Polypropylene composite insulating material with thermal self-recovery property as well as preparation method and application thereof

Abstract

The invention relates to a polypropylene insulating material, in particular to a polypropylene composite insulating material with thermal self-recovery performance and a preparation method and application thereof. The composite material is formed by blending maleic acid grafted polypropylene, active carbon fibers and polyvinylidene fluoride according to the mass ratio of (95-110): (0.1-0.5): (1-15). Polypropylene, maleic acid, the first initiator and part of the antioxidant are heated to a set temperature for melt blending; in the melt blending process, after the materials are completely molten, adding active carbon fibers, adding a second initiator, continuing melt blending until the melt blending time is 50-70% of the total melt blending time, adding polyvinylidene fluoride and the other part of the antioxidant, and then performing melt blending until the melt blending is finished. The polypropylene composite insulating material prepared by the invention can realize self-repairing through recombination of hydrogen bonds under a heating condition, and can maintain the original mechanical properties through self-repairing after mechanical damage, so that the service life of the material is remarkably prolonged.

Description

Technical Field

[0001] The present invention relates to a polypropylene insulating material, and in particular to a polypropylene composite insulating material with thermal self - recovery property, its preparation method and application. Background Art

[0002] Disclosing the information of this background art section is only intended to enhance the understanding of the overall background of the present invention, and it is not necessarily regarded as an admission or an implication in any form that this information constitutes the prior art already known to those of ordinary skill in the art.

[0003] The ability to heal mechanical damage and restore the original function is crucial for functional polymers in various application scenarios, as it determines their lifespan. Synthetic polymer materials capable of self - repairing by restoring mechanical properties and functions have attracted great interest because such materials can extend product lifespan and improve equipment performance. Polypropylene (PP), as a common thermoplastic polymer material, since its inception, has been widely used in fields such as electronic packaging, new energy, chemical industry, coatings, adhesives, machinery, construction, etc. due to its excellent mechanical properties, dimensional and chemical stability, and plays an important role. It has become one of the indispensable important materials in the industrial community. However, there are still many defects in practical applications. For example, after the polypropylene material is modified, a new polypropylene molecular structure will be formed, thus endowing it with new properties. For example, by adding glass fibers, metal additives or thermoplastic rubber, etc., the mechanical properties of polypropylene can be enhanced; by blending or copolymerizing with other polymers, the weather resistance, flame retardancy, etc. of polypropylene can be improved. Although today's modification technologies are very excellent, enabling the polypropylene material to have good mechanical properties and anti - aging lifespan, the new polypropylene network structure cannot be re - processed and restored after being broken or damaged. The accumulation of a large amount of polypropylene material waste has led to a waste of resources and caused environmental and economic loss problems. Summary of the Invention

[0004] In order to solve the deficiencies of the prior art, the purpose of the present invention is to provide a polypropylene composite insulating material with thermal self - recovery property, its preparation method and application. The polypropylene composite insulating material prepared by the present invention can achieve self - repair through the recombination of hydrogen bonds under heating conditions, and can maintain its original mechanical properties through self - repair after mechanical damage, thus significantly extending the service life of the material.

[0005] In order to achieve the above purpose, the technical solution of the present invention is as follows:

[0006] In the first aspect, a polypropylene composite insulating material with thermal self-recovery property is formed by blending maleic anhydride grafted polypropylene (PP-g-MA), activated carbon fiber (activated CF, ACF), and polyvinylidene fluoride (PVDF) in a mass ratio of 95 - 110:0.1 - 0.5:1 - 15.

[0007] The matrix material first adopted in the present invention is PP-g-MA. Introducing maleic anhydride into the polypropylene molecular chain by chemical grafting can significantly improve the polarity of polypropylene, enhance its compatibility with other polar materials (PVDF), and thus improve the interfacial bonding force and overall performance of the composite material. The grafted polypropylene not only has better interfacial compatibility and can effectively reduce the phase separation phenomenon, but also performs better in improving the mechanical properties such as tensile strength and elongation at break of the material. Especially in a high-temperature environment, it can maintain high strength and stability, and also has a high thermal self-recovery ability. Specifically, after the grafted polypropylene composite material is damaged by external force, it can achieve self-recovery through the recombination of hydrogen bonds at high temperature, and the recovery rate is significantly higher than that of the blend material of polypropylene and other maleic anhydride grafted polyolefins.

[0008] Secondly, polyvinylidene fluoride (PVDF) used in the present invention contains fluorine atoms, which can form strong hydrogen bond interactions with the carboxylic acid groups in PP-g-MA. This kind of hydrogen bond is dynamically reversible at high temperature, can break and recombine under the action of external force, so as to achieve the thermal self-recovery ability of the material. Although ethylene-tetrafluoroethylene copolymer (ETFE) also contains fluorine atoms, it is found in the experiment that after PVDF is blended with PP-g-MA, there is an obvious absorption peak shift near 3292 cm -1 and 3342 cm -1 in the infrared spectrum, indicating that stronger hydrogen bond interactions are formed between PVDF and PP-g-MA, that is, the vinylidene fluoride structure of PVDF is easy to form stronger hydrogen bond interactions with PP-g-MA, and this stronger hydrogen bond interaction is beneficial to improving the thermal self-recovery property of the polypropylene composite material.

[0009] Through the compound blending of PP-g-MA, ACF, and PVDF, the polypropylene composite insulating material of the present invention not only has good thermal self-recovery property, but also has high mechanical properties, which is beneficial to its application in cables.

[0010] In the second aspect, a preparation method of the above polypropylene composite insulating material with thermal self-recovery property is to heat polypropylene, maleic anhydride, a first initiator, and part of the antioxidant to a set temperature for melt blending, and carry out grafting reaction during the melt blending process; the set temperature is T - (T + 10°C), and T is the melting temperature of polypropylene;

[0011] During the melt blending process, after the materials are completely melted, activated carbon fibers are added, and a second initiator is added. Then, the melt blending is continued until 50-70% of the total melt blending time, polyvinylidene fluoride and another part of the antioxidant are added, and then the melt blending is continued until the end.

[0012] Among them, the melting temperature of polypropylene is 164-176 °C.

[0013] In order to make the polypropylene composite insulating material have better performance, it is necessary to disperse each material more evenly, have better compatibility, and avoid the aging of raw materials. Therefore, in the process of melt blending and graft modification of polypropylene and maleic acid, ACF and PVDF are added in the present invention. Adding ACF after polypropylene is completely melted can make it easy to combine with polypropylene, thereby improving the interfacial compatibility of the composite material, and at the same time facilitating the dispersion of ACF. Adding the second initiator at this time can promote the graft reaction of polypropylene and maleic acid, and at the same time initiate the formation of a dynamic covalent network to improve the self-healing efficiency. Since the melt blending temperature is too high, PVDF is prone to aging at this temperature for a long time, thereby reducing the performance of its composite material. Therefore, PVDF is added in the second half of the melt blending in the present invention, which can avoid the aging of PVDF and ensure its dispersibility at the same time. The method of adding materials in the present invention is more conducive to improving the interfacial compatibility of the composite material, the dispersibility of each material, and avoiding aging, thereby ensuring the thermal self-recovery and mechanical properties of the polypropylene composite insulating material.

[0014] In some embodiments, the antioxidant is antioxidant 168 and antioxidant 1010. Antioxidant 168 is the main antioxidant, and the function of the main antioxidant 168 is to change the oxidation reaction process and prevent the reaction cycle by providing hydrogen to make the active free radicals become stable free radicals. Antioxidant 1010 is the auxiliary antioxidant, and the function of antioxidant 1010 is to decompose the hydroperoxides generated during the reaction process, and it cannot change the oxidation reaction process, but only plays a role in reducing the reaction rate. Using the two antioxidants in combination in the formulation system, there is a synergistic effect between different types of antioxidants, which will greatly improve the heat and oxygen aging resistance effect of PP, which is of great significance for extending the product life and reducing the production cost.

[0015] In the third aspect, an application of the above-mentioned polypropylene composite insulating material with thermal self-recovery property in the preparation of cables.

[0016] In the fourth aspect, a cable includes a conductor and an insulating layer, and the material of the insulating layer is the above-mentioned polypropylene composite insulating material with thermal self-recovery property.

[0017] The beneficial effects of the present invention are:

[0018] On the basis of maintaining the original excellent properties of polypropylene, by grafting maleic acid and blending ACF and PVDF, the invention not only has high mechanical properties, but also can incorporate hydrogen bonds into the polypropylene material to develop self-healing. Utilizing the interaction force between hydrogen bonds, under the stimulation of a specific external temperature, the dynamically reversible covalent bonds in the new polypropylene structure will be reversibly broken and reorganized, thereby endowing the composite material with the ability of thermal self-recovery. Due to the reorganization of the dynamically reversible covalent bonds in the polypropylene structure under specific external temperature conditions, the material can maintain its original mechanical properties, such as strength, toughness, hardness, etc., when subjected to external forces. The composite material can recover these properties after damage to maintain its reliability in use. And this thermal self-recovery ability can repair the damage caused by long-term mechanical use, thereby significantly extending the service life of the material and reducing the maintenance cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings forming a part of this invention are used to provide a further understanding of the invention. The schematic embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0020] Figure 1 It is a flowchart for preparing the composite material according to an embodiment of the invention;

[0021] Figure 2 It is an infrared spectrogram of the composite material prepared according to an embodiment of the invention in the range of 800 - 4000 cm -1 ;

[0022] Figure 3 It is a DSC melting curve of the composite material prepared according to an embodiment of the invention;

[0023] Figure 4 It is a bar chart of the recovery amount of the composite material prepared according to an embodiment of the invention at different temperatures;

[0024] Figure 5 It is a thermal recovery diagram of pure PP of the invention at different temperatures. A is room temperature, B is 50 °C, C is 90 °C, D is 105 °C, E is 120 °C, F is 130 °C, and the scale is 500 μm for all;

[0025] Figure 6 It is a thermal recovery diagram of PP-g-MA prepared in Example 1 of the invention at different temperatures. A is room temperature, B is 50 °C, C is 90 °C, D is 105 °C, E is 120 °C, F is 130 °C, and the scale is 500 μm for all;

[0026] Figure 7Thermal recovery diagrams of PP-g-MA / 1PVDF prepared in Example 2 of the present invention at different temperatures. A represents room temperature, B represents 50 °C, C represents 90 °C, D represents 105 °C, E represents 120 °C, F represents 130 °C, and the scale is 500 μm for all;

[0027] Figure 8 Thermal recovery diagrams of PP-g-MA / 3PVDF prepared in Example 3 of the present invention at different temperatures. A represents room temperature, B represents 50 °C, C represents 90 °C, D represents 105 °C, E represents 120 °C, F represents 130 °C, and the scale is 500 μm for all;

[0028] Figure 9 Thermal recovery diagrams of PP-g-MA / 5PVDF prepared in Example 4 of the present invention at different temperatures. A represents room temperature, B represents 50 °C, C represents 90 °C, D represents 105 °C, E represents 120 °C, F represents 130 °C, and the scale is 500 μm for all;

[0029] Figure 10 Thermal recovery diagrams of PP-g-MA / 10PVDF prepared in Example 5 of the present invention at different temperatures. A represents room temperature, B represents 50 °C, C represents 90 °C, D represents 105 °C, E represents 120 °C, F represents 130 °C, and the scale is 500 μm for all;

[0030] Figure 11 Mechanical stress-strain curves of the composite material prepared in the example of the present invention at room temperature without pre-elongation (A) and after pre-elongation at 120 °C (B) and 130 °C (C). Detailed implementation manners

[0031] It should be noted that the following detailed description is exemplary and is intended to provide further illustration of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention belongs.

[0032] It should be noted that the terms used herein are only for describing specific implementation manners and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0033] In order to endow the polypropylene composite insulating material with thermal self-recovery property and thus extend its service life, the present invention proposes a polypropylene composite insulating material with thermal self-recovery property, its preparation method and application.

[0034] In a typical embodiment of the present invention, a polypropylene composite insulating material with thermal self-recovery property is provided, which is formed by blending maleic anhydride-grafted polypropylene, activated carbon fiber, and polyvinylidene fluoride in a mass ratio of 95-110:0.1-0.5:1-15.

[0035] In some embodiments, maleic anhydride-grafted polypropylene, activated carbon fiber, and polyvinylidene fluoride are in a mass ratio of 100-110:0.2-0.4:1-10.

[0036] Another embodiment of the present invention provides a preparation method of the above polypropylene composite insulating material with thermal self-recovery property. Polypropylene, maleic anhydride, a first initiator, and a part of antioxidant are heated to a set temperature for melt blending, and a grafting reaction is carried out during the melt blending process; the set temperature is T~

[0037] (T + 10°C), where T is the melting temperature of polypropylene;

[0038] During the melt blending process, after the materials are completely melted, activated carbon fiber is added, and a second initiator is added, and then the melt blending is continued until 50-70% of the total melt blending time. Polyvinylidene fluoride and another part of antioxidant are added, and then the melt blending is continued until the end.

[0039] In some embodiments, the antioxidant is antioxidant 168 and antioxidant 1010. Research shows that these two antioxidants can synergistically improve the heat-oxidative aging effect of PP, which is beneficial to extending the service life of the polypropylene insulating composite material.

[0040] In some embodiments, the mass ratio of antioxidant 168 to antioxidant 1010 is 1.5-2.5:1. Research shows that the effect is better under this condition.

[0041] In some embodiments, the added mass of the antioxidant is 0.8-1.0% of the mass of polypropylene.

[0042] In some embodiments, the first initiator is dicumyl peroxide (DCP). Specifically, the added mass of the first initiator is 0.2-0.4% of the mass of polypropylene.

[0043] In some embodiments, the second initiator is di-tert-butyl peroxide (DTBP). Research shows that DTBP can better promote the PP-g-MA grafting reaction and has a better effect on initiating the formation of a dynamic covalent network, thereby further improving the self-healing efficiency. Specifically, the added mass of the second initiator is 0.2-0.4% of the mass of polypropylene.

[0044] In some embodiments, the total time of melt blending is 15-20 minutes.

[0045] In some embodiments, activated carbon fiber is added when the melt blending is carried out for 5 to 6 minutes. Under this condition, it can be ensured that the materials are completely melted.

[0046] In some embodiments, polyvinylidene fluoride and another part of antioxidant are added when the melt blending is carried out for 8 to 9 minutes. Under this condition, better dispersion can be achieved on the premise of avoiding aging.

[0047] The third embodiment of the present invention provides an application of the above-mentioned polypropylene composite insulating material with thermal self-recovery property in the preparation of cables.

[0048] The fourth embodiment of the present invention provides a cable, which includes a conductor and an insulating layer, and the material of the insulating layer is the above-mentioned polypropylene composite insulating material with thermal self-recovery property.

[0049] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0050] In the following embodiments, before the formal preparation, polypropylene (PP), antioxidant 168, antioxidant 1010, maleic acid (MA), and polyvinylidene fluoride (PVDF) are placed in an oven and dried at a temperature of 50 °C for 12 hours to ensure that these raw materials are completely dry.

[0051] Example 1

[0052] A preparation method of a polypropylene composite insulating material with thermal self-recovery property is as Figure 1 shown, and the steps are as follows:

[0053] First, 100 parts by mass of polypropylene, 4 parts by mass of maleic acid, 0.4 parts by mass of antioxidant 168, 0.2 parts by mass of antioxidant 1010, and 0.3 parts by mass of DCP are put into a mixer for melt blending. The temperature is set at 180 °C and the rotation speed is 50 revolutions per minute.

[0054] When the blending time is 5 minutes, first 0.3 parts by mass of ACF is added, then 0.3 parts by mass of DTBP is added. Then, when the grafting reaction time is 8 minutes, 0.2 parts by mass of antioxidant 168 and 0.1 parts by mass of antioxidant 1010 are put in, and the blending is continued for 7 minutes. The total blending time is 15 minutes to obtain a PP graft blend (PP-g-MA).

[0055] Place the composite material in a mold, cover polyester films on both the upper and lower sides of the mold, and then clamp it between two thick steel plates. Align the steel plates and put them into a flat vulcanizing machine. First, hot press at a pressure of 0 MPa for 5 minutes to ensure that the material is fully melted and air bubbles are discharged. Then, increase the pressure step by step by 5 MPa to 15 MPa, and maintain hot pressing for 5 minutes under each pressure level. After the pressurization is completed, cool it to finally obtain a sheet specimen that meets the experimental requirements.

[0056] Example 2

[0057] A preparation method of a polypropylene composite insulating material with thermal self-recovery property is as Figure 1 shown, and the steps are as follows:

[0058] First, put 100 parts by mass of polypropylene, 4 parts by mass of maleic acid, 0.4 parts by mass of antioxidant 168, 0.2 parts by mass of antioxidant 1010, and 0.3 parts by mass of DCP into a mixer for melt blending. Set the temperature at 180 °C and the rotation speed at 50 revolutions per minute.

[0059] When the blending time is 5 minutes, first add 0.3 parts by mass of ACF, then add 0.3 parts by mass of DTBP. Then, when the graft reaction time is 8 minutes, put in 1 part by mass of PVDF, 0.2 parts by mass of antioxidant 168, and 0.1 parts by mass of antioxidant 1010, and continue blending for 7 minutes. The total blending time is 15 minutes to obtain a polypropylene composite material (PP-g-MA / 1PVDF).

[0060] Place the composite material in a mold, cover polyester films on both the upper and lower sides of the mold, and then clamp it between two thick steel plates. Align the steel plates and put them into a flat vulcanizing machine. First, hot press at a pressure of 0 MPa for 5 minutes to ensure that the material is fully melted and air bubbles are discharged. Then, increase the pressure step by step by 5 MPa to 15 MPa, and maintain hot pressing for 5 minutes under each pressure level. After the pressurization is completed, cool it to finally obtain a sheet specimen that meets the experimental requirements.

[0061] Example 3

[0062] A preparation method of a polypropylene composite insulating material with thermal self-recovery property is as Figure 1 shown, and the steps are as follows:

[0063] First, put 100 parts by mass of polypropylene, 4 parts by mass of maleic acid, 0.4 parts by mass of antioxidant 168, 0.2 parts by mass of antioxidant 1010, and 0.3 parts by mass of DCP into a mixer for melt blending. Set the temperature at 180 °C and the rotation speed at 50 revolutions per minute.

[0064] When the blending time is 5 minutes, first add 0.3 parts by mass of ACF, then add 0.3 parts by mass of DTBP. Then, when the grafting reaction time is 8 minutes, add 3 parts by mass of PVDF, 0.2 parts by mass of antioxidant 168, and 0.1 parts by mass of antioxidant 1010, and continue blending for 7 minutes. The total blending time is 15 minutes to obtain a polypropylene composite material (PP-g-MA / 3PVDF).

[0065] Place the composite material in a mold, cover polyester films on both the upper and lower sides of the mold, and then sandwich it between two thick steel plates. After aligning the steel plates, place them in a flat vulcanizing machine. First, hot press at a pressure of 0 MPa for 5 minutes to ensure that the material is fully melted and air bubbles are discharged. Then, increase the pressure step by step by 5 MPa to 15 MPa, and maintain hot pressing for 5 minutes at each pressure level. After the pressurization is completed, perform cooling to finally obtain a sheet specimen that meets the experimental requirements.

[0066] Example 4

[0067] A preparation method of a polypropylene composite insulating material with thermal self-recovery property, as Figure 1 shown, the steps are as follows:

[0068] First, put 100 parts by mass of polypropylene, 4 parts by mass of maleic acid, 0.4 parts by mass of antioxidant 168, 0.2 parts by mass of antioxidant 1010, and 0.3 parts by mass of DCP into a kneader for melt blending. The temperature is set at 180 °C and the rotation speed is 50 revolutions per minute.

[0069] When the blending time is 5 minutes, first add 0.3 parts by mass of ACF, then add 0.3 parts by mass of DTBP. Then, when the grafting reaction time is 8 minutes, add 5 parts by mass of PVDF, 0.2 parts by mass of antioxidant 168, and 0.1 parts by mass of antioxidant 1010, and continue blending for 7 minutes. The total blending time is 15 minutes to obtain a polypropylene composite material (PP-g-MA / 5PVDF).

[0070] Place the composite material in a mold, cover polyester films on both the upper and lower sides of the mold, and then sandwich it between two thick steel plates. After aligning the steel plates, place them in a flat vulcanizing machine. First, hot press at a pressure of 0 MPa for 5 minutes to ensure that the material is fully melted and air bubbles are discharged. Then, increase the pressure step by step by 5 MPa to 15 MPa, and maintain hot pressing for 5 minutes at each pressure level. After the pressurization is completed, perform cooling to finally obtain a sheet specimen that meets the experimental requirements.

[0071] Example 5

[0072] A preparation method of a polypropylene composite insulating material with thermal self-recovery property, as Figure 1 shown, the steps are as follows:

[0073] First, 100 parts by mass of polypropylene, 4 parts by mass of maleic acid, 0.4 parts by mass of antioxidant 168, 0.2 parts by mass of antioxidant 1010, and 0.3 parts by mass of DCP are put into a mixer for melt blending. The temperature is set at 180 °C and the rotation speed is 50 revolutions per minute.

[0074] When the blending time is 5 minutes, first add 0.3 parts by mass of ACF, then add 0.3 parts by mass of DTBP. Then, when the graft reaction time is 8 minutes, put in 10 parts by mass of PVDF, 0.2 parts by mass of antioxidant 168, and 0.1 parts by mass of antioxidant 1010, and continue blending for 7 minutes. The total blending time is 15 minutes to obtain a polypropylene composite material (PP-g-MA / 10PVDF).

[0075] Place the composite material in a mold, cover polyester films on both the upper and lower sides of the mold, and then sandwich it between two thick steel plates. After aligning the steel plates, put them into a flat vulcanizing machine. First, hot press at a pressure of 0 MPa for 5 minutes to ensure that the material is fully melted and air bubbles are discharged. Then, with a pressure increment of 5 MPa, gradually increase the pressure to 15 MPa, and maintain hot pressing for 5 minutes at each pressure level. After the pressurization is completed, cool it to finally obtain a sheet sample that meets the experimental requirements.

[0076] The composite materials prepared in each example are subjected to infrared polarization experiments, and the results are as Figure 2 shown in Table 1. An absorption peak shift appears at around a wavenumber of 3300 cm-1. In the infrared spectrum of this composite material (polymer containing alcohol), O-H and C-C stretching vibration absorption bands can be observed, and they are affected by the intramolecular hydrogen bond. As the concentration increases, the intermolecular hydrogen bond effect will enhance, causing the band to shift to a lower frequency, that is, the infrared spectrum undergoes a red shift and the wavenumber increases. Due to the formation of hydrogen bonds, the vibration of O-H is restricted to a certain extent, resulting in a change in the intensity of the absorption peak. In the figure, the composite material has a more obvious change in absorption intensity compared to pure polypropylene, especially in the absorption peak intensity at around 3292 cm -1 where there is an obvious weakening effect.

[0077] Table 1 Absorption peak data

[0078]

[0079]

[0080] All samples at 3292 cm -1 and 3342 cm -1Absorption peaks exist nearby and show regular shifts with the increase in PVDF content. The changes in these two absorption peaks are attributed to O-H stretching vibration, C-H vibration, and hydrogen bond interaction. Since polar groups (carboxylic anhydride, C-F) are introduced into MA and PVDF, the C-H vibration frequency can be changed through hydrogen bonds. After pure PP is grafted with MA, the wave number of the absorption peak begins to increase. With the increase in the proportion of PVDF, the wave numbers of the absorption peaks all increase to varying degrees, and the increase in PP-g-MA / 10PVDF is the largest.

[0081] Therefore, the infrared polarization experiment shows that rich hydrogen bonds will be generated after polypropylene grafted with maleic acid is blended with PVDF. By utilizing the interaction force between hydrogen bonds, the polypropylene composite material can have a certain thermal self-recovery ability, that is, the ability to return to the initial state of the material itself when heated after being damaged by external forces.

[0082] Use DSC to characterize the thermal properties of the materials and comparatively analyze the melting characteristics of different materials. The results are as Figure 3 shown. The melting temperature of pure PP is 146.333 °C. After being grafted with MA, the melting temperature remains unchanged at 146.333 °C, indicating that the grafting of maleic anhydride has little effect on the melting behavior of PP. However, after blending with PVDF, the melting temperature increases. The melting temperature of PP-g-MA / 1PVDF is 150.424 °C, an increase of nearly 3 °C. And with the increase in the blending ratio, PP-g-MA / 3PVDF shows two melting peaks, one at 148.974 °C and the other new peak at 169.885 °C, indicating that the increase in PVDF content has a greater impact on the melting behavior. The intensity of the new melting peak of PP-g-MA / 10PVDF further increases. This is because the crystallization and melting behavior of PVDF is different from that of PP-g-MA, resulting in the formation of a new phase structure in the blend. These new phase structures may affect the thermal stability and mechanical properties of the material. In addition, the blending of PP and PVDF may lead to the formation of chemical or physical crosslinks, which will change the mobility of polymer chains and the growth of crystal structures. Chemical crosslinks will form a stable network structure, improving the thermal stability and mechanical strength of the material. Physical crosslinks affect the thermal properties of the material through intermolecular interactions (such as hydrogen bonds, van der Waals forces, etc.).

[0083] Therefore, the DSC experiment shows that the thermal behavior differences between PP and PP-g-MA / PVDF should be attributed to the influence of the intermolecular structure, which changes the mobility of polymer chains and the growth of crystal structures, making it have higher thermal stability and thermal self-recovery performance. In addition, the determination of the melting temperature provides a basis for setting the temperature for subsequent thermal recovery.

[0084] Conduct a constant elongation heat self-recovery experiment on each composite material: First, press the polypropylene composite material into a dumbbell shape with an initial marked length of 55 mm. Use a constant elongation retainer to stretch the polypropylene composite material by 30 mm. Place the samples of each component after constant elongation at normal temperature, 50 °C, 90 °C, 105 °C, 120 °C, and 130 °C for self-recovery experiments for the same period of time, and measure the deformation recovery amount of the dumbbell sheet. Second, conduct recoveries for different periods of time at the temperature with the most obvious recovery effect, and measure the total self-recovery deformation amount of the dumbbell sheet. The results are as follows Figure 4 and shown in Table 2

[0085] Initial deformation: All materials are stretched to 85.00 mm (initial marked length 55.00 mm, stretching amount 30.00 mm). Define the recovery amount: Recovery amount = 85.00 mm - length after recovery, Recovery rate = Recovery amount / 30.00 mm × 100%.

[0086] Table 2 Data of constant length stretching and heat recovery of samples at normal temperature

[0087]

[0088]

[0089] Figure 4 Among them, at normal temperature, the recovery amount of pure PP is 14.00 mm (recovery rate 46.7%). The recovery amount of PP-g-MA / 10PVDF is 17.50 mm (recovery rate 58.3%). It shows that the addition of PVDF significantly improves the normal temperature recovery rate, indicating that its polar groups enhance the elastic memory ability of the molecular chain. The grafting of PP-g-MA further optimizes the interfacial compatibility and reduces the negative impact of phase separation on the recovery performance. At high temperature, the best recovery material is PP-g-MA / 10PVDF with a recovery amount of 27.70 mm (recovery rate 92.3%) at 130 °C. The recovery amount of PP-g-MA / 3PVDF at 130 °C is 27.50 mm (recovery rate 91.7%). Among them, the recovery rate of all materials increases significantly with the increase of temperature and reaches the peak at 120 - 130 °C. For example, the recovery rate of pure PP increases to 78.6% at 130 °C, while that of PP-g-MA / 10PVDF is as high as 92.3%. When the PVDF content is low, the recovery rate increases significantly at high temperature, but the normal temperature performance is general (such as the recovery rate of PP-g-MA / 3PVDF at normal temperature is 60%). When the PVDF content is high, the high temperature recovery rate exceeds 90%, and the processing window is wide. The recovery rate of PP-g-MA / 10PVDF is as high as 92.3% at 130 °C, showing excellent thermal responsiveness.

[0090] Therefore, the constant elongation heat self-recovery experiment shows that the total heat recovery of the polypropylene composite material has been significantly improved compared with that of pure polypropylene. Moreover, with the increase in temperature, the total heat recovery also increases significantly. And the copolymers with different components are significantly better than pure polypropylene materials, and the heat recovery effect is the most obvious at 120 and 130 °C. With the increase in the concentration of PVDF, the heat recovery effect will also be correspondingly improved. Comparing the heat recovery of polypropylene, after the graft modification and blending of the present invention, the polypropylene composite material has excellent heat self-recovery ability in terms of mechanics and can be applied to the cable insulation environment with higher temperature to reduce economic losses.

[0091] Thermal self-recovery experiments of each composite material under polarized light were carried out: First, polypropylene and the composite materials were pressed into films, scratched, and the samples of each component after scratch damage were placed at room temperature, 50 °C, 90 °C, 105 °C, 120 °C, and 130 °C for thermal self-recovery for the same time (24 h), and the scratch recovery under a polarized light microscope was observed. The recovery situation is as follows Figures 5 - 10 shown.

[0092] The recovery situation is as Figures 5 - 10 shown. For each ratio of composite materials, the 24-hour recovery at 50 °C and 90 °C is almost the same as that at room temperature, and the change is not very obvious. When the temperature reaches 110 °C, there will be some self-healing changes after mechanical damage. The transparency at the center of the scratch will become darker, indicating that the material will repair and self-heal towards the center. That is, the material has a certain thermal self-recovery ability. When the temperature rises to 120 °C, the polypropylene materials of each component recover to some extent, but the recovery effect is not obvious, and the change is minimal compared with 110 °C. However, when the temperature rises to 130 °C, there are obvious changes. It can be seen that there are still obvious scratch marks on pure polypropylene, while half of the marks on the polypropylene material grafted with maleic acid have disappeared, indicating that there is a certain thermal recovery ability after grafting. When the content of PVDF in the blend increases, its recovery effect is significantly improved, and the scratch damage it receives has been repaired by nearly 90%, and almost no scratches can be seen.

[0093] The influence of temperature on the self-recovery effect of the damaged material: Pure PP ( Figure 5 ), the scratch is obvious at room temperature, and the recovery effect is weak (<20%). When the temperature is greater than or equal to 90 °C, the scratch gradually becomes blurred, and the recovery rate is about 60%-70% at 130 °C. This is because the PP chain segments have enhanced mobility at high temperature, but lack dynamic bonds, and the recovery depends on physical relaxation. PP-g-MA ( Figure 6 ): The scratch recovery rate at room temperature is slightly higher than that of pure PP because the polar groups of MA promote local chain segment recombination. When the temperature rises to 120-130 °C, the scratch recovery rate significantly increases to 80%-85%. PP-g-MA / PVDF (Figures 7 - 10 ):At room temperature, the higher the PVDF content, the higher the recovery rate (e.g., the recovery rate of PP-g-MA / 10PVDF is ~50%). When the temperature reaches 120 or 130 °C, the recovery rate of PP-g-MA / 3PVDF( Figure 8 ) is greater than 90%, and the scratch of PP-g-MA / 10PVDF( Figure 10 ) almost completely disappears, with a recovery rate as high as 95%. This is because the polar fluorine atoms of PVDF form dynamic hydrogen bonds with the carboxylic acid groups of PP-g-MA, and the ability of bond recombination is enhanced at high temperatures.

[0094] Therefore, the thermal self-recovery experiment shows that the polypropylene composite material after grafting and blending by the present invention has obvious thermal self-recovery ability. Compared with pure polypropylene, its self-healing ability under external damage can be significantly improved, enabling it to have a longer service life, be widely applied to environments with higher temperatures, and avoid economic losses.

[0095] Perform mechanical tensile experiments on the unextended polypropylene composite material and the polypropylene composite material after extension at 120 and 130 °C, and compare the mechanical parameters and the recovery amount of pure polypropylene. The results are as Figure 11 shown in Table 3 and Table 4.

[0096] Table 4 Mechanical tensile data of sample before and after constant-length tensile at 120 °C

[0097]

[0098] Table 5 Mechanical tensile data of sample before and after constant-length tensile at 130 °C

[0099]

[0100]

[0101] From Figure 11 Table 3 and Table 4, it can be seen that mechanical tensile tests were carried out on different types of polypropylene and composite materials. After constant-length recovery at high temperature, the tensile strain and tensile yield stress of all samples will decrease to varying degrees. This is because constant-length extension destroys the internal structure of polypropylene and affects its performance indicators such as intermolecular forces. However, due to the self-repairing properties of PP-g-MA and PP-g-MA / PVDF composite materials with different contents, most of the tensile properties are retained. When the temperature is 120 °C, the difference in the recovery amount of mechanical properties between the polypropylene composite material and pure polypropylene is not very obvious. However, when the temperature rises to 130 °C, the tensile strain of pure polypropylene decreases by nearly 80%, while the polypropylene composite material can maintain nearly 90%.

[0102] Performance at 120°C (Table 4). The breaking tensile strain of pure PP is 754.7%. The elastic modulus is 1273.3 MPa. The yield stress is 21.16 MPa, and the performance deteriorates after a certain elongation. Among them, the elastic modulus drops to 390.5 MPa, and the yield stress is 18.74 MPa. The breaking tensile strain of PP-g-MA / 10PVDF is 535.5%, lower than that of pure PP but still maintaining good ductility. The elastic modulus is 643.5 MPa, significantly higher than 390.5 MPa of pure PP after a certain elongation. The yield stress is 16.27 MPa and remains 15.28 MPa after a certain elongation, with high stability. This is because the addition of PVDF significantly improves the stiffness and strength stability of the material at high temperatures but sacrifices some ductility.

[0103] Performance at 130°C (Table 5). The elastic modulus of pure PP drops sharply from 1273.3 MPa (without a certain elongation) to 106.4 MPa (after a certain elongation), indicating serious softening at high temperatures. The yield stress is 13.15 MPa, indicating a significant decrease in strength. The elastic modulus of PP-g-MA / 10PVDF after a certain elongation is 670.8 MPa, significantly higher than 106.4 MPa of pure PP. And after a certain elongation, the tensile yield stress of the polypropylene composite changes little or even increases. Compared with the tensile yield stress of pure PP, which decreases significantly from 21.16 MPa to 13.15 MPa. This shows that the material with a high PVDF content (10 wt%) still maintains excellent stiffness and strength at 130°C, demonstrating high-temperature stability.

[0104] Through the above analysis, it can be seen that this polypropylene composite has excellent high-temperature strength and stiffness: high elastic modulus: the elastic modulus of PP-g-MA / 10PVDF after a certain elongation at 130°C is 670.8 MPa, much higher than the elastic modulus of pure PP, which is 106.4 MPa, and the change rate is much lower than that of pure PP, indicating stronger resistance to deformation.

[0105] The tensile yield stress of PP-g-MA / 10PVDF remains stable at 130°C, far exceeding the stability of the tensile yield stress of pure PP, and is suitable for high-temperature load-bearing scenarios.

[0106] Therefore, the mechanical tensile test shows that the polypropylene composite retains about 90% of the tensile strain at 130°C, and even the tensile yield stress will increase to some extent, while the tensile strain of pure polypropylene decreases by about 80%. This shows that the polypropylene composite of the present invention has excellent thermal self-healing performance.

[0107] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A polypropylene composite insulating material with thermal self-recovery, characterized in that: The polypropylene grafted with maleic acid, activated carbon fiber and polyvinylidene fluoride are blended in a mass ratio of 95-110:0.1-0.5:1-15.

2. The polypropylene composite insulating material with thermal self-recovery according to claim 1, characterized in that: The mass ratio of maleic acid grafted polypropylene, activated carbon fiber and polyvinylidene fluoride is 100-110:0.2-0.4:1-10.

3. A method for preparing the polypropylene composite insulating material with thermal self-recovery according to claim 1, characterized in that: The polypropylene, maleic acid, the first initiator and part of the antioxidant are heated to a set temperature for melt blending, and a grafting reaction is performed during the melt blending process; the set temperature is T to (T+10° C.), where T is the melting temperature of the polypropylene; During the melt blending process, activated carbon fiber is added after the material is completely melted, and a second initiator is added, and melt blending is continued to 50-70% of the total melt blending time, polyvinylidene fluoride and another part of the antioxidant are added, and then melt blending is completed.

4. The preparation method according to claim 3, characterized in that: The antioxidants are antioxidant 168 and antioxidant 1010; Or, the mass ratio of antioxidant 168 to antioxidant 1010 is 1.5 to 2.5:1; Alternatively, the added amount of the antioxidant is 0.8 to 1.0% of the mass of the polypropylene.

5. The preparation method according to claim 3, characterized in that: The first initiator is dicumyl peroxide; or, the added mass of the first initiator is 0.2-0.4% of the mass of polypropylene; Or, the second initiator is di-tert-butyl peroxide; or, the added mass of the second initiator is 0.2-0.4% of the mass of the polypropylene.

6. The preparation method according to claim 3, characterized in that: The total melt blending time was 15 to 20 minutes.

7. The preparation method according to claim 3, characterized in that: When the melt blending lasts for 5 to 6 minutes, activated carbon fibers are added.

8. The preparation method according to claim 3, characterized in that: When the melt blending lasts for 8 to 9 minutes, polyvinylidene fluoride and another part of antioxidant are added.

9. Use of the polypropylene composite insulating material with thermal self-recovery according to claim 1 or 2 in the preparation of cables.

10. A cable comprising a conductor and an insulating layer, characterized in that: The material of the insulating layer is the polypropylene composite insulating material with thermal self-recovery properties as claimed in claim 1 or 2.

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