Radiation Transmissive Fillers Improving the Depth of Cure in High Optical

Density Light Curable Materials

Field

[01 ] The present disclosure relates generally to dark colored radiation curable adhesives. Brief Description of Related Technology

[02] Radiation curable materials contain photoinitiator(s) which will absorb specific wavelengths of radiation, i.e., UV or visible wavelength radiation from a source to form free radicals. The free radicals will then initiate a photopolymerization reaction with the other materials to cure the radiation curable materials. Radiation must be able to pass into the radiation curable materials to form free radicals to initiate the photopolymerization reaction. Reaction will not proceed when there is insufficient radiation intensity available to the photoinitiator. If the physical depth of the LED or UV radiation penetration is reduced the depth of cure will also be reduced.

[03] The following factors are well known to limit the depth of cure of radiation curable materials. When the material has color or opacity, the ability of the LED or UV light to penetrate into the material will be reduced or eliminated. When the material has a component, such as a dye, that can absorb radiation at the same or similar wavelength as required by the photoinitiator there is competition between the dye and photoinitiator. The resultant radiation intensity available to the photoinitiator will be lowered and the depth of cure and cure speed will also be lowered.

[04] Conventionally manufacturers would use bezels or overlays to cover the edges of mobile phones, tablets, computer displays, TV, etc. to prevent the light leaking out from the edges and for aesthetic reasons. However, the trend is away from large bezels toward a no bezel or covering look. Manufacturers have turned to black or dark colored adhesives to replace bezels and seal the display edges to prevent the light leaking out from the edges and for aesthetic reasons. Such adhesives typically require an optical density (for a 0.1 mm thick sample) greater than 3.0. However, known radiation curable adhesives possessing an optical density of 3.0 or greater have a limited depth of cure of less than 0.2 mm. Manufacturers require a depth of cure of at least 0.2 mm or desirably more and known radiation curable adhesives having an optical density greater than 3.0 cannot provide the depth of cure required by manufacturers. Further, it is desirable for an adhesive having an optical density about 3.0 or more to achieve this depth of cure in about 10 seconds or less. Conventional radiation curable adhesives having an optical density about 3.0 or more have not been able to meet these requirements. These limitations prevent their use in many applications.

Summary

[05] One aspect of the disclosure provides a dark colored, radiation curable adhesive having a desirable combination of fast cure and high optical density, high depth of cure. [06] One aspect of the disclosure provides a dark colored, radiation curable adhesive having a cure speed of 30 seconds or less, and preferably 10 seconds or less.

[07] One aspect of the disclosure provides a dark colored, radiation curable adhesive having an optical density of 3 or more at 0.1 mm thickness.

[08] One aspect of the disclosure provides a dark colored, radiation curable adhesive having a depth of cure of 0.25 mm or more.

[09] One aspect of the disclosure provides a dark colored, radiation curable adhesive having a cure speed of 30 seconds or less, preferably a cure speed of 10 seconds or less, an optical density of 3 or more at 0.1 mm thickness and a depth of cure of 0.25 mm or more.

Detailed Description

[10] As used herein for each of the various embodiments, the following definitions apply:

[11] Unless otherwise specifically defined, "alkyl" refers to a linear, branched or cyclic saturated alkyl group having from 1 to about 9 carbon atoms including, for example, methyl, ethyl, propyl, butyl, hexyl, octyl, isopropyl, isobutyl, tert-butyl, cyclopropyl, cyclohexyl, cyclooctyl, vinyl and allyl. Unless otherwise specifically limited, a cyclic alkyl group includes monocyclic, bicyclic and polycyclic rings, for example norbornyl, adamantyl and related terpenes.

[12] Unless otherwise specifically defined, an aromatic ring is an unsaturated ring structure having about 5 to about 6 ring members and including only carbon as ring atoms. Unless otherwise specifically defined, an aromatic ring can be substituted or unsubstituted.

[13] Unless otherwise specifically defined, "aryl" refers to an aromatic ring system substituted or unsubstituted, that includes only carbon as ring atoms, for example phenyl, biphenyl or naphthyl.

[14] Unless otherwise specifically defined, "halogen" refers to an atom selected from fluorine, chlorine, bromine and iodine.

[15] Unless otherwise specifically limited the term substituted means substituted by at least one below described substituent group in any possible position or positions. Substituent groups for the above moieties useful in the disclosed compounds are those groups that do not significantly diminish the biological activity of the disclosed compound. Substituent groups that do not significantly diminish the desired activity of the disclosed compound include, for example, H, halogen, N3, NCS, CN, NO2, NX1X2, OX3, C(X4)3, OAc, O-acyl, O-aroyl, NH-acyl, NH-aroyl, NHCOalkyl, CHO, C(halogen)3, COOX4, SOsH, PO3H2, SO2NX1X2, CONX1X2, C(0)CF ₃ , alkyl, alcohol, alkoxy, alkylmercapto, alkylamino, di-alkylamino, sulfonamide or thioalkoxy wherein Xi and X2 each independently comprise H or alkyl, orXi and X2 together comprise part of a heterocyclic ring having about 4 to about 7 ring members and optionally one additional heteroatom selected from 0, N or S, orXi and X2 together comprise part of an imide ring having about 5 to about 6 members and X4 comprises H, alkyl, loweralkylhydroxy, or alkyl-NXiX2. Unless otherwise specifically limited, a substituent group may be in any possible position or any possible positions if multiply substituted. Additionally, the term may also include a hetero atom such as 0 or N interrupting the alkyl chain.

[16] The term “(meth)acrylate” includes acrylate and methacrylate.

[17] The term “oligomer” means a defined, small number of repeating monomer units such as 10-25,000 units, and desirably 10-100 units which have been polymerized to form a molecule and is a subset of the term polymer; the term “polymer” any polymerized product greater in chain length and molecular weight than the oligomer, i.e. or degrees of polymerization greater than 25,000. [18] One or more”, as used herein, relates to at least one and comprises 1 , 2, 3, 4, 5, 6, 7, 8, 9 or more of the referenced species. Similarly, “at least one” means one or more, i.e. 1 , 2, 3, 4, 5, 6, 7, 8, 9 or more. “At least one”, as used herein in relation to any component, refers to the number of chemically different molecules, i.e. to the number of different types of the referenced species, but not to the total number of molecules. For example, “at least one polyol” means that at least one type of molecule falling within the definition for a polyol is used but that also two or more different polyol types falling within this definition can be present but does not mean that only one type of said polyol is necessarily present.

[19] The term “molecular weight” refers to the average number molecular weight M _n , if not explicitly stated otherwise. The number average molecular weight M _n can be calculated based on end group analysis (OH numbers according to DIN EN ISO 4629, free NCO content according to EN ISO 11909) or can be determined by gel permeation chromatography according to DIN 55672 with THF as the eluent. If not stated otherwise, all given molecular weights are those determined by gel permeation chromatography. The weight average molecular weight M _w can be determined by GPC, as described for M _n .

[20] The disclosed compounds include any and all isomers and stereoisomers. In general, unless otherwise explicitly stated the disclosed materials and processes may be alternately formulated to comprise, consist of, or consist essentially of, any appropriate components, moieties or steps herein disclosed. The disclosed materials and processes may additionally, or alternatively, be formulated to be devoid, or substantially free, of any components, materials, ingredients, adjuvants, moieties, species and steps used in the prior art compositions or that are otherwise not necessary to the achievement of the function and/or objective of the present disclosure.

[21 ] The word “about” or “approximately” as used herein in connection with a numerical value refer to the numerical value ± 10%, preferably ± 5% and more preferably ± 1 % or less.

[22] Unless explicitly indicated otherwise, all percentages that are cited in connection with the compositions described herein refer to weight percent (wt.%) with respect to final composition with all components, unless stated otherwise.

[23] The radiation curable adhesive comprises a radiation curable acylate portion, a radiation opaque material and radiation transmissive filler. The radiation acrylate portion can also comprise monofunctional (meth)acrylate monomer, multifunctional (meth)acrylate monomer, (meth)acrylate oligomer or any combination thereof.

[24] The monofunctional (meth)acrylate monomer can be selected from compounds having a single (meth)acrylate functional group. Monofunctional (meth)acrylate monomers include aromatic (meth)acrylate monomers, aliphatic (meth)acrylate monomers and cycloaliphatic (meth)acrylate monomers. Examples of some useful monofunctional (meth)acrylate monomers include 2-hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate ("HPMA"), isobornyl (meth)acrylate, acryloyl morpholine, benzyl(meth)acrylate and bisphenol-A mono (meth)acrylate, octyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, etc.

[25] In some embodiments the radiation curable adhesive can comprise a combination of two or more monofunctional (meth)acrylate monomers. The radiation curable adhesive can comprise 5 to 30 wt.% of monofunctional acrylate monomer based on the weight of the radiation curable adhesive.

[26] The multifunctional (meth)acrylate monomer can be selected from compounds having a two or more (meth)acrylate functional groups. Examples of multifunctional (meth)acrylate monomers include di-or tri-functional (meth)acrylates like polyethylene glycol di(meth)acrylates, tetrahydrofuran (meth)acrylates and di(meth)acrylates, hexanediol di(meth)acrylate, trimethylol propane tri(meth)acrylate (“TMPTMA”), diethylene glycol dimethacrylate, triethylene glycol dimethacrylate (“TRIEGMA”), tetraethylene glycol dimethacrylate, dipropylene glycol dimethacrylate, di-(pentamethylene glycol) dimethacrylate, tetraethylene diglycol diacrylate, diglycerol tetramethacrylate, tetramethylene dimethacrylate, ethylene dimethacrylate, neopentyl glycol diacrylate, trimethylol propane triacrylate, pentaerythritol tetraacrylate, bisphenol-A di(meth)acrylates, such as ethoxylated bisphenol-A di(meth)acrylate ("EBIPMA"), bisphenol-F di(meth)acrylates, such as ethoxylated bisphenol-F (meth)acrylate.

[27] In some embodiments the radiation curable adhesive can comprise a combination of two or more multifunctional (meth)acrylate monomers. The radiation curable adhesive can comprise 5 to 30 wt.% of multifunctional acrylate monomer based on the weight of the radiation curable adhesive. [28] The (meth)acrylate oligomer can be selected from radiation curable compounds having one or more (meth)acrylate functional groups. Examples of such (meth)acrylate oligomers include (meth)acrylate containing urethane compounds, (meth)acrylate containing polyester compounds, (meth)acrylate containing silicone compounds, (meth)acrylate containing polyolefin compounds, (meth)acrylate containing epoxy compounds, or a combination thereof.

[29] Exemplary (meth)acrylate containing urethane compounds include tetramethylene glycol urethane acrylate oligomer and propylene glycol urethane acrylate oligomer. Other (meth)acrylate-functionalized urethanes are urethane (meth)acrylate oligomers based on polyethers or polyesters, which are reacted with aromatic, aliphatic, or cycloaliphatic diisocyanates and capped with hydroxy acrylates. For instance, difunctional urethane acrylate oligomers, such as a polyester of hexanedioic acid and diethylene glycol, terminated with isophorone diisocyanate, capped with 2-hydroxyethyl acrylate (CAS 72121 - 94-9); a polypropylene glycol terminated with tolyene-2, 6-diisocyanate, capped with 2- hydroxyethylacrylate (CAS 37302-70-8); a polyester of hexanedioic acid and diethylene glycol, terminated with 4,4'-methylenebis(cyclohexyl isocyanate), capped with 2- hydroxyethyl acrylate (CAS 69011-33-2); a polyester of hexanedioic acid, 1 ,2-ethanediol, and 1 ,2 propanediol, terminated with tolylene-2, 4-diisocyanate, capped with 2-hydroxyethyl acrylate (CAS 69011-31-0); a polyester of hexanedioic acid, 1 ,2-ethanediol, and 1 ,2 propanediol, terminated with 4,4'-methylenebis(cyclohexyl isocyanate, capped with 2- hydroxyethyl acrylate (CAS 69011 -32-1 ); and a polytetramethylene glycol ether terminated with 4,4'-methylenebis(cyclohexylisocyanate), capped with 2-hydroxyethyl acrylate.

[30] Still other (meth)acrylate oligomers are monofunctional urethane acrylate oligomers, such as a polypropylene terminated with 4,4'-methylenebis(cyclohexylisocyanate), capped with 2-hydroxyethyl acrylate and 1-dodosanol.

[31 ] (Meth)acrylate oligomers also include difunctional urethane methacrylate oligomers such as polytetramethylene glycol ether terminated with tolulene-2, 4-diisocyanate, capped with 2-hydroxyethyl methacrylate; polytetramethylene glycol ether terminated with isophorone diisocyanate, capped with 2-hydroxyethyl methacrylate; polytetramethylene glycol ether terminated with 4,4'-methylenebis(cyclohexylisocyanate), capped with 2- hydroxyethyl methacrylate; and polypropylene glycol terminated with tolylene-2,4- diisocyanate, capped with 2-hydroxyethyl methacrylate.

[32] In some embodiments the radiation curable adhesive can comprise a combination of two or more (meth)acrylate oligomers. The radiation curable adhesive can comprise 10 to 80 wt.% of monofunctional acrylate monomer based on the weight of the radiation curable adhesive.

[33] In some embodiments the radiation curable adhesive comprises a combination of one or more monofunctional (meth)acrylate monomers; one or more multifunctional (meth)acrylate monomers and one or more (meth)acrylate oligomers.

[34] The radiation used to cure the radiation curable adhesive can be in the ultraviolet (UV) wavelength from about 200 nm to about 1 ,000 nm. Useful UV wavelengths include UVA (about 320 nm to about 410 nm), UVB (about 290 nm to about 320 nm), UVC (about 220 nm to about 290 nm) and combinations thereof. The radiation used to cure the radiation curable adhesive can be in the visible wavelength from about 450 nm to about 550 nm. The radiation curable adhesive comprises a photoinitiator that initiates a curing reaction in the composition when exposed to radiation in these frequencies.

[35] Many photoinitiators may be used, including acetophenone and derivatives thereof, such as dichloroacetophenone, trichloroacetophenone, dialkoxyacetophenone, 2,2- dimethoxy-2-phenylacetophenone and 4-dialkylaminoacetophenone; benzophenone and derivatives thereof, such as 4,4'-bis(dimethylamino)benzophenone (Michler's ketone) and 4,4'-bis(diethylamine)benzophenone; benzil; benzoin and derivatives thereof, such as benzoin alkyl ether; benzildimethylketal; benzoylbenzoate; alphaacyloxime esters; thioxanthone and derivatives thereof, such as 2-chlorothioxanthone and diethylthioxanthone; azo-compounds, such as azobisisobutyronitrile; benzoyl peroxide; camphoquinone; and phosphine oxides, such as diphenyl-2, 4, 6-trimethylbenzoylphosphine oxide, with and without 1-benzoyl-cyclohexanol.

[36] Other examples of useful photoinitiators include photoinitiators available commercially from BASF, under the “IRGACURE” and “DAROCUR” trade names, specifically “IRGACURE” 184 (1-hydroxycyclohexyl phenyl ketone), 907 (2-methyl-1-[4- (methylthio)phenyl]-2-morpholino propan-1 -one), 369 (2-benzyl-2-N,N-dimethylamino-1- (4-morpholinophenyl)-1-butanone), 500 (the combination of 1 -hydroxy cyclohexyl phenyl ketone and benzophenone), 651 (2,2-dimethoxy-2-phenyl acetophenone), 1700 (the combination of bis(2,6-dimethoxybenzoyl-2, 4, 4-trimethyl pentyl) phosphine oxide and 2- hydroxy-2-methyl-1 -phenyl-propan-1 -one), and 819 [bis(2, 4, 6-trimethyl benzoyl) phenyl phosphine oxide] and “DAROCUR” 1173 (2-hydroxy-2-methyl-1 -phenyl-1 -propan-1 -one) and 4265 (the combination of 2,4,6-trimethylbenzoyldiphenyl-phosphine oxide and 2- hydroxy-2-methyl-1 -phenyl-propan-1 -one); and the visible light [blue] photoinitiators, dl- camphorquinone and “IRGACURE” 784DC. Other photoinitiators useful herein include alkyl pyruvates, such as methyl, ethyl, propyl, and butyl pyruvates, and aryl pyruvates, such as phenyl, benzyl, and appropriately substituted derivatives thereof.

[37] Cationic photoinitiators can also be used, such as, Bis(4-tert- butylphenyl)iodonium perfluoro-1 -butanesulfonate, Bis(4-tert-butylphenyl)iodonium perfluoro-1 -butanesulfonate, Bis(4-tert-butylphenyl)iodonium perfluoro-1 - butanesulfonate.

[38] The radiation curable adhesive can comprise a combination of two or more photoinitiators. Photoinitiators can be employed in concentrations effective to initiate curing of the radiation curable adhesive at a desired exposure to actinic radiation and typically in concentrations of about 0.1 % to about 10% by weight of composition.

[39] The radiation curable adhesive can include a radiation opaque material. The radiation opaque material is any material that is added to the radiation curable adhesive to provide a desired optical density. In some embodiments the material will be added to the radiation curable adhesive to provide an optical density of 3 or more at 0.1mm thickness. Useful radiation opaque materials include carbon black, calcium carbonate, metal or metal oxide powders, alumina, silicas, pigments, etc. One useful radiation curable material is Elftex 12.

[40] In some embodiments the radiation curable adhesive can comprise a combination of two or more radiation opaque materials. The radiation curable adhesive can comprise 0.1 to 10 wt.% of radiation opaque material based on the weight of the radiation curable adhesive.

[41] The radiation curable adhesive can include a radiation transmissive filler. Radiation transmissive fillers are fillers that will transmit radiation in the cure wavelength. Useful radiation transmissive fillers include glass beads and glass spheres such as Q-CEL materials from Potters Industries. The glass beads or glass spheres should be free of constituents that inhibit or block transmission of UV radiation. Silica fillers such as colloidal or fumed silica are not always transparent to UV radiation and are preferably not used as a radiation transmissive filler.

[42] In some embodiments the radiation curable adhesive can comprise a combination of two or more radiation transmissive fillers. The radiation curable adhesive can comprise 0.1 to 10 wt.% of radiation transmissive filler based on the weight of the radiation curable adhesive.

[43] The radiation curable adhesive can optionally include one or more additives. Useful additives include one or more of adhesion promoter, thickener, thixotrope, colorant, toughening agent, plasticizer, and stabilizers.

[44] In some embodiments the radiation curable adhesive can comprise the following components and weight percentages where the total of will be 100 weight percent.

[45] The radiation curable adhesive will have one or more of the following properties. Preferably, the radiation curable adhesive will all of the following properties:

[46] The disclosed radiation curable adhesive can be used in applications where a Radiation curable adhesive having an optical density greater than 3 in the uncured state and a large cure through depth is desirable. Some applications include bonding components of articles such as televisions, computer displays, hand held devices and mobile phones. The cured adhesive retains an optical density greater than 3 providing the bonded area with a desirable dark colored or black appearance and also preventing the leakage of light from the bonded display or device.

[47] In use the uncured adhesive is disposed in a desired location on the bonding surface of a first component. A second component is disposed over the first component and in contact with the uncured adhesive. The adhesive is exposed to radiation to cure the adhesive so that cured reaction products of the adhesive bond the first and second components together.

Test methods

[48] Cure is tested by surface tack-free time and photorheometry.

Surface tack-free time

[49] Apply a sample of uncured product onto a microscope slide. Cure the liquid product by exposure to 405 nm wavelength radiation for a desired time in seconds. In these experiments the exposure time was 10 seconds. After exposure to radiation check the sample for tackiness of the surface with a wooden stick. Assessment is either tacky or not tacky.

Photorheometry

[50] Photorheometry was carried out on an Anton Paar rheometer MCR301 with UV accessory, Loctite® CureJet™ LED, 850 mW/cm ² at 405 nm. Dispense a sample of uncured product to a disposable plate on the bottom of the parallel plate tool on the rheometer. Lower the top plate of the parallel plate tool to achieve a gap of 0.2 mm between the top and bottom plates. Initiate testing with parallel plates oscillation for 15 seconds to establish a baseline followed by exposure of the sample to 850 mW/cm ² of UV radiation at 405 nm. Record the curing profile.

Optical Density

[51] Optical density in the uncured adhesive is tested by measuring the percentage of 550 nm radiation transmitted through an uncured sample. Optical density in the cured adhesive is tested by measuring the percentage of 550 nm radiation transmitted through a cured sample. Dispense a sample of uncured product to a glass plate. Place 0.1 mm spacers on the glass plate and sample and cover with another glass plate. Clamp both glass plates together. Measure the percentage of UV light (wavelengths from 300 nm to 700 nm) transmitted through the uncured sample. A DataColor 650 instrument was found useful. Calculate the optical density (OD) using the following equation and transmission percentage at 550 nm,

OD = Log 10(1 /T).

[52] Optical density in the cured adhesive is tested by measuring the percentage of transmission through a cured sample. Dispense a sample of uncured product to a glass plate. Place 0.1 mm spacers on the four corners of the glass plate and sample and cover with another glass plate. Clamp both glass plates together. Cure the sample by exposure to 850 mW/cm ² of UV radiation at 405 nm for 10 seconds. Measure the percentage of UV light (wavelengths from 300 nm to 700 nm) transmitted through the cured sample. A DataColor 650 instrument was found useful. Calculate the optical density (OD) using the following equation and transmission percentage at 550 nm,

OD = Log10(1/T).

Depth of cure.

[53] Dispense a liquid product into an aluminum weighing dish. The amount of liquid product must be sufficient so that the liquid product will not be cured through its entire depth. Cure the liquid product by exposure to 850 mW/cm ² of UV radiation at 405 nm for 10 seconds. Allow the sample to cool to room temperature if necessary. Remove the cured product from the weighing dish and carefully scrape away the bottom, uncured liquid product to leave only cured material. Cure any residual uncured material at the bottom of the cured product with the same UV exposure to make sure that the surface is dry. Measure the thickness of the cured sample.

[54] The following examples are included for purposes of illustration so that the disclosure may be more readily understood and are in no way intended to limit the scope of the disclosure unless otherwise specifically indicated.

Comparative example A

1 0.5% of Irgacure 819 and 1 % of Irgacure 907

Comparative example A did not include any radiation transmissive filler.

Example 1

Example 2

Portions of each Example material were formed into a 0.1 mm uncured film. Each sample was cured by exposure to 850 mW/cm ² of UV radiation at 405 nm for 10 seconds. All samples were found to be fully cured to the listed cure through depth after the 10 second exposure.

1 0.1 mm sample thickness

[55] Example A did not use radiation transmissive filler while examples 1 and 2 included 2 wt. % of radiation transmissive filler. Each of Examples A, 1 and 2 had an optical density greater than 3.0 in the uncured state.

[56] The depth of cure for comparative Example A, without radiation transmissive filler, was only 0.16 mm. This would be unacceptable for many applications. Longer exposure times might increase depth of cure, but the longer exposure time would also be unacceptable for many applications.

[57] Even though Examples 1 and 2 were compositionally similar to Example A and each had an uncured optical density greater than 3, addition of the radiation transmissive filler to those compositions increased the depth of cure for Examples 1 and 2 was 0.29 mm and 0.27 mm respectively. This result is surprising as the carbon black would be expected to mask the radiation transmissive filler, preventing transmission of UV radiation through both the composition and the filler. Further, the radiation transmissive filler, despite being optically transmissive, surprisingly did not lessen the optical density of the uncured or cured adhesive materials.

[58] The proceeding description is meant to be exemplary and it is to be understood that variations and modifications may be employed without departing from the concept and intent of the invention as defined in the following claims.