Twin screw extrusion of biobased polymers: a comparison of different process geometries.
The twin screw extruder processes material within the barrel using screws to convey, mix and pump. The barrels are modular to allow for staging of individual unit operations along the barrel length. Barrels are electrically heated and liquid cooled, and serve as individual temperature control zones. Segmented elements are assembled on high torque shafts to optimize the screw configuration depending on the process requirements. A typical process length to diameter ratio (L/D) ranges from 32 to 48:1, with up to 60 L/D (or longer) possible for reactive extrusion and/or multi-stage devolitilization. Electrical energy is converted to mechanical energy through the drive train (motor-gearbox) and applied to the material as shear through the rotating screws.
Modular barrels and segmented screws, along with the controlled wiping and pumping characteristics of the screws, make it possible to match the screw and barrel geometries to the process tasks. Solids conveying and plastication typically occur in the first part of the process section. Elements for mixing, downstream feeding, liquid injection and venting are then commonly used, as dictated by the process. Discharge elements build and stabilize pressure at the die. Screw configurations range from shear intensive to passive.
The twin screw extruder is a starve-fed device; the rate is determined by metering devices. These can be solid or liquid feeders. The independence of the screw speed to the output makes it possible to adjust the shear intensity within the twin screw extruder to optimize mixing efficiencies. Additionally, because the pressure gradient is controlled within the extruder, and zero for part of the process, materials can be introduced into downstream barrel sections. This is beneficial, for example, in compounding high loadings of starch, where it may not otherwise be possible to feed in all at one feed location; or in feeding temperature and/or shear sensitive additives, such as natural fiber, which degrade if processed over the entire barrel length.
Free volume, a common extrusion term, is the space available within the process geometry that is used to process material, and is directly related to the OD/ID ratio. The OD/ID ratio is defined by dividing the outer diameter (OD) by the inner diameter (ID) of each screw. The industry standard OD/ ID ratio for HSEI twin screw extruders is approximately 1.55. (In Leistritz nomenclature, this is the HP series.) The new MaXX series, introduced in 2004, features a 1.66 OD/ID ratio. This design has deeper screw flights and increased barrel diameters with the same screws centerline and the same (or higher) torque as the HP series. Oftentimes, more material can be processed per rpm with a higher free volume.
Torque is another critical process parameter when discussing HSEI twin screw extrusion, and is directly related to the diameter of the screw shaft. The larger the diameter of the screw shaft, the higher the torque capability. While deeper screw channels offer more free volume, it is at the expense of the torque capability of the shafts, since a smaller diameter shaft is necessary. Based on the use of a symmetrical, hammered splined shaft a 1.55 OD/ID ratio has generally been deemed to result in the best balance of torque and volume. With a decreased cross-sectional area for the shaft, increased free volume is possible, but less torque is transmitted into the process. To offset the reduction in torque capability, the MaXX series utilizes an asymmetrical splined shaft (patent pending) that maintains the same (or higher) torque as standard HSEI twin screw extruders, but with a smaller diameter shaft. This geometry transmits power more efficiently into the screw elements, which allows for deeper flight depths and more volume.
Experimental data were generated using a ZSE-27 to compare the HP series twin screw extruder to the MaXX series. Because the screw centerline for the HP and MaXX series is the same, it is possible to interchange the process sections. A 40 L/D process length and similar screw designs were used to duplicate the processes, determine limitations and compare performance. HDPE, LDPE and PLA were used as test materials to evaluate capacity differences between the two process geometries. Process conditions were held constant to make as close a comparison as possible. Process data analyzed were rate, melt temperature, torque and specific energy input.
The shear rate in the overflight gap in the ZSE-27 HP and MaXX models is indicated below:
Shear rate = ([[pi].sup.*][D.sup.*]n)/(h * 60)
D = screw dia. (HP is 27 and MaXX is 28.3 mm)
n = screw rpm
h = overflight gap (.1 mm for both HP and MaXX)
Assuming 600 rpm, the comparative peak shear rates are as follows:
HP = ([[pi].sup.*][27.sup.*]600)/(.1 * 60) = 8,478 [sec..sup.-1]
MaXX = ([[pi].sup.*][28.3.sup.*]600)/(.1 * 60) = 8,886 [sec..sup.-1]
Results and discussion
To date, the evaluations of the LDPE, HDPE and PLA have been completed.
LDPE powder feedstock with a 12 MFI was processed on ZSE-27 HP and MaXX process sections. For the MaXX design, the increased free volume made it possible to feed more material to the extruder before encountering feed limitation. In this instance, the rate limiting factor was the volumetric feed capability of the main feed throat. The process was not torque limited. Melt temperature was also lower for the MaXX, as measured at the discharge.
Fractional melt HDPE processed at nearly the same rate on the ZSE-27 HP and MaXX series at the low and mid range screw rpms. All samples were torque limited, not feed limited. At 1,200 rpms the throughput rates achieved on the MaXX were somewhat higher. The melt temperatures for the HP samples were higher, as the material appears to be more susceptible to shear heating from the higher average shear rate inherent with the shallower flights of the HP design.
PLA processed at slightly higher rates with lower melt temperatures on the ZSE-27 MaXX as compared to the HP. All samples were torque limited, not feed limited. Results for the PLA samples are indicated by the data shown in figures 1 and 2.
The design enhancements to the MaXX series offer significant advantages for feed limited processes, as well as for melt temperature sensitive materials. The combination of high torque shafts and deeper screw flight channels enables higher throughput rates because of improved feeding capability, as well as lower specific energy input and lower melt temperatures. Recent advancements have also increased torque an additional 30%. This combination of attributes makes it possible to process a wide variety of materials, some of which may not have been previously possible to process using an HSEI extruder. Prime candidates are bio-based materials, many of which are torque intensive, heat and shear sensitive, and/or low bulk density feedstocks.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
by Charlie Martin, Augie Machado and Stuart Kapp, Leistritz (email@example.com)
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|Title Annotation:||Tech Service|
|Author:||Martin, Charlie; Machado, Augie; Kapp, Stuart|
|Date:||May 1, 2009|
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