TW201823006A - Birefringent polyester film with low haze - Google Patents

Abstract

A birefringent polyester film includes naphthalate units, ethylene units, a titanate compound, a planar branching unit, and 1 to 80 ppm metal content. The birefringent polyester film has an out-of-plane birefringence of at least 0.1 at 633 nm.

Description

 Birefringent polyester film with low haze  

Polymeric membranes are used in a wide variety of applications. Multilayer polymeric optical films are widely used for a variety of purposes, including as mirrors and polarizers. These films often have extremely high reflectivity while being lightweight and resistant to breakage. Exemplary applications for these films include compact electronic displays, including liquid crystal displays (LCDs) placed in mobile phones, personal data assistants, computers, televisions, and other devices.

One type of polyester that can be used to create a polymer for a polarizer film or a mirror film. One example of a polyester based polarizer includes a stack of polyester layers of different compositions. The polyester is composed of one or more different dicarboxylate monomers (eg, compounds having two or more carboxylic acid or ester functional groups) and one or more different diol monomers (eg, having It is prepared by the reaction of a compound of two or more hydroxy functional groups. One example of a polyester that can be used in a multilayer optical film is polyethylene naphthalate (PEN), which can be produced, for example, by reaction of naphthalene dicarboxylic acid (or ester) with ethylene glycol.

The present disclosure relates to a birefringent polyester film that exhibits low light absorption, high birefringence, and low haze. These birefringent polyester films can also exhibit high melt elasticity similar to other polymer film layers in terms of co-extrusion into a multilayer film. The birefringent polyester film may include a naphthalate unit, an ethylene unit, a titanate compound, a planar branching unit, and 1 to 80 ppm of a metal content. The birefringent polyester film has an out-of-plane birefringence of at least 0.1 at 633 nm, and a tan δ value in the range of 3 to 8.

In one aspect, the birefringent polyester film comprises: a naphthalate unit; an ethylene unit; a titanate compound; 0.1 to 2 mol% of planar branching units based on the naphthalate units and The total mol% of the branching unit; and 1 to 80 ppm of metal content. The birefringent polyester film has an out-of-plane birefringence of at least 0.1 at 633 nm.

In another aspect, the multilayer film comprises: a first birefringent polyester layer having an out-of-plane birefringence of at least 0.10 at 633 nm; and a second polymer layer having a lower than the first birefringence The birefringence of the polyester layer is disposed on the first birefringent polyester layer. The polyester layer comprises: a naphthalate unit; an ethylene unit; a titanate compound; 0.1 to 2 mol% of a planar branching unit based on the naphthalate units and the total mol% of the branching unit And 1 to 80 ppm of metal content.

In another aspect, the multilayer optical film comprises: a plurality of first birefringent polyester layers each having an out-of-plane birefringence of at least 0.10 at 633 nm, and a metal content of less than 100 ppm, and having a Celsius of 280 And a first tan δ value at a shear rate of 10 sec ^-1 ; and a plurality of second isotropic acrylate layers alternating between the first birefringent polyester layers, and each of the second The isotropic acrylate layer has a second tan δ value at a shear rate of 280 degrees Celsius and 10 sec ^-1 . The first tan δ value is within 30% or less of the second tan δ value.

These and various other features and advantages will be apparent from the following detailed description.

The following detailed description is therefore not to be taken as limiting.

All scientific and technical terms used herein have the meanings common to the art to which the invention pertains, unless otherwise indicated. The definitions presented herein are intended to enhance the understanding of some of the terms used in this document and are not intended to limit the scope of the disclosure.

All numbers expressing size, quantity, and physical properties of the features in the specification and claims are to be understood as being modified by the term "about" in all instances. Accordingly, the numerical parameters set forth in the foregoing specification and the accompanying claims are approximations, which can be obtained in accordance with the teachings disclosed herein. The nature changes.

The recitation of numerical ranges by endpoints includes all numbers that are included within the range (eg, 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within the range.

As used in the specification and the appended claims, the singular forms "a", "the" and "the" Example.

As used in this specification and the appended claims, the word "or" is generally used to mean "and/or" and unless the context clearly dictates otherwise.

As used herein, "having, including, including, "comprise, comprising" or the like is used in its open sense and generally means "including but not Limited to (including, but not limited to)". It should be understood that "consisting essentially of", "consisting of", and similar terms are included in "comprising" and the like.

"Index of refraction (refractive index), "index", or "RI" means that the material is at 633 nm in the plane of the material and is perpendicular or nearly vertical (for example, 8 degrees or less). Shift) Incident refractive index, unless otherwise indicated.

"Birefringent" means that the refractive indices in the orthogonal x, y, and z directions are not all the same. For the polymer layers described herein, the axial system is selected such that the x-axis and the y-axis are in the plane of the layer, and the z-axis is normal to the plane of the layer and generally corresponds to the thickness or height of the layer. Birefringence values are reported for light and normal incidence at 633 nm unless otherwise indicated.

"High refractive index" and "low refractive index" are relative terms; when the two layers are compared in two in-plane directions of interest, they have a larger average in-plane refractive index. The layer is a high refractive index layer, and the layer having a lower average in-plane refractive index is a low refractive index layer.

Unless otherwise indicated, "polymer" refers to polymers and copolymers (ie, polymers formed from two or more monomers or comonomers, including, for example, terpolymers), and Copolymers or polymers formed in the form of miscible blends, for example, by coextrusion or reaction, including, for example, transesterification. Unless otherwise indicated, block polymers, random polymers, graft polymers, and alternating polymers are included.

"Polyester" means a polymer containing an ester functional group in the main polymer chain. Copolyesters are included in the term "polyester".

The "tan δ (Tan Delta)" value is the ratio of the viscosity modulus (G") to the elastic modulus (G') and refers to the viscoelasticity of the polymer, with the lower tan δ value indicating a more elastic material. The tan δ values reported or referred to herein are based on a Plate-on-Plate rheometer (available from TA Instruments, New Castle, DE, USA) at a shear rate of 10 sec ^-1 at Measured at 280 degrees Celsius.

The present disclosure relates to a birefringent polyester film that exhibits low light absorption, high birefringence, and low haze. These birefringent polyester films can also exhibit high melt elasticity similar to other polymer film layers in terms of co-extrusion into a multilayer film. These birefringent polyester films can have low metal content, such as less than 100 ppm metal, or less than 80 ppm metal, or less than 30 ppm metal. The birefringent polyester film may include a naphthalate unit, an ethylene unit, a titanate compound, and a planar branching unit. The birefringent polyester film has an out-of-plane birefringence of at least 0.1 and a tan δ value in the range of 3 to 8. Polyethylene naphthalate (PEN) is known to have several orders of magnitude higher absorption than other polymers utilized in multilayer film constructions, such as acrylates or olefins. PEN is also known to have a different melt elasticity than other polymers utilized in multilayer film construction, such as acrylates or olefins. These rheological differences tend to cause layer control and uniformity issues, especially in the lateral or banner direction of the multilayer film construction. The present disclosure describes PEN polymers and articles made therefrom, which also provide a high level of birefringence for a multilayer film application, and high transmission % (low absorption), low haze, and This property is provided in the case of a combination of properties of high melt elasticity. Although the disclosure is not so limited, an understanding of the various aspects of the disclosure will be obtained by the examples provided herein.

The birefringent polyester film comprises: a naphthalate unit; an ethylene unit; a titanate compound; 0.1 to 2 mol% of a planar branching unit based on the total mol% of the naphthalate unit and the branching unit; 1 to 80 ppm of metal content. The birefringent polyester film has an out-of-plane birefringence of at least 0.1, or at least 0.15, or at least 0.2 at 633 nm.

The birefringent polyester film exhibits a low metal content. The birefringent polyester film can exhibit less than 100 ppm, or less than 80 ppm, or less than 30 ppm, or less than 10 ppm, or less than 5 ppm total metal content. The birefringent polyester film may exhibit a range of from 1 to 100 ppm, or from 1 to 80 ppm, or from 1 to 30 ppm, or from 1 to 10 ppm, or from 1 to 5 ppm. Total metal content. The birefringent polyester film can exhibit a total metal content of 4 ppm, or 3 ppm, or 2 ppm, or 1 ppm. Birefringent polyester films and especially polyethylene naphthalate benefit from low metal content. One or both of the light absorption, haze and both are reduced by reducing the amount of metal in the polyester film.

In many embodiments, the metal content comprises a titanate compound or titanium. In some embodiments, the metal content is predominantly, retentively, or only a titanate compound or titanium. The titanate compound can be used as a polymerization catalyst for producing a polyester. Any useful titanate compound can be utilized. The titanate compound can be a titanium alkoxide. Examples of the titanium compound or titanium alkoxide include: tetra-n-propyl titanate, tetra-isopropyl titanate, tetra-n-butyl titanate, tetra-n-butyl titanate tetramer, titanate tetra- Tertiary butyl ester, tetracyclohexyl titanate, tetraphenyl titanate, and tetrabenzyl titanate. One useful titanate compound is tetra-n-butyl titanate (TBT).

The polyester polymeric material can be described herein with reference to the total composition, i.e., 100 mol% units are derived from 50 mol% carboxylic acid ester or acid units and 50 mol% diol or glycol units. The polyester polymeric material may also be described herein with reference to the mol% of the carboxylic acid ester or acid subunit and the mol% of the diol or diol subunit (ie, in the preparation of the copolyester, 100 mol% of the carboxylic acid) The ester or acid subunit reacts with 100 mol% of the diol or glycol subunit).

The polyester is typically produced by the reaction of naphthalene dicarboxylic acid (or ester) with ethylene glycol and at least one additional comonomer that contributes to the planar branching unit. In some embodiments, the polyester is typically produced by the reaction of naphthalenedicarboxylic acid (diol) with ethylene glycol and at least one additional comonomer that contributes to a branched or cyclic C4-C10 alkyl unit. In still other embodiments, the polyester is typically a comonomer of naphthalene dicarboxylic acid (or ester) with ethylene glycol and at least one contributing planar branching unit and a contributing branched or cyclic C4-C10 alkyl unit. The reaction of the comonomer is made.

Suitable naphthalene carboxylate monomer molecules for forming the naphthalate subunit of the polyester include naphthalene carboxylate monomers having two or more carboxylic acid or ester functional groups. The naphthalene carboxylate monomer may include a naphthalene dicarboxylic acid such as a 2,6-naphthalene dicarboxylic acid monomer and an isomer thereof. The naphthalene carboxylate monomer may include a naphthalene dicarboxylate such as a 2,6-naphthalene dicarboxylate monomer and isomers thereof.

The 2,6-naphthalenedicarboxylic acid (or ester) monomer and/or its isomer is employed at a concentration such that 95 to 100 mol% of the carboxylate subunit contains a naphthalate subunit. Preferably, at least 96, 97, 98, 99, 99.25, 99.5, or 99.75 mol% of the carboxylate subunit comprises a naphthalate subunit.

The birefringent polyester layer or film can include planar branching units. The planar branching unit increases the molecular weight of the polyester material while maintaining the orientation or birefringence of the birefringent polyester layer or film. The planar branching unit may be present in the birefringent polyester layer or film in an amount ranging from 0.1 to 2 mol% based on the total mol% of the naphthalate unit and the branching unit forming the polyester material. In many embodiments, the planar branching unit can be present in the birefringent polymerization in an amount ranging from 0.1 to 1 mol% based on the total mol% of the naphthalate functional unit and the branched functional unit forming the polyester material. In the ester layer or film. In some embodiments, the planar branching unit may be present in the birefringent polyester in an amount ranging from 0.2 to 0.8 mol% based on the total mol% of the naphthalate unit and the branching unit forming the polyester material. In the layer or film. The planar branching unit can be derived, for example, from trimesic acid or trimellitic acid.

Suitable diol monomer molecules for forming the ethylene subunit of the polyester include ethylene glycol. In some embodiments, the ethylene glycol is employed at a concentration such that from 95 to 100 mole percent of the diol secondary units comprise ethylene minor units. Preferably, at least 97.5, 98, 98.5, 99, or 99.5 mol% of the diol subunits comprise ethylene subunits.

The birefringent polyester layer or film may comprise a branched or cyclic C4-C10 alkyl unit derived from a branched or cyclic C4-C10 alkyl diol such as neopentyl glycol, cyclohexane dimethanol, And mixtures thereof. The branched or cyclic C4-C10 alkyl unit may be less than 2 mol% or less, or less than 1.5, based on the total mol% of the ethylene unit and the branched or cyclic C4-C10 alkyl unit used to form the polyester material. An amount of mol%, or less than 1 mol%, is present in the birefringent polyester layer or film.

The birefringent polyester layer or film can exhibit a reduced tan δ value. For example, a conventional PEN layer or film can exhibit a tan delta value in the range of 18 to 25 as measured, for example, at 280 degrees Celsius and at a shear rate of 10 sec ^-1 . The tan δ value can be reduced using the planar branching unit described herein, as measured, for example, at 280 degrees Celsius and at a shear rate of 10 sec ^-1 , which is in the range of 3 to 8.

The multilayer film may include: a first birefringent polyester layer having an out-of-plane birefringence at 633 nm of at least 0.10, or at least 0.15, or at least 0.2; and a second polymer layer having a lower than the first The birefringence of the birefringent polyester layer is disposed on the first birefringent polyester layer. The polyester layer is as described above.

The second polymer layer can be isotropic or have a birefringence of less than 0.05 or less than 0.01. The term "isotropic" means that the refractive indices (measured at 633 nm) in the orthogonal x, y, and z directions are within 0.02 of each other, or within 0.01 of each other, or substantially. Equal on.

In many embodiments, the second polymer layer comprises an acrylate unit. In some embodiments, the second polymer layer comprises olefin units. The first birefringent polyester layer and the second polymer layer may each have a shear rate of 10 sec ^-1 at 280 ° C, within 3 of each other, or within 2, or within 1 or in Tan δ values within 30% of each other, or within 25%, or within 20%, or within 10%.

An exemplary second polymer based polyacrylate, such as polymethyl methacrylate (PMMA) for coextrusion (available under the trade designation VO44 Plexiglas from Arkema, Paris, FR) having a temperature of 280 degrees Celsius At a shear rate of 10 sec ^-1 , a tan δ value of about 7.9. Another exemplary second polymer is co-extruded co-PMMA (available from Arkema, Paris, FR under the trade name Atoglas 510A) having a shear rate of 10 sec ^-1 at 280 degrees Celsius. A tan δ value of about 5.0. An exemplary second polymer based polyolefin, such as polypropylene (PP) for coextrusion (available under the trade designation SR549 PP from Lyondell-Bassell, Houston, TX), which has a temperature of 280 degrees Celsius at 10 sec ^- The shear rate of ¹ is a tan δ value of about 2.9. Thus, the first birefringent polyester layer can have a shear rate of 10 sec ^-1 at 280 degrees Celsius, in the range of 2 to 9, or in the range of 3 to 8, or in the range of 4 to 7. The tan δ value inside.

The multilayer film can have a low haze value. The multilayer film may have a haze value of 2% or less, or 1.5% or less, or 1% or less, or 0.5% or less. The multilayer film can have high visible light transmission. The multilayer film may have a high visible light transmission of 85% or more, or 90% or more.

The multilayer optical film includes: a plurality of first birefringent polyester layers each having an out-of-plane birefringence of at least 0.10, or at least 0.15, or at least 0.2 at 633 nm, and less than 100 ppm, or less than 80 ppm, or less than 30 ppm, Or a metal content of less than 10 ppm, or less than 5 ppm, and having a first tan δ value at a shear rate of 10 sec ^-1 at 280 ° C; and a plurality of second isotropic layers (which may include acrylate) And alternating between the first birefringent polyester layers, and each of the second isotropic layers has a second tan δ value at a shear rate of 10 sec ^-1 at 280 degrees Celsius. The first tan δ value is within 30% or less of the second tan δ value, or within 25% or less, or within 20% or less.

The second polymer layer can be isotropic or have a birefringence of less than 0.05. In many embodiments, the second polymer layer comprises an acrylate unit. The first birefringent polyester layer and the second isotropic polymer layer may each have a shear rate of 10 sec ^-1 at 280 ° C, within 3, or within 2, or within 1 of each other. Or tan δ values within 30% of each other, or within 25%, or within 20%, or within 10%. PMMA has a tan δ value of about 7.9 at a shear rate of 10 sec ^-1 at 280 ° C, and a tan δ value of about 5.0 at a shear rate of 10 sec ^-1 at 280 ° C, PP has a tan δ value of about 2.9 at a shear rate of 10 sec ^-1 at 280 °C. Thus, the first birefringent polyester layer can have a shear rate of 10 sec ^-1 at 280 degrees Celsius, in the range of 2 to 9, or in the range of 3 to 8, or in the range of 4 to 7. The tan δ value inside.

The reflective and transmissive properties of the multilayer optical film vary with the refractive index of the individual layers and the thickness and thickness distribution of the layers. The layers were characterized at least in localized positions in the film by in-plane of may (in-plane) refractive index n _x, the thickness of the axis n _y, and associated with the membrane of the refractive index n _z to be expressed. These rates represent the refractive indices of the target material for light along mutually orthogonal x-axis, y-axis, and z-axis, respectively.

In practical applications, the refractive index can be controlled by careful selection of materials and process conditions. The multilayer film is produced by co-extruding a plurality of alternating polymers A, B, for example tens or hundreds of layers, optionally followed by passing the multilayer extrudate through one or more layer multiplication devices (layer multiplication device) And casting the mold, thus casting into a web, and then stretching or otherwise orienting the extrudate to form the final film. The resulting film is typically composed of hundreds of individual layers or microlayers, the thickness and refractive index of which are tailored to provide one or more reflection bands in the desired region of the spectrum, such as in visible or near infrared.

The multilayer optical film can include at least 100 layers, or at least 300 layers, or at least 400 layers, or at least 500 layers. The multilayer optical film can include several layers in the range of 400 to 800, or 500 to 700, or 550 to 650. In some embodiments, the multilayer optical film can include at least 700 layers, or at least 800 layers, or at least 900 layers, or at least 1000 layers. The multilayer optical film can include several layers in the range of 800 to 1500, or 900 to 1200, or 1000 to 1150. The layers may alternate between the first birefringent polyester layers and the second isotropic layers.

Multilayer optical films can have low haze values. The multilayer optical film may have a haze value of 2% or less, or 1.5% or less, or 1% or less, or 0.5% or less. The multilayer optical film can form a mirror film. The mirror film can reflect at least 99.0%, or at least 99.1%, or at least 99.2%, or at least 99.3%, or at least 99.4%, or at least 99.5%, or at least 99.6% of incident light.

This reflective property can be described as "hemispheric reflectivity" (R _hemi (λ)), which means that light (in certain wavelengths or wavelength ranges of interest) is incident on a component from all possible directions (regardless of The total reflectivity of the component when the component is a collection of surfaces, films, or films. Thus, the assembly is exposed to light incident from all directions (and all polarization states, unless otherwise specified) in the hemisphere centered in a normal direction, and collects all light reflected into the same hemisphere. The ratio of the total flux of reflected light in the wavelength range of interest to the total flux of incident light is the hemispherical reflectivity or R _hemi (λ).

The first birefringent polyester layer may have an out-of-plane birefringence of at least 0.20 at 633 nm, and the second isotropic (eg, acrylate) layer may have an out-of-plane refractive index of the first birefringent polyester layer. Out-of-plane refractive index (measured at 633 nm) within 0.05, or within 0.04, or within 0.03, or within 0.02, or within 0.01.

The object and advantages of the present disclosure are further illustrated by the following examples, which are not to be construed as limiting the specific materials and the amounts thereof, and other conditions and details.

Instance

All parts, percentages, ratios and the like in the examples are by weight unless otherwise indicated. The solvents and other reagents used were obtained from Sigma-Aldrich Corp., St. Louis, Missouri, unless otherwise stated.

 Material Abbreviation - General Chemical Description - Supplier (Location)  

̇NDC-2,6-naphthalenedicarboxylate-BP Amoco (Naperville, Ill.)

̇Trimesic- trimesic acid-Alfa Aesar (Ward Hill, MA)

̇Trimellitic-trimellitic acid-Lancaster Synthesis, Inc. (Pelham, NH)

̇EG-ethylene glycol-ME Global(Midland,Mich.)

̇CHDM-cyclohexanedimethanol-Eastman (Kingsport, Tenn.)

̇TEPA-triethylphosphonium thioacetate-Rhodia (Cranbury, N.J.)

̇Na Acetate-Sodium Acetate-Alfa Aesar (Ward Hill, Mass.)

̇Co Acetate-Cobalt-Shepherd Chemical (Cincinnati, Ohio)

̇Mn Acetate-Manganese Acetate-Shepard Chemical (Middletown, OH)

̇Sb Acetate-Arkema-Arkema (Philadelphia, Pa.)

̇Ca Acetate-calcium acetate-Mallinckrodt Chemical Inc. (St.Louis, MO)

All of the polymers in the examples below were synthesized according to the following procedure: A room temperature 10 gallon stainless steel oil jacketed batch reactor was charged with the amounts of monomer and catalyst indicated in Table 1 below. The contents were pressurized to 239.2 kPa with nitrogen and heated to 257 °C. During the transesterification, the resulting methanol and water by-products are removed via a separation column. After the methanol and water were removed, the batch pressure was reduced to atmospheric pressure. In some cases, the stabilizer TEPA is now added to the batch. The batch was then heated to 280 ° C and the pressure was reduced (via vacuum) to below 500 Pa. The condensation reaction by-product ethylene glycol is continuously removed through the separation column until the resulting polymer resin achieves the desired viscosity build, which is in combination with a polyphthalene having an intrinsic viscosity of about 0.50 dL/g. The ethylene formate resin is associated as measured in 60/40 wt% phenol/o-dichlorobenzene at 30 °C.

Table 2 lists the resulting compositions of the polymers in these examples. The single system is represented on a molar basis in which the ester and acid functional monomers are grouped together to a total of 100%, and the diol functional monomers are grouped together to a total of 100%. Catalysts and stabilizers are listed on the basis of PPM.

The polymerization kinetics, thermal stability, viscoelasticity, and quench morphology of the resulting resin were evaluated and compared in Table 3 .

Resin kinetics was compared by the amount of time (in minutes) it took for the resin to achieve the desired viscosity build associated with the 0.48 IV target.

The thermal stability of the resin samples was determined by placing ~5 grams of the dried resin sample into a nitrogen-purged plated Ares rheometer (available from TA Instruments, New Castle, Del.) at 280 degrees Celsius. It measures the complex viscosity of the resin at the beginning of the test and compares this result to the complex viscosity after an additional 30 minutes in the rheometer at 280 degrees Celsius. The results are reported on a relative basis (100% = initial value). All measurements were made at a shear rate of 100 sec ^-1 .

The calculated tan δ values of these resins were used to compare the viscoelasticity of the resin, which was also generated by a plated Ares rheometer that was purged with nitrogen at a shear rate of 10 sec ^-1 of 280 °C. The tan δ value is calculated as the ratio of the viscous modulus (G" to the modulus of elasticity (G'), and is a usable quantifier (lower value = more elastic) of the presence and extent of elasticity in the fluid.

The quenched morphology was measured by a differential scanning calorimetry (DSC) test method. The material was tested using DSC (Q2000, available from TA Instruments, New Castle, Del.). For each resin, about 5 to 10 mg of the sample was used. In particular, the resin was heated (melted) to 300 degrees Celsius and then cooled at a controlled rate (at 5 degrees C/min or 10 degrees C/min). The quenched morphology can be characterized by measuring the energy deviation associated with the resulting crystallization peak during this cooling cycle (larger energy values are associated with greater crystallization).

Table 4 below provides tan δ values for exemplary polymeric resins (acrylates or olefins) that can be coextruded with the polyester resins described herein. Exemplary acrylates are commercially available under the trade designation VO44 Plexiglas PMMA from PMMA of Arkema (Paris, FR). Another exemplary acrylate is commercially available from Arkema (Paris, FR) as co-PMMA under the trade name Atoglas 510A co-PMMA. An exemplary olefinic PP is available from SR549 PP of Lyondell-Bassell (Houston, TX).

Accordingly, the polyester resins described herein can have a tan delta value in the range of from about 3 to about 8 to provide a similar melt or viscoelasticity in the form of a coextruded resin to provide a cross-extruded film. Uniform physical and optical properties in the machine direction and in the transverse direction.

The exemplary polyester was then coextruded through a twin screw extruder, neck tube, feed block, and mold with a progressive temperature profile that peaked at 282 °C. The extruded material was cast and quenched into a 20 mil film. The transmission properties and haze % of the cast strip film were evaluated.

These 20 mil cast strip films were oriented and annealed in a KARO IV laboratory stretcher (Brueckner Maschinenbau GmbH & Co. KG, Siegsdorf, Germany). Simultaneous biaxial stretching in the machine direction and transverse direction to 350% x 350% of the original length was achieved using a 140 degree Celsius oven with a 45 second warm-up time. These oriented films were subjected to an additional 10 seconds of annealing at 240 degrees Celsius. The haze and birefringence of the oriented film were evaluated.

Refractive Index (RI) Measurement: The refractive indices of the various samples were measured in the MD, TD, and TM directions using a Metricon(R) coupler (Metricon Corporation, Pennington, N.J.). The MD and TD are in the in-plane direction, and the TM system is normal to the film surface. The refractive indices of MD, TD, and TM are labeled as: nx, ny, and nz, respectively.

_Inner plane birefringence △ n: To measure the birefringence properties of the uniaxially stretched film by the use of in-plane birefringence. The in-plane birefringence relates to the difference in the ratio (nx and ny) in the direction in the orthogonal plane. More specifically, for a uniaxially stretched film, the in-plane birefringence refers to the difference between the stretching direction and the non-stretching direction. For example, suppose that a film based on uniaxially stretched in the MD direction, the birefringence in a plane expression system as follows: _the △ n = n _x -n _y wherein, based in NX stretching direction (in this case, based MD The refractive index on the ny is the non-stretching direction (in this case, TD).

For biaxially stretched films, the in-plane birefringence is relatively small and sometimes close to zero (if balanced). In contrast, the out-of-plane birefringence is more indicative of the birefringence characteristics of the stretched film.

_Outer-plane birefringence △ n: To measure the birefringence properties of the film after biaxially oriented, the use of plane birefringence. The out-of-plane birefringence relates to the difference between the average of the in-plane ratios (MD and TD) and the ratio of the normal to the film (TM). The out-of-plane birefringence can be expressed as follows: Δn _outer = ((n _x + n _y )/2) - n _z where nx is the refractive index (RI) in the machine direction (MD), and ny is in The refractive index (RI) in the transverse direction (TD), and nz is the refractive index (RI) in the thickness direction (TM).

The haze measurement was carried out using a BYK Haze-gard haze meter from BYK-Gardner GmbH, Geretsried, Germany.

The transmission system was measured at 600 nm using a Lambda 950 spectrophotometer (from Perkin-Elmer, Akron, OH).

Examples A-5 to A-8

All of the polymers of Examples A-5 through A-8 below were synthesized according to the following procedure: A 2 gallon stainless steel electrically heated and insulated batch reactor was charged with the amounts of monomer and catalyst indicated in Table 6 . The contents were pressurized to 239.2kPa using N ₂ and heated to 257 ℃. During the transesterification, the resulting methanol and water by-products are removed via a separation column. After the methanol and water were removed, the batch pressure was reduced to atmospheric pressure. The batch was then heated to 280 ° C and the pressure was reduced (via vacuum) to below 500 Pa. For Examples A-5, A-7, and A-8, the condensation reaction by-product ethylene glycol was continuously removed through the separation column until the resulting polymer resin achieved the desired viscosity build, which Polyethylene naphthalate having an intrinsic viscosity of 0.50 dL/g is associated as measured in 60/40 wt% phenol/o-dichlorobenzene at 30 °C. For batch A-6, the condensation reaction could not proceed to a viscosity build associated with the ~0.50 dL/g end point, with the folded viscosity as the apex, resulting in a polymer with reduced physical properties.

Table 7 lists the resulting compositions of the polymers in these examples. The single system is represented on a molar basis in which the ester and acid functional monomers are grouped together to a total of 100%, and the diol functional monomers are grouped together to a total of 100%. Catalysts and stabilizers are listed on a PPM (parts per million basis) basis.

The resin kinetics were also compared in Table 7 by the amount of time (in minutes) that the resin took to achieve the desired viscosity build associated with the 0.48 IV target. Since instance A-6 does not reach the endpoint, the result is na (not applicable). The complex viscosity as measured by the Ares plate viscometer at 280 ° C at 100 sec ^-1 is included for reference.

Therefore, an embodiment of a birefringent polyester film having a low haze is disclosed.

All references cited herein, as well as the entire contents of the publications, are hereby expressly incorporated by reference inso- While the invention has been described and described with respect to the specific embodiments the embodiments of the invention The scope of the disclosure. The application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, the disclosure is intended to be limited only by the scope of the claims and their equivalents. The embodiments disclosed herein are for illustrative purposes only and are not limiting.

Claims (20)

 A birefringent polyester film comprising: a naphthalate unit; an ethylene unit; a titanate compound; 0.1 to 2 mol% of a planar branching unit based on the total mol of the naphthalate unit and the branching unit %; 1 to 80 ppm of metal content; and at 633 nm, at least 0.1 out-of-plane birefringence.    The membrane of claim 1, further comprising a branched or cyclic C4-C10 alkyl unit, present in an amount of less than 2 mol% based on the total mol% of the ethylene unit and the branched or cyclic C4-C10 alkyl unit .    The film of claim 1, wherein the metal content is in the range of from 3 to 30 ppm.    The film of claim 1, wherein the titanate compound comprises tetrabutyl titanate.    The membrane of claim 1, wherein the planar branching unit is derived from trimesic acid or trimellitic acid.   The film of claim 1, wherein the film has a tan δ value in the range of 3 to 8 at a shear rate of 280 degrees Celsius and 10 sec ^-1 .  A multilayer film comprising: a first birefringent polyester layer having an out-of-plane birefringence of at least 0.10 at 633 nm, the polyester layer comprising: naphthalate units; ethylene units; titanate compounds 0.1 to 2 mol% of a planar branching unit based on the total mol% of the naphthalate unit and the branching unit; 1 to 80 ppm of the metal content; and a second polymer layer having a lower than the first double The birefringence of the refracting polyester layer is disposed on the first birefringent polyester layer.    The multilayer film of claim 7, wherein the second polymer layer comprises an acrylate unit.    The multilayer film of claim 7, wherein the second polymer layer is isotropic.    The multilayer film of claim 7, wherein the first birefringent polyester layer has an out-of-plane birefringence of at least 0.20 at 633 nm.    The multilayer film of claim 7, wherein the first birefringent polyester layer further comprises a branched or cyclic C4-C10 alkyl unit, based on the total mol% of the ethylene unit and the branched or cyclic C4-C10 alkyl unit. It is present in an amount of less than 2 mol%.    The multilayer film of claim 7, wherein the metal content of the first birefringent polyester layer is in the range of from 3 to 30 ppm.    The multilayer film of claim 7, wherein the titanate compound comprises tetrabutyl titanate.    The multilayer film of claim 7, wherein the planar branching unit is derived from trimesic acid or trimellitic acid.   The multilayer film of claim 7, wherein the first birefringent polyester layer and the second polymer layer each have a tan δ value within 3 of each other at a shear rate of 280 degrees Celsius and 10 sec ^-1 . A multilayer optical film comprising: a plurality of first birefringent polyester layers each having an out-of-plane birefringence of at least 0.10 at 633 nm, and a metal content of less than 100 ppm, and having a temperature of 280 degrees Celsius and 10 sec ^- a first tan δ value at a shear rate of ¹ ; and a plurality of second isotropic acrylate layers alternating between the first birefringent polyester layers, and each of the second isotropic acrylate layers has a second tan δ value at a shear rate of 280 degrees Celsius and 10 sec ^-1 ; and the first tan δ value is within 30% or less of the second tan δ value.  The multilayer optical film of claim 16, wherein the multilayer optical film reflects at least 99.0% of incident light.    The multilayer optical film of claim 16, wherein the first tan δ value is within 2 units or less of the second tan δ value.    The multilayer optical film of claim 16, wherein the metal content of the first birefringent polyester layer is in the range of from 3 to 30 ppm.    The multilayer optical film of claim 16, wherein the first birefringent polyester layers have an out-of-plane birefringence of at least 0.20 at 633 nm, and the second isotropic acrylate layers have the first The out-of-plane refractive index within 0.05 of the out-of-plane refractive index of the birefringent polyester layer.