FI113791B - Method and apparatus for surface treatment of a web - Google Patents

Abstract

In the method for treating the surface of the web, liquid is added on the surface of a paper or paperboard web (W), possibly with other constituents. The absorbency of the surface of the paper or paperboard web is increased before liquid is brought on the surface of the web or simultaneously with bringing the liquid on the surface of the web. The method is used for wetting the surface of the web before calendering, or for coating the web in surface sizing or pigmenting. The absorbency of the surface is increased before bringing the liquid for example by means of a corona treatment, a flame treatment, an ozonization or by adding steam.

Description

1 113791

Method and apparatus for surface treatment of a web

The invention relates to a method for treating the surface of a web, in which the liquid is optionally added to the surface of the paper or board web with other ingredients. The invention also relates to apparatus for carrying out the above method.

The invention relates to the manufacture and finishing of paper, the end product of which is finished paper. The invention relates in particular to the production of printing papers.

After drying the paper or board, the surface structure of the web is made desirable by mechanical treatment, calendering. There are many methods of calendering, but all of them have in common the introduction of a web through one or more nips formed between two surfaces, normally rotating roll surfaces. The purpose of calendering is to improve the quality of the paper by squeezing it to a certain constant final thickness and, in particular, smoothing its surface. It is known to improve the processability of the fibers contained in paper or paperboard during calendering, "plasticized-from" the web by increasing the temperature and / or humidity. It is often advantageous to limit the effect of temperature and humidity to either one or both of the surface layers of the web, whereby the flexibility of their fibers and the quality of their respective surface can be:. improves without affecting too much the middle layer of the web. As a result, it is known to moisten the surfaces of paper or paperboard before •• ί. web calendering.

The problem with wetting is that raising the water content to achieve the desired moisture gradient in the web thickness direction is difficult. For example, when steam is used, the water may be poorly sealed to the surface of the web, especially when its surface temperature is increased. support. A more common problem is the hydrophobicity of the web, i.e., when the web is wetted by any known method, the adhesion of water to the surface; This is insignificant. In order to obtain sufficient wetting on the surface of the web, a dosage which then results in an undesired increase in moisture also in the inner parts of the web must be used. The goal would be: however, to have a steep moisture gradient between the surface of the web and the • 113791 2. Hydrophobicity may be due to the raw materials used in the pulp, but there is also apparent hydrophobicity due to the air film carried by the fast moving web and the air introduced by the wetting liquid, which is particularly evident in spray wetting.

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Another problem encountered in finishing the web is the absorption of the composition used for surface sizing or pigmentation into a paper or board web. Since the solutions, pastes, and pastes used in these steps are water-based, the background to the problems is the same as that of wetting.

The object of the invention is to eliminate the above drawbacks and to provide a versatile method by which sufficient absorption of liquids, especially water, into the paper or board web can be achieved. To accomplish this purpose, the method of the invention wherein the surface absorbency of at least partially dried paper or board web is increased is characterized in that the surface absorbency is increased by a vacuum acting on the surface which is applied through one side while simultaneously applying liquid to the web.

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In order to increase the absorbency of the surface in this way, a suction box and / or at least vacuum pulse generating means may be used.

li »I ·> • · * *. . Paper and board are heterogeneous materials with surface energy.

Addition of mechanical pulp and / or hydrophobic chemical has been found to increase the amount of surface energy variation. The surface energy of the paper affects the absorption of the liquids, the wetting of the surface and the adhesion properties of the surface. The smoother the structure's surface energy, the smoother the effects of the aforementioned processes are. The treatments described below can be used to calculate and equalize: · surface energy.

• · · • · • · ·. Particularly when spraying water from below the paper, the surface energy of certain spray • • · parameters unambiguously determines the amount of water adhered to the paper surface: ···: 35. Fluctuations in surface energy appear as uneven, small-scale variations in "adhesion" and moisture profile. This can • result in an uneven calendering result, resulting in • · 113791 3 printing. motling-marking of. Uneven surface wetting results in areas with lower surface energy wetting and absorbing water faster than areas with higher surface energy. In practice, this may be due not only to the above reasons but also to irregularities in the moisture profile of the paper prior to wetting.

The surface W of the web is subjected to chemical changes in the form of new polar groups, such as carbonyl, amide, hydroxyl and nitro groups, through the energy generated by the oxygen treatment and the radicals, to the polymeric material of the web fiber material, particularly cellulose, hemicellulose and lignin. Subsequently, the addition of a liquid, such as moistening with water, surface sizing or pigmentation, is easier to accomplish.

Em. the energy that produces the chemical changes can be produced by means of corona-15 treatment, flame treatment, ozone treatment or a combination thereof.

Another way to get the surface energy to calculate is to add steam. The vapor may be introduced in the form of cold or hot vapor, and by suitable dosing, a nearly monomolecular liquid-20 layer may be applied to the surface of the web. The vapor adsorbs evenly onto the web surface and smoothes and maximizes the adhesion of a much larger amount of liquid to the web in the next step.

. It is also an object of the invention to provide an apparatus with which the tele- 25 fluid adhesion to the web. To accomplish this purpose, an apparatus comprising, in addition to a fluid addition device, a device for reducing the hydrophobicity of the web surface at a point where the web has already formed and at least partially dried, is characterized in that. the device is disposed substantially at the fluid application device on the opposite side of the web 30 and is arranged to cause at least a vacuum in the web surface.

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The invention will now be described in more detail with reference to the enclosed silicon

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Fig. 1 shows the principle of the method according to the invention, Fig. 3 shows a side view of the apparatus according to the first embodiment of the invention, Fig. 3 illustrates an experiment carried out with the first embodiment of the invention 5. Fig. 4 is a side view of the apparatus according to the second embodiment of the invention, and Fig. 5 is a side view of the apparatus according to the third embodiment of the invention.

Figure 1 illustrates the processing steps of a paper or board web W according to the invention. The hydrophobicity is reduced in step A to the surface of the web 15 by treatment on the same side from which the liquid is added in step B. It is also possible to reduce the hydrophobicity in step C from the opposite side of the web W by treatment to the surface of the web to which the liquid is added. This treatment step C may take place simultaneously with or shortly after step B. Finally, the web W is introduced into the surface finishing step D, which can follow immediately after steps A and B or steps B and C.

The need to reduce hydrophobicity arises after the press, when the fully formed web is at least partially dried and is * · 25 absorbent for re-wetting. For the first time, the dry matter content of the web surface is about 60-65%.

* * * »: T; By the added liquid is meant the liquid as such or the liquid in the form of a mixture with other substances, for example surface-gluing or pigmentation substances, whereby the liquid acts as a carrier for these substances. In these cases, the liquid is provided in the form of a surface-adhesive solution, an adhesive or a coating paste. Liquid is any liquid whose adhesion to the web can be improved by reducing the hydrophobicity of the surface of the web, particularly water. Thus, step B above may be wetting, surface sizing or pigmentation.

The treatment is carried out in a paper machine or in a paper finishing apparatus at a high speed on a web moving past the treatment sites W. After steps A and B or B and C, the finishing to the web in step D can be the immediately subsequent calendering, if step B is 5 wetting, or it may be drying and calendering if the preceding step B is surface sizing or pigmentation.

Fig. 2 schematically illustrates a paper or board web W from a drying section in a paper or board machine which is introduced into the calendar 10 K. Prior to the calender, the web W has an energy targeting device 1 to supply energy to the air layer bordering the web. to the surface of the web such that additional hydrophilic groups are formed. The device is followed by a wetting device 2 in the direction of travel of the web W, whereby the surface, the hydrophilicity of which has been added in the previous step 15, is wetted. Thereafter, the web W passes to the calender K, which is depicted in the drawing by two rolls 3 and 4, between which, the calender nip N is passed. The nip in question may be the only nip in the calendar, or it may have a plurality of consecutive calendering nips in the direction of web travel, the first 20 of which are illustrated in the drawing. The calender is not limited to any type and can be machine calender, soft calender, long nip calender or multi roll calender.

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The energy targeting device 1 may be a corona processing device having a electrode or series of electrodes at a small air pocket from the surface of the web W and a counter electrode on the other side of the web, for example a roll. Corona treatment is based on the fact that the air pocket between the electrode and the paper or board web has a lower dielectric breakdown voltage than the web itself. When a high frequency high voltage is applied between the electrode and the counter electrode, the air between the electrode and the surface of the web is ionized and forms a gaseous conductor, which is seen as a blue corona. The corona burst generated by the electrode generates the following crude oxidants: ozone, atomic oxygen, free oxygen radicals; ···. and high speed electrons whose energy in the corona burst reaches several keV, for example, about 10 keV. Energy: “it is enough to break the bonds on the fiber surfaces and between them. When the bonds are broken, highly reactive free radicals are formed on the surface of the web, which react with oxidants formed during corona treatment. The surface of the web is oxidized and new polar groups are formed in the polymeric material of the fibers on the surface, which are mainly carbonyl, amide, hydroxyl and nitro groups. These groups 5 provide new adsorption sites for water, increasing the hydrophilicity of the surface.

Em. in addition to the chemical effect, the corona treatment also has an electrical adhesion enhancing effect since the corona burst occupies the web surface.

Alternatively, the energy targeting device 1 may be a flame treatment device. Exposure to the surface of a paper or board web by exposure to a high heat oxidizing flame in a flame treatment causes reactions such as those described above on the paper or board web surface and an increase in hydrophilicity through these oxidation reactions. A further advantage of the flame treatment is that it improves the coating of the web by removing so-called. "standing" fibers.

Corona burst also produces ozone, which affects the surface of the web. The surface of the web can also be subjected to an oxidizing effect by 20 special ozone treatment, ozonation, in which a suitable amount of ozone is supplied directly to the web from the ozone generator. The device 1 can then be an ozonation device and it delivers energy to the surface of the web in the form of chemical energy contained in the gas, so to speak.

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In addition, it is possible to place, for example, two different devices, such as a corona treatment apparatus and a flame treatment apparatus, in succession before the humidifier 2. a flame treatment device and an ozonator, or an ozone device and a corona treatment device on the same side of the web. The combined effect of these treatments 30 can provide a strong oxidizing treatment. These different treatments can; * · ';;; be in either order. The invention also does not exclude the possibility that all three treatments are in succession: · in the appropriate order.

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> · »· · 35 It is also possible that the web W has already been surface glued before being subjected to the aforementioned energy treatment and wetting. The treatment then has the same effects as the exposed surface of the web. Polar groups are formed on the starch, modified starch, other natural polymers or their derivatives or the like such as synthetic polymers used in surface sizing 7, analogous to phenomena occurring in cellulose, hemicellulose and lignin.

Em. energy treatments have been described e.g. S. Greig in "Adhesion Promoters for Extrusion Coating & Cast Film - a Three-Pronged Attack" in Conference Paper "Extrusion Coating of Paper and Paper-10 Board", 1990, and Chi-Ming Chan, "Polymer Surface Modification and Characterization", ss. 265-279. These publications mainly deal with the application of adhesion to plastic coatings in extrusion coating.

The above treatments, or some combination thereof, may be performed either on one side of the web W or on both sides thereof. In this case, many options prior to wetting are possible, namely:

Corona treatment on one side only, opposite side 20 unprocessed corona treatment on both sides, flame treatment only on one side, opposite side: unprocessed, - flame treatment on both sides, 25 - ozonation only on one side, opposite side untreated, ozonation: on both sides on one side and flame treatment on the opposite side, 30 - corona treatment on the other side and ozonation on the opposite ·· ·: 'side, - ozonation on one side and flame treatment on the opposite side, - combination of corona treatment and flame treatment on one side only, corona treatment and ozonation combination for one side only: Λ · side, opposite side unprocessed, 113791 8 combination ozone and flame treatment only for one side, opposite side unprocessed, corona treatment and flame treatment combination on one side, on the opposite side only corona treatment, 5 - combination of corona treatment and ozonation only on one side, opposite side only corona treatment, combination of ozonation and flame treatment only on one side, opposite side only corona treatment, corona treatment and flame treatment combination 10 flame treatment, combination of corona treatment and ozonation on one side, on the opposite side only flame treatment, combination of ozonation and flame treatment on one side, on the opposite side only flame treatment, 15 - corona treatment and flame treatment combination on one side, opposite side, only ozone ozone treatment only, combination of ozonation and flame treatment on one side, 20 on the opposite side only Ozone treatment, combination of corona treatment and flame treatment on both sides, 0 '- combination of corona treatment and ozonation on both sides: j: 25 combination of ozonation and flame treatment on both sides.

• · - corona treatment and flame treatment on one side, on the opposite side corona treatment and ozonation on one side, 30 - corona treatment and ozonation on the other, · ;;; on the opposite side a combination of ozonation and flame treatment, • j · - a combination of ozonation and flame treatment on the other side, • »·». ···. on the opposite side, a combination of corona treatment and flame treatment 35.

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Alternatives that have a combination of two different types of treatment on the same side may also include a third type of treatment on the same side.

Naturally, the wetting is carried out on the side of the web W which has undergone hydrophilic group treatment in one of the ways mentioned above. If the treatment has been carried out on both sides of the web W, the wetting with the wetting device 2 also takes place on both sides, and this double-sided wetting treatment is illustrated in the drawing with dashed circuits) with another wetting device 2. sym-sizer etc.) 15 - humidification by adjusting the relative humidity of the air to steaming.

As a result of the pre-wetting treatment, the contact angle of the water on the paper surface is significantly reduced, i.e. the surface energy is reduced.

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Corona, flame, and ozonization treatments are preferred alternatives for treating the surface of the web because they have sufficient power to effect changes in the surface structure of the web. There are other: 'treatment options, which are discussed below.

'25. One possible pretreatment nozzle is to provide a surface charge to the surface of the web to be wetted at a later stage.

The pre-treatment according to the invention is particularly useful in the lower wetting of the web, i.e. spray wetting, in which the surface energy is the determining factor in the adhesion of the liquid to the paper. Thus, according to a preferred embodiment, the treatment is carried out at least on the underside of the web. · · ·. lella for spray wetting underneath.

..r 35: '* It is also possible to increase the amount of water that can be added to the • V · by steaming, for example.

10 113791

Naturally, different damping principles can be used for duplex wetting across the web.

5 Upon wetting, the web obtains the desired surface moisture and is introduced into the calender n of calender K at this moisture.

Instead of wetting, the liquid may be added as a carrier material for the composition to be applied to the surface of the paper web, such as surface sizing of the paper web or pigmentation of the surface. The water-based compositions added during these work steps will better adhere to the web due to pretreatment.

Pre-treatment is particularly well suited for coating methods where the composition is brought into contact with the web at low pressure: spray-15 coating, jet coating and also brush roll application.

Figure 3 shows the water sorption as a function of contact time in corona treatment at different power rates on fully bonded fine paper and on unglued and lightly bonded fine paper. The adhesive used for surface sizing 20 has been AKD adhesive. The fully glued sample, which also has an untreated 0 test, was subjected to corona treatment at three different power levels. Comparative samples were non-glued and lightly AKD: '. surface-treated fine paper which has not undergone any treatment.

Figure 3 shows that the corona treatment clearly increases the water sorption of the surface-glued paper, and the sorption is proportional to the power used.

Figure 4 shows another alternative in which the surface hydrophobicity is reduced by steam treatment of the surface. The treatment device 1 may then be a suitable steam dispensing device, such as a steam box, on the surface of the web. The steam can be applied in the form of hot or cold steam and is adsorbed uniformly on the surface. The vapor can be dispensed in such a small amount that an almost monomolecular liquid is obtained on the surface. ···. floor. Such amounts do not substantially increase the moisture content of the paper and can be applied, for example, in the form of hot steam i on a hot web.

As a result of the treatment, the water contact angle on the surface of the paper decreases to zero throughout. This smooths and maximizes the adhesion of a much larger amount of liquid (e.g., "bulk stone") to the web in the next step.

Steaming can generally be applied to a maximum of 2 g / m 2 of water on a cold paper web which has not been pre-treated to increase the adsorption of the vapor.

Steaming treatment is particularly useful in spray wetting, i.e., spray wetting from below the web, since in this wetting mode, surface energy plays an important role in the fluid sticking to the surface of the paper web. Figure 4 illustrates the treatment device 1 located below the paper web W and then the spray-15 moistening device 2 located below the web web. However, the invention is not limited to this arrangement, but the corresponding devices can also be successively over the paper web W. Furthermore, it is possible that the aforementioned sequential treatments are performed both above and below.

After wetting, the web is introduced into the calender nip of the calender with substantially the same surface moisture.

: *: Wetting may be replaced by one of the above coating methods, · in which the liquid is present as a carrier material in a composition to be added to the surface.

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Figure 5 shows an embodiment of the invention. This is a spray wetting under the web W with a wetting device 2.

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The high speed web W carries with it an air film 30 having a thickness proportional to the running speed and the paper roughness.

This air film is schematically depicted by arrow 11. When spraying liquid onto the paper surface using air-sprayed spray nozzles as shown in Figure 5, the dispersion air is generally in the order of 4 bar and the pressure of the liquid is! generally between 0.5 and 2 bar. In this case, the • · 35 scattering air that comes with the jet fills the paper pores P with air. In the picture below: '* · where the pore size is exaggerated for clarity, this air is

Vi denoted by reference I2.

12 113791

The scattering air supplied by the jet and the air film carried by the web W make the liquid stick to the web, since the scattering air entering the pores slows down the absorption of the paper and the air film already initially reduces the contact between the liquid and the web.

Em. In order to reduce the apparent hydrophobicity due to the phenomenon, which is not due to the surface structure and materials of the web, on the opposite side of the web, in the case of Figure 5, a treatment device 1 is provided which is substantially air-permeable a web of support straps, such as wire 5, and exerts at least a vacuum through the web to the surface to which the wetting is applied. This device improves the physical adhesion of the liquid and the web by drawing air through the web W out of the water front. Vacuum can be obtained with a suction box of known construction, previously known only at the wet end of a papermaking machine, where they absorb water from the web at high water content. The apparatus actually shown in Fig. 5 comprises at least one stationary element 1a, generating at least a vacuum pulse extending across the web 20 substantially across the entire track width, which produces its vacuum shape pulse shape and / or position and the rapid motion of the web. A foil known from the wet end of the body, preferably two or more in succession in the direction of web travel. These release lists provide vacuum in the form of vacuum pulses. They also provide .. also the pre-vacuum pulses, and these are illustrated schematically with • ·:: vacuum profiles placed at the corresponding positions at the top of Fig. 5, where the X-axis corresponds to the normal pressure.

»· 30 The release list (s) may be perpendicular to the surface of the web, or may be located as a cover structure for the suction box. The suction box and the resulting change in vacuum profiles are depicted in dashed lines in Figure 5.

35 Of course, it is also possible to position the suction box: ** · foili / foils or both below the web and spray wetting • ·: Λ i from above.

13 113791

After wetting, the web is introduced into the calender nip of the calender in substantially the same surface moisture.

When calendering a surface wetted web in any of the above described ways, many known solutions can be used as calender K. The calender nip N shown in the figures may be a hard nip, a soft nip (soft nip) or a long nip provided with a strap or a shoe. The aforementioned nip variations can be accomplished by means of two hard rolls, two soft rolls 10, soft and hard rolls, roll and belt, or roll and shoe. The calender may also have a plurality of consecutive nipples, two or more of which may be of the same type or of a different type. Likewise, it is possible that the calender is a multi-roll calender in which successive nipples are formed between the rolls of the roll stack.

15

Calendering preferably involves heating the surface of the web by means of a heated smooth calender surface in the calender nip N. It is also possible that only the top of the web, just below the web, or both sides of the web are subjected to heating and smoothing in the calender. Em. the treatment is, of course, carried out at least on the side whose hydrophilicity is initially increased by one of the methods described above and subsequently moistened.

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The invention is not limited to the paper or cardboard web path only to the 25 treatment points described above, but can be used at any point after the occurrence of hydrophobicity. The reduction of the hydrophobicity ..... according to the invention can also be carried out with a drying section: where, for example, between the drying groups, the web is subsequently rewetted to adjust the moisture profile. It has been known to rehydrate the web with a drying section by controlling the wetting across the machine from the humidifying devices. According to the invention, the wetting can be profiled by adjusting the hydrophobicity-reducing pretreatment in the cross machine direction. The effect of the device providing the corona treatment and / or the flame treatment and / or the ozonation treatment is then adjustable at different points in the machine direction or in the machine direction: '·· may be separate, independently adjustable devices. This allows certain points in the web to be sensitized to absorb more water than the action. In this case, the wetting does not necessarily need to be separately profiled, i.e. the wetting device may be "uniformly" spreading. Em. Profiling opportunities also exist naturally in pre-calender wetting. Profiling has particular advantages in spray wetting, especially in under-spray spray wetting, and in steaming wetting.

The method and apparatus may be used in conjunction with a coating station and / or machine-sheeting process as a direct extension of the paper or board manufacturing process, or in a separate working step for coating and / or calendering intermediate stored paper or board for dry re-wetting.

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Claims (10)

A method of treating the surface of the web during manufacture and post-processing of paper or paperboard, in which process any liquid with other ingredients is added to the surface of an already formed and at least partially dried paper or paperboard web (W), and the absorbency of the surface of the at least partially dried paper or paperboard web is increased for this purpose, characterized in that the absorbency of the surface is increased by the surface-acting pressure developed by the web (W) on the other side of the web when liquid can be simultaneously added to the surface. of the track. 2. A method according to claim 1, characterized in that the negative pressure is induced at least with a negative pressure pulse, which is achieved by the position and / or the shape of a body placed against the web (W) or in the vicinity of the same means, such as a drainage strip ie. foil. 3. A method according to claim 1 or 2, characterized in that the absorbency is increased by successive overpressure and suppression pulses. Method according to any of claims 1 to 3, characterized in that the liquid is added to the surface of the web to moisten the web, after which the web: calendered. ; 25. 5. A method according to any of the preceding claims, characterized in that the absorption capacity of the surface is increased in a profiled manner in * · '; ; ' the transverse direction of the web (W). 6. Apparatus for treating the surface of the web during manufacture and post-processing of paper or paperboard, which apparatus comprises an additive existing on the paper or paperboard web (W). device (2) of liquid to bring liquid possibly with other constituents to the surface of the web, the device comprising a hydrophobic reducing device (1) of the surface which is arranged to increase the absorption capacity of the surface of the web. such a position on the tread of the web in which the web has already been formed and at least partially dried, characterized in that the hydrophobic reducing device (1) of the surface is positioned substantially at the addition device (2) of liquid on the opposite side of the web (W) and it is arranged to produce at least suppression on the surface of the web (W). 5 Device according to claim 6, characterized in that the device (1) comprises at least one member which is positioned against the web (W) or in the vicinity thereof, and whose position and / or shape produces at least a suppression pulse. 10 8. Apparatus according to claim 7, characterized in that the device produces successive overpressure and suppression pulses. Device according to any of the preceding claims 6-8, characterized in that liquid additive device (2) is a humidifier. 10. Device according to claim 9, characterized in that the humidifier is a spray humidifier located below the web, 20. · »· * · • · *« »