High-temperature-resistant preservation box cover and preparation method thereof

Abstract

The invention relates to the technical field of high polymer materials, in particular to a high-temperature-resistant preservation box cover and a preparation method thereof. The high-temperature-resistant preservation box cover is prepared from the following components in parts by weight: 50 to 70 parts of polypropylene, 6 to 10 parts of modified polyimide, 10 to 15 parts of nano cellulose, 2 to 4 parts of plasticizer, 0.5 to 1 part of antioxidant and 0.5 to 1 part of ethylene bis stearamide. The preparation method of the modified polyimide comprises the steps of blending modification and post-treatment. The high-temperature-resistant fresh-keeping box cover prepared by the invention has excellent high-temperature resistance and excellent tensile property, is convenient to process and is beneficial to mass production.

Description

Technical Field

[0001] The invention relates to the technical field of polymer materials, and in particular to a high temperature resistant fresh-keeping box cover and a preparation method thereof. Background Art

[0002] In the field of food preservation, fresh-keeping box covers play a vital role as key components. With the increasing requirements of consumers for food preservation and the continuous enhancement of environmental awareness, traditional fresh-keeping box cover materials have gradually failed to meet market demand.

[0003] The existing high-temperature resistant fresh-keeping box lids on the market are mostly made of PVC material, but PVC contains vinyl chloride residues and plasticizers, which are harmful to human health. Long-term contact may interfere with the human endocrine system, affect the reproductive system, and cause damage to the liver and urinary system. Although the heat-resistant glass fresh-keeping box lid is resistant to high temperatures and can withstand rapid temperature changes, the tempered glass fresh-keeping box lid may explode when encountering extreme cold and heat. At the same time, the traditional fresh-keeping box lid also has the problems of insufficient pressure resistance and easy aging. These performance limitations limit the application of the fresh-keeping box lid and affect the safety and reliability of the product. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a high temperature resistant fresh-keeping box cover and a preparation method thereof, so as to solve the problem that the ordinary plastic material used in the traditional fresh-keeping box cover has poor heat resistance and is easy to soften and deform at high temperature.

[0005] Based on the above purpose, the present invention provides a high temperature resistant fresh-keeping box lid, which is prepared by the following raw materials in parts by weight: 50-70 parts of polypropylene, 6-10 parts of modified polyimide, 10-15 parts of nanocellulose, 2-4 parts of plasticizer, 0.5-1 part of antioxidant and 0.5-1 part of ethylene bisstearamide.

[0006] The modified polyimide is prepared by the following steps:

[0007] (1) using 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and pyromellitic dianhydride as reaction monomers, reacting at 0-5° C. for 2-4 hours to obtain polyamic acid, then adding anhydrous toluene and condensing and refluxing at 160-200° C. for 4-8 hours to obtain hydroxy polyimide;

[0008] (2) under nitrogen protection, hydroxy polyimide is added to N,N-dimethylformamide and stirred for 30 minutes, 17-octadecynoic acid is added, and a catalyst is added at the same time, and the temperature is raised to 70-90° C. and stirred for reaction for 6-10 hours. After the reaction is completed, the reaction mixture is poured into deionized water to precipitate, and the intermediate A is obtained after filtering, washing, and drying;

[0009] (3) Under nitrogen protection, the intermediate A is added to N,N-dimethylformamide and stirred for 30 minutes, anhydrous toluene and a catalyst are added, and phenyldimethylsilane is added dropwise. After the addition is completed, the temperature is raised to 50-60° C. and stirred for reaction for 7-9 hours. After the reaction is completed, the reaction mixture is filtered and poured into deionized water to precipitate. The precipitate is filtered, washed, and dried to obtain a modified polyimide.

[0010] Preferably, the weight average molecular weight of the polypropylene is 3000-5000.

[0011] Preferably, the diameter of the nanocellulose is 50-60 nm.

[0012] Preferably, the plasticizer is acetyl tributyl citrate or epoxidized soybean oil.

[0013] Preferably, the antioxidant is one of antioxidant 1010, antioxidant 1076 and antioxidant 168.

[0014] Preferably, in step (1), the weight ratio of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, pyromellitic dianhydride and anhydrous toluene is 0.8-1.2:0.8-1.2:3-5.

[0015] Preferably, in step (2), the weight ratio of N,N-dimethylformamide, hydroxypolyimide and 17-octadecynoic acid is 15-25:1-3:1.2-3.6.

[0016] Preferably, the catalyst in step (2) is sulfuric acid or p-toluenesulfonic acid with a concentration of 98%.

[0017] Preferably, in step (3), the weight ratio of N,N-dimethylformamide, intermediate A, phenyldimethylsilane and anhydrous toluene is 15-25:1-3:1.1-3.6:1.1-3.6.

[0018] Preferably, the catalyst in step (3) is one of palladium carbon, aluminum chloride, and diethylaluminum chloride.

[0019] Furthermore, the present invention also provides a preparation process of the above-mentioned high temperature resistant fresh-keeping box cover, and the specific preparation process is as follows:

[0020] S1: mixing polypropylene, modified polyimide, nanocellulose, plasticizer, antioxidant and ethylene bis stearamide to obtain a mixture;

[0021] S2: adding the mixed material into a twin-screw extruder for melt extrusion;

[0022] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure-maintaining and cooling, the material is formed to obtain a finished product of a high-temperature resistant fresh-keeping box cover.

[0023] Preferably, the temperature of each zone in the twin-screw extruder in step S2 is: feeding zone: 210-230°C, melting zone: 230-250°C, mixing zone: 250-270°C, metering zone: 270-290°C.

[0024] Preferably, in step S2, the head temperature of the twin-screw extruder is 280° C., and the screw speed is 180-220 rpm.

[0025] Preferably, in step S3, the injection pressure of the injection molding machine is 70-90 MPa, the injection speed is 40-60 mm / s, the holding time is 8-12 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 40-60°C.

[0026] Beneficial effects of the present invention:

[0027] 1. Excellent heat resistance: Modified polyimide plays a key role. After hydroxy polyimide is modified with 17-octadecynoic acid and phenyldimethylsilane, the molecular structure changes significantly. Alkyne groups will undergo cross-linking reactions under high temperature conditions, prompting the polyimide molecular chains to build a tight and stable three-dimensional network structure. This unique structure can effectively limit the thermal motion of the molecular chains, greatly enhancing the structural stability of the material at high temperatures, making it less likely to deform. In addition, the silicon-oxygen bond formed by the silane group introduced by phenyldimethylsilane combined with the molecular chain has a higher bond energy, and the introduced benzene ring increases the rigidity of the molecular chain. The two work together to improve the heat resistance of the material. Although nanocellulose is slightly inferior in heat resistance, it can maintain a certain physical stability at high temperatures due to its high crystallinity and fibrous structure. After being combined with modified polyimide, the modified polyimide resists most of the thermal shock, while nanocellulose disperses the thermal stress to prevent local overheating from causing the material to soften and deform, thereby ensuring the structural integrity of the fresh-keeping box lid, making it reliable when storing food after high-temperature cooking and preserving food in high-temperature outdoor environments.

[0028] 2. Excellent tensile properties: First, the modified polyimide undergoes ingenious chemical changes during the preparation process, forming a unique molecular structure. These molecules are like a precisely woven "invisible network", intertwined and tightly linked with each other. When stretched by external forces, they are like a group of closely cooperating partners, able to efficiently and collaboratively disperse the tension. This synergistic mechanism enables the material to withstand large tensile forces without easily causing structural damage during the stretching process, fundamentally ensuring that the fresh-keeping box cover has excellent tensile strength.

[0029] Secondly, the addition of nanocellulose, due to its natural fiber properties, allows the entire material system to maintain a certain rigidity while also having good flexibility. When the material is subjected to tensile force, nanocellulose can undergo elastic deformation like a spring, absorbing and buffering external forces, effectively avoiding excessive local concentration of stress. This greatly reduces the risk of sudden breakage of the material due to stress concentration, allowing the fresh-keeping box cover to exhibit excellent ductility during the stretching process, significantly improving the elongation at break.

[0030] Furthermore, the secondary modification of phenyldimethylsilane is like adding a "double insurance" to the tensile properties of the material. After the silane group is tightly combined with the molecular chain, a stable molecular connection bridge is constructed, which greatly strengthens the interaction between the molecular chains. During the stretching process, this strengthened connection is like a strong rope, firmly holding the molecular chain, so that it can remain stable when subjected to external forces, effectively preventing the molecular chain from slipping or breaking prematurely, and further consolidating the overall tensile properties of the material.

[0031] 3. Excellent processing performance: The present invention exhibits good adaptability and operability during the processing. Reasonable addition of plasticizers and precisely set parameters of the twin-screw extruder and injection molding machine ensure that the materials can be melted smoothly and mixed evenly during the processing. This not only reduces the difficulty of production and reduces the scrap rate in the production process, but also helps to achieve efficient and stable industrial large-scale production, improves production efficiency, reduces production costs, and meets the market's large demand for high-quality fresh-keeping box lids, which has important economic significance. At the same time, stable processing performance also helps to ensure the consistency of product quality, so that every fresh-keeping box lid produced can meet the expected performance standards. DETAILED DESCRIPTION

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0033] Example 1: Preparation of modified polyimide:

[0034] (1) 80 g of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 80 g of pyromellitic anhydride are used as reaction monomers, reacted at 0° C. for 2 h to obtain polyamic acid, then 3 kg of anhydrous toluene is added and condensed and refluxed at 160° C. for 4 h to obtain hydroxy polyimide; the weight ratio of N,N-dimethylformamide, hydroxy polyimide and 17-octadecynoic acid is 15-25:1-3:1.2-3.6.

[0035] (2) Under nitrogen protection, 80 g of hydroxy polyimide was added to 1.2 kg of N, N-dimethylformamide and stirred for 30 min, 96 g of 17-octadecynoic acid was added, and 4 g of 98% sulfuric acid was added at the same time, the temperature was raised to 70° C. and stirred for reaction for 6 h. After the reaction was completed, the reaction mixture was poured into 800 g of deionized water, precipitated, filtered, washed, and dried to obtain intermediate A;

[0036] (3) Under nitrogen protection, 80 g of intermediate A was added to 1.2 kg of N,N-dimethylformamide and stirred for 30 min. 88 g of anhydrous toluene and 0.4 g of palladium carbon were added, and 88 g of phenyldimethylsilane was added dropwise. After the addition was completed, the temperature was raised to 50 ° C. and stirred for 7 h. After the reaction was completed, the reaction mixture was filtered and poured into 800 g of deionized water to precipitate. After filtering, washing and drying, the modified polyimide was obtained.

[0037] The specific preparation process of a high temperature resistant fresh-keeping box cover is as follows:

[0038] S1: 500 g of polypropylene with a weight average molecular weight of 3000, 60 g of modified polyimide, 100 g of nanocellulose with a diameter of 50 nm, 20 g of acetyl tributyl citrate, 5 g of antioxidant 1010 and 5 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0039] S2: adding the mixed material to a twin-screw extruder for melt extrusion, the temperature of each zone in the twin-screw extruder is: feeding zone: 210°C, melting zone: 230°C, mixing zone: 250°C, metering zone: 270°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 180rpm;

[0040] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 70MPa, the injection molding speed is 40mm / s, the pressure holding time is 8 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 40℃.

[0041] Example 2: Preparation of modified polyimide:

[0042] (1) 100 g of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 100 g of pyromellitic dianhydride were used as reaction monomers, reacted at 3° C. for 3 h to obtain polyamic acid, then 4 kg of anhydrous toluene was added and condensed and refluxed at 180° C. for 6 h to obtain hydroxy polyimide;

[0043] (2) Under nitrogen protection, 100 g of hydroxy polyimide was added to 100 g of N, N-dimethylformamide and stirred for 30 min, 120 g of 17-octadecynoic acid was added, and 5 g of 98% sulfuric acid was added at the same time, the temperature was raised to 80° C. and stirred for reaction for 8 h. After the reaction was completed, the reaction mixture was poured into 1 kg of deionized water, precipitated, filtered, washed, and dried to obtain intermediate A;

[0044] (3) Under nitrogen protection, 100 g of intermediate A was added to 1 kg of N,N-dimethylformamide and stirred for 30 min. 120 g of anhydrous toluene and 0.5 g of palladium carbon were added, and 120 g of phenyldimethylsilane was added dropwise. After the addition was completed, the temperature was raised to 55 ° C. and the reaction was stirred for 8 h. After the reaction was completed, the reaction mixture was filtered and poured into 1 kg of deionized water. The precipitate was filtered, washed, and dried to obtain the modified polyimide.

[0045] The specific preparation process of a high temperature resistant fresh-keeping box cover is as follows:

[0046] S1: 600 g of polypropylene with a weight average molecular weight of 4000, 80 g of modified polyimide, 125 g of nanocellulose with a diameter of 55 nm, 30 g of acetyl tributyl citrate, 7.5 g of antioxidant 1010 and 7.5 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0047] S2: adding the mixed material to a twin-screw extruder for melt extrusion, the temperature of each zone in the twin-screw extruder is: feeding zone: 220°C, melting zone: 240°C, mixing zone: 260°C, metering zone: 280°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 200rpm;

[0048] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 80MPa, the injection molding speed is 50mm / s, the pressure holding time is 10 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 50℃.

[0049] Example 3: Preparation of modified polyimide:

[0050] (1) 120 g of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 120 g of pyromellitic dianhydride were used as reaction monomers, reacted at 5° C. for 4 h to obtain polyamic acid, then 5 kg of anhydrous toluene was added and condensed and refluxed at 200° C. for 8 h to obtain hydroxy polyimide;

[0051] (2) Under nitrogen protection, 120 g of hydroxy polyimide was added to 1 kg of N, N-dimethylformamide and stirred for 30 min, 144 g of 17-octadecynoic acid was added, and 6 g of 98% sulfuric acid was added at the same time, the temperature was raised to 90° C. and stirred for 10 h. After the reaction was completed, the reaction mixture was poured into 1.2 kg of deionized water, precipitated, filtered, washed, and dried to obtain intermediate A;

[0052] (3) Under nitrogen protection, 120 g of intermediate A was added to 1 kg of N,N-dimethylformamide and stirred for 30 min. 144 g of anhydrous toluene and 0.6 g of palladium carbon were added, and 144 g of phenyldimethylsilane was added dropwise. After the addition was completed, the temperature was raised to 60° C. and stirred for reaction for 9 h. After the reaction was completed, the reaction mixture was filtered and poured into 1.2 kg of deionized water to precipitate. The precipitate was filtered, washed, and dried to obtain a modified polyimide.

[0053] The specific preparation process of a high temperature resistant fresh-keeping box cover is as follows:

[0054] S1: 700 g of polypropylene with a weight average molecular weight of 5000, 100 g of modified polyimide, 150 g of nanocellulose with a diameter of 60 nm, 40 g of acetyl tributyl citrate, 10 g of antioxidant 1010 and 10 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0055] S2: adding the mixed material to a twin-screw extruder for melt extrusion, wherein the temperature of each zone in the twin-screw extruder is: feeding zone: 230°C, melting zone: 250°C, mixing zone: 270°C, metering zone: 290°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 220rpm;

[0056] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 90MPa, the injection molding speed is 60mm / s, the pressure holding time is 12 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 60℃.

[0057] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that no modified polyimide is added, and the specific preparation process is as follows:

[0058] S1: 600 g of polypropylene with a weight average molecular weight of 4000, 125 g of nanocellulose with a diameter of 55 nm, 30 g of acetyl tributyl citrate, 7.5 g of antioxidant 1010 and 7.5 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0059] S2: adding the mixed material to a twin-screw extruder for melt extrusion, the temperature of each zone in the twin-screw extruder is: feeding zone: 220°C, melting zone: 240°C, mixing zone: 260°C, metering zone: 280°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 200rpm;

[0060] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 80MPa, the injection molding speed is 50mm / s, the pressure holding time is 10 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 50℃.

[0061] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the modified polyimide is replaced by hydroxy polyimide, and the specific preparation process is as follows:

[0062] Preparation of hydroxy polyimide: 100 g of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 100 g of pyromellitic dianhydride were used as reaction monomers, reacted at 3° C. for 3 h to obtain polyamic acid, then 4 kg of anhydrous toluene was added and condensed and refluxed at 180° C. for 6 h to obtain hydroxy polyimide;

[0063] The specific preparation process of a high temperature resistant fresh-keeping box cover is as follows:

[0064] S1: 600 g of polypropylene with a weight average molecular weight of 4000, 80 g of hydroxy polyimide, 125 g of nanocellulose with a diameter of 55 nm, 30 g of acetyl tributyl citrate, 7.5 g of antioxidant 1010 and 7.5 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0065] S2: adding the mixed material to a twin-screw extruder for melt extrusion, the temperature of each zone in the twin-screw extruder is: feeding zone: 220°C, melting zone: 240°C, mixing zone: 260°C, metering zone: 280°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 200rpm;

[0066] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 80MPa, the injection molding speed is 50mm / s, the pressure holding time is 10 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 50℃.

[0067] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the polyimide is only modified with 17-octadecynoic acid without secondary modification with phenyldimethylsilane. The specific preparation process is as follows:

[0068] The preparation process of modified polyimide is as follows:

[0069] (1) 100 g of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and 100 g of pyromellitic dianhydride were used as reaction monomers, reacted at 3° C. for 3 h to obtain polyamic acid, then 4 kg of anhydrous toluene was added and condensed and refluxed at 180° C. for 6 h to obtain hydroxy polyimide;

[0070] (2) Under nitrogen protection, 100 g of hydroxy polyimide was added to 100 g of N, N-dimethylformamide and stirred for 30 min, 120 g of 17-octadecynoic acid was added, and 5 g of 98% sulfuric acid was added at the same time, the temperature was raised to 80° C. and stirred for reaction for 8 h. After the reaction was completed, the reaction mixture was poured into 1 kg of deionized water to precipitate, and the modified polyimide was obtained after filtering, washing, and drying;

[0071] The specific preparation process of a high temperature resistant heat preservation box cover is as follows:

[0072] S1: 600 g of polypropylene with a weight average molecular weight of 4000, 80 g of modified polyimide, 125 g of nanocellulose with a diameter of 55 nm, 30 g of acetyl tributyl citrate, 7.5 g of antioxidant 1010 and 7.5 g of ethylene bisstearamide were mixed and stirred to obtain a mixture;

[0073] S2: adding the mixed material to a twin-screw extruder for melt extrusion, the temperature of each zone in the twin-screw extruder is: feeding zone: 220°C, melting zone: 240°C, mixing zone: 260°C, metering zone: 280°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 200rpm;

[0074] S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure holding and cooling, it is formed to obtain a high-temperature resistant fresh-keeping box cover product. The injection molding pressure of the injection molding machine is 80MPa, the injection molding speed is 50mm / s, the pressure holding time is 10 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 50℃.

[0075] Performance Testing

[0076] 1. High temperature stability test:

[0077] In order to test the high temperature resistance of the fresh-keeping box cover, the following tests were performed on Examples 1-3 and Comparative Examples 1-4, respectively. The samples were placed in a high temperature test box preheated at 100°C, and the temperature was gradually increased from 100°C to 180°C at intervals of 10°C. Each temperature point was maintained for 2 hours. After each temperature point, the samples were taken out to observe whether there was deformation, discoloration, softening, etc. in their appearance, and the changes in their compressive strength were tested. The deformation amount Changes in compressive strength

[0078]

[0079] The experimental results are shown in Table 1:

[0080] Table 1 High temperature stability

[0081]

[0082] Data analysis: It can be seen from Examples 1-3 in Table 1 that the high temperature resistant fresh-keeping box cover prepared by the present invention has excellent high temperature resistance and stability.

[0083] As can be seen from Example 2 and Comparative Example 1 in Table 1, the modified polyimide used in the present invention plays a key role in heat resistance. After its molecular structure is modified by 17-octadecynoic acid and phenyldimethylsilane, a stable cross-linked structure is formed. The silicon-oxygen bond introduced by the silane group, the alkynyl cross-linking and the benzene ring on the side chain can effectively limit the thermal motion of the molecular chain at high temperature, and enhance the overall heat resistance of the material. However, Comparative Example 1 lacks this core heat-resistant component and relies only on polypropylene, nanocellulose, etc. The heat resistance of polypropylene itself is limited. The molecular chain is easy to move and break at high temperature, and it cannot withstand high temperatures, resulting in rapid softening and yellowing of the material, and a significant decrease in heat resistance.

[0084] It can be seen from Example 2 and Comparative Example 2 in Table 1 that although the ordinary unmodified polyimide has a certain initial heat resistance, compared with the modified polyimide in Example 2, it lacks the additional thermal stability improvement brought by 17-octadecynoic acid and phenyldimethylsilane, and cannot form a stable cross-linked structure and strong chemical bonds at high temperature like the modified polyimide. In the temperature range of 130-180°C, as the temperature increases, the thermal motion of the molecular chain intensifies, the material gradually softens and deforms, and the heat resistance is significantly inferior to that of Example 2.

[0085] It can be seen from Example 2 and Comparative Example 3 in Table 1 that the heat resistance of Comparative Example 3 is inferior to that of Example 2. This may be because the modified polyimide in Example 2 is modified in two steps by 17-octadecynoic acid and phenyldimethylsilane. The silane group introduced by phenyldimethylsilane can combine with the molecular chain to form a high-bond energy silicon-oxygen bond, providing additional stability guarantee for the molecular chain at high temperature to prevent thermal decomposition. However, the modified polyimide in Comparative Example 3 is only modified by 17-octadecynoic acid, lacking the silicon-oxygen bond reinforcement given by phenyldimethylsilane and the intermolecular interaction of the benzene ring, resulting in insufficient molecular chain stability and aggravated softening and deformation of the material in the high temperature range of 160-180°C.

[0086] 2. Tensile test:

[0087] In order to test the stretchability of the fresh-keeping box cover, the following tests were performed on Examples 1-3 and Comparative Examples 1-4 respectively:

[0088] (a) Take three fresh-keeping box covers prepared in Examples 1-3 and Comparative Examples 1-3, measure their width, length, and thickness, and record them;

[0089] (b) All samples were clamped in a universal material testing machine, the tensile rate was set to 5 mm / min, and force and displacement data were collected every 0.1 s until the test material broke.

[0090] The results are shown in Table 2:

[0091] Table 2 Tensile properties

[0092]

[0093] Data analysis: It can be seen from Examples 1-3 in Table 2 that the high temperature resistant fresh-keeping box cover prepared by the present invention has excellent degradability.

[0094] It can be seen from Example 2 and Comparative Example 1 in Table 2 that in Comparative Example 1, the tensile properties of the material are greatly reduced. This may be because there is a lack of modified polyimide in Comparative Example 1, and the internal part of the material is like lacking a "skeleton" support, and the intermolecular force is weak. It is difficult to withstand a large tensile force relying only on basic components such as polypropylene and nanocellulose, resulting in a decrease in tensile strength; the elongation at break is also far from that of Example 2 due to the loose structure.

[0095] It can be seen from Example 2 and Comparative Example 2 in Table 2 that the material with modified polyimide added is significantly better than the material with unmodified polyimide added in terms of tensile strength and elongation at break. This may be due to the modified polyimide selected in the present invention. The introduction of long-chain molecules during the modification process plays a key role in improving tensile properties, because there is a strong entanglement between long-chain molecules, just like ropes intertwined with each other. When subjected to force, the molecular chains are difficult to slip off each other easily, thereby effectively transmitting stress and improving tensile strength. In addition, the long-chain structure with a certain degree of flexibility can buffer the impact of external forces by stretching and bending itself during the stress process, avoiding stress concentration from causing sudden breakage of the material, thereby improving elongation at break.

[0096] As can be seen from Example 2 and Comparative Example 3 in Table 2, Comparative Example 3 is slightly inferior to Example 2 in tensile strength and elongation at break. This may be because the preparation process of the modified polyimide of Comparative Example 3 lacks the secondary modification of phenyldimethylsilane. The modified polyimide in Example 2 is modified in two steps. The silane group introduced by phenyldimethylsilane can combine with the molecular chain to form a high-bond energy silicon-oxygen bond, which greatly improves the stability of the molecular chain when subjected to force, prevents premature breakage, and optimizes the interaction between molecules, enhancing the toughness of the material. However, Comparative Example 3 is only modified with 17-octadecynoic acid, and the molecular chain stability is insufficient. During the stretching process, the ability to resist external force deformation is weak.

[0097] It should be understood by those skilled in the art that the discussion of any of the above embodiments is merely illustrative and is not intended to imply that the scope of the present invention is limited to these examples. Under the concept of the present invention, the technical features in the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and there are many other variations of the different aspects of the present invention as described above, which are not provided in detail for the sake of simplicity.

Claims

1. A high temperature resistant fresh-keeping box cover, characterized in that: The method comprises the following raw materials in parts by weight: 50-70 parts of polypropylene, 6-10 parts of modified polyimide, 10-15 parts of nanocellulose, 2-4 parts of plasticizer, 0.5-1 part of antioxidant and 0.5-1 part of ethylene bis stearamide; The modified polyimide is prepared by the following steps: (1) using 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane and pyromellitic dianhydride as reaction monomers, reacting at 0-5° C. for 2-4 hours to obtain polyamic acid, then adding anhydrous toluene and condensing and refluxing at 160-200° C. for 4-8 hours to obtain hydroxy polyimide; (2) under nitrogen protection, hydroxy polyimide is added to N,N-dimethylformamide and stirred for 30 minutes, 17-octadecynoic acid is added, and a catalyst is added at the same time, and the temperature is raised to 70-90° C. and stirred for reaction for 6-10 hours. After the reaction is completed, the reaction mixture is poured into deionized water to precipitate, and the intermediate A is obtained after filtering, washing, and drying; (3) Under nitrogen protection, the intermediate A is added to N,N-dimethylformamide and stirred for 30 minutes, anhydrous toluene and a catalyst are added, and phenyldimethylsilane is added dropwise. After the addition is completed, the temperature is raised to 50-60° C. and stirred for reaction for 7-9 hours. After the reaction is completed, the reaction mixture is filtered and poured into deionized water to precipitate. The precipitate is filtered, washed, and dried to obtain a modified polyimide.

2. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: The weight average molecular weight of the polypropylene is 3000-5000.

3. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: The diameter of the nanocellulose is 50-60nm and the specific surface area is 60-80m 2 / g.

4. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: The plasticizer is acetyl tributyl citrate or epoxidized soybean oil.

5. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: The antioxidant is one of antioxidant 1010, antioxidant 1076 and antioxidant 168.

6. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: In the step (1), the weight ratio of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane, pyromellitic dianhydride and anhydrous toluene is 0.8-1.2:0.8-1.2:3-5.

7. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: In the step (2), the weight ratio of N,N-dimethylformamide, hydroxypolyimide and 17-octadecynoic acid is 15-25:1-3:1.2-3.6; the catalyst is sulfuric acid or p-toluenesulfonic acid with a concentration of 98%.

8. The high temperature resistant fresh-keeping box cover according to claim 1, characterized in that: In the step (3), the weight ratio of N,N-dimethylformamide, intermediate A, phenyldimethylsilane and anhydrous toluene is 15-25:1-3:1.1-3.6:1.1-3.6; the catalyst is one of palladium carbon, aluminum chloride and diethylaluminum chloride.

9. A method for preparing a high temperature resistant fresh-keeping box cover according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1: mixing polypropylene, modified polyimide, nanocellulose, plasticizer, antioxidant and ethylene bis stearamide to obtain a mixture; S2: adding the mixed material into a twin-screw extruder for melt extrusion; S3: The extruded material is directly introduced into the barrel of the injection molding machine through a pipeline and filled into the cavity of the fresh-keeping box cover mold. After pressure-maintaining and cooling, the material is formed to obtain a finished product of a high-temperature resistant fresh-keeping box cover.

10. The method for preparing a high temperature resistant fresh-keeping box cover according to claim 9, characterized in that The temperature of each zone in the twin-screw extruder is as follows: feeding zone: 210-230°C, melting zone: 230-250°C, mixing zone: 250-270°C, metering zone: 270-290°C, the head temperature of the twin-screw extruder is 280°C, and the screw speed is 180-220rpm; the injection pressure of the injection molding machine is 70-90MPa, the injection speed is 40-60mm / s, the holding time is 8-12 seconds, and the temperature of the fresh-keeping box cover mold is controlled at 40-60°C.

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