WO2016044078A1

WO2016044078A1 - Multilayer oled cover sheet

Info

Publication number: WO2016044078A1
Application number: PCT/US2015/049579
Authority: WO
Inventors: Richard Y. Liu
Original assignee: 3M Innovative Properties Company
Priority date: 2014-09-15
Filing date: 2015-09-11
Publication date: 2016-03-24

Abstract

A substantially transparent OLED cover sheet having first and second protective layers and a low modulus layer disposed between the first and second protective layers. The protective layers include polyesters having at least about 70 mole percent naphthalenedicarboxylate groups based on total carboxylate groups. The low modulus layer has a shear storage modulus less than about 10 MPa at 25 °C and 1 Hz.

Description

MULTILAYER OLED COVER SHEET

Background Polyethylene terephthalate (PET) has been used as a flexible substrate for bendable organic light emitting diode (OLED) displays. OLED displays are highly susceptible to damage from moisture. To protect from moisture damage, the substrate should be resistant to mechanical damage since any puncture in the substrate would allow moisture into the OLED device. To provide a sufficient toughness with a single PET layer, the PET layer needs to be thick and this can make the display undesirably stiff.

Accordingly, an improved solution to the problem of protecting bendable OLED displays is needed.

Summary

In some aspects of the present description, an OLED cover sheet is provided that includes a first protective layer, a second protective layer adjacent the first protective layer, and a first low modulus layer disposed between the first and second protective layers. The first protective layer includes a first polyester that includes at least about 70 mole percent naphthalenedicarboxylate groups based on total carboxylate groups, and the second protective layer includes a second polyester that includes at least about 70 mole percent naphthalenedicarboxylate groups based on total carboxylate groups. The first low modulus layer has a shear storage modulus less than about 10 MPa at 25 °C and 1 Hz. The OLED cover sheet is substantially transparent.

Brief Description of the Drawings

FIG. 1 is a cross-sectional view of a multilayer OLED cover sheet;

FIG. 2 is a cross-sectional view of a multilayer OLED cover sheet; and

FIG. 3 is a cross-sectional view of an OLED device. Detailed Description

In the following description, reference is made to the accompanying drawings that forms a part hereof and in which 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 disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. As used herein, layers, components, or elements may be described as being adjacent one another. Layers, components, or elements can be adjacent one another by being in direct contact, by being connected through one or more other components, or by being held next to one another or attached to one another. Layers, components, or elements that are in direct contact may be described as being immediately adjacent.

Flexible OLED devices can be susceptible to damage from routine use and a cover sheet may be applied to a flexible OLED display to protect the display. OLED devices are sensitive to attack from moisture and any physical punctures, crack or tears in the cover sheet can lead to moisture damage to the OLED device. It is desired that the cover sheet provide a useful level of protection for an OLED device, while having a sufficiently low bending stiffness that it does not significantly degrade the flexibility of a bendable OLED device. The desired level of protection may include protection against punctures or scratches or the like, and may include providing a water and/or oxygen barrier which can protect the OLED display from moisture, water, and/or oxidation damage. Conventional protective films cannot simultaneously provide the desired high puncture resistance, the desired low bending stiffness, and the desired low thickness. This is because increasing the puncture resistance in a conventional construction requires the modulus and/or the thickness of the protective film to be increased resulting in an increase in bending stiffness.

According to the present description, it has been discovered that a cover sheet having two or more protective layers made from certain types of polyesters attached together through a low modulus layer or through low modulus layers can provide a desired degree of protection of an OLED display while simultaneously maintaining high flexibility and a thin profile. The polyesters used in the protective layers may have an elastic storage modulus higher than that of PET and at the same time the multilayer OLED cover sheet may have a bending stiffness much lower than that of a monolithic PET layer having the same overall thickness as the cover layer. As used herein, elastic storage modulus refers to the real part of the complex elastic modulus in tension (commonly denoted E') determined using dynamic mechanical analysis (DMA) at 25 °C and at a frequency of 1 Hz, unless specified differently. A suitable DMA unit is the MK II unit commercially available from Polymer Laboratories, Amherst, MA. As used herein, shear storage modulus refers to the real part of the complex shear modulus (commonly denoted G') determined at 25 °C and at a frequency of 1 Hz, unless specified differently. The complex shear modulus can be determined using a parallel plate rheometer in shear mode, for example. As used herein, bending stiffness refers to the bending force per unit of vertical moving distance (N/mm) determined using the short beam shear (SBS) test described in the Examples and conducted at 25 °C, unless specified differently. In some cases the cover layer may have a bending stiffness less than 50 percent, or less than 35 percent, or less than 25 percent of the bending stiffness of a monolithic PET layer having the same overall thickness.

FIG. 1 is a schematic cross-sectional view of OLED cover sheet 100 that includes first protective layer 1 10, second protective layer 1 12 adjacent first protective layer 1 10, a low modulus layer 120 disposed between the first and second protective layers 1 10 and 1 12, and a hard-coat layer 130 adjacent second protective layer 112 opposite first protective layer 1 10. In the illustrated embodiment, low modulus layer 120 is immediately adjacent first and second protective layers 1 10 and 1 12, and hard-coat layer 130 is immediately adjacent second protective layer 1 12. In other embodiments, one or more additional layers may be included.

FIG. 2 is a schematic cross-sectional view of OLED cover sheet 200 that includes first protective layer 210, second protective layer 212 adjacent first protective layer 210, third protective layer 214 adjacent second protective layer 212 opposite first protective layer 210, a first low modulus layer 220 disposed between the first and second protective layers 210 and 212, a second low modulus layer 222 disposed between the second and third protective layers 212 and 214, and a hard-coat layer 230 adjacent third protective layer 214 opposite second protective layer 212.

Any of the protective layers (e.g., protective layers 1 10, 1 12, 210, 212, or 214) of the OLED cover sheets of the present description may include a polyester, which may be a copolyester. The polyester used in the protective layers may include carboxylate groups and diol groups and may include at least about 70 mole percent, or at least about 80 mole percent, or at least about 85 mole percent, or at least about 90 mole percent naphthalenedicarboxylate (NDC) groups based on total carboxylate groups.

Suitable polyesters include polyethylene naphthalate (PEN). Other suitable polyesters include

copolyesters of polyethylene naphthalate (PEN), such as CoPEN9010, a copolyethylene

naphthalate/terephthalate copolymer which includes about 90 mole percent NDC groups based on total carboxylate groups and which is further described in the Examples. In some embodiments, all protective layers in the OLED cover sheet include the same or substantially the same polyester. In other

embodiments, different polyesters are used in different protective layers.

Copolyethylene naphthalate/terephthalate copolymers which include from about 70 mole percent to about 100 mole percent NDC groups based on total carboxylate groups (and which include up to about 30 mole percent terephthalate groups based on total carboxylate groups) all share the property of being appropriately crystallizable in nature to be effectively orientable during stretching at typical stretching conditions during film manufacture. Such copolymers may therefore be useful in the manufacture of protective layers of the OLED cover sheets of the present description. Suitable copolyethylene naphthalate/terephthalate copolymers can be prepared as described in "Preparation of CoPEN9010" in the Examples by adjusting the relative amounts of dimethyl naphthalene dicarboxylate and dimethyl terephthalate to give the desired mole percent of NDC groups based on total carboxylate groups.

The OLED cover sheets can be prepared using known coextrusion techniques. For example, multiple polymeric flow streams can be combined in a die or feedblock in a layered fashion to provide a multilayer film. Alternatively, the OLED cover sheets can be prepared by lamination of pre-formed component layers or by a combination of coating and lamination. For example, OLED cover sheet 100 can be prepared by coating a resin, which when cured becomes a low modulus layer, onto a major surface of first protective layer 1 10, then applying second protective layer 1 12 onto the resin, and curing the resin, either before or after the application of second protective layer 112 onto the resin, for example, by applying actinic radiation (e.g., UV-radiation or electron beam radiation).

Any of the low modulus layers (e.g., low modulus layer 120, 220, or 222) of the OLED cover sheets of the present description may include an optically clear adhesive and/or may include a

(meth)acrylic polymer, a polyolefin polymer, or a polyolefin hybrid polymer in which olefin blocks are combined with other chemical modifying elements through either physical mixing or chemical bonding. The modifying elements may include, but are not limited to, (meth)acrylics, styrenics, acrylonitriles, ionomers, amides, imides, and mixtures thereof. As used herein, (meth)acrylic polymer refers to an acrylic polymer and/or a methacrylic polymer.

As used herein, a low modulus material or layer refers to a material or layer having a shear storage modulus at 25 °C and 1 Hz of no more than about 10 MPa. The low modulus layer or layers may have a shear storage modulus of less than about 10 MPa, or less than about 3 MPa, or less than about 1 MPa, or less than about 0.5 MPa, or less than about 0.2 MPa, all at 25 °C and 1 Hz. The low modulus layer or layers may have a shear storage modulus of greater than about 0.001 MPa at 25 °C and 1 Hz, or greater than about 0.01 MPa at 25 °C and 1 Hz. For example, the low modulus layer may have a shear storage modulus in the range of about 0.01 MPa to about 10 MPa at 25 °C and 1 Hz. Suitable low modulus layers include optically clear adhesives such as DELO-PHOTOBOND and DELO-DUALBOND adhesives (available from DELO Industrial Adhesives, Sudbury, MA) which have a reported shear modulus in the range of about 0.04 MPa to about 1 MPa.

Any or all of the protective layers may be uniaxially or biaxially stretched film, and stretching steps may be performed with or without orthogonal restraint, and biaxial stretching may be done equally or unequally in the two stretching directions, and may be done simultaneously or sequentially in the two stretching directions. Any or all of the protective layers may have a sum of an elastic storage modulus in a machine direction and an elastic storage modulus in a transverse direction in a range of about 8 GPa, or 9 GPa, or 10 GPa, or 1 1 GPa to about 15 GPa, or 16 GPa, or 18 GPa, all at 25 °C and 1 Hz. Any or all of the protective layers may have a sum of an elastic storage modulus in the machine direction and an elastic storage modulus in the transverse direction that is at least about 1,000 times larger, or at least about 5,000 times larger, or at least about 10,000 times larger than a shear storage modulus of a low modulus layer. Biaxially stretched PEN may have an elastic storage modulus at 25 °C and 1 Hz in the machine direction of about 7.3 GPa and an elastic storage modulus at 25 °C and 1 Hz in the transverse direction of about 6.0 GPa. In this case, the sum of the elastic storage modulus in the machine direction and the elastic storage modulus in the transverse direction is about 13.3 GPa. For comparison, under the same conditions, PET may have an elastic storage modulus in the machine direction of about 4.7 GPa and an elastic storage modulus in the transverse direction of about 5.2 GPa. In this case, the sum of the elastic storage modulus in a machine direction and the elastic storage modulus in a transverse direction is about 9.9 GPa.

The low modulus layer may have a glass transition temperature (Tg) less than about 60 °C, or less than about 50 °C, or less than about 25 °C. In some embodiments, the low modulus layer has a Tg greater than about -50 °C, or greater than about -25°C. In some embodiments, one or more protective layer(s) has a Tg greater than about 80 °C, or greater than about 90 °C, or greater than about 100 °C, and less than about 140 °C, or less than about 130 °C, or less than about 120 °C. For example, each of the protective layers may have a Tg in a range of about 100 °C to about 130 °C. Biaxially stretched PEN may have a Tg of about 125 °C, while biaxially stretched PET may have Tg in the range of 80 °C to 90 °C. Glass transition temperatures may be determined using standard dynamic mechanical analysis techniques as described in ASTM El 640 - 13.

Any of the OLED cover sheets described herein may include a hard-coat layer as an outer layer of the OLED cover sheet. The hard-coat layer can be applied as a coating and cured by applying actinic radiation, for example. The hard-coat layer may be an acrylic polymer or methacrylic polymer including a plurality of nanoparticles. The nanoparticles, which may be inorganic nanoparticles such as silica, zirconia, or titania, may be included in the hard-coat layer at a concentration of about 40 to about 95 weight percent. The nanoparticles may have a size distribution such that 90 weight percent of the nanoparticles have a diameter in the range from about 2 nm to about 400 nm. The (meth)acrylic polymer may include one or more of hexafluoropropylene oxide urethane acrylate, silicone polyether acrylate, 2- phenoxy ethyl methacrylate, and difunctional urethane acrylate. The hard-coat layer may have a pencil hardness higher than H or higher than 2H and may have a pencil hardness less than 1 OH. Suitable hard- coat layers are described in U.S. Pat. App. Pub. No. 2013/0302594 (Sugiyama et al.), for example.

Any of the OLED cover sheets of the present description may be substantially transparent so that light output from an OLED display is substantially unaffected by the cover sheet. An OLED cover sheet may be said to be substantially transparent if most (e.g., 90 percent or 95 percent) visible light (e.g., light having a wavelength between about 400 nm and about 700 nm) incident on the OLED cover sheet is transmitted through the OLED cover sheet or Fresnel reflected from a surface of the OLED cover sheet.

The OLED cover sheet may have a haze of less than 5 percent, or less than 3 percent, or less than 2 percent, or even less than 1 percent. Haze may be defined as specified in ASTM D 1003- 13 as the percent of transmitted light through a specimen that is scattered so that its direction deviates more than 2.5 degrees from the direction of the incident beam. A suitable device for measuring the haze of a cover sheet is the HAZE-GARD PLUS haze meter available from BYK-Gardner, Columbia, Md.

As described in the Examples, the OLED cover sheet may be characterized in terms of puncture resistance against a probe having a 200 micrometer diameter tip. In some embodiments, the OLED cover sheet has a puncture resistance of at least about 0.8 kgf, at least about 1.0 kgf, or at least about 1.2 kgf, or at least about 1.3 kgf against a probe with a tip having a 200 micrometer diameter. In some embodiments, an OLED cover sheet may have a puncture resistance of less than about 10 kgf or less than about 8 kgf against a probe with a tip having a 200 micrometer diameter. For example, in some embodiments, an OLED cover sheet may have a puncture resistance in a range of about 1.0 kgf to about 10 kgf against a probe having a 200 micrometer diameter tip. As described in the Examples, the OLED cover sheet may be characterized in terms of a short beam shear bending stiffness. In some embodiments, the OLED cover sheet has a short beam shear bending stiffness of less than about 15 N/mm, or less than about 12 N/mm, or less than about 10 N/mm, or less than about 8 N/mm, or less than about 7 N/mm, or less than about 6 N/mm. In some embodiments, an OLED cover sheet may have a short beam shear bending stiffness greater than 1 N/mm, or greater than 0.1 N/mm, or greater than 0.01 N/mm. For example, an OLED cover sheet may have a short beam shear bending stiffness in the range of about 1 N/mm to about 8 N/mm.

In some embodiments, the OLED cover sheet may have an overall thickness in the range of about 65 micrometers, or 100 micrometers, to about 350 micrometers, or about 400 micrometers. In some embodiments, each protective layer may have a thickness in a range of about 30 micrometers, or about 50 micrometers, to about 125 micrometers, or about 150 micrometers. In some embodiments, each low modulus layer may have a thickness in the range of about 15 micrometers, or about 25 micrometers, to about 75 micrometers, or about 100 micrometers.

In some aspects of the present description, an OLED device is provided that includes an OLED display and an OLED cover sheet attached thereto. The OLED cover sheet may be any of the OLED cover sheets described herein. Any type of OLED display can be used, including flexible or bendable displays.

FIG. 3 shows OLED device 301 including OLED cover sheet 300 attached to OLED display 340 having output surface 341. The OLED cover sheet 300 is attached to output surface 341 with optically clear adhesive 350. The OLED cover sheet 300 may be any of the OLED cover sheets described herein.

The first protective layer of the OLED cover sheet may be disposed facing the OLED display 340 and the opposite side of the OLED cover sheet may include a hard-coat layer. The OLED cover sheet 300 can be attached to any OLED display. In the illustrated embodiment, OLED display 340 includes substrate 342, transparent cathode 343, emissive layer 344, semi-conductive layer 345, and anode 346. The OLED device 301 may be bendable and the flexibility of OLED device 301 may be similar to the flexibility of OLED display 340.

Examples

All parts, percentages, ratios, etc. in the Examples are by weight, unless noted otherwise. Solvents and other reagents are available from Sigma-Aldrich Chemical Company (Milwaukee, WI), unless specified differently.

Puncture Test Method

Samples were tested for puncture using an INSTRON model 1122 (available from Instron, Norwood, MA). A steel probe having a tip diameter of about 200 micrometers was used to puncture specimens using a constant probe speed of 0.2 mm/second. The peak force required to puncture the test specimen was reported as the puncture resistance.

Short Beam Shear (SBS) Bending Stiffness Test

A three-point bending test was conducted on an INSTRON testing system using BLUEHILL software (available from Instron, Norwood, MA) and a three-point flexure fixture. The fixture included lower support anvils having diameters of 4 mm and an upper anvil with a 10 mm diameter. The span between the center points of the lower anvils was fixed at 8.76 mm, and the upper anvil was centered and aligned with the lower support anvils. A test specimen was placed on the lower support anvils and the upper anvil was lowered towards the test specimen and then pressed into the test specimen at a test rate of 0.5 mm/minute. A force curve was determined (force versus displacement). The slope of the linear portion of the force curve was used to calculate bending stiffness of the test specimen in N/mm.

Haze Measurements

Haze was determined using a HAZE-GARD PLUS haze meter available from BYK-Gardner, Columbia, Md.

Preparation of PEN

Polyethylene naphthalate (PEN) was synthesized in a batch reactor with the following raw material charge: dimethyl naphthalene dicarboxylate (136 kg), ethylene glycol (73 kg), manganese (II) acetate (27g), cobalt (II) acetate (27g) and antimony (III) acetate (48g). Under a pressure of 20 psig (138 kPa), this mixture was heated to 254 °C while removing methanol (a transesterification reaction byproduct). After 35 kg of methanol was removed, triethyl phosphonoacetate (49g) was charged to the reactor and the pressure was gradually reduced to 1 torr (131 N/m²) while heating to 290 °C. The condensation reaction by-product, ethylene glycol, was continuously removed until a polymer with an intrinsic viscosity of 0.48 dL/g (as measured in 60/40 wt. % phenol/o-dichlorobenzene at 23 °C) was produced. Preparation of CoPEN9010

Copolyethylene naphthalate/terephthalate copolymer (CoPEN9010) was synthesized in a batch reactor with the following raw material charge: 126 kg dimethyl naphthalene dicarboxylate, 1 1 kg dimethyl terephthalate, 75 kg ethylene glycol, 27 g manganese (II) acetate, 27 g cobalt (II) acetate, and 48 g antimony (III) triacetate. Under pressure of 20 psig (138 kPa), this mixture was heated to 254 °C while removing methanol. After 36 kg of methanol was removed, 49 g of triethyl phosphonoacetate was charged to the reactor and then the pressure was gradually reduced to 1 torr while heating to 290 °C. The condensation reaction by-product, ethylene glycol, was continuously removed until a polymer with an intrinsic viscosity of 0.50 dL/g, as measured in 60/40 wt. % phenol/o-dichlorobenzene at 23°C, was produced.

Comparative Example C-l

A single layer OLED cover sheet was made using PET film. The monolithic PET film was made by melt extruding pellets of PET resin using a twin screw extruder at a rate of 300 lbs/hr (136 kg/hr). The melt was cast on a chilled roll (also referred to as a "casting wheel") to form a cast web. The casting wheel speed was adjusted so the resulting cast web was 100 mils (2.54 mm) thick. The cast web was then sequentially stretched in a length orienter and a tenter orienter at a draw ratio of about 3.3-3.5 in each direction. Heat was applied in the draw gap of the length orienter so that the draw force during the stretching was in the range of 100- 1000 lbs (440-4400 N). Heat was applied in the tenter orienter so that the stretching temperature in the stretching zones was at about 95-100 °C. The stretching rate of was between about 50%/second and about 100%/second. The resulting film was then heat set at 230 °C in the heat set zone for about 5-30 seconds. The resulting film was about 10 mils (254 micrometers) thick and it had an elastic storage modulus of 4.7 GPa and 5.2 GPa in machine direction (MD) and transverse direction (TD), respectively, as determined using conventional DMA techniques. The PET film had a haze of 1%, an SBS bending stiffness of 25.3 N/mm, a puncture resistance against a 200 micrometer diameter tip probe of 1.49 kgf, and a tensile force of 30 kgf for 2% elastic deformation with 1 inch (2.54 cm) wide strip.

Example 1

A three layer OLED cover sheet was made via laminating two PEN sheets with a layer of optically clear adhesive using a roll laminator.

The PEN film was made by melt extruding pellets of PEN resin using a twin screw extruder at rate of 300 lbs/hr (136 kg/hr). The melt was cast on a chilled roll to form a cast web. The casting wheel speed was adjusted so the resulting cast web was about 40 mils (1 mm) thick. The cast web was then sequentially stretched in a length orienter and a tenter orienter at a draw ratio of about 3.3-3.5 in each direction. Heat was applied in the draw gap of the length orienter so that the draw force during the stretching was in the range of 100- 1000 lbs (440-4400 N). Heat was applied in the tenter orienter so that the stretching temperature in the stretching zones was at about 135-155 °C. The stretching rate was between about 50%/second and about 100%/second. The resulting film was then heat set at 230 °C for about 5-30 seconds. The resulting film was about 4 mil (102 micrometers) thick and it had elastic storage moduli of 7.3 GPa and 6.0 GPa in the machine direction (MD) and the transverse direction (TD), respectively, as determined at 25 °C and 1 Hz using a Dynamic Mechanical Analysis Tester. The PEN film had a haze of about 1%.

A low modulus acrylic optically clear adhesive, OCA8171, was obtained from 3M Company (St. Paul, MN) under the trade name "3M Optically Clear Adhesive 8171". The adhesive had a thickness of 2 mils (51 micrometers) and a haze of 0.6%. The PEN film was fed through a nip on a laminator with the OCA8171 to form a two layer laminate. The output was then laminated to another PEN film to form the three-layer OLED cover sheet.

The resulting three-layer OLED cover sheet was about 254 micrometers thick and had an SBS bending stiffness of 6.3N/mm, a haze of 1.8%, a puncture resistance against a probe having a 200 micrometer diameter tip of 1.34 kgf, and a tensile force of 34 kg for 2% elastic deformation with 1 inch (2.54 cm) wide strip. The three-layer cover sheet had a high puncture resistance comparable to that of monolithic PET of the same film thickness, but had a lower bending stiffness of 6.3 N/mm. Example 2

A three-layer OLED cover sheet was made via laminating two sheets of CoPEN9010. In between the two CoPEN9010 layers, a layer of optically clear adhesive, OCA8171, was deposited using a roll laminator.

The copolyester film was made by melt extruding pellets of CoPEN9010 using a twin screw extruder at rate of 300 lbs/hr (136 kg/hr). The melt was cast on a chilled roll to form a cast web. The casting wheel speed was adjusted so the resulting cast web was 40 mils (1 mm) thick. The cast web was then sequentially stretched in a length orienter and a tenter orienter at a draw ratio of about 3.3-3.5 in each direction. Heat was applied in the draw gap of the length orienter so that the draw force during the stretching is in the range of 100-1000 lbs (440-4400 N). Heat was applied in the tenter orienter so that the stretching temperature in the stretching zones was at about 125- 145 °C. The stretching rate was between about 50%/second and about 100%/second. The resulting film was then heat set at around 230 °C for about 5-30 seconds. The resulting film was about 4 mils (102 micrometers) thick and it had elastic storage moduli of 7.0 GPa and 6.1 GPa in the machine direction (MD) and the transverse direction (TD), respectively, as determined at 25 °C and 1 Hz using a DMA tester. The CoPEN9010 film had a haze of about 1%.

The CoPEN9010 film was fed through a nip on a laminator with the OCA8171 to form a two layer laminate. The output was then laminated to another CoPEN9010 film to form the three-layer OLED cover sheet. The resulting three-layer OLED cover sheet had excellent optical transparency and was very flexible and easy to bend.

Example 3

A three-layer OLED cover sheet was made via coextrusion using two twin-screw extruders and an ABA feedblock. The CoPEN9010 was fed by one twin screw extruder into the two A layers. In the other twin screw extruder, a mixture of KRATON 1730 (a block copolymer having a styrenic block and an ethylene/propylene block available from Kraton Performance Polymers, Inc., Houston, TX), PvEGALREZ 1094 available from Eastman Chemical Company, Kingsport, TN) and CLEARON PI 50 (a hydrogenated terpene resin available from Yasuhara Chemical Co., Ltd., Japan) were used to form layer B through weight loss feeders at a weight ratio of 50/35/15. The temperature for the extruder that forms A layers was set at 520 °F (271 °C). The temperature for the extruder that forms the B layer was set at 350 °F (177 °C). The die and feedblock were set at 450 °F (232 °C). The feed rate for the A layer extruder was 50 lbs/hr (23 kg/hr) and the feed rate for the B layer extruder was at 5 lbs/hr (2.3 kg/hr).

The melt stream of the A layer extruder was divided and combined with the melt stream of the B layer extruder in a three-layer feedblock to form the three-layer stack. The three-layer melt stack was then sent through the film die and was cast on a chilled drum to form a cast web. The casting speed was adjusted so that the cast web thickness was about 50 mils (1.3 mm). The resulting cast web was then stretched in a batch stretcher (KARO IV from Bruckner, Germany) to a stretch ratio of 3.5 x 3.5. The stretching temperature was 135 °C and stretching rate was about 50%/second. The film was then subsequently heat set at 230 °C in a dispatch oven for 30 seconds.

The resulting three-layer OLED cover sheet had a 5 mil (127 micrometer) thickness, excellent optical transparency, and was very flexible and easy to bend. The following is a list of exemplary embodiments of the present description.

Embodiment 1 is an OLED cover sheet comprising:

a first protective layer comprising a first polyester including at least about 70 mole percent

naphthalenedicarboxylate groups based on total carboxylate groups;

a second protective layer adjacent the first protective layer, the second protective layer comprising a second polyester including at least about 70 mole percent naphthalenedicarboxylate groups based on total carboxylate groups; and

a first low modulus layer disposed between the first and second protective layers, the first low

modulus layer having a shear storage modulus less than about 10 MPa at 25 °C and 1 Hz, wherein the OLED cover sheet is substantially transparent.

Embodiment 2 is the OLED cover sheet of embodiment 1 , wherein the first protective layer is immediately adjacent the first low modulus layer and the second protective layer is immediately adjacent the first low modulus layer opposite the first protective layer.

Embodiment 3 is the OLED cover sheet of embodiment 1 , wherein the first low modulus layer is an optically clear adhesive.

Embodiment 4 is the OLED cover sheet of embodiment 1 , wherein the first low modulus layer comprises a (meth)acrylic polymer. Embodiment 5 is the OLED cover sheet of embodiment 1 , wherein the first low modulus layer comprises an olefin block.

Embodiment 6 is the OLED cover sheet of embodiment 1 , further comprising a hard-coat layer adjacent the second protective layer opposite the first protective layer.

Embodiment 7 is the OLED cover sheet of embodiment 6, wherein the hard-coat layer comprises a (meth)acrylic polymer. Embodiment 8 is the OLED cover sheet of embodiment 1, further comprising:

a third protective layer adjacent the second protective layer opposite the first protective layer; and a second low modulus layer disposed between the second and third protective layers, the second low modulus layer having a shear storage modulus less than about 10 MPa at 25 °C and 1 Hz. Embodiment 9 is the OLED cover sheet of embodiment 1 , wherein each of the first and second polyesters include at least about 85 mole percent naphthalenedicarboxylate groups based on total carboxylate groups.

Embodiment 10 is the OLED cover sheet of embodiment 1 , wherein each of the first and second protective layers has a sum of an elastic storage modulus in a machine direction and an elastic storage modulus in a transverse direction in a range of about 9 GPa to about 16 GPa at 25 °C and 1 Hz.

Embodiment 1 1 is the OLED cover sheet of embodiment 1 , wherein each of the first and second protective layers have a Tg in a range of about 100 °C to about 130 °C.

Embodiment 12 is the OLED cover sheet of embodiment 1, wherein the first low modulus layer has a shear storage modulus less than about 1 MPa at 25 °C and 1 Hz.

Embodiment 13 is the OLED cover sheet of embodiment 1 , wherein each of the first and second protective layers has a sum of an elastic storage modulus in a machine direction and an elastic storage modulus in a transverse direction that is at least about 5,000 times larger than the shear storage modulus of the first low modulus layer.

Embodiment 14 is the OLED cover sheet of embodiment 1, wherein the OLED cover sheet has a haze of less than about 3 percent. Embodiment 15 is the OLED cover sheet of embodiment 1 , wherein the OLED cover sheet has a puncture resistance of at least about 1.0 kgf against a probe having a 200 micrometer diameter tip.

Embodiment 16 is the OLED cover sheet of embodiment 1 , wherein the OLED cover sheet has a short beam shear bending stiffness less than about 8 N/mm.

Embodiment 17 is an OLED device comprising:

an OLED display having an output surface; and

the OLED cover sheet of any of embodiments 1 to 16 attached to the output surface with an optically clear adhesive, the first protective layer of the OLED cover sheet facing the OLED display.

Embodiment 18 is the OLED device of embodiment 17, wherein the OLED device is bendable.

Embodiment 19 is the OLED device of embodiment 17, wherein the OLED cover sheet further comprises a hard-coat layer opposite the OLED display.

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 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. An OLED cover sheet comprising:

naphthalenedicarboxylate groups based on total carboxylate groups;

2. The OLED cover sheet of claim 1, wherein the first protective layer is immediately adjacent the first low modulus layer and the second protective layer is immediately adjacent the first low modulus layer opposite the first protective layer.

3. The OLED cover sheet of claim 1, wherein the first low modulus layer is an optically clear adhesive.

4. The OLED cover sheet of claim 1, wherein the first low modulus layer comprises a (meth)acrylic polymer.

5. The OLED cover sheet of claim 1, wherein the first low modulus layer comprises an olefin block.

6. The OLED cover sheet of claim 1, further comprising a hard-coat layer adjacent the second protective layer opposite the first protective layer.

7. The OLED cover sheet of claim 6, wherein the hard-coat layer comprises a (meth)acrylic polymer.

8. The OLED cover sheet of claim 1, further comprising:

a third protective layer adjacent the second protective layer opposite the first protective layer; and a second low modulus layer disposed between the second and third protective layers, the second low modulus layer having a shear storage modulus less than about 10 MPa at 25 °C and 1 Hz.

9. The OLED cover sheet of claim 1, wherein each of the first and second polyesters include at least about 85 mole percent naphthalenedicarboxylate groups based on total carboxylate groups.

10. The OLED cover sheet of claim 1, wherein each of the first and second protective layers has a sum of an elastic storage modulus in a machine direction and an elastic storage modulus in a transverse direction in a range of about 9 GPa to about 16 GPa at 25 °C and 1 Hz.

1 1. The OLED cover sheet of claim 1 , wherein each of the first and second protective layers have a Tg in a range of about 100 °C to about 130 °C.

12. The OLED cover sheet of claim 1, wherein the first low modulus layer has a shear storage modulus less than about 1 MPa at 25 °C and 1 Hz.

13. The OLED cover sheet of claim 1, wherein each of the first and second protective layers has a sum of an elastic storage modulus in a machine direction and an elastic storage modulus in a transverse direction that is at least about 5,000 times larger than the shear storage modulus of the first low modulus layer.

14. The OLED cover sheet of claim 1, wherein the OLED cover sheet has a haze of less than about 3 percent.

15. The OLED cover sheet of claim 1, wherein the OLED cover sheet has a puncture resistance of at least about 1.0 kgf against a probe having a 200 micrometer diameter tip.

16. The OLED cover sheet of claim 1, wherein the OLED cover sheet has a short beam shear bending stiffness less than about 8 N/mm.

17. An OLED device comprising:

an OLED display having an output surface; and

the OLED cover sheet of claim 1 attached to the output surface with an optically clear adhesive, the first protective layer of the OLED cover sheet facing the OLED display.