Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate

Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate

Polymer Degradation and Stability xxx (2013) 1e9 Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.e...

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Polymer Degradation and Stability xxx (2013) 1e9

Contents lists available at ScienceDirect

Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab

Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate Zhu-Bao Shao a, b, Cong Deng a, b, *, Yi Tan a, b, Ming-Jun Chen a, b, Li Chen a, b, Yu-Zhong Wang a, b, * a 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 b Analytical and Testing Center, Sichuan University, Chengdu 610064, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 September 2013 Received in revised form 5 October 2013 Accepted 7 October 2013 Available online xxx

Ammonium polyphosphate (form I APP) was modified via ion exchange reaction with ethylenediamine, and the resulting modified ammonium polyphosphate (MAPP) was used alone to prepare intumescent flameretardant (IFR) polypropylene (PP) via melt blending. The flame retardancy of PP containing MAPP was investigated by limiting oxygen index (LOI), vertical burning test (UL-94) and cone calorimeter (CC). The LOI value of PP containing 40 wt% of MAPP reached 32.5%, which increased by 56.9% compared with that of PP with the same content of APP, and the UL-94 rating was V-0 in the case of specimen thickness of 1.6 mm, while the latter had no rating. CC test results showed that the heat release rate (HRR), the mass loss rate (MLR) and the smoke production rate (SPR)of PP/MAPP system decreased significantly compared with neat PP and PP/ APP systems. Especially the fire growth rate (FGR) and SPR peak of PP containing 35 wt% MAPP decreased by 89.1% and 63.2% respectively compared with those of PP containing 35 wt% APP. These results demonstrated that only by incorporating the MAPP without additional charring agents, could PP be successfully flame retarded. Fourier transform infrared spectroscopy (FTIR) etc. were used to investigate the flame retardant mechanism of MAPP, and it was found that both the generation of carbonecarbon double bonds after the scission of CeN bonds and the residue consisting of some stable structures such as PeNeC and CeN etc. caused the charring ability to increase dramatically, which must be the principal reason for the much better flame retardancy of PP/MAPP system without any additional charring agent compared with APP. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Ammonium polyphosphate Ethylenediamine Flame retardant Polypropylene

1. Introduction PP has been widely used in many fields due to its excellent properties, such as electrical insulation, chemical corrosion resistance, machinability and so on [1e4]. However, the flammability of PP restricts its application in flame-retarded material field [5,6]. In order to expand its application in this field, much work has been performed to improve the flame retardancy of PP [7,8], in which IFR is considered to be one of the most promising candidates for its merits, such as low damage to the properties of polymer matrix, low loading, environmental friend, low smoke etc. [9]. Generally, IFR is composed of an acid source, a charring agent and a blowing agent [10]. Ammonium polyphosphate (APP) and * Corresponding authors. 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. Tel./fax: þ86 28 85410259. E-mail addresses: [email protected] (C. Deng), [email protected] (Y.-Z. Wang).

charring agent are the typical components of intumescent halogenfree flame retardant system. Ammonium polyphosphate (APP) is used as the experimental acid source and blowing source due to its high contents of phosphorus (P) and nitrogen (N). During the combustion process, the carbonization agent is engaged in charring process under the catalytic effect of acid source, and the char formed can be expanded under the action of blowing source to form an intumescent char layer. The char layer formed reduces heat transfer between the heat source and the polymer surface, and also limits fuel transfer from the polymer toward the flame as well as the diffusion of oxygen into the material, consequently promoting the flame retardancy of polymer composite. Almost in all IFR systems, the carbonization agent is necessary to form expandable char layer. However, as we all know, the preparation processes of carbonization agents are very complicated [11e13], and the solvent is harmful to the environment, so the ideal pathway is to find some new IFRs which are composed of a single material which gathers all the advantages of acid source, carbonization agent and blowing agent. At present, no was reported on how to apply a single material

0141-3910/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2013.10.005

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to prepare intumescent flame-retardant polymer composite. APP derivatives should be the best candidate since APP has gathered two advantages as acid source and blowing agent. For APP, currently the most important method to enhance its application in IFR is microencapsulation at the surface. The main aim of this method focuses on enhancing the water resistance and thermal stability of APP, or improving the compatibility between APP and polymer matrix [9,14]. Hu et al. [15e18] found that microencapsulated ammonium polyphosphate with melamineeformaldehyde resin, ureaeformaldehyde resin, polyurethane or epoxy resin shell, etc. could lead to a decrease in the water solubility of APP particles. Lei et al. [4] had studied microencapsulated ammonium polyphosphate with hydroxyl silicone oil (HSO) and melamine formaldehyde (MF) resin, and found that the compatibility of HSOe MFAPP with PP was better than that of unmodified APP besides the better water resistance. Bourbigot et al. [19] found the replacement of APP by a coated APP (APP-THEIC) was enhancing the performances of the epoxy based intumescent coating. However, no literature reported that APP modified through chemical reaction could act as an efficient charring agent besides the acid source and blowing agent, consequently might promote the flame retardancy of polymer composite. Based on the idea that APP can be involved in charring during the combustion process, a novel kind of modified APP was prepared by incorporating ethylenediamine in our experiment. So far, it has been found that ethylenediamine phosphate (EDAP) could enhance the flame retardancy of PP [20]. However, no literature reported that ethylenediamine could contribute to improve the flame retardant effect of APP in polymer matrix. So ethylenediamine was incorporated into APP through chemical reaction to enhance the flame retardant effect of APP in this work. In current experiment, the flame retardancy of PP/MAPP composite was studied with the aid of different combustion tests, and the flame retardant mechanism of MAPP as a separate IFR was also investigated in detail. 2. Experimental section 2.1. Materials Commercial APP (form I) was supplied by Changfeng Fire Retardants Co., Ltd. (Sichuan, China); ethylenediamine (AR, 99.0%) and ethanol (AR, 99.7%) were purchased from Kelong Chemical Reagent Co., Ltd. (Sichuan, China); polypropylene (T30S) was obtained from Petro China Lanzhou Petrochemical Co., Ltd. (Lanzhou China). 2.2. Preparation of MAPP A certain volume ratio of ethanol and water (800 mL: 30 mL) was poured into a three-neck flask equipped with a stirrer under a nitrogen atmosphere. Half an hour later, ethylenediamine (18 g) was injected into the flask, and the solution was stirred. Then 100 g APP was added to the flask. After that, the mixture was heated up to 90  C for 4 h. When the reaction completed, the reaction mixture was cooled down to room temperature. Then the white solid was filtered, washed with ethanol, and then dried to a constant weight. And the average sizes of APP and MAPP particles were investigated by Master Sizer 2000 (Malvern Instruments Ltd., UK), which are about 13.3 mm and 20.7 mm, respectively. 2.3. Sample preparation 2.3.1. Samples for combustion test Both the Commercial APP and the prepared MAPP were dried in a vacuum oven at 80  C for 12 h. Then the PP blends filled with different ratios of APP or MAPP were prepared via a twin-screw

extruder (CTE 20, Kebeilong Keya Nanjing Machinery Co., Ltd, Nanjing, China) with the rotation speed of 150 rpm at the following temperature protocol from the feed zone to the die: 175, 180, 190, 185, 180 and 170  C. Finally the extruded pellets were hot-pressed into different samples by plate vulcanizer (Qingdao Yadong Rubber Viachinery Co. Ltd. China). 2.3.2. Residual char samples for FTIR test and XPS test The samples were heated to the corresponding temperature at a heating rate of 10  C/min under a nitrogen atmosphere and retained 10 min at a series of temperature in TG 209 F1 (NETZSCH, Germany). Then the residual char samples were obtained. 2.4. Measurements The FTIR spectra were recorded by a Nicolet FTIR 170 SX spectrometer (Nicolet, America) using the KBr disk, and the wave number range was set from 4000 to 500 cm1. 1 H NMR spectra were recorded on a Bruker AV II-400 MHz spectrometer (Bruker, Switzerland) by using D2O as a solvent. The XRD patterns using Cu Ka radiation (l ¼ 1.542  A) were performed with power DX-1000 diffractometer (Dandong Fangyuan, China) at the scanning rate of 0.02 per second in the 2q range of 5e50 . The surface morphologies of the APP and MAPP were observed by using a JEOL JSM 5900LV scanning electron microscopy (SEM) (JEOL, Japan) at the accelerating voltage of 5 kV. The contents of carbon (C), nitrogen (N) and hydrogen (H) in APP and MAPP were measured by elemental analysis (EA) on CARLO ERBA1106 instrument (Carlo Erba, Italy). The LOI value was measured using an HC-2C oxygen index instrument (Jiangning, China) according to ASTM D2863-97 with a sheet dimension of 130 mm  6.5 mm  3.2 mm. The UL-94 vertical burning level was tested on a CZF-2 instrument (Jiangning, China) according to ASTM D3801, the dimensions of samples were 130 mm  13 mm  3.2 mm and 130 mm  13 mm  1.6 mm, respectively. The flammability of the sample was measured by a cone calorimeter device (Fire Testing Technology, UK). The samples with the dimension of 100 mm  100 mm  3 mm were exposed to a radiant cone at a heat flux of 50 kW/m2. ThermogravimetryeFourier transform infrared spectroscopy (TGeFTIR) consists of TG 209 F1 (NETZSCH, Germany) coupled with 170 SX FTIR spectrometer (Nicolet). The sample (about 6 mg) was heating at a rate of 10  C/min in the temperature range from 40 to 700  C under the nitrogen flow of 50 mL/min. XPS spectra were recorded by a XSAM80 (Kratos Co, UK), using Al Ka excitation radiation (hv-1486.6 eV). 3. Results and discussion 3.1. Characterization of MAPP The FTIR spectra of MAPP and APP are shown in Fig. 1. The peaks located at 3400e3030 cm1 were ascribed to the NH4 þ asymmetry stretching vibration [21], and the new absorption peaks appeared at the 2917 cm1, 2850 cm1 and 1541 cm1, which are the characteristic stretching absorption peaks of eCH2eCH2e and eNH3 þ : The appearance of the peaks corresponding to eCH2eCH2e and eNH3 þ in MAPP proved the EDA salt ðeNH3 þ  OePeÞ instead of ammonium salt ðeNH4 þ  OePeÞ was formed. The 1H NMR spectra of APP and MAPP are shown in Fig. 2. Obviously, there is no 1H NMR peak other than the water peak at 4.80 ppm. But for MAPP, the peak attributed to eCH2e protons appeared at 3.40 ppm, indicating the structure of NH3 þ eCH2 eCH2 eNH3 þ was

Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005

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Fig. 1. FTIR spectra of APP and MAPP.

Fig. 3. The XRD curves of APP and MAPP.

formed. In addition, there is no more peak other than the peaks at 3.40 and 4.80 ppm for MAPP, meaning that no NH2eCH2eCH2eNH2 was left. The results of 1H NMR further proved that the ethylenediamine-modified ammonium polyphosphate, MAPP was obtained successfully. The XRD spectra of MAPP and APP were showed in Fig. 3. Compared with APP, the diffraction peaks of MAPP almost appeared at the same positions except for one new peak at about 11.72 which was attributed to the characteristic peak of (NH4)5P3O10$H2O, suggesting that the crystalline structures of APP were not affected by the reaction between EDA and APP.

the MAPP, and the results are listed in Table 1. For APP, the contents of the C, N and H were 1.26, 13.2 and 3.48%, respectively. Here, it should noted that the carbon in APP was detected, which might be resulting from the following two types of matters. The first one was from CO2 absorbed on the surface of APP; the second one was from trace additives containing carbon. After incorporating the ethylenediamine, the contents of C and H increased to 7.19% and 3.92%, respectively; while the content of N reduced slightly. Compared with APP, the increases of C and H in MAPP also demonstrated that the ethylenediamine reacted with APP successfully.

3.2. SEM analysis

3.4. Water solubility

Fig. 4 shows the morphological surfaces of commercial APP and the MAPP. The commercial APP particles exhibited smooth surface. After being modified by ethylenediamine, the surfaces of MAPP were quite rough, and it seemed that the particles clung each other. Apparently, the morphological surfaces of MAPP are rather different with those of unmodified APP, suggesting that APP reacted with EDA during the synthesis process.

The water solubility was tested at 25  C according to HG/T 27702008, and the results are listed in Table 2. For APP, the water solubility was about 1.80 g/100 mL water. After being modified, the water solubility of MAPP decreased to 1.24 g/100 mL water. Obviously, the water solubility of APP was improved after being modified by EDA. 3.5. Flame retardancy

3.3. EA test EA was used to distinguish the differences of carbon (C), nitrogen (N) and hydrogen (H) contents between commercial APP and

The LOI and UL-94 tests were used to evaluate flame retardant performances of neat PP, PP/APP and PP/MAPP composites, and the results are shown in Table 3. Neat PP was a flammable polymer with

Fig. 2. The 1H NMR spectra of APP and MAPP.

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Fig. 4. SEM micrographs of APP (a) and MAPP (b).

a low LOI value of 18.0%, and it showed no rating in UL-94 test. When the addition of the APP was 25 wt%, the LOI value of the PP/ APP composite increased to 20.0%. But with increasing the APP, the LOI value of the PP/APP composite only increased slightly. When the content of the APP was 40 wt%, the LOI value of the PP/APP composite was only 20.9%, indicating that APP was not very effective to promote the flame retardancy of PP. For PP/MAPP composite, when the addition of the MAPP was 25 wt%, the LOI value of the PP/ MAPP composite reached 28.0%. In LOI test, the char layer formed during combusting could be observed obviously, which is different from that occurred in neat PP and PP/APP composite. With further increasing the MAPP, the LOI value of PP/MAPP composite increased significantly. When the content of MAPP was 40 wt%, the LOI value of the PP/MAPP composite reached 32.5%, which increased by 62.5% compared with APP at the same loading. Obviously, MAPP is much more effective to promote the LOI value of PP compared with APP in LOI test. Table 3 shows that the PP/APP composite failed to pass the UL94 rating and had an obvious dripping even though the addition of the APP reached 40 wt%. However, when the content of MAPP was 30 wt%, the PP/MAPP composite could pass UL-94 V-0 rating (3.2 mm) with a small amount of dripping, but no flammable dripping. Furthermore, when the content of MAPP increased to 40 wt%, the PP/MAPP composite could pass UL-94 V-0 rating (1.6 mm) while PP/APP composite was still no rating at the same loading. Obviously, as an intumescent flame retardant the efficiency of MAPP is much higher than that of APP in UL-94 test. Cone calorimeter is an effective method to study the flammability of materials. The heat release rate (HRR), total release rate (THR), smoke production rate (SPR) and mass loss rate (MLR)

curves of neat PP and PP composites are shown in Fig. 5, and the corresponding data are presented in Table 4. For neat PP, the peak of HRR (PHRR) was 841.6 kW/m2, and the THR was 89.1 MJ/m2. At the loading of 35 wt% APP, the PHRR and THR of PP/APP composite were 435.9 kW/m2 and 83.9 MJ/m2 respectively, which were lower than the corresponding 841.6 kW/m2 and 89.1 MJ/m2 of neat PP. At the same loading of MAPP, the PHRR and THR of PP/MAPP composite significantly reduced, which were 156.1 kW/m2 and 60.5 MJ/m2 respectively, and correspondingly decreased by 64.2% and 27.9% compared with PP/APP. Based on the HRR curves, fire growth rate (FGR) has been calculated to assess the fire hazard of the composite according to the following equation [22,23]:

FGR ¼ PHRR=tPHRR Generally, a lower FGR value indicates that the time to flashover is delayed, which allows enough time to evacuate for person in distress and/or arrive for fire extinguishers [24]. When 35 wt% APP was introduced into PP, the FGR was increased from 7.32 kW/m2 s to 8.72 kW/m2 s. With the incorporation of 35 wt% MAPP, the tPHRR was prolonged to 165 s, so the FGR of PP/MAPP composite significantly decreased to 0.95 kW/m2 s, which greatly decreased by 89.1%. The results mentioned above proved that the modification through EDA greatly reduced the FGR of PP/APP, consequently could significantly extent the time to escape in real accident. The SPR and MLR curves of PP and PP composites are presented in Fig. 6, and the corresponding data are shown in Table 4. It is noticeable that the SPR peak values of both PP/APP and PP/MAPP composites significantly reduced compared with neat PP. The SPR peak of neat PP was 0.095 m2/s. Compare with neat PP, the SPR peak of PP/APP composite decreased to 0.067 m2/s, and the value of PP/ MAPP composite further reduced to 0.035 m2/s. These results illustrated that both MAPP and APP could play a role in restraining

Table 1 C, N and H contents of APP and MAPP.

APP MAPP

C (%)

N (%)

H (%)

1.26 7.19

13.2 13.1

3.48 3.92

Table 2 Water solubility of APP and MAPP. Water solubility (g/100 mL water)

APP MAPP

1

2

3

4

5

6

1.77 1.17

1.79 1.26

1.80 1.30

1.70 1.28

1.83 1.24

1.90 1.20

Average (g/ 100 mL water) 1.80 1.24

Table 3 LOI and UL-94 results of neat PP, PP/APP and PP/MAPP systems. Component (wt%) PP

APP

MAPP

100 75 70 65 60 75 70 65 60

0 25 30 35 40 0 0 0 0

0 0 0 0 0 25 30 35 40

LOI/%

18.0 20.0 20.4 20.4 20.9 28.0 29.5 30.5 32.5

UL-94 (3.2 cm)

UL-94 (1.6 cm)

Rating

Dripping

Rating

Dripping

NR NR NR NR NR V2 V0 V0 V0

Yes Yes Yes Yes Yes Yes Yes No No

NR NR NR NR NR NR NR V2 V0

Yes Yes Yes Yes Yes Yes Yes Yes Yes

Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005

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Fig. 5. HRR and THR plots of neat PP, PP/35 wt% APP and PP/35 wt% MAPP composites.

Table 4 Cone calorimeter data of neat PP, PP/APP and PP/MAPP. Sample

PP

PP/App

PP/Mapp

TTI (s) PHRR (kW/m2) Time to PHRR (s) FGR (kW/m2 s) THR (MJ/m2) Peak SPR (m2/s) Average MLR (g/s) Char mass (%)

25 841.6 115 7.32 89.1 0.095 0.067 2.0

11 435.9 50 8.72 83.9 0.067 0.055 25.1

11 156.1 165 0.95 60.5 0.035 0.025 33.9

the produce of smoke. Moreover, the smoke suppression effect of MAPP was significantly better than that of APP. Fig. 6 shows that the average MLR of PP/APP and PP/MAPP composites also decreased markedly at 35 wt% flame retardant. For PP/APP composite, the average MLR decreased to 0.055 g/s from 0.067 g/s of neat PP, and it further reduced to 0.025 g/s for PP/MAPP composite. The digital photos of the residues for PP/APP and PP/MAPP after CC test are shown in Fig. 7. There was no residue left for neat PP after CC test; while a small amount of residue was left for PP/APP, and a poor and unexpanded char layer was formed after CC test. For PP/MAPP composite, a continuous intumescent char layer was formed after the combustion. Generally, the intumescent char layer could slow the heat and mass transfer between gas and condensed phases, and could also protect the underlying materials from further burning. So the formation of intumescent char layer should be the most important factor to achieve the much better flame retardancy of PP/MAPP. Fig. 8 shows the SEM micrographs of the residue surfaces of PP/ APP and PP/MAPP after CC test. PP/MAPP composite left more

continuous and compact char layer after burning compared with PP/APP composite, the char layer formed could effectively provide as a barrier between PP/MAPP composite and fire, consequently protect the underlying materials. 3.6. Flame retardant mechanism of MAPP Fig. 9 shows the thermal degradation curves of APP and MAPP under N2 atmosphere. Their degradation behaviors had apparent differences. First, For APP, there were two severe decomposing processes which located at about 320  C and 640  C, respectively. However, for MAPP, only one dramatic ML peak appeared at about 400  C during the degradation process. The thermal degradation process of APP could be divided into two steps. The first step was in the range of 200e450  C, the weight loss should be attributed to the elimination of NH3 and H2O in the thermal degradation process of polyphosphate. The second step was beyond 450  C, this weight loss was attributed to the release of phosphoric acid, polyphosphoric acid, and metaphosphoric acid with APP decomposition [25]. The thermal degradation process of MAPP is more complicated, which not only contained the same thermal degradation of ammonium polyphosphate as APP, but also included the thermal behavior of EDA salt. After incorporating the EDA, the residue of MAPP was much higher than that of APP between 300 and 400  C. The formation of more char residue at this stage contributed to the following formation of intumescent char layer, so it can be observed that the introduction of NH3 þ eCH2 eCH2 eNH3 þ in APP effectively enhanced the flame retardancy in the combustion tests. To further study the effect of EDA on PP/APP, the FTIR test of gas phases and condensed phases for APP and MAPP at different

Fig. 6. SPR and ML curves of neat PP, PP/35 wt% APP and PP/35 wt% MAPP composites.

Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005

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Fig. 7. Digital photographs of residues of neat PP (a, a1), PP/APP (b, b1) and PP/MAPP (c, c1) composites after CC test.

Fig. 8. SEM images of PP/APP (a  1000, a1  10,000) and PP/MAPP (b  1000, b1  10000) of the residues obtained from CC tests.

Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005

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Fig. 9. TG (a) and DTG (b) curves of APP and MAPP at a heating rate of 10  C/min under N2 atmosphere.

temperatures were performed. Fig. 10a shows the FTIReTG spectra of the gaseous phases of APP during the thermal degradation. The absorbing peaks of ammonia at 930 and 965 cm1 appeared at 300  C. Both the intensities of the two absorbing peaks decreased with increasing the temperature in the range from 300 to 400  C. Obviously, the highest thermal decomposition rate of APP was at near 300  C. Here, the release of NH3 should be from NH4 þ : However, for MAPP, as showed in Fig. 10b, the intensities of the absorbing peaks for NH3 increased with increasing the temperature in the range from 40 to 405  C, which was quite different from the decrease for APP in the range from 300 to 400  C, indicating the existence of another pathway to produce ammonia in the range of 300e405  C besides the thermal degradation of NH4 þ : Meanwhile, the new peak ascribed to the stretching vibration of the eCN appeared at 2305 cm1 at 405  C. Both phenomena demonstrated that another pathway to produce ammonia should be from the thermal decomposition of and NH3 þ eCH2 eCH2 eNH3 þ : In order to further demonstrate the flame retardant mechanism of MAPP, the condensed phases of MAPP were investigated by FTIR test. Fig. 11 shows the FTIR spectra of the condensed phases at different temperatures. The peaks of NH4 þ located at 3400e 3030 cm1 decreased gradually, and disappeared until the temperature increased to 400  C, indicating the occurrence of the thermal degradation of NH4 þ : This process is consistent with the release process of ammonia in the range from 40 to 405  C mentioned above [26]. The peak of NH3 þ located at 1541 cm1 also gradually decreased, and disappeared until the temperature increased to 400  C. Meanwhile, the peaks of PeNeC were

observed at 1104 and 714 cm1 with increasing the temperature. As Nguyen and Kim [27] claimed that the PeNeC rich charred residue was more thermally stable, resulting in more char residue. This finding illustrated why it could be observed that the residue of MAPP was higher than APP between 300 and 400  C in TG test. The absorbing peaks of eCH2eCH2e at 2927 and 2850 cm1 disappeared gradually from 300 to 400  C, and the absorbing peaks at 1632 cm1 become wider, suggesting the appearance of some structures containing eC]Ce. XPS data for the condensed products of MAPP at different temperatures can be used to further interpret the thermally decomposing behaviors. The XPS results are shown in Table 5, and the contents of carbon (C), nitrogen (N), oxygen (O), and phosphorus (P) were investigated. The content of O decreased first from 36.5% to 27.7% when the temperature increased from 200  C to 320  C, which might come from the condensation dehydration between ammonium salt and PeOH. Then the content of O increased, which might be due to the decrease of water release and the increase of other gases release. The content of N had the converse change tendency. It increased first due to the release of a little NH3 and a large number of other gases, then it decreased with increasing the release of NH3. The release of H2O and NH3 resulted in the increase of the content of C in the condensed phase. Subsequently, it was reduced because of the oxidization of unstable C, and then it increased slightly again due to the formation of stable C structures such as C]C etc. The content of P maintained increasing from 200 to 560  C, which should be due to the release of gases and the formation of PeOeP during the thermally decomposing

Fig. 10. FTIR spectra of the gaseous products of APP (a) and MAPP (b) during the thermal degradation.

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Fig. 11. FTIR spectra for the condensed products of MAPP at different temperatures.

Table 5 XPS data of the condensed products of MAPP during the thermal decomposition. Temperature 200 320 420 560



C  C  C  C

C (wt%)

N (wt%)

P (wt%)

O (wt%)

35.2 39.8 22.8 23.4

8.8 14.7 8.2 2.7

19.5 17.8 24.4 27.3

36.5 27.7 44.6 46.5

Scheme 1. Mechanism on the charring during the combustion process for MAPP.

formation of the intumescent, compact and stable char layer, consequently leading to the better flame retardant performance of MAPP than APP. process. XPS results demonstrated that the change tendency of the contents of main elements was consistent with the thermally decomposing process in TGeFTIR test. According to the TG, FTIR and XPS test results, the flame retardant mechanism can be concluded as follows: in the initial stage, with the release of NH3 and H2O, PeNeC structure was formed during the combustion process, so the more stable char residue was formed. In this stage, the intumescent char layer began to form. Then, with increasing the temperature, part of PeNeC structures decomposed under the catalytic effect of PeOH, accompanied by the release of H2O and NH3, which led the formation of intumescent, compact and stable char layer. Finally, the excellent flame retardant performance of MAPP was achieved. The possible mechanism on charring during the combustion of MAPP is shown in Scheme 1. 4. Conclusion Ethylenediamine modified APP (MAPP) was prepared successfully. PP/MAPP systems showed more effective flame retardancy than PP/APP systems, At 40 wt% of MAPP content, PP/MAPP system reached 32.5% of LOI value, and could pass the UL-94 V-0 rating (1.6 mm). The CC results showed MAPP had better flame retardant contribution in PP matrix than APP. The HRR, MLR and SPR of the PP/MAPP system decreased largely compared with neat PP and PP/ APP system. In addition, SEM micrographs and digital photos directly illustrated that the more compact and stable carbon layer was formed during the combustion process for PP/MAPP system in comparison with PP/APP. The study on flame retardant mechanism of MAPP showed that both the formation of the stable char layer and the production of a large of inflammable gases led to the

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Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005

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Please cite this article in press as: Shao Z-B, et al., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate, Polymer Degradation and Stability (2013), http://dx.doi.org/10.1016/ j.polymdegradstab.2013.10.005