Phosphorus and Nitrogen-Containing Polyols: Synergistic Effect on

Article pubs.acs.org/IECR

Phosphorus and Nitrogen-Containing Polyols: Synergistic Effect on the Thermal Property and Flame Retardancy of Rigid Polyurethane Foam Composites Yao Yuan,† Hongyu Yang,‡ Bin Yu,† Yongqian Shi,§ Wei Wang,† Lei Song,† Yuan Hu,*,† and Yongming Zhang*,† †

State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, PR China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, PR China § College of Environment and Resources, Fuzhou University, Fuzhou 350002, PR China ‡

S Supporting Information *

ABSTRACT: In this work, the investigation mainly focused on the synergistic effect of phosphorus-containing polyol (BHPP) and nitrogen-containing polyol (MADP) in improving the flame retardancy of EG/rigid polyurethane foam (RPUF). BHPP and MADP were synthesized through dehydrochlorination and Mannich reaction, respectively. The influence of the weight ratio of BHPP and MADP was studied by thermogravimetric analysis and limiting oxygen index (LOI) tests. The results demonstrated that the optimal weight percentage of BHPP and MADP in flame-retarding RPUF was 1/1. In addition, the incorporation of expandable graphite (EG) into the RPUF/ BHPP/MADP system could greatly improve the flame-retardant properties of RPUF composites. When the content of EG was 15 wt %, LOI value of RPUF composites reached 33.5%. Furthermore, the value of the peak of the heat release rate was reduced by 52.4% compared to that of pristine RPUF. Based on the analysis and discussion, a condensed flame-retardant mechanism was primarily proposed.

1. INTRODUCTION

approaches is considered as the preferred way to improve the fire safety of RPUF. Expandable graphite (EG), a kind of additive physically dispersed into the RPUF matrix which works principally in the condensed phase, can effectively improve the flame retardancy of RPUF. EG has a flake-like graphite structure, where sulfuric and nitric acid are intercalated into this structure.6,7 It is well expected that a smaller amount of FR is required to reach a better flame retardancy and a smaller influence upon the physical and mechanical properties of RPUF.8 Meanwhile, a series of research projects have demonstrated that most of the modified polyols show high efficiency in improving the combustion resistance of rigid foams and simultaneously do not cause a significant deterioration of physical and thermomechanical properties. In general, polyols contain two or more hydroxyl groups. To meet RPUF application needs for fire resistance, polyols with a wide range of chemical structures have been synthesized experimentally and commercially. Currently, modified polyols

Rigid polyurethane foam (RPUF), produced from the polyaddition between polyols and PMDI, is one material in an enormous family of foams. One of the main concerns regarding RPUFs is to improve or modify their intrinsic properties such as flammability, degradability, good adhesion, compatibility, insulation, etc.1−3 Currently, RPUFs are widely utilized as thermal-insulating materials which generate lots of heat in a moment after ignition.4,5 Thus, it is of great importance to explore the flame-retardant RPUFs. The main methods used are to incorporate additives or reactive flame retardants into the RPUF matrix. According to the previous literature, additive type flame retardants were incorporated by physical methods, resulting in poor compatibility5 and reduced mechanical properties. However, this obviously provides the most economical way. Generally, the application of reactive type flame retardants involves the design and modification of polyol with different chemical structures. A majority of them are mainly organic compounds which are endowed with a flame retarding portion that can form covalent bonds with RPUF and thus result in the improved compatibility efficiently between polymer and the flame retardants. Therefore, the synergy between the additive type and reactive type © XXXX American Chemical Society

Received: August 1, 2016 Revised: September 21, 2016 Accepted: September 27, 2016

A

DOI: 10.1021/acs.iecr.6b02942 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Synthesis of Reactive Flame Retardant Polyol (a) BHPP and (b) MADP

methylene isocyanate (PAPI) (PM-200, NCO weight percent, 30.5−32.0%; viscosity at 25 °C, 150−250 mPa·s) was obtained from Wanhua Chemical Group Co., Ltd., China. Dibutyltin dilaurate (LC) was purchased from Air Products and Chemicals, Inc. Triethanolamine (TEOA) was obtained from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Synthesis of BHPP. The synthetic route of the modified polyol is illustrated in Scheme 1a. Bis(4-hydroxybutyl) phenylphosphonate (BHPP) was synthesized through dehydrochlorination reaction of BPOD and BDO. BDO (0.2 mol, 18 g), TEA (0.21 mol, 21.25 g), and dry THF (60 mL) were casted into a 250 mL three-necked round-bottomed flask equipped with a nitrogen inlet, dropping funnel, and mechanical stirrer. BPOD (0.1 mol) was dissolved into 50 mL of THF, followed by the dropwise addition of the BPOD/ THF mixture into the reaction flask above at 0 °C. Subsequently, the reaction remained at 0 °C for 6 h with continuous agitation under a nitrogen atmosphere and then warmed up to room temperature with stirring for another 8 h. After filtering the precipitate, the triethylamine hydrochloride salt was removed, and the filtrate was rotary evaporated by distillation in vacuum. Finally, a thus-obtained yellow liquid as crude product was redissolved in THF in which the residue was filtered by micropore filtering film (200 μm) to remove the small amount of triethylamine hydrochloride salt and the filtrate rotary evaporated to remove THF. After the purification process, which occurred more than three times, the final product BHPP (a light yellow liquid) was obtained. The theoretical hydroxyl number of BHPP was 371.5 mg of potassium hydroxide (KOH) equiv/g. 2.3. Synthesis of MADP. The synthetic route of the MADP is illustrated in Scheme 1b. A three necked reaction vessel equipped with reflux condenser, thermometer, and mechanical stirrer was charged with 12.6 g of melamine and 9 g of paraformaldehyde dissolved in 180 mL of deionized water. A slight TEA solution was added, and the pH value was maintained in the range of 8.5−9. The reaction went on with continuous stirring at 65 °C for 2 h. Afterward, diethanol amine (10.7 g) at 60 °C for 3 h was added dropwise. A total of 0.1 mol of diethanol amine dissolved into 20 mL of deionized water was slowly dropped. Finally, the nonreacted triethylamine and deionized water were separated by rotary evaporation under vacuum to obtain a melamine-derived polyol (a yellow viscous liquid). The theoretical hydroxyl number of MADP was 705.7 mg of potassium hydroxide (KOH) equiv/g.

are mainly produced from phosphorus-containing compounds and nitrogen-containing compounds which contain several reactive functional groups. Phosphorus-containing flame retardants can promote the formation of char and reduce the flammable gases which act in the condensed phase.9−11 For instance, phosphorus-containing polyols such as ethylene glycol, ethylene glycol bisphosphate, etc. have been investigated.12,13 In recent years, melamine derivatives have been used as nonhalogenated FRs, such as RPUF, polyamide 6, and poly(vinyl alcohol). During combustion, melamine derivatives can absorb the heat of the RPUF, and the generation of nitrogen-containing vapors dilutes the gaseous combustion derived from the decomposition of the RPUF composites.14−17 The purpose of this work is to discuss the synergistic effect between bis(4-hydroxybutyl) phenylphosphonate (BHPP) and melamine-derived polyol (MADP) in improving the flame retardancy and thermal stability for RPUF composites during combustion. The intumescent flame retardant (IFR) system consists of BHPP and MADP which act as an acid source and blowing agent, while RPUF acts as the carbon source. The optimum weight ratio of BHPP and MADP and the factor of EG were systematically analyzed by thermogravimetric analysis (TGA) and other flame retardancy tests. TGA, SEM, and Raman spectra of the char layer for RPUF/IFR/EG demonstrated the formation of quality of the char residue. Additionally, the flame retardancy of RPUF composites was investigated by the LOI and Cone. Also, the char layer of RPUF composites was characterized by digital photos, SEM, and FTIR spectra, which deduced the flame retardant mechanism of BHPP/MADP/EG in RPUF composites.

2. EXPERIMENTAL SECTION 2.1. Materials. Benzene phosphorus oxidichloride (BPOD) was provided by Sun Chemical Technology (Shanghai) Co., Ltd. (Shanghai, China). Triethylamine (TEA), tetrahydrofuran (THF), 1,4-butanediol (BDO), diethanol amine (DEA), paraformaldehyde (PA), and melamine (MEL) were purchased from the Sinopharm Chemical Reagent Co. (Shanghai, China). TEA used as the acid absorbing agent was purified by distillation, and THF was refluxed with sodium follow by distillation before use. Polyether polyol (LY-4110, hydroxyl number: 430 mg of potassium hydroxide (KOH) equiv/g; viscosity at 25 °C: 2500 mPa·s), silicone surfactant (Si-Oil), and triethylenediamine (A33, 33%) were received from Jiangsu Luyuan New Materials Co., Ltd., China. Polyaryl polyB


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Industrial & Engineering Chemistry Research

Figure 1. 1H NMR spectrum (a) and 31P NMR spectrum (b) of BHPP.

2.4. Preparation of Modified PU Foams. The waterblown RPUFs were prepared by the free-rise method, and the formulas of RPUF and FR-RPUFs are shown in Table S1. A 500 mL plastic beaker was charged with the raw materials by using a high speed mechanical stirrer, followed by the addition of PAPI with vigorous stirring for 12 s. The resultant product was immediately emptied into a mold (250 × 150 × 60 mm3) and RPUFs with similar density by different amounts of water were produced. The amount of PAPI was calculated by the isocyanate index (moles of NCO/mol of OH). Finally, the foams were put into an oven at 70 °C for 48 h to complete the polymerization reaction. 2.5. Characterization. The 1H nuclear magnetic resonance 1 ( H NMR) and 31P nuclear magnetic resonance (31P NMR) spectra were recorded on an AVANCE 400 Bruker spectrometer at room temperature using chloroform-d and D2O as the solvent. Fourier transform infrared (FTIR) spectra were obtained by a Nicolet 6700 spectrometer (Nicolet Instrument Company, U.S.A.) using KBr pellets. The wavenumber range was 400− 4000 cm−1, and the resolution was 4 cm−1. Elemental analysis was recorded via the Vario EL III elemental analyzer. The thermal conductivities of samples were tested by a hotdisk thermal analyzer (TC 3000E, Xia Xi Technology, China) at 25 °C, adopting the transient plane source technique. The size of the specimen for the measurement was 50 × 50 × 20 mm3. The compressive properties of RPUF and FR-RPUFs were characterized with a universal testing machine (Instron 1185) at temperature of 25 + 2 °C according to GB/T8813-2008. The size of the specimens for the measurement was 50 × 50 × 50 mm3. The average values were obtained from at least five repeating tests. A real time Fourier transform infrared (RT-FTIR) method was recorded in the range of room temperature (RT) to 600 °C at a heating rate of around 10 °C/min under air on a MAGNAIR 750 spectrometer (Nicolet Instrument Company, U.S.A.) equipped with a heating device and a temperature controller. Powders of samples were mixed with KBr powders, and the mixture was pressed into a tablet, which was then placed in a ventilated oven. The dynamic FTIR spectra were obtained in situ during the thermo-oxidative degradation processes.

Density of the foam was measured according to ASTM D1622. The size of the specimen was 30 × 30 × 30 mm3, and at least three specimens were tested to obtain average density. The thermal stability was determined by thermogravimetric analysis (TGA) which was performed using a Q5000IR (TA Instruments) thermo-analyzer instrument at a linear heating rate of 20 °C/min from room temperature to 650 °C under an air flow. The limiting oxygen index (LOI) test was conducted using a HC-2 oxygen index meter (LOI analysis instrument company, Jiangning, China). The test was measured according to ASTM D2863. Size of the specimens for the measurement was 127 × 10 × 10 mm3. Cone calorimeter (Stanton Redcroft, U.K.) tests were performed according to the ISO 5660 standard. Each specimen with size of 100 × 100 × 25 mm3 was wrapped with aluminum foil and burned at an external heat flux of 35 kW/m2. The char layers of the samples after the cone calorimetry test were investigated by using a scanning electron microscope (SEM; KYKY1010B, Shanghai Electron Optical Technology Institute, China). The samples were coated with a gold/ palladium alloy. Raman spectra of the char were obtained using a SPEX-1403 laser Raman spectrometer (SPEX Co., U.S.A.) from 500 to 2000 cm−1.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of BHPP and MADP. Figure 1 shows the (a) 1H NMR spectrum and (b) 31P NMR spectrum of BHPP. The multiplets between 7.4 and 7.8 ppm (Figure 1a) are assigned to the aromatic rings (6). Signals from 3.9 to 4.4 ppm are attributed to the methylene proton of  CCH2OH (2). The chemical shift at 3.64 ppm is ascribed to the protons of OPOCH2 (5). The peak at 3.02 ppm is characteristic of a hydroxy proton (1). These peaks at 1.7−2.1 ppm are associated with OPOCCH2 (4), and the methylene proton peak of CH2COH (3) appears at 1.66 ppm. Moreover, Figure 1b reveals that only one sharp signal for phosphorus connected with benzene is observed at 19.48 ppm. These results confirm the successful synthesis of BHPP. The 1H NMR spectrum of MADP is shown in Figure 2. It shows the signal at 4.71 ppm, corresponding to the N CH2N protons (a) adjacent to the melamine structure. C


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which is the typical feature of melamine and diethanol amine.19 The peak at 2910 cm−1 corresponds to the CH2 stretching band. Additionally, the peak at 1631 cm−1 is due to the deformation vibrations of NH, and the 1056 cm−1 is assigned to a rock vibration of NH. The absorption bands at 1441 and 771 cm−1 are attributed to absorptions of triazine rings.20 It is worth noting that the peaks of NH2 at 3469 and 3420 cm−1 disappear completely, confirming the successful linkage between melamine and formaldehyde. Elemental analysis results of BHPP and MADP are listed in Table S2. The calculated values are based on the chemical structure formula. It can be found that C, H, O, and P exist in the BHPP while MADP consists of C, H, O, and N. However, the content of phosphorus cannot be measured directly and is only estimated based on the other measured values. The elemental analysis of BHPP and MADP shows the mol ratio C/ O/H/P = 14.01/5.02/23.01/1.0 and C/O/H/N = 18.01/6.02/ 39.01/9, which is similar to the calculated values. Thus, the structures of BHPP and MADP are further confirmed. 3.2. Physical and Mechanical Properties. Foam density is one of the desirable physical properties and has great influence on the thermal stability and flame retardant properties of the RPUFs.21 Generally, for RPUFs, the density is dependent on the foaming formula and the process of foaming. Therefore, in the recent work, RPUF and flame-retarded RPUFs (FRRPUFs) were prepared by the same foaming process, while the concentration of distilled water used as chemical blowing agent was kept varied in order to obtain the similar foam density of FR-RPUFs. In addition, the isocyanate index is constant (moles of NCO/OH = 1.1). Compared with the data from the Table S1, FR-RPUFs have density similar to the pristine RPUF. In the foaming formula, the as-synthesized polyols (BHPP, MADP) and EG are used as the “reactive” and “additive” type FRs, respectively, to prepare flame-retarded RPUFs with EG at invariable proportions and FR−polyols replacing part of the LY4110 with the purpose of similar viscosity for the systematic investigation of their synergistic effect of DHPP and MADP in EG/PURF system. Low thermal conductivity is an important property for RPUF especially in the thermal insulation material industry. Table S1 shows that the thermal conductivity of pristine RPUF was 0.0360 W/mK, which has been slightly below other FR-RPUFs. In general, thermal conductivity is enhanced with solid EG,

Figure 2. 1H NMR spectrum of MADP.

The peak at 4.18 ppm is ascribed to the proton in the secondary amino groups: NHCH2NR2 (b). Signals from 3.53 to 3.60 ppm correspond to the methylene proton of CCH2OH (c). Peaks from 2.88 to 2.93 ppm appear in the spectra of diethanol amine derivatives due to the N CH2C (d) group. It can be observed that the signals between 2.55 and 2.58 ppm come from protons of hydroxyl groups (e). These results prove that the target product MADP is synthesized successfully. FTIR was used to further dissect the structure of BHPP. It can be clearly observed from Figure S1 that the existence of a variety of functional groups, like OH (3414 cm−1), PPh (1537 cm−1), PO (1240 cm−1), and POC (1050 cm−1) confirms the successful linkage between benzene phosphorus oxidichloride and 1,4-butanediol.18 The peaks at 2930 and 2865 cm−1 correspond to asymmetric and symmetric stretching absorptions of CH2. Additionally, the absorption at 3062 cm−1 is attributed to the stretching mode of CH groups in aromatic rings. Moreover, the absorptions at 1634, 1596, and 1474 cm−1 are associated with the skeletal vibration of the aromatic ring. Figure S1 also presents the FTIR spectrum of the synthesized MADP. The absorption band at around 3445 cm−1 is attributed to stretching vibration of NH and OH,

Figure 3. TG and DTG curves of pristine RPUF and FR-RPUFs under nitrogen atmosphere. D


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Figure 4. TG and DTG curves of pristine RPUF and FR-RPUFs under air atmosphere.

air show a two-stage and three-stage process, respectively. The thermal pyrolysis of polymers during a fire is characterized by anaerobe decomposition, while the char yield under nitrogen controls the fire behavior.22 As presented in Figure 3a, the pristine RPUF initially decomposes at around 233 °C, and its thermal degradation process involves two steps that occur in the temperature ranges of 233−400 °C and 400−640 °C. The first step is due to the decomposition of the hard segment. The weak bond CNH in RPUF brings about the formation of isocyanate primary or secondary amine and alcohol.4 Compared with RPUF, FR-RPUFs exhibit higher T−50%, indicating higher thermal stability. However, the initial decomposition temperature of FR-RPUFs modified with phosphorus-containing and melamine-derived polyols is below that of the pristine RPUF. The thermal stability of these three samples follows the sequence RPUF/BHPP/MADP > RPUF/MADP > RPUF/BHPP. This is probably due to RPUF/MADP/BHPP forming the IFR system. MADP is a reactive FR with multifunctional groups that can enhance the extent of cross-link density and form a stable structure. In general, polyurethane with high cross-linking possesses excellent thermal stability under oxidative and inert environments.23,24 The second step is attributed to the thermal degradation of the soft segment. Figure 3a shows that the second degradation temperature of FR-RPUFs is higher than that of pristine RPUF. The residual yield of FR-RPUFs is enhanced with the addition of BHPP or MADP, and 39.7 wt % is obtained for RPUF-4 while only a residue of 17.8 wt % is left for pristine RPUF. The synergistic effect between BHPP and MADP promotes the formation of additional char residues. As showed in Figure 4a, the initial decomposition temperature of all the FR-RPUFs is lower than that of the pristine RPUF. The thermo-oxidative decomposition of FR-RPUF follows a sequence similar to that under nitrogen atmosphere. This result indicates that the sample RPUF-4 with the BHPP/ MADP ratio of 1:1 performs the best synergetic effect, improving the thermal stability of the EG/RPUFs system. Furthermore, the enhanced thermal stability could also be certified by suppressed mass loss rate from DTG curves, suggesting that the thermal stability of FR-RPUF is significantly improved with the coaddition of BHPP and MADP. 3.4. Flame Retardancy and Combustion Behavior. The LOI test is used to evaluate the flame retardancy of the FRRPUF. The influence of the incorporated flame retardant polyols (BHPP and MADP) on the oxygen index of RPUF was

which may be owing to the phenomenon that the solid EG destroyed the structure of cells in RPUFs. Compressive strength is another desirable property for RPUF and relies greatly on the density. As it is shown in Figure S2, Addition of the solid EG has a small impact on compressive strength due to the similar density under control. The compression strength of RPUF decreases after being modified by the EG/BHPP/MADP system. The decrease in compressive strength might due to a large number of replacements of LY 4110. It is worth noting that the compressive strength of RPUF-7 decreases sharply, and thus no research about RPUF-7 is needed. 3.3. Thermal Stability. TGA is one of the most useful tools to evaluate the thermal properties of the FR-RPUF. Figures 3 and 4 show the thermal curves of pristine RPUF and FRRPUFs under (a) nitrogen and (b) air atmosphere, respectively, and the typical thermal parameters are summarized in Tables 1 Table 1. TGA Results of Pristine RPUF and FR-RPUFs under Nitrogen Atmosphere samples

T−5% (°C)

T−50% (°C)

residue (wt %)

RPUF-0 RPUF-1 RPUF-2 RPUF-3 RPUF-4 RPUF-5 RPUF-6

233 229 163 189 185 177 175

342 353 386 363 405 381 362

17.8 32.4 31.5 33.9 39.7 33.8 36.7

and 2. It is noticeable that the degradation processes under nitrogen and thermo-oxidative decomposition of RPUF under Table 2. TGA Results of Pristine RPUF and FR-RPUFs under Air Atmosphere Tmax (°C) samples

T−5% (°C)

T−50% (°C)

step I

step II

residue (wt %)


207 208 179 177 187 178 179

344 430 431 456 522 496 525

297 294 269 244 226 239 242

540 537 536 539 536 535 537

0.8 12.9 13.1 10.1 26.0 10.4 22.5 E


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nitrogen atmosphere, which may be dependent primarily on cross-link concentration and formation of a protective charred layer of RPUF-6.26 The IFR/EG system also significantly decreases the THR of RPUF composites, and THR has a similar trend to PHRR. FR-RPUFs reduce the PHRR and THR values of the RPUF composites. These results obviously reveal that the IFR system can dramatically inhibit the combustion and thereafter improve the fire safety of RPUF. Based on these facts derived from LOI and cone calorimetry tests, it is concluded that the reactive flame retardants BHPP and MADP improve the flame retardancy of RPUF. On one hand, MADP enhances the cross-linking concentration and forms a protective charred layer, thereby improving the thermal stability and flame retardancy of RPUF. On the other hand, the combination of BHPP and MADP as an IFR system dramatically increases the LOI value and promotes char layer formation. Moreover, flame retardant polyols limit flame spread during combustion. 3.5. Morphology of Char Residues. To further study the role of BHPP and MADP during combustion of FR-RPUF systems, the char residues after the cone calorimeter test were studied in-depth from both a macroscopic and a microcosmic view. Figure 6 presents the digital photos of char residues of RPUF-0, RPUF-1, RPUF-2, RPUF-4, and RPUF-6. It is found that little residual char remains after burning of pristine RPUF, whereas more residues are left for the RPUF-2 (Figure 6a,b). The presence of EG promotes the formation of a loose, wormlike, and hardened char layer. Figure 6c reveals that a large number of cracks appears on the char layers of RPUF, caused by the formation of phosphate acids or phosphoric acid analogues through the decomposition of BHPP.27 Moreover, the content of the residues of RPUF-4 and RPUF-6 is much higher than that of other samples, which is in good accordance with TGA. These results suggest that the IFR/EG system can effectively improve the charring ability of the RPUF composite. These phenomena show the compact expanded char contributed by BHPP and MADP in combination with EG resulting in the formation of a firm char layer and the improvement flame retardancy of RPUF. SEM images of residual chars for pristine RPUF and FRRPUF are presented in Figure 7. The char residues of pristine RPUF and FR-RPUFs present distinct morphological discrepancies. As can be observed from Figure 7, a relatively loose structure of the char with large numbers of cracks is visible for pristine RPUF and RPUF-1, indicating that EG cannot act as effective char layer. The heat can transfer through the cracks in

investigated. As shown in Table 3, the LOI value of pristine RPUF is 20.0% and increases to 27.0% when 15 wt % EG is Table 3. LOI Values and Cone Calorimeter Data for Pristine RPUF and FR-RPUFs samples

TTI(s)

Td(s)

PHRR (kW/m2)

THR (MJ/m2)

CY (%)

LOI (%)


2 4 5 6 8 5 6

451 467 291 354 355 319 393

286 188 166 152 136 142 141

47.7 37.6 29.8 27.1 24.8 24.6 18.2

19.5 26.6 29.3 25.5 29.8 28.8 38.4

20 27 30 30.5 33.5 32.5 30

incorporated into RPUF. The improvement owes to the phenomenon that EG can expand instantaneously and hinder heat transfer. Based on this fact, the mass fraction of EG was constant, and BHPP/MADP (1:0, 2:1, 1:1, 1:2 and 0:1) was separately incorporated into the RPUF step by step. It is worth noting that the LOI values of FR-RPUF increase from 27.0% to 33.5%, and RPUF-4 exhibits the highest LOI values. Therefore, the excellent synergistic effect between BHPP and MADP is revealed and can be further confirmed. The cone calorimeter test is one of the most useful tools to evaluate the combustion behavior of FR-RPUF. To investigate the synergistic effect of BHPP and MADP on the combustion behaviors of the RPUF matrix, the HRR and THR curves are shown in Figure 5, and related combustion data is recorded in Table 3; it is noted that two sharp peaks of HRR appear in RPUF composites, but the second PHRR is lower than the first one. RPUF burns very rapidly and thus reaches the first main peak at 35 s.25 Then, HRR evolves into a narrow peak which is responsible for the decomposition of the protective charred layer.26 In Figure 5a, it can be observed that the HRR value of pristine RPUF significantly increases and reaches the maximum immediately. The PHRR of pristine RPUF is 286 kW/m2; however, the corresponding value of RPUF-1 is only 188 kW/ m2 and RPUF-2 is 166 kW/m2, which may be caused by the charring effects of BHPP during combustion. Moreover, the incorporation of MADP leads to remarkable reduction in the PHRR values of the RPUF composite. For instance, the PHRR of RPUF-4 is 136 kW/m2, a 52.4% reduction compared with that of pristine RPUF. Besides, RPUF-6 shows a lower PHRR than the RPUF-2, in accordance with TGA results under air or

Figure 5. HRR and THR curves of pristine RPUF and FR-RPUFs. F


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Figure 6. Residual char digital photos of (a) pristine RPUF, (b) EG/RPUF, (c) BHPP/EG/RPUF, (d) BHPP/MADP/EG/RPUF, and (e) BHPP/ MADP/EG/RPUF.

Figure 7. SEM images of the char formed after combustion: (a) pristine RPUF, (b) RPUF/EG, (c) RPUF/BHPP/EG, (d) RPUF/BHPP/MADP/ EG, and (e) RPUF/MADP/EG.

wonderful effect of condensed phase flame retardant during combustion. 3.6. Char Residue Analysis. Raman spectroscopy is a useful measurement to analyze the condensed phase products of RPUF composites. The Raman spectra of the residual chars (RPUF-1, RPUF-2, RPUF-4, and RPUF-6) are shown in Figure 8. The spectra reveal two strong and broad peaks with intensive maxima at approximately 1585 and 1360 cm−1. The peak at 1360 cm−1 (D band) is ascribed to the disordered graphite carbons, while the peak at 1585 cm−1, named as the G band, is assigned to the ordered carbon. Generally, the graphitization degree of the char was calculated by the area ratio of the D and G bands (ID/IG).28−30 The lower the ratio of ID/IG, the higher graphitization degree of the char layer. Gaussian bands were

the destroyed structure, leading to reduced flame retardance. However, RPUF-2 exhibits a slightly loose and holey char layer but more firm than pristine RPUF. It is caused by phosphinic or phosphate acids generated from BHPP during combustion. The acids promote the formation of a slightly compact carbonization zone by dehydration. It is interesting to note that RPUF-4 forms continuous and compact char layers, which may be attributed to the explanation that the acids generated by BHPP react with a melamine derivative to generate the salt covering the surface of the carbon residue.5 Furthermore, the RPUF-4 presents the highest char yields and the most compact char residue among the FR-RPUF systems. These results confirm that the combination of BHPP and MADP contributes to the formation of a continuous char layer, implying a G


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spectra of the RPUF-0, RPUF-1, RPUF-2, RPUF-4, and RPUF6 at different temperatures are displayed in Figure 9. The sharp peaks and bands of modified RPUF are exhibited in Table S3. The thermal degradation of the pristine RPUF is divided into two stages: the degradation of the hard segment and the soft segment. As can be shown from Figure 9, it is noted that the peaks at 2279 and 2137 cm−1 disappear above 200 °C, indicating the complete reaction of the isocyanates during the heating process. Moreover, the peak at 1724 cm−1, ascribed to the CO stretching vibration in the PU molecular chain, is still observed at 330 °C for the control sample, whereas it disappears at 280 °C for RPUF-2 and 300 °C for RPUF-4 and RPUF-6. This result indicates that the presence of BHPP and MADP promotes the degradation of the urethane bonds, which is consistent with the results of TG curves under air. By contrast, a number of peaks appear at around 1280 and 1090 cm−1 for RPUF-2. These peaks could be assigned to the absorptions of the OPO vibration in the PPh structure,33,34 indicating that phosphorus-containing fragments are retained in the char residue. For RPUF-6, three new peaks at 815, 1440, and 1680 cm−1, corresponding to the stretching vibration of the triazine rings35 and the CN bond, respectively, appear above 500 °C. As shown in Section 3.3, the first thermal degradation process of the RPUF composite is attributed to the decomposition of the hard segment like C NH leading to the formation of isocyanate, and the second step is due to the thermal degradation of the soft segment. On the basis of these phenomena, the flame retardant properties of RPUF composites were significantly improved, and the feasible condensed flame retardant mechanism is presented. 3.8. Flame-Retardant Mechanism. Based on these abovementioned facts, the potential flame retardant mechanism of the BHPP/MADP/EG in RPUF composite is presented. First,

Figure 8. Raman spectra of the residual char of pristine RPUF and FRRPUFs.

obtained from peak fitting by using the curve fitting software Origin 8.5/Peak Fitting Module. Figure 8 reveals that the ID/IG ratio follows the order of RPUF-1 (3.13) > RPUF-2 (2.83) > RPUF-4 (2.56) > RPUF-6 (2.43), which may be due to decomposition products being converted to char by the combination of the cross-linking effect of MADP.31,32 MADP can promote amorphous char into a graphitic structure after burning of the RPUF. Therefore, the char residue becomes hard and more firm. Generally, a higher graphitization degree of the char residue leads to well protected materials from thermal oxidation.29 3.7. Thermal Degradation Process. To investigate the thermally oxidative degradation process of FR-PURF samples, RT-FTIR was used to investigate the chemical structural changes during thermal decomposition. The changes in the IR

Figure 9. Real time FTIR spectra for the degradation process of pristine RPUF and FR-RPUFs. H


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of China (51303167), and Fundamental Research Funds for the Central Universities (WK2320000027).

the degradation products of FR-PURF can catalytically convert into char by the catalytic effect of phosphoric acid or phosphate acids resulting from decomposition of BHPP. Simultaneously, the chemical cross-linking reaction between BHPP/MADP and PAPI leads to the formation of OPO and triazine ring group, which can be confirmed by RT-FTIR spectra. Additionally, the NH3 gases released from MADP can accelerate the char layer to expand during the process.36 As a consequence of the swelling influence of EG, the higher quality char residue on the surface, as shown in Figure 7d, is further generated. The char layer acts as an effective barrier, restricts the thermal transmission, and thus can protect the substrate from further thermal degradation during combustion. Therefore, the flame retardant properties of RPUF composites are dramatically improved.

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(1) Ding, H.; Xia, C.; Wang, J.; Wang, C.; Chu, F. Inherently flameretardant flexible bio-based polyurethane sealant with phosphorus and nitrogen-containing polyurethane prepolymer. J. Mater. Sci. 2016, 51, 5008−5018. (2) Zhao, X.-l.; Chen, C.-k.; Chen, X.-l. Effects of carbon fibers on the flammability and smoke emission characteristics of halogen-free thermoplastic polyurethane/ammonium polyphosphate. J. Mater. Sci. 2016, 51, 3762−3771. (3) Zheng, K.; Zhang, J.; Cheng, J. Morphology, Structure, Miscibility, and Properties of Wholly Soy-Based Semi-interpenetrating Polymer Networks from Soy−Oil−Polyol-Based Polyurethane and Modified Soy Protein Isolate. Ind. Eng. Chem. Res. 2013, 52, 14335− 14341. (4) Chattopadhyay, D.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34, 1068−1133. (5) Zhang, M.; Zhang, J.; Chen, S.; Zhou, Y. Synthesis and fire properties of rigid polyurethane foams made from a polyol derived from melamine and cardanol. Polym. Degrad. Stab. 2014, 110, 27−34. (6) Yang, H.; Wang, X.; Song, L.; Yu, B.; Yuan, Y.; Hu, Y.; Yuen, R. K. Aluminum hypophosphite in combination with expandable graphite as a novel flame retardant system for rigid polyurethane foams. Polym. Adv. Technol. 2014, 25, 1034−1043. (7) Feng, F.; Qian, L. The flame retardant behaviors and synergistic effect of expandable graphite and dimethyl methylphosphonate in rigid polyurethane foams. Polym. Compos. 2014, 35, 301−309. (8) Levchik, S. V.; Weil, E. D. Thermal decomposition, combustion and fire-retardancy of polyurethanesa review of the recent literature. Polym. Int. 2004, 53, 1585−1610. (9) Chung, Y.-j.; Kim, Y.; Kim, S. Flame retardant properties of polyurethane produced by the addition of phosphorous containing polyurethane oligomers (II). J. Ind. Eng. Chem. 2009, 15, 888−893. (10) Joseph, P.; Tretsiakova-Mcnally, S. Reactive modifications of some chain-and step-growth polymers with phosphorus-containing compounds: effects on flame retardancea review. Polym. Adv. Technol. 2011, 22, 395−406. (11) Bourbigot, S.; Duquesne, S. Fire retardant polymers: recent developments and opportunities. J. Mater. Chem. 2007, 17, 2283− 2300. (12) Park, H.; Keun, J.; Lee, K. Syntheses and physical properties of two-component polyurethane flame-retardant coatings using chlorinecontaining modified polyesters. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 1455−1464. (13) Park, H. S.; Hahm, H. S.; Park, E. K. Preparation and characteristics of two-component polyurethane flame retardant coatings using 2, 3-dibromo modified polyesters. J. Appl. Polym. Sci. 1996, 61, 421−429. (14) Modesti, M.; Lorenzetti, A. Flame retardancy of polyisocyanurate−polyurethane foams: use of different charring agents. Polym. Degrad. Stab. 2002, 78, 341−347. (15) Levchik, S.; Balabanovich, A.; Levchik, G.; Costa, L. Effect of melamine and its salts on combustion and thermal decomposition of polyamide 6. Fire Mater. 1997, 21, 75−83. (16) Feng, D.; Zhou, Z.; Bo, M. An investigation of the thermal degradation of melamine phosphonite by XPS and thermal analysis techniques. Polym. Degrad. Stab. 1995, 50, 65−70. (17) Braun, M.; Sun, Y. Antimicrobial polymers containing melamine derivatives. I. Preparation and characterization of chloromelaminebased cellulose. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3818− 3827. (18) Liu, Y.-L.; Tsai, S.-H. Synthesis and properties of new organosoluble aromatic polyamides with cyclic bulky groups containing phosphorus. Polymer 2002, 43, 5757−5762. (19) Shimizu, I.; Okabayashi, H.; Hattori, N.; Taga, K.; Yoshino, A.; O’Connor, C. 13C-and 1H-NMR and FTIR spectroscopic evidence

4. CONCLUSIONS In the work, two types of novel phosphorus-containing and melamine-derived polyol (BHPP and MADP) were successfully synthesized, and the effects of BHPP and MADP on the properties of the EG/RPUF system were systematically investigated. When the weight ratio of BHPP/MADP was 1:1, the synergistic effect between BHPP and MADP significantly reduced the PHRR and THR of RPUF composites. For instance, the PHRR value was reduced by 52.4%, compared to that of pristine RPUF. In addition, the LOI value increased from 27.0% for RPUF/EG to 33.5% for RPUF/IFR/EG. The thermal decomposition and thermal oxidation further illustrated the synergistic effect between BHPP and MADP in promoting the char forming of RPUF composites. Additional, presence of OPO and triazine ring groups, as well as NH3, contributed to the compact and continuous char residue for RPUF composites. Finally, a condensed-phase flame-retardant mechanism and the synergistic effect were proposed for improved thermal stability and flame retardancy.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02942. The formulas, densities, and physical properties of flameretardant RPUF samples; elementary analysis; assignments of the peaks in the FTIR spectrum of the RPUF composites; FTIR spectrum; and the compressive strength test results of pristine RPUF and FR-RPUFs (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 551 63601664. Fax: +86 551 63606457 E-mail: [email protected] (Y. Hu). *Tel.: +86 551 63601664. Fax: +86 551 63606457 E-mail: [email protected] (Y.M. Zhang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (51323010), Fundamental Research Funds for the Central Universities (WK2320000032), the National Natural Science Foundation I


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Phosphorus and Nitrogen-Containing Polyols: Synergistic Effect on

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