High-low melting point hdpe synergistic bonding composite prepreg tape and preparation method and application thereof

Abstract

The application discloses a high-low melting point HDPE synergic bonding composite prepreg tape and a preparation method and application thereof. The prepreg tape is a three-layer composite structure, comprising a carbon fiber reinforced high melting point high density polyethylene middle layer and low melting point high density polyethylene film layers arranged on both sides of the middle layer. The high melting point high density polyethylene has a melting point of 130-145 DEG C. The low melting point high density polyethylene has a melting point of 105-125 DEG C. The preparation method comprises the following steps: stacking the three layers of materials, and then quenching and annealing after twice hot pressing. The low melting point HDPE is introduced on both sides of the carbon fiber reinforced high melting point HDPE, and the synergic melting behavior of the HDPE with different melting points is regulated through a two-step heating and quenching process, so that the interface is fully wetted and the crystalline structure is controllable, and the interlayer bonding performance and the gas barrier performance are considered, the prepreg tape is used in a hydrogen conveying pipeline, and the mechanical bearing capacity and the hydrogen barrier performance of the pipeline are improved.

Description

Technical Field

[0001] This invention relates to a prepreg tape, and more particularly to a high- and low-melting-point HDPE synergistic bonding composite prepreg tape, its preparation method, and its application. Background Technology

[0002] With the development of the hydrogen energy industry, hydrogen pipelines and storage and transportation equipment have placed higher demands on the barrier properties, mechanical properties and long-term service stability of materials. In particular, under high pressure and complex environmental conditions, the structural integrity of materials has become critical.

[0003] Thermoplastic composites have gained widespread attention in the field of hydrogen pipelines due to their advantages such as corrosion resistance, weldability, and recyclability. Among them, carbon fiber reinforced high-density polyethylene (HDPE) prepreg tape, with its high specific strength, resistance to environmental stress cracking, and good processing adaptability, can effectively improve the load-bearing capacity of pipelines and is gradually becoming an important material to replace the reinforcement layer of traditional metal pipelines. However, existing carbon fiber reinforced HDPE prepreg tapes still have certain limitations in practical applications: on the one hand, HDPE matrix is ​​a semi-crystalline polymer, and its crystal structure and interface state have a significant impact on the mechanical properties and gas barrier properties of the material; on the other hand, the interfacial compatibility between carbon fiber and HDPE is weak, and interface defects or pores are easily formed during processing, thereby reducing interlayer bonding strength and increasing the risk of hydrogen permeation. In addition, traditional hot pressing or single heating processes usually use a complete melting method above the melting point of HDPE. Although this is beneficial for interface wetting, it can easily lead to excessive molecular chain flow, coarsening of the crystal structure, and increased residual stress, making it difficult to simultaneously achieve interlayer bonding performance and gas barrier performance. Therefore, how to achieve crystal structure control and interface densification while ensuring sufficient interface wetting has become a key technical issue in the application of carbon fiber reinforced HDPE composites in hydrogen pipelines.

[0004] Existing HDPE-based composite materials for hydrogen transportation typically achieve structural construction through multi-layer layup and hot pressing. However, the following problems still exist during processing and service: Thermoplastic composite materials have received widespread attention in the field of hydrogen transportation pipelines due to their advantages such as corrosion resistance, weldability, and recyclability. Among them, carbon fiber reinforced high-density polyethylene (HDPE) prepreg tape, with its high specific strength, resistance to environmental stress cracking, and good processing adaptability, can effectively improve the load-bearing capacity of pipelines and is gradually becoming an important material to replace the reinforcement layer of traditional metal pipelines. However, existing carbon fiber reinforced HDPE prepreg tapes are usually processed using a complete melting method above the melting point of HDPE. Although this is beneficial for interface wetting, it can easily lead to excessive molecular chain flow and coarsening of the crystalline structure, making it difficult to simultaneously achieve interlayer bonding performance and gas barrier performance. Therefore, how to effectively control the crystalline structure and densify the interface while ensuring sufficient interface wetting, thereby balancing mechanical properties and gas barrier performance, has become a key technical problem that urgently needs to be solved. Summary of the Invention

[0005] Objectives of the Invention: The first objective of this invention is to provide a high- and low-melting-point HDPE synergistic bonding composite prepreg tape. By controlling the synergistic melting behavior of HDPEs with different melting points, sufficient interfacial wetting and controllable construction of the crystal structure are achieved, thereby balancing interlayer bonding performance and gas barrier performance. The second objective of this invention is to provide a method for preparing the high- and low-melting-point HDPE synergistic bonding composite prepreg tape. Through a two-step heating and quenching control process, the interface structure can be controlled and adjusted. The third objective of this invention is to provide applications of the high- and low-melting-point HDPE synergistic bonding composite prepreg tape.

[0006] Technical solution: The high- and low-melting-point HDPE synergistic bonding composite prepreg tape of the present invention is characterized in that the prepreg tape is a three-layer composite structure, including a carbon fiber reinforced high-melting-point high-density polyethylene (HDPE) intermediate layer, and low-melting-point high-density polyethylene film layers covering both sides thereof; the melting point of the high-melting-point high-density polyethylene is 130~145℃; the melting point of the low-melting-point high-density polyethylene is 105~125℃.

[0007] The low-melting-point HDPE film layer molecular chains have high chain segment mobility, thus possessing: low melting temperature, good fluidity and extensibility, and excellent interfacial wetting and filling capabilities.

[0008] The low-melting-point HDPE film layer preferentially melts during the composite molding process and forms an interfacial transition layer with the high-melting-point HDPE matrix. This interfacial transition layer is a crystalline structure formed by molecular chain interdiffusion. Preferably, the melting point of the high-melting-point high-density polyethylene is 132~140℃.

[0009] Preferably, the melting point of the low-melting-point high-density polyethylene is 110~120℃.

[0010] Preferably, the thickness of the low-melting-point high-density polyethylene (HDPE) film layer is 5% to 40% of the total thickness of the prepreg tape. More preferably, the thickness of the low-melting-point HDPE film layer is 20% to 40% of the total thickness of the prepreg tape. The thickness of the low-melting-point HDPE film layer has a significant impact on the interfacial wetting degree and the compactness of the composite structure: when the film layer thickness is low, the interfacial filling capacity is insufficient, and micropores are easily left behind, thereby reducing the interlayer bonding strength and increasing the gas permeation channels; when the film layer thickness is high, although it is beneficial to interfacial wetting, the excessively thick low-melting-point phase will weaken the continuity of the high-melting-point HDPE skeleton structure, thus affecting the overall mechanical properties. Therefore, by controlling its thickness within the above range, a synergistic optimization between interfacial bonding strength and gas barrier performance can be achieved.

[0011] Preferably, the carbon fiber volume fraction in the carbon fiber reinforced high-melting-point high-density polyethylene interlayer is 40-65%. The carbon fiber content is within a suitable range to ensure both mechanical load-bearing performance and interface wetting and processing performance.

[0012] Preferably, the carbon fibers exist in a unidirectional or multidirectional layup within a high-melting-point high-density polyethylene matrix.

[0013] The preparation method of the high and low melting point HDPE synergistic adhesive composite prepreg tape of the present invention includes the following steps:

[0014] (1) The low melting point high density polyethylene film is respectively attached to the upper and lower surfaces of the carbon fiber reinforced high melting point high density polyethylene intermediate layer to form a three-layer stacked structure.

[0015] (2) Apply heat and pressure to the upper and lower surfaces respectively to soften the low-melting-point HDPE film layer and form an initial interface bond with the intermediate layer; the heating temperature is a temperature range below or close to the melting point of the low-melting-point HDPE.

[0016] (3) The composite structure is further heated by laser heating or overall heating to a temperature of 135~165℃, and held at a pressure of 0.05~1 MPa for 2~20 min.

[0017] (4) The composite material after secondary heating is subjected to low-temperature quenching treatment at a quenching temperature of -30℃ to -50℃.

[0018] (5) The quenched composite material is subjected to interface crystallization regulation and annealing treatment. The annealing temperature is 20~30℃ and the holding time is 10~60 min.

[0019] In step (2), the heating temperature is 100℃~130℃, the pressure is 0.05~0.3 MPa, and the holding time is 30s~5 min. More preferably, in step (1), the heating temperature is 110℃~125℃. In this step, the heating temperature is controlled to ensure that the low melting point HDPE is in a softened or partially melted state and to achieve initial wetting of the interface.

[0020] Preferably, in step (3), the secondary heating temperature is 140~155℃, and the temperature is maintained for 2~10 min under a pressure of 0.1~0.5 MPa. During the heating process, the low-melting-point HDPE film layer melts preferentially and flows, while the high-melting-point HDPE gradually reaches a molten or partially molten state. Under pressure, the interface is filled and fused, thereby forming an overall composite structure. The secondary heating temperature in this step is used to control the difference in the melting state of the high and low melting-point HDPE, thereby achieving controllable construction of the interface structure. When the temperature is above the melting temperature of the low-melting-point HDPE, the low-melting-point HDPE melts preferentially and forms a low-viscosity mobile phase, which is beneficial for interface wetting and filling of microscopic defects. When the temperature is further close to or reaches the melting range of the high-melting-point HDPE, the high-melting-point HDPE gradually melts or partially melts, causing the interface region to form a molecular chain interdiffusion structure. When the temperature is controlled within the supercooled range, the high-melting-point HDPE retains a partial crystal structure, forming a "mobile phase-skeleton phase" synergistic system, which maintains structural stability while ensuring interface compactness. Therefore, by controlling this temperature range, a balance can be achieved between interface wetting, molecular chain diffusion, and structural support, thereby optimizing interlayer bonding performance and gas barrier performance.

[0021] In step (4), the quenching temperature is controlled to increase the crystallization nucleation rate and refine the grain structure.

[0022] In step (5), annealing is performed to improve crystallization perfection and interface stability.

[0023] The application of the high and low melting point HDPE synergistic bonding composite prepreg tape described in this invention in hydrogen pipeline structures.

[0024] Preferably, the composite prepreg tape is used to form a pressure-bearing reinforcement layer and / or a hydrogen barrier layer for the hydrogen transport pipeline, so as to improve the mechanical load-bearing capacity and hydrogen barrier performance of the pipeline.

[0025] Preferably, the composite prepreg tape can be wound, laid or stacked to form a hydrogen transport pipeline reinforcement layer, and combined with the inner lining layer and the outer protective layer to form a multi-layer pipeline structure.

[0026] Preferably, during the molding process, since the low-melting-point HDPE film layer is located on the surface, interfacial bonding between the multilayer structures can be achieved at a relatively low temperature. Preferably, the bonding is hot pressing, and the hot pressing temperature is 90℃~120℃.

[0027] Invention Mechanism: This invention introduces a low-melting-point HDPE film layer onto the surface of a high-melting-point HDPE prepreg tape, enabling the material to form the following synergistic mechanism during processing and service:

[0028] Processing stage: The low-melting-point layer is preferentially melted to achieve interface wetting, filling and defect elimination;

[0029] Cooling stage: A stable interfacial transition structure is formed through crystallization regulation;

[0030] Service life: The low-melting-point layer provides an interface buffer, reduces stress concentration and inhibits delamination propagation, thereby improving the brittle failure behavior dominated by traditional crystallization interfaces.

[0031] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention introduces low-melting-point HDPE on both sides of carbon fiber reinforced high-melting-point HDPE, regulates the synergistic melting behavior of HDPE with different melting points, realizes full wetting of the interface and controllable construction of crystal structure, thereby taking into account both interlayer bonding performance and gas barrier performance; (2) The preparation method of the present invention realizes controllable adjustment of interface structure through two-step heating and quenching control process; (3) The high and low melting-point HDPE synergistic bonding composite prepreg tape of the present invention is used in hydrogen pipelines, which improves the mechanical bearing capacity and hydrogen barrier performance of the pipelines. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the high and low melting point HDPE synergistic bonding composite prepreg tape structure of the present invention;

[0033] Figure 2 The hot melt curve of low-melting-point HDPE in Example 1;

[0034] Figure 3 The hot melt curve of high melting point HDPE in Example 1;

[0035] Figure 4 The images show X-ray diffraction patterns of high-melting-point HDPE at different quenching temperatures: original sample (a), Example 1 (b), Example 2 (c), and Comparative Example 1 (d).

[0036] Figure 5 The images show X-ray diffraction patterns of low-melting-point HDPE at different quenching temperatures: original sample (a), Example 1 (b), Example 2 (c), and Comparative Example 1 (d).

[0037] Figure 6 X-ray diffraction intensity of high melting point HDPE in Comparative Example 2;

[0038] Figure 7 Flowchart for the preparation of three-layer composite prepreg hot pressing;

[0039] Figure 8 The graph shows a comparison of the interlayer shear forces of the three-layer composite samples after different quenching temperatures in Examples 1, 2, 1, and 2.

[0040] Figure 9The graph shows a comparison of the air permeability curves of the three-layer composite samples after different quenching temperatures for Examples 1, 2, 1, and 2. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to the embodiments.

[0042] Example 1

[0043] The high and low melting point HDPE synergistic bonding composite prepreg tape of the present invention, such as Figure 1 As shown, the prepreg tape has a three-layer composite structure, including a carbon fiber reinforced high-melting-point high-density polyethylene intermediate layer, and low-melting-point high-density polyethylene film layers covering both sides thereof.

[0044] The low-melting-point high-density polyethylene film has a melting point of 119°C and a molecular weight of 7×10⁻⁶. 4 (Grade LD 105.BR, purchased from ExxonMobil), prepared to a thickness of 0.5 mm.

[0045] In the carbon fiber reinforced high-melting-point high-density polyethylene interlayer, the high-melting-point high-density polyethylene has a melting point of 135℃ and a molecular weight of 3.0 × 10⁻⁶. 5 (ExxonMobil HD 49100HL, purchased from ExxonMobil); carbon fiber volume fraction is 55%; the thickness of the interlayer is 3 mm.

[0046] The preparation method includes the following steps:

[0047] (1) The low melting point high density polyethylene film is respectively attached to the upper and lower surfaces of the carbon fiber reinforced high melting point high density polyethylene intermediate layer to form a three-layer stacked structure.

[0048] (2) The first heating is at 125℃, the pressure is 0.1 MPa, and the pressure holding time is 1 min;

[0049] (3) The second heating is at 140℃, the pressure is 0.5 MPa, and the pressure holding time is 2 min;

[0050] (4) The composite material after secondary heating is subjected to low-temperature quenching treatment at a quenching temperature of -20℃;

[0051] (5) The quenched composite material was subjected to interface crystallization regulation and annealing treatment. The annealing temperature was 25℃ and the holding time was 60 min.

[0052] After the above hot-pressing, quenching and annealing processes, the thickness of the carbon fiber reinforced high-melting-point high-density polyethylene interlayer remains basically unchanged at 3.0 mm due to the support of the 55% volume fraction of carbon fiber skeleton in the carbon fiber reinforced high-melting-point high-density polyethylene interlayer. However, the low-melting-point high-density polyethylene film layer melts and flows under secondary heating (140℃) and pressure (0.5MPa), and some of the melt flows into the micropores at the interface of the interlayer, causing its thickness to be reduced from the initial 0.50 mm to about 0.3 mm, with a total thickness of about 3.6 mm.

[0053] Example 2

[0054] Based on Example 1, the quenching temperature was changed to -40℃, while the other conditions remained unchanged.

[0055] Comparative Example 1

[0056] Based on Example 1, the quenching temperature was changed to 0°C, while the other conditions remained unchanged.

[0057] Comparative Example 2

[0058] Based on Example 1, instead of using a high- and low-melting-point HDPE film layer co-bonding structure, the carbon fiber reinforced high-melting-point HDPE prepreg tape is directly bonded by high-temperature hot pressing using the traditional hot-melt method, while keeping the composition of the remaining materials consistent.

[0059] The specific preparation method is as follows:

[0060] (1) Three layers of carbon fiber reinforced high melting point HDPE prepreg tape are directly stacked to form a laminated structure;

[0061] (2) The traditional integral hot melt process is used for one-time hot pressing molding. The heating temperature is set at 160℃, which is higher than the complete melting temperature of high melting point HDPE; the pressure is 0.5 MPa and the holding time is 10 min.

[0062] (3) After hot pressing, the composite material after secondary heating is subjected to low-temperature quenching treatment at a quenching temperature of -20℃.

[0063] (4) The quenched composite material was subjected to interface crystallization regulation and annealing treatment. The annealing temperature was 25℃ and the holding time was 60 min.

[0064] The high-melting-point HDPE of the low-melting-point HDPE in Example 1 was characterized, and the results are as follows: Figures 2-3 As shown.

[0065] Figure 2 and Figure 3The images show the differential scanning calorimetry (DSC) melting curves of low-melting-point HDPE and high-melting-point HDPE, respectively. The DSC curves indicate that low-melting-point HDPE reaches its melting peak around 120℃ and rapidly transforms into a complete melt, while high-melting-point HDPE reaches its peak at 124℃ and retains residual crystals at 130℃ until it completely melts at 136℃. This provides a thermodynamic basis for constructing a synergistic interface between the low-melting-point "mobile phase" and the high-melting-point "skeleton phase" within the melting window.

[0066] Figure 4 The X-ray diffraction (XRD) patterns of the original sample, Example 1, Example 2, Comparative Example 1, and high-melting-point HDPE after step (5) treatment are shown in the figure. It can be seen that the high-melting-point HDPE maintains characteristic diffraction peaks of the (110) and (200) crystal planes around 21.5° and 24.0° after each process step, confirming that its orthorhombic crystal form did not fundamentally change during hot processing. As the quenching temperature decreased from 0°C (Comparative Example 1) to -20°C (Example 1) and -40°C (Example 2), the intensity of the characteristic diffraction peaks of the (110) and (200) crystal planes generally showed a gradual weakening trend. This indicates that the lower the quenching temperature, the greater the undercooling of the melt during the cooling process. The extreme undercooling inhibits the diffusion of chain segments into the ordered arrangement of the crystal lattice, thereby reducing the overall crystallinity of the matrix to a certain extent and retaining more amorphous phase regions. Careful observation of the diffraction peak morphology reveals that as the quenching temperature decreases, the full width at half maximum (FWHM) of the characteristic diffraction peaks exhibits a significant broadening trend. A wider diffraction peak indicates a smaller microcrystalline size within the matrix. The physical mechanism lies in the fact that deep low-temperature quenching greatly increases the rate of heterogeneous nucleation. A large number of crystal nuclei erupt instantaneously, causing them to collide and overlap before they can grow larger, thus forming a "high-density, fine-grained" microcrystalline structure at the interface and in the matrix. Although low crystallinity is generally unfavorable for gas barrier properties, this invention, through the extreme refinement of microcrystalline grains induced by low quenching temperatures (-20℃ to -40℃), results in a geometric increase in the number of grain boundaries within the matrix. These numerous, randomly arranged, densely packed small grains interweave with a highly cross-linked / entangled amorphous network, blocking hydrogen permeation. When hydrogen molecules attempt to permeate, they must bypass countless tiny crystal particles, making their diffusion paths in amorphous regions and grain boundaries extremely tortuous, thus microscopically blocking the formation of continuous gas diffusion channels. In summary, although the orthorhombic crystal structure of each sample remained fundamentally unchanged, by controlling the low-temperature quenching temperature, this invention successfully achieved nanometer / micrometer-level fine control of the matrix grain size without damaging the basic crystal structure of HDPE. This "microcrystalline" interface and matrix structure resulting from low-temperature quenching is the core physical mechanism by which the prepreg tape of this invention achieves hydrogen barrier performance far exceeding that of traditional processes (Comparative Example 2) while maintaining excellent interlayer bonding.

[0067] Figure 5 The X-ray diffraction (XRD) patterns of the original sample, Example 1, Example 2 and Comparative Example 1 after step (5) treatment of low-melting-point HDPE are shown in the figure. As can be seen from the figure, the low-melting-point HDPE exhibits a sharp (110) crystal plane diffraction peak at 21.5°, but the (200) crystal plane diffraction peak is missing or extremely weak. This indicates that although the material maintains an orthorhombic crystal system structure, the crystal development is incomplete and the crystal orientation is highly concentrated on the (110) plane. Combined with the DSC data, it can be seen that the low-melting-point HDPE reaches the melting peak and melts completely at 120°C. The thermal history under this temperature window inhibits crystal growth during rapid crystallization, resulting in smaller grain size and lower crystallinity.

[0068] Figure 6 The X-ray diffraction pattern of high-melting-point HDPE in the sample prepared in Comparative Example 2 is shown. Figure 6 and Figure 4 A horizontal comparison reveals that the changes in peak intensity and half-maximum width (WHM) significantly regulate the residual crystalline skeleton of high-melting-point HDPE by the hot-pressing heating temperature (secondary heating temperature). In this embodiment, the secondary heating temperature is controlled at 140°C (within the partial melting window). This temperature allows the low-melting-point layer to completely melt and form a mobile phase, while the high-melting-point HDPE intermediate layer only undergoes partial melting, retaining a large amount of residual crystalline skeleton within the matrix. These unmelted residual crystals act as highly efficient "uniform nucleation centers," guiding molecular chains to rapidly align around them during cooling and crystallization. Therefore, their diffraction peak morphology is sharper, and the crystal perfection is higher. In contrast, Comparative Example 2 uses a traditional integral hot-melt process with a heating temperature set at 160°C, which is much higher than the complete melting temperature of high-melting-point HDPE. The original crystalline structure of the high-melting-point HDPE intermediate layer is completely destroyed, and the polymer chain segments are completely unentangled and in a highly disordered amorphous molten state. During the subsequent natural cooling process, the material undergoes a recrystallization process after complete melting. Due to the loss of nucleation constraints from the residual crystal framework and the slower natural cooling rate, molecular chain segments have ample time for long-range diffusion and rearrangement, resulting in significant coarsening and recrystallization of the grains. Experimental results show that, in Comparative Example 2, the large micrograins and increased interfacial free volume caused by complete melting and coarsening and recrystallization easily form continuous gas permeation microchannels within the material and at the interface during cooling and contraction. This structurally explains why, although Comparative Example 2 has slightly higher interlayer shear strength, its hydrogen permeability is significantly increased and its hydrogen barrier performance is severely deteriorated. This further demonstrates the originality and necessity of the present invention's method of "high and low melting point synergy + 140℃ two-step heating (preserving the residual crystal framework)" for controlling crystal morphology and balancing mechanical and hydrogen barrier performance.

[0069] Performance testing

[0070] like Figure 8As shown, the three-layer composite prepreg tape structures of Examples 1-2 and Comparative Examples 1-2 were hot-pressed together. The composite method is as follows:

[0071] The composite prepreg tapes prepared in Examples 1-2 and Comparative Example 1 were laid in three layers and placed in a hot press for hot pressing composite at a temperature of 110°C.

[0072] The temperature (110℃) was determined based on DSC analysis results: the melting peak of low-melting-point HDPE is located in the range of 100~120℃. At 110℃, low-melting-point HDPE has entered the molten state and has sufficient fluidity, which can effectively wet and fill the micro-defects at the interface; at the same time, the melting peak of high-melting-point HDPE is located in the range of 112~130℃. At 110℃, high-melting-point HDPE has not yet entered the main melting stage and maintains a complete crystal skeleton structure, thus forming a synergistic system of "low-melting-point mobile phase - high-melting-point skeleton phase", which maintains the structural stability and dimensional accuracy of the intermediate layer while achieving dense bonding at the interface.

[0073] During the hot-pressing process, a pressure of 0.1 MPa was applied and held for 1 minute to allow the low-melting-point HDPE melt to fully spread and penetrate to the surface of the carbon fiber impregnation tape and the interlayer interface under pressure. After cooling, an integrated multilayer composite structure was formed. The shear strength and air permeability of the hot-pressed composite sample were tested.

[0074] The carbon fiber reinforced high-melting-point high-density polyethylene three-layer laminate sample (excluding the low-melting-point layer) prepared in Comparative Example 2 was directly used for testing. Subsequently, the interlaminar shear strength and gas permeability coefficient of each group of samples were compared and tested.

[0075] Shear force testing was conducted on short beams according to GB / T 3357-2012 standard, specifically as follows:

[0076] After hot pressing and bonding, the sample is cut into rectangular specimens with a length of 100 mm, a width of 16 mm, and a thickness of 4 mm, with the span-to-thickness ratio fixed at 4:1, i.e., s = 4h. The specimens are placed symmetrically on a three-point bending fixture, and the loading radii of the upper pressure head and the lower support should meet the requirement of R = 3.0 ± 0.1 mm.

[0077] A compressive load was applied at a constant beam displacement rate of 1.0 ± 0.2 mm / min on a universal testing machine, and the load-displacement curve was recorded. The interlaminar shear strength (ILSS) was calculated using the maximum load F (N) at which interlaminar failure occurred in the specimen.

[0078] The air permeability test was conducted according to the GB / T 1038.1-2013 standard, using a differential pressure method for gas permeation testing, specifically as follows:

[0079] After hot-pressing composite, the sample is cut to a size with an effective permeation area of ​​not less than 38 cm².2 A circular sample is placed between the upper and lower test chambers of the gas permeameter and sealed. First, the system is evacuated until the pressure in the low-pressure chamber does not exceed 27 Pa, and degassing continues for more than 3 hours to remove the gas adsorbed on the sample.

[0080] After reaching the specified vacuum level, test gas (helium) is introduced into the high-pressure chamber to create a constant pressure difference of 0.1 MPa across the sample. The pressure change in the low-pressure chamber over time is monitored and recorded in real time until the permeation reaches a steady state (i.e., the pressure-time curve is a straight line).

[0081] The results are as follows Figures 8-9 As shown.

[0082] Depend on Figure 8 It was found that in the three-layer composite samples of Example 1 (quenched at -20℃), Example 2 (quenched at -40℃), and Comparative Example 1 (quenched at 0℃), the peak shear strength of the 0℃ sample was approximately 60–63 MPa, the peak shear strength of the -20℃ sample was approximately 53–56 MPa, the peak shear strength of the -40℃ sample was approximately 48–52 MPa, and the peak shear strength of the Comparative Example 2 sample prepared by the conventional hot-melt method was approximately 65–68 MPa. This indicates that the interlayer shear strength decreases with decreasing quenching temperature. The reason is that although low-temperature quenching increases the crystallization nucleation rate, it inhibits the diffusion and entanglement of molecular chains at the interface. In contrast, the chain segment rearrangement is more complete during 0℃ quenching, which is conducive to the formation of a more perfect interface entanglement structure. The traditional hot melting method is in a completely molten state, and the degree of interdiffusion of interface molecular chains is the highest, so its shear strength is further improved. However, at the same time, high-temperature complete melting will lead to grain coarsening and an increase in the free volume of the interface, which is easy to form gas permeation channels, thus causing a decrease in gas barrier performance and making it difficult to simultaneously take into account the interlayer bonding performance and hydrogen barrier performance.

[0083] Depend on Figure 9 Therefore, the permeability of the sample at 0℃ is approximately 1.1~1.3E-13 cm⁻¹. 3 The permeability of the sample at -20℃ is approximately 9.9E-14 to 1.2E-13 cm² / (cm²·s·Pa). 3 ·cm / (cm 2 The permeability of the sample at -40℃ is approximately (1.5~1.7)×10⁻⁻ s·Pa. 13 The permeability of the Comparative Example 2 sample prepared using the traditional hot-melt method is approximately (2.3–2.8) × 10⁻ 13 cm 3 ·cm / (cm 2(·s·Pa). Compared with shear force, hydrogen permeability shows an inverse relationship: the pressure increase rate of the -40℃ quenched sample is lower, corresponding to a permeability lower than that of the 0℃ sample, indicating superior barrier performance. This difference stems from the fact that low-temperature quenching increases nucleation density and refines grains, making the interface structure denser and the diffusion path more tortuous, thus effectively inhibiting gas molecule migration. While the sample prepared by the traditional hot-melt method has higher interlaminar shear strength, the high-melting-point HDPE completely melts and undergoes coarse-grained recrystallization at high temperatures, increasing the free volume of the interface region. Simultaneously, micropores and grain boundary defects easily form during cooling, leading to an increase in hydrogen diffusion channels. Therefore, its hydrogen permeability is significantly higher than that of the example sample, exhibiting the worst barrier performance.

[0084] Therefore, by controlling the quenching temperature, a balance can be achieved between the degree of interfacial molecular chain entanglement and structural density, thereby obtaining synergistic optimization of interlaminar shear strength and gas barrier performance. The interfacial bonding of Examples 1 and 2 using the hot-pressing composite process described in this invention is dense, with high gas permeation path tortuosity, and interlaminar shear strengths reaching 58 MPa and 60 MPa, respectively, while hydrogen permeability coefficients range from 9.9E-14 to 1.2E-13 cm⁻¹. 3 ·cm / (cm 2 ·s·Pa) and 7.0E-14~1.0E-13cm 3 ·cm / (cm 2 The high-melting-point HDPE layer (·s·Pa) combines excellent mechanical properties with gas barrier properties. However, in Comparative Example 1, insufficient wetting and molecular chain diffusion at the interface between the low-melting-point HDPE layer and the high-melting-point HDPE interlayer resulted in microscopic defects at the interface, allowing gas molecules to rapidly permeate along the interface. The hydrogen permeability coefficient was as high as 1.1~1.3E-13 cm⁻¹. 3 ·cm / (cm 2 The barrier performance (·s·Pa) is significantly inferior to that of Examples 1 and 2, and does not meet the requirements for use in high-pressure hydrogen pipelines.

[0085] Furthermore, in Comparative Example 2, a traditional high-temperature complete hot-melt bonding method was used for overall bonding. Although the interlaminar shear strength could reach 65–68 MPa due to the full diffusion of the interfacial molecular chains, the high-temperature complete melting led to coarsening of high-melting-point HDPE grains and an increase in interfacial free volume. At the same time, grain boundary defects and microporous structures were easily formed during the cooling process, which significantly increased the hydrogen diffusion channels, and the hydrogen permeability coefficient reached (2.3–2.8) × 10⁻⁶. -13 The concentration of 1000 ppm (cm³·cm / (cm²·s·Pa)) is significantly higher than that of the embodiments of the present invention. This indicates that although the traditional hot-melt process can improve the interfacial bonding strength, it is difficult to balance the interfacial density and gas barrier properties, and thus cannot meet the requirements for the long-term service stability of composite materials under high-pressure hydrogen transportation environments.

Claims

1. A high and low melting point HDPE synergistically bonded composite prepreg tape, characterized in that, The prepreg tape has a three-layer composite structure, including a carbon fiber reinforced high-melting-point high-density polyethylene intermediate layer and low-melting-point high-density polyethylene film layers covering both sides thereon; the melting point of the high-melting-point high-density polyethylene is 130~145℃; the melting point of the low-melting-point high-density polyethylene is 105~125℃.

2. The high and low melting point HDPE synergic bonding composite prepreg tape according to claim 1, characterized in that, The melting point of the high-melting-point high-density polyethylene is 132~140℃.

3. The high and low melting point HDPE synergistic bonding composite prepreg tape according to claim 1, characterized in that, The melting point of the low-melting-point high-density polyethylene is 110~120℃.

4. The high and low melting point HDPE synergistic bonding composite prepreg tape according to claim 1, characterized in that, The thickness of the low-melting-point high-density polyethylene film layer is 5-40% of the total thickness of the prepreg tape.

5. The high and low melting point HDPE synergistic bonding composite prepreg tape according to claim 1, characterized in that, The carbon fiber volume fraction in the carbon fiber reinforced high-melting-point high-density polyethylene interlayer is 40-65%.

6. The method for preparing high and low melting point HDPE synergistic adhesive composite prepreg tape according to any one of claims 1 to 5, characterized in that, Includes the following steps: (1) The low melting point high density polyethylene film is respectively attached to the upper and lower surfaces of the carbon fiber reinforced high melting point high density polyethylene intermediate layer to form a three-layer stacked structure. (2) Apply heat and pressure to the upper and lower surfaces respectively to soften the low-melting-point HDPE film layer and form an initial interface bond with the intermediate layer; the heating temperature is a temperature range below or close to the melting point of the low-melting-point HDPE. (3) The composite structure is further heated by laser heating or overall heating to a temperature of 135~165℃, and held at a pressure of 0.05~1 MPa for 2~20 min. (4) The composite material after secondary heating is subjected to low-temperature quenching treatment at a quenching temperature of -20℃ to -40℃. (5) The interface crystallization of the quenched composite material is controlled and annealed.

7. The method for preparing high and low melting point HDPE synergistic adhesive composite prepreg tape according to claim 6, characterized in that, In step (2), the heating temperature is 100℃~130℃, the pressure is 0.05~0.3 MPa, and the pressure holding time is 30 s~5 min.

8. The method for preparing high and low melting point HDPE synergistic adhesive composite prepreg tape according to claim 6, characterized in that, In step (3), the secondary heating temperature is 140℃~155℃, and the temperature is maintained for 2~10 min under a pressure of 0.1~0.5 MPa.

9. The application of the high and low melting point HDPE synergistic bonding composite prepreg tape according to any one of claims 1 to 5 in hydrogen pipeline structures.

10. The application according to claim 9, characterized in that, The composite prepreg tape is used to form a pressure-bearing reinforcement layer and / or a hydrogen barrier layer for hydrogen transportation pipelines.

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