Flame retardancy and mechanical properties of thermoplastic polyurethane/polyvinylidene fluoride composite materials

[Objective] This study investigates the influence of polyvinylidene fluoride(PVDF)content on the flame retardancy and mechanical properties of thermoplastic polyurethane(TPU)/PVDF composite materials, aiming to establish a balance between enhanced fire resistance and structural integrity for practical applications.[Methods] TPU/PVDF composite materials with varying PVDF mass fractions(0-50%)were fabricated via melt-blending at 185 ℃ using a twin roller mill, followed by hot pressing at 185 ℃ under 10 MPa.Microstructural features were analyzed using scanning electron microscopy(SEM), Fourier transform infrared spectroscopy(FTIR), and X-ray diffraction(XRD), while thermal stability was evaluated via thermogravimetric analysis(TGA).Flame retardancy was assessed through cone calorimetry test and char residue characterization(Raman, SEM).Mechanical performance was quantified using universal testing for tensile strength and elongation at break.[Results] Microstructurally, PVDF included progressive phase separation in TPU.When the PVDF mass fraction was low(10%-30%), PVDF was distributed in the form of micrometer sized droplets in TPU, and some wavy small protrusions appeared on the surface of TPU/PVDF composite materials.Furthermore, PVDF incorporation induced progressive surface roughening in brittle-fractured composites, culminating in macro-scale cracks and pores at higher loadings(40%-50% by mass).FTIR and XRD results showed that TPU/PVDF composite materials were successfully prepared, and hydrogen bonding was formed between TPU and PVDF.The TGA results revealed char residue rate increased with PVDF addition, rising by 2.43 times for the 50% PVDF composite material compared to pure TPU.The partial overlap between the decomposition peak of PVDF and the second stage decomposition peak of TPU implies that the two phases may undergo a synergistic crosslinking reaction at high temperatures, promoting the formation of a carbon layer.This indicates that PVDF can improve the thermal stability of TPU/PVDF composite material.Flame retardancy improved consistently:critical parameters decreased markedly in the 50% PVDF system, including peak heat release rate(PHRR, 55.1%), total heat release(THR, 40.3%), peak smoke production rate(PSPR, 36.4%), and total smoke production(TSP, 47.4%).Microstructural analysis revealed progressively denser and more continuous char layers with elevated PVDF mass fraction, accompanied by enhanced graphitization(as validated by Raman spectroscopy), which improved thermal shielding and oxygen barrier efficiency.Based on the above analysis, a possible flame-retardant mechanism for TPU/PVDF composite materials has been proposed.The fluorine-containing substances produced by the decomposition of PVDF reacted with the degradation products of TPU to form fluorine-containing carbides, which serve as the skeleton of the char layer to promote the rapid formation of a dense char layer and effectively block the transfer of smoke and heat.Meanwhile, the fluorine-containing free radicals released during the decomposition process of PVDF could capture the free radicals in the combustion chain reaction and interrupt the combustion reaction.In addition, PVDF also released non-combustible gases during the decomposition process, diluting the concentration of combustible gases during combustion.In summary, PVDF significantly enhanced the flame retardancy of TPU by catalyzing the formation of graphitized char layers, physical barrier effects, and free radical quenching.Mechanically, tensile strength exhibited non-monotonic behavior, initially increasing marginally at low PVDF mass fraction(10%)before declining sharply by 62.0% at 50% PVDF.In contrast, elongation at break deteriorated monotonically by 88.4% across the entire PVDF range, indicating embrittlement.[Conclusion] PVDF significantly enhances the flame retardancy of TPU composite materials by promoting dense, graphitized char formation, and suppressing heat and smoke emissions.However, excessive PVDF(>30% by mass)severely compromises mechanical performance due to interfacial incompatibility and stress-concentrating defects.The antagonistic relationship between fire resistance and mechanical integrity highlights a critical trade-off in composite design.An optimal PVDF mass fraction(20%-30%)balances moderate flame retardancy improvements(PHRR reduction of 15.3%-27.4%)with acceptable strength retention(64.2%-70.0% of pure TPU).Future studies should focus on compatibilizer integration or PVDF topological modification to mitigate phase separation and mechanical degradation at higher loadings.These findings provide actionable guidelines for tailoring TPU/PVDF composite materials to meet sector-specific requirements in fire-prone environments where material durability remains paramount.

【研究目的】本研究探讨聚偏氟乙烯（polyvinylidene fluoride, PVDF）含量对热塑性聚氨酯（thermoplastic polyurethane, TPU）/PVDF复合材料的阻燃性能与力学性能的影响，旨在为实际应用中实现增强耐火性能与结构完整性的平衡提供理论支撑。【研究方法】本研究通过熔融共混法制备PVDF质量分数范围为0~50%的TPU/PVDF复合材料：先于185℃下采用双辊混炼机进行混炼，随后在185℃、10MPa压力下进行热压成型。采用扫描电子显微镜（scanning electron microscopy, SEM）、傅里叶变换红外光谱（Fourier transform infrared spectroscopy, FTIR）以及X射线衍射（X-ray diffraction, XRD）对复合材料的微观结构进行表征；通过热重分析（thermogravimetric analysis, TGA）评估其热稳定性能；借助锥形量热测试及残炭表征（拉曼光谱、SEM）评价其阻燃性能；通过万能试验机测试拉伸强度与断裂伸长率，量化其力学性能。【研究结果】微观结构层面，TPU基体中PVDF相逐渐发生相分离：当PVDF质量分数较低（10%~30%）时，PVDF以微米级液滴形式分散于TPU基体中，且TPU/PVDF复合材料表面出现部分波浪状微小凸起；此外，PVDF的引入使脆断试样的表面逐渐粗糙化，当PVDF质量分数较高（40%~50%）时，试样表面最终出现宏观裂纹与孔隙。FTIR与XRD测试结果表明，本研究成功制备了TPU/PVDF复合材料，且TPU与PVDF之间形成了氢键作用。TGA测试结果显示，残炭率随PVDF添加量的增加而升高，PVDF质量分数为50%的复合材料残炭率较纯TPU提升2.43倍。PVDF的分解峰与TPU的第二阶段分解峰部分重叠，表明两相在高温下可能发生协同交联反应，促进炭层的形成，这说明PVDF能够提升TPU/PVDF复合材料的热稳定性能。阻燃性能随PVDF添加量增加持续提升：PVDF质量分数为50%的试样，其关键阻燃参数均显著下降，包括热释放速率峰值（peak heat release rate, PHRR，下降55.1%）、总热释放量（total heat release, THR，下降40.3%）、烟释放速率峰值（peak smoke production rate, PSPR，下降36.4%）以及总烟释放量（total smoke production, TSP，下降47.4%）。微观结构分析表明，随着PVDF质量分数升高，残炭层逐渐变得更加致密与连续，且石墨化程度提升（经拉曼光谱验证），这有助于增强热屏蔽与阻氧效率。基于上述分析，本研究提出了TPU/PVDF复合材料可能的阻燃机制：PVDF分解产生的含氟物质与TPU的降解产物反应生成含氟碳化物，作为炭层骨架促进致密炭层的快速形成，有效阻隔烟气与热量的传递；同时，PVDF分解过程中释放的含氟自由基可捕获燃烧链式反应中的自由基，中断燃烧反应；此外，PVDF在分解过程中还会释放不可燃气体，稀释燃烧过程中可燃气体的浓度。综上，PVDF通过催化石墨化炭层的形成、物理阻隔作用以及自由基淬灭作用，显著提升了TPU的阻燃性能。力学性能方面，拉伸强度呈现非单调变化规律：在PVDF质量分数较低（10%）时略有提升，当PVDF质量分数达到50%时，拉伸强度大幅下降62.0%；与之相反，断裂伸长率在整个PVDF质量分数范围内持续下降，降幅达88.4%，表明复合材料发生了脆化。【研究结论】PVDF通过促进致密石墨化炭层的形成、抑制热量与烟气释放，显著提升了TPU复合材料的阻燃性能。然而，当PVDF添加量过高（质量分数>30%）时，由于界面相容性不足与应力集中缺陷，复合材料的力学性能会严重受损。阻燃性能与结构完整性之间的拮抗关系，凸显了复合材料设计中关键的平衡取舍问题。最优的PVDF质量分数范围为20%~30%，该比例下阻燃性能可获得适度提升（PHRR下降15.3%~27.4%），同时力学强度保留率可达纯TPU的64.2%~70.0%。未来的研究可聚焦于引入相容剂或对PVDF进行拓扑改性，以缓解高添加量下的相分离与力学性能退化问题。本研究结果可为定制化制备TPU/PVDF复合材料提供可落地的指导方案，以满足易火灾环境下对材料耐久性有较高要求的行业特定需求。