Blend trends: compounding machinery in the 90's.

Blend Trends Compounding Machinery In the '90s

Compounders can expect most machinery refinements in the next 10 years to occur in continuous compounding equipment, on which this article will focus. Flexibility and versatility seem to be the buzzwords among major equipment suppliers when speaking about next-generation compounding machinery. Processors want to be able to run a wider variety of compounds on the same equipment, with a minimum of downtime for mechanical adjustments. New variable barrel restrictors or "throttle" technologies, and some quick-change improvements for screws and barrels will add to equipment flexibility. Look also for advancements in equipment productivity through new heating methods that achieve faster melting, and systems that bypass the pelletizing stage and combine steps in-line.

In twin-screw compounding, some suppliers see a movement ahead to counterrotating instead of corotating designs. But also watch for single-screw manufacturers to further optimize their extruders as a competing alternative.

More demanding applications, in turn, are creating a need for more accurate, on-line monitoring devices. Systems that were previously confined to the laboratory are now finding their way to the shop floor. A better understanding of flow characteristics is also paving the way for improved process control.

And of course, recycling is on the minds of compounders, who will attempt to upgrade the value of reclaim with the addition of fillers, reinforcements and other modifiers. Here, too, on-line control will be an important quality tool.

FLEXIBILITY IS KEY

Output rates are climbing, according to Gene Stroupe, product manager of compounding equipment for Berstorff Corp., Charlotte, N.C. At the same time, he sees two contrary trends: 1) that use of finer-particle-size fillers is requiring longer mixing times to wet their higher surface areas; and 2) that more and more compounders are buying smaller machines in an effort to achieve production flexibility. He sees a general downsizing of machinery among both small custom compounders and large-volume compounders trying to be responsive to smaller "niche" markets.

The trend toward smaller batches and high-value-added compounds underscores the need for equipment versatility. Machinery manufacturers say they're working to meet this demand. Several years ago, for example, Berstorff introduced radial barrel throttle valves on its twin-screw machines. Stroupe predicts rapidly increasing use of these throttles among U.S. compounders. The device is said to accommodate a maximum range of flow characteristics without changing the screw profile.

Werner & Pfleiderer Corp., Ramsey, N.J., has expanded on throttle technology with the introduction of a radial valve. W&P's axial barrel valves must be supplied as original equipment and can be cumbersome to operate on larger compounding units, because they require sliding the screws, gears and motor back and forth. The mechanically simpler radial version (see illustration) can be retrofitted to all sizes and is intended as original equipment on machines over 133 mm diam.

W&P's radial throttle is less expensive than its axial counterpart but also more sensitive, with a steeper gain characteristic, as shown in the graphs accompanying the schematic. This extra sensitivity makes it necessary to set the position of the radial valve more precisely, such as with computer feedback control. The radial version has so far been subjected to extensive lab tests on 70- to 90-mm twin screws. Meanwhile, W&P has also modified its axial valve design to make it easier to implement. The valve section is now composed of top and bottom plates that can move apart to allow the screws to be pulled easily, eliminating the need to further disassemble the throttle device, as was necessary in the past.

While most twin- and some single-screw compounders already utilize a modular, "building-block" design, screw and barrel configurations permitting easier changeovers are likely to play a more important role in the '90s. For example, Berstorff reportedly has simplified the task of replacing its twin-screw liners to adapt a machine for handling abrasive and corrosive materials.

Handling the materials of the '90s will be a challenge for compounding machinery suppliers, agrees Vincent Boreas, sales manager of continuous mixers for Farrel Corp., Ansonia, Conn. Farrel is focusing on handling new alloys and blends, particularly of engineering resins. New materials of construction will be needed to handle more abrasive materials. According to Boreas, one of the most intriguing is ceramic barrel liners. He says they have been used successfully in the Farrel continuous mixer, and the latest versions are more ductile, minimizing risk of shattering. Yet he sees a need for the cost of ceramics to come down before they will achieve widespread use.

Farrel also recently commercialized new rotors for its LCM. The #22 rotor was developed for the addition of glass fibers downstream for minimal breakage of the glass fibers. It also has advantages with high filler loadings by the split-feed method.

A prototype compounding machine that's said to show promising versatility is now being tested in Europe by the Italian parent of Pomini Inc., Brecksville, Ohio. The CM-AX 55, designed for engineering resins, is said to combine the advantages of the company's Long Continuous Mixer and twin-screw extruder in one machine. The screws are co-rotating and interpenetrating except in the mixing zone, where they are tangential with a special profile. The unit is said to be flexible because it combines three basic operations: mixing and blending, extrusion, and pelletizing.

Added flexibility for PVC compounding is claimed for Battenfeld's new segmented barrel and rotor design for its Planetary extruder, introduced at K'89 (see PT, Jan. '90, p. 79). It offers the possibility of alternating single- and planetary-screw sections for melting, mixing and degassing zones, which David Hart, v.p. of marketing for Purnell International, Houston, Battenfeld's U.S. representative, says makes the planetary extruder more flexible for increasingly sophisticated PVC compounds. Battenfeld originally developed the new design to present a novel alternative for compounding engineering resins. The planetary system's large screw surface area for mixing is thought to be a potential advantage.

OPTIMIZING SINGLE SCREWS

Although twin screws have been in the forefront of most recent compounding technology developments, some interesting work is being done to optimize single-screw extruders for that purpose. Single-screw machines can do a better job over a wider application range than is generally believed, says Charles Martin, v.p. of marketing for Killion Extruders, Cedar Grove, N.J.

Killion is seeking to increase the versatility of single screws by applying an approach that has worked with twin screws. Killion has developed a prototype segmented single-screw extruder that can use different combinations of screw elements to optimize mixing efficiency, and can function in 24:1, 28:1, 32:1, 36:1, or 40:1 L/D arrangements. The machine is designed primarily for laboratory use and specialty compounding applications with throughputs of less than 200 lb/hr. "We are not equaling twin-screw performance, but we are doing a better job on a wider range of resins with the single screw," says Martin. The machine is now in place at the Polymer Processing Institute at Stevens Institute of Technology in Hoboken, N.J., where it is being used as part of a three-year, industry-sponsored polymer mixing study.

TOWARD HIGHER OUTPUTS

Besides greater versatility, higher productivity remains a key theme in machinery development. Werner & Pfleiderer introduced two developments at K'89 that are still too new to have proven their potential (see PT, Jan. '90, p. 79). One innovation that could promote faster melting in twin-screw compounders is the use of induction heating bands. For any given size extruder, the method is said to provide from 67% to 320% greater wattage than conventional heater bands (see table, p. 67). Induction heating acts uniformly throughout the barrel; thus, higher temperatures reportedly can be achieved with less stress on the barrel because there is less of a temperature gradient from the outer-to-inner barrel surface.

W&P recommends induction heating for the initial melting zone, where it reportedly boosts throughput. Trials with ABS powder have shown increases of 33%, say company spokesmen, who add that the technology has its greatest application with fine powders, which can be entrained by fluidized air, leading to poor melting. Other applications include harder-to-melt materials such as highly filled compounds, and very-high-melt-temperature resins. With conventional crystalline engineering resins such as nylon, faster melting with induction heating is said to reduce barrel wear in the melting zone and provide more uniform melt while allowing fewer unmelted solids to get through.

With higher outputs and more viscous polyolefins comes the potential for output limitations due to excessive die pressures. W&P claims to have made progress in reducing pressure build-up with a new die-plate design. The new low-pressure die plate keeps pressure to a minimum through tapered melt-feed channels that distribute melt to as many as 30 outlet bores each. This arrangement is said to allow processing of polyolefins at 50,000 lb/hr with pressures as low as 1150 psi.

W&P engineers note that without intensively heated underwater pelletizing die plates, freeze-off is a problem leading to high pressures. This can be a problem with "mini pellets" of only 1.0-1.2 mm diam., which are believed to be especially attractive for masterbatches. W&P claims to have produced red HDPE color concentrate on a 90-mm ZSK-90 with a die plate having 1120 mini holes at up to 1100 lb/hr with pressures only up to 1740 psi. Blue LLDPE concentrate was run at 1210 lb/hr and 1960-2030 psi.

COMBINING STEPS IN-LINE

Several machine builders and compounders say that the possibility of bypassing the pelletizing stage altogether, and directly producing semi-finished sheet or profiles on a compounding line, is becoming more feasible. Berstorff's Stroupe comments that combining steps eliminates the need for a single-screw extruder in the forming stage, and that quality of the material is higher because of reduced heat history. On a corotating twin-screw machine, the material is reportedly subjected to lower and more uniform thermal stresses during the melting process than on a single-screw extruder. Joseph Scuralli, operations manager of Wayne/ICMA, Wayne, N.J., comments that such a line might require a melt pump.

Michael Irish, product manager for PVC/wire and cable at Buss (America), Inc., Elk Grove Village, Ill., sees a similar potential for direct pelletizing from the Buss Kneader without a secondary discharge device. Increasing the rpm of the Kneader has been shown to permit direct pelletizing of glass-reinforced polymers such as nylon 66 and PP.

In-line compounding and extrusion may be a factor driving greater use of counterrotating instead of corotating twin-screw machines in the coming decade, predicts William Thiele, general manager of American Leistritz Extruder Corp., Somerville, N.J. "Counter-rotating extruders have potential in dispersion compounding, reacting, and devolitizing in combination with finished product extrusion," he says. Driving their increased popularity will be a more positive conveying mechanism of "locked flight chambers" instead of drag flow. Thiele claims that because they tend to reach steady-state quicker than their drag-flow counterparts, counter-rotating machines are suited for specialty products where fast changeover may be required. Counter-rotating machines are also useful in compounding slippery materials, he adds.

Christopher Tucker, manager of process technology for Welding Engineers, Blue Bell, Pa., also sees increased potential for counterrotating, nonintermeshing machines, because of their generally acknowledged devolitilizing efficiency. Tucker sees a trend toward "cleaner" polymers with much lower residual monomer and solvent levels, down from 2000-3000 ppm 10 years ago to under 500 ppm today.

NEW UNDERSTANDING OF MIXING

A better understanding of mixing in the extruder will lead to a new generation of on-line monitoring equipment, which in turn has implications for tighter process control, according to Dr. Costas Gogos, director of new initiatives at the Polymer Processing Institute. For example, the thrust of PPI's mixing study is to gain insight into rheological behavior of a polymer before and after passing through specific mixing elements in an extruder.

John Curry, Werner & Pfleiderer's manager of process development, says that new capabilities in process simulation and material science are coming together to provide better process modeling and equipment design. Evidence of this was seen in papers presented at this year's SPE ANTEC conference in Dallas. For example, a three-dimensional numerical model to describe the flow between pairs of fully intermeshing, corotating kneading disks and the barrel in a twin-screw extruder was presented by A.D. Gotsis, Z.Ji, and D.M. Kalyon of Stevens Institute. Of all the various screw element types, kneading disks are said to present special difficulties in process modeling. The results of this new model are thought to give a better understanding of the dynamics of the flow in the kneading disk section.

Also, a more precise visualization of the flow within twin-screw extruders that is equally applicable to mixing and conveying sections in corotating and counterrotating machines (photo, p. 62) was described by D.H. Sebastian of Stevens Institute and R. Rakos of PPI. Current flow-analysis methods tend to simplify the complex screw geometry, and overlook important information about the actual flow field, the authors said. Their new method does not rely on extending single-screw visualizations to a twin-screw extruder, and leads to a set of equations that can be handled without a high-speed super-computer.

MORE ON-LINE MONITORING

One area of technology that holds especial promise for compounding in the next decade is sophisticated methods of on-line monitoring of what is happening to the material in the machine. On-line rheological analysis has been used for many years, but has been improved recently to increase its practical utility. Both Rheometrics Inc., Piscataway, N.J., and Goettfert, Inc., Rock Hill, S.C., have newer version of capillary rheometers that reportedly give results in as little as 1 min, vs. up to 30-40 min in the past (see PT, Sept. '88, p. 81). These devices also permit simultaneous on-line measurements of other properties in the same sample cell, such as uv or infrared spectroscopy.

Other types of on-line monitoring that are now available include dynamic mechanical testing of viscoelastic properties; Rheometrics has offered an on-line version for several years, though it has so far been utilized mainly for R&D. On-line infrared analysis in production is available with the IROS 100 system from Automatik Machinery Corp., Charlotte, N.C., which can handle temperatures up to 752 F and pressures of 4500 psi. In the near future, Automatik expects the monitoring software to be capable of being used for feedback control of the process. Dispersion quality can also be measured quantitatively on-line by systems from Flow Vision, Inc., Little Falls, N.J., which optically inspect the melt stream and count numbers and size distribution of particles or agglomerates.

At ANTEC this year, nuclear magnetic resonance imaging (NMRI) was reported to show promise as another means of quantitatively measuring dispersion, especially in opaque mixtures. S.W. Sinton and others of Lockheed Palo Alto Research Laboratory, Palo Alto, Calif., reported on two studies of continuous twin-screw extrusion using NMRI, one measured dispersion of a solid filler in a polymer melt, and the second the mixing of two polymeric phases in a TP elastomer blend. The authors conclude that NMRI is potentially useful as a non-intrusive, quantitative measure of dispersion monitoring method that possibly can be used on-line.

Another potential "handle" on the compounding process, which is still an active area of R&D, is specific energy input analysis. Since what is done to a polymer in a mixer can be reduced fundamentally to the concept of putting mechanical and thermal energy into it, monitoring and controlling energy input per gram (specific energy input) beckons as a promising avenue of investigation (see PT, Feb. '89, p. 65). Firms such as Buss and W&P are working actively in this area, and some Japanese firms are reported to be developing compounding systems around this concept.

The latest development from W&P in this area is what's described by manager of process development John E. Curry as a "self-learning" energy-input or viscosity control. In what could be a forerunner of the sort of "artificial intelligence" systems that are predicted for the future, this Optimax control reportedly can select the appropriate screw speed and material feed rate--the two chief determinants of specific energy input, according to Curry--to close in on a preset viscosity target, which is sensed by an on-line rheometer.

An alternative approach to the problem, says Curry, is to focus on mechanical shear stress imparted to the polymer as the key process variable related to specific energy input. This is affected by the polymer's inherent viscoelastic properties, as well as the screw speed, feed rate, and barrel temperature. One way to develop an integrated stress value is to measure drive amps or barrel pressure together with rpm rate and process residence time, allowing you to vary screw speed or temperature in response.

Another difficulty, Curry points out, is that you can't really measure external convective and radiative heat losses from the barrel, so you don't have a full accounting of energy input from all sources. However, Buss has a control scheme based on what it calls "total energy balance," which is said to take into account heat transfer to and from the barrel.

WASTE RECLAIM

CHALLENGES

A major growth area for compounders in the next decade will no doubt be recycling. According to Alan Haisser, product manager for compounding and reclaim at APV Chemical Machinery, South Plainfield, N.J., special difficulties in this task will be accommodating inconsistent feedstock and relatively "dirty" material. APV is exploring the use of various types of on-line feedback control to ensure consistent quality of reclaim during compounding.

APV is also developing an extrusion reclaim system that will segregate functions of reclaiming from that of compounding. The first stage would include steps of melting, devolitilizing, homogenizing, and filtering of cleaned and shredded material. Compounding would take place during a second separate step. Haisser says that such a two-stage system would be a departure from the norm of trying to perform all functions in one machine. The system is still in the design phase, and a prototype has yet to be built.

Welding Engineers is reportedly testing technology to "wash" dirty reclaim in the extruder, possibly by stripping it with steam or water. Tucker of Welding Engineers says that the technology has potential to eliminate a separate washing step, which could result in significant cost savings. That system, too, is being tested and is not yet ready for commercial introduction.