Substrate comprising a self-supporting three-layer stack

Abstract

A substrate comprising a self-supporting three-layer stack is described. The three-layer stack comprises a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer. The biaxially oriented layer can comprise a first polyester having greater than 45 mole percent of naphthalate units and greater than 45 mole percent of ethylene units. Each of the first outer layer and the second outer layer comprises a second polyester, which can comprise 40 to 50 mole percent of naphthalate units, at least 25 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10 alkyl units.

Description

BACKGROUND

[0001] Substrates are used in a variety of display applications. For example, an emissive layer of a display can be disposed on a substrate. As another example, electrodes for a touch sensor in a display can be disposed on a substrate. SUMMARY

[0002] In some aspects of the present description, a substrate including a self-supporting three-layer stack is provided. The three-layer stack includes first and second outer layers and a biaxially oriented layer disposed between and in direct contact with the first and second outer layers. The biaxially oriented layer includes a first polyester having greater than 45 mole percent naphthalate units and greater than 45 mole percent ethylene units. Each of the first and second outer layers includes a second polyester including 40 to 50 mole percent naphthalate units, at least 25 mole percent ethylene units, and 10 to 25 mole percent branched or cyclic C4-C10 alkyl units.

[0003] In some aspects of the present description, a substrate including a self-supporting three-layer stack is provided. The three-layer stack includes first and second outer layers and a biaxially oriented layer disposed between and in direct contact with the first and second outer layers. The biaxially oriented layer includes a first polyester including a plurality of first monomer units and each of the first and second outer layers includes a second polyester. The first polyester has a glass transition temperature of at least 90 °C. The second polyester is a copolyester including a plurality of the first monomer units and a plurality of second monomer units. The second monomer units hinder crystallization of the second polyester.

[0004] In some aspects of the present description, a substrate including a self-supporting three-layer stack is provided. The three-layer stack includes first and second outer layers and a biaxially oriented layer disposed between and in direct contact with the first and second outer layers. Each of the first and second outer layers has an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02. The biaxially oriented layer includes a first polyester including a plurality of first monomer units and each of the first and second outer layers includes a second polyester. The first polyester has a glass transition temperature of at least 90 °C. The second polyester is a copolyester including a plurality of the first monomer units. BRIEF DESCRIPTION OF DRAWINGS

[0005] Figure 1 schematic cross-sectional view of a substrate;

[0006] Figure 2 schematic view of a display including an emissive layer and a substrate;

[0007] Figure 3is a schematic cross-sectional view of a touch sensor; and

[0008] Figure 4 is a schematic cross-sectional view of a curved display. DETAILED DESCRIPTION

[0009] In the following description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration various embodiments. The drawings are not necessarily to scale. It is to be understood that other embodiments can be utilized and structural or logical changes can be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense.

[0010] According to some embodiments of the present description, substrates have been developed that are easily processed by extrusion, orientation, and conventional web handling techniques and are suitable for use in display applications and other applications. In some embodiments, the substrates provide high modulus and high toughness and are suitable for use, for example, in flexible display applications. In some embodiments, the substrates also provide low haze and, in some cases, high UV blocking and are suitable for display applications in which the substrate covers the viewable portion of the display, such as, for example, touch sensors. It has been found that some polyesters can be used to provide substrates, but the mechanical properties of individual isolated layers of the polyesters are not acceptable for many applications. In some cases, it is desirable for the substrate to have a high modulus (e.g., at least 4.5 GPa) and it has been found difficult to provide a monolithic single layer substrate with a suitably high modulus without sacrificing other desirable mechanical properties (e.g., toughness). For example, biaxially oriented polyethylene naphthalate (PEN) can be used as a high modulus substrate (e.g., a Young's modulus of about 5 GPa), but a monolithic single layer of biaxially oriented PEN can be too brittle for many applications. However, according to some aspects of the present description, it has been found that PEN or other polyesters can be used in a self-supporting three layer stack that includes coPEN or other copolyesters in the outer layers in direct contact with the PEN or other polyester layer, where the three layer stack is mechanically robust (e.g., high modulus and significantly less brittle than a monolithic biaxially oriented PEN layer of the same thickness) and can be used, for example, in flexible and / or curved display applications.

[0011] In some embodiments, the substrate is made by co-extrusion followed by biaxial stretching and optional heat setting. The biaxial stretching can be performed in equal or approximately equal (e.g., within 20%, or within 10%, or within 5%) stretch ratios in orthogonal first and second directions (e.g., machine direction and cross direction). The heat setting can be performed at a temperature above the melting point of the co-polyester of the outer layers. In some embodiments, a heat setting temperature in the range of 200 °C to 240 °C, or 210 °C to 230 °C is used. In some embodiments, the heat setting temperature is about 220 °C. It has been unexpectedly found that the isotropy of the outer layers of the substrate can be improved by heat setting, and that this improved isotropy provides improved durability (e.g., toughness) of the substrate.

[0012] Figure 1 For illustrative cross-sectional views of a substrate 100 including a self-supporting three-layer stack 105 including first and second outer layers 110, 112 and a biaxially oriented layer 101 disposed between and in direct contact with the first and second outer layers 110, 112. In some embodiments, the substrate 100 includes additional layers (e.g., one or more of a coating layer, an additional polymer layer outside the three-layer stack 105, and an adhesive layer on an outer surface of the three-layer stack 105), and in some embodiments, the substrate 100 does not include additional layers. In some embodiments, the biaxially oriented layer 101 comprises a first polyester including a plurality of first monomer units. In some embodiments, the first monomer units are naphthalate units and the first polyester further includes ethylene units. For example, in some embodiments, the first polyester includes greater than 45 mole percent naphthalate units and greater than 45 mole percent ethylene units. In some embodiments, the first polyester includes greater than 47 mole percent, or greater than 48 mole percent, or greater than 49 mole percent naphthalate units and greater than 47 mole percent, or greater than 48 mole percent, or greater than 49 mole percent ethylene units. For example, the first polyester can be polyethylene naphthalate (PEN), which can also be referred to as poly(2,6-ethylene naphthalate) and which includes 50 mole percent naphthalate units and 50 mole percent ethylene units. Such a polyester can be made by a condensation reaction between naphthalic acid and ethylene glycol. More generally, one or more diacids and one or more diols can be used to produce the first polyester. As an illustrative example, the first polyester can be the reaction product of 46 mole percent naphthalic acid, 4 mole percent terephthalic acid, 49 mole percent ethylene glycol, and 1 mole percent 1,4-butanediol. In some embodiments, a diester is used in place of a diacid in forming the first polyester.

[0013] A self-supporting three-layer stack is a three-layer stack that does not require additional layers for support. A self-supporting three-layer stack can be used as a substrate without additional layers, although the substrate can include the three-layer stack and additional coatings or layers. For example, coatings and polymer layers having a thickness of less than about one micron are generally not self-supporting because additional layers are needed to support such thin coatings or layers. Generally, the three-layer stacks of the present description are at least 10 microns thick in order to be self-supporting.

[0014] In some embodiments, each of the first outer layer 110 and the second outer layer 112 comprises a second polyester, wherein the second polyester is a copolyester comprising a plurality of first monomer units and a plurality of second monomer units. In some embodiments, the first monomer units are naphthalate units and the second monomer units are branched or cyclic C4-C10 alkyl units, which can be included to hinder crystallization of the second polyester. For example, in some embodiments, the second polyester comprises 40-50 mole% naphthalate units, at least 25 mole% ethylene units, and 10-25 mole% branched or cyclic C4-C10 alkyl units. The second polyester can comprise additional units, such as, for example, terephthalate units. In some embodiments, the second polyester is the reaction product of a composition comprising 40-50 mole% 2,6-naphthalene dicarboxylic acid or naphthalate diester (e.g., 2,6-naphthalene dicarboxylic acid dimethyl ester); at least 25 mole% ethylene glycol; and 10-25 mole% branched or cyclic C4-C10 alkyl diol, and / or up to 10 mole% branched or cyclic C4-C10 alkyl diacid, and / or up to 10 mole% branched or cyclic C4-C10 alkyl diester. Unless otherwise specified or clear from context, the mole% of diacid or diester and diol add up to 100 mole%. Additional components that do not form part of the resulting polyester (e.g., catalysts) are not included in these mole%. A catalyst can also be included in the composition, such as sodium acetate and / or tetrabutyl titanate. In some embodiments, the composition further comprises additional diols, diacids, or diesters. For example, in some embodiments up to 10% dimethyl isophthalate sodium sulfonate is included. In some embodiments, the second polyester comprises 40-50 mole%, or 45-50 mole% naphthalate units; 25-50 mole%, or 25-40 mole% ethylene units; and 10-25 mole% branched or cyclic C4-C10 alkyl units.

[0015] In some embodiments, the second polyester comprises 50 mole percent of naphthalate units, 25 to 40 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10 alkyl units, wherein the mole percent of ethylene units and the mole percent of branched or cyclic C4-C10 alkyl units add up to 50 mole percent. In some embodiments, the mole percent of ethylene units and the mole percent of branched or cyclic C4-C10 alkyl units add up to less than 50 mole percent, and comprise additional units formed from one or more other diol monomer molecules. Suitable diol monomer molecules include, for example, propylene glycol; 1,4-butanediol and its isomers; 1,6-hexanediol; polyethylene glycol; diethylene glycol; tricyclodecane glycol; its isomers; norbornanediol; dicyclooctanediol; trimethylolpropane; pentaerythritol; 1,4-benzenedimethanol and its isomers; bisphenol A; 1,8-dihydroxydiphenyl and its isomers; and 1,3-bis(2-hydroxyethoxy)benzene. Typically, the amount of monomer units derived from other diol monomer molecules (i.e., other than ethylene glycol and C4-C10 alkyl diols, such as neopentyl glycol or cyclohexanediol) is no greater than 5 mole percent. In some embodiments, the units derived from other diol monomer molecules are no greater than 1 mole percent or 2 mole percent. In the absence of other diol monomer molecules in the synthesis, the copolyester polymer can in some cases contain about 0.5 to 3 mole percent of diethylene glycol as a side reaction byproduct.

[0016] In some embodiments, the second polyester comprises less than 50 mole percent of naphthalate units. In some embodiments, the second polyester is formed from 2,6-naphthalene dicarboxylic acid or its isomers in combination with one or more other (i.e., different from naphthalene dicarboxylic acid monomers and their isomers) carboxylate monomer molecules. For embodiments in which the second polyester contains more than one type of carboxylate unit, the second polyester can be a block or random copolyester. The total amount of other carboxylate monomer molecules can range up to 10 mole percent of the copolyester. Typically, the total amount of other carboxylate monomer molecules is no greater than 8 mole percent, 6 mole percent, 4 mole percent, 3 mole percent, or 2 mole percent of the copolyester. Suitable other carboxylate monomer molecules include, for example, terephthalic acid; isophthalic acid; phthalic acid; azelaic acid; adipic acid; sebacic acid; norbornene dicarboxylic acid; dicyclooctane dicarboxylic acid; t-butyl isophthalic acid, trimellitic acid, sulfonated sodium isophthalic acid; 4,4'-biphenyldicarboxylic acid and its isomers; and lower alkyl esters of these acids, such as methyl or ethyl esters. The term "lower alkyl" herein refers to C1-C10, preferably C1-C4, and more preferably C1-C2 straight chain or branched alkyl groups.

[0017] Suitable branched or cyclic C4-C10alkyl diols include, for example, cyclohexane dimethanol, neopentyl glycol, and mixtures thereof. Corresponding branched or cyclic C4-C10alkyl diacids or branched or cyclic C4-C10alkyl diesters can be used in place of, or in addition to, branched or cyclic C4-C10alkyl diols. For example, dimethyl 1,4-cyclohexanedicarboxylate or 1,4-cyclohexanedicarboxylic acid can be used in place of, or in place of a portion of, cyclohexane dimethanol. In some embodiments, branched or cyclic C4-C10alkyl diols are used such that the second polyester can comprise 50 mole percent or near 50 mole percent of naphthalate units.

[0018] Often preferably, the composition of the second polyester of the first outer layer 110 is the same as the composition of the second polyester of the second outer layer 112. However, in some embodiments, the composition of the second polyester of the first outer layer 110 and the second polyester of the second outer layer 112 can be different while remaining within similar composition ranges. For example, the second polyester of the first outer layer 110 and the second polyester of the second outer layer 112 can be different, but both can comprise 40 to 50 mole percent of naphthalate units, at least 25 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10alkyl units. In some embodiments, the second polyester of the first outer layer 110 and the second polyester of the second outer layer 112 nominally have the same composition, but the composition can differ due to, for example, normal manufacturing variations.

[0019] In some embodiments, the second polyester comprises second monomer units that hinder crystallization of the second polyester. Suitable second monomer units for this purpose include branched or cyclic C4-C10alkyl units as further described elsewhere herein. Use of second monomer units that hinder crystallization can provide the first outer layer 110 and the second outer layer 112 that are amorphous or substantially amorphous and / or isotropic or substantially isotropic. The degree of isotropy can be characterized by the birefringence of the outer layer. In-plane birefringence refers to nx-ny, where nxand nyare the refractive indices in the x and y directions, respectively, as measured by ellipsometry. Out-of-plane birefringence refers to nz-nx, where nzis the refractive index in the z direction, as measured by ellipsometry. In some embodiments, the first outer layer 110 and the second outer layer 112 have an in-plane birefringence of less than 0.01, less than 0.005, or less than 0.001. In some embodiments, the first outer layer 110 and the second outer layer 112 have an out-of-plane birefringence of less than 0.01, less than 0.005, or less than 0.001. Figure 1The in-plane birefringence is greater than or equal to zero. Unless otherwise specified, the refractive indices are determined at a wavelength of 633 nm. The refractive indices can be determined in accordance with test standard ASTM D542-14 “Standard Test Method for Index of Refraction of Transparent Organic Plastics.” The out-of-plane birefringence refers to ½(nx+ny)-nz, where nz is the refractive index of light having an electric field along the z-axis. The out-of-plane birefringence is typically greater than or equal to zero. In some embodiments, each of the first and second outer layers has an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02. In some embodiments, each of the first and second outer layers has an in-plane birefringence of less than 0.01 and an out-of-plane birefringence of less than 0.01. In some embodiments, the substrate is a curved film and / or a flexible film used in curved displays and / or flexible displays. If the substrate is curved, the in-plane and out-of-plane birefringence refers to the birefringence components determined with respect to a plane tangent to the curvature of the substrate.

[0020] The melting point of the first and / or second polyester can be determined by differential scanning calorimetry (DSC). In some embodiments, the second polyester has a melting transition temperature of less than 220 °C, or less than 210 °C, or less than 200 °C after inducing crystallization. The crystallization can be induced by heating or stretching. In some embodiments, the melting transition temperature is determined by differential scanning calorimetry and the crystallization is induced by heating during the differential scanning calorimetry measurement. In some embodiments, the second polyester does not have a melting point detectable by differential scanning calorimetry. This is the case if there is no peak in the DSC curve associated with annealing-induced crystallization and no peak associated with melting of the induced crystalline phase. In some embodiments, the melting transition temperature, if present, is greater than 150 °C.

[0021] The melting enthalpy can be determined using differential scanning calorimetry as described in test standard ASTM E793-06 (2012) “Standard Test Method for Enthalpies of Fusion and Crystallization by Differential Scanning Calorimetry.” In some embodiments, the second polyester has a melting enthalpy of less than 10 J / g, or less than 5 J / g, or less than 3 J / g, or less than 1 J / g. The second polyester can be described as substantially amorphous if it does not have a melting point detectable by differential scanning calorimetry, or has a melting transition temperature of less than 220 °C after induced crystallization, or has a melting enthalpy of less than 10 J / g.

[0022] In some embodiments, the first polyester has a glass transition temperature of at least 100 °C. In some embodiments, the second polyester has a glass transition temperature of at least 90 °C or at least 100 °C. In some embodiments, each of the first polyester and the second polyester has a glass transition temperature of at least 90 °C or at least 100 °C. In some embodiments, one or both of the first polyester and the second polyester has a glass transition temperature of less than 160 °C. The glass transition temperature can be determined by differential scanning calorimetry as described in test standard ASTM E1356-08 (2014) “Standard Test Method for Assignment of the Glass Transition Temperatures by Differential Scanning Calorimetry.”

[0023] Biaxially orienting the polymer layer can increase the Young's modulus of the layer. In some embodiments, the Young's modulus of the biaxially oriented layer 101 is at least 4.5 GPa, or at least 5 GPa. In some embodiments, the Young's modulus of the biaxially oriented layer 101 is no greater than 15 GPa, or no greater than 10 GPa. Unless otherwise indicated, the Young's modulus is determined at 25 °C. The Young's modulus can be determined as described in test standard ASTM E111-04 (2010) "Standard Test Method for Young's Modulus, Tangent Modulus, and Chord Modulus." Biaxially orienting the polymer layer can result in a relatively low in-plane birefringence and a relatively high out-of-plane birefringence. In some embodiments, the biaxially oriented layer 101 has an in-plane birefringence of less than 0.1, or less than 0.05, or less than 0.03, or less than 0.02, or less than 0.01. In some embodiments, the biaxially oriented layer 101 has an out-of-plane birefringence of at least 0.18 or at least 0.2.

[0024] In some embodiments, the three-layer stack 105 has a thickness Ts in a range from 10 micrometers or 20 micrometers or 25 micrometers to 200 micrometers or 125 micrometers. In some embodiments, the thickness Tb of the biaxially oriented layer 101 is at least 80% or at least 90% of the thickness Ts of the three-layer stack 105. In some embodiments, the thickness Tb of the biaxially oriented layer 101 is no greater than 99% of the thickness Ts of the three-layer stack 100. It has been found that using thicknesses and thickness ratios within these ranges provides desirable mechanical properties for the total thickness Ts of the three-layer stack 105. In some embodiments, the substrate 100 does not include any additional layers and has a total thickness of Ts. In other embodiments, the substrate 100 can include additional layers, and thus can have a total thickness greater than Ts.

[0025] It has been found that using a common first monomer unit (e.g., a naphthalate unit) for the first and second polyesters can result in better delamination performance (separation of the outer layers 110 and / or 112 from the biaxially oriented layer 101) compared to using first and second polyesters that do not share a common first monomer unit. Without wishing to be bound by theory, it is believed that this occurs due to the similarity in the coefficient of thermal expansion of the layers in the absence of chemical bonds between the outer layers 110, 112 and the biaxially oriented layer 101.

[0026] In some embodiments, an additional coating or layer is applied to the outer surface of one or both of outer layer 110 and outer layer 112. In some embodiments, the outer surface of one or both of outer layer 110 and outer layer 112 is optically smooth. That is, any surface roughness present can have a small peak-to-valley height (e.g., small compared to 550 nm) compared to the wavelength of visible light. In some embodiments, the outer surface has a surface roughness Ra of less than 100 nm, or less than 50 nm, or less than 20 nm, or even less than 10 nm. Ra refers to the arithmetic average of the absolute values of the difference between the surface height and the average surface location. Ra can be determined according to the American Society of Mechanical Engineers (ASME) B46.1-2009 test standard.

[0027] For example, it can be desirable for substrate 100 to have a low haze such that the substrate does not degrade image quality when used in a display at a location where light produced by the display is transmitted through substrate 100. In some embodiments, substrate 100 has a haze of less than 2%, or less than 1%. Such low haze can be obtained using the polyester compositions described elsewhere herein. Haze can be determined according to test standard ASTM D1003-13 “Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics.”

[0028] It can be desirable for the substrate to block ultraviolet radiation in order to protect other layers included in, for example, a display. Utilization of naphthalate monomer units in the first polyester (e.g., using PEN) can provide the desired degree of ultraviolet absorption. Whether utilizing naphthalate monomer units or other monomer units, it can be desirable to include one or more ultraviolet absorbers in one or both of the first polyester and the second polyester to increase the ultraviolet absorption of substrate 100. Suitable ultraviolet absorbers include, for example, triazines, benzotriazoles, and benzophenones.

[0029] The polyesters described herein can be used in other applications, such as for retarder films as described in co-pending U.S. Provisional Patent Application 62 / 592,545, entitled “Retarder,” filed November 30, 2017, and incorporated by reference herein to the extent not contradictory to this specification.

[0030] Figure 2This is a schematic diagram of a display 250 including an emitter layer 260 and a substrate 200. The substrate 200 can be any substrate described elsewhere herein. For example, substrate 200 and triple-layer stack 205 can correspond to substrate 100 and triple-layer stack 105, respectively. The emitter layer 260 can be a display panel such as a liquid crystal display (LCD) panel or an organic light-emitting diode (OLED) display panel, or the emitter layer 260 can be a component of the display panel. For example, the emitter layer 260 can be the emitter layer of an OLED display panel. The emitter layer 260 is in... Figure 2 The xyz coordinate system is used as a reference for emitting light in the z-direction. In some embodiments, the substrate 200 is positioned opposite to the light output direction of the emitting layer 260. In some embodiments, the substrate of this specification is positioned to receive light from the emitting layer 260. In some embodiments, the substrate 200 is separated from the emitting layer 260, and in some embodiments, the substrate 200 is attached directly or indirectly (e.g., via one or more additional layers) to the emitting layer 260 (e.g., via an optically clear adhesive). In some embodiments, the display 250 may include... Figure 2 Additional layers or components not shown. For example, a touch sensor may be disposed near an emission layer 260 opposite to substrate 200. The touch sensor may include one or more substrates as described in this specification.

[0031] Figure 3 This is a schematic cross-sectional view of a touch sensor 310 including a first substrate 300a, which has a first plurality of electrodes 330a disposed on a first main surface 325a of the first substrate 300a. The touch sensor 310 also includes a second substrate 300b, which has a second plurality of electrodes 330b disposed on a first main surface 325b of the second substrate 300b. Figure 3 Only one of the electrodes 330b is visible in the cross-section. The first plurality of electrodes 330a may be along a first direction (e.g., with...). Figure 3 The first plurality of electrodes 330a and the second plurality of electrodes 330b extend along an orthogonal second direction (e.g., the x direction), such that the first plurality of electrodes 330a and the second plurality of electrodes 330b form a cross-grid as is conventional for electrodes used in touch sensors. In an alternative embodiment, the second substrate 300b is omitted and the second plurality of electrodes 330b are disposed on the second main surface 327a of the first substrate 300a. For example, the first substrate 300a may correspond to substrate 100 and may include a three-layer stack ( Figure 3 (Not shown in the text), as further described elsewhere herein. Similarly, for example, the second substrate 300b may correspond to substrate 100 and may include a three-layer stack ( Figure 3The first substrate 300a and the second substrate 300b can each correspond to the substrate 100, while in other embodiments only one of the first substrate 300a and the second substrate 300b corresponds to the substrate 100. In some embodiments, the first substrate 300a and the second substrate 300b each correspond to the substrate 100, while in other embodiments only one of the first substrate 300a and the second substrate 300b corresponds to the substrate 100.

[0032] The first plurality of electrodes and / or the second plurality of electrodes can be any type of electrode suitable for use in a touch sensor. Examples include transparent conductor electrodes (e.g., indium tin oxide (ITO)) and metal mesh electrodes, such as those described in U.S. Patent 8,933,906 (Frey), which is incorporated herein by reference to the extent not contradictory with this specification. The electrodes can be deposited onto the substrate by utilizing a sputtering or etching process as is known in the art. Other useful touch sensors and methods of making touch sensors can utilize the substrates of this specification described in U.S. Patents 8,384,961 (Frey et al.), 8,865,027 (Alden et al.), and 9,023,229 (Sebastian et al.) and U.S. Patent Application 2015 / 316955 (Dodds et al.), each of which is incorporated herein by reference to the extent not contradictory with this specification.

[0033] In some embodiments, the touch sensor 310 is placed over the display such that light emitted from the display transmits through the touch sensor 310. For example, the touch sensor 310 can be placed over the display 250.

[0034] In some embodiments, a display including one or more substrates of this specification is flexible and / or curved. Figure 4 A schematic cross-sectional view of a display 450 that is curved to a radius of curvature R. The display 450, which is schematically illustrated by the solid curved line, includes at least one substrate of this specification. In some embodiments, the display 450 can be bent to a radius of curvature R of 20 cm without visible damage, which should be understood to mean no visible damage to a person having 20 / 20 vision and without visual aids in ordinary indoor lighting with the display turned on or off. The display can be bent to a smaller radius of curvature (e.g., to a radius of curvature R of 10 cm) without visible damage. In some embodiments, the display can be bent from flat to a radius of curvature R of 20 cm and back to flat at least 100 times without visible damage.

[0035] The following is a list of exemplary embodiments of this specification.

[0036] Embodiment 1 is a substrate comprising a self-supporting three-layer stack comprising a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer, wherein the biaxially oriented layer comprises a first polyester having greater than 45 mole percent of naphthalate units and greater than 45 mole percent of ethylene units, wherein each of the first outer layer and the second outer layer comprises a second polyester comprising 40 to 50 mole percent of naphthalate units, at least 25 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10 alkyl units.

[0037] Embodiment 2 is the substrate according to Embodiment 1, wherein the biaxially oriented layer has a Young’s modulus of at least 4.5 GPa.

[0038] Embodiment 3 is the substrate according to Embodiment 1 or 2, wherein biaxially oriented layer has an out-of-plane birefringence of at least 0.18, or at least 0.2.

[0039] Embodiment 4 is the substrate according to any one of Embodiments 1 to 3, wherein the three-layer stack has a thickness in the range of 10 to 200 micrometers, or 20 to 200 micrometers, or 25 to 125 micrometers.

[0040] Embodiment 5 is the substrate according to any one of Embodiments 1 to 4, wherein the thickness of the biaxially oriented layer is at least 80% or at least 90% of the thickness of the three-layer stack.

[0041] Embodiment 6 is the substrate according to any one of Embodiments 1 to 5, wherein the second polyester is substantially amorphous.

[0042] Embodiment 7 is the substrate according to any one of Embodiments 1 to 6, wherein the second polyester does not have a melting point detectable by differential scanning calorimetry.

[0043] Embodiment 8 is the substrate according to any one of Embodiments 1 to 6, wherein the second polyester has a melting transition temperature of less than 220°C after induced crystallization.

[0044] Embodiment 9 is the substrate according to any one of Embodiments 1 to 6, wherein the second polyester has a melting enthalpy of less than 10 J / g, or less than 5 J / g, or less than 3 J / g.

[0045] Embodiment 10 is the substrate according to any one of Embodiments 1 to 9, wherein the first outer layer and the second outer layer each have an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02.

[0046] Embodiment 11 is the substrate of any one of embodiments 1 to 9, wherein the first and second outer layers each have an in-plane birefringence of less than 0.01 and an out-of-plane birefringence of less than 0.01.

[0047] Embodiment 12 is the substrate of any one of embodiments 1 to 11, wherein the second polyester comprises 25 to 50 mole percent ethylene units or 25 to 40 mole percent ethylene units.

[0048] Embodiment 13 is the substrate of any one of embodiments 1 to 12, wherein the first polyester is polyethylene naphthalate (PEN).

[0049] Embodiment 14 is the substrate of any one of embodiments 1 to 13, wherein for the second polyester, the mole percent of naphthalate units, the mole percent of ethylene units, and the mole percent of branched or cyclic C4-C10 alkyl units add up to 100 mole percent, and for the first polyester, the mole percent of naphthalate units and the mole percent of ethylene units add up to 100 mole percent.

[0050] Embodiment 15 is the substrate of any one of embodiments 1 to 14, having a haze of less than 2 percent, or less than 1 percent.

[0051] Embodiment 16 is a substrate comprising a self-supporting three-layer stack comprising a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer, wherein the biaxially oriented layer comprises a first polyester comprising a plurality of first monomer units and each of the first and second outer layers comprises a second polyester, the first polyester having a glass transition temperature of at least 90 °C, the second polyester being a copolyester comprising a plurality of the first monomer units and a plurality of second monomer units, the second monomer units hindering crystallization of the second polyester.

[0052] Embodiment 17 is the substrate of embodiment 16, wherein the first and second outer layers each have an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02.

[0053] Embodiment 18 is the substrate of embodiment 16 or 17, wherein the first monomer unit is a naphthalate unit.

[0054] Embodiment 19 is the substrate of any one of embodiments 16 to 18, wherein the first polymer further comprises ethylene units.

[0055] Embodiment 20 is the substrate of any one of embodiments 16-19, wherein the second monomer unit is a branched or cyclic C4-C10 alkyl unit.

[0056] Embodiment 21 is the substrate of any one of embodiments 16-20, wherein the first polyester comprises greater than 45 mole percent of naphthalate units and greater than 45 mole percent of ethylene units.

[0057] Embodiment 22 is the substrate of any one of embodiments 16-21, wherein the second polyester comprises 40 to 50 mole percent of naphthalate units, at least 25 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10 alkyl units.

[0058] Embodiment 23 is the substrate of embodiment 22, wherein the second polyester comprises 25 to 50 mole percent of ethylene units or 25 to 40 mole percent of ethylene units.

[0059] Embodiment 24 is a substrate comprising a self-supporting three-layer stack comprising first and second outer layers and a biaxially oriented layer disposed between and in direct contact with the first and second outer layers, each of the first and second outer layers having an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02, wherein the biaxially oriented layer comprises a first polyester comprising a plurality of first monomer units and each of the first and second outer layers comprises a second polyester, the first polyester having a glass transition temperature of at least 90 °C, the second polyester being a copolyester comprising a plurality of the first monomer units.

[0060] Embodiment 25 is the substrate of embodiment 24, wherein the second polyester comprises a plurality of second monomer units that hinder crystallization of the second polyester.

[0061] Embodiment 26 is the substrate of embodiment 25, wherein the second monomer unit comprises a cyclic C4-C10 alkyl unit.

[0062] Embodiment 27 is the substrate of any one of embodiments 24-26, wherein the first monomer unit is a naphthalate unit.

[0063] Embodiment 28 is the substrate of any one of embodiments 24-27, wherein the first polyester comprises greater than 45 mole percent of naphthalate units and greater than 45 mole percent of ethylene units.

[0064] Embodiment 29 is the substrate of any one of embodiments 24 to 28, wherein the second polyester comprises 40 to 50 mole percent of naphthalate units, at least 25 mole percent of ethylene units, and 10 to 25 mole percent of branched or cyclic C4-C10 alkyl units.

[0065] Embodiment 30 is the substrate of any one of embodiments 24 to 29, wherein the second polyester comprises 25 to 50 mole percent of ethylene units or 25 to 40 mole percent of ethylene units.

[0066] Embodiment 31 is the substrate of any one of embodiments 16 to 30, further characterized according to any one of embodiments 1 to 15.

[0067] Embodiment 32 is the substrate of any one of embodiments 1 to 31, wherein the first polyester has a glass transition temperature of at least 100 °C.

[0068] Embodiment 33 is the substrate of any one of embodiments 1 to 32, wherein each of the first polyester and the second polyester has a glass transition temperature of at least 90 °C or at least 100 °C.

[0069] Embodiment 34 is a display comprising an emissive layer disposed on the substrate of any one of embodiments 1 to 33.

[0070] Embodiment 35 is the display of embodiment 34, which is bendable to a radius of curvature of 20 cm without visible damage.

[0071] Embodiment 36 is the display of embodiment 34 or 35, which is curved.

[0072] Embodiment 37 is a touch sensor comprising the substrate of any one of embodiments 1 to 33 and a plurality of electrodes disposed on the substrate.

[0073] Embodiment 38 is a display comprising the touch sensor of embodiment 37.

[0074] Example

[0075] Examples 1-3 and Comparative Examples C1-C3 Manufacture and characterization of copolyesters

[0076] A series of polyester copolymers were prepared using the following procedure:

[0077] To a stainless steel 10 gallon reactor (equipped with hot oil temperature control, overhead separation column, and vacuum pump) were added the following components at room temperature:

[0078] 2,6-Naphthalene dicarboxylic acid dimethyl ester (NDC)

[0079] (Indorama Ventures, Decatur, AL)

[0080] Ethylene glycol (EG)

[0081] (Huntsman Petrochemical, The Woodlands, TX)

[0082] Cyclohexane dimethanol (CHDM)

[0083] (Eastman Chemical, Kingsport, TN)

[0084] Tetrabutyl titanate (TBT)

[0085] (Dorf Ketal, Houston, TX)

[0086] Cobalt acetate (CoAc)

[0087] (Shepherd Chemical, Cincinnati, OH)

[0088] Zinc acetate (ZnAc)

[0089] Mallinckrodt Baker, Phillipsburg, NJ.

[0090] Antimony triacetate (SbAc)

[0091] (Arkema, Philadelphia, PA)

[0092] Table 1 below provides the amount of each component used in each of the examples and comparative examples. After addition, the materials were heated and mixed at 125 rpm under 138 kPa (20 psig) N2. The temperature of the transesterification reaction was increased to 257°C (495°F) over a period of about 2 hours. The methanol was removed through a knockout column and collected in a receiver. The pressure in the kettle was slowly released to atmospheric pressure. In the case of comparative examples CI and C2, a stabilizer was added. The stabilizer was triethyl phosphonoacetate (TEPA) (available from Rhodia, Cranbury, NJ).

[0093] For these examples, a vacuum was applied to the kettle and increased as the batch viscosity allowed. Excess ethylene glycol was removed. After about 2 hours at a temperature of about 285°C (545°F) and a vacuum as low as about 0.13 kPa (1 mm Hg), the reaction proceeded to the desired endpoint (an intrinsic viscosity or IV of about 0.48 dL / g). The kettle was evacuated and the reaction product resin was cooled to room temperature, after which the resin was ground into small pieces for further evaluation. Table 1 provides the amount of each component used in each of the examples and comparative examples. The CHDM % in the header row refers to the mole % of CHDM in the diol portion of the polyester. For example, PEN w / 30% CHDM means that the diol portion of the polyester contains 30 mole % CHDM. Based on total diol and acid / ester, PEN w / 30% CHDM contains 50 mole % naphthalate units, 35 mole % ethylene units, and 15 mole % CHDM units.

[0094] Table 1.

[0095]

[0096] The resin samples of each example and comparative example were then placed in an oven maintained at 150°C for 48 hours to cold crystallize. Each of these crystallized materials was tested using a differential scanning calorimeter or DSC (available under the trade designation "Q2000" from TA Instruments, New Castle, DE). The test employed a 3-stage heating-cooling-heating temperature ramp over a temperature range of 30°C to 290°C. After the first heating ramp, the sample was held at 290°C for 3 minutes. The ramp rate for each of the heating and cooling ramps was 20°C / min. Both the first heating scan and the second heating scan were analyzed. The melting point and associated heat of fusion, as well as the glass transition temperature, were determined for each test sample and recorded.

[0097] Table 2 presents the results for melting point and associated heat of fusion (AH) and glass transition (Tg).

[0098] Table 2.

[0099]

[0100]

[0101] As can be seen from Table 2, Examples 1, 2, and 3 (PENg30, PENg40, and PENg50, respectively) exhibit a melting point of 210 °C or less and little to no enthalpy of fusion (less than 3 J / g). The Tg of all samples was found to be between 115 °C and 120 °C.

[0102] Examples 4-8 and Comparative Examples C4-C6 Manufacture and characterization of films

[0103] A series of 3-layer films were prepared by co-extrusion, stretch co-orientation, and annealing. The resins used as input materials were as follows:

[0104] PEN refers to 0.48 IV polyethylene naphthalate glycol resin

[0105] (Internal manufacture by methods known to those of ordinary skill in the art of polyester manufacture)

[0106] PETg refers to 0.73 IV copolyester

[0107] (Available under the trade designation "GN071" from Eastman Corporation, Kingsport, TN)

[0108] PENg30, PENg40, PENg50 and PENg60

[0109] (The materials described in Examples 1, 2, and 3, and Comparative Example C3 above, respectively)

[0110] For each of these examples, the details of the input materials for each layer of the ABA film stack are provided in Table 3.

[0111] Table 3.

[0112]

[0113] The outer layer (or skin, or "A" layer) was prepared by extruding the above resin using a 27 mm twin-screw extruder (TSE), conveyed via a neck tube using a gear pump, and fed into the outer layer of a 3-layer feedblock. The melt train used a progressive temperature extrusion profile with a peak temperature of 282°C. The inner layer (or core, or "B" layer) was prepared by extruding a PEN resin using a 27 mm TSE with a progressive temperature profile peaking at about 282°C, conveyed via a neck tube using a gear pump, and fed into the middle layer of a 3-layer feedblock. The feedblock and 20.3 cm (8 inch) film die were each maintained at a target temperature of 282°C, while the film casting wheel was maintained at about 50°C. A 914 micrometer (36 mil) thick film cast web was prepared by this process.

[0114] Samples of each cast web prepared from the extrusion and casting process were then stretched (oriented) and annealed using a laboratory stretcher (obtained under the trade designation "KARO IV" from Brueckner Maschinenbau GmbH & Co. KG, Siegsdorf, Germany). Stretching of each film was performed in an oven maintained at 140°C. The preheat time prior to stretching was 45 seconds. Each film was biaxially stretched to a final size of 350% of the original size in the machine direction (MD) and 350% of the original size in the transverse direction (TD), resulting in a 76 micrometer (3 mil) finished film. The stretched film was then conveyed to an annealing oven maintained at 225°C and held there for 15 seconds. Each stretched and oriented annealed film was then evaluated for haze, refractive index, delamination peel force, and Graves tear.

[0115] Haze was tested using a hazemeter (obtained under the trade designation "HAZE-GARD" from BYK-Gardner USA, Columbia, MD). Haze was measured according to ASTM D-1003 and reported as "% haze".

[0116] Refractive indices of film samples were measured in the machine direction (MD), transverse direction (TD), and thickness (TM) directions using a prism coupler (obtained from Metricon Corporation, Pennington, N.J.). The refractive indices in the MD, TD, and TM are denoted Nx, Ny, and Nz, respectively. The out-of-plane birefringence is the difference between the average in-plane refractive index (i.e., the average of Nx and Ny) and the refractive index normal to the film (Nz).

[0117] The notch maximum load values and notch area values were obtained according to ASTM D1004-13 on an Instron 5500-R from Instron, Norwood, MA.

[0118] The delamination peel force test values were obtained using an IMASS SP-2100 from IMASS, Inc., Accord, MA, in which the base film was adhered to a rigid flat glass plate with tape. The peel force measurements were made as follows: 90 degree peel; a slip speed of 60 inches / minute; and the peel force was averaged over the distance of travel of the peel. The resulting peel force values were recorded, given in grams / inch.

[0119] The results of these tests are reported in Table 4.

[0120] Table 4.

[0121]

[0122] Each of the examples and comparative examples provided a low haze film (less than 1% haze). Each of comparative example C5 and examples 4-8 exhibited a low out-of-plane birefringence (less than 0.01), which can be indicative of a desirable amorphous (coating receptive and flexible) surface of these films. In addition, each of these films exhibited a transmittance at 375 nm of less than 5%.

[0123] Except for comparative example C5, each of the films exhibited high delamination peel performance (i.e., they did not fail the test by peeling apart, or“were not separable”). Except for comparative example C6, all of the samples exhibited excellent notch tear (a maximum load of greater than 20 Newtons (4.5 lbs f ) and a notch area of greater than 62 Newtons*% (14 lbs f *%).

[0124] Unless otherwise indicated, the description of elements according to the figures should be understood to apply equally to corresponding elements in other figures. While specific embodiments have been illustrated and described, it will be appreciated that various alternate and / or equivalent modifications can be made of the embodiments described without deviating from the scope of the disclosure. This application is intended to cover any adaptations or variations of the specific embodiments discussed. Therefore, it is intended that the disclosure be protected by the patent as patentably set forth in the following claims, and that their equivalents be so included.

Claims

1. A substrate useful for flexible displays, the substrate comprising a self-supporting three-layer stack that is at least 10 micrometers thick and consists of a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer, wherein the biaxially oriented layer has a Young's modulus of at least 4.5 GPa and comprises a first polyester having greater than 45 mole percent naphthalate units and greater than 45 mole percent ethylene units, wherein each of the first outer layer and the second outer layer comprises a second polyester consisting of 40 to 50 mole percent naphthalate units, 25 to 40 mole percent ethylene units, and 10 to 25 mole percent branched or cyclic C4-C10 alkyl units.

2. The substrate of claim 1, wherein the biaxially oriented layer has a Young's modulus of at least 5 GPa.

3. The substrate of claim 1, wherein the biaxially oriented layer has an out-of-plane birefringence of at least 0.

18.

4. The substrate of claim 1, wherein the thickness of the biaxially oriented layer is at least 80 percent of the thickness of the self-supporting three-layer stack.

5. The substrate of claim 1, wherein the second polyester has no melting point detectable by differential scanning calorimetry.

6. The substrate of claim 1, wherein the second polyester has a melting enthalpy of less than 10 J / g.

7. The substrate of claim 1, wherein the first outer layer and the second outer layer each have an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.

02.

8. The substrate of claim 1, wherein the mole percent of the naphthalate units, the mole percent of the ethylene units, and the mole percent of the branched or cyclic C4-C10 alkyl units add up to 100 mole percent for the second polyester, and the mole percent of the naphthalate units and the mole percent of the ethylene units add up to 100 mole percent for the first polyester.

9. The substrate of claim 1, having a haze of less than 2 percent.

10. A substrate useful for flexible displays, the substrate comprising a self-supporting three-layer stack that is at least 10 micrometers thick and consists of a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer, wherein the biaxially oriented layer has a Young's modulus of at least 4.5 GPa and comprises a first polyester comprising a plurality of first monomer units, and each of the first outer layer and the second outer layer comprises a second polyester, the first polyester having a glass transition temperature of at least 90 °C, the second polyester being a copolyester comprising a plurality of the first monomer units and a plurality of second monomer units, the second monomer units hindering crystallization of the second polyester, the first monomer units being naphthalate units, wherein the second polyester consists of 40 to 50 mole percent naphthalate units, 25 to 40 mole percent ethylene units, and 10 to 25 mole percent branched or cyclic C4-C10 alkyl units.

11. A substrate useful for flexible displays, the substrate comprising a self-supporting three-layer stack that is at least 10 micrometers thick and consists of a first outer layer and a second outer layer and a biaxially oriented layer disposed between and in direct contact with the first outer layer and the second outer layer, each of the first outer layer and the second outer layer having an in-plane birefringence of less than 0.02 and an out-of-plane birefringence of less than 0.02, wherein the biaxially oriented layer has a Young's modulus of at least 4.5 GPa and comprises a first polyester comprising a plurality of first monomer units, and each of the first outer layer and the second outer layer comprises a second polyester, the first polyester having a glass transition temperature of at least 90 °C, the second polyester being a copolyester comprising a plurality of the first monomer units, the first monomer units being naphthalate units, wherein the second polyester consists of 40 to 50 mole percent naphthalate units, 25 to 40 mole percent ethylene units, and 10 to 25 mole percent branched or cyclic C4-C10 alkyl units.

12. A display comprising an emissive layer disposed on the substrate of any one of claims 1 to 11.

13. The display of claim 12, which is bendable to a radius of curvature of 20 cm without visible damage.

14. A touch sensor comprising the substrate of any one of claims 1 to 11 and a plurality of electrodes disposed on the substrate.

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