Flame retardancy and mechanical properties of epoxy thermosets modified with a novel DOPO-based oligomer

Synthesis of a reactive lignin-based flame retardant and its application in phenolic foam

To improve the flame retardancy of phenolic foam from the perspective of sustainable development, it is a feasible way to add bio-based flame retardants into phenolic foam. Lignin has a similar structure to phenol, which provides a possibility to replace part of phenol. In this paper, we prepared a kind of reactive bio-based flame retardant based on enzymatic hydrolyzed lignin, in which side chain was chemically grafted with phosphorus and nitrogen and benzene ring would participate in the phenolic condensation reaction. According to elemental analysis and ICP-OES data, the content of nitrogen and phosphorus in modified lignin (NP-L) increased to 2.95% and 3.55% respectively. Compared with original lignin, the carbon residue rate of NP-L increased from 3.25% to 12.13% because of the presence of flame retardant elements N and P. Then lignin-based flame retardant was used to replace phenol for modifying phenolic foams (NPLPFX). The limited oxygen index (LOI) and compressive strength of phenolic foam were improved effectively by adding modified lignin when the substitution rate was less than 25%. The LOI and compressive strength of the modified phenolic foam with 5% replacement amount (NPLPF5) are 55.6% and 0.24 MPa, which increased by 88% and 60% compared with pure phenolic foam. The cone calorimetric data also showed that NPLPF5 had good flame retardancy, and the peak heat release rate and total heat release were significantly lower than PF. This work suggests a novel green strategy for improving the flame retardancy performance of phenolic foam and promoting the utilization of lignin.

High-Performance Li-Organic Batteries Based on Conjugated and Nonconjugated Schiff-Base Polymer Anode Materials

In recent years, organic materials have been increasingly studied as anode materials in lithium-ion batteries (LIBs) due to their remarkable advantages, including abundant raw materials, low prices, diverse structures, and high theoretical capacity. In this paper, three types of aromatic Schiff-base polymer materials have been synthesized and examined as anode materials in LIBs. Among them, the polymer [C6H4N = CHC6H4CH=N]n (TTD-PDA) has a continuous conjugated backbone (label as conjugated polymer), while polymers [(CH2)2N=CHC6H4CH=N]n (TTD-EDA) and [C6H4N=CH(CH2)3CH=N]n (GA-PDA) have discontinuous conjugated back-bones (label as nonconjugated polymer). The organic anodes based on TTD-PDA, TTD-EDA, and GA-PDA for LIBs are discovered to represent high reversible specific capacities of 651, 492, and 416 mAh g-1 at a current density of 100 mA g-1 as well as satisfactory rate capabilities with high capacities of 210, 90, and 178 mAh g-1 and 105, 57, and 122 mAh g-1 at current densities of 2 and 10 A g-1, indicating that these Schiff-base polymers are all promising anode materials for LIBs, which broadens the design of organic anode materials with high specific capacity, superior rate performance, and stable cycling stability.

Enhancing Flame Retardancy and Smoke Suppression in Epoxy Resin Composites with Sulfur-Phosphorous Reactive Flame Retardant

The presence of massive amounts of toxic volatiles and smoke during combustion is a very serious problem facing epoxy resin (EP) composites. Therefore, flame retardants (FRs) can simultaneously enhance flame retardancy and reduce the release of smoke and fatal gases. Herein, a novel sulfur-phosphorous reactive flame retardant (SPMS) was synthesized for epoxy resin. The high efficiency of smoke suppression and flame retardancy of the EP/SPMS-APP hybrid was investigated using a cone calorimeter, a vertical burning test, and limited oxygen index measurements. Compared with those of pure EP, the composite with 20 wt% SPMS-APP reduced the peak heat release rate (pHRR), the peak smoke production rate (SPR), and total smoke production rate (TSR) by 82%, 94%, and 84%, respectively. The results showed a remarkable suppressed effect of alleviating the fire hazard of EP using a sulfur-phosphorus flame retardant.

A DOPO-Based Compound Containing Aminophenyl Silicone Oil for Reducing Fire Hazards of Polycarbonate

A novel P/N/Si-containing flame retardant (marked as DASO) was synthesized through an Atherton-Todd reaction between 9,10-dihydro-9-oxa-10-phospha-phenanthrene-10-oxide and aminophenyl silicone oil, and further used for reducing fire hazards of polycarbonate (PC). The chemical structure of DASO was verified via FTIR, ¹H, and ³¹P NMR. Upon the incorporation of 2 wt% DASO, the FRPC composite achieved a high limiting oxygen index (LOI) of 32.2% and a desired UL-94 V-0 rating. In this case, the peak heat release rate (PHRR) and total smoke production (TSP) were reduced by 26% and 44% as compared with the pure PC, respectively. The improved fire safety contributed to the flame retardant roles of DASO in both the condensed phase and gas phase. The presence of DASO promoted the formation of dense and highly graphited char layer in the condensed phase, and released non-combustible gases and phosphorus-containing radicals in the gas phase. Furthermore, the FRPC composites displayed comparable elongation at break but a slightly reduced tensile and impact strength.

A Bridge-Linked Phosphorus-Containing Flame Retardant for Endowing Vinyl Ester Resin with Low Fire Hazard

The high flammability of vinyl ester resin (VE) significantly limits its widespread application in the fields of electronics and aerospace. A new phosphorus-based flame retardant 6,6'-(1-phenylethane-1,2 diyl) bis (dibenzo[c,e][1,2]oxaphosphinine 6-oxide) (PBDOO), was synthesized using 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and acetophenone. The synthesized PBDOO was further incorporated with VE to form the VE/PBDOO composites, which displayed an improved flame retardancy with higher thermal stability. The structure of PBDOO was investigated using Fourier transformed infrared spectrometry (FTIR) and nuclear magnetic resonances (NMR). The thermal stability and flame retardancy of VE/PBDOO composites were investigated by thermogravimetric analysis (TGA), vertical burn test (UL-94), limiting oxygen index (LOI), and cone calorimetry. The impacts of PBDOO weight percentage (wt%) on the flame-retardant properties of the formed VE/PBDOO composites were also examined. When applying 15 wt% PBDOO, the formed VE composites can meet the UL-94 V-0 rating with a high LOI value of 31.5%. The peak heat release rate (PHRR) and the total heat release (THR) of VE loaded 15 wt% of PBDOO decreased by 76.71% and 40.63%, respectively, compared with that of untreated VE. In addition, the flame-retardant mechanism of PBDOO was proposed by analyzing pyrolysis behavior and residual carbon of VE/PBDOO composites. This work is expected to provide an efficient method to enhance the fire safety of VE.

Enhanced fire-proofing performance and crystallizability of bio-based poly(L-lactic acid): Dual functions of a Schiff base-containing synergistic flame retardant

Poly(L-lactic acid) (PLA) is a kind of important bio-macromolecule which can be prepared via fermentation of starch of maize and sweet potato. Flammability and extremely poor crystallizability limited its wide application. In this work, a novel Schiff base derivate (CP) was synthesized and, combined with ammonium polyphosphate (APP) as a synergistic flame retardant and nucleating agent to investigate its effects on LOI, UL-94 rating, thermal stability, combustion behavior and crystallizability of PLA. With loading of 5%CP/10%APP, PLA showed a significantly enhanced LOI and passed V-0 fire-safety rating with self-extinguish effect. PLA/5%CP/10%APP presented the lowest pHRR, THR and TSR, and highest char residue yield, FPI and FRI in cone calorimetry test, indicating an excellent flame retardancy effect, enhanced fire safety and longer escaping time in the fire. A continuous, compact and thick char layer structure formed as a protective barrier in combustion process, to enhance heat-insulating and oxygen resistance property, thermal stability and smoke-suppressing capacity of PLA. Flame retardancy mechanism was proposed and discussed based on comprehensive and in-depth characterization techniques. Also, 5%CP/10%APP presented a good nucleation effect to enormously increase crystallizability and shorten crystallization time of PLA.

The P/Si synergistic effect enduing epoxy resin with improved flame retardancy and outstanding mechanical properties

The bisphenol F epoxy resin (DGEBF) reacted with 10-(2,5-Dihydroxyphenyl)-10H-9-oxa-10-phospha-phenantbrene-10-oxide (ODOPB) and phenyltrimethoxysilane (PTMS) to obtain a novel epoxy resin containing both phosphorus and silicon (EP-P/Si). EP-P/Si exhibited evidently improved flame retardancy, with a limited oxygen index value of 33.4% and UL-94 V-1 rating acquired. In cone calorimeter test, its peak heat release rate (PHRR), total heat release (THR), average effective heat of combustion (av-EHC), and total smoke production (TSP) were reduced by 36.0%, 19.5%,11.5%, and 7.2% compared with neat epoxy resin (EP), respectively, indicating that the P/Si synergistic effect not only improved the flame retardancy but also inhibited the smoke release. The flame retardancy mechanism was studied by analysis of char residue and pyrolysis behavior in gas phase. Scanning electron microscopy (SEM) results exhibited that EP-P/Si formed a dense and compact carbon layer acting as a barrier to inhibit further combustion. And the Fourier transform infrared (FTIR) spectra, laser Raman spectroscopy (LRS), and X-ray photoelectron spectroscopy (XPS) results indicated that it had good thermal stability. In addition, the pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) results suggested that the phosphorus-containing radicals (·PO2) that had quenching effect existed in the gas phase. While the flame retardancy got improved, EP-P/Si also exhibited excellent mechanical properties, with an improvement of 31.8%, 6.2%, and 369.7% in tensile strength, flexural strength, and impact strength compared with EP, respectively.

Transparent low-flammability epoxy resins using a benzoguanamine-based DOPO derivative

We successfully prepared a highly effective flame-retardant additive called hsalbenzoguanamine phosphaphenanthrene (HDPD) through salicylaldehyde and nitrogen-rich benzoguanamine. The introduction of HDPD into epoxy resin (EP) sharply enhanced the flame retardancy of EP/HDPD thermosets. The introduction of 6 wt% HDPD into EP succeeded in reaching the V-0 rating. Limited oxygen index results revealed the high flame-retarding performance of HDPD. Cone calorimeter test data revealed that heat and smoke released from EP/6 wt% HDPD thermoset were significantly restrained. In addition, EP/6 wt% HDPD thermoset demonstrated excellent transmittance and mechanical strength. The transmittance of EP/6 wt% HDPD was assessed from 520 to 800 nm. The results showed that transmittance of EP/6 wt% HDPD were nearly 90% of the control group.

Thermal Stability, Smoke Density, and Flame Retardance of Ecotype Bio-Based Flame Retardant Agricultural Waste Bagasse/Epoxy Composites

In the study, agricultural waste bagasse was used as a bio-based flame retardant for reducing the flammability of epoxy. Specifically, an interpenetrating network (IPN) was formed through a ring opening reaction between the hydroxyl functional group of bagasse and the epoxy group of triglycidyl isocyanurate (TGIC), forming Bagasse@TGIC. Next, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) was mixed with Bagasse@TGIC, inducing a reaction between the active hydrogen of DOPO and the epoxy group of TGIC, ultimately forming Bagasse@TGIC@DOPO with an IPN structure. Finally, the novel flame retardant was added to epoxy to create a composite. The integral procedural decomposition temperature (IPDT) of pure epoxy is 619 °C; after the introduction of the 30 wt% flame retardant, the IPDT of the resultant composite material increased to 799 °C, greatly increasing the thermal stability by 29%. After the addition of the Bagasse@TGIC@DOPO flame retardant, the limiting oxygen index increased from 21% for the pure epoxy to 29% for the composite, and the UL-94 rating improved from failing rating for the pure epoxy and V-0 rating for the composite. The Raman spectrum indicated that the addition of Bagasse@TGIC@DOPO IPN substantially increased the biochar yield during the burning process, increasing thermal stability. These results confirmed that the epoxy/Bagasse@TGIC@DOPO composite had substantial flame retarding effects.

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.

Synthesis of a Flame Retardant for Epoxy Resins: Thermal Stability, Flame Retardancy, and Flame-Retardant Modes

A polyphosphonate (PDPA) flame retardant that contains phenyl phosphonic dichloride and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide groups, has been synthesized. The flame retardant was introduced into epoxy resins (EP) and cured by 4,4’-diamino diphenylmethane. The vertical burning, limited-oxygen index and cone calorimeter tests reveal that the PDPA can enhance the flame-retardant properties of the EP significantly. With only a 4 wt% PDPA loading, the EP composites achieved a limited-oxygen index value of 33.4% and a V-0 rating in the vertical burning test, and the peak heat release rate and total heat release were decreased by 40.9% and 24.6%, respectively. The thermal properties and gas pyrolysis products of the EP composites were evaluated by thermogravimetric analysis and thermogravimetric analysis-Fourier transform infrared spectroscopy, and the morphology and structure of residual char were characterized by scanning electron microscopy, Flourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. To explain the combined effects of the condensed and gas phases, modes of the flame-retardant action are proposed.

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.

Synthesis of a Novel Flame Retardant Containing Phosphorus, Nitrogen, and Silicon and Its Application in Epoxy Resin

A novel flame retardant (TDA) containing phosphorus, nitrogen, and silicon was synthesized successfully via a controllable ring-opening addition reaction between 1,3,5-triglycidyl isocyanurate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and 3-aminopropyltriethoxysilane, and TDA was then blended with diglycidyl ether of bisphenol A to prepare flame-retardant epoxy resins (EPs). The chemical structure and components of TDA were confirmed by Fourier transform infrared (FTIR) spectra, ³¹P nuclear magnetic resonance, and X-ray photoelectron spectroscopy. Thermogravimetric analysis results indicated that after the introduction of TDA, cured EP maintained good thermal stability with a minimum initial decomposition temperature of 337.6 °C, and the char yields of a EP/TDA-5 sample significantly increased by 76.2% compared with that of the neat EP thermoset. Additionally, with the addition of 25.0 wt % TDA (1.05 wt % phosphorus loading), the limited oxygen index value of cured EP increased from 22.5% of pure EP to 33.4%, and vertical burning V-0 rating was easily achieved. Meanwhile, after the incorporation of TDA, the total heat release and total smoke production of the EP/TDA-5 sample obviously reduced by 28.9 and 27.7% in the cone calorimeter test, respectively. Flame-retardant performances and flame-retardant mechanisms were further analyzed by scanning electron microscopy, FTIR, energy-dispersive spectrometry, and pyrolysis gas chromatography/mass spectrometry. The results reveal that the synergistic effect of phosphorus, nitrogen, and silicon plays an excellent flame-retardant role in both gaseous and condensed phases. In addition, the mechanical and dynamic mechanical properties of cured EP thermosets are well maintained rather than destroyed. All the results demonstrate that TDA endows epoxy resin with excellent flame retardancy and possesses great promise in the industrial field.

The Preparation, Thermal Properties, and Fire Property of a Phosphorus-Containing Flame-Retardant Styrene Copolymer

A 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) acrylate, (6-oxidodibenzo [c,e][1,2] oxaphosphinin-6-yl) methyl acrylate (DOPOAA), has been prepared. Copolymers of styrene (St) and DOPOAA were prepared by emulsion polymerization. The chemical structures of copolymers containing levels of DOPOAA were verified using Fourier transform infrared (FT-IR) spectroscopy and 1H nuclear magnetic resonance (1H-NMR) spectroscopy. The thermal properties and flame-retardant behaviors of DOPO-containing monomers and copolymers were observed using thermogravimetric analysis and micro calorimetry tests. From thermogravimetric analysis (TGA), it was found out that the T5% for decomposition of the copolymer was lower than that of polystyrene (PS), but the residue at 700 °C was higher than that of PS. The results from micro calorimetry (MCC) tests indicated that the rate for the heat release of the copolymer combustion was lower than that for PS. The limiting oxygen index (LOI) for combustion of the copolymer rose with increasing levels of DOPOAA. These data indicate that copolymerization of the phosphorus-containing flame-retardant monomer, DOPOAA, into a PS segment can effectively improve the thermal stability and flame retardancy of the copolymer.

Synthesis of a DOPO-triazine additive and its flame-retardant effect in rigid polyurethane foam

A DOPO (9,10-dihydro-9-oxa-10-phosphaphen-anthrene-10-oxide)-based halogen-free flame retardant (ODOPM-CYC) was synthesized and incorporated in rigid polyurethane foam (RPUF). The structure of ODOPM-CYC was characterized by Fourier transform infrared spectra (FTIR), 1H NMR and 31P NMR. The effects of ODOPM-CYC on the flame resistance, mechanical performances, thermal properties and cell structure of RPUF were also investigated. The results showed that the incorporation of ODOPM-CYC strikingly enhanced flame retardant properties of RPUF. The flame retarded RPUF acquired a limiting oxygen index (LOI) value of 26% and achieved UL-94 V-0 rating with the phosphorus content of 3 wt%. The smoke production rate (SPR) also showed an obvious decrease and total smoke release (TSR) was 39.8% lower than that of neat RPUF. Besides, the results demonstrated that the incorporation of ODOPM-CYC provided RPUF better thermal stability but did not show any obvious influence on its thermal conductivity.

Improved thermal properties of epoxy resin modified with polymethyl methacrylate-microencapsulated phosphorus-nitrogen-containing flame retardant

Epoxy resin (EP) composites with improved thermal resistance were fabricated. To solve the problem of low thermal resistance derived from phosphazene flame-retardant additives, we designed a system based on flame-retardant microcapsules P(H), with hexaphenoxycyclotriphosphazene as the core and polymethyl methacrylate as the shell. The core–shell structure was characterized and confirmed. The thermal resistance of the cured EP composites containing 1 wt% P(H) microcapsules was improved because of the increased glass transition temperatures. The P(2.75H)/EP composites can reach a limited oxygen index of 30.5% and V-1 rating in UL-94 tests. Heat and gas release rates were reduced during combustion tests. Residual images implied that the P(H) microcapsules may promote the formation of a flame-retardant char layer. Pyrolysis analysis demonstrated that the P(H) microcapsules can decompose in two procedures to produce flame-retardant gas components. Therefore, the flame-retardant mechanism involved the flame inhibition effect in the gas phase, and the charring effect in the condensed phase.

Epoxy resin composites with improved thermal resistance and flame retardancy were fabricated based on P(H) microcapsules.

Synthesis of a lignin-based phosphorus-containing flame retardant and its application in polyurethane

In this work, new lignin-based flame retardant LHDs were successfully synthesized through the reaction between lignin, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexamethylene diisocyanate (HDI). The chemical structure of LHD was characterized by FTIR, ¹H NMR, ³¹P NMR. The thermal stability of LHD was studied by TGA. The results showed that the residual carbon content of L15HD (15% of lignin in LHD) at 600 °C reached 16.55%, indicating that this prepared flame retardant can be a type of good char forming agent. LHDs were then applied to prepare flame-retardant lignin-based polyurethane (FLPU). Lignin-based polyurethane (LPU) was synthesized by the reaction between lignin, polyethylene glycol 200 (PEG 200) and hexamethylene diisocyanate (HDI). The limiting oxygen index (LOI) value of the FLPU reached 30.2% when the addition content of L15HD (15% lignin in LHD) in L₂₀PU (20% lignin in LPU) was 25%, exhibiting excellent flame-retardant properties. Scanning electron microscopy (SEM) analysis of the FLPU char residual showed that there was a continuous dense outer carbon layer on the residue surface, and the inner carbon layer had many expansion bubbles, indicating the LHDs have an excellent flame retardant effect for PU. In addition, FLPU presented better hardness and adhesion than PU. The hardness of F_L15-25L₂₀PU (lignin content in LPU was 20%, and added content of L15HD in LPU was 25%) reached 4H, and its adhesion was 0. These excellent properties illustrated that the LHDs are ideal flame retardants and reinforcing agents for LPU because of the co-curing and strong interface between LHD and LPU.

In this work, new lignin-based flame retardant LHDs were successfully synthesized through the reaction between lignin, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and hexamethylene diisocyanate (HDI).