Vulcanization acceleration based on melamine derivatives, rubber compositions containing melamine derivatives, and a process for the vulcanization of rubber
This invention relates to vulcanization accelerators based on melamine derivatives, optionally in combination with thiazole accelerators and other conventional accelerators, rubber compositions containing melamine derivatives, and a process for the vulcanization of rubbers, using melamine derivatives, optionally in combination with thiazole accelerators and other conventional accelerators, as vulcanization accelerators. 
     It is known in the art to use basic accelerators, alone or preferably in combination with thiazole accelerators such as 2-mercaptobenzothiazole (MBT), 2-mercapto-benzothiazyl disulphide (MBTS) or 2-benzothiazyl-cyclohexyl-sulphenamide (CBS), for the sulphur vulcanization of diene rubbers. The following are examples of basic accelerators used in practice: diphenyl-guanidine (DPG), di-o-tolyl guanidine (DOTG) and hexamethylene-tetramine (HEXA) (see &#34;Ullmann&#39;s Enzyklopadie der Technischen Chemie&#34;, 4th Edition, Volume 13, page 641 et seq.) Less effective than the above mentioned products are simple amines such as dibutylamine. 
     DPG, DOTG and HEXA have the following disadvantages. For the industrial production of DPG and DOTG, it is necessary to use an extremely toxic material, hydrocyanic acid, as a starting material as well as the highly toxic substance, cyanogen chloride, as an intermediate stage (see &#34;Ullmann&#39;s Enz. der Techn. Chemie&#34;, 4th Edition, Volume 12, page 416 et seq.) This gives rise to considerable problems and necessitates elaborate safety precautions. DOTG is relatively toxic (see J. Ind. Hyg. a. Tox. 13, 87, 1931) while HEXA has a highly irritant effect on the skin (see &#34;J. Am. Med. Ass.&#34; 128, 1209, 1945). DPG, DOTG and HEXA cause discolourations in pale or transparent mixtures. 
     It has now been found that melamine derivatives corresponding to the general formula (I): ##STR1## wherein R denotes hydrogen or the group ##STR2##  and mixtures are suitable as vulcanization accelerators for the sulphur vulcanization of diene rubbers. 
     They are free from the above mentioned disadvantages. Neither hydrocyanic acid nor cyanogen chloride are used for their preparation. The compounds have little toxicity and no irritant effect on the skin and can easily be prepared from melamine, formaldehyde and corresponding amines. They do not discolour the vulcanizates. 
     This invention thus provides vulcanization accelerators based on melamine derivatives corresponding to the general formula (I): ##STR3## wherein each R independently denotes hydrogen or the group ##STR4## and R 2  and R 3 , which may be identical or different, denote hydrogen, a straight chain or branched chain C 1  -C 4  alkyl or alkenyl group or a C 3  -C 7 , preferably a C 5  -C 6  cycloalkyl group, and R 2  and R 3  may also be linked through a C 2  -C 12 . preferably a C 2  -C 6  alkylene bridge which may contain a hetero atom such as oxygen, sulphur or N-C 1  -C 4 , preferably C 1  alkyl, 
     optionally in combination with conventional vulcanization accelerators taken from the group of thiazole accelerators and optionally other conventional accelerators. Where R denotes hydrogen or ##STR5## the compounds may belong to the following classes of compounds: ##STR6## 
     R preferably denotes the group ##STR7## R 2  and R 3  preferably have the following meaning: methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, tertiary butyl, propenyl and butenyl, a tetramethylene, pentamethylene, hexamethylene, oxadiethylene, thiadiethylene and N-alkylazadiethylene bridge in which the N-alkyl group may be methyl, ethyl, propyl or n-butyl. 
     The following compounds are mentioned as examples: 
     tris-(dimethylamino-methylene)-melamine 
     tris-(diethylamino-methylene)-melamine 
     tris-(di-n-propylamino-methylene)-melamine 
     tris-(di-iso-propylamino-methylene)-melamine 
     tris-(di-n-butylamino-methylene)-melamine 
     tris-(di-iso-butylamino-methylene)-melamine 
     tris-(di-sec-butylamino-methylene)-melamine 
     tris-(di-tert-butylamino-methylene)-melamine 
     tris-(methyl-cyclohexylamino-methylene)-melamine 
     tris-(ethyl-cyclohexylamino-methylene)-melamine 
     tris-(n-propyl-cyclohexylamino-methylene)-melamine 
     tris-(methyl-allylamino-methylene)-melamine 
     tris-(ethyl-allylamino-methylene)-melamine 
     tris-(di-allylamino-methylene)-melamine 
     tris-(n-butylamino-methylene)-melamine 
     tris-(tert-butylamino-methylene)-melamine 
     tris-(di-n-butyleneamino-methylene)-melamine 
     tris-(di-cyclohexylamino-methylene)-melamine 
     tris-(di-cyclopentylamino-methylene)-melamine 
     tris-(di-cycloheptylamino-methylene)-melamine 
     tris-(methyl-isopropylamino-methylene)-melamine 
     tris-(ethyl-isopropylamino-methylene)-melamine 
     tris-(methyl-tert-butylamino-methylene)-melamine 
     tris-(ethyl-tert-butylamino-methylene)-melamine 
     tris-(methyl-cyclohexanylamino-methylene)-melamine 
     tris-(cyclohexanylamino-methylene)-melamine 
     tris-(pyrrolidino-methylene)-melamine 
     tris-(piperidino-methylene)-melamine 
     tris-(hexamethyleneimino-methylene)-melamine 
     tris-(oxadiethyleneimino-methylene)-melamine 
     tris-(thiadiethyleneimino-methylene)-melamine 
     tris-(N-methyldiethyleneimino-methylene)-melamine 
     tris-(cyclohexylamino-methylene)-melamine 
     di-(methyl-cyclohexylamino-methylene)-melamine 
     di-(cyclohexylamino-methylene)-melamine 
     di-(tert.-butylamino-methylene)-melamine 
     dicyclohexylamino-methylene-melamine 
     di-(dimethylamino-methylene)-melamine 
     Mixtures of melamine derivatives may, of course, also be used. 
     The following are preferred: tris-(diethylamino-methylene)-melamine, tris-(dimethylamino-methylene)-melamine (hereinafter referred to as MFD), and tris-(pyrrolidyl-methylene)-melamine. 
     MFD, which is the substance preferably used, is relatively non-toxic, its LD 50  value (rats, per os) is 1,900 mg/kg and it has no irritating effect on the skin, mucous membranes or eyes. It does not discolour the vulcanizates. 
     The compounds corresponding to the general formula (I) may be used alone but they are preferably used in combination with conventional vulcanization accelerators. 
     The compounds mentioned above may be prepared by a method analogous to that of U.S. Pat. No. 2,210,831, by mixing melamine, formaldehyde and amine in water, optionally with the introduction of gaseous components. The reaction temperature is generally from 0° to 100° C. 
     The accelerators are advantageously used in the following quantities (phr, parts by weight based on 100.0 parts by weight of rubber): 
     (a) melamine derivatives corresponding to the general formula (I): approximately 0.05-5, preferably 0.1-2.0 phr; 
     (b) thiazole accelerators: approximately 0-5, preferably 0.3-2.0 phr; 
     (c) other conventional accelerators: approximately 0-2, preferably 0.01-1.0 phr. 
     The following are examples of thiazole accelerators (b) (see list of abbreviations in Table I): MBT, MBTS, ZMBT, CBS, TBBS, MBS and DCBS. 
     The following are examples of conventional accelerators 
     (c): TMTD, TMTM, ZDEC and OTOS. 
     The quantity of sulphur used for vulcanization is generally from 0.2 to 4 parts by weight per 100 parts by weight of rubber. 
     Sulphur donors such as N,N&#39;-dithio-bis-morpholine, dipentamethylene-thiuramic tetrasulphide, N,N&#39;-dithio-bis-hexahydro-2H-azepinone-(2) and 2-benzothiazyl-dithio-N-morpholide may be used in the usual quantities. 
     The accelerators and accelerator combinations according to the invention are suitable for the vulcanization of diene rubbers. The invention therefore also relates to diene rubber compositions containing the accelerator (combination) according to the invention in addition to the usual components of the mixture and to a process for the vulcanization of diene rubbers using the accelerators or accelerator combinations according to the invention. 
     The following diene rubbers are suitable: styrene-butadiene rubber (SBR), natural rubber (NR), nitrile rubber (NBR), polybutadiene (BR), polyisoprene (IR) and polychloroprene (CR). 
     Mixtures of various diene rubbers may also be used. 
     The indiviaul components of the accelerator system may be added to the diene rubbers or rubber mixture before vulcanization, either separately or in the form of a mixture or a preliminary mixture of rubber and accelerator (see Ullmann&#39;s Enzykl. d. Techn. Chemie, 3rd Edition, published by Urban &amp; Schwarzenberg, Munich-Berlin 1957, Volume 9, page 364). 
     The diene rubbers may contain all the usual additives such as fillers, in particular carbon blacks, mineral oils, plasticizers, substances which render the rubbers adhesive, accelerator activators, in particular stearic acid, waxes, age protective agents, ozone protective agents, blowing agents, dyes and pigments. 
     Fillers such as the types of carbon black used in the rubber processing industry, silica, particularly finely divided silica obtained in the gaseous phase and hydrophobic silica, and finely divided metal oxides, including mixed oxides and oxide mixtures, are valuable components for a mixture. 
     Vulcanization of the diene rubbers is general carried out at temperatures within the range of about 100° C. to about 300° C., perferably at 140°-240° C. Any of the usual industrial vulcanization processes may be employed, such as heating in a press or heating with super-heated steam, hot air, a salt bath, a fluidized bed, ultra-high frequency and a steam pipe. 
     The data given in the Examples which follow were obtained as follows: 
     Rubber mixtures were prepared in an internal mixer in the usual manner from the test formulations indicated. Sulphur and vulcanization accelerator were mixed again on rollers. Test flaps 4 mm in thickness were produced from the mixtures by pressure vulcanization (step-wise heating at the temperatures indicated in the Examples). The individual data were obtained by the test methods indicated in Table 1. The abbreviations used are also summarized in Table 1. 
    
    
     EXAMPLE 1 
     (phr=parts by weight based on 100 parts by weight of rubber) 
     
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                     phr                                                  
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Natural rubber (smoked sheets)                                            
                     100.0                                                
Carbon black N 330 (HAF)                                                  
                     45.0                                                 
Zinc oxide           5.0                                                  
Stearic acid         3.0                                                  
aromatic mineral oil 2.5                                                  
naphthenic mineral oil                                                    
                     2.5                                                  
TMQ                  1.0                                                  
IPPD                 1.5                                                  
sulphur              2.35                                                 
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Vulcameter      HEXA 1.0 phr  MFD 1.0 phr                                 
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prevulcanization time                                                     
t.sub.s (15) 120° C.                                               
                above 30      above 30                                    
t.sub.s (15) 130° C.                                               
                above 30      21.3                                        
Flow time t.sub.s (15) 150° C.                                     
                5.9           4.9                                         
final vulcanization time                                                  
t.sub.90 /150° C.                                                  
                24.5          22.7                                        
reaction time                                                             
t.sub.90 -t.sub. s /150° C.                                        
                18.6          17.8                                        
final vulcanization time                                                  
t.sub.90 /180° C.                                                  
                3.4           3.4                                         
Stepwise heating                                                          
M 300 150° C./10 min.                                              
                3.3           4.1                                         
20 min.         4.9           6.5                                         
30 min.         6.2           7.7                                         
45 min.         7.4           9.0                                         
F 150° C./30 min.                                                  
                22.2          22.9                                        
D 150° C./30 min.                                                  
                605           605                                         
H 150° C./30 min.                                                  
                54            59                                          
E 150° C./30 min.                                                  
                49            51                                          
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     The accelerator MFD according to the invention is found to have the following advantages over HEXA which is used for comparison: a shorter final vulcanization time t 90 , a shorter reaction time t 90  -t s , higher tension values M 300  and a higher hardness H. The elasticity E is also slightly higher. 
     EXAMPLE 2 
     (formulation as indicated in Example 1) 
     
         ______________________________________                                    
             HEXA 0.5 phr                                                 
                         MFD 0.5 phr                                      
             MBTS 0.5 phr                                                 
                         MBTS 0.5 phr                                     
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Vulkameter                                                                
prevulcanization time                                                     
t.sub.s (15) 120° C.                                               
               17.5          13.5                                         
t.sub.s (15) 130° C.                                               
               9.2           7.1                                          
flow time t.sub.s (15)/150° C.                                     
               3.3           2.8                                          
final vulcanization time                                                  
t.sub.90 /150° C.                                                  
               13.5          10.2                                         
reaction time                                                             
t.sub.90 -t.sub. s /150° C.                                        
               10.2          7.4                                          
final vulcanization time                                                  
t.sub.90 /180° C.                                                  
               2.7           2.4                                          
Stepwise heating                                                          
M 300 150° C./10 min.                                              
               8.9           10.1                                         
20 min.        9.6           10.4                                         
30 min.        9.1           9.7                                          
45 min.        8.2           8.6                                          
F 150° C./30 min.                                                  
               24.5          23.7                                         
D 150° C./30 min.                                                  
               590           590                                          
H 150° C./30 min.                                                  
               60            64                                           
E 150° C./30 min.                                                  
               53            54                                           
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     The accelerator MFD according to the invention used in combination with MBTS is found to have the following advantages over the comparison combination, HEXA/MBTS: a shorter prevulcanization time t 90 , a shorter reaction time t 90  -t s , higher tension values M 300  and a higher hardness H. 
     EXAMPLE 3 
     (formulation as indicated in Example 1) 
     
         ______________________________________                                    
             HEXA 0.5 phr                                                 
                         MFD 0.5 phr                                      
             MBT 0.5 phr MBT 0.5 phr                                      
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Vulkameter                                                                
prevulcanization time                                                     
t.sub.s (15) 120° C.                                               
               12.8          9.7                                          
t.sub.s (15) 130° C.                                               
               6.8           5.6                                          
flow time t.sub.s (15)/150° C.                                     
               2.4           2.2                                          
final vulcanization time                                                  
t.sub.90 /150° C.                                                  
               11.8          9.1                                          
reaction time t.sub.90 -t.sub. s /150° C.                          
               9.4           6.9                                          
final vulcanization time                                                  
t.sub.90 /180° C.                                                  
               2.5           2.3                                          
Stepwise heating                                                          
M 300 150° C./10 min.                                              
               8.6           10.6                                         
20 min.        9.1           10.1                                         
30 min.        9.1           9.9                                          
45 min.        8.6           9.4                                          
F 150° C./30 min.                                                  
               24.0          23.5                                         
D 150° C./30 min.                                                  
               575           535                                          
H 150° C./30 min.                                                  
               62            63                                           
E 150° C./30 min.                                                  
               52            54                                           
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     The accelerator MFD according to the invention used in combination with MBT shows the advantages over the comparison combination of HEXA/MBT already mentioned in Examples 1 and 2. 
     EXAMPLE 4 
     This demonstrates the influence of the accelerator MFD according to the invention on the colour of a transparent vulcanizate which has been prepared from the following components: 
     
         ______________________________________                                    
styrene-butadiene rubber (SBR 1507)                                       
                         100.0  phr                                       
silica                   50.0   phr                                       
dicyclohexylamine (filler activator)                                      
                         1.0    phr                                       
naphthenic mineral oil   5.0    phr                                       
stearic acid             2.0    phr                                       
phenolic age resistor.sup.1                                               
                         1.0    phr                                       
zinc oxide               1.8    phr                                       
sulphur                  1.8    phr                                       
ZMBT                     1.2    phr                                       
TMTD                     0.8    phr                                       
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                   1.0 phr    1.0 phr                                     
                   HEXA       MFD                                         
______________________________________                                    
colour of the vulcanizate                                                 
after vulcanization at                                                    
150° C./30 min.                                                    
                   brown      light                                       
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 .sup.1 Vulkanox.sup.(Trade Mark) DS, Bayer AG was used.                  
 
    
     It should be noted that DPG and DOTG are known from experience to discolour even more strongly than HEXA. 
     EXAMPLE 5 
     This shows results in a mixture based on styrene-butadiene rubber/polybutadiene having the following composition: 
     
         ______________________________________                                    
SBR 1712                68.5   phr                                        
polybutadiene.sup.1     68.5   phr                                        
carbon black N 220      95.0   phr                                        
aromatic mineral oil    20.0   phr                                        
resin to impart adhesiveness.sup.2                                        
                        2.0    phr                                        
stearic acid            2.0    phr                                        
ozone protective wax    1.5    phr                                        
IPPD                    1.5    phr                                        
TMQ                     1.0    phr                                        
Zinc oxide              3.0    phr                                        
sulphur                 1.8    phr                                        
MBTS                    1.0    phr                                        
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                HEXA 0.50 phr MFD 0.5 phr                                 
______________________________________                                    
prevulcanization time                                                     
t.sub.s (15)/130° C.                                               
                12.5          10.0                                        
flow time t.sub.s (15)/160° C.                                     
                1.8           1.8                                         
final vulcanization time                                                  
t.sub.90 /160° C.                                                  
                9.1           6.6                                         
reaction time                                                             
t.sub.90 -t.sub. s /160° C.                                        
                7.3           4.8                                         
tension M 300 160° C./                                             
10 min.         4.6           5.7                                         
20 min.         5.0           5.4                                         
30 min.         4.8           5.3                                         
45 min.         4.6           5.0                                         
tension M 300 180° C./                                             
5 min.          4.5           5.1                                         
10 min.         4.1           5.7                                         
15 min.         3.9           4.5                                         
F 160° C./20 min.                                                  
                13.7          12.1                                        
D 160° C./20 min.                                                  
                600           525                                         
H 160° C./20 min.                                                  
                62            62                                          
H 160° C./20 min.                                                  
                25            25                                          
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  The substances used were                                                
 .sup.1 Buna.sup.(Trade Mark) CB 30,                                      
 .sup.2 Koresin                                                           
 
    
     The accelerator MFD according to the invention affords the following advantages compared with HEXA which is used as comparison: a shorter final vulcanization time t 90 , a shorter reaction time t 90  -t s  and higher tension values M 300. 
     
                       TABLE 1                                                     
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Test methods                                                              
Vulkameter  Bayer Vulkameter, heating for 1 minute,                       
            measuring range 3/20 mV, test                                 
            duration at 160° C./30 min, rate of feed               
            600 mm/h                                                      
t.sub.s (15)                                                              
            prevulcanization time from Vulkameter                         
            measurement at 120° C. or 130° C., time         
            taken for the modulus of shear curve                          
            to rise above the minimum by 15 or                            
            30 mm.                                                        
t.sub.90 (min)                                                            
            complete heating time, time until                             
            90% of the maximum modulus of shear                           
            is reached (Bayer Vulkameter)                                 
t.sub.90 -t.sub. s (min)                                                  
            reaction time, difference between                             
            complete heating time t.sub.90 and pre-                       
            vulcanization time t.sub.s measured at                        
            the same temperature (160° C.)                         
Stepwise heating                                                          
            4 mm flaps, heating in press.                                 
M.sub.300 (Mpa)                                                           
            tension at 300% and 500% elongation,                          
            DIN 53504                                                     
F (MPa)     tear resistance, DIN 53504, standard                          
            ring R I                                                      
D (%)       elongation at break, DIN 53504,                               
            standard ring R I                                             
H (Shore A) hardness Shore A, DIN 53504, standard                         
            Ring R I                                                      
E (%)       recoil elasticity, DIN 53512, 4 mm                            
            flaps                                                         
Abbreviations                                                             
MBT      =         2-mercaptobenzothiazole                                
MBTS     =         dibenzothiazyl disulphide                              
HEXA     =         hexamethylene tetramine                                
MFD      =         tris-(dimethylamino-methylene)-                        
                   melamine                                               
DPG      =         diphenyl guanidine                                     
DOTG     =         di-ortho-tolyl guanidine                               
ZMBT     =         zinc salt of 2-mercapto benzo-                         
                   thiazole                                               
CBS      =         2-benzothiazyl-cyclohexyl                              
                   sulphenamide                                           
TBBS     =         2-benzothiazyl-tertiary butyl-                         
                   sulphenamide                                           
MBS      =         2-benzothiazyl-sulphene                                
                   morpholide                                             
DCBS     =         2-benzothiazyl-dicyclohexyl                            
                   sulphenamide                                           
TMTD     =         tetramethylthiuramic disulphide                        
TMTM     =         tetramethylthiuramic mono-                             
                   sulphide                                               
ZDEC     =         zinc-N--diethyl-dithiocarbamate                        
TMQ      =         polymerized 2,2,4-trimethyl-                           
                   1,2-dihydroquinoline                                   
IPPD     =         N--phenyl-N&#39;--isopropyl-p-                             
                   phenylene diamine                                      
OTOS     =         N--oxydiethylene-dithiocarbamyl-                       
                   N&#39;--oxydiethylene sulphenamide                         
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