Sizing Pigment Particles with Laser Diffraction
Pigments and paints are widely used across numerous industries. The properties of a given pigment system, such as tinctorial strength or color depth are primarily determined by particle size distribution. Physical factors such as these can play an essential role in final product integrity and quality making the measurement of particle size critical to the performance of a wide variety of pigment applications.
Laser Diffraction Analysis
Laser diffraction is the most commonly used technology for measuring particle size distributions in paint systems. With a typical analysis time of less than one minute, this method is widely used for many process control applications. But laser diffraction poses challenges for sizing submicron materials, such as those found in pigment systems. Small particles below 1 μm pose unique measurement challenges due to weak scattering signals and smooth angular patterns that offer no distinguishable feature in the scattering pattern to determine actual particle size.
However, with a shorter wavelength, the particle size and light wavelength ratio increase - allowing the accurate measurement of smaller particles. Derived from the Mie Theory, PIDS or Polarization Intensity Differential Scattering employs the polarization effects of scattered light . Using PIDS, the intensity vs. scattering angle information from the PIDS signals combines with the intensity vs. scattering angle data to provide a continuous size distribution from submicron to millimeter.
The Problem with Pigments
Pigments present their own set of unique challenges when using laser diffraction for particle size measurement. To accurately size colored pigment samples, you must know the actual refractive index as well as the predictive component. The challenge is not in the real refractive index value but in determining the ‘imaginary’ component. White or transparent materials will show no absorbance where pigments preferentially absorb certain wavelengths. For example, a blue pigment with an absorbance maximum at 640 nm will interact with a helium neon laser at 633 nm to render as a black body. To calculate particle size, this must be considered, especially if the particles are small.
Detect Small Particles
The imaginary component of a pigment can be determined using a UV/Vis spectrophotometer, which measures the relative absorbency of a material per given wavelength. A liquid that dissolves pigment particles into molecules should be used and scattering must be minimized to accurately measure absorption. For colored materials, you’ll need to determine the imaginary component for each wavelength and use selectively to calculate a complete Mie theory optical model for your sample. Additional sources for correlating and confirming your results include photomicrographs from optical or electron microscopes. These can detect small but problematic amounts of oversized materials. A suitable optical model for a pigment is best determined by tracking a milling process over time.
The Question of Shape
One drawback with laser diffraction is that it makes no allowance for the shape of the materials tested. Underlying assumptions used in size distribution calculations assume that all distributions are equivalent spherical distributions of the material being studied. This is often adequate except in cases where the material is long and thin, not rounded.
Summary
If the right approach is taken, enhanced multi-wavelength laser diffraction can be employed successfully to size particulate pigment samples. The refractive index can be determined using different means. It is also worthwhile considering using other techniques to initially corroborate the results obtained.
