METHOD FOR MEASURING THE COMPONENTS OF A COATING ON A MOVING BASE MATERIAL

Abstract: The invention relates to a method for measuring paper coating components by infrared measurement whereby the paper components are determined by reflection measurement of a middle infrared range in such a manner that the amount of calcium carbonate in the coating is determined in the wavelength range 2.5-8 mu m.

Description: METHOD FOR MEASURING THE COMPONENTS OF A COATING ON A MOVING BASE MATERIAL The invention relates to a method for measuring components of a coating on a moving base material by infrared measurement. The measurement of the amount of coating on coated paper is one of the most important paper quality measurements; in this description paper means conventional paper and/or cardboard. The amount of coating has conventionally been measured (US No. 5 338 361) continuously by absorption measurement of a near infrared range (NIR). In absorption measurement of an IR range the intensity of an IR beam absorbed by the coating or a quantity proportional thereto is determined as a function of the wavelength of the beam; the intensity of the beam absorbed by the IR beam measured by a wavelength corresponding to an absorption peak of the component to be measured correlates with the amount of coating. Measuring paper coating components by infrared measurement is known in the art and will not be described here in greater detail. The coating component to be measured is usually kaolin having absorption peaks in the NIR range. However, NIR measurement cannot be used for measuring the other important coating component, calcium carbonate, since calcium carbonate has no absorption peak in the NIR range. The total amount of coating can then be calculated on the basis of kaolin measurement assuming that the ratio between the amounts of kaolin and calcium carbonate in the coating is constant. In reality, however, the ratio between the amounts of kaolin and calcium carbonate is not always constant, but may vary. Thus, the prior art method described above does not provide accurate results particularly in measuring the amount of calcium carbonate. In addition, the method can only be employed when kaolin is used; when kaolin is not used as a coating component said method cannot be employed at all. US patent (No.) 5 455 422 describes a method in which the amount of coating is measured by measuring, for example, the absorption peak of latex at the wavelength 2.30 micrometers and the absorption peak of clay at the wavelength 2.21 micrometers. Said patent further describes the measurement of calcium carbonate by measuring the amount of backscattering at the wave- length 2.09 micrometers. However, for measuring the amount of calcium carbonate said method is unreliable an inaccurate. The amount of calcium car- bonate 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 the coating is constant. However, this is not always the case and problems are created particularly if the kaolin content is low i.e. below ap- proximately 20 % and the carbonate content correspondingly high i.e. approximately 80 %. EP publication (No.) 0 332 018 shows a method in which the amount of kaolin in paper is measured by transmission measurement, for example, at approximately the wavelengths 1.4 and 2.2 micrometers. However, it is very difficult to determine by transmission measurement what the portion of the coating in the measurement result is. Furthermore, the portion of calcium carbonate has to be approximated in a manner described in the previous chapter. GB publication (No.) 2 127 541 shows how the amount of additives in paper is measured by transmission measurement. The publication describes the measurement of the amount of calcium carbonate by measuring the absorption peaks at the wavelengths 1 1.54 micrometers and 1 1.77 micrometers. 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. An object of the present invention is to eliminate the drawbacks described above. A particular object of the invention is to introduce a new method for measuring the components of a coating on a moving base material by infrared measurement in such a manner that the measurement is better applicable than previous measurements particularly for determining various coating components and that the fillers on the base material do not cause problems in the measurement. A further object of the invention is to introduce a method for measuring coating components on a moving base material, for example, paper coating components in such a way that the measurement is not disturbed by a high absorption of the base material, for example, paper. The method of the invention is characterized in that the components of a coating are determined by reflection measurement in the wavelength range 2.5-12 μm in such a manner that the amount of calcium carbonate in the coating is determined in the wavelength range 2.5-8 μm. The basic idea of the invention is that the coating components are determined by reflection measurement of a middle infrared range. It is also essential that the amount of calcium carbonate in the coating is determined in the wavelength range 2.5-8 μm. The method of the invention provides an accurate and useful method for determining paper coating components using reflection measurement of the middle infrared range 2.5-12 μm. The method is particularly applicable to be used for measuring paper coating components i.e. for example measuring kaolin at the wavelength 2.5-11 μm, preferably at 8-11 μm and/or measuring calcium carbonate at the wavelength 2.5-8 μm, preferably at 3.0- 7.3 μm. The measurement can generally be carried out at any wavelength in the middle infrared range, for example, at 2.5-12 μm. The method of the invention is applicable for measuring the coating components on a moving base material also from the surface of a roller of a paper coating machine, a roller of a paper machine and/or generally from the surface of a metal plate. Reflection measurement i.e. measurement where a reflection source and a receiver are on the same side of the object to be measured, can be carried out using specular reflection measurement whereby the measuring beam is directed onto the surface of the base material as an oblique beam of parallel rays and a parallel reflected beam reflecting from the surface of the base material is detected by a detector i.e. the intensity of the reflected beam is determined as a function of the wavelength. The invention can be particularly advantageously implemented as diffusion reflection, in which case the measuring beam is directed towards the object to be measured and the intensity of the radiation diffusely reflecting from the object in all directions is determined as a function of the wavelength; the illumination of the object can also be diffusely implemented. Reflection measurement of the MIR range provides a very good cor- relation for the amount of coating components, particularly kaolin and carbonate and mixtures of these. The correlation is also very good for various base papers and/or cardboards in calibrating the measurement of their coatings. The MIR range has many advantages compared with the NIR range. In the MIR range high absorptions are achieved for all essential coating components, in addition the peaks are sharp and the scattering is insignificant. The sharp peaks are a desirable property as for the function of detectors of measuring apparatuses based on interference filters, and the small quantity of scattering allows the structural variation of the coating without the calibration suffering. The fact that the paper industry is willing to start using carbonate-based coatings, which cannot be directly measured in the NIR range, increases the sig- nificance of the result. The use of the method of the invention enables the implementation of measuring apparatuses, which can be used for on-line measurement of paper and/or cardboard coating components by paper and coating machines, for on-line measurement of coatings on rollers of paper coating machines or pa- per machines or generally on metal plates and as paper and coating research tools in laboratories. Particularly the use of the method of the invention enables the implementation of a small-scale measuring apparatus of the coating amount variation for laboratory use by designing the optics of the apparatus to suit small-scale measurement. In small-scale measurement the size of the measuring area can be 0.1-100 mm 2 , preferably 0.1-10 mm 2 , more preferably 0.1-2 mm 2 , and even smaller than this. Naturally the measuring area can also be larger, for example, of the size about 1 cm 2 (n = 1-10 or larger) as is known from prior art. The total amount of the coating to be measured or in small-scale measurement the coating amount to be measured may vary, for example, between 3-40 g/m 2 , preferably 17-25 g/m 2 . In the following the invention will be described in greater detail by means of the preferred embodiments with reference to the accompanying drawings, in which Figure 1 is a diagram showing diffusion reflection measurement, Figure 2 is a diagram showing specular reflection measurement, Figures 3 and 4 show reflectivity (%) i.e. the intensity of light measured as a function of a wavelength at the wavelength about 2-11 μm, Figures 5 - 9 show coating amounts determined according to the method of the invention as a function of coating amounts determined by labo- ratory measurements and Figure 10 shows small-scale variation of coating amount measured by the method of the invention, the measuring field being 1 mm 2 . In Figure 1 a parallel measuring beam B 1 is directed using a lens 1 obliquely to a paper 2 to be measured, from which a parallel reflected beam B 2 is directed using a lens system 3 to a detector 4. The measuring beam B., may consist of light rays having various wavelengths, for example, 2-12 μm. The detector 4 determines the intensity of the beam B 2 as a function of a different beam wavelength λ. The paper 2 may be stationary, for example, in laboratory measurement, or it may be in motion, for example in a paper machine. When the paper 2 moving in the paper machine is being measured the measuring apparatuses are preferably located in a measuring frame traversing in a cross direction in relation to the direction of the paper 2 in which case the measurement can be performed at the width of the entire paper 2. The illumination of the paper by the measuring beam B 1 and the detection of the reflected beam B 2 as a function of the wavelength λ of the light are known in the art, and will therefore not be described in more detail here. In measuring the MIR range specular optics can preferably be used, the specular optics being known per se and therefore not described in more detail here. Figure 2 is a diagram showing specular reflection measurement. An incoming beam B., is directed, for example, as a beam of parallel rays at an oblique angle towards the paper 2 and the beam B 2 of parallel rays reflected from the paper is detected in a manner known in the art using the lens system 3 and the detector 4, the intensity of the reflected beam is detected as a function of the wavelength λ of the light. Figures 3 and 4 graphically show the results of specular reflection measurement performed as shown in Figure 2 compared with the reflectivity of the base paper, a reflection spectrum of kaolin coated paper in Figure 1 and carbonate coated paper in Figure 2 being shown in relation to the spectrum of the base paper. The kaolin peak is shown in Figure 1 at the wavelength 8.5 - 10 μm and the carbonate peak in Figure 2 at the wavelength 6.5 - 7 μm. The peak of styrene butadiene used as a binder is seen around 3 μm (Figure 4). The peaks are very pronounced since the reflectivity of coated paper at the wavelengths of the peaks of the pigments is 5-11 times greater than the reflectivity of base paper. A particularly advantageous measuring method is to measure the amount of calcium carbonate by measuring the size of the absorption peaks located in its wavelength range 2.5-8 μm. Most preferably the size of an absorption peak or peaks located in the wavelength range 3 -7.3 μm is measured. The absorption peaks of calcium carbonate are located, for example, at the wavelengths about 5.55 μm and 3.95 μm. It is further preferable to measure the amount of kaolin by measuring the size of the absorption peaks located in its wavelength area 2,5 11 μm. An absorption peak of kaolin is located, for example, at the wavelength about 2.7 μm. A series of coating amount measurements of kaolin and carbonate coated papers was performed in order to find out how useful the measuring method is. The measurements showed that the measurement peak response increased to such an extent while the measurement angle of the coating amount increased that a choice had to be made between measurement dynamics and penetration depth. As a compromise the measurements were performed as diffusion reflection measurements, whereby the calibration showed a better result than specular reflection measurement on account of a good signal-to-noise ratio. Diffusion measurement is not either as sensitive to distance as specular reflection measurement. The coating amount measurements were performed by a Bomem FTIR spectrometer using a commercial diffusion reflection accessory (Harrick, The Praying Mantis). During the measurement a sample was moved over a measurement sample opening using an electric motor, the measuring spot being about 3 mm in diameter. Corresponding components were determined from the measured papers using laboratory measuring methods. In Figures 5- 9 the component amounts determined by the method of the invention are shown on y-axis and the corresponding component amounts determined by laboratory determinations on x-axis. Figure 5 shows the measurement of kaolin coating coated on wood-containing base paper, Figure 6 the measurement of kaolin coating coated on wood-free base paper, Figure 7 the measurement of carbonate coating coated on wood-containing base paper, Figure 8 the measurement of carbonate/kaolin coating coated on wood-containing base paper and Figure 9 the measurement of kaolin coating coated on wood- containing and wood-free base paper. A standard deviation (SEP) of the determinations performed by the method of the invention and a standard deviation (SEC) of the determinations performed by laboratory methods were cal- culated from the results; the number of factors describes the number of variables used in the determinations performed by laboratory methods. Figure 10 shows the variation of the measured small-scale coating amount and the deviation of coating amount measurement when the measurement was repeated twice from the same points of 1 mm in diameter. The preferred embodiments are meant to illustrate the invention without limiting it in any way.

Claims: CLAIMS 1. A method for measuring components of a coating on a moving base material by infrared measurement, characterized in that the components of the coating are determined by reflection measurement in the wavelength range 2.5-12 μm in such a manner that the amount of calcium carbonate in the coating is determined in the wavelength range 2.5-8 μm. 2. A method as claimed in claim 1, characterized in that the amount of kaolin in the coating is determined in the wavelength range 2.5- 11 μm. 3. A method as claimed in claim 2, characterized in that the amount of kaolin is determined by measuring the size of a kaolin absorption peak. 4. A method as claimed in any one of the previous claims, characterized in that the amount of calcium carbonate in the coating is de- termined in the wavelength range 3.0-7.3 μm. 5. A method as claimed in any one of the previous claims, characterized in that the amount of calcium carbonate is determined by measuring the size of a calcium carbonate absorption peak. 6. A method as claimed in any one of the previous claims, char- acterized in that a reflection spectrum is measured in the wavelength range 2.5-12 μm. 7. A method as claimed in any one of the previous claims, characterized in that the coating components are measured in the measurement area 0.1-100 mm 2 . 8. A method as claimed in any one of the previous claims, characterized in that the reflection measurement is implemented using specular reflection measurement. 9. A method as claimed in any one of claims 1-7, characterized in that the reflection measurement is implemented using diffusion re- flection measurement. 10. A method as claimed in any one of the previous claims, characterized in that the moving base material, the coating components of which are measured, is paper or cardboard.

Inventors: KAENSAEKOSKI MARKKU, SUMEN JUHA ANTERO, TENHUNEN JUSSI, TORNBERG JOUNI SAKARI, KAENSAEKOSKI, MARKKU, SUMEN, JUHA, ANTERO, TENHUNEN, JUSSI, TORNBERG, JOUNI, SAKARI
Applicants: KAENSAEKOSKI MARKKU, SUMEN JUHA ANTERO, TENHUNEN JUSSI, TORNBERG JOUNI SAKARI, VALMET AUTOMATION INC, KAENSAEKOSKI, MARKKU, SUMEN, JUHA, ANTERO, TENHUNEN, JUSSI, TORNBERG, JOUNI, SAKARI, VALMET AUTOMATION INC.
Application year: 1998
Main classification: G01N21
IPC codes: G01N21, G01N33