MICROPELLETIZATION AND THEIR APPLICATION TO MANUFACTURE POROUS PLASTIC PARTS

MICROPELLETIZATION AND THEIR APPLICATION TO MANUFACTURE POROUS PLASTIC PARTS Christian Schäfer and Tim A. Osswald, University of Wisconsin-Madison, Polymer Engineering Center (PEC), USA Florian Ammon, University of Erlangen-Nuremberg, Institute of Polymer Technology (LKT), Germany Vanessa Araujo Rivas, Simón Bolívar University, Caracas, Venezuela Abstract A novel micropelletization technique yields micropellets with a controlled morphology and narrow particle size distribution which can be used for sintering applications and additive manufacturing processes such as laser sintering. A polymer melt is extruded through a capillary and the extruded thread is stretched with a hot air stream until flow instabilities cause it to breakup into small droplets. This work focuses on an improved experimental setup with additional temperature control for the production of micropellets. By performing a variety of test series, options for further optimization of the process have been worked out. This is another important step towards an economical, ready-to-use-process that can provide ideally shaped and size-distributed micropellets using a wide range of polymers. Furthermore, sintered parts were produced to demonstrate possible utilization of these micropellets for industrial and commercial applications. Introduction Traditional powder based processes, which include additive manufacturing, have shown an increased demand in modern processing industries. Laser sintering techniques and specialized sintering processes, for instance, offer solutions for many challenges of the fast moving product-life-cycle [1]. These processes demand powders with specific properties to ensure a certain quality of the product [2]. Typical requirements include small particle size (< 250 μm), narrow size distribution, defined geometry, cost-competitiveness and the availability of a variety of polymers [3]. Current methods of polymer powder production cannot meet all of these requirements. For example, precipitation processes are only possible with select polymers and cryogenic milling can lead to edged pellets without a uniform particle surface [4]. Micropelletization offers solutions to these problems, since the particles have a controlled geometry and size distribution and are at least ten times smaller than standard thermoplastic pellets [5]. Yet, there is no commercially available technique that economically yields spherical micropellets in industrial quantities. Therefore, a new manufacturing process based on the physical principle of Rayleigh disturbances is being developed by the Polymer Engineering Center (PEC) at the University of Wisconsin-Madison [5, 6]. Thereby, a polymer strand is stretched with a hot air stream until flow instabilities cause it to breakup into small droplets. Cursory work led to promising results, which led to an optimized experimental setup with advanced parameter control [5, 6]. In this work, the new experimental setup was validated and characterized regarding the improved temperature control and novel die design. In addition to the production of micropellets, their further utilization is part of this research. Sintering or fusing these micropellets into films, sheets or even more complex plastic parts, for instance, opens new possibilities for producing parts used by the construction or agriculture industry such as dams, insulations and covers. Depending on the size and refinement of the processed micro granules, porous sheets are possible and can be used for a variety of filtration applications [7, 8, 9]. Experimental Setup The micropelletizer system consists of a 0.75 in (19 mm) Brabender model 2503 lab extruder with a length-to-diameter-ratio of 25:1 extruder, a custom extrusion die, and a downstream collection system (Figure 1). At the orifice of the nozzle, the polymer melt strand is stretched by an air stream causing instabilities on its surface, which eventually leads to a breakup of the polymer strand into droplets, the micropellets. Test series were performed using an improved nozzle design, which allowed for independent control of the airstream temperature and of the melt temperature [5, 10]. The nozzle consists of a mandrel, collar, and insulation component (Figure 2). The inner airstream is heated by a heat torch and controls the mandrel temperature, and consequently the polymer melt temperature. The second air stream, which creates the Rayleigh disturbances (Rayleigh air stream), is guided between the insulation and the collar and is heated by a second heat torch. The insulation component is made of a machinable Polytetrafluoroethylene, Teflon, and separates the mandrel air stream from the Rayleigh air stream. SPE ANTEC Indianapolis 16 / 832

Heating band T 4 Heating zones T 3 T 2 T 1 Polymer ω Materials High density polyethylene (HDPE), SCLAIR 2906, NOVA Chemicals, USA, was used as feedstock for the micropelletization process. T 5 Mandrel Air stream T Air Rayleigh Air stream Figure 1. Schematic of the micropelletizer with indicated parameter settings (V, ω, T 1 - T 5 and T Air,). Micropelletization Results In order to provide micropellets made from virgin and recycled polymers, the first step involves the study of the micropelletization process for virgin plastics which includes the study of the processing capability followed by the investigation of the break-up and the evaluation of the produced micropellets with regard to their size distribution and shape. Information about parameter optimization was carried out for the virgin HPDE by varying the following process parameters: the Rayleigh air volumetric flow rate, the Rayleigh air temperature, and the extruder rotational speed. The experiments discussed in this paper consider variations of the Rayleigh air stream temperature and the rotational speed of the extruder while keeping the extruder temperature profile constant (Table 1). The varying process parameters are listed in Table 2. Table 1. Extruder temperature profile T 1 [ C] T 2 [ C] T 3 [ C] T 4 [ C] T 5 [ C] 1 160 170 170 170 Figure 2. Schematic of the improved micropelletization die. The experiments explored varying the Rayleigh air stream temperature and the rotational speed of the extruder. Optical microscopy and digital photography were used to visually analyze the micropellets. An inhouse particle analysis system was used to determine the particle size and their L/D ratio. The micropellets were deposited on a carrier film using compressed air. A scanner was used to generate an 8-bit grey-scale image with a resolution of 9600 dpi in order to capture small objects. Depending on the quality of the image, it had to be filtered. The image was transformed into a binary image using a thresholding technique. The binary image was then processed using the Image Processing Toolbox in MATLAB. The algorithm is able to compute the size and aspect ratio distribution. Table 2. Process parameters Sample # T Air [ C] ω [rpm] V [L/min] T1_R6_A160 1 6 160 T160_R6_A160 160 6 160 T0_R6_A160 0 6 160 T1_R10_A100 1 10 100 Figure 3 shows a cumulative distribution function (CDF) plot of the particle size, where each micropellet is represented by a data point. For example, sample T1_R6_A160 consists of 1163 data points or 1163 micropellets, respectively. The results show that for T1_R6_A160, 12 % of the micropellets are smaller than 0.3 mm and around 80 % of all the pellets are smaller than 0.4 mm. It should be noted that the cumulative distributions, given in Figure 3 and Figure 6, represent a particle count. A weighted cumulative distribution will be shifted toward the right of the curve, accentuating the larger particles. For the sintering tests, the micropellets were sieved to separate them into three different particle size distributions (larger than 500 μm, between 355 and 500 μm and between 250 and 355 μm). SPE ANTEC Indianapolis 16 / 833

100 HDPE SCLAIR 2906 90 00 μm 80 Cumulative percent [%] 70 60 50 T1_R6_A160 T160_R6_A160 10 T0_R6_A160 T1_R10_A100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Diameter [mm] Figure 3. The CDF of the pellet diameter for four samples at different parameter settings. The first three samples in Figure 3 are for three different Rayleigh air stream temperatures. The results suggest there is no significant dependency of the temperature on the micropellet diameter for these parameter conditions. Since the samples for this test series are selected by sieving, all data points are expected to be smaller than 500 μm. This is in agreement with the plots shown in Figure 3. The scattered data points beyond 500 μm are a result of micropellet agglomerates that are not completely separated from each other due to a not perfectly performed dispersion mechanism in the particle analyzing system. An optical microscopy image of sample T1_R6_A160 is shown in Figure 4. In addition to the test series conducted at an extruder rotational speed of 6 rpm and a Rayleigh air stream volume flow rate of 160 L/min, sample T1_R10_A100 is shown, an extruder rotational speed of 10 rpm and a Rayleigh air stream volume flow rate of 100 L/min. This curve is shifted to the right which shows that these pellets have larger diameters. However, the micropellets obtained at these process conditions have a more spherical shape (Figure 5). The shape of the micropellets can be related to the length to diameter (L/D) ratio. Figure 6 shows a CDF plot of the L/D ratio of the pellets obtained with the process conditions listed in Table 2. Figure 4. HDPE (SCLAIR 2906) micropellets, sample T1_R6_A160. 00 μm Figure 5. HDPE (SCLAIR 2906) micropellets, sample T1_R10_A100. Every curve starts at an L/D ratio of one, since this is the minimum possible value and describes the shape of a perfect sphere. The curves for air temperatures at 1 C, 160 C and 0 C are very similar to each other and were all generated at a constant screw rotational speed of 6 rpm and a Rayleigh air stream volume flow rate of 160 L/min. With a higher Rayleigh air stream temperature, the L/D ratio is smaller and therefore the pellets tend to be more spherical. This does not generally mean that pellets are rounder with higher Rayleigh air stream temperatures. It can be shown that the interaction of different process parameters have a different influence on the L/D ratio and on the pellet size itself. The curve for sample T1_R10_A100 has the smallest L/D ratio and consequently the most spherical shape. This curve was generated at an air temperature of 1 C, a screw rotational speed of 10 rpm and a Rayleigh air stream volume flow rate of 100 L/min. SPE ANTEC Indianapolis 16 / 834

Cumulative percent [%] 100 90 80 70 60 50 HDPE SCLAIR 2906 T1_R6_A160 T160_R6_A160 10 T0_R6_A160 T1_R10_A100 0 1 2 3 4 5 6 L/D ratio Figure 6. Cumulative distribution functions (CDF) of the length to diameter (L/D) ratio of four samples at different parameter settings. There are different process windows existing for each parameter setup. A change of one single parameter can cause a significant change of the process window. One main reason for the process fluctuations within the improved nozzle design is caused by the PTFE insulation component. The thermal expansion of the PTFE part is much larger than the thermal expansion of the surrounding metal components [11]. This leads to a different expansion of the insulation component and the mandrel. This causes a shift of these two components relative to each other and therefore the dimensions of the inner channel for the Rayleigh air stream channel changes with different temperatures. To avoid this effect, the material of the insulation piece needs to be changed to a material with a similar thermal expansion coefficient as the surrounding metal components. Applications This section describes the fabrication and characterization of sintered HDPE micropellets into cylindrical samples. The effect of particle size and sintering time on the porosity and morphology was analyzed. Sample Fabrication The sieved micropellets, 100 mg, were sintered in cylindrical molds with an inner diameter of 10 mm. As the mold was filled mild vibration was used to settle the micropellets and eliminate large voids. The mold was then placed into a 0 C oven for 5 s, 570 s or 600 s and then air cooled to room temperature. The thickness of the sintered sample was measured, as it varied based on the particle size of the original micropellets. The described sintering conditions were evaluated during a comprehensive series of experiments with different sintering times at different temperatures. The results presented in this paper are an extract of the overall test series in order to show the relevant tendencies and possibilities of sintering micropellets into porous parts. Porosity The porosity of the sintered part was calculated from the density of the HDPE and the final sample dimensions, given by the formula: Porosity = 1 m Sample / V Sample ρ HDPE (1) where porosity is the volume fraction of pores in the sintered part, m Sample is the mass of the sintered part, V Sample is the volume of the sintered part and ρ HDPE is the density of the HDPE. The final porosity of the sintered samples as a function of sintering time for three different particle sizes is shown in Figure 7. The highest porosity, which is around %, was achieved for the largest particle size and the shortest oven time. Porosity [%] 45 35 25 15 5 5 550 560 570 580 590 600 610 Sintering time [s] Figure 7. Porosity of sintered HDPE micropellets as a function of sintering time for three different particle sizes. Morphology >500 μm 355-500 μm 250-355 μm The morphology of the sintered samples was studied using Zeiss Metrotom 800 x-ray micro computed tomography (micro-ct) with a resolution of 10 μm. Figure 8 shows images of four different samples for two different particle sizes at two different sintering times. For smaller particle sizes, the number of pores increases. However, the overall porosity itself is higher when sintering the larger sized micropellets (Figure 7). When using smaller particles the neck growth during sintering occurs more quickly, resulting in more complete sintering. SPE ANTEC Indianapolis 16 / 835

Figure 8 also shows the development of the sintered morphology by comparing two different sintering times. It can be seen that a time difference of only 60 s has a significant influence on the morphology and therefore on the porosity of the sintered part. Sintering time: Particle size: 250-355 μm Particle size: > 500 μm 00 μm t = 5 s Figure 8. X-ray micro-computed tomography images of four different samples for two different particle sizes and two different sintering times. Conclusion and Outlook t = 600 s With the optimized parameters and upgraded nozzle design, 90 % of the micropellets were between 250 and 500 μm. Specifically, improvements stemmed from the control of the Rayleigh air stream and the melt temperature. The results suggest that this additional parameter has influence on the micropelletizing process and is an important step towards understanding the process. However, test series showed that there are strong interdependencies between the parameters. Lastly, the insulation material, machinable Polytetrafluoroethylene, Teflon, was found to introduce issues due the greater thermal expansion compared to the surrounding metal components. The significant thermal expansion influenced the reproducibility of the process. References 1. D. Eyers, and K. Dotchev, Technology review for mass customisation using rapid manufacturing, Assembly Automation,, pp. 39-46 (10) 2. D. Drummer, D. Rietzel, and F. Kühnlein, Development of a characterization approach for the sintering behavior of new thermoplastics for selective laser sintering, Physics Procedia 5, 5, pp. 533 542 (10) 3. J.P. Kruth, G. Levy, F. Klocke, and T.H. Childs, Consolidation phenomena in laser and powder-bed based layered manufacturing, CIRP Annals - Manufacturing Technology 56, 2, pp. 7 759 (07) 4. D. Rietzel, F. Kühnlein, and D. Drummer, Characterization of new thermoplastics for additive manufacturing by selective laser sintering, Proceedings of the Annual Technical Conference of the Society of Plastics Engineers, ANTEC. Orlando, USA (10) 5. W. Aquite, A novel polymer extrusion micropelletization process, University of Wisconsin- Madison, dissertation (15) 6. W. Aquite, M. Launhardt, and T.A. Osswald, Manufacturing Polymer Micropellets and Powders using Rayleigh Disturbances, 44th CIRP Conference on Manufacturing Systems, University of Wisconsin- Madison (11) 7. M.E. Davis, Ordered porous materials for emerging applications, Nature 417, pp. 813-821 (02) 8. M.O. Adebajo, R.L. Frost, J.T. Kloprogge, O. Carmody, and S. Kokot, Porous materials for oil spill cleanup: A review of synthesis and absorbing properties, Journal of Porous Materials 10, 3, pp. 159 170 (03) 9. G.V. Salmoria, E.A. Fancello, C.R.M. Roesler, and F. Dabbas, Functional graded scaffold of HDPE/HA prepared by selective laser sintering: microstructure and mechanical properties, International Journal of Advanced Manufacturing Technology 65, 9 12, pp. 1529 1534 (13) 10. J. Puentes, Manufacturing of Micropellets Using Rayleigh Disturbances: Building the Optimized Micropelletizer Die, Bogotá, Colombia (11) 11. H. Domininghaus, Die Kunststoffe und ihre Eigenschaften, Berlin: Springer-Verlag (05) Additionally, this study characterized porous sintered HDPE samples made of micropellets by measuring the porosity and analyzing the morphology. The effect of sintering time on the porosity was analyzed. Micropellets, produced on the Micropelletizer, can be used to create porous plastics parts. Depending on the particle size and sintering time, porosity and pore size can be controlled. SPE ANTEC Indianapolis 16 / 836