A SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane and its preparation method

Abstract

The present application belongs to the field of high polymer materials, and particularly relates to a SiO2 synergistic bio-based PAPP / SPI intumescent flame-retardant thermoplastic polyurethane and a preparation method thereof. The present application combines pyrophosphoric acid piperazine and soybean protein isolate to construct a new type of intumescent flame retardant, and uses nano-silicon dioxide as a synergistic agent to melt blend with thermoplastic polyurethane to prepare SiO2 synergistic bio-based PAPP / SPI intumescent flame-retardant thermoplastic polyurethane. The flame-retardant thermoplastic polyurethane has obvious flame-retardant effect at a low SiO2 addition amount, and the mechanical properties are improved. In addition, the soybean protein isolate used is derived from soybeans and belongs to renewable raw materials, and is a green and environmentally friendly high-efficiency flame retardant. The present application provides a potential direction for preparing intumescent flame-retardant thermoplastic polyurethane with higher flame-retardant performance and excellent mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials, and specifically relates to a SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane and its preparation method. Background Technology

[0002] Thermoplastic polyurethane (TPU) is one of the most widely used plastics in the modern world. As a special material combining high elasticity and easy processing, it is widely used in automotive parts, electronic equipment, wires, and cables due to its good mechanical properties, abrasion resistance, and biocompatibility. However, TPU has a limiting oxygen index (LOI) of approximately 22.1%, making it highly flammable and releasing large amounts of toxic fumes when burning, causing personal injury, property damage, and environmental pollution. Therefore, research on improving the flame retardant properties of thermoplastic polyurethane is of great significance.

[0003] Additive flame retardant technology is simple to implement, low in cost, and easy to scale up for industrial production, making it the most widely used method in the TPU flame retardant field. Intumescent flame retardants (IFRs) have become one of the most promising flame retardants in flame retardant modification due to their advantages of low toxicity, low smoke production, and high flame retardant efficiency. However, the high flame retardant efficiency of traditional IFRs often requires a high addition amount, which can easily lead to a decrease in the mechanical properties of the matrix material. Adding synergistic flame retardants can effectively solve the above problems.

[0004] Silica (SiO2) is an environmentally friendly inorganic flame retardant with good thermal stability. The coating layer formed by SiO2 in the condensed phase can isolate combustibles and effectively reduce the heat release rate of combustibles. Generally, SiO2 is often used in combination with other flame retardants as a synergist. In the prior art, patent CN119331353B discloses a nano-silica synergistic flame-retardant polypropylene composite material and its preparation method. This invention introduces the silane coupling agent KH-570 on the surface of nano-silica, endowing the modified nano-silica with good compatibility and enhanced flame-retardant synergy. Then, it works synergistically with phosphorus-nitrogen flame retardants containing double bond structures to achieve uniform dispersion and interface strengthening in the polypropylene matrix, significantly improving the flame-retardant performance and comprehensive mechanical properties of the composite material. Patent CN118271957B discloses a nano-silica synergistic flame-retardant modified coating and its preparation method. A phosphorus-containing flame retardant is grafted onto nano-silica surface-treated with a silane coupling agent, improving the interfacial compatibility between the nano-silica and the polyurethane matrix. Simultaneously, through the phosphorus-silica synergistic flame-retardant mechanism and the compounding with unmodified nano-silica, the flame retardancy, water resistance, and weather resistance of the coating are significantly improved. Although the technology of using nano-silica as a flame-retardant synergist has been extensively studied, conventional surface modification (such as silane coupling agent treatment) mainly improves the compatibility between silica and the polymer matrix and enhances the flame retardancy and mechanical properties of the composite material to a certain extent. However, excessive or poorly dispersed nano-silica is prone to agglomeration in the matrix, affecting the material's processability and the balance of final properties. Therefore, developing a flame-retardant system with both high-efficiency flame retardancy and good mechanical properties is of great significance for promoting the practical application of TPU in environmentally friendly polymer materials. Summary of the Invention

[0005] To address the existing problems, the present invention aims to provide a flame-retardant thermoplastic polyurethane composite material with high flame retardant properties and good mechanical properties, and its preparation method. Specifically, the intumescent flame-retardant system composed of the synergist SiO2, PAPP, and SPI in the present invention plays a good synergistic flame-retardant role.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention discloses a SiO2-synergistic bio-based PAPP / SPI intumescent flame-retardant thermoplastic polyurethane, with the following composition by mass percentage:

[0008] Thermoplastic polyurethane (TPU): 73-74%; Piperazine pyrophosphate (PAPP): 20.83%; Soy protein isolate (SPI): 4.17%; Flame retardant synergist SiO2: 1-2%;

[0009] The flame retardant synergist SiO2 is a hydrophilic fumed nano-silica.

[0010] Preferably, the SiO2-synergistic bio-based PAPP / SPI intumescent flame-retardant thermoplastic polyurethane has the following composition by mass percentage: thermoplastic polyurethane: 74%; piperazine pyrophosphate: 20.83%; soy protein isolate: 4.17%; flame retardant synergist SiO2: 1%.

[0011] This invention provides a method for preparing SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane, the specific steps of which are as follows:

[0012] (a) Set the mixing temperature of the internal mixer to 165℃-170℃, and start feeding after the temperature stabilizes.

[0013] (b) The dried thermoplastic polyurethane is slowly poured into the internal mixer through the feed port, and after melting, dried piperazine pyrophosphate, soy protein isolate and flame retardant synergist SiO2 are slowly added. The above mixture is fully stirred, extruded and mixed under the action of dual rotors.

[0014] (c) After running for 10 minutes, samples were taken to obtain SiO2 synergistic bio-based PAPP / SPI expandable flame retardant thermoplastic polyurethane composite material.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0016] (1) The process of this invention is simple and straightforward, and has environmental advantages. SPI is derived from soybeans, which are abundant, non-toxic, and biodegradable. Introducing it into materials will not cause environmental pollution and is in line with the concepts of green chemistry and sustainable development.

[0017] (2) PAPP, as a three-in-one system, is combined with SPI and SiO2 as a synergist to form a new expansion flame retardant system. After melt blending and mixing with TPU, it is pressed into tablets to prepare flame retardant TPU composite material. The preparation process is simple and efficient, and does not require complicated reaction conditions and reaction steps. It is a simple flame retardant preparation strategy.

[0018] (3) As a flame retardant synergist, SiO2 nanoparticles are embedded in the carbon layer structure and are well dispersed in the matrix to form a thicker, denser, and more thermally stable expanded carbon layer. It physically isolates oxygen and heat transfer and produces a significant synergistic enhancement and toughening effect with the PAPP / SPI flame retardant system, further enhancing the mechanical properties of TPU composite materials. Attached Figure Description

[0019] Figure 1The formulations included pure TPU (TPU-1), 75wt% TPU / 25wt% PAPP (TPU-2) (Example 1), and a mixture of 75wt% TPU / 20.83wt% PAPP / 4.17wt% SPI (TPU-3). Vertical combustion test diagrams for (Example 2), 74wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1wt%SiO2 mixture (TPU-4) (Example 3, 1SiO2 / IFR / TPU), 73.5wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1.5wt%SiO2 mixture (TPU-5) (Example 4, 1.5SiO2 / IFR / TPU), and 73wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 2wt%SiO2 mixture (TPU-6) (Example 5, 2SiO2 / IFR / TPU): a1 corresponds to TPU-1, a2 corresponds to TPU-2, a3 corresponds to TPU-3, a4 corresponds to TPU-4, a5 corresponds to TPU-5, and a6 corresponds to TPU-6.

[0020] Figure 2 The TG(a) and DTG(b) curves are for pure TPU (TPU-1), 75wt%TPU / 25wt%PAPP (TPU-2) (Example 1), 75wt%TPU / 20.83wt%PAPP / 4.17wt%SPI mixture (TPU-3) (Example 2), 74wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1wt%SiO2 mixture (TPU-4) (Example 3, 1SiO2 / IFR / TPU), 73.5wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1.5wt%SiO2 mixture (TPU-5) (Example 4, 1.5SiO2 / IFR / TPU), and 73wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 2wt%SiO2 mixture (TPU-6) (Example 5, 2SiO2 / IFR / TPU).

[0021] Figure 3 The HRR(a), THR(b), SPR(c) and TSR(d) curves are for pure TPU (TPU-1), 75wt%TPU / 25wt%PAPP (TPU-2) (Example 1), 75wt%TPU / 20.83wt%PAPP / 4.17wt%SPI mixture (TPU-3) (Example 2), and 74wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1wt%SiO2 mixture (TPU-4) (Example 3, 1SiO2 / IFR / TPU).

[0022] Figure 4 Carbon residue images for pure TPU (TPU-1), 75wt% TPU / 25wt% PAPP (TPU-2) (Example 1), 75wt% TPU / 20.83wt% PAPP / 4.17wt% SPI mixture (TPU-3) (Example 2), and 74wt% TPU / 20.83wt% PAPP / 4.17wt% SPI / 1wt% SiO2 mixture (TPU-4) (Example 3, 1SiO2 / IFR / TPU): (a1, a2): TPU-1; (b1, b2): TPU-2; (c1, c2): TPU-3; (d1, d2): TPU-4, where a1, b1, c1, and d1 are front views of carbon residue, and a2, b2, c2, and d2 are side views of carbon residue.

[0023] Figure 5 SEM images of pure TPU (TPU-1), 75wt%TPU / 25wt%PAPP (TPU-2) (Example 1), 75wt%TPU / 20.83wt%PAPP / 4.17wt%SPI mixture (TPU-3) (Example 2), and 74wt%TPU / 20.83wt%PAPP / 4.17wt%SPI / 1wt%SiO2 mixture (TPU-4) (Example 3, 1SiO2 / IFR / TPU): (a): TPU-1; (b): TPU-2; (c): TPU-3; (d1, d2): TPU-4. Detailed Implementation

[0024] Materials used in the following examples:

[0025] The TPU was purchased from Yangzi Petrochemical Co., Ltd.

[0026] PAPP was purchased from Wuhan Smike Biotechnology Co., Ltd.

[0027] SPI was purchased from Shanghai McLean Biotechnology Co., Ltd.

[0028] The SiO2 was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0029] The following examples illustrate the test methods for various properties of the materials obtained in the embodiments:

[0030] Flammability Performance UL94: According to the GB / T2408-2008 test standard, the vertical flammability level of UL-94 was measured using a vertical flammability tester (ZY6017), and the sample size was 130mm×13mm×3mm.

[0031] Limiting Oxygen Index (LOI): The limiting oxygen index (LOI) value was measured using a limiting oxygen index meter (ZY6155A) according to the test standard GB / T2406.2-2009. The sample size was 80mm×100mm×4mm.

[0032] Mechanical property testing: The tests were conducted according to GB / T1040-2006 standard. Tensile strength and elongation at break were tested using a general performance testing machine (CMT5004). The tensile speed was 100 mm / min, and the sample size was 125 mm × 10 mm × 4 mm.

[0033] Amount of residual material at 800℃: Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (NETZSCH TG 209 F1) under a nitrogen atmosphere, with a heating rate of 10℃ / min, a gas flow rate of 40mL / min, and a heating range of 40~800℃.

[0034] Flue gas emission: Combustion behavior was measured using a cone calorimeter (VOUCH 6810) according to ISO 5660 testing standard, with a thermal radiation power of 50 kW / m³. 3 The sample size is 100mm×100mm×6mm.

[0035] The present invention will be further described below with reference to specific embodiments.

[0036] Example 1 (TPU-2)

[0037] The flame-retardant thermoplastic polyurethane is composed of the following percentages by weight: TPU: 75%, PAPP (piperazine pyrophosphate): 25%. The preparation process is as follows:

[0038] (1) Place the weighed TPU and PAPP (piperazine pyrophosphate) in an oven at 70°C for 12 hours to dry and remove moisture.

[0039] (2) Set the mixing temperature of the internal mixer to 165℃-170℃. After the temperature stabilizes, start feeding.

[0040] (3) First, slowly pour the dried TPU into the mixer through the feed port. After melting, slowly add PAPP (piperazine pyrophosphate) and mix thoroughly under the action of dual rotors.

[0041] (4) After running for 10 minutes, a sample was taken to obtain flame-retardant thermoplastic polyurethane PAPP / TPU.

[0042] The formulation of the materials in this embodiment is shown in Table 1.

[0043] The properties of the material in this embodiment are shown in Table 2.

[0044] Example 2 (TPU-3)

[0045] The flame-retardant thermoplastic polyurethane is composed of the following percentages by weight: TPU: 75%, PAPP (piperazine pyrophosphate): 20.83%, SPI (soy protein isolate): 4.17%. The preparation process is as follows:

[0046] (1) Place the weighed TPU, PAPP (piperazine pyrophosphate) and SPI (soy protein isolate) in an oven at 70°C for 12 hours to dry and remove moisture.

[0047] (2) Set the mixing temperature of the internal mixer to 165℃-170℃. After the temperature stabilizes, start feeding.

[0048] (3) First, slowly pour the dried TPU into the mixer through the feed port. After melting, slowly add the well-mixed PAPP and SPI. Under the action of the dual rotors, stir and extrude to mix thoroughly.

[0049] (4) After running for 10 minutes, a sample was taken to obtain flame-retardant thermoplastic polyurethane IFR / TPU.

[0050] The formulation of the materials in this embodiment is shown in Table 1.

[0051] The properties of the material in this embodiment are shown in Table 2.

[0052] Example 3 (TPU-4)

[0053] The flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 74%, PAPP: 20.83%, SPI: 4.17%, SiO2: 1%. The preparation process is as follows:

[0054] (1) Place the above materials in an oven at 70°C for 12 hours to dry and remove moisture.

[0055] (2) Set the mixing temperature of the internal mixer to 165℃-170℃. After the temperature stabilizes, start feeding.

[0056] (3) First, slowly pour the dried TPU into the mixer through the feed port. After melting, slowly add the well-mixed PAPP, SPI and SiO2. Under the action of the dual rotors, stir and extrude to mix thoroughly.

[0057] (4) After running for 10 minutes, a sample was taken to obtain flame-retardant thermoplastic polyurethane 1SiO. 2 / IFR / TPU.

[0058] The formulation of the materials in this embodiment is shown in Table 1.

[0059] The properties of the material in this embodiment are shown in Table 2.

[0060] Example 4 (TPU-5)

[0061] The difference between Example 4 and Example 3 is that the flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 73.5%, PAPP: 20.83%, SPI: 4.17%, SiO2: 1.5%, and other components are the same as in Example 3, resulting in flame-retardant thermoplastic polyurethane 1.5SiO2. 2 / IFR / TPU.

[0062] The formulation of the materials in this embodiment is shown in Table 1.

[0063] The properties of the material in this embodiment are shown in Table 2.

[0064] Example 5 (TPU-6)

[0065] The difference between Example 5 and Example 3 is that the flame-retardant thermoplastic polyurethane is composed of the following mass percentages: TPU: 73%, PAPP: 20.83%, SPI: 4.17%, SiO2: 2%, and the rest is the same as in Example 3, resulting in flame-retardant thermoplastic polyurethane 2SiO2 / IFR / TPU.

[0066] The formulation of the materials in this embodiment is shown in Table 1.

[0067] The properties of the material in this embodiment are shown in Table 2.

[0068] Table 1 Formulation of Flame-Retardant Thermoplastic Polyurethanes in Examples 1-5

[0069] sample TPU / % PAPP / % SPI / % <![CDATA[SiO2 <!-- 4 -->]]> Pure TPU 100.00 0 0 0 Example 1 75.00 25.00 0 0 Example 2 75.00 20.83 4.17 0 Example 3 74.00 20.83 4.17 1 Example 4 73.50 20.83 4.17 1.5 Example 5 73.00 20.83 4.17 2

[0070] Table 2 Summary of performance test results for pure TPU and flame-retardant thermoplastic polyurethanes of Examples 1-5

[0071] Performance parameters Pure TPU Example 1 Example 2 Example 3 Example 4 Example 5 UL94 flammability rating NR V-1 V-0 V-0 V-0 V-0 Limiting Oxygen Index (LOI) (%) 22.1 31.2 30.8 33.6 31.6 33.1 Tensile strength (MPa) 15.43 10.74 24.37 38.13 29.21 25.91 Elongation at break (%) 462.30 218.90 186.42 1192.14 875.16 746.31 Amount of residual substance at 800℃ (%) 8.82 15.11 8.73 21.51 22.65 22.85 <![CDATA[Flue gas emission (m 2 )]]> 1111.34 373.81 226.95 299.41 - -

[0072] Table 2 shows that when PAPP is used alone, the LOI is significantly improved to 31.2%, but Example 1 still has a UL-94 rating of V-1 and dripping occurs. After introducing the intumescent flame retardant, the flame retardant's compatibility with the TPU matrix is ​​poor, affecting the distribution of the TPU molecular chains. Although the flame retardant performance of Example 2 is improved, the mechanical properties are significantly reduced. After introducing SiO2, the LOI of Examples 3, 4, and 5 are all improved while maintaining a V-0 rating without dripping, and the mechanical properties are also improved to varying degrees. When the SiO2 addition amount is 1wt%, Example 3 (1SiO2 / IFR / TPU) achieves the best flame retardant and mechanical properties, with an LOI value of 33.6%, classifying it as a non-combustible material. This indicates that SiO2 and the intumescent flame retardant system produce a good synergistic flame retardant effect, and the optimal addition amount is 1wt%.

[0073] Figure 1 The figures show the vertical burning test results for pure TPU and flame-retardant thermoplastic polyurethane (Examples 1-5). TPU-1 (pure TPU) had an LOI of only 22.1% and produced dripping during vertical burning, failing to self-extinguish, thus classifying it as a flammable material. Example 1 showed a significantly increased LOI of 31.2%, but the UL-94 rating was V-1, and dripping occurred. This indicates that when PAPP is used alone, the strength of the char layer formed during combustion is insufficient to completely suppress dripping and achieve rapid self-extinguishing. Example 2 showed an LOI of 30.8%, a flame-retardant rating of V-0, and no dripping. This indicates that when PAPP is compounded with SPI, the phosphorus-nitrogen synergistic effect promotes the formation of a denser expanded char layer, effectively blocking heat and oxygen, and preventing melt dripping, meeting the stringent requirements of V-0 rating for self-extinguishing time and no dripping. Example 3 showed an LOI of 33.6%, the highest among all samples, while maintaining a V-0 rating with no dripping. The results show that the addition of 1 wt% SiO2 effectively enhances the thermal stability of the char layer, enabling it to act as a barrier more effectively in the early stages of combustion. Although the LOI values ​​of Examples 4 and 5 are lower than those of TPU-4, they are still higher than those of TPU-3 (Example 2), and maintain a V-0 rating. This indicates that the addition of 1-2 wt% SiO2 can effectively synergize the effect, but excessive addition will inhibit the synergistic char formation behavior of the flame-retardant system, reducing the density and integrity of the expanded char layer.

[0074] Figure 2The TG(a) and DTG(b) curves are shown for pure TPU and flame-retardant thermoplastic polyurethane (Examples 1-5). As can be seen from the TG(a) curve, the initial decomposition temperatures of Examples 1-5 are all lower than those of TPU-1. TPU-1 begins to lose significant weight at 300℃, with the maximum weight loss stage corresponding to the thermal decomposition of the hard and soft segments; the char rate at 800℃ is only 8.82%. Example 1 has an earlier initial decomposition temperature, indicating that PAPP begins to decompose and exert its flame-retardant effect at a lower temperature. Simultaneously, the char rate significantly increases to 15.11%, indicating that PAPP effectively promotes char formation. The char rate of Example 2 is lower than that of TPU-2 because most of the SPI volatilizes during heating, and its low char formation characteristic reduces the overall char rate of the system. As the SiO2 addition increases from 1wt% to 2wt%, the char rate of Examples 3, 4, and 5 increases stepwise at 800℃. This is because SiO2 can act as a physical crosslinking point at high temperatures to participate in the construction of the char layer skeleton, making the char layer more robust and dense. As seen in the DTG(b) curves, TPU-1 exhibits a single, sharp main decomposition peak, corresponding to rapid polymer chain breakage and extremely low char residue. In Example 1, the decomposition peak temperature shifts significantly forward, and the decomposition process broadens; a new decomposition stage appears at 367.94℃, because the phosphorus-containing compounds generated by PAPP decomposition further promote char layer formation and gas-phase dilution. In Example 2, the curves generally shift towards lower temperatures, and the intensity of the main weight loss peak is significantly lower than that of TPU-1, indicating a slower thermal decomposition rate, improved thermal stability, and a more complex decomposition stage. After adding the synergist SiO2, the decomposition rates in Examples 3, 4, and 5 become smoother, reflecting the physical barrier and stabilizing effect of SiO2 on the thermal decomposition process.

[0075] Figure 3 The HRR(a), THR(b), SPR(c), and TSR(d) curves are shown for pure TPU and flame-retardant thermoplastic polyurethane (Examples 1-3). The pHRR of TPU-1 is 928.74 kW / m². 2 In Example 1, the pHRR decreased by 70.2% compared to TPU-1, demonstrating the significant effect of PAPP as a phosphorus-nitrogen flame retardant. In Example 2, the pHRR decreased by 74.0% compared to TPU-1, indicating a positive synergistic effect from the 5:1 blend, improving the efficiency of char formation in the condensed phase and dilution in the gas phase. In Example 3, the pHRR decreased by 70.7% compared to TPU-1, indicating that this flame retardant system has excellent heat release inhibition effects. The THR of Examples 1-3 decreased significantly compared to TPU-1, indicating that the flame retardant effectively reduced the total heat release during combustion. The THR of Example 3 was slightly higher than that of Example 2, but the overall thermal hazard remained at a low level. All of the above demonstrates that the addition of flame retardant significantly reduced both pHRR and THR in Examples 1-3, improving flame retardant performance. The pSPR value of TPU-1 was 0.10m. 2The pSPR of Examples 1-3 all decreased to 0.04 m² / s, a 60.0% reduction compared to TPU-1, indicating that the flame-retardant system can effectively suppress the smoke generation rate in the early stages of combustion. The TSR of Example 3 was only 299.41 m² / m², a 73.0% decrease compared to TPU-1, demonstrating the excellent synergistic effect of this flame-retardant system in promoting combustion and suppressing smoke. The results show that the flame retardant added to the flame-retardant composite material plays a flame-retardant role during the temperature rise process. Piperazine pyrophosphate (PAPP), as a phosphorus-nitrogen flame retardant, works synergistically with the condensed phase and the gas phase: upon thermal decomposition, it produces phosphoric acid or polyphosphoric acid, promoting the dehydration and char formation of the TPU matrix; at the same time, it releases nitrogen-containing inert gases (such as NH3), diluting oxygen and combustible gases, thereby simultaneously suppressing heat release and smoke release. When combined with SPI, the phosphorus-nitrogen synergistic effect is further stimulated. The nitrogen in SPI and the phosphorus in PPAP synergistically promote the formation of a denser and more complete char layer, effectively capturing smoke particles and reducing smoke generation. At the same time, it enhances the gas phase dilution effect, making combustion more complete and reducing smoke volume. SiO2 promotes char formation and produces a significant synergistic enhancement effect with the PAPP / SPI flame retardant system.

[0076] Figure 4 The images show the char residue of pure TPU and flame-retardant thermoplastic polyurethane (Examples 1-3). TPU-1 left only a very small amount of residue after combustion. Example 1 formed an expanded char layer of a certain thickness after combustion, with a porous structure on the surface due to the release of a large amount of gas. Example 2 showed increased expansion height and a larger char layer volume. The density and integrity of the char layer were significantly improved, with smaller and fewer surface pores, forming a relatively robust char barrier. This confirms the synergistic charring effect between PAPP and SPI, which is consistent with... Figure 3 The CCT analysis results of (c) and (d) are consistent. In Example 3, the expansion height of the char residue was reduced, the char layer surface was harder, the overall density was higher, and the structure was more compact. Furthermore, the char layer color was lighter than in Example 2, due to the embedding of white SiO2 particles. SiO2 reduces the generation of combustible smoke particles by promoting char formation, thus having a certain smoke-suppressing effect. The addition of SiO2 improved the mechanical properties and integrity of the char layer, making it less prone to cracking or pulverization; however, it reduced the air layer (pores) inside the char layer used for heat insulation, resulting in a decrease in overall heat insulation efficiency, which is consistent with… Figure 3 The CCT analysis results of (a) and (b) are consistent.

[0077] Figure 5SEM images of pure TPU and flame-retardant thermoplastic polyurethane (Examples 1-3) are shown. The char residue from TPU-1 exhibits a loose, amorphous structure, failing to form a covering and protective char layer framework. Example 1, compared to TPU-1, has a smoother surface and fewer pores. Example 2 shows a wrinkled, dense char layer surface, which is more compact, continuous, and uniform, providing better barrier properties. Example 3 has a smoother, more continuous char layer surface with numerous embedded nanoparticles. This reflects SiO2 acting as crosslinking points to participate in char formation and enhance the continuity of the char layer. The abundant SiO2 particles provide rich nucleation sites, allowing gas to foam simultaneously at more locations, resulting in an increased number of bubbles but smaller individual sizes, forming a finer microporous structure. At high resolution, the char layer framework is visible as a composite material composed of a carbon matrix and uniformly dispersed SiO2 nanoparticles. The two are tightly bonded with no significant detachment.

[0078] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane, characterized in that, The composition by weight percentage is as follows: thermoplastic polyurethane: 73-74%; piperazine pyrophosphate: 20.83%; Soy protein isolate: 4.17%; Flame retardant synergist SiO2: 1-2%.

2. The SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane as described in claim 1, characterized in that, The composition by weight percentage is as follows: thermoplastic polyurethane: 74%; piperazine pyrophosphate: 20.83%; soy protein isolate: 4.17%; flame retardant synergist SiO2: 1%.

3. The SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane as described in claim 1, characterized in that, The flame retardant synergist SiO2 is a hydrophilic fumed nano-silica.

4. A method for preparing the SiO2-synergistic bio-based PAPP / SPI expandable flame-retardant thermoplastic polyurethane as described in claim 1, characterized in that, The specific steps are as follows: (a) Set the mixing temperature of the internal mixer to 165℃-170℃ and wait for the temperature to stabilize; (b) The dried thermoplastic polyurethane is slowly poured into the internal mixer through the feed port, and after melting, dried piperazine pyrophosphate, soy protein isolate and flame retardant synergist SiO2 are slowly added. The above mixture is fully stirred, extruded and mixed under the action of dual rotors to obtain SiO2 synergistic bio-based PAPP / SPI expanded flame retardant thermoplastic polyurethane composite material.

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