Silane-rubber coupling in sulfur, peroxide and metal oxide curing systems.

For industrial rubber goods used in many different applications, carbon black is usually the filler of choice. For white and colored compounds, natural fillers such as clay and silicates are common, but in applications that require a high degree of reinforcement, precipitated silica in combination with a silane coupling agent is of interest. This article focuses on reinforcement of the silica-silane filler systems in three different elastomer systems. Carbon black compounds were included for comparison. First, sulfur functionalized silanes in a silica-filled NBR model formulation for roll applications were investigated, with focus on static and dynamic properties. Second, EPDM and EPM compounds using a peroxidecure system were studied with regard to crosslink efficiency, using a vinyl silane and an activator, respectively. Finally, chloroprene rubber compounds, with and without silane coupling agent and with metal oxide-cure, were examined.

It is known that silanes can provide coupling between silica and rubber to enhance reinforcing properties of silica-filled rubber compounds (ref. 1). While one part of the silane, most commonly a trialkoxysilyl group, provides a coupling to the silica surface, the other part is an organo-functional group which couples to the rubber during vulcanization. The use of TESPT (bis-[triethoxysilylpropyl]polysulfide) as a coupling agent in sulfur-cured compounds is well established and has been reviewed recently (refs. 2 and 3). The main application for TESPT as a coupling agent is in the manufacture of tire treads with improved wet handling behavior and reduced fuel consumption (refs. 4 and 5). Besides this dominating use of TESPT in the tire industry, silane coupling agents are also valuable for polymers different t'rom the common diene rubbers (ref. 6). The type of silane should be properly selected according to the type of rubber and vulcanization system in the compounds in order to obtain optimum reinforcement (ref. 7).

The objective of this study is to understand the silica-rubber coupling effect on the degree of reinforcement using various chemical silanes in three different types of elastomers with different curing systems: NBR, EPDM/EPM and CR. The effectiveness of each silane in providing silica- rubber coupling is demonstrated by static and dynamic vulcanizate proper resistance data and demonstrate the effect of silanes on silica-rubber coupling in the compounds.

In the first part of this article, the bifunctional sulfur silanes containing polysulfide (-[S.sub.x]-), mercapto (-SH) and thiocyanate (-SEN) groups were examined in NBR compounds with sulfur cure. Due to the reduced amount of double bonds in NBR and the much higher polarity of this rubber compared to, e.g., BR, differences from common diene rubbers were expected

The second part deals with the use of a vinylsilane as an activator and a coupling agent in peroxide-cured compounds. For such compounds, it has been reported that a double bond contained in a silane like the vinyltriethoxysilane (VP Si 225) is best suited to provide a chemical link between the silica and rubber (ref. 8). In this investigation, peroxide-cured, silica-filled EPDM and EPM compounds with the vinyl silane were compared to those with TAC monomer (triallylcyanurate), an activator often used in peroxide-cured compounds (mE 9). The effect of both TAC and the vinyl silane on the crosslinking efficiency was studied in the two different polymers: EPDM with a termonomer providing a double bond and EPM without such a crosslinkable double bond as a side chain.

In the third part, mercapto-functional silanes were studied in metal oxide-cured CR compounds and compared to the more often used chloro-functional silane, chloropropyltriethoxysilane (ref. 10). In all three polymer systems, monofunctional alkyl silanes were included. They act as processing aids to reduce compound viscosity but do not provide rubber-silica coupling. A carbon black-filled compound was used in each system for comparison.

Experimental

Materials

The different types of liquid silanes used in this study are commercially available. Their chemical names, common abbreviations and characteristics are listed in table 1. The precipitated silica has a BET surface area of 175 m2/g, CTAB surface area of 165 [m.sup.2]/g. The carbon blacks N330 and N550 were used for comparison. Polymers studied were NBR, CR and EPDM/EPM. EPM and EPDM contain 64% and 71% by weight of ethylene, respectively. Diene (ENB) content in the EPDM is 5% by weight. Mooney viscosity at 125[degrees]C is 63 MU for the EPM and 65 MU for EPDM.

Compound formulations

The formulations of the NBR, EPDM/EPM and CR compounds are shown in the tables 2-4, respectively.

In the NBR formulation, the silanes Si 69, VP Si 263, VP Si 163, Si 264 and VP Si 208 were added on an equimolar basis, and the sulfur content was adjusted in such a way that the compounds had a comparable amount of "free" sulfur (refs. 11 and 12). Additionally, one compound with a reduced amount of the highly active VP Si 263 was included. The compounds were prepared in an internal mixer using three-stage mixing for a total mixing time of 8.7 minutes for the first and second stage. The dump temperature was 150-155[degrees]C.

The EPDM and EPM formulations contained silica in combination with VP Si 225, VP Si 208 and TAC. The TAC and VP Si 225 were also included in N550-filled EPDM/EPM compounds for comparison. The VP Si 208 only makes the silica surface hydrophobic and reduces effects caused by the absorption and/or deactivation of the initiator on the polar silica surface. A chemical linkage between silica and rubber is not provided by this alkylsilane. The compounds were mixed in an internal mixer using two-stage mixing for a total mixing time of six minutes for the first stage. The dump temperature was 160[degrees]C.

In the CR formulations, the silanes VP Si 203, Si 230, VP Si 263 and VP Si 163 were added on an equimolar basis, and in the case of VP Si 263, a compound with a reduced silane dosage was also prepared. The indices "H" and "L" in the following tables indicate the equimolar addition of the silanes and reduced amount, respectively. The CR compounds were prepared using two-stage mixing for a total mixing time of 4.4 minutes for the first stage. The first stage was mixed in an internal mixer and the second stage was done on an open mill. The mixing temperature for the CR compounds in the first stage was below 120[degrees]C, because these compounds are very sensitive to pre-crosslinking of the CR at high temperature (ref. 13). Due to the low mixing temperature, it can be assumed that some of the silanization took place during mixing and some during vulcanization.

Test methods

Cure properties were measured according to ASTM 5289 using a MDR at 160[degrees]C for NBR, 180[degrees]C lor EPDM/EPM and 170[degrees]C for CR compounds. Mooney viscosity, ML 1+4, was determined according to ASTM D1646 at 100[degrees]C. The dispersion index (DI) was determined using a dispersion analyzer according to ASTM 2663, Method C. The following mechanical properties were determined: Stress-strain properties (ASTM D 412), tear (Die C) properties (ASTM 624) and hardness (durometer A). For aging properties, tensile specimens were aged in an air circulating oven (ASTM D573) and in oil #3 (ASTM D471). After aging, tensile properties of aged samples were measured at room temperature.

Abrasion resistance was determined using the Goodyear angle test and compression set was performed according to ASTM 395, Method B. Abrasion fractured surfaces of rubber specimens were studied using a scanning electron microscope (SEM). Test specimens, 0.5 mmx 0.5 mm in size. were cut from the abraded surface of Goodyear angle wheel samples and coated with carbon under high vacuum prior to the SEM observations. Back-scattered electron images were obtained for all photographs.

Resilience was measured with the Zwick rebound test method, and heat build up was obtained using a Goodrich Flexometer. Tan delta, E' and E" were measured with an MTS 831 Elastomer Test System at 0[degree] and 60[degrees]C, 2% and 8% double strain amplitude (DSA) and a frequency of 12 Hz.

Results and discussion

NBR compounds with sulfur cure

In this model, NBR roll formulation, the sulfur-functional silanes Si 69, VP Si 263, VP Si 163 and Si 264, as well as the mono-functional alkylsilane, VP Si 208, were used on an equimolar basis, with the exception of the compound 3A, where the amount of VP Si 263 was reduced by half. Si 69 is mainly polysulfidic, whereas VP Si 263, VP Si 163, and Si 264 are monosulfidic silanes. These sulfur-functional groups can react with NBR during the vulcanization stage.

With regard to the reactivity, the mercapto group (VP Si 163 and VP Si 263) is the most reactive, whereas the thiocyanate group (blocked mercaptosilane of Si 264) and also the potysulfides ([S.sub.2]-[S.sub.5]) (Si 69) have to be activated prior to the coupling reaction (refs. 11 ands 12). Due to the different reaction mechanisms, differences between these silanes in coupling efficiency, as well as structure, can be expected. For Si 69, using the conventional curing system, polysufidic structures are mainly formed. In the case of Si 264 and VP Si 263, the structure of the silane-rubber bond has not yet been reported. The alkylsilane VP Si 208 contains an octyl hydrocarbon chain and is known to reduce filler-filler networking without providing coupling to the rubber (ref. 14).

Due to the trimethoxysilyl group of VP Si 163, this silane is more active toward the silica than the VP Si 263 with the triethoxysilyl group, but the methanol split off during the silanization can cause problems. In the case of Si 264, it has to be considered that the thiocyanate group can release traces of HCN at high temperatures (165[degrees]C and higher), which does not pose a hazard if a ventilation system is installed in mixing and vulcanization facilities.

NBR--cure characteristics and processability

As can be seen in table 5, the use of the mercapto silanes, VP Si 163 and VP Si 263 at low and high dosage (VP Si [263.sub.L] and VP Si [263.sub.H], respectively), resulted in higher compound viscosities and shorter scorch times than the other silanes and the carbon black reference. The VP Si 163 showed the shortest scorch time and the highest Mooney viscosity, while VP Si 263 at a lower dosage showed improved compound viscosity and scorch time compared to VP Si 163 and VP Si 263 at higher dosage. These data indicate that a certain degree of precrosslinking in the mercapto silane compounds already occurred during the mixing, but with low amounts of these silanes, processing is possible.

The delta torque ([M.sub.H]-[M.sub.L]) measured at low strain (0.5[degrees]arc) represents the compound modulus, which can be attributed to matrix crosslinks (sulfur crosslinks), filler-filler networking, and silica-rubber coupling. It should be noted that Si 264 exhibited the highest delta torque, whereas VP Si 163 gave the lowest delta torque, probably due to very low filler-filler networking. The fastest cure rate (100/t90-tS1) was observed in the compound with VP Si 163 and N-550, while Si 69 exhibited the slowest cure rate.

NBR--static vulcanizate properties Table 6 shows the stress-strain data, and stress-strain plots are depicted in figure 1. Hardness was similar for all compounds, but significant differences were observed in the moduli. A high ratio of 300% modulus to 100% modulus can be used as an indication of a high degree of reinforcement, due to a strong filler-rubber interaction (ref. 15). VP Si 163 and Si 264 exhibited a high ratio of 300% modulus to 100% modulus compared to the other silanes and the carbon black reference; while VP Si 208 resulted in the lowest reinforcement, due to the absence of a coupling possibility. The high degree of reinforcement of VP Si 163 can also be seen in high tear strength, excellent abrasion resistance (figure 2) and low compression set. Si 264 also provided low compression set and good abrasion resistance. The high reinforcement seen in the case of the thiocyanate silane Si 264 was attributed to strong silica-rubber coupling. This higher reinforcement with Si 264 compared to the Si 69 may be due to a higher coupling efficiency in this polar rubber, since the reinforcement in, e.g., BR and SBR was reported to be similar for both silanes (ref. 2). Nevertheless, using Si 69 or VP Si 263 still provided a very good reinforcement (moduli, abrasion resistance) comparable to the carbon black-filled compound, but the good compression set of the black-filled compound was not reached.

[FIGURES 1-2 OMITTED]

NBR--SEM study on abrasion fracture surfaces

Abrasion resistance is one of the most important reinforcing properties of rubber compounds and is used to predict the service life of rubber products such as the treads, shoe soles and rolls. Abrasion loss is due to material removal from the surface of the test specimen when it experiences both tearing and chunking out of material by the abrasive surface of the rotating wheel (ref. 16). Abrasion resistance is increased by strong polymer-filler interaction, and the formation of an abrasion pattern can give us insight into polymer-filler interaction (silica-rubber coupling) (ref. 17).

Figure 3 shows the SEM photographs of the abraded surfaces of the silica-filled NBR compounds containing alkylsilane VP Si 208, Si 264, and of the N550-filled NI3R compound. A ribbed structure can be seen in all compounds, but the abraded surface of the compound with VP Si 208 is roughest (coarse ribbed structure). High magnification observation shows deep grooves between the ribs, and big chunks of the rubber compound were removed. Extensive removal of material in the compound with alkylsilane indicates poor abrasion resistance resulting from lack of silica-rubber coupling by VP Si 208. The extent of the rubber compound removal is less in the case of Si 264, as seen from the smoother surface and shallower grooves. This suggests that Si 264 offers stronger silica-rubber coupling to the rubber compound and higher resistance to abrasive rupture than in the VP Si 208 compound. The abraded surface of the carbon black compound is smoothest of all and grooves between the ribs are very shallow, indicating less extensive abrasion. This is associated with the lowest angle abrasion loss and suggests that carbon black-rubber interaction is very strong. All the morphological information obtained from the SEM agrees well with the Goodyear angle abrasion test data in figure 2.

[FIGURE 3 OMITTED]

NBR--effect of aging

Aging results can be seen in table 7. Air aging hardened all NBR compounds. Hardness remained nearly unchanged when aging the NBR compounds in oil. Si 264 performed the best regarding aging resistance as seen from the lowest change in tensile, while the shortening of elongation at break was high for all tested compounds.

NBR--dynamic mechanical properties

With regard to dynamic properties, MTS, Zwick rebound and heat build-up measurements were performed. From MTS at 60[degrees]C, the delta E' [E'(2%DSA)-E'(8%DSA)] was calculated to study the filler-filler networking (it has to be considered that at 2% DSA, some part of the network has already broken down). The higher the delta E', the more pronounced is the breakdown of the filler network under the chosen deformation (ref. 18). It appears that VP Si 163 resulted in the lowest delta E', indicating low filler-filler networking compared to other silanes, as seen in table 8. This was in line with the low delta torque discussed above and may be explained by the fact that the trimethoxysilyl group in the VP Si 163 is more reactive and gives therefore better silanization under the given mixing conditions than the triethoxysilyl group of the other silanes (ref. 19). Also, the tan delta (hysteresis) (ref. 20) at 60[degrees]C was lowest with VP Si 163, followed by Si 264 and VP Si [263.sub.L]; whereas the tan delta for the compound with Si 69 was the highest (figure 4, table 8). These were in line with the results of the heat build-up measurements and the rebound values at 60[degrees]C for the compounds with silane coupling agents. For MTS at the low temperature measurement of 0[degrees]C, the results were different. Si 69 exhibited the lowest E' and tan delta values, followed by the compounds with the other coupling agents. Si 264 showed the highest stiffness at low temperature. The carbon black compound had the lowest hysteresis at 60[degrees]C, due to its low surface area and hence low filler networking. The compound with VP Si 208 also showed different dynamic behavior, caused by the absence of silica-rubber coupling.

[FIGURE 4 OMITTED]

NBR--conclusion

All silane coupling agents except the monofunctional VP Si 208 provide chemical linkage between the silica and the NBR rubber, resulting in higher reinforcement than without such a bond. Poor abrasion resistance and the deep groove formation observed by SEM is evidence of the silica-rubber coupling deficiency in the VP Si 208 compound. With regard to the static vulcanizate data, including compression set and aging, the thiocyanate-functional silane Si 264 performed best, followed by the mercaptosilanes. Si 69 gave high reinforcement as well, but the level was lower than Si 264. With regard to processability, Si 264 and Si 69 were good choices, whereas the VP Si 163 and VP Si 263 were a little scorchy and should be used at a lower dosage. The dynamic properties at high temperatures favored Si 264 and VP Si 163, but at low temperature, Si 69 showed the lowest stiffness and hysteresis loss. This indicates a different filler network and/or crosslink structure and density, respectively, between the silanes. Therefore, the choice of the optimum silane for each application depends on the compound performance requirements.

EPDM/EPM compounds with peroxide cure

In peroxide-cured EPDM and EPM model compounds, the effectiveness of the vinyl silane VP Si 225 as a coupling agent and activator in comparison to the conventional activator TAC (triallylcyanurate) was investigated. From the mechanism of the peroxide-cure, it is known that a crosslink in a fully saturated polymer like EPM is formed by the combination of two radicals of the polymer (figure 5a) (ref. 9). Due to the fact that these active species can undergo various side reactions, resulting in deactivation, crosslink density is lower than the amount of initiator, and the amount of crosslinks correlates directly with the amount of initiator. However, with the addition of double bonds, either by the use of partly unsaturated polymer like EPDM, or the presence of activators like TAC, crosslink density increases significantly. This is due to the fact that a crosslink can take place without termination of the radical (ref. 9). This is also valid for vinyl silanes, which can additionally provide silica-rubber coupling (figure 5b). In order to get a better insight into the effect of the double bond in the peroxide-cure on the crosslink density and reinforcement, silica and carbon black-filled compounds with and without the vinyl silane and with TAC were investigated. This was performed in the two polymers, EPM and EPDM. The silica-filled compound with VP Si 208 was used as a reference, providing silanized silica without the possibility to couple to the rubber. Additionally, a carbon black-filled compound with an activator and the vinyl silane were used for comparison.

[FIGURE 5 OMITTED]

EPDM/EPM--cure characteristics and processability

The compound Mooney viscosity of all silica-filled compounds was higher than those filled with carbon black, due to stronger silica-silica networking in this nonpolar rubber (figure 6). Nevertheless, using the alkylsilane, VP Si 208, and bifunctional vinyl silane, VP Si 225, in the silica-filled compounds (1B, 2B, 7B and 8B) led to a reduction in silica-silica networking and hence lower Mooney viscosities compared to compounds containing only TAC.

[FIGURE 6 OMITTED]

Cure characteristics of the EPDM and EPM compounds are compared in figures 7 and 8, respectively. As mentioned earlier, the delta torque (MH-ML) is an indicator of the crosslink density. It can be seen that the silica and carbon black-filled EPDM compounds gave higher crosslink densities (higher [M.sub.H]-[M.sub.L]) than the corresponding EPM compounds. This can be attributed to the fact that, in the case of EPDM, the radical at a polymer chain can provide coupling to the double bond of the third monomer without deactivation of the radical. Additionally, higher ethylene content of the EPDM compared to EPM may contribute to higher delta torque as well.

[FIGURES 7-8 OMITTED]

Shorter scorch times were observed in EPDM compounds containing both silica and carbon black compared to the EPM compounds. This is probably due to the fact that the double bond contained in EPDM resulted in the faster formation of a critical amount of crosslinks responsible for the torque increase. Addition of vinyl silane and TAC further shortened the scorch time with the vinyl silane being more efficient.

Beside this "polymer effect," it is observed that the delta torque in the silica-filled compounds was higher than that in the carbon black-filled compounds. This must be attributed to the stronger silica networking due to the stronger H-bonding compared to Van der Waals forces in carbon black.

In silica-filled EPDM and EPM compounds, TAC provided higher delta torque values than the vinyl silane and the alkylsilane. In the case of the silanes, the silica-silica networking was reduced due to hydrophobation of the surface, which was not the case for TAC. Therefore, a part of the high maximum torque for the TAC compounds may be attributed also to the higher filler-filler network. The higher delta torque for VP Si 225 compared to VP Si 208 was due to chemical coupling to polymer (figure 9a), which did not exist in the compound containing VP Si 208.

[FIGURE 9 OMITTED]

For the carbon black-filled compounds, no significant increase of delta torque was observed for the added vinyl silane because VP Si 225 can only couple to the polymer chain without terminating the radical, providing no multifunctional cross-link as with TAC, where increased crosslink density is observed (figure 9b). The effects of TAC were more pronounced in the EPM compounds, having no double bonds, so that the additional vinyl groups were efficient in increasing the total crosslink density.

EPDM/EPM--static vulcanizate properties

The static rubber data of EPDM and EPM compounds are given in table 9, and the differences in the reinforcing indices (modulus 300%/100%) are visualized in figure 10. In the EPDM compounds, the compound containing silica with the vinyl silane demonstrated stress-strain properties and reinforcement similar to the carbon black-filled compounds with/without TAC. In silica-filled compounds, TAC led to a lower ratio of 300% modulus to 100% modulus compared to VP Si 225, since no filler-rubber coupling was provided and only the matrix crosslink density was affected. In the case of carbon black, the added vinyl silane is not as effective as the TAC with regard to increasing crosslink density, because vinyl silane cannot act as a multi-functional crosslink.

[FIGURE 10 OMITTED]

This also indicates that silica-rubber coupling provided a stronger contribution to the reinforcement than the crosslinking by TAC. As expected, VP Si 208 demonstrated the lowest reinforcement, due to lack of silica-rubber coupling.

The same results of stress-strain behavior were seen in the EPM compounds, but the EPDM compounds resulted in higher moduli, which agrees with the higher delta torques. Hardness values of the silica compounds were higher than for the carbon black compound, which is in line with the higher Mooney viscosity and higher dynamic stiffness (filler network).

Aside from the high modulus, VP Si 225 also demonstrated the best abrasion resistance among all EPDM and EPM compounds and the lowest compression set (only among the silica filled compounds). This shows that silica-rubber coupling plays a major role in reinforcement of the vulcanizates.

EPDM/EPM--dynamic mechanical properties

The degree of the silica networking can be observed from the delta E' (E' at 2% DSA--E' at 8% DSA) as seen in table 10 for the EPDM compounds. The silica-filled EPDM compounds showed higher delta E' at 0 and 60[degrees]C compared to carbon black. This can be attributed to the stronger silica-silica networking and the low surface area of the carbon black used. Due to the hydrophobation of the silica by the added vinyl silane, the delta E' values were slightly lower than that of TAC, which is not surface active. In the case of carbon black, the differences between with and without added VP Si 225 were minor, due to the ineffectiveness of VP Si 225. Hysteresis loss of VP Si 225 and TAC in the silica-filled compounds was lower than all carbon black-filled compounds, as seen in tan delta values at 0 and 60[degrees]C. This was due to strong silica-rubber coupling from the vinyl silane and strong matrix crosslinking from TAC. The same tendencies were also observed in the case of EPM.

EPDM/EPM--summary

This study demonstrated clearly that a high reinforcement of peroxide-cured and silica-filled compounds can only be achieved by the use of the proper amount of a vinyl silane like VP Si 225. Furthermore, it has been shown that the vinyl silane in silica-filled compounds acts not only as a coupling agent, but also as an activator to increase crosslink density, similar to TAC. In carbon black-filled compounds, this pronounced activating effect is not observed because the silane cannot act as a multifunctional crosslinker like TAC.

CR compounds with metal oxide cure

Silica-rubber coupling in chloroprene rubber is usually achieved using chloropropyltriethoxysilane, Si 230, as described by Wolff (ref. 10). Using metal oxide-cure with ZnO, coupling should take place at the 3,4 units with the release of HCl. This study included the mercaptosilanes, VP Si 163 and VP Si 263, in order to investigate their coupling efficiency compared to Si 230. It is well known that mercaptosilanes are highly reactive in sulfur-cure systems, but it has been reported (ref. 21) that the nucleophilic mercapto group can also couple efficiently to the chloroprene rubber under formation of a monosulfidic crosslink. As references, the alkylsilane VP Si 203 was used, which cannot provide silica-rubber coupling, as well as a carbon black-filled compound.

CR--cure characteristics and processability

Table 11 shows the cure data and compound viscosity of all CR compounds. The silica-filled compounds containing the methoxy and ethoxy mercapto silanes, VP Si 163 and VP Si 263, respectively, had slightly higher compound viscosities compared to those with alkyl and chlorosilanes. This was most likely due to a certain amount of pre-scorch during mixing. Also, the Mooney scorch times were shorter for these compounds.

Figure 11 depicts the delta torque ([M.sub.H]-[M.sub.L]) and cure time t90%. Cure rate was slower and delta torque was higher in the silica-filled CR compounds compared to the N330-filled compound. In silica-filled compounds, VP Si 203 and VP Si 163 resulted in the lowest delta torque; whereas the compounds with VP Si 263 gave the highest torque values.

[FIGURE 11 OMITTED]

CR--static vulcanizate properties

The static data for the CR compounds are shown in table 12. Stress-strain plots of all CR compounds are shown in figure 12. It can be seen that the use of the mercapto silanes resulted in a higher modulus ratio (300% modulus/100% modulus) compared to chlorosilane Si 230 and alkylsilane VP Si 203, respectively. The low modulus of VP Si 203 was expected, but the significantly higher modulus of VP Si 263 compared to the Si 230 must be attributed to a much higher coupling efficiency. Comparing the compounds with the mercapto silanes, it appears that a low dosage of VP Si 263 (VP Si [263.sub.L]) resulted in balanced in-rubber properties, and the higher amount of VP Si 263 (VP Si [263.sub.H]) increased reinforcement only slightly. This may be due to the limited amount of 3,4-units that are accessible for a coupling reaction. Tear strength of all silane compounds was higher than N330, except for VP Si 203 that is without the provided silica-rubber coupling. Abrasion resistance and compression set improvements were also seen for the compounds with the mercapto silanes. Si 230 also showed significantly higher reinforcement than the alkylsilane. The carbon black reference compound exhibited very short elongation and quite high moduli and hardness, due to the higher surface area of N330. Most likely, N550 would have been the better carbon black choice for this compound.

[FIGURE 12 OMITTED]

CR--conclusion

This investigation demonstrated that mercapto silanes, especially VP Si 263, are best suited for the reinforcement of chloroprene rubber. The only obstacle to the use of these highly active silanes may be their tendency to pre-crosslink during mixing, which can lead to problems in further processing of the compounds. With Si 230, the coupling efficiency is lower than that of mercapto silanes, but processing is easy and reinforcement is probably high enough for most applications. Worth noticing is the fact that the mercapto silane apparently reacts directly with the 3,4-chloroprene unit with release of HCl and formation of a monosulfidic bond; which makes a silica-rubber crosslink possible, even without curatives, and hence without a matrix cross]ink. Such a unique network structure is not possible with sulfur- or peroxide-cure (ref. 2).

Summary

In silica-filled NBR compounds with sulfur-cure, the thiocyanate silane Si 264 gave the best balance between high reinforcement, hysteresis loss and processability. The mercaptosilane VP Si 263 and the polysulfidic silane Si 69 also provided good reinforcement, but differences were observed in the dynamic behavior. The highest reinforcement and lowest silica network were achieved with the mercapto silane VP Si 163, but problems in processability and the release of methanol may limit its use. Nevertheless, the choice of the optimum silane lot each application depends on the specific requirements for processability, reinforcement and dynamic behavior.

In the silica-filled EPDM/EPM compounds with peroxide-cure, the vinylsilane VP Si 225 acts as a silica-rubber coupler and activator to increase crosslink density at the same time. This results in better reinforcing properties than an activator like TAC, which only increases crosslink density by providing a multifunctional crosslink. Only with a silane coupling agent can reinforcement of silica-filled compounds, similar to carbon black reinforcement, be achieved.

In the case of silica-filled chloroprene rubber with metal oxide-cure, a silica-rubber coupling effect is achieved by the chlorosilane Si 230. Nevertheless, use of more active mercaptosilanes like VP Si 263 results in higher reinforcement, but processing is more difficult. This may limit the use of this silane in practice.

Table 13 summarizes the performance of each silane in the NBR and CR compounds.

Table 1--types and characteristics of silanes used

Chemical name                        Structure

Bis-(triethoxysilylpropyl            [([C.sub.2][H.sub.5]O).sub.3]
polysulfide, <x> = 3.8 (TESPT)       [S.sub.i][(C[H.sub.2]).sub.3-
                                     [S.sub.x]-[(C[H.sub.2]).sub.3]
                                     Si[(O[C.sub.2][H.sub.5]).sub.3]

3-mercaptopropyltriethoxysilane      [([C.sub.2][H.sub.5]O).sub.3]Si
(MTEO)                               [(C[H.sub.2]).sub.3]-SH

3-mercaptopropyftrimethoxysilane     [(C[H.sub.3]O).sub.3]Si
(MTMO)                               [(C[H.sub.2]).sub.3]-SH

3-thiocyanatopropyltriethoxysilane   [(C.sub.2][H.sub.3]O).sub.3]Si
(TCPTES)                             [(C[H.sub.2]).sub.3]-SCN

Vinyltriethoxysilane (VTEO)          [(C.sub.2][H.sub.3]O).sub.3]
                                     SiCH=C[H.sub.2]

3-chloropropyltriethoxysilane        [([C.sub.2][H.sub.3]O).sub.3]Si
(CIPTES)                             [(C[H.sub.2]).sub.3]-CI

Octyftriethoxysilane (OCTEO)         [([C.sub.2][H.sub.3]O).sub.3]Si
                                     [(C[H.sub.2]).sub.7]C[H.sub.3]

Propyltriethoxysilane (PTEO)         [([C.sub.2][H.sub.5]O).sub.3]Si
                                     [(C[H.sub.2]).sub.2]-C[H.sub.3]

                                     Trade       Molecula      Density
Chemical name                        name        r weight       (g/
                                                            [cm.sup.3])

Bis-(triethoxysilylpropyl            Si 69         532           1.10
polysulfide, <x> = 3.8 (TESPT)

3-mercaptopropyltriethoxysilane      VP Si 263     238           0.93
(MTEO)

3-mercaptopropyftrimethoxysilane     VP Si 163     196           1.05
(MTMO)

3-thiocyanatopropyltriethoxysilane   Si 264        263           1.03
(TCPTES)

Vinyltriethoxysilane (VTEO)          VP Si 225     190           0.90

3-chloropropyltriethoxysilane        Si 230        241           1.01
(CIPTES)

Octyftriethoxysilane (OCTEO)         VP Si 208     276           0.88

Propyltriethoxysilane (PTEO)         VP Si 203     206           0.89

Table 2--formulations for the NBR compounds

                        1A          2A

                                   VP Si
1st and 2nd stage     Si 69     [263.sub.H]

Perbunan NT3445C       100          100
Ultrasil 7000 GR        45          45
Corax N550              --          --
Si 69                  2.50         --
VP Si 263               --         2.24
VP Si 163               --          --
Si 264                  --          --
VP Si 208               --          --
Other chemicals: ZnO 3; stearic acid 1; Vulkanol
85 5; Vulkanox HS 1.5; Vulkanox MB 1.5

3rd Stage

MBTS                   1.8          1.8
Sulfur                 2.00        2.27

                           3A            4A

                          VP Si
1st and 2nd stage      [263.sub.L]    VP Si 163

Perbunan NT3445C           100           100
Ultrasil 7000 GR           45             45
Corax N550                 --             --
Si 69                      --             --
VP Si 263                 1.25            --
VP Si 163                  --            1.84
Si 264                     --             --
VP Si 208                  --             --
Other chemicals: ZnO 3; stearic acid 1; Vulkanol
85 5; Vulkanox HS 1.5; Vulkanox MB 1.5

3rd Stage

MBTS                       1.8           1.8
Sulfur                    2.40           2.27

                        5A         6A         7A

1st and 2nd stage     Si 264    VP Si 208    N550

Perbunan NT3445C        100        100        100
Ultrasil 7000 GR        45          45        --
Corax N550              --          --        45
Si 69                   --          --        --
VP Si 263               --          --        --
VP Si 163               --          --        --
Si 264                 2.47         --        --
VP Si 208               --         2.59       --
Other chemicals: ZnO 3; stearic acid 1; Vulkanol
85 5; Vulkanox HS 1.5; Vulkanox MB 1.5

3rd Stage

MBTS                    1.8        1.8        1.8
Sulfur                 2.27        2.27      2.27

Note: VP Si [263.sub.H] and VP Si [263.sub.L] refer to
VP Si 263 at equimolar and reduced level, respectively.

Table 3 for the EPM and EPDM
formulations compounds

1st stage          1B/7B       2B/8B     3B/9B
                   U7K/        U7K/      U7K/
                 VP Si 208   VP Si 225    TAC

EPM or EPDM         100         100       100
Utasil 7000 GR      50          50        50
Corax N550          --          --        --
VP Si 225           --          1.5       --
VP Si 208          1.50         --        --
Other chemicals: Zn-stearate 0.5; Carbowax 3350 2; Sunpar 2280 40

2nd stage

Peroxide (40%)       7           7         7
TAC                 --          --       0.75

1st stage        4B/10B   5B/11B    6B/12B
                  N550    N550/      N550/
                           TAC     VP Si 225

EPM or EPDM       100      100        100
Utasil 7000 GR     --       --        --
Corax N550         50       50        50
VP Si 225          --       --        1.5
VP Si 208          --       --        --
Other chemicals: Zn-stearate 0.5; Carbowax 3350 2; Sunpar 2280 40

2nd stage

Peroxide (40%)     7        7          7
TAC                --      0.75       --

Table 4--formulations for the CR compounds

                     1C         2C         3C
1st stage         VP Si 203   Si 230      VP Si
                                       [263.sub.L]

Baypren 210          100       100         100
Ultrasil7000 GR      45         45         45
N330                 --         --         --
VP Si 203            1.7        --         --
Si 230               --         2          --
VP Si 263            --         --         1.0
VP Si 163            --         --         --

Other chemicals: Stearic acid 0.5; MgO 4; Santicizer 148 15;
Sundex 790 5; Vulkanox 4010NA/LG 1; petroleum jelly 2

2nd stage

ZnO                   5         5           5
PB(ETU)-75            1         1           1

                      4C           5C        6C
1st stage            VP Si      VP Si 163   N330
                  [263.sub.H]

Baypren 210           100          100      100
Ultrasil7000 GR       45           45        --
N330                  --           --        50
VP Si 203             --           --        --
Si 230                --           --        --
VP Si 263             1.5          --        --
VP Si 163             --          0.82       --

Other chemicals: Stearic acid 0.5; MgO 4; Santicizer 148 15;
Sundex 790 5; Vulkanox 4010NA/LG 1; petroleum jelly 2

2nd stage

ZnO                    5            5        5
PB(ETU)-75             1            1        1

Note: VP Si [263.sub.H] and VP Si [263.sub.L] refer to VP
Si 263 at equimolar and reduced level, respectively.

Table 5--compound viscosity and cure characteristics for the NBR
compounds

                                            Compound

Property                        1A         2A            3A
                               Si 69      VP Si         VP Si
                                       [263.sub.H]   [263.sub.L]

ML 1+4 @ 100[degrees]C, (MU)    64                       72
[M.sub.H]-[M.sub.L], (dNm),
  160[degrees]C                27.4        77           28.9
tS1, (min.)                     1.1       27.1           0.8
t 20%, (min.)                   2.5        0.6           1.6
t 90%, (min.)                  17.7        1.2          14.5
Cure race index, (min.-1)       6.0       15.9           7.3
M. scorch, (min.)               7.5        6.5           5.2

                                    Compound

Property                          4A         5A
                               VP Si 163   Si 264

ML 1+4 @ 100[degrees]C, (MU)      78         64
[M.sub.H]-[M.sub.L], (dNm),
  160[degrees]C                  22.5       29.8
tS1, (min.)                       0.5       1.2
t 20%, (min.)                     0.9       2.2
t 90%, (min.)                    11.3       15.0
Cure race index, (min.-1)         9.3       7.3
M. scorch, (min.)                 3.0       7.5

                                   Compound

Property                          6A        7A
                               VP Si 208   N550

ML 1+4 @ 100[degrees]C, (MU)      62        58
[M.sub.H]-[M.sub.L], (dNm),
  160[degrees]C                  25.5      22.4
tS1, (min.)                       1.2      1.0
t 20%, (min.)                     2.3      1.3
t 90%, (min.)                    13.5      11.9
Cure race index, (min.-1)         8.1      9.2
M. scorch, (min.)                 8.0      5.0

Table 6--static vulcanizate properties for the NBR compounds

                                   Compound

Property                         1A      2A
                                Si 69   VP Si
                                         263

Hardness                         70
Modulus 100%, (MPa)              2.3     71
Modulus 300%, (MPa)              8.3     2.2
M 300%/M 100%                    3.6     8.4
Tensile strength, (MPa)         18.1     3.8
Elongation at break, (%)         490    18.3
Angle abrasion loss, (g/hr.)    20.2     490
Comp. set 70 h/100[degrees]C,   75.0    20.2
(%)                                     75.7
Dispersion index                 96

                                         Compound

Property                            3A            4A
                                   VP Si         VP Si
                                [263.sub.L]   [263.sub.H]

Hardness                            70            70
Modulus 100%, (MPa)                 2.2           2.6
Modulus 300%, (MPa)                 8.0          11.9
M 300%/M 100%                       3.6           4.7
Tensile strength, (MPa)            19.6          18.9
Elongation at break, (%)            510           390
Angle abrasion loss, (g/hr.)       21.9          11.0
Comp. set 70 h/100[degrees]C,      77.6          71.7
(%)
Dispersion index                    96            94

                                         Compound

Property                          5A        6A        7A
                                Si 264   VP Si 208   N550

Hardness                          72        69        69
Modulus 100%, (MPa)              2.6        1.6      3.9
Modulus 300%, (MPa)              11.0       3.7      14.3
M 300%/M 100%                    4.2        2.3      3.7
Tensile strength, (MPa)          16.0      18.3      16.1
Elongation at break, (%)         380        620      330
Angle abrasion loss, (g/hr.)     19.4      31.3      15.5
Comp. set 70 h/100[degrees]C,    70.5      66.1      51.2
(%)
Dispersion index                  93        74        99

Table 7--aging properties for the NBR compounds

Property                                     Compound

                                     1A         2A            3A
                                    Si 69      VP Si         VP Si
                                            26[3.sub.H]   26[3.sub.L]
Air Oven: 72 h. @ 125[degrees]C
Hardness change                      14                       13
Tensile change, (%)                  13         11            19
Elongation change, (%)               -43        -7            -51
                                                -47
ASTM #3 oil 70 h. @ 100[degrees]C
Hardness change                       0                        0
Tensile change, (%)                  -25         0            -34
Elongation change, (%)               -41        -14           -41

Property                                      Compound

                                     4A       5A      6A      7A
                                    VP Si   Si 264   VP Si   N550
                                     163              208
Air Oven: 72 h. @ 125[degrees]C
Hardness change                       7       11      14      9
Tensile change, (%)                  -9       -3      -23     6
Elongation change, (%)               -41     -42      -44    -39

ASTM #3 oil 7O h. @ 100[degrees]C
Hardness change                      -3       -1      -6      0
Tensile change, (%)                  -31      3       -32     -6
Elongation change, (%)               -36     -26      -23    -24

Table--8 dynamic mechanical properties for the NBR compounds

Property                                     Compound

                                     1A         2A            3A
                                    Si 69      VP Si         VP Si
                                            26[3.sub.H]   26[3.sub.L]

Zwick rebound @ 60[degrees]C, (%)   58.4                     59.0
Heat build-up, ([degrees]C)         16.1       58.8           5.6
                                               10.6
MTS (compression) @ 0[degrees]C
E', 2% DSA, (MPa)                   20.83                    23.65
E (2% DSA)-E' (8% DSA),             6.00       22.24         7.90
  (MPa)                                        7.09
Tan [delta], 2% DSA                 0.421                    0.512
                                               0.463
MTS (compression) @ 60[degrees]C
E', 2% DSA, (MPa)                   11.75                    11.14
E' (2% DSA)-E' (8% DSA),            2.06       11.01         1.91
  (MPa)                                        1.89
Tan [delta], 2% DSA                 0.149                    0.137

Property                                          Compound

                                       4A         5A        6A      7A
                                    VP Si 163   Si 264     VP Si   N550
                                                            208

Zwick rebound @ 60[degrees]C, (%)     61.4       60.2      56.5    64.8
Heat build-up, ([degrees]C)            8.9       10.6      15.6     5.0

MTS (compression) @ 0[degrees]C
E', 2% DSA, (MPa)                     22.90     25.05      22.45   22.46
E (2% DSA)-E' (8% DSA),               7.04       8.60      8.28    5.82
  (MPa)
Tan [delta], 2% DSA                   0.599     0.533      0.570   0.671

MTS (compression) @ 60[degrees]C
E', 2% DSA, (MPa)                     9.61      11.40      9.56    9.33
E' (2% DSA)-E' (8% DSA),              1.00       2.05      1.91    0.72
  (MPa)
Tan [delta], 2% DSA                   0.133     0.135      0.145   0.125

Table 9--static vulcanizate properties for the (a) EPDM and
(b) EPM compounds

(a) EPDM                                 1B          2B        3B
Property                                U7K/        U7K/      U7K/
                                     VP Si 208    VP Si 225   TAC

Hardness                                 65          70        70
Modulus 100%, (MPa)                     1.4          2.4      2.1
Modulus 200%, (MPa)                     2.3          4.9      3.9
Modulus 300%, (MPa)                     3.4          9.4      6.3
M 300%/M 100%                           2.4          3.9      2.9
Tensile strength, (MPa)                 22.9        19.5      21.2
Elongation at break, (%)                650          430      570
Tear strength, (kN/m)                   31.5        38.5      40.3
Angle abrasion loss, (g/hr.)            30.4        14.5      25.4
Comp. set 72 h./150[degrees]C, (%)      36.6        24.1      33.2
Dispersion index                         98          92        93

(b) EPM                                  7B          8B        9B
Property                                U7K/        U7K/      U7K/
                                     VP Si 208    VP Si 225   TAC

Hardness                                 55          60        61
Modulus 100%, (MPa)                     1.0          1.7      1.5
Modulus 200%, (MPa)                     1.4          3.0      2.6
Modulus 300%, (MPa)                     2.1          5.2      4.1
M 300%/M 100%                           2.1          3.1      2.7
Tensile strength, (MPa)              > 16.0 (a)     16.8      19.1
Elongation at break, (%)             > 830 (a)       600      730
Tear strength, (kN/m)                   31.5        35.0      35.0
Angle abrasion loss, (g/hr.)            32.7        19.4      25.1
Comp. set 72 h./150[degrees]C, (%)      50.3        41.4      56.4
Dispersion index                         80          77        87

(a) EPDM                              4B     5B        6B
Property                             N550   N550/     N550/
                                             TAC    VP Si 225

Hardness                              60     60        60
Modulus 100%, (MPa)                  2.4     2.5       2.2
Modulus 200%, (MPa)                  5.8     6.5       5.5
Modulus 300%, (MPa)                  9.9    11.8       9.5
M 300%/M 100%                        4.1     4.8       4.3
Tensile strength, (MPa)              19.6   17.5      18.1
Elongation at break, (%)             470     410       460
Tear strength, (kN/m)                43.8   40.3      43.8
Angle abrasion loss, (g/hr.)         18.5   16.5      17.6
Comp. set 72 h./150[degrees]C, (%)   17.8   13.2      30.7
Dispersion index                      99     99        99

(b) EPM                              10B     11B       12B
Property                             N550   N550/     N550/
                                             TAC    VP Si 225

Hardness                              50     52        49
Modulus 100%, (MPa)                  1.3     1.7       1.3
Modulus 200%, (MPa)                  2.9     4.3       3.2
Modulus 300%, (MPa)                  5.2     7.4       5.7
M 300%/M 100%                        4.0     4.5       4.3
Tensile strength, (MPa)              16.4   13.6      15.3
Elongation at break, (%)             750     490       660
Tear strength, (kN/m)                38.5   35.0      36.8
Angle abrasion loss, (g/hr.)         22.3   21.5      22.9
Comp. set 72 h./150[degrees]C, (%)   31.8   21.6      44.5
Dispersion index                      98     99        97

* Tensile specimens did not break

Table 10--dynamic mechanical properties for the EPDM compounds

EPDM                                   1B          2B        3B
Property                              U7K/        U7K/      U7K/
                                    VP Si 208   VP Si 225    TAC

Zwick rebound @ 23[degrees]C, (%)     66.8        69.0      67.6

MTS (compression) @ 0[degrees]C
E', 2% DSA, (MPa)                     13.09       12.62     14.12
E (2% DSA)-E' (8% DSA), (MPa)         2.72        2.37      2.88
Tan L, 2% DSA                         0.117       0.104     0.110

MTS (compression) @ 60[degrees]C
E', 2% DSA, (MPa)                     7.02        8.72      8.23
E (2% DSA)-E (8% DSA), (MPa)          1.05        1.06      1.19
Tan L, 2% DSA                         0.097       0.089     0.084

EPDM                                 4B      5B        6B
Property                            N550    N550/     N550/
                                             TAC    VP Si 225

Zwick rebound @ 23[degrees]C, (%)   71.4    71.6      72.0

MTS (compression) @ 0[degrees]C
E', 2% DSA, (MPa)                   9.83    10.95     11.62
E (2% DSA)-E' (8% DSA), (MPa)       1.16    1.42      1.54
Tan L, 2% DSA                       0.110   0.113     0.110

MTS (compression) @ 60[degrees]C
E', 2% DSA, (MPa)                   4.62    4.73      4.90
E (2% DSA)-E (8% DSA), (MPa)        0.36    0.35      0.37
Tan L, 2% DSA                       0.104   0.094     0.102

Table 11--compound viscosity and cure characteristics for the CR
compound

Property                                       Compound

                                       1C         2C         3C
                                    VP Si 203   Si 230      VP Si
                                                         26[3.sub.L]

ML 1+4 @ 100[degrees]C, (MU)           54         53         68

Rheometer @ 170[degrees]C
[M.sub.H]-[M.sub.L], (dNm)            22.9       24.6       26.3
tS1, (min.)                            0.3       0.3         0.3
t 90%, (min.)                         12.2       14.4       14.7
Cure rate index, (min. 1)              8.4       7.1         6.9
M. scorch @ 135[degrees]C, (min.)      5.5       5.6         3.2

Property                                         Compound

                                        4C           5C        6C
                                       VP Si      VP Si 163   N330
                                    26[3.sub.H]

ML 1+4 @ 100[degrees]C, (MU)            69           72        43

Rheometer @ 170[degrees]C
[M.sub.H]-[M.sub.L], (dNm)             25.6         23.6      18.5
tS1, (min.)                             0.4          0.4      0.7
t 90%, (min.)                          14.8         14.7      8.8
Cure rate index, (min. 1)               6.9          7.0      12.3
M. scorch @ 135[degrees]C, (min.)       3.1          2.3      3.0

Table 12--static vulcanizate properties for the CR compound

Property                                        Compound

                                        1C         2C         3C
                                     VP Si 203   Si 230      VP Si
                                                          26[3.sub.L]

Hardness                                59         60         65
Modulus 100%, (MPa)                     1.2       1.5         2.3
Modulus 200%, (MPa)                     2.1       3.3         6.0
Modulus 300%, (MPa)                     3.4       5.9        10.6
M 300%/M 100%                           2.8       3.9         4.5
Tensile strength, (MPa)                19.4       19.9       22.3
Elongation at break, (%)                850       760         580
Tear strength, (kN/m)                  43.8       56.0       56.0
Angle abrasion lass, (g/hr.)           46.0       33.0       26.6
Comp. set 70 h./100[degrees]C, (%)     40.3       43.6       34.6
Zwick rebound @ 23[degrees]C, (%)      47.4       46.9       54.0
Dispersion index                        82         78         70

Property                                          Compound

                                         4C           5C       6C
                                        VP Si      VP Si 163   N330
                                     26[3.sub.H]

Hardness                                 65           64        68
Modulus 100%, (MPa)                      2.6          2.3      4.8
Modulus 200%, (MPa)                      7.0          5.6      13.0
Modulus 300%, (MPa)                     12.3         10.0       --
M 300%/M 100%                            4.7          4.4       --
Tensile strength, (MPa)                 22.6         23.8      20.4
Elongation at break, (%)                 510          610      270
Tear strength, (kN/m)                   56.0         56.0      45.5
Angle abrasion lass, (g/hr.)            23.8         25.1      14.6
Comp. set 70 h./100[degrees]C, (%)      32.4         31.3      22.7
Zwick rebound @ 23[degrees]C, (%)       55.8         54.0      43.8
Dispersion index                         86           81        87

Table 13--performance of each silane in NBR and CR
(a excellent, b good, c fair)

NBR

Silane              Processability   Reinforcement    Dynamic
                                                     porperties

Si 69                     a                b             b
VP Si 26[3.sub.H]         c                b             b
VP Si 26[3.sub.L]         c                b             a
VP Si 163                 c                a             a
Si 264                    a                a             a
VP Si 208                 a                c             b

Silane              Processability   Reinforcement    Dynamic
                                                     porperties

CR

Si 230                    a                b             b
VP Si 26[3.sub.L]         c                a             a
VP Si 26[3.sub.H]         c                a             a
VP Si 163                 c                a             a
VP Si 203                 a                b             b

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