A novel Schiff-base polyphosphate ester: Highly-efficient flame retardant for polyurethane elastomer

N-P doped lignin flame retardant: preparation and application for thermoplastic polyurethane

A nitrogen‑phosphorus co-doped lignin-based flame retardant (L-N-Zn-ATMP) was successfully synthesized and incorporated into thermoplastic polyurethane (TPU) to enhance its flame retardancy. L-N-Zn-ATMP contained abundant benzene rings, alkoxy, and phosphate groups. Incorporation of this additive significantly improved the residual char rate of TPU composites, which increased from 0.19 % (pure TPU) to 7.52 %. Compared with pure TPU, the peak heat release rate and peak smoke production rate of TPU/10phr L-N-Zn-ATMP were reduced by 55.86 % and 50.00 %, respectively. The flame-retardant mechanism was proposed based on the comprehensive analysis of the residual char's structure and composition and thermal decomposition products. The synergistic effect between nitrogen and phosphorus promoted TPU carbonization and enhanced char formation, thereby providing thermal insulation and oxygen barrier effects. The non-combustible gases generated from L-N-Zn-ATMP effectively diluted the concentration of flammable volatiles generated during combustion. Although the TPU composites showed a slight decline in mechanical properties, they still met the practical application requirements. This study provided insights into the design of high-performance lignin-based flame retardants.

Synergistic atmospheric influence on the co-pyrolysis of antibiotic sludge and waste bicycle tires: Optimal drivers, products, and pathways

Effective management of antibiotic sludge (AS) is essential for disease prevention. This study investigated the co-pyrolysis of AS with polyurethane (PU) and rubber tires (RT), focusing on its key drivers, synergies, resulting products, and atmospheric (N2 versus CO2) dependency. Composite pyrolysis index indicated superior co-pyrolysis properties of AS with PU or RT in the CO2 atmosphere compared with those in the N2 atmosphere. The strongest synergistic effect occurred at an optimal ratio of 75 % AS to 25 % PU (AP31) or 25 % RT (AR31), regardless of the atmosphere. Real-time gas analysis revealed greater product diversity in N2 than in CO2, with AS-derived products predominating. The co-pyrolysis altered AS nitrogen groups, promoting pyrrolic-N and pyridinic-N formation, and accelerated organic sulfur decomposition. Experimental results combined with univariate and multivariate joint optimizations identified the co-pyrolysis pathways of AP31 (650 - 800 °C) and AR31 (600 - 800 °C), respectively, in the CO2 atmosphere as synergistically optimal for maximizing resource recovery while minimizing waste and pollutant generation. This study provides actionable insights into the synergistic co-pyrolysis of AS with PU or RT, facilitating optimized gas emissions, energy recovery, and resource reuse.

Self-Organization of Polyurethane Ionomers Based on Organophosphorus-Branched Polyols

Based on organophosphorus branched polyols (AEPAs) synthesized using triethanolamine (TEOA), ortho-phosphoric acid (OPA), and polyoxyethylene glycol with MW = 400 (PEG), vapor-permeable polyurethane ionomers (AEPA-PEG-PUs) were obtained. During the synthesis of AEPAs, the reaction of the OPA etherification with polyoxyethylene glycol was studied in a wide temperature range and at different molar ratios of the starting components. It turned out that OPA simultaneously undergoes a catalytically activated etherification reaction with triethanolamine and PEG. After TEOA is fully involved in the etherification reaction, excess OPA does not react with the terminal hydroxyl groups of AEPA-PEG or the remaining amount of PEG. The ortho-phosphoric acid remaining in an unreacted state is involved in associative interactions with the phosphate ions of the AEPA. Increasing the synthesis temperature from 40 °C to 110 °C leads to an increase in OPA conversion. However, for the AEPA-PEG-PU based on AEPA-PEG obtained at 100 °C and 110 °C, ortho-phosphoric acid no longer enters into associative interactions with the phosphate ions of the AEPA. Due to the hydrophilicity of polyoxyethylene glycol, the presence of phosphate ions in the polyurethane structure, and their associative binding with the unreacted ortho-phosphoric acid, the diffusion of water molecules in polyurethanes is enhanced, and high values of vapor permeability and tensile strength were achieved.

Synthesis of cerium-based flame retardant containing phosphorus and its impact on the flammability of polylactic acid

The synthesis and characterization of [Ce2(PPPA)4(OH)2]·4H2O, wherein PPPA denotes 3-(hydroxy(phenyl)phosphoryl)propanoate, were conducted. Its potential as a flame-retardant additive for poly(L-lactic acid) (PLA) in conjunction with ammonium polyphosphate (APP) was investigated. Remarkably, with just incorporation of the 1 % Ce-complex and 4 % APP, the resulting PLA composite (PLA-8) meets the V-0 standard, exhibiting an impressive limiting oxygen index (LOI) of 29.4 %. Moreover, the introduction of the Ce-complex leads to a significant extension of ignition time (TTI), a significant 24.1 % decrease in total heat release (THR) compared to pure PLA, and a notable increase in residual carbon rate from 0.3 % to 3.51 %. Although PLA-8 exhibits a minor decline of 8.7 % in tensile strength and 3.4 % in elongation at break, respectively, compared to pure PLA, there is a substantial improvement of 32.2 % in Young's modulus and 29.9 % in impact resistance. These results emphasise the potential of cerium-based phosphorus-containing flame retardants, with cerium playing a key role in enhancing the flammability characteristics of PLA. This study contributes to the development of sustainable and fire-resistant materials in polymer chemistry.

A Comprehensive Review of Reactive Flame Retardants for Polyurethane Materials: Current Development and Future Opportunities in an Environmentally Friendly Direction

Polyurethanes are among the most significant types of polymers in development; these materials are used to produce construction products intended for work in various conditions. Nowadays, it is important to develop methods for fire load reduction by using new kinds of additives or monomers containing elements responsible for materials' fire resistance. Currently, additive antipyrines or reactive flame retardants can be used during polyurethane material processing. The use of additives usually leads to the migration or volatilization of the additive to the surface of the material, which causes the loss of the resistance and aesthetic values of the product. Reactive flame retardants form compounds containing special functional groups that can be chemically bonded with monomers during polymerization, which can prevent volatilization or migration to the surface of the material. In this study, reactive flame retardants are compared. Their impacts on polyurethane flame retardancy, combustion mechanism, and environment are described.

Realizing balanced flame retardancy and electromagnetic interference shielding in hierarchical elastomer nanocomposites

The combination of electromagnetic interference (EMI) shielding performance and flame-retardant property is essential for applications in the field of electronics and electrics. To date, there have been few successful cases in achieving such portfolios, due to the different mechanisms and even mutual exclusivity of these two attributes. Herein, an ammonium polyphosphate@chitosan@carbon nanotube (APP@CS@MWCNT) core-multishell hybrid was synthesized by microencapsulation technology. Then, the hybrid was introduced into TPU matrix to fabricate TPU composites, acting as surface layer. Meanwhile, MXene film was used as intermediate layer to construct hierarchical TPU composites. The obtained results showed that after introduction of 1 wt% APP@CS@MWCNT hybrid, the peak of heat release rate (PHRR) and the peak of smoke produce rate (PSPR) of TPU composites decreased by 67.4% and 35.6%, respectively, compared with those of pure TPU. Owing to multiple reflection losses, interface polarization losses, and charge carrier movement-induced thermal dissipation, TPU/15AC@4M-SW exhibited the highest EMI shielding performance, and obtained shielding effectiveness values of 35.7 dB and 38.9 dB in X band and K band, respectively.

Strategy for Constructing Phosphorus-Based Flame-Retarded Polyurethane Elastomers for Advanced Performance in Long-Term

Polyurethane elastomer (PUE), which is widely used in coatings for construction, transportation, electronics, aerospace, and other fields, has excellent physical properties. However, polyurethane elastomers are flammable, which limits their daily use, so the flame retardancy of polyurethane elastomers is very important. Reactive flame retardants have the advantages of little influence on the physical properties of polymers and low tendency to migrate out. Due to the remarkable needs of non-halogenated flame retardants, phosphorus flame retardant has gradually stood out as the main alternative. In this review, we focus on the fire safety of PUE and provide a detailed overview of the current molecular design and mechanisms of reactive phosphorus-containing, as well as P-N synergistic, flame retardants in PUE. From the structural characteristics, several basic aspects of PUE are overviewed, including thermal performance, combustion performance, and mechanical properties. In addition, the perspectives on the future advancement of phosphorus-containing flame-retarded polyurethane elastomers (PUE) are also discussed. Based on the past research, this study provides prospects for the application of flame-retarded PUE in the fields of self-healing materials, bio-based materials, wearable electronic devices, and solid-state electrolytes.

Impact of a Novel Phosphoramide Flame Retardant on the Fire Behavior and Transparency of Thermoplastic Polyurethane Elastomers

In many application fields of thermoplastic polyurethane (TPU), excellent flame retardancy and transparency are required. However, higher flame retardancy is often at the expense of transparency. It is difficult to achieve high flame retardancy while maintaining the transparency of TPU. In this work, a kind of TPU composite with good flame retardancy and light transmittance was obtained by adding a new synthetic flame retardant named DCPCD, which was synthesized by the reaction of diethylenetriamine and diphenyl phosphorochloridate. Experimental results showed that 6.0 wt % DCPCD endowed TPU with a limiting oxygen index value of 27.3%, passing the UL 94 V-0 rating in the vertical burning test. The cone calorimeter test results showed that the peak heat release rate (PHRR) of the TPU composite was dramatically reduced from 1292 kW/m² (pure TPU) to 514 kW/m² by adding only 1 wt % DCPCD. With the increase of DCPCD contents, the PHRR and total heat release gradually decreased, and the char residue gradually increased. More importantly, the addition of DCPCD has little effect on the transparency and haze of TPU composites. In addition, scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were carried out to investigate the morphology and composition of the char residue for TPU/DCPCD composites and explore the flame retardant mechanism of DCPCD in TPU.

Vanillin-Derived Thermally Reprocessable and Chemically Recyclable Schiff-Base Epoxy Thermosets

The paradigm shift from traditional petroleum-based non-recyclable thermosets to biobased repeatedly recyclable materials is required to move toward circular bioeconomy. Here, two mechanically and chemically recyclable extended vanillin-derived epoxy thermosets are successfully fabricated by introduction of Schiff-base/imine covalent dynamic bonds. Thermoset 1 (T1) is based on linear monomer 1 (M1) with two alcohol end groups and one imine bond, while thermoset 2 (T2) is based on branched monomer 2 (M2) with three alcohol end-groups and three imine-groups. Thermosets are obtained by reaction of monomer 1 (M1) and monomer 2 (M2) with trimethylolpropane triglycidyl ether. The structure of the monomers and thermosets is confirmed by nuclear magnetic resonance and Fourier transform infrared spectroscopic techniques. Both thermosets exhibit good thermal and mechanical properties and they are stable in common organic solvents. Furthermore, they can be thermally reprocessed through compression molding with good recovery of the mechanical properties. Last but not least, the fabricated thermosets can be rapidly and completely chemically recycled to water-soluble aldehydes and amines by imine hydrolysis at room temperature in 0.1 m HCl solution. This is promising for development of future materials with multiple circularity by different routes.

Stretchable, Ultratough, and Intrinsically Self-Extinguishing Elastomers with Desirable Recyclability

Advanced elastomers are increasingly used in emerging areas, for example, flexible electronics and devices, and these real-world applications often require elastomers to be stretchable, tough and fire safe. However, to date there are few successes in achieving such a performance portfolio due to their different governing mechanisms. Herein, a stretchable, supertough, and self-extinguishing polyurethane elastomers by introducing dynamic π-π stacking motifs and phosphorus-containing moieties are reported. The resultant elastomer shows a large break strain of ≈2260% and a record-high toughness (ca. 460 MJ m-3 ), which arises from its dynamic microphase-separated microstructure resulting in increased entropic elasticity, and strain-hardening at large strains. The elastomer also exhibits a self-extinguishing ability thanks to the presence of both phosphorus-containing units and π-π stacking interactions. Its promising applications as a reliable yet recyclable substrate for strain sensors are demonstrated. The work will help to expedite next-generation sustainable advanced elastomers for flexible electronics and devices applications.

Investigation of the Flame Retardant Properties of High-Strength Microcellular Flame Retardant/Polyurethane Composite Elastomers

Flame retardants (FRs) often reduce the mechanical properties of polymer materials, and FR/microcellular polyurethane elastomer (MPUE) composite materials have not been systemically studied. Hence, we conducted this study on FR/MPUE composites by using multiple liquid FRs and/or expandable graphite (EG). Compared with liquid flame retardants, the LOI of an expandable graphite/dimethyl methylphosphonate (EG/DMMP) (3:1) combination was significantly increased (~36.1%), and the vertical combustion grade reached V-0 without a dripping phenomenon. However, the corresponding tensile strength was decreased by 17.5%. With the incorporation of EG alone, although the corresponding LOI was not a match with that of DMMP/EG, there was no droplet phenomenon. In addition, even with 15 wt% of EG, there was no significant decline in the tensile strength. Cone calorimeter test results showed that PHRR, THR, PSPR, and TSR were significantly reduced, compared to the neat MPUE, when the EG content surpassed 10 wt%. The combustion process became more stable and thus the fire risk was highly reduced. It was found that flame retardancy and mechanical properties could be well balanced by adding EG alone. Our proposed strategy for synthesizing FR/MPUE composites with excellent flame retardancy and mechanical properties was easy, effective, low-cost and universal, which could have great practical significance in expanding the potential application fields of MPUEs.

The improvement of fire safety performance of flexible polyurethane foam by Highly-efficient P-N-S elemental hybrid synergistic flame retardant

Herein, three different phosphorus-containing compounds (methyl phosphoryl dichloride, phenyl phosphoryl dichloride and phenyl dichlorophosphate) were reacted with 2-aminobenzothiazole respectively, and a series of synergistic flame retardants with phosphorus, nitrogen and sulfur elements were synthesized, named MPBT, PPBT and POBT respectively. Then, they were added to prepare flame-retardant flexible polyurethane foam (FPUF). Through the analysis of thermal stability, pyrolysis, heat release and smoke release behavior, the influence of different phosphorus-containing structures on the flame-retardant performance of FPUF was studied, and their flame-retardant mechanism was explored in detail. Among them, MPBT had the highest flame retardant efficiency with the same addition amount (10 wt%). The limiting oxygen index (LOI) value of PU/10.0% MPBT reached 22.5 %, and it successfully passed the vertical burning test. Subsequently, the addition amount of MPBT was increased and the best comprehensive performance of flame-retardant FPUF was explored. The results showed that the LOI value of PU/15.0% MPBT was increased to 23.5%. As for PU/15.0% MPBT, the peak heat release rate (PHRR) was 453 KW/m², which was reduced by 46.64 %; and the flame retardancy index (FRI) value was also increased to 6.88. At the same time, the mechanical properties of flame-retardant FPUF were studied. The tensile strength of PU/15.0% MPBT reached 170 KPa, and the permanent deformation of FPUF/10% MPBT was only 4 %, showing its excellent resilience. The above results show that this phosphorus-containing element hybrid synergistic flame retardant (MPBT) has a very good application prospect in the field of flame-retardant polymer materials.

Preparation of Flame-Retardant Polyurethane and Its Applications in the Leather Industry

As a novel polymer, polyurethane (PU) has been widely applied in leather, synthetic leather, and textiles due to its excellent overall performance. Nevertheless, conventional PU is flammable and its combustion is accompanied by severe melting and dripping, which then generates hazardous fumes and gases. This defect limits PU applications in various fields, including the leather industry. Hence, the development of environmentally friendly, flame-retardant PU is of great significance both theoretically and practically. Currently, phosphorus-nitrogen (P-N) reactive flame-retardant is a hot topic in the field of flame-retardant PU. Based on this, the preparation and flame-retardant mechanism of flame-retardant PU, as well as the current status of flame-retardant PU in the leather industry were reviewed.

Novel Cast Polyurethanes Obtained by Using Reactive Phosphorus-Containing Polyol: Synthesis, Thermal Analysis and Combustion Behaviors

Phosphorus-containing polyol applications in polyurethane synthesis can prevent volatilization of flame retardants and their migration on the surface of a material. In this work, novel cast polyurethanes were prepared by a one-step method with the use of different amounts of phosphorus-containing polyol, 4,4′–diphenylmethane diisocyanate and 1,4-butanediol. The chemical structure, thermal, physicochemical and mechanical properties and flame resistance of the prepared materials were investigated. The results obtained for cast flame-retarded polyurethanes were compared with cast polyurethane synthesized with commonly known polyether polyol. It has been shown that with an increasing amount of phosphorus content to polyurethane’s chemical structure, an increased flame resistance and char yield were found during combustion tests. Phosphorus polyol worked in both the condensed (reduced heat and mass exchange) and gas phase (inhibition of flame propagation during burning). The obtained materials contained phosphorus polyol, indicating higher thermal stability in an oxidative environment than an inert atmosphere.

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.

Flame Retardation of Natural Rubber: Strategy and Recent Progress

Natural rubber (NR) as a kind of commercial polymer or engineering elastomer is widely used in tires, dampers, suspension elements, etc., because of its unique overall performance. For some NR products, their work environment is extremely harsh, facing a serious fire safety challenge. Accordingly, it is important and necessary to endow NR with flame retardancy via different strategies. Until now, different methods have been used to improve the flame retardancy of NR, mainly including intrinsic flame retardation through the incorporation of some flame-retarding units into polymer chains and additive-type flame retardation via adding some halogen or halogen-free flame retardants into NR matrix. For them, the synergistic flame-retarding action is usually applied to simultaneously enhance flame retardancy and mechanical properties, in which some synergistic flame retardants such as organo-montmorillonite (OMMT), carbon materials, halloysite nanotube (HNT), etc., are utilized to achieve the above-mentioned aim. The used flame-retarding units in polymer chains for intrinsic flame retardation mainly include phosphorus-containing small molecules, an unsaturated chemical bonds-containing structure, a cross-linking structure, etc.; flame retardants in additive-type flame retardation contain organic and inorganic flame retardants, such as magnesium hydroxide, aluminum hydroxide, ammonium polyphosphate, and so on. Concerning the flame retardation of NR, great progress has been made in the past work. To achieve the comprehensive understanding for the strategy and recent progress in the flame retardation of NR, we thoroughly analyze and discuss the past and current flame-retardant strategies and the obtained progress in the flame-retarding NR field in this review, and a brief prospect for the flame retardation of NR is also presented.

Novel Organophosphate-Derived Dual-Layered Interface Enabling Air-Stable and Dendrite-Free Lithium Metal Anode

Lithium (Li) metal, as a promising candidate for next-generation energy storage systems, suffers from an extremely unstable interface that is prone to crack, causing serious corrosion of Li metal and dendrite growth. To address this, a novel dual-layered interface on the Li metal anode is reported, which is featured with organics (COPO₃ , (CO)₂ PO₂ , and (CO)₃ PO) on the top and inorganics (Li₃ PO₄ ) at the bottom. The flexible organic layer with reduced Young's modulus (≈550 MPa) contributes to maintain structural integrity, while the rigid inorganic layer with improved Young's modulus of ≈12 GPa is beneficial to suppress the Li dendrite growth. Accordingly, the protected Li is stabilized to maintain successive electrodeposition over 800 cycles of plating/stripping process at a current density of 2 mA cm^-2 . Furthermore, the uniform dual-layered interface tends to prevent the corrosion of air to Li metal, exhibiting almost the same performance as the Li metal treated in the inert atmosphere.

A biobased Schiff base from protocatechualdehyde and its application in flame-retardant, low-smoke epoxy resin systems

Herein, a new renewable Schiff base flame retardant 4,4'-((1E,1'E)-((oxybis(4,1-phenylene))bis(azanylylidene))bis(methanylylidene))bis(benzene-1,2-diol) (PH-ODA) was prepared by the reaction of protocatechualdehyde with 4,4'-diaminodiphenyl ether (ODA). PH-ODA (acting as a carbonization agent) combined with ammonium polyphosphate (APP) were used as intumescent flame retardants for commercial bisphenol A epoxy resin (DGEBA). For the cured epoxy resin containing 7.5% APP and 2.5% PH-ODA, the limiting oxygen index (LOI) reached 29.9% (with the V-0 rating in UL-94 test), and the peak heat release rate and total smoke production were respectively decreased by 88.1% and 68.3%, compared with pure epoxy resin. The enhancement of fire-safety performance was due to PH-ODA/APP promoting the formation of a compact intumescent char structure. It was also found that the synergism between PH-ODA and APP was helpful to enhance the fire resistance of the epoxy matrix. This work provides a facile and sustainable route for synthesizing Schiff base compounds from biomass-derived resources, possessing great potential for application in highly-effective intumescent flame retardants.

Chain Extension and Synergistic Flame-Retardant Effect of Aromatic Schiff Base Diepoxide on Polyamide 6/Aluminum Diethylphosphinate Composites

A way to suppress the deterioration in mechanical properties of polyamide 6 (PA6) is required, especially with high loading of flame retardants in the matrix. In this study, a novel aromatic Schiff base diepoxide (DES) was synthesized. It exhibited an efficient chain extension effect on PA6 and a synergistic flame-retardant effect with aluminum diethylphosphinate (AlPi) for PA6. The PA6 composite with 16 wt.% AlPi only passed UL-94 V-0 rating at 1.6 mm thickness, while the combination of 1.5 wt.% DES with 13 wt.% AlPi induced PA6 to achieve a UL-94 V-0 rating at 0.8 mm thickness. The tensile, flexural, and Izod notched impact strengths were increased by 16.2%, 16.5%, and 24.9%, respectively, compared with those of V-0 flame-retarded PA6 composites with 16 wt.% AlPi. The flame-retarded mechanism of PA6/AlPi/DES was investigated by cone calorimetry and infrared characterization of the char residues and pyrolysis products. These results showed that DES had a synergistic effect with AlPi in condensed-phase flame retardation by promoting the production of aluminum phosphorus oxides and polyphosphates in the char residues.