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How can single particle optical sizing determine the best processing time for powders?
By Patrick O’Hagan, Ph.D.
Agilent Sales Manager
Many companies are seeking less expensive ways to process powders to a tightly specified range of particle sizes. Properties of powders – such as viscosity, porosity, hardness, strength, taste, appearance, and stability – are all dependent upon the particle size distribution (PSD) of the final product. Because some particle-sizing techniques are fraught with error, much time and money is currently wasted by overprocessing powders. Single particle optical sizing (SPOS) is a more reliable technique that delivers the sensitivity and resolution that you need to accurately determine when processing is sufficient to meet particle-size specifications.
The problem with laser diffraction
Laser diffraction (LD) is an ensemble technique used to measure PSDs. It is based on a combination of two distinct effects – Mie scattering and Fraunhofer diffraction. To avoid significant errors in the computed PSD, the instrument must successfully merge the independent results from these two analyses. In laser diffraction, particles of all sizes contribute simultaneously to the signal, requiring ill-conditioned mathematical algorithms to extract what is at best, only an approximate PSD.
On the other hand, single particle optical sizing, as used in the Agilent 7080 AccuSizer, counts and sizes particles one at a time with great statistical accuracy. It requires no complex mathematical algorithms – just a standard calibration curve. SPOS affords high sensitivity, resolution, and accuracy – all desirable attributes when you must be sure that your process yields high-quality product with a minimum expenditure.
Sensitivity to see the large-particle impurities that affect performance
For many processes, the concentration of large-particle impurities is an important predictor of product performance. For example, in the kaolin clays that are used to coat and fill paper, these impurities affect the gloss of the paper. Figure 2 shows the SPOS measurements of the PSDs of clays with known concentrations of impurities at or below the 5% level by weight. These curves demonstrate that the Agilent 7080 AccuSizer, which counts and sizes particles one at a time, can quantitatively determine these low-level impurities. Below the 5% level, Fraunhofer diffraction showed no differences in the particle size distributions between the various samples.
Resolution to give the true PSD
When you need to determine the end point of a process based on a particle size measurement, resolution is another critical factor. Figure 3 compares the resolution of the 7080 AccuSizer with that of a laser diffraction instrument. The 7080 AccuSizer produced a very narrow peak that was consistent with the manufacturer’s specification for this sample – a very expensive latex standard that is used to calibrate and check the accuracy of particle-sizing equipment. The laser diffraction instrument showed a broad peak with artifacts at both ends of the size distribution. If this were an actual production situation, considerable time could have been wasted to “fix” a problem that was actually an artifact of the measurement technique.
Accurately determine a process end point
When materials are processed in powder form, an accurate measure of particle size ensures that you spend just the right amount of time to produce a consistent, quality product. If you need the sensitivity and resolution to accurately determine the end point of a process, consider the Agilent 7080 AccuSizer. To learn more, visit our product page.
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