CN117362760A

CN117362760A - Starch-based biodegradable film and preparation method thereof

Info

Publication number: CN117362760A
Application number: CN202311323492.3A
Authority: CN
Inventors: 钱少平; 孔英祺; 朱昌法; 李心雨
Original assignee: Ningbo University
Current assignee: Ningbo University
Priority date: 2023-10-12
Filing date: 2023-10-12
Publication date: 2024-01-09

Abstract

The starch/PBAT/nano-cellulose biodegradable film disclosed by the invention comprises the following components in percentage by mass: 100 parts of starch, 100-150 parts of PBAT, 1-10 parts of modified NFC, 15-20 parts of glycerol, 30-40 parts of deionized water, 0.5-4 parts of lubricant and 0.5-4 parts of stabilizer; the modified NFC is esterified modified NFC. According to the invention, through adding the esterified modified NFC, a stable bridge structure is constructed between the starch and the PBAT, so that the interface compatibility of the starch and the PBAT is effectively improved, and the mechanical property of the composite material is enhanced. In addition, the dispersibility of NFC in the mixture matrix is improved through high-energy ball milling treatment, the interface bonding strength of NFC and the matrix is improved, and the mechanical property of the composite material is further enhanced. The preparation method is simple, efficient and low in cost, is easy to realize industrial production, and has wide market application prospect and commercial value.

Description

Starch-based biodegradable film and preparation method thereof

Technical Field

The invention belongs to the technical field of degradable plastic films, and particularly relates to a starch/PBAT/nanocellulose biodegradable film and a preparation method thereof.

Background

The starch source is rich, renewable and low in cost, and the starch can be used for producing starch-based biodegradable plastics through plasticizing modification. Starch-based biodegradable plastics are typically blends of modified starch with biodegradable polyesters such as poly (adipic acid)/terephthalic acid/butylene glycol (PBAT), poly (lactic acid) (PLA), poly (butylene succinate) (PBS), and the like. It can be completely biodegraded, has no pollution to the environment, and can treat wastes by means of composting, landfill and the like. The starch-PBAT is taken as an important product in starch-based biodegradable plastics, and can be applied to various fields such as disposable plastic bags, agricultural mulching films, preservative films and the like. However, two fatal disadvantages of starch-PBAT blends severely limit their performance. First, the lack of compatibility between hydrophilic starch and hydrophobic polyester results in poor stress transfer between phase interfaces, resulting in a significant decrease in mechanical properties of the composite after the starch addition exceeds the critical point (30%). On the other hand, starch is easy to regenerate, and the shelf life and the service performance of starch-based biodegradable plastic products are seriously influenced.

The addition of nanofibrillated cellulose (NFC) is an effective method to improve the compatibility of thermoplastic starch (TPS) with PBAT without affecting starch biodegradability. NFC can reduce the crystallinity of TPS, thereby inhibiting retrogradation of starch. Some studies report the reinforcing effect of nanocellulose on polyester-based composites. For example, lang et al (Polymers 2022, 14 (21), 4517) reported a PBAT/TPS/Cellulose Nanocrystalline (CNC) composite, which found a 30% improvement in tensile strength of the composite after the addition of 4wt% CNC. Morae et al (Mat. Sci. Eng. C-Mater.2017, 78, 932-941) found that the tensile strength and Young's modulus of TPS/PBAT films were significantly improved and the water resistance of the material was also improved after addition of Plasticized Cellulose Acetate (PCA). However, lee et al (Compos. Sci. Technology.2014, 105, 15-27) indicate that when the NFC content exceeds 30%, NFC will agglomerate under the surface energy and thus lose nanoreinforcement. Therefore, the dispersibility of NFC in the matrix is an important factor affecting the TPS/PBAT composite properties.

The high-energy ball milling method can generate mechanochemical action under the repeated collision of grinding ball media, so that the powder is fully uniform and refined. The patent application document with publication number CN113121888A discloses a method for preparing plasticized starch by ball milling, which finds that under the extremely strong physical force generated by ball milling, the intramolecular and intermolecular hydrogen bonds of starch are destroyed in a large amount, the crystal lattice is damaged, the crystallinity is reduced, and thus the retrogradation resistance of thermoplastic starch is improved. The patent application document with publication number of CN113248798A discloses a starch/cellulose/PBAT composite film and a preparation method thereof, wherein starch, cellulose and PBAT are uniformly mixed by ball milling, then granulated and then blown. Through comparison, the ball milling is helpful to promote the dispersibility of cellulose, starch and PBAT so as to improve the mechanical properties of the composite material, however, the interfacial compatibility of common cellulose and PBAT is poor, and the improvement of the mechanical properties of the composite material is limited. Therefore, the invention provides a method for improving TPS and PBAT by using esterified NFC, and further improving the dispersibility of NFC by high-energy ball milling treatment.

Disclosure of Invention

The invention aims to solve the problems of poor interfacial compatibility, low starch filling amount, easy regeneration of the film and low strength of the existing starch/PBAT composite film, and provides a TPS/PBAT/NFC high-performance composite film and a preparation method thereof.

The technical scheme adopted for solving the technical problems is as follows: the TPS/PBAT/NFC composite film comprises the following components in parts by mass: 100 parts of starch, 100-150 parts of PBAT, 1-10 parts of esterification modified NFC, 15-30 parts of glycerol, 30-40 parts of deionized water, 0.5-1 part of lubricant and 0.5-1 part of stabilizer.

Preferably, the starch is one or more than two of corn starch, tapioca starch, pea starch, potato starch, wheat starch, sweet potato starch and lotus root starch.

Preferably, the lubricant is one or a combination of more than two of stearic acid, ethylene bis-stearic acid amide, oleic acid amide and erucic acid amide. Preferably, the stabilizer is one or more than two of antioxidant 1010, antioxidant 168, antioxidant 1076 and antioxidant 1790.

The preparation method of the TPS/PBAT/NFC composite film comprises the following steps:

1. preparation of esterification modified NFC

1) Mixing 5-10 parts of corn stalk powder with 70-80 parts of sodium hydroxide solution, heating and stirring for 4-7 hours at 85-95 ℃, washing to be neutral by pure water, adding a mixed solution of 60 parts of sodium chlorite solution and 3-4 parts of glacial acetic acid, heating and stirring for 1-2 hours at 70-80 ℃, and washing to be neutral by pure water;

2) Placing all solute samples obtained in the step 1) in a beaker, adding 50-60 parts of oxalic acid solution, stirring and heating at 60-70 ℃ for 0.5-1 h, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 1-2 h by using a homogenizer with the rotating speed of 10000-16000 rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

Preferably, the mesh number of the corn stalk powder in the step 1) is 100-150 meshes, the concentration of the sodium hydroxide solution is 10-20wt% and the concentration of the sodium chlorite solution is 10-15wt%; the concentration of the oxalic acid solution in the step 2) is 30 to 40 weight percent.

The surface of the nanocellulose is rich in hydrophilic hydroxyl groups, and the nanocellulose and a hydrophobic PBAT matrix generate stronger phase separation phenomenon in the fusion preparation process of the composite material. In the step 2) of the method, oxalic acid is adopted to carry out esterification modification on NFC, so that hydrophilic hydroxyl on the surface of NFC is converted into hydrophobic ester group. The ester group introduced by the esterification reaction and the ester group in the PBAT form a similar chemical structure, so that the compatibility between the nanocellulose and the PBAT can be effectively increased. According to the method disclosed by the invention, the step 2) realizes the preparation and esterification modification of NFC at the same time, and the process is simple, convenient and efficient.

Preparation of TPS/PBAT/NFC composite film

a: adding 1-10 parts of modified NFC into 30-40 parts of pure water, and processing for 10-15 min in an ultrasonic crusher with the power of 300-350W to obtain a pre-dispersed NFC aqueous solution;

b: mixing the NFC aqueous solution obtained in the step a with 100 parts of starch and 15-20 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 50-80 ℃, the ball milling rotating speed is 100-900 rpm, and the ball milling time is 0-1 h, so that NFC highly dispersed starch slurry is obtained;

c: gelatinizing the starch obtained in the step b for 0.5-1 h at the temperature of 65-75 ℃ to obtain gelatinized starch, and drying the gelatinized starch at the temperature of 105 ℃ for 8h in an oven to obtain NFC enhanced TPS;

d: mixing the NFC enhanced TPS obtained in the step c with 100-150 parts of PBAT, 0.5-1 part of lubricant and 0.5-1 part of stabilizer in a double-screw extruder for 2-5min at the temperature of 100-140 ℃ and the screw rotating speed of 50-100 rpm to obtain a TPS/PBAT/NFC mixture;

e: and d, drying the mixture obtained in the step in a vacuum drying oven at 80 ℃ for 2-5 hours, and processing the dried material into the TPS/PBAT/NFC composite film by using a flat vulcanizing machine or a film blowing machine.

Preferably, the pre-pressing pressure of the flat vulcanizing machine in the step e is 0.3-0.7 MPa, the pre-pressing time is 150-180 s, the pressing pressure is 2.5-3.5 MPa, and the pressing time is 150-180 s.

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

According to the method, in the step b, the starch particles are subjected to effective refining treatment by using mechanical ball milling, so that the particle size of the starch particles is reduced. The smaller particle size is beneficial to increasing the contact area of the starch and other components and improving the plasticizing effect. At the same time, the collision and extrusion action of the high-energy ball mill exposes more nano-microfibers from the NFC, and uniformly disperses the NFC into TPS, and a large number of hydrogen bonds are formed. The starch and NFC are fully mixed, the enhancement function of NFC is fully exerted, and the overall tensile strength of the composite film is improved. In addition, the mechanical ball milling can carry out physical denaturation on starch molecules through shearing force and friction force, change the molecular structure of the starch, crush the original crystal lattice of the starch, greatly reduce the crystallization degree, effectively prevent retrogradation of the starch and prolong the service life of starch-based products.

The method of the invention passes through the step d, and the NFC after surface esterification has a plurality of ester bonds which are the same as ester groups in the PBAT, so that the NFC has strong affinity. NFC enhances the affinity with PBAT and simultaneously forms hydrogen bond interaction with TPS, thereby playing a bridging role. This is why the composite film is reinforced. This interface enhancement effect can increase the stress transfer efficiency between NFC and PBAT, thereby increasing the strength of the film.

Compared with the prior art, the invention has the following advantages:

1. according to the invention, the esterified and modified NFC is adopted as an enhancement phase, so that the interface compatibility, the mechanical property and the resilience resistance of the TPS/PBAT composite film are improved. Compared with the traditional nanocellulose, the content of hydrophilic hydroxyl on the NFC surface after esterification modification is reduced, and meanwhile, the ester group is newly introduced, so that the interaction between NFC and a polyester matrix is effectively improved.

2. According to the invention, the ball milling treatment is adopted to enhance the dispersibility of the modified NFC in TPS particles, which is beneficial to the full contact of starch particles and NFC and improves the tensile strength of the material. Meanwhile, the original crystal lattice of the starch is destroyed under the action of mechanical force, the crystallinity is reduced, retrogradation behavior of the starch-based product in the use process can be effectively prevented, the stability is excellent, and the application of the starch-based material in biodegradable materials is enlarged.

3. Compared with the traditional TPS/PBAT/NFC composite film preparation method, the method disclosed by the invention has the advantages of convenience in processing, low equipment requirement, less modification auxiliary agent consumption, extremely high mixing uniformity of modified NFC and starch particles for mechanical property reinforcement, capability of enhancing the mechanical property of TPS while preventing TPS retrogradation, improvement of the starch addition amount and reduction of the cost of biodegradable materials.

Drawings

Fig. 1: the tensile properties of TPS/PBAT/NFC composite films with different NFC addition amounts;

fig. 2: SEM images of TPS/PBAT/NFC composite films with different NFC additions (a: example 1; b: example 2; c: example 3; d: example 4;e; example 5);

fig. 3: crystallinity and grain size of TPS/PBAT/NFC composite films with different NFC addition amounts;

fig. 4: the moisture absorption performance of TPS/PBAT/NFC composite films with different NFC addition amounts;

fig. 5: ultraviolet blocking performance of TPS/PBAT/NFC composite films with different NFC addition amounts;

fig. 6: stretching performance of TPS/PBAT/NFC composite films with different ball milling treatment time;

fig. 7: SEM images of TPS/PBAT/NFC composite films with different ball mill treatment times (a: example 6;b: example 7);

fig. 8: crystallinity and grain size of TPS/PBAT/NFC composite films with different ball milling treatment time;

fig. 9: moisture absorption performance of TPS/PBAT/NFC composite films with different ball milling treatment time;

fig. 10: ultraviolet blocking performance of TPS/PBAT/NFC composite films with different ball milling treatment time;

fig. 11: stretching performance of TPS/PBAT/NFC composite films with different ball milling treatment rotating speeds;

fig. 12: tensile properties of TPS/PBAT/NFC composite films with different glycerol addition amounts.

Detailed Description

The invention is described in further detail below with reference to the embodiments of the drawings.

The mass part array compositions of the TPS/PBAT/NFC composite films of examples 1 to 5 are shown in Table 1, and the TPS/PBAT/NFC composite films of examples 1 to 5 are respectively denoted by the numbers C1, C2, C3, C4 and C5, and a single factor experiment is performed.

In examples 1 to 5: the starch is corn starch, and the lubricant is stearic acid, ethylene bis stearamide 1:1, and an antioxidant 1010 is used as a stabilizer.

TABLE 1

The preparation method of the TPS/PBAT/NFC composite film of the embodiment 1 comprises the following steps:

1. preparation of esterification modified NFC

1) Mixing 10 parts of corn stalk powder with 80 parts of 20wt% sodium hydroxide solution, heating at 95 ℃ for 6 hours under stirring, washing with pure water to be neutral, adding 60 parts of mixed solution of 15wt% sodium chlorite solution and 3 parts of glacial acetic acid, heating and stirring for 2 hours, heating at 80 ℃, and then washing with deionized water to be neutral;

2) Placing all solute samples obtained in the step 1) in a beaker, adding 60 parts of 40wt% oxalic acid solution, stirring and heating for 1h at 70 ℃, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 2h by using a homogenizer with the rotating speed of 16000rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

Preparation of TPS/PBAT/NFC composite film

a: adding 0 part of modified NFC into 35 parts of deionized water, and processing in an ultrasonic crusher for 15min, wherein the crusher power is 350W, so as to obtain a pre-dispersed NFC aqueous solution;

b: mixing the pre-dispersed NFC aqueous solution in the step a with 100 parts of starch and 20 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 60 ℃, the ball milling rotating speed is 300rpm, and the ball milling time is 1h, so that NFC highly dispersed starch slurry is obtained;

c: gelatinizing the starch obtained in the step b for 2 hours at the temperature of 75 ℃ to obtain gelatinized corn starch, and drying the gelatinized corn starch at the temperature of 105 ℃ for 8 hours in an oven to obtain NFC enhanced TPS;

d: mixing the NFC enhanced TPS obtained in the step c with 100 parts of PBAT, 0.5 part of lubricant and 0.5 part of stabilizer in a double-screw extruder for 10min at the temperature of 140 ℃ and the screw rotating speed of 70rpm to obtain a TPS/PBAT/NFC mixture;

e: and d, drying the mixture obtained in the step in a vacuum drying oven at 80 ℃ for 1h, and pressing 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 0.7MPa, the pre-pressing time is 180s, the pressing pressure is 3.5MPa, and the pressing time is 180s. The thickness of the composite film was about 0.25mm.

The preparation methods of the TPS/PBAT/NFC composite films of examples 2 to 5 are basically the same as example 1, except that in the preparation step a of examples 2 to 5, the mass fractions of the esterified modified NFC are 1 part, 4 parts, 7 parts, and 10 parts, respectively, to prepare the TPS/PBAT/NFC composite films of examples 2 to 5.

For the TPS/PBAT/NFC composite films of examples 1-5, a tensile test sample was processed with a cutter. The mechanical properties of TPS/PBAT/NFC composite films of examples 1-5 were tested in groups according to the national standard GB/T1040.1-2018. The tensile test sample was a dumbbell-shaped specimen having a length of 50mm, a gauge length of 30mm, a middle width of 4mm, and a thickness of 0.24mm. The tensile speed was 20mm/min, and at least 5 samples were tested for each group, and the average values of tensile strength, tensile modulus, and tensile elongation at break were sampled, and the results are shown in FIG. 1.

As can be seen from fig. 1, after the esterified modified NFC is added, the tensile strength and the elastic modulus of the TPS/PBAT/NFC composite film are in an ascending trend, and the esterified modified NFC content reaches the maximum value when 7 parts. The tensile strength of C4 is increased from 3.9MPa to 6.2MPa by 59% compared with the case that the NFC addition amount is 0 part (C1); the elastic modulus increased from 93.95MPa to 263MPa by 180%. These results indicate that the esterified modified NFC has excellent reinforcing effect on the PBAT/TPS composite film. Its enhanced mechanism can be attributed to the network-like rigid NFC forming a large number of physical interlocks in the matrix. In addition, the esterified modified NFC has amphipathy between a starch phase and a PBAT phase, can serve as a bridging agent, and promotes the transfer of internal stress and strain when the film is stressed. However, when the NFC addition amount reaches 10 parts, the agglomerated NFC particles may generate stress concentration, resulting in significant decrease in tensile strength, elastic modulus and elongation at break.

For the TPS/PBAT/NFC composite films of examples 1-5, after quenching with liquid nitrogen, the observations were amplified 5000-fold at 6 kV. Fig. 2 shows a scanning electron microscope image of the fracture surface of the PBAT/TPS/NFC composite film. The fracture surface (C1) of the film without NFC added appears to be neither flat nor rough, presenting many particles and grooves. This roughness indicates a lack of compatibility between the hydrophilic TPS and the hydrophobic PBAT interfaces. Therefore, the weak bonding forces between the TPS and PBAT phases prevent efficient transmission of stresses and strains, which adversely affect the mechanical properties of the film. In C2, after adding 1 part of esterified modified NFC, particles and cracks are slightly reduced, and a phase interface starts to become blurred. This means that the esterification modified NFC has improved compatibility with PBAT/TPS composites. Meanwhile, the esterification modification treatment enhances the affinity of NFC with PBAT and TPS, so that NFC becomes a bridge between two phases. However, when the NFC content reaches 10 parts, the agglomeration phenomenon of NFC starts to occur. The enhancement of the interfacial force between NFC/TPS and TPS/PBAT effectively improved the tensile properties of the film, which is consistent with the improvement in mechanical properties observed in the tensile test.

XRD testing was performed using an X-ray diffractometer for TPS/PBAT/NFC composite films of examples 1-5, with samples scanned at room temperature at a rate of 6/min over a region of 2 theta of 5-60 deg.. And the grain size of the sample was calculated using the scherrer formula, and the XRD test results are shown in fig. 3. As can be seen from the figure, the 4 crystal diffraction peaks of the film sample, which appear at 17.07 °, 20.34 °, 22.64 ° and 24.43 ° in 2θ, correspond to the 011, 101, 100 and 111 crystal planes of PBAT, respectively. The crystal diffraction peak of TPS/PBAT/NFC composite film is narrow and sharp under 0 part of NFC content, especially on the 101 crystal face. With the addition of esterification modified NFC, the intensity of the diffraction peak decreased and became wider, indicating that NFC impeded the movement of grain boundaries, slowing down the crystallization behavior in the PBAT/TPS matrix. However, no significant crystalline diffraction peaks (2θ=15.5 °, 17.1 °, 18.0 °, 23.0 °) of the starch were observed in the XRD test. This is attributable to the overlap of the diffraction peaks of the starch with those of the PBAT which are strong and similarly located. This phenomenon may be due to three reasons: 1) the gelatinization process increases the amorphous area of the starch, 2) the addition of the esterification modified NFC forms a new hydrogen bond with the starch, so that the starch is difficult to recrystallize due to the interference of intermolecular and intramolecular hydrogen bonds, 3) the ball milling process mechanically breaks the original crystal lattice of the starch, and the crystallinity of the starch is further reduced. 1 part of NFC was added and the crystallinity of the film was reduced from 54.8% to 34.5%. This may be due to the esterification modified NFC hindering ordered movement of the PBAT molecular chains, resulting in a reduced crystallinity. The reduced crystallinity effectively prevents retrogradation of the starch, which is beneficial for the lifetime of the starch-based product. Fig. 3 presents the crystallinity and grain size of the PBAT/TPS/NFC composite film at a 2θ angle of 20.34 °. Through the gelatinization process of the starch and the addition of NFC, the crystallization behavior of the starch is significantly reduced. The crystal growth process involves a gradual displacement of grain boundaries as the grains phagocytose each other, and the dispersed rigid esterified modified NFC phase impedes the displacement of grain boundaries, thereby limiting crystal growth in the thin film. This may be the reason for the reduced grain size after adding NFC. The relatively smaller crystalline material, when subjected to external forces, results in more uniform plastic deformation, reduced stress concentrations, and improved tensile strength, consistent with the results of tensile testing.

The water absorption performance of TPS/PBAT/NFC composite films of examples 1 to 5 was tested in groups according to the national standard GB/T1034-70 method. The water vapor permeability of TPS/PBAT/NFC composite films of examples 1 to 5 is detected in groups according to the national standard GB/T1037-2008 method; the ultraviolet blocking performance of TPS/PBAT/NFC composite films of examples 1 to 5 was tested in groups according to the national standard GB/T1038.1-2022 method. The water absorption test samples were square samples of 1cm×1cm, and the water absorption was measured after standing at room temperature for 72 hours under 50% relative humidity, at least 5 samples were tested in each group and averaged. The water vapor transmission performance test sample was a circular sample having a diameter of 5cm, 30g of deionized water was added to a 50mL beaker, the sample was covered on the beaker port, the water vapor transmission rate was measured every 12 hours, and 6 times was repeated, and at least 5 samples were tested for each group and averaged. The film was tested for ultraviolet absorption using an ultraviolet-visible spectrophotometer with a wavelength set to 200-400nm and a transmission spectrum obtained using air as a reference.

Fig. 4 shows the water absorption and water vapor transmission rates of examples 1 to 5. The water vapor transmission rate of the PBAT/TPS/NFC film was slightly higher than that of the film without the esterification modified NFC. This may be due to the formation of hydrogen bonds between NFC and starch, reducing the crystallinity of starch, thereby promoting the diffusion of moisture within the film. Furthermore, XRD test results show that esterification modified NFC impedes crystallization behavior of TPS, resulting in a reduction of crystallization area. Amorphous regions allow more readily the passage of water vapor molecules than crystalline regions, thereby increasing the water vapor transmission rate of the film. In addition, the water absorption value of the PBAT/TPS/NFC composite film also has a similar trend, which can be explained by the reduction of TPS crystallization area.

Fig. 5 shows the ultraviolet transmittance spectra of examples 1 to 5, and the ultraviolet transmittance of the films showed a general decrease trend after NFC addition. Especially at a wavelength of about 300nm, the transmittance is reduced from 77% with an esterification modified NFC addition of 0 parts to 46% with 10 parts, by 40.26%. This decrease in transmittance is likely to be an absorption of UV energy, involving a transition of the free electrons of oxygen to the LUMO orbitals of the c=o bonds, indicating that NFC has relatively good UV isolation properties. Another reason for the reduced uv transmittance is the nano-size of NFC, which ranges in size from 5nm to 100nm, much smaller than the wavelength of uv light (200-400 nm). Therefore, when the ultraviolet light interacts with the NFC, the ultraviolet light undergoes multiple reflections and refractions, preventing the ultraviolet light from penetrating the material, and producing an ultraviolet isolation effect.

The mass parts and compositions of TPS/PBAT/NFC composite films of examples 6 to 7 are shown in Table 2, and example 6 is directed to

The TPS/PBAT/NFC composite films of example 7 are denoted by the numbers S1 and S2, respectively, and a single factor experiment was performed.

TABLE 2

Examples 6 to 7: the starch is wheat starch, and the lubricant is oleamide and erucamide 1:1, and antioxidant 1076 is used as stabilizer.

The preparation method of the TPS/PBAT/NFC composite film of the embodiment 6 comprises the following steps:

1. preparation of esterification modified NFC

1) Mixing 8 parts of corn stalk powder with 70 parts of 20wt% sodium hydroxide solution, heating at 85 ℃ for 7h under stirring, washing with pure water to neutrality, adding 60 parts of mixed solution of 15wt% sodium chlorite solution and 3 parts of glacial acetic acid, heating at 80 ℃ under stirring for 2h, and washing with pure water to neutrality;

2) Placing all solute samples obtained in the step 1) in a beaker, adding 55 parts of 40wt% oxalic acid solution, stirring and heating for 0.5h at 70 ℃, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 1h by using a homogenizer with the rotating speed of 14000rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

Preparation of TPS/PBAT/NFC composite film

a: adding 1 part of esterified modified NFC into 35 parts of deionized water, and treating in an ultrasonic crusher for 10min, wherein the crusher power is 300W, so as to obtain a pre-dispersed NFC aqueous solution;

b: mixing the pre-dispersed NFC aqueous solution in the step a with 100 parts of starch and 15 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 60 ℃, the ball milling rotating speed is 300rpm, and the ball milling time is 0h, so that NFC highly dispersed starch slurry is obtained;

c: gelatinizing the starch slurry obtained in the step b in a water bath kettle for 1.5 hours at the temperature of 65 ℃ to obtain gelatinized wheat starch, and drying the gelatinized wheat starch in an oven at the temperature of 105 ℃ for 8 hours to obtain NFC enhanced TPS;

d: mixing the NFC enhanced TPS obtained in the step c with 100 parts of PBAT, 0.6 part of lubricant and 0.8 part of stabilizer in a double-screw extruder for 10min at 120 ℃ and with the screw speed of 50rpm to obtain a TPS/PBAT/NFC mixture;

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

The preparation method of the TPS/PBAT/NFC composite film of example 7 is basically the same as that of example 6, except that in the preparation step b of example 7, the ball milling treatment time is 1h.

For the TPS/PBAT/NFC composite films of examples 6-7, tensile test samples were processed with a cutter. And carrying out grouping detection on the mechanical properties of the TPS/PBAT/NFC composite films of the embodiments 6-7 according to the method of national standard GB/T1040.1-2018. The tensile test sample was a dumbbell-shaped specimen having a length of 50mm, a gauge length of 30mm, a middle width of 4mm, and a thickness of 0.24mm. The tensile speed was 20mm/min, and at least 5 samples were tested for each group, and the average values of tensile strength, tensile modulus, and tensile elongation at break were sampled, and the results are shown in FIG. 6.

S2 showed better mechanical properties after ball milling for 1h than sample S1 without ball milling. Specifically, the tensile strength is increased from 3.3MPa to 4.3MPa, by 30.3%, the elastic modulus is increased from 128.85MPa to 201.71MPa, and by 56.6%. This improvement can be attributed to the enhanced dispersion of NFC in TPS matrix and the ball milling process causing the formation of new hydrogen bonds. Subsequently, the highly dispersed NFC creates a more efficient bridging effect between TPS and PBAT phases, thereby improving film performance. In contrast, non-ball-milled PBAT/TPS/NFC films lack dispersion of NFC, which explains the lower mechanical strength of S1 compared to S2.

For the TPS/PBAT/NFC composite films of examples 6-7, after quenching with liquid nitrogen, a 5000-fold magnification was observed at 6 kV. Fig. 7 shows a scanning electron microscope image of the fracture surface of the PBAT/TPS/NFC composite film. The PBAT/TPS/NFC composite film of fig. 7a shows a heterogeneous phase structure, indicating that NFC is not uniformly distributed in the matrix in the non-ball milled sample, which results in limited improvement of two-phase compatibility. After ball milling, as shown in fig. 7b, the presence of particles and grooves is slightly reduced due to the uniform dispersion of NFC in the matrix, and the phase interface starts to become blurred, which means that the compatibility effect of NFC on PBAT/TPS composite material is improved.

XRD testing was performed using an X-ray diffractometer for TPS/PBAT/NFC composite films of examples 6-7, with samples scanned at room temperature at a rate of 6/min over a region of 2 theta of 5-60 deg.. And the grain size of the sample was calculated using the scherrer formula, and the XRD test results are shown in fig. 8. After ball milling for 1h, the crystallinity of the composite material is subject to the action of mechanical force in the ball milling process, so that the abrasion and fracture of a crystallization area can be generated. This results in the destruction of the starch crystal structure in the composite film, and the reduction of the crystal grain size, which leads to a decrease in crystallinity.

The water absorption performance of TPS/PBAT/NFC composite films of examples 6 to 7 was tested in groups according to the national standard GB/T1034-70 method. Performing grouping detection on the water vapor permeability of the TPS/PBAT/NFC composite films of the embodiments 6 to 7 according to the national standard GB/T1037-2008 method; the ultraviolet blocking performance of TPS/PBAT/NFC composite films of examples 6 to 7 was tested in groups according to the national standard GB/T1038.1-2022 method. The water absorption test samples were square samples of 1cm×1cm, and the water absorption was measured after standing at room temperature for 72 hours under 50% relative humidity, at least 5 samples were tested in each group and averaged. The water vapor transmission performance test sample was a circular sample having a diameter of 5cm, 30g of deionized water was added to a 50mL beaker, the sample was covered on the beaker port, the water vapor transmission rate was measured every 12 hours, and 6 times was repeated, and at least 5 samples were tested for each group and averaged. The film was tested for ultraviolet absorption using an ultraviolet-visible spectrophotometer with a wavelength set to 200-400nm and a transmission spectrum obtained using air as a reference.

Fig. 9 shows the water absorption and water vapor transmission rates of examples 6 to 7. As can be seen from the figure, the ball milling results in an increase in the water absorption and water vapor transmission rate of the TPS/PBAT/NFC composite film, which may be due to the collision, friction and shearing of starch particles due to mechanical forces during the ball milling, resulting in the destruction, fracture or reorganization of the polymer chain structure in the starch-based composite film. These structural changes increase the porosity or defects of the material, making it easier for moisture to penetrate into the film interior, resulting in increased water absorption and water vapor transmission rates. Furthermore, as can be seen from fig. 10, the ball-milled samples had a better uv blocking effect. This may be that the ball milling process reduces the material particle size, while resulting in a more uniform distribution of NFC in the film. The uniformly distributed particles further scatter light during the light propagation process, and the reflection and absorption of the material on ultraviolet rays are increased, so that the transmittance of the ultraviolet rays is reduced.

The mass part array compositions of the TPS/PBAT/NFC composite films of examples 8 to 12 are shown in Table 3, and the TPS/PBAT/NFC composite films of examples 8 to 12 are respectively denoted by the numbers K1, K2, K3, K4 and K5, and a single factor experiment was performed.

TABLE 3 Table 3

In examples 8 to 12: the starch is pea starch, and the lubricant is ethylene bis-stearamide, oleamide and erucamide 1:2:1, and antioxidant 1076 is used as stabilizer.

The preparation method of the TPS/PBAT/NFC composite film of the embodiment 8 comprises the following steps:

1. preparation of esterification modified NFC

1) Mixing 5 parts of corn stalk powder with 75 parts of 18wt% sodium hydroxide solution, heating and stirring for 7 hours at 85 ℃, washing to be neutral by pure water, adding 60 parts of mixed solution of 13wt% sodium chlorite solution and 3 parts of glacial acetic acid, stirring and heating for 2 hours at 80 ℃, and then washing to be neutral by pure water;

2) Placing all solute samples obtained in the step 1) in a beaker, adding 60 parts of 40wt% oxalic acid solution, stirring and heating for 0.5h at 70 ℃, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 1h by using a homogenizer with the rotating speed of 15000rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

Preparation of TPS/PBAT/NFC composite film

a: adding 7 parts of esterified modified NFC into 30 parts of deionized water, and treating in an ultrasonic crusher for 10min, wherein the crusher power is 300W, so as to obtain a pre-dispersed NFC aqueous solution;

b: mixing the pre-dispersed NFC aqueous solution in the step a with 100 parts of starch and 20 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 60 ℃, the ball milling rotating speed is 100rpm, and the ball milling time is 1h, so that NFC highly dispersed starch slurry is obtained;

c: gelatinizing the starch slurry obtained in the step b in a water bath kettle for 1.5 hours at the temperature of 65 ℃ to obtain gelatinized pea starch, and drying the gelatinized pea starch in an oven at the temperature of 105 ℃ for 8 hours to obtain NFC enhanced TPS;

d: mixing the NFC enhanced TPS obtained in the step c with 125 parts of PBAT, 0.7 part of lubricant and 0.5 part of stabilizer in a double-screw extruder for 10min at 120 ℃ and a screw rotating speed of 50rpm to obtain a TPS/PBAT/NFC mixture;

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

The preparation method of TPS/PBAT/NFC composite films of examples 9-12 is basically the same as example 8, except that in the preparation step b of examples 9-12, the ball milling rotational speeds are 300, 500, 700 and 900rpm, respectively.

For the TPS/PBAT/NFC composite films of examples 8-12, a tensile test sample was processed with a cutter. The mechanical properties of TPS/PBAT/NFC composite films of examples 8-12 were tested in groups according to the national standard GB/T1040.1-2018. The tensile test sample was a dumbbell-shaped specimen having a length of 50mm, a gauge length of 30mm, a middle width of 4mm, and a thickness of 0.24mm. The tensile speed was 20mm/min, and at least 5 specimens were tested for each group, and the average values of tensile strength, tensile modulus, and tensile elongation at break were sampled, and the results are shown in FIG. 11.

As can be seen from fig. 11, the ball milling speed improved the tensile strength and elastic modulus of the TPS/PBAT/NFC composite film, and the rotational speed (K3) reached the maximum value at 500 rpm. When the rotating speed is 500rpm, the tensile strength of the composite film reaches 7.81MPa, and is improved by 55.89% compared with 5.01MPa of 100rpm (K1); the elastic modulus is improved from 200.25MPa to 337.49MPa, and 68.53 percent is improved. These results demonstrate that the ball milling speed has a significant improvement effect on the mechanical properties of the TPS/PBAT/NFC composite film, probably because as the ball milling speed increases, the material particles are subjected to greater mechanical and frictional forces, causing the particles to become progressively finer. The fine particles have larger surface area, can increase the contact area of the material and other components, and improve the interface bonding strength. In addition, the larger mechanical force can more effectively improve the dispersion uniformity of the particles in the starch-based composite material, the uniform dispersion of the particles is beneficial to improving the mechanical property of the material, the internal defects and the porosity of the material are reduced, and the compactness of the material are improved. However, when the ball milling rotation speed reaches 700rpm, the mechanical properties of the TPS/PBAT/NFC composite film are reduced, which may be caused by the fact that the high-speed ball milling causes intense collision and friction between particles, so that the surfaces of the particles are excessively abraded and even peeled off, and the particles are deformed or even broken.

The mass part array compositions of the TPS/PBAT/NFC composite films of examples 13 to 17 are shown in table 4, and the TPS/PBAT/NFC composite films of examples 13 to 17 are denoted by the numbers F1, F2, F3, F4, and F5, respectively, and a single factor experiment was performed.

TABLE 4 Table 4

In examples 13 to 17: the starch adopts tapioca starch, and the lubricant adopts ethylene bisstearamide, oleamide and erucamide 2:1:1, and an antioxidant 1010 is used as a stabilizer.

The preparation method of the TPS/PBAT/NFC composite film of the embodiment 13 comprises the following steps:

1. preparation of esterification modified NFC

1) Mixing 6 parts of corn stalk powder with 70 parts of 20wt% sodium hydroxide solution, heating and stirring for 7 hours at 85 ℃, washing to be neutral by pure water, adding 60 parts of mixed solution of 15wt% sodium chlorite solution and 3 parts of glacial acetic acid, stirring and heating for 2 hours at 80 ℃, and then washing to be neutral by pure water;

2) Placing all solute samples obtained in the step 1) in a beaker, adding 60 parts of 40wt% oxalic acid solution, stirring and heating for 0.5h at 70 ℃, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 1h by using a homogenizer with the rotating speed of 13000rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

Preparation of TPS/PBAT/NFC composite film

b: mixing the pre-dispersed NFC aqueous solution in the step a with 100 parts of starch and 10 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 60 ℃, the ball milling rotating speed is 500rpm, and the ball milling time is 1h, so that NFC highly dispersed starch slurry is obtained;

c: gelatinizing the starch slurry obtained in the step b in a water bath kettle for 1.5 hours at the temperature of 65 ℃ to obtain gelatinized tapioca starch, and drying the gelatinized tapioca starch at the temperature of 105 ℃ for 8 hours in an oven to obtain NFC enhanced TPS;

d: mixing the NFC enhanced TPS obtained in the step c with 150 parts of PBAT, 0.8 part of lubricant and 1 part of stabilizer in a double-screw extruder for 10min at 120 ℃ and with the screw speed of 80rpm to obtain a TPS/PBAT/NFC mixture;

e: and d, drying the mixture obtained in the step in a vacuum drying oven, and pressing the dried material into the TPS/PBAT/NFC degradable composite film in a flat vulcanizing machine, wherein the pre-pressing pressure of the flat vulcanizing machine is 0.6MPa, the pre-pressing time is 180s, the pressing pressure is 3.5MPa, and the pressing time is 180s. The thickness of the composite film was about 0.35mm.

The preparation methods of TPS/PBAT/NFC composite films of examples 14 to 17 are basically the same as example 14, except that in the preparation steps b of examples 14 to 17, the glycerol addition amounts are 15, 20, 25 and 30 parts, respectively.

For the TPS/PBAT/NFC composite films of examples 13-17, a tensile test sample was processed with a cutter. The mechanical properties of TPS/PBAT/NFC composite films of examples 13-17 were tested in groups according to the national standard GB/T1040.1-2018. The tensile test sample was a dumbbell-shaped specimen having a length of 50mm, a gauge length of 30mm, a middle width of 4mm, and a thickness of 0.24mm. The tensile speed was 20mm/min, and at least 5 specimens were tested for each group, and the average values of tensile strength, tensile modulus, and tensile elongation at break were sampled, and the results are shown in FIG. 12.

As can be seen from fig. 12, the increase in the glycerol content significantly improved the mechanical properties of the TPS/PBAT/NFC composite film, and the tensile strength of the film reached a maximum at a glycerol content of 20 parts. When the glycerol content is 20 parts (F3), the tensile strength of the TPS/PBAT/NFC composite film reaches 7.80MPa, and compared with the glycerol content of 10 parts (F1), the TPS/PBAT/NFC composite film is improved from 6.36MPa to 7.80MPa, so that the TPS/PBAT/NFC composite film is improved by 22.64%; the elastic modulus is improved from 232.97MPa to 337.52MPa, and 44.87 percent is improved. The improvement of the mechanical properties benefits from the plasticizing effect of glycerol on starch. The increase of the glycerol content leads the original crystal structure of the starch to be largely destroyed, leads the molecular structure to be more disordered, and finally realizes the transformation from crystalline state to amorphous state. The intermolecular and intramolecular hydrogen bonding of starch is further broken, thereby achieving better mechanical properties. However, too much plasticizer may impair the mechanical properties of the composite, which may be that the plasticizer has limited solubility in the matrix, and when the plasticizer is used in an amount exceeding its solubility, the plasticizer may precipitate or separate out, forming aggregates or voids. This can lead to increased voids within the material and uneven structure, thereby reducing the compactness and mechanical properties of the material. In addition, excessive amounts of plasticizer may cause the material to become too soft to effectively withstand external loads or form stable structures. This reduces the strength and stiffness of the material and reduces the mechanical properties. The elongation at break of the composite material always keeps the rising trend along with the rising of the glycerol content, and the softening effect of the plasticizer on the material and the molecular chain structure is also caused.

Claims

1. The TPS/PBAT/NFC composite film is characterized by comprising the following components in parts by mass: 100 parts of starch, 100-150 parts of PBAT, 1-10 parts of esterification modified NFC, 15-30 parts of glycerol, 30-40 parts of deionized water, 0.5-1 part of lubricant and 0.5-1 part of stabilizer; the lubricant is one or the combination of more than two of stearic acid, ethylene bisstearamide, oleamide and erucamide; the stabilizer is one or the combination of two of antioxidant 1010 and antioxidant 1076; the preparation method of the TPS/PBAT/NFC composite film comprises the following steps:

1) Mixing 5-10 parts of corn stalk powder with 70-80 parts of 10-20 wt% sodium hydroxide solution, heating and stirring for 4-7 h at 85-95 ℃, washing to neutrality by pure water, adding 60 parts of mixed solution of 10-15 wt% sodium chlorite solution and 3-4 parts of glacial acetic acid, stirring and heating for 1-2 h at 70-80 ℃, and then washing to neutrality by pure water;

2) Placing all solute samples obtained in the step 1) into a beaker, adding 50-60 parts of 30-40 wt% oxalic acid solution, stirring and heating for 0.5-1 h at 60-70 ℃, and carrying out high-speed shearing treatment on the cooled solid-liquid mixture for 1-2 h by using a homogenizer with the rotating speed of 10000-16000 rpm; and centrifugally washing the homogenized sample to be neutral, and then freeze-drying to obtain the esterified modified NFC.

3) Adding 1-10 parts of the esterified modified NFC obtained in the step 2) into 30-40 parts of deionized water, and treating for 10-15 min in an ultrasonic crusher with the power of 300-350W to obtain a pre-dispersed NFC aqueous solution;

4) Mixing the NFC aqueous solution obtained in the step 3) with 100 parts of starch and 15-20 parts of glycerol, adding the mixture into a planetary ball mill for dry ball milling, wherein a grinding medium adopts zirconia or ceramic balls, the temperature is 50-80 ℃, the ball milling rotating speed is 100-900 rpm, and the ball milling time is 0-1 h, so that NFC highly dispersed starch slurry is obtained;

5) Gelatinizing the starch slurry obtained in the step 4) in a water bath kettle for 0.5-1 h at the temperature of 65-75 ℃ to obtain gelatinized starch, and drying the gelatinized starch in an oven at the temperature of 105 ℃ for 8h to obtain NFC enhanced TPS;

6) Mixing the NFC enhanced TPS obtained in the step 5) with 100-150 parts of PBAT, 0.5-4 parts of lubricant and 0.5-4 parts of stabilizer in a double-screw extruder for 2-5min at the temperature of 100-140 ℃ and the screw rotating speed of 50-100 rpm to obtain a TPS/PBAT/NFC mixture;

7) Drying the mixture obtained in the step 6) in a vacuum drying oven at 80 ℃ for 2-5 hours, and processing the dried material into the TPS/PBAT/NFC composite film by using a flat vulcanizing machine or a film blowing machine.

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

3. The TPS/PBAT/NFC composite film as claimed in claim 1, wherein the mesh number of the corn stalk powder is 100-150 mesh.

4. The TPS/PBAT/NFC composite film according to claim 1, wherein the lubricant is one or a combination of two or more of stearic acid, ethylene bisstearamide, oleamide, erucamide. The stabilizer is one or more of antioxidant 1010, antioxidant 168, antioxidant 1076, and antioxidant 1790.

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

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