CONTROLLED DENSITY THERMOPLASTIC MULTILAYER FILMS

Abstract

Biaxially oriented multilayer films comprise a stack (AB)n of n AB layer units, where n is greater than or equal to 32. Each AB layer unit has individual layers comprising a first layer A and a second layer B superimposed on the first layer A. Biaxially oriented multilayer films include gaps between layers within a portion of the individual layers in the stack. The gaps between layers have aspect ratios of width-to-height from 20:1 to 500:1. The first layers of the layer units have a first composition, and the second layers of the layer units have a second composition. The first composition includes a first thermoplastic and a first polymer immiscible with the first thermoplastic. The second composition includes a second thermoplastic, which may be the same as or different from the first thermoplastic.

Description

CONTROLLED DENSITY THERMOPLASTIC MULTILAYER FILMS Technical field of the invention Cross-reference to related applications This application claims priority over US Application Serial No. 62 / 882,821, filed on August 5, 2019, the full description of which is incorporated herein by reference. Background of the invention The embodiments of the present invention generally relate to biaxially oriented multilayer films and more specifically to biaxially oriented multilayer films exhibiting lower densities and improved transparency. Thermoplastic film materials are well-known in the field. These materials are commonly used in the construction of flexible packaging, solar control window films, substrates for electronics manufacturing, and a wide variety of other valuable and important applications. Manufacturers have been seeking to produce substantial quantities of these plastic materials for commercialization due to the performance and cost advantages achieved with the technology. The polymers and resins used to manufacture such film materials can be obtained from many commercial sources. For example, they can be produced by converting petroleum-based sources, such as polyester, nylon, and polycarbonate materials; by converting natural gas-based sources, such as polyethylene, polypropylene, butadienes, and other materials; or through natural processes, such as for carbohydrate polymers like cellulose derivatives, starch derivatives, polylactic acid, and polyethylene furonate. Furthermore, combinations of material sources are also used to produce thermoplastic resinous materials as a way to balance cost and supply considerations. In addition to the type of material used in the production of thermoplastic films, the processing technology is fundamental to producing films with different desired properties. The properties of thermoplastic film materials can be modified by altering the molding and stretching processes. These alterations are common in biaxial orientation lines, blown film lines, and extrusion molding and sheeting lines. In the case of thermoplastic materials such as polypropylene or polyethylene, film materials with poor crystallinity and low density can be produced under low-extrusion and low-stretch molding conditions. These materials are easily bonded to other materials through heat.However, high stretch ratios significantly increase the crystallinity of materials, resulting in increased thermal resistance, barrier properties, and solvent resistance. It is well known in the field to vary processing temperatures, stretch ratios, heat setting conditions, process speeds, and other related machinery processes to achieve the desired end-use properties. Density is a well-described attribute for polymer materials and blends. Cavitation or blending with miscible materials of lower apparent density can be used to reduce the density of thermoplastic film materials. In the case of highly oriented films, cavitation can reduce the film density. Density reduction is sought for many reasons, including using less material to form an equivalent thickness, potentially reducing costs. Furthermore, the formation of micrometric voids, or cavitation, can produce highly opaque materials. Using less material is also a desirable goal for lightweight packaging structures, offering potential environmental benefits. In the production of high-transparency, low-density thermoplastic film materials, the only practical solution currently available is to rely on resins with inherently low density. For example, PET films are frequently replaced with biaxially oriented nylon (BON) films. A nylon resin with a lower intrinsic density than PET will produce materials with better performance or a higher surface area per unit mass at an equivalent thickness. However, other attributes of PET film materials, such as barrier properties and thermal resistance, may have to be sacrificed to utilize the lower-density BON materials. Although transparent film materials and low-density film materials exist, there remains a need for film materials that have reduced density without sacrificing transparency. Brief description of the invention The embodiments of this invention are directed to biaxially oriented multilayer films. The biaxially oriented multilayer films include a stack (A—B) of n A—B layer units, where n is greater than or equal to 32. Each A—B layer unit has individual layers comprising a first layer A and a second layer B superimposed on the first layer A. The biaxially oriented multilayer films include gaps between layers within a portion of the individual layers in the stack. The gaps between layers have width-to-height aspect ratios of 20:1 to 500:1. The first layers of the layer units have a first composition, and the second layers of the layer units have a second composition. The first composition includes a first thermoplastic and a first polymer immiscible with the first thermoplastic. The second composition includes a second thermoplastic, which may be the same as or different from the first thermoplastic. Brief description of the drawings Figure 1 is an illustrative embodiment of a stack according to one or more embodiments of the invention. Figure 2 is an illustrative embodiment of a unit according to one or more embodiments of this invention. Figure 3 is a cross-sectional SEM micrograph of a multilayer film according to an embodiment of this invention. Figure 4 is a cross-sectional SEM micrograph of a multilayer film from the figure 3. Figure 5 is a schematic representation of the gaps between layers present in multilayer films according to the embodiments of this invention. Figure 6 is an SEM micrograph of a multilayer film prepared as a comparative example for multilayer films according to the embodiments of this invention. Detailed description of the invention The embodiments of this invention include biaxially oriented multilayer films. The biaxially oriented multilayer films include a stack (A—B) of n A—B layer units, where n is greater than or equal to 32. Each A—B layer unit has individual layers comprising a first layer A and a second layer B superimposed on the first layer A. The biaxially oriented multilayer films include gaps between layers within a portion of the individual layers in the stack. The gaps between layers have width-to-height aspect ratios of 20:1 to 500:1. The first layers of the layer units have a first composition, which includes a first thermoplastic and a first polymer immiscible with the first thermoplastic. The second layers of the layer units have a second composition, which comprises a second thermoplastic. The second thermoplastic is the same as or different from the first thermoplastic. In several embodiments, biaxially oriented multilayer films include a stack (A—B) of n A—B layer units, where n is greater than or equal to 32. Each A—B layer unit has individual layers comprising a first layer A and a second layer B superimposed on the first layer A. Biaxially oriented multilayer films include gaps between layers within a portion of the individual layers in the stack. The gaps between layers have width-to-height aspect ratios of 20:1 to 500:1. The first layers of the layer units have a first composition, and the second layers of the layer units have a second composition. The first composition includes a thermoplastic primer and a gap-forming agent. The gap-forming agents create gaps in the thermoplastic primer. It should be understood that there are several gap-forming agents suitable for creating gaps in the thermoplastic primer.The second composition includes a second thermoplastic that may be the same as or different from the first thermoplastic. The term "comprising" is used in the present invention and in the claims as an open-ended term that does not exclude other unspecified elements. Unless otherwise specified, the terms "includes" and "having" are equivalent to "comprising." As used in the specification and in the appended claims, singular nouns preceded by an article a, an or the, the, should be regarded as including the plural forms, unless the context clearly states otherwise. In the various configurations, the multilayer film includes a stack. Figure 1 provides an illustration of the stack. Stack 100 includes multiple units 120. Figure 2 provides an illustration of unit 120. Each unit 120 includes a first layer 121 and a second layer 122. In one or more embodiments, the first layer includes a first thermoplastic. In several embodiments, the second layer includes a second thermoplastic. In one or more embodiments, the first and second thermoplastics may be selected from a homopolymer, copolymer, terpolymer of a C2a α-olefin, a polyamide, a polyacetate, a polyester, a polycarbonate, a polystyrene, a polyvinyl chloride, a polyvinyl alcohol, a polyvinyl acetate, a polyacrylic acid, and a polyethylene terephthalate, or a mixture of any of the foregoing materials. In some embodiments, the first thermoplastic, the second thermoplastic, or both, is a polypropylene-based homopolymer or copolymer. In other embodiments, the first thermoplastic, the second thermoplastic, or both, is a polyethylene-based homopolymer or copolymer (e.g., an LLDPE). In several embodiments, the first thermoplastic may include polyethylene terephthalate (PET). In some embodiments, the second thermoplastic may include PET, and in one or more embodiments, both the first and second thermoplastics may include PET. In some embodiments, the first thermoplastic includes polybutylene terephthalate (PBT). In some embodiments, the second thermoplastic includes PBT, and in one or more embodiments, both the first and second thermoplastics include PBT. In some configurations, the first thermoplastic includes PET, and the second thermoplastic includes PBT. In embodiments of biaxially oriented multilayer film, the first composition also includes a first polymer immiscible with the first thermoplastic. In some embodiments, the second composition also includes a second polymer immiscible with the second thermoplastic, wherein the second polymer may be the same as or different from the first polymer. The first polymer may be selected from various polymers having a lower density than the first thermoplastic. The second polymer, when present, may be selected from polymers having a lower density than the second thermoplastic. In one or more embodiments of the biaxially oriented multilayer film, the first polymer may include one or more cycloolefin polymers. The cycloolefin polymers may be homopolymers or copolymers containing polymerized cycloolefin units and, optionally, acyclic olefin comonomers. Suitable cycloolefin polymers include 0.1% to 100% by weight of cycloolefin units, or 10% to 99% by weight of cycloolefin units, or 50% to 95% by weight of cycloolefin units, based in each case on the total mass of the cycloolefin polymer. In some embodiments, the cycloolefin polymers may have a structure according to any of Formulas I, II, III, IV, V, or VI: In formulas (I), (II), (III), (IV), (V), and (VI), R1, R2, R3, R4, R5, R6, R7, and R8 are either identical or different and independently denote either a hydrogen atom or a C1-C30 hydrocarbyl. The term “hydrocarbyl” refers to a hydrocarbon radical. Optionally, any two or more of the radicals R1 through R8 from any of formulas I-VI may be cyclically linked to each other. Examples of C1-C30 hydrocarbyl radicals include linear or branched Ci-Cs alkyl, Ce-Os aryl, C7-C20 alkyleneryl, and C3-C20 cycloalkyl. The term alkyl refers to either a branched or an unbranched alkane radical. The term aryl refers to an aromatic ring structure that has a substituent on the carbon atom of the aromatic ring. The term alkylenearyl refers to an alkyl-aromatic structure that has a substituent on a carbon of the alkyl group. Cycloolefin polymers may contain from 0% to 45% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of at least one monocyclic olefin of formula Vil: HC=CH \ / ' (Vil) (CHA In formula (Vil), n is an integer from 3 to 10. Cycloolefin polymers may also contain from 0% to 99% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of an acyclic olefin of formula VIII: \ / / = \ (VIII) In formula (VIII), R9, R10, R11, and R12 are independently selected from a hydrogen atom (-H), from C1-C10 hydrocarbyl, Ci-Cs alkyl, or Ce-Cu aryl. Similarly, cycloolefin polymers obtained by ring-opening polymerization of at least one of the monomers of formulas I to VI and subsequent hydrogenation are suitable in principle. Cycloolefin homopolymers are synthesized from a monomer of any of the formulas (I) to (VI). The fraction of polymerized units of acyclic olefins of formula VIII is up to 99% by weight, 5% to 80% by weight, or 10% to 60% by weight, based on the total weight of the respective cycloolefin copolymer. In several embodiments, cycloolefin copolymers (COCs) include polymerized units of polycyclic olefins of formula (VIII), wherein one of R9, R10, R11, and R12 is a cyclohydrocarbyl with a norbornene parent structure of formula (I). In some embodiments, the COC includes polymerized units of norbornene or tetracyclododecene, formula (III), wherein R1, R2, R3, R4, R5, R6, and R7 are hydrogen. In other embodiments, the first polymer includes COCs containing polymerized units of acyclic olefins, such as ethylene. In some embodiments, the first polymer includes norbornene / ethylene and tetracyclododecene / ethylene copolymers containing 5% to 80% by weight of ethylene units, or 10% to 60% by weight of ethylene units, based on the copolymer weight. Co-octamalized compounds (COCs) can include compounds prepared by heterogeneous or homogeneous catalysis with organometallic compounds, according to any known synthetic route. Appropriate catalyst systems are based on cocatalysts of titanium compounds and / or vanadium compounds along with aluminum organyls. Cycloolefin copolymers can be incorporated into the biaxially oriented multilayer film as the starting material in the form of granules or a granulated concentrate, as is well known in the field. The polyester granules, and any other components for this purpose, are premixed, and the premix is ​​then fed into the extruder. Inside the extruder, the components undergo further mixing and are heated to the processing temperature. For the process of preparing the invention, it is convenient that the extrusion temperature be above the glass transition temperature (Tg) of the cycloolefin copolymer (COC), generally at least 30°C, from 40°C to 230°C, or from 50°C to 200°C above the glass transition temperature of the cycloolefin copolymer (COC). In several embodiments, the first composition comprises from 0.5% to 10% by weight of the first polymer, based on the total weight of the first composition. In one or more embodiments, the first composition comprises from 0.5% to 5% by weight of the first polymer, based on the total weight of the first composition. In some forms, the first composition includes 80% to 95% by weight of PET as the first thermoplastic and 5% to 20% of COC as the first polymer, based on the total weight of the first composition, and the second composition includes 80% to 100% of PET as the second thermoplastic and 0% to 20% of COC as the second polymer, based on the total weight of the second composition. In embodiments of the biaxially oriented multilayer film, the first composition comprises 80% to 95% by weight of PET as the first thermoplastic and 20% to 5% of PBT as the first polymer, based on the total weight of the first composition, and the second composition comprises 80% to 100% by weight of PET as the second thermoplastic and 0% to 20% of PBT as the second polymer, based on the total weight of the second composition. In some embodiments, the first composition comprises 85% to 95% by weight of PET as the first thermoplastic and 15% to 5% of PBT as the first polymer or 90% to 95% by weight of PET as the first thermoplastic and 10% to 5% of PBT as the first polymer, based on the total weight of the first composition. In several forms, the second composition includes 90% to 100% by weight of PET as the second thermoplastic and 0% to 10% of PBT as the second polymer, based on the total weight of the second composition.In one or more configurations, the biaxially oriented multilayer film has a volumetric turbidity of less than or equal to 20%, as measured by ASTM D 1003. The term volumetric turbidity refers to the percentage of incident incandescent light that is transmitted through the film, which is deflected or scattered at more than 2.5 degrees from the direction of incoming light. The term light transmission, on the other hand, refers to the percentage of incident light that passes through a film. The volumetric turbidity of a biaxially oriented multilayer film can be measured with a spectrophotometer or haze meter using the ASTM D 1003 method. In one or more configurations, the volumetric turbidity of the biaxially oriented multilayer film is less than or equal to 20%.In other modalities, turbidity is less than or equal to 18%, less than or equal to 15%, less than or equal to 12%, less than or equal to 10%, less than or equal to 8%, less than or equal to 5%, less than or equal to 38%, or less than or equal to 2% or / o. Biaxially oriented multilayer films include interlayer voids within a portion of the individual layers in the stack. These interlayer voids comprise the first polymer, which is immiscible with the first thermoplastic. Because the first polymer is immiscible with the first thermoplastic, it forms voids within the thermoplastic layer where no thermoplastic polymer is present. The voids formed by the first polymer immiscible with the first thermoplastic become interlayer voids once the desired number of A-B units are formed. An interlayer void is a void confined to a single layer, such as the first layer A, the second layer B, or a single A-B unit. In one or more forms of biaxially oriented multilayer film, the gaps between layers have heights greater than 0 and less than 0.4 pm. In some forms, the gaps between layers have heights greater than 0 and less than 0.3 pm. When the height of the gap between layers is less than the wavelength range of visible light, the biaxially oriented multilayer film is considered to have volumetric turbidity. Furthermore, the higher the visible light transmittance, the greater the height of the gaps will be compared to the wavelength range of visible light. In one or more forms of biaxially oriented multilayer film, the gaps between layers have width-to-height aspect ratios of 20:1 to 500:1. In some forms, the gaps between layers have width-to-height aspect ratios of 100:1 to 500:1. In some forms, the gaps between layers have width-to-height aspect ratios of 250:1 to 500:1. Figure 6 is an SEM micrograph of a multilayer film prepared as a comparative example. The comparative example in Figure 6 is a single-layer film comprising an immiscible polymer and a thermoplastic. The immiscible polymer forms voids in the thermoplastic. The voids in the comparative example have a height greater than 0.4 pm. In one or more embodiments, the stack includes multiple (A—B)n units, where n describes the number of layers of A—B units. In some embodiments, the subscript n of (A—B)ns is greater than or equal to 50. In some embodiments, n is from 55 to 600, from 100 to 550, or from 200 to 550. For example, when the subscript n of (A—B)ns is 50, and there is only a first layer A and a second layer B in the A—B unit, then the stack will comprise 100 individual layers, where there is a repeating pattern ABAB. In one or more embodiments, the A—B unit comprises three layers, so that the stack has a repeating pattern of ACB, AAB, or ABB, where C is a third thermoplastic. In one or more configurations, at 260 °C, the first composition has a melt viscosity ηi and the second composition has a second melt viscosity η2, where |ηi - η2| / ηi is greater than 0 to less than or equal to 0.2. In some configurations, |η1 - η2| / ηi is greater than 0 to 0.10, 0.01 to 0.1, 0.01 to 0.05, or 0.001 to 0.02. The extensive variation between viscosity layers is well known to impose flow instabilities in multilayer systems. Flow instabilities can result in a poor thickness profile in the transverse or machine direction in a typical film-forming process. Viscosity profiles must be managed appropriately to maintain the flat film characteristic within the multilayer process. Without intending to be limited to any particular theory, it is believed that the two polymers or polymer blends should have relatively small differences in melt viscosity. For example, with a 5% relative viscosity difference under typical extrusion melting conditions and with a shear rate of approximately 10 to 100 s⁻¹, they have the capacity to create a biaxially oriented, multilayer film with low density and low turbidity. This relatively small difference in viscosity restricts the layering of the blend when the polymer or polymer blends are in a molten state (so that the temperature of the polymer is higher than the melting temperature of the polymer or polymer blend). This relatively small difference in viscosity allows the two polymers or polymer blends to separate and stack to form a stack of A—B units.The splitting and stacking process can occur multiple times to form 16, 32, 64, 128, 256, 512, or 1024 individual layers of repeating A-B layers. When the desired number of layers is reached, the melt is forced through an extrusion die and molded onto a water-cooled laminator. It is then wound onto a rotating core and passed on for further processing, such as for a thick plastic film. Materials with melt viscosity differences greater than 5% at similar melt temperatures may exhibit rheological stresses between the individual submicron layers. These stresses can deform and relax void structures, causing agglomerates to form, which increases the volumetric turbidity of the film. There are several methods for producing multilayer films. One example of a method for forming a multilayer film involves combining two separately molten streams to form a double-layer system. The double-layer system is passed through feed blocks that divide the double-layer system perpendicular to a stacking plane. The divided double-layer system is then guided vertically and passed through a die that flattens and stacks the layers on top of each other. This effectively doubles the number of alternating layers. Multiple feed blocks can be added in series to double the number of layers each time one is passed through. Examples of this technique can be found in U.S. Patent Nos. 3,557,265; 3,565,985; 3,759,647; 5,380,479; 3,328,003; 3,565,985; 3,479,425; and in the pre-granted USA publication 2012 / 0288696. All numerical ranges in this invention, for example, from 1 to 10, should be interpreted as describing all individual values ​​within the ranges, as well as all subranges within the ranges, as separate categories. For example, the range from 1 to 10 describes as separate categories each individual value from 1 to 10, including 1 and 10, as well as the subranges including, but not limited to, 5 to 10, 2 to 8, 5 to 8, and 3 to 9. Examples It should be understood that Examples 1 through 7 are provided to illustrate the embodiments described in this invention and are not intended to limit the scope of this invention or the appended claims. Examples 1 through 3 are prepared using Method A, wherein each example has a different number of layers. In Method A, the films produced had an A / B / A / B layer, where layer A was PET and layer B was PET / COC. Examples 4 through 6 are prepared using Method B, wherein each example has a different number of A / B / A / B layers. In Method B, the films were 7PCQ Ln / 77n7 / q / YIAI were prepared, wherein layer A was PET / COC and layer B was PET / COC. Example 7 was prepared using Method C, wherein the film produced by Method C included layers of PET and PET / PBT. Method A - Preparation of Examples 1 to 3 A multilayer thermoplastic film was prepared by extruding a stack of A-B units of thermoplastic materials. The thermoplastic materials included a layer of PET and another layer of PET blended with 10 wt% COC. After molding, the film was oriented in a Karo IV offline film extender. This equipment is commercially available from Brukner Incorporated. The selected polyester resin had an intrinsic viscosity (IV) of 0.69 and was purchased commercially from Polyquest Industries as Grade PCQ69. The selected COC was produced by Topas as Grade 5013-F. Prior to extrusion, the PET resins were dried to remove moisture in order to maintain melt strength during extrusion. Although not strictly necessary, polyethylene coating layers were extruded on both sides of the double stack to improve melt flow during testing. The molded multilayer film was then stretched in both directions along its major axis using one of two methods: offline and online stretching and orientation. In the case of smaller-scale offline stretching, this method is convenient for testing and evaluating new film formulations. In this case, the film frames can be oriented simultaneously or sequentially by a system of mechanically linked clamps inside an oven (such as in the Bruckner Karo IV test machine). With in-line drawing, the molded film was first heated and drawn out in the direction of the machine by a pair of rollers. Generally, the film was held under pressure, or contracted, between the rollers. The second set of rollers rotated two to four times faster than the first, so the molded film was drawn out into a longer, thinner sheet moving at a higher speed. The film then moved into a static oven, where pairs of clips mounted on moving chains continuously gripped the film edges and were forcibly separated after a short heating time. As the chains were forced apart, the film's width increased by a factor of three to four, and the film thinned by the same amount.This two-axis stretching dramatically increases the strength of the film, typically through stress-induced crystallization of the polymeric materials in the film. The layer film of Example 1 was prepared by followed after 9 square mixing elements. The layer film of Example 2 was prepared by followed after 8 square mixing elements. The layer film of Example 3 was prepared by coextrusion as described in the Method Method Method TO, TO, A, followed by 7 square mixing elements. Method B - Preparation of Examples 4 to 6 When preparing the multilayer films of Examples 4 through 6, the multilayer thermoplastic films were prepared by extruding a double stack of thermoplastic materials consisting of one layer of PET blended with 8% COC and another layer of PET blended with 10% COC by weight. After molding, the film was oriented on a Karo IV offline film extender. The selected polyester resin had an intrinsic viscosity (IV) of 0.69 and was purchased from Polyquest Industries as Grade PCQ69. The selected COC is produced by Topas as Grade 5013-F. Prior to extrusion, the PET resins were dried to remove moisture in order to maintain melt strength during extrusion. Although not strictly necessary, polyethylene cap layers were placed on both sides of the double stack to improve melt flow during testing. were extruded in The layer film of Example 4 was prepared by followed after 9 square mixing elements. The layer film of Example 5 was prepared by followed after 8 square mixing elements. The layer film of Example 6 was prepared by followed after 7 square mixing elements. Method C - Preparation of Example 7 The multilayer films of Example 7 were prepared by coextrusion as described in the double stack. Method Method Method B, B, B, thermoplastic materials consisting of one layer of PET and another layer of PET blended with 10 wt% polybutylene terephthalate (PBT). The selected PBT had a melt viscosity similar to PET and was supplied by DuPont. After molding, the film was oriented using an offline Karo IV film extender. The selected polyester resin had an intrinsic viscosity (IV) of 0.69 and was purchased from Polyquest Industries as Grade PCQ69. Prior to extrusion, the PET resins were dried to remove moisture in order to maintain melt strength during extrusion. Although not strictly necessary, polyethylene cap layers were extruded on both sides of the double stack to improve melt flow during testing. The layer film of Example 7 was prepared by coextrusion as described in Method C, followed afterward by 8 square mixing elements. Comparative example C1 The multilayer film in Comparative Example C1 was prepared by extruding a double stack of thermoplastic materials consisting of one layer of PET and another layer of PET blended with 10 wt% PBT and calcium carbonate. The PBT / calcium carbonate masterbatch is commercially available from many sources. After molding, the film was oriented on a Karo IV offline film extender. The selected polyester resin had an intrinsic viscosity (IV) of 0.69 and was purchased from Polyquest Industries as Grade PCQ69. Prior to extrusion, the PET resins were dried to remove moisture in order to maintain melt strength during extrusion. Although not strictly necessary, polyethylene cap layers were extruded on both sides of the double stack to improve melt flow in the test. The film in Comparative Example C1 was prepared by coextrusion followed by 8 square mixing elements. Comparative example C2 The thermoplastic multilayer film was prepared by extruding a double stack of thermoplastic materials consisting of one layer of PET and another layer of PET blended with 10 wt% polypropylene. A series of square mixers followed by a molten stream and the resinous materials were molded in a frozen laminator. After molding, the film was oriented using an offline Karo IV film extender. The selected polyester resin had an intrinsic viscosity (IV) of 0.69 and was purchased from Polyquest Industries as Grade PCQ69. The selected PP (Pinnacle 1703) was used. Before extrusion, the PET resins were dried to remove moisture in order to maintain melt strength during extrusion. Although not strictly necessary, polyethylene coating layers were extruded on both sides of the double stack to help draw the polymer melt out of the die. The layer film in Comparative Example C2 was prepared by coextrusion as described in E, followed by seven square mixing elements. In this sample, very high volumetric turbidity was observed. SEM analysis of the layer structure shows that the PP resin crosses the interfacial boundaries of the multilayer stack 254, failing to produce the circular voids necessary for high transparency at low density. Comparative example C3 A monolayer film containing 92% PET with a VI of 0.62 was blended with 8 wt% COC. Extrusion molding and tempering of the material were oriented in the MD and TD directions using conventional methods, as described in US 9580798 and US 10131122. High turbulence and high opacity were observed along the reduced film density. Comparative example C4 A three-layer film was prepared with virgin PET content using conventional methods described in US 9580798 and in USA Application 1013 / 001122. The properties of Inventive Examples 1 to 7 and Comparative Examples C1 to C4 are listed in Table 1. Table 1: Multilayer Film Data Example Number of Layers Layer A Material Layer B Material Volumetric Turbidity Density (g / m²) Example 1 1024 PET PET / COC 10.6 1.20 Example 2 512 PET PET / COC 6.17 1.06 Example 3 254 PET PET / COC 8.02 1.06 Example 4 1024 PET / COC PET / COC 16.5 1.14 Example 5 512 PET / COC PET / COC 6.67 1.25 Example 6 254 PET / COC PET / COC 10.3 1.18 Example 7 512 PET PBT / PET 1.63 1.36 Comparison C1 512 PET PBTVA / PET 55.3 1.17 Comparison C2 254 PET PP / PET 92.2 0.86 Comparison C3 1 PET / COC N / A 86 1.23 Comparative C4 3 PET PET 4 1.41 The multilayer films in Examples 1 through 7 had at least 512 layers and a volumetric turbidity of less than 20. The multilayer films in Examples 1 through 7 had densities lower than 1.40 g / cm². Although the multilayer film in Comparative Example C1 had a low density of 1.17 g / cm², its volumetric turbidity was 55.3, and the film was opaque. Similarly, although the multilayer film in Comparative Example C2 had a low density of 0.86 g / cm², it also had a high volumetric turbidity of 92.2, higher than the volumetric turbidity of the multilayer film in Comparative Example C1. The film in Comparative C3 was a single-layer film. Although the density was low, measuring 1.23 g / cm2, the volumetric turbidity of the single-layer film was high, measuring 86. In contrast, the multi-layer film in Comparative C4, a three-layer film, had good optical quality with a volumetric turbidity of 4, but the film density was higher than 1.40 g / cm2, specifically 1.41 g / cm2. MEASUREMENT METHODS Turbidity measurements can be performed using the standard ASTM D1003 or equivalent techniques. Density can be measured using any standard method, such as ASTM D792, or with a helium or nitrogen pycnometer.

Claims

1. A biaxially oriented multilayer film comprising: a stack (A—B)n of n A—B layer units, wherein n is greater than or equal to 32, each A—B unit having individual layers comprising a first layer A and a second layer B overlaid on layer A; and gaps between layers within a portion of the individual layers in the stack, the individual gaps having width-to-height aspect ratios of 20:1 to 500:1; wherein: the first layers of the layer units have a first composition, the first composition comprising a first thermoplastic and a first polymer immiscible with the first thermoplastic; and the second layers of the layer units have a second composition, the second composition comprising a second thermoplastic; and the second thermoplastic is the same as or different from the first thermoplastic.

2. The biaxially oriented multilayer film according to claim 1, wherein at 260 °C, the first composition has a first melt viscosity ηι and the second composition has a second melt viscosity η2, wherein |ηι - η2| / ηι is from 1 to 1.

1.

3. The biaxially oriented multilayer film according to claim 1 or claim 2, wherein the first composition comprises more than approximately 0 to 10% by weight of the first polymer, based on the total weight of the first composition.

4. The biaxially oriented multilayer film according to the preceding claims, comprising at least 250 layer units.

5. The biaxially oriented multilayer film according to the preceding claims, comprising at least 500 layer units.

6. The biaxially oriented multilayer film according to the preceding claims, comprising at least 1000 layer units.

7. The biaxially oriented multilayer film according to any any any any any of the preceding claims, wherein the first thermoplastic is polyethylene terephthalate (PET).

8. The biaxially oriented multilayer film according to any of the preceding claims, wherein the first thermoplastic and the second thermoplastic are PET.

9. The biaxially oriented multilayer film according to any of the preceding claims, wherein the first polymer comprises a cyclo-olefinic copolymer (COC) or polybutylene terephthalate (PBT).

10. The biaxially oriented multilayer film according to any of the preceding claims, wherein the thermoplastic film has a volumetric turbidity of less than 20, as measured by ASTM D1003.

11. The biaxially oriented multilayer film according to any of the preceding claims, wherein the second composition also comprises a second polymer immiscible with the second thermoplastic.

12. The biaxially oriented multilayer film according to claim 11, wherein: the first composition comprises 80% to 95% by weight of PET and 5% to 20% of COC, based on the total weight of the first composition; and the second composition comprises 80% to 100% by weight of PET and 0% to 20% of COC, based on the total weight of the second composition.

13. The biaxially oriented multilayer film according to claim 11, wherein: the first composition comprises 80% to 95% by weight of PET and 5% to 20% of PBT, based on the total weight of the first composition; and the second composition comprises 80% to 100% by weight of PET and 0% to 20% of PBT, based on the total weight of the second composition.

14. The biaxially oriented multilayer film of any of the preceding claims, wherein the gaps between layers have heights less than 500 nm.