Abstract:
A method of forming a coated printing paper product is provided. A printing paper product is first coated in a non-contact coating process. The printing paper product is then final calendered with a surface conditioning device. The coated printing paper product is formed for offset printing with a bulk of between about 1.15 m 3 /kg and about 1.3 m 3 /kg, and with the top side of the coated printing paper product having a PPS-s10 roughness of between about 0.7 μm and about 1.5 μm and a Hunter gloss of between about 30% and about 80%.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of the present invention relate to an LWC paper product and a method of producing the same.  
         [0003]     2. Description of Related Art  
         [0004]     In the field of producing paper products, one ongoing goal is to improve the quality of a paper product, especially LWC paper, and the economy of producing the same.  
         [0005]     Paper is required to have a certain surface quality for ensuring a desired gloss and print quality, a low transparency and a sufficient stiffness and tear resistance. Additionally, since paper is produced in large quantities in a paper mill, the efficient use of raw material is important. However, these demands are somewhat contradictory to each other. Paper can be provided with a sufficient gloss by calendering the paper by compressing it in a nip, often moistened and heated in a certain manner. The surface fibers and coating of paper are preferably pressed smooth by this compression, yet without compacting the middle layer of paper. The compaction of a middle layer undermines the stiffness of paper and reduces tear resistance. At the same time, the transparency of paper increases. This compaction of a middle layer is often referred to as a loss of bulk. In this case, bulk is understood as being the inverse value of density and a loss thereof is thus equal to a densifying compaction of paper or board.  
         [0006]     Since the process of making paper is highly raw material intensive, even a minor saving in raw material provides a major advantage over competitors. In this respect, a saving of just one percent can be considered a major competitive edge and the investment restitution time is short. Saving raw material is also desirable for environmental reasons. By virtue of a paper grade of lower weight, the beneficial multiplicative effects of the paper of this invention cover the product&#39;s entire life span, the reduced consumption of raw material resulting in a lighter product which ultimately creates savings also in shipping operations and in the way of a reduced amount of waste. The improved bulk and opacity do not cause the consumer any practical adversities.  
         [0007]     A machine calender is often used together with other calenders, the machine calender referring to a hard calender with no elasticity in its rolls. The use of a machine calender as the sole surface treatment method is not advisable. A soft calender refers to a soft-nip calender, wherein the calender roll has a surface which is elastic, the surface having possibly a hardness in the same order as the surface hardness of wood, yet being elastic.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008]     The above and other needs are met by the present invention which, in one embodiment, provides a method of making a paper product having a flat printing surface, a high gloss and stiffness in the printing paper with a lesser-than-before consumption of material, and avoiding bottlenecks as well as improving runnability in the production process. Generally, the pretreatment of a paper surface prior to a coating process is performed with a machine calender and the finishing treatment with a supercalender. The function of a machine calender is to provide the web with a uniform thickness profile. After machine calendering, paper is coated and final calendering is usually performed with a supercalender. The coating method is typically blade coating or film coating.  
         [0009]     The quality values of thus produced offset LWC printing paper are within the following range:  
                                                                         Bulk   0.90-1.1   cm 3 /g           PPS-s10 roughness   0.8-1.6   μm                Gloss   50-70%                      
 
         [0010]     According to the invention, printing paper is treated with a long-nip calender after a coating process in order to upgrade the paper qualities over what is known before and, in addition, the production runnability is improved and the production method is not subject to a speed restraint the same way as a supercalender. A long-nip calender suitable for making paper of the invention has been described, for example, in U.S. Pat. No. 6,164,198 also assigned to the assignee of the present invention.  
         [0011]     More particularly, a calender suitable for the surface treatment of paper includes a fixed support element, around which is a tubular jacket. A heated counter element is disposed on the other side of the tubular jacket from the support element, such that a web passes through between said counter element and the tubular jacket. The fixed support element is provided with load elements, pressing the jacket against the heated counter element and thereby enabling a calendering process between the jacket and the counter element. The jacket has its opposite ends secured to end walls mounted rotatably relative to the support element, the rotary motion of the end walls being delivered by a separate drive motor, which is independent of a motion of the fiber web in order to avoid overheating of the jacket.  
         [0012]     A method of the invention for conditioning the surface of coated or uncoated paper with a surface conditioning device comprises feeding a fiber web through a long nip established by a roll and a counter-roll, the former being in the form of a tubular-shaped flexible jacket. Across the extent of the nip the jacket deflects or bends and thereby presses into contact with the counter-roll over a long stretch. The paper treated with the method is lighter than currently available paper grades, while stiffness and surface properties are equal to those of currently available papers.  
         [0013]     The solution enables a running speed substantially higher than what is accomplished with a supercalender. In addition, the runnability is better, which also contributes to improved quality and reduces waste.  
         [0014]     Web speed in the calender may be higher than 600 m/min, preferably higher than 800 m/min, and still more preferably 1000 m/min, and even as high as about 4000 m/min. Thus, the calender neither restricts the speed of a paper machine nor is there a need for several calenders in parallel. The above-mentioned heated roll has a temperature of 150-350° C., preferably higher than 170° C., most preferably about 200-250° C. Linear pressure in the nip is within the range of 100-500 kN/m, preferably less than 400 kN/m, most preferably about 320-380 kN/m. Maximum pressure in the nip is 3-15 MPa, preferably less than 13 MPa, most preferably about 8-12 MPa.  
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0015]     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:  
         [0016]      FIG. 1  is a sectional view of a long-nip calender, provided with an extended nip between a shoe calender and a counter-roll;  
         [0017]      FIG. 1A  is a partial enlargement of  FIG. 1 ;  
         [0018]      FIG. 2A  is a partial sectional view of the device shown in  FIG. 1 , along the roll axis and depicting a drive mechanism; and  
         [0019]      FIG. 2B  shows the operation of press shoes in a longitudinal section. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The present inventions now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.  
         [0021]     In  FIG. 1 , a paper  80  travels through an extended and heated nip  1 . The nip  1  is established by an enclosed shoe roll  10  present under the web  80 . Above the web  80  is a heatable counter-roll  22 . The enclosed shoe roll comprises a flexible jacket  12  impervious to liquid. The jacket consists for example of fiber-reinforced polyurethane. A stationary fixed support element  14  carries at least one load shoe  18 . Between the load shoe  18  and the support element is an actuator  20 , such as a hydraulic cylinder, for urging the concave load shoe  18  and thereby also the flexible jacket  12  against the counter-roll  22 . Thus, the jacket  12  is forced out of its normal unloaded position  11  in a direction away from the center of the enclosed shoe roll. The jacket  12  is fastened at both ends thereof to end walls  24 ,  26 , thus creating a sealed compartment  13  (see  FIG. 2 ). As also shown in  FIG. 1 , at least one detector device  99  is mounted in communication with the web  80  for detecting web breaks. The detector device  99  is connected to a control device  98  for controlling the operation of a calendering process depending on whether the web is broken or not.  
         [0022]     As shown in  FIG. 1 , the heatable counter-roll  22  is accompanied by a disengagement mechanism, comprising a lever  95  pivotable by a hydraulic cylinder assembly  94  and provided with a pivot point  96  for pivoting the lever thereon. The disengagement mechanism presses the counter-roll  22  to an engagement with the nip  1  and disengages it from the nip  1 . Between the load shoe  18  and the jacket  12  is supplied pressurized oil, which develops a hydrostatic pressure throughout the nip and presses the jacket to an engagement with the counter-roll  22  over the entire extent of the nip  1 . At the same time, the oil protects the jacket from being damaged by lumps and a temperature rise.  
         [0023]     In  FIG. 2A , it is shown that the end walls  24 ,  26  are rotatably mounted on stub shafts  16 ,  17  of the support element  14  (The end walls are preferably not integral but divided into a static part and a rotating part, as shown in  FIG. 2B ). On one end of the stub shaft, a cylindrical shaft  32  is arranged rotatably via bearings  34 . A support column  36  is arranged to the cylindrical shaft via self-aligning bearings  38 , which allow spherical movement to allow the deformation/bending of the support element  14  in response to a heavy load. One of the end walls  24  is fixedly attached to the cylindrical shaft. A drive transmission  40 , in the present embodiment a cog wheel, is fixedly attached to the cylindrical shaft outside the end wall. The cog wheel is connected to a transmission  42  and in turn a drive  44 . A cog wheel  46  is fixedly attached to the cylindrical shaft inside the end wall. A drive shaft  48  is arranged inside the jacket and parallel to the support element  14 . The drive shaft  48  is supported by bearings  50  arranged in bearing houses  52  attached to the support element. At each end of the drive shaft, cog wheels  54  are arranged. Preferably these cog wheels have a prolonged toothed portion to allow axial movement of the intermeshing cog wheel which is attached to the end wall. A further cog wheel  56  is fixedly attached to the second end wall  26  inside the jacket. Both cog wheels inside the jacket mesh with the corresponding cog wheel on the drive shaft. The second end wall  26  is rotatably arranged on the second stub shaft  17 . The second stub shaft is in turn fixedly attached to a second support column  58 .  
         [0024]     The operation proceeds as follows. During normal operation, the driven heated roll  22  is in interaction with the fiber web and the flexible jacket  12  by a desired pressure being exerted by the load shoe  18 , thereby causing a friction based drive of both the fiber web and the flexible jacket. Accordingly, during normal operation, the forces exerted in the nip provide for rotation of the enclosed shoe roll.  
         [0025]     Only in specific occasions, it will normally be desirable to operate the independent drive of the enclosed shoe roll  10 , for example when starting up the calender. If the calender should be started without first speeding up the flexible jacket  12 , this would inevitably cause damage to the flexible jacket due to overheating. Furthermore, it would also be deteriorating for the fiber web, since at the moment of start it would develop exceptional tension forces in the fiber web. Accordingly, the independent drive arrangement of the enclosed shoe roll is to be used for instance at the start-up of the calendering surface. At the start, the nip gap is not closed, but the roll  22  has been moved out of contact with the nip  1 . Before moving the heated counter-roll  22  into the nip, the drive arrangement  44  of the enclosed shoe roll  10  is activated to accelerate the first end wall  24  via transmissions. The rotation of the end wall causes the inner first cog wheel  46  to rotate, and subsequently the drive shaft  48 . The drive shaft transmits the rotation to the second end wall  26  via the second inner cog wheel  56 . The both end walls are thus accelerated and rotate at the same speed until a desired peripheral speed is obtained, which is normally equal to the speed of the fiber web. The nip is closed by activating the hydraulic piston  94  to pivot the lever  95  and thereby moving the counter-roll  22  into the nip and subsequently the load shoe  18  is urged against the heated roll  22  by its actuators  20 . Once the calender functions in the desired manner, the drive arrangement of the enclosed shoe roll can be deactivated and the press roll driven in a conventional manner by friction within the nip  1 .  
         [0026]     In  FIG. 2B  there is shown an alternative embodiment of the drive arrangement for an enclosed shoe roll. This embodiment uses friction for the transmission of rotational forces.  
         [0027]      FIG. 2B  also shows a design of arranging the support element and the end walls. The end walls are divided into inner parts  24 A,  26 A connected non-rotatably to the support element  14 , a rotational part  24 B,  26 B, and a bearing assembly  24 C,  26 C therebetween. The support element  14  is at each end thereof arranged with self-aligning bearings  23 ,  25  to allow a deflection of the support element  14 .  
         [0028]     In the figure there is shown a drive  44  having a shaft  19 B. On the shaft  19 B is mounted a disc  19  having a rubber layer at its peripheral end  19 A. The outer ends of the flexible jacket  12  are fixedly attached between an annular ring  15 , acting as a replaceable force transmitting device, and the periphery of each end wall. The ring  15  is fixedly attached to the end wall. On the inside of the rotational part  24 B,  26 B of each end wall there is fixedly attached a cog wheel  46 ,  56 . The drive arrangement  44 ,  19  is movable in and out of contact with the force transmitting device  15 . When it is desired to accelerate the enclosed shoe roll  10 , the drive arrangement is moved such that the rubber layer  19 A comes into frictional engagement with the force transmitting device  15 . The cog wheel  46  and the drive shaft  48  transmit the rotation of the end wall  24  to the other end wall  26  by the cog wheels  54 ,  55  and  56 , which at the same time function as a synchronizing device. Hence, both end walls  24 ,  26  are operated as described in reference to  FIG. 2A .  FIG. 2B  further illustrates in a schematic view one functional embodiment of the load shoe  18 . Generally, the load shoe  18  is not disposed diametrically relative to the drive shaft, but perpendicularly as in  FIG. 2A .  
         [0029]     Tests conducted by the assignee indicated that, in test batches manufactured by a long-nip calender as described above, the batches of paper could be provided with a ratio of bulk and smoothness better than in currently available grades of paper. Thus, according to measurements, the goals of the invention are achieved.  
         [0030]     Shoe calenders can be driven at high speeds and, furthermore, by the application of an elevated temperature, e.g. about 250° C., and by taking into account a long dwell time in the calendering zone, the resulting gloss finish will be equal to what is achieved in a slower solution using a supercalender. In addition, the paper is provided with improved bulk. In addition to aspects contributing directly to the quality of paper, the results include savings of production space in a mill, the elimination of a production limiting supercalender, and the provision of a more manageable, more easily controlled system.  
         [0031]     In view of producing paper of the invention, there is used a non-contact coating process prior to glazing final calendering. Suitable coating methods include, for example, curtain or spray coating.  
         [0032]     The quality values of thus produced paper in pilot conditions were as follows:  
                                                                         Bulk   1.15-1.3   cm 3 /g           PPS-s10 roughness   1.0-1.5   μm                Gloss   40-50%                      
 
         [0033]     Compared with prior known grades, the obtained paper is higher in bulk and smooth and, in addition, the production method has a production capacity which is higher that what is achieved by a single supercalender. The method provides saving in paper manufacture and improves economy. Especially, the increase of capacity is possible with the same paper machine by on-line calendering. When compared to using several supercalenders, more space can be saved in the case of a new mill or the operation of an old mill can be rationalized. The provision of higher bulk represents a direct saving in terms of the amount of material and energy needed for production and, likewise, lighter printing paper saves energy over its service life and ultimately produces less waste to be handled.  
         [0034]     Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.