Abstract:
An optical film stack includes a plurality of optical films held together by electrostatic force. Liners, which may also be held to the optical film stack by electrostatic force, protect the optical film stack to form an optical film unit. Multiple optical films may be stacked onto each other and efficiently assemble into an optical display. In addition, the optical film stack has no exposed adhesive, which tends to collect dust and particles or transfer to other parts of the display if the edges of the optical films are misaligned relative to each other.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to optical displays. In particular, the present invention relates to pre-stacked optical films for assembly into an optical display.  
         [0002]     Optical displays, such as backlit liquid crystal displays (LCDs), are used in a wide variety of applications including mobile telephones, personal digital assistants (PDAs), electronic games, laptop computers, monitors and television screens. Optical films are stacked within an optical display in order to enhance brightness and improve display performance without sacrificing battery life.  
         [0003]     Currently, films used in displays are provided as individual films to display manufacturers. During assembly of a display, the cover films of the optical films are removed, and the films are deionized to remove dust particles. Each optical film is then stacked, one by one, into a frame that fits between a back light assembly and an LCD panel. In some cases, each optical film is laminated onto an adjacent optical film by an adhesive layer, but misalignment of optical films resulting from positioning inaccuracies of the laminating machine results in areas of exposed adhesive. The exposed adhesive easily becomes covered with dust and may be transferred to other sets of optical films. In addition, differences in thermal expansion ratios between different optical films may result in warping when laminated together with adhesive. Thus, resolving these problems would increase product output and reduce the number of damaged products.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The present invention is an optical film unit and a method of making the optical film unit in which stacked optical films and liners are held together by electrostatic force. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a front view of a representative embodiment of an optical film unit for use in a display in accordance with the present invention.  
         [0006]      FIG. 2  is a perspective view illustrating a method of making optical film units in accordance with the present invention.  
         [0007]      FIG. 3  is a schematic diagram of a representative device for charging optical films. 
     
    
     DETAILED DESCRIPTION  
       [0008]      FIG. 1  is a front view showing an optical film unit  10  in accordance with the present invention. Optical film unit  10  includes optical film stack  12  (including optical films  12   a  and  12   b ), liner  14  and liner  16 . Liners  14  and  16  are protective coverings and can be made from the same or different materials, which are known in the art.  
         [0009]     Optical film  12   a  is placed on liner  14  followed by optical film  12   b  and then liner  16 . As will be described in more detail below, components of unit  10  are held together by electrostatic force so that adhesive need not be applied to optical films  12   a  and  12   b.    
         [0010]     In use, liner  16  is removed from optical film stack  12  and liner  14 . Optical film stack  12  is subsequently removed from liner  14  and assembled into an optical display, such as an LCD, between the optical light guide and liquid crystal panel. Holding the optical films together prior to installation into an optical display decreases the time and cost associated with assembly of the optical display by avoiding individually inserting each film.  
         [0011]     Although optical film unit  10  is shown having two optical films,  12   a  and  12   b , there is no limit as to the number of films that may be stacked, and the number of optical films will vary depending on the display in which it is used. Some or all of the optical films may be of the same type, again, depending on the optical display. Examples of the types of optical films that may be used in the present invention include light directing films, turning films, multi-layer polymer films, diffuser-type films, reflective films, etc.  
         [0012]     In addition, during placement of optical film  12   b  onto optical film  12   a , the edges of optical films  12   a  and  12   b  may be misaligned relative to each other due to positioning inaccuracies of the robotic devices used to stack the films. But because there is no adhesive placed on the optical films, exposed edges of the films are not covered with dust, and adhesive is not transferred to other optical films or parts of the display, which may interfere with viewing.  
         [0013]     Installation of optical film stack  12  into an optical display can be performed using an automated assembly line and may reduce the amount of debris between and damage to the films, because the manufacturer does not individually install the films. Reduced thickness of the stacked optical films is also an advantage of the present invention. A recent trend in backlit displays is toward thinner modules. The lack of adhesive provides for thinner stacked films, which allows for a thinner design.  
         [0014]     In addition, differences in thermal expansion ratios of the various types of optical films can lead to warping within the display. The free-flow properties of optical films stacked without adhesive will alleviate warping.  
         [0015]     It should be noted that embodiments of the present invention also encompass only charging one or some of the optical films of the stack or the use of adhesive between some optical films. In some instances, it may be desirable to have greater adhesion between some of the stacked optical films. In those cases, an adhesive can be applied only to those films requiring greater adhesion, while using electrostatic force to hold the remaining optical films.  
         [0016]      FIG. 2  is a representative embodiment of the method of making the present invention. Optical films  12   a  and  12   b  are shown attached to liners  14   a  and  14   b , respectively. Initially, a third liner also covered films  12   a  and  12   b  on liners  14   a  and  14   b . The third liner was removed just prior to the stage of the method shown in  FIG. 2 .  
         [0017]     Next, optical film  12   a  is removed from liner  14   a . Typically, optical film  12   a  acquires a charge during removal of the third liner, liner  14   a  or both, which is described in more detail below. Optical film  12   a  is then placed on liner  14   c  either manually with a gloved hand or by a robotic arm with a silicone rubber pick-up head to reduce charge dissipation from film  12   a . When charged optical film  12   a  is placed on neutral liner  14 , the charge within optical film  12   a  and liner  14  moves to create a neutral charge at the interface between film  12   a  and liner  14 . So for example, when film  12   a  acquires a negative charge and is placed on liner  14 , the negative charge will migrate toward liner  14 . In turn, at the interface liner  14  takes on a positive charge so that film  12   a  and liner are attracted to each other and hold together.  
         [0018]     In an alternate embodiment, optical film  12   a  may be charged after it is picked from liner  14   a  instead of using the charge gained during removal of the liners. With either embodiment, if desired, equipment may be engineered to control the amount of charge gained by the films.  
         [0019]     Next, liner  14   b  is removed from optical film  12   b , and film  12   b  is placed onto film  12   a  to form optical film stack  12 . Optical film  12   b  acquires a charge and is picked and placed as described for film  12   a . Film  12   b  is held to film  12   a  by migration of charge. When film  12   b  acquires its negative charge and is placed on film  12   a , its negative charge will move toward the interface between films  12   a  and  12   b , because the surface of film  12   a  at this interface has taken on a relatively more positive charge.  
         [0020]     Lastly, liner  16  is placed over optical film  12   b  such that it covers optical film stack  12  and liner  14   c . Again, the drive to create a neutral interface between the layers forces migration of charges such that film  12   b  and liner  16  hold together.  
         [0021]      FIG. 3  illustrates a representative method of charging optical films. As briefly described regarding  FIG. 2 , the optical films typically acquire a charge by removing liners from the individual films prior to forming optical film stack  12 .  
         [0022]      FIG. 3  includes device  18  for removing the liners from the optical films, liner  14   a , optical film  12   a , optical film strip  20 , weeding tape  22  and weed  24 . Device  18  further includes support  26 , idlers  28 ,  30  and  32 , laminating station  34  and stripper bar  36 . Optical film strip  20  is essentially a web of optical film material between liner  14   a  and a top liner that has been kiss-cut through the top liner and optical film material to form optical films  12   a.    
         [0023]     In operation, optical film strip  20  is fed into device  18  and supported through device  18  by support  26 . Idler  28  guides strip  20  and weeding tape  22  into laminating station  34  to laminate weeding tape  22  onto strip  20 . Stripper bar  36  then removes weed  24 , which includes weeding tape  22 , the top liner and waste optical film material. Lastly, idlers  30  and  32  guide liner  14   a  such that it is removed from optical film  12   a . Optical film  12   a , at this point, has acquired a charge through the process of removing the liners. In the embodiment shown in  FIG. 3 , film  12   a  is not deionized to remove dust and debris prior to stacking. In order to prevent contamination by dust and debris during the charging process, charging must be performed in a clean area free of contaminants.  
         [0024]     The resulting charge on film  12   a  is generated by friction of the liner and weeding tape materials during the dispensing process. To illustrate this phenomenon, voltage measurements were taken and the results are shown at various points in  FIG. 3 . As strip  20  was unwound, charges of +20 kv and +10 kv were generated by the unwinding process. As weeding tape  22  was unwound, charges of +50 kv, +20 kv, +10 kv and +7 kv were generated during unwinding. The charge measured at laminating station  34  was −30 kv and −80 kv. Charges of +50 kv, +30 kv and +60 kv were measured at points where weed  24  passed around stripper bar  36  and was subsequently wound. As liner  14   a  was removed from optical film  12   a  and wound, charges of 0 kv, +40 kv and +60 kv were measured. The resulting charge on optical film  12   a  was −20 kv.  
       EXAMPLE  
       [0025]     An optical film unit having two optical films was produced by the method of the present invention. The liners were both LD-1010-75 by Hitach Chem., and the two optical films were thin-BEF by 3M Company. The optical film unit was fed into a 3M dispenser and tested for dispensability and parts handling. No issues regarding contamination, alignment, warping or loss of electrostatic properties were identified.  
         [0026]     The present invention provides several advantages. The process simplifies the manufacture of optical displays resulting in gains in efficiency with respect to cost, including fewer damaged films, and time. In addition, the optical films, without adhesive, result in thinner optical film stacks and have free flow properties after assembling into the backlight unit that reduce warping. In addition, it alleviates issues surrounding misalignment of stacked optical films.  
         [0027]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.