Abstract:
In order to make available a crosslinking agent for ethylene acrylate (AEM) and polyacrylate (ACM) elastomers, which is equivalent to the crosslinking agents containing guanidine that have been used until now, but which does without the teratogenically acting guanidines which are toxologically hazardous, it is proposed that the crosslinking agent for diaminically crosslinkable AEM and ACM elastomers comprise a diamine crosslinker, an accelerator selected from 1,8-diazabicyclo-5,4,0-undec-7-ene (DBU), derivatives and salts thereof, and a moderator of the pyrrolidone type.

Description:
CROSSLINKING AGENT 
     The present disclosure relates to the subject matter disclosed in German application number 10 2008 020 196.0 of Apr. 15, 2008, which is incorporated herein by reference in its entirety and for all purposes. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a crosslinking agent for diaminically crosslinkable ethylene acrylate and polyacrylate elastomers. Ethylene acrylate elastomers are referred to hereinbelow as AEM elastomers and polyacrylate elastomers as ACM elastomers. 
     AEM and ACM elastomers are relatively inexpensive elastomeric materials which are usable in a large number of applications, in particular, also in the area of seals. The two elastomers may also be used in mixtures with one another as an elastomeric material. 
     Those AEM and ACM elastomers which are accessible to diaminic crosslinking comprise so-called cure site monomer units whose content in the elastomer is normally approximately 1 to approximately 5 phr. 
     The possibilities for use of these elastomeric materials in the area of seals range from static seals such as, for example, cylinder head, valve cover or oil sump gaskets, seals for manual transmissions and automatic transmissions, O-rings, seals in the area of intake manifolds and exhaust gas recirculations to dynamically stressed sealing elements such as, for example, piston sealing rings, in particular, also so-called bonded piston seals. 
     AEM elastomers are obtainable, for example, from DuPont under the trade name VAMAC®. ACM elastomers are sold by Zeon Chemicals, for example, under the trade names Nipol® and HyTemp®. 
     In addition, Tohpe Corporation, Unimatec and Nippon Mectron offer diaminically crosslinkable elastomers of the ACM type. 
     Conventionally, diaminically crosslinkable AEM and ACM elastomers are crosslinked with crosslinking agents such as, for example, hexamethylene diamine carbamate, N,N-dicinnamylide diamine carbamate, 4,4-diaminodicyclohexylmethane, m-xylene diamine, 4,4-diaminodiphenylmethane, 4,4-diaminodiphenyl ether and 2,2-bis[4-(4-amino-phenoxy)phenyl]propane, which can be used individually or in mixtures. Other suitable diamine crosslinking agents belong to the group of hexamethylene diamines. 
     The aforementioned crosslinking systems are used together with an accelerator, which is preferably taken from substances of the guanidine group, to which, in particular, the accelerators OTBG, DOTG, DPG and/or biguanidine belong. 
     For some time, these accelerators have been classified as toxicologically hazardous. They also act teratogenically upon contact with the skin. 
     In view of the European Union REACH Regulation, availability of such accelerators in the future appears questionable. 
     Tertiary amines such as, for example, 1,8-diazabicyclo-5,4,0-undec-7-ene (DBU), have been suggested as alternatives to these accelerators, but they do not constitute an equivalent alternative to the guanidine accelerators since they result in inferior elastic properties of the crosslinked elastomer, with the same crosslinker component content, and, in particular, the compression set at elevated temperatures suffers considerably. 
     Furthermore, the storage stability of these accelerators in comparison with the guanidine accelerators in the finished mixture of the elastomer with the crosslinking system is significantly reduced. 
     The object of the invention is to propose a crosslinking agent for AEM and ACM elastomers, which is equivalent to the crosslinking agents containing guanidine that have been used until now, but which does without the teratogenically acting guanidines which are toxicologically hazardous. 
     SUMMARY OF THE INVENTION 
     This object is accomplished in accordance with the invention by a crosslinking agent for diaminically crosslinkable ethylene acrylate (AEM) and polyacrylate (ACM) elastomers, comprising a diamine crosslinker, an accelerator selected from 1,8-diazabicyclo-5,4,0-undec-7-ene (DBU), derivatives and salts thereof, and a moderator of the pyrrolidone type. 
     Surprisingly, it has been found that the diamine crosslinkers can be successfully used with DBU accelerators when the reaction speed of the DBU accelerators is reduced by a substance with a moderating effect. In particular, pyrrolidones have proven to be of value as moderator with a retarding effect. By using pyrrolidones as moderator, it is possible to lower the reaction speed of the DBU accelerators to such an extent that an inhomogeneous crosslinking structure can be avoided, while at the same time still maintaining a sufficiently high reaction speed for the crosslinking reaction. 
     Furthermore, an extended storage stability of the crosslinking agents comparable to the previously obtained storage stabilities of the crosslinking agents containing guanidine is achieved by adding pyrrolidones. 
     Typical representatives of DBU accelerators, which may be used in conjunction with the present invention, are Vulcofac® ACT 55 of the company Safic Alcan, Rhenogran® XLA 60 of the company Rhein Chemie, Alcanpoudre® DBU 70 of the company Safic Alcan and LUVOMAXX Safe Cure® CA of the company Lehmann &amp; Voss &amp; Co. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It is recommended that pyrrolidones be added to the crosslinking agent, in particular, in a weight ratio of DBU to pyrrolidone of approximately 1:1 to approximately 7:1, preferably approximately 1.5:1 to approximately 5:1. These ranges apply for the particularly preferred 2-pyrrolidone as moderator. 
     Also possible are N-methylpyrrolidone or polyvinylpyrrolidone (PVP) with a higher dosage. 
     The representatives mentioned at the outset, i.e., in particular, hexamethylene diamine carbamate, N,N-dicinnamylide diamine carbamate, 4,4-diaminodicyclohexylmethane, m-xylene diamine, 4,4-diaminodiphenylmethane, 4,4-diaminodiphenyl ether, 2,2-bis[4-(4-amino-phenoxy)phenyl]propane and hexamethylene diamines, are preferably used as diamine crosslinker. 
     Typically, the accelerator is used in a weight ratio to the diamine crosslinker of approximately 1:1 to approximately 3:1. In particular, the weight ratio ranging from approximately 1.5:1 to approximately 2.5:1 is used. 
     The invention also relates to the use of a crosslinking agent in accordance with the present invention for crosslinking diaminically crosslinkable AEM elastomers. 
     Herein the crosslinking agent is preferably used in a proportion of approximately 0.8 phr to approximately 3 phr, in particular, approximately 1 phr to approximately 2.5 phr (phr is parts per hundred of rubber). 
     Typical, preferred accelerator contents range from approximately 1.5. to 6 phr. 
     The invention also relates to the use of the crosslinking agent according to the invention for crosslinking diaminically crosslinkable ACM elastomers. ACM elastomers can be crosslinked under comparable conditions. This applies, in particular, to the crosslinking agent contents. 
     Furthermore, ACM and AEM elastomers may be optionally used in mixtures with one another, and proportions of crosslinking agent in the elastomer mixture deducible from the above-mentioned ranges are then recommended. 
     The invention also relates to diaminically crosslinked AEM elastomer and/or ACM elastomer, produced using a crosslinking agent according to the invention. 
     In addition, the diaminically crosslinked elastomers according to the invention may contain conventional processing agents, synthetic plasticizers, fillers etc. 
     Finally, the invention relates to a seal, comprising an elastomeric sealing element, the elastomeric sealing element being produced from an elastomer according to the invention or an elastomer mixture according to the invention. 
     The seals according to the invention may consist substantially completely of the elastomeric sealing element, i.e., may be produced completely from the elastomeric material according to the invention, for example, in the form of a flat material with a thickness of from one to several millimeters, which as such is easy to handle. 
     In other applications, the seal according to the invention comprises a substrate, in particular, a plastic substrate, an inorganic substrate or a substrate in the form of a metallic support, for example, a metal sheet. 
     The substrate in the seals according to the invention is embedded, in accordance with one of the embodiments at least in areas thereof, and in accordance with another embodiment completely, in the elastomeric sealing element. 
     Here the substrate assumes a reinforcing function for the elastomeric sealing element. In these cases, the layer thickness of the elastomeric materials can be minimized. Typical layer thicknesses lie in a wide range of approximately 5 μm to approximately 10 mm or more. 
     In the applications in which the substrate is substantially completely embedded in the elastomeric material, the substrate is practically not freely accessible at any point on the surface. The substrate is, therefore, completely enclosed by the elastomeric material and, for example, protected against aggressive chemicals. 
     When substrates are used in the seals according to the invention, in order to obtain better adhesion to the substrate, in particular, when a metallic support is used as substrate, an adhesive is applied to the substrate surface before the coating with the elastomeric material is carried out. 
     Various methods are suitable for producing the elastomeric sealing elements in the seals according to the invention. In particular, the elastomeric materials can be formed by injection molding methods, the transfer molding method or also a compression molding method. 
     Other suitable forming methods are extrusion and calendering, which may also be used for producing the seals or elastomeric sealing elements according to the invention. 
     These and further advantages of the invention will be explained in greater detail hereinbelow with reference to the Examples. 
     Examples 1 to 9 
     The VAMAC GLS type is used as AEM elastomer. All of the values contained in the following Table 1 are parts by weight and relate to 100 parts by weight of this elastomer. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 7 
                 Example 8 
                 Example 9 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Component 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Filler (carbon 
                 75 
                 75 
                 75 
                 75 
                 75 
                 75 
                 75 
                 75 
                 75 
               
               
                 black) 
               
               
                 Processing 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 agent 
               
               
                 Flow agent 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Release agent 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Age-protecting 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 agent 
               
               
                 Plasticizer 1 
                 17 
                   
                 17 
                   
                 17 
                   
                 17 
                 17 
                 17 
               
               
                 Plasticizer 2 
                   
                 17 
                   
                 17 
                   
                 17 
               
               
                 Crosslinker 1 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
                 1.2 
               
               
                 Crosslinker 2 
                   
                   
                   
                   
                   
                   
                   
                   
                 1.87 
               
               
                 Accelerator 
                 1.50 
                 1.50 
                 2.50 
                 2.50 
                 3.50 
                 3.50 
                 4.50 
                 4.50 
                 4.50 
               
               
                 Moderator 
                 0.8 
                 0.8 
                 0.8 
                 0.8 
                 1 
                 1 
                 1 
                 1.2 
                 0.8 
               
               
                 Properties 
               
               
                 Hardness 
                 62.3 
                 62.3 
                 61.3 
                 65.0 
                 61.1 
                 64.2 
                   
                 62.5 
                 66.5 
               
               
                 [Shore A] 
               
               
                 Compression 
                 49.1 
                 55.2 
                 43.2 
                 53.3 
                 38.8 
                 44.0 
                 33.8 
                 41.0 
                 31.8 
               
               
                 set [%] 
               
               
                   
               
             
          
         
       
     
     The hardness values contained in Table 1 are Shore A values measured in accordance with DIN EN ISO 868. 
     The values of the compression set, so-called compression set values, result from a compression of the sample by 25% at 150° C. and a holding time of 24 hours at this temperature. The samples were taken out of the testing apparatus in the cold state (RT). The compression set values were determined after a relaxation time of 30 min (ISO 815-1-2). 
     The following specific products were used for the components given in Table 1: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Filler (carbon black) 
                 Corax N772 
               
               
                   
                 Processing agent 
                 Stearic acid 
               
               
                   
                 Flow agent 
                 Armeen 18 D 
               
               
                   
                 Release agent 
                 Vanfre VAM 
               
               
                   
                 Age-protecting agent 
                 Dusantox 86 
               
               
                   
                 Plasticizer 1 
                 Santicizer 409A 
               
               
                   
                 Plasticizer 2 
                 Bisoflex T810T 
               
               
                   
                 Crosslinker 1 
                 Vulcofac HDC 
               
               
                   
                 Crosslinker 2 
                 RT Advancure HD 
               
               
                   
                 Accelerator 
                 Vulcofac ACT55 (DBU) 
               
               
                   
                 Moderator 
                 2-pyrrolidone 
               
               
                   
                   
               
             
          
         
       
     
     The following Table 2 gives further test results from samples of Examples 1 to 9, which were obtained from tensile tests (ISO 730) and compression stress relaxation tests. 
     In order to determine the compression stress relaxation values, the sample body is first compressed to 75% of its original thickness. The force required to do this is the so-called static preload. Owing to relaxation processes in the sample body, the force required for the compression to 75% of the original thickness decreases in the course of time. The force required at the end of the 30-minute holding time is stated in the last line of Table 2. The entire test was carried out at 23° C. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Measured 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 value 
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
                 Example 6 
                 Example 7 
                 Example 8 
                 Example 9 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Tear strength 
                 11.1 
                 14.3 
                 12.6 
                 12.6 
                 13.5 
                 13.8 
                 13.6 
                 14.5 
                 8.4 
               
               
                 [MPa] 
               
               
                 Elongation at 
                 262 
                 274 
                 257 
                 242 
                 272 
                 258 
                 252 
                 284 
                 175 
               
               
                 break [%] 
               
               
                 Tension 
                 1.10 
                 1.20 
                 1.10 
                 1.20 
                 1.10 
                 1.20 
                 1.30 
                 1.20 
                 1.20 
               
               
                 modulus M 
               
               
                 25% [MPa] 
               
               
                 Tension 
                 1.71 
                 1.95 
                 1.84 
                 2.07 
                 1.81 
                 2.04 
                 2.07 
                 1.85 
                 2.03 
               
               
                 modulus M 
               
               
                 50% [MPa] 
               
               
                 Tension 
                 3.30 
                 4.00 
                 3.80 
                 4.40 
                 3.90 
                 4.50 
                 4.50 
                 3.70 
                 4.50 
               
               
                 modulus M 
               
               
                 100% [MPa] 
               
               
                 Tension 
                 8.05 
                 10.04 
                 9.43 
                 10.88 
                 9.71 
                 10.90 
                 10.71 
                 9.79 
               
               
                 modulus M 
               
               
                 200% [MPa] 
               
             
          
           
               
                 Compression stress relaxation 
               
             
          
           
               
                 Static preload 
                 240.3 
                 268.1 
                 259.8 
                 257.5 
                 210.8 
                 240.3 
                 204.5 
                 171.1 
                 221.7 
               
               
                 [N] 
               
               
                 After 30 min 
                 118.7 
                 135.2 
                 131.5 
                 131.8 
                 119.0 
                 118.7 
                 122.4 
                 99.3 
                 137 
               
               
                 relaxation [N] 
               
               
                   
               
             
          
         
       
     
     Example 10 
     An AEM elastomer of the VAMAC GLS type is used here as elastomer. The formulation components are given in the following Table 3. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Component 
                   
                 Parts by weight [phr] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Filler (carbon black) 
                 Corax N772 
                 54 
               
               
                 Processing agent 
                 Stearic acid 
                 2.00 
               
               
                 Flow agent 
                 Armeen 18 D 
                 0.75 
               
               
                 Release agent 
                 Vanfre VAM 
                 1.00 
               
               
                 Age-protecting agent 
                 Dusantox 86 
                 2.00 
               
               
                 Plasticizer 1 
                 Santicizer 409A 
               
               
                 Plasticizer 2 
                 Bisoflex T810T 
                 17 
               
               
                 Crosslinker 1 
                 Vulcofac HDC 
                 2.00 
               
               
                 Accelerator (DBU) 
                 Vulcofac ACT55 
                 1.50 
               
               
                 Moderator 
                 2-pyrrolidone 
                 0.80 
               
               
                   
               
             
          
         
       
     
     The tear strength of the elastomer was 11.2 MPa, the elongation at break was determined at 314%, and the hardness was determined in accordance with Shore A at 54.6. 
     The compression set of the elastomers is summarized for several holding times of the samples in Table 4. The measuring conditions corresponded to those of Examples 1 to 9. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Measured value/Holding time [h] 
                 24 
                 72 
                 168 
                 336 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Compression set [%] 
                 31.1 
                 41.0 
                 49.1 
                 55.4 
               
               
                   
               
             
          
         
       
     
     Examples 11 to 13 
     The use of various accelerators and their effect on the mechanical properties of the elastomers obtained are shown with reference to Examples 11 to 13. The formulations of the Examples and some important mechanical properties are summarized in Table 5. The proportions of the components in the respective formulation are given in parts by weight per 100 parts by weight of VAMAC GLS elastomer. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Component 
                   
                 Example 11 
                 Example 12 
                 Example 13 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Filler (carbon black) 
                 Corax N772 
                 56 
                 56 
                 56 
               
               
                 Processing agent 
                 Stearic acid 
                 2 
                 2 
                 2 
               
               
                 Flow agent 
                 Armeen 18 D 
                 0.5 
                 0.5 
                 0.5 
               
               
                 Release agent 
                 Vanfre VAM 
                 1.5 
                 1.5 
                 1.5 
               
               
                 Age-protecting agent 
                 Dusantox 86 
                 2 
                 2 
                 2 
               
               
                 Plasticizer 1 
                 Santicizer 409A 
                 17 
                 17 
                 17 
               
               
                 Plasticizer 2 
                 Bisoflex T810T 
               
               
                 Crosslinker 1 
                 Vulcofac HDC 
                 1.2 
                 1.2 
                 1.3 
               
               
                 Crosslinker 2 
                 RT Advancure HD 
               
               
                 Accelerator 1 
                 DOTG 
                 8 
               
               
                 Accelerator 2 (DBU) 
                 Vulcofac ACT 55 
                   
                 5.1 
                 4.5 
               
               
                 Moderator 
                 2-pyrrolidone 
                   
                   
                 1 
               
               
                 Properties 
               
               
                 Compression set [%] 
                   
                 30.0 
                 41.5 
                 28.0 
               
               
                 Tear strength [MPa] 
                   
                 10.8 
                 9.6 
                 13.6 
               
               
                 Elongation at break [%] 
                   
                 420 
                 290 
                 360 
               
               
                   
               
             
          
         
       
     
     The results show that the compression set parameters (Example 13) obtainable in accordance with the invention are comparable to those of conventional formulations (Example 11). This means that the accelerator type DOTG which is hazardous to health is successfully replaced without having to tolerate disadvantages for the important mechanical properties of the elastomers as is the case when DBU accelerator is used (Example 12). This finding can also be proven by long-term tests (cf. the values of Table 6). 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
             
             
               
                   
                   
               
               
                   
                 Example 11 
                 Example 12 
                 Example 13 
               
             
          
           
               
                   
                 Maximum 
                 Maximum 
                 Maximum 
                 Maximum 
                   
                 Maximum 
               
               
                   
                 volume 
                 flow speed 
                 volume 
                 flow speed 
                 Maximum 
                 flow speed 
               
               
                 Months 
                 [mm 3 ] 
                 [mm 3 /s] 
                 [mm 3 ] 
                 [mm 3 /s] 
                 volume [mm 3 ] 
                 [mm 3 /s] 
               
               
                   
               
             
          
           
               
                 0 
                 2577 
                 188.3 
                 2413 
                 151.2 
                 2067 
                 159.3 
               
               
                 1 
                 1952 
                 125.2 
                 1760 
                 98.2 
                 1805 
                 106.0 
               
               
                 2 
                 1683 
                 106.2 
                 1442 
                 76.1 
                 1550 
                 99.0 
               
               
                 4 
                 644 
                 34.1 
                 337 
                 16.0 
                 525 
                 28.0 
               
               
                 5 
                 375 
                 21 
                 216 
                 12 
               
               
                   
               
             
          
         
       
     
     The flow behavior summarized in Table 6 was determined in a Rheovulkameter long-term study at 20 bar, 190° C. and a warming-up time of 5 seconds.