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Nov. 26, 1968 A. J. DANIELSON Etal 3,413,168
ADHESIVE BONDING METHOD PERMITTING PRECISE POSITIONING
Filed May 3, 1967
United States Patent Office P_d JTMZ
ADHESIVE BONDING METHOD PERMITTING
Alton J. Danielson, Stillwater, and Harold A. Berg, North
St. Paul, Minn., assignors to Minnesota Mining and 5
Manufacturing Company, St. Paul, Minn., a corpora-
tion of Delaware
Continuation-in-part of application Ser. No. 258,487,
Feb. 14, 1963. This application May 3, 1967, Ser.
7 Claims. (CI. 156—71) 10
ABSTRACT OF THE DISCLOSURE
Small protrusions having a non-adhesive exposed sur- 15 face and which are collapsible under hand pressure, e.g., fragile microspheroids, are sparsely randomly uniformly distributed over the pressure-sensitive-adhesive-coated surfaces of various articles to permit sliding non-adherent contact and precise placement of the article on an ad- 2^ hesive-receptive surface prior to final pressure-bonding.
This application is a continuation-in-part of copending application Ser. No. 258,487, filed Feb. 14, 1963, now U.S. Patent No. 3,331,729.
This invention relates to articles, particularly sheet materials, made from or coated with adhesives, and to meth- 30 ods of making and using the same; and is particularly, although not exclusively, concerned with adhesive films and coatings wherein the adhesive material is normally tacky and pressure-sensitive.
Pressure-sensitive adhesives as employed on masking 35 tape products and the like are characterized by a "quickgrab" property which permits the tape to adhere on mere contact with adhesive-receptive surfaces to which applied. Once the adhesive tape or sheet contacts the surface it can no longer be moved about without being first stripped from 40 the surface. The application of pressure-sensitive adhesive sheet materials in larger sections to specific surface areas is thereby made unduly difficult. The problem is still more severe where the pressure-sensitive adhesive forms a strong permanent bond with the surface, or where the carrier 45 sheet is easily wrinkled or torn or otherwise distorted.
As an example of the difficulties involved, large sheets of figured or patterned paper or fabric wall-coverings coated with pressure-sensitive adhesive have heretofore been virtually impossible to apply because of the difficulty 50 of matching the pattern while preventing the coated surface from becoming tightly bonded to the wall, and the tendency of the adhesive surface to cohere permanently to itself when once placed in contact. The difficulty has previously been overcome primarily by providing the coated 55 sheet with a removable cover-sheet or liner which may be progressively stripped from the adhesive surface just prior to pressing the latter against the wall. The tension required to remove the liner frequently causes dislocation or wrinkling of previously applied areas, or distortion of 60 the coated covering. Wall coverings accordingly have not ordinarily been bonded in place with pressure-sensitive adhesives.
Even with coverings of much smaller areas, much difficulty has been encountered where extremely accurate 65 positioning or registering of the adhesive-coated covering is required. The locating on a flat sheet-metal base of numbers of small oddly shaped segments of differently colored plastic films or beaded reflex-reflective sheet materials in close-fitting patterns is often required in the man- 70 ufacture of sign-boards and markers. Precise placing of the segments is difficult when they are adherent on contact.
Coating the base surface with a liquid adhesive which for a time permits removal and relocation of the segments has not been found fully effective since the adhesive contains volatile solvents which must first be removed and the dried layer soon cures to a state in which it no longer forms an adherent bond to the film or sheet.
Medallions, labels, face-plates, printed circuit components and other attachments are frequently desired on metal or other substrates. Adhesives containing volatile solvents or vehicles in many instances cannot be used for such purposes because of the difficulty of removing the solvent after the vapor-impermeable covering is in place. Coatings of normally non-tacky heat-activatable adhesives or of fusible solder or other metallic bonding agents permit precise positioning of the coated coverings but are not satisfactory on bulky metal substrates having high heat capacity or on substrates which are not sufficiently heatresistant. Pressure-sensitive adhesives, even though providing ample bonding power and ease of application, have not heretofore been found desirable for these applications because of their "quick-grab" properties which prevent repositioning of the member when once tentatively applied.
The present invention overcomes these and other deficiencies and disadvantages. In ;a preferred form of the invention there is provided an adhesive coating which is pressure-sensitive and capable of forming a strong adhesive bond with a substrate surface on being pressed thereagainst under moderate hand pressure, but which on light contact with said surface remains unbonded thereto and which may therefore be moved around over the surface and slid into position as desired. Neither drying nor heating of the assembly is required. A firm bond is attained directly, and merely by localized application of pressure.
The invention also provides a method of making these adhesive-coated sheet materials in a form in which the coating is effectively protected from pressure-activation even against severe localized application of pressure, for example in printing or embossing the reverse surface of the coated sheet or in cutting or stamping labels or other cutouts from stacks of the sheets, or in otherwise processing the sheet material for decorative or other applications.
These and other unique and advantageous properties and results are obtained by supplying over the adhesive coating a sparse uniform distribution of very small hollow thin-walled fragile microspheroids or "Microballoons," preferably embedded somewhat beyond their centers in the adhesive layer and protected from premature collapse by a close-fitting removable protective covering, all as will be hereinafter described and illustrated.
In the drawing,
FIGURE 1 is a schematic representation outlining a presently preferred method and apparatus for making the adhesive sheet material, and
FIGURE 2 represents in cross-section the several stages in the manufacture of the adhesive sheet material by the method and apparatus of FIGURE 1;
FIGURES 3 and 4 show a cross-sectional view of a protected adhesive film as applied and put to use on a rigid substrate; and
FIGURE 5 illustrates a doubly protected adhesive film also in cross-section.
In FIGURE 1, a web of thermoplastic polymer coated paper 10 from a supply roll 11 is first drawn past a source of thin-walled hollow fragile small spheroidal particles. An aspirator 12 supplied with compressed air as indicated by the arrow 13 and with microspheroids 14 from container 15 provides a convenient source. The web in horizontal position is heated in an oven 16, and is then drawn beneath a coating hopper or spreader bar 17 where it is evenly coated with a solution of pressuresensitive adhesive 18. The coated web next passes through
an oven 19 for removal of solvent, the dried product being indicated at 20. A protective and decorative film or web 21, for example a pigmented plastic film, from supply roll 22 is united with the dry adhesive surface by pressure applied between squeeze rolls 23 and 24. 5 The completed sheet product 25 is then wound up into storage roll 26 for subsequent conversion to sizes and shapes as desired.
The several stages of manufacture are illustrated in FIGURE 2. The first stage illustrates the micropheroids 10 14 held by electrostatic attraction to the flat smooth surface of the polymeric coating 27 of the web 10. In the second stage the microspheroids have penetrated, desirably for about one-fourth to about one-third of their diameter, into the heat-softened coating, the latter being 15 displaced thereby to provide corresponding rounded socket-like depressions 28. The dried pressure-sensitive adhesive coating 29 added at stage 3 has a gently globuliferous pebbled surface appearance. The application in stage 4 of the plastic film 21 under pressure partially 20 flattens the adhesive protuberances and provides a strong adherent bond between the adhesive surface and the surface of the plastic film, but does not flatten or shatter the microspheroids 14 in their close-fitting sockets.
In a specific illustrative example the web 10 consists 25 of 70 lb. kraft paper (per ream of 3000 sq. ft.) smoothly and uniformly coated with 18 lbs. per ream of polyethylene.
The walls or shells of the hollow thin-walled fragile microspheroids are composed of urea-formaldehyde resin 30 and the screen size is defined as "through 140 mesh, on 270 mesh." Thus the particles have an average diameter of between about 55 and about 105 microns. For an illustrative preparation of such hollow particles, reference is made to Veatch et al. U.S. Patent No. 2,797,201. 33
The hollow particles are applied as uniformly as possible over the surface of the polyethylene coating at a rate of approximately one-half lb. per thousand square yards, which is about one-third the amount needed to provide a compact monolayer. The electrostatic charges 40 produced are sufficient to adhere the dry particles to the plastic surface and to maintain good particle distribution.
The web is heated for about 30-45 seconds at 275° F. in the horizontal position, during which time the poly- 45 ethylene softens and the particles sink into the plastic layer. Heating conditions are controlled so that the particles penetrate the plastic to about one-fourth to onethird of their diameter and without damage to the particles. The web is then cooled. 50
A coating of pressure-sensitive adhesive composition is next applied, the spreader bar being set at ten mils above the exposed tips of the partially embedded microspheroids. The composition, as described in Ulrich Reissue Patent No. 24,906, consists of a copolymer of 95 55 parts of fusel oil acrylate and 5 parts of acrylic acid, in solution in a mixture of ethyl acetate and heptane at a coatable viscosity and at a concentration of about 20-25 percent. The coating is dried by heating at 150° F. for approximately ten minutes, forming a pebbly sur- go face which adheres well to a variety of surfaces on contact.
A plastic decorative and protective film is separately produced by calendering to a thickness of four mils a pre-milled composition containing essentially 500 parts @5 by weight of "VYNW" vinyl chloride-vinyl acetate copolymer, 100 parts of "Paraplex G-40" polymeric plasticizer, 75 parts of dioctyl phthalate, and 100 parts of titanium dioxide pigment, together with small amounts of stabilizers and lubricants where desired. As described in 70 Oace et al. Reissue Patent No. 23,843, such films are characterized as being capable of remaining in stable equilibrium with rubber-resin type pressure-sensitive adhesives applied thereto. The smooth shiny white film is combined with the adhesive-coated web by pressing be- 75
tween rubber-covered roller 23 and steel roller 24, the pressure being sufficient to at least partially smooth out the adhesive layer and to form a firm bond with the vinyl film but insufficient to cause disruption of the hollow particles 14. Where desired, the film may first be provided with a thin coating of an adhesive primer composition for improving the bond with the pressuresensitive adhesive.
The product can now be wound into rolls, or cut into sheets and stacked for storage and shipment. The handling and pressing incident to these operations does not flatten or fracture the microspheroids, due to the presence of the close-fitting polymeric protective layer which equalizes the forces over the entire surface.
When it is desired to apply the adherent protective film to a substrate surface, for example a section of wall or of metal signboard, the film and protective carrier are first stripped apart. The polyethylene readily releases the adhesive and the microspheroids, so that the film 21 is not stretched or pulled out of shape and the protruding portions of the hollow particles provide spacing means for holding the adhesive slightly away from the substrate surface. The film is placed lightly on the surface and gently slid into the exact position desired, the microspheroids continuing to support the film and to prevent its adhesion to the substrate. When the desired placement has been achieved, the film is pressed into adhesive contact by localized pressure applied with a small hard roller, such as the wooden roller used by paper-hangers, or with the fingertips, or by forcefully drawing a blunt-edged scraper bar across the film surface. As is customary in the application of wallpaper or the like, the application of pressure is initiated at or near the central area of the sheet, and is extended progressively toward the edge areas so as to avoid any entrapment of air bubbles between the two surfaces. Similarly, initiating the pressing action at raised areas, e.g. along seams or over rivet-heads in the substrate surface, and extending toward the lower or edge areas serves to provide maximum smoothness of application and to prevent the formation of bubbles or blisters. Under the pressure applied, the hollow particles collapse, permitting the adjoining adhesive surfaces to contact and adhere to the surface of the substrate.
The residual shells of the collapsed particles remain in the adhesive layer and reduce the force of adhesion' between the adhesive-coated film and the substrate, as measured by the stripping force necessary to separate the two, by an amount closely proportional to the area of the adhesive surface occupied by the microspheroids. As an example, a one-inch wide strip of the coated film prepared as described above and applied to a clean smooth aluminum plate required a force of 4.5 lbs. for removal by stripping back over itself at a standard speed of 12 inches per minute. A strip otherwise identical but containing no microspheroids exhibited a removal of 5.5 lbs. In the first sample, the particles were present to the extent of about one-third the concentration required for close packing, and therefore covered approximately one-fifth to onefourth of the total surface.
An impressive demonstration of the effectiveness of the principles just illustrated may be made with an adhesive-coated sheet material prepared in the same manner with the single exception that the outer heavy vinyl film is replaced with a thin and easily torn film or treated paper. The resulting sheet is stripped from the supporting liner and placed, adhesive side down, on a flat clean metal panel, where it is easily slid about by pulling on the edges. A central strip is then pressed firmly against the panel by drawing a finger-tip along the upper surface under firm pressure. The edge of the sheet is then lifted and an attempt made to remove the sheet by lifting or stripping. The pressed area remains firmly bonded to the metal. The remainder is not adhered to the metal and is easily torn away and removed.
Although the polyethylene-coated paper protective liner is removable from the adhesive surface by hand stripping, the force necessary for the removal may be found to be undesirably high, particularly where larger areas are concerned. This difficulty may be avoided by applying over 5 the flamed or otherwise suitably treated polyethylene surface a very thin well-bonded coating of a release agent or low-adhesion back-size. A surface coating of "DC-23" or other methylsilicone polymer release agent is exemplary, such a layer being shown at 30 in FIGURE 3 as applied ^ to the surface of the polymeric coating 27. The silicone composition is conveniently applied from solution and cured by preliminary heating. The amount employed is sufficient to permit easy removal of the subsequently applied adhesive but is so slight as to have no apparent 15 effect on the partial penetration of the spheroids into the softened plastic surface layer.
The procedure employed in the preceding example and illustrated in FIGURE 1 is particularly useful with such easily collapsed fragile microspheroids as the urea-formal- 2o dehyde resin capsules there described, and may also be used with less fragile microspheriods. A somewhat modified process, which is preferred with the latter class of spheroids, involves coating and drying the pressuresensitive adhesive directly on the plastic film or other 25 permanent backing and then simultaneously embedding the hollow particles partly within the dry adhesive layer and partly within the plastic release layer of the removable liner. The spheroids are conveniently applied to either the liner or the adhesive surface by spray application as 39 a slurry in water, the water being substantially completely evaporated during the spray process. The liner is passed over a heated drum, where the plastic layer is softened sufficiently to permit the desired depth of penetration of the spheroids; and the adhesive-coated film, also at ele- 35 vated temperature, is pressed against the plastic surface with the layer of particles between.
With some film materials it is not possible on a commercial basis to apply the liquid adhesive composition directly to the plastic film. The following example there- 40 fore illustrates a further modification in which the adhesive is first dried separately and is subsequently applied to the film, after which the hollow microspheres are placed on the adhesive surface and the sheet pressed against the plastic surfaced liner. ^_
A temporary carrier web of heavy kraft paper having on one surface a smooth layer of polyethylene and sized with a cured polymethylsilicone release coat is coated with an adhesive composition, the formula being as follows: 50
Acrylate copolymer 566
Phenol-aldehyde resin 141
Salicylic acid 28
Zini resinate ("Zirex") 85
Heptane 6690 55
Alcohol : 226
The acrylate copolymer is a copolymer of three parts by weight of 2-ethyIbutyl acrylate and one part of ethyl acrylate. The phenolic resin is a heat-advancing resin which is 60 compatible with the copolymer.
The adhesive is dried at 150° F., the dry coating weighing 5-6 grains per 24 sq. in. A 4-mil vinyl resin film as hereinbefore described is combined with the adhesive coating under squeeze-roll pressure. The carrier web is 65 stripped away. Small fragile hollow glass spheroids are formed into a slurry in about eight times their weight of water and the slurry is sprayed onto the adhesive surface. Essentially all of the water evaporates during the coating procedure. About one-fourth to one-third of the 70 surface is covered with the spheroids. The carrier is then passed over a steam-heated roller where it is heated to approximately 270° F., at which point the spheroid-coated adhesive film is returned into pressure contact with the softened plastic release layer. The pressure, applied with 75
a rubber-covered roller having a Shore durometer hardness reading of about 60, is maintained at approximately one pound per inch of width. The spheroids become embedded to approximately an equal depth in the plastic layer and in the adhesive layer. The product is then cooled and wound up in roll form.
Thin-walled hollow small glass spheres or spheroids represent a class of microspheroids which have sufficient strength and rigidity to withstand the moderate pressures applied through the heat-softened liner and adhesive layers in the modified process just described while still being sufficiently fragile to be collapsed under application pressures applied to the unprotected exposed particle surfaces. The fragility of such particles may be controlled by controlling the thickness or composition of the shell wall, or by introducing strains or areas of weakness in the shell as by surface etching or the presence of inclusions, or in other ways.
The more fragile resinous microcapules, if previously partially embedded in the polyethylene layer as described in the earlier example, may be adhered to or partially embedded in the heat-softened dried adhesive layer in the same manner as just described, to provide a product which initially possesses the desired free-sliding characteristics but which subsequently loses this property because of the gradual collapse of the protruding portions of the hollow particles.
FIGURE 3 illustrates the application of the adhesivecoated web 20', corresponding to the web 20 of FIGURE 1 but including the adhesive release layer 30, to a rigid substrate 31 such as a smooth concrete or wood subfloor. The liner is then stripped away in successive small areas and replaced, as shown in FIGURE 4, by the permanent floor surfacing material 32, which may for example be a vinyl floor tile. The tile is slid into the desired position and is then bonded in place by the application of localized pressure sufficient to collapse the microspheroids 14. By placing boards or other flat-surfaced walkways or supports over the protective liner at areas not in process, a workman is able to reach all parts of the floor area without affecting the microspheroids, which remain as fully effective spacers until purposely collapsed by localized pressure on the flexible vinyl tile. Ceramic tile and other rigid attachments are similarly positioned and adhered in place where the tile area is sufficiently small and the available pressure sufficiently large to provide unit pressure sufficient to collapse the separating hollow particles.
A further application of the principles of the invention is illustrated in FIGURE 5 wherein is shown in crosssection a self-sustaining strip of adhesive 33, preferably a pressure-sensitive adhesive, which has been sparsely coated on both surfaces with partially embedded microspheroids 14. The adhesive strip is first positioned in a desired location by sliding into place, and one or more pieces of decorative, protective, or functional sheet material applied thereover and also slid into desired position. Adhesive bonding is then accomplished by application of localized pressure sufficient to collapse the particles. The adhesive strip may be prepared as described hereinbefore, i.e. on a liner, or between two liners, carrying the partially embedded microspheroids. Where two liners are employed, it is convenient to employ a polysilicone coating on only one of them, or different coatings on the two, so that they require a differential removal effort.
The same principles may be applied to freshly prepared coatings of temporarily or permanently tacky or selfadherent adhesive materials. As an example, in bonding "Micarta" cured resin sheet material to desk or counter tops, both the substrate and the sheet are coated with a high strength self-bonding adhesive cement which is then dried. The cement as generally used comprises a blend of a synthetic rubbery polymer, a compatible phenolic resin, and an alkaline earth oxide. On contact, the two layers bond together so firmly that they cannot be separated. Thus while the adhesive cement is not properly