Phosphinates and layered silicates in charring polymers: The flame retardancy action in polyurethane foams

It Takes Two to Tango: Synergistic Expandable Graphite–Phosphorus Flame Retardant Combinations in Polyurethane Foams

Due to the high flammability and smoke toxicity of polyurethane foams (PUFs) during burning, distinct efficient combinations of flame retardants are demanded to improve the fire safety of PUFs in practical applications. This feature article focuses on one of the most impressive halogen-free combinations in PUFs: expandable graphite (EG) and phosphorus-based flame retardants (P-FRs). The synergistic effect of EG and P-FRs mainly superimposes the two modes of action, charring and maintaining a thermally insulating residue morphology, to bring effective flame retardancy to PUFs. Specific interactions between EG and P-FRs, including the agglutination of the fire residue consisting of expanded-graphite worms, yields an outstanding synergistic effect, making this approach the latest champion to fulfill the demanding requirements for flame-retarded PUFs. Current and future topics such as the increasing use of renewable feedstock are also discussed in this article.

Clays as Inhibitors of Polyurethane Foams' Flammability

Polyurethanes are a very important group of polymers with an extensive range of applications in different branches of industry. In the form of foams, they are mainly used in bedding, furniture, building, construction, and automotive sectors. Due to human safety reasons, these applications require an appropriate level of flame retardance, often required by various law regulations. Nevertheless, without the proper modifications, polyurethane foams are easily ignitable, highly flammable, and generate an enormous amount of smoke during combustion. Therefore, proper modifications or additives should be introduced to reduce their flammability. Except for the most popular phosphorus-, halogen-, or nitrogen-containing flame retardants, promising results were noted for the application of clays. Due to their small particle size and flake-like shape, they induce a "labyrinth effect" inside the foam, resulting in the delay of decomposition onset, reduction of smoke generation, and inhibition of heat, gas, and mass transfer. Moreover, clays can be easily modified with different organic compounds or used along with conventional flame retardants. Such an approach may often result in the synergy effect, which provides the exceptional reduction of foams' flammability. This paper summarizes the literature reports related to the applications of clays in the reduction of polyurethane foams' flammability, either by their incorporation as a nanofiller or by preparation of coatings.

Stable electrically conductive, highly flame-retardant foam composites generated from reduced graphene oxide and silicone resin coatings

To acheive flexible polyurethane (PU) foam composites with stable electrical conductivity and high flame retardancy involved first coating of graphene oxide (GO) onto PU foam surfaces and then chemically reducing the GO with hydrazine to form reduced GO (RGO). The RGO-coated PU foam is then dipped into a solution containing silicone resin (SiR) and silica nano-particles and cured. The resulting composites (PU-RGO-SiR) show superior flame retardancy, thermal stability and mechanical stability relative to the PU starting materials or PU coated with either RGO or SiR alone. The electrical conductivity of the PU-RGO-SiR composites (as high as 118 S m^-1 at room temperature) could almost be retained but with small loss of 9.5% of the original value after 150 cyclic compression. When the samples were subjected to a temperature range from -50 to 400 °C, the electrical conductivity could remain constant at -50 °C, 25 °C, 100 °C, 200 °C, and even at 300 °C and 400 °C; the electrical-conductivity exhibited mild vibration but the vibration range was not beyond 5.6%. Flame retardancy tests show that the limiting oxygen index (LOI) increases from 14.7% for the pure foam to 31.5% for PU-RGO-SiR, and the PU-RGO-SiR composites exhibit a 65% reduction in the peak heat release rate (pHRR) and a 30% reduction in total smoke release (TSR). Thus, stable electrically conductive and highly flame-retardant foam composites have potential applications even in a variety of harsh conditions like high temperature, flame, organic solvents, and external compression.

Fire behavior of innovative alginate foams

A new biosourced composite foam (AF, associating foamed alginate matrix and orange peel filler) is successfully tested for fire-retardant properties. This material having similar thermal insulating properties and density than fire-retardant polyurethane foam (FR-PUF, a commercial product) shows promising enhanced properties for flame retardancy, as assessed by different methods such as thermogravimetric analysis (TGA), pyrolysis combustion flow calorimetry (PCFC) and a newly designed apparatus called RAPACES for investigating large-scale samples. All these methods confirm the promising properties of this alternative material in terms of fire protection (pHRR, THR, EHC, time-to-ignition, flame duration or production of residue), especially for heat flux not exceeding 50 kW m^-2. At higher heat flux (i.e., 75 kW m^-2), flame retardant properties tend to decrease but maintain at a higher level than FR-PUF. The investigation of the effect of AF thickness shows that the critical thickness (CT) is close to 1.5-1.7 cm: heat diffusion and material combustion are limited to the CT layer that protects the underlying layers from combustion. A multiplicity of factors can explain this behavior, such as: (a) negligible heat conduction, (b) low heat of combustion, (c) charring formation, and (d) water release. Water being released from underlying layers, dilutes the gases emitted during the combustion of superficial layers and promotes the flame extinction.

Fire Phenomena of Rigid Polyurethane Foams

Rigid polyurethane foams (RPUFs) typically exhibit low thermal inertia, resulting in short ignition times and rapid flame spread. In this study, the fire phenomena of RPUFs were investigated using a multi-methodological approach to gain detailed insight into the fire behaviour of pentane- and water-blown polyurethane (PUR) as well as pentane-blown polyisocyanurate polyurethane (PIR) foams with densities ranging from 30 to 100 kg/m³. Thermophysical properties were studied using thermogravimetry (TG); flammability and fire behaviour were investigated by means of the limiting oxygen index (LOI) and a cone calorimeter. Temperature development in burning cone calorimeter specimens was monitored with thermocouples inside the foam samples and visual investigation of quenched specimens’ cross sections gave insight into the morphological changes during burning. A comprehensive investigation is presented, illuminating the processes taking place during foam combustion. Cone calorimeter tests revealed that in-depth absorption of radiation is a significant factor in estimating the time to ignition. Cross sections examined with an electron scanning microscope (SEM) revealed a pyrolysis front with an intact foam structure underneath, and temperature measurement inside burning specimens indicated that, as foam density increased, their burning behaviour shifted towards that of solid materials. The superior fire performance of PIR foams was found to be based on the cellular structure, which is retained in the residue to some extent.

Molecular Firefighting-How Modern Phosphorus Chemistry Can Help Solve the Challenge of Flame Retardancy

The ubiquity of polymeric materials in daily life comes with an increased fire risk, and sustained research into efficient flame retardants is key to ensuring the safety of the populace and material goods from accidental fires. Phosphorus, a versatile and effective element for use in flame retardants, has the potential to supersede the halogenated variants that are still widely used today: current formulations employ a variety of modes of action and methods of implementation, as additives or as reactants, to solve the task of developing flame-retarding polymeric materials. Phosphorus-based flame retardants can act in both the gas and condensed phase during a fire. This Review investigates how current phosphorus chemistry helps in reducing the flammability of polymers, and addresses the future of sustainable, efficient, and safe phosphorus-based flame-retardants from renewable sources.

Lightweight and Ultrastrong Polymer Foams with Unusually Superior Flame Retardancy

High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame retardants are toxic to the environment and/or lack the physical properties required for use in many contemporary applications. Here, we show that isocyanate-based polyimide (PI) foam, a flammable material, can exhibit unusually superior flame retardancy as well as other excellent properties, such as being lightweight and displaying high mechanical strength, by incorporating red phosphorus (RP)-hybridized graphene. The covalent bonds formed between the graphene platelets and the PI matrix provide the resultant PI foam with a specific Young's modulus (83 kNm kg^-1) that is comparable to or even higher than those displayed by state-of-the-art foams, including silica aerogels, polystyrene foams, and polyurethane foams. In addition, even a low content of the RP-hybridized graphene (2.2 wt %) results in an exceptionally higher limiting oxygen index (39.4) than those of traditional flame-retardant polymer-based materials (typically 20-30). The resultant PI foam also exhibits thermal insulation properties that are similar to that of air. Moreover, the RP-hybridized graphene is prepared using a one-step ball milling process in 100% yield, and does not require solvent or produce waste. The preparation of the flame-retardant PI foams can be scaled as the starting materials are commercially available and the techniques employed are industrially compatible.

Formulation of intumescent flame retardant coatings containing natural-based tea saponin

Natural product tea saponin (TS), extracted from the nutshell of camellia (Camellia oleifera Abel, Theaceae), was introduced into intumescent flame retardant formulations as blowing agent and carbon source. The formulations of the flame retardant system were optimized to get the optimum proportion of TS, and intumescent flame retardant coatings containing tea saponin (TS-IFRCs) were then prepared. It was found that TS can significantly affect the combustion behavior and the thermal stability of TS-IFRCs evaluated by cone calorimetry and simultaneous thermal analyzer, respectively. It was shown that TS, degraded to water vapor and carbon at high temperatures, can combine with other components to form a well-developed char layer. The char layer was supposed to inhibit erosion upon exposure to heat and oxygen and enhance the flame retardancy of TS-IFRCs. In addition, the smoke release of TS-IFRCs was also studied, which provided a low amount of smoke production.