An overview of variables affecting batch mixing in a tangential mixer.

The tangential batch mixer is the primary mixing device in many compounding lines. Within the batch mixer, raw materials such as rubber, fillers, oils and chemicals are combined to form a mixed compound that is subsequently formed into slabs, strips or pellets by either a mill, extruder or mill extruder combination. The product from this "compounding line" is then formed into usable goods by fabrication machinery of varying types.

The tangential batch mixer is composed of a mixing chamber containing two counter rotating non-intermeshing mixing blades (commonly referred to as rotors). Above the mixing chamber is a feeding section (commonly referred to as a hopper) which contains a ram (commonly referred to as a weight) which forces unmixed compound components into the mixing chamber. The bottom of the mixing chamber is equipped with a door (except in tilt mixer designs) which opens allowing the mixed product to be discharged from the mixing chamber. The critical mixing components of the mixer are temperature controlled to allow an efficient mixing process.

Variables affecting batch mixing

The goal of understanding the variables of any batch mixer is to be able to apply the knowledge so that one can produce the highest quality product with the lowest possible cost. The manner in which the mixer is run can have a dramatic effect on the cost of compounding. The focus of this article is a discussion of a number of variables that can affect the efficiency of the batch mixer based compounding line and it is not intended to present a cost analysis of the compounding operation.

Currently the approach to so called "mixer operation optimization" varies significantly from compounder to compounder. At one end of the compounding spectrum are the high volume producers who have a limited number of compounds that are produced in long production runs. At the other extreme is the specialty compounder who may have as many as one thousand active formulations with production runs consisting of only one or two batches. The approach to optimization must be different. The mixing operation can be effectively analyzed and/or optimized only after the variables affecting the mixing process have been identified and are controlled. What all compounders have in common is that the same type of mixer is required to accomplish the mixing task. In each case the variables affecting the way in which the compounds are mixed are the same. These variables are as follows:

* Compound formulation;

* batch weight/fill factor;

* applied batch pressure;

* mix steps and procedures;

* mixer speed (rpm);

* mixer temperature (temperature of the circulating heating/cooling medium);

* compound component considerations (ingredient dependent):

   - feed form; - feed temperature; - feed quality/purity; and

* environmental affects:

   - temperature - humidity.

Compound formulation

One of the most significant changes that will affect the process of mixing is when a change in formulation occurs. A highly oil extended EPDM formulation is not expected to mix under the same mixing parameters as a highly filled natural rubber compound. Changes in compound composition can have a dramatic effect on the process of mixing. Additives that can affect the mixing process are polymer type (molecular weight/molecular weight distribution), filler type (particle size and/or structure), the amount and temperature of the oils, waxes and meltable ingredients. It must be kept in mind that the accuracy of weighments in any given compounding operation must be defined and that critical components in the formulation may require tight quality and weighment tolerances in order for consistent mixing to occur. However, fluctuations in raw material quality do occur and it is not uncommon for minor variance in formulation to be required in order to maintain consistency in the final product. Changes in formulation may also be required due to changes in the operation or condition of the compounding machinery.

Batch weight/fill factor

Having proper batch weight is critical to achieve efficient compounding. There is an operating tolerance that will allow efficient compounding to occur. If the batch is too small, the mixer is under filled and the ram bottoms immediately (goes to the full down position). Under this condition the ram is not used effectively to pressurize the mixing chamber to create effective shear mixing. If the batch is too large, the mixer is overfilled. In this case the ram never seats or bottoms. The fed material is lodged in the throat of the mixer hopper during the cycle and is never forced into the mixing chamber. Monitoring ram position during the mixing cycle is an excellent way to determine whether the batch weight is either excessively large or small.

Most often, the ram position can be seen and documented by the observation of a "telltale rod" or by the use of a ram position indicator (RPI) (ref. 4). The telltale rod is usually a pipe or heavy cable that is attached to the mixer's weight which protrudes through the top of the mixer's hopper cover. The pipe or cable is exposed to the process and is not considered to be mechanically reliable. The RPI produces an output signal specifying the ram position at any time during the mixing cycle and is located within the mixer's air cylinder assembly isolated from the process.

Figure 1 shows the output from the RPI documented on a continuous recording device. Ram position versus time for three different batch weights are specified. In la, the batch size is undersized (notice how quickly the ram seats or bottoms after the ram is put down after loading). In 1b, the batch weight is considered optimum. For efficient compounding the ram should bottom just before peak power is reached. This assures high shear mixing is occurring when all compound components are within the mixing chamber. In 1c, the batch weight is too large (notice how the ram never seats or bottoms during the mixing cycle).

[Figure 1 ILLUSTRATION OMITTED]

All tangential batch mixer,; are volumetrically specific and batch weight is based on that volume. The batch weight is primarily dependent on the composite viscosity of the product being mixed. The amount of the material that can be mixed in one batch must be based on the net volume of the mixer and the targeted viscosity of that product. In some cases, minor adjustments may be necessary to compensate for the bulk density of the fed materials.

Two commonly asked questions are "How can I determine batch weight for a newly developed compound?" and "How can I size a batch for a first time run on a production mixer?" The following procedure has been developed to answer these questions.

For a majority of compounds, an equation using net mixer chamber volume, compound density and a variable referred to as fill factor (FF) has been successfully used to calculate the approximate batch weight for tangential batch mixers. The batch weight calculated in this manner is referred to as the theoretical batch weight.

Theoretical batch weight = (net mixer volume) x (compound density) x (FF)

Net mixer volume "liters"

To use the equation successfully one must know the net volume of the mixer to be used. Depending on the type of rotor installed, the mixer size classification and to what extent the mixer is worn, the net chamber volume of a specific mixer may vary. Table 1 specifies the net chamber volume of various sized mixers equipped with different rotor designs.

Table 1

Rotor type      Br- 1600      1D       F-50
2 wing            1.60      16.50        -
4 wing ST          -          -         50

Rotor type        F-80      F-200      F-270     F-370    F-620
2 wing             80        200        270                 -
4 wing            70.5       156        257       414      652
4 wing ST         70.5       156        257       414      704

Compound density (KG/liter)

A value of compounded product density must be obtained. The calculated product density or preferably the product density at 100 [degrees] C has been used.

Fill factor

Fill factor specifies the percent fill of the mixing chamber necessary for effective circulation within the mixing chamber. Data presented in figure 2 specify the percent fill of the mixer for any compound based on the targeted Mooney viscosity of the compound. The figure is based on empirical testing and optimization studies on a broad range of material. From this figure one can easily see that at a specific Mooney there are a few combinations of ram or applied batch pressure and fill factors that fall within the possible operating range specified by the graph. There is, however, a fill factor at a specific applied batch pressure that will yield the best results. As previously stated, there is an operating tolerance around this ideal fill factor. This tolerance can be broad or narrow based on the specific formulation being mixed and to date must be determined empirically.

[Figure 2 ILLUSTRATION OMITTED]

Empirical studies involving changes in fill factor and applied batch pressures have produced a graph that can specify an effective applied batch pressure for a compound where the approximate ML4 Mooney viscosity of the product is known (figure 3).

[Figure 3 ILLUSTRATION OMITTED]

A typical example of the application of the equation and the two figures is presented in figure 4. In this case study, the targeted ML4 Mooney was obtained from mill mixing the product and was found to be approximately 40. From figure 3, at 40 ML4 viscosity, the targeted applied batch pressure is 25 psi. From figure 2, at 40 ML4 Mooney and at approximately 25 psi pressure, the theoretical fill factor is approximately 72%. Fill factors between 61% and 80% were evaluated, and as seen on the chart, the most effective shear mixing as suggested by the Mooney of the product was 72%.

[Figure 4 ILLUSTRATION OMITTED]

As previously stated, the calculation of the theoretical batch weight using this method is intended to serve as a guide. A way to confirm that the theoretical batch weight is correct and to establish an operating range is to mix and document product quality for a series of batches using the theoretical batch weight as a center point. The evaluation of a series of batches 7% above and 7% below the theoretical batch is recommended.

Applied batch pressure

As previously shown there are different combinations of ram pressure and fill factor to allow an acceptable product to be produced. In order to allow optimum distributive and dispersive mixing to occur in the mixing chamber, the degree of compaction of the material in the mixing chamber (during compounding) is important. The higher the Mooney viscosity of a product the greater is the compaction force required to mix the product efficiently in the mixing chamber (as previously presented in figure 3). This compaction force is referred to as applied batch pressure or ram pressure. Inadequate batch pressure may not develop adequate dispersive shear mixing and can result in excessively long mixing times. Excessive batch pressure can create "tunneling" within the mixing chamber. Tunneling occurs when the material being mixed turns with the rotor, thus eliminating compound rotor to rotor exchange within the mixing chamber (a critical component of distributive mixing). Further, excessive ram pressure could result in a rapid temperature rise of the mix causing the cycle to be ended before adequate dispersive and distributive mixing has been achieved. Correct batch pressure and batch weight will allow the containment of the batch within the mixing chamber. It will also permit efficient compound movement throughout the mixing chamber, thus achieving optimum dispersion and distribution of all compound components.

Applied batch pressure (ram or weight pressure) must not be confused with cylinder pressure. Applied batch pressure is the pressure being applied directly to the batch by the weight or ram. Cylinder pressure is the pressure being applied to the mixer air cylinder. The two are different due to the difference in area between the air cylinder on top of the mixer and the area of the hopper opening where the weight or ram pressurizes the product mix. Ram pressure is a term that is loosely applied and is sometimes used to describe air pressure to the mixer's cylinder and not necessarily batch pressure as the name implies.

For each machine there is a chart in the operator's manual that specifies the difference between the pressure applied to the batch and the pressure applied to the air cylinder. Over the years, mixers of the same size have been supplied with different diameter air cylinders and different sized weights. The machine size, the hopper cylinder diameter and the size of the weight must be known if the relationship between cylinder air pressure and applied batch or ram pressure is to be accurately determined. When batch size is being calculated, the applied batch pressure is the pressure specified on the reference graphs and is referred to as ram pressure.

Tables 2 and 3 specify the relationships between cylinder pressure and ram pressure for selected Farrel machinery. A ram pressure range and the associated cylinder pressure for specific machines are specified.

Table 2

                    Br1600      1D          F-50         F-80
Standard air cyl.
  diameter          6.0 inch    8.0 inch    14 inch      16 inch
Std. hopper(in.)    6.0x3.25    15x6.0      20.1x10.4    24.0x10.5

                    F-200       F-270       F-370        F-620
Standard air cyl.
  diameter          20 inch     22 inch     26 inch      26 inch
Std. hopper(in.)    30x18       35x18       37.4x24.0    37.4x24.0
Table 3

Batch    Batch
press    pressure    Cylinder air pressure (psi) for standard
range    (psi)       hoppers and air cylinders only

#1       10-20       10-20     17-34     13-26     12-23
#2       20-40       14-28     34-67     26-52     23-47
#3       40-60       28-42     61-101    52-79     47-70

Batch    Batch
press    pressure    Cylinder air pressure (psi) for standard
range    (psi)       hoppers and air cylinders only

#1       10-20       16-23     16-31     16-31     16-31
#2       20-40       33-65     33-63     31-63     31-63
#3       40-60       65-98     63-94     63-94     63-94

From the previous discussion about the calculation of the theoretical batch weight, the relationship between fill factor and applied batch pressure is defined. It should be remembered that the targeted Mooney viscosity used in the calculation is that of the completed mix. The batch calculation is based on the viscosity of the mix at 100 [degrees] C as is specified by the ML4 specification. At the start of the mix cycle, the apparent viscosity is high. This is one of the reasons the ram does not seat early in the mixing cycle. The use of multiple ram pressures in a mixing cycle is now becoming commonplace. In these cases, high ram pressure is used to initially force the ram down at the beginning of a mixing cycle. As mixing progresses, the temperature of the mix increases and the viscosity drops. At this point the ram pressure is lowered to promote improved circulation of the mix within the mixing cases.

Mixing steps and procedures

The art of rubber mixing is apparent when one begins to study the options available to establish the best mixing procedures for a given compound. Procedures specifying the following options are commonly used:

* Number of passes through the mixer (discrete mixing cycles);

* loading procedures for each pass through the mixer (i.e., order of addition of ingredients);

* brushing events (the cleaning of the ram);

* changes of the mixer rpm;

* changes of batch pressure; and

* specific discharging procedures.

Over the years, a series of standard mixing procedures has been developed. These mixing procedures may vary in the way they are implemented and controlled. However, the basic procedures defining the number of steps required to produce a given class of products have remained the same. The typical nomenclature used to define these general mixing procedures are as follows:

* Single step mix - a completed compound is acquired in one pass through the mixer. (This procedure is typical of the mechanical goods industry). Types:

- upside down mix - all ingredients are added to the mixer first followed by the polymer;

- standard mix - polymer is added fast, followed by the remaining ingredients.

* Multiple pass mix - typical of both the tire and mechanical goods industry:

- premastication (optional) - mastication of the polymer(s) in a separate mixing step;

- masterbatch - mixing of all formulation components except the curing package (may be a standard or upside down procedure and/or one or more may be necessary to incorporate high levels of fillers or oils);

- remill - reworking of the masterbatch to lower viscosity or improve dispersion (optional or may be more than one);

- final mix - incorporation of the cure package in the masterbatch or remill.

In each of the prementioned mixing steps there is usually a set of mixing instructions that detail:

* The means of cycle control, i.e., time, energy and indicated temperature;

* the order of addition of materials;

* the point in the cycle individual ingredients must be added;

* the point that the ram should be lowered, raised and/or cleaned;

* the procedure for discharging the batch such as either, i.e., ram up, ram down or ram float discharge and the time the drop door should be left open for batch removal; and

* set operating parameters for the mixer itself:

- applied batch pressure(s);

- mixer speed(s);

- mixer temperature (temperature of the circulating heating/cooling median).

These instructions can be implemented manually or can be set up to occur automatically when a computer control system is used. The following are general application guidelines for these mixing procedures.

Single step mixing procedure

Single step mixing is the most desirable mixing procedure. However, this procedure cannot be applied successfully to all mixing situations. If quality levels of the product cannot be achieved at a reasonable production rate (mix cycle), then multiple pass mixing is adopted. The single step mixing procedure normally includes all of the functions in a multiple step mix combined in one pass through the mixer. This procedure is common to the mechanical goods industry. The mixing procedure is normally complex and somewhat time consuming, normally requiring a sophisticated loading sequence and or a variable speed mixer. These mixing procedures are best completed with a computer control system adapted to the mixer for cycle control. The following mixing chronology is typically used:

* Polymer mastication and/or blending;

* filler incorporation (multiple additions possible);

* the brushing or cleaning of the ram (multiple possible);

* oils and miscellaneous chemical incorporation (multiple additions possible); and

* cure package incorporation.

Upside down single step mix

This mix procedure is quite common to the mechanical goods industry. When polymers are being mixed that are temperature sensitive or exhibit a dramatic change in viscosity with temperature, the upside down mixing procedure is best suited. A typical procedure is as follows:

* Add fillers, chemicals and oils (oil addition at this point is optional);

* blend for a short period of time and add polymer;

* multiple brushes and possibly delayed oil addition procedures possible; and

* product discharge.

The pre-mentioned mixing procedures can be presented in a pie chart. This allows the entire mixing procedure to be presented and examined. The pie chart identifies each phase of the mixing cycle that must be addressed. A typical example is presented in figure 5. Data defining the mixing cycle (table 4) were obtained by the use of a computer mix cycle control and data acquisition system. The data allowed each phase of the mixing cycle to be controlled and defined.

[Figure 5 ILLUSTRATION OMITTED]

Table 4

Mixer report

Max time:         167
Batch temp:       310
KWP:              4.31
Peak Kw:          179
Recipe:
Name:             125
Batch wt.:        2
Batch no.:
cycle time:
Avg. probe temp.: 308.8 [degrees] F
Standard dev.:    [+ or -] 443
Variance:         303/316

Step   Time      Temp.      Energy   Ram up   Rotor    Cyl.
no.    sec.   [degrees] F    KWH      time    speed   press.

 1      0         113        0.09      17      41       73
 2      33        200        1.66      10      41       73
 3      72        254        3.12      19      41       73
 4     106        310        4.31      4       42       72
 5      0          0                   0        0       0
 6      0          0                   0        0       0
 7      0          0                   0        0       0
 8      0          0                   0        0       0
 9      0          0                   0        0       0
 10     0          0                   0        0       0

Step    I.R.M.   Instructions
no.

 1       18.4    Add 1
 2       20.1    Add oil
 3       36.3    Brush
 4       48.2    Discharge
 5               ram
 6               Add 1
 7               1st rubber
 8               2nd black
 9
 10              Rubber @
                 68 [degrees] F

Total ram func.:     48.2
Ram down time:      106
Metal start temp.:  105
Rotor temp.:         87
Side temp.:         120
Door temp.:          86

Multiple step compounding Polymer mastication

The mastication of the polymer prior to the incorporation of the compound components is accomplished in this step. This mastication minimizes the effects of aging, temperature, moisture, residue solvents, feed form and minor variations of polymer non-uniformity. The pre-mastication usually is completed at a single mixer speed to allow for uniform homogenization of the polymer. The following sequence is typical:

* All feed rubber is fed in one charging step at the beginning of the cycle; and

* the mix is brought to a preset indicated temperature and the batch discharged.

Polymer mastication with chemicals

The incorporation of a chemical additive(s) is included with the benefit of the mastication step. Typical additives such as peptizers or antioxidants are used. If peptizers are being used to chemically reduce rubber viscosity, the mix has to be brought to a threshold temperature in order to activate the peptizer. There is usually a time-temperature relationship which will dictate the speed and/or mixing time required to accomplish the task (usually controlled viscosity reduction). In cases where antioxidants are used, the objective is to keep mix temperature at a minimum to maximize the benefit of the antioxidant in the final formulation. The mixer is generally operated at slow to medium speed when antioxidants are being incorporated. The following mixing procedure is typically used:

* Add approximately one half of the polymer, then the chemical additive, followed by the remaining polymer in a single addition sequence. This addition procedure is commonly referred to as sandwiching.

* The ram is then put down, the mix brought to a critical indicated temperature and the batch is then discharged. There are a few applications where the peptizer is used, in which the mixer rpm is dropped at a specific indicated mix temperature to allow the chemical action of the peptizer to continue for a preset time within a temperature window prior to batch discharge. A variable speed drive is required in this procedure.

Polymer mastication and blending

The homogenization (blending) of polymers of different types or of viscosity can require a separate mixing procedure. Depending on the difficulty of the task, the procedure can be very simple or one involving a very specific mixing sequence. The speed of the mixer, as well as the indicated mix temperature achieved during the cycle, are critical. The speed has to be one that will allow both polymers to be blended uniformly. The most difficult mixing task is one where the polymers are dramatically different in viscosity. The following mixing sequence has been used successfully with polymers of similar viscosity:

* Add all polymers usually at high mixer speed;

* bring the mix to the appropriate indicated mix temperature and discharge.

The mixing sequence for polymers of significantly different viscosity is:

* Add the higher viscosity polymer first and masticate to a predetermined indicated mix temperature;

* add the lower viscosity polymer (an adjustment may be required); and

* discharge at the appropriate mix temperature.

Masterbatching

The simplest masterbatch mix sequence involves the following:

* Single addition of polymer followed by the addition of miscellaneous chemicals and fillers at the beginning of the cycle.

* The addition of oil (at point later in the cycle where a portion of the filler and polymer has been incorporated. The point of oil addition is critical. If the oil is added too early, agglomerates of oil and filler may be created that are very difficult to disperse. If the oil is added too late, the batch (mix) may go to "sleep" (a loss of power) causing an excessively long cycle. The heating of the oil prior to its addition has in many cases helped mix quality and efficiency. The oil addition can be based on indicated mix temperature, energy (KWH), time or power (KW).

* The discharge of the batch (sometimes preceded with a ram raise or "brush") of which either or both can be based on indicated mix temperature energy (KWH), time, or in a few specialized cases, mixer torque.

Complex masterbatch

When high loadings of reinforcing or reactive fillers are being incorporated into a polymer formulation, the filler may have to be added in stages in a single mixing cycle or in increments using a series of simple masterbatching steps. The following multiple filler addition procedure has been used successfully:

* Add all polymer, one half of filler, chemicals and mix;

* add remaining filler and mix;

* add oil; and

* mix and discharge.

Masterbatch with polymer mastication

Eliminating a separate pre-masticating step for the polymer is a time saving measure. The following mix procedure has improved productivity:

* Add all polymer and selected chemicals (where applicable) and mix;

* add all fillers and mix (multiple filler additions may be required;

* add oils and mix; and

* discharge.

Upside down masterbatch

Upside down mixing is common in cases where there is a dramatic change in polymer viscosity with mix temperature during initial compounding or in situations where soft (low structure) carbon blacks are being mixed. In these cases, the blending of all of the fillers., chemicals and, sometimes, oils prior to the addition of the polymer is a significant advantage. A typical upside down mix sequence is as follows:

* Add fillers, chemicals and, sometimes, oils, and blend for a short time prior to the addition of polymer followed by ram down mixing;

* brush or clean the ram and add oils if not added already and mix; and

* discharge batch.

Remilling

This step used primarily for viscosity reduction and or dispersion improvement involves the addition of a completed masterbatch in either slab or pellet form, followed by ram down mixing and product discharge. Pellet or cut slab feed form is generally preferred. There are a number of cases where multiple remill steps are required to obtain the desired levels of dispersion and/or viscosity reduction.

Final mixing

The final mix involves the incorporation of the curing package of a given formula into the completed masterbatch or remilled batch. As with the remill stage, the desired feed form is either pellets or cut slab stock. Typically, there is a single addition of polymer mix that includes the chemicals at the beginning of the cycle. The chemicals are usually sandwiched (added in the middle of the rubber addition) within the pellets or slab as they are being fed to the mixer. When high viscosity slab rubber is used, indicated mix temperature at the beginning of the mix cycle may be inaccurate due to the frictional effects of the slab rubber against the tip of the thermocouple. In these situations the initial indicated temperature spikes should be ignored if indicated mix temperature is being used as the mix control criterion. As long as feed stock temperature to the mixer is controlled, energy can be used as the mixer control criterion. In either case, the actual temperature of the batch should be documented (by using a needle pyrometer). The following is a summary of the most common mix procedure:

* Load one half of the rubber masterbatch, then all the chemicals, followed by the remainder of the masterbatch;

* bring the ram down and mix; and

* discharge the batch.

Mixer RPM

Single speed mixers

There are many mixers that are operating with single speed motors. In these cases, a significant degree of versatility is lost. Single speed mixers are normally used in situations where the the mixer is dedicated to a specific product or group of products when the speed has been selected based on a history of producing that product. The rpm used is one that will permit adequate mixing to occur prior to the point the product mix is:

* Thermally degraded;

* reduced to a viscosity where high shear stress mixing cannot occur; and

* chemically reacted making the product unacceptable.

Variable speed mixers

Speed specific mixing procedures are common. In these cases, a specific mixer rpm is selected to produce a specific product based on empirical testing. The ability to select a specific mixing speed allows the compounder to get the best balance between product quality and productivity. There is an operating window that must be defined. A typical example can be viewed in figure 6. In this case, carbon black dispersion was identified as the critical quality base. A series of batches was produced varying only mixer rpm. The product dispersion was analyzed and an operating window was established. The mixer rpm selected for production was one that fell within the operating window and produced the highest productivity.

[Figure 6 ILLUSTRATION OMITTED]

In variable speed mixing, the mixer rpm is adjusted (up or down) during the mixing cycle. In order to select the optimum speeds, each phase of the cycle must be identified and the optimum rpm for each must be identified. Some questions that must be asked are as follows:

* What is the best speed for polymer homogenization, and/or polymer breakdown?

* What is the best speed(s) for filler and/or chemical incorporation?

* What is the best speed for oil incorporation?

* Does the mix have to be cooled during the mixing cycle?

Figure 7 presents a mixing cycle that was developed to meet a single stage mixing requirement. Each phase of the mixing cycle was identified and a specific rpm was selected for that phase.

[Figure 7 ILLUSTRATION OMITTED]

Mixer temperature (tempering)

The control of metal temperature is paramount to successful compounding in a tangential batch mixer. Areas which should be controlled are as follows:

* Mixer sides;

* mixer door (for non tilt mixer designs);

* mixer weight;

* mixer rotors.

Every compound being mixed has a characteristic temperature profile that will permit optimum flow of the compound within the mixing chamber. Figure 8 presents data from a study (ref. 5) where all mixing parameters and procedures were the same except for mixer metal temperature. In this study, batches were produced with the "mixer temper" at 27 [degrees] C, 43 [degrees] C, 60 [degrees] C and 77 [degrees] C. The most efficient mixing occurred at 60 [degrees] C.

[Figure 8 ILLUSTRATION OMITTED]

Compound component considerations and environmental effect

Feed form

All batch mixers are sensitive to the geometric size and form of the material being fed to it. Depending on the compound component, the changes may or may not be significant. In any case, the form of the feed materials (polymers, fillers and chemicals) should be consistent. If for any reason the feed form is changed the process should be monitored for potential changes.

One example of the effect of feed on mixing cycle can be seen in figure 9. In this study (ref. 5), the form of the natural rubber being fed to the mixer to produce a masterbatch was purposely changed in a controlled experiment. At one extreme, full bales were fed to the mixer for compounding the masterbatch. At the other extreme, granulated rubber was fed to the mixer to produce the same product. Data clearly showed that a chopped bale feed form size between 0.5 kg, and 2.0 kg produced the most efficient mixing cycle. Another such example is the use of the friable bale to improve mixing efficiency. The friable bale is a loosely compacted bale which breaks apart easily when fed to the internal batch mixer.

[Figure 9 ILLUSTRATION OMITTED]

Feed material temperature and environmental effects

The temperature of the feed rubber to the mixer can significantly affect the efficiency of a mixing cycle. One example can be seen in figure 10. Under controlled conditions, natural rubber was fed to the mixer for mastication purposes at different temperatures. All other mixing procedures and operating parameters were unchanged.

[Figure 10 ILLUSTRATION OMITTED]

In this study (ref. 3), the rubber was preconditioned to selected temperatures between -10 [degrees] C and 32 [degrees] C prior to compounding. This temperature range was selected to simulate the expected temperature variation in the warehousing area of a compounding facility. The rubber was mixed under controlled conditions to a temperature of about 115 [degrees] C in the mixing chamber. The most efficient mastication occurred when the rubber was fed to the mixer between 4 [degrees] C and 21 [degrees] C. In that temperature range, the viscosity reduction of the feed stock per unit mixing time was the highest.

Feed quality and environmental effects

The presence of moisture on the surface of the rubber can be considered contamination created by the environment. High humidity, as well as insufficient drying after the first stage of a multiple stage mix, can create significant changes in the mixing characteristics.

As with any batch mixer, the quality of the feed components of the formulation relates directly to the quality of the product.

Conclusion

There are a multitude of variables that can affect the mixing process. Within the text of this article a number were identified and discussed. When optimizing a mixing operation or when troubleshooting a mixing problem, a knowledge of these variables is required. Within this article a number of variables were discussed with the specific hope of creating a degree of awareness of these variables, and how such tools as the ram position indicator, a computer control and data acquisition system can be used to document and help define the mixing process.

References

(1.) "Batch mixer and design operation" by W.A. Rapetski, International Rubber Conference, Nurenberg, Germany, July 1979.

(2.) R.T. Vanderbilt Rubber Handbook, 30 Winfield St., P.O. Box 5150, Norwalk, CT.

(3.) Tempered Rubber Study - by Richard Jenkins, Chicago Rubber Group, Nov. 1982.

(4.) "A technical seminar in polymer mixing" by M.A. Melotto, Connecticut Rubber Group technical seminar on mixing, 1996.

(5.) "Influences of higher energy cost on rubber compounding" by F.J. Borzenski, Akron Rubber Group 1976 Winter Symposium.

Acknowledgements

"An overview of variables affecting batch mixing in a tangential mixer" is based on a paper given at the October, 1997 Rubber Division meeting.

"Optimizing mixing performance through filler dispersion control" is based on a paper given at the September, 1998 Rubber Division meeting.

"Enhanced mixing in the intermeshing batch mixer" is based on a paper given at the September, 1998 Rubber Division meeting.