WO2009105429A1

WO2009105429A1 - Multilayer optical compensation film

Info

Publication number: WO2009105429A1
Application number: PCT/US2009/034310
Authority: WO
Inventors: Robert D. Taylor; Richard J. Thompson; Adam D. Haag; John M. Klaeser; Brandt K. Carter; Richard C. Allen
Original assignee: 3M Innovative Properties Company
Priority date: 2008-02-20
Filing date: 2009-02-17
Publication date: 2009-08-27

Abstract

A multilayer optical compensation film is described. The multilayer optical compensation film includes a biaxially oriented polymer film having two or more co-extruded polymer layers and one of the co-extruded polymer layers comprises a crystallization modifier. Also described is a process for making a multilayer optical compensation film and an optical stack comprising an optical element and a multilayer optical compensation film.

Description

MULTILAYER OPTICAL COMPENSATION FILM

FIELD

[01] The present disclosure relates to multilayer optical compensation films and particularly to co-extruded multilayer optical compensation films.

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 multilayer polymeric optical film useful for a variety of applications including, for example, optical compensators for displays, such as liquid crystal displays, as well as the displays and other devices containing the multilayer optical compensators.

[08] In a first aspect, a multilayer optical compensation film includes a biaxially oriented polymer film having two or more co-extruded layers, where one of the co-extruded layers includes a crystallization modifier. The biaxially oriented polymer film is substantially non-absorbing and non-scattering for at least one polarization state of light. The biaxially oriented polymer film has x, y, and z orthogonal indices of refraction where at least two of the orthogonal indices of refraction are not equal, and 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 defining a principle axis of orientation. Each of the two or more co-extruded layers has a thickness of greater than 200 nm and the in-plane retardance is substantially uniform across a length or a width of the multilayer optical compensation film.

[09] In another aspect, a process for making a multilayer optical compensation film includes co-extruding two or more polymer layers to form a multilayer polymer film and one of the co-extruded layers includes a crystallization modifier; stretching the multilayer polymer film in a first direction; and stretching the multilayer polymeric film in a second direction different than the first direction forming a biaxially stretched multilayer optical compensation film. The biaxially stretched multilayer 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 wherein at least two of the orthogonal indices of refraction are not equal, and 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 defining 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.

[10] In a further aspect, an optical stack includes an optical element, and a multilayer optical compensation film disposed on the optical element. The multilayer optical compensation film includes a biaxially oriented polymer film having two or more co-extruded layers and one of the co-extruded layers includes a crystallization modifier. The biaxially oriented polymer film is substantially non-absorbing and non-scattering for at least one polarization state of light. The biaxially oriented polymer film has 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 greater than 10 nm and less than 550 nm and an absolute value of an out- of-plane retardance greater than 55 nm, and a slow axis defining a principle axis of orientation. Each of the two or more co-extruded layers has a thickness of greater than 200 nm. The in-plane retardance is substantially uniform across a length or a width of the multilayer optical compensation film.

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 top schematic view of a tenter apparatus for use to form the optical film element;

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

[15] FIG. 4 is a schematic cross-sectional view of another illustrative multilayer optical compensation film; 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 herein 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] This disclosure relates to multilayer optical compensation films and particularly to multilayer optical compensation films where the film layers are formed by co-extrusion and the layers are tailored to improve the optical properties, processability, and/or handlability of the resulting multilayer 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.

[35] FIG. 1 illustrates an axis system for use in describing the multilayer 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 multilayer 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 multilayer optical compensation film.

[36] The multilayer optical compensation film 100 includes a biaxially oriented polymer film 100 having two or more co-extruded layers 105, 106. The biaxially oriented polymer film 100 is substantially non-absorbing and non-scattering for at least one polarization state of light.

[37] The illustrated biaxially oriented polymer film 100 includes a first polymer layer 105 and a second polymer layer 106 with an interface 107 disposed between the first polymer layer 105 and a second polymer layer 106. In many embodiments, the first polymer layer 105 and a second polymer layer 106 are formed of different materials or are different polymer formulations or include different additives. The layers 105, 106 have a thickness of 200 nm or greater, or 400 nm or greater, or 1 micrometer or greater. In many embodiments, the layers 105, 106 have 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. The layers 105, 106 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.

[38] In some embodiments, the biaxially oriented polymer film 100 includes three layers, four layers, five layers, or more than five layers, where each layer is co-extruded and then oriented to form the biaxially oriented polymer film 100. As described above, each layer has a thickness of 200 nm or greater, or 500 nm or greater, or 1 micrometer or greater. In many embodiments, the layers have 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. Each co-extruded and oriented layer 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.

[39] This multilayer optical compensation film provides increased functionality since each of the two or more co-extruded layers can be customized according to the requirements of each multilayer optical compensation film. For example, each layer can be customized to provide improved cleanliness, handability, surface defect reduction, processability, compatibility with other surfaces, and the like.

[40] A variety of materials and methods can be used to make multilayer optical compensation film described herein. For example, the multilayer optical compensation film can include 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 multilayer 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 multilayer 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 multilayer optical compensation film can have a total thickness (z direction) of 0.5 micrometers or greater. In many embodiments, the multilayer 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] Techniques for manufacturing optical compensation film have been developed. These techniques include co-extruding two or more polymer layers to form a multilayer polymer film, stretching the multilayer polymer film in a first direction and stretching the multilayer polymer film in a second direction different than the first direction forming a biaxially stretched multilayer polymer film. In some embodiments, at least a portion of the stretching in the second direction occurs simultaneously with the stretching in the first direction. These techniques form a multilayer optical compensation film with the properties and attributes described above.

[47] Monolayer optical compensation films have been previously formed. However, precise processing is needed to achieve a monolayer optical compensation film with uniform optical properties, especially when the monolayer optical compensation film polymer crystallizes and a crystallization modifier is utilized to reduce haze within the monolayer optical compensation film. In particular, when a monolayer optical compensation film is formed (e.g., via extrusion), it is cooled on a casting wheel before it is stretched in a tenter apparatus (see, e.g., FIG. 2). These crystalline monolayer films are often polyolefm (e.g., polypropylene) that can detach or detack from the casting wheel prematurely and/or violently, resulting in artifacts in the film or unwanted stress induced birefringence.

[48] Uncontrolled and/or premature detack is believed to be correlated with rates of crystallization of polymer resin on the casting wheel. Without being limited or bound to any particular theory, it is believed that when the rate of crystallization is too high for the specific process being used, premature release from the casting wheel can occur. The addition of additives such as a crystallization modifier (whose purpose is generally to control haze) further increases the rate of crystallization. Hence, monolayer extrusion processes are difficult when crystallization modifiers are needed to reduce haze within the monolayer optical compensation film.

[49] Monolayer optical compensation films that crystallize rapidly can experience problems where the edges of the cast web curls as it is cooled on the casting wheel. Cast web edge curl can cause the cast web to wander or come out of the edge trim knives, for those cases where cast web is trimmed prior to subsequent orientation or stretching process(es). Preventing cast web edge curl can reduce or limit yield loss from excessive edge trim. Cast web edge curl can cause difficulty in keeping the web in the tenter clips (see FIG. 2). If the tenter clip(s) do(es) not clip the web properly, the web can pull out from one or more clips, which may cause the film to break. Cast web edge curl can also facilitate premature detack of cast web from the casting wheel, thus making it more difficult to control the defects described above. The addition of additives such as crystallization modifiers in monolayer optical compensation films can make edge curl even worse.

[50] Certain additives, such as some crystallization modifiers, are volatile at extrusion temperatures. These additives can migrate and/or bloom to the surface of the cast web. Volatile additives once vaporized can condense on cooler surfaces or in the surrounding cooler atmosphere. Thus, particulate debris can fall onto the molten web, which is a source of yield loss. If condensate collects on the pinning system (such as an air knife used to pin the cast web to the casting wheel), local changes in pinning force can occur, resulting in defects such as die lines and de-wets (defined as non-flat surface of the cast web nearest the casting wheel, which are caused when insufficient pinning forces do not displace air behind the cast web).

[51] FIG. 2 illustrates a top schematic view of a tenter apparatus for biaxially stretching or orientating the co-extruded multilayer polymer films to form the multilayer optical compensation films 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 stretched multilayer optical compensation 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 stretched multilayer optical compensation 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. 2.) The stretched multilayer optical compensation film 26 exiting the tenter may be wound up for later processing or use, or may be processed further.

[52] The multilayer polymer film can be cast into a sheet form to prepare a web suitable for stretching to arrive at the multilayer optical compensation film described above. The multilayer 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 multilayer web is then biaxially stretched on the tenter as illustrated in FIG. 2 or stretched using a length orienter and tenter as used in bi-axial oriented film manufacturing. The tenter device described in FIG. 2 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. [53] 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.

[54] 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.

[55] The amount of stretching in the MD may be 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 multilayer optical compensation film with substantially uniform in-plane retardance.

[56] The multilayer optical compensation film may be propelled through the post-treatment section 20. In this section, the multilayer 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 multilayer 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.

[57] 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 multilayer optical compensation film, multilayer film casting and quenching parameters, the time- temperature history while preheating the multilayer 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.

[58] The cooling of the biaxially stretched multilayer 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 multilayer 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 multilayer optical compensation films when applied in an effective amount. Cooling may be provided by forced air convection.

[59] FIG. 3 is a schematic cross-sectional view of an illustrative bi-layer optical compensation film 300 and FIG. 4 is a schematic cross-sectional view of another illustrative multilayer optical compensation film. FIG. 3 illustrates a bi-layer optical compensation film 300 having a first polymer layer 305 co-extruded with a second polymer layer 306 and having an interface 307 disposed between the first polymer layer 305 and the second polymer layer 306. The illustrative bi-layer optical compensation film 300 possesses the optical properties described above in relation to FIG. 1. The first polymer layer 305 and the second polymer layer 306 are different. FIG. 4 illustrates a tri-layer optical compensation film 400 having an inner polymer layer 405 co-extruded between two outer polymer layers 409, 408. The illustrative multilayer optical compensation film 400 possesses the optical properties described above in relation to FIG. 1. [60] The first polymer layer 305 and the second polymer layer 306 are formed of different polymers or are formed of different polymer formulations. The outer polymer layers 409, 408 can be formed of the same or different polymer or polymer formulation and the inner polymer layer 405 is formed of a different polymer or is formed of different polymer formulation than at least one of the outer polymer layers 409, 408.

[61] In some embodiments, one polymer layer is birefringent and the other polymer layer is isotropic. Different polymer formulations refer to, for example, each polymer layer formed of the same polymer but one layer additionally including an additive not found in the other layer, or a first polymer layer is a homopolymer and the second polymer layer is a co-polymer of the homopolymer. Any number of additives may optionally be added to one or both polymer layers forming the multilayer polymer film. A partial listing of additives includes, for example, stabilizers, processing aids, crystallization modifiers, tackifiers, stiffening agents, nano-particles, and the like.

[62] In some embodiments, one or more polymer layers forming the multilayer optical compensation film are "skin" layers forming the multilayer optical compensation film. The skin layer(s) 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 skin layer is isotropic so that the skin layer does not alter the optical properties of the protected optical compensation layer.

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

[64] For polymer film layers comprising 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.

[65] Optical films such as retarders (i.e., optical compensation films) are ideally uniform. 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.

[66] 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 multilayer optical compensation films can be made with lower variations in the slow axis.

[67] 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 multilayer optical compensation film.

[68] 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 multilayer optical compensation film. A partial listing of these polymers includes, for example, polyolefin, polyacrylates, polyesters, polycarbonates, fluoropolymers and the like. [69] Polyolefin 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).

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

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

[72] Stabilizers include, for example, anti-oxidants, anti-ozone agents, anti-static agents, UV absorbers, and light stabilizers. Processing aids include, for example, lubricants, extrusion aids, blocking agents, and electrostatic pinning aids.

[73] Crystallization modifiers include, for example, clarifying agents and nucleating agents. Crystallization modifiers aid in reducing "haze" in the multilayer 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.

[74] In some embodiments, the multilayer 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 multilayer optical compensation film includes a crystallization modifier. It has been found that having the outer polymer layer of the multilayer polymer film that is 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 multilayer optical compensation film.

[75] 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 a multilayer optical compensation film 501 disposed on or adjacent to an optical element 510. The illustrative multilayer optical compensation film 500 possesses the optical properties described above in relation to FIG. 1, and can include more than the two layers 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.

[76] 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 multilayer optical compensation films described above.

[77] 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 polarizer which is thinner than a conventional polarizer not containing additional multilayer optical compensation film.

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

[79] 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 multilayer optical compensation films or stacks described herein. Additional films may also include touch components.

[80] 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.

[81] EXAMPLES

[82] Example 1

[83] A co-extruded three layer (ABA) construction was prepared having skins formed of Total PP 3376 (an isotactic polypropylene resin containing free of crystallization modifier) and a core of capacitor grade polypropylene containing 1200ppm MilladTM 3988 (available from Milliken Chemical, Spartanburg, S. C). The skins layers accounted for about 13% of total thickness of the cast multilayer polymer web. The thickness of the multilayer cast web was about 770 micrometers. The cast web was formed using conventional extrusion and casting processes and the tenter apparatus described herein. The process parameters were as follows.

[84] Table 1

The out-of-plane retardance of the resulting biaxially oriented multilayer film was measured using an Axometrics Polarimeter (available from Axometrics, Inc.). Approximately 20 replicates were measured to determine an average Rth. Custom made equipment was used to measure the in-plane retardance and slow axis orientation. Individual measurements were made in increments of 2.5 millimeters over a distance of 100 meters. The average value and standard deviation were determined from the resulting data.

The thickness of the biaxially oriented film was ~12 micrometers. The out-of-plane retardance was — 130 nm. The average in-plane retardance was -110 nm with a standard deviation of 1.85 nm. The standard deviation of the slow axis orientation was ~0.l l degrees.

The visual appearance of the resulting biaxially oriented multilayer film was assessed between crossed polarizers. It was rated using a visual analogue scale from 1 to 5, where 1 indicates no optical non-uniformities. The visual appearance of the resulting film was rated 1.

[85] Example 2

[86] Example 2 was made in a manner similar to Example 1, except the MD stretch ratio was varied. The MD stretch ratio was 6.0 : 1 (following a 3% relax after an initial stretch to 6.18 : 1).

The thickness of the biaxially oriented film was ~12 micrometers. The out-of-plane retardance was — 130 nm. The average in-plane retardance was -115 nm with a standard deviation of 1.80 nm. The standard deviation of the slow axis orientation was -0.13 degrees. The visual appearance was rated 1.

Comparative Example 1 :

[87] Comparative Example 1 was an extruded monolayer cast web comprising Total EOD0523 (a clarified isotactic polypropylene including a crystallization modifier). The cast web was formed using conventional extrusion and casting processes and the tenter apparatus described herein. The extrusion and casting process parameters were similar to those in Example 1, except the casting wheel speed was 8 meters per minute. The resulting cast web thickness was 535 micrometers. The orientation process conditions were similar to Example 1, except the TD stretch ratio was 6.6 : 1, the tenter preheat temperature was 163°C, the tenter stretch cool was 100⁰C, and the tenter anneal temperature was 140⁰C.

[88] The thickness of the biaxially oriented film was ~12 micrometers. The out-of-plane retardance was — 112 nm. The average in-plane retardance was ~33 nm with a standard deviation of 0.93 nm. The standard deviation of the slow axis orientation was -0.46 degrees. The visual appearance was rated 3.

[89] 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 multilayer optical compensation film comprising: a biaxially oriented polymer film having two or more co-extruded layers, the biaxially oriented polymer film being substantially non-absorbing and non- scattering for at least one polarization state of light and one of the co-extruded layers comprises a crystallization modifier; the biaxially oriented polymer film 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 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 defining a principle axis of orientation; and each of the two or more co-extruded layers having a thickness of greater than 200 nm; wherein the in-plane retardance is substantially uniform across a length or a width of the multilayer optical compensation film.

2. A multilayer optical compensation film according to claim 1, having a standard deviation of the in-plane retardance of no more than 2.5 nm.

3. A multilayer optical compensation film according to claim 1, having a standard deviation of the slow axis of no more than 0.2 degrees when measured along a first direction.

4. A multilayer optical compensation film according to claim 1, wherein the absolute value of the out-of-plane retardance is less than the in-plane retardance.

5. A multilayer optical compensation film according to claim 1, wherein the two or more co-extruded layers are each formed of different polymers.

6. A multilayer optical compensation film according to claim 1, wherein at least one of the two or more co-extruded layers are formed of a polyolefm.

7. A multilayer optical compensation film according to claim 1, wherein the two or more co-extruded layers are formed of a non-aromatic polyolefm.

8. A multilayer optical compensation film according to claim 1, wherein at least one of the two or more co-extruded layers comprises polypropylene.

9. A multilayer optical compensation film according to claim 8, wherein at least one of the two or more co-extruded layers comprises a polypropylene co-polymer, and at least one of the two or more co-extruded layers comprises a polypropylene homopolymer.

10. A multilayer optical compensation film according to claim 8, wherein at least one of the two or more co-extruded layers is isotropic, and at least one of the two or more co- extruded layers is birefringent.

11. A multilayer optical compensation film according to claim 8, wherein at least one of the two or more co-extruded comprises polyolefm and a crystallization modifier, and at least one of the two or more co-extruded layers comprises polyolefm and is substantially free of a crystallization modifier.

12. A process for making a multilayer optical compensation film comprising: co-extruding two or more polymer layers to form a multilayer polymer film where one of the co-extruded polymer layers comprises a crystallization modifier; stretching the multilayer polymer film in a first direction; stretching the multilayer polymeric film in a second direction different than the first direction forming a biaxially stretched multilayer optical compensation film; wherein the biaxially stretched multilayer 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 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 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.

13. A process according to claim 12, wherein the stretching the multilayer polymer film in a first direction and the stretching the multilayer polymeric film in a second direction different than the first direction occurs simultaneously.

14. A process according to claim 12, wherein each co-extruded polymer layer is formed of a different polymer.

15. A process according to claim 12, wherein at least one of the co-extruded polymer layers comprises polypropylene.

16. A process according to claim 12, wherein at least one of the co-extruded polymer layers comprises a polypropylene co-polymer, and at least one of the other co-extruded polymer layers comprises a polypropylene homopolymer.

17. A process according to claim 12, wherein at least one of the co-extruded polymer layers is isotropic, and at least one other of the co-extruded polymer layers is birefringent.

18. A process according to claim 12, wherein at least one of the co-extruded polymer layers is an isotropic skin layer, and at least one of the other co-extruded polymer layers is birefringent.

19. An optical stack comprising; an optical element; and a multilayer optical compensation film disposed on the optical element, the multilayer optical compensation film comprising: a biaxially oriented polymer film having two or more co-extruded layers, the biaxially oriented polymer film being substantially non-absorbing and non- scattering for at least one polarization state of light and where one of the co-extruded polymer layers comprises a crystallization modifier; the biaxially oriented polymer film 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 of 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 defining a principle axis of orientation; and each of the two or more co-extruded polymer layers having a thickness of greater than 200 nm; wherein the in-plane retardance is substantially uniform across a length or a width of the multilayer optical compensation film.

20. An optical stack according to claim 19, wherein the optical element is a polarizing element.

21. An optical stack according to claim 19, wherein the optical element is an absorbing polarizing element.

22. An optical stack according to claim 19, wherein the optical element is a reflecting polarizing element.

23. An optical stack according to claim 19, wherein the optical element is birefringent such that the optical properties of the optical element are different from the multilayer compensation film.