Dimensionally stable biodegradable film

Abstract

A method for improving the dimensional stability of a biodegradable film, the method comprising providing a poly(hydroxyalkanoate)-based blown polymer film to a nip roller, feeding the poly(hydroxyalkanoate)-based blown polymer film to an annealing roller, and heating the blown film by contact with the annealing roller at a temperature ranging from about 80 to about 150°C for a predetermined period of time.

Description

[Technical Field]

[0001] Related Applications This application claims priority to Provisional Application No. 63 / 369,997, filed August 1, 2022.

[0002] Technical Field The present disclosure relates to biodegradable films, and in particular to methods for improving the dimensional stability of poly(hydroxyalkanoate)-based polymer films. [Background technology]

[0003] Blown polymer films are the primary processing technique for film manufacturers to convert polymer films for food packaging applications. Blown polymer films are typically used in bag manufacturing and in heat-sealing processes, which are widely used in flexible packaging. The stability of blown polymer films is crucial to maintaining product quality and preventing food quality deterioration during storage. Bag manufacturing and heat-sealing processes are generally performed at high temperatures. Semi-crystalline polymer films tend to shrink as they cool from high temperatures. The degree of shrinkage is highly dependent on the composition of the film material and the processing conditions used to manufacture the film. Semi-crystalline materials shrink more than amorphous materials. Dimensional instability or shrinkage of films when cooled from high temperatures plays a major role in the overall quality of the finished product. Significant shrinkage after the bag manufacturing or heat-sealing process can result in imperfections in the finished product, including leakage, risk of pinhole formation, and broken seal areas.

[0004] Biodegradable poly(hydroxyalkanoate)-based polymer films also shrink when cooled from elevated temperatures to room temperature. The shrinkage characteristics of poly(hydroxyalkanoate)-based films pose significant problems during coating, metallizing, laminating, printing, bag-making, and sealing processes. Conventional blown film processes produce poly(hydroxyalkanoate)-based polymer films with shrinkage of 45% to 50% measured in the machine direction (MD) at 110°C. The temperature selected for shrinkage measurement is the temperature used for most of the equipment components that post-process the film. The shrinkage values ​​of poly(hydroxyalkanoate)-based polymer films are significantly higher than the industry standard of less than 5% for other polymer films. Therefore, what is needed is an improved process for producing poly(hydroxyalkanoate)-based polymer films that provides films with significantly reduced shrinkage when used in packaging applications. Summary of the Invention

[0005] In view of the above, embodiments of the present disclosure provide a method for improving the dimensional stability of a machine direction oriented (MDO) biodegradable film, the method including providing a poly(hydroxyalkanoate)-based blown polymer film to a nip roller, feeding the poly(hydroxyalkanoate)-based blown polymer film to an annealing roller, and heating the blown film by contact with the annealing roller at a temperature ranging from about 80 to about 150° C. for a predetermined period of time.

[0006] In one embodiment, the method comprises heating the blown film by contacting it with an annealing roller at a temperature ranging from about 100 to about 120°C.

[0007] In one embodiment, the breathable film has a thickness ranging from about 7 microns to about 260 microns.

[0008] In another embodiment, the predetermined time period according to the method ranges from about 1 second to about 1 minute.

[0009] In another embodiment, provided is a machine direction oriented (MDO) poly(hydroxyalkanoate)-based film made by providing a poly(hydroxyalkanoate)-based blown polymer film to nip rollers, feeding the poly(hydroxyalkanoate)-based blown polymer film to an annealing roller, and heating the blown film by contact with the annealing roller at a temperature ranging from about 80 to about 150°C for a predetermined period of time.

[0010] In another embodiment, provided is a dimensionally stable biodegradable packaging film made by providing a poly(hydroxyalkanoate)-based blown polymer film to nip rollers, feeding the poly(hydroxyalkanoate)-based blown polymer film to an annealing roller, and heating the blown film by contact with the annealing roller at a temperature ranging from about 80 to about 150°C for a predetermined period of time.

[0011] In another embodiment, provided is a method for improving the dimensional stability of a biodegradable film, the method comprising providing a poly(hydroxyalkanoate)-based biaxially oriented polymer film having a variable machine direction orientation (MDOP) and a variable transverse direction orientation (TDO), feeding the biaxially oriented film to an annealing roller, and heating the biaxially oriented film by contact with the annealing roller at a temperature ranging from about 80°C to about 150°C for a predetermined period of time.

[0012] In another embodiment, the biodegradable film is stretched from about 0.3 to about 10 times in the machine direction.

[0013] In another embodiment, the biodegradable film is stretched in the transverse direction by about 0.3 to about 10 times.

[0014] As described in more detail below, certain blown film annealing conditions can provide optimal dimensional stability or minimize shrinkage of poly(hydroxyalkanoate)-based films. According to the present disclosure, applying a heat treatment process relieves the inherent stresses generated during the blown film extrusion process. When properly annealed, poly(hydroxyalkanoate)-based films should not change dimensions over time or when exposed to high temperatures. Thus, embodiments of the present disclosure provide blown film annealing conditions that can result in poly(hydroxyalkanoate)-based films with shrinkage of 4% or less at high temperatures (100°C). Dimensionally stable poly(hydroxyalkanoate)-based films made according to the present disclosure may be particularly useful in the food packaging industry. DETAILED DESCRIPTION OF THE INVENTION

[0015] In one embodiment, the present disclosure provides polymeric film compositions suitable, among other things, for packaging consumer goods.

[0016] Preferably, the polymeric film composition is biodegradable and / or compostable. More particularly, the polymeric film composition is both biodegradable and compostable.

[0017] As used herein, the term "biodegradable" refers to plastic or polymeric materials that will undergo biodegradation by living organisms (microorganisms) in anaerobic and aerobic environments (as specified by ASTM D5511), soil environments (as specified by ASTM 5988), fresh water environments (as specified by ASTM D5271 (EN 29408)), or marine environments (as specified by ASTM D6691). The biodegradability of biodegradable plastics can also be specified using ASTM D6868 and European EN 13432.

[0018] The polymeric film compositions of the present disclosure are preferably "compostable" as specified by ASTM D6400, either for industrial compostability or for home compostability.

[0019] Specifically, the biodegradable polymer film composition includes a poly(hydroxyalkanoate) as the first biodegradable polymer. The composition generally comprises from about 5 weight percent to about 95 weight percent poly(hydroxyalkanoate). More preferably, the composition comprises from about 35 weight percent to about 90 weight percent poly(hydroxyalkanoate). Even more preferably, the polymer composition comprises from about 40 weight percent to about 70 weight percent poly(hydroxyalkanoate).

[0020] In some examples, the poly(hydroxyalkanoate) in the composition preferably comprises a mixture of a first copolymer and a second copolymer. The first copolymer comprises about 90 to about 99.9 mole percent of 3-hydroxybutyrate monomer residues and about 0.1 to about 10 mole percent of a second 3-hydroxyalkanoate monomer residues having 5 to 12 carbon atoms. The second copolymer comprises at least 70 mole percent of 3-hydroxybutyrate monomer residues and an amount of the second 3-hydroxyalkanoate monomer residues having 5 to 12 carbon atoms that is at least 6 mole percent less than the amount of the second 3-hydroxyalkanoate in the first copolymer.

[0021] It has been found that when the difference in the molar percentage of the second 3-hydroxyalkanoate in the first copolymer and the second copolymer is at least 6 mole percent, the first copolymer and the second copolymer are no longer miscible with each other and instead separate into different poly(hydroxyalkanoates) within the composition. Advantageously, this can be exploited to modify the properties of the final composition.

[0022] In such cases, the inclusion of a relatively small amount of the second poly(hydroxyalkanoate) copolymer improves the impact properties or toughness of the composition, providing an article with improved performance.

[0023] The polymer composition also includes a second biodegradable polymer selected from the group consisting of poly(butylene succinate), poly(butylene succinate-co-adipate), poly(lactic acid), cellulose esters (e.g., cellulose acetate), thermoplastic starch, and mixtures thereof. The amount of this second biodegradable polymer is typically from about 5 to about 95 weight percent of the total composition.

[0024] In some embodiments, the second biodegradable polymer can include poly(butylene succinate) in an amount of about 5 weight percent to about 50 weight percent of the polymer composition. More preferably, the polymer composition includes about 10 weight percent to about 30 weight percent poly(butylene succinate).

[0025] According to some embodiments, the second biodegradable polymer can include poly(butylene succinate)-co-butylene adipate in an amount of about 5 weight percent to about 50 weight percent of the polymer composition. More preferably, the polymer composition includes about 10 weight percent to about 30 weight percent of poly(butylene succinate)-co-butylene adipate.

[0026] In some instances, the second biodegradable polymer can comprise poly(lactic acid) in an amount of about 10 weight percent to about 70 weight percent of the polymer composition. More preferably, the polymer composition comprises about 20 weight percent to about 60 weight percent poly(lactic acid).

[0027] In certain embodiments, the second biodegradable polymer can comprise cellulose acetate or another cellulose ester in an amount of about 5 weight percent to about 50 weight percent of the polymer composition. More preferably, the polymer composition comprises about 10 weight percent to about 30 weight percent of cellulose acetate or another cellulose ester.

[0028] In each of the various compositions discussed above, the poly(hydroxyalkanoate) polymer can be a homopolymer, composed of only a single type of monomer residue. Generally, the poly(hydroxyalkanoate) polymer is a copolymer, composed of at least two different types of monomer residues. In some instances, the poly(hydroxyalkanoate) polymer can be a terpolymer, composed of at least three different types of monomer residues.

[0029] For example, in some embodiments, the at least one poly(hydroxyalkanoate) polymer is preferably a copolymer made up of from about 75 to about 99.9 mole percent of 3-hydroxybutyrate monomer residues and from about 0.1 to about 25 mole percent of a second 3-hydroxyalkanoate monomer residues having 5 to 12 carbon atoms.

[0030] In another embodiment, the at least one poly(hydroxyalkanoate) polymer is preferably a terpolymer composed of from about 75 to about 99.9 mole percent of monomer residues of 3-hydroxybutyrate, from about 0.1 to about 25 mole percent of monomer residues of 3-hydroxyhexanoate, and from about 0.1 to about 25 mole percent of monomer residues of a tertiary 3-hydroxyalkanoate having 5 to 12 carbon atoms.

[0031] Generally, the at least one poly(hydroxyalkanoate) polymer has a weight average molecular weight of from about 50,000 daltons to about 7,500,000 daltons, more preferably a weight average molecular weight of from about 300,000 daltons to about 3,000,000 daltons, as determined by ASTM D6474-20.

[0032] In certain embodiments, the poly(hydroxyalkanoate) and at least one biodegradable polymer preferably react with each other via transesterification. In such cases, a small amount of a catalyst (e.g., tin ethylhexanoate) can optionally be incorporated into the composition to promote the transesterification reaction. The transesterification of the poly(hydroxyalkanoate) and at least one biodegradable polymer leads to a branched structure that, for example, results in better processability with less energy consumption. Transesterification can also provide improved physical properties, as the transesterified polymer can act as an interfacial agent and improve the compatibility of other polymer molecules in the composition that have not fully reacted.

[0033] Most commonly, the above-mentioned transesterification reaction is carried out by reacting the poly(hydroxyalkanoate) and at least one biodegradable polymer together in a reactive extrusion process.

[0034] A nucleating agent is also typically present in the polymer composition in an amount of about 0.1 weight percent to about 5 weight percent. In certain embodiments, the core layer nucleating agent is preferably selected from the group consisting of erythritol, pentaerythritol, dipentaerythritol, artificial sweeteners, stearic acid compounds, polysaccharides, sorbitol, mannitol, inositol, polyester waxes, nanoclays, behenamide, erucamide, stearamide, oleamide, polyhydroxybutyrate, thymine, cyanuric acid, cytosine, adenine, uracil, guanine, boron nitride, and mixtures thereof.

[0035] The polymer composition can also include an optional plasticizer material. Suitable materials for the plasticizer are typically selected from the group consisting of sebacic acid compounds, citric acid compounds, fatty esters of adipic acid, fatty esters of succinic acid, and fatty esters of glucaric acid, lactic acid compounds, alkyl diesters, citric acid compounds, alkyl methyl esters, dibenzoic acid compounds, propylene carbonate, caprolactone diol having a number average molecular weight of 200 to 10,000 g / mol as specified by ASTM D6474-20, poly(ethylene glycol) having a number average molecular weight of 400 to 10,000 g / mol as specified by ASTM D6474-20, vegetable oil esters, long-chain alkyl acids, adipic acid compounds, glycerol, isosorbide derivatives, or mixtures thereof, polymeric plasticizers, poly(hydroxyalkanoate) copolymers containing at least 18 mole percent of hydroxyalkanoate monomer residues other than hydroxybutyrate, and mixtures thereof.

[0036] The amount of plasticizer in the polymer composition can be up to about 15 weight percent. More preferably, the polymer composition comprises from about 5 weight percent to about 15 weight percent plasticizer.

[0037] Optionally, the polymer composition can also contain a filler material. Materials suitable for the filler are typically selected from the group consisting of calcium carbonate, talc, nanoclay, nanocellulose, hemp fiber, kaolin, carbon black, wollastonite, glass fiber, carbon fiber, graphite fiber, mica, silica, dolomite, barium sulfate, magnetite, halloysite, zinc oxide, titanium dioxide, montmorillonite, feldspar, asbestos, boron, steel, carbon nanotubes, cellulose fiber, flax, cotton, starch, polysaccharides, aluminum hydroxide, magnesium hydroxide, modified starch, chitin and chitosan, alginate compounds, gluten, zein, casein, collagen, gelatin, polysaccharides, guar gum, xanthan gum, succinoglycan, natural rubber, rosin acid, lignin, natural fibers, jute, kenaf, hemp, ground nut shells, wood flour, and mixtures thereof.

[0038] The amount of filler in the polymer composition can be up to about 50 weight percent. More preferably, the core layer polymer composition comprises from about 5 weight percent to about 40 weight percent filler.

[0039] Additionally, the polymeric composition may also include up to 20 weight percent of an impact modifier. More preferably, the polymeric composition includes from about 5 weight percent to about 15 weight percent of the impact modifier. Impact modifiers suitable for the polymeric composition are preferably selected from the group consisting of acrylic resins and emulsions, isosorbide derivatives, natural rubber, aliphatic polyesters, or mixtures thereof.

[0040] Additionally, in some examples, the polymer composition may comprise up to 50 weight percent of one or more additives selected from the group consisting of poly(vinyl alcohol), poly(vinyl acetate), poly(vinyl laurate), poly(ethylene vinyl acetate), poly(glycolic acid), furandicarboxylic acid-based polyesters, cellulose, nanocellulose, glucan, and mixtures thereof.

[0041] In each of the various polymer compositions discussed above, the poly(hydroxyalkanoate) polymer can be a homopolymer, which is composed of only a single species of monomer residue. Generally, the poly(hydroxyalkanoate) polymer is a copolymer, which is composed of at least two different species of monomer residues. In some instances, the poly(hydroxyalkanoate) polymer can be a terpolymer, which is composed of at least three different species of monomer residues.

[0042] For example, in some embodiments, the at least one poly(hydroxyalkanoate) polymer is preferably a copolymer composed of from about 75 to about 99.9 mole percent of 3-hydroxybutyrate monomer residues and from about 0.1 to about 25 mole percent of a second 3-hydroxyalkanoate monomer residues having 5 to 12 carbon atoms.

[0043] In another embodiment, the at least one poly(hydroxyalkanoate) polymer is preferably a terpolymer composed of from about 75 to about 99.9 mole percent of monomer residues of 3-hydroxybutyrate, from about 0.1 to about 25 mole percent of monomer residues of 3-hydroxyhexanoate, and from about 0.1 to about 25 mole percent of monomer residues of a tertiary 3-hydroxyalkanoate having from 5 to 12 carbon atoms.

[0044] Generally, the at least one poly(hydroxyalkanoate) polymer has a weight average molecular weight of from about 50,000 daltons to about 7,500,000 daltons, more preferably a weight average molecular weight of from about 300,000 daltons to about 3,000,000 daltons, as determined by ASTM D6474-20.

[0045] In a further aspect, the present disclosure also provides product packaging for consumer goods products, which uses the above-described polymer composition. Specifically, the product packaging has at least one biodegradable packaging portion, which portion includes the above-described polymer composition. The product packaging can be used to package clothing, household goods, food, and beauty and health products.

[0046] In certain embodiments, the biodegradable packaging portion can be formed by a method selected from the group consisting of injection molding, compression molding, thermoforming, cast and blow film formation, extrusion coating, extrusion blow molding, injection stretch blow molding, and extrusion profiling.

[0047] Crystallinity The volume percent crystallinity (Φc) of a semi-crystalline polymer (or copolymer) often determines what kind of end-use properties the polymer will retain. For example, highly crystalline (greater than 50%) polyethylene polymers are strong and rigid, making them suitable for products such as plastic milk containers, while low-crystalline polyethylenes are flexible and robust, making them suitable for products such as food wraps and garbage bags. Crystallinity can be determined in a number of ways, including X-ray diffraction, differential scanning calorimetry (DSC), density measurements, and infrared absorption. The optimal method will depend on the material being tested.

[0048] The volume percent crystallinity (Φc) of a poly(hydroxyalkanoate) copolymer can vary depending on the molar percentage of poly(3-hydroxyhexanoate) in the poly(hydroxyalkanoate) copolymer. Addition of poly(3-hydroxyhexanoate) effectively reduces the volume percent crystallinity, crystallization rate, and melting point of the poly(hydroxyalkanoate) copolymer, while increasing the flexibility and degradability of the copolymer. Nucleating agents, as described herein, can be used to accelerate the crystallization process of poly(hydroxyalkanoate) copolymers.

[0049] In general, the poly(hydroxyalkanoates) of the present invention preferably have a crystallinity of from about 0.1% to about 99%, more preferably from about 2% to about 80%, and even more preferably from about 20% to about 70%, as measured by X-ray diffraction.

[0050] When the poly(hydroxyalkanoates) of the present invention are to be processed into molded articles or films, the amount of crystallinity of such poly(hydroxyalkanoates) is more preferably from about 10% to about 80%, more preferably from about 20% to about 70%, and even more preferably from about 30% to about 60%, as measured by X-ray diffraction.

[0051] Melting point Preferably, the biodegradable poly(hydroxyalkanoates) of the present invention have a melting point (T m ) is about 30°C to about 170°C, more preferably about 90°C to about 165°C, and even more preferably about 130°C to about 160°C.

[0052] Film Manufacturing Method The films of the present invention used as container labels with improved biodegradability and / or compostability can be processed using conventional procedures to produce monolayer or multilayer films on conventional film manufacturing equipment. In a film extruder, pellets of the poly(hydroxyalkanoates) of the present invention can be dry-blended and then melt-mixed. Alternatively, if insufficient mixing occurs in the film extruder, the pellets can be dry-blended in a pre-compounding extruder, then melt-mixed, and subsequently re-pelletized before film extrusion.

[0053] The poly(hydroxyalkanoates) of the present invention can be melt-processed into films using either cast or blown film extrusion methods, both of which are described in PLASTICS EXTRUSION TECHNOLOGY—2nd Ed., by Allan A. Griff (Van Nostrand Reinhold—1976). In the cast film process, a molten polymer mixture is extruded through a linear slot die. Typically, a flat web is cooled on a large, moving, polished metal roll. The web is rapidly cooled and peeled from this initial roll, passing through one or more pre-chilling rolls, then through a set of rubber-coated pull or "take-up" rolls, and finally to a winder.

[0054] In the blown film extrusion process, a molten polymer blend is forced upward through a thin, annular die opening. The blown film process is also known as tubular film extrusion. Air is introduced through the center of the die to expand and expand the tube. This creates a moving bubble, which is maintained at a constant size by controlling the internal air pressure. The film tube is cooled by air blown through one or more cooling rings surrounding the tube. The tube is then pulled through a pair of pull rolls into a flattening frame and into a winder, where it is collapsed. For label applications, the flattened tubular film is subsequently split open and slit to widths suitable for label use.

[0055] Both cast and blown film processes can be used to produce either monolayer or multilayer film structures. When producing monolayer films from a single thermoplastic material or a blend of only thermoplastic components, a single screw extruder and a single manifold die are required.

[0056] When producing multilayer films, a coextrusion process is preferably employed, which requires more than one extruder and either a coextrusion feedblock or a multi-manifold die system, or a combination of the two, to obtain the multilayer film structure.

[0057] Quite surprisingly, it has been discovered that the annealing temperature of the annealing roller during blown film and cast film manufacturing processes can control or minimize shrinkage of poly(hydroxyalkanoate)-based polymer films. An improved annealing process for poly(hydroxyalkanoate)-based polymer films can be achieved by heating the film above its glass transition temperature and below its melting point for a certain period of time, and then allowing the film to cool to room temperature to allow the material to relax. Annealing semi-crystalline film materials at relatively high temperatures appears to improve film quality in terms of microscopic and macroscopic structural properties such as crystal morphology, density, and grain size. Therefore, the improved annealing process can provide film dimensional stability and / or minimize shrinkage by relieving intrinsic stresses within the film material. The overall improvement in shrinkage depends on the annealing temperature and annealing time.

[0058] The following non-limiting examples illustrate a process for controlling shrinkage of poly(hydroxyalkanoate)-based polymer films having machine direction orientation (MDO) in the case of a blown film process, and having both machine direction orientation (MDO) and transverse direction orientation (TDO) in the case of a biaxially oriented process. [Example]

[0059] Example 1 The compounded poly(hydroxyalkanoate)-based polymeric material was converted into barrier print films using a conventional blown film process, which were then stretched 3x in the machine direction (MDO). Experiments were conducted using the same extruder and MDO profile at various annealing roller temperatures. Film samples were prepared at annealing roller temperatures of 60°C, 80°C, 100°C, 110°C, and 120°C. Using the method described in ASTM D2732, shrinkage in both the machine direction (MD) and transverse direction (TD) was investigated at 45°C, 80°C, 100°C, and 110°C. The film annealing temperature and shrinkage data at 45°C, 80°C, 100°C, and 110°C are shown in Table 1. Shrinkage measurement temperatures were selected from the storage temperature of 45°C for finished product storage and transport and post-processing temperatures, such as coating, metallization, and sealing temperatures (80°C, 100°C, and 110°C). [Table 1]

[0060] As shown in the table above, poly(hydroxyalkanoate) film shrinkage values ​​were very high compared to the industry standard of less than 5% at low annealing temperatures (60°C). The shrinkage values ​​decreased significantly when annealing temperatures exceeded 100°C, reaching the industry standard of less than 5% at an annealing temperature of 120°C.

[0061] The following non-limiting examples illustrate a process for controlling shrinkage of poly(hydroxyalkanoate)-based polymer films made by a biaxially oriented film process.

[0062] Example 2 Sample poly(hydroxyalkanoate)-based films were prepared using the same extruder and MDO profile at various transverse direction orientation (TDO) annealing temperatures. The transverse direction (TD) annealing temperatures used were 60°C, 93°C, and 129°C. Shrinkage measurement temperatures were selected based on the storage / transport temperature of the finished product (45°C) and the maximum temperatures expected for post-processing processes, such as coating, metallization, and sealing (80°C, 100°C, and 110°C). Both machine direction (MD) and transverse direction (TD) shrinkage were determined by heating the finished film in a 110°C oven for 10 minutes. ISO 14616 was used to determine shrinkage values. The results are shown in the table below. [Table 2]

[0063] Poly(hydroxyalkanoate) film shrinkage values ​​were very high at low annealing temperatures (60°C) compared to the industry standard of less than 5%. Shrinkage values ​​decreased significantly at higher annealing temperatures, reaching the industry standard of less than 5% at an annealing temperature of 129°C.

[0064] Poly(hydroxyalkanoate)-based polymer films have been prepared by modifying poly(hydroxyalkanoate) with melt strength enhancers, chain extenders, and other processing aids. Poly(hydroxyalkanoate)-based films made according to the present disclosure may contain about 50 to 80 weight percent of a poly(hydroxyalkanoate) copolymer and about 20 to about 50 weight percent of a polymeric modifier. In some embodiments, the poly(hydroxyalkanoate) copolymer is poly-3-hydroxybutyrate-co-3-hydroxyhexanoate.

[0065] Exemplary formulations that can be used to make biodegradable films according to the present disclosure are shown in the table below. [Table 3]

[0066] Poly(hydroxyalkanoate) films have been formulated for both oriented and unoriented film production on both cast and blown film lines. Poly(hydroxyalkanoate) films provide excellent barrier for use in packaging applications and high dyne levels for excellent printability for use in labels. Multiple formulations have been tested for the production of poly(hydroxyalkanoate) films, and these formulations can be modified and optimized for specific applications and equipment.

[0067] The foregoing description of preferred embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments have been chosen and described as part of an effort to provide the best explanation of the principles of the invention and its practical application, thereby enabling those skilled in the art to utilize the invention in various embodiments, with various modifications as may be appropriate to the particular use contemplated. All such modifications and variations are within the scope of the present invention, as defined by the appended claims, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Claims

1. 1. A method for improving the dimensional stability of a biodegradable film, comprising: providing a poly(hydroxyalkanoate)-based blown polymer film to a nip roller; feeding the poly(hydroxyalkanoate)-based blown polymer film to an annealing roller; and heating the blown film by contact with the annealing roller at a temperature ranging from about 80 to about 150°C for a predetermined period of time.

2. The method of claim 1, wherein the breathable film is heated to a temperature in the range of about 100 to about 120°C.

3. The method of claim 1 , wherein the breathable film has a thickness ranging from about 7 microns to about 250 microns.

4. The method of claim 1 , wherein the predetermined time period ranges from about 1 second to about 1 minute.

5. The method of claim 1, wherein the poly(hydroxyalkanoate)-based blown polymer film is stretched in the machine direction by about 0.3 to about 10 times.

6. The method of claim 1, wherein the poly(hydroxyalkanoate)-based blown polymer film is stretched in the transverse direction by about 0.3 to about 10 times.

7. A poly(hydroxyalkanoate)-based film made by the method of claim 1.

8. A dimensionally stable, biodegradable packaging film comprising the poly(hydroxyalkanoate)-based film of claim 7.

9. 1. A method for improving the dimensional stability of a biodegradable film, comprising: providing a poly(hydroxyalkanoate)-based biaxially oriented polymer film having a varying machine direction orientation (MDO) and a varying transverse direction orientation (TDO); feeding the biaxially oriented film to an annealing roller; and heating the biaxially oriented film by contact with the annealing roller at a temperature ranging from about 80 to about 150°C for a predetermined period of time.

10. The method of claim 9, wherein the biaxially oriented polymer film is heated to a temperature in the range of about 100 to about 120°C.

11. The method of claim 9, wherein the biaxially oriented polymer film has a thickness ranging from about 7 microns to about 250 microns.

12. The method of claim 7, wherein the predetermined time period ranges from about 1 second to about 1 minute.

13. 10. A biaxially oriented poly(hydroxyalkanoate) based film made by the method of claim 9.

14. 14. A dimensionally stable, biodegradable packaging film comprising the biaxially oriented poly(hydroxyalkanoate)-based film of claim 13.

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