Abstract:
A method and apparatus for two-side coating of a light base weight printing paper web containing mechanical pulp or recycled fiber. The first side of a web ( 5 ) is coated in a first coating station ( 1 ), the coating applied to the first side is dried at least partially in a first dryer unit ( 3 ). The second side of the web ( 5 ) is coated subsequent to the drying of the first side in a second coating station ( 2 ), and the coating applied to the second side is dried at least partially in a second dryer unit ( 4 ). Both coatings are formed by applying a required amount of coating mix onto the perimeter of respective soft film-coating rolls ( 22 ) and subsequently transferring the coat film to the web ( 5 ) in a nip (N 1 ) formed between a respective backing roll ( 24 ) and the respective soft film-coating roll ( 22 ).

Description:
This is a division of application Ser. No. 08/573,570, filed Dec. 15, 1995 now U.S. Pat. No. 5,650,010 which is a continuation of application Ser. No. 08/132,037, filed Oct. 5, 1993, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for two-side coating of a thin printing paper web containing mechanical pulp, e.g., a paper web made of recycled fiber. 
     The invention further relates to an apparatus suited to two-side coating of a thin printing paper web made of mechanical pulp. 
     BACKGROUND OF THE INVENTION 
     Thin printing paper webs containing mechanical pulp are conventionally coated on subsequent coating stations equipped with short-dwell coaters as the coater units. A combination of two subsequent coater stations is necessary as low base paper weight and high content of groundwood make single-run coating on both sides impossible. Two-side coating with conventional methods would excessively wet the web and thus impair its runnability. Furthermore, the measurement of coat weight in a two-side coating operation is difficult. 
     Despite their multiple benefits, short-dwell coaters also have several drawbacks. Air entrapped in the coat paste easily causes mottling. Because of the small linear application pressure and short application distance, wetting of the base web and subsequent fiber swelling occurs even after the web has passed the doctor blades, thereby impairing the smoothness of the coat being applied. As a rule, doctor blade coating methods become critical with thin webs and particularly with light coat weights. 
     Paper grades containing a high proportion of mechanical pulp and a high percentage of coat fillers such as, e.g., SC paper (wood containing high filler content super-calendered printing paper) make doctor blade coating impossible because of the fragility of the base paper web. 
     Web defects leading to brittleness result in low production yield and inferior runnability. 
     Base paper grades containing recycled fiber have posed unexpected problems in doctor blade coaters; specifically, the coat is easily marked during coating by streaks caused by defective doctor blades. 
     The darker color of recycled-fiber containing base paper grades make the opacifying power of the coat mix more critical. Because of the way in which doctor blade coating is accomplished, a smooth coat is formed, not a coat of uniform coat weight. As the base paper is not necessarily smooth, the opacifying power obtained in doctor blade coating is insufficient, resulting in mottling of the coated web. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the disadvantages of the above-described prior-art technology and to achieve an entirely novel method and apparatus for two-side coating of a thin printing paper web containing mechanical pulp such as, for example, a paper web grade made of recycled fiber. 
     The present invention is based on performing the coating operation using a two-step compressive film-lamination technique. 
     The present invention relates a method and apparatus in which a coating is applied onto both sides of a light base weight printing paper web containing mechanical pulp or recycled fiber in which one side of the web is coated in a first coating station and then dried, at least partially, in a first dryer unit, and then the second side of the web is coated in a second coating station and then dried, at least partially, in a second dryer unit. Both of the two coating layers are formed by applying the required amount of a coating mix onto the perimeter of respective soft film-coating rolls which rotate with a peripheral speed approximately equally to the speed of the paper web. The coating mix is subsequently transferred to the paper web in a nip formed between a respective backing roll and the respective soft film-coating roll. 
     The invention provides significant benefits. 
     The present invention cuts in half the amount that a web is wetted in comparison with single-run two-side coating. Thus, the present invention provides good runnability. Further, good coat quality is attained at light coat weights. Particular benefit is achieved with base paper grades containing recycled fibers because the formation of streaks associated with doctor blade coating can be avoided. Since the compressive film-lamination technique forms a coat of uniform weight on the web, the coat has a high opacifying power. For the same reason, a higher burst index relative to doctor blade coating is attained. Moreover, the measurement of coat weight by means of rupturing testers is easy. The present invention imposes minimal mechanical stress on the web. While two-side single-run coating requires a long path of unsupported pulling of the web prior to the web support roll to give the coat a possibility of drying prior to touching the support roll, according to the present invention, the uncoated side can be supported by a roll immediately after the coating of the other side, thus achieving a significant reduction in coater unit size. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation view of a part of a paper machine incorporating a coater according to the present invention; 
     FIG. 2 is a side view of an alternative embodiment of the coating step of the present invention; and 
     FIG. 3 is a side view of a detail of a nozzle assembly according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In conjunction with the present invention, the term compressive film-lamination technique is used to refer to a technique in which during coating the web is subjected to a linear pressure in a nip formed between a film-coating roll and a backing roll so that the peripheral speeds of both the film-coating roll and the backing roll are approximately equal to the web speed. 
     In conjunction with the present invention, the term light web material refers to web materials having a base weight of less than 65 g/m 2 . 
     With reference to FIG. 1, the apparatus according to the present invention comprises two compressive film-lamination stations: a first coating station  1  and a second coating station  2 . A first drying unit  3  is placed between the two coating stations. The path of web  5  is configured so that subsequent to the first drying unit  3  there is the second coating station  2 , followed by a second drying unit  4 . Basically, both drying units have a similar construction. The first coating station  1  is comprised of a film-coating roll  22 , a coater bar  20  and a backing roll  24  of the film-coating roll  22 . The coater bar  20  meters a desired amount of coating mix onto the film-coating roll  22 , wherefrom the coating mix is subsequently transferred in the nip N 1  to the web  5 . To make the coating mix adhere to the web with the greatest smoothness without exhibiting a peeling off effect, such as, for example, when an orange is peeled, the backing roll  24  has advantageously a smaller diameter than the film-coating roll, whereby the angle at which the web  5  conforming to the backing roll  24  exits the nip N 1  is maximized. In an alternative embodiment the web  5  is guided toward the film-coating roll  22 , whereby also a good coat surface quality is obtained. 
     The first drying unit  3  is comprised of infra-red dryers  6  and dryer drums  7 . 
     Basically, the second coating station  2  has a similar construction to that of the first coating station  1  and components common to both coating stations have the same reference numerals distinguished by the letter “a”. Because of the routing of the web  5 , the first station  1  is a mirror image of second coat station  2 . The first station  2  is analogously followed by the second drying unit  4  comprising an infra-red dryer  8  with subsequent drying cylinders (not shown). 
     With reference to the embodiment of the present invention shown in FIG. 2, the coating bar  20  can be replaced by a so-called gate roll coater in which the coating mix is transferred from a coating mix fountain  34  located above a nip N2, which is formed by a transfer roll  30  and a metering roll  32 , via the nip N 2  onto the perimeter of said metering roll  32  and further via a nip N 3  onto the perimeter of a film-coating roll  36 . From the perimeter of the film-coating roll  36 , the coating mix is applied to a web  38  in a nip N 4  formed between the film-coating roll  36  and a backing roll  39 . According to the present invention, the diameter of the backing roll  39  can be smaller than the diameter of the film-coating roll  36 . 
     Typically, the film-coating roll  36  has a diameter of 1000 mm, and the backing roll  39  has a diameter of 800 mm. 
     Characterizingly, in both of the above-described exemplifying embodiments of the present invention the surface of the film-coating roll  22 , 36  is adjusted to carry a film of the coating mix of approximately 7 to 15 μm in thickness, a major portion (typically approximately 80%) of which is adhered to the web in the nip between the film-coating roll and the backing roll. Thus, the web is coated with a coat of 7 to 15 g/m 2  dry weight. 
     The adsorption of the coating mix paste and the water contained therein is related to the magnitude of the nip pressure and the duration of said pressure, that is, the width of the nip. On the other hand, the nip width is solely determined by the diameters of the rolls and their hardness, while the magnitude of the nip pressure is principally determined by the linear loading of the nip, and additionally, by the web speed. Accordingly, the good penetration of the coat into the web is achieved by means of a high linear application pressure imposed in a relatively wide nip. 
     The maximum nip pressure typically is 1000 kPa gauge and the nip width is in excess of 15 mm. Good results according to the present invention have been obtained by keeping the nip pressure above 500 kPa and the nip width greater than 10 mm. Such desirable nip widths can be attained by means of hard rolls with diameters in excess of 600 mm. Particularly the film-coating roll must have a diameter greater than 600 mm, whereby the backing roll must also have a diameter in excess of 600 mm. To achieve the minimum pressure limit of 500 kPa, the linear nip loading must be at least 20 kN/m for a typical coating mix paste. In conventional kiss roll coating, the application pressure is only approximately 50 to 100 kPa, while a conventional doctor blade coater can achieve a pressure of 1000 kPa over a nip width of less than 1 mm. In a conventional short-dwell coater the encountered levels of application pressure are even lower. The technique according to the invention is typically suitable for web speeds of 400 to 1500 m/min. The linear nip loading is typically in the range of 20 to 50 kN/m, advantageously approximately 35 kN/m. The coating material of both the backing roll  39  and the film-coating roll  36  is polyurethane, rubber or any suitable resilient material. The P&amp;J numbers of the rolls are typically in the range of approximately 0 to 40. The film-coating roll employed in the embodiments according to the invention is invariably a so-called soft roll with a surface material of polyurethane, for instance. 
     With reference to FIG. 3, a nozzle assembly for a film-coating roll  40  is comprised of a coater blade  42  tilted to an acute angle and mounted to a frame structure  41 . Between the frame structure  41  and the blade  42  is placed a loading hose  43  suited to controlling the linear pressure and position of the coater blade  42 . The coating mix  46  is contained in a metering fountain formed between a front wall  44  and the coater blade  42 , wherefrom the mix is transferred in a controlled manner onto the perimeter of the coating roll  40 . The front wall  44  is mounted by means of support elements  45  onto the chassis of the apparatus. Such a nozzle assembly is known in the art and its construction is described, for example, in greater detail in U.S. Pat. No. 4,839,201, which is incorporated herein by reference. The coater blade  42  can alternatively be replaced by a doctoring bar. 
     In a preferred embodiment of the present invention, the backing roll has a metal surface. The metal surface can be of chromium, for instance. Also ceramic or polymer covered backing rolls are usable. A metal or ceramic covered backing roll performs initial calendering of the coated web. Such coated rolls with a steel core can also be chilled whereby condensation of moisture onto the roll perimeter is attained, which in turn aids keeping the roll clean. Because of the improved thermal and wear resistance of a metal or ceramic covered roll, steam cleaning means and scraper blades shown schematically in FIG. 1 with a common reference numeral  50 , can also be used for keeping the rolls clean. 
     According to the present invention, the nozzle assembly employed for metering the coating mix onto the film-coating roll can also be a slot-orifice die metering assembly or a spraying apparatus capable of spraying the coating mix onto the roll surface. 
     The table below gives exemplifying compositions of coating mixes suitable for advantageous use according to the present invention: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Coating mix 
                 Coating mix 
               
               
                   
                 Coating color component 
                 composition 1 
                 composition 2 
               
               
                   
                   
               
             
             
               
                   
                 Calcium carbonate 
                 100 parts 
                   
               
               
                   
                 pigment 
               
               
                   
                 Kaolin pigment 
                   
                 100 parts 
               
               
                   
                 Starch binder 
                  10 parts 
                  6 parts 
               
               
                   
                 Synthetic binder 
                  4 parts 
                  6 parts 
               
               
                   
                 Additives 
                  2 parts 
                     0.5 parts 
               
               
                   
                 Solids content 
                 55% 
                 58% 
               
               
                   
                 Viscosity (Brookfield 
                 500 cP 
                 800 cP 
               
               
                   
                 100) 
               
               
                   
                   
               
             
          
         
       
     
     Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.