Low VOC silanes reduce ethanol emissions
|Date Published||September 5, 2005|
Traditional polysulfide silanes used as coupling agents in silica-filled tread compounds-such as bis-3-triethoxysilyl propyltetrasulfide (TESPT) and bis-triethoxysilylpropyl disulfide (TESPD)-lead to substantial ethanol emissions during tire manufacturing. These coupling agents have a very narrow processing window during tire compounding because of their polysulfidic nature, which can cause premature vulcanization during mixing.
To avoid scorch, ensure complete reaction of silane with silica, and efficiently remove the ethanol generated during mixing, these silanes typically need to be mixed in multiple non-productive steps. In addition, polysulfide silanes slow down the cure kinetics of tread compounds to undesirably long cure times. As the tire industry moves toward reducing air pollutants released into the atmosphere, low VOC silanes will become critically important.
In 2003, GE introduced NXT silane. This coupling agent helps eliminate the need to mix silica rubber compounds in multiple non-productive mix steps. It improves the vulcanization kinetics, reduces ethanol emissions and leads to an improved balance between rolling resistance and wet traction when compared to polysulfide silanes.
In this paper, we introduce product line extensions of NXT silane: NXT LowV silane and NXT Ultra-LowV silane. These silanes help eliminate the need to mix silica compounds in multiple steps and provide significant reductions in total ethanol evolved during tire manufacturing. They also lead to faster cure rates and improve the overall performance of the compound. Data are presented on VOC (ethanol) emissions with NXT LowV silane and NXT Ultra-LowV silane, and their processing characteristics during compounding and performance in the fully cured tread compounds. For detailed information on NXT silane, reference is made to the publication1 in Tire Technology International 2003, Annual Review, Section 1, ``The next generation of silane coupling agents for silica/silane-reinforced tread compounds,'' pages 44-51.
Description of low VOC silanes
Several approaches to solve the ethanol emissions issue have been attempted in the recent past. These approaches include modifying elastomers with pendant internal silanol groups,˛ and using oligomeric sulfur silane compositionsł and non-silane based coupling agents4 that do not emit ethanol during compounding. Our chemical approach to reducing ethanol emissions is depicted in Fig. 1. This figure shows the idealized structures of NXT LowV (monomer) and NXT Ultra-LowV (dimer) silanes. In practice, these silanes are mixtures of monomers, dimers and higher oligomers.
The monomer content is relatively high in NXT LowV silane, where in the case of NXT Ultra-LowV silane, the dimer and oligomer contents are relatively high. Both these silanes are blocked mercaptosilanes (thiocarboxylate functional), with an octanoyl group blocking the mercapto part of the molecule. The silicon end of the monomer bears a cyclic di-alkoxy group and an ethoxy group. The dimer consists of two monomer species that are bridged together with a di-alkoxy group. The di-alkoxy and ethoxy groups are released during mixing and facilitate rapid silica-silane coupling.
The diol resulting from silane hydrolysis is not volatile and remains in the compound. Due to its polar nature, the diol may tend to hydrogen bond to the silanol groups on the silica surface. Based on its chemistry and composition, NXT LowV silane reduces the ethanol evolved during tire manufacturing by more than 66 percent. With even fewer ethoxy groups, NXT Ultra-LowV silane can eliminate greater than 90 percent of the ethanol evolved during tire manufacturing.
NXT LowV silane and NXT Ultra-LowV silane offer the same easy high-temperature, single-pass mixing available with NXT silane, along with lower ethanol emission during compounding and manufacturing operations. They facilitate high-temperature (170 to 180°C) mixing without causing viscosity increase or premature vulcanization. As with NXT silane, because of the hydrophobic nature of the octanoyl group, these silanes continue to provide superior silica dispersion with similar coupling efficiencies of silica to rubber as that of polysulfide silanes. NXT LowV silane and NXT Ultra-LowV silanes also minimize re-agglomeration of silica aggregates and improve the shelf stability of the green compound. After the tire is assembled, NXT LowV and NXT Ultra-LowV silanes react with the polymer during vulcanization.
The octanoyl-blocking group is removed by cure-heat in the presence of a proton-donor5 that is usually a part of the vulcanization ingredients, to yield the mercaptosilane that rapidly bonds with the rubber. NXT LowV and NXT Ultra-LowV silane compounds show faster cure rates when compared to NXT silane and polysulfide silane compounds. One to two phr of additional proton-donor may be added to some low-vinyl content SBR compounds to optimize deblocking.
Table I shows typical material formulation used to mix rubber compounds containing various silanes. Mixing was carried out in a 1.6 liter ``B'' type Banbury (Farrel Co.) with tangential rotors. Silquest A-1289 (TESPT) silane and Silquest A-1589 (TESPD) silane were chosen as controls. For processing and compound performance evaluation, mixing with control silane Silquest A-1589 was carried out in two non-productive mix steps, which are separated by a cooling step. The NXT, NXT LowV and NXT Ultra-LowV silane compounds were mixed in one-step, without any intermediate cooling. Detailed mix procedures are shown in Table II. For the ethanol emissions testing, only the non-productive mix ingredients in the formulation were mixed in a single non-productive mix step for 12 minutes at 160°C with Silquest A-1289 silane and Silquest A-1589 silane and at 170°C with NXT silane and NXT LowV silane. Vapors emitted during compounding were collected from the vent lines onto charcoal absorption tubes and analyzed for ethanol.
Processing and performance of NXT LowV silane and NXT Ultra-LowV silanes
Based on the ethanol emission rates shown in Table III, NXT silane reduces ethanol emission by up to 10 percent, whereas NXT LowV silane reduces it by up to 71 percent. These numbers compare very well with theoretical predictions.
Processing and performance
The control compound with Silquest A-1589 silane was mixed at 160°C in two non-productive mix steps, whereas compounds containing NXT, NXT LowV and NXT Ultra-LowV silanes were mixed at 170°C in only one non-productive mix step. As shown in Table IV, both NXT LowV silane and NXT Ultra-LowV silane show excellent processing characteristics similar to those of NXT silane, even when mixed at 170°C in one non-productive step. The total mixing time with NXT silane and new NXT LowV silane and NXT Ultra-LowV silane was 12 minutes with one mixing step, whereas the total mixing time with Silquest A-1589 silane was 18 minutes with two non-productive steps.
Table V shows a comparison of performance characteristics with rubber compounds made with Silquest A-1589, NXT, NXT LowV and NXT Ultra-LowV silanes. Based on the data, NXT LowV silane and NXT Ultra-LowV silane show similar reductions in Mooney viscosity as NXT silane, with about 40 percent reduction in cure times. While providing dynamic hysteresis properties similar to NXT silane, these silanes show a further improvement in heat buildup of compounds. The reinforcement indices of NXT LowV silane and NXT Ultra-LowV silane are similar to those of NXT silane and Silquest A-1589 silane. The higher tand values at low temperatures (0 to -15°C) observed in Fig. 2 indicate a strong potential to improve wet traction properties of tire treads made with NXT LowV silane and NXT Ultra-LowV silane. The low tand values at 60°C indicate a decrease in rolling resistance for tires made with these silanes. XT LowV silane or NXT Ultra-LowV silane lead to higher static and dynamic hardness in the rubber compound compared to NXT silane.
The large improvement in silica dispersion with NXT silane leads to a decrease in filler-filler interactions (lowered G', DG' and Shore A hardness) compared to polysulfide silanes, thus creating an opportunity to expand the performance envelope. To take full advantage of improved silica dispersion, certain reformulations to the compounds were recommended 1,6 (viz. bound rubber reformation, thixotropic or hydrodynamic routes) with adjustments targeted toward equal hardness/modulus. These reformulations include either addition of precipitated silica, fumed silica or carbon black to increase the hardness of NXT silane compounds. Since NXT LowV silane and NXT Ultra-LowV silane do not show as great a decrease in hardness, only minor reformulations, if any, of the type described above may be needed.
NXT LowV and NXT Ultra-LowV silanes may provide substantial ethanol emissions reduction during tire mixing processes. In addition, they lead to improved vulcanization kinetics with faster curing of tread compounds. These silanes need only one non-productive mix step, reduce Mooney viscosities and provide a broader processing window compared to polysulfide silanes. Rubber compounds containing NXT LowV and NXT Ultra-LowV silane predict improved hysteresis of a tire tread under rolling conditions, and offer a strong potential to improve wet and snow skid characteristics of tire tread compounds.
1. P.G. Joshi, R.W. Cruse, R.J. Pickwell, K.J. Weller, W.E. Sloan, M. Hofstetter, E.R. Pohl, M.F. Stout and F.D. Osterholtz, Tire Technology International 2003, Annual Review, Section 1, ``The next generation of silane coupling agents for silica/ silane-reinforced tread compounds,'' pg. 44-51.
2. D.J. Zanzig, G. Thielen and J. Verthe, EP1400559A1 to Goodyear Tire & Rubber Co., March 2004.
3. Hans-Detlef Luginsland, A. Hasse, M. Radcziwill, R. Krafczyk, U.S. 6727339 B2 to Degussa A.G., April 2004.
4. K.J. Pyle, L.G. Wideman, P.H. Sandstrom and R.R. Smith, to Goodyear Tire & Rubber Co., October 2002.
5. R.W. Cruse, M. Hannon, R.J. Pickwell, P. Joshi and F.D. Osterholtz, to GE U.S. 6635700, ``Mineral filled elastomer compositions,'' October 2004.
6. P.G. Joshi, A. Chaves, L. Hwang, M. Hofstetter, L. Panzer and M. Stout, WO 04005395A3, ``Silica rubber mixtures having improved hardness,'' January 2004.
Presented at the spring 2005 ACS Rubber Division meeting, held May 16-18 in San Antonio.
Technical Notebook Edited by Harold Herzlich
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Ethanol emissions have become an environmental concern related to tires and tire manufacturing. Ethanol generated from silanes used as coupling agents in silica-reinforced tire components can be a significant VOC emission component. GE has responded by introducing novel low-VOC silane coupling agents. Detailed chemistry, compound mixing and tread-compound performance data will be presented along with laboratory VOC measurements.