A new silane for future requirements--lower rolling resistance, lower VOCs.
To address this issue, VP Si 363, which can be seen as a successor to Si 69, has been developed recently, and is now being introduced to the tire industry. With this new silane, a further reduction of rolling resistance by more than 10% is achievable, and the emission of VOCs is reduced by up to 80%.
This article introduces VP Si 363 as a new rubber silane for the tire industry. Detailed in-rubber investigations are discussed and results of a tire test are given.
Chemical structure of VP Si 363
The molecular structure of VP Si 363 is shown schematically in figure 1. VP Si 363 is characterized by a free mercapto group on the rubber-active site and a combination of ethoxy groups and polymeric, amphiphilic substituents on the silica-active site. On average, the Si-unit bears one ethoxy and two polymeric substituents, where a polar polyethylene part and a non-polar alkyl part build up the polymeric substituents.
[FIGURE 1 OMITTED]
This structure, displayed in figure 1, leads to reaction mechanisms different from state-of-the-art silanes and is responsible for the improved performance, which can be gained by the use of this silane.
The silica-silane reaction proceeds via the ethoxy substituent on the Si-unit. It is cleaved during the mixing process, and the silane bonds covalently to the silica surface. Due to the steric hindrance given by the bulky polymeric substituents, this reaction can be slowed down on the one hand, but on the other hand the polar part of these substituents ensures an efficient silica affinity which guarantees a fast absorption and reaction on the silica surface. Furthermore, free silanol groups on the silica surface are also shielded by these non-volatile, long-chain substituents, leading to an excellent hydrophobation of the silica. Additionally, the polar glycol part acts as an intrinsic activator function comparable to ethylene glycols.
The mercapto group is responsible for the efficient coupling of this silane to the polymer chains, resulting in a significantly enhanced performance. The use of sulfide silanes leads in general to a coupling yield silane-polymer of approximately 50% (refs. 4 and 5), but a free mercapto group yields nearly 100% due to a changed coupling mechanism. For this reason, on a molar basis only half of the amount of silane (calculated in Si-units) is needed to reach a superior level of reinforcement.
The scorchy behavior often observed for silanes containing a free mercapto group (ref. 6) is prevented by a changed cure system, which will be described later, and by the polymeric substituents on the Si-unit. Due to their long-chain nature, a special shielding effect is provided as depicted in figure 2.
[FIGURE 2 OMITTED]
Besides the additional hydrophobation of the silica surface, the long-chain substituents generate a certain steric hindrance for the mercapto group. Its reaction with the accelerator and the sulfur is slowed down. A higher scorch safety is obtained. Nevertheless, this reaction still proceeds via the free mercapto group pathway, leading to a quantitative coupling yield silica--silane--polymer. As a consequence, excellent reinforcement is obtained, as shown in the following section.
Formulations and mixing procedure
The investigated rubber compounds were prepared according to the formulations and the mixing procedure given in tables 1 and 2.
Regarding the mixing procedure given in table 2, no adjustment was made. The same mixing procedure was applied for both silane compounds. In general, mixing procedures optimized for Si 69 compounds can be applied for VP Si 363 compounds as well.
Adjustments were made regarding the formulations given in table 1. VP Si 363 and Si 69 differ in molecular weight and coupling efficiency, resulting in different requirements for the dosage. And as already mentioned, the use of VP Si 363 requires a change in the cure system.
The first compound in table 1 acts as a reference. It is a typical green tire compound based on an S-SBR/BR blend filled with the HD performance silica Ultrasil 7000 GR, together with 6.4 phr Si 69 (8 phf--parts per hundred filler). Zinc oxide is introduced in the first step; and in the final mixing step, an SEV cure system with DPG as co-activator is applied.
The second formulation is the test compound based on VP Si 363. Here, the necessary adjustments are shown. The recommended silane dosage is in the region of 10 to 12 phf. In the test formulation, VP Si 363 is used at 9 phr, which corresponds to 11 phf. The cure system applied for the test compound shows the strongest differences. As known when comparing Si 69 to other silanes like, e.g., the disulfide silane Si 266, a sulfur adjustment has to be made and the main accelerator CBS has to be kept at the same level. This measure guarantees the same overall crosslink density and has been reviewed recently (refs. 4-7).
When VP Si 363 is used, besides this sulfur correction, a further adjustment has to be made. Due to the fact that VP Si 363 bears a free mercapto group and an intrinsic activator function in terms of the glycol units, the activator composition of the rubber compound has to be changed. Regarding the relationship of the guanidine activator DPG to the thiuram activator TBzTD, an "inverse activator composition" (IAC) has to be established. In the VP Si 363 compound, only small amounts of DPG are used in contrast to the reference. Instead, a certain amount of TBzTD (tetrabenzyl thiuram disulfide) is introduced. In general, silica-filled compounds require a high amount of DPG, e.g., 2 phr, and only a very small amount of "kickers" like TBzTD (0-0.2 phr). For VP Si 363 compounds, only a small amount of DPG (0-0.3 phr) and a higher amount of TBzTD (0.2-0.5 phr) are applied.
In summary, the overall amount of activators can be reduced due to the activator function of VP Si 363, and the relationship of DPG to TBzTD has to become an inverse relationship. This IAC, together with the intrinsic shielding effect of the polymeric Si-substituents, would lead to an improved vulcanization behavior. A sufficient incubation period, together with a stable torque plateau, is the consequence.
Figure 3 shows the rheometer curves for the reference and test compounds. The VP Si 363 compound exhibits a sufficient incubation time, a steep slope and a clear plateau of the rheometer curve. Compared to the reference compound, the incubation period is even more pronounced and the vulcanization speed is increased.
[FIGURE 3 OMITTED]
Silica-filled compounds containing VP Si 363 develop only a weak silica network. The strong hydrophobation ability of VP Si 363 prevents the silica from reagglomeration. A consequence of this weak silica network is a strongly reduced Payne-effect, typical for VP Si 363 compounds, as shown by the RPA curves for the raw compounds in figure 4.
[FIGURE 4 OMITTED]
This behavior becomes even more evident in the cured state. Together with the high coupling efficiency of VP Si 363, this strongly reduced silica network leads to a very low hysteresis loss of the compound. Figure 5 shows the corresponding RPA curves of the vulcanizates for [G.sup.*], and figure 6 the curves for the tan [delta] values.
[FIGURES 5-6 OMITTED]
It is obvious that the silica-network is strongly reduced in the VP Si 363 compound. The stress-softening effect, expressed as the G x difference between low and high strain, has nearly vanished. At high strains, the curves for VP Si 363 and Si 69 coincide, indicating a similar in-rubber structure. Comparing the tan [delta] curves, it can be seen that a reduction of the tan [delta] maximum by 45% is reached, only due to the influence of the new silane.
The high coupling efficiency and the reduced silica network lead also to a changed temperature behavior. It is well known that an unfilled compound shows, over a wide temperature range, a nearly constant dynamic modulus [E.sup.*] with a steep increase of [E.sup.*] near the glass transition temperature ([T.sub.g]), representing the characteristic temperature behavior of the polymer. Filled compounds show also the influence of the filler system used. The temperature behavior of [E.sup.*], for the silica-filled compounds with Si 69 and VP Si 363, is shown in figure 7.
[FIGURE 7 OMITTED]
Taking these curves as a measure for the temperature dependency of the filler-filler network, the following can be stated: In the temperature range of about -30[degrees]C to 80[degrees]C, the Si 69 compound exhibits a higher [E.sup.*] value than the VP Si 363 compound. This can be attributed to the formation of a silica net--the the compound mixed with Si 69. At temperatures higher than 80[degrees]C, the dynamic modulus is on a similar level for both compounds. The silica network is nearly eliminated and both compounds show a comparable overall crosslink density.
As a consequence, compounds mixed with VP Si 363 display a lower temperature dependence of the dynamic modulus over a wide temperature range. A tire tread produced with VP Si 363 will show a nearly stable dynamic behavior, and therefore a more constant driving characteristic at low, as well as at high temperatures, offering new opportunities for all kinds of tires, regardless if they are summer, winter or all season tires.
The temperature dependency of the loss factor tan [delta], which gives a picture of the hysteresis, is shown in figure 8.
[FIGURE 8 OMITTED]
Comparable effects are observed. At lower temperatures, there is a pronounced silica effect for the Si 69 compound. Its strong silica network leads to a virtual broadening of the [T.sub.g]-peak, where at high temperatures the influence of the silica network is less pronounced. Here, the increased coupling efficiency of VP Si 363 becomes more evident, securing the reduction of tan [delta] even for highest temperatures.
A summary of the in-rubber properties of the reference and the VP Si 363 compounds is given in table 3. The in-rubber data in table 3 reflect the before-mentioned properties of VP Si 363 compounds. On the one hand, compared to the Si 69 reference compound, the silica network is decreased. This leads to a reduced Mooney viscosity after the final mixing stage and a lower hardness. On the other hand, the coupling efficiency of the silane towards the polymer is increased, resulting in an increased modulus at 300% strain and also an increased reinforcing index (modulus 300%/modulus 100%). The improvements in ball rebound and DIN abrasion are further consequences.
The dynamic data show the influence of both effects, as already discussed. Once more a reduction in tan [delta] at 60[degrees]C of more than 40% is reached. But the low hysteresis loss is only one of the major advantages that can be obtained with this new silane. Additionally, a strong reduction in volatile organic compounds (VOCs) is gained, as shown in the following section.
On average the Si-unit ofVP Si 363 bears one ethoxy and two polymeric substituents where the latter ones are not volatile. Together with the reduced molar amount needed for this silane, a VOC reduction of approximately 80% compared to the use of state-of-the-art silanes is achieved.
It is recommended to use VP Si 363 in the region between 10 to 12 phf (parts per hundred filler) which, e.g., corresponds to 8 to 10 phr (parts per hundred rubber) for a compound with 80 phr silica. Compared to 6.4 phr of Si 69, the upper limit of 10 phr for VP Si 363 corresponds to a molar amount of Si-units of only 42%. The Si-units ofVP Si 363 carry only 33% as much of volatile ethoxy groups. Hence, the expected reduction of VOCs can be calculated to 86%.
Figure 9 shows the measured VOCs emitted during the mixing process for compounds filled with 80 phr of silica. Three different compounds were investigated, including a reference compound with no silane, a compound with 6.4 phr Si 69 and a compound with 10 phr VP Si 363.
[FIGURE 9 OMITTED]
Compared to the Si 69 compound, a strong reduction of the VOC emission is observed for the VP Si 363 compound. In this case, an 80% reduction of VOC is measured, which is in good accordance with the theoretically expected reduction of 86%. This drastic reduction of VOCs should reduce the requirements on the VOC emission control during the mixing process. Furthermore, even the emission out of the resulting rubber articles should be reduced, fulfilling future requirements for environmentally friendly products.
To lower the emission of VOC's is only one of the requirements ofVP Si 363. The second requirement is to lower rolling resistance of resulting tires. A tire test was performed to confirm the results of the lab studies. Once more, except for the adjusted cure system necessary for VP Si 363 compounds, no further adjustment or optimization was done. 6.6 phr Si 69 was replaced by 9.0 phr VP Si 363 in a full silica compound. The mixing routine was not modified. Processing was similar, e.g., compound viscosity, sheet appearance, extrusion process. The improvements in the resulting vulcanizate data are summarized as ratings in percentage in table 4.
All indicators for hysteresis loss were strongly improved, (tan 5 by 43%, heat build-up by 23% and ball rebound by 25%). Regardless of the conditions applied--load controlled, strain controlled or energy controlled--hysteresis loss was always significantly improved. A reduced rolling resistance of the corresponding tire could be expected. Consequently, tires were built where the treads consisted of the Si 69 and the VP Si 363 compounds. The results obtained can be seen in figure 10, and are also summarized as percentage ratings against the Si 69 control.
As shown in figure 10, the rolling resistance was improved by 13% in combination with balanced tire properties. Taking into account that the shift from carbon black to the silica/silane system as filler system counted for a reduction of rolling resistance by approximately 20% (ref. 1), the further improvement of 13%, given by the use of VP Si 363, can be seen as a further performance jump. Replacing Si 69 by VP Si 363 changes the properties of a resulting tire significantly. Future requirements like lower rolling resistance and reduced VOC emissions can be fulfilled.
VP Si 363, which can be seen as a successor of Si 69, has been developed recently and is now being introduced to the tire industry. In-rubber data indicate that when replacing Si 69 in a tread compound with this new silane, the resulting tire should exhibit a strongly improved rolling resistance. Tires were built and tested to verify this hypothesis. Except for the adjusted cure system (IAC and sulfur adjustment), no further adjustment or optimization was made. Balanced tire properties were obtained and a performance jump for rolling resistance of 13% was verified. In addition, VOC emissions were reduced by 80%. Temperature stability of the dynamic modulus, as well as a reduction in the amount of co-activators like DPG needed so far for silica compounds, are further advantages confirmed by this test. Using VP Si 363 targets fulfilling the requirements of the future, including lower rolling resistance and lower VOCs.
(1.) R. Rauline, EP 0501227, US 5,227,425, Compagnie Gdndrale des Establissements Michelin.
(2.) S. Wolff, Kautschuk Gummi Kunststoffe, 34, 280 (1981).
(3.) S. Wolff, Rubber Chem. Technol., 69, 325 (1996).
(4.) A. Hasse, O. Klockmann, A. Wehmeier and H.-D. Luginsland, Kautschuk Gummi Kunststoffe, 55, 236 (2002).
(5.) H.-D. Luginsland, Kautschuk Gummi Kunststoffe, 53, 10 (2000).
(6.) F. Thurn and S. Wolff, Kautschuk Gummi Kunststoffe, 28, 733 (1975).
(7.) O. Klockmann and A. Hasse, paper no. H, Rubber Division, ACS, San Antonio, TX, May 16-18, 2005.
Table 1--formulations 1. Stage Phr Phr Buna VSL 5025-1 96 96 Buna CB 24 30 30 Ultrasil 7000 GR 80 80 Si 69 6.4 -- VP Si 363 -- 9.0 ZnO 3 3 Stearic acid 2 2 Aromatic oil 10 10 6PPD 1.5 1.5 Wax 1 1 2. Stage Batch 1. Stage 3. Stage Batch 2. Stage DPG 2 0.25 TBzTD -- 0.5 CBS 1.5 1.5 Sulfur 1.5 2.2 Table 2--mixing procedures 1. Stage Intermix 1.5E 0-1' Polymers 1-3' 1/2 filler, silane, ZnO, oil, stearic acid 3-4' 1/2 filler, antioxidant, wax 4 Lift ram and clean 4-5' Continue mixing 5 Lift ram 5-6' Mix and dump (150[degrees]C) 2. Stage 0-2' Plastify batch stage 1 2-5' Mix and target temperature 5 Dump (150[degrees]C) 3. Stage 0-2' Plastify batch 2, sulfur, accelerators Dump on open mill (110[degrees]C) Homogenize Table 3--in-rubber data obtained on compounds prepared according to tables 1 and 2 Si 69 VP Si 363 Silane amount [Phr] 6.4 9.0 ML(1+4) @ 100[degrees]C, 3. stage [ME] 68 59 Mooney-scorch (130[degrees]C) t5 [min.] 29.0 23.7 t35 [min.] 37.6 29.6 MDR, 165[degrees]C, 0.5 Fmax-Fmin [dNm] 16.3 15.3 t 10% [min.] 1.7 2.1 t 90% [min.] 7.5 5.2 t 80%-t 20% [min.] 2.8 1.5 Stress-strain (ring) Tensile strength [MPa] 14.6 14.8 M 100% [MPa] 1.8 1.8 M 300% [MPa] 9.6 11.8 M 300%/100% [MPa] 5.3 6.6 Elongation @ break [%] 390 345 Hardness (duro. A) [-] 61 57 Ball-rebound, 60 [degrees]C [%] 64 72 DIN-abrasion (abr. loss) [[mm.sup.3]] 85 77 MTS, 16 Hz, 50N pre-force 25N Ampl. E*, 0[degrees]C [MPa] 16.2 11.2 Tan [delta], 0[degrees]C [-] 0.471 0.438 E*, 60 [degrees]C [MPa] 7.6 6.7 Tan [delta], 60 [degrees]C [-] 0.110 0.064 Dispersion topography Peak area [%] 0.7 1.1 Table 4--rating of vulcanizate data (higher indicates better, Si 69 = 100%). Si 69 VP Si 363 Silane amount [Phr] 6.6 [Phr] 9.0 Tensile strength 100 98 M 300% 100 111 M300%/100% 100 115 Elongation @ break 100 88 Ball-rebound, 60[degrees]C 100 125 DIN-abrasion 100 118 Heat build-up 100 123 Tan [delta], 70[degrees]C 100 143 Figure 10--results of tire test (higher indicates better, Si 69 = 100%) Braking distance dry [GRAPHIC OMITTED] Si 69 VP Si 363 Rolling resistance [%] 100 113 Braking distance wet [%] 100 97 Braking distance dry [%] 100 100 Wear 1%] 100 96
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|Date:||Aug 1, 2006|
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