EP1099025B1

EP1099025B1 - Method and apparatus for treating the surface of a web

Info

Publication number: EP1099025B1
Application number: EP99931318A
Authority: EP
Inventors: Antti Heikkinen; Mikko Tani; Pekka Linnonmaa
Original assignee: Metso Paper Oy
Current assignee: Metso Paper Oy
Priority date: 1998-06-22
Filing date: 1999-06-22
Publication date: 2010-01-20
Anticipated expiration: 2019-06-22
Also published as: EP1099025A1; CA2335631A1; WO1999067466A1; US20050098284A1; FI981429A; CA2335631C; AU4786999A; ATE455903T1; DE69941955D1; FI981429A0; FI113791B

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

The invention relates to a method for treating the surface of a web, in which method liquid is added on the surface of a paper or paperboard web, possibly with other constituents. The invention also relates to an apparatus for implementing the aforementioned method.
The invention relates to the manufacture and finishing of paper, the end product of which is a finished paper. In particular, the invention relates to the manufacture of printing papers, intented to be printed at a separate stage after their surface has been finished and the web has been reeled.
After the paper or paperboard has been dried, the surface structure of the web is made suitable by means of a mechanical treatment, calendering. There are several calendering methods, but it is common to all of them that the web is passed through one or several nips which are formed between two surfaces, typically between rotating roll surfaces. The purpose of the calendering is to improve the paper quality by pressing the paper into a fixed final thickness, and especially by smoothing its surface. As is well known, the mouldability of the fibres contained in the paper or paperboard, the "plasticization" of the web in connection with the calendering, can be improved by increasing the temperature and/or moisture. It is often advantageous to restrict the impact of the temperature and moisture only on either surface layer or on both surface layers of the web, wherein the mouldability of their fibres can be improved without affecting the central layer of the web excessively. As a result of this; a known procedure is the wetting of the surfaces of paper or paperboard webs before the calendering of the web (cf. US-A-4 653 395 ).
When wetting is utilized, the problem is that it is difficult to increase the water content to attain the desired moisture gradient in the thickness direction of the web. When for example steam is used, water can condensate poorly on the surface of the web, especially when its surface temperature is high. The most common problem is the hydrophobicity of the web, i.e. when the web is wetted by means of a known method, the adhesion of the liquid to the surface is poor. To attain a sufficient wetting on the surface of the web, it is thus necessary to use such a dosage which results in an undesirable increase in the moisture also in the inner parts of the web. The aim is, however, to attain a steep moisture gradient between the surface and the inner part of the web. Hydrophobicity may result from the raw materials used in the pulp, but there is also an apparent hydrophobicity which results from the air film carried by the rapidly moving web, and the air brought along by the wetting liquid, which is perceivable especially in spray wetting.
Another problem occurring in the finishing of the web is the absorption of the composition used in surface sizing or pigmenting into the paper or paperboard web. Since the solutions, sizes and pastes used in these working stages are aqueous, the problems are based on the same facts as in the wetting.
A method and an apparatus for treating the surface of an already formed and at least partly dried paper or paper board web comprising the features summarized in the preambles of claims 1 and 10, respectively, are known from document US-A-4 653 395 .
Document DE 3 115 958 A1 discloses a method and an apparatus, in which a paper web is subjected to an energy treatment by means of a corona treatment device before a liquid id added to the surface of the web by means of a wetting device. The corona treatment device issues energy to the web, increases the hydrophilicity, and increases the absorbency of the web. After the wetting device, the web can be reeled up.
Document DE 4 324 337 A1 discloses a method and an apparatus for rewetting a printed paper web. In this known method and apparatus, the printed paper web is subjected to an energy treatment by means of a corona treatment device before a liquid is added to the surface of the web by means of a wetting device. By means of the corona treatment, the hydrophilicity and the absorbency of the web is increased. The rewetted printed web travels to a folding and cutting device.
Document EP 464775 A2 discloses a method and an apparatus for subjecting a web to an energy treatment by means of a corona treatment device before a coating liquid is added to the web by means of a coating device. The corona treatment is effected for electrically charging the web, and the web is electrically charged for improving the adhesion between the web and the coating liquid.
It is an object of the present invention to provide a method and an apparatus by means of which it is possible to attain a steep moisture gradient between the surface and the inner part of the web by the wetting before the web is calendered, so as to efficiently restrict the impact of the moisture on the surface layer(s) during calendering.
According to the invention, this object is achieved by the method defined in claim 1 and the apparatus defined in claim 10.
Papers and paperboards are materials with heterogeneous surface energy. It has been discovered that the increase in the proportion of mechanical pulp and/or the act of adding a hydrophobic chemical increases the amount of fluctuation in the surface energy. The surface energy of the paper affects the absorbtion of liquids, the wettability of the surface and the adhesion properties of the surface. The more uniform the structure in view of its surface energy is, the more uniform are the effects of said processes. By means of the method of the invention it is possible to reduce and equalize the surface energy.
Especially when water is sprayed from underneath the paper with determined spaying parameters, the surface energy unequivocally determines the amount of water attached on the surface of the paper. The fluctuations in the surface energy appear in an irregular, small-scale fluctuation in the "adhesion" and moisture profile. This can cause an uneven calendering result, which causes e.g. mottling in printing. An uneven wetting of the surface results in that the sections which have a lower surface energy become wet and absorb water faster than sections with a higher surface energy. In practice, this can, in addition to the aforementioned reasons, result from irregularities in the moisture profile of the paper before wetting.
With the oxygen and radical reactions produced by means of an energy treatment, chemical changes are effected in the surface of the web W in the form of new polar groups, such as carbonyl, amide, hydroxyl and nitro groups, in the polymer material of the fibrous raw material of the web, especially in cellulose, hemicellulose and lignin. Thereafter it is easier to add the liquid, i.e. to perform the wetting with water, the surface sizing or the pigmenting.
The energy effecting the aforementioned chemical changes can be produced by means of a corona treatment, a flame treatment, an ozone treatment or a combination of these.
In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1: shows the principle of the method according to the invention,
Fig. 2: shows a side-view of an apparatus according to an embodiment of the invention, and
Fig. 3: illustrates an experiment performed by means of the first embodiment of the invention.

Fig. 1 illustrates the treatment stages of a paper or paperboard web W according to the invention. The web in question travels in a paper or paperboard machine or in a finishing machine for paper or paperboard, and its surface is subjected to finishing whereafter it is reeled to form a reel. Hydrophobicity is reduced at a stage A by means of a treatment effected on the surface of the web on the same side from which liquid is added at a stage B. It is also possible to reduce the hydrophobicity additionally at a stage C by means of a treatment effected from the opposite side of the web W on the surface of the web on which liquid is added. This treatment stage C can take place simultaneously with the stage B or slightly thereafter. Finally, the web W is passed to a finishing stage D of the surface which can follow immediately after the stages A and B or after the stages B and C.
The need to reduce the hydrophobicity arises after a press at that stage when a completely formed web is at least partly dry, and it is absorbent for re-wetting. When the hydrophobicity occurs for the first time, the dry matter content is approximately 60 to 65%.
The liquid to be added denotes liquid as such or liquid in a formulation
in which other substances are mixed therein, for example substances used in surface sizing and pigmenting, wherein the liquid functions as a carrier for these substances. In these cases the liquid is issued in the form of a surface sizing solution, a surface sizing paste, or a coating colour. The liquid is any liquid whose adhesion to the web can be improved by reducing the hydrophobicity on the surface of the web, especially water. The above-described stage B can thus be wetting, surface sizing or pigmenting.
In the paper machine or finishing apparatus for paper, the treatment is effected on the web W which moves at a high speed past the treatment sections. After the stages A and B or B and C the finishing effected on the web at the stage D can be immediately succeeding calendering, if the stage B is wetting, or it can be drying and calendering, if the preceding stage B is surface sizing or pigmenting.
Fig. 2 schematically shows the paper or paperboard web issued from a drying section in a paper or paperboard machine, which web W is passed to a calender K. Before the calender in the travel direction of the web, there is a device 1 for applying energy, by means of which device 1 energy is issued into the layer of air bordered by the surface of the web travelling past the device, the energy also affecting the surface of the web in such a way that more hydrophilic groups are produced. In the travel direction of the web W, the device is succeeded by a wetting apparatus 2 by means of which the surface whose hydrophilicity was increased at the previous stage, is wetted. Thereafter the web W is passed to the calender K, which in the drawing is illustrated by two rolls 3 and 4 between which there is a calender nip N through which the web is passed. The nip in question can be the only nip in the calender, or there may be several successive calender nips in the travel direction of the web, wherein the drawing shows the first one of them. The calender is not restricted to any type, and it can be a machine calender, a soft calender, a long-nip calender or a multiple-roll calender.
The device 1 applying energy on the web can be a corona treatment device, which is provided with an electrode or a series of electrodes, located at a distance of a small air gap from the surface of the web W, and on the opposite side of the web with a counter-electrode, e.g. a roll. The corona treatment is based on the fact that the air gap between the electrode and the paper or paperboard web has a lower dielectric discharge voltage than the web itself. When a high frequency high voltage is produced between the electrode and the counter electrode, the air between the electrode and the web is ionized, and generates a gaseous conductor which can be seen as a blue corona. The corona burst generated with the electrode produces the following oxidants: ozone, atomic oxygen, free oxygen radicals and high-speed electrons, whose energy in the corona burst often reaches the amount of several keV, e.g. the value of approximately 10 keV. The energy is sufficient for breaking the bonds present on the surfaces of the fibres and between the fibres. When the bonds break, very reactive free radicals are produced on the surface of the web, which free radicals react with the oxidants produced in the corona treatment. The surface of the web becomes oxidized, and new polar groups are produced in the polymer material of the fibres on the surface, the polar groups being primarily carbonyl, amide, hydroxyl and nitro groups. These groups provide new adsorption sites for water, thus increasing the hydrophilicity of the surface.
In addition to the aforementioned chemical effect the corona treatment also has an electrical effect which improves adhesion, because the corona burst charges the surface of the web.
Alternatively, the device 1 directing energy can be a flame treatment device. In the flame treatment, the act of subjecting the surface of a paper or paperboard web W to an oxidizing flame with high thermal energy produces reactions similar to those described above on the surface of the paper or paperboard web, as well as an increase in the hydrophilicity via these oxidizing reactions. A further advantage of the flame treatment is that it improves the coatability of the web by removing so-called "standing" fibres from the surface of the web.
In the corona burst ozone is also produced, which has an effect on the surface of the web. The surface of the web can also be subjected to an oxidizing effect by means of a special ozone treatment, ozonization, in which a suitable amount of ozone is directed as such to the web from an ozone generator. The device 1 can thus be an ozonizer, by means of which energy is issued on the surface of the web in the form of chemical energy contained in gas. In this context it is also possible to use the term energy treatment device.
Furthermore, it is possible to place e.g. two different devices such as a corona treatment device and a flame treatment device, a flame treatment device and an ozonizer, or an ozonizer and a corona treatment device successively before the wetting apparatus 2 on the same side of the web. With the combined effect of these treatments it is possible to attain a strong oxidizing treatment. These different treatments can take place in either order. Furthermore, the invention does not exclude the possibility that all the three treatments take place succesively in a suitable order.
The aforementioned treatment is effected on the surface of the web on which the fibres are exposed. It is also possible, that the web W has already been surface sized before it is passed to the aforementioned energy treatment and wetting. Thus, the treatment has the same effects as on the bare surface of the web. In the starch, modified starch, other natural polymers or their derivatives or in corresponding substances, such as synthetic polymers; used in the surface sizing, polar groups are produced analogously with the phenomena occurring in the cellulose, hemicellulose or lignin.
The aforementioned energy treatments are described for example in an article by S. Greig "Adhesion promoters for extrusion coating & cast film - a three-pronged attack", in a conference publication "Extrusion coating of paper and paperboard", 1990 and in the book by Chi-Ming Chan: "Polymer Surface Modification and Characterization", pp. 265-279. These publications primarily discuss the increase of adhesion of plastic coatings in extrusion coating.
The aforementioned treatments or a combination of them can be performed either on one side or on both sides of the web W. Thus, there are many alternatives that are possible before wetting, namely:

The corona treatment only on one side, the opposite side untreated,
the corona treatment on both sides,
the flame treatment only on one side, the opposite side untreated,
the flame treatment on both sides,
the ozonization only on one side, the opposite side untreated,
the ozonization on both sides,
the corona treatment on one side and the flame treatment on the opposite side,
the corona treatment on one side and the ozonization on the opposite side,
the ozonization on one side and the flame treatment on the opposite side,
a combination of the corona treatment and the flame treatment only on one side, the opposite side untreated,
a combination of the corona treatment and the ozonization only on one side, the opposite side untreated,
a combination of the ozonization and the flame treatment only on one side, the opposite side untreated,
a combination of the corona treatment and the flame treatment on one side, only the corona treatment on the opposite side,
a combination of the corona treatment and the ozonization only on one side, only the corona treatment on the opposite side,
a combination of the ozonization and the flame treatment only on one side, only the corona treatment on the opposite side,
a combination of the corona treatment and the flame treatment only on one side, only the flame treatment on the opposite side,
a combination of the corona treatment and the ozonization on one side, only the flame treatment on the opposite side,
a combination of the ozonization and the flame treatment on one side, only the flame treatment on the opposite side,
a combination of the corona treatment and the flame treatment on one side, only the ozonization treatment on the opposite side,
a combination of the corona treatment and the ozonization treatment on one side, only the ozonization treatment on the opposite side,
a combination of the ozonization and the flame treatment on one side, only the ozonization treatment on the opposite side
a combination of the corona treatment and the flame treatment on both sides,
a combination of the corona treatment and the ozonization on both sides,
a combination of the ozonization and the flame treatment on both sides
a combination of the corona treatment and the flame treatment on one side, a combination of the corona treatment and the ozonization on the opposite side,
a combination of the corona treatment and the ozonization on one side, a combination of the ozonization and the flame treatment on the opposite side,
a combination of the ozonization and the flame treatment on one side, a combination of the corona treatment and the flame treatment on the opposite side.

The alternatives which entail a combination of two different types of treatments on the same side can also contain a treatment of a third type on the same side.
Naturally, the wetting is effected on that side of the web W which has been subjected to a treatment of adding hydrophilic groups in any of the aforementioned ways. If the treatment has been conducted on both sides of the web W, the wetting by means of the wetting device 2 is effected on both sides of the web. The wetting device can function on any of the following principles:

spray wetting
film transfer wetting (LAS, sym-sizer, etc.)
moistening by adjusting the relative humidity of air
steaming

As a result of the treatment conducted before wetting, the contact angle of water on the surface of the paper is considerably reduced, i.e. the surface energy is reduced.
The corona, flame and ozonization treatments are the advantageous alternatives for treating the surface of the web, because they are efficient enough for producing changes in the structure of the surface of the web.
The pretreatment according to the invention is helpful especially in spray wetting underneath the web, in which the surface energy is the determining factor in the adhesion of the liquid on the paper. According to the preferred embodiment, the treatment is conducted at least underneath the web for a bottom-side spray wetting.
Similarly, the treatment also makes it possible to increase the amount of water that can be added e.g. by means of steaming in the web.
Naturally, in the two-sided wetting it is possible to utilize different wetting principles on different sides of the web.
In the wetting the web obtains the desired surface moisture, and it is passed in this moisture into the calender nip N of the calender K.
The wetting can also be replaced by adding liquid as a carrier material contained in the composition to be applied on the surface of the paper web, such as the surface sizing of the paper web or the pigmenting of the surface. Thanks to the pretreatment, the aqueous compositions added at these working stages adhere better on the web.
The pretreatment is especially well suited for coating methods in which the composition is brought in contact with the web at a low pressure: spray coating, jet coating as well as spreading roll application.
Fig. 3 shows water sorption as a function of the contact time for fully sized fine paper with different powers in the corona treatment, and for unsized and soft sized fine paper. The size used in the surface sizing is AKD glue. The fully sized sample, whose comparative test effected without a treatment is also shown, was subjected to the corona treatment using three different powers. The reference samples were an unsized fine paper and a fine paper soft surface sized with AKD, which were not subjected to the treatment.
Fig. 3 shows that the corona treatment clearly increases the water sorption of the surface sized paper, and the sorption is proportional to the power used.
When a web which has been surface wetted in any of the above-described manners is calendered, it is possible to use many known solutions as the calender K. The calender nip N shown in the drawings can be a hard nip, a soft nip, or a long nip which has been produced with a belt or a shoe. The aforementioned nip variations can be implemented by means of two hard rolls, two soft rolls, a soft and a hard roll, a roll and a belt, and a roll and a shoe. The calender can also be provided with several successive nips, of which two or more successive nips can be of the same type or of a different type. Similarly, it is possible that the calender is a multiple-roll calender in which successive nips are formed between the rolls in a stack of rolls.
The calendering advantageously entails the heating of the surface of the web by means of a heated, smooth calender surface in the calender nip N. It is also possible that heating and smoothing is effected in the calender only on the upper side of the web, only on the lower side of the web or on both sides of the web. Naturally, the aforementioned treatment is effected at least on that side whose hydrophilicity has been initially increased in a way described above, and which has been wetted thereafter.
According to the invention, the wetting can be profiled by adjusting the pretreatment reducing the hydrophobicity in the cross machine direction. The effect of the device producing the corona treatment and/or flame treatment and/or ozonization treatment is thus adjustable in different points in the cross machine direction, or there may be separate devices adjustable independently of each other in the cross machine direction. Thus, certain portions of the web can be rendered sensitive to absorb more water than others. Thus, the wetting need not necessarily be profiled separately, i.e. the wetting device can be "evenly" spreading. The aforementioned profiling possibilities naturally exist in the wetting effected before calendering. The profiling is advantageous especially in spray wetting, especially in sparay wetting underneath the web, and in the wetting effected by means of steaming.
The method and the device can be used in connection with an on-line calender functioning as a direct extension to the process of making paper or paperboard, or in a separate working stage when temporarily stored paper or paperboard is calendered, the surface of such paper or paperboard being already initially dry for re-wetting.

Claims

Method for treating the surface of an already formed and at least partly dried paper or paperboard web (W), comprising the steps of:
adding liquid on the at least partly dried surface of the web (W) to wet the web (W), and

thereafter calendering the web (W),
characterized
in that, before adding the liquid to the surface of the web (W), the absorbency of the least partly dried surface of the web (W) is increased by subjecting it to an energy treatment, with which new polar groups are produced on the material constituting the surface of the web (W).
The method according to claim 1, characterized in that the energy treatment is a corona treatment.
The method according to claim 1, characterized in that the energy treatment is a flame treatment.
The method according to claim 1, characterized in that the energy treatment is an ozonization.
The method according to claim 1, characterized in that the surface of the web (W) is subjected to at least two different types of energy treatment selected from the group consisting of corona treatment, flame treatment and ozonization.
The method according to one of claims 1 to 5, characterized in that the absorbency of the surface is increased in a profiled manner in the transverse direction of the web (W), by profiling the effect of the energy treatment or energy treatments.
The method according to one of claims 1 to 6, characterized in that the liquid is added on the surface of the web (W) along with other constituents intended as a coating of the web (W).
The method according to claim 7, characterized in that the liquid along with the other constituents is added on the surface of the web (W) by spray coating, jet coating or spreading roll application.
The method according to one of claims 1 to 8, characterized in that the calendering is performed on-line as a direct extension of a process of making paper or paperboard.
Apparatus for treating the surface of an already formed and at least partly dried paper or paperboard web (W) travelling along a travel path, comprising
a wetting device (2) for adding liquid on the surface of the web (W), the wetting device (2) being located on the travel path of the web (W), and
a calender (K) for calendering the web (W), the calender (K) being located on the travel path of the web (W) after the wetting device (2) in the travel direction of the web (W),
characterized
by an energy treatment device (1), which is located on the travel path of the web (W) before the wetting device (2) in the travel direction of the web (W), the treating device (1) being arranged to issue energy at least to the immediate vicinity of the at least partly dried surface of the web (W), in order to produce new polar groups on the surface of the web (W) and to increase the absorbency of the surface of the web.
The apparatus according to claim 10, characterized in that the energy treatment device (1) is a corona treatment device.
The apparatus according to claim 10, characterized in that the energy treatment device (1) is a flame treatment device.
The apparatus according to claim 10, characterized in that the energy treatment device (1) is an ozonization device.
The apparatus according to claim 10, characterized in that the energy treatment device (1) comprises at least two different types of devices selected from the group consisting of a corona treatment device, a flame treatment device, and an ozonization treatment device.
The apparatus according to one of claims 10 to 14, characterized in that the wetting device (2) is a spray wetting device, a film transfer wetting device, a moistening device based on the adjustment of the relative humidity of air, or a steaming device.
The apparatus according to one of claims 1 to 14, characterized in that the wetting device (2) is a coating device arranged to add the liquid on the surface of the web (W) together with other constituents intended as a coating of the web (W).
The apparatus according to claim 16, characterized in that the coating device is a spray coating device, a jet coating device or a spreading roll.
The apparatus according to one of claims 10 to 17, characterized in that the calender (K) is an on-line calender functioning as a direct extension of a paper or paperboard machine.