# Patent

## Title
Guarded cover film for LCD polarizers

## Abstract

The invention generally relates to polymer films used as protective cover sheets for polarizer plates, a method for producing polarizing plates, and a Liquid Crystal Display employing the same. More particularly, the invention relates to cover sheet composites having a removable, carrier substrate and, optionally, a strippable, protection layer that protects the cover sheet from abrasion and dirt during the manufacture of the cover sheet and polarizer plates.

## Background
FIELD OF THE INVENTION
The present invention generally relates to polymer films used as protective cover sheets for polarizer plates, a method for producing polarizing plates, and a Liquid Crystal Display employing the same. More particularly, the invention relates to cover sheet composites having a removable, carrier substrate and, optionally, a strippable, protection layer that protects the cover sheet from abrasion and dirt during the manufacture of the cover sheet and polarizer plates.
BACKGROUND OF THE INVENTION
Transparent resin films are used in a variety of optical applications. In particular, resin films are used as protective cover sheets for light polarizers in variety of electronic displays, particularly Liquid Crystal Displays (LCD).
LCDs contain a number of optical elements that may be formed from resin films. The structure of reflective LCD's may include a liquid crystal cell, one or more polarizer plates, and one or more light management films. Liquid crystal cells are formed by dispersing liquid crystals such as twisted nematic (TN) or super twisted nematic (STN) materials between two electrode substrates. Polarizer plates are typically a multi-layer element of resin films and are comprised of a polarizing film sandwiched between two protective cover sheets. Polarizing films are normally prepared from a transparent and highly uniform amorphous resin film that is subsequently stretched to orient the polymer molecules and stained with a dye to produce a dichroic film. An example of a suitable resin for the formation of polarizer films is fully hydrolyzed polyvinyl alcohol. Because the stretched polyvinyl alcohol films used to form polarizers are very fragile and dimensionally unstable, protective cover sheets are normally needed to offer both support and abrasion resistance. Protective cover sheets of polarizer plates are required to have high uniformity, good dimensional and chemical stability, and high transparency. Originally, protective coversheets were formed from glass, but a number of resin films are now used to produce lightweight and flexible polarizers. Although many resins have been suggested for use in protective cover sheets including, cellulosics, acrylics, cyclic olefin polymers, polycarbonates, and sulfones, acetyl cellulose polymers are most commonly used in protective cover sheets for polarizer plates. Polymers of the acetyl cellulose type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hyroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, triacetyl cellulose (TAC) is commonly used to manufacture resin films for use in protective cover sheets for polarizer plates.
Some LCD devices may contain a protective cover sheet that also serves as a compensation film to improve the viewing angle of an image. Compensation films (i.e. retardation films or phase difference films) are normally prepared from amorphous films that have a controlled level of birefringence either by uniaxial stretching or by coating with discotic dyes. Suitable resins suggested for formation of compensation films by stretching include polyvinyl alcohols, polycarbonates and sulfones. Compensation films prepared by treatment with dyes normally require highly transparent films having low birefringence such as TAC and cyclic olefin polymers.
Protective cover sheets may require the application of other functional layers (herein also referred to as auxiliary layers) such as an antiglare layer, antireflection layer, anti-smudge layer, or antistatic layer. Generally, these functional layers are applied in a process step that is separate from the manufacture of the resin film.
In general, resin films are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270-300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have proprionate groups in place of acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally used to manufacture these films.
Resin films for optical applications are manufactured almost exclusively by casting methods. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40-200 microns. In general, thin films of less than 40 microns are very difficult to produce by casting methods due to the fragility of wet film during the peeling and drying processes. Films having a thickness of greater than 200 microns are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods, and problems associated with decomposition at high temperature are avoided.
Examples of optical films prepared by casting methods include: 1.) Cellulose acetate sheets used to prepare light polarizers as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Cael as well as more recent disclosures in U.S. patent application. Ser. No. 2001/0039319 A1 to Harita and U.S. patent application. Ser. No. 2002/001700 A1 to Sanefuji, 2.) Cellulose triacetate sheets used for protective covers for light polarizers as disclosed in U.S. Pat. No. 5,695,694 to Iwata, 3.) Polycarbonate sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani, and 4.) Polyethersulfone sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. Nos. 5,759,449 and 5,958,305 both to Shiro.
Despite the wide use of the casting method to manufacture optical films, there are however, a number of disadvantages to casting technology. One disadvantage is that cast films have significant optical birefringence. Although films prepared by casting methods have lower birefringence when compared to films prepared by melt extrusion methods, birefringence remains objectionably high. For example, cellulose triacetate films prepared by casting methods exhibit in-plane retardation of 7 nanometers (nm) for light in the visible spectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Polycarbonate films prepared by casting methods exhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani. U.S. patent application Ser. No. 2001/0039319 A1 to Harita claims that color irregularities in stretched cellulose acetate sheets are reduced when the difference in retardation between widthwise positions within the film is less than 5 nm in the original unstretched film. For many applications of optical films, low in-plane retardation values are desirable. In particular, values of in-plane retardation of less than 10 nm are preferred.
Commonly-assigned U.S. Patent Application Publication Nos. 2003/0215658A, 2003/0215621A, 2003/0215608A, 2003/0215583A, 2003/0215582A, 2003/0215581A, 2003/0214715A describe a coating method to prepare resin films having low birefringence that are suitable for optical applications. The resin films are applied onto a discontinuous, sacrificial substrate from lower viscosity polymer solutions than are normally used to prepare cast films.
Birefringence in cast or coated films arises from orientation of polymers during the manufacturing operations. This molecular orientation causes indices of refraction within the plane of the film to be measurably different. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. The absolute value of birefringence multiplied by the film thickness is defined as in-plane retardation. Therefore, in-plane retardation is a measure of molecular anisotropy within the plane of the film.
During a casting process, molecular orientation may arise from a number of sources including shear of the dope in the die, shear of the dope by the metal support during application, shear of the partially dried film during the peeling step, and shear of the free-standing film during conveyance through the final drying step. These shear forces orient the polymer molecules and ultimately give rise to undesirably high birefringence or retardation values. To minimize shear and obtain the lowest birefringence films, casting processes are typically operated at very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line speeds generally produce the highest quality films.
Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.
Another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and degas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks and may create substantial waste.
In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.
Cast films may exhibit undesirable cockle or wrinkles. Thinner films are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the film. In particular, the preparation of polarizer plates from resin films requires a lamination process involving application of adhesives, pressure, and high temperatures. Very thin films are difficult to handle during this lamination process without wrinkling. In addition, many cast films may naturally become distorted over time due to the effects of moisture. For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of polarizer plates. Finally, resin films used as protective cover sheets for polarizer plates are susceptible to scratch and abrasion, as well as the accumulation of dirt and dust, during the manufacture and handling of the cover sheet. The preparation of high quality polarizer plates for display applications requires that the protective cover sheet be free of defects due to physical damage or the deposition of dirt and dust.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the limitations of prior art polarizer cover sheets prepared by conventional casting methods and provide a new cover sheet that is thinner and provides improved handling and processing during the steps necessary for the fabrication of polarizer plates.
It is another object to provide a new cover sheet that is less susceptible to physical damage such as scratch and abrasion and is more dimensionally stable during its manufacture, storage and final handling steps necessary in the fabrication of polarizer plates.
It is a further object to provide a new cover sheet that is less prone to the accumulation of dirt and dust during its manufacture, storage and final handling steps necessary in the fabrication of polarizer plates.
It is a still further object to provide an improved process for the fabrication of polarizer plates using the new cover sheets of the invention.
These and other objects of the invention are accomplished by a guarded cover sheet composite comprising a carrier substrate and a cover sheet comprising a low birefringence polymer film and one or more auxiliary layers on the same side of said carrier as the low birefringence polymer film. Auxiliary layers useful in the present invention include an abrasion-resistant layer, antiglare layer, low reflection layer, antireflection layer, antistatic layer, viewing angle compensation layer, and moisture barrier layer. The guarded cover sheet composite may also have a strippable, protection layer on the side of the low birefringence polymer film opposite to the carrier substrate.
The fabrication of very thin cover sheets is facilitated by the carrier substrate that supports the wet cover sheet coating through the drying process and eliminates the need to peel the sheet from a metal band or drum prior to a final drying step as required in the casting methods described in prior art. Rather, the cover sheet is completely dried before separation from the carrier substrate. In fact, the composite comprising the cover sheet and carrier substrate are preferably wound into rolls and stored until needed for the fabrication of polarizer plates.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an exemplary coating and drying apparatus that can be used in the practice of the method of the present invention.
FIG. 2 is a schematic of an exemplary coating and drying apparatus of FIG. 1 including a station where the cellulose acetate web separated from the substrate is separately wound.
FIG. 3 is a schematic of an exemplary multi-slot coating apparatus that can be used in the practice of the present invention.
FIG. 4 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a three-layer cover sheet and a partially peeled carrier substrate.
FIG. 5 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a four-layer cover sheet and a partially peeled carrier substrate.
FIG. 6 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a five-layer cover sheet and a partially peeled carrier substrate.
FIG. 7 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a four-layer cover sheet, a strippable, protection layer and a partially peeled carrier substrate.
FIG. 8 shows a cross-sectional representation of a guarded cover sheet composite of the invention comprising a four-layer cover sheet and a partially peeled carrier substrate wherein the carrier substrate has a release layer formed thereon.
FIG. 9 shows a schematic of a method to fabricate a polarizer plate using the guarded cover sheet composites of the invention.
FIG. 10 shows a cross-sectional representation of a liquid crystal cell with polarizer plates on either side of the cell.
FIG. 11 is a schematic of a casting apparatus as used in prior art to cast cellulose acetate films.



DETAILED DESCRIPTION OF THE INVENTION
The following definitions apply to the description herein:
In plane phase retardation, Rin, of a layer is a quantity defined by (nx−ny)d, where nx and ny are indices of refraction in the direction of x and y. x is taken as a direction of maximum index of refraction in the x-y plane and y direction is perpendicular to it. x-y plane is parallel to the surface plane of the layer. d is a thickness of the layer in the z-direction. The quantity (nx−ny) is referred to as in-plane birefringence, Δnin. The value of Δnin is given at a wavelength λ=550 nm.
Out of-plane phase retardation, Rth, of a layer is a quantity defined by [nz−(nx+ny)/2]d. nz is the index of refraction in the z-direction. The quantity [nz−(nx+ny)/2] is referred to as out-of-plane birefringence, Δnth. If nz>(nx+ny)/2, Δnth is positive (positive birefringence), thus the corresponding Rth is also positive. If nz<(nx+ny)/2, Δnth is negative (negative birefringence) and Rth is also negative. The value of Δnth is given at λ=550 nm.
Intrinsic Birefringence Δnint of a polymer refers to the quantity defined by (ne−no), where ne, and no are the extraordinary and the ordinary index of the polymer, respectively. The actual birefringence (in-plane Δnin or out-of-plane Δnth) of a polymer layer depends on the process of forming it, thus the parameter Δnint.
Amorphous means a lack of long-range order. Thus an amorphous polymer does not show long-range order as measured by techniques such as X-ray diffraction.
Transmission is a quantity to measure the optical transmissivity. It is given by the percentile ratio of out coming light intensity Iout to input light intensity Iin as Iout/IIn×100.
Optic Axis refers to the direction in which propagating light does not see birefringence.
Uniaxial means that two of the three indices of refraction, nx, ny, and nz, are essentially the same.
Biaxial means that the three indices of refraction, nx, ny, and nz, are all different.
Cover sheets employed in Liquid Crystal Displays are typically polymeric sheets having low optical birefringence that are employed on each side of a dichroic film in order to maintain the dimensional stability of the dichroic film and to protect it from moisture and UV degradation. In the following description a guarded cover sheet means a cover sheet that is disposed on a removable, protective carrier substrate. A strippable, protective film may also be employed on the side of the cover sheet opposite to the carrier substrate so that both sides of the cover sheet are protected prior to its use in a polarizer plate.
The present invention is directed to an improved cover sheet used in the fabrication of polarizer plates for Liquid Crystal Displays. In particular, the present invention provides a guarded cover sheet composite comprising a carrier substrate, a low birefringence polymer film, and one or more auxiliary layers on the same side of said carrier substrate as the low birefringence polymer film. Suitable auxiliary layers for use in the present invention include abrasion resistant hardcoat layer, antiglare layer, anti-smudge layer or stain-resistant layer, antireflection layer, low reflection layer, antistatic layer, viewing angle compensation layer, and moisture barrier layer. Optionally, the guarded cover sheet composite of the invention also comprises a strippable, protection layer on the side of the cover sheet opposite to the carrier substrate. The cover sheet composite of the invention has improved handling compared to cover sheets of the prior art and is less susceptible to physical defects and the accumulation of dirt and dust during the manufacture of the cover sheet and its subsequent use in the fabrication of polarizer plates. The present invention is also directed to an improved polarizer plate and its method of fabrication using the guarded cover sheet composites of the invention.
Turning now to FIG. 1 there is shown a schematic of an exemplary and well-known coating and drying system 10 suitable for preparing the guarded cover sheet composites of the present invention. The coating and drying system 10 is typically used to apply very thin films to a moving carrier substrate 12 and to subsequently remove solvent in a dryer 14. A single coating apparatus 16 is shown such that system 10 has only one coating application point and only one dryer 14, but two or three (even as many as six) additional coating application points with corresponding drying sections are known in the fabrication of composite thin films. The process of sequential application and drying is known in the art as a tandem coating operation.
Coating and drying apparatus 10 includes an unwinding station 18 to feed the moving substrate 12 around a back-up roller 20 where the coating is applied by coating apparatus 16. The coated substrate 22 then proceeds through the dryer 14. In the practice of the present invention the final guarded cover sheet composite 24 comprising a cover sheet on substrate 12 is wound into rolls at a wind-up station 26.
As depicted, an exemplary four-layer coating is applied to moving web 12. Coating liquid for each layer is held in respective coating supply vessel 28, 30, 32, 34. The coating liquid is delivered by pumps 36, 38, 40, 42 from the coating supply vessels to the coating apparatus 16 conduits 44, 46, 48, 50, respectively. In addition, coating and drying system 10 may also include electrical discharge devices, such as corona or glow discharge device 52, or polar charge assist device 54, to modify the substrate 12 prior to application of the coating.
Turning next to FIG. 2 there is shown a schematic of the same exemplary coating and drying system 10 depicted in FIG. 1 with an alternative winding operation to apply a strippable protection layer. Accordingly, the drawings are numbered identically up to the winding operation. In the practice of the present invention the guarded cover sheet composite 24 comprising a carrier substrate (which may be a resin film, paper, resin coated paper or metal) with a cover sheet applied thereto is taken between opposing nip rollers 56, 58. The guarded cover sheet composite 24 is adhesively adhered or electrostatically adhered to a preformed strippable protection layer 60 which is supplied from unwind station 62 and the guarded cover sheet composite containing the strippable protection layer is wound into rolls at wind-up station 64. In a preferred embodiment of the present invention, polyolefin or polyethylene phthalate (PET) is used as the preformed, strippable protection layer 60. Either the cover sheet/carrier substrate composite 24 or the protection layer 60 may be pretreated with an electric charge generator to enhance the electrostatic attraction of the protection layer 60 to the cover sheet/carrier substrate composite 24.
The coating apparatus 16 used to deliver coating fluids to the moving substrate 12 may be a multi-layer applicator such as a slide bead hopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, or a slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes. Alternatively, the coating apparatus 16 may be a single layer applicator, such as slot die bead hopper or jet hopper. In a preferred embodiment of the present invention, the application device 16 is a multi-layer slide bead hopper.
As mentioned above, coating and drying system 10 includes a dryer 14 that will typically be a drying oven to remove solvent from the coated film. An exemplary dryer 14 used in the practice of the method of the present invention includes a first drying section 66 followed by eight additional drying sections 68-82 capable of independent control of temperature and air flow. Although dryer 14 is shown as having nine independent drying sections, drying ovens with fewer compartments are well known and may be used to practice the method of the present invention. In a preferred embodiment of the present invention the dryer 14 has at least two independent drying zones or sections.
Preferably, each of drying sections 68-82 each has independent temperature and airflow controls. In each section, temperature may be adjusted between 5° C. and 150° C. To minimize drying defects from case hardening or skinning-over of the wet layers, optimum drying rates are needed in the early sections of dryer 14. There are a number of artifacts created when temperatures in the early drying zones are inappropriate. For example, fogging or blush of cellulose acetate films is observed when the temperature in zones 66, 68 and 70 are set at 25° C. This blush defect is particularly problematic when high vapor pressures solvents (methylene chloride and acetone) are used in the coating fluids. Aggressively high temperatures of 95° C. in the early drying sections 66, 68, and 70 are found to cause premature delamination of the cover sheet from the carrier substrate. Higher temperatures in the early drying sections are also associated with other artifacts such as case hardening, reticulation patterns and blistering of the cover sheet. In preferred embodiment of the present invention, the first drying section 66 is operated at a temperature of at least about 25° C. but less than 95° C. with no direct air impingement on the wet coating of the coated web 22. In another preferred embodiment of the method of the present invention, drying sections 68 and 70 are also operated at a temperature of at least about 25° C. but less than 95° C. It is preferred that initial drying sections 66, 68 be operated at temperatures between about 30° C. and about 60° C. It is most preferred that initial drying sections 66, 68 be operated at temperatures between about 30° C. and about 50° C. The actual drying temperature in drying sections 66, 68 may optimize empirically within these ranges by those skilled in the art.
Referring now to FIG. 3, a schematic of an exemplary coating apparatus 16 is shown in detail. Coating apparatus 16, schematically shown in side elevational cross-section, includes a front section 92, a second section 94, a third section 96, a fourth section 98, and a back plate 100. There is an inlet 102 into second section 94 for supplying coating liquid to first metering slot 104 via pump 106 to thereby form a lowermost layer 108. There is an inlet 110 into third section 96 for supplying coating liquid to second metering slot 112 via pump 114 to form layer 116. There is an inlet 118 into fourth section 98 for supplying coating liquid to metering slot 120 via pump 122 to form layer 124. There is an inlet 126 into back plate 100 for supplying coating liquid to metering slot 128 via pump 130 to form layer 132. Each slot 104, 112, 120, 128 includes a transverse distribution cavity. Front section 92 includes an inclined slide surface 134, and a coating lip 136. There is a second inclined slide surface 138 at the top of second section 94. There is a third inclined slide surface 140 at the top of third section 96. There is a fourth inclined slide surface 142 at the top of fourth section 98. Back plate 100 extends above inclined slide surface 142 to form a back land surface 144. Residing adjacent the coating apparatus or hopper 16 is a coating backing roller 20 about which a web 12 is conveyed. Coating layers 108, 116, 124, 132 form a multi-layer composite which forms a coating bead 146 between lip 136 and substrate 12. Typically, the coating hopper 16 is movable from a non-coating position toward the coating backing roller 20 and into a coating position. Although coating apparatus 16 is shown as having four metering slots, coating dies having a larger number of metering slots (as many as nine or more) are well known and may be used to practice the method of the present invention.
For the purpose of the present invention, the coating fluids are comprised principally of a polymer binder dissolved in an organic solvent. In a particularly preferred embodiment, the low birefringence polymer film is a cellulose ester. These are commercially available in a variety of molecular weight sizes as well as in the type and degree of alkyl substitution of the hydroxyl groups on the cellulose backbone. Examples of cellulose esters include those having acetyl, propionyl and butyryl groups. Of particular interest is the family of cellulose esters with acetyl substitution known as cellulose acetate. Of these, the fully acetyl substituted cellulose having a combined acetic acid content of approximately 58.0-62.5% is known as triacetyl cellulose (TAC) and is generally preferred for preparing cover sheets used in electronic displays.
In terms of organic solvents for TAC, suitable solvents, for example, include chlorinated solvents (methylene chloride and 1,2 dichloroethane), alcohols (methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, diacetone alcohol and cyclohexanol), ketones (acetone, methylethyl ketone, methylisobutyl ketone, and cyclohexanone), esters (methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, isobutyl acetate, n-butyl acetate, and methylacetoacetate), aromatics (toluene and xylenes) and ethers (1,3-dioxolane, 1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and 1,5-dioxane). In some applications, small amounts of water may be used. Normally, TAC solutions are prepared with a blend of the aforementioned solvents. Preferred primary solvents include methylene chloride, acetone, methyl acetate, and 1,3-dioxolane. Preferred co-solvents for use with the primary solvents include methanol, ethanol, n-butanol and water.
Coating formulations may also contain plasticizers. Appropriate plasticizers for TAC films include phthalate esters (dimethylphthalate, dimethoxyethyl phthalate, diethylphthalate, dibutylphthalate, dioctylphthalate, didecylphthalate and butyl octylphthalate), adipate esters (dioctyl adipate), and phosphate esters (tricresyl phosphate, biphenylyl diphenyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, trioctyl phosphate, tributyl phosphate, and triphenyl phosphate), glycolic acid esters (triacetin, tributyrin, butyl phthalyl butyl glycolate, ethyl phthalyl ethyl glycolate, and methyl phthalyl ethyl glycolate). Plasticizers are normally used to improve the physical and mechanical properties of the final film. In particular, plasticizers are known to improve the flexibility and dimensional stability of cellulose acetate films. However, plasticizers are also used here as coating aids in the converting operation to minimize premature film solidification at the coating hopper and to improve drying characteristics of the wet film. In the method of the present invention, plasticizers are used to minimize blistering, curl and delamination of TAC films during the drying operation. In a preferred embodiment of the present invention, plasticizers are added to the coating fluid at a total concentration of up to 50% by weight relative to the concentration of polymer in order to mitigate defects in the final TAC film.
The coating formulation for the low birefringence polymer may also contain one or more UV absorbing compounds to provide UV filter element performance and/or act as UV stabilizers for the low birefringence polymer film. Ultraviolet absorbing compounds are generally contained in the polymer in an amount of 0.01 to 20 weight parts based on 100 weight parts of the polymer containing no ultraviolet absorber, and preferably contained in an amount of 0.01 to 10 weight parts, especially in an amount of 0.05 to 2 weight parts. Any of the various ultraviolet light absorbing compounds which have been described for use in various polymeric elements may be employed in the polymeric elements of the invention, such as hydroxyphenyl-s-triazine, hydroxyphenylbenzotriazole, formamidine, or benzophenone compounds. As described in copending, commonly assigned U.S. patent application Ser. No. 10/150,634, filed May 5, 2002, the use of dibenzoylmethane ultraviolet absorbing compounds in combination with a second UV absorbing compound such as those listed above have been found to be particularly advantageous with respect to providing both a sharp cut off in absorption between the UV and visible light spectral regions as well as increased protection across more of the UV spectrum. Additional possible UV absorbers which may be employed include salicylate compounds such as 4-t-butylphenylsalicylate; and [2,2′thiobis-(4-t-octylphenolate)]n-butylamine nickel(II). Most preferred are combinations of dibenzoylmethane compounds with hydroxyphenyl-s-triazine or hydroxyphenylbenzotriazole compounds.
Dibenzoylmethane compounds which may be employed include those of the formula (IV)




where R1 through R5 are each independently hydrogen, halogen, nitro, or hydroyxl, or further substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy, ester, carboxyl, alkyl thio, aryl thio, alkyl amine, aryl amine, alkyl nitrile, aryl nitrile, arylsulfonyl, or 5-6 member heterocylce ring groups. Preferably, each of such groups comprises 20 or fewer carbon atoms. Further preferably, R1 through R5 of Formula IV are positioned in accordance with Formula IV-A:





Particularly preferred are compounds of Formula IV-A where R1 and R5 represent alkyl or alkoxy groups of from 1-6 carbon atoms and R2 through R4 represent hydrogen atoms.

Representative compounds of Formula (IV) which may be employed in accordance the elements of the invention include the following:
(IV-1): 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (PARSOL® 1789)
(IV-2): 4-isopropyl dibenzoylmethane (EUSOLEX® 8020)
(IV-3): dibenzoylmethane (RHODIASTAB® 83)
Hydroxyphenyl-s-triazine compounds which may be used in the elements of the invention, e.g., may be a derivative of tris-aryl-s-triazine compounds as described in U.S. Pat. No. 4,619,956. Such compounds may be represented by Formula V:
(V)




wherein X, Y and Z are each aromatic, carbocylic radicals of less than three 6-membered rings, and at least one of X, Y and Z is substituted by a hydroxy group ortho to the point of attachment to the triazine ring; and each of R1 through R9 is selected from the group consisting of hydrogen, hydroxy, alkyl, alkoxy, sulfonic, carboxy, halo, haloalkyl and acylamino. Particularly preferred are hydroxyphenyl-s-triazines of the formula V-A:

(V-A)





wherein R is hydrogen or alkyl of 1-18 carbon atoms.

Hydroxyphenylbenzotriazole compounds which may be usedin the elements of the invention, e.g., may be a derivative of compounds represented by Formula VI:
(VI)




wherein R1 through R5 may be independently hydrogen, halogen, nitro, hydroxy, or further substituted or unsubstituted alkyl, alkenyl, aryl, alkoxy, acyloxy, aryloxy, alkylthio, mono or dialkyl amino, acyl amino, or heterocyclic groups. Specific examples of benzotriazole compounds which may be used in accordance with the invention include 2-(2′-hydroxy-3′-t-butyl-5′-methylphenyl)-5-chlorobenzotriazole; 2-(2′-hydroxy-3′,5′-di-t-amylphenyl)benzotriazole; octyl 5-tert-butyl-3-(5-chloro-2H-benzotriazole-2-yl)-4-hydroxybenzenepropionate; 2-(hydroxy-5-t-octylphenyl)benzotriazole; 2-(2′-hydroxy-5′-methylphenyl)benzotriazole; 2-(2′-hydroxy-3′-dodecyl-5′-methylphenyl)benzotriazole; and 2-(2′-hydroxy-3′,5′-di-t-butylphenyl)-5-chlorobenzotriazole.

Formamidine compounds which may be used in the elements of the invention, e.g., may be a formamidine compound as described in U.S. Pat. No. 4,839,405. Such compounds may be represented by Formula VII or Formula VIII:




wherein R1 is an alkyl group containing 1 to about 5 carbon atoms; Y is a H, OH, Cl or an alkoxy group; R2 is a phenyl group or an alkyl group containing 1 to about 9 carbon atoms; X is selected from the group consisting of H, carboalkoxy, alkoxy, alkyl, dialkylamino and halogen; and Z is selected from the group consisting of H, alkoxy and halogen;





wherein A is —COOR, —COOH, —CONR′R′′, —NR′COR, —CN, or a phenyl group; and wherein R is an alkyl group of from 1 to about 8 carbon atoms; R′ and R′′ are each independently hydrogen or lower alkyl groups of from 1 to about 4 carbon atoms. Specific examples of formamidine compounds which may be used in accordance with the invention include those described in U.S. Pat. No. 4,839,405, and specifically 4-[[(methylphenylamino)methylene]amino]-ethyl ester.

Benzophenone compounds which may be used in the elements of the invention, e.g., may include 2,2′-dihydroxy-4,4′dimethoxybenzophenone, 2-hydroxy-4-methoxybenzophenone and 2-hydroxy-4-n-dodecyloxybenzophenone.
Coating formulations may also contain surfactants as coating aids to control artifacts related to flow after coating. Artifacts created by flow after coating phenomena include mottle, repellencies, orange-peel (Bernard cells), and edge-withdraw. Surfactants used control flow after coating artifacts include siloxane and fluorochemical compounds. Examples of commercially available surfactants of the siloxane type include: 1.) Polydimethylsiloxanes such as DC200 Fluid from Dow Corning, 2.) Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from Dow Corning, and 3.) Polyalkyl substituted polydimethysiloxanes such as DC190 and DC1248 from Dow Corning as well as the L7000 Silwet series (L7000, L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkyl substituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 from General Electric. Examples of commercially available fluorochemical surfactants include: 1.) Fluorinated alkyl esters such as the Fluorad series (FC430 and FC431) from the 3M Corporation, 2.) Fluorinated polyoxyethylene ethers such as the Zonyl series (FSN, FSN100, FSO, FSO100) from Du Pont, 3.) Acrylate:polyperfluoroalkyl ethylacrylates such as the F series (F270 and F600) from NOF Corporation, and 4.) Perfluoroalkyl derivatives such as the Surflon series (S383, S393, and S8405) from the Asahi Glass Company. In the method of the present invention, surfactants are generally of the non-ionic type. In a preferred embodiment of the present invention, non-ionic compounds of either the siloxane or fluorinated type are added to the uppermost layers.
In terms of surfactant distribution, surfactants are most effective when present in the uppermost layers of the multi-layer coating. In the uppermost layer, the concentration of surfactant is preferably 0.001-1.000% by weight and most preferably 0.010-0.500%. In addition, lesser amounts of surfactant may be used in the second uppermost layer to minimize diffusion of surfactant into the lowermost layers. The concentration of surfactant in the second uppermost layer is preferably 0.000-0.200% by weight and most preferably between 0.000-0.100% by weight. Because surfactants are only necessary in the uppermost layers, the overall amount of surfactant remaining in the final dried film is small. In the method of the present invention, a practical surfactant concentration in the uppermost layer having a wet thickness of 20 μm and a density of 0.93 g/cc is 0.200% by weight which after drying gives a final surfactant amount of approximately 37 mg/sq-m.
Although surfactants are not required to practice the method of the current invention, surfactants do improve the uniformity of the coated film. In particular, mottle nonuniformities are reduced by the use of surfactants. In transparent cellulose acetate films, mottle nonuniformities are not readily visualized during casual inspection. To visualize mottle artifacts, organic dyes may be added to the uppermost layer to add color to the coated film. For these dyed films, nonuniformities are easy to see and quantify. In this way, effective surfactant types and levels may be selected for optimum film uniformity.
The preparation of the guarded cover sheet composites of the present invention may also include the step of coating over a previously prepared composite of low birefringence polymer film and carrier substrate. For example, the coating and drying system 10 shown in FIGS. 1 and 2 may be used to apply a second multi-layer film to an existing low birefringence polymer film/substrate composite. If the film/substrate composite is wound into rolls before applying the subsequent coating, the process is called a multi-pass coating operation. If coating and drying operations are carried out sequentially on a machine with multiple coating stations and drying ovens, then the process is called a tandem coating operation. In this way, thick films may be prepared at high line speeds without the problems associated with the removal of large amounts of solvent from a very thick wet film. Moreover, the practice of multi-pass or tandem coating also has the advantage of minimizing other artifacts such as streak severity, mottle severity, and overall film nonuniformity.
The prior art method of casting resin films is illustrated in FIG. 11. As shown in FIG. 11, a viscous polymeric dope is delivered through a feed line 200 to an extrusion hopper 202 from a pressurized tank 204 by a pump 206. The dope is cast onto a highly polished metal drum 208 located within a first drying section 210 of the drying oven 212. The cast film 214 is allowed to partially dry on the moving drum 208 and is then peeled from the drum 208. The cast film 214 is then conveyed to a final drying section 216 to remove the remaining solvent. The final dried film 218 is then wound into rolls at a wind-up station 220. The prior art cast film typically has a thickness in the range of from 40 to 200 μm.
Coating methods are distinguished from casting methods by the process steps necessary for each technology. These process steps in turn affect a number of tangibles such as fluid viscosity, converting aids, substrates, and hardware that are unique to each method. In general, coating methods involve application of dilute low viscosity liquids to thin flexible substrates, evaporating the solvent in a drying oven, and winding the dried film/substrate composite into rolls. In contrast, casting methods involve applying a concentrated viscous dope to a highly polished metal drum or band, partially drying the wet film on the metal substrate, stripping the partially dried film from the substrate, removing additional solvent from the partially dried film in a drying oven, and winding the dried film into rolls. In terms of viscosity, coating methods require very low viscosity liquids of less than 5,000 cp. In the present invention the viscosity of the coated liquids will generally be less than 2000 cp and most often less than 1500 cp. Moreover, in the present invention the viscosity of the lowermost layer is preferred to be less than 200 cp. and most preferably less than 100 cp. for high speed coating application. In contrast, casting methods require highly concentrated dopes with viscosity on the order of 10,000-100,000 cp for practical operating speeds. In terms of converting aids, coating methods generally involve the use of surfactants as converting aids to control flow after coating artifacts such as mottle, repellencies, orange peel, and edge withdraw. In contrast, casting methods do not require surfactants. Instead, converting aids are only used to assist in the stripping operation in casting methods. For example, n-butanol is sometimes used as a converting aid in casting TAC films to facilitate stripping of the TAC film from the metal drum. In terms of substrates, coating methods generally utilize thin (10-250 μm) flexible supports. In contrast, casting methods employ thick (1-100 mm), continuous, highly polished metal drums or rigid bands. As a result of these differences in process steps, the hardware used in coating is conspicuously different from those used in casting as can be seen by a comparison of the schematics shown in FIGS. 1 and 11, respectively.
Turning next to FIGS. 4 through 8, there are presented cross-sectional illustrations showing various guarded cover sheet composite configurations possible with the present invention. In FIG. 4, a guarded cover sheet composite 151 comprising a three-layer cover sheet 171 having lowermost layer 162, intermediate layer 164, and outermost layer 168 is shown partially peeled from a carrier substrate 170. In this illustration, layer 162 could be a low birefringence film, layer 164 could be a moisture barrier layer, and layer 166 could be a viewing angle compensation layer, for example. Layers 162, 164, and 166 may be formed either by applying and drying three separate liquid layers on the carrier substrate 170 or by simultaneously applying a multiple layer composite coating and then drying the layers in a single drying operation.
FIG. 5 illustrates another guarded cover sheet composite 153 comprising a cover sheet 173 that is comprised of, for example, four compositionally discrete layers including a lowermost layer 162 nearest to the carrier support 170, two intermediate layers 164 and 166, and an uppermost layer 168. FIG. 5 also shows that the entire multiple layer cover sheet 173 may be peeled from the carrier substrate 170. In this illustration, layer 162 could be a low birefringence polymer film, layer 164 could be a moisture barrier layer, layer 166 could be an antistatic layer, and layer 168 could be an abrasion-resistant layer, for example.
FIG. 6 shows another guarded cover sheet composite 155 comprising cover sheet 175 that is comprised of a lowermost layer 162 nearest to the carrier substrate 170, three intermediate layers 164, 165, and 166, and an uppermost layer 168, being peeled from the carrier substrate 170. In this illustration, layer 162 could be a low birefringence polymer film, layer 164 could be a moisture barrier layer, layer 165 could be an antistatic layer, layer 166 could be an abrasion-resistant layer, and layer 168 could be a low reflection layer, for example.
FIG. 7 shows yet another guarded cover sheet composite 157 comprising cover sheet 177 that is comprised of a lowermost layer 162 nearest to the carrier substrate 170, two intermediate layers 164 and 166, and an uppermost layer 168, and strippable protection layer 169 that is adhesively adhered to the uppermost cover sheet layer 168 by pressure sensitive adhesive layer 167. The cover sheet 177 with strippable protection layer 169 is being peeled from the carrier substrate 170.
FIG. 8 illustrates a further guarded cover sheet composite 159 comprising a cover sheet 179 that is comprised of, for example, four compositionally discrete layers including a lowermost layer 174 nearest to the carrier support 182, two intermediate layers 176 and 178, and an uppermost layer 180. The carrier substrate 182 has been treated with a release layer 184 to modify the adhesion between the cover sheet lowermost layer 174 and substrate 182. Release layer 184 may be comprised of a number of polymeric materials such as polyvinylbutyrals, cellulosics, polyacrylates, polycarbonates and poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid). The choice of materials used in the release layer may be optimized empirically by those skilled in the art.
FIGS. 4 through 8 serve to illustrate some of the guarded cover sheet composites that may be constructed based on the detailed teachings provided hereinabove, they are not intended to be exhaustive of all possible variations of the invention. One skilled in the art could conceive of many other layer combinations that would be useful as guarded cover sheet composites for use in the preparation of polarizer plates for LCDs.
Turning now to FIG. 9, a schematic representation of a method to fabricate a polarizer plate from guarded cover sheet composites of the invention is illustrated. Guarded cover sheet composite 151 (see FIG. 4) comprising cover sheet 171 and carrier substrate 170 and guarded cover sheet composite 153 (see FIG. 5) comprising cover sheet 173 and carrier substrate 170 are supplied from supply rolls 232 and 234, respectively. A PVA-dichroic film 238 is supplied from supply roll 236. Prior to entering a lamination nip between opposing pinch rollers 242 and 244, the carrier substrate 170 is peeled from guarded cover sheet composites 151 and 153 to expose a lowermost layer (in the case of FIGS. 4 and 5, this is layer 162, which for the purpose of example could be a low birefringence polymer film such as TAC). The peeled carrier sheet 170 is wound into rolls at take-up rolls 240. A glue solution may be optionally applied to both sides of the PVA-dichroic film or to the lowermost layer of cover sheets 171 and 173 prior to the sheets and film entering the nip between pinch rollers 232 and 234. Cover sheets 171 and 173 are then laminated to either side of PVA-dichroic film 238 with the application of pressure (and, optionally, heat) between the opposing pinch rollers 242 and 244 to give the polarizer plate 250. Polarizer plate 250 may then be dried by heating and wound into rolls until needed. Depending on the particular layer configuration for the guarded cover sheet composites employed, a wide variety of polarizer plates having cover sheets with various combinations of auxiliary layers may be fabricated.
FIG. 10 presents a cross-sectional illustration showing a liquid crystal cell 260 having polarizer plates 252 and 254 disposed on either side. Polarizer plate 254 is on the side of the LCD cell closest to the viewer. Each polarizer plate employs two cover sheets. For the purpose of illustration, polarizer plate 254 is shown with an uppermost cover sheet (this is the cover sheet closest to the viewer) comprising low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, antiglare layer 268, and low reflection layer 270. The lowermost cover sheet contained in polarizer plate 254 comprises low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, and viewing angle compensation layer 272. On the opposite side of the LCD cell, polarizer plate 252 is shown with an uppermost cover sheet, which for the purpose of illustration, comprises low birefringence polymer film 262, moisture barrier layer 264, antistatic layer 266, and viewing angle compensation layer 272. Polarizer plate 252 also has a lowermost cover sheet comprising low birefringence polymer film 262, moisture barrier layer 264, and antistatic layer 266.
Carrier substrates suitable for the use in the present invention include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate, polystyrene, and other polymeric films. Additional substrates may include paper, laminates of paper and polymeric films, glass, cloth, aluminum and other metal supports. Preferably, the carrier substrate is a polyester film comprising polyethylene terephthalate (PET) or polyethylene naphthalate (PEN). The thickness of the carrier substrate is about 20 to 200 micrometers, typically about 40 to 100 micrometers. Thinner carrier substrates are desirable due to both cost and the weight per roll of guarded cover sheet composite. However, carrier substrates less than about 20 micrometers may not provide sufficient dimensional stability or protection for the cover sheet.
The carrier substrate may be coated with one or more subbing layers or may be pretreated with electrical discharge devices to enhance the wetting of the substrate by coating solutions. Since the cover sheet must ultimately be peeled from the carrier substrate the adhesion between cover sheet and substrate is an important consideration. Subbing layers and electrical discharge devices may also be employed to modify the adhesion of the cover sheet to the carrier substrate. Subbing layers may therefore function as either primer layers to improve wetting or release layers to modify the adhesion of the cover sheet to the substrate. The carrier substrate may be coated with two subbing layers, the first layer acting as a primer layer to improve wetting and the second layer acting as a release layer. The thickness of the subbing layer is typically 0.05 to 5 micrometers, preferably 0.1 to 1 micrometers.
Cover sheet/substrate composites having poor adhesion might be prone to blister after application of a second or third wet coating in a multi-pass operation. To avoid blister defects, adhesion should be greater than about 0.3 N/m between the first-pass layer of the cover sheet and the carrier substrate. As already mentioned, the level of adhesion may be modified by a variety of web treatments including various subbing layers and various electronic discharge treatments. However, excessive adhesion between the cover sheet and substrate is also undesirable since the cover sheet may be damaged during subsequent peeling operations. In particular, cover sheet/substrate composites having too great an adhesive force may peel poorly. The maximum adhesive force that allows acceptable peel behavior is dependent on the thickness and tensile properties of the cover sheet. Typically, an adhesive force between the cover sheet and the substrate greater than about 300 N/m may peel poorly. Cover sheets peeled from such excessively well-adhered composites exhibit defects due to tearing of the cover sheet and/or due to cohesive failure within the sheet. In a preferred embodiment of the present invention, the adhesion between the cover sheet and the carrier substrate is less than 250 N/m. Most preferably, the adhesion between the cover sheet and the carrier substrate is between 0.5 and 25 N/m.
In a one preferred embodiment of the invention, the carrier substrate is a polyethylene terephthalate film having a first subbing layer (primer layer) comprising a vinylidene chloride copolymer and second subbing layer (release layer) comprising polyvinyl butyral. In another preferred embodiment of the invention the carrier substrate is polyethylene terephthalate film that has been pretreated with a corona discharge prior to application of the cover sheet.
Substrates may also have functional layers such as antistatic layers containing various polymer binders and conductive addenda in order to control static charging and dirt and dust attraction. The antistatic layer may be on either side of the carrier substrate, preferably it is on the side of the carrier substrate opposite to the cover sheet.
On the side of the substrate opposite to the cover sheet a backing layer may also be employed in order to provide a surface having appropriate roughness and coefficient of friction for good winding and conveyance characteristics. In particular, the backing layer comprises a polymeric binder such as a polyurethane or acrylic polymer containing matting agent such a silica or polymeric beads. The matting agent helps to prevent the sticking of the front side of the guarded cover sheet composite to the backside during shipping and storage. The backing layer may also comprise a lubricant to provide a coefficient of friction of about 0.2 to 0.4. Typical lubricants include for example (1) liquid paraffin and paraffin or wax like materials such as carnauba wax, natural and synthetic waxes, petroleum waxes, mineral waxes and the like; (2) higher fatty acids and derivatives, higher alcohols and derivatives, metal salts of higher fatty acids, higher fatty acid esters, higher fatty acid amides, polyhydric alcohol esters of higher fatty acids, etc., disclosed in U.S. Pat. Nos. 2,454,043; 2,732,305; 2,976,148; 3,206,311; 3,933,516; 2,588,765; 3,121,060; 3,502,473; 3,042,222; and 4,427,964, in British Patent Nos. 1,263,722; 1,198,387; 1,430,997; 1,466,304; 1,320,757; 1,320,565; and 1,320,756; and in German Patent Nos. 1,284,295 and 1,284,294; (3) perfluoro- or fluoro- or fluorochloro-containing materials, which include poly(tetrafluoroethylene), poly(trifluorochloroethylene), poly(vinylidene fluoride, poly(trifluorochloroethylene-co-vinyl chloride), poly(meth)acrylates or poly(meth)acrylamides containing perfluoroalkyl side groups, and the like. However for lasting lubricity a polymerizable lubricant such as Additive 31, a methacryloxy-functional silicone polyether copolymer (from Dow Corning Corp.) is preferred.
Low birefringence polymer films suitable for use in the present invention comprise polymeric materials having low Intrinsic Birefringence Δnint that form high clarity films with high light transmission (i.e., >85%). Preferably, the low birefringence polymer film has in-plane birefringence, Δnin of less than about 1×10−4 and an out-of-plane birefringence, Δnth of from 0.005 to −0.005.
Exemplary polymeric materials for use in the low birefringence polymer films of the invention include cellulose esters (including triacetyl cellulose (TAC), cellulose diacetate, cellulose acetate butyrate, cellulose acetate propionate), polycarbonates (such as Lexan® available from General Electric Corp.), polysulfones (such as Udel® available from Amoco Performance Products Inc.), polyacrylates, and cyclic olefin polymers (such as Arton® available from JSR Corp., Zeonex® and Zeonor® available from Nippon Zeon, Topas® supplied by Ticona), among others. Preferably, the low birefringence polymer film of the invention comprises TAC, polycarbonate, or cyclic olefin polymers due their commercial availability and excellent optical properties.
The low birefringence polymer film has a thickness from about 5 to 100 micrometers, preferably from about 5 to 50 micrometers and most preferably from about 10 to 40 micrometers. Films having thickness of 10 to 40 micrometers are most preferred due to cost, handling, ability to provide thinner polarizer plates, improved light transmission, and freedom from curl on the carrier substrate. Polarizer plates fabricated from conventional cover sheets comprising a low birefringence polymer film thickness of about 80 micrometers have a total thickness of at least 180 micrometers. In a preferred embodiment of the current invention, polarizer plates assembled from cover sheets of the invention have a total thickness of less than 120 micrometers, and most preferably less than 80 micrometers.
In a preferred embodiment the guarded cover sheet composite comprises a strippable, protection layer on the surface of the cover sheet opposite to the carrier substrate. The strippable, protection layer may be applied by coating the layer or it may be applied by adhesively adhering or by electrostatically adhering, a preformed protection layer. Preferably, the protection layer is a transparent polymer layer. In one particular embodiment, the protection layer is a low birefringence layer that allows optical inspection of the cover sheet without the need to remove the protection layer. Particularly useful polymers for use in the protection layer include: cellulose esters, acrylics, polyurethanes, polyesters, cyclic olefin polymers, polystyrene, polyvinyl butyral, polycarbonate, and others. When a preformed protection layer is used, it is preferably a layer of polyester, polystyrene, or polyolefin film.
The strippable, protection layer is typically 5 to 100 micrometers in thickness. Preferably, the protection layer is 20 to 50 micrometers thick to insure adequate resistance to scratch and abrasion and provide easy handling during removal of the protection layer.
When the strippable, protection layer is applied by coating methods it may be applied to an already coated and dried cover sheet or the protection layer may be coated simultaneously with one or more layers comprising the cover sheet.
When the strippable, protection layer is a preformed layer it may have a pressure sensitive adhesive layer on one surface that allows the protection layer to be adhesively laminated to the guarded cover sheet composite using conventional lamination techniques. Alternatively, the preformed protection layer may be applied by generating an electrostatic charge on a surface of the cover sheet or the preformed protection layer and then bringing the two materials into contact in a roller nip. The electrostatic charge may be generated by any known electric charge generator, e.g., a corona charger, a tribocharger, conducting high potential roll charge generator or contact charger, a static charge generator, and the like. The cover sheet or the preformed protection layer may be charged with a DC charge or a DC charge followed by an AC charge in order to create an adequate level of charge adhesion between the two surfaces. The level of electrostatic charge applied to provide a sufficient bond between the cover sheet and the preformed protection layer is at least more than 50 volts, preferably at least more than 200 volts. The charged surface of the cover sheet or the protection layer has a resistivity of at least about 1012 Ω/square, preferably at least about 1016 Ω/square in order to insure that the electrostatic charge is long lasting.
Liquid Crystal Displays typically employ two polarizer plates, one on each side of the liquid crystal cell. Each polarizer plate, in turn, employs two cover sheets, one on each side of the PVA-dichroic film. Each cover sheet may have various auxiliary layers that are necessary to improve the performance of the Liquid Crystal Display. Useful auxiliary layers employed in the cover sheets of the invention include: abrasion resistant hardcoat layer, antiglare layer, anti-smudge layer or stain-resistant layer, antireflection layer, low reflection layer, antistatic layer, viewing angle compensation layer, and moisture barrier layer. Typically, the cover sheet closest to the viewer contains one or more of the following auxiliary layers: the abrasion resistant layer, anti-smudge or stain-resistant layer, antireflection layer, and antiglare layer. One or both of the cover sheets closest to the liquid crystal cell typically contain a viewing angle compensation layer. Any or all of the four cover sheets employed in the LCD may optionally contain an antistatic layer and a moisture barrier layer.
The guarded cover sheet composite of the invention may contain an abrasion resistant layer on the same side of the carrier substrate as the low birefringence polymer film. Preferably, the abrasion resistant layer is located on the side of the low birefringence polymer film opposite to the carrier.
Particularly effective abrasion resistant layers for use in the present invention comprise radiation or thermally cured compositions, and preferably the composition is radiation cured. Ultraviolet (UV) radiation and electron beam radiation are the most commonly employed radiation curing methods. UV curable compositions are particularly useful for creating the abrasion resistant layer of this invention and may be cured using two major types of curing chemistries, free radical chemistry and cationic chemistry. Acrylate monomers (reactive diluents) and oligomers (reactive resins and lacquers) are the primary components of the free radical based formulations, giving the cured coating most of its physical characteristics. Photo-initiators are required to absorb the UV light energy, decompose to form free radicals, and attack the acrylate group C═C double bond to initiate polymerization. Cationic chemistry utilizes cycloaliphatic epoxy resins and vinyl ether monomers as the primary components. Photo-initiators absorb the UV light to form a Lewis acid, which attacks the epoxy ring initiating polymerization. By UV curing is meant ultraviolet curing and involves the use of UV radiation of wavelengths between 280 and 420 nm preferably between 320 and 410 nm.
Examples of UV radiation curable resins and lacquers usable for the abrasion layer useful in this invention are those derived from photo polymerizable monomers and oligomers such as acrylate and methacrylate oligomers (the term “(meth)acrylate” used herein refers to acrylate and methacrylate), of polyfunctional compounds, such as polyhydric alcohols and their derivatives having (meth)acrylate functional groups such as ethoxylated trimethylolpropane tri(meth)acrylate, tripropylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylate and mixtures thereof, and acrylate and methacrylate oligomers derived from low-molecular weight polyester resin, polyether resin, epoxy resin, polyurethane resin, alkyd resin, spiroacetal resin, epoxy acrylates, polybutadiene resin, and polythiol-polyene resin, and the like and mixtures thereof, and ionizing radiation-curable resins containing a relatively large amount of a reactive diluent. Reactive diluents usable herein include monofunctional monomers, such as ethyl (meth)acrylate, ethylhexyl (meth)acrylate, styrene, vinyltoluene, and N-vinylpyrrolidone, and polyfunctional monomers, for example, trimethylolpropane tri(meth)acrylate, hexanediol (meth)acrylate, tripropylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6-hexanediol di(meth)acrylate, or neopentyl glycol di(meth)acrylate.
Among others, in the present invention, conveniently used radiation curable lacquers include urethane (meth)acrylate oligomers. These are derived from reacting diisocyanates with a oligo(poly)ester or oligo(poly)ether polyol to yield an isocyanate terminated urethane. Subsequently, hydroxy terminated acrylates are reacted with the terminal isocyanate groups. This acrylation provides the unsaturation to the ends of the oligomer. The aliphatic or aromatic nature of the urethane acrylate is determined by the choice of diisocyanates. An aromatic diisocyanate, such as toluene diisocyanate, will yield an aromatic urethane acrylate oligomer. An aliphatic urethane acrylate will result from the selection of an aliphatic diisocyanate, such as isophorone diisocyanate or hexyl methyl diisocyanate. Beyond the choice of isocyanate, polyol backbone plays a pivotal role in determining the performance of the final the oligomer. Polyols are generally classified as esters, ethers, or a combination of these two. The oligomer backbone is terminated by two or more acrylate or methacrylate units, which serve as reactive sites for free radical initiated polymerization. Choices among isocyanates, polyols, and acrylate or methacrylate termination units allow considerable latitude in the development of urethane acrylate oligomers. Urethane acrylates like most oligomers, are typically high in molecular weight and viscosity. These oligomers are multifunctional and contain multiple reactive sites. Because of the increased number of reactive sites, the cure rate is improved and the final product is cross-linked. The oligomer functionality can vary from 2 to 6. Among others, conveniently used radiation curable resins include polyfunctional acrylic compounds derived from polyhydric alcohols and their derivatives such as mixtures of acrylate derivatives of pentaerythritol such as pentaerythritol tetraacrylate and pentaerythritol triacrylate functionalized aliphatic urethanes derived from isophorone diisocyanate. Some examples of urethane acrylate oligomers used in the practice of this invention that are commercially available include oligomers from Sartomer Company (Exton, Pa.). An example of a resin that is conveniently used in the practice of this invention is CN 968® from Sartomer Company.
A photo polymerization initiator, such as an acetophenone compound, a benzophenone compound, Michler's benzoyl benzoate, α-amyloxime ester, or a thioxanthone compound and a photosensitizer such as n-butyl amine, triethylamine, or tri-n-butyl phosphine, or a mixture thereof is incorporated in the ultraviolet radiation curing composition. In the present invention, conveniently used initiators are 1-hydroxycyclohexyl phenyl ketone and 2-methyl-1-[4-(methyl thio) phenyl]-2-morpholinopropanone-1.
The abrasion resistant layer is typically applied after coating and drying the low birefringence polymer film. The abrasion resistant layer of this invention is applied as a coating composition that typically also includes organic solvents. Preferably the concentration of organic solvent is 1-99% by weight of the total coating composition.
Examples of solvents employable for coating the abrasion resistant layer of this invention include solvents such as methanol, ethanol, propanol, butanol, cyclohexane, heptane, toluene and xylene, esters such as methyl acetate, ethyl acetate, propyl acetate and mixtures thereof. With the proper choice of solvent, adhesion of the abrasion resistant layer can be improved while minimizing migration of plasticizers and other addenda from the low birefringence polymer film, enabling the hardness of the abrasion resistant layer to be maintained. Suitable solvents for TAC low birefringence polymer film are aromatic hydrocarbon and ester solvents such as toluene and propyl acetate.
The UV polymerizable monomers and oligomers are coated and dried, and subsequently exposed to UV radiation to form an optically clear cross-linked abrasion resistant layer. The preferred UV cure dosage is between 50 and 1000 mJ/cm2.
The thickness of the abrasion resistant layer is generally about 0.5 to 50 micrometers preferably 1 to 20 micrometers, more preferably 2 to 10 micrometers.
The abrasion resistant layer is preferably colorless, but it is specifically contemplated that this layer can have some color for the purposes of color correction, or for special effects, so long as it does not detrimentally affect the formation or viewing of the display through the overcoat. Thus, there can be incorporated into the polymer dyes that will impart color. In addition, additives can be incorporated into the polymer that will give to the layer desired properties. Other additional compounds may be added to the coating composition, including surfactants, emulsifiers, coating aids, lubricants, matte particles, rheology modifiers, crosslinking agents, antifoggants, inorganic fillers such as conductive and nonconductive metal oxide particles, pigments, magnetic particles, biocide, and the like.
The abrasion resistant layer of the invention tyically provides a layer having a pencil hardness (using the Standard Test Method for Hardness by Pencil Test ASTM D3363) of at least 2H and preferably 2H to 8H.
The guarded cover sheet composite of the invention may contain an antiglare layer, a low reflection layer or an antireflection layer on the same side of the carrier substrate as the low birefingence polymer film. Preferably, the antiglare layer, low reflection layer or antireflection layer is located on the side of the low birefringence polymer film opposite to the carrier. Such layers are employed in an LCD in order to improve the viewing characteristics of the display, particularly when it is viewed in bright ambient light. The refractive index of an abrasion resistant, hard coat is about 1.50, while the index of the surrounding air is 1.00. This difference in refractive index produces a reflection from the surface of about 4%.
An antiglare coating provides a roughened or textured surface that is used to reduce specular reflection. All of the unwanted reflected light is still present, but it is scattered rather than specularly reflected. For the purpose of the present invention, the antiglare coating preferably comprises a radiation cured composition that has a textured or roughened surface obtained by the addition of organic or inorganic (matting) particles or by embossing the surface. The radiation cured compositions described hereinabove for the abrasion resistant layer are also effectively employed in the antiglare layer. Surface roughness is preferably obtained by the addition of matting particles to the radiation cured composition. Suitable particles include inorganic compounds having an oxide, nitride, sulfide or halide of a metal, metal oxides being particularly preferred. As the metal atom, Na, K, Mg, Ca, Ba, Al, Zn, Fe, Cu, Ti, Sn, In, W, Y, Sb, Mn, Ga, V, Nb, Ta, Ag, Si, B, Bi, Mo, Ce, Cd, Be, Pb and Ni are suitable, and Mg, Ca, B and Si are more preferable. An inorganic compound containing two types of metal may also be used. A particularly preferable inorganic compound is silicon dioxide, namely silica.
Additional particles suitable for use in the antiglare layer of the present invention include the layered clays described in commonly-assigned U.S. patent application Ser. No. 10/690,123, filed Oct. 21, 2003. The most suitable layered particles include materials in the shape of plates with high aspect ratio, which is the ratio of a long direction to a short direction in an asymmetric particle. Preferred layered particles are natural clays, especially natural smectite clay such as montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, halloysite, magadiite, kenyaite and vermiculite as well as layered double hydroxides or hydrotalcites. Most preferred clay materials include natural montmorillonite, hectorite and hydrotalcites, because of commercial availability of these materials.
The layered materials suitable for this invention may comprise phyllosilicates, for example, montmorillonite, particularly sodium montmorillonite, magnesium montmorillonite, and/or calcium montmorillonite, nontronite, beidellite, volkonskoite, hectorite, saponite, sauconite, sobockite, stevensite, svinfordite, vermiculite, magadiite, kenyaite, talc, mica, kaolinite, and mixtures thereof. Other useful layered materials may include illite, mixed layered illite/smectite minerals, such as ledikite and admixtures of illites with the layered materials named above. Other useful layered materials, particularly useful with anionic matrix polymers, may include the layered double hydroxide clays or hydrotalcites, such as Mg6Al3.4(OH)18.8(CO3)1.7H2O, which have positively charged layers and exchangeable anions in the interlayer spaces. Preferred layered materials are swellable so that other agents, usually organic ions or molecules, may splay, that is, intercalate and/or exfoliate, the layered material resulting in a desirable dispersion of the inorganic phase. These swellable layered materials include phyllosilicates of the 2:1 type, as defined in the literature (for example, “An introduction to clay colloid chemistry,” by H. van Olphen, John Wiley & Sons Publishers). Typical phyllosilicates with ion exchange capacity of 50 to 300 milliequivalents per 100 grams are preferred. Generally, it is desirable to treat the selected clay material to separate the agglomerates of platelet particles to small crystals, also called tactoids, prior to introducing the platelet particles to the antiglare coating. Predispersing or separating the platelet particles also improves the binder/platelet interface. Any treatment that achieves the above goals may be used. Examples of useful treatments include intercalation with water soluble or water insoluble polymers, organic reagents or monomers, silane compounds, metals or organometallics, organic cations to effect cation exchange, and their combinations.
Additional particles for use in the antiglare layer of the present invention include polymer matte particles or beads which are well known in the art. The polymer particles may be solid or porous, preferably they are crosslinked polymer particles. Porous polymer particles for use in an antiglare layer are described in commonly-assigned U.S. patent application Ser. No. 10/715,706, filed Nov. 18, 2003.
Particles for use in the antiglare layer have an average particle size ranging from 2 to 20 micrometers, preferably from 2 to 15 micrometers and most preferably from 4 to 10 micrometers. They are present in the layer in an amount of at least 2 wt percent and less than 50 percent, typically from about 2 to 40 wt. percent, preferably from 2 to 20 percent and most preferably from 2 to 10 percent.
The thickness of the antiglare layer is generally about 0.5 to 50 micrometers preferably 1 to 20 micrometers more preferably 2 to 10 micrometers.
Preferably, the antiglare layer used in the present invention has a 60° Gloss value, according to ASTM D523, of less than 100, preferably less than 90 and a transmission haze value, according to ASTM D-1003 and JIS K-7105 methods, of less than 50%, preferably less than 30%.
In another embodiment of the present invention, a low reflection layer or antireflection layer is used in combination with an abrasion resistant hard coat layer or antiglare layer. The low reflection or antireflection coating is applied on top of the abrasion resistant or antiglare layer. Typically, a low reflection layer provides an average specular reflectance (as measured by a spectrophotometer and averaged over the wavelength range of 450 to 650 nm) of less than 2%. Antireflection layers provide average specular reflectance values of less than 1%.
Suitable low reflection layers for use in the present invention comprise fluorine-containing homopolymers or copolymers having a refractive index of less than 1.48, preferably with a refractive index between about 1.35 and 1.40. Suitable fluorine-containing homopolymers and copolymers include: fluoro-olefins (for example, fluoroethylene, vinylidene fluoride, tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoro-2,2-dimethyl-1,3-dioxol), partially or completely fluorinated alkyl ester derivatives of (meth)acrylic acid, and completely or partially fluorinated vinyl ethers, and the like. The effectiveness of the layer may be improved by the incorporation of submicron-sized inorganic particles or polymer particles that induce interstitial air voids within the coating. This technique is further described in U.S. Pat. No. 6,210,858 and U.S. Pat. No. 5,919,555. Further improvement in the effectiveness of the low reflection layer may be realized with the restriction of air voids to the internal particle space of submicron-sized polymer particles with reduced coating haze penalty, as described in commonly-assigned U.S. patent application Ser. No. 10/715,655, filed Nov. 18, 2003.
The thickness of the low reflection layer is 0.01 to 1 micrometer and preferably 0.05 to 0.2 micrometer.
An antireflection layer may comprise a monolayer or a multi-layer. Antireflection layers comprising a monolayer typically provide reflectance values less than 1% at only a single wavelength (within the broader range of 450 to 650 nm). A commonly employed monolayer antireflection coating that is suitable for use in the present invention comprises a layer of a metal fluoride such as magnesium fluoride (MgF2). The layer may be applied by well-known vacuum deposition technique or by a sol-gel technique. Typically, such a layer has an optical thickness (i.e., the product of refractive index of the layer times layer thickness) of approximately one quarter-wavelength at the wavelength where a reflectance minimum is desired.
Although a monolayer can effectively reduce the reflection of light within a very narrow wavelength range, more often a multi-layer comprising several (typically, metal oxide based) transparent layers superimposed on one another is used to reduce reflection over a wide wavelength region (i.e., broadband reflection control). For such a structure, half wavelength layers are alternated with quarter wavelength layers to improve performance. The multi-layer antireflection coating may comprise two, three, four, or even more layers. Formation of this multi-layer typically requires a complicated process comprising a number of vapor deposition procedures or sol-gel coatings, which correspond to the number of layers, each layer having a predetermined refractive index and thickness. Precise control of the thickness of each layer is required for these interference layers. The design of suitable multi-layer antireflection coatings for use in the present invention is well known in the patent art and technical literature, as well as being described in various textbooks, for example, in H. A. Macleod, “Thin Film Optical Filters,” Adam Hilger, Ltd., Bristol 1985 and James D. Rancourt, “Optical Thin Films User's Handbook”, Macmillan Publishing Company, 1987.
The guarded cover sheet composite of the invention may contain a moisture barrier layer on the same side of the carrier substrate as the low birefringence polymer film. The barrier layer may be located on one or both sides of the low birefringence polymer film. The moisture barrier layer comprises a hydrophobic polymer such as a vinylidene chloride polymer, vinylidene fluoride polymer, polyurethane, polyolefin, fluorinated polyolefin, polycarbonate, and others, having a low moisture permeability. Preferably, the hydrophobic polymer comprises vinylidene chloride. More preferably, the hydrophobic polymer comprises 70 to 99 weight percent of vinylidene chloride. The moisture barrier layer may be applied by application of an organic solvent-based or aqueous coating formulation. To provide effective moisture barrier properties the layer should be at least 1 micrometer in thickness, preferably from 1 to 10 micrometers in thickness, and most preferably from 2 to 8 micrometers in thickness. The cover sheet of the invention comprising a moisture barrier layer has a moisture vapor transmission rate (MVTR) according to ASTM F-1249 that is less than 1000 g/m2/day, preferably less than 800 g/m2/day and most preferably less than 500 g/m2/day. The use of such a barrier layer in the cover sheet of the invention provides improved resistance to changes in humidity and increased durability of the polarizer comprising the cover sheet, especially for TAC cover sheets having a thickness less than about 40 micrometers.
The guarded cover sheet composite of the invention may contain a transparent antistatic layer on the same side of the carrier substrate as the low birefringence polymer film. The antistatic layer may be located on either side of the low birefringence polymer film. The antistatic layer aids in the control of static charging that may occur during the manufacture and use of the cover sheet composite. Effective control of static charging reduces the propensity for the attraction of dirt and dust to the cover sheet composite. The guarded cover sheet composite of the invention may be particularly prone to triboelectric charging during the peeling of the cover sheet from the carrier substrate. The so-called “separation charge” that results from the separation of the cover sheet and the substrate can be effectively controlled by an antistatic layer having a resistivity of less than about 1×1011 Ω/square, preferably less than 1×1010 Ω/square, and most preferably less than 1×109 Ω/square.
Various polymeric binders and conductive materials may be employed in the antistatic layer. Polymeric binders useful in the antistatic layer include any of the polymers commonly used in the coating art, for example, interpolymers of ethylenically unsaturated monomers, cellulose derivatives, polyurethanes, polyesters, hydrophilic colloids such as gelatin, polyvinyl alcohol, polyvinyl pyrrolidone, and others.
Conductive materials employed in the antistatic layer may be either ionically-conductive or electronically-conductive. Ionically-conductive materials include simple inorganic salts, alkali metal salts of surfactants, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts). Of these, ionically-conductive polymers such as anionic alkali metal salts of styrene sulfonic acid copolymers and cationic quaternary ammonium polymers of U.S. Pat. No. 4,070,189 and ionically-conductive colloidal metal oxide sols which include silica, tin oxide, titania, antimony oxide, zirconium oxide, alumina-coated silica, alumina, boehmite, and smectite clays are preferred.
The antistatic layer employed in the current invention preferably contains an electronically-conductive material due to their humidity and temperature independent conductivity. Suitable materials include:
1) electronically-conductive metal-containing particles including donor-doped metal oxides, metal oxides containing oxygen deficiencies, and conductive nitrides, carbides, and bromides. Specific examples of particularly useful particles include conductive SnO2, In2O, ZnSb2O6, InSbO4, TiB2, ZrB2, NbB2, TaB2, CrB, MoB, WB, LaB6, ZrN, TiN, WC, HfC, HfN, and ZrC. Examples of the patents describing these electrically conductive particles include; U.S. Pat. Nos. 4,275,103; 4,394,441; 4,416,963; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; and 5,368,995.

2) fibrous electronic conductive particles comprising, for example, antimony-doped tin oxide coated onto non-conductive potassium titanate whiskers as described in U.S. Pat. Nos. 4,845,369 and 5,166,666, antimony-doped tin oxide fibers or whiskers as described in U.S. Pat. Nos. 5,719,016 and 5,0731,119, and the silver-doped vanadium pentoxide fibers described in U.S. Pat. No. 4,203,769

3) electronically-conductive polyacetylenes, polythiophenes, and polypyrroles, preferably the polyethylene dioxythiophene described in U.S. Pat. No. 5,370,981 and commercially available from Bayer Corp. as Baytrone® P.

The amount of the conductive agent used in the antistatic layer of the invention can vary widely depending on the conductive agent employed. For example, useful amounts range from about 0.5 mg/m2 to about 1000 mg/m2, preferably from about 1 mg/m2 to about 500 mg/m2. The antistatic layer has a thickness of from 0.05 to 5 micrometers, preferably from 0.1 to 0.5 micrometers to insure high transparency.
Contrast, color reproduction, and stable gray scale intensities are important quality attributes for electronic displays, which employ liquid crystal technology. The primary factor limiting the contrast of a liquid crystal display is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. Furthermore, the leakage and hence contrast of a liquid crystal display are also dependent on the direction from which the display screen is viewed. Typically the optimum contrast is observed only within a narrow viewing angle range centered about the normal incidence to the display and falls off rapidly as the viewing direction deviates from the display normal. In color displays, the leakage problem not only degrades the contrast but also causes color or hue shifts with an associated degradation of color reproduction.
Thus, one of the major factors measuring the quality of LCDs is the viewing angle characteristic, which describes a change in contrast ratio from different viewing angles. It is desirable to be able to see the same image from a wide variation in viewing angles and this ability has been a shortcoming with liquid crystal display devices. One way to improve the viewing angle characteristic is to employ a cover sheet having a viewing angle compensation layer (also referred to as a compensation layer, retarder layer, or phase difference layer), with proper optical properties, between the PVA-dichroic film and liquid crystal cell, such as disclosed in U.S. Pat. Nos. 5,583,679, 5,853,801, 5,619,352, 5,978,055, and 6,160,597. A compensation film according to U.S. Pat. Nos. 5,583,679 and 5,853,801 based on discotic liquid crystals which have negative birefringence, is widely used.
Viewing angle compensation layers useful in the present invention are optically anisotropic layers. The optically anisotropic, viewing angle compensation layers may comprise positively birefringent materials or negatively birefringent materials. The compensation layer may be optically uniaxial or optically biaxial. The compensation layer may have its optic axis tilted in the plane perpendicular to the layer. The tilt of the optic axis may be constant in the layer thickness direction or the tilt of the optic axis may vary in the layer thickness direction.
Optically anisotropic, viewing angle compensation layers useful in the present invention may comprise the negatively birefringent, discotic liquid crystals described in U.S. Pat. Nos. 5,583,679, and 5,853,801; the positively birefringent nematic liquid crystals described in U.S. Pat. No. 6,160,597; the negatively birefringent amorphous polymers described in commonly assigned U.S. Patent Application Publication 2004/0021814A and U.S. patent application Ser. No. 10/745,109, filed Dec. 23, 2003. These latter two patent applications describe compensation layers comprising polymers that contain non-visible chromophore groups such as vinyl, carbonyl, amide, imide, ester, carbonate, sulfone, azo, and aromatic groups (i.e. benzene, naphthalate, biphenyl, bisphenol A) in the polymer backbone and that preferably have a glass transition temperature of greater than 180 degree C. Such polymers are particularly useful in the compensation layer of the present invention. Such polymers include polyesters, polycarbonates, polyimides, polyetherimides, and polythiophenes. Of these, particularly preferred polymers for use in the present invention include:

    1)poly(4,4′-hexafluoroisopropylidene-bisphenol) terephthalate-co-isophthalate,
    2)poly(4,4′-hexahydro-4,7-methanoindan-5-ylidene bisphenol) terephthalate,
    3) poly(4,4′-isopropylidene-2,2′6,6′-tetrachlorobisphenol) terephthalate-co-isophthalate, 4) poly(4,4′-hexafluoroisopropylidene)-bisphenol-co-(2-norbornylidene)-bisphenol terephthalate, 5) poly(4,4′-hexahydro-4,7-methanoindan-5-ylidene)-bisphenol-co-(4,4′-isopropylidene-2,2′,6,6′-tetrabromo)-bisphenol terephthalate, 6) poly(4,4′-isopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate or poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene)bisphenol) terephthalate-co-isophthalate or copolymers of any of the foregoing. A compensation layer comprising these polymers typically has an out-of-plane retardation, Rth, that is more negative than −20 nm, preferably Rth is from −60 to −600 nm, and most preferably Rth is from −150 to −500 nm.


Another compensation layer suitable for the present invention includes an optically anisotropic layer comprising an exfoliated inorganic clay material in a polymeric binder as described in Japanese Patent Application 11095208A.
The auxiliary layers of the invention can be applied by any of a number of well known liquid coating techniques, such as dip coating, rod coating, blade coating, air knife coating, gravure coating, microgravure coating, reverse roll coating, slot coating, extrusion coating, slide coating, curtain coating, or by vacuum deposition techniques. In the case of liquid coating, the wet layer is generally dried by simple evaporation, which may be accelerated by known techniques such as convection heating. The auxiliary layer may be applied simultaneously with other layers such as subbing layers and the low birefringence polymer film. Several different auxiliary layers may be coated simultaneously using slide coating, for example, an antistatic layer may be coated simultaneously with a moisture barrier layer or a moisture barrier layer may be coated simultaneously with a viewing angle compensation layer. Known coating and drying methods are described in further detail in Research Disclosure No. 308119, Published December 1989, pages 1007 to 1008.
The cover sheets of the invention are suitable for use with a wide variety of LCD display modes, for example, Twisted Nematic (TN), Super Twisted Nematic (STN), Optically Compensated Bend (OCB), In Plane Switching (IPS), or Vertically Aligned (VA) liquid crystal displays. These various liquid crystal display technologies have been reviewed in U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,410,422 (Bos), and U.S. Pat. No. 4,701,028 (Clerc et al.).
As should be obvious based on the preceding detailed description, a wide variety of guarded cover sheet composites having various types and arrangements of auxiliary layers may be prepared. Some of the configurations possible in accordance with the present invention are illustrated by the following non-limiting examples.
Guarded Cover Sheet Composite C1:












abrasion resistant layer


TAC


carrier substrate









A 100 micrometer thick polyethylene terephthalate carrier substrate is corona treated and then coated with a triacetyl cellulose (TAC) formulation. The dried TAC layer is 20 micrometers in thickness and contains 11 wt % triphenyl phosphate plasticizer, 1 wt % TINUVIN® 8515 UV absorber (a mixture of 2-(2′-Hydroxy-3′-tert-butyl-5′-methylphenyl)-5-chloro benzotriazole and 2-(2′-Hydroxy -3′,5′-ditert-butylphenyl)-benzotriazole, available from Ciba Specialty Chemicals.) and about 0.1 wt % PARSOL® 1789 UV absorber (4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane, available from Roche Vitamins Inc.). Onto the TAC layer is applied an abrasion resistant layer, prepared by coating, drying and then UV curing a urethane acrylate oligomer, CN 968® from Sartomer Company.

Guarded Cover Sheet Composite C2:













low reflection layer


abrasion resistant layer


TAC


carrier substrate









Guarded cover sheet composite C2 is prepared in an analogous manner to composite C1 except a low reflection layer having a thickness of 0.1 micrometers, comprising a fluorinated olefin polymer is applied onto the abrasion resistant layer.

Guarded Cover Sheet Composite C3:













abrasion resistant layer


TAC


release layer


primer layer


carrier substrate









Guarded cover sheet composite C3 is prepared in an analogous manner to composite C1 except the polyethylene terephthalate carrier substrate has a first subbing layer that is a 0.1 micrometer thick primer layer comprising poly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) and a second subbing layer that is a 0.5 micrometer thick release layer comprising polyvinyl butyral.

Guarded Cover Sheet Composite C4:













abrasion resistant layer


antistatic layer


moisture barrier layer


TAC


carrier substrate









Guarded cover sheet composite C4 is prepared in an analogous manner to composite C1 except that a 5 micrometer thick moisture barrier layer comprising poly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) containing 78 wt % vinylidene chloride is applied onto the TAC layer. An antistatic layer comprising Baytron® P (polyethylene dioxythiophene/polystyrene sulfonate, available from Bayer Corp) in a poly(vinylidene chloride-co-acrylonitrile-co-acrylic acid) binder is applied onto the moisture barrier layer. The antistatic layer contains 3 mg/m2 Baytron® P and has a surface resistivity of about 1×108 Ω/square. The abrasion resistant layer employed in composite C1 is applied onto the antistatic layer.

Guarded Cover Sheet Composite C5:













strippable, protection layer


pressure sensitive adhesive layer


abrasion resistant layer


antistatic layer


moisture barrier layer


TAC


carrier substrate









Guarded cover sheet composite C5 is prepared from composite C4 by adhesively laminating a strippable protection layer that has a precoated, pressure sensitive adhesive layer on one side of a protection layer. The protection layer is a preformed, 25 micrometer thick polyester film. The pressure sensitive adhesive layer is suitably a thin (1 to 5 micrometer thick) acrylic-based adhesive, which are well known in the art and readily available commercially.

Guarded Cover Sheet Composite C6:













abrasion resistant layer


antistatic layer


moisture barrier layer


cylic olefin polymer


carrier substrate









Guarded cover sheet composite C6 is prepared in an analogous manner to composite C4 except that a 20 micrometer thick layer of a cyclic olefin polymer, Zeonor® available from Nippon Zeon, is used in place of TAC as the low birefringence polymer film.

Guarded Cover Sheet Composite C7:













viewing angle compensation layer


TAC


carrier substrate









Guarded cover sheet composite C7 is prepared in an analogous manner to composite C1 except that in place of an abrasion resistant layer, a viewing angle compensation layer is applied onto the TAC layer. The viewing angle compensation layer is a 3 micrometer thick layer comprising poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate.

Guarded Cover Sheet Composite C8:













viewing angle compensation layer


antistatic layer


moisture barrier layer


TAC


carrier substrate









Guarded cover sheet composite C8 is prepared in an analogous manner to composite C4 except that in place of an abrasion resistant layer, a viewing angle compensation layer is applied onto the TAC layer. The viewing angle compensation layer is a 3 micrometer thick layer comprising poly(4,4′-hexafluoroisopropylidene-bisphenol-co-4,4′-(2-norbornylidene) bisphenol) terephthalate-co-isophthalate.

Guarded Cover Sheet Composite C9:













viewing angle compensation layer


cyclic olefin polymer


carrier substrate









Guarded cover sheet composite C9 is prepared in an analogous manner to composite C7 except that a 20 micrometer thick layer of a cyclic olefin polymer, Zeonor® available from Nippon Zeon, is used in place of TAC as the low birefringence polymer film.

Guarded Cover Sheet Composite C10:













viewing angle compensation layer


antistatic layer


moisture barrier layer


cyclic olefin polymer


carrier substrate









Guarded cover sheet composite C10 is prepared in an analogous manner to composite C8 except that a 20 micrometer thick layer of a cyclic olefin polymer, Zeonor® available from Nippon Zeon, is used in place of TAC as the low birefringence polymer film.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
PARTS LIST


    10 drying system
    12 moving substrate/web
    14 dryer
    16 coating apparatus
    18 unwinding station
    20 back-up roller
    22 coated web
    24 cover sheet composite
    26 wind up station
    28 coating supply vessel
    30 coating supply vessel
    32 coating supply vessel
    34 coating supply vessel
    36 pumps
    38 pumps
    40 pumps
    42 pumps
    44 conduits
    46 conduits
    48 conduits
    50 conduits
    52 discharge device
    54 polar charge assist device
    56 opposing rollers
    58 opposing rollers
    60 preformed protection layer
    62 winding station
    64 winding station
    66 drying section
    68 drying section
    70 drying section
    72 drying section
    74 drying section
    76 drying section
    78 drying section
    80 drying section
    82 drying section
    92 front section
    94 second section
    96 third section
    98 fourth section
    100 back plate
    102 inlet
    104 metering slot
    106 pump
    108 lower most layer
    110 inlet
    112 2nd metering slot
    114 pump
    116 layer
    118 inlet
    120 metering slot
    122 pump
    124 form layer
    126 inlet
    128 metering slot
    130 pump
    132 layer
    134 incline slide surface
    136 coating lip
    138 2nd incline slide surface
    140 3rd incline slide surface
    142 4th incline slide surface
    144 back land surface
    146 coating bead
    151 guarded cover sheet composite
    153 guarded cover sheet composite
    155 guarded cover sheet composite
    157 guarded cover sheet composite
    159 guarded cover sheet composite
    162 lowermost layer
    164 intermediate layer
    165 intermediate layer
    166 intermediate layer
    167 pressure sensitive layer
    168 uppermost layer
    169 preformed protection layer
    170 carrier support
    171 coversheet
    173 cover sheet
    175 cover sheet
    177 cover sheet
    179 cover sheet
    174 lowermost layer
    176 intermediate layer
    178 intermediate layer
    180 uppermost layer
    182 carrier substrate
    184 release layer
    200 feed line
    202 extrusion hopper
    204 pressurized tank
    206 pump
    208 metal drum
    210 drying section
    212 drying oven
    214 cast film
    216 final drying section
    218 final dried film
    220 wind-up station
    232 guarded cover sheet composite supply roll
    234 guarded cover sheet composite supply roll
    236 PVA-dichroic film supply roll
    238 PVA-dichroic film
    240 carrier substrate take-up roll
    242 opposing pinch roll
    244 opposing pinch roll
    250 polarizer plate
    252 polarizer plate
    254 polarizer plate
    260 LCD cell
    262 low birefringence polymer film
    264 moisture barrier layer
    266 antistatic layer
    268 antiglare layer
    270 low reflection layer
    272 viewing angle compensation layer

## Claims

1. A method of forming a polarizing plate comprising (a) providing two guarded cover sheet composites each comprising a carrier substrate and a low birefringence polymer film comprising polymeric material having an in-plane birefringence Δnin of less than about 1×10−4 and an out-of-plane birefringence Δnth of from 0.005 to −0.005, (b) providing a dichroic film, and (c) simultaneously bringing said low birefringence polymer films into adhesive contact on each side of said dichroic film.

2. The method of claim 1 wherein said carrier substrate that is removed prior to bringing the low birefringence film into contact with said dichroic film.

3. The method of claim 1 wherein said carrier substrate that is removed after bringing the low birefringence film into contact with said dichroic film.

4. The method of claim 1 wherein said guarded cover sheet composite further comprises a protection layer on the side opposite to said carrier substrate and said protection layer is removed after bringing the low birefringence film into contact with said dichroic film.

5. The method of claim 1 wherein said guarded cover sheet composite further comprises a protection layer on the side opposite to said carrier substrate and said protection layer is removed prior to bringing the low birefringence film into contact with said dichroic film.

6. The method of claim 1 wherein adhesive is applied simultaneously with bringing together said dichroic film and said low birefringence films into contact.

7. The method of claim 6 wherein said adhesive comprises a polyvinyl alcohol solution.

8. The method of claim 1 wherein pressure is applied as said dichroic film and low birefringence films are brought into contact.

9. The method of claim 1 wherein after said dichroic film and low birefringence films are brought into contact the polarizing plate is dried.