Multilayer reflection polarizer having a crystalline low refractive index layer

Abstract

Multilayer reflective polarizers are described. In particular, multilayer reflective polarizers that include both crystalline high and low refractive index layers are disclosed. These reflective polarizers may be particularly suitable for automotive, architectural, and industrial applications.

Description

Background Art

[0001] A multilayer reflective polarizer is generally an optical film formed of alternating polymer layers, and due to the difference in refractive index between the alternating polymer layers, one of the orthogonally polarized lights is substantially reflected and the other light is substantially transmitted, and the alternating polymer layers are oriented in an optical film. By designing the laminate and selecting the materials, the multilayer reflective polarizer can polarize light over a desired range of visible and infrared wavelengths.

Summary of the Invention

[0002] In one aspect, the present specification relates to a multilayer reflective polarizer. The multilayer reflective polarizer includes a plurality of alternating first polymer layers 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. For at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer is at least 0.04. For a second in-plane direction orthogonal to at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer is less than 0.04. The multilayer reflective polarizer has at least four edges, and for either the first polymer layer or the second polymer layer, the refractive index measured at 550 nm does not exceed 1.7.

Brief Description of the Drawings

[0003]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

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

[0005] Multilayer optical films have also been demonstrated by coextruding 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 polymer multilayer optical films, polymer materials are used almost exclusively or solely in the production of the individual layers. Such films are suitable for mass production processes and can be produced as large sheets and rolls.

[0006] Multilayer optical films include individual micro-layers having different refractive index characteristics, whereby some light is reflected at the interfaces between adjacent micro-layers. Since the micro-layers are sufficiently thin, the light reflected at multiple interfaces undergoes constructive or destructive interference, giving the multilayer optical film the desired reflection or transmission characteristics. In multilayer optical films designed to reflect ultraviolet, visible, or near-infrared wavelength light, each micro-layer generally has an optical thickness (physical thickness × refractive index) of less than about 1 μm. Thicker layers, such as protective boundary layers (PBLs) placed within the multilayer optical film to separate the skin layer on the outer surface of the multilayer optical film or the coherent groups of micro-layers that can interfere (hereinafter referred to as "packets") with each other, may be included.

[0007] For polarizing applications, such as reflective polarizers, at least a portion of the optical layer is formed using a birefringent polymer, and the refractive index of the polymer has different values along the axes of the orthogonal coordinate system of the polymer. Generally, the micro-layers of the birefringent polymer have axes of an orthogonal coordinate system defined by the normal to the layer plane (z-axis), with the x-axis and y-axis existing within the layer plane. Birefringent polymers can also be used in non-polarizing applications.

[0008] In some cases, the micro-layers have thicknesses and refractive index values corresponding to a quarter-wave stack, i.e., consist of an optical repeating unit or unit cell having two adjacent micro-layers each with equal optical thickness (f-ratio = 50%), and such an optical repeating unit is effective for reflection by constructive interference light where the wavelength λ is twice the total optical thickness of the optical repeating unit. Other layer configurations are also known, such as a multi-layer optical film having two types of micro-layer optical repeating units with an f-ratio different from 50%, or a film containing micro-layers with more than two types of optical repeating units. The design of these optical repeating units can be configured to reduce or increase specific higher-order reflections. See, for example, U.S. Patent Nos. 5,360,659 (Arends et al.) and 5,103,337 (Schrenk et al.). Using a thickness gradient along the thickness axis of the film (e.g., the z-axis), an extended reflection band can be provided that extends across the entire human visible region and into the near infrared, such that the micro-layer stack continues to reflect across the entire visible spectrum when the reflection band shifts to shorter wavelengths at an oblique angle of incidence. A thickness gradient adjusted to sharpen the band edges, i.e., the wavelength transition between high reflection and high transmission, is described in U.S. Patent No. 6,157,490 (Wheatley et al.).

[0009] Multilayer optical films, and further details of related designs and structures, are described in U.S. Patent No. 5,882,774 (Jonza et al.), U.S. Patent No. 6,531,230 (Weber et al.), PCT Publication No. 95 / 17303 (Ouderkirk et al.), PCT Publication No. 99 / 39224 (Ouderkirk et al.), and the publication titled "Giant Birefringent Optics in Multilayer Polymer Mirrors", Science, Vol. 287, March 2000 (Weber et al.). Multilayer optical films, and related articles, may include additional layers and coatings selected based on optical, mechanical, and / or chemical properties. For example, an ultraviolet absorbing layer can be added to the incident side of the film to protect components from degradation caused by ultraviolet light. The multilayer optical film can be attached to a mechanical reinforcement layer using an ultraviolet curable acrylate adhesive or other suitable material. Such a reinforcement layer may include a polymer such as PET or polycarbonate, and may also include a structured surface that provides an optical function such as light diffusion or collimation, for example, by using beads or prisms. Additional layers and coatings can also include a scratch resistant layer, a tear resistant layer, and a hardener. 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 reflection and transmission characteristics of the multilayer optical film are functions of the refractive index of each micro-layer, and the thickness and thickness distribution of the micro-layers. Each micro-layer has an in-plane refractive index n x , n y , and a refractive index n zcan be defined by these. These refractive indices respectively represent the refractive indices of the target material for light polarized along the x-axis, y-axis, and z-axis that are orthogonal to each other. To facilitate the description in this patent application, unless otherwise specified, the x-axis, y-axis, and z-axis are the axes of a local orthogonal coordinate system applicable to any target point on the multilayer optical film, the micro-layers extend parallel to the x-y plane, and the x-axis is where Δn x is oriented in the plane of the film so as to maximize the magnitude. Therefore, the magnitude of Δn y can be made not greater than the magnitude of Δn x . Further, in the calculation of the difference Δn x , the difference Δn y , and the difference Δn z , the choice of which material layer to start from is defined by requiring that Δn x is non-negative. In other words, the refractive index difference 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 selected such that n 1x ≧n 2x , that is, Δn x ≧0.

[0011] In practice, the refractive index is controlled by well-considered material selection and processing conditions. Conventional multilayer films are produced by co-extruding a large number of layers, for example, dozens or hundreds of layers of two alternating polymers A and B, and in some cases, this multilayer extrudate is then passed through one or more multiplication dies, and then the extrudate is stretched or oriented in another way to form the final film. The resulting film typically consists of a large number, that is, hundreds of individual micro-layers whose thickness and refractive index are adjusted to provide one or more reflection bands in a desired region of the spectrum such as visible or near-infrared. To obtain the desired reflectivity with a reasonable number of layers, adjacent micro-layers typically have a refractive index difference (Δn x) is exhibited. In some embodiments, the material is selected such that the refractive index difference for light polarized along the x-axis is as high as possible after alignment. When reflectivity is desired for two orthogonal polarizations, the adjacent micro-layers also exhibit a refractive index difference (Δn y ) of at least 0.04 for light polarized along the y-axis.

[0012] The above-referenced '774 patent (Jonza et al.) describes, among other things, a method of adjusting the refractive index difference (Δn z ) between adjacent micro-layers for light polarized along the z-axis to obtain the desired reflectivity characteristics for the p-polarized component of obliquely incident light. To maintain a high reflectivity of p-polarized light at an oblique angle of incidence, the z-refractive index mismatch Δn z between the micro-layers is controlled to be substantially smaller than the largest in-plane refractive index difference Δn x , such that Δn z ≦0.5×Δn x or Δn z ≦0.25×Δn x . A z-refractive index mismatch of zero or close to zero results in an interface between the micro-layers where the reflectivity for p-polarized light is constant or nearly constant as a function of the angle of incidence. Further, the z-refractive index mismatch Δn z can be controlled to have the opposite polarity compared to the in-plane refractive index difference Δn x , i.e., Δn z <0. This condition results in an interface where the reflectivity for p-polarized light increases with increasing angle of incidence, similar to the case of s-polarized light.

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

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

[0015] Reflection polarizers are often used in visual display systems such as liquid crystal displays. These systems, which are currently found in a variety of electronic devices including computers such as mobile phones, tablets, notebooks, and sub-notebooks, and some flat panel televisions, use a liquid crystal (LC) panel that is illuminated from behind using an extended area backlight. The reflection polarizer is placed on top of the backlight or incorporated into the backlight in another way to transmit light in a polarized state that can be used by the LC panel from the backlight to the LC panel. Light in an orthogonal polarization state that cannot be used by the LC panel is reflected back into the backlight, where the light is reflected and finally returned towards the LC panel, at least partially converted into a usable polarization state, "recycling" the light that would normally be lost and increasing the resulting brightness and overall efficiency of the display.

[0016] In certain embodiments, the multilayer reflection polarizer may be useful in automotive applications. For example, the multilayer reflection polarizer may be used at least in part on or near the front windshield of a vehicle. For safety reasons, this application is significantly different from conventional liquid crystal display applications as the driver still needs to be able to observe the road or the surrounding environment through the multilayer reflection polarizer. Additionally, it is necessary to prevent other drivers from being dazzled or having their vision impaired by bright reflections from the driver's front windshield. High-reflectivity (for one polarization state), high-performance conventional reflection 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, the reflective polarizer may be used with polyvinyl butyral (PVB), processed with polyvinyl butyral (PVB), or laminated to polyvinyl butyral (PVB) for the fracture resistance of safety glass. The components of PVB-based materials can penetrate and deteriorate reflective polarizers fabricated and designed in the conventional manner under the high-temperature processing used to form the laminated front glass component. As another example, polyethylene naphthalate (PEN), used as a polymer and / or copolymer in many commercially available reflective polarizers, particularly polyethylene naphthalate containing NDC (dimethyl-2,6-naphthalenedicarboxylate), turns yellow when exposed to ultraviolet light. The vehicle environment results in a large amount of exposure to solar radiation, which will deteriorate the reflective polarizer over time. In such ambient environments, spontaneous large-scale crystallization may occur, generating 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 refractive index of the reflective polarizers described herein, measured at 550 nm, does not exceed 1.7 in any layer and along any direction.

[0018] A multilayer optical film is typically formed from alternating layers of two different polymers. One layer is a layer that can generate birefringence when oriented. Since almost all polymers used in the formation of multilayer optical films have an increasing refractive index upon stretching, this layer is also typically known as a high refractive index layer (or "high refractive index optical system", i.e., HIO). The other layer of the alternating polymer layers is typically an isotropic layer having a refractive index below that of the high refractive index layer. For this reason, this layer is typically called a low refractive index layer (or "low refractive index optical system", i.e., LIO). Usually, the high refractive index layer is crystalline or semi-crystalline, and the low refractive index layer is amorphous. This is at least based on the idea that an amorphous material would be required to obtain a sufficiently high block axis reflectivity (based on the mismatch between the high refractive index layer and the low refractive index layer along a particular in-plane direction) and a sufficiently low pass axis reflectivity (based on the match between the high refractive index layer and the low refractive index layer along a second direction orthogonal to the in-plane direction).

[0019] Surprisingly, it has been found that a multilayer reflective polarizer having both a high refractive index layer and a low refractive index layer with a certain degree of crystallinity, which occurs during stretching due to the low stretching temperature of polyethylene terephthalate, is particularly suitable for these automotive applications. Additionally, surprisingly, it has been found that a multilayer reflective polarizer in which both a high refractive index optical system and a low refractive index optical system produce an asymmetric increase in refractive index by stretching can be useful in automotive applications. In some embodiments, each of the high refractive index layer and the low refractive index layer can produce or have an in-plane birefringence of at least 0.01. In some embodiments, along one in-plane direction, the difference between the high refractive index layer and the low refractive index layer can be at least 0.04, but along a second orthogonal in-plane direction, the difference can be less than 0.04. During a particular intermediate stretching step, a particular multilayer optical film can have similar birefringence characteristics, but these films are subsequently subjected to a heat set process to minimize birefringence in at least one of the layers (typically the low refractive index layer, or an isotropic layer) in order to maximize the block axis (stretching axis) reflectivity, which means that the final film (i.e., the film in roll form or the converted film having at least four edges) does not exhibit these characteristics.

[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 a 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 (again, polyethylene terephthalate in which cyclohexanedimethanol is used as a glycol modifier, but with the modifier doubled 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 layer described herein.

[0021] Reflection polarizers containing materials such as the exemplary sets above showed surprisingly better suppression of haze due to crystallization progressing gradually during processing rather than occurring spontaneously (with larger crystal sites) during exposure to radiation or heat after exposure to high temperatures. Furthermore, aesthetic and appearance issues such as fine wrinkles or delamination between layers appear to occur at significantly lower frequencies with the combinations of crystalline materials exemplified herein.

[0022] The shrinkage rate may be greater than that of conventional reflective polarizers, particularly along the maximum elongation direction. However, the amount of shrinkage can be controlled by a heat setting process, and in the automotive manufacturing and assembly process, a specific shrinkage rate is desired. For example, a reflective polarizer for automotive use may include or be laminated with a film for automotive windows, i.e., a film that reflects infrared light without substantially reflecting light within the visible spectrum. Films for automotive windows, such as those available from 3M Company, are typically alternating layers of PET and co-poly(methyl methacrylate) (PMMA). Since the shrinkage rates are similar between the two films, the lamination of the two films has a low tendency to wrinkle or warp after a temperature change. Reflective polarizers having crystallinity in both the high refractive index layer and the low refractive index layer also function well with respect to the chemical resistance and permeability (edge intrusion) of other materials.

[0023] The reflective polarizers described herein may also have an f-ratio greater than 0.5. In some embodiments, the f-ratio may 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. The shift at an f-ratio greater than 0.5 preferentially attenuates the primary reflection band over the higher-order reflection bands of the multilayer reflective polarizer, effectively reducing the reflectivity of the polarizer for the designed wavelength range. Similar optical effects are observed for f-ratios less than 0.5, for example, 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. When combined with the resulting lower birefringence (compared to PEN or coPEN) arising from the stretching of PET, these reflective polarizers require more layers to reach a sufficient level of reflectivity. Counterintuitively, this is a design feature. In weak reflective polarizers such as those described herein, variations in the caliper of the micro-layers can have an adverse and non-uniform effect on the entire spectrum of the film. By further weakening individual micro-layer pairs, the reflection bands of adjacent micro-layer pairs can be reinforced, and layers that overlap the reflection bands of adjacent micro-layer pairs can be added to the design. This results in a smoother spectrum and more consistent performance regardless of the 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 may have resistance to haze even after exposure to heat. In some embodiments, the reflective polarizer may have a haze of no more than 1% when measured after 100 hours of exposure to 85 °C, 95 °C, or even 105 °C. In some embodiments, the reflective polarizer 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 polarizer 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 not be affected even by short exposure to extreme heat, such as in an annealing process. In some embodiments, the transmission spectrum from 400 nm to 800 nm may decrease by only 10% or even only 5% after a 30-second annealing process at 232 °C (450 °F).

[0025] Reflective polarizers as described herein are useful for automotive applications, but may also be used or may be 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 superimposed over the wearer's field of view. Many advantages that may be suitable for a head-up display for automotive applications may also be desirable in these augmented reality applications. The reflective polarizers described herein are also useful in architectural applications (both exterior and interior windows) and industrial windows and eyewear applications. In some architectural applications, a reflective polarizer as described herein may be configured to prevent viewing a polarized display surface (such as a television or laptop computer) from outside the room.

[0026] When a multilayer reflective polarizer as described in this specification is combined with or laminated onto automotive glass, the multilayer reflective polarizer may be configured to reflect light polarized parallel to the road surface so as to reduce the perceived glare from the road surface, or to reflect light polarized perpendicular to the road surface so as to be visible through polarized sunglasses.

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

[0028] Next, the film of Example 1 was placed in a frame to restrict shrinkage and heat-treated in an oven at 450°F for 30 seconds. This heat treatment is presumed to result in annealing sufficient to remove residual crystallinity in the low refractive index layer. Therefore, by comparing the transmission spectra before and after exposure to this stress, a change in residual crystallinity in the low refractive index layer is expected to be shown. The transmittance in the p-polarized blocked state at 60 degrees before and after exposure to stress is shown in Figure 3.

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

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

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

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

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

[0034] The films of Examples 1 to 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 film transmittance values to the calculated transmittance values using a 4×4 Berriman optical stack code. In each example, significant birefringence exists in the LIO layer, meaning there is significant crystallinity.

[0035] Even though each of Examples 1 to 3 has a similar birefringent LIO layer, it is notable and surprising that in Example 3, the transmittance did not change after heat-set annealing at 450°F. In Examples 1 and 2, the change in transmittance from before to after annealing means a change in crystallinity.

[0036] [Table 1]

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

[0038] [Table 2]

[0039] Examples 7 - 9 Examples 1 to 6 were stretched by a conventional straight tenter process. Examples 7 to 9 were prepared under the same extrusion conditions as Examples 1 to 6, but using a parabolic tenter process as described in Invited Paper 45.1, authors Denker et al., title "Advanced Polarizer Film for Improved Performance of Liquid Crystal Displays", presented at the Society for Information Displays (SID) International Conference in San Francisco, Calif., Jun. 4 - 9, 2006, or stretched at the same temperature and draw ratio as described in U.S. Patent Application Publication No. 20070047080(A1) (Stover et al.).

[0040]

Table 3

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

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

[0043]

Table 4

[0044]

Table 5

[0045] The above embodiments have been described in detail for the purpose of facilitating the description of various aspects of the present invention. Therefore, the present invention should not be regarded as being limited to the specific examples and embodiments described above. Rather, the present invention is understood to encompass all aspects of the present invention, including various modifications, equivalent processes, and alternative devices included within the scope of the present invention as defined by the appended claims and their equivalents. Exemplary embodiments are shown below. [Item 1] A multilayer reflective polarizer including a plurality of alternating first polymer layers and second polymer layers, each of the first polymer layer and the second polymer layer having an in-plane birefringence of at least 0.01, for at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer being at least 0.04, for a second in-plane direction orthogonal to the at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer being less than 0.04, the multilayer reflective polarizer having at least four edges, a multilayer reflective polarizer in which, for either the first polymer layer or the second polymer layer, the refractive index measured at 550 nm does not exceed 1.7. [Item 2] 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, the multilayer reflective polarizer according to Item 1. [Item 3] The f-ratio is at least 0.65, the multilayer reflective polarizer according to Item 2. [Item 4] The f-ratio is at least 0.75, the multilayer reflective polarizer according to Item 2. [Item 5] The first polymer layer includes polyethylene terephthalate, and the second polymer layer includes glycol-modified co-(polyethylene terephthalate), the multilayer reflective polarizer according to Item 1. [Item 6] The second polymer layer further includes polycyclohexyl dimethyl terephthalate, the multilayer reflective polarizer according to Item 5. [Item 7] The second polymer layer further includes a second copolyester, the multilayer reflective polarizer according to Item 6. [Item 8] Each of the first polymer layer and the second polymer layer exhibits crystallinity, the multilayer reflective polarizer according to Item 1. [Item 9] An optical laminate including the multilayer reflective polarizer according to Item 1 and a mirror film laminated to the multilayer reflective polarizer, wherein the mirror film reflects less than 20% of visible light and at least 80% of light in the range of 900 nm to 1200 nm, the optical laminate. [Item 10] An automotive glass laminate comprising an automotive glass layer and the optical laminate according to item 9. [Item 11] The automotive glass laminate according to item 10, wherein the mirror film is disposed on the outside of the automotive glass layer, and the multilayer reflective polarizer is disposed on the inside of the automotive glass layer. [Item 12] A glass laminate comprising the multilayer reflective polarizer according to item 1 and a glass layer, wherein the multilayer reflective polarizer is laminated to the glass layer. [Item 13] The glass laminate according to item 12, 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 so as to be visible through polarized sunglasses. [Item 14] The glass laminate according to item 12, wherein the glass layer is an automotive glass layer, and the multilayer reflective polarizer is configured to reflect light polarized parallel to the road surface so as to reduce the perceived glare from the road surface. [Item 15] The glass laminate according to item 12, wherein the glass layer is an architectural glass layer. [Item 16] The glass laminate according to item 12, wherein the glass layer is an industrial glass layer. [Item 17] The multilayer reflective polarizer according to item 1, further comprising at least one non-optical layer, wherein the at least one non-optical layer is a skin layer or a protective boundary layer. [Item 18] The multilayer reflective polarizer according to item 17, wherein each of the at least one non-optical layer exhibits crystallinity. [Item 19] The multilayer reflective polarizer according to item 1, wherein the multilayer reflective polarizer does not include any non-optical protective boundary layers within the plurality of alternating first and second polymer layers. [Item 20] The multilayer reflective polarizer according to item 1, wherein the multilayer reflective polarizer does not include any non-optical skin layers adjacent to the plurality of alternating first and second polymer layers.

Claims

1. A multilayer reflective polarizer comprising a plurality of alternating first polymer layers and second polymer layers, each of the first polymer layer and the second polymer layer having an in-plane birefringence of at least 0.01, in each of the first polymer layer and the second polymer layer, the refractive index in the block axis direction being higher than the refractive index in the transmission axis direction, for at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer being at least 0.04, for a second in-plane direction orthogonal to the at least one in-plane direction, the difference in refractive index between each of the first polymer layer and the second polymer layer being less than 0.04, the multilayer reflective polarizer having at least four edges, a multilayer reflective polarizer in which the refractive index measured at 550 nm does not exceed 1.7 for either the first polymer layer or the second polymer layer.

2. 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, the multilayer reflective polarizer according to claim 1.

3. The f-ratio is at least 0.65, the multilayer reflective polarizer according to claim 2.

4. The f-ratio is at least 0.75, the multilayer reflective polarizer according to claim 2.

5. The first polymer layer contains polyethylene terephthalate, and the second polymer layer contains glycol-modified co-(polyethylene terephthalate), the multilayer reflective polarizer according to claim 1.

6. Each of the first polymer layer and the second polymer layer exhibits crystallinity, the multilayer reflective polarizer according to claim 1.

7. An optical laminate comprising the multilayer reflective polarizer according to claim 1 and a mirror film laminated to the multilayer reflective polarizer, wherein the mirror film reflects less than 20% of visible light and at least 80% of light in the range of 900 nm to 1200 nm.

8. An automotive glass laminate comprising an automotive glass layer and the optical laminate according to claim 7.

9. A glass laminate comprising the multilayer reflective polarizer according to claim 1 and a glass layer. The multilayer reflective polarizer is laminated to the glass layer, a glass laminate.

10. The glass layer is an automotive glass layer, and the multilayer reflective polarizer is configured to reflect light polarized perpendicular to the road surface so that it can be seen through polarized sunglasses. The glass laminate according to claim 9.

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

12. The multilayer reflective polarizer according to claim 1, further comprising at least one non-optical layer, wherein the at least one non-optical layer is a skin layer or a protective boundary layer.

13. The multilayer reflective polarizer according to claim 12, wherein each of the at least one non-optical layer exhibits crystallinity.

14. The multilayer reflective polarizer according to claim 1, does not include any non-optical protective boundary layer within the plurality of alternating first polymer layers and second polymer layers.

15. The multilayer reflective polarizer according to claim 1, does not include any non-optical skin layer adjacent to the plurality of alternating first polymer layers and second polymer layers.

Images