Abstract:
This invention relates to calendering systems. Such structures of this type, generally, employ the use of hard and soft nips to provide excellent smoothness without gloss mottle.

Description:
This is a divisional of copending application Ser. No. 08/980,781 filed on Dec. 1, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to calendering systems. Such structures of this type, generally, employ the use of hard or soft nips to provide excellent smoothness without gloss mottle. 
     2. Description of the Related Art 
     It is well known in calendering systems, particularly heated soft roll calendering systems, to employ a soft roll at high pressures. Exemplary of such prior art is U.S. Pat. No. 4,624,744 (&#39;744) to J. H. Vreeland, entitled “Method of Finishing Paper Utilizing Substrata Thermal Molding”. While the &#39;744 patent does achieve calendering, the use of the high nip pressures, namely, pressures above 2000 psi, reduce the bulk of the paper. Consequently, such use of a calendering device is, typically, employed when calendering fine papers. Consequently, a more advantageous calendering system, then, would be employed if calendering could be done at lower nip pressures in order to reduce bulk loss. 
     It is apparent from the above that there exists a need in the art for a calendering system which is able to calender as well as the known calendering systems, while providing excellent smoothness without gloss mottle (an uneven pattern of gloss or reflectance), but at the same time is able to calender at lower nip pressures. 
     It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure. 
     SUMMARY OF THE INVENTION 
     Generally speaking, this invention fulfills these needs by providing a substantially gloss mottle-free calendered paper with significantly increased smoothness consisting of a coated paper produced by a method comprising, passing the coated paper through a first nip formed between a substantially harder calendering roll and a heated roll means, passing the coated paper through a second nip formed between a substantially softer calendering roll and the heated roll means to produce a substantially gloss mottle-free calendered paper having significantly increased smoothness and operating the method at nip pressures between the first and second nip of substantially less than 2000 psi. 
     In certain preferred embodiments, the harder calendering roll has a surface hardness of greater than 80 shore D. The heated roll is a polished metallic roll. The softer calendering roll has a surface hardness of less than or equal to 80 shore D. Also, calcium carbonate (CaCO 3 ) is added to the coating placed upon the paper. The coating is applied at a coat weight of approximately 8-24 lbs/3900 ft 2 . The coating contains at least 40% solids and at least 30% CaCO 3 . 
     In another further preferred embodiment, the use of the harder-softer roll combination allows one to produce a paper which is substantially gloss mottle-free and has a significantly increased smoothness. 
     The preferred calendering system, according to this invention, offers the following advantages: good stability; good durability; substantially reduced gloss mottle; significantly increased smoothness; reduced operating nip pressures; increased operating capacity; reduced converting problems; and excellent economy. In fact, in many preferred embodiments, these factors of improved gloss mottle, improved smoothness, reduced nip pressures, increased capacity, and reduced converting problems are optimized to an extent that is considerably higher than heretofore achieved in prior, known calendering systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying FIGURE, in which the FIGURE is a schematic illustration of a calendering system using hard and soft rolls, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As discussed earlier, the &#39;744 patent adequately calenders fine papers, but at higher nip pressures. Typically, these nip pressures are greater than 2000 psi as measured by Equation (1) below as set forth by H. L. Schmidt, Rubber Roll Hardness-Another Look, Pulp and Paper, Mar. 18, 1968, pp 30-32. The Equation (1) is:                nip                 width     ,     n   =       (       4        LTD   1          D   2         E        (       D   1     +     D   2       )         )       1   m                 (   1   )                                
     m=exponent which is dependent on roll diameter 
     L=line load (pli) 
     T=thickness of cover (inches) 
     D 1 =diameter of harder roll (inches) 
     D 2 =diameter of softer roll (inches) 
     E=elastic modulus 
     However, in today&#39;s modern paper manufacturing machines, it is desirable to run at lower nip pressures, i.e., substantially less than 2000 psi. These lower nip pressures reduce bulk loss of the calendered paper and allow paper with greater caliper or thickness to be produced. Using Equation (1), nip pressures in the present invention have been measured from 900 to 1400 psi. 
     Along with reducing bulk loss, there are several other desired qualities that a paper manufacturer wants the paper to achieve after calendering. From past studies, it has been determined that a Parker Print-Surf (a measurement of surface roughness) of 1.0 or less and a gloss (or reflectance) of greater than or equal to 60 based upon a 75° Hunter gloss are currently acceptable parameters for determining whether or not a paper is calendered to achieve the best results. 
     With reference first to the FIGURE, there is illustrated an advantageous environment for use of the concepts of the invention. In particular, as shown in the FIGURE, there is illustrated calendering system  2 . System  2 , includes in part, harder or backing roll  4  having a hard resiliently yieldable surface, conventionally treated, polished metal roll  6 , softer or backing roll  8  having a soft resiliently yieldable surface, conventional paper  10 , coating  12 , and nips  14  and  16 . It is to be understood that softer roll  8  may also be located ahead of harder roll  4 . Also, roll  6  may be a series of heated rolls such that substrate  10  does not wrap around roll  6  and nips  14  and  16  located in a series. 
     Harder roll  4 , preferably, is any roll constructed of natural or synthetic materials having a surface hardness of greater than 80 shore D measured by conventional techniques. Softer roll  8 , preferably, is any suitable roll constructed of natural or synthetic materials having a surface hardness of less than or equal to 80 shore D. 
     Paper substrate  10  of the present invention is coated by coating  12  on at least one side surface and frequently on both sides. The paper trade characterizes a paper web or sheet that has been coated on one side as C1S and C2S if sheet coated on both sides. 
     Compositionally, coating  12  is a fluidized blend of coating clay, calcium carbonate (CaCO 3 ), and/or titanium dioxide with binders and additives which is smoothly applied to the traveling web surface. In particular, CaCO 3  is added to the fluidized blend of minerals such that the CaCO 3  comprises greater than 30% by weight of the minerals. Also, the mixture includes at least 40% by weight of solids in order to reduce gloss mottle and increase smoothness. 
     Coating  12  is applied to paper  10  at a rate of 8-24 lbs/3000 ft 2  by conventional techniques. Preferably, coating 12 is applied by a means of a rod coater, air knife or blade by conventional techniques. 
     The following test results prove the novelty of the present invention and its application as a desired calendering system. 
     Using coated basestock with a starting Parker Print-Surf value of 1.9 and a caliper value of 0.012″, the following results were achieved as shown below in TABLE 1: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Caliper 
                   
                   
                   
               
               
                 Load (pli) 
                 Roll Hardness 
                 (in) 
                 PPS 
                 Sheffield 
                 Gloss 
               
               
                   
               
             
             
               
                 348 
                 Softer 
                 11.9 
                 1.4 
                 15 
                 61 
               
               
                 417/417 
                 Harder/Softer 
                 11.9 
                 1.2 
                  6 
                 68 
               
               
                 348 
                 Harder 
                 12.0 
                 1.1 
                  8 
                 68 
               
               
                   
               
               
                 where PPS = Parker Print-Surf, Softer = Softer roll 8, and Harder = Harder roll 4  
               
             
          
         
       
     
     The above data demonstrate a more profound effect of the harder polymer roll (88 Shore D) on the larger scale roughness (Sheffield) than on the fine scale roughness (measured by PPS). There was an obvious visual improvement in surface uniformity of the harder/softer roll combination condition as compared to the harder roll only condition. 
     Using coated basestock with a starting PPS value of 2.4 and a caliper value of 0.11″, the following results were achieved as shown below in TABLE 2: 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Caliper 
                   
                   
                   
               
               
                 Load (pli) 
                 Roll Hardness 
                 (in) 
                 PPS 
                 Sheffield 
                 Gloss 
               
               
                   
               
             
             
               
                 348 
                 Harder 
                 10.9 
                 1.9 
                 10 
                 64 
               
               
                 417/417 
                 Harder/Harder 
                 10.7 
                 1.7 
                 10 
                 71 
               
               
                 417/417 
                 Harder/Softer 
                 10.8 
                 1.7 
                 13 
                 71 
               
               
                   
               
             
          
         
       
     
     Again, the harder/softer roll combination provides reduced PPS values and higher gloss values than a single hard roll. Also, the harder/softer roll combination gives better gloss uniformity than the harder/harder roll combination. 
     Based upon the favorable results from TABLE 1 and TABLE 2, calendering system  2  was placed on a conventional papermaking machine. The paper was calendered using a harder roll (Shore D hardness of greater than 80), two softer rolls (Shore D hardness of less than or equal to 80) and the harder/softer roll combination of the present invention. The results of the three runs are shown below in TABLE 3: 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Roll Hardness 
                 PPS 
                 Sheffield 
                 Gloss 
                 Mottle 
               
               
                   
               
             
             
               
                 Harder 
                 1.2 
                 N/A 
                 62 
                 Unacceptable Gloss 
               
               
                   
                   
                   
                   
                 Uniformity 
               
               
                 Softer/Softer 
                 1.3 
                 6 
                 56 
                 Acceptable Gloss Uniformity 
               
               
                 Harder/Softer 
                 0.8 
                 4 
                 68 
                 Acceptable Gloss Uniformity 
               
               
                   
               
             
          
         
       
     
     Clearly, the use of the harder/softer calendering roll combination creates a paper having a Parker Print-Surf of 1.0 or less, a gloss of greater than or equal to 60, and reduced gloss mottle. 
     Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.