MICROPELLETIZATION OF VIRGIN AND RECYCLED THERMOPLASTIC MATERIALS

Transcription

1 MICROPELLETIZATION OF VIRGIN AND RECYCLED THERMOPLASTIC MATERIALS Christian Schäfer, Jackson S. Bryant and Tim A. Osswald, University of Wisconsin-Madison, Polymer Engineering Center (PEC), USA Stefan P. Meyer, University of Erlangen-Nuremberg, Institute of Polymer Technology (LKT), Germany Abstract Traditional polymer powder and micropellet based processes, such as powder bed fusion and rotational molding, have been in increased demand in modern processing industries. These processes require polymer powders and micropellets with a small particle size, narrow size distribution and defined geometry for a variety of polymer resins. Therefore, micropelletization technologies, where particles in the size range of 50 to 1000 μm are generated, have been attracting growing attention over the past decade. A new technique, developed at the Polymer Engineering Center, yields micropellets with a controlled morphology and narrow particle size distribution. In this process, a polymer melt is extruded through a capillary and is subsequently stretched with a hot air stream until flow instabilities cause it to break up into particles. Small changes in process conditions result in different size distributions and particle shapes, such as lentil-like pellets, fibers and thread segments. This work shows how material properties and processing parameters influence the produced micropellets. Besides the processing of virgin thermoplastic material, recycled high density polyethylene flakes are used as feedstock for the micropelletization process in order to show the capability of this process to contribute to current polymer recycling efforts. Micropellets are typically in the size range of 0.5 mm to 1.0 mm and offer advantages over standard sized pellets [6,8,10]. Some of the major advantages and benefits of micropellets are a higher bulk density, increased flow ability, and higher surface area. Compared to conventional molding pellets, polymer powders are at least ten times smaller, in the size range of 50 μm to 500 μm. Mechanically ground powders, which are used in processes such as Selective Laser Sintering (SLS), typically consist of spherulite particles with a narrow diameter size distribution of 60 μm (+/- 40 μm) [5,10,11]. Particle size is an important characteristic that affects the choice of a powder for a certain application. Therefore, obtaining the correct particle size and morphology is necessary since the powder properties influence both processing ability and characteristics of the final part [12]. A comparison of the size range for standard sized pellets, micropellets, and powders is shown in Figure 1. PA12 Powder Conventional Molding Pellets 3 mm Introduction Micropelletization is a technology that provides a variety of potential applications in the compounding and masterbatch industry as well as for rotomolding and sintering applications [1 3]. For the purpose of producing micropellets, conventional pelletizing techniques deal with many design and operational challenges. This results in the development of more advanced processing methods. The increasing demand for micropellets and powders in the field of polymer processing represents a challenge for current production techniques and therefore requires alternative manufacturing methods. Present methods of polymer powder production are restricted to a limited selection of polymer resins and in most cases cannot fulfill the requirements with regard to size distribution and particle shape [4,5]. As a result, much work has gone into the development and study of micropelletization processes due to the advantage of using powders and micropellets in polymer processing applications [3,6 9]. Micropellets Figure 1. Size range comparison for conventional molding pellets, micropellets and polyamide 12 (SLS) powder. In 2010 [13] and 2012 [14], Osswald et al. conceived different ideas of manufacturing polymer micropellets by using compressed hot air to impose surface disturbances on an extruded polymer strand. Based on the physical principle of Rayleigh disturbances, a new manufacturing process is being developed by the Polymer Engineering Center (PEC) at the University of Wisconsin-Madison [15,16]. Thereby, a polymer strand is stretched with a hot air stream until flow instabilities cause it to break up into small particles. Cursory work led to promising results, which led to an optimized experimental setup with advanced parameter control. This work shows how material properties and processing parameters influence the production of micropellets. SPE ANTEC Anaheim 2017 / 1112

2 Experimental Setup The micropelletization system consists of a 0.75 in (19 mm) Brabender model 2503 lab extruder with a lengthto-diameter-ratio of 25:1, a custom-made extrusion die, and a downstream collection system (Figure 2). At the orifice of the nozzle, the polymer melt strand is stretched by a hot air stream causing instabilities on the strand surface, which eventually leads to breakup into micropellets. A schematic drawing of the micropelletization die is shown in Figure 3. The die consists of three major components; the mandrel, the collar, and the separation piece. The separation piece allows the nozzle to have two independently controllable temperature zones. The inner airstream (air stream for heating the capillary, Figure 2) is responsible for controlling the temperature of the mandrel and is therefore directly linked to the polymer melt temperature. The second air flow (surrounding air stream for imposing deviatoric stresses, Figure 2) is responsible for causing the surface disturbances on the polymer strand and is guided through the gap between separation piece and collar. T 5 Air stream for heating the capillary Figure 2. Schematic drawing of the micropelletizer with indicated parameter settings (V, ω, T 1 - T 5 and T Air). Heat sensor Heating band Insulation (separation) T 4 Mandrel T Air T 3 V Heating zones T 2 T 1 Surrounding air stream for imposing deviatoric stresses Heating band Polymer ω Polymer melt Extruder The modular design of the extrusion die allows the exchange of the mandrel with different capillary diameters. The mandrel used in this work has a capillary diameter of 1 mm (Figure 3). The micropelletization setup used in this work allows the adjustment of the following process parameters (Figure 2): The temperature of the surrounding air stream, T Air, the rotational speed of the extruder screw, ω, and the volumetric flow rate of the surrounding air stream, V. Additional process parameters are the temperatures of the three heating zones of the extruder T 1, T 2 and T 3. The heating band on the block is referred to as T 4. The temperature of the mandrel air stream is indicated as T 5. Process variables and ranges are listed in Table 1. Table 1. Controlled variables and ranges in the micropelletization process. Process variable Range Unit Airflow, V SLPM Screw rotational speed, ω 1-30 RPM Extruder heating zones, T 1- T C Heating band on die block, T C Nozzle temperature, T C Air flow temperature, T Air C Each heat torch is controlled by a 1/16 DIN Ramp/Soak controller number CN7853, Omega. It has an accuracy of ± 0.25 % at span and ± 1 at the least significant digit as well as a resolution of 0.1 C for thermocouples. The controller evaluates the process and selects the PID (proportional-integral-derivative) values to maintain good control. This setup has proven to be very effective in keeping the temperature provided by the heat torches constant after setting it with a deviation of ± 1 C. The volumetric flow rate of the air is measured by the Aalborg GFC57 thermal mass flow meter in combination with red lion digital panel meter c48 series. This system allows to control and indicate the flow rate of the air, displayed in units of SLPM (Standard Liters Per Minute) for standard conditions (1.013 bar, 20 C). Particle Analysis Compressed air Heat torch Collar Capillary (Ø 1 mm) Heat torch Collecting pipe Micropellets Figure 3. Schematic drawing of the extrusion die. Compressed air Optical microscopy and digital photography were used to visually analyze the micropellets. To measure the size and shape of the particles produced in the micropelletization process, a MATLAB program that analyzed scanned images of the particles was used. Prior to analyzing the particle image, certain steps were followed to generate an image of each sample. For this purpose, particles were distributed uniformly across a glass plate before being scanned and converted into a binary image. SPE ANTEC Anaheim 2017 / 1113

3 The scanned image was converted into a binary image using a standardized threshold conversion. In order to determine a coherent edge between particle and surrounding material (e.g. glass plate) the ISO50%-Global- Threshold calculation was utilized. This standardized threshold calculation is known from computer tomography analysis for industrial measurements [17]. First, the histogram of a grayscale picture (e.g. provided by the scanner) has to be determined. In the grayscale area (dark) the maximum value needs to be found and is defined as the surrounding material. Furthermore, the maximum between 128 and 255 is defined as particles. To determine the threshold, the mean value of the gray value of the surrounding material and of the particles is calculated. With the calculated threshold value, the grayscale was converted into a binary picture. The binary image was then subjected to a noise reduction using the bwareaopen function in MATLAB. This function removes all connected components that are smaller than a chosen pixel size (e.g. 8 pixels). This allows the removal of scanning errors, pixel errors, inaccurately small particles (0-50 μm), dust particles, fibers and other impurities. The noise reduction limits this analyzing tool to particle sizes that are larger than 75 μm. The noise filtered binary image is then visually analyzed in Adobe Photoshop to further remove areas of the image where the program would inaccurately measure the size of the particles. With the particle distributing technique available, some of the particles will overlap on the glass plate. Overlapping particles need to be removed prior to utilizing the MATLAB program for calculating diameter and aspect ratio, since these would distort the size measurement. If particles overlap, the program views them as only one particle. Additionally, outlier particles, such as long filaments and dirt particles are removed. Recent research activities involve the improvement of the MATLAB program to automatically filter out overlapping particles in order to eliminate the step of manually analyzing each image in Photoshop. The MATLAB program measures the size of each particle using ellipses. For each particle, an ellipse was sized to contain the whole particle while avoiding blank areas with no particle in them. The minor axis of the fitted ellipse was put along the smallest diameter of the particle. Using ellipses is necessary to measure particles produced by the micropelletization process because the particles have varying geometries. The program also measured the length of the major axis of the ellipse. By taking the ratio of the major and minor axes of each measurement, the aspect ratio was calculated for each particle. An aspect ratio of one would represent a perfectly spherical particle. A ratio larger than one represents an ellipsoid particle, or other elongated particle. The MATLAB program measures the smallest diameter of some particles inaccurately. If a particle is crescent shaped for example, the program overlays an ellipse to contain the area of the particle, but is unable to accurately measure the smallest diameter. Though most of the particles are ellipsoidal, some are longer, and bent. The error due to geometry of the particle becomes larger as its aspect ratio increases. Figure 12 shows a microscope image of a sample made from recycled HDPE. It shows the kinds of non-ellipsoid geometries that are obtained under certain processing conditions. Some of these particles, such as long, curved filaments, were removed from the sample image before being measured with the MATLAB code. This error skews the data in some samples to show a larger diameter of the pellets than there actually is. To mitigate the error, the diameter measured by the program is described as the effective particle diameter. The effective particle diameter is useful in that it takes into account the non-ellipsoid geometries that some particles have. It represents how these particles would act in a process where their size is significant and better represents their surface area to volume ratio. For example, if particles were being sieved for a limited amount of time those with non-ellipsoid geometries would have a lower probability of passing through the sieve which matched the particle s smallest diameter. An increased aspect ratio will further decrease the probability of a non-ellipsoid particle to pass a certain sieve size. The results of the sieving would be time dependent. By looking at the effective particle size of the particles, a short time scale is used, which more accurately reflects how size may matter in an industrial setting. The most desirable shape for particles produced in the micropelletization process is a sphere. Spheres have a high surface area to volume ratio and therefore provide improved flow ability and sintering characteristics [1,18,19]. As a particle shifts from being an ellipsoid to a non-ellipsoid, the surface area to volume ratio decreases. Therefore, the effective particle diameter better represents the shift in this significant property than a strict measurement of the smallest diameter of a particle does. Materials Virgin high density polyethylene (HDPE), as well as recycled HDPE were used as feedstock for the micropelletization process. Furthermore, polypropylene (PP), and polyethylene terephthalate (PET) were used to analyze their process ability and their feasibility to produce micropellets. Material information, including the melt flow index (MFI), is given in Table 2. SPE ANTEC Anaheim 2017 / 1114

4 Table 2. Material information of the selected materials used in the micropelletization process. Condition Material MFI Temperature (Manufacturer) [g/10min] [ C] and Weight [kg] HDPE SCLAIR 2906 (NOVA Chemicals) HDPE Recycled, EcoStar (Placon) PP Profax TM 6523 (LyondellBasell) PET Eastapak TM 9921 (Eastman) Results Results for Virgin HDPE / / / / 2.16 The results plotted in Figure 4, Figure 5, and Figure 6 compare size distribution and aspect ratio of particles produced at different processing temperatures, extruder screw rotational speeds, and volumetric air flow rates. Additionally, microscope images of the produced particles are presented in Figure 7 and Figure 8. The focus of this section is to reveal the effect of processing temperature and extruder throughput (screw rotational speed) on the shape and size distribution of the produced micropellets. Therefore, two different processing temperatures, 160 C and 190 C, and two different screw rotational speeds, 5 and 10 rpm are evaluated. An overview of sample nomenclature and processing parameters is given in Table 3. Table 3. Process parameters for virgin HDPE. Sample Name Temperature T 4=T 5=T Air [ C] RPM [rev/m in] Air- Flow [SLPM] TA160_R05_F TA160_R10_F TA190_R05_F TA190_R10_F Characterizing the size distribution of particles by indicating the mean diameter of an entire batch can be a helpful and fast measure. However, this value is not particularly helpful when looking at batches that do not show a typical Gaussian distribution. Since most of the batches, produced with the micropelletization process, show a mixed distribution, it is of interest to plot the relative frequency of the particle diameters as shown in Figure 4. This representation allows the processor to make statements about whether certain process conditions yield even particle size distributions or if pronounced peaks are present. Sample TA190_R10_F150, for instance, shows a fairly even particle size distribution, with values ranging from 80 to 1000 μm (Figure 4). On the other hand, sample TA160_R10_F150 exhibits two distinct peaks. One peak with particles ranging from 80 to 300 μm and the second peak with particles ranging from 300 to 1050 μm. The mean diameter of this batch is 630 μm. By just giving this value it would be completely unknown, what particle sizes are present in one batch. Therefore, when providing information about the produced micropellets, particle diameter distribution (relative frequency) is necessary. Figure 5 shows the cumulative distribution function of the particle diameter for micropellets produced at four different process conditions. The data representation in Figure 5 allows an easy comparison of the influence and effect of different process parameters on the particle diameter. Micropellets produced at 5 rpm show significantly smaller particles when processed at higher processing temperatures (Figure 7). However, when looking at the shape of the micropellets, they seem to be more elongated. In order to characterize the shape of the micropellets, Figure 6 presents the cumulative distribution function of the particle aspect ratio (L/D). It clearly shows that sample TA190_R05_F130 has a larger aspect ratio compared to sample TA160_R05_F130. For micropellets produced at 10 rpm a similar effect can been seen. Summing up, the following conclusions can be drawn from the experimental data: Relative Frequency With increasing processing temperature, micropellets decrease in size. With increasing screw rotational speed, micropellets increase in size. Higher processing temperatures result in micropellets with larger aspect ratio. 1 9 % TA160_R05_F130 8 % TA160_R10_F150 7 % TA190_R05_F130 6 % TA190_R10_F150 5 % 4 % 3 % 2 % 1 % Particle Diameter [μm] Figure 4. Particle diameter distribution (rel. frequency) for four different process conditions. SPE ANTEC Anaheim 2017 / 1115

5 Cumulative Percentage TA160_R05_F130 3 TA160_R10_F150 2 TA190_R05_F130 1 TA190_R10_F Particle Diameter [μm] Figure 5. Cumulative percentage of the particle diameter for four different process conditions. Cumulative Percentage TA160_R05_F130 TA160_R10_F150 2 TA190_R05_F130 1 TA190_R10_F Particle Aspect Ratio (L/D) Figure 6. Cumulative percentage of particle aspect ratio (L/D) for four different process conditions. 1 mm 160 C 190 C Figure 7. Microscope images of micropellets produced at 5 RPM and 130 SLPM for two different processing temperatures (Material: HDPE, SCLAIR 2906, Nova Chemicals). 1 mm 160 C 190 C Results for Recycled HDPE The results plotted in Figure 9, Figure 10, and Figure 11 compare size distribution and aspect ratio of particles produced at two different extruder screw rotational speeds for a constant processing temperature of 160 C and a constant air volumetric flow rate of 170 SLPM. An overview of sample nomenclature and processing parameters is given in Table 4. In addition to the data analysis for particle size and aspect ratio, Figure 12 shows microscope images of recycled HDPE particles produced at two different extruder screw rotational speeds. These images provide further information about the shape and surface morphology of the produced particles. Table 4. Process parameters for recycled HDPE. Sample Name Temperature T 4=T 5=T Air [ C] RPM [rev/m in] Air- Flow [SLPM] TA160_R05_F TA160_R10_F Compared to the results for virgin HDPE, presented in the previous section, the recycled HDPE shows a narrower particle diameter distribution with values ranging from 70 to 500 μm. Additionally, when looking at Figure 10, 8 of the particles are in the size range between 70 and 250 μm. Therefore, processing the recycled HDPE will result in much smaller particles compared with the virgin HDPE. It is important to point out, that the viscosity of the recycled HDPE is about a power of ten lower than the viscosity of the virgin HDPE. In general, a lower material viscosity will lead to thinner thread segments during processing and therefore in smaller particle diameters. However, the aspect ratio of those particles tends to be significantly larger (Figure 11) in contrast to those produced with virgin HDPE (Figure 6). The aspect ratio of the recycled HDPE particles varies between values of 1 and 5 showing a very broad distribution. An increase of the extruder screw rotational speed does not have any noticeable effect on the aspect ratio (Figure 11). On the other hand, the aspect ratio of the virgin HDPE particles varies between values of 1 and 3 showing a fairly narrow distribution. In the case of recycled HDPE, a change in extruder rotational speed only affects particle diameter. Similar to the trend seen for virgin HDPE, an increase in screw rotational speed will result in larger particles (Figure 10). Figure 8. Microscope images of micropellets produced at 10 RPM and 150 SLPM for two different processing temperatures (Material: HDPE, SCLAIR 2906, Nova Chemicals). SPE ANTEC Anaheim 2017 / 1116

6 Relative Frequency 25 % 2 15 % 1 5 % TA160_R05_F170 TA160_R10_F170 The overall objective of the micropelletization technique is to mainly produce spherical particles for sintering and 3D printing applications. However, small particles with irregular shape might still be interesting for sintering applications, such as filtration and insulation, especially when produced from recycled materials. 1 mm 5 RPM 10 RPM Particle Diameter [μm] Figure 9. Particle diameter distribution (rel. frequency) for particles produced at two different extruder screw rotational speeds (5 rpm and 10 rpm). Figure 12 shows microscope images of the recycled HDPE particles produced at two different extruder screw rotational speeds, 5 and 10 rpm. Compared to the particle shape of virgin HDPE particles (Figure 7 and Figure 8), the recycled HDPE particles show a larger variety of different particle shapes. Cumulative Percentage Particle Diameter [μm] Figure 10. Cumulative percentage of particle diameter for particles produced at two different extruder screw rotational speeds (5 rpm and 10 rpm). Culumative Percentage TA160_R05_F170 TA160_R10_F TA160_R05_F170 TA160_R10_F Particle Aspect Ratio (L/D) Figure 11. Cumulative percentage of particle aspect ratio (L/D) for particles produced at two different extruder screw rotational speeds (5 rpm and 10 rpm). Figure 12. Microscope images of micropellets produced at two different extruder screw rotational speeds (Material: HDPE Recycled, EcoStar, Placon). Results for PP and PET Experimental testing of the micropelletization process has shown that not only pellets but also fibers and threads of different size distributions and shapes are obtained under a wide range of process conditions for different polymer resins. The micropelletization of both PP and PET had a tendency to result in filaments and micropellets with an elongated shape rather than leading to the desired spherical micropellets. Selected results are presented below (Figure 13). Figure 13. Microscope images of fibers produced with the micropelletization process for PP and PET. Processing of PP resulted in thin, long fibers with polymer pearls attached to those fibers. It shows that the airstream caused disturbances, but no breakup occurred. The elastic property of the viscoelastic PP seems to dominate at this point. Besides the PP, a PET material was tested in the micropelletization process. This material resulted in very thin, continuous fibers, independent of the air flow rate. No pellets were collected. Further investigations with different types of PP and PET have not yet been performed and is part of future work. SPE ANTEC Anaheim 2017 / 1117

7 Discussion The setup of the micropelletization die is similar to the commercial melt blowing process for the production of nonwoven fabrics [20]. Even though the melt blowing technology was developed over 60 years ago [20], it is still part of intensive research because of its complexity regarding material behavior, fiber motion and process parameters. The micropelletization process, studied in this work, is a fairly new process technique and therefore only little information is known about the effect of different material properties. Properties that might affect and promote the occurrence of breakup are molecular structure, molecular weight, and molecular weight distribution. In melt blowing applications polymers of low molecular weight and melt viscosity are used. Typical melt flow rates for melt blowing grade polypropylene, for instance, are 12 to 1500 g/10min [20]. The rheological properties of polymer melts vary considerably with molecular structure and has a significant influence on spin ability and also on the development of Rayleigh disturbances [21,22]. In shear flow the viscosity of polymer melts usually decreases with increasing deformation rate. In an extensional flow, seen in processes such as fiber spinning, melt blown and film blowing, the viscosity frequently increases with increasing extensional rate [23]. The fluid is extensional thickening. In the micropelletization of HDPE, the breakup occurs close to the nozzle outlet and no significant stretching is observed. Figure 14 shows high speed camera footage of the micropelletization process for HDPE using 7200 fps at a maximum resolution of 512 x 600 pixels. a c Figure 14. High speed camera footage of the micropelletization process for HDPE (7200 fps, 512x600). Breakup scenarios at different process conditions: 150 SLPM, 8 RPM, and 190 C (a-b); 150 SLPM, 10 RPM, and 160 C (c-d). b d Various breakup scenarios for two different process conditions are illustrated. However, when processing other materials, such as the PP and PET shown in Figure 13, significant stretching can be observed and extensional thickening might influence the flow behavior and ultimately breakup of the polymer strand. Conclusion and Outlook Micropellets made from both virgin and recycled HDPE were successfully produced with the micropelletization process. The effect of processing temperature, and extruder screw rotational speed on particle size distribution and aspect ratio was evaluated. In conclusion, particle size decreased with increasing processing temperature and decreasing extruder screw rotational speed. Compared to the virgin HDPE, the low viscosity, recycled HDPE resulted in particles with a larger and broader distributed aspect ratio but significantly smaller in particle diameter. PP and PET resulted in thin, long fibers and no breakup was observed. Further studies will consider other types of PP, as well as LDPE and LLDPE. This will allow to gather additional information on how different types of materials behave in the micropelletization process. References [1] Parkin, J., Micropellet technology comes of age, Plast. Addit. Compd., 9 (5), pp , 2007 [2] Free, D., Pelletizing your compound: what are your options?, Plast. Addit. Compd., 8 (1), pp , 2006 [3] Thompson, M. R., Xi, C., Takacs, E., Tate, M., and Vlachopoulos, J., Experiments and flow analysis of a micropelletizing die, Polym. Eng. Sci., 44 (7), pp , 2004 [4] Schmidt, J., Sachs, M., Blümel, C., Winzer, B., Toni, F., Wirth, K.-E., and Peukert, W., A novel process route for the production of spherical SLS polymer powders, Proceedings of the 30th International Conference of the Polymer Processing Society (PPS-30), Cleveland, USA, 2015 [5] Rietzel, D., Kuehnlein, F., and Drummer, D., Characterization of new thermoplastics for additive manufacturing by selective laser sintering, Proceedings of the 68th Annual Technical Conference (ANTEC) of the Society of Plastics Engineers (SPE), Orlando, USA, 2010 [6] Xi, C., Thompson, M. R., Vlachopoulos, J., Takacs, E., and Tate, M., Study of the micropelletization process, Proceedings of the 61st Annual Technical Conference (ANTEC) of the Society of Plastics Engineers (SPE), Brookfield, USA, 2003 SPE ANTEC Anaheim 2017 / 1118

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