Patent Application
20050098284
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 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.
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 web 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.
The purpose of the invention is to eliminate the aforementioned drawbacks and to present a versatile method by means of which it is possible to attain a sufficient absorption of liquids, especially water, on the paper or paperboard web. To attain this purpose, the method according to the invention is primarily characterized in that 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 liquid on the surface of the web. The absorbency of the surface can be improved e.g. by means of a preceding energy treatment, with which polar groups are produced in the material constituting the surface of the web, by means of a preceding addition of steam or an underpressure effecting simultaneously through the surface of the web.
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 above-described treatments it is possible to reduce and equalize the surface energy.
Especially when water is sprayed from underneath the paper with determined spraying 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.
Another way of reducing the surface energy is to add steam. Steam can be brought in the form of hot or cold steam, and by means of a suitable dosage it is possible to attain a nearly monomolecular liquid layer. The steam is adsorbed evenly on the surface of the web, and it equalizes and maximizes the adhesion of a considerably larger amount of liquid brought at the next stage on the web.
The third way is to remove the apparent hydrophobicity of the web by means of an underpressure acting on the surface simultaneously with the addition of liquid, the underpressure being effected through the web from the other side. To increase the absorbency of the surface in this way, it is possible to use a suction box and/or means producing at least underpressure impulses.
Furthermore, the purpose of the invention is to present an apparatus by means of which it is possible to produce a better adhesion of the liquid on the web. To attain this purpose, the apparatus is characterized in that in addition to the device for adding the liquid it comprises a device for reducing the hydrophobicity. This device is provided for bringing energy at least to the immediate vicinity of the surface of the web, to produce new polar groups on the surface of the web, for applying steam on the surface of the web, or for exerting direct underpressure on the surface of the web.
In the following, the invention will be described in more detail with reference to the appended drawings, in which
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.
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 I 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 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, and this two-sided wetting treatment is illustrated with a second wetting apparatus depicted with broken lines in the drawing. The wetting device can function on any of the following principles:
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. There are also other treatment alternatives which will be discussed hereinbelow.
One possible pretreatment method is the production of a surface charge on the surface of the web on the side which is wetted at a later stage.
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.
As a result of the treatment, the contact angle of the water on the surface of the paper is reduced uniformly into zero. This equalizes and maximizes the adhesion on the web of a considerably larger quantity of liquid (e.g. “bulk water”), which is introduced at the next stage.
By means of steaming it is possible to apply the maximum quantity of 2 g/m3 of water on such a cold paper web whose surface has not been pretreated to increase the adsorbtion of steam.
The steam treatment is especially advantageous in spray wetting from underneath the web, because in this wetting method the surface energy has a great significance in the adhesion of the liquid on the surface of the paper web. Thus,
After the wetting the web is guided substantially in the same surface moisture in the calender nip of the calender.
The wetting may be replaced with any of the aforementioned treatment methods in which the liquid functions as a carrier material in the composition to be added on the surface.
The dispersing air accompanying the spray and the air carried along by the web W impair the adhesion of the liquid on the web, because the dispersing air entering the pores decelerates the absorption of liquid in the paper and the air film already initially impairs the contact of the liquid and the web.
To reduce the apparent hydrophobicity resulting from the aforementioned phenomenon and not from the surface structure or materials of the web, a treatment device 1 is placed on the opposite side of the web, in the case of
The foil/s may be placed as such against the surface of the web, or they can be placed as a covering structure of a suction box. The suction box and the change it produces in the vacuum profiles is illustrated in
Naturally, such a placement is also possible that the suction box, foil/s or both are located below the web, and the spray wetting is effected from above.
After the wetting the web is guided substantially in the same surface moisture in the calender nip of the calender.
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.
The invention is not restricted solely to the aforementioned treatment points in the travel path of the paper or paperboard web, but it can be used in all points after the occurrence of hydrophobicity. According to the invention, the reduction in hydrophobicity can also be conducted in the drying section, in which the web is later rewetted e.g. between drying groups to adjust the moisture profile. It is a known procedure to re-wet the web in the drying section by controlling the wetting in the transverse direction of the web from the wetting devices. 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 as well. 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 a coating station and/or 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 coated and/or calendered, the surface of such paper or paperboard being already initially dry for re-wetting.
| Number | Date | Country | Kind |
|---|---|---|---|
| 981429 | Jun 1998 | FI | national |
This is a divisional of U.S. patent application Ser. No. 09/720,642, filed Mar. 5, 2001 and entitled “Method and apparatus for treating the surface of a web”, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09720642 | Mar 2001 | US |
| Child | 10923559 | Aug 2004 | US |
