Using Morphological Analysis for the Quantification of Multiple Components in Reflective Glass Beads

Highway stripes and signs use paint with a specific mix of reflective glass beads to provide a safety coating, as well as sand to increase friction and provide traction when the coated surfaces are subject to rain or ice. The beads must comply with quality control specifications regarding their shape and size and strength.

These parameters are all essential for their proper function. The size is specified so that the beads project above the surface enough to capture and reflect sufficient light, while the shape must be round enough to allow it to act as a lens which will reflect the light in the desired direction with adequate intensity.

It is known that any mix of glass beads will show the presence of a few fused beads because of manufacturing process artefacts. These beads reduce the overall reflective efficacy of the mixture. If there is too much sand, on the other hand, too much light will be absorbed while too little sand reduces traction below safe limits. These are all therefore parameters which need to be measured and a quantitative report issued for quality control purposes.

Laser diffraction (LD) has been the most widely used particle size distribution measurement technology in industry since it was brought out in the 1970s, but digital image analysis (DIA) is a newer development which is now preferred for assessment of particle shape, including sphericity, as one of 30 different shape and size values.

Combining LD and DIA Technologies

Microtrac has brought out the innovative Sync device, an instrument which uses both LD and DIA to carry out paired measurements on the same sample at the same time, using a single sample cell. The system in the wet analysis setup is shown below with the dry feeder module, both being interchangeable.

The internal configuration for both wet and dry analysis modules is shown in the following diagrams:

LD Size Distribution Reports

Figure 3 shows the Microtrac Sync report obtained on size distribution from an LD analysis of a glass bead mixture. The detailed report includes graphical and tabular reporting of size distribution, summary of central tendency and span, the size percentiles and the SOP conditions under which the measurement was performed.

DIA Reports

Figure 4 shows some results from the video file generated by a measurement of glass beads, displaying sand, fused beads and good beads, which are the major components of the mix.

The DIA offers post-analysis software capabilities which allow different components to be detected as well as quantified, as Table 1 shows in relation to the glass bead mix.

The View Particles feature allows the analyst to see the whole image video file and to sort the images using any parameter, in either ascending or descending order. The selected parameter is shown. The file can be converted into spreadsheet form to list all the size and shape parameters for each of the parameters in the sample. Either the image or the data files can be printed as PDF versions.

A Search Query Function is available to separate certain subsets of particles selected by applying limits to particular parameters. Another useful feature is the Scatter Diagram which shows particle distribution with respect to any two parameters, represented on the x-axis and y-axis. It is also possible to re-run the whole video file, once saved, after changing the SOP parameters.

The Filter Function allows the quantification of seven components or less, using distinguishable differences in one or more size or shape values, and also allows the quantification of all particles which have gone beyond specification limits in the sample.

The X-Y graph and the Table display data in the form of frequency or cumulative distributions for up to six parameters using any of the standard formats, whether volume percentage, count or number percentage, logarithmic percentage or linear. Once the Filter Function is selected during an analysis, various filters may be used for different parameter distributions to be studied.

The use of the View Particles feature is seen in Figure 5.

This demonstrates how useful this feature is in method development software. The upper left shows a camera frame while the lower left shows the image file view which can be scrolled through all the thousands of particles which are measured in the selected sample. Any parameter can be selected to sort the images, with its value being shown for each image. The file can be converted to a spreadsheet enabling all 36 parameters for each of the particles to be listed.

The upper left shows a glass bead agglomerate in the blue box, with the upper right showing all its size and shape parameters in list format.

The left shows a Search Particles button which opens a particle query window, as seen below the list of parameters on the right side. This is a very useful window which allows any selection criteria to be entered to launch a search for matching component types within the sample. The figure shows how, for instance, it is possible to search for all particles whose length is over 25 microns. Once this query is selected, the image file will show only images of particles within the sample which are more than 25 microns long. The Scatter Diagram view can be selected here.

The Particle Query window was used to configure three separate searches which were added to the Search Particles window. The second of these was then selected for calculation, resulting in the display of only those images of particles that match the criteria of the search, in the new image file, as well as showing what proportion these particles bear to the whole sample.

Figure 7 shows how the View Particles display can be used to open the Scatter Diagram, which shows how any selected parameter is distributed, based on the selected distribution or sub-distribution in View Particles. The illustration uses the Scatter Diagram to show the distribution for the whole sample of glass beads.

The Scatter Diagram is seen at lower left, with blue dots representing where each particle in the sample is located, along the top red X-axis and right red Y-axis graphs. The greater density of particles is shown by a darker blue.

It is possible to select any of the 30 parameters, as shown in callout 1, to plot on either graph. In the illustration, F or Feret Length is on the X-graph and Transparency on the Y-graph. Transparency is plotted on a scale of 0 to 1 with 1 representing perfect transparency and 0 being total opacity.

A whole sample view helps to select the right particle queries. The following example shows three locations with high population density: one which has small particle size and has high transparency values (callout 2, upper left), one which is larger but is equally transparent (callout 3) and a third which is just as large but is significantly less transparent (callout 4).

These distributions may be reported in Volume percentage (callout 5) or in Number percentage as here. If the volume format is used, it is clear that the small size mode comprises less than 1% of total sample volume, which confirms that it is a very insignificant mode consisting of abraded and very small particles or dust, which are unlikely to affect sample quality. The Summary Data on the right side is for both graphs.


The differences in Transparency of the two larger modes are useful in separating, identification and assessing the quantity of each of these components.

The figure below shows the Filter Function being used to accomplish this, with the result showing that the mode with high Transparency values probably consists of glass beads, good or fused, while the lower mode is probably sand. Using the W/L Aspect Ratios to set a shape filter will also isolate the fused beads from the good ones.

If desired, these filter parameters can now be selected to incorporate them into the SOP, and the application of the different filters will help isolate and quantify each of the populations that make up the sample. Filter F3 gives the quantity of Bad or fused Beads, F4 of Good Beads, F5 of Sand, while F2 gives the quantity of transparent beads, Good or Bad.

Figure 10 gives some segments from the image files generated for each of the components.

Summary

The use of reflective glass beads for highway stripes and signs mandates their compliance with size distribution and shape specifications.

LD is the industry standard for particle size measurement.

DIA is the latest industry technology for aspect ratio, transparency and many other size and shape particle parameters.

The Microtrac Sync combines these two technologies in one powerful instrument to measure the same sample in the same sample cell at the same time, using both technologies. The final report is comprehensive as to size distribution, using up to 30 different size and shape values.

The component sub-populations of the measured sample can be separated and quantified using the Sync post-analysis software.

The ratio of good to fused glass beads, and the proportion of sand particles in the mix, can all be calculated using the same sample, as in this experiment.

This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.

For more information on this source, please visit Microtrac, Inc.

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