Conductive polypropylene composite material and method for producing the same

Abstract

The present application relates to the technical field of resin, in particular to a conductive polypropylene composite material and a preparation method thereof.In view of the problems of poor processing fluidity, easy shear damage of conductive network and deterioration of mechanical properties caused by high filling of conductive filler in the prior art, the present application realizes the low volume resistivity, high melt flow rate and excellent mechanical toughness by the solid phase pre-coating technology, and effectively balances the conductivity, processability and strength.

Description

Technical Field

[0001] This invention relates to the field of resin technology, and in particular to a conductive polypropylene composite material and its preparation method. Background Technology

[0002] Conductive polymer composites are widely used in antistatic and electromagnetic shielding applications in electronics, automotive, and aerospace industries. Polypropylene, as a general-purpose plastic, requires functionalization through the addition of conductive fillers to achieve its insulating properties. Traditional techniques primarily rely on highly filled carbon black or metal powders. While carbon black is relatively inexpensive, the required addition amount to reach the conductivity percolation threshold is typically as high as 15wt%-20wt%, severely degrading the melt flow properties of the material, leading to injection molding difficulties, increased internal stress in the product, and significantly sacrificing the toughness of the matrix, resulting in obvious embrittlement of the composite material. Metal fillers, while offering high conductivity, are dense, prone to oxidation, and expensive. Furthermore, they are susceptible to orientation or migration during high-shear processing, leading to unstable conductivity.

[0003] To lower the percolation threshold, novel carbon nanomaterials such as carbon nanotubes and graphene have been introduced. These materials, with their high aspect ratio or large specific surface area, can theoretically construct conductive networks at relatively low addition levels. However, dispersing these carbon nanomaterials in polypropylene melt is a major challenge. Strong van der Waals forces make them highly prone to agglomeration, and the shear forces of conventional melt blending are insufficient to achieve uniform dispersion at the nanoscale, often resulting in locally enriched agglomerates rather than ideal three-dimensional interconnected networks. This not only limits the improvement of conductivity but also allows these agglomerates to become stress concentration points, further deteriorating mechanical properties.

[0004] To address the dispersibility issue, existing technologies often employ surface modification of the filler or the addition of compatibilizers. For example, acid oxidation introduces carboxyl groups onto the surface of carbon nanotubes, or maleic anhydride-grafted polypropylene is used as a compatibilizer, aiming to improve the interfacial affinity between the filler and the polypropylene matrix. However, during melt blending, the filler remains primarily dispersed within the bulk phase of the polymer melt. In subsequent processing (such as injection molding and extrusion), intense shear and tensile flows can disrupt the already formed fragile conductive network, leading to broken conductive pathways and poor product performance reproducibility. Furthermore, excessive compatibilizers may introduce polar interfaces, negatively impacting the crystallization behavior and long-term thermal stability of polypropylene itself.

[0005] A deeper problem lies in the fact that current technological approaches are mostly limited to optimizing the bulk uniform dispersion of fillers in the molten state, while neglecting the strong correlation between the final properties of composite materials and the processing history. How to construct a stable conductive network structure that can self-organize during processing and effectively resist shear failure without significantly compromising the inherent excellent processing flowability and mechanical toughness of polypropylene is the core challenge that current technologies have not adequately addressed. This requires collaborative innovation across multiple dimensions, including the spatial distribution design of fillers, precise control of interfacial interactions, and timing matching of processing techniques, rather than simply adding components or making localized improvements. Summary of the Invention

[0006] In view of this, the purpose of this invention is to propose a conductive polypropylene composite material and its preparation method, so as to solve the problems of existing conductive polypropylene composite materials, which rely on the uniform dispersion of high content conductive fillers in the melt, resulting in a significant decrease in processing fluidity, and the shear force during the molding process easily destroys the fragile conductive network, making it difficult to achieve low resistivity, excellent processability and mechanical toughness.

[0007] To achieve the above objectives, the present invention provides a method for preparing a conductive polypropylene composite material, comprising the following steps:

[0008] (1) Prepare polypropylene powder with a particle size of 75-150 μm;

[0009] (2) Polypropylene powder is mixed with stearic acid, carboxylated multi-walled carbon nanotubes and graphene nanosheets under heating conditions to obtain solid-phase pre-coated composite powder.

[0010] (3) Solid pre-coated composite powder is mixed with bifunctional grafted modified graphite phase carbon nitride and maleic anhydride grafted polypropylene to obtain a mixture with pre-constructed interface layer.

[0011] (4) The mixture with the pre-constructed interface layer is melt-mixed with polypropylene resin and polyolefin elastomer at a first temperature. Then the mixing temperature is raised to a second temperature higher than the first temperature. An initiator is added at the second temperature and the mixing is continued. After that, an antioxidant is added and the mixture is discharged and granulated to obtain a conductive polypropylene composite material.

[0012] Preferably, in step (1), the polypropylene powder is obtained by drying polypropylene particles, pre-cooling them in liquid nitrogen, pulverizing them at low temperature, and then sieving them. The particle size of the polypropylene powder is 75-150 μm.

[0013] Preferably, in step (1), the polypropylene powder is homopolymer polypropylene with a melt flow rate of 12 g / 10 min at 230℃ / 2.16 kg.

[0014] Preferably, in step (2), the solid phase pre-coating mixing is carried out in a jacketed high-speed mixer with a jacket temperature of 70-80°C. After pre-mixing at 500-700 rpm for 4-6 minutes, the mixture is stirred at 1000-1400 rpm for 12-18 minutes, and the material is cooled to below 40°C before being discharged.

[0015] Preferably, in step (2), the mass ratio of polypropylene powder, stearic acid, carboxylated multi-walled carbon nanotubes and graphene nanosheets is (567-593):(8-12):(16-24):(4-6).

[0016] Preferably, in step (3), solid-phase mixing is carried out in a jacketed high-speed mixer with a jacket temperature of 125-145℃, a stirring speed of 700-900rpm, a mixing time of 6-10min, and the material is cooled to below 40℃ before being discharged.

[0017] Preferably, in step (3), the mass ratio of solid pre-coated composite powder, bifunctional grafted modified graphite phase carbon nitride and maleic anhydride grafted polypropylene is (609-621):(4-6):(15-25).

[0018] Preferably, in step (4), the polypropylene resin is a random copolymer of polypropylene with a melt flow rate of 11 g / 10 min at 230℃ / 2.16 kg.

[0019] Preferably, in step (4), the melt flow rate of the polyolefin elastomer at 190℃ / 2.16kg is 0.5g / 10min.

[0020] Preferably, in step (4), the mass ratio of the pre-constructed interface layer mixture, polypropylene resin, polyolefin elastomer, initiator and antioxidant is 640:(260-300):(70-110):(2-4):3.

[0021] Preferably, in step (4), the first temperature is 150°C and the mixing is carried out for 2-4 minutes, and the second temperature is 170°C and the mixing continues for 1-3 minutes after the initiator is added.

[0022] Preferably, in step (4), the initiator is a polymer-coated granule of dicumyl peroxide, model Perkadox BC-EP40.

[0023] Preferably, the bifunctional grafted modified graphitic carbon nitride is prepared by the following method: graphitic carbon nitride is mixed with N,N-dimethylformamide, dehydrated under reduced pressure at 50°C for 20-40 min, then protected with dry nitrogen gas and stirred and ultrasonically dispersed at 700-900 rpm for 20-40 min to form a suspension. Triethylamine is added and the system is lowered to 0°C. Methacryl chloride is added dropwise at 0°C over 20-40 min and stirred at 0°C for 45-75 min, then the temperature is raised to 30°C and reacted for 2-4 h. Stearoyl chloride is then added dropwise and reacted at 40°C for 1-3 h. After the reaction is completed, the mixture is filtered, washed, and vacuum dried to obtain bifunctional grafted modified graphitic carbon nitride.

[0024] Preferably, the mass ratio of the graphitic carbon nitride, methacrylamide chloride and stearoyl chloride is 20:(18-22):(25-35).

[0025] Preferably, the graphitic carbon nitride is obtained by heating melamine to 540-560°C at 4-6°C / min and holding it at that temperature for 3-5 hours, then naturally cooling it to room temperature, grinding it, and passing it through a 200-mesh sieve.

[0026] Furthermore, the present invention also provides a conductive polypropylene composite material, obtained by the above-described method for preparing conductive polypropylene composite material.

[0027] The beneficial effects of this invention are:

[0028] This invention utilizes a solid-phase pre-coating technique to preferentially position carboxylated multi-walled carbon nanotubes and graphene nanosheets onto the surface of polypropylene powder particles through melt bridging with stearic acid. This method preliminarily constructs a conductive network based on particle boundaries before the material is melt-blended. During subsequent melt processing, this pre-formed network structure more effectively resists shear failure, facilitating the formation of continuous and stable conductive paths. This results in excellent conductivity with a relatively low total filler content, while minimizing interference with the continuous phase of the polypropylene matrix, ensuring good melt flowability and processing capabilities.

[0029] By synergistically introducing one-dimensional carboxylated multi-walled carbon nanotubes and two-dimensional graphene nanosheets in the same pre-coating step, the complementary morphological advantages of the two are utilized. One-dimensional carbon nanotubes are easily stacked to form a long-range conductive framework, while two-dimensional graphene nanosheets can effectively cover and bridge the gaps between particles, preventing the conductive pathway from being interrupted at the interface. This multi-scale synergistic effect results in a conductive network with higher connectivity and structural stability, better adapting to deformation during processing and use, and exhibiting more robust conductivity.

[0030] Graphitic carbon nitride grafted with bifunctional groups of methacryloyl chloride and stearoyl chloride introduces unique interfacial functions into the system. The reactive double bond provided by the methacryloyl group lays the foundation for subsequent interfacial chemical anchoring via free radical reaction, while the long-chain stearoyl alkyl group significantly improves the compatibility of this component with the polypropylene matrix, acting as an interfacial lubricant. This design enhances interfacial bonding, promotes good dispersion and positioning of the filler in the interfacial region, reduces interfacial defects, and helps to achieve effective stress transfer under external forces, thereby maintaining or even enhancing the mechanical properties of the material while achieving electrical conductivity.

[0031] Adding maleic anhydride-grafted polypropylene during the solid-state construction stage allows the anhydride groups on its molecular chain to interact strongly with the polar groups such as carboxyl groups on the surface of carboxylated multi-walled carbon nanotubes, resulting in an anchoring effect. Simultaneously, the polypropylene backbone exhibits good compatibility with the matrix resin. This structure effectively bridges the conductive filler and the polypropylene matrix, enhancing interfacial adhesion strength and reducing the tendency for filler migration and detachment during processing and use. This contributes to the long-term stability of the conductive network and the maintenance of the overall mechanical properties of the composite material.

[0032] The step-by-step processing technology employed in this invention, particularly the timing control strategy of first performing low-temperature melting and welding followed by high-temperature initiation of the reaction, has significant beneficial effects. First, the material is initially melted and the particle interfaces are welded at a relatively low temperature, which helps to fix the conductive network spatial structure formed by the solid-phase pre-coating. Then, the temperature is increased to initiate the interfacial chemical reaction, which concentrates the reaction mainly in the interfacial region, effectively avoiding premature or excessive bulk cross-linking. This ensures that the material maintains a wide processing window and good flowability during processing, ultimately obtaining a composite material with balanced properties.

[0033] Polymer-coated dicumyl peroxide was selected as the initiator. Its inert polymer coating layer enabled uniform dispersion and controllable thermal decomposition behavior of the initiator in the polypropylene matrix. This controllability avoids violent reactions or excessive cross-linking in localized areas due to excessively high initiator concentrations, ensuring the uniformity and mildness of the interfacial reaction. This facilitates the formation of a well-structured and consistent interfacial layer, improving the stability of the production process and batch consistency of the products.

[0034] Adding the antioxidant after the interfacial initiation reaction is complete avoids premature consumption of free radicals by the antioxidant, which would affect the efficiency of the interfacial grafting reaction. This arrangement ensures the full progress of the interfacial chemical anchoring process, while the subsequent addition of the antioxidant effectively inhibits thermal oxidative degradation of the material during subsequent processing and use, protecting the polymer's molecular chain structure, thus balancing the interfacial modification effect with the long-term stability of the material. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0036] Figure 1 The image shows the infrared spectrum of the conductive polypropylene composite powder of Example 2 of the present invention. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0038] Example 1:

[0039] Step (1): Preparation of graphitic carbon nitride

[0040] Weigh 50g of melamine and place it in a covered alumina crucible. Place the crucible in a muffle furnace and heat it to 540℃ at 4℃ / min. Hold the temperature for 3 hours and then let it cool naturally to room temperature. Take out the block product and grind it through a 200-mesh sieve to obtain graphitic carbon nitride.

[0041] Step (2): Preparation of bifunctional grafted modified graphitic carbon nitride

[0042] 20g of graphitic carbon nitride and 350g of N,N-dimethylformamide were weighed and added to a dry three-necked flask equipped with a mechanical stirrer and a dropping funnel. The mixture was dehydrated under reduced pressure at 50℃ for 20min, then protected with dry nitrogen gas and stirred and ultrasonically dispersed at 700rpm for 20min to form a uniform suspension. Subsequently, 35g of triethylamine was weighed and added to the system, and the system was cooled to 0℃ in an ice-water bath. 18g of methacryloyl chloride was added dropwise over 20min at 0℃ and stirred at 0℃ for 45min. The temperature was then raised to 30℃ and reacted for 2h. 25g of stearoyl chloride was then added dropwise and reacted at 40℃ for 1h. After the reaction was completed, the mixture was filtered and washed three times each with N,N-dimethylformamide, anhydrous ethanol, and deionized water. The mixture was then dried under vacuum at 60℃ for 10h to obtain bifunctional grafted modified graphitic carbon nitride.

[0043] Step (3): Preparation of polypropylene powder

[0044] Weigh 593g of polypropylene homopolymer particles (Lyonder Basell, Moplen HP500N, melt flow rate 12g / 10min (230℃ / 2.16kg), density 0.90g / cm³). 3 After drying in a vacuum drying oven at 80℃ for 6 hours, the dried polypropylene particles were pre-cooled in liquid nitrogen for 5 minutes and then immediately subjected to low-temperature pulverization. The pulverized product was graded by 100-mesh and 200-mesh sieves to obtain polypropylene powder with a particle size of 75μm-150μm.

[0045] Step (4): Solid-phase pre-coating of carboxylated multi-walled carbon nanotubes / graphene nanosheets

[0046] Weigh out 593g of polypropylene powder, 8g of stearic acid, 16g of carboxylated multi-walled carbon nanotubes (XFNANO, model XFM03, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., outer diameter 5nm-15nm, length 10μm-30μm, carboxyl content 3.86wt%), and 4g of graphene nanosheets (XFNANO, model XF022, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., thickness 8nm-15nm, sheet diameter 5μm-20μm, specific surface area 90m²). 2 / g-130m 2 / g) is added to a jacketed high-speed mixer, the jacket is heated to 70°C and premixed at 500 rpm for 4 min, then stirred at 1000 rpm for 12 min, and then cooled to below 40°C to discharge the material, thus obtaining solid pre-coated composite powder.

[0047] Step (5): Solid phase construction of reactive lubrication anchoring interface layer

[0048] Weigh 621g of solid pre-coated composite powder, 4g of bifunctional grafted modified graphite phase carbon nitride and 15g of maleic anhydride grafted polypropylene (Synthomer G-3003 Polymer, acid value 8mgKOH / g, melt viscosity 60000cP at 190℃) and add them to a jacketed high-speed mixer. Mix at 700rpm for 6min at a jacket temperature of 125℃. Then cool the material to below 40℃ to obtain a mixture with pre-constructed interface layer.

[0049] Step (6): Melt blending and granulation

[0050] Weigh out 640g of the interface layer pre-constructed mixture, 300g of polypropylene random copolymer particles (LyondellBasell, LyondellBasell, Moplen RP340N, melt flow rate 11g / 10min (230℃ / 2.16kg), Vicat softening temperature 130℃), and 70g of polyolefin elastomer (Dow Chemical, Engage 8150, density 0.868g / cm³). 3The product (melt index 0.5 g / 10 min (190℃ / 2.16 kg), melting point 55℃) was premixed in a drum at room temperature for 2 min and then added to the mixing chamber of an internal mixer or torque rheometer. The rotor speed was set to 25 rpm and the chamber temperature was stabilized at 150℃ for 2 min. Then the chamber temperature was raised to 170℃ and 2 g of dicumyl peroxide polymer-coated granules (Nouryon, Perkadox BC-EP40, 40% inert polymer-coated formulation, Assay 39.0%-41.0%, safe processing temperature 130℃, typical crosslinking temperature 170℃) were added and mixed for another 1 min. Finally, 2 g of antioxidant 1010 and 1 g of antioxidant 168 were added and mixed at 170℃ for 1 min. The mixture was then discharged, tableted, cooled, and pulverized into granules to obtain a conductive polypropylene composite material.

[0051] Example 2:

[0052] Step (1): Preparation of graphitic carbon nitride

[0053] Weigh 50g of melamine and place it in a covered alumina crucible. Place the crucible in a muffle furnace and heat it to 550℃ at 5℃ / min. Hold the temperature for 4 hours and then let it cool naturally to room temperature. Take out the block product and grind it through a 200-mesh sieve to obtain graphitic carbon nitride.

[0054] Step (2): Preparation of bifunctional grafted modified graphitic carbon nitride

[0055] 20g of graphitic carbon nitride and 400g of N,N-dimethylformamide were weighed and added to a dry three-necked flask equipped with a mechanical stirrer and a dropping funnel. The mixture was dehydrated under reduced pressure at 50℃ for 30 min, then protected with dry nitrogen gas and stirred and ultrasonically dispersed at 800 rpm for 30 min to form a uniform suspension. Subsequently, 40g of triethylamine was weighed and added to the system, and the system was cooled to 0℃ in an ice-water bath. 20g of methacryloyl chloride was added dropwise over 30 min at 0℃ and stirred at 0℃ for 60 min. The temperature was then raised to 30℃ and reacted for 3 h. 30g of stearoyl chloride was then added dropwise and reacted at 40℃ for 2 h. After the reaction was completed, the mixture was filtered and washed three times each with N,N-dimethylformamide, anhydrous ethanol, and deionized water. The mixture was then dried under vacuum at 60℃ for 12 h to obtain bifunctional grafted modified graphitic carbon nitride.

[0056] Step (3): Preparation of polypropylene powder

[0057] Weigh 580g of polypropylene homopolymer granules (Lyonder Basell, Moplen HP500N, melt flow rate 12g / 10min (230℃ / 2.16kg), density 0.90g / cm³). 3After drying in a vacuum drying oven at 80℃ for 8 hours, the dried polypropylene particles were pre-cooled in liquid nitrogen for 5 minutes and immediately subjected to low-temperature pulverization. The pulverized product was graded by 100-mesh and 200-mesh sieves to obtain polypropylene powder with a particle size of 75μm-150μm.

[0058] Step (4): Solid-phase pre-coating of carboxylated multi-walled carbon nanotubes / graphene nanosheets

[0059] Weigh out 580g of polypropylene powder, 10g of stearic acid, 20g of carboxylated multi-walled carbon nanotubes (XFNANO, model XFM03, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., outer diameter 5nm-15nm, length 10μm-30μm, carboxyl content 3.86wt%), and 5g of graphene nanosheets (XFNANO, model XF022, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., thickness 8nm-15nm, sheet diameter 5μm-20μm, specific surface area 90m²). 2 / g-130m 2 / g) is added to a jacketed high-speed mixer, the jacket is heated to 75°C and premixed at 600 rpm for 5 min, then stirred at 1200 rpm for 15 min, and then cooled to below 40°C to discharge the material, thus obtaining solid pre-coated composite powder.

[0060] Step (5): Solid phase construction of reactive lubrication anchoring interface layer

[0061] Weigh 615g of solid pre-coated composite powder, 5g of bifunctional grafted modified graphite phase carbon nitride and 20g of maleic anhydride grafted polypropylene (Synthomer G-3003 Polymer, acid value 8mgKOH / g, melt viscosity 60000cP at 190℃) and add them to a jacketed high-speed mixer. Mix at 800rpm for 8min at a jacket temperature of 135℃. Then cool the material to below 40℃ to obtain a mixture with pre-constructed interface layer.

[0062] Step (6): Melt blending and granulation

[0063] Weigh out 640g of the interface layer pre-constructed mixture, 280g of polypropylene random copolymer particles (LyondellBasell, LyondellBasell, Moplen RP340N, melt flow rate 11g / 10min (230℃ / 2.16kg), Vicat softening temperature 130℃), and 90g of polyolefin elastomer (Dow Chemical, Engage 8150, density 0.868g / cm³). 3The product (melt index 0.5 g / 10 min (190℃ / 2.16 kg), melting point 55℃) was premixed in a drum at room temperature for 3 min and then added to the mixing chamber of an internal mixer or torque rheometer. The rotor speed was set to 30 rpm and the chamber temperature was stabilized at 150℃ for 3 min. Then the chamber temperature was raised to 170℃ and 3 g of dicumyl peroxide polymer-coated granules (Nouryon, Perkadox BC-EP40, 40% inert polymer-coated formulation, Assay 39.0%-41.0%, safe processing temperature 130℃, typical crosslinking temperature 170℃) were added and mixed for another 2 min. Finally, 2 g of antioxidant 1010 and 1 g of antioxidant 168 were added and mixed at 170℃ for 1 min. The mixture was then discharged, tableted, cooled, and pulverized into granules to obtain a conductive polypropylene composite material.

[0064] Example 3:

[0065] Step (1): Preparation of graphitic carbon nitride

[0066] Weigh 50g of melamine and place it in a covered alumina crucible. Place the crucible in a muffle furnace and heat it to 560℃ at 6℃ / min. Hold the temperature for 5 hours and then let it cool naturally to room temperature. Take out the block product and grind it through a 200-mesh sieve to obtain graphitic carbon nitride.

[0067] Step (2): Preparation of bifunctional grafted modified graphitic carbon nitride

[0068] 20g of graphitic carbon nitride and 450g of N,N-dimethylformamide were weighed and added to a dry three-necked flask equipped with a mechanical stirrer and a dropping funnel. The mixture was dehydrated under reduced pressure at 50℃ for 40 min, then protected with dry nitrogen gas and stirred and ultrasonically dispersed at 900 rpm for 40 min to form a uniform suspension. Subsequently, 45g of triethylamine was weighed and added to the system, and the system was cooled to 0℃ in an ice-water bath. 22g of methacryloyl chloride was added dropwise over 40 min at 0℃ and stirred at 0℃ for 75 min. The temperature was then raised to 30℃ and reacted for 4 h. 35g of stearoyl chloride was then added dropwise and reacted at 40℃ for 3 h. After the reaction was completed, the mixture was filtered and washed three times each with N,N-dimethylformamide, anhydrous ethanol, and deionized water. The mixture was then dried under vacuum at 60℃ for 14 h to obtain bifunctional grafted modified graphitic carbon nitride.

[0069] Step (3): Preparation of polypropylene powder

[0070] Weigh 567g of polypropylene homopolymer particles (Lyonder Basell, Moplen HP500N, melt flow rate 12g / 10min (230℃ / 2.16kg), density 0.90g / cm³). 3After drying in a vacuum drying oven at 80℃ for 10 hours, the dried polypropylene particles were pre-cooled in liquid nitrogen for 5 minutes and then immediately subjected to low-temperature pulverization. The pulverized product was graded by 100-mesh and 200-mesh sieves to obtain polypropylene powder with a particle size of 75μm-150μm.

[0071] Step (4): Solid-phase pre-coating of carboxylated multi-walled carbon nanotubes / graphene nanosheets

[0072] Weigh out 567g of polypropylene powder, 12g of stearic acid, 24g of carboxylated multi-walled carbon nanotubes (XFNANO, model XFM03, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., outer diameter 5nm-15nm, length 10μm-30μm, carboxyl content 3.86wt%), and 6g of graphene nanosheets (XFNANO, model XF022, Nanjing Xianfeng Nanomaterials Technology Co., Ltd., thickness 8nm-15nm, sheet diameter 5μm-20μm, specific surface area 90m²). 2 / g-130m 2 / g) is added to a jacketed high-speed mixer, the jacket is heated to 80°C and premixed at 700 rpm for 6 min, then stirred at 1400 rpm for 18 min, and then cooled to below 40°C to discharge the material, thus obtaining solid pre-coated composite powder.

[0073] Step (5): Solid phase construction of reactive lubrication anchoring interface layer

[0074] Weigh 609g of solid pre-coated composite powder, 6g of bifunctional grafted modified graphite phase carbon nitride and 25g of maleic anhydride grafted polypropylene (Synthomer G-3003 Polymer, acid value 8mgKOH / g, melt viscosity 60000cP at 190℃) and add them to a jacketed high-speed mixer. Mix at 900rpm for 10min at a jacket temperature of 145℃. Then cool the material to below 40℃ to obtain a mixture with a pre-constructed interface layer.

[0075] Step (6): Melt blending and granulation

[0076] Weigh out 640g of the interface layer pre-constructed mixture, 260g of polypropylene random copolymer particles (LyondellBasell, LyondellBasell, Moplen RP340N, melt flow rate 11g / 10min (230℃ / 2.16kg), Vicat softening temperature 130℃), and 110g of polyolefin elastomer (Dow Chemical, Engage 8150, density 0.868g / cm³). 3The product (melt index 0.5 g / 10 min (190℃ / 2.16 kg), melting point 55℃) was premixed in a drum at room temperature for 4 min and then added to the mixing chamber of an internal mixer or torque rheometer. The rotor speed was set to 35 rpm and the chamber temperature was stabilized at 150℃ for 4 min. Then, the chamber temperature was raised to 170℃ and 4 g of dicumyl peroxide polymer-coated granules (Nouryon, Perkadox BC-EP40, 40% inert polymer-coated formulation, Assay 39.0%-41.0%, safe processing temperature 130℃, typical crosslinking temperature 170℃) were added and mixed for another 3 min. Finally, 2 g of antioxidant 1010 and 1 g of antioxidant 168 were added and mixed at 170℃ for 1 min. The mixture was then discharged, tableted, cooled, and granulated to obtain a conductive polypropylene composite material.

[0077] Comparative Example 1:

[0078] The difference between Comparative Example 1 and Example 2 is that in step (4), the jacket temperature was changed from 75°C to 40°C, and the jacket temperature was kept at 600 rpm for 5 min and 1200 rpm for 15 min. Then the temperature was lowered to below 40°C and the solid pre-coated composite powder was discharged. The other conditions were the same as in Example 2.

[0079] Comparative Example 2:

[0080] The difference between Comparative Example 2 and Example 2 is that: in step (4), 5g of graphene nanosheets are not added, and the amount of carboxylated multi-walled carbon nanotubes is adjusted from 20g to 25g (so that the total amount of conductive filler is still 25g). The process is still carried out by heating the jacket to 75°C, premixing at 600rpm for 5min, stirring at 1200rpm for 15min, and cooling down to below 40°C before discharging. The other conditions are the same as in Example 2.

[0081] Comparative Example 3:

[0082] The difference between Comparative Example 3 and Example 2 is that the 5g of bifunctional grafted modified graphite phase carbon nitride added in step (5) is replaced by 5g of graphite phase carbon nitride prepared in step (1), and step (5) is still mixed at 800rpm for 8min at a jacket temperature of 135℃ and then cooled to below 40℃ before discharge; the other conditions are the same as in Example 2.

[0083] Comparative Example 4:

[0084] The difference between Comparative Example 4 and Example 2 is that: in step (2), 30g of stearyl chloride is not added, but 20g of methacryloyl chloride is added dropwise over 30min at 0℃ and stirred at 0℃ for 60min, then the temperature is raised to 30℃ and reacted for 3h, and then reacted at 40℃ for 2h. After the reaction is completed, the mixture is filtered and washed three times each with N,N-dimethylformamide, anhydrous ethanol and deionized water, and then vacuum dried at 60℃ for 12h before being discharged; the other conditions are the same as in Example 2.

[0085] Comparative Example 5:

[0086] The difference between Comparative Example 5 and Example 2 is that: in step (5), 20g of maleic anhydride-grafted polypropylene is not added, but 20g of polypropylene powder obtained in step (3) is added instead (to keep the total mass of the material fed in step (5) consistent with that in Example 2), and it is still mixed at 800rpm for 8min at a jacket temperature of 135℃ and then cooled to below 40℃ before being discharged; the other conditions are the same as those in Example 2.

[0087] Comparative Example 6:

[0088] The difference between Comparative Example 6 and Example 2 is that in step (6), 3g of dicumyl peroxide polymer-coated particles are not added after the chamber temperature is raised to 170°C, but are added to the mixing chamber along with 640g of interface layer pre-constructed mixture, 280g of polypropylene random copolymer particles and 90g of polyolefin elastomer before the start of mixing. Then, the mixing is carried out at a chamber temperature of 150°C for 3 minutes, and then raised to 170°C for 2 minutes. Finally, 2g of antioxidant 1010 and 1g of antioxidant 168 are added and mixed at 170°C for 1 minute before discharge, tableting, cooling and granulation. The remaining conditions are the same as in Example 2.

[0089] Comparative Example 7:

[0090] The difference between Comparative Example 7 and Example 2 is that: in step (6), 3g of dicumyl peroxide polymer-coated particles are not added, and the mixture is still mixed at a stable chamber temperature of 150°C for 3 minutes, then raised to 170°C and mixed for another 2 minutes (without adding an initiator during these 2 minutes), and finally 2g of antioxidant 1010 and 1g of antioxidant 168 are added and mixed at 170°C for 1 minute before being discharged, tableted, cooled, and pulverized into granules; the remaining conditions are the same as in Example 2.

[0091] Performance testing:

[0092] Before testing, all conductive polypropylene composite materials were dried in a vacuum drying oven at 80°C for 4 hours. Test tablets were then prepared using a flat plate hot pressing method: the particles were placed in a metal mold, preheated at 190°C for 3 minutes, and then hot-pressed at 10MPa for 5 minutes. After cooling to 25°C at 10MPa, the tablets were demolded to obtain tablets with thicknesses of 1.0 mm and 4.0 mm, respectively. Subsequently, the tablets were machined or cut to the dimensions specified in the corresponding national standards. The samples used for infrared spectroscopy and thermogravimetric analysis were powders obtained by cutting the corresponding tablets into small pieces, grinding them, and passing them through a 200-mesh sieve. This ensured that the different embodiments and comparative examples were consistent in terms of sample source, thermal history, and sample preparation pressure conditions.

[0093] Fourier transform infrared spectroscopy characterization: 2 mg of the conductive polypropylene composite powder sample from Example 2 was taken, and the spectrum was acquired at room temperature. The results are as follows: Figure 1 As shown.

[0094] Thermogravimetric analysis characterization: 8.0 mg of each powder sample corresponding to the examples and comparative examples were placed in an alumina crucible and heated from 30 °C to 800 °C under a nitrogen atmosphere (flow rate 60 mL / min) at a heating rate of 10 °C / min. The 5% weight loss temperature, the temperature corresponding to the maximum weight loss rate, and the residual mass fraction at 800 °C were recorded. The results are shown in Table 1.

[0095] Melt flow rate test: The test was conducted according to GB / T 3682.1-2018 "Determination of melt flow rate (MFR) and melt volumetric flow rate (MVR) of thermoplastic plastics - Part 1: Standard method". 20g of sample granules from both the example and comparative examples were taken and dried at 80℃ for 2 hours before the test. The samples were then tested at 230℃ and 2.16kg. After adding the sample to the barrel and preheating for 4 minutes, the sample was cut every 30 seconds for a total of 5 times and weighed. The melt flow rate was calculated. Each sample was tested three times, and the average value was taken. The results are shown in Table 1.

[0096] Volume resistivity and surface resistivity testing: Volume resistivity and volume resistivity were tested according to GB / T 31838.2-2019, and surface resistivity and surface resistivity were tested according to GB / T 31838.3-2019. The 1.0 mm thick plates from the examples and comparative examples were cut into 100 mm × 100 mm samples. After conditioning in an environment of 23℃ and 50% relative humidity for 24 hours, the samples were tested. A DC voltage of 100 V was applied and electrolyzed for 1 minute to read the resistance value. The volume resistivity and surface resistivity were calculated according to the standard electrode size and sample thickness. Three samples were taken from each sample and tested separately, and the average value was taken. The results are shown in Table 1.

[0097] Tensile property testing: Tests were conducted according to GB / T 1040.2-2022 "Determination of tensile properties of plastics - Part 2: Test conditions for molded and extruded plastics". The 4.0 mm thick sheets from the examples and comparative examples were processed into Type 1A dumbbell specimens. The tests were conducted at 23℃ and 50% relative humidity, with a gauge length of 50 mm and a tensile speed of 50 mm / min. Tensile strength and elongation at break (%) were recorded. Five specimens were tested for each sample, and the average value was calculated. The results are shown in Table 1.

[0098] Cantilever beam notched impact strength test: The test was conducted according to GB / T 1843-2008 "Determination of Impact Strength of Plastic Cantilever Beams". The 4.0 mm thick sheets from the examples and comparative examples were processed into 80 mm × 10 mm × 4 mm specimens with a notch depth of 2.0 mm and a notch bottom radius of 0.25 mm. The test was conducted at 23℃ using a 2J pendulum calibrated according to standard methods. Ten specimens were tested for each sample, and the average value was taken. The results are shown in Table 1.

[0099] Bending performance test: The test was conducted according to GB / T 9341-2008 "Determination of bending properties of plastics". The 4.0 mm thick sheets from the examples and comparative examples were processed into 80 mm × 10 mm × 4 mm specimens. The three-point bending support span was 64 mm (span to thickness ratio was 16). The test was conducted at 23℃ with a loading speed of 2 mm / min. The bending strength was recorded. Five specimens were tested for each sample, and the average value was taken. The results are shown in Table 1.

[0100] Table 1 Performance Test Results

[0101]

[0102] Data Analysis:

[0103] As can be seen from the data in Examples 1-3 in Table 1, the conductive polypropylene composite material prepared by this invention exhibits more stable decomposition initiation and more obvious high-temperature residue in thermogravimetric characterization. This indicates that the carbon skeleton composed of carboxylated multi-walled carbon nanotubes, graphene nanosheets, and graphitic carbon nitride, along with the layered shielding effect, can inhibit the diffusion of thermal decomposition products. At the same time, both the volume resistivity and surface resistivity are in the low resistance range that can meet the requirements of conductive / static dissipation applications, indicating that the solid pre-coating of the particle surface promotes the preferential enrichment of conductive fillers on the surface and boundaries of polypropylene particles, thereby making it easier to form continuous conductive pathways. In terms of mechanics, the tensile and flexural strengths remain within the effective load-bearing range of the polypropylene matrix, while the elongation at break and impact toughness can still maintain a high level. This is presumably due to the fact that the bifunctional grafted modified graphitic carbon nitride provides reactive sites and lubricating long chains at the interface, and achieves interface anchoring under the action of maleic anhydride grafted polypropylene and dicumyl peroxide, so that the conductive skeleton no longer sacrifices toughness, thus exhibiting a synergistic improvement in strength, toughness, and conductivity.

[0104] As can be seen from the data in Table 1 for Example 2 and Comparative Example 1, when the jacket temperature drops to 40°C, making it difficult for stearic acid to fully melt and form a film, the preferential coating of carboxylated multi-walled carbon nanotubes and graphene nanosheets on the surface of polypropylene powder is weakened. The conductive filler is more easily diluted by the bulk phase in the subsequent melting stage, the continuity of the conductive pathway at the particle boundaries decreases, and the volume resistivity and surface resistivity increase significantly. At the same time, due to local stress concentration and insufficient interface wetting, the tensile, bending, and impact performances weaken simultaneously. These results indicate that solid-phase pre-coating is not only a dispersion method but also a key step that determines the spatial distribution of the conductive network.

[0105] As can be seen from the data in Example 2 and Comparative Example 2 in Table 1, when only carboxylated multi-walled carbon nanotubes are used without graphene nanosheets, both conductivity and strength-toughness balance decline. The main reason is that carboxylated multi-walled carbon nanotubes are primarily one-dimensional frameworks, which, while beneficial for long-range connectivity, lack sufficient surface-to-surface bridging / overlapping ability at particle boundaries. The introduction of graphene nanosheets provides two-dimensional spreading and cross-boundary connectivity at the boundaries, making it easier for the one-dimensional framework to form a multi-scale interconnected network. Therefore, the combined use of one-dimensional and two-dimensional conductive fillers produces a synergistic drag reduction effect that is difficult to extrapolate linearly using the content of a single filler.

[0106] As can be seen from the data in Table 1 for Example 2 and Comparative Example 3, when the bifunctional grafted modified graphitic carbon nitride is replaced with unmodified graphitic carbon nitride, both conductivity and mechanical properties tend to be unfavorable. The presumed reason is that unmodified graphitic carbon nitride lacks the reactive sites introduced by methacryloyl chloride, making it difficult to form an effective interface anchor with maleic anhydride-grafted polypropylene under the initiation of dicumyl peroxide. The interface layer is more likely to become a loose filling layer, contributing little to the fixation of the conductive framework and stress transfer. In Example 2, however, the reactive sites are coupled with the conductive framework and the interface compatibilization path, thus simultaneously achieving a synergistic effect of low resistance and high strength and toughness.

[0107] As can be seen from the data in Example 2 and Comparative Example 4 in Table 1, when stearoyl chloride is omitted, resulting in graphitic carbon nitride exhibiting only single reactivity and lacking lubricating long chains, the processing flow and overall mechanics of the system decrease, while the resistivity increases. This may be because the lack of lubricating long chains weakens the interfacial wetting / slip regulation effect of the interface layer on the carboxylated multi-walled carbon nanotubes and graphene nanosheets, making it more difficult to achieve uniform coating during the solid-phase construction stage. Consequently, the conductive framework is more easily sheared and destroyed or re-embedded by the polypropylene matrix during subsequent melt mixing, leading to a decrease in the continuity of the conductive pathway. This comparative example demonstrates the necessity of a dual function of reactive sites and lubricating long chains.

[0108] As can be seen from the data in Table 1 for Example 2 and Comparative Example 5, when maleic anhydride-grafted polypropylene is replaced by polypropylene powder, the tensile and flexural load-bearing capacity of the material decreases, the resistivity increases, while the elongation at break and impact toughness remain at a high level, exhibiting an explainable anomalous combination. The presumed reason is that in Example 2, maleic anhydride-grafted polypropylene not only acts as a compatibilizer but also provides polypropylene phase anchors for the bifunctional grafted modified graphitic carbon nitride, allowing the conductive framework to be fixed at the particle boundaries. Without this anchor, the interface layer is more prone to physical adsorption, and the conductive framework is more likely to slip and rearrange under stress and processing shear, thereby reducing strength and weakening stable conductive pathways. However, the weakened interface constraint may also release the toughening contribution of the polyolefin elastomer, preventing a simultaneous deterioration in elongation and impact. This further demonstrates that the combined use of interface reaction anchoring and a conductive framework can achieve a synergistic effect between strength and conductivity.

[0109] As can be seen from the data in Table 1 for Example 2 and Comparative Example 6, when dicumyl peroxide is added to the mixer immediately, rather than rapidly after the temperature reaches 170°C, the melt flow rate decreases significantly and the resistivity increases markedly. Simultaneously, the elongation at break and impact toughness also decrease. The presumed reason is that premature participation of dicumyl peroxide in the reaction causes the polyolefin elastomer and interfacial layer to solidify prematurely or undergo local cross-linking, leading to a rapid increase in melt viscosity and limiting the migration and redistribution of conductive fillers on the particle surface / particle boundaries. The conductive network is locked before it is fully constructed, resulting in discontinuous conductive island structures. This result demonstrates that the sequential combination of solid-phase pre-coating and subsequent interfacial anchoring is crucial for achieving low resistance and processability, and is significantly unpredictable.

[0110] As can be seen from the data in Table 1 for Example 2 and Comparative Example 7, when dicumyl peroxide is not added, the fluidity and ductility of the system are improved, but the resistivity increases and the strength and flexural capacity are not as good as the overall level of Example 2. The main reason is that without the initiation of dicumyl peroxide, it is difficult to form a sufficient interfacial reaction anchor between the bifunctional grafted modified graphite phase carbon nitride and maleic anhydride grafted polypropylene. The conductive framework relies more on physical contact for maintenance and is more easily re-encapsulated by the polypropylene matrix or slips at the interface under the influence of processing shear and usage stress, resulting in a decrease in the stability of the conductive pathway. It can be seen that dicumyl peroxide is not just an additive, but the key to achieving interfacial layer reaction locking and amplifying the synergistic effect of the one-dimensional and two-dimensional conductive framework.

[0111] from Figure 1 It can be seen that in the infrared spectrum of the pressed conductive polypropylene composite powder of Example 2, the values ​​at 2950, ​​2918, and 2838 cm⁻¹ are... -1 Characteristic absorptions of stretching vibrations of the polypropylene backbone (-CH3 and -CH2) were observed nearby, at 1456 and 1376 cm⁻¹. -1 -CH3 / -CH2 bending vibration peaks can be observed at 1167, 998, 973 cm⁻¹, and approximately 841 / 809 cm⁻¹. -1 The positions then show skeletal vibrations and rocking vibrations related to the polypropylene crystal form, respectively; approximately 1730 cm. -1 The presence of strong and slightly broadened carbonyl stretching vibration absorption nearby indicates that both the maleic anhydride-grafted polypropylene and the stearic acid pre-coating layer in the system are retained and participate in the interface construction; in the 1630-1240 cm⁻¹ region... -1 Multiple sets of C=N / CN skeletal vibration peaks are visible within the range, with a peak at approximately 810 cm⁻¹. -1The presence of typical triazine ring breathing vibrations indicates that bifunctional grafted modified graphite carbon nitride has been successfully introduced and enriched in the interfacial region. Combined with the flat baseline and high intensity of carbonyl and nitrogen-containing framework peaks in the spectrum, it can be inferred that the conductive framework and the interfacial layer formed a continuous and stable interfacial functional structure during the solid-phase pre-coating and subsequent melting reaction anchoring process, which is beneficial to the subsequent construction of the conductive network and the synergistic maintenance of mechanical properties.

[0112] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a conductive polypropylene composite material, characterized in that, Includes the following steps: (1) Prepare polypropylene powder with a particle size of 75-150 μm; (2) Polypropylene powder is mixed with stearic acid, carboxylated multi-walled carbon nanotubes and graphene nanosheets under heating conditions to obtain solid-phase pre-coated composite powder. (3) Solid pre-coated composite powder is mixed with bifunctional grafted modified graphite phase carbon nitride and maleic anhydride grafted polypropylene to obtain a mixture with pre-constructed interface layer. (4) The mixture with the pre-constructed interface layer is melt-mixed with polypropylene resin and polyolefin elastomer at a first temperature. Then the mixing temperature is raised to a second temperature higher than the first temperature. An initiator is added at the second temperature and mixing is continued. After that, an antioxidant is added and the mixture is discharged and granulated to obtain a conductive polypropylene composite material. The bifunctional grafted modified graphitic carbon nitride was prepared by the following method: graphitic carbon nitride was mixed with N,N-dimethylformamide, dehydrated under reduced pressure at 50°C for 20-40 min, then protected with dry nitrogen gas and stirred and ultrasonically dispersed at 700-900 rpm for 20-40 min to form a suspension. Triethylamine was added and the system was lowered to 0°C. Methacryl chloride was added dropwise at 0°C over 20-40 min and stirred at 0°C for 45-75 min. The temperature was then raised to 30°C and reacted for 2-4 h. Stearoyl chloride was then added dropwise and reacted at 40°C for 1-3 h. After the reaction was completed, the mixture was filtered, washed, and vacuum dried to obtain bifunctional grafted modified graphitic carbon nitride. The mass ratio of the graphitic carbon nitride, methacryloyl chloride, and stearoyl chloride is 20:(18-22):(25-35); The graphitic carbon nitride is obtained by heating melamine to 540-560℃ at 4-6℃ / min and holding it at that temperature for 3-5 hours, then naturally cooling it to room temperature, grinding it, and passing it through a 200-mesh sieve. In step (2), the solid phase pre-coating mixing is carried out in a jacketed high-speed mixer with a jacket temperature of 70-80℃. After pre-mixing at 500-700 rpm for 4-6 minutes, the mixture is stirred at 1000-1400 rpm for 12-18 minutes, and the material is cooled to below 40℃ before being discharged. In step (2), the mass ratio of polypropylene powder, stearic acid, carboxylated multi-walled carbon nanotubes and graphene nanosheets is (567-593):(8-12):(16-24):(4-6); In step (3), the mass ratio of solid pre-coated composite powder, bifunctional grafted modified graphite phase carbon nitride and maleic anhydride grafted polypropylene is (609-621):(4-6):(15-25). In step (4), the mass ratio of the pre-constructed interface layer mixture, polypropylene resin, polyolefin elastomer, initiator and antioxidant is 640:(260-300):(70-110):(2-4):

3. In step (4), the first temperature is 150°C and the mixture is mixed for 2-4 minutes, and the second temperature is 170°C and the mixture is continued for 1-3 minutes after the initiator is added.

2. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (1), the polypropylene powder is obtained by drying polypropylene particles, pre-cooling them in liquid nitrogen, pulverizing them at low temperature, and then sieving them. The particle size of the polypropylene powder is 75-150 μm.

3. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (1), the polypropylene powder is homopolymer polypropylene, and the melt flow rate at 230℃ / 2.16kg is 12g / 10min.

4. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (3), solid-phase mixing is carried out in a jacketed high-speed mixer with a jacket temperature of 125-145℃, a stirring speed of 700-900rpm, a mixing time of 6-10min, and the material is cooled to below 40℃ before being discharged.

5. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (4), the initiator is a polymer-coated granule of dicumyl peroxide, model Perkadox BC-EP40.

6. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (4), the polypropylene resin is a random copolymer of polypropylene, and the melt flow rate is 11 g / 10 min at 230℃ / 2.16 kg.

7. The method for preparing the conductive polypropylene composite material according to claim 1, characterized in that, In step (4), the melt flow rate of the polyolefin elastomer at 190℃ / 2.16kg is 0.5g / 10min.

8. A conductive polypropylene composite material, characterized in that, It is obtained by the method for preparing the conductive polypropylene composite material according to any one of claims 1-7.

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