Improving silane coupling to rubber.

The use of the alkoxy silanes bis (triethoxysilylpropyl)tetrasulfide (TESPT) and bis (triethoxysilylpropyl) disulfide (TESPD) to modify rubber properties when mineral fillers are compounded has been of great commercial interest for several decades (refs. 1 and 2). This field is still extremely active. In mid 2004, the U.S. patent office had the key word listing of 66 applications for TESPT and 22 for TESPD (using only the initials for the search). The work of Rauline (ref. 2), using a three-pass mix with solution polymers, has become the standard for a passenger tread with improved dynamic properties. Mixing procedures have been developed where bases such as zinc oxide are delayed during the initial silica silane reaction period (refs. 3 and 4). It is thought that the zinc components react with the silica surface, which decreases hydrophobation with the silanol. The emphasis of these works has been on the silane and silanol reaction pathways. It appears that many of the conclusions about these reactions are tempered by the presence of water, or the molar limitation of water, present during mixing (refs. 5 and 6). After the silanols have condensed on the silica surface, the sulfur portion of the silane is envisioned to be available for reactions with the vulcanization system to tie the filler to the rubber (figure 1).

[FIGURE 1 OMITTED]

Zinc chemistry (ref. 7) and the control of concentration and the structure of the fatty acids that are used is an important variable in the vulcanization process (ref. 8). Mechanisms that use expanding d orbits of the zinc ligand complexes have been used to explain this role of zinc (ref. 9). The TESPT has a polysulfide chain with S ranking of about 3.8. The TESPD has a nominal S ranking of slightly more than 2. From a vulcanization standpoint, the polysulfide should be easily incorporated into reactive coupling intermediates. It should make sulfur available for normal crosslinking as well as coupling. It is more problematical that the disulfides would incorporate easily in coupling intermediates. In the disulfide, little sulfur will become free to join the sulfur pool. In fact, interesting compounding changes are required for substitution of TESPT by TESPD (ref. 4). When combining the issues of zinc interactions with silica/silanols and vulcanization chemistry, along with the complexities of mixing, this system is extremely complicated in physics and chemistry. In this work, a NR compound with mixed fillers based on CB and silica, treated with TESPD, in a two pass mix, was treated with a zinc soap compound, ZB. This material is a proprietary blend of fatty acid derivatives. The material contains 10% zinc. The additives were all added during the first pass, which has the potential to give the most problems with non-productive interactions.

Experimental

Compound recipe

The formulations tested are shown in table 1. The variations tested include a control with no silane or added ZB. The silane was added at a 5% level of the silica (Si), and ZB was added at 2.5 and 5 phr (2.5 ZB and 5 ZB).

The N220 was obtained from Harwick Standard. The silica was from Degussa. Struktol A 86 is a peptizer, 40MS is a resin and Struktol SCA 985PL is a 50% pelletized TESPD. The other ingredients are standard rubber additives.

The compounds were mixed in a Fan'el BR 1600 lab mixer at 77 rpm with a 70% fill factor in a two pass process. In the first pass, the NR was mixed with the peptizer for 40 seconds, and the rest of the ingredients were added. The first of two sweeps was made after 90 seconds, and the total mix time was 240 seconds. The curatives were added in a second pass, which was 80 seconds.

Standard testing done on the stocks included Mooney viscosity (ASTM D 1646), cure rheometer (ASTM D 2084), tensile testing (ASTM D 412), rebound (ASTM D 2632) and dynamic testing (ASTM D 623). Cured viscoelastic testing was done in compression using a mechanical energy resolver (MER-1100B), manufactured by Instrumentors.

Discussion

The combination of the silane TESPD and the zinc ZB showed both processing and physical property synergism. The mixing curves showed that most of the processing improvements came from the addition of ZB. There was faster incorporation and significantly lower mixing torques. There was a concentration dependence on the mix (figures 2-5). Depending on mixing criteria, the data suggest that mixing times and work inputs could be significantly reduced with the zinc compound. In this work, it is not possible to determine whether the influence of the additive was strongest on one filler or the other. The shape of the mixing curves and the faster incorporation peaks imply better uniformity and dispersion with the combination of additives. The drop temperatures were high enough to obtain a decent level of silanization.

[FIGURE 2-5 OMITTED]

Secondary processing steps also were more influenced by the zinc compound. The Mooney viscosity (figure 6) showed large changes for the initial viscosity. The long-term storage viscosity showed a storage effect with an increase in ML (1+4) of over 120 for the control and the silane, while the ZB-containing stock only increased five units. Although only limited extrusion trials (cold feed) were done, the aged stock showed a 20% reduction in torque for the silane and a 25% reduction for the silane and ZB combination. A similar drop in pressure was seen, with no change in output rate, at fixed RPM. The surface and edge ratings went from B6 to A9 with the combination. These results suggest that filler flocculation is not occurring in the presence of the ZB additive, but is occurring with the silane alone (ref. 10).

[FIGURE 6 OMITTED]

Both the silane and the zinc compound influenced the scorch delay. The best (longest) scorch delay was with the combination at the high level of zinc. The cure time was also extended slightly (figure 7). Some of the reversion resistance of these compounds comes from the silane and some from the zinc chemical, but the most was obtained with the combination (table 2).

[FIGURE 7 OMITTED]

The efficiency of the crosslinking, as measured by 100% modulus, showed that silane contributed more than the zinc chemical. However, the combination had the highest state of cure (table 3). Oxidative aging showed a lower percent change for the combination (table 4). Tear resistance was maintained (figure 8).

[FIGURE 8 OMITTED]

The dynamic properties as measured by rebound (table 5), shear (table 6) and compression (table 7) all showed significant synergism between the silane and the ZB. The biggest change was seen in the long-term dynamic distortion on the flexometer, where the combination was shut off before the sample failed. All these results are consistent with each other and with better filler dispersion and coupling efficiency.

Conclusion

The combination of processing effects and physical properties of this type of NR, coupled silica compound show that significant improvements can be made by the use of zinc chemistry. Although this work does not answer whether the coupling of silica to rubber is enhanced by an increased efficiency of the silane sulfur to enter into the vulcanization mechanism, it does suggest that this occurs. It also suggests that more work is required to understand why the viscosity of the zinc enriched systems did not show significant storage hardening effects.

Table 1--formulations

                                                    2.5ZB,   5.0ZB,
Ingredient          -       Si    2.5ZB    5.0ZB        Si       Si

SMR 5              100      100      100      100      100      100
A 86               .25      .25      .25      .25      .25      .25
N220                40       40       40       40       40       40
Silica VN3          20       20       20       20       20       20
Stearic acid       2.5      2.5      2.5      2.5      2.5      2.5
ZnO                  4        4        4        4        4        4
6PPD               2.5      2.5      2.5      2.5      2.5      2.5
TMQ                  1        1        1        1        1        1
Struktol 40MS        6        6        6        6        6        6
SCA985PL             0        2        0        0        2        2
ZB47                 0        0      2.5        5      2.5        5
2nd pass
TBBS               1.5      1.5      1.5      1.5      1.5      1.5
Sulfur             1.5      1.5      1.5      1.5      1.5      1.5
Total           179.25   181.25   181.75   184.25   183.75   186.25

Table 2--reversion

Compound      T-2 reversion

Control       13.29
Si            13.71
2.5ZB         15.29
5.0ZB         18.00
2.5ZB, Si     18.00
5.0ZB, Si     25.92

Table 3--modulus data

Compound    100% modulus   300% modulus   300% modulus
               (MPa)          (MPa)       100% modulus

Control         1.6            8.1            5.1
Si              2.0            9.8            4.9
2.5ZB           1.8            8.3            4.6
5.0ZB           1.7            7.6            4.5
2.5ZB, Si       2.1            9.8            4.7
5.0ZB, Si       2.3            10.2           4.4

Table 4--aged 70 hours @ 100[degrees]C

Compound      % change       % change
            100% modulus   300% modulus

Control         106             69
Si               90             55
2.5ZB           117             82
5.0ZB           100            72.3
2.5ZB, Si        90            54.1
5.0ZB, Si       78.2            47

Table 5--rebound

Compound    0[degrees]C   Room temp.   100[degrees]C

Control         18            35            50
Si              16            34            53
2.5ZB           16            33            51
5.0ZB           15            33            52
2.5ZB, Si       17            34            54
5.0ZB, Si       15            32            55

Table 6--Firestone flexometer heat build-up and blowout

250 lb. weight; .325" throw; heat build-up 45 min.
Blowout run until machine proximity switch activates.

            Heat build-up
Compound    (C[degrees])    Blow-out (min.)

Control          164              65
Si               129              680
2.5ZB            141              230
5.0ZB            139              882
2.5ZB, Si        133            >6,180
5.0ZB, Si        127            >8,631

Table 7--tan delta

            Room   temp.          100[degrees]C
Compound    1 Hz   10 Hz   1 Hz   10 Hz

Control     .163   .165    .131   .134
Si          .135   .132    .112   .113
2.5ZB       .167   .164    .152   .143
5.0ZB       .149   .147    .119   .121
2.5ZB, Si   .145   .147    .102   .108
5.0ZB, Si   .131   .141    .095   .103

References

1. U.S. Patent 3,873,489, F. Thurn, et.al. (Degussa).

2. U.S. Patent 5,227,425, R. Rauline (Michelin).

3. L. Reuvekamp, S. Debnath, J. Ten Brinke, P. Van Swaaij and J. Noordermeer, Rubber Chem. and Tech., vol. 77-1, 34, 2004.

4. C. Stone, K. Meriting and M. Hensel, paper 59, Rubber Division meeting, Oct. 2000.

(5.) K.J. Kim and J. Vander Kooi, J. Appl. Polym. Sci. (to be published).

(6.) K.J. Kim and J, Vander Kooi, paper 78, Rubber Division meeting, Oct. 2003.

(7.) G. Heideman, J. Noordermeer, R. Datta and B. Van Baarle, Rubber Chem. and Tech., vol. 77-3, 512, 2004.

(8.) U.S. Patent 5,302,315, H. Umland, (Schill & Seilacher).

(9.) J. Vanderkooi, J. Sherritt, H. Umland and M. Hensel, paper 121, Rubber Division meeting, Oct. 1993.

(10.) A. Hasse, A. Wehmeier and H-D. Luginsland, Rubber World, 230, 1, 22, April 2004.