Degradable film material for packaging and preparation method thereof

Abstract

The invention provides a degradable film material for packaging and a preparation method thereof, and the degradable film material is prepared from the following raw materials in parts by mass: 50 parts of polylactic acid, 30-50 parts of poly (butylene terephthalate)-adipate, 2-5 parts of sulfonate-containing polyester with double epoxy-terminated ends, 0.2-1 part of organic divalent metal salt and 0.1-0.5 part of an antioxidant. According to the application, sulfonate-containing polyester with double-end epoxy end sealing is introduced into a degradable single-layer film system formed by polylactic acid and poly (butylene adipate-co-terephthalate), and the synergistic effect of organic divalent metal salt is matched, so that under the condition of not depending on a multi-layer composite structure or an inorganic filler, the degradable single-layer film system has the advantages that the mechanical property of the degradable single-layer film system is greatly improved; the water vapor transmission rate and the oxygen transmission rate of the film material are effectively reduced, meanwhile, the good melt processing stability and film forming adaptability of the material are kept, and therefore the problems that an existing degradable single-layer packaging film is insufficient in barrier property and poor in stability are solved.

Description

Technical Field

[0001] This application relates to the field of packaging materials technology, specifically to a biodegradable film material for packaging and its preparation method. Background Technology

[0002] With the increasing demand for green and low-carbon packaging materials, the application of biodegradable polymer materials in food, daily chemical, and industrial packaging has attracted widespread attention. Polylactic acid (PLA) and polybutylene terephthalate (PPBAT) are considered to be promising polymer systems among current biodegradable packaging film materials due to their relatively sustainable sources, biodegradability, and good processing adaptability.

[0003] PLA possesses high mechanical strength and good transparency, but its toughness is insufficient, its thermal stability and impact resistance are limited, and its water vapor and oxygen barrier properties are insufficient for some packaging applications when used alone. PBAT, on the other hand, has good flexibility and processing performance, but its gas barrier properties are relatively poor. Therefore, existing technologies commonly employ a blending of PLA and PBAT to achieve a balance between mechanical and processing performance to some extent.

[0004] However, PLA / PBAT blends still face several problems when used as packaging film materials. First, due to the differences in molecular structure and polarity between PLA and PBAT, micro-phase separation structures are easily formed in their blends, making it difficult to further reduce water vapor transmission rate (WVTR) and oxygen transmission rate (OTR), especially in packaging applications with high barrier performance requirements.

[0005] Secondly, to improve barrier performance, some existing technologies attempt to enhance performance through multilayer composite structures, coating barrier layers, or introducing inorganic fillers. However, multilayer structures typically require additional coating, lamination, or bonding processes, which are complex and costly. Furthermore, they can easily introduce components that are difficult to degrade synergistically into the degradable system, hindering the overall degradability and recycling of the material. The introduction of inorganic fillers may lead to decreased transparency of the membrane material, poorer processing flowability, and even film-forming defects, affecting the packaging appearance and performance.

[0006] In addition, some technologies improve the compatibility between polymers by introducing polar modifiers or reactive additives. However, in the existing modification methods, the modified structure is prone to migration or failure during melt processing and use, making it difficult to form a stable and durable dense structure inside the membrane material. This results in the barrier performance fluctuating with changes in processing conditions and usage environment, making it difficult to meet the stability requirements for long-term use.

[0007] Therefore, existing biodegradable packaging film materials still generally have the following shortcomings: while maintaining a single-layer structure and good processability, it is difficult to effectively reduce the permeability of water vapor and oxygen at the same time; the barrier performance improvement methods are not stable enough and are easily affected by processing and usage conditions; some modification methods increase the complexity of the process or weaken the biodegradable properties of the material.

[0008] To address the aforementioned issues, there is an urgent need to develop a packaging film material that can effectively enhance barrier properties, maintain structural stability, and be industrially feasible within a single-layer biodegradable system. Summary of the Invention

[0009] This application provides a biodegradable film material for packaging and a method for preparing the same, aiming to solve the problem that existing biodegradable packaging film materials cannot effectively balance water vapor and oxygen barrier properties while maintaining a single-layer structure and good processing performance, thereby obtaining a biodegradable packaging film material that has both low water vapor permeability and oxygen permeability and is suitable for industrial film formation processing.

[0010] In a first aspect, this application provides a biodegradable film material for packaging, comprising the following parts by weight of raw materials: 50 parts polylactic acid, 30-50 parts polybutylene terephthalate adipate, 2-5 parts double-ended epoxy-capped sulfonate polyester, 0.2-1 parts organic divalent metal salt, and 0.1-0.5 parts antioxidant.

[0011] According to this application, by introducing a sulfonate polyester with double-ended epoxy end capping into a biodegradable single-layer film system composed of polylactic acid and polybutylene terephthalate-adipate, and with the synergistic effect of organic divalent metal salts, the water vapor permeability and oxygen permeability of the film material are effectively reduced without relying on multilayer composite structures or inorganic fillers, while maintaining good melt processing stability and film-forming adaptability of the material. This solves the problems of insufficient barrier performance and poor stability of existing biodegradable single-layer packaging films.

[0012] Specifically, polylactic acid (PLA) and polybutylene terephthalate (PET) are used as the main matrix of the membrane material to jointly construct a biodegradable continuous polyester phase. However, the two differ in molecular structure and polarity, and simple blending easily leads to the formation of micro-phase interfaces within the membrane, causing gas molecules to diffuse relatively rapidly along the interface region. This application introduces a sulfonate-containing polyester with double-ended epoxy end caps, enabling this polyester to participate in the structural construction of the matrix polyester segments during melt processing. This allows it to play a structural connecting and transitional role between different polyester phases, weakening interfacial discontinuities and reducing the number of micro-defects caused by phase separation.

[0013] Meanwhile, the sulfonate groups introduced into the double-ended epoxy-capped sulfonate polyester form stable ion-associated or coordinated structures in the presence of organic divalent metal salts. These ions act as dispersed, polar, dense nodes in the polymer system. These dense nodes do not simply fill the polyester matrix but rather form a synergistic confinement effect with the surrounding polymer segments, inhibiting local chain movement and thus reducing the overall free volume of the membrane material. Gas molecules must bypass these dense nodes and confined regions when passing through the membrane material, elongating their diffusion path and reducing the diffusion rate. This mechanism achieves simultaneous inhibition of water vapor and oxygen permeability.

[0014] It should be noted that after the sulfonic acid groups in the sulfonate-containing polyester form ionic coordination or associative structures in the presence of organic divalent metal salts, the proportion of their existence in the form of free ions is significantly reduced. The polar effect of the sulfonic acid groups is partially shielded or constrained, so that they no longer exhibit highly free hydrophilic groups. Through the above coordination effect, the sulfonate structure changes from an ionic state that is originally prone to interacting with water molecules to a stable coordination node embedded in the polyester matrix. Thus, while exerting a densification effect, it weakens its impact on the overall hydrophilicity of the membrane material, which is beneficial to reducing the migration and adsorption of water in the membrane and minimizing the adverse effects on water vapor barrier performance.

[0015] Furthermore, due to the organic divalent metal salt's organic ligand structure, it exhibits good dispersibility and compatibility within the system. It can participate in the coordination of the sulfonate structure without disrupting the continuous polyester phase, avoiding the local aggregation or processing instability issues that might arise from the direct addition of inorganic salts. This ensures the relative stability of the ion-densified microstructure during melt processing and subsequent use. The introduction of antioxidants effectively inhibits the thermo-oxidative degradation of polylactic acid and polybutylene terephthalate (PET) during melt processing, maintaining the polymer's molecular weight and structural integrity. This, in turn, helps maintain the stability of the membrane material's densified microstructure and the consistency of its barrier properties.

[0016] In summary, through the synergistic effect of sulfonate-containing polyesters with double-ended epoxy end capping and organic divalent metal salts in the polyester matrix, this application constructs a stable, uniform, and durable dense microstructure in a single-layer biodegradable film system. The constructed dense microstructure mainly originates from the ionic coordination interaction between sulfonate groups and organic divalent metal salts. This type of ionic interaction is reversible and thermosensitive, capable of dissociation and rearrangement under melt processing conditions without forming a permanent covalent crosslinking network. Therefore, effective control of water vapor and oxygen permeability can be achieved without increasing processing complexity, resulting in a film material that combines barrier properties, processability, and biodegradability, making it suitable for practical applications in the packaging field.

[0017] In some embodiments, the method for preparing the double-ended epoxy-capped sulfonate polyester includes the following steps: S1: Sulfonate-containing dicarboxylic acid, aliphatic dicarboxylic acid and diol undergo polycondensation reaction under catalytic conditions to obtain sulfonate-containing polyester with double-ended hydroxyl groups; S2: The sulfonate polyester with double hydroxyl end capping is mixed with epichlorohydrin, and the hydroxyl groups of the sulfonate polyester with double hydroxyl end capping and the epoxy groups of epichlorohydrin are etherified to generate chlorohydrin end groups, so as to obtain sulfonate polyester with double chlorohydrin end capping. S3: The chlorohydrin groups in the sulfonate polyester with double-terminated chlorohydrin groups are dehydrochlorinated and cyclized under alkaline conditions to obtain a sulfonate polyester with double-terminated epoxy groups.

[0018] In some of the above embodiments, by first constructing a sulfonate-containing polyester backbone structure with double-ended hydroxyl groups and then gradually converting the end groups, the reactivity of the end groups can be introduced without destroying the integrity of the polyester backbone and the distribution of the sulfonate structure. This preparation route avoids the side reactions or structural runaway problems caused by directly introducing highly reactive epoxy groups during polycondensation, and is beneficial for obtaining functional polyesters with a narrow molecular weight distribution and well-defined end group structures.

[0019] Specifically, in step S1, by controlling the polycondensation reaction conditions, the sulfonate-containing structure is uniformly embedded in the polyester backbone, providing a continuous and controllable ion site distribution for the subsequent formation of a stable ion-associated structure with organic divalent metal salts. Simultaneously, the double-ended hydroxyl-terminated structure provides clear reaction sites for subsequent end-group modification, which is beneficial for improving the uniformity of end-group conversion. In steps S2 and S3, through the etherification reaction of hydroxyl groups with epichlorohydrin and the subsequent dehydrochlorination cyclization reaction, the originally less chemically active hydroxyl end groups are converted into more reactive epoxy end groups. This makes the resulting sulfonate-containing polyester more likely to react or combine with polyester matrices such as polylactic acid and polybutylene terephthalate during melt processing, thereby forming a more stable structural connection relationship within the membrane material. Compared to polyesters without epoxy end-capping or with only a single-ended active structure, this double-ended epoxy-terminated structure is more conducive to multi-point anchoring in the matrix, enhancing the continuity and stability of the densified microstructure.

[0020] Compared to other possible end-group introduction methods, the above-described implementation method of gradually constructing a double-ended epoxy end-capped structure has significant advantages in terms of end-group structure controllability, polyester main chain stability, and synergistic effect of ionic functional units. It is beneficial to achieve a stable improvement in barrier performance while maintaining good melt processing performance.

[0021] In some embodiments, in step S1, the ratio of the total molar amount of the sulfonate dicarboxylic acid and the aliphatic dicarboxylic acid to the molar amount of the diol is 1:1.05~1.2.

[0022] In some of the above embodiments, by setting the molar amount of diol in appropriate excess relative to dicarboxylic acid, it is beneficial to obtain a sulfonate polyester structure with hydroxyl groups as the main end groups during the polycondensation reaction, thereby providing stable and controllable reaction sites for subsequent end-group modification reactions. This molar ratio range can avoid problems such as uneven end groups or incomplete reactions caused by insufficient diol, while also preventing excessive diol from causing excessively low polyester molecular weight or overly flexible structure, thus helping to balance end-group controllability and polyester structural stability.

[0023] In some embodiments, in step S1, the molar amount of the sulfonate dicarboxylic acid is 3% to 10% of the total molar amount of the sulfonate dicarboxylic acid and the aliphatic dicarboxylic acid.

[0024] In some of the above embodiments, by controlling the amount of sulfonate-containing dicarboxylic acid within the specified range, an appropriate amount and uniformly distributed sulfonate functional units can be introduced into the polyester backbone. This allows the resulting sulfonate-containing polyester to maintain the continuity of the polyester backbone and processing stability while possessing sufficient ionic functional sites for subsequent synergistic effects. This dosage range avoids insufficient functional effects due to excessively low sulfonate unit content, and also prevents excessively high content from adversely affecting the regularity of the polyester backbone, melt processing performance, and water vapor barrier properties. By limiting the molar amount of sulfonate-containing dicarboxylic acid within the above range, a good balance can be achieved between functionality and structural stability, resulting in a sulfonate-containing polyester with stable performance and good processing adaptability, achieving optimized water vapor barrier effects.

[0025] In some embodiments, in step S1, the sulfonate-containing dicarboxylic acid includes sulfonate-containing aromatic dicarboxylic acids.

[0026] In some of the above embodiments, the sulfonate-containing aromatic dicarboxylic acids have sulfonate groups introduced into the polyester backbone structure via an aromatic ring skeleton. Due to the high structural rigidity and planarity of the aromatic ring, the spatial position of the sulfonate groups within the polyester chain segments is relatively fixed, which is beneficial for forming a stable and predictable distribution in the polyester system. Compared to flexible segments, this type of aromatic skeleton can effectively restrict the conformational freedom of the sulfonate groups, reducing the possibility of migration or reorientation during use.

[0027] In the presence of organic divalent metal salts, the sulfonate groups can form ionic coordination or associative structures with metal ions. Due to the spatial confinement of the sulfonate groups by the aromatic ring skeleton, this coordination structure more easily forms relatively stable coordination nodes in the polyester matrix and maintains high structural integrity after film formation. These stable coordination nodes can act as "ionic anchors" in the polyester matrix, causing surrounding polymer segments to form locally confined regions in their vicinity, thereby facilitating the construction of uniformly distributed, dense microstructures.

[0028] Therefore, the ion coordination network formed with the aromatic sulfonate structure as the core is beneficial to prolonging the diffusion path of gas molecules in the membrane material, providing a stable structural basis for reducing water vapor and oxygen permeability.

[0029] In some embodiments, in step S1, the diol includes aliphatic diols and alicyclic diols, and the alicyclic diol accounts for 10% to 20% of the molar percentage of the diol.

[0030] In some of the above embodiments, the inventors discovered that by compounding aliphatic and alicyclic diols, both compliant and conformationally restricted segments can be introduced into the sulfonate-containing polyester backbone, thereby achieving a balance between the flexibility and rigidity of the polyester structure. This may be because aliphatic diol segments have higher flexibility, which is beneficial for maintaining the processing flowability and structural continuity of the polyester oligomer; while alicyclic diols have a cyclic structure and lower conformational freedom, and their introduction can improve the local rigidity and structural stability of the polyester segments.

[0031] When alicyclic diols are introduced at a molar ratio of 10%–20%, their cyclic structures can form dispersed conformationally confined units in the polyester backbone, restricting the movement of surrounding chain segments, thereby reducing the free volume of the polyester system and inhibiting excessive chain relaxation. Without significantly affecting overall flexibility and processing properties, these locally rigid units help improve the stability of the polyester structure in subsequent systems, making it less prone to structural collapse or migration during melt processing and use.

[0032] Meanwhile, by combining it with aliphatic diols, the increased rigidity provided by alicyclic diols is not concentrated in a localized area of ​​the polyester backbone, but rather distributed more evenly throughout the entire polyester chain. This avoids the increased brittleness or processing instability caused by excessively high rigidity unit content. Compared to using only aliphatic diols, this compounding method is more conducive to improving the density of the polyester structure; and compared to cases with excessively high alicyclic diol content, it maintains good melt flowability and film-forming adaptability.

[0033] Furthermore, compared to aromatic diols, alicyclic diols, while increasing the local rigidity of polyester segments, also exhibit a non-planar conformation and a lower tendency for π–π stacking, making them less likely to interfere with the coordination between sulfonate groups and organic divalent metal salts. This is more conducive to forming stable and continuous ionic densification microstructures within the polyester matrix. Therefore, by combining aliphatic and alicyclic diols, the segmental structure of sulfonate-containing polyesters can be rationally controlled without weakening the ionic coordination effect. This helps to construct a stable and uniform densification structure and further improves the barrier properties and stability of biodegradable membrane materials.

[0034] In some embodiments, in step S2, the molar ratio of the hydroxyl groups in the sulfonate polyester with double-ended hydroxyl groups to the molar ratio of the epichlorohydrin is 1:5 to 20.

[0035] In some of the above embodiments, setting the amount of epichlorohydrin in excess relative to the hydroxyl groups is beneficial to improving the sufficiency of the etherification reaction between the hydroxyl and epoxy groups. This allows the end groups of the sulfonate polyester with double-ended hydroxyl groups to be more completely converted into chlorohydrin structures, thus providing a stable reaction basis for the subsequent dehydrochlorination cyclization reaction. This dosage range avoids the problem of incomplete end group conversion due to insufficient epichlorohydrin dosage. At the same time, the excess epichlorohydrin can also act as a reaction solvent during the reaction process and can be removed in subsequent processing without affecting the structure and properties of the resulting double-ended epoxy-capped sulfonate polyester.

[0036] In some embodiments, the organic divalent metal salt includes zinc stearate and zinc lactate, wherein the mass ratio of zinc stearate to zinc lactate is 1:1 to 3.

[0037] By combining zinc stearate and zinc lactate, the distribution of divalent zinc ions in the polyester matrix can be synergistically regulated, taking into account the structural and compatibility differences between polylactic acid (PLA) and polybutylene terephthalate (PET). The ligand structure of zinc lactate has high structural similarity to the repeating units of the PLA backbone, resulting in good compatibility in the PLA phase and facilitating the stable dispersion of zinc ions in PLA-enriched regions. Meanwhile, zinc stearate, with its long-chain fatty acid structure, exhibits high compatibility with the aliphatic segments in PET, making it easier to distribute uniformly within the polyester phase.

[0038] Through the aforementioned compounding method, zinc ions can obtain stable dispersion environments in both the polylactic acid (PLA) and polybutylene terephthalate (PET) phases. This allows them to effectively participate in the ionic coordination or association with sulfonate groups in sulfonate-containing polyesters in different polyester phases and at phase interfaces, thereby forming a more uniformly distributed and more stable ion-interaction structure throughout the polyester matrix. Compared to using only a single organic zinc salt, this compounding method is more effective in preventing excessive concentration or absence of ion-interaction structures in local phase regions, thus contributing to improved continuity of the densified microstructure of the membrane material.

[0039] In some embodiments, the polylactic acid has a melt flow rate of 5-10 g / 10 min at 190°C / 2.16 kg. Based on the above embodiments, polylactic acid within this melt flow rate range has moderate fluidity during melt processing, which ensures sufficient mixing with polybutylene terephthalate (PET) and functional components, while also preventing insufficient melt strength or decreased mechanical properties of the film during film formation due to excessively low molecular weight. This is beneficial for obtaining a biodegradable membrane material with stable processing and uniform film formation.

[0040] In some embodiments, the polybutylene terephthalate (PET) has a melt flow rate of 2-6 g / 10 min at 190°C / 2.16 kg. Based on the above embodiments, PET within this melt flow rate range can provide good flexibility and melt flow buffering for the system during melt blending, effectively improving the film-forming adaptability and processing stability of the membrane material without significantly increasing the system viscosity, which is conducive to the continuous extrusion and stable molding of monolayer membrane structures.

[0041] In some embodiments, the antioxidant includes at least one of antioxidant 1010 and antioxidant 1076. Based on the above embodiments, by introducing the above antioxidants, the thermo-oxidative degradation of polylactic acid and polybutylene terephthalate during melt processing can be effectively inhibited, the molecular chain breakage and molecular weight reduction can be slowed down, and the integrity of the polyester matrix structure can be maintained, thereby maintaining the stability of the barrier properties and mechanical properties of the membrane material during processing and use.

[0042] Secondly, this application provides a method for preparing a biodegradable film material for packaging, comprising: Provide the raw materials included in the biodegradable membrane material according to any embodiment of the first aspect; The raw materials are melt-co-extruded to obtain a biodegradable film material for packaging.

[0043] According to this application, the raw materials can achieve stable blending and continuous film formation under conventional melt extrusion equipment conditions, without the need for complex multilayer co-extrusion or post-processing, thus producing biodegradable film materials with good film-forming properties and processing stability. This method has a simple process flow, is applicable to existing packaging film production lines, and is beneficial for the industrial preparation and practical application of the biodegradable film materials.

[0044] Compared with the prior art, the beneficial effects of this application are at least as follows: 1) This application introduces a double-ended epoxy-capped sulfonate polyester and works in conjunction with an organic divalent metal salt into a biodegradable single-layer film system composed of polylactic acid and polybutylene terephthalate adipate. This achieves a simultaneous reduction in water vapor permeability and oxygen permeability without relying on a multilayer composite structure or inorganic filler, thus solving the problem of insufficient barrier performance of existing biodegradable single-layer packaging films.

[0045] 2) The dense microstructure constructed in this application mainly originates from the ionic coordination or association between sulfonate groups and organic divalent metal salts. This type of ionic interaction is reversible and thermosensitive, and it is not easy to form a permanent cross-linked structure during the melt processing. Therefore, while improving the barrier performance of the membrane material, it will not significantly affect the melt processing stability and film formation adaptability of the material.

[0046] 3) By rationally selecting the structural form of sulfonate-containing polyesters and the type and ratio of organic divalent metal salts, the ion interaction structure can be uniformly distributed in different polyester phases and their interface regions, avoiding the problem of local concentration or absence of ion structure, thereby improving the continuity and long-term stability of the dense structure of the membrane material, which is conducive to maintaining the barrier performance during use.

[0047] 4) The raw material system used in this application is mainly composed of common biodegradable polyester. The preparation method only involves conventional melt blending and extrusion film forming processes. There is no need to introduce additional complex post-processing steps or special equipment. The process flow is simple, applicable to existing packaging film production lines, and has good industrial feasibility. Detailed Implementation

[0048] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0049] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0050] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0051] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0052] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0053] Polylactic acid has a melt flow rate of approximately 8 g / 10 min at 190°C / 2.16 kg. The melt flow rate of polybutylene terephthalate (PET) at 190°C / 2.16 kg is approximately 4 g / 10 min.

[0054] Preparation Example 1 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: S1: Sodium isophthalic acid-5-sulfonate and adipic acid are added to a reaction vessel equipped with mechanical stirring, reflux condensation, temperature control and pressure reduction devices, wherein the molar amount of sodium isophthalic acid-5-sulfonate accounts for 5% of the total molar amount of the dicarboxylic acid.

[0055] Subsequently, diols are added to achieve a total molar ratio of 1:1.1 between the total molar amount of the dicarboxylic acid and the total molar amount of the diol. The diols include 1,4-butanediol and 1,4-cyclohexanediethanol, wherein the molar amount of 1,4-cyclohexanediethanol accounts for 15% of the total molar amount of the diols. Tetrabutyl titanate is added to the reaction system as a polycondensation catalyst, at an amount equal to 0.3% of the total molar amount of the dicarboxylic acid.

[0056] Under nitrogen protection, the reaction system was heated to 200°C and subjected to a polycondensation reaction. During the reaction, low-molecular-weight byproducts such as water were continuously discharged through a condenser. After 4 hours of reaction, the system pressure was gradually reduced to -0.08 MPa, and the reaction was continued at this pressure for another hour. After the reaction was completed, the system was cooled to room temperature under nitrogen protection to obtain a sulfonate polyester with hydroxyl-terminated ends.

[0057] S2: Add the sulfonate polyester with double-ended hydroxyl groups obtained in step S1 to the reactor, and add epichlorohydrin to make the ratio of the total molar amount of hydroxyl groups in the sulfonate polyester with double-ended hydroxyl groups (tested by acetylation-back titration method) to the molar amount of epichlorohydrin 1:10.

[0058] Under nitrogen protection, the reaction system was heated to 65°C and stirred continuously for 6 hours to induce etherification of the polyester terminal hydroxyl groups with epichlorohydrin, generating chlorohydrin-terminated structures. After the reaction was completed, unreacted epichlorohydrin was removed by vacuum distillation to obtain a sulfonate polyester with dichlorohydrin-terminated ends.

[0059] S3: Dissolve the sulfonate polyester with dichlorohydrin end-capped structure obtained in step S2 in toluene, so that the mass ratio of the sulfonate polyester with dichlorohydrin end-capped structure to toluene is 1:5. Under stirring conditions, add 10wt% sodium hydroxide aqueous solution to the reaction system, and the mass ratio of sodium hydroxide aqueous solution to toluene is 1:10.

[0060] Under the above alkaline conditions, the temperature of the reaction system was controlled at 50°C, and the reaction was continued to be stirred for 2 hours, so that the chlorohydrin end group underwent a dehydrochlorination reaction in the alkaline environment and cyclized to generate an epoxy structure.

[0061] After the reaction was complete, deionized water was added to the system for washing. After standing and separating the layers, the organic phase was separated. The washing was repeated until the washing solution was neutral. Subsequently, the solvent was removed by vacuum distillation, and finally, a sulfonate polyester A with double-ended epoxy resin was obtained.

[0062] Preparation Example 2 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: The preparation method is largely the same as in Example 1, except that in step S1, the molar amount of sodium isophthalic acid-5-sulfonate accounts for 15% of the total molar amount of the dicarboxylic acid, ultimately yielding a sulfonate polyester B with double-ended epoxy end capping.

[0063] Preparation Example 3 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: The preparation method is largely the same as in Example 1, except that in step S1, the molar amount of sodium isophthalic acid-5-sulfonate accounts for 1% of the total molar amount of the dicarboxylic acid, ultimately yielding a sulfonate polyester C with double-ended epoxy capping.

[0064] Preparation Example 4 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: The preparation method is largely the same as in Example 1, except that in step S1, the diol is only 1,4-butanediol, and the final product is a sulfonate polyester D with double-ended epoxy.

[0065] Preparation Example 5 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: The preparation method is largely the same as in Example 1, except that in step S1, the diol comprises 1,4-butanediol and 1,4-cyclohexanediethanol, wherein the molar amount of 1,4-cyclohexanediethanol accounts for 25% of the total molar amount of the diol, and finally a sulfonate polyester E with double-ended epoxy end capping is obtained.

[0066] Preparation Example 6 Preparation of sulfonate-containing polyesters with double-ended epoxy capping: The preparation method is largely the same as in Example 1, except that in step S1, the diol comprises 1,4-butanediol and terephthalic acid, wherein the molar amount of terephthalic acid accounts for 15% of the total molar amount of the diol, and finally a sulfonate polyester F with double-ended epoxy end capping is obtained.

[0067] Comparative Preparation Example 1 Preparation of double-ended epoxy-capped polyester: The preparation method is largely the same as in Example 1, except that in step S1, the dicarboxylic acid includes isophthalic acid and adipic acid, wherein the molar amount of isophthalic acid accounts for 5% of the total molar amount of the dicarboxylic acid, and finally a polyester with double-ended epoxy capping is obtained.

[0068] Example 1 Preparation of biodegradable film materials for packaging: Weigh out polylactic acid (PLA), polybutylene terephthalate (PET), a double-epoxy-terminated sulfonate polyester, an organic divalent metal salt, and an antioxidant as raw materials. The raw materials include 50 parts PLA, 40 parts PET, 3 parts double-epoxy-terminated sulfonate polyester A, 0.5 parts organic divalent metal salt (a mixture of zinc stearate and zinc lactate in a 1:2 mass ratio), and 0.2 parts antioxidant. The PLA and PET are dried separately before use and then sealed for later use.

[0069] Dried polylactic acid, polybutylene terephthalate (PET), double-ended epoxy-capped sulfonate polyester, organic divalent metal salt, and antioxidant were premixed in a mixing device to obtain a raw material mixture. This raw material mixture was then fed into a twin-screw extruder for melt blending and extrusion. The extruder's temperature zones along the material conveying direction were sequentially set as follows: Zone 1 150°C, Zone 2 160°C, Zone 3 170°C, Zone 4 180°C, Zone 5 180°C, with the die head temperature set at 180°C. Vacuum exhaust was activated during extrusion. The extrudate was water-cooled and then pelletized to obtain biodegradable membrane material blend particles.

[0070] After further drying, the resulting blended particles are used to form a single-layer film using a single-screw extruder. During film formation, the temperature zones of the single-screw extruder barrel are sequentially set to 155℃, 165℃, 175℃, and 180℃, with the die head temperature controlled at 180℃ and the screw speed maintained at a medium range to ensure continuous and stable melt extrusion. The melt is then blown into a film through an annular die, with the blow-up ratio controlled within the range of 2.0 to 2.5. The traction speed is matched to the extrusion volume to ensure stable film bubble formation. During film formation, the film bubble is cooled and shaped using an air ring. Following traction, pressure rollers, and winding processes, a single-layer biodegradable packaging film is obtained, with a film thickness of approximately 40μm.

[0071] Example 2 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that: double-ended epoxy-capped sulfonate polyester B is used instead of double-ended epoxy-capped sulfonate polyester A in the raw materials.

[0072] Example 3 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that: double-ended epoxy-capped sulfonate polyester C is used instead of double-ended epoxy-capped sulfonate polyester A in the raw materials.

[0073] Example 4 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that: double-ended epoxy-capped sulfonate polyester D is used instead of double-ended epoxy-capped sulfonate polyester A in the raw materials.

[0074] Example 5 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that: double-ended epoxy-capped sulfonate polyester E is used instead of double-ended epoxy-capped sulfonate polyester A in the raw materials.

[0075] Example 6 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that: double-ended epoxy-capped sulfonate polyester F is used instead of double-ended epoxy-capped sulfonate polyester A in the raw materials.

[0076] Example 7 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that only zinc lactate is used as an organic divalent metal salt in the raw materials.

[0077] Example 8 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that only zinc stearate is used as an organic divalent metal salt in the raw materials.

[0078] Comparative Example 1 Preparation of biodegradable film materials for packaging: Similar to Example 1, except that the raw materials used were double-epoxy-terminated polyester from Comparative Preparation Example 1 instead of double-epoxy-terminated sulfonate polyester A.

[0079] Comparative Example 2 Preparation of biodegradable film materials for packaging: It is largely the same as Example 1, except that organic divalent metal salts are not used in the raw materials.

[0080] Test section The biodegradable film materials for packaging obtained from the above embodiments and comparative examples were placed in an environment of 23±2℃ and 50±5%RH for 24 hours and then subjected to the following tests.

[0081] Water vapor transmission rate (WVT) test: Referring to GB / T1037-2021 "Determination of Water Vapor Transmission Performance of Plastic Films and Sheets - Cup Method for Weight Gain and Loss", the water vapor transmission rate (g / (m³)) was tested under conditions of 38℃ / 90%RH. 2 ·d)).

[0082] Oxygen Transmission Rate (OTR) Test: The oxygen transmission rate (cm) is measured using an oxygen transmission rate tester at one atmosphere and 23°C / 0%RH. 3 / (m 2 ·d)). The results are shown in Table 1.

[0083] Table 1

[0084] According to Table 1, compared with Comparative Example 1 and Comparative Example 2, the water vapor transmission rate (WVT) and oxygen transmission rate (OTR) of the biodegradable film materials for packaging prepared in each embodiment were significantly reduced. This indicates that by introducing a sulfonate polyester with double-ended epoxy end capping into a biodegradable monolayer film system composed of polylactic acid and polybutylene terephthalate, and with the synergistic effect of organic divalent metal salts, this application can effectively improve the barrier performance of the film material and achieve simultaneous suppression of water vapor and oxygen transmission behavior without relying on a multilayer composite structure. The reason may be that, in Comparative Example 1, the double-ended epoxy-capped polyester used does not contain sulfonate structures. It mainly plays a certain role in end-group reaction or compatibility in the system, and it is difficult to form a stable ionic dense structure in the polyester matrix. Therefore, its restriction effect on polymer chain segment movement and free volume is limited, resulting in a relatively short diffusion path of water vapor and oxygen in the membrane material and a limited degree of improvement in barrier performance. In Comparative Example 2, although sulfonate structures were introduced into the system, no organic divalent metal salt was added. The sulfonic acid groups mainly exist in the form of free ions, which have strong polarity and hydrophilicity. They easily promote the adsorption and migration of water in the membrane material, and it is also difficult to form stable and uniform ion association nodes. As a result, the water vapor permeability and oxygen permeability of the membrane material are both at a high level.

[0085] As shown in Examples 1-3, the sulfonate content in the double-ended epoxy-capped sulfonate-containing polyester has a certain impact on the barrier properties of the membrane material. When the sulfonate content is moderate (Example 1), the sulfonate structure can form relatively uniform and stable ionicly dense nodes under the action of organic divalent metal salts, effectively restricting the movement of polymer chain segments, thereby achieving lower water vapor permeability and oxygen permeability. When the sulfonate content is further increased (Example 2), the barrier properties of the membrane material decrease, indicating that when the sulfonate content is too high, its structural distribution and polarity characteristics may have a certain impact on the densification effect. When the sulfonate content is further decreased (Example 3), the effect on improving the barrier properties of the membrane material is limited.

[0086] As shown in Examples 1 and 4-6, the different diol structures in the sulfonate-containing polyester with double-ended epoxy end caps also have a certain impact on the barrier properties of the membrane material. When the diol is only an aliphatic diol (Example 4), the water vapor permeability and oxygen permeability of the resulting membrane material are relatively high; while after introducing a certain proportion of alicyclic or aromatic diols into the diol (Examples 5 and 6), the barrier properties of the membrane material are significantly improved. This indicates that by controlling the rigidity and flexibility of the chain segment structure in the polyester, it is beneficial to enhance the restrictive effect of ion-dense nodes on the movement of polymer chain segments, thereby further extending the diffusion path of gas molecules and making the membrane material more suitable for packaging applications with high barrier performance requirements.

[0087] As demonstrated in Examples 1, 7, and 8, the type and combination of organic divalent metal salts also have a certain impact on the barrier performance of membrane materials. When only zinc lactate or only zinc stearate is used, the water vapor permeability and oxygen permeability of the membrane material are higher than when both organic divalent metal salts are used simultaneously. When zinc lactate and zinc stearate are used in combination (Example 1), the membrane material exhibits superior barrier performance. This indicates that the compatibility differences of different organic divalent metal salts in the polylactic acid phase and the polybutylene terephthalate-adipate phase help to make the ion-associated structure more uniformly and stably distributed in the polyester matrix, thereby further improving the overall barrier effect of the membrane material.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A biodegradable film material for packaging, characterized in that, Includes the following quantities of raw materials: 50 parts polylactic acid, 30-50 parts polybutylene terephthalate adipate, 2-5 parts double-ended epoxy-capped sulfonate polyester, 0.2-1 parts organic divalent metal salt, and 0.1-0.5 parts antioxidant.

2. The biodegradable membrane material according to claim 1, characterized in that, The method for preparing the double-ended epoxy-capped sulfonate polyester includes the following steps: S1: Sulfonate-containing dicarboxylic acid, aliphatic dicarboxylic acid and diol undergo polycondensation reaction under catalytic conditions to obtain sulfonate-containing polyester with double-ended hydroxyl groups; S2: The sulfonate polyester with double hydroxyl end capping is mixed with epichlorohydrin, and the hydroxyl groups of the sulfonate polyester with double hydroxyl end capping and the epoxy groups of epichlorohydrin are etherified to generate chlorohydrin end groups, so as to obtain sulfonate polyester with double chlorohydrin end capping. S3: The chlorohydrin groups in the sulfonate polyester with double-terminated chlorohydrin groups are dehydrochlorinated and cyclized under alkaline conditions to obtain a sulfonate polyester with double-terminated epoxy groups.

3. The biodegradable membrane material according to claim 2, characterized in that, In step S1, the ratio of the total molar amount of the sulfonate dicarboxylic acid and the aliphatic dicarboxylic acid to the molar amount of the diol is 1:1.05~1.

2.

4. The biodegradable membrane material according to claim 2, characterized in that, In step S1, the molar amount of the sulfonate dicarboxylic acid is 3% to 10% of the total molar amount of the sulfonate dicarboxylic acid and the aliphatic dicarboxylic acid.

5. The biodegradable membrane material according to claim 4, characterized in that, In step S1, the sulfonate-containing dicarboxylic acid includes sulfonate-containing aromatic dicarboxylic acids.

6. The biodegradable membrane material according to claim 2, characterized in that, In step S1, the diol includes aliphatic diols and alicyclic diols, and the alicyclic diol accounts for 10% to 20% of the total diol.

7. The biodegradable membrane material according to claim 2, characterized in that, In step S2, the molar ratio of the hydroxyl groups in the sulfonate polyester with double-ended hydroxyl groups to the molar ratio of the epichlorohydrin is 1:5~20.

8. The biodegradable membrane material according to claim 1, characterized in that, The organic divalent metal salt includes zinc stearate and zinc lactate, wherein the mass ratio of zinc stearate to zinc lactate is 1:1 to 3.

9. The biodegradable membrane material according to any one of claims 1 to 8, characterized in that, The raw materials meet at least one of the following conditions: 1) The melt flow rate of the polylactic acid at 190℃ / 2.16kg is 5~10g / 10min; 2) The melt flow rate of the polybutylene terephthalate at 190℃ / 2.16kg is 2~6g / 10min; 3) The antioxidants include at least one of antioxidant 1010 and antioxidant 1076.

10. A method for preparing a biodegradable film material for packaging, characterized in that, include: Provide the raw materials included in the biodegradable membrane materials according to claims 1 to 9; The raw materials are melt-co-extruded to obtain a biodegradable film material for packaging.

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