Low VOC Silanes for Silica Tires

Transcription

1 Low VC Silanes for Silica Tires SILANES - TIRE & RUBBER Marketing Bulletin Abstract 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 VC emission component. Momentive Performance Materials has responded by introducing novel low-vc silane coupling agents. Detailed chemistry, compound mixing and tread-compound performance data will be presented along with laboratory VC measurements. Presented at the Spring th Technical Meeting of the Rubber Division, American Chemical Society, San Antonio, TX, May 16-18, ISSN: Introduction Traditional polysulfide silanes used as coupling agents in silica filled tread compounds - e.g. bis-3-triethoxysilyl propyltetrasulfide (TESPT) and bistriethoxysilylpropyl disulfide (TESPD) lead to substantial ethanol emissions during tire manufacturing. These coupling agents have a very narrow processing window during tire compounding due to their polysulfidic nature, which can cause premature vulcanization during mixing. In order 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 towards reducing air pollutants released into the atmosphere, low VC silanes will become critically important. In 2003, Momentive Performance Materials, Silicones 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 VC (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 publication (1) in Tire Technology International 2003, Annual Review, Section 1, The Next Generation of Silane Coupling Agents for Silica/Silane-Reinforced Tread Compounds, pg by Prashant G. Joshi NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc.

2 Low VC Silanes for Silica Tires 2.0 Description of Low VC Silanes Figure 1: NXT LowV* silane and NXT Ultra-LowV* silane CH 3 S CH 2 CH 3 Si S Si CH 3 S Si CH 3 Momomer Dimer + ligomers 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 (2), and using oligomeric sulfur silane compositions (3) and nonsilane based coupling agents (4) that do not emit ethanol during compounding. ur chemical approach to reducing ethanol emissions is depicted in Figure 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-Low V 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%. With even fewer ethoxy groups, NXT Ultra-LowV silane can eliminate greater than 90% 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, due to 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-donor (5) 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. ne to two phr of additional proton-donor may be added to some low-vinyl content SBR compounds to optimize deblocking. NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 2 of 12

3 Low VC Silanes for Silica Tires 3.0 Experimental Table 1 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 Company) 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 all mixed in one-step, without any intermediate cooling. Detailed mix procedures are shown in Table 2. 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. Table 1: Typical Silica-Reinforced Tire Tread Formulation Used to Evaluate NXT LowV silane and NXT Ultra-LowV silane PHR Ingredient ssbr - (~50% vinyl content) 25 BR - (high cis butadiene rubber) 80 Silica (precipitated) variable Silquest A-1589 silane, NXT silane, NXT LowV silane or NXT Ultra-LowV silane (Momentive Performance Materials) 5.0 il - (Aromatic or non-aromatic) 2.5 Zinc xide 1.0 Stearic Acid PPD - (antioxidant/antiozonant) 1.5 Wax Final Mix Ingredients 1.4 Sulfur 1.7 CBS - (primary accelerator) 2.0 DPG - (secondary accelerator) Silquest, NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 3 of 12

4 Low VC Silanes for Silica Tires 3.0 Experimental (continued) Table 2: Mixing Procedures for Polysulfide Silanes versus NXT* Silane Two-Step Mix Procedure for Polysulfide Silanes First Banbury Pass: Cooling with water at 25 C Size B mixer at 120 rpm, full cooling t 0 min Add polymers, ram down mix (RDM) 30 seconds t 0.5 min Add 50% silica, all silane RDM 30 seconds t 1 min Add remaining silica, oil RDM 30 seconds t 1.5 min Dust down, RDM 15 seconds t 1.75 min Dust down, RDM 15 seconds t 2 min Dust down, higher speeds to 160 C t 3 min Dump, sheet off C roll mill, cool to near room temperature Mix time for first pass: 5-6 minutes Second Banbury Pass t 0 min Add compound from first pass, RDM 30 seconds t 0.5 min Add remainder of ingredients, RDM 30 seconds t 1 min Dust down, RDM to 160 C by increasing rotor speed t 1.25 min Hold temperature while mixing for eight minutes t 9.25 min Dump, sheet off C roll mill, cool to near room temperature Mix time for second pass: minutes Total non-productive mix time: minutes plus cool down time Productive Mix Add curatives on C roll mill ne-step Mix Procedure for NXT based silanes First Banbury Pass: Cooling with water at 25 C Size B mixer at 120 rpm, full cooling t 0 min Add polymers, ram down mix (RDM) 30 seconds t 0.5 min Add 50% silica, all silane RDM 30 seconds t 1 min Add remaining silica, oil RDM 30 seconds t 1.5 min Dust down, RDM 15 seconds t 1.75 min Add remainder of ingredients, RDM 30 seconds t 2.25 min Dust down, RDM to 170 C by increasing rotor speed t 2.5 min Hold temperature while mixing for eight minutes t 10.5 min Dump, sheet off C roll mill, cool to near room temperature Total non-productive mix time: minutes Productive Mix Add curatives on C roll mill Curing and testing was done according to ASTM standards. Small strain dynamic tests were carried out on a Rheometrics Dynamic Analyzer (ARES - TA Instruments). Compound Testing Standards Mooney viscosity and scorch ASTM D1646 Stress-strain properties ASTM D412 scillating disc rheometer ASTM D2084 Heat build-up ASTM D623 Curing of test plaques ASTM D3182 Dynamic Mechanical Tests Payne Effect Strain Sweeps: 0.01% - 25% shear strain amplitude -10 Hz, 60 C Characteristics G initial, G, and tan max Temperature Sweep: -40 C to + 80 C, 1% DSA. NXT is a trademark of Momentive Performance Materials Inc. Page 4 of 12

5 Low VC Silanes for Silica Tires 4.0 Processing and Performance of NXT LowV* silane and NXT Ultra-LowV* silane Ethanol Emissions Based on the ethanol emission rates shown in Table 3, NXT* silane reduces ethanol emission by up to 10%, whereas NXT LowV silane reduces it by up to 71%. These numbers compare very well with theoretical predictions. Table 3. Data on Ethanol Emission During Mixing with Silquest* A-1289, Silquest A-1589, NXT and NXT LowV Silanes Theoretical Measured % of % Reduction from Silane Loading, phr Ethanol (mg) Ethanol (mg) Theoretical Silquest A-1289 Silquest A ,911 9,910 59% Silquest A ,000 9,320 55% 6% NXT , % 10% NXT LowV 8.2 4,875 2,907 60% 71% Silquest, NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 5 of 12

6 Low VC Silanes for Silica Tires 4.0 Processing and Performance of NXT LowV* silane and NXT Ultra-LowV* silane (continued) 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 4, 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 4: Processing Characteristics with NXT silane, NXT LowV silane and NXT Ultra-LowV silane State of Silquest A-1589 NXT NXT LowV NXT Ultra-LowV Compound Silane Silane Silane Silane Non-Productive Mixing Step 1 ok Excellent Excellent Excellent Mix Temp/time 160 C, 6 min 170 C, 12 min 170 C, 12 min 170 C, 12 min Non-Productive Mixing Step 2 Mix Temp/time Productive Mixing ok Skip Skip Skip 160 C, 12 min ok Excellent Excellent Excellent NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 6 of 12

7 Low VC Silanes for Silica Tires 4.0 Processing and Performance of NXT LowV* silane and NXT Ultra-LowV* silane (continued) Table 5 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% reduction in cure times. While providing dynamic hysteresis properties similar to NXT silane, these silanes show a further improvement in heat build-up 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 tan values at low temperatures (0 to -15 C) observed in Figure 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 tan values at 60 C indicate a decrease in rolling resistance for tires made with these silanes. NXT 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, G and Shore A hardness) compared to polysulfide silanes, thus creating an opportunity to expand the performance envelope. In order 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. Table 5: Comparison of Performance Characteristics with Silquest A-1589 silane, NXT silane, NXT LowV silane and NXT Ultra-LowV silane Silquest A-1589 NXT NXT LowV NXT Ultra-LowV Ingredient (phr) silane (TESPD) silane silane silane Solution SBR Butadiene Rubber Silica Silquest A-1589 silane 6.2 NXT silane 8.2 NXT LowV silane 8.2 NXT Ultra-LowV silane 8.2 Number of Mixing Steps Mixing Temperature 160 C 170 C 170 C 170 C Silanization Time (min) Compound Properties Processing Mooney Viscosity Scorch Time (min) Cure Time t90 (min) ML (dnm) MH (dnm) Properties in the Cured State - Non-linearity 60 C G initial (MPa) G (MPa) G max (MPa) tan max Heat Build Up ( T C) Wet-Skid Indicator, 10 Hz, 1% DSA tan 0 C Properties in the Cured State - Reinforcement Hardness (Shore A) M 25% (MPa) M 100% (MPa) M 300% (MPa) M 300%/M100% Elongation at Rupture (%) Stress at Rupture (MPa) Abrasion Loss (mm 3 ) DIN Silquest, NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 7 of 12

8 Low VC Silanes for Silica Tires 4.0 Processing and Performance of NXT LowV* silane and NXT Ultra-LowV* silane (continued) Figure 2: Temperature Response of Viscoelastic Properties of Silquest* A-1589 silane, NXT* silane, NXT LowV silane and NXT Ultra-LowV silane tan delta Conclusions Silquest A-1589 silane NXT silane NXT LowV silane NXT Ultra-LowV silane Temperature ( C) 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 nonproductive 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. References 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. sterholtz, Tire Technology International 2003, Annual Review, Section 1, The next generation of silane coupling agents for silica/silane- reinforced tread compounds, pg D.J. Zanzig, G. Thielen and J. Verthe, EP A1 to The Goodyear Tire & Rubber Company, March Hans-Detlef Luginsland, A. Hasse, M. Radcziwill, R. Krafczyk, US B2 to DeGussa AG, April K.J. Pyle, L.G. Wideman, P.H. Sandstrom and R.R. Smith, to The Goodyear Tire & Rubber Company, ctober R. W. Cruse, M. Hannon, R.J. Pickwell, P. Joshi and F.D. sterholtz, to Momentive Performance Materials US Mineral filled elastomer compositions ct P.G. Joshi, A. Chaves, L. Hwang, M. Hofstetter, L. Panzer and M. Stout, W A3 Silica rubber mixtures having improved hardness Jan Silquest, NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 8 of 12

9 Low VC Silanes for Silica Tires Patent Status NXT* silane, NXT LowV* silane and NXT Ultra-LowV* silanes are the subject of one or more pending U.S. patent applications. Nothing contained herein shall be construed to imply the non existence of any relevant patents or to constitute the permission, inducement or recommendation to practice any invention covered by any patent, without authority from the owner of the patent. Product Safety, Handling and Storage Customers considering the use of this product should review the latest Material Safety Data Sheet and label for product safety information, handling instructions, personal protective equipment if necessary, and any special storage conditions required. Material Safety Data Sheets are available at or, upon request, from any Momentive Performance Materials representative. Use of other materials in conjunction with Momentive Performance Materials products may require additional precautions. Please review and follow the safety information provided by the manufacturer of such other materials. Limitations Customers must evaluate Momentive Performance Materials products and make their own determination as to fitness of use in their particular applications. NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 9 of 12

10 Low VC Silanes for Silica Tires Emergency Service Momentive Performance Materials maintains an around-the-clock emergency service for its products. The American Chemistry Council (CHEMTREC) and CareChem24 International also maintain an around-the-clock emergency service for all chemical products: Location Momentive Performance Materials Products All Chemical Products Mainland U.S., Puerto Rico CHEMTREC: Alaska, Hawaii CHEMTREC: Canada CHEMTREC: Europe (0) (UK) (Albanian, Czech, Danish, Dutch, English, Finnish, French, German, Greek, Hungarian, Italian, Lithuanian, Norwegian, Polish, Portuguese, Romanian, Russian, Serbo-Croatian, Slovak, Spanish, Swedish, Turkish, Ukrainian) Middle East, All countries, except Israel (Lebanon) Middle East, Israel (0) (UK) Latin America, Asia/Pacific, CHEMTREC: (collect) all other locations worldwide At sea Radio U.S. Coast Guard, which can directly contact Momentive Performance Materials at or CHEMTREC at D NT WAIT. Phone if in doubt. You will be referred to a specialist for advice. NXT, NXT LowV and NXT Ultra-LowV are trademarks of Momentive Performance Materials Inc. Page 10 of 12

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