US20070281157A1

US20070281157A1 - Reflective polarizer, fiber, and process for making

Info

Publication number: US20070281157A1
Application number: US11/643,071
Authority: US
Inventors: Thomas M. Laney; Peter T. Aylward; Xiang-Dong Mi
Original assignee: Rohm and Haas Denmark Finance AS
Current assignee: SKC Hi Tech and Marketing Co Ltd
Priority date: 2006-06-05
Filing date: 2006-12-21
Publication date: 2007-12-06
Also published as: WO2007145812A3; KR20090028609A; EP2033027A2; WO2007145812A2; TW200745641A; JP2009540363A

Abstract

A fiber comprises a birefringent fibril discontinuous polymeric phase dispersed in a continuous polymeric phase with prescribed matched and mis-matched refractive indices. A diffusely polarizing organic film, optical element, display, and method of making such a film are also disclosed.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from provisional application 60/810,888 filed Jun. 5, 2006.

FIELD OF THE INVENTION

This invention relates to the field of diffusely reflecting polarizers and polarizing displays and to a fiber useful therein.

BACKGROUND OF THE INVENTION

The performance potential and flexibility of polarized displays, especially those utilizing the electro-optic properties of liquid crystalline materials, has led to a dramatic growth in the use of these displays for a wide variety of applications. Liquid crystal displays (LCD's) offer the full range from extremely low cost and low power performance (e.g. wristwatch displays) to very high performance and high brightness (e.g. AMLCD's for avionics applications, computer monitors and HDTV LCD's). Much of this flexibility comes from the light valve nature of these devices, in that the imaging mechanism is decoupled from the light generation mechanism. While this is a tremendous advantage, it is often necessary to trade performance in certain categories such as luminance capability or light source power consumption in order to maximize image quality or affordability. This reduced optical efficiency can also lead to performance restrictions under high illumination due to heating or fading of the light-absorbing mechanisms commonly used in the displays.
In portable display applications such as backlit laptop computer monitors or other instrument displays, battery life is greatly influenced by the power requirements of the display backlight. Thus, functionality must be compromised to minimize size, weight and cost. Avionics displays and other high performance systems demand high luminance but yet place restrictions on power consumption due to thermal and reliability constraints. Projection displays are subject to extremely high illumination levels, and both heating and reliability must be managed. Head mounted displays utilizing polarized light valves are particularly sensitive to power requirements, as the temperature of the display and backlight must be maintained at acceptable levels.
Previously disclosed displays suffer from low efficiency, poor luminance uniformity, insufficient luminance and excessive power consumption which generates unacceptably high levels of heat in and around the display. Previously disclosed displays also exhibit a non-optimal environmental range due to dissipation of energy in temperature sensitive components. Backlight assemblies are often excessively large in order to improve the uniformity and efficiency of the system.
Several areas for efficiency improvement are readily identified. Considerable effort has gone into improving the efficiency of the light source (e.g. fluorescent lamps) and optimizing the reflectivity and light distribution of backlight cavities to provide a spatially uniform, high luminance light source behind the display. Pixel aperture ratios are made as high as the particular LCD approach and fabrication method will economically allow. Where color filters are used, these materials have been optimized to provide a compromise between efficiency and color gamut Reflective color filters have been proposed for returning unused spectral components to a backlight cavity.
When allowed by the display requirements, some improvement can also be obtained by constricting the range of illumination angles for the displays via directional techniques.
Even with this previous disclosure optimization, lamp power levels must be undesirably high to achieve the desired luminance. When fluorescent lamps are operated at sufficiently high power levels to provide a high degree of brightness for a cockpit environment, for example, the excess heat generated may damage the display. To avoid such damage, this excess heat must be dissipated. Typically, heat dissipation is accomplished by directing an air stream to impinge upon the components in the display. Unfortunately, the cockpit environment contains dirt and other impurities which are also carried into the display with the impinging air, if such forced air is even available. Presently available LCD displays cannot tolerate the influx of dirt and are soon too dim and dirty to operate effectively.
Another drawback of increasing the power to a fluorescent lamp is that the longevity of the lamp decreases dramatically as ever higher levels of surface luminance are demanded. The result is that aging is accelerated which may cause abrupt failure in short periods of time when operating limitations are exceeded.
Considerable emphasis has also been placed on optimizing the polarizers for these displays. By improving the pass-axis transmittance (approaching the theoretical limit of 50%), the power requirements have been reduced, but the majority of the available light is still absorbed, constraining the efficiency and leading to polarizer reliability issues in high throughput systems as well as potential image quality concerns.
A number of polarization schemes have been proposed for recapturing a portion of the otherwise lost light and reducing heating in projection display systems. These include the use of Brewster angle reflections, thin film polarizers, birefringent crystal polarizers and cholesteric circular polarizers. While somewhat effective, these previous disclosure approaches are very constrained in terms of illumination or viewing angle, with several having significant wavelength dependence as well. Many of these add considerable complexity, size or cost to the projection system, and are impractical on direct view displays. None of these previous disclosure solutions are readily applicable to high performance direct view systems requiring wide viewing angle performance.
Also taught in the previous disclosure (U.S. Pat. No. 4,688,897) is the replacement of the rear pixel electrode in an LCD with a wire grid polarizer for improving the effective resolution of twisted nematic reflective displays, although this reference falls short of applying the reflective polarizing element for polarization conversion and recapture. The advantages which can be gained by the approach, as embodied in the previous disclosure, are rather limited. It allows, in principle, the mirror in a reflective LCD to be placed between the LC material and the substrate, thus allowing the TN mode to be used in reflective mode with a minimum of parallax problems. While this approach has been proposed as a transflective configuration as well, using the wire grid polarizer instead of the partially-silvered mirror or comparable element, the previous disclosure does not provide
means for maintaining high contrast over normal lighting configurations for transflective displays. This is because the display contrast in the backlit mode is in the opposite sense of that for ambient lighting. As a result, there will be a sizable range of ambient lighting conditions in which the two sources of light will cancel each other and the display will be unreadable. A further disadvantage of the previous disclosure is that achieving a diff-usely reflective polarizer in this manner is not at all straightforward, and hence the reflective mode is most applicable to specular, projection type systems.
Disclosed in the previous disclosure (U.S. Pat. No. 2,604,817) and later in the previous disclosure(U.S. Pat. No. 5,999,239) is a means to produce a diffusely reflective polarizer utilizing polymeric fibers dispersed in a continuous polymer matrix. Typical monofilament birefringent fibers(ex, polyester) were demonstrated to create such a diffuse reflective polarizer in (U.S. Pat. No. 2,604,817). These fibers are embedded into an isotropic polymer matrix. The manufacturability and optical performance of such a reflective polarizer utilizing even the smallest typical monolithic birefringent fibers, however, is not sufficient enough to enable such a diffuse reflective polarizer to be cost effective. It is, therefore, an object of the present invention to improve the optical efficiency of polarized displays, especially direct view liquid crystal displays (LCDs) and to simplify manufacture and reduce the costs thereof.
It is a further object of the present invention to provide this efficiency increase while retaining wide viewing angle capability and minimize the introduction of chromatic shifts or spatial artifacts.
It is a further object of the present invention to reduce the absorption of light by polarized displays, minimizing heating of the displays and degradation of the display polarizers.
It is a further object of the present invention to provide an LCD having increased display brightness.
It is yet a further object of the present invention to reduce the power requirements for LCD backlight systems.
It is yet a further object of the present invention to improve display backlight uniformity without sacrificing performance in other areas.
It is still a further object of the present invention to achieve these objects by using a process that enables a cost-effective means to produce an efficient diffusely reflective polarizer for use in LCD backlight systems.
Cost-effectiveness is achieved by utilizing a unique island-in-the sea fiber design and a unique extrusion process to create a diffusely reflective polarizer.

SUMMARY OF THE INVENTION

The invention provides a fiber comprising a birefringent fibril discontinuous polymeric phase dispersed in a continuous polymeric phase wherein the refractive indices of the discontinuous and continuous phases in the X and Y directions are substantially matched to each other, the refractive indices of the discontinuous and continuous phases in the X and Y directions are substantially mismatched from the refractive index of the fibril discontinuous polymeric phase in the Z direction, and wherein the extrusion melting temperature of the continuous phase is less than the onset melting range of the discontinuous phase.
The invention also includes a process for making the fiber and an optical element employing the fiber. The invention enables one to improve the optical efficiency of polarized displays, especially direct view liquid crystal displays (LCDs) and to simplify manufacture and reduce the costs thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an end view of an island-in-the-sea fiber (10) with a continuous phase (20) and internal fibrils (discontinuous phase) 30.

FIG. 1B is a 3D view of an island-in-the-sea fiber 10 with the projection of the fibril 30 is the length direction with a continuous phase polymer (sea) 20 between the fibrils. FIG. 2 is a perspective view of the island-in-the-sea fiber 10 with fibril 30 and sea polymer 20. The sea polymer and fibrils comprise 3 dimensions of Refractive index. The fiber is stretched in the length (Z) direction and therefore there is an ordinary refractive index for the sea polymer and fibrils in the X and Y plane and an extraordinary index in the length (Z) direction as shown by symbol 40 & 41. The ordinary and extraordinary indices of the fibril may be different than the sea polymer indices.

FIG. 3A is a circular island-in-the-sea fiber 10 with elliptical fibrils 31.

FIG. 3B is a circular island-in-the-sea fiber 10 with circular fibrils 30

FIG. 3C is an elliptical island-in-the-sea fiber 11 with radial fibrils 30

FIG. 3D is an elliptical island-in-the-sea fiber 11 with mixed shapes and

size fibrils

30 and 31

FIG. 3E is a rectilinear shaped island-in-the-sea fiber 12 with elliptical shaped fibrils 31

FIG. 3F is a rectilinear shaped island-in-the-sea fiber 12 with random rectilinear shaped fibrils 32

FIG. 3G is a circular shaped island-in-the-sea fiber 10 with triangular shaped fibrils 33.

FIG. 4A is a section view of several island-in-the-sea fiber 10 entering the feed port of an extruder barrel 50 prior to melting the sea polymer.

FIG. 4B is a section view of the melt stream in a pipe exiting the extruder 60 of several fibrils 31 that have been dispersed into the melted sea polymer 20.

FIG. 5A is a cross machine cross section of the composite sheet (1) showing the fibrils (10) dispersed throughout.

FIG. 5B is a down machine cross section of the composite sheet (1) showing fibrils 31 dispersed and aligned throughout.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for producing a diffusely reflecting polarizer film made up of a composite of birefringent polymeric fibrils dispersed in an isotropic polymeric phase. The birefringent fibrils are created by producing multi-component island-in-the-sea fibers whereby the birefringent fibrils are islands in a sea of a continuous polymeric phase and wherein the refractive indices of the continuous phase in the X and Y directions (see FIG. 2) are substantially matched and wherein the extrusion melting temperature of the continuous phase is less than the onset melting range of the discontinuous phase. These fibers are then cut to short lengths and either solely extruded or extruded with additional resin pellets comprising either the same polymer as that of the continuous polymeric phase of the island-in-the sea fiber or a polymer with very similar optical and processing properties. The extrusion is done at a temperature sufficient to melt the continuous polymeric phase and additional resin pellets but not high enough to initiate melting of the birefringent fibrils. The fibrils are mixed and uniformly dispersed in the melted continuous polymeric phase. The melted mix is then pumped through a filming extrusion die with narrow enough die lands to produce high pressure and high shear forces on the fibrils thus orienting and aligning the fibrils in the machine direction. The extruded film is then cooled and a resulting diffusely reflecting polarizer film is formed.
Definitions:
The term fibril is defined as a material phase in a fiber that is discontinuous in the cross sectional plane of the fiber but either continuous in the fiber length direction or otherwise elongated to a dimension in the fiber length direction at least 100 times greater than the largest dimension in the cross section plane.
Extrusion melting temperature is defined here as a temperature at which the viscosity of the melted polymer is in a range that enables processing at reasonable pressures, and will be defined here as 100 degrees C. above the glass transition temperature of the polymer.
Onset melting temperature is defined here as the temperature near the melting point of the polymer at which thermal energy is first observed to be seen imparted to the birefringent polymer fibril during a standard differential scanning calorimeter measurement.
In order to make the fiber composite film of the present invention effective as a reflective polarizer it is desirable to create many small fibrils within a fiber such that many more optical interfaces can be created in a given thickness of film when dispersed by the process of the present invention into a composite film. Processes to create fibers with many small fibrils, also known as, island-in-the sea fiber making processes are well known in the trade. In particular the processes as described in U.S. Pat. Nos. 5,162,074 and 5,466,410 utilizing photo-etched plates to control flow of the different polymer melts in the multi-component fiber are very suitable. The cross sectional shape of the fibers can be of any geometry such as circular, rectilinear, elliptical, triangular, tri-lobal, or trapezoidal. Typically the fiber cross sectional shape will be circular or elliptical with the most common cross sectional shape being circular. Similarly, the cross sectional shape of the fibrils can be of any geometry such as circular, rectilinear, elliptical, triangular, tri-lobal, or trapezoidal. Again, typically the fibril cross sectional shape will be circular or elliptical with the most common cross sectional shape being circular.
It should be noted that polymeric surfactants also referred to as compatibilizers may be added to either one or both polymer of the discontinuous and continuous phases of the fibers. Typical materials may include blocked or grafted copolymers where segments of the copolymer matches that of either or both the discontinuous and or continuous phases in the polymeric fiber. The copolymers may be added in a weight ratio of 0.05 to 10 percent. This range may vary depending on the degree of substitution on the copolymer.
While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the designs and methods disclosed herein may be made without departing from the scope of the invention, which is defined in the appended claims.
Fiber description
Many items are made from synthetic fibers. Conventionally, two processes are used to manufacture synthetic fibers: a solution spinning process and a melt spinning process. The solution spinning process is generally used to form acrylic fibers, while the melt spinning process is generally used to form nylon fibers, polyester fibers, polypropylene fibers, and other similar type fibers. As is well known a polyester fiber comprises a long-chain synthetic polymer having at least 85 percent by weight of an ester of a substituted aromatic carboxylic acid unit.
The melt spinning process is of particular interest as since a large portion of the synthetic fibers that are used in the textile industry are manufactured by this technique and the process is ubiquitous at production scale. Also, since the present invention also requires unique down stream extrusion processing of the fibers to produce a composite film with oriented fibrils, melt spun fibers are desirable. The melt spinning process generally involves passing a molten polymeric material through a device that is known as a spinneret to thereby form a plurality of individual synthetic fibers. Once formed, the synthetic fibers are typically collected into a strand or cut into staple fibers. Synthetic fibers are typically used to make knitted, woven, or non-woven fabrics, or alternatively, synthetic fibers can be spun into a yarn to be used thereafter in a weaving or a knitting process to form a synthetic fabric. Multi-component fibrils have been well demonstrated in previous disclosures. Such fibers comprise two or more polymers and typically are designed to either split apart due to incompatibility of the polymers or one polymer is dissolved in solvent such that smaller fibrils of the other polymer are left. This method results in much smaller fibers or fibrils than can be traditionally produces via mono-component fiber processes and offers a wider range of final properties of the fiber-based article in which the fibers are used. The present invention relates to a multi-component fiber having both a birefringent polymeric fibril component as well as a continuous polymeric phase component with a melt processing temperature lower than the onset melting temperature of the birefringent fibril.
The birefringent fibrils in the island-in-the-sea fiber of the present invention can comprise any polymer in the general class of polyesters. Typical polyesters for such use can be polyethylene(terephlatate), polyethylene(naphthalate), or any copolymers of either. The most suitable polyester for the birefringent fibril is polyethylene(naphthalate).
The continuous polymeric phase in the island-in-the-sea fiber of the present invention can comprise any polymer in the general classes of polyesters, acrylics, or olefins. Typical polymers for such use can be polyethylene(terephlatate), poly(methyl-methacrylate), poly(cyclo-olefin), or any copolymers of either. The most suitable polymers for the continuous phase is poly(1,4-cyclohexylene dimethylene terephthalate) or poly(ethylene-terephthalate/isophthalate) copolymer.
As mentioned previously the extrusion melting temperature of the continuous polymeric phase of the fibers should be less than the onset melting temperature of the birefringent fibrils. Typically this difference will be greater than 10° C. but is preferred to be greater than 40° C. Most preferably the extrusion melting temperature of the continuous polymeric phase is greater than 75° C. below the onset melting temperature of the birefringent fibrils.
The island-in-the sea fibers of the present invention are cold drawn after being melt spun as is typical for such a fiber process. The cold draw is done with the fibers heated to just above the glass transition temperature (Tg) of the fibrils polymer. Typically the cold draw is done at 2 to 20° C. above Tg.
The amount of draw or draw ratio, which is the ratio by which the fiber is lengthened relative to its initial length, is important in attaining a high level of birefringence of the fibril. This is important as it creates a large difference in the Z direction (see FIG. 2) extraordinary index of the fibril and the eventual Z direction (see FIG. 5B) ordinary index of the continuous phase of the composite film. The continuous phase of the fiber is melt relaxed during film processing and therefore retains the ordinary index in the Z direction of the final composite film resulting in an isotropic continuous phase. The large difference in Z direction index of the fibril and the continuous phase is desired as it results in a high degree of reflection of light that passes through the film that is approaching the film orthogonal to the film surface and is linearly polarized parallel to the length of the fibril. The draw ratio should be greater than 2 to 1 and preferably greater than 3 to 1. Most preferably the draw ratio is greater than 3.5 to 1 to maximize the degree of crystallinity and thus birefringence of the fibrils.
The continuous polymeric phase may also become birefringent in the drawing process but this is not critical. Any birefringence of the continuous phase polymer will be eliminated during the subsequent extrusion process when making the composite polarizing film. Therefore drawing temperature is only critical for the continuous phase polymer to the degree that the polymer will stretch at the draw temperature without cracking and/or sticking to the draw rollers.
As mentioned previously, a large number of smaller fibrils in the fibers is preferable as this will ultimately result in many more optical interfaces in the final composite film reflective polarizer. The number of fibrils in the fiber is determined by the design of the spin pack. For a given spin pack design the size of the fibrils is then determined by the relative weight ratio of fibril polymer to continuous phase polymer when melt spinning. Typical weight ratios of fibril polymer to continuous phase polymer is less than 2 to 1 and preferably less than 0.8 to 1. Most preferably the weight ratio of fibril polymer to continuous phase polymer is less than 0.3 to one.
As previously mentioned the size, as measured by cross sectional area, of the fibrils is important because more smaller fibrils can be packed into a fiber lending to more optical interfaces. Typical cross sectional areas of fibrils is less than 3.0 square microns. Preferably the cross sectional area is less than 0.6 square microns and most suitably the cross sectional area of the fibrils is less than 0.2 square microns.
The number of fibrils that are in the fiber is important as discussed previously and is determined by the spin pack design. Typically the number of fibrils in a fiber is greater than 50. Preferably the number of fibrils is greater than 500. Fibers with greater than 1000 fibrils have been demonstrated and are most preferred.
The X and Y directions are orthogonal and in the plane of the cross section of the fibers and fibrils, see FIG. 2. The refractive indices in the X and Y directions of the fibrils (ordinary indices) and of the continuous polymeric phase of the fibers should be substantially matched (i.e., differ by less than about 0.05) in either the X or Y axes (see FIG. 2). The refractive indices in the X and Y directions of the fibrils (ordinary indices) and of the continuous polymeric phase of the fibers are substantially mismatched (i.e., differ by more than about 0.07) from the refractive index of fibrils in the Z axis (see FIG. 2). Preferably, the indices of refraction of the continuous and discontinuous phases differ by less than about 0.03 in the match direction, and most preferably, less than about 0.02. Preferably, the indices of refraction of the fibrils(ordinary indices) and of the continuous polymeric phase of the fibers differ from that of the index of refraction of the fibrils in the Z direction by more than 0.1, and most preferably by more than 0.2.
Filming Process Description
The first step to converting the fibers described previously into a diffusely reflective polarizing film is to cut the fibers into short lengths. This is important as fibers with shorter aspect ratios, defined as length of fibers divided by cross sectional area, can be dispersed in the melted continuous polymeric phase and be oriented through a shear force field much more readily. Typically the fibers are cut to a length less than 5 mm. Preferably the fibers are cut to a length less than 1 mm and most suitably the fibers are cut to a length of less than 0.4 mm.
Next the cut fibers are fed into the feed port of a typical single screw or twin screw extruder and processed at the extrusion melting temperature of the continuous polymeric phase of the fibers. The cross section of the fibers entering the feed port are illustrated in FIG. 4A. The fibrils will not melt at this temperature as the continuous polymer is chosen as to have an extrusion melting temperature below the onset melting temperature of the fibrils. Typically the extrusion melting temperature will be more than 10 C. lower than the onset melting temperature of the fibrils. Preferably the extrusion temperature will be more than 40 C. lower than the onset melting temperature of the fibrils. Most suitably the extrusion temperature will be more than 75 C lower than the onset melting temperature of the fibrils.
The fibrils are already wetted out buy the continuous polymeric phase by the nature of the fiber design. This results in very good dispersion quality of the fibrils in the molten continuous phase polymer as a result of further mixing in the extruder or subsequently to the extruder via any know melt mixing devices.
FIG. 4B illustrates the fibrils being dispersed in the molten mix via a cross section of a pipe exiting the extruder or downstream mixing device.
Next the dispersed mixture is subjected to high shear forces via pumping of the mixture through small die gaps in extrusion filming die. These high shear forces result in the fibrils being oriented parallel to each other with the length direction of the fibrils parallel to the flow direction through the die. This results in the composite film having the fibrils aligned parallel to each other and parallel to the machine direction of the film. The high shear forces are created by attaining melt pressures in the die greater than 1000 psi. Preferably the die pressures are greater than 2000 psi and most suitably the die pressures are greater than 3000 psi. These high shear forces result in the fibrils being aligned with the machine direction of the film such that the angle between the Z direction axis, see FIG. 2, and the machine direction of the film is less than 45 degrees. Preferably the angle between the Z direction axis and the machine direction of the film is less than 15 degrees and most suitably the angle between the Z direction axis and the machine direction of the film is less than 5 degrees.
The aligned fibrils are illustrated in FIGS. 5A and 5B showing the cross machine direction and down machine direction section views of the composite film.
The continuous phase of the dispersed mixture in the final composite film comprises at least the continuous polymeric phase of the fibers. Additionally the dispersed mixture can comprise any additional resin or polymer that is added to the fibers in the extrusion process. This added polymer must meet all of the requirements of the continuous phase polymer of the fiber and can comprise the same resins that have been described for the fiber continuous phase.
The indices of refraction of the continuous and discontinuous phases of the composite film are substantially matched (i.e., differ by less than about 0.05) along at least a first of three mutually orthogonal axes (X axis in FIG. 5A), and are substantially mismatched (i.e., differ by more than about 0.07) along a second of three mutually orthogonal axes (Z axis in FIG. 5B). Preferably, the indices of refraction of the continuous and discontinuous phases differ by less than about 0.03 in the match direction, and most preferably, less than about 0.02. The indices of refraction of the continuous and discontinuous phases preferably differ in the mismatch direction by at least about 0.07, more preferably, by at least about 0.1, and most preferably, by at least about 0.2.
The mismatch in refractive indices along a particular axis has the effect that incident light polarized along that axis will be substantially scattered, resulting in a significant amount of reflection. By contrast, incident light polarized along an axis in which the refractive indices are matched will be spectrally transmitted or reflected with a much lesser degree of scattering. This effect can be utilized to make a variety of optical devices, including reflective polarizers and mirrors.
Effect of Index Match/Mismatch
The magnitude of the index match or mismatch along a particular axis directly affects the degree of scattering of light polarized along that axis. In general, scattering power varies as the square of the index mismatch. Thus, the larger the index mismatch along a particular axis, the stronger the scattering of light polarized along that axis. Conversely, when the mismatch along a particular axis is small, light polarized along that axis is scattered to a lesser extent and is thereby transmitted specularly through the volume of the body.
Skin Layers
A layer of material which is substantially free of a discontinuous phase may be disposed on one or both major surfaces of the composite film, i.e., the extruded composite the discontinuous phase and the continuous phase. The composition of the layer, also called a skin layer, may be chosen, for example, to protect the integrity of the discontinuous phase within the extruded blend, to add mechanical or physical properties to the final film or to add optical functionality to the final film. Suitable materials of choice may include the material of the continuous phase or the material of the discontinuous phase.
A skin layer or layers may also add physical strength to the resulting composite or reduce problems during processing, such as, for example, reducing the tendency for the film to split during the orientation process. Skin layer materials which remain amorphous may tend to make films with a higher toughness, while skin layer materials which are semi-crystalline may tend to make films with a higher tensile modulus. Other functional components such as antistatic additives, UV absorbers, dyes, antioxidants, and pigments, may be added to the skin layer, provided they do not substantially interfere with the desired optical properties of the resulting product.
The skin layers may be applied to one or two sides of the extruded blend at some point during the extrusion process, i.e., before the extruded blend and skin layer(s) exit the extrusion die. This may be accomplished using conventional coextrusion technology, which may include using a three-layer coextrusion die. Lamination of skin layer(s) to a previously formed film of an extruded blend is also possible. Total skin layer thicknesses may range from about 2% to about 50% of the total film thickness.
A wide range of polymers are suitable for skin layers. Predominantly amorphous polymers include copolyesters based on one or more of terephthalic acid, 2,6-naphthalene dicarboxylic acid, isophthalic acid phthalic acid, or their alkyl ester counterparts, and alkylene diols, such as ethylene glycol. Examples of semicrystalline polymers are 2,6-polyethylene naphthalate, polyethylene terephthalate, and nylon materials.
Antireflection Layers
The films and other optical devices made in accordance with the invention may also include one or more anti-reflective layers. Such layers, which may or may not be polarization sensitive, serve to increase transmission and to reduce reflective glare. An anti-reflective layer may be imparted to the resulting film of the of the present invention through appropriate surface treatment, such as coating or sputter etching.
In some embodiments of the present invention, it is desired to maximize the transmission and/or minimize the specular reflection for certain polarizations of light. In these embodiments, the optical body may comprise two or more layers in which at least one layer comprises an anti-reflection system in close contact with a layer providing the continuous and discontinuous phases. Such an anti-reflection system acts to reduce the specular reflection of the incident light and to increase the amount of incident light that enters the portion of the body comprising the continuous and discontinuous layers. Such a function can be accomplished by a variety of means well known in the art. Examples are quarter wave anti-reflection layers, two or more layer anti-reflective stack, graded index layers, and graded density layers. Such antireflection functions can also be used on the transmitted light side of the body to increase transmitted light if desired.
More Than Two Phases
The composite films made in accordance with the present invention may also consist of more than two phases. Thus, for example, an optical material made in accordance with the present invention can consist of two different discontinuous phases within the continuous phase. The second discontinuous phase could be randomly or non-randomly dispersed throughout the fibrils or can be a separate discontinuous phase from the fibrils, and can be aligned along a common axis.
Composite films made in accordance with the present invention may also consist of more than one continuous phase. Thus, in some embodiments, the optical body may include, in addition to a first continuous phase and a discontinuous phase, a second phase which is co-continuous in at least one dimension with the first continuous phase
Multilayer Combinations
If desired, one or more sheets of a continuous/disperse phase film made in accordance with the present invention may be used in combination with, or as a component in, a multilayered film (i.e., to increase reflectivity). Suitable multilayered films include those of the type described in WO 95/17303 (Ouderkirk et al.). In such a construction, the individual sheets may be laminated or otherwise adhered together or may be spaced apart with the polymeric sheet of this invention. If the optical thicknesses of the phases within the sheets are substantially equal (that is, if the two sheets present a substantially equal and large number of scatterers to incident light along a given axis), the composite will reflect, at somewhat greater efficiency, substantially the same band width and spectral range of reflectivity (i.e., “band”) as the individual sheets. If the optical thicknesses of phases within the sheets are not substantially equal, the composite will reflect across a broader band width than the individual phases. A composite combining mirror sheets with polarizer sheets is useful for increasing total reflectance while still polarizing transmitted light.
Additives
The composite films of the present invention may also comprise other materials or additives as are known to the art. Such materials include pigments, dyes, binders, coatings, fillers, compatibilizers, antioxidants (including sterically hindered phenols), surfactants, antimicrobial agents, antistatic agents, flame retardants, foaming agents, lubricants, reinforcers, light stabilizers (including UV stabilizers or blockers), heat stabilizers, impact modifiers, plasticizers, viscosity modifiers, and other such materials. Furthermore, the films and other optical devices made in accordance with the present invention may include one or more outer layers which serve to protect the device from abrasion, impact, or other damage, or which enhance the processability or durability of the device.
Suitable lubricants for use in the present invention include calcium stearate, zinc stearate, copper stearate, cobalt stearate, molybdenum neodocanoate, and ruthenium (III) acetylacetonate.
Antioxidants useful in the present invention include 4,4′-thiobis-(6-t-butyl-m-cresol), 2,2′-methylenebis-(4-methyl-6-t-butyl-butylphenol), octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate, bis-(2,4-di-t- butylphenyl)pentaerythritol diphosphite, Irganox™ 1093 (1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)-dioctadecyl ester phosphonic acid), Irganox™ 1098 (N,N′-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamide), Naugaard™ 445 (aryl amine), Irganox™ L 57 (alkylated diphenylamine), Irganox™ L 115 (sulfur containing bisphenol), Irganox™ LO 6 (alkylated phenyl-delta-napthylamine), Ethanox 398 (flourophosphonite), and 2,2′-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite.
A group of antioxidants that are especially preferred are sterically hindered phenols, including butylated hydroxytoluene (BHT), Vitamin E (di-alphatocopherol), Irganox™ 1425WL(calcium bis-(O-ethyl(3,5-di-t-butyl-4hydroxybenzyl))phosphonate), Irganox™ 1010 (tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane), Irganox™ 1076 (octadecyl 3,5-di-tert-butyl-4-hydroxyhydrocinnamate), Ethanox™ 702 (hindered bis phenolic), Etanox 330 (high molecular weight hindered phenolic), and Ethanox™ 703 (hindered phenolic amine).
Dichroic dyes are a particularly useful additive in some applications to which the optical materials of the present invention may be directed, due to their ability to absorb light of a particular polarization when they are molecularly aligned within the material. When used in a film or other material which predominantly scatters only one polarization of light, the dichroic dye causes the material to absorb one polarization of light more than another. Suitable dichroic dyes for use in the present invention include Congo Red (sodium diphenyl-bis-oc-naphthylamine sulfonate), methylene blue, stilbene dye (Color Index (CI)=620), and 1,1′-diethyl-2,2′-cyanine chloride (CI=374 (orange) or CI=518 (blue)). The properties of these dyes, and methods of making them, are described in E. H. Land, Colloid Chemistry (1946). These dyes have noticeable dichroism in polyvinyl alcohol and a lesser dichroism in cellulose. A slight dichroism is observed with Congo Red in PEN.
Other suitable dyes include the following materials: [CHEM-1] The properties of these dyes, and methods of making them, are discussed in the Kirk Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661 (4th Ed. 1993), and in the references cited therein.
When a dichroic dye is used in the optical bodies of the present invention, it may be incorporated into either the continuous or discontinuous phase. However, it is preferred that the dichroic dye is incorporated into the discontinuous phase.
Dychroic dyes in combination with certain polymer systems exhibit the ability to polarize light to varying degrees. Polyvinyl alcohol and certain dichroic dyes may be used to make films with the ability to polarize light. Other polymers, such as polyethylene terephthalate or polyamides, such as nylon-6, do not exhibit as strong an s ability to polarize light when combined with a dichroic dye. The polyvinyl alcohol and dichroic dye combination is said to have a higher dichroism ratio than, for example, the same dye in other film forming polymer systems. A higher dichroism ratio indicates a higher ability to polarize light.
Molecular alignment of a dichroic dye within a composite film made in accordance with the present invention is preferably accomplished by stretching the composite film after the dye has been incorporated into it. However, other methods may also be used to achieve molecular alignment. Thus, in one method, the dichroic dye is crystallized, as through sublimation or by crystallization from solution, into a series of elongated notches that are cut, etched, or otherwise formed in the surface of a film., either before or after the composite film has been oriented. The treated surface may then be coated with one or more surface layers, may be incorporated into a polymer matrix or used in a multilayer structure, or may be utilized as a component of another optical body. The notches may be created in accordance with a predetermined pattern or diagram, and with a predetermined amount of spacing between the notches, so as to achieve desirable optical properties.
In a related embodiment, the dichroic dye may be disposed within one or more hollow fibers or other conduits, either before or after the hollow fibers or conduits are disposed within the composite film. The hollow fibers or conduits may be constructed out of a material that is the same or different from the surrounding material of the composite film.
In yet another embodiment, the dichroic dye is disposed along the layer interface of a multilayer construction, as by sublimation onto the surface of a layer before it is incorporated into the multilayer construction. In still other embodiments, the dichroic dye is used to at least partially backfill the voids in a microvoided film made in accordance with the present invention.
Functional Layers
Various functional layers or coatings may be added to the composite films of the present invention to alter or improve their physical or chemical properties, particularly along the surface of the film. Such layers or coatings may include, for example, slip agents, low adhesion backside materials, conductive layers, antistatic coatings or films, barrier layers, flame retardants, UV stabilizers, abrasion resistant materials, optical coatings, or substrates designed to improve the mechanical integrity or strength of the film or device.
The films of the present invention may be given good slip properties by treating them with low friction coatings or slip agents, such as polymer beads coated onto the surface. Alternately, the morphology of the surfaces of these materials may be modified, as through manipulation of extrusion conditions, to impart a slippery surface to the film; methods by which surface morphology may be so modified are described in U.S. Ser. No. 08/612,710.
In some applications, as where the composite film of the present invention are to be used as a component in adhesive tapes, it may be desirable to treat the films with low adhesion backsize (LAB) coatings or films such as those based on urethane, silicone or fluorocarbon chemistry. Films treated in this manner will exhibit proper release properties towards pressure sensitive adhesives (PSAs), thereby enabling them to be treated with adhesive and wound into rolls. Adhesive tapes made in this manner can be used for decorative purposes or in any application where a diffusely reflective or transmissive surface on the tape is desirable.
The films and optical devices of the present invention may also be provided with one or more conductive layers. Such conductive layers may comprise metals such as silver, gold, copper, aluminum, chromium, nickel, tin, and titanium, metal alloys such as silver alloys, stainless steel, and intone, and semiconductor metal oxides such as doped and undoped tin oxides, zinc oxide, and indium tin oxide (ITO).
The composite film of the present invention may also be provided with antistatic coatings or films. Such coatings or films include, for example, V 2 O 5 and salts of sulfonic acid polymers, carbon or other conductive metal layers.
The optical films and devices of the present invention may also be provided with one or more barrier films or coatings that alter the transmissive properties of the optical film towards certain liquids or gases. Thus, for example, the devices and films of the present invention may be provided with films or coatings that inhibit the transmission of water vapor, organic solvents, O 2, or CO 2 through the film. Barrier coatings will be particularly desirable in high humidity environments, where components of the film or device would be subject to distortion due to moisture permeation.
The composite films of the present invention may also be treated with flame retardants, particularly when used in environments, such as on airplanes, that are subject to strict fire codes. Suitable flame retardants include aluminum trihydrate, antimony trioxide, antimony pentoxide, and flame retarding organophosphate compounds.
The composite film of the present invention may also be provided with abrasion-resistant or hard coatings, which will frequently be applied as a skin layer. These include acrylic hardcoats such as Acryloid A-11 and Paraloid K-120N, available from Rohm & Haas, Philadelphia, Pa.; urethane acrylates, such as those described in U.S. Pat. No. 4,249,011 and those available from Sartomer Corp., Westchester, Pa.; and urethane hardcoats obtained from the reaction of an aliphatic polyisocyanate (e.g., Desmodur N-3300, available from Miles, Inc., Pittsburgh, Pa.) with a polyester (e.g., Tone Polyol 0305, available from Union Carbide, Houston, Tex.).
The composite film of the present invention may further be laminated to rigid or semi-rigid substrates, such as, for example, glass, metal, acrylic, polyester, and other polymer backings to provide structural rigidity, weatherability, or easier handling. For example, the composite film of the present invention may be laminated to a thin acrylic or metal backing so that it can be stamped or otherwise formed and maintained in a desired shape. For some applications, such as when the optical film is applied to other breakable backings, an additional layer comprising PET film or puncture-tear resistant film may be used.
The composite film and devices of the present invention may also be provided with shatter resistant films and coatings. Films and coatings suitable for this purpose are described, for example, in publications EP 592284 and EP 591055, and are available commercially from 3M Company, St Paul, Minn.
Various optical layers, materials, and devices may also be applied to, or used in conjunction with, the films of the present invention for specific applications. These include, but are not limited to, magnetic or magneto-optic coatings or films; liquid crystal panels, such as those used in display panels and privacy windows; photographic emulsions; fabrics; prismatic films, such as linear Fresnel lenses; brightness enhancement films; holographic films or images; embossable films; anti-tamper films or coatings; IR transparent film for low emissivity applications; release films or release coated paper; and polarizers or mirrors.
Multiple additional layers on one or both major surfaces of the composite film are contemplated, and can be any combination of aforementioned coatings or films. For example, when an adhesive is applied to the composite film, the adhesive may contain a white pigment such as titanium dioxide to increase the overall reflectivity, or it may be optically transparent to allow the reflectivity of the substrate to add to the reflectivity of the composite film.
In order to improve roll formation and convertibility of the film, the composite film of the present invention may also comprise a slip agent that is incorporated into the film or added as a separate coating. In most applications, slip agents will be added to only one side of the film, ideally the side facing the rigid substrate in order to minimize haze.
Thickness of Composite Film
The thickness of the composite film is also an important parameter which can be manipulated to affect reflection and transmission properties in the present invention. As the thickness of the composite film increases, diffuse reflection also increases, and transmission, both specular and diffuse, decreases. Thus, while the thickness of the composite film will typically be chosen to achieve a desired degree of mechanical strength in the finished product, it can also be used to directly to control reflection and transmission properties. Thickness can also be utilized to make final adjustments in reflection and transmission properties of the composite film. Thus, for example, in film applications, the device used to extrude the film can be controlled by a downstream optical device which measures transmission and reflection values in the extruded film, and which varies the thickness of the film (i.e., by adjusting extrusion rates or changing casting wheel speeds) so as to maintain the reflection and transmission values within a predetermined range.

EXAMPLE 1

Polyethylene(naphthalate), PEN (VFR-40102 from M&G Group) was first dried in a desicant dryer at 140 C for 12 hours. Also, an isophthalic acid modified Co-PET, Crystar® Merge 3991 by DuPont was dried in a desicant dryer at 55 C for 12 hours.
These polymers were then fed into two separate extruders and melt extruded at 300° C. and 270° C., respectively.
Island in the sea fibers were produced by feeding the two melt streams into a specially designed spinneret that created 1410 fibrils within each fiber. The Co-PET was fed as the continuous phase of the fiber and the PEN was fed as the discontinuous fibrils. 72 fibers were produced simultaneously by the spinner. The fibers were air cooled upon exiting the orifices of the spinneret and then heated and stretched 4 times their original length at a temperature of 120° C. The final diameter of the fibers was nominally 40 μm.
The fibers were wound on bobbins. The fibers were then unwound from the bobbins and fed into a cutter and cut to nominally 0.25 mm in length. The cut fibers were then dried in a desiccant dryer at 55° C. for 12 hours. Also, neat Co-PET 3991 pellets were dried in a desiccant dryer at 55° C. for 12 hours. The dried cut fibers were then dry blended with the dried Co-PET pellets at a 15 to 85 ratio, respectively. The blend was fed into a 19mm diameter twin screw extruder where it was melted, mixed and extruded. The melted extrudate was then pumped through an extrusion die with narrow die slots at an inlet pressure of 2500 psi. The fibrils within the melted resin substantially aligned in the machine direction due to high shear stresses in the flow. A 7″ wide film was extruded from the extrusion die and cooled on a chill roll at 65° C. The resulting film was 43 um thick.

EXAMPLE 2

This example was made identically to example 1 except the chill roll speed was slowed down to thicken the film to a thickness of 76 μm.

Optical Testing

The two samples from above were tested optically to determine if they performed as reflective polarizers. The test equipment used was a integrating sphere attached to a spectrophotometer (Perkin Elmer Lambda 650S). A reflectance is determined at a wavelength of 55 0nm. A light transmission value is also obtained at a wavelength of 550 nm. In order to determine the performance of the films as a reflective polarizer, polarized light was first directed onto the films with the polarization being perpendicular to the machine direction of the film. The percentage of light transmitted is measured as Tmax. The percentage of light reflected is measured as Rmin. Next, polarized light was directed onto the films with the polarization being parallel to the machine direction of the film. The percentage of light reflected is measured as Rmax. The percentage of light transmitted is measured as Tmin.
Table 1 shows the results of the above described optical testing on the example films. In order for the films to demonstrate performance as a reflective polarizer Tmax should be greater than Tmin and Rmax should be greater than Rmin. The larger these differences are the higher the performance of the film as a reflective polarizer.

TABLE 1

Example	Tmax	Tmin	Rmax	Rmin

1	86.1	72.9	25.1	13.5
2	80.1	66.5	29.9	18

It can be seen in Table 1 that indeed both samples show a degree of reflective polarization. It is expected that upon further optimization the performance could be significantly improved to even higher performance.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The entire contents of the patents and other publications referred to in this specification are incorporated herein by reference.

PARTS LIST

1—composite sheet
10—circular fiber
11—elliptical fiber
12—rectilinear fiber
20—continuous phase
30—fibrils/discontinuous phase
31—elliptical fibril
32—rectilinear fibril
33—triangular fibril
40–ordinary refractive indices in X and Y directions
41—extraordinary refractive index in Z direction
50—extruder barrel
60—extruder exit

Claims

1. A fiber comprising a birefringent fibril discontinuous polymeric phase dispersed in a continuous polymeric phase wherein the refractive indices of the discontinuous and continuous phases in the X and Y directions are substantially matched to each other, the refractive indices of the discontinuous and continuous phases in the X and Y directions are substantially mismatched from the refractive index of the fibril discontinuous polymeric phase in the Z direction, and wherein the extrusion melting temperature of the continuous phase is less than the onset melting range of the discontinuous phase.

2. The fiber of claim 1 wherein the cross-sectional shape of the fiber is circular, rectilinear, elliptical, triangular, tri-lobal, or trapezoidal.

3. The fiber of claim 1 wherein the cross-sectional shape of the fiber is circular or elliptical.

4. The fiber of claim 1 wherein the birefringent fibril discontinuous polymeric phase has a cross-sectional shape that is circular, rectilinear, elliptical, triangular, tri-lobal, or trapezoidal.

5. The fiber of claim 1 wherein the birefringent fibril discontinuous polymeric phase has a cross-sectional shape that is circular or elliptical.

6. The fiber of claim 1 wherein the birefringent fibril discontinuous polymeric phase comprises a polyester.

7. The fiber of claim 6 wherein the polyester comprises polyethylene(terephthalate) , polyethylene(naphthalate) , or any copolymers of either.

8. The fiber of claim 6 wherein the polyester comprises polyethylene(naphthalate).

9. The fiber of claim 1 wherein the continuous polymeric phase comprises a polyester, an acrylic, or an olefin.

10. The fiber of claim 1 wherein the continuous phase comprises polyethylene(terephthalate), poly(methyl-methacrylate), poly(cyclo-olefin), or any copolymers of either.

11. The fiber of claim 1 wherein the number of fibrils in the fiber is greater than 50.

12. The fiber of claim 1 wherein fibrils each have a cross sectional area of less than 3 square microns.

13. The fiber of claim 1 wherein the ratio of discontinuous phase to continuous phase on a weight basis is less than 2 to 1

14. The fiber of claim 1 wherein the fiber has been cold drawn at a temperature just above the Tg of the fibril polymer to achieve a high level of birefringence of the discontinuous phase.

15. The fiber of claim 14 wherein the fiber temperature is between 2 and 20° C.

16. The fiber of claim 14 wherein the fiber has been cold drawn at least 2 to 1.

17. A process for making a diffusely reflective polarizer film comprising the steps of:

a) providing fibers that comprise two components distributed as (1) a continuous phase and (2) a discontinuous phase in the form of fibrils, respectively, wherein at least the phase 2 is birefringent;

b) cutting the fibers to a desired length;

c) extrusion melting the cut fibers at a temperature sufficient to melt phase 1 but not melt phase 2;

d) extrusion mixing the melted phase 1 with the unmelted phase 2 at the same time as step c) or after step c) to uniformly disperse the phase 2 fibrils as a mixture within phase 1;

e) subjecting the dispersed mixture to shear forces whereby the fibrils of phase 2 become oriented within the mixture; and

f) forming a composite film from the mixture comprising the oriented phase 2 fibrils.

18. The process of claim 17 wherein the discontinuous phase in the form of fibrils has a cross-sectional shape that is circular, rectilinear, elliptical, triangular, tri-lobal, or trapezoidal.

19. The process of claim 17 wherein the discontinuous phase in the form of fibrils has a cross-sectional shape that is circular or elliptical.

20. The process of claim 17 wherein the discontinuous phase comprises a polyester.

21. The process of claim 20 wherein the polyester comprises polyethylene(naphthalate).

22. The process of claim 17 wherein the continuous phase 1 comprises a polyester, an acrylic, or an olefin.

23. The process of claim 17 wherein the continuous phase 1 comprises poly(1,4-cyclohexylene dimethylene terephthalate) or poly(ethylene-terephthalate/isophthalate) copolymer.

24. The process of claim 17 wherein the fibers that comprise two components have greater than 1000 phase 2 fibrils.

25. The process of claim 17 wherein the phase 2 fibrils each have a cross sectional area of less than 0.2 square microns.

26. The process of claim 17 wherein the refractive indices of the discontinuous and continuous phase in the X and Y directions of the fibers that comprise two components are substantially matched, the refractive indices of the discontinuous and continuous phase in the X and Y directions of the fibers that comprise two components are substantially mismatched from the refractive index of the discontinuous phase of the fibers in the Z direction and wherein the extrusion melting temperature of the continuous phase is less than the onset melting range of the discontinuous phase.

27. An optical element comprising a film containing a layer including continuous phase and discontinuous phase materials, wherein the discontinuous phase materials are cut fibrils and include a material having a different refractive index in the orthogonal X and Y directions in a plane perpendicular to the direction of light travel.

28. The optical element of claim 27 wherein the cut fibrils are cut to a length of less than 5 mm.

29. The optical element of claim 27 wherein the cut fibrils are cut to a length of less than 1 mm.

30. The optical element of claim 27 wherein the cut fibrils are cut to a length of less than 0.4 mm.

31. A display comprising the optical element of claim 27.

32. The display of claim 31 further comprises at least one function selected from the group consisting of image viewing screen, antireflection layer, ambient light suppression, color filter array, light valve, illumination enhancement, light collimation, light directing, light diffusion, stiffening, resistance to thermal expansion, light spreading, a light source, image algorithm, image storage, image buffer, optical brightener, IR reflection and a power source.