Multilayer reflective polarizer with crystalline low refractive index layer

Abstract

To describe a multilayer reflective polarizer, specifically, disclose a multilayer reflective polarizer including both a crystalline high-refractive index layer and low-refractive index layer.SOLUTION: A multilayer reflective polarizer includes a plurality of alternative first polymer layers and second polymer layers. Each of the first polymer layer and second polymer layer has in-plane birefringence of at least 0.01. A difference in a refractive index between each of the first polymer layer and the second polymer layer is at least 0.04 with respect to at least one in-plane direction. A difference in the refractive index between each of the first polymer layer and the second polymer layer is less than 0.04 with respect to a second in-plane direction orthogonal to the at least one in-plane direction. The multilayer reflective polarizer has at least four edges, in which a refractive index measured at 550 nm concerning any of the first polymer layer or the second polymer layer does not exceed 1.7.EFFECT: These reflective polarizers can be particularly suitable for automotive, architectural, and industrial applications.SELECTED DRAWING: Figure 1

Description

[Background technology]

[0001] Multilayer reflective polarizers are generally optical films formed of alternating polymer layers oriented such that differences in refractive index between the alternating polymer layers result in substantial reflection of one of the orthogonal polarizations and substantial transmission of the other. Depending on the design of the layer stack and the selection of materials, multilayer reflective polarizers can polarize light over a desired range of visible and infrared wavelengths. Summary of the Invention

[0002] In one aspect, the present disclosure relates to a multilayer reflective polarizer. The multilayer reflective polarizer includes a plurality of alternating first and second polymer layers. Each of the first and second polymer layers has an in-plane birefringence of at least 0.01. In at least one in-plane direction, the difference in refractive index between each of the first and second polymer layers is at least 0.04. In a second in-plane direction perpendicular to the at least one in-plane direction, the difference in refractive index between each of the first and second polymer layers is less than 0.04. The multilayer reflective polarizer has at least four edges, and the refractive index of neither the first nor second polymer layer exceeds 1.7, measured at 550 nm. [Brief explanation of the drawings]

[0003] [Figure 1] 1 is a graph of the layer thickness profile for Example 1. [Figure 2] 1 is a graph of the transmission spectrum for Example 1. [Figure 3] 1 is a graph of p-polarized light block state transmission at 60 degrees incidence before and after exposure to thermal stress for Example 1. [Figure 4] 1 is a graph of the layer thickness profile for Example 2. [Figure 5] 10 is a graph of the transmission spectrum for Example 2. [Figure 6]10 is a graph of p-polarized light block state transmission at 60 degrees incidence before and after exposure to thermal stress for Example 2. [Figure 7] 1 is a graph of the layer thickness profile for Example 3. [Figure 8] 10 is a graph of the transmission spectrum for Example 3. [Figure 9] 10 is a graph of p-polarized light block state transmission at 60 degrees incidence before and after exposure to thermal stress for Example 3. [Figure 10] 1 is a graph of the transmission spectrum for Example 4 between glass sheets. [Figure 11] 1 is a graph of the transmission spectrum for Example 5 between glass sheets. [Figure 12] 1 is a graph of the transmission spectrum of a double-glazed glass with an interlayer PVB. DETAILED DESCRIPTION OF THE INVENTION

[0004] Multilayer optical films, i.e., films that provide desired transmission and / or reflection properties at least in part through an arrangement of microlayers with different refractive indices, are known. It is known to fabricate such multilayer optical films by depositing a series of inorganic materials as optically thin layers ("microlayers") onto a substrate in a vacuum chamber. Inorganic multilayer optical films are described, for example, in textbooks such as "Thin-Film Optical Filters," 2nd Edition, by H.A. Macleod, Macmillan Publishing Co. (1986), and "Design of Optical Interference Filters," by A. Thelan, McGraw-Hill Inc. (1989).

[0005] Multilayer optical films have also been demonstrated by coextrusion of alternating polymer layers. See, for example, U.S. Patent Nos. 3,610,729 (Rogers), 4,446,305 (Rogers et al.), 4,540,623 (Im et al.), 5,448,404 (Schrenk et al.), and 5,882,774 (Jonza et al.). In these polymeric multilayer optical films, polymer materials are used predominantly or exclusively to fabricate the individual layers. Such films are compatible with mass production processes and can be fabricated as large sheets and rolls.

[0006] Multilayer optical films contain individual microlayers with different refractive index properties, causing some light to be reflected at interfaces between adjacent microlayers. The microlayers are thin enough that light reflected at multiple interfaces undergoes constructive or destructive interference, giving the multilayer optical film its desired reflective or transmissive properties. In multilayer optical films designed to reflect ultraviolet, visible, or near-infrared wavelength light, each microlayer typically has an optical thickness (physical thickness x refractive index) of less than about 1 μm. Thicker layers, such as skin layers on the outer surfaces of the multilayer optical film or protective boundary layers (PBLs) disposed within the multilayer optical film to separate coherent groups of microlayers (hereinafter referred to as "packets"), may also be included.

[0007] In polarizing applications, such as reflective polarizers, at least a portion of the optical layers are formed using birefringent polymers, where the refractive index of the polymer has different values ​​along the axes of the polymer's Cartesian coordinate system. Generally, birefringent polymer microlayers have Cartesian coordinate axes defined by the normal to the layer plane (z-axis), with the x- and y-axes lying within the layer plane. Birefringent polymers can also be used in non-polarizing applications.

[0008] In some cases, the microlayers are composed of optical repeat units or unit cells with thicknesses and refractive index values ​​corresponding to a quarter-wave stack, i.e., two adjacent microlayers each having equal optical thickness (f-ratio = 50%), and such optical repeat units are effective for constructive interference light reflection, where the wavelength λ is twice the total optical thickness of the optical repeat unit. Other layer configurations are also known, such as multilayer optical films with two-microlayer optical repeat units having f-ratios different from 50%, or films in which the optical repeat unit contains more than two microlayers. The design of these optical repeat units can be configured to reduce or enhance specific high-order reflections. See, for example, U.S. Pat. Nos. 5,360,659 (Arends et al.) and 5,103,337 (Schrenk et al.). A thickness gradient along the thickness axis (e.g., z-axis) of the film can be used to provide an extended reflection band, for example, across the human visible range and into the near infrared, so that the microlayer stack continues to reflect across the visible spectrum as the reflection band shifts to shorter wavelengths at oblique angles of incidence. Thickness gradients tailored to sharpen the band edges, i.e., the wavelength transition between high reflection and high transmission, are described in U.S. Patent No. 6,157,490 (Wheatley et al.).

[0009] Further details of multilayer optical films and related designs and structures are described in U.S. Pat. Nos. 5,882,774 (Jonza et al.) and 6,531,230 (Weber et al.), PCT Publication Nos. WO 95 / 17303 (Ouderkirk et al.) and WO 99 / 39224 (Ouderkirk et al.), and a publication entitled "Giant Birefringent Optics in Multilayer Polymer Mirrors," Science, Vol. 287, March 2000 (Weber et al.). Multilayer optical films and related articles may contain additional layers and coatings selected for their optical, mechanical, and / or chemical properties. For example, a UV-absorbing layer can be added to the incident side of the film to protect components from degradation caused by UV light. UV-curable acrylate adhesives or other suitable materials can be used to attach multilayer optical films to mechanical reinforcement layers. Such reinforcing layers may include polymers such as PET or polycarbonate and may also include structured surfaces that provide optical functionality such as light diffusion or collimation, for example, by using beads or prisms. Additional layers and coatings may also include scratch-resistant layers, tear-resistant layers, and hardeners. See, for example, U.S. Patent No. 6,368,699 (Gilbert et al.). Methods and apparatus for making multilayer optical films are described in U.S. Patent No. 6,783,349 (Neavin et al.).

[0010] The reflective and transmissive properties of a multilayer optical film are a function of the refractive index of each microlayer and the thickness and thickness distribution of the microlayers. Each microlayer has an in-plane refractive index, n x , n y , and the refractive index n associated with the thickness axis of the film zThese indices represent the refractive index of the subject material for light polarized along mutually orthogonal x-, y-, and z-axes, respectively. For ease of description in this patent application, unless otherwise specified, the x-, y-, and z-axes are axes of a local Cartesian coordinate system applicable to any point of interest on the multilayer optical film, with the microlayers extending parallel to the xy plane and the x-axis representing Δn x It is assumed that the molecules are oriented in the plane of the film so as to maximize the magnitude of Δn y The magnitude of Δn x Furthermore, the difference Δn x , difference Δn y , and the difference Δn z The choice of which material layer to start with in the calculation of Δn x In other words, the difference in refractive index between two layers forming an interface is Δn j =n 1j -n 2j where j=x, y, or z, and the layer designations 1, 2 are n 1x ≧n 2x , i.e., Δn x ≧0.

[0011] In practice, the refractive index is controlled by judicious material selection and processing conditions. Conventional multilayer films are made by coextruding many layers, e.g., tens or hundreds of layers, of two alternating polymers A and B, optionally followed by passing the multilayer extrudate through one or more multiplication dies, and then stretching or otherwise orienting the extrudate to form the final film. The resulting film is typically composed of many, i.e., hundreds, of individual microlayers whose thicknesses and refractive indices are tailored to provide one or more reflection bands in a desired region of the spectrum, such as the visible or near-infrared. To obtain the desired reflectivity with a reasonable number of layers, adjacent microlayers typically have a refractive index difference (Δn x) for light polarized along the x-axis. In some embodiments, the materials are selected so that the refractive index difference for light polarized along the x-axis is as high as possible after orientation. If reflectivity for two orthogonal polarizations is desired, adjacent microlayers also exhibit a refractive index difference (Δn y ) can also be displayed.

[0012] The above-referenced '774 (Jonza et al.) patent, among other things, focuses on the refractive index difference (Δn z ) to obtain the desired reflectivity characteristics for the p-polarized component of obliquely incident light. To maintain high reflectivity for p-polarized light at oblique incidence angles, the z-index mismatch Δn between the microlayers is z is the largest in-plane refractive index difference Δn x By controlling the Δn z ≦0.5×Δn x or Δn z ≦0.25×Δn x A z-index mismatch of zero or near-zero magnitude results in interfaces between microlayers whose reflectivity for p-polarized light is constant or nearly constant as a function of the angle of incidence. Furthermore, the z-index mismatch Δn z is the in-plane refractive index difference Δn x has the opposite polarity compared to Δn z <0. This condition results in an interface whose reflectivity for p-polarized light increases with increasing angle of incidence, just as it does for s-polarized light.

[0013] The '774 (Jonza et al.) patent also describes appropriate design considerations for multilayer optical films configured as polarizers, called multilayer reflective or reflective polarizers. For many applications, an ideal reflective polarizer has high reflectance along one axis (the "extinction" or "block" axis) and zero reflectance along the other axis (the "transmission" or "pass" axis). For purposes of this application, light whose polarization state is substantially aligned with the pass or transmission axis is referred to as passed light, and light whose polarization state is substantially aligned with the block or extinction axis is referred to as blocked light. Unless otherwise specified, passed light at a 60° angle of incidence is measured with p-polarized passed light. If any reflectance occurs along the transmission axis, the efficiency of the polarizer at off-normal angles will be reduced, and color may be introduced into the transmitted light if the reflectance for various wavelengths differs. Furthermore, in some multilayer systems, exact matching of the two y- and two z-indices is not possible, and when the z-axis indices are mismatched, it may be desirable to introduce a slight mismatch in the in-plane indices n1y and n2y. In particular, by configuring the y-indices mismatch to have the same sign as the z-indices mismatch, a Brewster effect occurs at the microlayer interfaces, minimizing off-axis reflectance along the transmission axis of the multilayer reflective polarizer, and therefore off-axis color.

[0014] Another design consideration discussed in '774 (Jonza et al.) relates to surface reflections at the air interfaces of multilayer reflective polarizers. Unless the polarizer is double-sided laminated to an existing glass component or another existing film with a clear optical adhesive, such surface reflections reduce the transmission of the desired polarization through the optical system. Thus, in some cases it may be useful to add an antireflection (AR) coating to the reflective polarizer.

[0015] Reflective polarizers are often used in visual display systems such as liquid crystal displays. Currently found in a variety of electronic devices, such as computers, including mobile phones, tablets, notebooks, and subnotebooks, as well as some flat-panel televisions, these systems use liquid crystal (LC) panels that are illuminated from behind with an extended-area backlight. The reflective polarizer is placed over or otherwise incorporated into the backlight to transmit light of a polarization state usable by the LC panel from the backlight to the LC panel. Light of an orthogonal polarization state that cannot be used by the LC panel is reflected back into the backlight, where it may ultimately be reflected back toward the LC panel and at least partially converted to a usable polarization state, "recycling" light that would otherwise be lost and increasing the resulting brightness and overall efficiency of the display.

[0016] In certain embodiments, multilayer reflective polarizers can be useful in automotive applications. For example, multilayer reflective polarizers may be used on or near at least a portion of a vehicle's windshield. This application differs significantly from traditional liquid crystal display applications because, for safety reasons, the driver must still be able to observe the road or surrounding environment through the multilayer reflective polarizer. Furthermore, bright reflections from the driver's windshield must not dazzle or impair the vision of other drivers. Highly reflective (for one polarization state), high-performance traditional reflective polarizers do not meet these requirements.

[0017] Furthermore, previously known reflective polarizers are sensitive to the processing and environmental exposure associated with automotive assembly and general use. For example, reflective polarizers may be used with, processed with, or laminated to polyvinyl butyral (PVB) for safety glass shatter resistance. Components of PVB-based materials can permeate and degrade conventionally fabricated and designed reflective polarizers under the high-temperature processing used to form laminated windshield components. As another example, polyethylene naphthalate (PEN), particularly polyethylene naphthalate containing NDC (dimethyl-2,6-naphthalenedicarboxylate), used as a polymer and / or copolymer in many commercially available reflective polarizers, yellows when exposed to ultraviolet light. The vehicle environment provides significant exposure to solar radiation, degrading reflective polarizers over time. Spontaneous large-scale crystallization can also occur in such ambient environments, causing haze within the reflective polarizer. In some embodiments, the reflective polarizers described herein do not contain polyethylene naphthalate. In some embodiments, the reflective polarizers described herein do not contain naphthalene-2,6-dicarboxylic acid. In some embodiments, the reflective polarizers described herein have a refractive index, measured at 550 nm, of no more than 1.7 in any layer and along any direction.

[0018] Multilayer optical films are typically formed from alternating layers of two different polymers. One layer is capable of generating birefringence when oriented. Because almost all polymers used to form multilayer optical films experience an increase in refractive index upon stretching, this layer is typically also known as a high refractive index layer (or "high refractive index optics," or HIO). The other layer of the alternating polymer layers is typically an isotropic layer with a refractive index equal to or lower than that of the high refractive index layer. For this reason, this layer is typically referred to as a low refractive index layer (or "low refractive index optics," or LIO). The high refractive index layer is usually crystalline or semi-crystalline, and the low refractive index layer is amorphous. This is based at least in part on the idea that an amorphous material would be necessary to obtain sufficiently high block axis reflectivity (based on mismatch between the high and low refractive index layers along a particular in-plane direction) and sufficiently low pass axis reflectivity (based on matching between the high and low refractive index layers along a second direction perpendicular to the in-plane direction).

[0019] It has now been surprisingly discovered that multilayer reflective polarizers having both high and low refractive index layers with some degree of crystallinity during stretching due to the low stretching temperature of polyethylene terephthalate are particularly suitable for these automotive applications. Additionally, it has been surprisingly discovered that multilayer reflective polarizers in which both the high and low refractive index optics undergo an asymmetric increase in refractive index upon stretching can be useful in automotive applications. In some embodiments, each of the high and low refractive index layers may exhibit or have an in-plane birefringence of at least 0.01. In some embodiments, the difference between the high and low refractive index layers along one in-plane direction may be at least 0.04, while the difference along a second, orthogonal in-plane direction may be less than 0.04. While certain multilayer optical films may have similar birefringence properties during certain intermediate stretching steps, these films subsequently undergo a heat-setting process to maximize block axis (stretch axis) reflectivity and minimize birefringence in at least one of the layers (typically the low refractive index layer, or the isotropic layer), meaning that the final film (i.e., the film in roll form or the converted film with at least four edges) did not exhibit these properties.

[0020] In some embodiments, the high refractive index layer is selected to be polyethylene terephthalate (PET) and the low refractive index layer is selected to be a copolyester of polyethylene terephthalate in which cyclohexanedimethanol is used as the glycol modifier (PETG, such as available from Eastman Chemicals, Knoxville, Tenn.). In some embodiments, the high refractive index layer is selected to be PET and the low refractive index layer is selected to be a 50:50 blend of PETG and PCTG (also polyethylene terephthalate in which cyclohexanedimethanol is used as the glycol modifier, but doubles the modifier in PETG, available from Eastman Chemicals, Knoxville, Tenn.). In some embodiments, the high refractive index layer is selected to be PET, and the low refractive index layer is selected to be a 33:33:33 blend of PETG, PCTG, and an "80:20" copolyester having 40 mol% terephthalic acid, 10 mol% isophthalic acid, 49.75 mol% ethylene glycol, and 0.25 mol% trimethylpropanol. Other copolyesters may be useful as or within the low refractive index layers described herein.

[0021] Reflective polarizers containing materials such as the exemplary set above surprisingly exhibit better suppression of haze after exposure to elevated temperatures, due to the fact that crystallization occurs gradually during processing, rather than spontaneously (with larger crystalline sites) during radiation or heat exposure. Furthermore, aesthetic and appearance issues such as microwrinkling or delamination appear to occur significantly less frequently with the crystalline material combinations exemplified herein.

[0022] The shrinkage may be greater than that of conventional reflective polarizers, especially along the maximum stretch direction. However, the amount of shrinkage can be controlled by the heat-setting process, and a specific shrinkage is desired in automotive manufacturing and assembly processes. For example, a reflective polarizer for automotive applications may include or be laminated with an automotive window film, i.e., a film that reflects infrared light without substantially reflecting light in the visible spectrum. Automotive window films, such as those available from 3M Company, are typically alternating layers of PET and co-poly(methyl methacrylate) (PMMA). Because the shrinkage rates between the two films are similar, a laminate of the two films is less likely to wrinkle or warp after temperature changes. Reflective polarizers with crystallinity in both the high and low refractive index layers also perform well in terms of chemical resistance and transparency (edge ​​penetration) of other materials.

[0023] The reflective polarizers described herein can also have an f-ratio greater than 0.5. In some embodiments, the f-ratio can be greater than 0.55, greater than 0.6, greater than 0.65, greater than 0.7, greater than 0.75, greater than 0.8, or even greater than 0.85. A shift in the f-ratio greater than 0.5 attenuates the first-order reflection band in favor of higher-order reflection bands in the multilayer reflective polarizer, effectively reducing the reflectivity of the polarizer over the designed wavelength range. Similar optical effects are observed for f-ratios less than 0.5, such as for f-ratios less than 0.45, less than 0.4, less than 0.35, less than 0.3, less than 0.25, less than 0.2, or even less than 0.15. Combined with the resulting lower birefringence (compared to PEN or coPEN) that arises from the stretching of PET, these reflective polarizers require more layers to reach sufficient levels of reflectivity. While counterintuitive, this is a design feature. In weakly reflective polarizers such as those described herein, variations in the caliper of the microlayers can have a detrimental and disproportionate effect on the overall spectrum of the film. By further weakening individual microlayer pairs, the reflection bands of adjacent microlayer pairs can be reinforced, and layers that overlap the reflection bands of adjacent microlayer pairs can be added to the design. This smooths the spectrum and allows for more consistent performance regardless of position on the film web or between rolls. The reflective polarizers described herein may have more than 100 layers, more than 150 layers, more than 200 layers, more than 250 layers, or even more than 300 layers.

[0024] The reflective polarizers described herein can be resistant to haze, even after heat exposure. In some embodiments, the reflective polarizers may not have a haze greater than 1% when measured after 100 hours of exposure to 85°C, 95°C, or even 105°C. In some embodiments, the reflective polarizers may have a haze of 2% or less after 100 hours of exposure to 105°C or even 120°C. In some embodiments, the reflective polarizers may have a haze of 3% or less or 3.5% or less after 100 hours of exposure to 120°C. In some embodiments, the transmission of these reflective polarizers may be unaffected by brief exposure to extreme heat, such as during an annealing process. In some embodiments, the transmission spectrum from 400 nm to 800 nm degrades by 10% or less, or even 5% or less, after a 30-second annealing process at 232°C (450°F).

[0025] Reflective polarizers such as those described herein are useful in automotive applications, but may also be used or suitable for certain polarizing beam splitter / view combiner applications. For example, in certain augmented reality displays or display devices, the generated and projected image may be overlaid on the wearer's field of view. Many advantages that may be suitable for head-up displays, for example, for automotive applications, may be similarly desirable in these augmented reality applications. Reflective polarizers described herein are also useful in architectural applications (both exterior and interior windows) and industrial window and eyewear applications. In some architectural applications, reflective polarizers such as those described herein may be configured to prevent polarized display surfaces (such as televisions or laptop computers) from being viewed from outside the room.

[0026] When a multilayer reflective polarizer as described herein is combined or laminated to an automotive glass, the multilayer reflective polarizer can be configured to reflect light polarized parallel to the road surface to reduce perceived glare from the road surface, or to reflect light polarized perpendicular to the road surface so that it can be seen with polarized sunglasses.

[0027] Example Example 1 A birefringent reflective polarizer was prepared as follows. Two polymers were used in the optical layers. The first polymer (first optical layer) was EASTAPAK PET 7352, available from Eastman Chemicals (Knoxville, TN). The second polymer (second optical layer) was polyethylene terephthalate glycol (PETG) GN071, also manufactured by Eastman Chemicals. The ratio of the feed rates of the first polymer to the second polymer was selected so that the optical layers had an f-ratio of 0.75. The polymer used for the skin layers was EASTAPAK PET 7352. The materials were fed from separate extruders into a multilayer coextrusion feedblock, where they were assembled into packets of 275 alternating optical layers, with thicker protective boundary layers of the first optical layers on both sides, for a total of 277 layers. Skin layers of second optical layer material were added to both sides of the structure in a manifold specifically designed for that purpose, resulting in a final structure with 279 layers. This multilayer melt was then cast through a film die onto a chill roll and quenched in the conventional manner for polyester film. The cast web was then stretched in an industrial-scale linear tenter in the stretching section at a draw ratio of approximately 6:1 and a temperature of 225°F. The heat-setting section had a temperature of 350°F. The layer thickness profile is shown in Figure 1. The layer profile, first and second polymeric materials, and selected process conditions resulted in the resulting transmittance spectra in the pass and block states shown in Figure 2. The resulting physical thickness of this film was approximately 29.2 μm, as measured with a capacitance gauge. The shrinkage measured at 302°F was 2.1% in the machine direction (MD) of the coextrusion unit and 1.9% in the transverse direction (TD) of the coextrusion unit. The shrinkage of the film was measured by heating a 1-inch by 9-inch piece of film to the desired temperature and measuring the shrinkage along the length of the sample after 15 minutes. The sample was under very slight tension, sufficient to keep the film flat during testing. In some end use applications, the film may have approximately the same shrinkage in the orthogonal directions.

[0028] The film of Example 1 was then placed in a frame to limit shrinkage and heat-treated in a 450°F oven for 30 seconds. This heat treatment is expected to provide sufficient annealing to remove any residual crystallinity in the low index layer. Therefore, comparing the transmission spectra before and after exposure to this stress is expected to indicate any changes in the residual crystallinity in the low index layer. The transmission in the p-polarized blocking state at 60° before and after exposure to stress is shown in Figure 3.

[0029] Example 2 A birefringent reflective polarizer was prepared as follows. Two polymers were used in the optical layers. The first polymer (first optical layers) was EASTAPAK PET 7352, available from Eastman Chemicals. The second polymer (second optical layers) was a 50:50 wt% blend of polyethylene terephthalate glycol (PETG) GN071, also from Eastman, and VM318D PCTg, also from Eastman. The ratio of the feed rates of the first polymer to the second polymer was selected so that the optical layers had an f-ratio of 0.65. The polymer used for the skin layers was EASTAPAK PET 7352. The materials were fed from separate extruders into a multilayer coextrusion feedblock, where they were assembled into a packet of 275 alternating optical layers, plus thicker protective boundary layers of the first optical layers on both sides, for a total of 277 layers. Skin layers of second optical layer material were added to both sides of the structure in a manifold specifically designed for that purpose, resulting in a final structure with 279 layers. This multilayer melt was then cast through a film die onto a chill roll and quenched in the conventional manner for polyester film. The cast web was then stretched in an industrial-scale linear tenter in the stretching section at a draw ratio of approximately 6:1 and a temperature of 225°F. The heat-setting section had a temperature of 350°F. The layer thickness profile is shown in Figure 4. The layer profile, first and second polymeric materials, and selected process conditions resulted in the resulting transmission spectra in the pass and block states shown below in Figure 5. The resulting physical thickness of this film was approximately 26.9 μm, as measured with a capacitance gauge. The shrinkage measured at 302°F was 2.3% in the MD and 2.4% in the TD. In some end-use applications, the film may have nearly identical shrinkage in the orthogonal directions.

[0030] As in Example 1, the film of Example 2 was framed to limit shrinkage and heat-treated in an oven at 450° F. for 30 seconds. The transmittance in the p-polarized light blocking state at 60° before and after heat treatment is shown in Figure 6.

[0031] Example 3 A birefringent reflective polarizer was prepared as follows. Two polymers were used in the optical layers. The first polymer (first optical layers) was EASTAPAK PET 7352 available from Eastman Chemicals. The second polymer (second optical layers) was a 33:33:33 blend of polyethylene terephthalate glycol (PETG) GN071 from Eastman, VM318D PCTG from Eastman Chemicals (Knoxville, Tenn.), and 80:20 CoPET. The 80:20 CoPET was pelletized from an amorphous copolyester containing the following molar ratios: 40 mol% terephthalic acid 10 mol% isophthalic acid 49.75 mole % ethylene glycol 0.25 mol% trimethylpropanol

[0032] The ratio of the feed rates of the first polymer to the second polymer was selected so that the optical layers had an f-ratio of 0.65. The polymer used for the skin layers was EASTAPAK PET 7352. The materials were fed from separate extruders into a multilayer coextrusion feedblock, where they were assembled into packets of 275 alternating optical layers, with thicker protective boundary layers of the first optical layers added on both sides for a total of 277 layers. Skin layers of second optical layer material were added to both sides of the structure in a manifold dedicated to that purpose, resulting in a final structure with 279 layers. This multilayer melt was then cast through a film die onto a chill roll and quenched in the conventional manner for polyester film. The cast web was then stretched in the stretching section of an industrial-scale linear tenter at a stretch ratio of approximately 6:1 and a temperature of 225°F. The heat-setting section had a temperature of 350°F. The layer thickness profile is shown in Figure 7. The layer profile, first and second polymer materials, and selected process conditions resulted in the resulting transmission spectra in the pass and block states shown in Figure 8. The resulting physical thickness of this film is approximately 28.2 μm as measured with a capacitance gauge.

[0033] The film of Example 3 was then placed in a frame to limit shrinkage and heat-treated in a 450°F oven for 30 seconds. The transmittance in the p-polarized blocking state at 60 degrees before and after thermal stress is shown in Figure 9. The lack of evidence of a shift after heat treatment for Example 3 indicates a negligible shift in the crystalline state of the low index layer, which appears to correlate with improved thermal robustness of the resulting multilayer film.

[0034] The films of Examples 1-3 were evaluated for the refractive index of each layer. The PET layer was measured directly on the outer film surface by Metricon. The refractive index of the LIO layer was calculated by matching the measured transmittance of the film to the calculated transmittance from a 4x4 Berriman optical stack code. In each example, significant birefringence was present in the LIO layer, indicating significant crystallinity.

[0035] It is notable and surprising that, even though Examples 1-3 each have a similar birefringent LIO layer, Example 3 showed no change in transmittance after heat-set annealing at 450° F. In Examples 1-2, the change in transmittance from before to after annealing indicates a change in crystallinity.

[0036] [Table 1]

[0037] Examples 4 to 6 Examples 4 to 6 were produced by the same process as Examples 1 to 3, but with the following differences.

[0038] [Table 2]

[0039] Examples 7 to 9 Examples 1-6 were stretched using a conventional linear tenter process. Examples 7-9 were made using the same extrusion conditions as Examples 1-6, except that they were stretched using a parabolic tenter process as described in Invited Paper 45.1, by Denker et al., entitled "Advanced Polarizer Film for Improved Performance of Liquid Crystal Displays," presented at the Society for Information Displays (SID) International Conference in San Francisco, Calif., June 4-9, 2006, or at temperatures and stretch ratios similar to those described in U.S. Patent Application Publication No. 20070047080(A1) (Stover et al.).

[0040] [Table 3]

[0041] The films of Examples 1-9 were then laminated between 1 / 8 inch thick soda-lime glass sheets using the PVB layer as an adhesive. The transmission spectra of this glass-wrapped construction for Examples 4 and 5 are shown in Figures 10 and 11, respectively. Figure 12 shows a comparable spectrum for a glass laminate using only a PVB layer between the glass sheets.

[0042] High temperature test The example films were aged in an oven at elevated temperatures of 85° C., 95° C., and 100° C. Haze was measured after 100 and 1000 hours and compared to films aged at room temperature (RT); the results are shown in Tables 4 and 5, respectively. Comparing similar materials, films with higher crystalline content showed less increase in haze with aging due to thermal exposure.

[0043] [Table 4]

[0044] [Table 5]

[0045] Because the above-described embodiments have been described in detail to facilitate explanation of various aspects of the invention, the present invention should not be deemed limited to the particular examples and embodiments described above. Rather, the present invention should be understood to encompass all aspects of the invention, including various modifications, equivalent processes, and alternative devices, included within the scope of the invention as defined by the appended claims and their equivalents.

Claims

1. 1. A multilayer reflective polarizer comprising a plurality of alternating first and second polymer layers, each of the first polymer layer and the second polymer layer has an in-plane birefringence of at least 0.01; In each of the first polymer layer and the second polymer layer, a refractive index in a block axis direction is higher than a refractive index in a transmission axis direction; each of the first polymer layer and the second polymer layer exhibits crystallinity; a difference in refractive index between each of the first polymer layer and the second polymer layer for at least one in-plane direction is at least 0.04; a difference in refractive index between each of the first polymer layer and the second polymer layer in a second in-plane direction orthogonal to the at least one in-plane direction is less than 0.04; the multilayer reflective polarizer has at least four edges; the refractive index measured at 550 nm for either the first polymer layer or the second polymer layer does not exceed 1.7; The multilayer reflective polarizer, wherein the first polymer layer comprises polyethylene terephthalate and the second polymer layer comprises glycol-modified co(polyethylene terephthalate).

2. 10. The multilayer reflective polarizer of claim 1, wherein the f-ratio of the multilayer reflective polarizer, defined as the ratio of the average optical thickness of the first polymer layer to the total optical thickness of both the first polymer layer and the second polymer layer, is at least 0.

55.

3. 3. The multilayer reflective polarizer of claim 2, wherein the f-ratio is at least 0.

65.

4. 3. The multilayer reflective polarizer of claim 2, wherein the f-ratio is at least 0.

75.

5. The multilayer reflective polarizer of claim 1; a mirror film laminated to the multilayer reflective polarizer, The optical laminate wherein the mirror film reflects less than 20% of visible light and reflects at least 80% of light between 900 nm and 1200 nm.

6. A cover glass laminate comprising a cover glass layer and the optical laminate of claim 5.

7. The multilayer reflective polarizer of claim 1; a glass layer; and a glass laminate comprising: The multilayer reflective polarizer is laminated to the glass layer.

8. 8. The glass laminate of claim 7, wherein the glass layer is an automotive glass layer and the multilayer reflective polarizer is configured to reflect light polarized perpendicular to the road surface, such as can be seen with polarized sunglasses.

9. 8. The glass laminate of claim 7, wherein the glass layer is an automotive glass layer and the multilayer reflective polarizer is configured to reflect light polarized parallel to a road surface so as to reduce perceived glare from the road surface.

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