Flame-Retardant Flexible Polyurethane Foams with Highly Efficient

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Flame retardant flexible polyurethane foams with highly efficient melamine salt Wen-Hui Rao, Zai-Yin Hu, Hua-Xiu Xu, Ying-Jun Xu, Min Qi, Wang Liao, Shimei Xu, and Yu-Zhong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 02 Jun 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Industrial & Engineering Chemistry Research

Flame retardant flexible polyurethane foams with highly efficient melamine salt Wen-Hui Rao, Zai-Yin Hu, Hua-Xiu Xu, Ying-Jun Xu, Min Qi, Wang Liao*, Shimei Xu and Yu-Zhong Wang*

Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China.

ABSTRACT: A highly efficient flame retardant melamine salt (DPPMA) was synthesized with diphenylphosphinic acid and melamine. For DPPMA, the chemical structure was characterized by Fourier transform infrared spectroscopy (FTIR), Nuclear magnetic resonance spectroscopy analysis (NMR) and elemental analysis. and the thermal degradation behavior was investigated by thermogravimetric analysis (TGA). Subsequently, flexible polyurethane foam (FPUF) were prepared with DPPMA. Limiting oxygen index (LOI) tests, vertical burning tests and cone calorimetric tests were carried out to investigate the combustion of the materials. The results showed that the FPUF with only 5 php DPPMA passed the vertical burning test and had an increased LOI value of 22%. The corresponding flame-retardant mechanism was systematically investigated by FTIR, TGA and Energy dispersive X-ray spectrometer (EDX). The results revealed that DPPMA released NH3 and H2O to dilute the combustible gas and phosphorus-containing radicals in combustion, which devoured those active free radicals produced by the FPUF. 1. INTRODUCTION Polyurethane (PU) materials have been designed with various formats and functionality to raise people’s happiness. Among which, flexible polyurethane foams (FPUFs) have the highest output and takes ca. 40% of the PU market. The main applications of FPUFs are in the automobiles and electric products as cushions and interior decorations. In addition, booms of regional construction markets, such as China and India, will also promote FPUF consumption in sofas, mattresses, etc.

ACS Paragon Plus Environment


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However, pristine FPUFs are highly flammable with a limiting oxygen index (LOI) as low as ca. 18%. Without flame retardant treatment, FPUFs catch fire and spread it easily. According to statistics, fires caused by mattresses or furniture mats in the United States cause 330 deaths, 2070 injuries and $300 million loss per year.1 Effective and commercially available flame retardants (FRs) for FPUFs are chlorinated phosphorus alternatives, such as tris(2-chloroethyl) phosphate (TCEP), tris(2-chloro-1-methylethyl) phosphate (TCPP) and tris(1,3-dichloro-2-propyl) phosphate (TDCPP). However, for the environmental and health consideration, halogenated flame retardants are regrettably limited or even banned in many districts.2-8 Use of additive halogen-free flame retardents (FRs) is a simple and green alternative to impart high fire safety to FPUFs. Melamine and its derivatives tend to form char structures and may also release inert gas in a combustion environment, and therefore are widely used as FR additives.9-13 On designing melamine additives with higher FR efficiency, Costa and Camino10 carefully revealed the thermal behaviour of melamine. Lv et al.14 investigated the flammability and thermal degradation of flame retarded polypropylene (PP) composites containing melamine phosphate (MP) and pentaerythritol (PER) derivatives. Their results indicated that the fire safety of the trinary PP/MP/PER composite was better than the PP/MP binary composite, which is an established case that one-component melamine salt can hardly reach the expected effect. Faced with this problem, Chen et al.15 prepared flame-retarded FPUF with a monocomponent 2-carboxyethyl(phenyl)phosphinic acid melamine salt (CMA), and the resultant FPUFs with the CMA dosage higher than 10 php possess self-extinguishing capability. Despite monocoponent melamine and its derivatives are economic in cost and environmentally benign, the flame retardant efficiency are not very high and hence high addition amounts are required, which results in a poor compatibility and a compromise on mechanical properties. Herein, a novel melamine salt named DPPMA is designed by the reaction between melamine and diphenylphosphinic acid. The synthetic route of DPPMA is quite simple but the fire retardant capacity of the product is very high. More importantly, addition of DPPMA maintains the mechanical properties of FPUFs. This work also systematically revealed the corresponding mechanisms.

2. EXPERIMENTAL SECTION


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2.1 Material Polyether polyols (GEP-560s, number average molecular weight of 3000, average functionality of 3.0, OH content of 56 mg of KOH/g, technical pure grade) was obtained from Gaoqiao Petrochemical Company, Shanghai, China. Mixture of eighty percent of 2, 4-toluene diisocynate and twenty percent of 2, 6-toluene diisocynate (TDI 80/20, technical pure grade) was obtained from Chongqing Weiteng Polyurethane Products Factory, Chongqing, China. Diphenylphosphinic acid (DPPA) was supplied by Energy Chemical, Shanghai, China. Catalyst (Modified triethylenediamine, A-33) were technical pure grade and were supplied by Yutian Chemical Company, Liyang, China. Melamine (MA) and Catalyst (dibutyltindilaurate, DBTDL) were analytical reagent grade and were supplied by Kelong Chemical Reagent Factory, Chengdu, China. Surfactant (AK 6680, technical pure grade) was supplied by Jiangsu Maysta Chemical Technology Chemical Co.,Ltd., Beijing, China. Distilled water was used as a blowing agent and ethanol (analytical reagent grade) were supplied by Kelong Chemical Regent Factory, Chengdu, China. 2.2. Synthesis of DPPMA Diphenylphosphinic acid (DPPA, 21.8 g) was dispersed in mixture solvents (ethanol and water) with the temperature was kept at 90 °C. After that, Melamine MA (12.6 g) was added into the solution and reacted for 6 hours. Thereafter, the temperature was decreased to 50 °C, and the solution was filtrated, with the white product (named as DPPMA) obtained. Finally, the product was dried in vacuum oven at 80 °C for 12 h. The yield is around 96%. The corresponding synthetic procedure was demonstrated in Scheme 1. The structure analysis was performed through FTIR spectrum, 1H NMR spectrum and 31P NMR spectrum in Figure 1 and Figure 2, respectively.

Scheme 1. The synthetic route of DPPMA

2.3. Preparation of FPUF



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The pure and flame-retardant FPUF samples were prepared by a one-pot and free rise method. Briefly, Polyols (GEP-560s), distilled water, catalysts (A-33 and DBTDL), surfactant (AK6680), and flame retardant (DPPMA) were well mixed in a 1 L plastic beaker through mechanical stirring. TDI 80/20 was subsequently added into the beaker with vigorous stirring for 5 s. The mixture was swiftly poured into an open plastic mold (30× 20 ×15 cm3) to from free-rise foam. The foam was cured for 24 h at 80°C. The formulations of the FPUFs in this work were shown in Table S1 (Supporting Information). The NCO/OH ratio was 1.05. 2.4. Measurements Fourier transform infrared spectra (FTIR) were obtained from a Nicolet FTIR 170SX spectrometer over the wave number range from 500 to 4000 cm-1 by the KBr disk method. Nuclear magnetic resonance spectroscopy analysis (NMR): 1H NMR and

31

P NMR were

recorded on a Bruker AV 400 spectrometer using DMSO-d6 as the solvent. The contents of carbon (C), hydrogen (H) and nitrogen (N) in DPPMA were measured by elemental analysis (EA) on CARLO ERBA1106 instrument (CarloErba, Italy). Limiting oxygen index (LOI) testing was carried out with a HC-2C oxygen index instrument, which was performed according to ISO 4589-1:1996. The samples were used with 150 × 10 × 10 mm3. The density of FPUF samples was measured according to ISO 845: 2006. The size of the specimen was no less than 100 cm3, and the average values of five samples were recorded. Scanning electron microscopy (SEM) images were obtained using an INSPECTF spectrometer. The morphologies of the cross-sectional with a thin gold layer was observed under a high vacuum at a voltage of 20 kV. Energy dispersive X-ray spectrometer (EDX) was equipped for the elemental analysis in the surface scanning model. Mechanical properties of the samples were measured by tensile measurements with a ZBC1400-2 testing machine according to ISO 1798: 2008. Thermogravimetric analysis (TGA) was performed using a Netzsch 209 F1 thermal analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere at temperatures ranging from room temperature to 800 °C. Cone calorimeter (CC) tests were performed by a Cone calorimeter at a heat flux of 25 kW/m2 according to ISO 5660-1 standard. The samples used for the test were 100 × 100 × 25 mm3.


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In addition, we offer a table to summarize full names and corresponding abbreviations of the materials and testings in this article (Table S8).

3. RESULTS AND DISCUSSION Characterization of DPPMA. To prove a successful synthesis of DPPMA, its chemical structure was characterized by FTIR, 1H NMR and 31P NMR. Figure 1 shows the FTIR spectra of the reactants and product. The four absorption peaks at 3468 cm-1, 3418 cm-1, 3331 cm-1 and 3123 cm-1 of MA are attributed to the vibration absorption of -NH2. A wide vibration absorption at 2614 cm−1 is attributed to O=P-OH in DPPA, which shifts to 2485 cm−1 after the reaction between -NH2 and O=P-OH occurred.16 In addition, in the spectrum of DPPMA, the vibration absorption of -NH3+ are at 3154 cm-1 and 1509 cm-1.15

Figure 1. FTIR spectra of DPPA, MA and DPPMA

Figure 2 shows the 1H NMR and

31

P NMR spectra of DPPA and DPPMA. In Figure 2a, the

chemical shifts at 7.3~8.0 ppm indicate the aromatic hydrogens of DPPA. A new peak at 7.1 ppm emerges for DPPMA which is belonging to the hydrogen of -NH2. In Figure 2b, the single peak for phosphorous at 19.5 ppm for DPPA shifts to 23.3 ppm for that of DPPMA.15 In addition, elemental analysis (EA) test was carried out to investigate the element contents of DPPMA (Table S2), which also demonstrate a successful synthesis of DPPMA. All the abovementioned results indicate a successful synthesis of DPPMA based on our design.



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Figure 2. 1H NMR spectra (a) and 31P NMR spectra (b) of DPPA and DPPMA

The thermogravimetric behavior of DPPMA was measured in N2 with a heating rate of 10 °C/min. Both the thermogravimetric analysis (TGA) curve and its corresponding derivative curve (DTG) are shown in Figure 3. There are three steps in the thermal decomposition of DPPMA. The onset degradation temperature, defined as the temperature at which the sample loses 5% of the weight (T5%), of DPPMA is 260 °C. This temperature is lower than that of pristine FPUF,17 implying that nonflammable gas and its residue from degradation of the DPPMA are available before the degradation of FPUF. In addition, the maximum weight loss temperatures (Tmax) for DPPMA are distinguished at 105 °C, 284 °C and 375 °C, respectively. The first maximum weight loss temperatures (Tmax1) at 105 °C indicates that it is the elimination of adsorbed water. Finally, the residue of DPPMA at 600 °C is 10 wt%.

Figure 3. TGA/DTG curves of DPPMA in N2


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Microstructure and mechanical properties of FPUFs with DPPMA. SEM images of pristine FPUF and FPUF with different DPPMA contents are presented in Figure 4. The cell structures of the pristine FPUF are smooth, as shown in Figure 4a series. The b series and c series of Figure 4 further confirm DPPMA are uniformly dispersed in the foams when the loading amount is no more than 10 php and the cellular structure of the foam maintains. In contrast, when the contents of DPPMA exceed 20 php, the cellular structures are affected and wrinkles can be recognized. As d series of Figure 4 shows, 30 php DPPMA results in obvious aggregates and rupture of cell struts. The density of FPUFs with and without DPPMA were listed in Table S1. The density values slightly increase when the content of DPPMA is less than 20 php. In contrast, the density of FPUF-30 (30 php of DPPMA) increased considerably, which probably dues to that the increase of DPPMA results in some collapse of the cellular structures.

Figure 4. SEM surface photographs of pristine FPUF (a-1~a-3), FPUF-10 (b-1~b-3), FPUF-20 (c-1~c-3) and FPUF-30 (d-1~d-3)



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The mechanical properties of a polymeric foam is critical in its practical applications. Among which, the tensile strength and the tear strength are the most important properties. As shown in Table S3, the tensile strengths of FPUFs with DPPMA are comparable with the pristine one within the error range. For the FPUF-10, the tensile strength even remarkably increases. However, when the content of DPPMA further increases, the cellular structures were damaged as abovementioned, and consequently resulted in lower tensile strength. Elongation at break shows the same trend that the value increases at a low dosage of the FR and further addition makes decrease of the elongation. The highest elongation is 380 % for FPUF-5. Fire behavior and flame retardant mechanism of FPUF with DPPMA. The index of limiting oxygen index (LOI) is a direct indicator for the combustion capability of material. A value at or low than the oxygen content in air (21%) means the material burns easily. Table S1 shows that increase of the DPPMA amount in the foam effectively increases the corresponding LOI values, which could achieve a high value of 25% for easily flammable FPUF when DPPMA is 30 php. Simulated combustion behaviors of the samples were investigated by the cone calorimeter (CC) tests according to the ISO 5660-1 standard. Figure 5 shows the resultant heat release rate (HRR) and total heat release (THR) curves. The corresponding characteristic parameters, including time to ignition (TTI), the peak of HRR (PHRR), time to the PHRR (tP), total smoke production (TSP), the maximum average rate of heat emission (MARHE) and the average effective heat of combustion of volatiles (Av-EHC, the ratio of average of heat release rate to the average mass loss rate) are summarized in Table S4. Fire growth rate (FIGRA) is a derived parameter of the ratio between PHRR and tp. A lower value implies a lower maximum heat release or a prolonged time to a flashover phenomenon, and therefore relates to a higher fire safety of the testing sample. In the figure, FPUFs with DPPMA had lower PHRR values than the pristine one, indicating that DPPMA could effectively depress the thermal oxidation of the matrix. Increase of DPPMA also extends the tp values. The increase of the THR and the TSP of FPUF-30 is attributed to the vapor phase flame retardancy of DPPMA. Table S4 shows the Av-EHC values of FPUF-DPPMA are lower than that of pristine one. In addition, polyphoshoric acid formed by DPPMA also contributes to higher residue yield.18 However, pristine PU foam burns out in this test.


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Figure 5. HRR (a) and THR (b) curves of pristine FPUF, FPUF-10, FPUF-20 and FPUF-30 under an external heat

ﬂux of 25 kW/m2

Generally, a flame retardant is designed to minimize the flame spread rate and prevent sustained burning.19 To study the flame spreading behavior of the FPUFs, vertical burning tests are straightforward and strict, in which the testing standard of Cal. TB117-2000 is more difficult to pass than Cal. TB117-2013. The vertical burning tests in this study were carried out according to Cal. TB117-2000. Figure 6 shows that the fire burned intensely across the entire pristine PU foam which leaded to burn out finally. The smoke and melt dripping was as also very serious (Figure 6 (a-1~a-3)). In contrast, flammability of FPUF with DPPMA was depressed significantly. Merely 5 php DPPMA made FPUF self-extinguishable (Figure 6 (b-1~b-3)). Table S5 listed the characteristic parameters of the FPUFs in the vertical burning tests. The char length was gradually decreased with increase of DPPMA.



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Figure 6. The digital photos of pristine FPUF (a-1~a-3), and FPUF-5 (b-1~b-3) at different ignition time

Flame retardant mechanism of FPUF with DPPMA. The flame retardant mechanism of DPPMA was revealed by a systematically study of substances in the vapor phase and the condensed phase during the pyrolosis. Thermogravimetry analysis (TGA) was used to characterize thermal degradation as measured by volatilization of degradation products. The resulted TG (Figure 7a) and differential thermogravimetry (DTG) (Figure 7b) curves of pristine FPUF and FPUF/DPPMAs were shown, and the characteristic data were listed in Table S6. Two main stage thermal degradation steps could be distinguished for all FPUF samples. The first maximum mass loss was the degradation of hard segments, resulting in the release of isocyanate, alcohol, primary and/or secondary amine, olefin as well as carbon dioxide.19, 20 The second maximum degradation step corresponded to the thermal decomposition of the soft segments17. For pristine FPUF, almost no residue left at a temperature higher than 410 °C. An important thermal stability index is T 5% (at which the sample has a 5 wt% weight loss), which is accepted the indication of degradation onset. After a combination of DPPMA, T5% and the first maximum weight loss temperature (Tmax1) both shifted to lower temperatures, which is attributed to the earlier decomposition of DPPMA. Furthermore, compared to the pristine foam, the weight loss rates (both RTmax1 and RTmax) of flame-retardant samples decreased.


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Figure 7. TGA and DTG curves of pristine FPUF, FPUF-10, FPUF-20 and FPUF-30 at a heating rate of 10 °C/min

in N2

A combination of FTIR to TGA measurement is used to analyze the gaseous products during pyrolysis. Figure 8 shows the FTIR spectra of volatilized pyrolysis products emitted from DPPMA at its maximum decomposition rate. The bands at 3500-3600 cm-1 are attributed to the vibration absorption of hydroxide groups, indicating the water vapor. Besides, the peak at around 2360 cm-1 is due to the stretching vibration of CO2, and the peak at around 960 cm-1 is due to the stretching vibration of NH3. The bands at 3390 cm-1 and 3333 cm-1 are attributed to the stretching vibration of -NH2. When heated to above 375 °C, the characteristic absorption peaks of phenyl were observed (3069 cm-1 and 1435 cm-1). The sharp characteristic peak at 1180 cm-1 is assigned to the stretching vibration of P=O group. When the temperature reached to 442 °C, a strong absorption band at 2283 cm-1 and 2245 cm-1 appeared, attributing to the stretching vibration of C≡N bond. As a result, the fragments at gas phase are mainly composed of phosphorus and nitrogen-containing products, which exert a quenching effect in the vapor phase during the combustion of DPPMA21.



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Figure 8. The TG-IR spectra of gas phase in the thermal degradation of DPPMA at the maximum decomposition

rates

The fire behavior in the condensed phase was investigated by morphology analysis and component analysis of the residues after the CC tests. SEM photographs of the residue are shown in Figure 9. The external surface of the char residue of FPUF-30 char residues was dense (Figure 9a), which thus could work as a shell to hinder the transfer of heat, oxygen and combustible gas. In addition, the internal surface promoted this effect by its intumescent structure (Figure 9b). Components of the residue was studied by IR and the spectrum was shown in Figure 10. The bands at 2963 cm-1 are attributed to the stretching vibration of -CH2. The characteristic absorption peaks of P=O (1261 cm-1) and P-O-C (1095 cm-1) could be observed.22 Energy dispersive X-ray spectrometer (EDX) was utilized to investigate the element concentration of carbon (C), nitrogen (N), oxygen (O), and phosphorus (P) for char residue of FPUF. The element content of char residue and FPUF-30 was showed in Table S7. The percentage of phosphorus was only 0.39 wt% in the char residue. And phosphorus (P) of FPUF-30 was 0.14 wt%. The percentage of phosphorus retained in the char compared to that in the foam was 21%. indicated that most of phosphorus was released into the vapor phase.9


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Figure 9. SEM microphotographs of char residues of FPUF-30: (a) external surface and (b) internal surface.

Figure 10. FTIR spectra of FPUF-30 char residues.

Based on above analysis, a possible flame-retardant model of DPPMA in FPUF is summarized in Figure 11. DPPMA began to decompose after 284 °C, And the main decomposition products were diphenylphosphinic acid and melamine, With the temperature enhancing, the diphenylphosphinic acid generate the corresponding gaseous products containing phosphorus and benzene,

which

could

produce

polyphosphoric

acid

in

condensed

phase

and

phosphorus-containing radicals to capture the free radicals produced by the FPUF1. Meanwhile, during heating, melamine was known to undergo progressive endothermic condensation and the formation of polymeric products such as melam, melem, and melon and produce NH3 to diluted the combustible gas.23 Besides, the condensation products further cross-link diphenylphosphinic acid to form phosphocarbonaceous structure, which could protect the foam from burning.24-28



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Figure 11. A possible flame-retardant mechanism model of DPPMA in FPUF.

4. CONCLUSION A novel phosphorus and nitrogen-containing melamine derivatives named DPPMA was synthesized and applied in FPUF. The combustion tests, including LOI, CC and vertical burning tests, indicated that the flame retardancy of FPUF was improved significantly with the incorporation of DPPMA. Particularly, the vertical burning test showed that FPUF could self-extinguishing when FPUF containing merely 5 php DPPMA. All these results present that the flame retardant efficiency of DPPMA was outstanding for FPUF. The corresponding flame retardant mechanism was analyzed by FTIR, TG-IR and EDX, indicating that DPPMA mainly played its role in the vapor phase. In addition, the mechanical properties of resulting FPUFs were improved.

AUTHOR INFORMATION Corresponding Author

* Tel. & Fax: +86-28-85410755; E-mail: [email protected] (Y. Wang); [email protected] (W. Liao)

ACKNOWLEDGMENT


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This work was financially supported by the National Natural Science Foundation of China (Grants 51320105011 and 51121001) and Program for Changjiang Scholars, Innovative Research Team in University (IRT. 1026) and Sichuan Province Youth Science and Technology Innovation Team (No. 2017TD0006).

Supporting Information: Elemental analysis of DPPMA, formulations of pristine FPUF and flame retardant FPUF with different DPPMA contents, detailed characterization data about the vertical burning test, mechanical properties data of foam, TGA data of foam, CC data of foam, the EDX data of char residue and foam, and the Abbreviation and Full name in article. This material is available free of charge via the Internet at http://pubs.acs.org.

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2017, 56, 4649-4658.



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TOC graph 82x43mm (300 x 300 DPI)


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Flame-Retardant Flexible Polyurethane Foams with Highly Efficient

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