Thermal degradation behaviors and flame retardancy of epoxy resins with novel silicon‐containing flame retardant

A novel macromolecular silicon‐containing intumescent flame retardants (Si‐IFR) was synthesized, and its structure was a caged bicyclic macromolecule containing phosphorus‐silicon characterized by IR. Epoxy resins (EP) were modified with Si‐IFR to get the flame retardant EP, whose flammability and burning behavior were characterized by UL 94 and limiting oxygen index (LOI). Twenty percentage of weight of Si‐IFR was doped into EP to get 27.5% of LOI and UL 94 V‐0. The degradation behavior of the flame retardant EP was studied by thermogravimetry, differential thermogravimetry, scanning electron microscopy, and X‐ray photoelectron spectroscopy analysis. The experimental results exhibited that when EP/Si‐IFR was heated, the phosphorus‐containing groups firstly decompose to hydrate the char source‐containing groups to form a continuous and protective carbonaceous char, which changed into heat‐resistant swollen char by gaseous products from the nitrogen‐containing groups. Meanwhile, SiO₂ reacts with phosphate to yield silicophosphate, which stabilizes the swollen char. The barrier properties and thermal stability of the swollen char are most effective in resisting the transport of heat and mass to improve the flame retardancy and thermal stability of EP. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis of a novel cross‐linked organophosphorus‐nitrogen containing polymer and its application in flame retardant epoxy resins

A novel cross‐linked organophosphorus–nitrogen polymetric flame retardant additive poly(urea tetramethylene phosphonium sulfate) defined as PUTMPS was synthesized by the condensation polymerization between urea and tetrahydroxymethyl phosphonium sulfate. Its chemical structure was well characterized by Fourier transform infrared (FTIR) spectroscopy, ¹³C and ³¹P solid‐state nuclear magnetic resonance. The synthesized PUTMPS and curing agent m‐phenylenediamine were blended into epoxy resins to prepare flame retardant epoxy resin thermosets. The effects of PUTMPS on fire retardancy and thermal degradation behavior of EP/PUTMPS thermosets were investigated by limiting oxygen index (LOI), vertical burning test (UL‐94), cone calorimeter measurement, and thermalgravimetric analysis (TGA) tests. The surface morphologies and chemical compositions of char residues for cured epoxy resins were investigated by scanning electron microscopy and X‐ray photoelectron spectroscopy (XPS), respectively. Water resistant properties of epoxy resin thermosets were evaluated by putting the samples into distilled water at 70°C for 168 h. The results demonstrated that the EP/12 wt% PUTMPS thermosets successfully passed UL‐94 V‐0 flammability rating and the LOI value reached 31.3%. The TGA results indicated that the incorporation of PUTMPS promoted epoxy resin matrix decomposed and char forming ahead of time, which led to a higher char yield and thermal stability for epoxy resin thermosets at high temperature. The morphological structures and analysis of XPS for the char residues of the epoxy resin thermosets shown that PUTMPS benefited to the formation of a sufficient, more compact, and homogeneous char layer with rich flame retardant elements on the materials surface during burning, which prevented the heat transmission and diffusion, limited the production of combustible gases, inhibited the emission of smoke, and then led to the reduction of the heat release rate and smoke produce rate. After water resistance tests, EP/12 wt% PUTMPS thermosets still remained excellent flame retardancy. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of novel triazine charring agent and its effect in intumescent flame‐retardant polypropylene

A novel charring agent (CNCA‐DA) containing triazine and benzene ring, using cyanuric chloride, aniline, and ethylenediamine as raw materials, was synthesized and characterized. The effects of CNCA‐DA on flame retardancy, thermal degradation, and flammability properties of polypropylene (PP) were investigated by limited oxygen index (LOI), vertical burning test (UL‐94), thermogravimetric analysis (TGA), and cone calorimeter test (CCT). The TGA results showed that CNCA‐DA had a good char forming ability, and a high initial temperature of thermal degradation; the char residue of CNCA‐DA reached 18.5% at 800°C; Ammonium polyphosphate (APP) could improve the char residue of APP/CNCA‐DA system, the char residue reached 31.6% at 800°C. The results from LOI and UL‐94 showed that the intumescent flame retardant (IFR) containing CNCA‐DA and APP was very effective in flame retardancy of PP. When the mass ratio of APP and CNCA‐DA was 2 : 1, and the IFR loading was 30%, the IFR showed the best effect; the LOI value reached 35.6%. It was also found that when the IFR loading was only 20%, the flame retardancy of PP/IFR can still pass V‐0 rating in UL‐94 tests, and its LOI value reached 27.1%. The CCT results demonstrated that IFR could clearly change the decomposition behavior of PP and form a char layer on the surface of the composites, consequently resulting in efficient reduction of the flammability parameters, such as heat release rate (HRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), and mass loss (ML). © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Synthesis of a novel DOPO‐based polyphosphoramide with high char yield and its application in flame‐retardant epoxy resins

A novel and highly effective flame retardant (FR), DOPO‐TPMP oligomer, was synthesized by a simple condensation of 4‐(hydroxymethyl)‐2,6,7‐trioxa‐1‐phosphabicyclo[2.2.2]octane‐1‐oxide and phosphorus oxychloride followed by a polycondensation reaction with 6‐(2,5‐dihydroxyphenyl)‐6H‐dibenzo[c,e][1,2]oxaphosphinine‐6‐oxide. The chemical structure of DOPO‐TPMP was well characterized using Fourier transform infrared and NMR spectra. DOPO‐TPMP was used as an additive‐type FR for epoxy resin (EP). The FR properties of the resultant EP composites were investigated by limiting oxygen index (LOI) test, UL‐94 vertical burning test and cone calorimeter measurements. Specifically, the EP composite containing 10.0% DOPO‐TPMP achieved a LOI value of 36.1%, V‐0 rating in the UL‐94 test and a 58% reduction in peak heat release rate. Further mechanism analysis attributed the enhanced flame retardancy to the increased char yield on the addition of DOPO‐TPMP. © 2019 Society of Chemical Industry     

Study on flame‐retardancy and thermal degradation behaviors of intumescent flame‐retardant polylactide systems

Two intumescent flame‐retardant (IFR) additives, IFR‐I and IFR‐II, were synthesized and their structure was confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance. Polylactide (PLA) was modified by the two IFRs to obtain flame‐retardant composites. The flammability of the PLA/IFR composites was characterized by the vertical burning test UL‐94 and limiting oxygen index. The limiting oxygen index values of the PLA composites increased with increase of IFR content. The PLA composite with 20 wt% IFR‐I could pass the UL‐94 V0 rating, while the composite with 30 wt% IFR‐II could not. The results of pyrolysis combustion flow calorimetry showed that the heat release capacity of PLA composites with 30 wt% IFR‐I decreased 43.1% compared with that of pure PLA. The thermal degradation and gas products of PLA/IFR‐I systems were monitored by thermogravimetric analysis and thermogravimetric analysis infrared spectrometry. Scanning electron microscopy was used to investigate the surface morphology of the char residue. Copyright © 2011 Society of Chemical Industry     

A novel intumescent flame‐retardant system for flame‐retarded LLDPE/EVA composites

A new intumescent flame retardant (IFR) system consisting of ammonium polyphosphate (APP) and charing‐foaming agent (CFA) and a little organic montmorillonite (OMMT) was used in low‐density polyethylene (LLDPE)/ethylene‐vinyl acetate (EVA) composite. According to limiting oxygen index (LOI) value and UL‐94 rating obtained from this work, the reasonable mass ratio of APP to CFA was 3 : 1, and OMMT could obviously enhance the flame retardancy of the composites. Cone calorimeter (CONE) and thermogravimetric analysis (TGA) were applied to evaluate the burning behavior and thermal stability of IFR‐LLDPE/EVA (LLDPE/EVA) composites. The results of cone calorimeter showed that heat release rate peak (HRR‐peak) and smoke production rate peak (SPR‐peak) and time to ignition (TTI) of IFR‐LLDPE/EVA composites decreased clearly compared with the pure blend. TGA data showed that IFR could enhance the thermal stability of the composites at high temperature and effectively increase the char residue. The morphological structures of the composites observed by scanning electron microscopy (SEM) and X‐ray diffraction (XRD) demonstrated that OMMT could well disperse in the composites without exfoliation, and obviously improve the compatibility of components of IFR in LLDPE/EVA blend. The morphological structures of char layer obtained from Cone indicated that OMMT make the char layer structure be more homogenous and more stable. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A novel flame retardant of spirocyclic pentaerythritol bisphosphorate for epoxy resins

A novel flame retardant for epoxy resins, bisdiglycol spirocyclic pentaerythritol bisphosphorate (BDSPBP) was synthesized from the reaction of diglycol with spirocyclic pentaerythritol bisphosphorate diphosphoryl chloride, which was obtained from the reaction of phosphoryl chloride with pentaerythritol. Flammability of the cured epoxy resin systems consisted of diglycidyl ether of bisphenol A (DGEBA), low‐molecular‐weight polyamide and BDSPBP are investigated by vertical burning test (UL‐94) and limiting oxygen index test (LOI). The results indicate that BDSPBP has good flame retardance on epoxy. The thermogravimetric analysis (TGA) shows that the epoxy resin containing BDSPBP has a high yield of residual char at high temperatures, indicating that BDSPBP is an effective charring agent. From the SEM observations of the residues of the flame retardant systems burned, the compact charred layers can be seen, which form protective shields to protect effectively internal structure, and inhibit the transmission of heat and heat diffusion during contacting fire. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4978–4982, 2006     

含磷阻燃剂阻燃环氧树脂热降解动力学

以2-二苯基膦酰基-1,4-苯二酚(DPO-HQ)为阻燃剂制备了阻燃环氧树脂,利用动态热重分析法(TGA)研究了纯环氧树脂(EP)和阻燃环氧树脂(FR-EP)在不同升温速率下的热稳定性,建立了EP和FR-EP体系的动力学模型和非模型动力学(MFK),并对比分析了模型动力学和非模型动力学对于描述EP体系和FR-EP体系的适用性.结果表明:阻燃剂的引入降低了环氧树脂初始降解温度,但增加了残炭率.由Flynn-Wall-Ozawa方法和Coats-Redfern方法建立的模型动力学表征EP和FR-EP体系高温降解过程中误差较大,而非模型动力学能更准确地预测和描述EP和FR-EP体系的高温降解行为.     

Novel flame‐retardant epoxy based on zinc methylethyl phosphinate

A novel flame retardant zinc methylethylphosphinate (Zn(MEP)) was used to fill epoxy resins (EPs). The structure of Zn(MEP) was conformed with Fourier transform infrared, hydrogen nuclear magnetic resonance and phosphorus nuclear magnetic resonance, and X‐ray fluorescent and X‐ray diffraction. The flammability, decomposition behavior, and glass transition temperature (T_g) of cured EP/Zn(MEP) were investigated. Zn(MEP) is stable below 406°C. EP containing 20 phr of Zn(MEP) achieves 27.5% of limiting oxygen index and UL‐94 V0 rating. Scanning electron microscopy‐energy‐dispersive X‐ray and Fourier transform infrared spectroscopy investigations show that a condensed char layer with carbon‐rich and phosphorus‐rich components was formed during heating Zn(MEP)/EP, the atomic ratio of P to Zn on the surface of the char is reduced compared with the initial sample. The P‐rich components and lower atomic ratio of P/Zn on the char surface implies that the Zn(MEP) acts in both condensed phase and gas phase. TGA investigation shows that there are interactions between Zn(MEP) and EP when they are copyrolyzed. The interactions lead to a modification in degradation process and promote the char forming. Compared with aluminum diethylphosphinate Zn(MEP) filled EP shows lower limiting oxygen index but higher T_g. In addition, the interactions between polymer and additive are different when aluminum diethylphosphinate instead of Zn(MEP) is added into EP. Copyright © 2013 John Wiley & Sons, Ltd.     

用于环氧树脂不饱和聚酯的聚合物型磷系阻燃剂

研究了1种新型富芳香结构的含磷聚合物阻燃剂聚苯基磷酸(9,10-二氢-9-氧杂-10-膦酰杂菲)苯撑酯(PDPPP)的合成及其对环氧树脂(ER)、不饱和聚酯树脂(USPE)的阻燃性能的影响。这种阻燃剂是由2-(6-氧-6氢-膦酰杂菲)1,4-对苯二酚(ODOPB)与苯膦酰二氯通过熔融缩聚获得。将磷含量为13.8%的PDPPP添加到环氧树脂、不饱和树脂中,结果表明该聚合物对环氧树脂、不饱和树脂具有良好的阻燃性能,PDPPP含量只需达到2%即磷含量只需达到0.28%时氧指数LOI可达28,阻燃性UL-94可达到V0级。     

Synthesis of polysiloxane‐type multifunctional flame retardant and its application in epoxy systems

Polymethyl(3‐glycidyloxypropyl)siloxane (PMGS) was synthesized as a flame‐retardant additive, which were cocured with diglycidyl ether of bisphenol‐A (DGEBA) using 4,4′‐diaminodiphenylsulfone as a curing agent. The structure of PMGS was confirmed through Fourier transform infrared and ¹H‐NMR spectra. The cured products were characterized with dynamic mechanical thermal analysis, thermogravimetric analysis, and oxygen index analyzer. With PMGS incorporated, the cured epoxy resin showed better thermal stability, higher limited oxygen index, and higher char yield. At moderate loading of PMGS, the storage modulus and glass transition temperature of the cured epoxy resin based on neat DGEBA were obviously improved. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

新型含氮环氧树脂研究进展

氮系阻燃剂具有高效、低毒(包括其分解产物)等优点,故已成为当今阻燃剂的发展方向。综述了氮系阻燃剂的特点及其阻燃机理,以及目前国内外对含氮环氧树脂(EP)和含氮阻燃固化剂的最新研究进展,并对其发展前景作了展望。     

Synergistic flame‐retardant effect of halloysite nanotubes on intumescent flame retardant in LDPE

Synergistic flame‐retardant effect of halloysite nanotubes (HNTs) on an intumescent flame retardant (IFR) in low‐density polyethylene (LDPE) was investigated by limited oxygen index (LOI), vertical burning test (UL‐94), thermogravimetric analysis (TGA), cone calorimeter (CC) test, and scanning electronic microscopy (SEM). The results of LOI and UL‐94 tests indicated that the addition of HNTs could dramatically increase the LOI value of LDPE/IFR in the case that the mass ratio of HNTs to IFR was 2/28 at 30 wt % of total flame retardant. Moreover, in this case the prepared samples could pass the V‐0 rating in UL‐94 tests. CC tests results showed that, for LDPE/IFR, both the heat release rate and the total heat release significantly decreased because of the incorporation of 2 wt % of HNTs. SEM observations directly approved that HNTs could promote the formation of more continuous and compact intumescent char layer in LDPE/IFR. TGA results demonstrated that the residue of LDPE/IFR containing 2 wt % of HNTs was obviously more than that of LDPE/IFR at the same total flame retardant of 30 wt % at 700°C under an air atmosphere, and its maximum decomposing rate was also lower than that of LDPE/IFR, suggesting that HNTs facilitated the charring of LDPE/IFR and its thermal stability at high temperature in this case. Both TGA and SEM results interpreted the mechanism on the synergistic effect of HNTs on IFR in LDPE, which is that the migration of HNTs to the surface during the combustion process led to the formation of a more compact barrier, resulting in the promotion of flame retardancy of LDPE/IFR. In addition, the mechanical properties of LDPE/IFR/HNTs systems were studied, the results showed that the addition of 0.5–2 wt % of HNTs could increase the tensile strength and the elongation at break of LDPE/IFR simultaneously. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40065.     

含磷本质阻燃环氧树脂的研究进展

综述了含磷本质阻燃环氧树脂(包括含磷协同本质阻燃体系)的发展、现状和未来趋势。与添加型阻燃剂阻燃环氧树脂相比,通过含磷环氧化合物和/或含磷固化剂把磷元素嵌入环氧树脂结构中制得的含磷本质阻燃环氧树脂,具有阻燃效率高、阻燃持久、物理力学性能不受影响、燃烧过程中毒性腐蚀性挥发物质的生成量低等优势。利用协同阻燃效应,可以进一步提高阻燃性能。但是,含磷本质阻燃环氧树脂和含磷协同本质阻燃体系存在制备工艺复杂、生产成本较高等不足。     

Flame retardant aircraft epoxy resins containing phosphorus

As part of a program to develop fire resistant exterior composite structures for future subsonic commercial and general aviation aircraft, flame retardant epoxy resins are under investigation. Epoxies and their curing agents (aromatic diamines) containing phosphorus were synthesized and used to prepare epoxy formulations. Phosphorus was incorporated within the backbone of the epoxy resin and not used as an additive. The resulting cured neat epoxy formulations were characterized by thermogravimetric analysis, propane torch test, elemental analysis, microscale combustion calorimetry, and fire calorimetry. Several formulations showed excellent flame retardation with phosphorous contents as low as 1.5% by weight. The fracture toughness and compressive strength of several cured formulations showed no detrimental effect due to phosphorus content. The chemistry and properties of these new epoxy formulations are discussed.     

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     

Synergistic barrier flame‐retardant effect of aluminium poly‐hexamethylenephosphinate and bisphenol‐A bis(diphenyl phosphate) in epoxy resin

Two flame retardants, aluminium poly‐hexamethylenephosphinate (APHP) and bisphenol‐A bis(diphenyl phosphate) (BDP), were incorporated into diglycidyl ether of bisphenol A (DGEBA) thermoset with 4,4′‐diaminodiphenyl sulfone (DDS) as curing agent, and then the synergistic flame‐retardant behaviors of the cured thermosets were investigated. Compared with thermosets containing 10 wt% APHP and 10 wt% BDP alone, the sample with 3.3 wt% APHP and 6.7 wt% BDP (3.3%APHP/6.7%BDP/EP; EP is DGEBA/DDS) possessed a better flame‐retardant effect since its limited oxygen index reached 35.0% and in the UL94 test it passed the V‐0 rating. The cone calorimeter test revealed that the 3.3%APHP/6.7%BDP/EP sample generated less gaseous fragments and more smoke particles instead of fuels and verified that APHP and BDP exhibited an outstanding synergistic effect on the barrier effect. Macroscopic digital photos and micrographs from scanning electron microscopy further disclose that BDP facilitated the formation of a flexible film covering holes in the residue. The flexible film was combined with aluminium phosphate particles which were produced by decomposed APHP, thereby forming a char layer with increased barrier effect. The synergistic barrier effect from APHP and BDP imposed a better flame‐retardant performance for epoxy thermosets. © 2017 Society of Chemical Industry     

Preparation and investigation of flame‐retardant epoxy resin modified with a novel halogen‐free flame retardant containing phosphaphenanthrene,triazine‐trione,and organoboron units

In this article, a novel flame retardant (coded as BNP) was successfully synthesized through the addition reaction between triglycidyl isocyanurate, 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide and phenylboronic acid. BNP was blended with diglycidyl ether of bisphenol‐A to prepare flame‐retardant epoxy resin (EP). Thermal properties, flame retardancy, and combustion behavior of the cured EP were studied by thermogravimetric analysis, limited oxygen index (LOI) measurement, UL94 vertical burning test, and cone calorimeter test. The results indicated that the flame retardancy and smoke suppressing properties of EP/BNP thermosets were significantly enhanced. The LOI value of EP/BNP‐3 thermoset was increased to 32.5% and the sample achieved UL94 V‐0 rating. Compared with the neat EP sample, the peak of heat release rate, average of heat release rate, total heat release, and total smoke production of EP/BNP thermosets were decreased by 58.2%–66.9%, 27.1%–37.9%, 25.8%–41.8%, and 21.3%–41.7%, respectively. The char yields of EP/BNP thermosets were increased by 46.8%–88.4%. The BNP decomposed to produce free radicals with quenching effect and enhanced the charring ability of EP matrix. The multifunctional groups of BNP with flame retardant effects in both gaseous and condensed phases were responsible for the excellent flame retardancy of the EP/BNP thermosets. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45291.     

Synergistic effects of fumed silica on intumescent flame‐retardant polypropylene

The synergistic effects of fumed silica on the thermal and flame‐retardant properties of intumescent flame retardant (IFR) polypropylene based on the NP phosphorus‐nitrogen compound have been studied by Fourier transfer infrared (FTIR) spectroscopy, cone calorimeter test (CCT), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), and UL‐94 tests. The LOI and UL‐94 data show that when ≤1 wt % fumed silica substituted for the IFR additive NP can increase 2 to 4% LOI values of the PP blends and keep the V‐0 rating. The data obtained from the CCT tests indicate the heat release rates (HRR) reduce by about 23% for the PP/NP sample with 0.5 wt % fumed silica, whereas the mass loss rates (MLR) and total heat release (THR) values are much lower than those of the PP/NP samples without fume silica. The TGA data demonstrate that a suitable amount of fumed silica can increase the thermal stability and charred residue of the PP/IFR/SiO₂ blends after 500°C. The morphological structures of charred residues observed by SEM give positive evidence that a suitable amount of fumed silica can promote the formation of compact intumescent charred layers and prevent the charred layers from cracking, which effectively protects the underlying polymer from burning. The dynamic FTIR spectra reveal that the synergistic flame‐retardant mechanism of a suitable amount of fumed silica with IFR additive is due to its physical process in the condensed phases. However, a high loading of fumed silica restricts the formation of charred layers with P? O? P and P? O? C complexes formed from burning of polymer materials and destroys the swelling behavior of intumescent charred layers, which deteriorates the flame retardant and thermal properties of the PP/IFR blends. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis and properties of flame‐retardant epoxy resins based on DOPO and one of its analog DPPO

Two phosphorus‐containing heterocyclic flame retardants ‐9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO) and 2,8‐dimethyl‐phenoxaphosphin‐10‐oxide (DPPO) ‐ and their derivatives were characterized and incorporated in the backbone of epoxy novolac to obtain flame‐retardant epoxy resins. The structures and spectroscopic data including high‐resolution mass spectroscopy of these flame retardants were determined. Flame‐retardant epoxy resins with a phosphorus content of up to 2% based on heterocyclic DOPO and DPPO were cured with 4,4′‐diaminodiphenylmethane (DDM), and their features were examined by UL 94, LOI, and DSC. In this manner, high‐performance polymers with glass transition temperatures around 190°C and the UL 94 rating V0 were obtained. These polymers were compared with epoxy resins incorporating diphenyl phosphite and diphenyl phosphate, which are nonheterocyclic and do not pass the UL 94 test up to 2% phosphorus. DPPO has a similar flame retardancy like the commercially available DOPO. Furthermore, to explain the difference in the efficiency of the tested flame retardants, key experiments for the determination of the active species during the flame‐retarding process were performed and the PO radical was identified. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007.