Heat-resistant high-toughness food-grade PS modified sheet material and preparation and application thereof

Abstract

The application discloses a heat-resistant high-toughness food-grade PS modified sheet and preparation and application thereof, and comprises the following components in mass fractions: 75-88 parts of styrene homopolymer, 8-15 parts of styrene-butadiene-styrene block copolymer, 2-5 parts of ethylene-octene copolymer grafted maleic anhydride and 1-3 parts of organic modified nano-montmorillonite; in the styrene-butadiene-styylene block copolymer, the styrene content is 35wt%-45wt%, and the 1,2-vinyl structure content in the butadiene block is greater than or equal to 35wt%; through synergistic effect of specific components, the application successfully breaks the technical bottleneck of the three trade-offs of toughening, heat resistance and food safety, so that the PS modified sheet has excellent impact toughness, high-temperature deformation resistance and extremely low migration out risk, and can meet the strict requirements of high-temperature / high-fat scenes such as hot food trays, microwave lunch boxes and fried food packaging.

Description

Technical Field

[0001] This invention relates to the field of PS modification technology, and in particular to a heat-resistant, high-toughness food-grade PS modified sheet and its preparation and application. Background Technology

[0002] Polystyrene (PS) sheets are widely used in food packaging, such as hot food trays, microwave-safe lunch boxes, and fried food packaging, due to their excellent rigidity, processability, and transparency. However, polystyrene itself is brittle and has poor impact resistance, making it prone to cracking during transportation and use, which severely limits its application in high-end food packaging. To address this issue, the art typically employs toughening modification by adding elastomers. However, the introduction of elastomers significantly lowers the Vicat softening temperature of the material, leading to a decrease in heat resistance and making it difficult to meet the requirements of high-temperature packaging scenarios. Simultaneously, low-molecular-weight residues in the elastomers are prone to migrate and precipitate from weak interfacial areas under high-temperature and high-oil environments, making it difficult to pass the migration amount test of national food safety standards. Therefore, how to maintain high heat resistance and high food safety while achieving toughening has become a long-standing technical challenge in this field.

[0003] In existing technologies, toughening modification often involves adding styrene-butadiene-styrene block copolymers (SBS) with low styrene content (<30%) and high cis-butadiene structure. While this approach improves the toughness of PS to some extent, low-styrene SBS has poor compatibility with the PS matrix, weak interfacial bonding, and its low glass transition temperature significantly reduces the heat resistance of the blend system. Simultaneously, the high cis-butadiene segments exhibit good flexibility and strong mobility, making them prone to deentanglement and migration under high-temperature, high-oil environments, leading to excessive total migration. Furthermore, the introduction of elastomers further complicates the material system, and effective technical means are lacking for simultaneously toughening while suppressing low-molecular-weight migration and maintaining heat resistance.

[0004] To address the aforementioned issues, researchers have attempted improvements by adding inorganic fillers, using compatibilizers, or adjusting processing techniques. However, it has consistently been difficult to simultaneously meet the comprehensive requirements of high toughness, high heat resistance, and high food safety. The reason for this lies in the inherent trade-offs among toughness, heat resistance, and safety: increasing the amount of elastomer improves toughness but leads to decreased heat resistance and increased migration risk; simply increasing the styrene content improves compatibility and heat resistance but sacrifices toughness; adding fillers enhances rigidity and barrier properties but easily causes interface defects and toughness loss. Therefore, how to overcome the performance constraints among these three aspects through the synergistic design of material structure and processing to achieve a balance of high toughness, high heat resistance, and high safety remains a pressing technical challenge in this field. Summary of the Invention

[0005] In view of this, this application provides a heat-resistant, high-toughness food-grade modified PS sheet and its preparation and application, in order to solve the problem of how to balance the safety and toughness of PS sheets in high-heat environments.

[0006] To achieve the above technical objectives, this application adopts the following technical solution: In a first aspect, this application provides a heat-resistant, high-toughness food-grade modified PS sheet, comprising the following components in parts by weight: 75-88 parts of styrene homopolymer, 8-15 parts of styrene-butadiene-styrene block copolymer, 2-5 parts of ethylene-octene copolymer grafted with maleic anhydride, and 1-3 parts of organically modified nano-montmorillonite; wherein the styrene-butadiene-styrene block copolymer has a styrene content of 35wt%-45wt%, and the 1,2-vinyl structure content in the butadiene block is ≥35wt%.

[0007] Preferably, the styrene-butadiene-styrene block copolymer has a linear structure.

[0008] Preferably, the weight ratio of styrene blocks to butadiene blocks in the styrene-butadiene-styrene block copolymer is 35-45:55-65; and the number average molecular weight of the styrene-butadiene-styrene block copolymer is 80,000-150,000.

[0009] Preferably, the weight-average molecular weight of the styrene homopolymer is 180,000-250,000, and the melt index (200℃ / 5kg) is 3-8g / 10min.

[0010] Preferably, the ethylene-octene copolymer grafted onto maleic anhydride is an ethylene-octene copolymer with an octene content of 20%-30%.

[0011] Preferably, the organically modified nano-montmorillonite is montmorillonite modified with octadecyl dimethyl benzyl ammonium chloride.

[0012] Secondly, this application provides a method for preparing a heat-resistant, high-toughness food-grade PS modified sheet, comprising the following steps: Organically modified nano-montmorillonite was heated and mixed with a portion of styrene homopolymer to obtain montmorillonite masterbatch; The remaining styrene homopolymer, styrene-butadiene-styrene block copolymer, ethylene-octene copolymer grafted with maleic anhydride and the montmorillonite masterbatch are melt-blended, extruded and granulated to obtain modified alloy granules. The modified alloy granules were subjected to extrusion casting to obtain thick sheets; The thick sheet is stretched longitudinally in one direction to obtain a stretched sheet, which is then heat-set and subjected to back pressure tension. After cooling and winding, the heat-resistant, high-toughness food-grade PS modified sheet is obtained.

[0013] Preferably, the longitudinal unidirectional stretch ratio is 3.5-4:1, and the stretching speed is 10-20m / min.

[0014] Preferably, the longitudinal unidirectional stretching method involves 4-6 pairs of stretching rollers performing gradient stretching, with the speed difference between adjacent rollers increasing progressively.

[0015] Thirdly, this application provides an application of a heat-resistant, high-toughness food-grade modified PS sheet in high-temperature / high-oil food packaging containers.

[0016] The beneficial effects of this application are as follows: This application utilizes a specific SBS structure with a styrene content of 35%-45% and a 1,2-vinyl structure content ≥35%, resulting in better compatibility with the PS matrix. Simultaneously, the rigidity of the butadiene segments effectively reduces the negative impact on the matrix's heat resistance and inhibits high-temperature migration. Furthermore, a chemical bonding network formed by the in-situ reaction of POE-g-MAH with organically modified nano-montmorillonite physically entangles with the SBS, further restricting the disentanglement and migration of butadiene segments at high temperatures. The dispersion of the montmorillonite sheets also extends the diffusion path of small molecules. Finally, a low-temperature, high-stretching process induces oriented crystallization in the PS matrix, forming physical cross-linking points to compensate for heat resistance. The synergistic effect of these three factors successfully overcomes the technical bottleneck of the trade-off between toughening, heat resistance, and food safety, enabling the modified PS sheet to possess excellent impact toughness, high-temperature deformation resistance, and extremely low risk of migration, meeting the stringent requirements of high-temperature / high-oil applications such as hot food trays, microwave-safe lunch boxes, and fried food packaging. Detailed Implementation

[0017] The embodiments of the technical solution of this application will be described in detail below. The following embodiments are only used to illustrate the technical solution of this application more clearly, and are therefore only examples, and should not be used to limit the scope of protection of this application.

[0018] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0019] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0020] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0021] Unless otherwise explicitly defined and specified herein, all technical and scientific terms used in this application shall have the generally accepted meanings understood by one of ordinary skill in the field of chemical and chemical materials technology (including but not limited to polymer chemistry, inorganic chemistry, organic synthesis, catalysis chemistry, materials processing, and chemical unit operations) based on their professional knowledge and conventional practice. The use of any terminology herein is intended to describe the specific embodiments of this application in the clearest and most accurate manner, so as to fully disclose the technical solution. Such use shall not in any way be construed as a limitation on the scope of the claims, nor does it imply the exclusion of equivalent technical solutions that could be reasonably known by one of skill in the art based on the concept of this application.

[0022] The terms "comprising," "including," "having," "containing," and any grammatical variations or similar expressions used in the specification and claims of this application are all open-ended and non-exhaustive descriptive terms. Their purpose is to explicitly describe the existence of the stated technical features, components, steps, or parts, while explicitly allowing and covering the possibility that other features, components, steps, parts, or any combinations thereof not explicitly listed may exist or be added to the technical solution, as long as such additions do not destroy the integrity and inventiveness of the original technical solution.

[0023] When the terms "embodiments," "some embodiments," or "specific embodiments" are mentioned in the specification, they refer to examples that, in conjunction with the specific parameters, materials, steps, and results described in that section, constitute one or a group of examples for implementing the technical solutions of this application. These embodiments are used for full disclosure and illustrative purposes, not for exhaustive enumeration. Those skilled in the art should understand that, without departing from the overall inventive concept of this application, the various technical features disclosed in different embodiments can be combined, substituted, modified, or deleted to form other implementation methods that are not listed one by one in the specification but also fall within the protection scope of this application.

[0024] Unless otherwise expressly specified and limited, all terms related to chemical process operations, material preparation, processing and analytical testing involved in this application shall be interpreted in the broadest sense based on the conventional understanding of those skilled in the art.

[0025] Regarding performance testing and structural characterization, all testing and characterization methods involved in this application, unless otherwise specified, refer to conventional methods known in the art. Specific testing conditions may be selected and adjusted according to the sample properties and relevant national standards, international standards, or industry-standard methods. Test items may include mechanical properties (such as tensile, bending, and impact strength), thermal properties (such as DSC and TGA analysis), and chemical stability (such as solvent resistance and acid / alkali corrosion resistance). Structural characterization methods may include FT-IR, NMR, XRD, SEM, TEM, and BET. All test results should be understood to be within the allowable range of conventional experimental errors.

[0026] Regarding numerical values ​​and ranges, all parameter ranges expressed in this application in the form of "from a certain value to a certain value" should be understood as explicitly disclosing the endpoints of the range, each specific numerical point between the endpoints, and all sub-ranges formed by any two numerical points within the range. For example, "30℃ to 80℃" discloses 30, 31, ..., 80℃, as well as sub-ranges such as 30-50℃, 45-70℃, etc. When a numerical value is preceded by "about," "approximately," or similar words, it indicates that the numerical value is allowed to have reasonable errors recognized in the art under the measurement or control conditions, which can generally be understood as the deviation allowed by relevant standards or a normal fluctuation range of ±5% or ±10%.

[0027] In the modification research of polystyrene (PS) sheets, how to simultaneously achieve high toughness, high heat resistance, and high food safety has always been a pressing problem to be solved in this field. Especially for high-temperature / high-fat food packaging scenarios, such as hot food trays, microwaveable lunch boxes, and fried food packaging, the material must be able to withstand high temperatures without deformation, have sufficient toughness to prevent brittleness during transportation, and ensure that harmful substances do not migrate into the food. All three are indispensable.

[0028] The aforementioned problems arise because polystyrene itself is brittle and must be toughened by adding elastomers. However, the introduction of elastomers lowers the Vicat softening temperature of the material, leading to a decrease in heat resistance. Simultaneously, low-molecular-weight residues in the elastomers are prone to migrate and precipitate from weak points at the interface under high-temperature oily conditions, making it difficult to meet national food safety standards. There is a trade-off between toughening, heat resistance, and safety.

[0029] Existing technologies typically employ SBS elastomers with low styrene content (<30%) and high cis-butadiene structure for toughening. While this approach improves the toughness of PS to some extent, it fails to address the technical challenge of simultaneously improving toughness while maintaining heat resistance and suppressing high-temperature migrations. This is because low-styrene SBS has poor compatibility with PS, resulting in weak interfacial bonding. Furthermore, its low glass transition temperature significantly reduces the heat resistance of the matrix, and at high temperatures, butadiene segments are prone to untangling and migrating out.

[0030] Based on this, this application was created.

[0031] This application provides a heat-resistant, high-toughness food-grade modified PS sheet, comprising the following components in parts by weight: 75-88 parts of styrene homopolymer (pure PS resin particles), 8-15 parts of styrene-butadiene-styrene block copolymer, 2-5 parts of ethylene-octene copolymer grafted with maleic anhydride, and 1-3 parts of organically modified nano-montmorillonite; wherein the styrene-butadiene-styrene block copolymer has a styrene content of 35wt%-45wt%, and the 1,2-vinyl structure content in the butadiene block is ≥35wt%.

[0032] In the above components, styrene homopolymer serves as the matrix resin, providing the material's basic rigidity, heat resistance, and processing foundation. The styrene-butadiene-styrene block copolymer, as the toughening phase, has a styrene content of 35%-45%, higher than that of conventional SBS, resulting in better compatibility with the PS matrix. Its butadiene blocks contain ≥35% 1,2-vinyl structures, a structural feature that gives the butadiene segments more side groups and greater molecular chain rigidity. The ethylene-octene copolymer grafted with maleic anhydride has a dual function: its ethylene-octene copolymer backbone has an affinity for the butadiene blocks of SBS, and its maleic anhydride groups can react with the active groups on the surface of the organically modified nano-montmorillonite to form a chemically bonded structure. The organically modified nano-montmorillonite participates in the above reactions to build the network; on the other hand, its layered structure, after dispersion in the material, can extend the diffusion path of small molecules.

[0033] The principle behind this solution's ability to address the technical problem is as follows: SBS with a styrene content of 35%-45% and a 1,2-vinyl structure content of ≥35% was selected to improve the compatibility between SBS and the PS matrix. Simultaneously, the high rigidity of the butadiene blocks minimized the negative impact on the heat resistance of the PS matrix, and the weak mobility of butadiene segments at high temperatures reduced the risk of migration. The chemical bond structure formed by POE-g-MAH and organically modified montmorillonite in the in-situ reaction physically entangled with SBS, limiting the disentanglement and migration of butadiene segments in SBS at high temperatures. This chemical bond structure also acts as a physical cross-linking point in the material, providing some compensation for heat resistance. The montmorillonite sheets, after dispersion in the material, form tortuous paths, extending the diffusion path of small molecules and further reducing migration. The low-temperature, high-strength stretching process caused oriented crystallization in the PS matrix, and the resulting crystalline regions acted as physical cross-linking points, contributing to both the material's strength and heat resistance.

[0034] In this application, if the 1,2-vinyl structure content in the butadiene block is too low, the butadiene chain segment exhibits good flexibility and strong toughening ability as an isolated elastomer particle. However, its strong molecular chain mobility reduces the heat resistance of the blend system and makes it prone to disentanglement and migration at high temperatures. This application selects SBS with a high 1,2-vinyl structure. While its toughening ability as an isolated particle decreases, toughening is achieved at the overall material level through the synergistic effect of subsequent chemical bonding structure and oriented crystallization, while maintaining heat resistance and low migration.

[0035] In some embodiments, the styrene-butadiene-styrene block copolymer has a linear structure.

[0036] In this embodiment, the linear SBS structure exhibits better compatibility with the PS matrix, enabling the formation of a finer dispersed phase during melt blending. This provides more interfacial contact points for subsequent entanglement between the chemically bonded structure and the SBS. Conversely, if a star-shaped SBS structure is used, its high degree of molecular chain branching results in poor compatibility with PS, easily forming a coarse dispersed phase, which is detrimental to interfacial bonding and network entanglement.

[0037] In some embodiments, the weight ratio of styrene blocks to butadiene blocks in the styrene-butadiene-styrene block copolymer is 35-45:55-65; the number average molecular weight of the styrene-butadiene-styrene block copolymer is 80,000-150,000.

[0038] In this embodiment, the weight ratio of styrene blocks to butadiene blocks is within this range, ensuring that the styrene content in SBS remains stable at 35%-45%; the number-average molecular weight of 8-150,000 ensures sufficient chain entanglement between SBS and the PS matrix while avoiding processing difficulties due to excessively high molecular weight. If the molecular weight is too low, the entanglement is insufficient, and the bond between SBS and the network is weak; if the molecular weight is too high, the dispersed phase size is coarse, affecting the material uniformity.

[0039] In some embodiments, the weight-average molecular weight of the styrene homopolymer is 180,000-250,000, and the melt index (200℃ / 5kg) is 3-8g / 10min.

[0040] In this embodiment, a weight-average molecular weight of 180,000-250,000 provides sufficient molecular chain length to ensure effective orientation and crystalline structure formation during stretching; a melt index of 3-8 g / 10 min ensures the flowability of the extrusion casting process, matching the subsequent stretching process. If the molecular weight is too low, the stretching orientation effect is poor, and the crystallinity is insufficient; if the molecular weight is too high, the melt flowability is poor, and extrusion is difficult.

[0041] In some embodiments, the ethylene-octene copolymer grafted onto maleic anhydride is an ethylene-octene copolymer with an octene content of 20%-30%.

[0042] In this embodiment, POE with an octene content of 20%-30% exhibits good affinity with SBS butadiene blocks, effectively participating in network construction while maintaining a certain degree of rigidity. If the octene content is too low, the POE segments are too stiff, resulting in poor compatibility with SBS; if the octene content is too high, the POE is too soft, leading to insufficient strength.

[0043] In some embodiments, the organically modified nano-montmorillonite is montmorillonite modified with octadecyl dimethyl benzyl ammonium chloride.

[0044] In this embodiment, octadecyl dimethyl benzyl ammonium chloride is used as an organic modifier. Its quaternary ammonium salt group can react with the maleic anhydride group of POE-g-MAH to form a chemical bond. If other organic modifiers are used, their reactivity is insufficient, making it difficult to form an effective chemical bond.

[0045] This application provides a method for preparing a heat-resistant, high-toughness food-grade PS modified sheet, comprising the following steps: Organically modified nano-montmorillonite was heated and mixed with a portion of styrene homopolymer to obtain montmorillonite masterbatch; The remaining styrene homopolymer, styrene-butadiene-styrene block copolymer, ethylene-octene copolymer grafted with maleic anhydride and the montmorillonite masterbatch are melt-blended, extruded and granulated to obtain modified alloy granules. The modified alloy granules were subjected to extrusion casting to obtain thick sheets; The thick sheet is stretched longitudinally in one direction to obtain a stretched sheet, which is then heat-set and subjected to back pressure tension. After cooling and winding, the heat-resistant, high-toughness food-grade PS modified sheet is obtained.

[0046] In the above preparation method, the first step adopts a two-step pre-dispersion method. First, montmorillonite and a portion of PS are made into a masterbatch, which further expands the interlayer spacing of montmorillonite under high-temperature shear, providing better interfacial conditions for the subsequent reaction with POE-g-MAH. In the second step, the melt blending stage, POE-g-MAH and montmorillonite undergo an in-situ reaction to form a chemically bonded structure. At the same time, this structure is combined with SBS through physical entanglement. The third step, the low-temperature high-stretching process, is carried out at 105-115℃. At this time, the molecular chain movement is restricted, and the formed chemically bonded structure is oriented along the stretching direction. At the same time, the PS matrix undergoes strain-induced crystallization. In the fourth step, the heat setting stage, back pressure tension is applied to eliminate internal stress while maintaining the oriented structure.

[0047] Specifically, the preparation method of this application is as follows: Organically modified nano-montmorillonite and a portion of styrene homopolymer are mixed at a weight ratio of 1:2-1:4 and mixed at 80-100℃ and 800-1200 rpm for 10-20 minutes to obtain montmorillonite masterbatch. The remaining styrene homopolymer, styrene-butadiene-styrene block copolymer, ethylene-octene copolymer grafted with maleic anhydride, and the montmorillonite masterbatch are added to a twin-screw extruder and melt-blended at 180-220℃ with a screw speed of 300-500 rpm and an aspect ratio of 48-56:1. The mixture is then extruded. The modified alloy granules are obtained by granulation. The modified alloy granules are added to a single screw extruder and extruded through a T-die at 200-220℃ to cast into a sheet with a thickness of 0.3-0.8mm. The sheet is preheated at 105-115℃ and then stretched longitudinally in one direction at a stretch ratio of 3.5:1 to 4.0:1 and a stretching speed of 10-20m / min to obtain a stretched sheet. The stretched sheet is heat-set at 95-105℃ for 20-40 seconds while applying a back pressure tension of 0.2-0.4MPa. After cooling to room temperature, it is wound up.

[0048] In some embodiments, the longitudinal unidirectional stretch ratio is 3.5-4:1, and the stretching speed is 10-20 m / min.

[0049] In this embodiment, a stretching ratio of 3.5-4:1 is a range that matches the formulation system of this application. Below 3.5, the PS matrix exhibits insufficient orientation and crystallization, and the orientation degree of the chemically bonded structure is also inadequate; above 4.0, excessive stretching may lead to the destruction of the dispersed phase structure, resulting in stress whitening. A stretching speed of 10-20 m / min ensures the stability of the stretching process.

[0050] In some embodiments, the longitudinal unidirectional stretching method involves 4-6 pairs of stretching rollers performing gradient stretching, with the speed difference between adjacent rollers increasing progressively.

[0051] In this embodiment, gradient stretching results in more uniform molecular chain orientation, avoiding localized stress concentration caused by high-speed stretching in a single step. The design of gradually increasing speed difference allows for the gradual application of tensile stress, giving the chemically bonded structure and dispersed phase sufficient time to respond.

[0052] This application provides an application of a heat-resistant, high-toughness food-grade modified PS sheet in high-temperature / high-oil food packaging containers.

[0053] The following specific embodiments further illustrate this solution.

[0054] Raw material preparation: Styrene-butadiene-styrene block copolymers are prepared according to the following steps: In a 5L polymerization reactor fully purged with nitrogen, 3L of cyclohexane is added as a solvent, followed by the sequential addition of 200g styrene, 0.00833mol n-butyllithium, and 0.083mol tetrahydrofuran. Polymerization is carried out at 60°C for 1 hour to complete the synthesis of the first polystyrene block. Then, 600g of butadiene monomer is added to the reaction system, and polymerization continues at 60°C for 2 hours. At this time, due to the regulating effect of tetrahydrofuran, the 1,2-vinyl structure content in the polybutadiene block can reach more than 35%. Then, 200g of styrene is added, and polymerization is carried out at 60°C for 1 hour to complete the synthesis of the second polystyrene block. After the reaction is completed, a small amount of water is added to terminate the reaction, and an antioxidant is added. The polymer solution is coagulated in hot water to remove the solvent. The resulting granules are washed, dried, and extruded to obtain a linear SBS product with a styrene content of 40%, a 1,2-vinyl structure content of ≥35%, and a number average molecular weight of 120,000.

[0055] Example 1 A heat-resistant, high-toughness food-grade modified PS sheet is prepared as follows: Weigh 2.5 kg of organically modified nano-montmorillonite (Fenghong FH-OMMT-1827) and 7.5 kg of styrene homopolymer (Sigma-Aldrich 430102), add the above materials to a high-speed mixer, set the mixing temperature to 90℃ and the stirring speed to 1000 rpm, and continue mixing for 15 minutes, controlling the material temperature at 90±5℃ during the mixing process. After mixing, discharge and cool to obtain montmorillonite masterbatch for later use; 74.5 parts of styrene homopolymer, 12 parts of styrene-butadiene-styrene block copolymer (linear structure, styrene content 40%, 1,2-vinyl structure ≥35%), 3 parts of ethylene-octene copolymer grafted with maleic anhydride (Dow FUSABOND N493), and 10 parts of montmorillonite masterbatch were added to a twin-screw extruder for melt blending. The twin-screw extruder parameters were set as follows: screw length-to-diameter ratio (L / D) of 52:1, screw diameter of 65 mm; extrusion temperature from feed port to die head of 180℃, 190℃, 200℃, 210℃, 200℃, and die head of 200℃ (overall within the range of 180-220℃); screw speed of 400 rpm (within the range of 300-500 rpm); and feed rate of 50 kg / h. After the material is melt-blended, extruded, cooled and pelletized, modified alloy granules are obtained. The granules are dried in a forced-air drying oven at 80°C for 4 hours to remove moisture. The dried modified alloy granules were added to a single-screw extruder and extruded into thick sheets through a T-die. The extruder parameters were as follows: screw diameter 90mm; extrusion temperature: feeding section 180℃, compression section 200℃, metering section 220℃, die temperature 210℃ (die temperature within the range of 200-220℃); die width 800mm; cooling roller temperature 40℃; traction speed 3m / min; by adjusting the die lip gap and traction speed, a cast sheet with a thickness of 0.5mm (within the range of 0.3-0.8mm) and a width of 700mm was obtained. The cast sheet is fed into a longitudinal stretching unit for unidirectional stretching. The stretching unit consists of preheating rollers, five pairs of stretching rollers, and heat-setting rollers. During the preheating stage, the sheet passes sequentially through five preheating rollers at temperatures of 90℃, 100℃, 110℃, 110℃, and 105℃, respectively, to uniformly raise the sheet temperature to the stretching temperature. During the stretching stage, five pairs of stretching rollers are used for gradient stretching, with the speed difference between adjacent rollers increasing progressively. The speeds of each pair of stretching rollers are set as follows: the first pair of stretching rollers has a speed of 5 m / min, the second pair has a speed of 8 m / min (stretch ratio 1.6), and so on. The stretching rollers have a speed of 12 m / min for the third pair (stretch ratio 1.5), a speed of 17 m / min for the fourth pair (stretch ratio 1.4), and a speed of 19 m / min for the fifth pair (stretch ratio 1.1), with a total stretch ratio of 3.8:1. The roller temperature is maintained at 110℃ during the stretching process, and the final stretching speed (speed of the fifth roller) is 19 m / min. In the heat setting stage, the stretched sheet enters the heat setting roller and is heat-set at 100℃ for 30 seconds. At the same time, a back pressure tension of 0.3 MPa is applied at the exit of the heat setting roller to maintain the orientation structure of the sheet. After heat setting, the sheet is cooled to room temperature (25°C) by cooling rollers and then wound up to obtain a heat-resistant, high-toughness food-grade PS modified sheet with a final sheet thickness of 0.14 mm.

[0056] Example 2 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the longitudinal tensile ratio is 1:1.

[0057] Example 3 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the longitudinal tensile ratio is 2.5:1.

[0058] Example 4 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the longitudinal tensile ratio is 4.5:1.

[0059] Example 5 A heat-resistant, high-toughness food-grade modified PS sheet is described, with other contents being the same as in Example 1, except that the stretching method is a single stretching, completed in one step with a ratio of 3.8:1.

[0060] Comparative Example 1 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the styrene content in the styrene-butadiene-styrene block copolymer is 30 wt%.

[0061] Comparative Example 2 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the styrene content in the styrene-butadiene-styrene block copolymer is 50 wt%.

[0062] Comparative Example 3 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that the 1,2-vinyl content in the styrene-butadiene-styrene block copolymer is 20wt%.

[0063] Comparative Example 4 A heat-resistant, high-toughness food-grade modified PS sheet is the same as in Example 1, except that POE-g-MAH is not added.

[0064] Comparative Example 5 A heat-resistant, high-toughness food-grade modified PS sheet is identical to that in Example 1, except that montmorillonite is not added.

[0065] Testing and Evaluation The impact strength of different embodiments and comparative examples was tested according to GB / T 1043.1-2008; the Vicat softening temperature of different embodiments and comparative examples was tested according to GB / T 1633-2000; and the total migration of olive oil simulants of different embodiments and comparative examples was tested according to GB 31604.8-2021. The results are shown in Table 1.

[0066] Table 1 Test Results

[0067] This application utilizes SBS with a specific styrene content and a 1,2-vinyl structure as the toughening phase, combined with a chemical bonding network formed by the in-situ reaction of POE-g-MAH and organically modified montmorillonite, and incorporates optimized longitudinal gradient stretching and heat setting processes. This successfully achieves a synergistic improvement in the high toughness, high heat resistance, and high food safety of modified PS sheets. This technical solution effectively solves the industry problems of decreased heat resistance and easy migration and precipitation of small molecules at high temperatures caused by toughening modification in existing technologies. It is particularly suitable for high-temperature / high-fat food packaging applications such as hot food trays, microwaveable lunch boxes, and fried food packaging.

[0068] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations 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 heat-resistant, high-toughness food-grade modified PS sheet, characterized in that, The product comprises the following components in parts by weight: 75-88 parts styrene homopolymer, 8-15 parts styrene-butadiene-styrene block copolymer, 2-5 parts ethylene-octene copolymer grafted with maleic anhydride, and 1-3 parts organically modified nano-montmorillonite; wherein the styrene-butadiene-styrene block copolymer contains 35wt%-45wt% styrene, and the 1,2-vinyl structure content in the butadiene block is ≥35wt%.

2. The heat-resistant, high-toughness food-grade PS modified sheet according to claim 1, characterized in that, The styrene-butadiene-styrene block copolymer has a linear structure.

3. The heat-resistant, high-toughness food-grade PS modified sheet according to claim 1, characterized in that, The weight ratio of styrene blocks to butadiene blocks in the styrene-butadiene-styrene block copolymer is 35-45:55-65; the number average molecular weight of the styrene-butadiene-styrene block copolymer is 80,000-150,000.

4. The heat-resistant, high-toughness food-grade PS modified sheet according to claim 1, characterized in that, The weight-average molecular weight of the styrene homopolymer is 180,000-250,000, and the melt index (200℃ / 5kg) is 3-8g / 10min.

5. The heat-resistant, high-toughness food-grade PS modified sheet according to claim 1, characterized in that, The ethylene-octene copolymer grafted onto maleic anhydride is an ethylene-octene copolymer with an octene content of 20%-30%.

6. The heat-resistant, high-toughness food-grade PS modified sheet according to claim 1, characterized in that, The organically modified nano-montmorillonite is montmorillonite modified with octadecyl dimethyl benzyl ammonium chloride.

7. A method for preparing a heat-resistant, high-toughness food-grade PS modified sheet as described in any one of claims 1-6, characterized in that, Includes the following steps: Organically modified nano-montmorillonite was heated and mixed with a portion of styrene homopolymer to obtain montmorillonite masterbatch; The remaining styrene homopolymer, styrene-butadiene-styrene block copolymer, ethylene-octene copolymer grafted with maleic anhydride and the montmorillonite masterbatch are melt-blended, extruded and granulated to obtain modified alloy granules. The modified alloy granules were subjected to extrusion casting to obtain thick sheets; The thick sheet is stretched longitudinally in one direction to obtain a stretched sheet, which is then heat-set and subjected to back pressure tension. After cooling and winding, the heat-resistant, high-toughness food-grade PS modified sheet is obtained.

8. The preparation method according to claim 7, characterized in that, The longitudinal unidirectional stretch ratio is 3.5-4:1, and the stretching speed is 10-20 m / min.

9. The preparation method according to claim 7, characterized in that, The longitudinal unidirectional stretching method involves 4-6 pairs of stretching rollers performing gradient stretching, with the speed difference between adjacent rollers increasing progressively.

10. The application of a heat-resistant, high-toughness food-grade PS modified sheet as described in any one of claims 1-6 in high-temperature / high-oil food packaging containers.

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