Reflective polarizer and method of shaping same

Abstract

A reflective polarizer includes a plurality of alternating first and second layers. For a substantially normally incident light, the plurality of first and second layers has an average reflectance Rs0 of at least 50% when the incident light is polarized along a first direction and an average reflectance Rp0 of greater than 1% and at most 40% when the incident light is polarized along an orthogonal second direction. When the reflective polarizer is heated to a temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second, a birefringence of the second layers, but not the first layers, is reduced by at least 20% and / or Rp0 decreases by at least a factor of 1.3 while Rs0 remains at least 50%. The reflective polarizer can be shaped via thermoforming.

Description

[0001] REFLECTIVE POLARIZER AND METHOD OF SHAPING SAME

[0002] TECHNICAL FIELD

[0003] The present description relates generally to reflective polarizers capable of being shaped via thermoforming.

[0004] BACKGROUND

[0005] A reflective polarizer can include alternating first and second layers and can have a higher reflectance for one polarization state than for an orthogonal polarization state. A polymeric reflective polarizer can be thermoformed into a curved shape.

[0006] SUMMARY

[0007] In some aspects, the present description provides a reflective polarizer including a plurality of alternating polymeric first and second layers numbering at least 10 in total. Each first and second layer of the plurality of alternating polymeric first and second layers can have an average thickness of less than about 500 nm. The first layers have indices of refraction nix and nly in respective orthogonal first and second in-plane directions of the reflective polarizer. The second layers have indices of refraction n2x and n2y in the respective first and second in-plane directions. In-plane first and second birefringences of the first and second layers are nix - nly and |n2x - n2y|, respectively. For at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007; nix - n2x > 0.05; and 0.005 < |nly - n2y| < nix - n2x, such that for a substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance of at least 50% when the incident light is polarized along the first in-plane direction and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along the second in-plane direction. When the reflective polarizer is heated to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the second birefringence, but not the first birefringence, is reduced by at least 20%; |nly - n2y| is reduced by at least 20%; and nix - n2x remains greater than 0.05.

[0008] In some aspects, the present description provides a reflective polarizer including a plurality of alternating polymeric first and second layers numbering at least 10 in total. Each first and second layer of the plurality of alternating polymeric first and second layers can have an average thickness of less than about 500 nm. The first and second layers having respective in-plane first and second birefringences. For at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007, such that for a substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance RsO of at least 50% when the incident light is polarized along a first in-plane direction of the reflective polarizer and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along an orthogonal second in-plane direction of the reflective polarizer. When the reflective polarizer is heated to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second, RpO decreases by at least a factor of 1.3, and RsO remains at least 50%.

[0009] In some aspects, the present description provides a method of shaping a reflective polarizer that includes a plurality of alternating polymeric first and second layers having respective in-plane first and second birefringences. The method includes thermoforming the reflective polarizer into a shape curved about two orthogonal axes. The thermoforming includes heating the reflective polarizer to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time greater than about 1 second. Prior to thermoforming, for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007; and for a substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance RsO of at least 50% when the incident light is polarized along a first inplane direction of the reflective polarizer and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along an orthogonal second in-plane direction of the reflective polarizer. After thermoforming, for the substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance Rs 1 when the incident light is polarized along the first in-plane direction of the reflective polarizer and an average reflectance Rp 1 when the incident light is polarized along the second in-plane direction of the reflective polarizer. Rpl / Rsl is less than 0.75 times RpO / RsO.

[0010] These and other aspects will be apparent from the following detailed description. In no event, however, should this brief summary be construed to limit the claimable subject matter.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic cross-sectional view of a reflective polarizer, according to some embodiments.

[0013] FIG. 2 is a schematic plot of reflectance versus wavelength for a plurality of alternating polymeric first and second layers of a reflective polarizer, according to some embodiments.

[0014] FIGS. 3-4 are plots of pass-state reflectance versus wavelength of alternating polymeric first and second layers of exemplary reflective polarizers.

[0015] FIGS. 5-7 are plots of in-plane birefringence of low index layers of various bilayer films versus heat set temperature for various stretch conditions and heat set times, according to some embodiments. FIG. 8 is a plot of in-plane birefringence of low index layers of various bilayer films versus heat set time for a heat set temperature of 140 deg. C, according to some embodiments.

[0016] FIG. 9 is a schematic illustration of thermoforming a reflective polarizer, according to some embodiments.

[0017] DETAILED DESCRIPTION

[0018] In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

[0019] As is known in the art, multilayer optical films including alternating polymeric layers can be used to provide desired reflection and transmission in desired wavelength ranges by suitable selection of layer thicknesses and refractive index differences. Multilayer optical films and methods of making multilayer optical films are described in U.S. Pat. Nos. 5,882,774 (Jonza et al.); 6,783,349 (Neavin et al.); 6,949,212 (Merrill et al.); 6,967,778 (Wheatley et al.); 9,162,406 (Neavin et al.); and 11,493,677 (Haag et al.), for example.

[0020] An optical system can utilize a reflective polarizer and a partial reflector with a retarder therebetween to provide a folded optical path as generally described in U.S. Pat. No. 10,678,052 (Ouderkirk, et al.), for example. In such optical systems, it can be desired to minimize pass state reflectance of the reflective polarizer. It has been found that even a small pass state reflectance can result in undesired ghost images, for example. The reflective polarizer used in such optical systems often have a curved shape. Polymeric reflective polarizers can be thermoformed into curved shapes as generally described in U.S. Pat. Nos. 9,599,761 (Ambur et al.) and 11,543,572 (Jennings et al.), for example. Such reflective polarizers typically include alternating birefringent (typically higher average refractive index) first layers and isotropic (typically lower average refractive index) second layers where the refractive indices of the birefringent and isotropic layers are substantially matched along a pass axis of the reflective polarizer and substantially mismatched along a block axis of the reflective polarizer. The indices are typically matched along the pass axis to minimize the pass state reflectance. However, it has been found that the refractive indices (e.g., of the birefringent first layers) can shift during thermoforming resulting in an undesirably high pass state reflectance for the thermoformed reflective polarizer even when the reflective polarizer had desirably low pass state reflectance prior to thermoforming. It has been found that even a small shift in the birefringence of the first layers can result in an undesirably high pass state reflectance.

[0021] It has been found, according to some embodiments, that a low pass state reflectance of a postthermoformed reflective polarizer can be achieved by deliberately mismatching the indices along the pass axis in the un-thermoformed reflective polarizer such that the mismatch is reduced upon thermoforming. This is achieved, according to some embodiments, by utilizing lower index second layers that are initially somewhat birefringent (e.g., a birefringence greater than 0.007 but less than the birefringence of the first layers) such that this initial birefringence is substantially reduced upon thermoforming (e.g., the birefringence relaxes or “melts out” under typical thermoforming conditions) resulting in substantial index matching along the pass-axis in the thermoformed reflective polarizer. Conventionally, the isotropic layers of reflective polarizers are isotropic due to being made from polymers that do not become birefringent under typical reflective polarizer stretching conditions. Such layers can be modified to become somewhat birefringent by mixing in other polymers that can become birefringent. For example, the birefringent first layers can be formed from polyethylene naphthalate (PEN) or a copolymer thereof (coPEN) and the second layers can be formed from a (e.g., copolymerized) blend of conventional low index polymers, such as glycol-modified polyethylene terephthalate (PETG), and PEN or coPEN. In some embodiments, the (e.g., copolymerized) blend includes a copolyester having isosorbide groups as such copolyesters have been found to aid in achieving the desired pass-state index match in the reflective polarizer after thermoforming.

[0022] FIG. 1 is a schematic cross-sectional view of a reflective polarizer 100, according to some embodiments. The reflective polarizer 100 includes a plurality of alternating polymeric first and second layers 10 and 11. In some embodiments, the plurality of alternating polymeric first and second layers 10, 11 number at least 10, 20, 30, 40, 50, 80, 100, 150, 200, 300, 400, 500, 600, or 650 in total. The total number of layers in the plurality of alternating polymeric first and second layers 10, 11 can be up to 10000, 5000, 2000, 1000, or 800, for example. Each of the first and second layers in the plurality of alternating polymeric first and second microlayers 10, 11 can have an average thickness (mean over an area of the layer of the layer thickness) less than about 500, 450, 400, 350, 300, 250, or 200 nm, for example. The average thicknesses can be at least about 20, 30, 40, 50, or 60 nm, for example. The layers 10, 11 may be described as interference layers, optical layers, or microlayers. Such layers may be described as reflecting or transmitting light primarily by optical interference when the reflectance and transmittance of the interference layers can be reasonably described by optical interference or reasonably accurately modeled as resulting from optical interference. The first layers 10 can have a higher average (e.g., average over two orthogonal in-plane directions or average over three mutually orthogonal directions) refractive index than the second layers 11. Accordingly, the first layer 10 may be referred to as high index optical (HIO) layers and the second layers 11 may be referred to as low index optical (LIO) layers.

[0023] In some embodiments, the plurality of alternating polymeric first and second layers 10, 11 is disposed on at least one macrolayer 124, 126. A macrolayer is generally a layer too thick to reflect or transmit light in a wavelength range of interest primarily by optical interference. In the illustrated embodiment, the plurality of alternating polymeric first and second layers 10, 11 is disposed between first and second macrolayers 124 and 126 (e.g., skin layers). The reflective polarizer 100 may optionally include other macrolayers (e.g., protective boundary layers separating first and second packets of the alternating first and second layers 10, 11) as would be appreciated by those of ordinary skill in the art. In some embodiments, each macrolayer of the at least one macrolayer 124, 126 has an average thickness of greater than about 600, 700, 800, 900, 1000, 1250, 1500, 1750, or 2000 nm. The average thickness of each of the macrolayers can be up to about 150, 100, 50, 30, 20, or 10 microns, for example.

[0024] In some embodiments, a reflective polarizer 100 includes a plurality of alternating polymeric first and second layers 10 and 11, where the first layers have indices of refraction nix and nly in respective orthogonal first and second in-plane directions (e.g., x- and y-directions) of the reflective polarizer, and where the second layers have indices of refraction n2x and n2y in the respective first and second in-plane directions. In-plane directions are directions in the plane of the reflective polarizer (e.g., the x-y plane of FIG. 1 or a tangent plane in the case of a curved reflective polarizer). The first and second layers can be characterized by in-plane birefringences of the layers. In-plane birefringence of a layer can be described as the difference in refractive index along principal directions in the plane of the layer. In-plane first and second birefringences of the first and second layers are nix - nly and |n2x - n2y|, respectively. The x- axis can be taken along the higher index direction of the first layers so that nix > nly. n2x - n2y may be positive or negative. In some embodiments, n2x > n2y. In some embodiments, for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007, 0.008, 0.009, 0.01, 0.012, 0.014, 0.016, 0.018, 0.02. In some such embodiments, or in other embodiments, for the at least one wavelength in the visible wavelength range, nix - n2x > 0.05, 0.07, 0.09, 0.11, 0.13, 0.15, 0.17, 0.19, or 0.2. In some such embodiments, or in other embodiments, for the at least one wavelength in the visible wavelength range, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, 0.012, 0.014, 0.016, 0.018, or 0.02 < |nly - n2y| < nix - n2x. In some such embodiments, or in other embodiments, the first birefringence, nix - nly, is greater than 0.05, 0.07, 0.09, 0.11, 0.13, 0.15, 0.17, 0.19, or 0.2. In some such embodiments, or in other embodiments, |n2x - n2y|, is less than 0.9, 0.7, 0.5, 0.4, 0.3, 0.25, or 0.2 times the first birefringence. In some such embodiments, or in other embodiments, the first birefringence is less than 0.35, 0.3, or 0.25. The first and second layers can have respective indices of refraction nlz and n2z along a thickness direction of the reflective polarizer orthogonal to each of the first and second directions. In some embodiments, for the at least one wavelength, |nlz - nly| is less than 0.5, 0.25, 0.15, 0.1, 0.075, 0.05, 0.025 or 0.01 times |nlx - nly|. In some such embodiments, or in other embodiments, for the at least one wavelength, |n2z - n2y| is less than 1, 0.8, 0.6, 0.5, 0.4, 0.3, or 0.2 times |nlx - nly|. Each refractive index of each of the first and second layers can be in a range of about 1.4 to about 2, or about 1.45 to about 1.9 for example.

[0025] FIG. 2 is a schematic plot of reflectance versus wavelength for the plurality of alternating polymeric first and second layers 10, 11, according to some embodiments. The plurality of alternating polymeric first and second layers 10, 11, before thermoforming, has an average reflectance in a wavelength range of I (e.g., 400 nm, 420 nm, or 450 nm) to 2 (e.g., 700 nm, or 680 nm, or 650 nm) of RsO and RpO for substantially normally incident (e.g., within about 20, 15, 10, or 8 degrees of normally incident) light 30 polarized along the respective first (e.g., along x-axis or polarization state 141) and second (e.g., along y-axis or polarization state 142) in-plane directions. The reflectance RpO may be undesirably high for some applications. However, after thermoforming, the average reflectances shift from RsO and RpO to Rsl and Rpl, respectively. Rpl may be in a desired range for many applications, even when RpO is not. RsO and Rsl can be the same or about the same. The reflectance (e.g., RsO, RpO) of the plurality of alternating polymeric first and second layers 10, 11 may differ from the reflectance of the reflective polarizer 100 due to Fresnel reflections at the outermost surfaces of the reflective polarizer 100. The reflectance of the plurality of alternating polymeric first and second layers 10, 11 can be determined by subtracting out these Fresnel reflections from measured reflectance of the reflective polarizer (e.g., in air) and / or can be determined from optical modeling, as would be appreciated by those of ordinary skill in the art.

[0026] In some embodiments, the reflective polarizer 100 is such that for a substantially normally incident light 30 and for the visible wavelength range, the plurality of alternating polymeric first and second layers 10, 11 has an average reflectance RsO of at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, or 98% when the incident light is polarized along the first in-plane direction (e.g., along x-axis or polarization state 141) and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along the second in-plane direction (e.g., along y-axis or polarization state 142). In some embodiments, RpO is greater than 2%, 3%, 4%, 5%, or 6%. In some such embodiments, or in other embodiments, RpO is at most 35%, 30%, 25%, 20%, or 15%.

[0027] FIG. 3 is a plot of pass-state reflectance (in percent) versus wavelength of optical layers (plurality of alternating polymeric first and second layers 10, 11) of a reflective polarizer made from polymers conventionally used in polymeric reflective polarizers. FIG. 4 is a plot of pass-state reflectance (in percent) versus wavelength of optical layers (plurality of alternating polymeric first and second layers 10, 11) of a reflective polarizer according to some embodiments of the present description. Each of the reflective polarizers of FIGS. 3 and 4 included about 650 alternating first and second layers 10, 11. The polymers used in the reflective polarizers of FIGS. 3 and 4 are described further elsewhere herein. The reflectance results were obtained by measuring the reflectance in air and subtracting Fresnel reflections at the outermost surfaces of the reflective polarizer to determine the reflectance of the optical layers. Error in the measurements resulted in a negative reflectance for some wavelengths. It will be understood that that the actual reflectance for these wavelengths is at least 0%. Results are shown in FIGS. 3-4 for different heat set conditions. The label “Stretch_135C_No_heatset” indicates that the film was stretched at 135 deg. C and that no heat set was applied after stretching. Such conditions are typical for conventional polymeric reflective polarizers. The data labeled “Stretch_135C_heatset_155C” and “Stretch_135C_heatset_160C” indicate that a heat set step was utilized at 155 and 160 deg. C, respectively. The heat set time was 5 seconds for each of the plots of FIGS. 3-4. The heat set was applied to mimic effects of thermoforming on the optical layers. In FIG. 3, only a small shift in the average passstate reflectance was observed, while a substantial reduction in average pass-state reflectance was observed in FIG. 4. With appropriate selection of the materials of the high and low index layers, the refractive indices of the high and low index layers along the pass state can be made to substantially match so that the average pass state reflectance for visible light after thermoforming can be lower than shown in FIGS. 3 and 4. For example, the HIO material can be adjusted (e.g., by blending with another polymer) to have a refractive index along the pass state that substantially matches the pass state refractive index of the LIO material after the LIO material shifts from thermoforming, and / or the LIO material can be adjusted (e.g., by blending with another polymer) to have a pass state refractive index after thermoforming that substantially matches the pass state index of the HIO material.

[0028] A change in the reflective polarizer upon thermoforming can be characterized by a shift in a property of the reflective polarizer (reflectance of the optical layers, birefringence of the optical layers, or both) upon heating the reflective polarizer to at least one temperature in a specified range for a specified time.

[0029] In some embodiments, when the reflective polarizer 100 is heated to at least one temperature that is greater than a glass transition temperature of the second layers 11 and less than a melting temperature of the first layers 10 for a time of at least about 1 second, RpO decreases by at least a factor of 1.3 (i.e., the final reflectance, which may be referred to as Rpl, is less than or equal to the corresponding initial reflectance divided by 1.3); and RsO remains at least 50% (i.e., the final reflectance, which may be referred to as Rpl, is at least 50% when the corresponding initial reflectance is at least 50%). In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second, RpO decreases by at least a factor of 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, or 2.6. In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second, RsO remains at least 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, or 98%. In some embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second, RpO / RsO decreases by at least a factor of 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, or 2.6. In some embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second, RsO does not decrease or decreases by no more than a factor of 1.1, 1.07, 1.05, 1.04, 1.03, 1.02, or 1.01.

[0030] In order to test reduction of birefringence of LIO layers with thermoforming, various bilayer (a layer of HIO polymer coextruded and co-stretched with a layer of LIO polymers) films were prepared, stretched on a lab scale stretcher, and heat set at various temperatures for various times. The bilayer films had a total thickness of about 2 mils with the lower index layer having a thickness of about 0.5 mils. The refractive indices of the two layers of the bilayer films were measured at a wavelength of 632 nm using a prism coupler from Metricon Corporation (Pennington, NJ). FIGS. 5-7 are plots of in-plane birefringence of low index layers of various bilayer films versus heat set temperature (HS Temp) for various stretch conditions and heat set times, according to some embodiments. FIG. 8 is a plot of in-plane birefringence of low index layers of various bilayer films versus heat set time for a heat set temperature of 140 deg. C, according to some embodiments. The in-plane birefringence of the low index layers is denoted TD-MD referring to the difference between the index in a transverse direction (TD, stretch direction which can correspond to the x-direction, for example) and the index in a machine direction (MD which can correspond to the y-direction, for example). The draw (stretch) ratios in the respective MD and TD directions were 0.45 and 5 for FIGS. 5 and 7 and were 0.5 and 5 in FIGS. 6 and 8. The stretch temperature was 135 deg. C for each of FIGS. 5-8.

[0031] In some embodiments, when the reflective polarizer is thermoformed, the second birefringence (in-plane birefringence of the second layers) is substantially reduced, but any reduction in the first birefringence (in-plane birefringence of the first layers) is substantially less (e.g., as a percent) than the reduction in the second birefringence. This may occur due to the thermoforming conditions (e.g., due to a larger difference between the thermoforming temperature and a glass transition temperature of the second layers than between the thermoforming temperature and a glass transition temperature of the first layers) such that the birefringence of the second layers is “melted out” while the birefringence of the first layers is substantially maintained. For example, the birefringence of the second layers may be reduced by 20% or more while the birefringence of the first layers may be reduced by only about 5% or even less.

[0032] In some embodiments, when the reflective polarizer 100 is heated to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the second birefringence (in-plane birefringence of the second layers), but not the first birefringence (in-plane birefringence of the first layers), is reduced by at least 20%; |nly - n2y| is reduced by at least 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75%; and nix - n2x remains greater than 0.05. In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second and for the at least one wavelength in the visible wavelength range, nix - n2x remains greater than 0.05, 0.07, 0.09, 0.11, 0.13, 0.15, 0.17, 0.19, or 0.2. In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to the at least one temperature for the time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the first birefringence is reduced by no more than 15, 12, 10, 8, 6, or 5%. In some such embodiments, or in other embodiments, when the reflective polarizer is heated to the at least one temperature for the time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the second birefringence is reduced by at least 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, or 75%.

[0033] The temperature and time used for either or both of the reflectance change and refractive index change tests can be selected to mimic useful thermoforming conditions. As described elsewhere herein, the at least one temperature can be greater than a glass transition temperature of the second layers 11 and less than a melting temperature of the first layers 10. The glass transition temperature of the first layers can be greater than the glass transition temperature of the second layers. In some embodiments, the at least one temperature is greater than a glass transition temperature of the first layers 10 and less than a melting temperature of the first layers 10. In some embodiments, the at least one temperature is or includes a temperature in a range of 130 to 165 degrees C. As described elsewhere herein, the at least one temperature can be held for at least about 1 second. In some embodiments, the time of at least about 1 second is no more than about 15, 12, 10, 8, or 6 seconds. In some embodiments, the time of at least about 1 second is at least about 1.2, 1.4, 1.6, 1.8, or 2 seconds.

[0034] Suitable materials for the various layers of the reflective polarizer 100 include polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polycarbonate, copolyesters, and blends or copolymers thereof. Other suitable materials are described in the multilayer optical fdm references provided elsewhere herein. In some embodiments, the HIO layers (e.g., the first layers 10) can be coPEN 90 / 10 (also referred to as Low Melt PEN or LMPEN) layers as described in U.S. Pat. No. 6,946,188 (Hebrink et al.), for example, and the LIO layers (e.g., the second layers 11) can be a blend of coPEN 90 / 10 and at least one other polyester or other polymer, for example.

[0035] CoPEN 90 / 10 can be described a copolyester where carboxylate units of the copolyester comprises 90 mole percent naphthalate units and 10 mole percent terephthalate units. More generally, the first layers can be formed from a copolyester that includes other ratios of naphthalate and terephthalate units. For example, in some embodiments, carboxylate units of the copolyester can include about 80 to 95 mole percent naphthalate units and about 5 to 20 mole percent terephthalate units. Such copolyesters may be denoted coPEN N / 100-N where N is the mole percent naphthalate units and 100-N is the mole percent terephthalate units.

[0036] The at least one other polyester or other polymer of the LIO layers can include at least one glycol- modified PET (PETg). Gly col-modified PET can be described as PET with at least some of the glycol units replaced with different units such as those derived from cyclohexanedimethanol. Suitable gly col- modified PETs include PCTg and EASTAR GN071 (both available from Eastman Chemical Company, Knoxville, TN), and include PETgs that include plant-derived biomass components (e.g., isosorbide) as such glycol-modified PETs have been found, according to some embodiments, to result in improved index match (compared to PCTg and EASTAR GN071, for example) with the HIO layers over the visible wavelength range, for example, when birefringent material (e.g., coPEN 90 / 10) is included in the LIO layers. Suitable glycol-modified PETs that comprise isosorbide groups include those available from SK Chemicals (Gyeonggi-do, South Korea) under the tradename ECOZEN, such as the ECOZEN T-series which includes ECOZEN T120. Such glycol-modified PETs can include isosorbide-derived groups and can have glass transition temperatures (e.g., about 120 degrees C) higher than those of PCTg and EASTAR GN071 (e.g., about 80 degrees C), for example.

[0037] The reflective polarizer Cl of FIG. 3 included HIO layers formed from coPEN 90 / 10 and LIO layers formed from a blend of polycarbonate and copolyesters. The reflective polarizer Cl was made by coextrusion followed by substantially uniaxial stretching using a lab scale stretcher. The reflective polarizer of FIG. 4 and various bilayer films of FIGS. 5-8 are denoted N / M / 100-N-M which indicates the composition of the lower index layers. The reflective polarizer 65 / 15 / 25 of FIG. 4 was made as generally described for the reflective polarizer Cl with coPEN 90 / 10 HIO layers but with LIO layers formed from a blend of ECOZEN T 120 at 65 wt%, coPEN 90 / 10 at 15 wt%, and EASTAR GN071 at 25 wt%. The bilayers N / M / 100-N-M of FIGS. 5-8 used a coPEN 90 / 10 higher index layer and used a lower index layer formed from a blend of ECOZEN T120 (N weight percent), coPEN 90 / 10 (M weight percent), and EASTAR GN071 (100-N-M weight percent). The bilayer films 90 / 10 of FIGS. 5-7 used a coPEN 90 / 10 high index layer and a low index layer formed from a blend of ECOZEN T 120 at 90 wt% and coPEN 90 / 10 at 10 wt%. The bilayer films Bl of FIGS. 5-7 used coPEN 90 / 10 high index layers and used a low index layer formed of the same material as the LIO layers of reflective polarizer C 1. It will be understood that blends of polymers and copolymers can be copolymerized blends (e.g., a blend of first and second copolyesters may undergo a transesterification reaction during an extrusion process resulting in a copolymer of the first and second copolyesters; such a blend may be referred to as a copolymerized blend).

[0038] In some embodiments, each of the first and second layers 10 and 11 comprises a copolyester comprising naphthalate groups, terephthalate groups and glycol groups. In some such embodiments, or in other embodiments, the copolyester of each of the second layers further comprises isosorbide groups. In some embodiments, the copolyester of each of the second layers comprises isosorbide groups at greater than about 5, 10, 15, 20, or 25 mole percent based on total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each of the second layers comprises isosorbide groups at about 5 to about 35, 30, or 25 mole percent based on total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each of the second layers further comprises cyclohexanedimethanol groups. In some embodiments, the copolyester of each of the second layers comprises cyclohexanedimethanol groups at about 10, 15 or 20 to about 55, 50, 45, or 40 mole percent based on total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each of the second layers comprises ethylene glycol groups at about 5, 10, or 15 to about 60, 50, 40, or 35 mole percent based on total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each of the first layers comprises the naphthalate groups at about 80 to 95 mole percent based on a total moles of carboxylate groups. In some such embodiments, or in other embodiments, the copolyester of each of the first layers comprises the terephthalate groups at about 5 to 20 mole percent based on a total moles of carboxylate groups. In some such embodiments, or in other embodiments, the copolyester of each of the seconds layers comprises the terephthalate groups at about 75 to 95 mole percent based on a total moles of carboxylate groups. In some such embodiments, or in other embodiments, the copolyester of each of the seconds layers comprises the naphthalate groups at about 5 to 25 mole percent based on a total moles of carboxylate groups.

[0039] FIG. 9 is a schematic illustration of thermoforming a reflective polarizer 100, according to some embodiments. Edge portions 408 of the reflective polarizer 100 is held in clamps 473 and a mold 460 having a curved mold surface 462 is schematically illustrated. The reflective polarizer 100 and / or the mold 460 can be heated (e.g., to a temperature T which can correspond to the at least one temperature described elsewhere herein that is greater than a glass transition temperature of the second layers 11 and less than a melting temperature of the first layers 10). The reflective polarizer can be at the at least one temperature for the time greater than about 1 second (or a range described elsewhere herein). The clamps 473 can pull the reflective polarizer 100 down over the curved mold surface 462 so that the resulting thermoformed reflective polarizer has a shape (e.g., corresponding to the shape of surface 462) curved about two orthogonal axes (e.g., x- and y-axes). An alternative to pulling the reflective polarizer 100 down over the mold surface, the reflective polarizer 100 could be pushed (e.g., via pressurization) against a mold surface. Useful pull-down and pressurization processes that may be utilized are described in U.S. Pat. No. 11,543,572 (Jennings et al.), for example.

[0040] In some embodiments, a method of shaping a reflective polarizer 100 is provided. The reflective polarizer 100 includes a plurality of alternating polymeric first and second layers 10 and 11 having respective in-plane first and second birefringences, as described further elsewhere herein. The method includes thermoforming the reflective polarizer 100 into a shape curved about two orthogonal axes. The thermoforming includes heating the reflective polarizer 100 to at least one temperature that is greater than a glass transition temperature of the second layers 11 and less than a melting temperature of the first layers 10 (or in a range described elsewhere herein) for a time greater than about 1 second (or in a range described elsewhere herein). The thermoforming can include conforming the reflective polarizer to a curved mold surface 462. In some embodiments, prior to thermoforming, for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007 (or in a range described elsewhere herein); and for a substantially normally incident light 30 (see, e.g., FIG. 1) and for the visible wavelength range, the plurality of alternating polymeric first and second layers 10, 11 has an average reflectance RsO of at least 50% (or in a range described elsewhere herein) when the incident light is polarized along a first in-plane direction (e.g., having polarization state 141 polarized along the x-axis) of the reflective polarizer 100 and an average reflectance RpO of greater than 1% and at most 40% (or RpO can be in another range described elsewhere herein) when the incident light is polarized along an orthogonal second in-plane direction (e.g., having polarization state 142 polarized along the y-axis) of the reflective polarizer 100. In some embodiments, after thermoforming, for the substantially normally incident light 30 and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance Rsl when the incident light is polarized along the first in-plane direction of the reflective polarizer 100 and an average reflectance Rpl when the incident light is polarized along the second in-plane direction of the reflective polarizer 100. In some embodiments, Rpl / Rsl < 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, or 0.4 x RpO / RsO (e.g., in some embodiments, Rpl / Rsl < 0.6 x RpO / RsO). In some such embodiments, or in other embodiments, RpO / Rpl is at least 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6. In some such embodiments, or in other embodiments, |Rs0-Rsl| / Rs0 < 0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, or 0.02. Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1, means that the quantity has a value between 0.9 and 1. 1, and that the value could be 1.

[0041] Terms such as “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “substantially” with reference to a property or characteristic is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description and when it would be clear to one of ordinary skill in the art what is meant by an opposite of that property or characteristic, the term “substantially” will be understood to mean that the property or characteristic is exhibited to a greater extent than the opposite of that property or characteristic is exhibited.

[0042] All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control.

[0043] Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations, or variations, or combinations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A reflective polarizer comprising a plurality of alternating polymeric first and second layers numbering at least 10 in total, each first and second layer of the plurality of alternating polymeric first and second layers having an average thickness of less than about 500 nm, the first layers having indices of refraction nix and nly in respective orthogonal first and second in-plane directions of the reflective polarizer, the second layers having indices of refraction n2x and n2y in the respective first and second in-plane directions, in-plane first and second birefringences of the first and second layers being nix - nly and |n2x - n2y|, respectively, wherein for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm: the second birefringence is less than the first birefringence and greater than 0.007; nix - n2x > 0.05; and 0.005 < |nly - n2y| < nix - n2x, such that for a substantially normally incident light and for the visible wavelength range: the plurality of alternating polymeric first and second layers has an average reflectance of at least 50% when the incident light is polarized along the first in-plane direction and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along the second in-plane direction, wherein when the reflective polarizer is heated to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second and for the at least one wavelength in the visible wavelength range: the second birefringence, but not the first birefringence, is reduced by at least 20%; |nly - n2y| is reduced by at least 20%; and nix - n2x remains greater than 0.05.

2. The reflective polarizer of claim 1, wherein when the reflective polarizer is heated to the at least one temperature for the time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the second birefringence is reduced by at least 25%.

3. The reflective polarizer of claim 1, wherein when the reflective polarizer is heated to the at least one temperature for the time of at least about 1 second and for the at least one wavelength in the visible wavelength range, the first birefringence is reduced by no more than 15%.

4. The reflective polarizer of claim 1, wherein the at least one temperature comprises a temperature in a range of 130 to 165 degrees C.

5. The reflective polarizer of claim 1, where the time of at least about 1 second is no more than about 15 seconds.

6. The reflective polarizer of claim 1, wherein each of the first and second layers comprises a copolyester comprising naphthalate groups, terephthalate groups and glycol groups.

7. The reflective polarizer of claim 6, wherein the copolyester of each of the second layers further comprises isosorbide groups.

8. A reflective polarizer comprising a plurality of alternating polymeric first and second layers numbering at least 10 in total, each first and second layer of the plurality of alternating polymeric first and second layers having an average thickness of less than about 500 nm, the first and second layers having respective in-plane first and second birefringences, wherein for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007, such that for a substantially normally incident light and for the visible wavelength range: the plurality of alternating polymeric first and second layers has an average reflectance RsO of at least 50% when the incident light is polarized along a first in-plane direction of the reflective polarizer and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along an orthogonal second in-plane direction of the reflective polarizer, wherein when the reflective polarizer is heated to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time of at least about 1 second:RpO decreases by at least a factor of 1.3; andRsO remains at least 50%.

9. The reflective polarizer of claim 8, wherein when the reflective polarizer is heated to the at least one temperature for the time of at least about 1 second, RpO / RsO decreases by at least a factor of 1.3.

10. The reflective polarizer of claim 8, wherein RsO is at least 80% and when the reflective polarizer is heated to the at least one temperature for the time of at least about 1 second, RpO decreases by at least a factor of 1.5; and RsO remains at least 80%.

11. The reflective polarizer of claim 8, wherein the at least one temperature comprises a temperature in a range of 130 to 165 degrees C, and the time of at least about 1 second is no more than about 15 seconds.

12. A method of shaping a reflective polarizer, the reflective polarizer comprising a plurality of alternating polymeric first and second layers having respective in-plane first and second birefringences, the method comprising thermoforming the reflective polarizer into a shape curved about two orthogonal axes, the thermoforming comprising heating the reflective polarizer to at least one temperature that is greater than a glass transition temperature of the second layers and less than a melting temperature of the first layers for a time greater than about 1 second, wherein prior to thermoforming: for at least one wavelength in a visible wavelength range of about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007; and for a substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance RsO of at least 50% when the incident light is polarized along a first in-plane direction of the reflective polarizer and an average reflectance RpO of greater than 1% and at most 40% when the incident light is polarized along an orthogonal second in-plane direction of the reflective polarizer, wherein after thermoforming: for the substantially normally incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers has an average reflectance Rs 1 when the incident light is polarized along the first in-plane direction of the reflective polarizer and an average reflectance Rp 1 when the incident light is polarized along the second in-plane direction of the reflective polarizer, wherein: Rpl / Rsl < 0.75 x RpO / RsO.

13. The method of claim 12, wherein RpO / Rpl is at least 1.3.

14. The method of claim 12, wherein |Rs0-Rsl| / Rs0 < 0.1.

15. The method of claim 12, wherein the at least one temperature comprises a temperature in a range of 130 to 165 degrees C, and the time of at least about 1 second is no more than about 15 seconds.

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