HIGH OTR POLYOLEFIN FILMS

Abstract: Oriented polyolefin films employ an olefin heteropolymer and a high modulus polypropylene. The high modulus polypropylene can be blended with the olefin heteropolymer in a single layer film or to form the core of a multilayer film. Alternatively, the high modulus polypropylene can be added as a surface layer to a core of the olefin heteropolymer. These films are characterized by an oxygen and moisture vapor transmission rate greater than that of conventional polypropylene films and a tensile modulus great enough to permit their use as packaging films. This combination of properties makes these films suitable for packaging fresh produce items which continue to respire after they are harvested.

Description: HIGH OTR POLYOLEFIN FILMS Specification This invention relates to polyolefin films having certain unique properties. Specifically, it relates to clear polyolefin films that exhibit an increased oxygen transmission property as compaied to conventional polypropylene films known to the art. Background Information It is known that, for many applications, particularly packaging applications, polypropylene is less than totally satisfactory due to a relatively high permeability to oxygen. A great amount of effort has been expended in recent years to decrease the oxygen transmission characteristics of polypropylene. The best results to date have been obtained by metallization of the films. There are, however, some applications in which a greater and controllable oxygen transmission rate is desired. This is particularly the case when fresh fruits, vegetables and flowers are to be packaged as these products continue to respire after they are packaged. Absence or insufficient levels of oxygen, which occurs as the oxygen originally present in the package is consumed, leads to premature senescence and spoilage of the products. At the same time, respiration leads to a build-up of moisture in the package, which can also lead to spoilage of the product if the moisture cannot escape from the package. For many fresh vegetables, fruits and flowers, conventional polypropylene films, whether monolayer or multilayer, of a thickness sufficient to be self-supporting, have oxygen and moisture vapor permeability values that are not sufficient to allow the optimum oxygen and moisture levels to be maintained in a sealed package. It is thus desirable to provide packaging films wherein the oxygen content can be replenished as necessary and from which moisture and carbon dioxide can escape. It is known to prepare microporous films based on an opaque polymer mixture comprised of about 45% to 55% homopolypropylene and 55% to 45% of a copolymer of propylene and ethylene containing about 2% to 5% ethylene by weight, which films have greater oxygen permeability than films made of plain polypropylene. This polymer mixture is blended into a mixture comprised of about 40 to 60% of the polymer mixture and 60 to 40% of certain inorganic filler materials and is subjected to biaxial orientation. The filler material selected is one that causes voiding of the polymer matrix during the drawing operation. Exemplary of such voiding pigments are barium sulfate, calcium carbonate, silica, diatomaceous earth and titania. Calcium carbonate is said to be the preferred filler material. Anderson, U.S. Patent 4,842,875, teaches the use of such films in the preparation of controlled atmosphere containers for use with fresh vegetables, fruits and flowers. Microporous films known to the ait as described above function reasonably well for preparing films having increased and, to a degree, controllable oxygen and moisture vapor permeability. However, the voiding pigments employed in the prior art are of a particle size large enough and are employed in concentrations great enough to result in formation of voids of such a size that the resultant films are almost totally opaque. It is also known to prepare high OTR polyolefin films for some applications by laniinating a polyethylene film of about one mil thickness to a polypropylene film of about 0.4 to 0.5 mil thickness. The polypropylene film acts as a stiffener to give the polyethylene sufficient stiffness that it can be used as a produce wrapper. The polyethylene, which has an inherently high OTR, then acts as a high OTR sealant layer. It is an object of this invention to provide transparent, high modulus polyolefin films based on polypropylene having, among other useful properties, increased oxygen and moisture vapor properties. It is a further object of the invention to provide polyolefin films based on propylene that are useful in modified atmosphere packaging applications for packaging fresh vegetables, fruits and flowers. It is yet another object of this invention to provide novel polyolefin films for use for packaging wherein a high modulus film is desired. Summary of the Invention In accordance with this invention, there are provided transparent biaxially oriented polyolefin films employing olefin heteropolymers and high modulus polypropylene, which films have a stiffness (i.e. tensile modulus) sufficient to have self- supporting, stand-alone characteristics such that they can be employed as packaging films. These films have increased oxygen and moisture vapor permeability, as compared with conventional, unmodified polypropylene film, and are substantially clearer than are films possessing these permeability characteristics that have heretofore been known to the art. In accordance with certain preferred embodiments of this invention, there are provided transparent composite, oriented polyolefin films comprised of a core layer and at least one skin layer, said core layer being a heteropolymer of at least two α- monoolefins, and having a thickness sufficient to give the composite structure standalone characteristics; said at least one skin layer being a high modulus polypropylene of a thickness such that its oxygen transmission rate (OTR) is higher than that of the core layer and the total composite structure having an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. in/day/atmosphere, a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. in./day, and a tensile modulus of at least about 180,000 psi and most preferably at least about 185,000 psi. The above-identified composite films of the invention are characterized by having oxygen and moisture vapor permeability substantially greater than those of conventional films based on polypropylene wherein the thickest layer of the film, i.e., the core layer, is polypropylene. The films of the invention are also clear and transparent so that a product packaged therein can be visually inspected for its quality without opening the package. At the same time, they have tensile properties that make them suitable for use as packaging films. In accordance with other embodiments of the invention, there are provided transparent oriented polyolefin films comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two α- monoolefins and 75 to 10% by weight of a high modulus species of polypropylene, said film being characterized by a thickness of about 40 to 80 gauge, preferably about 50 to 70 gauge, a tensile modulus of at least about 180,000 psi, an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. In/day/atmosphere and a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. In./day. Preferred films according to these embodiments of the invention are prepared with blends comprised of 40 to 90% heteropolymer and 60 to 10% high modulus polypropylene. The films of these embodiments of the invention have increased oxygen and moisture vapor permeability as compared with conventional, unmodified polypropylene films of similar thickness and are substantially clearer than are polypropylene films heretofore known to the art and possessing similar oxygen and moisture vapor transmission characteristics. In certain embodiments of this invention composite polyolefin films comprise a core layer carrying a functional skin layer on one or both of its surfaces, said core layer being comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two α-monoolefins and 75 to 10% by weight of a high modulus polypropylene. These latter composite films have excellent properties for use in other packaging applications where a film of high modulus is desired. Detailed Description of the Invention In the following discussion, gas permeability properties of the films of the invention will be discussed in terms of their OTR and MVTR. Carbon dioxide transmission rate is also an important parameter of these films affecting their utility as packaging materials for fresh produce items. Carbon dioxide transmission rates can be estimated using the generally accepted factor of a 4 to 1 weight ratio of carbon dioxide to oxygen transmitted per unit of time. When used in this disclosure, the term "α-monoolefin" (sometimes "alpha- monoolefins) refers to a linear unsaturated hydrocarbon monomer having one carbon- carbon double bond, which double bond is located at the end of the linear chain. The term is intended to include any such monomer having 6 carbon atoms or less, including ethylene and propylene. The term "heteropolymer" as used throughout this disclosure means an olefin polymer containing at least two -monoolefins. Typical of such heteropolymers are ethylene-propylene copolymers having about 4.5 to 6% by weight of ethylene, copolymers of ethylene containing about 5 to 25% by weight of other α-monoolefins of 4 to about 8 carbon atoms, butene-1 -propylene copolymers containing about 5 to 34% by weight of butene-1 and ethylene--propylene~butene-l terpolymers. Such heteropolymers can be converted to films and can be oriented within a temperature range comparable to the temperature range within which polypropylene is satisfactorily drawn to effect orientation. The term "self supporting" in reference to a film means that the film can support itself without the aid of a supporting layer to provide the necessary strength and body and it can readily be handled without being adhered to another substrate. Such a film may also be referred to as a "stand-alone" film. The term" transparent", when employed in connection with the films of this invention, is intended to indicate a film having a haze value less than about 5% as measured with a conventional haze measuring instrument such as the BYK Gardner Haze-Gard Plus.) References to a polypropylene or polypropylene species or composition having a "high modulus" means a polypropylene or a polypropylene composition, a film of which has a modulus greater than that of a conventional propylene homopolymer film of the same thickness drawn to the same draw ratio using the same drawing technique, i.e. bubble or tenter. For example, the modulus of a 70 gauge (0.7mil thickness) film of conventional polypropylene, drawn 7X by IX on simultaneous draw equipment is on the order of 400,000 psi. The modulus of a high modulus polypropylene of the same thickness, similarly drawn, is at least about 450,000 psi. Heteropolymers such as those described are well known in the polyolefin art and are widely used as heat seal layers on commercial polypropylene composite films. In those applications, the heteropolymers comprise very thin layers on the surface of a relatively thick polypropylene core. The layers thus employed are so thin that, unless they are adhered to a core layer, they cannot be wound into a roll, nor can they be used for any purpose that requires them to be handled except when they are adhered to a supporting substrate., i.e., they are not self supporting. As a general rule, when the heteropolymers are intended to serve as sealing layers, they are applied to the polypropylene core in layers of no more than about 0.1 mil in thickness, whereas the thicker polypropylene layer is usually about 0.70 to about 0.80 mils. The heteropolymers are inherently more permeable to oxygen and moisture than is polypropylene per se. For example, a one mil layer of a typical biaxially oriented polypropylene exhibits OTR, measured according to ASTM D3985-81, on the order of about 125 to 150 cc/100 sq. in./atmosphere/day and MVTR on the order of about 0.3 to 0.4 g/100 sq.in./day. A one mil layer of a copolymer of propylene containing 6% ethylene exhibits OTR on the order of about 300 to 350 cc/100 sq. in./atmosphere/day and MVTR on the order of about 0.7 to 1.0 g/100 sq. in./day. However, when used as sealing layers, these latter materials are in such thin layers that they have virtually no effect on the OTR and MVTR of a film to which they may be applied since the lower OTR and MVTR of the polypropylene core does not permit all the oxygen and moisture that can pass through the sealant layer to pass through the polypropylene layer. It has not heretofore been possible, however, to take advantage of the greater permeability properties of the heteropolymers in packaging of fresh produce items since the heteropolymers have not heretofore been shown to be amenable to the production of packaging films. These materials lack the higher degree of stereoregularity characteristic of polypropylene and thus they do not possess sufficient tensile strength and stiffness to form stand-alone films that can be employed as packaging films. In accordance with this invention, it has been found that, by combining the heteropolymer with a high modulus polypropylene species, a clear, self supporting film can be prepared that has stand-alone properties and a sufficiently high OTR and MVTR required for use in packaging fresh fruits, flowers and vegetables that require a continuous refreshing of the oxygen supply in the closed package. In certain embodiments of the invention, the high modulus polypropylene species is combined with the heteropolymer by layering a core of the heteropolymer on either or both of its surfaces with a skin of the high modulus polypropylene species. Such composite films are clear, self-supporting films having a sufficiently high OTR that they can be used for packaging fresh fruits, flowers and vegetables that require a continuous refreshing of the oxygen supply in the closed package. In accordance with other embodiments of this invention the high modulus polypropylene species is combined with the heteropolymer by blending the heteropolymer with the high modulus polypropylene species within specific blending ratios and thickness limitations to prepare films that have a sufficient tensile modulus to be self-supporting while exhibiting the OTR and MVTR required for use in packaging fresh produce items that continue to respire after they are harvested and packaged for the market. It has been found that films having the desired properties can be prepared from polyolefin blends consisting essentially of about 30 to 90% by weight of the heteropolymer and 70 to 10% by weight of the high modulus polypropylene based on the total weight of the blend. A preferred blend composition is one consisting essentially of about 40 to 90% by weight of the heteropolymer and about 60 to 10% by weight of the high modulus polypropylene. While the embodiments of this invention employing blends of the high modulus polypropylene and the heteropolymer preferably are not composite structures employing additional functional layers, as will be explained in greater detail later in this application such functional layers can be employed in accordance with the broadest aspects of the invention. As stated above, when the heteropolymers employed in this invention are applied as heat seal layers on a conventional polypropylene core, their greater OTR has virtually no effect on the OTR of the composite, i.e., the composite OTR is unchanged from that of a polypropylene core of the same thickness without the skins. This appears to be a rather straight-forward observation, readily explainable by the substantially greater thickness of the core and its substantially lower OTR, which does not permit all of the oxygen that can pass through the skin in a specified time period to continue on through the core in that same time period. It is known that OTR is, to a degree, dependent on the thickness of a polyolefin film and increases in almost a stiaight line relationship to the thickness as the thickness decreases. Thus, by decreasing the thickness of a polypropylene layer and increasing that of a heteropolymer layer, a point will be reached at which the OTR of the polypropylene layer will be greater than that of the heteropolymer layer. At this point, based on the observation of the OTR effect of heteropolymer skins on a polypropylene core, one would anticipate that the OTR of the composite would be that of the heteropolymer core. However, upon examining the results of the OTR testing of the films prepared in carrying out the work that led to certain embodiments of the invention disclosed in this application, it was found that, when a thin polypropylene layer (or layers) is (are) applied to a high OTR core, the OTR of the composite is not the same as would have been expected of the core layer without the polypropylene skin(s). The OTR of the composite is less than that of the core layer alone even though the polypropylene skin layers are so thin as to have an OTR greater than that of the core. This effect is explainable by the fact that, even though the OTR of the polypropylene skin layers is now greater than that of the heteropolymer core, the difference between the OTR values of the core and the skins is not nearly so great as that between the polypropylene core and the heteropolymer skins of conventional films. In all composite films according to this invention, the time required for the oxygen to pass through the skin layer becomes a factor with respect to the composite OTR of the film. Since the OTR of a heteropolymer skin on prior art films is so high, the time required for oxygen to pass through is so small that it has a negligible effect on the composite OTR. Since OTR is a rate, it can be represented by distance (i.e., film thickness) divided by time required for oxygen to pass through the film, i.e., OTR = thickness/time Accordingly, the time required for oxygen to pass through the film is equal to the thickness divided by the OTR, i.e., Time = thickness/OTR For a three layer film, the composite OTR is the composite film thickness divided by the sum of the times required for oxygen to pass through the individual layers, i.e., .. ΛTΌ thickness composite OTR = TJOTR, + T 2 /OTR 2 + T 3 /OTR 3 Where T j and T 3 represent the thicknesses of the skin layers and T 2 represents the thickness of the core layer of the composite while OTRj and OTR 3 represent the OTR values of the corresponding skin layers and OTR 2 represents the OTR value of the core layer. Since the OTR and the thickness of the individual layers can be easily measured, it is not necessary to know the absolute time required for the oxygen to pass through each layer. With the above knowledge, it is now possible to tailor films to have a predetermined OTR for packaging specific items of produce. If one has sufficient information concerning the respiration rate of a product, one can vary the layer thicknesses to approximate that respiration rate sufficiently accurately for successful commercial use. This is true for all composite films within the scope of this invention. Preferred heteropolymers for use as the core material in the films of the invention are the copolymers of ethylene and propylene containing about 4.5 to 6% by weight of ethylene, copolymers of ethylene containing about 5 to 25% by weight of other α- monoolefins of 4 to about 8 carbon atoms, copolymers of propylene and butene-1 containing about 5 to 34% butene-1 by weight and terpolymers of ethylene, propylene and butene-1. Particularly preferred are the 4.5 to 6% copolymers of ethylene and propylene and the copolymers of propylene and 5 to 34% butene-1. Composite Films Employing a Core Being a Heteropolymer of at least Two α- Monoolefins and at least One Skin Layer Being a High Modulus Polypropylene In order for composite films of this invention having a core layer being a heteropolymer of at least two α-monoolefins to have a stiffness sufficient to permit their use as packaging films, a high modulus polypropylene or polypropylene composition is employed in the skin layer. Suitable and preferred polypropylenes for use in the skin layer, which exhibit the required high modulus characteristics include the newly developed high crystallinity polypropylene. This material, available under several trade names, is defined as having an isotactic index of at least 93%, i.e., the polymer contains no more than about 7%, and preferably no more than about 2 to 6% xylene soluble materials. Typical high crystallinity polypropylene is further characterized by higher stiffness, greater surface hardness, lower heat deflection at high temperatures, lower heat shrinkage and better creep properties than conventional isotactic polypropylenes, which have isotactic index generally less than 93%. Typical high crystallinity polypropylenes that can be employed include ACCPRO 9117, ACCPRO 9119 and ACCPRO 9218 (all available from Amoco Polymers, Alpharetta, GA); and also Chisso HF 5010 and Chisso XF 2805 (all available from Chisso Chemical Co. Ltd., Tokyo, Japan. High modulus polypropylenes also include nucleated polypropylenes. These are conventional isotactic polypropylenes that have been nucleated to increase their crystallinity. Exemplary commercially available nucleated polypropylenes include Aristech FF035C (Aristech Chemical Co. Pittsburgh, PA). High modulus polypropylenes also include blends of conventional Ziegler-Natta isotactic polypropylene with low molecular weight hydrocarbon resins. Inclusion of about 5% or more of the low molecular weight hydrocarbon resin in a conventional polypropylene increases the modulus to the desired high modulus range. The upper limit of low molecular weight resin is about 25% by weight. The low molecular weight hydrocarbon resins are hydrogenated or unhydrogenated resins derived from olefin monomers, such as the resins derived from terpene monomers, coal tar fractions and petroleum feedstocks. In general, the low molecular weight resins are characterized by a molecular weight less than about 5000, a T g of about 50 to 100 C and a softening point less than about 140 C. Suitable resins include those prepared from terpene monomers (e.g., limonene, alpha and beta pinene, such as Piccolyte resins from Hercules Incorporated, Wilmington, DE, and Zonatac resins from Arizona Chemical Company, Panama City, FL.). Other low molecular weight resins are prepared from hydiocaibon monomers and mixtures thereof, such as C 5 monomers (e.g., piperylene, cyclopentene, cyclopentadiene, and isoprene), oligomerized C 5 monomers, particularly the thermally oligomerized C 5 monomers such as the hydrogenated thermally oligomerized cyclopentadiene resins sold under the trade name Escorez (for example Escorez 5300) by Exxon Chemical Co. of Baytown, TX. Others are prepared from C 9 monomers, particularly the monomers derived from C 9 petroleum fractions which are mixtures of aromatics, including styrene, methyl styrene, alpha methyl styrene, vinyl naphthalene, the indenes and methyl indenes and, additionally, pure aromatic monomers, including styrene, α-methyl-styrene and vinyltoluene. Examples of these resins include hydrogenated oc -methyl styrene—vinyl toluene resins sold under the trade name Regahez by Hercules Incorporated of Wihriington, DE. The hydrogenated C 9 and pure monomer resins are preferred. Particularly preferred are the hydrogenated cyclopentadiene resins and the hydrogenated aromatic resins derived from pure aromatic monomers, e.g., the hydrogenated α -methyl styrene— vinyltoluene copolymers. Another polypropylene having the high modulus required of the skin layers of the films of this invention can be prepared by blending conventional commercial isotactic polypropylene prepared via Ziegler-Natta catalysis with a polypropylene prepared by use of a metallocene catalyst. Such blends and films prepared therewith are described in copending and commonly assigned U.S. Patent Application number 09/055389, filed April 6, 1998. In the referenced application, it is shown that very significant increases in tensile modulus are realized when these polymer types are blended. Such modulus improvements are realized with blends containing 10 to 90% of the Ziegler-Natta polymer. The composite films of this invention described herein are generally of a total thickness from about 0.5 to about 2 mil. A high modulus polypropylene skin layer can be applied to one or both surfaces of the heteropolymer core. Preferably, it will be applied to both surfaces and, most preferably, the skin thickness will be the same on both sides. Although the results vary with different heteropolymer cores, the combination of increased OTR and satisfactory tensile modulus sought in the above-described composite films of the invention is generally realized when the polypropylene skin layer or layers constitute about 14 to 56% of the thickness of the composite structure. Preferably a skin layer will comprise about 30 to 56% of the thickness of the composite structure. It is surprising to find that a skin layer of high modulus polypropylene as thin as that employed in the above-described composite films of this invention can impart to the entire film the modulus required of a packaging film. When a skin layer is applied to both surfaces, the two layers should each be of a thickness such that its OTR is greater than that of the core layer. As suggested hereinabove, the above-described composite films of this invention have several properties that make them unique and desirable for the packaging of fresh produce products. First, they are of a stiffness required of packaging materials generally. Thus, they exhibit a tensile modulus of at least about 180,000 psi and most preferably greater than about 185,000 psi. While this is not as high as the modulus of conventional polypropylene films in which the polypropylene is the primary component, it is sufficient for many packaging applications. In addition, the above-described composite films exhibit an OTR greater than 200 cc/100 sq. in./day/atmosphere; a level at which the packaged product can continue to respire for a reasonable time after it has been harvested, thus assuring a reasonable shelf life for the product. Thirdly, the above- described composite fiims are found to have a moisture vapor transmission rate greater than that of conventional polypropylene films, whereby they permit the escape of moisture generated by the produce product's continued respiration after it is packaged. Finally, the above-described composite films of this invention retain a high degree of clarity and transparency, so that the packaged product can be inspected for its appearance by the potential purchaser The above-described multilayer composite films of the invention can be prepared by coextrusion, extrusion coating or lamination. The preferred method is coextrusion wherein the various layers are extruded simultaneously through a multi-layer die and immediately brought together in the molten state so that they are permanently bonded upon solidifying . . Film forming and drawing to effect orientation can be carried out by conventional techniques, i.e. the well known tubular (bubble) process or the equally well known tenter process can be employed. When the films are prepared by the bubble process, the draw is effected simultaneously and uniformly in the machine and cross directions to about 3X to 7X and preferably 5X to 7X. Using the tenter process, drawing is carried out sequentially to about 3X to 7X in the machine direction and to about 7X to 1 IX in the cross direction. As noted earlier, the heteropolymer core of the composite film of this invention can have the high modulus skin on one or both of its surfaces. In the embodiment of the invention wherein the high modulus polypropylene skin is present on only one surface of the heteropolymer core, the other surface can have no skin at all or it can have a different functional layer. Functional layers that can be employed as the other layer include such layers as, e.g, a heat seal layer. Such a layer will be of a material of lower melting point than the core so that when heat is applied to effect the seal, the orientation of the core layer will not be disturbed. A commonly used heat seal layer is comprised of a terpolymer of propylene, ethylene and butene-1, unless, of course, that is the core heteropolymer. Other polymers that can be employed as a heat seal layer include polyvinyl or polyvinylidene chloride. Another frequently used functional layer is a cold seal layer. In this layer, the polymer component will frequently be the same as that of the core layer, though, here again, it need not be the same. To prepare a cold seal film, a cold seal adhesive, such as, e.g., a rubber adhesive is applied to the surface of the oriented film. Another commonly used functional layer is a slip layer to facilitate handling of the film during later converting operations. Such a layer is comprised of a polymer containing a slip agent such as a high molecular weight fatty acid amide. A functional layer may also contain an antiblock additive to facilitate unwinding of the film after it has been wound at the teiminus of the film manufacturing process. These types of layers can be comprised of polypropylene if the layer is more oxygen permeable than the heteropolymer core and accordingly will not decrease the OTR of the total film structure. Preferably, such layers will be made of the same heteropolymer as is employed in the core layer. The above-described composite film embodiments of this invention are illustrated in the following Examples. EXAMPLE 1 Three-layer films having a core comprised of a 6% ethylene copolymer with propylene (Fina 9470 from Fina Chemical Co., Houston, TX) and two surface layers, each comprised of a nucleated isotactic polypropylene (Aristech FF 035 C from Aristech Chemical Co. Pittsburgh, PA) were prepared. At the same time, two control films comprised of a single layer of the 6% ethylene copolymer (Control 1) and the nucleated polypropylene (Control 2) were prepared. The films were extruded through a three- layer die onto a casting roll maintained between about 211 and 213 °F at a casting speed of about 3.1 feet/minute. All of the films were drawn 6X by 6X using a T.M. Long stretcher. The three layer film was drawn at 150°C, the copolymer film (Control 1) at 115°C and the nucleated polypropylene film (Control 2) at 150°C. Relevant physical properties of the films are presented in Table 1 below. Table 1 1 : As percentage of total film structure 2: psi by Instron Tensile Tester 3: cc/100 sq. in/atmosphere/day (ASTM D3985-81) 4: grams/100 sq. in./24 hrs. (ASTM D372) The significant increase in both the OTR and the MVTR in films having an acceptable tensile modulus are clearly seen from the data presented in Table 1. The increased stiffness as compared to that of the film comprised only of the heteropolymer is also clearly seen. The film denoted as Example 1, while it is marginal in stiffness for use for some packaging purposes, would serve effectively as a material for making the panel on a modified atmosphere package according to the Anderson patent referenced above. The low haze values indicate a high degree of transparency. EXAMPLE 2 The procedure substantially according to Example 1 was repeated with a copolymer of ethylene and propylene containing 4.5% ethylene (Fina 8573 from Fina Chemical Co.). The polypropylene was the same as was used in Example 1. Three-layer films having a core comprised of the 4.5% ethylene copolymer with propylene (Fina 8573 from Fina Chemical Co., Houston, TX) and two surface layers comprised of a nucleated isotactic polypropylene (Aristech FF035C) were prepared. Similarly, two control films comprised of a single layer of the nucleated polypropylene and the 4.5% ethylene copolymer, respectively, were prepared. The three-layer films and the polypropylene control were cast on a casting roll maintained between about 211 and 213 °F at a casting speed of about 3.1 feet/minute. The copolymer film was cast at 133 °F. All of the films were drawn 6X by 6X using a T.M. Long stretcher. The three layer films, designated 2a, 2b and 2c, were drawn at 125°C, 125°C and 135 °C, respectively. The nucleated polypropylene control film (Control 1) was drawn at 145 °C and the copolymer control (Control 2) at 125 °C. Relevant physical properties are presented in Table 2, below. TABLE 2 EXAMPLE 3 A third series of three layer films was prepared wherein the heteropolymer core layer was a copolymer of butene-1 and propylene containing 34% of the butene-1. The skin layers were of nucleated isotactic polypropylene. Two such films were prepared, of which one, designated 3a, had skin layers equal to 34.8% of the total film thickness. The other, designated 3b, had skin layers equal to 52.2% of total film thickness. Both films were cast at 211 °F and 3.1 feet/minute. Both films were oriented 6X by 6X, 3a at 140°C and 3b at 145 °C. No controls were run with these films since the copolymer alone does not draw sufficiently well to produce any meaningful data. Property data for these films are presented in Table 3. TABLE 3 Here again, it is seen that the films exhibit increased oxygen and moisture vapor permeability. At the same time, they exhibit sufficient stiffness to be useful as packaging materials. Films Employing Blends of Heteropolymer of at least Two α-Monoolefins and High Modulus polypropylene. Either with or without Skin Layer(s) In accordance with the embodiments of this invention employing blends of a heteropolymer of at least two α-monoolefins and a high modulus polypropylene, it has been found that a blend of the heteropolymer with a high modulus polypropylene within specific blending ratios and thickness limitations can be employed to prepare films that have a sufficient tensile modulus to be self-supporting while exhibiting the OTR and MVTR required for use in packaging fresh produce items that continue to respire after they are harvested and packaged for the market. It has been found that films having the desired properties can be prepared from polyolefin blends consisting essentially of about 30 to 90% by weight of the heteropolymer and 70 to 10% by weight of the high modulus polypropylene based on the total weight of the blend. A preferred blend composition is one consisting essentially of about 40 to 90% by weight of the heteropolymer and about 60 to 10% by weight of the high modulus polypropylene. In these embodiments of the invention a separate skin layer is not required; the high modulus polypropylene being combined with the heteropolymer by being blended therewith. The heteropolymers preferred for use in these embodiments of the invention are the same as those identified for use in the earlier-described embodiments of this invention. Films prepared with the blends just described, when cast and drawn to a typical packaging film thickness, i.e., 0.7 to 0.8 mil, exhibit several properties that make them unique and desirable for the packaging of fresh produce products. First, they are of a stiffness required of packaging materials generally, i.e. a tensile modulus of at least about 180,00 psi and most preferably at least about 185,000 psi and significantly higher as the high modulus polypropylene content is increased. In addition, such films exhibit an OTR greater than 200 cc/100 sq. in./day/atmosphere; a level at which the packaged product can continue to respire for a reasonable time after it has been harvested, thus assuring a reasonable shelf life for the product. Thirdly, the films are found to have a moisture vapor transmission rate (MVTR) greater than that of conventional polypropylene films, whereby they permit the escape of moisture generated by the produce product's continued respiration after it is packaged. Finally, the films retain a high degree of clarity and ttansparency, so that the packaged product can be inspected for its appearance by the potential purchaser. In order for the films to have a stiffness sufficient to permit their use as packaging films, a high modulus polypropylene or polypropylene composition, as described above in connection with the previously disclosed embodiments of this invention is employed in the blend. Film forming and drawing to effect biaxial orientation can be carried out by conventional techniques, i.e. the well known tubular (bubble) process or the equally well known tenter process can be employed. When the films are prepared by the bubble process, the draw is effected simultaneously and uniformly in the machine and cross directions to about 3X to 7X and preferably about 5X to 7X. Using the tenter process, drawing is carried out sequentially to about 3X to 7X in the machine direction and to about 7X to 1 IX in the cross direction. Films according to the embodiments of the invention employing a blend of the heteropolymer and the high modulus polypropylene, and which exhibit the high OTR and MVTR characteristics, can be either monolayer or composite films wherein the heteropolymer/polypropylene blend is a core layer having a functional skin layer on either or both of its surfaces. Skin layers must have OTR and MVTR greater than that of the core. In either the monolayer or composite embodiment, the films are generally of a thickness from about 0.5 to 1.0 mil and preferably from about 0.5 to 0.8 mil. Functional layers that can be employed as the skin layer include such layers as, e.g„ a heat seal layer. Such a layer will be of a material of lower softening point than the core so that when heat is applied to effect the seal, the orientation of the core layer will not be disturbed. A commonly used heat seal layer is a terpolymer of propylene, ethylene and butene-1, unless, of course, that is the core heteropolymer. Other polymers that can be employed as a heat seal layer include polyvinyl or polyvinylidene chloride. Another commonly used functional layer is a slip layer to facilitate handling of the film during later converting operations. Such a layer is comprised of a polymer containing a slip agent such as a high molecular weight fatty acid amide. A functional layer may also contain an antiblock additive to facilitate unwinding of the film after it has been wound at the terminus of the film manufacturing process. Preferably, such layers will be made of the same heteropolymer blend as is employed in the core layer. A slip layer can also be comprised of polypropylene since such a layer will be sufficiently thin as to be more oxygen permeable than the heteropolymer core. The films according to these embodiments of the invention, in either the monolayer or the composite embodiment thereof, can also be provided with a cold seal coating. Such a coating is typically a rubber adhesive applied to the polymer surface. The functional skin layer can also be a layer of polypropylene provided that the skin layer must be sufficiently thin that its OTR and its MVTR are greater than those of the blended heteropolymer/high modulus polypropylene core. A polypropylene skin layer can serve several functions when applied to one or both surfaces of the blended heteropolymer/high modulus polypropylene core. The use of polypropylene in slip and antiblock layers has already been mentioned. Polypropylene skins can also be employed to provide printable surfaces to the films of the invention by subjecting the skins to an oxidative medium according to known methods. A preferred oxidative medium is corona discharge. Another preferred oxidative technique is flame treatment. Another, but less preferred technique is the use of an oxidizing acid. One skilled in the film art can readily determine the degree of oxidative treatment required for a particular application. Another example of the use of polypropylene as a skin layer is in a case wherein the blended heteropolymer/high modulus polypropylene core is not sufficiently stiff or if its stiffness is marginal. The modulus of such a film can be increased into a more desirable range by application of a polypropylene skin. This layer can be of either a conventional polypropylene or of a high modulus polypropylene species so long as the modulus of the skin layer is greater than that of the blended heteropolymer/high modulus polypropylene core. The high modulus species is preferred for this purpose. The high modulus polypropylene species can be one of those identified hereinabove as being useful in the core blend or as a skin layer in the earlier-described embodiments of this invention. The composite films can be prepared by any conventional method, such as coextrusion, lamination or extrusion coating. All of these techniques are well known in the film art. Preparing blends of heteropolymer and polypropylene Pellets of the desired polypropylene and heteropolymer in the desired ratios were dry mixed and melt blended in a Leistritz twin screw extruder (American Leistritz Extruder Corporation, Somerville, NJ). The blended polymer mass was extruded into a water bath at room temperature to quench it and the quenched strands were chopped into pellets. EXAMPLE 4 A series of blends of a high modulus polypropylene (Aristech FF035C) and a copolymer of ethylene and propylene containing about 4.5% by weight ethylene (Fina 8573, Fina Chemical Co. Houston, TX) were prepared and pelletized as described in the previous paragraph. All were extruded at 170 RPM and at a temperature between 242 and 244 °C. The blend compositions thus prepared were cast at a temperature of about 246 °C onto a casting roll maintained at about 60°C. Extrusion was carried out at about 3.1 to 3.4 fpm. A cast film of about 23 mil thickness was recovered. The cast film, in each case was drawn about 6X by 6X on a T.M.Long Stretcher (T.M. Long Corporation, Somerville, NJ) and heat set 10%. Film orientation conditions are recorded in Table A. Table A The films thus prepared were characterized as to their modulus, measured on an Instron tensile tester, their OTR according to ASTM D-3985-81 and their haze value measured according to ASTM D-1003, using a Gardner Haze Meter. The results of this testing are set forth in Table 4, below. Table 4 * Unit is cc/100 sq. in/atmosphere/day It is readily seen from the data recorded in Table 4 that as the heteropolymer concentration is increased in the range of about 50 to 90 %, the OTR increases to a point at which the film becomes satisfactoiy for use in modified atmosphere packaging applications and its modulus remains within a useable range. EXAMPLE 5 Following the same procedures as were followed in Example 4, a series of films were prepared using a blend of the same high modulus polypropylene and an ethylene- propylene copolymer containing about 6% ethylene by weight (Fina Z9470). Orientation conditions for these films are recorded in Table B, below. Table B The properties of the films thus prepared are set forth in Table 5, below. Table 5 * Unit is cc/100 sq. In./atmosphere/day ** Unit is gms/100 sq. In./day Here again, it is readily seen that a heteropolymer content from about 25 to 75% provides a film having a satisfactoiy modulus and both OTR and MVTR suitable for use in modified atmosphere packaging. EXAMPLE 6 Another series of films were prepared as described in Example 4 employing, as the high modulus polypropylene species, Accpro 9117 from Amoco Polymers and the 6% ethylene copolymer employed in Example 5. The polymer and the heteropolymer were pelletized as previously described, using substantially the same processing conditions. The blend compositions were cast at about 3.1 to 3.2 fpm and a temperature of about 245 °C onto a casting roll maintained at about 60 °C. A cast film of about 23 mil thickness was recovered. The cast film, in each case, was drawn about 6X by 6X on the T.M. Long stretcher. Film orientation conditions are recorded in Table C. Table C The resulting films were characterized as to their modulus, their OTR and their Haze value with the results recorded in Table 6 below. Table 6 Here again, it is readily seen that a heteropolymer content in the blends from about 25 to 75% provides a film suitable for use in modified atmosphere packaging. EXAMPLE 7 In this example, two composite film structures were prepared consisting, in one case,(sample 7a), of a core layer comprised of a 50/50 blend of a high modulus polypropylene (Aristech FF035C) and a 4.5% ethylene copolymer with propylene (Fina 8573).having a skin layer of the same polypropylene on both of its surfaces. In the other case (Sample 7b), the skin layers were the same polypropylene on one surface and the 4.5% ethylene copolymer on the other surface. A control (Sample 7c), was also prepared having no surface layers. These films were all cast onto a 103 °C casting roll at about 3.1 fpm, then drawn 6X by 6X at about 130°C. Property data for these films are recorded in Table7, below. TABLE 7 It is readily seen from Example 7 that the addition of a polypropylene skin or skins is an effective way to decrease or otherwise control the oxygen permeability of the film. Sample No. 7a provides the maximum reduction of the OTR for a case in which the product requires a lower oxygen permeability than that afforded by the polypropylene/heteropolymer blend, but greater than that of polypropylene alone. In the case of Sample 7b, the OTR value is little changed from that of the polypropylene/heteropolymer blend, but this embodiment has the advantage of being readily heat sealable. EXAMPLE 8 Example 7 was repeated except that the heteropolymer employed in the core layer was a 6% ethylene copolymer with propylene (Fina Z9470). In this example, the films were cast at 3.3 fpm on a casting roll maintained at 99 °C. Films were drawn 6X by 6X. Properties measured for these films are recorded in Table 8. TABLE 8 Here again, the control over the OTR of a film that results from the presence of a polypropylene skin or skins is readily seen. The following examples demonstrate the preparation of thicker composite films according to the invention. These films, due to their thickness, exhibit lower OTR values than do those of Examples 4 tluough 8. This makes them less suitable for modified atmosphere packaging, but they are suitable for other polypropylene packaging film applications where higher OTR is not necessary or desired. EXAMPLE 9 A three layer composite film was cast on a pilot scale tenter film line fitted with a three layer composite die. This 45 mil cast film consisted of a core layer comprised of a 25/75 blend of nucleated high modulus polypropylene (Aristech FF035C from Aristech Chemical Co. Pittsburgh, PA) and an ethylene-propylene copolymer containing about 4.5% ethylene. On one of its surfaces it carried a skin layer of the same ethylene- propylene copolymer containing 0.1% of a high molecular weight aliphatic acid amide slip agent (Crodamide BR from Croda, Inc). On the other surface, it carried a skin layer of the same ethylene-propylene copolymer containing 0.2% of a cross-linked polydimethylsiloxane antiblocking agent (Tospearl T-120 from Sumitomo Chemical Corp.). This layer can also serve as a heat seal layer. The three layer cast film was drawn 6X in the machine direction at 110°C and 8.6X in the transverse direction at 152 °C using tenter equipment. A film of 100 gauge (1 mil) thickness was recovered. This film consisted of a 96 gauge core with a 2 gauge skin on each of its surfaces. Physical properties were measured for this film with the results recorded in Table 9, below. In addition to the measurements recorded in Table 9, this film was also tested for its heat sealability. The sealability was found to meet commercial requirements. Example 10 A second, thicker, composite film was prepared using the same polymers and copolymers as were used in Example 9. This cast film was drawn 6X in the machine direction at 110°C and 9.5X at 151 °C in the transverse direction. The core layer of the drawn film was 116 gauge and the skins were each 2 gauge. Physical properties measured for this film are recorded in Table 9, below. Example 11 Another 3 layer film was prepared in the same manner as the film prepared in Example 9. This film consisted of a core layer comprising a 50/50 blend of high modulus polypropylene (Aristech FF 035 C) and a copolymer of ethylene and propylene containing 6% ethylene by weight. The cast film was drawn 6X in the machine direction at 110°C and 9.4X in the transverse direction at 153 °C. The core layer of the drawn film was 96 gauge and the skin layers were each 2 gauge. Physical properties of this film are recorded in Table 9, below. Example 12 Another 3 layer film was prepared in the same manner as the film prepared in Example 9. This film was identical to that of Example 9 except that the core layer contained as the high modulus component Accpro 9117 (Amoco Polymers, Alpharetta, GA) The cast film was drawn 6X in the machine direction at 110 °C and 10.2X in the transverse direction at 151 °C. The core layer of the drawn film was 96 gauge and the skin layers were each 2 gauge. Physical properties measured for this film are recorded in Table 9, below. Table 9 The film of Example 9 has OTR suitable for use in many modified atmosphere packaging applications. The films of Examples 10, 11 and 12 would have very limited use in modified atmosphere packaging. This is due to their greater than normal thickness, which causes their OTR to be too low for this application. However, each of these films can be used in any other application where a polyolefin film of greater than normal thickness is desired.

Claims: CLATMS 1. Transparent composite, oriented polyolefin films comprised of a core layer and at least one skin layer, said core layer being a heteropolymer of at least two α- monoolefins, and having a thickness sufficient to give the composite structure standalone characteristics; said at least one skin layer being a high modulus polypropylene of a thickness such that its oxygen transmission rate (OTR) is higher than that of the core layer and the total composite structure having an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. In./day/atmosphere, a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. in./day, and a tensile modulus of at least 185,000 psi. 2. A film according to Claim 1 wherein the at least one high modulus polypropylene skin comprises about 14 to 56% of the thickness of the total composite film structure. 3. A film according to Claim 2 wherein the high modulus polypropylene is selected from polypropylene species having an isotastic index greater than about 93%, nucleated polypropylenes, blends of polypropylene with about 5 to 25% by weight of a low molecular weight hydrocarbon resin, and blends of Ziegler-Natta polypropylene with metallocene-catalyzed polypropylene. 4. A film according to Claim 3 wherein the heteropolymer core layer is a copolymer of ethylene and propylene containing about 4.5 to 6% by weight of ethylene. 5. A film according to Claim 3 wherein the heteropolymer is a propylene— butene-1 copolymer containing about 5 to 34% by weight of butene-1. 6. A film according to Claim 3 wherein the heteropolymer is an ethylene— propylene-butene-1 terpolymer. 7. A transparent composite, oriented polyolefin film comprised of a core layer having a skin layer on each of its surfaces, said core layer being a heteropolymer of at least two α-monoolefins, and having a thickness sufficient to give the composite structure stand-alone characteristics; said skin layers being a high modulus polypropylene of a thickness such that their oxygen transmission rate is higher than that of the core layer and the total composite structure having an oxygen transmission rate of at least about 200 cc/100 sq. In./day/atmosphere, a moisture vapor transmission rate of at least about 0.5 gram/sq. in./day, and a tensile modulus of at least 185,000 psi. 8. A film according to Claim 7 wherein the high modulus skin layers comprise about 14 to 56% of the thickness of the total composite film structure. 9. A film according to Claim 8 wherein the high modulus polypropylene is selected from polypropylene species having an isotastic index greater than about 93%, nucleated polypropylenes, blends of polypropylene with about 5 to 25% by weight of a low molecular weight hydrocarbon resin, and blends of Ziegler-Natta polypropylene with metallocene-catalyzed polypropylene. 10. A film according to Claim 9 wherein the heteropolymer core layer is a copolymer of ethylene and propylene containing about 4.5 to 6% by weight of ethylene. 11. A film according to Claim 9 wherein the heteropolymer is a propylene— butene-1 copolymer containing about 5 to 34% by weight of butene-1. 12. A film according to Claim 9 wherein the heteropolymer is an ethylene- propylene— butene-1 terpolymer. 13. A transparent biaxially oriented polyolefin film comprised of apolyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two oc-monoolefms and 75 to 10% by weight of a high modulus polypropylene.. 14. A film according to Claim 13 wherein the heteropolymer is selected from the group consisting of a copolymer of ethylene and propylene containing about 4.5 to 6% ethylene by weight, a copolymer of propylene and butene-1 containing about 5 to 34%butene-l by weight and a terpolymer of ethylene, propylene and butene-1. 15. A transparent oriented polyolefin film comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two alpha-monoolefins and 75 to 10% by weight of a high modulus polypropylene, said film being characterized by a thickness of about 40 to 80 gauge, a tensile modulus of at least about 180,000 psi, an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. In/day/atmosphere and a moisture vapor transmission rate (MVTR) of at least about 0.5 gram sq. in./day. 16. A film according to Claim 15 wherein the polyolefin blend is comprised of about 40 to 90% by weight of the heteropolymer and 60 to 10% by weight of the high modulus polypropylene. 17. A film according to Claim 15 wherein the heteropolymer is selected from the group consisting of a copolymer of ethylene and propylene containing about 4.5 to 6% ethylene by weight, a copolymer of propylene and butene-1 containing about 5 to 34% butene-1 by weight and a terpolymer of ethylene, propylene and butene-1. 18. A film according to Claim 15 wherein the heteropolymer is selected from the group consisting of a copolymer of ethylene and propylene containing about 4.5 to 6% ethylene by weight, a copolymer of propylene and butene-1 containing about 5 to 34% butene-1 by weight and a terpolymer of ethylene, propylene and butene-1. 19. A transparent biaxially oriented composite polyolefin film comprised of a core layer carrying a functional skin layer on one or both of its surfaces, said core layer being comprised of a polyolefin blend consisting essentially of about 25 to 90% by weight of a heteropolymer of at least two α-monoolefins and 75 to 10% by weight of a high modulus polypropylene. 20. A film according to Claim 19 characterized by a thickness of about 40 to 80 gauge, a tensile modulus of at least about 180,000 psi, an oxygen transmission rate (OTR) of at least about 200 cc/100 sq. In/day/atmosphere and a moisture vapor transmission rate (MVTR) of at least about 0.5 gram/sq. in./day.. 21. A film according to Claim 19 wherein the functional layer on at least one of its surfaces is a layer of a high modulus polypropylene. 22. A film according to Claim 20 wherein the functional layer on at least one of its surfaces is a layer of a high modulus polypropylene.

Inventors: DEMEUSE MARK T, DEMEUSE, MARK, T.
Applicants: APPLIED EXTRUSION TECHNOLOGIES, APPLIED EXTRUSION TECHNOLOGIES, INC.
Application year: 1999
Main classification: B32B27
IPC codes: B32B27, C08L23