Multilayer optical film

Abstract

A multilayer optical film includes a plurality of alternating first and second microlayers. The first microlayers comprise a first copolyester comprising naphthalene dicarboxylate and terephthalate units totaling greater than 92 mole percent based on total moles of dicarboxylate units. The second microlayers comprise a second copolyester comprising: terephthalate units at greater than 60 mole percent based on total moles of dicarboxylate units; isosorbide units at greater than 15 mole percent based on total moles of diol units; and at least one of ethylene-glycol units and cyclohexanedimethanol units at greater than 15 mole percent in total based on total moles of diol units. The optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

Description

[0001] MULTILAYER OPTICAL FILM

[0002] TECHNICAL FIELD

[0003] The present application generally relates to optical films, and more specifically, to multilayer optical films.

[0004] BACKGROUND

[0005] A multilayer optical film can include a plurality of polymeric interference layers for providing reflection for at least one polarization state in a desired wavelength range.

[0006] SUMMARY

[0007] In some aspects, the present description provides a multilayer optical film including a plurality of polymeric layers including at least one microlayer and a plurality of alternating first and second microlayers disposed on the at least one macrolayer. Each macrolayer of the at least one macrolayer can have an average thickness greater than about 600 nm. The plurality of alternating first and second microlayers number at least 10 in total. Each first and second microlayer has an average thickness less than about 500 nm. The first microlayers comprise a first copolyester comprising naphthalene dicarboxylate and terephthalate units totaling greater than 92 mole percent based on total moles of dicarboxylate units. The second microlayers comprise a second copolyester comprising: terephthalate units at greater than 60 mole percent based on total moles of dicarboxylate units; isosorbide units at greater than 15 mole percent based on total moles of diol units; and at least one of ethylene-glycol units and cyclohexanedimethanol units at greater than 15 mole percent in total based on total moles of diol units. The optical film can be such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

[0008] In some aspects, the present description provides a multilayer optical film including a plurality of polymeric layers including at least one microlayer and a plurality of alternating first and second microlayers disposed on the at least one macrolayer. Each macrolayer of the at least one macrolayer can have an average thickness greater than about 600 nm. The plurality of alternating first and second microlayers number at least 10 in total. Each first and second microlayer has an average thickness less than about 500 nm. The first microlayers comprise a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent and polyethylene terephthalate (PET) at greater than 2 weight percent. The first copolyester can comprise naphthalene dicarboxylate and terephthalate units totaling greater than 92 mole percent based on total moles of dicarboxylate units. The second microlayers comprise a second copolyester comprising: terephthalate units at greater than 35 weight percent; isosorbide units at greater than 8 weight percent; and at least one of ethyleneglycol units and cyclohexanedimethanol units at greater than 8 weight percent in total. The optical fdm can be such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

[0009] In some aspects, the present description provides a multilayer optical film including a plurality of polymeric layers including at least one microlayer and a plurality of alternating first and second microlayers disposed on the at least one macrolayer. Each macrolayer of the at least one macrolayer can have an average thickness greater than about 600 nm. The plurality of alternating first and second microlayers number at least 10 in total. Each first and second microlayer has an average thickness less than about 500 nm. The first microlayers include a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent, polyethylene terephthalate (PET) at greater than 2 weight percent, and glycol-modified polyethylene terephthalate (PETG) at greater than 1 weight percent. The second micro layers comprising a second copolyester comprising: terephthalate units at greater than 35 weight percent; isosorbide units at greater than 8 weight percent; ethylene-glycol units at greater than 1 weight percent; and cyclohexanedimethanol units at greater than 1 weight percent. The optical film can be such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

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

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic cross-sectional view of an optical film, according to some embodiments.

[0013] FIG. 2 is a schematic cross-sectional view of an optical film 200 undergoing a peel test, according to some embodiments.

[0014] FIG. 3 is a plot of transmittance versus wavelength for substantially normally incident light and for at least one polarization state, according to some embodiments. DETAILED DESCRIPTION

[0015] In the following description, reference is made to the accompanying drawings that form a part hereof and in which various embodiments are shown by way of illustration. The drawings are not necessarily to scale. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present description. The following detailed description, therefore, is not to be taken in a limiting sense.

[0016] As is known in the art, multilayer optical fdms including alternating polymeric layers can be used to provide desired reflection and transmission in desired wavelength ranges and polarization states by suitable selection of layer thicknesses and refractive index differences. Multilayer optical fdms and methods of making multilayer optical fdms are described in U.S. Pat. Nos. 5,882,774 (Jonza et al.); 6,783,349 (Neavin et al.); 6,949,212 (Merrill et al.); 6,967,778 (Wheatley et al.); and 9,162,406 (Neavin et al.), for example. A multilayer optical fdm can be a reflective polarizer substantially reflecting a first polarization state (block state) and substantially transmitting a second polarization state (pass state) or can be a mirror fdm substantially reflecting each of two orthogonal polarization states, for example. The multilayer optical fdm can include a plurality of alternating first and second microlayers. The first microlayers can be higher index (higher average in-plane refractive index) optical (HIO) layers and the second microlayers can be lower index (lower average in-plane refractive index) optical (LIO) layers.

[0017] The first and second microlayers of the optical fdm can include different respective first and second polyesters or copolyesters, for example. As is known in the art, polyesters and copolyesters can be made by polycondensation reactions of dicarboxylic acids (or diesters thereof) with diols. For example, polyethylene terephthalate can be made by reacting ethylene glycol with terephthalic acid or dimethyl terephthalate. Generally, the units of the chain of the polyester or copolyester derived from dicarboxylic acids or diesters thereof are referred to as dicarboxylate units and the units of the chain of the polyester or copolyester derived from diols are referred to as diol units. A copolyester can include a plurality of chemically different dicarboxylate units and / or a plurality of chemically different diol units. For example, a copolyester can be formed by reacting ethylene glycol with naphthalene dicarboxylic acid and terephthalic acid to result in a copolyester (coPEN) including naphthalene dicarboxylate units and terephthalate units as the dicarboxylate units of the copolyester and including ethylene-glycol units as the diol units of the copolyester. Such copolyesters may be denoted coPEN N / 100-N where N is the mole percent naphthalene dicarboxylate units and 100-N is the mole percent terephthalate units based on total moles of dicarboxylate units. For example, based on total moles of dicarboxylate units, coPEN 90 / 10 includes 90 mole percent naphthalene dicarboxylate units and 10 mole percent terephthalate units. CoPEN 90 / 10 is also known a low-melt PEN (LmPEN). As another example of a copolyester, a copolyester can be formed by reacting terephthalic acid with ethylene glycol and cyclohexanedimethanol (typically 1,4-cyclohexanedimethanol) to result in a copolyester (PETG) including terephthalate units as the dicarboxylate units of the copolyester and including ethyleneglycol units and cyclohexanedimethanol units as the diol units of the copolyester. More generally, glycol-modified PET (PETG) can be described as PET with at least some of the ethylene glycol units replaced with different diol units such as cyclohexanedimethanol units.

[0018] According to some embodiments, it has been found that polymers can be chosen for the first and second microlayers to substantially improve delamination resistance of the optical film. It has been found, according to some embodiments, that copolyesters for the LIO layers that include isosorbide units (units of the polymer chain of the polyester derivable from isosorbide) can substantially improve delamination resistance. The copolyester including the isosorbide units typically also includes terephthalate units and at least one of at least one of ethylene-glycol units and cyclohexanedimethanol units. The HIO layers can include coPEN N / 100-N (e.g., coPEN 90 / 10), for example, when the LIO layers include isosorbide units. The naphthalene dicarboxylate units of the coPEN N / 100-N can reduce bonding with adjacent layers compared to PET, for example, but it has been found, according to some embodiments, that including isosorbide units in the LIO layers can more than compensate for any such reduction in delamination resistance. As another example of suitable polymers, it has been found that when the HIO and LIO layers both include a glycol-modified polyethylene terephthalate (PETG) that the delamination resistance can be substantially improved, especially when the LIO layers also include isosorbide units. It may be desired to include PETG with coPEN N / 100-N (e.g., coPEN 90 / 10), for example, in the HIO layers in order to increase the delamination resistance even though this typically reduces the refractive index of the HIO layers resulting in an often undesired reduction in reflectance. When a blend of PETG and coPEN N / 100-N (e.g., coPEN 90 / 10) coextruded into the HIO layers, the PETG and coPEN N / 100-N are typically copolymerized via a transesterification reaction resulting in a copolyester including naphthalene dicarboxylate units, terephthalate units, ethylene-glycol units, and cyclohexanedimethanol units. In some embodiments, the delamination resistance is increased to such an extent that no delamination occurs for a peel force of 2000 g / in, for example. This can occur, for example, due to a sufficient quantity of isosorbide being included in the second copolyester and / or due to a combination of isosorbide being included in the second copolyester and PETG being included in both the first and second copolyesters.

[0019] In some embodiments, it is desired to use LIO layers with a high glass-transition temperature (e.g., greater than 100 degrees C). For example, LIOs with a high glass-transition temperature Tg can result in reduced microwrinkling in an optical stack including the multilayer optical film as described in U.S. Pat. Appl. Pub. No. 2022 / 0236466 (Haag et al.), for example. However, previous high Tg LIO layers have resulted in poor delamination resistance. It has been found, according to some embodiments, that high Tg copolyester for the LIO layers can be provided such that the LIO layers have a suitable refractive index for forming a reflective polarizer or mirror fdm when combined with HIO layers including coPEN N / 100-N, for example, and such that the resulting optical film has a high delamination resistance. In some embodiments, the copolyesters of the HIOs and LIOs also result in good stability (e.g., maintaining high reflectivity) against exposure to high temperature and / or high humidity environments so that the optical film can meet environmental stability specifications for a variety of applications. In some embodiments, the optical film has a high transmission of 5G signals (e.g., sub-mm to mmWave signals), so the optical film can be incorporated into an enclosure for an electronic device utilizing such signals (e.g., cell phone) to provide a decorative effect, for example, as generally described in Int. Appl. Pub. No. WO 2022 / 027589 (Long et al.), for example.

[0020] FIG. 1 is a schematic cross-sectional view of an optical film 200, according to some embodiments. The optical film 200 includes a plurality of alternating first and second microlayers 10 and 11. In some embodiments, the plurality of alternating first and second microlayers 10, 11 may number greater than 10, 20, 30, 40, 50, 80, 100, 150, 200, 300, 400, 500, 600, or 700 in total. The total number of layers in the plurality of alternating first and second microlayers 10, 11 can be up to 10000, 5000, 2000, 1000, or 800, for example. Each of the first and second microlayers in the plurality of alternating first and second microlayers 10, 11 can have an average thickness less than about 500, 450, 400, 350, 300, 250, or 200 nm, for example. The average thicknesses can be at least about 20, 30, 40, 50, or 60 nm, for example. The microlayers 10, 11 can be polymeric layers and / or can be interference layers (or optical layers). Interference layers or optical layers may be described as layers reflecting or transmitting light primarily by optical interference. Interference layers may be described as reflecting or transmitting light primarily by optical interference when the reflectance and transmittance of the interference layers can be reasonably described by optical interference or reasonably accurately modeled as resulting from optical interference.

[0021] In some embodiments, the plurality of alternating first and second microlayers 10, 11 is disposed on at least one macrolayer 124, 125, 126. A macrolayer is generally a layer too thick to reflect or transmit light in a wavelength range of interest primarily by optical interference. In the illustrated embodiment, the plurality of alternating first and second microlayers 10, 11 is disposed between first and second macrolayers 124 and 126 (e.g., skin layers). The optical film 200 may optionally include other macrolayers (e.g., protective boundary layer 125). In some embodiments, each macrolayer of the at least one macrolayer 124, 125, 126 has an average thickness of greater than 600, 700, 800, 900, 1000, 1250, 1500, 1750, or 2000 nm. The average thickness of each of the macrolayers can be up to about 150, 100, 50, 30, 20, or 10 microns, for example. In some embodiments, the plurality of alternating first and second microlayers 10, 11 comprise first and second pluralities 110, 111 of microlayers where the first and second pluralities 110, 111 of microlayers are separated by one or more macrolayers 125 of the at least one macrolayer 124, 125, 126. In some embodiments, each of the first and second pluralities of microlayers number at least 50, 100, 150, 200, or 250 in total.

[0022] The first and second microlayers 10 and 11 typically comprise respective first and copolyesters where the first copolyester comprises naphthalene dicarboxylate units and the second copolyester comprises terephthalate units and isosorbide units. The first and second copolyesters can include some common components. For example, the first copolyester can include terephthalate units and the second copolyester can include naphthalene dicarboxylate units, and / or each of the first and second copolyesters can include units derived from PETG. The second copolyester typically include a sufficient amount of isosorbide units (e.g., greater than 60 mole percent based on total moles of dicarboxylate units, or greater than 8 weight percent based on a total weight of the copolyester) to result in a desired delamination resistance (e.g., greater than 200 g / in under a 90 degree, 10 in / min peel test). The amount of isosorbide needed can depend on the other components of the first and second copolyesters. For example, when the first and second copolyesters include common units (e..g., cyclohexanedimethanol units) derived from PETG, less isosorbide is needed to achieve a same delamination resistance than when no common component is shared between the first and second copolyesters.

[0023] In some embodiments, the first microlayers 10 comprise a first copolyester comprising naphthalene dicarboxylate and terephthalate units totaling greater than 92, 94, 96, 98, or 99 mole percent based on total moles of dicarboxylate units. In some embodiments, based on total moles of dicarboxylate units, the first copolyester comprises greater than 80, 82, 84, 86, 88 mole percent naphthalene dicarboxylate units and greater than 2 mole percent terephthalate units. In some embodiments, based on total moles of dicarboxylate units, the first copolyester comprises greater than 3, 4, 5, 6, 7, 8, or 9 mole percent terephthalate units. In some embodiments, based on total moles of dicarboxylate units, the first copolyester comprises at greater than 95, 96, 97, 98, 99, 99.5, 99.6, 99.7, 99.8, or 99.9 mole percent in total: naphthalene dicarboxylate units; terephthalate units; and optionally, one or more of isophthalate units, sodium sulfoisophthalate units, sebacate units, or adipate units. In some embodiments, dicarboxylate units of the first copolyester consist essentially of naphthalene dicarboxylate units and terephthalate units. As used herein, “consist essentially” in this context means that the first copolyester comprises naphthalene dicarboxylate and terephthalate units totaling greater than 98 mole percent based on total moles of dicarboxylate units. In some embodiments, the first copolyester comprises ethylene-glycol units at greater than 70, 75, 80, 85, 90, 95 mole percent based on total moles of diol units. In some embodiments, the first copolyester comprises cyclohexanedimethanol units at greater than 1, 2, 3, 4, or 5 mole percent based on total moles of diol units.

[0024] In some embodiments, the first microlayers 10 comprise a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60, 65, 70, 75, 80, or 85 weight percent and polyethylene terephthalate (PET) at greater than 2 weight percent. In some embodiments, the copolymerized blend comprises the PET at greater than 3, 4, 5, 6, 7, or 8 weight percent. In some embodiments, the copolymerized blend of the first copolyester further comprises greater than 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 weight percent of glycol- modified polyethylene terephthalate (PETG). For example, in some embodiments, the first microlayers 10 comprise a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent, polyethylene terephthalate (PET) at greater than 2 weight percent, and glycol-modified polyethylene terephthalate (PETG) at greater than 1 weight percent. In some embodiments, the glycol-modified polyethylene terephthalate (PETG) comprises cyclohexanedimethanol units at greater than 10, 12, 14, 16, or 18 weight percent. In some embodiments, the PEN, PET and PETG in total comprises greater than 90, 92, 94, 96, 98, or 99 percent by weight of the first copolyester. In some embodiments, each of the first microlayers comprises the first copolymer at greater than 80, 85, 90, 92, 95, 96, 98, or 99 weight percent.

[0025] In some embodiments, the second microlayers 11 comprise a second copolyester comprising terephthalate units at greater than 60, 65, 70, 75, or 80 mole percent based on total moles of dicarboxylate units; isosorbide units at greater than 15 mole percent based on total moles of diol units; and at least one of ethylene-glycol units and cyclohexanedimethanol units at greater than 15 mole percent in total based on total moles of diol units. In some embodiments, the second copolyester comprises isosorbide units at greater than 18, 20, 22, 24, 26, 28, 30 mole percent based on total moles of diol units. In some embodiments, the second copolyester comprises ethyleneglycol units at greater than 10, 12, 14, 16, 18, 20, 22, or 24 mole percent based on total moles of diol units. In some embodiments, the second copolyester comprises cyclohexanedimethanol units at greater than 1, 2, 3, 4, 5, 20, 25, 30, 35, 40, or 45 mole percent based on total moles of diol units. In some embodiments, the second copolyester comprises terephthalate units at greater than 85, 90, 95, or 98 mole percent based on total moles of dicarboxylate units. In some embodiments, the second copolyester further comprises naphthalene dicarboxylate units at greater than 5, 10, 15, or 20 mole percent based on total moles of dicarboxylate units. In some embodiments, the second copolyester further comprises naphthalene dicarboxylate units at greater than 5, 10, 15, or 20 weight percent. The second copolyester can be a copolymerized blend comprising poly(ethylene- glycol isosorbide cyclohexanedimethanol terephthalate) (PEICT) and at least one of coPEN N / 100-N and PENG. PENG is glycol-modified PEN that can be made as described in U.S. Pat. Appl. No. 2020 / 0156355 (Johnson et al.), for example. PENG30, for example, includes 70 mole percent ethylene-glycol units and 30 mole percent cyclohexanedimethanol units based on total moles of diol units with the dicarboxylate units being naphthalene dicarboxylate units.

[0026] In some embodiments, the second microlayers 11 comprise a second copolyester comprising: terephthalate units at greater than 35, 36, 38, 40, 42, 44, 45, 46, 47, 48, 49, or 50 weight percent; isosorbide units at greater than 8 weight percent; and at least one of ethyleneglycol units and cyclohexanedimethanol units at greater than 8 weight percent in total. In some embodiments, the second copolyester comprises isosorbide units at greater than 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 weight percent. In some embodiments, the second copolyester comprises ethylene-glycol units at greater than 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, or 5 weight percent. In some embodiments, the second copolyester comprises cyclohexanedimethanol units at greater than 10, 12, 14, 16, 18, 20, 22, 24, or 25 weight percent. In some embodiments, the second copolyester comprises: terephthalate units at greater than 35 weight percent (or in a range described elsewhere herein); isosorbide units at greater than 8 weight percent; ethylene-glycol units at greater than 1 weight percent (or in a range described elsewhere herein); and cyclohexanedimethanol units at greater than 1 weight percent (or in a range described elsewhere herein). In some embodiments, the second copolyester further comprises naphthalene dicarboxylate units at greater than 5, 6, 7, 8, 9, or 10 weight percent. In some embodiments, the second copolyester comprises: terephthalate units at greater than 48 weight percent (or in a range described elsewhere herein); isosorbide units at greater than 10 weight percent; ethylene-glycol units at greater than 2 weight percent (or in a range described elsewhere herein); and cyclohexanedimethanol units at greater than 12 weight percent (or in a range described elsewhere herein).

[0027] In some embodiments, each of the second microlayers comprises the second copolymer at greater than 80, 85, 90, 92, 95, 96, 98, or 99 weight percent. In some embodiments, the second microlayers comprise the second copolyester at greater than 40, 50, 60, 70, or 80 weight percent and further comprises a polycarbonate blended with the second copolyester.

[0028] In some embodiments, the second copolyester has a glass transition temperature greater than 100, 102, 104, 106, or 108 degrees C. The glass transition temperature can be up to 125, 120 or 115 degrees C, for example. The glass transition temperature can be determined by differential scanning calorimetry (DSC), for example.

[0029] In some embodiments, the first copolyester comprises terephthalate units at greater than 2 mole percent (or in a range described elsewhere herein) based on total moles of dicarboxylate units, and each of the first and second copolyesters comprises cyclohexanedimethanol units at greater than 1, 2, 3, 4, or 5 mole percent based on total moles of diol units. In some embodiments, each of the first and second microlayers comprises the respective first and second copolymers at greater than 80, 85, 90, 92, 95, 96, 98, or 99 weight percent.

[0030] In some embodiments, first copolyester comprises a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent (or in a range described elsewhere herein), and polyethylene terephthalate (PET) at greater than 2 weight percent (or in a range described elsewhere herein). In some embodiments, the copolymerized blend of the first copolyester further comprises greater than 1 weight percent (or in a range described elsewhere herein) of glycol-modified polyethylene terephthalate (PETG). In some embodiments, the second copolyester comprises a copolymerized blend of at least poly(ethylene-glycol isosorbide cyclohexanedimethanol terephthalate) (PEICT) at greater than 60 weight percent and glycol- modified polyethylene terephthalate (PETG) at greater than 2, 4, 6, 8, 10, 12, 14, 16, 18, or 20 weight percent. In some embodiments, the PEICT comprising isosorbide units at greater than 15, 16, 17, or 18 weight percent. In some embodiments, the glycol-modified polyethylene terephthalate (PETG) of the first and second copolyesters have a same composition.

[0031] Suitable PETG include, for example, PCTg and EASTAR GN071 both available from Eastman Chemical Company, Knoxville, TN. Useful coPET including isosorbide units include those available from SK Chemicals (Gyeonggi-do, South Korea) under the tradename ECOZEN, such as the ECOZEN T-series which includes ECOZEN T120. CoPEN N / 100-N can be made as described in U.S. Pat. No. 6,946,188 (Hebrink et al.), for example. Other useful polymeric materials are described in the multilayer optical film references provided elsewhere herein.

[0032] The optical film is typically formed by coextruding, co-stretching and optionally heat setting the polymeric layers of the optical film. The pre-stretch temperature, stretch temperature, stretch rate, stretch ratio, heat set temperature, heat set time, heat set relaxation, and cross-stretch relaxation can be selected to adjust refractive indices of the optical layers as generally described in in U.S. Pat. No. 6,967,778 (Wheatley et al.), for example. The delamination resistance can depend on the temperature during stretching and on the heat set temperature. It has been found that the delamination resistance typically increases with increasing stretch temperature in a range of temperatures useful for stretching polyester films. For reflective polarizers, the delamination resistance typically decreases with increasing heat set temperature, while for mirror films, the delamination resistance typically increases with increasing heat set temperature. In some embodiments, the stretch temperature is in a range of 100 to 160 degrees C or 120 to 150 degrees C, for example. In some embodiments, the heat set temperature is in a range of 130 to 210 degrees C or 150 to 190 degrees C, for example. The compositions of the first and second microlayers 10, 11 can be chosen such that when the layers are coextruded, co-stretched and heat set, the first microlayers are substantially birefringent (e.g., birefringence greater than 0.06) and the second micolayers are substantially optically isotropic (e.g., birefringence less than 0.04).

[0033] In some embodiments, for at least one wavelength in a range of about 400 nm to about 2000 nm, the first and second microlayers have respective higher and lower average in-plane (in the plane of the layers - e.g., the xy-plane of FIG. 1) refractive indices. In some embodiments, for at least one wavelength in a range of about 400 nm to about 2000 nm, a birefringence of the first microlayers is greater than a birefringence of the second microlayers by at least 0.05, 0.06, 0.07, 0.08. 0.09, or 0. 1. Here, the birefringence of a layer refers to the difference between the largest and smallest refractive indices of the layer. In some embodiments, for the at least one wavelength, the birefringence of the second microlayers is less than 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01. In some embodiments, the second microlayers are substantially optically isotropic (e.g., birefringence less than 0.04). In some embodiments, for at least one wavelength in a range of about 400 nm to about 2000 nm and for at least a same one of an in-plane birefringence (difference between largest and smallest in-plane refractive indices) and an out-of-plane birefringence (magnitude of difference between the refractive index in the thickness direction and the average in-plane refractive indices), the birefringence is at least 0.05, 0.06, 0.07, 0.08. 0.09, or 0.1 higher for the first microlayers than for the second microlayers.

[0034] In some embodiments, the at least one wavelength (for which refractive indices and / or birefringence are determined) includes at one wavelength in a range of 420 nm to 680 nm (e.g., 532 nm, 550 nm, and / or 633 nm).

[0035] FIG. 2 is a schematic cross-sectional view of an optical film 200 undergoing a peel test, according to some embodiments. A 90 degree peel test with peel force F is schematically illustrated. The peel force F may be described as a delamination resistance along the xl-direction (referring to the illustrated xl-yl-z coordinate system) since the separated layers peel along the xl- direction. The xl-direction may correspond to either the x-direction or the y-direction of FIG. 1, for example. For a reflective polarizer, the delamination resistance can be determined for the peel direction xl being along the first polarization state (block state) of the reflective polarizer. The 90 degree peel test can be carried out at a peel speed of 10 inches per min, for example. The peel force generally decreases with decreasing peel speed. Conventionally, a peel speed of 60 inches per min has been used to characterize delamination resistance of polymeric multilayer optical films. However, it has been found, according to some embodiments, that the delamination resistance can be too high to measure using the conventional peel speed of 60 inches per minute. Accordingly, a peel speed of 10 inches per min may be used to reduce the peel force to more readily measurable values. In some embodiments, the delamination resistance is the average resistance-to-peel strength determined according to the ASTM D6862-11 (Reapproved 2016) test standard.

[0036] In some embodiments, the optical film is scored (a score 342 that can be formed by cutting with a razor blade, for example, is schematically illustrated) by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, and a tape 344 is attached to a major surface of the optical film and pulled in the peel test. In some embodiments, the plurality of scored polymeric layers includes at least some of the plurality of alternating first and second microlayers 10 and 11. In some embodiments, at least some of the plurality of polymeric layers of the scored optical film are not scored. In some embodiments, it can be determined in a peel test that the delamination resistance is greater than a specified limit (e.g., when no delamination is observed when a peel is attempted at the specified limit), but it can be difficult to determine a precise value of the delamination resistance due the difficulty in getting the layers to delaminate at all when the delamination resistance is high. In some embodiments, the optical film is such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance (along at least one direction) of no less than about 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600, 1800, or 2000 g / in under a 90 degree, 10 in / min peel test. In some embodiments, the optical film is a reflective polarizer, and the scored optical film has a delamination resistance along a first (block) polarization state of the reflective polarizer of no less than about 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600, 1800, or 2000 g / in under a 90 degree, 10 in / min peel test. In some embodiments, the scored optical film has a delamination resistance along each in-plane direction of no less than about 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1200, 1400, 1600, 1800, or 2000 g / in under a 90 degree, 10 in / min peel test.

[0037] The optical film 200 can be a reflective polarizer or a mirror film, for example. FIG. 3 is a plot of transmittance versus wavelength for substantially normally incident light 12 (see, e.g., FIG. 1) and for at least one polarization state (e.g., polarization state 141 and / or 142 schematically illustrated in FIG. 1), according to some embodiments. In some embodiments, any optical absorption of the optical film 200 is negligible so that the reflectance is 100 percent minus the transmittance to a good approximation. A reflectance R for a wavelength of about 550 nm is schematically illustrated. In some embodiments, the optical film 200 is a mirror film so that the transmittance of each of orthogonal first and second polarization states 141 and 142 (see, e.g., FIG. 1), the transmittance follows an illustrated transmittance curve. In some embodiments, the optical film 200 is a reflective polarizer so that the transmittance for the first polarization state 141 follows the illustrated transmittance curve while the reflective polarizer substantially transmits the second polarization state 142 (e.g., the transmittance can correspond to the schematically illustrated transmittance Tl).

[0038] In some embodiments, for substantially normally incident light 12, for a first polarization state 141, and for a predetermined wavelength range at least 20 nm wide, the optical film has an average optical reflectance of at least about 60, 70, 80, 85, 90, or 95 percent. In some embodiments, for the substantially normally incident light 12 and the predetermined wavelength range, and for a second polarization state 142 orthogonal to the first polarization state, the optical film 200 has an average optical reflectance of at least about 60, 70, 80, 85, 90, or 95 percent. In some embodiments, for the substantially normally incident light 12 and the predetermined wavelength range, and for a second polarization state 142 orthogonal to the first polarization state 141, the optical film 200 has an average optical transmittance of at least about 60, 65, 70, 75, 80, 85, or 90 percent. In some embodiments, for substantially normally incident light 12, for the first polarization state 141, and for a wavelength range extending at least from about 420 nm to about 680 nm, the optical film 200 has an average optical transmittance in a range of about 10 percent to about 30 percent. For example, in some embodiments, it is desired to increase delamination resistance by blending a polymer (e.g., PETG) having a same composition into the other polymer(s) of the first and second microlayers 10, 11 even when doing so increases the transmittance (reduces the reflectance) by reducing the refractive index difference between the layers. In some embodiments, for the first polarization state 141, and for a wavelength range extending at least from about 420 nm to about 680 nm, the optical film 200 has an average optical transmittance of less than about 10, 8, 6, 5, 4, 3, or 2 percent.

[0039] The predetermined wavelength range used for defining average optical transmittance or average optical reflectance can be from about 450 nm to about 650 nm, or from about 420 nm to about 680 nm, or from about 420 nm to about 850 nm, for example.

[0040] EXAMPLES

[0041] All parts, percentages, ratios, etc. in the examples are by weight, unless noted otherwise.

[0042] Materials

[0043] Delamination Resistance Test Method

[0044] A 2” piece of 3M 8898 tape was placed on the optical film sample to be tested. Three to six 5mm strips were cut from the film and tape. A 1” 3M 9589 double sided carpet tape was attached to an aluminum plate. The 5mm strips were laminated to the double sided tape. The optical film was scored at a 45 degree angle and a peel was initiated. The optical film was then peeled with a 90 degree, 10 in / min peel test. For reflective polarizer films, the peel direction was along the block axis of the reflective polarizer. The test was repeated for 5 samples of each optical film and the average of the peel forces was reported as the delamination resistance. Delamination resistance can be determined up to about 2200 g / in using this method. For higher delamination resistance it can be difficult to initiate a peel. In such cases, the delamination resistance is reported as > 2000 g / in.

[0045] Examples 1-3 and Comparative Examples CE1-CE2

[0046] Multilayer optical films (reflective polarizer films) were made by coextruding alternating high and low index optical layers (HIO and LIO) between outer protective boundary layers (PBLs) through a 151-layer feed block and die. The PBLs were produced by extruding LmPEN resin through a 27 mm TSE through a neck tube and gear pump into the outer layers of the 151 -layer feed block and die at a feed rate of 10 Ibs / hr. This melt train used a progressive temperature extrusion profile, with peak temperatures of -280 deg. C. The HIO layers were produced by extruding LmPEN resin through a 27mm TSE with a progressive temperature profile peaking at or around -280 deg. C through a neck tube and gear pump into the appropriate layers of the 151-layer feed block and die at a feed rate of 10 Ibs / hr. The LIO layers were produced by extruding the resin and resin blends listed in the table below through a 27mm TSE with a progressive temperature profile peaking at or around -280 deg. C through a neck tube and gear pump into the appropriate layers of the 151-layer feed block and die at the feed rates given in the table below. The multilayer melt was then cast through a film die onto a chill roll, in the conventional manner for polyester films, upon which it was quenched. The cast web was then stretched in a commercial scale linear tenter at a draw ratio approximately 6: 1 and a temperature indicated in the table below. The feed block / die was held at a target temp of 285 deg. C while the casting wheel was run at about 50 deg. C. Before the cast web was stretched, it was preheated at the temperature and time indicated in the table below. In some cases, the optical film was heat set at the temperature and time indicated in the table below after stretching. Examples 4-5 and Comparative Example CE3

[0047] Examples 4-5 and Comparative Example CE3 are multilayer optical films (mirror films) that were made as generally described for Examples 1-3 and Comparative Examples CE1-CE2 except that the coextruded layers were biaxially stretched with the layers described as follows. Examples 4-5 and Comparative Example CE3 each included alternating HIO and LIO layers arranged into two 325 layer packets with protective boundary layers (PBLs) on opposite sides of the optical layers of each packet. The materials of the HIO and LIO layers are given in the following table with weight ratios of the various polymers indicated in parentheses. Comparative Example CE3 also included PEN outer skin layers.

[0048] FIG. 3 is a plot of transmission versus wavelength for Comparative Example CE3 and Examples 4-5. For comparison, results are also shown for a Toray PICASUS100GH film available from Toray Industries, Inc. (Japan) and for a 3M mirror film ESR 80v2 available from 3M Company (St. Paul, MN).

[0049] Examples 6-8 and Comparative Example CE4

[0050] Examples 6-7 and Comparative Example CE4 are multilayer optical films (reflective polarizers) that were made as generally described for Examples 1-3 and Comparative Examples CE1-CE2 but the cast web was stretched in a parabolic tenter at a stretch temperature of 144 deg. C and the layers were as follows. Examples 6-8 and Comparative Example CE4 each included alternating HIO and LIO layers arranged into two 325 layer packets with protective boundary layers (PBLs) on opposite sides of the optical layers of each packet. Layered PBLs including 5 layers each were utilized between the two packets. LmPEN was used for the HIO layers. The materials of the LIO layers are given in the following table. A heat set temperature of 138 deg. C was used for Examples 6-8 and 134 deg. C for Comparative Example CE4. The films included PBL layers on each side of each of the packets and had a total average thickness of about 56 microns. The weight ratio of the plurality of HIO layers to the plurality of LIO layers was 42:58 for each of the optical films. Delamination resistance was measured for multiple samples of each film and results are provided in the following table.

[0051] Terms such as “about” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “about” as applied to quantities expressing feature sizes, amounts, and physical properties is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, “about” will be understood to mean within 10 percent of the specified value. A quantity given as about a specified value can be precisely the specified value. For example, if it is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description, a quantity having a value of about 1, means that the quantity has a value between 0.9 and 1.1, and that the value could be 1.

[0052] Terms such as “substantially” will be understood in the context in which they are used and described in the present description by one of ordinary skill in the art. If the use of “substantially” with reference to a property or characteristic is not otherwise clear to one of ordinary skill in the art in the context in which it is used and described in the present description and when it would be clear to one of ordinary skill in the art what is meant by an opposite of that property or characteristic, the term “substantially” will be understood to mean that the property or characteristic is exhibited to a greater extent than the opposite of that property or characteristic is exhibited.

[0053] All references, patents, and patent applications referenced in the foregoing are hereby incorporated herein by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control.

[0054] Descriptions for elements in figures should be understood to apply equally to corresponding elements in other figures, unless indicated otherwise. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and / or equivalent implementations can be substituted for the specific embodiments shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations, or variations, or combinations of the specific embodiments discussed herein. Therefore, it is intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is claimed is:

1. A multilayer optical film comprising a plurality of polymeric layers comprising: at least one macrolayer, each macrolayer of the at least one macrolayer having an average thickness greater than about 600 nm; and a plurality of alternating first and second microlayers disposed on the at least one macrolayer and numbering at least 10 in total, each first and second microlayer having an average thickness less than about 500, the first microlayers comprising a first copolyester comprising naphthalene dicarboxylate and terephthalate units totaling greater than 92 mole percent based on total moles of dicarboxylate units, the second microlayers comprising a second copolyester comprising: terephthalate units at greater than 60 mole percent based on total moles of dicarboxylate units; isosorbide units at greater than 15 mole percent based on total moles of diol units; and at least one of ethylene-glycol units and cyclohexanedimethanol units at greater than 15 mole percent in total based on total moles of diol units, such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

2. The multilayer optical film of claim 1, wherein the second copolyester comprises isosorbide units at greater than 18 mole percent based on total moles of diol units.

3. The multilayer optical film of claim 1, wherein the second copolyester comprises ethyleneglycol units at greater than 10 mole percent based on total moles of diol units.

4. The multilayer optical film of claim 1, wherein the second copolyester comprises cyclohexanedimethanol units at greater than 20 mole percent based on total moles of diol units.

5. The multilayer optical film of claim 1, wherein the second copolyester comprises terephthalate units at greater than 85 mole percent based on total moles of dicarboxylate units.

6. The multilayer optical film of claim 1, wherein, based on total moles of dicarboxylate units, the first copolyester comprises greater than 80 mole percent naphthalene dicarboxylate units and greater than 2 mole percent terephthalate units.

7. The multilayer optical film of claim 1, wherein the first copolyester comprises ethylene -glycol units at greater than 70 mole percent based on total moles of diol units.

8. The multilayer optical film of claim 7, wherein the first copolyester comprises cyclohexanedimethanol units at greater than 1 mole percent based on total moles of diol units.

9. The multilayer optical film of claim 1, wherein the first copolyester comprises terephthalate units at greater than 2 mole percent based on total moles of dicarboxylate units, and each of the first and second copolyesters comprises cyclohexanedimethanol units at greater than 1 mole percent based on total moles of diol units.

10. A multilayer optical film comprising a plurality of polymeric layers comprising: at least one macrolayer, each macrolayer of the at least one macrolayer having an average thickness greater than about 600 nm; and a plurality of alternating first and second microlayers disposed on the at least one macrolayer and numbering at least 10 total, each first and second microlayer having an average thickness less than about 500 nm, the first microlayers comprising a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent and polyethylene terephthalate (PET) at greater than 2 weight percent, the first copolyester comprising naphthalene dicarboxylate and terephthalate units totaling greater than 92 mole percent based on total moles of dicarboxylate units, the second microlayers comprising a second copolyester comprising: terephthalate units at greater than 35 weight percent; isosorbide units at greater than 8 weight percent; and at least one of ethylene-glycol units and cyclohexanedimethanol units at greater than 8 weight percent in total, such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

11. The multilayer optical film of claim 10, wherein the copolymerized blend of the first copolyester further comprises greater than 1 weight percent of glycol-modified polyethylene terephthalate (PETG).

12. The multilayer optical film of claim 11, wherein the second copolyester comprises a copolymerized blend of at least poly(ethylene-glycol isosorbide cyclohexanedimethanol terephthalate) (PEICT) at greater than 60 weight percent and glycol- modified polyethylene terephthalate (PETG) at greater than 2 weight percent, the PEICT comprising isosorbide units at greater than 15 weight percent.

13. A multilayer optical film comprising a plurality of polymeric layers comprising: at least one macrolayer, each macrolayer of the at least one macrolayer having an average thickness greater than about 600 nm; and a plurality of alternating first and second microlayers disposed on the at least one macrolayer and numbering at least 10 in total, each first and second microlayer having an average thickness less than about 500 nm, the first microlayers comprising a first copolyester comprising a copolymerized blend of at least polyethylene naphthalate (PEN) at greater than 60 weight percent, polyethylene terephthalate (PET) at greater than 2 weight percent, and glycol-modified polyethylene terephthalate (PETG) at greater than 1 weight percent, the second microlayers comprising a second copolyester comprising: terephthalate units at greater than 35 weight percent; isosorbide units at greater than 8 weight percent; ethylene-glycol units at greater than 1 weight percent; and cyclohexanedimethanol units at greater than 1 weight percent, such that when the optical film is scored by cutting through at least some of the plurality of polymeric layers to result in a plurality of scored polymeric layers, then the scored optical film has a delamination resistance of no less than about 200 g / in under a 90 degree, 10 in / min peel test.

14. The multilayer optical film of any one of claims 1 to 13, wherein each of the first and second microlayers comprises the respective first and second copolymers at greater than 80 weight percent.

15. The multilayer optical film of any one of claims 1 to 13, wherein the second microlayers comprise the second copolyester at greater than 40 weight percent and further comprises a polycarbonate blended with the second copolyester.

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