Refining method of bamboo fiber ultra-nano micro-powder and natural polymer blending material

Abstract

The present application relates to the technical field of high polymer composite material, and discloses a refining treatment method of bamboo fiber ultra-nano micro-powder and natural polymer blending material, which comprises the following steps: (a) mixing bamboo fiber ultra-nano micro-powder with a eutectic solvent to obtain a premix and realize in-situ activation; (b) adding the premix and a natural polymer matrix from a main feeding port of a double-screw extruder; (c) adding a dynamic interface locking agent through a side feeding port arranged downstream of the main feeding port during the melt blending process, and then extruding and granulating. The present application adopts a staged dynamic compatibilization process, uses a eutectic solvent to destroy the hydrogen bond network of bamboo fiber to promote its shear dispersion in the front section, and then builds a stable interface combination in the rear section through a dynamic interface locking agent after the dispersion is completed. The method cooperatively solves the conflict between dispersion and interface compatibilization, so that the composite material obtains good comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer composite materials, and particularly relates to a refining treatment method of bamboo fiber ultra-nano micro powder and natural polymer blending material. BACKGROUND

[0002] Natural polymer materials represented by polylactic acid (PLA) and thermoplastic starch (TPS) have been widely concerned due to their renewable and biodegradable properties. However, these materials generally have the problems of insufficient mechanical properties (such as impact toughness and strength) and heat resistance, which limit their application range.

[0003] In order to improve the performance, the method of adding reinforcing fillers is usually used. Bamboo fiber ultra-nano micro powder (BNF) as a kind of biomass nano filler has the advantages of high specific surface area, high modulus and low density, and is an ideal reinforcing material for natural polymer matrix.

[0004] However, there are serious technical challenges in preparing BNF reinforced composites. BNF is derived from cellulose, and its surface is rich in high-density hydroxyl groups, which has strong polarity and hydrophilicity. This leads to two main problems: first, BNF has strong hydrogen bonding between molecules, which makes it prone to agglomeration during storage and processing, and it is difficult to achieve uniform dispersion at the nanoscale in the hydrophobic natural polymer matrix (such as PLA); second, the hydrophilic surface of BNF has poor interfacial compatibility with the hydrophobic matrix, resulting in weak interfacial bonding between them.

[0005] The coexistence of these two problems (agglomeration and weak interface) leads to the fact that the stress between the filler and the matrix cannot be effectively transmitted, and the nanoreinforcement potential of BNF cannot be fully realized, and the macroscopic performance (especially the mechanical properties and heat resistance) of the composite material is limited.

[0006] In order to improve the interfacial bonding, the existing technology often adds interfacial compatibilizers such as maleic anhydride grafted polymers or silane coupling agents during melt blending. However, in the traditional “one-step” blending process, if these compatibilizers come into contact with BNF at the beginning of blending, they will often react with the surface of BNF agglomerates or form a coating. This premature interfacial reaction will solidify the agglomerates, which will hinder the subsequent melt shear force from dispersing the agglomerates, resulting in poor dispersion of the fillers.

[0007] If another approach is used, small molecule additives such as plasticizers or lubricants are used to assist the dispersion of BNF. Although this may improve the distribution of BNF in the melt to some extent, these additives often remain at the interface, forming a weak interfacial layer, which weakens the rigid connection between the filler and the matrix, is not conducive to the transmission of interfacial stress, and ultimately leads to a decrease in the strength and thermal stability of the material.

[0008] Therefore, the existing melt blending technology is difficult to solve the problems of BNF de-agglomeration-dispersion and strong interfacial bonding in a single process step. SUMMARY

[0009] The technical problem solved by the present application is that the bamboo fiber ultra-nano powder has strong internal hydrogen bonding due to its high specific surface area and high density of hydroxyl groups on the surface, which makes it difficult to achieve uniform dispersion at the nanoscale in a hydrophobic natural polymer matrix such as polylactic acid, and there is a problem of poor interfacial compatibility between the matrix.

[0010] The present application aims to provide a refining method for bamboo fiber ultra-nano powder and natural polymer blend material, which has a simple process flow and can achieve efficient dispersion and strong interfacial locking of bamboo fiber ultra-nano powder in a natural polymer matrix.

[0011] The bamboo fiber ultra-nano powder and natural polymer blend material refining method comprises the following steps:

[0012] (a) Pre-mixing bamboo fiber ultra-nano powder and deep eutectic solvent in a mixing device to obtain bamboo fiber ultra-nano powder pre-mix;

[0013] (b) Adding the bamboo fiber ultra-nano powder pre-mix and natural polymer matrix through the main feed port of a twin-screw extruder;

[0014] (c) Melting and blending in the extruder, and adding a dynamic interfacial locking agent through the side feed port downstream of the main feed port during the melting and blending process, then extruding and granulating the melt blend to obtain the blend material.

[0015] By adopting the above technical solution, the core of the present application is to utilize the process design of staged dynamic compatibilization to realize the ordered process of activation-dispersion-locking of bamboo fiber ultra-nano powder (BNF) in different zones of a twin-screw extruder. This design of segmented and synergistic action of multifunctional components in the same processing process is a breakthrough over the prior art. The specific mechanism is as follows:

[0016] Step (a): In-situ interfacial activation and de-agglomeration. The present application uses deep eutectic solvent (DES) as an interfacial activator, rather than a traditional solvent. DES molecules (such as choline chloride and urea) penetrate into the interior of BNF agglomerates in the pre-mixing stage, form transient and reversible hydrogen bonds with the hydroxyl groups on the surface of BNF through their high hydrogen bond breaking ability, which breaks and loosens the original strong hydrogen bond network in BNF, reduces the aggregation energy of BNF, and makes the agglomerates be preliminarily activated.

[0017] Step (b): construction of dynamic flexible interface and shear dispersion. BNF premix carrying DES and natural polymer matrix are fed into the extruder from the main feeding port. Under the action of high temperature melting and strong shear in the front section of the extruder (i.e. before the side feeding port), the presence of DES establishes a dynamic flexible interface, and the activated BNF agglomerates are easily broken up by mechanical shear force and uniformly dispersed in the molten matrix. At this stage, DES also helps to reduce the melt viscosity of the system and promote dispersion.

[0018] Step (c): dynamic interface locking and structure solidification. When BNF has been well dispersed (i.e. in the middle and rear sections of the extruder), a dynamic interface locking agent (such as citric acid) is added through the side feeding port. The introduction of this locking agent (usually an organic small molecule with multiple functional groups such as carboxyl and hydroxyl groups) achieves fine interface engineering. The locking agent can react (such as esterification) with the large number of hydroxyl groups exposed on the surface of BNF and the end groups or ester groups of the natural polymer matrix (such as polylactic acid) in the molten state or form multiple hydrogen bonds. This builds a stable semi-rigid interface network between the dispersed BNF and the matrix, achieves interface locking, and effectively improves the stress transfer efficiency.

[0019] The present application achieves the synergistic optimization of dispersibility and binding force by completing the dispersion (using DES and front section shear) and interface construction (using the locking agent added in the rear section) in different sections of the extruder. This design avoids the solidification of BNF by the locking agent before dispersion, and solves the problem of weak interface binding caused by the use of DES alone, ultimately obtaining a blended material with high dispersibility and strong interface binding.

[0020] Preferably, the natural polymer matrix is at least one of polylactic acid or thermoplastic starch.

[0021] By adopting the above technical solution, the main natural polymer matrix applicable to the present application is limited, and polylactic acid and thermoplastic starch are the most widely used materials in this field.

[0022] Preferably, the eutectic solvent is composed of a hydrogen bond donor and a hydrogen bond acceptor.

[0023] By adopting the above technical solution, the chemical nature of the eutectic solvent is clarified, providing a theoretical basis for subsequent specific selection.

[0024] Preferably, the hydrogen bond acceptor is choline chloride; the hydrogen bond donor is urea or glycerol; and the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:(1.5-2.5).

[0025] By adopting the technical scheme, two kinds of eutectic solvent combinations (choline chloride / urea and choline chloride / glycerol) with high efficiency, low cost and environmental friendliness are provided, and the molar ratio of 1:2 is the preferred ratio to achieve the lowest eutectic point, which ensures the liquid activity at the process temperature.

[0026] Preferably, the dynamic interfacial locking agent is an organic small molecule containing at least two functional groups selected from carboxyl or hydroxyl.

[0027] By adopting the technical scheme, the interfacial locking agent is limited from the chemical structure. The multi-functional groups, especially carboxyl and hydroxyl, are the basis for forming multi-point hydrogen bonds or esterification reactions with the BNF surface and the matrix molecules.

[0028] Preferably, the dynamic interfacial locking agent is citric acid or tartaric acid.

[0029] By adopting the technical scheme, two most preferred interfacial locking agents are provided. Both citric acid and tartaric acid are food-grade, bio-based raw materials, and contain multiple carboxyl and hydroxyl groups in the molecule, with high locking efficiency.

[0030] Preferably, in step (a), the weight ratio of the eutectic solvent to the bamboo fiber ultra-nano powder is (0.05-0.15):1.

[0031] By adopting the technical scheme, the optimal amount of the activator is limited. This range can ensure sufficient activation of the BNF surface, while avoiding excessive DES that is difficult to remove in subsequent extrusion.

[0032] Preferably, based on 100 parts by weight of the natural polymer matrix: the amount of bamboo fiber ultra-nano powder is 5-15 parts by weight; and the amount of the dynamic interfacial locking agent is 0.1-0.5 parts by weight.

[0033] By adopting the technical scheme, the ratio range of the core components is limited. This range is the preferred interval for balancing the mechanical properties (such as strength and toughness) and processing performance of the material.

[0034] Preferably, the pre-mixing in step (a) is carried out in a high-speed mixer, the mixing temperature is 70-80℃, and the mixing time is 15-25 minutes.

[0035] By adopting the technical scheme, the preferred process window of step (a) is limited. 75℃ is the common operating temperature of choline chloride / urea DES, and 20 minutes can ensure uniform pre-mixing.

[0036] Preferably, in step (c), a vacuum exhaust port is provided downstream of the side feeding port for removing the eutectic solvent and reaction by-products.

[0037] By adopting the technical scheme, the key process setting is clear. The vacuum exhaust port is crucial for removing small molecules (such as water) generated by DES and interfacial reaction, and is a prerequisite for ensuring stable final material performance and no pores.

[0038] In summary, the present application includes at least one of the following beneficial technical effects:

[0039] 1. The present application uses a eutectic solvent in step (a) to pre-activate the bamboo fiber ultra-nano micro powder, which destroys the internal hydrogen bond network; combined with the melting shear action in the front section of the extruder in step (b), the bamboo fiber ultra-nano micro powder is uniformly dispersed in the natural polymer matrix, effectively overcoming the technical problem of easy agglomeration of nano fillers in the matrix.

[0040] 2. The present application adds a dynamic interface locking agent at the side feeding port in step (c), which ensures that the interfacial reaction (or hydrogen bond construction) occurs after the bamboo fiber ultra-nano micro powder has been fully dispersed. This builds a stable interfacial bond between the filler and the matrix, improves the interfacial stress transfer efficiency, and thus improves the mechanical properties and thermal stability of the composite material.

[0041] 3. The present application realizes the two key steps of activation and dispersion and interface locking in the same twin-screw extruder through the partition function of the main and side feeding ports. This phased dynamic compatibilization process effectively solves the time sequence conflict between filler dispersion and interface compatibilization in traditional blending process, has high process integration, and realizes the control of the microstructure of the composite material in the continuous extrusion process. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 is the dynamic mechanical analysis (DMA) curve of the composite material measured according to the test example 3 of the present application; wherein (a) is the change curve of the storage modulus (E') with temperature; (b) is the change curve of the loss factor (tan δ) with temperature. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application will be further described in detail below with specific examples. It should be understood that the specific examples described herein are only used to explain the present application, and do not limit the protection scope of the present application.

[0044] The sources and specifications of the main raw materials and reagents used in the following examples and comparative examples are as follows: the reagents not specifically mentioned are commercially available analytical pure or higher grade products.

[0045] Bamboo fiber ultra-nano micro powder (BNF): self-made, the preparation method is described in detail in Preparation Example 1.

[0046] Polylactic acid (PLA): grade 4032D, number average molecular weight (Mn) of 50000-70000 g / mol, weight average molecular weight (Mw) of 100000-140000 g / mol, melt index of 6-8 g / 10 min (210℃, 2.16 kg), density of 1.24 g / cm 3 .

[0047] Corn starch: food grade, amylopectin content of 70-80 wt%.

[0048] Maleic anhydride grafted polylactic acid (PLA-g-MA): self-made, the preparation method is described in detail in Preparation Example 2.

[0049] Preparation Example 1:

[0050] The present preparation example provides a preparation method of bamboo fiber ultra-nano micro powder (BNF), comprising the following steps:

[0051] 1. Raw material pretreatment: cut the bamboo pulp board and soak it in deionized water for 24 h.

[0052] 2. Chemical purification: mix the soaked bamboo pulp with a 4wt% sodium hydroxide solution at a solid-liquid ratio of 1:20 (g / mL), and mechanically stir at 85℃ for 2 h. After the reaction is completed, filter and wash with deionized water until the filtrate is neutral.

[0053] 3. Bleaching treatment: mix the purified fiber with a bleaching solution composed of equal volumes of acetic acid and a 17wt% sodium chlorite solution at a solid-liquid ratio of 1:25 (g / mL), and react in the dark at 75℃ for 3 h. After the reaction is completed, filter and wash with deionized water until neutral.

[0054] 4. Mechanical defibration: dilute the bleached cellulose pulp to a solid content of 1.5wt%, and use a high-pressure homogenizer to treat it at a pressure of 100MPa for 20 cycles to obtain a nanocellulose water dispersion.

[0055] 5. Freeze-drying: pre-freeze the obtained water dispersion at -80℃ for 24 h, and then freeze-dry it under a vacuum degree of less than 10Pa for 60 h to obtain fluffy white bamboo fiber ultra-nano micro powder.

[0056] Preparation Example 2:

[0057] The present preparation example provides a preparation method of maleic anhydride grafted polylactic acid (PLA-g-MA), comprising the following steps:

[0058] 1. Raw material mixing: 100 parts by weight of polylactic acid (PLA) particles, 5 parts by weight of maleic anhydride (MA), and 0.5 parts by weight of dicumyl peroxide (DCP) were mixed uniformly in a high-speed mixer. Among them, the PLA particles have been vacuum dried at 80°C for 12h.

[0059] 2. Melt grafting: The above mixture was subjected to a melt grafting reaction by a co-rotating twin-screw extruder. The temperature settings of the extruder from the feeding port to the die were 160°C, 175°C, 185°C, 190°C, 190°C, 185°C. The screw rotation speed was 180 rpm.

[0060] 3. Purification treatment: After the extruded strip was cooled and cut into particles, it was dissolved in chloroform, then added dropwise into excess methanol for precipitation. The precipitate was collected by filtration and washed with methanol three times to remove unreacted MA and initiator residues.

[0061] 4. Drying: The purified product was dried in a vacuum oven at 60°C for 24h to obtain maleic anhydride grafted polylactic acid (PLA-g-MA).

[0062] Preparation Example 3:

[0063] The present preparation example provides a preparation method of γ-(2,3-epoxypropoxy) propyl trimethoxysilane modified bamboo fiber ultra-nano powder (BNF-Si), comprising the following steps:

[0064] 1. Preparation of silane hydrolysis solution: Prepare an ethanol / water mixed solution with a volume ratio of 90:10, add γ-(2,3-epoxypropoxy) propyl trimethoxysilane (KH-560) to make its mass fraction 2wt%, adjust the pH value of the solution to 4.5 with acetic acid, and magnetically stir for 1h to make it fully hydrolyzed.

[0065] 2. Grafting reaction: Add BNF prepared in Preparation Example 1 to the above hydrolysis solution to make the solid content of BNF in the solution 1wt%. Transfer the mixed system to a three-necked flask and mechanically stir at 60°C for 3h.

[0066] 3. Washing treatment: After the reaction is completed, the mixture is centrifuged to collect the solid product. Then wash it with an ethanol / water (90 / 10, v / v) mixed solution three times to remove unreacted silane coupling agent.

[0067] 4. Freeze-drying: Redisperse the washed product in a small amount of deionized water, pre-freeze at -80°C for 24h, and then freeze-dry under a vacuum degree lower than 10Pa for 60h to obtain silane-modified bamboo fiber ultra-nano powder.

[0068] Preparation Example 4:

[0069] The present preparation example provides a preparation method of thermoplastic starch (TPS), comprising the following steps:

[0070] 1. Raw material premixing: 100 parts by weight of corn starch, 30 parts by weight of glycerol and 5 parts by weight of deionized water were mixed in a high-speed mixer for 15 min to obtain a uniform premix. Among them, the corn starch has been vacuum dried at 80℃ for 6h.

[0071] 2. Melt plasticization: the above premix was melt plasticized by a co-rotating twin-screw extruder. The temperature of each zone of the extruder from the feeding port to the die was set as: 90℃, 115℃, 130℃, 135℃, 130℃, 120℃. The screw rotation speed was 150 rpm.

[0072] 3. Granulation and drying: the extruded strip was cut into particles after air cooling, and the obtained particles were dried in a vacuum oven at 70℃ for 12h to obtain thermoplastic starch (TPS) particles.

[0073] Unless otherwise specified, the composite materials in the examples and comparative examples were prepared by a twin-screw extruder. The extruder was a co-rotating twin-screw extruder (L / D = 48) equipped with a main feeding port (located in zone 1) and a side feeding port (located in zone 7). The screw rotation speed was uniformly set to 200 rpm. All the parts are parts by weight.

[0074] Example 1:

[0075] The present example provides a refining treatment method of bamboo fiber ultra-nano micro-powder and polylactic acid blended material, comprising the following steps:

[0076] 1. Pretreatment stage: in-situ activation of BNF

[0077] 10 parts of BNF powder prepared in Preparation Example 1 and 1.0 parts of liquid DES (preheated to 75℃) composed of choline chloride and urea (molar ratio 1:2) were added to a high-speed mixer. Mixed at a speed of 80 rpm for 20 min to obtain a BNF premix with a surface activated by DES.

[0078] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0079] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 parts of antioxidant 1010 and 0.5 parts of calcium stearate were preliminarily mixed uniformly in a mixing bag, and then added to the twin-screw extruder through the main feeding port.

[0080] The temperature profile of the extruder (from the feeding port to the die): 165℃, 180℃, 185℃, 185℃, 180℃, 175℃, 175℃, 170℃, 170℃, 170℃, 170℃.

[0081] When the extrusion is stable, 0.3 parts of citric acid powder is added to the 7th zone through the side feeding port.

[0082] A vacuum vent is set at the 9th zone, and the vacuum degree is maintained at -0.09 MPa.

[0083] The melt strip extruded from the die is cooled in a 20℃ water bath and then cut into particles by a granulator.

[0084] 3. Sample preparation:

[0085] The obtained particles are dried in a vacuum oven at 80℃ for 12h, and then a standard test sample strip is prepared using an injection molding machine (cylinder temperature 190℃, mold temperature 40℃, injection pressure 70MPa, holding pressure time 12s).

[0086] Example 2:

[0087] The present embodiment provides a refining treatment method of bamboo fiber ultra-nano micro-powder and polylactic acid blended material, comprising the following steps:

[0088] 1. Pretreatment stage: in-situ activation of BNF

[0089] 5 parts of BNF powder prepared in Preparation Example 1 and 0.5 parts of liquid DES (preheated to 75℃) composed of choline chloride and urea (molar ratio 1:2) are added to a high-speed mixer. Mix at a speed of 80rpm for 20min to obtain BNF premix with DES-activated surface.

[0090] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0091] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 parts of antioxidant 1010 and 0.5 parts of calcium stearate are preliminarily mixed uniformly in a mixing bag, and then added to a twin-screw extruder through the main feeding port.

[0092] Extruder temperature profile (from feeding port to die): 165℃, 180℃, 185℃, 185℃, 180℃, 175℃, 175℃, 170℃, 170℃, 170℃, 170℃.

[0093] When the extrusion is stable, 0.1 parts of citric acid powder is added to the 7th zone through the side feeding port.

[0094] A vacuum vent is set at the 9th zone, and the vacuum degree is maintained at -0.09 MPa.

[0095] The melt strip extruded from the die is cooled in a 20℃ water bath and then cut into particles by a granulator.

[0096] 3. Sample preparation:

[0097] The obtained particles were dried in a vacuum oven at 80°C for 12 h, and then used to prepare standard test bars using an injection molding machine (cylinder temperature 190°C, mold temperature 40°C, injection pressure 70 MPa, and dwell time 12 s).

[0098] Example 3:

[0099] The present example provides a refining treatment method for a bamboo fiber ultra-nano micro-powder and polylactic acid blended material, comprising the following steps:

[0100] 1. Pretreatment stage: in-situ activation of BNF

[0101] 15 parts of BNF powder prepared in Preparation Example 1 and 2.0 parts of a liquid DES composed of choline chloride and urea (molar ratio 1:2) preheated to 75°C were added to a high-speed mixer. After mixing at a speed of 80 rpm for 20 min, a BNF premix with a DES-activated surface was obtained.

[0102] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0103] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 parts of antioxidant 1010, and 0.5 parts of calcium stearate were preliminarily mixed uniformly in a mixing bag, and then fed into a twin-screw extruder through the main feeding port.

[0104] The temperature profile of the extruder (from the feeding port to the die) was 165°C, 180°C, 185°C, 185°C, 180°C, 175°C, 175°C, 170°C, 170°C, 170°C, and 170°C.

[0105] When the extrusion was stable, 0.5 parts of citric acid powder was added through the side feeding port to the 7th zone.

[0106] A vacuum exhaust port was set at the 9th zone, and the vacuum degree was maintained at -0.09 MPa.

[0107] The melt strip extruded from the die was cooled in a 20°C water bath, and then cut into particles by a granulator.

[0108] 3. Sample preparation:

[0109] The obtained particles were dried in a vacuum oven at 80°C for 12 h, and then used to prepare standard test bars using an injection molding machine (cylinder temperature 190°C, mold temperature 40°C, injection pressure 70 MPa, and dwell time 12 s).

[0110] Example 4:

[0111] The present example provides a refining treatment method for a bamboo fiber ultra-nano micro-powder and polylactic acid blended material, comprising the following steps:

[0112] 1. Pretreatment stage: in-situ activation of BNF

[0113] 10 parts of BNF powder prepared in Preparation Example 1 and 1.0 part of a liquid DES composed of choline chloride and glycerol (molar ratio 1:2) preheated to 75℃ were added into a high-speed mixer. After mixing at a rotation speed of 80 rpm for 20 min, a BNF premix with DES-activated surface was obtained.

[0114] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0115] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 part of antioxidant 1010 and 0.5 part of calcium stearate were preliminarily mixed uniformly in a mixing bag and fed into a twin-screw extruder through a main feeding port.

[0116] Extruder temperature profile (from feeding port to die): 165℃, 180℃, 185℃, 185℃, 180℃, 175℃, 175℃, 170℃, 170℃, 170℃, 170℃.

[0117] When the extrusion was stable, 0.3 parts of citric acid powder were added through a side feeding port into the 7th zone.

[0118] A vacuum vent was set at the 9th zone, and the vacuum degree was maintained at -0.09 MPa.

[0119] The melt strip extruded from the die was cooled in a 20℃ water bath and then cut into granules by a granulator.

[0120] 3. Sample preparation:

[0121] The obtained granules were dried in a vacuum oven at 80℃ for 12 h, and then standard test bars were prepared using an injection molding machine (cylinder temperature 190℃, mold temperature 40℃, injection pressure 70 MPa, and dwell time 12 s).

[0122] Example 5:

[0123] The present example provides a refining treatment method for a bamboo fiber ultra-nano micro powder and polylactic acid blended material, comprising the following steps:

[0124] 1. Pretreatment stage: in-situ activation of BNF

[0125] 10 parts of BNF powder prepared in Preparation Example 1 and 1.0 part of a liquid DES composed of choline chloride and urea (molar ratio 1:2) preheated to 75℃ were added into a high-speed mixer. After mixing at a rotation speed of 80 rpm for 20 min, a BNF premix with DES-activated surface was obtained.

[0126] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0127] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 parts of antioxidant 1010 and 0.5 parts of calcium stearate were preliminarily mixed uniformly in a mixing bag, and were fed into a twin-screw extruder through a main feeding port.

[0128] The temperature profile of the extruder (from the feeding port to the die) was 165°C, 180°C, 185°C, 185°C, 180°C, 175°C, 175°C, 170°C, 170°C, 170°C, and 170°C.

[0129] When the extrusion was stable, 0.3 parts of tartaric acid powder were added to the 7th zone through a side feeding port.

[0130] A vacuum exhaust port was arranged at the 9th zone, and the vacuum degree was maintained at -0.09 MPa.

[0131] The melt strip extruded from the die was cooled in a 20°C water bath and then cut into particles by a granulator.

[0132] 3. Sample preparation:

[0133] The obtained particles were dried in a vacuum oven at 80°C for 12 h, and then a standard test sample strip was prepared using an injection molding machine (cylinder temperature 190°C, mold temperature 40°C, injection pressure 70 MPa, and pressure holding time 12 s).

[0134] Example 6:

[0135] The present example provides a refining treatment method of bamboo fiber ultra-nano micro-powder and thermoplastic starch (TPS) blended material, comprising the following steps:

[0136] 1. Pretreatment stage: in-situ activation of BNF

[0137] 10 parts of BNF powder prepared in Preparation Example 1 and 1.0 parts of a liquid DES (preheated to 75°C) composed of choline chloride and urea (molar ratio 1:2) were added to a high-speed mixer. After mixing at a speed of 80 rpm for 20 min, a BNF premix with a surface activated by DES was obtained.

[0138] 2. Melt blending and extrusion stage: segmented dynamic compatibilization

[0139] The above BNF premix, 100 parts of polylactic acid (PLA), 0.5 parts of antioxidant 1010 and 0.5 parts of calcium stearate were preliminarily mixed uniformly in a mixing bag, and were fed into a twin-screw extruder through a main feeding port.

[0140] The temperature profile of the extruder (from the feeding port to the die) was 165°C, 180°C, 185°C, 185°C, 180°C, 175°C, 175°C, 170°C, 170°C, 170°C, and 170°C.

[0141] When the extrusion is stable, 0.3 parts of citric acid powder is added to the 7th zone through the side feeder.

[0142] A vacuum vent is set at the 9th zone, and the vacuum degree is maintained at-0.09 MPa.

[0143] The melt strip extruded from the die is cooled in a 20℃ water bath and then cut into granules by a cutter.

[0144] 3. Sample preparation:

[0145] The obtained granules are dried in a vacuum oven at 70℃ for 12h, and then a standard test sample strip is prepared using an injection molding machine (cylinder temperature 135℃, mold temperature 35℃, injection pressure 65 MPa, and dwell time 12s).

[0146] Comparative Example 1:

[0147] This comparative example provides a preparation method of pure PLA material, which is different from Example 1 in that no BNF, DES, citric acid, etc. are added, and the rest are the same.

[0148] Comparative Example 2:

[0149] This comparative example provides a preparation method of directly blending unmodified BNF with PLA, which is different from Example 1 in that no DES and citric acid are added, and the BNF is directly mixed with PLA before extrusion, and the rest are the same.

[0150] Comparative Example 3:

[0151] This comparative example provides a preparation method of blending silane-modified BNF (BNF-Si) prepared in Preparation Example 3 with PLA, which is different from Example 1 in that the BNF is BNF-Si prepared in Preparation Example 3, and no DES and citric acid are added, and the BNF-Si is directly mixed with PLA before extrusion, and the rest are the same.

[0152] Comparative Example 4:

[0153] This comparative example provides a preparation method of BNF / PLA composite material using maleic anhydride grafted polylactic acid (PLA-g-MA) prepared in Preparation Example 2 as a compatibilizer, which is different from Example 1 in that no DES and citric acid are added, but 5 parts of PLA-g-MA prepared in Preparation Example 2 are added, and the PLA-g-MA is mixed with BNF and PLA before extrusion, and the rest are the same.

[0154] Comparative Example 5:

[0155] This comparative example provides a preparation method of BNF / PLA composite material with only DES added and no interface locking agent, which is different from Example 1 in that no citric acid is added through the side feeder, and the rest are the same.

[0156] Comparative Example 6:

[0157] This comparative example provides a preparation method of BNF / PLA composite with only the addition of interfacial locking agent, without DES, which is different from Example 1 in that the in-situ activation pretreatment of BNF by DES is not performed, and no DES is added, and BNF is mixed with citric acid and PLA before extrusion, and the rest is the same.

[0158] Comparative Example 7:

[0159] This comparative example provides a preparation method of BNF / PLA composite with DES and citric acid added simultaneously at the main feed port, which is different from Example 1 in that neither DES nor citric acid is added at the pretreatment stage or side feed port, but is added from the main feed port after being mixed with BNF and PLA before extrusion, and the rest is the same.

[0160] Test Example 1:

[0161] This test example uses the fixed funnel method (referring to GB / T 1482-2010) to measure the repose angle of the powder to characterize its flowability.

[0162] The experimental steps are as follows

[0163] (1) A brass funnel with an inner diameter of 10 mm and a cone angle of 60° is fixed on a support, so that the lower opening of the funnel is 20 mm above the horizontal base plate.

[0164] (2) Take 100 g of the sample to be tested (BNF dry powder of Preparation Example 1 or BNF premix of Example 1) and pour it into the funnel at one time.

[0165] (3) The sample flows out of the lower opening of the funnel and naturally accumulates on the horizontal base plate to form a cone.

[0166] (4) After the sample completely stops flowing, use a vernier caliper to measure the average bottom diameter D of the formed cone accumulation (measure the diameters in two perpendicular directions and take the average, calculate the radius r = D / 2) and the height h of the cone.

[0167] (5) Calculate the repose angle θ by the formula tan(θ) = h / r.

[0168] (6) Repeat the measurement 5 times for each sample, and calculate the average and standard deviation.

[0169] The experimental results are shown in Table 1.

[0170] Table 1 Repose angle measurement data of BNF dry powder and BNF premix:

[0171]

[0172] According to the data in Table 1, the average angle of repose of the BNF dry powder (pure BNF) of Preparation Example 1 is 54.1°, and the data dispersion is relatively large. This indicates that the bamboo fiber ultra-nano micro powder in the dry state has strong inter-particle hydrogen bonding and van der Waals force due to its high specific surface area and rich hydroxyl groups on the surface, resulting in strong cohesion and poor flowability, and is prone to form stable agglomerates.

[0173] The average angle of repose of the BNF premix treated by DES in-situ activation in Example 1 is reduced to 38.8°.

[0174] From the mechanism analysis, the liquid deep eutectic solvent (DES) penetrates into the BNF agglomerates during high-speed mixing, and competes with the hydroxyl groups on the surface of cellulose to form hydrogen bonds as a hydrogen bond acceptor and donor. This interaction destroys the original hydrogen bond network between BNF fibers that maintains the structure of the agglomerates. The introduction of DES changes the BNF from a high-cohesion dry powder to a premix with a lubricated surface and a partially destroyed hydrogen bond network inside. This change in state is manifested macroscopically as a decrease in the angle of repose and an improvement in flowability.

[0175] Improved flowability helps to stabilize the feeding of the subsequent twin-screw extrusion process, and the pre-activation (dissociation) inside the agglomerates provides an easier-to-disperse initial state for the mechanical shear dispersion in the melt blending stage, reducing the energy barrier required for dispersion.

[0176] Test Example 2:

[0177] The experimental steps are as follows:

[0178] (1) Dry the composite particles of Comparative Example 2, Comparative Example 5 and Example 1 in a vacuum oven at 80°C for 12h to remove residual moisture.

[0179] (2) Use a melt flow rate instrument to test according to the ASTM D1238 standard.

[0180] (3) Preheat the instrument barrel to 210°C, and after cleaning the barrel, add about 6g of dried sample.

[0181] (4) After preheating the sample in the barrel for 5min, load a 2.16kg piston and weight.

[0182] (5) After the melt is extruded from the die, start timing and cut the extrudate strip at fixed time intervals (30s).

[0183] (6) Weigh the mass of the cut strip, and repeat the measurement at least 3 times.

[0184] (7) The mass of the strip was converted to the mass of extrusion in 10 min, with the unit of g / 10 min, and the average value and standard deviation were calculated.

[0185] The experimental results are shown in Table 2.

[0186] Table 2 Melt flow rate (MFI) data of the composite materials (210℃, 2.16 kg):

[0187]

[0188] According to the data in Table 2, the melt flow rate of Comparative Example 2 (unmodified BNF directly blended) is the lowest, with an average value of only 2.42 g / 10 min. This indicates that the untreated BNF has serious agglomeration in the PLA matrix, which hinders the flow of the polymer melt, resulting in high viscosity of the system.

[0189] Compared with Comparative Example 2, the melt flow rate of Comparative Example 5 (only DES added) is greatly improved to 9.01 g / 10 min. This change confirms the effective role of DES in the melt blending stage: on the one hand, DES as a low molecular weight liquid plays a plasticizing and internal lubricating role in the melt; on the other hand, the in-situ activation step improves the initial dispersion state of BNF, reducing the degree of agglomeration in the melt. The two work together to significantly reduce the melt viscosity of the composite material.

[0190] The melt flow rate of Example 1 (DES activation + citric acid locking) is 5.53 g / 10 min. This value is significantly higher than that of Comparative Example 2, but lower than that of Comparative Example 5. This data trend verifies the staged dynamic compatibilization mechanism of the present application. First, the comparison with Comparative Example 2 proves the effectiveness of DES activation and dispersion. Second, the comparison with Comparative Example 5 proves that the introduction of citric acid (dynamic interface locking agent) does not play a lubricating role. On the contrary, in the molten state, the multi-functional groups (carboxyl groups) of citric acid form a new hydrogen bond interaction network between the dispersed BNF (with hydroxyl groups on the surface) and PLA (ester groups). This interface locking effect enhances the intermolecular forces between the filler and the matrix, limiting the relative movement of the polymer chains, thus making the melt viscosity rise compared to Comparative Example 5 with only DES lubrication.

[0191] In summary, the data of Test Example 2 confirm the lubrication / dispersion effect of DES and the interface locking effect of citric acid, which together realize the regulation of the rheological properties of the composite material.

[0192] Test Example 3:

[0193] The experimental steps are as follows:

[0194] (1) The injection molded specimens (40mm×10mm×4mm) of Comparative Example 5 and Example 1 were dried in a vacuum oven at 80°C for 12 hours.

[0195] (2) Use a dynamic mechanical analyzer and load the sample in a single cantilever beam mode.

[0196] (3) Set test parameters: temperature scan range 30℃ to 130℃, heating rate 3℃ / min, test frequency 1Hz, strain amplitude 0.1%.

[0197] (4) Start the test and record the curves of the energy storage modulus (E'), loss modulus (E'') and loss factor (tanδ) as a function of temperature.

[0198] (5) Extract from the data curve: take the peak temperature of the tanδ curve as the glass transition temperature (Tg) and record the energy storage modulus (E') value at 60℃.

[0199] (6) Each group of samples was tested three times, and the mean and standard deviation were calculated.

[0200] The experimental results are shown in Table 3 and Figure 1 As shown.

[0201] Table 3. Dynamic mechanical property data of composite materials:

[0202]

[0203] According to Table 3 and Appendix Figure 1 The data, including Figure 1 As shown in (a) (the curve of storage modulus versus temperature), the storage modulus (E') of Example 1 (solid line) is higher than that of Comparative Example 5 (dashed line) throughout the entire test temperature range. Particularly in the rubbery plateau region above the glass transition temperature (T > 70°C), the E' value of Example 1 (Table 3 shows 254 MPa at 60°C) is significantly higher than that of Comparative Example 5 (181 MPa). In this temperature range, E' mainly reflects the efficiency of stress transfer at the interface. The higher E' of Example 1 indicates that citric acid, as an interface locking agent, constructs effective hydrogen bond interactions between the BNF and PLA matrix, forming a stronger interfacial bond and improving stress transfer efficiency. In contrast, the lower E' of Comparative Example 5 (DES only) indicates that DES mainly plays a lubricating or plasticizing role at the interface, resulting in weak interfacial bonding.

[0204] At the same time, attached Figure 1The tan delta curve (b) in FIG. 2 shows that the tan delta peak (i.e. glass transition temperature, Tg) of Example 1 (solid line) shifts to a higher temperature compared to that of Comparative Example 5 (dashed line). This is confirmed by the data in Table 3, where the Tg of Example 1 (66.1 °C) is higher than that of Comparative Example 5 (62.4 °C). The Tg is the temperature at which the alpha-relaxation of the polymer chains occurs. The shift to a higher temperature demonstrates that the motion of the polymer chains near the surface of the BNF in Example 1 is restricted. The source of this restriction is the hydrogen-bond network constructed by the citric acid at the interface.

[0205] In summary, the data of Test Example 3 validate the role of the dynamic interfacial locking agent in this application to construct a strong interface in a dispersed system.

[0206] Test Example 4:

[0207] The experimental procedure is as follows:

[0208] (1) All injection molded bars were conditioned at 23 °C, 50% RH for at least 40 h according to ASTM D618 standard prior to testing.

[0209] (2) Tensile property testing: The tensile strength, tensile modulus, and elongation at break were tested according to ASTM D638 standard using a universal testing machine with Type I dumbbell-shaped bars at a tensile rate of 5 mm / min.

[0210] (3) Flexural property testing: The flexural strength and flexural modulus were tested according to ASTM D790 standard using a universal testing machine with a three-point bending mode at a span of 64 mm and a test rate of 2 mm / min.

[0211] (4) Impact property testing: The notched impact strength of the injection molded bars was tested according to ASTM D256 standard using a Charpy impact testing machine.

[0212] (5) Each set of property data was tested on at least 5 bars, and the average value and standard deviation were calculated.

[0213] The experimental results are shown in Table 4.

[0214] Table 4. Macroscopic mechanical property data of the composites:

[0215]

[0216] According to the data in Table 4, the tensile strength of Comparative Example 1 (pure PLA matrix) is 60.1 MPa, and the notched impact strength is 2.5 kJ / m 2 . The mechanical properties of Comparative Example 2 (unmodified BNF directly blended) are 45.3 MPa in strength and 1.8 kJ / m 2) are all lower than Comparative Example 1, indicating that the untreated BNFs agglomerate in the matrix and act as defect points, deteriorating the material properties.

[0217] Comparative Examples 3 (silane modification) and 4 (PLA-g-MA compatibilization) as traditional modification methods, their properties are improved compared with Comparative Example 2, but the improvement of impact strength (3.0-3.2 kJ / m 2 ) is limited.

[0218] The data of Comparative Example 5 (DES only) and Comparative Example 6 (citric acid only) are key. The modulus (4.35 GPa) of Comparative Example 5 (DES only) is high, indicating that the dispersion of BNFs is good (consistent with the MFI data of Test Example 2), but its impact strength (2.8 kJ / m 2 ) and tensile strength (61.5 MPa) are low, proving that DES plays a lubricating role at the interface, and the interface bonding is weak. The properties of Comparative Example 6 (citric acid only) are similar to those of Comparative Example 2, indicating that the interface locking agent cannot play a role when the BNFs are not dispersed.

[0219] The tensile strength (80.1 MPa) and notched impact strength (5.0 kJ / m 2 ) of Example 1 (DES activation + citric acid locking) are higher than all the comparative examples. This data confirms the necessity of combining DES pre-activation (to achieve dispersion) with citric acid dynamic locking (to build a strong interface).

[0220] The properties (impact 2.6 kJ / m 2 ) of Comparative Example 7 (DES and citric acid are both added at the main port) are much lower than Example 1. This proves the importance of the staged process in this scheme: DES must pre-activate BNFs before melt shearing, and citric acid must be added after BNFs have been dispersed (side feeding port) to effectively lock the interface.

[0221] The data of Examples 1, 2, and 3 show that as the amount of BNFs increases, the rigidity (tensile modulus and bending modulus) and strength (tensile strength and bending strength) of the material increase. The data of Examples 4 and 5 show that using different types of DES (glycerol) or interface locking agents (tartaric acid) can achieve the technical effect of this scheme. The data of Example 6 (TPS matrix) (impact 8.1 kJ / m 2 ) confirms the applicability of this scheme to other natural polymer matrices.

[0222] Test Example 5:

[0223] The experimental steps are as follows:

[0224] (1) The injection molded bars (size 127 mm x 12.7 mm x 3.2 mm) of Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, Example 1 and Example 3 were dried in a vacuum oven at 80 °C for 12 h.

[0225] (2) The heat deflection temperature tester was used to load the samples according to the standard of ASTM D648 with flat mode.

[0226] (3) The samples were immersed in a silicone oil bath with a constant load of 0.455 MPa.

[0227] (4) After the samples were balanced under the load for 5 min, the temperature was started to increase at a rate of 2 °C / min.

[0228] (5) The oil bath temperature corresponding to the standard deformation of 0.25 mm at the center point of the sample was recorded, which was the heat deflection temperature (HDT).

[0229] (6) Each group of samples was repeated for 3 times, and the average value and standard deviation were calculated.

[0230] The experimental results are shown in Table 5.

[0231] Table 5 Heat deflection temperature (HDT, 0.455 MPa) of the composite materials:

[0232]

[0233] According to the data in Table 5, the heat deflection temperature (HDT) of Comparative Example 1 (pure PLA) was 56.0 °C. The HDT (57.2 °C) of Comparative Example 2 (BNF directly blended) did not significantly improve compared with Comparative Example 1, indicating that the undispersed BNF agglomerates could not effectively improve the heat resistance of the matrix.

[0234] The HDT of Comparative Example 3 (silane modification) and Comparative Example 4 (PLA-g-MA compatibilization) increased to 65.2 °C and 66.4 °C, respectively, indicating that the traditional compatibilization methods improved the interfacial bonding and the heat resistance of the materials.

[0235] The HDT of Example 1 (10 parts of BNF) reached 77.9 °C, and the HDT of Example 3 (15 parts of BNF) was further improved to 84.8 °C. The data of Example 1 was significantly higher than that of Comparative Examples 3 and 4, which was attributed to the good dispersion of BNF achieved by DES activation (as shown in Test Example 2) and the strong interfacial bonding constructed by the interfacial locking effect of citric acid (as shown in Test Example 3). This strong interface and well-dispersed filler network could more effectively limit the movement of high molecular chain segments at high temperature.

[0236] Example 3 has higher HDT than Example 1, which proves that under the premise of good interface combination, increasing the filling amount of BNF (from 10 to 15) can build a stronger three-dimensional filler network, further strengthening the stabilizing effect on the matrix.

[0237] Based on the data of Test Examples 1-5, the following conclusions can be drawn:

[0238] The data of Test Examples 1-5 collectively verify the effectiveness of the proposed dynamic compatibilization mechanism in stages.

[0239] Firstly, the data of Test Example 1 (rest angle) confirms that the in-situ activation step of DES effectively destroys the initial agglomerates of BNF, improving its flowability. The comparison of the data of Comparative Example 2 and Comparative Example 5 in Test Example 2 (MFI) further shows that this activation improves the dispersion state of BNF in the melt and reduces the viscosity of the system.

[0240] Secondly, the MFI rebound data of Test Example 2 (Comparative Example 5 vs. Example 1) and the increase of E' and Tg in Test Example 3 (DMA) collectively confirm that the dynamic interfacial locking agent (citric acid) builds strong interfacial interactions in the matrix where BNF has been dispersed (added through the side feed port).

[0241] The effectiveness of this mechanism is finally reflected in the macroscopic performance. The data of Test Example 4 (mechanical properties) shows that the comprehensive performance (strength and impact toughness) of Example 1 is significantly better than that of Comparative Example 2 (untreated), Comparative Example 3 (silane), and Comparative Example 4 (MA compatibilizer), etc. traditional schemes. At the same time, the performance comparison between Example 1 and Comparative Example 7 (non-staged) proves the necessity of the staged process (i.e. DES pre-activation and post-adding of locking agent).

[0242] Finally, the data of Test Example 5 (HDT) shows that this structural advantage brought by good dispersion and strong interface also translates into the improvement of material heat resistance (HDT is the highest).

[0243] Although embodiments of the present application have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made therein without departing from the principles and spirit of the application, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for refining a blended material of bamboo fiber ultra-nano micro-powder and natural polymer, characterized in that, The method comprises the following steps: (a) pre-mixing bamboo fiber ultra-nano powder and a deep eutectic solvent in a mixing device to obtain a bamboo fiber ultra-nano powder pre-mixture; the weight ratio of the deep eutectic solvent to the bamboo fiber ultra-nano powder is (0.05-0.15):1; (b) adding the bamboo fiber ultra-nano powder pre-mixture and a natural polymer matrix through a main feeding port of a twin-screw extruder; the natural polymer matrix is at least one of polylactic acid or thermoplastic starch; (c) melt blending in the extruder and adding a dynamic interface locking agent through a side feeding port arranged downstream of the main feeding port during the melt blending, and then extruding and granulating the melt blend to obtain the blended material; the dynamic interface locking agent is citric acid or tartaric acid.

2. The method according to claim 1, wherein the bamboo fiber ultra-nano-micropowder and natural polymer blended material is refined. The deep eutectic solvent is composed of a hydrogen bond donor and a hydrogen bond acceptor.

3. The method for refining bamboo fiber ultra-nano powder and natural polymer blend material according to claim 2, characterized in that, The hydrogen bond acceptor is choline chloride; and the hydrogen bond donor is urea or glycerol. The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:(1.5-2.5).

4. The method according to claim 1, wherein the bamboo fiber ultra-nano-micropowder and natural polymer blended material is refined. Based on 100 parts by weight of the natural polymer matrix: The amount of the bamboo fiber ultra-nano powder is 5-15 parts by weight; The amount of the dynamic interface locking agent is 0.1-0.5 parts by weight.

5. The method according to claim 1, wherein the bamboo fiber ultra-nano-micropowder and natural polymer blended material is refined. The pre-mixing in step (a) is performed in a high-speed mixer at a mixing temperature of 70-80°C and a mixing time of 15-25 minutes.

6. The method according to claim 1, wherein the bamboo fiber ultra-nano-micropowder and natural polymer blended material is refined. In step (c), a vacuum exhaust port is arranged downstream of the side feeding port for removing the deep eutectic solvent and reaction by-products.

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