WO1997040090A2

WO1997040090A2 - Process for radiation cross-linking polymers and radiation cross-linkable compositions

Info

Publication number: WO1997040090A2
Application number: PCT/US1997/006166
Authority: WO
Inventors: Peter A. Stark; Robin E. Wright; Chung I. Young
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1996-04-19
Filing date: 1997-04-16
Publication date: 1997-10-30
Also published as: EP0894112A2; KR20000005547A; WO1997040090A3; BR9708703A; JP2000509089A

Abstract

This invention relates generally to a process for radiation cross-linking polymers. A monochromatic radiation source (e.g. an excimer lamp or excimer laser) can be used to cross-link elastomers and thermoplastic polymers compounded with a radiation activatable cross-linking agent such as anthraquinone, substituted anthraquinone, acetophenones (of the multifunctional or copolymerizable type), benzophenones (of the multifunctional or copolymerizable type), or substituted triazine. The invention also relates to a radiation cross-linkable composition, especially those which combine polymerizable and nonpolymerizable radiation-activatable cross-linkers.

Description

PROCESS FOR RADIATION CROSSLINKING POLYMERS AND RADIATION CROSSLINKABLE COMPOSITIONS

CROSS-REFERENCE TO RELATED APPLICATIONS This a continuation-in-part of U.S. Patent Application Serial No. 08/635,276, filed April 19, 1996, the entire contents of which are incoφorated herein by reference.

BACKGROUND OF THE INVENTION

Field ofthe Invention

This invention relates generally to a process for radiation crosslinking polymers and, more specifically, to a process for radiation crosslinking polymers using a monochromatic light source such as an excimer laser or lamp. This invention further relates to radiation crosslinkable compositions and, more specifically, to compositions that include both a polymer having radiation-activitable crosslinking groups and non-polymerizable radiation-activitable crosslinkers.

Description ofthe Related Art Crosslinked polymers (i.e., polymer networks) have quite different mechanical and physical properties than their uncrosslinked linear or branched counterparts. For example, crosslinked polymers may show unique and highly desirable properties such as solvent resistance, high cohesive strength, and elastomeric character.

The crosslinking reaction can occur in situ during formation ofthe polymer. However, since further processing ofthe polymer is often necessary, it is more typical to start from the linear or branched polymer which is then crosslinked in the final processing step. The curing or crosslinking step is typically activated by moisture, thermal energy or radiation. The latter has found widespread application, particularly using ultraviolet light as the radiation source. Ultraviolet lamps are conventionally used as the ultraviolet light source for irradiating photo-treatable adhesives, coatings and the like. Most often, the lamp includes a mercury element bulb, although one or more additives may be used to accentuate a particular spectral range or output ofthe lamp. Thus, the spectrum ofthe light radiated by the lamp is the spectrum ofthe mercury element or that ofthe additive-modified mercury element. The spectrum produced by these lamps has radiation present over the entire, relatively wide range of 240-2000 nm. Because the radiation is distributed over the entire broad range, the lamp's output is relatively insignificant in any particular, narrow segment ofthe output spectrum. For some applications, however, it is desirable that the greatest part ofthe output radiation lie within a narrow range. In addition, the broad spectral distribution associated with conventional ultraviolet light sources may require long exposure times and may result in photochemical degradation ofthe polymer, undesirable side reactions, and deteriorated surface properties from overcuring ofthe polymer surface.

Recently, new ultraviolet light sources have become available which can deliver a monochromatic or narrow band output based upon excimer formation that occurs in certain noble gases or noble gas/halogen mixtures when exposed to high energy. The wavelengths ofthe emissions from these sources depend on the gases employed. For example, excimer sources containing xenon gas emit at a wavelength of 172 nm, xenon chloride excimer sources have a narrowband emission at 308 nm, and krypton chloride excimer sources generate ultraviolet radiation at 222 nm. Descriptions ofthe mechanisms by which these devices operate and the configurations of these devices are reviewed in Kitamura et al., Applied Surface Science. 79/80 (1994), 507-513; German Patent Appl. DE 4,302,555 Al (Turner et al ); and Kogelschatz et al., ABB Review. 3 (1991), 21-28.

Excimer lamps have been used in the modification and microstructuring of polymer surfaces and the photodeposition of various coatings on metal, dielectric and semiconductor surfaces. Examples of these applications can be found in Kogelschatz, Applied Surface Science. 54 (1992), 410-423, and Zhang et al., Journal of Adhesion Science and Technology. 8(10) (1994), 1179-1210.

European Patent Appl. EP 604738 Al (Nohr et al.) describes a method of preparing a laminate which involves coating a cationically curable adhesive composition onto the surface of a first sheet, exposing the adhesive composition to ultraviolet radiation from an excimer lamp having a narrow wavelength band within the range of about 260 to about 360 nm, and bringing the surface of a second sheet in contact with the adhesive composition-bearing surface ofthe first sheet. The adhesive composition includes about 94 to about 60 percent by weight of a cycloaliphatic diepoxide, from about 1 to about 10 percent by weight of a cationic photoinitiator, and from about 5 to about 30 percent by weight of a vinyl chloride-vinyl acetate- vinyl alcohol teφolymer (based on the weight of the adhesive composition).

International Patent Appl. WO 94/14853 (Blum et al.) describes a method for crosslinking a contact adhesive having integral photopolymerizable groups by using monochromatic radiation from an excimer laser ultraviolet light source in the 180 to 400 nm range. The contact adhesive is prepared from olefinic unsaturated monomers and at least one comonomer containing photopolymerizable groups. These photopolymerizable groups, such as acetophenones, benzophenones, benzil derivatives, benzoin derivatives, dialkoxy acetophenones, hydroxyalkyl phenones, α-acyloxime esters, α-halogen ketones, thioxanthones, fluoronone derivatives, anthraquinone derivatives, iron-arene complexes, dibenzosuberones, and Michlers ketone, are incoφorated into the contact adhesive. Due to their proximity to and incoφoration into the polymeric backbone, such crosslinkers provide efficient crosslinking when copolymerized at appropriate concentrations for the cured polymer's end-use. See, for example, U.S. Patent No. 4,737,559 (Kellen et al ), wherein aromatic ketone monomers (in particular, para- acryloxybenzophenone) are incoφorated with other acrylate and methacrylate monomers to form pressure-sensitive adhesive copolymers. By copolymerizing the aromatic ketone monomer into the polymer backbone prior to crosslinking, crosslinking efficiency is greatly increased by including the monomer in the copolymer as compared with addition of an aromatic ketone compound which is not initially copolymerized into the copolymer. Because ofthe increased efficiency, only small amounts ofthe aromatic ketone monomer are needed to achieve useful degrees of crosslinking.

Some inherent limitations exist, however, when adhesives and other polymeric materials having radiation activatable crosslinking groups are prepared from monomers containing photopolymerizable groups. For example, problems may arise if the formulator wishes to increase or decrease the degree of crosslinking in a polymer containing copolymerized crosslinking agents. The crosslinker level has been predetermined during the formation of these polymers and thus cannot be directly altered or adjusted. The photopolymerization conditions can also activate the radiation-activatable crosslinking groups, resulting in premature and undesired crosslinking during the preparation ofthe polymer. The latter can be particularly problematic when high levels of crosslinking are desired.

Crosslinking of polymers containing radiation-activatable crosslinking groups can also be affected by the subsequent addition of other components, e.g., oligomeric or polymeric materials such as rheological modifiers, plasticizers, tackifying resins, etc. Adding these components dilutes the concentration ofthe incoφorated crosslinking groups, decreasing the ability of radically-active sites to combine and form crosslinks with other sites on the same or different polymer chains. Moreover, dilution reduces the number of cross-links per unit volume which may impair the performance ofthe crosslinked polymer system. Furthermore, the added components may diminish the performance ofthe incoφorated crosslinker by absorbing light that is needed to activate the crosslinking agent, by reacting with the activated crosslinker (if the added component contains abstractable hydrogen atoms), and/or by providing unincoφorated low molecular weight fragments to the composition. In all of these cases, the added components reduce the crosslinking efficiency ofthe crosslinking groups that have been incoφorated in the polymer.

Alternatively, non-polymerizable crosslinking agents offer some advantages over the copolymerizable crosslinking compounds. Although typically more volatile and marginally less efficient than their copolymerizable counteφarts, non- polymerizable crosslinking compounds can be added at any desired level to a variety of polymer systems, either before, during or following polymerization. Examples of non-polymerizable crosslinking agents include anthraquinone, substituted anthraquinone, multifunctional acetophenones, multifunctional benzophenones, and triazines.

U.S. Patent Nos. 4,391,678 and 4,330,590 (each to Nesley) describe a class of fast curing, non-polymerizable triazine photocrosslinkers which are mixed with acrylic monomers and, optionally, ethylenically unsaturated copolymerizable monomers. When the triazine-containing polymerizable mixture is exposed to UV radiation a crosslinked polyacrylate is formed. Although effective crosslinking agents for polyacrylates, triazines can evolve corrosive gases during polymerization and/or crosslinking. U.S. Patent No. 5,407,971 (Everaerts et al.) describes the use of radiation- activatable polyfunctional acetophenones and benzophenones as crosslinking agents for elastomeric polymers. When compared to the use of conventional acetophenones, benzophenones and triazines, these radiation-activatable polyfunctional acetophenone and benzophenone crosslinking agents have lower volatility, increased compatibility, and decreased oxygen sensitivity, and avoid the evolution of toxic or corrosive by¬ products and discoloration ofthe final product.

WO 96/05249 discloses a syrup that can be cured to a crosslinked viscoelastomeric material. A composition that is disclosed is based on a mixture of free radically polymerizable, ethylenically unsaturated monomers that includes a small amount of an ethylenically unsaturated monomer that has a radiation-sensitive hydrogen abstracting group. This mixture is then exposed to energy so as to partially polymerize the monomer mixture and form a coatable syrup. An ethylenically unsaturated monomer having a radiation-sensitive hydrogen abstracting group or a polyethylenically unsaturated monomer can then be added to the syrup. The syrup can then again be exposed to energy to obtain the final crosslinked viscoelastomeric material.

SUMMARY OF THE INVENTION In a first aspect, this invention relates generally to a process for radiation crosslinking polymers. It has been discovered that a monochromatic radiation source (e.g., an excimer lamp or an excimer laser) can be used to crosslink elastomers and thermoplastic polymers compounded with a radiation activatable crosslinking agent such as anthraquinone, substituted anthraquinone, acetophenones (ofthe multifunctional or copolymerizable type), benzophenones (ofthe multifunctional or copolymerizable type), and substituted triazines. This results in a more efficiently crosslinked polymer as compared to conventional curing processes, such as those which use a high, medium or low pressure mercury vapor lamp. Thus, by the term "radiation-activatable crosslinking agent or group" in connection with this invention is meant that the crosslinking agent or group can be activated, i.e. becomes reactive, upon exposure to radiation and in particular light such as light emitted by a high, medium or low pressure mercury vapor lamp or an excimer lamp or laser.

Thus, in one embodiment ofthe first aspect ofthe present invention, there is provided a process for crosslinking a polymer, the process comprising the steps of:

(a) providing a radiation crosslinkable composition comprising: (i) a radiation crosslinkable polymer having abstractable hydrogen atoms (hereinafter also abbreviated as radiation crosslinkable polymer); and (ii) a non-polymerizable, radiation activatable crosslinking agent;

(b) providing a monochromatic radiation source having a wavelength sufficient to activate the crosslinking agent and to crosslink the polymer; and

(c) exposing the radiation-crosslinkable composition to the radiation emitted by the monochromatic radiation source for a time sufficient to crosslink the polymer.

The radiation crosslinkable polymer may be a thermoplastic polymer such as those selected from the group consisting of polyolefins, polystyrenes, vinyl plastics, polyacrylates, polymethacrylates, poly(vinyl esters), polyamides, polycarbonates, polyketones, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing polymers may be derived and a copolymerizable comonomer. Alternatively, the radiation crosslinkable polymer may be an elastomer such as those selected from the group consisting of polyurethanes, polydiorganosiloxanes, A-B-A-type block copolymers, synthetic rubber, natural rubber, ethylene-vinyl monomer polymers, poly(vinyl ethers), poly(vinyl esters), polyacrylates, polymethacrylates, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing polymers may be derived and a copolymerizable comonomer. In any event, it is preferred that the radiation crosslinkable polymer (exclusive ofthe crosslinking agent) absorb substantially no radiation emitted by the monochromatic radiation source. Although the monochromatic radiation source may be an excimer laser or an excimer lamp, the latter is preferred. The monochromatic radiation source preferably emits radiation having a wavelength of about 270 to 370 nm, more preferably about 300 to 360 nm, and even more preferably about 300 to 340 nm. The most preferred monochromatic radiation source is a XeCl excimer lamp that emits radiation having a wavelength of about 308 nm.

In another embodiment ofthe first aspect ofthe invention, the radiation activatable crosslinking agent may be of either the polymerizable type (i.e., the crosslinking agent is copolymerized with the monomers that are polymerized to form the polymer), the non-polymerizable type (i.e., the crosslinking agent was combined with the polymer subsequent to its formation, or was combined with the monomers prior to the polymerization but without reacting with the monomers), or a combination of both types, and the monochromatic radiation source is an excimer lamp. In still other embodiments ofthe first aspect ofthe invention, the radiation crosslinkable composition, subsequent to crosslinking, may be a pressure sensitive adhesive. Thus, the invention also provides for a process for making a pressure sensitive adhesive tape, the process comprising the steps of: (a) providing a flexible web suitable as a backing for a flexible pressure sensitive adhesive tape; (b) providing a radiation crosslinkable pressure sensitive adhesive composition comprising: (i) a radiation crosslinkable polymer having abstractable hydrogen atoms; and (ii) a radiation activatable crosslinking agent; (c) applying the radiation crosslinkable pressure sensitive adhesive composition to at least a portion of at least one surface of the flexible web; (d) providing an excimer lamp that can emit monochromatic radiation having a wavelength sufficient to activate the crosslinking agent and crosslink the polymer; and (e) exposing the radiation crosslinkable pressure sensitive adhesive composition to the radiation emitted by the excimer lamp for a time sufficient to crosslink the polymer and form a pressure sensitive adhesive tape. In a second aspect, this invention relates to radiation crosslinkable compositions comprising a radiation crosslinkable polymer having radiation- activatable crosslinking groups capable of abstracting hydrogens when activated, and a non-polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated. The non-polymerizable radiation activatable crosslinking agent is preferably selected from the group consisting of anthraquinone, substituted anthraquinone, multifunctional acetophenones, and multifunctional benzophenones. While triazines can in principle also be used as a non-polymerizable radiation activatable crosslinking agent, they are preferably not used in connection with the second aspect ofthe invention because they can evolve corrosive gases such as HCl upon activation and, may be subject to undesirable, premature activation during UV-radiation induced polymerization. Preferably, the polymer is a poly(meth)acrylate. Further preferred is a polymer in the form of beads prepared by suspension polymerization, in particular a poly(meth)acrylate in the form of suspension polymerized beads. Although the compositions ofthe second aspect of the invention are particularly suitable for use with a monochromatic radiation source such as an excimer lamp or laser, these radiation crosslinkable compositions can also be cured by other visible or ultraviolet light sources, including broader spectrum ultraviolet sources such as medium pressure mercury lamps, and the like.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a first aspect, this invention relates generally to a process for radiation crosslinking polymers and, more specifically, to a process for radiation crosslinking polymers by using a monochromatic radiation source. A monochromatic radiation source is one which emits radiation over a narrow spectral range; for example, radiation having a half width of no more than about 50 nm, preferably about 5 to 15 nm. Any source of monochromatic radiation may be used in the first aspect ofthe invention, although it is preferred that the radiation source be an excimer laser or an excimer lamp. Excimer lamps (i.e., an incoherent and pulsed source) are particularly preferred, even more so than excimer lasers (i.e., a coherent and pulsed source) because they offer many advantages. Excimer lamps are more efficient than excimer lasers, which can result in reduced operating costs. Lamp systems tend to be smaller and more easily handled. It is easier to change the wavelength of the radiation emitted by a lamp than by a laser. In addition, excimer lamps are more effective in delivering uniform radiation over a larger physical area than lasers.

An excimer source is typically identified or referred to by the wavelength at which the maximum intensity ofthe radiation occurs, a convention followed herein. The wavelength ofthe monochromatic radiation should be useful for crosslinking the polymer and will generally be from about 270 to 370 nm, preferably about 300 to 360 nm, and more preferably about 300 to 340 nm. Monochromatic radiation having a wavelength of about 308 nm has been found to be particularly useful. The wavelength ofthe emitted radiation is determined by the excimer source. While various inert gas-halogen mixtures are known and can be used as excimer sources, those based on xenon chloride (XeCl) are most preferred as the radiation of maximum intensity occurs at about 308 nm. The intensity ofthe light will generally be from about 5 to 20,000 mW/cm², more preferably from about 10 to 10,000 mW/cm², and most preferably from about 50 to 2,000 mW/cm². Excimer radiation sources are commercially available from Heraeus Noblelight, Hanau, Germany and have been described in various patent publications including

International Patent Application No. WO 94/14853, and German Patent Application No. DE 4,302,555 Al, as well as several literature references such as Kitamura et al., Applied Surface Science. 79/80 (1994), 507-513; Kogelschatz et al., ABB Review. 3 (1991), 21- 28; Kogelschatz, Applied Surface Science. 54 (1992), 410-423; and Zhang et al., Journal of Adhesion Science and Technology. 8(10) (1994), 1179-1210.

The monochromatic radiation source is used in conjunction with a radiation- crosslinkable composition that comprises and, more preferably, consists essentially of, one or more radiation crosslinkable polymers and one or more radiation activatable crosslinking agents. The polymers (exclusive ofthe radiation activatable crosslinking agent) contain abstractable hydrogen atoms and absorb substantially no radiation emitted by the monochromatic radiation source. By "absorb substantially no radiation" it is meant that while a small amount of radiation (less than 10%, preferably less than 1%) may be absorbed by the polymer, it is preferred that no radiation be absorbed as any absorbed radiation reduces the efficiency ofthe system because it is not employed for crosslinking. This also prevents any undesirable photochemically-induced reactions that may compromise performance ofthe polymer if it were to absorb the monochromatic radiation.

The abstractable hydrogen atoms may be present in the backbone and/or the side chains ofthe polymer in an amount sufficient to allow crosslinking ofthe polymer to the desired level upon exposure ofthe crosslinking agent/polymer composition to the monochromatic radiation source. As a general rule, hydrogen atoms are most easily abstracted from tertiary carbon atoms, allylic and benzylic groups, those hydrogens on carbon atoms in a position alpha to an oxygen or nitrogen atom (e.g., organic ethers and tertiary amines), and those carried by terminal or pendant mercapto groups. Among the abstractable hydrogen atom-containing polymers that can be used in the first aspect ofthe invention are thermoplastic polymers and elastic polymers ("elastomers"). Thermoplastic polymers are macromolecular materials that can be repeatedly melted and solidified by heating and cooling throughout a characteristic temperature range without exhibiting chemical change during the transformation process. Examples of useful abstractable hydrogen atom-containing thermoplastic polymers include polyolefins such as polyethylene, polypropylene, polymethylpentene, polybutylene, and ethylene-vinyl alcohol copolymers; polystyrene; vinyl plastics such as polyvinyl chloride, polyvinylidene chloride, and chlorinated polyvinyl chloride; polyacrylates and polymethacrylates having a high glass transition temperature such as po.y(methyl methacrylate); poly(vinyl esters) having a high glass transition temperature such as poly(vinyl acetate); polyamides; polycarbonates; polyimides; polyketones such as polyetheretherketone; and copolymers that are based on the polymerization of at least one ofthe monomers from which the foregoing polymers may be derived and a copolymerizable comonomer (which could also be a monomer from which the foregoing polymers are derived), said copolymers containing abstractable hydrogen atoms.

Even more preferred for use in the first aspect ofthe invention, however, are elastomers which are defined as macromolecular materials that return rapidly to their approximate initial dimensions and shape after substantial deformation by a weak stress and subsequent release of that stress as measured according to ASTM D 1456-86 ("Standard Test Method For Rubber Property-Elongation At Specific Stress"). Examples of abstractable hydrogen atom-containing elastomers useful in the present invention include polyurethanes; polydiorganosiloxanes (such as polydimethylsiloxanes); A-B-A type block copolymers such as styrene-isoprene-styrene block copolymers (SIS), and styrene-butadiene-styrene block copolymers (SBS); various synthetic rubbers such as ethylene-propylene-diene monomer rubbers (EPDM), styrene-butadiene rubber (SBR), polyisobutylene, synthetic polyisoprene, polybutadiene, acrylonitrile-butadiene copolymers, and polychloroprene; natural rubber; ethylene vinyl monomer polymers such as ethylene-vinyl acetate; poly(α-olefins); poly(vinyl ethers); poly(vinyl esters); polymethacrylates and polyacrylates such as poly(methyl methacrylate); and copolymers that are based on the polymerization of at least one ofthe monomers from which the foregoing elastomers may be formed and a copolymerizable comonomer (which could also be a monomer from which the foregoing polymers are derived), said copolymers containing abstractable hydrogen atoms. The preferred elastomers for use in the first aspect ofthe invention are polyacrylates, natural rubber, polybutadiene, polyisoprene, SBS block copolymers, and SIS block copolymers. Most preferred as the thermoplastic or elastomeric radiation crosslinkable polymer in the radiation curable compositions ofthe invention are poly(meth)acrylates which can be obtained as the polymerization product of acrylate or methacrylate monomers, often copolymerized with ethylenically unsaturated free radically polymerizable monomers. Acrylate and methacrylate monomers useful in preparing the radiation crosslinkable polymer ofthe present invention include compositions having the formula H2C=CRιCOO-Y, wherein Rj represents -H or -CH3, and Y represents a monovalent straight chain alkyl, branched alkyl or cycloalkyl group having from about 1 to about 24 carbon atoms. Examples of such acrylate and methacrylate monomers include but are not limited to those selected from the group consisting of methyl acrylate, methyl methacrylate, isooctyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate, isodecyl acrylate, 4-methyI-2-pentyl acrylate, 2-methylbutyl acrylate, isoamyl acrylate, sec-butyl acrylate, n-butyl acrylate, tert-butyl acrylate, isobornyl acrylate, butyl methacrylate, ethyl acrylate, dodecyl acrylate, octadecyl acrylate, cyclohexyl acrylate and mixtures thereof. Preferred acrylate monomers include those selected from the group consisting of isooctyl acrylate, isononyl acrylate, isoamyl acrylate, isodecyl acrylate, 2- ethylhexyl acrylate, isobornyl acrylate, n-butyl acrylate, sec-butyl acrylate, and mixtures thereof. Ethylenically unsaturated free radically reactive monomers which are readily copolymerizable with acrylate and methacrylate monomers can also be used in the preparation ofthe preferred polyacrylates. Such monomers include but are not limited to those selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, sulfoethyl methacrylate, N- vinyl pyrrolidone, N- vinyl caprolactam, acrylamide, t-butyl acrylamide, dimethyl amino ethyl acrylamide, N-octyl acrylamide, acrylonitrile, vinyl acetate, vinyl propionate, styrene, mixtures thereof, and the like. Preferred monomers include those selected from the group consisting of acrylic acid, methacrylic acid, N-vinyl pyrrolidone, styrene, vinyl acetate, and mixtures thereof. Some ofthe foregoing thermoplastic polymers and elastomers may be more effectively used in the first aspect ofthe invention if the monomers from which they are derived are copolymerized with monomers from which the other ofthe thermoplastic polymers and elastomers are derived. (Monomers for the thermoplastic polymers may be copolymerized with monomers for the elastomers.) In addition to the radiation crosslinkable polymer, the compositions used in the first aspect ofthe invention include a radiation activatable crosslinking agent ofthe copolymerizable or non-polymerizable type, although the latter are preferred. In general, radiation activatable crosslinking agents useful in the first aspect ofthe invention are those which become hydrogen abstractors after absorbing light having a wavelength of about 270 to 370 nm, preferably about 300 to 360 nm, more preferably about 300 to 340 nm. Copolymerizable radiation activatable crosslinking agents generally become randomly incoφorated into the backbone ofthe radiation crosslinkable polymer during the polymerization ofthe polymer. As a result, the copolymerizable crosslinking agent should preferably be compatible and miscible with the monomers from which the polymer is derived (i.e., there should be no gross phase separation upon mixing). Copolymerizable crosslinking agents tend to promote more efficient crosslinking than their non- polymerizable counteφarts and minimize concerns associated with volatility ofthe crosslinking agent.

Useful copolymerizable crosslinking agents include copolymerizable anthraquinones, copolymerizable acetophenones, copolymerizable benzophenones, copolymerizable triazines, and mixtures thereof. Copolymerizable crosslinking agents useful in the invention can be found in U.S. Patent Nos. 4,737,559 (Kellen et al.); 5,073,611 (Boettcher et al.); 5,128,386 (Auchter et al ); 5,202,483 (Bott et al ); 5,248,805 (Boettcher et al.); 5,264,533 (Boettcher et al ); 5,294,688 (Auchter et al ); and 5,389,699 (Boettcher et al ). Preferred copolymerizable crosslinking agents are the acrylate-functional aromatic ketones disclosed in U.S. Patent No. 4,737,559 (Kellen et al ), in particular 4-acryloxybenzophenone.

Non-polymerizable radiation activatable crosslinking agents are either mixed with or reacted with the radiation crosslinkable polymer subsequent to polymerization ofthe polymer, or are mixed with the monomer(s) for the polymer prior to polymerization but, in this event, do not react with the monomer(s). Included within this class are multifunctional crosslinking agents and graftable crosslinking agents. One advantage associated with the use of non-polymerizable crosslinking agents is that they are more versatile because they are added to the polymer subsequent to polymerization. In addition, the non-polymerizable type need not be miscible, compatible or reactive with the monomers from which the polymer is derived.

Preferred non-polymerizable crosslinking agents are anthraquinone, substituted anthraquinone, multi-functional acetophenones, multi-functional benzophenones, substituted triazines, and mixtures thereof. Specific examples of useful anthraquinone- type non-polymerizable crosslinking agents include anthraquinone, t-butyl anthraquinone and 2-ethyl anthraquinone. Particularly preferred as non-polymerizable radiation activatable crosslinking agents are multi-functional acetophenones and benzophenones having the following formula:

wherein:

X represents CH₃-; phenyl; or substituted-phenyl; W represents -O-,-NH-, or -S-;

Z represents an organic spacer selected from the group consisting of aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic groups free of esters, amides, ketones, urethanes, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms not intramolecularly accessible to the carbonyl group in formula (I); and n represents an integer of 2 or greater; preferably 2-6.

In one particularly preferred embodiment, X is phenyl; W is oxygen; Z is -(-CH₂-)-₂.ι₂; and n is 2.

Specific examples of preferred multi-functional benzophenones include l,5-bis(4- benzoylphenoxy)pentane, l,9-bis(4-benzoylphenoxy)nonane, and l,l l-bis(4- benzoylphenoxy)undecane.

Useful non-polymerizable substituted triazine crosslinking agents are also described in U.S. Patent Nos. 4,329,384 (Vesley et al ); 4,330,590 (Vesley); and

4,379,201 (Vesley). Specific examples of substituted triazine crosslinking agents include 2,4-bistrichloromethyl-6-(4-methoxyphenyl)-s-triazine, and 2,4-bistrichloromethyl-6-(3 ,4- dimethoxyphenyl)-s-triazine.

Combinations of copolymerizable and non-polymerizable radiation activatable crosslinking agents can also be used in the radiation crosslinkable compositions described above and this provides a second aspect ofthe invention. As mentioned above, copolymerizable crosslinkers are effective in many radiation curable compositions, but are less useful in some situations. For example, the initiation conditions used to form a polymer containing a certain level of polymerizable radiation activatable crosslinking agent can also activate the photoactive group present in the incoφorated crosslinker. One undesired result of activating the crosslinking agent during polymerization can be premature gelation. Reducing the copolymerizable crosslinking agent level can decrease or eliminate unwanted gelation, and may also reduce the degree of crosslinking when the polymer is subsequently irradiated during the intended crosslinking process.

To balance the objectives of preventing premature gelation during polymerization while retaining a sufficient degree of crosslinking in the cured product, non-polymerizable radiation-activatable crosslinking agents capable of abstracting hydrogen when activated can be combined with a radiation crosslinkable polymer having radiation activatable crosslinking groups capable of abstracting hydrogen when activated. In particular, it has been found that a broad spectrum of crosslinking levels can be obtained by adding a non- polymerizable radiation-activatable crosslinking agent of this type to a radiation crosslinkable polymer that includes radiation-activatable crosslinking groups likewise capable of abstracting hydrogen atoms when activated.

Combinations of copolymerizable and non-polymerizable crosslinking agents are also useful in curing radiation crosslinkable polymers that have been diluted with certain oligomeric or polymeric additives. Such additives can diminish the efficiency of incoφorated crosslinkers by a variety of mechanisms, including inhibition ofthe crosslinking reaction by reducing the concentration ofthe photoactive species in the diluted system and by absorbing the energy required to activate the crosslinker. Additives that have abstractable hydrogens can inhibit or terminate the crosslinking reaction through chain-transfer or chain-termination mechanisms and result in a decreased level of crosslinking.

In such diluted systems, the incoφorated crosslinking agent can be advantageously supplemented by adding a sufficient amount of a non-polymerizable crosslinking agent to overcome the deleterious effects ofthe polymeric or oligomeric additives. Not only can the addition of non-polymerizable crosslinking agents increase the concentration ofthe crosslinking agent for curing diluted systems, but can also allow the formulator the flexibility of adding crosslinking agents that have a different absoφtion or photoactivation character than those incoφorated into the radiation crosslinkable polymer. This latter benefit is particularly useful if the oligomeric or polymeric additive absorbs UV radiation at wavelengths that inhibit photoactivation ofthe incoφorated crosslinking agent. Poly(meth)acrylates are preferred for use as the abstractable hydrogen- containing radiation-crosslinkable polymer in the second aspect ofthe invention. Poly(meth)acrylates can be prepared by a variety of polymerization methods, including emulsion, suspension, solvent, solution, or bulk polymerization. These polymerization methods are described in Principles of Polymerization, 3rd ed. (G. Odian, Wiley-Interscience : New York, 1981, pp. 286-296). Those poly(meth)acrylates prepared using suspension polymerization are especially preferred for use in the second aspect ofthe invention and can be made according to the methods described in U.S. Patent Nos. 4,833,179 (Young et al.); 4,952,650 (Young et al.); and 5,292,844 (Young et al ). The suspension polymers are generally in the form of spherical or pearl-shaped beads typically having a diameter of at least 1 μm, with diameters of up to 1000 μm or more being possible.

In general, these polymerization methods first involve preparing a monomer premix. The premix typically comprises acrylate and/or methacrylate monomers, optional ethylenically unsaturated free radically polymerizable monomers, chain transfer agent, and free radical initiator. If used, a copolymerizable radiation activatable crosslinking agent is also added to the premix. The premix is then combined with a water phase comprising a suspending agent, water, and, optionally, a surfactant. A modifier moiety, such as hydrophobic silica, polystyryl methacrylate macromonomers and/or reactive zinc salt can be added to the suspension mixture before, during or after polymerization. Similarly, at any point prior to irradiation of the suspension polymer, a non-polymerizable radiation activatable crosslinking agent capable of abstracting hydrogens when activated can also be added to the radiation crosslinkable polymer made according to these methods. An example of how a radiation crosslinkable polymer having radiation-activatable crosslinking groups can be effectively combined with non-polymerizable radiation- activatable crosslinking agents is to crosslink poly(meth)acrylate adhesive polymers that have been diluted with tackifying resins. Tackifying resins typically employed in poly(meth)acrylate adhesives are low molecular weight organic compounds derived from natural sources and based on rosin acids, selected phenol-modified teφenes, and alpha- pinenes. These resins promote adhesion, but often at the expense ofthe adhesive's bulk or cohesive strength.

When ultraviolet light is used to crosslink tackified poly(meth)acrylate adhesives, the tackifying resin can provoke several ofthe deleterious effects mentioned above that are associated with oligomeric and polymeric additives (i.e., absoφtion of energy required to activate the incoφorated crosslinker, undesired chain transfer or chain termination reactions, etc.). To solve these problems, it has been discovered that poly(meth)acrylates having radiation-activatable crosslinking groups can be effectively combined with non-polymerizable radiation activatable crosslinking agents to yield the radiation crosslinkable composition. Such a combination may provide enhanced adhesive performance when compared to the sole use of an equivalent amount ofthe non- polymerizable radiation activatable crosslinking agents.

In the second aspect ofthe invention that uses a combination of a non- polymerizable radiation-activatable crosslinking agent and a radiation crosslinkable polymer having radiation-activatable crosslinking groups, the radiation-activatable crosslinking groups are preferably derived from radiation-activatable crosslinking agents having a polymerizable group. Useful polymerizable radiation-activatable crosslinking agents for use in this embodiment include polymerizable acetophenones, polymerizable benzophenones, polymerizable anthraquinones, and mixtures thereof. Further polymerizable crosslinking agents useful in the invention can be found in U. S. Patent Nos. 4,737,559 (Kellen et al.); 5,073,611 (Boettcher et al ); 5,128,386 (Auchter et al ); 5,202,483 (Bott et al ); 5,248,805 (Boettcher et al.); 5,264,533 (Boettcher et al.); 5,294,688 (Auchter et al.); and 5,389,699 (Boettcher et al.). Preferred polymerizable crosslinking agents are the (meth)acrylate-functional aromatic ketones disclosed in U.S. Patent No. 4,737,559 (Kellen et al ), in particular 4-acryloxybenzophenone. The latter type of polymerizable crosslinking agents are particularly preferred in the preparation of a poly(meth)acrylate polymer having radiation-activatable crosslinking groups that are capable of abstracting hydrogen atoms upon activation.

Preferred non-polymerizable crosslinking agents for use in the second aspect of the invention are those selected form the group consisting of non-polymerizable anthraquinone including non-polymerizable substituted anthraquinone, non-polymerizable multi-functional acetophenones, non-polymerizable multi-functional benzophenones, and mixtures thereof. Specific examples of useful anthraquinone-type non-polymerizable crosslinking agents include anthraquinone, t-butyl anthraquinone and 2-ethyl anthraquinone. Examples of useful non-polymerizable multifunctional acetophenones and benzophenones are those of Formula (I) set forth above.

The radiation-activatable crosslinking agents, whether they have been copolymerized into the radiation-crosslinkable polymer or they are ofthe non- polymerizable type (for either aspect ofthe invention), are typically employed, either singly or in combination, in an effective amount by which is meant an amount large enough to provide the desired ultimate properties. For example, in the context of manufacturing an adhesive, an effective amount of crosslinking agent is an amount sufficient to crosslink the polymer so that it has adequate cohesive strength but not in an amount so large that the polymer becomes overcured. The actual amount of crosslinking agent used will vary depending on the application, the type of polymer, the type of crosslinking agent, the ease of hydrogen abstraction from the polymer, the reactivity of the radicals formed, the intensity and length of exposure ofthe composition to irradiation, the polymer's molecular weight, and the desired final properties ofthe material. Within these guidelines, the total amount of crosslinking agent is preferably about 0.01 to 25 weight %, more preferably about 0.1 to 10 weight %, and most preferably about 0.1 to 1.0 weight %, based upon the total weight ofthe polymer.

Other useful materials which may be optionally utilized in either aspect ofthe present invention include thermally expandable polymeric microspheres, glass microspheres, fillers, pigments, foaming agents, stabilizers, fire retardants, and viscosity adjusting agents which do not interfere with light absoφtion and crosslinking. Crosslinked polymers may be readily produced according to the invention. In the case of using a copolymerizable radiation-activatable crosslinking agent, the crosslinking agent is blended or combined with the other monomers that will yield the polymer, the crosslinking agent being miscible, compatible, and reactive with these monomers. The monomers are then polymerized in any conventional manner such as anionic, cationic and free-radical techniques well known to those skilled in the art. In the case of a non- polymerizable radiation-activatable crosslinking agent, the crosslinking agent is mixed with the preformed polymer by dissolving in a solvent, extruding with the polymer, or other standard compounding techniques for combining a non-polymerizable crosslinking agent with a polymer. Alternatively, the non-polymerizable crosslinking agent may be combined with the monomer(s) prior to polymer formation but, in this event, does not react with the monomer(s).

In any event, once the crosslinking agent has been combined with the polymer, the crosslinking agent/polymer composition can be irradiated directly or applied to a substrate by methods well-known in the art, such as solvent coating, extrusion coating, (e.g., hot melt extrusion coating), solventless or waterbome coating. The compositions can be applied to at least a portion of at least one major surface of a suitable flexible or rigid substrate or surface. Useful flexible substrates or webs include paper, plastic films such as polypropylene), poly(ethylene), poly(vinyl chloride), polytetrafluoroethylene), polyester [e.g., poly(ethylene terephthalate)], polyimide film such as DuPont' s Kapton™, cellulose acetate, and ethyl cellulose. Substrates can also be woven fabric formed of threads of synthetic or natural materials such as cotton, nylon, rayon, glass, or ceramic material, or they can be of nonwoven fabric such as air-laid webs of natural or synthetic fibers or blends of these. In addition, suitable substrates can be formed of metal, metallized polymeric film, or ceramic sheet material. Suitable rigid substrates include glass, wood, metal, treated metal (such as those comprising automobile and marine surfaces), polymeric, and composite materials such as fiber-reinforced plastics. Once the composition has been applied to the substrate, it is exposed to a radiation source (preferably a monochromatic radiation source) ofthe wavelength described previously for a time sufficient to crosslink the polymer to the desired level.

While both excimer lasers and excimer lamps may provide monochromatic radiation source, the latter arc preferred for several reasons including more efficient irradiation of a larger physical area. An excimer lamp may comprise a single lamp or a bank of lamps, preferably arranged to irradiate an area somewhat larger than the target substrate so as to ensure uniform radiation exposure over the entire area. When an excimer laser is employed, it may be helpful to provide a lens system to distribute the radiation over a wider physical area. The radiation crosslinkable compositions that have been described herein are useful in various applications including as adhesives (especially, pressure sensitive adhesives and pressure sensitive adhesive-coated articles such as tapes, sheets, labels, etc.), coatings, sealants, photoresists and hardcoats.

TEST PROCEDURES The following test procedures were used to evaluate the compositions prepared in the examples. The compositions were useful as adhesives.

Room Temperature Shear Strength

Shear strength is a measure ofthe cohesiveness or internal strength of an adhesive. It is based upon the amount of force required to pull an adhesive strip (tape) from a standard flat surface in a direction parallel to the surface to which it has been affixed with a definite pressure. It is measured as the time in minutes required to pull a standard area of adhesive-coated sheet material from a stainless steel test panel under stress of a constant, standard load. This test follows the procedure described in ASTM D 3645M-88: "Holding Power of Pressure Sensitive Adhesive Tapes."

The tests were conducted at room temperature (about 22° C) on strips of adhesive-coated sheet material applied to a stainless steel panel which was cleaned and prepared as described above in the peel adhesion test for the glass plate. A 0.127 dm by 0.127 dm portion of each strip was in firm contact with the panel with one end portion of the tape being free. The panel with the adhesive-coated strip attached was held in a rack such that the panel formed an angle of 178° with the extended free end ofthe adhesive- coated strip which was tensioned by applying a force of 1000 grams as a hanging weight from the free end ofthe adhesive-coated strip. The 2° less than 180° was used to negate any peel forces, thus ensuring that only the shear forces were measured, in an attempt to more accurately determine the holding power ofthe adhesive being tested. The time elapsed for each adhesive-coated strip to separate from the test panel was recorded as the shear strength. The test was discontinued after 10,000 minutes, which is a measure of excellent shear strength. The reported results are an average of two samples. High Temperature Shear Strength

High temperature shear strength was measured as described in the shear strength test except that the test samples were first aged at 70° C for 15 minutes, and then tested at 70° C. The reported results are an average of two samples.

Gel Fraction

A known amount of crosslinked polymer was put in an excess of a solvent capable of dissolving the polymer, allowed to dissolve for 24 hours, and filtered. The recovered solid was dried and the amount recorded. The gel content was determined as follows: solid weight x 100% initial weight of sample

Results are an average of two numbers and are reported to the nearest whole number.

EXAMPLES

The following non-limiting examples further illustrate the present invention. In the examples, the following abbreviations are used:

ABP 4-acryloxybenzophenone

C5EBP 1 , 5-bis(4-benzoylphenoxy)pentane

C9EBP 1 , 9-bis(4-benzoylphenoxy)nonane

C 11 EBP 1,11 -bis(4-benzoylphenoxy)undecane

TBA t-butyl anthraquinone

TRIAZINE 2,4-bistrichloromethyl-6(4- methoxyphenyl)-s-triazine

EXAMPLE 1 - Preparation of C5EBP

4-hydroxybenzophenone (3000 g; 15.15 moles), NaOH (608 g; 15.15 moles), and ethylene glycol (5500 g) were placed in a 12 liter flask fitted with a condenser and mechanical stirrer. The reaction mixture was stirred at 85°C until the 4-hydroxybenzophenone and the NaOH were dissolved. The reaction mixture was then set to 135 °C and 2000 g (8.6 moles) of 1,5-dibromopentane was added. Excess NaOH (80 g, 2.05 moles) was added in portions to maintain a basic pH. After heating for 1.5 hours, the reaction was essentially complete. The mixture was cooled by the addition of 2500 g water and the precipitated product was filtered from the ethylene glycol/water mixture. The tan, solid precipitate was then mixed with 3608 g ethyl acetate to purify the product. The ethyl acetate purification step was repeated and, following air drying, 2982 g of purified C5EBP was obtained.

EXAMPLE 2 - Preparation of C11EBP C 11EBP was prepared in accordance with example 1 by replacing the 1,5- dibromopentane with an equimolar amount of 1,11-dibromoundecane.

EXAMPLES 3-8

A series of acrylate pressure sensitive adhesives was prepared according to the method of U.S. Patent Re. 24,906 (Ulrich), incoφorated by reference herein, in ethyl acetate using 90 wt.% isooctyl acrylate, 10 wt.% acrylic acid, and 0.1 wt.% carbon tetrabromide (CBr₄) chain transfer agent. The inherent viscosity of each of the resultant adhesives was 0.64 dl/g in ethyl acetate at 27° C.

A non-polymerizable radiation-activatable crosslinking agent ofthe type and amount (amt.) specified in Table 1 below was dissolved in a 40 wt.% solution of each adhesive in ethyl acetate. (The amount of crosslinking agent reported for examples 3 to 5 represents the weight % based on the weight ofthe polymer. The amounts reported for examples 6 to 8 are a calculated weight %, based on having actually used an equivalent molar percentage when compared to examples 3 to 5.) These mixtures were then knife coated onto a 38 μm thick aziridine-primed poly(ethylene terephthalate) film and dried for 15 minutes at 65° C. to yield 25 μm thick coatings. The coated films were passed through a curing chamber at a speed of approximately 1.6 meters per minute under the output of a 308 nm XeCl excimer lamp (Model Excimer Lamp 308, commercially available from Heraeus Noblelight, Hanau, Germany) in which the lamp was mounted on a conveyor belt system at a height of approximately 2.5 cm above the belt. Table 1 shows the total energy dose received by each example. (All excimer energy doses herein were measured in the UVB (280- 320 nm) range.) The cured samples were stored for 24 hours in a constant temperature room held at 22° C. and 50% relative humidity. The gel fraction for each example was measured as described above using ethyl acetate as the solvent. Room temperature shear strengths were also measured as described above. The gel fraction and shear strength data for each of these examples can be found in Table 1.

These examples show that shear strength performance increases as the amount of radiation activatable crosslinking agent increases or as the radiation dose increases. Even at relatively low amounts of crosslinking agent, good shear strength can be achieved by increasing the radiation dose. With moderate amounts of crosslinking agent, excellent shear strength is rapidly obtained. Examples 3 and 6 that demonstrated less than 10,000 minutes of shear strength failed cohesively (i.e., adhesive residue remained on both the test panel and the sheet material). EXAMPLES 9-11

The following examples demonstrate the use of a XeCl excimer laser to crosslink the acrylate pressure-sensitive adhesives of Examples 3-8 using different amounts of C5EBP non-polymerizable crosslinking agent and different adhesive thicknesses as shown below in Table 2. The mixtures were knife coated onto a 38 μm thick, flexible, aziridine-primed poly(ethylene terephthalate) web, dried at 65° C for 15 minutes, and cured using a Lambda Physik (Gόttingen, Germany) 300i Series XeCl excimer laser having a maximum intensity at 308 nm and a maximum pulse frequency of 150 Hz. The beam was optically spread using quartz optics to cover an area 0.03 dm high and 1.52 dm wide with an average energy per pulse of about 100 mJ/cm . Samples to be irradiated were attached to the platform of an xy-translation stage positioned in the vertical orientation, allowing the sample to be passed normal to the incident excitation pulses at variable speeds. Dose was controlled primarily by changing the speed ofthe substrate. Table 2 shows the total energy dose received by each example. The gel fraction for each example was measured as described above using ethyl acetate as the solvent and with the results reported below in Table 2.

TABLE 2

As can be seen from Table 2, compositions that incoφorate varying amounts of a non-polymerizable crosslinking agent can be crosslinked using an excimer laser as the monochromatic radiation source.

EXAMPLES 12-15 An emulsion polymerized acrylic pressure-sensitive adhesive comprising 94.5 parts isooctyl acrylate and 5.5 parts acrylic acid having an inherent viscosity of 2.05 dl/g in ethylacetate at 27° C was prepared according to U.S. Pat. No. Re. 24,906 (Ulrich). The adhesive was dried and then dissolved in a 70/30 toluene/isopropanol solvent mixture (25 wt% solids), along with the amounts and types of non- polymerizable crosslinking agents found in Table 3. These formulations were then knife coated onto a 38 μm thick, flexible, aziridine-primed poly(ethylene terephthalate) film, and dried for 15 minutes at 65°C to yield a dried coating thickness of 15 μm. The coated samples were then cured using the XeCl excimer lamp system of Examples 3-8, with the total energy dose reported in Table 3. Also reported in Table 3 are the gel fraction (in ethyl acetate) and the room temperature shear strength data obtained by the methods described above. TABLE 3

Examples which displayed less than 10,000 minutes of shear strength demonstrated at least some cohesive failure. At a constant amount of crosslinking agent, the gel fraction and room temperature shear strength increase with increasing amounts of excimer radiation dose.

EXAMPLES 16-19

Examples 16-19 illustrate the advantages gained when a copolymerizable crosslinking agent (ABP) is used alone and when it is combined with a non- polymerizable crosslinking agent (C5EBP) in a radiation crosslinkable polyacrylate pressure-sensitive adhesive. In these examples, a partially polymerized pre-adhesive composition was prepared by mixing 90 parts isooctyl acrylate, 10 parts acrylic acid, 0.15 % Irgacure 651 (photoinitiator commercially available from Ciba-Geigy), and 0.025 % CBr₄ chain transfer agent (the percentages for the latter two components being based on the combined amount of isooctyl acrylate and acrylic acid). The mixtures were irradiated with low intensity ultraviolet light until a partially polymerized pre-adhesive composition having a coatable viscosity was obtained. To the partially-polymerized pre-adhesive composition, 0.35 % Irgacure 651 photoinitiator, an additional 0.025% CBr₄, 0.1 % ABP (Examples 16-19), and 0.1 % C5EBP (Examples 17 and 19) were combined and mixed thoroughly. The compositions were knife-coated at a thickness of 2.5 mm between two sheets of 0.005 mm thick UV-transparent polyester films coated with a silicone release layer. The coated sandwich construction was passed through two irradiation zones where a total of 750 mJ/cm² of energy was expended. Zone 1 was at approximately 187 mJ/cm² of energy at a light intensity of 0.4 milliwatts/cm². Zone 2 was at an energy of approximately 563 mJ/cm² at a light intensity of 2.0 milliwatts/cm². During irradiation, the coated sandwich construction was cooled by air impingement to remove the heat of polymerization. After passing through the two exposure zones, the polyester sheets were removed from the sandwich construction, the polymerized composition was placed in a hot melt coater/extruder, and the melted composition was coated onto a silicone-treated release liner at a thickness of either 51 μm (Examples 16 and 17) or 127 μm (Examples 18 and 19), and then transfer laminated onto a 38 μm thick, flexible, aziridine-primed poly(ethylene terephthalate) film. The coated samples were then cured using the system described in Examples 3- 8 to provide the total energy dose shown in Table 5. The gel fraction (in ethyl acetate) and shear strength (both room temperature and high temperature) were measured for the examples as described above and with the results shown in Table 4.

TABLE 4

I. ca

I

Examples having less than 10,000 minutes of strength failed cohesively.

These examples demonstrate that copolymerizable crosslinking agents can be used in the practice ofthe invention. Furthermore, the addition of a non-polymerizable crosslinking agent to an elastomeric adhesive composition that included a copolymerizable crosslinking agent enhanced the degree of crosslinking as reflected by comparing examples 19 and 21 with, respectively, examples 18 and 20. These examples also demonstrate that the combination of an excimer light source and a radiation activatable crosslinking agent can be effectively used to cure a relatively thick film of elastomeric material without overcuring the surface ofthe material.

EXAMPLES 20-22

The emulsion polymerized acrylic pressure-sensitive adhesives of examples 20-22 were prepared and tested following the procedures described in conjunction with examples 12-15 except that the adhesives were coated to a dry thickness of 25 μm. The amount of crosslinking agent reported for example 20 represents the weight % based on the weight ofthe polymer. The amounts reported for examples 21 and 22 are a calculated weight % based on having actually used an equivalent molar percentage when compared to example 20. Table 5 shows the total energy dose received by each example, the gel fraction, and the room temperature shear strength values. TABLE 5

Room temperature shear strength values are reported as either an average of two samples if both failed prior to 10,000 minutes or as two individual samples if only one failed prior to 10,000 minutes. All samples having less than 10,000 minutes shear strength demonstrated at least some cohesive failure. These examples show that the methods ofthe invention are an effective means of crosslinking a radiation crosslinkable polymer. Since these examples either survived for more than 10,000 minutes in the room temperature shear strength test or demonstrated at least some cohesive failure, the polymer surface was not overcured.

EXAMPLES 23-40

Examples 23-40 illustrate the use of excimer lamps for crosslinking various elastomers (polybutadiene, polyisoprene, and triblock copolymer) combined with a range of radiation activatable crosslinking agents. Polybutadiene having a weight average molecular weight of 119,000 and polyisoprene having a weight average molecular weight of 263,000 were prepared by standard anionic polymerization techniques in cyclohexane using sec-butyl lithium as the initiator. Kraton™ 1107 is a styrene-isoprene-styrene triblock copolymer commercially available from Shell Co. The elastomers were then dissolved in toluene and combined with the amounts and types of non-polymerizable crosslinking agents found in Table 6. These 20 wt% solids formulations were coated onto a 38 μm thick, flexible, primed poly(ethylene terephthalate) film and dried for 15 minutes at 65°C to yield the dried coating thicknesses recorded in Table 6. The coated samples were then crosslinked using the XeCl excimer lamp system described in examples 3-8. Each sample received a total dosage of 800 mJ/cm². Table 6 shows the gel fraction (in toluene) for each example, obtained by the method described above. TABLE 6

The foregoing examples demonstrate that the methods ofthe invention can be applied to radiation crosslinkable compositions that comprise various elastomers and different amounts of various crosslinking agents and that these compositions can be coated in a range of thickness.

EXAMPLE 41 AND COMPARATIVE EXAMPLES C-1 AND C-2

These examples compare the use of unsubstituted anthraquinone, acetophenone, and benzophenone as crosslinking agents with a monochromatic radiation source. A solventborne acrylic pressure-sensitive adhesive was prepared, combined with the types and amounts of crosslinking agents specified in Table 7, knife-coated to a dried thickness of about 25 μm onto a 35 μm thick, aziridine- primed poly(ethylene terephthalate) film, and cured using an XeCl excimer lamp system according to the procedure described in conjunction with Examples 3-8. The room temperature shear strength was measured as described above and with the results reported below in Table 7.

Samples that demonstrated less than 10,000 minutes shear strength showed cohesive failure. Even though acetophenone and benzophenone are well-known radiation activatable crosslinking agents, their use in conjunction with the methods of the invention resulted in minimal crosslinking as evidenced by the low shear strength values. On the other hand, anthraquinone was a very efficient crosslinking agent. EXAMPLE 42

Example 42 shows the use of a monochromatic radiation source to crosslink a thermoplastic composition, poly(vinyl acetate). Poly(vinyl acetate) having an average molecular weight of 500,000 was obtained from Aldrich Chemical Company and dissolved in chloroform to provide a 25% solids solution. The solution was then knife coated onto a 38 μm thick, aziridine-primed poly(ethylene terephthalate) film and dried for 15 minutes at 65° C to yield a 25 μm thick coating, but did not adhere well to the film. The coated sample was then cured in a static condition for 15 seconds under the output ofthe 308 nm XeCl excimer lamp referred to in Examples 3-8. Total dose received was 889 mJ/cm².

Gel fraction was determined following the test procedure described above using chloroform as the solvent. The gel fractions were 0% and 16% for the control (i.e., no radiation exposure) and example 42, respectively. This example demonstrates the utility ofthe processes ofthe invention in crosslinking a thermoplastic polymer.

COMPARATIVE EXAMPLES C-3 AND C-4

Comparative examples C-3 and C-4 show the use of a conventional mercury lamp ultraviolet light source to process compositions similar to those of Examples 20-22. Emulsion polymerized acrylic pressure sensitive adhesives were prepared according to the procedure described in conjunction with Examples 20-22, using the types and amounts of non-polymerizable crosslinking agent shown in Table 8. The coated samples were then cured by the use of a Fusion Systems UV processor using an "H" bulb at full power and a conveyor speed of 22.9 meters/min. (75 feet/min ). The total energy dose received by each sample is recorded in Table 10 (measured in the UVA (320-390 nm) range), along with gel fraction and room temperature shear strength, as measured by the methods described above. Except for the non-irradiated examples (dose = 0 mJ/cm²) that failed cohesively, all shear failures were adhesive in nature (i.e., the adhesive sample separated cleanly from the test panel).

When compared to the examples processed using monochromatic radiation source, these comparative examples having similar gel fractions demonstrated a significant decrease in shear strength and a change in the shear failure mode. The diminished shear strength can be attributed to the undesired emissions inherent in the conventional, broad spectral output mercury bulbs used to prepare these examples.

EXAMPLE 43 AND COMPARATIVE EXAMPLE C-5 A 25 wt. % solventborne natural rubber-based adhesive composition was prepared by combining 50 parts natural rubber (a CV-60 Standard Malaysian Rubber (SMR) natural rubber), 50 parts by weight styrene-butadiene rubber (SBR 1011 A, commercially available from Ameripol/Synpol), 50 parts by weight aliphatic hydrocarbon tackifying resin (Escorez 1304, commercially available from Exxon Chemical Co.), 1 part by weight Irganox 1010 (a multi-functional hindered phenol antioxidant, commercially available from Ciba-Geigy Coφ.), and 0.1 part C9EBP crosslinking agent in toluene.

C9EBP crosslinking agent was prepared like C5EBP described in example 1 except that the 1,5-dibromopentane was replaced with an equimolar amount of 1,9- dibromononane. The adhesive composition was coated on a primed polyester film and dried to a thickness of 25 μm. In Example 43, the coated sample was then crosslinked using the XeCl excimer lamp system described in Examples 3-8 while in example C-5, the coated film was cured by a PPG high intensity UV processor (PPG Inc., Pittsburgh, PA) with two lamps at the normal setting and a conveyor speed of 18.3 meters/min. (60 feet per minute). (Energy doses for Example C-5 were measured in the UVA range while those for Example 43 were measured in the UVB range.) Table 9 shows the total energy dose received by each sample and the gel fraction (in toluene) and room temperature shear strength values ofthe adhesives as measured by the methods described above.

TABLE 9

These examples show the advantage of using the excimer lamp in processing this natural rubber-based pressure sensitive adhesive composition. At comparable gel fractions, the excimer processed materials demonstrated superior adhesive properties when contrasted with the samples exposed to the mercury bulb.

EXAMPLES 44-50 AND COMPARATIVE EXAMPLES C-6 - C-12

These examples illustrate the benefits of using a combination of crosslinking agents copolymerized in the radiation crosslinkable polymer (ABP) and non- polymerizable crosslinking agents (C5EBP) in tackified, suspension polymerized polyacrylate pressure sensitive adhesive formulations. Polyacrylate suspension pressure sensitive adhesives A1-A4 were prepared in accordance with U.S. Patent No. 4,833,179 (Young et al.) by the following method:

A 2 liter split reaction flask equipped with a condenser, thermometer, nitrogen inlet, motor driven agitator, and a heating mantle with temperature control was first charged with the ingredients ofthe dispersion medium described below in Table 10 and then heated to 58 °C. The dispersion medium was maintained at this temperature with agitation and nitrogen purging for one hour to remove oxygen. At this point, a premixed charge ofthe oil phase described below in Table 10 was added to the reactor under vigorous agitation (700 φm) to obtain a good suspension. The reaction was continued with nitrogen purging throughout the polymerization. After the exotherm ofthe reaction was reached, the reaction was continued at 75 °C for another 2 hours, and then the batch was cooled to room temperature.

TABLE 10

¹ Co loidal silica commercially available from Νalco Chemical Co.

² Isooctyl thioglycolate. ³ 2,2'-azobis(2-methylbutanenitrile), a thermal initiator commercially available from E.I. du Pont de Nemours Company. ⁴ Inherent viscosity in ethyl acetate at 27°C

The reaction mixture was then dried to remove the suspension medium and the dried suspension polyacrylate was dissolved in ethyl acetate at 30% concentration. 230 parts ofthe polyacrylate solution was then blended with 30 parts Foral™ 85 (rosin ester tackifying resin commercially available from Hercules Inc.) and, for examples 43-49, additionally combined with the amounts C5EBP non- polymerizable crosslinking agent specified in Table 11. The tackified polyacrylate blends were coated onto 38 μm thick aziridine-primed poly(ethylene terephthalate) film to a dried thickness of 25 μm and cured by the use of a PPG high intensity UV processor (PPG Inc., Pittsburgh, PA) with two lamps at the normal setting and a conveyor speed of 18.3 meters/min. (60 feet per minute). The total energy dose received by each example was 275 mJ/cm². The room temperature and high temperature shear strength of each cured sample was measured as described above, except that the high temperature measurements were made using a 500 gram hanging weight.

TABLE 11

* Examples having less than 10,000 minutes of strength failed cohesively. Example pairs C-6 and 44, C-7 and 45, C-8 and 46, C-9 and 47, C-10 and 48, C-11 and 49, and C-12 and 50 demonstrate a marked increase in shear strength found when a combination of polymerizable and non-polymerizable crosslinking agents is used to cure tackified suspension polymerized polyacrylate pressure sensitive adhesive formulations. In each example pair, the formulation containing only non- polymerizable crosslinking agent provided lower shear strengths than the corresponding example having an equivalent amount (on a molar basis) of combined crosslinking agents. It is further noted that increasing the amount of non- polymerizable crosslinking agent (comparative examples C-6 to C-12) in the absence of a copolymerized crosslinking agent hardly results in an increase in the shear strength and can even cause a decrease. Contrary to this, incoφorating a relatively small amount of a non-polymerizable radiation-activitable crosslinking group in the polymer results in an increase in shear strength.

EXAMPLES 51-59 AND COMPARATIVE EXAMPLES C-13 - C-15 A series of acrylate pressure sensitive adhesives (Adhesives C-13 - C-15) was prepared according to the method of U.S. Patent Re. 24,906 (Ulrich), in ethyl acetate using 90 wt.% isooctyl acrylate, 10 wt.% acrylic acid, 0.1 wt.% carbon tetrabromide (CBr₄) chain transfer agent, and the amounts of copolymerizable crosslinking agent (ABP) listed below in Table 12. Examples 51 to 59 were prepared by adding to the corresponding comparative example C5EBP non-copolymerizable radiation-activatable crosslinking agent in the amount specified in Table 12 below by dissolving in a 40 wt.% solution of each adhesive in ethyl acetate. All of these examples were then blended with 30 parts of Foral™ 85 (rosin ester tackifying resin commercially available Hercules Inc.) per one hundred parts adhesive solution, knife coated onto a 38 μm thick aziridine- primed poly(ethylene terephthalate) film, and dried for 15 minutes at 65°C to yield 25 μm thick coatings. The coated samples were then cured by the use of a Fusion Systems UV processor using an "FT bulb at full power and a conveyor speed of 30.5 meters/min. (100 feet/min.). Each sample was exposed to three different levels of UV radiation and the total energy dose received by each sample is recorded in Table 12 (measured in the UVA (320-390 nm) range), along with the room temperature shear strength as measured by the method described above.

TABLE 12

From the above table it can be seen that adding a relatively small amount of a non-polymerizable radiation-activitable crosslinking agent to a composition having a radiation-crosslinkable polymer that includes radiation-activitable crosslinking groups can substantially increase the shear strength ofthe crosslinked system. Accordingly, by adding varying amounts ofthe non-polymerizable radiation-activatable crosslinking agent to a composition having a radiation-crosslinkable polymer that has radiation-activatable crosslinkable groups, crosslinked polymers that have widely varying crosslink densities and properties and can be easily and advantageously prepared.

Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope ofthe present invention as defined in the claims.

Claims

CLAIMS WHAT IS CLAIMED IS:

1. A process for crosslinking a polymer, the process comprising the steps of:

(a) providing a radiation crosslinkable composition comprising: (i) a radiation crosslinkable polymer that contains abstractable hydrogen atoms; and (ii) a non-polymerizable, radiation activatable crosslinking agent;

2. A process according to claim 1 wherein the radiation crosslinkable polymer is a thermoplastic polymer.

3. A process according to claim 2 wherein the thermoplastic polymer is selected from the group consisting of polyolefins, polystyrenes, vinyl plastics, polyacrylates, polymethacrylates, poly(vinyl esters), polyamides, polycarbonates, polyketones, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing polymers may be derived and a copolymerziable comonomer.

4. A process according to claim 1 wherein the radiation crosslinkable polymer is an elastomer.

5. A process according to claim 4 wherein the elastomer is selected from the group consisting of polyurethanes, polydiorganosiloxanes, A-B-A-type block copolymers, synthetic rubber, natural rubber, ethylene- vinyl monomer polymers, poly( vinyl ethers), poly (vinyl esters), polyacrylates, polymethacrylates, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing elastomers may be derived and a copolymerizable comonomer.

6. A process according to claim 1 wherein the radiation crosslinkable polymer, exclusive ofthe radiation activatable crosslinking agent, absorbs substantially no radiation emitted by the monochromatic radiation source.

7. A process according to claim 1 wherein the non-polymerizable radiation activatable crosslinking agent is selected from the group consisting of anthraquinone, substituted anthraquinone, multifunctional acetophenone, multifunctional benzophenone, substituted triazine, and mixtures thereof.

8. A process according to claim 1 wherein the monochromatic radiation source is an excimer laser.

9. A process according to claim 1 wherein the monochromatic radiation source is an excimer lamp.

10. A process according to claim 9 wherein the excimer lamp is a XeCl lamp that emits radiation having a wavelength of about 308 nm.

11. A process according to claim 1 wherein the monochromatic radiation source emits radiation having a wavelength of about 300 to 360 nm.

12. A process according to claim 1 wherein the radiation crosslinkable composition, subsequent to crosslinking, is a pressure sensitive adhesive.

13. A process for crosslinking an elastomer, the process comprising the steps of: (a) providing a radiation crosslinkable composition comprising: (i) a radiation crosslinkable elastomer that contains abstractable hydrogen atoms; and (ii) a radiation activatable crosslinking agent selected from the group consisting of anthraquinone, substituted anthraquinone, acetophenones (ofthe multifunctional or copolymerizable type), benzophenones (ofthe multifunctional or copolymerizable type), substituted triazine, and blends thereof;

(b) providing a XeCl excimer lamp that emits monochromatic radiation having a wavelength of about 308 nm to activate the crosslinking agent and to crosslink the elastomer, the elastomer absorbing substantially no radiation emitted by the XeCl excimer lamp; and

(c) exposing the radiation-crosslinkable composition to the radiation emitted by the XeCl excimer lamp for a time sufficient to crosslink the elastomer.

14. A process according to claim 13 wherein the radiation crosslinkable composition, subsequent to crosslinking, is a pressure sensitive adhesive.

15. A process according to claim 14 wherein the radiation activatable crosslinking agent was combined with the radiation crosslinkable elastomer subsequent to the formation ofthe elastomer.

16. A process for crosslinking a polymer, the process comprising the steps of: (a) providing a radiation crosslinkable composition comprising: (i) a radiation crosslinkable polymer that contains abstractable hydrogen atoms; and (ii) a radiation activatable crosslinking agent; (b) providing an excimer lamp that can emit monochromatic radiation having a wavelength sufficient to activate the crosslinking agent and crosslink the polymer; and

(c) exposing the radiation crosslinkable composition to the radiation emitted by the excimer lamp for a time sufficient to crosslink the polymer.

17. A process according to claim 16 wherein the radiation crosslinkable polymer is a thermoplastic polymer.

18. A process according to claim 17 wherein the thermoplastic polymer is selected from the group consisting of polyolefins, polystyrenes, vinyl plastics, polyacrylates, polymethacrylates, poly(vinyl esters), polyamides, polycarbonates, polyketones, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing polymers may be derived and a copolymerizable comonomer.

19. A process according to claim 16 wherein the radiation crosslinkable polymer is an elastomer.

20. A process according to claim 19 wherein the elastomer is selected from the group consisting of polyurethanes, polydiorganosiloxanes, A-B-A-type block copolymers, synthetic rubber, natural rubber, ethylene-vinyl monomer polymers, poly(vinyl ethers), poly (vinyl esters), polyacrylates, and polymethacrylates, and copolymers comprising the polymerization product of at least one ofthe monomers from which the foregoing elastomers may be derived and a copolymerizable comonomer.

21. A process according to claim 16 wherein the radiation crosslinkable polymer, exclusive ofthe radiation activatable crosslinking agent, absorbs substantially no radiation emitted by the excimer lamp.

22. A process according to claim 16 wherein the radiation activatable crosslinking agent was copolymerized with the monomers that were polymerized to form the radiation crosslinkable polymer.

23. A process according to claim 22 wherein the radiation activatable crosslinking agent is 4-acryloxybenzophenone.

24. A process according to claim 16 wherein the radiation activatable crosslinking agent was either combined with the radiation crosslinkable polymer subsequent to the formation ofthe polymer, or was combined with the monomer(s) from which the polymer was derived but without reacting with the monomer(s).

25. A process according to claim 16 wherein the radiation activatable crosslinking agent is selected from the group consisting of anthraquinone, substituted anthraquinone, acetophenones (ofthe multifunctional or copolymerizable type), benzophenones (ofthe multifunctional or copolymerizable type), substituted triazine, and blends thereof.

26. A process according to claim 16 wherein the excimer lamp emits radiation having a wavelength of about 300 to 360 nm.

27. A process according to claim 16 wherein the excimer lamp is a XeCl lamp that emits radiation having a wavelength of about 308 nm.

28. A process according to claim 16 wherein the radiation crosslinkable composition, subsequent to crosslinking, is a pressure sensitive adhesive.

29. A process for making a flexible pressure sensitive adhesive tape, the process comprising the steps of:

(a) providing a flexible web suitable as a backing for a flexible pressure sensitive adhesive tape; (b) providing a radiation crosslinkable pressure sensitive adhesive composition comprising: (i) a radiation crosslinkable polymer that contains abstractable hydrogen atoms; and (ii) a radiation activatable crosslinking agent;

(c) applying the radiation crosslinkable pressure sensitive adhesive composition to at least a portion of at least one surface ofthe flexible web; (d) providing an excimer lamp that can emit monochromatic radiation having a wavelength sufficient to activate the crosslinking agent and crosslink the polymer; and

(e) exposing the radiation crosslinkable pressure sensitive adhesive composition to the radiation emitted by the excimer lamp for a time sufficient to crosslink the polymer to form a pressure sensitive adhesive tape.

30. A process according to claim 29 wherein the radiation crosslinkable polymer is a radiation-crosslinkable elastomer.

31. A process according to claim 30 wherein the radiation crosslinkable composition is applied to the flexible web by hot melt extrusion.

32. A process according to claim 31 wherein the excimer lamp is a XeCl lamp that emits radiation having a wavelength of about 308 nm.

33 A process according to claim 1 wherein the radiation crosslinkable polymer comprises radiation-activatable crosslinking groups capable of abstracting hydrogen atoms when activated, and the non-polymerizable radiation activatable crosslinking agent is capable of abstracting hydrogen atoms when activated.

34. A process according to claim 33 wherein the non-polymerizable radiation-activatable crosslinking agent is selected from the group consisting of non- polymerizable anthraquinone, non-polymerizable benzophenones, non-polymerizable acetophenones, and mixtures thereof.

35. A radiation crosslinkable composition comprising:

(a) a radiation crosslinkable polymer having abstractable hydrogen atoms and radiation-activatable crosslinking groups capable of abstracting hydrogen atoms when activated; and (b) a non-polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated.

36 A radiation crosslinkable composition according to claim 35 wherein the non-polymerizable radiation-activatable crosslinker is not a triazine

37 A radiation crosslinkable composition according to claim 35 wherein the non-polymerizable radiation-activatable crosslinking agent is selected from the group consisting of non-polymerizable anthraquinone, non-polymerizable benzophenones, non-polymerizable acetophenones, and mixtures thereof

38 A radiation crosslinkable composition according to claim 37 wherein the non-polymerizable radiation-activatable crosslinking agent is selected from the group consisting of anthraquinone, t-butyl anthraquinone, and 2-ethyl anthraquinone

39 A radiation crosslinkable composition according to claim 35 wherein the non-polymerizable radiation-activatable crosslinking agent is a multi-functional acetophenone or benzophenone corresponding to the following formula

wherein X represents CH₃-; phenyl; or substituted-phenyl, W represents -O-,-NH-, or -S-;

Z represents an organic spacer selected from the group consisting of aliphatic, aromatic, aralkyl, heteroaromatic, and cycloaliphatic groups free of esters, amides, ketones, urethanes, and also free of ethers, thiols, allylic groups, and benzylic groups with hydrogen atoms not intramolecularly accessible to the carbonyl group in formula (I); and n represents an integer of 2 or more

40. A radiation crosslinkable composition according to claim 39 wherein X is phenyl, W is -O-, and Z is an alkylene group having 2 to 12 carbon atoms, and n is 2.

41. A radiation crosslinkable composition according to claim 35 wherein the radiation crosslinkable polymer is a polyacrylate or a polymethacrylate.

42. A radiation crosslinkable composition according to claim 35 wherein the radiation-activatable crosslinking groups are derived from a polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated.

43. A radiation crosslinkable composition according to claim 42 wherein the polymerizable radiation-activatable crosslinking agent is selected from the group consisting of polymerizable acetophenones, polymerizable benzophenones, polymerizable anthraquinone, and mixtures thereof.

44. A radiation crosslinkable composition according to claim 42 wherein the polymerizable radiation-activatable crosslinking agent is an acrylate functional aromatic ketone or a methacrylate functional aromatic ketone.

45. A radiation crosslinkable composition according to claim 44 wherein the polymerizable radiation-activatable crosslinking agent is 4-acryloxybenzophenone.

46. A radiation crosslinkable composition according to claim 41 further comprising a tackifying resin.

47. A radiation crosslinkable composition according to claim 46 wherein the radiation-crosslinkable polymer is in the form of beads.

48. A radiation crosslinkable composition according to claim 47 wherein the radiation-crosslinkable polymer is a suspension polymer.

49. A radiation crosslinkable composition according to claim 41 wherein the radiation-crosslinkable polymer is in the form of beads.

50. A pressure sensitive adhesive obtainable by crosslinking the radiation crosslinkable composition of claim 35.

51. A pressure sensitive adhesive-coated article comprising a flexible web and a layer ofthe pressure sensitive adhesive of claim 50 disposed on a major surface ofthe web.

52. An article comprising a substrate and a layer ofthe radiation crosslinkable composition of claim 35 on the substrate.

53. A process according to claim 16 wherein the radiation crosslinkable polymer comprises radiation-activatable crosslinking groups capable of abstracting hydrogen atoms when activated, and the radiation-activatable crosslinking agent is a non-polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated.

54. A process according to claim 29 wherein the radiation crosslinkable polymer comprises radiation-activatable crosslinking groups capable of abstracting hydrogen atoms when activated, and the radiation-activatable crosslinking agent is a non-polymerizable radiation-activatable crosslinking agent capable of abstracting hydrogen atoms when activated.

55. A process according to claim 54 wherein the radiation crosslinkable polymer is a polyacrylate or a polymethacrylate.

56. A process according to claim 55 wherein the radiation crosslinkable pressure sensitive adhesive composition further comprises a tackifying resin.

57. A process according to claim 54 wherein the radiation crosslinkable polymer is in the form of suspension polymerized beads.

58. A process for crosslinking a polymer, the process comprising the steps of providing a radiation crosslinkable composition according to claim 35 and exposing the radiation crosslinkable composition to ultraviolet or visible light for a time and at a wavelength sufficient to crosslink the polymer.