A fiber-reinforced heat-shrinkable composite material and its preparation method

Abstract

This invention discloses a fiber-reinforced heat-shrinkable composite material and its preparation method, belonging to the technical field of heat-shrinkable materials. The raw materials for this composite material include modified reinforcing fibers, high-density polyethylene, epoxy resin, etc.; wherein the modified reinforcing fibers use chopped carbon fibers, glass fibers, or aramid fibers as the matrix, and the surface is grafted with graft monomers containing dynamic ester bonds through mercapto-olefin photopolymerization; the graft monomers are prepared by a two-step reaction of bisphenol A diglycidyl ether, 5-norbornene-2-carboxylic acid, and carboxylated polyethylene glycol monomethyl ether. The preparation process involves fiber modification, twin-screw segmented blending extrusion to form a preform, biaxial stretching, and heat preservation. This invention effectively improves the interfacial bonding force between the fiber and the matrix, achieving synergistic optimization of mechanical strength and heat-shrinkable function, exhibiting good shrinkage uniformity, low residual shrinkage rate, and excellent long-term stability, making it suitable for packaging and protective applications.

Description

Technical Field

[0001] This invention relates to the field of heat-shrinkable materials technology, and in particular to a fiber-reinforced heat-shrinkable composite material and its preparation method. Background Technology

[0002] Heat-shrinkable materials, as polymers with "shape memory" properties, are widely used in fields such as wire and cable joint protection, pipe wrapping, and packaging of various products because they can tightly wrap irregularly shaped workpieces after heating. Their core performance lies in whether the material can provide uniform, stable, and durable wrapping force after shrinkage, which directly depends on the material's mechanical strength, the controllability of its shrinkage behavior, and its dimensional stability during use.

[0003] However, commonly used polyolefin heat-shrinkable materials, such as high-density polyethylene (HDPE), while possessing good shrinkage properties, have limited inherent mechanical strength, making them unsuitable for applications requiring high puncture resistance, tear resistance, or long-term pressure resistance. Introducing reinforcing fibers (such as glass fiber and carbon fiber) is a direct and effective way to improve mechanical properties. However, the inherent poor compatibility and weak interfacial bonding between fibers and the polymer matrix often lead to low stress transfer efficiency. During heat shrinkage, uneven shrinkage, wrinkling, and even cracking can easily occur due to interfacial debonding, severely limiting the effectiveness of fiber reinforcement.

[0004] For fiber-matrix interface modification, existing technologies mostly employ silane coupling agents or physical coating of polymer layers onto the fiber surface. While these methods can improve wettability to some extent, the resulting interface layers are often static physical adsorption or weak chemical bonding. Under dynamic deformations such as thermal cycling, stretching, and shrinkage, the interface is prone to slippage or failure, failing to maintain a strong and tough interfacial bond over the long term. Furthermore, an ideal interface design should not only achieve strong bonding but also possess the ability to adaptively adjust to stress concentration during shrinkage, avoiding damage caused by localized stress accumulation. In recent years, dynamic covalent chemistry (such as dynamic ester bonds) has endowed materials with reversible rearrangement and stress relaxation capabilities, while flexible polymer segments (such as polyethylene glycol) can effectively improve the toughness and deformation compatibility of materials. How to synergistically introduce the stress dispersion characteristics of dynamic ester bonds and the toughening and buffering effects of flexible segments into the interface design of fiber-reinforced heat-shrinkable composites, thereby simultaneously achieving high strength, highly uniform shrinkage, and excellent long-term stability, has become a pressing problem in this field. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a fiber-reinforced heat-shrinkable composite material and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a fiber-reinforced heat-shrinkable composite material, comprising the following raw materials in parts by weight: modified reinforcing fiber: 15-30 parts, high-density polyethylene: 30-50 parts, epoxy resin: 15-30 parts, auxiliary additives: 0.5-1 parts, curing agent: 0.1-0.3 parts, crosslinking agent: 0.3-0.5 parts, initiator: 0.001-0.003 parts; The modified reinforcing fiber uses chopped carbon fiber, chopped alkali-free glass fiber or chopped aramid fiber as the fiber substrate, and grafted monomers containing dynamic ester bonds are grafted onto the surface through mercapto-olefin photopolymerization.

[0007] Preferably, the density of the high-density polyethylene is 0.941-0.965 g / cm³. 3 Crystallinity ≥65%, number average molecular weight 200,000-500,000; epoxy value of epoxy resin 0.48-0.52 eq / 100g.

[0008] Preferably, the auxiliary additive refers to a mixture of antioxidant 1010 and ultraviolet absorber UV-531 in a weight ratio of 1:1.

[0009] Preferably, the curing agent refers to at least one of diethylenetriamine, triethylenetetramine, or tetraethylenepentamine.

[0010] Preferably, the crosslinking agent refers to trimethylolpropane triacrylate.

[0011] Preferably, the initiator is dicumyl peroxide.

[0012] Preferably, the chopped carbon fibers have a length of 3-5 mm and a diameter of 7-10 μm; the chopped alkali-free glass fibers have a length of 3-5 mm and a diameter of 10-13 μm; and the chopped aramid fibers have a length of 3-5 mm and a diameter of 12-15 μm.

[0013] Preferably, the grafted monomer is prepared by the following method: (1) Under nitrogen protection, bisphenol A diglycidyl ether, 5-norbornene-2-carboxylic acid and triethylamine were added to N,N-dimethylformamide, heated to 90-110℃, and stirred for 12-16 h. After cooling to room temperature, ethyl acetate was added, and the mixture was washed twice with dilute hydrochloric acid solution, and then washed 3-5 times with deionized water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to obtain the intermediate. The chemical reaction equation is as follows: This step is a carboxyl-induced epoxy ring-opening reaction. Triethylamine (a tertiary amine) acts as a base catalyst, first reacting with the carboxyl group of 5-norbornen-2-carboxylic acid through hydrogen bonding or slight proton abstraction, thereby activating the carboxyl group. Subsequently, this activated carboxylic acid molecule acts as a nucleophile, attacking the less sterically hindered carbon atom on the epoxy ring of bisphenol A diglycidyl ether, leading to an SN2-type nucleophilic ring-opening of the epoxy bond, generating an alkoxy anion intermediate. This alkoxy anion rapidly abstracts a proton from a neighboring protonated triethylamine or another carboxylic acid molecule, transforming itself into a stable hydroxyl group, while simultaneously regenerating the free triethylamine catalyst, yielding the intermediate. (2) Under nitrogen protection, in a reaction vessel equipped with a water separator and a reflux condenser, the intermediate, carboxylated polyethylene glycol monomethyl ether, p-toluenesulfonic acid, and toluene were added. The mixture was heated to 100-120℃ with stirring and reacted for 6-10 hours. After the reaction was completed, the mixture was cooled to room temperature and washed successively with sodium carbonate solution and deionized water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was recrystallized in a mixed solution of ethyl acetate / petroleum ether to obtain the grafted monomer. The chemical reaction equation is as follows: This step is an esterification reaction catalyzed by toluenesulfonic acid. The hydroxyl and carboxyl groups of the intermediate polyethylene glycol monomethyl ether undergo esterification to yield the grafted monomer. The generated water azeotropically reacts with toluene and is discharged from the system through a water separator, driving the reaction forward. The product is then... 1 Characterized by 1H NMR.

[0014] More preferably, the grafted monomer is prepared by the following method: (1) Under nitrogen protection, bisphenol A diglycidyl ether, 5-norbornene-2-carboxylic acid and triethylamine were added to N,N-dimethylformamide, heated to 100°C, stirred for 14 h, cooled to room temperature, ethyl acetate was added, washed twice with dilute hydrochloric acid solution, and then washed 3-5 times with deionized water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to obtain the intermediate. (2) Under nitrogen protection, in a reaction vessel equipped with a water separator and a reflux condenser, the intermediate, carboxylated polyethylene glycol monomethyl ether, p-toluenesulfonic acid and toluene were added, and the temperature was raised to 110°C with stirring. The reaction was carried out for 8 hours. After the reaction was completed, the mixture was cooled to room temperature and washed with sodium carbonate solution and deionized water in sequence. The organic phase was dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was recrystallized in a mixed solution of ethyl acetate / petroleum ether to obtain the grafted monomer.

[0015] Preferably, the chemical structural formula of the grafted monomer is as follows: .

[0016] Preferably, the molar ratio of bisphenol A diglycidyl ether and 5-norbornene-2-carboxylic acid in (1) is 1:2-2.1.

[0017] More preferably, in (1), the molar ratio of bisphenol A diglycidyl ether and 5-norbornene-2-carboxylic acid is 1:2.05.

[0018] Preferably, in (1), the weight ratio of bisphenol A diglycidyl ether, triethylamine, N,N-dimethylformamide and ethyl acetate is 1:0.01-0.03:8-12:5-10.

[0019] More preferably, in (1), bisphenol A diglycidyl ether, triethylamine, N,N-dimethylformamide and ethyl acetate are in a weight ratio of 1:0.02:10:8.

[0020] Preferably, the concentration of the dilute hydrochloric acid solution in (1) is 0.1-0.5 mol / L.

[0021] More preferably, the concentration of the dilute hydrochloric acid solution in (1) is 0.3 mol / L.

[0022] Preferably, the molar ratio of the intermediate and carboxylated polyethylene glycol monomethyl ether in (2) is 1:2-2.3.

[0023] More preferably, in (2), the intermediate and carboxylated polyethylene glycol monomethyl ether have a molar ratio of 1:2.1.

[0024] Preferably, in step (2), the intermediate, p-toluenesulfonic acid and toluene are in a weight ratio of 1:0.01-0.03:10-20.

[0025] More preferably, in (2), the intermediate, p-toluenesulfonic acid and toluene are in a weight ratio of 1:0.02:15.

[0026] Preferably, the concentration of sodium carbonate solution in (2) is 5-10 wt%.

[0027] More preferably, the concentration of the sodium carbonate solution in (2) is 5 wt%.

[0028] Preferably, the volume ratio of ethyl acetate to petroleum ether in the ethyl acetate / petroleum ether mixed solution in (2) is 3:1.

[0029] Preferably, the molecular weight of the carboxylated polyethylene glycol monomethyl ether in (2) is 808.95, and the number of repeating units of ethylene glycol in its molecular structure is 16.

[0030] Furthermore, the present invention also provides a method for preparing a fiber-reinforced heat-shrinkable composite material, comprising the following steps: S1. Immerse the fiber substrate in a nitric acid solution, heat to 50-60℃, and react for 1-2 hours with stirring. After removal, rinse with deionized water until the pH of the washing solution is neutral. After drying, immerse in a 3-mercaptopropyltrimethoxysilane ethanol solution under nitrogen protection, heat to 40-60℃, and react with stirring for 3-6 hours. After removal, rinse with deionized water for 1-3 minutes and dry to obtain thiolized fibers. Mix the thiolized fibers, grafted monomers, and 2-hydroxy-2-methyl-1-phenyl-1-propanone, disperse in anhydrous ethanol at a solid-liquid ratio of 1:5-10, and irradiate with ultraviolet light for 10-20 minutes with stirring. After removal, rinse with deionized water for 3-5 minutes and dry to obtain modified reinforcing fibers. In this step, nitric acid, acting as a strong oxidizing agent, first attacks the inert structures on the fiber surface (such as the graphitized structure of carbon fibers and the siloxane structure of glass fibers), causing the surface C-C bonds and Si-O bonds to break, thereby generating polar active sites such as hydroxyl and carboxyl groups, providing reaction anchors for subsequent silanization grafting. The methoxy group of 3-mercaptopropyltrimethoxysilane first undergoes hydrolysis to generate silanol groups (-SiOH). Subsequently, the silanol groups undergo dehydration condensation with the hydroxyl groups generated by oxidation on the fiber surface, forming Si-O-fiber covalent bonds, anchoring the silane coupling agent to the fiber surface, ultimately... The active sites of thiol groups (-SH) are exposed to complete the thiolization modification of the fiber. Finally, the photoinitiator (2-hydroxy-2-methyl-1-phenyl-1-propanone) is cleaved under ultraviolet light to generate active free radicals. These active free radicals abstract protons from the thiol groups (-SH) on the fiber surface to generate thiol free radicals (-S·). The thiol free radicals attack the norbornene double bond (C=C) of the grafted monomer, undergoing an addition reaction to form a carbon free radical. The carbon free radicals abstract protons from another thiol group to generate an SC covalent bond (completing the grafting), thus grafting the grafted monomer onto the fiber surface. S2. Modified reinforcing fiber, high-density polyethylene, epoxy resin, curing agent, crosslinking agent, and initiator are added through the main feed port of a twin-screw extruder and reacted in the front section of the twin-screw extruder for 5-10 minutes. Then, the material is conveyed to the side feed port of the twin-screw extruder, and auxiliary additives are added through the side feed port. The material is then blended in the rear section of the twin-screw extruder for 10-20 minutes. Finally, the material is extruded through the die head equipped with a slit-type preform die to the cooling roller and cooled to room temperature to obtain the composite material preform. S3. After heating the composite material preform to 85-95℃, it is subjected to biaxial stretching. While maintaining the stretched state, the temperature is raised to 110-120℃ and held for 20-40 minutes. After cooling to room temperature, a fiber-reinforced heat-shrinkable composite material is obtained.

[0031] Preferably, the concentration of the nitric acid solution in S1 is 30-60 wt%; and the concentration of the 3-mercaptopropyltrimethoxysilane ethanol solution is 5-15 wt%.

[0032] Preferably, in S1, the mercapto-modified fiber, the grafted monomer mixture, and 2-hydroxy-2-methyl-1-phenyl-1-propanone are mixed in a weight ratio of 1:0.1-0.2:0.001-0.005.

[0033] Preferably, the wavelength of the ultraviolet light in S1 is 365 nm, and the intensity is 10-20 mW / cm². 2 .

[0034] Preferably, the twin-screw extruder in S2 is equipped with a main feed port and a side feed port, wherein the distance between the side feed port and the die head is two-thirds of the length of the entire machine.

[0035] Preferably, in S2, the front section of the twin-screw extruder, from the main feed port to the side feed port, has a temperature of 100-120℃.

[0036] Preferably, in S2, the rear section of the twin-screw extruder extends from the side feed port to the die head, with a rear section temperature of 140-160℃ and a screw speed of 40-70 rpm.

[0037] Preferably, the thickness of the composite material blank in S2 is 1-3 mm.

[0038] Preferably, the biaxial stretching ratio in S3 is 2.5-3.5 times.

[0039] Preferably, the mechanism of action of the fiber-reinforced heat-shrinkable composite material in this invention is explained as follows: The prerequisite for the thermal shrinkage of the composite material in this invention is the "stable construction of shrinkage memory." The PEG flexible segments introduced by carboxylated polyethylene glycol monomethyl ether (mPEG-COOH) in the grafted monomer play a crucial role. In the initial stage of preparation, the twin-screw extruder employs a low-to-high temperature gradient. The high-density polyethylene (HDPE), epoxy resin, modified reinforcing fibers, and crosslinking initiator / curing agent fed into the main feed undergo initial crosslinking within this temperature range. At this time, due to their good flexibility, the PEG flexible segments can move along with the HDPE... The epoxy dual matrix molecular chains soften synchronously, avoiding the initial cross-linking network being too brittle and affecting subsequent stretching. Then, during the biaxial stretching of the preform, the PEG flexible segments further enhance the plasticity of the matrix system, allowing the modified fiber and dual matrix molecular chains to smoothly and synchronously orient along the stretching direction. At the same time, the PEG segments themselves also store some shrinkage potential energy along with the oriented structure. Finally, during the heat preservation and shaping stage, the remaining cross-linking reaction is completed to form a complete three-dimensional cross-linking network. The PEG flexible segments are stably locked in the oriented structure, which not only ensures the stable storage of shrinkage potential energy, but also avoids the defect of easy cracking of a single rigid cross-linking network, providing a reliable memory basis for subsequent heat shrinkage.

[0040] The thermal shrinkage process during use is a synergistic process of "crosslink network relaxation - potential energy release - molecular chain deorientation". The PEG flexible segments introduced by mPEG-COOH work synergistically with the dynamic ester bonds, which is the core to ensure uniform shrinkage and avoid cracking. When the composite material is heated to the softening temperature, the three-dimensional crosslink network relaxes due to the intensified thermal motion of the molecular chains, and the shrinkage potential energy stored in the oriented structure is released. At this time, the PEG flexible segments, with their excellent flexibility, provide sufficient space for the molecular chain deorientation, driving the composite material to shrink smoothly in the opposite direction of stretching. At the same time, if local stress concentration occurs during the shrinkage process, the PEG flexible segments can buffer the stress through their own segment curling and stretching, and achieve stress dispersion in conjunction with the reversible transesterification reaction of the dynamic ester bonds, effectively avoiding material cracking or poor adhesion caused by stress concentration. After cooling, the PEG flexible segments re-stabilize with the crosslink network, allowing the composite material to regain rigidity while retaining a certain degree of flexibility, ensuring the durability of the adhesion to the workpiece.

[0041] The composite material of this invention uses a modified reinforcing fiber surface grafted with a monomer containing dynamic ester bonds. The PEG flexible segments can form a tight physical entanglement with the HDPE molecular chain, and the polar groups of the dynamic ester bonds can form stable hydrogen bonds with the epoxy crosslinking products. At the same time, the covalent bonds between the grafted monomer and the fiber ensure that the grafted layer will not fall off during stretching and shrinkage, achieving a high-strength bond between the fiber and the matrix. During the stretching energy storage and deorientation shrinkage stages of heat shrinkage, this strong interfacial bond can ensure that the molecular chains of the fiber and the matrix move synchronously, avoiding shrinkage potential energy loss or uneven shrinkage due to interfacial slippage. The reversible rearrangement characteristics of the dynamic ester bonds further alleviate the stress concentration at the interface during shrinkage, providing interfacial support for the stable realization of the heat shrinkage function.

[0042] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention, through the synthesis of a grafted monomer, simultaneously introduces dynamic ester bonds and flexible PEG segments onto the fiber surface, effectively solving the compatibility problem between reinforcing fibers and high-density polyethylene (HDPE) and epoxy resin matrices. The flexible PEG segments can form tight physical entanglement with HDPE molecular chains, while the dynamic ester bonds can form stable hydrogen bonds with epoxy resin crosslinking products. Combined with covalent anchoring on the fiber surface, this constructs a high-strength interfacial bonding system. Simultaneously, the reversible transesterification reaction of the dynamic ester bonds and the synergistic effect of the flexibility of the PEG segments can rapidly disperse localized stress during heat shrinkage, avoiding cracking or wrinkling caused by stress concentration, and significantly improving the shrinkage uniformity and bonding tightness of the composite material.

[0043] 2. This invention employs a synergistic system of high-density polyethylene and epoxy resin dual matrix, combined with the reinforcing effect of chopped reinforcing fibers, to achieve an optimized balance between mechanical properties and heat shrinkage function. The rigid structure of epoxy resin and the flexibility of high-density polyethylene complement each other, and together with the high modulus characteristics of fibers, significantly improve the tensile strength and deformation resistance of the composite material. Simultaneously, the dual matrix undergoes mild cross-linking control, ensuring the stable construction of the heat shrinkage memory structure without losing shrinkage plasticity due to excessive cross-linking. This allows the material to achieve efficient and smooth heat shrinkage while possessing excellent mechanical support, adapting to various packaging and protection scenarios.

[0044] 3. This invention endows the composite material with excellent long-term stability through a multi-structural design. The dense three-dimensional network formed by the synergistic cross-linking of the two matrices, combined with the rigid skeleton effect of the fibers, can firmly lock the tensile orientation structure, significantly reducing residual shrinkage and ensuring long-term adhesion without rebound after shrinkage. The dual benzene ring structure in the grafted monomers synergistically enhances the thermal stability of the material, and the synergistic effect of antioxidants and UV absorbers effectively delays the aging and degradation of the material during use. This allows the composite material to maintain stable mechanical properties and shrinkage characteristics even in complex usage environments, significantly expanding its application scenarios and service life. Attached Figure Description

[0045] Figure 1 The grafted monomer in the preparation example of this invention 1 H NMR spectrum. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0047] Preparation example: The specific preparation method of the grafted monomer includes the following steps: (1) Under nitrogen protection, 100g of bisphenol A diglycidyl ether, 83.21g of 5-norbornene-2-carboxylic acid and 2g of triethylamine were added to 1kg of N,N-dimethylformamide, heated to 100℃, stirred for 14h, cooled to room temperature, and 800g of ethyl acetate were added. The mixture was washed twice with 0.3mol / L dilute hydrochloric acid solution, and then washed 3-5 times with deionized water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to obtain the intermediate. (2) Under nitrogen protection, 100g of intermediate, 275.44g of carboxylated polyethylene glycol monomethyl ether, 2g of p-toluenesulfonic acid and 1.5kg of toluene were added to a reaction vessel equipped with a water separator and a reflux condenser. The mixture was heated to 110℃ with stirring and reacted for 8h. After the reaction was completed, the mixture was cooled to room temperature and washed successively with a 5wt% sodium carbonate solution and deionized water. The organic phase was dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was recrystallized in a mixed solution of ethyl acetate and petroleum ether (the volume ratio of ethyl acetate and petroleum ether was 3:1) to obtain the grafted monomer.

[0048] Comparative preparation example 1: The difference between comparative preparation example 1 and preparation example is that the intermediate obtained in (1) is used as the grafting monomer.

[0049] Comparative Preparation Example 2: The difference between Comparative Preparation Example 2 and the Preparation Example is that 5-norbornene-2-carboxylic acid is replaced with acrylic acid.

[0050] Example 1: A specific preparation method of a fiber-reinforced heat-shrinkable composite material, comprising the following steps: S1. Immerse 150g of short-cut carbon fibers (3-5mm in length and 7-10μm in diameter) in a 30wt% nitric acid solution, heat to 50℃, and react for 1h with stirring. After removal, rinse with deionized water until the pH of the washing solution is neutral, dry, and then immerse in a 5wt% 3-mercaptopropyltrimethoxysilane ethanol solution under nitrogen protection. Heat to 40℃ and stir for 3h. After removal, rinse with deionized water for 1min and dry to obtain mercapto-modified fibers. Mix 150g of mercapto-modified fibers, 15g of graft monomer prepared according to the preparation example, and 0.15g of 2-hydroxy-2-methyl-1-phenyl-1-propanone, disperse in anhydrous ethanol at a solid-liquid ratio of 1:5, and stir. Then, use a wavelength of 365nm and an intensity of 10mW / cm². 2 Irradiate with ultraviolet light for 10 minutes; after removal, rinse with deionized water for 3 minutes, and dry to obtain modified reinforced fiber; S2. Add 150g of modified reinforcing fiber, 300g of high-density polyethylene, 150g of epoxy resin, 1g of triethylenetetramine, 3g of trimethylolpropane triacrylate, and 0.01g of dicumyl peroxide through the main feed port of a twin-screw extruder (equipped with a main feed port and a side feed port, wherein the distance between the side feed port and the die head is two-thirds of the total length of the extruder). Maintain the screw speed at 40 rpm and extrude at 100°C in the front section of the twin-screw extruder (from the main feed port to the side feed port). After reacting for 5 minutes, the material is then fed to the side feed port of the twin-screw extruder. 5g of auxiliary additive (a mixture of antioxidant 1010 and UV absorber UV-531 in a 1:1 weight ratio) is added through the side feed port. The material is then blended at 140°C for 10 minutes in the rear section of the twin-screw extruder (from the side feed port to the die head). Finally, the material is extruded through the die head equipped with a slit preform die to the cooling roller. After cooling to room temperature, a composite preform with a thickness of 1mm is obtained. S3. The composite material preform is heated to 85°C and then subjected to biaxial stretching with a stretching ratio of 2.5 times. While maintaining the stretching state, the temperature is raised to 110°C and held for 20 minutes. After cooling to room temperature, a fiber-reinforced heat-shrinkable composite material is obtained.

[0051] Example 2: A specific preparation method of a fiber-reinforced heat-shrinkable composite material, comprising the following steps: S1. Immerse 200g of short-cut carbon fibers (3-5mm in length and 7-10μm in diameter) in a 40wt% nitric acid solution, heat to 55℃, and react for 1.5h with stirring. After removal, rinse with deionized water until the pH of the washing solution is neutral, dry, and then immerse in a 10wt% 3-mercaptopropyltrimethoxysilane ethanol solution under nitrogen protection. Heat to 50℃ and stir for 4.5h. After removal, rinse with deionized water for 2min and dry to obtain mercapto-modified fibers. Mix 200g of mercapto-modified fibers, 30g of graft monomer prepared according to the preparation example, and 0.6g of 2-hydroxy-2-methyl-1-phenyl-1-propanone, disperse in anhydrous ethanol at a solid-liquid ratio of 1:8, and stir. Then, use a wavelength of 365nm and an intensity of 15mW / cm². 2 Irradiate with ultraviolet light for 15 minutes; after removal, rinse with deionized water for 4 minutes, and dry to obtain modified reinforced fiber; S2. Add 200g of modified reinforcing fiber, 400g of high-density polyethylene, 200g of epoxy resin, 2g of triethylenetetramine, 4g of trimethylolpropane triacrylate, and 0.02g of dicumyl peroxide through the main feed port of a twin-screw extruder (equipped with a main feed port and a side feed port, wherein the distance between the side feed port and the die head is two-thirds of the total length of the extruder). Maintain the screw speed at 60 rpm and feed at 110°C in the front section of the twin-screw extruder (from the main feed port to the side feed port). After reacting for 8 minutes, the material is then fed to the side feed port of a twin-screw extruder. 8g of auxiliary additives (a mixture of antioxidant 1010 and UV absorber UV-531 in a 1:1 weight ratio) are added through the side feed port. The material is then blended at 150°C for 15 minutes in the rear section of the twin-screw extruder (from the side feed port to the die head). Finally, the material is extruded through the die head equipped with a slit preform die to the cooling roller. After cooling to room temperature, a composite preform with a thickness of 2mm is obtained. S3. After heating the composite material blank to 90°C, it is subjected to biaxial stretching with a stretching ratio of 3 times. While maintaining the stretching state, the temperature is raised to 115°C and held for 30 minutes. After cooling to room temperature, a fiber-reinforced heat-shrinkable composite material is obtained.

[0052] Example 3: A specific preparation method of a fiber-reinforced heat-shrinkable composite material, comprising the following steps: S1. Immerse 300g of short-cut carbon fibers (3-5mm in length and 7-10μm in diameter) in a 60wt% nitric acid solution, heat to 60℃, and react for 2h with stirring. After removal, rinse with deionized water until the pH of the washing solution is neutral, dry, and then immerse in a 15wt% 3-mercaptopropyltrimethoxysilane ethanol solution under nitrogen protection. Heat to 60℃ and stir for 6h. After removal, rinse with deionized water for 3min and dry to obtain mercapto-modified fibers. Mix 300g of mercapto-modified fibers, 60g of graft monomer prepared according to the preparation example, and 1.5g of 2-hydroxy-2-methyl-1-phenyl-1-propanone, disperse in anhydrous ethanol at a solid-liquid ratio of 1:10, and stir. Then, use a wavelength of 365nm and an intensity of 20mW / cm². 2 Irradiate with ultraviolet light for 20 minutes; after removal, rinse with deionized water for 5 minutes, and dry to obtain modified reinforced fiber; S2. Add 300g of modified reinforcing fiber, 500g of high-density polyethylene, 300g of epoxy resin, 3g of triethylenetetramine, 5g of trimethylolpropane triacrylate, and 0.03g of dicumyl peroxide through the main feed port of a twin-screw extruder (equipped with a main feed port and a side feed port, wherein the distance between the side feed port and the die head is two-thirds of the total length of the extruder). Maintain the screw speed at 70 rpm and reverse the feed at 120°C in the front section of the twin-screw extruder (from the main feed port to the side feed port). After 10 minutes, the material is fed to the side feed port of the twin-screw extruder. 10g of auxiliary additive (a mixture of antioxidant 1010 and UV absorber UV-531 in a 1:1 weight ratio) is added through the side feed port. The material is then blended at 160°C for 20 minutes in the rear section of the twin-screw extruder (from the side feed port to the die head). Finally, the material is extruded through the die head equipped with a slit preform die to the cooling roller. After cooling to room temperature, a composite preform with a thickness of 3mm is obtained. S3. The composite material preform is heated to 95°C and then subjected to biaxial stretching with a stretching ratio of 3.5 times. While maintaining the stretching state, the temperature is raised to 120°C and held for 40 minutes. After cooling to room temperature, a fiber-reinforced heat-shrinkable composite material is obtained.

[0053] Example 4: The difference between Example 4 and Example 2 is that short-cut carbon fiber is replaced with short-cut glass fiber, and triethylenetetramine is replaced with diethylenetriamine.

[0054] Example 5: The difference between Comparative Example 5 and Example 2 is that short-cut carbon fibers are replaced with short-cut aramid fibers, and triethylenetetramine is replaced with tetraethylenepentamine.

[0055] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the graft monomer prepared according to the preparation example is replaced with the graft monomer prepared according to Comparative Preparation Example 1.

[0056] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the graft monomer prepared according to the preparation example is replaced with the graft monomer prepared according to Comparative Preparation Example 2.

[0057] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the thiolized fiber obtained in S1 is used as the modified reinforcing fiber.

[0058] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that step S1 is omitted, and the modified reinforcing fiber in step S2 is replaced with short-cut carbon fiber.

[0059] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that epoxy resin is not added, the amount of high-density polyethylene added is increased to 600g, and triethylenetetramine is not added.

[0060] Performance testing: 1. Heat shrinkage rate and shrinkage uniformity test: Referring to GB / T 13519-2008 "Heat Shrinkable Film", 100mm×100mm square samples were cut from the finished products of Examples 1-5 and Comparative Examples 1-5. The original length (L0) of the sample in the circumferential direction (corresponding to the transverse stretching direction) and the axial direction (corresponding to the longitudinal stretching direction) were measured with a vernier caliper with an accuracy of 0.01mm. The sample was suspended in a constant temperature oven at 115℃ and kept at that temperature for 30min. After being taken out and allowed to cool naturally to room temperature, the final length (L1) in both directions was measured again. The heat shrinkage rate in the circumferential and axial directions was calculated ((L0-L1) / L0×100%). The shrinkage uniformity was evaluated by measuring the difference in shrinkage rate at different positions of each sample and by the maximum difference in shrinkage rate at different positions. The experimental results are shown in Table 1.

[0061] 2. Tensile strength and elongation at break test: Referring to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", dumbbell-shaped Type I specimens (length 150 mm, gauge length 50 mm, width 10 mm, thickness subject to actual conditions) were cut from the composite material products of each example and comparative example in both circumferential and axial directions. The specimens were installed on a universal testing machine, and the test environment temperature was set to 23℃, relative humidity 50%, and tensile speed 50 mm / min. The equipment was started until the specimen broke. The maximum tensile force and gauge length elongation at break were recorded for each specimen. The tensile strength and elongation at break were calculated. The experimental results are shown in Table 1.

[0062] 3. Residual shrinkage rate test: 50mm×50mm square samples were cut from the finished products of Examples 1-5 and Comparative Examples 1-5. The samples were heated to 115℃ and kept at that temperature for 30min according to the heat shrinkage rate test method. After cooling to room temperature, the shrinkage size (L1) was measured. The sample was then placed in an oven at 115℃ for 20min and kept at that temperature for 20min. After cooling, the final size (L2) was measured. The residual shrinkage rate ((L1-L2) / L1×100%) was calculated. The experimental results are shown in Table 1.

[0063] Table 1 Performance Test Results

[0064] Data Analysis: As can be seen from the performance test data in Table 1, the fiber-reinforced heat-shrinkable composite materials prepared by the technical solution of the present invention in Examples 1-5 have good heat-shrinkage function, excellent mechanical properties and extremely low residual shrinkage rate, among which Example 2 has the best comprehensive performance.

[0065] Example 2 exhibits excellent heat shrinkage performance, which may be due to the PEG flexible segments introduced by carboxylated polyethylene glycol monomethyl ether in the grafted monomer used in Example 2. These segments can form physical entanglements with the HDPE molecular chains, while the dynamic ester bonds can disperse shrinkage stress through reversible transesterification, avoiding local differences in shrinkage rates. The dual-matrix system of HDPE and epoxy resin is controlled by a "low at the beginning and high at the end" processing temperature gradient, achieving mild synergistic crosslinking. This ensures sufficient energy storage in the oriented structure without hindering shrinkage due to excessive crosslinking. The amount of chopped carbon fiber is in the optimal range of "saturated reinforcement effect - uniform dispersion," and its high thermal conductivity further promotes uniform temperature transfer, ensuring symmetrical bidirectional shrinkage. In contrast, Comparative Example 1, due to the replacement of the grafted monomer with an intermediate without PEG segments, lacked flexible buffering and stress dispersion units, resulting in a significant decrease in shrinkage uniformity. Comparative Example 2, by replacing 5-norbornene-2-carboxylic acid with acrylic acid, demonstrated the unique structural advantages of norbornene's ring-strained double bonds. The ring strain allows the double bonds to form a directional grafting structure during thiol-olefin photopolymerization, resulting in a symmetrical distribution of grafted monomers on the fiber surface. In contrast, the ordinary double bond grafting of acrylic acid lacks directionality, leading to uneven distribution of dynamic ester bonds and PEG segments on the fiber surface, resulting in an imbalance in stress transmission during shrinkage. Furthermore, the ring structure of norbornene can interact with bisphenol A. The diglycidyl ether forms a steric synergy of benzene rings, improving the compatibility of the fiber in polyethylene and epoxy resin matrices. Comparative Example 3 uses only mercapto-modified fibers, lacking the control of grafted monomers on the material. Slippage easily occurs between the fiber and high-density polyethylene and epoxy resin, disrupting shrinkage symmetry. Comparative Example 4 uses unmodified fibers, whose surface inertness leads to poor dispersibility and weak interfacial bonding, resulting in localized stress concentration during shrinkage. Comparative Example 5 lacks the constraint of epoxy resin, causing excessive disorientation of HDPE molecular chains, resulting in a higher shrinkage rate and affecting uniformity. Example 2 achieves a balance in mechanical properties because the short-cut carbon fiber itself has high modulus characteristics. Through the "PEG flexible segment entanglement + dynamic ester bond hydrogen bond + covalent bond anchoring" of the grafted monomer, it forms a high-strength interfacial bond with polyethylene and epoxy resin, ensuring that the fiber and the matrix bear the load synchronously during stretching, which significantly improves the tensile strength. At the same time, the combination of PEG flexible segments and the high elasticity of HDPE alleviates the rigid constraints brought by carbon fiber and epoxy resin, and retains sufficient toughness to adapt to the requirements of biaxial stretching film formation. All comparative examples exhibited insufficient mechanical properties due to the lack of core technical features: Comparative Example 1 lacked PEG segments, resulting in weakened interfacial bonding, inadequate fiber reinforcement, and decreased tensile strength; in Comparative Example 2, the SC bonds formed by the ring-strained double bonds of norbornene exhibited higher bond energy due to ring strain stabilization, and the grafted structure was symmetrically distributed, enabling more uniform stress transfer. In contrast, the ordinary double-bond grafted bonds of acrylic acid lacked this stabilizing effect, making them prone to bond breakage during stretching. Furthermore, the linear structure of acrylic acid could not form spatial coordination with the diphenyl ring of bisphenol A diglycidyl ether, leading to a decrease in the overall integrity of the interfacial bonding, resulting in reduced strength and toughness. None of them are as good as Example 2; Comparative Example 3 lacks complete interface regulation of grafted monomers, and the fiber and polyethylene and epoxy resin rely only on weak interaction of thiol groups. When stretched, interface slip is prone to occur, and both strength and toughness are affected; The unmodified fiber of Comparative Example 4 is only in physical contact with the matrix, and the interface bonding is extremely poor. The fiber cannot bear the load, the tensile strength is greatly reduced, and the insufficient toughness causes the elongation at break to be close to the film-forming critical value; Comparative Example 5 has no epoxy resin support and relies only on the crosslinking network of HDPE. The strength improvement is limited. At the same time, due to the lack of rigid constraints, the elongation at break is high, but it is easy to deform during use, which does not meet the actual application requirements; Example 2 showed the lowest residual shrinkage rate, which may be because the free radical crosslinking of HDPE and the epoxy-amine ring-opening crosslinking of epoxy resin form a synergistic crosslinking network with a dense and stable structure that can firmly lock the orientation structure after biaxial stretching; the rigid skeleton of the short-cut carbon fiber further restricts the secondary movement of the molecular chain and avoids rebound after shrinkage; the flexible PEG segments and dynamic ester bonds in the grafted monomer can buffer residual stress and reduce secondary shrinkage caused by network relaxation. Comparative Example 1, lacking PEG segments, suffers from insufficient flexibility in its crosslinking network, resulting in ineffective release of residual stress and a tendency for secondary shrinkage. Comparative Example 2, while the acrylic acid grafted monomer introduces dynamic ester bonds, benefits from the cyclic structure of norbornene, which provides spatial anchoring to the crosslinking network and enhances its stability. The linear structure of acrylic acid lacks this anchoring effect, weakening the crosslinking network's ability to lock the orientation structure and leading to increased residual shrinkage. Comparative Example 3, with only thiol-treated fibers, fails to form strong interfacial bonds, making the orientation structure prone to loosening due to interfacial slippage during stretching, thus increasing residual shrinkage. Comparative Example 4, with its weak interfacial bond between the unmodified fibers and the matrix, cannot effectively support the crosslinking network, resulting in unstable orientation structure locking. Comparative Example 5, lacking the synergistic crosslinking of epoxy resin, suffers from insufficient density and stability in its single HDPE crosslinking network, making it difficult to lock the orientation structure long-term, resulting in significantly higher residual shrinkage.

[0066] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A fiber-reinforced heat-shrinkable composite material, characterized in that, The raw materials include the following parts by weight: modified reinforcing fiber: 15-30 parts, high-density polyethylene: 30-50 parts, epoxy resin: 15-30 parts, auxiliary additives: 0.5-1 parts, curing agent: 0.1-0.3 parts, crosslinking agent: 0.3-0.5 parts, and initiator: 0.001-0.003 parts; The modified reinforcing fiber uses chopped carbon fiber, chopped alkali-free glass fiber, or chopped aramid fiber as the fiber matrix, and grafted monomers containing dynamic ester bonds are grafted onto the surface through mercapto-olefin photopolymerization; wherein, the chopped carbon fiber has a length of 3-5 mm and a diameter of 7-10 μm; the chopped alkali-free glass fiber has a length of 3-5 mm and a diameter of 10-13 μm; and the chopped aramid fiber has a length of 3-5 mm and a diameter of 12-15 μm; The grafted monomer was prepared by esterification of an intermediate obtained from the epoxide ring-opening reaction of bisphenol A diglycidyl ether and 5-norbornene-2-carboxylic acid via a carboxyl-induced reaction with carboxyl polyethylene glycol monomethyl ether. Its chemical structure is as follows: 。 2. The fiber-reinforced heat-shrinkable composite material according to claim 1, characterized in that, The density of the high-density polyethylene is 0.941-0.965 g / cm³. 3 Crystallinity ≥65%, number average molecular weight 200,000-500,000; epoxy value of epoxy resin 0.48-0.52 eq / 100g.

3. The fiber-reinforced heat-shrinkable composite material according to claim 1, characterized in that, The auxiliary additives refer to a mixture of antioxidant 1010 and ultraviolet absorber UV-531 in a weight ratio of 1:1; the curing agent refers to at least one of diethylenetriamine, triethylenetetraamine, or tetraethylenepentamine; the crosslinking agent refers to trimethylolpropane triacrylate; and the initiator refers to dicumyl peroxide.

4. The fiber-reinforced heat-shrinkable composite material according to claim 1, characterized in that, The method for preparing the grafted monomer is as follows: (1) Under nitrogen protection, bisphenol A diglycidyl ether, 5-norbornene-2-carboxylic acid and triethylamine were added to N,N-dimethylformamide, heated to 90-110℃, stirred for 12-16h, cooled to room temperature, ethyl acetate was added, washed twice with dilute hydrochloric acid solution, and then washed 3-5 times with deionized water. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to obtain the intermediate. (2) Under nitrogen protection, in a reaction vessel equipped with a water separator and a reflux condenser, the intermediate, carboxylated polyethylene glycol monomethyl ether, p-toluenesulfonic acid and toluene were added. The mixture was heated to 100-120℃ with stirring and reacted for 6-10 h. After the reaction was completed, the mixture was cooled to room temperature and washed with sodium carbonate solution and deionized water in sequence. The organic phase was dried with anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was recrystallized in a mixed solution of ethyl acetate / petroleum ether to obtain the grafted monomer.

5. The fiber-reinforced heat-shrinkable composite material according to claim 4, characterized in that, In (1), the molar ratio of bisphenol A diglycidyl ether and 5-norbornene-2-carboxylic acid is 1:2-2.1; the weight ratio of bisphenol A diglycidyl ether, triethylamine, N,N-dimethylformamide and ethyl acetate is 1:0.01-0.03:8-12:5-10; and the concentration of the dilute hydrochloric acid solution is 0.1-0.5 mol / L.

6. The fiber-reinforced heat-shrinkable composite material according to claim 4, characterized in that, In (2), the molar ratio of the intermediate and carboxylated polyethylene glycol monomethyl ether is 1:2-2.3; the weight ratio of the intermediate, p-toluenesulfonic acid and toluene is 1:0.01-0.03:10-20; the concentration of the sodium carbonate solution is 5-10wt%; the volume ratio of ethyl acetate and petroleum ether in the ethyl acetate / petroleum ether mixed solution is 3:1; the molecular weight of the carboxylated polyethylene glycol monomethyl ether is 808.95, and the number of repeating units of ethylene glycol in its molecular structure is 16.

7. The method for preparing the fiber-reinforced heat-shrinkable composite material according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Immerse the fiber substrate in a nitric acid solution, heat to 50-60℃, and react for 1-2 hours with stirring. After removal, rinse with deionized water until the pH of the washing solution is neutral. After drying, immerse in a 3-mercaptopropyltrimethoxysilane ethanol solution under nitrogen protection, heat to 40-60℃, and react with stirring for 3-6 hours. After removal, rinse with deionized water for 1-3 minutes and dry to obtain thiolized fibers. Mix the thiolized fibers, grafted monomers, and 2-hydroxy-2-methyl-1-phenyl-1-propanone, disperse in anhydrous ethanol at a solid-liquid ratio of 1:5-10, and irradiate with ultraviolet light for 10-20 minutes with stirring. After removal, rinse with deionized water for 3-5 minutes and dry to obtain modified reinforcing fibers. S2. Modified reinforcing fiber, high-density polyethylene, epoxy resin, curing agent, crosslinking agent, and initiator are added through the main feed port of a twin-screw extruder and reacted in the front section of the twin-screw extruder for 5-10 minutes. Then, the material is conveyed to the side feed port of the twin-screw extruder, and auxiliary additives are added through the side feed port. The material is then blended in the rear section of the twin-screw extruder for 10-20 minutes. Finally, the material is extruded through the die head equipped with a slit-type preform die to the cooling roller and cooled to room temperature to obtain the composite material preform. S3. After heating the composite material preform to 85-95℃, it is subjected to biaxial stretching. While maintaining the stretched state, the temperature is raised to 110-120℃ and held for 20-40 minutes. After cooling to room temperature, a fiber-reinforced heat-shrinkable composite material is obtained.

8. The method for preparing the fiber-reinforced heat-shrinkable composite material according to claim 7, characterized in that, The concentration of the nitric acid solution in S1 is 30-60 wt%; the concentration of the 3-mercaptopropyltrimethoxysilane ethanol solution is 5-15 wt%; the weight ratio of the mercapto-modified fiber, the grafted monomer mixture, and 2-hydroxy-2-methyl-1-phenyl-1-propanone is 1:0.1-0.2:0.001-0.005; the wavelength of the ultraviolet light is 365 nm, and the intensity is 10-20 mW / cm². 2 .

9. The method for preparing the fiber-reinforced heat-shrinkable composite material according to claim 7, characterized in that, The twin-screw extruder in S2 is equipped with a main feed port and a side feed port, wherein the distance between the side feed port and the die head is two-thirds of the total length of the machine; the front section of the twin-screw extruder is from the main feed port to the side feed port, and the front section temperature is 100-120℃; the rear section of the twin-screw extruder is from the side feed port to the die head, and the rear section temperature is 140-160℃; the screw speed is 40-70 rpm; the thickness of the composite material preform is 1-3 mm.

10. The method for preparing the fiber-reinforced heat-shrinkable composite material according to claim 7, characterized in that, The biaxial stretching ratio in S3 is 2.5-3.5 times.

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