Reflective polarizer and forming method thereof

Abstract

A reflective polarizer includes a plurality of alternating first and second layers. For substantially perpendicular incident light, the plurality of first and second layers have an average reflectance Rs0 of at least 50% when the incident light is polarized in a first direction and an average reflectance Rp0 of greater than 1% and at most 40% when the incident light is polarized in an orthogonal second direction. When the reflective polarizer is heated to a temperature above the glass transition temperature of the second layer and below the melting temperature of the first layer for a time of at least about 1 second, the birefringence of the second layer rather than the first layer is reduced by at least 20%, and / or Rp0 is reduced to at most 1 / 1.3 of the original value while Rs0 remains at least 50%. The reflective polarizer may be shaped via thermoforming.

Description

Technical Field

[0001] The present specification as a whole relates to a reflective polarizer that can be formed by thermoforming. Background Art

[0002] A reflective polarizer may include alternating first and second layers and may have a higher reflectivity for one polarization state than for an orthogonal polarization state. A polymeric reflective polarizer can be thermoformed into a curved shape. Summary of the Invention

[0003] In some aspects, the present specification provides a reflective polarizer that includes: a plurality of alternating polymeric first and second layers having a total of at least 10. Each of the plurality of alternating polymeric first and second layers may have an average thickness less than about 500 nm. The first layer has refractive indices n1x and n1y in respective orthogonal first and second in-plane directions of the reflective polarizer. The second layer has refractive indices n2x and n2y in the respective first and second in-plane directions. The first and second in-plane birefringences of the first and second layers are n1x–n1y and |n2x–n2y|, respectively. For at least one wavelength in the visible wavelength range of from about 420 nm to about 680 nm, the second birefringence is less than the first birefringence and greater than 0.007; n1x–n2x>0.05; and 0.005<|n1y–n2y|<n1x–n2x, such that for substantially vertically incident light and for the visible wavelength range, the plurality of alternating polymeric first and second layers have an average reflectivity of at least 50% when the incident light is polarized along the first in-plane direction, and an average reflectivity Rp0 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 higher than the glass transition temperature of the second layer and lower than the melting temperature of the first layer for a time of at least about 1 second, and for at least one wavelength in the visible wavelength range, the second birefringence rather than the first birefringence is reduced by at least 20%; |n1y–n2y| is reduced by at least 20%; and n1x–n2x remains greater than 0.05.

[0004] In some aspects, this specification provides a reflective polarizer comprising: a plurality of alternating polymer first and second layers, totaling at least 10. Each of the plurality of alternating polymer first and second layers may have an average thickness of less than about 500 nm. The first and second layers have corresponding in-plane first and second birefringences. For at least one wavelength in the 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 substantially perpendicular incident light and for the visible wavelength range, the plurality of alternating polymer first and second layers have an average reflectivity Rs0 of at least 50% when the incident light is polarized along the first in-plane direction of the reflective polarizer, and an average reflectivity Rp0 of greater than 1% and at most 40% when the incident light is polarized along the orthogonal second in-plane direction of the reflective polarizer. When the reflective polarizer is heated to at least one temperature greater than the glass transition temperature of the second layer and less than the melting temperature of the first layer for at least about 1 second, Rp0 decreases to at most 1 / 1.3 of its original value, and Rs0 remains at least 50%.

[0005] In some aspects, this specification provides a method for shaping a reflective polarizer comprising a plurality of alternating polymer first and second layers having respective in-plane first and second birefringences. The method includes thermoforming the reflective polarizer into a shape bent about two orthogonal axes. The thermoforming includes heating the reflective polarizer to at least one temperature above the glass transition temperature of the second layer and below the melting temperature of the first layer for a duration greater than about 1 second. Prior to thermoforming, for at least one wavelength in the 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 substantially perpendicular incident light and for the visible wavelength range, the plurality of alternating polymer first and second layers have an average reflectivity Rs0 of at least 50% when the incident light is polarized along the first in-plane direction of the reflective polarizer, and an average reflectivity Rp0 of greater than 1% and at most 40% when the incident light is polarized along the orthogonal second in-plane direction of the reflective polarizer. After thermoforming, for substantially perpendicular incident light and for the visible wavelength range, multiple alternating polymer first and second layers have an average reflectivity Rs1 when the incident light is directionally polarized along the first plane of the reflective polarizer, and an average reflectivity Rp1 when the incident light is directionally polarized along the second plane of the reflective polarizer. Rp1 / Rs1 is 0.75 times Rp0 / Rs0.

[0006] These and other aspects will become apparent from the detailed description that follows. However, in no way should this brief overview be construed as limiting the subject matter for which protection may be claimed. Attached Figure Description

[0007] Figure 1 This is a schematic cross-sectional view of a reflective polarizer according to some implementation schemes.

[0008] Figure 2 This is a schematic graph of the reflectivity versus wavelength of multiple alternating polymer first and second layers of a reflective polarizer according to some embodiments.

[0009] Figures 3 to 4 This is a graph showing the transmittance of the alternating polymer first and second layers of an exemplary reflective polarizer as a function of wavelength.

[0010] Figures 5 to 7 It is a graph showing the in-plane birefringence versus heat setting temperature of the low-refractive-index layer of various bilayer films under various stretching conditions and heat setting times, based on some implementation schemes.

[0011] Figure 8 It is a graph showing the in-plane birefringence of the low-refractive-index layer of various bilayer films versus the heat setting time at a heat setting temperature of 140°C, based on some implementation schemes.

[0012] Figure 9 This is a schematic diagram of thermoforming a reflective polarizer according to some implementation schemes. Detailed Implementation

[0013] Reference is made in the following description to the accompanying drawings, which form part of this disclosure and in which various embodiments are illustrated by way of example. The drawings are not necessarily drawn to scale. It should be understood that other embodiments may be conceived and practiced without departing from the scope or spirit of this specification. Therefore, the following detailed description should not be considered limiting.

[0014] As is known in the art, multilayer optical films comprising alternating polymer layers can be used to provide desired reflection and transmission within a desired wavelength range by appropriately selecting layer thicknesses and refractive index differences. Multilayer optical films and methods of manufacturing multilayer optical films are described, for example, in U.S. Patent No. 5,882,774 (Jonza et al.); U.S. Patent No. 6,783,349 (Neavin et al.); U.S. Patent No. 6,949,212 (Merrill et al.); U.S. Patent No. 6,967,778 (Wheatley et al.); U.S. Patent No. 9,162,406 (Neavin et al.); and U.S. Patent No. 11,493,677 (Haag et al.).

[0015] Optical systems can utilize reflective polarizers and partial reflectors, along with retarders between them, to provide folded optical paths, as broadly described, for example, in U.S. Patent No. 10,678,052 (Ouderkirk et al.). In such optical systems, minimizing the transmittance reflectivity of the reflective polarizer may be desirable. It has been found, for example, that even small transmittance reflectivity can lead to unwanted spurious images. Reflective polarizers used in such optical systems typically have a curved shape. Polymer reflective polarizers can be thermoformed into curved shapes, as broadly described, for example, in U.S. Patent Nos. 9,599,761 (Ambur et al.) and 11,543,572 (Jennings et al.). Such reflective polarizers typically comprise an alternating birefringent (typically with a higher average refractive index) first layer and an isotropic (typically with a lower average refractive index) second layer, wherein the refractive indices of the birefringent and isotropic layers are substantially matched along the transmission axis of the reflective polarizer and substantially mismatched along the blocking axis. The refractive indices are typically matched along the transmission axis to minimize transmittance reflectivity. However, it has been found that even when the reflective polarizer has a desired low transmittance reflectivity before thermoforming, the refractive index (e.g., the first birefringent layer) can shift during thermoforming, resulting in an undesirable high transmittance reflectivity for the thermoformed reflective polarizer. Even a small shift in the birefringence of the first layer has been found to lead to an undesirable high transmittance reflectivity.

[0016] According to some embodiments, it has been found that a low transmittance-state reflectivity of the thermoformed reflective polarizer can be achieved by intentionally creating a refractive index mismatch along the transmission axis in the unthermoformed reflective polarizer, thereby reducing the mismatch during thermoforming. According to some embodiments, this is achieved by utilizing a second layer with a lower refractive index, which is initially slightly birefringent (e.g., a birefringence greater than 0.007 but less than that of the first layer), such that this initial birefringence is significantly reduced during thermoforming (e.g., the birefringence relaxes or “melts” under typical thermoforming conditions), resulting in significant refractive index matching along the transmission axis in the thermoformed reflective polarizer. Typically, the isotropic layer of a reflective polarizer is isotropic because it is made of a polymer that does not become birefringent under typical reflective polarizer stretching conditions. Such layers can be modified to become slightly birefringent by incorporating other polymers that can become birefringent. For example, the birefringent first layer may be formed from polyethylene naphthalate (PEN) or its copolymer (coPEN), and the second layer may be formed from a blend of conventional low-refractive-index polymers (such as glycol-modified polyethylene terephthalate (PETG)) and PEN or coPEN (e.g., copolymers). In some embodiments, the blend (e.g., copolymers) comprises a copolyester having isosorbide groups, as such copolyesters have been found to facilitate achieving desired transmittance refractive index matching in reflective polarizers after thermoforming.

[0017] Figure 1This is a schematic cross-sectional view of a reflective polarizer 100 according to some embodiments. The reflective polarizer 100 includes a plurality of alternating polymer first layers 10 and polymer second layers 11. In some embodiments, the total number of the plurality of alternating polymer first layers 10 and polymer second layers 11 is at least 10, 20, 30, 40, 50, 80, 100, 150, 200, 300, 400, 500, 600, or 650. For example, the total number of layers in the plurality of alternating polymer first layers 10 and polymer second layers 11 can be as high as 10,000, 5,000, 2,000, 1,000, or 800. Each of the first and second layers in the plurality of alternating polymer first microlayers 10 and polymer second microlayers 11 may have an average thickness (average value over a region of the layer thickness) of, for example, less than about 500 nm, 450 nm, 400 nm, 350 nm, 300 nm, 250 nm, or 200 nm. For example, the average thickness can be at least about 20 nm, 30 nm, 40 nm, 50 nm, or 60 nm. Layers 10 and 11 can be described as interference layers, optical layers, or microlayers. Such layers can be described as reflecting or transmitting light primarily through optical interference when the reflectivity and transmittance of the interference layer can be reasonably described by optical interference or when the reflectivity and transmittance of the interference layer can be reasonably and accurately modeled as a result of optical interference. The first layer 10 can have a higher average refractive index than the second layer 11 (e.g., the average refractive index in two orthogonal in-plane directions or the average refractive index in three mutually orthogonal directions). Therefore, the first layer 10 can be referred to as a high refractive index optical (HIO) layer, and the second layer 11 can be referred to as a low refractive index optical (LIO) layer.

[0018] In some embodiments, a plurality of alternating polymer first layers 10 and polymer second layers 11 are disposed on at least one macroscopic layer 124, 126. Macroscopic layers are typically layers too thick to reflect or transmit light in the wavelength range of interest primarily through optical interference. In an illustrative embodiment, a plurality of alternating polymer first layers 10 and polymer second layers 11 are disposed between a first macroscopic layer 124 and a second macroscopic layer 126 (e.g., a surface layer). As will be understood by those skilled in the art, the reflective polarizer 100 may optionally include additional macroscopic layers (e.g., protective boundary layers separating first and second groups of alternating first layers 10 and second layers 11). In some embodiments, each of the at least one macroscopic layer 124, 126 has an average thickness greater than about 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, 1250 nm, 1500 nm, 1750 nm, or 2000 nm. For example, the average thickness of each macroscopic layer can be as high as approximately 150 micrometers, 100 micrometers, 50 micrometers, 30 micrometers, 20 micrometers, or 10 micrometers.

[0019] In some embodiments, the reflective polarizer 100 includes a plurality of alternating polymer first layers 10 and polymer second layers 11, wherein the first layers have refractive indices n1x and n1y in corresponding orthogonal first in-plane directions and second in-plane directions (e.g., x and y directions) of the reflective polarizer, and wherein the second layers have refractive indices n2x and n2y in corresponding first in-plane and second in-plane directions. The in-plane direction is the plane of the reflective polarizer (e.g., ...). Figure 1direction in the x-y plane or in the case of a curved reflective polarizer, the cut plane). The first and second layers can be characterized by the in-plane birefringence of the layer. The in-plane birefringence of the layer can be described as the refractive index difference along the principal directions in the plane of the layer. The first and second in-plane birefringences of the first and second layers are n1x–n1y and |n2x–n2y|, respectively. The x-axis can be taken along the higher refractive index direction of the first layer such that n1x>n1y. n2x–n2y can be positive or negative. In some embodiments, n2x>n2y. In some embodiments, for at least one wavelength in the visible wavelength range from 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 at least one wavelength in the visible wavelength range, n1x–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 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<|n1y–n2y|<n1x–n2x. In some such embodiments, or in other embodiments, the first birefringence n1x–n1y 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 0.9 times, 0.7 times, 0.5 times, 0.4 times, 0.3 times, 0.25 times 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 refractive indices n1z and n2z along the thickness direction of the reflective polarizer orthogonal to each of the first and second directions. In some embodiments, for at least one wavelength, |n1z–n1y| is 0.5 times, 0.25 times, 0.15 times, 0.1 times, 0.075 times, 0.05 times, 0.025 times or 0.01 times |n1x–n1y|. In some such embodiments, or in other embodiments, for at least one wavelength, |n2z–n2y| is 1 times, 0.8 times, 0.6 times, 0.5 times, 0.4 times, 0.3 times or 0.2 times |n1x–n1y|. Each refractive index of each of the first and second layers can be in the range of, for example, about 1.4 to about 2, or about 1.45 to about 1.9.

[0020] Figure 2 This is a schematic graph illustrating the reflectance versus wavelength of multiple alternating polymer first layers 10 and polymer second layers 11 according to some embodiments. Prior to thermoforming, for substantially perpendicularly incident (e.g., within approximately 20, 15, 10, or 8 degrees of perpendicular incident) light 30 polarized along corresponding first (e.g., along the x-axis or polarization state 141) and second (e.g., along the y-axis or polarization state 142) in-plane directions, the average reflectance of the multiple alternating polymer first layers 10 and polymer second layers 11 in the wavelength range of λ1 (e.g., 400 nm, 420 nm, or 450 nm) to λ2 (e.g., 700 nm, 680 nm, or 650 nm) wavelengths is Rs0 and Rp0. For some applications, the reflectance Rp0 may be undesirably high. However, after thermoforming, the average reflectance shifts from Rs0 and Rp0 to Rs1 and Rp1, respectively. For many applications, even if Rp0 is not within the desired range, Rp1 can be within the desired range. Rs0 and Rs1 can be the same or approximately the same. Due to Fresnel reflection at the outermost surface of the reflective polarizer 100, the reflectivities (e.g., Rs0, Rp0) of the plurality of alternating polymer first layers 10 and polymer second layers 11 can differ from the reflectivities of the reflective polarizer 100. The reflectivities of the plurality of alternating polymer first layers 10 and polymer second layers 11 can be determined by subtracting these Fresnel reflections from the measured reflectivities of the reflective polarizer (e.g., in air), and / or by optical modeling, as will be understood by those skilled in the art.

[0021] In some embodiments, the reflective polarizer 100 such that, for substantially perpendicular incident light 30 and for the visible wavelength range, a plurality of alternating polymer first layers 10 and polymer second layers 11 have an average reflectivity Rs0 of at least 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, or 98% when the incident light is polarized along a first in-plane direction (e.g., along the x-axis or polarization state 141), and an average reflectivity Rp0 greater than 1% and at most 40% when the incident light is polarized along a second in-plane direction (e.g., along the y-axis or polarization state 142). In some embodiments, Rp0 is greater than 2%, 3%, 4%, 5%, or 6%. In some such embodiments, or in others, Rp0 is at most 35%, 30%, 25%, 20%, or 15%.

[0022] Figure 3 This is a graph of the transmittance (in percentage) versus wavelength of the optical layers (multiple alternating polymer first layer 10 and polymer second layer 11) of a reflective polarizer made of a polymer commonly used in polymer reflective polarizers. Figure 4This is a graph of the transmittance (in percentage) versus wavelength of the optical layers (multiple alternating polymer first layer 10 and polymer second layer 11) of a reflective polarizer according to some embodiments of this specification. Figure 3 and Figure 4 Each reflective polarizer in the reflective polarizer comprises approximately 650 alternating first layers 10 and second layers 11. Figure 3 and Figure 4 The polymer used in the reflective polarizer is further described elsewhere in this document. The reflectivity of the optical layer is determined by measuring the reflectivity in air and subtracting the Fresnel reflection at the outermost surface of the reflective polarizer. Errors in the measurement result in negative reflectivity for some wavelengths. It will be understood that the actual reflectivity for these wavelengths is at least 0%. For different heat-setting conditions, the results are as follows: Figures 3 to 4 As shown in the image. The label “Stretch_135C_No_heatset” indicates that the film was stretched at 135°C and no heat setting was applied after stretching. Such conditions are typical for conventional polymer reflective polarizers. Data labeled “Stretch_135C_heatset_155C” and “Stretch_135C_heatset_160C” indicate that a heat setting step was used at 155°C and 160°C, respectively. For Figures 3 to 4 For each curve in the graph, the heat setting time is 5 seconds. Heat setting is applied to simulate the effect of thermoforming on the optical layer. Figure 3 In the middle, only a small shift in the average transmittance reflectance was observed, while in Figure 4 A significant reduction in average transmittance reflectance was observed. By appropriately selecting the materials of the high-refractive-index layer and the low-refractive-index layer, the refractive indices of the high-refractive-index layer and the low-refractive-index layer can be essentially matched along the transmittance state, allowing the average transmittance reflectance of visible light after thermoforming to be lower than [a certain value]. Figure 3 and Figure 4 As shown. For example, the HIO material can be adjusted (e.g., by blending with another polymer) to have a refractive index along the transparent state that substantially matches the transparent state refractive index of the LIO material after the LIO material is offset from the thermoforming process, and / or the LIO material can be adjusted (e.g., by blending with another polymer) to have a transparent state refractive index that substantially matches the transparent state refractive index of the HIO material after thermoforming.

[0023] The variation of a reflective polarizer during thermoforming can be characterized by the shift in the properties of the reflective polarizer (the reflectivity of the optical layer, the birefringence of the optical layer, or both) when the reflective polarizer is heated to at least one temperature within a specified range for a specified time.

[0024] In some embodiments, when the reflective polarizer 100 is heated to at least one temperature above the glass transition temperature of the second layer 11 and below the melting temperature of the first layer 10 for at least about 1 second, Rp0 decreases to at most 1 / 1.3 of its original value (i.e., the final reflectivity (which may be referred to as Rp1) is less than or equal to the corresponding initial reflectivity divided by 1.3); and Rs0 remains at least 50% (i.e., when the corresponding initial reflectivity is at least 50%, the final reflectivity (which may be referred to as Rp1) is at least 50%). In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to at least one temperature for at least about 1 second, Rp0 decreases to at most 1 / 2.6, 1 / 2.5, 1 / 2.4, 1 / 2.3, 1 / 2.2, 1 / 2.1, 1 / 2, 1 / 1.9, 1 / 1.8, 1 / 1.7, 1 / 1.6, 1 / 1.5, or 1 / 1.4 of its original value. In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to at least one temperature for at least about 1 second, Rs0 remains at least 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, or 98% of its original value. In some embodiments, or in other embodiments, when the reflective polarizer 100 is heated to at least one temperature for at least about 1 second, Rp0 / Rs0 decreases to at most 1 / 2.6, 1 / 2.5, 1 / 2.4, 1 / 2.3, 1 / 2.2, 1 / 2.1, 1 / 2, 1 / 1.9, 1 / 1.8, 1 / 1.7, 1 / 1.6, 1 / 1.5, 1 / 1.4, or 1 / 1.3 of its original value. In some embodiments, or in others, when the reflective polarizer 100 is heated to at least one temperature for at least about 1 second, Rs0 does not decrease or decreases to at least 1 / 1.01, 1 / 1.02, 1 / 1.03, 1 / 1.04, 1 / 1.05, 1 / 1.07 or 1 / 1.1 of its original value.

[0025] To test the effect of thermoforming on the reduction of birefringence of the LIO layer, various bilayer films (co-extruded and co-stretched with the LIO polymer layer and the HIO polymer layer) were prepared, stretched on a laboratory-scale stretching machine, and heat-set at different temperatures for different durations. The total thickness of the bilayer films was approximately 2 mils, with the lower refractive index layer having a thickness of approximately 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. Figures 5 to 7 It is a graph of in-plane birefringence versus heat setting temperature (HSTemp) of the low-refractive-index layer of various bilayer films under various stretching conditions and heat setting times, based on some implementation schemes. Figure 8This is a graph showing the in-plane birefringence versus heat setting time for various bilayer films with low refractive index layers at a heat setting temperature of 140°C, according to some implementation schemes. The in-plane birefringence of the low refractive index layer is denoted as TD-MD, which refers to the difference between the refractive index in the transverse direction (TD, the stretching direction, which may correspond to, for example, the x-direction) and the refractive index in the longitudinal direction (MD, which may correspond to, for example, the y-direction). The stretching ratio in the corresponding MD and TD directions is... Figure 5 and Figure 7 The values ​​are 0.45 and 5, and... Figure 6 and Figure 8 The values ​​are 0.5 and 5. For Figures 5 to 8 Each of them has a stretching temperature of 135℃.

[0026] In some implementations, when the reflective polarizer is thermoformed, the second birefringence (the planar birefringence of the second layer) decreases significantly, but any decrease in the first birefringence (the planar birefringence of the first layer) is significantly less than the decrease in the second birefringence (e.g., in percentage terms). This may occur due to thermoforming conditions (e.g., because the difference between the thermoforming temperature and the glass transition temperature of the second layer is greater than the difference between the thermoforming temperature and the glass transition temperature of the first layer), causing the birefringence of the second layer to "melt," while the birefringence of the first layer is essentially maintained. For example, the birefringence of the second layer may decrease by 20% or more, while the birefringence of the first layer may decrease by only about 5% or even less.

[0027] In some embodiments, when the reflective polarizer 100 is heated to at least one temperature above the glass transition temperature of the second layer and below the melting temperature of the first layer for at least about 1 second, and for at least one wavelength in the visible wavelength range, the second birefringence (the in-plane birefringence of the second layer) instead of the first birefringence (the in-plane birefringence of the first layer) decreases by at least 20%; |n1y–n2y| decreases by at least 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or 75%; and n1x–n2x remains greater than 0.05. In some such embodiments, or in other embodiments, when the reflective polarizer 100 is heated to at least one temperature for at least about 1 second, and for at least one wavelength in the visible wavelength range, n1x–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 at least one temperature for at least about one second, and for at least one wavelength in the visible wavelength range, the first birefringence decreases 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 at least one temperature for at least about one second, and for at least one wavelength in the visible wavelength range, the second birefringence decreases by at least 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, or 75%.

[0028] The temperature and time used for either or both of the reflectivity change and refractive index change tests can be selected to simulate available thermoforming conditions. As described elsewhere herein, at least one temperature may be greater than the glass transition temperature of the second layer 11 and less than the melting temperature of the first layer 10. The glass transition temperature of the first layer may be greater than the glass transition temperature of the second layer. In some embodiments, at least one temperature is greater than the glass transition temperature of the first layer 10 and less than the melting temperature of the first layer 10. In some embodiments, at least one temperature is or is included in the range of 130°C to 165°C. As described elsewhere herein, at least one temperature may be held for at least about 1 second. In some embodiments, the duration of at least about 1 second does not exceed about 15 seconds, 12 seconds, 10 seconds, 8 seconds, or 6 seconds. In some embodiments, the duration of at least about 1 second is at least about 1.2 seconds, 1.4 seconds, 1.6 seconds, 1.8 seconds, or 2 seconds.

[0029] 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 film references provided elsewhere herein. In some embodiments, the HIO layer (e.g., the first layer 10) may be, for example, a coPEN 90 / 10 layer (also known as low-melting-point PEN or LMPEN) as described in U.S. Patent No. 6,946,188 (Hebrink et al.), and the LIO layer (e.g., the second layer 11) may be, for example, a blend of coPEN 90 / 10 and at least one other polyester or other polymer.

[0030] CoPEN 90 / 10 can be described as a copolyester, wherein the carboxylate ester units of the copolyester comprise 90 mol% of naphthalene ester units and 10 mol% of terephthalate ester units. More generally, the first layer may be formed of a copolyester comprising other ratios of naphthalene ester units and terephthalate ester units. For example, in some embodiments, the carboxylate ester units of the copolyester may comprise about 80 mol% to 95 mol% of naphthalene ester units and about 5 mol% to 20 mol% of terephthalate ester units. Such a copolyester may be represented as coPEN N / 100-N, where N is the molar percentage of naphthalene ester units and 100-N is the molar percentage of terephthalate ester units.

[0031] At least one other polyester or other polymer in the LIO layer may include at least one diol-modified PET (PETg). Diol-modified PET can be described as PET in which at least some of the diol units are replaced by different units (such as those derived from cyclohexanediethanol). Suitable diol-modified PETs include PCTg and EASTAR GN071 (both available from Eastman Chemical Company, Knoxville, TN), and include PETg containing a plant-derived biomass component (e.g., isosorbide), because according to some embodiments, such diol-modified PETs have been found to result in improved refractive index matching with the HIO layer in the visible wavelength range (e.g., compared to PCTg and EASTAR GN071), for example, when a birefringent material (e.g., coPEN 90 / 10) is included in the LIO layer. Suitable diol-modified PETs containing isosorbide groups include those available under the trade name ECOZEN from SK Chemicals, Gyeonggi-do, South Korea, such as the ECOZEN T series including ECOZEN T120. For example, such diol-modified PETs may include isosorbide-derived groups and may have a glass transition temperature (e.g., about 120°C) higher than, for example, that of PCTg and EASTAR GN071 (e.g., about 80°C).

[0032] Figure 3 The reflective polarizer C1 comprises an HIO layer formed of coPEN 90 / 10 and an LIO layer formed of a blend of polycarbonate and copolyester. The reflective polarizer C1 is prepared by co-extrusion followed by substantially uniaxial stretching using a laboratory-scale stretching machine. Figure 4 Reflection polarizer and Figures 5 to 8 Various bilayer films are denoted as N / M / 100-NM, which indicates the composition of the lower refractive index layer. Figure 4 The reflective polarizer 65 / 15 / 25 is prepared in a manner generally described for the reflective polarizer C1, having a coPEN 90 / 10 HIO layer, but with an LIO layer formed of a blend of 65 wt% ECOZEN T120, 15 wt% coPEN90 / 10 and 25 wt% EASTAR GN071. Figures 5 to 8 The bilayer N / M / 100-NM uses a higher refractive index layer of coPEN 90 / 10 and a lower refractive index layer formed by a blend of ECOZEN T120 (N wt%), coPEN 90 / 10 (M wt%) and EASTAR GN071 (100-NM wt%). Figures 5 to 7 The bilayer film 90 / 10 uses a high refractive index layer of coPEN 90 / 10 and a low refractive index layer formed by a blend of 90 wt% ECOZEN T120 and 10 wt% coPEN 90 / 10. Figures 5 to 7 The bilayer film B1 uses a high refractive index layer of coPEN 90 / 10 and a low refractive index layer formed of the same material as the LIO layer of the reflective polarizer C1. It should be understood that blends of polymers and copolymers can be copolymer blends (e.g., blends of a first copolyester and a second copolyester can undergo transesterification during the extrusion process to produce a copolymer of the first and second copolyesters; such blends may be referred to as copolymer blends).

[0033] In some embodiments, each of the first layer 10 and the second layer 11 comprises a copolyester containing naphthalene dicarboxylate groups, terephthalate groups, and diol groups. In some such embodiments, or in other embodiments, the copolyester of each second layer further comprises isosorbide groups. In some embodiments, the copolyester of each second layer comprises isosorbide groups greater than about 5 mol%, 10 mol%, 15 mol%, 20 mol%, or 25 mol% based on the total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each second layer comprises isosorbide groups from about 5 mol% to about 35 mol%, 30 mol%, or 25 mol% based on the total moles of diol groups. In some such embodiments, or in other embodiments, the copolyester of each second layer further comprises cyclohexanediethanol groups. In some embodiments, each copolyester of the second layer comprises about 10 mol%, 15 mol%, or 20 mol% to about 55 mol%, 50 mol%, 45 mol%, or 40 mol% of cyclohexanediethanol groups based on the total molar number of diol groups. In some such embodiments, or in other embodiments, each copolyester of the second layer comprises about 5 mol%, 10 mol%, or 15 mol% to about 60 mol%, 50 mol%, 40 mol%, or 35 mol% of ethylene glycol groups based on the total molar number of diol groups. In some such embodiments, or in other embodiments, each copolyester of the first layer comprises about 80 mol% to 95 mol% of naphthalene ester groups based on the total molar number of carboxylic acid ester groups. In some such embodiments, or in other embodiments, each copolyester of the first layer comprises about 5 mol% to 20 mol% of terephthalate ester groups based on the total molar number of carboxylic acid ester groups. In some such embodiments, or in other embodiments, each copolyester of the second layer comprises about 75 mol% to 95 mol% of terephthalate groups based on the total molar number of carboxylate groups. In some such embodiments, or in other embodiments, each copolyester of the second layer comprises about 5 mol% to 25 mol% of naphthalene ester groups based on the total molar number of carboxylate groups.

[0034] Figure 9This is a schematic diagram of thermoforming a reflective polarizer 100 according to some embodiments. The edge portion 408 of the reflective polarizer 100 is held in a jig 473, and a mold 460 with a curved die surface 462 is schematically illustrated. The reflective polarizer 100 and / or the mold 460 can be heated (e.g., to a temperature T, which may correspond to at least one temperature described elsewhere herein, greater than the glass transition temperature of the second layer 11 and less than the melting temperature of the first layer 10). The reflective polarizer can be held at at least one temperature for a duration greater than about 1 second (or within the range described elsewhere herein). The jig 473 can pull the reflective polarizer 100 down onto the curved die surface 462, such that the resulting thermoformed reflective polarizer has a shape curved about two orthogonal axes (e.g., the x-axis and the y-axis) (e.g., corresponding to the shape of surface 462). As an alternative to pulling the reflective polarizer 100 down onto the die surface, the reflective polarizer 100 can be pushed against the die surface (e.g., via pressure). Useful pull-down and pressurization processes that can be utilized are described, for example, in U.S. Patent No. 11,543,572 (Jennings et al.).

[0035] In some embodiments, a method is provided for shaping a reflective polarizer 100. The reflective polarizer 100 comprises a plurality of alternating polymer first layers 10 and polymer second layers 11 having corresponding in-plane first and second birefringences, as further described elsewhere herein. The method includes thermoforming the reflective polarizer 100 into a shape bent about two orthogonal axes. The thermoforming includes heating the reflective polarizer 100 to at least one temperature above the glass transition temperature of the second layer 11 and below the melting temperature of the first layer 10 (or within the range described elsewhere herein) for a duration greater than about 1 second (or within the range described elsewhere herein). The thermoforming may include conforming the reflective polarizer to a bending mold surface 462. In some embodiments, prior to thermoforming, for at least one wavelength in the 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 within the range described elsewhere herein); and for substantially perpendicular incident light 30 (see, for example...), the second birefringence is greater than 0.007 (or within the range described elsewhere herein) for... Figure 1For the visible wavelength range, the plurality of alternating polymer first layers 10 and polymer second layers 11 have an average reflectivity Rs0 of at least 50% (or within the range described elsewhere herein) when the incident light is directionally polarized along a first plane of the reflective polarizer 100 (e.g., having a polarization state 141 polarized along the x-axis), and an average reflectivity Rp0 of greater than 1% and at most 40% (or Rp0 may be within another range described elsewhere herein) when the incident light is directionally polarized along a second plane orthogonal to the reflective polarizer 100 (e.g., having a polarization state 142 polarized along the y-axis). In some embodiments, after thermoforming, for substantially perpendicular incident light 30 and for the visible wavelength range, the plurality of alternating polymer first and second layers have an average reflectivity Rs1 when the incident light is directionally polarized along a first plane of the reflective polarizer 100, and an average reflectivity Rp1 when the incident light is directionally polarized along a second plane of the reflective polarizer 100. In some embodiments, Rp1 / Rs1 < 0.75, 0.7, 0.65, 0.6, 0.55, 0.5, 0.45, or 0.4 × Rp0 / Rs0 (e.g., in some embodiments, Rp1 / Rs1 < 0.6 × Rp0 / Rs0). In some such embodiments or in others, Rp0 / Rp1 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, or 2.6. In some such embodiments or in others, |Rs0-Rs1| / Rs0 < 0.1, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, or 0.02.

[0036] Terms such as “about” will be understood in the context in which they are used and described in this specification by those skilled in the art. If the use of “about” to express quantities of characteristic size, quantity, and physical properties is unclear to those skilled in the art in the context in which it is used and described in this specification, then “about” will be understood to mean within 10% of the specified value. A quantity given a specified value as “about” can be precisely the specified value. For example, if it is unclear to those skilled in the art in the context in which it is used and described in this specification, 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 can be 1.

[0037] The term “substantially” will be understood by those skilled in the art in the context of its use and description in this specification. If, in the context of its use and description in this specification, the use of “substantially” regarding a property or characteristic is not readily apparent to those skilled in the art, and when the opposite meaning of such property or characteristic is clear to those skilled in the art, the term “substantially” will be understood to mean that the property or characteristic is more pronounced than its opposite meaning.

[0038] All cited references, patents, and patent applications are incorporated herein by reference in their entirety in a consistent manner. In the event of any inconsistency or contradiction between the incorporated references and this application, the information in the foregoing description shall prevail.

[0039] Unless otherwise indicated, the description of elements in the accompanying drawings should be understood to apply equally to corresponding elements in the other drawings. While specific embodiments have been illustrated and described herein, those skilled in the art will recognize that various alternative and / or equivalent embodiments may be used instead of the illustrated and described embodiments without departing from the scope of this disclosure. This application is intended to cover any modifications, variations, or combinations of the specific embodiments discussed herein. Therefore, this disclosure is intended to be limited only by the claims and their equivalents.

Claims

1. A reflective polarizer comprising a plurality of alternating first and second polymer layers, the total number of which is at least 10, each of the plurality of alternating first and second polymer layers having an average thickness less than about 500 nm, the first layer having refractive indices n1x and n1y in respective orthogonal in-plane first and second directions of the reflective polarizer, the second layer having refractive indices n2x and n2y in the respective in-plane first and second directions, the in-plane first birefringence and in-plane second birefringence of the first and second layers being n1x–n1y and |n2x–n2y|, respectively, wherein for at least one wavelength in the visible wavelength range of from about 420 nm to about 680 nm: the second birefringence is less than the first birefringence and greater than 0.007; n1x–n2x>0.05; and 0.005<|n1y–n2y|<n1x–n2x, such that for substantially perpendicularly incident light and for the visible wavelength range: the plurality of alternating first and second polymer layers have an average reflectivity of at least 50% when the incident light is polarized along the in-plane first direction and an average reflectivity Rp0 of greater than 1% and at most 40% when the incident light is polarized along the in-plane second direction, wherein when the reflective polarizer is heated to at least one temperature that is higher than the glass transition temperature of the second layer and lower than the melting temperature of the first layer for a time of at least about 1 second, and for the at least one wavelength in the visible wavelength range: the second birefringence, rather than the first birefringence, is reduced by at least 20%; |n1y–n2y| is reduced by at least 20%; and n1x–n2x remains greater than 0.

05.

2. The reflective polarizer according to claim 1, wherein when the reflective polarizer is heated to the at least one temperature for a 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 according to claim 1, wherein when the reflective polarizer is heated to the at least one temperature for a 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 according to claim 1, wherein the at least one temperature includes a temperature in the range of 130°C to 165°C.

5. The reflective polarizer according to claim 1, wherein the time of at least about 1 second does not exceed about 15 seconds.

6. The reflective polarizer according to claim 1, wherein each of the first and second layers comprises a copolyester containing naphthalenedicarboxylate groups, terephthalate groups, and diol groups.

7. The reflective polarizer according to claim 6, wherein the copolyester of each second layer in the second layer further comprises isosorbide groups.

8. A reflective polarizer comprising at least 10 alternating polymer first and second layers, each of the alternating polymer first and second layers having an average thickness of less than about 500 nm, the first and second layers having corresponding in-plane first and second birefringences, wherein for at least one wavelength in the 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. This makes it possible for substantially perpendicular incident light and for the visible wavelength range: The plurality of alternating polymer first and second layers have an average reflectivity Rs0 of at least 50% when the incident light is directionally polarized along the first plane of the reflective polarizer, and an average reflectivity Rp0 of greater than 1% and at most 40% when the incident light is directionally polarized along the orthogonal second plane of the reflective polarizer. When the reflective polarizer is heated to at least one temperature above the glass transition temperature of the second layer and below the melting temperature of the first layer for at least about 1 second: Rp0 is reduced to at most 1 / 1.3 of its original value; and Rs0 should remain at least 50%.

9. The reflective polarizer of claim 8, wherein when the reflective polarizer is heated to the at least one temperature for at least about 1 second, Rp0 / Rs0 decreases to at most 1 / 1.3 of its original value.

10. The reflective polarizer of claim 8, wherein Rs0 is at least 80%, and when the reflective polarizer is heated to the at least one temperature for at least about 1 second, Rp0 decreases to at most 1 / 1.5 of its original value; and Rs0 remains at least 80%.

11. The reflective polarizer of claim 8, wherein the at least one temperature is in the range of 130°C to 165°C, and the time of at least about 1 second does not exceed about 15 seconds.

12. A method for shaping a reflective polarizer, the reflective polarizer comprising a plurality of alternating polymer first and second layers having corresponding in-plane first and second birefringences, the method comprising thermoforming the reflective polarizer into a shape bent about two orthogonal axes, the thermoforming comprising heating the reflective polarizer to at least one temperature above the glass transition temperature of the second layer and below the melting temperature of the first layer for a time greater than about 1 second. Before thermoforming: For at least one wavelength in the visible wavelength range of approximately 420 nm to approximately 680 nm, the second birefringence is less than the first birefringence and greater than 0.007; and For substantially perpendicular incident light and for the visible wavelength range, the plurality of alternating polymer first and second layers have an average reflectivity Rs0 of at least 50% when the incident light is directionally polarized along the first plane of the reflective polarizer, and an average reflectivity Rp0 of greater than 1% and at most 40% when the incident light is directionally polarized along the orthogonal second plane of the reflective polarizer. After thermoforming: For the substantially perpendicular incident light and for the visible wavelength range, the plurality of alternating polymer first and second layers have an average reflectivity Rs1 when the incident light is polarized in the direction within the first plane of the reflective polarizer, and an average reflectivity Rp1 when the incident light is polarized in the direction within the second plane of the reflective polarizer, wherein: Rp1 / Rs1<0.75×Rp0 / Rs0.

13. The method of claim 12, wherein Rp0 / Rp1 is at least 1.

3.

14. The method of claim 12, wherein |Rs0-Rs1| / Rs0<0.

1.

15. The method of claim 12, wherein the at least one temperature is in the range of 130°C to 165°C, and the time of at least about 1 second does not exceed about 15 seconds.

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