Analytical Properties of Silica ± a Key for Understanding Silica Reinforcement 1

ROHSTOFFE UND ANWENDUNGEN RAW MATERIALS AND APPLICATIONS Rolling resistance Highly dispersible silica Filler network Silanol group density surface activity For a long time simultaneous improvements of seemingly contradictory tire properties, e. g. rolling resistance, wet grip, winter performance and service life have been the major request from automobile manufactures especially for original equipment (OE) tires. It was only in the beginning of the nineties that these demands could be met using tread compounds with special polymers and high loadings of silica (together with silanes) instead of carbon black. The properties of these compounds strongly depend on the precipitated silica used. An optimum specific surface area could be derived by critically monitoring Mooney viscosity and dynamic stiffness. The effect of silanol group density, moisture content and surface activity on important in-rubberproperties is shown. Analytische Eigenschaften von KieselsaÈuren ± ein SchluÈssel zum VerstaÈndnis der VerstaÈrkerwirkung Rollwiderstand hoch dispergierbare KieselsaÈure FuÈ llstoffnetzwerk Silanolgruppendichte OberflaÈ chenaktivitaèt Seit langem wurde von der Automobilindustrie eine gleichzeitige Verbesserung von scheinbar gegensaè tzlichen Reifeneigenschaften wie Rollwiderstand, Naûrutschverhalten und Lebensdauer, vor allem in der ErstausruÈ stung, gefordert. Zu Beginn der 90er Jahre wurden diese Anforderungen durch den Einsatz von LaufflaÈ chencompounds mit speziellem Polymer und einer sehr gut dispergierbaren KieselsaÈure (zusammen mit Silan) als Ersatz fuèr Ruû erfuè llt. Die Eigenschaften dieser Compounds haè ngen stark von denen der eingesetzten KieselsaÈure ab. In einer umfangreichen Studie wurden eine bestgeeignete spezifische OberflaÈ che sowie ein optimaler DBP-Bereich ermittelt. Ferner wird der Einfluû der Silanolgruppendichte, der PartikelgroÈ ûenverteilung und der OberflaÈ chenaktivitaè t diskutiert. Analytical Properties of Silica ± a Key for Understanding Silica Reinforcement 1 A. Blume, HuÈ rth (Germany) A modern tire has to fulfil several requirements: good dry and wet grip, secure braking, reasonable service life and low rolling resistance. The development of tires with these desirable properties has been achieved [1] using a newly developed, highly dispersible silica together with silanes and a special solution SBR [2 ± 4]. Based on this development the ªGreen Tireº, introduced in 1992, provides an approximately 20 % lower rolling resistance when compared with carbon black and a high wet traction without compromising treadwear. The morphology of the silica used is an important parameter. In several publications [5, 6] the analytical properties of the silica have been reported to have a large influence on the physical performance of the rubber compound. Wagner [7] indicated that some important silica properties such as silanol content, adsorbed water, 1 based on A. Blume, ACS Rubber Division Meeting, Chicago, April 13±16, 1999, Paper 73 structure and surface area affect the viscosity, cure rate, modulus and abrasion resistance. Voet, Morawski and Donnet [8] correlated the silica surface area and the structural index with the rubber performance characteristics of a 50 phr silica-filled SBR-compound. Hewitt [9] reported that the silica surface area is inversely related to compound cure rate and dynamic properties and directly related to compound viscosity, compression set and reinforcement. Experimental 37 silicas were selected for this study to obtain the widest range in physical properties (Fig. 1). The particle size distribution has been determined by laser diffraction measurements. The deagglomeration of the silica was carried out by ultrasonic treatment of the silica in suspension, monitored simultaneously with laser diffraction. By doing this particle sizes between 40 nm and 500 lm were detected. Fig. 1. DBP and BET of silicas used 338 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000

Fig. 2. Time to reach 95 % cure versus BET Fig. 3. Mooney Viscosity versus BET Fig. 4. Modulus at 100 % elongation versus BET Fig. 5. Modulus at 300 % elongation versus BET Fig. 6. Shore A Hardness versus BET Fig. 7. Dispersion Coefficient versus BET The specific silanol group density was determined by its reaction with LiAlH 4. The silica was dried for one hour at 120 8C under vacuum, then mixed with LiAlH 4 at room temperature. By measuring the volume of the gas evolved, the amount of hydrogen formed has been determined [10] (equation 1): 4 R 3 SiOH LiAlH 4! R 3 O 3 Al R 3 Si-OLi 4 H 2 1 The resulting concentration of silanol groups is presented as [mmol OH/g], the silanol group density as [mol OH/ m 2 ]. The specific silanol group density is calculated using equation 2: CSIOH 1000 d SIOH ˆ BET 2 d SIOH ˆ specific silanol group density C SIOH ˆ concentration of silanol groups BET ˆ nitrogen specific surface area KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000 339

For the characterisation of the dispersion behaviour the dispersion coefficient was determined according to the literature [11]. The vulcanizates were cut into thin pieces using a razor blade. These test pieces were then investigated with the help of an optical microscope. The resulting images depicting a wide scale of grey tints, were then reduced to binary black and white images. The black areas represented the silica agglomerates, the white area the background. The dispersion coefficient was calculated from the size of the black areas. Usually 40 different regions of the razor cut were taken into account. The dispersion coefficient was calculated using equation 3. D %Š ˆ f ˆ V 100 0:78 2 R n 10 000 f V A D ˆ dispersion coefficient R ˆ sum of particle area n ˆ number of images f ˆ Medalia factor V ˆ filler volume A ˆ area of the image 3 The surface activity was determined by measuring the so called electrokinetic sonic amplitude [12, 13] (ESA). The characterization of the double layer on the silica surface (formed by the cloud of surrounding cations) was carried out using ultrasonic vibration. The negatively charged silica particles and the surrounding positively charged ionic atmosphere are attracted by the positive and negative electrode, respectively. Due to the alternating current signal they then begin to oscillate. The motion of the larger silica particles is lower than that of the ions. Due to the fact that the density of the silica particles and the surrounding fluid is different these oscillations cause periodic density changes, resulting in sound waves. These waves with a frequency of approximately 1 MHz can be detected by employing a highly sensitive detector after passing a delay line made from nonconductive material. The resulting ESAsignal has the unit [mv]. During the measurement small amounts of sulphuric acid were added to the silica solution. With increasing acid concentration, the ESAsignal approaches ªzeroº, which is called the isoelectric point (iep) or zeta potential [14]. In this work, a green tire tread compound with 80 phr silica was used. For the determination of compound and vulcanisate properties standard ISO, DIN or ASTM test procedures have been employed (Mooney test according DIN 53523, tensile test according DIN 53505 and dynamic tests with MTS, 16 Hz, preload 50 N, amplitude load 25 N) Results and discussion Many different correlations exist between single analytical properties of the silica and in-rubber-properties [5 ± 9]. For the development of an improved green tire tread the use of a tailor-made silica is necessary. The purpose of this study is to determine certain ranges in the silica properties where a favourable effect on important in-rubber-properties can be found. Both the processing characteristics of the compound and the static and dynamic properties of the vulcanisates were investigated. The viscosity of the green compound should be low to ensure better processability. The time to reach 95 % cure should be short. The relation of modulus 300 % to modulus 100 % is called the reinforcing index and should be high. The rolling resistance has been found to correlate very well with tan d at 60 8C; the dynamic (or cornering) stiffness with the complex modulus E* at 60 8C; the wet traction is said to correlate with tan d at 0 8C [2]. The lower tan d at 60 8C, the higher the E* at 60 8C and the higher the tan d at 0 8C the better the dynamic in-rubber-properties of the green tire tread compound. Promising BET/DBP-Combination In the first step, the BET surface area and the DBP-value of the silicas have been determined. These selected analytical properties were correlated with two important rheological properties (Mooney viscosity and cure rate) as a measure of processing behaviour and seven vulcanisate parameters of a green tire tread compound (modulus 100 %, modulus 300 %/100 %, Shore A hardness, dispersion coefficient, dynamic stiffness E* at 60 8C, tan d at 60 8C and tan d at 0 8C) (Figs. 2 ± 10). A BET surface area of more than approximately 200 m 2 /g leads to a low cure rate (Fig. 2). Due to this, the high surface area silicas exhibit a higher concentration of silanol groups which, upon reaction with silane, resulted in a higher amount of free silanol groups. Moreover, a stronger interaction of free silanol groups with the accelerator is possible leading to a lower cure rate. A silica with a DBP-value lower than 235 g/ 100 g shows a high cure rate in most cases. Consequently, to obtain a high cure rate of the green tire tread compound, the use of a silica with a BET surface area lower than 200 m 2 /g and a DBP value lower than 235 g/100 g is necessary. The study of the parameters influencing the Mooney viscosity (Fig. 3) reveals the following relation: a low BET surface area and in most cases a DBP-value lower than 235 g/100 g lead to a low Mooney viscosity. For a Mooney viscosity lower than 80 the BET surface area should be lower than 200 m 2 /g. This correlation can be attributed to a stronger filler-filler-interaction of the silica with increasing surface area and structure. Thus, the same combination of lower DBP-value and lower BET surface area results in good rheological properties. On the other hand, the modulus 100 % (Fig. 4) and the modulus 300 % (Fig. 5) increases respectively when using silicas with a DBP-value higher than 235 g/ 100 g and higher BET surface areas. In the case of low strains of approximately 10 to 50 %, the filler-filler network plays the dominant role. As the filler network is strain dependent and is destroyed at larger strain, the dominant parameter at high strain is the polymer-filler interaction. With higher surface areas and higher structure, the polymer-filler interaction increases resulting in increasing moduli. The same explanation is also valid for the Shore A hardness (Fig. 6). The dispersion coefficient depends mainly on the DBP-value (Fig. 7). DBPvalues higher than 235 g/100 g lead to good dispersion behaviour in most of the cases (dispersion coefficient greater than 90 %). The improved dispersion characteristics of silicas with a higher DBP-value and hence a higher void volume can be explained as follows: during the beginning of the mixing process, the polymer penetrates into the voids of 340 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000

Fig. 8. Dynamic stiffness E* at 60 8C versus BET Fig. 9. tan d at 60 8C versus BET action increases because of the increasing possibility to form hydrogen-bonds. On the other hand, the usage of a silica with a DBP-value lower than 235 g/ 100 g and a BET surface area lower than 220 m 2 /g leads to a low tan d at 60 8C (Fig. 9) and a high tan d at 0 8C (Fig. 10). In conclusion, the same combination of higher DBP-value and higher BET surface area results in most of the cases in good vulcanisate properties with the exception of tan d at 60 8C and tan d at 0 8C as mentioned before. This behaviour is contradictory to the desired rheological properties. Using a statistical analysis and evaluation program (STATISTICA) a promising combination of BET surface area and DBP value was calculated which best fulfils these contradictory requirements. The areas of silica properties which have a favourable effect on important in-rubber-properties are a BET surface area lower than 180 m 2 /g and a DBP-value higher than 235 g/100 g. Fig. 10. tan d at 0 8C versus BET the silica (incorporation process). If the amount of voids is high more polymer will penetrate the silica agglomerates. During the following deagglomeration a break down of the silica agglomerates occurs (which were partly penetrated by the polymer). The higher the penetration of the silica the better its dispersion in the polymer. To obtain a high dynamic stiffness (E*) it is necessary to use a silica with a high BET surface area and a DBP-value larger than 235 g/100 g (Fig. 8). The dynamic modulus E* at low strain depends on crosslinking density as well as on filler-filler interaction. The strain levels used in this study were not sufficient to permanently destroy the silica network. As mentioned previously, an increase in BET surface area is also accompanied by an increase in the total amount of silanol groups. Consequently, the filler-filler inter- Improvement of the Dispersion In order to study the dispersion behaviour by varying another physical property (maintaining the selected BET/DBP combination) of the silica, the particle size distribution was taken into account [15]. The particle size distribution of three selected silicas is depicted in Fig. 11. The resulting particle size distribution of U 3370, U 7000 and as well as that of the reference shows a bimodal distribution. The main peak at approximately 10 lm is attributed to the initial structure with large agglomerates of silica. This higher structure is partly destroyed during the ultrasonic treatment. The extent of this breakingup of the higher structure can be correlated to the size of the first peak at 0.5 lm. Upon keeping the ultrasonic energy input constant, the deagglomeration (dispersion) of U 7000 is higher than that of U 3370. The ratio of the peak height of the original agglomerates to the peak height of the decomposed agglomerates is defined as the WK-coefficient (equation 4). WK ˆ Ho H d H o ˆ peak height of original agglomerates H d ˆ peak height of decomposed agglomerates 4 342 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000

There is a good correlation between this WK-coefficient and the dispersion coefficient (Fig. 12). The lower the WKcoefficient (which means a larger and higher peak of decomposed agglomerates) the better the dispersion. Therefore, this WK-coefficient is a good analytical measure to predict the dispersion behaviour of the silica. Improvement of the dynamic stiffness E* Fig. 11. Particle Size Distribution The variation of the specific silanol group density has an influence on the dynamic stiffness E* at 60 8C. The silanol groups fulfil two important functions: the formation of the filler network and, to a limited extent, the reaction with the silane. In order to exclude the influence of the BET surface area (the higher the BET surface area the higher the total amount of silanol groups) only the specific silanol group density was taken into account. The lower the specific silanol group density the higher the dynamic stiffness at 60 8C (Fig. 13). This result is somewhat unexpected. At a higher silanol group density a higher filler-filler interaction could also be expected. Consequently, the E* at 60 8C should be higher, too. But this is not the case. To understand this relationship, two factors need to be taken into consideration: Firstly, the reaction between the silane and the silanol groups, secondly, the correlation of the Mooney viscosity with the degree of hydrophobation. Each silica was treated with the same quantity of silane. The higher the specific silanol group density the lower the Mooney viscosity (Fig. 14). This means that the degree of hydrophobation is higher at a higher specific silanol group density. If the silica is more hydrophobic the filler-filler interaction decreases. This is clearly shown by plotting the ratio of the modulus 300 % to modulus 100 measure of the reinforcement, versus the Mooney viscosity (Fig. 15). The lower the Mooney viscosity the higher the reinforcement and the lower the filler-filler interaction. Consequently, the dynamic stiffness E* at 60 8C will be higher at a lower specific silanol group density with the obvious disadvantage of a lower reinforcing index. On the one hand, decreasing the specific silanol group density is a good way to increase the dynamic stiffness E* at KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000 343

Fig. 12. Correlation between the WK ± and the Dispersion Coefficient Fig. 13. Dynamic stiffness E* at 60 8C versus specific silanol group density Fig. 14. Mooney Viscosity versus specific silanol group density Fig. 15. Reinforcing Index versus Mooney Viscosity Fig. 16. Time to reach 95 % cure versus Zeta Potential Fig. 17. ESA-signal at ph 5 versus silanol group concentration 60 8C, but on the other hand it has a negative influence on the reinforcing index. Therefore, the specific silanol group density needs to be finely adjusted to achieve an optimum balance between these two important properties. Improvement of the vulcanisation behaviour The cure rate is a further important in-rubber property that needs to be improved. At a given BET/DBP/WK-coefficient/specific silanol group density ratio, another physical property of the silica, namely the surface activity, was varied. The surface chemistry and surface activity of the silica has the largest influence on the cure reaction. The higher the total amount of silanol groups, the higher the adsorption of accelerators. If the BET surface area 344 KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000

and also the specific silanol group density are kept constant, the only variable is the surface activity. The dispersed silica particles are surrounded by hydrogen ions, originating from the dissociation of the silanol groups, which form a permanent double layer. The larger the dissociation the lower the isoelectric point (iep), i. e. the zeta potential. Seemingly, the zeta potential has an influence on the cure reaction, as can be seen in Fig. 16. The lower the iep the faster the cure reaction. The silanol group concentration can also be determined by ESA measurement. At the beginning of the titration the ESA-signal remains constant. This value correlates quite well with the specific silanol group concentration (Fig. 17) and is independent of the accessibility of different silica pores. The latter means, this method has an advantage over other silanol group measurements, such as the titration with LiAlH 4 or the Sears titration. Therefore, it is possible to characterise the surface activity and the specific silanol group density of a silica using a single ESA measurement. Conclusion An extensive study was carried out to investigate the connection of different silica properties and their influence on rubber performance. A promising combination of BET surface area and DBP value has been found. The WK-coefficient, the specific silanol group density and the surface activity are further important parameters for the improvement of the dispersion behaviour, the dynamic stiffness and the cure rate. This new knowledge led to the development of the highly dispersible silica U 7000 and can speed up further developments. References [1] EP 0501 227, US 5.227.425. [2] S. Wolff, ACS ± Rubber Division, New York, April 8±11, 1986, paper 46. [3] G. Agostini, J. Berg, Th. Materne, New Compound Technology. Oct. 1994, Akron, Ohio/ USA. [4] G. Heinrich, Workshop ªReifenº Deutsches Institut fuè r Kautschuktechnologie, May 25±26, 1998. [5] T. A. Okel, W. H. Waddell, ACS ± Rubber Division, Denver, Colorado, May 18±21, 1993, paper 37. [6] F. Bomo, ITEC 1996, Paper # 21B. [7] M. P. Wagner, Rubber Chem. Technol. 49 (1976) 703. [8] A.Voet, J.C.Morawski, J.B.Donnet, Rubber Chem. Technol. 50 (1977) 342. [9] N. Hewitt, ACS ± Rubber Division, Educational Symposium # 4, Cleveland, 1979. [10] R. Bode, H. Ferch, H. Fratzscher, Kautsch., Gummi, Kunstst. 20 (1967) 578. [11] R. H. Schuster, H. Geisler, D. Buûmann, 2nd Conference on Carbon Black, Mulhouse (France) Sept. 1992. [12] J. SchroÈ der, farbe lack 97 (1991) 957. [13] A. J. Babchin, R. S. Chow, R. P. Sawatzky, Advances in Colloid and Interface Science 30 (1989) 111. [14] R. K. Iler, The Chemistry of silica, John Wiley & Sons, New York (1979) 660. [15] A. Blume, S. Uhrlandt, GAK 2 (1999). The author Dr. Anke Blume is Manager of Silica Development at the Applied Technology Department of the Advanced Filler and Pigments Division of Degussa-HuÈ ls AG in HuÈ rth (Germany). Corresponding author Dr. A. Blume, Degussa-HuÈ ls AG, Kalscheurener Str. 11, D-50354 HuÈ rth (Germany) KGK Kautschuk Gummi Kunststoffe 53. Jahrgang, Nr. 6/2000 345