Intelligent response type flame-retardant polypropylene foamed beads, and preparation method and application thereof

Abstract

This invention discloses a smart responsive flame-retardant polypropylene foamed beads, its preparation method, and its application, specifically relating to the field of flame-retardant polymer foam materials technology. The preparation method includes firstly, co-extruding a skin melt and a core melt into a composite die to form a core-skin structure melt strip with the core melt as the core and the skin melt as the skin. The core-skin structure melt strip is then pelletized to obtain composite PP substrate microparticles. These microparticles are then placed in an EPP autoclave foaming equipment, where they undergo impregnation and stirring, programmed pressure relief, crushing, centrifugal drying, and fluidized bed drying to obtain composite EPP foamed beads. This invention, through the synergistic design of core-skin functional partitioning and smart responsive components, transforms the traditional passive and dispersed flame-retardant mode into an active and centralized high-efficiency barrier mode, achieving intelligent and efficient flame retardancy.

Description

Technical Field

[0001] This invention relates to the field of flame-retardant polymer foam materials technology, specifically to a smart responsive flame-retardant polypropylene foam beads, its preparation method, and its application. Background Technology

[0002] Expanded polypropylene (EPP) beads are widely used in automotive, packaging, and energy storage safety fields due to their advantages such as light weight, high resilience, heat resistance, and recyclability. However, polypropylene is flammable and produces significant dripping during combustion, making it difficult to meet high-level flame retardant requirements (such as UL-94 V-0), which limits its application in high-safety scenarios.

[0003] Traditional methods achieve flame retardancy by directly blending flame retardants into the polypropylene matrix. However, this homogeneous modification inevitably interferes with the resin's rheological behavior, leading to difficulties in foaming, uneven cell structure, and significant deterioration of mechanical properties. For example, although Chinese patents CN119613801A and CN113185777A improve the flame retardancy rating by compounding fillers or flame retardant systems, they still belong to the whole-component uniform dispersion mode and fail to resolve the inherent contradiction between performance and flame retardancy.

[0004] To overcome this bottleneck, recent studies have attempted to find solutions from the perspective of material structure design. For example, Chinese patent CN111073131A proposes a "post-coating" process: coating the surface of foamed EPP beads with a functional slurry containing flame retardants and conductive fillers to form a core-shell composite structure. This method effectively avoids interference from fillers on the internal cell structure by confining the functional components to the bead surface, thus endowing the material with both flame retardant and antistatic functions while better preserving the foaming ratio and mechanical properties of the EPP itself. However, the coating layer constructed by this patent is essentially a static, non-responsive functional layer. Its flame retardant components are already solidified on the bead surface under normal conditions, unable to detect external fire signals, and lack the ability to be triggered and rapidly form a dense, continuously expanding char layer in the early stages of a fire (on a timescale of seconds). Furthermore, this "post-coating" process relies on wet coating and drying steps, resulting in uneven coating thickness, weak interfacial bonding with the substrate, and poor batch stability, making it difficult to adapt to the needs of continuous industrial production of EPP.

[0005] Therefore, there is an urgent need to develop a new generation of flame-retardant EPP technology based on advanced structural design and response mechanism integration, so as to truly achieve a synergistic leap in safety performance and comprehensive physical properties. Summary of the Invention

[0006] Therefore, this invention provides a smart responsive flame-retardant polypropylene foamed beads, its preparation method, and its application to solve the problems in the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: According to a first aspect of the present invention, a method for preparing intelligent responsive flame-retardant polypropylene foam beads is provided, the method comprising: Step 1: Preparation of composite PP substrate microparticles The skin layer melt and the core layer melt are respectively introduced into a composite die for co-extrusion to form a skin-core structure melt strip with the core layer melt as the core and the skin layer melt as the skin. The skin-core structure melt strip is then pelletized to obtain composite PP substrate microparticles. Step 2: Preparation of composite EPP foamed beads Composite PP substrate microparticles are placed in an EPP autoclave foaming equipment, and after impregnation and stirring, programmed depressurization, crushing, centrifugal drying, and fluidized bed drying, composite EPP foam beads are obtained, which are intelligent responsive flame retardant polypropylene foam beads. The skin melt comprises a first polypropylene resin, a thermoresponsive polymer, and an intumescent flame retardant (IFR); the melting point or decomposition temperature of the thermoresponsive polymer is lower than that of polypropylene; the core melt comprises a second polypropylene resin and a flame retardant.

[0008] The intelligent responsive flame-retardant polypropylene foam beads of this invention feature a core-skin structure with different functional zones. The core layer uses polypropylene resin as a matrix and contains a basic flame retardant, primarily serving as a basic flame retardant, structural support, and foaming agent. The skin layer, as an intelligent responsive functional layer, uses polypropylene resin as a continuous phase and uniformly disperses three key components: a low-melting-point thermosensitive responsive polymer, an intumescent flame retardant (IFR), and a thermosensitive catalyst. Under normal conditions, this skin layer and core layer work together to ensure the material's processability and mechanical properties; upon contact with fire, it responds rapidly, triggering and completing a rapid expansion and charring reaction to generate a dense char layer for ultra-efficient barrier properties. Furthermore, due to the influence of the low-melting-point phase, the molding capability of the intelligent responsive flame-retardant polypropylene foamed parts is also significantly improved.

[0009] Furthermore, the thermosensitive polymer is a biodegradable polyester. As an example, one or more of polycaprolactone (PCL), polylactic acid (PLA), or poly-β-hydroxybutyric acid (PHB) are preferred. The thermosensitive polymer accounts for 10-40% of the mass of the skin melt. More preferably, it is polycaprolactone (PCL) because it has suitable compatibility with polypropylene and its melting point (approximately 60°C) best matches the typical activation temperature window of IFR, resulting in the most sensitive response. More preferably, its proportion is 20-35%. Within this range, sufficient liquid medium is provided to promote the full expansion of IFR into char during a fire, while avoiding excessive addition that would cause the skin to become too soft at room temperature, affecting the rigidity of the substrate beads and the foaming process.

[0010] While intumescent flame retardants (IFRs) are commonly used flame retardants, in this invention, the flame-retardant effect of the skin layer is not a simple reproduction of the known functions of IFRs themselves. Instead, it achieves a revolutionary improvement in the flame-retardant performance of IFRs through a core-skin structure design and a smart response chain integrating PCL melting, catalyst activation, and IFR expansion. In existing technologies, IFRs are static flame-retardant components. However, in this invention, the IFR is a key execution unit embedded within a dynamic emergency response system. The presence of this system allows the IFR to remain inert under normal conditions, without affecting material properties, while being instantly and efficiently detonated in a fire, producing a barrier effect far exceeding that of traditional applications.

[0011] In this invention, PCL, PLA, and PHB primarily function as thermosensitive responsive phases. Their role is not to provide intrinsic flame retardancy, but rather to rapidly melt in the early stages of a fire, providing a reaction medium and triggering environment for the IFR and catalyst in the skin layer, thus achieving emergency expansion. This is fundamentally different from the prior art where they are used as blended flame retardant components.

[0012] Furthermore, the intumescent flame retardant is a compound system of acid source, carbon source, and gas source. As an example, ammonium polyphosphate, pentaerythritol, and melamine are preferred. More preferably, the intumescent flame retardant (IFR) is a compound system of ammonium polyphosphate (APP), pentaerythritol (PER), and melamine (MEL). The preferred mass ratio is (2-3):1:1. This compound system has high char formation efficiency and good synergistic effect with thermosensitive polymers.

[0013] Furthermore, the skin melt also contains a thermosensitive catalyst, which is one or more of tin benzoic acid, p-toluenesulfonamide, chloroplatinic acid, or Lewis acids. As an example, tin benzoic acid is preferred. It exhibits extremely high catalytic efficiency for the APP-PER-MEL system, significantly reducing the activation energy of the char formation reaction and achieving a "second-level" rapid response.

[0014] This invention utilizes a thermosensitive catalyst, whose main function goes beyond simply using catalysts to improve the carbonization efficiency of IFR (Integrated Fusion Reactor). Instead, it verifies a smart, responsive core-skin structure and specifically screens and defines a class of thermosensitive catalysts for this structure. The core technical characteristic of this type of thermosensitive catalyst lies in its low-temperature, high-activity nature. It can be synchronously activated with the melting behavior of the low-melting-point polymer in the core layer, thereby triggering the expansion reaction of the IFR. This is fundamentally different in functional positioning and working mechanism from existing carbonization catalysts that activate slowly at higher temperatures.

[0015] The amount of the thermosensitive catalyst added accounts for 0.5-5% of the total mass of the skin melt. More preferably, the amount added is 1-2%. Excessive catalyst may lead to pre-decomposition during processing, while too little catalyst will not achieve a rapid catalytic effect.

[0016] The core layer melt comprises a second polypropylene resin and a flame retardant. Preferably, the flame retardant in the core layer melt is one of an intumescent flame retardant, a brominated flame retardant, or a phosphorus-nitrogen flame retardant, and its addition amount accounts for 15-35% of the total mass of the core layer melt. More preferably, its addition amount is 20-30%. This addition amount can provide sufficient basic flame retardancy while minimizing the impact on the strength of the core layer polypropylene melt, thereby ensuring a high foaming ratio. More preferably, to balance environmental protection and performance requirements, the core layer flame retardant is preferably a halogen-free intumescent flame retardant (IFR) system.

[0017] Furthermore, the flame retardant in the core melt is an intumescent flame retardant, a brominated flame retardant, or a phosphorus-nitrogen flame retardant, and its addition amount accounts for 15-35% of the total mass of the core melt. The intumescent flame retardant, brominated flame retardant, and phosphorus-nitrogen flame retardant mentioned in this invention are common classifications based on different flame retardant mechanisms or chemical compositions, and their categories do not overlap.

[0018] The addition of 15-35% flame retardant to the core layer in this invention is a key component of the "skin-core synergistic function" design concept, and its purpose and function are as follows: 1. Complementary functions enhance reliability: The outer layer serves as a rapid-response layer, while the core layer is intrinsically flame-retardant, forming a "second-level emergency response" that ensures high reliability and a V-0 rating under any fire conditions.

[0019] 2. Structural support to prevent failure: The core layer provides a heat-resistant layer for the expanded carbon layer formed by the skin layer, preventing the carbon layer from being burned through due to the pyrolysis of the substrate under prolonged flame exposure.

[0020] 3. Performance Optimization, Resolving Contradictions: This structural design cleverly confines the "high flame retardant content," which could potentially impair mechanical and foaming properties, to the core layer, thereby protecting the processability of the skin layer, the fusion of the beads, and the overall foaming capacity. Ultimately, while maintaining a total flame retardant content comparable to or even better than traditional methods, this invention, through structural innovation, simultaneously achieves ultra-high flame retardancy, high mechanical strength, and excellent foaming properties, resolving a long-standing technical contradiction in this field.

[0021] Furthermore, both the first and second polypropylene resins are high melt strength polypropylene (HMSPP). HMSPP has higher melt strength and strain hardening behavior, which is crucial for stabilizing the core-sheath co-extrusion process, inhibiting cell coalescence, and obtaining a uniform and fine cell structure, and is a prerequisite for preparing high-performance foamed beads.

[0022] Furthermore, in step one, the composite die head is a concentric cylindrical multi-layer composite die head; the co-extrusion process parameters are: the skin layer melt processing temperature is 150-190℃, and the core layer melt processing temperature is 180-220℃. More preferably, the skin layer processing temperature is 160-180℃ to ensure that the heat-sensitive component does not decompose; the core layer processing temperature is 190-210℃ to ensure sufficient plasticization.

[0023] Furthermore, in step one, the pelletizing method is underwater hot cutting at a water temperature of 30-60℃. More preferably, the water temperature is controlled at 40-50℃ to ensure a smooth and flat cut surface and prevent the core-skin structure from deforming or peeling during pelletizing.

[0024] The substrate microparticles are placed in a high-pressure foaming autoclave, a physical foaming agent is injected, and foaming is carried out under the target foaming conditions.

[0025] The physical blowing agent is an inert gas, selected from carbon dioxide (CO2), nitrogen (N2), or mixtures thereof. More preferably, carbon dioxide (CO2) is used as the blowing agent because it has high solubility in polypropylene, making it easier to obtain a higher foaming ratio and a more uniform cell structure.

[0026] The foaming impregnation pressure is 4.0-8.0 MPa. More preferably, the impregnation pressure is 4.5-6.0 MPa. Preferably, the foaming impregnation temperature is 140-165°C. More preferably, the impregnation temperature is 150-160°C. This temperature range is the suitable foaming window for polypropylene, ensuring that the substrate is sufficiently softened to accommodate the foaming agent while maintaining sufficient melt strength to inhibit cell collapse and coalescence. After impregnation, a rapid depressurization process is performed to obtain smart-responsive flame-retardant polypropylene foam beads. The smart-responsive flame-retardant polypropylene foam beads are then subjected to air pressure, steam molding, and drying to produce smart-responsive core-shell structure flame-retardant EPP molded products.

[0027] According to a second aspect of the present invention, a smart responsive flame-retardant polypropylene foamed bead has a continuous skin layer and a core layer, the skin layer comprising a thermo-responsive polymer and an intumescent flame retardant, and the melting initiation temperature of the skin layer being lower than the melting initiation temperature of the core layer; the thickness of the skin layer is 5-25 μm.

[0028] Skin thickness is a designable structural feature that is precisely controlled through the core process parameter of "skin / core extrusion rate ratio," rather than being simply proportional to the feed rate. The thickness parameter has a dual impact on material properties: 1. Flame retardant properties: The thickness directly affects the volume of the 'smart material' that can be used to form an expanded char layer during a fire, thus affecting the thickness and strength of the protective layer.

[0029] 2. Foaming and Basic Properties: Thickness is crucial in determining the foaming agent's retention capacity, the uniformity of cell nucleation, and the final foaming ratio. Simultaneously, as the outer shell of the beads, it profoundly affects the quality of the fusion interface between beads during steam molding, thus determining the mechanical strength of the final product.

[0030] According to a third aspect of the present invention, a molded article prepared from the above-mentioned smart-responsive flame-retardant polypropylene foam beads is provided, wherein the mold is prepared by steam molding of the smart-responsive flame-retardant polypropylene foam beads. The composite EPP beads are subjected to air pressure, steam molding, and drying to obtain a smart-responsive flame-retardant polypropylene foam molded article.

[0031] Specifically, when the molded product is exposed to an open flame or high temperature environment, its surface skin can quickly melt and catalyze the intumescent flame retardant to form a dense intumescent char layer. The flame retardant rating of the molded product reaches UL-94 V-0 level, and the peak heat release rate (PHRR) is reduced by more than 20% compared with conventional homogeneous foam materials with the same flame retardant content.

[0032] The present invention has the following advantages: This invention utilizes co-extrusion technology to separate the skin layer and core layer formulations. The core layer incorporates a high content of flame retardant, while the skin layer formulation includes a low-melting-point, heat-sensitive polymer, flame retardant, and a heat-sensitive catalyst. This improves the processing performance, foaming properties, and bead interface adhesion during steam molding. By leveraging the rapid expansion effect of the heat-sensitive polymer, catalyst, and intumescent flame retardant in the skin layer under open flame conditions, a robust intumescent carbon layer and excellent flame retardancy in the core layer are formed, resulting in EPP molded parts with a V0 flame retardancy rating. The flame-retardant EPP beads produced by this invention have a high bead ratio, high tensile strength in the EPP molded products, and a flame retardancy rating up to V0. The processing method is convenient and efficient, offering broad application prospects.

[0033] This invention transforms the traditional passive and dispersed flame-retardant mode into an active and centralized high-efficiency barrier mode through the synergistic design of core-skin functional zoning and intelligent response components, achieving intelligent and efficient flame retardancy. In the event of a fire, the skin can rapidly form a dense, expanded char layer within seconds, significantly reducing the peak heat release rate (PHRR) of the material by more than 20%, and even more than 30%, compared to traditional homogeneous flame-retardant foam materials, resulting in a qualitative leap in flame-retardant efficiency.

[0034] The co-extrusion-underwater pelletizing-autoclaving-steam molding process adopted in this invention consists of mature and precisely controllable unit operations in the field of polymer processing. Through optimization and coordinated control of process parameters at each stage (such as extrusion temperature, foaming temperature / pressure / time, steam pressure, etc.), it is possible to stably and efficiently produce intelligent flame-retardant foamed beads and their molded products with consistent structure and excellent performance, providing a solid foundation for large-scale industrialization. Finally, the microparticles prepared by this invention exhibit excellent foaming performance: the unique core-skin structure design protects the rheological properties of the PP matrix, and combined with the optimized autoclaving process (preferably CO2 foaming agent, 150-160℃, 4.5-6.0MPa), the foaming ratio can easily reach over 20 times, achieving ultra-lightweight material with uniform and fine pores; the foamed beads possess superior fusion properties and mechanical strength: the continuous PP phase in the skin layer ensures excellent fusion performance of the foamed beads during molding. The steam molding process enables the final product to have mechanical properties such as compressive strength that are far superior to those of homogeneous flame-retardant EPP materials with the same overall flame retardant content. Steam-molded products have top-notch flame retardancy: the intelligent response mechanism ensures that the molded products can easily achieve the highest flame retardant rating of UL-94 V-0, and the peak heat release rate (PHRR) is significantly reduced.

[0035] This invention, through ingenious "structural design" and "material design," successfully solves the technical contradictions that have long plagued the field of flame-retardant foam materials, and provides a high-performance, high-safety, and industrially producible intelligent flame-retardant foam material solution with extremely broad application prospects.

[0036] This invention solves the efficiency bottleneck problem of passive flame retardant materials, achieving the intelligent effect of "no interference in normal times and strong protection in emergencies", and is widely used in aerospace, transportation and construction fields with extremely high safety requirements. Attached Figure Description

[0037] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0038] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0039] Figure 1 This is a physical image of a smart responsive flame-retardant polypropylene foam bead provided in Embodiment 1 of the present invention; Figure 2 This is an electron microscope image of a smart responsive flame-retardant polypropylene foam bead provided in Embodiment 1 of the present invention; Figure 3 This is an electron microscope image of a smart responsive flame-retardant polypropylene foam bead provided in Embodiment 2 of the present invention; Figure 4 This is an electron microscope image of a smart responsive flame-retardant polypropylene foamed bead provided in Embodiment 3 of the present invention. Detailed Implementation

[0040] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Polypropylene (PP): High melt strength polypropylene (HMSPP), grade HMS 20Z, Sinopec.

[0042] Thermosensitive polymers: polycaprolactone (PCL, melting point approximately 60°C); polylactic acid (PLA, grade 4032D).

[0043] Intumescent flame retardant (IFR): Ammonium polyphosphate (APP) / pentaerythritol (PER) / melamine (MEL) compound (mass ratio 3:1:1), commercially available.

[0044] Thermosensitive catalysts: tin benzoic acid (SnBz); p-toluenesulfonamide (TSA), commercially available.

[0045] Other flame retardants: decabromodiphenyl ethane (DBDPE); antimony trioxide (Sb2O3, compounded with DBDPE 3:1); magnesium hydroxide (MH), commercially available; MPP is melamine polyphosphate, commercially available.

[0046] Foaming agent: carbon dioxide (CO2) gas, commercially available.

[0047] Performance testing methods: Cell size and density: After taking cross-sectional images with a scanning electron microscope (SEM), statistical analysis was performed.

[0048] Tensile strength: Tested according to GB / T 10654 "Determination of tensile strength and elongation at break of flexible foam polymer materials" using a universal testing machine.

[0049] Apparent density: determined according to GB / T 6343-2009 "Determination of apparent density of foamed plastics and rubber".

[0050] Crust thickness: The cross-section of the beads was observed using a scanning electron microscope (SEM) to directly measure the crust thickness.

[0051] The vertical flammability rating is tested according to UL-94 standard.

[0052] A cone calorimeter (CONE, thermal radiation power 50kW / m²) was used. 2 Test combustion performance and record peak heat release rate (PHRR).

[0053] Test the compressive strength (10% deformation) according to ASTM D1622.

[0054] According to the formulations in Tables 1 and 2, the raw materials for the skin layer and core layer were mixed at high speed for 5 minutes, and then the mixtures were added to two twin-screw extruders respectively. The processing temperature for the skin layer was 150-190℃; the processing temperature for the core layer was 180-220℃. The melt was co-extruded through a concentric cylindrical composite die and pelletized underwater (water temperature 40℃) to obtain substrate microparticles. These substrate microparticles were then placed in an autoclave and impregnated to saturation at 4.5 MPa CO2 and 150℃, followed by programmed depressurization (depressurization rate 0.1-5 MPa / s) to foam, resulting in composite EPP foamed beads. Finally, the foamed beads were steam molded into standard specimens to obtain intelligent responsive flame-retardant polypropylene foamed beads. A physical image of the intelligent responsive flame-retardant polypropylene foamed beads obtained in Example 1 is shown below. Figure 1 As shown, the electron microscope image is as follows: Figure 2 As shown; the electron micrograph of the smart responsive flame-retardant polypropylene foam beads of Example 2 is shown below. Figure 3 As shown; the electron micrograph of the smart responsive flame-retardant polypropylene foam beads of Example 3 is shown below. Figure 4 As shown. By Figure 1 It can be seen that the intelligent responsive flame-retardant polypropylene foam beads prepared by autoclaving have a uniform and complete appearance and a smooth surface; the scanning electron microscope (SEM) cross-sectional images of the intelligent responsive flame-retardant polypropylene foam beads obtained in Examples 1-3 of this invention are shown below. Figure 2-4 .Depend on Figure 2-4 As can be seen, all beads exhibit a clear core-skin structure, with a uniform skin layer thickness that is tightly bonded to the core layer without obvious interface separation. The cell structure is uniform and fine, predominantly closed-cell.

[0055] Table 1. Formulations (parts by weight) and process parameters for the examples

[0056] Table 2 Comparative Formulations (parts by mass) and Process Parameters

[0057] Experimental Example 1 The smart responsive flame-retardant polypropylene foam beads obtained in Examples 1-8 and Comparative Examples 1-10 were steam-molded into standard specimens and their performance was tested. The results are shown in Table 3.

[0058] Table 3 Performance Tests of Examples and Comparative Examples

[0059] Note: "Skin thickness" refers to the thickness of the functionalized skin layer of the co-extruded foamed beads; Comparative Examples 1 and 6 have homogeneous structures with no clear skin-core layering; the skin layers of Comparative Examples 3 and 4 do not contain smart responsive components and cannot form an effective functional skin layer. The PHRR reduction ratio is based on Comparative Example 1 (traditional homogeneous flame retardant material) at 202 kW / m. 2 Calculations were performed using the baseline, and the complete combustion of Comparative Example 6 is not meaningful for comparison.

[0060] Performance testing revealed that all embodiments achieved a V-0 rating and a significant decrease in PHRR, demonstrating the success of the "PCL melting-catalyst activation-IFR expansion" smart response chain. However, Comparative Example 2 (no catalyst) and Comparative Example 5 (no IFR in the skin layer) showed a significant performance decline, proving that both the catalyst and IFR are necessary conditions for forming an effectively expanded char layer, and neither can be omitted. Furthermore, Comparative Example 4 (skin layer only PP) completely failed, further demonstrating the core function of the skin layer structure. Combining Example 1 and Comparative Example 1, it is evident that the core-skin structure offers significant advantages in protecting the matrix, exhibiting higher mechanical strength. Under identical molding conditions, the molded product of Example 1 of this invention has a significantly higher compressive strength than the homogeneous flame-retardant material of Comparative Example 1. This indicates that the core-skin structure design, while achieving an ultra-high flame-retardant rating, fundamentally improves the processing and bonding properties of the flame-retardant beads, thereby obtaining a final product with much higher mechanical strength. Furthermore, comparing with Comparative Example 8, it can be seen that Comparative Example 8 (no flame retardant core layer) only achieves a V-1 rating, proving that the core layer flame retardant provides important backup flame retardancy and burn-through resistance, complementing the "emergency response" function of the skin layer. Without the flame retardant support of the core layer, protection will inevitably fail. The selection and content of key components affect foaming and flame retardancy: Example 2 (high PCL) has the highest flame retardant efficiency but a slightly lower foaming ratio; Example 3 (low PCL) has a high foaming ratio but a slightly lower flame retardant efficiency, therefore the content of the heat-sensitive polymer needs to be selected according to the actual scenario. Example 4 (TSA) is effective, but its overall performance is slightly inferior to Example 1 (SnBz), indicating that the choice of catalyst type affects the intensity of the reaction. Examples 6 and 8 (halogen-free) can achieve V-0, but the smoke release is large (the bromine-antimony system of Comparative Example 5 has huge smoke release). Example 7 (MH) has good smoke suppression effect but poor char formation, resulting in a decrease in the flame retardant rating. In summary, Examples 1, 3, 6, and 8 demonstrate the best overall performance balance, achieving V-0 flame retardancy and a significant reduction in PHRR (>24%) while maintaining a high foaming ratio (>20 times) and good mechanical strength (>170 kPa). Comparative Example 7 (excess PCL) shows that excessively pursuing flame retardancy efficiency at the expense of the skin's mechanical integrity is undesirable and will lead to a loss of overall material strength.

[0061] In the intelligent system of this invention, without this thermosensitive catalyst with specific response characteristics, even with the presence of IFR and thermosensitive polymer, it is impossible to achieve second-level response and V-0 level high-efficiency flame retardancy. As shown in Example 1 and Comparative Example 9, the thermosensitive responsive polymer and the thermosensitive catalyst have a synergistic effect. This enables the skin IFR to rapidly char, achieving a transition from V-2 to V-0, and increasing the PHRR reduction from 5% to 28%. Without this intelligent response system, even with the skin-core structure retained, the flame retardant effect is only comparable to conventional homogeneous flame retardant materials, and the active flame retardant mode of this invention cannot be achieved. Comparative Example 10 demonstrates that the skin-core intelligent response system of this invention has excellent flame retardant compatibility. It can be flexibly selected according to environmental requirements, cost, or specific application scenarios (such as low smoke, halogen-free), while maintaining stable overall flame retardant performance.

[0062] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing intelligent responsive flame-retardant polypropylene foam beads, characterized in that, The method includes: Step 1: Preparation of composite PP substrate microparticles The skin layer melt and the core layer melt are respectively introduced into a composite die for co-extrusion to form a skin-core structure melt strip with the core layer melt as the core and the skin layer melt as the skin. The skin-core structure melt strip is then pelletized to obtain composite PP substrate microparticles. Step 2: Preparation of composite EPP foamed beads Composite PP substrate microparticles are placed in an EPP autoclave foaming equipment, and after impregnation and stirring, programmed depressurization, crushing, centrifugal drying, and fluidized bed drying, composite EPP foam beads are obtained. The skin melt comprises a first polypropylene resin, a thermo-responsive polymer, and an intumescent flame retardant; the core melt comprises a second polypropylene resin and a flame retardant.

2. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, In step one, the thermosensitive polymer is a biodegradable polyester, and the mass percentage of the thermosensitive polymer in the skin melt is 10-40%.

3. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, The intumescent flame retardant is a compound system of acid source, carbon source and gas source.

4. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, The skin melt also contains a thermosensitive catalyst, which is one or more of tin benzoic acid, p-toluenesulfonamide, chloroplatinic acid, or Lewis acid.

5. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, The flame retardant in the core melt is an intumescent flame retardant, a bromine-based flame retardant, or a phosphorus-nitrogen-based flame retardant, and its addition amount accounts for 15-35% of the total mass of the core melt.

6. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, Both the first polypropylene resin and the second polypropylene resin are high melt strength polypropylene.

7. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, In step one, the composite die head is a concentric cylindrical multi-layer composite die head; the co-extrusion process parameters are: the skin layer melt processing temperature is 150-190℃, and the core layer melt processing temperature is 180-220℃.

8. The method for preparing intelligent responsive flame-retardant polypropylene foam beads according to claim 1, characterized in that, In step one, the pelleting method is underwater hot cutting at a water temperature of 30-60℃.

9. A smart responsive flame-retardant polypropylene foamed bead prepared by any one of the preparation methods described in claims 1-8, characterized in that, The foamed beads have a continuous skin and a core layer. The skin layer contains a thermo-responsive polymer and an intumescent flame retardant, and the melting initiation temperature of the skin layer is lower than that of the core layer. The thickness of the skin layer is 5-25 μm.

10. A molded article prepared from the intelligent responsive flame-retardant polypropylene foam beads according to claim 9, characterized in that, The mold is prepared by steam molding of smart responsive flame-retardant polypropylene foam beads.

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