Thermal expansion foam molding web reinforced sandwich composite structure material and preparation method thereof

Abstract

The invention relates to the technical field of sandwich composite materials, and discloses a web-reinforced sandwich composite structure prepared by using a thermal expansion compression molding process and a method of the web-reinforced sandwich composite structure. Firstly, a light core (hard foam, balsa wood, honeycombs and the like) is cut into a required shape, an expandable foam prepreg film is cut according to a designed web structure, the interior and exterior of the web structure and the outer contour of the whole product are coated with expandable foam glue, then composite prepreg is laid on the product according to design requirements, and the composite prepreg is formed. And the web plate reinforced sandwich composite material structure is formed by utilizing internal expansion force generated by limited foaming in a thermal expansion foam pre-impregnated glue film mold. The maximum expansion force of the thermal expansion compression molding process can be maintained after the epoxy resin gel point, so that the performance of the web reinforced sandwich composite material is fully ensured, and the structural performance of the prepared web is enhanced.

Description

Technical Field

[0001] The invention relates to the technical field of sandwich composite materials, in particular to a thermal expansion foam formed web reinforced sandwich composite structural material and a preparation method thereof. Background Art

[0002] Sandwich composite materials have great development potential and application space in the field of lightweighting because they are lighter and can increase structural bending moment and thus improve the mechanical properties of the structure. However, when sandwich composite materials are directly prepared using lightweight cores, it is easy for the core and the skin to debond when subjected to external forces. Therefore, more current research is focused on Z-direction reinforced sandwich composite materials, especially structures such as grids, corrugated plates, and fiber columns. However, the molding process of this type of structure is relatively complicated. Generally, the web structure is prepared first, and then the secondary molding is performed to bond it to the skin. The process cost is high, and the interface mechanical properties are still the biggest problem, which seriously limits the application value of various new web reinforcement structures.

[0003] Combining a lightweight core with a web reinforcement structure can improve the inadequate Z-direction strength and interface performance defects of traditional sandwich composite materials. At the same time, due to the presence of the foam core, the buckling of the web structure can be hindered, thereby improving the overall mechanical properties of the structure and the stability under external forces. From a technological point of view, the current vacuum bag pressing-autoclave molding process requires multi-step molding. The web reinforcement structure and the upper and lower skins are molded separately and then connected together by gluing, and the cost is also high. The traditional compression molding process also requires multi-step molding and requires the design of a more complex mold.

[0004] The present invention uses a heat expansion foam prepreg to form a web-reinforced foam sandwich composite material structure using a heat expansion molding process. The foam glue and the resin in the composite material prepreg belong to the same system, so that they can be integrally formed, and the expansion foam is used to generate expansion force inside the structure to form a complex reinforced sandwich composite material. At the same time, the curing of the composite material prepreg and the decomposition of the foaming agent are carried out simultaneously, and a chemical cross-linking reaction occurs at the interface of the foam and the composite material, thereby improving the interface mechanical properties of the structure. This integrated molding process scheme better combines new materials, structures and processes, and has greater development space in the field of web-reinforced sandwich composite materials. Summary of the invention

[0005] The technical problem solved by the present invention is: the present invention uses a heat-expanding foam pre-impregnated film such as a heat-expanding epoxy foam to prepare a web-reinforced sandwich composite material structure through a heat-expanding molding process. The present invention is a typical method of combining materials, processes and structural design together, thereby using new materials and new processes to solve the molding problems and performance defects of typical structures of complex reinforced sandwich composite materials, and can be applied to lightweight fields such as automobiles, aerospace, and navigation.

[0006] The technical solution of the present invention is:

[0007] A method for preparing a heat expansion foam formed web reinforced sandwich composite structure material:

[0008] (1) Preparation of lightweight core: According to the designed web reinforcement structure, lightweight foam, balsa wood or honeycomb is pre-cut or carved as a lightweight core as the carrier of the web reinforcement structure, or expandable foam pre-impregnated film is used to achieve in-mold foaming.

[0009] (2) Preparation of heat-expandable foam prepreg film: Design a cutting plan for the heat-expandable foam prepreg film according to the cutting structure of the lightweight core, cut the heat-expandable foam prepreg film, and then lay the heat-expandable foam prepreg film on the lightweight core, including the inner and outer sides of the web structure.

[0010] (3) Preparation of composite prepreg: The composite prepreg whose curing temperature matches the foaming temperature of the thermal expansion foam prepreg film is cut into a desired shape and then laid on the prefabricated product in the previous step.

[0011] (4) The prefabricated product of the previous step is placed in a mold and locked, and is directly heated by a press or an oven to allow the expandable foam prepreg film to foam and expand, thereby generating an internal expansion force to allow the resin to fully infiltrate the fiber and further solidify the gel. The expansion force of the foam prepreg film is used to form the web structure of the foam sandwich composite material. The mold is demolded after cooling to obtain a heat-expandable foam molded web reinforced sandwich composite structure material.

[0012] Furthermore, (1) the mid-web reinforcement structure is a Z-direction reinforcement structure, a stiffening structure designed in a lightweight core, including a grid, a corrugated plate or a fiber column.

[0013] Furthermore, the heating in (3) raises the temperature to 100-180°C, the viscosity of the resin system in the composite prepreg is between 0.06-0.1 Pa.s, the foaming agent in the expandable foam prepreg film decomposes to generate an internal expansion force, the expansion force is directionally controlled between 4MPa-10MPa, and the maximum expansion force can be maintained between 10-30min.

[0014] Furthermore, the heat-expandable foam prepreg is prepared by adding a reinforcement to any one of the resin systems of composite epoxy resin system, polyurethane, phenolic resin, cyanate resin, bismaleimide resin or unsaturated polyester resin, and then adding a chemical foaming agent or expanding microspheres to form the foam prepreg.

[0015] Furthermore, the composite epoxy resin system is composed of phenolic epoxy resin, solid epoxy resin NPES-904H and E51 epoxy resin, and the composite epoxy resin system contains a latent curing agent; wherein the phenolic epoxy resin is any one of phenol-type phenolic epoxy resin, o-cresol-type phenolic epoxy resin or bisphenol A-type phenolic epoxy resin.

[0016] Furthermore, the chemical foaming agent is any one of azo compounds, sulfonylhydrazide compounds and nitroso compounds; the resin system and the foaming agent are evenly mixed, prepolymerized at low temperature and then prepared through a dipping device to form an expanded foam prepreg film with an initial thickness of 1-10 mm.

[0017] Furthermore, the expanded microspheres are a material with a core-shell structure, the shell is a thermoplastic resin with high plastic deformation ability, and the core is a chemical foaming agent or compressed gas, which can decompose at high temperature to produce gas molecules or expand the compressed gas to form a foam structure.

[0018] Furthermore, the reinforcement is one or more combinations of epoxy-modified aramid fiber, glass fiber, carbon fiber, and basalt fiber.

[0019] Further, the preparation method of epoxy modified aramid fiber is as follows:

[0020] (1) The aramid fiber is immersed in acetone, ultrasonically cleaned, the fiber is taken out and dried, and then added into a phosphoric acid solution, immersed and activated, the fiber is taken out, washed with water, and dried to obtain a hydroxyl-modified aramid fiber.

[0021] (2) Add hydroxy-modified aramid fiber and tetrabutylammonium bromide to epichlorohydrin, pre-react at a temperature of 50-65°C, then add sodium hydroxide, continue the reaction, take out the fiber, wash it with water and acetone, and dry it to obtain epoxy-modified aramid fiber.

[0022] Furthermore, the dosage of tetrabutylammonium bromide, epichlorohydrin and sodium hydroxide is 1.5-3%, 300-480% and 100-180% of the hydroxyl-modified aramid fiber respectively.

[0023] The beneficial technical effects of the present invention are:

[0024] The present invention uses a mixture of foam and web structure to better improve the interface mechanical properties of the structure, while preventing the web structure from buckling under the action of external force, thereby improving the structural strength. This overcomes the problem in traditional processes that grids, corrugated plates, and fiber columns cannot be integrated into one, and have poor interface mechanical properties, making it easy for the web structure to buckle under the action of external force.

[0025] The present invention can integrate the web structure and the skin by using the heat-expanding epoxy foam prepreg film, and the internal expansion force generated by the expansion foam can provide sufficient molding pressure for the composite material from the inside to the outside. At the same time, the heat-expanding foam and the composite material prepreg used belong to the same epoxy resin system, and during the integrated molding process, a chemical reaction can occur to form a chemical cross-linking structure at the interface of the composite material and the foam, thereby improving the interface mechanical properties of the structure.

[0026] The invention uses a heat-expandable epoxy foam as an expansion source, bisphenol A novolac epoxy resin, solid epoxy resin NPES-904H and E51 epoxy resin to form a composite epoxy resin body, epoxy-modified aramid fiber as a reinforcement, and diethyltetramethylimidazole as a latent curing agent. During the curing process, the curing agent can undergo a cross-linking reaction with the epoxy resin and the epoxy groups of the modified aramid fiber, thereby improving the interface bonding force of the aramid fiber and the epoxy resin expansion foam system, so that the aramid fiber effectively enhances the mechanical properties of the epoxy resin expansion foam. During the process of forming a web-reinforced sandwich composite material, a molding pressure of 4-10MPa can be generated from the inside to the outside. For a web-reinforced sandwich composite material structure, defects caused by insufficient pressure during the molding process are easily generated at the R corner position, while the use of the heat-expandable epoxy foam can generate a relatively uniform pressure inside, thereby forming a web-reinforced sandwich composite material structure with higher molding quality.

[0027] From the perspective of structural performance, the use of a mixture of foam and web reinforcements can prevent the web structure from buckling when the structure is subjected to external forces to a certain extent, thereby improving the mechanical properties of the structure. In addition, the resins in the expandable foam and composite prepreg belong to the same epoxy resin system, so during the curing and foaming process, a chemical cross-linking reaction occurs at the interface of the foam and the composite material, which can better improve the interface mechanical properties of the structure.

[0028] From the process point of view, compared with the traditional compression molding process, the thermal expansion molding process can more reliably mold complex reinforced structures; compared with the vacuum bag molding process, it can more effectively control the molding pressure and has higher adjustability; compared with the autoclave molding process, the cost is lower. Therefore, this is a molding process with great potential in the field of composite materials.

[0029] Typical structural parts such as web-reinforced sandwich composites can be used in the field of lightweight automobiles, such as new energy vehicle battery boxes, automobile bottom protection plates, automobile anti-collision beams, etc.; aircraft wings, aircraft elevators, satellite fairings, etc. in the aerospace field; wind power field; various sports equipment, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a schematic diagram of the thermal expansion molding process: 1-upper mold; 2-upper and lower mold locking bolts; 3-lower mold.

[0031] Figure 2 It is a schematic diagram of the thermal expansion molding process, showing the front and back changes of the web structure and the expansion of the foam.

[0032] Figure 3 The product before and after expansion: 1-upper skin; 2-expandable epoxy foam glue; 3-lightweight core; 4-composite material web structure.

[0033] Figure 4 This is a schematic diagram of a typical I-beam product.

[0034] Figure 5 This is a schematic diagram of typical corrugated plate structure ply and specimen.

[0035] Figure 6 This is a schematic diagram of a foam sandwich T-beam: 1-composite material; 2-composite material; 3-expanded expandable epoxy foam.

[0036] Figure 7 This is a schematic diagram of the R angle position. DETAILED DESCRIPTION

[0037] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0038] Example 1

[0039] The aramid fiber was immersed in acetone, ultrasonically cleaned, taken out and dried, then added into a phosphoric acid solution with a mass fraction of 40%, immersed and activated at a temperature of 50°C for 3 hours, and the fiber was taken out, washed with water, and dried to obtain a hydroxyl-modified aramid fiber.

[0040] Add hydroxy-modified aramid fiber and tetrabutylammonium bromide to epichlorohydrin, pre-react at 60°C for 0.5h, then add sodium hydroxide, and control the dosage of tetrabutylammonium bromide, epichlorohydrin and sodium hydroxide to 2%, 400% and 150% of the hydroxy-modified aramid fiber, and continue the reaction for 3h. Take out the fiber, wash it with water and acetone, and dry it to obtain epoxy-modified aramid fiber.

[0041] Bisphenol A novolac epoxy resin, solid epoxy resin NPES-904H and E51 epoxy resin are used to form a composite epoxy resin body, epoxy modified aramid fiber, latent curing agent diethyltetramethylimidazole and EFKA2723 defoaming agent are added, mixed evenly, and azobisisobutyronitrile is used as a chemical foaming agent to prepare a thermal expansion foam prepreg film system.

[0042] According to the designed web reinforcement structure, the pre-cut lightweight polyvinyl chloride foam is used as a lightweight core and as a carrier of the web reinforcement structure.

[0043] Design the cutting plan of the heat expansion foam prepreg according to the cutting structure of the lightweight core, cut the heat expansion foam prepreg, and then lay the heat expansion foam prepreg on the lightweight core, including the inside and outside of the web structure. When laminating, pay attention to not stretching the R angle position, and reserve space appropriately to avoid laying it too tightly, as the internal expansion force will cause the foam to completely fill the gap.

[0044] The composite material prepreg whose curing temperature matches the foaming temperature of the heat expansion foam prepreg film is cut into the required shape, and then laid on the prefabricated product in the previous step. The composite material prepreg can be laid evenly without obvious pores, and there is no need to lay it tight.

[0045] The prefabricated product in the previous step is placed in the mold and locked. The mold is closed in the X and Y directions and free in the Z direction. The size of the mold cavity is controlled according to the expansion ratio. The mold is locked and heated to 130°C in an oven for 2 hours. The expandable foam prepreg film foams and expands. The foaming agent in the heat-expandable epoxy foam glue will reach the maximum expansion force within the range of 3-10 minutes, and the maximum expansion force can be maintained for more than 30 minutes, generating an internal expansion force to allow the resin to fully infiltrate the fiber, further gelling and solidifying. The expansion force of the foam prepreg film is used to form the web structure of the foam sandwich composite material. The mold is demolded after cooling to obtain a heat-expandable foam molded web reinforced sandwich composite structure material.

[0046] The present invention also provides the following embodiments:

[0047] Example 2

[0048] A typical corrugated sheet reinforced foam sandwich composite structure, the expanding foam expands 4 times, the initial thickness is 1mm, and the expansion force is 0.5MPa.

[0049] The upper and lower skins are each laid with 4 layers of CFRP prepreg, and the middle corrugated plate structure uses 2 layers of CFRP prepreg.

[0050] Expandable foams of different widths are alternately laid on both sides of the middle two layers of CFRP prepreg. The width of the expandable foam can be adjusted. Figure 5 As shown. The expandable foam is a composite epoxy resin body composed of bisphenol A phenolic epoxy resin, solid epoxy resin NPES-904H and E51 epoxy resin, epoxy modified aramid fiber, latent curing agent diethyltetramethylimidazole, EFKA2723 defoamer are added, mixed evenly, and azobisisobutyronitrile is used as a chemical foaming agent to configure a system of thermal expansion foam prepreg film (the same as Example 1); cut the composite material prepreg, and lay the composite material prepreg on the prefabricated product in the previous step, note that the web needs to consider that the size will change after expansion, and the composite material prepreg can be properly laid evenly without obvious pores, and it does not need to be stretched and laid.

[0051] After the materials are laid, they are placed in a square mold with controllable thickness. In this embodiment, the expansion foam expands 4 times, and the thickness of the corrugated board core after molding is 4 mm.

[0052] After the mold has cooled, the product is taken out. Since it is a thermosetting epoxy foam, no secondary expansion will occur, and the interface performance between the foam matrix and the composite resin matrix is ​​good.

[0053] Example 3

[0054] A typical T-shaped reinforced sandwich composite material, the thermal expansion epoxy foam prepreg film expands 1-2 times, and the expansion force is 0.5-1MPa.

[0055] (1) Cut composite prepreg according to the structure of the T-beam, and lay out the skin and web structure separately, such as Figure 6 Middle area 1.

[0056] (2) Considering that the R corner position is the most vulnerable to damage in the T-beam structure, an expandable foam filling method is adopted to lay an expandable epoxy foam prepreg film at the R corner position. The expandable epoxy foam prepreg film is a composite epoxy resin body composed of bisphenol A phenolic epoxy resin, solid epoxy resin NPES-904H and E51 epoxy resin, epoxy modified aramid fiber, latent curing agent diethyltetramethylimidazole, EFKA2723 defoaming agent are added, mixed evenly, and azobisisobutyronitrile is used as a chemical foaming agent to prepare a heat-expandable foam prepreg film system (the same as Example 1).

[0057] (3) Composite material prepreg is laid on the inner side of the T-beam according to the design, and the laid product is placed in a mold, so that a T-beam with a foam structure can be formed. Compared with a pure composite material structure T-beam, this T-beam structure is less likely to be damaged at the R corner position and has higher designability and process feasibility.

[0058] The mechanical properties of the composite materials were tested using a universal material testing machine and a compression strength tester.

[0059] Comparative Example 1 is different from Example 1 in that unmodified aramid fiber is used in the system of the heat expansion foam pre-impregnated film instead of epoxy modified aramid fiber.

[0060] Comparative Example 2: The difference from Example 1 is that epoxy-modified aramid fiber is not added to the system of the thermal expansion foam pre-impregnated film.

[0061] The mechanical properties of the composite materials were tested using a universal material testing machine and a compression strength tester.

[0062]

[0063]

[0064] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for preparing a heat expansion foam molded web reinforced sandwich composite structure material, which requires: The preparation method is as follows: (1) Preparation of lightweight core: According to the designed web reinforcement structure, pre-cut or carve lightweight foam, balsa wood or honeycomb as a lightweight core as the carrier of the web reinforcement structure, or use expandable foam pre-impregnated film to achieve in-mold foaming; (2) Preparation of heat-expandable foam prepreg film: designing a cutting scheme of the heat-expandable foam prepreg film according to the cutting structure of the lightweight core, cutting the heat-expandable foam prepreg film, and then laying the heat-expandable foam prepreg film on the lightweight core, including the inner and outer sides of the web structure; (3) Preparation of composite prepreg: cutting the composite prepreg whose curing temperature matches the foaming temperature of the thermal expansion foam prepreg film into a desired shape, and then laying it on the prefabricated product in the previous step; (4) The prefabricated product of the previous step is placed in a mold and locked, and is directly heated by a press or an oven to allow the expandable foam prepreg film to foam and expand, thereby generating an internal expansion force to allow the resin to fully infiltrate the fiber and further solidify the gel. The expansion force of the foam prepreg film is used to form the web structure of the foam sandwich composite material. The mold is demolded after cooling to obtain a heat-expandable foam molded web reinforced sandwich composite structure material.

2. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 1, characterized in that: (1) The mid-web reinforcement structure is a Z-direction reinforcement structure, a stiffening structure designed in a lightweight core, including grids, corrugated plates or fiber columns.

3. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 1, characterized in that: (3) The temperature is raised to 100-180°C, the viscosity of the resin system in the composite material prepreg is between 0.06-0.1Pa.s, and the foaming agent in the expandable foam prepreg film is decomposed to generate an internal expansion force, and the expansion force is directionally controlled between 4MPa-10MPa, and the maximum expansion force can be maintained between 10-30min.

4. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 1, characterized in that: The heat-expandable foam prepreg is formed by adding a reinforcement to any resin system of composite epoxy resin system, polyurethane, phenolic resin, cyanate resin, bismaleimide resin or unsaturated polyester resin, and then adding a chemical foaming agent or expanding microspheres to form a foam prepreg.

5. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 4, characterized in that: The composite epoxy resin system is composed of phenolic epoxy resin, solid epoxy resin and E51 epoxy resin, and the composite epoxy resin system contains a latent curing agent; wherein the phenolic epoxy resin is any one of phenol-type phenolic epoxy resin, o-cresol-type phenolic epoxy resin or bisphenol A-type phenolic epoxy resin.

6. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 4, characterized in that: The chemical foaming agent is any one of azo compounds, sulfonylhydrazide compounds and nitroso compounds; the resin system and the foaming agent are evenly mixed, prepolymerized at low temperature and then prepared through a dipping device to form an expanded foam prepreg film with an initial thickness of 1-10mm.

7. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 4, characterized in that: Expanded microspheres are a material with a core-shell structure. The shell is a thermoplastic resin with high plastic deformation ability, and the core is a chemical foaming agent or compressed gas, which can decompose at high temperature to produce gas molecules or expand compressed gas to form a foam structure.

8. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 4, characterized in that: The reinforcement is one or more combinations of epoxy-modified aramid fiber, glass fiber, carbon fiber and basalt fiber.

9. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 2, characterized in that: The preparation method of epoxy modified aramid fiber is as follows: (1) soaking the aramid fiber in acetone, ultrasonically cleaning it, taking out the fiber and drying it, then adding it to a phosphoric acid solution, soaking it for activation, taking out the fiber, washing it with water, and drying it to obtain a hydroxyl-modified aramid fiber; (2) Add hydroxy-modified aramid fiber and tetrabutylammonium bromide to epichlorohydrin, pre-react at a temperature of 50-65°C, then add sodium hydroxide, continue the reaction, take out the fiber, wash it with water and acetone, and dry it to obtain epoxy-modified aramid fiber.

10. The method for preparing the heat expansion foam formed web reinforced sandwich composite structure material according to claim 9, characterized in that: The dosage of tetrabutylammonium bromide, epichlorohydrin and sodium hydroxide is 1.5-3%, 300-480% and 100-180% of the hydroxyl-modified aramid fiber respectively.

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