Process advantages by improved rotor cooling, part 2.

Last month's Process Machinery column (Rubber World, September 2005) began with a discussion of the overall energy balance, temperature influence and rotor design, presented an investigation of HESC rotor cooling capacity and set up an experiment, with results described below.

Results and discussion

Carbon black compounds (A, B, C)

Two mixing cycles showing typical examples of the mixing behavior with standard full-four-wing rotors and with HESC rotors are compared. In order to be sure to exclude first batch effects, a number of batches was run to reach steady operating conditions. In the first mixing step, run with high rotor speed, the HESC rotor showed significantly higher energy consumption. After a short ram lift, the rotor speed was reduced. At the beginning of the second mixing step, the difference in power consumption between the standard full-tour-wing and HESC were about equal. This difference decreased in the course of the second mixing step and disappeared by the end of this step. After a ram sweep, the power consumption was again higher with the HESC rotor.

It was also seen that the batch temperature remained almost constant during the complete mixing cycle, irrespective of the type of rotors. This is due to the steady state process, where the mixer has reached its operating temperature. The batches were mixed directly after one another. At the beginning of every cycle, there is a compensating effect: The feeding of the ingredients leads to a cooling of the mixing chamber/compound, whereas the friction within the compound caused by the motion of the rotors leads to an increase of the mixing chamber/batch temperature.

In spite of the higher energy consumption caused by the HESC rotors, there is no significant temperature increase in comparison to the full-four-wing rotors. This means that a higher energy input into the compound can be provided at comparable batch temperature due to the better cooling capacity of the HESC rotors. A higher energy input into a compound is usually accompanied by improved properties, e.g., carbon black dispersion (ref. 4). Despite the fact that a final batch is evaluated here, it could be shown that the physical properties (standard deviation of the maximum torque) of the compound improve through use of the HESC rotors. Due to the higher energy consumption, the batch temperature was kept on a higher level, and the dump temperature was reached earlier. Therefore, the mixing time was shorter.

Significant differences of energy input can be seen during the ram sweeps. During the first ram sweep, the energy input for the full-four-wing rotor is nearly zero, and the energy input for the HESC rotor is 250 kW. The difference during the second ram sweep is about the same.

An explanation for the effects observed can be seen as follows: If the machine is operated at too high temperatures, a thin layer of low viscous material will be generated between the batch and the mixer's steel contact surfaces, and slippage reduces the shearing stress [T.sub.w].

Conversely, it can be said that the surface temperature of the standard full-four-wing rotor rises to a higher temperature level due to lower cooling efficiency. Another problem, which reinforces the effect, is the poorer temperature distribution on the surface of this rotor. Due to the less uniform wall thickness of this conventional rotor, it stores more thermal energy in the wing area.

From the view of a control loop, the rotor acts as a dead time control element. When the machine is cold in the start-up phase of the production, the rotor needs some mixing cycles to reach thermal equilibrium. During this phase, heat will be accumulated inside the rotor mass. In the rubber industry, this effect is well known as the first batch effect. Because of the lower mass of the HESC rotor in the wing areas, the thermal equilibrium is reached much earlier. Therefore, the rotor temperature in the transient state of mixing processes with some interruptions can be controlled better with an HESC rotor.

During the mixing process, the energy input for the HESC rotors is clearly higher. The temperature rises faster, which reduces the mixing time. The mixing process with HESC rotors needs less time in all cases. A reduction of mixing time between 5% for the hardest compound C and 22% for the medium-hard compound B is seen. Consequently, the throughput of a mixing line can be increased significantly with use of the HESC rotors versus full-four-wing rotors.

In the case of final batch mixing, the deviation range of the curing characteristics, as provided by the analysis of rheometer tests, is an important criterion. Usually, the rheometer test is the only 100% quality check in a mixing room of the tire industry. The vulcanization kinetic was determined with a moving die rheometer. From each batch, two rheometer curves have been recorded, meaning that in all, data of 20 rheometer tests are in hand for each compound. The standard deviation of the maximum torque of all rheometer curves was calculated. For all compounds which have been mixed with HESC rotors, the standard deviation is much lower than in the case of the standard full-four-wing rotors. This example shows clearly that a higher mixing quality or a higher productivity at constant quality can be achieved with the HESC rotors.

In order to find an explanation for the more constant compound quality in the case of HESC rotors, the respective energy inputs have been calculated for all compounds. This analysis shows that the power consumption of the two rotor types is significantly different. For all recipes, the HESC rotors consume 10-20% more energy than full-four-wing rotors. As mentioned, the power consumption during the mixing cycle is often accompanied by an improved dispersion quality of the compound (refs. 4 and 6). Herewith, the improved rheometer results in the case of the HESC rotors can be explained in a better way. The higher energy demand is based on a higher batch motion along the flights and through the gap between the rotors and the mixing chamber.

Silica tread compound (D)

Currently, silica is replacing carbon black in tire tread compounds to a high degree in any markets. The silanization reaction has to take place within the mixing chamber. This means that the mixer becomes a reactor. For the silanization during mixing of tire tread compounds, bifunctional organosilanes are used. One of the different available types is TESPT (TESPT = Bis-(triethoxysilylpropyl)-tetrasulfane) (ref. 3). Silanization (sometimes called water repellent finishing) occurs when the triethoxysilyl group of the organosilane reacts with the silanol groups on the silica surface (refs. 8-10). The silanization reaction generates reaction products such as water and ethanol, which must be removed from the mixer by repeated ram sweeps and an effective venting system. From previous publications (refs. 11-13), silanization with sufficient results is achieved at a batch temperature from 145-155 [degrees]/ 293-311 [degrees]F. Above this temperature, an additional reaction can occur. The sulfur/sulfur bonds are weak and can be broken by the shear forces and heat during mixing. This leads to free sulfur in the compound. At temperatures higher than 160 [degrees]C/320 [degrees]F, the free sulfur can react with the polymer. This pre-crosslinking has to be avoided. A comprehensive list of literature can be found in the review article by Gorl (ref. 14).

For this reason, intermeshing machines are also becoming even more interesting for the tire industry because they are known to generally provide better cooling behavior at the same batch weight. The batch temperature can be controlled very well, so the temperature history can therefore be ensured reproducible. While the advantages of intermeshing mixers for the production of silica compounds and some basic requirements of the machine have been discussed (ref. 5), an improved cooling capacity, as provided by the tangential style HESC rotors is, of course, of great interest for manufacturing silica compounds, too. These rotors fit into the 270-liter standard machine, which is in widespread use in the tire industry.

The heat history of a silica compound is of great importance. This means that the silanization reaction can take place at a comparably low temperature for a longer time, or vice versa. It is historically more difficult to run the mixer at a higher temperature because of the risk of hot spots. This risk is reduced with the HESC rotors due to the better temperature distribution on the rotors' surfaces. Therefore, tests have been made running the mixer with a different heat history. Power consumption and batch temperature were examined for a mixer which was operated with fullfour-wing rotors at first, and then with HESC rotors afterwards. The mixing procedure of the full-four-wing experiment was used as the master reference curve. The rotor speed of the experiment with HESC rotors was maintained at a higher level after the first ram lift. Consequently, the batch temperature rose faster than the one with the standard full-four-wing rotors. In the ongoing mixing process, the batch temperature was kept constant in order to allow the reaction. This was done by adjustments of the rotor speed, controlled by a software routine, focusing the silanization reaction. In the case of the HESC rotors, the rotor speed had to be increased at the end of the mixing cycle to keep the temperature on the same level. Otherwise, it would have decreased because of it superior cooling characteristics.

Due to the better temperature distribution provided by the HESC rotors, the batch can be run at a higher temperature level. The risk of hot spots in the compound when using the HESC rotors is reduced. This means that mixing times can be reduced significantly if a constant temperature history is assumed. It is known that silica compounds need to have a similar temperature history during the silanization phase. The silanization reaction can be accomplished at a low temperature in combination with long mixing times and vice versa.

It can be seen that the power consumption of the batch mixed with HESC rotors is comparable to the standard full-four-wing rotors, but in about half of the elapsed mixing time. It was possible to drop the batch mixed with the HESC rotors much earlier because the same temperature history was reached as for the standard full-four-wing rotor. This results in almost one third of the total mixing time being saved.

Conclusions and prospects

A significant improvement of cooling efficiency and temperature control is provided by the HESC rotor design (HESC = high efficiency super cooling). Certain modifications of the construction have led to a highly effective mixing system with excellent temperature control capabilities. It has been demonstrated in production that use of the HESC rotor leads to a higher energy input into a compound, resulting in reduced mixing times.

Investigations have also shown that the standard deviation of the maximum torque values inside a batch and from batch-to-batch can be reduced about 50%. This offers more possibilities to reduce mixing times with the same quality.

Advantages of the HESC rotors can be summarized as follows: The use of HESC rotors results in higher productivity and improved compound quality. Furthermore, the improved cooling capacity of the HESC rotors provides significant advantages for the mixing of silica compounds.

by Fabian A. Schmahl, Andrea Limper and Harald Keuter, HF Rubber Machinery