Patterned adhesive

Abstract

Provided is a patterned adhesive useful in foldable electronic display applications. The patterned adhesive includes a central adhesive layer and a pair of peripheral adhesive layers. The pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer in an alternating stripe pattern. Optionally, the ratio of shear modulus between central adhesive layer and each peripheral adhesive layer is from 2:1 to 10:1 or vice versa, the shear modulus measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C. Further, the central adhesive layer displays a refractive index within 0.015 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm, and each of the central adhesive layer and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

Description

[0001] PATTERNED ADHESIVE

[0002] Field of the Invention

[0003] Provided are patterned adhesive compositions and articles. The adhesive compositions and articles can be useful for bonding optical films in electronic display applications.

[0004] Background

[0005] Optically clear adhesives (OCAs) have diverse applications in consumer, industrial and automotive display technologies. These adhesives have become preferred bonding solutions for many reasons, including high adhesion and cohesion, protection, and enhanced performance of touchscreens as devices transitioned from resistive to capacitive touch technology. Desirable properties of OCAs generally include optical clarity and high interfacial adhesion over a wide range of temperatures.

[0006] Foldable touchscreen displays generally have multilayered configurations, with some layers providing protection from impacts and mechanical forces during bending. Some layers have more than one function — for example, such layers can include polarizers, structural films, thin glass, and metal layers. The properties of these functional films during repeated bending can lead to highly specialized mechanical property requirements unique to the display industry.

[0007] When a film is bent, the inner surface experiences compression while the outer surface is in tension. The location within the film where these forces are in balance — w here the strain is neutral — is known as the neutral plane. Typically, the neutral plane will be located at the midpoint of the film’s thickness, but the precise location or locations in a multilayer system can be controlled through the use of OCAs.

[0008] Summary

[0009] Certain layers of the display can withstand a limited range of bending strain prior to yielding or fracturing, such as the OLED encapsulation, transistor control plane, polarizer, or touch panel. OCAs with low modulus undergo large shear displacement during bending, rather than forcing the shear and bending stresses to the stiffer layers which the adhesives bond together. In this way, OCAs with low modulus can create multiple neutral planes in a laminated system, allowing the stiffer layers (e.g., functional films) to each bend more independently while the OCAs shear around them. Therefore, OCAs of appropriate stiffness are useful to adjust the location of neutral planes closest to these sensitive layers. Minimizing bending strain on the sensitive layers can improve the durability and reliability of the whole laminate (e.g., the electronic display) during folding. To meet these technical requirements, foldable OCAs tend to have more extreme property requirements than traditional requirements such as low haze and high adhesion. For example, the OCA should be highly elastic to avoid deformation. It must also demonstrate a low shear modulus. Particularly, it should mechanically decouple adherend layers from each other while bending over a wide temperature range, also suggesting the need for low glass transition temperature materials. The Tgof the adhesive defines the range where modulus, and therefore bending stiffness, increase steeply with cooling. Somewhat in opposition to the above, the adhesive must also retain high interfacial adhesion over a wide temperature range, even though this is often very challenging with low-Tgand low-modulus pressure sensitive adhesives. Further, stiffer OCAs can benefit aspects of folding performance by resisting buckling and cavitation. The challenge of neutral plane balance for folding thus relates to minimizing the difference in adhesive modulus over the operating temperature range, without decreasing modulus to the extent that buckling and cavitation can induce other failure modes.

[0010] These design constraints can be balanced against each other by using multi-material constructions. One such construction would be the patterning of differing materials or differing mechanical properties within one material to better meet the material challenges of a dynamic folding device. However, it is highly difficult to design and pair material sets that both have the complimentary mechanical properties required to achieve the desired behavior in the device, as well as to process and position the materials such that the spatial pattern is not visible to the eye upon viewing the device.

[0011] The provided adhesive articles use disparate adhesive compositions within a spatially patterned adhesive article to provide surprising technical benefits for foldable electronic display constructions. Having a patterned array of differing mechanical properties within a singular layer of adhesive may have utility in balancing the needed properties for a complex folding display element. In some instances, the degree of elasticity or stiffness / modulus of the material is manipulated through a patterned exposure curing step. In other instances, dissimilar materials are coated or processed together in such a way to yield the desired patterned effect. The provided adhesives underscore that it is not sufficient to only have a differing modulus in targeted regions of a foldable stack; rather it is important to also maintain a low Tgacross the materials of varying modulus to enable the device to exhibit good folding performance across a wide range of operating temperature.

[0012] The ability to manipulate the general modulus of a polymeric material with very little change in the Tg, particularly when the Tgregime is far below ambient temperature, is technically challenging. In addition, devising a scheme where the modulus of the patterned material sets may be systematically varied while also retaining a relatively consistent and matching refractive indices (in addition to Tg) can be even more difficult. The provided adhesive articles use material sets in which multiphasic and monophasic adhesive compositions are integrated, enabling a wide range of moduli while minimizing variation in other properties such as Tgand refractive index. Ultimately, this enables a high level of folding performance across a wide range of use temperatures while minimizing haze and optical defects. The deployment of these material sets in a patterned adhesive is unique and has great utility in solving some of the challenging design constraints of adhesives for foldable display constructions.

[0013] In a first aspect, a patterned adhesive is provided comprising: a central adhesive layer; a pair of peripheral adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer in an alternating stripe pattern; optionally, wherein the ratio of shear modulus between central adhesive layer and each peripheral adhesive lay er is from 2 : 1 to 10 : 1 or vice versa, the shear modulus measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C; wherein the central adhesive layer displays a refractive index within 0.015 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm; and further wherein each of the central adhesive layer and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

[0014] In a second aspect, a patterned adhesive is provided, comprising: a central adhesive layer; a pair of intermediate adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer; a pair of peripheral adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the intermediate adhesive layers in an ABCBA stripe pattern; optionally, wherein the ratio of shear modulus between central adhesive layer and each peripheral adhesive layer is from 2: 1 to 10: 1 or vice versa, the shear modulus measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C; wherein the central adhesive layer displays a refractive index within 0.015 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm; wherein the intermediate adhesive layers each display a refractive index that is between that of the adjacent central adhesive layer and peripheral adhesive layer; and further wherein each of the central adhesive layer, intermediate adhesive layers, and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

[0015] Brief Description of the Drawings

[0016] FIG. 1 is a plan view of an adhesive article according to one exemplary embodiment.

[0017] FIG. 2 is a perspective view of an assembly showing the article of FIG. 1 in a folded configuration.

[0018] FIGS. 3 and 4 are perspective views of assemblies showing respective adhesive articles in folded configurations, according to various embodiments. FIG. 5 is a schematic view of a process useful for making the articles of FIGS. 1-4.

[0019] FIG. 6 is a perspective view of an assembly showing an adhesive article according to another exemplary embodiment.

[0020] FIG. 7 is a side view of an assembly showing the article of FIG. 6 in a folded configuration.

[0021] FIG. 8 is a schematic view of a process useful for making the assembly of FIG. 7.

[0022] FIG. 9 is a side view of an assembly showing an adhesive article in both its folded and unfolded configurations.

[0023] FIG. 10 is an enlarged side view of the assembly of FIG. 9, identifying certain dimensions in relation to the adhesive layers therein.

[0024] FIGS. 11A-C show the equivalent strain and hydrostatic stress profiles for three different assemblies having identical geometries and differing only in their respective adhesive compositions.

[0025] 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 principles of the disclosure. The figures are not drawn to scale.

[0026] DEFINITIONS

[0027] As used herein:

[0028] “alkyl” refers to a monovalent group that is a radical of an alkane and includes straightchain, 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 orpolycyclic 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.

[0029] “allyl” refers to a functional group having the formula CH2=CH-CH2-.

[0030] "ambient conditions” means at a temperature of 23°C and a pressure of 103.4 kilopascals.

[0031] “ambient temperature” means 23 °C.

[0032] “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” may be used interchangeably. A cured or crosslinked polymer is generally characterized by insolubility but may be swellable in the presence of an appropriate solvent.

[0033] “curable” refers to a composition that can be cured. “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.

[0034] “(meth)acrylate group” 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-.

[0035] Detailed Description

[0036] As used herein, the terms “preferred” and “preferably” refer to embodiments described herein that can afford certain benefits, under certain circumstances. However, other embodiments may 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.

[0037] 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 may 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.

[0038] 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 may 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.

[0039] 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.

[0040] Layer constructions

[0041] An exemplary adhesive article incorporating the provided adhesive compositions is illustrated in FIG. 1 and hereafter denoted by the numeral 100. The article 100 is comprised of a composite layer 101 made up of three spatially separated layers in an alternating stripe pattern. The layers include a central adhesive layer 102 and a pair of peripheral adhesive layers 104, forming a center stripe and outer stripes, respectively. Each of the peripheral adhesive layers 104 extends alongside and directly contacts the central adhesive layer 102 along shared edges 106. The article 100 is optically transparent with a haze value of equal to or less than 0.5 % for a 50-micrometer thick adhesive.

[0042] In the depicted embodiment, the adhesive article 100 is intended for use in a foldable electronic display or other foldable assembly, and thus characterized by folding axis 108. Optionally and as shown, the central and peripheral adhesive layers 104, 102, 104 are symmetrically arranged about the folding axis 108.

[0043] In the provided striped configuration, shear modulus differs significantly between central adhesive layer and each peripheral adhesive layer. In some embodiments, the ratio of shear modulus between central adhesive layer and each peripheral adhesive layer is in the range of from 2: 1 to 10: 1, or alternatively, the ratio of shear modulus between each peripheral adhesive layer and the central adhesive layer is in the range of from 2:1 to 10: 1. As referenced above, shear modulus is measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C.

[0044] Preferably, the central adhesive layer displays a refractive index preferably within 0.015, more preferably within 0.010, and most preferably within 0.005 that of the peripheral adhesive layers, as measured for a light wavelength of 532 nm. The provided compositions further enable each of the central adhesive layer and peripheral adhesive layers to achieve the aforementioned differences in shear modulus with each of the layers displaying a low Tg. In various embodiments, the Tgof each of the central adhesive layer and peripheral adhesive layers can be at most -30°C, at most -35°C, or at most -40°C.

[0045] A surprising technical benefit achieved by this configuration is the ability to tailor the properties of the OCA in a foldable device to optimize the mechanical properties over both folded and unfolded regions of the electronic display without inducing observable visual defects. For example, a folded region can be provided with a multiphasic adhesive with a relatively low shear modulus having multiple neutral planes can be created along the thickness dimension, while unfolded regions can be provided with higher shear modulus with only a single neutral plane.

[0046] The multiple neutral planes can enable multiple layers of films within one region to bend almost independently, minimizing strain on mechanically sensitive layers. At the same time, the single neutral plane can benefit the non-bending regions of the display by resisting deformation in buckling, cavitation, and projectile impact.

[0047] The stripes represented by the central and peripheral adhesive layers 104, 102 can have any transverse width suitable for the application at hand. To accommodate the bending of the adhesive as the device is folded, the central adhesive layer 102 can have a transverse width that is from 2 mm to 100 mm, from 4 mm to 50 mm, from 6 mm to 30 mm, or in some embodiments, less than, equal to, or greater than 2 mm, 3, 4, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mm. The peripheral adhesive layers 104 are generally sized based on the overall dimensions of the display.

[0048] The relative sizes of the central and peripheral adhesive layers 104, 102 also need not be particularly limited. Relative to each other, the central adhesive layer 102 can have a transverse width that is from 1% to 99%, 10% to 50%, 10% to 20%, or in some embodiments, less than, equal to, or greater than 1%, 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 95, 97, or 99% of the overall composite layer 101, with the balance accounted for by the peripheral adhesive layers 104. As used above, the term “transverse width” represents stripe width and is thus measured in plan view perpendicular to the folding axis 108.

[0049] For display applications, it is generally desirable for the thicknesses of the adhesive layers 102, 104 to be consistent with each other such that the composite layer has a uniform thickness across its area. In various embodiments, these layers can have a thickness of from 5 micrometers to 200 micrometers, from 15 micrometers to 150 micrometers, from 25 micrometers to 75 micrometers, or in some embodiments, less than, equal to, or greater than 5 micrometers, 10, 15, 20, 25, 30, 35, 40, 45, 50, 75, 100, 150, or 200 micrometers.

[0050] FIG. 2 shows a fragmentary view of the adhesive article 100 with the composite layer 101 partially peeled and folded back on itself at an angle of 180°. In this illustration, the composite layer 101 is part of an assembly 150 in which the composite layer 101 is disposed between upper and lower release liners 110, 112. The upper and lower release liners 110, 112 can directly contact the major surfaces of the composite layer 101 to facilitate handling and storage of the adhesive.

[0051] FIG. 3 shows an assembly 250 comprised of an adhesive article 200. The adhesive article 200 includes two layers, a composite layer 201 and a monolithic layer 203, that are laminated to each other. The composite layer 201 has a configuration analogous to that of the composite layer 101, being comprised of a central adhesive layer 202 flanked by peripheral adhesive layers 204. The monolithic layer 203 is coextensive with the composite layer 201 and has a uniform composition across the layer. In a preferred embodiment, the composition of the monolithic layer is equivalent to that of the peripheral adhesive layers 204, such that the composition does not change along the thickness dimension along the areas corresponding to the peripheral adhesive layers 204.

[0052] As shown in the figure, an upper release liner 210 remains along the top major surface of the adhesive layer 200, but the lower release liner has been removed and the adhesive article 200 has been adhesively bonded to a substrate 214. The substrate 214 can be any of a number of suitable adherends, such as a glass layer or functional fdm. Useful functional films can be made from polyethylene terephthalate, polyimide, cycloolefin polymer (COP), a multilayer optical film (MOF), or a polarizer film.

[0053] FIG. 4 shows an assembly 350 that is a variant of that provided in FIG. 3, except the order of composite layer 301 and monolithic layer 303 within adhesive article 300 is reversed. While the composite layer 201 contacts substrate 214 in assembly 250, the monolithic layer 303 contacts substrate 314 in the assembly 350. Other aspects of the assembly 350 are analogous to those already described and need not be repeated. In these embodiments, the monolithic carrier layer can increase the stability of the composite coating of dissimilar formulations or fluids, particularly in the region between the die exit and web.

[0054] FIG. 5 illustrates an exemplary apparatus 160 capable of making a composite layer, as described above, using a continuous process. In this process, polymer coating solutions 170, 172 corresponding to the central adhesive layer 102 and peripheral adhesive layers 104, respectively, are directed into a feedblock and then expelled through an extrusion die 162 and onto a moving release liner 112 threaded around a casting roll 166, as shown. The coated web is then conveyed to a curing station 168, which exposes the coated web to ultraviolet light or other actinic radiation to functionally cure the polymer coating solutions. As previously described, the polymer coating solutions may themselves include components that are partially or fully cured to adjust viscosity and facilitate casting of a uniform film onto the release liner 112.

[0055] FIG. 6 shows a variant of the prior assemblies. In this figure, an assembly 450 in which the adhesive layer is a composite layer 400 is disposed on a release liner 412. While the composite layer 400 has a striped pattern with a central layer 402 located between a pair of peripheral layers 404, it further includes an additional pair of stripes. These additional stripes are represented by intermediate layers 420 located between the central and peripheral layers 402, 404, as illustrated, to provide an ABCBA stripe pattern. It can be beneficial for the intermediate layers 420 to have a blended composition obtained by mixing the compositions used to make the central and peripheral layers 402, 404 prior to curing.

[0056] It is expected that some degree of mixing of coating compositions will occur at the stripe boundaries following casting and prior to curing. Nonetheless, where even slight differences of refractive index could be visually perceptible, inclusion of the intermediate layers 420 can be advantageous in providing a smoother transition in optical and / or mechanical properties between the central and peripheral layers 402, 404.

[0057] Alternative embodiments are also possible. These embodiments can use adhesive material sets that take advantage of a reinforcing adhesive composition in the transition zone represented by the intermediate layers 420. Surprisingly, it was discovered that using higher modulus, reinforcing adhesives in this transition region while using center and outer regions comprised of relatively lower modulus adhesives provides an enhanced decoupling effect between layers along areas where it is most needed, while still reinforcing against buckling or deflection in the direction normal to the composite thickness. This technical effect shall be exemplified later in forthcoming sections referencing FIGS. 11A-C.

[0058] FIG. 7 shows an assembly 550 in which the adhesive article represented by composite layer 400 is sandwiched between release liners 410, 412 and folded upon itself at a folding angle of 180°. As shown, the folded region resides entirely within the central layer 402, with both the intermediate layers 420 and peripheral layers 404 generally extending along planar regions of the assembly 550 while in its folded configuration.

[0059] FIG. 8 shows an apparatus and process 560 for making the assembly 550. In an exemplary method, polymer coating solutions are provided in reservoirs 570, 572, 574 corresponding to the central layer 402, peripheral layers 404, and intermediate layers 420, respectively. As shown, these polymer coating solutions are then conveyed using pumps 575 into a feedblock and die 562 that divides the flows into a pre-determined stripe pattern. The die 562 dispenses the solutions onto a release liner 564 backed by cast roll 566 as shown to provide coated web 567. Optionally and as shown, a top release liner 510 can be applied on the exposed top surface of the coated web 567 using nip roll 576 prior to the coated web 567 being cured through the upper release liner 510 by actinic radiation as it passes under curing station 568. The pressure applied by the nip roll 576 against the cast roll 566 can be adjusted as appropriate to obtain a desired caliper for the adhesive article.

[0060] FIG. 9 shows an adhesive assembly 950 in both folded and unfolded configurations. As shown, the assembly 950 is comprised of an adhesive article 900 bonded to a pair of fixed plates 914. The fixed plates 914 are bonded along end regions of the adhesive article 900, while unbonded along a central region. In its folded configuration, the fold region of the assembly 950 has a teardrop shape, characterized by a half-gap thickness “d.” It was discovered that the bond interface between the adhesive and adherend experiences the greatest degree of hydrostatic tension near the inflection point of the assembly 950, defined as the point where the layers transition from being concave to convex. Further, using an adhesive having a comparatively higher modulus along these regions (relative to those of the central and outer regions) was found to provide a reinforcement effect to enable enhanced resistance to cavitation and bubbling defects through repeated folding and unfolding of the assembly 950.

[0061] FIG. 10 shows a fragmentary view of the assembly 950 defining three spatial regions along the assembly 950 — “A,” “B,” and “C,” corresponding to a central region, intermediate region, and outer region, respectively. The assembly 950 is comprised of an adhesive article 900 bonded to a fixed plate 914, which extends along only a part of the length of the article 900. In this figure, “W” represents the lateral dimension of the intermediate adhesive layer and “d” represents the degree of overlap between the intermediate adhesive layer and the fixed plate.

[0062] In a preferred embodiment, each of the central adhesive layer and peripheral adhesive layers displays a shear modulus of from 10 kPa to 100 kPa, or in some embodiments, less than, equal to, or greater than 10 kPa, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 kPa as measured by dynamic mechanical analysis at ambient conditions and 1 Hz.

[0063] In preferred embodiments, each intermediate adhesive layer can display a shear modulus that independently exceeds that of each of the central adhesive layer and peripheral adhesive layers by a factor of from 2 to 500, from 5 to 100, from 10 to 50, or in some embodiments, less than, equal to, or greater than 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 150, 170, 200, 250, 300, 350, 400, 450, or 500, as measured by dynamic mechanical analysis at ambient conditions and 1 Hz.

[0064] When bent, the composite adhesive assemblies display a neutral plane which is the location along the thickness where axial strain is zero. At this location the shear strain is typically at its maximum. When low-modulus layers are added, additional neutral planes can form when the structure is bent that partially decouple the regions above and below the low-modulus layers. Advantageously, this allows for shear decoupling between layers.

[0065] FIGS. 11A-C show three columns of figures representing three respective adhesive assemblies. Each assembly is comprised of four adhesive layers laminated to five adherend layers in alternating fashion. The three assemblies differ in the composition of the adhesive layers but are otherwise identical. Each column shows three vertically-arranged figures, where the top figure shows equivalent strain profile along the layers, the middle figure shows hydrostatic stress profile along the layers, and the bottom figure shows bending strain plotted as a function of distance from the bottom major surface of the adhesive assembly.

[0066] The equivalent strain profile, hydrostatic stress profile, and bending strain were calculated using a two-dimensional plane finite element analysis (FEA) model simulating a folded panel configuration. Adhesive modulus was fitted to a finite strain viscoelastic model, a model that captures both viscoelastic (relaxation) and hyperelastic (nonlinear large strain) characteristics. A reduced polynomial hyperelastic function was used to represent the instantaneous modulus function that varies with strain. As defined in FIG. 10, W is 5 millimeters and d is 1 millimeter in this FEA model.

[0067] FIG. 11A shows an adhesive assembly having a uniform adhesive composition across the entire major surface of each adhesive layer (i.e., a non-pattemed adhesive), the adhesive having a given baseline modulus. FIG. 11B shows an adhesive assembly including an intermediate adhesive along region B (region 916) that is 5 millimeters wide and a modulus that is 10 times higher (i.e., stiffer) than the baseline modulus of the adjacent central and outer adhesive layers along regions A and C, respectively. FIG. 11C shows an adhesive assembly in which a monolithic adhesive extending along regions A and B (collectively, region 918) has a modulus greater by a factor of 10 than the baseline modulus of the outer adhesive along region C.

[0068] As revealed in the profiles of FIGS. 11A-C, the degree of strain and hydrostatic stress is far greater in the transition zone for the monolithic configuration (FIG. 11 A) and the configuration having the stiffened transition zone (FIG. 1 IB) than for the configuration having the stiffened central and transition zones (FIG. 11C). The superior performance afforded by the former two adhesives is also evident in the bending strain plots, which show a less negative bending strain for these compositions. Differences are especially significant in areas remote from the bottom surface of the assembly (e.g., region 920). These data profiles evince a strong decoupling effect between layers in the configurations of FIGS. 11 A and 11B. A significant benefit of the configuration of FIG. 11B over that of FIG. 11 A is the enhanced modulus in the transition zone, which can enable a stronger adhesive bond.

[0069] Adhesive compositions

[0070] The central and peripheral adhesive layers can be made from any number of various adhesive compositions known in the art.

[0071] In some embodiments, the central adhesive layer has a multiphasic composition, while each of the peripheral adhesive layers has a monophasic composition. Alternatively, the peripheral adhesive layers each has a multiphasic composition, while the central adhesive layer has a monophasic composition. The multiphasic and monophasic compositions can generally be derived from acrylic polymers with an optional alternative polymer additive to serve as a secondary phase.

[0072] Useful multiphasic compositions include dual-phase compositions. The dual-phase composition can have a majority phase and minority phase, where the minority phase has a size scale in the micrometer range. For instance, the minority phase can reside in discrete domains that are uniformly dispersed within a continuous majority phase. The discrete domains can have a median transverse dimension in the range of from 0.1 micrometers to 25 micrometers, from 0.5 micrometers to 10 micrometers, from 1 micrometers to 5 micrometers, or in some embodiments, less than, equal to, or greater than 0.1 micrometers, 0.2, 0.3, 0.4, 0.5, 0.7, 1, 2, 5, 10, 15, 20, or 25 micrometers. While less common, the minority phase could also be bicontinuous with the majority phase.

[0073] The minority phase can represent from 0.5% to 40%, from 1% to 30%, from 2% to 15%, or in some embodiments, less than, equal to, or greater than 0.5%, 1, 2, 5, 10, 15, 20, 25, or 30% by weight relative to the overall weight of the dual-phase composition. During a folding or unfolding event, it is expected that the composite layer will undergo significant deformation and cause stresses. The resistance forces to these stresses will be in part determined by the modulus and thickness of the layers of the folding display, including the composite layer.

[0074] To ensure a low resistance to folding as well as adequate performance, generation of minimal stress and good dissipation of the stresses involved in a bending event, the composite layer has a sufficiently low storage or elastic modulus, often characterized as shear storage modulus (G’). To further ensure that this behavior remains consistent over the expected use temperature range of such devices, there is minimal change in G’ over a broad and relevant temperature range. In one embodiment, the relevant temperature range is between about -30°C to about 90°C. In one embodiment, the shear modulus is less than about 2 MPa, particularly less than about 1 MPa, more particularly less than about 0.5 MPa over the relevant temperature range. Therefore, it is preferred to position the Tg, the temperature at which the material transitions to a glassy state, with a corresponding change in G’ to a value typically greater than about 107 Pa, outside and below this relevant operating range. In one embodiment, the Tgof the composite layer in a flexible display is less than about -20°C, particularly less than about -30°C, and more particularly less than about - 35°C.

[0075] The composite layer in one embodiment also exhibits relatively low creep to avoid lasting deformations in the multilayer composite of a display following repeated folding or bending events. Material creep may be measured through a simple creep experiment in which a constant shear stress is applied to a material for a given amount of time. Once the stress is removed, the recovery of the induced strain is observed. In one embodiment, the shear strain recovery within 1 minute after removing the applied stress at room temperature is at least about 50%, particularly at least about 60%, about 70% and about 80%, and more particularly at least about 90% of the peak strain observed at the application of the shear stress. The test is normally performed at room temperature but could also be performed at any temperature relevant to the use of the flexible device.

[0076] Multiphasic adhesive compositions

[0077] In some embodiments, the multiphasic composition is based on a crosslinked network of polyvinyl acetal and acrylic copolymer obtained by reacting a functionalized polyvinyl acetal and an acrylic monomer. In some embodiments, the functionalized polyvinyl acetal can be reacted with a mixture of two or more different acrylic monomers.

[0078] Polyvinyl acetal is a useful class of polymers derived from the condensation reaction between polyvinyl alcohol and an aldehyde, typically formaldehyde. The resulting material exhibits excellent chemical resistance, good mechanical properties, and exceptional thermal stability, making it applicable for different technical purposes.

[0079] Polyvinyl acetal is known for use in production of various engineering plastics. Its excellent thermal stability allows it to withstand high temperatures without significant degradation, making it suitable for applications where heat resistance is essential. Additionally, it exhibits good mechanical strength and dimensional stability, making it valuable in the production of components and parts for various industries, including automotive, electrical, and consumer goods.

[0080] The polyvinyl acetal can be prepared by saponifying polyvinyl acetate to prepare polyvinyl alcohol and then acetalizing the polyvinyl alcohol with an aldehyde in the presence of a catalyst. The degree of saponification of the polyvinyl alcohol is not particularly limited and is commonly within a range of 70 to 99.9 mol%. The degree of saponification is preferably 70 to 99.9 mol%, and more preferably 80 to 99.8 mol%.

[0081] For acetalization of the polyvinyl alcohol with an aldehyde in the presence of a catalyst, a solution containing the polyvinyl alcohol may be used. An exemplary solvent used for the solution containing the polyvinyl alcohol is water.

[0082] The aldehyde is not particularly limited. Commonly, a C1-C10 aldehyde is favorably used. The Cl -CIO aldehyde is not particularly limited and may be either a linear aldehyde or a branched aldehyde. Examples thereof can include n-butyraldehyde, isobutyraldehyde, n-valeraldehyde, 2- ethylbutyraldehyde, n-hexylaldehyde, n-octylaldehyde, n-nonylaldehyde, n-decylaldehyde, formaldehyde, acetaldehyde, and benzaldehyde. Aldehydes may be used alone or in combination of two or more thereof. Preferred among these are n-butyraldehyde, n-hexylaldehyde, and n- valeraldehyde, and more preferred is n-butyraldehyde.

[0083] In some embodiments, the polyvinyl acetal is comprised of polyvinyl butyral (where the aldehyde is n-butyraldehyde, the polyvinyl acetal is referred to as polyvinyl butyral). The use of the polyvinyl butyral allows appropriate adhesion force to glass, leading to better light resistance and weather resistance. Two or more types of polyvinyl acetals may be optionally used in combination.

[0084] In some embodiments, the polyvinyl acetal can be functionalized by reacting the polyvinyl acetal with isocyanatoethyl (meth)acrylate or allyl isocyanate in the presence of a suitable catalyst and an acrylic monomer. This reaction provides a polyvinyl acetal backbone with pendent (meth)acrylate or allyl groups. These pendent functional groups, in turn, are copolymerizable with any number of other acrylic monomers. The isocyanatoethyl (meth)acrylate or allyl isocyanate can be present in any wt% based on the total weight of the polyvinyl acetal as appropriate to create the desired degree of functionality. For example, the isocyanatoethyl (meth)acrylate or allyl isocyanate can be present in an amount of from 0.1% to 15%, from 0.3% to 13%, from 0.5% to 10%, or in some embodiments, less than, equal to, or greater than 0.1%, 0.3, 0.5, 1, 3, 6, 10, 13, or 15% by weight, relative to that of the polyvinyl acetal.

[0085] If the degree of reactive functionalization of the polyvinyl acetal is too great, the multiphasic adhesive may become highly crosslinked leading to a loss of adhesion performance. Adhesive compositions in which the polyvinyl acetal is too low, however, can decrease compatibility between the acrylic and PVA polymers which may create unwanted haze. Additional hydroxyl reactive compounds that also contain co-polymerizable or crosslinkable functional groups may be used. Pendent crosslinkable functionality of a sufficient degree can enable the functionalized polyvinyl acetal to function as a crosslinking agent to form a crosslinked network even when reacted with acrylic monomers that are monofunctional.

[0086] Once functionalized, the polyvinyl acetal can then be reacted with one or more acrylic monomers to form a crosslinked network comprised of the functionalized polyvinyl acetal and acrylic copolymer. The acrylic copolymer is generally a random copolymer, but suitable copolymers can also include acrylic block copolymers and tapered block copolymers.

[0087] Useful acrylic monomers include both alkyl and polar monomers. Such polar monomers are inclusive of acid-functional monomers, hydroxy-functional monomers, nitrogen-containing monomers, and combinations thereof. Hydroxy-functional monomers include, for example, 2-hydroxyethyl (meth)acrylate, and 2-hydroxy-propyl (meth)acrylate, 4-hydroxybutyl(meth)acrylate. Advantageously, polar monomers can help compatibilize the polyvinyl acetal (e.g., butyral) polymer with high glass transition temperature (high-Tg) and low glass transition temperature (low-Tg) alkyl (meth)acrylate monomers to produce a multiphasic adhesive with relatively lower haze. In some embodiments, these polar monomers can have a Tggreater than 0°C, while the Tgcan be less than that of the high-Tgmonofunctional alkyl (meth)acrylate monomer. In a preferred embodiment, the multiphasic adhesive is derived from precursors comprising from 40 wt% to 90 wt% of alkyl (meth)acrylate monomers having a homopolymer Tgof less than 0°C. When characterized using DMA, the multiphasic adhesive preferably displays at least one Tgless than 10°C.

[0088] Acid-functional monomers include, but are not limited to, those selected from ethylenically unsaturated carboxylic acids, ethylenically unsaturated sulfonic acids, ethylenically unsaturated phosphonic acids, and mixtures thereof. Examples of such compounds include those selected from acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, citraconic acid, maleic acid, oleic acid, P-carboxyethyl (meth)acrylate, 2-sulfoethyl methacrylate, styrene sulfonic acid, 2- acrylamido-2 -methylpropanesulfonic acid, vinylphosphonic acid, and mixtures thereof.

[0089] Nitrogen-containing monomers include, but are not limited to, N-vinylpyrrolidone; N- vinylcaprolactam; acrylamide; mono- or di-N-alkyl substituted acrylamide such as N, N- dimethylacrylamide; t-butyl acrylamide; dimethylaminoethyl acrylamide; acryloylmorpholine, and N-octyl acrylamide. In some embodiments, the crosslinked network of polyvinyl acetal and acrylic copolymer comprises from 0% to 20%, 0.5% to 10%, from 1% to 5%, or in some embodiments equal to or greater than 0%, or less than, equal to, or greater than 0.1, 0.5, 1, 2, 3, 4, 5, 7, 10, 12, 15, or 20% polymerized units of nitrogen-containing monomers by weight, relative to the overall weight of the multiphasic adhesive composition.

[0090] Other polar monomers include alkoxy -functional (meth)acrylate monomers. Representative examples 2-(2-ethoxyethoxy) ethyl (meth)acrylate, 2-ethoxyethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-(methoxyethoxy) ethyl, 2-methoxyethyl methacrylate, and polyethylene glycol mono(meth)acrylates.

[0091] In some embodiments, the crosslinked network of polyvinyl acetal and acrylic copolymer includes from 0.5% to 45%, from 1% to 30%, or in some embodiments, less than, equal to, or greater than 0.5, 1, 2, 3, 4, 5, 7, 10, 12, 15, 17, 20, 25, 30, 35, 40 and 45% polymerized units of alkoxyfunctional (meth)acrylate monomers by weight, relative to the overall weight of the crosslinked network. In other embodiments, the crosslinked network comprises polymerized units of less than 20%, less than 15%, or less than 10% of alkoxy -functional (meth)acrylate monomers by weight or is free of polymerized units of alkoxy-functional (meth)acrylate monomers. When the polyvinyl acetal (e.g., butyral) polymer displays a lower crosslinkable functionality, then higher amounts of polar monomer may be necessary to provide a compatibilized mixture. Conversely, when the polyvinyl acetal possesses a relatively higher degree of functionality, then less polar monomer may be needed for compatibilization.

[0092] In certain embodiments, the crosslinked network includes polymerized units of one or more polar monomers of 2-hydroxyethyl (meth)acrylate or N,N-dimethyl acrylamide. The multiphasic adhesive can include polymerized units of one or more of these monomers in an amount of from 5% to 55%, from 7% to 45%, from 10% to 35%, or in some embodiments, less than, equal to, or greater than 5%, 7, 10, 15, 20, 25, 30, 35, 40, 45, 50, or 55% relative to the overall weight of the multiphasic adhesive composition.

[0093] In some embodiments, the crosslinked network includes polymerized units of one or more low-Tg (meth)acrylate monomers, i.e., a (meth)acrylate monomer that, when reacted to form a homopolymer, has a Tgno greater than 0°C. In some embodiments, the low-Tgmonomer has a homopolymer Tgof from -80°C to -5°C, from -70°C to -10°C, or in some embodiments, less than, equal to, or greater than -80°C, -70, -60, -50, -40, -30, -20, -10, or 5°C. The low-Tg monomer can, for example, have the formula:

[0094] H2C=CR1C(O)OR8

[0095] , wherein R1is H or methyl and R8is an alkyl with 1 to 22 carbons or a heteroalkyl with 2 to 20 carbons and 1 to 6 heteroatoms selected from oxygen or sulfur. The alkyl or heteroalkyl group can be linear, branched, cyclic, or a combination thereof.

[0096] Examples of low-Tgmonomers include but are not limited to ethyl acrylate, n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-pentyl acrylate, isoamyl acrylate, n- hexyl acrylate, 2-methylbutyl acrylate, 2-ethylhexyl acrylate, 4-methyl-2-pentyl acrylate, n-octyl acrylate, 2-octyl acrylate, isooctyl acrylate, isononyl acrylate, decyl acrylate, isodecyl acrylate, lauryl acrylate, isotridecyl acrylate, octadecyl acrylate, and dodecyl acrylate.

[0097] In some embodiments, the crosslinked network comprises polymerized units of at least one low-Tg monomer(s) having an alkyl group with 6 to 24 carbon atoms. In some embodiments, the low-Tg monomer has an alkyl group with 7 or 8 carbon atoms. Exemplary monomers include, but are not limited to, 2-ethylhexyl (meth)acrylate, isooctyl (meth)acrylate, n-octyl (meth)acrylate, 2- octyl (meth)acrylate, isodecyl (meth)acrylate, and lauryl (meth)acrylate. In some embodiments, the monomer is an ester of (meth)acrylic acid with an alcohol derived from a renewable source, such as 2-octyl (meth)acrylate. Other suitable monomers include branched long chain acrylates such as those described in U.S. Patent No. 8,137,807 (Clapper et al.). Additional suitable alkyl monomers include secondary alkyl acrylates such as those described in U.S. Patent No. 9,102,774 (Clapper et al.).

[0098] The multiphasic adhesive can comprise at least 20%, 30, 40, 50, 60, 70 or 80% by weight of polymerized units of monofunctional alkyl (meth)acrylate low-Tgmonomer (i.e., having a homopolymer Tgof less than 0°C), based on the total weight of the multiphasic adhesive. The multiphasic adhesive typically comprises no greater than 60%, 70, 80 or 90% by weight of polymerized units of monofunctional alkyl (meth)acrylate monomer having a Tgof less than 0°C, based on the total weight of the multiphasic adhesive composition.

[0099] The Tgof the homopolymer of various monomers is known and is reported in various handbooks. The Tgof some illustrative monomers is also reported in International Patent Application No. WO2016 / 094277 (Janoski et al.).

[0100] The polyvinyl acetal can be present in an amount of from 85.0 wt% to 99.9 wt%, from 87.0 wt% to 99.7 wt%, from 90.0 wt% to 99.5 wt%, or in some embodiments less than, equal to, or greater than 85.0 wt%, 87.0, 90.0, 91.0, 92.0, 94.0, 96.0, 97.0, 98.0, 99.0, 99.5, 99.7 or 99.9 wt% relative to the overall weight of the functionalized polyvinyl acetal. Relative to the overall weight of the multiphasic adhesive, the functionalized polyvinyl acetal can represent a weight fraction of from 1% to 20%, from 2% to 15%, from 3% to 12%, or in some embodiments, less than, equal to, or greater than 1%, 2, 3, 4, 5, 8, 10, 12, 15 or 20%.

[0101] The acrylic copolymer can be present in a weight fraction of from 60% to 99%, from 70% to 98%, from 80% to 97%, or in some embodiments, less than, equal to, or greater than 60%, 70, 80, 85, 90, 93, 95, 97, 98, or 99%, relative to the overall weight of the multiphasic adhesive.

[0102] The functionalized polyvinyl acetal and acrylic monomers can be combined with any number of additional additives, including catalysts, crosslinkers, ultraviolet light absorbers, dyes and pigments.

[0103] Often for these blended systems, two or more Tgvalues (as defined by peak tan delta values observed when performing a DMA temperature sweep at 1 Hz) can be detectable for respective phases or domains within the multiphasic adhesive. The lowest Tgwill typically be associated with the acrylic copolymer phase of the crosslinked network.

[0104] Other aspects of the multiphasic adhesive are described in co-pending patent application, filed under U.S. Patent Application No. 63 / 531,032 (Beagi et al.).

[0105] Monophasic adhesive compositions

[0106] The monophasic adhesive compositions can be obtained by directly polymerizing any number of select acrylic monomer mixtures. These compositions can be crosslinked at different levels to provide a range of elastic properties, while still generally meeting requirements for optical clarity. In one embodiment, the monophasic adhesive compositions are derived from reactive precursors including at least one alkyl(meth)acrylate ester having between about 1 to about 24 carbon atoms in the alkyl group and a free-radical generating initiator.

[0107] Examples of suitable alkyl acrylates (i.e., acrylic acid alkyl ester monomers) include, but are not limited to, linear or branched monofunctional acrylates or methacrylates of non-tertiary alkyl alcohols, where the alkyl groups have from 1 to 24 carbon atoms. Examples of suitable monomers include, but are not limited to: 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (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), isostearylacrylate 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 (Clapper et al.). Additional suitable alkyl monomers include secondary alkyl acrylates such as those described in U.S. Patent No. 9,102,774 (Clapper et al.).

[0108] In the interest of adjusting the optical properties of the monophasic adhesive composition relative to that of the multiphasic adhesive composition, higher refractive index (RI) monomers can be added to the acrylic monomer mixtures. For example, biphenylmethyl acrylate is a relatively high-RI can be added to raise the overall RI of the monophasic adhesive composition. Advantageously, various monomers can be added in small amounts to help in matching the RI between the multiphasic and monophasic adhesive compositions and yet preserve the low shear modulus and low-Tgproperties of the monophasic adhesive.

[0109] In some embodiments, the monomer composition includes only alkyl (methjacrylate monomers with optional vinyl ester or styrenic monomers. In such cases, the modulus and Tgof the composition can be adjusted by selecting combinations of low and high Tgyielding monomers. In another embodiment, the monomer composition includes between about 60 to about 99 parts by weight of the alkyl(meth)acrylate ester having between about 1 to about 24 carbon atoms in the alkyl group, particularly between about 65 and about 95 parts, and more particularly between about 70 and about 95 parts.

[0110] 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: (methjacrylamide, N-morpholino (methjacrylate, N-vinyl pyrolidone and N-vinyl caprolactam. Other suitable polar monomers may include hydroxyl containing monomers such as 2 -hydroxy ethyl (methjacrylate, and 2-hydroxy-propyl (methjacrylate, 4-hydroxybutyl(meth)acrylate, and ether containing monomers such as 2-ethoxyethoxyethyl (methjacrylate, and 2- methoxyethoxyethyl(meth)acrylate, and 2-ethylhexyl ethoxylated acrylate (e.g., M1086 Miwon Specialty Chemicals, KR). In some embodiments, the monomer composition 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.

[0111] The monomer composition of the monomer composition can also include a vinyl ester, such as a Cl to CIO vinyl ester. An example of commercially available suitable vinyl esters include, but are not limited to: vinyl acetate, VeOVA 9 or VeOVA 10, available from Momentive Specialty Chemicals, New Smyrna Beach, FL. The vinyl ester can be added to the monomer mixtures in an amount of between about 1 part and about 20 parts by weight, particularly between about land about 15 parts, and more particularly between about 1 and about 10 parts. Other monomers, such as styrenic monomers, may also be used.

[0112] Examples of free-radical generating initiators include, but are not limited to, thermal or photoinitiators. Examples of thermal initiators include, but are not limited to, peroxides such as benzoyl peroxide and its derivatives or azo compounds. An example of a commercially available azo compound includes VAZO 67, available from E. I. du Pont de Nemours and Co. located in Wilmington, Del., which is 2,2'-azobis-(2-methylbutyronitrile). A variety of peroxide or azo compounds are available that can be used to initiate thermal polymerization at a wide variety of temperatures. A photoinitiator may also be used, either replacing the thermal initiator or used in combination with the thermal initiator. Particularly useful photoinitiator includes IRGACURE 651 and DAROCUR 1173, both available from BASF, located in Tarrytown, NY. The initiators are typically added to the precursor mixtures in an amount of between about 0.01 parts to about 2 parts by weight, particularly between about 0.02 and about 1 part, and more particularly between about 0.02 and about 0.5 parts.

[0113] 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 DESMODUR N3300 from Bayer, located in Cologne, Germany. Ultraviolet, or “UV” activated crosslinkers can also be used to crosslink the precursors of the monophasic adhesive composition. 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 5 parts by weight based, particularly between about 0.01 and about 4 parts, and more particularly between about 0.01 and about 3 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 macromere or polystyrene macromer, may also be used. Macromers may be used in an amount of about 1 to about 20 parts by weight of the total monomer components.

[0114] The monomer composition may be inherently tacky. If desired, tackifiers can be added to the precursor mixture before formation of the monomer composition. Useful tackifiers include, for example: rosin ester resins, aromatic hydrocarbon resins, aliphatic hydrocarbon resins, terpene, 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 part 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.

[0115] Other materials can be added to the monomer mixture for special purposes, including, for example: molecular weight control agents, coupling agent, oils, plasticizers, antioxidants, UV stabilizers, UV absorbers, pigments, curing agents, polymer additives, nanoparticles, and other additives. In cases where the monomer composition needs to be optically clear, other materials can be added to the monomer mixture, provided that they do not significantly reduce the optical clarity of the monophasic adhesive composition after polymerization and coating.

[0116] The monomeric components can be blended into a precursor mixture, which can then be pre-polymerized by exposure to heat or actinic radiation (to decompose initiators in the mixture). This can be done prior to the addition of a crosslinker and other components to form a coatable syrup to which, subsequently, one or more crosslinkers, other additives, and additional initiators can be added. The compounded syrup is then coated on a liner or directly on a substrate and completely polymerized under inert atmosphere by additional exposure to UV. Alternatively, the crosslinker, optional additives, and initiators can be added to monomers and the mixture can be both polymerized and cured in one step (for example, as a liquid OCA). The desired coating method and viscosity will determine which procedure is used.

[0117] In another process, the monomeric components can be blended with a solvent to form a mixture. The mixture can be polymerized by exposure to heat or actinic radiation (to decompose initiators in the mixture). A crosslinker and additional additives such as tackifiers and plasticizers may be added to the solvated polymer which may then be coated on a liner and run through an oven to dry off solvent to yield the coated adhesive film. Solventless polymerization methods, 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.) can also be used.

[0118] Further options and advantages associated with these adhesive compositions are described in U.S. Patent No. 10,640,689 (Behling et al.). EXAMPLES

[0119] Objects and advantages of this disclosure are further illustrated by the following nonlimiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure. Unless otherwise noted, all parts, percentages, and ratios in the Examples and the rest of the specification are by weight.

[0120] Materials

[0121] Table 1 lists components used in preparing the Examples and Comparatives described herein.

[0122] Table 1.

[0123] Test methods

[0124] Haze Test

[0125] Haze measurements were made using a HunterLab (Reston, VA) UltrascanPro Spectrophotometer in transmission mode. 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. The aperture of the instrument is approximately 30 mm in diameter. For an approximate measure of haze of the striped adhesive, the stripe was placed in such a way that it was centered in front of the aperture of the HunterLab instmment.

[0126] Dynamic Mechanical Analysis (DMA)

[0127] Dynamic mechanical analysis was used to probe the modulus as a function of temperature as well as to determine the Tgof the material. 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 -50°C to 150°C at 3°C / minute. During this ramp, the sample was 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 peaks in the tan delta vs. temperature profile.

[0128] Refractive Index Analysis

[0129] Refractive index was measured using a Metricon 2010 prism coupler equipped with a 532nm laser. Measurement is accomplished with the adhesive applied directly to the prism. All refractive index measurements were conducted at room temperature.

[0130] Peel Test:

[0131] Peel tests were conducted on the air side of boro-float glass. Prior to testing, the glass surface was cleaned with an isopropanol-moistened wipe, then a dry wipe. One release liner was removed from the OCA sample which was then manually laminated to a 2-mil-thick primed PET backing film (Skyrol SH81, SKC, South Korea), with the primer facing the OCA. Then the release liner on the opposite side of the adhesive was removed, and the OCA sample laid on the glass slide with light tension. The OCA was manually laminated to the glass with a roller and cut to strips either 1 cm or 1 in wide, extending longer than the glass substrate. The free end of the sample was attached to the clamp of a peel arm for an IMASS-SP2100 Peel Tester. Then the peel arm was carefully set in place on the IMASS load cell. The test commenced with the IMASS platen moving at 12 in / min and the arm peeling back the OCA at an angle close to 180 degrees. When recording the peel force data from the IMASS, the first two seconds of each peel were excluded as the transient initiation force. Following that, the subsequent 10 seconds of each peel were averaged to provide the values in the table.

[0132] Folding Test -20°C and 25°C

[0133] OCA specimens were cut to a sample size of 6 cm by 15 cm, aligning the center OCA stripe to run parallel to the short axis of each part and positioning that stripe in the middle of the long axis of each part. After removing the release liners from the OCA and treating both sides with air plasma on both sides to optimize adhesion, the OCA specimens were laminated to 50 mm polyimide film (GL30, SKC-Kolon, South Korea) and 30 mm glass (Joongwoo M-tech, South Korea).

[0134] Laminated specimens were adhered with double-sided tape to the folding platforms of a Covotech bending machine set up with a bending radius of 3 mm. The laminates aligned the center stripe with the bending axis region of the machine. After time to equilibrate temperature in the environmental chamber around the bending machine to either -20°C or 25°C, the machine began to fold the laminates at a continuous rate of 30 cycles / min, with no pauses in the open or closed state.

[0135] Sample preparation

[0136] Preparatory Example PVB0.6-IEM

[0137] 430 g of 30HH grade PVB was mixed with 2050 g of 2EHA in a vessel utilizing a mixing blade and applied heat. This solution was mixed for 90 minutes at a temperature of approximately 50°C until the PVB was well solvated by the 2EHA. 2.5 g of Bismuth catalyst (BICAT 8108) was added and allowed to mix for 10 minutes. 2.57 g of IEM was added to the vessel and the solution was mixed for 20 hours at a temperature of approximately 60°C. Preparatory Polymer Coating Solutions

[0138] Polymer coating solutions (PCSs) were prepared according to the following procedure with the amounts provided in Table 2 below. The components listed in the premix section of Table 2 were first charged to a clear jar and allowed to mix for 30 minutes. PCS-1 to PCS-7 were then exposed to approximately 350 nm UV irradiation until a viscous solution was obtained. Additional components as listed in the compounding section of Table 2 were then added to the jar and allowed to mix on a jar roller for greater than 4 hours. PCS samples 8 and 9 were mixed for greater than 4 hours in one step with both premix and compounding components simultaneously without requiring a viscosity - increasing, UV-irradiation step. The compositions of the polymer coating solutions PCS-1 to PCS-9 are provided in Table 2 below. Amounts shown are in wt%.

[0139] Table 2. The polymer solutions for both the inner and outer stripes were simultaneously delivered by pump to a slot coating die. The center stripe location and width were determined by shims within the slot die. In the examples contained herein the stripes were 16 mm wide.

[0140] Upon exiting the die, the patterned polymer solutions were coated onto a siliconized PET release liner (RF02N SKC Hass, KR). A second siliconized PET release liner was laminated on top of the construction and the laminated construction was exposed to UV irradiation with an output wavelength centered around 365nm and an overall dose of approximately 2,500 mJ / cm2.

[0141] The physical properties of patterned constructions represented by Examples 1-6 and Comparative Examples CE1 and CE2 are provided in Table 3 below.

[0142] Table 3.

[0143] For Examples 1-3 in Table 3 above, the outer stripes of the adhesive are kept at a constant low modulus, low Tgconfiguration while the center stripe is designed to have a higher modulus. Utilizing the unique material scheme, the center stripe modulus is systematically increased while keeping the Tgbelow -35°C and without significant increases in the refractive index of the material. By keeping the refractive index increase to a minimum, the differential between the RI of the center and outer stripes is also minimized in order to also decrease the visual appearance of the stripe pattern. Example 4 demonstrates the inverse design in which the higher modulus formulation is located in the outer stripes while the lower modulus material set is in the center stripe. The matching Tgand RI between center stripe and outer stripes are maintained in Examples 4, 5, and 6. Examples 5 and 6 show an even greater reduction in the RI differential from outer to center stripe can be achieved through the addition of a biphenyl type monomer in the outer stripe formulations.

[0144] CE1 represents a configuration where a higher modulus center stripe is provided with an increased Tgformulation along the folding axis. Not only is there a significant mismatch in Tg between the two regions of the adhesive, but there is also a large differential between the refractive indices of the material pair as indicated in Table 4 below. Finally, CE2 illustrates the case in which there is no differential of modulus between the center and outer stripe regions, analogous to a more traditionally coated, non-pattemed material.

[0145] Performance data for Examples 1-6 and Comparative Examples CE1 and CE2 are provided in Table 4 below. Provided in this table are the number of folding cycles and failure mode (e.g., delamination or cracking, if present) at two different temperatures, RI differential, % Haze, and peel performance.

[0146] Table 4.

[0147] Examples 1-6 in Table 4 generally show better performance than either of the Comparative Examples CE1 and CE2, taking into account the collective analysis of number of folding cycles passed, type of failure mode observed, the desire for a low differential in RI between the materials, low haze, and adhesion.

[0148] 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

CLAIMSWhat is claimed is:

1. A patterned adhesive comprising: a central adhesive layer; a pair of peripheral adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer in an alternating stripe pattern; optionally, wherein the ratio of shear modulus between central adhesive layer and each peripheral adhesive layer is from 2: 1 to 10: 1 or vice versa, the shear modulus measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C; wherein the central adhesive layer displays a refractive index within 0.015 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm; and further wherein each of the central adhesive layer and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

2. The patterned adhesive of claim 1, wherein the central adhesive layer displays a refractive index within 0.010 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm.

3. The patterned adhesive of claim 2, wherein the central adhesive layer displays a refractive index within 0.005 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm.

4. The patterned adhesive of any one of claims 1-3, wherein each of the central adhesive layer and peripheral adhesive layers displays a glass transition temperature of at most -35°C.

5. The patterned adhesive of claim 4, wherein each of the central adhesive layer and peripheral adhesive layers displays a glass transition temperature of at most -40°C.

6. The patterned adhesive of any one of claims 1-5, wherein die central adhesive layer comprises a multiphasic composition and each peripheral adhesive layer comprises a monophasic composition.- 27 -RECTIFIED SHEET (RULE 91) ISA / EP7. The patterned adhesive of any one of claims 1-5, wherein the central adhesive layer comprises a monophasic composition and each peripheral adhesive layer comprises a multiphasic composition.8.. The patterned adhesive of 6 or 7, wherein the multiphasic composition comprises a majority phase and minority phase uniformly dispersed in the majority phase, the minority phase present in discrete domains having a median transverse dimension of from 0.1 micrometers to 5 micrometers.

9. The patterned adhesive of any one of claims 6-8, wherein the monophasic composition is derived from an alkyl(meth)acrylate ester having 1 to 24 carbon atoms in the alkyl group and a free-radical generating initiator.

10. The patterned adhesive of any one of claims 6-9, wherein the multiphasic composition comprises a crosslinked network of polyvinyl acetal and acrylic copolymer.

11. The patterned adhesive of claim 10, wherein the functionalized polyvinyl acetal is present in a weight fraction of from 1% to 20% of the overall weight of the adhesive and is comprised of a polyvinyl acetal backbone with pendent functional groups reactive with the acrylic monomers; and further wherein the acrylic copolymer is present in a weight fraction of from 60% to 99% of the overall weight of the adhesive.

12. The patterned adhesive of claim 11, wherein the polyvinyl acetal backbone is a polyvinyl butyral backbone.

13. The patterned adhesive of claim 11 or 12, wherein the polyvinyl acetal backbone with pendent functional groups is obtained by reacting polyvinyl acetal with isocyanatoethyl (methjacrylate or allyl isocyanate.

14. A patterned adhesive comprising: a central adhesive layer; a pair of intermediate adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer;28RECTIFIED SHEET (RULE 91) ISA / EPa pair of peripheral adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the intermediate adhesive layers in an ABCB A stripe pattern; optionally, wherein die ratio of shear modulus between central adhesive layer and each peripheral adhesive layer is from 2:1 to 10:1 or vice versa, the shear modulus measured by dynamic mechanical analysis at a frequency of 1 Hz and a temperature of 25°C; wherein the central adhesive layer displays a refractive index within 0.015 that of the peripheral adhesive layers as measured for a light wavelength of 532 nm; wherein the intermediate adhesive layers each display a refractive index that is betw een that of the adjacent central adhesive layer and peripheral adhesive layer; and farther wherein each of the central adhesive layer, intermediate adhesive layers, and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

15. A patterned adhesive comprising: a central adhesive layer; a pair of intermediate adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the central adhesive layer; and a pair of peripheral adhesive layers, the pair of peripheral adhesive layers extending alongside, and adjacent to, opposing sides of the intermediate adhesive layers in an ABCB A stripe pattern; wherein each of the central adhesive layer and peripheral adhesive layers displays a shear modulus of from 10 kPa to 100 kPa as measured at ambient conditions and 1 Hz; wherein each intermediate adhesive layer displays a shear modulus that independently exceeds that of each of the central adhesive layer and peripheral adhesive layers by a factor of from 2 to 500 as measured at ambient conditions and 1 Hz; wherein each of the intermediate adhesive layers and the peripheral adhesive layers displays a refractive index within 0.015 that of the central adhesive layer as measured for a light wavelength of 532 nm; and farther wherein each of the central adhesive layer, intermediate adhesive layers, and peripheral adhesive layers displays a glass transition temperature of at most -30°C.

16. The patterned adhesive of claim 15, wherein each intermediate adhesive layer displays a shear modulus that independently exceeds that of the central adhesive layer and peripheral adhesive layers by a factor of from 5 to 100 as measured at ambient conditions and 1 Hz.29RECTIFIED SHEET (RULE 91) ISA / EP17. The paterned adhesive of claim 16, wherein each intermediate adhesive layer displays a shear modulus that independently exceeds that of the central adhesive layer and peripheral adhesive layers by a factor of from 10 to 50 as measured at ambient conditions and 1 Hz.

18. The patterned adhesive of any one of claims 15-17, wherein the central adhesive layer and peripheral adhesive layers have the same composition.

19. The paterned adhesive of any one of claims 1-18, wherein the patterned adhesive is optically transparent with a haze value of equal to or less than 0.5 % for a 50-micro meter thick adhesive.30RECTIFIED SHEET (RULE 91) ISA / EP

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