Flame‐retardant properties and mechanisms of epoxy thermosets modified with two phosphorus‐containing phenolic amines

Two phosphorus‐containing phenolic amines, a 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO)‐based derivative (DAP) by covalently bonding DOPO and imine (SB) obtained from the condensation of p‐phenylenediamine with salicylaldehyde, and its analog (AP) via the addition reaction between diethyl phosphite and SB, were used to prepare flame‐retardant epoxy resins. The burning behaviors and dynamic mechanical properties of epoxy thermosets were studied by limited oxygen index (LOI) measurement, UL‐94 test, and dynamic mechanical analysis. The flame‐retardant mechanisms of modified thermosets were investigated by thermogravimetric analysis, Py‐GC/MS, Fourier transform infrared, SEM, elemental analysis, and laser Raman spectroscopy. The results revealed that epoxy thermoset modified with DAP displayed the blowing‐out effect during UL‐94 test. With the incorporation of 10 wt % DAP, the modified thermoset showed an LOI value of 36.1% and V‐0 rating in UL‐94 test. The flame‐retardant mechanism was ascribed to the quenching and diluting effect in the gas phase and the formation of phosphorus‐rich char layers in the condensed phase. However, the thermoset modified with 10 wt % AP only showed an LOI value of 25.7% and no rating in UL‐94 test, which was possibly ascribed to the mismatching of charring process with gas emission process during combustion. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43953.     

Synthesis and properties of a novel phosphorous‐containing flame‐retardant hardener for epoxy resin

An aryl phosphinate dianhydride 1,4‐bis(phthalic anhydride‐4‐carbonyl)‐2‐(6‐oxido‐6H‐dibenz[c,e][1,2]‐oxaphosphorin‐6‐yl)‐phenylene ester (BPAODOPE) was synthesized and its structure was identified by FTIR and ¹H‐NMR. BPAODOPE was used as hardener and flame retardant for preparing halogen‐free flame‐retarded epoxy resins when coupled with another curing agent. Thermal stability, morphologies of char layer, flame resistance and mechanical properties of flame‐retarded epoxy resins were investigated by thermogravimetric analysis, SEM, limiting oxygen index (LOI), UL‐94 test, tensile, and charpy impact test. The results showed that the novel BPAODOPE had a better flame resistance, the flame resistance and char yield of flame‐retarded epoxy resins increased with an increase of phosphorus content, tensile strength and impact strength of samples gradually decreased with the addition of BPAODOPE. The flame‐retarded sample with phosphorus contents of 1.75% showed best combination properties, LOI value was 29.3, and the vertical burning test reached UL‐94 V‐0 level, tensile strength and impact strength were 30.78 MPa and 3.53 kJ/m², respectively. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Flame retardancy and dielectric properties of dicyclopentadiene‐based benzoxazine cured with a phosphorus‐containing phenolic resin

A dicyclopentadiene‐based benzoxazine (DCPDBZ) was prepared and separately copolymerized with melamine–phenol formaldehyde novolac or phosphorus‐containing phenolic resin (phosphorus‐containing diphenol) at various molar ratios. Their curing behaviors were characterized by differential scanning calorimetry. The electrical properties of the cured resins were studied with a dielectric analyzer. The glass‐transition temperatures were measured by dynamic mechanical analysis. The thermal stability and flame retardancy were determined by thermogravimetric analysis and a UL‐94 vertical test. These data were compared with those of bisphenol A benzoxazine and 4,4′‐biphenol benzoxazine systems. The effects of the diphenol structure and cured composition on the dielectric properties, moisture resistance, glass‐transition temperature, thermal stability, and flame retardancy are discussed. The DCPDBZ copolymerized with phosphorus‐containing novolac exhibited better dielectric properties, moisture resistance, and flame retardancy than those of the melamine‐modified system. The flame retardancy of the cured benzoxazine/phosphorus‐containing phenolic resins increased with increasing phosphorus content. The results indicate that the bisphenol A and 4,4′‐biphenol systems with a phosphorus content of about 0.6% and the dicyclopentadiene system with a phosphorus content of about 0.8% could achieve a flame‐retardancy rating of UL‐94 V‐0. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis of a novel phosphorus‐containing dicyclopentadiene novolac hardener and its cured epoxy resin with improved thermal stability and flame retardancy

A novel phosphorus‐containing dicyclopentadiene novolac (DCPD‐DOPO) curing agent for epoxy resins, was prepared from 9,10‐dihydro‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) and n‐butylated dicyclopentadiene phenolic resin (DCPD‐E). The chemical structure of the obtained DCPD‐DOPO was characterized with FTIR, ¹H NMR and ³¹P NMR, and its molecular weight was determined by gel permeation chromatography. The flame retardancy and thermal properties of diglycidyl ether bisphenol A (DGEBA) epoxy resin cured with DCPD‐DOPO or the mixture of DCPD‐DOPO and bisphenol A‐formaldehyde Novolac resin 720 (NPEH720) were studied by limiting oxygen index (LOI), UL 94 vertical test and cone calorimeter (CCT), and differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. It is found that the DCPD‐DOPO cured epoxy resin possess a LOI value of 31.6% and achieves the UL 94 V‐0 rating, while its glass transition temperature (T_g) is a bit lower (133 °C). The T_g of epoxy resin cured by the mixture of DCPD‐DOPO and NPEH720 increases to 137 °C or above, and the UL 94 V‐0 rating can still be maintained although the LOI decreases slightly. The CCT test results demonstrated that the peak heat release rate and total heat release of the epoxy resin cured by the mixture of DCPD‐DOPO and NPEH720 decrease significantly compared with the values of the epoxy resin cured by NPEH720. Moreover, the curing reaction kinetics of the epoxy resin cured by DCPD‐DOPO, NPEH720 or their mixture was studied by DSC. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44599.     

Effect of triphenyl phosphate flame retardant on properties of arylamine‐based polybenzoxazines

Three types of arylamine‐based benzoxazine resins modified with both condensed‐phase and gas‐phase action flame retardant, i.e. triphenyl phosphate (TPP) at various weight ratios were investigated. From rheological study, it was found that the viscosity of benzoxazines/TPP mixtures were significantly lower than that of the neat benzoxazine monomers suggesting flow property enhancement. Furthermore, differential scanning calorimetry results revealed that the onset and the maximum temperatures of the exothermic peak, due to the ring opening polymerization of benzoxazine resins, shifted to lower temperatures with increasing TPP. In addition, all polybenzoxazines possessed relatively high char yield, which increased as the TPP content increased thus enhancing their flame retardancy. The limiting oxygen index values of the flame retarded polybenzoxazines also increased with TPP addition. The maximum flame retardancy of UL94 V‐0 class was obtained with an addition of only few percents of TPP in the polybenzoxazines. Flexural strength, flexural modulus, and glass transition temperature of those polybenzoxazines tended to decrease with an addition of TPP mainly due to its plasticizing effect. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1074‐1083, 2013     

A phosphorus‐containing diamine for flame‐retardant,high‐functionality epoxy resins. I. Synthesis,reactivity, and thermal degradation properties

A phosphorus‐containing amine, bis(4‐aminophenoxy)phenyl phosphine oxide (BAPP), suitable for curing epoxy resins with improved fire performance was synthesized and characterized with Fourier transform infrared spectroscopy and nuclear magnetic resonance. The reactivity of the amino group was evaluated by differential scanning calorimetry of the epoxy–amine mixture and by proton nuclear magnetic resonance of the amino unit. With BAPP as a curing agent, a range of high‐functionality, aerospace epoxy resins were cured, and the dynamic kinetic parameters calculated from Kissinger's and Ozawa's models were compared with those from the more widely used amines. The thermal degradation properties of the phosphorus‐containing epoxy resins were studied by thermogravimetric analysis, the degradation activation energy was calculated, and a multistep thermal degradation process was observed. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2093–2100, 2004     

Synthesis and properties of novel phosphorus‐containing bismaleimide/epoxy resins

A novel soluble phosphorus‐containing bismaleimide (BMI) monomer, bis(3‐maleimidophenyl)phenylphosphine oxide (BMIPO), was synthesized by the imidization of bis(3‐aminophenyl) phenylphosphine oxide, in which its structural characterization was identified with ¹H‐NMR, ¹³C‐NMR, and Fourier transform infrared spectra. The BMIPO resin, with five‐membered imide rings and high phenyl density, was an excellent flame retardant with a high glass‐transition temperature (T_g), onset decomposition temperature, and limited oxygen index. In phosphorus‐containing BMI/epoxy/4,4′‐methylene dianiline (DDM)‐cured resins, homogeneous products were obtained from all proportions without phase separation. Because of the higher reactivity of BMIPO/DDM relative to that of 4,4′‐bismaleimidodiphenylmethane (BMIM)/DDM, the increase in the BMIPO/BMIM ratio in this blending resin increased the recrosslinking hazards of the postcuring stage and so lowered the T_g value and thermal stability. The thermal stability of the BMI/epoxy‐cured system was lower than that of the epoxy‐cured system because of the introduction of a phosphide group into BMIPO, whereas for the T_g value and flame retardancy, the former was significantly higher than the latter: the higher the BMIPO content in the blend, the higher the flame retardancy. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2080–2089, 2002; DOI 10.1002/app.10607     

Transparent epoxy resin material with excellent fire retardancy enabled by a P/N/S containing flame retardant

An epoxy resin (EP) with excellent fire retardancy, good transparency, and satisfactory thermal stability has been obtained by introducing a new N/P/S containing flame retardant (HBD) into EP composites. When the phosphorus content was 0.48 wt%, EP/HBD reached V-0 rating with the limiting oxygen index of 33.5%. The cone calorimeter test (CC) indicated that the incorporation of HBD resulted in 1.5 times increase in ignition time, a 50% decrease in the maximum of heat release rates, 40% reduction of total heat release, and 50.7% decrease in total smoke production compared with EP. Besides, the fire-resistant behavior of EP/8% HBD is much better than the EP materials modified by similar P/N/S flame retardants reported in literature. The fire-retardant mechanism of HBD on EP was also analyzed by Raman, scanning electron microscope, Py-GC/MS, and Fourier transform infrared spectroscopy. The results show that HBD plays an important role in the formation of a dense intumescent carbon layer and gas phase quenching.     

Synthesis and characterization of water‐dispersed flame‐retardant polyurethane resin using phosphorus‐containing chain extender

In this research, a flame‐retardant water‐dispersed polyurethane resin was synthesized through incorporating phosphonate groups into the polyurethane structure in the chain‐extension step. A phosphorus‐containing reactive flame‐retardant compound was synthesized for this purpose. First, bis(4‐nitrophenyl)phenyl phosphine oxide was synthesized and then converted to bis(4‐amino phenyl)phenyl phosphine oxide (BAPPO) by reducing its nitro groups into amines. The obtained products were characterized by IR, ¹H‐NMR, and ³¹P‐NMR, and the thermal properties of the polymers were determined by DSC analysis. The BAPPO‐containing polyurethane showed physical properties that were almost similar to those of phosphorus‐free polyurethane and exhibited good flame resistance with a limiting oxygen index value of 27. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1314–1321, 2004     

Flame‐retardant mechanism of silica: Effects of resin molecular weight

The effects of resin molecular weight on the flame‐retardant mechanism of silica were studied with two different molecular weights of poly(methyl methacrylate) (PMMA), 122,000 and 996,000 g/mol, and two silicas, fused silica with a small surface area and silica gel with a large surface area. A total of six different samples were studied, with a mass fraction of 10% silica. The mass loss rate of the six samples in nitrogen and the heat release rate from burning in air were measured at an external radiant flux of 40 kW/m². The addition of silica gel to the low‐molecular‐weight PMMA significantly reduced the mass loss rate and heat release rate; addition to the high‐molecular‐weight PMMA provided the largest reductions of these quantities in this study. For fused silica, some reduction in mass loss rate and heat release rate was observed when it was added to the high‐molecular‐weight PMMA; addition to the low‐molecular‐weight PMMA did not reduce either loss rate. Chemical analysis of the collected residues and observation of the sample surface during gasification reveal the accumulation of silica near the surface; the larger its coverage over the sample surface was, less the mass loss rate and heat release rate were. Both the level of accumulation and its surface coverage depended strongly not only on the silica characteristics but also on the melt viscosity of the PMMA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1541–1553, 2003     

Excellent role of Cu2O on fire safety of epoxy resin with ammonium polyphosphate based on the construction of self-intumescent flame retardant system

This work reports an effective self-intumescent flame retardant system for epoxy resin (EP) based on the remarkable synergistic effect between Cu₂O and ammonium polyphosphate (APP). The effect of Cu₂O/APP on improving EP's fire performance was evaluated by limited oxygen index (LOI), UL-94, and cone calorimeter test. The optimal mass ratio of Cu₂O: APP was shown to be 2:8. With 15 wt% total flame retardant loading, the EP with optimum Cu₂O/APP formulation reached V-0 classification and high LOI (33.5%), while the EP with APP only got NR and low LOI (26.5%). Additionally, the pHRR, total heat release, total smoke production, CO production of the EP with optimum Cu₂O/APP formulation were primarily decreased. All the improvements were ascribed to the formation of the self-intumescent char layer of EP resulted from the catalyzing effect of Cu₂O for char formation and CO to CO₂ conversion. These findings will consolidate approaches for conferring flame retardancy to flammable polymers or their blends.     

Flame‐retardant,glass‐fabric‐reinforced epoxy resin laminates fabricated through a gradient distribution mode

In general, epoxy resin (EP) glue mixed with a high content of flame retardants is used to coat glass fabrics layer by layer to prepare fire‐retardant printed circuit boards (PCBs). However, the addition of the flame retardants not only increases the cost but also greatly deteriorates the processability and mechanical properties of the PCBs. In this study, a gradient distribution mode of composite flame retardants was designed and applied in EP‐based PCB composites. Unlike the traditional uniform distribution mode, in which flame retardants are evenly distributed in every resin layer, the gradient mode concentrates a higher content of the flame retardants on the surface layer, and the concentrations are gradually reduced along the thickness. In this way, the surface resin can quickly form a condensed charring barrier to hold back fire; this effectively protects the underlying resin, which has lower contents of flame retardant. The results of this study show that PCB prepared by the gradient mode obtained satisfactory flame retardance (a UL94 V‐0 rating) with only a 3.5 wt % total amount of flame retardant; this value was much lower than that (6.3 wt %) of composites featuring a uniform distribution. Additionally, the gradient mode also maintained the mechanical properties of PCB better. The tensile, impact, and flexural strengths of the gradient distribution system were obviously higher than those of the uniform distribution one with the same content of flame retardant. On the basis of the mode, a more economic and efficient technology was developed to manufacture flame‐retardant layered PCB. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44369.     

Flame‐retardant cyanate ester resin with suppressed toxic volatiles based on environmentally friendly halloysite nanotube/graphene oxide hybrid

Development of high‐performance thermosetting resins by adding environmentally friendly flame retardant to heat‐resistant resins without deteriorating their outstanding thermal stability is an important research direction. Here, a unique hybrid (GHNT) consisting of graphene oxide (GO) and halloysite nanotubes (HNT) was synthesized, and then a series of composites based on cyanate ester (CE) resin were fabricated. The effects of GHNT on the heat resistance, flame retardancy, and smoke suppression of GHNT/CE composites were intensively investigated. The GHNT/CE composite with 5.0 wt % GHNT not only has about 15.1 °C higher initial degradation temperature, but also shows 54.6% or 37.9% lower peak heat release rate or maximum smoke density than CE resin. These results clearly demonstrate that GHNT is not the simple combination of GO and HNT; instead, it obviously shows positive synergistic effects in simultaneously improving the flame retardancy and thermal resistance of CE resin. The improved flame retardancy could be attributed to condensed‐phase mechanisms, including increasing char yield, building a dense char layer, and free radical scavenging. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46587.     

Synthesis of phosphorus‐containing flame‐retardant antistatic copolymers and their applications in polypropylene

By adjusting the molar ratios of antistatic monomer of octyl phenol ethylene oxide acrylate (denoted as AS), rigid monomer of methyl methacrylate (denoted as MMA), and flame‐retardant monomer of 2‐(phosphoryloxymethyl oxyethylene) acrylate (denoted as FR), a series of flame‐retardant antistatic copolymers poly (octyl phenol ethylene oxide acrylate‐co‐methyl methacrylate‐co‐phosphoryloxymethyl oxyethylene acrylate) (donated as AMF) were synthesized through radical polymerization. Among the obtained copolymers, two copolymers, AMF162 (the feed molar ratio of AS, MMA, and FR as 1 : 6 : 2) and AMF1104 (the feed molar ratio of AS, MMA, and FR as 1 : 10 : 4) with different concentrations were added into polypropylene (PP) to prepare PP‐AMF162 and PP‐AMF1104 series of composites. The thermal stability, limiting oxygen index, the antistatic property, and mechanical properties of PP composites were tested and analyzed. PP‐AMF162 series composites have excellent antistatic effect. When the AMF162 content was equal to or <15 wt %, the impact strength of PP‐AMF162 composites was higher than that of pure PP. The results indicated that copolymer AMF162 was a suitable flame‐retardant and antistatic additive for PP. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41677.     

Phosphorus‐containing polyhedral oligomeric silsesquioxane/polyimides hybrid materials with low dielectric constant and low coefficients of thermal expansion

Novel phosphorus‐containing polyhedral oligomeric silsesquioxane (POSS)/polyimides (PI) hybrid materials with low dielectric constant and low linear coefficients of thermal expansion (CTE) were prepared and characterized. The POSS/PI hybrid materials were synthesized with octa(aminopropyl)silsesquioxane (OAPS) and a series of phosphorus‐containing polyamide acids(PAA). The PAAs were synthesized with bis(4‐aminophenoxy) phenyl phosphine oxide (BAPPO), 4,4’‐diaminodiphenyl ether (ODA) and 3,3',4,4'‐biphenyl tetracarboxylic diandhydride (BPDA). The structures and properties of the hybrid materials were characterized. And the effect of the phosphorus‐containing structure on the POSS/PI hybrid materials was discussed. The dielectric constants and CTE of the hybrid materials were remarkably lower than that of the unmodified POSS/PI films. The lowest values of dielectric constant and CTE could achieve as low as 2.64 (1 MHz) and 27.45 ppm/K. Besides, the hybrid materials also had excellent thermal properties. The highest 5% weight loss temperature of the hybrid materials was as high as 580°C under air. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42611.     

Joint flame‐retardant effect of triazine‐rich and triazine/phosphaphenanthrene compounds on epoxy resin thermoset

To obtain a more efficient flame‐retardant system, the extra‐triazine‐rich compound melamine cyanurate (MCA) was coworked with tri(3‐9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide‐2‐hydroxypropan‐1‐yl)?1,3,5‐triazine‐2,4,6‐trione (TGIC–DOPO) in epoxy thermosets; these were composed of diglycidyl ether of bisphenol A (DGEBA) epoxy resin and 4,4′‐diaminodiphenyl methane (DDM). The flame‐retardant properties were investigated by limited oxygen index measurement, vertical burning testing, and cone calorimeter testing. In contrast to the DGEBA/DDM (EP for short) thermoset with a single TGIC–DOPO, a better flame retardancy was obtained with TGIC–DOPO/MCA/EP. The 3% TGIC–DOPO/2% MCA/EP thermoset showed a lower peak heat‐release rate value, a lower effective heat of combustion value, fewer total smoke products, and lower total yields of carbon monoxide and carbon dioxide in comparison with 3% TGIC–DOPO/EP. The results reveal that MCA and TGIC–DOPO worked jointly in flame‐retardant thermosets. The dilution effect of MCA, the quenching effect of TGIC–DOPO, and their joint action inhibited the combustion intensity and imposed a better flame‐retardant effect in the gas phase. The 3% TGIC–DOPO/2% MCA/EP thermoset also exhibited an increased residue yield, and more compositions with triazine rings were locked in the residues; this implied that MCA/TGIC–DOPO worked jointly in the condensed phase and promoted thermoset charring. The results reveal the better flame‐retardant effect of the MCA/TGIC–DOPO system in the condensed phase. Therefore, the joint incorporation of MCA and TGIC–DOPO into the EP thermosets increased the flame‐retardant effects in both the condensed and gas phases during combustion. This implied that the adjustment to the group ratio in the flame‐retardant group system endowed the EP thermoset with better flame retardancy. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43241.     

Flame‐retardant mechanism of expandable polystyrene foam with a macromolecular nitrogen–phosphorus intumescent flame retardant

Expandable polystyrene (EPS) foam is largely used as the thermally insulating external wall in buildings and constructions, but it is extremely flammable because of the presence of almost 98% air into its porous structure, its high surface‐area‐to‐mass ratio, and its elemental composition. Lots of serious fire disasters caused by EPS foam have posed great threats to people's properties and lives in recent years. Thus, a halogen‐free, flame‐retardant EPS is urgently needed, and its preparation is still a global challenge. To solve the problem that it is easy for EPS foam to form melt dripping and difficult for it to generate a char layer during the combustion process, a macromolecular nitrogen–phosphorus intumescent flame retardant (MNP) was selected to prepare flame‐retardant EPS foam and good mechanical and flame‐retardant properties were obtained. The scanning electron microscopy characterization revealed that MNP could penetrate into the gap between the beads, and a thin physical coating layer formed on the surface of the bead. The data from the thermogravimetry–Fourier transform infrared test indicated that a nitrogenous noncombustible gas was generated by the pyrolysis of MNP. When the MNP content increased to 30%, the limiting oxygen index and the smoking density rate of the EPS–MNP foam were 28.8 and 23.6, respectively, and a UL94 V‐0 classification was achieved. In addition, the heat‐release rate, total heat‐release, smoke produce rate, and carbon dioxide production of the EPS–MNP foams all decreased obviously; this was attributed to the flame‐retardant effects of MNP in both the condensed and gas phases. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44356.     

Synthesis of a phosphorus‐ and nitrogen‐containing flame retardant and evaluation of its application in waterborne polyurethane

A phosphorus‐ and nitrogen‐containing intumescent flame retardant, pentaerythritol di‐N‐hydroxyethyl phosphamide (PDNP), was synthesized with phosphorus oxychloride, pentaerythritol, and ethanolamine as raw materials. Using the prepared PDNP as a chain extender, a series of flame‐retardant waterborne polyurethanes (WPU) were prepared, and their structures were characterized using NMR and Fourier transform infrared spectroscopy (FTIR). Additionally, the thermal properties and flame retardancy of WPU films were investigated by thermogravimetric analysis, limiting oxygen index (LOI) tests, cone calorimeter tests, and thermogravimetry‐FTIR. These results indicated that PDNP materials exhibit good char‐forming ability at high temperature and that PDNP‐modified waterborne polyurethane obtained an LOI value of 26.0% for a PDNP content of 9 wt %. Finally, the morphology and the element distributions of char residues of WPU were analyzed by scanning electron microscopy and energy dispersive spectrometry after combustion. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46093.