Regulating Effects of Nitrogenous Bases on the Char Structure and

Research Article pubs.acs.org/journal/ascecg

Regulating Effects of Nitrogenous Bases on the Char Structure and Flame Retardancy of Polypropylene/Intumescent Flame Retardant Composites Zhijing Wang,†,‡ Yinfeng Liu,† and Juan Li*,‡ †

School of Materials and Science Engineering, Shanghai University, Shanghai 200444, P. R. China Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P. R. China

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‡

S Supporting Information *

ABSTRACT: In this paper, four kinds of nitrogenous bases, adenine (A), guanine (G), cytosine (C), and uracil (U), were used as biobased gas sources to regulate the efficiency of an intumescent flame retardant (IFR) in polypropylene (PP). The flame retardant properties of PP composites were evaluated by using the limiting oxygen index (LOI), the vertical burning (UL-94) test, an infrared thermal imager, etc. The thermal degradation behaviors and char morphology were studied by using thermogravimetric analysis (TGA) and scanning electron microscopy. It is found that U and C play great roles in improving the flame retardancy of PP/IFR composites. The PP sample containing 17 wt % IFR and 1 wt % U (or C) achieves the UL-94 V0 rating without melt-dripping and has a LOI value of >27.9%, while the samples with equal amounts of A or G are not classified in the UL-94 test. TGA results showed that U (or C) can react with an IFR, but the interaction between A (or G) and an IFR is weak. U (or C) accelerates the formation of char and regulates its space structure at the right content. They induce the formation of a cellular and intumescent char layer that decreases the surface temperature quickly after ignition and protects the underlying resin from flame, thus improving the efficiency of PP/IFR composites. KEYWORDS: Nitrogenous bases, Intumescent flame retardant, Char structure, Microporous, Polypropylene

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INTRODUCTION Flame retardancy is necessary for most polymers used in industry and daily life because of their structure. Generally, introducing flame retardants into a polymer matrix is a simple and feasible method for modifying their flammability. So far, thousands of flame retardants have been used in different polymers. With the improvement of safety and environmental protection requirements, halogen-free flame retardants for polymer materials have attracted a great deal of attention.1−3 Among them, an intumescent flame retardant (IFR) is an environmentally friendly and promising flame retardant with excellent comprehensive properties, such as being low smoke, nontoxic, and halogen-free, generating few corrosive gases, etc.4−6 A conventional IFR system is usually composed of three components: an acid source [e.g., ammonium polyphosphate (APP)], a carbon source [e.g., pentaerythritol (PER)], and a gas source (e.g., melamine).7The three components of the IFR react at the right time and temperature to form an intumescent and cellular char layer that could prevent the matrix from burning effectively. Therefore, the synergistic effect of the three sources for the IFR is one of the most important factors for good flame retardancy. However, the flame retardant efficiency of the commercial IFR system is unsatisfactory; usually no less © 2017 American Chemical Society

than 25 wt % APP/PER in PP (polypropylene) or 30 wt % APP/PER in PE (polyethylene) is needed to achieve good flame retardant performance.8,9 Moreover, the larger the amount of IFRs, the poorer the physical and mechanical properties of the polymers. In addition, the cost of the materials will increase. Therefore, it is necessary to improve the flame retardant efficiency to reduce the effect of the IFR on the mechanical performance of polymer materials. To overcome the challenge, both chemical and physical methods were used to increase the retardant efficiency of the IFR system. For example, Wang et al.10 prepared a novel monocomponent polymeric IFR, PA-APP. The results showed that the PP sample with 20 wt % PA-APP achieves the UL-94 V0 rating. Introducing catalysts and/or synergists into commercial IFRs is another simple method for increasing the flame retardant efficiency. Many materials were used as synergists in the IFR system in the past, such as zeolite,11,12 clay,13,14 sepiolite,15,16 organoboron siloxane,17,18 and metallic compounds.19,20 Tang et al.21 explored the effect of a nickel catalyst Received: November 9, 2016 Revised: January 16, 2017 Published: January 16, 2017 2375

DOI: 10.1021/acssuschemeng.6b02712 ACS Sustainable Chem. Eng. 2017, 5, 2375−2383

Research Article

ACS Sustainable Chemistry & Engineering on the flame retardant properties of PP/IFR composites. The results showed that the PP composites containing 19 wt % IFR and 1 wt % nickel catalyst can pass the UL-94 V0 test. Nie et al.22 synthesized a nanoporous nickel phosphate VSB-1 and studied its synergistic effect on the flame retardancy of PP/IFR composites. With 2 wt % VSB-1 and 18 wt % IFR, the sample reaches the UL-94 V0 rating. Chen et al.10,23 found that phosphomolybdic acid-based imidazolium ([BMIm]3PMo) has an excellent synergistic effect on the flame retardancy of PP/IFR composites. The PP composites with 14.5 wt % IFR and 0.5 wt % [BMIm]3PMo may achieve the UL-94 V0 rating. The fact that many researches focused on acid sources, carbon sources or synergists should be mentioned, as few novel gas sources have been developed except melamine and its derivatives, dicyandiamide, urea, etc. It is well-known that synergistic reactions among the three sources of IFR are the key factors for the formation of intumescent char and good flame retardancy. Therefore, a suitable gas source may regulate the cellar structure of char at the right time, which can modify the efficiency of IFR to some extent. Because the thermal stability of dicyandiamide or urea is not good enough for most polymer processing, only melamine and its derivatives are used universally. However, given that every polymer has its degradation characteristics, melamine is not suitable for all polymers. Therefore, developing novel gas sources is necessary for modifying the efficiency of IFR. Nitrogenous bases are kinds of nitrogenous substances from nature that have good thermal stability. They can release gas during combustion like melamine, so they should be good candidates as gas sources of an IFR. On this basis, in this paper four nitrogenous bases were used as gas sources in the IFR system. PP was used as the matrix because PP is a popular and flammable polymer, and IFR is the right candidate for PP to achieve good flame retardancy.24,25 The regulating effects of nitrogenous bases on the char structure and flame retardancy of PP/IFR composites were investigated. Flame retardancy and thermal degradation behaviors of PP composites were investigated by using the limiting oxygen index (LOI), the vertical burning (UL-94) test, a cone calorimeter, thermogravimetric analysis (TGA), etc. In addition, the surface temperature was detected by an infrared thermal imager during the test of UL-94 and the char morphology was observed by scanning electron microscopy (SEM).

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Table 1. Flame Retardancy of PP Composites UL-94 sample

PP (wt %)

IFR (wt %)

bases (wt %)

LOI (%)

dripping/ ignition

t1 (s)/t2 (s)

rating

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12

100 100 82 82 82 82 82 82 82 82 82 82

0 25 18 17 17 17 17 16 16 16 16 15

0 0 0 1(A) 1(G) 1(C) 1(U) 2(A) 2(G) 2(C) 2(U) 3(C)

17.5 28.3 24.6 27.3 27.6 27.9 28.7 26.8 27.6 28.0 28.6 27.2

Y/Y N/N Y/Y Y/Y Y/Y N/N N/N Y/Y Y/Y N/N N/N Y/Y

−/− 0/0 0/>30 0/>30 0/>30 0/2 0/1 0/>30 0/>30 0/1 0/1 0/>30

NC V0 NC NC NC V0 V0 NC NC V0 V0 NC

The UL-94 vertical burning rating was measured on an AG5100B vertical burning tester (Zhuhai Angui Testing Instrument Co. Ltd.). The specimen dimensions were 100 mm × 13 mm × 3.2 mm according to ASTM D3801. A thermal infrared imager (FLUKE Ti400) was used to detect the temperature of the specimens during the UL-94 vertical burning test. The LOI was tested on a 5801 digital oxygen index analyzer (Kunshan Yangyi Testing Instrument Co. Ltd.) according to ASTM D2863-97; the dimensions of the samples were 100 mm × 6.5 mm × 3.2 mm. Combustion behaviors were investigated by means of a cone calorimeter (Fire Testing Technology Co. Ltd.), according to standard ISO 5660. The samples with dimensions of 100 mm × 100 mm × 3.2 mm were placed on a holder and irradiated at a heat flux of 35 kW/m2 in a horizontal configuration. The morphology of the char residues was observed by a scanning electron microscope (S4800 Hitachi Corp.) after being sputter-coated with a conductive gold layer.

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RESULTS AND DISCUSSION Flame Retardancy of PP Composites. Synergistic effects of nitrogenous bases on the flame retardant properties of PP composites were evaluated via the UL-94 vertical burning test and LOI, and the relative data are listed in Table 1. Neat PP is flammable accompanied by heavy dripping. Its LOI value is only 17.5%, so it is not classified in the UL-94 test. S3, which is PP containing 18% IFR, has an increased LOI value of 24.6%. However, S3 is still not classified in the UL-94 test. When the content of the IFR in PP composites is increased to 25 wt %, S2 could reach the UL-94 V0 rating with a higher LOI value of 28.3%. When the bases were introduced into the IFR system, the flame retardant properties of PP composites with a total IFR content of 18 wt % are clearly improved. LOI values for all PP composites containing 1−3 wt % bases are improved more or less. For instance, the LOI value of S7 is 28.7%, which is 4.1% higher than that of S3. The flame retardancy of the PP sample containing C (or U) is better than the flame retardancy of those containing A and G. For example, the PP sample with 17 wt % IFR and 1 wt % C (or U) achieves the UL-94 V0 rating, while the samples with an equal content of A or G fail in the UL-94 test. When the formulation is changed to 2 wt % C (or U) and 16 wt % IFR, S10 and S11 can also pass the UL-94 V0 test. However, upon addition of 3 wt % C (or U) and 15 wt % IFR to PP, S12 (or S13) cannot be classified in the UL-94 test. According to the results presented above, C and U are more suitable for PP/IFR composites than A and G. Although A and G both improve the LOI values of PP/IFR composites,

EXPERIMENTAL SECTION

Materials. APP (n > 1500; Preniphor TMEPFR-APP231) was provided by Presafer Phosphor Chemical Co. Ltd. (Qingyuan, China). PER and nitrogenous bases were provided by Aladdin Industrial Inc. (Shanghai, China). PP (F401) was obtained from Sinopec Yangzi Petrochemical Co. Ltd., with a melt index of 2.0 g/min (230 °C/2.16 kg). All reagents were used without further purification. The used bases are adenine (A), guanine (G), cytosine (C), and uracil (U). Preparation of PP Composites. PP composites were prepared by melt blending on a Brabender mixer at 200 °C with a rotor speed of 50 rpm, and the processing time was 10 min. Then the PP composites were hot pressed into a 100 mm × 100 mm × 3.2 mm plate at 200 °C under 10 MPa for 5 min. The formulations of PP composites are listed in Table 1; the IFR that was used was a mixture of APP and PER at a weight ratio of 3/1. All the raw materials were dried in a vacuum oven at 80 °C for 12 h before being used. Measurements. TGA tests were performed by using a TGA/ DSC1 analyzer (Mettler Toledo International Inc., Greifensee, Switzerland). An approximately 3.0−5.0 mg sample was placed in an alumina crucible at a heating rate of 10 °C/min in nitrogen (N2) or an air atmosphere in the temperature range from 50 to 800 °C. 2376


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ACS Sustainable Chemistry & Engineering

intumescent char of S4 and S5 looks like that of S6 and S7 in LOI tests, but they are not classified in the UL-94 tests. On one hand, the quantity and quality of char residues are not sufficient to act as an effective barrier for the underlying resin. On the other, the macro intumescent char is not a sufficient condition for good flame retardant performance. The micro char structure may play a more important role in the flame retardant properties of PP composites. Infrared Thermal Imaging Analysis. The combustion process of PP composites was recorded by an infrared thermal imager during the UL-94 test. As shown in Figure 3, the highest temperature after the first ignition (T1) of the PP/IFR composite (S3) is 363 °C. When 1 wt % nitrogenous bases are added to the PP/IFR system, the T1 values of S4−S7 are 45, 18, 22, and 33 °C lower than that of S3, respectively. Moreover, the highest temperature after the second ignition (T2) of S3 is 512 °C. The T2 values of S6 and S7 are 383 and 379 °C, respectively, which are 129 and 133 °C lower than that of S3, respectively. As can be seen, S4 and S5 have T2 values higher than those of S6 and S7, while the T2 for S4 is higher than that for S3. The results suggest that the char formed by S4 after the first ignition has better thermal conductivity, so it has the lowest T1. Here the char is just formed and does not expand evidently. After the second ignition, a large amount of cellar char has been produced. The surface temperature of samples is affected by the heat barrier properties of char, which may relate to its microstructure. The char residues of S6 and S7 prevent the transfer of heat from outside to resin; therefore, the surface temperature of the composites decreases evidently, and the flame is extinguished quickly after ignition. Thus, the samples described above achieve the UL-94 V0 rating. Thermal Degradation Behaviors. To investigate the roles of nitrogenous bases in the IFR system, TGA was used to explore the reactions between different components. The experimental (Exp.) and calculated (Cal.) TGA curves of the samples under a N2 atmosphere are illustrated in Figure 4, and the detailed data are listed in Table 2. Here the Exp. curves are obtained from TGA testing, and the Cal. curves are calculated according to their weight percentage in the blend. The temperatures at 1−50 wt % degradation are marked as T1 wt %, T5 wt %, T10 wt %, T20 wt %, and T50 wt %. The weight ratio of IFR and nitrogenous base is 17/1. All bases make the IFR degrade at a lower temperature according to Figure 4. T1 wt %, T5 wt %, and T10 wt % for all IFR/bases-Exp. are lower than their Cal. values. For example, the T1 wt % values of IFR/C-Exp. and IFR/U-Exp. are 180 and 159 °C, respectively, which are 13 and

they have few effects on the UL-94 rating, suggesting that they have a stronger effect on the LOI than on UL-94 vertical burning. The results mean that different nitrogenous bases play different roles in the IFR system; some of them act as good gas sources to enhance the efficiency of the IFR in PP, and some of them are not suitable for PP/IFR composites. This may relate to their thermal degradation behaviors, which will be discussed below. The photos of the samples after LOI and UL-94 tests are presented in Figures 1 and 2. Neat PP produces few char residues

Figure 1. Photos of PP composites after the LOI test.

Figure 2. Photos of PP composites after the UL-94 test.

after the LOI test and burned completely after the UL-94 test. With the addition of 18% IFR, a few intumescent char residues are observed on the surface of S3 and the sample burned to the clamp after the UL-94 test. However, the expansion volume of char for the samples containing both IFR and nitrogenous bases after the LOI test is larger than the expansion volumes of those samples without nitrogenous bases. The char residues after LOI tests for S4−S7 look similar, and their LOI values change from 27 to 29%. Most S4 and S5 are burned, while S6 and S7 keep good shapes after the UL-94 test. It is interesting that the

Figure 3. Photos of infrared thermal imaging for different PP composites. 2377


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Figure 4. TGA curves of IFR and nitrogenous bases under a N2 atmosphere.

T1 wt %, T5 wt %, T10 wt %, and T50 wt % of both IFR/C-Exp. and IFR/U-Exp. are higher than those of IFR/A-Exp. and IFR/G-Exp., which means that IFR/C and IFR/U are more stable in air. Moreover, the char residues of IFR/C-Exp. and IFR/U-Exp. at 800 °C are 12.4 and 12.1%, respectively, which are higher than those of IFR/A-Exp. and IFR/G-Exp. All the results suggest that bases can react with IFR and promote IFR to degrade at a lower temperature. Compared with that of A and G, the antioxidant activity of C and U is better. Though C and U promote the decomposition of IFR early, the thermal stability of char is good enough that the level of char residues is the same as that of IFR at 800 °C, while A and G make IFR degrade so much that the char residue is clearly decreased at 800 °C. Hence, C and U are more suitable for use in the PP/IFR system. Then the thermal degradation behaviors of PP composites with different nitrogenous bases in air and N2 atmospheres were investigated as shown in Figure 6. The related data are listed in Table 4. Under an air atmosphere, the curve for neat PP shows a two-step decomposition and has few char residues in the temperature range from 600 and 800 °C. Compared to PP, introducing IFR into PP causes the T1 wt % to shift to a lower temperature. Via incorporation of nitrogenous bases into the PP/IFR system, the thermal stability of the samples increases further. S6 and S7 have the same T1 wt % of 189 °C, which is 44 °C higher than that of S3. Though T1 wt % values of S4 and S5 are also 25 and 4 °C higher than that of S3, respectively, the difference is smaller than that of S6 and S7. The results mean that S6 and S7 are more stable than S4 and S5 at the early decomposition stage. However, the difference decreases while the temperature increases. The T50 wt % values of S6 and S7 are even 6 and 9 °C lower than that of S3, respectively. In addition, all PP composites degrade earlier than neat PP at lower temperatures and show better thermal stability at higher temperatures (>350 °C) because of the formation of char. In addition, the char residues at 800 °C for S6 and S7 are greater than that for S3 and S4. The results suggest that C or U promotes the degradation of PP composites during the early

Table 2. Exp. and Cal. Data of IFR and Nitrogenous Bases under a N2 Atmosphere sample

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

residues at 600 °C

residues at 800 °C

A G C U IFR IFR/A-Exp. IFR/G-Exp. IFR/C-Exp. IFR/U-Exp. IFR/A-Cal. IFR/G-Cal. IFR/C-Cal. IFR/U-Cal.

232 184 264 219 193 191 190 180 159 194 193 194 193

280 435 311 268 230 216 213 213 201 233 232 233 232

294 454 315 283 270 243 236 241 222 276 279 279 274

328 499 434 318 530 506 495 500 469 513 523 528 459

0 19.9 40.3 0 33.4 32.9 28.4 29.9 27.4 29.7 32.0 34.1 29.9

0 2.4 29.6 0 26.6 23.0 18.4 22.8 22.1 23.7 24.0 26.9 23.9

34 °C lower than those of IFR, respectively. Moreover, T1 wt %, T5 wt %, T10 wt %, and T50 wt % for IFR/C-Exp. and IFR/U-Exp. are not only lower than their Cal. values but also lower than those of IFR/A-Exp., which means strong interactions among them. It is worth mentioning that IFR/G-Exp. also has T5 wt %, T10 wt %, and T50 wt % values lower than those of IFR/A-Exp. and IFR/C-Exp. However, the char residue of IFR/G-Exp. at 800 °C is 18.4%, which is the lowest among the samples. This suggests that although G promotes IFR degradation at a lower temperature, the char produced by IFR/G is not stable at high temperatures. All the results suggest that some reactions exist among bases and IFR. U and C are more suitable for the IFR system to improve the flame retardant properties of PP composites. The reactions between IFR and bases in an air atmosphere are also investigated by using TGA as shown in Figure 5, and the related data are summarized in Table 3. First, the degradation behaviors for bases and IFR under air are different from that under N2. The TGA curves shift to a lower temperature when bases are introduced into the IFR system. However, 2378


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Figure 5. TGA curves of IFR and nitrogenous bases under an air atmosphere.

Table 3. Exp. and Cal. Data of IFR and Nitrogenous Bases under an Air Atmosphere sample

T1 wt % (°C)

T5 wt % (°C)

T10 wt % (°C)

T50 wt % (°C)

residues at 600 °C

residues at 800 °C

A G C U IFR IFR/A-Exp. IFR/G-Exp. IFR/C-Exp. IFR/U-Exp. IFR/A-Cal. IFR/G-Cal. IFR/C-Cal. IFR/U-Cal.

233 153 212 183 180 164 147 188 177 182 180 180 180

276 411 311 245 225 206 203 217 214 228 225 228 227

291 441 315 266 261 236 228 246 273 268 261 271 262

325 492 427 304 578 524 521 552 538 566 578 574 565

0 2.6 22.2 0 33.5 20.0 18.9 26.9 24.7 29.8 33.5 32.3 29.4

0 0 0 0 12.1 6.3 7.2 12.4 12.1 10.8 12.1 10.6 10.4

Table 4. TGA Data of Different PP Composites under Air and N2 Atmospheres sample

T1 wt % (°C)

T5 wt % (°C)

S1 S3 S4 S5 S6 S7

261 145 170 149 189 189

278 263 258 252 265 265

S1 S3 S4 S5 S6 S7

363 142 235 178 222 230

409 341 363 346 359 363

T10 wt % (°C) Air 288 272 271 263 273 273 N2 425 417 406 406 412 415

T50 wt % (°C)

residues at 800 °C

326 325 321 322 319 316

0.7 2.5 2.2 1.2 2.8 2.8

455 469 458 458 457 457

0 5.0 3.7 3.7 4.0 4.6

PP/IFR composite, the degradation temperatures from T1 wt % to T10 wt % for PP composites are clearly improved compared to those of the PP/IFR composites. For example, T1 wt % values for S6 and S7 are 80 and 88 °C higher than that of S3, respectively. It is noteworthy that S3 has the most char residues among all samples. This is the opposite of the case under air, indicating that char formation promoted by C or U may depend on the participation of oxygen. Furthermore, the good

period and strengthens the thermal stability of the char system at later stages, which helps to protect the matrix from burning. The degradation of PP under a N2 atmosphere is one-step degradation. T1 wt % of PP is clearly higher than that in air, and no char residues are kept at high temperatures. The IFR makes T1 wt %, T5 wt %, and T10 wt % of PP shift to lower temperatures, but T50 wt % increases. Moreover, S3 has 5.0% residues at 800 °C. Upon addition of nitrogenous bases to the

Figure 6. TGA curves of different PP composites in (a) air and (b) N2 atmospheres. 2379


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in accordance with its poor flame retardant behavior in the UL-94 test. Moreover, few large holes are observed in the char of both S7 and S11. It helps to explain why they achieve the UL-94 V0 rating but S5 fails. When the content of U is changed, the micromorphology of char is also different. S7 containing 1 wt % U does not show a rough morphology. Its surface is smooth without large pores under high magnification. When the content of U is increased to 3 wt %, some dendritic char residues appear on the surface of S13. Moreover, some defects or pores are found around the branch. Because the compatibility between bases and PP is not very good, excess U may accumulate in the local area, resulting in the formation of the dendritic structure of char. In addition, excess gas flows may cause more and larger pores, and the char layer will become imperfect. The imperfect char leads to the poor flame retardancy of S13. Therefore, with good intumescent char layers, S7 and S11 exhibit flame retardant performances in LOI and UL-94 tests better than those of S3 and S13. All the results presented above indicate that right bases and appropriate content induce the formation of intumescent char with a proper porosity for PP/IFR composites. It is the special micromorphology of char that contributes to good flame retardant properties. The Fourier transform infrared (FT-IR) spectra of char residues are presented in Figure 8. The strong absorption at

flame retardancy of S6 and S7 may not only relate to the amount of char but also mainly rely on the microstructure of char. Micromorphology of Char Residues. The micromorphology of the char layer after the UL-94 test was observed by using SEM as illustrated in Figure 7 and Figure S1. The char

Figure 8. FT-IR spectra of char layers of (a) S3, (b) S5, and (c) S7.

3500−3000 cm−1 corresponds to the valence vibrations of O−H and N−H bonds in hydroxyl and amino compounds. The peaks at 1623 and 1626 cm−1 correspond to CC stretching vibrations in the aromatic compounds. The peaks near 1242 and 1174 cm−1 are attributed to the P−O−C stretching mode of the phosphocarbonaceous complex. The peak at 1083 cm−1 is the symmetric vibration of the P−O bond in P−O−P. The peaks at 997 and 981 cm−1 belong to the symmetric vibration of PO3. All of these assignments indicate that the char layer is formed through a series of chemical reactions and the char is a compound of products containing P−O−C, P−O−P, PO3, etc.26,27 All these peaks can be found in the char of S5 and S7, which indicates that introducing bases into the IFR system does not change the char structure. The bases may just regulate the cooperation among the acid source, carbon source, and gas source and promote the formation of microporous and intumescent char layers, which is the main factor for enhancing the flame retardant properties of PP composites. Cone Calorimetry Tests. The combustion performance of PP composites was investigated by means of a cone calorimeter, which is always used to evaluate the actual fire burning behavior of materials. It can provide the following principle fire

Figure 7. SEM photographs of char for PP composites: (a1 and a2) S3, (b1 and b2) S5, (c1 and c2) S7, (d1 and d2) S11, and (e1 and e2) S13.

of S3 seems compact, and the expansion ratio is low, which is supported by the photos after the LOI tests. Several large holes are observed on the surface as shown in Figure 7a1, indicating that it is imperfect. However, nitrogenous bases change the micromorphology of the char greatly. First, the char residues of S7 and S11 become rough. Second, the char is not dense but cellular as described in the magnified photos. Intumescent and cellular char is a good barrier that can effectively prevent the exchange of heat and gas between outside and inside and help samples to achieve good flame retardancy. While several large holes are observed on the surface of the char layer of S5, this is 2380


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Figure 9. Relationship between (a) HRR, (b) THR, and (c) ML and time for different PP composites at a heat flux of 35 kW/m2.

Table 5. Combustion Performance Parameters from a Cone Calorimetric Test sample

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

tPHRR (s)

FPI (m s2 kW−1)

FGR (kW m−2 s−1)

S1 S3 S5 S7

51 45 46 46

888 439 324 293

125 111 105 105

195 320 368 376

0.057 0.103 0.142 0.157

4.55 1.37 0.88 0.78

composites decreases slowly during the combustion and char residue increases. Though the ML curve of S7 looks like that of S5, its char residues are clearly greater than that of S5. These results indicate that U is better than G for promoting the formation of the intumescent char layer and decreases the rate of heat release, which protects the material from burning. Photographs of PP composites after the cone calorimetric test are presented in Figure 10. There are few char residues for PP.

parameters: time to ignition (TTI), heat release rate (HRR), peak HRR (PHRR), time to PHRR (tPHRR), total heat release (THR), and mass loss (ML). Figure 9 shows the relationship between HRR, THR, and ML and time for PP composites at a heat flux of 35 kW/m2, and the corresponding data are listed in Table 5. PP (S1) burns rapidly, and its HRR shapes a single peak with a PHRR of 888 kW/m2. When 18 wt % IFR is introduced into PP, the PHRR of S3 decreases to 439 kW/m2. After the addition of nitrogenous bases to the PP/IFR composite, the PHRRs of S5 and S7 decrease further, which are 324 and 293 kW/m2, respectively. In addition, the HRR curves of all PP composites show two peaks, in which the first is assigned to the development of the intumescent protective char and the second to the degradation of the charring layer. PP has a THR of 125 MJ/m2, while introducing IFR into PP decreases the THR of PP composites. Nitrogenous bases decrease the THR further, but S5 and S7 have the same THR of 105 MJ/m2. The tPHRR for all PP composites is longer than that of PP, and the longest tPHRR is that of S7. The results mean that IFR delays the combustion effectively and bases enhance the effect on tPHRR. Moreover, the FPI that equals TTI/PHRR and the FGR that equals PHRR/tPHRR are used to analyze the fire hazard of PP composites further. Generally, a higher FPI value and a lower FGR value indicate a lower burning risk. S7 achieves the highest FPI and smallest FGR, suggesting its good flame retardancy. In addition, the relationship between ML and time is shown in Figure 9c. It is evident that the mass of neat PP decreases quickly and nothing is left after combustion. Upon introduction of IFR or IFR/bases into the PP matrix, the ML of PP

Figure 10. Photographs of PP composites after the cone calorimetric test: (a) S1, (b) S3, (c) S5, and (d) S7. 2381



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Upon addition of 18% IFR, the char layer of S3 covers the whole substrate. However, some large holes or cracks are also observed at the center of the char layer, indicating that it is not good enough to protect the underlying resin from burning. After the introduction of G into the PP/IFR composite, a continuous and compact char layer is formed for S5, but some crevasses and holes are also seen in the surface of the char. Upon addition of U to the PP/IFR system, S7 forms a smooth and compact char layer, which is perfect, and few cracks and holes are found. A good char layer can act as a barrier between the resin and flame, withstand the erosion of heat and gas, and protect the matrix from combustion. It is the good char layer that makes PP composites exhibit good flame retardant performance.

CONCLUSIONS In this work, four nitrogenous bases were used as novel gas sources in PP/IFR composites. The flame retardancy, thermal degradation, charring behaviors, and heat release of PP composites were investigated. The results showed that all nitrogenous bases clearly improve the LOI values of PP composites. However, only U or C plays a positive role in improving the UL-94 rating of PP composites. Adding 17 wt % IFR and 1 wt % U (or C) makes PP composites achieve the UL-94 V0 rating, while ≥25 wt % single IFR is needed in PP composites to reach the same UL-94 rating. TGA results showed that U and C induce the PP composites to degrade at a lower temperature and improve the thermal stability and char residues at higher temperatures. Most importantly, U and C as gas sources promote the formation of intumescent and cellular char, which is a good barrier for preventing the exchange of heat and gas between the resin and outside. Therefore, U and C not only reduce the temperature after the first ignition but also decrease the temperature after the second ignition in the UL-94 test. Moreover, the cone calorimetric tests revealed that the addition of U to the PP/IFR system decreases the PHRR and THR and prolongs the tPHRR, suggesting a lower burning risk for the PP/IFR/U composite. In conclusion, a base can be used as a novel and green gas source of IFR when its thermal and chemical performance is suitable. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02712. SEM morphology of the char residues for more samples after the UL-94 test (Figure S1) (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-574-86685256. Fax: +86-574-86685186. E-mail: [email protected]. ORCID

Juan Li: 0000-0001-9075-4209 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (51473178) and the Program for Ningbo Science and Technology Innovative Team (2015B11005). 2382


Research Article

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Regulating Effects of Nitrogenous Bases on the Char Structure and

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