Car paint, house paint, thermoses, computer housing, decorative coatings for furniture, plastics for window frames, white and colored paper, lipstick, frosted chocolate drops – these are a number of sample products where finely ground titanium dioxide is used.
Why use titanium dioxide?
What makes this seemingly insignificant white powder of such interest for various branches of industry? Dr. Frank Mersch of KRONOS International has this to say: "Titanium dioxide has the highest refractive index of all white pigments and thus has an extraordinary capacity for dispersing light in the visible part of the light spectrum." The higher the visible light dispersal the better the pigment covers up the surface. This means that less pigment is required for obtaining a sufficient coating. So what does a white pigment have to do with color paints? Mersch remarks, "Because this pigment refracts light so effectively and simultaneously covers the surface involved so excellently, color pigments look much more brilliant." This results in lush, saturated colors. Dr. Frank Mersch is the Head of Microscopy in the Physics group of the Department for Product Control and Quality Management at KRONOS, one of the largest producers of titanium dioxide worldwide. KRONOS produces about 450,000 tons annually at 6 different sites around the world. This amount represents approximately 11% of the global annual market of more than 4 million tons.
The dispersion factor of the pigments does not only depend on the refractive index of the material. It is very much dependent on the size distribution of the particles. The critical dimensions involved here are the mean particle diameter and width distribution. If the mean deviates even just a little from optimal particle size or if distribution is too broad, this may result in a significant loss in dispersion. Methods for determining particle-size distribution are therefore indispensable tools for any pigment producer.
Conventional techniques for measuring particles
KRONOS uses various methods for analyzing particle size. Pigment particles are first distributed finely and evenly in a liquid – usually water. The dispersion of the pigment particles is then examined – using a sedimentation or laser dispersal technique, for example. Mersch: "These techniques usually have the drawback that before actually conducting measurements you have to know a number of things about the particles. If, for example, the density or refractive index is not known, it is not possible to make a physical measurement that makes any sense." Furthermore, in some cases it is not possible to obtain a stable watery dispersion with the particles. This makes it difficult to make a particle size analysis of such pigments, such as those used in plastics that are produced extremely hydrophobically
What a difference – being able to see it right away
Electron microscopy plus digital image analysis offers an alternative here that transcends the above-mentioned limitations. Without having to bother with dispersion, the pigments are immediately shown through the electron microscope and then analyzed by the software. KRONOS’ department for product control and quality management has a FEI Tecnai20 transmission electron microscope (TEM), which produces their results. A MegaView II digital camera is side mounted on the wide-angle port of the electron microscope. To acquire the image, the prism of the camera is moved into the electron beam path of the microscope via pressurized air and intercepts the electrons at the intermediate image plane. The camera transfers the image to the connected PC that has the analySIS software installed, both the camera and image-analysis system are produced by Soft Imaging System. "Evaluating the images by hand is completely out of the question", says Mersch with emphasis. "That would take way too much time and effort." On the other hand however, electron microscopy is not suitable for routine investigations.
The more particles, the merrier...
The most important criteria for successful particle-size determination using digital image analysis is sufficient image contrast, spatial separation of the particles and large numbers of particles – to ensure the resulting statistics are reliable. Via the live image, image contrast can be optimized interactively before acquisition. To improve image quality during and after the acquisition there are numerous functions available, such as shading correction for eliminating uneven illumination. The "Separator" filter function ensures that any adjacent particles within the image are recognized and detected correctly as separate particles. In order to obtain statistically significant results, Frank Mersch has the software evaluate multiple images one after the other and then has the results all put into a single data sheet. Integrated statistical evaluation provides the desired numerical values for the mean particle diameter and standard deviation. Size distribution can be shown graphically. At the end of every particle-size determination a measurement report is generated using the report generator. The acquired images are archived in a database along with the evaluation and the report.
KRONOS has several analySIS, Scandium and iTEM licenses. They use the software for some of their light microscopes as well. These have been 'digitized' via Olympus Soft Imaging Solutions own ColorView 12 or CC-12 digital cameras. The image data of the scanning-electron microscope (SEM) is read out by this software and stored in the database. Frank Mersch is currently working on automating SEM image acquisition in a more advanced way. He uses the mia (multiple image alignment) software extension and the integrated Imaging C programming environment for controlling the microscope and the stage.
Due to the large surface areas of the samples and the ability to directly investigate grain-size distribution – eg, in thin layers of paint – the SEM opens up opportunities for practical applications with pigments. To reveal all facets of the pigments requires using the light microscope, SEM and TEM together. The first thing done with these samples is visually documenting the pigments using the stereo viewer and the ColorView 12. In order to better understand the electron microscopical results, images of the areas of interest are then acquired via the light microscope and the CC-12. This makes it much easier to interpret the SEM images because SEM images show electronic properties of the sample. These SEM images show surfaces or materials on the nano level. TEM images go right through the material, taking a step onto the sub-nano level.