Photo-curable compositions

Abstract

A photo-curable composition includes a radiation curable resin component, a photoinitiator to initiate substantial curing of the radiation curable resin component on exposure to actinic radiation, a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing a pH value of the composition from a first pH value to a second pH value when exposed to actinic radiation, and a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.

Description

PHOTO-CURABLE COMPOSITIONSBACKGROUND

[0001] Radiation curable adhesives are used in various industries where product assembly requires rapid bonding of parts. For example, radiation curable adhesives may be utilized for bonding parts by interposing the adhesive precursor between the parts and exposing the precursor to actinic radiation. In some cases, strong bonds can be obtained within seconds of exposure to actinic radiation. Although conventional radiation curable adhesives have developed a good reputation for industrial bonding applications due to their rapid cross-linking, quality assurance protocols can sometimes have difficulty ascertaining whether complete polymerization and desired bonding potential has been achieved. For example, actinic radiation shadowing during cure processing can sometimes limit the extent of cure of the adhesive.

[0002] Currently, only part inspection using specialized equipment (e.g., UV fluorescence) and physical performance testing are used to assure the application and cure of the adhesive. These conventional testing methods are inefficient, typically require specialized equipment and sometimes destroy the tested part. Accordingly, it would be beneficial to receive reliable assurance of sufficient cure after a bonded assembly has undergone exposure to actinic radiation. Therefore, it is desired to provide a photo-curable composition that is capable of indicating the extent of cure by exhibiting an appearance change (e.g., real-time visual appearance change) to verify that the desired cure conditions have been met.SUMMARY

[0003] According to one aspect, a photo-curable composition includes: (1) a radiation curable resin component, (2) a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing a pH value of the composition from a first pH value to a second pH value when exposed to actinic radiation, and (3) a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.

[0004] According to another aspect, a photo-curable composition includes: (1) a radiation curable resin component, (2) a photoinitiator to initiate substantial curing of the radiation curableresin component on exposure to actinic radiation, (3) a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing a pH value of the composition from a first pH value to a second pH value when exposed to actinic radiation, and (4) a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.

[0005] According to another aspect, a method of curing a photo-curable composition includes applying actinic radiation to a photo-curable composition sufficient to change a pH value of the composition from a first pH value to a second pH value, wherein the appearance of the photo-curable composition is different at the first pH value and the second pH value, wherein the photo-curable composition includes: (1) a radiation curable resin component, (2) a photoinitiator to initiate substantial curing of the radiation curable resin component on exposure to actinic radiation, (3) a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing the first pH value of the composition to a second pH value when exposed to actinic radiation, and (4) a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.BRIEF DESCRIPTION OF DRAWING

[0006] This written disclosure describes illustrative embodiments that are non-limiting and non-exhaustive. Reference is made to illustrative embodiments that are depicted in the figures, in which:

[0007] FIG. 1 illustrates examples of some pH-sensitive dyes.DETAILED DESCRIPTION

[0008] The objects and advantages enumerated above together with other objects, features, and advances represented by the present disclosure will now be presented in terms of detailed embodiments described with reference to the attached drawing figures which are intended to be representative of various possible configurations. Other embodiments and aspects are recognized as being within the grasp of those having ordinary skill in the art.

[0009] Embodiments of the present disclosure provide novel photo-curable compositions for indicating the extent of a cure reaction upon exposure to actinic radiation. Often, radiation curable resins are used to bond objects together, such as plastic or glass materials. While conventional radiation curable resins have been utilized for many applications, these conventional resins are incapable of quickly and efficiently indicating application of the composition on a material and the extent of cure reaction upon exposure to actinic radiation. The compositions of the present disclosure are capable of providing verification that the photo-curable composition has been properly applied to a material (to be bonded) and are capable of indicating the extent of cure reaction during and / or after exposure to sufficient actinic radiation.

[0010] Photo-curable compositions of the present disclosure may include radiation curable resins, a photoinitiator for initiation of crosslinking reactions, and various additives, such as acrylate monomers, sensitizers, etc. These photo-curable compositions may be exposed to actinic radiation to initiate cross-linking. Actinic radiation is electromagnetic radiation capable of initiating photochemical reactions. Accordingly, actinic radiation is capable of transmitting sufficient energy to cause photoinitiators to fracture into free radicals. These free radicals can initiate the curing or polymerization process to produce long polymer chains from monomers. The production of long polymer chains at least partially begins or completes the curing process.

[0011] In one embodiment, the photo-curable composition includes a radiation curable resin component, a photoinitiator, a latent pH adjusting agent, and a pH-sensitive dye. These photo-curable compositions may further include various additives to assist with polymerization and / or cure efficiency. For example, the photo-curable composition may further include one or more sensitizers. Compositions of the present disclosure may be non-aqueous compositions. Accordingly, these compositions may be non-polar, organic compositions.

[0012] In one aspect, the radiation curable resin component comprises a resin component having one or more (meth)acrylate functional moieties. Examples of a (meth)acrylate- functionalized resin include (meth)acrylate functional resin formed from various monofunctional (meth)acrylate monomers, such as homopolymers of monofunctional Ci-io alkyl(meth)acrylates and copolymers of monofunctional Ci-io alkyl(meth)acrylates; polyester acrylates; urethane acrylates; polyether acrylates; (meth)acrylate-functionalized urethanes, (meth)acrylate- functionalized polyesters, and poly(isobutylene) di(meth)acrylates.

[0013] (Meth)acrylate-functionalized urethanes (or urethane (meth)acrylate resins) suitable as the (meth)acrylate-functionalized resin component include those disclosed in U.S. Patent Nos. 4,018,851, 4,295,909 and 4,309,526 to Baccei, and U.S. Patent Nos. Re 33,211, 4,751,273, 4,775,732, 5,019,636 and 5,139,872 to Lapin et al., and U.S. Patent Nos. 10,745,590 for instance.

[0014] The (meth)acrylate-functionalized resin component may be a multi- (such as di- or tri-) functional urethane acrylate oligomer, more desirably an aliphatic polyether urethane acrylate. An example of a suitable (meth)acrylate-functionalized resin component is BR-582-E8 (commercially available from Dymax Corporation, Torrington, CT), which is described as an aliphatic urethane acrylate oligomer having a polyether backbone. Dymax also makes available commercially a series of other (meth)acrylate-functionalized urethanes, which have a functionality of between about 1 and about 3 and demonstrate a percent elongation of greater than about 50. One such (meth)acrylate-functionalized urethane from Dymax is a tri -functional urethane acrylate oligomer, more specifically an aliphatic polyether urethane triacrylate, known as BR-990.

[0015] The (meth)acrylate-functionalized resin component can be a (meth)acrylate- functionalized urethane based on polyesters or polyethers, which are reacted with aromatic, aliphatic, or cycloaliphatic diisocyanates and capped with hydroxy acrylates. As an example, the (meth)acrylate-functionalized urethane may be made according to the following reaction scheme:

[0016] Another example of a useful (meth)acrylate-functionalized urethane is a block resin made from a saturated polyester diol (such as one sold under the tradename DESMOPHEN S- 1011-35) and dicyclohexylmethane-4,4’-diisocyanate (available commercially as DESMODUR W), and capping with 2-hydroxy ethyl acrylate, the block resin being diluted with IBOA.

[0017] The (meth)acrylate-functionalized resin component can be a hydrophobic (meth)acrylate-functionalized urethane selected from aliphatic urethane (meth)acrylates, aromatic urethane (meth)acrylates and mixtures thereof, such as polybutadiene based urethane (meth)acrylates, polyisobutylene based urethane (meth)acrylates, polyisoprene based urethane (meth)acrylate, polybutyl rubber based urethane (meth)acrylates and the mixtures thereof. Suitable commercially available hydrophobic urethane (meth)acrylates include UT-4462 and UV36301B90 available from Nippon Gohsei; CN 9014 available from Sartomer; and SUO- H8628 available from SHIIN-A T&C.

[0018] The (meth)acrylate-functionalized resin component can be a (meth)acry late- functionalized urethanes based on a polyurethane block copolymer having a backbone of alternating hard and soft segments and at least two ends. The ends each may be terminated with a vinyl ether, alkenyl ether or (meth)acrylate group. Such polyurethane block copolymers may be represented by the following general formula:wherein A is a hard segment, such as the reaction product of a poly isocyanate and an aromatic, heterocyclic or cycloaliphatic polyol;B is a divalent soft segment and X is a q-valent soft segment, such as where B and X may be a divalent and a multivalent group, respectively, derived from a polyether polyol, polyester polyol or hydrogenated hydrocarbon elastomer, such as polybutadiene;D is a vinyl ether or (meth)acrylate group, such as where the vinyl ether may be derived from hydroxy functional vinyl ethers, for instance 2-hydroxyethyl vinyl ether, 4- hydroxybutyl vinyl ether, cyclohexanedimethanol monovinyl ether, diethylene glycol monovinyl ether, 1,6-hexanediol monovinyl ether and 3-aminopropyl vinyl ether, or the vinyl ether terminal groups may be derived from an amino functional vinyl ether, in which case vinyl ether urea capped polyurethanes may be obtained; p is 0-10; and q is 2-6.

[0019] The (meth)acrylate-functionalized resin component can be a (meth)acrylate- functionalized urethane having a polyurethane backbone. In some embodiments at least a portion of the polyurethane backbone includes a urethane linkage formed from isophorane diisocyanate. For instance, such a (meth)acrylate-functionalized urethane is made from an alkylane glycol (such as polypropylene glycol), isophorane diisocyanate and hydroxy alkyl(meth)acrylate (such as hydroxyl ethyl acrylate). Other examples include a polyester of hexanedioic acid, diethylene glycol, terminated with isophorone diisocyanate, capped with 2- hydroxyethyl acrylate; a polytetramethylene glycol ether terminated with isophorone diisocyanate, capped with 2-hydroxyethyl methacrylate; and a hydroxy terminated polybutadiene terminated with isophorone diisocyanate, capped with 2-hydroxyethyl acrylate.

[0020] In one aspect the radiation curable resin component comprises a cycloaliphatic epoxide and / or oxetane. In another example, the radiation curable resin component includes an epoxy system. Epoxy resin may react with itself in the presence of a photoinitiator and sufficient actinic radiation to form a cured network. Generally, the epoxy system includes one or more epoxide groups, such as cycloaliphatic epoxies. These epoxides include organic three-membered cyclic oxygen compounds. One example of a cycloaliphatic epoxy is 3,4-epoxycyclohexylmethyl- 3,4-epoxy-cyclohexanecarboxylate (EEC).

[0021] The number average molecular weight (Mn) of the radiation curable resin component may be 1000 to 100,000, more desirably 2000 to 50,000.

[0022] The radiation curable resin component can be prepared using standard techniques known in the art or obtained from suitable commercial sources. Preparation techniques include controlled radical polymerization processes, including Single Electron Transfer Living Radical Polymerization (SET-LRP), by stable free radical polymerization (SFRP) such as reversible deactivation by coupling, or by degenerative transfer (DT). Once the polymerization is complete, the method may include further reacting the resultant polymer to form functional end groups onto the polymer. Forming functional ends on the polymer may be done, for example, by performing either an end-capping reaction or a substitution reaction.

[0023] The weight percentage of the radiation curable resin component in the photo-curable composition may range from about 10 wt.% to about 99.9 wt.%. In one example, the weight percentage of the radiation curable resin component in the photo-curable composition ranges from about 20 wt.% to about 99.9 wt.%. In another example, the weight percentage of the radiation curable resin component in the photo-curable composition ranges from about 98 wt.% to about 99.9 wt.%. The weight percentage of the radiation curable resin component in the photo-curable composition may be greater than about 95 wt.%, greater than about 97 wt.%, greater than about 98 wt.%, and / or greater than about 99 wt.%.

[0024] The photo-curable composition may optionally include one or more acrylate monomers. Some exemplary acrylate monomers include, for example, (meth)acrylic acid, methyl(meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tert-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-heptyl (meth)acrylate, n-octyl (meth)acrylate, 2- ethylhexyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate), phenyl (meth)acrylate, tolyl (meth)acrylate, benzyl (meth)acrylate, carboxyethyl (meth)acrylate, methoxyethyl (meth)acrylate, 3-methoxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2- hydroxypropyl (meth)acrylate, stearyl (meth)acrylate, glycidyl (meth)acrylate, 2-aminoethyl (meth)acrylate, gamma-(methacryloxoxypropyl) trimethoxysilane, (meth)acrylic acid-ethylene oxide adduct, trifluoromethylmethyl (meth)acrylate, 2-trifluoromethylethyl (meth)acrylate, 2- perfluoroethylethyl (meth)acrylate, 2-perfluoroethyl-2-perfluorobutylethyl (meth)acrylate, 2- perfluoroethyl (meth)acrylate, perfluorom ethyl (meth)acrylate, diperfluoromethylmethyl (meth)acrylate, 2-perfluoromethyl-2-perfluoroethylmethyl (meth)acrylate, 2-perfluorohexylethyl (meth)acrylate, 2-perfluorodecylethyl (meth)acrylate, 2-perfluorohexadecylethyl (meth)acrylate, isobornyl acrylate, dimethylacrylamide and combinations thereof.

[0025] In one example, the weight percentage of acrylate monomers in the radiation curable resin component ranges from about 20 wt.% to about 70 wt.%. In another example, the weight percentage of acrylate monomers in the radiation curable resin component ranges from about 30 wt.% to about 60 wt.%. For example, the weight percentage of acrylate monomers in the radiation curable resin component may range from about 25 wt.% to about 50 wt.%. In one example, the weight percentage of acrylamide monomers in the radiation curable resin component ranges from about 5 wt.% to about 50 wt.%. In another example, the weight percentage of acrylamide monomers in the radiation curable resin component ranges from about 10 wt.% to about 40 wt.%.

[0026] The photo-curable composition may further include a photoinitiator. The photoinitiator is capable of initiating substantial curing of the radiation curable resin component on exposure to actinic radiation. For example, the photoinitiator is capable of initiating chain growth polymerization and may undergo dissociation upon exposure to sufficient actinic radiation. In one example, the photoinitiator is selected based on the absorbency characteristics and the spectral output of an actinic radiation source. The polymerization and / or crosslinking effectiveness may be governed by the nature of monomers or oligomers used and the effectiveness of the photoinitiator. Examples of photoinitiators include benzophenone, benzoin-ether, 2- (dimethylamino)ethanol (DMAE), hydroxyacetophenones, 2-hydroxy-2-methyl-l-phenylpropan-1-one, camphor derivatives, and hydroxyl -phenyl-ketone. The weight percentage of photoinitiator in the photo-curable composition may range from about 0.1 wt.% to about 10 wt.%. In one example, the weight percentage of photoinitiator in the photo-curable composition ranges from about 1 wt.% to about 5 wt.%.

[0027] The photo-curable composition will include a latent pH adjusting agent. The latent pH adjusting agent is capable of increasing or decreasing a pH value of the photo-curable composition from a first pH value to a second pH value when (or after being) exposed to actinic radiation. The latent pH adjusting agent is capable of producing in situ photo-generated acids or bases when exposed to sufficient actinic radiation. For example, the latent pH adjusting agent is capable of producing acid (e.g., superacid) when the photo-curable composition is exposed to actinic radiation. Accordingly, the latent pH adjusting agent is capable of decreasing a pH value of the photo-curable composition when exposed to sufficient actinic radiation. In one example, applying actinic radiation in the UV range of wavelengths to the latent pH adjusting agent produces a superacid and a radical. For example, a produced superacid may be a stronger acid compared to traditional strong mineral acids such as sulfuric acid and hydrochloric acid. In one example, the latent pH adjusting agent is a photo-latent acid. A photo-latent acid is capable of producing acid in response to exposing the latent pH adjusting agent to actinic radiation.

[0028] The photo-latent acid may include an onium salt. For example, the onium salt may be a salt or salt-like compound where the central atom is of an element with nonmetallic character. In one example, the onium salt includes one or more of nitrogen, phosphorous, oxygen, sulfur, chlorine, bromine, and iodine. In another example, the onium salt includes one or more of Group 15 onium cations, Group 16 onium cations, and Group 17 onium cations. Accordingly, the onium salt may include ammonium onium cations, phosphonium onium cations, oxonium onium cations, sulfonium onium cations, fluoronium onium cations, chloronium onium cations, bromonium onium cations, and iodonium onium cations. An example of a suitable onium salt including an iodonium cation is shown below:

[0029] The onium salt may include one or more of lodonium salts and Sulfonium salts. Upon exposure to sufficient actinic radiation, the lodonium salt or Sulfonium salt may produce a Bronsted acid. In one example, the lodonium salt may include one or more salts in the lodonium borate family. One example of a salt in the iodonium borate family is 4-isopropyl-4’- methyldiphenyliodonium tetrakis (pentafluorophenyl)borate. The chemical structure of 4- isopropyl-4’-methyldiphenyliodonium tetrakis (pentafluorophenyl)borate is shown below.Additional examples of iodonium salts include lodonium hexafluoroantimonate salt and lodonium hexafluorophosphate salt. Examples of sulfonium salts include Sulfonium hexafluorophosphate salt and Sulfonium hexafluoroantimonate salt.

[0030] Alternatively, the latent pH adjusting agent may include a photo-latent base. The photo-latent base may be capable of providing the properties of an amidine base when exposed to sufficient actinic radiation. Therefore, the photo-latent base may be capable of increasing the pH value of the photo-curable composition when the photo-latent base is exposed to sufficient actinic radiation. In one example, the photo-latent base includes guanidinium 2-(3- benzoylphenyljpropionate (CAS No. 1418139-48-6).

[0031] The latent pH adjusting agent may be selected based on the solubility of the latent pH adjusting agent in the formed mixture, such as the solubility in the radiation curable resin component and / or the pH-sensitive dye. For example, the latent pH adjusting agent may be substantially soluble in the radiation curable resin component and / or the pH-sensitive dye. Further, the latent pH adjusting agent may be selected based on the responsiveness to actinic radiation, and the latent pH adjusting agent may be selected to comply with health and safety standards. For example, the latent pH adjusting agent may be selected to comply with health and safety standards for medical device bonding applications.

[0032] In one example, the weight percentage of the latent pH adjusting agent in the photo- curable composition ranges from about 0.00001 wt.% to about 5 wt.%. More can be used in some embodiments but may not be needed or economical. In another example, the weight percentage of the latent pH adjusting agent in the photo-curable composition ranges from about 0.0001 wt.% toabout 2 wt.%. In yet another example, the weight percentage of the latent pH adjusting agent in the photo-curable composition ranges from about 0.0001 wt.% to about 1 wt.%. The weight percentage of the latent pH adjusting agent in the photo-curable composition may be less than about 2 wt.%, less than about 1.5 wt.%, or less than about 1 wt.%.

[0033] The photo-curable composition will include a pH-sensitive dye. In one embodiment, the pH-sensitive dye is spectrally -responsive to the acid or base produced when the photo-curable composition is exposed to actinic radiation. Since the latent pH adjusting agent is capable of producing the acid or base, the pH value of the photo-curable composition may change from a first pH value to a second pH value. The second pH value may be higher or lower compared to the first pH value. According to one embodiment, the pH-sensitive dye exhibits a first spectral wavelength at the first pH value, and a second spectral wavelength at the second pH value, wherein the first spectral wavelength is different than the second spectral wavelength. In one example, the pH- sensitive dye does not exclusively react with a radical formed from the radiation curable resin component. Typically, the spectral wavelength is a function of the light wavelength reflected by the pH-sensitive dye and seen by an observer.

[0034] Spectral wavelengths may include any wavelength on the electromagnetic spectrum. For example, spectral wavelengths may include visible color wavelengths of the present disclosure and / or may include other wavelength ranges such as 1 nm to 400 nm. In one example, one or more of the first spectral wavelength and the second spectral wavelength is in the visible spectrum. In another example, the first spectral wavelength and / or the second spectral wavelength do not require a color that is perceptible / visible to the naked human eye. Accordingly, the first spectral wavelength and / or the second spectral wavelength may be detectible with the aid of instruments or radiation techniques known to one of ordinary skill in the art.

[0035] In one embodiment, the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance. The extent of appearance change from the first appearance to the second appearance may indicate the occurrence of cure reaction and the extent of cure reaction. In one example, the first appearance and / or the second appearance may be a spectral wavelength in the visible spectrum. The first appearance and / or the second appearance may be a visible color of the present disclosure, such as blue, green, red, or orange. In another example, the first appearance and / or the second appearance may be substantially transparent. Importantly, the pH-sensitive dyeis capable of exhibiting a different appearance depending on the pH of the photo-curable composition. Therefore, the pH-sensitive may change from a visible color appearance to substantially transparent, from substantially transparent to a visible color appearance, or from one visible color appearance to a separate, distinct visible color appearance. This change in appearance may be irreversible.

[0036] Visible color may include light bands of the electromagnetic spectrum that are visible to the human eye. Visible colors of the present disclosure include light with wavelengths ranging from 380 nm to 450 nm (violet), 450nm to 485 nm (blue), 485 nm to 500 nm (cyan), 500 nm to 565 nm (green), 565 nm to 590 nm (yellow), 590 nm to 625 nm (orange), and / or 625 nm to 750 nm (red). In one example, as used herein, substantially transparent may refer to a light transmittance of over 80%, over 85%, or over 90%. In yet another example, as used herein, substantially transparent may refer to a light transmittance of over 90%, over 95%, or over 97%.

[0037] The first appearance and / or the second appearance may be an appearance detectible by an instrument. For example, even if the naked human eye cannot detect an appearance change between the first appearance and the second appearance, an instrument, emitting UV-A wavelengths between about 320 nm and 400 nm, may be utilized to show a detectable appearance change. In one example, in component bonding applications, an instrument may be utilized to detect an appearance change, or to assist the human eye in detecting an appearance change to indicate the occurrence of cure reaction and the extent of cure reaction.

[0038] The desired pH-sensitive dye may be selected according to the desirable spectral wavelength and / or appearance at the first pH value or the second pH value. In one example, the photo-curable composition is substantially transparent at the first pH value or the second pH value. Further, a pH-sensitive dye may be selected based on the incorporation of an acid or base generating latent pH adjusting agent. For example, for medical device bonding and similar applications, a change from a visible color (such as blue or green) to a substantially transparent appearance is beneficial for verifying the extent of cure for the adhesive. Alternatively, the photo- curable composition may exhibit an appearance change from a substantially transparent appearance to a visible color to verify the extent of cure for the adhesive. Additionally, or alternatively, the desired pH-sensitive dye may be selected based on the initial or first pH value of the photo-curable composition. By selecting the pH-sensitive dye based on the initial or first pHvalue of the photo-curable composition, a pH-sensitive dye that exhibits a desired appearance at the first pH value may be selected.

[0039] The first pH value may range from about pH 2 to about pH 12. In one example, the first pH value ranges from about pH 4 to pH 10. The second pH value may range from about pH 2 to about pH 12. In one example, the second pH value ranges from about pH 4 to pH 10. In one non-limiting example, the first pH value ranges from about pH 5 to pH 7, and the second pH value ranges from about pH 3 to about pH 5. In another non-limiting example, the first pH value ranges from about pH 5 to pH 7, and the second pH value ranges from about pH 6 to about pH 8. If the latent pH adjusting agent produces an acid when exposed to sufficient actinic radiation, the first pH value may be greater than the second pH value. If the latent pH adjusting agent produces a base when exposed to sufficient actinic radiation, the first pH value may be less than the second pH value.

[0040] In one example, the pH-sensitive dye includes one or more of an anionic dye and a cationic dye. In another example, the pH-sensitive dye consists of an anionic dye. Anionic dyes may include diaminotriphenylmethane anionic dyes, triaminotriphenylmethane anionic dyes, or ring-closed triphenylmethane anionic dyes. One example of an anionic dye is a triarylmethane dye with at least two sulfonic acid groups. In another example, an anionic dye may be used with an acid producing latent pH adjusting agent, while a cationic dye may be used with a base producing latent pH adjusting agent. In yet another example, the pH-sensitive dye includes one or more of Crystal Violet, Cresol Red, Cresolphthalein (meta), Cresol Purple, Thymol Blue, Methyl Orange - Xylene Cyanol, Bromophenol Blue, Congo Red, Methyl Orange, Alizarin Red S, Bromocresol Green, Dichlorofluorescein, Methyl Red, Bromocresol Green, Methyl Red, Bromocresol Purple, Chlorophenol Red, Bromothymol Blue, Phenol Red, Naphtholphthalein (alpha), Phenolphthalein, Cresolphthalein, and Indigo Carmine. One example, Bromocresol green, is shown below.

[0041] The acidic form (left) and basic form including two resonance forms (right) of Bromocresol Green are shown below:

[0042] Relatively low levels of pH-sensitive dyes may be required to effectively demonstrate an appearance change upon curing. The weight percentage of the pH-sensitive dye in the photo- curable composition may range from about 0.00001 wt.% to about 2 wt.%. More can be used in some embodiments but may not be needed or economical. In one example, the weight percentage of the pH-sensitive dye in the photo-curable composition ranges from about 0.0001 wt.% to about 2 wt.%. In another example, the weight percentage of the pH-sensitive dye in the photo-curable composition ranges from about 0.0001 wt.% to about 1 wt.%. In yet another example, the weight percentage of the pH-sensitive dye in the photo-curable composition ranges from about 0.0001 wt.% to about 0.1 wt.%. The weight percentage of the pH-sensitive dye in the photo-curable composition may be less than about 2 wt.%, less than about 1 wt.%, or less than about 0.1 wt.%.

[0043] FIG. 1 illustrates examples of pH-sensitive dyes, according to some embodiments. As discussed, the desired pH-sensitive dye may be selected according to the desirable spectral wavelength and / or appearance at the first pH value or the second pH value. The pH-sensitive dye may be selected based on the starting pH value of the photo-curable composition prior to any exposure of actinic radiation. For example, it may be desirable to select a pH-sensitive dye that exhibits a visible color at a first pH value or starting pH value of the photo-curable composition prior to exposure to actinic radiation. In one example, the Bromocresol Green pH-sensitive dye may be selected with an onium salt to exhibit a blue or green color at the first pH value and a yellow color or substantially transparent appearance at the second pH value. In another example, the Bromothymol Blue pH-sensitive dye may be selected with a base generating latent pH adjusting agent to exhibit a yellow or substantially transparent appearance at the first pH value and a green or blue color at the second pH value.

[0044] The photo-curable composition may further include one or more sensitizers. Sensitizers may be utilized to alter the photoinitiator activation wavelength, improve reaction rate,or improve adhesion. In one example, sensitizers may be utilized with onium salts. Examples of sensitizers include thioxanthones, anthracenes, benzophenones, acetophenones, and titanocene. In one example, the weight percentage of the sensitizer in the photo-curable composition ranges from about 0.0001 wt.% to about 5 wt.%. More can be used in some embodiments but may not be needed or economical. In another example, the weight percentage of the sensitizer in the photo-curable composition ranges from about 0.0001 wt.% to about 1 wt.%. In yet another example, the weight percentage of the sensitizer in the photo-curable composition ranges from about 0.0001 wt.% to about 0. 1 wt.%.

[0045] The latent pH adjusting agent is capable of producing in situ photo-generated acids or bases when exposed to sufficient actinic radiation. The latent pH adjusting agent is capable of reacting with the pH-sensitive dye during and / or after exposure to sufficient actinic radiation to generate an observable color change. This chemistry mechanism allows for a tunable response by the pH-sensitive dye. Neither the pH adjusting agent nor the pH-sensitive dye are catalytic to the curing mechanism or the photo-curable composition. Further, the latent pH adjusting agent may be effectively added to the photo-curable composition without inhibiting the curing efficiency of the radiation curable resin component.

[0046] In one example, the photo-curable composition is non-aqueous, and the radiation curable resin component substantially cures to form an adhesive when exposed to sufficient actinic radiation. In another example, the photo-curable composition is non-aqueous and substantially free of water and / or is free of a reactive alkylating species after exposure to actinic radiation. In yet another example, the photo-curable composition is free of lanthanides and fluorescing materials. Significantly, the photo-curable compositions may be used for securing and bonding two or more distinct objects together, coatings, and for gap-filling. For example, the photo-curable compositions may be used for medical device bonding applications to verify the extent of cure reaction upon exposure to actinic radiation.

[0047] Importantly, since the photo-curable composition may be non-aqueous, it is not necessarily intuitive that an acid-base chemistry mechanism would operate correctly in this nonaqueous system. Specifically, it is not expected for acid-base chemistry of the present disclosure to occur in a photo-curable composition that may be an organic composition with less than ideal pH change conditions during crosslinking and polymerization. Importantly, acid-base dye technology can be tailored for appearance and / or visual color change according to formulationchemistry. Further, the initial or final appearance of the resin / adhesive may be tuned by selecting appropriate latent pH adjusting agents and pH-sensitive dyes. This tunability gives an increased formulating flexibility for use with a wide range of resins. Significantly, the photo-curable composition can illustrate the extent of curing reaction in addition to the occurrence of cure reaction.

[0048] The photo-curable composition can be prepared by mixing the components in the absence of relevant actinic radiation. The mixed photo-curable composition should be stored in a container opaque to actinic radiation to prevent unwanted curing.

[0049] The photo-curable composition can be disposed on a surface of a first substrate to be bonded. A surface of a second substrate is disposed over the adhesive. Actinic radiation is applied to the photo-curable composition. The applied actinic radiation will cure the radiation curable resin component and bond the first and second substrates together. The applied actinic radiation will also change a pH value of the composition from a first pH value of the present disclosure to a second pH value of the present disclosure, causing the pH-sensitive dye to change from the first appearance to the second appearance indicating the occurrence of cure reaction and in some embodiments the extent of cure reaction. This appearance may be instantly detectable by a naked human eye.

[0050] Sufficient actinic radiation may include electromagnetic radiation capable of initiating photochemical reactions. Actinic radiation, such as in the UV or visible region, may be used to interact with matter to generate chemical reactions. Actinic radiation may be provided by a light emitting diode (LED). In one example, actinic radiation includes UV radiation such as UVA (315nm to 400nm), UVB (280nm to 315nm), UVC (200nm to 280nm), and VUV (200nm to lOOnm). In another example, actinic radiation includes visible light such as with wavelengths ranging from 400nm to 480nm. In yet another example, actinic radiation includes radiation having a wavelength from about 390 nm to about 410 nm. Cure depth may be increased at longer wavelengths, such as greater than 350 nm. Actinic radiation is capable of transmitting sufficient energy to cause photoinitiators to split into free radicals. These free radicals can initiate the curing or polymerization process to produce long polymer chains from monomers.

[0051] In one example, the intensity of actinic radiation sufficient to initiate curing and / or cause sufficient curing may range from about 10 mW / cm2to about 500 mW / cm2. In another example, the intensity of actinic radiation sufficient to initiate curing and / or cause sufficient curingmay range from about 200 mW / cm2to about 450 mW / cm2. In yet another example, the intensity of actinic radiation sufficient to initiate curing and / or cause sufficient curing may range from about 300 mW / cm2to about 450 mW / cm2.

[0052] The length of time for actinic radiation exposure is determinable by one skilled in the art and depends on the selected components in the photo-curable composition. Exposure time may range from about 1 second to about 2 minutes, from about 3 seconds to about 30 seconds, or from 5 seconds to about 20 seconds. In one example, the appearance change of the photo-curable composition is permanent. In another example, heat may be applied to the photo-curable composition before, during, or after exposure of actinic radiation. For example, after activation of a photoinitiator, the polymerization reaction may be thermally driven. Therefore, cure speed may be increased with increasing temperatures. Heat may be applied to the photo-curable composition using hot air blowers and / or IR lamps.

[0053] The photo-curable composition has excellent tunability for a wide range of radiation curable resins and applications. For example, the mechanism of the present disclosure can be used with a variety of resins including acrylate monomers, acrylamide monomers, and / or epoxy systems. Further, the photo-curable composition can be tuned according to the desired appearance change after exposure to actinic radiation. For example, the pH-sensitive dye may be selected based on a desired appearance change. Additionally, the pH-sensitive dye may be selected based on the initial pH of the radiation curable resin component.

[0054] The photo-curable composition provides a reliable and cost-effective liquid to solid cure assurance. For example, the pH-sensitive dye in the photo-curable composition can act as indicator of application on a substrate, sufficient actinic radiation exposure, and / or the extent of complete cure. In bonding applications, the visibly perceived real-time appearance changes (such as a visually perceptible color change) can provide assurance of application and verification that an adhesive has received the desired amount of actinic energy to assure that the photo-curable composition is sufficiently cured.Prophetic Example 1

[0055] In one prophetic example, an onium salt can be added to a (meth)acrylate photo- curable composition and used as the latent pH adjusting agent. The mechanism during / after applying sufficient actinic radiation to an onium salt latent pH adjusting agent to produce an acid is shown below.

[0056] In the mechanism shown, R denotes alkyl and A denotes anion. As shown, sufficient actinic radiation is applied to the onium salt such that the onium salt has a higher energy state (shown as *). Upon reaching the higher energy state, a superacid (see radical cation) and radical are formed. Once a superacid and radical are formed, decomposition occurs to generate R» and AH. Accordingly, the onium salt may decrease the pH value of the photo-curable composition by generating a superacid upon exposure to sufficient actinic radiation. This can initiate a color change in a pH-sensitive dye in the photo-curable composition.Example 2

[0057] In one example, a pH-sensitive dye Bromocresol Green) was added to the radiation curable (meth)acrylate adhesive, LOCTITE AA 3921. Comparison composition A had no latent pH adjusting agent. Inventive composition B included the latent pH adjusting agent 4-isopropyl- 4’-methyldiphenyliodonium tetrakis (pentafluorophenyl)borate). Composition A (comparison composition) and Composition B are shown in Table 1.Table 1. Composition A and Composition B.

[0058] A few drops of each formulation were placed on a glass microscope slide. The color of the composition prior to actinic radiation exposure and after actinic radiation exposure was examined. The actinic radiation exposure was 400 mW / cm2for 10 seconds with 405 nm LED light. The appearance of composition A and composition B before and after actinic radiation exposure is shown in Table 2. As shown, the appearance of Composition A did not change from actinic radiation, while the appearance of Composition B changed from actinic radiation.Table 2. Appearance of Compositions before and after Cure.

[0059] While the disclosure has been escribed with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made, and equivalents may be substituted for elements thereof without departing from the scope of the embodiment(s). In addition, many modifications may be made to adapt a particular situation or material to the teachings of the embodiment(s) without departing from the essential scope thereof. Therefore, it is intended that the disclosure is not limited to the disclosed embodiment(s), but that the disclosure will include all embodiments falling within the scope of the appended claims. Various examples have been described. These and other examples are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A photo-curable composition, the composition comprising: a radiation curable resin component; a photoinitiator to initiate substantial curing of the radiation curable resin component on exposure to actinic radiation; a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing a pH value of the composition from a first pH value to a second pH value when exposed to actinic radiation; and a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.

2. The photo-curable composition of claim 1, wherein the radiation curable resin component is an organic resin including at least one of acrylate monomers and acrylamide monomers.

3. The photo-curable composition of claim 2, wherein the acrylate monomers include isobornyl acrylate.

4. The photo-curable composition of any one of claims 1 to 3, wherein the radiation curable resin component includes an epoxy.

5. The photo-curable composition of any one of claims 1 to 4, wherein the latent pH adjusting agent is capable of producing in situ photo-generated acids or bases when exposed to actinic radiation sufficient to initiate substantial curing of the radiation curable resin component.

6. The photo-curable composition of any one of claims 1 to 5, wherein the latent pH adjusting agent is a photo-latent acid.

7. The photo-curable composition of any one of claims 1 to 6, wherein the latent pH adjusting agent is a photo-latent base.

8. The photo-curable composition of any one of claims 1 to 7, wherein the latent pH adjusting agent includes an onium salt and produces an acid in response to exposing the latent pH adjusting agent to actinic radiation.

9. The photo-curable composition of any one of claims 1 to 8, wherein the latent pH adjusting agent is an lodonium Borate.

10. The photo-curable composition of any one of claims 1 to 9, wherein the weight percentage of the latent pH adjusting agent in the composition is less than about 1 wt.%.

11. The photo-curable composition of any one of claims 1 to 10, wherein the pH-sensitive dye is an anionic dye.

12. The photo-curable composition of any one of claims 1 to 11, wherein the composition is non-aqueous, and the radiation curable resin component substantially cures to form an adhesive when exposed to sufficient actinic radiation.

13. The photo-curable composition of any one of claims 1 to 12, wherein one or more of the first appearance and the second appearance is a spectral wavelength in the visible spectrum.The photo-curable composition of any one of claims 1 to 13, wherein the composition is substantially transparent at the second pH value.The photo-curable composition of any one of claims 1 to 14, wherein the composition is non-aqueous and substantially free of water.The photo-curable composition of any one of claims 1 to 15, wherein the composition is free of a reactive alkylating species after exposure to actinic radiation.

17. The photo-curable composition of any one of claims 1 to 16, wherein the composition is free of lanthanides and fluorescing materials.

18. A method of curing a photo-curable composition, the method comprising: applying actinic radiation to a photo-curable composition sufficient to change a pH value of the composition from a first pH value to a second pH value, wherein the appearance of the photo-curable composition is different at the first pH value and the second pH value, wherein the photo-curable composition includes: a radiation curable resin component; a photoinitiator to initiate substantial curing of the radiation curable resin component on exposure to actinic radiation; a latent pH adjusting agent, wherein the latent pH adjusting agent is capable of increasing or decreasing the first pH value of the composition to a second pH value when exposed to actinic radiation; and a pH-sensitive dye, wherein the pH-sensitive dye exhibits a first appearance at the first pH value, and a second appearance at the second pH value, wherein the first appearance is different than the second appearance.

19. The method of claim 18, wherein the latent pH adjusting agent includes an onium salt and applying actinic radiation in the UV range of wavelengths to the photo-curable composition produces a superacid and a radical.

20. The method of claim 18 or 19, wherein the radiation curable resin component is an organic resin including at least one of acrylate monomers and acrylamide monomers.

21. The photo-curable composition of any one of claims 18 to 20, wherein the extent of appearance change from the first appearance to the second appearance indicates the occurrence of cure reaction and the extent of cure reaction.exposure of a (meth)acrylic adhesive to curing radiation.22

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