Nanocomposites: A Single Screw Mixing Study of Nanoclay-filled Polypropylene By Jae Whan Cho, Jason Logsdon, Scott Omachinski, Guoqiang Qian, Tie Lan - Nanocor, Inc. Timothy W. Womer and Walter S. Smith - New Castle Industries, Inc. Abstract Polypropylene (PP) nanocomposites were prepared by two steps: a predispersed organoclay masterbatch was first prepared by using a twin screw extruder; the masterbatch was then letdown into base PP by using a single screw extruder. The effect of single screw mixing type on organoclay dispersion and nanocomposite properties was evaluated. The results indicated that the composites obtained from the masterbatch letdown with a single screw extruder showed better dispersion and better mechanical properties than the composite obtained from the direct compounding with twin screw extruder. Furthermore, the mechanical properties of these composites from masterbatch single screw letdown process is as good as the composite obtained from masterbatch letdown with a twin screw extruder. A rheological study also shows PP nanocomposite has the same flow characteristics as neat PP, indicating the new technology can drop in the current machine set up, without adding additional cost to end users. Introduction Nanocomposites are here. It is a new technology for enhancing the properties of commodity resins by means of dispersing nano-size particles of montmorillonite clay in neat resin (1). These new nanocomposites have improved mechanical properties, such as stiffness, heat distortion temperature (HDT), dimensional stabilities, and enhanced barrier to gas permeation. PP nanocomposites are a hot area of current interest, due to huge commercial opportunities in both automotive and packaging. However, the low polarity of the resin limits the interaction between organoclay and the polymer (2). Recently, a PP nanocomposite employing a masterbatch route has been developed (3-4). Good dispersion and mechanical properties have been achieved, however, a twin screw letdown is required, which causes increased compounding cost for customers. One possible approach to improve mixing in a single screw extruder is to introduce a mixing element (5). Generally, the conventional mixing elements are classified to the distributive mixing elements such as a distributive mixing section (6) and the dispersive mixing elements such as a Union Carbide mixing section (7). In this study, a new mixing element was introduced to a specially designed single screw extruder. This special extruder is modular so that several existing mixing elements were also introduced for comparison. The following are the results of nanocomposites made from a single screw extruder with different types of mixing elements. The same formulation was used for all of the mixer evaluations and the final extrudate was pelletized using a strand die, water trough and pelletizer to produce a compound that could be further evaluated. Formulation Most of the polypropylene (PP) nanocomposites were prepared as a two-step compounding process. First, a predispersed organoclay masterbatch concentrate (Nanomer C.30P) was prepared by using a co-rotating twin screw extruder. Then, the masterbatch was letdown into the PP/compatibilizer mixture by using a single screw extruder. As shown in Table 1, the masterbatch contains 50 wt.% organoclay with 25 wt.% compatibilizer and 25 wt.% homo PP. Octadecyl amine (ODA) modified montmorillonite (Nanomer I.30P) was used as an organoclay and a polypropylene-graftmaleic anhydride (PP-g-MA) was used as a compatibilizer. For comparisons, PP nanocomposites were also prepared from the masterbatch letdown into PP with a twin screw extruder or direct compounding of PP, PP-g-MA and organoclay with a twin screw extruder. All the final PP nanocomposite formulations are 6% organoclay/ 5% PP-g-MA/ 89% PP by weight %.
Equipment and Process Both single screw and twin screw extruders were used for this study. Table 2 shows the general processing parameters for all of extrusion trials that were completed on the various extruders and mixing sections that were evaluated. Four different mixing sections were tested on the single screw extruder. They were a standard flight section, UCC mixing section, a distributive mixing section, and a newly developed dispersionary mixing section. All mixing sections had approximately two L/D of flights after the mixing section to allow for re-orientation mixing afterwards and before exiting the end of the screw. The various mixing configurations are shown in Table 3. In the case of the twin screw extruder, three kneading disc blocks were introduced with proper kneading element configurations to optimize dispersion and temperature control. Low barrel temperature with high screw speed was maintained in order to obtain better dispersion. Rheological Evaluation After the trials were completed, a rheological evaluation was preformed to compare the four primary materials; pure homo PP, C.30P, PP-g-MA, and the final compounded PP nanocomposite. The shear viscosities of all four materials were evaluated at 221 C as shown in Figure 1. It can be seen that the viscosity of C.30P melt shows shear thinning non-newtonian behavior. On the other hand, it is interesting to note that with a 6% loading of organoclay, the flow characteristics of the nanocomposite is practically the same as the pure polypropylene. This is a very good finding because this leads us to believe that this new material will be a drop-in additive which will result in greater physical properties in the end product. Dispersion Study The extruded pellets were injection molded into standard test bars according to relevant ASTM. These bars were used for evaluating the amount of dispersion and measuring mechanical and thermal properties. Figure 2 shows the X-ray diffraction (XRD) of the organoclay particles. Based on the XRD data, the clay inter-layer distance (Å) was determined for each nanocomposite sample. A 22Å basal d-spacing was observed for the organoclay prior to compounding. For the organoclay concentrate, the masterbatch shows a basal d-spacing of approximately 24Å. In the case of PP nanocomposites, a 28Å d-spacing was observed regardless of the type of mixing screws. This d-spacing is almost same as the one for the composites obtained from the masterbatch letdown with a twin screw extruder or composite obtained from direct compounding with twin screw extruder. In order to evaluate the optical microscopic dispersion of the nanocomposites, the injection molded bars were cut in the flow perpendicular direction and polished with a diamond paste by using a surface polisher. The surface polishing was carried out by using a series of successively finer abrasives. Figure 3 (a)~(d) shows optical photomicrographs (magnification=200x) of PP nanocomposites containing 6 wt.% organoclay obtained from various mixing screws. An optical microscopic dispersion was also done on the composite directly compounded through the twin screw extruder and it is shown in Figure 3(e). In order to complete the comparison for the evaluation of organoclay dispersion, a typical nanocomposite prepared from the masterbatch letdown through a twin screw extruder was used as a reference point and its optical microscopic dispersion is shown in Figure 3(f). These two points of reference inform us of where we started from and where we would like to go in the single screw extruder. As shown in this figure, the nanocomposite prepared from the masterbatch letdown with a twin screw extruder showed a very good dispersion indicating microscopically homogeneous phase, but the composites prepared from direct compounding with a twin screw extruder shows numerous big tactoids. On the other hand, the composites prepared from the masterbatch letdown with a single screw extruder showed reasonably good dispersion over all the area with some localized tactoids. These tactoids are much smaller in size and much less in particle density than from direct compounding. As can be seen from Figure 3 (a)~(d), the effect of the different mixing screws on the particle dispersion is not significant. However, the particle size of the composite obtained from Dipersionary Mixer is slightly smaller than that of Union Carbide mixer. In this case, the Dispersionary Mixer has slightly less mixing length than Union Carbide Mixer. There are no considerable differences in particle size between the composites obtained from the Distributive Mixer and from the Dispersive Mixer. It is believed that the reason for the closeness of the values for the Distributive Mixer and Dispersionary Mixer is due to the increased length of the Distributive Mixer versus the Dispersive Mixer. Therefore, in future testing, a new Dispersionary Mixer with increased length will allow for more divisions of the polymer and concentrate to be dispersed. As expected, the particle size of the composite obtained from the Flighted End, which has no mixing screw, is much bigger than from the other mixing screws. However, this composite has much smaller particle in size than the composite from the direct compounding with a twin screw extruder.
The mechanical properties of PP nanocomposites made by single screw letdown are shown in Table 4. The mechanical properties of the neat polypropylene was also evaluated as a base data point and compared with the composites made by twin screw extrusion letdown or direct compounding. As can be seen in Table 4, PP nanocomposites made from masterbatch letdown with two steps showed better mechanical properties than the composite made from the direct compounding. It is very interesting to note that the mechanical properties of the composites made from masterbatch letdown with a single screw extruder were as good as the composite made from masterbatch letdown with a twin screw extruder. The effect of different types of the mixing screws on the mechanical properties is not very significant. Conclusion Nanocomposites do allow for improved mechanical properties to the base resin, but very intensive dispersionary mixing is required to make sure that organoclay particles are uniformly and thoroughly dispersed and distributed throughout the molten polymer before it enters the die. As expected, a masterbatch letdown process with a twin screw extruder provided nearly complete dispersion and superior mechanical properties. On the other hand, a masterbatch letdown process with a specially designed single screw extruder showed a reasonably good dispersion and comparable mechanical properties as a master batch letdown process with a twin screw extruder. Future work on this project includes additional improved mixing sections based on the results found in this study. Also planned is the evaluation of barriertype screws to insure that a complete melt is obtained before the dispersionary mixing takes place. 6. U.S.Patent 4,639,143. 7. Maddock, B. H., SPE J., July, 23 (1967). 8. Chung, Extrusion of Polymers Theory and Practice, Hanser Gardner Publishing, Inc., Cincinnati, Ohio. 9. Bernhardt, "Processing of Thermoplastic Materials", Robert E. Krieger Publishing Company. 10. Rauwendaal, "Polymer Extrusion", Hanser Publishers. 11. Tadmor and Gogos, Principles of Polymer Processing, John Wiley and Sons, New York. 12. Cheremisinoff, Polymer Mixing and Extrusion Technology, Marcel Dekker, Inc., New York. 13. Osswald, Polymer Processing Fundamentals, Hanser/Gardner Publications, Inc., Cincinnati. Apparent Shear Viscosity (Pa*s) 10000 1000 100 10 Masterbatch(Nanomer C.30P) 6% I.30P in PP 6523 PP-g-MA 10 100 1000 10000 Shear Rate (1/sec.) PP 6523 Figure 1. Shear viscosity of masterbatch concentrate and PP nanocomposites. These further developments will be reported during the oral presentation. References 1. Okada, A., Fukushima, Y., Kawasumi, M., Inagaki, S., Usuki, A., Sugiyama, S., Kurauchi, T., Kamigato, O., U.S. Patent 4,739,007 (1988). 2. Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A., and Okada, A., Macromolecules, 30, 6333(1997). 3. T. Lan and G. Qian, Proceeding of Additive 00 Clearwater Beach, FL, April 10~12, 2000. 4. G. Qian, J. W. Cho and T. Lan, Polyolefins 2001, Houston, TX, February 25~28,2001. 5. Rauwendaal, C., Mixing in Single Screw Extruders p129 Marcel Dekker, Inc., New York. Figure 2. X-ray diffraction patterns of organoclay, masterbatch, and PP nanocomposite.
(a) (b) (c) (d) (e) (f) Figure 3. Optical photomicrographs of PP nanocomposites (Magnification=200X). (a) Flighted End (c) Dispersionary Mixer (e) Direct Compounding (b) Distributive Mixer (d) Union Carbide Mixer (f) Masterbatch letdown with a twin screw
Table 1. Detailed compounding formulation for this study. Compounding Process Masterbatch Organoclay PP PP-g-MA Masterbatch 50 25 25 Letdown 12 0 86.2 1.8 Direct Compounding 0 6 89 5 Table 2. Detailed extruder configuration. Extruder Screw Diameter L/D Barrel Temperature ( C) Screw Speed (rpm) Feed Rate (kg/hr) Single Screw 30.8 25 205~240 90 Flood Feed Twin Screw 27 36 165~170 500 11.32 Table 3. Mixing section configuration. Mixer Removable Screw Length Mixing Element Length Mixer Barrier Gap Flighted End 436.63 0 - Distributive 436.63 190.5 0.02 Dispersionary 436.63 177.8 0.02 Union Carbide 436.63 358.9 - Table 4. Mechanical properties of the PP nanocomposites. Extruder Mixer Organoclay Flexural Strength Flexural Modulus (MPa) HDT ( C) XRD (Å) (%) (MPa) PP 6523 0 33.7 1080 85 - Single Screw Flighted End 6 44.0 1680 102 27.8 Single Screw Distributive 6 44.6 1720 104 28 Single Screw Dispersionary 6 45.1 1740 102 28 Single Screw Union Carbide 6 44.2 1680 102 27.8 Twin Screw Twin Screw Masterbatch letdown Direct Compounding 6 45.9 1810 105 28 6 43.8 1570 100 27