Shape and Size Analysis for Quality Control of Additive to Particulate Systems

The mineral Wollastonite (CaSiO3) possesses a host of unique properties which have promoted its use as a filler or additive to other products, to improve their attributes. It is the product of calcite reacted with silica (producing the byproduct carbon dioxide). The products it is used for include plastics, ceramics, construction products, paints and metallurgical processes.

This mineral is extremely useful in the ceramics industry because of its shape, which is acicular, its whiteness, and the way it improves the flow of the particles. These properties reduce shrinkage during the manufacturing process, enhance the brightness of the product after firing is complete, and boost the green and fired strength of the end product. When used in paints, it makes them last longer, while improving suspension properties and flatting.

Plastics acquire greater tensile strength, with a lower resin content, and better stability to heat and dimensional stress. The acicular nature of Wollastonite gives it attributes comparable to glass or fiber among synthetic products, and to kaolin, mica, barite and gypsum among natural non-fiber substances used in these processes. When used as a filler, smaller Wollastonite particles produce more reinforcement strength, as well as a decreased width/length (W/L) aspect ratio.

The acicular shape of the crystals gives Wollastonite a unique W/L aspect ratio which is unlike other particle systems. Thus, this mineral is detected and measured rather easily by looking for its morphological characteristics in the sample mix, using Dynamic Image Analysis (DIA).

The exact quantification of the material ensures that this additive or filler can be added in specific proportions to a given mix, to ensure the optimal improvement in the final product properties, and this proportion can be confirmed for quality control inspections. This article describes an innovative machine from Microtrac which uses dual techniques to obtain data on shape and size distribution of Wollastonite added in a mix.

Laser Diffraction (LD)

LD technology was introduced in the seventies by Microtrac. Its use for particle size analysis has now become ubiquitous in the industry, both at the supplier end and the user end. This is because it is fast, simple and reliable. This is now the standard quality control method in particle systems, reporting comprehensive particle size distribution data in Equivalent Spherical Diameter (Da) or volume percentage.

Dynamic Image Analysis (DIA)

DIA made its commercial entry into particle characterization applications in the 1980s. As its technological core advanced, including computing speeds, increased computer memory, digital camera speed and resolution, optical lenses and strobe lighting with increased brightness and speed, accompanied by rapid enhancements of the software which analyzes the measurement data, it became an efficient tool to measure the attributes of particles in at least 30 sizes and shapes. It has been adopted in a host of particle systems

The latest instrument combines LD and DIA to provide simultaneous measurements of the same sample passing through the same sample cell in a single analyzer.

Quality Control Laser Diffraction Size Distribution Data

Figure 1 shows a Wollastonite sample which has been analyzed using LD and DIA. The resulting report is shown in Figure 2, including the columns Summary Data, Size Percent, Percentiles, Peaks Summary, Tabular Data, and Distribution Graph on the left, and the measurement conditions on the right. The graph shows a log-normal plot by volume % with an Equivalent Spherical Diameter (ESD) at mean volume of 52 microns. The result is an almost Gaussian or near-normal distribution.

Quantifying Wollastonite as Additive With DIA Size/Shape Data

The use of Wollastonite as a filler or additive to improve material properties in various particle systems mandates the use of precise proportions. This can be assured taking advantage of its acicular shape, which is unlike the general particle shape of the systems it is incorporated into. The W/L Aspect Ratio of the Wollastonite particles is reported by DIA, and is capable of achieving a precise identification and quantitative assessment of this mineral in the particle mix.

Figure 3 shows an image file, part of a file of 47000 images which is available for analysis. It can be scrolled through, and any image can be viewed, magnified and either ascending or descending order selected for sorting them, according to any size or shape parameter.  It can even be converted into a spreadsheet which details size and shape for each of the particles. To illustrate the utility of the image file, the image enclosed in a blue line was selected from this file because it fitted the size and shape specifications listed on the right of the image.

DIA is a tool which measures each particle using all these values, so that the final frequency distribution of number and volume has infinite resolution.

Figure 4 shows the X-Y Distribution Graph and Tabular List to illustrate how the measurement results are reported. It is possible to select up to six separate sizes or shapes to be displayed using differential or cumulative percentage, as the number or volume or shape, or as linear or logarithmic formats.

The solid curves represent (from the right to the left) the cumulative percentage of finer particles in the form of Feret length, area equivalent diameter and Feret width. The blue triangle shows the W/L Aspect Ratio or differential shape for each sub distribution of size shown on the left Y axis.

In short, the acicular shape ensures that the W/L aspect ratio can be used to identify, separate and measure the amount of the mineral in the mix, in order to detect batches with off-specified amounts of Wollastonite. This is important as excessive additive amounts could push up the cost and impede free flow, while too little may defeat the purpose of its use.

The image file will help to identify images with increased acicular rather than block-shaped images. These images enable a reasonable estimate to be made of the W/L value that can help quantify the proportion of Wollastonite in the mix.

Two size segments showing different Wollastonite FWidths are shown in Figure 6. Particle size is not valuable in particle sorting, identification and quantification when the particles have varying shapes, because of the strong possibility that particles can have the same size but different shapes. The View Particles Search enables particles with W/L Aspect Ratio below 0.400 to be captured, since acicular shapes are predominant in the image file around this point. Figure 7 shows segments taken from the file, sorted according to this aspect ratio.

The result of a View Particles search targeting images with a W/L Aspect Ratio below 0.4 is shown in Figure 8. This set of images comprises 13% of the sample by number or count percentage. The availability of the whole image file ensured that the W/L value could be used to separate and measure the Wollastonite crystals alone from the plastic resin base which has low resin content, but high stability to heat and dimensional stress.

Such an analysis can be made use of to apply quality control measures to ensure the presence of the right quantity or proportion of Wollastonite additive in mixes for optimal sample properties.

Summary

The use of additives in ceramics, paints and plastics improves their properties. Wollastonite is one such additive which increases the whiteness, brightness, duration of use, strength, flatting and stability in suspension or dimensional stability of the final product.

The use of morphological parameters to analyze the sample mix is useful in quality control by enabling the measurement of the amount or proportion of Wollastonite in the powder, using the acicular shape as the identifier. This helps to make sure the right amount has been added.

The use of both particle size and shape as measurement parameters in the same automated instrument at the same time on the same sample results returns fast results for quality control checks to ensure the mix meets the specifications for additive proportion.

The Microtrac Sync uses LD and DIA to measure size and shape of the particles at the same time, in the stream using 30 different parameters, within a few minutes, and using a single sample.

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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