon atoms and 
     m stands for an integer from 2 to 5. 
     The compounds of formulae I and II are generally solid, non-corrosive substances which can easily be incorporated in rubber mixtures. 
     The amidamine is preferably used in a quantity of from 1 to 5% by weight, in particular from 2 to 4.5% by weight, based on the rubber. BDTD is preferably used in a quantity of from 1 to 4% by weight, in particular from 2 to 3% by weight, based on the rubber. 
     The invention further relates to the use of bis-(2,5-dithio-1,3,4-thiadiazole) as cross-linking agent for halogenated polymers. 
     For information on the properties and fields of application of chlorinated polyethylene, known as CM according to ASTM-D 1418, see E.Rohde, Kautschuk + Gummi, Kunststoffe 35, 1982, pages 478 et seq). 
     The halogenated polymer used may be CM, i.e. a chlorinated polyethylene with a chlorine content of about 25 to 45% by weight prepared by suspension chlorination of low pressure polyethylene. 
     Other halogenated elastomers, such as CSM, CO/ECO, CR and CIIR, may be halogenated with BDTD in the same manner. 
    
    
     EXAMPLE 1 
     Demonstrates the influence of increasing quantities of amidamine on the activation of cross-linking of a CM mixture with BDTD. No true cross-linking takes place without the addition of amidamine. An optimum cross-linking characterstic is in most cases obtained with 3 to 4.5 phr of amidamine. Sufficiently long initial vulcanization times (T 10) and a short final vulcanization time (T 90) are obtained. 
     
         ______________________________________                                    
              A      D       E       F                                    
______________________________________                                    
CM              100                                                       
MgO             10                                                        
Carbon black    50                                                        
Kaolin          60                                                        
Alkyl sulphonic acid ester                                                
                20                                                        
Aromatic mineral oil                                                      
                20                                                        
BDTD            2.5                                                       
Amidamine       --       1.5     3.0   4.5                                
Elastograph at 180° C.                                             
F min (Nm)      0.051    0.046   0.032 0.031                              
F max (Nm)      0.246    0.334   0.491 0.568                              
T 10 (min)      0.70     1.00    1.40  1.40                               
T 50 (min)      2.70     6.30    4.90  3.30                               
T 90 (min)      16.70    18.20   13.60 9.20                               
______________________________________                                    
 
    
     EXAMPLE 2 
     Demonstrates a comparison between the above-mentioned activation by powerfully corrosive liquids such as an aniline-condensation product and the cyclohexylamine, the optimum dose of which is about 1.5 phr. By comparison, the amidamine which has a substantially higher molecular weight must also be added in higher doses (see Example 1). 
     A more reliable initial vulcanization (T 10) with more rapid final vulcanization (T 90) is obtained with amidamine in spite of the considerably higher dosage. In addition, the viscosity of the mixture is lowered by the amidamine, whereby extrusion is facilitated and more advantageous properties are obtained for IM processing. The vulcanizate values of the mixture with amidamine are distinguished by their exceptionally good elongations at break. This advantage is also found after testing of the ageing in hot air. 
     
         ______________________________________                                    
               B       C       F                                          
______________________________________                                    
CM               100                                                      
MgO              10                                                       
Carbon black N-550                                                        
                 50                                                       
Kaolin           60                                                       
Alkyl sulphonic acid ester                                                
                 20                                                       
Aromatic mineral oil                                                      
                 20                                                       
BDTD             2.5                                                      
Aniline condensation product                                              
                 1.5       --      --                                     
Cyclohexylamine  --        1.5     --                                     
Amidamine        --        --      4.5                                    
Elastograph at 180° C.                                             
F min (Nm)       0.047     0.049   0.031                                  
F max (Nm)       0.483     0.581   0.568                                  
T 10 (min)       0.80      0.90    1.40                                   
T 50 (min)       3.30      2.70    3.30                                   
T 90 (min)       15.70     12.60   9.20                                   
Viscosity of mixture:                                                     
ML 1 + 4/100° C.                                                   
                 36        36      30                                     
Vulcanization 30 min/170° C.:                                      
Hardness (ShA)   82        82      83                                     
Elasticity (%)   13        13      13                                     
σ100 (MPa) 6.7       8.0     5.3                                    
σ300 (MPa) --        --      8.0                                    
Tensile strength (MPa)                                                    
                 7.2       8.1     8.1                                    
Elongation at break (%)                                                   
                 190       190     305                                    
Hot air ageing 7d/125° C.:                                         
                 B         C       F                                      
Hardness (ShA)   86        86      88                                     
Elasticity (%)   13        13      13                                     
Tensile strength (MPa)                                                    
                 10.2      11.4    9.8                                    
Elongation at break (%)                                                   
                 95        85      160                                    
______________________________________                                    
 
    
     EXAMPLE 3 
     Demonstrates the cross-linking of chlorinated polyethylene (CM) with increasing quantities of BDTD. In the present mixture, the optimum dose is about 2.0 parts by weight. For other formulations, doses of 0.5 to 4.5 parts by weight may be used and are customary. 
     
         ______________________________________                                    
(Example 1)      A       B       C     D                                  
______________________________________                                    
CM               100     100     100   100                                
MgO              10      10      10    10                                 
Carbon black N-550                                                        
                 70      70      70    70                                 
HA mineral oil   30      30      30    30                                 
Ester plasticizer                                                         
                 10      10      10    10                                 
Amidamine activator                                                       
                 3.5     3.5     3.5   3.5                                
BDTD             1.0     1.5     2.0   2.5                                
Vulcanization 20 min/170° C.                                       
Hardness (ShA)   77      80      82    82                                 
Elasticity (%)   16      16      15    15                                 
σ100 (MPa) 5.1     6.2     6.7   7.1                                
σ300 (MPa) 12.1    13,2    13,6  13.7                               
Tensile strength (MPa)                                                    
                 14.9    14.7    14.3  13.9                               
Elongation at break (%)                                                   
                 430     390     340   320                                
______________________________________                                    
 
    
     EXAMPLE 4 
     Demonstrates the cross-linking of epichlorohydrin rubber with BDTD by comparison with lead oxide and ethylene thiourea. With the doses used here, a somewhat more favourable initial vulcanization time combined with a substantially higher degree of cross-linking (see Hardness, o, Elongation at break) is achieved with BDTD. 
     
         ______________________________________                                    
(Example 2)             E       F                                         
______________________________________                                    
Epichlorohydrin elastomer                                                 
                        60      60                                        
Epichlorohydrin/ethylene oxide copolymer                                  
                        40      40                                        
2-Mercapto-benzimidazole                                                  
                        1       1                                         
Nickel dibutyl dithiocarbamate (80%)                                      
                        1.5     1.5                                       
Carbon black N-330      15      15                                        
Carbon black N-550      25      25                                        
Barium carbonate (80%)  4       --                                        
Urea activator          2       --                                        
Ethylene thiourea (70%) --      1.5                                       
Red lead (70%)          --      6                                         
BDTD                    2.5     --                                        
Initial vulcanization at 120° C.                                   
t.sub.5 (min)           5.4     4.4                                       
T.sub.35 (min)          7.5     6.3                                       
Vulcanization 20 min/180° C.                                       
Hardness (ShA)          71      67                                        
Elasticity (%)          25      23                                        
σ100 (MPa)        6.1     3.5                                       
Tensile strength (MPa)  10.2    13.9                                      
Elongation at break (%) 155     335                                       
Compression set         24      22                                        
______________________________________                                    
 
    
     EXAMPLE 5 
     Demonstrates a comparison of the cross-linking of CM with peroxide and with BDTD. The higher tear propagation resistance and improved elongation at break obtained when cross-linking is carried out with BDTD are particularly noteworthy. 
     
         ______________________________________                                    
(Example 3)           G       H                                           
______________________________________                                    
CM (chlorinate polyethylene)                                              
                      100     100                                         
Magnesium oxide       10      10                                          
Carbon black N-330    60      60                                          
Ester plasticizer     30      30                                          
Age resistor          0.3     0.3                                         
Amide-amine activator 5.5     --                                          
BDTD                  2.5     --                                          
Bis-(t-butyl-peroxy-isopropyl)benzene                                     
                      --      7                                           
Triallyl cyanurate    --      2.5                                         
Vulcanization 15 min/170° C.                                       
Hardness (ShA)        74      71                                          
Elasticity (%)        23      24                                          
Tear propagation                                                          
resistance (N/mm)                                                         
σ100 (MPa)      5.1     4.8                                         
σ300 (MPa)      15.2    17.3                                        
Tensile strength (MPa)                                                    
                      16.8    17.9                                        
Elongation at break (%)                                                   
                      365     320                                         
Compression set (%)                                                       
24 h/125° C.   12      11                                          
24 h/150° C.   29      32                                          
______________________________________                                    
 
    
     EXAMPLE 4 
     Demonstrates that BDTD is very suitable for cross-linking chlorine-containing elastomers. The use of DMTD and BMTD for such vulcanization is already known, as mentioned above. An intermediate position between these two cross-linking agents is obtained with BDTD. Thus the initial vulcanization time is longer with BDTD than with BMTD but shorter than with DMTD. 
     
         ______________________________________                                    
 ##STR1##                                                                 
 ##STR2##                                                                 
 ##STR3##                                                                 
(Example 4)        I       K         L                                    
______________________________________                                    
CM                 100                                                    
MgO                10                                                     
Carbon black N-550 70                                                     
Aromatic mineral oil                                                      
                   30                                                     
Alkyl sulphonic acid ester                                                
                   10                                                     
Amidamine          3.5                                                    
DMTD               2.0     --        --                                   
BMTD               --      2.0       --                                   
BDTD               --      --        2.0                                  
Vulcanization at 180° C.                                           
ti (min)           5.3     2.6       3.8                                  
F 10 (dNm)         30.0    31.5      28.3                                 
F 24 (dNm)         49.0    37.0      43.5                                 
Vulcanization 20 min/170° C.                                       
Hardness (ShA)     77      79        82                                   
Elasticity (%)     16      14        15                                   
σ100 (MPa)   8.1     5.4       6.7                                  
σ300 (MPa)   14.0    12.7      13.6                                 
Tensile strength (MPa)                                                    
                   14.9    14.5      14.3                                 
Elongation at break (%)                                                   
                   305     400       340                                  
Compression set                                                           
24 h/100° C. (%)                                                   
                   10      9         11                                   
24 h/125° C. (%)                                                   
                   16      15        13                                   
______________________________________