Particle Size and Shape of Polytetrafluoroethylene (PTFE) powders using Static and Dynamic Image Analysis. Understanding of particle size and shape ...
1244 doi:10.1017/S1431927612008070
Microsc. Microanal. 18 (Suppl 2), 2012 © Microscopy Society of America 2012
Particle Size and Shape of Polytetrafluoroethylene (PTFE) powders using Static and Dynamic Image Analysis
Understanding of particle size and shape distribution of a material forms the basis for most process and production operations [1]. Size and shape can have a significant effect on the mechanical and thermal properties of a material and how the properties translate from powder state of starting raw material to its compounded form [1]. The focus of this study is to develop a robust method for the measurement of particle size and shape distribution of Polytetrafluoroethylene (PTFE) powders. PTFE is an ideal additive with inherent lubricity and low coefficient of friction to improve wear resistance when combined with engineering thermoplastics. As a result the engineered thermoplastics present excellent dimensional stability, low wear, enhanced mechanical and thermal properties, flame retardancy, and fatigue endurance. We studied two methods for particle size measurements of PTFE powders: (1) static imaging mode and (2) dynamic imaging. Conventional static imaging method is based on a combination of optical or scanning electron microscopy (SEM) imaging followed by image processing and analysis to return quantitative data. (Figure 1a) For dynamic imaging, using Camsizer XT air dispersion mode, dispersed powders were measured by the high speed optical camera system (Figure 2a). Validation of methods was carried out using NIST traceable DRI-CAL particle size secondary standard (mean diameter; 24.8±1.2µm). For static imaging, DRI-CAL standard and PTFE powders were dispersed on a glass slide and imaged using optical microscope with moving stage platform. Size measurements were recorded from three different regions on a glass slide and categorized as zone 1, zone 2 and zone 3 (Figure1a-g) (Table 1). Each part constituted about ~2500-3000 particles for measurement analysis. A pooled analysis combining zones 1, 2 and 3, constituting of ~8000 measurements represented the entire slide in accordance with static image analysis methods [2]. For dynamic image analysis, real time, high resolution distribution of not only size but also shape (aspect ratio=breadth/length) characteristics were generated in a fast and automated manner based on millions of particles (Figure 2b) [3]. Interestingly, particle size measurements based on static image analysis show variations in size statistics between parts 1, 2 and 3 and compared to the pooled dispersion analysis. ANOVA analysis of variance show significant statistical differences with p value=0.00. These variations can be attributed to sampling size, segregation or limited count of measurements for the static imaging method and can evidently lead to inaccuracy or bias in measurements. Measurements based on dynamic image analysis highlight the repeatability from the smallest to the largest sized particle and statistics generated from millions of data points (Figure 2d, Table 3). Static imaging approach works, but it is time consuming and dependent on sample preparation. As shown in data comparison Table 1 and Table 2, the pooled measurements based on static imaging (~8000 particles) are in good agreement with the particle size distribution based on dynamic image analysis [2,3]. Dynamic imaging stands out in robust statistics and versatile analysis of particle size and shape distribution in shorter times.
Microsc. Microanal. 18 (Suppl 2), 2012
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[1] Jillavenkatesa, A. et al., ³Particle Size Characterization´, in NIST Practice Guide. (2001). [2] ISO 13322-1 - Particle size analysis -- Image analysis methods -- Part 1: Static image analysis methods. (2004). [3] ISO 13322-2 - Particle size analysis -- Image analysis methods -- Part 2: Dynamic image analysis methods. (2006)
Figure 1 (a) Schematic of static imaging technique based on optical imaging and image analysis. Optical image of DRICAL particle size standard on a glass microscope slide (b) Equivalent circular diameter measurements of standard based on static imaging. (c) Optical image of PTFE powders dispersed on a glass microscope slide. (d), (e), (f) show the image analysis and particle size measurements (equivalent circular diameter) from different regions of a glass slide and (g) Pooled measurements from regions d, e, f on glass slide. Figure 2 (a) Schematic of dynamic imaging technique. Optical image of PTFE powders dispersed using air pressure modality of Camsizer XT. (b) Particle size (equivalent circular diameter) and shape (aspect ratio) distribution of DRICAL particle size standard based on dynamic image analysis. (c) Particle size (equivalent circular diameter) and shape (aspect ratio) distribution of PTFE powders. (d) Repeatability of measurements using dynamic image analysis.
