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

The study addresses the critical fire risk posed by flammable polymeric materials widely used in construction, transportation, and electronics. Conventional flame retardancy relies on incorporating elements such as halogens, phosphorus, nitrogen, boron, and silicon, which often provide limited efficacy, require high loadings that degrade mechanical properties, and can increase smoke toxicity. Flexible polyurethane foam (FPUF), in particular, ignites readily, decomposes around 250 °C into flammable volatiles, and produces toxic gases (CO, NOx, HCN). Despite the fact that CO₂ is a well-known fire extinguishing agent, polymers typically release CO₂ too late and in insufficient quantity during combustion to confer flame retardancy. The authors propose altering polymer decomposition to trigger rapid, early-stage CO₂ release prior to intense decomposition and ignition, thereby improving flame retardancy and reducing toxic smoke without adding traditional flame-retardant elements.

Prior works highlight the limitations and environmental concerns of conventional flame retardants, including halogenated and phosphorus-based systems that can increase smoke toxicity and require high loadings to be effective. CO₂ has a century-long record as an effective fire suppressing agent, but self-generated CO₂ from polymers typically emerges late during combustion and is insufficient for self-extinguishing. Reports indicate that alkali metal salts can facilitate low-temperature pyrolysis and promote CO₂ generation during polymer or biomass processing. However, before this study, no instance had demonstrated a catalyst-enabled, smart, early-stage CO₂ release from polymers to enhance fire safety. The authors build on these insights to explore organic potassium carboxylates as nucleophilic catalysts to reprogram polyurethane decomposition.

- Materials and additives: A series of potassium salts (sulfate, carbonate, formate, acetate, oxalate, succinate, malate, tartrate) with varying nucleophilicity were evaluated. Polyether polyol EP-330N and MDI-2412 were used to fabricate FPUF. - Foam preparation: K-salts were pre-mixed into the polyether polyol, water, triethanolamine, catalysts (A-1, A33B), and surfactant (DC-2525). After vigorous mixing for 2 min, MDI was added and stirred for 5–8 s, then the mixture was free-rise foamed in molds and cured 72 h. All foams used NCO index 0.85 and density 60 ± 2 kg/m³. - Fire testing: - Limiting Oxygen Index (LOI) per ASTM D2863; specimen 10×10×150 mm; n=5. - UL 94-HB (horizontal burning) per ISO 9772-2020; specimen 150×50×13 mm; n=5. - Cone calorimetry per ISO 5660-1 at 25 kW/m²; specimen 100×100×25 mm; n=3. Time to ignition (TTI), Fire Performance Index (FPI), Fire Growth Index (FGI) were calculated. - Cone calorimeter coupled with FTIR to quantify gaseous species (CO₂ and combustible gases such as C₂H₆, C₃H₈, CH₂O, C₃H₄O). - Smoke density (ISO 5659-2) and smoke toxicity (FTT0095) tests; n=3. Concentrations of CO₂, HCN, NOx and Conventional Index of Toxicity (CIT) were recorded. - Thermal/pyrolysis analyses: - TGA under air (40–650 °C) at multiple heating rates (10–25 °C/min). Mass loss in 260–320 °C window and initial decomposition temperature T5% were determined. - TGA-FTIR to monitor evolved gases; CO₂ absorbance tracked at 2300–2400 cm⁻¹; cumulative intensity in 260–320 °C quantified (a.u./g). - TGA-MS to detect m/z 44 (CO₂), 2 (H₂), 18 (H₂O), and fragments indicating formaldehyde and acetaldehyde. - TGA-DSC to capture thermal events; comparison of exothermic peaks. - Kinetic analysis using model-free Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods to estimate apparent activation energy Ea over conversion α=0.2–0.8. - Mechanical characterization: - Tensile tests per ISO 1798:2008 (speed 500 mm/min, 250 N load cell) for stress, strain, and toughness (area under curve). - Compression/resilience via Instron (500 N load cell), 50% strain at 20 mm/min for 100 cycles; strength and deformation recovery rates calculated. - SEM of fracture surfaces (ZEISS Sigma 300, 3 kV) after gold coating to assess cell morphology and fracture features.

- Ultra-low loading K-salts drastically improve fire safety of FPUF: - LOI increased from 18% (pure) to 26.5% with only 1.05 wt% K-formate and to 26.0% with 1.31 wt% K-malate; samples self-extinguished without melt dripping and achieved UL 94-HB HF-1. - Cone calorimetry: TTI increased from 11 s (pure) to 102–105 s (K-formate/K-malate), a 927% increase, providing significantly more escape time. - Fire indices: FPI increased by ~600% and FGI decreased by ~40% with K-formate/K-malate, indicating lower ignitability and reduced early fire growth. - Early gas evolution and CO₂ dominance: - K-salt-filled FPUF showed notable pre-ignition mass loss (~8%) vs pure (~0.2%), associated with early CO₂ release in the 260–320 °C window. - CO₂ concentration during combustion increased by ~72% with K-salts, reaching high ppm levels; combustible gases were suppressed relative to pure FPUF. - Smoke toxicity (CIT) drastically reduced: Pure FPUF CIT=0.381; K-formate CIT=0.019 (−95%); K-malate CIT=0.053. HCN/NOx in smoke were also reduced. - Thermal/kinetic evidence for catalytic CO₂ generation: - TGA-FTIR showed accelerated urethane cleavage near 200 °C with strong CO₂ bands at 2300–2400 cm⁻¹. In 260–320 °C, cumulative CO₂ intensity was highest for K-formate (1912 a.u./g) and K-malate (1762 a.u./g). - TGA-MS detected concurrent CO₂ (m/z 44), H₂ (m/z 2), H₂O (m/z 18), and aldehyde fragments around ~300 °C, consistent with a hydrogen-rich, catalytic oxidation pathway to CO₂. - TGA-DSC: distinct exothermic peak near 300 °C for K-formate/K-malate vs ~350 °C for pure, indicating accelerated thermal oxidation. - Kinetic analysis: Apparent activation energy for thermal oxidative decomposition of polyether segments reduced from ~242 kJ/mol (pure) to 61 kJ/mol (K-formate) and 65 kJ/mol (K-malate) over α=0.2–0.8. - Structure–activity insight among K-salts: - Stronger nucleophiles (e.g., K-carbonate, K-formate) promoted earlier decomposition and higher CO₂ generation in 260–320 °C; steric hindrance and electron-withdrawing groups reduced activity (e.g., K-oxalate). Hydroxylated anions (malate, tartrate) can form potassium alcoholates with enhanced nucleophilicity. - Mechanical performance preserved or enhanced: - Tensile strength increased from 46 kPa (pure) to 105 kPa (+42%) with K-formate and to 67 kPa (+19%) with K-malate. - Tensile toughness improved to 52 kJ/m² (+108%) with K-formate and 35 kJ/m² (+40%) with K-malate. - Open-cell morphology maintained; fracture surfaces became rougher with more tortuous paths, indicating improved toughness. Compression resilience remained high after cyclic loading.

The findings demonstrate that introducing a minute amount of nucleophilic potassium salts into FPUF reprograms its thermal decomposition to release substantial CO₂ early (200–320 °C), before vigorous mass loss and ignition. This early, in situ CO₂ generation acts like a built-in fire-extinguishing mechanism, drastically delaying ignition (TTI), boosting LOI, enabling self-extinguishing behavior, and suppressing toxic smoke species (lower CIT, reduced HCN/NOx). Coupled TGA-FTIR/MS/DSC and kinetic analyses support a two-stage catalytic mechanism: (1) K-salt–promoted urethane cleavage to carbamic acid, decomposing to CO₂ and amines; (2) accelerated thermal oxidation of polyether chains via K-mediated radical/peroxide chemistry, forming esters and aldehydes that ultimately yield carboxylic acids and CO₂ (including Baeyer–Villiger oxidations). The large reduction in apparent activation energy and the concentrated CO₂ evolution between 260–320 °C rationalize the improved fire metrics (FPI↑, FGI↓). Importantly, this catalytic strategy avoids traditional flame-retardant elements (halogen, phosphorus), maintains foam morphology, and even enhances tensile strength and toughness at ultra-low loadings, highlighting its practical and environmental advantages. Given that many polymers comprise C/H/O backbones, analogous catalysts may extend this early-CO₂ strategy to broader polymer systems.

This work presents a green, catalytic approach to endow polymers—demonstrated on flexible polyurethane foam—with intrinsic fire safety by triggering early, pre-combustion CO₂ release using ultra-low loadings of nucleophilic potassium salts (e.g., formate, malate). The method yields self-extinguishing behavior (LOI ≥26%, UL 94-HB HF-1), a ~9× increase in time to ignition, markedly improved fire indices, and up to 95% reduction in smoke toxicity, while enhancing mechanical properties. Mechanistic studies establish a two-stage decomposition pathway and a significant drop in activation energy for thermal oxidation. The K-salts are inexpensive, readily available, and compliant with environmental regulations. Future research can explore catalyst generality across different polymer chemistries, optimization of anion/cation combinations for tailored nucleophilicity, long-term durability and aging under service conditions, and scale-up for industrial foam manufacturing.