Bi-phase flame-retardant effect of hexa-phenoxy-cyclotriphosphazene on rigid polyurethane foams containing expandable graphite

Carbon-based Flame Retardants for Polymers: A Bottom-up Review

This state-of-the-art review is geared toward elucidating the molecular understanding of the carbon-based flame-retardant mechanisms for polymers via holistic characterization combining detailed analytical assessments and computational material science. The use of carbon-based flame retardants, which include graphite, graphene, carbon nanotubes (CNTs), carbon dots (CDs), and fullerenes, in their pure and functionalized forms are initially reviewed to evaluate their flame retardancy performance and to determine their elevation of the flammability resistance on various types of polymers. The early transition metal carbides such as MXenes, regarded as next-generation carbon-based flame retardants, are discussed with respect to their superior flame retardancy and multifunctional applications. At the core of this review is the utilization of cutting-edge molecular dynamics (MD) simulations which sets a precedence of an alternative bottom-up approach to fill the knowledge gap through insights into the thermal resisting process of the carbon-based flame retardants, such as the formation of carbonaceous char and intermediate chemical reactions offered by the unique carbon bonding arrangements and microscopic in-situ architectures. Combining MD simulations with detailed experimental assessments and characterization, a more targeted development as well as a systematic material synthesis framework can be realized for the future development of advanced flame-retardant polymers.

Flame-Retardant and Smoke-Suppressant Flexible Polyurethane Foams Based on Phosphorus-Containing Polyester Diols and Expandable Graphite

A liquid-phosphorus-containing polyester diol, PPE, was prepared via condensation polymerization using commercial reactive flame retardant 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10-phospha-phenanthrene-10-oxide, adipic acid, ethylene glycol, and 1,4-butanediol. PPE and/or expandable graphite (EG) were then incorporated into phosphorus-containing flame-retardant polyester-based flexible polyurethane foams (P-FPUFs). The structure and properties of the resultant P-FPUFs were characterized using scanning electron microscopy tensile measurements, limiting oxygen index (LOI), vertical burning tests, cone calorimeter tests, thermogravimetric analysis coupled with Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Unlike the FPUF prepared using regular polyester polyol (R-FPUF), PPE increased the flexibility and elongation at break of the resultant forms. More importantly, the peak heat release rate (PHRR) and total heat release (THR) of P-FPUF were reduced by 18.6% and 16.3%, respectively, via gas-phase-dominated flame-retardant mechanisms, compared with those of R-FPUF. The addition of EG further reduced the peak smoke production release (PSR) and total smoke production (TSP) of the resultant FPUFs while increasing the LOI and char formation. Interestingly, it was observed that EG noticeably improved the residual quantity of phosphorus in the char residue. When the EG loading was 15 phr, the resulting FPUF (P-FPUF/15EG) attained a high LOI value (29.2%) and exhibited good anti-dripping performance. Meanwhile, the PHRR, THR, and TSP of P-FPUF/15EG were significantly decreased by 82.7%, 40.3%, and 83.4%, respectively, compared with those of P-FPUF. This superior flame-retardant performance can be attributed to the combination of the bi-phase flame-retardant behavior of PPE and condensed-phase flame-retardant characteristics of EG.

Oligoetherols and polyurethane foams based on cyclotriphosphazene of reduced flammability

New oligoetherols with based on cyclotriphosphazene ring were synthesized by functionalization of hexachlorocyclotriphosphazene with glycidol followed by reaction with ethylene glycol and glycerol. Oligoetherols were characterized by IR, 1H-NMR, and MALDI-ToF and hydroxyl number as well as physical properties like density, viscosity and surface tension. The oligoetherols were further converted into polyurethane foams. The rigid foams of enhanced thermal stability and considerably diminished flammability were obtained and their apparent density, water uptake and polymerization shrinkage, thermal conductivity coefficient and thermal stability were determined. The flammability of foams was studied by microcalorimetric methods, horizontal flaming test and oxygen index. The obtained polyurethane foams with incorporated cyclotriphosphazene ring are self-extinguishing.

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Bio-Based Rigid Polyurethane Foams Modified with C-MOF/MWCNTs and TBPBP as Building Insulation Materials: Synergistic Effect and Corresponding Mechanism for Enhancing Fire and Smoke Safety

Rigid polyurethane foams (RPUFs) as building insulation materials quickly burn and release a lot of heat, smoke, and carbon monoxide, and cause human safety risk and severe environmental pollution. To mitigate these disadvantages, MOF/MWCNTs were fabricated via mixing Cu ions' partly substituted framework of ZIF-67 and MWCNTs, and further calcinated MOF/MWCNTs (C-MOF/MWCTs) was newly generated by calcinating MOF/MWCNTs in air. Then, MOF/MWCNTs and C-MOF/MWCNTs were respectively employed together with a phosphorus-nitrogen-containing reactive flame retardant (TBPBP) to prepare renewable bio-based rigid polyurethane foam, including RPUF-T/MOF/MWCNTs 2 and RPUF-T/C-MOF/MWCNTs 2. The characterization results showed that RPUF-T/C-MOF/MWCNTs 2 had better performance than RPUF-T/MOF/MWCNTs 2 and neat RPUF. Compared to neat RPUF, the compressive strength, limiting oxygen index value, and the mass char residue in cone calorimetry test of RPUF-T/C-MOF/MWCNTs 2, respectively, were increased by 105.93%, 46.35%, and 347.32%; meanwhile, the total heat release rate, total smoke production, total carbon monoxide product, and total carbon dioxide product were reduced by 47.97%, 50.46%, 41.38%, 43.37%, respectively. This study provides a referable method for preparing RPUFs with good physical properties, fire, and smoke safety, which is favorable for human safety and environmental protection as new building insulation materials.

A Systematic Review and Bibliometric Analysis of Flame-Retardant Rigid Polyurethane Foam from 1963 to 2021

Flame-retardant science and technology are sciences developed to prevent the occurrence of fire, meet the needs of social safety production, and protect people's lives and property. Rigid polyurethane (PU) is a polymer formed by the additional polymerization reaction of a molecule with two or more isocyanate functional groups with a polyol containing two or more reactive hydroxyl groups under a suitable catalyst and in an appropriate ratio. Rigid polyurethane foam (RPUF) is a foam-like material with a large contact area with oxygen when burning, resulting in rapid combustion. At the same time, RPUF produces a lot of toxic gases when burning and endangers human health. Improving the flame-retardant properties of RPUF is an important theme in flame-retardant science and technology. This review discusses the development of flame-retardant RPUF through the lens of bibliometrics. A total of 194 articles are analyzed, spanning from 1963 to 2021. We describe the development and focus of this theme at different stages. The various directions of this theme are discussed through keyword co-occurrence and clustering analysis. Finally, we provide reasonable perspectives about the future research direction of this theme based on the bibliometric results.

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.

Chemically Accelerated Stabilization of a Cellulose-Lignin Precursor as a Route to High Yield Carbon Fiber Production

The production of carbon fiber from bio-based or renewable resources has gained considerable attention in recent years with much of the focus upon cellulose, lignin, and cellulose-lignin composite precursor fibers. A critical step in optimizing the manufacture of carbon fiber is the stabilization process, through which the chemical and physical structure of the precursor fiber is transformed, allowing it to withstand very high temperatures. In this work, thermogravimetric analysis (TGA) is used to explore and optimize stabilization by simulating different stabilization profiles. Using this approach, we explore the influence of atmosphere (nitrogen or air), cellulose-lignin composition, and alternative catalysts on the carbon yield, efficiency, and rate of stabilization. Carbon dioxide and water vapor released during stabilization are analyzed by Fourier transform infrared (FTIR) spectroscopy, providing further information about the stabilization mechanism and the accelerating effect of oxygen and increased char yield (carbon content), especially for lignin. A range of different catalysts are evaluated for their ability to enhance the char yield, and a phosphorus-based flame retardant (H₃PO₄) proved to be the most effective; in fact, a doubling of the char yield was observed.

Bioinspired, Highly Adhesive, Nanostructured Polymeric Coatings for Superhydrophobic Fire-Extinguishing Thermal Insulation Foam

Lightweight polymeric foam is highly attractive as thermal insulation materials for energy-saving buildings but is plagued by its inherent flammability. Fire-retardant coatings are suggested as an effective means to solve this problem. However, most of the existing fire-retardant coatings suffer from poor interfacial adhesion to polymeric foam during use. In nature, snails and tree frogs exhibit strong adhesion to a variety of surfaces by interfacial hydrogen-bonding and mechanical interlocking, respectively. Inspired by their adhesion mechanisms, we herein rationally design fire-retardant polymeric coatings with phase-separated micro/nanostructures via a facile radical copolymerization of hydroxyethyl acrylate (HEA) and sodium vinylsulfonate (VS). The resultant waterborne poly(VS-co-HEA) copolymers exhibit strong interfacial adhesion to rigid polyurethane (PU) foam and other substrates, better than most of the current adhesives because of the combination of interfacial hydrogen-bonding and mechanical interlocking. Besides a superhydrophobic feature, the poly(VS-co-HEA)-coated PU foam can self-extinguish a flame, exhibiting a desired V-0 rating during vertical burning and low heat and smoke release due to its high charring capability, which is superior to its previous counterparts. Moreover, the foam thermal insulation is well-preserved and agrees well with theoretical calculations. This work offers a facile biomimetic strategy for creating advanced adhesive fire-retardant polymeric coatings for many flammable substrates.

Enhancing the Fire Safety and Smoke Safety of Bio-Based Rigid Polyurethane Foam via Inserting a Reactive Flame Retardant Containing P@N and Blending Silica Aerogel Powder

Rigid polyurethane foams (RPUFs) are widely used in many fields, but they are easy to burn and produce a lot of smoke, which seriously endangers the safety of people's lives and property. In this study, tetraethyl(1,5-bis(bis(2-hydroxypropyl)amino)pentane-1,5-diyl)bis(phosphonate) (TBPBP), as a phosphorus-nitrogen-containing reactive-type flame retardant, was successfully synthesized and employed to enhance the flame retardancy of RPUFs, and silica aerogel (SA) powder was utilized to reduce harmful fumes. Castor oil-based rigid polyurethane foam containing SA powder and TBPBP was named RPUF-T45@SA20. Compared with neat RPUF, the obtained RPUF-T45@SA20 greatly improved with the compressive strength properties and the LOI value increased by 93.64% and 44.27%, respectively, and reached the V-0 rank of UL-94 testing. The total heat release (THR) and total smoke production (TSP) of RPUF-T45@SA20 were, respectively, reduced by 44.66% and 51.89% compared to those of the neat RPUF. A possible flame-retardant mechanism of RPUF-T45@SA20 was also proposed. This study suggested that RPUF incorporated with TBPBP and SA powder is a prosperous potential composite for fire and smoke safety as a building insulation material.

The Impact of Hemp Shives Impregnated with Selected Plant Oils on Mechanical, Thermal, and Insulating Properties of Polyurethane Composite Foams

Polyurethane (PUR) foams reinforced with 2 wt.% hemp shives (HS) fillers were successfully synthesized. Three different types of HS fillers were evaluated—non-treated HS, HS impregnated with sunflower oil (SO) and HS impregnated with tung oil (TO). The impact of each type of HS fillers on cellular morphology, mechanical performances, thermal stability, and flame retardancy was evaluated. It has been shown that the addition of HS fillers improved the mechanical characteristics of PUR foams. Among all modified series, the greatest improvement was observed after the incorporation of non-treated HS filler—when compared with neat foams, the value of compressive strength increased by ~13%. Moreover, the incorporation of impregnated HS fillers resulted in the improvement of thermal stability and flame retardancy of PUR foams. For example, the addition of both types of impregnated HS fillers significantly decreased the value of heat peak release (pHRR), total smoke release (TSR), and limiting oxygen index (LOI). Moreover, the PUR foams containing impregnated fillers were characterized by improved hydrophobicity and limited water uptake. The obtained results confirmed that the modification of PUR foams with non-treated and impregnated HS fillers may be a successful approach in producing polymeric composites with improved properties.

New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams

The effect of the addition of new flame retardant systems on the properties of rigid polyurethane (RPUF) foams, in particular, reduction in flammability, was investigated. The modification included the introduction of a flame retardant system containing five parts by weight of expanded graphite (EG) (based on the total weight of polyol), one part by weight of pyrogenic silica (SiO2) and an ionic liquid (IL): 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim] [BF4]), in an amount of 3:1 with respect to the weight of added silica. The kinetics of the synthesis of modified foams—including the growth rate and the maximum temperature—were determined and the physicochemical properties, such as the determination of apparent density and structure by optical microscopy, mechanical properties such as impact strength, compressive strength and, three-point bending test were determined. An important aspect was also to examine the thermal properties such as thermal stability or flammability. It has been shown that for rigid polyurethane foams, the addition of expanded graphite in the presence of silica and ionic liquid has a great influence on the general use properties. All composites were characterized by reduced flammability as well as better mechanical properties, which may contribute to a wider use of rigid polyurethane foams as construction materials.

Preparation of Flame-Retardant Rigid Polyurethane Foams by Combining Modified Melamine-Formaldehyde Resin and Phosphorus Flame Retardants

In this work, ethylene glycol-modified melamine-formaldehyde resin (EMF) was synthesized from ethylene glycol, paraformaldehyde, and melamine, and then rigid polyurethane foams (RPUFs) were prepared using EMF, polyols and polyisocyanate. The effects of ammonium polyphosphate (APP) and dimethyl methylphosphonate (DMMP) on the flame retardancy, mechanical properties, thermal stability, and morphology of the prepared RPUFs were studied. It is shown that the flame-retardant performance of EMF-filled RPUFs can be enhanced by the addition of APP and DMMP. Thus, APP and DMMP can synergistically improve the flame retardancy of RPUFs. APP has good smoke suppression, while DMMP can increase the total smoke production and CO/CO2 weight ratio during the combustion of RPUFs.

Effects of Chemically Treated Eucalyptus Fibers on Mechanical, Thermal and Insulating Properties of Polyurethane Composite Foams

In this work, rigid polyurethane (PUR) foams were prepared by incorporating 2 wt% of eucalyptus fibers. The eucalyptus fibers were surface-modified by maleic anhydride, alkali, and silane (triphenylsilanol) treatment. The impact of the modified eucalyptus fibers on the mechanical, thermal, and fire performances of polyurethane foams was analyzed. It was observed that the addition of eucalyptus fibers showed improved mechanical and thermal properties and the best properties were shown by silane-treated fibers with a compressive strength of 312 kPa and a flexural strength of 432 kPa. Moreover, the thermal stability values showed the lowest decline for polyurethane foams modified with the silane-treated fibers, due to the better thermal stability of such modified fibers. Furthermore, the flame resistance of polyurethane foams modified with the silane-treated fibers was also the best among the studied composites. A cone calorimetry test showed a decrease in the peak of heat release from 245 to 110 kW∙m⁻² by the incorporation of silane-treated fibers. Furthermore, total heat release and total smoke release were also found to decrease remarkably upon the incorporation of silane-treated fibers. The value of limiting oxygen index was increased from 20.2% to 22.1%. Char residue was also found to be increased from 24.4% to 28.3%. It can be concluded that the application of chemically modified eucalyptus fibers has great potential as an additive to incorporate good mechanical, thermal, and fire properties in rigid polyurethane foams.

Improving the flame retardancy of waterborne polyurethanes based on the synergistic effect of P-N flame retardants and a Schiff base

A novel reactive intumescent fire retardant hexa-[4-[(2-hydroxy-ethylimino)-methyl]-phenoxyl]-cyclotriphosphazene (HEPCP), containing both cyclotriphosphazene and Schiff base structures, is successfully prepared. The chemical structures of HEPCP and flame-retardant waterborne polyurethane (WPU) (FR-WPU) were characterized via³¹P, ¹H NMR and FT-IR. Thermogravimetric (TG) analysis showed that HEPCP exhibited excellent thermal stability and produced rich char residue under high temperature compared with the control sample. The Schiff base and cyclotriphosphazene had a synergistic effect on the WPU. Limiting oxygen index (LOI) values of up to 26.7% were recorded; the dripping behavior was simultaneously improved and achieved a V-1 rating in the UL-94 test by incorporating 0.5 wt% phosphorus. In contrast to the pure WPU, the peak heat release rate (pHRR) of the FR-WPU/HEPCP5 decreased by 43.8%. The char residues increased from 0.63% to 6.96%, and scanning electron microscopy (SEM) showed a relatively continuous and membranous substance, with few holes. The results of TGA-FIR, Py-GC/MS and SEM indicated that HEPCP displayed a fire-retardant mechanism in the condensed-phase. In addition, the thermomechanical behaviors and the mechanical properties indicated that both mechanical properties and T_gh increased.

A novel reactive intumescent fire retardant hexa-[4-[(2-hydroxy-ethylimino)-methyl]-phenoxyl]-cyclotriphosphazene (HEPCP), containing both cyclotriphosphazene and Schiff base structures, is successfully prepared.

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.

Excellent Fireproof Characteristics and High Thermal Stability of Rice Husk-Filled Polyurethane with Halogen-Free Flame Retardant

The thermal stabilities, flame retardancies, and physico-mechanical properties of rice husk-reinforced polyurethane (PU–RH) foams with and without flame retardants (FRs) were evaluated. Their flammability performances were studied by UL94, LOI, and cone calorimetry tests. The obtained results combined with FTIR, TGA, SEM, and XPS characterizations were used to evaluate the fire behaviors of the PU–RH samples. The PU–RH samples with a quite low loading (7 wt%) of aluminum diethylphosphinate (OP) and 32 wt% loading of aluminum hydroxide (ATH) had high thermal stabilities, excellent flame retardancies, UL94 V-0 ratings, and LOIs of 22%–23%. PU–RH did not pass the UL94 HB standard test and completely burned to the holder clamp with a low LOI (19%). The cone calorimetry results indicated that the fireproof characteristics of the PU foam composites were considerably improved by the addition of the FRs. The proposed flame retardancy mechanism and cone calorimetry results are consistent. The comprehensive FTIR spectroscopy, TG, SEM, and XPS analyses revealed that the addition of ATH generated white solid particles, which dispersed and covered the residue surface. The pyrolysis products of OP would self-condense or react with other volatiles generated by the decomposition of PU–RH to form stable, continuous, and thick phosphorus/aluminum-rich residual chars inhibiting the transfer of heat and oxygen. The PU–RH samples with and without the FRs exhibited the normal isothermal sorption hysteresis effect at relative humidities higher than 20%. At lower values, during the desorption, this effect was not observed, probably because of the biodegradation of organic components in the RH. The findings of this study not only contribute to the improvement in combustibility of PU–RH composites and reduce the smoke or toxic fume generation, but also solve the problem of RHs, which are abundant waste resources of agriculture materials leading to the waste disposal management problems.

Synergistic Effects of Flame Retardants on the Flammability and Foamability of PS Foams Prepared by Supercritical Carbon Dioxide Foaming

Halogen-free flame-retardant polystyrene (PS) foams prepared by supercritical carbon dioxide (SC-CO₂) foaming have been achieved. The flame-retardants include expandable graphite (EG) and melamine phosphate (MP), and their influence on the foamability, decomposition behavior, fire performance, and mechanical properties of PS foams were investigated. It has been shown that flame retardants can generate inert gases and catalyze the char formation from PS, and the formed thick char layer with a notable barrier property can greatly decrease the heat release of PS foams. The addition of triphenyl phosphate (TPP) or hexaphenoxycyclotriphosphazene (HPCTP), which acts as a flame-retardant plasticizer, can obviously improve the foamability and fire performance of the foams. TPP or HPCTP can generate active phosphorous species and phenoxyl radicals to enhance the gas phase flame-retardant effect; therefore, the flame-retarded PS foams (with 25 wt % MP/EG) achieve HF1 and V-0 ratings, with limiting oxygen index (LOI) values of 30.1 or 29.6%, respectively. The numerical assessment of synergistic effects of TPP and HPCTP on further enhancing flame retardancy of PS foams has been provided by the microcalorimeter (MCC) test. Further X-ray photoelectron spectroscopy (XPS) investigation on char residues of PS foams demonstrates the formation of the P-O-C and other stable structures.

Preparation and Characteristics of an Environmentally Friendly Hyperbranched Flame-Retardant Polyurethane Hybrid Containing Nitrogen, Phosphorus, and Silicon

The NCO functional group of 3-isocyanatoproplytriethoxysilane (IPTS) and the OH functional group of 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phospha-phenantbrene-10-oxide (DOPO-BQ) were used to conduct an addition reaction. Following completion of the reaction, triglycidyl isocyanurate (TGIC) was introduced to conduct a ring-opening reaction. Subsequently, a sol–gel method was used to initiate a hydrolysis–condensation reaction on TGIC–IPTS–DOPO-BQ to form a hyperbranched nitrogen–phosphorous–silicon (HBNPSi) flame retardant. This flame retardant was incorporated into a polyurethane (PU) matrix to prepare a hybrid material. Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), limiting oxygen index (LOI), UV-VIS spectrophotometry, and Raman analysis were conducted to characterize the structure and analyze the transparency, thermal stability, flame retardancy, and residual char to understand the flame retardant mechanism of the prepared hybrid material. After the flame retardant was added, the maximum degradation rate decreased from −36 to −17 wt.%/min, the integral procedural decomposition temperature (IPDT) increased from 348 to 488 °C, and the char yield increased from 0.7 to 8.1 wt.%. The aforementioned results verified that the thermal stability of PU can be improved after adding HBNPSi. The LOI analysis indicated that the pristine PU was flammable because the LOI of pristine PU was only 19. When the content of added HBNPSi was 40%, the LOI value was 26; thus the PU hybrid became nonflammable.

Structure and Flame-Retardant Actions of Rigid Polyurethane Foams with Expandable Graphite

In this paper, rigid polyurethane foams that were filled with expandable graphite (RPUF/EG) composites were prepared by the liquid blending method, and then the structure and flame retardancy performance of materials were investigated through optical microscope, scanning electron microscope, limit oxygen index, cone calorimeter, thermogravimetric analysis coupled to fourier transform infrared spectrum, and X-ray photoelectron spectroscopy. The results showed that a large number of EG could be good to the exhibition of flame retardancy of RPUF, where the optimal material was found at loading 15 phr EG that showed an increased limit oxygen index value and a decreased calorific or fuming value. TGA coupled FTIR and XPS revealed that EG could disassembled before RPUF under heating treatment, and it could form a pyknotic and enahnced residual carbon layer on RPUF surface after the fire, which restricted the transfer of gas, like oxygen or heat into PU matrix, finally resulting in the promotion of flame retardancy of RPUF.

Flame Retardant Behavior of Ternary Synergistic Systems in Rigid Polyurethane Foams

In order to explore flame retardant systems with higher efficiency in rigid polyurethane foams (RPUFs), aluminum hydroxide (ATH), [bis(2-hydroxyethyl)amino]-methyl-phosphonic acid dimethyl ester (BH) and expandable graphite (EG) were employed in RPUF for constructing ternary synergistic flame retardant systems. Compared with binary BH/EG systems and aluminum oxide (AO)/BH/EG, ATH/BH/EG with the same fractions in RPUFs demonstrated an increase in the limited oxygen index value, a decreased peak value of heat release rate, and a decreased mass loss rate. In particular, it inhibited smoke release. During combustion, ATH in ternary systems decomposed and released water, which captured the phosphorus-containing products from pyrolyzed BH to generate polyphosphate. The polyphosphate combined with AO from ATH and the expanded char layer from EG, forming a char layer with a better barrier effect. In ternary systems, ATH, BH, and EG can work together to generate an excellent condensed-phase synergistic flame retardant effect.

Influence of the Characteristics of Expandable Graphite on the Morphology, Thermal Properties, Fire Behaviour and Compression Performance of a Rigid Polyurethane Foam

Three types of expandable graphite (EG) differing in particle size and expansion volume, are compared as flame retardant additives to rigid polyurethane foams (RPUFs). In this paper we discuss microstructure, thermal stability, fire behavior, and compression performance. We find that ell size distributions were less homogeneous and cell size was reduced. Furthermore, thermal conductivity increased along with EG loading. Thermogravimetric analysis (TGA) showed that EG only increased residue yield differently. The results indicate that a higher expansion of EG increased the limiting oxygen index (LOI) value, whereas a bigger particle size EG improved the rating of the vertical burning test (UL94). Results from the cone calorimeter test showed that a bigger particle size EG effectively reduced peak of heat release rate (pHRR). Furthermore, a higher expansion, led to a decrease in smoke production (TSP). The combination of both characteristics gives extraordinary results. The physical–mechanical characterization of the EG/RPUF foams revealed that their compression performance decreased slightly, mostly due to the effect of a bigger size EG.

Flame-retardant and smoke-suppressant flexible polyurethane foams based on reactive phosphorus-containing polyol and expandable graphite

In this manuscript, flame-retardant and smoke-suppressant flexible polyurethane foams (FPUFs) were designed and synthesized based on novel liquid phosphorus-containing polyol named as PDEO and expandable graphite (EG). The reactive PDEO can be chemically added into the chain of FPUF, while expandable graphite was blended into the matrix of foam through foaming process. Benefitting from the incorporation of reactive PDEO with a long chain, the resultant FPUF containing EG exhibited considerable mechanical properties. More importantly, the synergistic effect of PDEO and EG can endow FPUF with great flame retardancy, anti-driping performances. Furthermore, the resultant FPUF/EG/PDEO foams exhibit considerable smoke suppression performances. The vertical burning test revealed that the FPUF containing 5 php PDEO and 10 php EG extinguished quickly without dripping and kept the original shape after removing the igniter. The cone calorimeter results demonstrated that the synergistic effect of PDEO and EG can effectively reduce the heat release rate (HRR) and total release rate (THR) of the composite foam. Remarkably, the smoke production release (SPR), total smoke production (TSP), light transmission and specific optical density results indicated significantly smoke-suppressant properties of the composite foam. The mechanism analysis confirmed that the synergistic effect of gas-condensed bi-phase action from PDEO and EG contributed the great flame retardation of the composite foam. This novel FPUF provides a promising strategy for producing the polymer foam with flame retardation, smoke suppression and anti-dripping performances.

Polyurethane Foams: Past, Present, and Future

Polymeric foams can be found virtually everywhere due to their advantageous properties compared with counterparts materials. Possibly the most important class of polymeric foams are polyurethane foams (PUFs), as their low density and thermal conductivity combined with their interesting mechanical properties make them excellent thermal and sound insulators, as well as structural and comfort materials. Despite the broad range of applications, the production of PUFs is still highly petroleum-dependent, so this industry must adapt to ever more strict regulations and rigorous consumers. In that sense, the well-established raw materials and process technologies can face a turning point in the near future, due to the need of using renewable raw materials and new process technologies, such as three-dimensional (3D) printing. In this work, the fundamental aspects of the production of PUFs are reviewed, the new challenges that the PUFs industry are expected to confront regarding process methodologies in the near future are outlined, and some alternatives are also presented. Then, the strategies for the improvement of PUFs sustainability, including recycling, and the enhancement of their properties are discussed.

The pyrolysis behaviors of phosphorus-containing organosilicon compound modified ammonium polyphosphate with different phosphorus-containing groups, and their different flame-retardant mechanisms in polyurethane foam

Two phosphorus-containing organosilicon compounds (PCOCs) with similar structure but different phosphorus-containing groups (phenyl phosphate group, PCOC1; phenylphosphoryl group, PCOC2) were synthesized. They were used to modify ammonium polyphosphate (APP), and the products obtained were coded as MAPP1 and MAPP2. Then MAPP1 and MAPP2 were respectively incorporated into low-density rigid polyurethane foam (LD-RPUF). The pyrolysis behavior of these two kinds of MAPP was investigated. Results showed that PCOC2, with the phenylphosphoryl group, induced the decomposition of APP, leading to early and rapid decomposition of MAPP2 with the release of NH₃ in a short time and the formation of crosslinked structure quickly. Simultaneously, the phosphorus of MAPP2 was all retained in the condensed phase. In contrast, PCOC1, with the phenyl phosphate group, also induced the decomposition of APP. However, not all the phosphorus-containing groups of MAPP1 were retained in the condensed phase; some of the phosphorus was released into the gas phase in the form of PO₂· and PO· free radicals. Evaluation of the flame-retardant effect by means of the cone calorimeter test demonstrated that MAPP2 had better flame-retardant properties in the LD-RPUF system, including the reduction of peak heat release rate, total heat release, and total smoke release. Moreover, the char yield of LD-RPUF/MAPP2 was more than that of LD-RPUF/MAPP1. Macro and micro photographs showed that MAPP2 can promote the LD-RPUF matrix to form an intumescent char layer with more complete and stable foam during the combustion process compared with MAPP1. Finally, a possible flame-retardant mechanism of MAPP1 and MAPP2 in LD-RPUF is proposed.

The centralized release of nonflammable gas and quick formation of crosslinked structure increase the flame retardant properties of polyurethane foams.

Highly Efficient Flame Retardant Polyurethane Foam with Alginate/Clay Aerogel Coating

Highly efficient flame retardant polyurethane foams with alginate/clay aerogel coatings were fabricated using a freeze-drying method. The microstructure and the interaction of the samples were characterized with scanning electron and optical microscopy (SEM) and (OM). The results show that PU foam has a porous structure with pore sizes of several hundred microns, and that of aerogel ranges from 10 to 30 μm. The PU foam matrix and the aerogel coatings have strong interactions, due to the infusion of aerogel into the porous structure of the foam and the tension generated during the freeze-drying process. Both the PU foam and the aerogel exhibit good thermal stabilities, with onset decomposition temperatures above 240 °C. Combustion parameters, including LOI, TTI, HRR, TSR, FIGRA, CO, and CO₂, all indicate significantly reduced fire risk. Total heat release of all but one of the samples was maintained, indicating that the flame retardant mechanism is to decrease flame spread rate by forming a heat, oxygen, and smoke barrier, rather than by reducing fuel content. This facile and inexpensive post-treatment of PU foam could expand its fire safe applications.

Enhanced thermal stability and flame retardancy of polyurethane foam composites with polybenzoxazine modified ammonium polyphosphates

The influence of polybenzoxazine modified ammonium polyphosphate (BMAPP) on the thermal behavior and flame retardancy of a rigid polyurethane foam (PU) are investigated using various techniques.