Mechanism of fire retardancy of polyurethanes using ammonium polyphosphate

A novel modified nano-alumina composite sol for potential application in forest firefighting

Herein, modified ammonium polyphosphate wrapped nano-alumina (mAPP@Als) was first synthesized and then dispersed in traditional fire extinguishing solution (FES) to fabricate a FES-mAPP@Als composite sol. It was found that the phosphorus-silica containing units were attached onto the nano-alumina surface, and the mAPP@Als particles showed excellent dispersion level in FES with a single-domain particle size distribution range. Due to the synergistic effects of the phosphorus-nitrogen and silica-alumina flame retardant components, FES-mAPP@Als (5% concentration) coated wood exhibited improved limiting oxygen index (33.2%) and carbonization ability, and depressed heat release (41.9%) and smoke production (10.7%), as compared to the pristine wood. In addition, the FES-mAPP@Als composite sol showed enhanced fire-extinguishing and anti-reignition capacities compared to the FES. This research offers a novel composite sol fire extinguishing agent for fighting forest fires.

An intumescent flame-retardant system based on carboxymethyl cellulose for flexible polyurethane foams with outstanding flame retardancy, antibacterial properties, and mechanical properties

A novel and eco-friendly intumescent flame-retardant system based on sodium carboxymethyl cellulose (CMC) was established for wide-used flexible polyurethane foams (FPUFs). FPUF-(APP₆CMC₁)GN₁ with extremely uniform coatings extinguished and reached the UL-94 V-0 rating, and presented an improvement of thermal insulation properties. Moreover, there was a 58 % reduction in peak heat release rate for FPUF-(APP₆CMC₁)GN₁ compared with that of FPUF, and the microstructure analysis of char residues indicated that a perfect intumescent char layer had formed on the surface of FPUFs. Especially, CMC and GN enhanced the compactness and stability of char layers. Therefore, little volatile production was generated under the protection of physical layers in the high temperature as evaluated during the thermal degradation processes. Meanwhile, the flame-retardant FPUFs remained the ideal mechanical properties and obtained excellent antibacterial properties, and the antibacterial rates of E.coli and S.aureus were 99.9 % (FPUF-(APP₆CMC₁)GN₁). This work provides an eco-friendlier strategy for the design of multi-function FPUFs.

Polyimide Copolymers and Nanocomposites: A Review of the Synergistic Effects of the Constituents on the Fire-Retardancy Behavior

Carbon-based polymer can catch fire when used as cathode material in batteries and supercapacitors, due to short circuiting. Polyimide is known to exhibit flame retardancy by forming char layer in condensed phase. The high char yield of polyimide is attributed to its aromatic nature and the existence of a donor–acceptor complex in its backbone. Fabrication of hybrid polyimide material can provide better protection against fire based on multiple fire-retardancy mechanisms. Nanocomposites generally show a significant enhancement in mechanical, electrical, and thermal properties. Nanoparticles, such as graphene and carbon nanotubes, can enhance flame retardancy in condensed phase by forming a dense char layer. Silicone-based materials can also provide fire retardancy in condensed phase by a similar mechanism as polyimide. However, some inorganic fire retardants, such as phosphazene, can enhance flame retardancy in gaseous phase by releasing flame inhibiting radicals. The flame inhibiting radicals generated by phosphazene are released into the gaseous phase during combustion. A hybrid system constituted of polyimide, silicone-based additives, and phosphazene would provide significant improvement in flame retardancy in both the condensed phase and gas phase. In this review, several flame-retardant polyimide-based systems are described. This review which focuses on the various combinations of polyimide and other candidate fire-retardant materials would shed light on the nature of an effective multifunctional flame-retardant hybrid materials.

Acoustic Performance and Flame Retardancy of Ammonium Polyphosphate/Diethyl Ethylphosphonate Rigid Polyurethane Foams

Flame-retardant water-blown rigid polyurethane foams (RPUFs) modified by ammonium polyphosphate (APP) and diethyl ethylphosphonate (DEEP) were synthesized by a one-pot free-rising method. We performed scanning electron microscopy (SEM), compression strength tests, acoustic absorption measurements and thermogravimetric analysis, as well as limited oxygen index, vertical burning and cone calorimeter tests to investigate the mechanical properties, acoustic performance and flame retardancy of the foams. SEM confirmed that the open-cell structures of the foams were successfully constructed with the introduction of a cell-opening agent. Upon using 20 php APP, the average acoustic absorption coefficient of the foam reached 0.535 in an acoustic frequency range of 1500-5000 Hz. The results of thermogravimetric analysis demonstrated that the incorporation of APP and DEEP can effectively restrain mass loss of RPUFs during pyrolysis. In particular, the compressive strength of a foam composite containing 5 php APP and 15 php DEEP increased to 188.77 kPa and the LOI value reached 24.9%. In a vertical burning test and a cone calorimeter test, the joint use of APP and DEEP endowed RPUFs with a V-0 rating and they attained a THR value of 23.43 MJ/m². Moreover, the addition of APP improved the acoustic absorption performance of the foam, verified by acoustic absorption measurements. Considering potential applications, the formulation containing 15 php APP and 5 php DEEP could be used in the preparation of a new flame-retardant acoustic absorption rigid polyurethane foam.

Burning Behaviour of Rigid Polyurethane Foams with Histidine and Modified Graphene Oxide

Since rigid polyurethane (PU) foams are one of the most effective thermal insulation materials with widespread application, it is an urgent requirement to improve its fire retardancy and reduce the smoke emission. The current work assessed the fire behavior of PU foam with non-halogen fire retardants system, containing histidine (H) and modified graphene oxide (GO_A). For investigated system, three loadings (10, 20, and 30 wt.%) were used. The Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis, cone calorimetry (CC) and smoke density chamber tests as well as pre- and post-burning morphological evaluation using scanning electron microscope (SEM) were performed. Moreover, TGA combined with FT-IR was conducted to determine the substances, which could be evolved during the thermal decomposition of the PU with fire retardant system. The results indicated a reduction in heat release rate (HRR), maximum average rate of heat emission (MAHRE), the total heat release (THR) as well as the total smoke release (TSR), and maximum specific optical density (Ds_max) compared to the polyurethane with commercial fire retardant, namely ammonium polyphosphate (APP). A significantly improvement, especially in smoke suppression, suggested that HGO_A system may be a candidate as a fire retardant to reduce the flammability of PU foams.

Influence of Sepiolite and Lignin as Potential Synergists on Flame Retardant Systems in Polylactide (PLA) and Polyurethane Elastomer (PUE)

A comparison of the influence of sepiolite and lignin as potential synergists for fire retardant (FR) systems based on ammonium polyphosphate (APP) has been carried out in polyurethane elastomer and polylactide. Different ratios of kraft lignin and sepiolite were tested in combination with APP in both polymers. The thermal stability and the fire behavior of the corresponding composites were evaluated using Thermogravimetric Analysis (TGA), a Pyrolysis Combustion Flow Calorimeter (PCFC) and Cone Calorimeter (CC). The mechanisms of flame retardancy imparted by APP and other components were investigated. Synergistic effects were highlighted but only for specific ratios between APP and sepiolite in polyurethane elastomer (PUE) and polylactide (PLA) on one hand, and between APP and lignin in PLA on the other hand. Sepiolite acts as char reinforcement but through the formation of new phosphorus compounds it is also able to form a protective layer. Conversely, only complementary effects on fire performance were noted for lignin in PUE due to a dramatic influence on thermal stability despite its action on char formation.

Highly Effective Flame-Retardant Rigid Polyurethane Foams: Fabrication and Applications in Inhibition of Coal Combustion

The extemporaneous combustion of coal remains a major threat to safety in coal mines because such fire accidents result in casualties and significant property loss, as well as serious environmental pollution. This work proposed the fabrication of flame-retardant rigid polyurethane foam (RPUF) containing expandable graphite as char expander/sealant with melamine phosphosphate and 2-carboxyethyl (phenyl)phosphinic acid as char inducer and radical trapping agents. The as-prepared RPUF successfully inhibited coal combustion by forming thermally stable high graphitic content expandable intumescent char sealing over the coal. The RPUF achieved UL-94 V-0 rating in addition to significant reductions in peak heat release, total heat release, and CO and CO₂ yields. The external and the internal residual char structure was studied by X-ray photoelectron spectra, Raman spectroscopy, and real-time Fourier transform infrared spectra techniques, and a flame-retardant mode of action has been proposed. This work provides important insight into a facile fabrication of highly efficient and economical flame-retardant RPUF to inhibit the spontaneous combustion of coal.

Graphene platelets versus phosphorus compounds for elastomeric composites: flame retardancy, mechanical performance and mechanisms

Polymers are widely used but their flammability remains a serious issue causing fatalities and property damage. Herein we present an investigation into the effectiveness of graphene platelets (GnPs) to simultaneously improve the flame retardancy and mechanical properties of ethylene propylene diene monomer rubber (EPDM). EPDM was melt compounded respectively with GnPs and a commercial flame retardant (ammonium polyphosphate, APP) to produce two groups of composites. Although both fillers were well dispersed in EPDM, GnPs significantly improved the mechanical properties of EPDM whilst APP compromised some of the mechanical properties particularly at high fractions. This difference was attributed to the filler particle size and interfacial bonding. Through cone calorimetry testing, 21 wt% char yield was recorded for the EPDM/GnP composite at 12.0 vol%, in comparison with 8 wt% for the EPDM/APP composite. APP was able to lower the peak heat release rate (PHRR) and specific mass loss rate (MLR), but unfortunately it decreased the ignition time and fire performance index (FPI). By contrast, GnPs has been found to increase ignition time by 29% and FPI by 62%, while still achieved the same level of reductions in PHRR and specific MLR, demonstrating clear advantages over APP. During combustion the highly thermally stable GnPs bonded with the viscous, degraded EPDM macromolecules, forming a thick solid char layer which prevented the transport of heat and smoke, contributing to its superior flame retarding performance over APP.

Synthesis of Phosphated K-Carrageenan and Its Application for Flame-Retardant Waterborne Epoxy

In this paper, phosphated K-carrageenan (P-KC) was obtained by reacting POCl₃ with the renewable source K-carrageenan (KC). P-KC and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were added into waterborne epoxy (EP) to improve its flame retardancy. The structure of P-KC was studied comprehensively using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermo-gravimetric analysis (TGA), showing the successful synthesis of P-KC. The flame retardancy of the EP was evaluated by the cone calorimeter test. The results showed that different mass ratios of DOPO and P-KC affected the flame retardancy of EP. When the mass ratio of DOPO and P-KC was 2:1, total heat release (THR) and total smoke production (TSP) decreased by 48.7% and 37.4%, respectively. The microstructures of residue char were observed by FTIR and scanning electron microscopy (SEM), indicating that the flame-retardant waterborne epoxy (FR-EP) system held a more cohesive and denser char structure. The char inhibited the diffusion of heat and oxygen, which played a key role in the flame retardancy.

Extreme Heat Shielding of Clay/Chitosan Nanobrick Wall on Flexible Foam

Flexible polyurethane foam (PUF) is widely used in bedding, transportation, and furniture, despite being highly flammable. In an effort to decrease the flammability of the polymer, an environmentally friendly flame retardant coating was deposited on polyurethane foam (PUF) via layer-by-layer assembly. Treated foam was subjected to three different fire scenarios, 10 s torch test, cone calorimetry, and a 900 s burn-through test, to evaluate the thermal shielding behavior of an eight bilayer chitosan/vermiculite clay nanocoating. In each fire scenario, the nanocoating acts as a thermal shield from the flames by successfully protecting the backside of the PUF, whereas the side directly exposed to the flame results in a hollowed nanocoating that maintains the complex three-dimensional porous structure of the foam. Cone calorimetry reveals that the coating reduces the peak heat release rate and total smoke release by 53 and 63%, respectively, whereas a temperature gradient greater than 200 °C is observed across a 2.5 cm thick coated foam sample during the rigorous burn-through fire test. The thermal shielding behavior of this polymer/clay nanocoating makes this system very attractive in improving the fire safety of polyurethane foam used for insulating applications.

A novel, low surface charge density, anionically modified montmorillonite for polymer nanocomposites

A new, low surface charge density, anionically modified montmorillonite (MMT) has been prepared and its polyurethane nanocomposites are prepared.

Study on application behavior of pyrolysis char from waste tires in silicone rubber composites

This work deals, in the first part, with the preparation and characterization of a type of pyrolysis char (PC) from waste tires. Best conditions were chosen to obtain the optimal PC with the largest Brunauer-Emmett-Teller (BET) surface area. The structure of PC was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The second part focuses on preparation of novel room-temperature vulcanizated silicone rubber (SR)/PC composites. Properties such as thermal stability, flame-retardance, tensile strength, and elongation at break were researched and compared. Flame-retardance test showed that the vertical burning time and limiting oxygen index (LOI) values of different SR composites were increased. The last part evaluates the reinforcing and flame-retardant mechanisms of PC in SR composites. Polar light microscopy (PLM) and SEM analysis demonstrated that the mixture of PC and ammonium polyphosphate (APP) had better compatibility in SR matrix.

Salen based Schiff bases to flame retard thermoplastic polyurethane mimicking operational strategies of thermosetting resin

Resorcinol based Salen framework as a non-phosphorus and non-halogen fire retardant additive for thermoplastic polyurethane (TPU).