STRIPPABLE FILM FOR ADHESIVE COATING AND LAMINATING AND PROCESS FOR APPLYING ADHESIVE TO A BASE MEMBER

Abstract: A strippable film for adhesive coating and laminating comprises a base paper, and an extrusion coated thermoplastic resin layer. The film has the property that, upon coating an adhesive onto the resin layer and assembling the adhesive coated strippable film with a base member, the base paper can be stripped from said resin layer and the resin layer can be subsequently stripped from the adhesive coating.

Description: The present invention relates to a simplified process for applying an adhesive to a base member. It also relates to a strippable film useful in said process. In many applications, adhesives are applied to a base member as a pre-formed film which is carried on a polymeric release liner. Due to the nature of the adhesives required in many of these applications, the adhesive layer cannot be formed directly on the release liner. Many adhesives are extrusion coated and/or cured at elevated temperatures at which the release liners, which are usually thermoplastic, are not dimensionally stable. At the elevated temperatures used to extrude and/or cure the adhesives, the release liner softens and stretches under the weight of the adhesive. Currently in applications requiring the combination of a polymeric release liner and heat treated adhesives, the adhesive is coated onto a release paper and laminated to the film which functions as the release liner. The paper provides the necessary supporting substructure during the coating and curing of the adhesive. The release paper-adhesive-release liner composite is wound into a roll which may be stored until application to a base member whereupon the release paper is removed and the adhesive layer with the release liner is assembled with the base member in a transfer laminating operation. U.S. Patent No. 3,509,991 to Hurst teaches a release sheet and an adhesive structure embodying the same. This reference addresses the problem which occurs in marketing adhesive tape in roll form, i.e., the non-backed adhesive sticks to the backside of the tape already in roll form. The problem is overcome by providing a release sheet with different release values on each side. The release sheet comprises a sheet of paper which is extrusion coated with polyethylene and then both sides are coated with a release agent. Pressure sensitive adhesive is then applied to the surface with the more difficult release properties and the resulting laminate is wound into roll form. When pulling the laminate from the roll, the adhesive sticks to the surface having the more difficult release properties rather than to the backside of the tape already on the roll. When the tape is used, the adhesive is applied to a desired surface, and the film-paper laminate is removed from the adhesive. The reference teaches that the polyolefin film and paper layer are firmly bonded to each other. U.S. Patent No: 3 690 909 to Finley discloses a strippable release composition in which a silicone-alkyd resin is applied to a base substrate and cured thereon. A polyolefin film is applied to the cured resin layer. The polyolefin film adheres sufficiently to the alkyd resin for further processing such as the addition of subsequent layers but is releasable therefrom. U.S. Patent No. 4,398,985 to Eagon teaches a releasable self-detackifying laminate construction. The reference addresses the problem that once the paper stock is peeled off a laminate, a tacky pressure sensitive adhesive surface is exposed on the base stock. This tackiness makes further processing of the base stock by automated equipment next to impossible. The reference overcomes the problem by coating a film of a non-silicone containing polymer onto one side of the paper stock and then laminating this construction to an adhesive which has been coated onto a release liner. When the paper stock is peeled away, the polymer film will delaminate from the paper stock and be permanently affixed to the adhesive coating. The resulting substrate has a detackified adhesive surface which can be further used or processed either by hand or by automated equipment. The term "base member" as used herein refers to a number of products to which the adhesive layer and accompanying release liner may be applied including but not limited to signs, labels, tags, credit, business or membership cards, etc. The term "base paper" refers to the paper substrate upon which the release liner is formed and which supports the liner in the presence of the heated adhesive. The term "dual strippable" refers to the property of the release liner wherein the release liner is strippable from the base paper and subsequently strippable from the adhesive film after application of the adhesive film to a base member. The term "adhesive coated member" refers to the adhesive backed intermediate formed prior to application of the adhesive to a base member. The present invention provides, in a first aspect thereof, a strippable film adapted for use in adhesive coating and laminating, comprising a base paper, and an extrusion coated thermoplastic resin layer; said film being characterised in that the adhesion between the base paper and the resin layer is such that if an adhesive is coated on to said resin layer and the adhesive coated strippable film is then assembled with a base member, said base paper can be stripped from said resin layer leaving the said resin layer still adhered to the adhesive coating, and enabling the resin layer to be stripped subsequently from the adhesive coating. The invention also extends to an adhesive coated dual strippable film adapted for applying the said adhesive to a base member, characterised in comprising a film as aforesaid with the said adhesive coated on to the said resin layer. The present invention also provides, in a second and alternative aspect thereof, a process for applying an adhesive to a base member which comprises the steps of: extruding a thermoplastic resin layer onto the surface of a base paper to form a laminate while limiting the degree of penetration of said base paper by said thermoplastic resin layer such that said base paper can be stripped from said thermoplastic resin layer; applying an adhesive coating to said laminate such that said thermoplastic resin layer is interposed between said base paper and said adhesive coating; applying said adhesive coated laminate to said base member; and stripping said base paper from said thermoplastic resin layer. The adhesive coated members produced by following the teachings of this invention, in effect have a dual strippable release liner. The adhesive coated member comprises a base paper, an extrusion coated thermoplastic resin, and an adhesive coating. The release force between the adhesive coating and the thermoplastic resin layer in preferred embodiments is greater than the release force between the thermoplastic resin layer and the base paper. In use, this differential release capability allows the film to be coated with an adhesive and assembled with a base member, the base paper to be stripped from the film, and the film to be subsequently stripped from the adhesive layer. Selective release characteristics of the thermoplastic resin layer allowing the base paper to be stripped from the resin layer which itself can subsequently be stripped from the adhesive coating, are achieved by controlling a number of variables. Thus the thermoplastic resin layer can be prevented from infiltrating the pores of the base paper on processing, i.e., upon application of the thermoplastic resin film to the base paper and upon subsequent application of the adhesive coating to the thermoplastic resin layer. The selective release characteristics of the thermoplastic resin layer are to a large extent achieved through selection of the resin, limiting its extrusion temperature and controlling the penetration of the base paper by the coated resin. Extrusion coating conditions are discussed later. Useful thermoplastic resins include low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, polypropylene, polycarboxylate, poly(methyl methacrylate), poly(ethylene terephthalate), poly(butylene terephthalate), polyurethane, copolyester, ionomers, thermoplastic elastomers, and copolymers of many of these resins. A particularly useful thermoplastic resin is a polyolefin. Preferred polyolefins are polyethylene and polypropylene. These resins may contain coloured pigment for final product identification purposes. The selective release characteristics of the thermoplastic resin layer can be partially achieved by controlling the paper density. With a view to preventing the thermoplastic resin layer from infiltrating the pores of the base paper during processing to an extent which would prevent the base paper from being stripped from the resin layer, a useful paper has a high density or low porosity. We have found papers with a density of at least 0.75 g/cm³ to be useful. A preferred paper has a density of about0.95 to 1.12 g/cm³. Useful papers typically have a thickness of about 3.0 to 12 mils (0.00762 to 0.03048 cm). Particularly useful papers are 3-5 mils (0.00762 to 0.0127cm) thick with a density of 1.0 g/cm³. Adhesion of the thermoplastic resin layer to the base paper can also be controlled by the addition of a release agent or an antioxidant to the thermoplastic resin. The selective release characteristics of the thermoplastic resin layer from the base paper are also partially achieved through treating the base paper with various sizing agents and/or release agents which prevent the thermoplastic resin layer from penetrating the base paper upon processing. A typical sizing agent is starch but other sizing agents such as PVA, Scripset (a product of Monsanto Company), gelatin, and soy protein are also useful. Typically, the sizing agents are applied at a rate of about 60 to 120 pounds/ton (30 to 50 Kg/tonne). The base paper must be sized properly to ensure the proper release characteristics of the thermoplastic resin layer preferably without any fibre contamination in the thermoplastic resin layer. Fibre contamination represents an aesthetic fault which can lead to irregular stripping of the base paper during processing. Release agents can also be incorporated into the base paper to reduce the adherence of the thermoplastic resin layer to the base paper. Useful release agents include Quilon (Chrome Complexes) (a product of E. I. DuPont deNemours & Co.) and Scotchban fluorocarbons (a product of 3M). A preferred release agent is Quilon. Typically, the release agents are incorporated into the base paper at about 0.1 pound /3,000 sq. ft(0.162 g/m²). The selective release characteristics of the thermoplastic resin layer from the adhesive coating can be achieved by controlling the surface energy of the resin layer and/or addition of release agents into the thermoplastic layer. The aforementioned variables plus other process variables to be discussed later all contribute to achievement of selective release characteristics for the strippable film. The release force between the adhesive coating and the thermoplastic resin layer has to be greater than the release force between the thermoplastic resin layer and the base paper. The selective release characteristics allow the base paper to be stripped from the thermoplastic resin layer and the thermoplastic resin layer to be subsequently stripped from the adhesive coating. The resulting release force of the base paper from the thermoplastic resin layer is about 5 to 200 grammes force /in (19.3 to 772 N/m) (90° peel) while a preferred release force is about 20 to 80 grammes force/in (77.2 to 309 N/m). This release force is limited on the low end by premature separation of the thermoplastic resin layer and the base paper. The resulting release force of the thermoplastic resin layer from the adhesive coating is about 10 to 250 grammes force/in (38.6 to 965 N/m) while a preferred release force is about 25 to 100 grammes/in (96.5 to 386 N/m). This release force is limited on the low end by the release force of the resin layer from the base paper and on the upper end by adhesive tack and thermoplastic resin strength. The term "base member" refers to a wide variety of products to which an adhesive coated strippable film may be applied. Examples of base members include signs, labels, tags, and credit, business or membership cards. A release coating may be interposed between the thermoplastic resin layer and the adhesive coating to contribute to the achievement of selective release characteristics between the thermoplastic resin layer and the adhesive coating. A release coating is particularly useful when pressure sensitive adhesives are used. A preferred release coating is a conventional silicone release coating. The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings, in which:- Fig. 1 is a schematic illustration of successive stages 1A to 1E of a previously proposed process for coating an adhesive and laminating the adhesive to a base member; and Fig. 2 is a similar schematic illustration following the teachings of the the present invention. Referring first to the previously proposed process of Fig. 1 for coating an adhesive and laminating the adhesive to a base member, Figure 1A shows a paper 10 carrying a coating 12 (e.g. a polymer coating, a silicone coating, or a clay coating) after coating with an adhesive 14. Due to the nature of the adhesive required in many applications, the adhesive cannot be coated directly onto the release film but instead must be coated onto the coated paper as shown in Figure 1A. The release film 16 is applied after application of the adhesive 14 while the adhesive 14 is supported on the paper as shown in Figure 1B. Figure 1B illustrates the adhesive-coated coated paper after lamination of release film 16. The coated paper provides the necessary supporting substructure to prevent the release film 16 from stretching in the presence of the heated adhesive coating 14. If desired, the laminate of Figure 1B can be wound up and stored in the form of a roll which is unwound when transfer to the adhesive to the base member is desired. The coated paper is then stripped from the adhesive 14 as shown in Figure 1C which is then applied to a base member 18. Figure 1D illustrates the adhesive coated release film after application to base member 18. The release film 16 can be removed subsequently as shown in Figure 1D. The adhesive coated base member is ready for application to an appropriate substrate as shown in Figure 1E. In the process illustrated in Fig. 2, the adhesive is coated directly on the strippable film. Figure 2A illustrates a base paper 30 after extrusion coating of a thermoplastic resin layer 32. As mentioned earlier, the selective release characteristics of the strippable film are also achieved by controlling the coating conditions. The extrusion coating temperatures can be varied somewhat in contributing to the achievement of the controlled release forces. By employing low extrusion temperatures, the resin sets up faster on the surface of the base paper, and does not penetrate the base paper surface and increase adhesion. Useful extrusion coating temperatures will vary with the nature of the resin. For polyethylene, extrusion temperatures of about 425 to 575°F (218 to 302°C) are used. Of course, higher extrusion temperatures can be used if the resin is allowed to cool before it is brought into contact with the base paper. Thus, by allowing the extruded resin layer to fall a distance before contacting the base paper such that it cools, its penetration into the base paper can be controlled. Typically, the extrusion coated thermoplastic resin layer has a thickness of about 0.5 to 10 mils (0.00127 to 0.0254 cm). In a preferred embodiment, the extrusion coated thermoplastic resin layer has a thickness of about 1.5 to 3.5 mils (0.00381 to 0.00889 cm). Figure 2B illustrates the thermoplastic resin coated base paper after coating an adhesive 34 onto the thermoplastic resin layer 32. The present process allows direct adhesive coating of the strippable film. The strippable film and process can be used in conjunction with a variety of adhesives requiring elevated temperatures to apply or cure the adhesive. Both pressure-sensitive and hot melt adhesives can be used. Typical examples are Rhoplex (a product of Rohm and Haas Company) and Gelva acrylic based adhesives (products of Monsanto Co.). Useful adhesive drying and/or curing temperatures are about 225°F (107°C). Preferred adhesive drying or curing temperatures are about 180 to 250°F(82 to 121°C). The adhesive coated strippable film is laminated to a base member 36 as shown in Figure 2C and the base paper 30 is removed. Thereafter, at the time of application of the base member to a supporting substrate, thermoplastic resin layer 32 can be removed as shown in Figure 2D. Figure 2E illustrates the adhesive coated base member after removal of the thermoplastic resin layer 32 and which is ready for application to the desired substrate. In application of pressure-sensitive adhesives, a release coating may be desired between the thermoplastic resin layer 32 and adhesive coating 34. Such a coating can be applied in any conventional manner. The present invention is particularly useful in manufacturing printed adhesive overlays or "decals" for signs or the like as more fully illustrated by the following non-limiting example. Example A base paper was prepared by refining a mixture of 60% softwood Kraft and 40% hardwood Kraft to a Canadian Standard Freeness of 430 to 450, adding to the refined pulp 12 pounds per ton of a cationic starch (Cato 15) and the paper formed and dried in the paper machine. While still on the paper machine, the paper was run through two size presses followed each time by drying. At both size presses, the paper was treated with an aqueous solution of 6% paper coating starch. The total pick up of starch surface size on both size presses was about 5.2 g/m². The paper was machine calendered on the paper machine to give a smooth surface. The Sheffield smoothness was about 40 to 50. The basis weight of the paper was 110 g/m², the caliper 4.3 mils (0.010922 cm) and the density 1.0 g/cm³. The paper produced as above was extrusion coated at about 300 ft./min. (152 cm/S)on one side with 3 mils (0.00762 cm) of Eastman polypropylene resin 4E31. A masterbatch NC-3340-C containing a yellow pigment obtained from Plastic Color Chip Inc. was blended with the polypropylene at 1.5% loading imparting a yellow colour to the polypropylene film. The melt temperature from the extruder was 550°F (288°C), the glossy chill roll was kept at 60°F (15.5°C) and the nip pressure from the pressure roll was 120 pounds per lineal inch (210 N/lineal cm). The release force of the strippable film from the paper base as measured in the laboratory ranged from 32 to 44 grammes force/inch (123.5 to 170 N/m) and averaged 39 grammes force/inch (150.6N/m). In a second converting operation, the glossy side of the yellow film was coated with a solvent based silicone coating so that it would have suitable release from pressure sensitive adhesives. In a third converting operation, the silicone coated film was coated with Monsanto's solvent based Acrylic adhesive Gelva 737. The adhesive was applied with a knife over roll application and was dried and cured in a two zone oven at 150°F (65.5°C 225°F (107°C), with a dwell time of about 2 minutes in each zone. The dry coating weight was 24 g/m². When the adhesive coated strippable film leaves the drying oven, it is laminated to a poster or sign and the base paper is stripped from the film prior to winding the laminate into a roll.

Claims: A strippable film adapted for use in adhesive coating and laminating, comprising a base paper, and an extrusion coated thermoplastic resin layer; said film being characterised in that the adhesion between the base paper and the resin layer is such that if an adhesive is coated on to said resin layer and the adhesive coated strippable film is then assembled with a base member, said base paper can be stripped from said resin layer leaving the said resin layer still adhered to the adhesive coating, and enabling the resin layer to be stripped subsequently from the adhesive coating. A film according to Claim 1, further characterised in that said resin layer is a polyolefin, and preferably polypropylene. A film according to Claim 1 or Claim 2, further characterised in that said resin layer is pigmented. A film according to any preceding claim, further characterised in that said resin layer comprises a silicone coating which overcoats the extruded thermoplastic resin. A film according to any preceding claim, further characterised in that said base paper has a density greater than 0.75 g/cm³. A film according to any preceding claim further characterised in that said base paper is sized to reduce adherence of said resin layer to said base paper. An adhesive coated dual strippable film adapted for applying the said adhesive to a base member, characterised in comprising a film according to any preceding claim with the said adhesive coated on to the said resin layer. A film according to Claim 7, further characterised in that the release force between said adhesive coating and said resin layer is greater than the release force between said resin layer and said base paper. A process for applying an adhesive to a base member which comprises the steps of: extruding a thermoplastic resin layer onto the surface of a base paper to form a laminate while limiting the degree of penetration of said base paper by said thermoplastic resin layer such that said base paper can be stripped from said thermoplastic resin layer; applying an adhesive coating to said laminate such that said thermoplastic resin layer is interposed between said base paper and said adhesive coating; applying said adhesive coated laminate to said base member; and stripping said base paper from said thermoplastic resin layer. The process of Claim 9 which additionally comprises the step of stripping said thermoplastic resin layer from said adhesive coating to reveal said adhesive coating. The process of Claims 9 or 10, wherein said thermoplastic resin layer is polyolefin, and preferably polypropylene. The process of any of Claims 9, 10 or 11, wherein said thermoplastic resin layer is pigmented. The process of any of Claims 9 to 12, wherein the release force between said adhesive coating and said thermoplastic resin layer is greater than the release force between said thermoplastic resin layer and said base paper. The process of any of Claims 9 to 13, wherein said base paper has a density of at least 0.75 g/cm³. The process of any of Claims 9 to 14, wherein said base paper is sized to reduce adherence of said thermoplastic resin layer to said base paper.

Inventors: KASPER, KLAUS B., WILLIAMS, DAVID R.
Applicants: SCHOELLER TECHNICAL PAPERS, INC
Application year: 1987
Main classification: C09J7
IPC codes: B32B27, C09J7, B32B7