A starch-based biodegradable film and a method for preparing the same

Abstract

The starch / PBAT / nanocellulose biodegradable film disclosed by the application has the mass fraction composition of: starch 100 parts, PBAT 100-150 parts, modified NFC 1-10 parts, glycerol 15-20 parts, deionized water 30-40 parts, lubricant 0.5-4 parts, and stabilizer 0.5-4 parts; the modified NFC is esterification modified NFC. By adding the esterification modified NFC, a stable 'bridge' structure is constructed between the starch and the PBAT, the interface compatibility of the two is effectively improved, and the mechanical properties of the composite material are strengthened. In addition, by high-energy ball milling treatment, the dispersibility of the NFC in the mixture matrix is improved, the interface bonding strength of the NFC and the matrix is improved, and the mechanical properties of the composite material are further enhanced. The preparation method of the application is simple, efficient and low in cost, and industrial production is easy to realize, and has wide market application prospect and commercial value.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of degradable plastic films, and particularly relates to a starch / PBAT / nano-cellulose biodegradable film and a preparation method thereof. BACKGROUND

[0002] Starch is abundant, renewable and low in price, and can be used to produce starch-based biodegradable plastics through plasticization modification. Starch-based biodegradable plastics are generally blends of modified starch and biodegradable polyesters such as polybutylene adipate-co-terephthalate (PBAT), polylactic acid (PLA) and polybutylene succinate (PBS). It can be completely biodegraded, has no pollution to the environment, and the waste can be treated by composting, landfilling and the like. Starch-PBAT, as an important category of starch-based biodegradable plastics, can be applied to many fields such as disposable plastic bags, agricultural mulching films and preservative films. However, two fatal defects of the starch-PBAT blend seriously restrict its performance. Firstly, due to the lack of compatibility between hydrophilic starch and hydrophobic polyester, the stress transfer between the phase interfaces is poor, resulting in a significant decrease in the mechanical properties of the composite material when the starch addition amount exceeds the critical point (30%). On the other hand, starch is prone to retrogradation, which seriously affects the shelf time and use performance of the starch-based biodegradable plastic product.

[0003] Adding nano-microfibrillated cellulose (NFC) is an effective method to improve the compatibility of thermoplastic starch (TPS) and PBAT without affecting the biodegradability of starch. NFC can reduce the crystallinity of TPS, thereby inhibiting the retrogradation of starch. Some studies have reported the reinforcing effect of nano-cellulose on polyester-based composites. For example, Lang et al. (Polymers 2022, 14(21), 4517) reported a PBAT / TPS / cellulose nanocrystal (CNC) composite material, and they found that the tensile strength of the composite material was increased by 30% after adding 4wt% of CNC. Moraes et al. (Mat. Sci. Eng. C-Mater. 2017, 78, 932-941) found that the tensile strength and Young's modulus of the TPS / PBAT film were significantly improved after adding plasticized cellulose acetate (PCA), and the water resistance of the material was also improved. However, Lee et al. (Compos. Sci. Technol. 2014, 105, 15-27) pointed out that when the NFC content exceeds 30%, the NFC will agglomerate under the action of surface energy, thereby losing the nano-reinforcing effect. Therefore, the dispersibility of NFC in the matrix is an important factor affecting the performance of TPS / PBAT composites.

[0004] High-energy ball milling method can produce mechanical chemical action under repeated impact of grinding ball medium, so that the powder is fully uniform and refined. The patent application file with publication number CN113121888A discloses a method for preparing plasticized starch by ball milling. It is found that under the extremely strong physical force generated by ball milling, the hydrogen bonds within and between starch molecules are largely destroyed, the crystal lattice is damaged, and the crystallinity is reduced, thereby improving the anti-regeneration ability of thermoplastic starch. The patent application file with publication number CN113248798A discloses a starch / cellulose / PBAT composite film and a preparation method thereof. The starch, cellulose and PBAT are uniformly mixed by ball milling and then granulated, and then blown into a film. It is found by comparison that ball milling helps to improve the dispersibility of cellulose, starch and PBAT and thus improve the mechanical properties of the composite material. However, the interface compatibility of ordinary cellulose and PBAT is poor, and the improvement of the mechanical properties of the composite material is limited. Therefore, the present application proposes a method for improving the TPS and PBAT by using esterified NFC, and further improving the dispersibility of NFC by high-energy ball milling. SUMMARY

[0005] The problem to be solved by the present application is to solve the problems of poor interface compatibility, low starch filling amount, easy regeneration and low strength of the existing starch / PBAT composite film. The present application proposes a TPS / PBAT / NFC high-performance composite film and a preparation method thereof.

[0006] The technical scheme adopted by the present application to solve the above technical problems is: a TPS / PBAT / NFC composite film, the mass fraction composition of which is: starch 100 parts, PBAT 100-150 parts, esterified modified NFC 1-10 parts, glycerol 15-30 parts, deionized water 30-40 parts, lubricant 0.5-1 part, stabilizer 0.5-1 part.

[0007] As a preferred embodiment, the starch is one or a combination of two or more of corn starch, cassava starch, pea starch, potato starch, wheat starch, sweet potato starch, and lotus root starch.

[0008] As a preferred embodiment, the lubricant is one or a combination of two or more of stearic acid, ethylene bis-stearamide, oleic acid amide, and erucic acid amide. As a preferred embodiment, the stabilizer is one or a combination of two or more of antioxidant 1010, antioxidant 168, antioxidant 1076, and antioxidant 1790.

[0009] The preparation method of the above-mentioned TPS / PBAT / NFC composite film comprises the following steps:

[0010] 1. Preparation of esterified modified NFC

[0011] 1) 5-10 parts of corn straw powder is mixed with 70-80 parts of sodium hydroxide solution, heated and stirred at 85-95℃ for 4-7h, washed with pure water until neutral, then 60 parts of sodium chlorite solution and 3-4 parts of mixed solution of glacial acetic acid are added, heated and stirred at 70-80℃ for 1-2h, and then washed with pure water until neutral;

[0012] 2) The solute sample obtained in 1) is placed in a beaker, 50-60 parts of oxalic acid solution is added, heated and stirred at 60-70℃ for 0.5-1h, the solid-liquid mixture after cooling is treated by high-speed shearing using a homogenizer for 1-2h, the rotation speed of the homogenizer is 10000-16000rpm; the sample after homogenization is washed and centrifuged until neutral, and then freeze-dried to obtain esterified modified NFC.

[0013] Preferably, in step 1), the corn straw powder has a mesh size of 100-150 mesh, the concentration of sodium hydroxide solution is 10-20wt%, and the concentration of sodium chlorite solution is 10-15wt%; in step 2), the concentration of oxalic acid solution is 30-40wt%.

[0014] The surface of nanocellulose is rich in hydrophilic hydroxyl groups, which will cause strong phase separation with the hydrophobic PBAT matrix during the melt preparation of the composite material. In the present application, oxalic acid is used in step 2) of the above method to esterify and modify the NFC, so that the hydrophilic hydroxyl groups on the surface are converted into hydrophobic ester groups. The ester groups introduced by esterification form similar chemical structures with the ester groups in PBAT, thereby effectively increasing the compatibility between nanocellulose and PBAT. In the present application, the preparation and esterification modification of NFC are simultaneously realized in step 2) of the above method, which is simple and efficient.

[0015] 2. Preparation of TPS / PBAT / NFC composite film

[0016] a: 1-10 parts of modified NFC is added to 30-40 parts of pure water, treated in an ultrasonic crusher for 10-15min, the power of the crusher is 300-350W, to obtain a pre-dispersed NFC aqueous solution;

[0017] b: the NFC aqueous solution obtained in step a is mixed with 100 parts of starch and 15-20 parts of glycerol, added into a planetary ball mill for dry ball milling, the grinding medium is zirconia or ceramic ball, the temperature is 50-80℃, the ball milling speed is 100-900rpm, and the ball milling time is 0-1h, to obtain a highly dispersed starch slurry of NFC;

[0018] c: the starch slurry obtained in step b is gelatinized at 65-75℃ for 0.5-1h to obtain gelatinized starch, which is then dried in an oven at 105℃ for 8h to obtain NFC reinforced TPS;

[0019] d: NFC enhanced TPS obtained in step c is mixed with 100-150 parts of PBAT and 0.5-1 parts of lubricant and 0.5-1 parts of stabilizer in a twin-screw extruder for 2-5 min at a temperature of 100-140℃ and a screw speed of 50-100 rpm to obtain a TPS / PBAT / NFC mixture;

[0020] e: The mixture obtained in step d is dried in a vacuum drying oven at 80℃ for 2-5 h, and the dried material is processed into a TPS / PBAT / NFC composite film using a flat press or a blown film machine.

[0021] Preferably, in step e, the pre-pressing pressure of the flat press is 0.3-0.7 MPa, the pre-pressing time is 150-180 s, and the pressurizing pressure is 2.5-3.5 MPa, and the pressurizing time is 150-180 s.

[0022] Preferably, in step e, the thickness of the TPS / PBAT / NFC composite film is 0.05-0.5 mm.

[0023] In the method of the present application, the starch particles are effectively refined by mechanical ball milling in step b, so that the particle size is reduced. Smaller particle size is beneficial to increasing the contact area of starch with other components and improving the plasticizing effect. At the same time, the collision and extrusion effect of high-energy ball milling exposes more nanofibrils from NFC and uniformly disperses NFC into TPS, while forming a large number of hydrogen bonds. The sufficient mixing of starch and NFC will fully exert the reinforcing effect of NFC and improve the overall tensile strength of the composite film. In addition, mechanical ball milling can physically denature starch molecules through shear force and friction, change the structure of starch molecules, crush the original crystal lattice, and greatly reduce the degree of crystallinity, which will effectively prevent the retrogradation of starch and prolong the service life of starch-based products.

[0024] In the method of the present application, the NFC after surface esterification has many ester bonds similar to those in PBAT in step d, so they have strong affinity. NFC enhances the affinity with PBAT while forming hydrogen bond interactions with TPS, acting as a bridge. This is the reason for enhancing the composite film. This interfacial reinforcement effect can improve the stress transfer efficiency between NFC and PBAT, thereby increasing the strength of the film.

[0025] Compared with the prior art, the present application has the following advantages:

[0026] 1、The esterification modified NFC is used as the reinforcing phase in the application, which improves the interfacial compatibility, mechanical properties and resistance to retrogradation of the TPS / PBAT composite film. Compared with the traditional nanocellulose, the hydrophilic hydroxyl content on the surface of the esterification modified NFC is reduced, and the ester groups are newly introduced, which effectively improves the interaction between the NFC and the polyester matrix.

[0027] 2、The ball milling treatment is used in the application to enhance the dispersibility of the modified NFC in the TPS particles, which is beneficial to the full contact between the starch particles and the NFC, and improves the tensile strength of the material. At the same time, the original lattice of starch is destroyed under the action of mechanical force, and the crystallinity is reduced, which can effectively prevent the retrogradation behavior of the starch-based product during use, has excellent stability, and expands the application of the starch-based material in the biodegradable material.

[0028] 3、Compared with the preparation method of the traditional TPS / PBAT / NFC composite film, the method of the application is convenient to process, has low equipment requirements, and has less dosage of modified additives. The modified NFC for mechanical property strengthening has high mixing uniformity with the starch particles, can enhance the mechanical properties of TPS while preventing the retrogradation of TPS, and can increase the addition amount of starch, thereby reducing the cost of biodegradable materials. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 : Tensile properties of TPS / PBAT / NFC composite films with different NFC addition amounts;

[0030] Figure 2 : SEM images of TPS / PBAT / NFC composite films with different NFC addition amounts (a: Example 1; b: Example 2; c: Example 3; d: Example 4; e: Example 5);

[0031] Figure 3 : Crystallinity and grain size of TPS / PBAT / NFC composite films with different NFC addition amounts;

[0032] Figure 4 : Moisture absorption properties of TPS / PBAT / NFC composite films with different NFC addition amounts;

[0033] Figure 5 : Ultraviolet blocking properties of TPS / PBAT / NFC composite films with different NFC addition amounts;

[0034] Figure 6 : Tensile properties of TPS / PBAT / NFC composite films with different ball milling treatment times;

[0035] Figure 7SEM images of TPS / PBAT / NFC composite films with different ball milling treatment time (a: Example 6; b: Example 7);

[0036] Figure 8 Crystallinity and grain size of TPS / PBAT / NFC composite films with different ball milling treatment time;

[0037] Figure 9 Moisture absorption properties of TPS / PBAT / NFC composite films with different ball milling treatment time;

[0038] Figure 10 Ultraviolet barrier properties of TPS / PBAT / NFC composite films with different ball milling treatment time;

[0039] Figure 11 Tensile properties of TPS / PBAT / NFC composite films with different ball milling treatment speed;

[0040] Figure 12 Tensile properties of TPS / PBAT / NFC composite films with different glycerol addition amount. DETAILED DESCRIPTION

[0041] The application will be further described in detail below with reference to the accompanying drawings.

[0042] The mass fraction composition of the TPS / PBAT / NFC composite films of Examples 1-5 is shown in Table 1, and the TPS / PBAT / NFC composite films of Examples 1-5 are respectively represented by numbers C1, C2, C3, C4, and C5 for single factor experiment.

[0043] In Examples 1-5: corn starch is used as starch, a 1:1 mixture of stearic acid and ethylene bis-stearamide is used as lubricant, and antioxidant 1010 is used as stabilizer.

[0044] Table 1

[0045]

[0046] The preparation method of the TPS / PBAT / NFC composite film of Example 1 includes the following steps:

[0047] 1. Preparation of esterified modified NFC

[0048] 1) 10 parts of corn straw powder is mixed with 80 parts of 20wt% sodium hydroxide solution, heated and stirred at 95℃ for 6h, washed with pure water to neutral, then 60 parts of 15wt% sodium chlorite solution and 3 parts of mixed solution of glacial acetic acid are added, heated and stirred for 2h at 80℃, and then washed with pure water to neutral;

[0049] 2) Put all the solute sample obtained in 1) into a beaker, add 60 parts of 40 wt% oxalic acid solution, stir and heat at 70℃ for 1h, and then treat the cooled solid-liquid mixture with a homogenizer at a high speed for 2h at a rotation speed of 16000 rpm; centrifugally wash the sample after homogenization until neutral, and then freeze-dry to obtain the esterification-modified NFC.

[0050] 2. Preparation of TPS / PBAT / NFC composite film

[0051] a: add 0 parts of modified NFC to 35 parts of deionized water, and treat in an ultrasonic crusher for 15 min at a power of 350 W to obtain a pre-dispersed NFC aqueous solution;

[0052] b: mix the pre-dispersed NFC aqueous solution in step a with 100 parts of starch and 20 parts of glycerol, and add them into a planetary ball mill for dry ball milling, wherein zirconia or ceramic balls are used as the grinding medium, the temperature is 60℃, the ball milling speed is 300 rpm, and the ball milling time is 1h to obtain a highly dispersed starch slurry of NFC;

[0053] c: gelatinize the starch slurry obtained in step b for 2h at a temperature of 75℃ to obtain gelatinized corn starch, and then dry the gelatinized corn starch in an oven at 105℃ for 8h to obtain NFC-reinforced TPS;

[0054] d: mix the NFC-reinforced TPS obtained in step c with 100 parts of PBAT, 0.5 parts of lubricant and 0.5 parts of stabilizer in a twin-screw extruder for 10 min at a temperature of 140℃ and a screw rotation speed of 70 rpm to obtain a TPS / PBAT / NFC mixture;

[0055] e: dry the mixture obtained in step d in a vacuum drying box at 80℃ for 1h, and then press the dried material into a TPS / PBAT / NFC composite film in a flat vulcanizing machine, wherein the pre-pressing pressure of the flat vulcanizing machine is 0.7 MPa, the pre-pressing time is 180 s, the pressing pressure is 3.5 MPa, and the pressing time is 180 s. The thickness of the composite film is about 0.25 mm.

[0056] The preparation method of the TPS / PBAT / NFC composite film in Examples 2-5 is basically the same as that in Example 1, except that in step a of Examples 2-5, the mass fraction of the esterification-modified NFC is 1 part, 4 parts, 7 parts and 10 parts, respectively, to prepare the TPS / PBAT / NFC composite films in Examples 2-5, respectively.

[0057] The TPS / PBAT / NFC composite films of Example 1 to Example 5 were processed into tensile test samples with a cutter. The mechanical properties of the TPS / PBAT / NFC composite films of Example 1 to Example 5 were tested in groups according to the method of GB / T 1040.1-2018. The tensile test sample was a dumbbell-shaped sample with a length of 50 mm, a gauge length of 30 mm, a central width of 4 mm, and a thickness of 0.24 mm. The tensile speed was 20 mm / min, at least 5 samples were tested for each group, and the average value of the tensile strength, tensile modulus, and tensile elongation at break of the samples were taken, and the results are shown in Table 2. Figure 1

[0058] From Figure 1 It can be seen that after adding esterified modified NFC, the tensile strength and elastic modulus of the TPS / PBAT / NFC composite film showed an upward trend, and reached a maximum when the content of esterified modified NFC was 7 parts. Compared with the case where the NFC content was 0 parts (C1), the tensile strength of C4 increased from 3.9 MPa to 6.2 MPa, an increase of 59%; the elastic modulus increased from 93.95 MPa to 263 MPa, an increase of 180%. These results show that esterified modified NFC has excellent reinforcing effect on PBAT / TPS composite film. Its reinforcing mechanism can be attributed to the formation of a large number of physical interlocking in the matrix by the network-like rigid NFC. In addition, esterified modified NFC has amphiphilic properties between starch and PBAT phases, and can act as a bridging agent to facilitate the transmission of internal stress and strain when the film is stressed. However, when the NFC content reaches 10 parts, the stress concentration caused by the agglomeration of NFC particles leads to a significant decrease in tensile strength, elastic modulus, and elongation at break.

[0059] For the TPS / PBAT / NFC composite films of Example 1 to Example 5, after quenching with liquid nitrogen, they were observed at 6kV with a magnification of 5000 times. Figure 2 ​The scanning electron microscope images of the fracture surface of the PBAT / TPS / NFC composite films are shown. The fracture surface of the film without NFC (C1) looks uneven and rough, presenting many particles and grooves. This roughness indicates the lack of compatibility between the hydrophilic TPS and the hydrophobic PBAT interface. Therefore, the binding force between the two phases of TPS and PBAT is weak, which hinders the effective transmission of stress and strain, which has an adverse effect on the mechanical properties of the film. In C2, after adding 1 part of esterified modified NFC, the particles and cracks are slightly reduced, and the phase interface begins to become blurred. This means that the esterified modified NFC improves the compatibility of the PBAT / TPS composite material. At the same time, the esterification modification process enhances the affinity of NFC with PBAT and TPS, making NFC a "bridge" between the two phases. However, when the NFC content reaches 10 parts, the agglomeration phenomenon of NFC begins to appear. The enhancement of the interfacial force between NFC / TPS and TPS / PBAT effectively improves the tensile properties of the film, which is consistent with the improvement of the mechanical properties observed in the tensile test.

[0060] For the TPS / PBAT / NFC composite films of Examples 1-5, XRD tests were performed using an X-ray diffractometer, and the samples were scanned at a rate of 6° / min in the 2θ region of 5-60° at room temperature. The crystal size of the sample was calculated using the Scherrer formula, and the XRD test results are shown in Figure 3 As can be seen from the figure, the film sample appears 4 crystal diffraction peaks at 2θ of 17.07°, 20.34°, 22.64° and 24.43°, which correspond to the 011, 101, 100 and 111 crystal planes of PBAT, respectively. The crystal diffraction peak of the TPS / PBAT / NFC composite film is narrow and sharp at 0 parts of NFC content, especially at the 101 crystal plane. With the addition of esterified modified NFC, the intensity of the diffraction peak decreases and becomes wider, indicating that NFC hinders the movement of the grain boundary and slows down the crystallization behavior in the PBAT / TPS matrix. However, no obvious crystallization diffraction peak of starch (2θ = 15.5°, 17.1°, 18.0°, 23.0°) was observed in the XRD test. This can be attributed to the overlap of the diffraction peaks of starch with the strong and similar position diffraction peaks of PBAT. There can be three reasons for this phenomenon: 1) the gelatinization process increases the amorphous region of starch, 2) the addition of esterified modified NFC forms new hydrogen bonds with starch, making it difficult for starch to recrystallize due to the interference of intermolecular and intramolecular hydrogen bonds, 3) the ball milling process mechanically destroys the original lattice of starch, further reducing its crystallinity. With the addition of 1 part of NFC, the crystallinity of the film decreases from 54.8% to 34.5%. This can be due to the esterified modified NFC hindering the ordered movement of the PBAT molecular chain, resulting in a decrease in crystallinity. The decrease in crystallinity effectively prevents the retrogradation of starch, which is beneficial for the life of starch-based products. Figure 3The crystallinity and grain size of the PBAT / TPS / NFC composite films at 20.34° 2Q angle were presented. The crystallization behavior of starch was significantly reduced after the gelatinization process of starch and the addition of NFC. The gradual displacement of the grain boundaries during the crystal growth process involves the interpenetration of the grains. The dispersed rigid esterified NFC phase hinders the displacement of the grain boundaries, thus limiting the crystal growth in the film. This can be the reason for the decrease in grain size after the addition of NFC. The relatively small crystalline material causes more uniform plastic deformation when subjected to external forces, reducing stress concentration and improving tensile strength, which is consistent with the results of the tensile test.

[0061] The water absorption performance of the TPS / PBAT / NFC composite films of Examples 1-5 was tested according to the method of national standard GB / T 1034-70. The water vapor transmission performance of the TPS / PBAT / NFC composite films of Examples 1-5 was tested according to the method of national standard GB / T 1037-2008; the ultraviolet barrier performance of the TPS / PBAT / NFC composite films of Examples 1-5 was tested according to the method of national standard GB / T 1038.1-2022. The water absorption test sample was a square sample of 1 cm x 1 cm, and the water absorption test was carried out at room temperature, 50% relative humidity, and the water absorption rate was measured after 72 h, at least 5 samples were tested for each group and the average value was taken. The water vapor transmission performance test sample was a circular sample with a diameter of 5 cm, 30 g of deionized water was added to a 50 mL beaker, the sample was covered on the beaker opening, and the water vapor transmission rate was measured every 12 h, repeated for 6 times, at least 5 samples were tested for each group and the average value was taken. The ultraviolet-visible spectrophotometer was used to test the ultraviolet absorption of the film, the wavelength was set to 200-400 nm, and the transmission spectrum of the film was obtained using air as the reference.

[0062] Figure 4 The water absorption rate and water vapor transmission rate of Examples 1-5 were demonstrated. The water vapor transmission rate value of PBAT / TPS / NFC film was slightly higher than that of the film without esterified NFC. This can be due to the formation of hydrogen bonds between NFC and starch, which reduces the crystallinity of starch, thus promoting the diffusion of water in the film. In addition, the XRD test results show that the esterified NFC hinders the crystallization behavior of TPS, resulting in a decrease in the crystalline region. Compared with the crystalline region, the amorphous region is more likely to allow water vapor molecules to pass through, thereby increasing the water vapor transmission rate of the film. In addition, the water absorption rate value of the PBAT / TPS / NFC composite film also showed a similar trend, which can also be explained by the decrease in the TPS crystalline region.

[0063] Figure 5The UV transmission spectra of Examples 1-5 are shown, and after the addition of NFC, the UV transmittance of the film shows a general downward trend. Especially at about 300 nm wavelength, the transmittance decreases from 77% for 0 parts of esterified modified NFC to 46% for 10 parts, a decrease of 40.26%. This decrease in transmittance is likely to be the absorption of UV energy, involving the transition of free electrons of oxygen to the LUMO orbital of the C=O bond, which indicates that the NFC has relatively good UV isolation performance. Another reason for the decrease in UV transmittance is the nanosize of the NFC, which ranges from 5 nm to 100 nm, much smaller than the wavelength of UV light (200-400 nm). Therefore, when UV light interacts with NFC, it undergoes multiple reflections and refractions, hindering its penetration of the material, producing an UV isolation effect.

[0064] The mass fraction composition of the TPS / PBAT / NFC composite film of Examples 6-7 is shown in Table 2, and the TPS / PBAT / NFC composite film of Example 6 is denoted as S1, and the TPS / PBAT / NFC composite film of Example 7 is denoted as S2, and single factor experiments are carried out.

[0065] The TPS / PBAT / NFC composite film of Example 7 is denoted as S1, S2, respectively, and single factor experiments are carried out.

[0066] Table 2

[0067]

[0068] In Examples 6-7: the starch is wheat starch, the lubricant is a 1:1 mixture of oleic acid amide and erucic acid amide, and the stabilizer is antioxidant 1076.

[0069] The preparation method of the TPS / PBAT / NFC composite film of Example 6 includes the following steps:

[0070] 1. Preparation of esterified modified NFC

[0071] 1) Mix 8 parts of corn straw powder with 70 parts of 20 wt% sodium hydroxide solution, heat and stir at 85°C for 7h, wash with pure water until neutral, then add 60 parts of 15 wt% sodium chlorite solution and 3 parts of mixed solution of glacial acetic acid, stir and heat for 2h at 80°C, then wash with pure water until neutral;

[0072] 2) Put all the solute samples obtained in 1) into a beaker, add 55 parts of 40 wt% oxalic acid solution, stir and heat at 70°C for 0.5h, then use a homogenizer to shear at high speed for 1h, the homogenizer speed is 14000 rpm; centrifuge and wash the sample until neutral after homogenization, then freeze-dry to obtain esterified modified NFC.

[0073] 2. Preparation of TPS / PBAT / NFC composite film

[0074] a: 1 part of esterified modified NFC was added into 35 parts of deionized water, and treated in an ultrasonic disruptor for 10 min at a power of 300 W to obtain a pre-dispersed NFC aqueous solution;

[0075] b: the pre-dispersed NFC aqueous solution obtained in step a was mixed with 100 parts of starch and 15 parts of glycerol, and dry ball milling was performed in a planetary ball mill, with zirconia or ceramic balls as the grinding medium, at a temperature of 60°C, a ball milling speed of 300 rpm, and a ball milling time of 0 h, to obtain a highly dispersed starch slurry of NFC;

[0076] c: the starch slurry obtained in step b was gelatinized in a water bath for 1.5 h at a temperature of 65°C to obtain gelatinized wheat starch, which was dried in an oven at 105°C for 8 h to obtain NFC-enhanced TPS;

[0077] d: the NFC-enhanced TPS obtained in step c was mixed with 100 parts of PBAT, 0.6 parts of a lubricant, and 0.8 parts of a stabilizer in a twin-screw extruder for 10 min at a temperature of 120°C and a screw speed of 50 rpm to obtain a TPS / PBAT / NFC mixture;

[0078] e: the mixture obtained in step d was dried in a vacuum drying box, and the dried material was pressed into a TPS / PBAT / NFC degradable composite film in a flat vulcanizing machine, with a pre-pressing pressure of 0.7 MPa, a pre-pressing time of 180 s, a pressing pressure of 3.5 MPa, and a pressing time of 180 s. The thickness of the composite film was about 0.27 mm.

[0079] The preparation method of the TPS / PBAT / NFC composite film of Example 7 was basically the same as that of Example 6, except that the ball milling time in step b of Example 7 was 1 h.

[0080] The TPS / PBAT / NFC composite films of Examples 6-7 were processed into tensile test samples with a cutter. The mechanical properties of the TPS / PBAT / NFC composite films of Examples 6-7 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped samples with a length of 50 mm, a gauge length of 30 mm, a central width of 4 mm, and a thickness of 0.24 mm. The tensile speed was 20 mm / min, at least 5 samples were tested for each group, and the average values of the tensile strength, tensile modulus, and tensile elongation at break of the samples were taken, and the results are shown in Table 1. Figure 6

[0081] ​Compared with the sample S1 without ball milling, S2 showed better mechanical properties after ball milling for 1 h. Specifically, the tensile strength increased from 3.3 MPa to 4.3 MPa, an increase of 30.3%, and the elastic modulus increased from 128.85 MPa to 201.71 MPa, an increase of 56.6%. This improvement can be attributed to the enhanced dispersion of NFC in the TPS matrix and the formation of new hydrogen bonds caused by ball milling. Subsequently, the highly dispersed NFC produced a more effective bridging effect between the TPS and PBAT phases, thereby improving the performance of the film. In contrast, the non-ball milled PBAT / TPS / NFC film lacked the dispersion of NFC, which explained the lower mechanical strength of S1 compared to S2.

[0082] For the TPS / PBAT / NFC composite films of Example 6-Example 7, after quenching with liquid nitrogen, they were observed at 6 kV with magnification of 5000 times. Figure 7 The scanning electron microscope images of the fracture surface of the PBAT / TPS / NFC composite film are shown. Figure 7 The PBAT / TPS / NFC composite film of a showed an uneven phase structure, indicating that the NFC was not uniformly distributed in the matrix in the sample without ball milling, which resulted in limited improvement in the compatibility between the two phases. After ball milling, as shown in Figure 7 b, due to the uniform dispersion of NFC in the matrix, the presence of particles and grooves was slightly reduced, and the phase interface began to become blurred, which meant that the compatibility effect of NFC on the PBAT / TPS composite material was improved.

[0083] For the TPS / PBAT / NFC composite films of Example 6-Example 7, XRD tests were performed using an X-ray diffractometer, and the samples were scanned at a rate of 6° / min in the 2θ region of 5-60° at room temperature. The grain size of the samples was calculated using the Scherrer formula, and the XRD test results are shown in Figure 8 After ball milling for 1 h, the crystallinity of the composite material decreased. During the ball milling process, the material particles were subjected to mechanical forces, which caused the wear and fracture of the crystalline regions. This resulted in the destruction of the starch crystal structure in the composite film and the reduction of the grain size, thereby leading to a decrease in the crystallinity.

[0084] The water absorption performance of the TPS / PBAT / NFC composite films of Example 6 to Example 7 was tested according to the method of national standard GB / T 1034-70. The water vapor transmission performance of the TPS / PBAT / NFC composite films of Example 6 to Example 7 was tested according to the method of national standard GB / T 1037-2008; the ultraviolet barrier performance of the TPS / PBAT / NFC composite films of Example 6 to Example 7 was tested according to the method of national standard GB / T 1038.1-2022. The water absorption test sample was a square sample of 1 cm x 1 cm, and the water absorption test was carried out at room temperature under the condition of 50% relative humidity for 72 h, and then the water absorption rate was weighed, at least 5 samples were tested in each group and the average value was taken. The water vapor transmission performance test sample was a circular sample with a diameter of 5 cm, 30 g of deionized water was added in a 50 mL beaker, the sample was covered on the beaker mouth, the water vapor transmission rate was weighed every 12 h, repeated for 6 times, at least 5 samples were tested in each group and the average value was taken. The ultraviolet-visible spectrophotometer was used to test the ultraviolet absorption of the film, the wavelength was set to 200-400 nm, and the transmission spectrum of the film was obtained using air as the reference.

[0085] Figure 9 The water absorption rate and water vapor transmission rate of Example 6 to Example 7 are shown. As can be seen from the figure, ball milling leads to the increase of the water absorption rate and water vapor transmission rate of the TPS / PBAT / NFC composite film, which may be due to the fact that during the ball milling process, the starch particles are subjected to mechanical force, and collisions, friction and shearing occur between the particles, leading to the destruction, fracture or recombination of the polymer chain structure in the starch-based composite film. These changes in structure will increase the porosity or defects of the material, making it easier for water to penetrate into the interior of the film, thereby leading to the increase of the water absorption rate and water vapor transmission rate. In addition, as can be seen from the figure, the sample after ball milling has better ultraviolet barrier effect. This may be due to the fact that the ball milling process reduces the particle size of the material, and at the same time leads to the more uniform distribution of NFC in the film. Such uniformly distributed particles will further scatter light during the propagation of light, increasing the reflection and absorption of ultraviolet light by the material, thereby reducing the transmission rate of ultraviolet light. Figure 10

[0086] The mass fraction composition of the TPS / PBAT / NFC composite films of Example 8 to Example 12 is shown in Table 3, and the TPS / PBAT / NFC composite films of Example 8 to Example 12 are represented by the numbers K1, K2, K3, K4, K5 respectively, and single factor experiments are carried out.

[0087] Table 3

[0088]

[0089] ​In Examples 8-12: Starch is pea starch, lubricant is a mixture of ethylene bis-stearamide, oleic acid amide, and erucic acid amide at a ratio of 1:2:1, and stabilizer is antioxidant 1076.

[0090] The method for preparing the TPS / PBAT / NFC composite film of Example 8 includes the following steps:

[0091] 1. Preparation of esterified modified NFC

[0092] 1) 5 parts of corn straw powder was mixed with 75 parts of 18 wt% sodium hydroxide solution, heated and stirred at 85°C for 7h, washed with pure water to neutral, then 60 parts of 13 wt% sodium chlorite solution and 3 parts of glacial acetic acid mixed solution were added, stirred and heated for 2h at 80°C, and then washed with pure water to neutral;

[0093] 2) The solute sample obtained in 1) was placed in a beaker, 60 parts of 40 wt% oxalic acid solution was added, stirred and heated at 70°C for 0.5h, and the cooled solid-liquid mixture was treated with a homogenizer at a high speed for 1h at a speed of 15000 rpm; After centrifugal washing to neutral, freeze-drying was carried out to obtain esterified modified NFC.

[0094] 2. Preparation of TPS / PBAT / NFC composite film

[0095] a: 7 parts of esterified modified NFC was added to 30 parts of deionized water, treated in an ultrasonic disruptor for 10 min at a power of 300W to obtain a pre-dispersed NFC aqueous solution;

[0096] b: The pre-dispersed NFC aqueous solution in step a was mixed with 100 parts of starch and 20 parts of glycerol, and was put into a planetary ball mill for dry ball milling, with zirconium oxide or ceramic balls as the grinding medium, at a temperature of 60°C, a ball milling speed of 100 rpm, and a ball milling time of 1h, to obtain a highly dispersed starch slurry of NFC;

[0097] c: The starch slurry obtained in step b was gelatinized in a water bath for 1.5h at a temperature of 65°C to obtain gelatinized pea starch, which was dried in an oven at 105°C for 8h to obtain NFC reinforced TPS;

[0098] d: The NFC reinforced TPS obtained in step c was mixed with 125 parts of PBAT, 0.7 parts of lubricant, and 0.5 parts of stabilizer in a twin-screw extruder for 10 min at a temperature of 120°C and a screw speed of 50 rpm to obtain a TPS / PBAT / NFC mixture;

[0099] e: The mixture obtained in step d was dried in a vacuum drying box, and then the dried material was pressed into a TPS / PBAT / NFC degradable composite film in a flat vulcanizing machine. The pre-pressing pressure of the flat vulcanizing machine was 0.6 MPa, the pre-pressing time was 180 s, the pressing pressure was 3.2 MPa, and the pressing time was 180 s. The thickness of the composite film was about 0.29 mm.

[0100] The preparation method of the TPS / PBAT / NFC composite film of examples 9-12 was basically the same as that of example 8, except that in step b of the preparation of examples 9-12, the ball milling speed was 300, 500, 700, and 900 rpm, respectively.

[0101] For the TPS / PBAT / NFC composite films of examples 8-12, tensile test samples were processed with a cutter. The mechanical properties of the TPS / PBAT / NFC composite films of examples 8-12 were tested in groups according to the method of national standard GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped samples with a length of 50 mm, a gauge length of 30 mm, a central width of 4 mm, and a thickness of 0.24 mm. The tensile speed was 20 mm / min. At least 5 samples were tested for each group, and the average values of the tensile strength, tensile modulus, and tensile elongation at break of the samples were taken. The results are shown in Table 1. Figure 11

[0102] From Figure 11 ​It can be seen that the ball milling speed improves the tensile strength and elastic modulus of the TPS / PBAT / NFC composite film, and reaches the maximum at 500 rpm (K3). When the speed is 500 rpm, the tensile strength of the composite film reaches 7.81 MPa, which is 55.89% higher than 5.01 MPa at 100 rpm (K1); the elastic modulus increases from 200.25 MPa to 337.49 MPa, an increase of 68.53%. These results demonstrate that the ball milling speed has a significant improvement on the mechanical properties of the TPS / PBAT / NFC composite film, which may be due to the fact that as the ball milling speed increases, the particles are subjected to greater mechanical force and friction, making the particles gradually become smaller. Small particles have a larger surface area, which can increase the contact area between the material and other components, and improve the interfacial bonding strength. In addition, greater mechanical force can more effectively improve the uniformity of particle dispersion in the starch-based composite material, and the uniform dispersion of particles is beneficial to improve the mechanical properties of the material, and reduces the internal defects and porosity of the material, and improves the compactness and tightness of the material. However, when the ball milling speed reaches 700 rpm, the mechanical properties of the TPS / PBAT / NFC composite film decrease, which may be due to the fact that high-speed ball milling causes violent collision and friction between particles, resulting in excessive wear and even peeling of the particle surface, and thus the particles are deformed or even broken.

[0103] The mass fraction composition of the TPS / PBAT / NFC composite film of Example 13 to Example 17 is shown in Table 4, and the TPS / PBAT / NFC composite films of Example 13 to Example 17 are represented by the numbers F1, F2, F3, F4, F5, respectively, for single factor experiment.

[0104] Table 4

[0105]

[0106] In Example 13 to Example 17: starch is cassava starch, lubricant is a mixture of ethylene bis-stearamide, oleic acid amide and erucic acid amide 2:1:1, and stabilizer is antioxidant 1010.

[0107] The preparation method of the TPS / PBAT / NFC composite film of Example 13 comprises the following steps:

[0108] 1. Preparation of esterified modified NFC

[0109] 1) Mix 6 parts of corn straw powder with 70 parts of 20wt% sodium hydroxide solution, heat and stir at 85℃ for 7h, wash with pure water until neutral, then add a mixed solution of 60 parts of 15wt% sodium chlorite solution and 3 parts of glacial acetic acid, stir and heat for 2h at 80℃, and then wash with pure water until neutral;

[0110] 2) Put all the solute sample obtained in 1) into a beaker, add 60 parts of 40 wt% oxalic acid solution, heat and stir at 70℃ for 0.5h, and then treat the cooled solid-liquid mixture with a homogenizer at a high speed for 1h at a rotation speed of 13000 rpm; centrifugally wash the sample after homogenization until neutral, and then freeze-dry to obtain the esterification-modified NFC.

[0111] 2. Preparation of TPS / PBAT / NFC composite film

[0112] a: Put 7 parts of esterification-modified NFC into 30 parts of deionized water, and treat in an ultrasonic disruptor for 10 min at a power of 300 W to obtain a pre-dispersed NFC aqueous solution;

[0113] b: Mix the pre-dispersed NFC aqueous solution in step a with 100 parts of starch and 10 parts of glycerol, and add them into a planetary ball mill for dry ball milling, using zirconia or ceramic balls as the grinding medium, at a temperature of 60℃, a ball milling rotation speed of 500 rpm, and a ball milling time of 1h to obtain a highly dispersed starch slurry of NFC;

[0114] c: Paste the starch slurry obtained in step b in a water bath for 1.5h at a temperature of 65℃ to obtain pasted cassava starch, which is dried in an oven at 105℃ for 8h to obtain NFC-reinforced TPS;

[0115] d: Mix the NFC-reinforced TPS obtained in step c with 150 parts of PBAT, 0.8 parts of lubricant and 1 part of stabilizer in a twin-screw extruder for 10 min at a temperature of 120℃ and a screw rotation speed of 80 rpm to obtain a TPS / PBAT / NFC mixture;

[0116] e: Dry the mixture obtained in step d in a vacuum drying box, and then press the dried material into a TPS / PBAT / NFC degradable composite film in a flat vulcanizing machine at a pre-pressing pressure of 0.6 MPa for 180 s and a pressing pressure of 3.5 MPa for 180 s. The thickness of the composite film is about 0.35 mm.

[0117] The preparation method of the TPS / PBAT / NFC composite film in Examples 14-17 is basically the same as that in Example 14, except that in step b of Examples 14-17, the amount of glycerol added is 15, 20, 25 and 30 parts, respectively.

[0118] For the TPS / PBAT / NFC composite films of Example 13 to Example 17, a cutter was used to process tensile test samples. The mechanical properties of the TPS / PBAT / NFC composite films of Example 13 to Example 17 were tested in groups according to the method of GB / T 1040.1-2018. The tensile test samples were dumbbell-shaped samples with a length of 50 mm, a gauge length of 30 mm, a central width of 4 mm, and a thickness of 0.24 mm. The tensile speed was 20 mm / min, at least 5 samples were tested for each group, and the average values of the tensile strength, tensile modulus, and tensile elongation at break of the samples were taken, and the results are shown in Table 2. Figure 12

[0119] As can be seen from Figure 12 , the increase in glycerol content significantly improves the mechanical properties of the TPS / PBAT / NFC composite film, and the tensile strength of the film reaches a maximum value when the glycerol content is 20 parts. When the glycerol content is 20 parts (F3), the tensile strength of the TPS / PBAT / NFC composite film reaches 7.80 MPa, which is increased from 6.36 MPa to 7.80 MPa when the glycerol content is 10 parts (F1), an increase of 22.64%; the elastic modulus is increased from 232.97 MPa to 337.52 MPa, an increase of 44.87%. The improvement in mechanical properties is due to the plasticizing effect of glycerol on starch. The increase in glycerol content causes the original crystal structure of starch to be largely destroyed, making the molecular structure more disordered, and ultimately achieving a transition from crystalline to amorphous. The hydrogen bonding between and within starch molecules is further destroyed, thereby achieving better mechanical properties. However, too much plasticizer can damage the mechanical properties of the composite material, which may be due to the limited solubility of the plasticizer in the matrix. When the amount of plasticizer exceeds its solubility, the plasticizer will precipitate or separate out, forming aggregates or voids. This will cause the number of pores inside the material to increase, and the structure to become non-uniform, thereby reducing the density and mechanical properties of the material. In addition, too much plasticizer can cause the material to become too soft and unable to effectively withstand external loads or form a stable structure. This will reduce the strength and stiffness of the material and decrease the mechanical properties. The elongation at break of the composite material always remains on the rise with the increase in glycerol content, which is also due to the softening effect of the plasticizer on the material and molecular chain structure.​

Claims

1. A TPS / PBAT / NFC composite film, characterized in that, The mass fraction composition is: starch 100 parts, PBAT 100-150 parts, esterified modified NFC 1-10 parts, glycerol 15-30 parts, deionized water 30-40 parts, lubricant 0.5-1 part, stabilizer 0.5-1 part; the lubricant is one or a combination of two or more of stearic acid, ethylene bis-stearamide, oleic acid amide, erucic acid amide; the stabilizer is one or a combination of two of antioxidant 1010, antioxidant 168, antioxidant 1076, antioxidant 1790; the preparation method of the TPS / PBAT / NFC composite film comprises the following steps: 1) 5-10 parts of corn straw powder is mixed with 70-80 parts of 10-20 wt% sodium hydroxide solution, heated and stirred at 85-95°C for 4-7h, washed with pure water until neutral, then 60 parts of 10-15 wt% sodium chlorite solution and 3-4 parts of mixed solution of glacial acetic acid are added, stirred and heated for 1-2h at 70-80°C, and then washed with pure water until neutral; 2) the solute sample obtained in 1) is placed in a beaker, 50-60 parts of 30-40 wt% oxalic acid solution is added, stirred and heated at 60-70°C for 0.5-1h, the cooled solid-liquid mixture is treated by high-speed shearing using a homogenizer at 10000-16000 rpm; after centrifugal washing to neutral, freeze-drying is carried out to obtain esterified modified NFC; 3) 1-10 parts of esterified modified NFC obtained in step 2) is added to 30-40 parts of deionized water, treated in an ultrasonic crusher for 10-15 min, and a pre-dispersed NFC aqueous solution is obtained; 4) the NFC aqueous solution obtained in step 3) is mixed with 100 parts of starch and 15-30 parts of glycerol, and a planetary ball mill is added for dry ball milling, the grinding medium is zirconia or ceramic ball, the temperature is 50-80°C, the ball milling speed is 100-900 rpm, and the ball milling time is 0-1h, to obtain a highly dispersed starch slurry of NFC; 5) the starch slurry obtained in step 4) is gelatinized in a water bath for 0.5-1h at a temperature of 65-75°C to obtain gelatinized starch, which is then dried in an oven at 105°C for 8h to obtain NFC enhanced TPS; 6) the NFC enhanced TPS obtained in step 5) is mixed with 100-150 parts of PBAT, 0.5-1 parts of lubricant and 0.5-1 parts of stabilizer in a twin-screw extruder for 2-5 min at a temperature of 100-140°C and a screw speed of 50-100 rpm to obtain a TPS / PBAT / NFC mixture; 7) the mixture obtained in step 6) is dried in a vacuum drying oven at 80°C for 2-5h, and the dried material is processed into a TPS / PBAT / NFC composite film using a flat vulcanizing machine or a film blowing machine.

2. The TPS / PBAT / NFC composite film according to claim 1, characterized in that, The starch is one or a combination of two or more of corn starch, cassava starch, pea starch, potato starch, wheat starch, sweet potato starch, lotus root starch.

3. The TPS / PBAT / NFC composite film according to claim 1, characterized in that, The corn straw powder has a mesh size of 100-150.

4. The TPS / PBAT / NFC composite film according to claim 1, characterized in that, The pre-pressing pressure of the flat plate vulcanizing machine in step 7) is 0.3-0.7 MPa, the pre-pressing time is 150-180 s, and the pressurizing pressure is 2.5-3.5 MPa, and the pressurizing time is 150-180 s.

5. The TPS / PBAT / NFC composite film according to claim 1, characterized in that, The thickness of the TPS / PBAT / NFC composite film in step 7) is 0.05-0.5 mm.

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