Advanced rotor design: the four-wing N rotor--Part 2.

Rubber compounds have generally been mixed the same way for over 90 years since the introduction of the internal intensive mixer by Fernley H. Banbury in 1916. The internal intensive mixer has a number of advantages over the old two-roll mill mixing; the internal mixer is more productive, cleaner and safer. Over the years, improvements have been made to the quality and throughput (kilogram/hour) of the materials masticated with internal mixers via a number of mechanical changes. These include modified and/or new rotor designs, improved chamber designs, improved rotor cooling and improved product discharging, to name a few. This article is continued from the February 2009 issue of Rubber World.

Equipment and experimental

The data presented herein were obtained either from a 16 liter pilot plant sized intensive mixer located at Kobelco's pilot and test facility in Hudson, OH, or from 270 liter production mixers. The 16 liter mixer is controlled by a mixing control and monitoring system which allows the sample batches to be mixed in manual (controlled by the operator) or pre-programmed automatic mode (controlled by the unit). All the data presented in this article were obtained from mixes done in automatic mode (thus removing errors or variable response times from manual control).

The 16 liter mixer is driven by a 250 horsepower DC motor, allowing variable rotor speeds from 0-120 rpm. In addition, the BB-16 is equipped with a special gearbox that will allow either an even speed (1:1) or friction speed (1.15:1) ratio between the dual output shafts that are directly coupled to the mixer rotors. The mixer's sides, rotors and drop door can be cooled or heated using a single zone temperature control unit (water temperatures to as low as 7[degrees]C can be achieved via a chiller).

When the individual batches reached their programmed discharge point (either time, temperature, power or a combination of any of the three), a discharge door automatically opens and discharges the product from the mixer into a chute leading to a TSR 125 twin screw extruder/roller head fitted with twin 7" calender rolls. The sides, augers and rolls of the TSR can be heated or cooled with temperature controlled water. The twin augers, which put very little extra work into the material, are driven by a 10 horsepower DC motor, achieving speeds to 30 rpm. The calender rolls are driven by a 7.5 horsepower DC motor allowing roll speeds up to 25 rpm. Since the TSR imparts virtually no extra work into the samples, the properties achieved are due to the work imparted by the rotors and not by any extra mixing (such as milling).

The TSR gives a sheeted product approximately 12" wide with thicknesses ranging from 0.25" to 0.4". The subsequent sheets are either air cooled by hanging them directly on cooling racks or first water cooled by immersing the sheeted material in a stainless steel trough containing cold water (city water at temperatures ranging from 4[degrees]C to 27[degrees]C, depending on the time of year the work was done), followed by hanging on racks to dry.

The materials mixed with the various 270 liter mixers were either discharged into a TSR or onto a mill. From there, the material would go through a typical batch-off system.

Samples were taken from different sheets or sections of the same batch and tested as is (no further sample preparation such as milling) using a Vise Tech Mooney shearing disc viscometer or instruments of similar design. The Mooney test employed was with 100[degrees]C or higher temperature, with a one minute warm-up and four minute test using the large rotor. The viscosities are an average of a minimum of two samples per batch. Dispersion measurements were done on a Dispergrader+ reflected light microscope. Batch temperatures were measured using a hand-held thermocouple with digital readout.

Discussion

The four-wing N rotor has very good intra-batch and interbatch temperature uniformity. This is due to the way the rotors mix the material in the chamber. A comparison of the intra- and inter-batch temperature uniformity between the four-wing N and the four-wing standard rotors can be seen in table 1. A number of batches of natural rubber was masticated with each type of rotor. When the material was discharged, each batch was probed a number of times with a hand-held thermocouple, and for each probe, a temperature was recorded. The various probed temperatures for each batch mixed with that rotor were then tabulated, and the highest and lowest temperatures recorded were noted. The lowest temperature was subtracted from the highest temperature, which gave the temperature differential ([DELTA]T) between measurements of all the batches for that rotor.

The four-wing N rotor had only a 4[degrees]C difference in temperature between the lowest and highest probed temperature measurements, giving very uniform intra- and inter-batch mixing. The four-wing standard rotors, on the other hand, had a 23[degrees]C difference, which means it was not giving a very uniform mix nor uniformity between batches.

This example also demonstrates how the four-wing N rotor yields a lower batch temperature for an equivalent mix time compared to the four-wing standard rotor. When the four-wing standard samples were dropped at 154[degrees]C (with an average mix time of 125 seconds), the average batch temperature for the four-wing N mixes aider 125 seconds of mixing was 146[degrees]C, 8[degrees]C lower.

There are various ways that the rotor wings can be oriented to one another in an even speed mixer, and this can affect the way the material is pulled into the chamber, the ram seat time, the cycle time, the dispersion and distribution of additives. A natural rubber/high carbon black loaded compound (table 2) was mixed with 4WN rotors with the rotors in 0[degrees] orientation and with the rotors in the recommended 90[degrees] orientation.

The 90[degrees] recommended orientation gave a faster mix time and ram seat time, slightly lower Mooney, higher throughput and used less energy in the mix. In addition, the 0[degrees] orientation did not ingest the material as smoothly as with the 90[degrees] orientation, actually slowing the rotor speed momentarily as the material was ingested. No such slowdown was seen with the 90[degrees] orientation.

As previously mentioned, the 4WN rotor was initially designed for final mixing (addition of cure package). Table 3 compares the final mixes for a series of tire compounds mixed in 270 liter mixers with both the four-wing N and four-wing standard rotors.

For the examples listed in table 3, the throughput improvement information was accumulated after mixing anywhere from 150,000 to over 3 million kg of each of the materials. The N rotor not only has a faster cycle time, but it also has close to a 4% larger batch size than the S rotor to start with, and the quality of the final material was reported to be better than the quality of the materials mixed with the four-wing standard rotor.

The four-wing N rotor was also compared to the four-wing standard rotor for re-milling materials (simply mixing materials to further lower the viscosity). As can be seen in table 4, a tire compound masterbatch (all from the same batch mixed in a 270 liter mixer) was simply re-mixed in a 16 liter mixer with both types of rotors at 50 rpm. The four-wing standard rotors were in the preferred 0[degrees] orientation and the four-wing N rotors were in the preferred 90[degrees] orientation. The materials re-milled with the four-wing N rotors gave a significantly more uniform batch temperature (AC of 3[degrees] versus 19[degrees] for the four-wing standard rotor), a lower Mooney viscosity, lower energy consumption, lower drop temperature and a higher throughput for the same mix time.

The four-wing N rotor has also been found to be useful in certain masterbatch mixing. The N rotor was compared to the four-wing H swirl rotor for mixing pigment masterbatches. As can be seen from the comparisons in table 5, the N rotor gave a better dispersion (as measured on a Dispergrader dispersion tester), as well as a lower maximum temperature than that achieved with the H rotor for equivalent mix times and mix schemes.

Summary

As can be seen from the data presented herein, the four-wing N rotor, with its atypical rotor features, can have significant positive effects on the cycle time, Mooney viscosity, dispersion, energy consumption and output of not only the final mixing of rubber systems, but also the masterbatching and re-milling of various rubber systems.

This article is based on a paper presented at a meeting of the Rubber Division, ACS (www.rubber. org).

by Richard J. Jorkasky II, Kobelco Stewart Bolling (www.ksbiusa.com)

Table 1--comparing material uniformity--masticated
natural rubber

        Rotor       ML 1+4        [DELTA]T    Temperature
Rotor   speed    100[degrees]C   [degrees]C   @ 125 sec.
        (rpm)       Mooney         @ drop       of mix

4WN        60        38               4       146[degrees]C
4WS        60        40              23       154[degrees]C

Table 2--4W N rotor orientation--natural rubber/
carbon black

        Rotor    [degrees]   Cycle     Ram
        speed      Rotor      time     seat
Rotor   (rpm)     orien.     (sec.)   (sec.)

4WN       50        90        166      115
4WN       50         0        181      119

           ML 1+4       16 liter    Energy
        100[degrees]C    output       use
Rotor      Mooney       (kg/hr.)    (kwh/kg)

4WN          86            290       0.132
4WN          88            268       0.143

Table 3--270 liter mixer final mix

Material   Rotor   Drive   Cycle time      % output
                   speed       (sec.)   improvement

A           4N     Even            93          15.3
A           4S     Even           110             -
B           4N     Even            98          17.6
B           4S     Even           119             -
C           4N     Even            88          25.8
C           4S     Even           118             -
D           4N     Even            95          17.2
D           4S     Even           115             -
E           4N     Even            79          30.5
E           4S     Even           114             -

Table 4--otor comparison--re-mill of a tire compound
masterbatch

        Rotor   Cycle    Drop      Batch
        speed    time    temp.     temp.
Rotor   (rpm)   (sec.)    (C)    [DELTA] C

4WN       50       94     145           3
4WS       50       94     151          19

           ML 1+4        16 liter    Energy
        100[degrees]C     output       use
Rotor      Mooney        (kg/hr)     (kg/hr)

4WN             81          570      0.0752
4WS             83          550      0.0801

Table 5--four-wing N comparison in pigment masterbatch mixing

          Rotor     Mix            %            Mixing
Pigment           scheme   dispersion   temperature (C)

Red        4N          1           78               109
           4H          1           68               114
Red        4N          2           71               103
           4H          2           64               109
Blue       4N          1           78               115
           4H          1           67               118
Blue       4N          2           80                97
           4H          2           70               103