CONFORMABLE MULTILAYER FILMS

Abstract: Conformable multilayer films having alternating layers of a stiff polymeric material and a flexible polymeric material are described.

Description: CONFORMABLE MULTILAYER FILMS Technical Field This invention relates to conformable films, and more particularly, to multilayer films having alternating layers of a stiff polymeric material and flexible polymeric material. Background of the Invention There is an ongoing need for conformable, non-yielding films, which may be bonded to uneven and/or irregular surfaces and having a variety of new properties not possessed by current products. Polyvinyl chloride (PVC) films and tapes are conventionally used for a wide variety of applications. One prevalent use is for auto paint masking applications. PVC has many properties that are advantageous for such applications. For example, PVC films are known to be conformable to the varying topographies present on an automobile's exterior. One disadvantage of PVC films, however, is the accompanying use of plasticizers in PVC films. Plasticizers are typically needed in PVC films in order to make the films more flexible, lower the glass transition temperature of the films, and make the films more conformable. However, plasticizers can migrate to the substrate on which PVC films are adhered, leaving a residue or "ghosting" when removed. The ghosting is not removable with solvent wipes. Thus, the exterior appearance of the automobile may be detrimentally affected when PVC films are used as the tape backing in auto paint masking tapes. Furthermore, such plasticizers may degrade adjacent adhesive layers, reducing the adherence of the tape to the automobile. Multilayer films have also been described. For example, constructions having up to several thousand layers of alternating polymers with different refractive indices have been shown to have mirror-like properties. Also, films that are severable or have puncture resistance have been made by using alternating layers of stiff and ductile polymers as described in U.S. Patent Nos. 4,908,278 and 5,427,842 (Bland et al.). Pressure sensitive adhesive tapes are also known that have multilayered films as the backings on which the pressure sensitive adhesive is coated. Although a variety of properties have been accomplished with such multilayered constructions, there is still a need for conformable, non-yielding film products to replace poly(vinyl chloride) films. Summary of the Invention It is desired to have alternative compositions for use in tape backings and films. It is particularly desired to have alternative compositions for use in graphics applications, where stress relaxation and strain recovery properties are important and in auto paint masking tapes, where, in addition, minimal ghosting is also desired. For graphics film and paint masking applications, it is desirable to have an easy to apply, conformable, non-yielding film that has good stress relaxation and low strain recovery to enable the film to be applied over irregular surfaces without delaminating or releasing from the substrate surface ("popping up"). In addition the graphics films should be printable (i.e. to have a receptive surface for printing and/or graphics) and exhibit good weathering for outdoor applications. A disadvantage of PVC and flexible polyolefin films is that they have poor stress relaxation, strain recovery properties and/or ghosting. The present invention provides unified multilayer films having alternating layers of stiff and flexible polymeric materials. Preferably, multilayer films of the present invention are no greater than about 250 micrometers (μm) thick. In one embodiment, multilayer films have a construction of alternating layers of different stiff materials or different flexible materials. In any one construction of the alternating layers of stiff material and a flexible material, each of the stiff layers typically includes the same material or combination of materials, although they may include different materials or combinations of materials. Similarly, each of the layers that is flexible typically includes the same material or combination of materials, although they may include different materials or combinations of materials. Preferably, multilayer films of the present invention have a unified construction of at least 10 substantially contiguous layers of organic polymeric material, more preferably, at least 13 layers, even more preferably, at least 29 layers, although as few as 5 layers are possible or two layers with certain materials. In certain embodiments, there are at least three layers of the same stiff material and often, at least three layers of the same material that is a flexible material. The two outermost layers of multilayer films of the present invention can include one or more stiff materials, which may be the same or different in each of the two outermost layers. Alternatively, the two outermost layers can include one or more materials that are flexible materials, which may be the same or different in each of the two outermost layers. Furthermore, the inventive films include embodiments in which only one of the outermost layers includes a stiff material. Multilayer films of the present invention can also be oriented, either in one or two directions, if so desired. In addition, the films can be annealed by running the film over hot rolls, through an oven or heating the film with an infrared heater. In certain embodiments of the present invention, the multilayer films can include a layer that is flexible, a stiff material layer, and a tie layer therebetween. The present invention also provides a process of preparing a multilayer film. The process includes melt processing organic polymeric material to form a unified construction of at least 2, preferably at least 5, substantially contiguous layers of organic polymeric material, the construction comprising layers of stiff organic polymeric materials alternating with layers of flexible organic polymeric material. Preferably, all the layers are substantially simultaneously melt processed, and more preferably, all the layers are substantially simultaneously coextruded. Another aspect of the present invention provides a multilayer film having a unified construction and a total thickness of no greater than about 250 micrometers; the construction comprising layers comprising a stiff material alternating with layers comprising a material that is flexible. In another aspect, the present invention provides a multilayer film having alternating layers of stiff and flexible materials and further having a layer of pressure sensitive adhesive. In another aspect, the present invention provides a multilayer film having alternating layers of stiff and flexible materials and further having a layer of a material, such as a thermoplastic layer or primer layer, to improve the ink receptivity of the surface. A further aspect of the present invention provides a process of preparing a multilayer film, the process comprising melt processing organic polymeric material to form a unified construction of at least 2, preferably at least 5, substantially contiguous layers of organic polymeric material, the construction comprises layers comprising a stiff material alternating with layers comprising a material that is flexible. Herein, the following definitions are used: "Stiff materials" comprises thermoplastic polymers and blends of polymers having a Young's modulus greater than about 207 MPa (30,000 psi), more preferably greater than about 345 MPa (50,000 psi), even more preferably greater than about 517 MPa (75,000 psi). "Flexible materials " comprises thermoplastic polymers and blends of polymers having a Young's modulus less than about 172.4 MPa (25,000 psi), more preferably less than about 68.9 MPa (10,000 psi) and most preferably from about 0.69 to 68.9 MPa (100 to 10,000 psi). "Unified" means that the layers are not designed to be separated or delaminated as would a pressure sensitive adhesive tape in roll form. "Melt viscosity" means the viscosity of molten material at the processing temperatures and shear rates employed. "Conformability" means that the film accommodates curves, depressions or projections on a substrate surface so that the film may be stretched around curves or projections, or may be pressed down into depressions without breaking or delaminating the film and preferably with minimal necking. Detailed Description of Preferred Embodiments The present invention is directed to multilayer products (e.g., a single- or double- sided pressure sensitive adhesive tape or sheeting, a backing for a tape or sheeting or a pressure sensitive adhesive film) in the form of films of organic polymeric material, wherein the film layers that include stiff materials alternate with layers that include flexible materials. In other preferred embodiments there are alternating layers of different stiff materials (e.g., alternating layers of two different stiff materials). The two outermost layers of the films may include stiff materials, materials that are flexible, or one of the outermost layers may include a stiff material and the other a material that is flexible. Each layer of the construction is continuous and has a substantially contiguous relationship to the adjacent layers. Preferably, each layer is substantially uniform in thickness. The multiple layers in any one construction are "unified" into a single multilayer film such that the layers do not readily separate. The multilayer article has a Young's modulus of from about 10,000 to 150,000 psi (69 to 1034 MPa) at the use temperature, an elongation of 100% or more, and a strain recovery of less than 55% over 24 hours. By use temperature it is meant the temperature to which the film, after applied to a substrate, is exposed. In most cases the use temperature will be room temperature, however the film may be exposed to use temperatures considerably above or below this value, as when the film is applied to a substrate outdoors or other surfaces that are exposed to outdoor conditions. Preferably the multilayer article has a residual stress of 40% or less. Further, the multilayer article preferably has minimal necking, preferably 25% or less and more preferably 5% or less when tested according to ASTM D 882-95A and the necking test described herein. Necking refers to a film's tendency to undergo plastic deformation under strain and yield irrecoverably. When films are used as tape backings, necking can result in irregular tape lines during application. The Young's modulus of the multilayer film may be conveniently varied by appropriate selection of the flexible material and the stiff material and by selection of the relative proportions of stiff and flexible materials (the weight percent of the stiff and flexible layers relative to the film weight). To maximize the conformability of the multilayer film, it is preferred that the film has a Young's modulus in the range of about 68.9 to 344.7 MPa (10,000 to 50,000 psi) for a 3-4 mil film (-76-102 micrometers). It will be understood that the preferred Young's modulus is affected by the overall thickness of the multilayer film, and the relative thickness and number of stiff and flexible layers. For example, the conformability of the multilayer film may be maintained in relatively thinner films - in the 1-3 mil thickness range (25-76 micrometers), when using materials having a relatively higher Young's modulus. To maximize the relaxation behavior, strain recovery and the handleability (ease of manipulation, particularly when using large sheets) of the film, it is preferred that that the film has a Young's modulus in the range of about 344.7 to 1034.2 MPa (50,000 to 150,000 psi). Stiff materials useful in the present invention comprise amorphous and semicrystalline thermoplastic homo- and copolymers (and mixtures and blends thereof). Useful amorphous polymers generally have a glass transition temperature (T g ) of greater than 50°C (preferably greater than 70°C), such that the stiff layers have a Young's modulus greater than about 207 MPa (30,000 psi). Additionally they typically have an elongation of less than 100%. Examples of useful stiff materials include homo- and copolymers of methyl methacrylate, styrene, ring substituted alkyl styrenes such as α-methyl styrene, acrylonitrile and methacrylonitrile, copolymers of ethylene and vinyl alcohol (such as EVOH), polyesters, polyamides, polyurethanes and polycarbonates. They generally have a melt flow index of 5 or less and are amorphous, colorless materials. These should not, however, be considered to be limiting features, because stiff materials which are crystalline or which have higher melt flow indices may be used (and may in fact be desired when the stiff material constitutes the outer layer). A preferred stiff material is EVOH. Additionally, minor amounts of other materials may be added to the stiff polymer provided that the mixture meets the above described criteria. These additional materials may include flexible polymers, polymer additives such as plasticizers, antioxidants, colorants, flame retardants, UV stabilizers, heat stabilizers, and processing aids such as extrusion aids and lubricants. One may also use materials not normally considered stiff, such as flexible polymers, to which fillers have been added to increase the modulus. The quantity of the stiff material used in the film is dependent upon the specific properties desired of the multilayer film. However, it has been found that from 5 to 80 percent by weight (preferably from 45 to 70 percent by weight) of the stiff material is desired, relative to the weight of the multilayer film, when the materials are coextruded in a multilayer film. Flexible materials useful in the present invention are thermoplastic homo- or copolymers, or mixtures and blends thereof. The flexible materials have a Young's modulus of less than or equal to 25,000 psi (172 MPa). Typically the flexible polymers are polyolefinic semicrystalline thermoplastic materials. Additionally, minor amounts of other materials may be added to the flexible polymer provided that the mixture meets the above-described criteria. These additional materials may include stiff polymers, polymer additives such as plasticizers, antioxidants, colorants, flame retardants UV stabilizers, heat stabilizers, and processing aids such as extrusion aids and lubricants. Useful flexible materials have at least 100% elongation at a strain rate of 600% per minute when tested in tension at 25° C. The flexible materials are formulated to have minimal necking as defined in the test procedures below. The yield is that point at which a film of the material undergoes pronounced plastic deformation upon further application of strain. Thus the yield is manifested in a stress-strain plot at the first point where an increase in strain occurs without an increase in stress. A number of materials are useful as the flexible material. Examples of such materials include group homo- and copolymers of ethylene, propylenes, butene and blends thereof, including copolymers of ethylene with vinyl acetate, acrylic acid, methyl methylacrylate, methacrylic acid, hexene and octene; maleic anhydride grafted polyethylene or polypropylene; polyacrylates; polyamides, polyester resins and polyurethanes. Commonly a low modulus, flexible, preferably non-necking polyolefin is used as the flexible material such as polyethylene containing 1 -octene units, or polypropylene with a high level of atactic content. These flexible polyolefins can be blended with polyethylenes or isotactic or syndiotactic polypropylene. Preferred flexible materials includes blends of homo- and copolymers of propylene with maleic anhydride grafted polypropylene. The flexible materials can also include non-olefin polymers such as flexible polyamides, flexible polyester resins or flexible polyurethanes. Specific examples of these polymers and copolymers include, MACROMELT™ polyamides available from Henkel Inc., GRILON™ polyamides available from EMS American Grilon Inc, VITEL polyesters available from Bostik USA, HYTREL™ polyesters available from DuPont, MORTHANE™ polyurethanes available from Morton International, ESTANE™ polyurethanes available from B.F. Goodrich, KRATON™ styrene/isoprene or styrene/butadiene copolymers (and corresponding hydrogenated copolymers) available from Shell Chemical Products Inc., polyethylene copolymers and polypropylene copolymers, polyesters and the like. It has been found that from 20 to 95 percent by weight flexible material is generally useful. More preferably the film contains from about 30 to 55 percent by weight of the flexible material when extruded in a multilayer film. The quantity of flexible material utilized in the multilayer film is, of course, dependent upon the specific properties desired in the final film, and on the selection of a specific flexible and specific stiff material. For example, where the stiff material is brittle, such as polymethyl methacrylate, the resulting multilayer films tend to tear at high levels of the stiff material, so in such instances a relatively high level (50% or more) of flexible material is preferred. Conversely, when the stiff material is more ductile, such as with ethylene vinyl alcohol (EVOH), a relatively greater amount and a relatively broader range of compositions is useful. The materials of the stiff layer (A) and flexible layer (B) can be modified with one or more processing aids, such as plasticizers, to modify properties such as Young's modulus. Plasticizers useful with either polymeric materials are preferably miscible at the molecular level, i.e., dispersible or soluble in the thermoplastic material. Examples of plasticizers include, but are not limited to, polybutene, paraffinic oils, petrolatum, liquid rubbers, and certain phthalates with long aliphatic side chains such as tridecyl phthalate. When used, a processing aid is typically present in an amount of about 5 parts to about 300 parts by weight, and preferably up to about 200 parts by weight, based on 100 parts by weight of the polymeric material. If plasticizers are used, it is preferred that they not be incorporated into the outermost layers (whether the outer layer is a flexible or stiff material) of the multilayer film, so the effects of plasticizer migration may be avoided. These multilayer articles (i.e., multilayer films) are typically prepared by melt processing (e.g., extruding). In a preferred method, the layers are generally formed at the same time, joined while in a molten state, and cooled. That is, preferably, the layers are substantially simultaneously melt-processed, and more preferably, the layers are substantially simultaneously coextruded. Products formed in this way possess a unified construction and have a wide variety of useful, unique, and unexpected properties, which provide for a wide variety of useful, unique, and unexpected applications. Preferably, the films are no greater than about 250 micrometers (μm) thick (more preferably, no greater than about 150 μm, and most preferably, no greater than about 50 μm). Such multilayer products have a construction of at least 2 layers, preferably, at least 5 layers, more preferably, at least 13 layers, and even more preferably, at least 29 layers. Depending on the materials and additives chosen, thicknesses of the layers, and processing parameters used, for example, the multilayer films will typically have different properties at different numbers of layers. That is, the same property (e.g., tensile strength, fire retardancy) may go through maximum or a minimum at a different number of layers for two particular materials when compared to two other materials. Multilayer films can include an (AB) n construction, with either A and/or B layers as the outermost layers (e.g., (AB) n A, (BA) n B, or (AB) n ). In such constructions, each of the A layers are stiff as a result of the incorporation of a stiff material, which may be the same or different in each layer, and each of the B layers is flexible as a result of incorporation of a flexible material, which may be the same or different in each layer. Multilayer films can also include an (AA') n construction, with A and/or A' layers as the outermost layers (e.g., (AA') n A, (A'A) n A', or (AA') n ). In such constructions, each of the A and A' layers includes a different stiff material. In each of these constructions, n is preferably at least 2, and more preferably, at least 5, depending on the materials used and the application desired. The multilayer articles exhibit a desirable combination of conformability and high stress relaxation and low strain recovery. The flexible material (commonly a polyolefin or polyolefin blend) is especially desirable for use in graphics marking applications or paint masking applications because the flexible polyolefin is economical and possesses the desired conformability and non-necking behavior. The flexible materials, however, are relatively elastic and exhibit inadequate stress relaxation and high strain recovery. By layering the stiff thermoplastic polymer with the flexible material, however, one can greatly reduce the elastic character of the film. Also, with multilayer coextrusion, one can reduce the thickness of the stiff thermoplastic layers to the point that the stiff polymer becomes ductile. The resulting films exhibit a synergistic combination of conformability and high stress relaxation with low strain recovery. Preferred embodiments include one, prefereably three or more layers of the same stiff material and at least one, preferably three or more layers of the same flexible material in alternating layers. Whether both of the outer layers are stiff or flexible, or whether only one of the outer layers is stiff and one is flexible, the multilayer films can be used as backing for single- or double-sided pressure sensitive adhesive tapes, for example. For preferred embodiments, there are generally no more than about 500 layers, more preferably, no more than about 200 layers, and most preferably, no more than about 100 layers, although it is envisioned that constructions having many more layers can be made using the materials and methods described herein. The individual layers of multilayer films of the present invention can be of the same or different thicknesses. Preferably, each internal layer is no greater than about 5 micrometers (μm) thick, and more preferably, no greater than about 1 μm thick. Each of the two outermost layers can be significantly thicker than any of the inner layers, however. Preferably, each of the two outermost layers is no greater than about 150 μm thick. Typically, each layer, whether it be an internal layer or one of the outermost layers, is at least about 0.01 μm thick, depending upon the materials used to form the layer and the desired application. Thus, multilayer films of the present invention can be used as films for graphic applications. This is because they have advantageous conformability, exhibit less than 25% necking, have good stress relaxation and strain recovery, and are dimensionally stable. Such desirable properties are believed to result from the incorporation of alternating layers of stiff and flexible materials. The multilayer films are also useful as tape backings for paint masking applications, where it is additionally preferred that the films exhibit less than 5% necking. Suitable materials for use in preparing the films of the present invention, whether they are stiff or flexible materials, are melt processable. That is, they are fluid or pumpable at the temperatures used to melt process the films (e.g., about 50°C to about 300°C), and they are film formers. Furthermore, suitable materials do not significantly degrade or gel at the temperatures employed during melt processing (e.g., extruding or compounding). Preferably, such materials have a melt viscosity of about 10 poise to about 1 ,000,000 poise, as measured by capillary melt rheometry at the processing temperatures and shear rates employed in extrusion. Typically, suitable materials possess a melt viscosity within this range at a temperature of about 175°C and a shear rate of about 100 seconds "1 . In melt processing multilayer films of the present invention, the materials in adjacent layers need not be chemically or physically compatible or well matched, particularly with respect to melt viscosities, although they can be if so desired. That is. although materials in adjacent polymeric flowstreams can have relative melt viscosities (i.e., ratio of their viscosities) within a range of about 1 : 1 to about 1 :2, they do not need to have such closely matched melt viscosities. Rather, the materials in adjacent polymeric flowstreams can have relative melt viscosities of at least about 1 :5, and possibly up to about 1 :20. For example, the melt viscosity of a flowstream of polymer B (or A) can be similar or at least about 5 times, and even up to about 20 times, greater than the melt viscosity of an adjacent flowstream of polymer A (or B). In melt processing films of stiff and flexible materials, the differences in elastic stresses generated at the interface between the layers of different polymers is also important. Preferably, these elastic differences are minimized to reduce or eliminate flow instabilities that can lead to layer breakup. With knowledge of a material's elasticity, as measured by the storage modulus on a rotational rheometer over a range of frequencies (0.001 rad/sec. < ω < 100 rad/sec.) at the processing temperature, along with its viscosity at shear rates less than 0.01 second "1 , and the degree to which the material's viscosity decreases with shear rate, one of skill in the art can make judicious choices of the relative thicknesses of the layers, the die gap, and the flow rate to obtain a film with continuous, uniform layers. Significantly, relatively incompatible materials (i.e., those that typically readily delaminate as in conventional two layer constructions) can be used in the multilayer films of the present invention. Although they may not be suitable for all constructions, they are suitable for the constructions having larger numbers of layers. That is, generally as the number of layers increases, relatively incompatible materials can be used without delamination occurring. If desired a functional layer may be applied to one or both of the major surfaces of the film of the invention. For example, an adhesive layer may be applied to at least one of the major surfaces. The adhesive layer may be activated by pressure, heat, solvent or any combination thereof and may be of any type based on a poly(α olefin), a block copolymer an aery late, a rubber/resin, or a silicone. The adhesive may be applied at conventional coating weights (e.g., 0.0001 to 0.02 g/cm^) using any conventional coating means such a rotary rod die, slot die or a gravure roll. Other functional layers may also be employed. Thus, for example, an abrasive material (optionally in a binder), a light sensitive layer or an ink-receptive layer may be employed. Low adhesion back sizes (LAB's) which restrict adhesion of various types of surfaces to the film when it is wound as a coil or is stacked on itself may also be employed as a functional layer. Ink receptive surfaces comprise materials that have an affinity for the binder used in inks. With an ink receptive layer, the multilayer films of the invention may be used for graphics applications whereby images, graphics and/or text is transferred to the film by any conventional means such as screen printing or heat transfer techniques. Still other functional layers may be employed if desired. They may be employed singly or in combination with other functional layers on one or both sides of the film. The film may also be treated with a conventional primer coating, and/or activated by flame or corona discharge, and/or by other surface treatment to enhance adhesion of the functional layer thereto. When an additional pressure sensitive adhesive (psa) layer is used, pressure sensitive adhesives useful in the present invention can be self tacky or require the addition of a tackifier. Such materials include, but are not limited to, tackified natural rubbers, tackified synthetic rubbers, tackified styrene block copolymers, self-tacky or tackified acrylate or methacrylate copolymers, self-tacky or tackified poly-α-olefins, and tackified silicones. Examples of suitable pressure sensitive adhesives are described in U.S. Pat. Nos. Re 24,906 (Ulrich), 4,833,179 (Young et al.), 5,209,971 (Babu et al.), 2,736,721 (Dexter), and 5,461,134 (Leir et al.), for example. Others are described in the Encyclopedia of Polymer Science and Engineering, vol. 13, Wiley-Interscience Publishers, New York, 1988, the Encyclopedia of Polymer Science and Technology, vol. 1 , Interscience Publishers, New York, 1964 and Handbook of Pressure-Sensitive Adhesives, D. Satas, Editor, 2 nd Edition. Von Nostrand Reinhold, New York, 1989.. Other additives, such as fillers, pigments, crosslinking agents, flame-retardants, antioxidants, ultraviolet stabilizers, and the like, may be added to modify the properties of either the stiff (A or A') or the flexible (B) layers. Each of these additives is used in an amount to produce the desired result. Pigments and fillers can be used to modify cohesive strength and stiffness, as well as chemical resistance and gas permeability. For example, clays, hydrated silicas, calcium silicates, silico-aluminates, and the fine furnace and thermal blacks increase cohesive strength and stiffness. Platy pigments and fillers, such as mica, graphite, and talc, are preferred for acid and chemical resistance and low gas permeability. Other fillers can include glass or polymeric beads or bubbles, metal particles, fibers, and the like. Typically, pigments and fillers are used in amounts of about 0.1% to about 50% by weight, based on the total weight of the multilayer film. Pigments and fillers may also be used to modify the optical properties of the film such as the color, opacity and gloss. Crosslinkers such as benzophenone, derivatives of benzophenone, and substituted benzophenones such as acryloyloxybenzophenone may also be added, and may be used to increase the modulus of the polymer of either layer. Such crosslinkers are preferably not thermally activated, but are activated by a source of radiation such as ultraviolet or electron-beam radiation subsequent to forming the films. Typically, crosslinkers are used in amounts of about 0.1% to about 5.0% by weight, based on the total weight of the multilayer film. Flame retardants may be added to incorporate resistance to flame initiation or flame propagation in the constructions of the invention. Examples include brominated aromatic compounds, such as decabromodiphenyloxide available under the trade designation DE83R from Great Lakes, W. Lafayette, IN, antimony compounds, such as antimony trioxide or antimony pentoxide, and aluminum trihydrate, such as that available under the trade designation MICRAL ATH 1500 from Solem Ind., Norcross, GA. Typically, flame retardants are used in amounts of about 1% to about 50% by weight, based on the total weight of the multilayer film. A flame retardant polyethylene concentrate is commercially available under the trade designation PE concentrate 1 Nat- 2P-W from M.A. Hannah Corp., Elk Grove, IL, which contains a flame retardant blend of a brominated imide, antimony trioxide and polyethylene polymer. Flame retardants may be added to the multilayer films of the invention using the specific flame retardants and amounts described in WO 99/28128. Antioxidants and/or ultraviolet stabilizers, including hindered amine light stabilizers (HALS) may be used to protect against severe environmental aging caused by ultraviolet light or heat. These include, for example, hindered phenols, amines, and sulfur and phosphorus hydroxide decomposers. Typically, antioxidants and/or ultraviolet stabilizers are used in amounts of about 0.1% to about 5.0% by weight, based on the total weight of the multilayer film. Tie layers, which are typically hot melt adhesives (i.e., tacky when in the melt state), can also be used to enhance the adhesion between each of the layers if so desired. Materials useful in the tie layers include, but are not limited to, ethylene/vinyl acetate copolymer (preferably containing at least about 10% by weight of vinyl acetate units), carboxylated ethylene/vinyl acetate copolymer such as that available under the trade designation CXA 3101™, from E.I. DuPont de Nemours, Inc., copolymers of ethylene and methyl acrylate such as that commercially available under the trade designation POLY- ETH 2205 EMA™, from Gulf Oil and Chemicals Co., ethylene/(meth)acrylic acid copolymer such as that available under the trade designation SURLYN™ from E.I. DuPont de Nemours, Inc., maleic anhydride modified polyolefins and copolymers of polyolefins such as that commercially available under the trade designation MODIC™, from Mitsubishi Chemical Co., polyolefins containing homogeneously dispersed vinyl polymers such as those commercially available under the trade designation VMX™ from Mitsubishi Chemical Co. (e.g., FN-70, an ethylene/vinyl acetate based product having a total vinyl acetate content of 50% and JN-70, an ethylene/vinyl acetate based product containing dispersed polyme hylmethacrylate and having a vinyl acetate content of 23% and a methyl methacrylate content of 23%), POLYBOND™ (believed to be a polyolefin grafted with acrylic acid) from B.P. Chemicals Inc., Cleveland, OH. PLEXAR™ (believed to be a polyolefin grafted with functional groups) from Quantum Chemicals, Inc., Cincinnati, OH, a copolymer of ethylene and acrylic acid such as that commercially available under the trade designation PRIMACOR™ from Dow Chemical Co., Midland, MI, a copolymer of ethylene and methacrylic acid such as that commercially available under the trade designation NUCREL™ from E.I. DuPont de Nemours, Inc and a terpolymer containing ethylene, glycidyl methacrylate, methyl methacrylate available available as LOTADER™ AX 8900 from Elf Atochem North America. Philadelphia, PA. The multilayer films of the present invention can be used as the backings or substrates for single-sided or double-sided adhesive products, such as tapes. Such films can be prepared using extrusion techniques, then coated or co-extruded with a low- adhesion backsize (LAB) material, which restricts adhesion of various types of surfaces to the film when it is wound as a coil or is stacked on itself. A wide variety of known adhesive materials (e.g., any of the pressure sensitive materials described herein) and LAB materials (e.g., polyolefins, acrylates, urethanes, cured silicones, fluorochemicals) can be used as well as a wide variety of known coating techniques, including solvent coating and extrusion or co-extrusion coating techniques. Multilayer films of the present invention can be made using a variety of equipment and a number of melt-processing techniques (typically, extrusion techniques) well known in the art. Such equipment and techniques are disclosed, for example, in U.S. Pat. Nos. 3,565,985 (Schrenk et al.), 5,427,842 (Bland et al.), 5,589,122 (Leonard et al.), 5,599,602 (Leonard et al.), and 5,660,922 (Herridge et al.). For example, single- or multi-manifold dies, full moon feedblocks (such as those described in U.S. Pat. No. 5,389,324 to Lewis et al.), or other types of melt processing equipment can be used, depending on the number of layers desired and the types of materials extruded. For example, one technique for manufacturing multilayer films of the present invention can use a coextrustion technique, such as that described in International Publication No. WO 93/07228 or U.S. Pat. No. 5,660,922 (Herridge et al). In a coextrusion technique, various molten streams are transported to an extrusion die outlet and joined together in proximity of the outlet. Extruders are in effect the "pumps" for delivery of the molten streams to the extrusion die. The precise extruder is generally not critical to the process. A number of useful extruders are known and include single and twin screw extruders, batch-off extruders, and the like. Conventional extruders are commercially available from a variety of vendors such as Davis-Standard Extruders, Inc. (Pawcatuck, CT), Black Clawson Co. (Fulton, NY), Berstorff Corp. (NC), Farrel Corp. (CT), and Moriyama Mfg. Works, Ltd. (Osaka, Japan). Other pumps may also be used to deliver the molten streams to the extrusion die. They include drum loaders, bulk melters, gear pumps, and the like, and are commercially available from a variety of vendors such as Graco LTI (Monterey, CA), Nordson (Westlake, CA), Industrial Machine Manufacturing (Richmond, VA), and Zenith Pumps Div., Parker Hannifin Corp. (NC). Typically, a feedblock combines the molten streams into a single flow channel. The distinct layers of each material are maintained because of the laminar flow characteristics of the streams. The molten structure then passes through an extrusion die, where the molten stream is reduced in height and increased in width so as to provide a relatively thin and wide construction. This type of coextrusion is typically used to manufacture multilayer film constructions having about 5 layers or more. However, the use of a feedblock is optional, as a variety of coextrusion die systems are known. For example, multimanifold dies may also be employed, such as those commercially available from The Cloeren Company (Orange, TX). In multi-manifold dies, each material flows in its own manifold to the point of confluence. In contrast, when feedblocks are used, the materials flow in contact through a single manifold after the point of confluence. In multimanifold die manufacturing, separate streams of material in a flowable state are each split into a predetermined number of smaller or sub-streams. These smaller streams are then combined in a predetermined pattern of layers to form an array of layers of these materials in a flowable state. The layers are in intimate contact with adjacent layers in the array. This array generally comprises a stack of layers which is then compressed to reduce its height. In the multimanifold die approach, the film width remains constant during compression of the stack, while the width is expanded in the feedblock approach. In either case, a comparatively thin, wide film results. Layer multipliers in which the resulting film is split into a plurality of individual subfilms which are then stacked one upon another to increase the number of layers in the ultimate film may also be used. The multimanifold die approach is typically used in manufacturing multilayer film constructions having less than about 5 layers. In manufacturing the films, the materials may be fed such that either the stiff material or the flexible material forms the outermost layers. The two outermost layers are often formed from the same material. Preferably, although not necessarily, the materials comprising the various layers are processable at the same temperature. Significantly, although it has been generally believed that the melt viscosities of the various layers should be similar, this is not a necessary requirement of the methods and products of the present invention. Accordingly, residence times and processing temperatures may have to be adjusted independently (i.e., for each type of material) depending on the characteristics of the materials of each layer. Other manufacturing techniques, such as lamination, coating, or extrusion coating may be used in assembling multilayer films and products from such multilayer films according to the present invention. For example, in lamination, the various layers of the film are brought together under temperatures and/or pressures (e.g., using heated laminating rollers or a heated press) sufficient to adhere adjacent layers to each other. Continuous forming methods include drawing a pressure sensitive adhesive composition out of a film die and subsequently contacting a moving multilayer film. After forming, the pressure sensitive adhesive coatings are solidified by quenching using both direct methods, such as chill rolls or water baths, and indirect methods, such as air or gas impingement. The films of the present invention may also be annealed to minimize or eliminate necking in the film, to relieve asymmetric stresses in the film that give rise to shrinking, and to improve the dimensional stability. Commonly, the films are coextruded and then they are run over hot rolls, through a heated oven or subjected to an IR heater. It is desirable to heat treat the films under minimal tension so that the asymmetric stresses are relieved. The films of the present invention can be oriented, either uniaxially (i.e., substantially in one direction) or biaxially (i.e., substantially in two directions), if so desired. Such orientation can result in improved strength properties, as evidenced by higher modulus and tensile strength. Preferably, the films are prepared by co-extruding the individual polymers to form a multilayer film and then orienting the film by stretching at a selected temperature. For example, uniaxial orientation can be accomplished by stretching a multilayer film construction in the machine direction at a temperature of about the melting point of the film, whereas biaxial orientation can be accomplished by stretching a multilayer film construction in the machine direction and the cross direction at a temperature of about the melting point of the film. Optionally heat-setting at a selected temperature may follow the orienting step. The various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to that set forth herein for illustrative purposes only. Examples This invention is further illustrated by the following examples which are not intended to limit the scope of the invention. In the examples, all parts, ratios and percentages are by weight unless otherwise indicated. The following test methods were used to characterize the multilayer films: Test Methods Stress Relaxation Test In order to determine the tendency of a film to lose stress over a short period of time, a stress relaxation test was performed. For the first part of the test, a film sample was mounted in the jaws of a tensile testing machine and strained at a constant rate of 600% per minute until the sample reached 100% elongation. The jaws were then stopped and the stress was observed for one minute under constant strain. The percent residual stress at one minute was defined as 100 times the stress at one minute divided by the initial stress at 100% strain. Strain Recovery Test In order to determine the tendency of the film to recover after being strained, a strain recovery test was performed. A strip of film was cut and the length was noted. The film was then strained to 100% at a strain rate of 600% per minute and held at 100% strain for one minute. The stress was then removed and the film was allowed to dwell for 24 hours, after which time the sample length was measured. The percent strain recovery at 24 hours was defined as 100 times the (length at 100% strain minus the length at 24 hours) divided by the initial length before straining. Necking Test The amount of necking for each sample was determined by examination of the stress strain curve for the sample. The curve was generated with a standard tensile/elongation method on an Instron mechanical testing frame operating at 30.5 cm/minute (12 inches/minute). Sample were of 12.7 mm (0.5 inches) width and gauge length of 50.8mm (2 inches). Thickness of the samples depended on process conditions and were measured using a Ono Sokki Liner Thickness Gage. The necking percent was determined by noting the stress value (SI) at the point of initial maximum stress and the stress value (S2) at the following minimum stress. Necking is defined as 100 x (SI - S2)/S1 and is reported as percent necking. Shrinkage Unrestrained linear thermal shrinkage of plastic film was measured according to ASTM D 1204. A film sample having a width of about 25.4 mm and a length of about 101.6 mm was cut with a die. The length direction was parallel to the direction in which the film was made or the machine direction (MD). Notches, for reference points, were made about 75 mm apart in the length direction. Each film sample was placed unrestrained for 10 minutes in an oven that had been set at one of three temperatures, 163°C, 149°C or 135°C. Film shrinkage was measured upon removal for both the MD and cross-web direction (CD). MD % shrinkage was 100 times the change in distance between the notches divided by the initial distance between the notches. CD% shrinkage was 100 times the change in width divided by the initial width. The larger value was reported and was the MD% shrinkage unless otherwise indicated. Materials Used Material Description EVOH 105 Ethylene vinyl alcohol copolymer, 44 mole % ethylene, available from Eval Company of America, Lisle, Illinois. Rexflex™ WL101 Significantly atactic polypropylene available from Huntsman Polypropylene Corp., Woodbury, New Jersey. Bynel™ 50E555 Maleic anhydride graft polypropylene, available from Dupont Packaging and Industrial Polymers, Wilmington Delaware. Now available as Bynel™ 50E631 Filler A A blend of a brominated imide, antimony trioxide and polyethylene polymer at a 33.75/11.25/55 weight ratio, available as PE Cone. 1 Nat-2P-W from M.A Hannah, Elk Grove Village, Illinois. Fina™ 3374 Isotactic polypropylene available from Fina Oil & Chem, Dallas, Texas. Filler B A hindered amine light stabilizer concentrate, available as 10407, from Ampacet Corporation, Tarrytown New York. Filler C A pigment concentrate of carbon black in 50 weight percent polyethylene, available as 12085 from Standridge Color Corporation, Social Circle Georgia. Macromelt™ 6900 A flexible polyamide available from Henkel Adhesives, Elgin, Illinois. Lotader™ AX8900 A terpolymer containing ethylene, glycidyl methacrylate, methyl methacrylate available from Elf Atochem North America, Phil., PA. Engage™ 8200 A metallocene polymerized olefin, containing 24% octane comonomer available from Dow Chemical Co., Midland, Michigan. EVOH F 104 Ethylene vinyl alcohol copolymer, 32 mole % ethylene, available from Eval company of America, Lisle, Illinois. Material Description Lotader™ AX8840 A copolymer containing ethylene and glycidyl methacrylate available from Elf Atochem North America. HIPS 484 A high impact polystyrene available from Dow Plastics, Midland, Michigan. VM100 A polymefhylmethacrylate available from AtoHaas Americas Inc., Philadelphia, Pennsylvania. AMNO A Nylon 12 polyamide available from Elf Atochem, North America Lexan™ PC 11 IN A polycarbonate available from General Electric Company, Pittsfield, Massachusetts. PPSC 912 An ethylene-propylene copolymer with a melt index of 65, available as Profax SC 912 from Montell North America, Wilmington. Delaware. Vitel™ 4450 An aromatic, saturated polyester resin available fron Bostik Inc., Middleton, Massachusetts. LLDPE 6806 Linear low density polyethylene, available from Dow Chemical Co., Midland, Michigan. Bynel™ 41E558 Maleic anhydride graft linear low density polyethylene, available from Dupont Packaging and Industrial Polymers, Wilmington, Delaware Primacor™ 3440 Poly (ethylene acrylic acid), available from Dow Plastics, Midland, Michigan. EVOH Gl 56 Ethylene vinyl alcohol copolymer, 48 mole % ethylene, available from Eval Company of America, Lisle, Illinois. Examples 1-10, Comparative Examples 1-4 Examples 1-10 illustrate the effect of number of layers in multilayer films having a construction A(BA) n BA. In Example 1, the stiff layers were made of EVOH El 05 and conveyed in a Killion single screw extruder (KILLION Model KTS-125, 32 mm, having an L/D of 24/1, commercially available from Killion Extruder Inc., Cedar Grove, New Jersey), operating with zone temperatures increasing from 182°C to 221°C to the "A" slots of a feedblock having 91 slots. The feedblock, made as described in U.S. Pat. No. 4,908,278 (Bland et al.), allowed two flow streams fed into the 91 slots in an alternating manner to come together in a multilayer flow stream having layers arranged as A(BA) 44 B A. The temperature of both the feedblock and the die were set at 232°C. The flexible layers were made from a mixture of Rexflex™ WL101 and Bynel™ 50E555 premixed in a weight ratio of 40:60. The mixture was fed by a twin screw extruder (LEISTRITZ AG Model LSM 34 GL, 34 mm, having an L/D of 42/1, commercially available from American Leistritz Extruder Corp, Somerville, NJ.) operating with zone temperatures increasing from 121°C to 232°C into the "B" slots of the feedblock. The resulting multilayered flow stream was passed through a single orifice film die and drop cast onto a chrome chill roll set at a temperature of 24°C and collected. The line speed was 6.7 m/min, the individual flowrates of A and B were such that the material in the stiff and flexible layers were in a weight ratio of 30:70 and the overall thickness was measured at 102 micrometers. Some of the construction was then annealed by placing the multilayer film in an air circulating oven set at 135°C for 5 minutes unrestrained. Example 2 was made as in Example 1 , except the flow rates of the materials were adjusted to obtain a weight ratio of 50:50. Example 3 was made as in Example 1, except the flow rates of the materials were adjusted to obtain a weight ratio of 70:30 and the feedblock had only 13 slots available for use. Examples 4-6 were made as in Example 1 , except the feedblock had only 13 slots available for use and the flow rates of the materials for Examples 4-6 were adjusted to obtain a weight ratio of 30:70, 50:50 and 70:30, respectively. Examples 7-10 were made as in Example 1, except the stiff layers were made from EVOH G156 ethylene vinyl alcohol and the slots available for use in the feedblock were 5, 4, 3 and 2, respectively. Comparative Example 1 was a poly vinyl chloride film, thickness of about 51 micrometers, used in the manufacture of Controltac™ 180-10 Graphic Marking Film available from 3M Company, St. Paul, Minnesota. Comparative Example 2 was a poly vinyl chloride film, thickness of about 51 micrometers, used in the manufacture of Scotchcal™ 3650 Graphic Marking Film available from 3M Company, St. Paul, Minnesota. Comparative Example 3 was a polyurethane film, thickness of about 51 micrometers, used in the manufacture of Controltac™ 190-10 Graphic Marking Film available from 3M Company, St. Paul, Minnesota. Comparative Example 4 was polyolefin-based film, thickness of about 100 micrometers, used in the manufacture of Scotchcal™ 3540C Graphic Marking Film available from 3M Company, St. Paul, Minnesota. Examples 1-10 and Comparative Examples 1-4 were tested for Modulus, Residual Stress and Strain Recovery. The test results and number of film layers are shown in Table 1. Table 1 As seen, the films of the invention exhibited reduced strain recovery and comparable stress relaxation to that seen with conventional graphic marking films. Examples 4-7 and Comparative Examples 1-3 were tested for Necking. The test results are shown in Table 2. Table 2 As seen, annealing the film can substantially reduce necking when minimal necking is important. Examples 11-13 Examples 11-13 illustrate the effect of filler on the conformability properties of multilayer films having a construction A(BA) n BA. Example 1 1 was made in a manner similar to Example 1 except the materials were different and the process conditions were changed to make a multilayer film having 13 layers, a weight ratio of stiff material to flexible material of 80:20 and a overall thickness of about 114 micrometers. The stiff layer was made from Filler A and Rexflex™ WL101 that were premixed in a weight ratio of 75:25. Rexflex ™ WL101 is a normally flexible material rendered stiff by the addition of the Filler A. The flexible layer was made from Rexflex™ WL101 and Fina™ 3374 that were premixed in a weight ratio of 75:25. Process temperatures were adjusted to accommodate the melt characteristics of the various materials. Example 12 was made in a manner similar to Example 1 except the materials were different and the process conditions were changed to make a multilayer film having 13 layers, a weight ratio of stiff material to flexible material of 32:68 and a overall thickness of about 87 micrometers. The stiff layer was made from EVOH G156. The flexible layer was made from Rexflex™ WL101 and Bynel™ 50E555 that were premixed in a weight ratio of 40:60 and then augmented with Filler B in a weight ratio of flexible mixture to filler of 100:15. Process temperatures were adjusted to accommodate the melt characteristics of the various materials. Example 13 was made in a manner similar to Example 1 except the flexible material was different and the process conditions were changed to make a multilayer film having 91 layers, a weight ratio of stiff material to flexible material of 30:70 and an overall thickness of about 79 micrometers. The flexible layer was made from Rexflex™ WL101, Bynel™ 50E555, Filler B and Filler C that were premixed in a weight ratio of 40:48:8:4. Process temperatures were adjusted to accommodate the melt characteristics of the various materials. Examples 11-13 were tested for Modulus, Residual Stress and Strain Recovery. The test results and number of film layers are shown in Table 3. Table 3 As seen, fillers can be used without adversely affecting conformability of the film. Also, in some cases, fillers can have a significant effect on conformance. As previously noted, the stiff layer of Example 1 1 was a flexible polymer made stiff by the presence of a filler. Examples 14-15 Examples 14-15 illustrate the effect of tie-layers on the conformability properties of multilayer films having a construction AC(BCAC) n BCA, where layer C is a tie-layer Example 14 was made in a manner similar to Example 1 except a tie-layer layer was added, a third extruder was used to feed the tie-layer material to the C slots of a multilayer feedblock and the process conditions were changed. The tie-layer was made from Macromelt™ 6900. The tie-layer material was conveyed in a KILLION single screw extruder (KILLION, 19mm, having an L/D of 32/1, commercially available from Killion Extruders Inc., Cedar Grove, New Jersey, to C slots of a feedblock. Process temperatures were adjusted to accommodate the melt characteristics of the various materials. The process conditions were changed to make a multilayer film having 65 layers, a weight ratio of stiff material to tie-layer to flexible material of 50:15:35 and a overall thickness of about 115 micrometers. Example 15 was made in a manner similar to Example 14 except the tie-layer and flexible layer were made from different materials. The tie-layer was made from Lotader™ AX8900 epoxy-functional polyethylene. The flexible layer was made from Engage™ 8200 ethylene/octene copolymer. The overall film thickness was about 101 micrometers. Example 16 was made in a manner similar to Example 14 except the layers were made from different materials. The stiff layer, tie-layer and flexible layer were made from EVOH F104 with 32 mole % ethylene copolymer, Lotader™ AX8900 epoxy-containing polyethylene, and Engage™ 8200 ethylene/octene copolymer, respectively. The overall film thickness was about 109 micrometers. Examples 14-16 were tested for Modulus, Residual Stress and Strain Recovery. The test results and number of film layers are shown in Table 4. Table 4 As seen, tie-layers can be used without adversely affecting conformability properties. Each film was also tested for necking. The necking of the unannealed and annealed film of Example 14 was 14% and 2%, respectively. The necking of the unannealed and annealed film of Example 15 was 22% and 8%, respectively. The necking of the unannealed and annealed film of Example 16 was 19% and 10%, respectively. Examples 17-28 Examples 17-28 illustrate the effect of changing different variables on the conformability properties of multilayer films having a construction A(BA) n BA or B(AB) n AB. The Examples 17-28 were made in a manner similar to Example 1 except the type of material in the stiff "A" layers and flexible "B" layers, the number of layers and the weight ratio of stiff to flexible material were varied. Process temperatures were adjusted to accommodate the melt characteristics of the various materials. These variables are shown in Table 5. Table 5 Examples 17-28 were tested for Modulus, Residual Stress and Strain Recovery. The test results and number of film layers are shown in Table 6. Table 6 1 - film tore before it could be stretched to 100 percent elongation. As seen, satisfactory conformability performance can be obtained with constructions having a wide range of moduli. Weight ratios of stiff to flexible components tends to be lower when the material of the stiff layer is more brittle. Examples 29-31 Examples 29-31 illustrate the effect of shrinkage on the multilayer films of the invention. Samples of Examples 29-31 were made as Examples 1, 2 and 24, respectively. Comparative Example 5 was an extruded film made from a blend of Fina™ 3576 stiff polypropylene and Rexflex™ WL101 flexible polypropylene in a weight ratio of 60:40 according to USSN 09/119494, "Polymer Blends and Tapes Therefrom" (Kollaja, et al), Example 2 except Fina™ 3576 was used instead on Escorene™ 4792E1 ethylene/propylene copolymer. Comparative Example 6 was a calendered plasticized polyvinyl chloride film available as Renolit™ SK-M Signmask Blue from American Renolit Corp., Whippany, New Jersey. Comparative Example 7 was a calendered plasticized polyvinyl chloride film available as Renolit™ S from American Renolit Corp. All examples were tested for Shrinkage. The test results are shown in Table 7. Table 7 As seen, the shrinkage over the temperatures used was significantly less for Examples 29-31 than that observed for the comparative example polyvinyl chloride films. Also, the films of the invention were more dimensionally stable (less shrinkage) at 163°C than the comparative film made from a polypropylene blend (C5).

Claims: What Is Claimed Is: 1. A multilayer film having a unified construction of at least 2 substantially contiguous layers of organic polymeric material, comprising a stiff material alternating with layers comprising a flexible material, wherein said multilayer article exhibits a strain recovery of 55% or less, and a Young's modulus of 69 to 1034 MPa at room temperature and an elongation of 100% or greater. 2. The multilayer film of claim 1 having a residual stress of 40% or less. 3. The multilayer film of claim 1 wherein said flexible material comprises a polymeric material having a Young's modulus of less than 25,000 psi (68.9 MPa). 4. The multilayer film of claim 1 wherein said stiff material comprises a polymeric material having a Young's modulus of greater than 50,000 psi (345 MPa). 5. The multilayer film of claim 1 having necking of 25% or less as measured by ASTM D882-95A. 6. The multilayer film of claim 5 having necking of 5% or less. 7. The multilayer film of claim 1 further comprising a layer of pressure sensitive adhesive. 8. The multilayer film of claim 1 further comprising an ink-receptive layer. 9. The film of claim 1 having Young's modulus of 68.9 to 344.7 MPa (10,000 to 50,000 psi). 10. The film of claim 1 having a Young's modulus from 344.7 to 1034.2 MPa (50,000 to 150,000 psi).

Inventors: KOLLAJA RICHARD A, NGUYEN DAVID D, REE BUREN R, SOLOMONSON STEVEN D, STEELMAN RONALD S, KOLLAJA, RICHARD, A., NGUYEN, DAVID, D., REE, BUREN, R., SOLOMONSON, STEVEN, D., STEELMAN, RONALD, S.
Applicants: 3M INNOVATIVE PROPERTIES CO, 3M INNOVATIVE PROPERTIES COMPANY
Application year: 2000
Main classification: B32B27
IPC codes: B32B27, C09J7