Position: [FIGURE 1 OMITTED]
Design of the carousel blender/feeder
Rubber compounds in particulate blend form present an acute problem for supplying a consistent feedstock to the continuous mixing system. The obvious solution, separate metered feeds of each ingredient, is very expensive and unwieldy because of the number and variety of ingredients. This points to conventional batch weighing, followed by some form of pre-blending and metering to the mixer. For this route to be successful, two difficulties have to be overcome:
* The blending action must be gentle, to avoid damage to pelletized materials, particularly fillers such as carbon black and silica;
* segregation during blending and subsequent feeding to the continuous mixing system must be avoided.
The latter difficulty is particularly acute. The ingredients of a typical rubber compound have a wide range of particle shapes, sizes and densities. From figure 2, which shows a capability map for currently available powder blenders, it can be seen that the requirements of rubber compound blending fall in a region which is not serviced. In addition, even if a consistent powder blend can be produced, segregation in transport to the continuous mixer must be avoided to gain any benefit from this achievement.
[FIGURE 2 OMITTED]
The carousel blender/feeder (ref. 5) shown in figure 3 has been designed to overcome the difficulties outlined above. Its action is based on ordered subdivision to render it insensitive to segregation during blending, and as shown in figure 1, it can be mounted directly onto the continuous mixing system to avoid segregation during transport.
[FIGURE 3 OMITTED]
During blending, the contents of each cell are subdivided into two equal parts as they fall into the cell below. Sequenced rotation of the carousels and opening of the cells gives a consistent composition in each of the final cells, irrespective of the sequence of weighing or initial distribution of ingredients in the top carousel. The final carousel then delivers a pulsed feed as each cell is discharged. So, from a batch weighed input, a continuous feed is delivered to the mixer. Referring back to figure 2, it can be seen that the carousel blending system occupies the essential territory of "gentle action combined with insensitivity to segregation." The dependence on gravity for the blending action enables a light construction to be used and gives a low energy requirement. The prototype system has a diameter of 0.4 m, a height of 1.3 m and has a maximum design output of 250 kg/hr.
Design of the single rotor continuous mixing system (SRM)
From analysis of existing continuous mixers (refs. 3 and 4), it is clear that some separation of incorporation, distribution and filler dispersion functions is desirable for an efficient design. Similarly, separation of conveying and mixing is necessary for versatility, to bring residence or mixing time under operational control. A schematic cross-section of the prototype, which has a simple fixed die, is shown in figure 4.
[FIGURE 4 OMITTED]
The prototype system shown in figure 4 has a 90 mm diameter feed unit screw and a 200 mm diameter mixing unit rotor. Maximum speeds are 30 rev./min. for the former and 100 rev./min. for the latter. Simple water circulation temperature control is used for the screw, rotor and barrels.
Mixing trials and results
Two dissimilar compounds were selected for initial evaluation of the SRM. The purpose of the SBR compound shown in table 1 is to explore the dispersive mixing behavior of the SRM, while that of the EPDM compound in table 2 is to determine its ability to deal with high filler and oil loadings.
Union Carbide's Elastoflo EPDM is produced directly from polymerization with a spherical granule in the size range 0.5 to 1 mm, while the SBR is granulated bale material with an irregular particle in the size range 4 mm to 7 mm. No difficulties were experienced with the feeding of the SRM system. In the results which follow, rubber compound viscosity is used as a measure of filler dispersion (refs. 6 and 7).
A great deal has been learned from the mixing trials to date. The screw and rotor speeds of eight and 40 rev./min. for the SBR compound and 20 and 40 rev./min, for the EPDM compound are simply the first found to give a visually mixed rubber compound.
The results in tables 3 and 5 confirm that the mixing treatment can be varied over a wide range by independent adjustment of feed unit and mixing unit speeds. In addition, the outlet impedance (die resistance) of the mixing unit has been found to be a powerful and useful variable. A reduction of impedance reduces the effective fill factor in the mixing unit. The second result in table 3, marked by the *, was obtained with a reduced impedance. Unit work and rubber temperature are reduced, but there is an unacceptable deterioration in dispersion, indicated by the high viscosity. In contrast, table 5 shows that reducing the outlet impedance for the EPDM compound gives an overall improvement, reducing unit work and rubber temperature while maintaining a good dispersion. Outlet impedance clearly has a strong influence on the intensity of mixing; and a much lower intensity is needed for dispersive mixing of N550 carbon black than for N330.
The unsatisfactory filler dispersion of both the SBR compound and the EPDM compound at maximum speed is attributed to the high rubber temperature causing a substantial drop in the stress available for dispersion. There is substantial scope for improving cooling. The prototype SRM was designed for simplicity and ease of modification, which precluded the incorporation of efficient cooling. In both tables 3 and 5, the last row gives the best result that can be achieved with the current prototype--maximum output with satisfactory filler dispersion. This will be increased when the cooling efficiency is improved.
In tables 4 and 6, the results for satisfactory carbon black dispersion in a Shaw K1 Intermix (intermeshing rotor internal mixing system of 5.5 liters chamber volume) are given, together with the batch viscosity, which provides a measure of this state-of-mix. Comparison with the unit work results in tables 3 and 5 shows the high energy efficiency of the continuous mixer. For the SBR compound, the energy required is approximately a third that of the batch internal mixer and half for the EPDM compound. In addition in the obvious energy saving, this improvement enables the size of the whole mixing system to be reduced. With the simplicity and stiffness of the cylindrical feed and mixing unit barrels, the mass of the continuous mixing system can also be much lower than that of an equivalent batch mixer.
Physical properties
Tensile properties have been measured to determine if the state-of-mix results in tables 3 through 6 are carried forward into the vulcanizate. The SRM results in table 7 are derived from feed and mixing unit speeds of 8/40 for the SBR compound and 20/40 for the EPDM.
The similarity of results from the SRM and the Intermix for both compounds confirms that the former gives effective curative distribution and dispersion in addition to good filler dispersion.
Purging characteristics
Minimum wastage of rubber compound and time during a compound changeover are natural objectives of continuous mixer design. Experience with the prototype SRM has shown that the feed unit screw and the dispersive zone are completely self purging, but physical removal of rubber from the feed unit head and the distributive zone is necessary. Design modifications to improve the self-emptying of the distributive zone are being investigated. In addition, a trial to explore the extent to which a direct changeover is possible, without emptying the machine of rubber, has been carried out. The SBR and EPDM compounds, with and without curatives, were used to track the progress of purging. The feed hopper was run empty of the particulate blend with curatives and followed immediately with a blend which omitted the curatives. Samples of mixed compound were then taken at intervals for cure testing. The results, shown in figure 5, were obtained at an output rate of 35 kg/hr. for the SBR and 44 kg/hr. for the EPDM.
[FIGURE 5 OMITTED]
The reduction in the amount of material present with curatives is tracked by the change in crosslink density ([T.sub.max.] - [T.sub.min.]), until it is eliminated, at one minute for the SBR and two minutes for the EPDM. After each of these trials, the mixing system was opened and samples of compound removed from the head of the feed unit for further cure tests, to check for residual compound with curatives. These samples showed zero crosslink density and confirmed that the head is tree of regions of re-circulatory flow or stagnation.
Scale-up
Scaling rules have been developed for the SRM, using non-isothermal, non-Newtonian flow analyses to extrapolate from the prototype results. They are conservative in that they do not assume any increase in dispersive mixing efficiency resulting from the planned improvements in heat transfer efficiency and output pressure control. The predictions in tables 8 and 9 are for single-pass mixing with all the curatives included in the blended particulate feedstock. Consequently, the maximum output rate is dictated by robber temperature, and a target outlet value of 100[degrees]C has been used. The specific heat transfer rates have been set at levels expected for drilled cooling channels, except for the prototype characteristics in the top row, which are experimentally determined. For comparison, the experimentally determined heat transfer coefficient of the flood cooled Intermix used in the work reported here is 770 W/[m.sup.2]/C. Net power does not include allowances for drive and friction losses.
Conclusions and further development
Results from a prototype continuous rubber mixing system have been presented which provide proof of principle and show the way forward for improvement and for the design and build of production systems. In particular, the new system:
* is compact, simple and robust;
* has simple and versatile operational control to mix a wide range of compounds;
* feeding is insensitive to rubber particle size and shape within the range investigated;
* is energy efficient;
* can achieve complete mixing in a single pass;
* has good purging behavior for rubber compound changes.
Substantial improvements to the heat transfer efficiency of the mixing unit are possible, to combine high output with low rubber temperature. This will also improve the dispersive mixing capability. The prototype unit is relatively inefficient due to simplicity of construction and design for adaptability.
Table 1 - SBR compound Material Pphr SBR 1500 100 Carbon black N330 40 Zinc oxide 5 Stearic acid 2 Sulfur 2 CBS 1 Table 2 - EPDM compound Material Pphr EPDM (includes 20 phr CB)) 120 Carbon black N550 100 Oil 75 Zinc oxide 5 Stearic acid 2 Sulfur 2 CBS 1 Table 3 - single rotor continous mixer results for the SBR compound Speed Power Output Unit work (rev./min.) (kW) (kg/h.) (MJ/[m.sup.3]) 8/40 1.0/4.8 43 527 8/40 * 1.1/3.0 43 343 30/100 4.9/16 150 620 16/100 2.4/10.8 78 620 Speed Output Viscosity (rev./min.) temp. (kPa S) ([degrees]C) 8/40 97 97 8/40 * 88 120 30/100 153 121 16/100 121 98 Table 4 - Intermix results for the SBR compound Speed Power Mixtime Unit work (rev./min.) (kW) (kg/h.) (MJ/[m.sup.3]) 30 21.5(avg.) 4 1,718 Speed Output Viscosity (rev./min.) temp. (kPa S) ([degrees]C) 30 135 95 Table 5 - single rotor continous mixing system results for the EPDM compound Speed Power Output Unit work (rev./min.) (kW) (kg/h.) (MJ/[m.sup.3]) 20/40 2.9/8.0 73 537 20/40 * 2.7/3.7 73 315 30/100 4.2/12.6 130 471 25/100 3.5/7.4 105 372 Speed Output Viscosity (rev./min.) temp. (kPa S) ([degrees]C) 20/40 131 83 20/40 * 96 84 30/100 127 94 25/100 120 85 Table 6 - Intermix results for the EPDM compound Speed Mixing Output Unit work (rev./min.) time (kg/h) (MJ/[m.sup.3]) (min.) 32 19 2 754 Speed Output Viscosity (rev./min.) temp. (kPa S) ([degrees]C) 32 118 84 Table 7. comparison of tensile properties SBR M100 (MPs) EB (%) TS (MPs) SRM 2.8 494 21.4 K1 Intermix 2.6 436 20.0 EPDM SRM 4.3 428 13.6 K1 Intermix 5.1 361 14.1 Table 8 - scale-up predictions for the SBR compound Screw D Q Power Feed (mm) (kg/hr.) (kW-net) unit 90 80 1.7 120 178 4 150 333 8 180 560 13 210 871 21 230 1,149 28 260 1,569 41 Rotor D Power Rubber Heat trans. Mixing (mm) (kW-net) temp (C) (W/[m.sup.2]/C) unit 200 9 148 410 300 20 102 920 400 38 102 980 500 64 102 1,060 600 100 102 1,150 700 132 103 1,100 800 180 103 1,180 Table 9 - scale-up predictions for the EPDM compound Screw D Q Power Feed (mm) (kg/hr) (kW-net) unit 90 106 2.3 120 236 5.5 150 443 11 180 744 19 210 1,157 29 230 1,527 39 260 1,917 50 Rotor D Power Rubber Heat trans. Mixing (mm) (kW-net) temp (C) (W/[m.sup.2]/C) unit 200 9 100 460 300 14 100 590 400 27 101 630 500 45 99 700 600 69 101 750 700 92 100 730 800 115 100 710
References
(1.) C.W. Evans, Powdered and Particulate Rubber Technology, Applied Science Publishers, London, (1978).
(2.) J.L. White, Rubber Processing, Hanser/Gardner, Cincinnati (1995).
(3.) I. Manas-Zloczower and Z. Tadmor (Eds), Mixing and Compounding of Polymers, Hanser/Gardner Cincinnati (1994).
(4.) H. Ellwood, "A tale of continuous developments, " Eur. Rubb. J., 169 (3), 26-32 (1987).
(5.) J. Clarke and P.K. Freakley, Patent Application No. PCT/GB00/ 02882 (WO 01/07153).
(6.) J. Clarke and P.K. Freakley, "Reduction in viscosity of an SBR compound by mastication and dis-agglomeration during mixing, " Rub. Chem. Technol., 67, 4, 700-715 (1994).
(7.) P.K. Freakley and J. Clarke, "Comparisons of the mixing of rubber with carbon black in an internal mixer and in a biconical rotor rheometer," J. Appl. Polym. Sci., 53, 121-132 (1994).
(8.) J. Clarke and P.K. Freakley, "Modes of dispersive mixing and filler agglomerate size distribution in rubber compounds," Plast. Rub. and Comps., Proc. and Appl., 24, 5, 261-266 (1995).
P.K. Freakley, Loughborough University and J.B. Fletcher, Carter Bros. Ltd.
Continuous mixing holds out the promise of efficient and consistent rubber processing. This has been recognized for many years, with the last major development activity occurring in the 1970s (ref. 1). Then, the lack of a reliable supply of technologically and economically viable particulate rubber, coupled with a relatively undeveloped mixing technology, caused the movement to founder, despite the best efforts of enthusiasts. Now, continuous mixing is back on the agenda with major materials suppliers offering a range of elastomers in particulate form. There is also a much better understanding of mixing to draw upon for process design (refs. 2-4). It is also recognized that supplying a consistent feedstock to the continuous mixing system is a pre-requisite for success.
