An insulating and flame-retardant cross-linked polypropylene foam material and its preparation method

Abstract

This application provides an insulating and flame-retardant cross-linked polypropylene foam material and its preparation method, belonging to the field of polypropylene foam materials. A flame-retardant synergistic masterbatch is prepared by grafting maleic anhydride onto aluminum diethylphosphonate, melamine polyphosphate, modified montmorillonite, calcium borate, and a portion of EPDM rubber. An irradiation-sensitized masterbatch is prepared by grafting maleic anhydride onto a portion of ethylene-1-octene block copolymer, triallyl isocyanurate, and trimethylolpropane trimethacrylate. This masterbatch is then mixed with homopolymer polypropylene, the remaining components, and azodicarbonamide, and subjected to dynamic radiation cross-linking extrusion, low-energy electron beam gradient irradiation cross-linking, and staged high-temperature foaming to obtain the product. This application solves the problems of premature decomposition of foaming agents, inhibition of foaming by flame retardants, and irradiation degradation of polypropylene by using a zinc-free formulation, masterbatch pre-dispersion, and dynamic and gradient irradiation synergistic cross-linking, achieving a balance between high flame retardancy, high insulation, and good foaming performance in cross-linked polypropylene foam materials.

Description

Technical Field

[0001] This application relates to the field of polypropylene foam material technology, and in particular to an insulating and flame-retardant cross-linked polypropylene foam material and its preparation method. Background Technology

[0002] Cross-linked polypropylene foam, especially IXPP foam prepared through irradiation cross-linking technology, is a closed-cell foam material made primarily of polypropylene through electron beam irradiation cross-linking and high-density foaming processes. Compared to traditional polypropylene foam, irradiation cross-linking technology uses a high-energy electron beam to form a stable chemical cross-linking network between polypropylene molecular chains, giving the material higher melt strength and a wider foaming temperature window. This allows for the production of lightweight foam products with uniform cell structure, high closed-cell rate, and excellent mechanical properties. Benefiting from the excellent heat resistance and chemical stability of the polypropylene substrate, cross-linked polypropylene foam also exhibits long-term temperature resistance up to 120℃, short-term peak temperature resistance up to 140℃, and good oil and chemical resistance. It is increasingly widely used in fields such as battery separators for new energy vehicles, cushioning seals for consumer electronics, automotive interiors, and cold chain packaging.

[0003] In many applications, cross-linked polypropylene (IXPP) foam materials typically need to simultaneously meet multiple performance requirements, including high flame retardancy, high insulation, and good expansion ratio and closed-cell ratio. However, existing XPP foam materials that combine high flame retardancy, high insulation, and good foaming performance still have significant shortcomings. On the one hand, the introduction of flame retardant systems often significantly interferes with the foaming process: high filler content of flame retardants increases melt viscosity, inhibits cell nucleation and growth, leading to a decrease in expansion ratio, uneven cell structure, or reduced closed-cell ratio, and in severe cases, even cell collapse. Some commercially available IXPP products only have a UL94 HF-1 flame retardancy rating, which is insufficient for applications with more stringent flame retardancy requirements. On the other hand, the compatibility between the foaming agent and cross-linking aids also poses a technical obstacle: cross-linking aids such as zinc compounds significantly lower the initial decomposition temperature of azodicarbonamide foaming agents, causing premature decomposition of the foaming agent during the extrusion processing stage, resulting in pre-foaming defects in the master sheet and insufficient expansion ratio in the final product. Therefore, how to achieve good foaming ratio and closed-cell ratio while ensuring the excellent flame retardant and insulation properties of the material has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] This application provides an insulating and flame-retardant cross-linked polypropylene foam material and its preparation method to solve the following technical problem: how to provide a cross-linked polypropylene foam material that has both high flame retardancy rating, high insulation performance, and good foaming ratio and closed-cell ratio.

[0005] In a first aspect, this application provides a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, the method comprising the following steps: S1. Aluminum diethylphosphinate, melamine polyphosphate, modified montmorillonite, calcium borate and some EPDM rubber grafted with maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch. S2. Part of the ethylene-1-octene block copolymer, triallyl isocyanurate and trimethylolpropane trimethacrylate are melt-blended and granulated in a mixer to obtain radiation sensitization masterbatch. S3. Mix homopolymer polypropylene, the remaining portion of the ethylene-1-octene block copolymer, the remaining portion of the EPDM-grafted maleic anhydride, rare earth β nucleating agent, antioxidant, the flame retardant synergistic masterbatch and the radiation sensitizing masterbatch, and then add azodicarbonamide to obtain a mixture. S4. The mixture is added to a twin-screw extruder, and an electron beam irradiation window is set in the middle and rear section of the extruder. Dynamic radiation crosslinking extrusion is carried out under nitrogen protection, and then calendered to obtain a master sheet with a thickness of 0.8 to 1.2 mm. S5. Irradiate one side of the master sheet with a low-energy electron beam and control the penetration depth to be 0.10-0.25 mm. Then irradiate the other side under the same conditions to perform gradient irradiation crosslinking on the master sheet to obtain a crosslinked master sheet. S6. The cross-linked masterbatch is subjected to staged high-temperature foaming to obtain the cross-linked polypropylene foam material.

[0006] Optionally, by weight, the cross-linked polypropylene foam material is composed of the following raw materials: homopolymer polypropylene: 40-60 parts, ethylene-1-octene block copolymer: 15-35 parts, EPDM rubber grafted with maleic anhydride: 5-10 parts, rare earth β nucleating agent: 0.5-1.5 parts, aluminum diethylphosphonate: 6-12 parts, melamine polyphosphate: 5-10 parts, modified montmorillonite: 2-5 parts, calcium borate: 1-3 parts, azodicarbonamide: 5-10 parts, antioxidant: 0.5-3 parts, triallyl isocyanurate: 2-4 parts, trimethylolpropane trimethacrylate: 1-2 parts.

[0007] Optionally, in step S1, the portion of EPDM rubber grafted with maleic anhydride is one-third of the total weight. The portion of ethylene-1-octene block copolymer mentioned in step S2 is one-third of the total weight.

[0008] Optionally, in step S4, the process parameters for the dynamic radiation crosslinking extrusion include: extruder feed section temperature of 140–150°C, compression section temperature of 150–155°C, metering section temperature of 150–155°C, die temperature of 145–150°C, screw speed of 200–300 rpm, electron beam energy of 1.0–1.5 MeV, and irradiation dose of 30–50 kGy.

[0009] Optionally, in step S5, the energy of the low-energy electron beam is 150-200 keV, the single-sided irradiation dose is 8-12 kGy, and the mother film is placed on a circulating water-cooled plate during irradiation, with the surface temperature controlled below 50°C.

[0010] Optionally, in step S6, the specific process of the staged high-temperature foaming is as follows: first, preheat at 160-170℃ for 3-5 minutes, then foam at 190-210℃ for 4-8 minutes, and finally set at 180-190℃ for 2-3 minutes, followed by natural cooling.

[0011] Optionally, in step S3, the rare earth β nucleating agent is the rare earth compound WBGⅡ.

[0012] Optionally, in step S1, the modified montmorillonite is montmorillonite modified with octadecyltrimethylammonium chloride.

[0013] Optionally, in step S3, the antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite in a 1:1 mass ratio.

[0014] Secondly, this application provides an insulating and flame-retardant cross-linked polypropylene foam material prepared by any one of the methods described in the first aspect, wherein the cross-linked polypropylene foam material meets the following properties: limiting oxygen index ≥30%, UL94 vertical flammability rating of V-0, and volume resistivity ≥1×10⁻⁶. 15 Ω·cm, foaming ratio of 6 to 12 times, closed cell ratio ≥90%.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application achieves a synergistic improvement in high flame retardancy rating, high insulation performance, good foaming ratio, and closed-cell ratio through the systematic integration of zinc-free flame retardant synergistic masterbatch, irradiation sensitized masterbatch, dynamic and gradient irradiation crosslinking, and staged foaming process.

[0016] Regarding flame retardant performance, this application prepares a flame retardant synergistic masterbatch in step S1. This masterbatch comprises aluminum diethylphosphinate, melamine polyphosphate, modified montmorillonite, and calcium borate. Aluminum diethylphosphinate acts as an acid source, releasing acidic substances at high temperatures, while melamine polyphosphate acts as a gas source, decomposing to release non-combustible gases. Both components work together to promote the formation of an expanded char layer on the substrate surface. The nanosheets of modified montmorillonite migrate to the material surface during combustion, combining with the expanded char layer to form a denser physical thermal insulation barrier. Calcium borate acts as a smoke suppressant synergist, inhibiting smoke release and promoting char layer perfection. The four components work synergistically to achieve a high flame retardant rating with relatively low addition amounts.

[0017] In terms of high insulation performance, both homopolymer polypropylene and ethylene-1-octene block copolymer are nonpolar polymers with high intrinsic volume resistivity. After being uniformly dispersed in the matrix, the layered silicate structure of modified montmorillonite elongates the charge migration path, further enhancing insulation performance. The three-dimensional network structure constructed by irradiation crosslinking restricts the movement of polymer molecular chains, reducing charge migration opportunities and thus maintaining the material's high volume resistivity.

[0018] Regarding the foaming ratio and closed-cell ratio, this application achieves these through a combination of multi-stage crosslinking and foaming processes. The irradiation-sensitized masterbatch prepared in step S2 contains triallyl isocyanurate and trimethylolpropane trimethacrylate, both of which are multifunctional monomers. In step S4, the mixture undergoes dynamic radiation crosslinking extrusion in a twin-screw extruder with an electron beam energy of 1.0–1.5 MeV and an irradiation dose of 30–50 kGy, resulting in a uniform pre-crosslinked network on the masterbatch, significantly improving the melt strength of the polypropylene. In step S5, a low-energy electron beam with an energy of 150–200 keV is used to irradiate both sides of the masterbatch, controlling the penetration depth to be 0.10–0.25 mm and the single-sided irradiation dose to be 8–12 kGy, forming a gradient structure with high surface crosslinking density and low core crosslinking density. In step S6, the cross-linked masterbatch undergoes staged high-temperature foaming. During the foaming stage, azodicarbonamide decomposes and releases gas, which is effectively encapsulated by the gradient cross-linking network. The highly cross-linked surface layer inhibits cell rupture, while appropriate cross-linking in the core promotes uniform gas expansion, thereby obtaining a foam structure with high closed-cell ratio and uniform cell size. Simultaneously, the raw materials in this application do not contain zinc, avoiding the activation effect of zinc-containing substances on the decomposition temperature of azodicarbonamide. This ensures that the foaming agent does not decompose prematurely during the extrusion processing stage (140–165°C), guaranteeing the quality of the masterbatch and the final foaming ratio. Attached Figure Description

[0019] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0020] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 A schematic flowchart illustrating a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material according to an embodiment of this application; Figure 2 This is a SEM image of the cell structure of the cross-linked polypropylene foam material provided in Example 1 of this application. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0024] Figure 1 This is a schematic flowchart illustrating a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, as provided in an embodiment of this application.

[0025] like Figure 1 As shown, this application provides a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, which includes the following steps: S1. Aluminum diethylphosphinate, melamine polyphosphate, modified montmorillonite, calcium borate and some EPDM rubber grafted with maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch. S2. Part of the ethylene-1-octene block copolymer, triallyl isocyanurate and trimethylolpropane trimethacrylate are melt-blended and granulated in a mixer to obtain radiation sensitization masterbatch. S3. Mix homopolymer polypropylene, the remaining portion of ethylene-1-octene block copolymer, the remaining portion of EPDM grafted maleic anhydride, rare earth β nucleating agent, antioxidant, flame retardant synergist masterbatch and radiation sensitizing masterbatch, and then add azodicarbonamide to obtain the mixture. S4. Add the mixture to a twin-screw extruder, set an electron beam irradiation window in the middle and rear section of the extruder, and perform dynamic radiation crosslinking extrusion under nitrogen protection. Then, calender to obtain a master sheet with a thickness of 0.8 to 1.2 mm. S5. Irradiate one side of the master sheet with a low-energy electron beam and control the penetration depth to 0.10-0.25 mm. Then irradiate the other side under the same conditions to perform gradient irradiation crosslinking on the master sheet to obtain a crosslinked master sheet. S6. The cross-linked masterbatch is subjected to staged high-temperature foaming to obtain cross-linked polypropylene foam material.

[0026] It should be noted that the core function of step S1 (preparation of flame retardant synergistic masterbatch) is to pre-blend and granulate the components (aluminum diethylphosphinate, melamine polyphosphate, modified montmorillonite, and calcium borate) in the halogen-free flame retardant system with a portion of the compatibilizer (EPDM-grafted maleic anhydride). This masterbatch process has multiple benefits: First, the strong dispersing capability of the twin-screw extruder allows the nanosheets of modified montmorillonite to be fully exfoliated in the polymer matrix, while the phosphorus-based and nitrogen-based flame retardants are uniformly premixed with calcium borate, avoiding agglomeration caused by differences in particle size and surface polarity when added directly; second, the early addition of the compatibilizer improves the interfacial compatibility between the flame retardant and the subsequent matrix resin; finally, masterbatch ensures that the flame retardant components are spatially close together, laying the structural foundation for the formation of a continuous and dense expanded char layer during subsequent combustion. This step effectively solves the problem of uneven dispersion in the high-filler flame retardant system, while also preventing premature contact between the flame retardant and the foaming agent in subsequent steps.

[0027] The purpose of step S2 (preparation of irradiation-sensitized masterbatch) is to construct a highly efficient pre-dispersion system for irradiation crosslinking aids. Triallyl isocyanurate (TAIC) and trimethylolpropane trimethacrylate (TMPTMA) are both multifunctional monomers that can rapidly undergo grafting and crosslinking reactions with polypropylene macromolecular free radicals under electron beam irradiation. Premixing and granulating these two monomers with a portion of ethylene-1-octene block copolymer (OBC) in a Banbury mixer avoids excessively high local concentrations or agglomeration of the highly reactive monomers during subsequent mixing, ensuring molecular-level dispersion in the matrix resin. Simultaneously, OBC, as a carrier resin, exhibits good compatibility with the main polypropylene matrix and can also participate in the construction of the crosslinking network under irradiation. The preparation of this masterbatch is crucial for achieving efficient and uniform crosslinking under low irradiation doses, ensuring high melt strength in subsequent applications.

[0028] Step S3 (main mixing) is responsible for the final integration of all functional components with the matrix resin. First, homopolymer polypropylene (main matrix), the remaining OBC (secondary matrix, further adjusting flexibility and melt strength), the remaining EPDM grafted maleic anhydride (compatibilizer, improving the PP / OBC interface and the compatibility of the flame retardant with the matrix), rare earth β-nucleating agent (regulating polypropylene crystallization behavior, inducing the formation of β-crystals to improve toughness and heat resistance), antioxidant (preventing thermo-oxidative aging during processing and irradiation), and the flame retardant synergistic masterbatch and irradiation sensitization masterbatch prepared in the first two steps are thoroughly dry-mixed to ensure a preliminary uniform distribution of each component. Finally, azodicarbonamide foaming agent is added and briefly mixed. This timing is crucial; azodicarbonamide is extremely stable at room temperature, but may decompose prematurely when heated or in contact with certain metal ions. Adding it at the end of the mixing process minimizes the thermal history of the foaming agent before extrusion and its contact time with potential activators, thus ensuring the thermal stability of the foaming agent in the subsequent extrusion stage.

[0029] Step S4 (dynamic radiation crosslinking extrusion) is the first core innovation of this process. While the mixture is melted and plasticized in a twin-screw extruder, it is simultaneously irradiated online by a high-energy electron beam (1.0–1.5 MeV) at an electron beam irradiation window located in the latter part of the extruder. At this time, the material is still in a molten and flowing state. Under the action of the electron beam, TAIC and TMPTMA in the irradiated sensitized masterbatch rapidly generate free radicals, initiating a crosslinking reaction between polypropylene molecular chains and forming a preliminary three-dimensional network structure. This dynamic crosslinking method, where extrusion and irradiation occur simultaneously, has two major advantages: first, the crosslinking reaction occurs under conditions of uniform mixing and flow of the material, avoiding the crosslinking gradient caused by uneven dose distribution in the thickness direction in traditional static irradiation; second, online crosslinking immediately improves melt strength, resulting in better dimensional stability and strength of the extruded masterbatch, facilitating subsequent calendering into uniformly thick sheets (0.8–1.2 mm). Nitrogen protection effectively inhibits the oxidative degradation of polypropylene during irradiation.

[0030] It should be noted that for most materials, the relationship between the electron range R (μm) and the electron energy E (keV) can be approximately expressed as: R = 0.0276·E^1.67 / ρ, where ρ is the material density (g / cm³). 3This relationship demonstrates that the depth distribution of energy deposition can be precisely controlled by adjusting the electron beam energy. Step S5 (gradient irradiation crosslinking) is the second core innovation. The master sheet after dynamic irradiation crosslinking already possesses a certain overall crosslinking degree, but there is no difference between the crosslinking structure on the surface and inside. In this step, a low-energy electron beam (100-200 keV) is used to irradiate both sides of the master sheet. Since the penetration depth of the low-energy electron beam is precisely controlled within 0.10-0.25 mm, it can only act on the surface layer of the master sheet. Therefore, only a hard shell with a high crosslinking density is formed on the material surface, while the crosslinking degree of the core remains basically unchanged. The design intention of this gradient crosslinking structure is that during subsequent high-temperature foaming, the highly crosslinked surface layer has higher melt strength, which can effectively inhibit the rupture of bubbles on the surface, thereby obtaining a smooth, dense, and pore-free skin; while the relatively low crosslinking degree of the core is conducive to the uniform expansion of the gas generated by the decomposition of the foaming agent, forming small and uniform internal bubbles. Meanwhile, during the irradiation process, a circulating water-cooled plate was used to control the surface temperature of the mother sheet below 50°C, preventing premature decomposition of azodicarbonamide caused by irradiation heat.

[0031] Step S6 (staged high-temperature foaming) is crucial for transforming the cross-linked masterbatch into the final foam product. The staged heating design perfectly matches the decomposition characteristics of azodicarbonamide: first, preheating at 160–170℃ for 3–5 minutes ensures a uniform temperature rise in the masterbatch, but before reaching the violent decomposition temperature of azodicarbonamide (approximately 190℃); then, rapidly heating to 190–210℃ and holding for 4–8 minutes allows the azodicarbonamide to decompose, releasing large amounts of nitrogen and carbon monoxide, forming stable cells within the gradient cross-linking network; finally, setting at 180–190℃ for 2–3 minutes fixes the cell structure during cooling, preventing collapse or merging. After natural cooling, a cross-linked polypropylene foam material with high closed-cell ratio, smooth surface, and excellent flame-retardant and insulating properties is obtained.

[0032] In some embodiments, the cross-linked polypropylene foam material, by weight, is composed of the following raw materials: homopolymer polypropylene: 40-60 parts, ethylene-1-octene block copolymer: 15-35 parts, EPDM-grafted maleic anhydride: 5-10 parts, rare earth β-nucleating agent: 0.5-1.5 parts, aluminum diethylphosphinate: 6-12 parts, melamine polyphosphate: 5-10 parts, modified montmorillonite: 2-5 parts, calcium borate: 1-3 parts, azodicarbonamide: 5-10 parts, antioxidant: 0.5-3 parts, triallyl isocyanurate: 2-4 parts, trimethylolpropane trimethacrylate: 1-2 parts.

[0033] It should be noted that homopolymer polypropylene, as the main matrix resin, provides the material with basic mechanical strength and heat resistance, and its high crystallinity helps maintain the rigidity of the cell structure. Ethylene-1-octene block copolymer, as the secondary matrix resin, possesses a unique block structure that endows the material with excellent melt elasticity and flexibility, significantly improving melt strength during foaming and preventing cell coalescence and rupture. Ethylene propylene diene monomer (EPDM) rubber grafted with maleic anhydride acts as a compatibilizer; the maleic anhydride groups in its molecular chain can interact with the surfaces of modified montmorillonite and flame retardants. Simultaneously, the rubber segments have good compatibility with polypropylene and ethylene-1-octene block copolymer, effectively improving the interfacial bonding between inorganic fillers and the organic matrix and preventing phase separation. Rare earth β-nucleating agents are used to regulate the crystallization behavior of polypropylene, inducing the formation of a β-crystal form. This β-crystal form has higher impact toughness and heat distortion temperature, compensating for the potential decrease in rigidity caused by the addition of flame retardants and elastomers. Furthermore, the refined spherulites facilitate uniform cell nucleation during the foaming process.

[0034] In terms of the flame retardant system, aluminum diethylphosphonate and melamine polyphosphate constitute a phosphorus-nitrogen synergistic intumescent flame retardant. During combustion, they undergo an esterification reaction to generate a dense intumescent char layer, isolating oxygen and heat. Modified montmorillonite, as a nano-char-forming agent, has its layered silicate sheets migrate to the material surface at high temperatures and combine with the intumescent char layer to form a denser and stronger thermal barrier. At the same time, its nanosheets can also increase the volume resistivity of the material and enhance its insulation performance. Calcium borate, as a smoke suppressant synergist, dehydrates and absorbs heat at high temperatures, dilutes combustible gases, and promotes melt cross-linking to form char. Meanwhile, its decomposition product, calcium metaborate, can form a glassy coating layer to inhibit smoke release, forming a ternary synergistic system with the phosphorus-nitrogen flame retardant. The above four components can achieve the UL94 V-0 flame retardant rating and limiting oxygen index ≥30% with a relatively low total addition amount (14-30 parts), while reducing interference with the foaming process.

[0035] Azodicarbonamide is a chemical foaming agent with a decomposition temperature of approximately 190–210°C. It remains stable at extrusion processing temperatures (140–165°C), decomposing only during the high-temperature foaming stage to release gases such as nitrogen, forming a cellular structure. Triallyl isocyanurate and trimethylolpropane trimethacrylate are radiation crosslinking sensitizers. Both are multifunctional monomers that can rapidly graft and crosslink with polypropylene macromolecular free radicals under electron beam irradiation, constructing a three-dimensional network structure. Triallyl isocyanurate exhibits higher crosslinking efficiency and better scorch resistance, while trimethylolpropane trimethacrylate, as an auxiliary sensitizer, can further increase the crosslinking density. When used in a 2:1–3:1 ratio, efficient crosslinking can be achieved with a total addition of only 3–6 parts, avoiding degradation side reactions caused by excessive addition. The antioxidant utilizes a hindered phenol and phosphite complex system to capture free radicals during processing and irradiation, preventing thermal oxidative degradation of polypropylene and ensuring long-term material stability.

[0036] In some embodiments, in step S1, the portion of the EPDM rubber grafted with maleic anhydride is one-third of the total weight. In step S2, the ethylene-1-octene block copolymer constitutes one-third of the total weight.

[0037] In some embodiments, the process parameters for dynamic radiation crosslinking extrusion in step S4 include: extruder feed section temperature of 140–150°C, compression section temperature of 150–155°C, metering section temperature of 150–155°C, die temperature of 145–150°C, screw speed of 200–300 rpm, electron beam energy of 1.0–1.5 MeV, and irradiation dose of 30–50 kGy.

[0038] The process parameters for dynamic radiation crosslinking extrusion are primarily reflected in the temperature gradient design of each section of the extruder. The feed section temperature is controlled at 140–150°C, allowing the mixture to gradually melt and plasticize, preventing premature heating of azodicarbonamide due to sudden temperature increases. The compression and metering sections are maintained at 150–155°C, slightly higher than the feed section, ensuring complete polymer melting and thorough mixing. This temperature range is also significantly lower than the drastic decomposition temperature of azodicarbonamide (approximately 190°C), thus ensuring the thermal stability of the foaming agent during extrusion. The die temperature is reduced to 145–150°C, which helps reduce the expansion effect of the melt upon exiting the die, resulting in a calendered sheet with uniform thickness and a smooth surface. The screw speed is set at 200–300 rpm, ensuring sufficient residence time for the material in the barrel for melting, mixing, and dynamic irradiation, while avoiding excessive shear heat from excessively high speeds that could lead to localized overheating. The electron beam energy is selected from 1.0 to 1.5 MeV. This energy range can penetrate molten masterbatch with a thickness of 0.8 to 1.2 mm, ensuring uniform electron beam action in the thickness direction and achieving overall pre-crosslinking. The irradiation dose is controlled at 30 to 50 kGy, which is a balance point: too low a dose will result in insufficient crosslinking and cannot effectively improve melt strength; too high a dose may cause radiation degradation of polypropylene. Within this dose range, in conjunction with the multifunctional monomers in the irradiation-sensitized masterbatch, a moderate and uniform pre-crosslinked network can be formed, laying the foundation for subsequent gradient irradiation and high-temperature foaming.

[0039] In some embodiments, in step S5, the energy of the low-energy electron beam is 150-200 keV, the single-sided irradiation dose is 8-12 kGy, and the mother sheet is placed on a circulating water-cooled plate during irradiation, with the surface temperature controlled below 50°C.

[0040] In the gradient irradiation crosslinking step, the energy of the low-energy electron beam is set to 100–200 keV. Unlike the high-energy electron beam used in dynamic irradiation, the low-energy electron beam has significantly reduced penetration ability, with a penetration depth of only about 0.10–0.25 mm. Therefore, it mainly acts on the surface of the mother sheet and hardly affects the core. The single-sided irradiation dose is 8–12 kGy, which is sufficient to initiate additional crosslinking reactions on the surface without causing surface embrittlement due to excessive crosslinking. After irradiation on both sides, the mother sheet forms a gradient structure with high surface crosslinking density and low core crosslinking density. Placing the mother sheet on a circulating water-cooled plate and controlling the surface temperature below 50°C during irradiation is crucial: low-energy electron beam bombardment generates heat on the surface, and if heat is not dissipated in time, local temperature rise may cause premature decomposition of azodicarbonamide or softening and deformation of the polymer surface; the controlled temperature of 50°C is far below the decomposition initiation temperature of azodicarbonamide and does not affect the solid structure of the polypropylene matrix.

[0041] In some embodiments, the specific process of staged high-temperature foaming in step S6 is as follows: first, preheat at 160-170℃ for 3-5 minutes, then foam at 190-210℃ for 4-8 minutes, and finally set at 180-190℃ for 2-3 minutes, followed by natural cooling.

[0042] The process parameters for staged high-temperature foaming are precisely matched to the thermal decomposition characteristics of azodicarbonamide. First, preheating is performed at 160–170℃ for 3–5 minutes. This temperature range is slightly below the rapid decomposition temperature of azodicarbonamide (approximately 190℃). The purpose is to ensure a uniform temperature rise of the entire substrate to near the activation threshold of the foaming agent, while preventing excessive decomposition of the foaming agent during the preheating stage. Subsequently, the temperature is increased to 190–210℃ and held for 4–8 minutes. At this point, azodicarbonamide rapidly decomposes, releasing gases such as nitrogen and carbon monoxide. Under the constraint of the gradient crosslinking network, cells nucleate and grow. The temperature window of 190–210℃ ensures complete decomposition of the foaming agent without causing excessive reduction in the melt strength of the crosslinked polypropylene, which could lead to cell collapse. The holding time of 4–8 minutes is adjusted according to the substrate thickness and foaming ratio requirements. Too short a time results in insufficient foaming, while too long a time may cause cell coarsening or merging. Finally, the foaming agent is set at 180–190℃ for 2–3 minutes. During this stage, the foaming agent has completely decomposed. Appropriate cooling allows the viscosity of the polymer melt in the cell walls to rise again, thus fixing the cell structure and preventing internal stress caused by rapid cooling. After natural cooling, a finished product with uniform cells and a smooth surface is obtained. The entire staged heating strategy ensures efficient utilization of the foaming agent and achieves fine adjustment of cell morphology through coordinated control of temperature and time.

[0043] In some embodiments, in step S3, the rare earth β nucleating agent is the rare earth compound WBGⅡ.

[0044] In some embodiments, in step S1, the modified montmorillonite is octadecyltrimethylammonium chloride-modified montmorillonite.

[0045] In some embodiments, in step S3, the antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite in a 1:1 mass ratio.

[0046] Based on a general inventive concept, this application provides an insulating and flame-retardant cross-linked polypropylene foam material prepared by any of the above methods. The cross-linked polypropylene foam material meets the following properties: limiting oxygen index ≥30%, UL94 vertical flammability rating of V-0, and volume resistivity ≥1×10⁻⁶. 15 Ω·cm, foaming ratio of 6 to 12 times, closed cell ratio ≥90%.

[0047] The cross-linked polypropylene foam material of this application achieves a limiting oxygen index ≥30%, UL94V-0, and volume resistivity ≥1×10⁻⁶ through the synergistic combination of formulation design and process control. 15 It has excellent comprehensive properties, including Ω·cm, foaming ratio of 6 to 12 times, and closed-cell ratio of ≥90%.

[0048] The achievement of a limiting oxygen index (LOI) ≥30% and a UL94V-0 flame retardant rating is attributed to the multiple synergistic effects of the intumescent halogen-free flame retardant system. Aluminum diethylphosphonate, acting as an acid source, releases phosphoric acid-containing substances at high temperatures, while melamine polyphosphate, acting as a gas source, decomposes to release non-combustible gases. Together, these components induce the formation of an expanded char layer on the substrate surface. Modified montmorillonite migrates to the material surface during combustion; its nanolayered structure not only catalyzes the formation of a denser, more porous char layer but also constructs a dense and complete wrinkled char layer on the melt surface, effectively blocking heat and mass transfer between the bottom material and the combustion zone. Calcium borate, acting as a smoke suppressant, forms a glassy coating layer through its decomposition products, inhibiting smoke release and promoting further char layer refinement. These four components synergistically form multiple protective barriers—heat insulation, oxygen barrier, and smoke suppression—during combustion, enabling the material to achieve a V-0 flame retardant rating with a relatively low total addition amount, while maintaining an LIO exceeding 30%. Rare earth β nucleating agents regulate the crystallization behavior of polypropylene and induce the formation of β crystals. The refined spherulite structure provides a more ideal matrix environment for the uniform dispersion of flame retardants, thereby indirectly improving the flame retardant efficiency.

[0049] Volume resistivity ≥ 1×10 15 The high insulation performance of Ω·cm is ensured by the intrinsic insulating properties of the matrix resin and the synergistic effect of the nanofillers. Both homopolymer polypropylene and ethylene-1-octene block copolymer are non-polar polymers, and their molecular structures do not contain conductive ions or polar groups; their intrinsic volume resistivity can reach 10 Ω·cm. 16 Above Ω·cm. After the modified montmorillonite nanosheets are uniformly dispersed in the matrix, their layered silicate structure further extends the charge migration path, enhances the physical barrier effect, and effectively hinders the formation of conductive pathways. The three-dimensional network structure constructed by irradiation crosslinking restricts the movement of polymer molecular chains, reduces the chance of charge migration caused by chain segment activity, and enables the material to maintain a high level of insulation after foaming.

[0050] The foam structure with a foaming ratio of 6–12 times and a closed-cell ratio of ≥90% is the result of synergistic optimization of matrix modification, crosslinking control, and foaming process. The introduction of ethylene-1-octene block copolymer effectively improves the melt strength of the blend system, forming an island structure with PP as the continuous phase and OBC as the dispersed phase in the PP / OBC blend system. Notched impact strength and elongation at break are significantly improved. Simultaneously, the block structure of OBC provides excellent elastic support for the cell walls during foaming. Dynamic irradiation crosslinking endows the master sheet with a uniform pre-crosslinked network, significantly enhancing the melt strength of the crosslinked polypropylene and effectively encapsulating the gas released from the decomposition of azodicarbonamide, preventing cell merging or rupture. Gradient irradiation crosslinking constructs a high-crosslink density reinforcing layer on the surface of the master sheet, suppressing excessive expansion of surface cells during the high-temperature foaming stage, thus obtaining a smooth and dense skin and uniform internal cells. The decomposition of azodicarbonamide releases non-flammable gases such as nitrogen, which, under the constraint of the crosslinked network, form a fine and uniform closed-cell structure. The staged foaming process involves first preheating the substrate at 160–170℃ to ensure uniform heating, then foaming at 190–210℃ to fully decompose the azodicarbonamide, and finally stabilizing the cell structure at 180–190℃, further ensuring the controllability of the cell morphology. The synergistic effect of these three processes allows the material to achieve a high closed-cell rate of over 90% while maintaining a moderate foaming ratio of 6–12 times.

[0051] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0052] The raw material information for the cross-linked polypropylene foam materials of the examples and comparative examples is shown in Table 1.

[0053] Table 1 Raw material information for cross-linked polypropylene foam materials

[0054] Example 1 This embodiment provides a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, the specific steps of which are as follows.

[0055] Raw material composition (parts by weight): Homopolymer polypropylene: 50 parts, ethylene-1-octene block copolymer: 25 parts, EPDM grafted maleic anhydride: 8 parts, rare earth β nucleating agent (rare earth compound WBGⅡ): 1.0 part, aluminum diethylphosphinate: 9 parts, melamine polyphosphate: 7 parts, modified montmorillonite (octadecyltrimethylammonium chloride modified): 4 parts, calcium borate: 2 parts, azodicarbonamide: 8 parts, antioxidant (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a 1:1 mass ratio): 1.5 parts, triallyl isocyanurate: 3 parts, trimethylolpropane trimethacrylate: 1.5 parts.

[0056] The modified montmorillonite was prepared as follows: 10g of nano-montmorillonite was dispersed in 200mL of deionized water and stirred for 30min to form a uniform suspension. 4g of octadecyltrimethylammonium chloride was weighed, dissolved in 50mL of deionized water, and slowly added dropwise to the montmorillonite suspension. The mixture was stirred and reacted in an 80℃ water bath for 3h. After the reaction was complete, the mixture was filtered and repeatedly washed with deionized water until no chloride ions were detected. The filter cake was dried in an 80℃ oven to constant weight, ground, and sieved to obtain the final product.

[0057] Preparation method: S1. Preparation of flame retardant synergistic masterbatch: 9 parts of aluminum diethylphosphinate, 7 parts of melamine polyphosphate, 4 parts of modified montmorillonite, 2 parts of calcium borate, and one-third (2.7 parts) of the total 8 parts of EPDM-grafted maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch.

[0058] S2. Preparation of irradiation sensitization masterbatch: Take one-third (8.3 parts) of the total 25 parts of ethylene-1-octene block copolymer, and melt-blend and granulate it with 3 parts of triallyl isocyanurate and 1.5 parts of trimethylolpropane trimethacrylate in a mixer to obtain irradiation sensitization masterbatch.

[0059] S3. Main Mixing: Mix 50 parts of homopolymer polypropylene, the remaining ethylene-1-octene block copolymer (16.7 parts), the remaining EPDM-grafted maleic anhydride (5.3 parts), 1.0 part of rare earth β nucleating agent, 1.5 parts of antioxidant, the flame retardant synergistic masterbatch obtained in step S1 and the radiation sensitizing masterbatch obtained in step S2, and finally add 8 parts of azodicarbonamide and mix evenly to obtain the mixture.

[0060] S4. Dynamic Radiation Crosslinking Extrusion: The mixture is added to a twin-screw extruder. An electron beam irradiation window is set in the rear section of the extruder, and dynamic radiation crosslinking extrusion is performed under nitrogen protection. The extruder feed section temperature is 145℃, the compression section temperature is 152℃, the metering section temperature is 152℃, the die temperature is 148℃, the screw speed is 250 rpm, the electron beam energy is 1.2 MeV, and the irradiation dose is 40 kGy. After extrusion, a master sheet with a thickness of 1.0 mm is obtained by calendering.

[0061] S5. Gradient Irradiation Crosslinking: One side of the master sheet is irradiated with a low-energy electron beam at an energy of 180 keV, a single-sided irradiation dose of 10 kGy, and an effective crosslinking layer thickness of 0.20 mm. The master sheet is then flipped over and the other side is irradiated under the same conditions. During the irradiation process, the master sheet is placed on a circulating water-cooled plate, and the surface temperature is controlled below 45°C to obtain a crosslinked master sheet.

[0062] S6. Staged high-temperature foaming: The cross-linked masterbatch is first preheated at 165℃ for 4 minutes, then foamed at 200℃ for 6 minutes, and finally shaped at 185℃ for 2.5 minutes and naturally cooled to obtain cross-linked polypropylene foam material.

[0063] Example 2 This embodiment provides a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, the specific steps of which are as follows.

[0064] Raw material composition (parts by weight): Homopolymer polypropylene: 40 parts, ethylene-1-octene block copolymer: 35 parts, EPDM grafted maleic anhydride: 5 parts, rare earth β nucleating agent (rare earth compound WBGⅡ): 0.5 parts, aluminum diethylphosphinate: 6 parts, melamine polyphosphate: 10 parts, modified montmorillonite (octadecyltrimethylammonium chloride modified): 2 parts, calcium borate: 3 parts, azodicarbonamide: 5 parts, antioxidant (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a 1:1 mass ratio): 0.5 parts, triallyl isocyanurate: 2 parts, trimethylolpropane trimethacrylate: 2 parts.

[0065] Preparation method: S1. Preparation of flame retardant synergistic masterbatch: 6 parts of aluminum diethylphosphinate, 10 parts of melamine polyphosphate, 2 parts of modified montmorillonite, 3 parts of calcium borate, and one-third (1.7 parts) of the total 5 parts of EPDM-grafted maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch.

[0066] S2. Preparation of irradiation sensitization masterbatch: Take one-third (11.7 parts) of the total 35 parts of ethylene-1-octene block copolymer, and melt-blend and granulate it with 2 parts of triallyl isocyanurate and 2 parts of trimethylolpropane trimethacrylate in a mixer to obtain irradiation sensitization masterbatch.

[0067] S3. Main Mixing: Mix 40 parts of homopolymer polypropylene, the remaining ethylene-1-octene block copolymer (23.3 parts), the remaining EPDM-grafted maleic anhydride (3.3 parts), 0.5 parts of rare earth β nucleating agent, 0.5 parts of antioxidant, the flame retardant synergistic masterbatch obtained in step S1 and the radiation sensitizing masterbatch obtained in step S2, and finally add 5 parts of azodicarbonamide and mix evenly to obtain the mixture.

[0068] S4. Dynamic Radiation Crosslinking Extrusion: The mixture is added to a twin-screw extruder. An electron beam irradiation window is set in the rear section of the extruder, and dynamic radiation crosslinking extrusion is performed under nitrogen protection. The extruder feed section temperature is 140℃, the compression section temperature is 150℃, the metering section temperature is 150℃, the die temperature is 145℃, the screw speed is 200 rpm, the electron beam energy is 1.0 MeV, and the irradiation dose is 30 kGy. After extrusion, a master sheet with a thickness of 0.8 mm is obtained by calendering.

[0069] S5. Gradient Irradiation Crosslinking: One side of the master sheet is irradiated with a low-energy electron beam at an energy of 150 keV, a single-sided irradiation dose of 8 kGy, and an effective crosslinking layer thickness of 0.15 mm. The master sheet is then flipped over and the other side is irradiated under the same conditions. During the irradiation process, the master sheet is placed on a circulating water-cooled plate, and the surface temperature is controlled below 40°C to obtain a crosslinked master sheet.

[0070] S6. Staged high-temperature foaming: The cross-linked master sheet is first preheated at 160℃ for 3 minutes, then foamed at 190℃ for 8 minutes, and finally shaped at 180℃ for 3 minutes. After natural cooling, cross-linked polypropylene foam material is obtained.

[0071] Example 3 This embodiment provides a method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, the specific steps of which are as follows.

[0072] Raw material composition (parts by weight): Homopolymer polypropylene: 60 parts, ethylene-1-octene block copolymer: 15 parts, EPDM grafted maleic anhydride: 10 parts, rare earth β nucleating agent (rare earth compound WBGⅡ): 1.5 parts, aluminum diethylphosphinate: 12 parts, melamine polyphosphate: 5 parts, modified montmorillonite (octadecyltrimethylammonium chloride modified): 5 parts, calcium borate: 1 part, azodicarbonamide: 10 parts, antioxidant (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite in a 1:1 mass ratio): 3 parts, triallyl isocyanurate: 4 parts, trimethylolpropane trimethacrylate: 1 part.

[0073] Preparation method: S1. Preparation of flame retardant synergistic masterbatch: 12 parts of aluminum diethylphosphinate, 5 parts of melamine polyphosphate, 5 parts of modified montmorillonite, 1 part of calcium borate, and one-third (3.3 parts) of 10 parts of EPDM-grafted maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch.

[0074] S2. Preparation of irradiation sensitization masterbatch: Take one-third (5 parts) of the total 15 parts of ethylene-1-octene block copolymer, and melt-blend and granulate it with 4 parts of triallyl isocyanurate and 1 part of trimethylolpropane trimethacrylate in a mixer to obtain irradiation sensitization masterbatch.

[0075] S3. Main Mixing: Mix 60 parts of homopolymer polypropylene, the remaining ethylene-1-octene block copolymer (10 parts), the remaining EPDM-grafted maleic anhydride (6.7 parts), rare earth β nucleating agent (1.5 parts), antioxidant (3 parts), the flame retardant synergistic masterbatch obtained in step S1, and the radiation sensitizing masterbatch obtained in step S2. Finally, add 10 parts of azodicarbonamide and mix evenly to obtain the mixture.

[0076] S4. Dynamic Radiation Crosslinking Extrusion: The mixture is added to a twin-screw extruder. An electron beam irradiation window is set in the rear section of the extruder, and dynamic radiation crosslinking extrusion is performed under nitrogen protection. The extruder feed section temperature is 150℃, the compression section temperature is 155℃, the metering section temperature is 155℃, the die temperature is 150℃, the screw speed is 300 rpm, the electron beam energy is 1.5 MeV, and the irradiation dose is 50 kGy. After extrusion, a calendered sheet with a thickness of 1.2 mm is obtained.

[0077] S5. Gradient Irradiation Crosslinking: One side of the master sheet is irradiated with a low-energy electron beam at an energy of 200 keV, a single-sided irradiation dose of 12 kGy, and an effective crosslinking layer thickness of 0.25 mm. The master sheet is then flipped over and the other side is irradiated under the same conditions. During the irradiation process, the master sheet is placed on a circulating water-cooled plate, and the surface temperature is controlled below 50°C to obtain a crosslinked master sheet.

[0078] S6. Staged high-temperature foaming: The cross-linked master sheet is first preheated at 170℃ for 5 minutes, then foamed at 210℃ for 4 minutes, and finally shaped at 190℃ for 2 minutes. After natural cooling, cross-linked polypropylene foam material is obtained.

[0079] Comparative Example 1 This comparative example is modified from the one disclosed in Example 1 as follows: Without the addition of aluminum diethylphosphinate and melamine polyphosphate, the flame retardant synergistic masterbatch contains only modified montmorillonite, calcium borate, and EPDM-grafted maleic anhydride. The remaining components and preparation methods are the same as in Example 1.

[0080] Comparative Example 2 This comparative example is modified from the one disclosed in Example 1 as follows: Without the addition of modified montmorillonite and calcium borate, the flame retardant synergistic masterbatch contains only aluminum diethylphosphinate, melamine polyphosphate, and EPDM-grafted maleic anhydride. The remaining components and preparation methods are the same as in Example 1.

[0081] Comparative Example 3 This comparative example is modified from the one disclosed in Example 1 as follows: Without adding triallyl isocyanurate and trimethylolpropane trimethacrylate, i.e. without preparing radiation sensitization masterbatch, in step S3, only homopolymer polypropylene, ethylene-1-octene block copolymer, EPDM grafted maleic anhydride, rare earth β nucleating agent, antioxidant, and flame retardant synergistic masterbatch are mixed and then azodicarbonamide is added. The remaining components and preparation methods are the same as in Example 1.

[0082] Comparative Example 4 This comparative example is modified from the one disclosed in Example 1 as follows: No rare earth β nucleating agent was added; the remaining components and preparation methods were the same as in Example 1.

[0083] Comparative Example 5 This comparative example is modified from the one disclosed in Example 1 as follows: Without adding ethylene-1-octene block copolymer, i.e. the matrix resin is only homopolymer polypropylene, in step S2, instead of using ethylene-1-octene block copolymer to prepare radiation sensitization masterbatch, triallyl isocyanurate and trimethylolpropane trimethacrylate are directly mixed with homopolymer polypropylene, and the remaining components and preparation methods are the same as in Example 1.

[0084] Comparative Example 6 This comparative example is modified from the one disclosed in Example 1 as follows: The dynamic radiation crosslinking extrusion in step S4 is omitted. That is, the mixture is directly added to a twin-screw extruder for melt extrusion. No electron beam irradiation window is set, no radiation crosslinking is performed, and the master sheet is directly calendered. The remaining components and preparation methods are the same as in Example 1.

[0085] Comparative Example 7 This comparative example is modified from the one disclosed in Example 1 as follows: The gradient irradiation crosslinking in step S5 is omitted, that is, the mother sheet obtained in step S4 is not subjected to low-energy electron beam double-sided irradiation, and directly enters step S6 for staged high-temperature foaming. The remaining components and preparation methods are the same as in Example 1.

[0086] Comparative Example 8 This comparative example is modified from the one disclosed in Example 1 as follows: Without adding EPDM rubber grafted with maleic anhydride, i.e. without preparing flame retardant synergistic masterbatch, diethylphosphinate aluminum, melamine polyphosphate, modified montmorillonite, calcium borate, homopolymer polypropylene, ethylene-1-octene block copolymer, rare earth β nucleating agent, antioxidant, etc. are directly mixed, and the remaining components and preparation methods are the same as in Example 1.

[0087] The performance of the cross-linked polypropylene foam materials obtained in Examples 1-3 and Comparative Examples 1-8 was measured, and the results are shown in Table 2.

[0088] Table 2 Performance of Crosslinked Polypropylene Foam Materials

[0089] As shown in Table 2, the cross-linked polypropylene foam materials in Examples 1-3 all achieved a limiting oxygen index ≥30%, UL94V-0, and volume resistivity ≥1.0×10⁻⁶. 15 It has excellent comprehensive properties, including Ω·cm, foaming ratio of 6 to 12 times, and closed-cell ratio of ≥90%.

[0090] Comparative Example 1, lacking aluminum diethylphosphonate and melamine polyphosphate, could not form a phosphorus-nitrogen intumescent flame retardant system. It could not construct an effective char layer by relying solely on modified montmorillonite and calcium borate. Its limiting oxygen index was only 19.5%, close to that of pure polypropylene, and it could not pass the UL94 test.

[0091] Comparative Example 2, lacking modified montmorillonite and calcium borate, lost its flame-retardant synergistic effect, resulting in a less dense char layer, a limiting oxygen index of only 26.3%, and a rating of only V-2. Simultaneously, its insulation performance decreased, with the volume resistivity dropping to 0.8 × 10⁻⁶. 15 Ω·cm.

[0092] Comparative Example 3, lacking triallyl isocyanurate and trimethylolpropane trimethacrylate, could not construct an effective cross-linking network, resulting in extremely low polypropylene melt strength, significant gas escape during high-temperature foaming, a foaming ratio of only 3 times, and a closed-cell rate of only 65%.

[0093] Comparative Example 4, due to the lack of rare earth β nucleating agent, the polypropylene crystallization behavior was not optimized, the number of cell nucleation points was small and uneven, the foaming ratio was reduced to 5 times, and the closed cell rate was 80%.

[0094] Comparative Example 5, due to the lack of ethylene-1-octene block copolymer, has insufficient matrix melt strength. Even with crosslinking, it is difficult to maintain the cell structure, with a foaming ratio of only 4 times and a closed-cell rate of 70%.

[0095] Comparative Example 6 omitted the dynamic radiation crosslinking extrusion step, resulting in a lack of pre-crosslinking network in the mother sheet. Subsequent gradient irradiation alone could not provide core melt strength, and the gas could not be encapsulated during foaming, resulting in a foaming ratio of only 2 times and a closed-cell rate of only 50%.

[0096] Comparative Example 7 omitted the gradient irradiation crosslinking step, resulting in the absence of a highly crosslinked dense layer on the surface. During foaming, the surface pores ruptured, and the closed-cell rate dropped to 75%.

[0097] Comparative Example 8, due to the lack of a maleic anhydride compatibilizer grafted onto EPDM rubber, exhibited poor interfacial compatibility between the flame retardant and the matrix, resulting in uneven dispersion, decreased flame retardant efficiency, a limiting oxygen index of only 25.6% reaching the V-1 level, and negatively impacted volume resistivity and foaming performance.

[0098] Figure 2 This is a SEM image of the cell structure of the cross-linked polypropylene foam material provided in Example 1 of this application.

[0099] Depend on Figure 2 It can be seen that the cross-linked polypropylene foam material in Example 1 has a uniform and dense closed-cell structure. The cells are mostly polygonal and regular in shape. The cell boundaries are clear and complete, the cell walls are smooth and continuous, and there are no obvious defects such as broken holes, cell merging and open holes. The cell size of the material is between 50 and 150 μm, and the pore size distribution is uniform and narrow.

[0100] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0101] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

[0102] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing an insulating and flame-retardant cross-linked polypropylene foam material, characterized in that, The method includes the following steps: S1. Aluminum diethylphosphinate, melamine polyphosphate, modified montmorillonite, calcium borate and some EPDM rubber grafted with maleic anhydride are mixed, melt-blended and granulated by a twin-screw extruder to obtain flame retardant synergistic masterbatch. S2. Part of the ethylene-1-octene block copolymer, triallyl isocyanurate and trimethylolpropane trimethacrylate are melt-blended and granulated in a mixer to obtain radiation sensitization masterbatch. S3. Mix homopolymer polypropylene, the remaining portion of the ethylene-1-octene block copolymer, the remaining portion of the EPDM-grafted maleic anhydride, rare earth β nucleating agent, antioxidant, the flame retardant synergistic masterbatch and the radiation sensitizing masterbatch, and then add azodicarbonamide to obtain a mixture. S4. The mixture is added to a twin-screw extruder, and an electron beam irradiation window is set in the middle and rear section of the extruder. Dynamic radiation crosslinking extrusion is carried out under nitrogen protection, and then calendered to obtain a master sheet with a thickness of 0.8 to 1.2 mm. S5. Irradiate one side of the master sheet with a low-energy electron beam and control the penetration depth to be 0.10-0.25 mm. Then irradiate the other side under the same conditions to perform gradient irradiation crosslinking on the master sheet to obtain a crosslinked master sheet. S6. The cross-linked masterbatch is subjected to staged high-temperature foaming to obtain the cross-linked polypropylene foam material.

2. The method according to claim 1, characterized in that, The cross-linked polypropylene foam material, by weight, is composed of the following raw materials. Composition: Homopolymer polypropylene: 40-60 parts, ethylene-1-octene block copolymer: 15-35 parts, EPDM-grafted maleic anhydride: 5-10 parts, rare earth β-nucleating agent: 0.5-1.5 parts, aluminum diethylphosphinate: 6-12 parts, melamine polyphosphate: 5-10 parts, modified montmorillonite: 2-5 parts, calcium borate: 1-3 parts, azodicarbonamide: 5-10 parts, antioxidant: 0.5-3 parts, triallyl isocyanurate: 2-4 parts, trimethylolpropane trimethacrylate: 1-2 parts.

3. The method according to claim 1, characterized in that, In step S1, the portion of EPDM rubber grafted with maleic anhydride is one-third of the total weight. The portion of ethylene-1-octene block copolymer mentioned in step S2 is one-third of the total weight.

4. The method according to claim 1, characterized in that, In step S4, the process parameters for the dynamic radiation crosslinking extrusion include: extruder feed section temperature of 140-150℃, compression section temperature of 150-155℃, metering section temperature of 150-155℃, die temperature of 145-150℃, screw speed of 200-300rpm, electron beam energy of 1.0-1.5MeV, and irradiation dose of 30-50kGy.

5. The method according to claim 1, characterized in that, In step S5, the energy of the low-energy electron beam is 150-200 keV, the single-sided irradiation dose is 8-12 kGy, and the mother film is placed on a circulating water-cooled plate during irradiation, with the surface temperature controlled below 50°C.

6. The method according to claim 1, characterized in that, In step S6, the specific process of the staged high-temperature foaming is as follows: first, preheat at 160-170℃ for 3-5 minutes, then foam at 190-210℃ for 4-8 minutes, and finally set at 180-190℃ for 2-3 minutes, followed by natural cooling.

7. The method according to claim 1, characterized in that, In step S3, the rare earth β nucleating agent is the rare earth compound WBGⅡ.

8. The method according to claim 1, characterized in that, In step S1, the modified montmorillonite is montmorillonite modified with octadecyltrimethylammonium chloride.

9. The method according to claim 1, characterized in that, In step S3, the antioxidant is a mixture of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl)phosphite in a 1:1 mass ratio.

10. An insulating and flame-retardant cross-linked polypropylene foam material prepared by the method according to any one of claims 1 to 9, characterized in that, The crosslinked polypropylene foam material satisfies the following properties: limiting oxygen index ≥ 30%, UL94 vertical burning rating is V-0, volume resistivity ≥ 1 × 10 15 Ω·cm, foaming ratio is 6-12 times, and closed cell ratio ≥ 90%. The crosslinked polypropylene foam material satisfies the following properties: limiting oxygen index ≥ 30%, UL94 vertical burning rating is V-0, volume resistivity ≥ 1 × 10 15 Ω·cm, foaming ratio is 6-12 times, and closed cell ratio ≥ 90%.

Images