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Internal mixer automated monitoring, control.

Internal mixer automated monitoring, control

The Quabaug Corporation has been in the rubber industry since 1916. In 1984, Quabaug reviewed those market areas which would benefit from our expertise in producing high quality footwear components. Consistent quality in our product line demanded reproducible results in a variety of properties, including stress strain, abrasion, aging and flex life. This consistency was required over a large spectrum of elastomers from natural rubber through nitrile to ethylene vinyl acetate.

Fernely H. Banbury patented "A machine for treating rubber and other heavy plastic material" under patent #1,200,070 in 1916, the year of our incorporation. At the age of 35 he realized that he had created a machine of importance to the rubber industry. However, this practical engineer probably did not realize how very important the understanding and control of his device would become in the last quarter of our century.

Peter Johnson predicted in 1976 that rubber technology in the last quarter of the 20th Century would concentrate on the "physics and engineering of processing and products" (ref. 1).

The last 15 years have certainly proven that Dr. Johnson was correct with respect to rubber mixing. During that period we have begun to understand the physics and engineering of various divisions of the mixing process. Two of importance are temperature control and work input.

The importance of temperature control in mix consistency

In the early days of mixing, the rubber industry used artists to blend ingredients into a supposedly homogenous compound. At first, all ingredients would be thrown into the chamber, the ram would be pushed down, and the rotors would be run until the batch "sounded right" or until the vibration on the platform "felt right" (because the strain on the motor was reduced).

As the industry became more sophisticated, batches were mixed by time using state of the art technology like alarm clocks or wall units. Finally, our industry discovered that since the ingredients go in cool and come out hot, temperature could be a useful mixing parameter. As probing devices became more sophisticated, the industry determined that temperature was helpful not only for batch discharge, but for batch mix control through the total process.

In a definitive paper (ref. 2), W. Kemp Shepherd of Farrel showed that proper temperature control can accomplish the following: * Shorter mixing cycle by up to 50%; * Reduced power consumption by 10 to 20%; * Improved dispersion; * Improved batch to batch consistency; * Elimination of "spring and fall" variation and "first batch" effect.

Dr. Nakajima and company confirmed that temperature control was important since almost half of the total mechanical energy input into a mixed compound went to raise the temperature while almost half was removed by the cooling water (ref. 3). Temperature control of the internal mixer has proven to be beneficial in controlling batch consistency as reported in a number of studies including Jenkins (ref. 4), Wiedman et al (ref. 5), etc.

As Melotto of Parrel has shown (ref. 6), an IR probe could replace a thermocouple probe in either the door top or frame end to give an even better temperature measurement resulting in more improved control of mixing (table 1).
 Table 1
 Temperature Thermocouple
Actual/measured Frame Door Infrared
93 [Degrees] C --- 80 93
 121 87 115 121
 149 131 145 149
 177 163 174 177


The importance of energy control in mix consistency

The mechanical energy put into a rubber mix is converted either to thermal energy (which is removed by cooling water or used to increase the batch temperature) or it is used to produce a more thermodynamically stable mixture.

Since the definitive studies by Van Buskirk and company (ref. 7), our industry has recognized the importance of measuring and controlling work (or the integration of power over time) for producing more consistent rubber mixes. As O'Connor and Putman showed (ref. 8), the variation in % die swell can be reduced over 5-fold by dropping batches on work-input rather than temperature. Hetzel and company showed that work input control allows for (ref. 9): * Shorter mixing time, increased productivity; * No second pass; * Improved reproducibility.

Beyond temperature and energy:

Equipment design and control

High quality, reproducible batches are produced through the measurement and control of the mixing parameters, control which Monsanto shows (ref. 10) will: * Assure uniformity within a batch; * Assure batch to batch uniformity; and * Make the most efficient use of equipment, time and power.

Temperature and work input are important parameters for measuring and controlling rubber mixing. However, there are other areas which must be controlled in order to produce uniform mixes. Jack Byam and G.P. Colbert of Du Pont recognized that three basic parameters contribute to the quality, rate and economy of a process (ref. 11). (Process: quality rate & economy = f (Materials properties and operating, variables, equipment design)

Temperature and work input are two operating variables which impact the mixing process. Raw material variability likewise affects mixing consistency. In addition to operating variables and materials' properties, modifications in the equipment design can improve the quality of mixing. One of the equipment areas sometimes overlooked in controlling the mixing process is chamber loading.

Equipment design

Chamber loading of fillers For large users of few ingredients such as tire or roofing membrane manufacturers, pneumatic feeding of solid particulate fillers from silos is a reasonable method for chamber loading (ref. 12). Even if large storage silos are not used, day silos such as surge bins are required for these systems to operate (refs. 13 and 14). However, for job shop manufacturers and custom mixers, these systems are not practical since the number of fillers used on a typical day can be several times the number of economically available bins. This concern can be addressed by modifying the chamber loading system to allow for a rear hopper loading of fillers.

The polymer and "salt and pepper" additives are loaded through the normal front feed hopper door. The fillers are loaded through the rear hopper and can be added at a controlled rate according to the mixing instructions for conventional, upside-down or sandwich mixing.

Reproducible multiple adds of fillers are possible, allowing for more difficult to disperse fillers to be added first in a consistent manner. Mixing consistency can be seen in comparing the Mooney viscosity for a 52 pph n-330, 10 pph aromatic oil SBR masterbatch conventionally mixed by front vs. rear hopper addition of the carbon black (table 2).
 Table 2
 100 [Degrees] C V-min
 X S.D.
Front hopper 104 3.7
Rear hopper 79.5 1.9


The Mooney viscosity is lower and less variable, indicating a better, more consistently dispersed mixture.

Chamber loading of plasticizers A second area to consider when studying chamber loading is that of oil addition. Three sources of mixing inconsistency can occur during oil addition: * Temperature variations; * Weighing errors; * Discontinuous discreet additions.

When oil is added to a rubber mix, it will act to quench the temperature of the mix. If the temperature of the oil varies due to day/night or seasonal differences, then the temperature of the batch will vary accordingly. 45 pph of 20 [degrees] C oil added to 100 pph rubber with 45 pph filler at 66 [degrees] C can quench the mixture by over 10 [degrees] C.

A common industry practice is for production compounders to estimate the weight of an ingredient. They will note that a 20 1 pail of D.O.P. is 19.4 kg. Therefore, they will conclude that 20 1 = 19.4 kg. But, if D.B.P. is required, the error will be more than +5%, while if process oil is required, the error will be more than -5%.

Mixer operators experience fatigue over the working day, actually slowing down during the shift. The variation of the rate of addition of oil due to operator fatigue will affect the quality of the mix.

The goal is to minimize any variation in mixing imposed by individual internal mixer operators. This is achieved by controlling the mixing cycle so it is consistent from batch to batch.

To overcome these problems in the chamber loading of oil, we have installed a heated oil system in which up to 12 plasticizers can be heated to 66 [degrees] C, metered gravimetrically via a weighing balance to a holding tank and injected into the internal mixer.

The result of this consistent temperature controlled metering and injecting is improved reproducibility with better mix quality as measured by both the rate and state of cure. For example, in a black reinforced D.O.P. plasticized nitrile butadiene compound the parameters shown in table 3 were monitored before and after the controlled oil system was installed. The state of cure increased by 6.5% with no increase in variation, while the rate of cure increased 3.3 to 5.8% with up to 40% reduction in standard error. [Tabular Data Omitted]

A clay extended D.O.P. plasticized nitrile butadiene rubber compound is shown in table 4. In all parameters measured, the standard error with the oil system was less than the control without the heated oil system.
 Table 4
 M* M* M* T* E H
 100 200 300
No oil control 2.05 3.18 4.28 13.3 641% 64.9
Heated oil injection 2.09 3.28 4.35 13.6 662% 66.3


* MPa

Variable rotor speed A second area which we have controlled through equipment modification is the rotor speed. There are at least four stages recognized in the mixing process (ref. 15):

Exponential mixing [1 = ke.sup.N] (Viscosity reduction and pellet rupture; incorporation; distribution)

Dispersive mixing Dispersion

In step 1, the viscosity and elasticity of the rubber is reduced while the filler pellet size is reduced. During the step of incorporation, the ingredients are brought into the rubber phase. During step 3 (distribution), the agglomerates are evenly scattered through the rubber phase as the wetting out continues. During dispersion, the agglomerates are reduced towards the smaller aggregate size and wetting out may be reversed by the release of occluded rubber due to partially wetted agglomerate fracture.

The viscosity of the polymer is inversely related to the temperature. The rate of the temperature rise is directly related to the rotor speed.

Rapid comminution, incorporation and distribution insures a uniform gross mixture. However, if the rotor speed is too high, then the resulting rapid temperature rise will result in poor dispersion due to low polymer viscosity. As Philip Freakly says (ref. 16), "Dispersive and distributive mixing generate conflicting requirements with respect to rotor speed. The rate of distributive mixing is a function of rotor speed proceeding more rapidly as speed is increased; but to retain a high viscosity in the rubber for dispersive mixing it is desirable to run a mixer slowly, to minimize the rise in batch temperature".

This is why Dr. Johnson writes (ref. 17), "Thus there is a tradeoff between increased speed of mixing and less well dispersed or homogenous material."

We have installed continuous variable speed DC controlled rotors which allow for rapid breakdown, incorporation and distribution followed by better dispersion at a lower rotor speed. For example, a carbon black reinforced aromatic oil plasticized styrene butadiene sulfur cured compound had been mixed on a 2 wing 11D Banbury (R) (a registered trademark of the Farrel Corp.) with a fixed 27 rpm. This was converted to a 1 pass mix using variable speed and oil injection. The parameters were measured in table 5.

Table 5

Carbon black reinforced aromatic oil plasticized SBR
 2 Pass fixed speed 1 Pass variable speed
M100 2.80 [+ or -] .45 MPa 2.97 [+ or -] .41
M200 6.73 [+ or -] 1.12 7.09 [+ or -] .88
M300 11.13 [+ or -] 1.50 12.18 [+ or -] 1.10
T 16.77 [+ or -] .65 17.39 [+ or -] .72
E 420 [+ or -] 58% 413 [+ or -] 34


Clay extended, aromatic oil plasticized SBR
 2 Pass fixed speed 1 Pass variable speed
TS2 2.41 [+ or -] .14 2.08 [+ or -] .11
t'90 4.77 [+ or -] .46 4.51 [+ or -] .34
MH 5.62 [+ or -] .21 N.m 5.75 [+ or -] .21
M100 2.59 [+ or -] .31 MPa 2.78 [+ or -] .26
Shore A 49.7 [+ or -] 1.9 50.9 [+ or -] 1.9


With increased rate and state of cure in good control, a molder could speed up the curing process 20", resulting in a 5% increase in productivity for a 6' cycle.

Computer monitoring and control of mixing

The rear hopper, the variable speed rotor and the heated oil injection modules are integrated into an RADO engineered Werner & Pfleiderer PKS-21 mixing control system.

The system is responsible for the three ranges of material feed control, mixing control and production control.

A Digital Equipment PDP 11/73 Master Computer utilizes a multicomponent menu to administer the three applications by a W&P line computer coupled to a dedicated Allen-Bradley or Siemens system for controlling the mixing. The menu has four major areas: material administration, plant configuration, formula administration and order administration.

In material administration, the raw materials are assigned code numbers as they are grouped in logical species for purchasing, inventory and production control.

In plant configuration, the raw materials are assigned to one of four categories: rear hopper filler, heated oil injection, belt or manual.

The rear hopper filler and heated oil injection have been discussed. Both can be automatically fed into the internal mixer by time, temperature and/or energy. Belt materials include elastomers, polybagged fillers, masterbatches, etc., weighed, confirmed and released for automatic belt feeding of the front door at the start of the mix.

Manual adds include curatives, blowing agents, process aids, etc., added later in the mix cycle.

In formula administration, the compounds are specified by both their composition and their mixing sequence. A mix cycle can be analyzed into as many as 20 subcycles which can be controlled by time, temperature and energy.

For example, an N-330, S-790 1502 SBR masterbatch was mixed in steps 10, 11 and 12 according to the cycle shown in table 6.
 Table 6
 PKS-21 mixing step control
 Step 10 Step 11 Step 12
Time [sec] 20.000 10.000 20.000


Energy [KJ/lb]
Rotations[#] 9.000 4.500 9.000
Temp. [deg. c] 121.000 121.000 135.000
Reaction time [PSEC]
Front door Shut Open Shut
Drop door Shut Shut Shut
Lift ram Down Up Down
Ram [KPa] 517.000 0.000 517.000
Rotor [RPM] 27.000 27.000 27.000
Call number (1 = Rear hopper
 2 = Oil
 3 = Belt)


As you can see, even a supposedly simple 4' mix is really a complex series of subroutines.

In order administration, the short term schedules are generated from long term schedules to optimize productivity and reproducibility.

Conclusion

Temperature and work input are two important operating variables for controlling the mix consistency of rubber compounds. Filler addition, oil addition and rotor speed are three important design features which also affect mix quality. All of these variables can be monitored and controlled using a W&P PKS-21 system.

References

[1.] Peter S. Johnson, "The incentive for innovation in rubber processing - a review," CIC/Rubber Groups: Montreal and Toronto, May/June 1970, p. 3. [2.] W. Kemp Shepherd, "The advantages of the Farrel tempered water system when used with high heat transfer Banbury equipped with drilled sides," - Rubber Chemical Division of the Chemical Institute of Canada: Montreal, Quebec, Canada, May 4, 1976, pp. 17-18. [3.] N. Nakajima, E.R. Harrell, and D.A. Seil, "Energy balance and heat transfer in mixing of elastomer compounds with the internal mixer," RC&T, Vol. 55 (May/June, 1982) pp. 466-467. [4.] Richard Jenkins, "Tempered rubber study," Chicago Rubber Group, Nov. 8, 1979. [5.] W.M. Wiedmann and H.M. Schmid, "Optimization of rubber mixing in internal mixers," RC&T, Vol. 55 (May/June, 1982) p. 376. [6.] Michael A. Melotto, "Update on mixing in the rubber industry," Connecticut Rubber Group, Feb. 1989, pp. 6, 12. [7.] Paul Van Buskirk, S.B. Turetzky, P.F. Gunberg, "Practical parameters for mixing," RC&T, Vol. 28, no. 4, pp. 577-591. [8.] G.E. O'Connor and J.B. Putman, "Work input control of production batch mixing," RC&T, Vol. 51, no. 4, p. 807. [9.] F.W. Hetzel, C.K. Chen, P.T. Dolezal, P.S. Johnson, "Mixing optimization on a factory scale using power profiles," The Rubber Division, ACS, Las Vegas, Nevada, May 20-23, 1980. [10.] Monsanto, "Power integration, a more precise and efficient method for control of batch to batch rubber processing and property uniformity," Technical Bulletin, IE3. [11.] J.D. Byam and G.P. Colbert, "An integrated approach to efficient polymer processing," #371, Conference on practical rheology in polymer processing, Loughborough University, Loughborough, U.K., March 26-27, 1980, p. 3. [12.] Urs Maite, "Pneumatic conveying of carbon black as an aid to plant efficiency," Elastomerics, June 1986, pp. 33-36. [13.] Ibid, p. 38. [14.] Vincent A. DeLuca, Jr., "Automated carbon black systems for mechanical goods rubber compounding," Rubber World, Aug. 1986, pp. 25-28. [15.] Philip K. Freakley, "Rubber processing and production organization," Plenum Press, NY, 1986, pp. 46-48. [16.] Ibid, p. 58. [17.] Peter S. Johnson, "Development in mixing, science and technology," Elastomerics, Jan. 1983, p. 12.
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Title Annotation:rubber industry
Author:Lee, William B.
Publication:Rubber World
Date:Feb 1, 1990
Words:2868
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