Modified nanocellulose reinforced starch composite film and preparation method thereof

Abstract

The invention discloses a modified nanocellulose reinforced starch composite film and a preparation method thereof, the film comprises the following components in parts by mass: 100 parts of starch, 2-10 parts of modified nanofibrillated cellulose, 15-30 parts of glycerol, 1-5 parts of a cross-linking agent, 0.5-2 parts of a stabilizer and 0.1-0.5 part of an antioxidant, mNFC has a network-like or branched multi-dimensional structure, the diameter of a single fiber is 5-100 nm, the length is 1-10 [mu] m, and the diameter of the single fiber is 1-10 [mu] m. The mass percentage content of amylose in the starch is 20-25%. The film disclosed by the invention shows high mechanical strength, water resistance, thermal stability and thermoplasticity; according to the method disclosed by the invention, a one-pot preparation technology of pH regulation and control in-situ catalysis and a precise control casting molding technology are combined, so that efficient hydrophobic modification and enhanced dispersion of mNFC in a starch matrix are realized, the reinforcing and toughening effects of mNFC in amylose and amylopectin parts are fully exerted, and starch recrystallization is inhibited.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of biodegradable films, and particularly relates to a modified nanocellulose reinforced starch composite film and a preparation method thereof. BACKGROUND

[0002] Starch is a natural polymer material polymerized by α-glucose molecules, and has good usability, renewability, low cost, film-forming performance, and biodegradability. Starch can be modified by thermoplasticity, and compounded with degradable polymers such as polycaprolactone, polylactic acid, and polyvinyl alcohol, to prepare a starch-based film. Such a film exhibits ideal transparency, odorless characteristics, and excellent physical and chemical properties such as semi-permeability to water, oxygen, and carbon dioxide. Therefore, in recent years, starch has been applied in the field of film packaging and other fields, and is considered to be one of the most promising plastic substitutes.

[0003] There are a large number of hydroxyl groups between starch molecules, and due to the hydrogen bonding between the hydroxyl groups, the melting temperature is higher than the decomposition temperature, and the starch does not have thermoplastic processability. By modification or adding a plasticizer, the hydrogen bonds and the crystal structure of the starch can be destroyed, thereby imparting thermoplasticity. However, although the plasticized starch film has good processability, its mechanical strength, aging resistance, and other properties often cannot meet the requirements of actual applications, and therefore further modification by introducing a reinforcing body is required. Biomass fillers can improve the performance of materials while maintaining the full biodegradability of the materials, and are often used as starch reinforcing agents. Among them, nanocellulose has high aspect ratio and high crystallinity, and has become a very promising reinforcing filler. When nanocellulose is blended with starch, the two can interweave to form a stable three-dimensional network structure, and produce abundant physical entanglement, fiber bridging, and hydrogen bonding, thereby significantly improving the comprehensive performance of the film.

[0004] To solve the compatibility and dispersibility problems between nanocellulose and the starch matrix, chemical modification of nanocellulose such as carboxylation, amidation, esterification, and silanization is an effective way to improve the interfacial compatibility of biologic nanocomposites. Among them, silane grafting modification is a commonly used method for enhancing interfacial compatibility because of its convenient operation, high efficiency, and economic and environmental protection. For example, Wang et al. used silanized modified nanofibrillated cellulose (CNFs) to melt mix with PBAT, and the mechanical properties and degradation properties of the obtained composite film were improved. Specifically, after adding 1 wt% of CNFs filler, the tensile strength of the composite material was 26 MPa, the elongation was 372%, the toughness was 75 MJ / m 3 , and the tensile modulus increased by 82%. However, the preparation process of CNFs is relatively complex and the cost is relatively high.

[0005] In the prior art, patent application CN116394446A discloses a biodegradable thermoplastic starch film, which involves mixing decrystalline starch and plasticizers (glycerol and carbamide), followed by pre-plasticization and hot pressing to form a film. The resulting thermoplastic starch film exhibits a certain degree of anti-crystallization ability while maintaining biodegradability. However, the structure formed by the plasticizer and starch is unstable, and the plasticizing effect weakens over time, leading to starch retrogradation and a decrease in the film's mechanical properties. This makes it impossible to simultaneously meet the requirements for film strength and anti-aging properties during use. Therefore, this invention proposes a modified nanocellulose-reinforced starch composite film and its preparation method. Summary of the Invention

[0006] The technical problem to be solved by this invention is to provide a modified nanocellulose-reinforced starch composite film and its preparation method, addressing the shortcomings of existing technologies. The starch composite film of this invention exhibits high mechanical strength, water resistance, thermal stability, and thermoplasticity, and is a novel, non-toxic, environmentally friendly, biodegradable bio-nanocomposite film material. The preparation method of this invention combines a one-pot preparation technique with pH-controlled in-situ catalysis and precision casting molding technology to achieve efficient hydrophobic modification and enhanced dispersion of modified nanocellulose (mNFC) in a starch matrix. This fully utilizes the reinforcing and toughening effects of mNFC in the amylose and amylopectin fractions, inhibits starch recrystallization, and thus prepares a multifunctional and aging-resistant starch composite film.

[0007] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a modified nanocellulose-reinforced starch composite film, wherein the starch composite film comprises the following mass fractions: 100 parts starch, 2-10 parts modified nanocellulose, 15-30 parts glycerol, 1-5 parts crosslinking agent, 0.5-2 parts stabilizer, and 0.1-0.5 parts antioxidant, wherein the modified nanocellulose satisfies the following condition (a), and the starch satisfies the following condition (b): (a) The modified nanofiberized cellulose has a network or branched multidimensional structure, and the diameter of a single fiber is 5~100 nm and the length is 1~10 μm; (b) The starch contains 20-25% amylose by mass.

[0008] The mNFC used in this invention possesses a unique network or branched multidimensional structure with a high aspect ratio and high specific surface area, enabling it to physically entangle with starch molecules and form numerous hydrogen bonds, which is key to performance optimization. The starch used in this invention, with a amylose content of 20-25%, is easily gelatinized and plasticized. Its structure contains sufficient amylose as a structural framework, while also containing numerous branched starches to plasticize and inhibit retrogradation. Furthermore, mNFC can inhibit the recrystallization of amylose and has minimal impact on the aging of branched starch itself, while stabilizing its gel structure. Therefore, the introduction of mNFC gives the film excellent strength, barrier properties, and flexibility.

[0009] The network or branched multidimensional structure of mNFC and the diameter of individual fibers can be determined by scanning electron microscopy; the amylose content in starch can be determined by the iodine colorimetric method.

[0010] Preferably, the modifier for the modified nanofiberized cellulose is 3-aminopropyltrimethoxysilane (KH540). gamma -Aminopropyltriethoxysilane (KH550) gamma At least one of methacryloyloxypropyltrimethoxysilane (KH570) and vinyltrimethoxysilane (A171).

[0011] Preferably, the starch is at least one of waxy corn starch, high amylose corn starch, tapioca starch, sweet potato starch, and wheat starch.

[0012] Preferably, the crosslinking agent is at least one of calcium gluconate, glutaraldehyde, and ethylene glycol diglycidyl ether (EGDE).

[0013] Preferably, the stabilizer is at least one of butylated hydroxytoluene (BHT), bio-based phosphite, and bio-based polyphenol.

[0014] Preferably, the antioxidant is at least one of 1010, 1076, and 168. The Chinese names of antioxidants 1010, 1076, and 168 are pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and tris(2,4-di-tert-butylphenyl)phosphite, respectively.

[0015] Preferably, the thickness of the starch composite film is 0.08~0.25 mm; and the tensile strength of the starch composite film is 3~17 MPa at a stretching rate of 10 mm / min.

[0016] The preparation method of the above-mentioned modified nanocellulose-reinforced starch composite film includes the following steps: 1) Mix 5-10 parts of straw powder with 70-80 parts of 4 wt% sodium hydroxide solution, heat and stir at 50-60℃ for 5-6 h, then centrifuge and wash until neutral to remove surface impurities, hemicellulose and other extracts. Preferably, the straw powder is at least one of corn straw, wheat straw, rice straw and sugarcane bagasse. Then add 60 parts of 7 wt% hydrogen peroxide solution and 3-4 parts of 0.5 wt% magnesium sulfate solution, heat and stir at 70-80℃ for 1-2 h, then centrifuge and wash until neutral.

[0017] 2) Place all the solid samples obtained in step 1) into a container equipped with a mechanical stirrer and condenser, and add 50-60 parts of a 2 wt% silane coupling agent solution to the container. At 40-60℃, first catalyze the hydrolysis of the silane coupling agent at pH=4-5, then react the solid sample with the silane coupling agent at pH=4 for 1 h, and then promote the condensation reaction at pH=6-7. Finally, terminate the reaction by adding excess pure water. The cooled solid-liquid mixture is then subjected to high-speed shearing at 10000-16000 rpm for 1-2 h. The obtained sample is then centrifuged and washed until neutral, and freeze-dried to obtain modified nanofiberized cellulose.

[0018] Step 2) above describes the preparation of mNFC using a one-pot in-situ catalytic method with pH control. The basic principle is to achieve efficient in-situ grafting of silanes by dynamically controlling the pH of the aqueous system in stages: First, the silane coupling agent is hydrolyzed under acidic conditions of pH 4-5 (which can be achieved by adding dilute acetic acid); then, the solid sample is reacted with the silane coupling agent under pH 4 conditions (which can be achieved by adding dilute sodium hydroxide solution); subsequently, the system is adjusted back to neutral conditions of pH 6-7 to promote the completion of the condensation reaction.

[0019] Because nanofiberized cellulose (NFC) is rich in charged groups and hydrophilic hydroxyl groups on its surface, it suffers from poor mechanical strength, low thermal stability, and low water resistance, resulting in poor compatibility and dispersibility in nanocomposites. Therefore, in step 2), the method of this invention utilizes a one-pot, pH-controlled in-situ catalytic process—a dynamic "acid-initiated-base-controlled" strategy—to achieve high grafting rates and low side reactions. In step 2), the method employs a silane coupling agent to silanize and modify NFC. The silane coupling agent hydrolyzes to produce silanol groups, which combine with the hydroxyl groups on the NFC surface, reducing the number of hydrophilic hydroxyl groups. This invention achieves both efficient NFC preparation and green, efficient silanization modification.

[0020] 3) Add 2-10 parts of the modified nanofiberized cellulose obtained in step 2) to 30-40 parts of pure water, and sonicate at 300-350 W power for 10-15 min to obtain an aqueous dispersion of the modified nanofiberized cellulose.

[0021] 4) The modified sodium cellulose aqueous dispersion obtained in step 3) is mixed with 100 parts starch, 15-30 parts glycerol, 1-5 parts crosslinking agent, 0.5-2 parts stabilizer and 0.1-0.5 parts antioxidant, and then subjected to dry ball milling. The ball milling conditions are as follows: zirconium oxide or ceramic balls are used as the grinding medium, the temperature is 30-50℃, the rotation speed is 500-700 rpm, and the time is 1-2 h to obtain a pretreated starch mixture.

[0022] Starch's high hydrophilicity and poor mechanical properties limit its widespread application in starch films. This invention addresses this issue by using mNFC to reinforce the starch matrix in step 4). mNFC, through silanization modification to regulate the interface between NFC and starch, can self-assemble into an ordered structure within the starch film, thereby improving the film's mechanical properties, barrier properties, water resistance, and thermal stability. Simultaneously, the addition of a crosslinking agent effectively constructs a metal-organic coordination structure, further enhancing the interfacial interaction between mNFC and starch, while the ball milling process improves the reactivity of hydroxyl groups.

[0023] 5) The starch mixture obtained in step 4) is cast into a mold and dried sequentially at 70°C for 8 h, 50°C for 10 h, and 40°C for 30 h to obtain a starch composite film.

[0024] In step 5), the precision casting molding technology can obtain a uniform film thickness over a large film-forming area, ensuring the consistency of film thickness throughout the preparation process, reducing defects such as bubbles and cracks, and promoting the uniform dispersion of mNFC in the starch matrix, thereby improving the quality of the starch composite film.

[0025] 6) Place the starch composite film obtained in step 5) in a constant temperature and humidity environment of 25℃ and 58% relative humidity for 48 h to obtain the modified nanocellulose-reinforced starch composite film.

[0026] Compared with the prior art, the present invention has the following advantages: 1. This invention utilizes mNFC (molecularly synthesized non-linear cellulose) with a unique network or branched multidimensional structure and individual fiber diameters of 5-100 nm, along with starch containing 20-25% amylose. mNFC acts as a reinforcing phase, regulating the interfacial compatibility between NFC and starch, thereby improving the mechanical properties, water resistance, and aging resistance of the starch composite film, resulting in a film with excellent strength, barrier properties, and flexibility. The combination of mNFC and starch in this invention produces a synergistic effect. Combined with specific proportions of glycerol, crosslinking agents, stabilizers, and antioxidants, the interfacial bonding and toughening effects at the nanoscale are fully utilized, achieving an optimal balance between strength and toughness. The starch composite film of this invention exhibits high mechanical strength, water resistance, thermal stability, and thermoplasticity, making it a non-toxic, environmentally friendly, novel biodegradable bio-nanocomposite film material.

[0027] 2. The preparation method of this invention combines a one-pot preparation technique with pH-controlled in-situ catalysis and precise casting molding technology to achieve efficient hydrophobic modification and enhanced dispersion of mNFC in a starch matrix. This fully leverages the reinforcing and toughening effects of mNFC on both amylose and amylopectin, inhibiting starch recrystallization, thereby preparing a multifunctional and aging-resistant starch composite film. The key innovation of this invention is the pH-controlled in-situ catalysis one-pot method, specifically the dynamic "acid-initiated-alkali-controlled" strategy, used to prepare mNFC, achieving high grafting rates and low side reactions.

[0028] 3. The preparation method of this invention utilizes high-speed shearing and ball milling to break down the crystal structure of the solid material. The disruption of the crystal structure and hydrogen bond network leads to an increase in hydroxyl groups, thereby improving the accessibility and reactivity of starch. Simultaneously, the strong mechanical force can easily induce the uniform dispersion and embedding of metal salts into the starch, significantly improving the interfacial contact and interaction between starch and metal salts. Compared with traditional starch film preparation processes, the preparation method of this invention is simple, highly operable, requires less chemical reagents, and has lower equipment requirements. It can achieve highly uniform mixing of mNFC and starch and significantly increase the amount of mNFC added, helping to reduce costs and expand the practical application areas of starch composite films. Attached Figure Description

[0029] Figure 1 Tensile properties of starch composite films with different mNFC addition amounts; Figure 2 Water resistance of starch composite films with different mNFC addition amounts; Figure 3 Water barrier properties of starch composite films with different mNFC addition amounts; Figure 4 Thermal properties of starch composite films with different mNFC addition amounts; Figure 5SEM images of starch composite films with different mNFC addition amounts; Figure 6 Tensile properties of starch composite films modified with different silane coupling agents for NFC; Figure 7 Water resistance of starch composite films modified with different silane coupling agents for NFC; Figure 8 Water barrier properties of starch composite films modified with different silane coupling agents for NFC; Figure 9 Thermal properties of starch composite films modified with different silane coupling agents for NFC; Figure 10 SEM images of starch composite films of NFC modified with different silane coupling agents; Figure 11 Tensile properties of starch composite films with different amounts of crosslinking agent added; Figure 12 Water resistance of starch composite films with different amounts of crosslinking agent added; Figure 13 Water barrier properties of starch composite films with different amounts of crosslinking agent added; Figure 14 Thermal properties of starch composite films with different amounts of crosslinking agent added; Figure 15 Tensile properties of starch composite films with different amounts of stabilizer added; Figure 16 Water resistance of starch composite films with different amounts of stabilizer added; Figure 17 Water barrier properties of starch composite films with different amounts of stabilizer added; Figure 18 Thermal properties of starch composite films with different amounts of stabilizer added; Figure 19 SEM images of different morphologies of nanocellulose; Figure 20 Tensile properties of starch composite films prepared with different nanocellulose morphologies; Figure 21 Differential scanning calorimetry images of starches with different amylose contents; Figure 22 Tensile properties of starch composite films prepared with different amylose contents. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] The mass fraction composition of the starch composite films of Examples 1 to 6 is shown in Table 1. The starch composite films of Examples 1 to 6 are respectively represented by the numbers C1, C2, C3, C4, C5, and C6, and single-factor experiments were conducted.

[0032] Table 1

[0033] The method for preparing the starch composite film in Example 1 includes the following steps: 1) Mix 100 parts starch, 20 parts glycerol, 3 parts calcium gluconate and 0.5 parts stabilizer, add to a planetary ball mill, and dry ball milling is performed. Zirconia or ceramic balls are used as the grinding media. The temperature is 30℃, the ball milling speed is 500 rpm and the ball milling time is 2 h to obtain a pretreated starch mixture. 2) The starch mixture obtained in step 1) is cast into a mold and dried at 40°C for 48 h to obtain a starch composite film; 3) Place the starch composite film obtained in step 2) in a constant temperature and humidity environment of 25°C and 58% relative humidity for 48 h to obtain the starch composite film of Example 1.

[0034] The method for preparing the starch composite film in Example 2 includes the following steps: 1) Mix 10 parts of corn stalk powder with 80 parts of 4 wt% sodium hydroxide solution, heat and stir at 50°C for 6 h, centrifuge and wash until neutral to remove surface impurities, hemicellulose and other extracts; then add 60 parts of 7 wt% hydrogen peroxide solution and 3 parts of 0.5 wt% magnesium sulfate solution, heat and stir at 70~80°C for 2 h, centrifuge and wash until neutral; 2) Place all the solid samples obtained in step 1) into a container equipped with a mechanical stirrer and condenser, and add 60 parts of KH570 solution with a concentration of 2 wt% to the container. At 50°C, firstly, catalyze the hydrolysis of silane coupling agent under pH=4~5 conditions, then react the solid sample with silane coupling agent under pH=4 conditions for 1 h, and then promote the condensation reaction under pH=6~7 conditions. Finally, terminate the reaction by adding excess pure water. Place the cooled solid-liquid mixture into a homogenizer and shear it at high speed at 16000 rpm for 2 h. Then, centrifuge and wash the obtained sample at 12000 rpm until neutral, and freeze-dry it to obtain mNFC. 3) Add 2 portions of the mNFC obtained in step 2) to 40 portions of pure water, and sonicate in an ultrasonic homogenizer at a power of 300 W for 15 min to obtain an aqueous dispersion of mNFC. 4) The aqueous dispersion of mNFC obtained in step 3) is mixed with 100 parts starch, 20 parts glycerol, 3 parts calcium gluconate, 0.5 parts stabilizer and 0.2 parts antioxidant, and added to a planetary ball mill. Dry ball milling is performed under the following conditions: zirconium oxide or ceramic balls are used as the grinding media, the temperature is 30℃, the rotation speed is 500 rpm and the time is 2 h to obtain a pretreated starch mixture. 5) The starch mixture obtained in step 4) is cast into a mold and dried sequentially at 70°C for 8 h, 50°C for 10 h, and 40°C for 30 h to obtain a starch composite film; 6) Place the starch composite film obtained in step 5) in a constant temperature and humidity environment of 25°C and 58% relative humidity for 48 h to obtain the starch composite film of Example 2.

[0035] The preparation methods of the starch composite films in Examples 3 to 6 are basically the same as those in Example 2, except that in step 3) of the preparation of Examples 3 to 6, the mass fractions of mNFC are 4 parts, 6 parts, 8 parts and 10 parts, respectively, to prepare the starch composite films of Examples 3 to 6.

[0036] For the starch composite films of Examples 1 to 6, tensile test samples were prepared using a cutting tool. The mechanical properties of the starch composite films of Examples 1 to 6 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped specimens, 50 mm in length and 4 mm in width at the center. The tensile speed was 10 mm / min, and at least 5 specimens were tested in each group. The average value of the tensile strength and elongation at break of the samples was taken. The results are as follows: Figure 1 As shown.

[0037] from Figure 1It can be seen that the tensile strength of the starch composite film increases with the increase of mNFC content. The tensile strength of the starch composite film reaches its maximum when the mNFC content is 8 parts. Compared with the pure starch film of Example 1 (tensile strength 1.33 MPa), the highest tensile strength of the starch composite films with added mNFC in Examples 2-6 is increased to 10.64 MPa. This result indicates that mNFC has a significant reinforcing effect on the starch composite film. The reinforcing mechanism is as follows: high-strength mNFC binds to starch molecules through electrostatic attraction and hydrogen bonding, forming a reinforced network structure. This network structure has a certain load-bearing capacity and prevents the slippage of starch molecular chains, thereby effectively improving the tensile strength of the film. However, when the mNFC content exceeds 8 parts, the tensile strength of the film decreases significantly. This is attributed to the excessive mNFC content in the starch matrix causing agglomeration, resulting in poor mNFC dispersibility and weakening the reinforcing effect. Furthermore, as the tensile strength of the film increases, its elongation at break decreases. This is mainly because the strong interfacial bonding between mNFC and the matrix, while efficiently transferring stress and thus improving strength, also severely restricts the movement of polymer molecular chains, resulting in a decrease in the elongation at break of the film.

[0038] To investigate the effect of mNFC addition on the water resistance of starch composite films, water contact angle tests were performed on the starch composite films. Figure 2 The results showed that, compared with the pure starch film of Example 1, the water contact angle of the starch composite films of Examples 2 to 6 first increased and then decreased with the increase of mNFC addition, reaching a maximum of 87.8° when the mNFC addition was 8 parts. This is related to the special structure of mNFC and the surface roughness of the film. mNFC can better bind with the starch matrix to form hydrogen bonds, which reduces the ability of water molecules to penetrate through the composite material and stabilizes the starch matrix in a high humidity environment. At the same time, it can regulate the morphology of the film surface, promote the interfacial compatibility of the composite material, and make it more difficult for water molecules to enter the interior of the composite material. The effect of mNFC addition on the water barrier performance of the starch composite film was further evaluated by water vapor permeability (WVP) testing. Figure 3 The WVP value of the pure starch film in Example 1 was 1.94 g·mm / (m). 2 The water resistance (WVP) of the starch composite films in Examples 2 to 6 after adding different amounts of mNFC gradually decreased. This is mainly attributed to the high specific surface area and high mechanical strength of mNFC, which enables it to form a tight network structure when interacting with starch molecules, thereby effectively improving the water resistance of the film.

[0039] For the starch composite films of Examples 1 to 6, the thermal properties of the films were tested using a thermogravimetric analyzer. The tests were conducted under a nitrogen atmosphere at a flow rate of 20 mL / min, with a temperature range of 25–600 °C and a heating rate of 10 °C / min. The test results are as follows: Figure 4 As shown in the figure, the results indicate that the thermal decomposition initiation temperature of the starch composite film gradually increases with increasing mNFC content, suggesting enhanced thermal stability. Under a nitrogen atmosphere, all films exhibited initial mass loss before 100°C, due to moisture evaporation from the sample surface. The second stage of mass loss occurred at approximately 160°C, primarily related to the decomposition of glycerol in the sample. Above 300°C, the residual weight of all films decreased sharply, indicating the highest thermal degradation rate, mainly caused by the thermal degradation of the cellulose structure, including depolymerization, dehydration, and the decomposition of glucose units. The thermal stability of the starch composite film gradually improves with increasing mNFC content.

[0040] Figure 5 Scanning electron microscope (SEM) images of the starch composite film surfaces of Examples 2, 5, and 6 are shown. Figure 5 (a), (b), and (c) in the text correspond to Examples 2, 5, and 6, respectively. From Figure 5 As shown in (a), adding two parts of mNFC resulted in a smoother, continuous phase, demonstrating improved compatibility between mNFC and the starch matrix. Figure 5 As can be seen in (b), after adding 8 parts of mNFC, uniformly distributed nanofibers were visible on the surface of the starch composite film, with no obvious particle aggregation. Figure 5 As can be seen in (c), when the amount of mNFC added reaches 10 parts, the surface of the starch composite film is very rough, and some aggregated cellulose nanoparticles can be clearly observed. This can be attributed to the excessive addition of mNFC, which causes the nanofiller to self-aggregate, resulting in a decrease in the adhesion between mNFC and the starch matrix, stress concentration under stress, and a corresponding decrease in mechanical properties.

[0041] The mass fraction composition of the starch composite films of Examples 7 to 10 is shown in Table 2. The starch composite films of Examples 7 to 10 are respectively represented by the numbers E1, E2, E3 and E4, and single-factor experiments were conducted.

[0042] Table 2

[0043] In Examples 7 to 10: the starch used was sweet potato starch, the stabilizer was bio-based phosphite, and the silane coupling agents were KH540, KH550, KH570, and A171, respectively.

[0044] The preparation methods of the starch composite films in Examples 7 to 10 are basically the same as those in Example 2. The difference is that in step 2) of the preparation of Examples 7 to 10, the silane coupling agents are selected as KH540, KH550, KH570 and A171, respectively, to prepare the starch composite films of Examples 7 to 10.

[0045] For the starch composite films of Examples 7 to 10, tensile test samples were prepared using a cutting tool. The mechanical properties of the starch composite films of Examples 7 to 10 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped specimens, 50 mm in length and 4 mm in width at the center. The tensile speed was 10 mm / min, and at least 5 specimens were tested in each group. The average value of the tensile strength and elongation at break of the samples was taken. The results are as follows: Figure 6 As shown.

[0046] from Figure 6 As can be seen, the addition of mNFC effectively improves the mechanical strength of the starch composite film. The enhanced mechanical properties of the starch film by mNFC can be attributed to the chemical interaction and physical entanglement between the silane coupling agent and the starch molecular chains. The silane coupling agent reacts with water molecules, adding hydroxyl groups to the chain, and reacts with a large number of hydroxyl groups on the starch surface. Finally, the silane coupling agent binds to the starch surface, forming Si-O bonds through a dehydration condensation reaction. Under stress, the silane coupling agent forms an elastic transition layer between the matrix and the filler, effectively relieving and dissipating interfacial stress, improving energy absorption and dispersion capabilities, thereby promoting the formation of a strong and compatible interface between the filler and the matrix. Among the four silane coupling agents, the mNFC-starch composite film prepared using KH570 exhibits the best mechanical properties. Compared with the unmodified composite material, the tensile strength is increased by 8 times. The results showed that the methacryloxy group CH2=C(CH3)COO- in KH570 had the best compatibility with starch, forming a strong physical entanglement. Therefore, KH570 had the best effect on improving the interfacial compatibility of starch composites.

[0047] The water contact angles of the starch composite films from Examples 7 to 10 were tested to investigate the hydrophobic properties of starch composite films reinforced with different mNFC values. The results are as follows: Figure 7 As shown. From Figure 7 As can be seen, the addition of mNFC improves the water resistance of the starch-based composite film. The silane coupling agent promotes the dispersion of NFC in the starch matrix, forming a denser cross-linked network. The active groups no longer participate in the process of water adsorption, thus improving the hydrophobic properties of the film. Figure 8 The image shows the water barrier properties of different mNFC-enhanced starch films. From Figure 8As can be seen, silane modification further improves the interfacial compatibility between nanocellulose and starch, enhances their bonding force, and thus improves the density and structural stability of the film (as shown in the SEM image). This structure effectively restricts the diffusion path of water vapor molecules in the film, reducing the possibility of water vapor permeation.

[0048] Thermogravimetric analysis was performed on the starch composite films of Examples 7 to 10 to study the thermal properties of starch films reinforced with different mNFC. The results are as follows: Figure 9 As shown. From Figure 9 As can be seen, the addition of silane coupling agent increases the initial decomposition temperature of the composite material, and the maximum decomposition rate temperature also increases slightly. This indicates that the addition of silane coupling agent improves the interfacial compatibility between NFC and starch, thus resulting in a slight increase in thermal stability. This demonstrates that the addition of silane coupling agent can improve the thermal properties of the composite material; therefore, modifying the composite material by adding silane coupling agent is an effective method to improve the overall performance of the composite material.

[0049] Figure 10 Scanning electron microscope (SEM) images of the surface of the starch composite films of Examples 7-10 are shown. Figure 10 (a), (b), (c), and (d) in the text correspond to Examples 7, 8, 9, and 10, respectively. From... Figure 10 As can be seen, the starch composite film with 8 parts of mNFC exhibits a uniform and smooth surface. This is likely because the coupling agent establishes covalent bonds with the starch surface, generating significant steric hindrance and preventing particle aggregation. Simultaneously, the organic segments of the coupling agent enhance the compatibility of mNFC with starch, improving particle dispersibility and interfacial bonding. Ultimately, this enhances the strength and toughness of the composite material.

[0050] The mass fraction composition of the starch composite films of Examples 11 to 15 is shown in Table 3. The starch composite films of Examples 11 to 15 are respectively represented by the numbers M1, M2, M3, M4 and M5, and single-factor experiments were conducted.

[0051] Table 3

[0052] In Examples 11 to 15: sweet potato starch was used, calcium gluconate was used as the crosslinking agent, and KH570 was used as the coupling agent.

[0053] The preparation methods of the starch composite films in Examples 11 to 15 are basically the same as those in Example 2, except that in step 4) of the preparation of Examples 11 to 15, the mass parts of calcium gluconate are 2 parts, 2.5 parts, 3 parts, 3.5 parts and 4 parts, respectively, to prepare the starch composite films of Examples 11 to 15.

[0054] For the starch composite films of Examples 11 to 15, tensile test samples were prepared using a cutting tool. The mechanical properties of the starch composite films of Examples 11 to 15 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped specimens, 50 mm in length and 4 mm in width at the center. The tensile speed was 10 mm / min, and at least 5 specimens were tested in each group. The average value of the tensile strength and elongation at break of the samples was taken. The results are as follows: Figure 11 As shown.

[0055] from Figure 11 As can be seen, adding an appropriate amount of calcium gluconate can effectively improve the mechanical strength of the starch composite film. With the increase of the amount of calcium gluconate added, the tensile strength of the starch composite film in Examples 11 to 15 first increases and then decreases, with the tensile strength reaching the maximum value of 10.64 MPa when 3 parts of calcium gluconate are added. 2+ With activated electronic orbitals, mNFC can accept isolated charge pairs on the hydroxyl oxygen atoms of starch and glycerol, forming a double cross-linked network that enhances its mechanical strength. Simultaneously, this strong interaction disrupts the hydrogen bonds and crystal structure of the starch group, facilitating contact and interaction between glycerol and its macromolecules, thereby improving the compatibility of mNFC with starch and enhancing the plasticizing effect of glycerol. Low amounts of calcium gluconate result in weak metal-organic coordination structures; however, appropriate addition can improve the reactivity of hydroxyl groups and the dispersibility of calcium gluconate in mNFC and starch.

[0056] Material aging is mainly due to physical diffusion caused by differences in moisture concentration between the environment and the material. Water contact angle tests were conducted on Examples 11-15 to investigate the effect of different amounts of calcium gluconate added on the hydrophobic properties of the starch composite film. Figure 12 As shown, the water contact angle is largest when 3 parts of calcium gluconate are added. This is because Ca... 2+ Strong ligand interactions were formed between mNFC and starch. Figure 13 To investigate the effect of different calcium gluconate addition amounts on the water barrier properties of starch composite films, a calcium gluconate addition amount of 3 parts significantly reduced the water barrier performance (WVP) through the cross-linking effect of calcium ions, resulting in the film achieving optimal water barrier performance. Excessive addition amounts significantly increased WVP due to the introduction of too many hydrophilic groups, potential plasticizing effects, and possible structural degradation.

[0057] Thermogravimetric analysis was performed on the starch composite films of Examples 11-15 to investigate the thermal properties of starch composite films with different amounts of calcium gluconate added. Figure 14As shown. The improvement in the thermal properties of the starch film after adding calcium gluconate is due to the significant improvement in the compatibility of the blend, because Ca... 2+ The introduction of ball milling disrupts the ordered arrangement of mNFC and starch molecular chains, resulting in a tighter bond between calcium gluconate, glycerol, mNFC, and starch, thus improving plasticizing. The results also confirm the improvements in dispersion, pulverization, hydrogen bond activation, and interfacial compatibility, as well as the significantly enhanced thermal processing performance, caused by ball milling pretreatment.

[0058] The mass fraction composition of the starch composite films of Examples 16 to 19 is shown in Table 4. The starch-based composite films of Examples 16 to 19 are respectively represented by the numbers F1, F2, F3, and F4, and single-factor experiments were conducted.

[0059] Table 4

[0060] In Examples 16 to 19: the starch used was sweet potato starch, the stabilizer was bio-based polyphenol, and the coupling agent was KH570.

[0061] The preparation methods of the starch composite films in Examples 16 to 19 are basically the same as those in Example 2, except that in step 4) of the preparation of Examples 16 to 19, the mass parts of the stabilizer are 2 parts, 1.5 parts, 1 part, and 0.5 parts, respectively, to prepare the starch composite films of Examples 16 to 19.

[0062] For the starch composite films of Examples 16 to 19, tensile test samples were prepared using a cutting tool. The mechanical properties of the starch-based composite films of Examples 16 to 19 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped specimens, 50 mm in length and 4 mm in width at the center. The tensile speed was 10 mm / min, and at least 5 specimens were tested in each group. The average value of the tensile strength and elongation at break of the samples was taken. The results are as follows: Figure 15 As shown.

[0063] from Figure 15As can be seen, adding an appropriate amount of stabilizer can effectively improve the mechanical strength of starch films. With the increase of stabilizer addition, the tensile strength of the starch composite films in Examples 16 to 19 first increases and then decreases, with the tensile strength reaching its maximum when 1 part of stabilizer is added. This is because the stabilizer fills the gaps between starch molecular chains, forming a three-dimensional reinforcing network ("organic-inorganic" hybrid structure) together with mNFC, increasing the matrix density (reducing free volume) and hindering crack propagation. Excessive stabilizer (>1 part) will lead to agglomeration stress concentration or embrittlement, while less than 1 part of stabilizer has a moderate plasticizing effect (weak hydrogen bond relaxation) to maintain a certain toughness.

[0064] Water contact angle tests were conducted on the starch composite films of Examples 16-19 to investigate the effect of different stabilizer addition amounts on the hydrophobic properties of the starch composite films. Figure 16 As shown, the water contact angle is the largest when the amount of stabilizer added is 1 part. This is because the stabilizer tends to migrate to the surface of the film during the film formation process, and the hydrophobic long chain of the silane coupling agent is exposed to form a low surface energy layer, which improves its hydrophilicity. Figure 17 To investigate the effect of different stabilizer addition amounts on the water barrier properties of starch composite films, when the addition amount is 1 part, the surface-active hydroxyl groups (Si-OH) of the stabilizer form a strong hydrogen bond network with the -OH groups of starch / mNFC, restricting polymer chain movement and binding free water molecules. However, excessive addition can lead to the formation of defective channels due to aggregation, which in turn increases the water barrier performance (WVP).

[0065] Thermogravimetric analysis was performed on the starch composite films of Examples 16-19 to investigate the thermal properties of starch composite films with different amounts of stabilizer added. Figure 18 As shown, the stabilizer has a high thermal conductivity and forms an insulating layer inside the material, delaying heat transfer to the polymer chains. It crosslinks with starch / NFC through a silane coupling agent, forming Si-OC bonds with higher thermal stability, hindering high-temperature untangling and thermal decomposition reactions of the molecular chains, thus significantly improving the thermal properties of the starch composite film.

[0066] The mass composition of the starch composite films of Examples 20 to 22 is shown in Table 5. The starch composite films of Examples 20 to 22 are respectively represented by the numbers G1, G2, and G3, and single-factor experiments were conducted.

[0067] Table 5

[0068] The preparation methods of the starch composite films in Examples 20 to 22 are basically the same as those in Example 2, except that modified cellulose nanocrystals (mCNW) are used in step 3) of the preparation in Example 20, mNFC is used in step 3) of the preparation in Example 21, and modified microfibrillated cellulose (mMFC) is used in step 3) of the preparation in Example 22.

[0069] Figure 19 Scanning electron microscope images of cellulose nanofibers with different fiber morphologies. Figure 19 (a), (b), and (c) in the text correspond to Examples 20, 21, and 22, respectively. From Figure 19 As can be seen, the mCNW image exhibits a short rod-like structure, without a network structure, and shows a discrete distribution; the mNFC image shows a highly entangled three-dimensional network structure, forming a porous mesh; while the mMFC image shows an interwoven mesh but a loose structure, with coarse fiber bundles piled up in a disorderly manner.

[0070] For the starch composite films of Examples 20-22, tensile test samples were prepared using a cutting tool. The mechanical properties of the starch composite films of Examples 20-22 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped specimens, 50 mm in length and 4 mm in width at the center. The tensile speed was 10 mm / min, and at least 5 specimens were tested in each group. The average value of the tensile strength and elongation at break of the samples was taken. The results are as follows: Figure 20 As shown.

[0071] Figure 20 The tensile properties of starch composite films prepared from cellulose nanofibers with different fibrous morphologies. Figure 20 As can be seen, the G2 film prepared by mNFC has the highest tensile strength (15.64 MPa). This is because mNFC possesses both high strength and high toughness. Under stress, the long nanofibers of mNFC disperse local stress throughout the network through inter-fiber hydrogen bonds and mechanical interlocking, delaying crack propagation. In contrast, the short rod structure of mCNW cannot effectively overlap, and the short rod-shaped structure is prone to slipping from the matrix under stress, leading to reduced reinforcement efficiency. Therefore, the G1 film prepared by mCNW has the lowest tensile strength (8.26 MPa). The fiber bundles of mMFC are thicker and shorter, and the bonding between the thick fiber bundles and the starch interface is weaker, making stress concentration prone to fracture. Its network building ability is weaker than that of mNFC. Therefore, the tensile strength of the G3 film prepared by mMFC (10.26 MPa) is lower than that of the G2 film. Thus, mNFC has a significant reinforcing effect on starch composite films (an 89% improvement compared to mCNW).

[0072] The mass composition of the starch composite films of Examples 23 to 25 is shown in Table 6. The starch composite films of Examples 23 to 25 are respectively represented by the numbers H1, H2 and H3, and single-factor experiments were conducted.

[0073] Table 6

[0074] The preparation methods of the starch composite films in Examples 23 to 25 are basically the same as those in Example 2, except that in step 4) of preparation in Example 23, waxy corn starch is used; in step 4) of preparation in Example 24, high amylose corn starch is used; and in step 4) of preparation in Example 25, sweet potato starch is used.

[0075] Figure 21 Differential scanning calorimetry (DSC) images of starches with different amylose contents. From Figure 21 As can be seen, the melting temperature of waxy corn starch is 83°C, the melting temperature of high amylose corn starch is 166°C, and the melting temperature of sweet potato starch is 141°C. The mechanical properties of the starch composite films of Examples 23-25 ​​are as follows: Figure 22 As shown. From Figure 22The results show that the mechanical strength of the starch composite film made from sweet potato starch is 16.25 MPa. This is because sweet potato starch easily forms a uniform and fine paste at room temperature. This ensures the uniformity of the film-forming solution and results in fewer defects in the final film. While the amylopectin structure in starch, which accounts for 75-80%, is not conducive to the formation of dense crystalline regions, it can effectively play a role in "space filling" and "plasticizing" within the network structure formed by amylose. Its melting temperature is intermediate (around 100°C-140°C), indicating that the crystalline structure it forms has moderate stability. The mechanical strength of the waxy corn starch composite film is 8.58 MPa, which can be attributed to the fact that waxy corn starch has almost no amylose as a structural backbone, resulting in severely insufficient strength. Furthermore, waxy corn starch has a low melting temperature (below 100°C). This is directly related to its weak and short A-type microcrystalline structure and weak interactions between molecular chains. High-amylose corn starch has a high amylose content (55-70%), and its amylose molecules are very prone to rapid rearrangement (i.e., retrogradation), forming excessively strong crystalline regions. This leads to increased brittleness and decreased flexibility in starch composite films. High-amylose corn starch has the highest melting temperature (over 150°C). This is because its double-helix structure (B-type crystals) is tightly ordered and numerous, with extremely strong molecular chain entanglement and interaction forces. High energy (high temperature) is required to break these ordered structures, resulting in lower mechanical strength in its starch composite films compared to sweet potato starch. Therefore, sweet potato starch is the preferred choice for preparing high-performance starch composite films because its unique composition optimizes crystal stability and crystallinity, thus forming a uniform network structure with a good strength-toughness balance.

Claims

1. A modified nanocellulose-reinforced starch composite film, characterized in that, The starch composite film comprises the following mass fractions: 100 parts starch, 2-10 parts modified nanofiberized cellulose, 15-30 parts glycerol, 1-5 parts crosslinking agent, 0.5-2 parts stabilizer, and 0.1-0.5 parts antioxidant. The modified nanofiberized cellulose satisfies the following condition (a), and the starch satisfies the following condition (b): (a) The modified nanofiberized cellulose has a network or branched multidimensional structure, and the diameter of a single fiber is 5~100 nm and the length is 1~10 μm; (b) The starch contains 20-25% amylose by mass.

2. The modified nanocellulose-reinforced starch composite film according to claim 1, characterized in that, The modifier for the modified nanofiberized cellulose is 3-aminopropyltrimethoxysilane. γ -aminopropyltriethoxysilane, γ At least one of methacryloyloxypropyltrimethoxysilane and vinyltrimethoxysilane.

3. The modified nanocellulose-reinforced starch composite film according to claim 1, characterized in that, The starch is at least one of waxy corn starch, high amylose corn starch, tapioca starch, sweet potato starch, and wheat starch.

4. The modified nanocellulose-reinforced starch composite film according to claim 1, characterized in that, The crosslinking agent is at least one of calcium gluconate, glutaraldehyde, and ethylene glycol diglycidyl ether.

5. The modified nanocellulose-reinforced starch composite film according to claim 1, characterized in that, The stabilizer is at least one of butylated hydroxytoluene, bio-based phosphite, and bio-based polyphenol.

6. The modified nanocellulose-reinforced starch composite film according to claim 1, characterized in that, The antioxidant is at least one of 1010, 1076 and 168.

7. The modified nanocellulose-reinforced starch composite film according to any one of claims 1 to 6, characterized in that, The thickness of the starch composite film is 0.08~0.25 mm; under the condition of a stretching rate of 10 mm / min, the tensile strength of the starch composite film is 3~17 MPa.

8. A method for preparing the modified nanocellulose-reinforced starch composite film according to any one of claims 1 to 6, characterized in that, Includes the following steps: 1) Mix 5-10 parts of straw powder with 70-80 parts of 4 wt% sodium hydroxide solution, heat and stir at 50-60℃ for 5-6 h, then centrifuge and wash until neutral; then add 60 parts of 7 wt% hydrogen peroxide solution and 3-4 parts of 0.5 wt% magnesium sulfate solution, heat and stir at 70-80℃ for 1-2 h, then centrifuge and wash until neutral; 2) Place all the solid samples obtained in step 1) into a container equipped with a mechanical stirrer and condenser, and add 50-60 parts of a 2 wt% silane coupling agent solution to the container. At 40-60℃, first catalyze the hydrolysis of the silane coupling agent at pH=4-5, then react the solid sample with the silane coupling agent at pH=4 for 1 h, then promote the condensation reaction at pH=6-7, and then terminate the reaction by adding excess pure water. The cooled solid-liquid mixture is then subjected to high-speed shearing at 10000-16000 rpm for 1-2 h. The obtained sample is then centrifuged and washed until neutral, and freeze-dried to obtain modified nanofiberized cellulose. 3) Add 2-10 parts of the modified nanofiber cellulose obtained in step 2) to 30-40 parts of pure water, and sonicate at 300-350 W for 10-15 min to obtain an aqueous dispersion of the modified nanofiber cellulose; 4) The modified sodium cellulose aqueous dispersion obtained in step 3) is mixed with 100 parts starch, 15-30 parts glycerol, 1-5 parts crosslinking agent, 0.5-2 parts stabilizer and 0.1-0.5 parts antioxidant, and then subjected to dry ball milling. The ball milling conditions are as follows: zirconium oxide or ceramic balls are used as the grinding media, the temperature is 30-50℃, the rotation speed is 500-700 rpm, and the time is 1-2 h to obtain a pretreated starch mixture. 5) The starch mixture obtained in step 4) is cast into a mold and dried sequentially at 70°C for 8 h, 50°C for 10 h, and 40°C for 30 h to obtain a starch composite film; 6) Place the starch composite film obtained in step 5) in a constant temperature and humidity environment of 25℃ and 58% relative humidity for 48 h to obtain the modified nanocellulose-reinforced starch composite film.

9. The method for preparing the modified nanocellulose-reinforced starch composite film according to claim 8, characterized in that, The straw powder is at least one of corn straw, wheat straw, rice straw, and sugarcane bagasse.

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