A method of manufacturing of glassine paper

Technical field

0001The invention relates to a method of manufacturing of glassine paper.

Background art

0002Glassine release base paper is a special paper grade which strongly distinguish from common bulk grades such as newsprint paper, supercalendered uncoated magazine paper (SC-paper) and coated magazine paper printing paper (LWC, MFC).

0003Typically the glassine paper is produced so that the base paper including the surface closing with film-forming products is manufactured in a paper machine and is reeled up as machine rolls. Commonly the base paper is dried and then re- moisturized to the level needed for super calendaring. The moisture content in the base paper is typically is very high, about 15 - 20% of the solid contents of the paper. Due to the drying and re-moisturizing the paper cross directional moisture content profile becomes substantially even. The machine rolls are stored at least 2 hours, in order to even out of the moisture left in the paper. After the moisture has stabilized the base paper is treated in an off-line supercalender in order to provide desired properties of the glassine paper.

0004As an example of such an off-line supercalender solution it is referred to document <patcit id="pcit0001" dnum="JP2009155758A"><text>JP2009-155758A</text></patcit>.

0005The standard manufacturing process has several drawbacks. For example, it requires a process involving two separate steps; the paper machine and the super calender. In some cases it may be required that there are even two separate supercalenders per one paper machine in order that the supercalendering process can follow the machine production as the calendering speed is lower than the paper ma- chine speed. This increases space requirement and also investment costs. Additionally the intermediate stage for evening out the moisture in the paper consumes time, requires additional human resources and makes the process more complex. In view of the above prior art, it is the object of the invention to provide an improved method of manufacturing of glassine paper with properties meeting at least the values described above, which improved the efficiency of the prior art manufacturing process.

0006Further information of producing paper and related to this field can be found on <patcit id="pcit0002" dnum="GB551950A"><text>GB 551 950</text></patcit>, <patcit id="pcit0003" dnum="WO2001104427A"><text>WO 2001/104427</text></patcit> and <patcit id="pcit0004" dnum="US2211348A"><text>US 2 211 348</text></patcit>.

Disclosure of the Invention

0007The present invention is defined in the current claim 1. With regard to the method, this object is met with a method of manufacturing of glassine paper comprising following steps of paper by forming a paper web in a forming section and drying the web in a press section and in a pre-drying section; applying film-forming product on both sides of the web; removing moisture from the film-forming product in an after-drying section; passing the web having film-forming product on its surfaces through an on-line calendering process; controlling the CD thickness profile be a heat transfer system capable of heating a roll surface locally and in adjustable manner in cross direction of the machine;and reeling up the glassine paper.

0008According to an embodiment of the invention calendering process comprise calendering process comprises calendering treatment in several successive nips in at least one multi-nip calender.

0009According to an embodiment of the invention calendering process comprises at least a first calendering treatment in a first calendering section and a second calendering treatment in a second calendering section.

0010According to another embodiment of the invention calendering process comprises controlling web tension between the first and the second calendering treatment.

0011According to another embodiment of the invention calendering process comprises application of water by condensing steam onto the web before passing the web to anyone of the calendering sections.

0012According to another embodiment of the invention calendering process comprise cooling of the paper web and application of water by condensing steam onto the web before passing the web to anyone of the calendering sections.

0013According to still another embodiment of the invention calendering process comprise application of water by condensing steam onto the web before passing the web to anyone of the calendering sections.

0014According to still another embodiment of the invention, calendering process comprises at least a first calendering treatment in a first calendering section and a second calendering treatment in a second calendering section and the first calendering treatment comprises treating the paper web in at least four successive calendering nips.

0015According to still another embodiment of the invention the second calendering treatment comprises treating the paper web in at least two successive calendering nip.

0016According to still another embodiment of the invention, removing moisture from the film-forming product in an after-drying section comprises a step of controlling moisture CD profile of the web by transferring heat adjustably in CD direction while drying the web; and wherein controlling moisture CD profile is based on the moisture content profile of the web entering the calendering treatment.

0017According to still another embodiment of the invention removing moisture from the film-forming product in an after-drying section comprises a step of controlling moisture CD profile of the web by transferring heat adjustably in CD direction while drying the web; and wherein controlling moisture CD profile is based on the moisture content profile of the web before the reel-up.

0018According to still another embodiment of the invention removing moisture from the web in the pre-drying section and applying film-forming product on both sides of the paper comprises a step of controlling moisture CD profile of the web by transferring heat adjustably in CD direction to the web at the press section; and wherein controlling moisture CD profile is based on the moisture content profile of the web before film application and/or impregnation system.

0019According to still another embodiment of the invention drying the web by a press section comprises a step of controlling moisture CD profile of the web by transferring heat adjustably in CD direction to the web prior to entering into a press nip; and wherein controlling moisture CD profile is based on the moisture content profileof the web entering the process of applying film-forming product on both sides of the base paper.

0020According to an embodiment of the invention the film-forming product is selected from a group comprising starch, polyvinyl alcohol, carboxymethyl cellulose, alginate or any other product.

0021According to still another embodiment of the invention wherein the applying the film-forming product is practiced in a size-press, film press or any other coating/impregnating system.

0022According to still another embodiment of the invention the temperatures of the surface of the heating cylinders are controlled to be from 170 to 240 °C and nip loads of the first calendering section and the second calendering section are controlled to be 270 - 400kN/m.

0023According to an embodiment of the invention the temperatures of the surface of the heating cylinders are controlled to be from 170 to 240 °C and nip loads of the first calendering section and the second calendering section are controlled to be about300kN/m.

0024According to still another embodiment of the invention a target calendering process moisture content is set for the web and wherein the target moisture content of the web is achieved by drying the web in the after-drying section. This provides the effect that the web after the after-drying section is as such suitable for calendering in respect of its moisture content and thus no additional steam moisturizing is needed.

0025Advantageously the after-drying section is operated so that the target calendering moisture content is obtained by increasing web moisture content up to a point where the target moisture content is achieved.

0026According to still another embodiment of the invention the web is further dried in a post-calendering drying section resulting in final moisture content.

0027According to still another embodiment of the invention the calendering process is performed at elevated moisture content and the web is dried in a post- calendering drying section resulting in final moisture content.

0028According to still another embodiment of the invention the web is subjected to drying with cylinders having their surface roughness selected so that the surface quality obtained in the preceding calendering process will be remained or improved.

0029The present invention provides several benefits compared to prior art method of producing glassine paper, of which only few are mentioned.

0030By means of the invention it is possible to manufacture glassing paper which has excellent properties. At least an equivalent densification is reached with a lower calendering activity, with smaller number of nips and lower nip load meaning e.g. less energy consumption. Also evaporation rate needed during the calendering phase is smaller than in prior art solutions.

0031The present invention results in continuous process without a need of having separate machines and storage space to provide needed density of the paper. Consequently the present invention provides more effective overall process.

0032In this context the Glassine grade is defined as follows. Glassine is a very dense, highly resistant and smooth paper, and it is air, water and oil resistant. Typically the lower grammages (40 to 65 g/m² are designed to be translucent. Typica Iproperties for glassine paper are obtained by a high level of fiber refining, surface closing with film-forming products and calendering. Glassine is used for example as base for release liner because it is suitable for silicone coating. Glassine has to be chemically compatible with the silicone for insuring a proper curing and anchorage of the silicone coating. Glassine may be manufactured from 40 to 150g/m² and is typically used as base of release liner for self-adhesive materia! and label.

0033As an example, in the case of base of release liner for self-adhesive label, the usual specifications required by the market for converting and processing in the application are as follows. <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="82mm" /><colspec colnum="2" colname="col2" colwidth="56mm" /><thead valign="top"><row><entry>Density: Grammage (IS0536) / Thickness (IS0534)</entry><entry>Typical values between 1,1 and 1,2</entry></row></thead><tbody><row><entry>Smoothness of side to be silicone coated (ISO 5627)</entry><entry>900sec min</entry></row><row><entry>Transparency (ISO 2470)</entry><entry>48% min</entry></row><row><entry>Rizinus Cobb absorption (ISO 535)</entry><entry>1,1 g/m² max</entry></row><row><entry>Thickness standard deviation (ISO 534)</entry><entry>2,0µm max</entry></row><row><entry>Porosity (ISO 11004)</entry><entry>&lt; 15.000 pm/Pas</entry></row><row><entry>Breaking length (ISO 1924)</entry><entry>minimum 9,15Km</entry></row><row rowsep="0"><entry>Release forces at given conditions (coatweight: 1 g/m²</entry><entry>@ 10 m/min &lt; 50g</entry></row><row><entry valign="middle">FTM 4 (TESA TAPE 7475)</entry><entry>@ 300 m/min &lt; 90g</entry></row></tbody></tgroup></table></tables>

Brief Description of Drawings

0034In the following, the invention will be described with reference to the accompanying exemplary, schematic drawings, in which <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1</figref> shows a paper machine for manufacturing of glassine paper according to an embodiment of the invention,</li><li><figref idref="f0002">Figure 2</figref> shows a paper machine for manufacturing of glassine paper according to another embodiment of the invention,</li><li><figref idref="f0003">Figure 3</figref> shows a paper machine for manufacturing of glassine paper according to still another embodiment of the invention and</li><li><figref idref="f0004">Figure 4</figref> shows a calendering section of a paper machine for manufacturing of glassine paper according to still another embodiment of the invention.</li></ul>

Detailed Description of Drawings

0035<figref idref="f0001">Fig. 1</figref> shows a paper machine 10 which comprises in successive order i.e. in a running direction of the web, a forming section 12, a press section 14, a first or a pre-drying section 16, a film application and/or impregnating system 18, such as a size press or film press, a second or an after-drying section 20, a calendering section 22 and a reel-up 24. The forming section 12, the press section 14, and the pre-drying section 16 and their operation are known as such by a skilled person in the art.

0036A base paper for glassine paper is manufactured by forming a paper web in a forming section 12 and drying the web in a press section 14 and in a pre-drying section 16. The paper machine further comprises a steam box 26 arranged in connection with the press section. The steam box 26 is arranged to apply heat controllably in cross direction (later CD) to the web effecting on the removal of the water from the web. The operation of the steam box 26 is controlled based on CD moisture profile measured after the pre-drying section 16 and before the film application and/or impregnating system 18 by a measurement frame 28. The purpose is to provide CD moisture profile as even as possible so that the average CD moisture content is within a range of 2 to 7%. The term moisture content means a percentage of ratio of water weight to the weight of the web. This provides advantageous circumstances for the next step, in which a film forming product, which advantageously is for example a water based solution of starch, polyvinyl alcohol, carboxymethyl cellulose, alginate or any other suitable product and/or mixture thereof, is applied on both sides of the base paper in the film application and/or impregnating system 18 following the pre-drying section 16. The actual composition of the film forming product may be the same for both sides of the base paper, or alternatively different film forming product may be applied to each side of the paper. This provides the effect of resulting in good distribution of the film-forming product in the size press. There is a controlling unit 30 which is arranged to perform the above mentioned control procedure in order to practice the method.

0037Subsequently the web having its both surfaces applied with the film forming product is passed through the after-drying section 20. The web and/or the applied film forming product is dried in the after-drying section 20. The after drying section comprises a first drying section 20.1 and a second drying section 20.2, at least one of which is capable of controlling moisture CD profile of the web by transferring heat adjustably in CD direction while drying the web. Advantageously the first drying section is a contactless dryer, such as an infrared dryer, which makes it possible to control the moisture CD profile of the web by transferring heat adjustably in CD direction very efficiently.

0038In the embodiment of <figref idref="f0001">figure 1</figref> the infra-red dryer, preceded by a contactless air operated web guiding device, allows the applied film forming product to be dried enough before the web is brought into contact with any solid surface. The second drying section 20.2 is advantageously a cylinder dryer having steam and/or hot oil heated cylinders. The operation of the first 20.1 and the second dryer part 20.2 is controlled based on CD moisture profile measured after the after-drying section 20 and before the calendering treatment section 22 by a measurement frame 32. The first dryer part 20.1 is operated so that it generally dries the web and/or the applied film forming product and additionally transfers heat to the web so that moisture CD profile may be controlled. The second dryer part 20.2 must not include such profile control capability. Again, the purpose is to provide web CD moisture profile as even as possible at an average CD moisture content range of 6 - 10%, advantageously about 8 % before the calendering treatment section.

0039Operating the after-dryer accordingly provides most effective control of densification of the web in the successive calendering treatment. There is a controlling unit 34 which is arranged to perform the above mentioned control procedure in order to practice the method. After the second dryer part 20.2 and before the first calendering section 22.1 there is a web cooling section 40, which here comprises two cooling cylinders. The cooling section 40 is used as a cooling stage. The web is cooled so that, when the web is subjected to steam application stage 42' after the cooling, the water applied as steam is condensed on the surface of the web.

0040After the base paper and the applied film forming product has been dried in the after-drying section 20 and, if desired and/necessary, moisturized in the steam application stage 42', it is passed through an on-line calendering treatment section 22 in which calendering process is practiced. The calendering treatment section 22 comprises a first calendering section 22.1 in which the paper web is subjected to first calendering treatment, and a second calendering section 22.2 in which the paper web is subjected to second calendering treatment. Between the first and the second calendering sections there is also a web cooling section 40, which advantageously comprises cooling cylinders. The cooling section 40 is used as a cooling stage. In the embodiment of <figref idref="f0001">figure 1</figref> there are two successive cooling cylinders. Moisture content of the web coming from the first calendering section is at about 5 - 6%. Now, the web is cooled by the web cooling section so that, when the web is subjected to steam application stage 42 after the cooling, the water, which is applied as steam is condensed on the surface of the web. Preferably the web temperature after the cooling is below 47°C. But, in case the cooling capacity makes it possible, the web temperature may be cooled down to even lower temperature. The steam application stage may be also referred to as a steam moisturizer. According to an embodiment of the invention there may be another steam moisturized between the first calendering section and after the second dryer part 20.2. The total increase of the moisture content of the web by the steam moisturizer(s) is in the range of 2 - 3 %. This added moisture is evaporated by the calendering process so that the moisture content of the paper at the reel- up is about 6 - 5%.

0041The first calendering section 22.1 comprises a roll stack of a multi-nip calender in which several nips are arranged, advantageously four successive nips. Accordingly, the fist calendering treatment according to the invention comprises treating the paper web in at least four successive calendering nips. In order to result in the desired quality of the glassine paper the temperatures of the heating cylinders are con- trolled to be between 170 - 240°C, and nip loads of the first calendering section are controlled to be 270 - 400 kN/m preferably about 300KN/m. The nips of the first calendering section are arranged to control mainly the CD thickness profile of the paper and to increase the desired quality of the glassine paper i.e. increase density and transparency of the paper. The CD thickness profile controlled by a heat transfer system 44 capable of heating a roll surface locally and adjustable manner in cross direction of the machine (longitudinal direction of roll). Advantageously the heat transfer 30 system is an inductive heating system installed on the first steel cylinder of the calendering section. The operation of the heat transfer system is directly controlled by a thickness CD measurement arranged on a scanner 36.1 before the reel-up 24.

0042The second calendering section 22.1 comprises a stack of rolls in which at least two successive calendering nips are arranged. Accordingly, the second calendering treatment comprises treating the paper web in at least two successive calendering nips after the cooling stage and steam moisturizer. In order to result in the desired quality of the glassine paper the temperatures and nip loads of the second calendering section are controlled preferably so that the densification and smoothness are at the required levels. Accordingly, the second calendering treatment according to the invention comprises treating the paper web so that the temperatures of the heating cylinders are controlled to be between 170 - 240°C, and nip loads of the second calendering section are controlled to be 270-400 kN/m preferably around 300KN/m.

0043Thus according to the invention the calendering section are both provided with multi-nip calenders having successive hard surface and soft surface rolls of which the hard surface rolls are heatable.

0044According to an embodiment of the invention the method comprises a step of controlling web tension between the first and the second calendering treatment and calendering sections. Controlling the web tension controls the tension of the web be- tween the first and the second calendering treatment in separate calendering stacks makes it possible to effect on the smoothness of the paper by creating friction between the paper surface and the calender roll.. Smoothness, on the other hand, is beneficial in further processing of the glassine paper, for example silicone coating. The web tension is provided by the speed difference between the fist and the second calendering section.

0045There is a controlling unit 36 which is arranged to perform the above mentioned control procedure in order to practice the method controlling at least the operation of the first calendering section 22.1, second calendering section 22.2, web cooling section 40 and the steam moisturizer 42.The control units 30, 34 and 36 may be separate control units or they may be integral parts of a paper machine quality control system 38.

0046Since the moisturizer is a steam moisturize, the water will efficiently diffuse into the web, below its surface to inner structure of the web and provide increased densification of the web and smoothness. Increasing the moisture content of the web at the entrance of a calendering section increases also transparency and reduces oil absorption. These are the main indicators of good densification and siliconability. Additionally it is possible in some circumstances obtain good results with smaller nip loads in the calender.

0047A trial experiment that has been made by the inventors has shown that for example that standard deviation thickness value of the glassine paper (62 g/m² produced by the method according to the invention is less than 1,7µm (typically max. 2,µm) which is far below the commercial requirements. Density value was about 1,1 g/cm³ (typically 1,1 - 1,2 g/cm³₎. Also excellent transparency of 50,9% (typically min 48%) was obtained.

0048<figref idref="f0002">Fig. 2</figref> shows an embodiment of the invention. This embodiment comprises the paper machine 10 as shown in <figref idref="f0001">figure 1</figref> with an exception that after the calendering treatment section 22 there is a post-calendering drying section 26 arranged between the calendering treatment section 22 and the reel-up 24. The post-calendering drying section 26 is operated so that it generally dries the web and/or the remaining moisture in the web. This allows usage higher moisture content levels of the web in the calendering process, which in turn improves particularly the densification. Densification of the web is important to the process of manufacturing the glassine paper. There is a controlling unit 39, which is arranged in connection with the second after- drying section 26. The paper machine's 10 control system may also comprise a web measurement frame 41 before the post calendering drying section in the web running direction. Otherwise, the embodiment of <figref idref="f0001">figure 1</figref> comprises corresponding elements and operates similarly to that of <figref idref="f0001">figure 1</figref>.

0049<figref idref="f0003">Fig. 3</figref> shows another embodiment of the invention. A paper machine 10 comprises in successive order i.e. in a running direction of the web, a forming section 12, a press section 14, a pre-drying section 16, a film application and/or impregnating system 18, an after-drying section 20, a calendering section 22, a post-calendering drying section 26 and a reel-up 24. The forming section 12, the press section 14, and the pre-drying section 16 and their operation are known as such by a skilled person in the art.

0050In this embodiment a base paper for glassine paper is manufactured by forming a paper web in a forming section 12 and drying the web in a press section 14 and in a pre-drying section 16. The paper machine further comprises a steam box 26 arranged in connection with the press section. The steam box 26 is arranged to apply heat controllably in cross direction to the web effecting on the removal of the water from the web in the press section. The operation of the steam box 26 is controlled, e.g. based on CD moisture profile measured after the pre-drying section 16 and before the film application and/or impregnating system 18 by a measurement frame 28. The purpose is to provide CD moisture profile as even as possible at an average CD moisture content range of 2 to 7% after the pre-drying section. This provides advantageous circumstances for the next step, in which a film forming product is applied on both sides of the base paper in the film application and/or impregnating system 18. There is a controlling unit 30 in the paper machine 10 which is arranged to perform the above mentioned control procedure in order to practice the method.

0051Subsequently the web having its both surfaces applied with the film forming product is passed through the after-drying section 20. The after drying section comprises a first drying section 20.1 and a second drying section 20.2, at least one of which is capable of controlling moisture CD profile of the web by transferring heat adjustably in CD direction while drying the web. The web and/or the applied film forming product is dried in the after-drying section 20 while simultaneously controlling he CD moisture content of the web. Advantageously the first drying section is a contactless dryer, such as an infra-red dryer, which makes it possible to control the moisture CD profile of the web by transferring heat adjustably in CD direction very efficiently.

0052In the embodiment of <figref idref="f0003">figure 3</figref> the infra-red dryer, preceded by a contactless air operated web guiding device, allows the applied film forming product to be dried enough before the web is brought into contact with any solid surface. The second drying section 20.2 is advantageously a cylinder dryer. The operation of the first 20.1 and the second dryer part 20.2 is controlled advantageously based on CD moisture profile measured after the after-drying section 20 and before the calendering treatment section 22 by a measurement frame 32. The first dryer part 20.1 is operated so that it generally dries the web and/or the applied film forming product and additionally transfers heat to the web so that moisture CD profile may be controlled i.e. heating is controlled to be more intensive at the CD locations where the web has more moisture. The second dryer part 20.2 must not include such profile control capability. Again, the purpose is to provide CD moisture profile as even as possible so that an average CD moisture content is within a range of 7 - 11%, advantageously about 9 % before the calendering treatment section. In the method of manufacturing of glassine paper according to the embodiment of <figref idref="f0003">figure 3</figref> a target calendering process moisture content is set for the web. The target calendering process moisture content is 7 - 11%, advantageously about 9 %. It is monitored by a measurement frame 32 based on which the operation of the paper machine upstream in running direction of the web is controlled

0053The second drying section 20.2 is controlled and operated so that the web coming out from the second drying section is at the target calendering moisture content or within an acceptable window. In other words the web is dried to the target calendering moisture content so that adding of additional water is not required. The target moisture is stored or made available to the controlling unit 34 which controls the operation of the second drying section 20.2. Thus, advantageously the after-drying section is operated so that the target moisture content for calendering process 22 is obtained by raising web moisture content only up to the point where the target moisture content is reached. This way there is no need to add additional water into the web i.e. the web is led from the after-drying section 20 to the calendering process without actively moisturizing the web.

0054Operating the after-dryer accordingly provides most effective control of densification of the web in the successive calendering treatment. The controlling unit 34 is arranged to perform the above mentioned control procedure in order to practice the method.

0055Thus, subsequently the base paper and the applied film forming product has been dried in the after-drying section 20 and, it is passed through an on-line calendering treatment section 22 in which calendering process is practiced. Even if the calendering treatment section 22 comprises in <figref idref="f0003">figure 3</figref> only one calendering section 22.1 the actual physical composition may vary in the scope of the invention. For example, the calendering section may comprise two separate stacks, or event 3 or 4 stacks if so desired. In any case, the paper web is subjected to calendering treatment which provides the effect required by glassine paper.

0056The calendering section 22.1 comprises a roll stack of a multi-nip calender in which several nips are arranged. Accordingly, the calendering treatment according to the invention comprises treating the paper web in at least four successive calendering nips. In order to result in the desired quality of the glassine paper the tempera- tures of the heating cylinders are controlled to be between 170 - 240°C, and nip loads of the first calendering section are controlled to be 270 - 400 kN/m. The nips of the calendering section are arranged to control mainly the CD thickness profile of the paper and to increase the desired quality of the glassine paper i.e. increase density and transparency of the paper. The CD thickness profile controlled by a heat transfer system 44 capable of heating a roll surface locally and adjustable manner in cross direction of the machine (longitudinal direction of roll). Advantageously the heat transfer system is an inductive heating system installed on the first steel cylinder of the calendering section. The operation of the heat transfer system is directly controlled by a thickness CD measurement arranged on a scanner 37 before the reel-up 24 and/or the scanner 41 after the calendering section and/or the scanner 32 before the calendering section.

0057There is a controlling unit 34, 36 which is arranged to perform the above mentioned control procedure in order to practice the method controlling at least the operation of the calendering section 22.1, The control units 30, 34, 36 and 39 may be separate control units or they may be integral parts of a paper machine quality control system 38.

0058Since the target moisture is obtained only by drying the web, the water is advantageously manner distributed in the web, below its surface at inner structure of the web and provides increased densification of the web and smoothness. Maintaining the target moisture content of the web at the entrance of a calendering section increases also transparency and reduces oil absorption. These are the main indicators of good densification and siliconability. Additionally it is possible in some circumstances obtain good results with smaller nip loads in the calender.

0059A trial experiment that has been made by the inventors has shown that for example that standard deviation thickness value of the glassine paper (62 g/m²) produced by the method according to the invention is less than 1,7µm (typically max. 2,0 µm) which is far below the commercial requirements. Density value was about 1,1 g/cm³ (typically 1,1 - 1,2 g/cm³₎.Also excellent transparency of 50,9% (typically min 48%) was obtained.

0060After the calendering treatment section 22 there is a post-calendering drying section 26 arranged between the calendering treatment section 22 and the reel- up 24. The post-calendering drying section 26 is operated so that it generally dries the web and/or the remaining moisture in the web. This allows calendering at elevated moisture content i.e. usage higher moisture content levels of the web in the calendering process, which in turn improves particularly the densification. The elevated moisture content means such moisture content that calendering process alone will not evaporate all required amount of water from the web in order to meet the final moisture content for commercial product. Densification of the web is important to the process of manufacturing the glassine paper. There is a controlling unit 39 which is arranged in connection with the second after-drying section 26. The paper machine 10 control system may also comprise a web measurement frame 41 before the post calendering drying section in the web running direction.

0061In order to have the surface of the paper after calendering at desired quality and not degrade the surface, it is essential to have chromed drying cylinders in the post-calendering drying section 26 with very smooth surface (Roughness parameter Ra at least 0.1 µm) According to an embodiment of the invention the drying cylinder in the post-calendering section 26 have their roughness selected so that the surface quality obtained in the preceding calendering process will be remained or improved.

0062In <figref idref="f0004">figure 4</figref> there is shown a part of a paper machine illustrating two successive calender sections 22.1 and 22.2 in more detailed manner. The operation of a calendering process comprising at least a first calendering treatment in which the paper web is subjected to first surface treatment in a first calendering section 22.1 and a second calendering treatment in which the paper web is subjected to second surface treatment in a second calendering section 22.2. Each of the calender rolls 50.1, 50.2, 50.3, 50.4, 50.5 of the first stack as well as the rolls 52.1, 52.2, 52.3 of the second stack is provided with a drive 54. Operation of each drive 54 is controlled by the controlling unit 36.

0063In successful calendering process the chain of successive treatments to which the paper is subjected is critical. According to an embodiment of the invention the speed of the rolls 52.1 - 52.3 of the second calender section is slightly higher than the speed of the rolls 50.1 - 50.5 of the first calender section. The speed differential creates a slip of the paper web in the nip between the first calender rolls, thus creating the friction phenomenon.

0064Thus, by controlling the speed (tension) of web after a stack 22.1 of calen- dar rolls 50.1 - 50.5 by the second stack 22.2 it is possible to have friction as a control parameter enhancing the smoothening effect of the cylinders of the calender.