Multilayer Biodegradable Polymer Film Based on Starch and Agricultural By-products and Method for Manufacturing the Same

Abstract

The present invention discloses a multilayer biodegradable polymer film based on starch and agricultural byproducts and a method for manufacturing the same. The disclosed multilayer biodegradable polymer film based on starch and agricultural byproducts and the method for manufacturing the same may include an outer layer formed by a biodegradable resin composed of polylactic acid (PLA) or polybutylene adipate terephthalate (PBAT), an inner layer formed by a composite material in which starch is mixed with an agricultural byproduct comprising one or more of rice bran, chaff, or rice straw, and an adhesive layer formed by a biodegradable reactive resin or polyvinyl alcohol (PVA) and bonding the outer layer and the inner layer. Accordingly, by using starch and agricultural byproducts to bond the outer layer, the inner layer, and the adhesive layer into a multilayer structure, the present invention improves mechanical strength, flexibility, and barrier properties compared to a single-material film and provides an excellent biodegradability effect.

Description

Technology Field

[0001] The present invention relates to a multilayer biodegradable polymer film based on starch and agricultural by-products and a method for manufacturing the same. More specifically, the invention relates to a multilayer biodegradable polymer film based on starch and agricultural by-products having excellent biodegradability and eco-friendliness by forming a multilayer structure using starch, a biodegradable polymer resin, and natural fibrous agricultural by-products, and a method for manufacturing the same. Background Technology

[0002] Generally, polylactic acid (PLA) single-layer films are widely known as eco-friendly packaging materials. PLA is a bio-based polymer derived from renewable plant resources such as corn; it is utilized in food packaging due to its high transparency, gloss, and antibacterial properties. However, PLA single-layer films have limitations, including relatively high brittleness, low impact strength, and poor resistance to moisture and high-humidity environments. Due to these weaknesses, there were performance limitations when using PLA single-layer films for packaging materials requiring higher mechanical strength and durability. Furthermore, due to PLA's inherent high polarity, the effectiveness of conventional antistatic agents is minimal, causing static electricity to easily accumulate on the film surface. Consequently, static electricity phenomena, such as dust adhesion, can occur.

[0003] Meanwhile, polyethylene (PE) films, widely used in packaging plastic bags and agricultural mulching films, possess excellent mechanical strength and water resistance but are non-biodegradable, causing serious environmental problems. It has been reported that when used PE mulching films remain in the soil, they do not decompose for hundreds of years in the form of microplastics, contaminating the soil, and the disposal of such waste films poses a significant burden. Therefore, the recovery and disposal of PE-based films after use are essential, and due to their limitations in terms of sustainability, the development of eco-friendly materials to replace them is required.

[0004] In addition, research has been conducted on biodegradable composite films incorporating naturally derived agricultural byproducts such as starch, rice bran, rice straw, and rice husk. Starch-based films have the advantages of excellent biodegradability and low raw material costs, but generally have disadvantages such as low mechanical strength, poor transparency, and weak water resistance. Similarly, while improvements in tensile strength and other properties have been reported when natural fibrous materials are used as reinforcing materials, there is a concern that the uniformity and moisture resistance of the film may deteriorate if appropriate dispersion and interfacial treatment technologies are not employed.

[0005] In other words, conventional plastic films, particularly polyethylene (PE) films, are made from petroleum, take a long time to decompose in the natural environment, and remain as microplastics upon disposal, causing serious environmental problems. Prior art literature

[0006] Republic of Korea Registered Patent No. 10-1062012 (Registration Date: 2011.09.05) Republic of Korea Registered Patent No. 10-1000749 (Registration Date: 2010.12.13) Republic of Korea Registered Patent No. 10-2150566 (Registration Date: 2020.09.02) Republic of Korea Registered Patent No. 10-1823409 (Registration Date: 2018.01.31) Republic of Korea Registered Patent No. 10-1443020 (Registration Date: 2014.09.16) The problem to be solved

[0007] The present invention was created to solve the problems of the prior art described above, and aims to provide a multilayer biodegradable polymer film based on starch and agricultural by-products and a method for manufacturing the same, which has improved mechanical strength, flexibility, and barrier properties compared to a single-material film, as well as excellent biodegradability and eco-friendliness, by mutually bonding an outer layer, an inner layer, and an adhesive layer in a multilayer structure using starch and agricultural by-products.

[0008] In addition, the present invention has another objective of providing a multilayer biodegradable polymer film based on starch and agricultural by-products and a method for manufacturing the same, which includes rice bran, rice straw, and rice husk in the inner layer and adhesive layer to prevent a decrease in tensile strength and stiffness while promoting decomposition upon disposal, and capable of antibacterial properties, moisture resistance, antistatic properties, and rapid composting required for packaging materials. means of solving the problem

[0010] To achieve the above objectives, a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one aspect of the present invention may comprise an outer layer formed by a biodegradable resin composed of polylactic acid (PLA) or polybutylene adipate terephthalate (PBAT), an inner layer formed by a composite material in which starch is mixed with an agricultural byproduct comprising one or more of rice bran, chaff, or rice straw, and an adhesive layer formed by a biodegradable reactive resin or polyvinyl alcohol (PVA) and bonding the outer layer and the inner layer.

[0011] In addition, the present invention is characterized in that the thickness ratio of the outer layer, inner layer, and adhesive layer is 2 to 4: 5 to 7: 1, and the total thickness is 75 μm to 85 μm.

[0012] In addition, the inner layer in the present invention is characterized by comprising 70 parts by weight of a thermoplastic starch resin formed by mixing glycerin with starch and 30 parts by weight of an agricultural byproduct powder.

[0013] In addition, the thermoplastic starch resin in the present invention is characterized by being formed by adding a glycerin plasticizer to starch formed from one or more of corn, potatoes, or rice.

[0014] In addition, the agricultural byproduct in the present invention is characterized by being ground to a powder particle size of 50 to 125 μm.

[0015] In addition, the adhesive layer in the present invention is characterized by comprising one or more of the following: a polyurethane-based or epoxy-based biodegradable adhesive, a PLA or PBAT copolymer grafted with maleic anhydride (MA), or a reactive resin chemically modified by adding a multifunctional carboxylic acid to a starch resin.

[0016] In addition, the adhesive layer in the present invention is characterized by adding 0.51% by weight of an eco-friendly isocyanate-based binder, such as polymethylene diisocyanate (pMDI), based on the total weight.

[0017] In addition, to achieve the above objectives, a method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural byproducts according to another aspect of the present invention comprises: (a) a step of forming an outer layer by melt-molding a biodegradable resin such as polylactic acid (PLA) or PBAT; (b) a step of preparing a composition for forming an inner layer by mixing agricultural byproducts such as rice bran, rice straw, and rice husk in the form of fine powder with a starch-based resin and forming an inner layer; (c) a step of preparing a biodegradable adhesive resin (tie layer) to be located between the outer layer and the inner layer and forming an adhesive layer; (d) a step of forming an integral multilayer film by simultaneously laminating the compositions of the outer layer, the inner layer, and the adhesive layer through a co-extrusion or lamination process; and (e) a step of cooling the formed multilayer film and, if necessary, winding it to obtain a final product.

[0018] In addition, in the above invention, step (a) is characterized by drying and grinding the agricultural byproduct and mixing it. Effects of the invention

[0020] As described above, the starch and agricultural byproduct-based multilayer biodegradable polymer film and the method for manufacturing the same according to one aspect of the present invention, unlike the prior art, utilizes starch and agricultural byproducts to interconnect an outer layer, an inner layer, and an adhesive layer in a multilayer structure, thereby improving mechanical strength, flexibility, and barrier properties compared to a single-material film, and possessing excellent biodegradability and eco-friendly effects.

[0021] In addition, according to the starch and agricultural byproduct-based multilayer biodegradable polymer film and the method for manufacturing the same, mechanical properties such as tensile strength and elongation are improved due to the multilayer structure including PLA and PBAT, thereby enhancing strength and toughness, and has the effect of increasing oxygen and moisture barrier performance required for packaging materials.

[0022] In addition, according to the starch and agricultural byproduct-based multilayer biodegradable polymer film and the method for manufacturing the same, since the outer layer, inner layer, and adhesive layer are made of biodegradable materials, biodegradation can proceed effectively when discarded, making it eco-friendly and reducing environmental pollution. Brief explanation of the drawing

[0024] FIG. 1 is a schematic diagram illustrating a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention. FIG. 3 is a block diagram illustrating a method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention. FIG. 4 is a schematic diagram showing an example of use of a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention. FIG. 5 is a schematic diagram showing the operation of a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention. Specific details for implementing the invention

[0025] Hereinafter, preferred embodiments of a multilayer biodegradable polymer film based on starch and agricultural byproducts and a method for manufacturing the same according to the present invention will be described with reference to the attached drawings. In this process, the thickness of lines or the size of components shown in the drawings may be exaggerated for clarity and convenience of explanation.

[0026] Furthermore, the terms described below are defined in consideration of their functions in the present invention, and these may vary depending on the intent or practice of the user or operator. Therefore, the definitions of these terms should be based on the content throughout this specification.

[0027] Furthermore, the terms used in this specification are used merely to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, terms such as "comprising" or "having" are intended to indicate the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof. Terms such as "first," "second," etc., may be used to describe various components, but the components should not be limited by these terms. These terms are used solely for the purpose of distinguishing one component from another.

[0028] FIG. 1 is a schematic diagram illustrating a multilayer biodegradable polymer film based on starch and agricultural by-products according to one embodiment of the present invention, FIG. 2 is a cross-sectional view illustrating a multilayer biodegradable polymer film based on starch and agricultural by-products according to one embodiment of the present invention, and FIG. 3 is a block diagram illustrating a method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural by-products according to one embodiment of the present invention.

[0029] In addition, FIG. 4 is a schematic diagram showing an example of use of a multilayer biodegradable polymer film based on starch and agricultural by-products according to one embodiment of the present invention, and FIG. 5 is a schematic diagram showing the operation of a multilayer biodegradable polymer film based on starch and agricultural by-products according to one embodiment of the present invention.

[0030] With reference to FIGS. 1 to 5, a multilayer biodegradable polymer film based on starch and agricultural byproducts and a method for manufacturing the same according to one aspect of the present invention will be described.

[0031] The multilayer biodegradable polymer film (F) based on starch and agricultural byproducts according to the present embodiment may include an outer layer (10), an inner layer (20), and an adhesive layer (30), and the outer layer (10) and the inner layer (20) may be bonded together via the adhesive layer (30) to form a single integral.

[0032] The outer layer (10) is the outermost layer of the film and is formed by polylactic acid (PLA) or polybutylene adipate terephthalate (PBAT) alone or in a mixture thereof. This outer layer (10) prevents moisture and gas from passing through the film and can increase the strength of the entire film at the same time.

[0033] In addition, additives such as UV blockers or antioxidants may be included as needed, and it is desirable to form them so that they account for about 20 to 40 percent of the total thickness of the film.

[0034] The inner layer (20) is an inner layer of a film that is composed mainly of a thermoplastic starch resin made of corn starch, etc., and contains about 5 to 50 weight percent, for example, about 30 weight percent, of fine powder of agricultural by-products such as rice bran, rice straw, and rice husk in the total inner layer. The inner layer (20) occupies about 50 to 70 percent of the total thickness of the film, providing biodegradability and contributing to cost reduction at the same time. In addition, the natural components contained in the inner layer (20) are rapidly decomposed by microorganisms after disposal, which has the effect of increasing the biodegradation rate of the entire film.

[0035] Additionally, the inner layer (20) can be ground to a range of 100 to 500 μm, as agricultural by-products affect the physical properties of the film. For example, the rice husk powder can be used with a particle size of about 120 to 250 mesh (about 50 to 125 μm). The inner layer (20) occupies about 50 to 70% of the total thickness of the film, providing biodegradability and contributing to cost reduction at the same time. Furthermore, the natural components contained in the inner layer are rapidly decomposed by microorganisms after disposal, which has the effect of increasing the biodegradation rate of the entire film.

[0036] The adhesive layer (30) is a thin intermediate layer interposed between the outer layer (10) and the inner layer (20) to firmly bond the two layers. The adhesive layer (30) may use one or more of the following: a polyurethane-based or epoxy-based biodegradable adhesive, a PLA or PBAT copolymer grafted with maleic anhydride (MA), or a reactive resin chemically modified by adding a multifunctional carboxylic acid to a starch resin.

[0037] Additionally, an eco-friendly isocyanate-based binder, such as polymethylene diisocyanate (pMDI), may be added in an amount of about 0.51% by weight relative to the total weight of the adhesive layer (30). Although the adhesive layer (30) is formed as a very thin layer of less than about 5% of the total thickness of the film (F), it plays an important role in maintaining the structural integrity of the film by effectively preventing delamination between the outer layer (10) and the inner layer (20).

[0038] The multilayer biodegradable polymer film (F) based on starch and agricultural byproducts according to the present invention, configured as described above, provides excellent protective performance (blocking moisture and oxygen, ensuring strength) for the contents due to the outer layer (10), enables rapid decomposition after use due to the filling of natural materials in the inner layer (20), and maintains structural stability through the adhesive layer (30).

[0039] Therefore, when the product of the present invention is used as a packaging film or a disposable plastic bag, it can replace existing non-biodegradable composite films and perform a similar level of function, while being completely decomposed upon disposal, thereby effectively reducing environmental pollution.

[0040] A method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural byproducts according to one embodiment of the present invention as described above will be explained.

[0041] A method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural byproducts according to the present embodiment may include: (a) a step of forming an outer layer by melt-molding a biodegradable resin such as polylactic acid (PLA) or PBAT; (b) a step of preparing a composition for forming an inner layer by mixing agricultural byproducts such as rice bran, rice straw, and rice husk in the form of fine powder into a starch-based resin; (c) a step of preparing a biodegradable adhesive resin (tie layer) to be located between the outer layer and the inner layer; (d) a step of forming an integral multilayer film by simultaneously laminating the compositions of the outer layer, inner layer, and adhesive layer through a co-extrusion or lamination process; and (e) a step of cooling the formed multilayer film and, if necessary, winding it to obtain a final product.

[0042] Specifically, first, a biodegradable resin such as polylactic acid (PLA) or polybutylene adipate terephthalate (PBAT) is prepared to form the outer layer of the film (F) (step a). Then, an agricultural byproduct such as rice bran, rice straw, and rice husk in the form of fine powder is mixed with a starch-based resin to prepare a composition for forming the inner layer (step b).

[0043] Next, a biodegradable adhesive resin, i.e., a tie layer, is prepared for bonding between the outer layer and the inner layer (Step c). The prepared compositions of the outer layer, inner layer, and adhesive layer are simultaneously extruded through a co-extrusion process to form an integral multilayer film (Step d). The formed film can be cooled and wound to obtain the final product, or it can be manufactured through a lamination process if necessary (Step e).

[0044] In addition, in the step of preparing the inner layer composition (step b), agricultural by-products are dried and ground in advance and used, thereby optimizing the microstructure of the inner layer and the mechanical and biodegradable properties of the film. The film produced by this process integrates the outer layer, inner layer, and adhesive layer, enabling the simultaneous attainment of excellent barrier properties, mechanical strength, and rapid biodegradability, which are difficult to achieve with single-layer films.

[0045] As such, the multilayer biodegradable polymer film based on starch and agricultural by-products produced by the manufacturing method of the present invention can be realized as an eco-friendly film that exhibits excellent moisture and oxygen barrier properties and high mechanical strength, while rapidly decomposing in natural environments such as soil after use. The core technical features of the present invention are reflected in each manufacturing step.

[0046] For example, in the outer layer formation step (step a), polylactic acid (PLA) or PBAT is used to impart high barrier properties and mechanical strength to the film, and in the inner layer preparation step (step b), agricultural by-products in the form of fine powder are added to a starch-based resin to improve the biodegradation rate and reduce costs. Additionally, in the adhesive layer preparation step (step c), properties are realized that prevent delamination between the outer and inner layers and ensure interlayer integrity to maintain structural stability.

[0047] A final multilayer film is formed through co-extrusion (step d) and cooling and winding (step e), and the physical properties and range of applications of the film can be controlled by adding a lamination process as needed. Films manufactured by this process configuration can simultaneously achieve superior barrier properties, mechanical strength, and rapid biodegradability compared to existing single-layer or non-degradable composite films.

[0048] Meanwhile, the outer layer (10) formed in step (a) forms the outermost surface of the film and is composed of a biodegradable resin, such as polylactic acid (PLA) or polybutylene adipate-co-terephthalate (PBAT), in a single or mixed form. PLA has high tensile strength and transparency but is vulnerable to impact due to its high hardness, while PBAT has excellent flexibility and elongation but has relatively low moisture and oxygen barrier properties. Therefore, it is desirable to select either PLA or PBAT or blend them in an appropriate ratio to use as the outer layer material depending on the application of the film.

[0049] For example, in applications requiring high oxygen and moisture barrier properties, such as food packaging, PLA is primarily used, and PBAT can be mixed in as needed to supplement impact durability. Conversely, in applications where flexibility is important and long-term storage of contents is not a major concern, such as biodegradable garbage bags, the material can be applied by increasing the PBAT content and adding a small amount of PLA to ensure basic strength.

[0050] The main function of the outer layer (10) is to provide moisture and oxygen barrier properties and secure mechanical strength, while simultaneously providing printability and processing stability. In particular, applying PLA to the outer layer can significantly reduce the moisture permeability of the inner layer starch film, and according to the literature, a moisture barrier effect of more than 1300% and improved oxygen barrier properties can be expected compared to a single starch film. In addition, since the outer layer is directly exposed to the external environment (sunlight, oxygen, etc.) during use and protects the inner layer, stabilizing additives such as UV blockers can be included as needed to prevent deterioration of the inner layer during outdoor storage and extend the film life.

[0051] Preferably, the thickness of the outer layer is formed to be about 20 to 40% of the total film thickness, thereby ensuring excellent barrier properties and mechanical strength while suppressing unnecessary cost increases.

[0052] The inner layer (20) formed in step (b) forms an inner layer or an intermediate layer of the film and is formed with a structure in which fine powder of agricultural by-products such as rice bran, rice straw, and rice husk is filled, with the main component being a starch-based resin. The starch-based resin is a thermoplastic starch (TPS) resin manufactured by adding a plasticizer such as glycerin to starch such as corn, potato, and rice.

[0053] Since agricultural byproduct powders affect the physical properties of the film, they are generally ground to about 50–125 μm or less for use; in the case of rice husk powder, sieving it to about 120–250 mesh (about 50–125 μm) allows for a uniform surface. The filler content can be adjusted to about 5–50 weight% of the total inner layer; if the content is low, the biodegradation-promoting effect is negligible, and if it is excessively high, the mixing and co-extrusion processes become difficult. For example, rice husk powder can achieve a balance of biodegradation promotion and manufacturing stability in the range of 20–50%, while rice bran powder is effective in the range of about 5–15%.

[0054] When natural byproduct powder is included in the inner layer (20), it is preferentially decomposed by microorganisms during composting after use, exhibiting a synergistic effect that accelerates the decomposition rate of the entire film. However, since the addition of natural fillers may partially reduce mechanical properties such as tensile strength and elongation, it is desirable to optimize the type of powder, particle size, and content according to the application. If necessary, PLA or PBAT may be mixed into the inner layer resin to reinforce mechanical strength and reduce the moisture sensitivity of starch, and hydrophilic plasticizers such as sorbitol or compatibility-enhancing additives may be used together with glycerin.

[0055] The inner layer (20) accounts for about 50-70% of the total film thickness, and increases economic efficiency by using low-cost materials and provides additional effects such as acting as nutrients in the soil after film decomposition to promote microbial activity.

[0056] The adhesive layer (30) formed in step (c) is a thin intermediate layer interposed between the outer layer (10) and the inner layer (20) to firmly bond the two layers. As the adhesive layer material, polyurethane-based or epoxy-based biodegradable adhesives, or tie layer resins in which reactive functional groups are introduced into a polymer, etc., may be used. In a preferred embodiment, if a PLA or PBAT copolymer grafted with maleic anhydride (MA) is used as the adhesive layer material, the compatibility and adhesion between the outer layer and the inner layer are improved.

[0057] In addition, it is also possible to increase adhesion by chemically modifying the inner layer starch resin. For example, if a small amount of multifunctional carboxylic acid (MA) is added to the inner layer resin, chemical bonds such as ester bonds are formed at the interface with the outer layer, which can effectively prevent delamination. Along with this, if an eco-friendly isocyanate-based binder such as polymethylene diisocyanate (pMDI) is added at about 0.51 weight%, chemical crosslinking is formed at the polymer interfaces that have low compatibility with each other, maximizing adhesion, improving the tensile strength of the film, and suppressing layer separation.

[0058] The adhesive layer (30) is very thin, less than about 5% of the total thickness of the film, but it integrally combines the outer layer and the inner layer to maintain stable behavior during handling and use. A multilayer structure including such an adhesive layer has significantly higher interfacial adhesion strength compared to the case without an intermediate layer, providing stable physical properties without interlayer delamination during actual processing and use.

[0059] In step (d), a co-extrusion process for forming a multilayer film can be carried out. For co-extrusion, at least three extruders are used, including one for the outer layer, one for the inner layer, and one for the adhesive layer, and each extruder controls the temperature and pressure conditions individually according to the type of resin fed in.

[0060] Extruder for outer layer: Feeds PLA or PBAT resin (a mixture of the two resins if necessary) and heats and melts it. Extruder for inner layer: Heats and melts a starch-based compound resin mixed with starch, agricultural byproduct powder, plasticizers, etc. Extruder for adhesive layer: Feeds the prepared adhesive layer material (e.g., MA-modified PLA resin) in a molten state.

[0061] Each molten resin is simultaneously introduced through a T-die or a slot die to form a molten resin film with a three-layer structure. Inside the die, an adhesive layer material is injected between the outer layer (outer) and the inner layer (inner) to create a triple-layered structure. Afterward, when the molten resin film is extruded from the die exit in the form of a flat film, it immediately passes through a cooling roller or a water-cooled bath, where the temperature is rapidly lowered and the film solidifies.

[0062] The cooled film is continuously wound onto a winding roll to be recovered as a final roll product, or, if necessary, can be cut into appropriate sizes to be produced in the form of a sheet. In the co-extrusion process, the relative thickness of each layer is adjusted according to the application. The outer layer is formed to be approximately 20–40% of the total thickness to provide barrier properties and tensile strength, while the inner layer is composed to be relatively thick, at approximately 50–70%, to reduce costs and improve biodegradability. The adhesive layer is distributed very thinly at approximately 5% or less, but is supplied uniformly to ensure sufficient interlayer strength.

[0063] As the three layers are bonded together in a molten state, a multilayer film with excellent interlayer adhesion can be obtained without a separate bonding process. The co-extrusion process has the advantage of high productivity and uniform control of layer thickness, as it can manufacture a multilayer film in a single continuous process.

[0064] The present invention will be explained in more detail below through the following examples. However, the following examples are merely illustrative of the present invention and do not limit the scope of the present invention.

[0065] After preparing the films of the examples and comparative examples, the biodegradation rate, mechanical properties (strength), and barrier performance were evaluated. The biodegradation rate test was conducted under industrial composting conditions (temperature 58°C, humidity 90%, etc.) in accordance with ASTM D6400 standards, and the process involved measuring the mass loss rate of the film after a certain period. Mechanical properties were evaluated by measuring tensile strength and elongation using a universal testing machine and assessing interlayer adhesion strength (peel strength). Barrier performance was compared by measuring the water vapor evaporation rate (WVTR) and oxygen permeability (OTR) of the film.

[0067] <Example 1>

[0068] In the multilayer film according to the present invention, the outer layer is composed of 100% PLA, and the inner layer is formed with a composition in which 30% by weight of rice husk powder is mixed into a starch / glycerin matrix. A PLA-based resin with maleic anhydride adhesive functional groups introduced was used as the adhesive layer. A three-layer film with a total thickness of approximately 80 μm was manufactured through a co-extrusion process, and the thickness ratio of the outer layer:inner layer:adhesive layer was adjusted to approximately 3:6:1.

[0070] <Comparative Example 1>

[0071] The composition of the outer layer and adhesive layer was the same as in Example 1, but a three-layer film was prepared composed solely of a pure starch-based resin without adding natural byproduct powder to the inner layer (i.e., inner layer filler content 0%). The total thickness and the ratio of each layer were maintained identically to the example.

[0073] <Comparative Example 2>

[0074] A single-layer film was prepared using a composition containing 30% rice husk powder in a starch composite material for the inner layer. That is, as a comparative example consisting only of a single layer of starch film without an outer layer and an adhesive layer, the thickness was formed to a level of 80 μm, similar to the total thickness of Example 1. This comparative example is intended to evaluate the film performance in the case where there is no outer layer in the multilayer structure of the present invention.

[0076] <Experimental Example 1> Biodegradation Rate Test

[0077] The biodegradation rates of the films in Example 1 and Comparative Example were evaluated under industrial composting conditions (temperature 58°C, relative humidity 90% RH). Each film specimen was stored under the corresponding conditions according to ASTM D6400 standards, and the remaining mass ratio was measured at 4, 8, and 12 weeks.

[0078] [Table 1] presents the residual mass ratios for each film. The film in Example 1 showed rapid degradation, with a mass reduction of approximately 60% from the initial mass (residual mass approximately 40%) after 4 weeks, and after 12 weeks, the residual mass decreased to less than 5%, indicating over 90% degradation. In contrast, the film in Comparative Example 1, which did not have a filler added to the inner layer, showed a mass reduction of only about 30% after 12 weeks, with approximately 70% of the initial mass remaining in film form. Additionally, in the case of Comparative Example 2 (a starch monolayer film without an outer layer), the initial degradation rate was faster due to the absence of a PLA outer layer, but it showed a significant decrease in the mechanical strength and barrier performance of the film.

[0079] Meanwhile, differences in results were also confirmed in natural soil burial tests (room temperature outdoor environment 20–30°C). The film of Example 1 was almost completely decomposed after 6 months of burial in soil, to the point where it was difficult to find any residue with the naked eye, whereas the PLA / starch 3-layer film of Comparative Example 1 remained largely intact even after 6 months. Through this, the effect of enhancing the biodegradation rate by adding agricultural byproduct powder (rice husk) to the inner layer was proven even in a natural environment.

[0080] Sample Remaining mass (%) after 4 weeks Remaining mass (%) after 8 weeks Remaining mass (%) after 12 weeks Example 1 (film of the present invention, 3 layers) 40% 15% < 5 (almost decomposed) Comparative Example 1 (3 layers without filler) 95% 85% 70% Comparative Example 2 (Starch monolayer film) 30% 10% ~ 0 (complete decomposition)

[0082] <Experimental Example 2> Evaluation of Mechanical Properties

[0083] The mechanical properties of each film were evaluated using a universal testing machine (UTM). Tensile strength was measured by pulling the specimen in the mechanical direction (MD) and transverse direction (TD) according to standard test methods until fracture, and interlayer adhesion strength was evaluated by a peel test in which two layers were separated by 180°.

[0084] As shown in the mechanical property comparison results of [Table 2], the tensile strength of the three-layer film of the present invention (Example 1) was measured to be approximately 25 MPa in the MD direction and approximately 27 MPa in the TD direction. This is about 80-85% of the tensile strength of a single-layer PLA film of the same thickness (Comparative Example 3, assumed value), demonstrating that practical strength is maintained due to the reinforcing effect of the outer PLA layer even when a filler is added to the inner layer. On the other hand, the starch single-layer film without an outer layer (Comparative Example 2) showed very low tensile strength of 10 MPa or less in all directions, confirming that the PLA outer layer is essential for film strength in a multilayer structure.

[0085] In addition, the interlayer adhesion of the film in Example 1 was excellent, as shown in the 180° peel test, to the extent that no layer separation occurred even under a load of 5 N or more per inch (25.4 mm) width. However, in the case of a two-layer film simply laminated without an adhesive layer, layer separation occurred easily even under a small force of less than 0.5 N, clearly demonstrating the effect of introducing an adhesive layer.

[0086] Sample Tensile strength (MD direction, MPa) Tensile strength (TD direction, MPa) Interlayer adhesion strength 180° peel, N / inch) Example 1 (Three-layer film of the present invention) 25 27 > 5 (no layer separation) Comparative Example 2 (Starch monolayer film) < 10 < 10 - (Single-layer structure) (Note) Non-adhesive 2-layer film - - < 0.5 (separates easily) Comparative Example 3 (PLA monolayer film, assumption) ~30 ~32 - (Single-layer structure)

[0088] <Experimental Example 3> Evaluation of Barrier Properties

[0089] To verify the moisture and gas barrier performance of the film of the present invention, water vapor evaporation rate (WVTR) and oxygen permeability (OTR) tests were conducted. WVTR measured the amount of moisture (g / m²·day) evaporating through a film of a certain thickness in an environment of 40°C and 90% relative humidity, and OTR measured the amount of oxygen gas (cc / m²·day) passing through the film under conditions of 23°C.

[0090] As can be seen from the results in [Table 3], the film of Example 1 demonstrated excellent barrier properties against moisture and oxygen compared to the starch monolayer film (Comparative Example 2). The WVTR of Example 1 was approximately 38 g / m²·day, which was only about 1 / 13 of the WVTR of Comparative Example 2 (approx. 500 g / m²·day). Similarly, regarding oxygen barrier properties, the OTR of Example 1 was approximately 80 cc / m²·day, which was more than 10 times lower than that of the film of Comparative Example 2 (approx. 800 cc / m²·day). Due to these dramatic improvements in moisture and oxygen barrier properties, it was confirmed that packaging contents such as food with the film of Example 1 extended the shelf life by 2 to 3 times compared to the comparative film.

[0091] Sample WVTR (40℃, 90%RH,g / ㎡·day) OTR (23℃,cc / ㎡·day) Example 1 (PLA / starch 3-layer film) 38 80 Comparative Example 2 (Starch monolayer film) 500 800

[0093] In summary, the experimental results above showed that the multilayer biodegradable film according to the present invention exhibited superior or equivalent performance compared to conventional films in all aspects, including biodegradation rate, mechanical strength, and barrier properties. In particular, a synergistic effect was confirmed in which the decomposition rate was significantly improved by adding agricultural byproduct powder (rice husk) to the inner layer, and moisture and oxygen barrier properties were greatly enhanced by introducing PLA to the outer layer. These results demonstrate the eco-friendliness of the film of the present invention, as it enhances the storage stability of contents while rapidly decomposing in the environment after disposal.

[0094] The multilayer biodegradable polymer film based on starch and agricultural by-products and the method for manufacturing the same, configured as described above, minimizes environmental burden by enabling complete biodegradation upon disposal after use, as all layers are made of biodegradable materials. In particular, it is eco-friendly and distinctly different from existing fossil fuel-based plastic films because it completely decomposes in nature without leaving any residues such as microplastics.

[0095] In addition, the multilayer structure design of the present invention overcomes the limitations of single-material films. For example, while conventional PLA single-layer films decompose under industrial composting conditions, their decomposition rate is very slow in natural environments at room temperature, and their low moisture and oxygen barrier properties have limited their use as food packaging materials. On the other hand, the film of the present invention has a structure in which a PLA outer layer and a starch / fiber inner layer are combined, so moisture and gas barrier properties are significantly improved, allowing the freshness retention period (shelf life) of the contents to be extended by more than 2 to 3 times compared to existing starch-based films. In fact, it was confirmed that when the outer layer is composed of PLA, the rate of moisture evaporation is significantly reduced, decreasing to a level of less than 1 / 10 compared to a pure starch film, which corresponds to a performance improvement of more than 1000% in terms of moisture barrier properties.

[0096] Furthermore, the structure of the present invention, in which natural fiber powder is filled into the inner layer, accelerates the decomposition rate and significantly shortens the survival period in the environment after disposal. While pure PLA film takes several months to completely decompose under industrial composting conditions and can remain in natural soil for several years or more, the film of the present invention decomposed at a rate approximately twice as fast under the same conditions, and it was confirmed that a significant portion decomposed within one year even when buried in natural soil. In particular, when approximately 30% of agricultural byproduct powder was included in the inner layer, the biodegradability was significantly improved compared to the case without agricultural byproducts under the same composition, and it was confirmed that there was extremely little film residue after a certain period. This is believed to be because natural organic matter is consumed first by microorganisms, which finely decomposes the film structure and consequently accelerates the decomposition of polymer parts such as PLA.

[0097] For example, the multilayer structure according to the present invention simultaneously achieves three effects—high barrier properties, sufficient mechanical strength, and rapid biodegradation—thereby securing superior performance and differentiation compared to existing biodegradable films or composite material films. Furthermore, by optimizing the combination of the outer layer and the inner layer, it provides physical durability capable of withstanding high-temperature and high-pressure processes (HPP, etc.) used in food sterile packaging, and by designing all components in an eco-friendly manner, it provides an eco-friendly packaging solution throughout the entire lifecycle, from the product usage stage to the final disposal stage after disposal. This technical excellence is demonstrated by specific numerical values ​​through the embodiments described below.

[0098] Although the present invention has been described with reference to the embodiments illustrated in the drawings, this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the true technical scope of protection of the present invention should be determined by the claims below. Explanation of the symbols

[0099] 10 : Outer layer 20 : Inner layer 30 : Adhesive layer

Claims

Claim 1 An outer layer formed by a biodegradable resin composed of polylactic acid (PLA) or polybutylene adipate terephthalate (PBAT); an inner layer formed by a composite material in which starch is mixed with an agricultural byproduct comprising one or more of rice bran, chaff, or rice straw; and an adhesive layer formed by a biodegradable reactive resin or polyvinyl alcohol (PVA) and bonding the outer layer and the inner layer; wherein the thickness ratio of the outer layer, the inner layer, and the adhesive layer is 2 to 4 : 5 to 7 : 1 and the total thickness is 75 μm to 85 μm; the inner layer comprises 70 parts by weight of a thermoplastic starch resin formed by mixing glycerin with starch and 30 parts by weight of agricultural byproduct powder; the thermoplastic starch resin is formed by adding a glycerin plasticizer to starch formed from one or more of corn, potatoes, or rice; the agricultural byproduct is ground to a powder particle size of 50 to 125 μm; and the adhesive layer comprises one or more of a polyurethane-based or epoxy-based biodegradable adhesive, a PLA or PBAT copolymer grafted with maleic anhydride (MA), or a reactive resin chemically modified by adding a multifunctional carboxylic acid to the starch resin. A starch and agricultural byproduct-based multilayer biodegradable polymer film characterized by including, and adding 0.5 to 1 weight percent of an eco-friendly isocyanate-based binder such as polymethylene diisocyanate (pMDI) to the total weight of the adhesive layer. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 A method for manufacturing a multilayer biodegradable polymer film based on starch and agricultural by-products according to claim 1, comprising: (a) a step of forming an outer layer by melt-molding a biodegradable resin such as polylactic acid (PLA) or PBAT; (b) a step of preparing a composition for forming an inner layer by mixing agricultural by-products such as rice bran, rice straw, and rice husk in the form of fine powder with a starch-based resin and forming an inner layer; (c) a step of preparing a biodegradable adhesive resin (tie layer) to be located between the outer layer and the inner layer and forming an adhesive layer; (d) a step of forming an integral multilayer film by simultaneously laminating the compositions of the outer layer, inner layer, and adhesive layer through a co-extrusion or lamination process; and (e) a step of cooling the formed multilayer film and winding it as needed to obtain a final product; wherein, in step (b), the agricultural by-products are dried and ground before use.

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