WO2000011099A2

WO2000011099A2 - Tackified acrylic adhesive emulsion and transfer tape containing his

Info

Publication number: WO2000011099A2
Application number: PCT/US1999/015559
Authority: WO
Inventors: Thomas C. Epple
Original assignee: Avery Dennison Corporation
Priority date: 1998-08-25
Filing date: 1999-07-08
Publication date: 2000-03-02
Also published as: CA2341541A1; WO2000011099A3

Abstract

In one embodiment, the present invention relates to an emulsified pressure-sensitive adhesive, comprising: from about 50 % to about 95 % by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5 % to about 50 % by weight of an emulsified tackifier, the emulsified tackifier having a softening point from 5 °C to about 90 °C and a solids content from about 40 % to about 95 % solids, with the proviso that the emulsified tackifier does not contain low boiling components.

Description

Title: TACKIFIED EMULSION ACRYLIC ADHESIVE FOR LOW

FOGGING APPLICATIONS

Technical Field

This invention relates to emulsified pressure-sensitive adhesives and transfer tapes and double sided tapes containing the emulsified pressure- sensitive adhesives. More particularly, the invention relates to emulsified pressure-sensitive adhesives made with emulsified tackifiers that exhibit good adhesion to low energy surfaces while not fogging.

Background of the Invention

The automotive, aerospace, consumer appliance and other industries require transfer tapes and double coated tapes with adhesives that have a combination of properties such as high durability, resistance to continuous exposure to high temperatures, low fogging properties and good peel adhesion to a wide range of substrates. Transfer tapes and double coated tapes are used in applications requiring a relatively high coat weight of an adhesive and/or for joining dissimilar materials (for instance, bonding to metal, paints, plastic, enamel, paper, fabric, glass and wood) and/or where durability, ultraviolet (UV) light and environmental resistance is needed. Transfer tapes and double coated tapes are also used in applications which require bonding a rough, irregular surface to either a flat surface or another rough, irregular surface.

Transfer tapes generally contain a layer of tacky, pressure-sensitive adhesive carried on a release liner. In operation, the exposed surface of the adhesive is applied to one substrate, after which the release liner is removed, exposing the opposite surface of the adhesive. The newly exposed adhesive surface is then placed in contact with a second substrate, thereby joining the two substrates. Double coated tapes generally contain two layers of a tacky, pressure-sensitive adhesive carried on the two sides of a facestock with a release liner over one side of the adhesive covered facestock. In operation, the exposed adhesive layer is applied to one substrate, after which the release liner is removed, exposing the other adhesive layer. The newly exposed adhesive layer is then placed in contact with a second substrate, thereby joining the two substrates.

Acrylic adhesives are use in transfer tapes and double coated tapes because they have many desirable adhesive properties. However, acrylic adhesives do not strongly adhere to low surface energy plastics. Examples of low surface energy plastics include acrylonitrile-butadiene-styrene (ABS) terpolymer, low density polyethylene and isotactic polypropylene. In order to increase the adhesion between an acrylic adhesive and a low surface energy plastic, one or more tackifiers are incorporated into the acrylic adhesive. This is accomplished by combining an organic solvent containing a tackifier to an acrylic adhesive. When conventional transfer tapes or double coated tapes are used to adhere various automotive components fogging problems occur. Fogging involves the outgassing of low boiling and/or volatile components present in the tackified acrylic adhesive. Thus, while adding a tackifier to an acrylic adhesive desirably improves adhesion to low energy surfaces, the tackifier undesirably causes fogging problems. What is desired is a durable adhesive that can be used in transfer tapes or double coated tapes that is characterized by both good low surface energy adhesion and low fogging properties.

Summary of the Invention

The present invention provides emulsified pressure-sensitive adhesives which can strongly adhere to a variety of surfaces including low energy surfaces while not causing any fogging problems. Accordingly the present invention also provides transfer tapes and double coated tapes containing the emulsified pressure-sensitive adhesives. In one embodiment, the present invention relates to an emulsified pressure-sensitive adhesive, comprising: from about 50% to about 95% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5% to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5 °C to about 90°C and a solids content from about 40% to about 95% solids, with the proviso that the emulsified tackifier does not contain low boiling components.

In another embodiment, the present invention relates to an emulsified pressure-sensitive adhesive, comprising: from about 55% to about 90% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 1 0% to about 45% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5 °C to about 90°C, a pH from about 7 to about 9 and a solids content from about 40% to about 95 % solids, with the proviso that the emulsified tackifier does not contain low boiling components.

In yet another embodiment, the present invention relates to a transfer tape comprising: a release liner, and an emulsified pressure-sensitive adhesive on at least one side of the release liner, the emulsified pressure-sensitive adhesive comprising: from about 50% to about 95% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5% to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5°C to about 90°C and a solids content from about 40% to about 95% solids, with the proviso that the emulsified tackifier does not contain low boiling components.

In still yet another embodiment, the present invention relates to a method of bonding a tape to a low energy surface, comprising applying the tape to the low energy surface, the tape comprising a substrate layer and a layer of an emulsified pressure-sensitive adhesive, the emulsified pressure- sensitive adhesive comprising: from about 50% to about 95% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5 % to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5°C to about 90°C and a solids content from about 40% to about 95 % solids, with the proviso that the emulsified tackifier does not contain low boiling components.

Detailed Description of the Invention

The present invention relates to emulsified pressure-sensitive adhesives and transfer tapes containing the emulsified pressure-sensitive adhesives which exhibit high adhesion to a diverse array of surfaces including low energy surfaces. Additionally, the emulsified pressure-sensitive adhesives and transfer tapes containing the emulsified pressure-sensitive adhesives do not exhibit fogging thereby facilitating their use in automotive applications. The emulsified pressure-sensitive adhesives are made by combining an emulsion pressure-sensitive adhesive polymer and an emulsified tackifier.

Low energy surfaces are typically non-polar polymeric surfaces. In one embodiment, the low energy surface is made of a polyolefin. Specific examples of low energy surfaces include surfaces of plastics made of polyethylene (especially low density polyethylene), polypropylene (especially isotactic polypropylene), and acrylonitrile-butadiene-styrene terpolymer. Other surfaces to which the emulsified pressure-sensitive adhesives of the present invention bond well to include fiberglass, appliance enamel, glass, formica, manila tags and recycled corrugated cardboard. Emulsion pressure-sensitive adhesive polymers and methods of making emulsion pressure-sensitive adhesive polymers are described in U.S. Patents 4,925,908; 5, 1 64,444; 5, 1 89, 1 26; 5,221 ,706; 5,623,01 1 and 5,723, 1 91 which are hereby incorporated by reference to the extent they describe emulsion pressure-sensitive adhesive polymers and methods of making and using the same. Such emulsion pressure-sensitive adhesive polymers are distinct from solvent polymers or bulk polymers.

The amount of emulsion pressure-sensitive adhesive polymers used in the present invention is dependent upon the identity of the emulsion pressure-sensitive adhesive polymer and emulsified tackifier used. In one embodiment, the emulsified pressure-sensitive adhesive prepared in accordance with the present invention comprises from about 95% to about 50% by weight total of one or emulsion pressure-sensitive adhesive polymers. In another embodiment, the emulsified pressure-sensitive adhesive comprises from about 90% to about 55 % by weight total of one or more emulsion pressure-sensitive adhesive polymers. In yet another embodiment, the emulsified pressure-sensitive adhesive comprises from about 85% to about 60% by weight total of one or more emulsion pressure-sensitive adhesive polymers.

Generally speaking, the emulsion pressure-sensitive adhesive polymers are acrylic based polymers. Acrylic based polymers contain at least monomers of acrylates, methacrylates, alkyl acrylates and alkyl methacrylates as well as one more of other monomers, copolymerizable compounds, and additives. In one embodiment, the alkyl groups of the alkyl acrylates and alkyl methacrylates contain from 1 to about 1 2 carbon atoms. Examples of acrylic based polymers include those containing monomers of acrylate, methacrylate, methyl acrylate, ethyl acrylate, methylmethacrylate, 2-ethyl hexyl acrylate, isoctyl acrylate, butyl acrylates such as n-butyl acrylate, sec- butyl acrylate, and iso-butyl acrylate, methyl butyl acrylate, 4-methyl-2-pentyl acrylate, heptyl acrylate, isodecyl methacrylate and the like. The amount of emulsified tackifier used in the present invention is dependent upon the identity of the emulsion pressure-sensitive adhesive polymer and emulsified tackifier used. In one embodiment, the emulsified pressure-sensitive adhesive prepared in accordance with the present invention comprises from about 5% to about 50% by weight total of one or more emulsified tackifiers. In another embodiment, the emulsified pressure- sensitive adhesive comprises from about 1 0% to about 45% by weight total of one or more emulsified tackifiers. In yet another embodiment, the emulsified pressure-sensitive adhesive comprises from about 1 5% to about 40% by weight total of one or more emulsified tackifiers.

The emulsion pressure-sensitive adhesive polymers are combined in any suitable manner (mixed, blended, etc.) with an emulsified tackifier to provide the emulsified pressure-sensitive adhesives according to the present invention. For example, the emulsified pressure-sensitive adhesives of the present invention are generally formulated by first preparing an emulsion pressure-sensitive adhesive polymer as described above, adjusting its pH to between about 6 and about 7, and adding the emulsified tackifier. Transfer tapes and double sided tapes are then prepared as follows: the formulated emulsified pressure-sensitive adhesive is coated on a substrate such as a release liner from about 25 to about 1 50 g/m² coat weight, dried at elevated temperatures such as from about 50°C to about 90°C for about 1 0 to 1 5 minutes, and laminated to a paper or plastic (polymeric films) backing.

Although numerous embodiments of emulsified pressure-sensitive adhesives and emulsion pressure-sensitive adhesive polymers exist, six specific embodiments of emulsified pressure-sensitive adhesives and/or emulsion pressure-sensitive adhesive polymers are described below. In one embodiment, the emulsified pressure-sensitive adhesives of the present invention contain from about 90 to about 99.5 parts, i.e. percent by weight. The alkyl acrylate monomers preferably have from 1 to about 1 2 carbon atoms in the alkyl chain and more preferably most of the alkyl acrylate monomer comprises from about 4 to about 8 carbon atoms in the alkyl chain. Noncrystalline copolymers including alkyl acrylates having more than 1 2 carbon atoms in the alkyl chain may be used as desired.

In one embodiment, emulsion pressure-sensitive adhesive polymers in which at least the majority of the alkyl acrylate monomers have from about 4 to about 8 carbon atoms in the alkyl chain are preferred as providing the optimum balance of hardness, adhesion and removability. If the polymer contains less than about 97 or 98 parts alkyl acrylate monomers, it is preferred that the polymer further contain up to about 1 0 parts of any other monomer such as vinyl acetate or the like.

In addition to the alkyl acrylate monomer, the emulsion pressure- sensitive adhesive polymers of the first embodiment comprise up to about 2 parts of one or more polar monomers such as methacrylic acid, acrylic acid, itaconic acid, maleic acid, or acrylamide. The polar monomers impart mechanical stability and cohesive strength of the polymer.

The emulsion pressure-sensitive adhesive polymers of the first embodiment further comprise up to about one part and preferably from about 0.1 part to about 0.3 part of at least one multifunctional acrylate monomer.

Including the multifunctional acrylate monomers enables the desired gel contents of the polymer to be obtained. That is, without the multifunctional acrylate monomer, the gel content of the polymer tends to be too low. The multifunctional acrylate monomer may be present in an amount of at least about 0.1 percent to obtain the desired gel content.

The amount of multifunctional acrylate monomer should not exceed about one percent as such amounts tend to reduce the stability of the acrylic- based emulsion from which the polymers are prepared, resulting in coagulation of the emulsion particles during preparation. Amounts of the multifunctional acrylate monomer above about 0.3 percent are not preferred as no additional benefit, i.e. no additional increase in gel content is typically observed. Examples of suitable multifunctional acrylates include polyethylene glycol diacrylate, hexane diol diacrylate, ethoxylated trimethylpropane triacrylate, pentaerythritol triacrylate and polypropylene glycol diacrylate. Hexane diol diacrylate is presently preferred.

An initiator induces polymerization of the monomers. Any suitable initiator may be used. Initiators include peroxides and peroxydisulfates. The initiator is preferrably present in an amount of about 0.1 percent to about 0.2 percent of the monomer weights. The presently preferred initiators are potassium persulfate and the t-butyl hydroperoxide, particularly in combination with each other in a ratio of from about 1 : 1 to about 1 :3. Such combinations are preferred as they tend to provide optimum control of the rate of initiation and of the percentage conversion, i.e. the amount of polymer formed at a given time.

The emulsion pressure-sensitive adhesive polymers of the first embodiment have a gel content of from about 70 to about 90 percent by weight and preferably from about 75 to about 83 percent. As used herein, "gel content" refers to the percentage by weight of the polymer that is insoluble in tetrahydrofuran, toluene or other similar organic solvent.

In this embodiment, it is preferred that the glass transition temperature (Tg) of the polymers be in the range from about -30°C to about -60°C and preferably from about -50°C to about -55 °C. Also in this embodiment, the soluble portion weight average molecular weight of the polymers is also not critical, but is preferably at least about 200,000.

In addition to the emulsion pressure-sensitive adhesive polymers, the polymers contain one or more adhesion stabilizers to minimize peel adhesion buildup over time and/or with elevated temperatures. Preferred adhesion stabilizers include one or more plasticizers and one or more protective colloids. The plasticizer may be present in an amount of up to about 3 percent by weight based on the total weight of the monomers and is preferably present in an amount from about 1 percent to about 3 percent by weight. Any suitable plasticizer may be used. Preferred plasticizers are monomeric plasticizers including dialkyl esters of dibasic acids such as dioctyl phthalate and dioctyl azelate.

A protective colloid, also known in the art as a steric stabilizer, is preferably present in the emulsion pressure-sensitive adhesive polymer composition in an amount of up to about 1 percent by weight of the monomers and preferably from about 0.5 percent to about 1 percent by weight of the monomers. If present, it is preferred that the amount not exceed about 1 percent by weight. Any suitable protective colloid may be used. A preferred protective colloid is a high molecular weight polyvinyl alcohol composition sold by Air Products under the trade name Vinol 540.

This product is approximately 87 to 89% hydrolyzed, i.e. polyvinyl acetate which has undergone about 89% hydrolysis to form polyvinyl alcohol, and has, according to the manufacturer, a molecular weight of about 1 05,000 to about 1 1 0,000. Other suitable protective colloids may include polyethylene oxide and hydroxy ethyl cellulose.

To obtain emulsion pressure-sensitive adhesive polymers having the above parameters, it preferable that the particle size of the emulsion from which the polymers are synthesized does not exceed about 21 0 nm. Such a particle size tends to stabilize the emulsion without decreasing the gel content of the polymerized product. In one embodiment, preparing of acrylic- based emulsion polymers having a gel content of from about 70 to about 90 percent with the desired peel adhesion involves polymerization in the presence of a combination of anionic and non-ionic emulsifiers or surfactants. In one embodiment, the total amount of anionic and non-ionic emulsifiers is preferably maintained below about 4.0 percent by weight and more preferably below about 3.7 percent by weight based on the weight of the monomers. In one embodiment, the amount of anionic emulsifier is from about 1 .5 to about 3.0 percent by weight and preferably from about 1 .8 to about 2.1 percent by weight based on the weight of the monomers. In one embodiment, the amount of non-ionic emulsifiers is in the range from about

0.5 to about 2.0 percent by weight and preferably from about 1 .3 to about 1 .7 percent by weight based on the weight of the monomer.

The preferred anionic emulsifier is sodium sulfonate of ethoxylated octyl phenol sold by Rohm & Haas under the tradename Triton X-200. Such an emulsifier contains about 20 moles of ethylene oxide per mole of ethoxylated octyl phenol. Other suitable anionic emulsifiers include alkyl polyether sulfosuccinates, alkylaryl polyether sulfosuccinates, alkylpolyether sulfonates and alkyl aryl polyether sulfonates. The preferred non-ionic emulsifier is ethoxylated nonyl phenol sold by Rohm & Haas under the tradename Triton N-1 1 1 . This emulsifier contains about 1 0 moles of ethylene oxide per mole of ethoxylated nonyl phenol. Other suitable non- ionic emulsifiers include alkyl aryl polyether alcohols.

In addition to the anionic and non-ionic emulsifiers, the gel content and particle size is preferably controlled by the inclusion of a small amount of electrolyte such as tetrasodium pyrophosphate, ammonium phosphate di- or mono-basic or the like in the emulsion. In one embodiment, the amount of electrolyte is from about 0.1 5 to about 0.3 percent by weight based on the weight of the monomers.

Methods for preparing emulsion pressure-sensitive adhesive polymers of the first embodiment comprise first preparing a catalyst feed and a pre- emulsion feed. The catalyst feed comprises water and catalyst. Any suitable catalyst may be utilized. For example, sodium formaldehyde sulfoxylate may be used. The pre-emulsion comprises the alkyl acrylate monomers, polar monomers, multifunctional acrylate monomers, plasticizer initiator, anionic and non-ionic emulsifiers and water. In the method, catalyst and pre-emulsion feeds are added simultaneously to a suitable reactor. The feed rates for the catalyst and pre- emulsion feeds are adjusted so that the monomers and catalyst are added to the reactor over a period of from about 2 to about 4 hours. After about 20% to about 60%, and preferably about 40% of the pre-emulsion feed and catalyst feed have been added, the pH of the resulting emulsion is adjusted to about 6.3 to about 6.5 by the addition of sodium bicarbonate or the like. At this point, the protective colloid is mixed with the catalyst feed. The catalyst feed and the pre-emulsion feeds are again added simultaneously to the reactor at the desired rates preferably over a period of about 1 to about 2 hours whereby the feeds end simultaneously. Polymerization results in the formation of a latex having a viscosity typically in the range of from about 2500 to about 3500 centipoise.

In a second embodiment, the emulsion pressure-sensitive adhesive polymers of the present invention contain a first component of, on a percent by weight basis from about 35 to about 60 percent by weight total, one or more alkyl acrylates containing about 4 to about 8 carbon atoms in the alkyl groups, and preferably total alkyl acrylate concentration, including mixtures of alkyl acrylates, preferably present in a total amount of from about 40 to about 50 percent by weight of the monomers.

In this second embodiment, a second monomeric component is one or more vinyl esters present in a total amount of from about 1 5 to about 35 percent by weight, preferably from about 20 to about 25 percent by weight based on total weight of the monomers, said vinyl esters containing from 2 to about 1 6 carbon atoms in the alkyl group of the acid. Vinyl esters include vinyl acetate, vinyl butyrate, vinyl propionate, vinyl isobutyrate, vinyl valerate, vinyl versitate, and the like.

In this second embodiment, a third component is one or more diesters of a dicarboxylic acid and mixtures thereof, present in a total amount of from about 20 to about 35 percent by weight based on the total weight of the monomers. Each ester group of the diester of the dicarboxylic acid independently contains from about 8 to about 1 6, preferably from about 8 to about 1 2, carbon atoms. Examples of diesters include di-2-ethyl hexyl maleate, di-2-ethyl hexyl fumarate and mixtures thereof.

In this second embodiment, a fourth component is at least one unsaturated carboxylic acid containing from about 3 to about 5 carbon atoms and present in a total amount of up to about 5 percent by weight of the polymer, preferably from 1 to about 3 percent by weight. Unsaturated carboxylic acids include, among others, acrylic acid, methacrylic acid, itaconic acid, and the like.

The emulsion pressure-sensitive adhesive polymers may be prepared in the presence of a reactive surfactant which polymerizes during formation of the polymer and becomes an integral part of the polymer. Preferred reactive surfactants include anionic vinyl functional monomers such as sodium vinyl sulfonate and sodium styrene sulfonate and the like. The reactive surfactant is present as part of the total surfactant system and in an amount up to about 0.4 percent by weight of the total monomers, preferably about 0.1 to about 0.25 percent by weight.

The monomer proportions may be adjusted in such a way that the adhesive has a glass transition temperature less than about -30°C, preferably less than about -34°C, giving a good balance of adhesion and tack at room temperature and low temperatures. The emulsion pressure-sensitive adhesive polymers of this embodiment have a very broad glass transition temperature range of from 1 5 °C to 30°C, e.g. -22 °C to -52 °C, which is unusual among acrylic polymers.

In this second embodiment, gel content or percent insolubles are in the range from about 50 to about 70 percent by weight which provides excellent cohesive strength without the use of multifunctional monomers. In this regard, gel represents the amount of polymer which is insoluble in tetrahydrofuran expressed in percent by weight and determined by the membrane gel partitioning method. In this method, about 600 to 800 milligrams of 1 00 percent solid polymer is weighed onto a millipore membrane disk of 5 micrometer porosity. The disk is heat sealed and transferred to a scintillation vial. About 20 milliliters of tetrahydrofuran is added to the vial and the vial is rotated on a shaker for 1 6 to 24 hours. The sealed disk is then removed, washed with tetrahydrofuran, and dried first by placing it on a Whatman No. 1 filter paper, followed by exposure to 1 00°C in the oven for a few hours. The dried disk is weighed and the insoluble portion of the polymer determined by the equation: percent insoluble = [(b - c) x 100]/a + % gel wherein a = total weight of 1 00 percent solids polymer b = weight of the polymer plus membrane before tetrahydrofuran treatment c = polymer plus membrane remaining after tetrahydrofuran treatment. Polymer properties can be further modified to fit end use applications by inclusion of multifunctional monomers and the use of other chemical cross-linking agents. Other aids which may be used to develop cross-linking include thermal cross-linking and cross-linking by actinic and electron beam radiation.

In this embodiment, emulsion pressure-sensitive adhesive polymers are prepared by emulsion polymerization under conditions of agitation in an autogenous atmosphere in the presence of suitable polymerization initiators such as peroxydisulfate and peroxides. Depending on desired polymer properties including gel content, the preferred levels of these initiators are in the range of from about 0.5 to about 1 .0 percent by weight based on the total weight of the monomers. Initiators include potassium persulfate, t-butyl hydrogen peroxide, and the like. Level of agitation varies depending on the system and influences conversion. Typically, about 30 to 50 percent of the total initiator is added along with an initial monomer charge to the reactor, and the rest is added along with the balance monomers during polymerization over a period of from about 4 to about 5 hours. In some embodiments, it is desirable to maintain the pH of the emulsion during polymerization between from about 2 to about 4, preferably from about 2.5 to about 4. This can be achieved by the use of buffers such as sodium bicarbonate and sodium acetate, typically in amounts up to 0.3 percent by weight based on the weight of the monomer. The stabilizer system used during polymerization contains a combination of anionic and nonionic surfactants present in an amount up to about 3.5 percent by weight based on the weight of the monomers. A suitable anionic surfactant is the sodium salt of an ethoxylated nonylphenol sulfate, and a suitable nonionic surfactant is ethoxylated nonylphenol. Emulsion pressure-sensitive adhesive polymers of this embodiment are produced at high solids level content, typically about 50 to about 70 percent by weight. Reaction is carried out at temperatures from 70°C to 85 °C with an initial charge of up to about 10 percent by weight of the total monomers, with the balance of the monomers being added to the emulsion reaction system over a period of about 4 to about 5 hours, with total monomer conversion approaching 1 00 percent.

In a third embodiment, the emulsion pressure-sensitive adhesive polymers of the present invention contain, on a percent by weight basis from about 35 to about 60 percent by weight total, of at least one of the alkyl acrylates containing about 4 to about 8 carbon atoms in the alkyl group and at least four other components. Preferably, a mixture of alkyl acrylates are employed with the total alkyl acrylate concentration, in an amount of from about 40 to about 50 percent by weight of the monomers.

A second monomeric component of the third embodiment is one or more vinyl esters present in a total amount of from about 1 5 to about 35 percent by weight, preferably from about 20 to about 25 percent by weight based on total weight of the monomers, said vinyl esters containing from 2 to about 1 6 carbon atoms in the alkyl group of the acid. Representative of the vinyl esters include vinyl acetate, vinyl butyrate, vinyl propionate, vinyl isobutyrate, vinyl valerate, vinyl versitate, and the like.

A third component of the emulsion pressure sensitive adhesive polymers are one or more diesters of a dicarboxylic acid and mixtures thereof, present in a total amount of from about 1 5 to about 30 percent by weight based on the total weight of the monomers. Each ester group of the diester of the dicarboxylic acid independently contains from about 6 to about 1 2, preferably from about 8 to about 1 2, carbon atoms. Preferred diesters include di-2-ethylhexyl maleate, di-2-ethylhexyl fumarate and mixtures thereof.

A fourth component of the emulsion pressure sensitive adhesive polymers is a reactive multifunctional monomer which is an acrylate and/or methacrylate having a cyanurate or phosphate moiety and present in an amount of from 0.1 to about 1 percent by weight of the monomers. Preferred multifunctional monomers include tris-(2-hydroxyethyl)isocyanurate triacrylate and bis-(methacryloxyethyl) phosphate. A fifth component of the emulsion pressure sensitive adhesive polymers is at least one unsaturated carboxylic acid containing from about 3 to about 5 carbon atoms and present in a total amount of up to about 1 0 percent by weight of the polymer, preferably from 1 to about 5 percent by weight. The unsaturated carboxylic acid includes, among others, acrylic acid, methacrylic acid, itaconic acid, mixtures thereof and the like.

In this embodiment, the emulsion pressure sensitive adhesive polymers are preferably prepared in the presence of a reactive surfactant which polymerizes during formation of the polymer and becomes an integral part of the polymer. Reactive surfactants include anionic vinyl functional monomers such as sodium vinyl sulfonate and sodium styrene sulfonate and the like.

The reactive surfactant is present as part of the total surfactant system and in an amount up to about 0.4 percent by weight of the total monomers, preferably about 0.1 to about 0.25 percent by weight.

The emulsion pressure sensitive adhesive polymers have a very broad glass transition temperature range of from 1 5 °C to 30°C, which is unusual among acrylic polymers. Depending on polymerization conditions, copolymers showing two distinct glass transition temperatures, have been observed. Gel content or percent insolubles are in excess of 60 percent by weight which provides excellent cohesive strength. In this regard, gel content represents the amount of polymer which is insoluble in tetrahydrofuran expressed in percent by weight and determined by the membrane gel partitioning method.

Polymer properties can be further modified to fit end use applications by inclusion of multifunctional monomers and the use of other chemical cross-linking agents. Other aids which may be used to develop cross-linking include thermal cross-linking and cross-linking by actinic and electron beam radiation.

In this embodiment, the emulsion pressure sensitive adhesive polymers are prepared by emulsion polymerization under conditions of agitation in an autogenous atmosphere in the presence of suitable polymerization initiators such as peroxydisulfate and peroxides. Depending on desired polymer properties including gel content, the preferred levels of these initiators are in the range of from about 0.5 to about 1 .0 percent by weight based on the total weight of the monomers. Initiators include potassium persulfate, t-butyl hydrogen peroxide, and the like. Level of agitation will vary depending on the system and will influence conversion. Typically, about 30 to 50 percent of the total initiator is added along with an initial monomer charge to the reactor, and the rest is added along with the balance monomers during polymerization over a period of from about 4 to about 5 hours. It is desirable to maintain the pH of the emulsion during polymerization between from about 2 to about 4, preferably from about 2.5 to about 4. This can be achieved by the use of buffers such as sodium bicarbonate and sodium acetate, typically in amounts up to 0.3 percent by weight based on the weight of the monomer.

The stabilizer system used during polymerization contains a combination of anionic and nonionic surfactants present in an amount up to about 3.5 percent by weight based on the weight of the monomers. A suitable anionic surfactant is the sodium salt of an ethoxylated nonylphenol sulfate, and a suitable nonionic surfactant is ethoxylated nonylphenol.

In this embodiment, the emulsion pressure sensitive adhesive polymers are produced at high solids level content, typically about 50 to about 70 percent by weight. Reaction is carried out at temperatures from 70 °C to 85 °C with an initial charge of up to about 1 0 percent by weight of the total monomers, with the balance of the monomers being added to the emulsion reaction system over a period of about 4 to about 5 hours, with total monomer conversion approaching 100 percent.

In a fourth embodiment, the emulsion pressure-sensitive adhesive polymers of the present invention are based on polymers which contain, on a percent by weight basis, from about 95% to about 97.5% by weight total of at least one alkyl ester of acrylic acid containing about 1 to about 10 carbon atoms in the alkyl group and at least one other component. A second monomeric component of this embodiment is a mixture of α, β unsaturated carboxylic acids present in a total amount of at least about 2.5 % by weight, preferably from about 2.5% to about 5% by weight. The unsaturated carboxylic acid may contain from about 3 to about 5 carbon atoms and includes among others, acrylic acid, methacrylic acid, itaconic acid and the like. Mixtures of acrylic acid and methacrylic acid in a respective weight ratio of about 1 : 1 to about 1 :3, preferably from about 1 : 1 to about 1 :2, may be used. Other monomers are not required to provide broad based emulsion adhesive systems but may be included. Styrenic monomers are, however, to be avoided in this embodiment. Gel content or percent insolubles of the polymer are at least about

50% by weight and typically range from about 50% to about 65% by weight which provides excellent cohesive strength without internal cross-linking or the use of multifunctional monomers.

In this fourth embodiment, the emulsion pressure-sensitive adhesive polymers have a glass transition temperature less than about -1 0°C, preferably less than about -30°C, and give, when combined with the anionic surfactant system, a good balance of adhesive properties at ambient and low temperatures.

In this fourth embodiment, the emulsion pressure-sensitive adhesive polymers are prepared by emulsion polymerization under conditions of agitation in an autogenous atmosphere in the presence of suitable polymerization initiators such as water soluble persulfates. Chain transfer agents such as alkyl mercaptans can be employed. Electrolytes can be used to stabilize the emulsion. Solids content varies depending upon the selected polymerization conditions. Polymerization conditions are desirably chosen to maintain grit levels low. Polymerization typically occurs under acid conditions and the emulsion formed is neutralized typically with ammonia to a final pH of from about 6 to about 6.5.

Polymers of this embodiment can be produced at a high solids level, at temperatures from about 70°C to about 85 °C, with use of an initial batch of monomers followed by addition of the balance of the monomers being added to the emulsion reaction system over a period of time.

The emulsion polymers are preferably formed in the presence of an anionic surfactant system present in an amount of from up to about 2.5% by weight based on the weight of the monomers and anionic surfactant system. In this embodiment, the anionic surfactant is present in a total amount of from about 1 % to about 2% by weight of the mixture and is a sodium alkyl ether sulfate of the formula:

CH₃-(CH₂)-_nCH₂O(CH₂CH₂O)_x SO₃Na

wherein n represents the number of — (CH₂)— groups normally in the range from about 6 to about 1 2 and x represents the number of repeating ethoxy groups normally in the range from 2 to about 30, preferably from about 20 to about 30. The anionic surfactant may be used in admixture with from about 0.1 % to about 0.2% by weight of a sodium dialkyl sulfosuccinate having from about 6 to about 1 8 carbon atoms in the alkyl group. The third anionic surfactant may be used and present in an amount from about 0.2% to about 0.3% by weight and is a disodium ethoxylated alkyl alcohol half ester of an sulfosuccinic acid in which the alkyl group contains from about 1 0 to about 1 2 carbon atoms and the like.

In a fifth embodiment, the emulsion pressure-sensitive adhesive polymers of the present invention contain, on a polymerized basis and based on the total weight of the polymer, the following monomers: (i) from about 35 to about 60 percent by weight of at least one alkyl acrylate containing from about 4 to about 8 carbon atoms in the alkyl group; (ii) from about 1 5 to about 35 percent by weight of at least one vinyl ester containing from 2 to about 1 6 carbon atoms in the alkyl chain of the ester; (iii) from about 1 5 to about 35 percent by weight of at least one diester of a dicarboxylic acid, wherein each alkyl group of the diester independently contains from about 6 to about 1 2 carbon atoms; and (iv) from 0 to about 5 percent by weight of an unsaturated carboxylic acid containing from about 3 to about 5 carbon atoms.

In this embodiment, the first monomeric component comprises at least one alkyl acrylate, and preferably a mixture of alkyl acrylates, present in a total amount of from about 35 to 60 percent by weight. The second monomeric component of the fifth embodiment comprises vinyl esters present in a total amount of from about 1 5 to about 35 percent by weight, more preferably from about 1 5 to about 20 percent by weight, with the vinyl esters containing from 2 to about 1 6 carbon atoms in the alkyl group of the ester. Suitable vinyl esters include vinyl acetate, vinyl butyrate, vinyl propionate, vinyl isobutyrate, vinyl valerate, vinyl versitate, and the like.

The third monomeric component of the fifth embodiment comprises one or more diesters of a dicarboxylic acid, wherein each alkyl group of the diester independently contains from about 6 to about 1 2 carbon atoms, the diesters being present in a total amount of from about 1 5 to about 35 percent by weight. Diesters include dioctyl esters such as di-2-ethylhexyl maleate, di-2-ethylhexyl fumarate, and mixtures thereof.

The fourth monomeric component of the fifth embodiment comprises up to about 5 percent by weight, preferably from about 1 to about 3 percent by weight, of at least one unsaturated carboxylic acid containing from 3 to about 5 carbon atoms. Such unsaturated carboxylic acids include acrylic acid, methacrylic acid, itaconic acid, β-carboxyethyl acrylate, mixtures thereof and the like.

Preferably, the emulsion pressure-sensitive adhesive polymers used in the emulsified pressure-sensitive adhesives of the present invention are prepared in the presence of a reactive surfactant which polymerizes during formation of the polymer and becomes an integral part of the polymer. Reactive surfactants include anionic vinyl functional monomers such as sodium vinyl sulfonate, sodium styrene sulfonate, and the like. The reactive surfactant is present as part of the total surfactant system in an amount up to about 0.5 percent by weight of the total monomers, preferably about 0.1 to about 0.25 percent by weight.

The proportions of monomers are adjusted in such a way that the polymer has a glass transition temperature (T_g) less than about -30°C, giving a good balance of adhesion and tack at room and low temperatures. The above-described emulsion pressure-sensitive adhesive polymers have a gel content (% insolubles) in the range of 50 to 70% by weight, which provides good cohesive strength without the use of multifunctional monomers. In this regard, gel represents the amount of polymer which is insoluble in tetrahydrofuran, expressed in % by weight and determined by the membrane gel partitioning method.

In a sixth embodiment, the emulsified pressure-sensitive adhesives of the present invention are prepared by combining certain acrylic based copolymers (as the emulsion pressure-sensitive adhesive polymer) with an emulsified tackifier and initially curing the combination by exposure to heat, chemical crosslinking agent, actinic radiation or electron beam radiation. This is known herein as primary cure.

The acrylic based copolymer is prepared by copolymerizing a mixture of monomers comprising at least one monomer selected from the group consisting of alkyl acrylate esters and alkyl methacrylate esters containing from about 4 to about 1 2 carbon atoms in the alkyl group and mixtures thereof; a glycidyl monomer selected from the group consisting of allyl glycidyl ethers; glycidyl acrylate esters, glycidyl methacrylate esters, and mixtures thereof, and an unsaturated carboxylic acid.

On a copolymerized basis, the copolymers typically comprise from about 55% to about 85% by weight of an alkyl acrylate and/or alkyl methacrylate ester containing 4 to about 1 2 carbon atoms in the alkyl group; from 0% to about 35% by weight of an alkyl acrylate and/or alkyl methacrylate ester containing 1 to about 4 carbon atoms in the alkyl group, from about 0.01 % to about 2% by weight of a glycidyl monomer; and a positive amount up to about 1 5%, preferably from about 5% to about 1 3% by weight of an unsaturated carboxylic acid.

In one embodiment, the uncured copolymers have a weight average molecular weight of at least about 200,000, preferably from about 200,000 to about 1 ,000,000, as determined by size exclusion chromatography using polystyrene as the calibrator. In another embodiment, the uncured copolymers have a weight average molecular weight from about 300,000 to about 900,000.

Examples of unsaturated carboxylic acids include acrylic, methacrylic, fumaric acid, mixtures thereof and the like. The emulsion copolymers of this sixth embodiment may be synthesized using emulsion polymerization techniques. Solids content during polymerization may typically range from about 40% to about 60% in order to achieve the desired weight average molecular weight. Reaction occurs in the presence of free radical initiators such as peroxide and azo compounds, preferably of the azo type, for example, 2-2'-azo-bis-isobutyronitrile.

The emulsified tackifier may be a waterborne tackifier dispersion. In one embodiment, the emulsified tackifier has a softening point from about 1 °C to about 1 00°C or below about 1 00°C, as determined by ASTM E 28-67 (ring and ball). In another embodiment, the emulsified tackifier has a softening point from about 5 °C to about 90°C or below about 90°C. In yet another embodiment, the emulsified tackifier has a softening point from about 5 °C to about 85 °C or below about 85°C. In still yet another embodiment, the emulsified tackifier has a softening point from about 25 °C to about 45 °C or below about 45°C. In one embodiment, the emulsified tackifier has a pH from about 6 to about 1 0. In another embodiment, the emulsified tackifier has a pH from about 7 to about 9. In yet another embodiment, the emulsified tackifier has a neutral or an alkaline pH.

In one embodiment, the emulsified tackifier is a waterborne tackifier dispersion containing from about 40% solids to about 95% solids. In another embodiment, the emulsified tackifier is a waterborne tackifier dispersion containing from about 50% solids to about 90% solids. In yet another embodiment, the emulsified tackifier is a waterborne tackifier dispersion containing from about 55 % solids to about 85% solids.

Examples of emulsified tackifiers include those under the trade designation Aquatac^®, and specifically Aquatac^® 41 74, Aquatac^® 41 88,

Aquatac^® 4201 , Aquatac^® 5527, Aquatac^® 5590, Aquatac^® 6025, Aquatac^® 6085, Aquatac^® 8005, Aquatac^® 9027 and Aquatac^® 9041 available from Arizona Chemical of Panama City, Florida.

The particular emulsified tackifier or the amount employed is selected to be compatible or substantially compatible with the emulsion pressure- sensitive adhesive polymer. As used herein, the term "substantially compatible" means that when the emulsified tackifier and emulsion pressure- sensitive adhesive polymer are combined, the resulting combination is substantially transparent, as opposed to cloudy, to normal vision. The emulsified pressure-sensitive adhesive and/or the emulsified tackifier of the present invention may be characterized by the absence of one or more of low boiling point components (boiling at less than about 75 °C, preferably less than about 85 °C), volatile components, organic solvents, low molecular weight tackifiers or tackifier components (less than 500 number average molecular weight), and other components which are believed to cause fogging.

The emulsified pressure-sensitive adhesive of the present invention may further comprise additives such as coloring agents, e.g., such as carbon black, titanium dioxide, organic dyes and pigments, fillers, plasticizer, diluents, thickening agents, and the like. Pigment, if desired, is provided in an amount sufficient to impart the desired color to the emulsified pressure- sensitive adhesive. Examples of pigments include solid inorganic fillers such as carbon black, titanium dioxide and the like, and organic dyes.

The emulsified pressure-sensitive adhesives of the present invention may be cured or primarily cured by exposure to heat, chemical crosslinking agent, actinic radiation, or electron beam radiation. The result is excellent tack and peel and a good balance of high temperature shear. In one embodiment of the invention, the adhesives are primarily cured by exposure to heat under drying conditions; i.e., the adhesives are cured at elevated temperatures sufficient to evaporate solvent(s) form the composition. Such temperatures typically range from about 70°C to about 1 20°C. In embodiments where the emulsified pressure-sensitive adhesives are primarily cured, "secondary cure" of the adhesives occurs when the adhesive is subsequently exposed to elevated temperatures, typically under use conditions. It should be understood that "secondary cure" can also occur, at least in part, under the conditions of the primary cure, when the latter is effectuated by exposing the adhesive to heat.

The activation temperature at which secondary cure occurs can be lowered by the incorporation into the copolymer of an "activator moiety" . As described above, it is preferable that activator moiety comprise a vinyl lactam, and more preferably N-vinyl pyrolidone, N-vinyl caprolactam, or mixtures thereof, but the invention is not limited solely thereto. Any chemical species which has the effect of lowering the activation temperature of the secondary cure and which is compatible with the tackified acrylic based adhesives of the present invention may be used. Typically, the activation temperature is no greater than about 1 20°C. By adjusting, the amount and type of activator moiety used, it is possible to "dial in" the temperature of the secondary cure.

In addition to the adhesive compositions described above, the present invention further provides both a transfer tape and tapes of layered construction, the latter consisting of a core coated on or both sides with a skin layer comprised of a tackified acrylic based copolymer of the instant invention.

Transfer tapes prepared in accordance with the present invention comprise a self-supporting film of the emulsified pressure-sensitive adhesive as described above, and at least one release liner. Thus, the emulsified pressure-sensitive adhesives may be coated on a suitable release liner, such as a silicone release coated paper, and stored as a roll. The release coated liner may contain one or more layers of a backing sheet (made of a polymeric film including polyolefin, polyester, etc.) and a silicon coating thereon. Various release coated liners are known in the art. Alternatively, a tape of layered construction can be prepared by coating a core, such as an acrylic based polymer matrix, on or both sides with a "skin layer" or tackified acrylic based pressure-sensitive adhesive of the type described above. The core may be a pressure-sensitive adhesive. The skin layer can be cured before or after being coated on or laminated to the acrylic core. One or more release liners "sandwich" the tape until removal prior to use.

The adhesives and tapes of the present invention exhibit excellent peel adhesion to a variety of substrates such as metals, plastics, and paints, and especially low energy plastics. While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore, it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.

Claims

What is claimed is:

1 . An emulsified pressure-sensitive adhesive, comprising: from about 50% to about 95% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5% to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5 °C to about 90°C and a solids content from about 40% to about 95% solids, with the proviso that the emulsified tackifier does not contain low boiling components.

2. The emulsified pressure-sensitive adhesive according to claim 1 , wherein the emulsified tackifier has a pH from about 6 to about 1 0.

3. The emulsified pressure-sensitive adhesive according to claim 1 , wherein the emulsified tackifier is a waterborne tackifier dispersion having a solids content from about 55% to about 85% solids.

4. The emulsified pressure-sensitive adhesive according to claim 1 , wherein the emulsion pressure-sensitive adhesive polymer comprises monomers of at least one of acrylate, methacrylate, methyl acrylate, ethyl acrylate, methylmethacrylate, 2-ethyl hexyl acrylate, isoctyl acrylate, butyl acrylates such as n-butyl acrylate, sec-butyl acrylate, and iso-butyl acrylate, methyl butyl acrylate, 4-methyl-2-pentyl acrylate, heptyl acrylate, and isodecyl methacrylate.

5. An emulsified pressure-sensitive adhesive, comprising: from about 55% to about 90% by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 10% to about 45% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5 °C to about 90°C, a pH from about 7 to about 9 and a solids content from about 40% to about 95% solids, with the proviso that the emulsified tackifier does not contain low boiling components.

6. The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 90 to about 99.5% by weight of an alkyl acrylate monomer comprising from about 4 to about 8 carbon atoms in the alkyl group, up to about 2% by weight of a polar monomer and from about 0.1 to about 0.3% by weight of a multifunctional acrylate monomer.

7. The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 35 to about 60% by weight of an alkyl acrylate monomer comprising from about 4 to about 8 carbon atoms in the alkyl group, from about 1 5 to about 35% by weight of an vinyl ester monomer, from about 20 to about 35% by weight of an diester of a dicarboxylic acid monomer, and from about 1 to about 5% by weight of an unsaturated carboxylic acid monomer containing from about 3 to about 5 carbon atoms.

8. The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 35 to about 60% by weight of an alkyl acrylate monomer comprising from about 4 to about 8 carbon atoms in the alkyl group, from about 1 5 to about 35% by weight of an vinyl ester monomer, from about 1 5 to about 30% by weight of an diester of a dicarboxylic acid monomer, from about 0.1 to about 1 % by weight of a reactive multifunctional monomer and from about 1 to about 1 0% by weight of an unsaturated carboxylic acid monomer containing from about 3 to about 5 carbon atoms.

9. The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 95 to about 97.5% by weight of an alkyl ester of acrylic acid monomer comprising from about 1 to about 10 carbon atoms in the alkyl group, from about 2.5% to about 5% by weight of an α,β-unsaturated carboxylic acid monomer.

1 0. The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 35 to about 60% by weight of an alkyl acrylate monomer comprising from about 4 to about 8 carbon atoms in the alkyl group, from about 1 5 to about 35% by weight of an vinyl ester monomer comprising from about 2 to about 1 6 carbon atoms in the alkyl chain, from about 1 5 to about 35% by weight of an diester of a dicarboxylic acid monomer having an alkyl chain containing about 6 to about 1 2 carbon atoms, and from about 0 to about 5% by weight of an unsaturated carboxylic acid monomer containing from about 3 to about 5 carbon atoms.

1 1 . The emulsified pressure-sensitive adhesive according to claim 5, wherein the emulsion pressure-sensitive adhesive polymer comprises from about 55 to about 85% by weight of an alkyl acrylate and/or alkyl methacrylate ester monomer comprising from about 4 to about 8 carbon atoms in the alkyl group, from about 0.01 to about 2% by weight of a glycidyl monomer and from about 5 to about 1 3% by weight of an unsaturated carboxylic acid monomer.

1 2. The emulsified pressure-sensitive adhesive according to claim 5, wherein the low boiling components boil at less than about 75°C.

1 3. A transfer tape comprising: a release liner, and an emulsified pressure-sensitive adhesive on at least one side of the release liner, the emulsified pressure-sensitive adhesive comprising: from about 50% to about 95 % by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5% to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5°C to about 90°C and a solids content from about 40% to about 95% solids, with the proviso that the emulsified tackifier does not contain low boiling components.

14. The transfer tape of claim 1 3, wherein the release liner is a silicone coated release liner.

1 5. The transfer tape of claim 1 3, wherein the release liner comprises from about 25 to about 1 50 g/m² of the emulsified pressure- sensitive adhesive on each coated side.

1 6. The transfer tape according to claim 1 3, wherein the low boiling components boil at less than about 85°C.

1 7. A method of bonding a tape to a low energy surface, comprising: applying the tape to the low energy surface, the tape comprising a substrate layer and a layer of an emulsified pressure-sensitive adhesive, the emulsified pressure-sensitive adhesive comprising: from about 50% to about 95 % by weight of an emulsion pressure-sensitive adhesive polymer, wherein the emulsion pressure-sensitive adhesive polymer is an acrylic based polymer; and from about 5% to about 50% by weight of an emulsified tackifier, the emulsified tackifier having a softening point from about 5°C to about 90°C and a solids content from about 40% to about 95 % solids, with the proviso that the emulsified tackifier does not contain low boiling components.

14. The method of claim 1 7, wherein the substrate layer is one of a release liner and a polymeric film.

1 5. The method of claim 1 7, wherein the low energy surface is a polyolefin.

20. The method of claim 1 7, wherein the low energy surface is one of polyethylene, polypropylene, and acrylonitrile-butadiene-styrene terpolymer.