Silica/silane - a winning reinforcement formula.

The reinforcement of polymers by fillers is of great practical and technical importance. Today, rubber products fulfill many important functions in almost all areas of life, in the household just as much as in the aviation and space industries. Their performance depends on the right combination of polymers, rubber chemicals and reinforcing filler systems.

Carbon blacks are the classical reinforcing fillers for use in rubber products. Approximately 90% of the world's carbon black production is used in rubber compounds to improve tensile strength, abrasion resistance, skid resistance or further enhance physical properties. A typical rubber compound used for car tires contains, for example, approximately 30-35% by weight of carbon black.

Because carbon blacks can only be used in those rubber articles which are black in color, the search for alternative active fillers which permit the production of highly durable colored products led to the development of precipitated silicas.

Precipitated silica in rubber applications

In 1951, the first commercial precipitated silica product was marketed under the name Ultrasil VN3 in Europe. In the beginning, the uses of silica as a reinforcing filler instead of carbon black were limited due to various problems:

* Higher compound viscosity due to high filler/filler interaction;

* more difficult mixing and processing;

* longer vulcanization times; and

* lower crosslinking density.

Therefore, for a long period of time, precipitated silica-based rubber compounds could only find market share in special niche applications. Besides applications in light-colored or transparent articles, e.g., shoe soles, precipitated silicas were also used in small amounts in tire treads for commercial vehicles to improve tear propagation resistance.

A further extension of the use of precipitated silicas in rubber compounds could be achieved using these fillers, together with cord bonding agents, for compounds in the belt/carcass region of tires. The well-known RFS-bonding system is based on three components: Resorcinol, a formaldehyde donor (methylene component) and reinforcing silica filler.

Apart from the above mentioned success of precipitated silicas in rubber compounds, the limitation was principally given by the surface chemistry of the filler and its ability to interact with the polymer. Figure 1 exhibits the differences in the surface chemistry of carbon black and precipitated silica.

[Figure 1 ILLUSTRATION OMITTED]

General purpose rubbers, such as natural rubber, SBR and BR, are non-polar materials. Silicas have polar silanol groups on their surface, which leads to different physical properties in these rubber compounds in comparison to carbon black. It was, therefore, essential to modify the hydrophilic fillers via a chemical reaction on the surface.

Surface modification by silanes

The bifunctional organosilane, Si 69, was introduced in 1971 with the following chemical formula (xbar = 3.8):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The ethoxy groups on this molecule are able to react with the silanol groups on the silica surface, thereby creating a stable bond between the silane and the filler. The second functional group permits the formation of crosslinks with the polymer, which results in filler/rubber crosslinks via the organosilane.

With the use of the organosilane Si 69, the reinforcement with silica in non-polar polymer compounds could be improved considerably. This was the beginning of the chemical reinforcement of fillers. With this breakthrough in reinforcement by a silica/silane system, the first practical competition towards carbon black began. In tires, as well as in mechanical rubber goods applications, this new filler system began to replace carbon black in growing amounts. Especially in applications where reduced heat generation and improved wet traction were requested, the silica/silane system offers the best solution.

Furthermore, silica/silane was used in certain amounts by major tire producers in cap and base formulations of truck tread compounds to improve heat generation. In the tread compounds of motorcycle and winter tires, where good wet grip is highly desirable, the silica/silane filler system partly replaces carbon black.

But the further extension of silica/silane in the tire industry, especially in tread compounds, was still limited due to:

* Weakness in abrasion resistance;

* poor dispersibility; and

* more difficult processing.

Silica/silane - part of the green tire concept

In 1992, a well known European tire manufacturer introduced the so-called "green tire concept" (ref. 1). This development of passenger car tire treads based upon a highly dispersible silica/silane system and a special solution-SBR polymer allowed a considerable progress in the performance of the tire. With this filler system, simultaneous improvements of seemingly contradictory tire properties, e.g., rolling resistance and wet grip, are now possible while still maintaining the service life (refs. 2-5) expressed as abrasion resistance. This leads to a considerable extension of tire performance. Figure 2 illustrates the improvement in tire performance which could not be achieved previously with carbon black alone.

[Figure 2 ILLUSTRATION OMITTED]

Since that time, the silica/silane filler system has become a winning formula in the reinforcement of rubber compounds, especially in the tire industry.

In Europe, the silica/silane filler system has already replaced carbon black to a significant extent in car tire tread compounds for the original equipment market (OEM) and for winter tires. Particularly the better wet traction brought silica/silane into a winning position in comparison to carbon black.

Such modern car tire tread compounds require the usage of advanced silica grades with excellent dispersibility. Degussa-Huls now offers a tailor-made silica product for this application. Tailor-made in this respect means not only product performance, but also that the production process of a modern silica is controlled accurately, which results in a very narrow variation range for the analytical properties of the silica.

This in turn leads to a very high reproducibility of the specific surface and moisture content, which are best suitable for the chemical reaction with the organosilane Si 69.

Tailor-made highly dispersible silica

Dispersion 

It is known from different investigations that silica dispersion is the most important precondition for excellent treadwear. Due to a strong filler/filler network, the main problem faced by using traditional silicas is a poor dispersion behavior. The elimination of this weakness is a major goal for development. To reach this, development of a suitable method for dispersion measurement is also necessary.

Among different methods, the easiest way to determine the dispersion is the optical microscopy (Phillips method). The conventional silica Ultrasil VN 3 GR shows a lot of undispersed particles in the green tire tread compound. The use of the HD-silica Ultrasil 7000 GR results in extremely high dispersion levels with modern S-SBR/BR tread compounds (figure 3).

[Figure 3 ILLUSTRATION OMITTED]

In another test with the Surfanalyzer, a tiny needle scans the vulcanizate surface and monitors its roughness (figure 3). The comparison of the topography of the conventional silica and the HD-silica (ref. 6) shows quite clearly the different dispersion behavior in robber.

A dispersion coefficient factor has been developed in order to detect even small differences in the dispersion behavior. Testing was carded out according to the method described in the literature (ref. 7). The vulcanizates are cut into thin glossy pieces using a razor blade and investigated under an optical microscope. The resulting photos with a wide scale of grey tints are then reduced to black and white. The black areas represent the silica, the white area the background. The dispersion coefficient can be calculated on the basis of the size of the black and white areas usually on 40 different photos. For the calculation of the dispersion coefficient, the filler volume and a correction with the Medalia factor (ref. 7) are taken into account.

D(%) = sum of particle area/number of photos [multiplied by] 10 000 [multiplied by] Medalia factor/filler volume [multiplied by] area of the photos

Medalia factor = filler volume/100 + 0.78/2

Figure 4 shows such dispersion coefficients for various silicas. The conventional silica, the granulated Ultrasil VN3 GR, gave a poor dispersion coefficient of only 72%; the best dispersion would have been 100%. Ultrasil 3370 GR shows improved dispersion behavior of about 86%. Only the use of Ultrasil 7000 GR in the tire tread leads to a dispersion coefficient of 97%. Results of the abrasion index of actual tire tests on the road confirm this ranking of Ultrasil 7000 GR (figure 4).

[Figure 4 ILLUSTRATION OMITTED]

Normally, mixing processes are optimized to achieve the best balance between dispersion, i.e., distribution of the ingredients and the economy. For compounds highly loaded with silica and silane, the silanization step is the determining factor, which needs excellent control of temperature.

The better dispersion of the silica gives the advantage that the filler surface becomes accessible faster for the silane and provides more flexibility in adjusting the mixing processes to the chemical reaction of the silanol groups with the triethoxy groups of the silane.

Monitoring the quality

Dispersion

The quality and especially the dispersibility of a high-tech product like Ultrasil 7000 GR has to be monitored carefully. Degussa-Huls has developed a new measurement technique to predict the dispersion behavior of a silica in rubber compounds, based upon a single analytical method (ref. 8). The use of the laser diffraction measurements for the determination of particle size distribution allows the detection of very small differences in the dispersion behavior (figure 5).

[Figure 5 ILLUSTRATION OMITTED]

The deagglomeration of the silica using this method is achieved by ultrasonic treatment of the silica in suspension and detection by simultaneous laser diffraction. It is possible to detect particle sizes between 40 nm and 500 [micro]m. The resulting particle size distribution of Ultrasil 3370 GR and of Ultrasil 7000 GR shows a bimodal distribution. The main peak in the range of about 10 [micro]m describes the initial structure with large agglomerates of silica. This higher structure is partly destroyed during ultrasonic treatment which simulates the energy input during the mixing process of the rubber compound. The easier this deagglomeration, the larger and bigger the first peak in the range of 0.5 [micro]m. At constant ultrasonic energy input, the deagglomeration (dispersion) of the HD-silica is more advanced than with the conventional grade, and the portion of deagglomerated small particles is significantly higher. The relation between the peak height of the original agglomerates and the peak height of the reduced agglomerates is defined as the WK-coefficient (figure 6).

[Figure 6 ILLUSTRATION OMITTED]

There is a good correlation between this WK-coefficient and the dispersion coefficient (figure 6). The lower the WK-coefficient (which means a larger and higher peak after deagglomeration) the better the dispersion. With this, a suitable analytical method was developed to monitor the quality of the HD-silica (figure 7).

[Figure 7 ILLUSTRATION OMITTED]

The excellent quality of the HD-silica offers the perfect solution for the improvement of rolling resistance as well as wet grip without any compromise in treadwear.

High consistency in specific surface and moisture content

Most of the HD silicas now being sold are used in tire tread compounds. To achieve the best performance of these compounds, highly loaded with silicas and silanes, it is necessary to achieve a high level of hydrophobation to enable best processing and coupling efficiency, and finally performance of the rubber article.

High consistency of the specific surface area (CTAB) is important for a compound viscosity with narrow tolerances and therefore controlled extrudability. Basic investigations have demonstrated that the moisture content of the silica has a significant influence on the hydrophobation reaction, and therefore narrow tolerances in moisture content are a prerequisite for the uniformity of the mixing procedure. HD-silica Ultrasil 7000 GR, produced to far narrower tolerances than standard grades, has the best qualification for the use in modern car tire tread compounds highly loaded with silica and silane.

Future innovations

The replacement of carbon black by the silica/silane filler system in car tire tread compounds has already reached a high level in the original equipment market in Europe due to requirements of wet traction and rolling resistance by the automotive industry. Further extension of silica/silane systems in the replacement market are to be expected. In modern winter tires, silica/silane has already replaced carbon black to a high extent.

In truck tire tread compounds, the use of silica/silane is limited due to the advantage of carbon black in abrasion resistance, especially under high severity wear conditions.

Consequently, further extension of the silica/silane filler system depends on innovations, tailor-made for the different tire compounds.

Further reduction of rolling resistance

The automotive industry additionally requires a further reduction of the rolling resistance of 20% for car tires, which would lead to additional fuel savings of approximately 5%.

The possibility of achieving this level of reduction of rolling resistance just by a further optimization of the tread compound is very limited.

By looking at a tire as a whole, other potentials can be identified. The different parts of a passenger car tire contribute to overall rolling resistance in different amounts (figure 8). The components of the truck tire body influence the rolling resistance to a much greater degree than those of passenger car tires.

[Figure 8 ILLUSTRATION OMITTED]

This means that the tire body components contribute approximately 50% to the rolling resistance of a passenger car tire and about 70% to that of a truck tire. Traditionally, most compounds of the tire body are filled with carbon black.

Other important properties mainly associated with truck tires are the service life and the durability of the casing to guarantee effective retreading. An improvement in this area can be obtained by reducing the heat build-up of the tire.

Degussa-Huls has developed a new, highly dispersible silica for use in different compounds of the tire body. This experimental low surface area product EXP 7006 has a CTAB surface area of about 120 [m.sup.2]g. With the use in different compounds of the tire body, the following behavior can be achieved:

* Excellent processing properties;

* good dispersion behavior; and

* good dynamic properties.

The WK-coefficient (refs. 5 and 6) is very low. This suggests that the dispersion behavior (ref. 7) of the silica inside the rubber compound is better than 95%.

In-rubber properties

EXP 7006 was tested in a basic carcass compound (NR/ESBR). The newly developed silica was compared with traditionally used carbon black types and conventional precipitated silicas. A three-stage mixing procedure was employed for each compound, and for the determination of compound and vulcanizate properties, standard ISO, DIN or ASTM test procedures were adopted. Dynamic testing was conducted on an MTS 831 tester according to DIN 53513. Figure 9 shows the result of the NR/E-SBR compound.

[Figure 9 ILLUSTRATION OMITTED]

In comparison to the conventional compounds, the sample made with EXP 7006 shows clear advantages in heat build-up (increase of the temperature during dynamical strain), in dispersion behavior and tan [Delta] 60 [degrees] C (which correlates with the rolling resistance). The static data (not shown here) are on the same level.

Only with this newly developed silica (together with appropriate amounts of silane) such a low level of tan [Delta] is achievable, a feat which cannot presently be matched by carbon black.

New silicas for tread compounds

The abrasion resistance of HD silicas with a CTAB specific surface area of 160-170 [m.sup.2]/g meets the requirements of car tire tread compounds. In truck tire tread compounds, silica/ silane cannot compete with high surface carbon blacks concerning abrasion resistance. Silicas with higher surface area are now in development to meet the requirements of truck tread compounds and to establish silica technology in track tires.

Silica/silane has been established in winter tires due to superior wet grip. New silanes with reduced filler/filler network are in development to lower the dynamic stiffness to improve snow traction. With tailor-made silicas, a higher penetration of this filler system in winter tires will occur in the near future.

Conclusion

The market introduction of the green tire concept brought the silica/silane filler system in direct competition with carbon black. Due to the outstanding performance regarding wet traction and rolling resistance, the silica/silane system is a winning formula in reinforcement, and therefore a component which becomes more significant in rubber compounds.

New silicas with adjusted specific surface area, surface chemistry and agglomerate structure are in development. This can lead to further displacement of carbon black in different tire compounds. The competition between silica/silane and carbon black is now the driving force for further innovations.

References

(1.) EP 0501 227, U.S. 5.227.425.

(2.) S. Wolff, "The influence of fillers on rolling resistance," presented at the 129th Meeting of the Rubber Division, American Chemical Society, New York, April 8-11,1986.

(3.) G. Agostini, J. Bergh and Th. Materne, New Compound Technology. Oct. 1994, Akron, OH.

(4.) G. Heinrich, Materialforschung am Reifen, presented at the workshop "Reifen" of the Deutsches Institut fur Kautschuktechnologie, May 25-26, 1998.

(5.) S. Wolff, "Silica based tread compounds: Background and performance," TyreTech, Basel, Switzerland, Oct. 1993.

(6.) A. Blume, paper 73, presented at a meeting of the Rubber Division, American Chemical Society, Chicago, Illinois, April 13-16, 1999.

(7.) R.H. Schuster, H. Geisler and D. Bu[Beta]mann, 2nd Conference on Carbon Black, Mulhouse (France), Sept. 1992.

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