A thermoreversible cross-linked polyethylene material based on diels-alder synthesis and a method for its preparation

Abstract

The present application relates to the technical field of high polymer material modification, and particularly relates to a kind of heat-reversible crosslinking polyethylene material based on Diels-Alder synthesis and a preparation method thereof.The preparation method comprises: maleic anhydride polyethylene is melt grafted with excess furan methylamine, then is treated by high temperature and high pressure dehydration ring closure to form stable furan functionalized polyethylene; then melt blend with bismaleimide, and by low temperature long time annealing treatment, to promote Diels-Alder reaction to proceed fully, so as to build a perfect heat-reversible crosslinking network.The present application has the beneficial effects that the obtained heat-reversible crosslinking polyethylene material has excellent temperature mechanical strength, solvent resistance and clear heat-reversible characteristics, and can be uncrosslinked at higher than 150 DEG C to realize melt reprocessing, and the network is reconstructed after cooling, and the strength retention rate is still higher after two times of recycling.The present application solves the problem of non-recyclable traditional crosslinking polyethylene, and is particularly suitable for recyclable products such as packaging and pipe.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a thermally reversible crosslinked polyethylene material synthesized based on Diels-Alder and its preparation method. Background Technology

[0002] Polyethylene (PE), as one of the most common synthetic resins, is widely used in packaging and storage due to its low cost, good sealing properties, ease of processing, and low-temperature resistance. However, most polyethylene packaging is currently for single use, and it is difficult to clean, especially after use, if it is contaminated with residues such as asphalt, resulting in ineffective recycling. Most of it is disposed of by incineration or landfill, causing serious environmental pollution.

[0003] To improve the recyclability of polyethylene, existing technologies typically employ physical modification or chemical crosslinking methods. While physical modification, such as blending and filling, can improve performance to some extent, it cannot solve the fundamental recyclability problem. Although chemical crosslinking can enhance material properties, it often results in materials that are no longer reprocessable, lose their thermoplasticity, and are difficult to recycle.

[0004] The development of dynamic covalent chemistry has provided new insights into solving this problem. The Diels-Alder (DA) reaction, as a thermally reversible cycloaddition reaction, can undergo a reverse decrosslinking reaction at high temperatures, restoring the material's processability. Upon cooling, the crosslinked network reforms, restoring its mechanical properties. Previous studies have applied the DA reaction to the reversible crosslinking of biodegradable materials such as polycaprolactone (e.g., Chinese invention patent application CN113429553A). However, this technology primarily targets low molecular weight polycaprolactone, aiming to reduce melt viscosity and improve processing performance. Its material system, application scenarios, and the technical problems it addresses are fundamentally different from those of polyethylene.

[0005] Currently, there are no reports of successfully applying DA thermal reversible crosslinking technology to polyethylene systems and achieving high performance that allows for recycling and reprocessing. Summary of the Invention

[0006] This invention aims to provide a thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis and its preparation method. By introducing Diels-Alder thermally reversible covalent bonds, polyethylene can maintain good mechanical properties and solvent resistance while possessing the characteristics of high-temperature processability and low-temperature reversible crosslinking, thereby realizing the material's reprocessability and recyclability.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis is provided, comprising the following steps: S1 Grafting reaction: Maleic anhydride-modified polyethylene (PE-g-MA) and furanyl methylamine (FA) are melt-blended and grafted to obtain grafting intermediates; S2 Dehydration and ring-closing treatment: The grafting intermediate obtained in step S1 is subjected to dehydration and ring-closing treatment to obtain furan-functionalized polyethylene (PE-g-FA). S3 Crosslinking reaction: The furan-functionalized polyethylene obtained in step S2 is melt-blended with bismaleimide (BMI) and a crosslinking network is formed by Diels-Alder reaction to obtain a crosslinking intermediate product; S4 Annealing treatment: The cross-linking intermediate product obtained in step S3 is annealed to obtain thermally reversible cross-linked polyethylene material (PE-g-DA).

[0008] Furthermore, in step S2 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the dehydration and ring-closing treatment is carried out at 180℃~220℃ and a pressure of 5 MPa~15 MPa for 20~40 minutes.

[0009] As described above, temperatures below 180℃ may lead to incomplete ring closure and poor product stability; temperatures above 220℃ may trigger PE degradation. Applying pressure helps to expel the generated moisture and promotes close contact of molecular chains, ensuring efficient and complete formation of a stable furfuralimide structure.

[0010] Furthermore, in the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the dehydration and ring-closing treatment is carried out using a hot press at 200°C and 10 MPa for 30 minutes.

[0011] As described above, the above constraints provide the optimal dehydration and ring-closing conditions. Parameters such as 200℃ and 10 MPa are experimentally verified to be the best parameters for achieving efficient dehydration and ring-closing without damaging the PE matrix. This ensures the stability and high grafting rate of furan groups in PE-g-FA, laying the foundation for subsequent successful DA crosslinking.

[0012] Furthermore, in step S4 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the annealing treatment is carried out at a temperature of 110℃~130℃ for a treatment time of 20~28 hours.

[0013] As described above, the optimized conditions for constructing a stable, thermally reversible cross-linked network are defined. This temperature range is higher than the typical temperature of the DA reaction but much lower than the reverse DA reaction temperature, which promotes the forward DA reaction to perfect the cross-linked network without causing network dissociation. Prolonged annealing ensures the reaction reaches equilibrium, resulting in a uniform and stable dynamic covalent network.

[0014] Furthermore, in step S1 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the amount of furanylmethylamine (FA) used is 2 to 5 times the molar amount of maleic anhydride (MA) functional groups in the PE-g-MA.

[0015] As described above, the above improvement adopts an excess FA strategy to overcome the problem of low grafting rate caused by side reactions or moisture due to equimolar feeding, ensuring that the anhydride groups are fully reacted, maximizing the introduction of furan functional groups, thereby improving the density of crosslinkable sites and the upper limit of performance of the final material.

[0016] Furthermore, in step S1 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the amount of furanylmethylamine (FA) used is 3 times the molar amount of maleic anhydride (MA) functional groups in the PE-g-MA.

[0017] Furthermore, in step S1 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the temperature of the melt blending grafting reaction is 155℃~165℃, and the reaction time is 5~15 minutes.

[0018] As described above, the improvements optimize the grafting reaction kinetics. This temperature range ensures that PE-g-MA melts fully and FA has sufficient reactivity, while avoiding excessively high temperatures that could lead to FA volatilization or PE thermal oxidation.

[0019] Furthermore, in step S3 of the above-mentioned method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the melt blending is carried out at 165℃~175℃, a rotation speed of 40-60 rpm, and a reaction time of 25~35 minutes.

[0020] As described above, the melt blending temperature is slightly higher than the typical DA reaction temperature. This is intended to utilize the high melt temperature to promote the diffusion of BMI in the viscous PE melt and the efficiency of its collision reaction with furan groups. At the same time, the presence of some reverse DA reactions at high temperatures may be beneficial for obtaining a more uniform network structure.

[0021] Furthermore, the specific steps of the above-mentioned method for preparing thermally reversible crosslinked polyethylene materials based on Diels-Alder synthesis include the following: S1 Grafting reaction: Maleic anhydride-modified polyethylene and furanyl methylamine with an equivalent amount of 3 times the maleic anhydride functional group content in maleic anhydride-modified polyethylene were mixed in an internal mixer at 160°C and 50 rpm for 10 min to obtain a grafting intermediate product. S2 Dehydration and Ring-Closed Treatment: The grafting intermediate obtained in step S1 is subjected to dehydration and ring-closed treatment to obtain furan-functionalized polyethylene; the dehydration and ring-closed treatment is carried out in a hot press at 200°C and 10 MPa for 30 minutes. S3 Crosslinking reaction: The furan-functionalized polyethylene obtained in step S2 is melt-blended with bismaleimide, and a crosslinking network is formed through the Diels-Alder reaction to obtain a crosslinking intermediate; the melt blending is carried out at 170°C, 50 rpm, and for 30 minutes. S4 Annealing treatment: The cross-linked intermediate product obtained in step S3 is annealed at a temperature of 120°C for 24 hours to obtain thermally reversible cross-linked polyethylene material.

[0022] Another technical solution of the present invention is to provide a thermally reversible cross-linked polyethylene material, which is prepared by the above-mentioned method for preparing thermally reversible cross-linked polyethylene material based on Diels-Alder synthesis.

[0023] The beneficial effects of this invention are as follows: This invention, through a four-step preparation method comprising "grafting reaction - dehydration and ring-closure treatment - cross-linking reaction - annealing treatment," effectively solves the technical challenge of constructing a stable and efficient thermally reversible cross-linked network on an inert polyethylene matrix. In particular, the method ensures the stable grafting of furan groups through the crucial "dehydration and ring-closure treatment," and promotes the perfection and balance of the dynamic covalent network through "annealing treatment." This successfully yields thermally reversible cross-linked polyethylene that possesses excellent abnormal temperature mechanical properties, clear thermally reversible characteristics (high-temperature decrosslinking, low-temperature recrosslinking), and good reprocessing and recycling capabilities, providing a green and circular solution for traditionally non-recyclable cross-linked polyethylene materials. Attached Figure Description

[0024] Figure 1 The FTIR spectra of PE-g-MA, PE-g-FA, and PE-g-DA in Example 1 of this invention are shown. Figure 2 a, b, and c in Example 1 of this invention show the comparison of the solubility of PE-g-MA, PE-g-FA, and PE-g-DA in o-dichlorobenzene before and after 24 hours at 125°C. Figure 3 The solubility change of PE-g-DA in Example 1 of this invention is shown in the sol-gel process in DCB. Figure 4 The crosslinking density of PE-g-DA products with different BMI addition amounts in Example 1 of the present invention; Figure 5 This refers to the crosslinking and decrosslinking reaction processes of PE-g-MA and PE-g-DA with different BMI addition amounts in Example 1 of the present invention; Figure 6 This refers to the DSC test of PE-g-DA heating-cooling-heating in Example 1 of the present invention; Figure 7 SEM images of PE-g-MA (a) and PE-g-DA (b) of Example 1 of the present invention at 120°C for 0, 2 h and 4 h; Figure 8 Tensile tests were conducted on PE-g-MA and PE-g-DA with different BMI additions in Example 1 of this invention. Figure 9 The PE-g-DA recycling tensile test of Example 1 of the present invention; Figure 10 The FTIR spectra of PE-g-MA, PE-g-FA, and PE-g-DA are those of Comparative Example 1 of the present invention; Figure 11 Another FTIR spectrum of PE-g-MA, PE-g-FA, and PE-g-DA from Comparative Example 1 of the present invention; Figure 12 This is yet another FTIR spectrum of PE-g-MA, PE-g-FA, and PE-g-DA from Comparative Example 1 of the present invention; Figure 13 This is yet another FTIR spectrum of PE-g-MA, PE-g-FA, and PE-g-DA from Comparative Example 1 of the present invention; Figure 14 The graph shows the solubility variation of PE-g-DA in Comparative Example 1 of this invention. Figure 15 The DSC test results for PE-g-MA and PE-g-DA in Comparative Example 1 of this invention are shown. Detailed Implementation

[0025] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0026] Unless otherwise specified, all raw materials used in the examples were commercially purchased. PE-g-MA was purchased from a commercially available product with a maleic anhydride grafting rate of 1.2 wt.%. Furan methylamine (FA, >99%), 1,10-bismaleimide (BMI), o-dichlorobenzene, and other reagents were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0027] Example 1

[0028] A method for preparing a thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, the reaction formula of which is as follows:

[0029] Specifically, the steps include the following: S1 Grafting reaction: Maleic anhydride-modified polyethylene (PE-g-MA, 1.2 wt.%) and furanyl methylamine, which is 3 times the equivalent of the maleic anhydride functional group content in maleic anhydride-modified polyethylene, were mixed in an internal mixer at 160°C and 50 rpm for 10 min to obtain a grafting intermediate product; the product was named PE-g-FA. S2 Dehydration and Ring-Closed Treatment: The grafting intermediate obtained in step S1 is subjected to dehydration and ring-closed treatment to obtain furan-functionalized polyethylene; the dehydration and ring-closed treatment is carried out in a hot press at 200°C and 10 MPa for 30 minutes. S3 Crosslinking reaction: The furan-functionalized polyethylene obtained in step S2 is melt-blended with bismaleimide (BMI) to form a crosslinking network through a Diels-Alder reaction to obtain a crosslinking intermediate; the melt blending is carried out at 170°C, 50 rpm, and for 30 minutes. S4 Annealing treatment: The crosslinking intermediate obtained in step S3 is subjected to annealing treatment, which is performed by annealing at 120°C in an oven for 24 hours to obtain a thermally reversible crosslinked polyethylene material, named PE-g-DA.

[0030] Tests and Results Analysis: 1. Testing Method The chemical structures of PE-g-MA, PE-g-FA, and PE-g-DA were characterized using Fourier transform infrared spectroscopy (FTIR), confirming the success of their functionalization and crosslinking. The thermal reversible behavior of PE-g-DA was investigated using DSC and solubility tests, allowing for the evaluation of the reversibility of the dynamic network under different thermal conditions.

[0031] FTIR spectrum Fourier transform infrared (Thermo Scientific Nicolai 6700) using attenuated total internal reflection (ATR) mode, from 400 to 4000 cm⁻¹ -1 Thirty-two scans were performed under the wave range. Measurements were used to identify characteristic functional groups in PE-g-MA, PE-g-FA, and PE-g-DA.

[0032] Solubility test Solubility tests were performed to determine the degree of crosslinking or decrosslinking of PE-g-MA, PE-g-FA, and PE-g-DA, and to evaluate their thermal reversibility. The sample (0.5 g) and o-dichlorobenzene (DCB, 15 ml) as a solvent were added to a 25 ml glass vial and heated in an oven at 125 °C for 24 h. The degree of solubility of the product before heating (t=0) and after heating (t=24 h) was photographed and recorded. To further understand the reversibility of DA crosslinking, PE-g-DA (0.5 g) was placed in o-dichlorobenzene (15 ml) at 160 °C for different time intervals, and then cooled to below 60 °C to observe the recombination of the crosslinked network.

[0033] Crosslinking density test Crosslinking density of the material (S, mol / cm) 3 The crosslinking density was determined using an o-dichlorobenzene swelling test. Approximately 0.5 g of dried crosslinked sample PE-g-DA was weighed into a 25 ml glass bottle, and 15 ml of o-dichlorobenzene was added for soaking for 3 days until swelling equilibrium was reached. The weight at this point was recorded (W1). Subsequently, the swollen sample was dried in an 80°C oven until constant weight was achieved, and the weight was recorded (W2). The swollen and dried weights were used to calculate the crosslinking density using the Flory-Rehner equation.

[0034]

[0035] Among them, V R V is the volume fraction of PE-g-DA in the expanded sample. S χ is the molar volume of the solvent (113.3 mL / mol for o-dichlorobenzene at room temperature), χ is the interaction parameter (0.4 for the o-dichlorobenzene-anhydride-modified polyethylene system), and ρ is the density (0.95 g / cm³ for the crosslinked product). 3 o-Dichlorobenzene 1.3 g / cm³ 3 ).

[0036] DSC Test The thermal reversibility of PE-g-MA and cross-linked PE-g-DA was determined using a DSC Q2000 differential scanning calorimeter from TA Instruments, USA. The heating rate was set to 5℃ / min, the measurement range was 70℃-200℃, a nitrogen atmosphere was used with a nitrogen flow rate of 50 mL / min and a nitrogen purity ≥99.999%, and an aluminum crucible was covered.

[0037] Mechanical properties and recycling tests Mechanical property testing was performed on a universal tensile testing machine at room temperature for PE-g-MA and PE-g-DA. Dumbbell-shaped specimens manufactured according to standard testing protocols were tested at a speed of 50 mm / min, with 10 samples per measurement. For each set of samples, a median chart was selected as the characteristic of the entire series. The tests yielded stress-strain relationships for ultimate tensile strength and elongation at break, providing a quantitative assessment of the mechanical property improvements achieved through DA crosslinking. Recycling experiments were conducted on a hot press, where tensile test samples were cut into small pieces, placed in a mold, and reprocessed at 200°C and 10 MPa for approximately 30 minutes. After cooling to room temperature, the samples were tested again.

[0038] SEM (Scanning Electron Microscopy) The self-healing behavior of scratches in the specimens was observed using scanning electron microscopy at an accelerating electron energy of 5 kV. First, the crosslinked membranes of PE-g-MA and the crosslinking product PE-g-DA were cut, and then re-contacted with the separated membrane surfaces at a thermal stage of 120 °C for approximately 0, 2, and 4 hours, respectively.

[0039] 2. Results

[0040] 2.1. Grafting PE-g-MA using FA like Figure 1 The successful introduction of furan groups and subsequent network formation were confirmed by Fourier transform infrared spectroscopy (FTIR). For PE-g-MA, at 1718 cm⁻¹... -1 A distinct carbonyl absorption peak can be clearly observed at 737 cm⁻¹, which is a characteristic peak of the anhydride ring group. After reaction with FA, the peak is observed at 737 cm⁻¹. -1 (Furan ring CH bending vibration) and 1145cm -1 The appearance of a new peak in (COC stretching vibration) proves that FA was successfully grafted onto anhydride-modified PE.

[0041] 2.2. DA Crosslinking of Furan-Bismaleimide Following the furan grafting reaction, the product of the functionalized PE-g-MA further reacts with BMI. For example... Figure 1 The presence of BMI in the product is indicated by a concentration of 1513 cm⁻¹. -1 The absorption peak at 1187 cm⁻¹ confirms that it corresponds to the stretching vibration of the BMI aromatic ring (CH=CH). Simultaneously, at 1187 cm⁻¹... -1 A new peak appeared, corresponding to the vibration of the DA ring (CO / CN coupling mode in the succinimide-furan adduct). These results indicate that a DA reaction occurred between PE-g-FA and BMI, forming a DA crosslinking network.

[0042] 2.3. Solubility test of cross-linked products like Figure 2 This shows a comparison of the solubility of PE-g-MA, PE-g-FA, and PE-g-DA in o-dichlorobenzene at 125℃ before and after 24 hours. The PE-g-MA sample dissolves rapidly and completely in DCB, while the PE-g-FA dissolves slightly more slowly but also completely after 24 hours. PE-g-DA, however, only partially dissolves in DCB after 24 hours. This is because PE-g-MA and PE-g-FA lack the DA crosslinking bonds that form an insoluble covalent network.

[0043] When a solution of DCB and PE-g-DA is heated to 160°C and then cooled to 60°C, the change in solubility of PE-g-DA is as follows: Figure 3 As shown, when heated to 160°C, the sample gradually transformed into a homogeneous solution within approximately 2 hours. This phenomenon is attributed to the reverse DA reaction at high temperature, which cleaves the DA adduct and releases polymer chains from the network, thereby restoring solubility. Subsequent cooling of the solution to 60°C resulted in the reappearance of insoluble portions and macroscopic gels, indicating the reformation of DA crosslinks and network reconstruction. The reversible dissolution-gel cycle provides direct macroscopic evidence for the dynamic covalent bonding and network reconstruction of PE-g-DA. These observations are consistent with FTIR analysis, which revealed the expected modulation of diagnostic bands associated with the formation and dissociation of furan-maleimide DA adducts throughout the thermal cycle. In summary, the solubility test and IR spectral data confirm that PE-g-DA possesses a thermally switchable crosslinked network: high-temperature decrosslinking driven by RDA enables dissolution and processing, while recoupling of DA upon cooling restores the crosslinked state. This reversible network behavior supports the recyclability and tunable processability of the material without sacrificing crosslinker-derived properties.

[0044] 2.4. Crosslinking density test Approximately 0.5 g of the dried crosslinked sample was weighed into a vial and immersed in 15 mL of o-dichlorobenzene until swelling equilibrium was reached after 3 days. The crosslinking density was then calculated using the Flory-Rehner equation. Figure 4 As the BMI content increases, the crosslinking density of the crosslinking product also increases.

[0045] 2.5. DSC Test To further verify the temperature range of the DA and RDA reactions (crosslinking and decrosslinking reactions), the thermal reversibility of the PE-g-DA sample was verified using DSC. Figure 5When BMI was added at amounts of 0.003 mol and 0.006 mol, a new endothermic peak (around 150 °C) was observed during heating, and a new exothermic peak (around 130 °C) was observed during cooling. However, no new peaks were observed when the BMI addition was low. To determine whether the new peaks were due to the characteristic of the DA crosslinking network breaking and re-crosslinking reaction, the inventors performed a heating-cooling-reheating DSC test on the crosslinked product (e.g., [temperature information missing]). Figure 6 A new peak was observed during both heating and one cooling processes. This allows us to determine the temperature range for the DA and RDA reactions.

[0046] 2.6. SEM Testing Due to the presence of reversible dynamic covalent bonds in PE-g-DA, the crosslinked product possesses potential self-healing properties. To investigate its self-healing behavior, a small crack was cut into the surface of the specimen with a blade. The scratched specimen was then heated in an oven at 120°C for different times, and the crack healing process was observed using a scanning electron microscope. (See scanning electron microscope image for example.) Figure 7 As shown in the figure, the cracks on the PE-g-DA sample film gradually decreased in size and eventually almost disappeared. However, the cracks on the PE-gMA sample film showed virtually no healing over time; this result indicates that cross-linked samples can heal at around 120℃.

[0047] 2.7. Tensile Test The stress-strain curves of PE-g-MA and PE-g-DA samples with different amounts of BMI are shown in the figure. Figure 8 As shown. Figure 8 Tensile tests were conducted on PE-g-MA and PE-g-DA with different BMI additions. The results showed that as the BMI addition decreased, the tensile strength of PE-g-DA decreased, while the elongation at break increased. Compared to PE-g-MA, the material with 0.0006 mol of BMI showed a 94% increase in tensile strength and a 12% decrease in elongation at break; the material with 0.003 mol of BMI showed a 72% increase in tensile strength and a 43% increase in elongation at break; and the material with 0.001 mol of BMI showed a 62% increase in tensile strength and an 80% increase in elongation at break. This is mainly attributed to the changes in crosslinking density caused by different BMI additions.

[0048] Furthermore, due to the reversibility of the DA reaction, PE-g-DA also exhibits recyclability. Taking the crosslinked product with 0.003 mol of BMI as an example, the stress-strain curves after hot-pressing the recovered sample are as follows: Figure 9 As shown. Figure 9 PE-g-DA recycling tensile test The results showed that the reshaped specimens still possessed good mechanical properties. The tensile strength retention rate was approximately 93% after the first cycle and remained around 80% after the second cycle, indicating that the cross-linked product exhibited good thermal reversibility. Although the elongation at break decreased after cycling, the elongation at break of the sample was still higher than that of the original material after two cycles.

[0049] Compare with Example 1 Referring to the steps and proportions in Example 1, the following are unoptimized failure cases from the early stages of the development of this invention.

[0050] 1. Grafting was performed by mixing PE-g-MA (50g) and FA (one equivalent of MA content in PE-g-MA) in an internal mixer at 120°C and 50 rpm for 10 min. The resulting product was named PE-g-FA. Then, PE-g-FA (50g) and different equivalents of BMI (based on the theoretical amount of furan in the product) were mixed in an internal mixer at 170°C and 50 rpm for 30 min to obtain PE-g-DA. However, through… Figure 10 Fourier transform infrared spectroscopy (FTIR) revealed that the infrared spectra of PE-g-MA, PE-g-FA, and PE-g-DA were essentially the same, and the desired characteristic peaks did not appear. The first step of grafting FA failed.

[0051] The inventors then began adjusting the reaction temperature of FA, conducting comparisons at four gradients: 130℃, 140℃, 150℃, and 160℃. Finally, at a reaction temperature of 160℃, they observed significant changes in the infrared spectra of PE-g-FA and PE-g-MA. Figure 11 However, no characteristic peaks have yet appeared to prove that the FA graft was successful.

[0052] After a period of continuous experimentation and literature review, it was discovered that PE-g-MA and FA required further dehydration and ring-closure after the ring-opening reaction. Therefore, the inventors added a dehydration step using a hot press at 10 MPa and 200°C after the reaction of FA and PE-g-MA. Figure 12 After dehydration and ring closure, 745 cm⁻¹ appeared in PE-g-FA. -1 (Furan ring CH bending vibration) and 1141cm -1 The novel characteristic peak of the (COC stretching vibration) confirms the successful grafting of FA onto the anhydride-modified PE. Following the furan (FA) grafting reaction, the product of the functionalized PE-g-MA further reacts with BMI. Figure 12 The presence of BMI in the product is indicated by a concentration of 1509 cm⁻¹. -1The absorption peaks were used to verify this, which correspond to the stretching vibration of the aromatic ring of BMI (CH=CH). However, the characteristic peaks of the DA adduct did not appear, indicating that a DA crosslinking network may not have been formed between PE-g-FA and BMI.

[0053] After further experimentation and literature review, the inventors discovered that the product required annealing after the reaction of PE-g-FA with BMI, as the subsequent DA reaction was difficult to carry out at room temperature. Following experiments, the inventors selected an annealing temperature of 120℃ and an annealing time of 24 hours. Figure 13 It can be found that PE-g-DA is at 1187cm. -1 A new peak appeared, which corresponds to the vibration of the DA ring (CO / CN coupling mode in the succinimide-furan adduct).

[0054] The inventors subsequently conducted swelling tests and DSC tests, finding that when a solution of DCB and PE-g-DA was heated to 160°C and then cooled to 60°C, the solubility of PE-g-DA changed as follows: Figure 14 As shown, the sample completely dissolved when heated to 160°C. Upon cooling the solution to 60°C, it was found that some of the solution gelled, but the gelation was not complete. This indicates that the cross-linked network may only have partially formed.

[0055] DSC testing of PE-g-MA and PE-g-DA as follows Figure 15 It can be observed that a new peak appeared in PE-g-DA during the heating process, but no new peak appeared during the cooling process. Based on the swelling test results, the inventors speculate that the crosslinking density is insufficient and that the DA bond content in the material needs to be increased.

[0056] Therefore, the inventors decided to add an excess of FA, adding FA at a molar equivalent of 3 times the anhydride group content of PE-g-MA. The final experimental procedure was as follows: PE-g-MA (50g) and FA at a molar equivalent of 3 times the MA content in PE-g-MA were mixed in an internal mixer at 160°C and 50 rpm for 10 min for grafting. The resulting product was reacted in a hot press at 10 MPa and 200°C for 30 min and named PE-g-FA. Then, PE-g-FA (50g) and different equivalents of BMI based on the theoretical amount of furan in the product were mixed in an internal mixer at 170°C and 50 rpm for 30 min. Finally, the product was annealed in an oven at 120°C for 24 h to obtain a thermally reversible DA crosslinked product, named PE-g-DA.

[0057] In summary, this thesis prepared a thermally reversible crosslinked PE via a solvent-free melt method, grafting fatty acids (FAs) onto maleic anhydride-modified polyethylene (MA), followed by crosslinking with bismaleimide using a Diels-Alder chemical method. FTIR confirmed the success of the grafting and DA crosslinking. The resulting PE-g-DA exhibits a reversibly crosslinked network with significant thermal reversibility: DA chains dissociate upon heat treatment at temperatures above 150°C and reform near 60°C, enabling repeatable network reconstruction and reprocessing. The material exhibits higher solvent resistance, better thermal stability, and tunable mechanical properties. With increasing BMI content, the ultimate strength increases while ductility decreases, reflecting the expected strength-ductility tradeoff. Overall, this study establishes an efficient, economical, and environmentally friendly strategy for producing recyclable, high-performance PE plasmons, combining thermally reversible crosslinking with tunable mechanics to promote the design and use of sustainable plasmons. The crosslinked PE-g-DA can be recycled and reprocessed into new materials via high-temperature hot pressing. This recyclability makes it a potential replacement for single-use polyethylene plastics in sustainable packaging materials, thereby reducing environmental pollution. Furthermore, its reprocessability makes it suitable for manufacturing recyclable parts for the automotive or electronics industries, supporting the development of a circular economy.

[0058] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, characterized in that, Includes the following steps: S1 Grafting reaction: Maleic anhydride-modified polyethylene and furanyl methylamine were melt-blended and grafted to obtain a grafting intermediate product. S2 Dehydration and ring-closing treatment: The grafting intermediate obtained in step S1 is subjected to dehydration and ring-closing treatment to obtain furan-functionalized polyethylene. S3 Crosslinking reaction: The furan-functionalized polyethylene obtained in step S2 is melt-blended with bismaleimide, and a crosslinking network is formed by Diels-Alder reaction to obtain a crosslinking intermediate product; S4 Annealing treatment: The cross-linking intermediate product obtained in step S3 is annealed to obtain thermally reversible cross-linked polyethylene material.

2. The method for preparing the thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, In step S2, the dehydration closed-loop treatment is carried out at 180℃~220℃ and a pressure of 5 MPa~15 MPa for 20~40 minutes.

3. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 2, characterized in that, The dehydration closed-loop treatment is carried out using a hot press at 200°C and 10 MPa for 30 minutes.

4. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, In step S4, the annealing process is carried out at a temperature of 110℃~130℃ for 20~28 hours.

5. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, In step S1, the amount of furanylmethylamine used is 2 to 5 times the molar amount of maleic anhydride functional groups in the maleic anhydride-modified polyethylene.

6. The method for preparing the thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 5, characterized in that, In step S1, the amount of furanylmethylamine used is three times the equivalent of the molar amount of maleic anhydride functional groups in the maleic anhydride-modified polyethylene.

7. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, In step S1, the temperature of the melt blending grafting reaction is 155℃~165℃, and the reaction time is 5~15 minutes.

8. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, In step S3, the melt blending is carried out at 165℃~175℃, a rotation speed of 40-60rpm, and a reaction time of 25~35 minutes.

9. The method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis according to claim 1, characterized in that, Specifically, the steps include the following: S1 Grafting reaction: Maleic anhydride-modified polyethylene and furanyl methylamine with an equivalent amount of 3 times the maleic anhydride functional group content in maleic anhydride-modified polyethylene were mixed in an internal mixer at 160°C and 50 rpm for 10 min to obtain a grafting intermediate product. S2 Dehydration and Ring-Closed Treatment: The grafting intermediate obtained in step S1 is subjected to dehydration and ring-closed treatment to obtain furan-functionalized polyethylene; the dehydration and ring-closed treatment is carried out in a hot press at 200°C and 10 MPa for 30 minutes. S3 Crosslinking reaction: The furan-functionalized polyethylene obtained in step S2 is melt-blended with bismaleimide, and a crosslinking network is formed through the Diels-Alder reaction to obtain a crosslinking intermediate; the melt blending is carried out at 170°C, 50 rpm, and for 30 minutes. S4 Annealing treatment: The cross-linked intermediate product obtained in step S3 is annealed at a temperature of 120°C for 24 hours to obtain thermally reversible cross-linked polyethylene material.

10. A thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis, characterized in that, It is prepared by the method for preparing thermally reversible crosslinked polyethylene material based on Diels-Alder synthesis as described in any one of claims 1-9.

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