Digital Imaging Solutions

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Whiter than white with lush colorsDigital particle-size analysis of TiO2 white pigments

Dr. Peter Stallknecht, Olympus Soft Imaging Solutions GmBH

Car paint, house paint, thermoses, computer housing, de-corative 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 pig-ments 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 pig-ment have to do with color paints? Mersch remarks, „Be-cause this pigment refracts light so effectively and simul-taneously covers the surface involved so excellently, color pigments look much more brilliant.“ This results in lush, sa-turated 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 produ-ces 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.

Particle-size distribution The dispersion factor of the pigments does not only depend on the refractive index of the material. It is very much de-pendent 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 indispen-sable tools for any pigment producer.

APPlICATION RePORT: DIgITAl PARTICle-SIze ANAlySIS OF TIO2 WHITe PIgMeNTS

Whiter than white with lush colorsTitanium dioxide powder is the best selling white pigment for paints, lacquers, plas-tics, paper and cosmetics. One decisive factor indicating the quality of the product is the size distribution of the powder particles. Alongside various particle-size analysis methods, the titanium dioxide producer KRONOS Titan uses electron microscopes and Olympus Soft Imaging Solutions image analysis system to determine size distribution. The integrated automatic particle analysis feature takes care of evaluating the images

What do these products all have in common? All of them require tita-nium dioxide powder. It gives them a shiny surface and brilliant colors. Titanium dioxide is the most important white pigment in the paint and plastics industry. This is due to its refractive index for light. It is higher than that of diamonds and any other known materials.

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 partic-les is then examined – using a sedimentation or laser dis-persal 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 measure-ment 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

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pigments are immediately shown through the electron mic-roscope and then analyzed by the software. KRONOS’ de-partment for product control and quality management has a FeI Tecnai20 transmission electron microscope (TeM), which produces their results. A MegaView II digital came-ra 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 interme-diate image plane. The camera transfers the image to the connected PC that has the iTeM software installed, both the camera and image-analysis system are produced by Olympus Soft Imaging Solutions. „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.on the respective methane pressure. At a pressure of about 37.5 kPa, a layer of minimum grain boundary density is formed. There is a correlation between this minimum grain boundary density and a maximum degree of layer crystallization. This is one example of how digital image analysis can help clarify and optimize deposition parameters.

How well the titanium dioxide powder fulfills its task as a white pigment depends on the size distribution of the particles. What is the mean size of the particles? How many particles are how much larger or smaller than the mean size? These are questions quality control has to answer. Electron microscopical investigations using digital image analysis offer a number of advantages in contrast with standard conventional methods.

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 opti-mized interactively before acquisition. To improve image quality during and after the acquisition there are numerous functions available, such as shading correction for elimi-nating 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 stan-dard deviation. Size distribution can be shown graphically. At the end of every particle-size determination a measure-ment report is generated using the report generator. The acquired images are archived in a database along with the evaluation and the report.

How large are the titanium dioxide particles? The particles are shown electron microscopically (image segment a) and automatically detected, measured and classified according to size by the iTEM software. The software places the measurement data into a sheet, evaluates it statis-tically and makes a diagram of the results. The bar graph (b) shows the resulting size class distribution.

APPlICATION RePORT: DIgITAl PARTICle-SIze ANAlySIS OF TIO2 WHITe PIgMeNTS

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APPlICATION RePORT: DIgITAl PARTICle-SIze ANAlySIS OF TIO2 WHITe PIgMeNTS

Digitized microscopes KRONOS has several analySIS licenses. They use this 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 Scandium, the SeM software by the same company 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 envi-ronment 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 lay-ers of paint – the SeM opens up opportunities for practical applications with pigments. To reveal all facets of the pig-ments 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 becau-se 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.

Titanium oxide hydrate, a preliminary stage of the titanium dioxide pig-ment, is also measured using image analysis at KRONOS. The titanium oxide hydrate particles can be labeled in respective colors according to their size in the electron microscope image. The software draws the colors on the image overlay, which is like a digital transparency covering the image.

ACKNOWleDgemeNTS

Text written in close collaboration with:

Dr. Frank Mersch, Product Control and Quality Manage-ment, Physics – Microscopy, KRONOS INTERNATIONAL Inc., Leverkusen, Germany

Image source: KRONOS INTERNATIONAL Inc. Electron microscope images acquired using a MegaView II mounted on a Tecnai20 TEM.

All rights including translation are reserved. Any and all forms of reproduction must be authorized by Olympus Soft Imaging Solutions. We are unable to assume any liability for unrequested consignments received.

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