Abstract:
The present invention is directed to an electrophoretic display film comprising: a) microcups each having a first shape, and b) remaining microcups which take up at least 10% of the total number of microcups and have shapes different from the first shape. The microcup designs of the present invention not only may reduce the Moiré pattern, but it may also make the defects of the microcup walls much less noticeable, and therefore increase the cutting yield.

Description:
This application claims priority to U.S. Provisional Application No. 61/598,725, filed Feb. 14, 2012; the content of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to microcup designs for electrophoretic displays, aiming to both reduce noticeable defects of display images and avoid Moirè. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 6,930,818 discloses an electrophoretic display based on the microcup technology. The patent describes the manufacture of microcups as display cells, by microembossing or imagewise exposure. The microcups are then filled with an electrophoretic fluid comprising charged pigment particles dispersed in a solvent or solvent mixture. 
     The top openings of the microcups traditionally may have the same size and shape and such microcups spread across the entire display surface. For example, all of the microcups may have a top opening of a square shape or all of the microcups may have a top opening of a hexagonal shape, on the viewing side. 
     For this traditional type of design, because the microcups are not randomized, the Moiré pattern can occur when the microcup film is laminated to a TFT backplane which also has a repeatable regular pattern. If a color filter is utilized in such a display device, the Moiré pattern is even more severe because the color filter also has a repeatable regular pattern. 
     By rotating the microcups to a less severe angle, the Moiré phenomenon may be reduced. However, such structural changes could reduce the cutting yield and also because the rotation angle must be precise, it could increase the processing cost. 
    
    
     
       BRIEF DISCUSSION OF THE DRAWINGS 
         FIG. 1  depicts microcups having (a) a square opening and (b) a hexagon opening, respectively. 
         FIGS. 2 and 3  illustrate how the opening shapes of microcups may be randomized. 
         FIG. 4  illustrates the “fill factor” of a microcup-based display film. 
         FIGS. 5(   a )- 5 ( c ) illustrate an alternative aspect of the present invention. 
         FIG. 6  shows a TFT backplane laminated to a display film with microcups (a) having the same shape, and (b) having randomized shapes. 
         FIG. 7  is a three-dimensional view of a microcup film with wavy partition walls. 
         FIG. 8  is a top view of a microcup film with wavy partition walls. 
         FIG. 9  depicts the pitches and amplitudes of a wavy partition wall. 
         FIG. 10  illustrates microcups with wavy partition walls. 
     
    
    
     SUMMARY OF THE PRESENT INVENTION 
     One aspect of the present invention is directed to an electrophoretic display film comprising: 
     a) microcups which have a first shape, and 
     b) remaining microcups which take up at least 10% of the total number of microcups and have shapes which are different from the first shape. 
     In one embodiment, the remaining microcups take up at least 30% of the total number of microcups. In another embodiment, the remaining microcups take up at least 50% of the total number of microcups. 
     In one embodiment, the microcups of (b) are randomly interspersed among the microcups of (a). 
     In one embodiment, the microcups of (b) are formed by removing partition walls of a predetermined number of microcups of (a), and replacing the removed partition walls with new partition walls. 
     In another embodiment, the microcups of (b) are formed by independently shifting each of the apex points in a predetermined number of microcups of (a) within a defined area, and reconnecting the shifted apex points. 
     In one embodiment, the defined area is a circle. In one embodiment, an apex points are independently shifted by Δx in the X direction and Δy in the Y direction. In one embodiment, the absolute value of Δx or Δy does not exceed the radius of a circle as the defined area. 
     In one embodiment, an electrophoretic display film comprises microcups wherein all microcups have individually different shapes. 
     A further aspect of the invention is directed to an electrophoretic display comprising microcups wherein at least one of the microcups comprises a wavy partition wall. 
     In one embodiment, pitches of the wavy partition walls are different. 
     In one embodiment, amplitudes of the wavy partition walls are different. 
     In one embodiment, pitches of the wavy partition walls are different and amplitudes of the wavy partition walls are the same. 
     In one embodiment, amplitudes of the wavy partition walls are different and pitches of the wavy partition walls are the same. 
     In one embodiment, amplitudes of the wavy partition walls are different and pitches of the wavy partition walls are different. 
     In one embodiment, at least one of the microcups has a square opening and at least one set of the parallel partition walls is wavy. 
     In one embodiment, at least one of the microcups has a hexagonal opening and at least one set of the parallel partition walls is wavy. 
     In one embodiment, the wavy partition wall has a pitch ranging from 5 μm-2000 μm. 
     In one embodiment, the wavy partition wall has an amplitude ranging from 0.1 μm to 20 μm. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to minimize the Moiré pattern or visual defects in a display device by randomizing the shapes of microcups or partition walls of microcups. 
     The term “shape”, in the present invention, refers to the shape of the top opening of a microcup, on the viewing side. For example, a microcup may have a square opening (i.e., square shape) as shown in  FIG. 1(   a ) or a hexagonal opening (i.e., hexagonal shape) as shown in  FIG. 1(   b ). 
     In one aspect of the present invention, the microcups may be randomized by removing partition walls (dotted lines) of microcups having the same shape and replacing the removed partition walls with new partition walls (solid dark lines), as shown in  FIGS. 2 and 3 . A new partition wall may be formed by connecting two apex points which are not the same two apex points connecting a removed wall. 
     Starting with all microcups having the same size and shape, it is preferred that at least 10%, and more preferred that at least 30%, of the microcups are altered in this method. In addition, the altered microcups are preferably randomly interspersed among the unaltered microcups. 
     As to which partition walls to remove and where to add new partition walls, one criterion is that in the final design, the fill factor must be substantially maintained. More specifically, the fill factor of the altered microcup-based film can not be altered by more than 30% of that of the original microcup-based film having microcups having the same size and shape. 
     The fill factor is determined by the area other than the wall area divided by the entire area. As shown in  FIG. 4 , the fill factor is the sum of area A (where the display fluid is present) divided by the sum of the area A and the area W (the wall area). 
     In another aspect, the shapes of the microcups may be randomized as shown in  FIGS. 5(   a )- 5 ( c ). In  FIG. 5(   a ), the original microcups have the same shape, a hexagon. In randomizing the shape, each of the apex points (P) of the hexagons may be independently shifted within a defined area. The defined areas have the same size and shape around each apex point. In the example of  FIG. 5(   a ), a circle as the defined area is shown around each apex point. 
     In  FIG. 5(   b ), the original apex point (P) is shown to be shifted by a distance of Δx in the X direction and a distance of Δy in the Y direction, to point P′. The values of Δx and Δy may be positive or negative, depending on the direction of the shift. 
     When the defined area is a circle as shown, Δx or Δy is greater than zero; but its absolute value cannot exceed the radium of the circle. 
     An example is given here to illustrate this method. It is assumed that an original arrangement has regular hexagon-shaped microcups which have a nominal pitch of 100 μm. The term “nominal pitch” is intended for the original X and Y coordinates of a regular hexagon-shaped microcup. 
     In this example, it is further assumed that the absolute values of both Δx and Δy range between 10 μm and 25 μm, which means that the original apex point may move a distance of at least 10 μm but not exceeding 25 μm in either the X or the Y direction. As stated above, the Δx and Δy may be positive or negative, depending on the direction of the shift. 
     After shifting, the apex points are then reconnected. 
     This method is carried out with the aid of a computer program, such as CAD or equivalent by feeding the predetermined parameters into the program. The randomized microcups resulted from this example are shown in  FIG. 5(   c ). 
     Since the sum of Δx for all apex points should be substantially zero, the nominal pitch of the resulting microcups remains to be about 100 μm, on average. This also applies to Δy. The fill factor will also remain substantially the same before and after randomization. 
     It is also noted that greater Δx or Δy would result in a higher degree of randomness in the altered microcups. The maximum of Δx or Δy should be controlled to be no greater than the original side length of a hexagon, preferably no greater than 50% of the original side length of the hexagon. 
     This method can be applied to microcups of other shapes, such as square, triangle or octagon. 
       FIG. 6   a  shows a microcup-based film in which the microcups having the same hexagon shape are laminated to a TFT backplane. In this case, a Moirè pattern is visible.  FIG. 6   b  shows a microcup-based film in which the microcups having random shapes are laminated to a TFT backplane and no Moirè pattern is observed. 
     A further aspect of the present invention is directed to altering the partition walls of the microcups.  FIG. 7  is a three-dimensional view in which the partition walls along one of the axes of the microcups (e.g., the X-axis) are pseudo-random while the partition walls along the Y axis are still straight lines. 
     This alteration only occurs on the X/Y plane; but not on the Z axis, which means the height of the microcup walls will be kept unchanged. 
       FIG. 8  is the top view of this microcup design and it can be seen that the pitch may vary from one curvature in a microcup wall to another curvature in the same microcup wall. This is further illustrated in  FIG. 9  which depicts a wavy microcup wall ( 91 ) which is the wall ( 81 ) depicted in  FIG. 8 . 
     For illustration purpose, there is a vertical reference line ( 92 ) which intersects the wavy wall ( 91 ) at four points, “a”, “b”, “c” and “d”. The distance between point “a” and “b” is a first pitch, Pit 1 ; the distance between point “b” and “c” is a second pitch, Pit 2 ; and the distance between point “c” and “d” is a third pitch, Pit 3 . In the context of the present invention, Pit 1 , Pit 2  and Pit 3 , are preferably different. In another embodiment, at least two of them may be equal. 
     Another parameter defining the wavy microcup wall is the amplitude, which is the distance between the reference line  92  and the outer-most point on a curvature in the wavy wall  91 . As shown in  FIG. 9 , there are three outer-most points on the wave wall, “e”, “f” and “g”. The distance between the reference line  92  and point “e” is a first amplitude, Amp; the distance between the reference line  92  and point “f” is a second amplitude, Amp′; and the distance between the reference line  92  and point “g” is a third amplitude, Amp″. In the context of the present invention, Amp, Amp′ and Amp″, are preferably different. In another embodiment, at least two of them may be equal. 
     In one embodiment, the pitches Pit 1 =Pit 2 =Pit 3  and Amp≠Amp′≠Amp″. In another embodiment, Amp=Amp′=Amp″ and Pit 1 ≠Pit 2 ≠Pit 3 . In a further embodiment, Pit 1 ≠Pit 2 ≠Pit 3  and Amp≠Amp′≠Amp″. 
     Some of the partition walls are shown to be wavy in a top view in  FIG. 8 . However it is noted that the curvature extends throughout the depth of the entire wall area, as shown in  FIG. 7 . 
     In the present design, the pitch could be ranged from 5 μm-2000 μm, while amplitude could be varied from 0.1 μm to 20 μm. The pitch and the amplitude, as explained above, may be independently varied. 
     In one embodiment of the present design, the wavy walls only occur in one axis. However they may also occur in both axes.  FIG. 10   a  depicts a microcup having a square opening. In this case, there are two sets of partition walls, set  1  and set  2 . Each set has two partition walls which are parallel to each other. According to the present invention, set  1  of both partition walls, W 1  may be changed to be wavy (as shown), or set  2  of both partition walls, W 2 , may be changed to be wavy (not shown), or both sets of the partition walls may be changed to be wavy (not shown). 
     The top opening of the microcups may be hexagonal or any other shapes.  FIG. 10   b  depicts a microcup having a hexagonal top opening. In this case, there are three sets of parallel partition walls, set  1 , set  2  and set  3 . According to the present invention, at least one set of the partition walls is changed to be wavy. In other words, there may be only one set of partition walls, which is changed to be wavy (as shown), or two of the three sets of the partition walls are changed to be wavy (not shown), or all three sets of the partition walls are changed to be wavy (not shown). 
     The designs of the present invention may reduce the Moiré pattern. In addition, the randomized microcups or wavy partition walls will result in a fuzzy appearance that the regular shaped microcups do not have, and therefore any small defects (such as scratch or dust) are less noticeable. 
     The microcups of the present invention may be manufactured by the microembossing process described in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety. The formed microcups may then be filled and sealed also according to disclosures in this US patent. 
     While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.