Multilayer optically clear adhesive

Abstract

Provided is a multilayer adhesive for bonding components within a foldable electronic device. The adhesive includes an elastic layer having opposed first and second major surfaces, where a first viscoelastic skin layer disposed on the first major surface and a second viscoelastic skin layer disposed on the second major surface. The elastic layer has a Tan Delta at 70ºC of from 0.02 to 0.20 and each of the first and second viscoelastic skin layers independently has a Tan Delta at 70ºC of from 0.20 to 0.55, with the Tan Delta of each of first and second viscoelastic skin layers being at least 125 percent that of the elastic layer. Advantageously, the multilayer adhesive enables a high degree of crease recovery, while preserving strong adhesion and decoupling bending stresses to avoid damage to functional layers within the foldable electronic device.

Description

[0001] MULTILAYER OPTICALLY CLEAR ADHESIVE

[0002] Field of the Invention

[0003] Provided are adhesive articles, along with related assemblies and methods. The adhesive articles and assemblies can be useful for bonding layered components in electronic display applications.

[0004] Background

[0005] Foldable electronic displays have been transformative in consumer electronics by combining the portability of smartphones with the larger screens of tablets. This innovation has required advancements across materials science, engineering, and semiconductor technology.

[0006] The realization of foldable displays came about with the availability of resilient, transparent conductive materials and ultra-thin glass capable of folding up to 180 degrees. Optically clear adhesives (OCAs) are also critical components used to bond these components to each other. It is desirable for OCAs to have a high degree of optical clarity and high interfacial adhesion over a wide range of temperatures. Foldable smartphones, foldable laptops and wearable screens have been enabled by these technologies, which continue to expand the possibilities of mobile computing and personal electronics.

[0007] Summary

[0008] To meet the mechanical requirements of these applications, it is preferable for the OCA to be highly elastic to avoid deformation and aid in recovery during the folding and unfolding of the device. It is advantageous to have a low Tgto ensure similar mechanical properties across a wide range of operating temperatures. It is also advantageous for such an adhesive to have a low shear modulus to better mechanically decouple the more rigid layers of the display from each other in bending while retaining a high interfacial adhesion over a wide temperature range (-20°C to 85°C).

[0009] It was further recognized that flexible displays should also have a smooth surface profde with nearly zero crease or depression in the center over the folding hinge. Displays need OCAs to assemble multiple functional layers, and in flexible and foldable displays, the OCAs must decouple bending stresses to avoid breaking functional layers. To minimize the top surface crease, the adhesives should recover quickly from elastic deformation, and they must also resist hydrostatic stresses to avoid bubble or cavitation failures.

[0010] For stress decoupling and rapid recovery, OCAs can be made from low-modulus, highly elastic materials. However, soft and elastic OCAs also tend to adhere poorly, so methods and constructions are sought to combine the disparate properties of elasticity, low modulus, and adhesion in one material.

[0011] In display materials for foldable consumer electronics, one of the significant market needs is in preventing the typical crease that occurs in the display in the bending region. Within these types of displays, multiple optical adhesives are employed to enhance both the optical and mechanical performance of the displays, particularly through stress decoupling of the rigid layers of the display during repeated folding / bending of the display. Through material evaluation and simulation of the crease formation of the display stack, we have greater understanding of the potential adhesive mechanical properties that will help facilitate greater crease recovery and overall crease prevention.

[0012] A solution can be provided by a multilayer optical adhesive including a highly elastic core layer and of sufficiently low modulus to aid in crease recovery combined with outer skin layers that are viscoelastic to improve bonding to adjacent substrates. As a further advantage, this construction can be made using a simultaneous coating and curing process. These processes allow for excellent interlayer strength while preserving differential properties of elasticity in the core and skin layers. Optionally, high molecular weight crosslinkers can be used to reduce the propensity of the component responsible for high core elasticity to diffuse from the core to skin layers before curing. By reducing diffusion before curing, it is possible to achieve even greater differentiation of mechanical and adhesive properties between these layers.

[0013] In a first aspect, a multilayer adhesive for bonding components within a foldable electronic device is provided. The multilayer adhesive comprises: an elastic layer having opposed first and second major surfaces; a first viscoelastic skin layer disposed on the first major surface; and a second viscoelastic skin layer disposed on the second major surface, wherein the elastic layer has a Tan Delta at 70°C of from 0.02 to 0.2 and each of the first and second viscoelastic skin layers independently has a Tan Delta at 70°C of from 0.2 to 0.55, with the Tan Delta of each of first and second viscoelastic skin layers being at least 125 percent that of the elastic layer.

[0014] In a second aspect, an adhesive assembly is provided, comprising: the aforementioned multilayer adhesive, and a pair of release liners in direct contact with respective outward-facing major surfaces of the first and second viscoelastic skin layers.

[0015] In a third aspect, a foldable electronic device is provided, comprising: a display stack including a flexible display panel; and the aforementioned multilayer adhesive, wherein the multilayer adhesive bonds the flexible display panel to a display cover layer whereby crease formation in a bending region of the flexible display panel is reduced during repeated folding and unfolding of the foldable electronic device relative to bonding with the elastic layer alone.

[0016] Brief Description of the Drawings

[0017] FIG. 1 is an isometric view of a multilayered adhesive between opposing substrates according to one exemplary embodiment.

[0018] FIG. 2 is an elevational side view of the multilayered adhesive of FIG. 1, showing interdiffusion at the layer interfaces.

[0019] FIG. 3 is a schematic showing a process of making the provided multilayered adhesives.

[0020] FIG. 4 is a plot showing combinations of peel adhesion performance versus Tan Delta for various adhesives, including the provided multilayered adhesives.

[0021] Repeated use of reference characters in the specification and drawings is intended to represent the same or analogous features or elements of the disclosure. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the disclosure. The figures are not drawn to scale. DEFINITIONS

[0022] As used herein:

[0023] “alkyl” refers to a monovalent group that is a radical of an alkane and includes straight-chain, branched, cyclic, and bicyclic alkyl groups, and combinations thereof, including both unsubstituted and substituted alkyl groups. Unless otherwise indicated, the alkyl groups typically contain from 1 to 30 carbon atoms. In some embodiments, the alkyl groups contain 1 to 20 carbon atoms, 1 to 10 carbon atoms, 1 to 6 carbon atoms, 1 to 4 carbon atoms, or 1 to 3 carbon atoms. Cyclic groups can be monocyclic or polycyclic and typically have from 3 to 10 ring carbon atoms. Examples of “alkyl” groups include methyl, ethyl, n-propyl, n-butyl, n-pentyl, isobutyl, t-butyl, isopropyl, n-octyl, n-heptyl, ethylhexyl, cyclopentyl, cyclohexyl, cycloheptyl, adamantyl, and norbomyl.

[0024] “allyl” refers to a functional group having the formula CH2=CH-CH2-.

[0025] “cure” refers to the joining of polymer chains together by covalent chemical bonds, usually via crosslinking molecules or groups, to form a network polymer. Therefore, in this disclosure the terms “cured” and “crosslinked” can be used interchangeably. A cured or crosslinked polymer is generally characterized by insolubility but can be swellable in the presence of an appropriate solvent.

[0026] “curable” refers to a composition capable of being cured.

[0027] “glass transition temperature” (or “Tg”) refers to a temperature at which an amorphous polymer changes from a hard / glassy state to a more pliable rubbery state, or vice versa, and can be determined by performing a Dynamic Mechanical Analysis (or “DMA”) temperature sweep at a given frequency. From this technique, the Tgcan be defined as the temperature at which the tan 5 peaks.

[0028] “(meth)acrylate” refers to a functional group that is either an acrylate group of the formula CH2=CH-C(O)O- or a methacrylate group of the formula CH2=C(CH3)-C(O)O-.

[0029] “molecular weight” refers to weight average molecular weight, unless otherwise indicated.

[0030] “polymeric” refers to a chemical compound comprised of many repeat units covalently bonded to each other in end-to-end fashion, for example having more than 5, 10, 20, and / or 50 repeat units. “weight average molecular weight” is a parameter reflecting the weight fraction of individual polymer chains in a polymer sample and measured using known gel permeation chromatography (GPC) techniques.

[0031] Detailed Description

[0032] As used herein, the terms “preferred” and “preferably” refer to embodiments described herein that can afford certain benefits, under certain circumstances. However, other embodiments can also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[0033] As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” or “the” component can include one or more of the components and equivalents thereof known to those skilled in the art. Further, the term “and / or” means one or all of the listed elements or a combination of any two or more of the listed elements. It is noted that the term “comprises,” and variations thereof do not have a limiting meaning where these terms appear in the accompanying description. Moreover, “a,” “an,” “the,” “at least one,” and “one or more” are used interchangeably herein. Relative terms such as left, right, forward, rearward, top, bottom, side, upper, lower, horizontal, vertical, and the like can be used herein and, if so, are from the perspective observed in the particular drawing. These terms are used only to simplify the description, however, and not to limit the scope of the invention in any way.

[0034] Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described relating to the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Disclosed herein are adhesives and adhesive assemblies that are optically clear and suitable for deployment in foldable electronic devices. These devices can include a display stack including a flexible display panel and the aforementioned multilayer adhesive, wherein the multilayer adhesive bonds the flexible display panel to a display cover layer.

[0035] A bonded assembly is shown in FIG. 1 and hereinafter designated by the numeral 50. Bonded assembly 50 includes an adhesive 100 disposed between substrates 102, 104. As illustrated, the adhesive 100 has a multilayered structure that comprises a plurality of discrete layers — a core layer 106, sandwiched between skin layers 108, 110. In preferred embodiments, each of the core layer 106 and skin layers 108, 110 are individually and collectively pressure-sensitive adhesives.

[0036] The core layer 106 is an elastic layer, providing the material with a high degree of resilience. This resilience enables the overall multilayer adhesive 100 to recover quickly from deformation that results from being folded. Faster recovery, in turn, reduces the degree of surface creasing that would normally occur when the bonded assembly 50 is folded and unfolded.

[0037] The elasticity of the core layer 106 can be characterized by a Tan Delta at 70°C of from 0.02 to 0.20, from 0.03 to 0.15, from 0.04 to 0.10, or in some embodiments, less than, equal to, or greater than 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0. 15, 0. 17, or 0.2, as measured by dynamic mechanical analysis (described in the Examples). By contrast, the viscoelasticity of the skin layers 108, 110 can independently be characterized by a Tan Delta at 70°C of from 0.20 to 0.55, from 0.25 to 0.45, from 0.25 to 0.40, or in some embodiments, less than, equal to, or greater than 0.20, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or 0.55. Optionally, the skin layers 108, 110 have the same composition, such that the Tan Delta values are identical.

[0038] The Tan Delta of each of first and second viscoelastic skin layers 108, 110 can be significantly greater than that of the elastic layer 106. For example, the Tan Delta of each of first and second viscoelastic skin layers 108, 110 is at least 125 percent, 130, 135, 140, 145, 150, 155, 160, 170, 180, 190, 200, 210, 220, 250, or even 300 percent that of the elastic layer 106.

[0039] The core layer 106 can have a Storage Modulus (G’) at 25°C of from 5 kPa to 80 kPa, from 8 kPa to 45 kPa, or in some embodiments, less than, equal to, or greater than 5 kPa, 6, 7, 8, 9, 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 kPa. Each of the skin layers 108, 110 can independently have a Storage Modulus at 25°C of from 10 kPa to 80 kPa, from 10 kPa to 50 kPa, or in some embodiments, less than, equal to, or greater than 10 kPa, 12, 15, 17, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 kPa. The glass transition temperature (Tg) for both the core layer 106 and skin layers 108, 110 can independently be less than -25°C, less than -30°C, or less than -40°C. The substrates 102, 104 are not particularly limited and can be either temporarily or permanently bonded. In an exemplary embodiment, one or both of the substrates 102, 104 are release liners removed immediately prior to using the adhesive 100 in a bonding application. In a manufacturing process, it can be beneficial to dispose the adhesive 100 between opposing release liners that have different release forces so they can be removed sequentially. For example, substrate 102 can be an easy release liner, while substrate 104 can be a tight release liner.

[0040] Useful release substrates are known in the art, and can include for example liners constructed of silicone-coated polyester or silicone-coated paper. Alternatively, the substrate 102 could be coated onto a functional film along one of its major surfaces with a release substrate disposed on its opposing major surface. Useful functional films can be made from polyethylene terephthalate, polyimide, cyclo olefin polymer (COP), a multilayer optical film (MOF), glass, or a polarizer film.

[0041] The thicknesses of the core layer 106 and skin layers 108, 110 need not be particularly restricted. Depending on the application, the thickness of the core layer 106 can be from 5 micrometers to 150 micrometers, or in some embodiments, less than, equal to, or greater than 5 micrometers, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120, 130, 140, or 150 micrometers. The skin layers 108, 110 can have a total thickness of from 10 micrometers to 300 micrometers, or in some embodiments, less than, equal to, or greater than 10 micrometers, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, or 300 micrometers. The thicknesses of the skin layers 108, 110 can be the same or different from each other.

[0042] Related to thickness is the coating weight of the core layer 106 and skin layers 108, 110. The coating weight of the core layer 106 can be from 4.6E-04 grams per square meter (gsm) to 1.4E-02 gsm, from 6.4E-04 gsm to 8.2E-3 gsm, from 1.1E-03 gsm to _7.3E-03 gsm, or in some embodiments, less than, equal to, or greater than 4.6E-04 gsm, 6.4E-04, 1.1E-03, 2.7E-03, or 4.6E-03 gsm. The skin layers 108, 110 can have a total coating weight of from 4.6E-04 gsm to 1.8E-02 gsm, from 6.4E-04 gsm to 1.4E-02 gsm, from 9. IE-04 gsm to 9. IE-03 gsm, or in some embodiments, less than, equal to, or greater than 1.1E-03 gsm, 1.4E-03, 3.2E-03, 4.1E-03, or 5.0E-03 gsm. The thicknesses of the skin layers 108, 110 can be the same or different from each other.

[0043] The thickness of the skin layers 108, 110 relative to the core layer 106 can also be adjusted as appropriate for the given application. For example, the total thickness of the skin layers 108, 110 can be from 80 percent to 300 percent, or in some embodiments, less than, equal to, or greater than, 80 percent, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270, 280, 290, or 300 percent of the core layer 106 thickness. The above percentages also apply with respect to relative differences in coating weight between the skin layers 108, 110 and core layer 106.

[0044] In various embodiments, the core and skin layers 106, 108, 110 can be derived from a precursor composition that includes an alkyl (meth)acrylate having 1 to 24 carbons in the alkyl group. The alkyl (meth)acrylate can include aliphatic, cycloaliphatic, or aromatic alkyl groups. Useful alkyl (meth)acrylates (i.e., (meth)acrylic acid alkyl ester monomers) include linear or branched monofunctional acrylates or methacrylates of non- tertiary alkyl alcohols, the alkyl groups of which have from 1 up to 24, or from 1 up to 18 carbon atoms.

[0045] In some instances, the alkyl (meth)acrylate can be present in an amount of from 40 percent to 95 percent, from 55 percent to 90 percent, from 60 percent to 80 percent, or in some embodiments, less than, equal to, or greater than 50 percent, 55, 60, 65, 70, 75, 80, 85, 90 or 95 percent by weight relative to the overall weight of the adhesive layer.

[0046] Useful alkyl (meth)acrylates can include, for example, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, pentyl (meth)acrylate, n- octyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, hexyl (meth)acrylate, n-nonyl (meth)acrylate, isoamyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, dodecyl (meth)acrylate, isobomyl (meth)acrylate, cyclohexyl (meth)acrylate, phenyl meth(acrylate), benzyl meth(acrylate), and 2-methylbutyl (meth)acrylate, and combinations thereof. Other suitable monomers include branched long chain acrylates such as those described in U.S. Patent No. 8,137,807, which is hereby incorporated by reference. Additional suitable alkyl monomers include secondary alkyl acrylates such as those described in U.S. Patent 9,399,724, which is hereby incorporated by reference. In one embodiment, the acrylic -based assembly layer includes only alkyl (meth)acrylate monomers with optional vinyl ester or styrenic monomers. In such cases, the modulus and glass transition temperature (Tg) of the composition can be adjusted by selecting combinations of low and high Tgyielding monomers.

[0047] In some embodiments, the precursor composition includes a polar copolymerizable monomer. Examples of suitable polar copolymerizable monomers include, but are not limited to: acrylic acid (AA), methacrylic acid, itaconic acid, fumaric acid, methacrylamide, n-alkyl substituted and n,n-dialkyl substituted acrylamides or methacrylamides where the alkyl group has up to 3 carbons, and n-vinyl lactams. Examples of suitable monomers include, but are not limited to: (meth)acrylamide, n- morpholino (meth)acrylate, n-vinyl pyrolidone and n-vinyl caprolactam. Other suitable polar monomers may include hydroxyl containing monomers such as 2-hydroxyethyl (meth)acrylate, and 2-hydroxy-propyl (meth)acrylate, 4-hydroxybutyl(meth)acrylate, and ether containing monomers such as 2-ethoxyethoxyethyl (meth)acrylate, 2-methoxyethoxyethyl(meth)acrylate, and the like. In one embodiment, the acrylic-based assembly layer includes between about 1 and about 40 parts by weight of the polar copolymerizable monomer, particularly between about 5 and about 35 parts, and more particularly between about 5 and about 30 parts.

[0048] In some embodiments, the precursor composition includes a combination polar and alkyl monomer. Examples of such monomers include, but are not limited to, 2-ethylhexyl diglycol acrylate and similar structures.

[0049] The adhesive precursor composition can further include a hydrophilic polymeric compound. This polymeric compound can have a weight average molecular weight (Mw) of at least 500, or at least 1000, or even higher. Suitable hydrophilic polymeric compounds include poly(ethylene oxide) segments, hydroxyl functionality, or a combination thereof. The combination of polyethylene oxide) and hydroxyl functionality in the polymer needs to be high enough to make the resulting polymer hydrophilic. By “hydrophilic,” it is meant that the polymeric compound can incorporate at least 25 weight percent of water without phase separation. The hydrophilic polymeric compound can be present in an amount of from 1 percent to 20 percent, relative to the combined weight of the alkyl (meth)acrylate and the copolymerizable polar monomer.

[0050] Copolymerizable, hydrophilic polymer compounds include, for example, CD552, available from Sartomer Company, Exton, PA, which is a monofunctional methoxylated polyethylene glycol (550) methacrylate, or SR9036, also available from Sartomer, that is an ethoxylated bisphenol A dimethacrylate that has 30 polymerized ethylene oxide groups between the bisphenol A moiety and each methacrylate group. Other examples include phenoxypolyethylene glycol acrylate available from Jarchem Industries Inc., Newark, N.J. Other examples of polymeric hydrophilic compounds include poly acrylamide, poly-n,n- dimethylacrylamide, and poly-n-vinylpyrrolidone.

[0051] In a preferred embodiment, the adhesive is derived from a precursor composition including from 40 percent to 95 percent by weight of an alkyl (meth)acrylate having 1 to 24 carbons in the alkyl group and from 0 percent to 25 percent by weight of a copolymerizable polar monomer, relative to the overall weight of the precursor composition.

[0052] The adhesive precursor composition can also include a hydrophilic, hydroxyl- functional monomer distinct from the alkyl (meth)acrylate and copolymerizable polar monomer. The hydroxyl -functional monomer optionally has a hydroxyl equivalent weight of less than 400, the hydroxyl equivalent molecular weight being defined as the molecular weight of the monomer divided by the number of hydroxyl groups in the monomer. The hydrophilic, hydroxyl-functional monomer can be present in an amount of from 2 percent to 40 percent by weight relative to the combined weight of the alkyl (meth)acrylate and the copolymerizable polar monomer(s) above.

[0053] Useful monomers of this type include 2-hydroxyethyl acrylate and methacrylate, 3- hydroxypropyl acrylate and methacrylate, 4-hydroxybutyl acrylate and methacrylate, 2- hydroxyethylacrylamide, and n-hydroxypropyl acrylamide. Additionally, hydroxy functional monomers based on glycols derived from ethylene oxide or propylene oxide can also be used. An example of this type of monomer includes a hydroxyl-terminated polypropylene glycol acrylate, sold under the trade designation BISOMER PPA 6 from Cognis, Germany. Diols and triols having hydroxyl equivalent weights of less than 400 are also contemplated for the hydrophilic monomer. The hydroxyl-functional monomer can be present in an amount of from 0 to 25 percent, from 1 percent to 20 percent, from 2 percent to 15 percent, or in some embodiments, less than, equal to, or greater than 1 percent, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 percent by weight relative to the overall weight of the adhesive layer. It is preferred to have the sum of all hydroxyl monomer concentration to be less than 25% to ensure that the dielectric constant remains in a useful range and does not have too great of environmental sensitivity. Similarly, it is preferred to have the sum of all polar monomer concentration including hydroxyl monomer to be less than 30% to ensure that the dielectric constant remains in a useful range and does not have too great of environmental sensitivity.

[0054] In some embodiments, the monomer mixture includes a multifunctional crosslinker. For example, the precursor mixture may include thermal crosslinkers which are activated during the drying step of preparing solvent coated adhesives and crosslinkers that copolymerize during the polymerization step. Such thermal crosslinkers may include, but are not limited to: multifunctional isocyanates, multi-functional aziridines, and epoxy compounds. Exemplary crosslinkers which can be copolymerized include difunctional acrylates such as 1,6-hexanediol diacrylate or multifunctional acrylates such as are known to those of skill in the art. Useful isocyanate crosslinkers include, for example, an aromatic triisocyanate available as DESMODURN3300 from Bayer, located in Cologne, Germany. Ultraviolet (or “UV”) activated crosslinkers can also be used to crosslink the precursors of the assembly layer. Such UV crosslinkers may include non-copolymerizable photocrosslinkers, such as benzophenones and copolymerizable photocrosslinkers such as acrylated or methacrylated benzophenones like 4-acryloxybenzophenones. Typically, the crosslinker, if present, is added to the monomer mixture in an amount of between about 0.01 parts and about 10 parts by weight based, particularly between about 0.01 and about 8 parts, and more particularly between about 0.01 and about 6 parts. Other crosslinking methods, such as ionic crosslinking, acid-base crosslinking, or the use of physical crosslinking methods, such as by copolymerizing high Tgmacromers, such as, for example, polymethylmethacrylate or polystyrene macromer, may also be used. When included, macromers may be used in an amount of about 1 to about 20 parts by weight of the total monomer components in the assembly layer composition. In some embodiments, multifunctional oligomers or polymeric crosslinkers may be utilized to crosslink the acrylic based assembly. Exemplary high-molecular weight crosslinkers include multifunctional urethane (meth)acrylates, such as aliphatic urethane diacrylates sold under the trade designations EBECRYL 230 (Allnex, Brussels, BE) and X6127 (Eternal Materials Co., Kaohsiung City, TW).

[0055] In some embodiments, the precursor mixture includes a crosslinker that has a (meth)acrylate functionality greater than 1. Such crosslinker can have a functionality of at least 1.5, 2, 2.5, or even 3.

[0056] In preferred embodiments, the crosslinker has a weight average molecular weight that is from 1000 g / mol to 100,000 g / mol, from 2000 g / mol to 60,000 g / mol, from 8000 g / mol to 40,000 g / mol, or in some embodiments, 1000 g / mol; 2000; 3000; 4000; 5000; 6000; 7000; 8000; 10,000; 15,000; 20,000; 25,000; 30,000; 35,000; 40,000; 45,000; 50,000; 60,000; 70,000; 80,000; 90,000; or 100,000 g / mol.

[0057] It can be a significant technical advantage to use crosslinkers that have a higher molecular weight relative to conventional crosslinkers. Molecular weight effects can be especially relevant in a manufacturing process where adhesive layers are co-extruded and integrated in molten form immediately after being expelled from the extrusion die. With larger molecules diffusing more slowly than smaller molecules, this help minimize differences in crosslink density between the core and skin layers near layer interfaces.

[0058] The crosslinker(s) can be present in an amount of from 0.05 percent to 20 percent by weight or in some embodiments, less than, equal to, or greater than, 0.05 percent, 0.1, 0.2, 0.5, 0.7, 1, 2, 3, 4, 5, 7, 10, 12, 15, 17, or 20 percent by weight relative to the overall weight of the monomer mixture. The amount, however, can be significantly different between the core and skin layers. In some embodiments, the amount of crosslinker in the core layer is 150 percent, 175, 200, 300, 400, 500, 600, 700, 800, 900, or even 1000 percent that in found the skin layers to provide greater elasticity and enhanced crease recovery performance in the core layer.

[0059] Retaining such a crosslinker in the core layer 106 helps achieve high core elasticity while still benefiting from improved interlayer adhesion created by diffusion of smaller monomers at the interface. While not drawn to scale, the phenomenon of diffuse layer interfaces is illustrated in the bonded assembly 60 in FIG. 2. As shown, the core layer 206, and the skin layers 208, 210 have diffuse boundaries as a result of interdiffusion of monomeric species proximate to where adjacent layers 208, 206 and layers 206, 210 contact each other. The resulting intermingling and entanglements of polymer chains can provide superior interlayer adhesion after curing.

[0060] For clarity, the values of Tan Delta as disclosed herein can be defined based on measurements on the individual layers 206, 208, 210 absent any possible interlayer diffusion effects.

[0061] Further, by virtue of using high-molecular weight crosslinkers in the provided multilayer adhesives, the high core elasticity allows crease formation in a bending region of the flexible display panel to be reduced during repeated folding and unfolding of the foldable electronic device relative to a monolithic adhesive consisting of the elastic layer alone.

[0062] In addition, the precursor mixtures for the provided adhesive compositions can include a thermal initiator or photoinitiator. Examples of thermal initiators include peroxides such as benzoyl peroxide and its derivatives or azo compounds such as VAZO 67, available from E. I. du Pont de Nemours and Co. Wilmington, Del., which is 2,2'- azobis-(2 -methylbutyronitrile), or V-601, available from Wako Specialty Chemicals, Richmond, VA, which is dimethyl-2,2'-azobisisobutyrate. A variety of peroxide or azo compounds are available that can be used to initiate thermal polymerization at a wide variety of temperatures.

[0063] Useful photoinitiators include, for example, IRGACURE 651, sold by Ciba Chemicals, Tarrytown, NY, which is 2, 2-dimethoxy-2 -phenylacetophenone. Typically, the crosslinker, if present, is added to the precursor mixtures in an amount of from 0.05 parts by weight to 5 parts by weight based upon the other constituents in the mixture. The initiator(s) can be added to precursor compositions in the amount of from 0.05 parts by weight to 2 parts by weight. The precursor mixtures can be polymerized and / or crosslinked using actinic radiation or heat to form the adhesive composition.

[0064] Useful photoinitiators also include diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO). By adding TPO to the pressure sensitive adhesive composition, the ultraviolet irradiation dose necessary for ultraviolet cross-linking can be decreased. As a result, for example, shortening of tact time or energy saving becomes possible and the process of laminating the adherend can be more efficient. Addition of TPO can be advantageous when the adherend contains an ultraviolet absorber and an ultraviolet ray is irradiated on the pressure sensitive adhesive through the adherend.

[0065] In some embodiments, one or more of the foregoing initiators can be used to initiate a primary polymerization step used in directly or indirectly preparing a curable adhesive. Alternatively, one or more of the foregoing initiators can be used to initiate a secondary polymerization step following a primary polymerization step, where a curable adhesive is fully cured in its final application, such as in a display device.

[0066] Each of the primary or secondary initiator(s) can be present in an amount of from 0.01 percent to 5 percent, from 0.1 percent to 2 percent, or in some embodiments less than, equal to, or greater than 0.01 percent, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.2, 2.5, 2.7, 3, 3.5, 4, 4.5, or 5 percent by weight, relative to the overall weight of the adhesive layer.

[0067] The acrylic-based assembly layer may be inherently tacky. If desired, tackifiers can be added to the precursor mixture before formation of the acrylic-based assembly layer. Useful tackifiers include, for example: rosin ester resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, terpene, acrylic resins or polymers, and terpene phenolic resins. In general, light-colored tackifiers selected from hydrogenated rosin esters, terpenes, or aromatic hydrocarbon resins are preferred. When included, the tackifier is added to the precursor mixture in an amount of between about 1 parts and about 50 parts by weight, more particularly between about 5 and about 45 parts, and most particularly between about 10 and about 30 parts, out of 100 parts of the precursor mixture

[0068] In some embodiments, the precursor monomer mixture may also include reactive plasticizers such as but not limited to LD-301 (AGC Inc.) or other monofunctional urethane acrylates, particularly those of sufficiently low Tg.

[0069] Other materials can be added for special purposes, including, for example, oils, plasticizers, antioxidants, UV stabilizers, pigments, curing agents, polymer additives, adhesion promoters, and other additives, provided that they do not significantly reduce the optical clarity of the pressure sensitive adhesive.

[0070] As an example, adhesion promoting additives such as silanes and titanates can be incorporated therein. Such additives can promote adhesion between the adhesive and the substrates, such as the glass and cellulose triacetate of a liquid crystal display (LCD) by coupling to silanol, hydroxyl, or other reactive groups in the substrate. The silanes and titanates may have only alkoxy substitution on the silicon or titanium atom connected to an adhesive copolymerizable or interactive group. Alternatively, the silanes and titanates may have both alkyl and alkoxy substitution on the silicon or titanium atom connected to an adhesive copolymerizable or interactive group.

[0071] The adhesion promoting additive can include a copolymerizable group that is generally an acrylate or methacrylate group, but vinyl and allyl groups may also be used. Alternatively, the silanes or titanates may also react with functional groups in the adhesive, such as a hydroxyalkyl (meth)acrylate. In addition, the silane or titanate may have one or more groups providing strong interaction with the adhesive matrix. Examples of this strong interaction include hydrogen bonding, ionic interaction, and acid-base interaction. An example of a preferred silane is (3-glycidyloxypropyl) trimethoxysilane.

[0072] In some embodiments, a silane adhesion promoter is present in an amount of from 0.02 percent to 1 percent, from 0.04 percent to 0.5 percent, or in some embodiments less than, equal to, or greater than 0.02 percent, 0.04, 0.05, 0.1, 0.2, 0.5, 1, 2, or 5 percent by weight, relative to the overall weight of the curable adhesive.

[0073] Further options and advantages associated with the adhesive compositions herein can be found, for example, in U.S. Patent Nos. 8,663,811 (Everaerts et al.) and 8,911,873 (Suwa et al.) and co-pending U.S. Patent Application No. 63 / 535,011 (Xia, et al.).

[0074] Advantageously, the provided multilayer adhesives can be made using a solventless curing process. Solventless polymerization processes, such as the continuous free radical polymerization method described in U.S. Patent Nos. 4,619,979 and 4,843,134 (Kotnour et al.); the essentially adiabatic polymerization methods using a batch reactor described in U.S. Patent No. 5,637,646 (Ellis); and the methods described for polymerizing packaged pre-adhesive compositions described in U.S. Patent No. 5,804,610 (Hamer et al.) may also be utilized to prepare coatable compositions.

[0075] In a preferred embodiment, curing is initiated by irradiating the curable adhesive composition to actinic radiation, such as ultraviolet light. In general, preparation of the adhesives can take place according to the following steps:

[0076] 1) preparing a plurality of coatable compositions, each of the coatable compositions comprising at least one photopolymerizable monomer; at least one of the coatable compositions being curable to a pressure-sensitive adhesive state, monomers of each of the coatable compositions being copolymerizable when blended and subjected to curing conditions;

[0077] 2) simultaneously coating the coatable compositions onto a substrate (such as a release surface) to provide a plurality of superimposed layers with contiguous layers defining an interface therebetween, with one composition which is curable to a pressure-sensitive adhesive state being coated as a first or last layer;

[0078] 3) permitting diffusion of photopolymerizable monomers through the interface between adjacent layers; and

[0079] 4) subjecting the superimposed layers to actinic radiation to simultaneously cure the monomers in each layer, and to provide polymeric chains comprised of copolymers of photopolymerizable monomers originating from contiguous layers extending through the interface therebetween, thereby to produce adhesive constructions comprised of layers that resist delamination.

[0080] FIG. 3 shows an exemplary coating apparatus 70 useful in manufacturing the provided adhesives and adhesive assemblies. The coating apparatus 70 obtains a multilayer adhesive from die-coated coatings 12, 14, and 16, each comprised of a monomer blend syrup which is curable — in this example, photopolymerizable by UV radiation. The coatings 12, 16 are photopolymerizable to a pressure-sensitive adhesive having high elasticity, while the coating 14 is polymerizable to a pressure-sensitive adhesive having comparatively low elasticity and high viscoelasticity.

[0081] The compositions are concurrently coated onto an ultraviolet-transparent, low- adhesion carrier 10 having a release surface is concurrently coated by means of a multiple manifold co-extrusion die 20. The co-extrusion die 20 is adjacent to a back-up roller 18 as shown. Three manifolds 22, 24, 26 used to extrude the compositions of photopolymerizable coatings 12, 14, and 16 onto the low-adhesion carrier 10. The coating layers are then simultaneously subjected to UV radiation from a bank of lamps 28 to photopolymerize the monomers and provide a multilayer pressure-sensitive adhesive comprised of an elastic core layer between two viscoelastic skin layers. EXAMPLES

[0082] Materials

[0083] Table 1 below enumerates materials used in preparing the inventive and comparative examples herein.

[0084] Table 1. Materials, abbreviations, and suppliers Haze Test

[0085] Haze measurements were made using a HunterLab (Reston, VA) UltrascanPro Spectrophotometer in transmission mode. 100-micrometer multilayer adhesive constructions were prepared between siliconized PET as described above. One of the carrier liners was removed and the sample was laminated to a clear piece of 0.7 mm thick LCD glass (Swift Glass, Elmira Heights, New York). The sample was placed in the UltrascanPro Spectrophotometer to measure transmission and % Haze through the OCA / glass assembly.

[0086] Dynamic Mechanical Analysis (DMA)

[0087] Dynamic mechanical analysis was used to probe the modulus as a function of temperature as well as to determine the glass transition temperature (Tg) of the material. 75 micrometer or 100 micrometer thick adhesive composites between siliconized PET were prepared as described above. One liner was removed, and adhesive layers were stacked to form a total thickness of approximately 1000 micrometers. An 8 mm diameter by about 1 mm thick disk of laminated assembly layers was placed between the probes of a DHR parallel plate rheometer (TA Instruments, New Castle, DE). A temperature scan was performed by ramping from -70°C to 100°C at 3°C / minute. During this ramp, the samples were oscillated at a frequency of 1 Hz and a strain of approximately 0.4%. The shear storage modulus (G’), loss modulus (G”) and Tan Delta were recorded at select temperatures during this scan. The Tgof the material was also determined as the peak in the Tan Delta versus temperature profile. In particular, the Tan Delta intensity at a set temperature of 70°C was utilized to approximate the degree of crosslinking within the sample elastomer.

[0088] Peel Adhesion Test

[0089] 75 micrometer or 100 micrometer adhesive composites between siliconized PET were prepared as described above. The first liner was stripped away and the OCA was laminated to plasma treated PET liner. A 1.27 cm x 15.2 cm (0.5 inch x 6 inch) strip was cut from this construction, the second release liner was removed, and the strip was laminated to the air side of a standard float glass plate using a 2 kg (4.5 lb) roller. The construction of glass / OCA / PET was placed in an autoclave with applied settings of 50°C and 5 kg of pressure for 20 minutes. The sample was then aged for over 12 hours in an environment of 25°C and 55% humidity prior to testing. Peel adhesion force was acquired using an I-mass SP2100 peel tester (Strongsville, OH) with a 10 N load cell. Peel tests were conducted with approximately a 180 degree angle of the debonding substrate with respect to the glass substrate and at a rate of approximately 6 cm / min.

[0090] Crease Recovery Testing

[0091] After being folded and unfolded, the central region of the sample was analyzed using spectral domain-optical coherence tomography (SD-OCT) for crease measurements. SD-OCT can be used to non-destructively obtain cross sectional images of the surface over a region of interest. From the cross-sectional images, surface slopes were calculated over select intervals. The SD-OCT system has an axial resolution specification of about 6 micrometers in air and has lateral resolution specifications between 4 and 12 micrometers.

[0092] A time series of cross-sectional images of the sample were taken in 2 minutes interval for a total of 15 hours. After collecting the time-series images, a python -based image processing program was then used to extract the surface profile of the sample and the maximum slope (pm / mm) was calculated as the metric for crease. A smaller slope means smaller crease.

[0093] Preparatory Examples S1-S4 and C1-C6

[0094] For the outer skin layers, preparatory coating solutions S1-S4 were prepared according to the formulations listed below in Table 2. The components as denoted by part A were charged to a glass jar, purged with nitrogen for 300 s, and then irradiated with 365 nm irradiation with an intensity of 0.3 mW / cm2until the solution reached a viscosity of about 500-3000 cp as measured by a Brookfield viscometer (Brookfield AMETEK, Middleboro, MA). Following this step, the additional components listed for part B were added to the solutions and mixed thoroughly. Table 2. Coating Solutions for Outer Composite Layers

[0095] For the core layers, preparatory coating solutions C1-C6 were prepared according to the formulations listed below in Table 3. The components as denoted by part A were charged to a quart glass jar, purged with nitrogen for 300 seconds, and then irradiated with 365 nm UV irradiation with an intensity of 0.3 mW / cm2until the solution reached a viscosity of about 500-3000 cp as measured by a Brookfield viscometer. Following this step, the additional components listed under part B were added to the solutions and mixed thoroughly. Table 3. Preparatory Coating Solutions for Core Composite Layers and Single Layers

[0096] Examples EX1-EX5 and Comparatives CE1-CE5

[0097] Coating solutions were combined and coated to provide layered adhesives. DMA test results on individual adhesive layers corresponding to S1-S4 and C1-C6 are provided in Table 4. Construction details for finished samples EX1-EX5 and CE1-CE5 are provided in Table 5. Haze, Peel Test, and DMA results for finished samples EX1-EX5 and CE1-CE5 are provided in Table 6.

[0098] For examples utilizing co-extruded coating methods (designated “ML” below), solutions were coated using a process similar to that described in European Patent No. 0 305 161 (Zimmerman et al.) and depicted in FIG. 3. A die with three cavities, each fed with a separate reservoir and pump, deposited the multilayer construction between siliconized polyethylene terephthalate (PET) release liners SKC RF02N and SKC RF12ASW (SKC Haas, KR). Pump rates were used to control target caliper for each layer. Constructions were cured down line using a UV irradiation dose of approximately 2,500 mJ / cm2at a wavelength of approximately 365 nm.

[0099] Layered constructions generated through a lamination process (designated “Lam” in Table 5) were made using the same equipment. However, each layer was coated and UV irradiated separately as opposed to the simultaneous three layer coating. Each individual layer was then stripped of its release liner and laminated together under hand roller pressure to form the three-layer composite. Single-layered adhesives are designated “SL” in Table 5 and represent comparative examples and were exposed to the same UV dose as used for the multilayer constructions. Also shown in Table 6 are failure modes for each sample. The label “AD” indicates adhesive failure between the glass substrate and the OCA composite, while the label “IL” indicates interlayer failure between the individual OCA layers of the laminated composite.

[0100] Table 4. Dynamic Mechanical Analysis of Layers S1-S4 and C1-C6 Table 5. Examples 1-5 and Comparative Examples 1-5 Construction Details

[0101] Table 6. Examples 1-9 and Comparative Examples 1-5 Adhesion and DMA Results

[0102] Haze Test Results

[0103] All examples and comparative examples demonstrated a degree of optical haze (<0.5% haze) that would be sufficiently low to characterize them as acceptable optical display adhesives.

[0104] Adhesion vs. Elasticity Test Results

[0105] 180 degree peel test results on glass, as shown in FIG. 4, demonstrate the benefits of using a co-extruded multilayer process to obtain both high adhesion and high elasticity as indicated by the lower Tan Delta value space. The Tan Delta parameter as measured using the Dynamic Mechanical Analysis Test Method described above is a ratio of the viscous to elastic modulus contributions within the elastomer at a given temperature and frequency. When studied at elevated temperatures for soft, low-Tgelastomers, this value can be highly correlated with the degree of network crosslinking and elasticity with more elastic materials exhibiting very low Tan Delta values at 70°C in particular.

[0106] CE3 is a single layer material that is sufficiently elastic as indicated by the very low Tan Delta value, but also displays low adhesion. CE4 and CE5 have high adhesion but also high Tan Delta values, indicating insufficient elasticity. CE1 and CE2 are multilayer samples with elastic cores and higher adhesion outer layers generated through the process of making each layer independently and then laminating the layers into a composite adhesive. These samples were found to have weak interfaces between the elastic core and viscoelastic skin layers and tended to fail with low adhesion values. EX1- EX5 are multilayer samples with sufficiently elastic cores and high adhesion skins generated through the co-extrusion process. These examples all fall above the desired line of adhesion while also possessing high elasticity, with a dotted line separating the Examples and Comparative Examples of this disclosure.

[0107] Creep Recovery Test Results

[0108] When subjected to dynamic fold testing, both EX4 and CE5 showed high adhesion and durability over 40,000 fold cycles with a radius of curvature of less than 4 mm. However, when the extent of creasing and crease recovery were studied under static folding tests, EX4 showed a significant improvement at multiple points in time with reduced crease slopes in the bending composite, indicative of reduced creasing in an actual display module. Table 7 shows crease slope values in the bending region of the composites over time for EX4 and CE5.

[0109] As evidenced by the static fold test data in Table 7, EX4 displays significantly greater crease recovery compared to CE5 across multiple points in time. This indicates again that the greater elasticity in EX4 compared with CE5 can reduce creasing in static folding tests.

[0110] Table 7. Crease slope recovery.

[0111] All cited references, patents, and patent applications in the above application for letters patent are herein incorporated by reference in their entirety in a consistent manner. In the event of inconsistencies or contradictions between portions of the incorporated references and this application, the information in the preceding description shall control. The preceding description, given in order to enable one of ordinary skill in the art to practice the claimed disclosure, is not to be construed as limiting the scope of the disclosure, which is defined by the claims and all equivalents thereto.

Claims

CLAIMS:What is claimed is:

1. A multilayer adhesive for bonding components within a foldable electronic device, comprising: an elastic layer having opposed first and second major surfaces; a first viscoelastic skin layer disposed on the first major surface; and a second viscoelastic skin layer disposed on the second major surface, wherein the elastic layer has a Tan Delta at 70°C of from 0.02 to 0.2 and each of the first and second viscoelastic skin layers independently has a Tan Delta at 70°C of from 0.2 to 0.55, with the Tan Delta of each of first and second viscoelastic skin layers being at least 125 percent that of the elastic layer.

2. The multilayer adhesive of claim 1, wherein the elastic layer has a Tan Delta at 70°C offrom 0.03 to 0.15.

3. The multilayer adhesive of claim 2, wherein the elastic layer has a Tan Delta at 70°C offrom 0.04 to 0.10.

4. The multilayer adhesive of any one of claims 1-3, wherein the elastic layer has a Storage Modulus (G’) at 25°C of from 5 kPa to 80 kPa.

5. The multilayer adhesive of claim 4, wherein the elastic layer has a Storage Modulus (G’) at 25°C of from 8 kPa to 45 kPa.

6. The multilayer adhesive of any one of claims 1-5, wherein each of the first and second viscoelastic skin layers independently has a Tan Delta at 70°C of from 0.25 to 0.45.

7. The multilayer adhesive of claim 6, wherein each of the first and second viscoelastic skin layers independently has a Tan Delta at 70°C of from 0.25 to 0.40.

8. The multilayer adhesive of any one of claims 1-7, wherein each of the first and second viscoelastic skin layers independently has a Storage Modulus at 25°C of from 10 kPa to 80 kPa.

9. The multilayer adhesive of claim 8, wherein each of the first and second viscoelastic skin layers independently has a Storage Modulus at 25°C of from 10 kPa to 50 kPa.

10. The multilayer adhesive of any one of claims 1-9, wherein each of the elastic layer and first and second viscoelastic skin layers is a reaction product of a mixture comprising one or more alkyl (meth)acrylates, optionally one or more copolymerizable polar monomers, one or more hydrophilic polymeric compounds, one or more hydrophilic hydroxyl-functional monomers, a crosslinker, and one or more primary initiators.

11. The multilayer adhesive of claim 10, wherein crosslinker has a weight average molecular weight of from 1000 g / mol to 100,000 g / mol.

12. The multilayer adhesive of claim 11, wherein the crosslinker has a weight average molecular weight of from 2000 g / mol to 60,000 g / mol.

13. The multilayer adhesive of claim 12, wherein the crosslinker has a weight average molecular weight of from 8000 g / mol to 40,000 g / mol.

14. The multilayer adhesive of any one of claims 11-13, wherein the crosslinker comprises a multifunctional urethane (meth)acrylate.

15. The multilayer adhesive of any one of claims 10-14, wherein each mixture comprises:40 percent to 95 percent by weight of the one or more alkyl (meth)acrylates;0 percent to 30 percent by weight of the one or more copolymerizable polar monomers;2 percent to 25 percent by weight of the one or more hydrophilic hydroxyl- functional monomers;0.05 percent to 15 percent by weight of the crosslinker; and0.1 percent to 2 percent by weight of the one or more primary initiators, in each case relative to the overall weight of the mixture.

16. The multilayer adhesive of any one of claims 1-15, wherein the elastic layer has a thickness of from 5 micrometers to 150 micrometers and each of the first and second viscoelastic skin layers has a thickness of from 5 micrometers to 150 micrometers.

17. An adhesive assembly comprising: the multilayer adhesive of any one of claims 1-16; and a pair of release liners in directly contact with respective outward-facing major surfaces of the first and second viscoelastic skin layers.

18. A method of making the multilayer adhesive of any one of claims 1-16, wherein the elastic layer and the first and second viscoelastic skin layers are cured by actinic radiation.

19. The method of claim 18, wherein the elastic layer and the first and second viscoelastic skin layers are co-extruded and integrated in molten form.

20. A foldable electronic device comprising: a display stack including a flexible display panel; and the multilayer adhesive of any one of claims 1-16, wherein the multilayer adhesive bonds the flexible display panel to a display cover layer whereby crease formation in a bending region of the flexible display panel is reduced duringrepeated folding and unfolding of the foldable electronic device relative to bonding with the elastic layer alone.

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