Novel Multifunctional Organic-Inorganic Hybrid Curing Agent with High Flame-Retardant Efficiency for Epoxy Resin

Monitoring Ammonium Polyphosphate (APP) Biodegradation by Acinetobacter nosocomialis D-3 Using DAPI

Ammonium polyphosphate (APP), a pivotal constituent within environmentally friendly flame retardants, exhibits notable decomposition susceptibility and potentially engenders ecological peril. Consequently, monitoring the APP concentration to ensure product integrity and facilitate the efficacious management of wastewater from production processes is of great significance. A fluorescent assay was devised to swiftly discern APP utilizing 4',6'-diamino-2-phenylindole (DAPI). With increasing APP concentrations, DAPI undergoes intercalation within its structure, emitting pronounced fluorescence. Notably, the flame retardant JLS-PNA220-A, predominantly comprising APP, was employed as the test substrate. Establishing a linear relationship between fluorescence intensity (F-F0) and JLS-PNA220-A concentration yielded the equation y = 76.08x + 463.2 (R2 = 0.9992), with a LOD determined to be 0.853 mg/L. The method was used to assess the degradation capacity of APP-degrading bacteria. Strain D-3 was isolated, and subsequent analysis of its 16S DNA sequence classified it as belonging to the Acinetobacter genus. Acinetobacter nosocomialis D-3 demonstrated superior APP degradation capabilities under pH 7 at 37 °C, with degradation rates exceeding 85% over a four-day cultivation period. It underscores the sensitivity and efficacy of the proposed method for APP detection. Furthermore, Acinetobacter nosocomialis D-3 exhibits promising potential for remediation of residual APP through environmental biodegradation processes.

Comparison study on enhancement of phosphorus recovery from low-strength wastewater treated with different magnesium-based electrochemical constructed wetland

Phosphorus (P) recovery from wastewaters treated with constructed wetlands (CWs) could alleviate the current global P crisis but has not received sufficient attention. In this study, P transformation in different magnesium-based electrochemical CWs, including micro-electrolysis CW (M-CW), primary battery CW (P-CW), and electrolysis CW (E-CW), was thoroughly examined. The results revealed that the P removal efficiency was 53.0%, 75.8%, and 61.9% in the M-CW, E-CW, and P-CW, respectively. P mass balance analysis showed that P electrode deposition was the main reason for the higher P removal in the E-CW and P-CW. Significant differences were found between the E-CW and P-CW, P was distributed primarily on the magnesium plate in the P-CW but was distributed on the carbon plate in the E-CW. The E-CW had excellent P recovery capacity, and struvite was the major P recovery product. More intense magnesium plate corrosion and alkaline environment increased struvite precipitation in the E-CW, with the proportion of 61.6%. The results of functional microbial community analysis revealed that the abundance of electroactive bacteria was positively correlated with the deposition of struvite. This study provided an essential reference for the targeted electrochemical regulation of electric field processes and microorganisms in CWs to enhance P recovery.

Neighboring Molecular Engineering in Diels-Alder Chemistry Enabling Easily Recyclable Carbon Fiber Reinforced Composites

Although a variety of dynamic covalent bonds have been successfully used in the development of diverse sustainable thermosetting polymers and their composites, solving the trade-off between recovery efficiency and comprehensive properties is still a major challenge. Herein, a "one-stone-two-birds" strategy of lower rotational energy barrier (Er ) phosphate-derived Diels-Alder (DA) cycloadditions was proposed for easily recyclable carbon fiber (CF)-reinforced epoxy resins (EPs) composites. In such a strategy, the phosphate spacer with lower Er accelerated the segmental mobility and dynamic DA exchange reaction for network rearrangement to achieve high-efficiency repairing, reprocessing of the EPs matrix and its composites and rapid nondestructive recycling of CF; meanwhile, incorporating phosphorus-based units especially reduced their fire hazards. The resulting materials simultaneously showed excellent thermal/mechanical properties, superb fire safety and facile recyclability, realizing the concept of recycling for high-performance thermosetting polymers and composites. This strategy is of great significance for understanding and enriching the molecular connotation of DA chemistry, making it potentially applicable to the design and development of a wide range of dynamic covalent adaptable materials toward practical cutting-edge-tech applications.

Bio-Inspired Iron-Loaded Polydopamine Functionalized Montmorillonite as an Environmentally Friendly Flame Retardant for Epoxy Resin

As an important thermosetting material, flame-retardant epoxy resin has various applications in the aerospace, chemical, and electronics industry, and other fields. However, the flame retardancy of epoxy resins is often improved at the expense of mechanical performance. The contradiction between flame retardancy and mechanical properties seriously impedes the practical applications of epoxy resin (EP). Herein, iron-loaded polydopamine functionalized montmorillonite (D-Mt-Fe³⁺), which was prepared by dopamine, iron chloride and montmorillonite in an aqueous solution, was introduced to prepare iron-loaded polydopamine functionalized montmorillonite/epoxy resin composites (D-Mt-Fe³⁺/EP). As expected, D-Mt-Fe³⁺/EP-10 with 10 phr of D-Mt-Fe³⁺ passed the UL-94 V-0 rating, achieved a limiting oxygen index (LOI) value of 31.0% and reduced the smoke production rate (SPR) and total smoke production (TSP), indicating that the introduction of D-Mt-Fe³⁺ could endow EP with satisfactory flame retardancy through the radical scavenging function of dopamine in the gas phase and the catalytic charring effect of iron ions, respectively. Encouragingly, the mechanical property was also enhanced with the flexural strength increased by 25.5%. This work provided an attractive strategy for improving both the mechanical properties and fire resistance of EP, which greatly broadened their applications in the chemical industry and electronics field, etc.

A Novel P/N/Si-Containing Vanillin-Based Compound for a Flame-Retardant, Tough Yet Strong Epoxy Thermoset

It is still extremely challenging to endow epoxy resins (EPs) with excellent flame retardancy and high toughness. In this work, we propose a facile strategy of combining rigid-flexible groups, promoting groups and polar phosphorus groups with the vanillin compound, which implements a dual functional modification for EPs. With only 0.22% phosphorus loading, the modified EPs obtain a limiting oxygen index (LOI) value of 31.5% and reach V-0 grade in UL-94 vertical burning tests. Particularly, the introduction of P/N/Si-containing vanillin-based flame retardant (DPBSi) improves the mechanical properties of EPs, including toughness and strength. Compared with EPs, the storage modulus and impact strength of EP composites can increase by 61.1% and 240%, respectively. Therefore, this work introduces a novel molecular design strategy for constructing an epoxy system with high-efficiency fire safety and excellent mechanical properties, giving it immense potential for broadening the application fields of EPs.

Phosphorus biorecovery from wastewater contaminated with multiple nitrogen species by a bacterial consortium

Recovering finite and non-substitutable phosphorus from liquid waste streams through bio-mediated techniques has attracted increasing interest, but current approaches are incredibly dependent on ammonium. Herein, a process to recover phosphorus from wastewater under multiple nitrogen species conditions was developed. This study compared the effects of nitrogen species on the recovery of phosphorus resources by a bacterial consortium. It found that the consortium could not only efficiently utilize ammonium to enable phosphorus recovery but also utilize nitrate via dissimilatory nitrate reduction to ammonium (DNRA) to recover phosphorus. The characteristics of the generated phosphorus-bearing minerals, including magnesium phosphate and struvite, were evaluated. Furthermore, nitrogen loading positively influenced the stability of the bacterial community structure. The genus Acinetobacter was dominant under nitrate and ammonium conditions, with a relatively stable abundance of 89.01% and 88.54%, respectively. The finding may provide new insights into nutrient biorecovery from phosphorus-containing wastewater contaminated with multiple nitrogen species.

Flame Retardancy of Epoxy Resins Modified with Few-Layer Black Phosphorus

Few-layer black phosphorus (BP)- and red phosphorus (RP)-modified diglycidyl ether of bisphenol A-based epoxy resins (EP) was prepared with 4,4′-diaminodiphenylsulfone as a curing agent. The thermal stability and flame-retardant properties of the modified EPs were compared. Both BP and RP were able to improve the flame-retardant properties of EPs, while the BP showed higher flame-retardant efficiency than RP. As a two-dimensional nanomaterial, BP exhibited good compatibility, high flame-retardant efficiency, and negligible impact on the mechanical and thermal stability of EP. Pyrolysis-gas Fourier-transform infrared spectroscopic analysis of EP showed that the addition of BP significantly inhibited the release of pyrolysis products in the gas phase. The modes of action for flame-retardant BPs in gas phase and condensed phase were proposed.

Design of P-decorated POSS towards flame-retardant, mechanically-strong, tough and transparent epoxy resins

Epoxy resins (EPs) are known for their durability, strength, and adhesive properties, which make them a versatile and popular material for use in a variety of applications, including chemical anticorrosion, small electronic devices, etc. However, EP is highly flammable due to its chemical nature. In this study, phosphorus-containing organic-inorganic hybrid flame retardant (APOP) was synthesized by introducing 9, 10-dihydro-9-oxa-10‑phosphaphenathrene (DOPO) into cage-like octaminopropyl silsesquioxane (OA-POSS) via Schiff base reaction. The improved flame retardancy of EP was achieved by combining the physical barrier of inorganic Si-O-Si with the flame-retardant capability of phosphaphenanthrene. EP composites containing 3 wt% APOP passed the V-1 rating with a value of LOI of 30.1% and showed an apparent reduction in smoke release. Additionally, the combination of the inorganic structure and the flexible aliphatic segment in the hybrid flame retardant provides EP with molecular reinforcement, while the abundance of amino groups facilitates a good interface compatibility and outstanding transparency. Accordingly, EP containing 3 wt% APOP increased in tensile strength, impact strength, and flexural strength by 66.0 %, 78.6 %, and 32.3 %, respectively. The EP/APOP composites had a bending angle lower than 90°, and their successful transition to a tough material highlights the potential of this innovative combination of the inorganic structure and the flexible aliphatic segment. In addition, the relevant flame-retardant mechanism revealed that the APOP promoted the formation of a hybrid char layer containing P/N/Si for EP and produced phosphorus-containing fragments during combustion, showing flame-retardant effects in both condensed and vapor phases. This research offers innovative solutions for reconciling flame retardancy & mechanical performances and strength & toughness for polymers.

Co-curing preparation of flame retardant and smoke-suppressive epoxy resin with a novel phosphorus-containing ionic liquid

Phosphorus-containing ionic liquid derivatives have been proven to be effective flame retardants for epoxy resin (EP). Flame retardants can accelerate the curing process and improve flame retardancy and smoke suppression of EP composites, which is challenging. In this paper, a novel phosphorus-containing ionic liquid (TPP-PF₆) was synthesized and used both as a co-curing agent with 4,4'-diaminodiphenylmethane (DDM) and as a highly effective flame retardant for EP. It has been found that TPP-PF₆ was conducive to improve the char formation of EP to inhibit the smoke release at high temperatures. For EP/TPP-PF₆ composites, the flame-retardant performance was enhanced rapidly with the increase of TPP-PF₆. With only 2 wt% of TPP-PF₆, EP/2.0TPP-PF₆ reached a UL-94 V-0 rating and a limiting oxygen index of 30.3%. The peak heat release rate, total heat release, and total smoke production values of EP/2.0TPP-PF₆ were reduced by 36.32%, 45.81%, and 15.1% compared with those of pure EP, respectively. The thermal degradation products and flame retardant mechanism in gas and condensed phases were studied. It was found that TPP-PF₆ had flame retardant effect in the barrier effect of the condensed phase and the quenching effect of the gas phase. This work explores the high-efficiency flame retardant and smoke-suppressive structures with co-curing properties for EP, thus promoting the wide application of EP materials.

Interfacial engineering to construct P-loaded hollow nanohybrids for flame-retardant and high-performance epoxy resins

Nano flame retardants, as one of the key flame retardants in recent years, have been limited by poor efficiency and weak compatibility. In this study, we propose an interfacial hollow engineering strategy to tackle this problem by assembling P-phytic acid into the hollow cavity of mesoporous SiO₂ grafted with a polydopamine transition metal. In this design, the grafted polydopamine-metal coatings on the hybrids can greatly improve their interface compatibility with the polymer matrix, while the loaded phytic acid in the cavity contributes to enhance flame retardancy. Consequently, the resultant hierarchical P-loaded nanohybrids show both high flame retardancy and mechanical reinforcement for the polymer. Taking epoxy resin (EP, a typical thermosetting resin used in large quantities) as a representative, at only 1 wt% loading of the nanohybrids, the impact strength of the nanocomposites improved by 35.7% compared to pure EP. Remarkably, the hybrids can simultaneously endow EP with high flame retardancy (low heat release rate) and satisfactory smoke inhibition. Additionally, the flame-retardant mechanism analysis confirmed that the nanohybrid had a better catalytic carbonization effect on promoting the highly graphitized carbon layer, thereby suppressing the fire hazard of epoxy resins. This research offers a new interfacial hollow engineering method for the construct and design of high-performance EP with nanohybrids.

Advanced Flame-Retardant Methods for Polymeric Materials

Most organic polymeric materials have high flammability, for which the large amounts of smoke, toxic gases, heat, and melt drips produced during their burning cause immeasurable damages to human life and property every year. Despite some desirable results having been achieved by conventional flame-retardant methods, their application is encountering more and more difficulties with the ever-increasing high flame-retardant requirements such as high flame-retardant efficiency, great persistence, low release of heat, smoke, and toxic gases, and more importantly not deteriorating or even enhancing the overall properties of polymers. Under such condition, some advanced flame-retardant methods have been developed in the past years based on "all-in-one" intumescence, nanotechnology, in situ reinforcement, intrinsic char formation, plasma treatment, biomimetic coatings, etc., which have provided potential solutions to the dilemma of conventional flame-retardant methods. This review briefly outlines the development, application, and problems of conventional flame-retardant methods, including bulk-additive, bulk-copolymerization, and surface treatment, and focuses on the raise, development, and potential application of advanced flame-retardant methods. The future development of flame-retardant methods is further discussed.

Epoxy Compositions with Reduced Flammability Based on DER-354 Resin and a Curing Agent Containing Aminophosphazenes Synthesized in Bulk Isophoronediamine

A method for the synthesis of an amine-containing epoxy resin curing agent by dissolving hexakis-[(4-formyl)phenoxy]cyclotriphosphazene in an excess of isophoronediamine was developed. The curing agent was characterized via NMR and IR spectroscopy and MALDI-TOF mass spectrometry, and its rheological characteristics were studied. Compositions based on DER-354 epoxy resin and the synthesized curing agent with different amounts of phosphazene content were obtained. The rheological characteristics of these compositions were studied, followed by their curing. An improvement in several thermal (DSC), mechanical (compression, tension, and adhesion), and physicochemical (water absorption and water solubility) characteristics, as well as the fire resistance of the obtained materials modified with phosphazene, was observed, compared with unmodified samples. In particular, there was an improvement in adhesive characteristics and fire resistance. Thus, compositions based on a curing agent containing a 30% modifier were shown to fulfill the V-1 fire resistance category. The developed compositions can be processed by contact molding, winding, and resin transfer molding (RTM), and the resulting material is suitable for use in aircraft, automotive products, design applications, and home repairs.

Improvement the Flame Retardancy and Thermal Conductivity of Epoxy Composites via Melamine Polyphosphate-Modified Carbon Nanotubes

Surface chemical modification of carbon nanotubes can enhance the compatibility with polymers and improve flame retardancy performances. In this work, the double bond active sites were constructed on the surface of carbon nanotubes modified by the γ-methacryloyloxypropyl trimethoxysilane (KH570). Glycidyl methacrylate (GMA) was further grafted onto the surface of carbon nanotubes via free radical polymerization. Finally, the flame retardant melamine polyphosphate (MPP) was bonded to the surface of carbon nanotubes by the ring-opening reaction. This modification process was proved to be achieved by infrared spectroscopy and thermogravimetric test. The carbon nanotubes modified by flame retardant were added into the epoxy matrix and cured to prepare flame retardant and thermal conductive composites. The flame retardancy of composites were studied by cone calorimetry, UL94 vertical combustion test and limiting oxygen index. The thermal conductivity of composites was characterized by laser thermal conductivity instrument. The results showed that when the addition amount of flame retardant MPP-modified carbon nanotubes in composites was 10 wt%, the flame retardant level of UL94 reached to V2, the limiting oxygen index increased from 25.1 of pure epoxy resin to 28.3, the PHRR of pure epoxy resin was reduced from 800 kW/m² to 645 kW/m² of composites and thermal conductivity of composites was enhanced from 0.21 W/m·K⁻¹ of pure epoxy resin to 0.42 W/m·K⁻¹ of the composites.

Recyclable, malleable and intrinsically flame-retardant epoxy resin with catalytic transesterification

Flame retardancy and recyclability are two important issues in the research field of thermosets, particularly for epoxy resin (EP) with the biggest market share. It is of great importance, but rarely achievable, to integrate these properties simultaneously into EP. Herein, we report a facile way to prepare intrinsically flame-retardant epoxy vitrimers combining rapid recycling and multiple shape memory effects by introducing dynamic ester-linkages with catalytic transesterification activity into the crosslinking networks of EP. The flame-retardant epoxy vitrimers exhibited high T_g (∼110.7 °C), desirable thermal stability and excellent flame retardancy with UL-94 V-0 rating, and high LOI of ∼34%. Also, the value of the peak heat release rate (PHRR) and the total heat release (THR) showed 63% and 32% reduction, respectively. Meanwhile, flame-retardant epoxy vitrimers showed high malleability that could be reprocessed in 15 min at 200 °C without sacrificing the mechanical properties and flame retardancy. Moreover, the dynamic transesterification network allowed flame-retardant EP to access multiple shape memory effect. The design of flame-retardant epoxy vitrimers provide a prime example to foster the cyclic utilization of flame-retardant thermosetting polymers.

Surface Modification of Ammonium Polyphosphate for Enhancing Flame-Retardant Properties of Thermoplastic Polyurethane

Currently, the development of efficient and environmentally friendly flame-retardant thermoplastic polyurethane (TPU) composite materials has caused extensive research. Ammonium polyphosphate (APP) is used as a general intumescent flame retardant to improve the flame retardancy of TPU. In this paper, we developed a functionalized APP flame retardant (APP-Cu@PDA). Adding only 5 wt% of APP-Cu@PDA into TPU can significantly improve the flame-retardant's performance of the composite material, reflected by a high LOI value of 28% with a UL-94 test of V-0 rating. Compared with pure TPU, the peak heat release rate, total heat release, peak smoke release rate, and total smoke release were reduced by 82%, 25%, 50%, and 29%, respectively. The improvements on the flame-retardant properties of the TPU/5%APP-Cu@PDA composites were due to the following explanations: Cu²⁺-chelated PDA has a certain catalytic effect on the carbonization process, which can promote the formation of complete carbon layers and hinder the transfer of heat and oxygen. In addition, after adding 5% APP-Cu@PDA, the tensile strength and elongation at the break of TPU composites did not decrease significantly. In summary, we developed a new flame-retardant APP-Cu@PDA, which has better flame-retardant properties than many reported TPU composites, and its preparation process is simple and environmentally friendly. This process can be applied to the industrial production of flame retardants in the future.

A Study on the Synthesis, Curing Behavior and Flame Retardance of a Novel Flame Retardant Curing Agent for Epoxy Resin

In this work, a flame retardant curing agent (DOPO-MAC) composed of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide DOPO and methyl acrylamide (MAC) was synthesized successfully, and the structure of the compound was characterized by FT-IR and 1H-NMR. The non-isothermal kinetics of the epoxy resin/DOPO-MAC system with 1% phosphorus was studied by non-isothermal DSC method. The activation energy of the reaction (Ea), about 46 kJ/mol, was calculated by Kissinger and Ozawa method, indicating that the curing reaction was easy to carry out. The flame retardancy of the epoxy resin system was analyzed by vertical combustion test (UL94) and limiting oxygen index (LOI) test. The results showed that epoxy resin (EP) with 1% phosphorus successfully passed a UL-94 V-0 rating, and the LOI value increased along with the increasing of phosphorus content. It confirmed that DOPO-MAC possessed excellent flame retardance and higher curing reactivity. Moreover, the thermal stability of EP materials was also investigated by TGA. With the DOPO-MAC added, the residual mass of EP materials increased remarkably although the initial decomposition temperature decreased slightly.

Design of Intrinsically Flame-Retardant Vanillin-Based Epoxy Resin for Thermal-Conductive Epoxy/Graphene Aerogel Composites

Vanillin, as a lignin-derived mono-aromatic compound, has attracted increasing attention due to its special role as an intermediate for the synthesis of different biobased polymers. Herein, intrinsically flame-retardant and thermal-conductive vanillin-based epoxy/graphene aerogel (GA) composites were designed. First, a bifunctional phenol intermediate (DN-bp) was synthesized by coupling vanillin with 4, 4'-diaminodiphenylmethane and DOPO, and the epoxy monomer (MEP) was obtained by the epoxidation reaction with DN-bp and epichlorohydrin. Then, various amounts of MEP and diglycidyl ether of bisphenol A (DER) were mixed and cured. Interestingly, the flexural strength and modulus were greatly enhanced from 72.8 MPa and 1.3 GPa to 90.3 MPa and 2.8 GPa, respectively, at 30 wt % MEP, due to the rigidity of MEP and strong intermolecular N-H hydrogen bonding interactions. Meanwhile, the cured epoxy achieved a UL-94 V0 rating with a low P content of 1.06%. The flame-retardant vanillin-based epoxy was then impregnated into the thermal conductive 3D GA networks. The obtained epoxy/graphene composite showed excellent flame retardancy and thermal conductivity [λ = 0.592 W/(m·K)] with only 0.5 wt % graphene in the system. Based on these results, we believe that this work would represent a novel solution for the preparation of high-performance biobased flame-retardant multipurpose epoxies.

Facile construction of lamellar-like phosphorus-based triazole-zinc complex for high-performance epoxy resins

Epoxy resins (EP) possessing superior flame retardancy, mechanical properties and glass transition temperature are urgently needed to meet the ever-increasing requirement of high performance for the practical application of EP. Herein, lamellar-like phosphorus-based triazole-zinc complex (Zn-PT) was firstly constructed through coordination reaction in a facile condition to address the above issue. The results revealed that Zn-PT was well dispersed in epoxy matrix, and with 3 wt% Zn-PT, the tensile strength, flexural strength and modulus of epoxy composites were remarkably increased from 71, 112 and 2982 MPa of neat epoxy resin (EP) to 80, 162 and 3482 MPa respectively. The glass transition temperature was higher than EP. Besides, the limiting oxygen index (LOI) increased to 28.3%, and UL-94V-1 level was available. Meanwhile, the cone calorimeter test (CCT) results showed that epoxy composites displayed less heat release and smoke production. Generally, this work provides a feasible strategy to prepare high-performance epoxy composites, which has the potential to satisfy the requirement of epoxy in the practical application.

Fabrication and Mechanism Study of Cerium-Based P, N-Containing Complexes for Reducing Fire Hazards of Polycarbonate with Superior Thermostability and Toughness

A superior comprehensive performance is essential for the extensive utilization of polymers. Current flame-retardant strategies for polycarbonates (PCs) usually realize satisfied fire resistance at the cost of thermostability, toughness, and/or mechanical robustness. Thus, we report a rare-earth-based P, N-containing complex with a lamellar aggregated structure [Ce(DPA)3] by a coordination reaction between a tailored ligand and cerium(III) nitrate. The results indicate that incorporating 3 wt % Ce(DPA)3 enables the resultant PC composite to achieve UL-94 V-0 rating, with a 55% reduction in the peak heat release rate. Besides, the initial (T5) and maximum (Tmax1 and Tmax2) decomposition temperatures are significantly increased by 21, 19, and 27 °C, respectively, in an air atmosphere. Moreover, the impact strength and elongation at break of the PC composite containing 3 wt % Ce(DPA)3 are greatly increased by 20 and 59%, respectively, relative to pristine PC, while its tensile strength (57 MPa) is still close to that of bulk PC (60 MPa). Notably, this work provides a novel methodology for revealing the evolution mechanisms of chemical structures of vapor and residual products during thermal decomposition, which is conducive to guiding fire and heat resistance modification of PC in the future.

Flame Retardant Properties of a Guanidine Phosphate-Zinc Borate Composite Flame Retardant on Wood

This work combines guanidine dihydrogen phosphate (GDP) and zinc borate (ZB) to modify wood via microwave-ultrasonic impregnation for realizing favorable flame retardancy and thermal stability, which were investigated by the limiting oxygen index (LOI), thermogravimetric analysis (TGA), and cone calorimetry tests (CONE). The treated samples show better performance in fire retardancy with the LOI value increasing to 47.8%, and the results of TGA indicate the outstanding thermal stability of wood. In addition, the decline of heat release rate, total heat release, smoke production rate, and total smoke production examined by CONE further demonstrates the achievement of excellent flame retardancy and smoke suppression properties of the GDP/ZB-modified wood.

Synergistic Effects of Ladder and Cage Structured Phosphorus-Containing POSS with Tetrabutyl Titanate on Flame Retardancy of Vinyl Epoxy Resins

The cage and ladder structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) have been synthesized through the hydrolytic condensation of 9,10-dihydro-9-oxa-10-phosphenanthrene-10-oxide (DOPO)-vinyl triethoxysilane (VTES). The unique ladder and cage–ladder structured components in DOPO-POSS endowed it with good solubility in vinyl epoxy resin (VE), and it was used with tetrabutyl titanate (TBT) to construct a phosphorus-silicon-titanium synergy system for the flame retardation of VE. Thermal stabilities, mechanical properties, and flame retardancy of the resultant VE composites were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), three-point bending tests, limiting oxygen index (LOI) measurement, and cone calorimetry. The experimental results showed that with the addition of only 4 wt% DOPO-POSS and 0.5 wt% TBT, the limiting oxygen index value (LOI) increased from 19.5 of pure VE to 24.2. With the addition of DOPO-POSS and TBT, the peak heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) were decreased significantly compared to VE-0. In addition, the VE composites showed improved thermal stabilities and mechanical properties comparable to that of the VE-0. The investigations on pyrolysis volatiles of cured VE further revealed that DOPO-POSS and TBT exerted flame retardant effects in gas phase. The results of char residue of the VE composites by SEM and XPS showed that TBT and DOPO-POSS can accelerate the char formation during the combustion, forming an interior char layer with the honeycomb cavity structure and dense exterior char layer, making the char strong with the formation of Si-O-Ti and Ti-O-P structures.