Mixing of silica compounds from the view of a mixer supplier.

New developments in tire performance have led to the use of silica instead of carbon black in tire stocks (refs. 1-4). A lower rolling resistance (reduced fuel consumption) and better skid properties (improved safety) are arguments for the use of tires made from silica compounds.

In tire compounds, carbon black as the reinforcing filler is substituted partially or totally by silica. The silica itself does not lead to better properties in comparison with carbon black. Silane as an additional chemical ingredient in the compound is used as a coupling agent between silica and polymer. During mixing, the silane reacts with the silica and develops open movable chemical bridges which are able to react later with the polymer during the vulcanization process.

Because of the chemical reaction during the mixing process, the internal mixer acts as a chemical reactor in which the batch temperature control must be very accurate. In addition to the dispersion of filler in the compound, the reaction must be achieved. On the basis of this knowledge we have to consider the actual process technology. Do we have the optimal equipment to produce silica compounds?

This article gives an overview of today's mixing equipment available to produce silica compounds and the additional demands placed on the mixing process. Different mixer systems will be compared, and further requirements of the mixer itself and the control system will be discussed.

Silanization

A typical formulation for a tire compound with silica as filler is shown in table 1. Here the carbon black is 100% substituted by silica. The main difference in comparison with a carbon black compound is the amount of silane (6 phr) in the formulation. Silane, or more precisely organosilane, is a fluid which can be injected into an internal mixer during the mixing process. For this purpose, special equipment for storing and transport is necessary because silane is a very reactive material which may not come in contact with oxygen and water. To maintain the normal method of raw ingredient handling, a second possibility exists. Organosilane is also offered with carbon black as a base material (ratio: 1:1). In this case, organosilane can be handled as a minor chemical ingredient and pre-weighed in plastic bags. However, also with this material, longer contact with oxygen and water must be avoided.

Table 1 - typical
100% silica
compound
formulation

Polymer        100 phr
Silica          80 phr
Silane         6.4 phr
ZnO              3 phr
Stearic acid     2 phr
Aromatic oil    35 phr
Wax              1 phr
Antioxidant    1.5 phr
Curatives      3.5 phr

For the silanization during mixing of tire tread compounds, bi-functional organosilanes are used. One of the different available types is TESPT (Bis-[triethoxysilyl-propyl]-tetrasulfane). Silanization, or sometimes called water repellent finishing, occurs when the triethoxysilyl group of the organosilane reacts with the silanol groups on the silica surface (refs. 5-7). In figure 1, the different steps during silanization are shown in detail. First, the organosilane has to be transferred to the surface of the silica particles. There, the silanization reaction takes place, and reaction products like water and ethanol are generated. These products must be removed from the mixer by repeated ram sweeping and an effective venting system.

[ILLUSTRATION OMITTED]

From previous publications (refs. 8-11), silanization is best achieved between a batch temperature of 145 to 155 [degrees] C. Above this temperature an additional reaction can occur. The sulfur/sulfur bonds are weak and can be broken by shear and heat in mixing. This leads to free sulfur in the compound. At temperatures higher than 160 [degrees] C, the free sulfur can react with the polymer. This pre-crosslinking has to be avoided. A comprehensive list of literature will be found in the review article by U. Gorl (ref. 8).

The compound viscosity decreases substantially during the mixing process (ref. 12). At the beginning of mixing, the Mooney value is much higher than 100 (values above 200 are possible). During silanization, the bonded organosilanes at the surface of silica act as a slip additive and reduce the viscosity of the batch. At the end of the silanization process, the Mooney value is reduced to 50 or less. This means for the mixing process that the main dispersion of silica takes place in the beginning, so that all silica has to be fed very early (ref. 2). During dispersion of silica, the silica agglomerates will be destroyed and the open surface area increases substantially. Then is the best time to feed the organosilane into the mixer.

Chemical reactions are temperature dependent. This means the higher the temperature the faster the reaction. Figure 2 shows the qualitative relationship between batch temperature and mixing time. With higher batch temperature, the mixing time can be shortened and the productivity of the mixing line can be increased. On the other hand, when the batch temperature is too high, the sulfur of the bonded organosilane reacts with the polymer chains. The ideal situation would be to have the same batch temperature everywhere inside the batch, and the temperature as near as possible under the limit of the maximum allowable batch temperature where no sulfur reaction occurs. But these conditions do not exist. In reality there is a temperature distribution inside the batch. The batch temperature can vary up to 20 [degrees] C and more, depending upon the kind of rotor geometry and the process conditions. Therefore, it is necessary to have a safety region between the average batch temperature and the maximum temperature. The lower the temperature variation inside the batch, the closer the process can be run to the limit, resulting in maximum throughput.

[GRAPH OMITTED]

To get an idea about the quantity of ethanol and water formed during the silanization process, a quantitative determination is shown in table 2.

Table 2

Assumed batch weight      : 270 kg (mixer volume = 330 [dm.sup.3];
                            fill factor = 70%)
Portion of silica         : 95 kg (80 phr)
Portion of organosilane   : 7.6 kg (6.4 phr)
Reaction conversion       : 1 mole organosilane creates 2
                            moles ethanol
Ethanol                   : 1.3 [dm.sup.3] [congruent to approximately
                            equals] 1[m.sup.3] ethanol vapor at
                            150 [degrees] C
Water                     : 4.8 kg (4-6 mass-% of silica)
                            [congruent to approximately equals]
                            9 [m.sup.3] water vapor at 150 [degrees] C

If silanization is finished inside the mixer, nearly 1 [m.sup.3] ethanol has to be removed from the mixer. Because it is not clear if all the water vaporizes during silanization, the amount of water vapor can be less than the above calculated value.

For an optimal silanization inside an internal mixer, the following conditions need to be fulfilled during the mixing process:

* Uniformity of batch temperature;

* control of batch temperature;

* control of chemical reaction; and

* reaction products (ethanol, water) have to be removed.

Tangential/intermeshing rotor systems

Basically two kinds of internal mixer types can be distinguished: Tangential systems and intermeshing systems.

Tangential system

The distinguishing characteristic of this type of machine is that the paths of the blade tips only just fail to touch (figure 3). This means that in machines of this type, the rotors can mm at different speeds (1:1.1 as a rule). Various rotor shapes are available for different mixing jobs. The basic difference between them is the different way in which energy is applied: That is, rotors which mix more dispersively and rotors which mix more distributively. In the tangential system there is a lot of space between the rotors. This leads to excellent filling and discharge behavior. Mixers of the tangential type are manUfactured with empty volumes ranging from around one to 650 liters. The effective volumes during mixing may generally range from 75% to 85% of the empty volumes, depending on the kind of compound.

[ILLUSTRATION OMITTED]

Intermeshing system

Further developments in mixing equipment lead to the intermeshing system (see figure 4). In this system, the center line distance of the rotors is less than the rotor diameter. Therefore, the paths of the rotor tips cross the working area of the other rotor and both rotors must be driven with even speed. Only a small space remains between the rotors. When the blades of one rotor pass the area of the second rotor, there is only a small gap between the wing tip and the body of the other rotor. This gap ensures a strong additional mixing effect. Because of the small space between the rotors in intermeshing internal mixers, the fill factor is approximately 5% lower than with tangential mixers. Intermeshing internal mixers range in size from one to 320 liters.

[ILLUSTRATION OMITTED]

Comparing tangential and intermeshing systems

The properties of both systems are shown in figure 5. Benefits of the tangential internal mixer are its good feeding and discharging speeds. In contrast, the intermeshing machine offers primarily qualitative advantages (better dispersion) because of the additional mixing effect between the rotors, and the better batch temperature control because of larger cooling surface inside the mixer (at same empty volume). Figure 6 presents a comparison of the empty volumes and internal surface areas of the tangential and intermeshing inteRNal mixers. With an empty volume of around 255 liters, the tangential internal mixer GK 255 N has an internal surface area of 6 [m.sup.2]. On the other hand, with an empty volume which is almost the same, the intermeshing mixer GK 250 E has an internal surface area of 7.2 [m.sup.2]. This yields an internal surface area which is about 20% larger. Once this is known, it should be evident that intermeshing internal mixers can have better temperature behavior than tangential mixers.

Figure 5 - tangential/intermeshing internal mixer comparison

Machine type    Energy      Batch temp.  Batch temp.
               efficiency     control    uniformity
Tangential         +             +           -
Intermeshing       ++           +++          ++

Machine type     Mixing      Intake    Discharge
               efficiency   behavior   behavior
Tangential         +           ++         ++
Intermeshing      +++          +          +

+++ very good, ++ good, + satisfactory, - not satisfactory

[GRAPH OMITTED]

Traditionally, tangential internal mixers are used in the tire industry. However, views have changed somewhat since the arrival of silica compounds. This type of mix calls for a defined and reproducible temperature history, which is much more readily achieved with the intermeshing system, which is more sensitive to batch temperature than tangential systems.

Requirements of internal mixer design

For the design of an internal mixer for the production of silica compounds, several demands must be met.

Optimum cooling efficiency

All inner parts of the internal mixer which come in direct contact with the stock have to be cooled. Therefore, the chambers and the drop door should have drilled passages close to the inner surface (see figures 3 and 4). The rotors, which are a big part of cooling in an internal mixer, especially in intermeshing machines, should be of two-piece design. That means a rotor body with a cooling channel near the surface, and mounted on a shaft. A cored type, cooled ram and rotor end plates with drilled passages complete the picture of a heat sensitive machine. To obtain the best cooling efficiency, it is also desirable that the thickness of the walls between the inner surface of the mixer and the cooling water passages in the chamber sides be as small as possible.

Variable rotor speed

Variable rotor speed is one of the main demands of a modern internal mixer. In particular for the manufacturing of silica compounds, such variable rotor speed is an essential tool for optimal processing performance. Given a specific mixer, the rotor speed is the main process parameter to control the batch temperature.

Corrosion resistant hopper assembly

During production of silica compounds, water vapor arises due to the silanization reaction inside the mixer. This has to be removed from the mixer body by ram sweeping and from the hopper by suction. When water vapor condenses on the cooler surfaces of the hopper assembly, corrosion may occur and contaminate the stock with mst. Because of corrosion attack, the clearance between ram and hopper walls will increase and material will stick more and more in these opened gaps. Increasing production disturbances will be the result. To avoid such production problems, a stainless steel or chromed hopper assembly is deemed necessary.

Hydraulic hopper

Pneumatically operated rams are influenced by pressure variations within the plant's compressed air network. These changes in air pressure influence the mixing process and, at the very least, the compound quality.

When a separate hydraulic power unit is devoted exclusively to drive each mixer, negative influence from the operation of different machines is not present. The hydraulically driven ram is therefore much more precise in ram force adjustment than a pneumatically driven ram. An additional advantage is the possibility of rapid process dependent adjustments of the ram force.

In most cases, it is much more economical to utilize a hydraulic ram system than a pneumatic system.

Precise batch temperature measurement

For optimal controlling of the batch temperature, a precise measurement is necessary. It is not important which temperature sensor type is used (thermocouple or Pt 100). But it is required that the compound flows over and around the tip of the temperature sensor so effective heat exchange between stock and sensor tip is ensured. Therefore, the flow characteristics inside the mixer must be very well known so that the sensor position inside the mixing chamber can be optimized. It is helpful to have several temperature sensors at different positions in the mixing chamber, and for batch temperature control, to use the sensor with the highest temperature indication. This is especially important when the flow behavior of the mix changes during the mixing cycle. This happens during the silanization process when the compound viscosity decreases considerably. It is well known that the optimal fill factor of the mixer is dependent on the viscosity of the compound. The lower the viscosity, the higher the fill factor. This means for silica mixing that the mixer will tend to be under-filled with more advanced silanization.

Hard-coating inside mixer

For wear protection, internal mixers are provided with a hard coating. For this purpose, different types of welding materials are in use. Typically, these materials are building hard (e.g., chrom-) carbides as base for wear protection embedded in a softer matrix. With the required hardness for sufficient wear life, these coatings develop cracks down to the base material. This can lead to corrosion of the base material through the cracks and ultimately destruction of the hard coating.

With a new crack-free hard coating, the corrosion of base material can be avoided, and the wall thickness between mixing chamber inner surface and cooling water can be reduced. With crack-free hard coating, the heat sensitivity of the machine is therefore enhanced.

Dust stop design

There are zones inside internal mixers where material may remain. These zones are the axial gaps between rotors and rotor end plates and the radial gaps between rotor shafts and side walls, when silica compound remains in these gaps over extended time, uncontrollable reactions can occur and a batch can be contaminated with improper material. From the view of the process engineer, there will be no contamination until no gaps are present. In reality, however, this is not possible. Therefore, for silica mixing, the better alternative among the various types of dust stop arrangements is the hydraulic system, where the sealing face is directly at the rotor ends. No radial gaps between rotors shafts and end plates exist, and the remaining volume where material can stagnate is very small.

Process control system

For the production of silica compounds, the process has to be run quite differently compared with carbon black compounds. When the maximum batch temperature is reached, the stock cannot be discharged. Now the mixer and control system have to regulate the silanization process. That means keeping the batch temperature nearly constant, and discharging of the stock when the silanization is finished. For this purpose, the reaction kinetics of the silanization process must be described and implemented as a separate module in the mixer control system.

Tangential rotors with better cooling efficiency

HESC (high efficiency super cooling)

There are several hundred 270 liter tangential internal mixers in operation in the tire industry. The rotors of these mixers are very slim and can only be manufactured in monobloc style. This construction with relatively thick walls and a spray cooling system inside is not very efficient in cooling.

To improve the poor cooling behavior of such rotors, a new rotor with superior cooling efficiency was developed. This HESC rotor was specially designed for 270 liter machines. The shape of the rotor remains the same as before, it is just the cooling system inside of the rotor which is newly designed. Figure 8 shows a direct comparison. Both rotors were heated to a temperature of about 90 [degrees] C, and then cooled down with a cooling water temperature of about 18 [degrees] C. The surface temperature distribution was recorded utilizing infrared thermography. On the left side of figure 8, the snapshots of the rotors at the beginning of the cooling phase are shown, and on the right side after five minutes of cooling-down time. The two top snapshots show the standard rotor with spray cooling, while the lower snapshots show the new HESC rotor. After a cooling time of five minutes, the HESC rotor had cooled down quite uniformly to a temperature of 20-30 [degrees] C, while the conventionally configured rotor did not cool uniformly. It is interesting to note that inside the wings, where the wall is very thick, that heat was retained for a long time.

[GRAPH OMITTED]

Comparison of the cooling power during production has shown that the HESC rotor has more than double the cooling effect of the old rotor design (figure 9). Because of improved cooling rate and the more uniform surface temperature, HESC rotors are better suited for the production of silica compounds in a 270 liter mixer than conventional rotors used as standard in the industry.

[GRAPH OMITTED]

Replacement of a 270 liter tangential machine with an intermeshing machine

To make use of the advantages of an intermeshing internal mixer, a 270 liter machine can be replaced with a 250 liter intermeshing machine. The empty volume of the intermeshing machine is slightly greater than the 270 liter machine outfitted with four-wing rotors. On the other hand, the intermeshing machine has a lower fill factor. Therefore, both machines can be run with approximately 230-240 kg batch weight (assumed sp. gr. of 1.2 kg/liter). With nearly the same power requirement, gear box and motor can be retained when swapping out the 270.

The rotor center distances of both machines are not equal; there is a difference of only 10 mm. For this reason, two different ways of replacement are possible. If the space between gear box and mixer is sufficient, the difference in the center distances can be easily compensated for by proper couplings. Alternately, in some cases it is possible to change the spacing of one drive shaft in the gear box. Both possibilities were already implemented in practice. Regardless of which kind of replacement is used, the output shafts of the gear box have to operate with even speed.

It is also necessary to check the structural strength of the mezzanine, since the weight of an intermeshing machine is more than a comparable tangential machine. In general, the mezzanine may require some stiffening.

Conclusions

The production of silica compounds requires optimal temperature control during the mix cycle. Accurate batch temperature control and a high temperature uniformity inside the batch have to be achieved. With controlled uniform batch temperature, a close approach to the temperature limit is possible, and the cycle times can be reduced to a minimum, yielding maximum productivity. Intermeshing internal mixers with the newest rotor technology (PES 5 rotors) meet these requirements the best.

In comparison with tangential systems, the stages of the silica compound production process can be reduced with an intermeshing internal mixer. The production of silica compounds for the tire and technical rubber industries can be carried out more economically with intermeshing machines.

(This article is based on a paper given at the October, 2000 meeting of the Rubber Division)

References

(1.) S. Wolff, Silanes in tire compounding after 10 years - review, Third Annual Meeting and Conference on Tire Science and Technology. The Tire Society, March 28-29, 1984, Akron, OH.

(2.) L.R. Evans, S.D. Patkar and W.H. Waddell, presented at a meeting of The Polymer Processing Society, Akron, Obi, November 14-16, 1995.

(3.) L.R. Evans and W.C. Fultz, paper no. 15, presented at the 150th Technical Meeting of the Rubber Division, ACS, Anaheim, CA, May 6-9, 1997.

(4.) J.-A.E. Bice, S.D. Patkar and T.A. Okel, paper no. 17, presented at the 150th Technical Meeting of the Rubber Division, ACS, Anaheim, CA, May 6-9, 1997.

(5.) Degussa-Huls AG, Technical Report 5/1999.

(6.) Degussa-Huls AG, Technical Report 6/1999.

(7.) Degussa-Huls AG, Applied Technology Advanced Fillers, Product Information Si69, X50-S and VP Si75.

(8.) U. Gorl and A. Parkhouse, Gummi Fasern Kunststoffe 51 (7-8), 416 (1998).

(9.) U. Gorl and A. Parkhouse, Kautschuk Gummi Kunststoffe 52 (7-8), 493 (1999).

(10.) U. Gorl, J. Munzenberg, D. Luginsland and A. Muller, Kautschuk Gummi Kunststoffe 52 (9), 588 (1999).

(11.) B. Schwaiger, H.-D. Luginsland and A. Hasse, presented at VDI Jahrestagung Elastomerverarbeitung, Braunschweig, Germany, February 22-23, 2000.

(12.) G. Nijman and R. Luscalu, Tire Technology International (June), 56 (1999).