Intrinsically flame retardant epoxy resin - Fire performance and background - Part II

The flame retardant mechanism of a newly synthesized phosphorus-containing reactive amine, which can be used both as crosslinking agent in epoxy resins and as flame retardant, was investigated. The mode of action and degradation pathway were investigated by in situ analysis of the gases evolved during the degradation by thermogravimetric measurements coupled online with infrared (TG-EGA-FTIR) and mass spectroscopy (TG/DTA-EGA-MS) and by solid residue analysis by infrared (ATR) spectroscopic methods and X-ray photoelectron spectroscopy (XPS). It was observed that the main difference in the degradation of the reference and the flame retardant system is that the degradation of the latter begins at lower temperature mainly with the emission of degradation products of the phosphorus amine, which act as flame retardants in the gas phase slowing down the further degradation steps. At the high temperature degradation stage the solid phase effect of the phosphorus prevails: the formation of phosphorocarbonaceous intumescent char results in a mass residue of 23.4%. The ratio of phosphorus acting in gas phase and solid phase, respectively, was determined on the basis of thermogravimetric and XPS measurements.     

Synthesis of phosphorus-based flame retardant systems and their use in an epoxy resin

Methods are proposed to synthesize efficient organophosphorous compounds and combine them with montmorillonite nanoparticles. The chemical-physical structure and mechanism of action of the new systems were studied in epoxy resin. Best results were achieved using the fully phosphorylated calixresorcinarene derivative: the heat release rate peak could be decreased by 61% and the LOI value was increased from 21 to 28. The salt form of additives in case of phosphorylated phloroglucine derivatives was not advantageous in epoxy resin, because the additives could not participate in the crosslinking process effectively due to their inhomogenous distribution in the matrix. The incorporation of the nanoparticles did not create the desired flame retardant effect which can be explained by the increased heat conductivity and lower mobility of the nanoparticles due to the crosslinked structure.     

The non-halogen flame retardant epoxy resin based on a novel compound with phosphaphenanthrene and cyclotriphosphazene double functional groups

A novel flame retardant additive hexa-(phosphaphenanthrene -hydroxyl-methyl-phenoxyl)-cyclotriphosphazene (HAP-DOPO) with phosphazene and phosphaphenanthrene double functional groups has been synthesized from hexa-chloro-cyclotriphosphazene, 4-hydroxy-benzaldehyde and 9,10-dihydro-9-oxa-10- phosphaphenanthrene 10-oxide(DOPO). The structure of HAP-DOPO was characterized by Fourier transformed infrared (FT-IR) spectroscopy and ¹H nuclear magnetic resonance (¹H NMR) and ³¹P nuclear magnetic resonance (³¹P NMR). The additive HAP-DOPO was blended into diglycidyl ether of bisphenol-A (DGEBA) to prepare flame retardant epoxy resins. The flame retardant properties and thermal properties of the epoxy resins cured by 4, 4′-Diamino-diphenyl sulfone (DDS) were investigated from the differential scanning calorimeter (DSC), the thermogravimetric analysis (TGA), UL94 test, the limiting oxygen index (LOI) test and Cone calorimeter. Compared to traditional DOPO-DGEBA and ODOPB-DGEBA thermosets, the HAP-DOPO/DGEBA thermosets have higher T_gs at the same UL94 V-0 flammability rating for their higher crosslinking density and have higher char yield and lower pk-HRR at same 1.2 wt.% phosphorus content which confirm that HAP-DOPO has higher flame retardant efficiency on thermosets. The scanning electron microscopy (SEM) results shows that HAP-DOPO in DGEBA/DDS system obviously accelerate formation of the sealing, stronger and phosphorus-rich char layer to improve flame retardant properties of matrix during combustion.     

六苯氧基环三磷腈的热解及其对环氧树脂的阻燃机理

采用极限氧指数仪和锥形量热仪测试了以六苯氧基环三磷腈(HPCP)阻燃环氧树脂的燃烧性能,结果显示,与纯环氧树脂相比,阻燃环氧树脂的极限氧指数值(LOI)明显提高、热释放速率峰值(pk-HRR)和总热释放量(THR)明显下降、环氧树脂的点燃时间提前以及分解速度加快.采用热失重(TGA)、热重红外联用(TGA-FTIR)、X射线光电子能谱(XPS)和热裂解气相色谱质谱联用(Py-GC/MS)研究了HPCP及其阻燃环氧树脂的热解路线和阻燃机理.结果表明,在阻燃环氧树脂过程中,一方面,HPCP分子中的苯氧基团首先解离并发生歧化反应,由此产生的苯氧基及其歧化产物的焠灭效应在环氧树脂中发挥气相阻燃作用,剩余的磷腈环和苯环基团会进一步裂解产生小分子碎片;另一方面,环氧树脂基体在HPCP的作用下提前分解,产生了基于双酚A结构的大分子碎片并在HPCP裂解产物作用下加速炭化,从而使更多的基体组分以残炭的形式被固定在凝聚相中,提高了阻燃环氧树脂的残炭产率,发挥了凝聚相阻燃作用.     

Thermal degradation of a reactive flame retardant based on cyclotriphosphazene and its blend with DGEBA epoxy resin

Hexaglycidyl cyclotriphosphazene (HGCP) was synthesized, and characterized by FTIR, ³¹P, ¹H, and ¹³C-NMR. This compound was used as a reactive flame retardant to blend with commercial epoxy resin DGEBA (Diglycidyl ether of bisphenol A). Its effect on the DGEBA decomposition pathways was characterized by studying both gas and solid phases produced during thermogravimetric analysis (TGA). The gases evolved during TGA in air were studied by means of thermogravimetry coupled with Fourier transform infrared spectroscopy (TG–FTIR), while the solid residues were analysed by FTIR and scanning electron microscopy (SEM). The results showed that HGCP presents a good dispersion in DGEBA, and the blend thermoset with 4,4′-methylene-dianiline (MDA) curing agent leads to a significant improvement of the thermal stability at elevated temperature with higher char yields compared with pure DGEBA thermoset with the same curing agent. Improvement has also been observed in the fire behaviour of blend sample.     

The effect of a novel phosphorus-nitrogen reactive flame retardant curing agent on the performance of epoxy resin

A phosphorus-nitrogen reactive flame retardant curing agent poly-(isophorondiamine spirocyclic pentaerythritol bisphosphonate) (PIPSPB) was synthesized. The chemical structure of the obtained compound was identified by FTIR, ¹HNMR, and mass spectroscopies. Different proportions of DDS and PIPSPB were compounded as the curing agents to prepare a series of flame retardant epoxy resins with different phosphorus contents. The curing behavior of E-44/PIPSPB?+?DDS system was studied by DSC. A series of tests were conducted to characterize E-44/PIPSPB?+?DDS thermosetting system’s performance. The result demonstrates that the residual carbon content of EP/PIPSPB?+?DDS system is obviously higher than that of EP/DDS system after 500?°C with the increase of phosphorus content in the system, and the heat release rate of the system during combustion is significantly reduced. The generated phosphorus-containing carbon layer is obviously foamed, which shows that the flame retardancy of the system is the result of the combined action of condensed phase and gas phase. When the phosphorus content is 1.77wt%, EP-3 successfully passed UL94 V-0 flammability rating, the LOI value was as high as 29%, the impact strength and tensile strength of it were 6.08KJ/m² and 49.10MPa respectively, the adhesive strength could reach 13.89?MPa, the system presents a good overall performance.     

Flame retardancy of fibre-reinforced epoxy resin composites for aerospace applications

The applicability of phosphorus-containing reactive amine, which can be used in epoxy resins both as crosslinking agent and as flame retardant, was compared in an aliphatic and an aromatic epoxy resin system. In order to fulfil the strong requirements on mechanical properties of the aircraft and aerospace applications, where they are mostly supposed to be applied, carbon fibre-reinforced composites were prepared. The flame retardant performance was characterized by relevant tests and mass loss type cone calorimeter. Besides the flame retardancy, the tensile and bending characteristics and interlaminar shear strength were evaluated. The intumescence-hindering effect of the fibre reinforcement was overcome by forming a multilayer composite, consisting of reference composite core and intumescent epoxy resin coating layer, which proved to provide simultaneous amelioration of flame retardancy and mechanical properties of epoxy resins.     

Vegetable oil-based flame retardant epoxy/clay nanocomposites

Flammability of epoxy appears to be one of the greatest threats and hence limits its advanced applications. The present investigation, therefore, reports on vegetable oil-based self-extinguishing epoxy/clay nanocomposites for the first time. These nanocomposites were prepared by the ex-situ technique using mechanical shearing and ultrasonication at different loadings (1, 2.5 and 5 wt%) of nano-clay. Monoglyceride of Mesua ferrea L. seed oil, bisphenol-A and tetrabromobisphenol-A based epoxy resin was used as the matrix. XRD, TEM, SEM, FTIR and rheological studies confirmed partially exfoliated nanocomposites formation. The study demonstrates two fold improvements of tensile strength and scratch hardness, three-fold increase in adhesive strength and 20 units increase in gloss value without any change in impact resistance through nanocomposite formation. TG studied confirmed the enhancement of thermal stability of the nanocomposite by 25 °C. The limiting oxygen index values and UL 94 test indicated the self-extinguishing characteristic of the nanocomposites.     

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.     

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.     

A vanillin-derived flame retardant based on 2-aminopyrimidine for enhanced flame retardancy and mechanical properties of epoxy resin

In order to give epoxy resin good flame retardance, a novel bio-based flame retardant based on 2-aminopyrimidine (referred to as VAD) was synthesized from renewable vanillin as one of the starting materials. Its structure was confirmed by NMR and mass spectra. The epoxy resins containing VAD were prepared by utilizing 4,4-diaminodiphenylmethane (DDM) as a co-curing agent, and their flame-retardant, mechanical and thermal properties and corresponding mechanisms were studied. VAD accelerated the cross-linking reaction of DDM and E51 (diglycidyl ether of bisphenol A). 12.5 wt% VAD made the epoxy resin achieve UL-94 V-0 rating and its limited oxygen index (LOI) value increase from 22.4% to 32.3%. The cone calorimetric testing results revealed the decline in the values of total heat release (THR) and peak of heat release rate (pk-HRR) and the obvious enhancement of residue yield. A certain amount of VAD enhanced the flame inhibition, charring and barrier effects, resulting in good flame retardance of the epoxy resin. Furthermore, the tensile strength, flexural strength and flexural modulus of the epoxy resin with 12.5 wt% loading of VAD were 6.5%, 14.9%, 15.2% higher than those of EP, indicating the strengthening effect of VAD. This work guarantees VAD to be a promising flame retardant for enhancing the fire retardancy of epoxy resin without compromising its mechanical properties.     

Photopolymerization and thermal behaviors of acrylated benzenephosphonates/epoxy acrylate as flame retardant resins

Di(acryloyloxyethyl) benzenephosphonate (DABP) and acryloyloxyethyl phenyl benzenephosphonate (APBP) were synthesized starting from phenylphosphonic dichloride, and characterized by FT-IR and ¹H NMR. DABP and APBP were blended in the ratios of 10-50 wt.% with a commercial epoxy acrylate oligomer (EB600) to obtain a series of flame retardant UV-curable formulations. The viscosity of the formulations greatly reduced by the addition of the reactive monomers, whereas the photopolymerization rate according to the photo-DSC analysis increased. The thermal degradation behavior and flame retardancy of the UV-cured films were investigated by thermogravimetric analysis and the limiting oxygen index (LOI). The results revealed that the blended epoxy acrylates with DABP or APBP possess improved thermal stability at elevated temperature and have higher char yields, together with higher LOI values. The data from dynamic mechanical thermal analysis showed that DABP and APBP have good miscibility with EB600. The crosslink density increased along with the content of DABP or APBP in the blend, whereas the glass-transition temperatures of the blended resins decreased compared to pure cured EB600.     

Synthesis and thermal decomposition of a novel hyperbranched polyphosphate ester used for flame retardant systems

A novel hyperbranched polyphosphate ester (HPPE) was synthesized via the polycondensation of bisphenol-A as an A₂ monomer and phosphoryl trichloride as a B₃ monomer at 100 °C, without gelation. The initial molar ratio of A₂ to B₃ was set to be 1.5:1. The final product was precipitated from methanol. ³¹P NMR spectroscopy was used to monitor the reaction. The formed HPPE was characterized by FTIR and ¹H NMR to confirm its end groups. Differential scanning calorimetry data revealed that the cured bisphenol-A epoxy resin with HPPE as a curing agent possessed improved glass transition temperature. Dynamic mechanical thermal analysis also showed the increase in the glass transition temperature. The thermal degradation properties and flame retardancy were investigated by thermogravimetric analysis and limiting oxygen index (LOI). The results showed that the incorporation of HPPE into bisphenol-A epoxy resin increased its thermal stability and char yield during the decomposition by raising the second stage decomposition temperature. The LOI value increased from 23 to 31 when HPPE, instead of bisphenol-A, was used as a curing agent.     

Silicon-containing flame retardant epoxy resins: Synthesis, characterization and properties

Epoxy resins with different silicon contents were prepared from silicon-containing epoxides or silicon-containing prepolymers by curing with 4,4′-diaminodiphenylmethane. The reactivity of the silicon-based compounds toward amine curing agents was higher than that of the conventional epoxy resins. The T_g of the resulting thermosets was moderate and decreased when the silicon content increased. The onset decomposition temperatures decreased and the char yields increased when the silicon content increased. Epoxy resins had a high LOI value, according to the efficiency of silicon in improving flame retardance.     

Preparation of phosphorus‐containing phenolic resin and its application in epoxy resin as a curing agent and flame retardant

For the sake of improving the flame retardancy of epoxy resin (EP), a novel phosphorus‐containing phenolic resin (PPR) synthesized in our group instead of conventional phenolic resin (PR) was used to cure EP in the present research. The curing processes and the corresponding crosslinking structure and mechanical performance were investigated by differential scanning calorimeter and dynamic mechanical thermal analysis. Because of the introduction of flame‐retarding elements including P and Si, PPR exhibited higher charring capacity in the condensed phase, which is helpful to construct a char layer of higher quality. Correspondingly, PPR‐cured EP displayed remarkably improved flame retardance as compared to conventional PR‐cured EP through the related evaluations including limiting oxygen index, vertical burning test, and microscale combustion colorimeter. As a multifunction agent, it is believable that PPR possesses potential commercial value to prepare flame‐retardant EP with high performance.     

Highly efficient multielement flame retardant for multifunctional epoxy resin with satisfactory thermal,flame‐retardant,and mechanical properties

Multifunctional epoxy resins with excellent, thermal, flame‐retardant, and mechanical properties are extremely important for various applications. To solve this challenging problem, a novel highly efficient multielement flame retardant (PMSBA) is synthesized and the flame‐retardant and mechanical properties of modified epoxy resins are greatly enhanced without significantly altering their and thermal properties by applying the as‐synthesized PMSBA. The limiting oxygen index value reaches up to 29.6% and could pass the V‐0 rating in the UL‐94 test with even low P content (0.13%). Furthermore, cone calorimetry results demonstrate that 30.3% reduction in the peak heat release rate for the sample with 10.0 wt% PMSBA is achieved. X‐ray photoelectron spectroscopy and scanning electron microscopy indicate that Si‐C, Si‐N, and phosphoric acid derivative can be transformed into a multihole and intumescent char layer as an effective barrier, preserving the epoxy resin structure from fire. More importantly, mechanical properties such as impact strength, tensile strength, and flexural strength are also increased by 63.86%, 33.54%, and 15.65%, respectively, which show the incorporation of PMSBA do not deteriorate the mechanical properties of modified epoxy resins. All the results show that PMSBA is a promising strategy for epoxy resin with satisfactory, thermal, flame‐retardant, and mechanical properties.     

Flame retardant epoxy polymers based on all phosphorus-containing components

A phosphorus-containing epoxy resin, bis(3-t-butyl-4-glycidyloxyphenyl-2,4-di-t-butylphenyl)resorcinol diphosphate, was synthesized and subsequently cured with non-phosphorus containing amines, and/or novel phosphorus-containing aromatic or polyoxyalkylene amines. Chemical structures of these materials were characterized with FTIR, NMR, elemental analysis, and amine titration. The introduction of soft -P-O- linkage, polyoxyalkyene, or hard aromatic group into the backbones of the synthesized phosphorus-containing amines provides epoxy polymers with high phosphorus contents and tailored flexibility. Thermal analysis of differential scanning calorimeter and thermal gravimetric analysis (TGA) reveals that these resulted epoxy polymers possess moderate T_gs and thermal stability. Furthermore, high char yields in TGA analysis and high limited oxygen index values indicate that these phosphorus-containing epoxy polymers possess excellent flame retardant properties.     

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.     

Synthesis and characterization of P/Si flame retardant and its application in epoxy systems

In this report, a novel phosphorus/silicon‐containing reactive flame retardant, hexa(3‐triglycidyloxysilylpropyl)triphosphazene (HGPP), was synthesized and characterized by Fourier transform infrared spectrometry and nuclear magnetic resonance spectra (¹H, ³¹P, and ²⁹Si), respectively. To prepare cured epoxy, HGPP had been co‐cured with diglycidyl ether of bisphenol‐A (DGEBA) via 4,4‐diaminodiphenylsulfone as a curing agent. The mechanical, thermal, and flame retardant properties of the cured epoxy were evaluated by dynamic mechanical analysis, thermogravimetric analysis, and limiting oxygen index (LOI). According to these results, it could be found that incorporation of HGPP in the cured epoxy system showed good thermal stability, high LOI values, and high char yield at high temperature. As moderate loading of HGPP in the epoxy system, its storage modulus and glass transition temperature were higher than those of neat DGEBA. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis and properties of a phosphorus-containing flame retardant epoxy resin based on bis-phenoxy (3-hydroxy) phenyl phosphine oxide

A reactive phosphorus-containing compound, bis-phenoxy (3-hydroxy) phenyl phosphine oxide (BPHPPO) was first successfully synthesized to produce the phosphorus-containing flame retardant epoxy resin (BPHPPO-EP). The chemical structures were characterized from FTIR, MS, NMR spectra and elemental analyses. Thermal degradation behaviors and flame retardant properties of the cured epoxy resins were investigated from the thermogravimetric analysis (TGA) and the limiting oxygen index (LOI) test using 4,4′-diaminodiphenylsulfone (DDS) as curing agent. The high char yields and the high limiting oxygen index values were found to certify the great flame retardancy of this phosphorus-containing epoxy resin.