Method and apparatus for the measurement of colour properties of paint

Abstract: Measurement of a colour property of wet paint, such as lightness by colorimetry or photospectrometry, is carried out on a sample volume contained in a vessel (1) and while the sample volume is subjected to shearing forces and advantageously also turbulence. The shearing and turbulence are created by a rotating disc (5) having a spiral rib configuration (7) on one surface which faces a window (4) through which the measurements are made by a spectrophotometer (8).

Description: This invention relates to a method of and apparatus for measurement of one or more colour properties (e.g. lightness) of paint. Colour measurement of paint is usually carried out by taking an aliquot of paint, applying it as a coating to a test area, allowing it to dry and then measuring one or more colour properties of the dried coating using a spectrophotometer or a colorimeter. Colour measurement by this method is very time consuming because of the paint drying time. However, other problems arise in that some paints are difficult to apply repeatedly and special precautions are required to be taken by operators when applying certain paints. The aim within the industry for some time has been to measure the colour properties of paint in a wet state and in a way which predicts the colour of the paint when applied and dried. The theoretical benefits are mainly associated with time saving although some are associated with the increased likelihood of an automated paint plant. However simply causing a sample of wet paint to flow through a measuring cell as disclosed in EP-A-094218 or spreading it for example by a lamina flow over a plate as disclosed in DE-A-2525701 and measuring its colour properties produces inconsistent results even if when flowing through the cell or prior to flowing over the plate the sample is under shear We have discovered that this lack of consistency arises because of the instability of wet paint to shear. This results in part from the fact that in the wet sample, pigments tend to be flocculated and tend to deflocculate on application of shearing forces. An object of the invention is to provide a method and apparatus for colour measurement of wet paint that produces acceptably consistent results. According to the invention there is provided a method of measuring a colour property of paint comprising supplying a sample volume of liquid paint to a vessel and measuring the colour property of the sample volume while it is under shear, characterised in that shearing forces are applied to the sample volume by means within the vessel, which means simultaneously create turbulence in the sample and the colour property of the sample is measured during the application of shearing forces by said means and/or within a predetermined time after the cessation of the application of shearing forces by said means and while some effect of applying shearing forces is still present Advantageously the shearing forces applied produce a shear rate in the range 10⁶-10¹⁰ reciprocal seconds and preferably about 10⁸ reciprocal seconds. Deriving from this method the invention also provides a colour measurement apparatus comprising a vessel for holding a sample volume of paint, a device within the vessel for causing the sample volume to be subjected simultaneiously to shearing forces and turbulence creating forces and measuring means arranged to measure a colour property of the sample volume in the vessel. The means for measuring a colour property within the vessel may comprise a window in the holder through which colour can be measured by, for example, a spectrophotometer, or one or more optical fibres extending from within the vessel to a spectrophotometer. In one embodiment of the invention, a volume of a paint sample is sheared at a predetermined speed for a predetermined length of time and while shearing is still in progress the selected colour property of the paint is measured using a spectrophotometer or a colorimeter. Experimental results obtained in this manner give both good repeatability and consistent correspondence with measurements of dry paint. In another embodiment of the invention, a colour property of a volume of paint is measured immediately before, and at frequent intervals during shearing and at frequent intervals after shearing has ceased. This enables the shear stability of the measured colour property to be characterised so that the techniques used to disperse and stabilise pigments in paint can be optimised. In a still further embodiment of the invention one or more colourants are added to a volume sample of paint while the paint is being sheared and until the colour measurements agree with predetermined specified ones, i.e. have a desired value. In one form of apparatus according to the invention the shearing device comprises a rotatable disc disposed in the vessel (preferably with its plane substantially parallel to the window and) the disc being formed with protrusions, for example a spiral rib configuration milled or otherwise formed on one surface. In the case where the measurements of colour property are made through a window in the vessel the disc is preferably arranged with its plane parallel to the window and with the protusions formed preferably on that surface facing the window. It has been found that with a spiral configuration in addition to its shearing function the disc also creates a turbulent flow to cause movement of the paint over the glass window so that the layer of paint against the glass does not form a non-sheared lamina. The invention will now be further described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a first apparatus in accordance with the invention, Figure 2 is a plan view of one surface of the shearing device used in the apparatus of Figure 1, Figure 3 shows a graph of measured colour property against time. Figure 4 is a schematic view of an apparatus in which colorants can be padded during shearing, Figures 5a-5e show graphs explaining the method of using the apparatus of Figure 4. Referring now to Figures 1 and 2 of the drawings, the apparatus comprises a vessel in the form of a paint cell 1 having an annular side wall 2 and a bottom 3 in which is fitted a glass window 4 providing a viewing aperture. Disposed within the cell 1 is a paint shearing device in the form of a disk 5 which is connected for rotation to an electric motor 6. The disk 5 is disposed with its plane substantially parallel to the window 4 and on its face adjacent the window has a spiral groove milled into it as shown in Figure 2 to provide protrusions in the form of a downwardly projecting spiral rib 7. A spectrophotometer 8 is positioned so that it makes colour measurements through the window 4 by reflectance. In the use of the apparatus a sample volume of paint is supplied to the cell and the device 5 set in rotation to produce the desired shear rate of about 10⁸ reciprocal seconds in the sample. Factors affecting the operation of the apparatus are the configuration of the disk 5, its speed of rotation, the gap between it and the glass window 4 and the time for which the paint is sheared. A typical set of factors is: 1. spiral configuration as shown 2. gap 0.19 - 0.6 mm. 3. time of shear 5 mins. These factors can be varied providing a number of constraints related to the properties of the paint are observed. It has been found that in making colour measurements using this apparatus, there is a substantially straight line correlation between the colour properties of the wet and dry paint, the slope of this straight line being determined by the amount of shearing applied. Ideally the slope is 45° so that a change in for example lightness of the wet paint corresponds to an equal change in lightness of the dry paint. Thus in colour matching bulk volumes of paint to produce a predetermined dry paint coating consistently, the necessary colorant additions to the basic bulk mix to correct for any colour deficiencies can be readily calculated from the colour measurements made. Referring now to Figure 3 this is a graph showing how the sample volume varies in lightness L with Time as shearing is taking place. For consistency between one sample and another a reading of lightness would be taken on the stable plateau say after about 150 secs. Figure 3 also illustrates how the apparatus of Figure 1 or similar apparatus described hereafter can be used to determine other properties of the paint which affect its colour and are therefore embraced herein by the term "colour properties". The line FG represents a perfectly stable paint which is unaffected by shear. The line passing through points O,A,B,C,D and E represents the variation in lightness with time of a shear unstable paint during a cycle of operation of the apparatus of Figure 1. Shearing is indicated as starting at t=o and ceasing when t=480 secs. The various portions of the line can be interpreted as follows: OA Flooding - pigment moves to glass or air interface AB Remixing - flooding removed as a result of shear being started BC Deflocculation - absorbing pigments deflocculating due to shear thus increased absorption of light thus L value decreases CD Flooding - pigment moves to air or glass interface on cessation of shear DE Gradual reflocculation Thus the effect of adding dispersion and stabilising agents to the paint can be observed. In Figure 3 the Y axis represents lightness, but it could, for reflectance at a single wavelength, represent any of the following: Referring now to Figure 4 this shows basically the same apparatus as in Figure 1, and the same reference numerals have been used to designate corresponding parts. Additionally however a plurality of colorant dispensing valves 12 with nozzles 13 are shown for making colorant additions to the paint in the cell 1. This dispensing system can advantageously, as far as space saving is concerned, take the form described in UK Patent Application 8705482. As described in that Patent Application, a dispensing system comprises a circulating path for the paint to and from a reservoir, means for pressurising the paint to cause it to circulate around the path, a first valve means operable to divert the paint from the circulating path to a second valve means which is biased to a closed position and is openable by the hydraulic pressure of the paint to cause the liquid to be dispensed therefrom through a nozzle in the second valve means into the vessel 1. One such dispensing system is provided for each basic colour. The first and second valve means need not be positioned adjacent each other but could be interconnected by a liquid feed line. The second valve means, i.e. the valves 12,13 can be designed as relatively slim valves and do not have to carry bulky acutating means. Consequently, a relatively large number of them can be disposed close together in an inverted conical form for feeding paint into the cell 1 with the nozzles directed towards the apex of the cone. The addition of the colorants may be done in response to a request from a computer as a result of the colour measurements made by the spectrophotometer 8. Figures 5a and 5b represents the volumes V A and V B of the added colorants A and B with time while the paint is being sheared and Figures 5C, 5D and 5E represent the corresponding changes in colour properties resulting from the colorant additions.

Claims: A method of measuring a colour property of paint comprising supplying a sample volume of liquid paint to a vessel and measuring the colour property of the sample volume while it is under shear, characterised in that shearing forces are applied to the sample volume by means within the vessel, which means simultaneously create turbulence in the sample and the colour property of the sample is measured during the application of shearing forces by said means and/or within a predetermined time after the cessation of the application of shearing forces by said means and while some effect of applying shearing forces is still present. A method according to claim 1 characterised in that the shearing forces applied produce a shear rate in the range 10⁶-10¹⁰ reciprocal seconds. A method according to claim 2, characterised in that the shearing force applied to the wet paint produces a shear rate of about 10⁸ reciprocal seconds. A method according to any one of claims 1 to 3 characterised in that the measurement of the colour property is made a predetermined time after the initiation of the shearing force and before the cessation thereof. A method according to any one of claims 1 to 3, characterised in that a series of measurements of the colour property are made both during the application of the shearing force and after the cessation of the application of the shearing force. A method according to claim 5, characterised in that at least one measurement is also made prior to the initiation of the shearing force. A method according to any one of claims 1 to 4 wherein a colorant is added to the sample volume during shearing until the colour measurements made have a desired value. Apparatus for carrying out the method of any of the preceding claims, comprising a vessel (1) for holding a sample volume of paint, a device (5) within the vessel for causing the sample volume to be subjected simultaneously to shearing forces and turbulence creating forces and measuring means (4,8) arranged to measure a colour property of the sample volume in the vessel. Apparatus according to claim 8, characterised in that said device (5) comprises a rotatable disc disposed in said vessel and has protrusions (7) on one surface thereof whereby in addition to causing shearing said device (5) also creates a turbulent flow in the sample volume. Apparatus according to claim 9, characterised in that said protrusions (7) are provided by a spiral rib configuration on said one surface. Apparatus according to claim 9 or 10, characterised in that the measuring means includes a window (4) in the vessel (1) through which the colour property can be measured and said rotatable disc (5) is disposed with its plane substantially parallel to said window (4) and with said protrusions (7) on the surface of the disc (5) facing the window (4). Apparatus according to any one of claims 8 to 11 and including means (12,13) for feeding colourant into said vessel.

Inventors: COLCLOUGH MICHAEL LEONARD, SHEPHERD ELIZABETH ZOE, TAYLOR CELIA CHARLOTTE, COLCLOUGH, MICHAEL LEONARD, SHEPHERD, ELIZABETH ZOE, TAYLOR, CELIA CHARLOTTE
Applicants: ICI PLC, IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1988
Main classification: G01J3
IPC codes: G01J3, G01N33