Continuous tangential mixing of elastomers.

The continuous mixing of rubber dates back to the early 1950s, and many attempts have been made to develop a machine more capable than the conventional batch mixer. Today, there are several technical developments challenging both the economics and the mindset of the industry. The use of baled feedstocks is no longer mandatory. The machinery has been developed to granulate baled feedstock for many years, but long-term storage is still not practical for many materials. Today, more robber materials are available in particulate forms, and thermoplastic materials are extending their range of properties to compete with thermosets. Manufacturing technology is evolving to make the most of these new materials. There is growing economic and environmental pressure on the rubber industry, and these pressures are part of the reason the industry needs to find ways to improve the cost and quality of mixing (ref. 1).

The compounding process itself can be performed on various types of available compounding equipment, such as single and twin screw extruders, continuous mixers, two-roll mills, and batch mixers at different levels of efficiency and cost (ref. 2). The compounding process may involve some or all basic process functions--mastication/melting, mixing (extensive and intensive), degassing and pumping. To select the proper machine for the application, a thorough understanding of the principles of operation of the polymer compounding equipment is necessary (ref. 3).

In order to realize the full potential of these materials, efficient dispersion of the additives is of paramount importance, and therefore the mixing characteristics of the actual processing machinery must be carefully considered for a given application. While most laboratory tests are performed in small machinery, it is critical that the large-scale production can be done with identical thermal characteristics.

The Farrel Continuous Mixer (FCM) (ref. 4), developed in the late 1950s and introduced to the market in the early 1960s, has been extensively used in this area for several decades. It has undergone several changes and refinements, including new rotor designs for controlled residence time and processing temperatures, sequential incorporation of additives and improved energy efficiency by means of total process control. In this article, the design and operating characteristics of the FCM in elastomeric compounding will be outlined.

An obscure device, the Spiramax mixer/extruder, found limited success in the application. The device is a single rotor mixer/extruder with a stationary scroll to provide conveyance. The rotor had an extension to a vented extruder screw for discharge. The high rotor torque (up to 10,000 N-M) allowed low discharge temperatures. The smaller sizes worked well and produced good dispersion in viscous materials. However, it was learned that the larger machines did not work well.

The Mixing Venting Extruder (MVX), developed in 1975 and introduced to the market in the early 1980s, has been in commercial production at rates up to 5,000 kgs/hr. (ref. 4). There were some technical challenges in operating the MVX with the control technology available at the time. In addition, the feeding technique was prone to problems. Modern digital control systems have the power to overcome these limitations and extend the range of operation with tighter energy control variations. This machine did not have the scale or thermal limitations of the single rotor mixing machines. Farrel is currently undertaking a project to re-evaluate the MVX with technical improvements developed since it was last manufactured. It has been established as the only large-scale continuous mixer for robber compounds that can be scaled from small to large sizes. Examples of applications will be established.

More recently, Farrel has introduced a complete line of twin screw extruders (FTX) with unique mixing technology based on continuous mixer technology (ref. 6). There are several studies underway to establish the performance of the FTX for continuous compounding of rubber compounds. The design and operating characteristics of this twin screw are presented and discussed in some detail.

Structure and principles of FCM operation

The FCM is a starve-fed, counter-rotating, non-intermeshing, twin-rotor mixer, as illustrated in figure 1. Each rotor is composed of a conventional screw section followed by a mixing section. The screw section is used to convey feed materials to the mixing section. Each rotor employs two helically twisted wings, a half turn apart from each other, with the wings of each rotor twisted in opposite directions. Each wing is composed of two sections, a forward pumping section followed by a reverse pumping section, the two sections of each wing being of unequal length and using different helix angles. The apex of each wing, the point at which opposing sections of each wing intersect with each other, is offset from the apex of the complementary wing of the same rotor.

[FIGURE 1 OMITTED]

The rotors are housed in cylindrical enclosures known as right and left chamber halves, usually split at the horizontal plane, which communicate with each other along the entire mixing section. Feed and exit openings are located at the intersection of the two chamber halves. Figure 2 shows the entire assembly of rotors and chamber with the top of the chamber hinged open, exposing the position of the rotors in the bottom housing half and the position of the feed opening relative to the rotors. Both rotors and housing are temperature controlled, although quite often they are operated neutral, i.e., under adiabatic conditions.

[FIGURE 2 OMITTED]

The forward wings of each rotor drag the material in a helical path, with the reverse helix wing opposing the forward movement of material. Forward motion is obtained due to the differential pressure generated in the forward and reverse sections of the mixer as a result of differences in the material viscosity. In the forward section of the rotor, the power dissipated is sufficient to fuse the solid particle together and initiate phase transition from solid into a molten stage. Freezing experiments (ref. 5) have shown that the melting process is completed by the time the material reaches the apex region of the rotors. An adjustable orifice gate is used to provide for added resistance to the forward motion of material in the mixer. The dynamic pressurization of the material in the window of interaction between the two chamber halves ensures, in addition to the forward motion, efficient exchange of material from one chamber half into the other, a prerequisite for good extensive mixing. The prevailing flow patterns in tiffs section of the mixer are clearly demonstrated in figure 3, which shows the operation of the mixer with the top housing removed.

[FIGURE 3 OMITTED]

Several trends are readily apparent from figure 3: The material conforms to the general shape of the rotors, there is no evidence of un-molten material, even in the forward section of the rotor (an indication of the efficient conversion of mechanical energy into thermal energy); and the formation of squeezing flow patterns and rolling pools.

These flow patterns promote extensive mixing. Dispersive or intensive mixing is obtained by passing the material repeatedly over the high shear region formed between the rotor tip and the housing wall. Figure 4a shows a cross-sectional view of the rotors and figure 4b the rotor tip region. All rotors, irrespective of their wing configuration, employ identical cross-sections.

[FIGURE 4 OMITTED]

The squeezing flow patterns of figure 3 can be further maximized by setting and maintaining during operation a preferred alignment of the tips of the two rotors as they cross each other in the window of interaction. Tip-to-tip rotor orientation was found to maximize this flow pattern and is now the usual mode of operation. In this configuration, the two rotors are operated at even speed. Another means by which the mixing intensity can be controlled is by inserting dams in the space between the two rotors, in the window of interaction of the two chamber halves. The dams provide for added resistance to forward motion and divert material away from the window of interaction into the two chamber halves. The FCM is operated in starve mode, i.e., the feed is metered to the mixer by means of a feeding device. The mode of operation permits setting the mixer rotor speed independently of the flow rate, and thus provides for an additional degree of freedom in controlling the mixing intensity.

Size of orifice opening, number and position of inserts, rotor and housing temperatures are the other controllable operational parameters. The FCM has limited pumping capabilities, and a single screw extruder or a gear pump accomplishes the final pressurization. In the former case, the material is gravitationally led to the extruder, and the two machines operate independent of each other. In the latter case, the gear pump is directly attached to the mixer (ref. 7), and the gear pump performs the dual function of controlling the mixer operation and generating the pressure needed for pelletizing or other finishing operations. Operation of the FCM/GP combination is governed by integral automatic process controls.

Many FCMs are in use in robber applications, some doing final mix of tread stock, others wanning stock for calender or press operations. Today, the FCM is commonly used to add rubber as an impact modifier to thermoplastic materials. Although designed to replace the internal mixer, the FCM does not have the capacity to do single pass mixing of most of the compounds for today's tire industry. The concept lead to the MVX with more robust rotors, a teed stuffer and a close-coupled extruder.

Structure and principles of Spiramax operation

The Spiramax was a simple solution to the apparent challenge. It seemed that a large, high torque single rotor was the way to overcome the problems with both twinscrews and twin rotor continuous mixers. It performed the mixing functions with very low pressure (300-1,800 Kpa) which reduced agglomeration of pigments. The system required temperature control of the shear surfaces to support the melting mechanism. If lubricants were added to the system, the reduced friction eliminated the ability to develop torque interrupting the melting function. The clearance between the end of the rotor and the faceplate restricted the material flow and allowed for devolatilizing to take place.

The added complexity of a "stuffer" (positive feeding mechanism) to keep pressure on the chamber was required. Two types of devices have been applied. Figure 5 shows a reciprocating pneumatic ram, which cycled to allow materials to collect under the ram and then be forced down into the rotor gap. An auger type of stuffer was also applied for more free flowing materials that did not segregate easily.

[FIGURE 5 OMITTED]

The rotor has internal fluid passages for added heating/ cooling capacity. The body of the machine also has heating cooling bands to help control melt temperature. The barrel has a limited surface area to introduce heat to the process. The 150 mm machine has produced quality SBR masterbatches. The surface area of the mixer barrel is a function of D, as was the area of the rotor. While the volume is a function of [D.sup.2], the thermal characteristics of the device did not allow the larger machines to operate with the same efficiency of the smaller machines. Farrel documented and accepted the limitations of the cylinder device and developed the superior disc type extruder. Similar drum type extruders have found success in thermoplastics in limited sizes.

Structure and principles of MVX operation

Developed in 1975 as the solution for continuous mixing, venting and extrusion of thermoplastic compounds, the machine was also accepted for specific robber applications. The FCM by this time had established itself in the plastics industry and was capable of operating in a starve-fed mode. The MVX evolved to fill the void the FCM was designed for. There were significant challenges with the feed preparation and handling system. The granulated rubber would separate from the powder additives if any significant amount collected in the feed hopper. Because there was plug flow in the mixer, there would be variation in curative distribution. The material "bunkering" system developed allowed for small volumes of consistent mix to feed into the machine as required (ref. 8). Data collected from trials and commercial production show advantages over batch processed robber compounds. There are both economic and quality advantages to the MVX compounding extrusion method. The energy required, as well as the peak load, could be reduced significantly. Once the feed problems were solved, the curative distribution was superior to the batch technology of the time.

The three lobe delta rotors allowed very high torque to be transferred for excellent dispersion and mastication at low temperatures. The straight wing flight required the ram to feed the material into the mixer and down to the extruder. The extruder speed had a significant effect on the mixer behavior and the control loop was very challenging with the technology available. There were several control loops to allow automatic operation of the machine. The feed rate, mixer speed and extruder speed were all variables controlled to allow extrusion of tire components. The machines produce directly from SBR compounds, including: bead, apex, sidewall and tread stock. NR compounds often required a secondary operation prior to MVX feed. Even in NR applications, the MVX uses less power and eliminates a complete pass from the final extrusion.

The output of the MVX models is listed in table 1. Output is based on single pass. direct profile extrusion.

Design considerations of the twin screw extruder

Mixing in conventional co-rotating twin screw extruders is usually performed by kneading blocks, as shown in figure 6a, which traditionally employ relatively tight clearances between the kneading disk tips and the inner housing wall. In recent years, kneading blocks with loose clearances, or kneading blocks with varying tip clearances between the kneading lobe tip and the inner housing wall, have been introduced to enhance their performance. The governing principle in these designs is to increase the fraction of material that flows in the high shear regions formed between the lobe tip and the inner housing wall. The CME tangential mixing principle imparts similar shear energy, but with a narrow range of shear exposure. By imparting all ordered flow over the apex for a predictable number of passes, there is a significant improvement in shear distribution. By reducing the maximum shear rate and increasing the minimum shear rate, the FTX produces cooler, better dispersed materials.

[FIGURE 6 OMITTED]

Mixing elements introduced by Farrel (refs. 5, 9-11), shown in figure 6b, utilize tangential mixing technology derived from continuous mixers. These elements utilize a cross-sectional geometry similar to the one exhibited in figure 4 and with relatively large tip clearances. These elements induce larger circumferential flows over the tips and subject the material to controlled shear rates and stresses. Exchange of material occurs as a result of these flows, as well as in the intermesh regions between mixing elements, which again provide sufficient clearance to induce relatively large local backflows. These elements are known as continuous mixer elements (CME) due to their design relationship with the rotor designs of continuous mixers. Figure 6 shows schematically the design of CME and conventional kneading block mixing elements, and figure 7 shows the corresponding cross-section designs associated with the two mixing technologies. The distinct design characteristics of the CME geometry are the relatively large clearance between the elements and the inner housing wall, the relatively narrow tip width and the different profiles of the leading and trailing faces of each lobe. The CME sections can be used alone or in conjunction with slotted-flight reverse pumping elements. They are available in various L/D ratios with varying ratios of land width to tip clearance ratio and can be configured in a number of different ways depending on the application.

[FIGURE 7 OMITTED]

Conclusions

Rubber can, in principle and practice, be compounded in a number of different continuous compounding machine designs. To maximize machine performance and product quality, an understanding of the interaction of machine configuration, operating conditions and compound final properties is required. The Mixing Venting Extruder (MVX) has proven the most successful for the rubber industry at this difficult application. A development at Farrel, since the last MVX design, is its unique twin screw extruder mixing technology. There is a team of engineers and technicians familiar with the machines undertaking a technical evaluation of the next MVX-based mechanism for continuous rubber compounding. It is expected to be a hybrid of the proven technologies, rather than a new complex concept. By taking advantage of the modern digital control technology for machine parameters, and advances in mixing technology, Farrel expects to be in a position to offer a commercial machine to the industry. The manufacturing environment will not cooperate with delicate, complex mechanisms, and therefore the machines must have the robust design and reliability of a batch mixer, like the MVX does now. In addition, it should take advantage of the modular design and superior mixing of the FTX twinscrew extruder. Important considerations are scale-up, stability of the process with lubricants and the reliability in a manufacturing facility. Without these basics in place, there can be no commercial benefit.

Table 1--output of MVX models

MVX size    Typical power (kW)   Typical output (kg/hr.)

134/120              75/55                  200-600
240/200             150/75                500-2,200
360/250            450/225              1,000-3,000
565/300          1,000/300              2,500-5,500

References

(1.) Nijman, G., "Continuous mixing, a challenging opportunity?" presented at "Continuous Mixing Developments and Future Trends," D.I.K. Hannover December 2001.

(2.) Melotto. M.A., The Mixing of Rubber, Richard F. Grossman (ed.), Chapman and Hall, p. 2 (1997).

(3.) Valsamis, L.N. and E.L. Canedo, International Polymer Processing, 3, Hanser Publishers, New York, pp. 225-232 (1994).

(4.) Sorcinelli, G.J., The Mixing of Rubber, Richard F. Grossman (ed.), Chapman and Hall, pp. 217-220 (1997).

(5.) Canedo, E.L. and L.N. Valsamis, "Mixing in the Farrel Continuous Mixer," in Mixing and Compounding: Theory and Practice, Manas-Zloczower, L. Tadmor: Z., (eds.). Progress in Polymer Processing vol. 4. Hanser Publishers. Munich, New York, pp. 761-830 (1994).

(6.) A. Amash, M. Bogun and R.H. Schuster, "Continuous mixing process of powder rubber," presented at Continuous Mixing--Developments and Future Trends. D.I.K. Hannover, December 2001.

(7.) Scharer, H.R., Adv. Polymer Tech., 5, (1985) 65.

(8.) H. Ellwood, "A tale of continuous developments," European Rubber Journal, 169, (3), pp. 26-32 (1987).

(9.) L.N. Valsamis and E.L. Canedo, "Dispersion studies in the Farrel twin-screw extruder," presented at the World Compounding Congress, Neuss, Germany, October 1994.

(10.) L.N. Valsamis and E.L. Canedo: "Co-rotating intermeshing twin-screw extruder with Polygon and Farrel asymmetric modular mixing element (FAMME)," presented at the 9th International Conference on Polyolefins, Houston, February 1995.

(11). E.L. Canedo and L.N. Valsamis, "Mixing in co-rotating intermeshing twin-screw extruders with Farrel asymmetric modular mixing element (FAMME). An engineering analysis of wear," presented at the 53rd SPE Antec, Boston, May 1995. SPE Technical Papers 41, 312-320 (1995).