Method for preparing high-performance bamboo fiber / PMMA (polymethyl methacrylate) composite material through cooperative regulation and control of high pulping and chain transfer agent

Abstract

The invention provides a method for preparing a high-performance bamboo fiber / PMMA composite material through cooperative regulation and control of high-beating pretreatment and a chain transfer agent, and belongs to the technical field of biomass composite materials. The method comprises the steps that bamboo fibers are subjected to high-beating-degree pretreatment, and the beating degree is controlled to be 60-90 degrees SR; the method comprises the following steps: adding a chain transfer agent into a Fenton reaction system, and initiating controllable graft polymerization on the surface of the bamboo fiber in situ to form a PMMA (Polymethyl Methacrylate) graft layer with uniform chain length and regular arrangement; afterwards, homopolymers are removed through post-treatment, and modified bamboo fibers are obtained; and finally, carrying out melt blending on the modified bamboo fiber and a PMMA matrix, and carrying out hot press molding to obtain the composite material. According to the preparation method, the conversion of an interface structure from'quantity 'to'quality' is realized by precisely regulating and controlling the dosage of the chain transfer agent and the synergistic effect of high pulping pretreatment, the interface stress transfer efficiency and tensile strength of the composite material are remarkably improved, and the process is green and mild and is suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of biomass composite materials technology, specifically relating to a method for preparing high-performance composite materials by using high pulping pretreatment and precise control of chain transfer agent dosage to synergistically construct a high-performance polymer interface layer on bamboo fiber. Background Technology

[0002] Bamboo fiber, as a green and renewable reinforcing material, has great potential to replace traditional glass fiber. However, the weak interfacial bonding between bamboo and hydrophobic polymer matrices (such as polypropylene and polymethyl methacrylate) is a core bottleneck leading to poor mechanical properties of composite materials. To address the interfacial compatibility problem of bamboo-plastic composites, many studies have explored various modification methods. Currently, the main methods for improving the interface include physicochemical treatment (such as alkali treatment and silane coupling), which mainly improves surface wettability but fails to establish strong chemical bonds, resulting in limited interface improvement. Grafting polymerization methods (such as radiation grafting and thermal initiation) suffer from problems such as random grafting sites, uncontrollable graft chain length, and uneven distribution, leading to a loose interfacial layer structure and low stress transfer efficiency. Among these, the Fenton reaction has been used for cellulose modification due to its mild and environmentally friendly conditions. However, the reaction process is mainly homogeneous polymerization, with a large amount of polymer generated in the solution rather than on the fiber surface, resulting in low grafting efficiency. Furthermore, the length and distribution of graft chains are difficult to control precisely, and the resulting grafted layer is often a brittle coating with a loose structure and low stress transfer efficiency.

[0003] Current technologies for pretreatment of bamboo fibers mostly involve simple mechanical dispersion or mild chemical treatment, failing to fully utilize the mechanochemical effect to deeply regulate the internal structure of the fiber and limiting the exposure of a large number of potential grafting sites. When introducing chain transfer agents, the technical starting point is usually based on their general principles in homogeneous free radical polymerization, and generally follows the mindset that "the higher the grafting rate, the better the performance of the composite material".

[0004] Therefore, developing a modification technology that synergistically regulates fiber pulping pretreatment and chain transfer agent to achieve a qualitative leap in interface structure rather than simply accumulating the quantity of grafted material is an urgent need to break through existing technological bottlenecks and prepare high-performance bamboo fiber composite materials. Summary of the Invention

[0005] This invention aims to overcome the aforementioned deficiencies of existing technologies by providing a method for preparing high-performance bamboo fiber / PMMA composites through the synergistic regulation of high beating degree pretreatment and chain transfer agent. The core objective is to synergistically construct a polymer graft layer with controllable chain length and regular arrangement on the surface of bamboo fibers through high beating degree pretreatment and precise control of the chain transfer agent. This fundamentally improves the interfacial stress transfer efficiency of the composite material. Simultaneously, the interlayer penetration effect of low molecular weight PMMA promotes the microfibrillation and dispersion of fibers in the composite material, achieving simultaneous optimization of interfacial properties and macroscopic mechanical properties.

[0006] This invention provides a method for preparing high-performance bamboo fiber / PMMA composite materials through the synergistic regulation of high pulping and chain transfer agents, comprising the following steps: 1. High beating degree pretreatment of bamboo fiber: Bamboo fiber is subjected to high-intensity pulping in water, with the pulping degree controlled at 60-90°SR, preferably above 80°SR.

[0007] 2. Initiating controlled grafting: a. Disperse the pulped bamboo fiber in deionized water, add ferrous salt (such as ferrous sulfate heptahydrate), adjust the pH to 3-4, stir to adsorb, and press to dehydrate.

[0008] b. Add methyl methacrylate monomer and a chain transfer agent. The chain transfer agent is preferably a thiol compound (such as dodecyl mercaptan), and the molecular weight of the grafted chain can be precisely controlled by adjusting its amount.

[0009] c. Finally, add hydrogen peroxide solution dropwise and react at 30-60℃ for 0.5-1 h to initiate the Fenton reaction.

[0010] 3. Post-processing: After the reaction is complete, the mixture is filtered, and the homopolymer is removed by Soxhlet extraction with acetone for 24 h. After drying, the modified bamboo fiber with surface-grafted controllable polymer grafted layer is obtained.

[0011] 4. Preparation of composite materials: The modified bamboo fiber and polymer matrix (such as PMMA) are melt-blended and granulated at a high fiber content (such as 50-100wt%), and then hot-pressed to obtain the final composite material.

[0012] Compared with the prior art, the significant advantages and innovations of this invention are as follows: 1. Regulation Effect of Chain Transfer Agent: In the bamboo fiber Fenton grafted PMMA system, the addition of chain transfer agent does not control the entire solution polymerization, but rather precisely regulates the length and termination of polymer chains grown at each active site in the fiber-solution interface layer, thereby forming a polymethyl methacrylate graft layer with uniform chain length and dense, uniform arrangement on the fiber surface. The effect of chain transfer agent dosage on the final tensile strength of the composite material is not a simple linear or monotonic relationship, but rather has a clear performance peak, with the optimal dosage range being 0.5%-0.9%. This establishes a new and more efficient interface design approach: "replacing the simple accumulation of graft material with optimized quality of interface structure." This is the core control method of this invention, making interface performance predictable and designable, thereby improving the interfacial compatibility and mechanical properties of the composite material.

[0013] 2. High Freezing Degree Synergistically Promotes Efficient Grafting: High freezing treatment not only increases the fiber's specific surface area and active site density, but also disrupts the interlayer hydrogen bond network within the fiber. This allows for full contact between methyl methacrylate monomers and the fiber, as well as CTA (carboxymethyl methacrylate) contact, resulting in uniform graft chains grafted onto more accessible sites, thus achieving highly efficient grafting. Both aspects constitute a complete, novel, and efficient process combination, neither of which can be omitted.

[0014] 3. Interlayer opening and microfibrillation effect of low molecular weight PMMA chains: Low molecular weight PMMA chains are prepared by regulating the chain transfer agent. During the grafting process, they can penetrate into the interlayer of bamboo fibers, destroy the hydrogen bond network inside the fibers, and promote the further dissociation of individual fibers into microfibrils during the subsequent melt mixing process. This greatly increases the contact area between the fiber and the matrix, achieving dual interface reinforcement of "nanoscale dispersion" and "molecular entanglement".

[0015] 4. Fundamental change in stress transmission mechanism: When composite materials are subjected to stress, stress is transmitted through millions of controllable chains that are covalently bonded to the fibers and deeply entangled in the matrix. The polymer molecular chains on the surface of modified bamboo fibers and the molecular chains of the matrix form a deep entangled network, realizing the transformation from surface bonding to molecular network interpenetration, thereby significantly improving the strength and toughness of composite materials and fundamentally changing the stress transmission mechanism.

[0016] 5. Green and sustainable: The entire process is carried out in an aqueous phase, with mild reaction conditions, low energy consumption, environmental friendliness, and easy to scale up production. Detailed Implementation

[0017] The embodiments described below are merely some embodiments of the present invention, and the technical means used therein are conventional means well known to those skilled in the art. Example 1

[0018] (1) Take 360 ​​g of bamboo fiber, beat it in a Valley refiner until the degree of beating is 80°SR, and spin dry for later use.

[0019] (2) Take 1 g of the pulped fiber (octane-dry pulp) and place it in a beaker containing 50 mL of deionized water. Add 0.5 g of ferrous sulfate heptahydrate, adjust the pH to 3 with dilute sulfuric acid, stir for 30 min, and press to dehydrate.

[0020] (3) Add 6 g of methyl methacrylate monomer and dodecyl mercaptan as a chain transfer agent (the amount of which is 0.3% of the molar amount of MMA monomer). Under nitrogen protection and constant temperature water bath at 60°C, slowly add a solution containing 2 mmol of hydrogen peroxide and react for 1 h.

[0021] (4) After the reaction was completed, the mixture was filtered and extracted with acetone by Soxhlet for 24 h to remove the homopolymer. After drying, the modified bamboo fiber (denoted as BF-g-PMMA-0.3CTA) was obtained.

[0022] (5) The modified bamboo fiber and polymethyl methacrylate (PMMA) were mixed in an internal mixer at 200°C for 20 min at a weight ratio of 75:25. After granulation, the mixture was hot-pressed at 180°C to form a standard test strip. Example 2

[0023] The steps are the same as in Example 1, except that the amount of chain transfer agent dodecyl mercaptan is changed to 0.5% of the molar amount of MMA monomer, and the resulting modified fiber is denoted as BF-g-PMMA-0.5CTA. Example 3

[0024] The steps are the same as in Example 1, except that the amount of chain transfer agent dodecyl mercaptan is changed to 0.7% of the molar amount of MMA monomer, and the resulting modified fiber is denoted as BF-g-PMMA-0.7CTA. Example 4

[0025] The steps are the same as in Example 1, except that the amount of chain transfer agent dodecyl mercaptan is changed to 1.0% of the molar amount of MMA monomer, and the resulting modified fiber is denoted as BF-g-PMMA-1.0CTA.

[0026] Comparative Example 1 The steps are the same as in Example 1, but without the addition of chain transfer agent (CTA), and the resulting modified fiber is denoted as BF-g-PMMA-noCTA.

[0027] Comparative Example 2 The steps are the same as in Example 1, but the bamboo fiber is not pulped and pretreated (only simple mechanical dispersion is performed) and the amount of chain transfer agent dodecyl mercaptan is changed to 0.7% of the molar amount of MMA monomer. The resulting modified fiber is denoted as UBF-g-PMMA-0.7CTA.

[0028] Comparative Example 3 Highly beaten but ungrafted MMA-modified fiber (BF) was mixed with PMMA at a weight ratio of 75:25 in an internal mixer at 200°C for 20 min. After granulation, it was hot-pressed at 180°C to form standard test strips.

[0029] The products obtained in Examples 1-4 and Comparative Examples 1-3 were tested for performance. The specific test methods are as follows. 1. Calculate the grafting rate using the gravimetric method.

[0030] 2. Tensile properties were tested according to the national standard GB / T1040.2-2022. The sample specifications were dumbbell-shaped standard specimens of 150mm×20mm×1mm, and the tests were conducted using a universal testing machine with a loading speed of 10 mm / min.

[0031] The results are as follows: sample Grafting rate (%) Tensile strength (MPa) Example 1 112 22.97 Example 2 102 25.39 Example 3 100 28.55 Example 4 133 15.79 Comparative Example 1 163 18.21 Comparative Example 2 95 16.65 Comparative Example 3 0 13.4 The results showed that, under the premise of maintaining high freeness pretreatment (80°SR), compared with the sample without chain transfer agent (Comparative Example 1), the grafting rate was 163% and the tensile strength was 18.21 MPa. Although the introduction of chain transfer agent generally led to a decrease in grafting rate, the tensile strength of the composite material was significantly improved. With the increase of chain transfer agent dosage, the grafting rate showed a trend of first decreasing and then increasing, while the tensile strength showed a trend of first increasing and then decreasing.

[0032] The tensile strength of the composite material reached its highest value (Example 3) when the chain transfer agent dosage was 0.7% of the monomer molar amount, representing a 56.78% increase compared to Comparative Example 1 and a 113.06% increase compared to Comparative Example 3. This indicates the existence of a precise chain transfer agent dosage range (approximately 0.5%-0.9%) within which the chain transfer agent is not merely used to suppress molecular weight as conventionally believed, but rather to construct a grafted chain with moderate length and more regular distribution on the bamboo fiber surface by regulating the kinetics of surface-initiated polymerization. Its shorter chain length, higher branching degree, and more uniform distribution achieve deeper entanglement and more efficient stress transfer with the PMMA matrix, thus achieving a stronger interfacial reinforcement effect with less grafted material. This structure, despite corresponding to a low to medium grafting rate, maximizes stress transfer efficiency.

[0033] Comparing Example 3 and Comparative Example 2, although the grafting rates are similar, the tensile strengths differ significantly. This demonstrates that high beating pretreatment is a necessary prerequisite for activating the fiber surface and allowing the subsequent chain transfer agent regulation effect to be fully realized; the two have an inseparable synergistic relationship.

[0034] This discovery breaks through the linear mindset that the higher the grafting rate, the better the composite material performance. It establishes the core design principle of improving the "quality" of the interface structure by synergistically controlling the beating degree and the amount of chain transfer agent, instead of simply increasing the "quantity" of grafting. This provides key theoretical and practical basis for the predictable design and precise optimization of the interface performance of bamboo fiber composite materials.

Claims

1. A preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agents, characterized in that, Includes the following steps: (1) Bamboo fiber is pretreated with high beating degree, with a beating degree of 60-90°SR, and then spun dry and broken up to obtain pulp for later use; (2) Take a certain amount of pulped bamboo fiber and disperse it in water, add ferrous salt, adjust the pH, stir for a period of time and then press to dehydrate; (3) Add methyl methacrylate monomer and chain transfer agent, then add hydrogen peroxide solution dropwise, and react at 30-60℃ for 0.5-1h to carry out Fenton graft polymerization; (4) After the reaction is complete, the homopolymer is removed by filtration, washing, and acetone soxhlet extraction, and then dried to obtain modified bamboo fiber with PMMA grafted on the surface. The modified bamboo fiber and PMMA matrix are melt-blended with a certain fiber content, granulated, and hot-pressed to obtain a bamboo fiber / PMMA composite material.

2. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent as described in claim 1, characterized in that, In step (1), the beating degree is controlled above 80°SR.

3. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent as described in claim 1, characterized in that, The ferrous salt mentioned in step (2) is ferrous sulfate heptahydrate.

4. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent as described in claim 1, characterized in that, The pH value mentioned in step (2) is 3-4.

5. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent according to claim 1, characterized in that, In step (2), the amount of hydrogen peroxide used is 2 mmol.

6. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent according to claim 1, characterized in that, In step (3), the chain transfer agent is a thiol compound, preferably dodecyl thiol.

7. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent according to claim 1 or 3, characterized in that, In step (3), the amount of chain transfer agent used is 0.3%-1% of the molar amount of methyl methacrylate monomer.

8. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent according to claim 1, characterized in that, In step (5), the mass ratio of modified bamboo fiber to PMMA matrix is ​​75:

25.

9. The preparation process for high-performance bamboo fiber / PMMA composite materials by synergistic regulation of high pulping and chain transfer agent according to claim 1, characterized in that, In step (5), the melt blending temperature is 200°C and the hot pressing temperature is 180°C.