TIDAS Paint Dispersion Assessment System using a Hegman gauge
Developed by Huntsman Tioxide and Labman Automation Ltd the TIDAS unit allows the degree of dispersion of pigment in a paint film to be determined objectively. The unit has the ability to electronically capture images of the state of dispersion for quality assurance or product improvement purposes.
Sold worldwide and recently upgraded; the new TIDAS features:
- Low Voltage Components
- New Ethernet Interface Connectivity
- Windows XP/Vista/7 32bit/64bit compatible
- LED array camera with Freshnel lens
- Reduced footprint
- Automated draw-down option with definable speeds
- Fast and consistent analysis (15 Seconds)
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.
Determining the fineness of a paint’s grind is important, because too coarse a grind may reduce the paint’s colour uniformity, gloss, and opacity. The Hegman test is widely used for this purpose because it requires minimal skill and only a few seconds work
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 6um and 21um.
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.
TIDAS – Results & Data Display
One of the strengths of the TIDAS system is the manner in which it not only collects the results but more importantly displays them for the end user examples of this can be seen below.
Figure 4 shows an example of results achieved on TIDAS. Here a paint of average dispersion is shown. On the left-hand side, the enhanced image of the grind gauge is displayed, with the nibs showing up as white speckles. The histogram to the right is derived from this image, with the dispersion rating and other details displayed beside it.
Figure 4 – Data output from TIDAS
The histogram displays the nib count from each 1mm band of the image as blue bars. However, if more than 50 nibs are counted in one band, the result will be a continuous red bar to warn that the counts are ‘off scale’. If very large nibs are encountered in a band, it will be deemed that these are not true nibs, but are more likely to be air bubbles or dust, and the bars will be displayed in green for each large nib found. The algorithm will ignore any of these oversize particles in the calculation of the dispersion rating. The example in Figure 4 shows one of the two drawdowns that were recorded to produce individual ratings of 16um and 17um, and an average rating of 17mm (quoted to the nearest whole number). The reference number and rating for each drawdown, the average rating, and the time and date of recording are displayed to the right of the histogram.
It can also be seen from this example that the algorithm requires a certain number of nibs within adjoining micron bands to determine a dispersion rating and not the first isolated particle.
The mathematical algorithm, which is based on the visual dispersion rating technique of four experienced Huntsman Tioxide technicians, enables TIDAS to give consistent results in good agreement with manual ratings of an experienced, well-trained operator as shown in Figure 5.
Figure 5 – TIDAS correlation with one experienced operator
The image-capture system enables an electronic record of each dispersion to be kept. Such a system is invaluable for quality assurance records, production quality comparisons, and helping with product and process improvement work. Images can also be sent instantaneously to different locations using the electronic mail system.
TIDAS – Grind Gauge Requirements
Though visually similar to commercially available standard gauges, each gauge supplied with TIDAS is manufactured so that the grooves and scale markings on the gauge are positioned to a tighter specification than normal. A calibration block is also provided to ensure the instrument is performing to specification.
The 0-50um grind gauge is used for general testing on TIDAS as this range covers the normal spread of results experienced for good to poor levels of dispersion. However, there are occasions when undispersed particles can exceed, or be close to 50um. In these instances, TIDAS can be used with a 0-100um gauge to allow imaging, and a rating to be obtained. Typically, water based paints or paints containing large particle extenders would be tested on the deeper channel gauge. Conversely, paints showing excellent levels of pigment dispersion may require a more discriminating examination of the nibs present in the paint; TIDAS can therefore also be used with a 0-25um gauge.
The following images produced from the same paint sample when examined on each of the three ranges of grind gauge. Agreement between the 0-25 and 0-50um gauges is good (both give a rating of 16um). The 0-100um gauge is under-rating the dispersion, clearly demonstrating the need to use the most suitable gauge for the level of grind of the paint.
TIDAS – Additional Data Output
Further information can be obtained from TIDAS by downloading the nib count into a spreadsheet or graphics programme, enabling additional data analysis to be undertaken. Figure 7 shows an example of data from TIDAS that has been downloaded for comparative analysis. The nib count is shown for each 1mm dispersion band from 0mm to 50mm for four different pigments. This way of presenting the results allows the dispersion performance for multiple pigments to be compared on the same graph.
Figure 7 – TIDAS dispersion data plotted from spreadsheet
A number count taken from the spreadsheet of the 8-50?m range on the gauge, i.e. the area of perhaps most interest to the paint manufacturer, gives the following results for each of the four pigments:
Pigment 1 (rated 10 um) – 25 nibs
Pigment 2 (rated 16 um) – 118 nibs
Pigment 3 (rated 26 um) – 355 nibs
Pigment 4 (rated 34 um) – 491 nibs
In other words, different types of information can be supplied by TIDAS depending on the requirements of the user.
As with any instrumental analysis the results obtained are dependent on the correct paint preparation. Using TIDAS will not guarantee correct or consistent readings unless the correct paint preparation and application techniques are used and a clean working environment is maintained
For more information please email email@example.com
Labman would like to thank and acknowledge the following people for their tireless work during the development of TIDAS:
- James Hatfield (Target Technical)
- John Lawson (Huntsman)
- Richard Upton (Huntsman)