WO1992002118A2

WO1992002118A2 - Radiation crosslinked conjugated diene butyl psas

Info

Publication number: WO1992002118A2
Application number: PCT/US1991/003145
Authority: WO
Inventors: Vincent Leo Hughes; Jay Douglas Audett; Natalie Ann Merrill
Original assignee: Exxon Chemical Patents Inc.
Filing date: 1991-05-07
Publication date: 1992-02-20

Abstract

PSA compositions based on conjugated diene butyl rubber may be radiation cured by relatively low levels of UV or EB radiation while retaining oxidation resistance characteristics of ordinary butyl rubber. PSAs cured to a gel content of 25 percent or better have greatly enhanced high temperature cohesive strength.

Description

RADIATION CR0B8LINKED CONJUGATED DIENB BUTYL PSAfl

Field of the Invention

This invention is directed to pressure sensitive adhesive compositions comprising radiation crosslinked conjugated diene butyl rubber.

Background of the nvention

Both high temperature performance and tack properties are particularly desirable qualities in a pressure sensitive adhesive (PSA) . In addition, PSAs should be resistant to oxidative attack and inexpensive to manufacture. It would be desirable to formulate a PSA comprising a "butyl rubber" type polymer having the tack properties of such elastomers, but having suitable modification to enable radiation crosslinking. Such PSA formulations would have the required high temperature application performance, tack properties, and oxidation resistance.

Simple elastomeric polymers which have good tack properties and an oxidation resistant saturated backbone, such as polyisobutylene homopolymers, generally lac. cohesive strength at elevated temperatures.

On the other hand, natural type elastomers, synthetic polyisoprene and copolymers high in isoprene content may be crosslinked by electron beam (EB) or actinic radiation, but these are vulnerable to oxidative attack because of the high concentration of residual unsaturations.

U. S. Patent 4,691,782 to Stine discloses articles composed of material including an irradiated blend of a C₂ - C₃ polyolefin or substituted C₂ - C₃ polymer and a conjugated diene butyl rubber. The articles possess vibration and shock impact damping characteristics and are suitable for damping dynamic forces.

U. S. Patent 4,628,073 to Fisher discloses a soft- rubbery matrix material composed of 0.3 - 70 micron particles of a 50,000 - 300,000 molecular weight cross- linkable polymer dispersed in a fluxable elastomer, where the polymer softening point temperature exceeds the elastomer fluxing temperature, and the polymer and elastomer are combined and mixed at a temperature maintained above the fluxing temperature of the elastomer but below the softening temperature of the polymer.

U. S. Patent 4,556,464 to St. Clair discloses a cured adhesive composition comprising a block copclymer of an ABA type where block A is a random copolymer of a monoalkenylarene and a conjugated diene and block B is a conjugated diene elastomer; a tackifier resin compatible with block B; and a crosslinking agent compatible with block A. Polymerization of the conjugated diene produces an unsaturated backbone for reaction with the crosslinking agent.

U. S. Patents 4,143,098 and 4,195,133 to Murphy disclose elastomer modified polystyrene or styrene- acrylonitrile thermoplastics. The elastomer is a conjugated diene copolymer containing 85-99.5% by weight of a C₄ - C₇ isoolefin and 0.5 - 15 weight percent of a conjugated diolefin containing 4 to 14 carbon atoms and has randomly distributed sites of diene unsaturation.

U. S. Patent 4,152,231 to St. Clair discloses a radiation cured polymer composition comprising a linear or radial conjugated diene block polymer where the diene polymer is selected from the group consisting of C - C₁₂ conjugated dienes and their hydrogenated derivatives; a tackifier resin; and di-to-tetra functional acrylate or methacrylate coupling agents which promote crosslinking of the base polymer.

U. S. Patents 4,068,051 to Baldwin et al.; 4,038,472 to Rae; and 3,867,270 to Malatesta et al. disclose various aspects concerning conjugated diene containing butyl rubber. The '051 patent is directed to an improved process for the dehydrohalogenation of halogenated butyl rubber. The ^,472 patent teaches a conjugated diene butyl elastomer copolymer having a curing system comprising dihydroxybenzene and an oxidant or oxidation catalyst where benzoquinone is formed in-situ and cures the elastomer at room temperature. The ^,270 patent discloses a conjugated diene butyl rubber which is cured with UV radiation with the aid of a photosensitizer.

8ummarv Of The Invention

It has been discovered that Pressure Sensitive Adhesive (PSA) compositions based on polymer'; of isobutylene and a minor amount of diene, commonly referred to as conjugated diene butyl (CDB) copolymer, can be efficiently cured utilizing low levels of radiation and, because of the mostly saturated butyl backbone, offer improved stability with enhanced resistance to oxidation. Requiring only low radiation exposure to cure the PSA to a desired gel content, PSAs of the present invention may be radiation crosslinked with enhanced throughput and reduced energy consumption.

In one embodiment, the present invention provides a radiation crosslinkable PSA composition comprising 100 parts by weight of a conjugated diene butyl elastomer and 50 to about 200 parts by weight of a tackifying resin. The conjugated diene butyl rubber is preferably radiation crosslinked to a gel content of at least 25 percent by weight of the elastomer and the PSA has a storage modulus of less than about 5 x 10⁶ dynes/cm², a SAFT of at least about 50*C and a holding power of at least 100 hours.

In a further embodiment, the PSA composition preferably comprises from about 1 to about 10 parts by weight photosensitizer when ultraviolet (UV) radiation curing is utilized. The PSA is crosslinked by low radiation exposure of less than about 2 joules/cm² (J/cm²) and contains gel structures.

In another aspect, the PSA composition preferably comprises from about 1 to about 10 parts by weight of a photocrosslinking agent when electron beam (EB) radiation is utilized. The PSA is crosslinked by low radiation exposure of less than about 5 Mrad EB radiation and contains gel structures. In a further embodiment, the present invention provides a method for applying a conjugated butyl diene rubber based pressure sensitive adhesive comprising the steps of: (a) blending 100 parts by weight of a conjugated diene butyl elastomer with from about 50 to about 200 parts by weight of a tackifier; (b) depositing a layer of the blend onto a substrate surface; and (c) irradiating the blend on the surface of the substrate to crosslink the elastomer into a gel structure comprising at least about 25 percent by weight of the elastomer, preferably having a modulus below about 5 x 10⁶ dynes/cm², a SAFT of at least about 50*C and a holding power of at least about 100 hours.

In a further aspect the present invention provides a method comprising the steps of casting the PSA composition from a solvent, removing the solvent to leave a residue layer, and irradiating the residue with less than about 2 J/cm² of UV radiation or less than about 5 Mrad EB radiation.

Detailed Description of The Invention

Radiation crosslinked pressure sensitive adhesive (PSA) compositions of the present invention utilize EB or actinic, UV rich, radiation to cure the adhesive. Reactive conjugated unsaturation in the backbone of conjugated diene butyl (CDB) elastomers incorporate sites at which crosslinking proceeds.

The PSA formulation may be applied to a substrate from a solvent which upon evaporation deposits a layer of the adhesive. Tacky upon deposition, the PSA may be highly crosslinked by relatively low exposure to actinic or EB radiation which improves the PSA high temperature performance while not substantially diminishing adhesive properties.

The PSA compositions which embody the present invention comprise an admixture of a CDB elastomer and one or more suitable tackifying resins. The tackifier is generally present in an amount effective to promote adhesion to a particular substrate. The tackifier typically comprises from about 50 to about 200 phr, preferably from about 50 to about 100 phr, of the composition.

5 CDB elastomer refers to one or more "butyl" elastomers which incorporate conjugated unsaturations along the butyl backbone. In a post-polymerization reaction procedure the as-polymerized "butyl" elastomer is first halogenated to form a brominated elastomer, for

10 example, then dehydrohalogenated to produce the conjugated diene unsaturations along the "butyl" backbone.

The expression "butyl" elastomer is well known in the art and generally refers to an elastomeric copolymer

15 of an isoolefin of about 4 to 7 carbon atoms, such as isobutylene, which is a preferred isoolefin, and a conjugated diolefin of about 4 to 14 carbon atoms, such as isoprene, which is a preferred diolefin. "Butyl" elastomers typically comprise from about 85 to about 99.5

20 percent by weight of the isoolefin and from about 0.5 to about 15 percent by weight of the diolefin. The preferred CDB elastomer incorporates conjugated diene units distributed along the isobutylene backbone as either:

The conjugated diene is preferably present in an amount between about 0.5 and about 6 mole percent, more 0 preferably about 0.5 and about 3 percent by mole, and most preferably in an amount of about 2 mole percent.

Typically, the CDB elastomer has a number average molecular weight between about 5000 and about 500,000, preferably between about 50,000 and about 300,000, and

35 more preferably between about 100,000 and about 300,000. The resulting CDB elastomer is represented generally by the structure:

and

or mixtures of (I) and (II) , wherein n and m represent a number of isobutylene repeat units and x and y represent a number of conjugated diene repeat units in both common representations mentioned above. Manufacture of butyl elastomers is well known in the art and is described, for example, in U. S. Patent 2,356,128 which is hereby incorporated herein by reference. A method for the halogenation of butyl elastomers is described in U. S. Patent 3,099,644 which is hereby incorporated herein by reference. Dehydrohalogenation of halogenated butyl elastomers to produce CDB elastomers is described in U. S. Patent 3,867,270 which is hereby incorporated herein by reference.

A second component of the radiation curable CDB based PSA formulations of the present invention is a tackifier for promoting wetting of the substrate and adhesion. The tackifying system may include one or more tackifier resins with different composition and molecular weight characteristics. Exemplary tackifier components suitable for use in this invention include aliphatic hydrocarbon resins obtained from Exxon Chemical Co. under the trade designation ESCOREZ, e.g., ECR-111 and ECR- 143H, hydrogenated aliphatic hydrocarbon resins, and ECR- 327, a hydrogenated cycloaliphatic hydrocarbon resin. Hydrogenated tackifiers are preferred because unsaturation present in the tackifier may reduce the conversion of polymer to gel through radiation energy absorption or thr ugh tackifier participation in crosslinking when the adhesive is cured. These tackifiers typically have a ring and ball softening point from about 10"C to about 180"C, preferably from about 15*C to about 75"C. The tackifiers obtained under the trade designations ECR-111, ECR-143H and ECR-327 have been found to be particularly preferred. ECR-143H resin is prepared by the cationic polymerization of a C₅ olefin/diolefin feed stream as described in U. S. 4,916,192 which is incorporated by reference herein. A third component of the PSA formulation based on CDB elastomers of the present invention which may be present is a photosensitizer to enhance the sensitivity of the PSA to UV radiation. Useful photosensitizers include benzophenone, 1-hydroxycyclohexyl phenyl ketone, propiophenone, cyclopropyl phenyl ketone, acetophenone, 1,3,5-triacetyl benzene, benzaldehyde, thioxanthane, anthraquinone β-naphthyl phenyl ketone, β- naphthaldehyde, β-acetonaphthone, 2,3-pentanedione, benzil, fluorenone, pyrene, benzanthrene, and anthracene. Of these photosensitizers, benzophenone is preferred. While most of these are well known photosensitizers, other photosensitizers responsive to UV radiation would work equally well in the present invention.

Photosensitizers are generally chosen according to the wavelength sensitivity of the compound and the degree to which it absorbs UV energy in competition with formation of the dimerization species by which the photo- crosslinking mechanism generally proceeds. The PSA may comprise from about 0.01 to about 10 parts of the UV photosensitizer per 100 parts by weight CDB elastomer, preferably from about 1 to about 5 parts by weight in the formulation. A fourth component which may be present in the PSA formulation based on CDB butyl elastomers is a photo- crosslinking agent comprising a multifunctional acrylate or methacrylate coupling agent which promotes crosslinking of the base elastomer during exposure to EB radiation. The crosslinking promoters are typically di-, tri- and tetra-functional acrylates and methacrylates selected from the group consisting of the acrylic and methacrylic esters of polyols. Examples of such crosslinking promoters include 1,6-hexanediol diacrylate (HDODA) , 1,6-hexanediol di ethacrylate (HDODM) , trimethylolpropane triacrylate (TMPTA) , trimethylolpropane trimethacrylate (TMPTM) , pentaerythritol tetracrylate (PTA) and tetramethacrylate (PTM) and the like. Preferred crosslinking promoters include TMPTA and TMPTM. The PSA may comprise from about 0.1 to about 10 parts by weight of the crosslinking promoter per 100 parts by weight CDB elastomer and preferably, about 0.5 to about 2 parts by weight. The PSA composition may further contain relatively minor amounts of ingredients such as waxes, oils, fillers, coupling agents, colorants, antioxidants, and other stabilizing additives which do not substantially adversely affect the system such as, for example, by adversely interfering with the radiation crosslinking or adhesion of the PSA to a substrate surface.

The antioxidant or stabilizer can be added to the PSA at from about 0.1 to about 1.5 percent by weight of the adhesive, preferably from about 0.1 to about 1 percent by weight, and typically about 0.5 percent by weight.

The optional oils which may be mentioned include refined hydrocarbon oils such as are commonly used in adhesives, including paraffinic, aromatic, and naphthenic oils available under the trade designations KAYDOL, TUFFLO, and the like. The refined oils serve to reduce viscosity and improve surface tack properties, and are generally present at 0 to about 50 phr CDB. A modifying polymer which may be present in the tackifier system is characterized as a low molecular weight butyl resin. One such suitable resin is available commercially under the trade name KALENE 800 and comprises an approximately 40,000 molecular weight degraded butyl rubber; another is a polyisobutylene of about 50,000 molecular weight sold under the trade name VISTANEX IM. Another modifying polymer which may be present in the tackifier systsm may be characterized as a butyl oligomer of very low molecular weight. One such suitable resin is available commercially under the trade name PARAPOL 950 and comprises chiefly polyisobutylene oligomers of about 950 molecular weight.

Particulated fillers which may be used for thickening and price reduction include glass, silica, amorphous Si0₂, fumed alumina, calcium carbonate, fibers and the like. These fillers generally may comprise from about 1 to about 50 percent by weight of the adhesive, and preferably from about 2 to about 40 percent by weight. Silicates are generally used at from 2 to 10 percent by weight while calcium carbonate can comprise up to 30 or 40 percent by weight of the adhesive. Suitable commercially available fillers are available under the trade designations CAB-O-SIL, ZER0SIL 35, AEROSIL R972, DUCRAL 10 and the like

The radiation curable PSA compositions based on CDB elastomers of this invention are prepared by blending together the components in a suitable solvent or aqueous emulsion medium, for example, until a homogeneous blend is obtained. Various methods of blending materials of this type are known to the art, and any method that produces a homogeneous blend is satisfactory. Typically blending equipment includes, for example, mixing extruders, roll mills, Banbury mixers, Brabenders, and the like.

The PSA composition may be applied to the substrate from a solution of up to about 40 percent by weight solids of the ingredients in a non-polar solvent such as toluene, xylene, hexane and the like. Prior to crosslinking, the solvent is removed by evaporation to leave the coated substrate. Alternatively, the ingredients may be mixed in a solvent, the mixture emulsified and the solvent evaporated, and the adhesive applied to the substrate as a 60-70 weight percent solids water-based emulsion, the water being removed by subsequent evaporation.

Crosslinking may be effected by EB or UV radiation. EB or high energy radiation can be obtained from suitable sources including a Van de Graaf electron accelerator, betatron, cyclotron or the like. Such energy sources produce an ionizing radiation including electrons, protons, neutrons, gamma rays. X-rays and the like. UV radiation may be filtered to exclude the shorter, higher energy wavelength portion of the UV spectrum which may cause degradation of the adhesive surface. Alternatively, one may employ a UV source having the appropriate output spectrum. Crosslinking density of the CDB based PSA composition of the present invention is adjusted to a level of gel content for optimized tack and temperature performance properties. The irradiation of the PSA is preferably sufficient to obtain a gel content of the PSA in a range of from about 25 to about 110 percent by weight of the elastomer, more preferably from about 40 to about 55 percent by weight. Furthermore, the crosslinked PSA should preferably have a storage modulus less than about 5 x 10⁶ dynes/cm². At this modulus value or lower the PSA composition falls within Dalquist criteria for having good substrate wettability. Additionally, the crosslinked PSA should preferably have temperature performance characterized by a shear adhesion fail temperature (SAFT) of at least about 50*C, more preferably at least about 75"C, most preferably at least about 105*C, and a holding power test result of at least about 100 hours. A surprising feature of the present invention is that the substantially increased SAFT obtained by crosslinking is not accompanied by a significant corresponding reduction in adhesive properties such as loop tack and peel strength.

PSA formulations based on CDB elastomer may be crosslinked by short exposure to low intensity radiation. PSAs may be crosslinked to the required gel content by EB exposure of from about 1 to about 5 Mrad, preferably from about 1 to about 3 Mrad. PSAs may also be crosslinked by UV exposure from about 0.1 to about 2 J/cm², preferably from about 0.2 to about 1 J/cm², and more preferably from about 0.25 to about 0.7 J/cm². Crosslinked PSA formulations based on CDB elastomers of the present invention may also be characterized as having gel structures. A preferred use of the present formulation is in the preparation of PSA tapes or labels. The PSA tape comprises a flexible backing sheet and a layer of the adhesive composition of the present invention coated on one major surface of the backing sheet. The backing sheet may be a plastic film, paper or any other suitable material and the tape may include various other layers or coatings, such as primers, release coatings and the like, which are used in the manufacture of PSA tapes.

In another embodiment of the present invention, a method by which a coated substrate may be made is offered. Such method comprises blending together the CDB elastomer based formulation with one or more appropriate tackifiers, depositing a solution of the formulation on a suitable substrate, irradiating the adhesive with a low dose of actinic or EB radiation to effect gel formation so that the PSA formulation has a storage modulus of less than 5 x 10⁶ dynes/cm², temperature performance characterized as a SAFT of 50*C or more and a holding power of 100 hours or more. The shear adhesion failure temperature (SAFT) is defined as a temperature at which a 1.0 kg weight drops in an oven after being attached to a substrate material by a 1" x 1" overlap of tape coated with the PSA and increasing oven temperature at the rate of 40*F per hour. Holding power (HP) is defined as a time required for a 1" x 1" area of label adhered to steel to fail under a load of 1 kg applied in shear at a 2 ' antipeel or a time required for a 0.5 x 1" area of tape adhered to steel to fail under a load of 1 kg applied in shear at a 2* antipeel. Unless otherwise noted, the HP as referred to in the specification and the claims is based on the 1" x 1" label method. The method is Pressure Sensitive Tape Council Method #7.

The peel strength is determined according to Pressure Sensitive Tape Council Method #1. A 1 in. wide tape is adhered to a clean stainless steel bar and the bar is mounted in an Instron tester. The free end of the tape is pulled away at a 180* angle at a rate of 2 in. per minute and the required force is recorded.

Loop tack refers to a test conducted by using a 1 in. wide tape formed into a loop, adhesive side out. The loop is fixed in the upper jaws of an Instron tester and lowered to meet a horizontal clean stainless steel plate clamped in position in the lower jaws of the Instron tester. When about 5 in. of tape are adhered to the steel plate, the upper jaws are raised at a rate of 2 in./min. The required force is recorded as the loop tack.

The storage modulus (G ) is determined according to a Polymer Laboratories, Inc. dynamic mechanical thermal analyzer (DMTA) procedures at ambient temperature. The PSA is cast in a 2 mm deep Teflon-coated mold, and 12 mm diameter disks are die cut for DMTA testing. G is understood in the art to be a measurement of the elastic or storage modulus (stress/strain) measured in phase with sinusoidial shear displacement of the material. Gel refers to the insoluble residue of the rubber in the adhesive and is determined by exhaustive solvent extraction in refluxing toluene for about 72 hours, and drying and weighing the remaining gel. Qualitatively, the gel structure can be evaluated by soaking irradiated CDB specimens on a polyester substrate in toluene for a few minutes. Using this qualitative technique, if the adhesive lacks gel structure the rating is 0; trace gel structure is rated 1; loose gel structure is rated 2; and tight gel structure is rated 3.

The invention is further illustrated by means of the following illustrative examples:

Examples 1-12 and Comparative wirwn-ipies 1-3

Several PSA formulations based upon a CDB elastomer were blended with a variety of tackifying systems to evaluate a crosslinking response in terms of gel formulation upon exposure to EB radiation. For each sample tightness of subsequent gel structures was evaluated using the qualitative procedure. ^* jults appear in Table I. The data indicate tight gel s „ture in those samples containing crosslinking promoter TMPTA at low EB exposure.

TABLE I EB CURED PSA FORMULATIONS COMPOSITION (phr)

QUALITATIVE EVALUATION

ECR-327 TMPTA OF GEL

1 2 2

Examples 13-35 and Comparative T-va-mpi«■»« Λ- SAFT tests were run on PSA formulations prepared as in Examples 1-12. High temperature performance for different tackifying systems was evaluated as a function of level of radiation exposure. Results show good SAFT values particularly in those samples having crosslinking promoter TMPTA. Temperature performance was superior to or comparable with a typical nonirradiated KRATON formulation (Comparative Example 4) commonly employed for

PSA applications (KRATON is a Shell trademark for SBS or

SIS block copolymer PSA compositions) . Furthermore, the inadequacy of standard "butyl rubber" (Comparative

Example 5) is evident.

The holding power performance of PSA formulation prepared as in Examples 13-35 and Comparative Examples 4- 5 was tested upon varying exposure to EB radiation. Nonirradiated samples were also tested for comparison. Results tabulated in Table III indicate greatly enhanced holding power of the cured samples.

Peel tests were performed on selected PSA formulations prepared as in Examples 13-35. Results indicate that peel strength was only modestly reduced with increasing concentration of gel "before" and "after"

EB curing. Results appear in Table IV.

Loop tack tests were performed on selected PSA formulations prepared as in Examples 13-35. Results indicated that loop strength was only moderately reduced with increasing gel concentration "before" and "after" EB curing. Results appear in Table V.

Holding power performance was evaluated for several UV cured PSA samples utilizing benzophenone as a photosensitizer. Results in Table VI indicate excellent holding power for PSA formulations having a gel concentration of about 40% or greater. A nonirradiated comparative sample had poor holding power.

Peel strength performance of two PSAs formulated as in Examples 62 and 65 were evaluated. Results in Table VII indicate that peel strength was enhanced by selection of the appropriate tackifier system.

TABLE VII UV CURED PSA FORMULATIONS

COMPOSITION (phr)

EXAMPLE

66 1.0 100 100 45 2.05

67 1.0 100 60 40 47.7 3.35

Examples 68-75 and Comparative Examples 15-18

Various PSA formulations were evaluated before and after UV exposure for SAFT, peel, loop tack and 1" x h Lll holding power. The photosensitizer was a 3:1 mixture of benzophenone and 1-hydroxycyclohexyl phenyl ketone. The results are presented in Table VIII. TABLE VIII UV CURED PSA FORMULATIONS COMPOSITION (Phr) UV HOLDING

EXPOSURE ECR- ECR- ECR- PARAPOL PHOTO- SAFT PEEL LOOP TACK POWER EXAMPLE (J/cw²) CDB 327 143H 111

Coβp.15 0 100 --- 60 40

68 0.25 100 --- 60 40 69 0.75 100 --- 60 40

Coπp.16 0 100 --• 60 40

70 0.25 100 --- 60 40

71 0.75 100 — 60 40 Cop.17 0 100 100 72 0.25 100 100

73 0.75 100 100

Coup.18 0 100 100

74 0.25 100 100

75 0.75 100 100

Examples 76-79 and Comparative Examples 19-22

Various PSA formulations were prepared and evaluated for SAFT, peel, loop tack and 1" x h" holding power before and after exposure to electron beam exposure. The results are presented in Table IX.

The foregoing description of the invention is illustrative and explanatory thereof. Various changes in the materials, apparatus, and particular parts employed will occur to those skilled in the art. It is intended that all such variations within the scope and spirit of the appended claims be embraced thereby.

Claims

Claims:

1. A radiation crosslinkable composition, comprising:

(a) 100 parts by weight of conjugated diene butyl elastomer; and

(b) from about 50 to about 200 parts by weight of a tackifier intimately blended therewith; and

2. A radiation crosslinked pressure sensitive adhesive composition, comprising:

(a) 100 parts by weight of conjugated diene butyl elastomer; and

(c) wherein said conjugated diene butyl elastomer is radiation cross-linked to a gel content of at least about 25 percent by weight of said elastomer.

3. The composition of claim 2, wherein said gel content comprises from about 25 to about 110 percent by weight of said elastomer.

4. The composition of claim 1, wherein said composition further comprises from about 1 to about 10 parts by weight photosensitizer.

5. The composition of claim 4, wherein said photosensitizer comprises benzophenone.

6. The composition of claim 1, wherein said composition further comprises from about 1 to about 10 parts by weight photocrosslinking promoter.

7. The composition of claim 6, wherein said photocrosslinking promoter is selected from the group consisting of trimethylolpropane triacrylate and trimethylolpropane trimethacrylate.

8. The composition of claim 2, wherein said composition is crosslinked by UV radiation of less than about 2 joules/cm².

9. The composition of claim 2, wherein said composition is crosslinked by electron beam radiation of less than about 5.0 Mrad.

10. The composition of claim 1, comprising from about 50 to about 100 parts by weight of said tackifier.

11. The composition of claim 1, wherein said tackifier has a ring and ball softening point from about 10^*C to about 180^βC.

12. The composition of claim 11, wherein said softening point is from about 15*C to about 75*C.

13. The composition of claim 4, comprising from about 1 to about 5 parts by weight of said photosensitizer.

14. A method for applying a conjugated diene butyl-based pressure sensitive adhesive, comprising the steps of:

(a) intimately blending 100 parts by weight of a conjugated diene butyl elastomer with from about 50 to about 200 parts by weight of a tackifier;

(b) depositing a layer of said blend onto a substrate surface; and

(c) irradiating said blend on said surface to crosslink said elastomer into a gel content of at least about 25 percent by weight of said elastomer.

15. The method of claim 14, further comprising the step of adhering said layer to a second surface opposite said substrate.

16. The method of claim 14, wherein said blend includes a crosslinking promoter and said radiation comprises electron beam radiation.

17. The method of claim 16, wherein said radiation is less than about 5.0 Mrad.

18. The method of claim 14, wherein said blend includes a photosensitizer and said radiation comprises ultraviolet radiation.

19. The method of claim 18, wherein said radiation is less than about 2 joules/cm².

20. The method of claim 14, wherein said blend includes a solvent and said deposition includes casting said layer onto said substrate surface.

21. The method of claim 20, wherein said casting includes removing said solvent from said blend to leave said layer as a residue on said substrate surface.

22. The method of claim 14, wherein said blend comprises from about 50 to about 100 parts by weight of said tackifier.

23. The method of claim 18, wherein said photosensitizer comprises from about 1 to about 5 parts by weight of said blend.