Abstract:
The present invention provides a method and device for the efficient production of a substantially wood free coated web. In particular, a method for producing an at least substantially wood free coated web includes the application of a double coating on at least one side of the web, whereby the last coating application occurs by means of a contactless coating process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a continuation of PCT application No. PCT/EP2008/057335, entitled “METHOD AND DEVICE FOR PRODUCING AN AT LEAST ESSENTIALLY WOOD-FREE COATED PAPER WEB”, filed Jun. 12, 2008, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method and a device for producing a substantially wood free, coated web. 
         [0004]    2. Description of the Related Art 
         [0005]    With hitherto known methods of producing a wood free or almost wood free double coated paper web at higher speeds of more than 1400 m/min, the following segments are provided: gap former; press section; pre-drying section pre-calender with softnip, hard nip or wide nip extended in direction of web travel; metered size press (MSP) for the application of glue or for the purpose of pre-coating; blade coater; blade coater; and an online or offline multi-nip calender for degrees of gloss. 
         [0006]    The fundamental disadvantage of the hitherto known concepts consists in that the overall efficiency (e.g. time and material) is considerably reduced by the blade coaters. In a conventional method with an offline calendar, the loss of time in the area of the two blade coaters is approximately 4 to approximately 7%. Due to this substantial loss of time, the maximum speed potential of the paper machine cannot be completely utilized. 
         [0007]    Currently, the furnish for the wood free or approximately wood free double coated quality levels includes the following: needle bleached Kraft pulp, long fiber (NBKP); leaf bleached Kraft pulp, short fiber (LBKP); bleached chemo-thermo mechanical pulp, short fiber (BCTMP SF); and filler, whereby the fiber content of the furnish consists of at least 85% pulp fibers NBKP and LBKP. 
         [0008]    In a method to produce a wood free coated mat or semi-mat web which is known from WO 2005/071158 A1, the web is coated after pre-calendering on at least one side by means of a device for the application of a liquid or highly viscous coating medium and is subsequently dried by means of at least one drying device. Prior to pre-calendering, the web is single coated on a first side by means of a film or curtain coater. 
         [0009]    What is needed in the art is an improved method and device for the production of a substantially wood free coated web with which the time efficiency potential for double coating on at least one web side is substantially improved within the paper manufacturing process. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides a method and device for the efficient production of a substantially wood free coated web. In particular, the method of the present invention for producing an at least substantially wood free coated web includes the application of a double coating on at least one side of the web, whereby the last coating application occurs by means of a contactless coating process. 
         [0011]    One important aspect of the present invention is the avoidance of direct application coating systems which operate with stationary doctor blade elements, for example so-called blade coaters. The stress upon the web resulting from the relative speed between doctor blade and web increases the number of web breaks. Therefore, at least the last coating device operates contactless according to the present invention. 
         [0012]    The last coating application is, for example, applied by means of a curtain coater and/or by means of a spray coater. 
         [0013]    According to one embodiment of the present invention,n the fibrous component of the furnish may consist of less than 85% pulp fibers NBKP and LBKP, for example, of less than 75%, or less than 60% pulp fibers. The remainder may consist of wood containing fibers (e.g. mechanical wood pulp and/or BCTMP-SF). A paper web which is produced from this furnish and which contains a component of pulp fibers of more than 55% is referred to below as a “substantially wood-free paper web”. Compared with the current state of the art the furnish of expensive fibrous material can be reduced at the same or better coating quality, with the inventive solution. 
         [0014]    Here, the fiber content in the web is essentially consistent with that in the furnish, whereby differences in the fiber content between furnish and paper web can occur due to retention. 
         [0015]    A second embodiment of the method of the present invention is characterized in that the pre-coating is applied by means of a film coater and/or by means of a contactless coating process. The coating medium is applied by means of a surface which moves together with the web, as is the case, for example, with a film coater. This reduces the stress on the web compared to the blade coating process. If, following a pre-drying of the web, a color coat is applied as a pre-coating application, then the web may be run through a pre-calender unit prior to the application of the color pre-coat. If, however, in contrast, a glue is applied as a pre-coat following a pre-drying of the web, then the web may be run through a calender following the glue application. A higher consistency of the applied glue has an effect on the coating quality of the last, contactless coating application. According to a further embodiment of the present invention, the consistency can be approximately &gt;12%, for example, &gt;15% or 20%. 
         [0016]    The web may be carried through the press section in a closed loop. In this instance therefore, a closed web routing is provided in the press section whereby the web is supported, for example, by a clothing and/or a felt which is provided in the press, a transfer belt, etc. The web may be carried in the press section through only one shoe press with only one wide nip. 
         [0017]    According to a third embodiment, the web can also be routed in the press section through several shoe presses arranged in tandem, each being equipped with a wide nip. 
         [0018]    The web may also be routed through at least one calender. For this purpose the web can, for example, be carried through at least one calender with a soft nip which is formed between two calender rolls, at least one of which is equipped with a soft synthetic material cover, especially compared to steel rolls or a chilled roll. 
         [0019]    Alternatively, or in addition, the web can also be routed through at least one calender with a hard nip which is formed between two calender rolls, for example, steel rolls and/or chilled rolls, each of which possess a hard roll surface, especially compared to a synthetic material cover. 
         [0020]    Alternatively, or in addition, the web can also be routed through at least one calender, especially a shoe calender or a metal belt calender with a wide nip that is extended in the direction of web travel. 
         [0021]    The device of the present invention is accordingly characterized in that devices are provided to apply a double coating onto at least one side of the web, whereby a contactless coating device is provided for the application of the final coating application. 
         [0022]    A curtain coater may, for example, be utilized. By using the curtain coating process, the time efficiency compared to the conventional doctor blade coating methods can be improved by approximately 3 to approximately 6%. Here it is assumed that the web breaks occurring in the after-drying sections occur at the same frequency as with the blade coaters. The additional advantage in connection with the curtain coating process exists in that already at low coating weights a substantially greater coating coverage can be achieved. This results in a significant improvement of the potential to relocate ash from the coating layer to the filler, thus bringing about considerable cost savings for the paper manufacturer. The paper quality can be optimized by appropriate pre-calendering of the paper prior to coating by means of the curtain coater or coaters. 
         [0023]    The optimum quality range for the concept which is operated entirely online is the range of approximately 60 to approximately 200 g/m 2 , for example, in the range of approximately 80 to approximately 150 g/m 2 . 
         [0024]    The final calendering of the paper can be implemented online or, if necessary, also offline. The fundamental advantage of the curtain coating process compared to a conventional blade coating process consists in that the efficiency of calendering is substantially higher while the danger of deposits on the calender rolls is reduced. 
         [0025]    The inventive process as well as the inventive device can be utilized for the production of an at least essentially wood free coated paper web with a filler content of &gt; approximately 8%, for example, &gt; approximately 10% or approximately 12% at a basis weight of ≦ approximately 90 g/m 2  and/or with a filler content of &gt; approximately 12%, for example, approximately 14% at a basis weight of ≧90 g/m 2  and/or with a coating weight of the outer coating layer of ≦ approximately 10, for example, ≦ approximately 9 and ≦ approximately 8 g/(m 2  per side) at a basis weight of &lt; approximately 90 g/m 2  and/or with a coating weight of the outer coating layer of ≦ approximately 16, for example, ≦ approximately 14 and ≦ approximately 12 g(/m 2  per side) at a basis weight of approximately 90 g/m 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the present invention taken in conjunction with the accompanying drawings, wherein: 
           [0027]      FIG. 1  is a schematic illustration of an exemplary design variation of a device for the production of an at least essentially wood free coated paper web; 
           [0028]      FIGS. 2   a  and  2   b  illustrate a simplified schematic partial depiction of an additional exemplary design variation of the device including a high efficiency drier; 
           [0029]      FIG. 3  is a schematic depiction of an arrangement which includes two curtain coaters for the application of coating color onto both sides of the web in a contactless process; and 
           [0030]      FIG. 4  is a diagram wherein the coverage characteristics of a curtain coater are compared with the coverage characteristics of a blade coater. 
       
    
    
       [0031]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Referring now to the drawings, and more particularly to  FIG. 1 , there is shown a schematic depiction of an exemplary embodiment of device  10  for the production of an at least substantially wood free coated paper web including wire section  12 , press section  14  of pre-drying section  16 , device  18  for the application of glue, after-drying section  20 , calender or pre-calender  22 , device  24  for the application of a color coating, drying device  26  and reel winder  28 . 
         [0033]    As can be seen from  FIG. 1 , device  18  for the application of glue and device  24  for the application of color respectively can, for example, be a film coater. 
         [0034]    In area “X” after drying device  26  and before reel winder  28 , contactless coating devices  30 ,  32  are provided as the last applicator device for a two-sided coating application. Final applicator devices  30 ,  32  can specifically be in the embodiment of curtain coaters. 
         [0035]    The optimum quality range for the concept which is operated entirely online is the range of approximately 60 to approximately 200 g/min 2 , for example, in the range of approximately 80 to approximately 150 g/m 2 . 
         [0036]    The final calendering of the paper can be implemented online or, if necessary, offline. The advantages of the curtain coating process compared to a conventional blade coating process is that the efficiency of calendering is substantially higher while the risk of deposits on the calender rolls is reduced. 
         [0037]    The device can, therefore, include devices for applying at least one double layer of coating onto at least one web side, whereby contactless applicator device  32  is provided for the final coating application. A curtain coater and/or a spray coater is to be provided to apply the final coating application. To apply the pre-coating a film coater and/or a contactless applicator device is provided. 
         [0038]    Following a device for pre-drying the paper web which, for example, may include pre-drying section  16  and after-drying section  20 , device  24  for the application of the color pre-coating can be provided, whereby pre-calender  22  is provided prior to device  24 . 
         [0039]    Device  18  for the application of the glue is located between pre-drying section  16  and after-drying section  20 . Calender  22  is provided following pre-drying section  20 . The paper web is may be carried through press section  14  in a closed loop. The paper web can be carried in the press section through only one shoe press with only one shoe press nip or also through several shoe presses with one shoe press nip each which are located in tandem. 
         [0040]    Apparatus  10  also includes, for example, at least one calender  34 , which in the existing example is located, for example, between device  24  for the application of the color coating or drying device  26  and reel winder  28 . 
         [0041]    For example, at least one calender can be equipped with a soft nip which is formed between two calender rolls, at least one of which is equipped with a soft synthetic material cover, for example, compared to steel rolls or chilled rolls. 
         [0042]    Alternatively or in addition, at least one calender can be equipped with a hard nip which is formed between two calender rolls, for example, steel rolls, each of which possess a hard roll surface. 
         [0043]    Alternatively, or in addition at least one calender, for example, a shoe calender or a metal belt calender with a wide nip that extends in the direction of web travel can be provided. Wire section  12  includes for example, a gap former. 
         [0044]    As can be seen in  FIG. 2 , at least one high efficiency dryer  36  can be provided after press section  14 . Here, such high efficiency dryer  36  may, for example, include at least one impingement dryer (compare  FIG. 2 ) or, for example, at least one drying cylinder whose surface temperature is &gt; approximately 120° C. and approximately 130° C. 
         [0045]    After high efficiency dryer  36  the paper web possess a dry content of preferably &gt; approximately 63%. 
         [0046]    As can also be seen in  FIG. 2 , a single row pre-drying section can, for example, be provided after high efficiency dryer  36 .  FIGS. 2   a  and  2   b  show a simplified schematic partial view of such an exemplary design form of apparatus  10  with high efficiency dryer  36  and single row pre-drying section  16  following. Wire section  12  can include a gap former. 
         [0047]    Press section  14  includes press  38  with suction roll  40  and a nip through which paper web  42  is guided together with top felt  44  and bottom felt  46 . Press  38  is followed by shoe press  48 , through whose wide nip paper web  42  is guided together with top felt  50  and lower belt  52  which can, for example, be a felt or a transfer belt. After shoe press  48 , high efficiency dryer  36  is provided which—in this example—includes an impingement dryer. As previously mentioned a drying cylinder can be provided alternatively or in addition, whose surface temperature is &gt; approximately 120° C. and &gt; approximately 130° C. 
         [0048]    High efficiency dryer  36  is followed, for example, by single row pre-drying section  16 . Following pre-drying section  16  is device  18  for the application of glue, which in this instance may comprise a film coater. 
         [0049]    As can be seen from  FIG. 2   b , following device  18  for the application of glue, drying device  54 , calender or pre-calender  22 , film coater  56 , dryer  58 , curtain coater  60  for the underside of the web, curtain coater  62  for the top side of the web, contactless turning device  64  such as, for example, an Airturn, impingement dryer  66 , optional calender  68 , cylinder group  70 , such as, for example, a draw and/or cooling group and reel winder  28  are provided in tandem, viewed in direction of web travel L. Prior to two curtain coaters  60 ,  62  in direction of web travel L, air boundary layer suction device  72  or respectively  74  can be provided in each case in order to produce a good coating quality. 
         [0050]      FIG. 3  is a schematic illustration of an exemplary design form of an arrangement including curtain coaters  30 ,  32  for the application of coating color on both web sides in a contactless process. As can be seen in  FIG. 3 , drying device  76  is provided following this arrangement. The arrangement according to  FIG. 3  can be provided, for example, in area “X” of device  10  according to  FIG. 1 . Here, drying device  76  may again be provided following curtain coaters  30 ,  32 . The inventive solution results in advantages at higher speeds, since the entire process is subject to fewer web breaks and therefore offers better runability. The following measures are contributory in this:
       Former: Gap former (twin wire former);   Shoe press: Tension free—in other words, the web is continuously supported, and accordingly there are no web breaks. The shoe press provides a gentle pressing which is advantageous regarding the bulk and sufficient dewatering at higher speeds;   Pre-drying: The press is followed by a high efficiency dryer (for example impingement dryer, drying cylinder with high surface temperature &gt; approximately 120° C. and &gt; approximately 130° C.), after the high efficiency dryer the web has a dry content of &gt; approximately 63%, resulting in a greater web strength in the first free draw. This is followed by a single row drying section.   Coating process: Fewer web breaks occur without doctor blades. In addition, better coverage results with a curtain coater and/or a spray coater, thereby reducing the coating weight while being able to increase the filler in the paper and rendering the process more cost effective and economical.         
         [0055]      FIG. 4  is diagram in which the coverage properties of a curtain coater are compared to the coverage properties of a blade coater. The diagram shows the coverage [%] relative to the coating weight [g/m 2 ] for a blade coater and a curtain coater. The coverage can be determined, for example, as a surface share of the coating application for which the thickness of the coating application exceeds a certain minimum thickness of, for example, 1 micrometer. 
         [0056]    Due to the better coverage properties the coating weight with a curtain coater compared to a blade coater can be reduced at 95% coverage from approximately 12 g/m 2  per side to approximately 8 g/m 2  per side. Here, 2 times 4 g/m 2  more filler can be added in the pulp for bulk compensation, thereby clearly reducing the costs. 
         [0057]    The inventive process as well as the inventive device can be utilized advantageously for the production of an at least essentially wood free coated paper web with a filler content of &gt; approximately 8%, for example, &gt; approximately 10% and approximately 12% at a basis weight of ≦ approximately 90 g/m 2  and/or with a filler content of &gt; approximately 12%, for example, approximately 14% at a basis weight of ≧ approximately 90 g/m 2  and/or with a coating weight of the outer coating layer of ≦ approximately 10, for example, ≦ approximately 9 and ≦ approximately 8 g/(m 2  per side) at a basis weight of &lt; approximately 90 g/m 2  and/or with a coating weight of the outer coating layer of ≦ approximately 16, for example, ≦ approximately 14 and ≦ approximately 12 g/(m 2  per side) at a basis weight of approximately 90 g/m 2 . 
         [0058]    The present invention hereby relates to the production process of at least essentially wood free coated papers, especially to the sequences of color coating process, glue coating process and calenders, as well as to filler contents in the paper, and coating weights. 
         [0059]    While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims. 
         [0060]      10  Device 
         [0061]      12  Wire section 
         [0062]      14  Press section 
         [0063]      16  Pre-drying section 
         [0064]      18  Device for the application of a glue coating 
         [0065]      20  After-drying section 
         [0066]      22  Calender, pre-calender 
         [0067]      24  Device for the application of an color coating 
         [0068]      26  Drying device 
         [0069]      28  Reel winder 
         [0070]      30  Contactless coater 
         [0071]      32  Contactless coater 
         [0072]      34  Calender 
         [0073]      36  High efficiency dryer 
         [0074]      38  Press 
         [0075]      40  Suction roller 
         [0076]      42  Paper web 
         [0077]      44  Top felt 
         [0078]      46  Bottom felt 
         [0079]      48  Shoe press 
         [0080]      50  Top felt 
         [0081]      52  Bottom belt 
         [0082]      54  Drying device 
         [0083]      56  Film coater 
         [0084]      58  Drying device 
         [0085]      60  Curtain coater 
         [0086]      62  Curtain coater 
         [0087]      64  Contactless turning device, Airturn 
         [0088]      66  Impingement drying device 
         [0089]      68  Calender 
         [0090]      70  Cylinder group 
         [0091]      72  Air boundary layer suction 
         [0092]      74  Air boundary layer suction 
         [0093]      76  Drying device 
         [0094]    L Direction of web travel 
         [0095]    X Area