METHOD AND DEVICE FOR MEASURING THE AMOUNT OF COATING ON A MOVING SUBSTRATE

Abstract: The invention relates to a method and a device for measuring the amount of coating on a moving substrate. Reflection measurement is used in the invention for measuring the properties of a coating (4a). The amount of calcium carbonate in the coating is measured by measuring an absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers and/or the amount of kaolin is measured by measuring an absorption peak of kaolin at a wavelength of about 2.7 micrometers.

Description: METHOD AND DEVICE FOR MEASURING THE AMOUNT OF COATING ON A MOVING SUBSTRATE The invention relates to a method for measuring the amount of coating on a moving substrate, the method comprising measuring the amount of at least one component in a coating by reflection measurement. The invention also relates to a device for measuring the amount of coating on a moving substrate, the device comprising a radiation source for producing a light beam, a detector for measuring the reflected light beam and means for processing a signal of the detector, the device being arranged to measure the amount of at least one component in a coating. In a manufacturing process it is important to monitor the amount of coating on a moving substrate, such as paper or cardboard web. Coating improves the printing quality of paper, and the amount of coating should, if pos- sible, be kept constant in one paper grade. The coating materials consist of binders and coating pigments. Most common coating pigments used in coating are kaolin, calcium carbonate and titanium dioxide. During a paper manufacturing process the amount of coating is in general continuously measured by measuring devices moving in cross direction of the paper perpendicularly over the paper web as the web moves forward. US patent 5 455 422 describes a method in which the amount of coating is measured by measuring, for example, the absorption peak of latex at a wavelength of 2.30 micrometers and the absorption peak of clay at a wavelength of 2.21 micrometers. Said patent further describes the measure- ment of calcium carbonate by measuring the amount of backscattering at a wavelength of 2.09 micrometers. However, for measuring the amount of calcium carbonate said method is unreliable and inaccurate, since the method is based on a weak cellulose absorption coverage effect caused by the coating and dependent on multiple factors and not on the absorption caused by car- bonate. The amount of calcium carbonate could also be determined, for example, on the basis of kaolin measurement assuming that the ratio between the amounts of kaolin and calcium carbonate in a coating is constant. However, this is not always the case, and problems are created particularly if the kaolin content is low i.e. below approximately 20% and the carbonate content correspondingly high i.e. approximately 80%. EP publication 0 332 018 shows a method in which the amount of kaolin in paper is measured by transmission measurement, for example, at wavelengths of about 1.4 and 2.2 micrometers. However, by transmission measurement the portion of coating in the measurement result is very difficult to determine. Furthermore, the portion of calcium carbonate has to be ap- proximated as shown in the previous chapter. GB publication 2 127 541 shows how transmission measurement is used for measuring the amount of additives in paper. The publication describes how the amount of calcium carbonate is measured by measuring the absorption peaks at wavelengths of 11.54 micrometers and 11.77 microme- ters. The amount of coating cannot be measured by said method, since the fillers in base paper are included in the results. Furthermore, the absorption of paper can be so high that measurement through paper is not possible. Moreover, in its entirety, the accuracy of the measurement results is not good enough. It is an object of the present invention to provide a method in which the above drawbacks can be avoided. The method of the invention is characterized by measuring the amount of calcium carbonate by measuring an absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers. The device of the invention is further characterized by being arranged to measure the amount of calcium carbonate from the coating by measuring an absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers. The essential idea of the invention is to measure the components of the coating by reflection measurement so as to measure the amount of calcium carbonate by measuring the absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers and/or the amount of kaolin by measuring the absorption peak of kaolin at a wavelength of about 2.7 micrometers. The invention has the advantage that the amount of calcium car- bonate and kaolin can be measured accurately and reliably, simultaneously in the same measurement or separately in different measurements. It is also possible to measure the total amount of calcium carbonate and kaolin from several applications on top of one another. The selectivity of the measurement for both calcium carbonate and kaolin is very good. In the following, the invention will be described in greater detail in the accompanying drawings, in which 3 Figure 1 is a schematic view showing a measuring device of the invention, Figure 2 shows spectra of base paper and paper coated with calcium carbonate, and Figure 3 shows spectra of base paper and paper coated with kaolin. Figure 1 shows a measuring arrangement where radiation reflected from an object is measured, i.e. the radiation source and the receiver are on the same side of the object to be measured. Said measurement is referred to as reflection measurement. Figure 1 shows a measuring device comprising a radiation source 1 producing a light beam 2. The radiation source 1 may be, for example, a halogen lamp or another suitable radiation source for producing an infrared beam. The light beam 2 is directed through a filter 3. The filter 3 filters the light so that only the light that is essential for the measurement and that is at the right wavelength band enters the measurement point. The filter 3 may be, for example, a rotating filter disc including several interference filters or another filter solution known per se. The structure of the filter 3 is as such known for those skilled in the art, and is therefore not explained in greater detail in this context. After the filter 3 the light beam 2 is directed through a window 5 to a paper or cardboard web 4 moving in the paper machine. The window 5 can be made of quartz glass or sapphire, for example. The paper or cardboard web 4 moves in the direction of arrow A. A coating 4a is arranged on the surface of the paper or cardboard web 4. Instead of the moving paper or cardboard web 4, the moving substrate, the coatings of which are measured, may also be, for ex- ample, a roll of a paper coating machine, a roll of a paper machine and/or generally a surface of a metal plate. The device also comprises a reference sample 6 which is moved at given intervals to the measurement point as indicated by arrow B. The sample 6 operates as a reflection reference and the measurement result obtained therefrom provides a picture of the condition of the light source 1 , the detector 7 and the window 5. In addition, reference measurement can, if desired, be used to correct the actual measurement result. The reflected light beam 2 is directed to a detector 7. From the detector 7 the signals are directed through a preamplifier 8 to a computer 9 for processing the measured data in a manner known per se. For the sake of clarity, Figure 1 does not show the optics possibly needed to direct the light 4 beam 2. The structure used for conducting/guiding the light may be, for example, visualizing optics, an optical fiber or a bundle of optical fibers. In Figure 2, curve C shows a reflection spectrum of base paper and curve D, indicated by a dashed line, shows the reflection spectrum of paper coated with calcium carbonate. The wavelength λ in micrometers is on the horizontal axis and the absorbancy is on the vertical axis. When the spectra were measured, an absorption peak E was unexpectedly found for calcium carbonate at a wavelength range of 3.95 micrometers. By arranging the device according to Figure 1 to measure the absorption peak at a wavelength of about 3.95 micrometers, the device can measure the amount of calcium carbonate. Suitable reference wavelengths for measuring calcium carbonate are, for example, 4.55 micrometers and/or 3.7 micrometers. The suitable reference wavelengths for kaolin are, in turn, for example 2.64 micrometers and/or 2.8 micrometers. It should be noted that any reference wavelengths close to the actual measurement peak can be used as reference wavelengths. What is essential is that the absorbancies of base paper or coated paper are equal or nearly equal at said wavelength range. The wavelength 3.7 micrometers is particularly advantageous, since it can be used as reference also for measuring the amount of water. It is advantageous to measure the amount of water at a wavelength of about 3.175 micrometers. The measurements performed show that the reflectivity increases in the range over 6.3 micrometers, when the basis weight of carbonate was increased. But in the range under 6.3 micrometers the reflectivity decreases, when the basis weight of carbonate was increased, the absorption measurement thus functioning reliably in this range. In Figure 3, curve F shows the reflection spectrum of base paper and curve G, indicated by a dashed line, shows the reflection spectrum of kaolin coated paper. Figure 3 also shows the wavelength λ in micrometers on the horizontal axis and the absorbancy on the vertical axis. When the spectra were measured, an absorption peak H was unexpectedly found for kaolin at a wavelength range of about 2.7 micrometers. By arranging the device according to Figure 1 to measure the absorption peak at a wavelength of about 2.7 micrometers, the amount of kaolin in the coating can be determined. As for the properties affecting the signal-to-noise ratio of the radiation source, the windows and the detectors and the price of the device, a 5 measurement wavelength of 3.95 micrometers is advantageous for measuring the amount of calcium carbonate. Furthermore, the measurement of the amount of carbonate at a wavelength range of 3.95 micrometers can be implemented in the same meter as the measurement of the absorption peak of kaolin at 2.7 micrometers. The drawing and the description associated thereto are merely intended to illustrate the idea of the invention. As for the details the invention may vary within the scope of the appended claims.

Claims: CLAIMS 1. A method for measuring the amount of coating on a moving substrate, the method comprising measuring the amount of at least one component in a coating by reflection measurement, characterized by meas- uring the amount of calcium carbonate by measuring an absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers 2. A method as claimed in claim 1, c h a r a c t e r i z e d by measuring a reference value for the measurement of the absorption peak of calcium carbonate at a wavelength of about 3.7 micrometers. 3. A method as claimed in claim 1 or 2, c ha racterized by measuring the amount of kaolin by measuring an absorption peak of kaolin at a wavelength of about 2.7 micrometers. 4. A method as claimed in claim 3, c h a r a c t e r i z e d by measuring a reference value for the measurement of the absorption peak of kaolin at a wavelength of about 2.64 micrometers. 5. A method as claimed in any one of the preceding claims, characte rized by measuring the amount of water at a wavelength of about 3.175 micrometers. 6. A method as claimed in claim 5, c h a r a c t e r i z e d by measuring a reference value for measuring the amount of water at a wavelength of about 3.7 micrometers. 7. A method as claimed in any one of the preceding claims, characterized by the moving substrate being a paper or cardboard web (4). 8. A method as claimed in any one of the preceding claims, char- acterized by continuously measuring the amount of coating. 9. A device for measuring the amount of coating on a moving substrate, the device comprising a radiation source (1) for producing a light beam (2), a detector (7) for measuring the reflected light beam (2) and means for processing a signal of the detector (7), the device being arranged to measure the amount of at least one component in a coating (4a), characterized by being arranged to measure the amount of calcium carbonate from the coating (4a) by measuring an absorption peak of calcium carbonate at a wavelength of about 3.95 micrometers. 10. A device as claimed in claim 9, characterized by being arranged to measure the amount of kaolin from the coating (4a) by measuring an absorption peak of kaolin at a wavelength of about 2.7 micrometers. 7 11. A device as claimed in claim 9 or 10, characterized by being arranged to continuously measure the amount of coating. 12. A device as claimed in any one of the claims 9-11, characterized by the moving substrate being a paper or cardboard web (4).

Inventors: KAENSAEKOSKI MARKKU, MAENTYLAE MARKKU, TENHUNEN JUSSI, KAENSAEKOSKI, MARKKU, MAENTYLAE, MARKKU, TENHUNEN, JUSSI
Applicants: KAENSAEKOSKI MARKKU, MAENTYLAE MARKKU, TENHUNEN JUSSI, VALMET AUTOMATION INC, KAENSAEKOSKI, MARKKU, MAENTYLAE, MARKKU, TENHUNEN, JUSSI, VALMET AUTOMATION INC.
Application year: 1999
Main classification: G01N21
IPC codes: G01N21