WO2025046451A1 - Curable adhesive - Google Patents

Abstract

A curable adhesive is provided. The curable adhesive comprises a poly(meth)acrylate or (meth)acrylate-containing polyurethane; one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers; a free radical initiator; and polyvinylpyrrolidone or copolymer thereof. The inclusion of polyvinylpyrrolidone and certain reactive oligomers can enable both high adhesive modulus and excellent mechanical performance to be achieved. Optical films coupled to the provided adhesive can also resist undesirable changes in flatness during assembly.

Description

CURABLE ADHESIVE

Field of the Invention

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

Specialized adhesives are used in the manufacture of electronic displays, such as those found in computer monitors, televisions, cell phones, automotive dashboards, appliances, wearables, and other consumer devices. Some are incorporated into flexible electronic displays that can be affixed to plastic, ultra-thin glass, or other pliable substrates. This capability can greatly expand the functionality of electronic displays by allowing them to be integrated into non-planar objects, conform to desired designs, and resiliently bend during use, giving rise to new applications.

These trends have increased demand for adhesives, and particularly for optically clear adhesives (OCA), to serve as an assembly layer or gap filling layer in electronic display assemblies. An OCA, for example, can be used to bond a display module to a cover lens or sheet made from glass, PET, PC, PMMA, polyimide, PEN, or cyclic olefin copolymer. The OCA can improve the performance of the display by increasing brightness and contrast, while also providing structural support to the assembly.

Given the technical requirements for flexible display assemblies, adhesives are needed that provide not only conventional performance attributes, such as optical clarity, adhesion, and durability, but also bendability and recoverability, while avoiding defects and delamination.

For conventional adhesives, including pressure sensitive adhesives, an increase in modulus generally correlates with certain desirable effects, such as an increase in the mechanical strength of the bond, along with an improvement in peel and tensile adhesion. Further technical benefits associated with increased modulus include improved die-cut stability for converting and storage, improving impact resistance and waviness control, and enhanced outgassing resistance of bubbles at elevated humidity and temperature.

Modifications to increase adhesive modulus without sacrificing other performance parameters such as adhesion have been explored, but many have not been successful. For example, the adhesive could be processed prior to curing as a pressure sensitive adhesive for good lamination and process control, and then modulus increased by adding reactive high glass transition temperature (high-T_g) oligomers or monomers to form a semi-tacky or non-tacky adhesive film. Yet, this approach can often degrade initial adhesion strength, create undesirable “sharkskin” peel from release liners, and require additional lamination steps to achieve adequate wetting of the adhesive to the adherend. Such steps can include, for example, the application of heat to close the bond. Matching these adhesives with release liners that securely couple to the adhesives while providing a clean release therefrom can also be a significant technical challenge.

These problems can be overcome using a curable adhesive based on a (meth)acrylate- containing polyurethane, one or more (meth)acrylate monomers and/or (meth)acrylate- containing oligomers, a free radical initiator, and polyvinylpyrrolidone (PVP) or a copolymer thereof. Optionally, the curable adhesive can further include a silane adhesion promoter, such as 3-glycidyloxypropyl trimethoxy silane. Without PVP, the modulus of the adhesive increases and the mechanical performance (the peeling adhesion) decreases with increasing the concentration of reactive high-T_g monomers and/or oligomers. By incorporating PVP and moderating the amount of reactive monomers/oligomers, both high post-curing modulus and excellent adhesive performance can be achieved simultaneously. As an added benefit, optical films coupled to the adhesive can resist undesirable changes in flatness during assembly by virtue of this decoupling between modulus and adhesive performance. Once film processing is complete, the provided adhesives can be cured to obtain high post-curing modulus while retaining the flatness of bonded optical films.

In a first aspect, a curable adhesive is provided. The curable adhesive comprises: a poly(meth)acrylate or (meth)acrylate-containing polyurethane; one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers; a free radical initiator; and polyvinylpyrrolidone or copolymer thereof.

In a second aspect, a method of making a curable adhesive is provided, the method comprising: providing a poly(meth)acrylate, or alternatively reacting an aliphatic polyisocyanate with an aromatic polyester polyol to obtain a (meth)acrylate-containing polyurethane; and mixing the poly(meth)acrylate or (meth)acrylate-containing polyurethane with one or more (meth) acrylate monomers and/or (meth)acrylate-containing oligomers, a free radical initiator, optionally an adhesion promoter comprised of a silane adhesion promoter, and polyvinylpyrrolidone or copolymer thereof to obtain the curable adhesive.

In a third aspect, a method of making a bonded assembly is provided, the method comprising: disposing the curable adhesive between opposing major surfaces of first and second adherends, wherein the free radical initiator comprises a photoinitiator; and exposing the curable adhesive to actinic radiation to obtain a cured adhesive, wherein the cured adhesive displays a tan d of from 0.2 to 1 when tested at 70°C and a frequency of 1 Hz and a storage modulus of at least 500 kPa under ambient conditions.

In a fourth aspect, a bonded assembly is provided using the method above.

FIGS. 1-4 are elevational side views of tape adhesives according to various exemplary embodiments.

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 may not be drawn to scale.

DEFINITIONS

As used herein:

“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 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 norbornyl.

“allyl” refers a functional group having the formula CH2=CH-CH2-.

“ambient conditions” means at 21°C and 101.3 kilopascals.

“ambient temperature” means 21 °C.

“curable adhesive” refers to an adhesive that can be cured.

“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.

“halogen” refers to a halogen atom or one or more halogen atoms, including chlorine, bromine, iodine, and fluorine atoms or fluoro, chloro, bromo, or iodo substituents.

“(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-.

“molecular weight” refers to weight average molecular weight, unless otherwise indicated. “oligomer” refers to a molecule that comprises at least two repeat units and that has a molecular weight less than its entanglement molecular weight; such a molecule, unlike a polymer, exhibits a significant change in properties upon the removal or addition of a single repeat unit.

“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.

Detailed Description

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.

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.

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.

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.

In various embodiments, the provided adhesive compositions include a poly(meth)acrylate or (meth)acrylate-containing polyurethane; one or more (meth)acrylate monomers or (meth)acrylate-containing oligomers; a free radical initiator; polyvinylpyrrolidone or a copolymer thereof, and optionally, a silane adhesion promoter. Poly(meth)acrylates are inclusive of acrylic polymers in general and need not be particularly limited. Useful poly(meth)acrylates can be homopolymers or copolymers polymerized alkyl (meth) acrylate monomers, such as an alkyl (meth)acrylate containing an alkyl group including from 4 to 18 carbon atoms.

To provide strong adhesion and/or flexibility to the fully cured adhesive and obtain good wettability to an adherend, it can be beneficial for the poly(meth)acrylate to contain polymerized units of one or more alkyl (meth)acrylate monomers whose respective homopolymers have a glass transition temperature of 25 °C or lower. Suitable alkyl (meth)acrylates can include, for example, n-butyl acrylate, isobutyl acrylate, isoamyl acrylate, hexyl acrylate, cyclohexyl acrylate, 2-ethylhexyl (meth) acrylate, isooctyl (meth) acrylate, isononyl (meth)acrylate, n-decyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, isomyristyl (meth)acrylate, isocetyl (meth)acrylate or isostearyl (meth)acrylate.

In combination with any of the monomers above, an alkyl (meth)acrylate monomer having an alkyl group of 4 to 18 carbon atoms whose homopolymer has a glass transition temperature of 25 °C or higher can also be used. Examples of the alkyl (meth)acrylate having an alkyl group of 4 to 18 carbon atoms whose homopolymer has a glass transition temperature (T_g) of 25°C or higher include linear or branched alkyl (meth)acrylates such as t-butyl (meth)acrylate, n-butyl methacrylate and isobutyl methacrylate; and alicyclic alkyl (meth)acrylates such as cyclohexyl methacrylate, 4-t-butylcyclohexyl (meth)acrylate and isobornyl (meth)acrylate. Having polymerized units of alkyl (meth)acrylate monomers associated with higher T_g as noted above can be beneficial because these monomers can impart enhanced mechanical behavior in the cured adhesive to impart greater resistance to debonding.

Other alkyl (meth)acrylates that can be included in the poly(methacrylate) copolymer are classified as high-T_g monomers based on the glass transition temperature of the corresponding homopolymers. The high-T_g monomers often have a Tg greater than 30°C, greater than 40°C, or greater than 50°C when homopolymerized (i.e., a homopolymer formed from the monomer has a Tg greater than 30°C, greater than 40°C, or greater than 50°C). Some suitable high-T_g alkyl (meth) acrylate monomers include, for example, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, sec-butyl methacrylate, tert-butyl (meth)acrylate, cyclohexyl methacrylate, isobornyl (meth)acrylate, stearyl (meth)acrylate, and 3,3,5 -trimethylcyclohexyl (meth)acrylate.

In some embodiments, it can be further advantageous for the poly(meth)acrylates to include polymerized units of one or more hydrophilic monomers whose homopolymer has a T_g of 10°C or lower. These monomers may enable greater association with substrates of interest, improved electrical properties and moisture management, or improved cohesive strength within the adhesive. Examples of the hydrophilic monomer whose homopolymer has a T_g of 10°C or lower.

Useful monomers include hydroxy alkyl acrylates having an alkyl group of 4 or fewer carbon atoms, and a (meth) acrylic compound having an oxy ethylene group or an oxypropylene group, or a polyoxyethylene group or a polyoxypropylene group. Particular examples include, but are not limited to, 2-hydroxyethyl acrylate and hydroxypropyl acrylate. Among these, in view of imparting flexibility to the transparent adhesive sheet, the hydrophilic monomer is preferably a hydrophilic monomer whose homopolymer has a T_g of 0°C or lower, and more preferably a hydrophilic monomer whose homopolymer has a glass transition temperature of - 5 °C or lower, such as 2-hydroxyethyl acrylate or 2-hydroxypropyl acrylate.

In some embodiments, the (meth)acrylate polymer may include a non-hydroxy functional polar copolymerizable monomer. Examples of suitable non-hydroxy functional polar copolymerizable monomers include, but are not limited to: acrylic acid, methacrylic acid, itaconic acid, fumaric acid, ether functional monomers such as 2-ethoxyethyl (meth)acrylate, 2-ethoxyethoxyethyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, nitrogen containing monomers such as acrylamide, 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 substituted amide monomers include, but are not limited to: N,N-dimethylacrylamide, N,N-diethyl acrylamide, N-morpholino (meth)acrylate, N-vinyl pyrolidone and N-vinyl caprolactam. In some embodiments, the (meth) acrylate polymer can include between 0 and 25 parts by weight of the polar copolymerizable monomer, particularly between 1 and 20 parts, and more particularly between 1 and 15 parts.

In some embodiments, the (meth) acrylate polymer may include a vinyl ester, and particularly a Cl to CIO vinyl ester. An example of commercially available suitable vinyl esters include, but are not limited to, vinyl acetate and VEOVA 9 or VEOVA 10 (available from Momentive Specialty Chemicals, New Smyrna Beach, Florida).

In some embodiments, the (meth) acrylate polymer may include a polar (meth)acrylate monomer. Examples of suitable polar (meth)acrylate monomers include, but are not limited to, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxylbutyl acrylate, tetrahydrofuryl acrylate, acrylamide, N,N-dimethyl acrylamide, N-vinyl pyrrolidone, and acrylic acid.

In some embodiments, the (meth) acrylate polymer may include a monofunctional non- (meth)acrylate vinyl monomer. Examples of suitable monofunctional non-(meth)acrylate vinyl monomers include but are not limited to: N-vinyl pyrrolidone, N-vinyl carbazole, vinyl acetate, and vinyl ether.

The poly(meth)acrylate may be a copolymer that also comprises pendant vinyl groups, such as pendent acrylate groups, that can undergo further free radical addition. In one embodiment, the functionalized copolymer may be formed by firstly polymerizing a mixture of monomers comprising at least one (Cl -Cl 8) alkyl (meth)acrylate monomer and a hydroxy containing (meth) acrylate monomer. After polymerization, a portion of pendant hydroxyl groups may be further converted to pendant unsaturated (meth) acrylate groups;

In one embodiment, unsaturated pendent groups may grafted through the reaction of isocyananatoethyl (meth) acrylate with the hydroxy groups of the copolymer. The IEM creates the pendant unsaturated groups on the copolymer after thermal processing. An example of a commercially suitable isocyananatoethyl (meth)acrylate includes 2-isocyanatoethyl acrylate and 2-isocyanatoethyl methacrylates sold under the trade designations KARENZAOI and KARENZMOI from Showa Denko, Toyko, Japan.

(Meth)acrylate-containing polyurethanes can have a polyurethane backbone. Polyurethanes are generally made by reacting a polyisocyanate component with a polyol component.

The polyol component comprises an aromatic and/or aliphatic polyester or polycaprolactone polyol that comprises at least two hydroxyl terminal groups. When the polyol averages two or three hydroxyl groups, it may be characterized as a diol or triol, respectively. In yet other embodiments, the polyol may include a mixture of one or more diols and one or more triols, wherein the number of hydroxyl groups averages greater than 2, yet less than 3. Other polyols can have 4, 5 or 6 hydroxyl terminal groups.

Polyester polyols can be obtained by, for example, an esterification reaction between a polyol component and an acid component. Examples of acid components include succinic acid, methylsuccinic acid, adipic acid, pimelic acid, azelaic acid, sebacic acid, 1,12 -dodecanedioic acid, 1,14-tetradecanedioic acid, dimer acid, 2-methyl-l,4-cyclohexanedicarboxylic acid, 2- ethyl-l,4-cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid, 1,4- naphthalenedicarboxylic acid, 4,4'-biphenyldicarboxylic acid, and acid anhydrides thereof.

The polyol component can be, in some embodiments, an aromatic polyester polyol. An aromatic polyester polyol can be produced by polymerizing an aromatic dicarboxylic acid with an aliphatic diol, as known in the art. In some embodiments, the aromatic dicarboxylic acid includes isophthalic acid or phthalic acid. The polyester polyol may optionally be produced from some amount of other aromatic dicarboxylic acid such as terephthalic acid. Optionally, the polyester polyol can be produced from cycloaliphatic dicarboxylic acids such as 1,3- cyclopentanedicarboxylic acid; 1 ,2-cyclohexanedicarboxylic acid; 1,4- cyclohexanedicarboxylic acid; or 2,5-norbornanedicarboxylic acid. These dicarboxylic acids are commonly provided in the form of acid anhydrides.

The aliphatic diol used to produce the aromatic or aliphatic (e.g., polyester or polycarbonate) polyol can include a straight-chain or branched alkylene group such as ethylene glycol, diethylene glycol, propylene glycol, 1,3 -propanediol, 1,3 -butanediol, 1 ,4-butanediol, 1,5-pentanediol, 1 ,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1 ,9-nonanediol, 1,10- decanediol, 2,4-dimethyl-2-ethylhexane-l,3-diol, 2,2-dimethyl-l,3-propanediol (neopentyl glycol), 2-ethyl-2-butyl-l,3-propanediol, 2-ethyl-2-isobutyl-l,3-propanediol, 3-methyl-l,5- pentanediol, 2,2,4-trimethyl-l,6-hexanediol, octadecanediol, and the like. At least one of the aliphatic diols can include a straight-chain or branched alkylene group comprising from 4 to 36 carbon atoms, or in some embodiments, less than, equal to, or greater than 4, 5, 6, 8, 10, 12, 15, 17, 20, 22, 24, 26, 28, 30, 32, or 36 carbon atoms.

In some embodiments, the polyol can comprise a polycaprolactone polyol. The polycaprolactone polyol can be obtained by subjecting a cyclic ester monomer such as epsilon- caprolactone or sigma-valerolactone to ring-opening polymerization. Polycaprolactone polyols comprise an alkylene group having 5 carbon atoms.

In some embodiments, the polyol component can comprise a polycarbonate polyol such as obtained from the reaction of aliphatic diols such as butanediol-(l,4) and/or hexanediol-(l,6) with phosgene, diaryl-carbonates such as diphenylcarbonate or with cyclic carbonates such as ethylene or propylene carbonate. Also suitable are polyester carbonates obtained from the above-mentioned polyesters or polylactones with phosgene, diaryl carbonates or cyclic carbonates. The preparation of the polyester or polycarbonate polyol generally includes utilizing at last one aliphatic diol as previously described. The alkylene group of the aliphatic diol and polyester or polycarbonate polyol may comprise hydrophobic substituents such halogen substituents. One illustrative polycarbonate polyol is sold from Covestro AG under the trade designation DESMOPHEN C2200.

In some embodiments, a single aliphatic diol is used to prepare the polyol. In this embodiment, the aliphatic diol comprises an alkylene group comprising at least 4, 5, or 6 carbon atoms as previously described. Alternatively, two or more aliphatic diol may be used in the preparation of the polyol wherein at least one of such diols comprises an alkylene group comprising at least 4, at least 5, or least 6 carbon atoms. When a mixture of aliphatic diols are used, at least 50, 60, 70, 80, 90 or 95 wt-% of the total amount of diol are alkylene groups comprising at least 4, 5, or 6 carbon atoms as previously described.

The polyol is typically a polymer. The polyol can have an equivalent weight (molecular weight per hydroxyl group) ranging from about 250 g/mol to about 30,000 g/mol. In some embodiments, the equivalent weight of the polyol is from 500 g/mol to 30,000 g/mol, from 2000 g/mol to 20,000 g/mol, from 2000 g/mol to 10,000 g/mol, from 2000 g/mol to 4000 g/mol, or in some embodiments, less than, equal to, or greater than 250 g/mol; 500; 1000; 2000; 3000; 3500; 4000; 5000; 6000; 7000; 8000; 10,000; 20,000; or 30,000 g/mol. For diols and triols, typical molecular weights for the polyol can be two or three times of the equivalent weight ranges above, respectively. In some embodiments, the polymeric polyol has a molecular weight of less than 4000, 3500, or 3000 g/mole. In some embodiments, the aliphatic polyester polyol includes repeat units comprised of an alkylene group and a terminal ester group or more than one alkylene group bonded by means of an ester linkage and a terminal ester group.

In other embodiments, the aliphatic polycarbonate polyol can include repeat units comprising an alkylene group and a terminal carbonate group or more than one alkylene group bonded by means of a carbonate linkage and a terminal carbonate group.

In still other embodiments, the aromatic polyester polyol may include polymerized units comprised of an aromatic group of the dicarboxylic acid bonded to the alkylene group of the aliphatic diol by ester linkages. In this embodiment, the molar ratio of six-member rings to alkylene groups having at least 4, 5, or 6 carbon atoms can be approximately 1:1 and can range from about 1.5:1 to 1:1.5.

In a preferred embodiment, an aromatic polyester polyol is used that can be obtained by reacting an aromatic ortho- or meta-dicarboxylic acid anhydride component and an aliphatic diol component. Thus, the polyol component comprises polymerized units of an ortho- or metaphthalate and comprises polymerized units of an alkylene group comprising at least 4 carbon atoms.

In some embodiments, the polyester polyol is prepared from isophthalic acid or phthalic acid, and represented by structure I below:

, wherein R1 is independently an alkylene group comprising at least 4 carbon atoms, n is at least 2, 3, 4 or 5, and the ester group substituents are bonded to the ring at an ortho- or metaposition.

In some embodiments, n is no greater than 25, 20, 15, or 10. When the aromatic polyester polyol includes ortho- or meta-ester moieties, the polyester polyol tends to have a relatively low glass transition temperature, such as less than 0°C, less than 5°C, or less than 10°C. Further, such aromatic polyester polyols tend to be amorphous viscous liquids at 25°C. In some embodiments, the aromatic polyester polyols have a viscosity of less than 10,000 cP, or even less than 5,000 cP at 80°C.

Aromatic polyester polyols derived from orthophthalic acid are commercially sold from Stepan Co. under the trade designation STEPANPOL. These can be represented, for example, by structure II below:

(ID

, wherein R1 and n have any of the values set out above.

When the aromatic polyester polyol is derived from isoterephthalic acid, the polyester polyol can be represented by structure III below:

, wherein R1 and n have any of the values set out above.

The polyisocyanate component can be any of various polyfunctional isocyanate compounds. Examples of such polyfunctional isocyanate compounds include polyfunctional aliphatic isocyanate compounds, polyfunctional aliphatic cyclic isocyanate compounds, and a polyfunctional aromatic isocyanate compounds. Examples of the polyfunctional aliphatic isocyanate compounds include trimethylene diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,3- butylene diisocyanate, dodecamethylene diisocyanate, and 2,4,4-trimethylhexamethylene diisocyanate.

Examples of polyfunctional aliphatic cyclic isocyanate compounds include 1,3- cyclopentene diisocyanate, 1,3 -cyclohexane diisocyanate, 1 ,4-cyclohexane diisocyanate, isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated tetramethylxylene diisocyanate, partially bio-based aliphatic isocyanate polymer sold under the trade designation TOLONATE X FLO 100 from Vencorex US, Inc., Freeport, TX, and bio-based polyfunctional aliphatic cyclic isocyanates, such as 2-heptyl-3,4-bis(9-isocyanatononyl)-l-pentylcyclohexane sold by BASF Corporation under the trade designation DDI 1410.

Examples of polyfunctional aromatic isocyanate compounds include phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-toluidine diisocyanate, 4,4'-diphenyl ether diisocyanate, 4,4'-diphenyl diisocyanate, 1,5 -naphthalene diisocyanate, and xylylene diisocyanate. In some embodiments, the polyfunctional isocyanate comprises a polyisocyanate that is a liquid at 25 °C, either alone or in combination with minor amount of a polyisocyanate that is solid at 25°C. In other embodiments, such as when the polyol is an aliphatic polyol, the polyfunctional isocyanate could be a solid at 25 °C.

In some embodiments, the polyfunctional isocyanate compound comprises an aliphatic isocyanate compound, such as hexamethylene diisocyanate. In other embodiments, the polyfunctional isocyanate compound comprises a ortho- or meta-aromatic isocyanate compound, such as 1,4 methylene diphenyl diisocyanate (MDI), m-tetramethylene diisocyanate (TMXDI), or mixtures thereof. Mixtures of aliphatic and aromatic polyfunctional isocyanate compounds are also possible.

The (meth)acrylate functionality of the (meth)acrylate-containing polyurethane can be provided by including a suitable (meth)acrylate-containing alcohol or isocyanate in the polymerization reaction used to obtain the (meth)acrylate-containing polyurethane. In some embodiments, the (meth)acrylate-containing polyurethane is a linear polyurethane containing pendent acrylate groups. The (meth)acrylate-containing polyurethane can be present in an amount of from 10 wt% to 99 wt%, from 40 wt% to 97 wt%, from 70 wt% to 95 wt%, or in some embodiments less than, equal to, or greater than 10 wt%, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 96, 97, 98, or 99 wt% relative to the overall weight of the curable adhesive.

The polymer architecture above can be synthesized by reacting a compound comprising one or more hydroxy groups and one or more ethylenically unsaturated groups together with the aforementioned polyisocyanate and polyol components in a reactive mixture. In a preferred embodiment, the polyisocyanate is an aliphatic polyisocyanate and the polyol is an aromatic polyester polyol.

From the reactive mixture, the hydroxyl group reacts with the polyisocyanate component, incorporating ethylenically unsaturated groups into the polyurethane. In some embodiments, compound having a single hydroxyl group and a (meth)acrylate monomer having a single ethylenically unsaturated group can be used, such as hydroxyethyl acrylate (HEA). In some embodiments, an isocyanate group is bonded to the polyurethane polymer backbone and the opposing end of the diisocyanate is bonded to the hydroxyl group of the compound resulting in a terminal ethylenically unsaturated group.

In other embodiments, the (meth)acrylate-containing polyol includes at least two hydroxy groups and at least two ethylenically unsaturated groups, such as bisphenol A glycerolate dimethacrylate (Bis-GMA). In this embodiment, the compound reacts as a polyol and is thereby incorporated into the polyurethane backbone, where the ethylenically unsaturated groups are pendent with respect to the polyurethane backbone. The one or more (meth)acrylate-containing polyols can be present, independently, in an amount of from 0.1 wt% to 20 wt%, from 0.2 wt% to 10 wt%, from 0.5 wt% to 5 wt%, or in some embodiments less than, equal to, or greater than 0.1 wt%, 0.2, 0.5, 1, 2, 2.5, 3, 4, 5, 7, 10, 11, 12, 15, 17, or 20 wt%, relative to the overall weight of the reactive mixture.

Various compounds comprising one or more hydroxy groups and one or more ethylenically unsaturated groups can be used during the preparation of the polyurethane. Such compound can be aliphatic or aromatic. Other representative compounds sold by Nagase ChemteX Corporation, Osaka, Japan include for example epoxy acrylate form 1,6 hexane diol, sold under the trade designation DA-212, or epoxy acrylate form 1,4 hexane diol, sold under the trade designation DA-214L.

The provided curable adhesive composition further incorporates one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers. These monomers and/or oligomers can be blended with the poly (meth) acrylate or (meth)acrylate-containing polyurethane to obtain a reactive mixture and eventually cured to form a crosslinked network at the time of bonding.

In some embodiments, the reactive mixture includes a urethane acrylate oligomer, such as sold under the trade designation CN983 by Arkema, Colombes, France. In other embodiments, the reactive mixture includes an ethoxylated triacrylate, such as sold under the trade designation SR415 by Arkema, Colombes, France. Both urethane acrylate oligomers and ethoxylated triacrylate can effectively function as crosslinkers, but the latter monomer is somewhat more hydrophilic and was found to improve haze performance after the cured adhesive is subjected to high temperature high humidity aging.

In some embodiments, the (meth)acrylate-containing oligomers are comprised of a polyester-based urethane diacrylate oligomer. Suitable (meth)acrylate-containing oligomers can have a homopolymer T_g of greater than 30°C, 40°C, or even 50°C.

Other monomers having multiple (meth) acryloyl groups can be combined with a (meth)acrylate copolymer or polyurethane with pendent (meth)acrylate groups. These monomers can be added to adjust the crosslink density and increase the modulus of the cured (meth)acrylate copolymer or polyurethane. These monomers can react with pendent (meth)acryloyl groups of the curable (meth)acrylate copolymer or polyurethane when exposed to ultraviolet or visible light radiation in the presence of a photoinitiator. If added, the amount of these monomers is typically in the range of 0 to 40 parts per hundred (pph) based on the weight of the curable (meth)acrylate copolymer. For example, this amount can be less than, equal to, or greater than 1 pph, 2, 5, 10, 15, 20, 25, 30, 35, or 40 pph.

Exemplary monomers having two (meth)acryloyl groups include bisphenol A diacrylate, diethylene glycol diacrylate, triethylene glycol diacrylate, tetraethylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate (e.g., commercially sold from Arkema under the trade designation SR-210, SR-252, and SR-603), polypropylene glycol diacrylate, ethoxylated (30) bisphenol A diacrylate (e.g., commercially sold from Arkema under the trade designation SR9038), polyethylene/polypropylene copolymer diacrylate, neopentylglycol hydroxypivalate diacrylate modified caprolactone, and polyurethane diacrylates (e.g., commercially sold by Arkema under the trade designation CN2920. CN9178, and CN983, and from Eternal Materials Co. Ltd. under trade designation ETERCURE 282).

Exemplary monomers having three or four (meth)acryloyl groups include, but are not limited to, trimethylolpropane triacrylate (e.g., commercially sold under the trade designation TMPTA-N from Surface Specialties, Smyrna, GA and under the trade designation SR-351 from Sartomer, Exton, PA), ethoxylated trimethylol propane tri acrylate (e.g.g commercially sold under the trade designation SR9035 from Sartomer), pentaerythritol triacrylate (e.g., commercially sold under the trade designation SR-444 from Sartomer), tris(2- hydroxyethylisocyanurate) triacrylate (commercially sold under the trade designation SR-368 from Sartomer), a mixture of pentaerythritol triacrylate and pentaerythritol tetraacrylate (e.g., commercially sold from Surface Specialties under the trade designation PETIA with an approximately 1:1 ratio of tetraacrylate to triacrylate, and under the trade designation PETA-K with an approximately 3:1 ratio of tetraacrylate to triacrylate), pentaerythritol tetraacrylate (e.g., commercially sold under the trade designation SR-295 from Sartomer), di-trimethylolpropane tetraacrylate (e.g., commercially sold under the trade designation SR-355 from Sartomer), and ethoxylated pentaerythritol tetraacrylate (e.g., commercially sold under the trade designation SR-494 from Sartomer). An exemplary crosslinker with five (meth)acryloyl groups includes, but is not limited to, dipentaerythritol pentaacrylate (e.g., commercially sold under the trade designation SR-399 from Sartomer).

Advantageously, the curable adhesive composition further incorporates homopolymers or copolymers of substantially polar and high-T_g monomeric units. In preferred embodiments, the homopolymers or copolymers derive from polyvinylpyrrolidinone (PVP), sometimes referred to as povidone. PVP can be in the form of polyvinylpyrrolidinone homopolymer, polyvinylpyrrolidinone copolymer, or a combination thereof. Polyvinylpyrrolidinone is a nonionic synthetic polymer composed of repeating 1 -vinyl-2-pyrrolidone monomers. The repeat unit for PVP is represented by structure IV below:

PVP is known for use in the pharmaceutical industry as a binder in tablet manufacturing, and is soluble in water as well as in many organic solutions. This property is the result of hydrophilic and hydrophobic functional groups that can interact with varying solvents, with viscosity being largely unaffected by electrolytes. It was discovered, surprisingly, that the addition of PVP in the provided curable polyurethane-based or poly(meth)acrylate-based adhesive compositions can yield a high adhesive modulus and substantially alleviate degradation of adhesive performance that normally occurs when the modulus of the adhesive composition is substantially increased upon curing.

PVP copolymers can include random and block copolymers of PVP. A useful PVP random copolymer is N-vinylpyrrolidone-co-vinyl acetate copolymer, also known as copovidone, and used widely as dry and wet binder in tablets in the pharmaceutical industry. Compared to povidone, copovidone is less hygroscopic and absorbs less water. Copolymers such as copovidone may also enable greater compatibility with the curable poly(meth)acrylate or curable polyurethane polymer.

The PVP or copolymer thereof can have a weight average molecular weight of from 1000 g/mol to 75,000 g/mol, from 1,500 g/mol to 60,000 g/mol, from 2,000 g/mol to 50,000 g/mol, or in some embodiments, less than, equal to, or greater than 1,000 g/mol; 1,500; 2,000; 5,000; 10,000; 20,000; 30,000; 40,000; 50,000; 60,000; 75,000; 100,000; 500,000, or 1,000,000 g/mol.

The PVP or copolymer thereof can be present in any suitable amount to obtain the desired adhesive modulus after curing. Typically, the amount of PVP or copolymer thereof is from 2 wt% to 30 wt%, from 5 wt% to 20 wt%, from 5 wt% to 15 wt%, or in some embodiments, less than, equal to, or greater than 2 wt%, 3, 4, 5, 7, 10, 12, 15, 17, 20, 25 or 30 wt%, relative to the overall weight of the curable adhesive.

Advantageously, the introduction of carboxylic acid functionalities to the polymer chains enables hydrogen bonding between PVP/PVP vinyl acrylates and the polyurethane components. Such hydrogen bonding can have the effect of providing physical crosslinking (also called “thermodynamic crosslinking”) amongst the polymeric chains, thereby increasing storage modulus while in the pre-cured state, particularly at high temperatures.

Increased storage modulus can be a critical benefit in certain applications. Examples include three-dimensional lamination and thermal forming lamination, which often require that the uncured adhesive be processed at elevated temperatures. Operations can include lamination, clean peel from release liners and die-cutting. The uncured adhesive also needs to have flow properties to wet curved surfaces adequately, while retaining its dimensional stability until it is fully cured. Physical crosslinking can help provide an adhesive solution that realizes the foregoing benefits while preserving high adhesion and high modulus performance properties after curing. In some embodiments, the reactive mixture further comprises a carboxylic acid containing polyol. An example of a useful carboxylic acid containing polyol is dimethylolpropionic acid. When the reactive mixture is polymerized, the resulting copolymer is provided with carboxylic acid groups along the polymer backbone. This can induce hydrogen bonding between the acid and PVP groups between polymer chains, as illustrated below:

Given the desired degree of physical crosslinking, the carboxylic acid containing polyol can be present in an amount to provide a suitable concentration of acid functionality on the copolymer backbone. The amount can be from 0.1 wt% to 20 wt%, from 0.2 wt% to 10 wt%, from 0.5 wt% to 5 wt%, or in some embodiments, less than, equal to, or greater than 0.1 wt%, 0.2, 0.5, 1, 2, 2.5, 3, 4, 5, 7, 10, 11, 12, 15, 17, or 20 wt%, relative to the overall weight of the reactive mixture.

The curable adhesive composition includes one or more free radical initiators enabling the curable adhesive composition to be cured.

In a preferred embodiment, the free radical initiator is a photoinitiator activated by actinic radiation. Useful photoinitiators include benzoin ethers such as benzoin methyl ether and benzoin isopropyl ether; substituted acetophenones such as 2,2-dimethoxy-2- phenylacetophenone photoinitiator, sold under the trade designation 1-651, sold by Merck KGaA, Darmstadt, Germany or ESACURE KB-1 photoinitiator, sold by Lehvoss Group, Hamburg, Germany, and dimethylhydroxyacetophenone; substituted a-ketols such as 2- methyl-2-hydroxy propiophenone; aromatic sulfonyl chlorides such as 2-naphthalene-sulfonyl chloride; photoactive oximes such as 1 -phenyl- l,2-propanedione-2-(O-ethoxy- carbonyl)oxime; mono- or bis-acrylphosphine oxides sold under the trade designations IRGANOX 819 from BASF SE, Ludwigshafen, Germany or LUCIRIN TPO from Merck KGaA.

Preferred photoinitiators are photoactive compounds that undergo a Norrish I cleavage to generate free radicals that can initiate by addition to the acrylic double bonds. The photoinitiator can be added to the mixture to be coated after the polymer has been formed. Exemplary polymerizable photoinitiators are described, for example, in U.S. Patent Nos.

5,902,836 and 5,506,279 (Gaddam et al.).

Thermal free radical initiators are also possible, for which activation occurs through the application of heat rather than through exposure to actinic radiation. Such initiators include, but are not limited to, azo, peroxide, persulfate, and redox initiators, and combinations thereof. Further options and associated advantages relating to free radical thermal and photopolymerization techniques are described in U.S. Patent Nos. 4,654,233 (Grant et al.); 4,855,184 (Klun et al.); and 6,224,949 (Wright et al.).

Free radical initiators can be present in an amount of from 0.1 wt% to 5 wt%, or in some embodiments, less than, equal to, or greater than 0.1 wt%, 0.2, 0.5, 1, 2, 3, 4, or 5 wt%, based on the overall weight of the uncured composition.

For improved adhesive performance, 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 an liquid crystal display (UCD) by coupling to the 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.

The adhesive copolymerizable group 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 group 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.

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

In a preferred method of making the curable adhesive, an aliphatic polyisocyanate is reacted with an aromatic polyester polyol to obtain a (meth)acrylate-containing polyurethane in a common solvent, as appropriate. Suitable solvents for the reactive components can include ethyl acetate and methyl ethyl ketone. The (meth)acrylate-containing polyurethane can then be mixed with one or more (meth) acrylate monomers and/or (meth)acrylate-containing oligomers, a free radical initiator, an adhesion promoter comprised of a silane adhesion promoter, and polyvinylpyrrolidone or similar copolymer thereof. As explained previously, a poly(meth)acrylate can be substituted for the (meth)acrylate-containing polyurethane above, where the poly (meth) acrylate can include unsaturated pendent groups to facilitate further crosslinking in the cured adhesive.

Where a solvent is used, the solution can be cast onto a release surface and then the solvent removed through a separate drying step at elevated temperatures to obtain a uniform curable adhesive film.

FIGS. 1-3 show exemplary transfer adhesives incorporating the provided adhesive compositions. A tape adhesive according to one exemplary embodiment is illustrated in FIG. 1 and hereafter denoted by the numeral 100. The tape adhesive 100 is embodied in a primary layer 102 composed of an adhesive composition as described herein and having opposed first and second major surfaces 104, 106. Advantageously, the primary layer 102 provides mechanical waviness resistance while preserving high impact performance. The provided adhesives, both before and after curing, can be optically transparent with a post-cure haze value of less than 2%.

FIG. 2 shows a tape adhesive assembly 150 representing a bonded assembly. The assembly 150 includes the tape adhesive 100 comprised of the primary layer 102, whose characteristics are described above. The assembly 150 further includes a pair of release substrates 152, 154 disposed on each of the respective opposing major surfaces 104, 106 of the primary layer 102. In some embodiments, the primary layer 102 directly contacts both of the release substrates 152, 154 thereby acting to adhesively couple these release substrates 152, 154 to each other. 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 primary layer 102 could be coated unto 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), or a polarizer film.

FIG. 3 shows a tape adhesive 200 according to yet another embodiment, bearing similarities to the construction of tape adhesive 100 except a pair of secondary layers 210, 210’ are interposed between a primary layer 202 and the release substrates 252, 254, as shown. The secondary layers 210, 210’ can function as skin layers made from acrylic OCAs that contain a lower weight fraction of PVP or copolymers thereof relative to that of the primary layer 202.

FIG. 4 shows a bonded assembly in which a pair of adhesive layers 302, 302’ are disposed on opposing major surfaces of a quarter wave plate 360, bonding the quarter wave plate 360 to a lens layer 362 on one side and a reflective polarizer 364 on its opposite side. The reflective polarizer 364 substantially reflects light having a first polarization state and substantially transmits light having an orthogonal second polarization state. Materials useful for the lens layer 362 are not particularly restricted, although this layer is typically made from glass or a hard polymer such as a cyclic olefin copolymer or polycarbonate. The pair of adhesive layers 302, 302’ have characteristics of the provided adhesive as shown and described herein.

The bonded assembly 300 is an example of an optical stack used in display devices. Details regarding the operation of an optical stack are described elsewhere, for example in U.S. Patent No. 11,630,290 (Yun et al.), U.S. Patent Publication No. 2020/0319388 (Ambur et al.), and International Patent Publication Nos. WO 2023/111739 (Le et al.) and WO 2022/043791 (Haag et al.).

In a preferred embodiment, one or both of the secondary layers 210, 210’ contain a zero, or essentially zero, amount of PVP or copolymers thereof. A potential advantage of this embodiment is the retention of high room temperature tack, which can be beneficial for certain applications. Another potential advantage is the possibility of introducing greater flowability at the surface, which can improve adhesive wetting of the substrate or topological features such as an ink step, where present. Further advantages can include the possibility of isolating certain functionalities, such as UV blocking, to a particular layer.

More generally, a bonded assembly can be made by disposing the curable adhesive between opposing major surfaces of first and second adherends, wherein the free radical initiator comprises a photoinitiator; and exposing the curable adhesive to actinic radiation to obtain a cured adhesive.

Prior to curing, the curable adhesive can display a tan 8 of from 0.5 to 3, from 0.5 to 2, or in some embodiments, less than, equal to, or greater than 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.5, 1.7, 2, 2.5, or 3, when tested at 70°C and a frequency of 1 Hz, to provide proper flow properties for the adhesive. After being cured, the adhesive can display a tan 8 of from 0.2 to 1, from 0.2 to 0.6, from 0.2 to 0.5, or in some embodiments, less than, equal to, or greater than 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1, when tested at 70°C and a frequency of 1 Hz. In various embodiments, the cured adhesive displays a storage modulus of at least 500 kPa under ambient conditions.

EXAMPLES

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.

All parts, percentages, ratios, etc. in the examples and the rest of the specification are by weight, unless noted otherwise. Solvents and other reagents used were obtained from MilliporeSigma Company; Burlington, MA unless otherwise noted. The following abbreviations are used: nm = nanometers; mm = millimeters; cm = centimeters; in = inch; Hz - Hertz; min - minutes; h - hours; mJ - milliJoules. Materials

Materials used in the examples are provided in Table 1 below.

TABLE 1.

Test Methods

Peel Test, 25°C and 85°C

This 180° peel adhesion test is similar to the test method described in ASTM A 3330- 90, except a glass substrate is substituted for the stainless steel substrate described in the test method.

The coated adhesive samples were first laminated onto a 2 mil (51 micrometer) primed PET backing (3SAB from Mitsubishi). They were then slit into 1 cm strips and rolled onto glass substrates with a Cheminstruments HR- 100 roller. The test specimens were then autoclaved and cured using 3J/cm² D bulb. The test specimens were allowed to condition in a CTH room for 18 h prior to peel adhesion analysis. For 25°C testing, a 6 cm/min peel rate at a 180° peel angle was used with an IMass SP-2000 peel tester. For 85°C testing, specimens were allowed to dwell in a Instron environmental chamber for at least 15 minutes at 85 °C and analyzed at 6 cm/min peel rate at a 180° peel angle using a Instron load frame and environmental chamber. Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis was used to probe the modulus as a function of temperature as well as to determine the glass transition temperature (T_g) of the material. An 8- mm diameter by approximately 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 -45°C to 150°C at 3°C/minute. During this ramp, the samples 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 8 was recorded at select temperatures during this scan. The T_g of the material was also determined as the peak in the tan 8 vs. temperature profile.

Haze Test

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.

Polyurethane (PU) Preparation

To a resin reaction vessel equipped with a mechanical stirrer, a condenser, and an air inlet, 200 g of Polyol, 17.26 g of DI, 1.1 g of Bis-GMA, 0.02 g of BHT, 0.11 g of DBTDA and 50 g of MEK were added. The solution was heated up to 75°C while stirring. The temperature was maintained at 75+2° C until the NCO signal disappeared under FT-IR spectroscopy. During the reaction, a total 170 g of MEK was added to dilute the viscosity of the system. The clear PU solution of 50% by weight was obtained with an intrinsic viscosity (IV) of 0.47.

Low IV Polyurethane (PU-Low IV) Preparation

200.43 gram (g) of Polyol, 17.02 g of DI, 0.55 g of Bis-GMA, 0.02 g of BHT, 0.11 g of DBTDA and 50 g of MEK were added to a resin reaction vessel equipped with a mechanical stirrer, a condenser, and an air inlet. The solution was heated up to 75°C while stirring. The temperature was maintained at a temperature of 75 ± 2°C until the isocyanate peak disappeared under FT-IR spectroscopy. During the reaction, a total 125 g of MEK was added to dilute the viscosity of the system. The clear PU solution of 56% by weight was obtained with an IV of 0.40.

Polyurethane with 0.2 Acid Functionality (PU-0.2 acid) Preparation

200.98 gram (g) of Polyol, 19.32 g of DI, 1.1 g of Bis-GMA, 0.44 g of DMPA, 0.02 g of BHT, 0.11 g of DBTDA and 50 g of MEK were added to a resin reaction vessel equipped with a mechanical stirrer, a condenser, and an air inlet. The solution was heated up to 75°C while stirring. The temperature was maintained at 75+2° C until the isocyanate peak disappeared under FT-IR spectroscopy. During the reaction, around 172 g of MEK was added to dilute the viscosity of the system. The clear PU solution of 50% by weight was obtained with an IV of 0.46.

Polyurethane with 0.5 Acid Functionality (PU-0.5 acid) Preparation

200 grams (g) of Polyol, 20.37 g of DI, 1.1 g of Bis-GMA, 1.11g of DMPA, 0.02 g of BHT, 0.11 g of DBTDA and 50 g of MEK were added to a resin reaction vessel equipped with a mechanical stirrer, a condenser, and an air inlet. The solution was heated up to 75 °C while stirring. The temperature was maintained at 75+2° C until the isocyanate peak disappeared under FT-IR spectroscopy. During the reaction, around 172 g of MEK was added to dilute the viscosity of the system. The clear PU solution of 50% by weight was obtained with an IV of 0.48.

Comparative Examples CE-1 to CE-2 and Examples EX-1 to EX-6

Preparation of Curable Formulations

All curable formulations were prepared by adding the polyurethane polymers (as 50% in MEK), (meth)acrylate-functionalized oligomers, polyvinylpyrrolidone-based oligomers or copolymers, adhesion promoters and photoinitiators in wt% as shown in Table 2. Typically, all materials were mixed together in an 8 oz. amber jars and roller mixed for at least 8 h until the formulations were fully homogeneous.

Preparation of Adhesive Coatings

The adhesive formulations were first coated on Liner-2 using a knife coater to control the coating caliper. The coating was dried at ambient temperature for 10 min and then 70°C for 15 min, followed by laminating Liner- 1 on the dried adhesive.

The samples were tested according to the test methods described above, and their data were summarized in Table 2. Comparing CE-2 to CE-1, more urethane diacrylate oligomer CN983 increased G’, but decreased peel, at 25°C. While comparing EX-1 with CE-1, use of PVP oligomer increased both G’ and peel at 25 °C. With the use of different molecular weight PVP oligomers (EX-2, EX-3, and EX-4), it was observed higher molecular weight increased both G’ and peel at 25°C. EX-5 with the use of PVP-VA copolymer resulted in highest peel at 25°C. EX-6 with the use of lower IV PU showed higher 70°C tan 8 before 3J UV curing, indicating better flowability of the curable adhesive.

Preparatory Examples PE-1 to PE-5

A series of preparatory adhesive solutions were prepared with the formulations shown in Table 3. In the Table 3 below, adhesive solutions were made by adding the indicated amount of acrylic monomer to a glass vessel along with the indicated amount of solvent EtOAC, thermal initiator (Vazo 52), and chain transfer agent (PEI). Afterbubbling the solution with nitrogen gas for 3 minutes, the vessel was sealed and heated to 60°C for 16 hours followed by 65°C for 4 hours. The vessel was opened and the B81O8 and IEM were added as indicated below in Table 3. The vessel was resealed and heated at 60°C for 12 hours.

TABLE 2.

TABLE 3.

Comparative Example CE-3 and Examples EX-7, EX-8

Along with 15 pph of CN983, 0.4 pph of photoinitiator 1651, and 0.1 pph of a silane KBM403, a polymer solution of PVP2 (M_w ~ 2,500 g/mol) was added in amounts indicated in Table 4 below to base polymers PE-3 and PE-4 and allowed to mix for at least 12 h. Samples were coated and tested for both DMA and peel as described above. Properties listed below represent the samples after applied 3 J/cm² irradiation of UVA. Table 4 illustrates the effect of adding PVP or PVP copolymer at M_w ~ 2,500 g/mol to increase both the 25°C and 85°C peel while maintaining a high modulus system > 1 MPa, even improving upon the modulus of the comparative example 1 when 10 parts of PVP2 was used.

TABLE 4.

Comparative Examples CE-4 and CE-5 and Examples EX-9 to EX-12

Polymer solutions of varying molecular weights of PVP and PVP copolymer were added at amounts indicated in Table 5 to base polymer PE-4 or PE-5 as indicated, along with 15 pph of CN983, 0.4 pph of photoinitiator 1-651 , and 0.1 pph of a silane KBM403 to prepare the compositions of EX-9 to EX-12, CE-4 and CE-5. These components were allowed to mix for at least 12 h. Samples were coated and tested for both DMA and peel as described above. Properties listed below represent the samples after applied 3 J/cm² irradiation of UVA. Again, trends of increasing 25°C modulus as well as increased peel at 25°C and 85°C were observed with the addition of PVP or PVP copolymer components. In particular, the PVP1 at 10,000 g/mol and PVP-VA at 50,000 g/mol displayed both high modulus and high adhesion compared to the comparatives and other examples listed in Table 5. Control adhesive with no added PVP or PVP copolymer is represented when testing modulus at 25°C and 85°C.

TABLE 5.

Comparative Examples CE-6, 7, 8, 9 and Examples EX-13, 14

Preparation of Curable Formulations

All curable formulations were prepared by adding amounts of the polyurethane polymers (50% in MEK), (meth)acrylate-functionalized oligomers, polyvinylpyrrolidone-based oligomers or copolymers, adhesion promoters and photoinitiators in wt% as shown in Table 6. All materials were mixed together in an 8 oz. amber jars and mixed for at least 8 h until the formulations were homogeneous.

Preparation of Adhesive Coatings

The adhesive formulations were first coated on Liner-2, using a knife coater to control the coating caliper. The coating was dried at ambient temperature for 10 min and then heated to 70°C for 15 min, followed by laminating Liner- 1 onto the dried adhesive.

As summarized in Table 7, comparing EX- 13 (acid containing PU+PVP) to CE-6 (no acid in PU, no PVP) and CE-7 (no acid in PU, with PVP), the acid groups were observed to increase 85°C G’ before and after cure significantly, indicating there was strong hydrogen bonding between acid-containing PU and PVP oligomers. When EX-14 (acid containing PU+PVP) was compared to CE-8 (no acid in PU, with PVP), EX-14 displayed a significant decrease of 70°C tan delta both before and after cure, and a significant increase of 85°C G’ both before and after cure, indicated there was interaction between acid containing PU and PVP oligomers. Comparing EX- 14 (acid containing PU + PVP) to CE-9 (acid containing PU, no PVP), the former displayed a decrease in 70°C tan delta before curing and an increase in 85°C G’. When comparing EX-14 to CE-9, the addition of PVP resulted in almost doubling 25 °C G’ after cure, while also showing significantly improved peel performance.

TABLE 6.

TABLE 7.

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 curable adhesive comprising: a poly (meth) acrylate or (meth)acrylate-containing polyurethane; one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers; a free radical initiator; and polyvinylpyrrolidone or a copolymer thereof.

2. The curable adhesive of claim 1, further comprising a silane adhesion promoter, optionally comprising 3-glycidyloxypropyl trimethoxysilane.

3. The curable adhesive of claim 1 or 2, wherein the (meth)acrylate-containing polyurethane is a linear polyurethane containing pendent acrylate groups.

4. The curable adhesive of any one of claims 1-3, wherein the poly(meth)acrylate comprises pendent acrylate groups.

5. The curable adhesive of any one of claims 1-4, wherein the free radical initiator comprises a photoinitiator.

6. The curable adhesive of any one of claims 1-5, wherein the (meth)acrylate-containing polyurethane is derived from a reactive mixture comprising: an aliphatic polyisocyanate; and an aromatic polyester polyol.

7. The curable adhesive of claim 6, wherein the aromatic polyester polyol is derived from orthophthalic acid.

8. The curable adhesive of claim 6 or 7, wherein the reactive mixture further comprises a bisphenol A glycerolate dimethacrylate.

9. The curable adhesive of any one of claims 6-8, wherein the reactive mixture further comprises a carboxylic acid containing polyol.

10. The curable adhesive of claim 9, wherein the carboxylic acid containing polyol comprises dimethylolpropionic acid.

11. The curable adhesive of any one of claims 1-10, wherein the one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers comprise an ethoxylated trimethylolpropane triacrylate.

12. The curable adhesive of any one of claims 1-11, wherein the one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers comprise an urethane acrylate oligomer.

13. The curable adhesive of claim 12, wherein the urethane acrylate oligomer comprises a polyester-based urethane diacrylate oligomer.

14. The curable adhesive of any one of claims 1-13, wherein the (meth)acrylate-containing oligomers have a homopolymer T_g greater than 50°C.

15. The curable adhesive of any one of claims 1-14, wherein the polyvinylpyrrolidone or copolymer thereof comprises polyvinylpyrrolidone homopolymer.

16. The curable adhesive of any one of claims 1-15, wherein the polyvinylpyrrolidone or copolymer thereof comprises N-vinylpyrrolidone-co-vinyl acetate copolymer.

17. The curable adhesive of any one of claims 1-16, wherein the polyvinylpyrrolidone or copolymer thereof is present in an amount of from 2 wt% to 30 wt% relative to the overall weight of the curable adhesive.

18. The curable adhesive of any one of claims 1-17, wherein the curable adhesive displays a tan 8 of from 0.5 to 2 at 70°C.

19. The curable adhesive of any one of claims 1-18, wherein the adhesive is optically transparent with a haze value of less than 2%.

20. A method of making a curable adhesive, the method comprising: providing a poly(meth)acrylate, or alternatively, reacting an aliphatic polyisocyanate with an aromatic polyester polyol to obtain a (meth)acrylate-containing polyurethane; and combining the poly(meth)acrylate or (meth)acrylate-containing polyurethane with one or more (meth)acrylate monomers and/or (meth)acrylate-containing oligomers, a free radical initiator, optionally an adhesion promoter comprised of a silane adhesion promoter, and polyvinylpyrrolidone or copolymer thereof to obtain the curable adhesive.

21. A method of making a bonded assembly, the method comprising: disposing the curable adhesive of any one of claims 1-19 between opposing major surfaces of first and second adherends, wherein the free radical initiator comprises a photoinitiator; and exposing the curable adhesive to actinic radiation to obtain a cured adhesive, wherein the cured adhesive displays a tan 8 of from 0.2 to 1 when tested at 70°C and a frequency of 1 Hz and a storage modulus of at least 500 kPa under ambient conditions.

22. A bonded assembly made using the method of claim 21 , wherein the first or second adherend comprises a multilayer optical film.