[1]Patented Aug. 31, 1M 2 @ 3 2 8 @ 0 5 7 UNITED STATES PATENT OFFICE 2,328,057 PRESSURE-SENSITIEVE ADHESI[VE SEIEET Robert isham Coulter, St. Paul, Minn., assignor to Mnnesota Mning & Manufacturing Company, St. Paul, Minn., a corporation of Dela. ware Application February 27, 1941, Serial No. 380,769 2 Claims. (Cl. 117-76) This Invention relates to pressure-sensitive adchemical affinity which, as has been pointed out, hesive sheets and tapes of the type having a nonilbrous hydropliflic fllm backjng, such as regenerated cellulose ("Cellophane"). An object is to secure Improved anchorage of 5 the tacky adhesive coating by employing polyvinyl alcohol as a priming means. A further object Is to Improve the strength @nd toughness of adhesive sheets or tapes wherein hydrophihc cellulosic film backings are used, by employing 10 an interposed film or coating comprising polyvinyl alcohol which serves also in producing a strong uniorl of the pressure-sensitive adhesive to the cellulosic flim. A further object is to provide pressure-sensitive adhesive tape having cer- 16 tain advantages possessed by tape wherein a polyvinyl alcohol film is employed as the sole backing, but which is less expensive and free from obJectionable stretchiness (since for some purposes stretchiness is not only not needed but Is un- 20 desirable). By @a pressure-sensitive adhesive is meant a normally tacky Gr non-drying adhesive which adheres by pressing without need of activation by water,. solvents or heat. Adhesives of this type, 25 as commonly employed in pressure-sensitive adhesive tapes, are comprised of a blend of 9, highly cohesive plastic or rubbery base (such as rubber) and a tack-producing material (such as a resin), and may contain a fuler or reinf6rcing 30 material (such as zinc oxide), and sometimes include softening and/or plasticizing agents. The adhesive is highly cohesive so that adhesive tape can be unwound from rolls or removed from surfaces to which temporarily appiled without split- 35 ting or offsetting of adhesive as the result of Internal yielding of the adhesive coating. 'Mese .adhesives are organol)hilic, hydrophobic and water-insoluble. A serious and difficult problem is Involved In 40 attempting to bond such water-insoluble hydrophobic adhesive coatings to non-flbrous hydrophille films having smooth, dense, non-porous surfaces. In the first place, the two types of materials necessarily have a low afftnity. Such 45 films-eontain adsorbed moisture and, 'm the case of regenerated cellulose fllms, for example, commonly include a hygroscopic water-soluble softening agent, such as glycerine, which is not c.ompatible with water-insoluble hydrophobic coat- 50 ings' In the second place, the sinooth, dense, non-porous surface of the cellulosic fllm provides no opportunity for penetration of the hydropho.ble coating and hence the absence of mechanical interlocking inakes bonding dependent upon 55 Is low In this case. Thus the problem is quite different froni that in the art of coating fibrous cellulosic fabrics (such as papers, felts and cloths) wherein , a fibrous gnd porous surface peirmits of the penetration and mechanical interlocking of an applied coating, and the fibres commonly contain less moisture and generally do not include glycerine or other like flexibilizer. In the case of normally tacky and pressuresensitive adhesive coatings of the rubberresin type which are water-insoluble and hydrophobic, the @dhesive adheres to a regenerated cellulose film with substantially the same strength as it adheres to other surfaces toward which it has no special. chemical affinity-such as glass surfaces. Hence there is no substantial preferential bonding to the fllm backing when th,e adhesive is directly applied to the surface thereof, and delamination of the adhesive coating may occur when the adhesive sheet or tape is stripped from surfaces to which temporarily applied, or is unwotind from rolls thereof without careful handling. In order to provide strong bonding of the pressure-sensitive adhesive coating to a hydrophilic film backing, an interposed primer coating or fllm is commonly employed. In this art, a "priiiier" is by deflnition an interposed layer which causes the pressure-sensitive adhesive layer to be more flrznly bonded to the film backing than would be the case if the primer were omitted. The primer may comprise a mixture of a plurality of components, and may be composed of a plurality of layers or stratums which are united together and collectively serve the function. An object of the present lrivention is to provide a way of priming non-fibrous hydrophilic films having smooth, derise, non-porous stirfaces, so that rubber-resin pressure-sensi tive adhesive coating s, and the like, can be thereby bonded to the backing at least about twlce as firnily as would be the case if no primer were used, so as to provide a product in which the adhesive coatIng is non-offsetting under severe or unusual conditi6ns of use. A feature of the invention is that highly transparent and ciear adhesive tapes can be made and that the primer is'highly resistant to aging, and does not decrease the aging life of the adhesive or of the adhesive tape biit on the contrary makes for adhesive tapes having an increased useful life. A further feature is that adhesive tape made in accordan'ce with this invention has improved resistance to the effect of exposure to high humidities.
[2]2,328,057 The Invention Is not limited to adhesive sheets or tapes having regenerated cellulose-film backings, but relates In general to the bonding of hydrophobic pressure-sensitive adhesives to smooth, dense, non-porous, hydrophilic film surfaces. 5 Other examples of hydrophille films are: Hydrophilic cellulose etheis, including hydroxy alkyl cellulose ethers (such as hydroxy ethyl ceuulose), methyl cellulose, and special water-soluble types of ethyl cellulose; which are to be contrasted with 10 the cellulose nitrate, cellulose acetate, and ordinary ethyl cellulose, fflms of commerce, which are hydrophobic. In addition to such cellulosic Mms, mention may be made of non-cellul(>sic fums made of plasticized starch, protein fdms 15 made of gelatine or casein, tllms made Gf "Mazein" ("prolamine" found in corn and extracted from corn gluten meal and purifled), and films of polyvinyl alcohol. 'Moreover, the Mm need not be the sole back- 20 Ing, but may be laminated (or formed in situ) upon another sheet, such as paper or cloth. Also, the Mm may be a composite, as for example a fum_of regenerated c4@llulose JGined on either or both faces with a hydrophilic film of a different 25 type, such as a egating of polyvinyl alcohol, or other fflm material mentioned above, for example. The primer of the present i?ivention is formed of a combination of po)yvinyl alcohol interbonded with an organophilic material which is 30 compatible with the pressure-sensitive adhesive. The polyvinyl alcohol firmly bonds to the hydrophilic film surface and the said organophilic ma- - terial firmly bonds to the overlying hydrophobic organophilic adhesive, so that a strong linking 35 together of the fUm and the adhesive is produced. Polyvinyl alcohol is commonly manufactured by hydrolyzing a polyvinyl e,.ster such as polyvinyl acetate. Hydrolysis Is probably not complete in any. case and may be deliberately kept substan- 40 tially short of completion in order to produce forxns which are of limited solubility in water, due to the unsubr-tituted ester radicals remaining. Also, polyvinyl alcohols may be treated In various ways to Jncorporate other radical-s than hydroxy 45 while still retaining hydrophilic properties, as by partial acetylation,,or treatment with an aldehyde subh as acetaldebyde, formaldehyde or butyraldehyde. Flor present purposes, the term "polyvinyl@ alcohol" applies in any case where 50' there are sufficient hydroxy radicals in the moleeWe to Impart the hydrophihe propetty which causes flrm bonding to hydrophilic films when an aqueous solution;or dispersion of the polyvinyl alcohol is applied to the film and dried, and the 55 term applies even though the polyvinyl alcohol has been treated or insolubilized after application to the fllin to render it more moisture-resistant. The Drganophilic material employed in securing union of primer to adhesive layer, may be any Go organop'hilic material which is compatible with the particular pressure-sensitive adhesive used. In the case of rubber-resin adbesives, either rubber or compat-ibie resin,.or both, may be utilized 1 in the primer, for exam. ple. I'he. "rubber" may 65 be natural rubber or syntbetic rubber or a rubbery synthetic polymer (each oi which is an elastomer). As to the usage of the term "elastomer," see-Industrial and Engineering Cbemistry, vol. 31, pages @941@945, August, 1939. The 7o organophil - ic material may be dispersed in the aqueous solution (if polyviny! alcohoi, or may be dissolved In a volatile organic solvent and the solution emulsified with the aqueous polyvinyl alcohol solution so that one or the other vewole 75 (preferably the water) constitutes the extemal phase and the other a dispersed lntemal liquid phase. In either case the resultant liquid, contain ing both the polyvinyl alcohol and the organophilic on mat in dispersed and intermingled state, is used to coat the hydrophflic film and upoli dryllig will be firmly anchored to the fLIm. In any case the range of proportions of the two essential components within which true priming action is secured can be readily determined bY simple trial. and will of course depend somewhat upon the particular fllm, the particular organophilic component, and the particular pressuresensitive adhesive, which are used. In generql, It has been found that 15 to 300 parts by weight (dry basis) of polyvinyl alcohol per 100 parts by weight of the organophilic boilding component may bl@ used to advantage to secure at least about a 50% irierease in anchorage of the adhesive coating, althdugh this Is not to be regarded as a limitation since proportions outside this range, have prgved satisfactory in various cases. Generally speaking, experience has shown that 10-600 parts of polyvinyl alcohol per iOO parts of the organophilic component represents the broad range within which sufficient priming action is obtained to justify the expense of priming the backing. A second way of forming the primer is to prepare an aqueous dispersion of the orgarlophflic bonding material separate and apart from the aqueous solution of PoIyvinyl alcohol. The film is first coated with the polyvinyl alcohol soiution and then, while this coating is still in a wet or moist condition, the aqueous dispersion of the organophiuc bgnding material is applied thereover, followed by drying. The two coatings merge together and 9,n interbgnding of the two componets of the composite primer regmts. The union may be strengthened, if desired, by employing a polyvinyl alcohol solution in which a minor proportion of the organophilic material is dispersed, and/or a dispersion of the organophilic material In which a minor proportion of pdlyvinyl aicohol is included. A further *ay, which may if desired be utu!zed in coniuction with the foregoint expedient, is to include in the organophilic dispersion a substance compatible with the polyvinyl alcohol coating and which exerts a solubiuzing action upon the polyvinyl alcohol, such as triethanolamiiie, foramide or acetamide. Such substance may also be included in the polyvinyl al-'@ cohol solution, or in both. A Preferred aqueous dispersion of organophilic bonding material which may be empioyed in carrying out this Invention is latex. The latex emulsion may be a concentrated latex mUk containing uncured latex rubber; but use may be made of vulcanized latex milk, such as is exemplifled by "Vultex" (latex milk vulcanized by a special process which avoids coagulation). Use may also be made of equivalent artificial aqueous disPersions of rubber or of artificiiil or synthetic rubber-like materials (many of which are customarily referred to as being artificial latexes). A further example is the use of an aqueous @bpersion of isobutylene polymer ("Vistanex"), this material being readily compatible with rubberresin Pressure-sensitive adhesives in either the lower pdlyraer form (which is plastic and tacky) or higher PolYmer form (which is rubbery and non-tacky). Various other organophilic-resins in water-dispersed form may be used as heretofore indicated. Thus dispersions of rosin, ester gura, hydrogen-
[3]2,828,057 3 ated MSin, hy&wmaed ester gum, hydrogenated eoumglrone and indene resins, can be readIIY PrePared, as Is well known in the resin art, and MaY be used to provide the Grganophilic bonding material compatible with the p ressuresensitive adhesive. The Inclusion of ammonia or an ethanolamine (such as tr iethanolamine), for example, will insure ready blending with polyvinyl alcohol solutions. Also mixtures of different suitable organophilic materials can be used by mixing dispersions thereof, or by simultaneously dispersing two or more different organophilic materials, or by blending two or more and dispersing the blend. Thus an ammonia-rosin emulsion may be mixed with latex milk, for example, and can be readily Incorporated with a polyvinyl alcohol solution. Or rubber and a resin, such as ester gum, may be combined on a rubber mtll and the resultant blend dispersed in,water. These are given as illustrative of combinations of any two or more of the various organophilic bonding materials which have been mentioned. Also, as heretofore Indicated, one or more of the organophilic materials -may, be dissolved in, or softened by, a volatile solvent and then emulsifled in water. The emulsion may also include one or more organopbilic materials dfrectly dispersed in water without being in a s:)Ivent phase. The polyvinyl alcohol may be substituted for in part, or augmented by, a hydrophilic collo'dal agglutinant of different kind, such as casein, isinglass, gelatine, soy bean protein glues, "Mazein" (corn prolamine), dextriri, metbyl cellulose, waier - soluble polyhydric - alcohol/polybasic - acid resin, urea-aldehyde resin, water-soluble alkyd resin salt, or mixttires thereof. This may be of true value in reducing cost while at the same time obtaining the advantages of pdlyvinyl alcohol to a substantial extent. Flarthermore, the polyvinyl aicbhol may serve a valuable protective function in obtaining stable dispersion. For example, polyvinyl alcohol used in conjunction with casein serves to inhibit putrefaction and spoilage of the primer solution, These other hydrophilic colloidal agglutinants bond to hydrophilic films, although when used alone are not as good a polyvinyl alcohol. The inclusion of ammonia or an ethanolamine may be desirable in some cases to insure stable adri@ixtures. The accompanying drawings illustrate various aspects of the invention in diagrammat,'c fashIon as follows: Mgure 1 shows a roll of pressure-sensitive adhesive tape, and Mgute 2 shows a schematic enlarged edge view of the tape, wherein a nonfibrous hydrophilic film backing (such as regenerated ceuulose film) is coated on one side with a polyvinyl alcohol-latex primer upon which there Is a coating of a normally tacky and pressuresensitive water-insoluble hydrophobic adhesive (such as a pressure-sensitive adhesiveof the rubber-resin type). The composite adhesive tape niay be transparent, so that it may be applied to surfaces without concealing the coloring or markings thereof, in Nvhich case a transparent adhesive will be used. An adhesive containing coloring material may be used, which will be visible through the fllm back:ng and primer, to make a colored adhesive tape, the optical effect being such that the backing itself will appear colored. Figure 3 shows a schematic edge view of an adhesive sheet or tape wherein the pressure-sensitive adhesive covers part or all of the area of the fUm backing. being bonded thereto by a'n interposed Polyvinyl alcohol-latex primer or the Ue, characterlzed by a removable liner (such as holland cloth or regenerated cellulose filhi) contacting and covering the adhesive surface. me liner may be grasped and be peeled or stripped off without danger of delamination of the adhesive from its backilig, owing to th'e flrm bondIng produced by the primer, notwithstanding that 10 the backing has a smooth, dbnse, nonporous surf ace. Fligure 4 shows a schematic edge view of a double-coated pressure-sensitive tape having a liner attached to one of the adhesive faces. The 15 composite may be wound In roll form, and de'sired lengths of tape may be unwound from time to time and the liner stripped off for use of the double-coated tape. The hydrophilic film backIng In this form Is coated on both faces with the 20 lirimer and a pressure-sensitive adhesive coating Is applied to each side. Such double-coated tape has utility for many purposes; as for binding two surfaces together, which may later be separated again,without transfer of the adhesive 25 from Its backing. By using a transparent adhesive, a composite adhesive tape may be made which is transparent, and may be used as an "Invisible" splice for transparent sheets, for exomple. In general, such tape may be used in 30 place of rubber cement for many sealing and joining purposes, having the advantage that it may be applied more conveniently and without the user being bothered with solvent fumes, and separation can later be effected, when desired, 35 without the adhesive remaining upon the Surfaces to which the tape was applied. It will be evident that the powerful bonding of the pressure-sensitive adhesive to its backing, by virtue of the primer, makes possible the success of this 40 tYPe of adhesive sheeting. Mgure 5 shows In diagrammatic fashion an 11Iustrative system for the continuous making of pressure-sensitive adhesive sheeting, involvingthe steps of coating a regenerated cellulose fllm 45 web (fGr example) 'with the aqueous primer solution;- flash-dryini the primer coating in a manner which produces a flat, unwrinkled and unwarped sheet" applying a c6ating of adhesive solution (such as rubber-resin pressure-sensitive 50 adhesive dissolved in a volatile hydrocarbon S3]vent) upon the primer surface,, passing the web through a drying oven to evaporate the solvent; and winding up the resultant adhesive sheeting Into a roll with a temporary liner wound in as a r)5 separator for the tuins of the roll. The roll may then, when desired, be unwound by pu]Iing out the liner, and the adhesive sheetin.a passed ta a slitting machine for slitting into desired ape widths and w;lndings Into rolls of desired 60 length for sale. Mgure 6 shows a schematic edge view of an adhesive sbeet or tape wherein a hydrophilic film backing is flrst coated with polyvinyl alcohol in water solution and then, while this coating is 65 still in a wet or moist state, a doating of latex or the like (in aqueous dispersion) is applied, followed by drying. The tWo coatings become bonded together in this way and constitute, in combination, the primer. A Pres sure-sensitive 70 adhesive is applied over the dried latex coating. Excellent adhesive tape of g(>od transparency can be made in this way. In addition to its priming function, the polyvinyl alcohol coating, if api)lied to a regenerated cellulose film (for ex15 ample) . serves to increase the toughness and
[4]4 2,328,057 strength of the composite tape, particillarly if it Is made of somewhat greater thickness than Is needed for mere priining purposes@ The polyvinyl alcohol solution may have a dye or pigment incorporated therein so that the dried coating will color the tape. A stronger union of the two primer coatings can be obtained, if found desirable, by inciuding a minor proportion of latex (or equiv,.tlent) in the polyvinyl alcohol solution, 9,nd/or including a minor proportion of polyvinyl alcohol (or other hydrophilic colloidal agglutinant) in the latex dispersion. Ms,,ure 7 shows a schematic edge view of an adhesive sheet or ta?je in which the backing is f6rmed of a thin film of regenerated cellulose carrying oti the back face a film of polyvinyl alcohol formed in situ by casting or co@ting, and carrying a similar film of polyvinyl alcohol on the face side. This composite may have a total thickness comparable to that of the regenerated ceuulose film commonly employed as the sole backing, so as to avoid thickening the tape. The re---enerated cellulose film serves as a casting or coating base for the polyvinyl alcobol, results in a backinla less costly than if the backing we:re composed solely of a polyvinyl alcohol film, and prevents the backtng from being stretchy; wwle the polyvinyl alcohol layers result in a great increa.@e of toti,,-hness and tear-resistance and make for greater flexibility, and the one on the face side also serves as a part of the primer. Instead of usin-, tl-ie ordinary type of regenerated cellulose film, use r.- ay be made of film having a much lower content of glycerine, since the film is thin and the integral polyvinyl alcohol films will protect the composite from cracking or breaking when bent or flexed. 'i,ne polyvinyl alcohol film on the face side, while still in a wet or moist state, has applied to it a latex coating and, after drying, a pressuresensitive adhesive coating is applied thereover, as described in connection with Fig. 6. This polyv.nyl alcohol film thus serves the dual role of being a part of the backing and being a Part of the primer serving to link the pressure-sensitive adhesive to the regenerated cellulose film. Either or both of the polyvinyl alcohol films may include coloring material. To further increase bonding between the inner polyvinyl alcohol film and the latex coating, the expedient described in connection with Fig. 6 may be used, i. e., the inclusion of latex or other organophilic bonding material. Figure 8 shows a schematic edge view of an adhesive sheet or tape having a composite backing film cofnprised of two hydrophilic film layers, the back film being a regenerated cellulose film and the inner film being composed of polyvinyl alcohol which has been cast or coated thereupon as an aqueous solution and then dried ' As described in connection with Mg. 7, in this case also the back side of the regenerated cellulose film may be additionally coated with a polyvinyl alcohol film. After formin.- of the composite backing film, the polyvinyl alcohol film may be coated with a polyvinyl aicohol-latex type of primer, and after drying of the primer, a pressuresensitive adhesive is applied thereover. Figure 9 shows a schematic edge view of an adhesive sheet or tape having a composite film backing formed of two thin fums of regenerated cellulose (for example of 0.9 mil thickness each) vinyl alcohol. This laminated film may be formed by applying a thin coating of polyvinyl alcohol to one or both of the cellulose filins and then pressing together, followed by heating. Drying of the interposed polyvinyl alcohol is facilitated by the water content being absorbed by the regenerated cellulose films and thence diffusing outwardly to the atmosphere. This expedient makes possible the use of thin regenerated cellulose films (which 10 are cheaper per pound than thick films) to secure a backing of desired thickness. In addition, a tougher tape is produced, and this feature can be increased by employing a polyvinyl alcohol film of several teiiths of a mil thickness. The 15 interposed polyvinyl alcohol fUm may include coloring material to prodlice colored tape, and may include modifying agents such as plasticizers, latex and resins. A polyvinyl alcohol-latex type primer is ap20 plied to the regenerated cellulose film and, after drying, a pressure-sensitive adhesive is applied thereover. All of the illustrative tape structures se for in the drawings may be utilized in making trans@-'5 parent pressure-sensitive adhesive tape by employin- a transparent pressure-sensitive adhesive. Colore@ tapes ean be made not only by including colorinp, material in polyvinyl alcohol layers as described (in which case the inclusion of zinc 30 oxide or other opacifier in the pressure-sensitive adhesive may be des@rable in order to back up the color where an opaque t-,tpe is being mad-), but by including coloring material (dye or pigment) in the pressuresensitive adhesive which will be @15 visible tbrough the backing and primer. ngure 10 show.-, i-n diagrammatic fashion an adhesive slieet or tape havin-- a transparent hydrophili@- film backing (such as regenerated ce!Iulose) which has been coated in certain areas 40 with a pigmented polyvinyl alcohol forming a design, symbols or viords, and which MI@.Y be applied by means of an inta,-Iio printin-I cylilider or by offset printin.- or in aiy other suitable 4- way, iising a pigmented polyvinyl alcohol water solution "ink." TI-@e use of such polyvinyl alcohol ink makes for a firm anchorage of the prin'@ed matter. Water vehicle inlcs are well known in tl-le printing art and the same types of pig50 ments and dispersing aids may be u7,ed, bein.mixed into the polyvinyl alcohol solution by use of a pebble or ball mill, for example. A transparent primer coating of polyvinyl alcohol type is applied over the printing and anchors 3 a- firmly to both t'@le printed areas and to the interposed exposed backing areas. A pigmented pressure-sensitive adh6sive coating is applied thereo@,er and provides @. background visible through the backing i,-i the unprinted areas. 6i) ThLs if a red printing ink has been used, and zinc oxide has been used for pigmenting the adhesive, the tape will show red priiiting against a white backgrou,.id, aiid in addition the white adhesive will "back up" the red printed -.ireas so 65 that they will appear more opaque a-@il deeper in color. Attractive package seals, and stickers and labels, may be made in this way, and because of the firm inter-bonding of tlzie constituent elements of the structure, such tape 70 may be unwound from rolls and removed from surfaces to which temporarily applied without the adhesive pulling off from the backing. The primer may also include coloring material, in which case a white Pigmented adhbsive may be borided together by a thin interposed film of poly- j'5 used to back up the underlying colored areas.
[5]2,328,057 5 This same principle may of course be ustd in various other. ways. 7be backing may be printed In several colors. The backing may be en-. tlrely covered with printing so as to leave no uncovered portions, in which case the printing serves as the sole priming means in contact with the backing. The polyvinyl alcohol ink may include latex or other organophilic bonding material, in which case no further primer is needed If the Ink covers all, or niost of, thd-backing surface. A backing such as a regenerated cellulose fUm may be precoated with a transparent polyvinyl alcohol fUm, or with a primer film containing both polyvinyl alcohol 6,nd an organiphilic bonding material, upon which the printIng is applied ' Referring now to Mgure 5 for a niore detailed description of an illustrative system adapted to the making of adhesive tape of the kind shown in Plgs. I and 2, a web of regenerated cellulose fUm sheeting (or the like) is drawn from supply roll IO to and around idler roller II and thence over idler roller 12 and dovm between coating rollers 13 and 14, roller i4 being located below roller 13 and having a rubber surface. Poller 14 dips Into a bath of the aqueous primer solution and transfers it to the lower face of the web. These rollers are tightly set so that only a t]@in coating of primer solution remains upon the web as It leaves. 7be web then passes to and around st@amheated drum 16, thence back to and around idler roller i 7, and thence back to and around steamheated drum 18, following which it moves for some distance through the air to the adhesive coater, to perniit of cooling, being supported and guided by idler rollers 19 and 20. The web contacts said steamheated drums on its unprimed face, and is thereby quickly heated so as to cause flashdrying of the primer on the outer face. In a particular embodiment which has been employed, it has been found that heating of the drums to produce in operation a drum surface temperature of about 1801-2401 P. has given satisfactory results, the web contacting the drums for a total length of 36 inches, and a total period of 12/2 to 3 seconds, depending upon the rate of travel of the web (which has been 20 to 40 yards per minute). The web is drawn through the mentioned equipment and is thus maintained under tension, and at the same time is held flat by the rollers and heating drums, and this with the flash-drying results in a primed web which is flat and unwrinkled and remains, this way. If a regenerated cellulose film web were merely coated with the aqueous primer solution and dried without special rneasures being taken, it would buckle and curl and wrinkle to an extent making it unusable, and could not afterwards be flattened out successfully. The primed web, having the primer upon the upper face, is then drawn through the adhesive coater 21 " where its upper face is contacted by a viscid mass of pressuresensitive rubber-resin adhesive dissolved in a volatilehydrocarbon Solvent (for example) and the coated web is drawn out betweed doctor blades 22 and 23 to give a uniform coating of desired thickness. The coated web then passes into drying oven 24 for solvent removal, moving to the back of the oven and down over idler roller 25 6 the lower part of the oven, ihence around idler roller 26 to the front of the oven and up around idler roller 27 to idler roller 28, thence back toward the rear of the oven and around cold roll 29 and thence passes to the front of the oven and out, following which it passes down over.cold roll 30. The adhesive face of the web contacts cold rolls 29 and 30 and is thus @ kept from becoming unduly soft from the heating. After leaving th e last-mentioned cold roll, the adhesive web, which bears a tacky and pressuresensitive adhesive coating from which the solvent has been removed, passes down to and around 10 idler roller 31 and thence up to and around driven PUU drum 32, which is contacted by the tacky adhesive face of the web. It is this drum which pulls the web through the entire precedent equipment. 'Itie adhesive sheeting then moves to 15 driven wind up roll 33, where a temporary liner 34 may be fed in to separate the tums if desired. The adhesive sheeting may thereafter be prepared for sale in tape rolls, by slitting and re;. winding. It will be obvious that when it Is desired to 'o prime 'the film web on both faces (as in the structure shown in Mg. 4). that this can be accomplished @ by using two priming units series, for example, so that primer solution is 25 applied to'one face, dried, and then to the other face and that dried. Or if only one unit is avaijable, the web can be primed on one face, wound up in a roll, and then sent through the unit again so as to prime the other face. The web, primed 30 on both faces, can be double-coated with adhesive by beiiig drawn through an adhesive bath and thence between a pair of doctor blades or a pair of squeeze rous, which are spaced to give the desired coating thickness If a double-coated 35 tape having a liner is to i@ made, the doublecoated web as it moves to the s,litting means may be joined by a liner. web moving from its supply roll into contact, and the composite then sht and the I*pes wound into suitable rolls with the liner 40 on the outside of the tums. EXAMPLE 1 Parts by weight Polyvinyl alcohol ------------------------ 1.87 Formaldehyde solution (40%) ----------- 2.33 45 Ammoniurn hydroxide (28% NIi3) -------- 2.80 Rubber latex (60% rubber) -------------- 6.25 Water---- ---------- ------------------- 18.75 This example illustrates a primer solution welIL 50 adapted for use in the coating system described in connection with Fig. 5, and which contains .50 parts polyvinyl alcohol per 100 parts rubber la.tex solids (dry basis). In this formula a suitable polyvinyl alcohol 65 which has been found to be very satisfactory is the cold-water-soluble type having a molecular weight such ti'lat a 4% aqueous solution at 201 C. has a viscosity of about 20 centipoises ("medium viscosity" form). Ai@ example of a commercially (;o available polyvinyl'alcohol of this kind is the RH-488 grade sold by E. 1. du Pont de Nemours & Co. (Inc.), of Wilmington, Delaware. This form has also been found very satisfactory in each of the succeeding examples. It will of course 65 be understood that this is given only by way of example and not limitation. Thus low viscosity and high viscosity forms may be used, or mixtures of different viscosity forms. The polyvinyl alcohol is dissolved in the water 70 bY churning overnight, but may be dissolved more quickly by using warm water. The formaldehyde and ammonium hydroxide are then added, reacting to 'form hexamethylene tetramine. The latex milk is then slowly stirred in. 75 The resultant prim6r solution is very stable and
[6]6 2,328,057 may be made uP in large batches because of its stability and excellent keeping qualities. There is no putrefaction such as occurs upon exposure to air for several days (in open containers or coating reservoirs) in the case of priming sDIutions of the casein-latex type heretofore used. Thus it has been noticed that even after several weeks standing in open bue ' kets or drums, the priniing solution was in excellent condition for use and had no taint. An equivalent primer solution can be made by directly adding hexamethylene tetramine to the polyvinyl alcohol solution. A rubber anti-oxidant, such as beta-naphthol, may be included if desired, but has not been found necessary in obtaining adhesive tape having a long aging life. Upon drying of the applied primer coa.ting (as in the Mg. 5 system), the hexameth3flene tetrainine decomposes to yield formaldehyde and this exerts an insolubilizing action upon th6 polyvinyl alcohol, probably through conversion of some hydroxy radicals to formal radicals, i. e. the polyvinyl alcohol is partly transformed into polyvinyl formal; but without any apparent weakening of the bond between primer and backing film. The result is that the primer is made much more resistant to moisture, which is of great importance in this type of adhesive tape owing to the presence of adsorbed moisture in the hydrophilic :film backing and to the permeability of the backing to moisture of humid atmospheres to which exposed. This feature can be easily demonstrated in !the following way: Two adhesive tapes were made up having regenerated ceuulose film backings of 1.8 mils thickness coated with a primer film of about 0.2 mil thickness, upon which has been applied a rubberresin pressure-sensitive adhesive coating (as per Adhesive Example 1 hereinafter set forth). In one case the primer of' the foregoing formula was used, in the other case the formaldehyde and ammonium hydroxide were omitted; both primers being applied using the Fig. 5 system; so that conditions were identical except for omission of the insolubilizing action in one case. Two days after being made, the two different adhesive tapes were applied to a tin plate and pressed down firnily with a roller. The backs were wiped for 20 seconds with a moist clo,th, and the tapes were then stripped off. In the case of the tape having the insolubilized primer, tl-ie tape came amray clean without any adhesive being left on the tin plate - but in the other case there was complete delamination so that the adhesive coating remaine'd on the tin plate. This feature of insolubilizing the priiner Is of great practical value since in many parts of'the country high humidities prevail during large parts of each year, and it is of great importance that the primer be resistant to 'the effects of such high humidities. The present invention provides such a primer and yet the insolubilizing procedure in no way interferes with the clarity of -transparent adhesive tape, either initially or upon exposure to sunlight, and does not deteriorate the rubber of the primer and of the pressure-sensitive adhesive coating. In fact adhesive tape having a backing of regenerated cellulose fllrn and a, rubber-resin adhesive has, for some unknown reason, been found even more resistant to discoloration and deterioration by ultra-violet light when this formaldehyde treatment of the priiner is used than when it is omitted. A desirable feature of the present type of. primer is that it can be used in making transparent composite sheets of great clarity, and that it can be made of minute thickness which does not add appreciably to the thickness of the composite, thus also making for low cost. The primer ffim is very flexible, tough, and extensible, and io anchors w ith great firniness. Thus a film of regeiierated cellulose, having such primer fUm applied thereto, can be bent, creased and flexed without cracking or rupturing the primer film and bond. The extensibihty of the primer is ]5 very advantageous in connection with tape backings wbich are stretchy, such as backings of polyvinyl Alcohol fLIm. In pn'm'mg regenerated cerulose films in the making of pressure-sensitive adhesive tape, for 20 e3tample, one pound of the primer solution set forth in this example WiU satisfactorily coat 100-200 square Yards of surface to yield a dried primer film of approximately 0.1 to 0.2 mil thickness. The surface of the backing film, when the primer of this example is used, is contacted by a two-phase composition of matter; that is, is contacted both by poly alcoho par s a , by rubber particles-, in addition to such other 30 phases or particles that may be present. EXAMPLE 2 Polyvinyl alcohol --- ---- 1.90 ---------------- Rubber latex (60% rubber) -------------- 6.33 35 Water -------------------------------- 21.77 The polyvinyl alcohol is of the same kind used 'in the preceding example formula and is made into solution in the same way, and the latex nlflk is then stirred in. It may be mentioned that the 40 P01YVinYl alcohol acts as a protective coiloid and further stabilizes the dispersion of latex particles. This primer solution may be applied in the same way as the one of the preceding example. 45 Although no insolubilizing agent is included, good adhesive tapes can be made and under ordinary room conditions the anchorage of adhesive will be fu.Uy adequate. However, insolubihzation can be produced by -50 subjecting the primer Mm (after drying or semidrying) to the action of a formaldehyde solution or gas, as by applying a formaldehyde solution to the coating or dipping the entire coated sheet in a formalde@yde bath, followed by drying be55 fore applying the adhesive coating. Other aldehydes can be used in place of part or aU of the formaldehyde, such as acetaldehyde and butyraldehyde. The invention is not limited to aldehydes as, for example, desirable results may 6o be obtained by substituting other insolubrizing agents in whole or in part for the aldehyde. However, the use of an aldehyde has been found of particular value b6cause of its effectiveness, ease of use, non-impairinent of the clarity of 65 transparent tapes, and contribution to the general aging life of)the tape. In order to show the degree of bonding secured, and the effect of varying the proportion of polyvinyl alcohol to latex, a series of tests 70 were made wherein various samples of pressuresensitive adhesive tape were used involving differing ratios and different hydrophilic backing films. The bonding strengths were quantitatively compared by using a testing machine pro75 vided with a screw driven platform. A length
[7]of the adhesive tape to be tested is secured to the platform with the adhesive side out. Another length of strong paper-backing rubber-resin adhesive tape Is placed with its adhesive face in contact with the adhesive face of the sample so that the two adhesive coatings cohere, being pressed firmly together with a roher. The end of the outer tape is-drawn back at an angle of approximately 1800'and is connected by a string to a device for measuring tension, such as a spring scale. The platform Is then driven at a constant rate (in these tests 7.5 feet per minute), with the result that the second tape is stripped away from the test sample tape, carrying with it the adhesive of the tape being tested. It the anchorage of the adliesive to the primer is greater than the anchorage of the primer to the tape backing, the primer will also be stripped off. If the anchorage of the primer to the adhesive and to the backing is greater than the cohesive strength of the adhesive, the adhesive will split, and it will only be known that the actual anchorage flgure is greater than that measured. A measure of pnming efficiency is obtained by making a comparison with tape having the same backing and adhesive but with no primer. In these tests, a pressure-sensitive adhesive of the kind set forth hereinafter as "Adhesive Example 1" was used, and a primer coating of 0.1-0.2 rnil thickness was employed, made in accordance with the formula of the present example, except that various rati6s of polyvinyl alcohol to latex were empl6yed. The following table sets forth the measured values of the force required for stripping, the flrst column giving the parts by weight of polyvinyl alcohol per 100 parts latex rubber,(dry basis), and the stripping ford6 being converted to ounces per inch width of the tape tested. In each case several tests were made and the highest consistent figure is given. I'he polyvinyl alcohol film was cast from a solution ofhot-water-soluble medium-viscosity polyvinyl alcohol, exemplified by du Pont RH-349 grade, and was stretchy. A cold-water-soluble, medium viscosity, polyvinyl alcohol was used in making up the primers. Stripping I force for each kind of film Parts polyvinyl fLIcohol per backing (ounces per inch width) 100 parts rubber in primer Regemerated Methvl Polyvi nyl cellulose cellulose alocho l - 15 ---------------------------- 38 34 34 30 ---------------------------- 52 56 56 100 --------------------------- 72 52 64 300 --------------------------- 40 36 34 600 --- ----------------------- 32 26 24 No primer --- , ----------------- 1 24 22 1 20 In each case where 15 parts polyvinyl alcohol were used, the adhesive a-nd primer were both pulled cff; when 30 or 100 parts were used there was splitting bf the adhesive (showing the primer bond to be higher than the figures given); aiid when 300 or 600 parts were used the adhesive was pulled away from the primer. This shows that the primer has greater affinity for the backing or for the adhesive depending upon the proportion of polyvinyl alcohol, and incidentally proves that anchorage of the primer to the film backing depends primarily upon the polyvinyl alcohol compoiient and that anchorage to the adhesive coating depends primarily up6n the latex rubber component. 2,328,057 7 EXA30LE 3 Polyvinyl alcohol ----------------------- (0.90 Casein --------------------------------- 1.00 Formaldehyde (40%) ------------- - ----- 2.33 Ammonium hydroxide (28%) ------------ 2.80 Beta-naphthol -------------------------- 0.05 Rubber latex (60%) ---------- - ------ 6.25 Water -------------- ------------------- 18.75 'rne Polyvinyl alcohol is dissolved In halt the 10 water and half the ammonium hydroxide is added. The casein is soaked in the other half I of the water for 1/2 hour at 100, P. then the other half of the ammonium hyrroxide is added ' the temperature is raised to 125' P., and the batch 1,5 mixed until the casein is thoroughly dissolved, and the beta-naphthol is added. The polyvinyl alcohol solution Is then stirred in and the formaldehyd6 is added. The latex is then slowly stirred in, with continued mixing to secure a jini20 form dispersion. This primer solution provides a primer having approximately the same bonding strength and moisture resistance as the primer of ExamPle 1. The polyvinyl alcohol aids In maintaining a 25 stable compos@ltion and decreases the tendency of the casein to putrefy. It makes for a tougher primer film and one which is more extensible. The casein reduces the cost, being consid6rably cheaper than pclyvinyl alcohol at this time. 30 The beta-naphthol acts both as a casein preservative and as an anti-oxidant for tie rubber, but may be omitted if desired. EXAMPLE 4 35 Part A Isobutylene polymer --------------------- 100 Stearic acid ---------- ------------------- 4 Triethariolamine ----- ------------------- 2 Water @@ ------------------ - ------------- 130 40 Part B Polyvinyl alcohol ----- ------------------- 100 Forrnaldehyde (40%) -------------------- 125 Ammonium hydroxide (28%) ------------- 150 4.-, Water ---------------------------------- 1000 Part A: Melt the stearic acid and triethanolamine together in a steam jacketed kettle and work In the isobutylene polymer slowly with good mixing until thoroughly homogenized, the tem50 perature of the melt being brought to about 190' P. and the stirrer speeded up to about 1000 R P. M. Heat the water to abo-ut 190' P. and a@d gradually while continuing vigorous agitation. During this process the emulsion will in55 vert so that the water forms the continuous phase. After a smooth emuision is formed, the steam is tumed off and the emulsion is stirred gently until cooled to room temperature. A suitable lsobutylene polymer is represented by 60 "Vistanex No. 6" (sold by Advance Solvents & Chemical Corp.; New York city) , which is plastic and sticky. Part B: This may be prepared by churning the polyvinyl alcohol overnight with the water and 65 then adding the formaldehyde and ammonium I hydroxide. Part B is then sIDwly stirred into Part A with continued mixing until homogeneous. As a variation of this formula, in making part 70 Al instead of using 100 parts 6f isobutylene polymer, use may be made of 50 parts isobutylene polymer and 50 parts of hydrogenated indene resin (a clear, transparent, solid resin exemplifled by "Nevillite Resin," sgld by the Neville Co., Pitts75 burgh).
[8]As a further variution, latex may be incorporated with either of the foregoing so as to include an elastomer in the primer and obtain greater cohesiveness. EXAMPLE 5 Polyvinyl Alcohol -------------------------- 10 ftrmaldehyde (40 %) ---------------------- 12 Ammoniuxii hydroxide (28%) --------------- 15 Water ------------------------------------ 90 This solution'is suitable for use in Providing Polyvinyl alcohol films for laminating or coating fams of regenerated cellulose or the like as indicated by Mgs. 6, 7, 8 and 9 (and accompanying descriptions). The water conterit can be varied to produce the desired coating consistency. Drying of the film, as by using steam heated rolls of the general type shown in Mg. 5, results in ti'l-C formaldehyde rendering the polyvinyl alcohol fam more moisture-resistant. Modifying agents may, be included, such as formamide or acetamide to serve as a plasticizer. Latex milk, stabilized with ammonia, may he readily included, and eveii iri small proportions will serve to give increased anchorage to a superposed coating of latex as in the structures irustrated in Figs. 6 and 7. The formaldehyde and ammonium hydroxide may be omitted; thus, for example, satisfactory results have been obtained with a straight 15% solution of pDIyvinyl alcohol and water, either of the.@low viscosity pr medium viscosity type. The low viscosity type has a molecuiar weight such that a 4% solution has a viscosity of 5 centipoises at 20. C., and is somewhat more satisfactory than the medium viscosity type for forming polyvinyl alcohol films upoli regenerated cellulose :Mjns. However, the use of a higher viscosity type, either alone or in admixture with a type of lower viscosity, is not precluded. EXAMPLE 6 (INx) A B Polyvinyl alcohol --------------------------------- 7.7 7.7 Water --------------------------------------------- 6q. 2 79.2 Titanium dioxide --------------------------- ----- 16.4 15.4 hn 'al Red (X-1148) or Imperial Green (A-- 8028) 7.7 7.7 im 'r it.9b.'r ite. (6011/@) ------------------------------- ioo.o 50. 0 Ammonium hydroxide (28,y.) ---------------------- -------- 5.0 The poiyvinyl alcohol is dissolved in the water, the pigments added, and thorough mixing is effected in a paint mill. The dispersion of pigments is aided by the polyvinyl alc(yhol. The latex is then graduary stirred in to secure a urtiform ink, the ammonium hydroxide being added first when used. The colored pigments listed iri the formula are sold by the Imperial Paper and Color Corp., of Glens Falls, New York; but it will be understood that they are given merely by way of illustration. This colored ink may be used for printing or coating the fEm backing (such as regenerated cellulose) , the latter first being preferably pirimed (such as with the primer set forth in Example 2)1 . and the colored ptinting or coating being pref ,erably covered with a further coating of transparent priiner befgre applying the adhesive. Formaldehyde and ammonia may be included to render the polyvinyl alcohol rnore water-reaistant as hereinbefore mentioned. ExAmPLE 7 (INic) Part A Polyvinyl alcoh6l ------------------------- 375 2,328,057 Part B Part A solution -------------------------- 500 Titm2lum dioxide- ---------- loo 5 IinPerial Red or Imperial Green ----------- 50 The Part A solutiori is made by dissolving the PolYvinYl Eklcohol in water. 500 parts of this are usecl in making part B, the pigments being ground in by use of a paint mill. Then to part B is IC) added the remainder of part A (3250 Parts), with thorough mixing to se'eure a homogeneous product. The resultant polyvinyl alcohol ink solution may be used for coating or printing the hydrophilic backing film as hereinbefore described (see 15 . Mg. 10 for example), and anchors firmly thereto. Formaldehyde and ammonia may he included to render the Polyvinyl alcohol more water-resistant. 20 Adhesive Example I Parts by weight Milled latex crepe ------------------- 250 Ester gui@a (acid number of 2-10) ----- 175 "Flectol B" (antioxidant) ------------ 1.25 25 KePtane (volatile solvent) ----------- 1550 Alcohol (viscosity reducer) ----------- 15.5-31 The raw latex crepe is milled for about 10 minutes, at a temperature of about 140-150' P., and is then cut into small Pieces. The ester gum 30 Is dissolved in an equal amount of the solvent, ancl the "Flectol B" is likewise dissolved in a portion of the solvent. The balance of the solvent is Placed in a suitable churn and the various ingredientsadded and churned until a homogeneous 35 solution is obtained. The resulting adhesive solution is then ready, for coating upon the primed cehulose film, to form, upon evaporation of the solverit, a normally tacky and p ressure-sensitive transparent adhesive coating of such high co40 hesive strength that internal yielding will not occur to cause adhesive offsetting, when the adhesive sheet is strip]@ed frorn surfaces to which temporarily applied, such As glass, cellulosic fllrns, 45 and wood. This adhesive coating is quite stretchy and hence adapted for use on stretchy backings such as polyvinyl alcohol films. The alcohol is employed as a viscosity reducer to decrease the amount of solvent kequired to pro50 duce a solution of desired viscosity, a-nd may be omitted with a compensatory increase of solvent. The alcohol may be denatured ethyl alcohol. or may be meth@l alcohol, for example. The "Fleetol B" (a liquid condensation product of acetone and aniline) is a well known rubber antioxidant 55 and may be substituted for by others, such as beta-naphthol and "Solux" (p-hydroxy-Nphenyl-morpholine). A feature of the present primer is that adhesive solutions of this type nlay be applied with60 out washing off or damaging the primer film. As previously mentioned, a coloring pigment may be incorporated in the adhesive. This may be co,iveniently accomplished by miring the pigment into the rubber during the preliminary mill65 ing of the latt6r. Thus para-toluidine red may be added to give a red color, or aluminum bronzing powder to give a silver color. Zinc oxide may be incorporated if desired. 70 Various other tack-prbducing resins compatible with rubber, may be used in place of the specified ester gum, such as for example, wood or gum rosin, hydrogenated rosin or ester gum, and hydrogenated coumarone and indene resins. Water ---------------------------------- 3375 75 Plastieizers and softening agents may be included.
[9]2,328,057 -Adhesive gxampk 2 Parts Rubbery isobutylene polymer resin (52,000 molecular weight) --- - --------------- 100 Hydi-6genated indene resin of 145@-150, C. . meltjng point ----------- - ----------- --- 40 Plwticizing agent such as inethyl abletate or hydrogenated methyl abietate --------- 20 Softener, such as butyl steai-gte - --------- 10 Heptane (volatile solvent) ---------------- 700 7be above example Mustmtes a transparent hydroi>hobic water-insoluble pressuresensitive adhesive which contains no rubber@ but which bonds firmly to the primer becausl- of compatibiuty with the latex, or rubber phase of the primer, or other organophflic bonding componefat such as iuustrated in lftslmple 4. These adhesives may be compounded without the solvent, by millin the h2gredierkto together, and then calendered or frictioned L Upon the primed backing. Having described vadous embodiments of my invention for purposes of Mustmtion, butwithout intent to be limited theretoi wbat I
