Method of manufacturing film for lamination
This is a continuation, of application Ser. No. 07/838,915, filed Feb. 21, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to laminating methods and particularly to a method for use in one-sided or single-sided lamination and resultant product thereof. 
     In many applications it is desireable to heat laminate a plastic-like protector film to an unprotected underlying sheet for protecting the text material, etc., on the underlying sheet. The sheet and film is subsequently cut into individual pages, covers, etc. However, problems arise in one sided laminations. 
     Paper is a hygroscopic substance, it picks up excess moisture from the surrounding atmosphere quickly. It&#39;s moisture content depends upon the relative humidity and the temperature of the air it is in contact with. An important consequence of the desorption and absorption of water by paper is the change in dimension that usually accompanies changes in moisture content. Dimensional changes in paper originate in the swelling and contraction of the individual paper fibers. It is common for paper to show dimensional changes, up to 1% in the cross direction and corresponding changes in the machine direction could range up to 5%. 
     Paper used for cover stock for books and magazine covers are typically laminated, and one process of lamination is thermal. The thermal laminating film used usually consists of one layer of polyethylene or copolymer of polyethylene and oriented polypropylene or polyester. 
     One sided thermal lamination of paper using polypropylene or polyester has some inherent drawbacks. For stock with only one side laminated, moisture is now only absorbed by the paper on one side because of the protection of the laminate film. This increases the dimension of the paper and causes the laminated paper to curl towards the film side or laminated side. 
     A &#34;perfect&#34; bound book is one in which a layer of adhesive holds the pages and cover together. The problem on typical perfect bound books, depending on the relative humidity of the atmosphere and type of cover stock, can be a slight curl on the cover, i.e., the cover rolls like a scroll. 
     The protector film can be applied to both sides of the underlying or unprotected product in order to protect text on both sides of the product and to prevent paper moisture growth on any one side compared with the lack of moisture growth of the protector film on the other side. In other words, by laminating on both sides, the various forces are balanced or offset and the resulting web (i.e., product and film) is substantially planar or flat. 
     However, in most applications it is necessary to protect one side of a product and only one protector film need be laminated to the product. A successful one side lamination is thus desireable. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for manufacturing a film suitable for use in thermal one side lamination which eliminates the inherent tendency of perfect bound books, soft cover books and other laminating films of polyester and polypropylene to curl. Specifically, in accordance with the invention, nylon is used as the base film because nylon has the property of absorbing moisture with a resultant increase in dimensions similar to paper. This absorption of moisture by nylon makes the film compensate for the moisture absorption of paper by more than one phenomena. When the moisture is absorbed by nylon, not only does the dimension increase, but the film moisture absorption acts like a plasticizer, causing an increase in elongation and a decrease in tensile strength. The film is made softer so that it can elongate with the paper with less force because of a decrease in tensile strength, making the laminated book cover remain flat. 
     As background information, Table 1, from Britt Pulp and Paper Technology 2d Ed. Chapter 9-2, shows the moisture content values of three different types of paper at several relative humidities. FIG. 1 shows the typical moisture sorption isotherms for pulp and paper. These figures show the moisture contents of paper products ranging from approximately 2% through 10% at typical room temperature and humidities. 
     
                       TABLE 1                                                     
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             Moisture Content (%)                                         
Temperature    100%      Bleached Ground-                                 
73°     Rag       Sulfite  wood                                    
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52.4% RH desorption      8.37                                             
52.6% RH desorption                                                       
               5.86                                                       
52.4% RH adsorption      7.88                                             
48.0% RH desorption      7.89                                             
48.0% RH adsorption      7.62                                             
50.0% RH adsorption               9.70                                    
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     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph of the moisture content for paper products; and 
     FIG. 2 is a graph of the moisture content of Nylon 6 biaxially oriented. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Thermal lamination is the process of combining a heated web to second webs or sheets. The process uses dry non-tacky film until it is heated. The combining station is typically comprised of two rollers. One of the rollers is steel internally heated and having a polished surface. The other is a rubber covered impression roller. 
     A typical single-sided laminator apparatus is disclosed in U.S. Pat. No. 4,960,484, wherein a reverse curling is attempted to offset curling. 
     FIG. 1, from Britt Pulp and Paper Technology, Chapters 9-2, show moisture sorption isotherms for a typical paper formed from relatively pure cellulosic fibers. The desorption of cellulose that has never been dried is shown in curve A. The corresponding adsorption is shown on curve B. Partial absorption to a relative humidity corresponding to point C, followed by desorption, results in the course of line CD and similarly, partial desorption to point E followed by adsorption, results in the course shown by dotted line EF. 
     At 73°, 24 hours, Nylon 6 materials show a moisture range between 5-9.5% which is substantially similar to the paper products of FIG. 1 and Table 1. Particularly, Nylon 6 Extruded has a moisture content of 9.5°%. Nylon 6 Biaxially Oriented has a moisture content of 5-6%. Nylon 6 Uniaxially Oriented also has a moisture content of 5-6%. By comparison, Oriented Polypropylene has a moisture content of less than 0.005% under similar conditions. Polyethylene Terephthalates (PET) has a moisture content of 0.8% under similar conditions. 
     Some cellulosic films will absorb moisture similar to paper. However, these materials can have the problem of lack of heat stability to be used as a thermal film and some absorb so much moisture that a resultant curl would be oriented the other way. 
     It has been found that by using a nylon for the laminating material, curling is reduced. In particular, Nylon 6 either extruded, biaxially oriented, or uniaxially oriented as the laminate provides this advantage. Preferably, 0.6 mil biaxially oriented Nylon 6 is used. 
     FIG. 2 and Table 2 describe some of the properties of biaxially oriented Nylon which demonstrates its moisture absorbing properties similar to the paper materials described in FIG. 1 and Table 1. 
     Although the present invention has been described with reference to a specific embodiment, those of skill in the art will recognize that changes may be made thereto without departing from the scope and spirit of the invention as set forth in the appended claims. 
     
                                           TABLE 2                                 
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PROPERTIES OF NYLON, ORIENTED                                             
                                   Typical Value                          
Properties     Test Method         #1200 (48 G)                           
                                          #1500 (60 G)                    
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Density g/cm.sup.3                                                        
               ASTM D1505-68 68° F. (20° C.)                
                                   1.15˜1.16                        
Melting point °F. (°C.)                                     
               ASTM D2117-64       419˜437 (215˜225)          
Service temperature °F. (°C.)                               
               ASTM D759-68        -76˜266 (-60˜130)          
Tensile strength                                                          
               ASTM D882-67 68° F. (20° C.) 65%             
                                   2.8 × 10.sup.4 ˜3.6        
                                   × 10.sup.4                       
psi (kg/cm.sup.3)                  (2.0 × 10.sup.3 ˜2.5       
                                   × 10.sup.3)                      
Elongation %   ASTM D882-67 68° F. (20° C.) 65%             
                                    70˜110                          
Tensile modulus                                                           
               ASTM D882-67 68° F. (20° C.) 65%             
                                   2.0 × 10.sup.3 ˜3.1        
                                   × 10.sup.3                       
psi (kg/cm.sup.3)                  (1.4 × 10.sup.4 ˜2.2       
                                   × 10.sup.4)                      
Tearing strength g                                                        
               ASTM D1004-66 68° F. (20° C.) 65%            
                                    70˜200                          
                                          200˜400                   
Tearing strength propagated g                                             
               ASTM D1922-67 68° F. (20° C.) 65%            
                                    7˜10                            
                                          10˜12                     
Shrinkage %    for 5 min. in an air circulating oven at                   
                                   less than 2.5                          
               320° F. (160° C.)                            
Water absorption %                                                        
               ASTM D570-63 68° F. (20° C.)                 
                                   7˜9                              
Water vapor permeability                                                  
               ASTM E96-66 104° F. (40° C.) 90%             
                                   17˜19                            
                                          16˜17                     
g/100 sq. in/24 hrs.               (260˜290)                        
                                          (250˜270)                 
(g/m.sup.3 /24 hrs.)                                                      
Gas permeability O.sub.2                                                  
               ASTM D1434-66 77° F. (25° C.)                
                                   3.5˜4.8                          
                                          2.6˜3.9                   
cc/100 sq. in/24 hrs. /atm         (55˜75)                          
                                          (40˜60)                   
(cc/m.sup.3 /24 hrs./atm)                                                 
Transparency % ASTM D1746-62T 68° F. (20° C.)               
                                   87˜90                            
               65% RH                                                     
Dielectric constant                                                       
               ASTM D150-68 68° F. (20° C.)                 
                                   3.4 (1 kHz)                            
                                          3.0 (1 MHz)                     
Dissipation factor                                                        
               ASTM D150-68 68° F. (20° C.)                 
                                   0.022 (1 kHz)                          
                                          0.036 (1 MHz)                   
Volume resistivity Ω-cm                                             
               ASTM D257-66 68° F. (20° C.)                 
                                   5 × 10.sup.15                    
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PROPERTIES OF LAMINATED NYLON, ORIENTED NYLON                             
             Test Conclusions        68 F. (20 C.) 65% RH                 
             EMBLEM 25μ                                                
                      EMBLEM 15μ                                       
                              Polyester 12μ                            
                                     Polymer-coated                       
Properties   PE 50μ                                                    
                      PE 50μ                                           
                              PF 50μ                                   
                                     Cellophane #300 PE                   
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                                     50μ                               
Tensile strength                                                          
             31.0     18.1    16.6   13.8                                 
lb/inch width                                                             
ASTM D882-67 (826)    (482)   (413)  (369)                                
(kg/15 mm width)                                                          
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