FI96895B - Method for optimising the quality of paper that is to be coated - Google Patents

Description

96895 5

Method for optimizing the quality of the paper to be coated For the purpose of optimizing the quality of the paper to be coated

The invention relates to a method for optimizing the quality of the paper to be coated, in which the paper web to be coated is passed through a coating station or stations in which a coating paste containing pigment 10 and a binder and an optical brightener bound to said binder is applied at least to some extent.

As printing papers are now required to have even better printing properties, the importance of paper coating and surface sizing has grown considerably. In the production of better-quality 15-paper, more and more precise information about the various stages of the process is needed so that appropriate adjustments can be made whenever a possible disturbance occurs. In paper coating and surface sizing, such information is required, e.g., on the thickness of the coating or adhesive layer on the paper and on coating defects at the earliest possible stage, in order to be able to react accurately and quickly to the disturbances that occur.

In the pigment coating of paper, the general goal is to provide the smoothest possible coating layer. The thickness of the paper coating layer and its uniformity are also affected by the formation of the base paper, i.e. the small-scale uniformity of the basis weight of the paper. The concept of formation usually includes a variation in basis mass with a wavelength ranging from zero to a few centimeters. Several different methods are known for characterizing and measuring the formation, most of which are based on the measurement of the variation of the light transmission of paper on a small measuring area with a diameter of about 0.1 ... 1.0 mm. The goodness of the formation is then described by the standard deviation or coefficient of variation of the light intensity values that have passed directly through the paper. The advantage of the optical method is the speed and ease of measurement. However, especially for filler-containing and calendered papers, there may be variations in optical density that impair the reliability of 2,96895 optical measurements. In this context, the concept of formation and its measurement have been omitted because it affects and relates to the flatness of the paper coating layer. The uniformity of the coating layer and its variation can be partially characterized by similar characteristics as the formation of the base paper.

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With regard to the state of the art related to the present invention, reference is first made to the applicant's FI patent application 902930, FI patents 89104, 87492 and 91025 and US-3,956,630, US-4,250,382, CA-1,038,192, CA-1,197,630 and EP ~ A3 0 268 029.

10

Em. the methods disclosed in the patents generally do not aim, and do not always, measure local variations in the amount of coating or analyze the nature of the unevenness of the coating and the causes that cause such coating defects.

15 It can therefore be stated that prior art measurement methods have not provided a comprehensive solution for analyzing and determining the evenness of pavement smoothness and its various defects, as well as for fast and accurate on-line measurement and correction of coating defects by various coating process control, regulation and control.

20

As is already known, in the pigment coating of paper, a coating paste is applied to the surface of the base paper. This paste contains a pigment and a binder dissolved in water. The most commonly used pigment is kaolin clay and the binder is a mixture of latex and starch. The proportion of binder is usually 10-15% of the proportion of pigment.

25

One known way (e.g. applicant's FI-application 933223) to measure variations in the amount of coating; is to illuminate the coated paper web with UV light and to consider the distribution of the color or intensity of the visible light produced as a result of the fluorescence of the optical brightener previously used in the paper coating materials. One starting point for this invention is the finding that optical brightener is often bound to the binder of the paste.

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3,96895 seen. The uneven migration of the binder to the surface of the coating layer is in turn caused by the so-called mottling phenomenon, i.e. spotted unevenness of the print.

The Mottling phenomenon can be characterized as a delicate spotting distinguished by the printed surface of the human eye, which is a gentle contrast and low frequency unevenness of the print density. Mottling is measurable as a small-scale variation in the density of a print, usually manifested as patches of 1 to 7 mm in diameter. Mottling occurs in printing units in compression, usually due to differences in ink repositioning, sometimes also through differences in the water absorption of the paper.

10 In LWC offset paper, the mottling phenomenon can be caused by several reasons, e.g. poor base paper formation or inappropriate coating drying program, which in turn results in uneven distribution of the binder along the surface of the coated paper and thus variations in ink absorption.

15

With regard to the state of the art most closely related to the invention, reference is made to the applicant's FI patent application No. 933223 (filed on 15 July 1993) and the following journal articles Koskinen T., Tiittanen J., Ebeling K., "Comparison of test methods for LWC offset paper with the aim of predicting mottling tendency "Paper and wood, Oh. 74 / No. 1/1992 pp. 45-20 55 and Engström G. "Formation and consolidation of a coating layer and the effect on offset-print mottle", Tappi Journal, April 1994, pp. 160-172. In addition, reference is made to the reference publications of the above-mentioned journal articles.

Em. With regard to the article "Paper and wood", reference is made in particular to the good correlation of the distribution of optical bright tea with the distribution of the binder content discussed therein. With regard to the Tappi Journal article, reference is made in particular to the connection between the binder distribution and mottling described in more detail therein. At the beginning of page 171 of the latter article, it is stated that a large amount of binder results in its even distribution and thus less mottling.

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According to the Tappi Journal article, mottling is due to the uneven liquid absorption of the paper and only occurs in offset printing. One manifestation is the so-called backtrap mottle, in which the ink solvent is unevenly absorbed into the paper. At the poorly absorbent point, the ink does not have time to solidify sufficiently before the next 5 printing units, where the rubber mat carries a thicker intermediate layer than elsewhere. Thus, the amount of color remaining on the paper varies, and this is reflected in the density difference as a spotting. Wet repellency staining, on the other hand, is due to differences in the unit of weight in the absorption of wetting water in the area without color. For the next unit, the wet spot receives the ink differently than the dry spot, resulting in 10 spotting.

Spotting is mainly due to two reasons: first, the uneven distribution of the binder on the surface of the paper due to differences in water absorption and the wrong drying strategy (the general perception is that drying should be fast before the first 15 pour points, then slow); and secondly, differences in the porosity of the coating, which are affected by the stiffness of the binder in question. temperature, drying and calendering.

The prior art described above does not disclose a method or apparatus by which a paper manufacturer could manually or automatically control the paper coating process so that the occurrence of 20 harmful mottlings is known in advance. Indeed, these problems only come to the attention of the paper manufacturer through customer complaints.

The problems and drawbacks of the prior art are discussed in more detail below: 25 - staining is detected only after printing, the distribution of the binder on the paper surface can only be measured in the laboratory, eg by ESCA measurement (Electron Spectroscopy for Chemical Analysis), 30 - no on-line method is used to warn or prevent the risk of staining,

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Large local variations in the amount of coating can cause differences in binder distribution, so in this respect the adjustment of the coating amount profile could help, the moisture profile is usually measured before the next coating or reel, which does not indicate any moisture development during drying and thus drying strategy staining.

It is therefore an object of the present invention to provide a new method for obtaining a warning of the occurrence of mottling in connection with the coating of paper, so that the user of the coating station can take the necessary corrective measures. It is a further object of the invention to provide a method in which a closed control system applying the method of the invention can be connected to the coating station in order to correct the causes of mottling.

In order to avoid the above drawbacks and to achieve the above-mentioned and later-described objects of the invention, the invention is mainly characterized in that ultraviolet light or the like is measured using the fluorescence of said optical brightener as a result of the light path P (UV) and that said profile P (UV) generates an alarm and / or setpoint signal or signals from the process control system to control those parameters of the coating process, finishing process and / or papermaking process that have an effect on the distribution of said binder.

Since the optical brightener is bound to the binder of the coating paste, the distribution of the optical brightener also provides information on the distribution of the binder, especially if it differs from, for example, the radiolabeled distribution of the coating or the actual known correlation between the UV profile and the radiolabelled profile.

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In connection with the invention, a previously known measuring system based on radioactive radiation is also available, which measures either the basis weight or the ash content before and after the coating unit and determines the amount of coating as the difference between these measurements. This is hereinafter referred to as the gauge frame profile.

5

In the first embodiment of the invention, e.g. the UV profile measured according to the above-mentioned FI application 933229 is compared with the above-mentioned measuring frame profile. Both profiles are preferably scaled so that at a given average coating surface binder content they show the same coating amount profile. Thus, said profiles 10 are of the same shape and at the same level of coating if the binder is evenly distributed on the surface of the coating.

In a preferred embodiment of the invention, the separation profile of the UV profile and the measuring frame profile is calculated and if it is not straight, it is known that the coating agent and the binder are not distributed in the same way on the surface of the paper. In an area where the amount of binder differs from that prevailing elsewhere, it can be assumed that the ink is absorbed differently on the paper. According to studies, since the level of the binder content also affects its dispersion, which in turn correlates with mottling, the aim is to influence the driving parameters of the coating machine so that the transport of the binder to the coating surface 20 is as even as possible. The running parameters of the coating and papermaking process with which the may be affected are at least the following: factors affecting the porosity and evenness of the base paper to be coated: grinding 25 - press line loads intermediate calendering line load factors influencing the level and profile of the coating amount: dry matter content of the coating agent 30; infrared / fluidized / cylinder drying profiled infrared dryers.

In another embodiment of the invention, only the UV profile is considered. The aim is to make its distribution as even as possible. The closed-loop actuators can be a profiling infrared dryer, a coating quantity profile actuator (in which case its response is known as the binder distribution with respect to uniformity) and / or devices affecting the porosity of the base paper discussed in the previous paragraph.

10

In a third embodiment of the invention, the user is warned of the mottling risk on the basis of the measurement according to the first or second embodiment of the invention.

In the following, the invention will be described in more detail with reference to some preferred embodiments of the invention shown in the block diagrams of the accompanying drawing 15, to which the invention is not limited.

Figure 1 is a block diagram showing an embodiment of the method of the invention in which the separation profile of a measuring frame profile and a UV profile is measured, by means of which the coating-20 and / or papermaking process is controlled.

Figure 2 shows, in a manner similar to Figure 1, a simpler embodiment of the invention in which the UV profile of coated paper alone is measured without a meter frame profile.

25

Figures 1 and 2 show very schematically the coating process and only with the precision necessary to understand the method of the invention. shown in the figures, the coating process of the paper machine is a separate off-line process in which the uncoated paper web W is passed pohjapaperirullalta direction of the arrow Win. In this connection, it should be noted that the intermediate calender is generally only on on-machine lines. The invention is suitable for both on-machine and off-machine use. On off-machine lines, the actuator is a paper calender of a paper machine.

In Figures 1 and 2, the paper web W passes first through an intermediate calender 10 and then through a first measuring frame 11, where the basis weight or ash content profile of the web W is measured before the coating unit 12, for example based on radioactive radiation. The measurement signal m2 obtained from the measuring frame 11 is fed to the measuring frame profile unit 23, to which a measuring signal m3 is also fed from the second measuring frame profile 16 according to Fig. 1. The second measuring frame 16 is located after the coating unit 12 and the drying units 13, 14a, b, c and 15. On the basis of the difference between said measurement signals m3 - m2, the cross-section of the amount of coating applied to the track W in the coating unit 12 is continuously determined, hereinafter referred to as the measuring frame profile P (BW). This profile P (BW) is obtained as the output signal of the measuring frame profile unit 23.

According to Figure 1, after the first measuring frame 11, the paper web W to be coated is fed to a coating station 12, where a coating paste is applied, which is an aqueous suspension of an optical brightener, a binder and a pigment as described above. After the coating station 12, the wet track W has to be dried. The coating dryer arrangement first comprises an infrared dryer 13, then three successive fluidized bed units 20a, 14b and 14c and finally a short multi-cylinder dryer unit 15. This is followed by the above-mentioned second measuring frame 16, after which a UV measuring unit 17 is arranged. and for a more detailed implementation of the action, reference is made to the applicant's above-mentioned FI application 933229. The FI application discloses a method in which the amount and distribution of a coating material, surface adhesive or the like is measured from a product by irradiating a web-like, sheet-like or similar product with ultraviolet light to excite optical brightness fluorescence light and locally determining the fluorescence light intensity and / or chromaticity. and a proportion of the amount of coating, surface adhesive or similar additive. Em. The FI application discloses an apparatus in which the radiation source 30 comprises one or more ultraviolet lamps operating in a narrow wavelength range, the wavelength range of which is in the absorption range of the optical brightener. Em. radiation source

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9,96895 is sequenced to provide a substantially constant level of irradiation across the track. The light receiving and determining devices comprise one or more camera units with a photodetector fixedly spaced from the moving product.

5 In Fig. 1, in the UV unit 17, a UV beam Lj is applied to the coated surface of the paper web W over the entire width of the web. The UV unit 17 examines the intensity distribution of visible light generated as a result of the fluorescence of the optical brightener in the coating paste, as illustrated by the arrow L 1.

The invention is based on the finding that the optical brightener of the coating paste is bound to the binder, so that the distribution of the optical brightener also provides information on the distribution of the binder and thus on the mottling risk.

According to Figures 1 and 2, a measurement signal m j is obtained from the UV unit 17, which is fed to a UV profile unit 20. The output signal of this unit 20 is a transverse UV profile P (UV) of the coated paper web W. Said UV profile P (UV) is fed to a calculation unit 21, to which in addition in Fig. 1 the measuring frame profile P (BW) obtained from the measuring frame profile unit 23 is derived. The cross-sections P (UV) and P (BW) are scaled so that at a certain average coating surface binder content 20 they show the same coating amount profile. Thus, said profiles are of the same shape and show the same level of coating with the binder evenly distributed:. on the surface of the pavement in the transverse direction of the track W.

By means of the calculation unit 21 and the separation profile unit 24, a separation profile P (E) is formed according to Fig. 1. If the separation profile P (E) is not straight, it is an indication that the coating paste of the track W and its binder are not distributed with the same profile in the transverse direction of the track W. In an area where the amount of binder differs significantly from that prevailing elsewhere, in the printing process, the ink will be absorbed differently into the paper. To correct this error, said separation profile P (E) is fed to a set unit 22 for calculating the setpoints Sj 30 (i = 1 ... N), from which setpoint signals Sj ... sN are obtained for the feedback control system, which control the coating process and / or the papermaking process. In the invention, the separation profile P (E) of the above-mentioned profiles is minimized by a closed control system or alternatively the separation profile is adjusted to correspond to the previously known correlation of said profiles P (BW) and P (UV).

5 If the papermaking process is to be controlled as a closed control, the coating station must be an in-line unit connected to the back of the paper machine. Also in an off-machine, the driving values of the paper machine and the pulp production can be changed on the basis of the binder distribution. With said setpoint signals Sj ... sN, in the papermaking process, for example, the porosity of the base paper and its uniformity can be influenced by adjusting the line load profile of the dewatering press of the paper machine 10 or by adjusting the pulp grinding. The porosity of the base paper Win can also be influenced by adjusting the line load profile of the intermediate calender 10 by what the control signal describes it.

According to the method of the invention, the above-mentioned setpoint signals 15 Sj ... sN obtained from the setpoint calculation unit 22 can also influence the level of the coating amount and its cross-section by adjusting the dry matter content of the coating material and / or the blade coating blade load. This mode of action is illustrated by the control signal sg acting on the coating station 12. In addition, in addition to or instead of the above-mentioned control methods, the separation profile P (E) can be influenced by adjusting the drying units 13, 14a, 14b, 14c, 15 after the coating station 12, which is described by the control signals sa, sb, sc, sd and sf acting on the above units. Said control signals can influence the mutual ratios of the drying portions of the different units 13,14,15. In addition, or instead of the above-mentioned control method, an infrared dryer 13 with a drying effect can be profiled by means of the control signal sf. Fluid dryers 14a, 14b, 14c with a transverse profile 25 can also be used.

: The setpoint signals Sj ... sN can be considered directly as the control signals sa ... Sf of the actuators or, alternatively, on the basis of the setpoint signals S (... sN and signals from other parts of the control system, a control signal sa ... sf.

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Figure 2 shows a simpler embodiment of the invention than Figure 1, in which the measuring frames 11 and 16 are not utilized at all and thus the measuring frame profile P (BW) is not required in the method at all. According to Fig. 2, a measurement signal m1 generated as described above is obtained from the UV unit 17, by means of which a UV profile P (UV) is formed in the UV profile unit 5, which is passed to the calculation unit 21. A profile signal is derived from this calculation unit 21 to the setpoint calculation unit 22. said setpoint signals Sj ... sN In the embodiment of Fig. 2, the setpoint signals Sj ... sN preferably control devices influencing the porosity of the base paper and / or the profiling devices of the infrared dryer 13, which is illustrated by the control signal sf.

Figures 1 and 2 also show a third embodiment of the invention in which the control signals Sj ... sN are not necessarily used for closed control of the papermaking process or the coating process, but an alarm signal a is obtained from the calculation unit 21, which is fed to the coating process control room. Alternatively or in addition, the separation profile P (E) obtained according to Fig. 1 or the UV profile P (UV) obtained according to Fig. 2 is fed to the display of the control room. On the basis of said profiles and / or the alarm signal a, a skilled operator takes the necessary manual control measures on the basis of the instructions so as to obtain process corrective measures corresponding to the automatic corrective measures described above with the set values Sj ... sN.

Figures 1 and 2, only the paper web W, one side of the coating is shown. If both sides of the paper is coated, the arrow Wout after a web W is passed to the corresponding shown in Figure 1 and 2 the web W through the second half process steps of coating, which connection 25 is applicable to the above-described invention, the paper web W with the quality optimoimisvaiheita.

It should be emphasized that the invention has been illustrated quite schematically above with the aid of application examples. From the above, it will be apparent to one skilled in the art to use a variety of methods and devices known per se in control technology. For example, the set value calculation unit 22 or the like can be arranged as a computer-controlled intelligent expert system, in which the process model of the coating process and also of the papermaking process is stored as necessary. This process model can also be traced to adaptability, e.g. by leaving certain parameters of the model open and adapting them based on the measurement signals from the process to better describe the coating and / or papermaking process in use and to take into account changes in process parameters and / or papermaking quality.

In the following, the claims are set out, within the scope of the inventive idea defined by the invention, the various details of the invention may vary and differ from those set forth above by way of example only.

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Claims (8)

A method of optimizing the quality of paper to be coated, wherein a paper web (W) to be coated is passed through one or more coating stations (12), applying coating paste over the paper web (W) containing binder pigments and optical clearing agents, which at least in some men are bonded to said binder, characterized in that in order to reduce spots in the printing of the paper produced by the method, the transverse profile P (UV) of the web (W) is measured in relation to the direction of movement. ) of the light formed in the web (W) as a result of the fluorescence of the optical brightening agent using ultraviolet light or the like and that by means of said profile P (UV) an alarm and / or setpoint signal or signals (a; Sj ... sN) for the control system of the process, using which signals to regulate the parameters of the coating process, finishing and / or the papermaking process by which the distribution of said binder can be affected. 2. A method according to claim 1, characterized in that with the setpoint signals (S! ... sN) formed according to the method, the functional devices are controlled by the coating process and / or the papermaking process by which one or more of the following factors are affected: factors affecting the porosity and uniformity of the base paper (W) to be coated, such as pulverization, the line pressures of the press and / or the line loads of the intermediate calendering; factors affecting the level and profile of the coating amount, such as the dry matter content of the coating material and / or the load of the sheet coating device; factors affecting the drying of the web (W) to be coated, such as the drying ratio, infrared drying / float drying / cylinder drying and / or profiling of the transverse drying effect, especially the infrared dryer, of the paper web (W). Method according to Claim 1 or 2, characterized in that in the method, the cross profile P (BW) of the amount of coating material in the web to be coated is also measured, which profile is compared with the transverse profile P (UV) of the optical fiber. the clearing agent and that said parameters of coating in the coating process and / or the papermaking process are controlled by a closed control system in such a way that the differential profile P (E) = P (BW) - P (UV) between said two profiles is fixed minimized or defined for the coating process to correspond the correlation of said profiles 5 P (BW) and P (UV). Method according to claim 3, characterized in that the coating blank profiles P (BW) of the method are measured with measuring frames (11, 16) based on known radioactive measuring methods provided (Figure 1) before and after the coating station. (12). Method according to claim 4, characterized in that the measurement signals (m2, m3) obtained from said measurement frames (11, 16) are fed to a measurement profile unit (23), whereby a profile P (BW) is obtained on the coating amount as the output signal. which is fed to a counter (21) and that as an output signal to said counter (21), a UV profile signal P (UV) from the UV profile unit (20) for which an input signal (mj) from the measuring unit (12) is also fed from the measuring unit (12) ) of the advantage of the distribution of the optical clarifier and that on the base of the output signal of said counter (21), a separation profile P (E) is formed in the separation profile unit (24), with which the counter unit (22) is controlled for the setpoints (S ... sN) of the system (Figure 1). Method according to any of claims 1-5, characterized in that, except for the closed control system according to the invention or instead of this, an alarm signal (a) is generated in the counter unit (21) or the equivalent of the system, which is fed to the control unit of the coating process. and / or that the differential profile P (E) and / or the UV profile P (UV) of said profiles P (BW) and P (UV) is guided to a printer by the controller and / or to a display and that the operating personnel manually control the process on the base of said alarm signal and / or views. 30 Method according to any of claims 1-6, characterized in that the counter (22) or equivalent for the set values of the control system according to the method is arranged so that it is an intelligent computer controlled expert system to which in the necessary men the process model of the coating process is registered. and the paper-making process. 5 8. A method according to claim 7, characterized in that said process model is most suitably adapted in such a manner, that given parameter models are left open and they are adapted on the basis of the measurement signals obtained from the process of describing the coating and / or paper making process. take into account changes in the process parameters and / or the paper quality to be produced. Il