Microporous polymeric films having structures that enable vapors to flow through them such that they are breathable or have breathability while at the same time inhibiting or stopping the flow of liquids through them have been known for some time. These types of films have been utilized in a wide variety of applications, such as in the preparation of synthetic leather, in the preparation of cloth laminates for use as synthetic shoes, raincoats, outer wear, camping equipment such as tents, and the like, and in combination with fabrics and other materials for preparation of medical applications such as surgical gowns, bandages, and the like, and applications such as house wrap, covers for automobiles and other motorized vehicles, and the like.
A variety of porous stretched polymeric films are known in the art. In general, there are three types of processes used to produce porous, melt-processed polymeric films including a stretching step. These three types are processes for stretching films of neat, unblended polymers containing no fillers, other than typical stabilizing additives; processes for making films from blends of two or more polymers, or from blends of polymer with mineral oil or an organic salt in which the dispersed phase is extracted with the film stretched before or after the extraction; and processes for producing films from blends of polymer with a filler such as calcium carbonate or barium sulfate with the film stretched after casting and without extraction.
Commonly assigned U.S. Pat. No. 4,975,469 discloses oriented porous polymeric films formed from a polypropylene-based resin and having a moisture vapor transmission rate (MVTR) determined according to ASTM E-96, procedure E, in the range of about 2,500 to about 7,500 g/m.sup.2 /24 hr, and a process for forming the films. The aesthetic characteristics of these polypropylene porous films have a somewhat stiffer nature or harsher hand relative to porous film made from synthetic materials other than polypropylene and that they have a tendency to produce a "rustling" noise when flexed.
Commonly assigned U.S. Pat. No. 4,386,129 discloses a porous polymeric film having pores with polygonal cross sections and an average pore diameter of about 3 to about 100 microns formed from a resinous polymer of propylene and a process for forming the film comprising the steps of forming a film containing beta-spherulites and selectively extracting the beta-spherulites. Such films have utility as filtration devices, raincoats and tents.
Japanese Patent No. 199742 (1988) discloses a method for manufacturing a microporous polypropylene film from a composition consisting of polypropylene, a polymer with a melt crystallization point above that of polypropylene, and a beta-crystalline nucleating agent which is melt-extruded and molded into a sheet at 90.degree. to 120.degree. C., in which the resulting sheet is stretched at least uniaxially with an area draw ratio of 2.25-48.
EP 0 273 582A discloses a process wherein polypropylene blended with mineral oil and a nucleating agent is cast into a film. The mineral oil phase separates as droplets within the polypropylene matrix, and the droplets are removed in an extraction bath. The nucleating agent is said to act to reduce the size of the droplets and thereby reduce the size of the holes in the final product. The patent also discloses that the extracted film may be biaxially stretched although no description is given regarding the moisture vapor transmission rate of the resulting material.
U.S. Pat. No. 3,844,865 discloses a process in which polymeric materials of stretch-orientable, non-elastomeric, at least partially crystalline polymers derived from polymerization of one or more unsaturated monomers and stretch orientable polyurethanes and an inorganic salt such as calcium carbonate are formed into films and then stretched uniaxiaily or biaxially to obtain high moisture vapor transmission rates.
U.S. Pat. No. 4,350,655 discloses a process for forming porous thermoplastic films by cold stretching at high stretch tension and low stretch ratios films formed from a blend of synthetic orientable thermoplastic polymer and at least 50 wt % of an inorganic filler such as calcium carbonate, clays and titanium oxide coated with a fatty acid ester of silicon or titanium.
U.S. Pat. No. 4,185,148 discloses a process for producing a polypropylene film having a surface layer of beta-form crystals wherein the film is extruded with a thermal gradient such that one side of the film cools at a much more rapid rate than the other, thereby producing beta-crystals on the cooled side. The resulting film is biaxially stretched to produce a film having a rough surface and which is air-tight.
U.S. Pat. Nos. 4,210,709, 4,331,622 and 4,335,193 disclose microporous films produced by extracting an organic substance from a film comprising 40 to 90 volume percent of a polyolefin having a number average molecular weight of 15,000 or more, 10 to 60 volume percent of an inorganic filler and 2 to 20 wt % based on the total weight of polyolefin and inorganic filler of an organic substance which is substantially insoluble in and inert to sulfuric acid and has a solubility parameter ranging from 7.3 to 8.4. Disclosed inorganic fillers are silica, calcium silicate, aluminum silicate, aluminum oxide, calcium carbonate, magnesium carbonate, kaolin clay, pulverized talc, titanium oxide and diatomaceous earth. The organic substance includes low molecular weight polyolefins such as polyethylene wax, polyisobutylene, polybutadiene, and atactic polypropylene.
U.S. Pat. No. 4,791,144 discloses a microporous polypropylene film prepared by melt-forming into a film a mixture comprising (a) 20 to 80 wt % of a propylene homopolymer, a copolymer of propylene with other copolymerizable monomer or a blend thereof; (b) 80 to 20 wt % of at least one siliceous filler; (c) 10 to 40 wt % of a polyester type plasticizer and/or an epoxy-type plasticizer; and (d) a silane-type dispersant in an amount of 0.01 to 5 wt % based on the total weight of components (a), (b) and (c), and stretching the sheet or film at an area stretching ratio of 1.5 to 30.
U.S. Pat. Nos. 4,777,073 and 4,929,303 disclose a breathable polyolefin film and method for preparing same from melt embossed polyolefin/filler precursor films.
U.S. Pat. Nos. 4,814,124 and 4,921,653 disclose gas-permeable films prepared by stretching a film from a mixture containing a polyolefin resin such as polypropylene, low-density polyethylene, linear low-density polyethylene, high-density polyethylene, and polybutylene, copolymers such as ethylene-propylene copolymer and ethylene-vinyl acetate copolymer or blends of these polymers, and an inorganic filler such as calcium carbonate and barium sulfate preferably added at 50 to 500 parts by weight based on 100 parts by weight of the polyolefin resin.
U.S. Pat. Nos. 4,824,718 and 4,902,553 disclose a method of making microporous film comprising forming a blend of a crystallizable thermoplastic polymer such as polypropylene, polyethylene and polyethylene-polypropylene, a rattle-reducing additive material and a nucleating agent system such as a solid organic acid and an inorganic compound, extruding the blend into a film and orienting the film by stretching at a temperature in the range from 10.degree. C. to a temperature 10.degree. C. below the melting temperature of the thermoplastic polymer.
U.S. Pat. No. 4,613,643 discloses a process for forming soft porous sheets comprising 40 to 80 wt % of an inorganic filler and 60 to 20 wt % of a polyolefin-type thermoplastic elastomer composition such as ethylene-propylene copolymer, ethylene-propylene-diene copolymer, ethylene-butene copolymer and propylene-butene copolymer, and stretching the sheet at a temperature lower than the softening temperature of the polyolefin type thermoplastic elastomer.
U.S. Pat. No. 4,833,172 discloses a method for producing microporous material comprising a matrix consisting essentially of linear ultra-high molecular weight polyolefin, a large proportion of finely divided water-soluble siliceous filler and interconnecting pores by stretching to increase the porosity and provide a region of stretch-induced molecularly oriented polymer in the matrix.
U.S. Pat. No. 5,008,296 discloses microporous films prepared by stretching a film cast from composition of a polyolefin and high levels of CaCO.sub.3 or glass beads and calcium stearate in two directions from 1.5 to 7 times in each direction and having Gurley porosities of from 0.1 to 85 seconds.
Despite the porous films and methods of preparing same disclosed in the patents above, there remains a need for polymeric compositions capable of being formed into microporous films with improved strength and breathability and for a facile processes for forming such microporous films. It is an object of this invention to provide improved polymeric compositions capable of being convened into oriented microporous polymeric films. Another object of this invention is to provide oriented microporous polymeric film and improved processes for preparing these films. Other objects of this invention will be apparent to persons skilled in the art from the following description and claims.
We have found that the objects of this invention can be attained by providing a polymeric composition comprising ethylene-propylene block copolymer, polypropylene homopolymer or random copolymer of polypropylene having up to 10 wt % of a comonomer such as ethylene or an .alpha.-olefin of 4 to 8 carbon atoms and one or more components selected from low molecular weight polypropylene, beta-spherulite nucleating agent and inorganic filler such as calcium carbonate from which oriented polymeric microporous films are produced which have a MVTR of about 500 g/m.sup.2 /24 hr or greater as determined according to ASTM E-96, procedure E. Compositions comprising about 5 to about 30 wt % of the ethylene-propylene block copolymer, about 70 to about 95 wt % of the polypropylene homopolymer or random copolymer and beta-spherulite nucleating agent are converted into oriented polymeric microporous films by a process including the extractive removal of beta-spherulites. Polymeric compositions comprising ethylene-propylene block copolymer and polypropylene homopolymer or random copolymer in a weight of about 30-95/70-5 and one or more components elected from low molecular weight polypropylene, beta-spherulite nucleating agent and calcium carbonate are convened into oriented polymeric microporous films by orienting films cast from the compositions without any extraction step.
Advantageously, we have found that the polymeric compositions of this invention provide oriented microporous films that exhibit improved breathability, strength, toughness and break elongation and are useful for apparel applications such as sportswear, ski clothing, lining material for sportswear and the like, medical apparel such as surgical drapes, protective gowns and the like, equipment covers, filters, housewrap, pressure sensitive labels and the like.