Optimizing mixing in the Farrel Banbury mixer with wing function.

The primary function of a high intensity batch mixer is incorporating difficult to mix ingredients into polymeric materials without destroying the basic polymer properties. This process requires dispersion of complex agglomerates with good distributive mixing and temperature control to ensure composition and thermal uniformity of the processed materials. In recent years, an increased activity in rotor design has been observed for both tangential and interlocking mixer designs aimed at improving their productivity and mixing efficiencies. A different approach in mixer rotor design has been used by the various machine manufacturers (ref. 1) to arrive at these goals. However, all rotor designs rely on the same basic principle of operation to achieve the desired level of mixer performance, and that is: Mixing is achieved by the repeated passage of the compound through regions of high shear followed by intimate mixing of the sheared material within the batch. The general approach by each machine manufacturer was to add additional wings or rearrange the existing rotor wings. In each case, the basic rotor wing geometry remained unchanged.

Farrel used a novel approach in developing a new line of rotor designs for the Banbury product line. The mixing process taking place in a batch mixer was reduced into the basic processing functions, i.e., dispersive and extensive mixing (refs. 2 and 3), and these functions were then assigned to different geometrical sections of the rotors. The basic concept of a rotor composed of wings originating from either end was retained, but each rotor wing was now assigned a specific process function, and its geometry was optimized for that process function. Under these design principles, one or more rotor wings was/were designed to induce flows with increased emphasis on dispersive mixing, i.e., shearing, kneading and elongational flows, (the shearing rotor wing), whereas the other(s) was/were designed to induce material distribution within the mixing chamber followed by material splitting and recombination (the pushing rotor wing). These rotors have been labeled the wing function technology (WFT) rotors as a result of the process functionality assigned to each rotor wing design. Figure 1 shows schematically the general wing layout of a four-wing WFT rotor design. For reference the layout of a Farrel ST rotor design is also included. Figure 2 shows further details of the two wings in the vicinity of the wing tip region, and figure 3 the actual WFT rotors for a Farrel F270 Banbury.

[FIGURES 1-3 OMITTED]

In an earlier publication (ref. 4), pressure profiles and tracer studies were used to verify the basic rotor design. In this presentation, final mixing studies based on a pelletized rubber masterbatch using a laboratory size Banbury mixer and data from a production size mixer, an F270, are used to quantify the performance of the WFT rotors. Mixing uniformity was established by sampling material from selected mixer areas and establishing cure uniformity via oscillating rheometer torque curves under varying mixer set-up and operating conditions.

Experimental set-up

A set of two-wing WFT rotors was designed, fabricated and installed in a laboratory size mixer. The rotors were driven by a 40 hp variable frequency drive with a 225-rpm maximum rotor speed. The body sides and rotor body were both temperature controlled by means of a closed loop water circulating system. The mixer was also equipped with a four-inch diameter air cylinder with integral (internal) ram position indicator (RPI). Net chamber volume was calculated, based on rotor geometry, to be 1.55 liters. Experimentally, the net chamber volume was found to be of the same order. The mixer was equipped with even speed gearing.

The compound formulation involved in the experimentation consisted of 229 parts of pelletized synthetic tread type rubber compound, 1.60 parts sulfur, 1.80 combined parts of accelerators. The final compound had a specific gravity of 1.13.

Using this particular rubber formulation, a set of experiments was performed, all designed to verily uniform distribution of the components within the rubber matrix. In the first set of experiments, composition uniformity was established by generating ODR curves from five samples randomly selected from the processed material. In the second, the batch weight was fixed at 1,200 gm, the components were mixed at different mixing times, the contents of the mixer were removed as a single piece and samples were taken from specific mixer areas. Test specimens were taken from the area above the door top, two at the rotor ends, and the others randomly from within the batch. These samples were then analyzed for curative distribution.

Results and discussion

The general performance characteristics of the new WFT rotors were established by final mixing of a solution SBR tread type compound with an initial Mooney viscosity of 80 (M14). Several batches were mixed, all at the same mixer operating conditions. The objectives in these series of tests were to establish product uniformity and consistency within the batch and from batch to batch. The batch was discharged from the mixer when the batch temperature reached 240[degrees]F. The rubber, the curatives and the accelerators were all charged into the mixer at the start of the mixing cycle. The rotor speed was set to and maintained at 75 rpm for all test conditions. The temperature of the sides was set at 110[degrees]F, and that of the rotors at 135[degrees]F. Batch size in all test runs was maintained at 1,200 gm, which corresponds to a mixer fill level on the order of 70%. For each test run, ram position, power input and mixing times were recorded. Ram position is a good indicator of the mixing characteristics of the mixer. The objective in monitoring ram pressure was to ensure that the materials were properly fed to the mixer, and that the same consistency in mixing procedures was maintained. Ram downtime for the test conditions reported here was approximately 50 seconds, with a mixing cycle of 60 seconds and a specific power input of 0.12-0.13 kwh/kg. The temperature of the discharged material was cross-checked with a hand pyrometer. Generally, approximately ten individual readings were collected, and the average was then cross-referenced to the value measured by the process control system. The material discharged from the mixer was passed through the mill once and was set aside for analysis. For each batch, curative uniformity was established by ODR measurements for five different samples randomly collected from the batch. The results for four batches generated during this study are shown in figure 4. One can easily conclude that excellent consistency is maintained within the batch and from batch to batch. The ODR curves for the five samples within each batch are indistinguishable from each other. The same trend is also observed when the ODR curves of each of the four batches are compared to one another. In all cases, excellent distribution of the accelerators and curatives is observed.

[FIGURE 4 OMITTED]

The mixer characteristics of the WFT rotors were further investigated in another set of experiments, whereby at the end of the mixing cycle, the mixer was stopped with its contents still in the mixing chamber, and the carcass was subsequently removed from the mixer as a single piece. Samples for analysis of curative uniformity were then collected from selected mixer regions. A total of five samples was taken for analysis, two from areas located over the door top at the two rotor ends, and the others randomly selected from within the batch. The results of this study are shown graphically in figure 5. The operating conditions were similar to the ones previously reported. The batch size was again maintained at 1,200 gm, the rotor speed was 75 rpm, and the mixing time 60 sec. Mixer thermal conditions were similar to the ones previously reported. Once again, an excellent distribution of the additives was observed. The five curves are superimposed over each other and indistinguishable from one another.

[FIGURE 5 OMITTED]

In a further attempt to quantify the mixing characteristics of the new rotors, a tracer study was performed at the operating and thermal conditions used in the previous experiments. A white compound was first introduced into the mixing chamber, and when it reached a batch temperature of 200[degrees]F, the mixer was momentarily stopped, and a small amount of black tracer was inserted between the two rotors at one extreme end of the chamber. The compound was then mixed for another 60 sec., at which time the mixer was stopped and the contents were extracted as a single piece. The results of this study are shown in figure 6. They again verify the excellent distributive mixing characteristics of the new rotors. A uniform color batch was generated.

[FIGURE 6 OMITTED]

From the three sets of experiments and the associated data presented thus far, it is apparent that the new rotor design possesses outstanding distributive mixing characteristics. The analysis of the compounded material indicates excellent uniformity of the additives. As is customary in compounds of this nature, the amount of accelerators added to the compound is rather small. The data clearly support uniform distribution of the curatives and accelerators throughout the entire rubber matrix. The tracer studies indicated intimate mixing of the black tracer into the white polymer compound.

The general mixing characteristics of the new rotor design were further investigated in another set of experiments whereby curative distribution was established for batches mixed for different cycle times. These experiments were designed to explore, in addition to mixing quality, the productivity aspects of the new rotors. Using the same formulation as previously described, batches were prepared at different mixer discharge temperatures, and the processed materials were again analyzed for curative distribution. In addition to the mixer equipped with the WFT rotors, identical experiments were also performed in a laboratory size mixer equipped with standard two-wing Banbury-type rotors. Batches were prepared at mixer discharge temperatures of 160[degrees]F, 180[degrees]F, 200[degrees]F, 220[degrees]F and 240[degrees]F. Batch weight was set at 900 gm, the rotor speed at 75 rpm and the mixer thermal conditions as previously described. Table 1 shows the relevant processing data for the two mixers equipped with the two different rotor designs. [DELTA]Mhf is used to quantify the difference in uniformity of the compounded samples. The values of [DELTA]Mhf were extracted from figures 7 and 8. which show curative distribution of the compounded materials for the two rotor designs. They represent the difference between the minimum and the maximum values in curative distribution after the six-minute time mark of the ODR curves. This approach was used, rather than the usual standard deviation, due to lack of sufficient samples for a statistical analysis. A [DELTA]Mhf value of "0" represents excellent distribution, with the cure curves superimposed on each other. The data of table 1 indicate shorter mixing cycles for the WFT rotors, while maintaining uniform distribution of curatives within the batch. The difference in the overall performance characteristics between the two rotor designs is significant.

[FIGURES 7-8 OMITTED]

From the curative distribution curves of figure 7 and the data of table 1, it is evident that an acceptable quality product can be produced using the WFT rotors at a batch discharge temperature of 200[degrees]F, with a mixing time on the order of 31 sec. To obtain an equivalent product quality using the standard two-wing Banbury-type rotors, a mixing time on the order of 95 sec. is required, with a batch discharge temperature of 240[degrees]F. Significant gains in overall productivity can thus be obtained by just being able to discharge the material at a lower batch temperature, while still maintaining product quality.

Similar results have also been obtained from a production size mixer, Model F270, equipped with WFT rotors. Figures 9 and 10 show product quality in terms of ODR values, for a tread type compound mixed at two different batch sizes. Optimizing mixer performance in terms of mixer fill level, i.e., reducing the mixer fill level from 76% to 72%, resulted in a significant improvement in product quality, while also improving mixer productivity through shorter mixing cycles. Standard deviation in ODR measurements was reduced by approximately 28%, accompanied by a 14% gain in mixer productivity. Equivalent levels of mixer batch discharge temperatures were used in both cases, as were mixer thermal conditions and rotor speeds. Further improvement in mixer productivity is expected to prevail when lower mixer batch discharge temperatures are used to control mixer operation. These studies are currently under way and will be presented in a later publication. Compared to two other mixers within the same production facility, all mixers equipped with 4-wing ST and 2-wing standard Banbury rotors, respectively, mixing the same tread type compound, the WFT rotors demonstrated an improvement on the order of 30% in product uniformity with a 10% improvement in mixer productivity.

[FIGURES 9-10 OMITTED]

Similar trends were also observed when the performance of the mixer equipped with the WFT rotor design was compared to the other mixers in mixing a natural rubber based tread compound. The related information is presented in table 2. The superior performance of the WFT rotors is evident. The product produced by the WFT rotors equipped mixer exhibits the narrowest standard deviation, a 65-69% improvement in ODR measurements of curative distribution at a higher production rate and at lower energy consumption.

The performance characteristics of the WFT rotors are currently being investigated in other applications, and the results from these studies will be reported at a later date.

Conclusions

Experimental data from final mixing studies in a Banbury mixer Model BR, using standard and the new WFT rotors, respectively, were presented and discussed. Product quality, established by ODR measurements, demonstrated the ability of the new WFT rotors to provide high levels of product uniformity within the batch and from batch to batch. In comparative studies between the new rotors and standard Banbury-type rotors for a laboratory size mixer, product uniformity was maintained with mixing times significantly reduced using the new WFT rotors.

Results from a production size mixer, an F270 mixer equipped with WFT rotors, also showed significant improvements in product uniformity, a reduction in standard deviation of ODR values on the order of 28% and 69% for a synthetic and a natural based rubber tread type compound, respectively. The increase in productivity was on the order of 14% and 10%, respectively. Further testing is under way, and the results from these studies will be reported at a later date.

Table 1--mixing times and product quality as a
function of mixer discharge temperature for the
standard two-wing Banbury and WFT rotor
designs

Mixer discharge         Mixing time (sec.)            [DELTA]Mhf
  temperature       2-wing std.       2-wing   2-wing std.    2-wing
 ([degrees]F)         Banbury          WFT       Banbury       WFT

      160              Poor             20          -           15
                     uniformity
      180                27             27          29           5
      200                41             31          22           2
      220                50             42          19           1
      240                95             55           1           0
Table 2--production data for natural based tread
type compound for three F270 mixers, equipped
with three different rotor geometries

                           Mixer #1      Mixer #2      Mixer #3
Rotor type                4-wing ST   2-wing standard    4-wing
                                          Banbury         WFT

Batch size (lb.)             480            460           480
Ram pressure (psi)         100/80         130/120       120/110
Rotor speed (rpm)
  (intial/final)            39/30           43             40
Average mix time (sec.)     63.3           68.4           58.3
Average drop (kwh)          10.6            --             9.0
Rate total (lb./hr.)       18,800         17,000         19,900
Mh (std. dev.)              0.94           1.09           0.33

References

(1.) Wood, P.R., Rubber Technology International 1997, p. 172.

(2.) Tadmor, Z. and Gogos, C.G., Principles of Polymer Processing, Wiley & Sons, New York (1979).

(3.) Inoue, K., "Internal mixers," in Mixing and Compounding of Polymers: Theory and Practice, Ica Manas-Zloczower and Z. Tadmor, Eds., Carl Hanser:

Munich (1993).

(4.) Valsamis, L.N. and Borzenski, F.J., "Wing function technology--A new rotor technology for the Farrel Banbury mixer," presented at the ACS Rubber Division Meeting, Cleveland, OH, October 2001.

This article is based on a paper given at the October, 2001 Rubber Division meeting.

F.J. Borzenski and L.N. Valsamis, Farrel