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Facile Synthesis of a Highly Efficient, Halogen-Free, and Intumescent Flame Retardant for Epoxy Resins: Thermal Properties, Combustion Behaviors, and Flame-Retardant Mechanisms Chao Ma,† Bin Yu,‡,∥ Ningning Hong,‡ Yang Pan,§ Weizhao Hu,*,‡ and Yuan Hu*,‡ †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China ‡ State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China § National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P.R. China ∥ Institute of Textiles & Clothing, Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong, P.R. China S Supporting Information *

ABSTRACT: A novel branched poly(phosphonamidate-phosphonate) (BPPAPO) oligomer was synthesized from the polycondensation of phenylphosphonic dichloride and trihydroxymethylphosphine oxide followed by end-capping with aniline in a one-pot synthesis. BPPAPO exhibited excellent flame-retardant efficiency in epoxy resins (EP). With only 5.0 wt % loading, the EP composite reached UL-94 V-0 rating with a limiting oxygen index (LOI) value of 35.5%. BPPAPO catalyzed the early degradation of EP and promoted the formation of more char residue. Glass transition temperatures were partially lowered. When 7.5 wt % BPPAPO was incorporated, the peak heat release rate and total heat release were decreased by 66.2% and 37.3%, respectively, with a delayed ignition and the formation of a highly intumescent char residue. Combination of gas-phase and condensed-phase flame-retardant mechanisms was verified.

1. INTRODUCTION Epoxy resins (EP) are highly valuable in industry because of their excellent properties, such as moisture, solvent, and chemical resistance; toughness; superior electrical and mechanical properties; and good adhesion to many substrates. However, their main disadvantage is that they are more flammable than similar thermosets because they have a reduced tendency to carbonize, and their applications in some fields are restricted. Therefore, improving the fire safety of EP is necessary. The incorporation of halogen-containing compounds into epoxy matrices, either as coreactants or additives, has been proven to be an extremely efficient approach.1 However, corrosive and toxic gases, such as hydrogen halide, dibenzo-p-dioxin, and dibenzofuran,2 and a large amount of smoke are produced during combustion. Thus, they are restricted by the current legislation and it is desirable to develop halogen-free flame retardants. Among the favored flame-retardant elements used in epoxy resins, including phosphorus, nitrogen, silicon, and boron,3 phosphorus is considered promising because phosphorus-containing compounds are highly efficient, present low toxicity, and are environment-friendly. Phosphorus-containing flame retardants can act either in the vapor phase by a radical mechanism to © 2016 American Chemical Society

interrupt the exothermic processes and suppress combustion or in the condensed phase to change the degradation path in which more char is produced and lower amounts of volatiles are released.4 Recently, 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10oxide (DOPO) and its derivatives have been intensively studied to improve the flame retardancy of EP.5−13 One feature of epoxy thermosets modified with DOPO derivatives is their high glass transition temperatures (Tg),14−21 which can be used in printed circuit boards for high-performance applications.18 Variable low molecular weight DOPO derivatives exhibited excellent flame-retardant efficiency. For practical applications, additives are favored for less change of manufacturing processes and lower cost. Besides, as is wellknown, leaching and volatility of flame retardant can be reduced with the increase of molecular weight. Thus, phenylphosphonic dichloride (PPDC) is an advantageous precursor to synthesize polymeric flame-retardant additive directly because of the two Received: Revised: Accepted: Published: 10868

May 17, 2016 August 28, 2016 September 29, 2016 September 29, 2016 DOI: 10.1021/acs.iecr.6b01899 Ind. Eng. Chem. Res. 2016, 55, 10868−10879

Article

Industrial & Engineering Chemistry Research

methane, triethylamine, aniline, anhydrous sodium sulfate, and 4,4′-diaminodiphenylmethane (DDM) were supplied by Sinopharm Chemical Reagent Co. Ltd., China. Diglycidyl ether of bisphenol A (DGEBA, E-44, epoxy value = 0.44 mol/100 g) was obtained from Hefei Jiangfeng Chemical Industry Co. Ltd., China. Dichloromethane, triethylamine, and aniline were purified to remove water by distillation over CaH2 before use. All other chemicals were used as received. 2.2. Synthesis of THPO. THPO was synthesized by a modified method according to a published procedure.37 Barium hydroxide octahydrate (60.2 g) was added in portions to a mixture of 101.3 g of 75% THPS aqueous solution and 300 mL of deionized water at room temperature. After the addition, the reaction mixture was stirred for 2 h. Then, the white precipitate was separated by centrifugation, and 37.6 g of 30% hydroperoxide aqueous solution was added dropwise to the supernatant in an ice bath until the potassium iodide starch test was positive. After being stirred for another 2 h, the solvent was evaporated under vacuum. The product was obtained as a colorless viscous liquid, and the yield was 93%. 1H NMR (400 MHz, DMSO-d6): 3.81 ppm (6H, OP−CH2−O), 5.30 ppm (3H, C−OH). 31P NMR (162 MHz, DMSO-d6): 44.9 ppm (1P, OP−CH2). HR-ESI-MS calcd for [M-H]− C3H8O4P: 139.0155. Found: 139.0167. 2.3. Synthesis of BPPAPO. Before the reaction, traces of water in THPO were removed by azeotropic distillation with benzene. Then, 3.59 g of THPO, 10 g of PPDC, and 100 mL of dried dichloromethane were charged into a 250 mL threenecked flask equipped with a mechanical stirrer and a dropping funnel. The reaction mixture was cooled in an ice bath, and 7.7 g of triethylamine was slowly added into the flask via the dropping funnel. After the completion of the addition, the mixture was further agitated for 5 h. Then, a mixture of 4.68 g of aniline and 2.54 g of triethylamine was added dropwise into the flask; after that, the reaction temperature was elevated to room temperature. The reaction was kept overnight. The reaction mixture was washed with water several times to remove triethylamine hydrochloride, dried with anhydrous sodium sulfate, filtered, and evaporated under vacuum. After the product was dried in a vacuum oven at 70 °C for 24 h, a light yellow solid was obtained; the yield was 78%. 2.4. Preparation of the Neat and Flame-Retardant Epoxy Resins. DGEBA and BPPAPO (0, 2.5, 5.0, 7.5 wt % of the total thermoset) were mixed with a mechanical stirrer at 100 °C until BPPAPO was dissolved. After DDM of stoichiometric amount (relative to DGEBA) was added and dissolved, the hybrid was degassed for 3 min in a vacuum oven at 100 °C. Then, it was poured into the preheated stainless steel mold, cured at 120 °C for 2 h, and postcured at 150 °C for another 2 h. Thereafter, the thermoset was cooled slowly to room temperature to prevent cracking. They are labeled as EP and EPX, where EP denotes the neat epoxy resin without flame retardant and EPX represents the epoxy resin with X wt % BPPAPO incorporated. 2.5. Characterization. Nuclear magnetic resonance spectra were obtained on a Bruker AV400 NMR spectrometer (400 MHz) at room temperature. Tetramethylsilane was used as an internal standard for 1H NMR, and 85% H3PO4 was used as an external standard for 31P NMR. High-resolution electrospray ionization mass spectrometry (HR-ESI-MS) analysis was performed on a Thermo LTQ Orbitrap XL mass spectrometer (Thermo Fisher Scientific, San

reaction sites of chlorine. PPDC is facile to react with active hydrogen containing compounds, for example, alcohols, phenols, amines etc. In comparison to phosphorus oxychloride, the P−C bond in a phosphonate is more stable to hydrolysis than the P−O−C bond in a phosphate,22 which maintains flame retardancy in use. Unfortunately, the flame-retardant efficiency of phenylphosphonate in EP is not satisfactory. Only when more than 14 wt % bis(2,6-dimethyphenyl)phenylphosphonate23 or oligophenylphosphonate24 was incorporated did the EP composite pass the UL-94 V-0 rating. Phosphine oxides contain no P−O−C bonds and are very resistant to hydrolysis. Phosphine oxides linked with aryl groups25−32 or alkyl groups22,33−36 were applied into EP as curing agents or additives. Braun et al.25 pointed out that bis(3aminophenyl)- phenylphosphine oxide as epoxy curing agent released phosphorus-containing volatiles. The gas-phase flameretardant effect endowed it UL-94 V-1 rating, while the analogous phosphinate-, phosphonate-, and phosphate-cured epoxy resins merely obtained HB rating. Alcón et al.22 synthesized isobutylbis(glycidylpropylether)- phosphine oxide and used it as epoxy monomer with catalysts and hardeners to prepare a series of V-0 rated thermosets. Trihydroxymethylphosphine oxide (THPO) with three hydroxyl groups can be simply synthesized from tetrakis(hydroxymethyl)phosphonium sulfate (THPS),37,38 which is an industrial product of low cost. Both of them are nontoxic and environment-friendly. Chen et al.37 utilized THPO as a reactive-type flame retardant for flexible polyurethane foam and deduced that THPO mainly played a role in the condensed phase because most phosphorus was reserved in the char. In this article, two kinds of flame-retardant monomers, PPDC and THPO, were creatively combined by nucleophilic substitution reaction and end-capped with aniline to form a branched poly(phosphonamidate-phosphonate) (BPPAPO) oligomer in a one-pot synthesis. 1H nuclear magnetic resonance (NMR) spectroscopy, 31P NMR, Fourier transform infrared (FTIR) spectroscopy, elemental analysis, and gel permeation chromatography (GPC) were performed to confirm the structure of the newly synthesized BPPAPO. We first introduced THPO into epoxy resins to improve their flameretardant performance. With only 5.0 wt % BPPAPO, the EP composite achieved UL-94 V-0 rating. Therefore, the flame retardancy of BPPAPO in EP is higher than those of other flame retardants synthesized from PPDC and comparable to those of DOPO-containing compounds (as low as 4 wt %11). The thermal properties of epoxy thermosets were studied by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their flame-retardant performance was investigated by limited oxygen index (LOI), UL-94, and cone calorimetry (CC). For further clarification of flame-retardant mechanisms, thermal decomposition products of BPPAPO and epoxy thermosets were detected by pyrolysis photoionization time-of-flight mass spectrometry (PY-PI-TOFMS), and the char residues after the CC test were analyzed in detail by scanning electron microscopy (SEM), FTIR, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).

2. EXPERIMENTAL SECTION 2.1. Materials. A 75% THPS aqueous solution was bought from Aladdin Chemistry Co. Ltd., China. PPDC was purchased from Energy Chemical (Shanghai, China). Barium hydroxide octahydrate, 30% hydroperoxide aqueous solution, potassium iodide starch test papers, chloroform, benzene, dichloro10869

DOI: 10.1021/acs.iecr.6b01899 Ind. Eng. Chem. Res. 2016, 55, 10868−10879

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ion signal was amplified with a VT120C preamplifier (Ortec, OakRidge, TN) and recorded by a P7888 multiscaler (Fast ComTec, Oberhaching, Germany).39,40 Scanning electron microscopy micrographs of samples were acquired on a FEI Sirion 200 scanning electron microscope under high-vacuum conditions at an acceleration voltage of 10 kV. Raman spectra were obtained on a LabRAM-HR Confocal Raman Microprobe (JobinYvon Instruments, France) with a 514.5 nm argon ion laser. X-ray photoelectron spectroscopy (XPS) was conducted using a VG Escalab Mark II spectrometer equipped with a Al Kα excitation radiation (hυ = 1486.6 eV).

Jose, CA). The instrument was operated using electrospray ionization in negative ion mode. Elemental analysis was recorded via the Vario EL III elemental analyzer. Fourier transform infrared spectroscopy was performed obtained on a Nicolet 6700 spectrometer (Nicolet Instrument Company, U.S.). Samples were mixed with KBr and pressed into pellets. Gel permeation chromatography (GPC) was measured with an instrument equipped with a G1310B ISO pump, a G1316A PLgel column, and a G1362A differential refractive index detector. The eluent was DMF with 1.0 g/L LiBr at a flow rate of 1.0 mL/min. A series of low-polydispersity polystyrene standards were employed for calibration. Thermogravimetric analysis was carried out under nitrogen and air atmosphere using a Q5000 thermal analyzer (TA Co., U.S.) from room temperature to 800 °C at a heating rate of 20 °C/min. Differential scanning calorimetry was performed with a DSC Q2000 (TA Instruments Inc., U.S.) at a heating rate of 10 °C/ min under a nitrogen atmosphere. Tg was recorded at the midpoint of the inflection curve from the second heating run. Limited oxygen index (LOI) was obtained according to ASTM Standard D 2863 using a HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China) by measuring the minimum oxygen concentration required to support the candlelike combustion of samples. The test specimens of dimension 100 × 6.5 × 3.0 mm3 were burned in a precisely controlled mixed atmosphere of nitrogen and oxygen. The vertical burning test (UL-94) was performed on a CFZ2 type instrument (Jiangning Analysis Instrument Co., China) with the sample dimension of 130 × 13 × 3 mm3. In the test, the specimens were subjected to two 10 s ignitions, and the self-extinguish times were recorded as t1 and t2. The polymer achieves UL-94 V-0 flammability rating if t1 + t2 of each sample does not exceed 10 s and the total burning time for five samples does not exceed 50 s without any dropping. The cone calorimetry test was conducted on a Fire Testing Technology apparatus according to ISO Standard 5660 under an external heat flux of 35 kW/m2. The samples were of dimension 100 × 100 × 3 mm3. The measurement for each specimen was repeated three times, and the error values of the typical cone calorimeter were reproducible within ±5%. The pyrolysis photoionization time-of-flight mass spectrometry consists of a tubular furnace, a transfer line, and a homemade photoionization time-of-flight mass spectrometry. The flow rate of carrier gas (nitrogen) was maintained as 200 standard cubic centimeters per minute (SCCM). A deactivated fused-silica capillary (inner diameter of 250 μm) was put inside the transfer line, which would introduce the gaseous products into the photoionization region. To prevent product condensation, the transfer line was heated to 250 °C by a heating tape. Pyrolysis products introduced into the photoionization region (0.75 Pa) would be ionized by the vacuum ultraviolet (VUV) light (10.0/10.6 eV) emitted from a krypton discharge lamp (PKS106, Heraeus, Ltd., Germany). Most of the products with ionization energy (IE) lower than 10.6 eV could be ionized, and corresponding molecular ions could be formed. To avoid the influence of light intensity, the ion intensity of ethylene (99.99%, Nanjing Special Gas Factory Co., Ltd., China) was monitored before and after each experiment to calibrate the light intensity. The formed ions of products would be transferred in the ion optics and detected by TOFMS. The

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structural Characterizations of THPO and BPPAPO. THPO was first synthesized from THPS Scheme 1. Synthetic Route of THPO and BPPAPO

and alkali before oxidation, as illustrated in Scheme 1. 1H and P NMR spectra of it are presented in Figure 1. Two peaks corresponding to OP−CH2−O (3.81 ppm) and C−OH (5.30 ppm) appeared in the 1H NMR spectrum, while a singlet at 44.9 ppm can be seen in the 31P NMR spectrum. The integrated intensity ratio of peak a to b (0.92:2) is nearly 1:2. Moreover, HR-ESI-MS analysis was performed in negative ion mode as shown in Figure S1 in the Supporting Information, and [M-H]− ions were detected at m/z 139.0167 (139.0155 calculated for C3H8O4P). Therefore, THPO was successfully obtained. The synthesis of BPPAPO was performed by starting from the polycondensation of THPO and PPDC in the molar ratio of 1:2. Then the residual phosphonic chloride groups were 31

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Figure 2. FTIR spectrum of BPPAPO.

1437 and 1205 cm−1 are attributed to the vibrations of P−Ph and PO stretching, while the peaks at 692 and 746 cm−1 are attributed to the vibration of P−CH2 stretching.42 The peaks at 1047 and 932 cm−1, which correspond to P−O−C43 and P− N44 stretching vibration, demonstrate the successful synthesis of BPPAPO. Elemental analysis results of BPPAPO are listed in Table 1. The calculated values were based on the analysis of NMR Table 1. Elemental Analysis Results of BPPAPO BPPAPO

C (%)

H (%)

N (%)

O (%)

P (%)

calculated found

54.15 54.39

4.77 4.88

3.60 3.45

19.00 19.29

18.41 17.99a

Figure 1. (a) 1H and (b) 31P NMR spectra of THPO (DMSO-d6) and BPPAPO (CDCl3).

a

reacted with the amino groups of aniline. The synthetic route is presented in Scheme 1. The structure of BPPAPO is confirmed by 1H NMR, 31P NMR, FTIR, elemental analysis, and GPC. Figure 1a shows the 1H NMR spectrum of BPPAPO. The chemical shifts between 4.06 and 4.80 ppm are attributed to the methylene protons adjacent to OP−O, and the broad signals may be due to the partially formed six-membered cycles. The chemical shifts around 3.65 ppm are due to the methylene protons of unreacted hydroxymethyl, and those between 6.50 and 8.10 ppm correspond to the protons of phenyl rings. The reaction between PPDC and THPO is verified by the emergence of the signals of Ph−P(O)−O−CH2−PO (4.06−4.80 ppm). In the 31P NMR spectroscopy (Figure 1b), the signal at 16.2 ppm is attributed to the phosphorus of O− (Ph)PO−O, and the peak at 7.1 ppm is due to the phosphorus of NH−PO(Ph)−O. Three peaks located at 42.5, 40.6, and 39.3 ppm are attributed to the phosphorus of OP−(CH2−)3 with one, two, and three hydroxyls reacted, respectively, and they are distinguished as terminal (T), linear (L), and dendritic (D) units.41 Therefore, the branched structure is verified by the formation of abundant dendritic units. The weak peak at 41.2 ppm may be because of the formation of six-membered cycles. The disappeared signal of THPO (44.9 ppm) and the new signals of phosphorus corresponding to BPPAPO further prove the successful synthesis of the target product. The FTIR spectrum of BPPAPO is depicted in Figure 2. The absorption around 3416 cm−1 is assigned to the vibration of N−H and O−H stretching. The peaks at 3053, 2963, and 2892 cm−1 are due to the C−H stretching vibration of Ph−H and −CH2−, and the intense absorptions at 1598 and 1501 cm−1 are relative to the benzene skeleton vibration. The peaks at

spectra. From the integrated intensity ratio of peaks d and e in Figure 1a, the reaction extent of hydroxyl was 90% and all residual phosphonic chloride groups were end-capped by aniline (only k and l detected for PPDC precursor in Figure 1b). Good consistency was found between the measured values and calculated values. Thus, the structure of BPPAPO was further confirmed. In addition, only C, H, N, O, and P exist in the product, and the phosphorus content was estimated to be 17.99% based on the measured values. The molecular weight of BPPAPO was acquired from GPC as presented in Figure S2, and the number-average molecular weight is 2500 g/mol with the polydispersity index of 1.2. The molecular weight of BPPAPO can be larger because of the hydrodynamic volume of branched polymer is smaller than that of the linear polymer with the same molecular weight.45 Therefore, the oligomer structure of BPPAPO is verified. 3.2. Thermal Properties of Cured Epoxy Resin and Its Composites. The TGA and DTG curves of epoxy thermosets under nitrogen and air are illustrated in Figure 3. The decomposition temperature at 5% weight loss (Td), the temperature at maximum weight loss rate (Tmax), and char yield at 800 °C (CY) are obtained from them, and the data are given in Table 2. The incorporation of BPPAPO reduced the thermal stabilities of the flame-retardant epoxy resins compared to EP. The Td values decreased both under nitrogen and air, and the Tmax values declined under nitrogen and in the firststage degradation under air. The depressed Td values may be attributed to the fact that the OP−O bond is less stable than the C−C bond6,9 and EP5.0 still exhibited Td values close to 300 °C both under nitrogen and air. However, in the secondstage degradation under air, the Tmax values shifted to higher 10871

Estimated by 1 − 54.39% − 4.88% − 3.45% − 19.29%.


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Figure 3. Thermal properties of epoxy thermosets: (a) TGA curves under nitrogen, (b) DTG curves under nitrogen, (c) TGA curves under air, and (d) DTG curves under air.

Table 2. Thermal and Flame-Retardant Properties of Epoxy Thermosets with Different BPPAPO Content nitrogen

air

sample

Td (°C)

Tmax (°C)

CY (%)

Td (°C)

EP EP2.5 EP5.0 EP7.5

349 311 300 299

362 341 335 333

14.2 19.7 20.4 21.6

347 306 294 279

Tmax (°C) 358, 341, 336, 334,

548 570 557 565

CY (%)

Tg (°C)

LOI (%)

UL-94

0.04 1.20 1.56 2.10

155 145 134 122

26.3 34.6 35.5 36.2

NR V-1 V-0 V-0

the Tg values are presented in Table 2. BPPAPO exhibited a Tg value of 54 °C, and the existence of Tg also proves the oligomer structure of BPPAPO. For the epoxy thermosets, with the increase of BPPAPO content, the Tg values decreased. This phenomenon is ascribed to the plasticizing effect of BPPAPO with lower Tg values,45 which enlarges the free volume of the epoxy resin composites.47 On the other hand, only a single Tg can be found in every epoxy resin composite in the DSC thermograms. The single and decreased Tg values of the blends indicate good compatibility between the epoxy resin matrix and BPPAPO.48,49 3.3. Flame-Retardant Properties of Epoxy Thermosets. 3.3.1. LOI and UL-94. The flame-retardant properties of epoxy thermosets were investigated initially by LOI and UL-94 tests. The results are listed in Table 2. EP was flammable with a low LOI value of 26.3% and received no rating in the UL-94 test. When 2.5 wt % BPPAPO was incorporated, the LOI value increased dramatically to 34.5%, while it increased slowly from EP2.5 to EP7.5. The samples of EP5.0 and EP7.5 reached UL94 V-0 rating. From the phosphorus content of BPPAPO estimated previously, that of EP5.0 is calculated to be 0.9% (5.0% × 17.99%). These results reveal that BPPAPO endows the epoxy resin composites with excellent flame retardancy with relatively low loading. Figure 5 shows the digital images of the thermosets after the LOI test. The char residue of neat epoxy resin was little and

temperatures and the corresponding weight loss rates decreased. Furthermore, the char yields of epoxy thermosets at 800 °C increased with the increment of BPPAPO loading both under nitrogen and air. These results indicate that BPPAPO enhances the thermal and thermo-oxidative stabilities of the char residues. The formation of more stable and richer char promoted by BPPAPO can endow the epoxy thermosets with better flame retardancy.11,46 The glass transition temperatures of BPPAPO and the epoxy thermosets were measured by DSC as shown in Figure 4, and

Figure 4. DSC thermograms of BPPAPO and cured epoxy resins. 10872


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Figure 5. Digital images of epoxy thermosets after LOI test: (a) EP, (b) EP2.5, (c) EP5.0, and (d) EP7.5.

easy to crack. With increasing content of BPPAPO, the char residues became thicker and stronger, especially for EP5.0. Further observation indicates that the mandrils of epoxy resin composites are surrounded by an abundant fluffy char layer. It is very apparent for EP5.0 and EP7.5. The intumescent char layers hindered heat transfer and oxygen from mixing with flammable gas and protected the matrix from decomposition to decline fuel supply. The condensed-phase flame-retardant effect of BPPAPO was responsible for the enhancement of LOI values. 3.3.2. Cone Calorimetry. Cone calorimetry provides a useful method to investigate the fire behaviors of the cured epoxy resins. The concerned combustion parameters include time to ignition (TTI), peak of heat release rate (p-HRR), total heat release (THR), char yield (CY), total CO production per total mass loss (TCOP/TML), total CO2 production per total mass loss (TCO2P/TML), and total heat release per total mass loss (THR/TML). The parameters are summarized in Table 3. The HRR and THR curves as a function of time are presented in Figure 6. It can be observed that the p-HRR and THR values of cured epoxy resins decreased obviously along with the increase of BPPAPO loading. When compared to EP, the p-HRR values decreased by 23.9% for EP2.5, 40.7% for EP5.0, and 66.2% for EP7.5, and the reduction ratios of THR values are 22.5% for EP2.5, 25.9% for EP5.0, and 37.3% for EP7.5, respectively. The variation tendency of p-HRR and THR is in accordance with that of LOI values and UL-94 ratings. The significant reduction of p-HRR and THR values at low loadings of BPPAPO suggests that it effectively suppressed the combustion of epoxy resin composites. For epoxy resin composites, the ignition occurred earlier than EP for the shortened TTI values (the longer TTI value of EP7.5 will be interpreted in the following). The earlier decomposition of BPPAPO promoted the epoxy resin matrix to degrade faster. As the previous TGA data indicate, BPPAPO catalyzed the formation of more thermo-oxidative stable chars. This kind of char both insulated the flammable volatiles from heat and oxygen and protected the underlying materials from decomposition to leave more residue. The later can be verified by the increased ratio of char yields of epoxy resin composites

Figure 6. (a) HRR and (b) THR curves of epoxy thermosets.

to EP: 48% for EP2.5, 59% for EP5.0, and 98% for EP7.5. Flammable volatiles were reduced significantly through the formation of abundant char residues. For these reasons, the pHRR and THR values of flame-retardant epoxy resins decreased. Thus, flame retardancy of epoxy resin composites is enhanced by the formation of more thermo-oxidative stable and richer char promoted by BPPAPO, i.e., BPPAPO exhibits the condensed-phase flame-retardant effect in epoxy resins. When the shapes of HRR curves are further observed, EP was characterized by a sharp peak from the start to the end of burning, and this kind of heat release characterization means reduced tendency to char.50 However, two obvious peaks can be distinguished from the HRR curves of EP2.5 and EP5.0. The first HRR peak represented the formation of “initial” char layer in the early stage combustion.23,51−53 For EP, there was a weak shoulder in the HRR curve because the formed char layer was so unstable that it could not provide protection. For EP2.5 and EP5.0, the more thermo-oxidative stable char layers provided more effective protection, which induced the decline and the emergence of the first peak in the HRR curves. After a while, the char layer was destroyed under the long-time exposure to high temperature. More gas was released and burned, and the carbonaceous residue was formed. Therefore, the HRR curves exhibited another peak. Interestingly, for EP7.5, the second HRR peak disappeared accompanied by longer time to reach the first peak of HRR and a very slow decrease toward the end

Table 3. Combustion Parameters of the Epoxy Thermosets Obtained from Cone Calorimeter Test samples

TTI (s)

p-HRR (kW/m2)

THR (MJ/m2)

CY (%)

TCOP/TML (kg/kg)

TCO2P/TML (kg/kg)

THR/TML (MJ/m2g)

EP EP2.5 EP5.0 EP7.5

58 46 50 64

1795 1366 1065 607

67.6 52.4 50.1 42.4

12.1 17.9 19.2 23.9

0.084 0.095 0.113 0.130

1.96 1.40 1.26 1.04

2.30 1.80 1.70 1.45

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Industrial & Engineering Chemistry Research of combustion. The changed shape of the HRR curve of EP7.5 means that the “initial” char layer was stable enough to resist destruction. It hindered the combustion gases from heat and oxygen and prevented the decomposition of the matrix to reduce the release of flammable volatiles. This “one-two-one” variation of peak numbers in HRR curves also elucidates the condensed-phase flame-retardant mechanism of BPPAPO in epoxy resins. The ratio of THR to TML is a measurement of effective heat of combustion of the volatiles.54,55 The reduction of THR/ TML values with the increase of BPPAPO fraction clarifies the gas-phase flame-retardant activation of BPPAPO in epoxy resins. The THR/TML values can be reduced in two ways: (i) The PO type radicals produced quench the highly active H and OH radicals in the flame and hamper the radical chain reactions of combustion, which reduces the combustion efficiency and is regarded as the flame inhibition mechanism. (ii) The heat release of the combustible volatiles decomposed from flameretardant composites is less than that from the neat polymer, that is the composition of the volatiles is varied because of the changed decomposition path, for example, the reduction of flammable gaseous products and the nonflammable gases released.11,12,23,51,56 TCOP/TML56 and TCO2P/TML also reveal the gas-phase combustion state. The decrement of TCO2P/TML values with the increase of BPPAPO content indicates that the burning intensity was weakened. However, TCOP/TML values exhibit an opposite variation tendency to TCO2P/TML. The increased ratios of TCOP/TML to TCO2P/TML mean that the combustion of volatiles was inhibited because more incomplete combustion products (CO) and less complete combustion products (CO 2 ) were produced.12,50 The variation of TCOP/TML and TCO2P/ TML are in accordance with the decrease of THR/TML values. These changes are similarly attributed to the flame inhibition effect and the reduction of flammable gaseous products released. The results stated above reveal that BPPAPO exhibits gas-phase flame-retardant effect in epoxy resins. As additional evidence of the gas-phase flame-retardant mechanism, the TTI values of the cured epoxy resins decreased first and then increased with more and more BPPAPO incorporated. TTI was influenced by two opposite factors: (i) The degradation of epoxy resin composites ahead of time promoted by BPPAPO lowered TTI. (ii) The production of less flammable gases and radical quenching effect delayed TTI.53,55 A major role was played by the stronger inhibition effect in EP7.5; thus, the ignition of it is longer than that of EP. According to the discussions about cone calorimetry, the enhanced flame retardancy of epoxy resin composites is imparted by the condensed-phase and gas-phase action of BPPAPO simultaneously. 3.4. Analysis of Gaseous Pyrolysis Products. To elucidate the gas-phase flame-retardant mechanism, the PYPI-TOFMS was adopted to identify the decomposed volatiles. Samples of a same weight were introduced into the tubular furnace, and the pyrolysis was conducted with nitrogen as the carrier gas at 400 °C, which was slightly higher than the Tmax values of epoxy thermosets measured by TGA under nitrogen. The accumulated signals were recorded until there were no volatiles released, and the mass spectra of BPPAPO, EP, and EP7.5 are depicted in Figure 7. In comparison to the conventional “hard” electron ionization (EI) method, the near-threshold “soft” photoionization enables the fragment-free mass spectra to be obtained. In these spectra, nearly all the

Figure 7. PY-PI-TOFMS of (a) BPPAPO and (b) EP and EP7.5.

mass peaks can be attributed to parent ions with little or no fragments.39,40 Table 4 lists the assignments of the peaks in the mass spectra. The predominant decomposition volatiles of BPPAPO were benzene (m/z = 78), aniline (m/z = 93), Nmethylaniline (m/z = 107), and N,N-dimethylaniline (m/z = 121). Three weak peaks located at m/z 179, 181, and 193 are attributed to tris(aminomethyl)phosphine oxides and bis(methoxymethyl)((methylamino)methyl)phosphine oxide. Table 4. Assignments of the Peaks in the PY-PI-TOFMS of BPPAPO, EP, and EP7.5

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Industrial & Engineering Chemistry Research The released phosphorus-containing species from BPPAPO are speculated to generate PO type radicals during combustion and contribute to flame inhibition. The PO type radicals of several hundred parts per million in the flaming zone is enough to trap active H and OH radicals.55 For EP, phenols, aromatic amines, and some unsaturated aromatic compounds were mainly released. The volatiles of EP7.5 consist of more phenols (peak h) and phenol derivatives (peak j), less compounds with two benzene rings in the structure (the disappearance of peaks n, p, q, and r), and new amine compounds (peaks b and c) originated from BPPAPO decomposition. More small molecular weight compounds and less large molecular weight compounds produced indicate that BPPAPO promoted the decomposition of the epoxy resin matrix. These results are coincident with those of CC. Furthermore, according to PY-PITOFMS results, probable thermal decomposition mechanisms of BPPAPO and EP are proposed in Schemes 2 and 3. For

Scheme 3. Proposed Pyrolysis Mechanism of EP

Scheme 2. Proposed Pyrolysis Mechanism of BPPAPO

BPPAPO, the formed aniline reacts with the residual or produced hydroxymethyl to eliminate one or two molecules of water, and the products decompose to release N-methylaniline and N,N-dimethylaniline, respectively. THPO may be produced by the cleavage of P−O bonds, and the dehydration reaction occurs between THPO and methylamine, dimethylamine, or methanol to form the detected phosphine oxides. For EP, the pyrolysis routes are similar to those in the reported papers.57,58 3.5. Analysis of the Char Residues after CC Test. 3.5.1. Macroscopic and Microscopic Morphologies. Digital photos of the char residues of cured epoxy resins after CC test are shown in Figure 8. The char residue of EP was little, and this kind of fragmentary, loose, and fragile char residue cannot serve as a protective layer effectively. However, obviously rich and intumescent char residues were formed for the flameretardant epoxy resins. The continuous, compact, and rigid char

Figure 8. Digital photos of the char residues after cone calorimetry test: (top view) (a) EP, (b) EP2.5, (c) EP5.0, (d) EP7.5, (e) EP7.5 after the outmost layer peeled, and (f) partly magnified photo of panel e; (side view) (g) EP, (h) EP2.5, (i) EP5.0, and (j) EP7.5.

layers hindered the flammable gases from exposure to heat and oxygen and protected the inner materials to reduce decomposition. Thus, the p-HRR and THR were weakened. For epoxy resin composites, it is observed that the outermost layers of the char residues for EP2.5 and EP5.0 were destroyed in the center partially while it was unbroken for EP7.5. These phenomena illustrate the interpretation for the one-two-one variation of peak numbers in HRR curves. Interestingly, after the outermost layer of the char residue of EP7.5 was peeled, a 10875


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2922 cm−1 are attributed to the typical absorption of C−H of aliphatic carbons. The absorption near 1600 cm−1 indicates the formation of polyaromatic carbons. For EP5.0, a new broad absorbance at 1250 cm−1 reveals the contribution of PO stretching vibration, and the peak at 1102 cm−1 is ascribed to P−O−C stretching vibration.6,11 Therefore, the carbonized aromatic networks linked by phosphates were formed for EP5.0 after combustion. Raman spectroscopy is usually used to characterize the graphitic structure of char residues, and the spectra of the exterior char residues of EP and EP5.0 are shown in Figure 11.

unique intumescent char layer was presented (Figure 8e; L2). Under the outermost layer (L1) there was a honeycomb-like layer with numerous big closed pores (L2), and a dense layer existed below L2 (L3). From the magnified photo of L2 (Figure 8f), it is obvious that most pore diameters concentrated in the range of 3−7 mm. The honeycomb-like char layers reported for epoxy resins were always companied by pores with diameters of several hundred micrometers,8,46,59 and this kind of perfectly closed millimeter-scale pore has not been covered. The pyrolysis gases were sealed by the high-viscosity melt formed during combustion, and they accumulated and expanded to form big holes and intumescent char. In consideration of the fact that the coefficients of heat conductivity of gases are generally lower than those of solid residues,8 L3 was protected very efficiently by the intumescent L2 containing abundant gases. These effects further contributed to the formation of the single peak in the HRR curve of EP7.5. SEM images of the exterior char residues of EP and EP5.0 are presented in Figure 9. For EP, the char is fluffy and

Figure 9. SEM images of exterior char residues: (a) EP-500× , (b) EP5.0-500× , (c) EP-1000×, and (d) EP5.0-1000×.

fragmentary. However, a continuous and compact structure is presented for the char of EP5.0. This kind of char can seal the combustible volatiles, prevent oxygen supply, and ultimately weaken the combustion intensity. 3.5.2. Structure and Element Compositions. Figure 10 displays the FTIR spectra of the exterior chars of EP and EP5.0. The broad absorption of N−H and O−H stretching vibration can be observed around 3420 cm−1, and two peaks at 2855 and

Figure 11. Raman spectra of the exterior char residues of (a) EP and (b) EP5.0.

There exist two bands in both spectra: the D band around 1360 cm−1 corresponding to disordered graphite or glassy carbons and the G band around 1595 cm−1 associated with the vibration of carbon atoms in graphite layers, which represents the ordered graphic structure.60 Generally speaking, the degree of graphitization can be calculated by the ratio of the integrated intensities of D to G band (ID/IG). The ID/IG value of the exterior char residue of EP5.0 (2.75) is lower than that of EP (2.85), and this illustrates the degree of graphitization of EP5.0 char residue is higher than that of EP. Therefore, the thermaloxidative stability and strength of EP5.0 char residue were enhanced. XPS was employed to analyze the elemental compositions and contents of the exterior and interior char residues of EP and EP5.0 after the cone test. The related data are listed in Table 5, and the XPS spectra are displayed in Figure 12. The overall spectra of EP5.0 char residues show increased signals for the existence of phosphorus atoms (133.8 eV, P2p; 191.3 eV, P2s) originating from BPPAPO in comparison to those of EP. The single peak at 133.8 eV can be attributed to

Figure 10. FTIR spectra of the exterior char residues of EP and EP5.0. 10876


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factor was dominated; thus, the O/C ratio of EP5.0 interior char is lower than that of EP. Summarizing all the discussions above, the exterior carbonaceous char with higher graphitization degree was formed for EP5.0 by the promotion of BPPAPO, and it was linked by P− O−C and P−O−P bonds. The thermal-oxidative stability and strength were enhanced.

Table 5. Element Relative Contents of the Exterior and Interior Char Residues of EP and EP5.0 Measured by XPS sample EP EP5.0

exterior interior exterior interior

C (wt %)

P (wt %)

N (wt %)

O (wt %)

84.7 85.1 79.5 85.2

0 0 1.86 2.25

4.07 4.06 5.81 4.65

11.3 10.9 12.8 8.0

4. CONCLUSION A new branched poly(phosphonamidate-phosphonate) (BPPAPO) oligomer was successfully prepared and applied into DGEBA/DDM epoxy resin systems. DSC results reveal the good compatibility between BPPAPO and the epoxy resin matrix. With only 5.0 wt % loading, UL-94 V-0 rating was achieved with a LOI value of 35.5%. When 7.5 wt % BPPAPO was incorporated, the p-HRR and THR values were decreased by 66.2% and 37.3%, respectively, with a delayed TTI. The unique intumescent char layer of EP7.5 char residue accounts for the broad single peak in its HRR curve. Thermal properties were partly sacrificed, as evidenced by the reduced thermal decomposition temperatures and lower Tg values. The CC test illustrates the gas-phase and condensed-phase flame-retardant mechanisms occurring simultaneously. Phosphine oxides released from BPPAPO thermal decomposition may generate PO type radicals during combustion and inhibit the flame. In the condensed phase, BPPAPO promoted the decomposition of epoxy resin matrix and changed the decomposition path. The formed carbonaceous char with higher graphitization degree was linked by P−O−C and P−O−P bonds; thus, its thermooxidative stability and strength were enhanced. The compact and intumescent char residue effectively hindered the flammable gases from exposure to heat and oxygen and protected the inner matrix to reserve more char and reduce fuel supply.

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Figure 12. XPS spectra of the char residues of EP and EP5.0: (a) overall spectra and (b) C1s spectra.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b01899. HR-ESI-MS spectrum of THPO (Figure S1) and GPC trace of BPPAPO (Figure S2) (PDF)

61

pyrophosphates and/or polyphosphates. Three bands with binding energies of 284.8, 286.2, and 289.0 eV appeared in the C1s spectra. The major band at 284.8 eV is assigned to the C−C and C−H of aliphatic and aromatic species. In addition, the bands at 286.2 and 289.0 eV reveal the contribution of C−O/ C−N/C−O−P and OC−O, respectively.8,24,62 The higher ratio of the band at 286.2 eV for EP5.0 char residue can be explained by the C−O−P structure formed. The relative contents of phosphorus in the exterior and interior char residues of EP5.0 are 1.86 and 2.25 wt %, respectively. More phosphorus reserved in the interior char could be interpreted by the restricted release of phosphoruscontaining species for the reduced decomposition. It is found that the relative contents of oxygen of the exterior char layers increase from 11.3% for EP to 12.8% for EP5.0. The oxygen content is increased because the phosphors in the char residue maintained more oxygen atoms in the form of pyrophosphate and/or polyphosphate.24 However, the O/C ratio of EP5.0 interior char is lower than that of EP in spite of phosphorus atoms being incorporated. This was controlled by another factor. The compact exterior char of EP5.0 effectively prevented oxygen supply, whereas the fluffy exterior char of EP could not (from Figure 9); therefore, the concentration of oxygen in EP5.0 interior matrix was lowered and oxidation declined. This

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AUTHOR INFORMATION

Corresponding Authors

*W.H.: e-mail, [email protected]. *Y.H.: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (21374111), National Key Research and Development Program of China (2016YFB0302104), USTC-NSRL Joint Funds (KY2320000007), Fundamental Research Funds for the Central Universities (WK2320000029, WK2320000032), and China Postdoctoral Science Foundation (2014M561837).

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