WO1996011992A1

WO1996011992A1 - Packaged, sterilized, pressure sensitive adhesive products

Info

Publication number: WO1996011992A1
Application number: PCT/US1995/012910
Authority: WO
Inventors: Charles W. Taylor
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1994-10-18
Filing date: 1995-10-05
Publication date: 1996-04-25
Also published as: KR970707252A; AU3828295A; JPH10507473A; EP0787171A1

Abstract

This invention is directed to pressure-sensitive adhesives which are coated on sheet materials and then are packaged and sterilized by gamma radiation. The sterilized materials may be used as medical, hospital and first aid products. Processes for the preparation of the coated materials and products are also described.

Description

Packaged, steri l i zed , pressure sen si tive adhesi ve products .

This invention relates to pressure-sensitive adhesives coated on sheet materials, such as tapes, which are packaged and sterilized by gamma radiation and used as medical, hospital or first-aid products. This invention also relates to processes for the preparation of such coated sheet materials and products.

Background

Pressure-sensitive adhesive products are widely used for medical, hospital, first-aid, athletic protection, and related applications. Examples of such products are tapes, dressings, and surgical drapes. Acrylic polymers dominate the pressure sensitive adhesive medical market because of their low level of allergenicity (Satas, D., ed., "Handbook of Pressure Sensitive Adhesive Technology," Van Nostrand Reinhold, New York, 1989, Chapter 25).

Acrylic adhesives usually consist of a "pure polymer," have molecular weights that make them inherently pressure-sensitive, and do not require compounding to develop the optimal combination of properties. However, the adhesive properties can be changed by compounding with or incorporating tackifying resins. Acrylics have typically been used in solvent systems with very little or no tackifying resin (Satas, supra, pp. 4, 396, 430-431, 553-554, 562, 567). Rubber-based adhesives, however, commonly are compounded with tackifying resins. See, for example, U.S. Pat. No. 5,143,972 (Groves). To increase the cohesiveness of rubber-resin type pressure-sensitive adhesives, U.S. Pat. No. 2,956,904 (Hendricks) describes using high-energy electrons to bombard such adhesives. U.S. Pat. No. 3,328,194 (Kasper) describes increasing the cohesive strength or creep resistance of a rubber-resin pressure-sensitive adhesive of a tape by exposing the adhesive to ultraviolet light. U.S. Pat. No. 4,847,137 (Kellen et al.) describes using ultraviolet radiation to crosslink an acrylate pressure-sensitive adhesive, such as that of a medical tape, comprising a copolymer of acrylate or methacrylate ester of non-tertiary alcohol(s), certain aromatic ketone monomer, and, optionally, comonomers such as acrylic acid, to obtain a pressure-sensitive adhesive having a degree of compliance sufficient to yield high adhesion to skin. U.S. Pat.

No. 3,725,115 (Christenson et al.) describes improving the creep resistance, strength, and solvent resistance of a pressure-sensitive adhesive composition comprising a tacky, low molecular weight polymer, such as an acrylic polymer (and the composition may further comprise various additives such as plasticizers, tackifiers, fillers, and the like) by subjecting the composition to ionizing radiation, such as x-rays, gamma radiation, and high energy electrons. U.S. Pat. No.

5,028,484 (Martin et al.) describes a pressure sensitive adhesive which comprises (a) certain ultraviolet radiation polymerized acrylate polymers, and (b) certain

(poly)tertiary-butyl styrene tackifying resins which are present during the polymerization of the acrylate polymer (a).

U.S. Pat. No. 3,121,021 (Copeland) describes certain surgical adhesive tapes having a porous backing carrying a microporous pressure-sensitive adhesive coating which can be made of pure viscoelastic polymers that are inherently aggressively tacky and highly cohesive, such as the pressure-sensitive acrylate polymers described in U.S. Pat. No. Re. 24,906 (Ulrich). The preferred adhesive of the latter type is a copolymer of isooctyl acrylate and acrylic acid in 94:6 ratio. This preferred adhesive is described as aggressively tacky and pressure-sensitive and thus requires no addition of tackifying resins or plasticizers. However, suitable blends of rubbery polymers and tackifying resins can be used. These adhesive tapes can be sterilized, as by the use of radiation, steam, or ethylene oxide gas.

Other reports of the use of tackifying resins for acrylic pressure sensitive adhesives include published European Patent Applications 196,844 (Jacob et al.) and 383,497 (Yeadon et al.) and U.S. Pat. Nos. 5,095,065 (Yang), 5,013,784 (Yang) and 5,164,441 (Yang). The reported tackifying resins are certain hydrocarbon resins having a certain aromatic content, derived, for example, from vinyl aromatic compounds, such as styrene, and commercially available under the trade designation of, for example, ESCOREZ, such as ESCOREZ ECR-109A and ESCOREZ 5300. European Patent Application 196,844 also describes a blend of an acrylic polymer generally in the form of a latex and a resin tac-άfier that can be applied as an adhesive composition to a substrate and then dried. After drying, the coated substrate is cut into strips and rolled to provide a finished product. The substrate may also be cut into shaped items to produce labels or medicinal tapes.

Gamma rays, gamma radiation or gamma irradiation has considerable penetrating power and bactericidal effect and have been used to sterilize medical products (Block, S.S. "Disinfection, Sterilization, and Preservation," 4th Ed., Lea & Febiger, Philadelphia, London, 1991, Chapter 32). But gamma rays can alter the properties of material. For example, polymers represent a diverse group of materials and, as such, exhibit a variety of physical changes resulting from the chemical changes induced by ionizing radiation. Crosslinking, chain scission, gas evolution, and double bond formation are the basic chemical processes that cause the physical modification of polymers. These processes can result in undesirable changes in a product's properties (Morrissey, R.F. et al., ed., "Sterilization Technology," Van Nostrand Reinhold, New York, 1993, pp. 201, 223, 224).

Summary of the Invention

We have found that gamma radiation sterilization of certain noncompounded acrylic-(or polyacrylate)-based, pressure-sensitive adhesive medical products, such as hospital or first aid tapes, adversely affects or damages their adhesive performance, and we have discovered that the incorporation of compatible synthetic oligomers (or low molecular weight polymers or resins) of ethylenically-unsaturated benzene compounds into the acrylic-based adhesive protects or stabilizes the adhesive performance of such medical products even when they are sterilized with gamma radiation.

Accordingly, this invention provides, in one aspect, a gamma radiation sterilized, packaged, pressure-sensitive adhesive sheet material, such as a tape, dressing, surgical drape, or other medical, hospital or first aid product, comprising a coating of pressure sensitive adhesive on a flexible backing, said adhesive comprising or, preferably consisting or consisting essentially of, a compatible mixture of (a) an alkyl acrylate copolymer, such as a copolymer of isooctyl acrylate, and a polar comonomer, such as acrylic acid, and (b) aromatic hydrocarbon oligomer of one or more mono-ethylenically unsaturated, substituted benzene compounds, such as styrene and indene or mixtures thereof. The relative amounts of components (a) and (b) are sufficient such that the adhesive is tacky and functions as a pressure sensitive adhesive notwithstanding its subjection to gamma radiation sterilization. Component (b) acts as a stabilizer which substantially retains inherent tackiness of component (a) or protects it from degradation, undesired changes or adverse consequences of the gamma radiation utilized to sterilize the coated sheet material after its fabrication. The gamma radiation sterilized, pressure sensitive adhesive products may be prepared by mixing components (a) and (b) in a suitable mixer, such as an extruder, or by mixing compatible organic solvent solutions of components (a) and (b); then coating the resulting mixture of (a) and (b) onto suitable sheet material or backing, such as a thin flexible sheet; if needed, drying the resulting coated sheet material to volatilize the solvent(s); cutting or slitting the coated sheet material to a desired size; packaging the dried sheet material in suitable bacteria-impermeable containers, such as sealed plastic film pouches; and finally gamma radiation sterilizing the packaged, coated sheet material.

Detailed Description

A gamma radiation sterilized, pressure sensitive adhesive product of the present invention, e.g., a medical tape, is tacky, that is, it has a sticky nature and immediately or readily adheres to a surface of an adherend when brought into contact therewith, such as a low surface energy surface like that of the skin of a patient, with light pressure, such as finger pressure, and it has sufficient cohesiveness that it holds in place but can be peeled by hand from such surface without leaving visible residue, that is, it can be cleanly separated from the adherend. The pressure sensitive adhesive product may be fabricated to be compliant with the surface of the adherend and may be made to retain its advantageous inherent transparency, colorlessness, water resistance, and does not yellow when exposured to sunlight, as well as its low or non-allergenic properties. Such product may be made in a form, for example, of a fastening or strapping tape, to maintain the adherend surface, such as a wound, in a sterile condition or used in an environment, such as a patient's or operating room in a hospital, that must be maintained in a sterile condition. Such uses can be accomplished with confidence in the desired adhesiveness of the product being maintained notwithstanding its subjection to gamma radiation which would otherwise adversely affect its adhesive performance.

The polyacrylate, used as a precursor to form component (a) of the adhesive coating of the gamma radiation sterilized, pressure sensitive adhesive products of this invention, may be a copolymer of (1) a predominant or major amount of (meth)acrylic acid ester of non-tertiary alcohol having 1 to 14 carbon atoms, or mixture of such non-tertiary alcohols with the average number of carbon atoms being about 4 to 12 carbon atoms, and (2) a minor amount of polar comonomer which is aliphatic and is mono-ethylenically unsaturated, said monomers (1) and (2) being the only essential monomers in making the copolymer. Examples of the (meth)acrylic acid ester (1) are acrylic acid or methacrylic acid esterified with non-tertiary alkanols, such as substituted or nonsubstituted butanols, pentanols, hexanols, heptanols, octanols, decanols, and dodecanols as well as mixtures, including isomeric mixtures, of such alkanols.

Examples of the polar comonomers (2) are acrylic acid, methacrylic acid, itaconic acid, acrylamide, rnethacrylamide, acrylonitrile, vinyl acetate, N-vinylpyrrolidone, and mixtures thereof. Such polyacrylates include those which are known and those which are widely used. They may be prepared, for example, by solution, emulsion or suspension polymerization of a copolymerizable monomer mixture of 60 to 98 weight percent, preferably 92 to 96 weight percent, of the (meth)acrylic acid ester component (1) and 2 to 40 weight percent, preferably 4 to 8 weight percent, of the polar comonomer component (2). Such polyacrylates as prepared are non-crosslinked, stretchy or elastic, solvent soluble, e.g. in ethyl acetate, heptane, and isopropanol, and inherently tacky or sticky to the touch at room O 96/11992 PC1YUS95/12910

temperature, e.g., 20°C. They may be prepared with high molecular weight, e.g., weight average molecular weights of 100,000 to 1,000,000-5,000,000 (as measured by gel permeation chromatography). Such higher molecular weight is also manifested by inherent viscosity , for example, in the range of about 0.5 to about 2.0, preferably 0.7 to 1.3 (as measured by a Cannon-Fenske 50 viscometer at 25°C on 10 ml of an ethyl acetate solution of 0.2 g polymer per decaliter of solution).

Such polyacrylates can be made to inherently have a good balance of tack, shear, and peel properties. Their preparation, and the preparation of sheet material coated therewith, are described, for example, in U.S. Pat. No. Re. 24,906 (Ulrich) and in U.S. Pat. Nos. 3,121,021 (Copeland), 4,693,776 (Krampe et al.), and 4,833,179

(Young et al.). Such polyacrylates, or acrylic polymers, are also described in the

Satas text, supra, in Chapter 15.

The aromatic hydrocarbon oligomer, component (b) of the adhesive coating used in the practice of this invention, is a synthetic substance which is solvent soluble, for example, in ethyl acetate. The oligomer is also soluble or miscible in the polyacrylate component (a); thus the mixture of the solvent solutions of components (a) and (b) is a single-phase mixture of compatible components with close solubility parameters. Generally, the aromatic hydrocarbon oligomer is preferably a solid, has a number average molecular weight (Mn) of about 500 to 1500, preferably about 750 to 1000 (as determined, for example, by gel permeation chromatography), a polydispersity of less than about 2.5, preferably less than 2.0, and a glass transition temperature above room temperature, e.g., 20°C. These aromatic hydrocarbon oligomers, used as stabilizers in this invention, are inclusive of those oligomers prepared by polymerizing aromatic petroleum streams from naphtha cracking and used as tackifiers for rubbery or elastomeric pressure sensitive adhesives, as described, for example, in the Satas text, supra, Chapter 20. Some of them, such as poly(tertiaιy-butyl styrene), are described as tackifying resins used in certain admixtures with alkyl acrylates that are photopolymerized. A general class of the aromatic hydrocarbon stabilizers used in this invention are those oligomers prepared by polymerizing substituted benzene or benzenoid compounds which can be represented by the formulas I and II:

where R is a lower alkyl, having, for example, 1 to 4 carbon atoms, such as methyl, ethyl, propyl, and isobutyl. Representative ethylenically unsaturated substituted benzenes within the scope of above formula I are styrene, methylstyrene, and tertiary-butylstyrene and those within the scope of formula II (which are also considered herein as substituted benzenes) are indene and methylindene.

A preferred class of aromatic tackifiers includes a a water white, hydrogenated, aromatic hydrocarbon tackifying resin having an aromaticity greater than 50 percent, and preferably greater than 70 percent, and more preferably greater than 80 percent, based on the amount of monomer in the feedstock. These useful tackifying resins have a Z-average molecular weight (Mz) ranging from about 500 to 1800, preferably between about 600 and 1500, and most preferably from about 650 to 1200. In a highly preferred embodiment of the invention, the tackifying resin has a Mz between about 700 and 1100.

These useful tackifying resins also have a narrow molecular weight distribution with a polydispersity (Mw/Mn) of less than 2.5, preferably less than about 2.1, and most preferably less than 1.9. The molecular weight characterizations are determined by gel permeation chromatography using a polystyrene calibration basis and then converting to a polyisobutylene calibration basis according to the following equation: (1) log (Mw_{poly obulylene}) = 1.1 x log (Mw _polyMyrene) - 0.517

The procedure is described in detail in WO-A-91-07472 (Luvinh).

The softening points (Ring and Ball softening points as determined by ASTM E-28) of these tackifying resins typically fall within the range of about 40-120°C, and preferably from about 65-100°C.

The resins, after hydrogenation, are water white. A "water white" resin is defined to be a resin having a molten Gardner color of less than about 2. An especially preferred resin has a molten Gardner color of less than 1. These resins also preferably exhibit an initial color after hydrogenation of greater than about 24 saybolt (10 percent toluene solution).

These resins are hydrogenated petroleum hydrocarbon resins typically produced by catalytic polymerization of principally aromatic monomers which after polymerization and hydrogenation, retain at least 20 percent aromatic protons as determined by nuclear magnetic resonance (NMR), and preferably at least 24 percent aromatic protons. The aromaticity is measured by proton NMR (Η-NMR) analysis by generally accepted procedures. The "retained aromaticity" is the ratio of the aromaticity by proton NMR of the hydrogenated resin to that of the polymerized resin prior to hydrogenation. The procedure is described in detail in WO-A-91-07472 (Luvinh).

The tackifying resins may be prepared by the following procedure:

(a) Polymerizing under Friedel-Crafts polymerization conditions steam cracked distillates, or fractions thereof, having boiling points between about 135-220°C, and containing at least 40 percent by weight vinyl aromatic monomer contents, in the presence of 0 to 40 percent by weight of the polymerization mixture of a chain transfer agent; and

(b) catalytically hydrogenating the results of (a) such that at least 75 percent of the aromaticity is retained.

The tackifying resins are preferably prepared by catalytic polymerization of petroleum fractions identified as Heartcut Distillates, or HCD, and 0 to 40 percent by weight of a chain transfer agent. The resins are typically prepared from vinyl aromatic streams comprising the following components:

Components Typical Range Typical Heartcut Distillate

Styrene 1-15 8

Alkyl derivatives of styrene 15-40 25

Indene 5-20 10

Alkyl derivatives of indene 3-15 10

Non-reactive components 15-76 47

Such streams can be derived from the steam-cracked petroleum distillates or fractions thereof, having boiling points between about 135-220°C, so long as they contain or are modified to contain sufficient vinyl aromatic contents. For example, an essentially pure styrene component may be added to commercially available petroleum distillate products that fit this description except for styrene. In this manner, a vinyl aromatic stream comprising, by weight percent, 11.4 percent styrene, 31.6 percent alkyl derivatives of styrene, 17.1 percent indene, 5 percent alkyl derivatives of indene, and 34.9 percent non-reactive components, was confirmed to be a suitable resin feedstock.

Polymerization of the hydrocarbon resin is generally accomplished according to the teachings of U.S. Pat. No. 4,078,132 (Lepert). According to this teaching, branched chain aliphatic olefins are introduced during polymerization as chain transfer agents to achieve both lowered softening point and narrowed molecular weight distribution. Although this document addresses the preparation of substantially non-aromatic unsaturated thermoplastic resins, the teaching therein is applicable to feed streams comprising heartcut distillates or feed streams comprising vinyl aromatic monomer(s) to yield a highly aromatic precursor resin, which when hydrogenated provides the desired aromatic tackifier resin. The feed streams should contain at least 40 percent, by weight of total polymerizable monomers, of vinyl aromatic monomers, and preferably at least 50 percent. The polymerization process of U.S. Pat. No. 4,078,132 is particularly suitable when practiced at polymerization temperatures between -20-100°C, and preferably between 30-80°C, in the presence of a Friedel-Crafts catalyst such as aluminum chloride (A1C1₃) and in the presence of the branched chain reactive olefin chain transfer agents, which are preferably isoamylenes, dimates, or mixtures thereof. Most of the branched chain reactive olefin compounds will be effective when used in the proper amounts as described in U.S. Pat. No. 4,078,132. Some compounds, such as isoamylenes, are more reactive and can be advantageously used in smaller amounts to control the softening point and molecular weight distribution. Useful amounts of the reactive chain transfer agents are typically in the range of 10 to 20 percent by weight based on the total weight of the heartcut distillate or vinyl aromatic feed stream, and preferably in the range of 10 to 15 percent. The process conditions described in U.S. Pat. No. 4,514,554 (Hughes et al.), also include a description of the polymerization of petroleum fraction feedstocks including isoamylenes.

Hydrogenation can generally be accomplished according to the teachings in U.S. Pat. No. 4,629,766 (Malatesta et al.), but other conventional means of hydrogenation may also be used. Typically, temperatures of 200-300 °C are used at pressures of 10 to 300 kg/cm², and hydrogenating or hydrotreating catalysts such as Group VIII metals such as nickel, palladium, cobalt, ruthenium, platinum, and rhodium, Group VI metals such as tungsten, chromium and molybdenum, and Group VII metals such as manganese and copper, are used. These metals may be used alone or in a combination of two or more metals, in the metallic form, or in an activated form, and may be used directly or on a solid support such as alumina or silica-alumina. A preferred catalyst is one comprising sulfided nickel-tungsten on a gamma-alumina support having a fresh catalyst surface area ranging from 120 to 300 meters²/gram, and containing 2 to 10 percent by weight nickel and 10 to 25 percent by weight tungsten as described in U.S. Pat. No. 4,629,766. The hydrogenation is typically carried out with a hydrogen pressure of 20 to 300 atmospheres (2.03 x 10⁵ to 3.09 x 10⁷ Newtons per square meter), and preferably 150 to 250 atmospheres (1.52 x 10⁷ to 2.53 x 10⁷ Newtons per square meter). Additional description of hydrogenation of aromatic resins is described in U.S. Pat. No. 3,926,878 and WO- A-91/07472.

Hydrogenation is performed at temperatures, pressures and times with the effective catalysts so as to retain at least 75 percent aromaticity, preferably at least 80 percent, more preferably at least 85 percent, and most preferably, at least 90 percent. Optimization of the hydrogenation process can be empirically accomplished according to the above described process.

A commercially available aromatic hydrocarbon oligomer which is useful as component (b) in the adhesive coatings of this invention is ESCOREZ Tackifier 7312, described in product bulletin 102-0492-034, April 1992, of Exxon Chemical Co.. Some other commercially available aromatic hydrocarbon resins are described by Satas, supra, pp. 531-532. Such aromatic hydrocarbon resins can be used in the form of solvent solutions, e.g., 50 weight percent ethyl acetate solutions, in preparing the adhesive solution which is to be coated on a backing in the fabrication of the pressure-sensitive adhesive products of this invention.

The amount of aromatic hydrocarbon oligomer component that is mixed with the polyacrylate component is that amount which is sufficient to protect or stabilize the polyacrylate component, that is, to essentially or substantially maintain or retain its desired adhesive performance after the adhesive-coated sheet material is packaged and subjected to the gamma radiation sterilization. Generally, the amount, on a dry or solids basis, of the adhesion stabilizer will be 1 to 50 weight percent, preferably 10 to 35 weight percent, of the sum of the weights of the two essential components (a) and (b) in the adhesive coating, and the amount of the polyacrylate component will be 50 to 99 weight percent, preferably 65 to 90 weight percent, of the adhesive coating on the same basis.

Other materials may be blended or mixed with the mixture of polyacrylate and stabilizer components, such as coloring agents, fire retardants, reinforcing agents, or foaming agents. An antioxidant may be included in the adhesive coating solution, e.g., as a 0.5 weight percent solution of IRGANOX-1010 (which does not by itself sufficiently protect the polyacrylate from deleterious effects of gamma radiation-sterilization). But such other materials may affect the performance of the gamma radiation-sterilized adhesive products. Thus, it may be necessary to conduct material compatibility tests. The pressure sensitive adhesive used in this invention may be prepared by mixing compatible solutions of the polyacrylate adhesive component and aromatic hydrocarbon adhesion-stabilizer component in the amounts sufficient to obtain or retain the desired pressure sensitive adhesive properties notwithstanding the exposure of the adhesive to the gamma radiation sterilization. Typically, the two components are admixed as solutions in common volatile, organic solvents, such as alkanes, alcohols, or esters, e.g., hexane, isopropanol, and ethyl acetate. The mixed solution of the two compatible components has, for example, a coatable viscosity, e.g., 200 to 20,000 cps, may then be coated in a conventional manner on various flexible web substrates, or backings, e.g., plastic films, paper, and woven or nonwoven fabrics, in an appropriate thickness and coating weight, e.g., thicknesses of about 0.2 to 5 mils and coating weights of about 1 to 40 grains/24 in².

Preferred backing for medical applications are those which permit high moisture vapor transmission and or tissue or wound exudate therethrough, e.g. nonwoven fabric, woven fabrics, perforated films, films having high moisture vapor transmission and knits. The coatings may be applied using, for example, a reverse roll coater, knife-over-roll coater, roll-over-roll coater, air-knife coater, bar coater, direct roll coater, gravure coater, or pressure-fed die coater. The wet adhesive coating may then be dried, if needed, in a convection dryer or zoned oven, to evaporate the solvent(s), leaving non-volatile solids on the web, the evolved solvent vapor being incinerated as required by environmental regulations or adsorbed on activated charcoal and recovered. The dried coated sheet material can be cut or slit to appropriate widths, e.g. 1 inch (2.5 cm), using a shear, score, or razor blade slitter or cutter. The slit material may be rolled or otherwise assembled and packaged. The backside of the backing may be coated in a conventional manner with a low adhesion backsize, such as a urethane or silicone backsize, or laminated to a release sheet or liner to protect the product from unintended adhesion. Such backsizes and release liners are described, for example, by Satas, supra, Chapters 23 and 24. The coated sheet materials may take the form of any article conventionally used with skin adhesives, such as tapes, patches, strips, wound dressings, monitoring or neuro-stimulati-ng electrodes, and drapes. Such articles are described by Satas, supra, Chapter 25. The articles can be packaged in bacteria-impermeable films or materials, such as polyethylene, polypropylene, and the like, in the form of sealed pouches, envelopes, bubble packs, or other conventional packages, which are readily opened when the adhesive product is to be applied. The packaged adhesive-coated products are then exposed to the gamma radiation sterilization, e.g., in a dosage range of usually 20 to 45 kGy. Equipment and processing conditions used to carry out the coating, drying, and slitting steps are described, for example, in Satas, supra, Chapters 34-37. Equipment which can be used to carry out the gamma radiation-sterilization is, for example, a ^wCo source irradiator, such as a Model JS 7500 Tote Shuttle Dwell Panoramic Wet Storage Irradiator manufactured by AECL (Atomic Energy of Canada, Ltd.).

Objects and advantages of this invention are illustrated in the following example, which should not be construed to unduly limit this invention.

Example_, 1

Four solutions, labeled 1, 2, 3 and 4, of different solvent-based acrylic pressure-sensitive adhesives were prepared and are described in Table 1. An aromatic tackifier having the physical characteristics listed in Table 2 and a commercially available aromatic hydrocarbon oligomer, EXCOREZ Tackifier 7312, was added to each of the solutions to prepare adhesive coating solutions containing 25 g of acrylic polymer and 6.25 g aromatic hydrocarbon oligomer. Each such coating solution was coated on 2 mils (0.0508 mm) thick polyester (polyethylene terephthalate) film using a 9-inch (22.9 cm) knife coater using a 10 mils (0.254 mm) --

thick knife-setting. The resulting coated tapes were then dried for 7 minutes at 230°F (110°C) to remove the solvents. After drying, adhesive-to-adhesive tape laminates were constructed by passing equal lengths of the adhesive-coated tapes through a laminator press at 20 psi and about 25°C to provide uniform, bubble-free, pressure-sensitive adhesive tape laminates. Half of the tape laminates were tested without being irradiating and half of the tape laminates were tested after being sterilized with a gamma radiation dosage of 34.6 to 34.9 kGy, using the above-described AECL Model JS 7500 Irradiator.

To measure the creep compliance of each of the adhesive tape laminates, two test pieces of equal area were die-cut and placed in a parallel-plate creep compliance rheometer, one piece being on each side of the center plate, with an outer plate contacting the exposed surface of each. The parallel plates were placed in horizontal arrangement, approximately 20 psi pressure was applied vertically to hold the assembly in place and one end of the center plate was connected to a chart recorder. A hook was attached to the opposite end of the center plate with a flexible wire extending horizontally from the hook and then downward over a pulley, the outer plates being held in a fixed position. A suitable weight (one sufficient to measurably deform the sample a distance no greater than its thickness) was attached to the free end of the wire; then the strip chart recorder was started. The weight typically used to exert the stress on the adhesive coatings was 500 grams. From the strip chart recorder, the time, three minutes in this example, and the displacement (strain) were read and the applied force (stress) was recorded. The shear creep compliance at a given temperature was then calculated using the equation:

7_W = 2άX

¥

where t is the time at which the measurement is taken, A is the area of one face of the adhesive samples, h is the thickness of the adhesive mass, X is the displacement at time t and f is the force due to the mass attached to the wire connected to the middle plate. Where A is expressed in cm², h in cm, X in cm, and f in dynes, the compliance value J_(t) is given in cm²/dyne.

Shear creep compliance properties of all of the samples of tapes were evaluated as shown in Table 3. The data of Table 3 show the stabilizing effect of the aromatic hydrocarbon oligomer, in comparison to the deleterious effect of the gamma radiation on the acrylic adhesives which did not contain an aromatic tackifier. The observed stabilizing effect is shown by the relatively smaller change in creep compliance after the sterilization.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention.

TABLE 2

Hydrocarbon Resin Properties

Softening Point 98°C

Gel Permeation Chromatography Molecular Weight

Mw (Weight Average) 520 Mn (Number Average) 330

Mw/Mn (Polydispersity) 1.6

Mz (Z-average) 900

Aromaticity ^(I), % 27

Glass Transition Temperature (DSC midpoint) 51°C

Saybolt Color (10% solution in toluene) 28

Aged Gardener Color (50% solution in solvent), 5 5 hrs at 175°C

Molten Gardner Color < 1

Wax Cloud Point ⁾ 70°C

Volatility, weight percent (5 hrs at 175°C) 1.2

(1) Percent aromatic protons (about 95 percent on an aromatic monomer basis)

(2) 20 Escorene EVA UL7750, 40 resin, 40 paraffin wax (65°C MP)

TABLE 3

00

Table 1 for composition and properties.

Claims

1. Packaged, gamma radiation sterilized, pressure sensitive sheet material comprising a coating of pressure sensitive adhesive on a flexible backing, said adhesive comprising a compatible mixture of

(a) copolymer of alkyl acrylate and polar comonomer, and

(b) aromatic hydrocarbon oligomer of one or more ethylenically unsaturated benzene compounds.

2. The article of claim 1 wherein said copolymer is a copolymer of (meth)acrylic acid ester of non-tertiary alcohol having 1 to 14 carbon atoms and a polar comonomer selected from the group consisting of acrylic acid, methacrylic acid, itaconic acid, acrylamide, methacrylamide, acrylonitrile, vinyl acetate, and N-vinylpyrrolidone and wherein said ethylenically unsaturated benzene compounds are represented by the formulas:

where R is a lower alkyl.

3. The article of claim 1, wherein said copolymer is substantially a copolymer of isooctyl acrylate and acrylic acid and said aromatic hydrocarbon oligomer is a copolymer of styrene, methylstyrene, indene, and methylindene.

4. The article of claim 1 , wherein said aromatic hydrocarbon oligomer has a Mn of 500 to 1500.

5. The article of claim 1, wherein said aromatic hydrocarbon oligomer is a water white, hydrogenated resin produced from a hydrocarbon feedstock having

(i) a retained aromaticity on a weight percent of monomers in the feedstock basis of greater than 50 weight percent; (ii) a softening point of 65-120°C; (iii) a Mz of less than 1800; and (iv) substantially no molecular weight fractions above 7000.

6. The article of claim 5, wherein said aromatic hydrocarbon oligomer has a retained aromaticity on a weight percent of monomers in the feedstock basis of 70 to 95 weight percent.

7. The article of claim 5, wherein said aromatic hydrocarbon oligomer has a softening point of 65-100°C.

8. The article of claim 5, wherein said aromatic hydrocarbon oligomer has a Mz of less than 1500.

9. The article of claim 5, wherein said aromatic hydrocarbon oligomer has a polydispersity of less than 2.5.

10. A method of making the article of claim 1 which comprises coating a mixture of the copolymer and the aromatic hydrocarbon oligomer onto a sheet material; packaging the dried coated sheet material in bacteria-impermeable container, and irradiating the packaged coated sheet material with gamma rays to sterilize the same.