Multilayer optical film

Abstract

The present invention discloses a multilayer optical film, which includes a plurality of polymer layers arranged adjacent to each other in sequence. The difference in thickness between the spaced-apart first polymer layer and the second polymer layer in the plurality of polymer layers is less than about 10%. Each polymer layer disposed between the first polymer layer and the second polymer layer has a thickness of less than about 400 nm. Each layer in a group of at least three polymer layers disposed between the first polymer layer and the second polymer layer in the plurality of polymer layers has a thickness greater than the average thickness of the first polymer layer and the second polymer layer by about 20% to about 500%. The group of at least three polymer layers includes at least one pair of immediately adjacent polymer layers.

Description

Background Art

[0001] Multilayer optical films can include groupings of microlayers separated by optically thick protective boundary layers. Summary of the invention

[0002] In some aspects of the present specification, a multilayer optical film is provided, the multilayer optical film comprising a plurality of polymer layers arranged adjacent to each other in sequence. The difference in thickness between the spaced-apart first polymer layer and the second polymer layer in the plurality of polymer layers is less than about 10%. Each polymer layer disposed between the first polymer layer and the second polymer layer has a thickness of less than about 400 nm. Each layer in a group of at least three polymer layers disposed between the first polymer layer and the second polymer layer in the plurality of polymer layers has a thickness greater than the average thickness of the first polymer layer and the second polymer layer by about 20% to about 500%. A group of at least three polymer layers includes at least one pair of immediately adjacent polymer layers.

[0003] In some aspects of the present specification, a multilayer optical film is provided, the multilayer optical film comprising a plurality of optical repeating units arranged in sequence adjacent to each other along at least a portion of the thickness of the multilayer optical film. Each optical repeating unit comprises at least two layers and has a corresponding bandwidth. The bandwidths of a first optical repeating unit and a second optical repeating unit spaced apart from each other in the plurality of optical repeating units overlap each other. At least one pair of adjacent optical repeating units disposed between the first optical repeating unit and the second optical repeating unit in the plurality of optical repeating units has a non-overlapping bandwidth. None of the optical repeating units disposed between the first optical repeating unit and the second optical repeating unit has a thickness that is less than an average thickness of the first optical repeating unit and the second optical repeating unit by more than about 10%. Each layer disposed between the first optical repeating unit and the second optical repeating unit in the multilayer optical film has an average thickness of less than about 400 nm.

[0004] In some aspects of the present specification, a multilayer optical film is provided, the multilayer optical film including a plurality of alternating layers of a first polymer layer and a second polymer layer arranged adjacent to each other in sequence. At least the first polymer layer to the fourth polymer layer arranged adjacent to each other in the plurality of alternating layers of the first polymer layer and the second polymer layer have a desired average thickness t1 to t4, respectively. Each of t1 to t4 is less than about 400 nm. One of t2 and t3 is at least 5% greater than the other of t1, t4, and t2 and t3.

[0005] In some aspects of the present specification, a multilayer optical film is provided, the multilayer optical film comprising a first multilayer stack, a second multilayer stack, and a third multilayer stack disposed between the first multilayer stack and the second multilayer stack. Each of the first multilayer stack, the second multilayer stack, and the third multilayer stack comprises a plurality of polymer layers. The total number of polymer layers in each of the first multilayer stack and the second multilayer stack is at least 50. The first multilayer stack and the second multilayer stack comprise respective first polymer layers and second polymer layers adjacent to the third multilayer stack, wherein the thickness difference between the first polymer layer and the second polymer layer is less than about 10%. The third multilayer stack comprises at least one pair of adjacent polymer layers, such that each polymer layer in the at least one pair of adjacent polymer layers has a thickness at least about 20% greater than the average thickness of the first polymer layer and the second polymer layer. The multilayer optical film is integrally formed, and the minimum average peel force between the first portion and the second portion of the multilayer optical film is greater than about 0.4 N / cm, wherein the first portion and the second portion respectively comprise at least one polymer layer in the first multilayer stack and the second multilayer stack. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 is a schematic cross-sectional view of a multilayer optical film;

[0007] Figure 2A is a graph of layer thickness versus number of layers for a multilayer optical film;

[0008] Figure 2B for Figure 2A an enlarged view of a portion of a graph of;

[0009] Figure 3 is a schematic diagram of the overlapping bandwidth of the optical repeating unit;

[0010] Figure 4 is a schematic diagram of the non-overlapping bandwidth of the optical repeating unit;

[0011] Figure 5 is a schematic cross-sectional view of an optical repeating unit comprising two layers;

[0012] Figure 6 is a schematic cross-sectional view of an optical repeating unit comprising four layers;

[0013] Figures 7 and 8 is a schematic graph of transmittance versus wavelength of a multilayer optical film;

[0014] Fig. 9 Schematic diagram of the peeling test;

[0015] Fig.10 To have FIG. 2A to FIG. 2B A graph of transmittance versus wavelength for a reflective polarizer having a layer thickness profile shown in;

[0016] Fig.11A is a graph of layer thickness versus number of layers for a multilayer optical film;

[0017] Fig. 11B for Fig.11A an enlarged view of a portion of a graph of;

[0018] Fig.12 To have FIG. 11A to FIG. 11B A graph of transmittance versus wavelength for a reflective polarizer having a layer thickness profile shown in;

[0019] Fig.13 is a graph of layer thickness versus number of layers for a multilayer optical film including thick spacers between optical packets;

[0020] Fig.14A is a graph of layer thickness versus number of layers for a multilayer optical film; and

[0021] Fig. 14B for Fig.14A An enlarged view of a portion of a graph. DETAILED DESCRIPTION

[0022] In the following description, reference is made to the accompanying drawings, which form a part of the present invention and in which various embodiments are shown by way of example. The drawings are not necessarily drawn to scale. It should be understood that other embodiments may be conceived and carried out without departing from the scope or essence of the present description. Therefore, the following specific embodiments should not be considered to have a limiting meaning.

[0023] Multilayer optical films that provide desired transmission and / or reflection properties at least in part by an arrangement of microlayers having different refractive indices are known. For example, such optical films have been demonstrated by extruding alternating polymer layers, casting the layers onto a chill roll via a film die, and then stretching the cast web. See, for example, U.S. Pat. Nos. 3,610,729 (Rogers), 4,446,305 (Rogers et al.), 4,540,623 (Im et al.), 5,448,404 (Schrenk et al.), 5,882,774 (Jonza et al.), 6,157,490 (Wheatley et al.), 6,783,349 (Neavin et al.), and 9,279,921 (Kivel et al.) and International Application Publication No. 2018 / 163009 (Haag et al.). In these polymeric multilayer optical films, the polymer material may be primarily or exclusively used to constitute each layer. Such films are suitable for high-volume manufacturing processes and can be made into large sheets and rolls. By selecting appropriate microlayers and an appropriate arrangement (e.g., thickness profile) of the microlayers, the multilayer optical film can be configured, for example, as a broadband (e.g., visible and / or near-infrared wavelength range) reflective polarizer, a broadband mirror, a notched (e.g., having relatively narrow, spaced-apart reflective bands) reflective polarizer, or a notched mirror.

[0024] In some cases, the multilayer optical film includes two or more optical stacks or optical groups of optical repeating units. The optical repeating unit includes two or more layers and is repeated on the optical repeating unit stack or grouping. The optical repeating unit has a first-order reflection for a wavelength equal to twice the optical thickness (thickness multiplied by the refractive index) of the optical repeating unit. Each layer in the optical repeating unit grouping may have a thickness of less than 400nm, or less than 300nm, or less than 250nm, or less than 200nm. Each layer may have a thickness greater than about 5nm or greater than about 10nm. For example, the multilayer optical film may include two optical repeating unit groups, one of which is configured to reflect blue wavelengths to green wavelengths, and the other grouping is configured to reflect green wavelengths to red wavelengths. One or more interlayers may be included between the optical repeating unit groups. Conventionally, one or two optically thick (too thick to substantially contribute to first-order visible (e.g., wavelengths in the range of about 400 nm to about 700 nm) or near infrared (e.g., wavelengths in the range of about 700 nm to about 2500 nm) light reflection by optical interference) interlayers or protective boundary layers (PBLs) have been included. These PBL layers are typically included to prevent flow profiles in the coextruded web of alternating polymer layers from creating optical defects in the alternating polymer layers. According to the present specification, it has been found that the use of thinner PBLs can provide improved delamination resistance for adjacent optical groupings, but thinner PBLs can also cause optical defects. According to some embodiments of the present specification, it has been found that the use of a larger number of thinner PBLs provides improved delamination resistance between optical groupings without causing optical defects. According to some embodiments, these thinner PBLs are preferably less than about 400 nm thick. In some embodiments, three or more PBL layers are included between adjacent optical repeat unit groupings. In some embodiments, at least some (and in some cases, all) of these PBL layers are optical layers. In this context, an optical layer is a layer having a thickness within a range within which the layer can significantly contribute to first-order visible or near-infrared light reflection by optical interference. In some embodiments, the thickness profile of the PBL is selected to prevent or reduce optical coherence from such reflection (e.g., different optical repeating units in the PBL can have non-overlapping bandwidths) so that the PBL does not substantially affect the reflectivity of the multilayer optical film.

[0025] Figure 11 is a schematic cross-sectional view of a multilayer optical film 100. The optical film 100 includes a plurality of polymer layers 102 arranged adjacent to each other in sequence. The difference in thickness between the spaced-apart first polymer layer 111 and the second polymer layer 112 in the plurality of polymer layers is less than about 10%, or less than about 7%, or less than about 5%. The difference in thickness percentage between the first layer 111 and the second layer 112 is |h1-h2| divided by the greater of h1 and h2 multiplied by 100%, where h1 is the thickness of the first layer 111 and h2 is the thickness of the second layer 112. In some embodiments, each layer in a group 120 of at least three polymer layers disposed between the first polymer layer 111 and the second polymer layer 112 in the plurality of polymer layers 102 has a thickness (e.g., t1, t2, t3) that is about 20% to about 500% greater than the average thickness ((h1+h2) / 2) of the first polymer layer and the second polymer layer. In some such embodiments, each layer in group 120 has a thickness that is at least about 30%, or at least about 50%, or at least about 100%, or at least about 150% greater than the average thickness of the first and second polymer layers. In some such embodiments or in other embodiments, each layer in group 120 has a thickness that is no more than about 400%, or no more than about 300%, or no more than about 250% greater than the average thickness of the first and second polymer layers. In some embodiments, a group 120 of at least three polymer layers has an average thickness that is about 50% to about 400%, or about 100% to about 300%, or about 150% to about 250% greater than the average thickness of the first and second polymer layers 111, 112 (e.g., (t1+t2+t3) / 3 in the illustrated embodiment).

[0026] Typically, each layer in a multilayer optical film has a constant or approximately constant thickness. Unless otherwise specified, if there is variation in the thickness of the layers in the multilayer optical film, the thickness of the layer refers to the average (unweighted average) thickness of the layer. The average thickness of a group of layers is the arithmetic mean of the thicknesses of the individual layers in the group. The expected average thickness of a layer is the designed or nominal thickness of the layer. In some embodiments, the expected average thickness of a layer is the same or substantially the same as the average thickness of the layer.

[0027] In some embodiments, a group 120 of at least three polymer layers includes at least one pair of immediately adjacent polymer layers (e.g., 124 and 122, or 122 and 119). In some embodiments, the polymer layers in a group 120 of at least three polymer layers are arranged adjacent to each other in sequence, such as Figure 1 In other embodiments, additional polymer layers may separate some of the polymer layers in group 120.

[0028] The optical film 100 may include Figure 1In some embodiments, the multilayer optical film 100 includes at least 50 layers, or at least 100 layers, or at least 200 layers. In some such embodiments or in other embodiments, the multilayer optical film 100 includes no more than 1000 layers, or no more than 800 layers.

[0029] Polymer layer 128 is a polymer layer disposed between first polymer layer 111 and second polymer layer 112 in the plurality of polymer layers. In some embodiments, the total number of polymer layers 128 disposed between first polymer layer 111 and second polymer layer 112 in the plurality of polymer layers is at least 3, or at least 4, or at least 5. In some embodiments, the total number of polymer layers 128 disposed between first polymer layer 111 and second polymer layer 112 in the plurality of polymer layers is no more than 30, or no more than 25, or no more than 20, or no more than 15, or no more than 12, or no more than 10.

[0030] As further described elsewhere, the multilayer optical film 100 may have desired optical transmittance and reflectivity for substantially normal incident light 144 having a first polarization state 145 (e.g., normal incident light, or light incident at 30 degrees from normal, or within 20 degrees from normal, or within 10 degrees from normal) and substantially normal incident light 146 having a second polarization state 147.

[0031] Figure 2A is a graph of layer thickness versus number of layers for a multilayer optical film that may correspond to multilayer optical film 100 . Figure 2B yes Figure 2A A portion of a graph showing a first polymer layer 211 and a second polymer layer 212 spaced apart and a group 220 of at least three polymer layers disposed between the first polymer layer 211 and the second polymer layer 212. Data points at integer number of layers are shown. The lines between the data points provide a guide for the eye. The first layer 211 has a thickness of 84.3 nm, the second layer 212 has a thickness of 84.4 nm, and the layers in the group 220 have thicknesses of 175.5 nm, 196.2 nm, 129.8 nm, and 112.3 nm, respectively. Each layer in the group 220 has a thickness that is about 33% to about 133% greater than the average thickness of the first layer 211 and the second layer 212 (e.g., (112.3 nm - 84.35 nm) / 84.35 nm x 100% is about 33%).

[0032] In some embodiments, a group 120 or 220 of at least three polymer layers in the plurality of polymer layers is a group of at least four polymer layers in the plurality of polymer layers. In some embodiments, the group of at least three polymer layers includes less than 20 polymer layers, or less than 15 polymer layers, or less than 10 polymer layers.

[0033] In some embodiments, each of the first polymer layer 111 or 211 and the second polymer layer 112 or 212 is disposed between the group 120 or 220 of at least three polymer layers and at least 50 other polymer layers in the plurality of polymer layers. Figure 1 The sets of layers 125 and 126 schematically shown in FIG. 1 may each include at least 50 layers.

[0034] In some embodiments, each layer disposed between the first polymer layer 111 or 211 and the second polymer layer 112 or 212 has a thickness (e.g., the average thickness of the layer) of less than about 1 micron, or less than about 700 nm, or preferably less than about 500 nm, or more preferably less than about 400 nm, or even more preferably less than about 300 nm or less than about 250 nm.

[0035] In some embodiments, none of the layers disposed between the first polymer layer 111 or 211 and the second polymer layer 112 or 212 has a thickness (e.g., the average thickness of the layer) that is less than the average thickness of the first polymer layer and the second polymer layer by more than about 10% or more than about 5%. In other words, in some embodiments, none of the layers disposed between the first polymer layer 111 or 211 and the second polymer layer 112 or 212 has a thickness that is less than about 0.9 times or less than about 0.95 times the average thickness of the first polymer layer and the second polymer layer.

[0036] In some embodiments, the multilayer optical film 100 includes a first group 140 of polymeric layers arranged sequentially adjacent to one another along at least a portion of the thickness of the multilayer optical film 100. In some embodiments, the first group 140 includes at least 200 polymeric layers ( Figure 1 For example, the first group 140 may correspond to a system including at least 200 to 400 layers. FIG. 2A to FIG. 2B The first group 140 of polymer layers includes a first polymer layer 111 and a second polymer layer 112 and a group 120 of at least three polymer layers. Each layer in the first group 140 of at least 200 polymer layers has a thickness (e.g., an average thickness of the layer) of less than about 1 micrometer, or less than about 700 nm, or preferably less than about 500 nm, or more preferably less than about 400 nm, or even more preferably less than about 300 nm or less than about 250 nm.

[0037] In some embodiments, multilayer optical film 100 includes a plurality of optical repeating units (e.g., layer pairs 103, 104) arranged sequentially adjacent to each other along at least a portion of the thickness of multilayer optical film 100. Each optical repeating unit includes at least two layers 103 and 104 and has a corresponding bandwidth. For example, a first optical repeating unit 111, 113 may have a bandwidth W1 between a left wavelength λ1L and a right wavelength λ1R, and a second optical repeating unit 112, 114 may have a bandwidth W2 between a left wavelength λ2L and a right wavelength λ2R, as shown in FIG. Figure 3 As shown schematically. The left wavelength and the right wavelength can be understood as the full width at half maximum (FWHM) band edge wavelengths. In some embodiments, the bandwidths of the spaced-apart first optical repeating units 111, 113 and the second optical repeating units 112, 114 in the plurality of optical repeating units overlap with each other. For example, λ2L is between λ1L and λ1R, and λ1R is between λ2L and λ2R. In some embodiments, at least one pair of adjacent optical repeating units (e.g., 124, 122 and 119, 117) disposed between the first optical repeating units 111, 113 and the second optical repeating units 112 and 114 in the plurality of optical repeating units has a non-overlapping bandwidth. This Figure 4 Schematically illustrated in FIG, which shows a bandwidth W3 between λ3L and λ3R of an optical repeating unit (e.g., 124, 122) and a bandwidth W4 between λ4L and λ4R of an adjacent optical repeating unit (e.g., 119, 117). The bandwidths do not overlap because there is no overlap in the range λ3L and λ3R and in the range λ4L and λ4R. In some embodiments, none of the optical repeating units disposed between the first optical repeating unit and the second optical repeating unit has a thickness that is less than more than about 10% or more than about 5% of the average thickness of the first optical repeating unit and the second optical repeating unit. The thickness of the optical repeating unit is the total thickness of at least two layers in the optical repeating unit. In some embodiments, each layer disposed between the first optical repeating unit and the second optical repeating unit in the multilayer optical film has a thickness (e.g., the average thickness of the layer) of less than about 1 micron, or less than about 700 nm, or preferably less than about 500 nm, or more preferably less than about 400 nm, or even more preferably less than about 300 nm or less than about 250 nm.

[0038] The bandwidth of an optical repeating unit (ORU) is the bandwidth of the first order reflection band that an infinite stack of ORUs of equal thickness would exhibit. This can be readily calculated from the matrix elements of the characteristic matrix M as defined in Born and Wolf, Principles of Optics, 5th edition, p. 67.

[0039] The optical repeat unit can include two layers or can include more than two layers. For example, in some embodiments, each optical repeat unit includes at least three layers or at least four layers. Figure 5 5 is a schematic cross-sectional view of an optical repeat unit 505 including two layers 503 and 504. In some embodiments, the optical thickness of each of layers 503 and 504 is one quarter of substantially the same wavelength. In some embodiments, one or both of layers 503 and 504 are replaced with two (or more) layers. In some embodiments, the combined optical thickness of the two (or more) layers is one quarter of the wavelength that the optical repeat unit is configured to reflect.

[0040] Figure 6 Schematic cross-sectional view of an optical repeating unit 605 including four layers 603, 607, 604, 608. In some embodiments, layers 607 and 608 are bonding layers that are included to improve bonding to adjacent layers. In some embodiments, layers 607 and 608 have a thickness in the range of, for example, about 5 nm to about 50 nm, or about 10 nm to about 30 nm. In some embodiments, layers 607 and 608 have the same composition. In some embodiments, each optical repeating unit 605 includes layers A (603), B (607 or 608) and C (604), and the layers A, B and C are arranged in the order of ABCB. In some such embodiments, the optical thickness of layers AB and CB is one quarter of the substantially same wavelength.

[0041] In some embodiments, the multilayer optical film 100 includes a plurality of alternating layers of first polymer layers 103 and second polymer layers 104 arranged sequentially adjacent to each other. At least the first to fourth layers (e.g., layers 124, 122, 119, and 117, respectively) of the plurality of alternating layers of first polymer layers 103 and second polymer layers 104 arranged sequentially adjacent to each other have a desired average thickness t1 to t4, respectively, wherein one of t2 and t3 is at least 5%, or at least 7%, or at least 10%, or at least 15%, or at least 20%, or at least 30% greater than the other of t1, t4, and t2 and t3. In some embodiments, one of t2 and t3 is about 5% to about 500% greater than the other of t1, t4, and t2 and t3; or about 10% to about 500%; or about 20% to about 500%, or about 400%, or about 300%, or about 250%. In some embodiments, each of t1 to t4 is less than about 1 micron, or less than about 700 nm, or preferably less than about 500 nm, or more preferably less than about 400 nm, or even more preferably less than about 300 nm, or less than about 250 nm.

[0042] In some embodiments, multilayer optical film 100 is a reflective polarizer. Figure 7Schematic graph of transmittance of an optical film versus wavelength for substantially perpendicular incident light of first polarization state 745 and second polarization state 747. Figure 7 The wavelength range depicted in can be at least 200 nm wide (e.g., at least 450 nm to 650 nm, or 400 nm to 700 nm). In some embodiments, for substantially normal incident light within a wavelength range of at least 200 nm, the multilayer optical film 100 has an average optical reflectivity of at least 60% for a first polarization state 745, and an average optical transmittance T2 of at least 60% for an orthogonal second polarization state 747. Typically, optical absorption is negligible, such that the average optical reflectivity is about 100% minus T1.

[0043] In some embodiments, at least some of the polymer layers (e.g., one of the two alternating polymer layers) are substantially uniaxially oriented. For example, in some embodiments, the multilayer optical film is a reflective polarizer that is a substantially uniaxially stretched film and has a degree of uniaxiality U of at least 0.7, or at least 0.8, or at least 0.85, where U = (1 / MDDR - 1) / (TDDR 1 / 2 –1), where MDDR is defined as the machine direction stretch ratio and TDDR is defined as the transverse direction stretch ratio. Such substantially uniaxially oriented multilayer optical films are described, for example, in US Pat. No. 2010 / 0254002 (Merrill et al.).

[0044] In some embodiments, the multilayer optical film 100 is a mirror film. In some embodiments, for substantially normal incident light within a wavelength range of at least 200 nm, the multilayer optical film 100 has an average optical reflectivity of at least 60% for each of the mutually orthogonal first polarization state and the second polarization state. For example, the transmittance of the first polarization state and the second polarization state can each follow Figure 7 The curve marked 747 in .

[0045] The transmittance can differ significantly from Figure 7 For example, the transmittance may vary with wavelength rather than being constant or substantially constant over a range of wavelengths.

[0046] In some embodiments, multilayer optical film 100 is a reflective polarizer having spaced-apart reflective bands. Figure 81 is a schematic graph of transmittance of an optical film versus wavelength for substantially perpendicular incident light of first polarization state 845 and second polarization state 847. In some embodiments, for substantially perpendicular incident light and for each of first wavelength λ1 and second wavelength λ2, the multilayer optical film 100 reflects at least 60% of incident light having the first polarization state 845 and transmits at least 60% of incident light having the orthogonal second polarization state 847. For at least a third wavelength λ3 disposed between the first wavelength λ1 and the second wavelength λ2, the multilayer optical film 100 transmits at least 60% of incident light for each of the first polarization state 845 and the second polarization state 847.

[0047] The transmittance can differ significantly from Figure 8 For example, the transmittance or reflectance in the reflection bands near λ1 and λ2 may vary within the band and may have a more gradual band edge transition. For another example, the transmittance or reflectance may be significantly different for the reflection band near λ1 and the reflection band near λ2.

[0048] The multilayer optical film of the present specification can be integrally formed. As used herein, a first element "integrated" with a second element means that the first element and the second element are manufactured together, rather than being manufactured separately and then combined. Forming as one includes manufacturing the first element and then manufacturing the second element on the first element. An optical film including multiple layers is integrally formed if the layers are manufactured together (for example, combined into a molten stream and then cast onto a cooling roller to form a cast film, and then the cast film is oriented), rather than being manufactured separately and then combined.

[0049] In some embodiments, the multilayer optical film 100 includes a first multilayer stack (layer 111 and a group of layers 124), a second multilayer stack (layer 112 and a group of layers 126), and a third multilayer stack (a group of layers 128) disposed therebetween. Each of the first multilayer stack, the second multilayer stack, and the third multilayer stack includes a plurality of polymer layers. In some embodiments, the total number of polymer layers in each of the first multilayer stack and the second multilayer stack is at least 50, or at least 100, or at least 150, or at least 200. In some embodiments, the total number of polymer layers in the third multilayer stack is at least 3, or at least 4, or at least 5, and in some embodiments is no more than 30, or no more than 25, or no more than 20, or no more than 15, or no more than 12, or no more than 10. In some embodiments, each polymer layer in the third multilayer stack has a thickness of less than 400 nm, or less than 300 nm, or less than 250 nm. Each of the first multilayer stack and the second multilayer stack, and optionally the third multilayer stack, can be an optical stack comprising a plurality of optical repeating units as described elsewhere. The first multilayer stack and the second multilayer stack include respective first polymer layers 111 and second polymer layers 112 adjacent to the third multilayer stack (a set of layers 128). The difference in thickness between the first polymer layer 111 and the second polymer layer 112 is less than about 10%. The third multilayer stack includes at least one pair of adjacent polymer layers (e.g., 124 and 122, or 122 and 119) such that each polymer layer in the at least one pair of adjacent polymer layers has a thickness that is at least about 20% greater than the average thickness of the first polymer layer and the second polymer layer. In some embodiments, each polymer layer in the at least one pair of adjacent polymer layers has a thickness that is no more than about 500% greater than the average thickness of the first polymer layer 111 and the second polymer layer 112. In some embodiments, each layer in the third multilayer stack (a set of layers 128) has a thickness in the range of 90% to 600%, or to 500%, or to 400%, or to 350% of the average thickness of the first polymer layer 111 and the second polymer layer 112. In some embodiments, the multilayer optical film is integrally formed, and the minimum average peel strength between the first portion and the second portion of the multilayer optical film is greater than about 0.4 N / cm, wherein the first portion and the second portion respectively include at least one polymer layer of the first multilayer stack and the second multilayer stack. In some embodiments, the minimum average peel strength is greater than about 0.6 N / cm, or greater than about 0.8 N / cm. In some embodiments, the minimum average peel strength is determined using a substantially 90 degree peel test at a peel speed of about 1.5 m / min, wherein the minimum average peel strength is the minimum value of the peel strength averaged over an averaging time of about 5 seconds.

[0050] Fig. 9Schematically illustrated is a peel test applied to an integrally formed multilayer optical film 400, which may correspond to, for example, optical film 100. Optical film 400 may be cut into standard sizes for testing (e.g., 1 inch (2.54 cm) wide x 12 inch (30 cm) strips). Double-sided tape 458 (e.g., 3M 665 Double-Sided Tape available from 3M Company, St. Paul, MN) is attached to a plate 455 (e.g., a metal plate), and film 400 is attached to double-sided tape 448. The film 400 is scored (e.g., using a razor blade) near the edge of the film along score line 444, which forms an angle α with the major surface of the plate 455 in the range of, for example, 20 to 60 degrees or 30 to 45 degrees. The tape 459 is applied to the film 400 so that the tape 459 covers at least the scored portion of the film 400 and so that the free end 479 of the tape 459 can be used for the peel test. For example, the tape 459 can be an approximately 1.5 inch (4 cm) strip of 3M 396 Tape available from 3M Company of St. Paul, Minnesota. The free end 479 used for gripping during the peel test can be folded over on itself to form a non-stick sheet (e.g., approximately 1 / 2 inch (1.3 cm) sheet). A substantially 90 degree peel test is then performed by peeling from the free end 479. For example, the pulling direction (given by Fig. 9 The angle β between the applied force F in (shown schematically) and the direction parallel to the top surface of the plate 455 may be about 90 degrees. The peel test is performed at a peel speed (speed of the free end 479 along the pulling direction) in the range of about 1.2m / min to about 1.8m / min (e.g., about 1.5m / min). The peel test can be performed using, for example, an IMASS SP-2000 peel tester (IMASS Inc., Accord, MA, USA). The peel strength is averaged over an average time of about 4 to about 6 seconds (e.g., about 5 seconds). The average peel strength can be determined for each sample in a plurality of samples (e.g., five film samples) for a single average time, or for a single (e.g., longer) sample for multiple intervals of the average time. The minimum value among these average peel strengths is referred to as the minimum average peel strength.

[0051] The peel strength is between two portions 434 and 435 of the optical film 400, wherein each of the two portions 434, 435 includes at least one layer of the multilayer optical film 400 (e.g., one of the outermost polymer layers of the film). For example, during the peel test, the optical film 400 may delaminate at an interface between one of the outermost layers and an adjacent layer, such that one of the two portions 434, 435 includes the delaminated outermost layer and the other of the two portions 434, 435 includes the remainder of the optical film 400. As another example, the optical film 400 may delaminate between the first optical stack and the second optical stack. For example, when a conventional thick PBL layer is included between the first optical stack and the second optical stack, the peeling may occur through the bulk of the layer or at the interface between the layer and the adjacent layer. According to some embodiments, this failure mode is eliminated or substantially reduced by including a plurality of thinner PBLs between the first optical stack and the second optical stack. As another example, the optical film 400 may delaminate at an interface between two inner layers within one of the optical stacks. As another example, delamination may initiate at the interface between the outermost layer and an adjacent layer and then propagate into the inner layers of optical film 400 such that each portion 434 and 435 includes a portion of an inner layer.

[0052] In some embodiments, the multilayer optical film includes a plurality of alternating high refractive index layers and low refractive index layers. In some embodiments, the low refractive index layer is formed from a blend of polycarbonate, PETG (copolyester of polyethylene terephthalate (PET) with cyclohexanedimethanol as a glycol modifier; available from Eastman Chemicals, Knoxville, TN) and PCTG (copolyester of PET with cyclohexanedimethanol as a glycol modifier, with twice the amount of cyclohexanedimethanol compared to PETG; available from Eastman Chemicals, Knoxville, TN). The ratio of the polycarbonate used can be selected to provide a desired glass transition temperature. In some embodiments, the glass transition temperature can be selected to improve micro-wrinkling of the optical film, as further described in co-owned provisional patent application No. 62 / 851,991, filed on May 23, 2019, and entitled "MULTILAYER OPTICAL FILM". In some embodiments, the high refractive index layer is formed of polyethylene naphthalate (PEN) or PEN / polyethylene terephthalate (PET) copolymer. Alternatively, other polymer materials known to be useful in polymer multilayer optical films can be used.

[0053] Example

[0054] Peel strength test method

[0055] Film samples were prepared and cut into 1 inch (2.54 cm) by 12 inch (30 cm) strips. Double-sided tape (3M665 double-sided tape purchased from 3M Company, St. Paul, MN) was attached to the metal plate, and the sample strip was attached to the double-sided tape. Excess film was cut from one end of the plate so that the film was flush with the edge of the plate, while the other edges were scored by cutting at an acute angle using a razor blade. One end of an approximately 1.5 inch (4 cm) strip of tape (3M 396 tape purchased from 3M Company, St. Paul, MN) was folded over itself to form a 1 / 2 inch (1.3 cm) non-stick sheet. The other end of the tape was applied to the scored edge of the film sample. Then, a 90 degree peel test was performed using an IMASS SP-2000 peel tester (Emerson, Inc., Accord, Massachusetts, USA) using a 5 second average time and a peel speed of 60 in / min (1.5 m / min). Five strips were tested for each film sample. For the results given in the examples, the minimum value is reported to compare the weakest or lowest force required to delaminate the layers from each other.

[0056] Example 1

[0057] A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were coextruded, each of which included 325 alternating layers of polyethylene naphthalate (PEN) and a low refractive index isotropic layer composed of a blend of polycarbonate and copolyester (PC:coPET) such that the refractive index was about 1.57 and remained substantially isotropic when uniaxially oriented, wherein the PC:coPET weight ratio was approximately 41 wt % PC and 59 wt % coPET, and the Tg was 105.8 degrees Celsius. This isotropic material was selected so that after stretching, its refractive index in both non-stretched directions remained substantially matched to the refractive index of the birefringent material in the non-stretched direction, while in the stretched direction, the refractive index between the birefringent layer and the non-birefringent layer was substantially mismatched. The PEN and PC / coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock where they were assembled into packets of 325 alternating optical layers, plus thicker PC / coPET protective boundary layers on the outside of the stacked optical packets, and between packets, 9 alternating inner protective boundary layers of varying optical thickness (see Figure 2B), 661 layers in total. The multilayer melt was then cast through a film die onto a chill roll where it was quenched in a conventional manner for polyester films. The cast web was then stretched in a parabolic tenter at a temperature of 320°F in the transverse direction at a ratio of approximately 6:1 as described in U.S. Pat. No. 6,916,440 (Jackson et al.).

[0058] The layer thickness distribution of the optical film of Example 1 is shown in FIG. 2A to FIG. 2B The outermost protective boundary layer is not included in the graph. The light transmission and light blocking transmission at normal incidence angles are determined and shown in Fig.10 The average transmittance from 450 nm to 650 nm was 0.011% and 86.7% for block polarization and pass polarization, respectively. The film of Example 1 had a resulting total thickness of about 58.9 μm as measured by a capacitance gauge. The minimum average peel force was 0.991 N / cm.

[0059] Example 2

[0060] A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were coextruded, each having 325 alternating layers of 90 / 10 coPEN (a polymer composed of 90% polyethylene naphthalate (PEN) and 10% polyethylene terephthalate (PET)) and a low refractive index isotropic layer made from a blend of polycarbonate and copolyester (PC:coPET) such that the refractive index was about 1.57 and remained generally isotropic when uniaxially oriented, wherein the PC:coPET weight ratio was approximately 61 wt % PC and 39 wt % coPET, and a Tg of 116.4 degrees Celsius. This isotropic material was selected so that after stretching, its refractive index in both non-stretched directions remained substantially matched to the refractive index of the birefringent material in the non-stretched direction, while in the stretched direction, there was a substantial mismatch in the refractive index between the birefringent layer and the non-birefringent layer. The PEN and PC / coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock where they were assembled into packets of 325 alternating optical layers, plus thicker PC / coPET protective boundary layers on the outside of the stacked optical packets, and between packets, 9 alternating inner protective boundary layers of varying optical thickness (see Fig. 11B ), 661 layers in total. The multilayer melt was then cast through a film die onto a chill roll where it was quenched in a conventional manner for polyester films. The cast web was then stretched in a parabolic tenter at a temperature of 300°F in the transverse direction at a ratio of approximately 6:1 as described in U.S. Pat. No. 6,916,440 (Jackson et al.).

[0061] The layer thickness distribution of the optical film of Example 2 is shown in FIG. 11A to FIG. 11B The outermost protective boundary layer is not included in the graph. The light transmission and light blocking transmission at normal incidence angles are determined and shown in Fig.12 The average transmittance from 450 nm to 650 nm was 0.021% and 89.2% for block polarization and pass polarization, respectively. The film of Example 2 had a resulting total thickness of about 58.7 as measured by a capacitance gauge. The minimum average peel force was 0.876 N / cm.

[0062] Comparative Example C1

[0063] A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were coextruded, each having 325 alternating layers of polyethylene naphthalate (PEN) and a low refractive index isotropic layer composed of a blend of polycarbonate and copolyester (PC:coPET) such that the refractive index was about 1.57 and remained substantially isotropic when uniaxially oriented, wherein the PC:coPET weight ratio was approximately 41 wt % PC and 59 wt % coPET, and a Tg of 105.8 degrees Celsius. This isotropic material was selected such that after stretching, its refractive index in both non-stretched directions remained substantially matched to the refractive index of the birefringent material in the non-stretched direction, while in the stretched direction, there was a substantial mismatch in the refractive index between the birefringent layer and the non-birefringent layer. The PEN and PC / coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock where they were assembled into groups of 325 alternating optical layers, plus thicker PC / coPET protective boundary layers on the outside of the stacked optical groups, and thicker PC / coPET protective boundary layers between the optical groups, for a total of 653 layers. The multilayer melt was then cast through a film die onto a chill roll where it was quenched in a conventional manner for polyester films. The cast web was then stretched in a parabolic tenter frame at a temperature of 327° F. in the cross direction at a ratio of approximately 6:1 as described in U.S. Pat. No. 6,916,440 (Jackson et al.).

[0064] The layer thickness distribution of the optical film of Comparative Example C1 is shown in Fig.13 The outermost protective boundary layer is not included in the graph. The central protective boundary layer thickness is 5 μm. The average transmittances from 450 nm to 650 nm are 0.009% and 88.49% for the transmittance polarization and the block polarization, respectively. The film of Comparative Example C1 has a resulting total thickness of about 64.9 μm as measured by a capacitance gauge. The minimum peel force across the entire film is present between the groups and is measured to be 0.12 N / cm.

[0065] Example 3

[0066] A birefringent reflective polarizer optical film was prepared as follows. Two multilayer optical packets were coextruded, each of which included 325 alternating layers of polyethylene naphthalate (PEN) and a low refractive index isotropic layer composed of a blend of polycarbonate and copolyester (PC:coPET) such that the refractive index was about 1.57 and remained substantially isotropic when uniaxially oriented, wherein the PC:coPET weight ratio was approximately 41 wt % PC and 59 wt % coPET, and the Tg was 105.8 degrees Celsius. This isotropic material was selected so that after stretching, its refractive index in both non-stretched directions remained substantially matched to the refractive index of the birefringent material in the non-stretched direction, while in the stretched direction, the refractive index between the birefringent layer and the non-birefringent layer was substantially mismatched. The PEN and PC / coPET polymers were fed from separate extruders to a multilayer coextrusion feedblock where they were assembled into groups of 325 alternating optical layers, plus thicker PC / coPET protective boundary layers on the outside of the stacked optical groups, and between groups, 9 alternating inner protective boundary layers of some optical thickness but not uniform, for a total of 661 layers. The multilayer melt was then cast through a film die onto a chill roll where it was quenched in a conventional manner for polyester films. The cast web was then stretched in a parabolic tenter frame at a temperature of 319° F. in the transverse direction at a ratio of approximately 6:1 as described in U.S. Pat. No. 6,916,440 (Jackson et al.).

[0067] The layer thickness distribution of the optical film of Example 1 is shown in FIG. 14A to FIG. 14B The outermost protective boundary layer is not included in the graph. The average transmittance from 450nm to 650nm is 0.016% and 88.89% for the transmittance polarization and the block polarization, respectively. The film of Example 3 has a resulting total thickness of about 61.6μm as measured by a capacitance gauge. The minimum peel force on the entire film exists near the outer layer and is 0.81N / cm.

[0068] Terms such as "about" will be understood by one of ordinary skill in the art in the context of use and description in this specification. If it is not clear to one of ordinary skill in the art in the context of use and description in this specification that "about" should be used to express the amount of feature size, quantity, and physical properties, then "about" will be understood to mean within 10% of the specified value. A quantity given as about a specified value may be exactly the specified value. For example, if it is not clear to one of ordinary skill in the art in the context of use and description 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 the value may be 1.

[0069] All references, patents, and patent applications cited above are hereby incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or conflicts between an incorporated reference portion and the present application, the information in the foregoing description shall prevail.

[0070] Unless otherwise indicated, the description of the elements in the drawings should be understood to apply equally to the corresponding elements in the other drawings. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that the specific embodiments shown and described may be replaced with a variety of alternative and / or equivalent implementations without departing from the scope of the present disclosure. The present application is intended to cover any modification or variation of the specific embodiments discussed herein. Therefore, the present disclosure is intended to be limited only by the claims and their equivalents.

Claims

1. A multilayer optical film, comprising a plurality of polymer layers arranged adjacent to each other in sequence, wherein the difference in thickness between a first polymer layer and a second polymer layer spaced apart from each other in the plurality of polymer layers is less than about 10%, the first polymer layer and the second polymer layer and each polymer layer disposed between the first polymer layer and the second polymer layer have a thickness of less than about 400 nm, each layer in a group of at least three polymer layers disposed between the first polymer layer and the second polymer layer in the plurality of polymer layers has a thickness greater than an average thickness of the first polymer layer and the second polymer layer by about 50% to about 500%, the group of at least three polymer layers comprising at least one pair of adjacent polymer layers, wherein the total number of polymer layers disposed between the first polymer layer and the second polymer layer in the plurality of polymer layers does not exceed 30.

2. The multilayer optical film of claim 1 comprising at least 50 layers.

3. The multilayer optical film of claim 1, wherein the polymer layers in the set of at least three polymer layers are arranged sequentially adjacent to one another. 4 . The multilayer optical film of claim 1 , wherein the set of at least three polymeric layers in the plurality of polymeric layers is a set of at least four polymeric layers in the plurality of polymeric layers.

5. The multilayer optical film of claim 1, wherein the set of at least three polymeric layers have an average thickness that is from about 150% to about 250% greater than the average thickness of the first and second polymeric layers.

6. The multilayer optical film of claim 1, wherein each of the first polymer layer and the second polymer layer is disposed between the set of at least three polymer layers and at least 50 other polymer layers in the plurality of polymer layers.

7. The multilayer optical film of claim 1, wherein no layer disposed between the first polymer layer and the second polymer layer has an average thickness that is more than about 10% less than the average thickness of the first polymer layer and the second polymer layer.

8. The multilayer optical film of claim 1 , comprising a first group of at least 200 polymer layers arranged sequentially adjacent to one another along at least a portion of the thickness of the multilayer optical film, the first group of polymer layers comprising the first polymer layer and the second polymer layer and the group of at least three polymer layers, each layer in the first group of at least 200 polymer layers having an average thickness of less than about 400 nm.

9. The multilayer optical film of claim 1, wherein the set of at least three polymeric layers comprises fewer than 20 polymeric layers.

10. The multilayer optical film of any one of claims 1 to 9, wherein the total number of polymer layers in the plurality of polymer layers disposed between the first polymer layer and the second polymer layer does not exceed 20.

11. The multilayer optical film of claim 1 , wherein the plurality of polymer layers further comprise a plurality of optical repeating units arranged sequentially adjacent to one another along at least a portion of the thickness of the multilayer optical film, each optical repeating unit comprising at least two layers and having a corresponding bandwidth, the bandwidths of a first optical repeating unit and a second optical repeating unit spaced apart from one another in the plurality of optical repeating units overlapping one another, at least one pair of adjacent optical repeating units in the plurality of optical repeating units disposed between the first optical repeating unit and the second optical repeating unit having a non-overlapping bandwidth, none of the optical repeating units disposed between the first optical repeating unit and the second optical repeating unit having a thickness that is less than an average thickness of the first optical repeating unit and the second optical repeating unit by more than about 10%, and each layer disposed between the first optical repeating unit and the second optical repeating unit in the multilayer optical film having an average thickness of less than about 400 nm.

12. The multilayer optical film of claim 11, wherein each optical repeating unit comprises at least four layers.

13. The multilayer optical film of claim 11, wherein each optical repeating unit comprises layers A, B, and C arranged in the order of ABCB, wherein the optical thickness of layers AB and CB is one quarter of the wavelength of substantially the same.

14. The multilayer optical film of claim 1 , wherein the plurality of polymer layers further comprises a plurality of alternating layers of first optical layers and second optical layers sequentially adjacent to one another, at least the first to fourth optical layers sequentially adjacent to one another of the plurality of alternating layers of the first optical layers and the second optical layers respectively having an expected average thickness t1 to t4, each of t1 to t4 being less than about 400 nm, wherein one of t2 and t3 is at least 5% greater than the other of t1, t4, and t2 and t3.

15. A multilayer optical film comprising a first multilayer stack, a second multilayer stack, and a third multilayer stack disposed therebetween, wherein each of the first multilayer stack, the second multilayer stack, and the third multilayer stack comprises a plurality of polymer layers, the total number of polymer layers in each of the first multilayer stack and the second multilayer stack being at least 50, the first multilayer stack and the second multilayer stack comprising respective first polymer layers and second polymer layers immediately adjacent to the third multilayer stack, the first polymer layers and the second polymer layers and each polymer layer in the third multilayer stack having a thickness of less than about 400 nm, the thickness between the first polymer layers and the second polymer layers being less than about 100 nm, The difference in degree is less than about 10%, the third multilayer stack includes at least one pair of adjacent polymer layers, so that each polymer layer in the at least one pair of adjacent polymer layers has a thickness that is at least about 50% to about 500% greater than the average thickness of the first polymer layer and the second polymer layer, wherein the multilayer optical film is integrally formed, and the minimum average peel force between the first part and the second part of the multilayer optical film is greater than about 0.4N / cm, the first part and the second part respectively include at least one polymer layer in the first multilayer stack and the second multilayer stack, and wherein the total number of polymer layers arranged between the first multilayer stack and the second multilayer stack does not exceed 30.

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