Abstract:
This invention relates to a novel composition suitable for use in the manufacture of electrophoretic display cells. The mechanical properties of the cells are significantly improved with this composition in which a rubber material is incorporated.

Description:
BACKGROUND 
     The electrophoretic display is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a solvent. This general type of display was first proposed in 1969. An electrophoretic display typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. One of the electrodes is typically transparent. A dispersion composed of a colored solvent and suspended charged pigment particles is enclosed between the two plates. 
     When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side by attraction to the plate of polarity opposite that of the pigment particles. Thus the color showing at the transparent plate may be determined by selectively charging the plates to be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of grey) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages. 
     There are several types of electrophoretic displays available in the art, for example, the partition-type electrophoretic display (see M. A Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol ED 26, No. 8, pp 1148-1152 (1979)) and the microencapsulated electrophoretic display (as described in U.S. Pat. No. 5,961,804 and U.S. Pat. No. 5,930,026). In a partition-type electrophorectic display, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of the particles such as sedimentation. The microencapsulated electrophoretic display has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. 
     Furthermore, an improved electrophoretic display (EPD) technology was recently disclosed in the co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 pending, U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001 pending, U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 pending and U.S. Ser. No. 09/784,972, filed on Feb. 25, 2001 pending, all of which are incorporated herein by reference. The improved electrophoretic display comprises cells formed from microcups of well-defined shape, size, and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. 
     SUMMARY OF THE INVENTION 
     Multifunctional UV curable compositions have been employed to fabricate the microcup array for the improved electrophoretic display. However, the microcup structure formed tends to be quite brittle. The internal stress in the cups due to the high degree of crosslinking and shrinkage often results in undesirable cracking and delamination of the microcups from the conductor substrate during demolding. The microcup array prepared from the multifunctional UV curable compositions also showed a poor flexure resistance. 
     It has now been found that resistance toward flexure or stress may be significantly reduced if a rubber component is incorporated into the microcup composition. Two other key properties: demoldability during microembossing and adhesion between the sealing layer and the microcups have also been considerably improved with the composition containing this additional rubber component. 
     Suitable rubber materials for this purpose include SBR (styrene-butadiene rubber), PBR (polybutadiene rubber), NBR (acrylonitrile-butadiene rubber), SBS (styrene-butadiene-styrene block copolymer), SIS (styrene-isoprene-styrene block copolymer), and their derivatives. Particularly useful are functionalized rubbers such as polybutadiene dimethacrylate (CN301 and CN302 from Sartomer, Ricacryl 3100 from Ricon Resins Inc.), graft (meth)acrylated hydrocarbon polymer (Ricacryl 3500 and Ricacryl 3801 from Ricon Resins, Inc.), and methacrylate terminated butadiene-acrylonitrile copolymers (Hycar VTBNX 1300×33, 1300×43 from BF Goodrich). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B show the basic processing steps for preparing the microcups involving imagewise photolithographic exposure through a photomask of conductor film coated with a thermoset precursor (“top exposure”). 
     FIGS. 2A and 2B show alternative processing steps for preparing the microcups involving imagewise photolithographic exposure of the base conductor film coated with a thermoset precursor, in which the base conductor pattern on a transparent substrate serves a substitute for a photomask and is opaque to the radiation (“bottom exposure”). 
     FIGS. 3A and 3B show alternative processing steps for preparing the microcups involving imagewise photolithographic exposure combining the top and bottom exposure principles, whereby the walls are cured in one lateral direction by top photomask exposure and in the perpendicular lateral direction by bottom exposure through the opaque base conductor film (“combined exposure”). 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Unless defined otherwise in this specification, all technical terms are used herein according to their conventional definitions as they are commonly used and understood by those of ordinary skill in the art. The terms “microcup”, “cell”, “well-defined”, “aspect ratio” and “imagewise exposure” in the context of the present application are as defined in the copending applications identified above, as are the dimensions of the microcups. 
     The microcups may be prepared by microembossing or by photolithography. 
     I. Preparation of Microcups by Microembossing 
     Preparation of the Male Mold 
     The male mold may be prepared by any appropriate method, such as a diamond turn process or a photoresist process followed by either etching or electroplating. A master template for the male mold may be manufactured by any appropriate method, such as electroplating. With electroplating, a glass base is sputtered with a thin layer (typically 3000 Å) of a seed metal such as chrome inconel. It is then coated with a layer of photoresist and exposed to UV. A mask is placed between the UV and the layer of photoresist. The exposed areas of the photoresist become hardened. The unexposed areas are then removed by washing them with an appropriate solvent. The remaining hardened photoresist is dried and sputtered again with a thin layer of seed metal. A master is then ready for electroforming. A typical material used for electroforming is nickel cobalt. Alternatively, the master can be made of nickel by electroforming or electroless nickel deposition as described in “Continuous manufacturing of thin cover sheet optical media”, SPIE Proc. Vol. 1663, pp.324 (1992). The floor of the mold is typically between about 50 to 400 microns. The master can also be made using other microengineering techniques including e-beam writing, dry etching, chemical etching, laser writing or laser interference as described in “Replication techniques for micro-optics”, SPIE Proc. Vol.3099, pp76-82 (1997). Alternatively, the mold can be made by photomachining, using plastics, ceramics or metals. 
     The male mold thus prepared typically has protrusions between about 1 to 500 microns, preferably between about 2 to 100 microns, and most preferably about 4 to 50 microns. The male mold may be in the form of a belt, a roller, or a sheet. For continuous manufacturing, the belt type of mold is preferred. 
     Micro-cup Formation 
     Micro-cups may be formed either in a batchwise process or in a continuous roll-to-roll process as disclosed in the co-pending application, U.S. Ser. No. 09/784,972, filed on Feb. 25, 2001 pending. The latter offers a continuous, low cost, high throughput manufacturing technology for production of compartments for use in electrophoretic or liquid crystal displays. Prior to applying a UV curable resin composition, the mold may be prepared with a mold release to aid in the demolding process, if desired. The UV curable resin may be degassed prior to dispensing and may optionally contain a solvent. The solvent, if present, readily evaporates. The UV curable resin is dispensed by any appropriate means, such as coating, dipping, pouring and the like, over the male mold. The dispenser may be moving or stationary. A conductor film is overlaid on the UV curable resin. Examples of suitable conductor films include transparent conductor ITO on plastic substrates such as polyethylene terephthalate, polyethylene naphthate, polyaramid, polyimide, polycycloolefin, polysulfone, epoxy and their composites. Pressure may be applied, if necessary, to ensure proper bonding between the resin and the plastic and to control the thickness of the floor of the micro-cups. The pressure may be applied using a laminating roller, vacuum molding, press device or any other like means. If the male mold is metallic and opaque, the plastic substrate is typically transparent to the actinic radiation used to cure the resin. Conversely, the male mold can be transparent and the plastic substrate can be opaque to the actinic radiation. To obtain good transfer of the molded features onto the transfer sheet, the conductor film needs to have good adhesion to the UV curable resin, which should have a good release property from the mold surface. 
     II. Preparation of Microcup Array by Photolithography 
     The photolithographic processes for preparation of the microcup array are described in FIGS. 1,  2  and  3 . 
     II(a) Top Exposure 
     As shown in FIGS. 1A and 1B, the microcup array  10  may be prepared by exposure of a radiation curable material  11   a  coated by known methods onto a conductor electrode film  12  to UV light (or alternatively other forms of radiation, electron beams and the like) through a mask  16  to form walls  11   b  corresponding to the image projected through the mask  16 . The base conductor film  12  is preferably mounted on a supportive substrate base web  13 , which may comprise a plastic material. 
     In the photomask  16  in FIG. 1A, the dark squares  14  represent the opaque area and the space between the dark squares represents the opening (transparent) area  15  of the mask  16 . The UV radiates through the opening area  15  onto the radiation curable material  11   a . The exposure is preferably directly onto the radiation curable material  11   a , i.e., the UV does not pass through the substrate  13  or base conductor  12  (top exposure). For this reason, neither the substrate  13  nor the conductor  12  needs to be transparent to the UV or other radiation wavelengths employed. 
     As shown in FIG. 1B, The exposed areas  11   b  become hardened and the unexposed areas  11   c  (protected by the opaque area  14  of the mask  16  are then removed by an appropriate solvent or developer to form the microcups  17 . The solvent or developer is selected from those commonly used for dissolving or reducing the viscosity of radiation curable materials such as methylethylketone, toluene, acetone, isopropanol or the like. 
     II(b) Bottom Exposure or Combined Exposure 
     Two alternative methods for the preparation of the microcup array of the invention by imagewise exposure are illustrated in FIGS. 2A and 2B and  3 A and  3 B. These methods employ UV exposure through the substrate web, using the conductor pattern as a mask. 
     Turning first to FIG. 2A, the conductor film  22  used is pre-patterned to comprise cell base electrode portions  24  corresponding to the floor portions of the microcups  27 . The base portions  24  are opaque to the UV wavelength (or other radiation) employed. The spaces  25  between conductor base portions  22  are substantially transparent or transmissive to the UV light. In this case, the conductor pattern serves as a photomask. The radiation curable material  21   a  is coated upon the substrate  23  and conductor  22  as described in FIG.  2 A. The material  21   a  is exposed by UV light projected “upwards” (through substrate  23 ) and cured where not shielded by the conductor  22 , i.e., in those areas corresponding to the space  25 . As shown in FIG. 2B, the uncured material  21   c  is removed from the unexposed areas as described above, leaving the cured material  21   b  to form the walls of the microcups  27 . 
     FIG. 3A illustrates a combination method which uses both the top and bottom exposure principals to produce the microcup array  30  of the invention. The base conductor film  32  is also opaque and line-patterned. The radiation curable material  31   a , which is coated on the base conductor  32  and substrate  33 , is exposed from the bottom through the conductor line pattern  32  which serves as the first photomask. A second exposure is performed from the “top” side through the second photomask  36  having a line pattern perpendicular to the conductor lines  32 . The spaces  35  between the lines  34  are substantially transparent or transmissive to the UV light. In this process, the wall material  31   b  is cured from the bottom up in one lateral orientation, and cured from the top down in the perpendicular direction, joining to form an integral microcup  37 . 
     As shown in FIG. 3B, the unexposed area is then removed by a solvent or developer as described above to reveal the microcups  37 . 
     The radiation curable material used in the processes described above is a thermoplastic or thermoset precursor, such as multifunctional acrylate or methacrylate, vinylether, epoxide and their oligomers, polymers and the like. Multifunctional acrylates and their oligomers are the most preferred. A combination of multifunctional epoxide and multifunctional acrylate is also very useful to achieve desirable physico-mechanical properties. 
     It has now been found that addition of a rubber component significantly improves the quality of the microcups, such as resistance toward flexure or stress, demoldability during the microembossing step, and adhesion between the sealing layer and the microcups. 
     Suitable rubber materials have a Tg (glass transition temperature) lower than 0° C. Unsaturated rubber materials are preferred and rubber materials having uncapped or side chain unsaturated groups such as vinyl, acrylate, methacrylate, allyl groups are particularly preferred. More specifically, suitable rubber materials include SBR (styrene-butadiene rubber), PBR (polybutadiene rubber), NBR (acrylonitrile-butadiene rubber), SBS (styrene-butadiene-styrene block copolymer), SIS (styrene-isoprene-styrene block copolymer), and their derivatives. Particularly useful are functionalized rubbers such as polybutadiene dimethacrylate (CN301 and CN302 from Sartomer, Ricacryl 3100 from Ricon Resins Inc.), graft (meth)acrylated hydrocarbon polymer (Ricacryl 3500 and Ricacryl 3801 from Ricon Resins, Inc.), and methacrylate terminated butadiene-acrylonitrile copolymers (Hycar VTBNX 1300×33, 1300×43 from B F Goodrich). 
     The percentage of rubber component in the UV curable formulation can be in the range from 1 wt-% to 30 wt-%, preferably from 5 wt-% to 20 wt-%, even more preferably from 8-15 wt-%. The rubber components can be soluble or dispersible in the formulation. Ideally, the rubber component is soluble in the formulation before UV curing and phase separates into microdomains after UV curing. 
     EXAMPLES 
     Example 1 
     Microcup Composition without Rubber 
     35 parts by weight of Ebercryl® 600 (UCB), 40 parts of SR-399 (Sartomer®), 10 parts of Ebecryl 4827 (UCB), 7parts of Ebecryl 1360 (UCB), 8 parts of HDDA (UCB), and 0.05 parts of Irgacure® 369 (Ciba Specialty Chemicals), 0.01 parts of isopropyl thioxanthone (Aldrich) were mixed homogeneously and used to prepare the microcup arrary by either the microembossing or photolithographic process. 
     Example 2-7 
     Rubber-containing Microcup Compositions 
     The same procedure as Example 1 was repeated except that 6, 7, 8, 10, 11 or 14 phr (parts per hundred resin) of Hycar® VTBNX 1300×33 were added to the compositions of Examples 2-7, respectively. 
     Comparison of Flexure Resistance 
     The microcup compositions of Examples 1-7 were coated onto 2 mil PET film with a targeted dry thickness of about 30 μm, covered by untreated PET, and then cured for 20 seconds under UV light at an intensity of ˜5 mW/cm 2 . The coated samples were then 90 degree hand bended to determine the flexure resistance, after the untreated PET was removed. It was found that the flexure resistance of formulations containing more than 8 phr of Hycar VTBNX 1300×33 (Examples 4, 5, 6, 7) was improved significantly (Table 1). 
     Comparison of Release Properties Between the Cured Microcup and the Ni—Co Microembossing Male Mold 
     The microcup compositions of the Example 1-7 were coated onto 2 mil PET film with a targeted thickness of about 50 μm, microembossed with a Ni—Co male mold of 60×60×35 μm with partition lines of 10 μm width, UV cured for 20 seconds, and removed from the mold with a 2″ peeling bar at a speed of about 4-5 ft/min. The formulations containing more than 6 phr of rubber (Examples 2-7) showed significantly improved demoldability (Table 1). Little defect or contamination on the mold was observed for formulations containing 10-15 phr of rubber (Examples 5, 6, 7) after at least 100 molding-demolding cycles 
     Comparison of Adhesion Between the Microcup and the Sealing Layers 
     The microcup compositions of Example 1-7 were coated onto 2 mil PET film with a targeted dry thickness of about 30 μm, covered by untreated PET, and then cured for 20 seconds under UV light at an intensity of ˜5 mW/cm 2 . The untreated PET cover sheet was removed. A 15 wt % solution of the sealing material (Kraton® FG-1901X from Shell) in 20/80 (v/v) toluene/hexane was then coated onto the cured microcup layer and dried in 60° C. oven for 10 minutes. The thickness of the dried sealing layer was controlled to be about 5 μm. A 3M 3710 Scotch® tape was laminated at room temperature onto the sealing layer by a Eagle® 35 laminator from GBC at the heavy gauge setting. The T-peel adhesion force was then measured by Instron® at 500 mm/min. The adhesion forces listed in Table 1 were the average of at least 5 measurements. It was found that adhesion between the sealing layer and the cured microcup layer was significantly improved by incorporating rubber into the microcup. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 T Peel Adhesion Between the Cured Microcup Material and the 
               
               
                 Sealing Layer 
               
             
          
           
               
                   
                 Hycar 
                 Adhesion 
                   
                   
               
               
                   
                 VTBNX 
                 (peel) to 
               
               
                 Example 
                 1300 × 33 
                 sealing layer 
                 Flexure 
                 Release 
               
               
                 Number 
                 (phr) 
                 (gm/12.5 mm) 
                 Resistance 
                 from the mold 
               
               
                   
               
             
          
           
               
                 1 
                 0 
                 431 +/− 33 
                 poor, bending 
                 fair, some 
               
               
                   
                   
                   
                 line 
                 defects 
               
               
                   
                   
                   
                 broke 
               
               
                 2 
                 6 
                 513 +/− 12 
                 fair, bending 
                 Good, no 
               
               
                   
                   
                   
                 line broke 
                 defect after 50 
               
               
                   
                   
                   
                   
                 cycles 
               
               
                 3 
                 7 
                 — 
                 fair-good, 
                 good 
               
               
                   
                   
                   
                 bending line 
               
               
                   
                   
                   
                 broke 
               
               
                 4 
                 8 
                 — 
                 Good, bending 
                 good- 
               
               
                   
                   
                   
                 mark 
                 excellent 
               
               
                 5 
                 10 
                 543 +/− 20 
                 Excellent, no 
                 Excellent, no 
               
               
                   
                   
                   
                 bending mark 
                 defect after 
               
               
                   
                   
                   
                   
                 100 cycles 
               
               
                 6 
                 11 
                 — 
                 excellent 
                 excellent 
               
               
                 7 
                 14 
                 536 +/− 12 
                 excellent 
                 excellent 
               
               
                   
               
             
          
         
       
     
     Example 8 
     Microcup Composition without Rubber 
     36 parts by weight of Ebercryl® 830 (UCB), 9 parts of SR-399 (Sartomer®), 1.2 parts of Ebecryl 1360 (UCB), 3 parts of HDDA (UCB), 1.25 parts of Irgacure® 500 (Ciba Specialty Chemicals), and 25 parts of MEK (Aldrich) were mixed homogeneously and used to prepare the microcup array by microembossing as described previously, except that the UV curing time was 1 minute. This example showed some defect on the microcup or contamination on a Ni—Co male mold of 60×60×50 μm with 10 μm partition lines after about 10 molding-demolding cycles. 
     Example 9 
     Microcup Composition with Rubber 
     The same procedure as in Example 8 was repeated except that 5.47 parts of poly(butadiene-co-acrylonitrile) diacrylate (Monomer-Polymer &amp; Dajac Labs, Inc.) was added to the composition. No observable defect on the microcup array or contamination on the Ni—Co male mold was found after about 10 molding-demolding cycles. 
     Example 10 
     Pigment Dispersion 
     6.42 Grams of Ti Pure R706 was dispersed with a homogenizer into a solution containing 1.94 grams of Fluorolink® D from Ausimont, 0.22 grams of Fluorolink® 7004 also from Ausimont, 0.37 grams of a fluorinated copper phthalocyanine dye from 3M, and 52.54 grams of perfluoro solvent HT-200 (Ausimont). 
     Example 11 
     Pigment Dispersion 
     The same as in Example 10, except the Ti Pure R706 and Fluorolink were replaced by polymer coated TiO 2  particles PC-9003 from Elimentis (Hihstown, N.J.) and Krytox® (Du Pont) respectively. 
     Example 12 
     Microcup Sealing and Electrophoretic Cell 
     The electrophoretic fluid prepared in Examples 10 was diluted with a volatile perfluoro cosolvent (FC-33 from 3M) and coated onto a microcup array containing 11 phr of Hycar® VTBNX 1300×33 (Example 6) on a ITO/PET conductor film. The volatile cosolvent was allowed to evaporate to expose a partially filled microcup array. A 7.5% solution of polyisoprene in heptane was then overcoated onto the partially filled cups by a Universal Blade Applicator with an opening of 6 mil. The overcoated microcups were then dried at room temperature. A seamless sealing layer of about 5-6 microns thickness with acceptable adhesion was formed on the microcup array. No observable entrapped air bubbles in the sealed microcups were found under microscope. The sealed microcup array was then post treated by UV radiation or thermal baking to further improve the barrier properties. A second ITO/PET conductor precoated with an adhesive layer was laminated onto the sealed microcups. The electrophoretic cell showed satisfactory switching performance with good flexure resistance. No observable weight loss was found after being aged in a 66° C. oven for 5 days. 
     Example 13 
     Microcup Sealing and Electrophoretic Cell 
     The electrophoretic fluid prepared in Example 11 was diluted with a volatile perfluoro cosolvent (FC-33 from 3M) and coated onto a microcup array containing 12 phr of Hycar® VTBNX 1300×33 on a ITO/PET conductor film. The volatile cosolvent was allowed to evaporate to expose a partially filled microcup array. A 7.5% solution of polyisoprene in heptane was then overcoated onto the partially filled cups by a Universal Blade Applicator with an opening of 6 mil. The overcoated microcups were then dried at room temperature. A seamless sealing layer of about 5-6 microns thickness with acceptable adhesion was form on the microcup array. No observable entrapped air bubbles in the sealed microcups were found under microscope. The sealed microcup array was then post treated by UV radiation or thermal baking to further improve the barrier properties. A second ITO/PET conductor precoated with an adhesive layer was laminated onto the sealed microcups. The electrophoretic cell showed satisfactory switching performance with good flexure resistance. No observable weight loss was found after aged in a 66° C. oven for 5 days. 
     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 true spirit and 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, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. For example, it should be noted that the method of the invention for making microcups may also be used for manufacturing microcup arrays for liquid crystal displays. Similarly, the microcup selective filling, sealing and ITO laminating methods of the invention may also be employed in the manufacture of liquid crystal displays. 
     It is therefore wished that this invention to be defined by the scope of the appended claims as broadly as the prior art will permit, and in view of the specification.