New intermeshing mixer vs. traditional mixers.

This article is written to update and assist compounders using the internal batch mixers and two-roll mills. An internal batch mixer has a figure-eight-like mixing enclosure bounded by the right and left chamber-cavities, door-top, ram-bottom and front and back rotor-endplates. The dominant circulation of the mixing material from the right chamber-cavity to the left chamber-cavity is described by the following words: Co-flow means completely two directions, uni-flow means completely one direction and partial-flow means partially one or two directions. The words co-flow, uni-flow and partial-flow will be used throughout for the three original fundamental mixing technologies available on the marketplace today.

In recent years, the accelerated discovery of different types of polymers, fillers, etc., by the tire, rubber and plastics industries puts high demand on the equipment manufacturers to come up with a better internal batch mixer to handle the new materials. In 1998, Nortey (ref. 1) introduced the new-traditional co-flow intermeshing rotor internal mixer, which is changing the face of compounding as we know it with the old traditional uni-flow and partial-flow internal mixers. From laboratory and field mixing trials, the co-flow intermeshing mixer has better mixing performance capabilities than the old traditional mixers. The co-flow mixer will soon be the standard mixer to be reckoned with based on the accelerated demands for different sizes of this new mixer.

The two old traditional internal mixers are the uni-flow and partial-flow mixers. In 1934, Cooke (ref. 2) introduced the uni-flow geared intermeshing rotor internal mixer that improved substantially the compounding quality of the polymers more than the partial-flow (tangential) rotor internal mixer. Developments (refs. 10-12) have been carried out to improve the productivity and heat transfer of the uni-flow geared intermeshing mixers. In 1916, Banbury (ref. 3) introduced the partial-flow (tangential) rotor internal mixer that was successful to compound polymers. The tangential mixers increased the productivity of compounding several times more than the external compounding with the two-roll mills. The productivity gain of the tangential mixers was very high, and to quantify the gain, the mixer sizes were expressed in terms of the number of mills the mixers would replace. For example, mixer sizes 3 and 27 mean the mixers would replace, respectively, 3 and 27 mills. Many developments (refs. 4-9) were carried out to improve the mixing performance of the tangential mixers. Many compounding equipment manufacturers have been trying to improve the old traditional partial-flow (1916) and uni-flow geared intermeshing (1934) mixer technologies to keep up with the demand from the tire, rubber and plastics industries. Field and laboratory results today show that the partial-flow rotor internal mixers still have higher productivity and lower mixer cost per productivity than the uni-flow geared intermeshing mixers. On the other hand, the uni-flow geared intermeshing mixers have better quality, improved heat transfer and better one-pass mixing capabilities than the partial-flow mixers.

Mixers

An internal batch mixer has a mixing chamber and a driving system that causes a pair of rotors to rotate to mix materials. The type of an internal mixer is dictated by the type of rotors installed in the mixer and also the interaction of the right and left rotor lobes at the center region of the mixer. A rotor, which is the rotating component of a mixer, has major and minor (i.e., root) diameters. Intermeshing occurs when the imaginary major-diameter cylinders of both the right and left rotors intercept. The term intermeshing does not go far enough to describe the interaction between the rotor lobes. Therefore, this article will use terminology that would precisely describe the interaction of the rotor lobes at the center of the mixer. The term non-intermeshing (i.e., tangential) describes when the imaginary major-diameter cylinders of both the right and left rotors do not intercept. To be precise, there are three unique internal batch mixers, as shown in figure 1. The term non-geared intermeshing mixer is used when the rotor lobes intermesh in a non-gear-like configuration. Mixing at the center region occurs only between a rotor major-diameter against the other rotor minor-diameter. The geared intermeshing mixer occurs when the rotor lobes intermesh in a gear-like configuration. Mixing at the center region occurs between a rotor major-diameter against the other rotor minor-diameter, and also between a rotor lead-surface against the other rotor trail-surface. In the non-intermeshing (i.e., tangential) mixer, the rotor lobes do not intermesh. Mixing does not occur at the center region of the mixer.

[FIGURE 1 OMITTED]

Original internal batch mixing technologies

There are three original mixing technologies in use today. The latest is the technology of co-flow mixing, introduced in 1998. In this mixing system, materials in the right and left mixing chambers are circulated in counterclockwise and clockwise directions, coupled with four squeeze and relief-flows in one revolution as shown in figure 2. The co-flow intermeshing mixer uses non-geared intermeshing four-wing rotors. The technology of uni-flow mixing was introduced in 1934. In this mixing system, materials in the right and left mixing chambers are circulated only in one direction, coupled with two squeeze-flows in one revolution. The uni-flow intermeshing mixer uses geared intermeshing three-nog rotors. The technology for partial-flow mixing was introduced in 1916. In this mixing system, materials in the right and left mixing chambers are circulated partially in one or two directions per a revolution. The partial-flow mixer uses non-intermeshing two, four or six-wing rotors. The pair of rotors for a co-flow or uni-flow mixer is driven via two gears with equal number of teeth, while the pair of rotors for a partial-flow mixer is driven via two gears with equal or unequal number of teeth.

[FIGURE 2 OMITTED]

Rotor L/D ratio and rotor types

The optimal constant rotor L/D (length-to-diameter) ratio of the co-flow mixer is based on mixing quality, rotors strength and motor power peak. The improved mixing quality and minimal motor power peak of the co-flow mixer are determined by analytical technique and verified by using different lab-size mixers. The rotor's strength is determined by using three-dimensional finite element analysis. The optimal constant rotor L/D ratio allows mixing scale-up to be performed with ease. The co-flow mixer retrofits the partial-flow and the uni-flow geared intermeshing mixers with bigger material batch weight. The rotor L/D ratio of the uni-flow intermeshing mixers varies from 1.3 to 1.4. One style of the uni-flow mixers has the option of varying the gap between the rotors during the mix cycle (ref. 11). The rotor L/D ratio of the partial-flow mixers varies from 1.4 to 2.0. The wide variation of this ratio is the root cause of mixing scale-up difficulties. There are two options for the partial-flow mixers. The first option is the partial uni-flow mixer that is fitted with two, four, five or six-wing rotors, as shown in figure 3. The second option is the partial co-flow mixer that is fitted with two- or four-wing rotors (refs. 4, 6, 7 and 9).

[FIGURE 3 OMITTED]

Compounding

Compounding or mixing is the process of putting together different raw materials to get a final compound that is substantially different from the initial raw materials, but absolutely homogeneous as far as the compound's physical and chemical properties are concerned. During the mix cycle of an internal mixer, the following tour basic operations occur:

* Dispersion: reduction in particle size of fill agglomerates (de-agglomeration) to their ultimate particle size.

* Incorporation: wetting of solid particles by the polymer.

* Plasticization: modifying the rheological properties of the mix by reducing viscosity.

* Distribution: uniformly distributing all the particles already dispersed in order to obtain homogeneous compound.

Mixing scale-up

It is essential to use a small mixer for mixing scale-up to save time and money in the production environment. The following parameters of the small mixer should be the same as for the production-size mixer in order to minimize the mixing scale-up difficulties:

* The length-to-diameter ratio of the rotor.

* Internal design of mixer and external design of rotors.

* Comparable heat-transfer capability for mixer and rotors.

The above diameter is the major or tip diameter of the rotor. A study performed by using this approach for mixing scale-up worked out successfully. The study shows that the size of the small mixer is critical. The size should be selected when the critical mixing parameters are still effective in the mixing process.

Heat transfer of mixers and rotors

Batch mixing occurs between the rotating rotors and the stationary surfaces of the chamber bores, door, ram, rotor-end-plates and dust-stop seals. Additional mixing occurs between the two rotating rotors in the case of intermeshing mixers. The parameters causing the mixing action depend on temperature, as shown by the following equations (ref. 14):

[eta] = A.[e.sup.B/T]

[gamma] = [pi].d.s.cos[THETA]/60[delta]

[tau] = [eta].[[gamma].sup.n]

Where:

[eta] = Compound viscosity (lb.- sec./square inch);

A = Activation energy constant of the compound;

B = A constant that depends on the shear rate and shear stress;

T = Temperature ([degrees]F);

[gamma] = Shear rate (reciprocal seconds);

s = Rotor speed (rpm);

d = Mean diameter of rotor-tip and chamber bore (inches);

[delta] = Gap between rotor-tip and chamber bore (inches);

[THETA] = Co-helix angle (i.e. complementary angle of helix angle of a rotor lobe);

[tau] = Shear stress (psi);

n = Power law index (n = 0.2-0.3 for elastomers).

Temperature control is very important during reinforcing of fillers (especially silica) and crosslinking of curatives in the mixer. Therefore, the ideal mixer should be able to cool or heat the process material quickly and should be able to maintain the same temperature for all the surfaces that come in contact with the material. Maintaining the same temperature for the mixer components depends directly on minimizing the change in the mixer's inlet and outlet temperatures. The ideal mixing action occurs when the process material adheres to the stationary surfaces so that the rotor lobes would be able to shear the material. Most process materials adhere to a warm surface rather than a cold surface. With this in mind, to optimize the mixing action, the rotor temperature is kept lower than that of the stationary surfaces to allow the material to adhere to the stationary surfaces with minimal slippage. This action allows the rotor lobes to efficiently shear the process material. The temperature selection for the rotors and stationary surfaces is not straightforward because it depends on the type of process material and also on the design of the internal mixer. All the surfaces that come in contact with the process material for co-flow mixers are designed with forced flow heat transfer passages in order to have a better temperature control on the mix. The dust-stop seals are either lubricated or non-lubricated. Customers feedback on the life of the seals varies from customer to customer. Figure 4 shows different schematics of rotor inner profiles and heat transfer rating.

[FIGURE 4 OMITTED]

Mix passes and steps

There are many mix procedures for the internal batch mixers, but we will focus only on the two main types. The standard mix occurs when the polymer is first introduced into the mixer before all ingredients are added. This mix procedure prolongs the wear life of the mixer and rotors. The upside down mix occurs when all ingredients are first introduced into the mixer before the polymer is added. This procedure is very hard on the mixer and shortens the wear life of the rotors and mixer. Each of these two mix procedures could use multiple-pass or single-pass mixing. The ram action, which depends on pressure, plays an important role to enhance the mixing performance of a mixer. The right pressure for the ram is part of the mixing parameters that allow the mixing process to be optimized. In a mix cycle, there are many mix steps and different ram actions. Temperature, energy and time are the three discharge parameters. Due to slippage during the mix cycle, a material discharged on temperature tends to give consistent physical properties of the compound from batch to batch. If two discharge parameters are used to control the mix, temperature should be included, as a check for assuring that the material is not scorched or degraded.

In some cases, temperature at the beginning of the mix cycle of a tangential mixer fitted with even-speed rotor alignment tends to be higher than the set discharge temperature. This phenomenon occurs due to the rapid movement of the material on the door thermocouples at the beginning of the mix cycle. This high temperature is not the true stock temperature. Use time delay, energy consumption or lower rotor speed for the mixing control program to overcome this problem. If you have a variable speed motor, lower the rotor speed to overcome the problem, and then increase the rotor speed again to a desirable level during the mix cycle.

In advanced mix procedure, the rotor speed and ram pressure should be varied during the mix cycle to further improve the performance of the mixer and batch temperature control. Medium rotor speed and high rain pressure are used at the beginning of the mix cycle to enhance intensive mixing. After the injection of oil, high rotor speed is used to minimize the time for slippage. Low rotor speed and low ram pressure are used to enhance extensive mixing during the latter part of the mix cycle.

Quality and productivity

Quality of the final compound from a mixer is dependent on the specification of raw materials and the mixing quality of the mixer. Check the specification of the raw materials to minimize quality problems. Mix quality of a mixer occurs when all the raw materials are well dispersed and distributed to form a homogeneons compound that meets the target specification or goal. A compound could be well mixed and homogeneous and still miss the target specification due to loss of material through the dust-stop seals. In order to meet the target specification, the co-flow mixer reduces over 50% of material loss through the dust-stop seals in comparison to the old traditional mixers (ref. 15). If the mixer does not meet the target goal, then downstream equipment will be used at an additional cost to get the compound in specification.

Productivity or production rate of a mixer depends on the number of mix passes to get the final compound. The actual productivity of a single-pass mix without material loss through the dust-stop seals is a function of the optimum batch weight and total mix cycle time. Determination of the optimum batch weight is based on the pressure and movement of the floating weight. The actual productivity is calculated as follows:

* Total mixing cycle time = (loading time) + (mixing time) + (discharging time).

* Optimum batch weight = (net mixer volume) x (compound specific gravity) x (fill factor).

* Actual productivity = (optimum batch weight) + (total mixing cycle time).

The loading of materials into a mixer could be done automatically or manually. The automatic load time is usually shorter than the manual load time. Ingestion time of a mixer is the time after material loading; it takes the ram to push materials in the hopper throat into the mixer's chamber. The old traditional raft-flow geared intermeshing mixers have longer ingestion time than the new traditional co-flow intermeshing mixer or the old traditional tangential mixers. The reason for the short ingestion time is the rotor lobes do not occur at the center of the mixer at all times during the mix cycle. This design feature allows materials to be ingested into the mixer quickly. The mix time of co-flow and uni-flow mixers is shorter than that of the partial-flow mixers due to additional mixing at the center of the mixers.

Actual productivity or production rate is the function of optimum batch weight and total mix time. For instance, the four-wing tangential rotors have smaller batch weight than the two-wing rotors, but the only reason why the four-wing tangential rotors have higher productivity than the two-wing tangential rotors is due to the shorter total mix cycle time. The co-flow mixer has bigger optimum batch weight than the four-wing tangential rotor mixer and also mixes at a shorter total mix cycle time than the four-wing tangential rotor mixer. For example, in a two-pass mixing arrangement, the 500-liter co-flow mixer retrofits the 414-liter tangential (i.e., S-370/F-370) mixer with a 45% increase in productivity for the first-pass or second-pass mix. In the two-pass mix arrangement, the old traditional uni-flow geared intermeshing mixers have less productivity than the partial-flow four-wing rotor mixers.

The uni-flow geared intermeshing mixers, in many cases, are able to combine the first- and second-pass mix of the partial-flow mixers into a single-pass mix. In this situation, the uni-flow mixers have higher productivity than the partial-flow mixers. The co-flow mixer in the same situation has higher productivity than the uni-flow geared intermeshing mixers.

Mixing analysis

Mixing behavior of an internal mixer is complex. Compounding equipment manufacturers have different techniques, including flow visualization, to evaluate the mixing behavior, but unfortunately, these techniques are not published due to competition. Some independent institutions have published mathematical models developed to simulate the complex mixing behavior, and videos are made to capture the behavior (ref. 13).

An in-depth technique is developed to quantify the separate independent mixing parameters and test the mixing effect experimentally. This technique is used to analyze different mixing technologies and new concepts. A simplified version of the technique is described by first dividing the complex mixing behavior into two dependent parameters as intensive and extensive mixing. The intensive mixing depends on two independent mixing parameters, while the extensive mixing depends on three independent mixing parameters.

Intensive mixing

High shear by fracturing causes high breakdown, dispersion and incorporation of the material. This mixing type results in high temperature and pressure. The surface areas in contact with the process material are small. This mixing type is similar to the mixing at the nip of a two-roll mill.

Low shear by stretching causes low breakdown, dispersion and incorporation of the material. This mixing type results in low temperature and pressure.

The surface areas in contact with the process material are large.

Extensive mixing

Extensive mixing involve circulation of materials from the right chamber-cavity to the left chamber-cavity. This type of mixing action causes global distribution that results in uniformity of the process material. This type of mixing is similar to the distributive mixing performed by the operator of a two-roll mill.

Blending is a localized distributive mixing that results in low uniformity and dispersion of the material. This type of mixing occurs in the material rolling on the leading surface of a rotor lobe. This type of mixing behavior is similar to the mixing of the material rolling on the two rolls of a mill.

Squeeze and relief is a localized distributive mixing that results in low uniformity and dispersion. This type of mixing occurs when long and short lobes of a rotor are located in the same quadrant and the lobes originate from opposite ends (i.e., water or drive-end). The axial components of process material being pushed by the two lobes collide to cause squeeze-flow mixing. The rotational components of process material being rotated by the two lobes are additive due to the axial space between the two lobes. This axial space creates a relief for the material to flow out without bottleneck effects. This mixing phenomenon minimizes loss of material and mixing forces on the dust-stop seals. Co-flow mixer rotors use this type of mixing.

The two dependent and five independent mixing parameters are used to compare the mixing types for the co-flow, uni-flow and partial-flow mixers, as shown in figure 5. To analyze the three original internal mixing technologies, unwrapped envelopes of the rotor-tips are used. Figures 6 through 8 show the three different rotor envelopes and the result of the mixing analysis is in table 1.

[FIGURES 5, 6&8 OMITTED]

Three lab-size internal mixers are built to allow companies to perform comparative mixing evaluation of their compounds free of charge. In order to compare accurately the mix results of the three original mixing technologies, the three lab-size mixers, as shown in figure 5, are built by maintaining the following parameters and components the same for all mixers: rotor centerline, endframes and width of hopper opening. The first and second lab-size mixers are, respectively, scaled-down versions of the 270-liter partial-flow mixer (S-270/F-270) and the 257-liter uni-flow geared intermeshing mixer (NTR-250/K6A). The third lab-size mixer is a scaled-down version of the 265-liter co-flow intermeshing mixer (Co-280). Note: The lab-size co-flow mixer represents all mixer sizes because all Coflow4 mixers have the same length-to-diameter ratio. Various tire, rubber, plastics and floor tile companies have run lab trials successfully on the following compounds: NR, SBR, NBR90, EPDM, nitrile, BR, polychloroprene, TPE, TPO, PVC, Hypalon, etc. Results of the trials by the companies showed that the Coflow4 mixer performed better than the two other mixers in all the compounds tested.

Mixing study

A test was performed at a compounding plant to compare the Mooney viscosity reduction and dispersion of a 265-liter Coflow4 intermeshing mixer against a 414-liter tangential mixer. Five batches of a 60 durometer SBR/BR tread stock were masterbatched by using each mixer. The masterbatches from each mixer were final mixed in a 272-liter tangential mixer. Some of the processing parameters for the masterbatch and the final-mix results are tabulated in table 2.

The test for general dispersion and presence of large agglomerates was made on a Dispergrader Model 1000 100x magnification. The five samples from each mixer were analyzed with five scans. The detail analysis indicates that the general overall dispersion is 6.0 on average for the intermeshing samples compared to 4.5 on average for the tangential samples. The presence of agglomerates 23 microns and larger is almost 9.6 on average for the intermeshing samples compared to 8.5 on average lot the tangential samples. The total white area percentages for the samples are on average of 3.4% and 7.7%. respectively, for the intermeshing and tangential mixers. The images shown in figure 9 clearly indicate that the sample from the intermeshing mixer is better dispersed than the sample from the traditional tangential mixer. From several data collected in the field and laboratory, the overall mixing results of co-flow, uni-flow and partial-flow mixers are shown in a bar graph in figure 10.

[FIGURES 9-10 OMITTED]

Conclusion

The Coflow4 intermeshing rotor internal mixer has been shown analytically and experimentally to compound various materials better than all types of traditional uni-flow geared intermeshing and partial-flow rotor internal mixers. From customer trials and mixing study, the mixer uses less power peak and specific energy than the traditional mixers, and also has the following mixing performance advantages:

* Better mixing quality;

* 20% to 45% increase in productivity over the traditional tangential 4-wing rotor and geared intermeshing 3-nog rotor internal mixers, depending on rotor length-to-diameter ratio;

* better control of rotor heat transfer;

* longer wear life for dust-stop seals;

* wide range of process materials;

* retrofit capabilities at lower purchase cost.

The design features attributed to the mixer's superior mixing capabilities are as follows:

* Intermeshing;

* co-flow circulation of process material coupled with the squeeze and relief-flow mixing;

* optimum rotor L/D ratio:

* self-cleaning;

* rotor heat-transfer passages.

Table 1

Mixing action                      Co-flow     Uni-flow     Partial-
                                    mixer       mixer      flow mixer
                                   (Nortey)    (Cooke)     (Banbury)

The process material is              Yes          No           No
pushed forward, back,
squeeze and relief by each
rotor.

The process material is               No         Yes          Yes
pushed only forward or
back by each rotor.

The process material is               No         Yes          Yes
pushed hard on the dust-stop
seals by rotor lobes
resulting in a lot of material
loss.

The process material that is         Yes          No           No
going through the high
shear-zone experiences
uniform shear rate across
the axial length of rotors
similar to a two-roll mill.

Each rotor is individually           Yes          No           No
(balanced)

Dominant process material          Co-flow     Uni-flow     Partial-
circulation from right chamber-                               flow
cavity to left chamber-cavity
Table 2 - mixing study

Mixer      Batch    Rpm      V       SG      FF       W
type        rpm
           pres.
           (psi)          (liter)                   (lbs.)

Co-280      50       50     265     1.128   .783     515
S-370       58       45     414     1.128   .753     774

Mixer       Discharge     Cycle    Production   Tensile   Viscosity
type          temp.        time       rate                reduction

          ([degrees]F)    (sec.)   (lbs./hr.)    (psi)     (ml+4)

Co-280         280         240        7,725      2,450       50
S-370          310         270       10,320      2,428       60

References

(1.) Nortey, N.O., "Internal batch mixing machines and rotors." U.S. Patent 6,402,360 (2002).

(2.) Cooke, R.T., "Improvements in rubber mixing or preparing machine," British Patent 431,012 (1934).

(3.) Banbury, F.H., "Machines for treating rubber and other heavy plastic material," U.S. Patent 1,200,070 (1916).

(4.) Nortey, N.O., "Optimized four-wing non-intermeshing rotors for synchronous drive at optimum phase relation in internal batch mixing machines," U.S. Patent 4,834,543 (1988).

(5.) Sato, N., Miyaoka, M., Yamasaki, S., Inoue, K., Kuriyama. A., Fukui. T., Asai, T. and Nakagawa, K., "Mixing and kneading machine," U.S. Patent 4,284,358 (1981).

(6.) Wiedmann, W. and Schmid. H.. "Mixing apparatus for kneading of plastic substance," U.S. Patent 4,234,259 (1980).

(7.) Nortey, N.O., "Internal batch mixing machines with non-intermeshing rotors of increased performance," U.S. Patent 4,744,668 (1986).

(8.) Nortey, N.O. and Borzenski F.J., "Mixing machine with non-intermeshing pair of rotors driven solely at the same rotor speed in opposite directions and having a predetermined rotational alignment relationship between the two counter-rotating rotors," U.S. Patent 4,893,936 (1986).

(9.) Nortey N.O., "Two-wing non-intermeshing rotors of increased performance for use in the internal batch mixing machines," U.S. Patent 4,714,350 (1986).

(10.) Johnson, F., Homann, H., Rother, H. and Weckerle G., "Mixer with streamlined nogs and cut back long nogs," European Patent 170,397 (1990).

(11.) Passoni, G.C., "Closed parallel rotor mixer with adjustable interaxial separation," U.S. Patent 4,775,240 (1988).

(12.) Lasch, A. and Stromer E., "Modeling clay machine for raw rubber or other similar malleable material," German Patent 641,685 (1937).

(13.) White, J.L., "Polymer Engineering Science," 19, 818 (1979).

(14.) Tadmor, Z. and Klein, L, "Engineering principles of plasticating extrusion," New York, Reinhold Publishing (1970).

(15.) Nortey, N.O., Rubber World magazine, page 48, March (1999).

Narku O. Nortey, Skinner Engine