WO2009105427A2

WO2009105427A2 - Optical compensation film including strippable skin

Info

Publication number: WO2009105427A2
Application number: PCT/US2009/034302
Authority: WO
Inventors: Adam D. Haag; Robert D. Taylor; Richard C. Allen; Matthew B. Johnson; Richard J. Thompson; Timothy J. Hebrink
Original assignee: 3M Innovative Properties Company
Priority date: 2008-02-20
Filing date: 2009-02-17
Publication date: 2009-08-27
Also published as: WO2009105427A3; TW200950956A

Abstract

A composite optical compensation film is described. The composite optical compensation film includes a biaxially oriented polyolefin film and strippable skin layers disposed on opposing major surfaces of the biaxially oriented polyolefin film. Also described is a process for making composite optical compensation films.

Description

OPTICAL COMPENSATION FILM INCLUDING STRIPP ABLE SKIN

FIELD

[01] The present disclosure relates to composite optical compensation films and particularly to optical compensation films including strippable skin layers.

BACKGROUND

[02] Liquid crystal displays such as, for example, twisted nematic (TN), single domain vertically aligned (VA), optically compensated birefringent (OCB), in-plane switching (IPS) liquid crystal displays and the like, have inherently narrow and non-uniform viewing angle characteristics. Such viewing angle characteristics can describe, at least in part, the optical performance of a display. Characteristics such as contrast, color, and gray scale intensity profile can vary substantially in uncompensated displays for different viewing angles. There is a desire to modify these characteristics from those of an uncompensated display to provide a desired set of characteristics as a viewer changes positions horizontally, vertically, or both and for viewers at different horizontal and vertical positions. For example, in some applications there may be a desire to make the viewing characteristics more uniform over a range of horizontal or vertical positions.

[03] The range of viewing angles that are important can depend on the application of the liquid crystal display. For example, in some applications, a broad range of horizontal positions may be desired, but a relatively narrow range of vertical positions may be sufficient. In other applications, viewing from a narrow range of horizontal or vertical angles (or both) may be desirable. Accordingly, the desired optical compensation for non-uniform viewing angle characteristics can depend on the desired range of viewing positions. One viewing angle characteristic is the contrast ratio between the bright state and the dark state of the liquid crystal display. The contrast ratio can be affected by a variety of factors.

[04] Another viewing angle characteristic is the color shift of the display with changes in viewing angle. Color shift refers to the change in the color coordinates (e.g., the color coordinates based on the CIE 1931 standard) of the light from the display as viewing angle is altered. Color shift can be measured by taking the difference in the chromaticity color coordinates (e.g., Δx or Δy) at an angle normal to the plane containing the screen and at any non-normal viewing angle or set of viewing angles. The definition of acceptable color shift is determined by the application, but can be defined as when the absolute value of Δx or Δy exceeds some defined value, for example, exceeds 0.05 or 0.10. For example, it can be determined whether the color shift is acceptable for a desired set of viewing angles. Because the color shift may depend upon the voltage to any pixel or set of pixels, color shift is ideally measured at one or more pixel driving voltages.

[05] Yet another viewing angle characteristic that can be observed is substantial non-uniform behavior of gray scale changes and even the occurrence of gray scale inversion. The nonuniform behavior occurs when the angular dependent transmission of the liquid crystal layer does not monotonically follow the voltage applied to the layer. Gray scale inversion occurs when the ratio of intensities of any two adjacent gray levels approaches a value of one, where the gray levels become indistinguishable or even invert. Typically, gray scale inversion occurs only at some viewing angles.

[06] Compensators have been proposed to address these issues. One concept includes a compensator film made of discotic molecules. One drawback of current discotic compensators is the typical occurrence of comparatively large color shifts. There is a need for new compensator structures to provide improved or desired viewing angle characteristics.

BRIEF SUMMARY

[07] The present disclosure relates to polymeric optical film useful for a variety of applications including, for example, optical compensators for displays, such as liquid crystal displays.

[08] In one aspect, a process for making a composite optical compensation film includes co- extruding two or more polymer layers to form a composite polymer film, stretching the composite polymer film in a first direction, and stretching the composite polymeric film in a second direction different than the first direction forming a biaxially stretched optical compensation film having a strippable skin layer. The biaxially stretched optical compensation film is substantially non-absorbing and non-scattering for at least one polarization state of visible light, and has x, y, and z orthogonal indices of refraction. At least two of the orthogonal indices of refraction are not equal, and an in-plane retardance is greater than 10 nm and less than 550 nm and an absolute value of an out-of-plane retardance is greater than 55 nm, and a slow axis defines a principle axis of orientation. Each of the two or more co-extruded layers has a thickness of greater than 200 nm, and a standard deviation of the in-plane retardance is no more than 2.5 nm and a standard deviation of the slow axis is no more than 0.2 degrees when measured along a first direction.

[09] In another aspect, a composite optical compensation film is described. The composite optical compensation film includes a biaxially oriented polyolefm film and strippable skin layers disposed on opposing major surfaces of the biaxially oriented polyolefm film. The biaxially oriented polymer film is substantially non-absorbing and non-scattering for at least one polarization state of light and having x, y, and z orthogonal indices of refraction. At least two of the orthogonal indices of refraction are not equal, and an in-plane retardance is greater than 10 nm and less than 550 nm and an absolute value of an out-of- plane retardance is greater than 55 nm, and a slow axis defines a principle axis of orientation.

[10] In a further aspect, a composite optical compensation film includes a biaxially oriented polyolefm film, a first pair of strippable skin layers disposed on opposing major surfaces of the biaxially oriented polyolefm film, and a second pair of strippable skin layers disposed on the first pair of strippable skin layers. The biaxially oriented polymer film is substantially non-absorbing and non-scattering for at least one polarization state of light and has x, y, and z orthogonal indices of refraction wherein at least two of the orthogonal indices of refraction are not equal, an in-plane retardance of greater than 10 nm and less than 550 nm and an absolute value of an out-of-plane retardance of greater than 55 nm, and a slow axis defines a principle axis of orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

[11] The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which: [12] FIG. 1 is a schematic illustration of a coordinate system with an optical film element;

[13] FIG. 2 is a schematic cross-sectional view of an illustrative optical compensation film;

[14] FIG. 3 is a schematic cross-sectional view of another illustrative optical compensation film;

[15] FIG. 4 is a top schematic view of a tenter apparatus for use to form the optical film element; and

[16] FIG. 5 is a schematic cross-sectional view of an optical compensator stack according to the present disclosure.

[17] The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number.

DETAILED DESCRIPTION

[18] In the following description, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense.

[19] All scientific and technical terms used herein have meanings commonly used in the art unless otherwise specified. The definitions provided are to facilitate understanding of certain terms used frequently herein and are not meant to limit the scope of the present disclosure.

[20] Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein.

[21] The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.

[22] As used in this specification and the appended claims, the singular forms "a", "an", and "the" encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

[23] The term "polymer" will be understood to include polymers, copolymers (e.g., polymers formed using two or more different monomers), oligomers and combinations thereof, as well as polymers, oligomers, or copolymers that can be blended.

[24] A "biaxial retarder" denotes a birefringent optical element, such as, for example, a plate or film, having different indices of refraction along all three axes (i.e., n_x≠n_y≠n_z). Biaxial retarders can be fabricated, for example, by biaxially orienting polymer films. Examples of biaxial retarders are discussed in U.S. Pat. No. 5,245,456. Examples of suitable films include films available from Sumitomo Chemical Co. (Osaka, Japan) and Nitto Denko Co. (Osaka, Japan). In-plane retardation and out-of-plane retardation are parameters used to describe a biaxial retarder. As the in-plane retardation approaches zero, then the biaxial retarder element behaves more like a c-plate. Generally, a biaxial retarder, as defined herein, has an in-plane retardation of at least 3 nm for 550 nm light. Retarders with lower in-plane retardation are considered c-plates.

[25] The term "polarization" refers to plane polarization, circular polarization, elliptical polarization, or any other nonrandom polarization state in which the electric vector of the beam of light does not change direction randomly, but either maintains a constant orientation or varies in a systematic manner. In plane polarization, the electric vector remains in a single plane, while in circular or elliptical polarization, the electric vector of the beam of light rotates in a systematic manner. [26] The term "biaxially stretched" refers to a film that has been stretched in two different directions, a first direction and a second direction, in the plane of the film.

[27] The term "simultaneously biaxially stretched" refers to a film in which at least a portion of stretching in each of the two directions is performed simultaneously.

[28] The terms "orient," "draw," and "stretch" are used interchangeably throughout this disclosure, as are the terms "oriented," "drawn," and "stretched" and the terms "orienting," "drawing," and "stretching".

[29] The terms "retardation" or "retardance" refer to the difference between two orthogonal indices of refraction times the thickness of the optical element.

[30] The term "in-plane retardation" refers to the product of the difference between two orthogonal in-plane indices of refraction times the thickness of the optical element.

[31] The term "out-of-plane retardation" refers to the product of the difference of the index of refraction along the thickness direction (z direction) of the optical element minus one in- plane index of refraction times the thickness of the optical element. Alternatively, this term refers to the product of the difference of the index of refraction along the thickness direction (z direction) of the optical element minus the average of in-plane indices of refraction times the thickness of the optical element. To simplify discussion of out-of- plane retardation, the numerical value cited may refer to the absolute value of that calculated by either of the two formulae cited above.

[32] The term "substantially non-absorbing" refers to the level of transmission of the optical element, being at least 80 percent transmissive to at least one polarization state of visible light, where the percent transmission is normalized to the intensity of the incident, optionally polarized light.

[33] The term "substantially non-scattering" refers to the level of collimated or nearly collimated incident light that is transmitted through the optical element, being at least 80 percent transmissive for at least one polarization state of visible light within a cone angle of less than 30 degrees. [34] The term "strippable skin" or "strippable skin layer" refers to a layer capable of remaining adhered to the optical film as long as desired, e.g., during initial processing such as stretching, storage, handling, packaging, transporting and subsequent processing, but can be removed and optionally reapplied as necessary in a particular application. The strippable skin layer can be separated from an optical film without applying excessive force, damaging the optical films, or contaminating the optical films with a substantial residue of particles from the strippable layers.

[35] This disclosure relates to optical compensation films and particularly to optical compensation films co-extruded with strippable skin layers and the skin layers are tailored to improve the optical properties, processability, and/or handlability of the resulting optical compensation film. While the present invention is not so limited, an appreciation of various aspects of the invention will be gained through a discussion of the examples provided below.

[36] FIG. 1 illustrates an axis system for use in describing the optical compensation film 100. Generally, for display devices, the x and y axes correspond to the width and length of the display and the z axis is typically along the thickness direction of a display. This convention will be used throughout, unless otherwise stated. In the axis system of FIG. 1, the x axis and y axis are defined to be parallel to a major surface 102 of the optical compensation film 100 and may correspond to width and length directions of a square or rectangular surface. The z axis is perpendicular to that major surface and is typically along the thickness direction of the optical compensation film.

[37] The optical compensation film 100 includes a biaxially oriented polymer film 100 having one or more co-extruded layers 105 that have specified in-plane and out-of-plane retardances. The biaxially oriented polymer film 100 is substantially non-absorbing and non-scattering for at least one polarization state of light.

[38] The illustrated biaxially oriented polymer film 100 includes a single polymeric layer 105, however, in some embodiments, the biaxially oriented polymer film 100 includes two, three, four or five polymeric layers 105 that can be formed by coextrusion and ultimately oriented to possess specified in-plane and out-of-plane retardances, such as those filed in copending application, Attorney Docket Number 62847US002, filed on even date herewith. In these biaxially oriented polymer film 100 embodiments, the polymeric layers 105 can be formed of different materials or are different polymer formulations or include different additives.

[39] The layer 105 has a thickness of 200 nm or greater, or 400 nm or greater, or 1 micrometer or greater. In many embodiments, the layer 105 has a thickness in a range from 200 nm to 10 micrometers, or from 500 nanometers to 10 micrometers, or from 1 to 10 micrometers, as desired. In the multilayer biaxially oriented polymer film 100 embodiments, the polymeric layers 105 can have the same or different thicknesses, as desired. In many embodiments, the biaxially oriented polymer film 100 has a thickness in a range from 1 to 25 micrometers, or from 5 to 20 micrometers.

[40] A variety of materials and methods can be used to make the optical compensation film described herein. For example, the optical compensation film can include one, two or more co-extruded polymer layers that are biaxially stretched (i.e., oriented) and is substantially non-absorbing and non-scattering for at least one polarization state of visible light. These optical compensation films have: x, y, and z orthogonal indices of refraction where at least two of the orthogonal indices of refraction are not equal; an in-plane retardance of greater than 10 nm and less than 550 nm; an absolute value out-of-plane retardance of 55 nm or greater; and a slow axis defining a principle axis of orientation.

[41] The in-plane retardance of the optical compensation film can be in a range from 30 nm to 550 nm, or from 50 nm to 550 nm, as desired. The absolute value of the out-of-plane retardance of the polymeric optical film may be 55 nm or greater, up to 550 nm.

[42] In many embodiments, the absolute value of the out-of-plane retardance is less than the in- plane retardance. In some embodiments, the in-plane retardance is in a range from 300 to 400 nm and the absolute value of the out-of-plane retardance is in a range from 200 to 300 nm. In other embodiments, the in-plane retardance is in a range from 100 to 125 nm and the absolute value of the out-of-plane retardance is in a range from 75 to 100 nm.

[43] In further embodiments, the in-plane retardance is in a range from 25 to 75 nm and the absolute value of the out-of-plane retardance is in a range from 125 to 175 nm or from 175 to 225 nm or from 225 to 275 nm. [44] In many embodiments, the optical compensation film can have a total thickness (z direction) of 0.5 micrometers or greater. In many embodiments, the optical compensation film can have a thickness (z direction) of 1 micrometer to 200 micrometers, or 5 micrometers to 100 micrometers, or 7 micrometers to 75 micrometers, or 10 micrometers to 50 micrometers.

[45] The optical properties (e.g., in-plane retardance, out-of-plane retardance, and/or slow axis) are substantially uniform across the length and width of the multilayer optical compensation film. For example, in many embodiments the multilayer optical compensation film standard deviation of the in-plane retardance can be no more than 2.5 nm and a standard deviation of the slow axis is no more than 0.3 degrees when measured along a first direction. The first direction measurement may be taken over, for instance, 0.1 meter, 1 meter, 10 meters, 20 meters, or even at least 100 meters. The measurements may be taken in increments of 1 cm, 5 cm, 10 cm, or even 20 cm. For example, measurements along a first direction may be taken on samples of at least 10 meters in increments of 20 cm or less or along a first direction; of at least 30 meters in increments of 20 cm or less or along a first direction; or of at least 100 meters in increments of 20 cm or less. In many embodiments, variation in the slow axis (measured along a first direction, as described above) is non-periodic. Further, the multilayer optical compensation film may have a standard deviation of the in-plane retardance of no more than 2.5 nm and a standard deviation of the slow axis is no more than 0.3 degrees when measured along a first and a second direction.

[46] One or more polymer layers 112, 114, 116, 118 disposed about the optical compensation film 105 forms a composite optical compensation film 101 as illustrated in FIG. 2 and FIG. 3. FIG. 2 illustrates a composite optical compensation film 101 having the optical compensation film 105 disposed between strippable skin layers 112, 114. FIG. 3 illustrates a composite optical compensation film 101 having the optical compensation film 105 disposed between a first strippable skin layers 112, 114 and a second set of strippable skin layers 116, 118.

[47] The one or more polymer layers 112, 114, 116, 118 are "strippable" from an adjacent polymer layer and/or the optical compensation film 105. The strippable co-extruded polymer layers 112, 114, 116, 118 remain adhered to the adjacent polymer layer and/or the optical compensation film 105 forming the composite optical compensation film 101 during initial processing, such as casting or stretching, or in some exemplary embodiments, also during subsequent storage, handling, packaging, transporting and/or conversion, but can be stripped or removed by a user when desired.

[48] The composite optical compensation film 101 can be formed of and number of useful materials such as, for example, homopolymers or copolymers or blends of polypropylene, polyethylene, poly butene, poly methyl pentene, cyclic olefins, polystyrene, poly vinylidene fluoride, poly methyl methacrylate, poly acrylates, polyesters, co-polyesters, polycarbonates, and a ionomer.

[49] Illustrative useful combinations of materials include, for example, polypropylene resin to form the compensation film 105, and a poly(ethylene-co-(meth)acrylic acid) ionomer material to form the strippable skin layers. Illustrative polypropylene resins are available under the trade designation "PP3376" from Total Petrochemicals USA, Houston, Texas. Poly(ethylene-co-(meth)acrylic acid) ionomers are commercially available under the trade designation "SURLYN" (e.g., lithium poly(ethylene-co-methacrylic acid) ionomers such as "SURLYN 7930" or "SURLYN 7940"; sodium poly(ethylene-co-methacrylic acid) ionomers such as "SURLYN 1601", "SURLYN 8020", "SURLYN 8120", "SURLYN 8140", "SURLYN 8150", "SURLYN 8320", "SURLYN 8527", "SURLYN 8660", "SURLYN 8920", "SURLYN 8940", or "SURLYN 8945"; zinc poly(ethylene-co- methacrylic acid) ionomers such as "SURLYN 1652", "SURLYN 1705", "SURLYN 1706", SURLYN 6101", SURLYN 9020", "SURLYN 9120", "SURLYN 9150", "SURLYN 9320W", "SURLYN 9520", "SURLYN 9650", "SURLYN 9720", "SURLYN 9721", "SURLYN 9910", "SURLYN 9945", "SURLYN 9950", "SURLYN 9970", or "SURLYN PC-100") by E. I. du Pont de Nemours & Company, Wilmington, Del.

[50] For example, the strippable co-extruded polymer layers (112 and 114 in FIG. 2 or 112, 114, and 116, 118 in FIG. 3) can be removed and the optical compensation film 105 separated shortly after stretching the composite optical compensation film 101 or shortly prior to installation of one or more of the optical compensation films 105 into a display device. In many embodiments, the one or more strippable co-extruded polymer layer and the optical compensation film are separated without applying excessive force, damaging the optical compensation film, or contaminating the optical compensation film with a substantial residue of particles from the strippable co-extruded polymer layer. The strippable skin layers can provide a protective outer layer to reduce surface defects of the optical compensation film and/or can provide a greater thickness to the optical compensation film to improve handling and the like. In many embodiments, the strippable skin layer is isotropic so that the optical properties of the protected optical compensation layer can be tested without removing the strippable skin.

[51] The strippable co-extruded polymer layers (112 and 114 in FIG. 2 or 112, 114, and 116,

118 in FIG. 3) can provide a barrier for volatile additives. The layer structure of the monolayer or multilayer optical compensation film may be uniform across the entire width of the cast web, or one may choose to deckle the strippable skin layers of the cast web such that the strippable skin layers do not extend to the edge of the cast web. Alternatively, strippable skin layers may be used to encapsulate a core layer in a three layer construction. Strippable skin layers that extend to the edge of the cast web or encapsulate the core layer provide a means for controlling edge curl.

[52] These co-extruded strippable skin layers (i.e., protective layers) add functionality, such as improved cleanliness, handling, and surface defect reduction to the composite optical compensation film 101. One or more of these co-extruded strippable skin layers may be controllably removed following simultaneous or sequential biaxial orientation process to expose the compensation film 105.

[53] Advantages of controllably removable skin layer(s) include: minimizing the need for large clean rooms (as airborne and other debris cannot attach itself directly to the surface of the inner compensation film layer(s)), reducing surface scratches and other defects to the surfaces of the compensation film, and improving handling of thin film webs through increased overall film thickness and associated beam strength.

[54] There is no limit to the number of outer protective skin layers, except as may be precluded by the extrusion, handling, and orientation processes. For example, one or two layers may be co-extruded with the compensation film 105. Alternatively, if the adhesion of a preferred outer skin layer to the inner compensation film layer(s) is too low, an intermediate skin layer with improved adhesion to both the compensation and outer skin layers may be desired. The construction may or may not be symmetrical, depending upon the preferred order of removal of outer skin layer(s).

[55] In yet another embodiment, it may be preferred that there is more than one strippable skin layer on each major face of the optical film in the center of a multi-layer construction. For example, one embodiment may include five layers, symmetrical about the center plane of the film construction (see FIG. 3). In this example, the layer construction is ABCBA. The inner most "C" layer is the compensation film 105 described herein, which itself may contain one or more layers as described above. The outer most "A" layers 112, 114 may provide means for improved handling, reduced scratching, or other advantage. The middle "B" layers 116, 118 may perform as a pre-mask, which both protects and allows an end- user to inspect the inner most "C" layer 105 without exposing it to dust or other debris in the environment until it is ready to be used in the intended application or process. While not intended to be limit the scope of the invention, the outer most "A" layer 112, 114 may be a polyolefm, which may be inexpensive and may have certain mechanical properties such as surface roughness to facilitate handling. As a pre-mask, the middle "B" layers 116, 118 may comprise polyester materials and may have certain optical properties such as low haze or controlled birefringence which facilitates visual and optical inspection of the inner most "C" layer 105. Thus, the outer most "A" layer 112, 114 are strippable from the middle "B" layers 116, 118 and the middle "B" layers 116, 118 are strippable from the inner most "C" compensation film layer 105. One skilled in the art will understand which set of properties are of most interest for each respective layer and a range of material choices which will result.

[56] Techniques for manufacturing optical compensation film have been developed. These techniques include co-extruding two or more polymer layers to form the composite film, stretching the composite film in a first direction and stretching the composite film in a second direction different than the first direction forming a biaxially stretched composite optical compensation film 101 having the retardances specified above. In some embodiments, at least a portion of the stretching in the second direction occurs simultaneously with the stretching in the first direction. In other embodiments, the stretching is preformed in a sequential manner. This technique forms a composite optical compensation film 101 with the properties and attributes described above.

[57] FIG. 4 illustrates a top schematic view of a tenter apparatus for biaxially stretching or orientating the co-extruded composite films to form the composite optical compensation film described herein. The tenter may be of the type disclosed in U.S. Pat. No. 5,051,225. Tenter apparatus 10 includes a first side rail 12 and a second side rail 14 on which the driven clips 22 and idler clips 24 ride. The driven clips 22 are illustrated schematically as boxes marked "X" while the idler clips 24 are illustrated schematically as open boxes. Between pairs of driven clips 22 on a given rail, there are one or more idler clips 24. As illustrated, there may be two idler clips 24 between each pair of clips 22 on a given rail. One set of clips 22, 24 travels in a closed loop about the first rail 12 in the direction indicated by the arrows at the ends of the rail. Similarly, another set of clips 22, 24 travels in a closed loop about the second rail 14 in the direction indicated by the arrows at the ends of the rail. The clips 22, 24 hold the film edges and propel film 26 in the direction shown by the arrow at the center of the film. At the ends of the rails 12, 14, the clips 22, 24 release the film 26. The clips then return along the outside of the rails to the entrance of the tenter to grip the cast web to propel it through the tenter. (For clarity of illustration, the clips returning to the entrance on the outside of the rails have been omitted from FIG. 4.) The stretched composite optical compensation film 26 exiting the tenter may be wound up for later processing or use, or may be processed further.

[58] The composite polymer film can be cast into a sheet form to prepare a web suitable for stretching to arrive at the composite optical compensation film described above. The composite polymer film can be cast by feeding polymer resin into a feed hopper of a single screw, twin screw, cascade, or other extrusion system having an extruder barrel with temperatures adjusted to produce a stable homogeneous melt for each polymer layer. Each polymer can then be co-extruded through a sheet die onto a rotating cooled metal casting wheel. The composite web is then biaxially stretched on the tenter as illustrated in FIG. 4 or stretched using a length orienter and tenter as used in bi-axial oriented film manufacturing. The tenter device described in FIG. 4 will be used for the purpose of describing the stretching process in this application, however any simultaneous or sequential method for orienting polymer film can be used, as desired. Those skilled in the art can appreciate the difference between the two stretching methods and can envision the use of a length orienter and tenter to achieve the desired optical characteristics provided by this method. The extruded multilayer web may be quenched, reheated and fed to the clips 22, 24 on the first and second rails 12, 14 to be propelled through the tenter apparatus 10. The optional heating and the gripping by the clips 22, 24 may occur in any order or simultaneously.

[59] The rails 12, 14 pass through three sections: preheat section 16; stretch section 18; and post-stretch treatment section 20. In the preheat section 16, the multilayer film is heated to within an appropriate temperature range to allow a significant amount of stretching without breaking. The three functional sections 16, 18, and 20 may be broken down further into zones. For example, in one embodiment of the tenter, the preheat section 16 includes zones Zl, Z2, and Z3, the stretch section 18 includes zones Z4, Z5, and Z6, and the post-stretch section 20 may include zones Z7, Z8, and Z9. It is understood that the preheat, stretch, and post-treatment sections may each include fewer or more zones than illustrated. Further, within the stretch section 18, the TD (Transverse Direction) component of stretch or the MD (Machine Direction) component of stretch may be performed in the same or in different zones. For example, MD and TD stretch each may occur in any one, two or three of the zones Z4, Z5, and Z6. Further, one component of stretch may occur before the other, or may begin before the other and overlap the other. Still further, either component of stretch may occur in more than one discrete step. For example, MD stretch may occur in Z4 and Z6 without an MD stretch occurring in Z5.

[60] Some stretching in the MD and/or TD may also occur in the preheat section or post-stretch treatment section. For example, in the embodiment illustrated, stretching may begin in zone Z3. Stretching may continue into zone Z7 or beyond. Stretching may resume in any of the zones after zones Z3, Z5, or Z6.

[61] The amount of stretching in the MD may be the same or different than the amount of stretching in the TD. The amount of stretching in the MD may be up to 10% or 25% or 50% or 100%, or 1000% greater than the amount of stretching in the TD. The amount of stretching in the TD may be up to 10% or 25% or 50% or 100%, or 1000% greater than the amount of stretching in the MD. This "unbalanced" stretching can assist in providing the optical compensation film with substantially uniform in-plane retardance. In many embodiments, the composite optical film is drawn in a range from 40 to 60 times or from 45 to 55 times an initial area (e.g., a 7x7 balanced draw equals a 49 times draw of an initial area).

[62] The composite optical compensation film may be propelled through the post-treatment section 20. In this section, the composite optical compensation film 26 may be maintained at a desired temperature while no significant stretching occurs. This treatment can be referred to as a heat set or anneal, and may be performed to improve the properties of the final composite film, such as dimensional stability. Also, a small amount of relaxation in either or both the TD and MD may occur in the post-treatment section 20. Relaxation here refers to a convergence of the rails in the TD and/or a convergence of the driven clips on each rail in the MD, or simply the reduction of stress on the film in the TD and/or MD.

[63] Biaxial stretching of films is sensitive to many process conditions, including but not limited to the composition of the polymer or resin in each layer of the composite optical compensation film, composite film casting and quenching parameters, the time- temperature history while preheating the composite film prior to stretching, the stretching temperature used, the stretch profile used, and the rates of stretching. With the benefits of the teaching herein, one of skill in the art may adjust any or all of these parameters and obtain films having the desired optical properties and characteristics.

[64] The cooling of the biaxially stretched composite optical compensation film can begin before or after the onset of stretching in the stretch section 18. The cooling can be "zone" cooling which refers to cooling substantially the entire width or TD of the web, from the edge portions 28 of the composite film through the center portion 30 of the multilayer film. Zone cooling immediately after the stretching zone has been found to improve uniformity of in-plane retardance of the composite optical compensation films when applied in an effective amount. Cooling may be provided by forced air convection.

[65] Any number of additives may optionally be added to the polymer layers forming the composite polymer film. A partial listing of additives includes, for example, stabilizers (such as antioxidants, antiozonants, antistats, UV absorbers, and light stabilizers), process aids (such as lubricants, extrusions aids, blocking agents, and electrostatic pinning aids), crystallization modifiers (such as clarifying agents and nucleating agents), and/or tackifers (including for example stiffening agents, and nano-particles).

[66] Crystallization modifiers include, for example, clarifying agents and nucleating agents. Crystallization modifiers aid in reducing "haze" in the optical compensation film layers including crystalline polymer. Crystallization modifiers can be present in any amount effective to reduce "haze", such as, for example, 10 ppm to 500,000 ppm or 100 ppm to 400,000 pm or 100 ppm to 350,000 ppm or 250 ppm to 300,000 ppm.

[67] For polymer film layers including materials of relatively high intrinsic birefringence, the thickness, of low out-of-plane retardance (Rth) films, can be very low. Thin optical compensation films are difficult to handle. The addition of one or more polymer layers having a first material, whose birefringence is lower than one or more layers having a second material, can provide for an increase in the overall thickness of the film, making the resultant product easier to handle.

[68] Optical films such as retarders (i.e., optical compensation films) are ideally uniform. As these films are often fabricated where the thickness is one of the primary process controls. Variations in thickness, especially for films having a more highly birefringent layer or layers, may manifest as perceivable areas of varying retardance. Equivalent variations in thickness in a thicker film as in a thinner film, especially for films comprising lower birefringent materials in one or more layers, may reduce the appearance of optical non- uniformities when viewed between polarizers.

[69] Control of the slow axis variability is dependent, in part, on the magnitude of the ratio of the primary to secondary stretch direction, where the primary stretch is larger than the secondary stretch. When one or more layers having a first material are co-extruded with one or more layers having a second material, where the layer(s) of the first material have a lower level of birefringence than the layer(s) having the second material, one may have to orient the film with a larger ratio of primary to secondary stretch than a film having only a layer of the second material. Thusly made optical compensation films can be made with lower variations in the slow axis. [70] It has also been observed that the variation of slow axis is at least partially dependent upon variations in the birefringence of the starting cast web. When uncontrolled or premature detack occurs, the birefringence of the cast web can vary (downweb and cross web). Controlling how the cast web detacks from the casting wheel enables one to control the birefringence of the cast web and hence of the final film, thus lowering the variation in slow axis of the final optical compensation film.

[71] Any combination of one or more polymeric materials where at least one polymer layer is capable of being biaxially stretched and possessing the optical properties described herein are contemplated for each layer forming the optical compensation film. A partial listing of these polymers includes, for example, polyolefm, polyacrylates, polyesters, polycarbonates, fluoropolymers and the like.

[72] Polyolefm includes for example: cyclic olefin polymers such as, for example, polystyrene, norbornene and the like; non-aromatic olefin polymers such as, for example, polypropylene; polyethylene; polybutylene; polypentylene; and the like. A specific polybutylene is poly(l-butene). A specific polypentylene is poly(4-methyl-l-pentene).

[73] Polyacrylate includes, for example, acrylates, methacrylates and the like. Examples of specific polyacrylates include poly(methyl methacrylate), and poly(butyl methacrylate).

[74] Fluoropolymer specifically includes, but is not limited to, poly(vinylidene fluoride).

[75] In some embodiments, the optical compensation film includes two to or more olefin polymer layers where at least one outer polymer layer is substantially free of a crystallization modifier and at least one layer of the optical compensation film includes a crystallization modifier. It has been found that having the outer polymer layer of the multilayer polymer film that is in contact with the casting wheel being substantially free of a crystallization modifier improves the adhesion and detack of the multilayer polymer film from the casting wheel. This embodiment shows improved adhesion to the casting wheel, reduced artifacts or surface defects and generally improved optical uniformity of the resulting optical compensation film.

[76] FIG. 5 is a schematic cross-sectional view of an optical compensator stack 500 according to the present disclosure. The optical compensator stack 500 includes an optical compensation film 501 disposed on or adjacent to an optical element 510. The illustrative optical compensation film 500 possesses the optical properties described above and can include more than the one layer illustrated in FIG. 5. In many embodiments, the optical element 510 is a polarizing element such as, for example, an absorbing polarizer or a reflective polarizer.

[77] The optical elements are configured in combinations as described below to form optical bodies or optical compensator stacks. Optical bodies or optical compensator stacks can be formed by disposing a polarizer layer or a cholesteric liquid crystal material on the optical compensation films described above.

[78] One or more optical compensation stacks can be laminated to a first major face and/or a second major face of a LCD panel in a manner similar to that which conventional dichroic polarizers are laminated. The optical compensation stacks described above provide a wider range of retarder, for example, a biaxial retarder or c-plate, birefringence that can be fabricated to make an optical compensation stack without dramatically increasing the thickness of the polarizer. With the teaching herein, it is possible to fabricate an optical compensation stack with a polarizer which is thinner than a conventional polarizer not containing additional optical compensation film.

[79] The optical bodies, stacks, or co-extruded compensators described above can be used in a variety of optical displays and other applications, including transmissive (e.g., backlit), reflective, and transflective displays.

[80] To minimize surface reflections, to enable cleaning of the front surface, to prevent scratching as well as to facilitate a number of other properties, different layers or combinations of materials can be disposed on the co-extruded optical compensation films or stacks described herein. Additional films may also include touch components.

[81] To improve brightness of a resulting display, a number of different types of films may be added to the back of the display or in to a back- light cavity. These films may include diffusers, protective shields, EMI shielding, anti-reflection films, prismatic structured films, such as BEF (available from 3M Company, Saint Paul, MN), or reflective polarizers, such as DBEF (available from 3M Company) or Nipocs, PCF, or APCF (available from Nitto Denko). When reflective polarizers operate by transmitting and reflecting circularly polarized light, such as Nipocs, additional retarder films are often needed, such as a quarter wave plate and the like.

[82] EXAMPLES

[83] The following examples include a monolayer compensation film of Total PP 3376 (an isotactic polypropylene resin). The cast web thickness of the inner compensation film layer was controlled so as to produce an inner compensation film layer whose final thickness was approximately 18 micrometers. The area draw in the simultaneous biaxially orientation process was approximately 49 times at approximately 154 degrees Celsius. Various materials, number of layers, and outer layer thicknesses were studied to demonstrate a range of potential peel forces.

[84] Example 1

[85] A three-layer ABA construction was co-extruded. The inner layer was PP3376 and the two major outer surfaces were Dupont Surlyn 8660. The final thickness of each respective outer protective layer varied from about 3 to about 18 micrometers. In all cases, it was observed that controlled peel was attained for the removal of the outer films from the inner PP3376.

[86] Example 2

[87] A three-layer construction was co-extruded in a manner similar to Example 1 , with the exception that the outer layers comprised either Dupont Surlyn 8670 or 9970. In all cases, it was observed that controlled peel was attained for the removal of the outer films from the inner PP3376.

[88] Example 3

[89] A three layer construction was co-extruded in a manner similar to Example 1 , with the exception that the outer layers comprised either Dow Dowlex 2035 or 2500 (low density polyethylene commercially available from Dow Chemical Co., Danbury, Conn.) In all cases, the outer layers of cast web could be controllably removed, but for the oriented film the adhesion strength of the outer to the inner layer was too great to conveniently remove the outer films from the inner PP3376.

Example 4

[90] A three layer construction was co-extruded in a manner similar to Example 1 , with the exception that the outer layers comprised Eastar PETG 6763 (a copolyester available from Eastman Chemical Company, Kingsport, TN). In all cases, the inter-layer adhesion of the cast web was insufficient to avoid delamination during process and handling even prior to film stretching.

[91] Example 5

[92] Similar results were observed as in Example 4, when the outer layers comprised co-PET-F and co-PET-WR. As in Example 4, the inter-layer adhesion of the cast web was insufficient to avoid delamination during process and handling even prior to film stretching.

[93] One skilled in the art will appreciate that embodiments other than those disclosed are envisioned. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.

Claims

What is claimed is:

1. A process for making a composite optical compensation film comprising: co-extruding two or more polymer layers to form a composite polymer film; stretching the composite polymer film in a first direction; stretching the composite polymer film in a second direction different than the first direction to form a biaxially stretched optical compensation film having a strippable skin layer; wherein the biaxially stretched optical compensation film is substantially non-absorbing and non-scattering for at least one polarization state of visible light, and having x, y, and z orthogonal indices of refraction wherein at least two of the orthogonal indices of refraction are not equal, and an in-plane retardance being greater than 10 nm and less than 550 nm and an absolute value of an out-of-plane retardance being greater than 55 nm, and a slow axis defining a principle axis of orientation, and each of the two or more co-extruded layers having a thickness of greater than 200 nm, and a standard deviation of the in-plane retardance is no more than 2.5 nm and a standard deviation of the slow axis is no more than 0.2 degrees when measured along a first direction.

2. A process according to claim 1, wherein the co-extruding step comprises co- extruding a polyolefin layer between two outer skin layers forming a composite polymer film.

3. A process according to claim 1, wherein the co-extruding step comprises co- extruding a polypropylene layer between two outer skin layers forming a composite polymer film, where after the stretching step, the outer skin layers form strippable skin layers.

4. A process according to claim 2, wherein the two outer skin layers are isotropic and the polyolefin layer is birefringent.

5. A process according to claim 1, further comprising stripping the strippable skin layer from the biaxially stretched optical compensation film.

6. A process according to claim 2, wherein the polyolefm layer comprises polypropylene and the two outer skin layers comprise an ionomer.

7. A process according to claim 2, wherein the polyolefm layer comprises polypropylene and the two outer skin layers comprise a poly(ethylene-co-(meth)acrylic acid) ionomer.

8. A process according to claim 1, wherein the stretching steps stretch the composite polymer film in a range from 40 to 60 times an initial area.

9. A process according to claim 1, wherein the stretching steps stretch the composite polymer film in a range from 45 to 55 times an initial area.

10. A composite optical compensation film comprising: a biaxially oriented polyolefm film, the biaxially oriented polymer film being substantially non-absorbing and non-scattering for at least one polarization state of light, and having x, y, and z orthogonal indices of refraction wherein at least two of the orthogonal indices of refraction are not equal, an in-plane retardance being greater than 10 nm and less than 550 nm and an absolute value of an out-of-plane retardance being greater than 55 nm, and a slow axis defining a principle axis of orientation; and at least two strippable skin layers, each disposed on opposing major surfaces of the biaxially oriented polyolefin film.

11. A composite optical compensation film according to claim 10, wherein the polyolefin comprises birefringent propylene and the at least two strippable skin layers are isotropic.

12. A composite optical compensation film according to claim 10, wherein the polyolefin layer comprises polypropylene and the at least two strippable skin layers comprise an ionomer.

13. A composite optical compensation film according to claim 10, wherein the polyolefin layer comprises polypropylene and the at least two strippable skin layers comprise a poly(ethylene-co-(meth)acrylic acid) ionomer.

14. A composite optical compensation film comprising: a biaxially oriented polyolefin film, the biaxially oriented polymer film being substantially non-absorbing and non-scattering for at least one polarization state of light and having x, y, and z orthogonal indices of refraction wherein at least two of the orthogonal indices of refraction are not equal, an in-plane retardance being greater than 10 nm and less than 550 nm and an absolute value of an out-of-plane retardance being greater than 55 nm, and a slow axis defining a principle axis of orientation; a first pair of strippable skin layers, each disposed on opposing major surfaces of the biaxially oriented polyolefin film; and a second pair of strippable skin layers, each disposed on the first pair of strippable skin layers.

15. A composite optical compensation film according to claim 14, wherein the polyolefin comprises birefringent propylene and the first pair of strippable skin layers are isotropic.

16. A composite optical compensation film according to claim 14, wherein the polyolefin layer comprises polypropylene and the first pair of strippable skin layers comprises an ionomer.

17. A composite optical compensation film according to claim 14, wherein the polyolefin layer comprises polypropylene and the first pair of strippable skin layers comprises a poly(ethylene-co-(meth)acrylic acid) ionomer.

18. A composite optical compensation film according to claim 14, wherein the polyolefin comprises birefringent propylene, the first pair of strippable skin layers is isotropic, and the second pair of strippable skin layers comprises a polyolefin.

19. A composite optical compensation film according to claim 14, wherein the polyolefin comprises birefringent propylene, the first pair of strippable skin layers comprises a polyester, and the second pair of strippable skin layers comprises a polyolefin.