Polylactide resin composition and biaxial stretching method for polylactide resin composition

Abstract

The polylactide resin composition comprises a mixture of 40 to 95 weight percent of a crystalline polylactide resin and 5 to 60 weight percent of a specific amorphous polylactide resin. The polylactide resin composition is useful for making oriented films, particularly biaxially oriented films. The composition can be stretched on commercially available biaxial stretching equipment due to its ability to be stretched to higher draw ratios than sheets made from crystalline polylactide resin alone.

Description

[Technical Field]

[0001] The present invention relates to a polylactide resin composition and a method for making an oriented film from the polylactide resin composition. [Background technology]

[0002] Oriented polymer films are produced in large quantities for use as flexible packaging materials, especially when properties such as optical clarity, moisture resistance, and high tensile strength are required. Biaxially oriented films are used in packaging many foods, beverages, pharmaceuticals, and personal care products, as well as many other applications. Such films are made by forming a sheet, which is then stretched to produce the final film. Summary of the Invention [Problem to be solved by the invention]

[0003] While polylactide (also known as polylactic acid or PLA) can be used in these applications, its penetration into these markets is limited by the inability of PLA sheets to be stretched to high stretch ratios, especially when biaxially stretched. Most commonly available industrial-scale biaxial stretching equipment operates at stretch ratios of 3-5 in the machine direction and 4-10 in the transverse direction. PLA sheets cannot be stretched to these ratios without fracture, and therefore most commonly available biaxial stretching equipment cannot be used to process PLA into biaxially oriented film without modification. Oriented film manufacturers have already invested in equipment that must operate at these high stretch ratios and is not easily adapted to operate at lower ratios, significantly limiting PLA's penetration into this market. Similar to methods for producing PLA biaxially oriented film at higher stretch ratios, polylactide compositions that can be stretched to higher stretch ratios while maintaining other useful characteristics, such as the ability to orient while producing films with acceptable mechanical properties, would be desirable. [Means for solving the problem]

[0004] The present invention provides a polylactide resin composition, (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 93:7 or ≤ 7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 20:80 or ≤ 80:20. and a polylactide resin composition comprising a melt or solution blend of: [Brief explanation of the drawings]

[0005] [Figure 1] 1 is an atomic force microscope image of the polylactide resin composition of the present invention (Example 5). [Figure 2] 3 is an atomic force microscope image of the second polylactide resin composition of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0006] Surprisingly, the polylactide resin composition of the present invention can itself be stretched to higher draw ratios than crystalline polylactide. As such, it can be processed on a wider variety of industrial-scale biaxial stretching equipment than conventional polylactide resins. In many cases, the polylactide resin composition can be processed on existing polypropylene and PET biaxial stretching lines with little or no equipment modifications.

[0007] In some embodiments, an amorphous polylactide grade, for purposes of this invention, is poly(meso-lactide), which is a polymer or copolymer of meso-lactide in which at least 80% of the lactic units are incorporated by polymerization of meso-lactide, and the poly(meso-lactide) has an average length of the blocks of L-lactic and D-lactic units equal to at least 1.1 and at most 2.0. Polylactide resin compositions of the invention in which the amorphous polylactide grade is poly(meso-lactide) exhibit several unique and advantageous characteristics. Surprisingly, it has been found that crystalline polylactide and poly(meso-lactide) are mutually immiscible at these blend ratios. Thus, melt or solution blends of these polylactides are typically phase-separated, with the crystalline polylactide (i) occupying one phase and the poly(meso-lactide) occupying a separate phase in the blend. At higher crystalline polylactide contents (e.g., above about 50% by weight), the crystalline polylactide tends to form a continuous phase in which poly(meso-lactide) exists as a discontinuous amorphous phase. At lower crystalline polylactide contents (e.g., below about 50% by weight), the crystalline polylactide and poly(meso-lactide) tend to form co-continuous phases, where again the poly(meso-lactide) phase is amorphous. The poly(meso-lactide) phase cannot crystallize and therefore remains amorphous even after the polylactide resin composition is stretched. On the other hand, the crystalline polylactide phase can be thermally and / or mechanically crystallized (such as by stretching). This allows the blend to exhibit desirable thermal and physical properties associated with crystallization.

[0008] In another aspect, the present invention is a method for producing an oriented polylactide film, comprising: (a) forming a polylactide resin composition by melt or solution mixing, the polylactide resin composition comprising: (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 93:7 or ≤ 7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 20:80 or ≤ 80:20. forming a (b) extruding the polylactide resin composition in a machine direction through an extrusion die to form a polylactide sheet; (c) stretching the sheet at a temperature of 50°C to 120°C to produce an oriented polylactide film; The method includes:

[0009] In another aspect, the present invention provides a method for producing a biaxially oriented polylactide film, comprising: (a) forming a polylactide resin composition by melt or solution mixing, the polylactide resin composition comprising: (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 93:7 or ≤ 7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 20:80 or ≤ 80:20. forming a (b) extruding the polylactide blend in a machine direction through an extrusion die to produce a polylactide sheet having a machine direction corresponding to the direction of travel of the polylactide blend through the extrusion die and a transverse direction perpendicular thereto; (c) sequentially or simultaneously stretching the sheet in orthogonal directions at a temperature between 50°C and 120°C to produce a biaxially oriented polylactide film; The method includes:

[0010] In yet another aspect, the present invention provides a method for producing a biaxially oriented polylactide film, comprising: (a) forming a polylactide resin composition by melt or solution mixing, the polylactide resin composition comprising: (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 93.7 or ≤ 7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 20:80 or ≤ 80:20. forming, including (b) extruding the polylactide resin composition to form a tube; (iii) quenching and shrinking the tube; (iv) heating the shrunk tube to a temperature of 50°C to 120°C and re-expanding the tube to elongate the tube in the machine direction, and stretching the tube in the transverse direction while stretching the tube in the machine direction to produce a biaxially oriented polylactide film. The method includes:

[0011] The crystalline polylactide has a number average molecular weight of at least 5000 g / mol, as measured by GPC (gel permeation chromatography) against polystyrene standards. The number average molecular weight can be, for example, up to 200,000 g / mol. Number average molecular weights of about 30,000 to 130,000 g / mol are generally preferred. The crystalline polylactide, in some embodiments, is characterized by having a relative viscosity of 1.1 to 6, e.g., 1.25 to 5 or 1.5 to 4.5, as measured on a capillary viscometer at 30°C using a 1% wt / vol solution of the polylactide resin in chloroform against chloroform standards.

[0012] Lactic acid units constitute at least 90% or at least 95% by weight of the crystalline polylactide. The crystalline polylactide may further contain repeat units formed from other monomers copolymerizable with lactide, such as alkylene oxides (including ethylene oxide, propylene oxide, butylene oxide, tetramethylene oxide, etc.), cyclic lactones, or carbonates. The repeat units derived from these other monomers may be present in block and / or random configuration. These other repeat units suitably constitute up to 10% by weight of the crystalline polylactide, preferably 0% to 5% by weight, and particularly about 0% to 2% by weight, and may be absent.

[0013] The crystalline polylactide (i) may also contain residues of initiator compounds, which are often used during the polymerization process to control molecular weight. Suitable such initiators include, for example, water, alcohols, various types of polyhydroxy compounds (e.g., ethylene glycol, propylene glycol, polyethylene glycol, polypropylene glycol, other glycol ethers, glycerin, trimethylolpropane, pentaerythritol, hydroxyl-terminated butadiene polymers, polyether polyols, polyesters, etc.), polycarboxyl-containing compounds, and compounds having at least one carboxyl and one hydroxyl group (e.g., lactic acid or lactic acid oligomers). Except when the initiator is a residue of lactic acid or a lactic acid oligomer, which can constitute any proportion of the crystalline polylactide, the initiator residues preferably constitute no more than 10% by weight of the crystalline polylactide, particularly no more than 5% or no more than 2%.

[0014] The lactic acid units in the crystalline polylactide consist of L-lactic acid units and D-lactic acid units in a ratio of ≧93:7 or ≦7:93. This ratio can be, for example, 93:7 to 100:0, 95:5 to 100:0, 7:93 to 0:100, or 5:95 to 0:100. The L-lactic acid units and D-lactic acid units are preferably arranged randomly.

[0015] The crystalline polylactide (i) is, in some embodiments, a homopolymer of L-lactide or a random copolymer of L-lactide with one or more of meso-lactide, D-lactide, and rac-lactide. In such copolymers, the proportions of the various lactides are selected to provide a ratio of L-lactic acid units to D-lactic acid units of 93:7 to 99.9:0.1 or 95:5 to 99.9:0.1.

[0016] The crystalline polylactide, in alternative embodiments, is a homopolymer of D-lactide or a random copolymer of D-lactide with one or more of meso-lactide, L-lactide, and rac-lactide. In such cases, the proportions of the various lactides are selected to result in a ratio of L-lactic to D-lactic units of 7:93 to 0.1:99.9 or 5:95 to 0.1:99.9.

[0017] Crystalline polylactides can crystallize to form semicrystalline polymers. By "crystalline," we mean that the polylactide contains at least 5 J / g, preferably at least 15 J / g or at least 20 J / g, of crystallites after being heated alone in air at 110°C for 1 hour. Polylactides that yield a crystallinity of less than 5 J / g under these conditions are "non-crystalline" for purposes of this invention. A sample is preheated to at least 220°C to melt the crystallites and then quenched by rapidly cooling to room temperature (23±3°C). The quenched sample is then heated at 110°C for 1 hour and quenched again by cooling to room temperature. The crystallinity is then conveniently measured using a differential scanning calorimetry (DSC) method. The amount of such crystallinity is expressed herein in terms of J / g, i.e., the enthalpy of fusion (Joules) of the polylactide crystals in the sample divided by the weight of the polylactide in grams in the sample. A convenient test protocol for performing DSC measurements is to heat a 5-10 milligram sample from 25°C to 225°C at 20°C / min under air on a Mettler Toledo DSC 3+ calorimeter or equivalent instrument running STARe V.16 software.

[0018] In some embodiments, the crystalline polylactide has a glass transition temperature of 55°C to 65°C and a crystalline melting temperature upon crystallization of 95°C to 195°C, particularly 120°C to 185°C.

[0019] The crystalline polylactide may have long-chain branches (having three or more carbon atoms). Long-chain branches can be introduced into the polylactide in various ways, such as by reacting carboxyl groups on the polylactide with epoxide groups present on a (meth)acrylate polymer or copolymer. The (meth)acrylate polymer or copolymer is characterized as being a solid at 23°C, containing an average of about 2 to about 15 free epoxide groups per molecule (e.g., about 3 to about 10 or about 4 to about 8 free epoxide groups per molecule), and preferably being the polymerization product of at least one epoxy-functional acrylate or methacrylate monomer copolymerized with at least one additional monomer. The (meth)acrylate polymer or copolymer suitably has a number average molecular weight of about 150 to about 700, e.g., 200 to 500 or 200 to 400 g / mol per epoxide group. The (meth)acrylate polymer or copolymer suitably has a number average molecular weight of 1000 to 6000, for example about 1500 to 5000 or about 1800 to 3000 g / mol. Other approaches to introducing long chain branching are described in U.S. Pat. Nos. 5,359,026 and 7,015,302, WO 06 / 002372 A2 and WO 2019 / 152264.

[0020] In a preferred embodiment, the crystalline polylactide (i) lacks long chain branching.

[0021] Two molecules of lactic acid can condense with the elimination of two molecules of water to form 3,6-dimethyl-1,4-dioxane-2,5-dione, referred to herein as "lactide." Lactide has the structure: [ka] Each lactic acid unit in a lactide molecule contains one chiral center and exists in either the D- or L-form. Lactide molecules can take one of three forms: 3S,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (L-lactide), 3R,6R-3,6-dimethyl-1,4-dioxane-2,5-dione (D-lactide), and 3R,6S-3,6-dimethyl-1,4-dioxane-2,5-dione (meso-lactide). These have the following structures: [ka] It has.

[0022] L-lactide and D-lactide are a pair of enantiomers, while meso-lactide is a stereoisomer with one L-lactic acid unit and one D-lactic acid unit. Furthermore, a mixture of about 50% L-lactide and 50% D-lactide forms a high-melting material known as racemic lactide (or "rac-lactide"). Hydrolysis of both meso-lactide and rac-lactide yields a mixture of 50% L-lactic acid and 50% D-lactic acid.

[0023] The crystalline polylactide (i) is preferably produced by polymerizing L-lactide or D-lactide itself, or by copolymerizing L- or D-lactide and meso-lactide, preferably in a random manner, in a ratio such that the ratio of L:D lactic units is ≧93:7 or ≦7:93. The polymerization can be carried out batchwise, semi-continuously, or continuously.

[0024] Suitable polymerization temperatures are preferably above the melting temperature of the monomer or monomer mixture, but below the temperature at which significant polymer degradation occurs. The temperature range may be, for example, as low as 60°C or as high as 225°C.

[0025] The molecular weight and conversion are controlled by the polymerization time and temperature, the equilibrium between free lactide and polymer, and the use of initiator compounds. Generally, increasing the amount of initiator compound on a molar basis tends to decrease the molecular weight of the product polymer. Molecular weight control agents, such as those described in U.S. Patent No. 6,277,951, can also be added to obtain the desired molecular weight.

[0026] The polymerization is preferably carried out in the presence of a polymerization catalyst. Examples of these catalysts include various tin compounds, such as SnCl, SnBr, SnCl, SnBr, SnO, tin(II) bis(2-ethylhexanoate), butyltin tris(2-ethylhexanoate), hydrated monobutyltin oxide, dibutyltin dilaurate, tetraphenyltin, and the like, PbO, zinc alkoxides, zinc stearate, aluminum alkoxides, antimony triacetate and antimony(2-ethylhexanoate), bismuth(2-ethylhexanoate), calcium stearate, magnesium stearate, certain yttrium and rare earth compounds as described in U.S. Pat. No. 5,208,667 to McLain et al., chiral (R)-(SalBinap)-AlOCH complexes as described in Macromol. Chem. Phys. 1996, 197, 2627-2637, JACS Examples of catalysts include single-site b-diimidate zinc alkoxide catalysts such as those described in Macromolecules 1999, 121, 11583-11584, lithium t-butoxide aggregates such as those described in Macromolecules 1995, 28, 3937-3939 and Polymer 1999, 40, 5455-5458, aluminum and yttrium-based catalyst complexes such as those described in JACS 2002, 124, 1316-1326, and binuclear indium catalysts such as those described in Macromolecules 2016, 49, 909-919. The catalyst is used in a catalytically effective amount, which depends to some extent on the particular catalyst, but typically ranges from 1 mole of catalyst for every 3,000 to 50,000 moles of monomer.

[0027] The crystalline polylactide thus obtained preferably contains metal catalyst residues which are deactivated by contacting the crystalline polylactide with a deactivating agent.

[0028] The residence time under polymerization conditions is selected to provide a desired molecular weight polymer and / or desired conversion of monomers.

[0029] Other methods for making crystalline polylactide include condensation polymerization of lactic acid and solid state polymerization methods starting from oligomeric polylactic acid.

[0030] The crystalline polylactide may contain residual lactide. If present, the lactide may constitute at most 1%, at most 0.5%, at most 0.3%, or at most 0.2% by weight of the polylactide (i).

[0031] The amorphous polylactide (ii) has a number average molecular weight of at least 5000 g / mole, as measured by gel permeation chromatography relative to polystyrene standards, and at least 90% by weight of lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≥ 20:80 or ≤ 80:20. In some embodiments, the lactic units are L-lactic and D-lactic units in a ratio of 20:80 to 40:60 or 80:20 to 60:40. The amorphous polylactide can be prepared in the same general manner as described for the crystalline polylactide, except that the lactide is selected to provide the above ratio of L-lactide to D-lactic units. The amorphous polylactide can be, for example, (a) a copolymer of L-lactide with one or more of D-lactide or meso-lactide, (b) a copolymer of D-lactide with one or more of L-lactide or meso-lactide, (c) a polymer of rac-lactide, (d) a copolymer of rac-lactide with meso-lactide, or (e) a homopolymer of meso-lactide.

[0032] In a preferred embodiment, the amorphous polylactide is poly(meso-lactide). For purposes of this invention, poly(meso-lactide), PMLA, is a homopolymer of meso-lactide or a copolymer of at least 80% meso-lactide and at most 20% another lactide, preferably at least 88% meso-lactide and at most 12% another lactide or at least 90% meso-lactide and at most 10% another lactide. In the case of a copolymer, the copolymer may be a random and / or block copolymer. The other lactide may be any other lactide, including L-lactide, D-lactide, or a mixture of any two or more thereof.

[0033] Meso-lactide is unique among these various forms of lactide because, as it homopolymerizes, the number of consecutive L-lactic and D-lactic units produced in the polymer is at least 1 or at most 2. Polymerization of a mixture of L-lactide and D-lactide incorporates segments of an even number of lactic units in the polymer, the average block length of which is determined by the ratio of the monomers present in the feedstock. When a molecule of meso-lactide is added to the end of a growing polymer chain during the polymerization process, it introduces a single L-lactic unit and a single D-lactic unit at the chain end. When meso-lactide polymerizes in a "head-to-tail" manner (i.e., a D-lactic unit is added to a terminal L-lactic unit on the polymer chain, or vice versa), it forms the form: D-(LDLD) n where D represents a D-lactic acid unit and L represents an L-lactic acid unit. PMLA with this configuration is sometimes referred to as "syndiotactic." In this configuration, the number of consecutive D- and L-lactic acid units is always 1. Conversely, when meso-lactide is polymerized in a "head-to-head" manner (i.e., when a D-lactic acid unit is added to a terminal D-lactic acid unit), a stereoregular polymer of the form: D-(DLLD) n Instead, a polymer having the structure is produced. PMLA with this structure is sometimes called "heterotactic" or "disyndiotactic." In this case, the number of consecutive D- and L-lactic acid units is always 2. When meso-lactide is polymerized randomly, the number of consecutive D- and L-lactic acid units is sometimes 1 and sometimes 2, with an average of 1 to 2.

[0034] PMLA can be heterotactic or partially syndiotactic and partially heterotactic. The selection of catalyst and polymerization temperature, respectively, can affect the stereochemical configuration of PMLA. In general, it has been found that selecting a higher polymerization temperature, particularly 120°C or higher, especially 150°C or higher, results in less stereospecificity in PMLA, resulting in an average block length greater than 1 and less than 2. Similarly, tin-based catalysts tend to favor lower stereospecificity. In some embodiments, PMLA is polymerized with a tin catalyst at a temperature of at least 120°C, preferably at least 150°C, and at most 225°C, more preferably at most 190°C.

[0035] The average length of the blocks of L-lactic acid units and D-lactic acid units in PMLA can be, for example, at least 1.1, at least 1.2, at least 1.25, or at least 1.3, and can be, for example, at most 2, at most 1.75, at most 1.5, or at most 1.4. The average block length can be determined by the P (polylactic acid) ratio as described by Coates et al., in J. American Chemical Society 2002, 124, 1316. m and the following relationship: Average block length = 1 + (P m / (1+(1-P m ))) can be determined by proton NMR using

[0036] At least 90% or at least 95% by weight of the amorphous polylactide is made up of lactic acid units.

[0037] The number average molecular weight of the amorphous polylactide (ii), as measured by GPC against polystyrene standards, can be, for example, in the range of 5000 to 250,000 g / mol. Number average molecular weights of about 30,000 to 130,000 g / mol are preferred.

[0038] The amorphous polylactide (ii), in some embodiments, is characterized by having a relative viscosity of 1.1 to 6, 1.25 to 5, or 1.5 to 3.5, as measured on a capillary viscometer at 30° C. using a 1% weight / volume solution of the polylactide resin in chloroform against a chloroform standard.

[0039] In some embodiments, the amorphous polylactide has a glass transition temperature of 38°C to 50°C.

[0040] Other characteristics of the amorphous polylactide (ii) and the manner in which it is produced are as described above for the crystalline polylactide (i).

[0041] The polylactide resin composition comprises a mixture of crystalline polylactide (i) and amorphous polylactide (ii). The polylactide resin is melt or solution blended rather than a physical mixture of separate particles of the component polylactides. The crystalline polylactide may comprise 40 to 95 weight percent of the total weight of all polylactides in the polylactide resin composition. In certain embodiments, the crystalline polylactide comprises at least 50%, at least 60%, or at least 65% and up to 90%, at most 85%, or at most 80% of the total weight of all polylactides in the polylactide resin composition.

[0042] When used in the method of making an oriented film according to the present invention, the amorphous polylactide may comprise 5 to 60 weight percent of the total weight of all polylactides in the polylactide resin composition. In some embodiments, the amorphous polylactide (ii) may comprise at least 10%, at least 15%, at least 20%, at least 25%, and up to 45%, up to 40%, or up to 35% of all polylactides in the polylactide resin composition.

[0043] Other polylactides different from the crystalline polylactide (i) and the amorphous polylactide (ii) may be present, but if present, preferably constitute no more than 10%, no more than 5%, or no more than 2% of the total weight of the component polylactides, and may be absent.

[0044] The polylactide resin composition may contain other materials that may be useful for the particular end use it is used in. These may include, for example, polymers other than polylactide, i.e., non-polylactide polymers.

[0045] If present at all, the non-polylactide polymer may comprise, for example, 0.1 to 50%, 1 to 25%, or 1 to 10% of the combined weight of the non-polylactide polymer and polylactide.

[0046] Other optional materials that may be present in the polylactide resin composition include crystallization nucleators, such as finely divided solids, colorants, impact modifiers, internal and / or external lubricants, antiblocking agents, other extrusion processing aids, additives to control PLA hydrolysis and / or biodegradation properties, and the like.

[0047] The polylactide resin composition can be formed by forming a solution of both polylactide (i) and polylactide (ii) in a suitable solvent and then removing the solvent. Separate solutions of polylactides (i) and (ii) can be formed as needed and combined to form the polylactide resin composition. Alternatively, polylactides (i) and (ii) can be dissolved together in a single solvent to form a solution from which the solvent is then removed.

[0048] A preferred method of making polylactide resin compositions is by melt mixing. Crystalline and amorphous polylactides are conveniently heated to a temperature above the crystalline melting temperature of the crystalline polylactide and mixed at such temperature, preferably under shear, to form a blend. While the polylactides can be heated separately, it is generally preferred to form a mixture of polylactide particles (pellets, powder) and melt them together. Preferred equipment is a single-screw or twin-screw extruder. Granular polylactides can be fed individually or as a mixture to the extruder, where they are melted and mixed by the action of the screws to form the polylactide resin composition. In a particularly preferred process, the melt mixing step can be integrated into all or part of a subsequent stretching process to form a biaxially oriented film.

[0049] The polylactide resin composition of the present invention finds particular application in the production of oriented films. In one method of producing such films, the polylactide resin composition is extruded to form a sheet. The sheet can have a thickness of, for example, at least 250 μm, at least 500 μm, or at least 750 μm, and at most 2000 μm, at most 1500 μm, at most 1250 μm, or at most 1000 μm. Although not necessarily, the sheet extrusion step is conveniently combined with a melt-mixing step, where the melt-mixing step is carried out in an extruder, and then the mixed polylactide thus formed is extruded through an extrusion die attached to the same extruder to form the polylactide resin composition and produce the sheet. However, it is within the scope of the present invention to melt-mix crystalline and amorphous polylactides and form the sheet in different equipment and / or at different times.

[0050] Sheets are formed by heating the polylactide resin composition above the crystalline melting temperature of the crystalline polylactide, preferably to a temperature between 190°C and 225°C, to form a melt, which is then forced through an annular or slit die to produce a sheet. The direction of travel of the polylactide resin composition through the die is referred to as the "machine" direction of both the process and the resulting sheet. The sheet is then cooled below the glass transition temperature of the crystalline polylactide. Unless the sheet is immediately stretched, it is generally preferred to "quench" the sheet by rapidly cooling it below the glass transition temperature of the crystalline polylactide to minimize crystallization.

[0051] The sheet is then stretched in at least one direction to produce an oriented film. In the case of uniaxial orientation only, stretching is carried out in only one direction, typically the machine direction. Biaxially oriented films are stretched in two orthogonal directions, typically the machine direction and the transverse direction (i.e., the direction perpendicular to the machine direction in the plane of the sheet). In the case of biaxial orientation, stretching can be carried out in both directions simultaneously or sequentially. When carried out sequentially, it is usually beneficial to stretch first in the machine direction and then in the transverse direction.

[0052] Stretching can be accomplished, for example, by passing the sheet through two successive sets of rollers, the second set operating at a faster speed than the first, thereby stretching and uniaxially orienting the sheet. Stretching can also be accomplished, for example, using a tenter frame or other device having grips attached to opposite sides of the sheet. The grips act to pull the opposite sides apart, thereby stretching the sheet.

[0053] Stretching can be integrated into the sheet extrusion step by performing the stretching step immediately after the sheet is extruded. The temperature of the freshly extruded sheet is adjusted to the temperature described below and then stretched uniaxially (preferably in the machine direction) or biaxially (typically sequentially, first in the machine direction, then in the transverse direction). After the stretching step is completed, the oriented film is cooled below the glass transition temperature of the crystalline polylactide. In such a process, machine direction stretching can be performed using two consecutive sets of rollers described above to produce a uniaxially oriented film. The first set of rollers in such a process can include or be preceded by a chill roll that cools the sheet to the stretching temperature described below. Subsequent transverse stretching is preferably performed continuously by attaching grips to the sides of the uniaxially stretched film and operating the grips to separate the opposing sides in the transverse direction. A cooling step can be performed after the first stretching step, followed, if necessary, by reheating to the temperature at which the second stretching step will be performed.

[0054] Alternatively, the sheet may be stretched simultaneously in both the machine and transverse directions, for example, by attaching clips to all four sides of the sheet and separating the opposing sides, or by a combination of a tenter that stretches the sheet in the transverse direction and a roller system that stretches it in the machine direction.

[0055] In other embodiments, the sheet extrusion and stretching (orientation) steps are not integrated, but instead occur in separate manufacturing steps. In such processes, the extruded sheet is conveniently prepared as described above, cooled after extrusion, preferably rapidly below the glass transition temperature of the crystalline polylactide to avoid excessive crystallization of the crystalline polylactide phase, and stretched in a subsequent operation. Apparatus such as those described above are useful.

[0056] Suitable commercially available equipment for biaxially stretching flat sheets is sold, for example, by Parkinson Technologies, Inc. (Marshall and Williams Plastics brand), Biax-Fiberfilm Corporation (Microspan® stretching equipment), Brueckner-USA, Andritz Bias SAS, and The Japan Steel Works, Ltd., among others.

[0057] The drawing step is carried out at a temperature of 50° C. to 120° C. Preferred lower temperatures are at least 55° C., at least 60° C. or at least 70° C. Preferred higher temperatures are up to 100° C.

[0058] In a sequential stretching process in which the machine direction stretching is carried out first followed by the transverse direction stretching, the machine direction stretching is preferably carried out at a temperature of 50°C to 120°C, preferably 55 to 100°C, and the subsequent transverse direction stretching is preferably carried out at a temperature of 70°C to 120°C, most preferably 70°C to 100°C.

[0059] The simultaneous biaxial stretching is preferably carried out at a temperature of 70°C to 120°C, most preferably 70°C to 100°C.

[0060] The crystalline polylactide phase of the sheet will crystallize if given sufficient time when brought to the above stretching temperatures. If it crystallizes excessively, it will be more difficult to stretch. To avoid excessive crystallization of polylactide (i), it is preferred that all stretching steps be performed on the sheet at the stretching temperature for a cumulative period of 2 minutes or less, preferably 1 minute or less or 30 seconds or less.

[0061] The stretching speed can be, for example, 10% to 500% per second of the starting sheet's pre-stretch dimension (in the stretching direction), i.e., the sheet can be stretched in the stretching direction a distance per second equal to 10% to 500% of its initial dimension. Preferred stretching speeds are 25 to 200% per second or 25 to 100% per second.

[0062] In stretching flat sheets, the stretch ratios can generally be 3 to 10 in each of the machine and transverse directions. Preferred stretch ratios in the machine direction are 3 to 6 or 3 to 5, and preferred stretch ratios in the transverse direction are 3 to 10, 4 to 10, 5 to 9, or 5.5 to 9. In some cases, somewhat lower stretch ratios are observed in the machine direction. The area of ​​the uniaxially oriented film can be 3 to 10 times the area of ​​the starting sheet, and the thickness of the uniaxially oriented film can be, for example, one-third to one-tenth the thickness of the starting sheet. The area of ​​the biaxially oriented film can be, for example, 9 to 100 times the area of ​​the starting sheet, preferably 9 to 64, 12 to 50, or 15 to 36 times the area of ​​the starting sheet. The thickness of the biaxially oriented film can be, for example, one-ninth to one-hundredth the thickness of the starting sheet. The oriented film thickness in the case of uniaxial or biaxial orientation may be, for example, at least 4 μm, at least 8 μm or at least 12 μm and at most 200 μm, at most 100 μm, at most 50 μm.

[0063] The draw ratios obtainable with this method of the invention depend at least in part on the proportion of amorphous polylactide in the polylactide resin composition: increasing the proportion of amorphous polylactide generally allows for greater draw ratios to be achieved.

[0064] Increasing the sheet temperature during the drawing process also tends to allow greater draw ratios to be achieved.

[0065] Alternatively, biaxially oriented films can be produced by the so-called "double bubble" process. In the double bubble process, a polylactide resin composition is extruded to form a tube. The tube is quenched and shrunk by cooling to near or below the glass transition temperature of the crystalline polylactide. The shrunk tube is then heated to a temperature of 50°C to 120°C, preferably 70°C to 120°C, and re-expanded with air or other gas, stretching the tube in the machine direction while stretching the tube in the transverse direction. The double bubble biaxial film process is described, for example, in WO 2001 / 070483.

[0066] After the stretching step is complete, the biaxially oriented film thus obtained may be thermally annealed, for example, at a temperature of 70°C to 150°C, preferably 120°C to 140°C or 120°C to 135°C. Annealing at such temperatures may last, for example, from 5 seconds to 5 minutes, particularly from 5 to 30 seconds. Annealing should be carried out under tension. Annealing in this manner reduces film shrinkage when exposed to high temperatures.

[0067] The following examples illustrate the invention but are not intended to limit it in any way. All parts and percentages are by weight unless otherwise indicated.

[0068] Amorphous PLA A is poly(mesolactide). It is a linear copolymer made by polymerizing a mixture of approximately 90% meso-lactide and 10% L-lactide at 160-180°C in the presence of a tin catalyst. Lactic acid units constitute more than 98% of the total weight of amorphous PLA A. 45% of the lactic acid units are D-lactic acid units and 55% are L-lactic acid units. Amorphous PLA A has an average block length of L-lactic acid units and D-lactic acid units of 1.1-1.75. It has a relative viscosity of 2.7.

[0069] Amorphous PLA B is a linear copolymer made by polymerizing a mixture of L-lactide, meso-lactide, and D-lactide. Lactic acid units make up more than 98% of the total weight of amorphous PLA B. 80% of the lactic acid units are L-lactic acid units and 20% are D-lactic acid units.

[0070] Crystalline PLA A is commercially available as Ingeo® 4032D resin from NatureWorks LLC, Plymouth, Minnesota, US. Lactic acid units comprise more than 98% of the total weight of crystalline PLA A. Approximately 1.4% of the lactic acid units are D-lactic acid units, and approximately 98.6% are L-lactic acid units. Crystalline PLA A has a relative viscosity of 4.0.

[0071] Crystalline PLA B is commercially available from NatureWorks LLC as Ingeo® 4043D resin. Lactic acid units comprise more than 98% of the total weight of crystalline PLA B. 4.25% of the lactic acid units are D-lactic acid units and 95.75% are L-lactic acid units. Crystalline PLA B has a relative viscosity of 4.0.

[0072] Crystalline PLA C is commercially available from NatureWorks LLC as Ingeo® 2500HP resin. Lactic acid units constitute more than 98% of the total weight of crystalline PLA C. 0.5% of the lactic acid units are D-lactic acid units and 99.5% are L-lactic acid units. Crystalline PLA C has a relative viscosity of 4.0. [Example]

[0073] Examples 1 to 8 and Comparative Samples A to B Cast sheets with thicknesses of 650-800 μm are prepared from blends of crystalline PLA A, crystalline PLA B, and amorphous PLA A with crystalline PLA A or crystalline PLA B as shown in Table 1. Pellets of PLA material are melted in a twin-screw extruder, where they are mixed, extruded, and quenched before being formed into sheets.

[0074] [Table 1]

[0075] The sample of Example 5 was examined by atomic force microscopy in tapping mode at 25°C and 95% RH using a Keysight 5500 instrument equipped with high-speed force curve mapping capabilities. Prior to AFM analysis, the sample was microtomed at -120°C to create a smooth, flat area for scanning. As shown in Figure 1, the crystalline PLA B and amorphous PLA A are imaged as separate phases, with the crystalline PLA B forming a continuous phase and the amorphous PLA A forming discrete domains, roughly 5-10 nm in size, dispersed within the continuous phase.

[0076] A 50 / 50 blend by weight of crystalline PLA B and amorphous PLA A is prepared in the manner described above and examined by atomic force microscopy in the manner just described. As shown in Figure 2, images of crystalline PLA B and amorphous PLA A as co-continuous phases with domain sizes up to the order of a few microns are shown.

[0077] The sheets shown in Table 2 below are sequentially stretched on a Brueckner Karo 5.0 biaxial stretching unit at various temperatures as shown in Table 2, first in the machine direction at a stretch ratio of 3.5 and then in the transverse direction until burst. The transverse stretch ratio at burst is as shown in Table 2.

[0078] [Table 2]

[0079] The sheets shown in Tables 3-11 below are then sequentially stretched first in the machine direction and then in the transverse direction on a Brueckner Karo 5.0 biaxial stretching unit at various temperatures and various stretch ratios as shown in Tables 3, 6, and 9. The preheat time is 30 seconds in each case. The stretched samples are annealed under tension at 120°C for 30 seconds immediately after stretching. The stretched sheets are evaluated for total haze % according to ASTM D1003-21 and for shrinkage according to ASTM D2732-14. For certain samples, internal haze is evaluated by immersing the sample in clear corn oil in a glass cuvette and then re-measuring the haze according to ASTM D1003-21. The haze of a blank (oil and cuvette) is subtracted from the measured sample to determine the internal haze of the sample. The difference between total haze (initial) and internal haze is due to surface haze. Mechanical properties are measured according to ASTM D882. The results of this test are shown in Tables 4, 5, 7, 8, 10 and 11.

[0080] [Table 3]

[0081] Table 4

[0082] Table 5

[0083] Table 6

[0084] Table 7

[0085] Table 8

[0086] Table 9

[0087] Table 10

[0088] Table 11

[0089] As shown by the data in the table above, sheets containing 10-60% amorphous polylactide can be stretched significantly more in the transverse direction than sheets of crystalline polylactide alone (machine direction stretching is held constant in these examples), resulting in significantly higher drawdown ratios. Exceptions are seen in the 90 / 10 blend of crystalline PLA A and amorphous PLA and the 80 / 20 blend of crystalline PLA B and amorphous PLA, which are due to imperfections in the sheet before stretching. Increasing the proportion of amorphous PLA generally allows for more transverse stretching before breakage.

[0090] Mechanical properties and appearance are generally adequate for most of the inventive examples. However, 40 / 60 blends of crystalline PLA with amorphous PLA tend to exhibit a significant increase in haze (especially surface haze) and a significant loss in elongation at break. From an enantiomer perspective (i.e., crystalline PLA A versus crystalline PLA B), these effects are more pronounced when the crystalline PLA is very pure and therefore crystallizes more rapidly under stretching conditions. While the present invention is not limited to any theory, the increased haze and decreased elongation seen in the 40 / 60 samples may be due to a change in the microphase structure of the polylactide resin composition as the proportion of amorphous PLA increases.

[0091] Polylactide resin composition Examples 1-8 are simultaneously stretched on a Brueckner Karo 5.0 biaxial stretching unit at various temperatures as shown in Table 12. The stretch ratios are as shown in Table 12. The stretch ratios are the highest that could be obtained under these stretching conditions without breaking the film.

[0092] [Table 12]

[0093] Additional biaxially stretched films are made using Example 2 in the same sequential stretching process as above, with stretching temperatures and ratios as shown in Table 13.

[0094] [Table 13]

[0095] Examples 9-10 and Comparative Sample C Cast sheets having thicknesses of 650-800 μm are prepared from crystalline PLA C and blends of crystalline PLA C with amorphous PLA A as shown in Table 14. Pellets of PLA material are melted in a twin-screw extruder where they are mixed, extruded and quenched before being formed into sheets.

[0096] [Table 14]

[0097] The sheets are then sequentially stretched at 85°C in the manner described in the previous examples to produce biaxially oriented films. When the machine direction stretch ratio is greater than 2.0, Comparative Sample C cannot be stretched in the transverse direction to a stretch ratio greater than 2.0. Therefore, Comparative Sample C is stretched in the machine direction only to a stretch ratio of 2.0 before attempting transverse stretching. Examples 9 and 10 are stretched in the machine direction to a stretch ratio of 3.5 before transverse stretching to a stretch ratio of 6.0 (Example 9) and 7.0 (Example 10). Film properties were measured as described above, and the results are shown in Table 15. Crystallinity was measured by differential scanning calorimetry.

[0098] [Table 15]

[0099] Examples 9 and 10 demonstrate the beneficial effects of the present invention, even though the crystalline PLA resin is enantiomerically very pure and crystallizes very rapidly when subjected to temperatures above its glass transition temperature (approximately 65°C). Crystalline PLA C by itself can only be stretched to low draw ratios in the machine direction with difficulty. The addition of 20-40% amorphous PLA A allows the sheet to be stretched to high draw ratios to produce a film with excellent properties.

[0100] Examples 11 to 13 and Comparative Sample D Biaxially oriented films are made on a continuous pilot coextrusion line with in-line sequential stretching elements. The pilot line contains three single-screw extruders with operating speeds of 4.54, 50, and 4.54 kg / hr, respectively. These extruders feed a 12" (30.5 cm) wide extrusion die. The line is designed to produce a transverse stretch ratio of 4 to 10. Stretching in the machine direction is achieved by passing the extruded film over a low-speed stretch roll maintained at 57-58°C, followed by a high-speed stretch roll maintained at 54-56°C, and then over an annealing roll maintained at 43-45°C. The machine-direction stretch ratios are shown in Table 16. The film then proceeds to a transverse stretching zone that involves preheating to 65-66°C, stretching at 79-80°C, and annealing at 121°C for approximately 30 seconds. The transverse stretch ratios are as shown in Table 16. The machine direction stretch ratio of Comparative Sample D is limited to 2.8 to allow the film to be stretched in the transverse direction within the operating limits of the equipment (i.e., a TD stretch ratio of at least 4x) without breaking. The final film thickness is shown in Table 16. Shrinkage and crystallinity were measured as described above, and the results are shown in Table 16.

[0101] The resins used in Examples 11 to 13 and Comparative Sample D are as follows.

[0102] [Table 16]

[0103] [Table 17]

[0104] The drawdown ratio for each of Examples 11-13 is more than twice that of Comparative D (which can be obtained using crystalline PLA A itself under these conditions with a drawdown ratio of 15.8). Examples 11 and 13 are notable in that the main blend components processed through Extruder 2 are fed as pellet mixtures. This means that no prior melt mixing is required, as this can be done in-line on a continuous extrusion / stretching device, and the single screw extruder ensures sufficient mixing.

[0105] Examples 14 to 17 Biaxially stretched films are made by the sequential stretching process described in Examples 1-8, except that the annealing conditions are varied as shown in Table 17. The stretch ratios are as shown in Table 17.

[0106] Example 14 is made from an 80 / 20 blend of crystalline and amorphous PLA A.

[0107] Example 15 is made from a 60 / 40 blend of crystalline and amorphous PLA A.

[0108] Example 16 is made from an 80 / 20 blend of crystalline PLA B and amorphous PLA A.

[0109] Example 17 is made from a 60 / 40 blend of crystalline PLA B and amorphous PLA A.

[0110] Shrinkage in both the machine and transverse directions is measured as described above. The results are shown in Table 17. It should be noted that crystallinity is at most only slightly affected by annealing under certain conditions. Crystallinity appears to arise almost entirely as a result of the orientation that occurs during the stretching step.

[0111] [Table 18]

[0112] Example 18 Cast sheets having thicknesses of 650-800 μm are prepared from a 60 / 40 mixture of crystalline PLA A with amorphous PLA B in the general manner described above. The sheets are then sequentially stretched at 80° C. and 85° C. in the manner described in the examples above to produce biaxially oriented films. Film properties were measured as described above, and the results are shown in Table 18.

[0113] [Table 19]

Claims

1. A polylactide resin composition comprising: (i) 40 to 95 weight percent, based on the weight of all polylactides in the composition, of a crystalline polylactide having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography versus polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≧93:7 or ≦7:93; (ii) 60 to 5 weight percent, based on the weight of all polylactides in the composition, of an amorphous polylactide having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against a polystyrene standard, and containing at least 90 weight percent lactic units, the lactic units being L-lactic units and D-lactic units in a ratio of ≧20:80 or ≦80:20; 1. A polylactide resin composition comprising a melt or solution blend of:

2. 10. The polylactide resin composition of claim 1, which is phase separated.

3. 3. The polylactide resin composition of claim 2, wherein the amorphous polylactide is present as a discontinuous amorphous phase dispersed within a continuous phase of the crystalline polylactide.

4. 4. The polylactide resin composition of claim 3, containing 65 to 90 weight percent of said crystalline polylactide, based on the weight of all polylactides in the composition, and 10 to 35 weight percent of said amorphous polylactide, based on the weight of all polylactides in the composition.

5. 5. The polylactide resin composition of any one of claims 1 to 4, containing 65 to 85 weight percent of the crystalline polylactide, based on the weight of all polylactides in the composition, and 15 to 35 weight percent of the amorphous polylactide, based on the weight of all polylactides in the composition.

6. 3. The polylactide resin composition of claim 2, wherein the amorphous polylactide and the crystalline polylactide exist as co-continuous phases, and the amorphous phase is amorphous.

7. 7. The polylactide resin composition of claim 6, containing 55 to 64 weight percent of said crystalline polylactide, based on the weight of all polylactides in the composition, and 45 to 36 weight percent of said amorphous polylactide, based on the weight of all polylactides in the composition.

8. 10. The polylactide composition of claim 1, containing from 55 to 64 weight percent of said crystalline polylactide, based on the weight of all polylactides in the composition, and from 45 to 36 weight percent of said amorphous polylactide, based on the weight of all polylactides in the composition.

9. 9. The polylactide composition of claim 1, wherein the amorphous polylactide is poly(mesolactide).

10. The polylactide resin composition of any one of claims 1 to 9 in the form of a uniaxially or biaxially oriented film.

11. 11. A method of producing an oriented polylactide film, the method comprising: (a) melt or solution blending a polylactide resin composition, the polylactide blend of any one of claims 1 to 10, to form a polylactide resin composition by extrusion in the machine direction through an extrusion die to form a polylactide sheet; and stretching the sheet at a temperature of from 50°C to 120°C to form the oriented polylactide film.

12. 1. A method for producing an oriented polylactide film, comprising: (a) forming a polylactide resin composition by melt or solution mixing, said polylactide resin composition comprising: (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≧93:7 or ≦7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≧20:80 or ≦80:

20. forming a (b) extruding the polylactide resin composition in a machine direction through an extrusion die to form a polylactide sheet; (c) stretching the sheet at a temperature of 50°C to 120°C to produce the oriented polylactide film; A method comprising:

13. 11. A method of producing a biaxially oriented polylactide sheet, comprising: (a) extruding the polylactide resin composition of any one of claims 1 to 10 in a machine direction through an extrusion die to produce a polylactide sheet having a machine direction corresponding to the direction of travel of the polylactide blend through the extrusion die and an orthogonal cross direction; (b) sequentially or simultaneously stretching the sheet in orthogonal directions at a temperature of 50°C to 120°C to produce the biaxially oriented polylactide film; A method comprising:

14. 1. A method for producing a biaxially oriented polylactide film, comprising: (a) forming a polylactide resin composition by melt or solution blending, said polylactide resin composition comprising: (i) 40 to 95 weight percent crystalline polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 95 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≧93:7 or ≦7:93; and (ii) 60 to 5 weight percent amorphous polylactide, based on the weight of all polylactides in the composition, having a number average molecular weight of at least 5000 g / mole as measured by gel permeation chromatography against polystyrene standards, and containing at least 90 weight percent lactic units, the lactic units being L-lactic and D-lactic units in a ratio of ≧20:80 or ≦80:

20. forming a (b) extruding the polylactide resin composition in a machine direction through an extrusion die to produce a polylactide sheet having a machine direction corresponding to the direction of travel of the polylactide blend through the extrusion die and a cross direction perpendicular thereto; (c) sequentially or simultaneously stretching the sheet in orthogonal directions at a temperature of 50°C to 120°C to produce the biaxially oriented polylactide film; A method comprising:

15. 15. The method of claim 14, wherein in step (c), the sheet is stretched in a machine direction and a cross-direction, the stretch ratio in the machine direction being from 3 to 5, and the stretch ratio in the cross direction being from 4 to 9.

16. 16. The method of claim 14 or 15, wherein in step (c) the sheet is first stretched in the machine direction at a temperature of from 50°C to 100°C and then stretched in the orthogonal transverse direction at a temperature of from 70°C to 120°C.

17. The method of any one of claims 11 to 16, wherein the sheet is stretched at a stretching rate of 25% to 100% per second.

18. 11. A method for producing a biaxially oriented polylactide film, comprising: (a) extruding a polylactide resin composition according to any one of claims 1 to 10 to form a tube; (b) quenching and shrinking the tube; (c) heating the shrunken tube to a temperature of 50°C to 120°C and re-expanding the tube to stretch the tube in the machine direction while stretching the tube in the transverse direction. A method comprising:

19. 19. The method of claim 18, wherein in step (c), the stretch ratio in the machine direction is from 3 to 8 and the stretch ratio in the transverse direction is from 3 to 8.

20. 20. The method of claim 18 or 19, wherein in step (c) the tube is stretched in both the machine direction and the transverse direction at a stretch rate of from 25% to 100% per second.

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