A method for producing natural fiberboard

Abstract

This invention relates to a natural fiber reinforced composite material sheet and its manufacturing method, applicable to the automotive, drone, architectural decoration, and consumer electronics industries. The sheet uses natural fiber fabrics such as flax, hemp, jute, sisal, or bamboo fiber as the reinforcing layer, which is cured with a resin matrix. The reinforcing layer can consist of 1 to 10 layers of natural fiber fabric, and plain weave, twill weave, satin weave, or multi-axial weave can be used as needed to obtain good tensile, bending, and interlayer bonding properties. This invention improves fiber impregnation uniformity and reduces moisture absorption through resin system modification, fiber surface treatment, and vacuum-assisted curing, overcoming the shortcomings of natural fiber composite materials in terms of interfacial bonding, dimensional stability, and process consistency. The resulting sheet is lightweight, has high damping, good electromagnetic wave transmission, and is environmentally friendly and low-carbon, and can be used in battery protection panels, drone arm tubes, sound insulation panels, and automotive interior cladding.

Description

Technical Field

[0001] This invention relates to the field of composite materials, characterized by thin-walled composite material sheets made of natural fibers and resins, having sheet-like, plate-like, or three-dimensional shell shapes. Other types of natural fiber composite products also fall within the scope of this invention. These composite materials are characterized by moderate weight, high specific strength, high damping, environmental friendliness, insulation, and recyclability. Such composite material products can be applied to automotive body parts, sporting goods, furniture parts, ship hulls, or aircraft parts, etc. Technical Background

[0002] Composite materials, due to their advantages such as lightweight, high strength, and corrosion resistance, have been widely used in aerospace, automotive manufacturing, rail transportation, architectural decoration, sporting goods, and electronic equipment structural components. Among them, high-performance chemical fibers such as carbon fiber, glass fiber, and aramid fiber (Kevlar) are the mainstream reinforcing materials in current industrial applications. Carbon fiber has extremely high specific strength and specific modulus, making it suitable for weight-sensitive components requiring high structural rigidity; glass fiber, due to its lower cost and good strength, is widely used in automotive exterior parts, appliance housings, wind turbine blades, and building profiles; and aramid fiber, with its excellent impact resistance and cut resistance, is used in protective equipment, impact-resistant structures, and weight-reduction-required equipment.

[0003] The application of these chemically reinforced fibers aims to address the problems of high density, poor corrosion resistance, and limited formability of traditional metal materials. However, as industrial applications increasingly demand higher comprehensive material performance, chemically-reinforced fiber materials are gradually revealing various limitations. For example, both carbon fiber and aramid fiber exhibit poor damping capabilities, making it difficult to provide good dynamic comfort and reduce noise in vibrating environments. Glass fiber and carbon fiber lack sufficient toughness in layered composite structures, making them prone to brittle fracture or interlaminar cracking, thus limiting their further application in impact resistance and fatigue resistance. Furthermore, these chemical fibers have strong shielding or reflection effects on electromagnetic waves, making them unsuitable for applications requiring high wireless signal transmittance, such as millimeter-wave radar housings, drone radomes, and smart device housings, thus restricting their design and placement.

[0004] Faced with these unmet needs, natural fiber composite materials have received increasing attention in recent years. Natural fibers, represented by flax (and also including pillars, sugarcane jute, ramie, sisal, etc.), are becoming a key complementary direction in the field of composite materials due to their low density, high intrinsic damping, excellent toughness, low specific dielectric constant, good electromagnetic wave penetration, and renewable and biodegradable material properties. These natural fibers demonstrate excellent shock absorption and structural energy absorption potential in automotive exterior panels, interior structural panels, appliance brackets, architectural interior decoration, sports equipment, and outdoor applications, making them particularly suitable for civilian applications that prioritize comfort, dynamic performance, and human-machine interface. Developing prepregs and structural panels based on natural fibers can provide material solutions that simultaneously balance mechanical performance, environmental friendliness, and functionality for lightweight automotive exterior panels, vehicle millimeter-wave radar brackets, intelligent equipment housings, and architectural interior panels.

[0005] It is important to emphasize that, regardless of whether it is a chemical fiber composite or a natural fiber composite, its final core properties such as strength, toughness, heat resistance, and moisture resistance still depend on the resin system and curing process used. For example, epoxy resin systems can achieve high strength, low water absorption, and excellent interfacial bonding; unsaturated polyester resin systems are more suitable for mid-range structural parts requiring large-scale rapid prototyping; and modified polypropylene systems are suitable for automotive interior parts with high requirements for impact resistance and recyclability. Different resin systems determine the structural level and service life of the final product, giving natural fiber prepregs a wider design scope in the ever-evolving process systems.

[0006] However, existing natural fiber reinforced composite materials generally suffer from the following problems in their engineering preparation: high moisture content and unstable surface activity of natural fibers lead to low resin wetting efficiency and insufficient interfacial bonding strength; diverse fabric structures lack a clear range of layup parameters, easily causing uneven layer thickness and fluctuations in fiber volume fraction; different curing processes significantly affect compaction and interlayer density, while a process control window suitable for natural fibers is lacking. These factors result in poor molding consistency in the mass production of existing natural fiber boards, limiting their widespread application in automotive structural components, consumer electronics casings, and industrial sheets. Summary of the Invention

[0007] The purpose of this invention is to provide a natural fiber reinforced composite board and its preparation method. By clarifying the reinforcing layer structure, resin impregnation method and curing conditions, the natural fiber fabric can form a uniform, continuous and stable interfacial laminate under controllable process parameters, thereby improving the molding stability and manufacturing repeatability of the material in thin-walled structure applications.

[0008] To achieve the above objectives, the present invention proposes the following technical solution:

[0009] 1) The natural fiber reinforced composite board provided by the present invention is composed of 1 to 10 layers of natural fiber fabric, wherein the natural fiber fabric is selected from flax, hemp, jute, ramie, sisal, bamboo fiber or a mixture thereof;

[0010] 2) The fabric can be plain weave, twill weave, satin weave or multi-axial weave structure, and the unit area mass of a single layer is 150–800 g / m².

[0011] 3) The resin matrix adopts an epoxy resin system, which is used to impregnate natural fiber fabrics through dry cloth impregnation or prepreg process and then cured to form a thin-walled laminated structure.

[0012] In the dry impregnation process, natural fiber fabrics are dried, cut, and laid out at a preset angle. Impregnation is achieved through methods such as vacuum-assisted resin infusion or resin film infusion, followed by curing at room temperature or under heating conditions to form a board. This process can effectively impregnate the fiber layers under low pressure and is suitable for the manufacture of medium-sized or multi-specification boards.

[0013] In the prepreg process, the mass ratio of natural fiber to resin is 50:50 to 60:40. Natural fiber fabric is pre-coated with resin (stage B) to form a storable and cut prepreg. After layup, it is placed in an autoclave under vacuum sealing and cured at 80–160°C. Positive pressure can be applied during curing to improve interlayer compaction, resulting in boards with higher thickness uniformity and lamination stability. This process is suitable for the mass production of thin sheet materials requiring high dimensional accuracy and surface quality.

[0014] The technical contributions of this invention are mainly reflected in the following aspects:

[0015] 1) A defined range of lay-up and fabric structures suitable for natural fiber boards is proposed. By limiting the number of layers, basis weight, and weaving method, the reinforcing layer can maintain a continuous and uniform fiber distribution during resin impregnation, thereby providing a structural basis for the basic load-bearing capacity and stable molding of the board.

[0016] 2) A resin impregnation and curing pathway tailored to the characteristics of natural fibers was established. Two feasible routes—dry cloth impregnation and prepreg processes—enable stable resin content and interfacial bonding quality in natural fibers under both low-pressure and hot-pressing conditions, solving the problems of low impregnation efficiency and unclear curing windows in existing processes.

[0017] 3) A controllable molding system for thin-walled composite panels has been developed. By coordinating the layup sequence, vacuum sealing, curing temperature, and pressure parameters, this invention enables the panels to achieve a continuous laminated structure and good dimensional consistency, thereby improving the engineering applicability of natural fiber panels in structural and aesthetic components.

[0018] 4) This invention provides a basic material system and process framework for the mass production of natural fiber composite materials. The plate definition and process flow of this invention can be implemented under existing composite material molding equipment conditions, possessing replicability and industrialization potential, and providing a technical foundation for the transition of natural fiber composite materials from demonstration applications to large-scale production. Detailed Implementation

[0019] The following examples further illustrate the natural fiber reinforced composite board and its preparation method provided by the present invention. Those skilled in the art should understand that the following examples are for illustrative purposes only and are not intended to limit the invention; process parameters and structural forms can be appropriately adjusted without departing from the spirit of the invention.

[0020] Example 1: Preparation of natural fiber reinforced composite boards using a dry cloth impregnation process

[0021] In this embodiment, a thin-walled composite board with a thickness of approximately 1.5 mm is prepared using a three-layer natural fiber fabric as the reinforcing layer and a dry fabric epoxy impregnation process.

[0022] Plain weave flax fiber fabric with a unit area mass of 300 g / m² was selected and dried in a 60℃ hot air environment for 2 hours to control its moisture content below 2 wt%. According to the size requirements of the board, the fabric was cut into the required specifications along the warp and weft directions.

[0023] The three layers of fabric are then laid on the mold surface at layup angles of 0° / 90° / 0° to ensure that the fabric is flat, adheres well, and is wrinkle-free.

[0024] Liquid epoxy resin, curing agent, and accelerator are mixed in proportion, and an appropriate amount of toughening agent is added to adjust the viscosity of the system to suit the impregnation process. The resulting resin system is in a low-viscosity state at room temperature, which facilitates its penetration into the fiber gaps under vacuum assistance.

[0025] Cover the mold with a vacuum bag and seal it. Connect the vacuum system to achieve a vacuum level of -0.095 MPa inside the mold cavity. Inject the prepared epoxy resin into the mold through the resin inlet, allowing the resin to penetrate and impregnate each layer of fabric under vacuum.

[0026] After the resin fills the mold cavity, it is kept under vacuum and allowed to stand to ensure uniform impregnation. It is then cured at room temperature for 12 hours. After curing, the seal is removed and the board is demolded. The resulting board has a smooth surface and the resin and natural fibers are evenly bonded.

[0027] After demolding, the edges of the sheet material are trimmed, and polishing, spraying, or other surface treatments can be performed as needed for the application. The sheet material obtained in this embodiment can be used for automotive interior structural parts, lightweight decorative panels, and other applications.

[0028] Example 2: Preparation of natural fiber reinforced composite boards using prepreg process

[0029] In this embodiment, a composite board with high dimensional consistency is prepared by using five layers of natural fiber fabric as the reinforcing layer and using resin B-stage prepreg and autoclave curing. The mass ratio of natural fiber to resin is 50:50 to 60:40.

[0030] A twill fabric of flax fibers with a unit area mass of 220 g / m² was selected, and epoxy resin was coated onto the fiber surface by roller impregnation. The resin system was in stage B, and the prepreg was in a semi-cured state that could be cut and stored. The mass ratio of natural fiber to resin was controlled at 54:46.

[0031] The resulting prepreg can be stored for more than 30 days at −18°C.

[0032] After the prepreg has been brought to room temperature, it is cut and laid in five layers in the order of 0° / ±45° / 0°, ensuring that each layer fully fits the mold cavity.

[0033] After covering the layup with a release film and breathable felt, the mold is vacuum-sealed to achieve a pressure of −0.09 MPa in the mold cavity. The sealed mold is then placed in an autoclave and cured at 120°C for 90 min, with a positive pressure of 0.4 MPa applied to increase compaction.

[0034] After curing, cool the temperature to below 60℃, release the vacuum, and demold.

[0035] The composite board obtained in this embodiment has a tight interlayer structure and high thickness stability, and can be used for thin sheet parts that require high dimensional consistency, such as drone shells or consumer electronics panels.

[0036] Example 3: Specific preparation example of natural fiber reinforced composite prepreg

[0037] To verify the processability of the prepreg of the present invention, a batch of typical natural fiber prepregs was prepared in this embodiment for subsequent hot pressing or molding.

[0038] Plain weave flax fiber fabric with a unit area mass of 200 g / m² was selected as the reinforcement. The resin matrix consisted of epoxy resin, acid anhydride curing agent, and accelerator, with a small amount of toughening agent added to adjust the initial flowability.

[0039] Resin was uniformly coated onto natural fiber fabric using a thin-film lamination method, allowing it to reach the B-stage state under controlled temperature. The resulting prepreg had a uniform thickness, with a fiber-to-resin mass ratio ranging from 35:65 to 55:45.

[0040] Prepreg rolls, when stored at −18°C after packaging, can maintain resin reactivity for up to 30 days. In practical use, they can be directly cut to size and used for thermoforming.

[0041] Example 4: Post-processing of sheet materials

[0042] The boards obtained in Examples 1 and 2 were subjected to edge removal and surface grinding. Localized surface pores were repaired with epoxy adhesive from the same system, and a transparent or matte coating was further sprayed to meet decorative requirements. The boards of this invention exhibit good processability and can adapt to secondary processing steps such as cutting, drilling, hot bending, and overmolding.

Claims

1. A method for preparing a natural fiber reinforced composite board, characterized in that, The natural fiber fabric is dried to control its moisture content below 2 wt%. Within a limited time window after drying, 1 to 10 layers of the natural fiber fabric are laid in a mold at a preset layup angle. The natural fiber fabric is impregnated with an epoxy resin system or through a prepreg process. The structure after layup is vacuum-sealed to achieve a vacuum degree of -0.09 to -0.098 MPa in the mold cavity, and the vacuum is maintained for at least 5 to 15 minutes after the resin impregnation is complete. The impregnated structure is cured and shaped under vacuum conditions or vacuum combined with external pressure to obtain a natural fiber reinforced composite board.

2. The method according to claim 1, wherein the defined time window does not exceed 24 hours.

3. The method according to claim 1, wherein the curing step is carried out at 80–160°C, and an external pressure of 0.2–0.6 MPa is applied during the curing process to increase the interlayer compaction.

4. The method of claim 1, wherein during the vacuum holding phase, the resin viscosity is in the low viscosity range that allows it to permeate natural fiber fabrics.

5. The method according to claim 1, wherein the thickness deviation of the resulting composite board is not greater than ±0.1 mm.

6. A natural fiber reinforced composite board, characterized in that, The composite material is prepared by the method described in any one of claims 1 to 5.