High-strength hemp fiber composite material and injection molding process thereof

Abstract

This invention relates to the field of material injection molding technology, specifically to a high-strength hemp fiber composite material and its injection molding process. This invention overcomes the problems of low interfacial weld strength and poor formability in existing fiber composite materials. The injection molding process of this invention first involves extruding and granulating a fiber mixture, treated carbon fibers, resin, and crosslinking agent under low-temperature control to obtain the composite material. Then, pre-impregnated carbon cloth is positioned within the mold cavity. During the injection molding stage, variable pressure wavefront control is implemented. The first stage uses low-speed injection to smoothly compact the high-viscosity melt into continuous carbon cloth. The second stage involves high-speed mold filling, utilizing high shear heat to precisely trigger the unsealing and ring-opening of the latent crosslinking agent, forming a three-dimensional covalent bond network. Finally, a stepped high-pressure holding process eliminates reactive micropores. This process solves the molding defects of natural fiber carbonization due to heat and the easy erosion of carbon cloth inserts, improving the interlayer bonding strength and mechanical strength of the injection-molded parts, and is suitable for the manufacture of large lightweight automotive structural components.

Description

Technical Field

[0001] This invention relates to the field of material injection molding technology, specifically to a high-strength hemp fiber composite material and its injection molding process. Background Technology

[0002] With the acceleration of automotive lightweighting and the demand for low VOC emissions, the use of low-density natural plant fibers, including hemp fiber, wood fiber and carbon fiber blends, combined with continuous carbon cloth for in-mold injection molding, has become the core development direction for manufacturing large lightweight load-bearing components such as door interior panels and seat frames. However, in actual injection molding processing, this type of multi-scale composite system faces process limitations such as poor formability and low interface weld strength.

[0003] On the one hand, large automotive structural components typically have thin walls and complex reinforcing ribs on the back. In order to fill the complex mold cavity with polymer melt, traditional injection molding processes require setting the injection molding machine barrel temperature above 200°C to reduce melt viscosity. However, this directly leads to severe carbonization and degradation of hemp and wood fibers, which have extremely poor heat resistance, due to prolonged heating in the barrel, producing volatile organic compounds and a burnt odor. If the barrel temperature is forcibly reduced, the melt fluidity deteriorates drastically, which not only easily leads to defects such as insufficient glue, but also generates huge fluid scouring forces due to the high-speed, high-pressure direct injection process commonly used for forced mold filling. This directly causes severe scouring displacement, wrinkling, and tearing of the carbon fiber skeleton pre-placed in the mold cavity, resulting in the failure of the load-bearing structure.

[0004] On the other hand, during the short filling and cooling cycle of conventional injection molding, the polar natural fibers, macroscopic continuous carbon cloth, and non-polar polypropylene matrix are only bonded by weak physical molecular chain entanglement. Especially at the multi-point injection points in complex mold cavities, the interfacial bonding force at the weld lines formed by the convergence of melt wavefronts is extremely weak. Due to the lack of thermodynamic and rheological depth induction in the molding process, the materials of each phase cannot form a dense chemical anchor across the interface in the mold cavity. As a result, when the composite injection molded part is subjected to alternating stress or impact, it is very easy for fibers to be pulled out and for interlayer delamination to occur from the phase interface and carbon cloth skeleton.

[0005] In summary, it is necessary to modify the injection molding process to resolve the process paradox of barrel temperature and fluid filling, and to suppress insert erosion through precise fluid dynamics control, thereby solving the technical problems of poor formability and low interface weld strength in existing composite material injection molding processes.

[0006] Therefore, the present invention provides a high-strength hemp fiber composite material and its injection molding process. Summary of the Invention

[0007] The purpose of this invention is to provide a high-strength hemp fiber composite material and its injection molding process. The injection molding process of this invention first involves extruding and granulating a fiber mixture, treated carbon fibers, resin, and crosslinking agent under low-temperature control to obtain the composite material. Then, pre-impregnated carbon cloth is positioned within the mold cavity. During the injection molding stage, variable pressure wavefront injection control is implemented. The first stage uses low-speed injection to smoothly compact the high-viscosity melt into continuous carbon cloth. The second stage involves high-speed mold filling, utilizing high shear heat to precisely trigger the desealing and ring-opening of the latent crosslinking agent, forming a three-dimensional covalent bond network. Finally, a stepped high-pressure holding process eliminates reactive micropores. This process solves the molding defects of natural fiber carbonization due to heat and the easy erosion of carbon cloth inserts, improving the interlayer bonding strength and mechanical strength of the injection-molded parts, and is suitable for the manufacture of large lightweight automotive structural components.

[0008] To achieve the above objectives, the present invention provides the following technical solution: This invention provides an injection molding process for high-strength hemp fiber composite materials, characterized by comprising the following steps: 15-25 parts of fiber mixture, 5-10 parts of treated carbon fiber, 55-65 parts of polypropylene resin, 4-6 parts of compatibilizer, 2-5 parts of latent crosslinking agent composition, and 0.5 parts of antioxidant compound are fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 48:1. The temperatures of each zone from the feeding section to the die head of the screw extruder are controlled to be 160℃, 165℃, 170℃, 172℃, 172℃, and 170℃ respectively. The main screw speed is set to 200-250 rpm. The average residence time of the material in the screw is controlled at 45-60s to obtain a mixed system. Through stepped cooling control of the temperature zone setting, the local mechanical shear friction heat brought by the high speed is fully absorbed and offset, thereby ensuring that the actual melt temperature is always below 175℃, and ensuring that the latent crosslinking agent composition remains completely inert and closed-loop under this shear field. After the composite material is drawn into strands by the melt extruder, it is water-cooled in a circulating water tank at 20℃-25℃ for 2-3s, then pelletized and air-dried to obtain the composite melt. In the in-mold pre-setting stage, a continuous plain-weave carbon cloth with a 0.05-0.1mm thick polypropylene hot melt adhesive film is pre-impregnated and coated on the surface. It is then hot-pressed and pre-formed according to the geometry of the load-bearing skeleton area in the injection mold, and then precisely positioned in the cavity on the moving mold side. The mold is closed, and the mold temperature of the moving mold and the fixed mold is kept constant at 45℃-55℃ by a mold temperature controller. The composite melt is added to the barrel of a standard injection molding machine. The barrel temperatures from the feed section to the nozzle section are set to 165℃, 170℃, 175℃, and 175℃ respectively. The injection molding machine screw plasticizing speed is set to 80-100 rpm, and the back pressure is controlled at 3-5 MPa. Strict variable wavefront control is implemented during the injection process. The first-stage injection is set with an absolute injection speed of 25-35 mm / s (corresponding to 15% to 20% of the injection molding machine system setting at a low speed), and an injection pressure of 40-50 MPa. This allows the low-temperature, high-viscosity mixed melt to slowly contact the continuous carbon cloth with an obtuse-angle wavefront. The high viscosity and flow resistance of the melt itself flatten and stably press the carbon cloth. It adheres firmly to the inner wall of the mold cavity, completely preventing erosion and wrinkling. The absolute viscosity of the system is maintained in the high viscosity range of 1200-1800 Pa·s. When the screw reaches the preset stroke switching position, it instantly switches to the second stage of mold filling and injection. At this time, the absolute injection speed increases to 120-150 mm / s (corresponding to 75% to 85% of the high speed setting of the injection molding machine system). The injection pressure is simultaneously increased to 95-110 MPa. The viscosity of the melt drops sharply under extremely high shear rate. Within 0.5-1.0 s, it quickly fills all the mesh reinforcements and narrow cavities to obtain the processed melt. At this time, the melt viscosity is reduced to the extremely low viscosity range of 45-85 Pa·s.

[0009] The treated melt is then subjected to a shear-thermal in-situ crosslinking process to obtain a crosslinked melt. The first stage is the shear-triggered unsealing crosslinking period, during which the mixed melt is forcibly injected through a cross-sectional area of ​​1.5-2.5 mm under a high-speed injection of 120-150 mm / s in the second stage. 2When the mold gate and narrow cavity are used, extremely strong internal fluid shear friction is generated, causing the local melt temperature on the flow front to rise to 195℃-205℃ within 0.5-1.5s. This instantaneous high shear temperature precisely breaks through the reaction threshold of the caprolactam-blocked diphenylmethane diisocyanate in the latent crosslinking agent composition, causing it to rapidly deblock within 0.2-0.5s. The released isocyanate groups immediately undergo a primary polyurethane bonding reaction with the hydroxyl groups on the surface of hemp and wood fibers and the oxygen-containing functional groups on the surface of carbon cloth, increasing the viscosity to the gel phase transition critical point of 15000-20000 Pa·s; the second stage is open During the ring-chain extension and crosslinking period, as the melt fills the cavity and enters a high-pressure state, the local melt temperature of the core layer at the intersection of the high-shear flow channel and the melt wavefront is maintained at a gel window of 185℃-190℃. At this time, the 1,3-phenylbisoxazoline in the latent crosslinking agent undergoes a ring-opening reaction, and the viscosity increases to an ultra-high crosslinking solid range of 2,500,000-3,800,000 Pa·s. Its ring-opening product undergoes targeted ester-amidation and chain extension crosslinking with the carboxyl groups of the maleic anhydride-grafted polypropylene compatibilizer in the system within 2-5 seconds before the melt surface is completely frozen, thoroughly weaving a dense three-dimensional covalent bond network across the microscopic phase interface.

[0010] The cross-linked melt enters the pressure holding and shaping stage. With the cavity filled and the two-stage cross-linking reaction in the gel phase, the equipment immediately implements stepped high pressure holding control to eliminate reaction gas release and micropores. The first-stage pressure holding is set at 85-90 MPa and maintained for 8 seconds. Subsequently, the system switches to the second-stage pressure holding, where the pressure is reduced to 45-50 MPa and maintained for 12 seconds to eliminate residual macromolecular orientation internal stress. After the pressure holding is completed, mold cooling water at a temperature of 15℃-20℃ is introduced to rapidly reduce the mold temperature to a safe demolding temperature within 15-20 seconds, terminating all thermodynamic and chemical cross-linking reactions. The mold is then opened, and the product is ejected using ejector pins to obtain the composite material.

[0011] Preferably, the preparation of the fiber mixture includes the following steps: Natural sisal fiber and coniferous fiber were mixed at a mass ratio of 1.5:1 and placed in a 5% sodium hydroxide aqueous solution. The mixture was stirred and soaked at 60℃-70℃ for 2 hours to remove pectin and lignin from the surface. The mixture was then repeatedly rinsed with deionized water until the washing solution was neutral. It was then dried in a vacuum oven at 80℃ for 15 hours to obtain dried fiber with a moisture content of less than 0.5%. The dried fiber was then placed in a high-speed mixer at 800-1000 rpm and 90℃. A 3-aminopropyltriethoxysilane ethanol aqueous solution (2.0% of the total fiber mass) was sprayed evenly through a spray device and continuously mixed for 15-20 minutes. The mixture was then activated and dried at 105℃ for 2 hours to obtain a fiber mixture.

[0012] Preferably, the preparation of treated carbon fibers includes the following steps: The waterborne polyurethane emulsion was diluted with deionized water to a sizing solution with a solid content of 5%. The carbon fibers were then immersed in the sizing solution, with the bath temperature controlled at 40℃-45℃ and the immersion time at 10-15 minutes, ensuring that the dry weight of the polyurethane accounted for 2.5% of the total mass of the carbon fibers. After removal, the carbon fibers were dehydrated using a centrifuge at 1500 rpm for 5 minutes. Finally, the carbon fibers were placed in a 120℃ forced-air drying oven for curing and crosslinking treatment for 45-60 minutes to obtain the treated carbon fibers.

[0013] The present invention also provides a high-strength hemp fiber composite material, the raw materials for which include a fiber mixture, treated carbon fibers, polypropylene resin, maleic anhydride-grafted polypropylene, and a latent crosslinking agent composition.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention uses a specific high-speed shearing molding process to trigger a chemical cross-linking reaction in situ inside the mold cavity, upgrading the traditional physical entanglement to chemical bonding. The unsealed highly active groups construct a high-density covalent bond network between the resin matrix, hybrid fibers and continuous carbon cloth. Compared with existing conventional processes, this invention effectively eliminates interface slippage defects under stress and achieves mechanical synergy between multi-scale short fiber toughening and continuous carbon cloth reinforcement.

[0015] 2. To address the existing problem of interlayer delamination caused by poor compatibility between natural fibers and resin matrix, this process introduces a dual-targeting latent crosslinking agent composition. The blockade of MDI and bisoxazoline acts as an interfacial suture. During the gelation window, the ring-opening chain extension reaction promotes the targeted anchoring of isocyanate groups with fiber hydroxyl groups and oxazoline groups with resin compatibilizers, eliminating the polar barriers of the multiphase interface. This not only ensures the compactness of the microstructure but also endows the injection-molded structural parts with excellent anti-fatigue delamination ability under complex high-frequency vibration conditions.

[0016] 3. This invention solves the critical problem of carbon fiber inserts being easily deformed by high-pressure melt by using a variable pressure wavefront control injection molding process. In the first stage of low-speed paving, the high-viscosity polymer melt slowly enters the mold cavity in an obtuse-angled wavefront shape. Relying on the high viscosity resistance of the fluid itself, the carbon fiber is flattened and firmly pressed against the inner wall of the mold. The displacement rate of the carbon fiber insert is significantly reduced, eliminating the need for complex in-mold vacuum-assisted fixing devices and ensuring the absolute integrity of the core load-bearing skeleton.

[0017] 4. This invention controls the entire extrusion granulation and injection molding barrel process within a low-temperature safe zone, cutting off the long-term thermal degradation path of cellulose and lignin. The energy required for the chemical reaction is supplied only by the physical shear friction heat generated instantaneously through the gate. Through the dormancy-transient activation mechanism, the injection molded parts have no heat damage black spots on their appearance, and the total VOC release is extremely low, meeting automotive-grade environmental protection standards.

[0018] 5. This invention introduces a stepped pressure holding and shaping technology with a high-to-low pressure during the gelation period after the mold is filled. The first stage of extreme pressure holding uses strong mechanical force to forcibly crush and expel all reaction gas release and microbubbles before the polymer crosslinks and cures. The subsequent second stage of pressure stabilization effectively releases residual stress, improves the density of injection molded products, and gives large automotive exterior parts excellent dimensional stability against extreme weather. Attached Figure Description

[0019] Figure 1 The graph shows the changes in water absorption rate and dimensional shrinkage rate of the composite materials obtained in Example 1 and Comparative Examples 5-8. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The polypropylene resin of this invention is a high-flowability polypropylene resin with a melt flow rate of 35-45 g / 10 min (230℃ / 2.16 kg standard), a weight-average molecular weight (Mw) of 150,000-200,000 g / mol, a degree of polymerization between 3500-4700, and an apparent melt viscosity of 100-150 Pa·s at 200℃. The carbon fiber has a length of 3-5 mm and is coated with a polyurethane sizing agent. The aqueous polyurethane emulsion is a semi-transparent, slightly blue liquid with a solid content of 30%-35%, a pH value of 7.5-8.5, and a viscosity of 50-200 mM at 25℃. Pa·s; the compatibilizer is maleic anhydride-grafted polypropylene, CAS number 25722-45-6, with a grafting rate of 1.2% and a weight-average molecular weight (Mw) of 50,000-80,000 g / mol; the antioxidant compound is composed of antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1; the latent crosslinking agent composition is composed of caprolactam-blocked diphenylmethane diisocyanate and 1,3-phenylbisoxazoline in a mass ratio of 3:1, and the initial unblocking and ring-opening reaction temperature of this composition is 185℃-195℃; the insert skeleton is a continuous plain-weave carbon cloth pre-impregnated with a polypropylene hot melt adhesive film on the surface; Please see Figure 1 This invention provides a high-strength hemp fiber composite material and its injection molding process, the technical solution of which is as follows: Example 1 20 parts of fiber mixture, 8 parts of treated carbon fiber, 60 parts of polypropylene resin, 5 parts of compatibilizer, 3.5 parts of latent crosslinking agent composition, and 0.5 parts of antioxidant compound were fed into a co-rotating twin-screw extruder with an aspect ratio of 48:1. The temperatures of each zone from the feeding section to the die head of the screw extruder were controlled to be 160℃, 165℃, 170℃, 172℃, 172℃, and 170℃ respectively. The main screw speed was set to 250 rpm, and the average residence time of the material in the screw was controlled to be 60s to obtain the mixed system. After being drawn into strands by a melt extruder, the mixture was water-cooled in a 25℃ circulating water tank for 3s, then pelletized and air-dried to obtain a composite melt. In the in-mold pre-setting stage, a continuous plain-weave carbon cloth with a surface pre-impregnated and coated with polypropylene hot melt adhesive film is hot-pressed and pre-formed according to the geometry of the load-bearing skeleton area in the injection mold, and then precisely positioned in the cavity on the moving mold side; the mold is closed, and the mold temperature of the moving mold and the fixed mold is kept constant at 50℃ by a mold temperature controller. The composite melt is added to the barrel of a standard injection molding machine. The temperatures of the barrel from the feed section to the nozzle section are set to 165℃, 170℃, 175℃, and 175℃ respectively. The plasticizing speed of the injection molding machine screw is set to 80 rpm, and the back pressure is controlled at 5 MPa. Strict variable pressure wavefront control is implemented during the injection process. The absolute injection speed of the first stage is set to 30 mm / s, and the injection pressure is 45 MPa. When the screw reaches the preset stroke switching position, it instantly switches to the second stage of mold filling injection. At this time, the absolute injection speed increases to 135 mm / s, and the injection pressure increases to 105 MPa simultaneously. The viscosity of the melt drops sharply under extremely high shear rates, and it quickly fills all the mesh reinforcements and narrow cavities within 1.0 s to obtain the processed melt.

[0022] The treated melt is then subjected to a shear-thermal in-situ crosslinking process to obtain a crosslinked melt; the first stage is the shear-triggered unsealing crosslinking period, during which the hybrid melt is forcibly injected at a high speed of 135 mm / s in the second stage through a cross-sectional area of ​​2 mm. 2 When the mold gate and narrow cavity are filled, extremely strong internal fluid shear friction is generated, causing the local melt temperature on the flow front to rise to 200℃ within 1.5s; the second stage is the ring-opening chain extension crosslinking period. As the melt fills the cavity and enters a high-pressure state, the system temperature is maintained at the gel window period of 185℃. At this time, the 1,3-phenylbisoxazoline in the latent crosslinking agent undergoes a ring-opening reaction. Its ring-opening product undergoes targeted esterification and chain extension crosslinking with the carboxyl groups of the maleic anhydride-grafted polypropylene compatibilizer in the system, thoroughly weaving a dense three-dimensional covalent bond network that spans the microscopic phase interface.

[0023] The cross-linked melt is brought into the holding and shaping stage. When the cavity is full and the two-stage cross-linking reaction is in the gelation period, the equipment immediately implements step-by-step high holding pressure control to eliminate reaction gas release and micropores. The first stage is high-pressure holding, with the holding pressure set at 88 MPa and maintained for 8 seconds. Then the system switches to the second stage of stable holding, with the holding pressure reduced to 46 MPa and maintained for 12 seconds. After the holding pressure is completed, mold cooling water at a temperature of 20°C is introduced to rapidly reduce the mold temperature to a safe demolding temperature within 18 seconds. The mold is then opened and the product is ejected using ejector pins to obtain the composite material.

[0024] Examples 2-4 follow the same preparation method and parameter conditions as Example 1, with differences shown in Table 1.

[0025] Table 1. Parameter changes in Examples 1-4 Comparative Example 1 is the same as Example 1, except that caprolactam-blocked diphenylmethane diisocyanate is not added to the crosslinking agent composition, 1,3-phenylbisoxazoline is retained, and polypropylene resin is added to make up the missing mass fraction, while the rest of the process remains unchanged.

[0026] Comparative Example 2 is the same as Example 1, except that 1,3-phenylbisoxazoline is not added to the crosslinking agent composition, caprolactam-blocked diphenylmethane diisocyanate is retained, and polypropylene resin is added to make up the missing mass fraction, while the rest of the process remains unchanged.

[0027] Comparative Example 3 is the same as Example 1, except that a latent crosslinking agent composition is not introduced, and the mass fraction is made up by polypropylene resin, while the rest of the process remains unchanged.

[0028] Comparative Example 4 is the same as Example 1, except that the sisal fiber and coniferous fiber are not modified and are directly melt-granulated using untreated mixed fibers, while the rest of the process remains unchanged.

[0029] Comparative Example 5 is the same as Example 1, except that the switching design between the first-stage low-speed, low-pressure padding injection and the second-stage high-speed, high-pressure mold filling is not used. Instead, the injection process is set to a high-speed, high-pressure direct injection process with an absolute injection speed of 135 mm / s and an injection pressure of 102 MPa. The rest of the process remains unchanged.

[0030] Comparative Example 6 is the same as Example 1, except that the low temperature barrel and formula are kept the same, the high-speed mold filling jump of 135mm / s in the second stage is cancelled, and the absolute injection speed of the entire injection molding process is locked at a low speed of 30mm / s, while the rest of the process remains unchanged.

[0031] Comparative Example 7 is the same as Example 1, except that the highest heating zone temperature of the twin-screw extruder and the set temperature of the injection molding machine barrel are both set to 210°C, which is the standard temperature for conventional processes, while the rest of the process remains unchanged.

[0032] Comparative Example 8 is the same as Example 1, except that during the gelation period after the cavity is filled, the high pressure of 88MPa for the first stage of pressure holding and shaping is cancelled, and only the conventional low pressure of 45MPa is used for pressure holding and maintenance.

[0033] Experimental Example 1: Mechanical Property Testing The mechanical properties of the composite materials prepared in Examples 1-4 and Comparative Examples 1-6 were tested. Tensile strength was tested according to GB / T 1040.2-2022. On a universal testing machine, a standard dumbbell-shaped specimen was clamped in upper and lower fixtures, and a tensile load of 5 mm / min was applied along the longitudinal principal axis of the specimen until fracture or yielding occurred. The maximum load at fracture was recorded and divided by the initial cross-sectional area of ​​the specimen to calculate the tensile strength. Flexural modulus was tested according to GB / T 9341-2008 using the three-point bending method. The load-deflection curve was recorded, and the ratio of stress increment to strain increment in the initial linear proportion of the curve was selected to calculate the flexural modulus. Notched impact strength was tested according to GB / T 1043.1-2008 using a simply supported beam impact testing machine. The energy consumed by the pendulum to break the specimen was measured and divided by the remaining original cross-sectional area at the notch of the specimen to calculate the notched impact strength. Interlaminar shear strength was tested according to GB / T... 1450.1-2005, using the short beam shear method, this test mainly induces horizontal shear failure of the specimen at the neutral plane rather than bending fracture, and records the maximum load at specimen failure; the specific test of carbon cloth insert erosion displacement rate: using optical image measurement method, the initial position coordinates and surface weaving mesh morphology of the continuous carbon cloth insert in the mold cavity are accurately located and recorded before injection molding; after injection molding and demolding, a high-precision optical scanner is used to perform 3D scanning and image recognition of the carbon cloth mesh on the surface of the product, and the mesh image after molding is compared with the initial state to calculate the percentage of the area of ​​carbon cloth fiber bundle slippage, twisting or wrinkling to the total exposed area of ​​the carbon cloth insert; the test results are shown in Table 2.

[0034] Table 2 Test results of the examples and comparative examples As shown in Table 2, the mechanical properties and interfacial adhesion properties of the composite material obtained in the comparative example, through adjustments to the components and process, are significantly different from those in the example. In the example, by introducing a latent crosslinking agent composition consisting of caprolactam-blocked diphenylmethane diisocyanate and 1,3-phenylbisoxazoline, and simultaneously employing a variable pressure wavefront controlled injection process, the carbon cloth can be protected by the first-stage low-speed compaction, and the melt temperature can be increased by the extremely high shear rate during the second-stage high-speed injection, breaking through the reaction threshold and triggering in-situ crosslinking. A three-dimensional covalent bond network across the interface is formed at the melt interface of the co-extrusion, tightly bonding the polypropylene resin, fiber mixture and continuous carbon cloth.

[0035] In Comparative Example 1, the system lost groups that could bond to the abundant hydroxyl groups on the surface of the fiber mixture, resulting in the inability to form a dense chemically encapsulated interface between the fiber and the resin matrix. This led to a break in the stress transmission path and a significant decrease in tensile strength and interlaminar shear strength. In Comparative Example 2, without the addition of interfacial chain-extending crosslinking bridges, the crosslinking system could not undergo targeted ester-amidation chain-extending crosslinking with the carboxyl groups of the maleic anhydride-grafted polypropylene compatibilizer in the system. The three-dimensional covalent network between the nonpolar polypropylene matrix and the rigid carbon skeleton could not complete the final ring closure, thus weakening the resin matrix's ability to disperse stress on the carbon fibers. In Comparative Example 3, without the introduction of a latent crosslinking agent, the multi-scale hybrid fibers and continuous carbon cloth are bonded solely by physical molecular chain entanglement during injection molding. Due to the enormous interfacial shear stress generated during the process, the physical entanglement is insufficient to maintain structural integrity, resulting in extremely low interlaminar shear force and easy delamination, leading to a significant reduction in mechanical properties. In Comparative Example 4, hemp and wood fibers are directly granulated without modification, and their surfaces are still tightly wrapped by pectin and lignin, lacking exposed active hydroxyl and amino groups. Even if the crosslinking agent is desealed at the back end, the released highly active groups cannot adhere to the fiber surface. Finding chemical anchoring points led to severe interfacial debonding, weakening the peel and tensile strength of the final product. In Comparative Example 5, the injection molding process was set to a single high-speed, high-pressure direct injection process. Due to the lack of physical buffering and compaction protection in the early stage of the high-viscosity melt, the huge fluid shear force and scouring force acted directly on the pre-placed continuous carbon cloth in the mold, causing severe scouring, wrinkling, and displacement of the carbon cloth insert. The macroscopic stress structure was completely destroyed, resulting in a sharp deterioration of the overall mechanical properties. Only using the first stage of low-speed injection to fill the cavity, the melt temperature could not exceed the 185°C unsealing threshold, and the interlayer shear strength showed The lower speed confirms that the first stage of low-speed, stable grounding is a necessary prerequisite for ensuring that the second stage of high-speed shear-triggered crosslinking can effectively play its interlayer anchoring role. In Comparative Example 6, when the absolute injection speed of the injection molding process is locked at a low speed, the melt fails to generate extremely strong internal fluid shear friction when passing through the narrow mold gate. The local temperature rise extreme value of the flow front cannot break through the reaction threshold of the latent crosslinking agent, resulting in the failure to trigger the multi-stage thermodynamic crosslinking reaction. As a result, its various strength performances are significantly reduced. In addition, pure low speed will cause partial folding and missing glue in the carbon layout due to excessive flow resistance, affecting the overall performance.

[0036] Experiment Example 2: Stability Performance Test The stability performance of the composite materials prepared in Examples 1-4 and Comparative Examples 5-8 was tested. The total release of volatile organic compounds was determined using headspace analysis of the release of organic matter from non-metallic materials according to the German Association of the Automotive Industry (VDA) standard 277:1995. The materials were incubated in a constant-temperature heating block at 120°C for 5 hours to promote the desorption of volatile organic compounds and achieve gas-liquid equilibrium. Subsequently, the gas in the headspace vial was extracted and injected into a gas chromatograph equipped with a flame ionization detector for analysis. The total carbon content released per gram of material was calculated using acetone as the calibration equivalent. The odor rating test was conducted according to the German Association of the Automotive Industry (VDA) standard 270:2022, and the sensory rating was based on a scale of 1 (odorless) to 6 (unbearable malodor). The results are shown in Table 3. The water absorption rate was determined according to GB / T 1034-2008. Standard-sized samples were placed in a 50℃ oven and dried completely for 24 hours. After cooling, the initial absolute dry mass (accurate to 0.1 mg) was measured on an analytical balance. The samples were then completely immersed in distilled or deionized water at 23℃ for 24 hours. Afterward, the samples were removed, and the surface free moisture was quickly wiped dry with filter paper or a lint-free cloth, and weighed again. The percentage of the difference between the two weighings relative to the initial dry mass was the water absorption rate. Dimensional shrinkage was determined according to GB / T [specific standard]. 25071-2010, the initial critical feature dimensions of injection molded parts at 23℃ were accurately measured using a coordinate measuring machine (CMM). The samples were then placed in a high-low temperature alternating test chamber and subjected to a specified thermal shock cycle, rapidly heating from -40℃ to 85℃, holding at each temperature for 2 hours, for ten cycles. After completing all the cycles, the samples were removed and allowed to recover for 24 hours at room temperature (23℃). The same feature dimensions were then measured again using a CMM. The absolute percentage of the dimensional change to the initial dimension was calculated, which is the dimensional shrinkage rate under the high-low temperature alternating environment. The test results are shown in Table 3. The immersion water absorption rate and dimensional shrinkage rate changes for Example 1 and Comparative Examples 5-8 are shown in Table 3. Figure 1 As shown.

[0037] Table 3. Test results of the examples and comparative examples Through Table 3, Figure 1 The results show that the composite material obtained in the comparative example, through adjustments to the process and processing parameters, exhibits significant differences in environmental emission performance, water absorption rate, and dimensional stability compared to the example. The example employs a constant low-temperature barrel temperature control combined with a multi-stage variable pressure wavefront injection process, which not only inhibits the thermal degradation of the fiber mixture and blocks the generation of volatile organic compounds through low-temperature dormancy, but also utilizes shear heat to trigger in-situ chemical cross-linking, constructing a dense cross-interface waterproof network. Furthermore, during the gelation period, high-pressure stepwise pressure is used to crush the reaction micropores, endowing the material with excellent environmental protection and dimensional stability.

[0038] In Comparative Example 5, the continuous carbon cloth was severely eroded and damaged by a single high-speed direct injection. As mentioned earlier, this mainly led to the collapse of the mechanical structure. In terms of stability, the tearing of the macroscopic structure also caused local resin enrichment and fiber exposure, resulting in more water intrusion channels and a slight increase in water absorption and shrinkage. In Comparative Example 6, due to the inability to generate sufficient shear heat from low-speed injection, the latent crosslinking agent failed to deseal. As mentioned earlier, the system relied solely on physical entanglement. These microscopic physical gaps lacking chemical bonds became capillary channels for water penetration, leading to a significant increase in the material's water absorption rate. Furthermore, it was highly susceptible to dimensional slip deformation under alternating thermal expansion and contraction conditions. In Comparative Example 7, under high-temperature conditions, the residual hemicellulose and a small amount of lignin in the natural hemp and wood fibers underwent severe thermal degradation and macromolecular chain breakage. The intense thermo-oxidative aging reaction released a large amount of small molecule volatile gases such as furfural and acetic acid. This leads to a significant increase in the total release of volatile organic compounds. In addition, the carbonization and degradation of the fiber itself also destroys its microstructure, generating a large number of internal thermal damage cracks, making it easy for moisture to penetrate and causing extremely high dimensional shrinkage and warping. In Comparative Example 8, the high-pressure first-stage holding and shaping step of 88MPa was canceled, and only the conventional low-pressure holding step of 45MPa was used. Although the chemical cross-linking reaction occurred normally, the reaction gas released during the cross-linking chain extension process and the trace amount of moisture from the natural fiber vaporization due to heat could not be forcibly crushed and discharged by external force. These residual microbubbles solidified inside the material to form sponge-like micropore defects. These micropores not only increase the water absorption rate, but also provide a huge free volume space for the free relaxation of polymer molecular chains under high and low temperature alternating environment, resulting in unbalanced internal stress in the product, increased dimensional shrinkage rate, and loss of the assembly precision requirements required for large automotive structural parts.

[0039] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An injection molding process for a high-strength hemp fiber composite material, characterized in that, Includes the following steps: After the plain weave carbon cloth pre-impregnated with hot melt adhesive film is hot-pressed and pre-formed, it is positioned in the load-bearing skeleton cavity of the injection mold and the mold is closed. The composite melt is added to the injection molding machine barrel and subjected to two-stage injection with variable pressure wave front to obtain a treated melt; the treated melt is subjected to a shear heat in-situ crosslinking process, and after two stages of crosslinking, a crosslinked melt is obtained; the crosslinked melt is subjected to a two-stage holding pressure shaping stage, and the mold is opened to eject the product to obtain a composite material; the composite melt is obtained by melt extrusion, pelletizing and drying of a composition of fiber mixture, treated carbon fiber, polypropylene resin, compatibilizer and latent crosslinking agent.

2. The injection molding process for a high-strength hemp fiber composite material according to claim 1, characterized in that, The two-stage injection process before the variable pressure wave includes the following steps: the composite melt is added to the barrel of a standard injection molding machine. The injection speed of the first stage is 25-35 mm / s, and the injection pressure is 40-50 MPa. When the screw reaches the stroke switching position, it switches to the second stage mold filling injection, with an injection speed of 120-150 mm / s and an injection pressure of 95-110 MPa. The melt fills the mesh reinforcing ribs and narrow cavities under shear rate to obtain the processed melt.

3. The injection molding process for a high-strength hemp fiber composite material according to claim 1, characterized in that, The two-stage crosslinking process includes the following steps: the processed melt is subjected to a first stage of shear-triggered unsealing crosslinking period, during which the melt temperature is raised to 195℃-205℃; the second stage is a ring-opening and chain-expanding crosslinking period, during which the melt fills the cavity and enters a high-pressure state, while the system temperature is maintained to obtain the crosslinked melt.

4. The injection molding process for a high-strength hemp fiber composite material according to claim 1, characterized in that, The two-stage pressure holding and shaping stage includes the following steps: the first-stage pressure holding pressure is 85-90MPa; then it switches to the second-stage pressure stabilization and holding pressure, which is 45-50MPa; after the pressure holding is completed, cooling water is introduced into the mold, and the composite material is obtained by demolding.

5. The injection molding process for a high-strength hemp fiber composite material according to claim 1, characterized in that, The preparation of the composite melt includes the following steps: adding 15-25 parts of the fiber mixture, 5-10 parts of the treated carbon fiber, 55-65 parts of the polypropylene resin, 4-6 parts of maleic anhydride-grafted polypropylene, 2-5 parts of the latent crosslinking agent composition, and an antioxidant compound into a co-rotating twin-screw extruder, and melting and mixing to obtain a mixed system; then drawing, water cooling, pelletizing, and drying to obtain the composite melt.

6. The injection molding process for a high-strength hemp fiber composite material according to claim 5, characterized in that, The fiber mixture is obtained by extracting sisal fiber and coniferous fiber by soaking in sodium hydroxide, activating and drying by spraying with an aqueous solution of aminopropyltriethoxysilane; the treated carbon fiber is obtained by immersing carbon fiber in a polyurethane emulsion, followed by drying and curing.

7. A high-strength hemp fiber composite material, characterized in that, The raw materials for preparation include a fiber mixture, treated carbon fibers, polypropylene resin, maleic anhydride-grafted polypropylene, and a latent crosslinking agent composition; the composite material is prepared by the injection molding process described in any one of claims 1-6.

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