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1. WO2002018508 - METHODS FOR PROTECTING A SURFACE

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

METHODS FOR PROTECTING A SURFACE

Background
Masking tapes that are exposed to elevated temperatures (i.e., temperatures above room temperature, and typically above 100°C) are used in various applications. For example, masking tapes known as lead frame tapes are used in the electronics industry to protect against epoxy potting compound flash, particularly in the bonding of composite articles. As another example, masking tapes known as flashbreaker tapes are used in the aerospace industry, both for metal-to-metal bonding and composite bonding to help control the spread of bonding adhesives to other surfaces.
A typical metal-to-metal bonding process involves preparing metal (e.g., aluminum) panels by etching and coating with a primer (e.g., epoxy primer coating) to provide corrosion protection to the metal. To increase the strength or thickness of a part, two or more of the primed panels are bonded together, typically using dry film bonding adhesives (e.g., epoxy adhesives). Such dry film adhesives are cut to shape to fit the geometry of the two surfaces being brought together and are positioned between the parts to be bonded. These adhesives are activated by heating, e.g., up to about 350°F (177°C), typically in an autoclave that allows for pressures of about 35 psi (24 KPa) to about 70 psi (48 KPa) to be applied to the part. Prior to curing, during this heating process the dry film adhesive becomes flowable. If the bonding adhesive flows onto areas surrounding the bonding region, the adhesive must typically be sanded or ground off, which can result in damage to the primer. This is a labor intensive operation, and adds significantly to cost. Thus, flashbreaker tapes are used to protect areas surrounding the bonding region of the surface from the flowing bonding adhesive (i.e., flash). Such areas are protected to allow for them to be painted or bonded in subsequent operations. When flashbreaker tapes are removed, they should remove substantially cleanly such that little or no adhesive residue is left behind due to cohesive failure or adhesive transfer.
Currently available flashbreaker tapes include a silicone adhesive. Silicone adhesives remove easily without visual residue; however, nonvisible residue can remain. Such nonvisible residue can interfere with subsequent bonding and/or painting unless laborious cleaning operations are performed. New rubber-based flashbreaker tapes have been developed that do not include silicone; however, these are not very widespread and are not very easy to use. Thus, there is still a need for other flashbreaker tapes and other adhesive articles that can be used to protect surfaces at elevated temperatures.

Summary
This invention relates generally to methods for protecting surfaces. Significantly, the adhesive articles described herein can be used at elevated temperatures after which they can be removed from the surface while leaving substantially no adhesive residue.
In one embodiment, the present invention provides a method of protecting a surface that includes: providing an adhesive article that includes a backing, an acrylate-based pressure sensitive adhesive outer layer, and at least one acrylate-based pressure sensitive adhesive inner layer disposed between the outer layer and the backing, wherein the acrylate-based pressure sensitive adhesive outer layer includes from 0 to about 6 wt-% of a copolymerized monofiinctional ethylenically unsaturated monomer whose homopolymer has a Tg of at least about 10°C; applying the adhesive article to the surface; exposing the surface to a temperature of at least about 100°C; and removing the adhesive article from the surface leaving substantially no adhesive residue on the surface. Herein, the articles "a" or "an" or "the" means "at least one" or "one or more."
Preferably, the acrylate-based pressure sensitive adhesive inner layer includes at least one copolymerized monofiinctional (meth)acrylic acid ester whose homopolymer has a Tg of less than about 0°C and at least one copolymerized monofiinctional ethylenically unsaturated monomer whose homopolymer has a Tg of at least about 10°C. In certain preferred embodiments, the acrylate-based pressure sensitive adhesive inner layer includes a crosslinked acrylate-based adhesive.
Preferably, the acrylate-based pressure sensitive adhesive outer layer includes at least one polymerized monofiinctional (meth)acrylic acid ester whose homopolymer has a Tg of less than about 0°C. Preferably, the acrylate-based pressure sensitive adhesive outer layer includes a crosslinked acrylate-based adhesive.
In another embodiment of the present invention, there is provided a method of protecting a surface that includes: providing an adhesive article that includes a backing, an acrylate-based pressure sensitive adhesive outer layer, and at least one acrylate-based pressure sensitive adhesive inner layer disposed between the outer layer and the backing, wherein the acrylate-based pressure sensitive adhesive outer layer includes from 0 to about 6 wt-% of a copolymerized monofiinctional ethylenically unsaturated monomer whose homopolymer has a Tg of at least about 10°C; applying the adhesive article to the surface; exposing the surface to a temperature of at least about 100°C for at least about 4 hours; and removing the adhesive article from the surface leaving substantially no adhesive residue on the surface.
In yet another embodiment, there is provided a method of protecting a surface that includes: providing an adhesive article that includes a backing, an acrylate-based pressure sensitive adhesive outer layer, and at least one acrylate-based adhesive inner layer disposed between the outer layer and the backing, wherein the acrylate-based pressure sensitive adhesive outer layer includes at least one polymerized monofiinctional (meth)acrylic acid ester whose homopolymer has a Tg of less than about 0°C and from 0 to about 6 wt-% of a co-polymerized monofiinctional ethylenically unsaturated monomer whose homopolymer has a Tg of at least about 10°C, and the acrylate-based pressure sensitive adhesive inner layer includes at least one copolymerized monofiinctional (meth)acrylic acid ester whose homopolymer has a Tg of less than about 0°C and at least one copolymerized
monofiinctional ethylenically unsaturated monomer whose homopolymer has a Tg of at least about 10°C; applying the adhesive article to the surface; exposing the surface to a temperature of at least about 100°C; and removing the adhesive article from the surface leaving substantially no adhesive residue on the surface.

Brief Description of the Drawings
FIG. 1 is a schematic representation of the cross-section of an adhesive article used in the methods of the present invention.

Detailed Description of Preferred Embodiments
This invention relates broadly to methods for protecting surfaces that are subjected to high temperatures using adhesive articles that can be removed from the surface while leaving substantially no adhesive residue. Such methods include applying an adhesive article, such as a tape, protective film, etc., to a surface, exposing the surface to a temperature of at least about 100°C (preferably, at least about 125°C, more preferably, at least about 150°C, and most preferably, at least about 175°C), and removing the adhesive article from the surface with substantially no adhesive residue remaining.
The adhesive articles include a backing and a pressure sensitive adhesive disposed on the backing. These articles are generally cleanly removable after exposure to high temperature and optionally high pressure such that the articles can be removed from a surface by hand with little or no residue (i.e., substantially no residue) remaining on the surface. As used herein, "substantially no residue" means that there is no or a relatively small amount of visible, invisible, or tactile adhesive residue remaining, such that there is no interference with subsequent processing steps that may occur. Preferably, there is no visible, invisible, or tactile adhesive residue remaining. Herein, "adhesive residue" means adhesive left on the substrate due to either adhesive or cohesive failure upon removal of the adhesive article from the substrate surface.
The surface can be any of a wide variety of surfaces, including materials such as polymers, metals, glasses, combinations thereof, etc. Typically, the surface can be any of a wide variety of surfaces that are used in the aerospace industry or electronics industry. Preferably, the surface is an epoxy coating disposed on an aluminum substrate, as used in the aerospace industry. The surface can also include a metal such as aluminum, copper, gold, silver, or other metals, or a polymeric material, which are typically used in the electronics industry. Various combinations of these materials may also form the surface of the substrate. Thus, the adhesive articles of the present invention can be used as flashbreaker tapes and as lead frame tapes, for example.
The surface on which the adhesive article is disposed is exposed to a target temperature of at least about 100°C, preferably, at least about 125°C, more preferably, at least about 150°C, and most preferably, at least about 175°C, after the adhesive article is applied thereto. Also, the surface on which the adhesive article is disposed is preferably exposed to a pressure of at least about 15 psi (103 KPa), more preferably, at least about 30 psi (206 KPa), and most preferably, at least about 35 psi (240 KPa), and even as high as about 90 psi (620 KPa), most often in a nitrogen atmosphere, after the adhesive article is applied thereto. In particularly preferred embodiments, the method includes exposing the surface to a reduced pressure (i.e., a pressure less than atmospheric pressure) after the adhesive article is applied thereto to get the parts to be bonded in intimate contact, with no void areas between them. Subsequently, the surface is exposed to an elevated temperature (i.e., a temperature above room temperature) and optionally elevated pressure (i.e., a pressure above atmospheric pressure) for a period of time. The surface is typically exposed to a temperature above room temperature for at least about 4 hours, and often at least about 6 hours, which typically includes both the time in ramping up to the target temperature (i.e., at least about 100°C) and the time in cooling down to room temperature. For certain preferred embodiments, the surface is exposed to the target temperature for at least about 4 hours, and often at least about 6 hours.
Typically, after heating and allowing the article to cool to room temperature, the adhesive article is removed. The adhesive articles are selected such that there is substantially no adhesive residue remaining (i.e., little or no visible, invisible, or tactile adhesive residue in an amount that would interfere with subsequent processing steps if they are carried out). This can be evaluated using the Removability Test Method described below. An adhesive article can be evaluated for its suitability for use in the methods of the present invention by visually inspecting for adhesive residue, and for particularly preferred performance, for ghosting and/or edge beading. Herein, "ghosting" means a whitening or brightening of color on the substrate surface; and "edge beading" means a thin line of adhesive residue, of variable length, along the original edge of the lengthwise border of the adhesive article. Although it is particularly desirable that adhesive articles exhibit none of these characteristics, edge beading and/or slight ghosting is acceptable. Edge beading is typically only the result of the backing shrinking; thus, the adhesive may be acceptable if used on a different backing material, such as, for example, heat stabilized backings such as KAPTON, TEFLON, and stabilized polyester. Such backings preferably exhibit a shrinkage at the temperature of interest of about 1.0% or less, and more preferably, 0.5% or less. In the examples presented below where polyester was used as the backing it was not heat stabilized, and possessed a heat shrinkage of about 1.8% to 2.5% according to product literature. Slight ghosting (i.e., resulting in only a very small color change) is acceptable because it is cosmetic. Thus, preferably, upon removal after heating, the adhesive article leaves substantially no adhesive residue, more preferably, no visible, invisible, or tactile residue, even more preferably, it produces no ghosting, and most preferably, no edge beading at either 90° or 180° peel.
Multiple layer pressure sensitive adhesive constructions are desirable because various properties can be designed into each layer producing product performance not easily obtained from single layer adhesive constructions. For example, an inner layer can be designed to be relatively stiff, and/or to anchor the outer layer to a backing, while maintaining sufficient pressure sensitive adhesive properties of the overall article through the formulation of a more pressure sensitive outer adhesive layer.
Preferred adhesive articles are shown in Figure 1 in cross-section. These include at least two layers - an exposed outer layer 10 and at least one inner layer 12 disposed between the exposed outer layer 10 and a backing 14. The exposed outer layer 10 includes a pressure sensitive adhesive. In particularly preferred embodiments, at least one of the inner layers 12 includes a pressure sensitive adhesive. More preferably, the adhesive article includes a dual layer pressure sensitive adhesive construction.
The backing 14 can include a wide variety of backing materials that can withstand the high temperatures and optionally the high pressures described herein without significant deterioration. Such backing materials preferably do not allow the material (e.g., epoxy adhesive) from which the surface is being protected to bond thereto: They can include paper, polymeric materials, cloth, metallic foils, etc. Examples of such materials include tear resistant backings, such as those disclosed in U.S. Pat. No. 6,048,431 (Clements et al.), and thermally stable backings (e.g., those that exhibit little or no shrinkage upon exposure to elevated temperatures), such as polyimides, polyamides,
poly(tetrafluoroethylenes), and heat stabilized polyesters, etc. Examples of such materials include those sold under the trade designations KAPTON, TEFLON, and NYLON.
Particularly preferred polymeric backing materials include heat stabilized poly(ethylene terephthalate) and poly(tetrafluoroethylene).
The backing can be modified before contact with the adhesive composition to promote adhesion between the adhesive composition and the backing, such as by applying a primer composition to the backing or by exposing the backing to a corona or flame treatment, as is well known in the art. An advantage of the present invention is that the backing material does not require a primer, except for the typical sodium etching of poly(tetrafluoroethylene). The backing can also include a low adhesion backsize material, as is well known in the art.
The adhesive article (e.g., tape) can be constructed to include a tab along a longitudinal edge of the backing. The presence of a tab makes it easier to lift an edge of the article during the removal process. The tab can be created in a variety of ways including, for example, by not coating adhesive composition on the finger lift portion, deadening the adhesive composition in the area of the finger lift portion, using a thicker film to act as a tab along the edge, folding a portion of the article on itself to cover the adhesive composition, or adhering additional backing to a portion of the exposed adhesive composition.

Acrylate-based Pressure Sensitive Adhesive Outer Layer
A pressure sensitive adhesive layer in accordance with the present invention includes a pressure sensitive adhesive material. One well known means of identifying pressure sensitive adhesives is the Dahlquist criterion. This criterion defines a pressure sensitive adhesive as an adhesive having a 1 second creep compliance of greater than 1 x

10"6 cm2/dyne as described in Handbook of Pressure Sensitive Adhesive Technology.
Donatas Satas (Ed.), 2nd Edition, p. 172, VanNostrand Reinhold, New York, NY, 1989.

Alternatively, since modulus is, to a first approximation, the inverse of creep compliance, pressure sensitive adhesives may be defined as adhesives having a Young's modulus of less than 1 x 106 dynes/cm2.
Another well known means of identifying a pressure sensitive adhesive is that it is aggressively and permanently tacky at room temperature and firmly adheres to a variety of dissimilar surfaces upon mere contact without the need of more than finger or hand pressure, and which may be removed from smooth surfaces without leaving a residue as described in Glossary of Terms Used in the Pressure Sensitive Tape Industry provided by the Pressure Sensitive Tape Council, August, 1985.
Another suitable definition of a suitable pressure sensitive adhesive is that it preferably has a room temperature storage modulus within the area defined by the following points as plotted on a graph of modulus versus frequency at 25°C: a range of moduli from approximately 2 x 10s to 4 x 105 dynes/cm2 at a frequency of approximately

0.1 radian/second (0.017 Hz), and a range of moduli from approximately 2 x 106 to 8 x 106 dynes/cm2 at a frequency of approximately 100 radians/second (17 Hz) (for example, see Figure 8-16 on p. 173 of Handbook of Pressure Sensitive Adhesive Technology (Donatas Satas, Ed.), 2nd Edition, Van Nostrand Rheinhold, New York, 1989). Any of these methods of identifying a pressure sensitive adhesive may be used to identify suitable pressure sensitive adhesives for use in the methods of the present invention.
Useful acrylate-based pressure sensitive adhesive materials for the exposed outer pressure sensitive adhesive layer include those that include at least one polymerized monofiinctional (meth)acrylic acid ester of a nontertiary alcohol (preferably in which the alkyl group contains about 4 to 14 carbon atoms (on average)) whose homopolymer has a Tg (glass transition temperature) of no greater than about 0°C (Monomer A) and, optionally, one or more polymerized monofiinctional ethylenically unsaturated (reinforcing) monomer whose homopolymer has a Tg of at least about 10°C (Monomer B).
The term "(meth)acrylic" as used herein refers to acrylic and methacrylic. The term "monofunctional" in the context of a "monofiinctional (meth)acrylic acid ester" refers to a mono-(meth)acrylic monomer or a monomer containing one (meth)acrylic functionality, although other functionality can be present. The term "monofunctional" in the context of a "monofunctional ethylenically unsaturated reinforcing monomer" refers to a
monoethylenically unsaturated monomer or a monomer containing one ethylenically unsaturated functionality, although other functionality can be present. As used herein, "reinforcing monomer" means monomers that increase the modulus of the adhesive and thereby its strength. The alkyl group of the nontertiary alcohol can optionally contain oxygen atoms in the chain, thereby forming ethers for example.
Examples of suitable monofunctional (meth)acrylic acid esters useful as Monomer A include, but are not limited to, 2-methylbutyl acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, lauryl acrylate, n-decyl acrylate, 4-methyl-2-pentyl acrylate, isoamyl acrylate, sec-butyl acrylate, and isononyl acrylate. Preferred (meth)acrylic acid esters that can be used include, but are not limited to, 2-ethylhexyl acrylate, isooctyl acrylate, lauryl acrylate, and 2-methylbutyl acrylate. Various combinations of Monomer A can be used.
Monofunctional reinforcing monomers useful as Monomer B include, but are not limited to, (meth)acrylic acid, a (meth)acrylamide, a (meth)acrylate, an alpha-olefin, a vinyl ether, an allyl ether, a styrenic monomer, or a maleate. Examples of suitable monofunctional reinforcing monomers include, but are not limited to, acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, 2-hydroxyethyl acrylate or methacrylate, cyclohexyl acrylate, t-butyl acrylate, phenyl acrylate, isobornyl acrylate, 2-phenoxyethyl acrylate, N-vinyl pyrrolidone, N- vinyl caprolactam, acrylamide,
methacrylamide, N-substituted and N,N-disubstituted acrylamides such as N-ethyl acrylamide, N-hydroxyethyl acrylamide, N-octyl acrylamide, N-t-butyl acrylamide, N,N-dimethyl acrylamide, N,N-diethyl acrylamide, and N-ethyl-N-dihydroxyethyl acrylamide. Preferred monofunctional reinforcing monomers include, but are not limited to, acrylic acid, t-butyl acrylate, N,N-dimethyl acrylamide, N-octyl acrylamide, isobornyl acrylate, and 2-phenoxyethyl acrylate. Various combinations of reinforcing monomers can also be employed.
The (meth)acrylic monomer is used in an amount of from about 94 weight percent (wt-%) to 100 wt-%, based on a total weight of polymer (excluding any crosslinker or other additives). The reinforcing monomer is used in an amount of 0 wt-% to about 6 wt-%, based on a total weight of polymer (excluding any crosslinker or other additives).

Acrylate-based Adhesive Inner Layer
Preferred acrylate-based pressure sensitive adhesive materials for the inner adhesive layer include those described for the exposed outer adhesive layer except at least one copolymerized monofunctional ethylenically unsaturated (reinforcing) monomer whose homopolymer has a Tg of at least about 10°C (Monomer B) is included in the acrylate copolymer. The (meth)acrylic monomer is preferably used in an amount of about 80 wt-% to about 96 wt-%, based on a total weight of copolymer (excluding any crosslinker or other additives). The reinforcing monomer is preferably used in an amount of about 4 wt-% to about 20 wt-%, based on a total weight of copolymer (excluding any crosslinker or other additives). Typically, the more reinforcing monomer used in the outer pressure sensitive adhesive layer, the more reinforcing monomer needed in the inner layer.

Crosslinking Agents
The acrylate-based adhesives of the outer and inner layers preferably contain at least one type of crosslinking agent. Preferably, one or more nonionic crosslinking agents can be combined with monomer A and optional monomer B, if desired. A crosslinking agent is referred to herein as component C. Typically, component C modifies the adhesion of the adhesive and improves its cohesive strength. The crosslinking agent typically produces chemical crosslinks (e.g., covalent bonds). Prior to application of the adhesive article to a substrate, the crosslinking functionality is consumed, i.e., it is essentially completely reacted with monomers A and/or B or copolymers thereof.
Four suitable types of crosslinking agents are described below, and are referred to as C1-C4. Depending on the desired application, different crosslinking agents can be used for different results. Crosslinking agents that can be used in the exposed outer layer are preferably of the Cl type, and those that can be used in the inner layer include the Cl, C2, C3, or C4 type, with those of the Cl type being-preferred.
Preferably, component (Cl) a copolymerizable olefinically unsaturated compound, which, in the excited state, is capable of abstracting hydrogen; (C2) a compound having at least two reactive functional groups reactive with carboxylic acid groups; or (C3) a noncopolymerizable compound which, in the excited state, is capable of abstracting hydrogen. Component Cl is a free-radically polymerizable monomer capable of
polymerizing with monomers A and/or B. Components C2 and C3 are, essentially free of olefinic unsaturation and thus typically not copolymerizable with monomers A and/or B.
One type of nonionic crosslinking agent (i.e., component Cl) is an olefinically unsaturated compound that is copolymerized with monomers A and B and generates free radicals on the polymer upon irradiation of the polymer. Examples of such a compound include an acrylated benzophenone as described in U.S. Pat. No. 4,737,559 (Kellen et al.), p-acryloxy-benzophenone, which is available from Sartomer Company, Exton, PA, and monomers described in U.S. Pat. No. 5,073,611 (Rehmer et al.) including p-N-(methacryloyl-4-oxapentamethylene)-carbamoyloxybenzophenone, N-(benzoyl-p-phenylene)-N'-(methacryloxymethylene)-carbodiimide, and p-acryloxy-benzophenone. U.S. Pat. No. 5,506,279 (Babu et al.) at columns 5-6, describes another suitable olefinically unsaturated crosslinking agent referred to therein as Formula 2, which is {2-[4-(2-hydroxy-2-methyl-propan- 1 -one)phenoxy] } ethyl (2-methyl-2-(2-methyl-2-propen- 1 -one)amino)propanoate. The olefinically unsaturated compound which, in the excited state, is capable of abstracting hydrogen preferably includes acrylic functionality.

A second type of nonionic crosslinking agent (i.e., component C2), is a crosslinking compound which is essentially free of olefinic unsaturation and is capable of reacting with the carboxylic acid groups of optional monomer B. It includes at least two functional groups reactive with carboxylic acid groups. It may be added to a mixture of monomers (A and/or B) prior to their polymerization, or after they have been formed into a partially polymerized syrup, or to a copolymer of monomers. Examples of such components include, but are not limited to, l,4-bis(ethyleneiminocarbonylamino)benzene; 4,4 bis(ethyleneiminocarbonylamino)diphenylmethane; 1,8-bis(ethyleneiminocarbonylamino)octane; 1,4-tolylene diisocyanate; and 1,6-hexamethylene diisocyanate as described in U.S. Pat. No. 5,604,034 (Matsuda). Another example is N,N'-bis-l,2-propyleneisophthalamide as described in U.S. Pat. No. 4,418,120 (Kealy et al.). Other such crosslinking agents are available from K. J. Quin and Co., Seabrook, NH, and EIT Inc., Lake Wyllie, SC. Other examples of C2 crosslinking agents include diepoxides, dianhydrides, bis(amides), and bis(imides).
A third type of nonionic crosslinking agent (i.e., component C3) is a compound which is essentially free of olefinic unsaturation, is noncopolymerizable with monomers A and B, and, in the excited state, is capable of abstracting hydrogen. It can be added to a copolymer of monomers (A and/or B), or a partially polymerized syrup of monomers. Upon irradiation of the mixture, component C3 generates free radicals on the polymer or partially polymerized material. Examples of such components include, but are not limited to, 2,4-bis(trichloromethyl)-6-(4-methoxy)phenyl)-s-triazine; 2,4-bis(trichloromethyl)-6-(3,4-dimethoxy)phenyl)-s-triazine; 2,4-bis(trichloromethyl)-6-(3,4,5-trimethoxy)phenyl)-s-triazine; 2,4-bis(trichloromethyl)-6-(2,4-dimethoxy)phenyl)-s-triazine; 2,4-bis(trichloromethyl)-6-(3-methoxy)phenyl)-s-triazine as described in U.S. Pat. No.
4,330,590 (Vesley), and 2,4-bis(trichloromethyl)-6-naphthenyl-s-triazine and 2,4-bis(trichloromethyl)-6-(4-methoxy)naphthenyl-s-triazine as described in U.S. Pat. No. 4,329,384 (Vesley). Because triazines are known to liberate HC1 and, as a result, can cause embrittlement of poly(ethyelene terephthalate) backings at elevated temperatures, this is not a preferred crosslinking agent, although it may not adversely affect other backings such as poly(tetrafluoroethylene).

Another type of crosslinking agent that can be used in addition to one or more of components C1-C3, is an acrylic crosslinking monomer (component C4) containing at least two acrylic moieties, which preferably has an average of less than about 12 atoms in the chain between acrylic groups as disclosed in U.S. Pat. No. 4,379,201 (Heilmann et al.). Examples of this type of crosslinking agent include, but are not limited to,
trimethylolpropane triacrylate, 1,6-hexanediol diacrylate, pentaerythritol tetraacrylate, 1,2-ethylene glycol diacrylate, dodecyl diacrylate, and the diacrylate of ethylene oxide modified bisphenol A. These crosslinking agents are not preferred because they can cause ghosting, particularly if used in the exposed outer pressure sensitive adhesive layer.
If used, the crosslinking agent is used in an effective amount, by which is meant an amount that is sufficient to cause crosslinking of the pressure sensitive adhesive to provide adequate cohesive strength to produce the desired final adhesion properties to the substrate of interest. Two or more crosslinking agents may be used in combination if desired.
Preferably, the crosslinking agent(s) can be used in an amount of about 0.005 part by weight to about 2 parts by weight, more preferably, from about 0.01 part to about 1.0 part, and most preferably, from about 0.05 part to about 0.5 part, per 100 parts by weight of polymer.

Preparation of Acrylate-based Adhesives
The acrylate pressure sensitive adhesives of the present invention can be synthesized by a free-radical polymerization process using irradiation. Polymerization of the monomers to form the copolymer useful in the pressure sensitive adhesive composition of the present invention is typically carried out using thermal energy, ultraviolet radiation, electron-beam radiation, gamma-radiation, and the like, with ultraviolet being preferred. Such
polymerizations can be facilitated by a polymerization initiator, which can be a thermal initiator or a photoinitiator. Examples of suitable photoinitiators include, but are not limited to, benzoin ethers such as benzoin methyl ether and benzoin isopropyl ether, substituted benzoin ethers such as anisoin methyl ether, substituted acetophenones such as 2,2-dimethoxy-2-phenylacetophenone, and substituted alpha-ketols such as 2-methyl-2-hydroxypropiophenone. Examples of commercially available photoinitiators include IRGACURE 651 and DAROCUR 1173, both available from Ciba-Geigy Corp., Hawthorne, NY, and LUCER N TPO from BASF, Parsippany, NJ. Examples of suitable thermal initiators include, but are not limited to, peroxides such as dibenzoyl peroxide, dilauryl peroxide, methyl ethyl ketone peroxide, cumene hydroperoxide, dicyclohexyl peroxydicarbonate, as well as 2,2-azo-bis(isobutryonitrile), and t-butyl perbenzoate.
Examples of commercially available thermal initiators include VAZO 64, available from ACROS Organics, Pittsburgh, PA and LUCTDOL 70, available from Elf Atochem North America, Philadelphia, PA. The polymerization initiator is used in an amount effective to facilitate polymerization of the monomers. Preferably, the polymerization initiator is used in an amount of about 0.1 part to about 5.0 parts, and more preferably, about 0.2 part to about 1.0 part by weight, based on 100 parts of the total monomer content.
If a photocrosslinking agent is used, the coated adhesive can be exposed to ultraviolet radiation having a wavelength of about 250 nanometers (ran) to about 400 ran. The radiant energy in this preferred range of wavelength required to crosslink the adhesive is about 100 milliJoules/cm^ to about 1,500 milliJoules/cm^, and more preferably, about 200 milliJoules/cm^ to about 800 milliJoules/cm^.
The polymers of the present invention can be prepared by a variety of techniques, which may or may not include chain transfer agents (e.g., CBr_ι) to control molecular weight. These techniques may involve the use of appropriate polymerization initiators. A preferred solvent-free polymerization method using monomers A, B, and a crosslinking agent is disclosed in U.S. Pat. No. 4,379,201 (Heilmann et al.). Initially, a mixture of monomers A and B is polymerized with a portion of a photoinitiator by exposing the mixture to UV radiation in an inert environment for a time sufficient to form a coatable base syrup, and subsequently adding a crosslinking agent, the remainder of the
photoinitiator, and any optional additives (described below). This final syrup containing a crosslinking agent (e.g., having a Brookfield viscosity of about 100 centipoise to about 6000 centipoise at 23°C, as measured with a No. 4 LTV spindle, at 60 revolutions per minute) is then coated onto a substrate, such as a backing. A variety of coating methods including knife coating, slotted knife coating, or reverse roll coating may be used. In one embodiment, two notched bars are employed, with one downstream from the other. The first notched bar is used to coat the inner adhesive layer, and the second (downstream) notched bar is used to coat the outer, exposed adhesive layer. Once the syrup is coated onto a backing, further polymerization and crosslinking is typically carried out in an inert environment (i.e., an environment that is nonreactive with the monomer mixture). Suitable inert environments include nitrogen, carbon dioxide, helium, and argon, which exclude oxygen. A sufficiently inert atmosphere can be achieved by covering a layer of the photoactive syrup with a polymeric film, such as silicone-treated poly(ethylene
terephthalate) (PET) film, that is transparent to UV radiation or e-beam and irradiating through the film in air. The thickness of the layer of adhesive may vary over a broad range of about 0.0005 inch (12.5 microns) to about 0.010 inch (250 microns), preferably, about 0.001 inch (25 microns) to about 0.005 inch (126 microns), and more preferably, about 0.001 inch (25 microns) to about 0.002 inch (51 microns).
Once the adhesive composition has been substantially fully cured and optionally crosslinked so as to provide an adhesive article, the adhesive surface of the article may, optionally, be protected with a temporary, removable release liner (i.e., protective liner) such as a paper liner or plastic films such as polyolefin (e.g., polyethylene or polypropylene) or polyester (e.g., poly(ethylene terephthalate)) film. Such paper or films may be treated with a release material such as silicones, waxes, fluorocarbons, and the like. Only after the adhesive composition has been substantially fully cured and optionally crosslinked such that there is substantially no unsaturation are the adhesive articles of the present invention applied to a substrate.

Optional Adhesive Additives
The acrylate pressure sensitive adhesive compositions can include conventional additives such as tackifiers, plasticizers, flow modifiers, neutralizing agents, stabilizers, antioxidants, fillers, dyes, colorants, and the like, as long as they do not interfere with desired performance characteristics of the adhesive. Initiators that are not
copolymerizable with the monomers used to prepare the acrylate copolymer can also be used to enhance the rate of polymerization and/or crosslinking. Such additives can be used in various combinations. If used, they are incorporated in amounts that do not materially adversely affect the desired properties of the pressure sensitive adhesives or their film-forming properties. Typically, these additives can be incorporated into these systems in amounts of about 0.05 weight percent to about 25 weight percent, based on the total weight of the acrylate-based pressure sensitive adhesive composition.

Removability Test Method
Evaluation of the removability of tape samples was conducted by applying a test strip to an aluminum panel having a nominal thickness of 0.08 inch (0.2 cm), exposing the taped panel to heat, then peeling back the test strip by hand to determine removability. More specifically, test strips having a length of approximately 6 inches (15.2 cm) and a width of either 0.5 inch (1.3 cm) or 1 inch (2.5 cm) were applied to a 0.08 inch (0.2 cm) thick aluminum panel having a protective epoxy primer coating on one side (obtained from BFG Aerospace, Riverside, CA), such that 4 inches (10.2 cm) of the test strip were adhered to the panel and 2 inches (5.1 cm) hung over the edge. The test strip was rubbed down using finger pressure to ensure intimate contact between the primed surface and the exposed outer adhesive layer.
Conditioning was carried out by placing a taped panel in an aluminum tray with the test strips facing up, positioning a second aluminum tray on top of the first panel, and taping the edges of the trays together. A hole was punched into the upper tray and a nitrogen source placed therein. The chamber thus constructed having the aluminum panel inside was placed in a forced air oven at room temperature and a nitrogen flush begun. After 20-30 minutes the oven was turned on and the temperature increased to 350°F (177°C) over a period of about 20-30 minutes. The temperature was maintained for 4 hours then allowed to cool down to about 120°F (49°C) at which time the taped tray construction was removed from the oven and the nitrogen purge discontinued. The tray assembly with the panel inside was stored at room temperature (approximately 70°F (21 °C)) for 2-16 hours before removing the panel.
The free end (hanging over the panel edge) of the test strip was peeled back by hand at an angle of 90° for a length of about 2 inches (5.1 cm), then at an angle of 180° for a length of almost 2 inches (5.1 cm). For each angle of peel the panel surface thus exposed was inspected visually for adhesive residue, ghosting, or edge beading. Different results can occur for the 180° and 90° peel tests because of the differing peel mechanics. By "ghosting" it is meant a whitening or brightening of color was observed on the panel surface; by "edge beading" it is meant that a thin line of adhesive residue, of variable length, was observed along the original edge of the lengthwise border of the test strip. Edge beading is believed to be due to shrinkage of the backing during the heat cycle. Samples exhibiting none of these characteristics were graded "Pass" (P for Pass and F for Fail in the tables below). Samples exhibiting edge beading, or slight ghosting were also graded as "Pass" (note: although edge beading is not desirable it is felt that use of a heat stable backing for these examples would have minimized/eliminated this result). By "heat stable" with respect to a backing, it is meant a shrinkage of less than 1% under whatever conditions are employed. The polyester backings used in the present examples are reported to exhibit a shrinkage of about 1.8% to 2.5%.

Examples
The invention will be more fully appreciated with reference to the following non-limiting examples. The examples were evaluated with respect to certain test methods which are described herein.

Example 1 (Comparative)
An adhesive composition was prepared and used to provide a pressure sensitive adhesive tape which was evaluated for removability after exposure to heat or heat and pressure.
More specifically, a tape having a dual layer adhesive construction was prepared in the following manner. Two syrups were prepared. The first syrup was made by adding 686.0 grams of isooctyl acrylate (IOA), 14.0 grams of acrylic acid (AA), 1.8 grams of IRGACURE 651 (2,2-dimethoxy-2-phenylacetophenone, available from Ciba-Geigy, Chicago, IL), and 0.7 gram of EBECRYL P 36 (an acrylated derivative of benzophenone, available from UCB Radcure Inc., Smyrna, GA) to a 1 quart (0.95 liter) glass jar. The jar was then capped and put on a roller mill having two rubber rollers, one of which was motor driven at about 60 revolutions per minute (rpm), for about 2 hours to dissolve the IRGACURE 651 and EBECRYL P 36 in the IOA and AA monomers. The cap was removed and replaced with a cap having an opening for a nitrogen gas source. The opening was slightly larger than the gas source to permit a steady flow of gas. The monomer solution was purged with dry nitrogen for about 60 minutes. While maintaining an inert atmosphere, the jar was then gently swirled and irradiated using two 15 Watt fluorescent lamps (Model F15T8-BLB, available from General Electric, Schenectady, NY) positioned parallel to each other one inch apart. The jar was held about 6 inches (15.2 centimeters) from the lamps, and irradiated for about 30 seconds to provide a coatable syrup having an estimated viscosity of about 800 to 3000 centipoise. The nitrogen source and cap were then removed and the jar left open to the air for about 10 minutes. Then 7.0 grams each of IRGANOX 1010 (an antioxidant) and IRGAFOS 168 (a supplemental antioxidant) (both available from Ciba-Geigy, Chicago, IL) and another 1.4 grams of EBECRYL P 36 were added to the syrup. The jar was then capped and placed on a roller mill, as described above, for 24 hours to dissolve the EBECRYL P 36 and provide a clear coatable syrup having an IOA:AA ratio of 98:2 (by weight) was obtained. This syrup was designated Syrup A.
A second syrup was prepared in the same manner except no acrylic acid was used. A clear coatable syrup having an IOAAA ratio of 100:0 (by weight) was obtained. This syrup was designated Syrup B.
These two syrups were coated sequentially onto a 0.002' inch (51" micrometer) thick untreated polyester film backing using a coater having two notched bars. The gap between the backing and the first notched bar was 0.002 inch (51 micrometers) and the gap between the backing and the second notched bar was 0.003 inch (76 micrometers). Syrup A was introduced at the leading edge (upstream side) of the first notched bar, and Syrup B was introduced at the leading edge of the second notched bar (which was downstream from the first bar). The result was a coating having an inner layer of Syrup A and an outer layer of Syrup B.
The coated film was passed through an irradiation chamber inerted with nitrogen where it was exposed (from the adhesive side) to a wavelength of emission between 300 nanometers (ran) and 400 ran and intensity settings of 1.5 milliWatts/centimeter2 and 7.0 milliWatts/centimeter2 for a time of 50 seconds and 175 seconds respectively to give a total calculated dose of about 1300 milli Joules/centimeter2. The film with a now polymerized adhesive coating was then exposed from the adhesive side to a medium pressure mercury lamp for 6 seconds to provide a total target dose of 600
milliJoules/centimeter2. The intensity and dose values are reported in National Institute of Standards and Technology (NIST) units. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 1 below.

Example 2
Example 1 was repeated with the following modification. Syrup A had a composition of IOA:AA of 96:4. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the
"Removability Test Method." The results are shown in Table 1 below.

Example 3
Example 1 was repeated with the following modification. Syrup A had a composition of IOA:AA of 94:6. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the
"Removability Test Method." The results are shown in Table 1 below.

Example 4
Example 1 was repeated with the following modification. Syrup A had a composition of IOAAA of 92:8. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the
"Removability Test Method." The results are shown in Table 1 below.

Example 5
Example 1 was repeated with the following modifications. Syrup A had a composition of IOA:AA of 90:10. The pressure sensitive adhesive tape obtained evaluated for removability as follows. The aluminum panel having a test strip thereon was placed in a vacuum bag and a vacuum applied for several minutes preceding placement in the autoclave, according to standard industry practice. A pressure of about 50 psi (345 KPa) was applied, followed by release of the vacuum. The autoclave was then heated at a rate of 5°F/minute (2.8°C/minute) to 350°F (177°C) and held at this temperatμre for 4 hours followed by cooling at a rate of 5°F/minute (2.8°C/minute) to room temperature. After removal of the panel from the bag the test strips were peeled and the aluminum panel evaluated as described above in the "Removability Test Method." The results are shown in Table 1 below.

Example 6
Example 1 was repeated with the following modifications. Syrup A had a composition of IOAAA of 85:15, and the backing was a 0.002 inch (51 micrometers) thick film of poly(tetrafluoroethylene) (PTFE) which had been treated by Acton
Technologies, Inc. (Pittston, PA) using their FLUOROETCH process. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 1 below.

Example 7
Example 1 was repeated with the following modification. Syrup A had a composition of IOA:AA of 80:20. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the
"Removability Test Method." The results are shown in Table 1 below.

Example 8 (Comparative)
Example 7 was repeated with the following modification. After the syrup having an IOAAA ratio of 80:20 was prepared, sufficient additional AA monomer was added, along with the antioxidant components and EBECRYL P 36, to give a final IOA.AA ratio of 70:30. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 1 below.

Example 9 (Comparative) Example 7 was repeated with the following modification. After the syrup having an IOA:AA ratio of 80:20 was prepared, sufficient additional AA monomer was added, along with the antioxidant components and EBECRYL P 36, to give a final IOA:AA ratio of 60:40. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 1 below.

TABLE 1



* = Comparative Example
** = TEFLON backing; all others are 2" mil (51" micron) thick poly(ethylene terephthalate)

Example 10 (Comparative)
Example 1 was repeated with the following modification. Syrup B had a composition of IOA:AA of 99: 1. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 2 below.

Example 11
Example 2 was repeated with the following modification. Syrup B had a composition of IOA:AA of 99:1. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 2 below.

TABLE 2



* = Comparative Example

Example 12 (Comparative)
Example 2 was repeated with the following modification. Syrup B had a composition of IOA:AA of 98:2. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 3 below.

Example 13
Example 1 was repeated with the following modifications. Syrup A had a composition of IOA:AA of 90:10, and Syrup B had a composition of IOA:AA of 98:2. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the ''Removability Test Method." The results are shown in Table 3 below.

TABLE 3


Example 14
Example 4 was repeated with the following modification. Syrup B had a composition of IOA:AA of 96:4. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 4 below.

Example 15
Example 1 was repeated with the following modifications. Syrup A had a composition of IOA:AA of 90: 10, and Syrup B had a composition of IOA:AA of 96:4. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in the "Removability Test Method." The results are shown in Table 4 below.

Example 16
Example 7 was repeated with the following modification. Syrup B had a composition of IOA:AA of 96:4. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in "Removability Test Method." The results are shown in Table 4 below.

TABLE 4




* = Comparative Example

Example 17 (Comparative)
Example 1 was repeated with the following modifications. Syrup A had a composition of IOA:AA of 90:10, and Syrup B had a composition of IOA:AA of 94:6. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in "Removability Test Method." The results are shown in Table 5 below.

Example 18
Example 17 was repeated. The pressure sensitive adhesive tape obtained was evaluated for removability after exposure to heat as described above in "Removability Test Method." The results are shown in Table 5 below.

TABLE 5



* = Comparative Example
Note: Examples 17 and 18 are duplicates. The finding that Example 18 "Passed" and Example 17 "Failed" indicates that the limits of the monomer ratio in the outer layer are being approached.

Example 19 (Comparative)
Example 7 was repeated with the following modification. Syrup B had a
composition of IOA:AA of 92:8. The pressure sensitive adhesive tape obtained evaluated for removability after exposure to heat as described above in "Removability Test Method." The results are shown in Table 6 below.

TABLE 6



* = Comparative Example

Example 20
Example 1 was repeated with the following modifications. Syrup A had a composition of IOA:AA of 90:10. The pressure sensitive adhesive tape obtained was evaluated for removability as follows. A sample of the tape measuring 1.5 inches (1.3 cm) x 4 inches (10.2 cm) was applied to a sheet of copper (Grade 194, available from Olin Company, Norwalk, CT) having a thickness of 2 mm and measuring 3 inches (7.6 cm) x 4 inches (10.2 cm) such that the tape was positioned lengthwise. This was then conditioned as described in the "Removability Test Method" described above, except the conditioning time was 60 minutes. Upon peeling the tape back at an angle of 90° then 180°, there was observed a slightly darker color on the previously taped copper surface than seen prior to conditioning. In contrast, after conditioning an untaped area was observed to have a significantly darker color than seen prior to conditioning. On a scale of 1 to 10, with 1 representing the original color prior to conditioning and 10 representing the color of the untaped areas after conditioning, the taped area was graded (after conditioning and removal of the tape) as between 2 and 3. There was no ghosting, edge beading, or adhesive residue observed upon removal of the tape after conditioning. This example demonstrates the utility of the tapes of the invention in electronic applications, such as lead frame tapes.

Each of the patents, patent applications, and publications cited herein are incorporated herein by reference, as if individually incorporated. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of the invention. It should be understood that this invention is not limited to the illustrative embodiments set forth herein.