Synthesis of three novel phosphorus-containing flame retardants and their application in epoxy resins

One symmetric diamine (4) and two symmetric phenols (5) and (6) were synthesized as phosphorus-containing flame retardants. The synthesis comprised a two-step procedure: the condensation of p-phenylenediamine with benzaldehyde, 4-hydroxybenzaldehyde and 2-hydroxybenzaldehyde respectively, followed by the addition of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide to the imine linkage. The structures of (4)-(6) were characterized by FTIR, NMR and mass spectra. (4)-(6) served as co-curing agents of diaminodiphenylmethane for epoxy resins, and epoxy thermosets exhibited excellent flame retardancy, moderate changes in glass transition temperature (T_g) and thermal stability. When the phosphorus content reached 1.0 wt.%, the epoxy resin system met the UL-94 V-0 classification and the limiting oxygen index (LOI) reached more than 35.6, probably because of the nitrogen-phosphorus synergistic effect.     

Metal compound-enhanced flame retardancy of intumescent epoxy resins containing ammonium polyphosphate

A series of intumescent flame-retardant epoxy resins (IFR-EPs) were prepared only by adding a 5 wt% total loading of ammonium polyphosphate (APP) and metal compounds. All the samples could achieve V-0 rating and did not generate dripping during UL-94 testing. The limiting oxygen index (LOI) values of the samples with 4.83 wt% APP and 0.17 wt% CoSA increase from 27.1 to 29.4, compared with epoxy resin containing 5 wt% APP. The samples also showed excellent water resistance of flame retardancy in 30 °C and 70 °C water for 168 h. The LOI results show that the composition of metal compounds (metal ions and ligands/anions) and the mass ratios of APP to metal compounds affect the flame retardancy of the samples. TG results indicate that the catalytic effect of CoSA on the decomposition of both APP and the epoxy resins containing APP is better than that of CuSAO. The fire behavior of epoxy resin and epoxy resins containing APP with/without CoSA were investigated by cone calorimeter. Cone calorimeter parameters of the samples such as HRR, THR, TSP and COP indicate that the addition of APP and CoSA improves the fire safety of epoxy resin significantly, and CoSA shows an obvious catalytic effect.     

Dispersion of graphene oxide and its flame retardancy effect on epoxy nanocomposites

Graphene oxide was prepared by ultrasonication of completely oxidized graphite and used to improve the flame retardancy of epoxy.The epoxy/graphene oxide nanocomposite was studied in terms of exfoliation/dispersion,thermal stability and flame retardancy.X-ray diffraction and transmission electron microscopy confirmed the exfoliation of the graphene oxide nanosheets in epoxy matrix.Cone calorimeter measurements showed that the time to ignition of the epoxy/graphene oxide nanocomposite was longer than that of neat epoxy.The heat release rate curve of the nanocomposite was broadened compared to that of neat epoxy and the peak heat release rate decreased as well.     

Synthesis, characterization, thermal properties and flame retardancy of a novel nonflammable phosphazene-based epoxy resin

Hexakis(4-hydroxyphenoxy)-cyclotriphosphazene (PN-OH) was synthesized through nucleophilic substitution of the chloride atoms of hexachlorocyclotriphosphazene and reduction of the aldehyde groups, and its chemical structure was characterized by elemental analysis, ¹H and ³¹P NMR, and Fourier transform infrared (FTIR) spectroscopy. A new phosphazene-based epoxy resin (PN-EP) was successfully synthesized through the reaction between diglycidyl ether of bisphenol-A (DGEBA) and PN-OH, and its chemical structure was confirmed by FTIR and gel permeation chromatography. Four PN-EP thermosets were obtained by curing with 4,4′-diaminodiphenylmethane (DDM), dicyandiamide (DICY), novolak and pyromellitic dianhydride (PMDA). The reactivity of PN-EP with the four curing agents presents an increase in the order of DDM, PMDA, novolak and DICY. An investigation on their thermal properties shows that the PN-EP thermosets achieve higher glass-transition and decomposition temperatures in comparison with the corresponding DGEBA ones while their char yields increase significantly. The PN-EP thermosets also exhibit excellent flame retardancy. The thermosets with novolak, DICY and PMDA achieve the LOI values above 30 and flammability rating of UL94 V-0, whereas the one with DDM reaches the V-1 rating. The nonflammable halogen-free epoxy resin synthesized in this study has potential applications in electric and electronic fields in consideration of the environment and human health.     

Pyrolysis and fire behaviour of epoxy resin composites based on a phosphorus-containing polyhedral oligomeric silsesquioxane (DOPO-POSS)

The pyrolysis and fire behaviour of epoxy resin (EP) composites based on a novel polyhedral oligomeric silsesquioxane containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-POSS) and diglycidyl ether of bisphenol A (DGEBA) have been investigated. The pre-reaction between the hydroxyl groups of DOPO-POSS and the epoxy groups of DGEBA at 140 °C is confirmed by FTIR, which means that DOPO-POSS molecules of hydroxyl group could easily disperse into the epoxy resin at the molecular level. The EP composites with the DOPO-POSS were prepared through a curing agent, m-phenylenediamine (m-PDA). The morphologies of the EP composites observed by SEM indicate that DOPO-POSS disperses with nano-scale particles in the EP networks, which implies good compatibility between them. The thermal properties and pyrolysis of the EP composites were analyzed by DSC and TGA, TGA-FTIR, and TGA-MS. The analysis indicates that the DOPO-POSS change the decomposition pathways of the epoxy resin and increase its residue at high temperature; moreover, the release of phosphorous products in the gas phase and the existence of Si-O and P-O structures in the residue Is noted. The fire behaviour of the EP composites was evaluated by cone calorimeter (CONE). The CONE tests show that incorporation of DOPO-POSS into epoxy resin can significantly improve the flame retardancy of EP composites. SEM and XPS were used to explore micro-structures and chemical components of the char from CONE tests of the EP composites, they support the view that DOPO-POSS makes the char strong with the involvement of Si-O and P-O structures.     

A flame-retardant epoxy resin based on a reactive phosphorus-containing monomer of DODPP and its thermal and flame-retardant properties

A flame-retardant epoxy resin (EP) was synthesized based on a novel reactive phosphorus-containing monomer, 4-[(5,5-dimethyl-2-oxide-1,3,2-dioxaphosphorinan-4-yl)oxy]-phenol (DODPP), and its structures were characterized by FTIR, ¹H NMR and ³¹P NMR spectra. The DODPP-EP₃/LWPA (low molecular weight polyamide), which contains 2.5% phosphorus, can reach UL-94 V-0 rating and a limiting oxygen index (LOI) value of 30.2%. The thermal properties and burning behaviours of cured epoxy resins were investigated by differential scanning calorimeter (DSC), thermogravimetry (TG), LOI, UL-94 tests and cone calorimetry. The morphologies of residues of cured epoxy resins were investigated by scanning electron microscopy (SEM). DSC shows that the glass-transition temperatures of cured epoxy resins decrease with increasing phosphorus content. TGA shows that the onset decomposition temperatures and the maximum-rate decomposition temperatures decrease, while char yields increase, with the increase of phosphorus content. The data from the cone calorimeter tests give the evidence that heat release rate (HRR), peak heat release rate (PHRR), average heat release rate (Av-HRR), average mass loss rate (Av-MLR) and the fire growth rate index (FIGRA) decrease significantly for DODPP-EP₃/LWPA. SEM shows that the DODPP-EP₃/LWPA forms lacunaris and compact charred layers which inhibit the transmission of heat during combustion.     

Preparation, thermal properties and flame retardancy of phosphorus- and silicon-containing epoxy resins

Phosphorus- and silicon-containing epoxy resins were prepared from (2,5-dihydroxyphenyl)diphenyl phosphine oxide (Gly-HPO), diglycidyloxy methylphenyl silane (DGMPS) and 1,4-bis(glycidyloxydimethyl silyl)-benzene (BGDMSB) as epoxy monomers and diaminodiphenylmethane (DDM), bis(3-aminophenyl)methyl phosphine oxide (BAMPO) and bis(4-aminophenoxy)dimethyl silane (APDS) as curing agents. Epoxy resins with different phosphorus and silicon content were obtained. Their thermal, dynamic mechanical and flame retardant properties were evaluated. The high LOI values confirmed that epoxy resins containing hetero-atoms are effective flame retardants, but a synergistic efficiency of phosphorus and silicon on flame retardation was not observed.     

Effects of zinc-based flame retardants on the degradation behaviour of an aerospace epoxy matrix

Flame behaviour is a fundamental requirement for advanced aerospace composites. In this work, a commercial, low-viscosity epoxy system, typically used in liquid infusion composite processes, and its mixtures with three different zinc-based flame retardants (ZB, ZS, ZHS) at different weight percentages has been investigated by cone calorimetry and thermogravimetric analysis.Cone calorimetry has been performed to verify the flame retardancy effects induced by each filler composition. Nevertheless manufacturability issues require the evaluation of the rheological changes induced by filler on the unloaded matrix system. Rheological tests have been, therefore, performed to identify the maximum concentration of filler. Based on these results thermogravimetric tests have been performed to investigate thermal degradation kinetics of selected systems. The feasibility of Kissinger and Flynn-Wall-Ozawa method for the determination of characteristic degradation kinetics parameters has been evaluated and results were analysed. A simplified decomposition model was assumed to analyse epoxy degradation behaviour; it was found that this model gives appreciable matching with experimental TGA curve trend for neat epoxy whereas for the filled compounds additional stages were assume to occur.     

The flame retardancy of alginate/flame retardant viscose fibers investigated by vertical burning test and cone calorimeter

The smoldering weakness of alginate fibers can be effectively suppressed by simple blending with flame retardant viscose fibers. The char formed by flame retardant viscose fibers can prevent the heat transmission and suppress the smoldering of alginate.     

A quantitative study of carbon monoxide and carbon dioxide evolution during thermal degradation of flame retarded epoxy resins

Thermogravimetric analysis (TGA) combined with infrared analysis of the evolved gases analysis (EGA) has been used to study the thermal degradation behaviour of epoxy resin both in air and nitrogen. The mass loss as a function of temperature has been correlated with the evolution of carbon monoxide (CO) and carbon dioxide (CO₂), and oxygen consumption as measured using an oxygen analyser. An analytical technique has been developed to quantitatively measure the carbon monoxide and dioxide gases evolved. The effect of a range of flame retardants containing phosphorus, nitrogen and halogen elements on CO and CO₂ evolution during thermal degradation of flame retarded epoxy resins has also been observed.     

Studies on the effect of different levels of toughener and flame retardants on thermal stability of epoxy resin

A thermoplastic toughener, polyether sulphone (PES) and a number of different types of flame retardants were blended in different ratios with a commercial epoxy resin triglycidyl-p-aminophenol (TGAP) and 4,4-diamino diphenyl sulphone (DDS) a curing agent. The effect of type and levels of flame retardants (FR) and the toughening agent on the curing, thermal decomposition and char oxidation behaviour of the epoxy resin was studied by the simultaneous differential thermal analysis and thermogravimetric techniques. It was observed that the toughener slightly increases the curing temperature (by up to 20 °C) but had minimal effect on the decomposition temperature of the resin. Flame retardants, however affected all stages depending upon the type of flame retardant used. The curing peak for samples containing tougher and flame retardants although slightly changed depending upon the type of FR, was not more than ± 20 °C compared to that of samples containing toughener only. All flame retardants lowered the decomposition temperature of the epoxy resin. Phosphorus- and nitrogen-containing flame retardants reduced the char oxidation leading to more residual char, whereas halogen- containing flame retardants had less effect on this stage.     

Novel DOPO-based flame retardants in high-performance carbon fibre epoxy composites for aviation

Two novel, halogen-free, phosphorus-based oligomeric flame retardants are investigated in the commercial epoxy resin RTM6 and ∼70 wt.% carbon fibre RTM6 composites (RTM6-CF) with respect to pyrolysis and fire behaviour. The flame retardants are based on 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO) units linked to the star-shaped aliphatic ground body tetra-[(acryloyloxy)ethyl] pentarythrit (DOPP), or heterocyclic tris-[(acryloyloxy)ethyl] isocyanurate (DOPI), respectively. The glass transition temperature is reduced by adding DOPP and DOPI, but the mechanical properties of the composites (e.g. interlaminar shear strength (ILSS) and G_c in mode I and II) remain unchanged. Decomposition models are proposed based on mass loss, evolved gas analysis (TG–FTIR) and condensed product analysis (hot stage cell within FTIR). The fire behaviour is investigated comprehensively (UL 94, limiting oxygen index (LOI) and cone calorimeter). Both flame retardants act in the gas phase through flame inhibition and in the condensed phase through charring. The UL 94 of RTM6 is improved from HB to V-1 and V-0; the LOI from 25% to 34–38%. Peak heat release rate (PHRR) and total heat evolved (THE) are lowered by 31-49% and 40–44%, respectively. Adding CF increases the residue, reduces the THE, but suppresses the charring due to RTM6 and flame retardants. Thus the THE of RTM6-CF is reduced by about 25% when DOPI and DOPP are added. However, UL 94: V-0 and LOI of 45% and 48% are achieved with ∼0.6 wt.% phosphorus.     

Facile synthesis of bio-based phosphorus-containing epoxy resins with excellent flame resistance

The development of high-performance biomass-derived epoxy thermosets with excellent flame resistance is vital to various applications (i.e., composites, coatings and adhesives). Herein, a difunctional epoxy monomer bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl) phenyl phosphate (BEU-EP) was synthesized from abundant and biobased eugenol. In addition, BEU-EP was cured by 4,4′-diaminodiphenyl methane (DDM) and the cured resin diglycidyl ether of bisphenol A (DGEBA)/DDM was used as a reference. Results indicated that BEU-EP/DDM not only showed a 58.1%, 28.8% and 35.1% increase in residual char (at 700 °C), flexural and storage modulus (at 30 °C) compared with DGEBA/DDM, but also exhibited excellent flame resistance and smoke suppression. BEU-EP/DDM passed V-0 rating (in UL-94 testing) with limiting oxygen index (LOI) of 38.4% and greatly decreased the peak heat release rate (pHRR) and total smoke production (TSP) by 84.9% and 80.5%, respectively. The mechanism analysis confirmed that the phosphorus-containing group and aromatic structure from BEU-EP contributed both the gas and condensed-phase flame retardation of BEU-EP/DDM network. This work provides an efficient and scalable route for synthesizing biobased epoxy thermosets with high integrated performance and superior flame resistance.     

Novel high Tg flame retardancy approach for epoxy resins

A novel halogen-free phosphorus- and nitrogen-containing heterocyclic flame retardant - 5,10-dihydro-phenophosphazine-10-oxide (DPPA) - was synthesised and characterised by NMR, MS, and XRD analyses. DPPA and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were successfully incorporated in the backbone of diglycidyl ether of bisphenol A (DGEBA). The resultant prepolymers were cured with 4,4′-diaminodiphenylmethane (DDM) and diethyldiamino toluene (DETDA 80). The burning behaviours of the thermosets obtained were studied by UL 94 and LOI and compared to each other. Also their glass transition temperatures (Tgs) were determined by differential scanning calorimetry (DSC). Due to the bridging incorporation of DPPA in DGEBA by fusion process, high T_gs for these samples were achieved.     

Effect of anionic organoclay with special aggregate structure on the flame retardancy of acrylonitrile-butadiene-styrene/clay composites

Two organoclays, octadecylammonium modified montmorillonite (COM) and an anionic surfactant modified montmorillonite (AOM), were used to prepare acrylonitrile-butadiene-styrene (ABS)/clay composites. The flammability of these composites was evaluated by cone calorimetry. COM enhanced the flame retardancy of ABS/COM composite due to the catalysis by acidic sites originating from COM. Interestingly, AOM presented a similar flame retardancy to ABS/AOM composite, although AOM did not form the acidic sites. The morphology observation showed that the dispersion degree of AOM in ABS resin was lower than that of COM, which was also confirmed by the rheological properties of ABS composites. The investigations on the surface of the residue after cone calorimeter tests showed that the better flame retardancy of ABS/AOM composite was mainly due to the special structure of AOM.     

A novel intumescent flame retardant imparts high flame retardancy to epoxy resin

Intumescent flame retardant (IFR) has received the considerable attention ascribed to the inherent advantages including non‐halogen, low toxicity, low smoke release and environmentally friendly. In this work, a novel charring agent poly (piperazine phenylaminophosphamide) named as PPTA was successfully synthesized and characterized by Fourier transform infrared spectra (FTIR) and X‐ray photoelectron spectroscopy (XPS). Then, a series of flame‐retardant EP samples were prepared by blending with ammonium polyphosphate (APP) and PPTA. Combustion tests include oxygen Index (LOI), vertical Burning Test (UL‐94) and cone calorimeter testing,these test results showed that PPTA greatly enhances the flame retardancy of EP/APP. According to detailed results, EP containing 10 wt% APP had a LOI value of 30.2%,but had no enhancement on UL‐94 rating. However, after both 7.5 wt% APP and 2.5 wt% PPTA were added, EP‐7 passed UL‐94 V‐0 rating with a LOI value of 33.0%. Moreover, the peak heat release rate (PHRR) and peak of smoke product rate (PSPR) of EP‐7 were greatly decreased. Meanwhile, the flame‐retardant mechanism of EP‐7 was investigated by scanning electron microscopy (SEM), thermogravimetric analysis/infrared spectrometry (TG‐IR) and X‐ray photoelectron spectroscopy (XPS). The corresponding results presented PPTA significantly increased the density of char layer, resulting in the good flame retardancy.     

Synthesis,Characterization of Phosphorus ContainingDiamide-diimide-tetraamines Based on L-TryptophanAmino Acid and Their Effect on Flame Retardancy ofEpoxy Resins简

The curing behavior of diglycidyl ether of bisphenol-A(DGEBA) with different phosphorus containing diamidediimide-tetraamines(DADITAs) was studied by DSC. Eight DADITAs of varying structures were synthesized by reacting 1 mole of pyromellitic anhydride(PMDA)/3,3′-benzophenone tetracarboxylic dianhydride(BTDA)/1,4,5,8-naphthalene tetracarboxylic dianhydride(NTDA)/4,4′-oxydiphthalic anhydride(ODPA) with 2 mole of L-tryptophan(T) in a mixture of acetic acid and pyridine(3:2 V/V) followed by activaton with thionyl chloride and then condensation with excess of phosphorus containing triamines tris(3-aminophenyl) phosphine(TAP) and tris(3-aminophenyl) phosphine oxide(TAPO). DADITAs obtained by reacting PMDA/BTDA/NTDA/ODPA with L-tryptophan followed by condensation with TAP/TAPO were designated as PTAP, PTAPO, BTAP, BTAPO, NTAP, NTAPO, OTAP and OTAPO respectively. The structural characterization of synthesized DADITAs was done by FTIR,1H-NMR,13C-NMR,31P-NMR spectroscopic techniques and elemental analysis. Thermal stability of the isothermally cured epoxy was investigated using dynamic thermogravimetry analysis. The glass transition temperature(Tg) was highest in DGEBA cured using PTAP. All epoxy thermosets exhibited excellent flame retardancy, moderate changes in Tg and thermal stability. Due to presence of phosphorus in curing agents, all epoxy resin systems met the UL-94 V-0 classification and the limiting oxygen index(LOI) reached up to 38.5, probably because of the nitrogen-phosphorus synergistic effect.     

Kinetics study of thermal decomposition of epoxy resins containing flame retardant components

Hyperbranched polyphosphate ester (HPPE) and phenolic melamine (PM) were blended in different ratios with a commercial epoxy resin to obtain a series of flame retardant resins. The thermal decomposition mechanism of their cured products in air was studied by thermogravimetric analysis and in situ Fourier-transform infrared spectroscopy. The degradation behaviours of epoxy resins containing various flame retardant components were found to be greatly changed. The incorporation of phosphorus and nitrogen compounds improved the thermal stability at elevated temperature. The kinetics of thermal decomposition was evaluated by Kissinger method, Flynn-Wall-Ozawa method and Horowitz-Metzger method. The results showed that the activation energy at lower degree of the degradation decreased by the incorporation of flame retardant components, while increased at higher degree of the degradation.     

Influence of POSS structure on the fire retardant properties of epoxy hybrid networks

Monofunctional Polyhedral Oligomeric Silsesquioxanes (POSS), differing in chemical structure, were introduced into an epoxy-amine formulation in order to obtain hybrid organic/inorganic epoxy networks. The process developed allows the POSS to be either covalently bonded to the network and organized in small amorphous domains, or completely dispersed in larger crystalline domains. Morphology, thermal and fire retardant properties were studied through electron microscopy, DSC, TGA, cone calorimeter experiments and UL94 tests. It was demonstrated that POSS nanoclusters induced an effective fire retardant effect, which was controlled by several factors. It was shown that POSS bearing phenyl ligands were far more effective than POSS with isobutyl ligands, and that the presence of a chemical linkage between the phenyl-based POSS clusters and the matrix favoured the dispersion of the nanoclusters, resulting in enhanced fire retardancy. The fire retardant effect was only slightly enhanced by increasing the amount of POSS nanoclusters.