Theoretically thousands of different silica, obtained by different precipitation routes and downstream processes are possible to be synthesized. In this paper a very simplified view on the processes is done by distiguishing between two different basic precipitation routes that are in use at Evonik plants around the world. These techniques result in characteristic particle size distributions (PaSD), which can be obtained in different specific surface area (SSA) regions. One method to measure the PaSD, the smallest cluster sizes after very strong ultrasonic treatment, is given by a disc centrifuge, similar to that used to obtain carbon black aggregate size distributions (ASD).
Over the last decades a lot of investigations have been performed to control the resulting analytical parameters of the final product during the manufacturing process of precipitated silica on very large scales. This know-how is used today to tailor the material SSA and PaSD in order to adjust them to the diverse requirements of the rubber industry. Focussed on SSA and PaSD of different precipitated silica the effect on the in-rubber data will be shown and discussed. This production process know-how is the basis to control the characteristics and therefore the rubber performance of precipitated silica – also called morphology control.
Development of Silica for Rubber Applications
Starting 65 years ago and ever since, Evonik is producing Ultrasil VN 3 GR, the first precipitated silica for rubber applications. Dr. Hans Verbeek and his laboratory assistant Peter Nauroth developed this grade as “white carbon black” for light-colored shoe soles. While the “V” stands for Verbeek, Nauroth’s work on the product is credited through the “N” in the product name . Approximately twenty years later Metzeler launched 1973 the first traction tire with blue tread on the market – with precipitated silica and the bi-functional silane Si 69 . Approximately another 20 years later in 1992 the use of the silica/silane system led to the Green Tire technology for tire tread applications, where Michelin developed the combination of S-SBR / BR blends and highly dispersible (HD) silica together with a bi-functional silane .
This is still the basis for low rolling resistance tires, which are reducing the fuel consumption and improving the wet grip behavior while matching the level of abrasion resistance of carbon black filled treads in modern tires. Since then, the synthesis and functionalization of polymers, the design of silica particles and the chemical functionalities of organosilanes towards the silica and / or to the rubber active side is being constantly developed. For rubber formulations, the full potential of silica can only be achieved in combinations with the necessary amount of the right silane. Therefore, always the silica/silane system and its right mixing process is decisive for the final performance.
Nowadays, for precipitated silica, it is possible to variate SSA within a very broad range, always maintaining the HD character in tire compounds e.g. typical S-SBR / BR blends . For the final tire performance, the most important parameter is the CTAB SSA and not the BET SSA. In addition, a conversion of BET SSA  measurements into so-called STSA values in order to describe the outer SSA as it is standard for the characterization of carbon black, is not suitable for silica, because there is no satisfying correlation to the CTAB values nor to the in-rubber properties. Although, the CTAB of silica can show its reinforcing potential it is not the only analytical information used to predict the reinforcement behavior of precipitated silica in rubber compounds. Another very important parameter is the width of the PaSD, assessed in a centrifuge after very strong ultrasonic treatment. The sample preparation and evaluation of test results is described in the literature . This method is currently also in the consolidating phase at the ISO.
Characterisation of the particle size distributions
In the following studies, either commercial or experimental products of Evonik are investigated. Most of the experimental products have been produced on plant scale. The PaSDs of these silica in the nanometer and micrometer scale are depicted in Fig. 1 and 2. In Fig. 1 silica from precipitation route I (PPT I) and in Fig. 2 of precipitation route II (PPT II) are illustrated. Additionally, the leading parameter, the CTAB SSA ranges are given for all investigated silica. The products have been tested in a so-called Green Tire compound, based on an oil-extended S-SBR / BR blend. The Bis[3-(triethoxysilyl)propyl] disulfide (TESPD) silane Si 266 has been applied for the investigations since for initial tests of silica with a CTAB higher than 160 m²/g the amount can easily be adjusted linear to the CTAB. In this way, the total sulfur content of the compound is hardly affected.
No further formulation adjustments have been done since more variations, e.g. the often-discussed accelerator adjustments, would lead to a more complex system, of which the interpretation would be even more complicated. Nevertheless, such compounding measures might be sensible in order to fine-tune the performance potential of a given silica. In these experiments the silane amount is set constant to 5,8 phr for silica with CTAB smaller than 160 m²/g, since an adjustment to lower amounts would led to deteriorated in-rubber data and finally in losses in abrasion resistance .
Fine-tuning the PaSD influences the processing and dynamic stiffness
The modes (maximum of the distribution curve) of the PaSD are very well correlated to the CTAB values for all silica obtained either by PPT I or II (R² = 0.92). For both precipitation routes, a higher CTAB leads to a narrower PaSD and the mode is shifted to smaller cluster sizes. This is in agreement with the smaller primary particle sizes that are forming the aggregates of the silica. Since also the gap between CTAB and BET SSA is important for the final properties, it is also ensured for all investigated silica that these gaps are kept as small and equal as possible. Other characteristics like pH and loss on drying have been kept constant, too.
In-rubber data are shown in Fig. 3 Silica of PPT I are chosen as references and set to 100 %. Relative to that the values for PPT II are depicted (no rating). As documented before the reinforcing potential of a silica is dominated by the CTAB SSA and the chemical functionality and quantity of the used silanes . Nevertheless, the results show that fine-tuning the PaSD influences the processing and dynamic stiffness. The Mooney viscosities in the early mixing stages are improved for PPT II, which is also in-line with the lower Payne-effect measured in the Rubber Process Analyzer (second strain sweep from 0.3 % to 42.0 % strain on vulcanizates at 1.6 Hz and 60 °C) and the hysteresis loss, measured as tan δ.
Tailored Products for specific Demands
In this study, two very basic precipitation routes resulting in different PaSDs have been compared for different SSA ranges. It can be concluded that, at constant SSA, route one (PPT I) tends to a more narrow PaSD in the nanometer/micrometer scale whereas route two (PPT II) to broadened distributions. A broad PaSD shows lower compound viscosities and hence improved processing, while a more narrow distribution leads to a higher dynamic stiffness at high temperatures. Translated into tire characteristics positive impact on hysteresis and the resulting rolling resistance of a tire can be expected for a broad PaSD while a narrower PaSD would lead to a higher handling rating. Currently new precipitation routes are under investigation and there are indications that combinations of different routes will lead to a superior performance that has not been achievable before.
Evonik uses the described know-how to tailor products for specific demands, especially for the tire industry, as it has been shown for product developments like Ultrasil 9100 GR or just recently for Ultrasil 7800 GR. The expertise of tailoring and fine tuning silica properties is summarized as morphology control. n
 For us, the future is a tradition; brochure: 125 years of Degussa AG, Frankfurt am Main
 Reifengeschichte im Überblick; 01.06.2011; https://www.k-online.de
 R. Rauline; EP 0501227 B1; Compagnie générale des etablissements Michelin
 S. Uhrlandt, A. Blume; Development of HD Silicas for Tires – Processes, Properties, Performance; Rubber World 4/2002, Volume 226, No. 1
 H.-D. Luginsland, J. Fröhlich, A. Wehmeier; Influence of different silanes on the reinforcement of silica-filled rubber compounds; Rubber Chemistry and Technology; Volume 75 No.4; Sept. 2002
 A. Wehmeier et al.; EP2262730 B1; Precipitated silica acids as a reinforcement filler for elastomer mixtures
 A. Wehmeier, W. Wolff; New silica with improved balance between rolling resistance and abrasion resistance; IRC 2010; Mumbai; Nov. 2010
 A. Hasse, A. Wehmeier, H.-D. Luginsland; Crosslinking and reinforcement of silica/silane-filled rubber compounds; Rubber World 4/2004, Volume 230, No. 1
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