TIDAS - Paint Dispersion Assessment System using a Hegman gauge
AN INSTRUMENT FOR ASSESSMENT OF PIGMENT DISPERSIONLabman have now completed their updated TIDAS paint pigment dispersion system. This system is now more compact so is more suited to bench mounting in the laboratory. See: New TIDAS
SUMMARY
The development of a new instrument by Huntsman Tioxide and Labman Automation Ltd allows the degree of dispersion of pigment in a paint film to be determined objectively, with the ability to produce images of the state of dispersion that can be retained for quality assurance or product improvement purposes
INTRODUCTION
Nibs which can be found in a white paint, and which can be the result of undispersed titanium dioxide pigment have little detrimental effect on opacity, but clearly have a major impact on the paint's decorative appearance. Consequently, titanium dioxide manufacturers put a significant amount of effort into ensuring that their pigment disperses easily and is 'nib-free'. Thus, measuring the degree of pigment dispersion during paint and pigment manufacture has long been a standard quality control test. At Huntsman Tioxide, as at most paint manufacturers, dispersion has traditionally been assessed on the commonly used Hegman grind gauge (see Figure 1) in accordance with ISO 1524:2000. While this test does not measure the particle size or size distribution of the pigment, it is a quick and convenient method for highlighting the presence of larger, undispersed agglomerates or nibs
The rating of pigment dispersion has always been recognised as subjective. In order to achieve consistent results, experienced, well-trained operators are required to produce paints, followed by preparation and rating of the Hegman grind gauge drawdowns. Variability of results between operators is an accepted limitation of the test. However, in quality control situations such differences can lead to unnecessary rejection of product and possibly worse, acceptance of material that is actually out of specification.
In Figure 2, an example of operator variability is shown. Five pigment samples have been given to eight different operators (A to H). Each operator has prepared paints to draw down on a Hegman grind gauge for dispersion assessment. The dispersion rating for Pigment Sample 2 shows some of the worst variability, with a rating varying from between 6µm and 21µm.
Figure 2 - Example of operator variability for dispersion assessment
TIDAS - operating principle
To help improve quality control in its factories and product development at its research facilities, Huntsman Tioxide has developed TIDAS (Tioxide Instrumental Dispersion Assessment System), for which a patent on the technology has been filed world-wide.
It was recognised early in the development process that the basic principle of dispersion measurement using a Hegman grind gauge was sound and that the method will continue to be used as the industry standard for the foreseeable future. It was therefore decided that the Hegman grind gauge should remain central to any new technique but that an instrument was required to produce a detailed image of the gauge that could be further enhanced and processed using a computer programme. The subjectivity of an operator Hegman grind gauge rating could thus be eliminated.
The development of TIDAS began in 1995 and proved to be a lengthy process, in particular with regards to achieving a sufficiently high quality image of the Hegman grind gauge and in developing a computer algorithm that could imitate the performance of a standard operator. Much of the development work therefore evolved around defining the term 'standard operator'.
The operating principle of TIDAS is relatively simple, and is shown diagrammatically in Figure 3. The gauge, with freshly applied paint, is viewed by a high-resolution line scan camera angled at 12 degrees to the horizontal. Illumination is such that specular reflected light from the glossy wet paint misses the camera, which only sees light scattered by defects in the surface. Undispersed pigment particles ('nibs' or 'grits') therefore show up as light spots on a dark background. To overcome depth of focus problems, the camera is focused on a narrow strip of paint and the gauge is scanned by moving it through the line of focus. In this way a sharp image of the grind gauge channel and the scale marks is collected in about three seconds.
Figure 3 - TIDAS operating principle
The image is transmitted to the computer where it is displayed on the screen and stored on the hard disk. Image enhancement techniques are applied to increase the contrast between the nibs and the background, removing variations in background intensity that could arise from, say, ripples in the paint. Parameters relating to the dimensions of the specific gauge in use are used to help the computer locate the scale marks and the edges of the measurement channel. The channel is divided into areas corresponding to 1mm steps on the scale, and the number of nibs in each area counted and displayed as a histogram. The instrument displays more nibs than most people could see with the naked eye, however, this is taken into account in deriving a dispersion rating from the grit count.
The process, which takes about 10 seconds in total, is repeated, after which, as long as there is no more than a predefined spread in the results, a mathematical algorithm is used to analyse the results and produce a dispersion rating. During the analysis of the images any abnormal points such as air bubbles or dust are disregarded by the algorithm, thus reducing the risk of rogue results. The ratings correlate well with a standard observer, but unlike most observers will always give the same rating from a given drawdown.
To see the output created by TIDAS, Results and Data Display Click Here.
CONCLUSIONS
TIDAS has now been installed at all the Huntsman Tioxide factories for quality control purposes. This automated system will ensure that pigment produced across the Group’s factories is rated consistently. Photographic images from the factory quality control work will also be retained in accordance with quality assurance procedure. In addition, the instrument is being successfully used to assist in process development projects.
ACKNOWLEDGEMENTS
Labman would like to acknowledge the tireless work of Dave Lancefield, John Lawson, Richard Upton and the Huntsman Tioxide team for their development of TIDAS.
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