Preparation method of high-density composite board material added with plant fibers

Abstract

The invention relates to the technical field of composite materials, and particularly discloses a high-density composite board material added with plant fibers and a preparation method thereof.The preparation method comprises the steps that S1, at least three premix layers with the plant fiber content and the particle size changing in a gradient mode are prepared; s2, applying an interface fusion agent composed of nanocellulose whiskers and a polymer emulsion between adjacent premix layers; and S3, hot press molding. Through combination of gradient structure design, an interface fusion and surface modification technology and an oscillation pressure molding process, the interface compatibility of plant fibers and a polymer matrix is effectively improved, and the problems that fibers are prone to agglomeration and thermal degradation, and finished products are large in internal stress and prone to warping are solved; the obtained composite board material has the advantages of high density, high internal bonding strength, high interlayer bonding force and low residual strain.

Description

Technical Field

[0001] This invention relates to the field of composite material technology, and specifically to a method for preparing a high-density composite board material with added plant fibers. Background Technology

[0002] High-density fiberboard (HDF) is widely used in furniture, construction, packaging, and decorative materials due to its dense structure, high strength, and good impact resistance. Traditional HDF is mostly made from wood chips or plant fibers, bonded together under high temperature and pressure using adhesives such as urea-formaldehyde resin and phenolic resin. This type of board presents two significant problems: first, the use of formaldehyde-containing adhesives easily causes indoor air pollution and harms human health; second, it relies excessively on wood resources, which does not meet the requirements of sustainable development.

[0003] To replace some wood-based materials and achieve waste utilization, existing technologies attempt to add agricultural waste, such as straw, bamboo powder, and hemp fiber, to composite boards. However, the main components of plant fibers are cellulose, hemicellulose, and lignin, and their surfaces contain a large number of hydrophilic hydroxyl groups, while common polymer matrices, such as polyolefins and polyesters, are hydrophobic, resulting in poor interfacial compatibility and weak bonding. This directly leads to low stress transfer efficiency and a significant decrease in the mechanical properties of the composite material, especially tensile and impact strength.

[0004] In addition, plant fibers face the following technical challenges during processing: Dispersion problem: Plant fibers have low density and are prone to agglomeration, making it difficult to disperse evenly in a polymer matrix, forming stress concentration points and becoming the source of material damage.

[0005] Thermal degradation risk: Hemicellulose and lignin in plant fibers have poor thermal stability and are prone to thermal degradation at polymer matrix processing temperatures (usually above 180°C), resulting in loss of fiber strength and discoloration of the material.

[0006] Residual stress and warping: The difference in thermal expansion coefficients between plant fibers and polymer matrix is ​​huge, which generates huge internal stress during processing and cooling, making the products prone to warping and deformation and poor dimensional stability.

[0007] Therefore, developing a preparation method that can effectively improve the interface between plant fibers and the matrix, and protect the integrity of plant fibers and reduce internal stress during high-density molding, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] To address the technical challenges in existing technologies, such as poor interfacial compatibility between plant fibers and polymer matrices, easy fiber agglomeration, susceptibility to thermal degradation during processing, and high internal stress leading to warping in finished products, this invention provides a method for preparing high-density composite board materials with added plant fibers. This invention aims to achieve uniform dispersion and firm bonding of plant fibers in the matrix through innovative material structure design and molding processes. While ensuring high material density, it effectively protects fiber integrity and eliminates internal stress, thereby obtaining an environmentally friendly composite board with excellent mechanical properties and dimensional stability.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A high-density composite board material with added plant fibers, characterized in that it comprises at least a surface layer and a core layer, with a structure of surface layer-core layer-surface layer; an interface bonding agent is applied between the surface layer and the core layer, and the thickness ratio of the surface layer to the core layer is 1:1.5-2.5; the high-density composite board material further comprises at least one intermediate layer, and the intermediate layer is located between the surface layer and the core layer, with an interface bonding agent applied between each layer, and the thickness ratio of the surface layer, intermediate layer, and core layer is 1:(0.8-1.2):(1.5- 2.5); The interface bonding agent comprises 3-10 parts by mass of nanocellulose whiskers and 90-97 parts by mass of polymer emulsion; by weight, the surface layer, intermediate layer and core layer each comprise 100 parts of polymer matrix, 2-8 parts of compatibilizer, and 0.5-3 parts of lubricant, corresponding to 15-25 parts of modified plant fiber with a particle size finer than 150 mesh, 26-39 parts of modified plant fiber with a particle size of 80-150 mesh and 40-60 parts of modified plant fiber with a particle size of 40-80 mesh, respectively.

[0010] Preferably, the polymer matrix is ​​one of polyethylene terephthalate, polypropylene, or polylactic acid; the compatibilizer is a maleic anhydride graft compatibilizer corresponding to the polymer matrix; and the lubricant is one of zinc stearate, calcium stearate, or polyethylene wax.

[0011] Preferably, the polymer emulsion in the interface fusion agent is one of an acrylate emulsion, a polyurethane emulsion, or an ethylene-vinyl acetate emulsion.

[0012] Preferably, the amount of the interface bonding agent applied is: .

[0013] Preferably, the plant fiber is derived from one or more of bamboo powder, hemp fiber, straw powder, sawdust, or coconut shell fiber.

[0014] A method for preparing a high-density composite board material with added plant fibers includes the following steps: Step 1: Surface modification treatment of plant fibers: Plant fibers were sieved into three particle size specifications: 40-80 mesh, 80-150 mesh, and finer than 150 mesh. After surface modification treatment with silane coupling agent, modified plant fibers were obtained for later use. Step 2: Prepare the premix: According to the gradient structure requirements of the surface layer, intermediate layer and core layer, the corresponding premixes are formulated respectively; Step 3: Shaping The premixed materials are stacked according to the desired gradient structure, and an interface fusion agent is applied between adjacent layers; then hot pressing is used to obtain the high-density composite board material.

[0015] Preferably, in step three, the required structure is one of surface layer-core layer-surface layer, surface layer-intermediate layer-core layer-surface layer, or surface layer-intermediate layer-core layer-intermediate layer-surface layer.

[0016] Preferably, in step one, the surface modification treatment includes the following steps: Alkali treatment steps: Treat plant fibers with a sodium hydroxide solution of 3-8% by mass at 60-85℃ for 0.5-2.5 hours; Silane coupling agent treatment steps: Impregnation or spraying with a silane coupling agent solution with a mass fraction of 1%-5% and a solvent of 95% ethanol aqueous solution; the silane coupling agent is KH-550 or KH-570.

[0017] Preferably, in step two, the process of preparing the premix includes: placing the modified plant fiber, polymer matrix, compatibilizer and lubricant in a high-speed mixer and mixing them at 75-95°C for 10-30 minutes to obtain the premix.

[0018] The design of using low-part-weight, fine-particle-size plant fibers in the surface layer aims to ensure that the polymer matrix can fully encapsulate the fibers, forming a dense surface layer with fewer defects. This is crucial for the surface quality, wear resistance, and environmental isolation of the material. Due to their small size, the fine fibers are less likely to puncture the matrix and form stress concentration points, further ensuring the integrity of the surface layer. If the fiber content is too high or the particle size is too coarse, the surface layer will become rough and prone to stress concentration points due to insufficient matrix wetting.

[0019] The use of high-component, coarse-grained fibers in the core layer maximizes the reinforcing potential of plant fibers. The high component content aims to maximize the reinforcing effect of plant fibers, giving the composite material higher rigidity and load-bearing capacity. Coarse fibers are chosen because, while providing effective reinforcement, they have a smaller specific surface area compared to fine fibers of the same content, significantly reducing the difficulty of matrix wetting and avoiding densification difficulties and internal pore defects caused by excessively high melt viscosity. This is consistent with the requirements of the high-pressure molding stage.

[0020] The composition and particle size parameters of the intermediate layer lie between the surface and core layers, and its core function is to achieve a smooth transition in performance. This gradient transition is not a simple linear interpolation; its core function is to buffer the interfacial stress caused by the difference in modulus and coefficient of thermal expansion between the surface and core layers, thereby ensuring the reliability of the interlayer bonding. If the fiber content or particle size of the intermediate layer fails to form an effective gradient, the interlayer bonding area is prone to become a path for crack propagation during the cooling and curing process.

[0021] The dual gradient structure design of plant fiber content and particle size achieves a mechanical property distribution from toughness to strength from the surface to the core through a "content gradient," while a "particle size gradient" solves the problems of "wetting" and "stress concentration" that accompany this content distribution. These two elements complement each other, ensuring that the final product possesses excellent surface quality and high strength, while also exhibiting good interlayer bonding and dimensional stability.

[0022] Alkali treatment of plant fibers effectively removes non-crystalline components and impurities from the surface, exposes more cellulose active sites, and increases surface roughness, providing a foundation for subsequent coupling agent reactions. Insufficient alkali concentration or treatment time results in limited cleaning and activation effects, while overtreatment may damage the fiber structure. Subsequent silane coupling agent treatment utilizes the silanol groups generated after hydrolysis to form covalent bonds with the hydroxyl groups on the fiber surface. Simultaneously, its organic functional groups interact with the polymer matrix, establishing a stable molecular bridge between the hydrophilic fiber and the hydrophobic matrix. Insufficient coupling agent concentration or uneven treatment leads to incomplete bridging, while excessive concentration may result in brittle multilayer coatings, reducing interfacial strength. High-speed mixing at 75-95℃ serves a dual purpose: this temperature range moderately softens the polymer matrix to promote fiber coating and wetting, while avoiding thermal degradation of the plant fibers due to excessive temperature. The mixing time must ensure sufficient dispersion of components while preventing fiber length loss due to excessive shearing. This pretreatment process directly determines the uniformity of material flow in subsequent compression molding and the interfacial bonding quality of the final composite material. Unmodified or unevenly dispersed fibers are prone to becoming sources of stress defects under oscillating pressure, while fully modified and uniformly dispersed systems can achieve synergistic effects of densification and interfacial fusion through the process.

[0023] The polymer matrix is ​​limited to polyethylene terephthalate, polypropylene, or polylactic acid (PLA) based on their respective defined melt processing windows and their matching heat resistance limits with plant fibers. This avoids fiber degradation or matrix decomposition during molding due to mismatched thermal histories. The selection of maleic anhydride graft compatibilizers follows the principle of complementary molecular structures. Its anhydride functional groups can form ester or hydrogen bonds with the hydroxyl groups on the plant fiber surface, while the grafted backbone forms molecular chain entanglement with the corresponding matrix. Using a non-corresponding compatibilizer will lead to interfacial bridging failure. The selection of lubricants must consider both internal lubrication to reduce melt viscosity and external lubrication to prevent sticking. Zinc stearate metal soaps are suitable for polyolefin systems, while polyethylene wax is more suitable for polyester systems. Excessive lubricant will migrate to the fiber-matrix interface, forming an insulating layer that weakens the bond strength.

[0024] Acrylic emulsions, with their polar backbone structure, exhibit intermolecular interactions with polyethylene terephthalate (PET) or polylactic acid (PLA). In polyurethane emulsions, urethane groups can form hydrogen bonds with various polymers, while ethylene-vinyl acetate emulsions achieve chain entanglement with the polyolefin matrix through amorphous regions. This chemical compatibility ensures that, at molding temperatures, the emulsion polymer can undergo molecular-level diffusion and interweaving at the interface of adjacent premix layers. Using incompatible emulsions would result in clear phase separation boundaries between layers, becoming a weak point in mechanical properties.

[0025] Preferably, in step three, the hot pressing molding specifically employs a molding process that uses oscillating pressure and dynamic temperature field coordinated control, which includes the following stages: Phase 1, impregnation and venting phase: Raise the mold temperature to 150-190℃, apply an initial pressure of 3-8MPa, and apply pressure oscillation at a frequency of 0.05-0.5Hz under this pressure for 2-8 minutes; Phase 2, Gradient Densification Phase: Raise the mold temperature to 10-30°C above the melting temperature of the polymer matrix, increase the pressure stepwise to 15-30 MPa, and apply pressure oscillation with a frequency of 0.5-2 Hz and an amplitude of 5-20% of the target pressure value of Phase 2 during this phase, with a holding time of 10-30 minutes. Phase 3, Stress Relief and Shaping Stage: After the pressure holding period ends, while maintaining the pressure, the mold temperature is reduced to below 60°C at a rate of 1-5°C per minute, and then the pressure is released and the mold is demolded. During the gradient densification stage, the mold temperature is raised to 250-270°C for the polymer matrix being polyethylene terephthalate; to 180-200°C for the polymer matrix being polypropylene; and to 170-190°C for the polymer matrix being polylactic acid.

[0026] During the impregnation and venting stage, the combined effect of temperatures below the polymer's melting point and low-frequency pressure oscillations promotes the initial softening of the matrix material. This is particularly effective for core layers with high fiber content and coarse particle size, as the oscillations at this stage can more effectively expel moisture and low-molecular-weight volatiles from the interfiber spaces, laying the foundation for subsequent densification. If the temperature is too high, causing premature resin curing, it will seal the complex venting channels formed by the coarse fibers in the core layer.

[0027] During the gradient densification stage, the temperature increase causes the matrix to completely melt. Stepped pressure increase and high-frequency pressure oscillation work together on the multilayer structure exhibiting a stiffness gradient: the high-frequency oscillation not only breaks up fiber agglomerations within each layer, but the shear force generated also helps the melt to fully wet and penetrate the high-content, coarse-particle-size core layer, and induces polymer molecular chains to penetrate the interlayer interface bonding network, strengthening the bonding at the interlayer interfaces defined by particle size differences. The pressure amplitude needs to be precisely controlled during this stage to overcome the high melt viscosity caused by the high fiber content of the core layer, while avoiding excessive amplitude that could shear and damage the existing interlayer bonds.

[0028] During the stress relief and shaping stage, programmed cooling under pressure is crucial. Due to the significant differences in thermal expansion coefficients between the surface and core layers caused by variations in fiber content and particle size, uneven internal stresses are generated during cooling. This process controls the cooling rate to slowly reduce the mobility of polymer chain segments, providing sufficient relaxation time for the internal stresses generated by differences in thermal shrinkage between different layers. This effectively prevents product warping and ensures the overall dimensional stability of the gradient structure.

[0029] The technical solution of this invention optimizes the macroscopic properties of composite materials through systematic control of component ratios, interfacial characteristics, and process parameters. In the premix preparation stage, the polymer matrix, as the continuous phase, bears the main load, while plant fibers, as the reinforcing phase, achieve a reasonable distribution of internal stiffness and toughness through gradient design of content and particle size. Low content of fine fibers in the surface layer facilitates the formation of a dense and smooth surface, while high content of coarse fibers in the core layer primarily provides rigid support. If the fiber content is too low, the reinforcing effect is insufficient; if it is too high, it is difficult to be fully wetted by the matrix, leading to agglomeration defects. The compatibilizer, through its amphiphilic molecular structure, forms a chemical bridge at the fiber-matrix interface, significantly improving interfacial bonding strength. Insufficient dosage results in limited interfacial modification, while excessive dosage may cause a decline in the matrix's own properties. The lubricant is used to reduce internal friction and adhesion to equipment during processing; excessive addition will cause it to migrate to the interface, forming a weak boundary layer. Nanocellulose whiskers, with their high specific surface area and abundant surface hydroxyl groups, form strong hydrogen bonds with plant fibers. Their nanoscale effect can also synergistically penetrate into adjacent premix layers with the polymer emulsion, establishing a robust three-dimensional network structure between layers. The viscosity and activity of the fusion agent are matched with the surface characteristics of the premix layer, ensuring uniform coating without excessive absorption. The oscillating pressure molding process can regulate the rheological behavior and stress evolution of the composite material during curing. The low-frequency oscillation stage promotes melt flow and bubble escape, while the high-frequency oscillation stage induces fiber orientation and strengthens interfacial interlocking. The cooling process relaxes the internal stress caused by the thermal shrinkage differences of different components through the slow freezing of molecular chain motion. The entire temperature-pressure process precisely matches the melt-curing characteristics of the polymer matrix and the thermal stability window of the plant fibers. Excessive temperature or excessive pressure oscillation may cause fiber thermal degradation or resin decomposition, while the opposite may lead to insufficient densification and poor interfacial fusion.

[0030] Compared with the prior art, the present invention has the following advantages: From a compositional perspective, this approach constructs a gradient structure of plant fiber content and particle size, creating a dense barrier on the surface while the core layer provides primary rigid support, effectively optimizing the stress distribution within the material. Synergistic surface modification using alkali treatment and silane coupling agents establishes stable chemical bonds between the fibers and the matrix, significantly improving interfacial compatibility. The selection of maleic anhydride graft compatibilizers and specific lubricants corresponding to the polymer matrix further ensures synergistic effects between components. The nanocellulose whiskers in the interfacial fusion agent form a three-dimensional network structure with the polymer emulsion between layers, achieving a robust bond between the multilayer premixed material.

[0031] From a technological perspective, the three-stage molding process, which coordinates the control of oscillating pressure and dynamic temperature field, achieves precise regulation of the molding process. The impregnation and venting stage effectively removes moisture and volatiles through low-frequency pressure oscillation; the gradient densification stage utilizes high-frequency pressure oscillation to promote melt flow and fiber orientation, strengthening interfacial fusion; and the stress relief stage uses programmed cooling to fully relax the internal stresses generated by components with different coefficients of thermal expansion. This process system is highly compatible with the component characteristics, achieving high densification and low residual stress in the material while protecting the integrity of the plant fibers. Detailed Implementation

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] General Implementation Examples A method for preparing a high-density composite board material with added plant fibers, comprising the following steps, by weight: Step 1: Surface modification treatment of plant fibers; Plant fibers were sieved into three particle size specifications based on the surface layer, middle layer, and core layer: surface layer plant fibers with a particle size finer than 150 mesh, middle layer plant fibers with a particle size of 80-150 mesh, and core layer plant fibers with a particle size of 40-80 mesh. Surface modification treatments were then applied to each of the three types of plant fibers. First, an alkaline treatment is performed by immersing plant fibers of various specifications in a 3-8% sodium hydroxide solution and treating them in a constant temperature water bath at 60-85℃ for 0.5-2.5 hours. After treatment, the fibers are filtered and separated, repeatedly washed with deionized water until neutral, and then dried in an oven at 80℃ until constant weight.

[0034] Next, silane coupling agent treatment is performed. A 1-5% KH-550 or KH-570 ethanol solution (95% ethanol by volume) is prepared, and the alkali-treated bamboo powder is immersed in it. The mixture is then treated with ultrasound for 45 minutes to ensure uniform distribution of the coupling agent. After treatment, the plant fibers are removed and dried at 85℃ for 2 hours to obtain modified plant fibers for later use.

[0035] Step 2: Prepare the premix; Three types of premixes were prepared according to gradient distribution requirements: Surface premix: 100 parts polyethylene terephthalate, polypropylene or polylactic acid, 15-25 parts modified bamboo powder, hemp fiber, straw powder, wood chips or coconut shell fiber (particle size finer than 150 mesh), 2-8 parts maleic anhydride graft compatibilizer corresponding to the polymer matrix, and 0.5-3 parts zinc stearate, calcium stearate or polyethylene wax. Intermediate layer premix: 100 parts polyethylene terephthalate, polypropylene or polylactic acid, 26-39 parts modified bamboo powder, hemp fiber, straw powder, wood chips or coconut shell fiber (particle size 80-150 mesh), 2-8 parts maleic anhydride graft compatibilizer corresponding to the polymer matrix, and 0.5-3 parts zinc stearate, calcium stearate or polyethylene wax; Core layer premix: 100 parts polyethylene terephthalate, polypropylene or polylactic acid matrix, 40-60 parts modified bamboo powder, hemp fiber, straw powder, wood chips or coconut shell fiber (particle size 40-80 mesh), 2-8 parts maleic anhydride graft compatibilizer corresponding to the polymer matrix, and 0.5-3 parts zinc stearate, calcium stearate or polyethylene wax; Each layer of raw material is separately fed into a high-speed mixer and mixed at 75-95℃ for 10-30 minutes to ensure that each component is fully and evenly mixed, resulting in three premixes with different properties.

[0036] Step 3: Layering, interface fusion, and oscillating pressure molding; The premixed material is laid in the mold in the following order: surface layer - intermediate layer - core layer - intermediate layer - surface layer. The thickness ratio of the surface layer, intermediate layer, and core layer is 1:(0.8-1.2):(1.5-2.5). After each layer of premixed material is laid, an interface bonding agent is evenly applied using a spraying device, with the application rate per unit area controlled at 80-120 g / m². 2 Ensure that the bonding agent is evenly distributed between each layer, without any accumulation or missing areas.

[0037] The mold containing the premixed material is fed into a hot press and molded according to a three-stage process: Phase 1: Immersion and Exhaust Raise the mold temperature to 150-190℃, apply an initial pressure of 3-8MPa, and simultaneously apply pressure oscillation at a frequency of 0.05-0.5Hz, holding the pressure for 2-8 minutes. During this stage, noticeable water vapor discharge is observed. Once the venting has essentially stopped, proceed to the next stage.

[0038] Phase Two: Gradient Compaction Raise the mold temperature to 10-30°C above the polymer matrix melt temperature (250-270°C for polyethylene terephthalate; 180-200°C for polypropylene; 170-190°C for polylactic acid), and gradually increase the pressure to 15-30 MPa. Simultaneously apply pressure oscillation at a frequency of 0.5-2 Hz and an amplitude of 5-20% of the target pressure value for stage two, holding the pressure for 10-30 minutes. During this stage, the material fully melts and flows, achieving densification.

[0039] Phase 3: Stress Relief and Shaping After the pressure holding period, the temperature is gradually reduced to below 60°C at a rate of 1-5°C / min while maintaining pressure, with the entire process lasting an appropriate amount of time. Once the temperature stabilizes, the pressure is released and the material is demolded to obtain the high-density composite board material.

[0040] Example 1 A method for preparing a high-density composite board material with added plant fibers, comprising the following steps, by weight: Step 1: Surface modification treatment of plant fibers; Bamboo powder was sieved into three particle size specifications based on the surface layer, middle layer, and core layer: bamboo powder for the surface layer had a particle size finer than 150 mesh, bamboo powder for the middle layer had a particle size of 100 mesh, and bamboo powder for the core layer had a particle size of 60 mesh. Surface modification treatments were then applied to each of the three types of bamboo powder. First, an alkali treatment was performed by immersing bamboo powder of various specifications in a 5% sodium hydroxide solution and treating the plant fibers in a constant temperature water bath at 70℃ for 1.5 hours. After treatment, the mixture was filtered and separated, repeatedly washed with deionized water until neutral, and then dried in an oven at 85℃ to constant weight.

[0041] Next, a silane coupling agent treatment was performed. A 3% KH-550 ethanol solution (95% ethanol by volume) was prepared, and the alkali-treated bamboo powder was immersed in it. The mixture was then treated with ultrasound for 45 minutes to ensure uniform distribution of the coupling agent. After treatment, the bamboo powder was removed and dried at 85°C for 2 hours to obtain modified plant fiber.

[0042] Step 2: Premix preparation; Three types of premixes were prepared according to gradient distribution requirements: Surface premix: 100 parts polyethylene terephthalate, 20 parts modified bamboo powder (particle size finer than 150 mesh), 5 parts maleic anhydride-grafted PET, and 1 part zinc stearate. Intermediate layer premix: 100 parts polyethylene terephthalate, 32 parts modified bamboo powder (100 mesh particle size), 5 parts maleic anhydride-grafted PET, and 1 part zinc stearate. Core layer premix: 100 parts polyethylene terephthalate, 50 parts modified bamboo powder (60 mesh), 5 parts maleic anhydride-grafted PET, and 1 part zinc stearate. Each layer of raw material was separately fed into a high-speed mixer and mixed at 85°C for 20 minutes to ensure that each component was fully and evenly mixed, resulting in three premixes with different properties.

[0043] Step 3: Layering, interface fusion, and oscillating pressure molding; Premixed materials are laid in the mold in the following order: surface layer - intermediate layer - core layer - intermediate layer - surface layer. The thickness ratio of the surface layer, intermediate layer, and core layer is 1:1:2. After each layer of premixed material is laid, an interface bonding agent composed of 5 parts of nanocellulose whiskers and 95 parts of acrylic emulsion is evenly applied using a spraying device. The application rate per unit area is controlled at 100g / m². 2 Ensure that the bonding agent is evenly distributed between each layer, without any accumulation or missing areas.

[0044] The mold containing the premixed material is fed into a hot press and molded according to a three-stage process: Phase 1: Immersion and Exhaust The mold temperature was raised to 170℃, and an initial pressure of 5MPa was applied, along with pressure oscillation at a frequency of 0.1Hz, which was maintained for 5 minutes. Significant water vapor was observed to be released during this stage. The next stage was initiated after the venting process had largely ceased.

[0045] Phase Two: Gradient Compaction The mold temperature is raised to 265℃, and the pressure is gradually increased to 20MPa. Simultaneously, pressure oscillation with a frequency of 1Hz and an amplitude of 15% of the target pressure value for stage two is applied and held for 20 minutes. During this stage, the material fully melts and flows, achieving densification.

[0046] Phase 3: Stress Relief and Shaping After the pressure holding period, the temperature is gradually reduced to 55°C at a rate of 3°C / min while maintaining the pressure, with the entire process lasting an appropriate amount of time. Once the temperature stabilizes, the pressure is released and the material is demolded to obtain the high-density composite board material.

[0047] Example 2 A method for preparing a high-density composite board material with added plant fibers, comprising the following steps, by weight: Step 1: Surface modification treatment of plant fibers; Plant fibers were sieved into three particle size specifications based on the surface layer, middle layer, and core layer: surface layer hemp fiber with a particle size finer than 150 mesh, middle layer straw powder with a particle size of 100 mesh, and core layer straw powder with a particle size of 50 mesh. Surface modification treatments were then applied to each of the three plant fibers. First, an alkali treatment was performed by immersing plant fibers of various specifications in a 4% sodium hydroxide solution and treating them in a constant temperature water bath at 75°C for 1 hour. After treatment, the fibers were filtered and separated, repeatedly washed with deionized water until neutral, and then dried in an oven at 85°C until constant weight.

[0048] Next, the plant fibers were treated with a silane coupling agent. A 2% (w / w) KH-570 ethanol solution (95% by volume) was prepared, and the alkali-treated plant fibers were immersed in the solution and treated with ultrasound for 45 minutes to ensure uniform distribution of the coupling agent. After treatment, the plant fibers were removed and dried at 85°C for 2 hours to obtain modified plant fibers.

[0049] Step 2: Premix preparation; Three types of premixes were prepared according to gradient distribution requirements: Surface premix: 100 parts polypropylene, 18 parts modified hemp fiber (particle size finer than 150 mesh), 3 parts maleic anhydride-grafted PP, and 2 parts polyethylene wax. Intermediate layer premix: 100 parts polypropylene, 32 parts modified straw powder (particle size 100 mesh), 3 parts maleic anhydride grafted PP, 2 parts polyethylene wax. Core layer premix: 100 parts polypropylene, 45 parts modified straw powder (50 mesh particle size), 3 parts maleic anhydride grafted PP, and 2 parts polyethylene wax. Each layer of raw material was separately fed into a high-speed mixer and mixed at 80°C for 15 minutes to ensure that each component was fully and evenly mixed, resulting in three premixes with different properties.

[0050] Step 3: Layering, interface fusion, and oscillating pressure molding; Premixed materials are laid in the mold in the following order: surface layer - intermediate layer - core layer - intermediate layer - surface layer. The thickness ratio of the surface layer, intermediate layer, and core layer is 1:0.9:1.8. After each layer of premixed material is laid, an interface bonding agent composed of 8 parts of nanocellulose whiskers and 92 parts of polyurethane emulsion is evenly applied using a spraying device. The application rate per unit area is controlled at 90 g / m². 2 Ensure that the bonding agent is evenly distributed between each layer, without any accumulation or missing areas.

[0051] The mold containing the premixed material is fed into a hot press and molded according to a three-stage process: Phase 1: Immersion and Exhaust The mold temperature was raised to 160℃, an initial pressure of 5MPa was applied, and pressure oscillation at a frequency of 0.2Hz was applied simultaneously, and the pressure was held for 3 minutes. During this stage, obvious water vapor was observed to be discharged. After the venting basically stopped, the next stage was initiated.

[0052] Phase Two: Gradient Compaction The mold temperature is raised to 190℃, and the pressure is gradually increased to 18MPa. Simultaneously, pressure oscillation with a frequency of 1.5Hz and an amplitude of 10% of the target pressure value for stage two is applied and held at pressure for 15 minutes. During this stage, the material fully melts and flows, achieving densification.

[0053] Phase 3: Stress Relief and Shaping After the pressure holding period, the temperature is gradually reduced to 50°C at a rate of 2°C / min while maintaining the pressure, and the entire process is carried out for an appropriate time. After the temperature stabilizes, the pressure is released and the material is demolded to obtain the high-density composite board material.

[0054] Example 3 A method for preparing a high-density composite board material with added plant fibers, comprising the following steps, by weight: Step 1: Surface modification treatment of plant fibers; Wood chips were sieved into three particle size specifications based on the surface layer, middle layer, and core layer: surface layer wood chips with a particle size finer than 150 mesh, middle layer wood chips with a particle size of 120 mesh, and core layer wood chips with a particle size of 70 mesh. Surface modification treatments were then applied to the three types of plant fibers respectively. First, an alkaline treatment was performed by immersing plant fibers of various specifications in a 6% sodium hydroxide solution and treating them in a constant temperature water bath at 65°C for 2 hours. After treatment, the fibers were filtered and separated, repeatedly washed with deionized water until neutral, and then dried in an oven at 85°C until constant weight.

[0055] Next, the plant fibers were treated with a silane coupling agent. A 4% KH-570 ethanol solution (95% ethanol by volume) was prepared, and the alkali-treated plant fibers were immersed in the solution and treated with ultrasound for 45 minutes to ensure uniform distribution of the coupling agent. After treatment, the plant fibers were removed and dried at 85°C for 2 hours to obtain modified plant fibers.

[0056] Step 2: Premix preparation; Three types of premixes were prepared according to gradient distribution requirements: Surface premix: 100 parts polylactic acid, 18 parts modified hemp fiber (particle size finer than 150 mesh), 6 parts maleic anhydride-grafted PLA, and 0.8 parts calcium stearate; Intermediate layer premix: 100 parts polylactic acid, 32 parts modified straw powder (100 mesh particle size), 6 parts maleic anhydride grafted PLA, and 0.8 parts calcium stearate; Core layer premix: 100 parts polylactic acid, 45 parts modified straw powder (50 mesh), 6 parts maleic anhydride-grafted PLA, and 0.8 parts calcium stearate; Each layer of raw material was separately fed into a high-speed mixer and mixed at 75°C for 25 minutes to ensure that each component was fully and evenly mixed, resulting in three premixes with different properties.

[0057] Step 3: Layering, interface fusion, and oscillating pressure molding; Premixed materials are laid in the mold in the following order: surface layer - intermediate layer - core layer - intermediate layer - surface layer. The thickness ratio of the surface layer, intermediate layer, and core layer is 1:1.1:2. After each layer of premixed material is laid, an interface bonding agent composed of 3 parts of nanocellulose whiskers and 97 parts of ethylene-vinyl acetate emulsion is evenly applied using a spraying device. The application rate per unit area is controlled at 110 g / m². 2 Ensure that the bonding agent is evenly distributed between each layer, without any accumulation or missing areas.

[0058] The mold containing the premixed material is fed into a hot press and molded according to a three-stage process: Phase 1: Immersion and Exhaust The mold temperature was raised to 155℃, and an initial pressure of 6MPa was applied, along with pressure oscillation at a frequency of 0.05Hz, which was maintained for 6 minutes. Significant water vapor was observed to be released during this stage. The next stage was initiated after the venting process had largely ceased.

[0059] Phase Two: Gradient Compaction The mold temperature was raised to 185℃, and the pressure was gradually increased to 22MPa. Simultaneously, pressure oscillation with a frequency of 0.8Hz and an amplitude of 8% of the target pressure value for stage two was applied, and the pressure was held for 25 minutes. During this stage, the material fully melted and flowed, achieving densification.

[0060] Phase 3: Stress Relief and Shaping After the pressure holding period, the temperature is gradually reduced to 58°C at a rate of 4°C / min while maintaining the pressure, and the entire process is carried out for an appropriate time. After the temperature stabilizes, the pressure is released and the material is demolded to obtain the high-density composite board material.

[0061] Example 4 A method for preparing a high-density composite board material with added plant fibers, comprising the following steps, by weight: Step 1: Surface modification treatment of plant fibers; Coconut husk fibers were sieved into three particle size specifications based on the surface layer, middle layer, and core layer: surface layer fibers with a particle size finer than 150 mesh, middle layer fibers with a particle size of 80 mesh, and core layer fibers with a particle size of 40 mesh. Surface modification treatments were then applied to each of the three plant fibers. First, an alkali treatment was performed by immersing plant fibers of various specifications in an 8% sodium hydroxide solution and treating them in a constant temperature water bath at 85°C for 0.5 hours. After treatment, the fibers were filtered and separated, repeatedly washed with deionized water until neutral, and then dried in an oven at 85°C until constant weight.

[0062] Next, the plant fibers were treated with a silane coupling agent. A 5% KH-550 ethanol solution (95% by volume) was prepared, and the alkali-treated plant fibers were immersed in the solution and treated with ultrasound for 45 minutes to ensure uniform distribution of the coupling agent. After treatment, the plant fibers were removed and dried at 85°C for 2 hours to obtain modified plant fibers.

[0063] Step 2: Premix preparation; Three types of premixes were prepared according to gradient distribution requirements: Surface premix: 100 parts polypropylene, 25 parts modified coconut shell fiber (particle size finer than 150 mesh), 8 parts maleic anhydride-grafted PP, and 3 parts zinc stearate. Intermediate layer premix: 100 parts polypropylene, 39 parts modified coconut shell fiber (80 mesh particle size), 8 parts maleic anhydride-grafted PP, and 3 parts zinc stearate. Core layer premix: 100 parts polypropylene, 60 parts modified coconut shell fiber (40 mesh particle size), 8 parts maleic anhydride-grafted PP, and 3 parts zinc stearate. Each layer of raw material was separately fed into a high-speed mixer and mixed at 95°C for 10 minutes to ensure that each component was fully and evenly mixed, resulting in three premixes with different properties.

[0064] Step 3: Layering, interface fusion, and oscillating pressure molding; The premixed material is laid in the mold in the following order: surface layer - intermediate layer - core layer - intermediate layer - surface layer. The thickness ratio of the surface layer, intermediate layer, and core layer is 1:0.8:2.5. After each layer of premixed material is laid, an interface bonding agent composed of 10 parts of nanocellulose whiskers and 90 parts of acrylic emulsion is evenly applied using a spraying device. The application rate per unit area is controlled at 120g / m². 2 Ensure that the bonding agent is evenly distributed between each layer, without any accumulation or missing areas.

[0065] The mold containing the premixed material is fed into a hot press and molded according to a three-stage process: Phase 1: Immersion and Exhaust The mold temperature was raised to 190℃, and an initial pressure of 8MPa was applied, along with pressure oscillation at a frequency of 0.5Hz, which was maintained for 2 minutes. Significant water vapor was observed to be released during this stage. The next stage was initiated after the venting process had largely ceased.

[0066] Phase Two: Gradient Compaction The mold temperature is raised to 200℃, and the pressure is gradually increased to 30MPa. Simultaneously, pressure oscillation with a frequency of 2Hz and an amplitude of 20% of the target pressure value for stage two is applied and held for 10 minutes. During this stage, the material fully melts and flows, achieving densification.

[0067] Phase 3: Stress Relief and Shaping After the pressure holding period, the temperature is gradually reduced to 60°C at a rate of 1°C / min while maintaining the pressure, and the entire process is carried out for an appropriate time. After the temperature stabilizes, the pressure is released and the material is demolded to obtain the high-density composite board material.

[0068] Comparative Example 1 The difference from Example 1 is that the plant fiber content of all premixed layers is uniformly 32 parts, and the particle size is uniformly 100 mesh, forming a homogeneous single-layer structure board without distinguishing between the surface layer, middle layer and core layer.

[0069] Comparative Example 2 The difference from Example 1 is that the plant fiber content and particle size of the surface mixture are the same as those of the core mixture, which is 50 parts and the particle size is 60 mesh.

[0070] Comparative Example 3 The difference from Example 1 is that the plant fiber content and particle size of the core layer mixture are the same as those of the surface layer mixture, with a content of 20 parts and a particle size finer than 150 mesh.

[0071] Comparative Example 4 The difference from Example 1 is that no interface bonding agent is applied during the layering process, and the premixed layers are in direct contact.

[0072] Comparative Example 5 The difference from Example 1 is that, during the multilayer laying, the application rate of the interface bonding agent per unit area is 40 g / m². 2 .

[0073] Comparative Example 6 The difference from Example 1 is that, during the multilayer laying, the application rate of the interface bonding agent per unit area is 130 g / m². 2 .

[0074] Comparative Example 7 The difference from Example 1 is that a conventional constant pressure process is used in the molding stage, that is, no pressure oscillation is applied in stage one and stage two, only a constant pressure is maintained.

[0075] Comparative Example 8 The difference from Example 1 is that all surface modification treatments in step one are omitted, and unmodified plant fibers are used directly.

[0076] Comparative Example 9 The difference from Example 1 is that the mold temperature is raised to 280°C during the gradient densification stage, which is higher than the 250-270°C range set for polyethylene terephthalate (PET) in Example 1.

[0077] Comparative Example 10 The difference from Example 2 is that the mold temperature is raised to 210°C during the gradient densification stage, which is higher than the 180-200°C range set for polypropylene (PP) in Example 2.

[0078] Comparative Example 11 The difference from Example 3 is that the mold temperature is raised to 200°C during the gradient densification stage, which is higher than the 170-190°C range set for polylactic acid (PLA) in Example 3.

[0079] Comparative Example 12 The difference from Example 1 is that the pressure is released directly after the hot pressing in stage three, and the mold is forced to cool to below 60°C for demolding.

[0080] Comparative Example 13 The difference from Example 1 is that the number of plant fiber parts in the core layer is increased to 70 parts.

[0081] Performance testing: Density and density deviation: Tested according to the methods specified in GB / T 31765-2015 "High-density fiberboard".

[0082] Internal bond strength: Tested according to the method specified in GB / T 17657 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels".

[0083] Interlaminar shear / peel strength: Tested according to the method specified in GB / T 2790 "Adhesives 180° Peel Strength Test Method Flexible Materials vs. Rigid Materials".

[0084] Internal residual stress / strain monitoring: monitored by embedded FBG sensor.

[0085] The test results are shown in Table 1.

[0086] Table 1 Performance test results of the examples and comparative examples

[0087] The embodiments, through the synergistic effect of gradient structure design, interface fusion technology, and oscillating pressure molding process, all exhibit higher density, excellent internal bonding strength and interlayer bonding force, and lower residual strain, with overall performance significantly superior to the comparative examples. Specifically, the gradient structure effectively optimizes the stress distribution within the material, the interface fusion agent strengthens the interlayer bonding through chemical bridging and physical entanglement, while the oscillating pressure and programmed cooling processes respectively promote material densification and internal stress relaxation.

[0088] Compared to the examples where all parameters were within the optimized range, the performance of each comparative example showed a predictable theoretical decline due to specific defects in material structure or processing. Comparative Example 1, lacking a gradient structure, resulted in uneven internal stress distribution, thus deteriorating overall mechanical properties and dimensional stability. Comparative Examples 2 and 3, due to flawed gradient design, either had excessively rigid surface layers or weak core layers, disrupting the material's rational mechanical distribution and stress buffering mechanism, significantly impairing interlayer bonding. Comparative Examples 4, 5, and 6 collectively demonstrated the crucial role of interfacial bonding agents; their complete absence, insufficient dosage, or excessive dosage directly led to deterioration of interlayer shear strength. Comparative Example 7, lacking oscillating pressure, resulted in insufficient material venting and melt penetration, affecting material density and uniformity. Comparative Example 8, omitting fiber surface modification, led to poor fiber-matrix interfacial compatibility, severely weakening internal bonding strength. Comparative Examples 9, 10, and 11 all experienced thermal degradation of the plant fiber or polymer matrix due to excessively high processing temperatures, directly causing damage and performance degradation to the material itself. Comparative Example 12, due to the use of forced cooling instead of programmed cooling, failed to effectively relax the internal stress caused by the difference in thermal expansion coefficients, resulting in a sharp increase in residual strain. Comparative Example 13, on the other hand, had an excessively high core fiber content, exceeding the wetting and covering capacity of the matrix, which instead introduced defects and damaged the material's density.

Claims

1. A high-density composite board material with added plant fibers, characterized in that, It includes at least a surface layer and a core layer, and its structure is surface layer-core layer-surface layer; An interface bonding agent is applied between the surface layer and the core layer, and the thickness ratio of the surface layer to the core layer is 1:1.5-2.5; The high-density composite board material further includes at least one intermediate layer, which is located between the surface layer and the core layer. An interface bonding agent is applied between each layer, and the thickness ratio of the surface layer, intermediate layer and core layer is 1:(0.8-1.2):(1.5-2.5). The interface fusion agent comprises 3-10 parts by weight of nanocellulose whiskers and 90-97 parts by weight of polymer emulsion; By weight, The surface layer, intermediate layer, and core layer each comprise 100 parts of polymer matrix, 2-8 parts of compatibilizer, and 0.5-3 parts of lubricant, and respectively comprise 15-25 parts of modified plant fiber with a particle size finer than 150 mesh, 26-39 parts of modified plant fiber with a particle size of 80-150 mesh, and 40-60 parts of modified plant fiber with a particle size of 40-80 mesh. The polymer matrix is ​​one of polyethylene terephthalate, polypropylene, or polylactic acid; the compatibilizer is a maleic anhydride graft compatibilizer corresponding to the polymer matrix; the lubricant is one of zinc stearate, calcium stearate, or polyethylene wax; and the polymer emulsion of the interface fusion agent is one of acrylate emulsion, polyurethane emulsion, or ethylene-vinyl acetate emulsion. The modified plant fiber is a plant fiber modified with a silane coupling agent.

2. The high-density composite board material as described in claim 1, characterized in that, The amount of interfacial bonding agent is 80-120 g / m³. 2 .

3. The high-density composite board material as described in claim 1, characterized in that, The plant fiber is derived from one or more of bamboo powder, hemp fiber, straw powder, sawdust, or coconut shell fiber.

4. A method for preparing a high-density composite board material as described in any one of claims 1-3, characterized in that, Includes the following steps: Step 1: Surface modification treatment of plant fibers: The plant fibers are sieved into three grades: 40-80 mesh, 80-150 mesh and finer than 150 mesh. After being modified by silane coupling agent, the modified plant fibers are obtained for later use. Step 2: Prepare the premix: Prepare the premix according to the corresponding proportions of the surface layer, intermediate layer, and core layer; Step 3: Molding: Stack the layers according to the required structure, apply an interface bonding agent between the layers, and then hot press to obtain a high-density composite board material with added plant fiber.

5. The method for preparing the high-density composite board material as described in claim 4, characterized in that, The desired structure is one of the following: surface layer-core layer-surface layer, surface layer-intermediate layer-core layer-surface layer, or surface layer-intermediate layer-core layer-intermediate layer-surface layer.

6. The method for preparing the high-density composite board material as described in claim 4, characterized in that, In step one, the surface modification treatment includes the following steps: Alkali treatment steps: Treat plant fibers with a sodium hydroxide solution of 3-8% by mass at 60-85℃ for 0.5-2.5 hours; Silane coupling agent treatment: Impregnation or spraying with a silane coupling agent solution with a mass fraction of 1%-5% and a volume fraction of 95% ethanol aqueous solution; the silane coupling agent is KH-550 or KH570.

7. The method for preparing the high-density composite board material as described in claim 4, characterized in that, The process of preparing the premix includes: placing modified plant fibers, polymer matrix, compatibilizer and lubricant in a high-speed mixer and mixing at 75-95℃ for 10-30 minutes to obtain the premix.

8. The method for preparing the high-density composite board material according to claim 4, characterized in that, Step three, hot pressing, specifically includes the following stages: Phase 1, Impregnation and Degassing Phase: Raise the mold temperature to 150-190℃, apply an initial pressure of 3-8MPa, and apply pressure oscillation at a frequency of 0.05-0.5Hz under this pressure for 2-8 minutes. Phase 2, Gradient Densification Phase: Raise the mold temperature to 10-30°C above the melting temperature of the polymer matrix, increase the pressure stepwise to 15-30 MPa, and apply pressure oscillation with a frequency of 0.5-2 Hz and an amplitude of 5-20% of the target pressure value of Phase 2 during this phase, with a holding time of 10-30 min. Phase 3, Stress Relief and Shaping Stage: After the pressure holding period ends, while maintaining the pressure, the mold temperature is reduced to below 60°C at a rate of 1-5°C / min, and then the pressure is released and the mold is demolded. During the gradient densification stage, the mold temperature is raised to 250-270°C for the polymer matrix being polyethylene terephthalate; to 180-200°C for the polymer matrix being polypropylene; and to 170-190°C for the polymer matrix being polylactic acid.

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