MXPA96005474A - Oriented polypropylene films biacsialme - Google Patents

Abstract

Provided are a biaxially oriented polypropylene (PPOB) film with improved barrier properties, and a method for forming the film. The RTVH of a multilayer PPOB film is improved by mixing wax in the matrix layer resin, and providing a polyolefin cover layer on each side of the matrix layer. Significant further improvements are observed, when the film is heated to a temperature below the melting point of the matrix layer resin, below the distortion temperature of the film, and above the melting point of the wax during a length of time that extends, usually from at least five minutes to several hours, depending on the temperature

Description

BIOXIALLY ORIENTED POLYPROPYLENE FILMS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a polyolefin film structure with improved barrier properties. In one aspect, the invention relates to a method for forming a multilayer biaxially oriented polypropylene film, incorporating in a matrix layer, a minor amount of wax to improve the moisture barrier and mist properties thereof. Description of the Related Art Biaxially oriented polypropylene (PPOB) films are widely used for packaging, since they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Good resistance to moisture transmission, as measured by the "wet steam transmission regime" ("RTVH"), is very convenient for packing food products that contain salt or sugar or ingredients that are inherently hygroscopic. Conversely, a packaged item with a specific, intended moisture content will be more able to maintain the required moisture and will not dry out if packaged with a barrier film with high humidity. An essential purpose of the PPOB barrier film is to extend the shelf life of the packaging.

Since the barrier properties increase with the thickness of the film, it can be expected that a thicker film made, either by extrusion and direct orientation of the film, or by lamination of two or more films, has an improved RTVH ( that is, reduced). Although these thicker films remain transparent, they may suffer cloudiness and brightness. The small improvements are achieved at a significant cost, since the costs of the film increase proportionally with the thickness. An important method to reduce the RTVH in PPOB packaging films is to coat the film with a thin layer of metal, v. gr. , aluminum, which can result in improvements of four to ten times in RTVH values. However, said metallized film then becomes opaque, and the contents of the package can not be seen. Resistance to moisture and especially to oxygen transmission can also be improved by coating a PPOB film with polyvinylidine chloride ("CPVD"), or less commonly by co-extruding a layer of CPVD or its copolymers with polypropylene, followed by for orientation of the film. Although such films are useful, they have several disadvantages. First, an adhesive or seat layer is often needed, often it is necessary to prevent delamination of the layers of these two incompatible materials. Second, relatively thin coatings of CPVD provide a reduction of RTVH of only a factor of two or better than three. Finally, the chloride in CPVD, inhibits the recycling of the films. Packaging films that incorporate layers of polyvinyl alcohol, ("APV") or ethylene vinyl alcohol copolymers ("EOHV") can provide excellent oxygen barrier properties, but are not suitable for the high content barrier. humidity . The APV or EOHV layers must be walled with polyolefin layers to protect the APV or EOHV from absorbing moisture and therefore, losing the oxygen barrier. As with CPVD films, this film also requires adhesive layers or seat layers to prevent delamination. Still another approach, as taught in the U. U.A. Patent. No. 4,921, 749, to Bossaert et al., Cedants of "Exxon Chemical Patents Inc." , has been added to a base layer of polypropylene, a low molecular weight modifier such as hydrogenated petroleum resin in an amount of 3 to 30% by weight. The film is then biaxially oriented and is such to achieve a 40% reduction in RTVH. However, this technique is subject to the evolution of smoke and formation of the plate on the equipment during production, and it is relatively expensive at the concentrations of the hydrocarbon queen. The most recent work in the Patent of E. U.A. No. 5, 155, 160, to Yeh et al., Shows that the barrier properties of polyolefin films can be reduced by a factor of four or more, if a partially incompatible wax is added, generally in an amount between about two and two percent by weight, for polyolefin. When the materials taught by Bossaert et al., '749 and by Yeh et al.,' 160, are added to the polypropylene film, particularly at the upper percentages taught herein, shrinkage of the film becomes an important point. . In most packaging applications, good dimensional stability of the film is desired, to avoid distortion of the package. These additives often migrate to the surface of the film where they can be transferred to "form a plate" on the packaging equipment or the articles themselves. In addition, the properties of the film surface, such as the coefficient of friction (slip), stiffness of the film, heat seal characteristics, and printability, may be adversely affected. COMPETITION OF THE I NVENTION An object of the present invention is to produce a biaxially oriented polypropylene film, which has good processing characteristics in combination with improved moisture barrier properties. According to the invention, a thermoplastic film is made, forming a mixture of a polypropylene resin and an incompatible wax, extruding and casting the mixture to form a matrix layer of a film, orienting the matrix layer in directions biaxial, and providing on each side of the matrix layer, a polyolefin cover layer to provide a multilayer film. The matrix layer can be oriented before or after the layers of the cover are provided. Preferably, the matrix layer is oriented in a first direction before the layers of the cover, the resulting multilayer film, are provided, then being oriented in a second direction transverse to the first direction. The wax can be incorporated in a smaller amount than previously shown to be effective, thus minimizing the problems associated with the formation of wax plate on the packaging equipment. Significant further improvements can be achieved when the biaxially oriented film is heated to a temperature below the melting point of the resin of the matrix layer, below the distortion temperature of the film, and above the melting point. Initial waxing for a length of time that generally ranges from, at least, five minutes to several hours or more, depending on the temperature. The additional objects and advantages of the invention may be apparent from a review of the following detailed description, taken in conjunction with the appended claims. DESCRIPTION OF PREFERRED MODALITIES The invention provides a method for forming a multilayer sheet or film, having at least three layers, as described below, and the products thereof. Matrix Layer Resin The inner layer, or one of the inner layers, if there is more than one, is called the "matrix" layer and comprises polypropylene. The term "polypropylene" as used herein, is a generic reference to a semicrystalline polymer with a majority of the polymerized propylene, specifically including, isotactic homopolymers of propylene with ethylene and butene, and mixtures thereof. Preferred resins are those selected from propylene homopolymers and propylene copolymers, with less than three weight percent co-monomer, such as ethylene or butene. Flow melt regimes of 1 to 15 dg / min, as measured in accordance with ASTM D1238-90b, Condition 230 / 2.16 (formerly Condition F), and preferably from 1.5 to 6 dg / min, are suitable for the blown film or film. Examples of suitable commercial polypropylenes include resins are polypropylenes "Quantum 7300", "Amoco 6314", "Solvay 2108", and "Fina 3275". There are no restrictions on the thickness of the matrix layer, other than the constraints of oriented polypropylene tensile processes, typically from about 12 microns to about 50 microns. Wax The polypropylene matrix layer contains an incompatible wax, and preferably a small amount thereof. By "incompatible" it is meant that wax has only limited solubility with polypropylene. Both natural and synthetic waxes can be employed, including petroleum waxes such as paraffin wax (predominantly straight-chain saturated hydrocarbons) and macrocrystalline wax (predominantly saturated cyclic hydrocarbons, with isoparaffins), vegetable waxes (e.g., carnauba), mineral waxes, and animal waxes (e.g., spermaceti). Paraffins and polyethylene waxes are preferred. Crystalline morphology waxes appear to provide a lower moisture barrier in this invention. Since it is not intended to be bound by theory, it is thought that a preferred wax such as highly crystalline linear polyethylene remains incompatible with the crystalline regions of polypropylene, and will concentrate in the amorphous regions of the resin. In addition, it is thought that the wax migrates through the polyolefin cover layers to the surface thereof in a controlled manner during film processing. It is believed that the wax on the surface of the film forms a continuous, highly crystalline layer, dramatically reducing the permeation of moisture. The wax should have a molecular weight of 300 to 1000 and preferably 500 to 800. It is expected that the melting points of said waxes are between 50 ° C and 140 ° C, depending on the length of the chain. An example of a polyethylene type wax is the POLIWAX brand wax, available from Petrolite Corporation, which is a mixture of synthetic polyethylenes of various molecular weights with a specified average molecular weight. The amount of wax in the matrix layer, preferably ranges from 0.25 percent to 15 percent of the total weight of the resin-wax mixture. Higher amounts are not practical given the difficulty in mixing large amounts of wax with polypropylene, and it is also inhibited to avoid problems with shrinkage of the film. It has been found that substantial improvements in RTVH can be achieved with minimal shrinkage, with wax amounts below five percent, preferably between 0.25 and three percent, more preferably, less than two percent, and even more preferably between 0.5. and two percent. Mixing of Resin v Wax The mixing of the wax and the resin of the matrix layer can be achieved by conventional methods; The objective is to produce a homogeneous mixture. The two components can be fed separately into two streams in the feed throat of an extruder. Alternatively, they can be premixed in a mixer (e.g., a Henschel mixer) and can be fed by weight to an extruder. Due to differences in volume density and melting behavior very different from the two ingredients, it is especially preferred to mix them by melting to produce a wax concentrate in a polyolefin carrier, which is then mixed by itself with the resin of the layer of matrix in the extruder feed tank. A concentrate of about 15 weight percent wax in a carrier resin (e.g., polypropylene "PETROTHENE PP7300" from "Quantum Chemical Company") was found to work well. Preferred mixing devices are twin screw extruders, kneaders, or similar intensive mixers, especially those equipped with underwater die face pelletizers. Cover Layers A polyolefin cover layer is provided on each side of the matrix layer. This layer may be the same as, but is preferably different from the polypropylene of the matrix layer. The two cover layers can be of the same or different material and thickness. The cover layers may be but do not need it, as thick as the matrix layer, and the thicknesses of the cover layers of two microns or less are sufficient and preferred. The cover layers are preferably immediately adjacent to the matrix layer, but can also be separated by one or more intermediate layers from another resin or an adhesive. Additional layers, for example, a seal layer, may also be added to the exterior of the formed film. The polyolefin of the cover layer is selected from the homopolymers, copolymers, terpolymers of propylene and ethylene, or mixtures thereof. Preferred polyolefins are selected from the group consisting of propylene homopolymers, ethylene homopolymers, ethylene and propylene copolymers, copolymers of propylene with butene, copolymers of ethylene and alpha-olefins of ten or fewer carbon atoms, copolymers of ethylene and a unsaturated carboxylic acid, copolymers of ethylene and vinyl acetate, terpolymers of propylene, ethylene, and butene, and ionomers of ethylene and methacrylic acid. Especially ethylene-propylene copolymers are randomly preferred, containing ten percent or less ethylene by weight, copolymers of propylene with butene, and terpolymers of propylene, ethylene, and butene. Commercial sources of said polyolefins are well known in the art. The cover layers initially do not contain added wax, although the wax migrates from the matrix layer in and through the polymer of the cover layer during the processing of the film. Any of the film layers in the structure may also include minor amounts of conventional additives such as antioxidants, pigments, slip agents, antiblocks, fillers, stabilizers, and the like. In some films, slip agents or friction coefficient ("COF") agents may comprise materials, which increase the "sliding ability" of the film surface, and reduce the coefficient of friction of the film, such that processing problems such as film breakage can be avoided. Examples of these are fatty acid amides, arucamides, oelamides, and silicones. Minor amounts (e.g., less than about 0.5 weight percent, preferably less than 0.25 weight percent) of said additives in the cover layer are not expected to destroy the effect of enhanced RTVH. Film The manufacture of biaxially oriented, multi-layered films can be accomplished by several different processes known to those in the art. In general, a multi-layered, biaxially oriented film is formed according to the invention by forming a mixture of a polypropylene resin and wax as described above, extruding and casting the mixture to form a matrix layer, orienting the matrix layer in biaxial directions, and providing on each side of the matrix layer with a polyolefin cover layer, to provide a multilayer film, whereby loss of wax from the film is avoided. The matrix layer can be oriented in one direction before the cover layers are added, or alternatively, the matrix layer and the cover layers can be oriented biaxially together. The biaxial orientation can be carried out sequentially or simultaneously. In one embodiment of the invention, the cover layers are provided to the matrix layer by coextrusion therewith. In this case, the multi-layer film (i.e., the matrix layers and the cover layers), are biaxially oriented together.

In another embodiment, the cover layers are added to the matrix layer by internal extraction coating or lamination, as described in U.S. Pat. No. 5,156,904, to Rice et al., The disclosure of which is incorporated herein by reference. In this method, the matrix layer is formed by extruding and casting the resin and wax mixture, orienting the matrix layer in a first direction, providing a layer of polyolefin coating on each side of the matrix layer oriented to provide a film of multiple layers, and orienting the multilayer film in a second transverse (and preferably perpendicular) direction to the first direction. In practice, the matrix layer (or a coextruded sheet of the matrix layer and the cover layers) can be cast on a roller maintained at a temperature in the range of, e.g., 10 ° C to 100 °. C, reheated on rolls heated to a temperature (e.g., 100 ° C to 145 ° C) high enough to melt the wax in the matrix layer (e.g., 88 ° C for wax having a weight molecular weight of 500) still below the melting point of the propylene polymer of the matrix layer or the coextruded cover layers (if present), and then oriented in a machine direction. After the subsequent addition of the cover layers (if none are present, or as additional cover layers), the resulting film is reheated to a temperature preferably higher than the melting point of the cover layers (e.g. 135 ° C) and above the melting point of the wax but somewhat below the melting point of the polypropylene of the matrix layer (e.g., 150 ° C to 165 ° C), and the film is oriented in a second transverse direction (and preferably perpendicular) to the direction of the machine. During the second reheating step, the wax migrates through the cover layers to the respective surfaces thereof, but avoids ejecting the wax from the film during orientation, which could be counterproductive to the desired improvement in the film. barrier properties. The biaxial orientation is an important aspect of the invention since the orientation results in an improvement in RTVH, compared to, for example, a cast film without orientation. Heat Treatment After the formation and orientation of the PPOB film, it can optionally be subjected to the final heat treatment, as in an oven or "hot room". It is thought that the heat treatment causes the wax to migrate further from the matrix layer to the adjacent cover layers and the surface of the film, especially with the wax having an average molecular weight on the scale of 500 to 1000, particularly from 800 to 1000. The temperature of the treatment should be below the melting point of the polypropylene and below that temperature at which distortion occurs in the film, typically below 150 ° C. The heat treatment is preferably carried out below a temperature which can cause excessive shrinkage of the film, or approximately 110 ° C, and must be carried out above the initial melting point of the wax, usually about 50. ° C for preferred waxes. The heat treatment should be at least five minutes to several hours or more in length, the time depending on the temperature. The RTVH improvement regime is initially fast, with additional improvement possible by extended treatment for one or more days. Increases in temperature generally decrease the time required. The chosen time is preferably sufficient to allow migration of the wax through the cover layers. The invention provides commercially effective, convenient means for improving the properties of the moisture barrier and other physical properties of polypropylene films. For example, the addition of the wax to the film of the invention, performs a beneficial reduction of oxygen transmission rate. The inventive process avoids the loss of wax originally present in the matrix layer by providing the cover layers that control the rate of wax migration during orientation. The following non-limiting examples illustrate the practice of benefits of the invention. EXAMPLES Experimental Conditions The films were formed of polypropylene containing various amounts of wax. Polypropylene was an isotactic homopolymer with a melt flow rate of 1.8 dg / in as a monolayer in the A-H Comparisons and as the matrix layer of the multilayer structures of the examples. The wax "POLYWAX 500" having a density of 0.93 g / cm 3, a melting point of 88 ° C, and a molecular weight of 500, was added as a weight percent of the resin / wax mixture in the examples in where the wax was present. The (outer) cover layers were formed from a random ethylene-propylene copolymer of 5 MFR, having an ethylene content of 6.5% by weight, without containing wax. During manufacturing, monolayers or multiple layers (including, in some cases, cover layers), were passed through a slot die 220 ° C, to form a thick sheet. The sheet was heated to 120 ° C by passing it through hot rollers, and then approximately 500 percent was removed in the machine direction, to form a monoaxially oriented film (In some cases, the cover layers were applied). The film was further heated to 160 ° C in a laying furnace and stretched approximately 900 percent in the transverse direction. The film was cooled, trimmed, and wound on a roll. The steam transmission humidity (RTVH) regime was measured in a "MOCON" Model 600 instrument. The data were reported in units of g / 645 cm2 / 24 hours at 37.7 ° C and 90% relative humidity ( ASTM F 1249). To compensate for variations in the size of the film, the recorded RTVH data were normalized to a thickness of 25.4 microns, assuming that RTVH is inversely proportional to the thickness. Comparisons of A with H The comparisons in Table I show the characteristics of RTVH of the PPOB films of a layer containing wax, having a caliber measured between 15.5 and 18 microns. No cover layer or seal was present in any of these examples. Films that contain various amounts of wax "POLYWAX 500", showed no improvement in the RTVH, on average, compared to the "A" control that contains zero percent wax. If Comparison H is omitted perhaps as an anomaly, the remaining films showed an improvement averaging only six percent. The brightness of the control was 94, while the brightness of the others varied from 85 to 93, as measured by the Glossgard I I Brightness Meter of 45 ° from Pacific Scientific using ASTM D2457. The control nebulosity was 0.4, compared with a scale of 0.7 to 2.3 for the others. The film nebulosity measurements followed ASTM D 1003 using a "Gardner Hazeguard" instrument. PICTURE L MOVIE THE ONE OF A LAYER Examples 1 to 8 These examples show the properties of RTVH of the back-layer films with cover layers on each side of a polypropylene matrix layer, which incorporate various amounts of "POLYWAX 500" wax. The film had the structure A / B * / A (where "*" denotes the presence of wax). The cover layers "A" did not contain wax. The films had a caliber measured between 17.28 and 20.82 microns. The films were prepared under the same process conditions as the one layer films of the AH Comparisons, but the cover layers were applied after the orientation of the matrix layer in the machine direction, followed by the transverse orientation of the film. the resulting film of three layers.

As shown in Table II, the average RTVH of films containing wax was within 77% compared to the control that did not contain wax. This change from RTVH from 0.24 to 0.059 (normalized to 25.4 microns) represents more than a quadruple improvement compared to the three-layer films without wax, and even a greater improvement over a PPOB film of an uncoated layer (refer to Table I ), with or without wax.

Examples 9 to 12 Examples 9 to 12 illustrate the effect of the wax on several layers of films, with the results shown in Table III: The films of Examples 9-11 were co-extruded films, while the film of Example 12 incorporated a monoaxially oriented, coextruded, three-layer matrix with cover layers added thereto prior to orientation in the transverse direction. Example 9 had structure B / B / A without wax in any of the layers. The overall structure was 20.32 microns thick, with the cover layer approximately 18 microns thick and the cover layers, each representing approximately one meter thick. Example 10 had a structure of B * / B / A, where the cover layer B * contained 2.5 percent wax "POLYWAX 500". Example 1 1 had a structure of B * / B * / A with each of the layers B * having wax "POLYWAX 500" of 2.5 weight percent. Example 12 had a symmetric film of five layers of A / C / B * / C / A of total gauge of 0.78, in which the two C layers adjacent to the matrix were propylene homopolymers of about 1 miera of thickness, without wax. The reduction effect of RTVH, still existed but was diminished. TABLE I I I VARIATION IS OF STRUCTURE Examples 13 to 16 Examples 13 to 16 (not tabulated), illustrate effective wax concentrations at reduced RTVH. The PPOB films of three layers of total caliber varying from 20.0 to 20.57 microns, were elaborated by means of lamination of internal extraction, the layers of matrix having wax concentrations "POLYWAX 500" of zero 0.5, 1 .0, and 2.0 percent in weight. The RTVH (normalized to 25.4 microns) was 0.30, 0.12, 0.07, and 0.06, respectively. Therefore, an improvement in RTVH was observed at a wax concentration of 0.5% by weight. Examples 17 to 19 These examples demonstrate improvement in RTVH when a propylene copolymer is replaced by the propylene homopolymer of the matrix layer. These three-layer films were made by internal extraction lamination. Symmetric three-layer films, were elaborated to total thicknesses of 20.32 to 20.82 microns, of which, the layers of cover were of 1.27 microns each one. The matrix layer contained an ethylene-propylene copolymer of 3 MFR (PETROTH ENO PP 7300 available from Quantum Chemical Company), which contained 3% ethylene. Example 17 did not contain wax, while Examples 18 and 19 had 1.5% and 3% "POLYWAX 500" wax, respectively, in the matrix layer. The RTVH data (un tabulated) normalized to 25.4 microns, showed the control (# 17) to 0.45, while the Example s 18 and 19 showed 0.1 1 and 0.074, respectively. Examples 20 to 27 These examples show the effect of using waxes with higher molecular weights under conventional PPOB laying processing conditions. The three layer films of these examples were made by internal extraction lamination.

The OPP films of three layers, symmetric of the structure A / B * / A, were made with thicknesses of 20.32 microns, containing 2.0 percent of linear polyethylene waxes, selected. Petrolite "POLYWAX" products designated 500, 650, 800, 1000, 2000, and 3000 were used, which had average lengths of carbon chains of 36, 46, 61, 71, 143, and 214, with melting points of about 88, 100, 108, 14, 125, and 127 ° C, respectively. Only the wax film "POLYWAX 500" showed reduction in RTVH, at 0.20. The others had RTVH equivalent to the control, at approximately 0.37. Examples 28 to 47 These examples show the further improvement in RTVH after the heat treatment for three layer films made by internal extraction lamination. For the three-layer films in Examples 28 to 47, the columns in Table IV show the weight percent of the wax in the matrix layer, the average molecular weight of the wax (POLYWAX 500), the temperature of the treatment with heat, RTVH data (normalized to 25.4 microns) obtained before treatment, RTVH measured after two days at the set temperature, and the percentage improvement in the RTVH measurements. Example 43 showed no improvement after two days, but improved by 4% for an RTVH of 0.348 after eight days. No initial RTVH data was available for Examples 45 and 46.

The above detailed description is given solely for clarity of understanding, and modifications within the scope of the invention should not be construed as unnecessary limitations thereof, since they will be obvious to those skilled in the art.

Claims (31)

1. REVIVAL NAME IS 1. A method for making a multilayer thermoplastic film having improved barrier properties, comprising the steps of: (a) forming a mixture of a polypropylene resin and an incompatible wax; (b) extruding and casting the mixture to form a matrix layer of a film; (c) orienting the matrix layer in biaxial directions; and, (d) providing on each side of the matrix layer, a polyolefin cover layer, to provide a multilayer film, whereby loss of wax from said film is prevented.
2. 2. The method of claim 1, wherein the crystalline wax has an average molecular weight of 300 to 1000 and is preferably selected from the group consisting of paraffins and polyethylene waxes.
3. 3. The method of claim 1, wherein the mixture is formed with 0.25 to 15 weight percent wax, preferably with 0.25 to 3 weight percent wax.
4. 4. The method of claim 3, wherein the mixture is formed with wax of at least two percent by weight, preferably with wax of at least 0.
5. 5 percent by weight. The method of claim 3, wherein the wax has an average molecular weight of 500 to 800.
6. 6. The method of claim 1, further comprising the step of heating the multilayer film to a temperature above the initial boiling point of said wax, below the melting point of said resin, and below the distortion temperature. of said film, for at least five minutes, the wax preferably has an average molecular weight between 500 and 1000.
7. 7. The method of claim 6, wherein the film is heated from 50 ° C to 150 ° C in said step of heating, preferably for at least one hour in said heating step.
8. The method of claim 6, wherein the film is heated for at least 24 hours in said heating step.
9. 9. The method of claim 1, wherein the matrix layer is oriented after the cover layers are provided.
10. The method of claim 1, wherein the cover layers are provided by co-extrusion with the matrix layer and the thickness of each cover layer is less than two microns. eleven .
11. The method of claim 1, wherein the resin of the matrix layer is selected from the group consisting of isotactic propylene homopolymers, copolymers of propylene with ethylene or butene, terpolymers of propylene with ethylene and butene, and mixtures thereof.
12. 12. The method of claim 1, wherein the matrix layer resin is an isotactic propiieno homopoiimer.
13. The method of claim 1, wherein the cover layers are formed of a material selected from the group consisting of random ethylene-propylene copolymers with up to ten weight percent ethylene, copolymers of propylene with butene, and terpolymers of propylene, ethylene, and butene.
14. A method for forming a multilayer thermoplastic film, having improved barrier properties, comprising the steps of: (a) forming a mixture of a polypropylene resin and an incompatible wax; (b) extruding and casting the mixture to form a matrix layer of a film; (c) orienting the matrix layer in a first direction; (d) providing on each side of the oriented matrix layer, a polyolefin cover layer, to provide a multilayer film; and • (e) orienting the film of step (d) in a second direction transverse to the first direction, whereby loss of wax from said film is avoided.
15. The method of claim 14, wherein the wax is crystalline, has an average weight of 300 to 1000 and is selected from the group consisting of paraffins and polyethylene waxes.
16. 16. The method of claim 14, wherein the mixture is formed with 0.25 to 15 weight percent waxes, preferably with waxes of 0.25 to three percent by weight.
17. 17. The method of claim 16, wherein the mixture is formed with waxes of less than two percent by weight.
18. 18. The method of claim 17, wherein the mixture is formed with waxes of at least 0.5 percent by weight.
19. The method of claim 16, wherein the wax has an average molecular weight of 500 to 800.
20. The method of claim 14, further comprising the step of heating the product from step (e) to a temperature of above the initial melting point of said wax, below the melting point of said resin, and below the distortion temperature of said film of step (d) for at least five minutes, the wax has an average molecular weight between 500 and 1000.
21. 21. The method of claim 20, wherein the product of step (e) is heated from 50 ° C to 150 ° C in said heating step, or by one hour in said heating step .
22. 22. The method of claim 20, wherein the product of step (e) is heated for at least 24 hours in said heating step.
23. 23. The method of claim 14, wherein the thickness of each cover layer is less than two microns.
24. 24. The method of claim 14, wherein the resin of the matrix layer is selected from the group consisting of isotactic propylene homopolymers, copolymers of propylene with ethylene or butene, terpolymers of propylene with ethylene and butene, and mixtures thereof.
25. 25. The method of claim 24, wherein the matrix layer resin is an isotactic propylene homopolymer.
26. 26. The method of claim 24, wherein the cover layers are formed of a material selected from the group consisting of random ethylene-propylene copolymers, propylene with butene, and terpolymers of propylene, ethylene, and butene.
27. 27. A thermoplastic film having improved barrier properties formed by the method comprising the steps of: (a) forming a mixture of a polypropylene resin and an incompatible wax; (b) extruding and casting the mixture to form a matrix layer of a film; (c) orienting the matrix layer in biaxial directions; and, (d) providing on each side of the matrix layer, a polyolefin cover layer, to provide a multilayer film, whereby loss of wax from said film is prevented.
28. 28. The film of claim 27, wherein the wax is crystalline and has an average molecular weight of 300 to 1000.
29. 29. The film of claim 27, in the lime the multilayer film is formed by orienting the matrix layer in a first direction, providing a polyolefin cover layer on each side of the oriented matrix layer, and orienting the resulting film in a second direction transverse to the first direction.
30. 30. The film of claim 27 or 29, wherein the mixture is formed with less than two weight percent wax. The film of claim 29, wherein the polypropylene resin is selected from propylene homopoiomers, isotactic copolymers of propylene with ethylene or butene, terpolymers of propylene with ethylene and butene, and mixtures thereof.