REMOVABLE PRESSURE-SENSITIVE ADHESIVE TAPE

Abstract: The pressure-sensitive adhesive layer of the novel tape, like that of Silver U.S. Patent No. 3,922,464, comprises a copolymer of alkyl acrylate such as isooctyl acrylate and a small amount of emulsifier monomer. The novel tape differs from that of Silver by incorporating tackifier resin, thus better resisting lifting forces at elevated temperatures while also being cleanly removable.

Description: Field of the Invention The invention concerns pressure-sensitive adhesive tapes, which usually are slit to narrow widths and then wound upon themselves for storage and shipment. Such tapes are originally produced in wide widths, and the term "tape" encompasses such wide widths. The invention specifically concerns removable pressure-sensitive adhesive tapes such as masking tapes, weatherstripping tapes, label tapes, and protective plating tapes. Background Art The problem of developing a masking tape having good heat resistance is discussed in a general way in Satas: "Handbook of Pressure-Sensitive Adhesive Technology", Van Nostrand Reinhold Co., 1982, pages 203-207. After noting that "conventional natural rubber adhesives" do not have the required temperature resistance for a masking tape, the handbook says: "To impart the required degree of heat resistance to the adhesive, it is usual to effect a degree of crosslinking to the natural rubber. We have seen earlier that the tack and adhesion of pressure-sensitive adhesives depend upon the viscoelastic properties of the rubber-resin blend. Consequently, the degree of crosslinking has to be carefully controlled; if the degree of curing is too high, then the adhesive will lose its ability to deform readily and hence wet the surface to which it is being applied, but it has to be sufficiently cured to have resistance to the paint solvent, and also not to be softened too much by the stoving cycle" (page 203). As pointed out in U.S Patent No. 3,922,464, "Early pressure-sensitive adhesives were based on organic solvent solutions of natural or synthetic rubber tackified by some type of resinous material. These so called 'rubber-resin' adhesives are still widely used in masking tape, one major reason being that they can be formulated so as not to display adhesion buildup, i.e., their adhesion to the membrane does not increase to the point where the tape cannot be removed cleanly, even after exposure to the heat of a drying or curing oven." (col. 1, lines 40-48). The patent then alludes to problems of batch-to-batch consistency of rubber-resin adhesives and to staining problems, indicating that these problems are avoided by a wholly synthetic adhesive system based on a copolymer of (a) vinyl monomer such as iso-octyl acrylate, (b) a small amount of vinyl-unsaturated, homopolymerizable emulsifier monomer which is a surfactant such as sodium-2-sulfoethylmethacrylate, and preferably (c) a small amount of zwitterion monomer having a free radical-polymerizable unsaturated group. The patent also recommends that the adhesive layer contain a small amount of external emulsifier such as sodium dodecyl benzene sulfonate. The patent reports "the absence of adhesion increase, or 'buildup' when tape made with such an adhesive is applied to an enameled or lacquered surface. The adhesion stability is observed even when the tape, after application, is subjected to temperatures of 250°F (about 120°C) for 1 hour or for longer periods of time at lower temperatures ..." (col. 3, lines 2-10). Preferred tapes of the patent have been cleanly removed from enameled or lacquered surfaces after one hour at 150°C, a temperature commonly used in automotive painting. In order to reduce solvent emissions, the automotive industry is extensively converting to basecoat/clearcoat paint systems (usually acrylic enamel systems) to which pressure-sensitive adhesives are less adherent. Preferred tapes of the aforementioned patent would not remain well adhered at 150°C when subjected to the sort of lifting forces that are encountered when a masking tape is applied to a curved surface and/or is used to hold an apron. Another widely used automotive paint involving lifting problems at 150°C of the aforementioned tapes and other prior masking tapes is "50J" acrylic enamel of Ford Motor Co. Both the pressure-sensitive adhesive of the above-cited patent and that of U.S. Patent Re. 24,906 are copolymers of nontertiary alkyl acrylates such as 2-ethylhexyl acrylate, but the latter adhesive is a copolymer of such an acrylate and a small proportion of a highly polar monomer such as acrylic acid which produces adhesion buildup. This adhesive is thus not suitable for masking tapes, even though U.S. Patent No. 3,008,850, which concerns acrylates copolymerized with small proportions of acrylonitrile or methacrylonitrile, reports that a strip of the pressure-sensitive adhesive tape of Example 1 was cleanly stripped from an enameled automotive panel after 30 minutes at 121°C. U.S. Patent No. 4,418,120 states: "It has long been recognized that adhesives that consist essentially of a copolymer of alkyl acrylate and a minor proportion of copolymerizable monomer such as acrylic acid do not require a tackifying resin and are able to resist aging; thus, such adhesives have advantages over the earlier and more traditional rubber-resin adhesives; cf. Ulrich U.S. Pat. No. 24,906" (col. 1, lines 54-60). Patent No. 4,418,120 is concerned with the need to improve both the adhesion and the heat resistance at 70°C of such an acrylate:acrylic acid adhesive when applied to low energy surfaces, doing so by employing a tackifying resin for better adhesion and by crosslinking the tackified pressure-sensitive adhesive for better heat resistance at 70°C. This patent does not suggest that its tapes would be removable. Neither do other patents and publications which mention that tackifiers can be used with acrylate adhesives. For example, see U.S. Patents No. 3,740,366 and 4,077,926. Also, see page 89 of Jones: "Elastomers and Resin Modifiers for Water-based Adhesives", Proceeding Today's Technology , Holiday Inn O'Hare, Rosemont, IL, June 23-24, 1981, which notes that copolymers of the U.S. Patent Re. No. 24,906 "have typically been used without tackifying resins or with quite low levels" but that tackifying resins can "contribute significantly to probe tack, peel and quick stick." U.S. Patents No. 3,931,087 and 4,012,560 are concerned with pressure-sensitive adhesive tape based on a copolymer like those of the above-cited Silver patent except employing a specific sulfonic acid as the emulsifier monomer. Each of these patents closes with a paragraph which says that the invention contemplates the use of various modifiers in the pressure-sensitive adhesive resins, including tackifiers. The function of those tackifiers is not mentioned. Disclosure of Invention The invention provides what is believed to be the first nonstaining automotive masking tape that reliably resists typical lifting forces at 150°C from automotive paint and yet can reliably be stripped off cleanly from automotive paint after one hour at 150°C in the "Removal Test" described below. By "typical lifting forces" are meant those simulated in the "Curved Panel Lifting Test at 150°C" described below. The novel tape is similar to that of the U.S. Patent No. 3,922,464 in that it comprises a backing sheet carrying a layer of pressure-sensitive adhesive which comprises a copolymer of monomers comprising (a) from 95 to 99.8 parts by weight of at least one terminally unsaturated vinyl monomer, 60 to 100 weight percent of said vinyl monomer being selected from the class of nontertiary alkyl acrylates wherein each alkyl group has at least half of its carbon atoms in a single chain and the average length of the alkyl chain is at least 4 and not more than 12, and (b) from 0.2 to 5 parts by weight of at least one vinyl-unsaturated, homopolymerizable emulsifier monomer which is a surfactant having both a hydrophobic and a hydrophilic moiety, contains at least 5 but not more than 40 carbon atoms, and is water-dispersible, the total parts by weight of (a) plus (b) being 100. The novel tape differs from that of the Patent No. 3,922,464 in that its layer of pressure-sensitive adhesive comprises from 5 to 50% by weight of tackifier resin selected from at least one of (a) hydrogenated rosin ester, (b) polyterpene, (c) polymerized alkyl styrene, and (d) polymerized petroleum-derived monomer resin. Preferably the tackifier resin comprises from 10 to 40% by weight of the pressure-sensitive adhesive layer, because when below 10%, the tape is less likely to pass the Curved Panel Lifting Test at 150°C, and when above 40%, the tape is less likely to be stripped off cleanly. Specific tackifier resin dispersions which have been shown to be useful in the invention are: Hydrogenated rosin esters Resin A Phthalate ester of technical hydroabietyl alcohol, having a ring and ball softening point of 63°C and an acid number of 8. Resin C Glycerol ester of highly stabilized rosin, having a ring and ball softening point of 82°C. and an acid number of 9. Resin D Pentaerythritol ester of hydrogenated rosin, having a ring and ball softening point of 104°C. and an acid number of 12. Polyterpenes Resin B Pale-colored, inert, low-molecular-weight, thermoplastic hydrocarbon resin produced from the terpene monomer, alpha-pinene, having a ring and ball softening point of 112-118°C. Polymerized alkyl styrenes Resin E Water-white, highly color-stable, nonpolar, low-molecular-weight, thermoplastic hydrocarbon polymer derived largely from alpha-methylstyrene. Polymerized petroleum-derived monomer resins Resin F Light-color, thermoplastic, low-molecular-weight, petroleum-derived monomer hydrocarbon resin derived largely from mixed monomers of petroleum origin. Preferably the amount of the emulsifier monomer (b) used in making the pressure-sensitive adhesive of the novel tape is from 1 to 4 parts by weight. Below one part, the tape would be less likely to pass the Removal Test. Above 4 parts might involve polymerization problems. Preferred as the emulsifier monomer (b) is a monovalent salt of a styrene sulfonate such as sodium styrene sulfonate. As compared to emulsifier monomers named in the U.S. Patent No. 3,922,464, a monovalent salt of a styrene sulfonate more reliably produces substantially coagulum-free latices; both the latices and polymers produced therefrom tend to have greater hydrolytic stability; and latices of higher solids can be obtained which are easier to coat. By substantially coagulum-free is meant a latex which neither hangs up on a stirrer nor prematurely plugs up a filter. U.S. Patent No. 3,922,464 teaches the desirability of also employing "small amounts of external emulsifier, especially where short chain emulsifier monomers are used. Such emulsifiers, which are commonly employed in the preparation of many copolymer latices, assist in initial suspension of the monomeric reactants and tend to result in a more homogeneous appearing emulsion than when they are omitted" (col. 8, lines 39-48). While also not needed in the present invention, at least 0.05 weight percent of external emulsifier would be preferred in commercial production for the same reason. Preferably the external emulsifier is noncationic. Also applicable to the present invention are the following teachings from the U.S. Patent No. 3,922,464: "The acrylic acid esters useful in the practice of this invention are the esters of alcohols which, on a mole basis contain an average of more than 4 to not more than 12 carbon atoms, preferably from 6 to 12 carbon atoms, the majority of the esters being derived from alcohols containing 4-12 carbon atoms. Examples of suitable esters are the acrylic acid esters of non-tertiary alcohols such as: 2-methylbutanol, 3-methylbutanol, 2-ethyl-butanol, 4-methylpentanol, 2-propyl-pentanol, n-hexanol, 2-methylhexanol, 4-methylhexanol, 2-hexanol, 2-ethylhexanol, 4-ethylhexanol, 4-methyl-2-pentanol, 2,3,4-triethylhexanol, 2-ethyl-3-methylhexanol, 2-heptanol, 3-heptanol, 3-methylheptanol, 2-ethylheptanol, 2-methyl-4-ethylheptanol, n-octanol, isooctanol, n-decanol, n-dodecanol, 10-chloro-decanol, 6-methoxyhexanol, etc., and mixtures thereof. "If the average number of carbon atoms in the longest alkyl chain of the alcohols from which the acrylate esters are derived does not exceed 4, the adhesive tends to be harsh, raspy, and difficult to adhere quickly. If, however, at least 70% of the terminally unsaturated vinyl monomer is a suitable acrylate, short chain methacrylate monomers can be incorporated. If the average number of carbon atoms in the longest alkyl chain exceeds 12, the adhesive tends to become waxy and lack sufficient adhesion. "For various reasons it may be desirable to incorporate, as part of the terminally unsaturated vinyl monomeric portion of the adhesive systems, any of several modifying comonomers which are not emulsifier monomers. For example, acrylonitrile imparts firmness and solvent resistance, styrene and alpha-methyl styrene impart firmness and improve peel adhesion, t-butyl-styrene improves tack, methylmethacrylate makes the adhesive firmer, octyl vinyl ether softens the adhesive, vinyl acetate improves adhesion to certain plastic surfaces, etc." (Col. 3, line 34, through col. 4, line 2). Unlike the adhesives of all but one of the examples in U.S. Patent No. 3,922,464, zwitterion monomer preferably is not used in making tapes of the invention because such monomers tend to be expensive to make, and most of the zwitterion emulsifier monomers used in the patent examples are not currently commercially available. Furthermore, at only the 2% weight level used in most of the Silver examples, a zwitterion emulsifier monomer might raise the price of a tape above what potential users would be willing to pay. The objectives of the invention are readily achieved without any need for zwitterion monomer. Testing Before carrying out the tests described below, the tape should be held at 22°C and 50% relative humidity for at least 24 hours and preferably more than 48 hours. Curved Panel Lifting Test at 150°C Used in this test is an aluminum panel having a radius of curvature of 23 cm and a length of 35.5 cm in the curved direction. An automotive paint covers the convex surface of the panel. In order to provide a rigorous test, the paint preferably is one to which masking tapes are difficultly adherent, e.g., an automotive basecoat/clearcoat (BC/CC) acrylic enamel paint system or the automotive "50J" acrylic enamel paint of Ford Motor Co. Tapes to be tested have a backing sheet commonly used in masking tapes, namely a creped paper weighing about 50 g/m² and saturated with an acrylic latex. A latex of the pressure-sensitive adhesive to be evaluated is knife-coated onto one face of the backing sheet using a 0.1 mm orifice, allowed to dry at room temperature for about 10 minutes, and then dried in an oven for about 5 minutes at 95°C. The resulting tapes are cut to 2.54 x 35 cm. A 5.08 x 30 cm strip of nonadhesive masking paper (such as is used for automotive paint aprons) is applied to cover 1.27 cm of the adhesive layer except for 2.5 cm at each end of the tape. This assembly is hand applied to the aluminum panel in its curved direction, using the uncovered 1.27 cm of the adhesive layer to adhere it to the painted surface. The assembly-bearing panel is then put into an air-circulating oven at 150°C for 10 minutes, allowed to cool, and examined for failures. A rating of "pass" means no lifting has occurred. Any lifting at either end of the strip is noted as "end failure", and the total length of tape which has lifted is noted. Any lifting along the length of the strip is noted as "edge lifting", and the cumulative lengths over which edge lifting is noted is reported in % of the total length of the strip or strips tested. Removal Test Used in this test is a tin-plate panel, the test surface of which has a white, glossy, stain-resistant acrylic enamel paint (E. I. duPont 963-AM-263), an automotive paint noted for difficult removability of masking tape, being far more difficult in this respect than either the basecoat/clearcoat paint system or the "50J" paint preferred for the Curved Panel Lifting Test. Tapes prepared as in the Curved Panel Lifting Test are cut to 1.27 x 10.16 cm and adhered at room temperature by their adhesives to the test surface, followed by two passes of a 2-kg rubber-coated metal roller. After one hour in an air-circulating oven at either 121°C or 150°C, the tape is peeled back at an angle of 45° at an approximate rate of 1.9 m/min., after which the panel is removed from the oven and examined. A nontacky deposit is reported as "R" for "residue". Tapes experiencing up to 100% residue are considered to be marginally acceptable. When there is a tacky deposit, the tape is examined to determine whether the adhesive split cohesively, reported as "S", or failed adhesively from the backing, reported as "A". Both are unacceptable, except that tapes which leave a deposit only at the start of the peel (up to 5%) are considered to be acceptable. Any tape having a value of 0 in the Removal Test would leave no residue if the test surface were either the basecoat/clearcoat paint system or the "50J" paint preferred for the Curved Panel Lifting Test. Copolymers used in making representative tapes of the invention include: Acrylic Copolymer A A 2,000-ml split-resin flask was fitted with a variable speed agitator, condenser, purging tube for introducing nitrogen, and a recording controller. The following materials were added to the flask while purging the flask with nitrogen: grams Deionized water 479.6 Sodium bicarbonate 0.48 Sodium dodecyl benzene sulfonate 1.18 Sodium styrene sulfonate 15.68 Isooctyl acrylate 329.21 N-tert-octylacrylamide 47.03 The solid N-tert-octylacrylamide was dissolved in the isooctyl acrylate before adding to the flask. The nitrogen purge was continued until the end of the run. The flask and its contents were heated to 40°C, at which temperature a first initiator charge of 0.24 g of potassium persulfate and 0.08 g of sodium meta-bisulfite was added. A change in color of the emulsion indicated the start of polymerization. The temperature was held near 40°C by cooling and heating during the remaining period of reaction. 1.6 ml of 5% aqueous solution of sodium meta-bisulfite was added four hours after the first initiator charge, and 2.4 ml of a 5% aqueous solution of sodium meta-bisulfite and 2.4 ml of a 5% aqueous solution of potassium persulfate were added 7 hours after the first initiator charge. The emulsion was heated another 7-1/2 hours to complete polymerization. Solids - 44.4% pH - 8.0 Viscosity - 0.051 Pa.s, Brookfield #1 spindle, 30 rpm. ACRYLIC COPOLYMER B To a 5000-ml split-resin flask equipped as in Example 1 were added the following: Grams Deionized water 1606.0 Sodium bicarbonate 2.4 Sodium styrene sulfonate 20.0 Isooctyl acrylate 1706.8 N-tert-octylacrylamide 240.0 Sodium dodecyl benzene sulfonate 6.0 The flask and its contents were purged with nitrogen while stirring and heating to 40°C, and a first initiator charge of 1.2 g potassium persulfate and 0.4 g sodium meta-bisulfite was added. Polymerization started in about 1/2 hour. The temperature was held at 40°C by cooling and heating as required during polymerization. 8 ml of a 5% aqueous solution of sodium meta-bisulfite was added 5 hours after the first initiator charge, and 12 ml of a 5% aqueous solution of sodium meta-bisulfite and 12 ml of a 5% aqueous solution of potassium persulfate were added 7 hours after the first initiator charge. The emulsion was heated another 9 hours to complete the polymerization. Solids - 54.8% pH - 8.77 Viscosity - 1.348 Pa.s, Brookfield #3 spindle, 30 rpm. ACRYLIC COPOLYMER C A 1000-ml split-resin flask was fitted with a variable speed agitator, condenser, purging tube for introducing nitrogen, and a recording controller. The following materials were added to the flask while purging the flask with nitrogen: The solid N-tert-octylacylamide and the methacrylimide were dissolved in the isononyl acrylate before adding to the flask to which the deionized water, sulfopropylmethacrylate, and sodium dodecyl benzene sulfonate had previously been added. A nitrogen purge was continued until the end of the run. The flask and its contents were heated to 40°C, at which time the solution of FeSO₄ was added, followed by addition of the mercaptan solution, after which 0.68 g of potassium persulfate and 0.34 g of sodium meta-bisulfite were added. A change in color of the emulsion indicated the start of polymerization. The temperature was held near 40°C by cooling and heating during the remaining period of reaction. The reaction was continued for four hours and twelve minutes. After this time, solids approached theoretical and reaction was judged to be essentially complete. Solids - 56.1% pH - 6.18 Viscosity - 1.520 Pa.s, Brookfield #3 Spindle 30 rpm ACRYLIC COPOLYMERS D-L Acrylic Copolymers D-L were prepared using the same procedure as used for preparing Acrylic Copolymer A and are listed in Table I which gives parts by weight of each monomer charged. In the following examples, all parts are by weight except as noted. Example 1 To the dispersion of Acrylic Copolymer A was added the above-identified Resin A dispersion in an amount comprising 5% by weight of solids. The mixture was blended by hand with a spatula for about fifteen seconds and then applied to biaxially oriented poly(ethyleneterephthalate) film backing (38-micrometer thickness) via a knife-over-web applicator using a 102-micrometer orifice. The coating was allowed to air dry for about ten minutes and then in a forced air oven at 104°C for five additional minutes. The dried, coated tape was removed from the oven and covered with polyvinylchloride liner until subjected to testing. Examples 2-140 Additional tapes were prepared as in Example 1 except as indicated in Table II. Examples C-1 through C-12 at the end of the table are comparative. Results of testing the tapes of Examples 1-140 and C-1 throgh C-12 are reported in TAble III.

Claims: Pressure-sensitive adhesive tape comprising a backing sheet carrying a layer of pressure-sensitive adhesive comprising a copolymer of monomers comprising (a) from 95 to 99.8 parts by weight of at least one terminally unsaturated vinyl monomer, 60 to 100 weight percent of said monomer being selected from the class of nontertiary alkyl acrylates wherein each alkyl group has at least half of its carbon atoms in a single chain and the average length of the alkyl chain is at least 4 and not more than 12, and (b) from 0.2 to 5 parts by weight of at least one vinyl-unsaturated homopolymerizable emulsifier monomer which is a surfactant having both a hydrophobic and a hydrophilic moiety, contains at least 5 but not more than 40 carbon atoms, and is water-dispersible, the total parts by weight of (a) plus (b) being 100, wherein the improvement comprises: the layer of pressure-sensitive adhesive comprises 5 to 50% by weight of tackifier resin selected from at least one of (a) hydrogenated rosin ester, (b) polyterpene, (c) polymerized alkyl styrene, and (d) polymerized petroleum-derived monomer resin. Pressure-sensitive adhesive tape as defined in claim 1 wherein the tackifier resin comprises from 10 to 40 percent by weight of said adhesive layer. Pressure-sensitive adhesive tape as defined in claim 1 wherein the amount of emulsifier monomer (b) is from 1 to 4 parts by weight. Pressure-sensitive adhesive tape as defined in claim 3 wherein the emulsifier monomer (b) is a monovalent salt of a styrene sulfonate. Pressure-sensitive adhesive tape as defined in claim 4 wherein the monovalent salt is sodium styrene sulfonate. Pressure-sensitive adhesive tape as defined in claim 3 wherein said monomers are free from zwitterion monomer. Pressure-sensitive adhesive tape as defined in claim 3 wherein the adhesive layer includes a small amount of an external emulsifier. The adhesive of a pressure-sensitive adhesive tape as defined in claim 1.

Inventors: BENNETT, RICHARD E. C/O MINNESOTA MINING AND, OVERSTREET, THOMAS S. C/O MINNESOTA MINING AND, WINSLOW, LOUIS E. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: C09J7
IPC codes: C09J133, C09J7