Compounding news at NPE hit the high points of current interest, especially wood-fiber, long-glass, and nanoclay composites. Among the novelties were a four-screw compounder, an ultrasonic monitor for mixing quality, and simulation software for twin-screw compounding. (For more NPE compounding news, see PT, July ’03, p. 54.)
Wood is still the buzz
Wood-plastic composites remain one of the most active areas of compounding R&D. Krauss-Maffei launched a new two-stage twin-screw wood-plastic extrusion line at NPE. Its model KMD 133-36 WPC combines undried wood flour and resin—PVC, PE, or PP powder or pellet—at the feed throat. The first stage is a corotating twin-screw drying extruder with 177-mm diam., 9:1 L/D, a cooled feed throat, and oil heating/cooling in the screws.
This stage is unvented, so moisture flashes off when the mix falls into a second twin-screw extruder, which is counter-rotating, 133-mm diam., and 36:1 L/D. This extruder has large double vents at two positions on the barrel—the first vent is atmospheric, the second has high vacuum (up to 28 in. of water). Output is said to be 2000 lb/hr for a mixture of 60% wood with PVC, 1800 lb/hr with HDPE, and 1600 lb/hr with PP.
Sino-Alloy Machinery of Taiwan also showed off a new corotating twin-screw for wood-filled composites at NPE. Two of these units were installed last year in the company’s development center in California, where they are used for customer trials and also for commercial production of wood-filled masterbatches (30% to 60% wood in PP or PE). The two PSM 72 corotating twin-screws have 72-mm diam. and 44:1 L/D. Wood flour alone is first fed into the machine, where it undergoes heating to drive off moisture. Then resin is side fed into the extruder to mix with the filler.
At NPE, Japan Steel Works (JSW) showed for the first time in the U.S. a degassing twin-screw barrel with one or two high-vacuum vents. JSW says it can double throughputs of wood-filled compounds.
Draiswerke, which builds the Gelimat high-speed batch mixer for PVC dryblend, has modified this design in recent years for wood-plastic compounds. Very high rotor speeds generate enough energy both to flux the compound and to drive off moisture from undried wood flour. There are now a dozen Gelimat units in the U.S. and Canada making composites with up to 80% wood filler.
Draiswerke’s most recent Gelimat modification is for another hot area of technical and market development—direct long-fiber thermoplastic (D-LFT) compounding and molding. One machine in North America is used for D-LFT with long glass.
News for nanocomposites
An unusual corotating compounder with four screws was shown for the first time in the U.S. at NPE. It comes from Technovel Corp. of Osaka, Japan, which has built twin-screw compounders for 12 years. Another Osaka firm, D-Tech Co. Ltd., distributes the machines. Two years ago they jointly developed the four-screw KZW compounder for nanocomposites and reactive compounding. It lines up four parallel corotating screws in a row. This alignment in one plane reportedly heats, mixes, and cools more evenly than do cylindrical multi-screw configurations.
The four-screw KZW gets 2.5 to five times higher output than a twin-screw, Technovel claims. Other advantages of four screws are said to include lower power consumption, lower operating temperatures, longer reaction times, better dispersion, and better venting than a twin-screw of comparable L/D. KZW L/D ranges from 15:1 to 120:1, making it by far the world’s longest extruder (72:1 being about the max for twin screws). It comes in 13 sizes from an 8 mm to 134 mm screw diam.
Technovel’s KZW also uses a higher-powered drive to achieve screw speeds of 1000 to 2000 rpm and even up to 3000 rpm. The KZW’s backpressure is also extreme—up to 5075 psi vs. only about 1450 psi for conventional twin-screws, Technovel says.
Recently, Technovel came up with a four-screw model with deeper roots to compound materials with low bulk density. It has a depth/diameter (D/d) ratio of 1.88 vs 1.55 for the standard four-screw KZW.
Futuresoft Technologies, a seven-year-old designer of dies and other equipment for wood composites, brought to NPE a single-screw static-mixing section for nanocomposites. Its Extensional Flow Mixer (EFM), developed by (and licensed from) the National Research Council of Canada (PT, Sept. ’99, p. 17), reportedly can double the flexural modulus of nanocomposites relative to the same compounds mixed on a twin-screw.
In this device, the flow travels first around a spiral mandrel, then through three patented flow channels separated by tight ridges, which delaminate nanoclay particles. As melt flows over each of the three ridges, it reduces particle thickness by a factor of 10. So when material has passed all three, thickness is reduced 1000-fold. This also allows the EFM to mix, for example, 40% UHMW-PE with 60% HDPE, a blend being tested for gas pipes.
More news in twin-screws
American Keya Corp., the U.S. office of Keya Corp. of China, came to NPE with the model TE-75 corotating twin-screw extruder. Not widely known in this country, this 10-year-old firm is reportedly the world’s largest supplier of twin-screw extruders. With sales of over 300 machines a year and over 2000 sold worldwide, it has nearly 70% of the Chinese market. Although Keya has technology agreements with Coperion, Farrel, B&P, and Century Extruders, among others, it is beginning to offer innovations of its own. One is a single-screw reciprocating extruder for rubber and plastic mixing; another is a two-stage process with a twin-screw feeding a single-screw for direct in-line extrusion and molding.
Meanwhile, Wexco Corp., which makes single- and twin-screw bimetallic barrels, is testing a new carbide barrel coating that’s said to be highly corrosion resistant. Wexco says it will commercialize the new barrel in the next six months.
JSW showed new process-simulation software called Tex-FAN that’s included with its twin-screw compounders. JSW also introduced a new kneading block called the Twisted Kneading Rotor/Forward, which is midway between a kneading block and a wing-type mixing rotor. Each segment of the kneading block is slightly offset from the one before it, creating a whorl (see photo) that increases elongational mixing and raises output by preventing backflow.
Also appearing at NPE was Bruckner Formtec of Germany, an 18-month-old company formed within Bruckner Maschinenfabrik, the oriented-film specialist, to focus on extruders for unoriented film and sheet. Formtec has begun building corotating twin-screw extruders to process undried APET.
Can you hear the mixing?
The National Research Council of Canada showed the first commercial ultrasound technology for melt monitoring, commercialized this year by NDT Technologies Inc. Called the In-Line Ultrasonic Polymer Properties Monitoring System, it took 20 years to develop at the Industrial Materials Institute of the NRCC with support from Dow, DuPont, 3M, and others.
It uses a series of five Dynisco ultrasonic buffer rods to monitor a wide variety of compounding parameters inside the extruder. Ultrasound monitoring uses changes in the velocity and attenuation of sound waves to indicate such resin characteristics as mixing quality, viscosity, filler levels, and polymer degradation. For a recycled PET compound, speed of ultrasound travel indicates I.V. level. PP “visc breaking” with peroxide can also be detected by measuring sound velocity. Degree of dispersion of additives can also be detected. In foam extrusion, ultrasound can be used to compare the effect of different foaming agents, melt temperatures, or other process conditions on bubble formation. Screw and barrel wear can even be monitored by measuring the time delay of sound signals between the probe and screw.