Catalytic polymer self-cleavage for CO₂ generation before combustion empowers materials with fire safety

Polymeric materials are ubiquitous but highly flammable, posing considerable risks to life and property. Annual fire-related losses are estimated at approximately 1% of global GDP, with substantial human and economic costs. Traditional fire safety strategies rely on flame retardants like halogens, phosphorus, nitrogen, boron, and silicon, which are integrated into polymers. However, these methods have drawbacks. Firstly, they primarily address combustion and cracking, resulting in limited fire resistance. Secondly, high retardant addition compromises material properties, especially mechanical strength. Thirdly, high-temperature decomposition often increases smoke toxicity. For flexible polyurethane foam (FPUF), achieving high fire safety (oxygen index ≥26%) using traditional methods is challenging. FPUF decomposes at around 250 °C, generating flammable gases and toxic byproducts like carbon monoxide (CO), oxides of nitrogen (NOx), and hydrogen cyanide (HCN). Existing flame retardants often exacerbate smoke toxicity. Thus, there's a strong need for innovative, eco-friendly solutions to enhance the inherent fire safety of polymers without compromising their essential properties. Carbon dioxide (CO₂), a proven fire suppressant, is generated during polymer combustion. However, its effectiveness is limited by its late release and insufficient quantity. The key concept is to modify polymer decomposition to release large amounts of CO₂ early. Alkali metal salts have been shown to aid in CO₂ release during low-temperature pyrolysis, but their application for fire safety hasn't been explored. This research introduces a catalytic approach where CO₂ is released proactively, creating a self-extinguishing effect and decreasing smoke toxicity.

The existing literature extensively covers traditional flame-retardant strategies for polymers, emphasizing the use of halogenated and phosphorus-based compounds. These studies often highlight the trade-off between fire safety and mechanical properties. The negative environmental impact of some flame retardants, especially halogenated ones, has also been widely documented. Some research explores the use of alternative flame retardants from bio-based sources. Existing research on alkali metal salts focuses on their roles in pyrolysis and recovery of polymers, indicating their potential for altering the decomposition process, and there's some evidence suggesting their potential to increase CO2 release. However, a direct application of these salts to create an internal fire extinguishing mechanism through catalytic CO2 generation has not been previously demonstrated.

The study employed a range of potassium salts (K-salts) with varying anions (sulfate, carbonate, formate, acetate, oxalate, succinate, malate, and tartrate) to investigate their catalytic effects on FPUF. These salts were pre-mixed with polyether polyols before FPUF foaming. The fire safety performance was evaluated using several methods: limiting oxygen index (LOI), horizontal burning test (UL 94-HB), cone calorimetry (TTI, FPI, FGI), and smoke toxicity tests. The thermal behavior of the modified FPUF was examined using thermogravimetric analysis (TGA) coupled with Fourier-transform infrared spectroscopy (FTIR), mass spectrometry (MS), and differential scanning calorimetry (DSC). The chemical mechanisms of CO₂ release were investigated using TGA-FTIR, TGA-MS, and TGA-DSC. The mechanical properties (tensile strength, toughness, compression resilience) of the modified FPUF were also assessed using standard testing methods. Scanning electron microscopy (SEM) was used to examine the microstructure of the samples. Kinetic parameters (apparent activation energy Ea) were calculated using Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods from TGA data. The specific formulations used in FPUF preparation are detailed in the Supplementary Information, along with control experiments using conventional flame retardants (expandable graphite and tris(1-chloro-2-propyl) phosphate).

The incorporation of K-salts, particularly K-formate and K-malate, significantly enhanced the fire safety of FPUF. Compared to pure FPUF (LOI = 18%), FPUF with 1.05 wt% K-formate exhibited an LOI of 26.5%, while FPUF with 1.31 wt% K-malate showed an LOI of 26.0%. Both K-formate and K-malate-filled FPUF achieved the highest rating (HF-1) in the UL 94-HB test without melting drippings, significantly outperforming conventional flame retardants at much lower loadings. Cone calorimetry revealed that the time to ignition (TTI) significantly increased for K-salt-filled FPUFs (927% increase for K-formate), and the fire performance index (FPI) improved while the fire growth index (FGI) decreased. A notable pre-ignition mass loss (up to 8%) was observed, with a significant release of CO₂ (>0.35 kg/kg) before ignition, unlike pure FPUF. FTIR analysis confirmed a significantly higher CO₂ concentration (72% increase) in the combustion gases compared to other gases. The smoke toxicity tests showed a 95% reduction in the general conventional index of toxicity. TGA-FTIR, TGA-MS, and TGA-DSC data elucidated the mechanism of CO₂ generation, involving the catalytic cleavage of urethane and polyether segments in FPUF, resulting in the accelerated decomposition of these chains, thereby improving the overall fire-retardant efficiency. The study found that potassium salts with stronger nucleophilicity exhibited greater catalytic activity in polyurethane decomposition. Kinetic studies showed that K-formate and K-malate significantly reduced the apparent activation energy (Ea) for the thermal oxidation decomposition of polyether segments. Furthermore, the mechanical properties of the FPUF were enhanced. The tensile strength improved (42% for K-formate, 19% for K-malate), and toughness increased significantly (108% for K-formate, 40% for K-malate), while retaining excellent compression resilience. SEM images showed good compatibility and dispersibility of K-salts within the FPUF matrix.

This study demonstrates a novel, green approach to fire safety in polymeric materials using catalytic polymer auto-pyrolysis. The key finding is that the proactive release of CO₂ before intense decomposition effectively self-extinguishes the fire. This contrasts with traditional flame retardants, which primarily act during combustion. The use of readily available and inexpensive potassium salts provides a cost-effective and environmentally friendly solution, avoiding the use of potentially harmful substances. The high catalytic efficiency of these salts ensures that significant fire safety benefits can be achieved with minimal additive loading, preserving the material's mechanical properties. The detailed mechanistic studies unveil the role of nucleophilicity and the stepwise decomposition of urethane and polyether segments, providing a comprehensive understanding of the enhanced fire safety. The results suggest a significant potential for expanding this approach to other polymer systems, given the widespread presence of carbon, hydrogen, and oxygen elements.

This research successfully introduces a novel catalytic flame-retardant strategy using potassium salts to trigger early CO₂ release in polyurethane foam, significantly enhancing fire safety without compromising mechanical properties or introducing harmful substances. This method offers a promising, sustainable solution for improving the fire resistance of various polymer materials. Future research could explore the applicability of this catalytic approach to other polymer types and investigate other potential catalysts for optimal performance and broader applicability across a variety of materials.

The current study focuses on flexible polyurethane foam. While the findings suggest broader applicability, further investigation is needed to confirm the effectiveness of this catalytic approach in other polymer types. The specific effects of varying potassium salt concentrations beyond the range tested here need additional exploration to optimize the balance between fire safety and material properties. The long-term stability of the modified FPUF under various environmental conditions should be further evaluated to assess its practical applicability.