BREATHABLE EXTRUDED POLYMER FILMS

Abstract: A polymer film comprising at least first and second contiguous and coextruded portions, wherein the first portion is extruded from a first polymer composition containing a filler material in an amount sufficient to increase the water vapor permeability of the first portion relative to the second portion, and the second portion is extruded from a second polymer composition such that a tensile strength of the second portion is greater than the tensile strength of the first portion. Such polymer films allow for highly vapor permeability, while providing portions with sufficient strength to allow for taping or attachment of fastening devices.

Description: BREATHABLE EXTRUDED POLYMER FTLMS FIELD OF THE INVENTION The invention relates to breathable extruded polymer films. More specifically, the invention is directed to polyolefin films coextruded to have two or more contiguous portions including a first portion that is formed of a filled polymer composition and is gas/vapor permeable and liquid impermeable, and at least one higher strength second portion formed of a polymer material in the absence of substantial amounts of filler material. BACKGROUND OF THE INVENTION The formation of "breathable" films that are gas/vapor permeable and liquid impermeable by stretching a precursor film containing a high level of filler material is known, and described, for example in U.S. Patent No. 4,472,328, assigned to Mitsubishi Chemical Industries, Ltd. The breathable polyolefin film of the Mitsubishi patent is prepared from a polyolefin/filler composition having from 20 percent to 80 percent by weight of a filler material, such as calcium carbonate. A liquid or waxy hydrocarbon polymer elastomer such as a hydroxy-terminated liquid polybutadiene was described as producing a precursor film that could be mono- axially or bi-axially stretched to render the film breathable. The breathable film of the Mitsubishi patent is further mentioned in Great Britain Patent No. 2,1 15,702, assigned to Kao Corporation. The Kao patent describes the use of the breathable film of Mitsubishi as a diaper backsheet that prevents the leakage of liquids. U.S. Patent No. 4,350,655, assigned to Biax Fiber Film, ("the Biax '655 patent") describes a porous polyolefin film containing at least 50 percent by weight of a coated inorganic filler. The precursor film is formed without the addition of an elastomer by employing an inorganic filler that is surface coated with a fatty acid ester of silicon or titanium. The precursor film is then stretched between horizontally grooved rollers. Cold stretching of the precursor film at a temperature below 70°C produces a porous film. The resulting films are generally both vapor and liquid permeable. However, Example 3 of the Biax patent describes a film that is vapor permeable and liquid impermeable. A problem with polymer films rendered breathable by incorporation of filler materials is that the use of fillers also reduces the strength of the films. This reduction in tensile strength is disadvantageous in situations where the film must have sufficient strength to prevent tearing, for example to allow for attachment of tapes or the incorporation of fastening systems or elastic leg bands. U.S. Patent No. 3,840,418 to Sabee ("the Sabee '418 patent") describes a method for forming a sanitary product provided with sections having increased pin- or tape-retention strength in which a single synthetic resin is extruded by a selective extrusion process that selectively thickens the film web of resin at localized points where strength is needed. The Sabee patent does not discuss the use of filled polymer compositions and does not adjust the thickness of the various portions of the sanitary product to control vapor permeability. One proposed manner of increasing the physical properties of filled and stretched polymer sheets is described in US Patent 4,777,073, assigned to Exxon Chemical Patents, Inc. ("the Exxon '073 patent"). The Exxon '073 patent describes the melt embossing of polymer/filler precursor films, such that portions of the film will have reduced thickness. When the films are activated by stretching, the portions of reduced thickness exhibit higher permeability and the portions with non reduced thickness exhibit higher tear strengths. Although the films described in the Exxon '073 patent provide high permeability and necessary strength for the attachment of fasteners and the like, they do so at relative high thickness. High thickness increases the cost of the films and reduces the films' flexibility and soft feel. Films described in the Exxon '073 patent are also still deficient in strength in certain applications in consumer disposables, where the maker(s) of these disposables subject these films to secondary deformation steps. With all the technical efforts focused on developing better film, there still remains a need for superior polymer film products. In view of this need, applicants have developed polymer films of substantially constant thickness, providing areas of improved vapor permeability and contiguous areas exhibiting increased tensile and tear strengths, which polymer films can be economically and simply formed in a single co-extrusion step. SUMMARY OF THE INVENTION The present invention provides a polymer film comprising at least first and second contiguous and coextruded portions, wherein the first portion is extruded from a first polymer composition containing a filler material in an amount sufficient to increase the vapor permeability of the first portion relative to the second portion, and the second portion is extruded from a second polymer composition such that the tensile and tear strengths of the second portion is greater than the tensile and tear strengths of the first portion. The present invention further provides for manufactured products, particularly diaper backsheets that are formed from such films. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The polymer films of the present invention can be formed by extrusion using a segmented extrusion die of the type disclosed in U.S. Patent No. 4,435,141, assigned to Polyloom Corporation of America ("the Polyloom '141 patent"), which is incorporated herein by reference for purposes of U. S. patent practice. The die of the Polyloom '141 patent is provided with a grooved insert which, together with the interior surface of the die body members define separate alternating passages or channels through which the polymer flows. When fed by streams of at least two distinct polymer compositions the die allows for the extrusion a continuous film of contiguous portions formed of different polymer compositions. The first portion of the extruded precursor film having the relatively high vapor permeability and relatively low tensile strength can be prepared from a first polymer composition comprising at least one polyolefin component, and a filler. The polyolefin composition can be any polyolefin suitable for film production including, but not limited to, polypropylene, copolymers of propylene, homopolymers and copolymers of ethylene including very low density polyethylene (VLDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), linear medium density polyethylene (LMDPE), high density polyethylene (HDPE) and mixtures and blends thereof. A particularly preferred polyolefin is LLDPE. The use of LLDPE as the polyolefin component of the first polymer composition of this invention is preferred because of the material's high tear strength, ease of compounding and low cost. Linear low density polyethylene (LLDPE) is produced by copolymerizing ethylene with C to do alpha-olefin. Generally, the preferred alpha-olefin includes those selected from butene-1, pentene-1, hexene-1, 4-methylpentene-l, heptene-1, and octene. The comonomers are present in amounts of up to 20 wt.%, usually between 3 and 14 wt.%. The polymerization may be conducted at low pressure, in the presence of a conventional Ziegler-Natta catalyst, a metallocene catalyst, or a combination thereof and may be carried out in a gas phase. Other polymerization schemes are also contemplated such as slurry, solution and high pressure. LLDPE produced by such a method will have a density of 0.900 to 0.935 g/cm 3 and a melt index (MI) of 0.1 to 5.0 grams per 10 minutes. Methods of manufacturing LLDPE are well known and described, for example, in U.S. Patent No. 4,076,698. Fillers suitable for incorporation into the first polymer composition used to form the first portion of the extruded precursor film which exhibits a relatively high vapor permeability may be any inorganic or organic material having a low affinity for, and a significantly lower elasticity than, the polyolefin component. The preferred filler is a rigid material having a hydrophobic surface or a material that is treated to have a hydrophobic surface. The preferred mean average particle size of the filler material is 0.1 to 10 microns for precursor films having a thickness of 2 to 6 mils. Examples of useful inorganic filler materials include calcium carbonate, talc, clay, kaolin, silica, glass, diatomaceous earth, magnesium carbonate, barium carbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide, zinc oxide, magnesium hydroxide, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, glass powder, zeolite, silica clay and like materials. Calcium carbonate is particularly preferred for low cost, whiteness, inertness and availability. The inorganic fillers, such as calcium carbonate, are preferably surface treated to be hydrophobic. A hydrophobic filler material repels water and therefore reduces the agglomeration ofthe filler particles. Preferably, the surface coating also improves the ability ofthe filler to bind with the polyolefin, while simultaneously allowing the filler particles to be pulled away from the polyolefin under stress. A preferred coating is calcium stearate which is readily available and FDA compliant. Organic filler materials such as wood powder, pulp powder and other cellulose-based powders may also be used. Polymer powders, such as Teflon® powder and Kevlar® powder (each of which are manufactured and sold by E.I. DuPont de Nemours Co.) are also suitable for use. The amount of filler added to the polyolefin depends on the desired vapor permeability of the vapor permeable portion ofthe polymer film. However, it is believed that the formation of a vapor permeable (breathable) film portion requires that the amount of filler in the first polymer composition be at least 15 volume %, based on the total volume of filler and polyolefin (e.g. LLDPE/CaCO 3 having at least 38 wt.% CaCO ). This minimum amount of filler is needed to insure the interconnection within the film of voids created at the situs ofthe filler particles during the "activation" ofthe precursor film, further described, infra. Further, it is believed that useful films cannot be made with an amount of filler in excess of 35 vol.% (e.g., LLDPE/CaCO:, having greater than 65 wt.% CaCO 3 ). Higher amounts of filler may cause difficulty in compounding ofthe first polymer composition. The polyolefin/filler-containing first polymer composition used to form the first vapor permeable portions ofthe inventive polymer films can be compounded in a number of conventional ways. The components may be brought into intimate contact by, for example, dry blending these materials and subsequently passing the overall composition through a compounding extruder. Alternatively, the polyolefin and filler material may be fed directly to a mixing device such as a compounding extruder, high shear continuous mixer, two roll mixer, or an internal mixer, such as a Banbury mixer. Overall, the objective is to obtain a uniform dispersion ofthe filler material in the polymer without agglomeration. This objective can be met by inducing sufficient shear and heat to cause the polyolefin to melt. At the same time, the time and temperature of mixing should be controlled in the conventional manner to avoid molecular weight degradation ofthe polymer. Compounding of LLDPE and calcium carbonate that is surface treated with calcium stearate has been found to be improved by vacuum drying the mixture within the compounding extruder. The tear strength and softness ofthe vapor permeable portion ofthe polymer sheet can be improved by adding small amounts of elastomer to the first polymer composition. Suitable elastomers include natural rubber, ethylene-alpha olefin rubber (EPM), ethylene, alpha olefin-diene monomer rubber (EPDM), styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS), butyl, and the like. The second portion ofthe extruded precursor film having the relatively low vapor permeability and relatively high tensile strength (relative to the first portion) can be prepared from a second polymer composition comprising at least one polyolefin component and a relatively low amount, or no amount, of filler material. By relatively low, we intend that the second portion ofthe film has less than 15% by volume, preferably less than 10%, more preferably less than 5%, even more preferably less than 3%. The polyolefin can be the same or different than the polyolefin used to form the first polymer composition. As with the first composition, the use of LLDPE as the polyolefin ofthe second polymer composition is preferred. The second polymer composition has a relatively low amount, or no amount, of filler material. However, the second polymer composition can include a small amount of filler material, particularly titanium dioxide, added to the second polymer composition not to render the second portions gas permeable but to match the opacity ofthe formed second portions to that ofthe first portions so that the two contiguous portions cannot be visually distinguished. The polyolefin ofthe second portion may also be blended with an elastomer. Each ofthe first polymer composition and the second polymer composition may also contain additional conventional additives, such as anti- oxidants, anti-static agents, colorants and the like. A precursor film having contiguous portions formed ofthe first polymer composition and second polymer composition (and, if desired, third, fourth, fifth compositions, etc.) can be extruded (e.g. by cast extrusion), by simultaneously feeding the first polymer compositions and second polymer composition to an extruder (e.g., a cast extruder) fitted with a sectioned extruder die ofthe type described in the Polyloom '141 patent supra. Advantageously, the temperature of each polymer stream is controlled so that the melt characteristics ofthe disparate feed stocks are similar. Because the first polymer composition contains a high amount of filler relative to the second polymer composition, it may be necessary to feed the first polymer composition and the second polymer composition to the extruder at different temperatures in order to obtain the desired melt characteristics and insure that the first and second portions ofthe precursor film are formed contiguously, and with a constant width. The melt temperature ofthe polymer compositions and the die temperature ofthe extruder will vary depending on the materials being extruded. Cast extrusion is preferred. When the preferred cast extrusion process is employed, the melt temperature of each ofthe polymer compositions and the extruder die temperature will preferably be 350°F (175°C) to 450°F (235°C). Confluence ofthe molten first polymer composition and second polymer composition feed stocks occurs in the die, and a multicomponent film having contiguous first portions and second portions, formed ofthe first polymer composition and second polymer composition, respectively, is extruded from the lip aperture ofthe extrusion die. The thickness ofthe precursor film, having contiguous first and second portions, can be varied by adjusting the die gap and/or the film takeaway speed. Preferably, the die gap for extruding a LLDPE resin will be set to provide a precursor film thickness of 1 to 6 mil, preferably from 2 to 4 mil. The extruded precursor film may be taken up on a flat or engraved casting roller, as is conventional in the art. An engraved roller will deliver an embossed film of patterns well known in the art. After extrusion, the precursor film can be "activated" by the stretching. Stretching ofthe precursor film causes voids and the interconnection within the sheet ofthe voids created at the situs ofthe filler particles, thereby increasing the vapor permeability ofthe material and reducing the thickness ofthe precursor polymer film. Because stretching generally only increases the vapor permeability of the material in the presence of particulate filler material, only the first portions of the precursor sheet formed ofthe filler-containing first polymer composition are rendered vapor permeable by stretching. The precursor film can be stretched either mono- or bi-axially, and stretching may be performed by tentering the film and/or by passing the precursor film around two rollers driven at different speeds, as described in the Exxon '073 patent, supra. Preferably, the precursor film is stretched by passing the precursor film through a nip portion formed between interdigitating grooved rollers. The stretching of a polymer web by passage through interdigitated grooved rollers is disclosed, for example, in U.S. Patent No. 4,223,059 assigned to Biax Fiberfilm Corporation ("the Biax '059 patent"), which is incorporated herein by reference for purposes of U. S. patent practice. The Biax '059 discloses interdigitating rollers provided with grooves having a generally sine-wave shaped cross-section. Similarly, U.S. Patent No. 4, 153,664 to Sabee ("the Sabee '664 patent"), which is also incorporated herein by reference for purposes of U.S. patent practice, discloses the stretching of a polymer film between a pair of ring-rollers having grooves with a variety of shapes. Stretching can be conducted at elevated temperatures below the melt point ofthe polymer compositions (e.g., from 45 to 65°C), in which case the vapor permeability ofthe first portion(s) ofthe film will be inversely related to the stretching temperature. For example, the first portion(s) of a polymer film stretched at 45°C will have a higher degree of vapor permeability than an identical film stretched at a temperature of 65°C. The stretched precursor film may then optionally be heat set as described in the Exxon '073 patent. Heat setting stabilizes the stretched precursor film for any subsequent processing steps performed at temperatures higher than the stretching temperature. Heat setting can be conducted at any temperature that is above the stretching temperature and below the softening temperature of the polymer being used (e.g., 80 to 95°C). Higher temperatures cause some stiffening ofthe polymer film (both the first portion(s) and the second portion(s)), and a slight reduction in the vapor permeability (ofthe first portion(s)). Therefore, heat setting at 80°C will result in an overall softer polymer film which has more vapor permeable first portion(s) compared to a similar material heat set at 95°C. The polymer films ofthe present invention can be used as a component in the formation of numerous commercial products, such as a backsheet for sanitary articles including diapers, training pants, incontinence pants, bed pads and the like. The films ofthe present invention can find use in the construction of medical and surgical supplies, feminine care products, apparel and household furnishings, tape and packaging, and filtration membranes. For example, the breathable polymer films ofthe invention can be used to replace porous films in applications such as burn dressings, sterile packaging, rain coats, shoe linings, non-fogging packaging film, bacteria filters, water purification filters, and wind insulation for houses and buildings. Such films can also be used to form light weight camping and backpacking materials. The size, number and configuration of relatively vapor permeable first portions and relatively high tensile strength second portions can be varied depending on the end use of the film. For example, when used as a diaper backsheet, the polymer film can be formed with a single, central first portion formed ofthe filler material-containing first polymer composition, and high tensile strength second and third portions formed ofthe second polymer composition contiguous to respective sides ofthe first portion in the machine direction ofthe film. The central first portion provides a vapor permeable region where needed, while the stronger side portions provide sufficient strength to accept fastening tapes, elastic gathers, or the like. Similarly, when used to form a bed pad, a central region corresponding in position to the upper surface of a mattress can be formed as a vapor permeable first portion while side portions that wrap around the mattress and are provided with elastic holding members can be formed ofthe second, stronger polymer composition. When used as a diaper backsheet, the first portion will have a high degree of vapor permeability relative to the second and third portions. The higher vapor permeability ofthe central first portion allows for the release of liquid vapor, decreasing the wet feel ofthe diaper interior and reducing the possibility of skin irritations, such as diaper rash. While the degree of vapor permeability can be set to a high level by use of a more highly filled first polymer composition, it is important that the first portion maintain a general resistance to liquid transmission. For diaper backsheet uses, the vapor permeability ofthe first portion measured as a water vapor transmission rate (WVTR) in accordance with ASTM F-1249 (incorporated herein by reference for purposes of U. S. patent practice) can be greater than 1000 to 4000 g/m 2 /day at 38°C, 90%RH (e.g., 2300 g/m 2 /day), while the WVTR ofthe unfilled polymer second portions will be less than 400 g/m 2 /day (e.g., less than 100 g/m 2 /day). The higher tensile strength ofthe second and third portions allows for the attachment of tapes or fasteners for securing the diaper, as well as elastic leg openings. The second and third portions ofthe polymer film can have a machine direction tensile strength at 5% strain (per ASTM D-882, incorporated herein by reference for purposes of U. S. patent practice) of greater than 400 g/in (e.g., 500 g/in.), a dart drop impact strength (measured in accordance with ASTM-4272, incorporated herein by reference for purposes of U. S. patent practice) greater than 500g (e.g., 800g), an ultimate tensile strength in the machine direction greater than 1750 g/in. (e.g., 2000 g/in.) (in accordance with ASTM-D-882), and an ultimate tensile strength in the transverse direction greater than 1300 g/in. (e.g., 2000 grams/in.). The filled, first portion, on the other hand, may have a machine direction tensile strength at 5% of less than 400 g/in (e.g., less than 250 g/in.), a dart drop impact strength of less than 250g (e.g., lOOg), an ultimate tensile strength in the machine direction less than 1400 g/in. (e.g., 1250 g/in.), and an ultimate tensile strength in the transverse direction less than 1300 g/in. (e.g., 1000 g/in.). With the foregoing description ofthe invention, those skilled in the art will appreciate that modifications may be made to the invention without departing from the spirit thereof. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described above.

Claims: CLAIMS We claim: 1. A polymer film comprising at least first and second contiguous portions, wherein said first portion is extruded from a first polymer composition containing a filler material in an amount sufficient to increase the water vapor permeability of said first portion relative to said second portion, and said second portion is extruded from a second polymer composition such that the tensile strength of said second portion is greater than the tensile strength of said first portion. 2. The polymer film of claim 1, wherein said first portion is coextruded with, and positioned between said second portion and a third portion extruded from said second polymer composition, said first portion being contiguous with each of said second portion and said third portion in the machine direction. 3. The polymer film of claim 1 , wherein said second polymer composition contains less filler material relative to said first polymer composition. 4. The polymer film of claim 1, wherein the polymer component of each of said first polymer composition and said second polymer composition is a polyolefin. 5. The polymer film of claim 4, wherein said polyolefin of said first polymer composition and said second polymer composition is the same. 6. The polymer film of claim 4, wherein said polyolefin is selected from the group consisting of: low density polyethylene, linear low density polyethylene, linear medium density polyethylene, high density polyethylene, polypropylene homopolymer, a polypropylene copolymer, and mixtures thereof. 7. The polymer film of claim 4 wherein said polyolefin is linear low density polyethylene. 8. The polymer film of claim 1, wherein said filler material is selected from the group consisting of: calcium carbonate, talc, clay, kaolin, silica, diatomaceous earth, magnesium carbonate, magnesium sulfate, barium sulfate, calcium sulfate, aluminum hydroxide, zinc oxide, calcium oxide, magnesium oxide, titanium oxide, alumina, mica, glass powder, zeolite, silica clay, wood powder, pulp powder, cellulose and mixtures thereof. 9. The polymer film of claim 8, wherein said filler material is calcium carbonate. 10. The polymer film of claim 1, wherein said first polymer composition comprises 15 to 35 vol% of said filler material. 11. The polymer film of claim 2, wherein said first portion has a water vapor transition rate of 1000 to 4000 g/m 2 /day, and each of said second portion and third portion have a water vapor transition rate of less than 400 g/m 2 /day. 12. The polymer film of claim 1 1, wherein said first portion has a machine direction tensile strength at 5% strain below 250 g/in. and each of said second portion and said third portion have a machine direction tensile strength at 5% strain greater than 400 g/in. 13. The polymer film of claim 1 1, wherein said first portion has a dart drop impact strength of less than 250 g, and each of said second portion and third portion has a dart drop impact strength greater than 500 g. 14. The polymer film of claim 12, wherein said first portion has an ultimate tensile strength in the machine direction less than 1400 g/in. and an ultimate tensile strength in the transverse direction less than 1 100g/in., and each of said second portion and said third portion have an ultimate tensile strength in the machine direction greater than 1750 g/in. and an ultimate tensile strength in the transverse direction greater than 1300g/in. 15. A diaper backsheet formed ofthe polymer film of claim 1.

Inventors: BRADY KEVIN A, HEYN HANS E, BRADY, KEVIN, A., HEYN, HANS, E.
Applicants: EXXON CHEMICAL PATENTS INC, EXXON CHEMICAL PATENTS INC.
Application year: 1999
Main classification: B32B5
IPC codes: A61F13, B32B27, C08K3