Analysing Pigment Sizing with Laser Diffraction Analysis

Paints and pigments are critical elements of industrial materials. The characteristics of any given paint or pigment are evaluated based on the particle size distribution of the materials. The properties such as color intensity and tinctorial strength are determined by the particle size.

The light absorption characteristics of a pigment increases with the reduction in the diameter of particle. This results in increased surface area such that the particles translucent to the incident light.

Beckman Coulter’s LS™ Series multi-wavelength particle size analyzers employ a complementary scattering technology for tweaking the size distribution of sub-micron particles. This article briefs about the sizing of pigment particles using the Polarization Intensity Differential Scattering (PIDS) system, which is shown in Figure 1.

Beckman Coulter LS 13 320 Particle Size Analyzer with the Aqueous Liquid Module (ALM) and the Auto Prep Station.

Figure 1. Beckman Coulter LS 13 320 Particle Size Analyzer with the Aqueous Liquid Module (ALM) and the Auto Prep Station.

Laser Diffraction Analysis for Pigment Sizing

Particle size distributions of pigment systems have been measured using a number of particle sizing technologies. In the recent times, laser diffraction technology has been increasingly used for the evaluation of particle size distributions owing to its flexibility and short analysis time.

However, standard laser diffraction instruments have failed to provide accurate data on the size range of pigment systems in sub-micron level. This is due to the fact that the scattering of light by small particles is weak and without any detectable maxima and minima in the scattering pattern. Figure 2 shows the weak scattering signal of particles below 1µm.

The weak scattering signal from particles below 1µm in size present difficulties in measuring the maxima or minima at high angles.

Figure 2. The weak scattering signal from particles below 1µm in size present difficulties in measuring the maxima or minima at high angles.

In order to overcome these limitations, a number of manufacturers have adopted various solutions. Beckman Coulter has developed a new method - PIDS to improve sub-micron sizing in standard laser diffraction systems. The method involved using additional wavelengths besides the main diffraction laser source.

PIDS

PIDS that operates based on the transverse nature of light, consists of an electric vector formed at right angles to a magnetic vector. With the illumination of light on a sample at a given wavelength and polarization, a dipole is established due to electric field. The plane of oscillation of electrons in the electric dipole is the same as that of polarization. The oscillating dipole radiates light in all directions, except in the direction of the irradiating light source.

PIDS works on the principle that the sample is illuminated by three wavelengths sequentially, using vertical and horizontal polarized light. The scattered or re-radiated light from the sample is then measured for different angles. The analysis of difference between the horizontal and vertical polarized light for each wavelength provides data on the particle size distribution of the sample.

Extremely accurate results of both spherical and non-spherical sub-micron particle distributions can be achieved using this technique.

Constraints Involved in Measurement of Pigment Size Distributions

Most of the samples measured using the commercially available instruments are not colored, which make the analysis simple. Both real refractive index of the material and its imaginary component need to be determined to calculate an accurate particle size.

Pigments, however, absorb certain wavelengths preferentially, and this property must be taken into account while calculating particle size distribution to avoid significant errors. The real refractive index serves as a key function that can be estimated from known constants.

Determination of Imaginary Component

The imaginary component of a pigment is determined using a UV/V’s spectrophotometer. Two things should be considered while determining the imaginary component. Firstly, it is necessary to ensure that no large particles are present in the spectrophotometer sample cell.

Secondly, the relative amount of absorption must be considered while calculating the optical model. Beer's Law can be employed for evaluating the allowance of the imaginary component, for each complementary wavelength.

Each manufacturer follows different techniques to calculate optical models. Beckman Coulter enables the users to calculate a complete Mie theory optical model for a given sample. It is possible to create the optical model within three seconds using the latest version of software.

Complementary Information for Pigment Analysis

While analyzing pigments, correlating other sources of information for an initial verification of the given optical model proves to be beneficial. Photomicrographs are suitable for correlating information. These can be images from simple microscopes to electron microscopes.

However, Beckman Coulter determines the applicability of the optical models for any given pigment by tracking the milling process over time. One of the major limitations of the laser-based particle sizing devices is that no allowance for the shape of the materials will be made irrespective of the particle size. The mathematical models used for calculating size distributions are based on the light scattering by a sphere.

The size of particles in milled pigments, however, will not be spherical. To overcome this issue, researchers from the University of Utrecht developed a variety of mono-dispersed, non-spherical materials having an aspect ration of 3:1. Figure 3 shows the results obtained from the analysis of sub-micron hematite spindles (spheroids).

The photomicrograph SEM shown is a representative sample of hematite spindles analyzed.

Figure 3. The photomicrograph SEM shown is a representative sample of hematite spindles analyzed.

LS™ Series Particle Size Distribution for Hematite Spindles

Based on the results obtained from the UV/V’s spectroscopic ellipsometry data for the real and imaginary components, the optical characteristics of the hematite spindles have been evaluated. Hematite being a colored material imitates a pigmented material. The results from the LS Series Analyzer showed that the mean size is 78nm, which is a function of all the possible orientations of the particles while propagating via the illuminated beam.

Conclusion

Enhanced multi-frequency laser diffraction can be applied effectively for determining the sizing of particulate pigment systems if a suitable method is employed. Steps are taken to calculate the imaginary component of the optical model for laser diffraction particle size analyzer. It is also beneficial to take into account other techniques to corroborate the results obtained.

This information has been sourced, reviewed and adapted from materials provided by Beckman Coulter, Inc. - Particle Characterization.

For more information on this source, please visit Beckman Coulter, Inc. - Particle Size Characterization.

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