Cost-effective modifiers for the preparation of BIIR based tire tread formulations.
[FIGURE 1 OMITTED]
For several decades, it has been well known that the incorporation of halobutyl elastomers (CIIR or BIIR) into tire tread formulations results in a significant improvement in wet traction. However, as a result of the low levels of unsaturation present in these materials (ca. 1.5 mol % unsaturation in HIIR; c.f. ca. 50 mol % in BR), the level of van der Waals interaction with CB is minimal. Consequently, the abrasion resistance of tread compounds utilizing HIIR is poor. This, in turn, precluded the use of HIIR in any practical tread application. The intrinsic interaction of HIIR with siliceous fillers is also poor. Specifically, the low energy, hydrophobic, HIIR elastomer is incompatible with the high energy, hydrophilic, silica surface.
Building on the seminal results of Degussa and Michelin, several groups have shown that it is possible to compatabilize HIIR type elastomers and silica by using the same small molecule amphiphiles (e.g., TESPT) currently used in conventional silica tread technology (refs. 3-7). In the case of BIIR, the oligosulfido moiety present in TESPT reacts quite readily with the allylic bromides of BIIR (figure 2). A hydrolysis reaction between the silylether groups of the TESPT fragment and the silica surface silanol groups results in the formation of a stable siloxane linkage. Overall, these coupling reactions result in the formation of a chemical link between the elastomer and filler surface (figure 3).
[FIGURES 2-3 OMITTED]
In essence, TESPT is acting as a small molecule amphiphile that can effectively temper the surface energy differences that exist between BIIR and silica. In addition, this species possesses surface specific functional groups which, when reacted, result in the formation of a covalent linkage between these surfaces. With this compatibilization model in mind, we have recently identified aminoalcohols as effective
compatibilizers for BIIR/silica compounds.
Aminoalcohols are an extremely versatile class of compounds which find application in coatings, emulsion stabilization, water treatment and chemical synthesis (ref. 8). These compounds are marked by the presence of at least one neutral amino group and one hydroxyl group. As it is well known that BIIR reacts readily with neutral amines, it would be expected that the treatment of BIIR with an aminoalcohol would result in the corresponding quaternization reaction (ref. 9). The now modified BIIR possesses a hydroxyl group bound to the main chain which should, in principle, be able to hydrogen bond with the polar silica surface (figure 4) (ref. 10). It is also important to note that intermittent coupling of the BIIR and silica could occur as a result of the base catalyzed condensation of the aminoalcohol hydroxyl groups and the silica surface silanol units.
[FIGURE 4 OMITTED]
If the level of interaction is sufficient, it should be possible to use these species to effectively compatabilize BIIR and silica and ultimately develop silica tread formulations containing BIIR. Importantly, this compatibilization method does not result in alcohol evolution (c.f. figure 3). The following sections detail our studies into the use of aminoalcohols as compatibilizers for silica in BIIR compounds.
All of the polymers, fillers and standard compounding ingredients which were used are commercially available materials. N,N-dimethylaminoethanol (2) and hexamethyldisilazane (5) were obtained from Aldrich Chemical and used as received. Brominated 2,2,4,8,8-pentamethyl-4-nonene (3) was prepared as described previously (ref. 11).
Mixing was done on an internal mixer, 1.5 L or 1.6 L, in conjunction with either a 6" x 12" or 10" x 20" mill. Dynamic properties were determined with the use of a GABO Eplexor equipped with an automated sampling system. Test strips wine cut from a 2 mm macro sheet that was cured for tc90+5 minutes at 160[degrees]C. Measurements were taken from -100[degrees]C to 100[degrees]C at a frequency of 10 Hz, a static deformation of 3% and a dynamic deformation of 0.1%. Stress-strain measurements were determined at 23[degrees]C according to ASTM 412 Method A. Samples for stress-strain measurements were cut from a 2 mm macro sheet, cured for tc90+5 minutes, using Die C. Hardness values were determined using an A scale durometer according to ASTM 2240. Compound abrasion resistance was measured with the use of a DIN abrader equipped with 60-grit emery paper according to DIN 53516. Sample buttons were cured for tc90+10 min. at 160[degrees]C. Compound Mooney scorch was measured at 135[degrees]C according to ASTM 1646. Cure characteristics were determined with the use of a moving die rheometer according to ASTM 5289. Cure profiles were determined at 160[degrees]C at a frequency of 1.7 Hz and a deformation of 1[degrees]. Payne effect measurements were made with a rubber process analyzer. Measurements were conducted at 100[degrees]C, a frequency of 30 cpm and at 0.5, 1, 2, 5, 10, 20, 50 and 90[degrees] strains. NMR spectra were recorded with a spectrometer (400.13 MHz [sup.1]H) in CD[Cl.sub.3] with chemical shifts referenced to tetramethylsilane.
Synthesis and isolation of (E/Z)-N,N-dimelhyl-N-(2-hvdroxyethyl)-6,6-dimethyl-2-(2,2-dimethylpropyl) hept-2-enylammonium bromide, 4
A solution of M3 (0.0914g, 0.332 mmol), M2 (0.031g, 0.348 mmol) and dodecane (0.2 mL) was heated at 100[degrees]C for 4 h yielding a brown solid. The reaction mixture was filtered, washed with hexanes, and the solid residue dissolved in [Et.sub.2]O and washed with distilled water (2 x 10 mL), saturated KHC[O.sub.3] (2 x 10 mL) and saturated NaCl (2 x 10 mL). The organic phase was isolated and the mixture was concentrated in vacuo. Volatile components were removed by Kugelrohr distillation (P = 0.6 torr, T = 80[degrees]C) to give the residue. High-resolution MS analysis; required for [C.sub.18][H.sub.38]O[N.sup.+] m/e 284.2953, found m/e 284.2953. [sup.1H] NMR (CD[Cl.sub.3]): [delta] 0.8-2.2 (m, 25.88H, 2 x -C[(C[H.sub.3]).sub.3], 3 x -C[H.sub.2]-), 3.26 (s, 2.45H, -C[H.sub.3]), 3.28 (s, 3.62H, -C[H.sub.3]), 3.67 (t, 0.48H, -N-C[H.sub.2]), 3.76 (t, 1.61 H, -N-C[H.sub.2]), 4.03 (s, 0.73H, = C-C[H.sub.2]), 4.05 (s, 1.27H, = C-C[H.sub.2]), 4.19 (m, 2.11H, C[H.sub.2]-OH), 5.77 (m, 0.97H, -OH), 5.89 (t, 0.78H, = C-H), 6.00 (t, 0.21 H, = C-H).
Preparation of surface functionalized silica
To 442 g of HiSil 233 suspended in 3,000 mL of hexane was added 24.9 mL of 2 and 14.0 mL of 5. The resulting mixture was agitated at RT for a period of 8 h. The surface modified silica was separated by filtration and dried at 60[degrees]C to a constant weight.
Results and discussion
Our initial studies were centered around the basal aminoalcohol, ethanolamine, M1, and its effect on the physical properties of simple BIIR-silica compounds. The first series of compounds studied (A through C) were prepared according to the recipe and mixing sequence given in table 1.
As can be seen from the data presented in table 2, the introduction of M1 into a simple BIIR-silica formulation results in a dramatic improvement in the degree of filler reinforcement. Specifically, the M300/M100 value for B is significantly higher than that observed for A, while the DIN abrasion volume loss measured for B is considerably lower than that measured for the control compound. Measurement of the strain dependence of the complex modulus, [G.sup.*], (Payne effect) suggests that M1 is effectively compatibilizing the BIIR and silica surfaces (figure 5) (refs. 12 and 13). However, as M1 contains a 1[degrees] (primary) amine center, the scorch safety of this compound (and therefore processability) was quite poor.
[FIGURE 5 OMITTED]
It is well known that BIIR compounds can be cured in the presence of amines. Recent work aimed at elucidating this mechanism has shown that the crosslinking process involves an initial N-alkylation reaction, followed by a deprotonation step. The resulting, neutral, amine substituted BIIR can then undergo an additional N-alkylation reaction to form a covalent crosslink (figure 6) (ref. 9).
[FIGURE 6 OMITTED]
As a result of the required deprotonation step, amine crosslinking can only occur with either a 1[degrees] (primary) or 2[degrees] (secondary) amine. For this reason, the methylated 3[degrees] (tertiary) analogue of M1, N,N-dimethylaminoethanol M2, was investigated as an alternative silica modifier. While compounds based on M2 possessed improved physical properties and increased levels of filler dispersion, the use of M2 resulted in no improvement in scorch safety.
Our results with M2 suggest that there is an additional crosslinking mechanism at work in these compounds. As a first step in establishing this mechanism, it is necessary to establish the role of both the amine groups and hydroxyl groups of M1 and M2 in their reaction with BIIR. To this end, the reaction between brominated 2,2,4,8,8-pentamethyl-4-nonene (BPMN, M3) and M2 was studied in solution (figure 7). The results of this study indicate that alkylation occurs exclusively at the nitrogen center to give the corresponding substitution product M4 (figure 9). Since there is no evidence for allylic ether formation, we can rule out the participation of the hydroxyl functionality (at the allylic bromide) in the crosslinking reaction.
[FIGURES 7 & 9 OMITTED]
The incorporation of silica into a rubber formulation results in the introduction of acidic Si-OH groups which can potentially affect the cure chemistry. While standard sulfur-accelerator cure systems are accelerated by basic species, the ZnO component of the BIIR cure is acid catalyzed (refs. 1, 14 and 15). Therefore, the presence of silica (i.e., SiOH groups) would be expected to increase the overall cure reactivity. For this reason, the effect of hexamethyldisilazane M5, a well known silylating reagent, on the scorch safety of BIIR-silica formulations was studied.
The silyl transfer reaction occurring between silica and M5 results in the substitution of polar silanol functionalities with non-polar trimethylsiloxane groups (figure 8) (refs. 10 and 16). This results in a surface modified silica which should be easier to disperse within BIIR and not accelerate the cure chemistry.
[FIGURE 8 OMITTED]
Compounds utilizing both M2 and M5 (see table 1) possessed comparable physical properties to those prepared with M2 alone. As anticipated, the introduction of M5 into the formulation resulted in a marked improvement in the scorch safety (table 2). It is important to note that the silylation reaction results in the formation of ammonia gas. As the reaction between M5 and the silica surface results in formation of free N[H.sup.3], one would expect to observe rapid BIIR crosslinking. However, since N[H.sub.3] is produced at the silica surface, it is reasonable to assume that it would immediately react with surface silanol groups to yield the corresponding ammonium silicate. The lack of free ammonia was subsequently verified through head space analysis.
Based on our initial work, there appears to be a secondary crosslinking reaction which is occurring in the presence of M2. We have been able to show that this reaction(s) proceeds even in the absence of silica. While work aimed at elucidating the nature of this crosslinking reaction is ongoing, the delayed release of M2 into the elastomer matrix and the resulting effect on compound scorch satiety were studied. To this end, a surface functionalized silica was prepared which contained levels of adsorbed M2 and M5 (compound E) which were comparable to those present in compound D. As can be seen from the data presented in table 2, the use of surface functionalized silica resulted in a further improvement in the t03 scorch time without compromising the other physical properties.
As a first step towards the development of a practical tread compound utilizing the above described technology, blends of BIIR silica (compound D without curatives) and BR-CB (compound F, table 3) masterbatches (MB) were investigated. Various compounds were prepared by taking the appropriate amount of each masterbatch and blending them on a 6" x 12" mill for 10 minutes at 100[degrees]C. The curatives were then added on a room temperature mill (table 4).
Analysis of the resulting formulations revealed a negative influence of BIIR-MB content on compound abrasion resistance. Conversely, an increase in BIIR-MB content resulted in an increase in the tan [delta] (0[degrees]C) value. If the value of tan [delta] (0[degrees]C) is taken to be an indication of wet traction performance, the results of this study can be taken as a confirmation of the positive influence of BIIR content on wet traction. In order to identify a BIIR/BR blend ratio for further optimization, a comparison of these compounds with two standard tread formulations (reinforced with either CB or silica) was made (compounds N and O, tables 5 and 6). On the basis of DIN abrasion volume loss, the 1:1 BIIR:BR blend compound was identified as the most promising candidate. Following a series of optimization studies, the final BIIR/BR formulation can be found as compound P. Analysis of compound P has shown this material to possess an acceptable hardness level (ca. 65 pts.), comparable (c.f. N and O) physical reinforcement and improved abrasion resistance. Importantly, the excellent physical properties did not come at the expense of the amiable dynamic properties inherent to BIIR. The temperature dependence of the loss factor was significantly broader than that observed for either witness compound, as shown in figure 10. The broader mechanical glass transition, along with the higher tan [delta] (0[degrees]C) value suggests that compound P will possess superior traction behavior than either of the control formulations (figure 10).
[FIGURE 10 OMITTED]
During formulation development, attention must be given to the cost implications associated with the employment of a new technology. With the results presented above, we have shown that it is possible to improve wet traction (predicted) performance by incorporating a significant amount of BIIR into a tread formulation. Importantly, this can be done without adversely affecting the physical properties (as determined in the laboratory) of the resulting compound. The incorporation of BIIR into a tread formulation would be expected to increase the compound cost. However, it is important to note that the modifiers used for the BIIR-based technology are far less expensive than those used in conventional silica tread technology (e.g., TESPT). With these factors taken into account, the cost per kg. (for elastomers, fillers and modifiers only) for each of the witness compounds and test compound P was determined using recent, publicly available, component list prices (table 7).
As can be seen from the data presented in table 7, the use of the inexpensive silica modifiers M2 and M5 allows the preparation of cost competitive BIIR tread formulations. Importantly, the cost of the BIIR test compound is significantly less expensive than that determined for the conventional silica tread formulation.
By using an affordable silica modifier system based on N,Ndimethylaminoethanol and hexamethyldisilazane, we have been able to significantly improve the interaction between BIIR and silica. This newly developed technology has been successfully applied in the preparation of a BIIR/BR-based tread compound. When compared to standard tread formulations, this compound was found to possess comparable physical properties and improved dynamic behavior. Specifically, the broad mechanical glass transition centered around -20[degrees]C was found to be more pronounced for test compound P than for either witness formulation. Based on this, the BII/BR compound would be expected to possess superior wet traction behavior. Importantly, this performance enhancement does not incur a cost penalty. In fact, the price per kg of the BIIR/BR compound was determined to be (based on current list pricing) less than that calculated for the silica-based witness compound.
Table 1--recipes and mixing sequence for compounds A-E Compound Tag A B C D E * BB2030 1a 100 100 100 100 100 HiSil 233 1b 60 60 60 60 60 1 1c 2.2 2 1c 3.2 3.2 3.2 * 5 1c 2.9 2.9 * MgO 1d 1 1 1 1 1 Sulfur 2a 0.5 0.5 0.5 0.5 0.5 Stearic acid 2a 1.0 1.0 1.0 1.0 1.0 ZnO 2a 1.5 1.5 1.5 1.5 1.5 Mixing sequence (1.5 L. internal mixer) t = 0 min. Add 1a, 1/2 of 1b and 1/2 of 1c t = 1 min. Add 1/4 of 1b t = 2 min. Add 1/4 of 1b and 1/2 of 1c and 1d t = 4 min. Sweep t = 6 min. Dump * Compounds 2 and 5 were pre-adsorbed into silica surface. Table 2--phycical properties for compounds A-E Compound A B C D E Hardness A2 Inst. (pts.) 77 72 65 54 58 Ultimate tensile (MPa) 13.9 17.5 21.5 18.3 18.4 Ultimate elongation (%) 736 343 353 585 498 25% modulus (MPa) 1.95 1.83 1.29 0.73 1.08 50% modulus (MPa) 1.93 2.31 1.85 1.05 1.33 100% modulus (MPa) 1.98 3.67 3.65 1.73 2.16 200% modulus (MPa) 3.13 8.24 10.50 4.42 5.69 300% modulus (MPa) 4.98 14.50 17.92 8.21 10.55 M300/M100 2.52 3.95 4.91 4.75 4.88 DIN abrasion loss 450 255 180 161 182 (m[m.sup.3]) Mooney scorch, 103 11.1 1.4 1.4 4.9 10.8 @ 135[degrees]C (min.) Table 3--BR-CB masterbatch recipe Compound Tag F Taktene 1203 1a 100 CB N234 1b 60 CalSol 8240 1b 15 Sunolite 160 prills 1b 1.5 Stearic acid 1b 2 Zinc oxide 1b 3 TMQ 1c 1 6PPD 1c 1 Mixing sequence (1.5 L internal mixer) t = 0 minute Add 1a t = 1 minute Add 1b t = 2 minute Add 1c t = 4 minute Sweep t = 6 minute Dump Table 4--preparation and physical properties of BIIR-RR blends Compound G H I J F (g) 400 320 240 200 D (g) 0 80 160 200 Curatives Sulfur (g) 3.27 2.86 2.45 2.25 Vulkacit NZ (TBBS) (g) 1.96 1.57 1.18 0.98 Stearic acid (g) 0.00 0.49 0.98 1.23 ZnO (g) 0.00 0.74 1.47 1.84 Physical properties Hardness A2 inst. (pts.) 55 48 50 51 Ultimate tensile (MPa) 18.1 13.9 14.0 16.9 Ultimate elongation (%) 614 667 617 587 25% modulus (MPa) 0.72 0.54 0.57 0.59 50% modulus (MPa) 0.98 0.73 0.80 0.86 100% modulus (MPa) 1.41 1.00 1.18 1.37 200% modulus (MPa) 3.24 2.13 2.67 3.55 300% modulus (MPa) 6.83 4.39 5.45 7.2 DIN abrasion loss ([mm.sup.3]) 53 87 88 114 Mooney scorch, t03 @ 135[degrees]C (min.) 21.7 17.1 8.9 6.4 Tan [delta] (0[degrees]C) 0.26 0.37 0.47 0.54 Compound K L M F (g) 16 80 0 D (g) 240 320 400 Curatives Sulfur (g) 2.04 1.63 1.23 Vulkacit NZ (TBBS) (g) 0.78 0.39 0.00 Stearic acid (g) 1.47 1.96 2.45 ZnO (g) 2.21 2.94 3.68 Physical properties Hardness A2 inst. (pts.) 50 52 53 Ultimate tensile (MPa) 14.8 17.5 22.8 Ultimate elongation (%) 527 414 478 25% modulus (MPa) 0.60 0.63 0.65 50% modulus (MPa) 0.91 0.99 0.99 100% modulus (MPa) 1.59 1.86 1.75 200% modulus (MPa) 4.55 5.61 5.7 300% modulus (MPa) 9.14 11.52 12.88 DIN abrasion loss ([mm.sup.3]) 116 237 210 Mooney scorch, t03 @ 135[degrees]C (min.) 6.4 5.4 4.5 Tan [delta] (0[degrees]C) 0.59 0.61 0.81 Table 5--preparation of witness compounds N and O and BIIR/BR tread compound P Compound Tag N O P BB2030 1a 70 70 50 Buna VSL 5025-O 1a 30 30 Taktene 1203 1a 80 50 HiSil 233 1b 6.4 30 Silane TESPT (Si69) 1b 2 1b 1.4 5 1b 0.73 CB N234 1c 80 50 Stearic Acid 1d 1 Calsol 8240 1d 7.5 Sundex 790 1d 9 9 Sunolite 160 prills id 1.5 1.5 0.75 Stearic acid 1e 1 1 (6PPD) 1e 1 1 0.5 (TMQ) 1e 1 1 0.5 Zinc oxide 1e 2.5 2.5 Sulfur 2a 1.4 1.4 1 (CBS) 2a 1.7 1.7 1 (DPG) 2a 2.0 Zinc oxide 2a 2 Mixing sequence (1.6 L internal mixer) For N and O For P t = 0.0 min. Add1a t = 0.0 min. Add 1a t = 1.0 min. Add 1/2 of 1b + 1/2 of 1c t = 0.5 min. Add 1b t = 2.0 min. Add 1/2 of 1b + 1/2 of 1c t = 2.0 min. Add 1c t = 3.0 min. Add 1d t = 3.0 min. Sweep t = 5.0 min. Add 1e t = 3.5 min. Add 1d + le t = 6.0 min. Dump t = 5.0 min. Sweep t = 6.0 min. Dump Add 2a on RT 10" x 20" mill Table 6--properties of witness compounds N and O and BIIR/BR tread compound P Compound N O P Hardness A2 Inst. (pts.) 76 80 65 Ultimate tensile (MPa) 16.3 12.4 14.1 Ultimate elongation (%) 301 212 423 25% modulus (MPa) 1.82 2.08 121 50% modulus (MPa) 2.53 2.96 1.72 100% modulus (MPa) 4.31 5.20 2.94 200% modulus (MPa) 10.6 11.6 6.16 300% modulus (MPa) 16.2 10.19 DIN abrasion loss ([mm.sup.3]) 140 140 118 Mooney scorch, t03 12.3 23.1 7.0 @ 135[degrees]C (min.) Table 7--price comparison for compounds N-P Cost Cost N O P U.S./lb. U.S./kg BB2030 1.35 2.98 50 Buna VSL 5025-0 0.82 1.81 70 70 Taktene 1203 0.80 1.76 30 30 50 HiSil 233 1.20 2.65 80 30 CB N234 0.56 1.23 80 50 Silane Si69 9.20 20.28 6.4 2 1.33 2.93 1.4 5 5.75 12.68 0.73 Total price 278.22 520.91 391.45 (U.S.)Price U.S./kg 1.55 2.79 2.15
(1.) Kerner, D., Kleinschmit, P., Parkhouse, A. and Wolff, S., U.S. patent Degussa Aktiengesellschaft 1987, 4, 704, 414.
(2.) Chevallier, Y., Micouin, J. and Micouin, J.M., Michelin 1998, 0D1999-001367 .
(3.) Waddell, W.H., Kuhr, J.H., Poulter, R.R. and Rouckhout, D.F., Rubber World 2002, September, 26.
(4.) Hopkins, W., von Hellens, W., Koski, A. and Rausa, J., Rubber World 2002, April, 37.
(5.) Hopkins, W., von Hellens, W., Koski, A. and Rausa, J., Rubber World 2002, September, 38.
(6.) Waddell, W.H. and Poulter, R.R., Rubber World 2000, September, 36.
(7.) Poulter, R.R., Foster, J.G., Napier, C., Waddell, W.H. and Webb, J.R., Rubber & Plastics News 2002, 20-24.
(8.) Frump, J. A. Chem. Rev. 1971, 71, 483.
(9.) Hopkins, W., Parent, J.S., White, G.D. and Whitney, R.A., Macromolecules 2002, 35, 3,374-3,379.
(10.) Gun'ko, V.M., Vedamuthu, M.S., Henderson, G.L. and Blitz, J.P., Journal of Colloid and Interface Science 2000, 228, 157-170.
(11.) Parent, J.S., Thom, D.J., White, G.D., Whitney, R.A. and Hopkins, W.J., Polym. Sci. A. Polym. Chem. 2001, 39, 2,019.
(12.) Payne, A.R., J. Polymer Sci. 1974, 48, 169-196.
(13.) Clement, F., Bokobza, L. and Monnerie, L., Rubber Chemistry and Technology 2001, 74, 847-870.
(14.) Vukov, R., ACS Rubber Division meeting 1983, paper no. 6.
(15.) Vukov, R. and Wilson, G., ACS Rubber Division meeting, paper no. 73, 1984.
(16.) Jones, F.R., Vedamuthu, M.S. and Blitz, J.P, Fundamental and Applied Aspects of Chemically Modified Surfaces, 173-182.
|Printer friendly Cite/link Email Feedback|
|Date:||Sep 1, 2003|
|Previous Article:||New reinforcing materials for rising tire performance demands.|
|Next Article:||Cleveland hosts Rubber Division.|