WO1997005100A1

WO1997005100A1 - Chromophoric photocrosslinking compound

Info

Publication number: WO1997005100A1
Application number: PCT/US1995/009600
Authority: WO
Inventors: Steven M. Heilmann; Gaddam N. Babu; Larry R. Krepski; Howell K. Ii Smith; Daniel E. Mickus
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-07-28
Filing date: 1995-07-28
Publication date: 1997-02-13

Abstract

A photoactive compound that is the reaction product of an alkenyl azlactone compound and a nucleophilic aromatic ketone is described. This compound can be used, for example, to cross-link acrylic polymers via a hydrogen abstracting mechanism.

Description

CHROMOPHORIC PHOTOCROSSLINKING COMPOUND

BACKGROUND OF THE INVENTION

1. Field ofthe Invention This invention relates to photoactive crosslinking compounds prepared by reacting an electrophilic 2-alkenyl azlactone compound and a nucleophilic aromatic ketone. These photoactive compounds can be used to crosslink, for example, acrylic polymers.

2. Background Information Pressure sensitive adhesives (PSAs) made by photopolymerizing an alkyl acrylate and a polar copolymerizable monomer are known in the art. See, e.g., U.S. Patent Nos. RE 24,906, 4,181,755, 4,364,972, and 4,243,500. Acrylic-based PSAs exhibit good adherence to high energy (i.e., polar) substrates.

Solvent-processed acrylic PSA compositions can be crosslinked by adding a polyfunctional crosslinking agent that reacts with a reactive group present in the polymer. See, e.g., Japanese Kokoku 58[1983]-046236 in which is described a solvent-processed crosslinked acrylic PSA wherein incorporated isocyanate groups are available for reaction with the crosslinking agent.

Hot melt coating a PSA composition eliminates the necessity of solvent processing. To hot melt process an adhesive composition, the composition must be uncrosslinked during the coating process; however, to achieve a PSA with balanced properties (i.e., peel and shear adhesion), the composition eventually must be crosslinked. In hot melt coating processes, this is usually done by exposure to high energy radiation (e.g., E-beam or high intensity ultraviolet radiation). Commonly, when high intensity ultraviolet radiation is used, a photoactive crosslinking species such as benzophenone is added to the composition.

A more efficient method of photocrosslinking involves incoφorating hydrogen abstracting moieties into the polymer backbone prior to coating. Such polymers can be hot melt coated and subsequently cured by conventional irradiation techniques. This process is typified by U.S. Patent No. 4,737,599 where a PSA with good adhesion to skin is described.

The cohesive strength of an acrylic PSA can be increased without unduly affecting its compliance by utilizing a photoactive crosslinking agent in conjunction with a photoinitiator. See, e.g., U.S. Patent Nos. 4,181,752, 4,329,384, 4,330,590, 4,391,687, and 5,202,361. Useful photoactive crosslinking agents include various aldehydes, quinones, and particularly certain chromophore- substituted halomethyl-.s-triazines (because they provide desirably shortened reaction times and somewhat greater tolerance to oxygen over the non- halomethyl-containing agents), although their use can result in evolution of HCl. Copolymerizable photoinitiators such as 2-[4-(2-hydroxy-2,2-dimethyl-l- oxopropyl)phenoxy]ethyl 2-propenoate and their use in the polymerization of ethylenically unsaturated compounds is disclosed in U.S. Patent No. 4,922,004. Japanese Kokai 2[1990]-248482 describes a photocurable PSA obtained by reacting (a) 30 to 50 parts by weight (pbw) of a copolymer of an acrylic acid alkyl ester, a copolymerizable ethylenically unsaturated monomer having a polar group, and a copolymerizable monomer with a photosensitizing group (such as 2- acryloyloxybenzophenone or 1 -acryloyloxy-2- [4-(4-chlorobenzoyl)benzoyl- oxy]ethane); (b) 40 to 60 pbw of an aryloxy acrylic monomer such as phenoxyethyl acrylate or nonylphenoxyethyl acrylate; and (c) a tackifying resin. The composition is cured using a total dose of energy of 300 to 800 mJ/cm² from a high pressure mercury lamp. Such high intensity ultraviolet radiation is likely to produce an adhesive that has a shear strength value less than 100 minutes. Similarly, DE 42 03 183 Cl (Germany) discloses a method for producing

PSA layers comprising the steps of thickening a monomer mixture that includes a photoinitiator with a separately made solvent-free saturated UN-reactive polyacrylate, coating the thickened mixture onto a substrate, and irradiating the coated substrate. The separately made polymer comprises side chains that, when irradiated, participate in crosslinking reactions. The sole example involves the addition of a commercially available polymer having a molecular weight of about 200,000 to a monomer mixture that is then polymerized.

The shear values of PSAs prepared by actinically irradiating acrylic monomers can be enhanced by the addition of polyacrylic crosslinking agents. See, e.g., U.S. Patent No. 4,379,201. Such PSAs involve networks and are sensitive to processing conditions.

An ultraviolet (UN) radiation-curable composition that includes a copolymer of ethylenically unsaturated monomers, ethylenically unsaturated monomers, and optionally one or more polyethylenically unsaturated compounds is described in U.S. Patent No. 5,180,756.

When attempting to photocrosslink acrylic PSA compositions, one of two broad categories of photoactive crosslinking agents is generally used: an α- cleaving agent or a hydrogen abstracting agent. Ofthe latter category, the most commonly used example is probably acryloylbenzophenone (ABP). This photocrosslinker is an efficient crosslinker, but it is not always soluble in the relatively non-polar monomers that make up PSA monomer formulations.

Acrylic derivatives of anthraquinone, benzophenone, xanthone, thioxanthone, and 9-fluorenone have been described previously, as has an acrylamide derivative of anthraquinone. However, none of these compounds has been described as being useful as a reactive crosslinker for PSA compositions. What has not been previously described is an easily prepared, effective hydrogen abstracting-type photocrosslinking agent that exhibits enhanced solubility in relatively non-polar monomers.

SUMMARY OF THE INVENTION

Briefly, the present invention provides an easily synthesizable photoactive crosslinking compound that has the general formula

wherein R¹ is H or a Ci to C₃ alkyl group, preferably H or a methyl group; R² and R³ are independently H, an alkyl group having 1 to 14 carbon atoms, a cycloalkyl group having 3 to 14 carbon atoms, an aryl group having 5 to 12 ring atoms, an arenyl group having 6 to 26 carbon and 0 to 3 S, N, and nonperoxidic O heteroatoms, or R² and R³ taken together with the carbon to which they are attached form a carbocyclic ring containing 4 to 12 ring atoms; n is O or 1;

A is XCR⁴R⁵, p^CH CHR¹)].,, or X— {(CHZCHR'Y)]. where X is O, S, NH, or NR⁴; Y is O, C(O)O, OC(O)NH, OC(O)O, or NHC(O)O; R⁴ and R⁵ are independently H, a Ci to Cβ alkyl group, or an aryl group; and m is 0 or 1; and Z is a moiety derived from an acetophenone, benzophenone, anthraquinone, 9- fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone.

In another aspect, the present invention provides a method of making the above photoactive crosslinking compound comprising the steps of solubilizing and allowing to react a z-alkenyl azlactone compound and a nucleophilic aceto¬ phenone, benzophenone, anthraquinone, 9-fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone. This reaction can be facilitated by the addition of a catalyst comprising a nitrogen-containing base, preferably a bicyclic amidine or guanidine, or a trivaient phosphorous compound. Unless otherwise indicated, the following definitions apply throughout this document: "group" or "compound" or "moiety" or "monomer" or "polymer" means, unless otherwise noted, a chemical species that can be substituted by conventional substituents that do not interfere with the desired product, e.g., alkyl, alkoxy, aryl, dialkylamino, halo, nitro, and cyano groups;

"alkyl" means the monovalent residue remaining after removal of one hydrogen atom from a saturated linear or branched chain hydrocarbon having 1 to 14 carbon atoms; "aryl" means the monovalent residue remaining after removal of one hydrogen atom from an aromatic or heteroaromatic compound that can consist of one ring or two fused or catenated rings having 5 to 12 ring atoms which can include up to 3 heteroatoms selected from S, N, and nonperoxidic O, and in which the carbon atoms can be substituted by up to three halogen atoms, Ci to C₄ alkyl groups, Ci to C₄ alkoxy groups, N,N-di(Cι to C alkyl)amino groups, nitro groups, cyano groups, and Cι-C₄ alkyl carboxylic ester groups; and

"azlactone" means a compound having the general formula

wherein n, R¹, R², and R³ are defined as before.

The photoactive crosslinking compound ofthe present invention can be used to crosslink, for example, acrylic adhesive compositions in much the same way as ABP. However, the synthesis ofthe photoactive crosslinking compound ofthe present invention involves a simple addition reaction of an electrophilic azlactone and a nucleophilic aromatic ketone with no side products being created.

These addition products are acrylamidoacetyl- (or propionyl-) functional and, accordingly, are very reactive in free radical-initiated mono- and copolymerization reactions. Also, the addition products are more hydrolytically stable than their acrylate counterparts.

A significant advantage of using the 2-alkenyl azlactone instead of acryloyl chloride as an acylating agent is that the azlactone nucleophile reaction involves ring-opening addition; no smaller by-product molecule (such as hydrogen chloride) is displaced or generated in the reaction. The acrylamide functionality can offer certain advantages as a polymerizable group over the acrylate. The amide group is known to be more difficult to hydrolyze than the ester group; therefore, amide-functional polymers are expected to be more environmentally stable. Additionally, according to information published in the Polymer Handbook, 2nd edition, edited by J.

Brandrup and E.H. Immergut, Wiley-Interscience, New York, 1975, pp. II 47-49, acrylamides enjoy rates of free radical polymerization substantially faster than corresponding acrylates or methacrylates. For example, N,N-Dimethylacrylamide exhibits a rate of bulk polymerization (k_p ²/k_t) at 50°C 1142 times faster than methyl acrylate and 457 times faster than methyl methacrylate.

The acrylamide derivatized compounds ofthe present invention also provide an advantage over the previously described acrylamide derivative of anthraquinone in that the compounds ofthe present invention are more soluble in non-polar monomers because ofthe longer chain length between the unsaturated group and the carbocycle moiety. The carbon atoms in the chain aid in solubilizing the compounds ofthe present invention when used in conjunction with non-polar monomers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The photoactive crosslinking compound ofthe present invention has the general formula

wherein n, R¹, R², R³, A, and Z are defined as above. Preferably, the nucleophilic group (from A) is separated from the ring system of Z by at least one, preferably two, methylene groups. The various compounds from which Z can be derived are all aromatic ketones. Such ketones are known to be "hydrogen abstracting agents". When activated by absoφtion of ultraviolet light, these Z groups can act to crosslink various polymer systems.

As mentioned previously, Z is a moiety derived from an acetophenone, benzophenone, anthraquinone, 9-fluorene, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone. These nucleophilic aromatic ketones can be substituted with any functional group that is not a nucleophile (which would interfere in the reaction ofthe nucleophilic group ofthe aromatic ketone with the electrophilic azlactone). Potentially useful functional groups include alkyl, alkoxy, aryl, dialkylamino, halo, nitro, and cyano groups. Preferred Z groups include those derived from an acetophenone, benzophenone, anthraquinone, thioxanthone, chromone, and benzil. Particularly preferred are benzophenone and anthraquinone. Preferred photoactive crosslinking compounds include those where X (in A) is oxygen or NH, and where n is 0. Examples of preferred crosslinking compounds have the general formula

where R¹ is H or a methyl group (preferably H), D is — (OCH₂CH₂O) — or — (NHCH₂CH₂O) — , and Z is a moiety derived from those compounds listed previously, preferably from an acetophenone, benzophenone, anthraquinone, thioxanthone, chromone, or benzil. Particularly preferred among those compounds ofthe above formula are those where D is — (OCH₂CH₂O) — -

The photoactive crosslinking compound ofthe present invention can be prepared by the ring-opening of an electrophilic 2-alkenyl azlactone compound and simultaneous reaction with a nucleophile-substituted aromatic ketone. Suitable nucleophiles include hydroxyl, primary amine, secondary amine, and thiol groups.

Alkenyl azlactones can be prepared by methods well known in the art. See, e.g., Iwakura et al., Tetrahedron, 23, 3363 (1967); Hubner et al., Makromol. Chem., 11, 109 (1970); Taylor et al., J. Poly. Sci., Poly. Let. Ed, 7, 597 (1969); and U.S. Patent Nos. 4,304,705 and 4,777,276. These methods involve subjecting an amino acid having the general formula H₂N(CH₂)_nCR²R³COOH (wherein n, R², and R³ are defined as above) to acylation with an ethylenically unsaturated acylating agent having the general formula H₂C=CR¹C(O)Cl (wherein R¹ is defined as above) using the method described by, for example, Kulkari et al., J. Poly. Sci., 54, 491 (1961) in which the acylating agent (preferably containing a polymerization inhibitor such as hydroquinone) and an equivalent amount of an acid absorber (e.g., aqueous NaOH) are added portionwise to a chilled (e.g., 0°C), vigorously stirred aqueous solution of an equimolar amount of an alkali metal salt ofthe amino acid, followed by neutralization with an aqueous acid (e.g., 6 N HCl), and isolation ofthe unsaturated peptide carboxylic acid product. This product is then dehydrated by introduction of a dehydrating agent (such as, for example, acetic anhydride, ethyl chloroformate, or dicyclohexylcarbodiimide) to give a 2-alkenyl azlactone.

Because ofthe wider availability of starting amino acids and their greater thermodynamic stability (reflected in higher synthetic yields), the 5-membered ring species are preferred. Examples of suitable 5-membered ring azlactones include 2- ethenyl- 1 ,3-oxazolin-5-one; 2-ethenyl-4-methyl- 1 ,3-oxazolin-5-one; 2- isopropenyl- l,3-oxazolin-5-one; 2-isopropenyl-4-methyl-l,3-oxazolin-5-one; 2- ethenyl-4,4-dimethyl-l,3-oxazolin-5-one; 2-isopropenyl-4,4-dimethyl-l,3- oxazolin-5-one; 2-ethenyl-4-methyl-4-ethyl- 1 ,3-oxazolin-5-one; 2-isopropenyl-4- methyl-4-ethyl-l,3-oxazolin-5-one; 2-ethenyl-4,4-dibutyl-l,3-oxazolin-5-one; 2- isopropenyl-4-methyl-4-butyl- 1 ,3-oxazolin-5-one; and 2-isopropenyl-4-methyl-4- dodecyl- 1, 3 -oxazolin-5-one, although other such compounds will be apparent to those skilled in the art. Preferred azlactones are 2-ethenyl-4,4-dimethyl-l,3- oxazolin-5-one and 2-isopropenyl-4,4-dimethyl-l,3-oxazolin-5-one.

Nucleophile-substituted aromatic ketones that can be used in the present invention include, but are not limited to, the following compounds:

anthrone

X = O (xanthone),

S (thioxanthone), or dibenzosuberone NH (acridone)

chromone flavone (a type of chromone)

benzil acetophenone wherein A is defined as above.

The ring-opening reaction ofthe electrophilic azlactone compound and the nucleophile-substituted aromatic ketone can be catalyzed by nitrogen-containing bases, such as bicyclic amidines and guanidines, or trivaient phosphorus compounds. Bases that have been found to be particularly useful catalysts are selected from the group consisting of

(a) bicyclic amidines and guanadines having the general formula

wherein R⁶ and R⁷ independently represent an alkylene group or an alkyl- or aryl- substituted alkylene group of 2 to 12 carbon atoms, R⁸ is an alkyl or aryl group, and m is 0 when the base is an amidine or 1 when the base is a guanidine;

(b) trivaient phosphorus compounds having the formula R^¹^¹^ wherein R⁹, R¹⁰, and R¹¹ are independently H, an alkyl group, an aryl group, an arenyl group, a lower (i.e., 2 to 8 carbon atoms) alkoxy group, or a lower dialkyl amino group; and

(c) polymer-bound amidines and phosphines.

Examples of useful amidines include l,5-diazabicyclo[4.3.0]non-5-ene

l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) An example of a useful guanidine is l,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)

DBN and DBU are available from Aldrich Chemical Co. (Milwaukee, Wis.) while TBD is available from Fluka Chemical Coφ. (Ronkonkoma, NY). These and other amidines can also be prepared by methods well known in the art.

Examples of useful trivaient phosphorus compounds include trimethylphosphine, triethylphosphine, triethylphosphite, tributylphosphine, trioctylphosphine, tris(dimethylamino)phospine, dimethylphenylphosphine, diphenylmethylphosphine diphenylphosphine, dipropylphosphine, l,2-bis(di-n- propylphosphino)ethane, l,3-bis(diphenylphosphino)propane, diethylmethoxyphosphine, and triphenylphosphine.

The unusual effectiveness of these catalysts is not well understood. The fact that both stronger and weaker bases are less effective as catalysts indicate that factors other than base strength might be important.

When used, the amount of catalyst utilized in the instant process can vary from about 0.1 mole percent (based on the amount of azlactone present) to about 50 mole percent or more. However, 0.5 to 5 mole percent is sufficient to provide a reasonable reaction rate in most instances.

After mixing the alkenyl azlactone and nucleophilic aromatic ketone (optionally in the presence of a catalyst), preferably in equimolar amounts, the reactants are preferably allowed to react at room temperature (about 25 °C) to form the photactive crosslinking compound ofthe present invention. As those skilled in the art will recognize, these conditions can be modified to maximize yield or rate. For example, reaction temperatures from about 0°C to about 100°C or so can be utilized to carry out the process ofthe instant invention.

In certain cases nonreactive solvents or diluents can be utilized to facilitate or mediate the reaction. By "nonreactive" is meant that the solvents do not contain functional groups that can react with either the azlactone, the aromatic ketone, or the catalyst (when present) under the conditions utilized. Suitable nonreactive organic solvents include, for example, ethyl acetate, toluene, xylene, acetone, methyl ethyl ketone, acetonitrile, tetrahydrofuran, hexane, heptane, dimethylformamide, dimethylacetamide, and combinations thereof. In some instances, addition to the reaction mixture of an effective amount (e.g., 0.00005 to 0.5 weight percent based on the combined weight of azlactone and aromatic ketone) of an antioxidant or free radical inhibitor, such as a hindered phenol, can be advantageous.

The photoactive crosslinking compounds ofthe present invention can be used in the preparation of viscoelastomeric materials, preferably PSAs. This can be accomplished by mixing from about 0.0001 to about 5 parts by weight (pbw) of a photoactive crosslinking compound into 95 to 99.9999 pbw ethylenically unsaturated monomer(s) (such as, for example, acrylic acid and isooctyl acrylate). This can be done either before or after the monomer(s) have been partially polymerized to form a monomer-polymer syrup. This syrup is preferably of a coatable viscosity and is polymerizable to a viscoelastomeric material that can be crosslinked directly or hot-melt coated (for example, when no polyethylenically unsaturated monomer is present) and then crosslinked. The viscoelastomeric material is preferably a PSA having high shear at both ambient and elevated temperatures. The syrup comprises a solute polymer in a solvent monomer mixture. The polymer preferably has a very high molecular weight (e.g., at least about 100,000), preferably at least 500,000, more preferably at least 750,000, even more preferably at least 1,000,000, most preferably at least 1,500,000. One or both ofthe polymer and monomer contains at least one radiation-sensitive hydrogen abstracting group (from the photoactive crosslinking compound that, upon exposure to UN radiation, is activated to enable curing. The cured product is a crosslinked viscoelastomeric material.

The polymer ofthe syrup contains side chains that comprise radiation- sensitive hydrogen abstracting groups activatable by UN radiation, resulting in a crosslinked viscoelastomeric product. Where no photoactive crosslinking compound is present in the initial monomer mixture, some polymer that includes side chains comprising the aforementioned radiation-sensitive hydrogen abstracting groups or some photoactive crosslinking compound must be added to the syrup prior to formation ofthe viscoelastomeric material therefrom, i.e., polymerization ofthe monomer(s) ofthe monomer mixture. Preferably, however, the solute polymer is prepared in situ, i.e., directly from the solvent monomer mixture. This eliminates the need for solubilizing a separately made polymer in a monomer mixture and allows very high molecular weight polymers to be formed and solubilized.

Crosslinked viscoelastomeric materials produced from such a syrup can be used as PSAs, vibration damping materials, transfer adhesives, structural adhesives, protective coatings, and the like. Advantageously, such a syrup can have a coatable viscosity and can therefore be applied to a substrate prior to curing, thus allowing for the simple production of articles comprising one or more layers ofthe aforementioned viscoelastomeric material.

Preferably, a saturated energy-activated initiator of polymerization is used in forming the polymer component ofthe syrup from the solvent monomer component. These energy-activated sources can be either heat- or UN radiation- activated. Examples of heat-activated sources include benzoyl peroxide, t-butyl perbenzoate, cumene hydroperoxide, azobis(isobutyronitrile), and methyl ethyl ketoperoxide. Useful UN radiation-activated initiators include the benzoin ethers such as benzoin methyl ether and benzoin ispropyl ether; substituted acetophenones such as 2,2-diethoxyacetophenone, commercially available as Irgacure™ 651 photoinitiator (Ciba-Geigy Coφ.; Ardsley, ΝY), 2,2-dimethoxy-2- phenyl-1-phenylethanone, commercially available as Esacure™ KB-1 photo¬ initiator (Sartomer Co.; West Chester, PA), and dimethoxyhydroxyacetophenone; substituted α-ketols such as 2-methyl-2-hydroxy propiophenone; aromatic sulfonyl chlorides such as 2-naphthalenesulfonyl chloride; and photoactive oximes such as l-phenyl-l,2-propanedione-2-(O-ethoxycarbonyl)oxime. Particularly preferred among these are the substituted acetophenones. A saturated energy-activated source of free radicals can be present in an amount from 0.0001 to about 3 pbw, preferably from about 0.001 to about 1.0 pbw, more preferably from about 0.005 to about 0.5 pbw, per 100 pbw ofthe solvent monomer mixture. When present and upon activation through introduction of appropriate energy, the saturated energy-activated initiator of polymerization initiates the polymerization ofthe free radically-polymerizable ethylenically unsaturated monomers. When a photoactive crosslinking compound is also present, it also can be incoφorated into the backbone chain ofthe polymer, resulting in radiation- sensitive hydrogen abstracting groups pendent from the backbone chain.

Where a saturated heat-activated initiator is used with a monomer mixture that includes at least one photoactive crosslinking compound, the syrup can be exposed to heat only or to heat and UN radiation so as to initiate polymerization ofthe monomer mixture.

One or more free radically-polymerizable polyethylenically unsaturated monomers can be included in the monomer mixture or, preferably, added to the syrup. Use ofsuch monomer(s) allows for a reduction in the amount of photoactive crosslinking compound necessary to produce a viscoelastomeric material.

Although viscoelastomeric films can be prepared directly from the solvent monomer mixture (by quickly polymerizing a coated layer ofthe monomer to a polymer/monomer mixture), increasing the viscosity ofthe monomer mixture to a level more suitable for coating is preferred. This is readily accomplished by exposing the monomer(s) to a source of energy until about 0.1 to 35% (by wt.), preferably about 1 to 10% (by wt.), more preferably about 3 to 7% (by wt.), ofthe monomers have polymerized. If the source of energy is heat, a heat-activated initiator of free radicals can be included in the composition. If the source of energy is UN radiation, a radiation-activated source of free radicals can be used but is not absolutely required where a monomer ofthe monomer mixture contains a radiation sensitive group that produces free radicals on exposure to suitable radiation. Use of a radiation-activated source of free radicals is preferred in such situations, however.

The syrup is preferably prepared in situ by mixing one or more free radically-polymerizable ethylenically unsaturated monomers and 0 to 3 pbw of one or more ofthe photoactive crosslinking compounds and then polymerizing the monomer(s) to form a solute polymer. The monomers can be added in any order. Where no radiation-sensitive hydrogen abstracting groups are present in either the solute polymer or the solvent monomer mixture, some of these groups must be introduced into the syrup prior to formation ofthe viscoelastomeric material. This can be done by adding photoactive crosslinking compound to the composition after formation ofthe solute polymer or by adding to the syrup a second polymer (made separately from the syrup) that contains mer units with the above-described radiation-sensitive hydrogen abstracting groups pendent therefrom. Adjuvants, when desired, can thereafter be blended into the mixture. A syrup of a coatable viscosity can be applied to a substrate, preferably a flexible carrier web, using any conventional coating means such as roller coating, dip coating, knife coating, and extrusion coating. The substrate can further comprise a release coating between the substrate and the syrup or on the side of the substrate opposite the side on which the syrup is coated. Once a syrup has been prepared, a crosslinked viscoelastomeric material can be prepared therefrom in a variety of ways. In each method, however, the remaining monomer(s) in the syrup are polymerized by exposure to radiation that activates the hydrogen abstracting groups and facilitates crosslinking.

One way to make the viscoelastomeric material from the remaining monomer(s) is to irradiate the syrup with both high and low intensity UN radiation. Low intensity radiation is defined as 10 mW/cm² or less (as measured in accordance with procedures approved by the United States National Institute of Standards and Technology as, for example, with a UVTMAP™ UM 365 L-S radiometer manufactured by Electronic Instrumentation & Technology, Inc., in Sterling, VA), preferably in the wavelength region of 200 to 600 nm, preferably 280 to 400 nm. High intensity radiation is defined as anything greater than 10 mW/cm², preferably between 15 and 450 mW/cm². When such radiation is used, the viscoelastomeric material can be formed directly from the syrup.

Other ways of making the viscoelastomeric material involve initially exposing the syrup to only low intensity radiation. Syrup formulations that produce high performance viscoelastomeric materials will depend on the particular crosslinker and its ability to be activated by the particular radiation used.

Polymerization is preferably performed in an inert (i.e., oxygen free) atmosphere, such as a nitrogen atmosphere. Tolerance to oxygen can be increased by including in the syrup an oxidizable tin compound, as is taught in U.S. Patent No. 4,303,485.

A monomer-polymer syrup can be cured in air by covering a layer ofthe photoactive coating with a plastic film that is substantially transparent to UV radiation but impervious to oxygen and irradiating the composition through that film using UV lamps that emit light in the wavelength range corresponding to the absoφtion maximum ofthe hydrogen abstracting groups and saturated photoinitiator. Several different commercially available lamps, including medium pressure mercury lamps and low-intensity fluorescent lamps, can be used. The radiation intensity of these lamps is preferably adjusted so that the radiation intensity at the surface ofthe coating is less than 20 mW/cm², preferably 0.5 to 6 mW/cm², each having emission maxima between 200 and 600 nm, preferably between 280 and 400 nm. Maximum efficiency and rate of polymerization is dictated by the relationship between emission properties ofthe radiation source and absoφtion properties ofthe photoactive compounds employed. Where the saturated energy-activated initiator in the syrup is heat- activated, the syrup preferably is exposed to a heat source either before or simultaneously with exposure to radiation of a wavelength that activates the hydrogen abstracting groups present in the monomer and/or the polymer ofthe syrup. Where the energy-activated initiator in the syrup is a saturated UV radiation-activated initiator, the syrup preferably is exposed first to a wavelength of radiation that activates the saturated initiator until the monomers polymerize to a coatable viscosity so that the syrup can be coated on a substrate. This coated composition is exposed to radiation of a wavelength to which at least the hydrogen abstracting group ofthe photoactive crosslinking compound is sensitive at an intensity of less than 10 mW/cm² (for a total dose of 30 to 800 mJ/cm²) so as to further polymerize the monomers as well as crosslink the polymer chains.

Further details of this syrup process can be found in assignee's copending

PCT application (Attorney Docket no. 48960PCT2A). Objects and advantages of this invention are further illustrated by the following examples. The particular materials and amounts, as well as other conditions and details, recited in these examples should not be used to unduly limit this invention.

EXAMPLES

Example 1 : Preparation of 4-[2-(N-2-propenyl)amino-2- methylpropanoyloxyjbenzophenone

To a 250 mL round bottom flask were added 6.95 g (0.05 mole) 2-vinyl-

4,4-dimethylazlactone (SNPE, Inc.; Princeton, NJ), 9.90 g (0.05 mole) 4-hydroxy- benzophenone (Aldrich Chemical Co.; Milwaukee, WI), and 50 mL ethyl acetate.

This solution was magnetically stirred at room temperature while 0.38 g (0.00025 mole) DBU was added.

The solution immediately turned yellow and, within 30 minutes, an insoluble powdery white solid formed. This solid was collected by filtration, washed with cold ethyl acetate, and dried in vacuo. A total of 12.45 g was collected (74% yield), a sample ofwhich was found to have a melting point of

131-131.5°C.

IR and NMR spectroscopy was used to identify the product as 4-[2-(N- propenoyl)amino-2-methylpropanoyloxy]benzophenone (i.e., AcBP).

Examples 2-21: Comparison of ABP and AcBP

To a series of glass jars were added 90 pbw IOA, 10 pbw AA, and 0.04 pph 2,2-dimethoxy-2-phenyl-l-phenylethanone (Ciba-Geigy Coφ.; Ardsley, NY).

To some of the jars were added varying amounts of acryloxybenzophenone (hereinafter "ABP") made according to the procedures known in the art

(Examples 2-6), or AcBP (Examples 12-16). (To other of the jars, ABP or AcBP was not added until after the contents of the jar had been partially polymerized so as to provide a syrup.) Each jar was purged with nitrogen and the contents exposed to low intensity UV radiation so as to partially polymerize the monomers and form coatable mixtures. To each mixture was added an additional 0.12 pph 2,2-dimethoxy-2-phenyl-l-phenylethanone, 0.05 pph HDDA, and, to the jars to which no ABP or AcBP had previously been added, varying amounts of ABP (Examples 7-11) or AcBP (Examples 17-21). (Although the amounts by weight of AcBP are higher than those of ABP, the moles of each initiator were identical.) Each mixture was coated on polyethylene-coated silicone treated paper release liner at a thickness of 0.13 mm while the oxygen level ofthe curing chamber was maintained at about 200 ppm, each coated mixture was exposed to low intensity radiation for about 104 seconds at an average intensity of 2.0 mW/cm². Both shear strength and peel strength measurements were then taken. A 200 mJ/cm² high intensity exposure at an average intensity of 18 mW/cm² was thereafter applied and the peel strength values were again measured.

The test procedures were the same as described above with the exception that the stainless steel peel test were performed 20 minutes after the adhesive films were applied to the substrates.

Table I

Ex. No. Initiator (pbw) Low Intensity Radiation Only Low and High Intensity

Radiation

Added to Added to Shear Strength Peel Shear Strength Peel monomer the syrup mixture (min) (N/dm) (min) (N/dm)

RT 70°C RT 70°C

2 0.0075 - 3793 50 218 4006 57 187

3 0.05 ~ 5528 57 191 10,000+ 477 184

4 0.10 - 7247 71 185 10,000+ 10,000+ 187

5 0.15 - 10,000+ 181 210 10,000+ 10,000+ 188

6 0.20 — 10,000+ 4911 214 10,000+ 10,000+ 175

7 - 0.0075 2257 38 187 3221 51 203

8 -- 0.05 3620 46 195 8680 79 182

9 ~ 0.10 90 6 134 1937 35 132

10 -- 0.15 4047 62 189 10,000+ 109 184

11 - 0.20 5364 48 208 10,000+ 93 195

12 0.01 - 4981 46 182 4180 41 178

13 0.067 - 3084 40 174 10,000+ 2883 166

14 0.133 - 7295 50 179 10,000+ 10,000+ 181

15 0.20 - 10,000+ 114 181 10,000+ 10,000+ 172

16 0.267 - 10,000+ 195 180 10,000+ 10,000+ 181

17 - 0.01 3705 41 168 4706 29 189

18 - 0.067 4314 36 176 10,000+ 47 182

19 - 0.133 5893 35 173 10,000+ 124 177

20 - 0.20 5432 56 167 10,000+ 84 180

21 - 0.267 5968 45 182 10,000+ 81 175

The data of Table I show that AcBP acts as an initiator of ethylenically unsaturated monomer systems in much the same way as does ABP.

Example 22: 3-[2-(Ν-2-propenyl)amino-2-methylpropanoyloxy]acetopheone (AcAc)

A mixture of 27.2 g (0.20 mol) 3-hydroxyacetophenone (Aldrich Chemical Co.), 27.8 g (0.20 mol) vinyldimethyl azlactone (SNPE, Inc.), and 0.50 g (3.3 mmol) DBU was heated at 90°C for 18 hours. The product was recrystallized from aqueous ethanol to afford 30.7 g (55%) ofthe acrylamide as a white solid, with a melting point of 115-117°C. IR and NMR spectra were consistent with those expected for the desired product.

Examples 23-28: Testing of AcAc

Syrups including AcAc were coated, polymerized, and tested in the same manner as described in Examples 2-21. The results are given below in Table II. (All samples were exposed to high intensity radiation).

Table II

Example No. AcAc (pbw) Shear Strength (min)

Added to Added to the RT 70°C monomers syrup

23 0.1 — 1105 25

24 0.3 — 1948 34

25 0.5 — 2595 49

26 — 0.1 4129 90

27 — 0.3 1883 105

28 — 0.5 10K + 4162

The data of Table II show that AcAc can act as a reactive photocrosslinker. Improvement of shear values is believed to be possible through optimization of light source. Various modifications and alterations that do not depart from the scope and spirit of this invention will become apparent to those skilled in the art. This invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims

We claim:

1. A photoactive crosslinking compound having the general formula:

wherein R¹ is H or a Ci to C₃ alkyl group; R² and R³ are independently H, an alkyl group having 1 to 14 carbon atoms, a cycloalkyl group having 3 to 14 carbon atoms, an aryl group having 5 to 12 ring atoms, an arenyl group having 6 to 26 carbon atoms and 0 to 3 S, N, and nonperoxidic O heteroatoms, or R² and R³ taken together with the carbon to which they are attached form a carbocyclic ring containing 4 to 12 ring atoms; n is O or l;

A is XCR R⁵, p^CHaCHR¹)],,, or X— [(CH-CHR¹ Y)]_m where X is O, S, NH, or NR⁴; Y is O, C(O)O, OC(O)NH, OC(O)O, or NHC(O)O; R⁴ and R⁵ are independently H, a C* to C₆ alkyl group, or an aryl group; and m is 0 or 1; and Z is a moiety derived from an acetophenone, benzophenone, anthraquinone, 9- fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone.

2. The compound of claim 1 wherein Z is a moiety derived from acetophenone, benzophenone, anthraquinone, thioxanthone, chromone, or benzil.

3. The compound of claim 1 wherein X is oxygen or NH.

4. The compound of claim 1 wherein n is 0.

5. The compound of claim 1 wherein R² and R³ are both methyl groups.

6. The compound of claim 1 having the formula

wherein R¹ is H or a methyl group, D is — (OCH₂CH₂O) — or — (NHCH₂CH₂O) — , and Z is a moiety derived from acetophenone, benzophenone, anthraquinone, 9-fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone.

7. The compound of claim 6 wherein R¹ is H.

8. The compound of claim 6 wherein D is — (OCH₂CH₂O) — -

9. The compound of claim 6 wherein Z is a moiety derived from an acetophenone, benzophenone, anthraquinone, thioxanthone, chromone, or benzil.

10. A method of making the photoactive crosslinking compound of claim 1 comprising the steps of solubilizing and allowing to react a 2-alkenyl azlactone compound and a nucleophilic acetophenone, benzophenone, anthraquinone, 9-fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone.

11. The method of claim 10 wherein said 2-alkenyl azlactone compound and said nucleophilic acetophenone, benzophenone, anthraquinone, 9- fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone are present in approximately equimolar amounts.

12. The method of claim 10 wherein said 2-alkenyl azlactone compound and said nucleophilic acetophenone, benzophenone, anthraquinone, 9- fluorenone, anthrone, xanthone, thioxanthone, acridone, dibenzosuberone, benzil, or chromone are reacted in the presence of a nitrogen-containing base or a trivaient phosphorus compound.

13. The method of claim 12 wherein said nitrogen-containing base is a bicyclic amidine or guanidine compound.

14. The method of claim 12 wherein said nitrogen-containing base or trivaient phosphorus compound is present in an amount up to about 5 mole percent relative to said 2-alkenyl azlactone compound.