Storm Window Film

The present invention relates to a stretchable and heat shrinkable, thermoplastic film which may be utilized as a storm window. A preferred embodiment of the present invention comprises a palindromic three layer film comprising an interior layer of a linear low density polyethylene and two surface layers comprising a four component blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene, (3) an ethylene vinyl acetate copolymer and (4) one or more ultraviolet light stabilizers whereby a desired combination of physical characteristics beneficially results.

The present invention is directed to new and useful multi-layer heat shrinkable film formulations. One distinguishing feature of a heat shrink film is the film's ability, upon exposure to a certain temperature, to shrink or, if restrained from shrinking, to generate shrink tension within the film.

The manufacture of shrink films, as is well known in the art, may be generally accomplished by extrusion (single layer films) or coextrusion (multi-layer films) of thermoplastic resinous materials which have been heated to their flow or melting point from an extrusion or coextrusion die in, for example, either tubular or planer (sheet) form. After a post extrusion quenching to cool by, for example, the well-known cascading water method, the relatively thick "tape" extrudate is then reheated to a temperature within its orientation temperature range and stretched to orient or align the crystallites and/or molecules of the material. The orientation temperature range for a given material or materials will vary with the different resinous polymers and/or blends thereof which comprise the material. However, the orientation temperature range for a given thermoplastic material may generally be stated to be below the crystalline melting point of the material but above the second order transition temperature (sometimes referred to as the glass transition point) thereof. Within this temperature range it is easy to effectively orient the material.

The terms "orientation" or "oriented" are used herein to generally describe the process step and resultant product characteristics obtained by stretching and immediately cooling a resinous thermoplastic polymeric material which has been heated to a temperature within its orientation temperature range so as to revise the molecular configuration of the material by physical alignment of the crystallites and/or molecules of the material to improve certain mechanical properties of the film such as, for example, shrink tension and orientation release stress. Both of these properties may be measured in accordance with ASTM D 2838-81. When the stretching force is applied in one direction uniaxial orientation results. When the stretching force is applied in two directions biaxial orientation results. The term oriented is also used herein interchangeably with the term "heat shrinkable" with these terms designating a material which has been stretched and set by cooling while substantially retaining its stretched dimensions. As oriented (i.e. heat shrinkable) material will tend to return to its original unstretched (unextended) dimensions when heated to an appropriate elevated temperature.

Returning to the basic process for manufacturing the film as discussed above, it can be seen that the film, once extruded (or coextruded if it is a multi-layer film) and initially cooled to by, for example, cascading water quenching, is then reheated to within its orientation temperature range and oriented by stretching. The stretching to orient may be accomplished in many ways such as, for example, by "blown bubble" techniques or "tenter framing". These processes are well known to those in the art and refer to orientation procedures whereby the material is stretched in the cross or transverse direction (TD) and/or in the longitudinal or machine direction (MD). After being stretched, the film is quickly quenched while substantially retaining its stretched dimensions to rapidly cool the film and thus set or lock-in the oriented (aligned) molecular configuration.

Of course, if a film having little or no orientation is desired, e.g. non-oriented or non-heat shrinkable film, the film may be formed from a non-orientable material or, if formed from an orientable material may be "hot blown". In forming a hot blown film the film is not cooled immediately after extrusion or coextrusion but rather is first stretched shortly after extrusion while the film is still at an elevated temperature above the orientation temperature range of the material. Thereafter, the film is cooled, by well-known methods. Those of skill in the art are well familiar with this process and the fact that the resulting film has substantially unoriented characteristics. Other methods for forming unoriented films are well known. Exemplary, is the method of cast extrusion or cast coextrusion which, likewise, is well known to those in the art.

The degree of stretching controls the degree or amount of orientation present in a given film. Greater degrees of orientation are generally evidenced by, for example, increased values of shrink tension and orientation release stress. That is, generally speaking, for films manufactured from the same material under otherwise similar conditions, those films which have been stretched, e.g. oriented, to a greater extent will exhibit larger values for free shrink, shrink tension and/or orientation release stress. As stated above, the last two values are to be measured in accordance with ASTM-D-2838-81. The first value should be measured in accordance with ASTM D 2732-70 (reapproved 1976).

After setting the stretch-oriented molecular configuration the film may then be stored in rolls and utilized to tightly package a wide variety of items. In this regard, the product to be packaged may first be enclosed in the heat shrinkable material by heat sealing the shrink film to itself where necessary and appropriate to form a pouch or bag and then inserting the product therein and closing the bag or pouch by heat sealing or other appropriate means such as, for example, clipping. If the material was manufactured by "blown bubble" techniques the material may still be in tubular form or it may have been slit and opened up to form a sheet of film material. Alternatively, a sheet of the material may be utilized to over-wrap the product which may be in a tray. These packaging methods are all well known to those of skill in the art. Thereafter, the enclosed product may be subjected to elevated temperatures by, for example, passing the enclosed product through a hot air or hot water tunnel. This causes the enclosing film to shrink around the product to produce a tight wrapping that closely conforms to the contour of the product. As stated above, the film sheet or tube may be formed into bags or pouches and thereafter utilized to package a product. In this case, if the film has been formed as a tube it may be preferable to first slit the tubular film to form a film sheet and thereafter form the sheet into bags or pouches. Such bag or pouch forming methods, likewise, are well known to those of skill in the art.

Another alternative use for heat shrink film is in the formation of low cost storm windows. In this application a sheet of the material may be attached to the window frame and thereafter heat shrunk, for example by using a hand held electric hair dryer, to tighten the film and improve the overall appearance of the window. Alternatively, the film may be stretched across the window casement or housing and attached thereto without post attachment heat shrinking.

The above general outline for manufacturing of films is not meant to be all inclusive since such processes are well known to those in the art. For example, see U.S. Pat. Nos. 4,274,900; 4,229,241; 4,194,039; 4,188,443; 4,048,428; 3,821,182 and 3,022,543. The disclosures of these patents are generally representative of such processes and are hereby incorporated by reference.

Alternative methods of producing films of this type are known to those in the art. One well-known alternative is the method of forming a multi-layer film by an extrusion coating rather than by an extrusion or coextrusion process as was discussed above. In extrusion coating a first tubular layer is extruded and thereafter an additional layer or layers is sequentially coated onto the outer surface of the first tubular layer or a successive layer. Exemplary of this method is U.S. Pat. No. 3,741,253. This patent is generally representative of an extrusion coating process and is hereby incorporated by reference.

Many other process variations for forming films are well known to those in the art. For example, multiple layers may be first coextruded with additional layers thereafter being extrusion coated thereon. Or two multi-layer tubes may be coextruded with one of the tubes thereafter being extrusion coated or laminated onto the other. The extrusion coating method of film formation may be preferable to coextruding the entire film when it is desired to subject one or more layers of the film to a treatment which may be harmful to one or more of the other layers. Exemplary of such a situation is a case where it is desired to irradiate one or more layers of a film containing an oxygen barrier layer comprised of one or more copolymers of vinylidene chloride and vinyl chloride. Those of skill in the art generally recognize that irradiation is generally harmful to such oxygen barrier layer compositions. Accordingly, by means of extrusion coating, one may first extrude or coextrude a first layer or layers, subject that layer or layers to irradiation and thereafter extrusion coat the oxygen barrier layer and, for that matter, other layers sequentially onto the outer surface of the extruded previously irradiated tube. This sequence allows for the irradiation cross-linking of the first layer or layers without subjecting the oxygen barrier layer or other sequentially added layers to the harmful effects thereof.

Irradiation of an entire film or a layer or layers thereof may be desired so as to improve the film's resistance to abuse and/or puncture and other physical characteristics. It is generally well known in the art that irradiation of certain film materials results in the cross-linking of the polymeric molecular chains contained therein and that such action generally results in a material having improved abuse resistance. When irradiation is employed to accomplish the cross-linking, it may be accomplished by the use of high energy irradiation using electrons, X-rays, gamma rays, beta rays, etc. Preferably, electrons are employed of at least about 10.sup.4 electron volt energy. The irradiation source can be a Van de Graaff electron accelerator, e.g. one operated, for example, at about 2,000,000 volts with a power output of about 500 watts. Alternatively, there can be employed other sources of high energy electrons such as the General Electric 2,000,000 volt resonant transformer or the corresponding 1,000,000 volt, 4 kilowatt, resonant transformer. The voltage can be adjusted to appropriate levels which may be, for example, 1,000,000 or 2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Other apparatus for irradiating films are known to those of skill in the art. The irradiation is usually carried out at between one megarad and 75 megarads, with a preferred range of 8 megarads to 20 megarads. Irradiation can be carried out conveniently at room temperature, although higher and lower temperatures, for example, 0.degree. C. to 60.degree. C. may be employed.

Cross-linking may also be accomplished chemically through utilization of peroxides as is well known to those of skill in the art. A general discussion of cross-linking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a Library of Congress Catalog Card Number of 64-22188.

Another possible processing variation is the application of a fine mist of a silicone or anti-fog spray to the interior of the freshly extruded tubular material to improve the further processability of the tubular material. A method and apparatus for accomplishing such internal application is disclosed in a European patent application under publication no. of 0071349A2. This document was published on or about Feb. 9, 1983.

The polyolefin family of shrink films and, in particular, the polyethylene family of shrink films provide a wide range of physical and performance characteristics such as, for example, shrink force (the amount of force that a film exerts per unit area of its cross-section during shrinkage), the degree of free shrink (the reduction in linear dimension in a specified direction that a material undergoes when subjected to elevated temperatures while unrestrained), tensile strength (the highest force that can be applied to a unit area of film before it begins to tear apart), heat sealability (the ability of the film to heat seal to itself or another given surface), shrink temperature curve (the relationship of shrink to temperature), tear initiation and tear resistance (the force at which a film will begin to tear and continue to tear), optics (gloss, haze and transparency of material), elongation (the degree the film will stretch or elongate at room temperature), elastic memory (the degree a film will return to its original unstretched (unelongated) dimension after having been elongated at room temperature), and dimensional stability (the ability of the film to retain its original dimensions under different types of storage conditions). Film characteristics play an important role in the selection of a particular film and they may differ for each film application.

In view of the many above-discussed physical characteristics which are associated with polyolefin films and films containing a polyolefin constituent and in further view of the numerous applications with which these films have already been associated and those to which they may be applied in the future, it is readily discernable that the need for ever improving any or all of the above described physical characteristics or combinations thereof in these films is great, and, naturally, ongoing. In particular, the quest for films which may be utilized as a low cost storm window material has been ongoing since such a film application could compete well with the much more expensive permanent glass storm windows which have been historically utilized. A low cost heat shrink storm window film should preferably possess (1) good optical characteristics so that the function of the window is not undesirably degraded, (2) high physical abuse resistance, (3) good resistance to degradation from light, (4) good elongation (so that it may be stretched tightly onto the window frame prior to attachment thereto), (5) good elastic memory (so that it will not readily permanently deform when subjected to the forces of nature--e.g. wind, rain, small debris) and (6) a low to moderate degree of orientation (so that, if desired, the film may be shrunk into tight configuration with the window frame without generating an undesirable degree of tension within the film). Orientation also provides the film with improved physical characteristics such as, for example, good tensile strength.

In particular, the present multilayer film is preferable to a presently manufactured monolayer storm window film which should be utilized only on the interior side of the window. This prior art monolayer film comprises a single layer of linear medium density polyethylene material having a polyorganosiloxane coating on one side thereof.

Other prior art films utilizing linear polyethylene materials and blends thereof are known to those of skill in the art. Exemplary multilayer prior art films having a core layer of linear low density polyethylene material are U.S. Pat. No. 4,364,981 to Horner which discusses a three layer film having a core layer of low pressure, low density polyethylene (LLDPE) and outer layers of high pressure, low density polyethylene (conventional low density polethylene) and U.S. Pat. No. 4,399,180 to Briggs which discusses a stretch-wrap film having a core layer of linear low density polyethylene with a layer, on at least one side, comprising a highly branched low density polyethylene. U.S. Pat. No. 4,399,173 to Anthony discusses a multilayer film comprising a core layer of low melt index, low pressure, low density polyethylene and two outer layers of a high melt index, low pressure, low density polyethylene. U.S. Pat. No. 4,425,268 to Cooper discloses a composition adapted for processing into stretch-wrap film. Generally, the Cooper composition comprises a blend of an ethylene vinyl acetate copolymer and a linear low density polyethylene material. The material may also contain a tackifier.

Accordingly, it is a general object of the present invention to provide a heat shrinkable film which is useful as a low cost storm window.

It is another object of the present invention to provide a heat shrinkable film having a desired new and improved combination of physical characteristics such as, for example, a low degree of orientation or heat shrinkability combined with good puncture and tear resistance along with good elongation and elastic memory (elasticity or snap-back).

Another object of the present invention is to provide a stretchable three layer heat shrink film having a core layer comprising a linear low density polyethylene and two surface layers comprising a four component blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene, (3) an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers.

Yet another object of the present invention is to provide a stretchable three layer film having a core layer consisting of essentially of a linear low density polyethylene and two surface layers consisting essentially of a four component blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene, (3) an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers.

An even further object of the present invention is to provide a stretchable three layer heat shrink film comprising a core layer comprising a linear low density polyethylene and two surface layers comprising a four component blend of (1) from about 40% to about 60%, by weight, of linear low density polyethylene, (2) from about 20% to about 30%, by weight, of linear medium density polyethylene, (3) from about 20% to about 30%, by weight, of an ethylene vinyl acetate copolymer and (4) from about 500 to about 3,000 ppm (parts per million) of one or more ultraviolet stabilizers.

One other object of the present invention is to provide a stretchable three layer heat shrink film comprising a core layer consisting essentially of a linear low density polyethylene and two surface layers consisting essentially of a four component blend of (1) from about 40% to about 60%, by weight, of linear low density polyethylene, (2) from about 20% to about 30%, by weight, of linear medium density polyethylene, (3) from about 20% to about 30%, by weight, of an ethylene vinyl acetate copolymer and (4) from about 500 to about 3,000 ppm of one or more ultraviolet stabilizers.

Yet a further object of the present invention is to provide a stretchable three layer heat shrink film adapted for use as a storm window which comprises a core layer consisting essentially of a linear low density polyethylene and two surface layers consisting essentially of a four component blend of (1) about 50%, by weight, linear low density polyethylene, (2) about 25%, by weight, of linear medium density polyethylene, (3) about 25%, by weight, of an ethylene vinyl acetate copolymer and (4) about 1,500 ppm of a hindered amine ultraviolet light stabilizer.

Still further objects and the broad scope of applicability of the present invention will become apparent to those of ordinary skill in the art from the details disclosed hereinafter. However, it should be understood that the following detailed description which indicates the presently preferred embodiment of the present invention is only given for purposes of illustration since various changes and modifications well within the scope of the present invention will become apparent to those of ordinary skill in the art in view of the following detailed description.

Unless specifically set forth and defined or otherwise limited, the terms "polymer" or "polymer resin" as used herein generally include, but are not limited to, homopolymers, copolymers, such as, for example block, graft, random and alternating copolymers, terpolymers etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited the terms "polymer" or "polymer resin" shall include all possible symmetrical structures of the material. These structures include, but are not limited to, isotactic, syndiotactic and random symmetries.

The term "melt flow" as used herein is the amount, in grams, of a thermoplastic resin which can be forced through a given orifice under a specified pressure and temperature within ten minutes. The value should be determined in accordance with ASTM D 1238-79. The term "melt flow index" refers specifically to the value obtained in accordance with condition E of ASTM D 1238-79.

The terms "surface" or "surface layer" or "skin" or "skin layer" as used herein means a layer of a multi-layer film which comprises a surface thereof.

The term "interior" or "interior layer" as used herein refers to a layer of a multi-layer film which is not a skin or surface layer of the film.

The term "core" or "core layer" as used herein refers to an interior layer of a multi-layer film having an odd number of layers wherein the same number of layers is present on either side of the core layer.

The term "intermediate" or "intermediate layer" as used herein refers to an interior layer of a multi-layer film which is positioned between a core layer and a surface layer of said film.

The term "palindromic" film as used herein refers to a multilayer film the layer configuration of which is substantially symmetrical. Examples of palindromic films would be films having the following layer configurations: (1) A/B/A, (2) A/B/B/A, (3) A/B/C/B/A, etc. An example of a non-palindromic film layer configuration would be a film having a layer configuration of A/B/C/A.

The term polyolefin as used herein refers to polymers of relatively simple olefins such as, for example, ethylene, propylene, butenes, isoprenes and pentenes; including, but not limited to, homopolymers, copolymers, blends and modifications of such relatively simple olefins.

The term "polyethylene" as used herein refers to a family of resins obtained by polymerizing the gas ethylene, C.sub.2 H.sub.4. By varying the catalysts and methods of polymerization, properties such as density, melt index, crystallinity, degree of branching and cross-linking, molecular weight and molecular weight distribution can be regulated over wide ranges. Further modifications are obtained by copolymerization, chlorination, and compounding additives. Low molecular weight polymers of ethylene are fluids used as lubricants; medium weights polymers are waxes miscible with paraffin; and the high molecular weight polymers (generally over 6,000) are resins generally used in the plastics industry. Polyethylenes having densities ranging from about 0.900 grams per cubic centimeter to about 0.925 grams per cubic centimeter are called low density polyethylenes with those having densities from about 0.926 grams per cubic centimeter to about 0.940 grams per cubic centimeter being called medium density polyethylenes; and those having densities of from about 0.941 grams per cubic centimeter to about 0.965 grams per cubic centimeter and over are called high density polyethylenes. Conventional low density types of polyethylenes are usually polymerized at high pressures and temperatures whereas the high density types are usually polymerized at relatively low temperatures and pressures. The molecular structure of conventional low density polyethylenes is highly branched. While conventional medium density polyethylenes possess a molecular structure which is branched, the degree of branching is less than that of conventional low density polyethylenes. The molecular structure of high density polyethylenes possess little or no side branching.

The terms "linear low" or "linear medium density polyethylene" as used herein refer to copolymers of ethylene with one or more comonomers selected from C.sub.4 to C.sub.10 alpha olefins such as butene-1, octene, etc. in which the molecules thereof comprise long chains with few side chains branches or cross-linked structures. This molecular structure is to be contrasted with conventional low or medium density polyethylenes which are more highly branched than their respective linear counterparts. Moreover, the side branching which is present in linear low or linear medium density polyethylenes will be short as compared to the respective non-linear polyethylenes. The molecular chains of a linear polymer may be intertwined, but the forces tending to hold the molecules together are believed to be physical rather than chemical and thus may be weakened by energy applied in the form of heat. Linear low density polyethylene as defined herein has a density usually in the range of from about 0.900 grams per cubic centimeter to about 0.925 grams per cubic centimeter and, preferably, the density should be maintained between 0.916 grams per cubic centimeter to 0.925 grams per cubic centimeter. Linear medium density polyethylene, as defined herein, has a density usually in the range of from about 0.926 grams per cubic centimeter to about 0.941 grams per cubic centimeter. The melt flow index of linear low and medium density polyethylenes generally ranges from between about 0.1 to about 10 grams per ten minutes and preferably between from about 0.5 to about 3.0 grams per ten minutes. Linear low and linear medium density polyethylene resins of this type are commercially available and are manufactured in low pressure vapor phase and liquid phase processes using transition metal catalysts.

The term "ethylene vinyl acetate copolymer" (EVA) as used herein refers to a copolymer formed from ethylene and vinyl acetate monomers wherein the ethylene derived units in the copolymer are present in major amounts and the vinyl acetate derived units in the copolymer are present in minor amounts.

An "oriented" or "heat shrinkable" material is defined herein as a material which, when heated to an appropriate temperature above room temperature (for example 96.degree. C.), will have a free shrink of 5% or greater in at least one linear direction.

All compositional percentages used herein are calculated on a "by weight" basis.

Density should be measured in accordance with ASTM D 1505-68 (reapproved 1979).

Free shrink should be measured in accordance with ASTM D 2732.

Shrink tension and orientation release stress should be measured in accordance with ASTM D 2838-81.

The tensile properties of the film should be measured in accordance with ASTM D 882-81.

The elongation properties of the film should be measured in accordance with ASTM D 638.

The haze and luminous transmittance of the film should be measured in accordance with ASTM D 1003-61 (reapproved 1971).

The specular gloss of the film should be measured in accordance with ASTM D 2457-70 (reapproved 1977).

The tear propagation of the film should be measured in accordance with ASTM D 1938-67 (reapproved 1978).

The impact resistance of the film should be measured in accordance with ASTM D 3420-80.

One method for determining whether a material is "cross-linked" is to reflux the material in boiling toluene or zylene, as appropriate, for forty (40) hours. If a weight percent residue of at least 5 percent remains the material is deemed to be cross-linked. A procedure for determining whether a material is cross-linked vel non is to reflux 0.4 gram of the material in boiling toluene or another appropriate solvent, for example zylene, for twenty (20) hours. If no insoluble residue (gel) remains the material may not be be cross-linked. However, this should be confirmed by the "melt flow" procedure below. If, after twenty (20) hours of refluxing insoluble residue (gel) remains the material is refluxed under the same conditions for another twenty (20) hours. If more than 5 weight percent of the material remains upon conclusion of the second refluxing the material is considered to be cross-linked. Preferably, at least two replicates are utilized. Another method whereby cross-linking vel non and the degree of cross-linking can be determined is by ASTM-D-2765-68 (Reapproved 1978). Yet another method for determining whether a material is cross-linked vel non is to determine the melt flow of the material in accordance with ASTM D 1238-79 at 230.degree. Centigrade while utilizing a 21,600 gram load. Materials having a melt flow of greater than 75 grams per ten minutes shall be deemed to be non-cross-linked. This method should be utilized to confirm the "gel" method described above whenever the remaining insoluble gel content is less than 5% since some cross-linked materials will evidence a residual gel content of less than 5 weight percent. If the cross-linking is accomplished by irradiation of the film the amount of ionizing radiation which has been absorbed by a known film material can be calculated by comparing the weight percent of insoluble material (gel) remaining after refluxing the sample to the weight percents of gel remaining after refluxing standards of the same material which have been irradiated to different known degrees. Those of skill in the art also recognize that a correlation exists between the amount of ionizing irradiation absorbed and the melt flow of a material. Accordingly, the amount of ionizing irradiation which a material has absorbed may be determined by comparing the melt flow of the material to the melt flow of samples of the same material which have been irradiated to different known degrees.

The term "crystalline" or "crystalline polymer" material, etc. as used herein refers to a polymeric material which is composed of molecular chains which are so constructed that they can pack together well in ordered arrangements. The finite volume throughout which the order extends is designated by the term "crystallite" with the surrounding disordered regions, if any, being designated by the term "amorphous". The crystallites are denser than the surrounding amorphous regions of the material and also have a higher refractive index. If a crystalline material is oriented the crystallites become generally aligned with each other. Three well known methods for determining the degree of crystallinity are by (1) (a) measuring the specific volume of the specimen (V), (b) measuring the specific volume of the crystallites (Vc) within the specimen and (c) measuring the specific volume of the amorphous region (Va) contained within the specimen and then utilizing the equation ##EQU1## (2) X-ray diffraction methods and (3) infrared absorption methods. All of these methods are well known to those in the art. A general discussion of crystallinity can be found at pages 449 to 527 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a Library of Congress Catalogue Card Number of 64-22188.

The term "gauge" is a unit of measure applied to the thickness of films of the layers thereof. 100 gauge is equal to 1 mil which is one thousandth of an inch.

A rad is the quantity of ionizing radiation that results in the absorption of 100 ergs of energy per gram of a radiated material, regardless of the source of the radiation. A megarad is 10.sup.6 rads. (MR is an abbreviation for megarad.)

It has been discovered that a flexible, heat shrinkable thermoplastic film having a desirable combination of physical characteristics such as, a low degree of orientation or heat shrinkability, good elongation, good puncture and tear resistance, good elastic memory e.g. snap-back or elastic recovery and good resistance to light degradation has been achieved by the flexible film of the present invention. The film may be preferably utilized as a low cost storm window. The film may also be used in high abuse resistant stretch/shrink bundling applications.

This film comprises an interior layer comprising a linear low density polyethylene and two surface layers comprising a four component blend of (1) a linear low density polyethylene, (2) a linear medium density polyethylene, (3) an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers. A preferred embodiment of the film comprises a core layer consisting essentially of a linear low density polyethylene and two surface layers consisting essentially of a four component blend of (1) from about 40% to about 60%, by weight, of a linear low density polyethylene, (2) from 20% to about 30%, by weight, linear medium density polyethylene, (3) from about 20% to about 30%, by weight, of an ethylene vinyl acetate copolymer and (4) from about 500-3,000 parts per million of one or more ultraviolet stabilizers. The most preferred embodiment of the present invention is a three layered film comprising a core layer which consists essentially of a linear low density polyethylene which is a copolymer of ethylene and octene having a density of about 0.920 grams per cubic centimeter. The two surface layers of the most preferred embodiment consist essentially of a four component blend of (1) about 50%, by weight, of a linear low density polyethylene which is a copolymer of ethylene and octene having a density of about 0.920 grams per cubic centimeter, (2) about 25%, by weight, of a linear medium density polyethylene having a density of about 0.935 grams per cubic centimeter, (3) about 25%, by weight, of an ethylene vinyl acetate copolymer having about 3.6% vinyl acetate derived units and a density of from about 0.9232-0.9250 grams per cubic centimeter and (4) about 1,500 parts per million of a hindered amine ultraviolet light stabilizer.

The film is both stretched, e.g. biaxially oriented, and cross-linked. Preferably the film is cross-linked by irradiation with from about 4.0 to about 8.0 MR. A more preferable degree of cross-linking is accomplished by irradiation in the range of from about 5 to about 7 MR. The most preferable degree of cross-linking is accomplished by irradiation with about 6 MR.

The degree of stretching to achieve the appropriate degree of biaxial orientation and associated physical characteristics is preferably in the range of from about 3.0 to about 4.0 times the original dimensions in both the transverse (TD) and longitudinal (MD) directions. More preferably the degree of stretching is from about 3.0 to about 3.5 times the original dimensions in both the transverse and longitudinal directions. The most preferred degree of stretching, i.e. orientation, is approximately 3.3 times the original dimension in both the transverse and longitudinal directions.

The preferred thickness ratio of a skin layer to the core layer ranges from about 1 to 1 to about 1 to 4. Preferably the thicknesses of the two skin layers are substantially equal to each other and the thickness of each skin layer is about one-half the thickness of the core layer. Preferably, the total thickness of the film may range from about 40 gauge to about 150 gauge. More preferably the total thickness of the film may vary from about 50 to about 100 gauge. Most preferably the film thickness is about 75 gauge.

The multi-layer film may be combined with other polymeric materials for specific applications. For instance, additional layers may be added on either or both sides of the film to improve various physical characteristics.

FIG. I is a cross-sectional view of a preferred three layered embodiment of the present invention.

Referring to FIG. I, which is a cross-sectional view of a preferred three layered embodiment of the present invention, it is seen that this embodiment comprises a core layer 1 and two skin or surface layers 2 and 3. The preferred thickness ratio of the three layers of 1/2/1 is demonstrated in FIG. I. Preferred core layer 1 formulations comprise one or more linear low density polyethylene materials. Preferably, core layer 1 consists essentially of a linear low density polyethylene material which is a copolymer of ethylene and octene having a density of from about 0.918 to about 0.922 grams per cubic centimeter. Most preferably the core layer 1 consists essentially of a linear low density polyethylene material which is a copolymer of ethylene and octene having a density of about 0.920 grams per cubic centimeter.

Experimentation has revealed an especially preferred core layer material which may be obtained from the Dow Chemical Company under the trade designation Dowlex 2045. This material is a copolymer of ethylene and octene and has a density at 23.degree. C. of about 0.920 grams per cubic centimeter and a flow rate (measured by ASTM-D-1238, E-28) of about 0.7-1.2 grams per ten minutes. Other linear low density polyethylene materials or blends thereof may be utilized to form core layer 1.

Returning to FIG. I, and in particular, to surface layers 2 and 3 experimentation has determined that a preferred surface layer formulation should comprise a four component blend of (1) a linear low density polyethylene material, (2) a linear medium density polyethylene material, (3) an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers.

Preferably the formulation of the two skin layers comprises a four component blend of (1) from about 40% to about 60%, by weight, of a linear low density polethylene material (2) from about 20% to about 30%, by weight, of a linear medium density polyethylene material, (3) from about 20% to about 30%, by weight, of an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers. Even more preferably the surface layers of the film should comprise a four component blend of (1) from about 45% to about 55%, by weight, of a linear low density polyethylene material, (2) from about 23% to about 27%, by weight, of a linear medium density polyethylene material, (3) from about 23% to about 27%, by weight, of an ethylene vinyl acetate copolymer and (4) one or more ultraviolet stabilizers. The most preferred skin or surface layer formulation of the present invention consists essentially of a four component blend of (1) about 50%, by weight, of a linear low density polyethylene material, (2) about 25%, by weight, of a linear medium density polyethylene material, (3) about 25%, by weight, of an ethylene vinyl acetate copolymer and (4) one or more hindered amine ultraviolet stabilizers.

The same group of linear low density polyethylene resins which were discussed with regard to the core layer 1 may be utilized as the linear low density polyethylene constituent of the skin layers 2 and 3. However, the linear low density polyethylene material used in the skin layers does not have to be the material used in the core layer. A preferred linear low density polyethylene for utilization in the skin layers is Dowlex 2045, discussed above. Preferably, the linear medium density polyethylene of the skin layer has a density at 23.degree. C. of from about 0.933 to about 0.937 grams per cubic centimeter. More preferably the linear medium density polyethylene material has a density at 23.degree. C. of about 0.935 grams per cubic centimeter. A preferred linear medium density polyethylene material for utilization in the surface layer formulation may be obtained from the Dow Chemical Company under the trade designation Dowlex 2037. This material is a copolymer of ethylene and octene and has a density at 23.degree. C. of about 0.935 grams per cubic centimeter and a flow rate (measured by ASTM-D-1238, condition E-28) of 2.55.+-.0.35 grams per ten (10) minutes. Other linear low density polyethylene and linear medium density polyethylene materials may be utilized as those of skill in the art would readily recognize.

Ethylene vinyl acetate copolymers comprising from about 2%, by weight, to about 18%, by weight, vinyl acetate derived units may be utilized. Preferably the ethylene vinyl acetate copolymer will comprise from about 2%, by weight, to about 10%, by weight, of vinyl acetate derived units. Even more preferably the ethylene vinyl acetate copolymer will comprise from about 2%, by weight, to about 5%, by weight, of vinyl acetate derived units. The most preferred ethylene vinyl acetate copolymer for utilization in the surface layer formulation may be obtained from the El Paso Polyolefins Company. This material has a density at 23.degree. C. of from 0.9232 to about 0.9250 grams per cubic centimeter and a melt flow (measure by ASTM D 1238, condition E-28) of about 2.0.+-.0.5 grams per ten (10) minutes. The material contains from about 3.3 to about 4.1% vinyl acetate derived units. The nominal percent of vinyl acetate derived units present in the material is about 3.6%. Other ethylene vinyl acetate copolymers may be utilized as those of skill in the art would readily recognize.

A preferred ultraviolet light stabilizer may be obtained from Ciba-Geigy under the trade designation TINUVIN.RTM.622. This material is a polymeric hindered amine, which is believed to have a molecular weight in excess of 2000 and a melting range of 130.degree.-145.degree. C. Published solubility data (grams per 100 grams of solution at 20.degree. C.). for this material are: acetone 2, benzene 30, chloroform 40, ethyl acetate 5, hexane 0.01, methanol 0.1, methylene chloride 40, water 0.01, xylene 8. Preferably both surface layers comprise from about 500 ppm (parts per million) to about 3,000 ppm of the ultraviolet light stabilizer. More preferably both layers comprise from about 1,000 to about 2,000 ppm of the light stabilizer. Most preferably both surface layers comprise about 1,500 ppm of the light stabilizer. Those of skill in the art would readily recognize that other ultraviolet light stabilizers may be utilized.

Preferably the composition and other parameters of skin layers 2 and 3 are substantially the same. However, different linear low density polyethylene, linear medium density polyethylene, ethylene vinyl acetate copolymers and ultraviolet stabilizers or blends thereof may be utilized for each skin layer.

Optionally a colorant or dye may be incorporated into the film and, preferably, incorporated only in the core layer. The presence of the colorant reduces light transmision into the room or other area that is enclosed by the storm window. This feature reduces cooling requirements in warm climates. The option also may reduce glare from sunlight entering the room. Appropriate dyes/colorants are well known and available to those of skill in the art.

Those skilled in the art will readily recognize that all of the by weight percentages disclosed herein are subject to slight variation. Additionally, these percentages may vary slightly as a result of the inclusion or application of additives to the surface layers such as the silicone mist discussed above or inclusion therein of agents such as slip, antioxidant and anti-block agents. A preferred anti-block agent is a diatomaceous silica, SiO.sub.2, which is available from McCullough & Benton, Inc. under the tradename Superfine Superfloss. This material has a wet density of about 29.0 lbs/ft.sup.3, a specific gravity of about 2.30 and a pH of about 9.5. Other well known antiblock agents may be utilized. A preferred slip agent is Erucamide (available from Humko Chemical under the tradename Kemamide E). This material is believed to have an average molecular weight of about 335 and a melting point range of from about 72.degree. C. to about 86.degree. C. Other slip agents such as Stearamide (available from the Humko Chemical Company under the tradename Kemamide S) and N,N-'Dioleoylethylenediamine (available from Glyco Chemical under the tradename Acrawax C) may be utilized. A preferred silicone spray for application to the inner surface of the extruded tube is a liquid polyorganosiloxane manufactured by General Electric under the trade designation General Electric SF18 polydimethylsiloxane. A preferred antioxidant and thermal stabilizing agent is tetrakis[methylene 3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane. This material is believed to be a symmetrical molecule which includes four sterically hindered phenolic hydroxyl groups and has a molecular weight of about 1178. This material is available from by Ciba-Geigy under the trade designation Irganox.RTM.1010.

The general ranges for inclusion of these agents into the surface layers 2 and 3 and, in the case of the silicone spray, the application of the spray mist onto the interior surface layer of a tubular extrudate are as follows:

When utilized within the specification and claims of the present application the term "consisting essentially of" is not meant to exclude slight percentage variations or additives and agents of this sort.

Additional layers and/or minor amounts of various additives of the types described above may be added to the film structure of the present invention as desired but care must be taken not to adversely affect the desired physical propertities and other characteristics of the inventive film. It should also be recognized that many resins obtained from their manufacturer already contain small amounts of additives of different types.

In the preferred process for making the multi-layer film of the present invention the basic steps are coextruding the layers to form a multilayer film, irradiating the film, and then stretching the film to biaxially orient. These steps and additional desirable steps will be explained in detail in the paragraphs which follow.

The process begins by blending, as necessary, the raw materials (i.e. polymeric resins) in the proportions and ranges desired as discussed above. The resins are usually purchased from a supplier in pellet form and can be blended in any one of a number of commercially available blenders as is well known in the art. During the blending process any additives and/or agents which are desired to be utilized are also incorporated. The additives may be incorporated into the blend by utilizing a masterbatch containing small percentages of the additives. For example, in the preferred embodiment of the present invention the light stabilizer and antioxidant are added to the blend which will form the surface layers by blending in a masterbatch available from Ampacet under the trade designation 10478. This material comprises a three component blend of (1) about 88%, by weight, conventional low density polyethylene having a density of about 0.918-0.922 g/cm and a melt index of about 6-10 gm/10 min., (2) about 10%, by weight, of hindered amine stabilizer (TINUVIN.RTM.662) and (3) about 2%, by weight, of antioxidant (Irganox.RTM.1010).

The resins and applicable additives and/or agents are then fed to the hoppers of extruders which feed a coextrusion die. For the preferred palindromic three layer film, having two substantially identical surface layers, at least two extruders need to be employed. One for the two substantially identical skin or surface layers and one for the core layer. Additional extruders may be employed if a film having non-identical surface layers is desired. The materials are coextruded as a relatively thick tube or "tape" which has an initial diameter and thickness dependent upon the diameter and die gap of the coextrusion die. The final diameter and thickness of the tubular film is dependent upon the racking ratio, e.g. the stretching ratio. Circular coextrusion dies are well known to those in the art and can be purchased from a number of manufacturers. As an alternative to tubular coextrusion, slot dies could be used to coextrude the material in sheet form. Well known single or multi-layer extrusion coating processes could also be utilized, if desired.

An additional process step which should be utilized to manufacture the preferred embodiment of the presently inventive film is to irradiate the tape or unexpanded tubing or sheet by bombarding it with high-energy electrons from an accelerator to cross-link the materials of the tube. Cross-linking greatly increases the structural strenth of the film and/or the force at which the material can be stretched before tearing apart when the film materials are predominately ethylene such as polyethylene or ethylene vinyl acetate. Irradiation may also improve the optical propertities of the film and change the propertities of the film at higher temperatures. A preferred irradiation dosage level is in the range of from about 4.0 MR to about 8.0 MR. An even more preferred range is from about 5.0 MR to about 7.0 MR. The most preferred dosage level is approximately 6.0 MR.

Following coextrusion, quenching to cool and solidify, and irradiation of the tape, the extruded tape is reheated to its orientation temperature range and inflated, by application of internal air pressure, into a bubble thereby transforming the narrow tape with thick walls into a wide film with thin walls of the desired film thickness and width. This process is sometimes referred to as the "trapped bubble technique" of orientation or as "racking". The degree of inflation and subsequent stretching is often referred to as the "racking ratio" or "stretching ratio". For example, a transverse racking or stretching ratio of 2.0 would mean that the film had been stretched 2.0 times its original extruded size in the transverse direction during transverse racking. After stretching, the tubular film is then collapsed into a superimposed lay-flat configuration and wound into rolls often referred to as "mill rolls". The racking process orients the film by stretching it transversely and, to some extent, longitudinally and thus imparts shrink capabilities to the film. Additional longitudinal or machine direction racking or stretching may be accomplished by revolving the deflate rollers which aid in the collapsing of the "blown bubble" at a greater speed than that of the rollers which serve to transport the reheated "tape" to the racking or blown bubble area. Preferred transverse and longitudinal stretching ratios of the present film range from about 3.0 transverse by about 3.0 longitudinal to about 4.0 transverse by about 4.0 longitudinal. A particularly preferred stretching ratio is about 3.3 transverse by about 3.3 longitudinal.

To further disclose and clarify the scope of the present invention to those skilled in the art the following test data are presented.

A preferred embodiment of the present invention was formed by coextrusion, irradiated and stretched (oriented) by application of internal air (bubble technique) in accordance with the teachings described above. This embodiment was a three layered film irradiated with approximately 5 MR (8.0 MA, milliamps) and had an approximate layer thickness ratio of 1/2/1. The embodiment comprised a layer structure of "A+B+C+D/A/A+B+C+D". A represents a linear low density polyethylene having a density of about 0.920 gm/cm.sup.3 (Dowlex 2045). B represents a linear medium density polyethylene having a density of about 0.935 gm/cm.sup.3 (Dowlex 2037). C represents an ethylene vinyl acetate copolymer having from about 3.3% to about 4.1% vinyl acetate derived units and a density of from about 0.9232-0.9250 g/cm.sup.3 (El Paso PE 204CS95). D represents a hindered amine stabilizer (TINUVIN.RTM.622).

Test results for this film are listed below in Table I.

The data above demonstrates that the present film has significantly improved values for (1) elongation at break, (2) tear propagation and (3) ball burst impact as compared to the prior art monolayer storm window film. Accordingly, the present film is more easily stretched onto the window frame during installation (e.g. improved elongation). Also, the film is generally able to withstand more abuse (e.g. improved tear propagation and ball burst impact).

It should be understood that the detailed description and specific examples which indicate the presently preferred embodiments of the invention are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those of ordinary skill in the art upon review of the above detailed description and examples.