A toughened heat-resistant fully biodegradable polylactic acid packaging material and a preparation method thereof

Abstract

The present application relates to a kind of toughened heat-resistant full biodegradable polylactic acid packaging material and its preparation method.Polylactic acid, PBAT, corn starch, stearic acid starch ester, mannitol, epoxy-terminated PBAT-PLA block oligomer, PDLA grafted white carbon black and processing aid are dried, premixed plasticized, melt reaction blending, extruded and granulated and formed, to obtain packaging material.Epoxy-terminated PBAT-PLA block oligomer improves interface compatibility and toughening, PDLA grafted white carbon black promotes crystallization and stereocomplex crystal formation, and both synergistically improve material toughness, impact resistance, heat resistance and hot water dimensional stability.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid, and particularly to toughened and heat-resistant fully biodegradable polylactic acid packaging materials and their preparation methods. Background Technology

[0002] With the widespread use of plastic packaging products, traditional petroleum-based plastics are difficult to degrade naturally after use, easily causing white pollution and resource waste. Fully biodegradable materials, which can be broken down by microorganisms into carbon dioxide, water, and biomass under certain conditions, have become an important development direction in the packaging materials field. Among them, polylactic acid (PLA), an aliphatic polyester derived from renewable resources, possesses good transparency, rigidity, biocompatibility, and biodegradability, and can be used in packaging products such as food bags, shopping bags, sheets, blister trays, lunch boxes, and cup lids.

[0003] However, polylactic acid (PLA) also has significant drawbacks. PLA molecules are highly rigid, resulting in low elongation at break and poor impact resistance. This makes it prone to brittleness, cracking, and breakage when used in the production of films, sheets, or thermoformed products. Furthermore, ordinary PLA crystallizes slowly, exhibiting low crystallinity under conventional processing conditions and typically low heat distortion temperature. This makes it susceptible to softening, warping, or dimensional deformation under conditions of hot water, hot food, hot filling, or high-temperature storage and transportation. Therefore, simultaneously improving the toughness and heat resistance of PLA packaging materials is a key issue that needs to be addressed for its further application in the packaging field.

[0004] In existing technologies, to improve the toughness of polylactic acid (PLA), it is typically blended with flexible biodegradable polyesters such as PBAT, PBS, and PCL. These flexible polyesters can improve the elongation at break and impact resistance of the material to some extent. However, the compatibility between PLA and flexible polyesters is limited. Direct blending easily leads to the formation of large dispersed phase particles and weak interfacial bonding, resulting in decreased material strength, performance fluctuations, and insufficient processing stability. Furthermore, the large addition of flexible polyesters may reduce the material's rigidity and heat resistance dimensional stability, making the material prone to deformation even under high-temperature operating conditions, despite improved toughness.

[0005] To reduce costs and increase bio-based content, existing technologies often incorporate starch, modified starch, and polyol plasticizers into polylactic acid (PLA) or PLA / PBAT systems. Starch has advantages such as wide availability, low price, and good biodegradability; however, its molecular structure contains numerous hydroxyl groups, making it highly hydrophilic and exhibiting poor compatibility with hydrophobic polyester matrices. If the starch-based components are unevenly dispersed or have insufficient interfacial bonding, defects can easily form within the material, leading to a decrease in tensile strength, impact toughness, water resistance, and heat stability. Therefore, starch-filled fully biodegradable packaging materials require further improvement in the interfacial compatibility between the starch phase and the polyester phase.

[0006] To improve the heat resistance of polylactic acid (PLA), existing technologies often use inorganic fillers such as talc, calcium carbonate, silica, and montmorillonite as nucleating agents to promote PLA crystallization. This method can improve the crystallization rate and thermal stability of PLA to some extent, but common inorganic fillers have strong surface polarity and are prone to agglomeration in multiphase systems such as PLA / PBAT, resulting in limited nucleation efficiency and a decrease in material toughness. Simply adding common inorganic nucleating agents such as silica usually makes it difficult to simultaneously achieve high toughness and high heat resistance.

[0007] In addition, some technologies incorporate PDLA to form stereocomposite crystals with PLLA, thereby improving the heat resistance of polylactic acid (PLA) materials. Stereocomposite crystals exhibit high thermal stability, which is beneficial for improving the heat distortion temperature and dimensional stability of PLA materials. However, if PDLA is added to the system solely as a conventional resin or through physical mixing, its dispersion and localization in the multiphase blend are difficult to control, hindering its full nucleation and stereocomposite effects. Furthermore, without an effective interfacial compatibilizing structure, the addition of heat-resistant reinforcing components may cause the material to become brittle, affecting the actual performance of the packaging material.

[0008] Therefore, existing polylactic acid (PLA) packaging material modification technologies still have the following shortcomings: when using flexible polyesters such as PBAT alone for toughening, the improvement in heat resistance is limited or even decreases; when using nucleating fillers such as silica alone to improve heat resistance, it is easy to cause filler agglomeration and reduced toughness; when using PDLA alone to form stereocomposite crystals, its dispersion stability and interfacial positioning are insufficient; the interfacial bonding between the starch-based filler phase and the polyester matrix is ​​weak, which easily affects the overall performance of the material. Current technologies lack a method for preparing PLA packaging materials that can simultaneously achieve the synergistic effects of reaction compatibilization, flexibility toughening, heat-resistant nucleation, and stereocomposite crystallization in a fully biodegradable system.

[0009] Based on this, it is necessary to develop a fully biodegradable polylactic acid (PLA) packaging material with dual additives for synergistic toughening and heat resistance, and its preparation method. By constructing a synergistic effect between reactive flexible toughening additives and heat-resistant nucleating additives, the interfacial bonding and phase dispersion stability of the PLA / PBAT / starch system can be improved, while promoting PLA crystallization and stereocomposite crystal formation. This results in a fully biodegradable PLA packaging material with good toughness, impact resistance, heat resistance, and hot water dimensional stability. Summary of the Invention

[0010] In view of the above analysis, the present invention aims to provide a toughened and heat-resistant fully biodegradable polylactic acid packaging material and its preparation method to solve the problems in the background art.

[0011] The objective of this invention is mainly achieved through the following technical solutions: The present invention provides a method for preparing a toughened and heat-resistant fully biodegradable polylactic acid packaging material, comprising the following steps: S1. Polylactic acid, PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica, mannitol, and processing aids are dried separately. S2. Corn starch, stearic acid starch ester and mannitol are premixed and plasticized to obtain a starch-based plasticized filler phase; S3. Polylactic acid, PBAT, starch-based plasticizing filler phase, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica and processing aids are added to a twin-screw extruder for melt reaction and blending to obtain a modified polylactic acid composite. S4. The modified polylactic acid composite is extruded and granulated to obtain fully biodegradable polylactic acid modified particles; S5. The fully biodegradable polylactic acid modified particles are blown into film, cast, extruded into sheet, or vacuum-formed to obtain fully biodegradable polylactic acid packaging materials. In this process, the epoxy-terminated PBAT-PLA block oligomer serves as a reactive flexible toughening additive, and the PDLA-grafted silica serves as a heat-resistant nucleating additive. During the melt reaction blending process, the epoxy groups of the epoxy-terminated PBAT-PLA block oligomer react with the carboxyl or hydroxyl groups at the ends of the polylactic acid chain, the hydroxyl groups in the starch-based plasticizing filler phase, and the hydroxyl groups on the surface of the PDLA-grafted silica, causing the PDLA-grafted silica to be distributed in the interface region between the polylactic acid matrix and the PBAT flexible phase. At the same time, the PDLA segments in the PDLA-grafted silica and the PLLA segments in the polylactic acid form stereocomposite crystals, thereby achieving a synergistic improvement in the toughness and heat resistance of the packaging material.

[0012] In one or more embodiments, in step S3, the raw materials are as follows by mass: 35-55 parts polylactic acid, 25-45 parts PBAT, 20-40 parts corn starch, 8-20 parts stearic acid starch ester, 10-35 parts mannitol, 4-10 parts epoxy-terminated PBAT-PLA block oligomer, 1-4 parts PDLA-grafted silica, and 1-8 parts processing aids.

[0013] In one or more embodiments, in step S3, the raw materials are as follows by mass: 45 parts polylactic acid, 35 parts PBAT, 30 parts corn starch, 12 parts stearic acid starch ester, 20 parts mannitol, 6 parts epoxy-terminated PBAT-PLA block oligomer, 2 parts PDLA-grafted silica, and 4 parts processing aids.

[0014] In one or more embodiments, the number average molecular weight of the epoxy-terminated PBAT-PLA block oligomer is 3000-20000 g / mol, the epoxy value is 0.05-0.50 mol / 100g, and the mass ratio of PBAT flexible segments to PLA compatible segments is 60-90:10-40.

[0015] In one or more embodiments, the PDLA-grafted silica is prepared by the following method: First, the surface of silica is activated or aminated with silane coupling agent. Then, the treated silica is used as the initiator to induce D-lactide to undergo ring-opening polymerization under the action of a catalyst, and PDLA segments are grafted onto the surface of silica to obtain PDLA-grafted silica. The grafting rate of PDLA segments in the PDLA-grafted silica is 10-45%.

[0016] In one or more embodiments, the processing aids include diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidants, and calcium oxide.

[0017] In one or more embodiments, the processing aid comprises, by weight: 0.5–3 parts of diphenylmethane diisocyanate, 0.5–2 parts of ethylene bis-stearamide, 0.2–1 part of erucamide, 0.1–0.5 parts of antioxidant 168, 0.1–0.5 parts of hindered phenolic antioxidant, and 0.2–1 parts of calcium oxide.

[0018] In one or more embodiments, in step S2, corn starch, stearic acid starch ester and mannitol are premixed at 60-100°C for 5-30 minutes to allow mannitol to enter between corn starch and stearic acid starch ester to form a plasticizing system; in step S3, the melt reaction blending temperature is 145-185°C, the screw speed is 120-300 rpm, and the vacuum degree is -0.04 to -0.09 MPa.

[0019] In one or more embodiments, step S5 is followed by a heat setting process, wherein the heat setting temperature is 85–115°C and the heat setting time is 20–150 s; during the heat setting process, PDLA-grafted silica induces the crystallization of the polylactic acid matrix and promotes the formation of stereocomposite crystals between PLLA segments and PDLA segments.

[0020] In a second aspect, the present invention provides a toughened and heat-resistant fully biodegradable polylactic acid (PLA) packaging material, wherein PBAT forms a flexible toughening phase, epoxy-terminated PBAT-PLA block oligomers are located between the PLA matrix and the PBAT flexible toughening phase and form an interfacial reaction connection, and PDLA-grafted silica is distributed in the interfacial region between the PLA matrix and the PBAT flexible toughening phase.

[0021] The principle of this invention: This invention addresses the problems of insufficient toughness, low heat resistance, and poor interfacial compatibility of polylactic acid (PLA) packaging materials, particularly the PLA / PBAT / starch multiphase system. It constructs a dual-additive synergistic modification system composed of epoxy-terminated PBAT-PLA block oligomers and PDLA-grafted silica. The epoxy-terminated PBAT-PLA block oligomers act as a reactive, flexible, and toughening additive, while the PDLA-grafted silica acts as a heat-resistant nucleating additive. Both work together during melt-blending and heat-setting processes to simultaneously improve the toughness and heat resistance of fully biodegradable PLA packaging materials.

[0022] The epoxy-terminated PBAT-PLA block oligomer contains flexible PBAT segments, compatible PLA segments, and epoxy reactive end groups. The flexible PBAT segments form a flexible energy-dissipating structure within the polylactic acid (PLA) matrix, inducing shear yielding, crack deflection, and energy dissipation under external forces, thereby improving the material's elongation at break and impact toughness. The compatible PLA segments enhance the compatibility between the oligomer and the PLA matrix, reducing the interfacial tension between the PLA matrix and the flexible PBAT phase, and promoting the refinement and uniform dispersion of the flexible PBAT phase. The epoxy reactive end groups can undergo ring-opening reactions with the carboxyl or hydroxyl groups at the PLA chain ends, the hydroxyl groups in the starch-based plasticized filler phase, and the hydroxyl groups on the surface of PDLA-grafted silica, thus forming an interfacial bridging structure between the PLA matrix, the flexible PBAT phase, the starch-based plasticized filler phase, and the PDLA-grafted silica.

[0023] In PDLA-grafted silica, silica acts as an inorganic nucleation core, providing heterogeneous nucleation sites, lowering the nucleation energy barrier for polylactic acid (PLA) crystallization, and promoting rapid crystallization of PLA during heat setting. The PDLA segments grafted onto the silica surface interact with PLLA segments in PLA to form stereocomposite crystals. These stereocomposite crystals exhibit high thermal stability, which is beneficial for improving the material's heat distortion temperature and dimensional stability in hot water. Compared to ordinary silica, PDLA-grafted silica not only leverages the heterogeneous nucleation effect of inorganic particles but also induces the formation of PLLA / PDLA stereocomposite crystals through PDLA segments, while simultaneously improving the dispersion stability of silica in the PLA system.

[0024] In this invention, the two additives do not simply work in parallel, but rather synergistically in the PLA / PBAT interface region. The epoxy-terminated PBAT-PLA block oligomer reacts with the hydroxyl groups on the surface of PDLA-grafted silica through its reactive end groups, resulting in a more stable distribution of PDLA-grafted silica at the interface between the polylactic acid matrix and the flexible PBAT phase. The PDLA-grafted silica located at the interface further stabilizes and refines the flexible PBAT phase, and also induces polylactic acid crystallization and stereocomposite crystal formation at the interface between the polylactic acid matrix and the flexible PBAT phase. Thus, the system simultaneously achieves flexible phase toughening, interfacial reaction compatibility, heterogeneous nucleation, and stereocomposite heat-resistant crystallization, overcoming the problems of decreased heat resistance after toughening of ordinary polylactic acid materials, insufficient toughness after heat-resistant enhancement, and weak interfacial bonding in multiphase blend systems.

[0025] The technical solution of this invention can achieve at least one of the following effects: Compared with existing technologies, this invention introduces epoxy-terminated PBAT-PLA block oligomers and PDLA-grafted silica into a PLA / PBAT / starch-based fully biodegradable system, constructing a dual-additive synergistic modification system that combines reaction compatibilization, flexibility enhancement, heat-resistant nucleation, and stereocomposite crystallization. This effectively overcomes the problems of insufficient toughness, low heat resistance, and weak interfacial bonding in ordinary polylactic acid packaging materials.

[0026] In this invention, the epoxy-terminated PBAT-PLA block oligomer not only functions as a flexible toughening component, but the PBAT flexible segments in its molecular structure can improve the energy dissipation capacity of the material during stress, while the PLA segments can improve its compatibility with the polylactic acid matrix. Furthermore, the epoxy end groups can react with the carboxyl or hydroxyl groups at the polylactic acid chain ends, the hydroxyl groups in the starch-based plasticized filler phase, and the hydroxyl groups on the surface of PDLA-grafted silica. Therefore, this oligomer can form an interfacial bridging structure between the polylactic acid matrix, the PBAT flexible phase, the starch-based plasticized filler phase, and the PDLA-grafted silica, reducing interfacial defects and making the PBAT flexible phase more uniformly dispersed, thereby improving the elongation at break and impact resistance of the material.

[0027] Meanwhile, PDLA-grafted silica combines the heterogeneous nucleation effect of silica with the stereocomplexation effect of PDLA segments in a single additive structure. The silica core provides nucleation sites for polylactic acid (PLA) crystallization, promoting rapid crystallization during heat setting. The PDLA segments grafted onto the silica surface can form stereocomplex crystals with the PLLA segments in PLA, improving the thermal stability and crystallization perfection of the crystals. Compared with ordinary silica, PDLA-grafted silica not only promotes crystallization but also induces the formation of stereocomplex crystals, thereby lowering the cold crystallization temperature, increasing crystallinity, raising the heat distortion temperature, and improving the dimensional stability after hot water treatment.

[0028] More importantly, the two additives in this invention are not simply superimposed, but rather form a synergistic effect at the PLA / PBAT interface. The epoxy-terminated PBAT-PLA block oligomer enhances the interfacial bonding between the polylactic acid matrix and the PBAT flexible phase through reactive end groups, and can stabilize PDLA-grafted silica in this interfacial region. The PDLA-grafted silica located at the interface further exerts nucleation and stereocomplexation induction effects, enabling the refinement of the flexible phase, enhanced interfacial bonding, and improved heat resistance of polylactic acid crystallization to occur simultaneously. Thus, this invention avoids the problems of "simple toughening leading to decreased heat resistance" or "simple heat resistance enhancement leading to brittleness" in traditional polylactic acid materials, achieving a simultaneous improvement in toughness and heat resistance.

[0029] Furthermore, this invention uses corn starch, starch stearate, and mannitol to form a starch-based plasticized filler phase. While increasing the bio-based content of the material and reducing costs, the starch stearate improves the affinity between the starch phase and the polyester phase. Additionally, the epoxy-terminated PBAT-PLA block oligomer interacts with the hydroxyl groups in the starch-based component, improving the dispersion stability of the starch phase in the PLA / PBAT system. This reduces interfacial defects and hydrothermal performance degradation caused by starch filling, enabling the material to maintain its fully biodegradable characteristics while also possessing good mechanical properties, processing performance, and service stability.

[0030] Through the aforementioned synergistic effects, the fully biodegradable polylactic acid packaging material obtained by this invention exhibits high elongation at break, notched impact strength, heat distortion temperature, and dimensional stability in hot water. After treatment with 90°C hot water, it is less prone to significant softening, curling, warping, or dimensional changes, thus better meeting the packaging requirements for hot meals, hot water, hot filling, and high-temperature storage and transportation conditions. This material can be prepared using conventional plastic processing techniques such as drying, premixing and plasticizing, twin-screw melt reaction blending, extrusion granulation, blown film, casting, sheet extrusion, or thermoforming. The process offers good continuity and strong equipment adaptability, making it suitable for the large-scale production of fully biodegradable packaging products such as food packaging bags, shopping bags, blister trays, lunch boxes, cup lids, sheets, films, or disposable packaging containers.

[0031] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0032] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0033] Figure 1 This is a schematic diagram of the preparation process. Detailed Implementation

[0034] In the following embodiments, unless otherwise stated, all raw materials used are commercially available; the listed brands or models are merely optional examples, and those skilled in the art can also select equivalent products that meet the same technical specifications.

[0035] The polylactic acid is an extrusion-grade L-lactic acid resin, such as NatureWorks 4032D or TotalEnergies Corbion Luminy LX175, with an L-lactic acid unit content of not less than 96%, a melt flow rate of 3-8 g / 10 min, test conditions of 190℃ and 2.16 kg, a number-average molecular weight of 80,000-150,000 g / mol, and a moisture content of not more than 500 ppm after drying.

[0036] The PBAT is film-grade or extrusion-grade polybutylene terephthalate (PET), and can be from Kingfa Science & Technology, Lanshan Tunhe, or BASF's Ecoflex series. Its melt flow rate is 2-6 g / 10 min, the test conditions are 190℃ and 2.16 kg, the tensile elongation at break is not less than 500%, and the moisture content after drying is not higher than 500 ppm.

[0037] The corn starch is industrial-grade or food-contact grade corn starch, with an amylose content of 20-30%, a particle size D50 of 10-25 μm, and a moisture content not exceeding 14%. Before use, it is dried at 80℃ for 4 hours to reduce its moisture content to below 5%. The stearic acid starch ester is obtained by esterification modification of starch, with a degree of substitution of 0.02-0.12 and a moisture content not exceeding 5%, which can improve the affinity between the starch phase and the polyester phase. The mannitol is an industrial-grade or food-contact grade product with a purity of not less than 98%, a melting point of 165-170℃, and a moisture content not exceeding 1%.

[0038] The epoxy-terminated PBAT-PLA block oligomer can be obtained commercially available custom-made or prepared using conventional polyester oligomer end-group epoxidation methods. Its number-average molecular weight is 3000–20000 g / mol, preferably 8000–15000 g / mol; epoxy value is 0.05–0.50 mol / 100g, preferably 0.15–0.25 mol / 100g; and the mass ratio of PBAT flexible segments to PLA compatible segments is 60–90:10–40, preferably 70–85:15–30. This oligomer should have good melt flowability at 180°C and be able to react with the terminal carboxyl or hydroxyl groups of polylactic acid, starch hydroxyl groups, and PDLA-grafted silica surface hydroxyl groups during melt blending.

[0039] The silica is either fumed silica or precipitated silica, with a silica content of not less than 98%, an average particle size of 10–50 nm, a specific surface area of ​​100–300 m² / g, and hydroxyl groups on its surface. In preparing PDLA-grafted silica, the silica is first surface-aminated using the silane coupling agent KH-550. Then, using the aminated silica as the initiator, D-lactide undergoes ring-opening polymerization under stannous octoate catalysis, grafting PDLA segments onto the silica surface. The purity of the D-lactide used is not less than 98%, and the purity of the stannous octoate used is not less than 95%. The grafting rate of PDLA segments in the obtained PDLA-grafted silica is 10–45%, preferably 15–35%. The grafted product is used after washing, filtering, and vacuum drying, and is dried at 60°C for 4 hours before use.

[0040] The diphenylmethane diisocyanate is industrial-grade MDI with an isocyanate group content of 28-33% and a purity of not less than 98%. It should be stored in a sealed container to prevent water absorption and loss of efficacy. In this invention, the diphenylmethane diisocyanate is used as a chain extender and interface strengthening agent. During melt reaction blending, it can react with the terminal hydroxyl or carboxyl groups of polylactic acid, the hydroxyl groups in the starch-based plasticizing filler phase, and other components containing active hydrogen groups, thereby improving the melt strength and interfacial bonding performance of the system. After melt reaction blending, vacuum degassing, and subsequent extrusion molding, the diphenylmethane diisocyanate does not exist in the resulting packaging material in the form of free isocyanate.

[0041] The ethylene bis-stearamide is an industrial-grade lubricating dispersant with a melting point of 140–150°C and an acid value not exceeding 10 mg KOH / g. The erucamide is an industrial-grade slip agent with a purity of not less than 95% and a melting point of 75–85°C. Antioxidant 168 is a phosphite antioxidant with a purity of not less than 98%; the hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] with a purity of not less than 98%. The calcium oxide is an industrial-grade or analytical-grade powder with a calcium oxide content of not less than 90% and a particle size not exceeding 10 μm. It should be sealed and dried before use to absorb trace amounts of moisture in the system.

[0042] All of the above raw materials should be dried before use, so that the moisture content of polylactic acid, PBAT, epoxy-terminated PBAT-PLA block oligomer and PDLA grafted silica is not higher than 500 ppm, and the moisture content of corn starch, stearic acid starch ester and mannitol is not higher than 5%.

[0043] Preparation Example 1: Preparation of epoxy-terminated PBAT-PLA block oligomers This preparation example provides a method for preparing epoxy-terminated PBAT-PLA block oligomers.

[0044] By weight, 100 parts of hydroxyl-terminated PBAT-PLA block oligomer were added to a drying reactor and heated to 90°C under nitrogen protection, and stirred until the material was completely melted. Then, 12 parts of epichlorohydrin and 0.4 parts of tetrabutylammonium bromide were added, and the mixture was stirred at 80-90°C for 4 hours. Then, 3 parts of anhydrous potassium carbonate were added, and the reaction was continued for 2 hours to allow the terminal hydroxyl groups of the hydroxyl-terminated PBAT-PLA block oligomer to undergo glycidyl etherification, introducing epoxy end groups.

[0045] After the reaction was completed, the reaction product was cooled to room temperature and washed twice with anhydrous ethanol to remove unreacted epichlorohydrin and small molecule byproducts. Then it was washed with deionized water until the washing solution was neutral, and washed once more with anhydrous ethanol. After filtration, it was vacuum dried at 60°C for 12 hours to obtain epoxy-terminated PBAT-PLA block oligomers.

[0046] The resulting epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 12000 g / mol and an epoxy value of 0.18 mol / 100 g, with a mass ratio of PBAT flexible segments to PLA compatible segments of 75:25. During melt blending, this oligomer can react with the terminal carboxyl or hydroxyl groups of polylactic acid, the hydroxyl groups in the starch-based plasticizing filler phase, and the hydroxyl groups, silanol groups, or amino groups on the surface of PDLA-grafted silica, thereby improving the interfacial compatibility and toughness of the PLA / PBAT / starch system.

[0047] In other embodiments, the number average molecular weight of the resulting epoxy-terminated PBAT-PLA block oligomer can be adjusted to be 3000–20000 g / mol, the epoxy value to be 0.05–0.50 mol / 100 g, and the mass ratio of PBAT flexible segments to PLA compatible segments to be 60–90:10–40 by adjusting the molecular weight of the hydroxyl-terminated PBAT-PLA block oligomer, the ratio of PBAT flexible segments to PLA compatible segments to be 60–90:10–40.

[0048] Preparation Example 2: Preparation of PDLA-grafted silica This preparation example provides a method for preparing PDLA-grafted silica.

[0049] Ten parts by weight of silica were placed in a vacuum drying oven and dried at 110°C for 4 hours to remove surface-adsorbed moisture. The dried silica was then added to 200 parts by weight of an ethanol / water mixture (ethanol to water mass ratio 95:5). Subsequently, 1.5 parts by weight of silane coupling agent KH-550 were added, and the pH of the system was adjusted to 4.5–5.5 with glacial acetic acid. The mixture was stirred at 70°C for 3 hours to allow KH-550 to hydrolyze and condense with the silanol groups on the silica surface. After the reaction, the solid was collected by filtration and washed sequentially with ethanol and deionized water, three times with each solvent, to remove unreacted silane coupling agent and byproducts. The resulting solid was then vacuum dried at 80°C for 8 hours to obtain aminated silica.

[0050] By weight, 10 parts of the above-mentioned aminated silica, 25 parts of D-lactide, and 0.15 parts of stannous octoate were added to a drying reactor. The reactor was heated to 130°C under nitrogen protection and stirred for 6 hours to allow D-lactide to undergo ring-opening polymerization on the surface of the aminated silica, forming PDLA grafted segments. After the reaction, the product was cooled to room temperature, washed twice with chloroform to remove unreacted D-lactide and free PDLA, washed twice with anhydrous ethanol, filtered, and then vacuum dried at 60°C for 12 hours to obtain PDLA-grafted silica.

[0051] The grafting rate of PDLA segments in the obtained PDLA-grafted silica was 25%. This grafting rate was determined by thermogravimetric analysis and calculated based on the proportion of organic components losing thermal weight in the grafted product. The obtained PDLA-grafted silica was dried at 60°C for 4 hours before use to ensure its moisture content did not exceed 500 ppm. This PDLA-grafted silica can provide heterogeneous nucleation sites in the polylactic acid (PLA) system and form stereocomposite crystals with PLLA segments in PLA through PDLA segments, thereby improving the material's crystallinity and thermal dimensional stability.

[0052] In other embodiments, the grafting rate of PDLA segments in PDLA-grafted silica can be 10-45% by adjusting the amount of silica, D-lactide and stannous octoate added.

[0053] If the indicators meet the above range, they can be used to implement this invention.

[0054] Example 1 This embodiment provides a method for preparing a toughened and heat-resistant fully biodegradable polylactic acid packaging material.

[0055] Weigh out 45 parts by weight of polylactic acid, 35 parts of PBAT, 30 parts of corn starch, 12 parts of stearic acid starch ester, 20 parts of mannitol, 6 parts of epoxy-terminated PBAT-PLA block oligomer, 2 parts of PDLA-grafted silica, and 4 parts of processing aids.

[0056] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.5 parts, ethylene bis-stearamide 1.0 part, erucamide 0.5 parts, antioxidant 168 0.25 parts, hindered phenolic antioxidant 0.25 parts, and calcium oxide 0.5 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0057] The epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 12000 g / mol and an epoxy value of 0.18 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 75:25. The grafting rate of PDLA segments in the PDLA-grafted silica is 25%.

[0058] The epoxy-terminated PBAT-PLA block oligomer was prepared according to the method of Preparation Example 1, with a number-average molecular weight of 12000 g / mol and an epoxy value of 0.18 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments was 75:25. The PDLA-grafted silica was prepared according to the method of Preparation Example 2, wherein the grafting rate of PDLA segments was 25%.

[0059] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica were dried at 60°C for 4 hours, and processing aids were dried at 50°C for 2 hours.

[0060] 30 parts of dried corn starch, 12 parts of stearic acid starch ester and 20 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 85°C for 15 minutes to allow mannitol to enter between corn starch and stearic acid starch ester to form a plasticized system, thus obtaining a starch-based plasticized filler phase.

[0061] Subsequently, 45 parts of polylactic acid (PLA), 35 parts of PBAT, the obtained starch-based plasticized filler phase, 6 parts of epoxy-terminated PBAT-PLA block oligomer, 2 parts of PDLA-grafted silica, and 4 parts of processing aids were added to a twin-screw extruder for melt reaction blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: Zone 1 145℃, Zone 2 155℃, Zone 3 165℃, Zone 4 175℃, Zone 5 180℃, Zone 6 180℃, and Die Head 175℃; the screw speed was 200 rpm, and the vacuum degree of the vacuum exhaust section was -0.07 MPa. After melt reaction blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable PLA-modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0062] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 160–180°C, resulting in a sheet thickness of 0.35 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 100°C for 90 seconds. After cooling, a toughened and heat-resistant fully biodegradable polylactic acid packaging material was obtained.

[0063] In the above preparation process, epoxy-terminated PBAT-PLA block oligomers, as reactive flexible toughening additives, can react with the carboxyl or hydroxyl groups at the ends of polylactic acid chains, the hydroxyl groups in the starch-based plasticizing filler phase, and the hydroxyl groups on the surface of PDLA-grafted silica; PDLA-grafted silica, as a heat-resistant nucleating additive, can form stereocomposite crystals with the PLLA segments in polylactic acid through its PDLA segments.

[0064] Example 2 This embodiment provides a method for preparing a toughened, heat-resistant, fully biodegradable polylactic acid packaging material.

[0065] Weigh out 52 parts by weight of polylactic acid, 28 parts of PBAT, 25 parts of corn starch, 10 parts of stearic acid starch ester, 15 parts of mannitol, 5 parts of epoxy-terminated PBAT-PLA block oligomer, 3.5 parts of PDLA-grafted silica, and 3.5 parts of processing aids.

[0066] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.2 parts, ethylene bis-stearamide 0.8 parts, erucamide 0.4 parts, antioxidant 168 0.2 parts, hindered phenolic antioxidant 0.2 parts, and calcium oxide 0.7 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0067] The epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 15000 g / mol and an epoxy value of 0.15 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 70:30. The grafting rate of PDLA segments in the PDLA-grafted silica is 35%.

[0068] The epoxy-terminated PBAT-PLA block oligomer can be prepared according to the method of Preparation Example 1, with a number-average molecular weight of 15000 g / mol and an epoxy value of 0.15 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 70:30. The PDLA-grafted silica can be prepared according to the method of Preparation Example 2, wherein the grafting rate of PDLA segments is 35%.

[0069] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica were dried at 60°C for 4 hours, and processing aids were dried at 50°C for 2 hours.

[0070] 25 parts of dried corn starch, 10 parts of stearic acid starch ester and 15 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 90°C for 20 minutes to allow mannitol to enter between corn starch and stearic acid starch ester to form a plasticized system, thus obtaining a starch-based plasticized filler phase.

[0071] Subsequently, 52 parts of polylactic acid, 28 parts of PBAT, the obtained starch-based plasticized filler phase, 5 parts of epoxy-terminated PBAT-PLA block oligomer, 3.5 parts of PDLA-grafted silica, and 3.5 parts of processing aids were added to a twin-screw extruder for melt reaction blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: zone 1 150℃, zone 2 160℃, zone 3 170℃, zone 4 178℃, zone 5 182℃, zone 6 182℃, and die head 180℃; the screw speed was 180 rpm, and the vacuum degree of the vacuum exhaust section was -0.08 MPa. After melt reaction blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0072] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 165–180°C, resulting in a sheet thickness of 0.45 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 110°C for 120 seconds. After cooling, a fully biodegradable polylactic acid packaging material with synergistic toughening and heat resistance from two additives was obtained.

[0073] In the above preparation process, a high content of polylactic acid (PLA) serves as the continuous phase, providing material strength and rigidity. PBAT forms a flexible and toughening phase. Epoxy-terminated PBAT-PLA block oligomers are located between the PLA matrix and the PBAT flexible phase and are connected through an interfacial reaction via epoxy groups. PDLA-grafted silica is distributed in the interfacial region between the PLA matrix and the PBAT flexible phase. Its PDLA segments and PLLA segments in PLA form stereocomposite crystals, which further induce PLA crystallization during the heat setting process, thereby improving the heat distortion temperature and hot water dimensional stability of the resulting packaging material while maintaining good toughness.

[0074] Example 3 This embodiment provides a method for preparing a toughened, heat-resistant, fully biodegradable polylactic acid packaging material.

[0075] Weigh out 38 parts by weight of polylactic acid, 42 parts of PBAT, 35 parts of corn starch, 15 parts of stearic acid starch ester, 30 parts of mannitol, 8 parts of epoxy-terminated PBAT-PLA block oligomer, 1.5 parts of PDLA-grafted silica, and 5 parts of processing aids.

[0076] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: 2.0 parts diphenylmethane diisocyanate, 1.5 parts ethylene bis-stearamide, 0.8 parts erucamide, 0.25 parts antioxidant 168, 0.25 parts hindered phenolic antioxidant, and 0.2 parts calcium oxide. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0077] The epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 8000 g / mol and an epoxy value of 0.25 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 85:15. The grafting rate of PDLA segments in the PDLA-grafted silica is 15%.

[0078] The epoxy-terminated PBAT-PLA block oligomer can be prepared according to the method of Preparation Example 1, with a number-average molecular weight of 8000 g / mol and an epoxy value of 0.25 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 85:15. The PDLA-grafted silica can be prepared according to the method of Preparation Example 2, wherein the grafting rate of PDLA segments is 15%.

[0079] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica were dried at 60°C for 4 hours, and processing aids were dried at 50°C for 2 hours.

[0080] 35 parts of dried corn starch, 15 parts of stearic acid starch ester and 30 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 75°C for 10 minutes to allow mannitol to enter between corn starch and stearic acid starch ester to form a plasticized system, thus obtaining a starch-based plasticized filler phase.

[0081] Subsequently, 38 parts of polylactic acid, 42 parts of PBAT, the obtained starch-based plasticized filler phase, 8 parts of epoxy-terminated PBAT-PLA block oligomer, 1.5 parts of PDLA-grafted silica, and 5 parts of processing aids were added to a twin-screw extruder for melt reaction blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: zone 1 145℃, zone 2 150℃, zone 3 160℃, zone 4 168℃, zone 5 175℃, zone 6 175℃, and die head 170℃; the screw speed was 240 rpm, and the vacuum degree of the vacuum exhaust section was -0.06 MPa. After melt reaction blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0082] The obtained fully biodegradable polylactic acid modified particles were added to a blown film machine for blown film forming. The blown film temperature was 155–175℃, the blow-up ratio was 2.5:1, and the draw ratio was 3.0:1, resulting in a film thickness of 0.04 mm. Subsequently, the film was heat-set at 90℃ for 60 seconds, and after cooling, a fully biodegradable polylactic acid packaging material with synergistic toughening and heat resistance from two additives was obtained.

[0083] In the above preparation process, the higher content of PBAT and mannitol improves the flexibility and film forming adaptability of the material. The PBAT flexible segments in the epoxy-terminated PBAT-PLA block oligomer have good compatibility with the PBAT flexible phase, and the PLA segments have good compatibility with the polylactic acid matrix. The epoxy groups can react with the carboxyl or hydroxyl groups at the ends of the polylactic acid chains, the hydroxyl groups in the starch-based plasticized filler phase, and the hydroxyl groups on the surface of PDLA-grafted silica, thereby enhancing the interfacial bonding between the polylactic acid matrix, the PBAT flexible phase, and the PDLA-grafted silica. The PDLA-grafted silica is distributed in the interfacial region between the polylactic acid matrix and the PBAT flexible phase. Its PDLA segments form stereocomposite crystals with the PLLA segments in the polylactic acid and induce the crystallization of the polylactic acid matrix during heat setting. This allows the resulting film packaging material to maintain high elongation at break and tear resistance while exhibiting superior heat resistance and dimensional stability compared to ordinary PLA / PBAT films.

[0084] Comparative Example 1 This comparative example provides a method for preparing a basic fully biodegradable polylactic acid packaging material, which differs from Example 1 in that it does not include epoxy-terminated PBAT-PLA block oligomers and PDLA-grafted silica.

[0085] Weigh out 45 parts polylactic acid, 35 parts PBAT, 30 parts corn starch, 12 parts stearic acid starch ester, 20 parts mannitol, and 4 parts processing aids by weight.

[0086] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.5 parts, ethylene bis-stearamide 1.0 part, erucamide 0.5 parts, antioxidant 168 0.25 parts, hindered phenolic antioxidant 0.25 parts, and calcium oxide 0.5 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0087] In the preparation process, polylactic acid, PBAT, corn starch, stearic acid starch ester, mannitol, and processing aids were dried separately. Specifically, polylactic acid and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, and processing aids were dried at 50°C for 2 hours.

[0088] 30 parts of dried corn starch, 12 parts of stearic acid starch ester and 20 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 85°C for 15 min to obtain a starch-based plasticized filler phase.

[0089] Subsequently, 45 parts of polylactic acid, 35 parts of PBAT, the obtained starch-based plasticizing filler phase, and 4 parts of processing aids were added to a twin-screw extruder for melt blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: zone 1 145℃, zone 2 155℃, zone 3 165℃, zone 4 175℃, zone 5 180℃, zone 6 180℃, and die head 175℃; the screw speed was 200 rpm, and the vacuum degree of the vacuum exhaust section was -0.07 MPa. After melt blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0090] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 160–180°C, resulting in a sheet thickness of 0.35 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 100°C for 90 seconds. After cooling, the fully biodegradable polylactic acid packaging material of Comparative Example 1 was obtained.

[0091] This comparative example illustrates that, without the addition of epoxy-terminated PBAT-PLA block oligomers and PDLA-grafted silica, the basic PLA / PBAT / starch system lacks an effective interfacial toughening structure and a stereocomposite heat-resistant nucleating structure, resulting in limited improvement in material toughness and heat resistance.

[0092] Comparative Example 2 This comparative example provides a method for preparing a fully biodegradable polylactic acid packaging material with the addition of a reactive flexible toughening additive. The difference between this method and Example 1 is that epoxy-terminated PBAT-PLA block oligomers are added, but PDLA-grafted silica is not added.

[0093] Weigh out 45 parts by weight of polylactic acid, 35 parts of PBAT, 30 parts of corn starch, 12 parts of stearic acid starch ester, 20 parts of mannitol, 6 parts of epoxy-terminated PBAT-PLA block oligomer, and 4 parts of processing aids.

[0094] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.5 parts, ethylene bis-stearamide 1.0 part, erucamide 0.5 parts, antioxidant 168 0.25 parts, hindered phenolic antioxidant 0.25 parts, and calcium oxide 0.5 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0095] The epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 12000 g / mol and an epoxy value of 0.18 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 75:25.

[0096] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, the epoxy-terminated PBAT-PLA block oligomer was dried at 60°C for 4 hours, and the processing aids were dried at 50°C for 2 hours.

[0097] 30 parts of dried corn starch, 12 parts of stearic acid starch ester and 20 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 85°C for 15 min to obtain a starch-based plasticized filler phase.

[0098] Subsequently, 45 parts of polylactic acid, 35 parts of PBAT, the obtained starch-based plasticized filler phase, 6 parts of epoxy-terminated PBAT-PLA block oligomer, and 4 parts of processing aids were added to a twin-screw extruder for melt reaction blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: zone 1 145℃, zone 2 155℃, zone 3 165℃, zone 4 175℃, zone 5 180℃, zone 6 180℃, and die head 175℃; the screw speed was 200 rpm, and the vacuum degree of the vacuum exhaust section was -0.07 MPa. After melt reaction blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0099] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 160–180°C, resulting in a sheet thickness of 0.35 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 100°C for 90 seconds. After cooling, the fully biodegradable polylactic acid packaging material of Comparative Example 2 was obtained.

[0100] This comparative example is used to illustrate that adding epoxy-terminated PBAT-PLA block oligomers alone can improve the interfacial compatibility between the PLA matrix and the PBAT flexible phase and increase the toughness of the material. However, due to the lack of the heat-resistant nucleation and stereocomposite crystallization induction effect of PDLA-grafted silica, the improvement in the heat resistance of the material is limited.

[0101] Comparative Example 3 This comparative example provides a method for preparing a fully biodegradable polylactic acid packaging material with PDLA-grafted silica heat-resistant nucleating additive. The difference between this method and Example 1 is that PDLA-grafted silica is added, but epoxy-terminated PBAT-PLA block oligomers are not added.

[0102] Weigh out 45 parts polylactic acid, 35 parts PBAT, 30 parts corn starch, 12 parts stearic acid starch ester, 20 parts mannitol, 2 parts PDLA-grafted silica, and 4 parts processing aids by weight.

[0103] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.5 parts, ethylene bis-stearamide 1.0 part, erucamide 0.5 parts, antioxidant 168 0.25 parts, hindered phenolic antioxidant 0.25 parts, and calcium oxide 0.5 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0104] The PDLA-grafted silica was prepared according to the method of Preparation Example 2, wherein the grafting rate of PDLA segments was 25%.

[0105] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, PDLA-grafted silica, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, PDLA-grafted silica was dried at 60°C for 4 hours, and processing aids were dried at 50°C for 2 hours.

[0106] 30 parts of dried corn starch, 12 parts of stearic acid starch ester and 20 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 85°C for 15 min to obtain a starch-based plasticized filler phase.

[0107] Subsequently, 45 parts of polylactic acid, 35 parts of PBAT, the obtained starch-based plasticizing filler phase, 2 parts of PDLA-grafted silica, and 4 parts of processing aids were added to a twin-screw extruder for melt blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: zone 1 145℃, zone 2 155℃, zone 3 165℃, zone 4 175℃, zone 5 180℃, zone 6 180℃, and die head 175℃; the screw speed was 200 rpm, and the vacuum degree of the vacuum exhaust section was -0.07 MPa. After melt blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0108] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 160–180°C, resulting in a sheet thickness of 0.35 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 100°C for 90 seconds. After cooling, the fully biodegradable polylactic acid packaging material of Comparative Example 3 was obtained.

[0109] This comparative example illustrates that adding PDLA-grafted silica alone can promote polylactic acid crystallization and induce the formation of stereocomposite crystals between PLLA and PDLA segments, thereby improving the heat resistance of the material. However, due to the lack of interfacial toughening and interfacial bridging effects of epoxy-terminated PBAT-PLA block oligomers, the compatibility improvement between the PLA matrix and the PBAT flexible phase is insufficient, resulting in limited improvement in the toughness of the material.

[0110] Comparative Example 4 This comparative example provides a method for preparing a fully biodegradable polylactic acid packaging material with the addition of a reactive flexible toughening additive and ordinary silica. The difference between this method and Example 1 is that ordinary silica is used instead of PDLA-grafted silica.

[0111] Weigh out 45 parts polylactic acid, 35 parts PBAT, 30 parts corn starch, 12 parts stearic acid starch ester, 20 parts mannitol, 6 parts epoxy-terminated PBAT-PLA block oligomer, 2 parts ordinary precipitated silica, and 4 parts processing aids by weight.

[0112] The processing aid is composed of diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidant, and calcium oxide, wherein the proportions are: diphenylmethane diisocyanate 1.5 parts, ethylene bis-stearamide 1.0 part, erucamide 0.5 parts, antioxidant 168 0.25 parts, hindered phenolic antioxidant 0.25 parts, and calcium oxide 0.5 parts. The hindered phenolic antioxidant is pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate].

[0113] The epoxy-terminated PBAT-PLA block oligomer has a number-average molecular weight of 12000 g / mol and an epoxy value of 0.18 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 75:25. The ordinary silica is ungrafted with PDLA segments.

[0114] In the preparation process, polylactic acid (PLA), PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, ordinary fumed silica, mannitol, and processing aids were dried separately. Specifically, PLA and PBAT were dried at 65°C for 6 hours, corn starch, stearic acid starch ester, and mannitol were dried at 80°C for 4 hours, the epoxy-terminated PBAT-PLA block oligomer was dried at 60°C for 4 hours, ordinary fumed silica was dried at 110°C for 4 hours, and the processing aids were dried at 50°C for 2 hours.

[0115] 30 parts of dried corn starch, 12 parts of stearic acid starch ester and 20 parts of mannitol were added to a high-speed mixer and premixed and plasticized at 85°C for 15 min to obtain a starch-based plasticized filler phase.

[0116] Subsequently, 45 parts of polylactic acid, 35 parts of PBAT, the obtained starch-based plasticized filler phase, 6 parts of epoxy-terminated PBAT-PLA block oligomer, 2 parts of ordinary silica, and 4 parts of processing aids were added to a twin-screw extruder for melt reaction blending. The temperatures of each zone of the twin-screw extruder were set sequentially as follows: Zone 1 145℃, Zone 2 155℃, Zone 3 165℃, Zone 4 175℃, Zone 5 180℃, Zone 6 180℃, and Die Head 175℃; the screw speed was 200 rpm, and the vacuum degree of the vacuum exhaust section was -0.07 MPa. After melt reaction blending, the mixture was extruded, water-cooled, drawn, and pelletized to obtain fully biodegradable polylactic acid modified particles. The obtained particles were then dried at 60℃ for 4 hours before use.

[0117] The obtained fully biodegradable polylactic acid modified particles were added to a sheet extruder for sheet extrusion molding at a temperature of 160–180°C, resulting in a sheet thickness of 0.35 mm. Subsequently, the sheet was placed in a heat-setting device for heat-setting treatment at a temperature of 100°C for 90 seconds. After cooling, the fully biodegradable polylactic acid packaging material of Comparative Example 4 was obtained.

[0118] This comparative example is used to illustrate that although ordinary silica can provide a certain heterogeneous nucleation effect, its surface is not grafted with PDLA segments, so it cannot effectively induce the formation of PLLA / PDLA stereocomposite crystals. Moreover, its dispersion stability and interfacial synergy in the PLA / PBAT system are weaker than those of PDLA-grafted silica. Therefore, its overall improvement effect on the heat resistance and toughness of the material is lower than that of Example 1.

[0119] Performance testing methods To evaluate the toughening, heat resistance, and crystallization behavior of the fully biodegradable polylactic acid (PLA) packaging material obtained in this invention, the fully biodegradable PLA modified particles obtained in the examples and comparative examples were prepared as test samples under the same conditions. The thickness of the sheet samples was controlled at 0.35 mm; the specimens used for impact performance and heat distortion temperature testing were prepared by injection molding or compression molding. All samples were conditioned for 24 hours at a temperature of 23±2℃ and a relative humidity of 50±5% before testing.

[0120] Tensile property testing: The sheets obtained in the examples and comparative examples were cut into dumbbell-shaped tensile specimens and tested according to the method for testing the tensile properties of plastics. The testing speed was 50 mm / min, and no less than 5 parallel specimens were tested for each group of samples. The average value was taken. The tensile strength and elongation at break of each sample were recorded. Among them, the tensile strength is used to evaluate whether the strength of the material is maintained after toughening modification, and the elongation at break is used to evaluate the effect of improving the toughness of the material.

[0121] Notched impact strength test: Standard impact specimens were prepared from the materials obtained in the examples and comparative examples, and a standard notch was machined in the middle of the specimens. The test was conducted according to the notched impact strength test method for plastic cantilever beams. At least five parallel specimens were tested for each group of samples, and the average value was taken. The result is expressed in kJ / m². Notched impact strength is used to evaluate the material's resistance to brittle fracture and its impact toughness under external impact.

[0122] Heat distortion temperature (HDT) test: The materials obtained in the examples and comparative examples were prepared as HDT test specimens and tested according to the method for testing the load deformation temperature of plastics. During the test, a specified bending stress condition was used, and the heating rate was 120℃ / h. The temperature at which the specimen reached the specified deformation was recorded as the HDT. At least three parallel specimens were tested for each group of samples, and the average value was taken. The HDT is used to evaluate the heat resistance and dimensional stability of the material under the combined action of heat and stress.

[0123] Dimensional change rate test in 90℃ hot water: The sheets obtained in the examples and comparative examples were cut into 100mm × 100mm square samples. The longitudinal length L0 and transverse length W0 of the samples before treatment were measured. The samples were then completely immersed in 90℃ hot water for 10 minutes, removed, and allowed to cool naturally to room temperature. Surface moisture was wiped off, and the longitudinal length L1 and transverse length W1 of the samples after treatment were measured again. At least three parallel samples were tested in each group, and the average value was taken. The longitudinal dimensional change rate and transverse dimensional change rate were calculated using the following formulas: Longitudinal dimensional change rate / % = |L1 - L0| / L0 × 100%; Lateral dimension change rate / % = |W1 - W0| / W0 × 100%.

[0124] Simultaneously record whether the sample exhibits obvious softening, curling, warping, or deformation. The dimensional change rate in 90℃ hot water is used to evaluate the actual heat resistance stability of packaging materials in hot water, hot meals, or hot filling environments.

[0125] DSC testing: Differential scanning calorimetry (DSC) was used to perform thermal analysis on the materials obtained in the examples and comparative examples. 5–10 mg of sample was weighed and placed in an aluminum crucible. The test was conducted under a nitrogen atmosphere with a flow rate of 50 mL / min. The test procedure was as follows: the temperature was first raised from room temperature to 200 °C at a rate of 10 °C / min and held for 3 min to eliminate thermal history; then the temperature was lowered to 30 °C at a rate of 10 °C / min; and then raised to 250 °C at a rate of 10 °C / min. The cold crystallization temperature Tcc, the melting peak temperature Tm of ordinary polylactic acid crystals, the melting peak temperature Tm-sc of stereocomposite crystals, the cold crystallization enthalpy ΔHcc, and the melting enthalpy ΔHm were recorded during the second heating process.

[0126] Crystallinity Xc is calculated using the following formula: Xc / %=(ΔHm-ΔHcc) / (wPLA×ΔHm0)×100%.

[0127] Wherein, ΔHm is the enthalpy of melting of the sample, ΔHcc is the enthalpy of cold crystallization of the sample, wPLA is the mass fraction of polylactic acid in the polymer component of the sample, and ΔHm0 is the theoretical enthalpy of melting of fully crystallized polylactic acid, taken as 93.7 J / g. Tcc is used to evaluate the ease of crystallization of polylactic acid; a decrease in Tcc indicates that the material's crystallization ability is enhanced. Tm-sc is used to determine whether PLLA segments and PDLA segments form a stereocomposite crystal. Xc is used to evaluate the crystallinity of the material; an increase in Xc indicates that the polylactic acid matrix crystallizes more fully.

[0128] The effects of epoxy-terminated PBAT-PLA block oligomers and PDLA-grafted silica on the toughness, heat resistance, and crystallization behavior of fully biodegradable polylactic acid packaging materials were evaluated by comparing the tensile strength, elongation at break, notched impact strength, heat distortion temperature, dimensional change rate in hot water at 90°C, and DSC test results of the examples and comparative examples.

[0129] The test results are shown in Table 1 below. Table 1 Detection data analysis As shown in Table 1, Comparative Example 1, without the addition of epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica, has a tensile strength of 25.6 MPa, an elongation at break of 58%, a notched impact strength of 3.4 kJ / m², a heat distortion temperature of 63℃, and a dimensional change rate of 8.6% after treatment with 90℃ hot water for 10 min. This indicates that although the basic PLA / PBAT / starch system has certain flexibility and processability, its toughness and heat resistance dimensional stability are relatively low due to the lack of effective interfacial toughening structures and heat-resistant nucleation structures.

[0130] In Comparative Example 2, the addition of epoxy-terminated PBAT-PLA block oligomers to the base system increased the elongation at break to 138% and the notched impact strength to 6.9 kJ / m², significantly higher than Comparative Example 1. This indicates that the epoxy-terminated PBAT-PLA block oligomers can improve the interfacial compatibility between the PLA matrix and the PBAT flexible phase, and enhance the material's toughness through flexible segment energy dissipation and interfacial reaction bonding. However, Comparative Example 2's heat distortion temperature was only 67℃, and the dimensional change rate in 90℃ hot water was still 6.7%. Furthermore, no obvious stereocomposite crystal melting peak was observed in the DSC test, suggesting that the addition of reactive flexible toughening additives alone mainly improves toughness, with limited improvement on polylactic acid crystallization and heat resistance.

[0131] In Comparative Example 3, the addition of PDLA-grafted silica to the basic system increased the material's heat distortion temperature to 83℃, reduced the dimensional change rate in hot water at 90℃ to 3.8%, decreased the cold crystallization temperature (Tcc) from 113.5℃ in Comparative Example 1 to 96.5℃, increased the crystallinity (Xc) from 15.8% to 27.4%, and resulted in a stereocomposite crystal melting peak at 216.8℃. This indicates that PDLA-grafted silica can induce polylactic acid matrix crystallization and promote the formation of stereocomposite crystals between PLLA and PDLA segments, thereby improving the material's heat resistance. However, Comparative Example 3 showed an elongation at break of only 76% and a notched impact strength of only 4.5 kJ / m², lower than Comparative Example 2 and Example 1. This suggests that while adding PDLA-grafted silica alone is beneficial for improving heat resistance, the lack of interfacial toughening and compatibility effects from reactive flexible toughening additives limits the improvement in material toughness.

[0132] Comparative Example 4 used ordinary silica instead of PDLA-grafted silica. With the addition of epoxy-terminated PBAT-PLA block oligomers, the material exhibited an elongation at break of 118%, a notched impact strength of 6.1 kJ / m², a heat distortion temperature of 75°C, a dimensional change rate of 5.1% in hot water at 90°C, and a crystallinity (Xc) of 22.3%. Compared to Comparative Example 2, Comparative Example 4 showed improved heat resistance and crystallinity, indicating that ordinary silica has a certain heterogeneous nucleation effect. However, its DSC test did not show a significant melting peak of stereocomposite crystals, and all its overall performance was lower than that of Example 1. This indicates that ordinary silica cannot effectively induce the formation of PLLA / PDLA stereocomposite crystals and is also unable to achieve interfacial synergy with epoxy-terminated PBAT-PLA block oligomers.

[0133] In Example 1, after simultaneously adding epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica, the tensile strength of the material increased to 34.8 MPa, the elongation at break increased to 165%, the notched impact strength increased to 9.4 kJ / m², the heat distortion temperature increased to 95℃, the dimensional change rate in hot water at 90℃ decreased to 1.9%, the cold crystallization temperature Tcc decreased to 88.6℃, the crystallinity Xc increased to 33.6%, and a stereocomposite crystal melting peak at 218.5℃ appeared. Compared with Comparative Example 1, Example 1 showed an increase in elongation at break of approximately 184.5%, notched impact strength of approximately 176.5%, heat distortion temperature of 32°C, and a decrease in dimensional change rate in 90°C hot water of approximately 77.9%. Compared with Comparative Example 2, Example 1 not only showed further improved toughness but also an increase in heat distortion temperature of 28°C and a decrease in dimensional change rate in 90°C hot water of approximately 71.6%. Compared with Comparative Example 3, Example 1 showed further improvement in heat resistance, while elongation at break and notched impact strength increased by approximately 117.1% and 108.9%, respectively. These results indicate that the epoxy-terminated PBAT-PLA block oligomer and PDLA-grafted silica do not simply work in parallel but rather produce a synergistic effect during melt reaction blending and heat setting.

[0134] Specifically, the flexible PBAT segments in epoxy-terminated PBAT-PLA block oligomers enhance material toughness, while the PLA segments improve compatibility with the polylactic acid (PLA) matrix. The epoxy end groups react with the carboxyl or hydroxyl groups at the PLA chain ends, the hydroxyl groups in the starch-based plasticized filler phase, and the hydroxyl groups on the surface of PDLA-grafted silica, forming an interfacial bridging structure. PDLA-grafted silica provides heterogeneous nucleation sites through the silica core and, through the formation of stereocomposite crystals by PDLA and PLLA segments, improves the crystallization rate and degree of perfection of PLA. The synergistic effect of these two additives results in a more stable interfacial structure between the PBAT flexible toughening phase, the PLA matrix, and the PDLA-grafted silica, thereby simultaneously improving the material's toughness, impact resistance, heat resistance, and dimensional stability in hot water.

[0135] In Example 2, the polylactic acid (PLA) content was higher, the PBAT content was relatively lower, the PDLA-grafted silica content was increased to 3.5 parts, and the heat setting temperature and time were increased to 110℃ and 120s, respectively. Test results showed that Example 2 had a tensile strength of 38.5 MPa, a heat distortion temperature of 105℃, a dimensional change rate of 1.2% in 90℃ hot water, a reduced Tcc to 84.2℃, and an increased Xc to 38.8%, all superior to Example 1. These results indicate that increasing the continuous phase ratio of PLA, increasing the amount of PDLA-grafted silica, and appropriately increasing the heat setting strength are beneficial for further promoting PLA crystallization and the formation of stereocomposite crystals, thereby obtaining higher heat resistance and dimensional stability. Example 2 is suitable for packaging products such as lunch boxes, trays, and cup lids that require high rigidity and heat resistance.

[0136] In Example 3, the amounts of PBAT, mannitol, and epoxy-terminated PBAT-PLA block oligomers were relatively high, while the amount of PDLA-grafted silica was relatively low. Test results showed that Example 3 achieved an elongation at break of 245% and a notched impact strength of 11.6 kJ / m. 2 The values ​​were 165% and 9.4 kJ / m² higher than those in Example 1, respectively. 2 This indicates that increasing the amount of flexible biodegradable polyester, plasticizing components, and reactive flexible toughening additives is beneficial for further improving the flexibility and impact toughness of the material. Meanwhile, the heat distortion temperature of Example 3 still reaches 88℃, and the dimensional change rate in hot water at 90℃ is 2.8%, indicating that even in a relatively high-toughness formulation, PDLA-grafted silica can still provide a certain degree of heat-resistant nucleation and stereocomposite effect, enabling the resulting film packaging material to maintain high toughness while exhibiting good heat-resistant dimensional stability.

[0137] In summary, the test results show that epoxy-terminated PBAT-PLA block oligomers mainly contribute to interfacial compatibilization and toughness improvement, while PDLA-grafted silica mainly contributes to polylactic acid crystallization, stereocomposite crystal formation, and improved heat resistance. When both are added together, they can produce a synergistic reinforcing effect through interfacial reaction, flexible phase refinement, heterogeneous nucleation, and stereocomposite crystallization, enabling fully biodegradable polylactic acid packaging materials to simultaneously achieve high elongation at break, notched impact strength, heat distortion temperature, and hot water dimensional stability.

[0138] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a toughened, heat-resistant, fully biodegradable polylactic acid packaging material, characterized in that, The steps include the following: S1. Polylactic acid, PBAT, corn starch, stearic acid starch ester, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica, mannitol, and processing aids are dried separately. S2. Corn starch, stearic acid starch ester and mannitol are premixed and plasticized to obtain a starch-based plasticized filler phase; S3. Polylactic acid, PBAT, starch-based plasticizing filler phase, epoxy-terminated PBAT-PLA block oligomer, PDLA-grafted silica and processing aids are added to a twin-screw extruder for melt reaction and blending to obtain a modified polylactic acid composite. S4. The modified polylactic acid composite is extruded and granulated to obtain fully biodegradable polylactic acid modified particles; S5. The fully biodegradable polylactic acid modified particles are blown into film, cast, extruded into sheet, or vacuum-formed to obtain fully biodegradable polylactic acid packaging materials. In this process, the epoxy-terminated PBAT-PLA block oligomer serves as a reactive flexible toughening additive, and the PDLA-grafted silica serves as a heat-resistant nucleating additive. During the melt reaction blending process, the epoxy groups of the epoxy-terminated PBAT-PLA block oligomer react with the carboxyl or hydroxyl groups at the ends of the polylactic acid chain, the hydroxyl groups in the starch-based plasticizing filler phase, and the hydroxyl, silanol, or amino groups on the surface of the PDLA-grafted silica, causing the PDLA-grafted silica to be distributed in the interface region between the polylactic acid matrix and the PBAT flexible phase. Simultaneously, the PDLA segments in the PDLA-grafted silica and the PLLA segments in the polylactic acid form stereocomposite crystals, thereby achieving a synergistic improvement in the toughness and heat resistance of the packaging material.

2. The preparation method according to claim 1, characterized in that, In step S3, the raw materials are expressed in parts by mass as follows: 35-55 parts polylactic acid, 25-45 parts PBAT, 20-40 parts corn starch, 8-20 parts stearic acid starch ester, 10-35 parts mannitol, 4-10 parts epoxy-terminated PBAT-PLA block oligomer, 1-4 parts PDLA-grafted silica, and 1-8 parts processing aids.

3. The preparation method according to claim 2, characterized in that, In step S3, the raw materials are expressed in parts by mass as follows: 45 parts polylactic acid, 35 parts PBAT, 30 parts corn starch, 12 parts stearic acid starch ester, 20 parts mannitol, 6 parts epoxy-terminated PBAT-PLA block oligomer, 2 parts PDLA-grafted silica, and 4 parts processing aids.

4. The preparation method according to claim 1, characterized in that, The number-average molecular weight of the epoxy-terminated PBAT-PLA block oligomer is 3000–20000 g / mol, and the epoxy value is 0.05–0.50 mol / 100 g, wherein the mass ratio of PBAT flexible segments to PLA compatible segments is 60–90:10–40.

5. The preparation method according to claim 1, characterized in that, The PDLA-grafted silica is prepared by the following method: First, the surface of silica is activated or aminated with silane coupling agent. Then, the treated silica is used as the initiator to induce D-lactide to undergo ring-opening polymerization under the action of a catalyst, and PDLA segments are grafted onto the surface of silica to obtain PDLA-grafted silica. The grafting rate of PDLA segments in the PDLA-grafted silica is 10-45%.

6. The preparation method according to claim 1, characterized in that, The processing aids include diphenylmethane diisocyanate, ethylene bis-stearamide, erucamide, antioxidant 168, hindered phenolic antioxidants, and calcium oxide.

7. The preparation method according to claim 6, characterized in that, The processing aids, by weight, comprise: 0.5–3 parts of diphenylmethane diisocyanate, 0.5–2 parts of ethylene bis-stearamide, 0.2–1 part of erucamide, 0.1–0.5 parts of antioxidant 168, 0.1–0.5 parts of hindered phenolic antioxidant, and 0.2–1 parts of calcium oxide.

8. The preparation method according to claim 1, characterized in that, In step S2, corn starch, stearic acid starch ester and mannitol are premixed at 60-100℃ for 5-30 minutes, so that mannitol enters between corn starch and stearic acid starch ester to form a plasticizing system; in step S3, the melt reaction blending temperature is 145-185℃, the screw speed is 120-300 rpm, and the vacuum degree is -0.04--0.09 MPa.

9. The preparation method according to claim 1, characterized in that, Step S5 is followed by a heat setting process, wherein the heat setting temperature is 85-115°C and the heat setting time is 20-150s; during the heat setting process, PDLA grafted silica induces the crystallization of polylactic acid matrix and promotes the formation of stereocomposite crystals between PLLA segments and PDLA segments.

10. A toughened, heat-resistant, fully biodegradable polylactic acid packaging material, characterized in that, Prepared by the preparation method according to any one of claims 1 to 9; In the packaging material, PBAT forms a flexible toughening phase, epoxy-terminated PBAT-PLA block oligomers are located between the polylactic acid matrix and the PBAT flexible toughening phase and form an interfacial reaction connection, and PDLA-grafted silica is distributed in the interfacial region between the polylactic acid matrix and the PBAT flexible toughening phase.

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