Abstract:
Pressure-sensitive adhesive tape is made by coating a sheet backing with a solution of iso-octyl acrylate:acrylic acid copolymer containing a tackifying rosin ester and an anti-oxidant, evaporating the solvent, and crosslinking the adhesive. The resultant adhesive has an excellent balance of the tack, peel adhesion, and shear properties, even at 70° C., and adheres well to low energy surfaces.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to pressure-sensitive adhesive-coated sheet material and is especially concerned with pressure-sensitive tapes, including adhesive transfer tape, which have excellent tack, excellent shear properties at both room temperature and elevated temperatures, and good adhesion to steel and a variety of polymeric substrates. 
     Three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack, peel, and shear. The first measures the adhesive&#39;s ability to adhere quickly, the second its ability to resist removal by peeling, and the third its ability to hold in position when shearing forces are exerted. Generally speaking, the first two are directly related to each other but are inversely related to the third; cf., e.g., Sanderson U.S. Pat. No. 4,077,926, Col. 1, lines 31-36. 
     Modern industry presents a number of situations where pressure-sensitive adhesive products having balanced properties (i.e., excellent tack, peel, and shear) would be highly desirable. For example, to attach gasketing and weatherstripping materials in automotive and other industrial applications, there is a need for a pressure-sensitive adhesive which possesses a high tack as well as a high shear under such environmental conditions as will be encountered in use as the result of high and low temperatures, humidities, etc. A typical application involves the application of neoprene door and trunk lid gaskets to automobiles; the gasketing is continually under stress, often at high temperatures, making it essential that the mounting adhesive possess both high shear and high adhesion. Another application involves the lamination of sound-absorbing foam in truck cabs and electronic equipment cabinets; the foam, which is curved to fit the available space, seeks to return to its original dimensions, thereby exerting a constant stress on the adhesive. Still other demanding applications for pressure-sensitive adhesives include the industrial processing of film, fabric, carpet, or paper webs which must be spliced so as to retain their integrity when thereafter passed through high temperature ovens, the lamination of non-slip pads to polyethylene surf boards, the bonding of polyvinyl fluoride wall coverings to bulkheads in passenger aircraft, the adhesion of polyolefinbacked carpet to metallic substrates, etc. As a result of these needs, many persons have tried to prepare a pressure-sensitive adhesive having balanced properties (i.e., excellent tack, peel and shear), especially at elevated temperatures. 
     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. Re. 24,906. Although acrylate pressure-sensitive adhesives can be prepared as either solutions or aqueous emulsions, the latter are typically employed because of the higher molecular weight and percentage solids attainable at a practical coating viscosity. 
     The early so-called acrylate adhesives were somewhat heat-sensitive (cf., e.g., Bemmels U.S. Pat. No. 3,617,362 and 3,707,518), and crosslinking has been proposed. Unfortunately, the improved heat resistance and holding power obtained by crosslinking have been generally offset by the loss in tack. Sanderson U.S. Pat. Nos. 3,740,366 and 4,077,926 discuss improving heat resistance of acrylate adhesives by crosslinking with, e.g., zinc octoate or trivalent chromium ion; the adhesion and shear values obtained are not high enough for use in, e.g., the automotive applications discussed above. These patents also suggest that such conventional additives as tackifiers may be included but do not enlarge on this point. 
     Maska U.S. Pat. No. 3,701,758 discloses a pressure-sensitive adhesive formed from cyanoacrylate polymers which is said to have a balance of tack, adhesion, and cohesion, even at temperatures of 50°-100° C. This adhesive may (but usually does not) include a tackifier, and no mention is made of crosslinking; its low tack and adhesion values render it unsatisfactory for bonding firmly to such low energy surfaces as neoprene or polyethylene. Jubilee U.S. Pat. No. 3,222,419 discloses a terpolymer of vinyl acetate, acrylate or methacrylate, and a crosslinkable resin, the resultant product being said to have a balance of tack, adhesion and cohesion, as well as the ability to adhere to a wide variety of substrates; unfortunately, however, the adhesive has low heat resistance. Hydrogenated rosin ester may be included as a tackifying agent; this adhesive does not, however, perform well at elevated temperatures. 
     Samour U.S. Pat. Nos. 3,299,010 and 3,400,103 disclose pressure-sensitive adhesives of the acrylate:acrylic acid family which may be cured or crosslinked, suggesting that any of a wide variety of tackifying resins might be included but failing to provide any details; it appears that these patents illustrate the general principle that it has heretofore been possible to improve one adhesive property only at the sacrifice of others. Published European Patent Application EPA 79900506.1 (based on PCT application U.S. 79/00288, filed Apr. 30, 1979, published Nov. 29, 1979) describes an acrylate:acrylic acid adhesive which is prepared in solution and crosslinked with 0.002-0.2 weight percent Cr +++  to obtain a product having good initial adhesion and internal strength at high temperatures. This adhesive, however, does not have sufficiently high adhesion for use on some or all of such low energy surfaces as acrylonitrile:butadiene:styrene (ABS) terpolymer, low density polyethylene and isotactic polypropylene. 
     In recent years acrylate-based adhesives that possess a balance of high tack, high peel, and high shear properties have been prepared by coating a blend of such monomers as iso-octyl acrylate and acrylic acid on a backing, maintaining an inert atmosphere, and polymerizing the blend in situ. The commercial preparation of such tape products, however, requires a substantial investment in unconventional manufacturing equipment. Further, even these products have not possessed the desired uniformly excellent adhesion to low energy polymeric substrates. 
     Prior to the present invention, then, there has been a need for pressure-sensitive adhesive tape products which not only have balanced properties at wide temperature ranges and adhere well to low energy surfaces but also can be prepared easily and economically on conventional equipment. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides pressure-sensitive adhesive products which not only display balanced properties at elevated temperature but also can be made simply and economically. These adhesive products have excellent adhesion to low energy surfaces. In accordance with the invention, there is provided normally tacky and pressure-sensitive adhesive tape comprising a sheet material having, on at least one surface, a thin layer of normally tacky and pressure-sensitive adhesive containing trace amounts of solvent and consisting essentially of a blend of 
     a. a crosslinked pressure-sensitive copolymer of 100 parts by weight of monomers consisting essentially of iso-octyl acrylate (or 2-ethyl hexylacrylate) and 3-7 parts by weight of acrylic acid, said copolymer, prior to crosslinking, being soft, tacky, and having an inherent viscosity on the order of 0.75-1.5 dl/g, 
     b. a small amount of an antioxidant that is based on either a hindered phenol or sulfur-containing organo-metal salt, and 
     c. a tackifying rosin ester, said adhesive being characterized by a rolling ball tack value of less than 6 cm; an ability to bond firmly to stainless steel, ABS terpolymer, low density polyethylene, and isotactic polypropylene; and a shear value of at least 5,000 minutes at 70° C. 
     A presently preferred type product according to the invention is made by 
     a. blending 
     (1) an organic solvent solution of pressure-sensitive iso-octyl acrylate:acrylic acid copolymer having an inherent viscosity on the order of 0.75-1.5 dl/g, 
     (2) an effective amount of hindered phenol antioxidant, and 
     (3) a tackifying rosin ester, 
     b. adding an organic solvent solution of N,N&#39;-bis-1,2-propyleneisophthalamide crosslinker and continuing to blend the ingredients to form a reactive adhesive solution, 
     c. coating a thin layer of the adhesive solution on a sheet backing, and 
     d. heating the coated sheet backing to remove most of the solvent and react the copolymer and crosslinker. 
     In products and processes of the invention, the adhesive is typically permanently bonded to the sheet backing. For some applications, however, the sheet backing may be a release liner, so that the finished product is a so-called transfer tape; in use, the exposed adhesive surface of this tape is placed in contact with a desired substrate, the release liner stripped away, and the newly exposed adhesive surface bonded to a second substrate. 
     As is evident from the process of the invention, the copolymer is provided in organic solvent solution rather than in aqueous emulsion. The latter contain surfactants, which complicate formulating the tape of the invention because of interference with certain crosslinking agents. Further, a surfactant in an adhesive tends to migrate to the surface, where it often reduces bond strength. Thus, adhesives used in preparing tapes of the invention may be characterized as at least substantially (preferably completely) surfactant-free. 
    
    
     DETAILED DESCRIPTION 
     Test Procedures 
     A number of industry-recognized tests are commonly employed to characterize the performance of normally tacky and pressure-sensitive adhesive tapes. A brief summary of the tests employed to evaluate adhesives herein is described below: 
     Peel Adhesion. The test is a modification of PSTC-1, described in &#34;Test Methods for Pressure-Sensitive Tapes,&#34; 7th Edition, Pressure-Sensitive Tape Council, Glenview, Ill., 1976. A solution of the adhesive to be tested is coated on a release liner in a quantity such that, upon evaporation of the solvent, the weight of adhesive remaining is approximately 100 g/m 2 . The exposed surface of the adhesive is then laminated to a 50-micrometer thick aluminum foil that has been primed by coating with epoxidized polybutadiene reacted with dimethylamine, thereby forming a pressure-sensitive adhesive tape. Strips 15 cm long and 2.54 cm wide are cut from the tape and placed in contact with the substrate to which adhesion is to be determined. The liner is removed and, using two passes of a 2.05 kg roller, the adhesive surface of the tape is forced into contact with the substrate. One end of the tape is doubled back on itself at 180°, the exposed portion of the substrate being clamped in the upper jaws of a tensile testing machine while the doubled-back end of the tape is clamped in the lower jaws. The jaws are then separated at a rate of 30.5 cm/minute, the force to effect separation being reported in g/cm width. Where the substrate is stainless steel, this force should be at least 800 (preferably 1,100) g/cm width for adhesives useful in practicing the present invention. Where the substrate is ABS terpolymer, low density polyethylene, or isotactic polypropylene, the force required to effect removal should be at least 700 (preferably 1,000) g/cm, 560 (preferably 670) g/cm, and 670 (preferably 780) g/cm respectively. 
     Tack Rolling Ball Test. In this test an 11-mm diameter steel ball is rolled down a plane having a length of 18 cm and inclined at an angle of 21°30&#39; to a horizontal surface on which is positioned, adhesive side up, the tape to be evaluated, the adhesive thickness being at least 25 micrometers. The distance the ball rolls along the horizontal adhesive surface is measured, the tack being inversely proportional to the distance. Further details of this test are found in the aforementioned Test Methods for Pressure-Sensitive Tapes. Adhesives used in the practice of this invention should have a rolling ball tack value of no more than 6.0 cm and preferably less than 2.5 cm. 
     Shear Adhesion (Holding Power). As used herein, this test is a modification of PSTC-7, described in the same publication referred to in the previous tests. A 1.27-cm×2.54-cm specimen of the adhesive described in the preceding two tests is mounted on a 7.5-cm×20-cm stainless steel panel and an aluminum foil, both primed as previously described, is adhered thereto. The panel is then positioned with its long dimension at 2° from vertical so that the back of the panel forms an angle of 178° with the extended piece of tape, the 2.54-centimeter dimension of the adhesive extending in the vertical direction. The assembly is then placed in a 70° C. oven and a 1 kg weight attached to the free end of the tape. The cohesive strength of the adhesive is less than its adhesion to either the primed steel panel or the primed web, and the time required for the adhesive to fail cohesively, as evidenced by the tape&#39;s falling free from the panel, is reported in minutes. Generally speaking, adhesives used in the practice of this invention should have a shear adhesion value of at least 5,000 minutes. 
     For simplicity in reporting test results, a number of abbreviations have been employed. The significance of these abbreviations is set forth below: 
     
         ______________________________________Acrylate CopolymerIOA          Iso-octyl acrylate, or 2-ethyl        hexyl acrylateAA           Acrylic acidI.V.         Inherent Viscosity, dl/gTackifierF85          Glycerol ester of highly        stabilized rosin, having an acid        number of 9 and a softening point        of 82° C., available from Hercules,        Inc. under the trade designation        &#34;Foral&#34; 85.F105         Pentaerythrithol ester of hydro-        genated rosin, having an acid        number of 12 and a softening point        of 104° C., available from        Hercules, Inc. under the trade        designation &#34;Foral&#34; 105.PEE          Glycerol ester of rosin, having an        acid number of 7, commercially        available from Hercules, Inc.        under the trade designation &#34;PE        Estergum&#34;PEH          Pentaerythrithol ester of        partially hydrogenated rosin,        having an acid number of 12 and a        softening point of 104° C.,        commercially available from        Hercules, Inc. under the trade        designation &#34;Pentalyn&#34; HSE10         Glycerol ester of hydrogenated        rosin, having an acid number of 8        and a softening point of 83° C.,        commercially available from        Hercules, Inc. under the trade        designation &#34;Staybelite&#34; Ester 10SE100        Stabilized rosin ester, having an        acid number of 7 and a softening        point of 100° C., commercially avail-        able from Arakawa Chemical        Industries, Ltd. under the trade        designation &#34;Super Ester&#34; 100Z 85         Glycerol-derived ester of        disproportionated tall oil rosin,        having an acid number of 7 and a        softening point of 83° C.,        commercially available from        Arizona Chemical Company under the        trade designation &#34;Zonester&#34; 85SY40N        Modified rosin ester, commercially        available from Sylvachem Corpora-        tion under the trade designation        &#34;Sylvatac&#34; 40N.Crosslinking AgentsBA           N,N&#39;--bis-1,2-propylenisophthalamideTBT          Tetrabutyl titanateCR8          Chromium octoateC303         Hexamethoxymethyl melamine resin,        commercially available from        American Cyanamid Co. under the        trade designation &#34;Cymel&#34; 303,        0.5 mole of p-toluene sulfonic        acid catalyst being added to 100        parts by weight of adhesive at the        time of us.SubstratesSS           Stainless steelABS          Acrylonitrile:butadiene:styrene        terpolymerLDPE         Low density polyethyleneIPP          Isotactic polypropylene______________________________________ 
    
     Each of the examples tabulated below (in which all parts are by weight unless otherwise noted) was prepared by weighing into a suitable container (1) a solution of IOA:AA adhesive containing 100 parts solids, (2) tackifying resin, (3) antioxidant, and (if desired to lower viscosity) (4) a small amount of toluene. The ingredients were blended with a propeller mixer for about 30 minutes, after which (5) a crosslinking agent was added and mixing carried out an additional 10 minutes. After any entrained air bubbles had cleared from the solution, the composition was coated on a backing in conventional manner and dried in a hot air oven to evaporate the solvent. (It will be recognized, of course, that where either the tackifier, the crosslinking agent, or both are omitted, as in some of the comparative examples, the procedure is modified accordingly.) 
     
         __________________________________________________________________________                        Test ValuesPressure Sensitive Adhesive Formulation                        Rolling                             Adhesion to various                                         Shear,Acrylate Polymer             Ball substrates, N/cm Width                                         minutesParts    Parts Tackifier                  Crosslinker                        Tack,      LD    to failExample IOA    AA I.V.          Type Parts                  Type                     Mols                        Cm   SS ABS                                   PE IPP                                         at 70° C.__________________________________________________________________________Acceptable -- -- -- --   -- -- -- 6.0  800                                700                                   560                                      670                                         1,000Preferred -- -- -- --   -- -- -- 2.5  1100                                1,000                                   670                                      780                                         5,000A (Comp.) 94 6  1.5          --   -- -- -- 1.9  1010                                910                                   350                                      520                                         1B (Comp.) 94 6  1.5          F85  50 -- -- 5.1  2020                                1490                                   1390                                      1440                                         1C (Comp.) 94 6  1.5          --   -- BA 0.37                        0.3  1070                                860                                   430                                      560                                         &gt;5,0001     94 6  1.5          F85  50 BA 0.37                        3.0  1400                                1110                                   760                                      1090                                         &gt;5,0002     94 6  1.47          F85  50 BA 0.37                        0.3  1540                                -- 980                                      1050                                         &gt;5,0003     94 6  1.26          F85  50 BA 0.37                        1.6  1430                                1160                                   960                                      1650                                         &gt;5,0004     94 6  1.05          F85  50 BA 0.37                        0.6  1540                                1010                                   890                                      1050                                         &gt;5,0005     94 6  0.85          F85  50 BA 0.37                        0.6  1680                                1010                                   890                                      1050                                         &gt;5,000D (Comp.) 94 6  0.85          F85  50 BA 0.29                        1.9  1460                                1250                                   1120                                      1120                                         1806     94 6  0.55          F85  50 BA 0.37                        1.3  810                                -- -- -- &gt;5,000E (Comp.) 94 6  0.55          F85  20 BA 0.37                        0.6  1060                                1050                                   630                                      1610                                         &gt;5,0007     94 6  0.55          F85  20 BA 0.37                        0.6  840                                800                                   690                                      740                                         &gt;5,0008     94 6  0.85          F85  30 BA 0.37                        5.7  1200                                1000                                   800                                      900                                         &gt;5,000F (Comp.) 94    0.85          F85  70 BA 0.37                        12.7 2020                                1660                                   1300                                      850                                         &gt;5,0009     94 6  0.85          F105 30 BA 0.37                        1.0  1050                                980                                   690                                      760                                         &gt;5,000G (Comp.) 94 6  0.85          F105 50 BA 0.37                        &gt;10  1960                                1790                                   1300                                      1190                                         &gt;5,000H (Comp.) 94 6  0.85          SY 40N               30 BA 0.37                        1.0  1570                                1520                                   940                                      940                                         8          F85  2010    94 6  0.85          SE 100               30 TBT                     2.0                        1.3  1460                                890                                   630                                      980                                         &gt;5,00011    94 6  0.85          PeH  30 TBT                     2.0                        2.5  1120                                830                                   690                                      740                                         &gt;5,000I (Comp.) 94 6     SE 10               50 TBT                     2.0                        9.2  1900                                1300                                   850                                      1230                                         &gt;5,000J (Comp.) 94 6  0.85          F85  50 TBT                     1.33                        1.0  1460                                1160                                   1210                                      940                                         60012    94 6  0.85          F85  50 TBT                     2.0                        1.3  1460                                940                                   690                                      890                                         &gt;5,00013    94 6  0.85          F85  50 TBT                     2.67                        1.6  1230                                790                                   670                                      880                                         &gt;5,00014    94 6  0.85          F85  50 TBT                     4.0                        4.1  880                                710                                   630                                      710                                         &gt;5,000K (Comp.) 94 6  0.85          Z85  50 CR8   12.7 1900                                1570                                   890                                      1570                                         &gt;5,00012    94 6  0.85          F85  50 CR8                     0.79                        0.6  1460                                890                                   710                                      890                                         &gt;5,000L (Comp.) 94 6  0.85       C303                     1.75                        2.9  1570                                1480                                   980                                      1480                                         513    94 6  0.85       C303                     3.49                        5.8  730                                -- -- -- &gt;5,000M (Comp.) 90 10 1.5          --   -- -- -- &gt;15  900                                990                                   220                                      550                                         5N (Comp.) 90 10 1.5          F85  50 -- -- &gt;15  1250                                1340                                   400                                      870                                         4O (Comp.) 90 10 1.5          --   -- BA    &gt;15  950                                820                                   170                                      410                                         &gt;5,000P (Comp.) 90 10 1.5          F85  50 BA    &gt;15  1680                                1400                                   540                                      990                                         &gt;5,000Q (Comp.) 98 2  1.4          --   -- -- -- 0    620                                570                                   310                                      520                                         2R (Comp.) 98 2  1.4          F85  50 -- -- 0    1120                                970                                   680                                      880                                         0S (Comp.) 98 2  1.4          --   -- BA    0    560                                -- -- -- &gt;5,000T (Comp.) 98 2  1.4          F85  50 BA    3.5  680                                600                                   450                                      560                                         &gt;5,000U (Comp.) 98 2     F85  50 TBT                     2.0                        0.3  670                                680                                   830                                      680__________________________________________________________________________ 
    
     It will be recognized that those skilled in the art will make numerous variations of the foregoing examples without departing from the spirit of the invention. To illustrate, while 94:6 iso-octyl acrylate:acrylic acid copolymers are especially preferred adhesives, copolymers having 93:7-97:3 monomer ratios are considered of excellent quality. Copolymers having a 90:10-98:2 monomer ratio may be satisfactory for many purposes, especially where it is desired to emphasize one property, e.g., shear, more than another, e.g., tack. Speaking in general terms, the higher the percentage of iso-octyl acrylate, the tackier the adhesive; conversely, the higher the percentage of acrylic acid, the higher the shear properties. 
     Changes in a given variable can also be compensated for by making appropriate changes in another variable. For example, the tackiness of either a high acrylate copolymer or a high molecular weight copolymer can be increased by reducing the amount of tackifier or using one of a lower molecular weight (lower softening temperature). 
     Although it has been found important to include both iso-octyl acrylate and acrylic acid as monomers in preparing the acrylate pressure-sensitive adhesive starting materials, additional monomers (e.g., itaconic acid, methacrylic acid, acrylamide, methacrylamide, vinyl acetate, n-butyl acrylate, etc., can be included in minor amounts; e.g., a 74:20:6 iso-octyl acrylate:n-butyl acrylate:acrylic acid terpolymer yields a useful adhesive product. Similarly, other acrylates derived from alcohols having 4-14 carbon atoms may be included, as is generally taught in U.S. Pat. No. Re. 24,906. 
     Any adhesive composition according to the invention must include an anti-oxidant to inhibit oxidation of the tackifying agent and consequent loss of tackiness as the adhesive composition ages. In this regard, it has been found important to use an anti-oxidant which is based on either (1) a hindered phenol or (2) a sulfur-containing organo-metal salt. Among the hindered phenols are the ortho-substituted or 2,5-di-substituted hindered phenols in which each substituent group is a branched hydrocarbon radical having 3-20 carbon atoms, e.g., tertiary butyl or tertiary amyl. Other hindered phenols useful in practicing the invention include para-substituted phenols where the substituent group is --OR, R being methyl, ethyl, 3-substituted propionic ester, etc. Among the sulfur-containing organo-metal salts are the nickel derivatives of dibutyl dithiocarbamate. 
     The foregoing examples have employed numerous crosslinking agents, but there are believed to be still others which would likewise be effective.