PARTICLE DISPERSION DETERMINATION

Abstract: Device for determination of the dispersion of particles in a liquid vehicle in which light is shone on a film of the liquid vehicle and the light reflected from particles projecting through the surface of the film is detected by a video camera which produce an electronic image.

Description: PARTICLE DISPERSION DETERMINATION This invention relates to a method and device for determining the degree of dispersion (otherwise commonly referred to as "fineness of grind") of particles in a particle - vehicle system such as liquid coatings and their intermediates and in particular the degree of dispersion of pigment in paint. Clearly, to obtain optimum properties from a coating, it is desirable that the particles should be of optimum size. Thus in paints the size of pigment particles can affect both the opacity and the glossiness and it is necessary to mill the pigment as it is being dispersed into the vehicle to achieve the optimum size and in particular to break up any agglomerates of particles. However, excessive milling, is commercially undesirable in terms of energy and time and, therefore, a test for the degree to which agglomerates have been broken down is desirable. Such a test is known as a "fineness of grind" test and it gives a measure of the number of oversize particles remaining in the system. The standard test method as has been used for many years is set out in ASTMS D 1210. The basic component of the test is a steel gauge block having a smooth upper surface which contains a path or two parallel paths, each path tapering uniformly in depth lengthways from zero depth at one end to a specified depth, e.g. 50 or 100 μm, at the other end with intermediate calibrations in accordance with the depth at those points. The gauge may also be calibrated in terms of Hegman units, an arbitrary scale that will not be referred to further in this specification. In use, a sample of the system to be tested is placed at the deep end of a path so that it overflows slightly and is then drawn down the path towards the shallow end using a steel scraper blade of specified dimensions. The gauge is then placed between the operator and a light source and observation made of the point where a definite speckled pattern (as distinct from isolated specks ) first appears. This point gives a measure of the fineness of grind. In addition the point at which "stragglers", ie odd particles as distinct from a definite speckled pattern, first appear can be assessed. The sample can then be given a specification based on these two readings. However, this test has obvious drawbacks. Firstly, it is subjective as it depends on a personal assessment by the operator and hence significant differences can arise if the same paint is assessed by different operators. Secondly, the assessment has to be carried out before the film dries to any significant extent (within 10 seconds according to the standard) as the film thickness decreases during drying and operator assessment in such a short time span is difficult. Thirdly, once the test has been completed and the gauge cleaned, no record exits other than the assessment figure. According to the present invention, there is provided a device for assessing the fineness of dispersion of particles in a liquid vehicle, comprising a light source, means for mounting a film of said vehicle of increasing thickness, and means for producing an electronic image of said film, said light source, said mounting means and said image producing means being positioned relative to one another so that said image producing means only receives light reflected from discontinuities in the surface of said film. The image producing means is preferably a video camera. In order to avoid depth of focus problems, the video camera is focussed on a narrow line and the film is moved relative to the camera at a rate dependent on the line scan rate of the camera so the image is built up line by line. Alternatively, the image producing device may be may be an array of photo diodes or charge control devices. In one arrangement, the means for mounting the film comprises a platform, which is movable relative to the image producing means and means for accurately positioning on the platform a fineness of grind gauge block containing a channel of varying depth. The gauge block, may be such as is described in the Standard. Whilst the relative movement could be produced by keeping the film fixed and moving the image producing means, it is preferred that the film is moved. Alternatively, the film and the image producing means could be fixed and a movable optical path, i.e. an arrangement of lenses and/or prisms could be provided. According to another aspect of the invention, a device for assessing the fineness of dispersion of particles in a liquid vehicle comprises a light source, a video line scan camera mounted at an acute angle to the horizontal so that the angle of view is below the angle of illumination, a horizontal platform lying between the light source and the camera and movable relative to the camera, means on said platform for accurately positioning a fineness of grind gauge and means for storing and/or displaying an image generated by the camera. According to a further aspect of the invention, there is provided a method for assessing the fineness of dispersion of particles in a liquid vehicle comprising drawing down a sample of the vehicle to form a film of increasing thickness, illuminating said film with a light source and generating an electronic image of said film. The invention will be more readily understood from the following description taken in conjunction with the accompanying drawings in which: Figure 1 shows a gauge block of the type specified in the standard; Figure 2 is a diagrammatic view of one embodiment of the device according to the invention; Figure 3 shows the speckle pattern produced during a test of a paint by the device shown in Figure 2 and an analysis of the pattern in the form of a bar chart As shown in Figure 2, a line scan video camera 1 is mounted so as to be inclined at an angle of approximately 12 s to the surface of a horizontal platform 2 which is driven by a synchronous electric motor (not shown) so as to be movable longitudinally relative to the camera. A light source 4 is mounted above the other end of the platform at a height such that the camera essentially looks into the 'dark field' below the light source. To ijikimise flicker an incandescent lamp or a biaxial fluorescent lamp driven at high frequency is used. In use, a sample to be tested is drawn down into one of the channels of a standard gauge block as shown in Figure 1 having a maximum depth of 50 μm (available, for example, from Sheen Instruments Ltd of Kingston, England under the designation 501/50) and the block is immediately placed on the platform (which is provided with suitable means for accurately locating the block in position) with the deep end of the channel away from the camera. Activation of the drive motor causes the gauge block to move at a constant speed past a line 6 on which the camera is focussed over a period of circa 5 seconds so an image is built up line by line, any particle projecting through or distorting the surface of the sample film deflecting light into the camera. Scanning in this manner avoids depth of focus problems and allows the camera to be used with a wide aperture, maximising the amount of light collected . The reason for scanning the shallow end of the channel first is to reduce any effect due to the film drying, which will, of course commence at the shallow end. The image may be electronically stored so it can be reproduced and, by using a suitably programmed computer may be analysed to provide speckle and straggler readings and an output in a different form, such as a bar chart. The following example illustrates the invention. Example A paint sample consisting of titanium dioxide pigment in an alkyd resin vehicle was dripped on to a 50μm gauge block and drawn down to form a film of decreasing thickness as specified in ASTMS D 1210. The block was immediately placed in the device shown in Figure 2 and the film was scanned as previously described. The image produced is shown reversed in Figure 3 and computer analysis gave a fineness of grind reading of 10-21. Assessment of the sample by three different operators using the conventional method gave readings of 10-20, 9-20 and 11-22 respectively. Further computer analysis produced the bar chart shown in Figure 3 and as can be seen, this gives an indication of the number of particles protruding through the film at a particular point on the gauge, for example there are 7 particles protruding where the gauge has a depth of 10 to 1 lμm.

Claims: CLAIMS 1. A device for assessing the fineness of dispersion of particles in a liquid vehicle, comprising a light source, means for mounting a film of said vehicle of increasing thickness, and means for producing an electronic image of said film, said light source, said mounting means and said image producing means being positioned relative to one another so that said image producing means only receives light reflected from discontinuities in the surface of said film. 2. A device according to claim 1 in which the image producing means is a video camera. 3. A device according to claim 2 in which the video camera is a line-scan camera. 4. A device according to claim 1 , in which the image producing means is an array of photo-diodes. 5. A device according to claim 1 , in which the image producing means is an array of charge control devices. 6 , A device for assessing the fineness of dispersion of particles in a liquid vehicle comprises a light source, a video line scan camera mounted at an acute angle to the horizontal platform lying between the light source and the camera and movable relative to the camera means on said platform for accurately positioning a fineness of grind gauge and means for storing and/or displaying an image generated by the camera. 7. A method for assessing the fineness of dispersion of particles in a liquid vehicle comprising drawing down a sample of the vehicle to form a film of increasing thickness, illuminating said film with a light source and generating an electronic image of said film.

Inventors: HATFIELD JAMES HENRY, HOWARD PETER BARRY, LAWSON EDMUND JOHN, MACKENZIE ALASTAIR ORR, HATFIELD, JAMES, HENRY, HOWARD, PETER, BARRY, LAWSON, EDMUND, JOHN, MACKENZIE, ALASTAIR, ORR
Applicants: HATFIELD JAMES HENRY, HOWARD PETER BARRY, LAWSON EDMUND JOHN, MACKENZIE ALASTAIR ORR, TIOXIDE GROUP SERVICES LTD, HATFIELD, JAMES, HENRY, HOWARD, PETER, BARRY, LAWSON, EDMUND, JOHN, MACKENZIE, ALASTAIR, ORR, TIOXIDE GROUP SERVICES LIMITED
Application year: 1998
Main classification: G01N15
IPC codes: G01N15