Preparation method of foamed composite board with natural fiber prefabricated three-dimensional skeleton

Abstract

The invention relates to a composite material manufacturing technology, and provides a natural fiber three-dimensional reinforced composite board and a manufacturing method thereof. The method comprises the steps that natural fiber woven cloth is laid in a forming mold with a corrugated, arch-shaped or rib-shaped outline, epoxy resin is applied for dipping and curing, and the woven cloth forms a hard framework with a fixed three-dimensional structure; the framework is placed in a foaming mold, and the spatial position of the framework is kept through a positioning structure; and injecting polyurethane, polyethylene, PET foaming materials or other polymer foaming systems into the mold, so that the filling material permeates and fills a gap between the mold and the skeleton in the expansion or curing process, and the filling material and the natural fiber skeleton are integrally formed to obtain the composite board. The plate internally comprises a stable natural fiber three-dimensional supporting structure, the out-of-plane compression performance and the shear bearing capacity of the foam core material can be improved, and the plate is suitable for the fields of sports equipment, vehicle interiors, light structural plates and the like.

Description

Technical Field

[0001] This invention relates to the field of composite material manufacturing technology, specifically to a three-dimensional reinforced composite board and its manufacturing method, which is formed by prefabricating a three-dimensional skeleton structure with natural fibers and integrally molding it with polymer foam material. Background Technology

[0002] Natural fiber-reinforced composites, due to their low density, renewability, and good specific strength and stiffness, show application potential in sports equipment, transportation, and building materials. Common natural fibers include flax, ramie, sisal, jute, and bamboo fiber, which are typically used in combination with polymer matrices in the form of woven fabrics, knitted fabrics, felts, or chopped fibers to improve the material's stiffness and strength. In addition, thermoplastic foam materials such as polypropylene (PP), polyethylene (PE), polyethylene terephthalate (PET), and ethylene-vinyl acetate copolymer (EVA) are widely used as core materials in sandwich structures and weight-reducing structures due to their lightweight, good processability, and recyclability.

[0003] Existing natural fiber reinforced composite materials mainly have two-dimensional laminated structures. These are typically formed by laying natural fiber reinforcement layers on the surface of polymer sheets or foam cores, followed by methods such as hot pressing, resin impregnation, or film lamination to create planar reinforced composite panels. In these structures, the fibers are mainly distributed along the plane of the panel, and their reinforcement is concentrated in the in-plane direction, with limited contribution to out-of-plane forces. To obtain higher out-of-plane compressive or shear strength, existing solutions often require increasing the number of fiber layers or the resin content, leading to increased material density and processing energy consumption.

[0004] On the other hand, since natural fiber weaves are usually located on the surface of the board or outside the core material, existing technologies make it difficult to form a continuous three-dimensional support structure of natural fibers inside the foam material. Under external forces, the lack of longitudinal or spatially connected reinforcing channels within the foam core material makes it prone to problems such as localized crushing, delamination, or interfacial peeling. To improve the out-of-plane properties of the foam core material, some technical solutions attempt to strengthen it by increasing foam density, using high-strength synthetic fibers, or employing additional mechanical bonding methods; however, these methods increase material costs, weight, or processing complexity.

[0005] In summary, existing natural fiber reinforced materials typically employ two-dimensional layered reinforcement methods, making it difficult to form a three-dimensional reinforced structure that integrates with foam materials. This results in limitations in the material's out-of-plane load-bearing capacity, shear resistance, and structural stability. Furthermore, there is currently a lack of structural design and manufacturing methods capable of constructing three-dimensional natural fiber reinforced layers during foam molding or processing. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a three-dimensional reinforced composite board made of natural fibers and its manufacturing method. This method involves pre-impregnating a natural fiber woven fabric with resin and curing it into a rigid skeleton structure with a specific three-dimensional shape, then integrally molding it with a foaming material in a mold, thereby giving the board an out-of-plane reinforced structure.

[0007] The manufacturing method of the present invention includes the following steps:

[0008] 1) Prefabrication of three-dimensional skeleton of natural fiber: Provide natural fiber woven fabric and lay it on a mold or forming frame with corrugated, arched, ribbed or other three-dimensional contours to form a stable multi-dimensional geometric structure in the thickness direction of the woven fabric; apply prepreg hot pressing or epoxy resin system to the woven fabric to allow the resin to penetrate into the fiber gaps and form a cured connection at the fiber intersection; heat and cure the resin-impregnated woven fabric to obtain the core layer of natural fiber rigid reinforced board with a fixed spatial shape.

[0009] 2) Integrated molding of foamed materials: The cured natural fiber structure is placed in a metal mold and its spatial position is fixed by positioning blocks or limiting design; foamable polyurethane (PU), polyethylene (PE), PET foam or other thermoplastic foaming systems are injected into the mold; during heating, expansion or curing, the foamed material flows and fills the gaps between the natural fiber skeleton and the mold, thereby forming an integrated composite structure with the skeleton.

[0010] 3) Finishing and shaping of the board: After the foaming system has cured, the board is demolded and then cut, milled, chamfered or sanded to achieve the required size and surface quality.

[0011] In optional embodiments, the natural fibers may be flax, ramie, sisal, jute, or blends thereof, and the resin may be epoxy resin, phenolic resin, or bio-based resin. The geometry of the three-dimensional skeleton can be achieved through a wavy structure, a ribbed structure, or a zigzag structure to improve out-of-plane load-bearing capacity. The foaming material may be PU, PE, PET, EVA, or other reprocessable thermoplastic materials.

[0012] The beneficial effects of this invention are as follows: By prefabricating a three-dimensional rigid skeleton of natural fibers before foaming, the natural fibers form a spatial support framework within the board, thereby significantly improving the out-of-plane compression performance, shear load-bearing capacity, and structural stability of the foam material. Since the skeleton has a stable geometric shape after resin curing, it can maintain its spatial position during the foaming process, avoiding the structural collapse problem caused by uneven density in traditional foamed boards. This method eliminates the need for multi-layer stacking or cutting, simplifies the manufacturing process, enables the preparation of composite boards with high structural design freedom, and combines lightweight, recyclability, and high mechanical properties. Detailed Implementation

[0013] The technical solution of the present invention will be further described below with reference to embodiments, but the present invention is not limited to the contents shown in the following embodiments. Those skilled in the art can make various modifications or equivalent substitutions to the implementation methods without departing from the spirit of the present invention, and all such modifications or substitutions should fall within the protection scope of the present invention.

[0014] Example 1: Prefabrication of a natural fiber woven skeleton

[0015] Natural fiber woven fabric is selected as the reinforcing substrate. The natural fibers include flax, ramie, sisal, jute, or blends thereof, and the surface density of the woven fabric is preferably 200–400 g / m². The natural fiber woven fabric is cut into sheets corresponding to the size of the target board and laid on a molding die or metal frame with an undulating profile, so that the woven fabric forms a corrugated, arched, or ribbed three-dimensional structure in the thickness direction, thereby having a certain geometric stiffness in both the plane and the thickness direction.

[0016] Low-viscosity epoxy resin is selected as the impregnation resin system, with a room temperature viscosity preferably of 100–500 mPa·s. The epoxy resin and curing agent are mixed evenly at a mass ratio of 100:25, and then applied to the natural fiber woven fabric by brushing, rolling, or impregnation, allowing the resin to penetrate into the fiber gaps and coat the yarn surface. The amount of resin added is controlled to ensure that the resin mass fraction in the skeleton is 20%–40% to guarantee sufficient cured bonding at fiber intersections while retaining 5–30 mm of open pores to facilitate the flow and filling of subsequent foaming materials.

[0017] After resin application, the resin-coated woven fabric is held in a predetermined three-dimensional shape and cured at 60–80°C for 1–3 hours to obtain a rigid natural fiber skeleton with a fixed three-dimensional shape. This skeleton has multiple ribs or crest structures composed of woven tendons in the thickness direction, which can provide out-of-plane support and shear load-bearing capacity during subsequent foaming.

[0018] Example 2: Integrated molding of natural fiber rigid skeleton and foam material

[0019] The natural fiber rigid skeleton prepared in Example 1 is placed into a metal mold. The skeleton is fixed to the inner wall of the mold by positioning blocks or limiting pins, so that it maintains a predetermined three-dimensional shape in the mold cavity and forms a cavity area to be filled between the skeleton and the inner wall of the mold.

[0020] A polyurethane (PU) foam system is selected as the core material. The polyol component and isocyanate component are mixed in a predetermined ratio and stirred to form a homogeneous reaction system, which is then injected into the mold cavity through the mold gate. As the foaming reaction proceeds, the foam system expands, flows, and gradually fills the skeleton and the cavity of the mold, while simultaneously coating the surface of the skeleton ribs. By controlling the foaming amount and the mold cavity volume, the foam, after curing, fills the internal space of the mold without significant overfilling or overflow.

[0021] After the foaming system has finished reacting and fully cured, the mold is opened, and the material is demolded to obtain a three-dimensional reinforced composite board with an integrated natural fiber rigid skeleton and polyurethane foam. In this board, the epoxy resin pre-cured natural fiber woven skeleton forms a spatial rib structure, providing thickness-direction support and shear load-bearing path for the foam core material; the foam material fills the spaces between and around the ribs, playing a role in weight reduction, energy absorption, and shape retention.

[0022] Example 3: Cutting and Repair Processing of Three-Dimensional Reinforced Composite Panels

[0023] After removing the three-dimensional reinforced composite board obtained in Example 2 from the mold, the edges and surface of the board are dimensionally trimmed. First, the board is cut around its perimeter using a band saw or circular saw to achieve the predetermined planar dimensions. Depending on the board thickness and application requirements, the cutting speed is preferably controlled between 5 and 20 m / min to avoid foam damage or fiber rib tearing caused by high-speed cutting.

[0024] During the cutting process, the ribbed structure of the natural fiber skeleton and the foamed material together form a composite cross-section. The cut surface should remain continuous and intact, without large-area delamination or fiber pull-out. To improve cutting quality, woodworking saw blades with a tooth pitch of 4–10 TPI can be used to obtain a smoother cutting edge.

[0025] After the sheet material is cut to its outer shape, it is placed on a CNC milling machine or a manual milling machine for edge chamfering, surface thickness setting, and local finishing. The surface of the sheet material is lightly milled to remove uneven areas from the foaming process, ensuring that the sheet thickness tolerance is controlled within ±0.2mm.

[0026] The milled surface is then sanded, preferably with sandpaper grit of 120–240, to achieve a surface roughness of Ra 0.8–1.5 μm, meeting the requirements of subsequent bonding or surface lamination processes. Excessive pressure should be avoided during sanding to prevent damage to the natural fiber reinforcement near the surface.

[0027] If necessary, a thin layer of thermoplastic film or resin primer can be sprayed or roller-coated onto the surface of the board to enhance the adhesion between the board surface and the outer panel material. The final product is a three-dimensional reinforced composite board with stable dimensions, a smooth surface, and good out-of-plane load-bearing capacity, which can be used in applications such as core materials for sports equipment, interior panels for vehicles, and structural sandwich panels.

Claims

1. A method for manufacturing a three-dimensional reinforced foamed composite board made of natural fibers, characterized in that, Includes the following steps: 1) Prefabrication steps of natural fiber three-dimensional rigid skeleton: lay natural fiber woven fabric on a forming mold or forming frame with a periodic undulating profile along the length and / or width direction of the board, so that the natural fiber woven fabric forms a continuous undulating three-dimensional spatial structure in the thickness direction of the board. A curable resin system is applied to the natural fiber woven fabric for impregnation, and the curable resin system is cured while maintaining the three-dimensional spatial structure, so that the natural fibers form a rigid connection at the intersection, thereby obtaining a three-dimensional rigid skeleton of natural fibers with a fixed spatial shape. The resin mass fraction in the three-dimensional rigid skeleton of natural fibers is 20% to 40%, and multiple interconnected open pores are formed inside the three-dimensional rigid skeleton of natural fibers. The characteristic size of the open pores is 5 to 30 mm. 2) Skeleton positioning step: Place the natural fiber three-dimensional rigid skeleton obtained in step A) into the foaming mold, and fix the natural fiber three-dimensional rigid skeleton in the predetermined spatial position in the foaming mold cavity through the positioning structure, so that the natural fiber three-dimensional rigid skeleton maintains its fixed spatial shape in the subsequent foaming molding process. 3) Integrated foaming molding step: Inject the foaming system into the foaming mold, so that during the foaming or curing process, the foaming system expands within the open pores defined by the three-dimensional rigid skeleton of natural fibers and forms a continuous foam structure, while filling the space between the three-dimensional rigid skeleton of natural fibers and the inner wall of the mold cavity of the foaming mold. During the foaming or curing process, the flow and expansion behavior of the foaming system is subject to the geometric constraints of the fixed spatial shape of the three-dimensional rigid skeleton of natural fibers and the distribution of its open pores, so as to form a foamed composite board structure integrating the three-dimensional rigid skeleton of natural fibers and the foaming material. 4) Demolding and finishing steps: Demold the molded foamed composite board and cut, mill, or finish the surface of the foamed composite board as needed to obtain the foamed composite board of the target size.

2. The manufacturing method according to claim 1, characterized in that, The fiber material of the natural fiber woven fabric is selected from one or more of flax fiber, ramie fiber, sisal fiber, and jute fiber.

3. The manufacturing method according to claim 1, characterized in that, The areal density of the natural fiber woven fabric is 200–400 g / m².

4. The manufacturing method according to claim 1, characterized in that, The three-dimensional spatial structure of the natural fiber three-dimensional rigid skeleton is a corrugated structure, an arched structure, or a ribbed structure that is periodically repeated along the length and / or width of the board.

5. The manufacturing method according to claim 1, characterized in that, The natural fiber three-dimensional rigid skeleton forms multiple spaced ribs or crests in the thickness direction of the board, so that the open pores are interconnected in the thickness direction of the board.

6. The manufacturing method according to claim 1, characterized in that, The curable resin system is an epoxy resin system with a room temperature viscosity of 100–500 mPa·s.

7. The manufacturing method according to claim 6, characterized in that, The epoxy resin and curing agent are mixed at a mass ratio of 100:20 to 100:30 and then applied to the natural fiber woven fabric.

8. The manufacturing method according to claim 1, characterized in that, The foaming system is selected from one of the following: polyurethane foaming system, polyethylene foaming system, polyethylene terephthalate foaming system, or ethylene-vinyl acetate foaming system.

9. The manufacturing method according to claim 1 or 8, characterized in that, During the foaming or curing process, the foaming system forms a continuous foam structure within the open pores of the natural fiber three-dimensional rigid skeleton, and the continuous foam structure is constrained by the geometry of the natural fiber three-dimensional rigid skeleton at the boundary of the open pores.

10. The manufacturing method according to claim 1, characterized in that, The foaming system is foamed or cured at a temperature of 60–180°C.