Stress and dispersive mixing revealed in real time during melt blending
Twin screw extrusion is a standard technique used to mix or react polymeric materials. Two screws of varying widths and shapes rotate within a barrel. They can spin at varying speeds, interlocking or not, and either clockwise or counterclockwise, to mix the material and extrude it in the desired manner. But the specific strength and duration of the stress experienced by material passing through a twin screw extruder is unique to a particular configuration of screws, forcing engineers to calculate a unique stress map for each new configuration.
There have been numerous attempts in the past to find a correlation between stress in an extruder and a residence-time distribution (RTD). The RTD is a measure of the history of the flow of a material through a mixing device. Researchers have tried to use the RTD to imply intensity of mixing by various methods, for example, with the Peclet number or a number of passes.1 Typical RTDs, however, only provide an axial history of the flow of the material through the extruder and give no information regarding stress at different points in time as the material was mixed. There have also been some attempts to quantify the stress inside an extruder. Curry et al. used the percentage breakage of glass beads as a way to quantify the stress under different conditions and screw designs.2, 3 The limitation of that approach is the large effort required to obtain just a few values along the RTD. Gallant used stress beads to quantify the maximum stress in an extruder for an energetic application.4 Cheng et al. used stress beads to characterize different mixing section designs.5 The ability to accurately model the stress induced by a given screw geometry and operating condition is important in determining the degree of mixing.
We propose an industrially practical method to measure the residence-stress distribution (RSD), a display of the amount of stress the material experiences as it travels through the device, and compare it to the RTD.6, 7 The method needs to be sensitive enough to distinguish the stress history of different screw geometries as a function of operating conditions. Such an approach will allow practitioners to design screws to fit their particular product needs, for example, to create sufficient dispersion with the minimum specific energy for sensitive additives or maximizing dispersion stresses without breaking up delicate nano/microfillers.
We used high-density polyethylene (HDPE) Alathon H6018 in pellet form as the base material for these experiments. We measured shear stress in the extruder during mixing with stress beads. When subjected to stress levels beyond the critical shear stress, the beads break and release dye, which stains the extruded material. We performed the experiments with a 28mm Coperion co-rotating twin screw extruder using two screw configurations. The designs were the same except for the mixing section. The mixing section of the narrow screw configuration contained two right-handed kneading blocks (15- and 45mm), one neutral block (20mm), and one left-handed kneading block (15mm). The wide screw configuration consisted of five right-handed kneading blocks (15mm), one neutral block (20mm), and one left-handed kneading block (15mm).
We measure the reflectance of the polymer with a reflective optical probe at the end of the extruder. Titanium oxide dye shots feed into the extruder initially to stain the otherwise clear polymer melt white. The light probe records the reflectance off the polymer and generates an RTD curve. The stress beads then feed into the extruder. The beads break when the critical stress is reached and stain the polymer melt red. The light probe at the end of the extruder measures the varying signal as the concentration of the red dye changes. The time-varied curve measured by the light probe is the RSD. Not all of the material experiences the critical stress, and the RSD curve is always smaller than or equal to the RTD curve. The ratio of the area of the RSD curve divided by the area under the RTD curve is the measure of the percentage of stress beads broken. Figures 1 and 2 show the percentage breakup of dye beads within the two screw configurations over a range of operating conditions for screw speed (N) and specific throughput (Q/N).
Based on the results, our novel method for directly generating an RSD using a stress-sensitive bead is an effective tool to measure the stress history in a complex geometry such as a twin screw extruder. We compared wide- and narrow-kneading-block screw configurations over a range of specific throughputs and screw speeds. The wide-kneading-block configuration showed a higher percentage breakup of the stress beads when compared with the narrow kneading blocks of equal length. We interpret that result to mean that a higher percentage of the stress beads experienced the critical stress in the wide kneading blocks than in the narrow kneading blocks under the same operating conditions. This confirms and quantifies the common understanding about wider kneading blocks being better for dispersion. For a given screw configuration, there was an increase in percentage bead breakup as a function of screw speed and specific throughput. The higher values of each resulted in a higher bead breakup percentage.
These insights can be used to help design screw configurations for breaking and distribution of many important additives, such as carbon nanotubes or pharmaceuticals, whether one needs a high degree of stress or a low degree of stress but good distribution. We plan to investigate the relationship between specific RSDs and polymer properties, and continue to develop this technique for practical application.
- Z. Tadmor and C. G. Gogos, Principles of Polymer Processing, pp. 648–650, Wiley, 2006.
- J. Curry and A. Kiani, Measurement of stress level in continuous melt compounders, SPE ANTEC Tech. Papers 36, pp. 1599–160, 1990.
- J. Curry and A. Kiani, Experimental identification of the distribution of fluid stresses in continuous melt
compounders. Part 2, SPE ANTEC Tech. Papers 37, pp. 114–118, 1991.
- M. Gallant, Continuously Graded Extruded Polymer Composites for Energetic Applications, 2003. University of Maryland
- J. Cheng, Y. Xie and D. Bigio, Characterization of kneading block performance in co-rotating twin screw
extruders, SPE ANTEC Tech. Papers, pp. 198–202, 1998.
- D. Bigio, W. Pappas, H. Brown II, B. Debebe and W. Dunham, Residence stress distributions in a twin screw extruder, SPE ANTEC Tech. Papers 57, pp. 1382–138, 2011.
- W. Pappas, H. Brown II, G. Fukuda, R. Adnew and D. Bigio, Variable strength stress break analysis in a twin screw extruder, SPE ANTEC Tech. Papers, 2012. Paper accepted at SPE ANTEC in Orlando, FL, 2–4 April 2012.