Table of contents 1 Why is particle size important?
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- X VS. Y AXIS
| INCLUDING A MEAN VALUE
Ultimately, the sophistication of the specification should be driven by how particle size influences product performance. Given that some people ask about the “average size”, it is not surprising that some specifications are based on a mean diameter. This approach is complicated by the fact that there are several mean values that can be calculated and reported in the result (ref. 8). The most common mean value noted when using laser diffraction is the volume mean, or D4,3. The D4,3 is very sensitive to the presence of large particles in the distribution. It is a good idea to use or include the D4,3 in the specification if product performance is sensitive to the presence of large particles. The other mean value occasionally used is the D3,2, or surface mean. This value is only typically used when the product is an aerosol or spray. 11 X VS. Y AXIS Other published specifications are based on the percent below a given particle size such as: 50% below 20µm and 90% below 100µm. This type of specification is based on points along the y axis (which reports frequency percent) as opposed to the x axis (which reports diameter) as in the previous examples. Although this approach has been used in many specifications, it is important to realize the difference between using the x (size) and y (percent) axes. All measurements include an error which should always be considered when setting a specification. For the example shown in Figure 14, the D50 is 100µm with an error of +/- 5% on the x (size) axis. This error includes all sources such as sampling and sample preparation. The same error becomes +/- 20% when translated to the y (percent) axis. Stating an error of +/- 5% is more attractive than +/- 20%, even when expressing the same actual error range. The degree to which the y axis error is exaggerated vs. the x axis depends upon the steepness of the distribution curve. There are applications where the percent below a given particle size is an important result. Recently there has been interest in the presence of “nanoparticles” (at least one dimension smaller than 100nm) in products such as cosmetics. The software which calculates the PSD should be capable of easily reporting the percent under any chosen size—in this case the percent below 100nm (Figure 15). In the LA-960 software this is displayed as “Diameter on Cumulative %”. In the example below the value for percent less than 100nm is reported as 9.155%. Several points are worth mentioning in regards to setting a specification on the percent below 100nm as in this example specifically and for sub-micron materials generally. The particle size distribution is dependent upon many factors including the sample preparation method. The laser diffraction technique works best within a certain particulate concentration range. This sometimes requires that samples undergo dilution. In some cases this dilution may change the state of the particles and affect the apparent size distribution. Additionally, ultrasonic energy can be applied to improve the dispersion of agglomerates which can significantly change the result. tải về 7.54 Mb. Chia sẻ với bạn bè của bạn:
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