A variety of techniques exist for preparing viscoelastic compositions useful, e.g., as adhesives. These techniques include solution polymerization, emulsion polymerization, and bulk polymerization. One particularly useful bulk polymerization process involves exposing an essentially solvent-free, pre- viscoelastic composition containing reactive monomers to a source of transmissive energy such as ultraviolet radiation or thermal conduction to cause the monomers to react and form the viscoelastic composition.

Summarv of the Invention In general, the invention features a process for preparing a viscoelastic composition that includes the steps of: (a) exposing an essentially solvent-free, pre- viscoelastic composition that includes a reactive monomer to transmissive energy to form a viscoelastic composition that includes an amount of reactive monomer; and (b) exposing the viscoelastic composition to a dose of gamma radiation sufficient to reduce the amount of reactive monomer.

In preferred embodiments, the transmissive energy includes ultraviolet radiation, thermal conduction, or a combination thereof. The dose of gamma radiation preferably is greater than about 5 kGy and less than about 60 kGy.

Examples of preferred viscoelastic compositions include adhesive compositions such as pressure sensitive adhesive compositions, hot melt adhesive compositions, and vibration damping materials. The pre-viscoelastic composition and the viscoelastic composition are preferably essentially free of crosslinking agents.

Examples of suitable pre-viscoelastic compositions include one or more monomers selected from the following: (a) an acrylic or methacrylic acid ester of a non-tertiary alcohol having between 4 and 14 carbon atoms, inclusive (e.g., isooctyl acrylate, 2-ethyl hexyl acrylate, butyl acrylate, and combinations thereof); (b) a polar ethylenically unsaturated monomer (e.g., acrylic acid, methacrylic acid,

itaconic acid, acrylamide, methacrylamide, lower alkyl-substituted acrylamides, N- vinyl pyrollidone, and combinations thereof); and (c) a hydrophilic, ethylenically unsaturated monomer different from the polar monomer (e.g., an acrylate- terminated poly(alkylene oxide), a methacrylate-terminated poly(alkylene oxide), or a combination thereof).

The invention provides viscoelastic compositions having a reduced reactive monomer content. Such compositions are useful in a variety of settings, including medical applications (e.g., adhesive bandages and wound dressings).

Other features and advantages of the application will be apparent from the following description of the preferred embodiments thereof, and from the claims.

Description of the Preferred Embodiments The method is useful for producing a variety of viscoelastic compositions having a reduced reactive monomer content. Examples of viscoelastic compositions which can be prepared include pressure sensitive adhesives, hot melt adhesives, hot melt processable sealants, vibration damping materials, and gels (e.g., for medical applications). The viscoelastic composition may be in the form of a microemulsion, as described in Dietz et al., WO 95/20634.

Such polymerized microemulsions are derived from concurrent polymerization of a free-radically polymerizable hydrophilic or amphiphilic monomer or oligomer in the aqueous phase of a microemulsion and a free-radically polymerizable hydrophobic monomer in the organic phase of the microemulsion.

The viscoelastic compositions are prepared by exposing an essentially solvent-free, reactive monomer-containing, pre-viscoelastic composition to a source of transmissive energy. The transmissive energy may be selected from ultraviolet radiation, visible radiation, thermal radiation, or thermal conduction, with ultraviolet radiation and thermal conduction being preferred. The reactive monomers contain at least one ethylenically unsaturated functional group. Upon exposure to transmissive energy, the monomers react with each other through the ethylenically unsaturated functional group to form a viscoelastic composition.

The selection of specific ethylenically unsaturated monomers, and the relative amounts thereof depends upon the desired properties of the final viscoelastic product, which, in turn, depends upon the use to which the product will be put. In general, however, the following groups of monomers are useful.

One group of reactive monomers which may be used includes acrylic and methacrylic acid ester monomers prepared by reacting acrylic or methacrylic acid with alcohols such as 1-butanol, 2-butanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-l-butanol, 1-hexanol, 2-hexanol, 2-methyl-l -pentanol, 3-methyl-1- pentanol, 2-ethyl-l-butanol, 3,5,5 -trimethyl- 1 -hexanol, 3 -heptanol, 1-octanol, 2- octanol, isooctyl alcohol, 2-ethyl-l-hexanol, 1-decanol, 1 -dodecanol, 1-tridecanol, 1-tetradecanol, and the like, as well as combinations thereof. Particularly preferred ester monomers include isooctyl acrylate, 2-ethyl hexyl acrylate, and n-butyl acrylate.

A second group of suitable monomers, particularly where the viscoelastic composition is an adhesive designed for incorporation in an adhesive bandage or wound dressing, includes ethylenically unsaturated hydrophilic monomers such as free radically reactive hydrophilic oligomers (a polymer having a low number of repeating units, generally 2 to 20) and/or polymers including poly(alkylene oxides) (e.g., poly(ethylene oxide)), poly(vinyl methyl ether), poly(vinyl alcohol), cellulose derivatives, and mixtures thereof. Other suitable ethylenically unsaturated hydrophilic monomers include macromonomers, e.g., acrylate-terminated poly(ethylene oxide), methacrylate-terminated poly(ethylene oxide), methoxy poly(ethylene oxide) methacrylate, butoxy poly(ethylene oxide) methacrylate, p-vinyl benzyl-terminated poly(ethylene oxide), acrylate-terminated poly(ethylene glycol), methacrylate-terminated poly(ethylene glycol), methoxy poly(ethylene glycol) methacrylate, butoxy poly(ethylene glycol) methacrylate, p- vinyl benzyl-terminated poly(ethylene glycol), poly(ethylene oxide) diacrylate, poly(ethylene oxide) dimethacrylate, and combinations thereof. Particularly preferred ethylenically unsaturated hydrophilic monomers include acrylate and methacrylate esters prepared from mono-hydroxyi-terminated poly (lower alkylene oxides) such as polyethylene and polypropylene glycols commercially available

under the trade designation Carbowax from Union Carbide Corp. in a variety of molecular weights (e.g., Carbowax 350, Carbowax 550, Carbowax 750, Carbowax 2000, and Carbowax 5000). An example of a preferred acrylate-terminated polyethylene glycol is commercially available from Shin-Nakamura Chemical Co., Ltd., Japan, under the designation "NK Ester AM-90G." Another group of suitable reactive monomers includes polar monomers such as acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, lower alkyl-substituted acrylamides (e.g., methyl, ethyl and t-butyl-substituted acrylamides), N-vinyl-pyrollidone, and combinations thereof. The preferred polar monomer is acrylic acid.

Preferably, the pre-viscoelastic composition is essentially free of multifunctional crosslinking monomers. However, for certain applications (e.g., where high cohesive strength is required), such monomers may be included as well. The term "multifunctional" as used herein refers to crosslinking monomers which have two or more free radically polymerizable, ethylenically unsaturated groups. Useful multi-functional crosslinking monomers include acrylic or methacrylic esters of diols such as butanediol diacrylate, triols such as glycerol, and tetraols such as pentaerythritol. Other useful multifunctional crosslinking monomers include polymeric multifunctional (meth)acrylates, e.g., poly(ethylene oxide) diacrylate or poly(ethylene oxide) dimethacrylate; polyvinylic crosslinking agents such as substituted and unsubstituted divinylbenzene; and difunctional urethane acrylates such as "EBECRYL" 270 and "EBECRYL" 230 (1500 weight average molecular weight and 5000 weight average molecular weight acrylated urethanes, respectively--both available from Radcure Specialties), and combinations thereof.

The pre-viscoelastic composition may also include an initiator. For example, where the viscoelastic composition is prepared by exposure to ultraviolet radiation, a photoinitiator is included in the pre-viscoelastic composition. Useful photoinitiators include substituted acetophenones such a benzyl dimethyl ketal and 1 -hydroxycyclohexyl phenyl ketone, substituted alpha-ketols such as 2-methyl-2- hydroxypropiophenone, benzoin ethers such as benzoin methyl ether, benzoin

isopropyl ether, substituted benzoin ethers such as anisoin methyl ether, aromatic sulfonyl chlorides, and photoactive oximes. The amount of photoinitiator typically ranges from about 0.001 to about 5.0 parts by weight per 100 parts of total monomer, preferably from about 0.01 to about 5.0 parts by weight, and more preferably from about 0.1 to about 0.5 parts by weight.

For thermal polymerization, a thermal initiator is included. Useful thermal initiators include azo, peroxide, persulfate, and redox initiators.

Suitable azo initiators include 2,2'-azobis(2,4-dimethylvaleronitrile) (VAZOTM 52); 2,2'-azobis(isobutyronitrile) (VAZOTM 64); 2,2'-azobis-2- methylbutyronitrile (VAZOTM 67); and (1,1 '-azobis(1 -cyclohexanecarbonitrile) (VAZOTM 88), all of which are available from DuPont Chemicals, and 2,2'- azobis(methyl isobutyrate) (V-601) and 2,2'-azobis(2-amidinopropane) dihydrochloride (V-50) available from Wako Chemicals. Also suitable is 2,2'- azobis(4-methoxy-2,4-dimethylvaleronitrile), formerly available from DuPont Chemicals as VAZOTM 33.

Suitable peroxide initiators include benzoyl peroxide, acetyl peroxide, lauroyl peroxide, decanoyl peroxide, dicetyl peroxydicarbonate, di(4-t- butylcyclohexyl) peroxydicarbonate (PERKADOXTM 16S, available from AKZO Chemicals), di(2-ethylhexyl) peroxydicarbonate, t-butylperoxypivalate (LupersolTM11, available from Atochem), t-butylperoxy-2-ethylhexanoate (TrigonoxTM 21-C50, available from Akzo Chemicals, Inc.), and dicumyl peroxide.

Suitable persulfate initiators include potassium persulfate, sodium persulfate, and ammonium persulfate.

Suitable redox (oxidation-reduction) initiators include combinations of the above persulfate initiators with reducing agents such as sodium metabisulfite and sodium bisulfite; systems based on organic peroxides and tertiary amines (for example, benzoyl peroxide plus dimethylaniline); and systems based on organic hydroperoxides and transition metals, for example, cumene hydroperoxide plus cobalt naphthenate.

Other useful initiators include materials such as tetraphenyl 1,1,2,2- ethanediol.

Preferred thermal free-radical initiators are selected from the group consisting of azo compounds and peroxides, e.g., LupersolTM 11 and PerkadoxTM 16, and mixtures thereof.

The thermal initiator is generally used in an amount ranging from about 0.01 to about 5.0 parts by weight per 100 parts of total monomer, preferably from 0.025 to 2 weight percent.

A combination of thermal and photo initiators may be included in the pre-viscoelastic composition. For example, the pre-viscoelastic composition may polymerized, e.g., in a reactive extruder, to a certain conversion using a thermal initiator, the resulting composition (still in a pre-viscoelastic state) combined with a photoinitiator, and the polymerization completed upon exposure to ultraviolet radiation. Conversely, the initial polymerization may be initiated by a photoinitiator, and the polymerization subsequently completed using a thermal initiator. The thermal and photoinitiator may also be used together, rather than being added sequentially.

Other materials which may be added to the pre-viscoelastic composition include chain transfer agents for controlling molecular weight (e.g., carbon tetrabromide, sulfur compounds such as isooctylthioglycolate ("IOTG"), or alcohols), tackifiers, plasticizers (e.g., polyethylene glycol, polypropylene glycol, or glycerin), perfumes, deodorants, antioxidants, hydrophobic or hydrophilic silica, pigments, glass or polymeric bubbles or beads (which may be expanded or unexpanded), fibers, reinforcing agents, calcium carbonate, toughening agents, fire retardants, antioxidants, finely ground polymeric partricles such as polyester, nylon, and polypropylene, stabilizers, and pharmacologically active ingredients such as drugs, antibiotics, and anti-microbial agents. Polymeric microspheres, e.g., as described in Silver, U.S. 3,691,140 and Baker et al., U.S. 4,166,152, may be included as well.

The polymerization may be effected in a variety of ways. For example, the pre-viscoelastic composition may be coated onto a backing and thereafter exposed to a source of transmissive energy. Alternatively, the pre-viscoelastic composition may be partially or completely surrounded by a packaging material,

and the resulting package (e.g., in the form of a pouch) exposed to a source of transmissive energy. Examples of suitable packaging materials include flexible thermoplastic polymeric films such as ethylene-vinyl acetate, ethylene-acrylic acid, polypropylene, polyethylene, polybutadiene, and ionomeric films.

Thermal polymerization can be effected by immersing the packaged composition in a heat exchange medium at temperatures between about 40"C and 100°C for a time sufficient to polymerize the composition. The heat exchange medium may be a forced or impinged gas or a liquid such as water, perfluorinated liquids, glycerine, or propylene glycol. The heat necessary for thermal polymerization may also be provided by a metal platen, heated metal rolls, or microwave energy.

The temperature at which the polymerization occurs depends upon the activation temperature of the initiator. For example, polymerization using VAZOTM64, a commercially available initiator from DuPont Company can be carried out at about 80"C, while VazoTM52, also available from DuPont Company, can be used at about 70"C. It is preferable to carry out the polymerization in an appropriate liquid heat exchange medium at a controlled temperature. A suitable liquid heat exchange medium is water, heated to the desired reaction temperature.

Commercially available heat transfer fluids may also be used.

Polymerization can also be effected by exposure to ultraviolet (UV) radiation as described in U.S. Patent No. 4,181,752 (Martens et al.). For example, the polymerization may be carried out with UV black lights having over 60 percent, and preferably over 75 percent of their emission spectra between 280 to 400 nanometers (am), with an intensity between about 0.1 to about 25 mW/cm2, under conditions such that the total energy ranges from 200 to 3000 mj/cm2.

Where the pre-viscoelastic composition is combined with packaging material, the temperature of the pre-viscoelastic composition can be controlled by blowing cooling air around the packaged composition, by running the packaged composition over a cooled platen, or by immersing the packaged composition in a water bath or a heat transfer fluid during polymerization. Preferably, the packaged compositions are immersed in a water bath, with water temperatures between about

5"C and 90"C, preferably below about 30"C. Agitation of the water or fluid helps to avoid hot spots during the reaction.

Following polymerization, the viscoelastic product is exposed to gamma radiation to reduce the amount of unreacted monomer in the composition.

The extent of reduction can be measured using, e.g., gas chromatography. The particular radiation dosage required is a function of the composition. In general, however, the dosage is greater than about 5 kGy and less than about 60 kGy.

The invention will now be described further by way of the following examples.

EXAMPLES Examples 1-4 Examples 1-4 illustrate the effect of exposure to gamma radiation on the amount of unreacted isooctyl acrylate monomer for various isooctyl acrylate- acrylic acid-polyethylene oxide acrylate ("IOA/AA/EOA") polymers. The polymers were prepared as follows.

A vial was charged with isooctyl acrylate, after which 0.04% by weight photoinitiator (benzyl dimethyl ketal) was added and stirred until dissolution.

Acrylic acid and, if present, polyethylene glycol acrylate ("NK Ester--AM-90G" from Shin-Nakamura Chemical Co., Ltd., Japan) were then added to the solution and the resulting solution reacted to form a syrup of coatable viscosity by exposing the solution to ultraviolet radiation. A second portion of photoinitiator was added to the syrup, after which the syrup was coated between two release liners; the total amount of photoinitiator used was 0.4% by weight of the total monomer weight.

One of the release liners was a clear, silicone-coated polyethylene terephthalate film and the other liner was either silicone-coated paper or a second silicone-coated polyethylene terephthalate film. The caliper was set at 1.01 - 1.14 mm (40-45 mils) for each sample during the coating process.

Following coating, the sample was cured by exposure to a source of ultraviolet radiation (Sylvania Corp.) having over 80% of its emissions between 280 and 400 nm, with a maximum at 350 nm, for a time sufficient to achieve a total energy of 1136 mj/cm2, as measured by a UVIMAPTM UM 365 L-S

radiometer (Electronic Instrumentation & Technology, Inc., Sterling, VA). The sample was then analyzed for unreacted isooctyl acrylate monomer content using a gas chromatograph as follows.

0.5 g of sample was placed in a clean vial to which 4.5 g of a stock solution (acetone containing 0.5 wt.% meta-xylene) was then added. 0.5 microliters of the resultting solution was then injected into a Hewlett-Packard gas chromatograph (model HP5890) equipped with a DB5 column and a flame ionization detector maintained at 3000C. The column had a length of 30 meters, an inner diameter of 0.25 mm, and was equipped with a 0.25 micron film. The sample was injected while maintaining the column at 500C. The column was held at 50"C for 5 minutes following injection, after which the temperature was raised to 3000C in increments of 12.5"C/min. Once the column had reached 300"C, it was held there for 12 minutes to vaporize the sample. The amount of unreacted isooctyl acrylate was then calculated against an internal standard.

Next, the sample was exposed to a 25 kGy dose of gamma radiation (measured using an AAMI-specified dosimeter) from a Cobalt-60 source at a dose rate ranging from 0.01 to 0.1 kGy/sec and then re-evaluated for unreacted isooctyl acrylate monomer content. The results are shown in Table I. All amounts are given in weight percent.

TABLE I EXAMPLE Gamma Dose 0 kGy 25 kGy IOA AA EOA IOA(ppm) IOA(ppm) 1 95 5 0 3035 ND 2 90.2 4.8 5 3526 ND 3 80.8 4.2 15 4890 ND 4 90 10 0 1065 ND IOA = Isooctyl acrylate<BR> AA = Acrylic acid<BR> EOA = Polyethylene glycol acrylate<BR> ND = Not detected

Examples 5-16 Examples 5-16 illustrate the effect of varying the gamma radiation dosage on the amount of unreacted isooctyl acrylate monomer for various IOA/AA/EOA polymers, prepared with and without an isooctyl thioglycolate ("IOTG") chain transfer agent. The polymers were prepared and analyzed following the procedure used in Examples 1-4, with the exception that in the case of Examples 6, 7, 10, 12, 13, 15, and 16, an IOTG chain transfer agent was included in the reaction mixture as well. The total exposure energy in each example was 1048 mj/cm2. The results are shown in Table II. All amounts are given in weight percent.

TABLE II EXAMPLE GAMMA DOSE IOA AA EOA IOTG IOA IOA IOA IOA IOA (ppm) (ppm) (ppm) (ppm) (ppm) O kGy 5 kGy 10 kGy 15 kGy 33 kGy 5 50 10 40 0 6455 1025 176 160 0 6 50 10 40 0.1 3453 471 104 0 38 7 50 10 40 0.2 2215 467 103 21 0 8 45 14 41 0 2614 224 104 0 0 9 45 14 41 0.1 3383 137 117 40 0 10 45 14 41 0.2 2775 188 95 0 0 11 40 18 42 0 1400 91 67 20 0 12 40 18 42 0.1 1421 18 137 19 0 13 40 18 42 0.2 960 0 17 0 0 14 35 20 45 0 992 86 57 0 0 15 35 20 45 0.1 1849 75 83 0 0 16 35 20 45 0.2 1528 61 184 0 0 IOA = Isooctyl acrylate AA = Acrylic acid<BR> EOA = Polyethylene glycol acrylate IOTG = Isooctyl thioglycolate

Examples 17-24 Examples 17-24 illustrate the effect of exposure to gamma radiation on the amount of unreacted isooctyl acrylate monomer for various IOA/AA/EOA microemulsions. The microemulsions were prepared as follows.

Acrylic acid, crosslinking agent, and polyethylene glycol acrylate ("NK Ester--AM-90G" from Shin-Nakamura Chemical Co., Ltd., Japan) were added to isooctyl acrylate. Photoinitiator ("PI;" benzyl methyl ketal) was then dissolved in the solution, followed by the addition of surfactant. The surfactants used were "Brij 76TM," "Brij 98," and "Tergitol 15-S-12" commercially available from ICI.

Following surfactant addition, an aqueous solution containing 4% by weight KCl was added to form a microemulsion. The resulting microemulsion was then cast in the form of a film having a thickness of 0.38 mm onto a silicone-treated paper substrate (polyethylene coated, moisture resistant Kraft paper #56) and covered with a silicone-treated polyester film about 0.05 mm thick. The microemulsion was then photopolymerized in air using a source of ultraviolet radiation (Sylvania Corp.) having over 80% of its emissions between 280 and 400 nm, with a maximum at 350 nm, for a time sufficient to achieve a total energy of 1229 mJ/cm2, as measured by a UVIMAP UM 365 L-S radiometer (Electronic Instrumentation & Technology, Inc., Sterling, VA).

Following polymerization, the sample was analyzed for unreacted isooctyl acrylate monomer content using a gas chromatograph. Next, the sample was exposed to a 30 kGy dose of gamma radiation (measured using an AAMI- specified dosimeter) and then re-evaluated for unreacted isooctyl acrylate monomer content. The results are shown in Table III. All amounts are given in weight percent.

TABLE III EXAMPLE GAMMA DOSE IOA AA EOA 4%KCl Surfactant PI Cross-linker IOA (ppm) IOA (ppm) Solution 0 kGy 30 kGy Brij 76 17 23.45 16.43 38.35 7.97 13.8 0.4 0.0 1165.0 18.8 Brij 98 NNMBA 18 21.69 15.18 35.41 14.96 12.77 0.4 0.2 1140.0 22.8 19 29.92 13.46 31.41 12 13.21 0.4 0.2 2829.6 68.3 20 37.38 11.22 26.19 12 13.21 0.4 0.2 2025.3 194.9 Tergitol 15-S-12 HDDA 21 21.94 15.35 35.81 14 12.91 0.4 0.2 1093.0 17.7 22 29.7 13.56 31.64 12 13.11 0.4 0.2 2188.8 47.3 23 38.23 11.48 26.79 9.98 13.53 0.4 0.2 2234.7 37.0 24 46.64 9.34 21.79 8.5 13.73 0.4 0.2 2756.2 177.7 IOA = Isooctyl acrylate AA = Acrylic acid EOA = Polyethylene glycol acrylate<BR> PI = Photoinitiator NNMBA N,N-Methylenebisacrylamide HDDA = Hexanedioldiacrylate