Unique production process and silica structure.

Since the beginning of the 1990s, silica in conjunction with special solution polymers and organosilanes acting as coupling agents has been used in passenger car tire treads. During the nearly 10 years of their application now, these silica products have been continuously improved and matched with the higher demands placed on them. At present, a variety of manufacturers supply such products, which can be divided into the following three groups:

* Products which can be described as "first-generation" silica or "standard" silica.

* Products belonging to the second generation of silica. Today, such products are frequently termed "easy dispersible silica" or "semi-highly dispersible (semi-HD) silica."

* The third and latest generation of silica comprises a product group characterized by very good dispersion. These products are described as "highly dispersible silica (HD silica)." They make it possible to not only achieve considerable improvements in the wet-skidding properties and rolling resistance of car tires, but also improve their wearing characteristics--thus far problematic in comparison with the classical filler, carbon black--to a reference level.

Currently available products, especially those of the third generation, are being continually improved to match individual requirements of the tire producers.

Despite the advantages achieved by using the silica/silane system in tire tread compounds, there are strong requests of the automotive industry to further extend the tire performance.

This is a challenge for silica research to develop products providing effective interaction with the polymer matrix to enhance the performance of tires regarding grip and handling characteristics. This can not be achieved by improvement of existing products. These performance demands require a new class of silica. In this respect, special variations in the production processes are the key to develop the next generation of silica.

Conventional manufacturing process

The various steps of manufacturing silica are displayed schematically in figure 1.

[FIGURE 1 OMITTED]

In the first step (precipitation), the raw materials consisting of water glass and sulfuric acid are dosed into a stirred vessel containing water. In many cases, once a defined pH value has been set, the components are fed continuously to the reactor, this process taking place simultaneously over a certain time interval. Another possibility is to first supply a particular quantity of water glass and initially just dose the sulfuric acid. Normally, this is followed by a second stage in which water glass and sulfuric acid are added simultaneously under defined reaction conditions. During the reaction time, primary particles are first formed in the reactor; later these particles react with each other, accompanied by dehydration, to form aggregates (ref. 4). Within the aggregates, the primary particles are linked together via siloxane bonds (ref. 5). During this process, the aggregates are deposited to form larger units, or agglomerates. In these agglomerates, the aggregates are held together by hydrogen bonding or van der Waals interactions which are considerably weaker than siloxane bonds. A state of equilibrium, which is dependent on the process conditions and can be easily influenced, is reached between the aggregates and agglomerates.

After that, the obtained suspension is filtered and the filter cake washed. It can then be re-suspended and spray-dried, or fed directly to a short-term drying process. Granulation might be necessary to transfer the product in a low-dust form.

Process for manufacturing silica with new characteristics

To produce silica with new and unusual characteristics or combinations thereof, the process of precipitation was changed fundamentally, use no longer being made of the familiar standard techniques described above. The first stage of the modified, two-stage process with special parameters involves a production of primary particles, aggregates and agglomerates which are representative of the parameter set used. This is followed directly by the second process stage in which certain essential parameters are varied. This second stage again gives rise to primary particles, aggregates and agglomerates typical for the selected conditions.

The conditions prevailing in the two different stages can be varied independently of each other.

Comparison of the structures

Whereas products manufactured using conventional methods contain primary particles, aggregates and agglomerates of a characteristic size, silica obtained from the new process has a different structure. The main differences are represented schematically in a model in figure 2.

[FIGURE 2 OMITTED]

The new products contain primary particles, aggregates and agglomerates of different sizes typical for the individual process stages. The particles differ in the individual stages but coalesce in the final product. This leads to a new and unusual structure with new properties.

A comparison of the surfaces or surface ratios, measured using CTAB and the method in accordance with Brunauer, Emmett and Teller (BET) (ref. 6) reveals clear differences. Table 1 provides an overview of a large variety of surfaces of conventional and new products.

Whereas in the case of presently available products, the surface ratio (BET/CTAB) normally lies between 0.8 and 1.2, this value is doubled in the case of the new products. The surface ratios obtained here lie between 2.0 and 2.4 (table 1). On the assumption that the CTAB surface is a measure of the "effective external rubberized" surface, and the BET surface is a measure of the "total" surface, i.e., the sum of the "external" and "internal" surfaces, the behavior of the new products in the polymer matrix should also be determined decisively by the CTAB surface, as the large internal surface should not constitute effective rubber. Experimental applications have shown, though, that this is not the case. It turns out that certain characteristics expectedly depend on the value of the CTAB surface, whereas others are clearly influenced by the very high value of the internal surface. For instance, the high value of the internal surface makes it possible to significantly raise the reinforcing characteristics of silica without having to incur higher mixture viscosity and, therefore, disadvantages related to process capability. The general statement that CTAB indicates the effective rubberized surface needs to be revised.

Consequently, the new performance-silica gives rise to a product class exhibiting a low "external" CTAB as well as a high BET surface. The question, "which surface does a silica possess, and which applications is it thus suitable for?" should be rephrased more precisely to, "which internal and external surfaces does the product possess, and what is the ratio between these surfaces?"

Porosimetric measurements of the new products using the Hg intrusion technique (ref. 7) also reveal interesting results compared with the reference products. The unusual ratio between the BET and CTAB surfaces is indicated by a much higher volume of mesopores and macropores compared with conventional silica. This results in a total intrusion volume much higher than that of conventional silicas. The new products have a considerably higher structure. Table 2 shows the measured intrusion volumes of the new products compared with standard products. Figure 3 shows the characteristic curve during Hg intrusion (cumulative) as a function of the pore diameter of a new product compared with a reference having the same CTAB surface.

[FIGURE 3 OMITTED]

The results of Hg-intrusion are confirmed by measurements of empty volumes, i.e., the structural response of the new products to rising pressure. In this technique, a sample is subjected to a defined pre-treatment, after which a fixed quantity of the sample is introduced into a cylindrical glass chamber possessing a volume scale. The chamber is closed by means of a movable piston. During measurement, a constant pressure is applied to the piston until the volume of the sample in the chamber stops changing, and the measured value is read. After that, the pressure on the sample is increased. Figure 4 shows the decrease in structure (or empty volume) as a function of rising pressure. Here, a new silica is compared with a reference having the same CTAB surface. As is clearly evident, the new products already have a much higher empty volume at the beginning of the measurement. The structure decreases as the pressure rises, yet always remains far above the reference level, i.e., the higher structure is retained even under heavy loads, such as those occurring in mixers. The more stable structure can be penetrated more easily by polymers.

[FIGURE 4 OMITTED]

The differences between conventional silica and silica manufactured using the new technique can also be demonstrated using a transmission electron microscope (TEM). Figure 5 shows the differences between a new structure and a reference structure of conventional silica. The rough texture of the particle surface is particularly noticeable here. This roughness is the reason for the high BET surface value of the products. Due to the size of the CTAB molecule, CTAB measurements are not able to detect these differences. The roughness of the particles might also be a reason for altered behavior in the polymer matrix.

[FIGURE 5 OMITTED]

Summary

For many years, silicas have been used successfully in the tire industry. Fundamental changes in manufacturing techniques have now allowed the creation of a new class of products. These products exhibit a new structure which makes it possible to improve certain characteristics of filled rubber which hitherto seemed incompatible. For example, it could be demonstrated that it is possible to significantly enhance the reinforcing characteristics of silica without having to incur higher mixture viscosities and, therefore, disadvantages in process capability. This new product class provides an opportunity for creating further innovative products in the future. Although the development of these products with their unusual properties is in the incipient stage, it holds promise for new and interesting solutions in known areas of application and opens exciting perspectives for new fields of application.

Table 1--comparison of the surfaces of
conventional and new products

Conventional silicas    New class of silicas

BET   CTAB   BET/CTAB   BET   CTAB   BET/CTAB
              ratio                   ratio

120    120     1.0      290   130      2.2
170    160     1.1      310   140      2.4
210    190     1.1      345   170      2.0

Table 2--intrusion volumes of new products
compared with standard products

          Conventional silicas

          Pore vol.     Pore vol.
         (D >30 nm)    (D >50 nm)

U7K     1.48 (m1/g)   1.00 (ml/g)
EXP
 7006   1.99 (ml/g)   1.58 (ml/g)
Z1165   2.53 (ml/g)   2.11 (ml/g)

              New class of silicas

             Pore vol.     Pore vol.
            (D >30 nm)    (D >50 nm)

Silica A   4.14 (ml/g)   3.71 (ml/g)
Silica B   4.68 (ml/g)   4.23 (ml/g)

Silica C   4.60 (ml/g)   4.19 (ml/g)

References

(1.) A. Blume, S. Uhrlandt, paper presented at a meeting of the Rubber Division, American Chemical Society, April 4-6, 2000.

(2.) EP 0501227, U.S. 5.227.425.

(3.) S. Uhrlandt, A. Blume, KGK Kautschuk Gummi Kunststoffe 54 (10/2001) 520.

(4.) R.K. Iler, Chemistry of Silica, John Wiley, & Sons, 1979.

(5.) A. Blume, S. Uhrlandt, GAK 2 (1999) 116.

(6.) S. Brunauer P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309.

(7.) H.L. Ritter, L.C Drake, Ind. Eng. Chem. An., 17(1945) 782.