Regulating Effects of Nitrogenous Bases on the Char Structure and

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Regulating effects of nitrogenous bases on char structure and flame retardancy of polypropylene/intumescent flame retardant composites Zhijing Wang, Yinfeng Liu, and Juan Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02712 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017

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ACS Sustainable Chemistry & Engineering

Regulating effects of nitrogenous bases on char structure and flame retardancy of polypropylene/intumescent flame retardant composites Zhijing Wang1,2, Yinfeng Liu1, Juan Li2* 1. School of Materials and Science Engineering, Shanghai University, Shanghai 200444, PR China. 2.Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, PR China. *Corresponding author: phone: +86-574-86685256; fax: +86-574-86685186; Email: [email protected]

ABSTRACT In this paper, four kinds of nitrogenous bases: Adenine (A), Guanine (G), Cytosine (C) and Uracil (U) were used as bio-based gas sources to regulate the efficiency of intumescent flame retardant (IFR) in polypropylene (PP). The flame retardant properties of PP composites were evaluated by using limiting oxygen index (LOI), vertical burning (UL-94) test and infrared thermal imager etc. The thermal degradation behaviors and char morphology were studied by using thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). 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 larger than 27.9 %, while the samples with equal amount of A or G are not classified in the UL-94 test. TGA results showed that U (or C) can react with IFR, but the interaction between A (or G) and IFR is weak. U (or C) accelerates the formation of char and regulates its space structure at right content. They induce the

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formation of cellular and intumescent char layer which decreases the surface temperature quickly after ignition, and protects the underlying resin from flame thus improves the efficiency of PP/IFR composites. KEYWORDS: Nitrogenous bases; intumescent flame retardant; char structure; microporous; polypropylene;

INTRODUCTION Flame retardancy is a necessary performance for most of polymers to be applied in the industrial fields and daily life due to their structure. Generally, introducing flame retardants into polymer matrix is a simple and feasible method to modify 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 lots of attentions1-3. Among all, intumescent flame retardant (IFR) is an environmentally friendly and promising flame retardant with excellent comprehensive properties, such as low smoke, non-toxicity, halogens free and few corrosive gas generation etc.4-6 A conventional IFR system is usually composed of three components: acid sources (e.g. ammonium polyphosphate (APP)), carbon sources (e.g. pentaerythritol (PER)) and gas sources (e.g. melamine) 7.The three components of IFR react at a right time and temperature to form an intumescent and cellular char layer which could prevent the matrix from burning effectively. So the synergistic effect of the three sources for IFR is one of the most important factors for good flame retardancy. However, the flame retardant efficiency of commercial IFR system is unsatisfactory, usually no less than 25 wt % APP/PER in PP (polypropylene) or 30 wt % APP/PER in PE (polyethylene) is needed to achieve a good flame retardant performance8-9.


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Moreover, the more the amount of IFRs is, the poorer the physical and mechanical properties of polymers will be. In addition, the cost of materials will increase. Therefore, it is necessary to improve the flame retardant efficiency to reduce the effect of IFR on mechanical performance of polymer materials. To overcome the challenge, both chemical and physical methods were used to increase the retardant efficiency of IFR system. For example, Wang et al10 prepared a novel mono-component polymeric IFR, PA-APP. The results showed that the PP sample with 20 wt % PA-APP achieves the UL-94 V-0 rating. Introducing catalysts/synergists into commercial IFRs is another simple method to increase the flame retardant efficiency. Many matters were used as synergists in IFR system in the past years, such as zeolite11-12, clay13-14, sepiolite15-16, organoboron siloxane17-18 and metallic compounds19-20. Tang et al21 explored the effect of nickel catalyst on the flame retardant properties of PP/IFR. The results showed that the PP composites containing 19 wt % IFR and 1 wt % nickel catalyst can pass the UL-94 V-0 test. Nie et al22 synthesized a nanoporous nickel phosphate VSB-1 and studied its synergistic effect on flame retardancy of PP/IFR. With 2 wt % VSB-1 and 18 wt % IFR, the sample reaches the UL-94 V-0 rating. Chen et al10,23 found that phosphomolybdic acid based imidazolium ([BMIm]3PMo) has excellent synergistic effect on flame retardancy of PP/IFR. The PP composites with 14.5 wt % IFR and 0.5 wt % [BMIm]3PMo may achieve the UL-94 V-0. What should be mentioned is that lots of researches focused on acid sources, carbon sources or synergists, 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



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source may regulate the cellar structure of char at 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. So developing novel gas sources is necessary for modifying the efficiency of IFR. Nitrogenous base is a kind of nitrogenous substances from nature and has good thermal stability. They can release gas during combustion like melamine so they should be good candidates as gas sources of IFR. Based on this, 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 a right candidate for PP to achieve good flame retardancy24-25. The regulating effects of nitrogenous bases on char structure and flame retardancy of PP/IFR composites were investigated. Flame retardancy and thermal degradation behaviors of PP composites were investigated by using limiting oxygen index (LOI) and vertical burning (UL-94) test, cone calorimeter, thermal gravimetric analysis (TGA) etc. In addition, the surface temperature was detected by infrared thermal imager during the test of UL-94 and the char morphology was observed by scanning electron microscopy (SEM).

EXPERIMENTAL 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


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obtained from Sinopec Yangzi Petrochemical Co. Ltd., with a melt index of 2.0g/min (230℃/2.16kg). 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 ºC with rotor speed of 50 rpm, and the processing time was 10 min. Then the PP composites were hot pressed into a 100×100×3.2 mm3 plate at 200 ºC under 10 MPa for 5 min. The formulations of PP composites are given in Table 1, the used IFR is a mixture of APP and PER at a weight ratio 3/1. All the raw materials were dried in a vacuum oven at 80 ℃ for 12 h before using. Measurements TGA tests were carried out by using a TGA/DSC1 Analyzer (METTLER TOLEDO International Inc., Switzerland). About 3.0−5.0 mg sample was put in an alumina crucible at a heating rate of 10 °C/min in nitrogen (N2) or air atmosphere at the temperature range from 50 ºC to 800 ºC. UL-94 vertical burning rating was measured on an AG5100B vertical burning tester (Zhuhai Angui Testing Instrument Co. Ltd., China). The specimens dimension is 100×13×3.2 mm3 according to ASTM D3801. Thermal infrared imager (FLUKE Ti400) was used to detect the temperature of the specimens during UL-94 vertical burning test. LOI was tested on a 5801 digital oxygen index analyzer (Kunshan Yangyi Testing Instrument Co. Ltd., China) according to ASTM D2863-97; the dimension of samples is 100×6.5×3.2 mm3.



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Combustion behaviours were investigated by means of cone calorimeter (Fire Testing Technology Co. Ltd., U.K.), according to ISO 5660 standard. The samples with a dimension of 100 × 100 × 3.2 mm3 were placed on a holder and irradiated at a heat flux of 35 kW/m2 in horizontal configuration. Morphology of the char residues was observed by a SEM (S4800 Hitachi Corp., Japan) after being sputter-coated with a conductive gold layer.

RESULTS AND DISCUSSION Flame retardancy of PP composites Synergistic effects of nitrogenous bases on 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 % and not be 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 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 IFR system, the flame retardant properties of PP composites with total 18 wt % IFR are improved evidently. 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 PP sample containing C (or U) has better flame retardancy than 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 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, adding 3 wt % C (or U) and 15 wt % IFR into PP,


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S12 (or S13) cannot be classified in the UL-94 test. Table 1. Flame retardancy of PP composites PP

IFR

Bases

(wt %)

(wt %)

(wt %)

S1

100

0

0

17.5

S2

100

25

0

S3

82

18

S4

82

S5

Samples

LOI

UL-94

（%） Dripping/Ignition

t1/t2(s)

Rating

Y/Y

-/-

NC

28.3

N/N

0/0

V0

0

24.6

Y/Y

0/>30

NC

17

1(A)

27.3

Y/Y

0/>30

NC

82

17

1(G)

27.6

Y/Y

0/>30

NC

S6

82

17

1(C)

27.9

N/N

0/2

V0

S7

82

17

1(U)

28.7

N/N

0/1

V0

S8

82

16

2(A)

26.8

Y/Y

0/>30

NC

S9

82

16

2(G)

27.6

Y/Y

0/>30

NC

S10

82

16

2(C)

28.0

N/N

0/1

V0

S11

82

16

2(U)

28.6

N/N

0/1

V0

S12

82

15

3(C)

27.2

Y/Y

0/>30

NC

S13

82

15

3(U)

27.5

Y/Y

0/>30

NC

According to the above results, 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, they have few effects on the UL-94 rating suggesting that they have greater effect on LOI than 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 IFR in PP, and some of them are not suitable for PP/IFR. This may relate to their thermal degradation behaviors which will be discussed in the following part. The photos of the samples after LOI and UL-94 tests are presented in Figure 1 and Figure 2. Neat PP produces few char residue after the LOI test, and burned completely after 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



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both IFR and nitrogenous bases after LOI test is larger than those samples without nitrogenous bases. The char residues after LOI tests for S4, S5, S6 and S7 look similar and their LOI values change form 27 % to 29 %. Most of S4 and S5 are burned, while S6 and S7 keep good shapes after the UL-94 test. It is interesting that the intumescent char for S4 and S5 looks like S6 and S7 in LOI tests, but they are not classified in the UL-94 tests. On the one hand, the quantity and quality of char residues are not good enough to act as effective barrier for underlying resin. On the other hand, 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.

Figure 1. Photos of PP composites after LOI test.

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

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


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first ignition (T1) of PP/IFR (S3) is 363℃. When 1 wt % nitrogenous bases are added into PP/IFR system, T1 of S4, S5, S6 and S7 is 45℃, 18℃, 22℃ and 33℃ lower than that of S3, respectively. Moreover, the highest temperature after the second ignition (T2) of S3 is 512℃. T2 of S6 and S7 is 383℃ and 379℃, which is 129℃ and 133℃ lower than that of S3, respectively. As can be seen S4 and S5 have a higher T2 than S6 and S7. While T2 for S4 is higher than S3. The results suggest that the char formed by S4 after the first ignition has better thermal conductivity, so it obtains 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 heat transfer from outside to resin, so the surface temperature of the composites decreases evidently and the flame extinguished quickly after being ignited. Thus, the above samples achieve the UL-94 V0.

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

Thermal degradation behaviors In order to investigate the roles of nitrogenous bases in IFR system, TGA was



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used to explore the reactions between different components. The experimental (Exp.) and calculated (Cal.) TGA curves of the samples under N2 atmosphere are illustrated in Figure 4 and the detailed data are given 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 wt%-50 wt% degradation are marked as T1wt %, T5wt %, T10wt %, T20wt % and T50wt %, respectively. The weight ratio of IFR and nitrogenous base is 17/1. All bases make IFR to degrade at a lower temperature according to Figure 4. The T1wt %, T5wt % and T10wt % for all IFR/bases-Exp. are lower than their Cal. values. For example, the T1wt

%

of

IFR/C-Exp. and IFR/U-Exp. is 180℃ and 159℃, respectively, which is 13℃ and 34℃ lower than that of IFR, respectively. Moreover, the T1wt %, T5wt %, T10wt %, T50wt % for IFR/C-Exp. and IFR/U-Exp. are not only lower than their Cal. values, but also lower than that of IFR/A-Exp., which means strong interactions among them. It is worthy to point out that IFR/G-Exp. also has a lower T5wt %, T10wt %, T50wt % than that of IFR/A-Exp. and IFR/C-Exp. However, the char residue of IFR/G-Exp. at 800℃ 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 temperature. All the results suggest that some reactions exist among bases and IFR. U and C are more suitable for IFR system to improve the flame retardant properties of PP composites. The reactions between IFR and bases in air atmosphere are also investigated by using TGA as shown in Figure 5 and the related data are summarized in Table 3. Firstly, the degradation behaviors for bases and IFR under air are different from that


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100

60 40 20 0

a

60 40 20 0

100 200 300 400 500 600 700 800

100

IFR C IFR/C-Exp. IFR/C-Cal.

60 40 20 0

IFR U IFR/U-Exp. IFR/U-Cal.

80

Weight(%)

80

b 100 200 300 400 500 600 700 800 Temperature(oC)

Temperature(oC)

100

IFR G IFR/G-Exp. IFR/G-Cal.

80

Weight(%)

Weight(%)

100

IFR A IFR/A-Exp. IFR/A-Cal.

80

Weight(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0

c 100 200 300 400 500 600 700 800 Temperature(oC)

d 100 200 300 400 500 600 700 800

o Temperature( C)

Figure 4. TGA curves of IFR and nitrogenous bases under N2 atmosphere. Table 2. Exp. and Cal. data of IFR and nitrogenous bases under N2 atmosphere. Samples

T1wt % （℃）

T5wt % （℃）

T10wt % （℃）

T50wt % （℃）

Residues at 600℃

Residues at 800℃

A

232

280

294

328

0

0

G

184

435

454

499

19.9

2.4

C

264

311

315

434

40.3

29.6

U

219

268

283

318

0

0

IFR

193

230

270

530

33.4

26.6

IFR/A-Exp.

191

216

243

506

32.9

23.0

IFR/G-Exp.

190

213

236

495

28.4

18.4

IFR/C-Exp.

180

213

241

500

29.9

22.8

IFR/U-Exp.

159

201

222

469

27.4

22.1

IFR/A-Cal.

194

233

276

513

29.7

23.7

IFR/G-Cal.

193

232

279

523

32.0

24.0

IFR/C-Cal.

194

233

279

528

34.1

26.9

IFR/U-Cal.

193

232

274

459

29.9

23.9



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Figure 5. TGA curves of IFR and nitrogenous bases under air atmosphere. Table 3. Exp. and Cal. data of IFR and nitrogenous bases under air atmosphere.

T1wt %

T5wt %

T10wt %

T50wt %

Residues

Residues

（℃）

（℃）

（℃）

（℃）

at 600℃

at 800℃

A

233

276

291

325

0

0

G

153

411

441

492

2.6

0

C

212

311

315

427

22.2

0

U

183

245

266

304

0

0

IFR

180

225

261

578

33.5

12.1

IFR/A-Exp.

164

206

236

524

20.0

6.3

IFR/G-Exp.

147

203

228

521

18.9

7.2

IFR/C-Exp.

188

217

246

552

26.9

12.4

IFR/U-Exp.

177

214

273

538

24.7

12.1

IFR/A-Cal.

182

228

268

566

29.8

10.8

IFR/G-Cal.

180

225

261

578

33.5

12.1

IFR/C-Cal.

180

228

271

574

32.3

10.6

IFR/U-Cal.

180

227

262

565

29.4

10.4

Samples


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in N2. The TGA curves shift to a lower temperature when bases are introduced into IFR system. However, T1wt %, T5wt %, T10wt % and T50wt % of both IFR/C-Exp. and IFR/U-Exp. are higher than that 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℃are 12.4%, 12.1%，respectively, which are

higher than that 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 A and G, 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 so the char residues are as much as IFR at 800℃. While A and G make IFR degrade so heavy that char residue is decreased evidently at 800℃. Hence, C and U are more suitable to be used in PP/IFR system. Then the thermal degradation behaviors of PP composites with different nitrogenous bases in air and N2 atmosphere are investigated as shown in Figure 6. The related data are listed in Table 4. Under air atmosphere, the curve for neat PP shows a two-step decomposition, and has few char residues at the temperature range from 600℃ and 800℃. Compared to PP, introducing IFR into PP causes the T1wt % to shift to a lower temperature. Incorporating nitrogenous bases into PP/IFR system, the thermal stability of the samples increases further. S6 and S7 have the same T1wt % of 189 ℃ which is 44 ℃ higher than that of S3. Though T1wt % of S4 and S5 are also 25℃ and 4℃ 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 that of S4 and S5 at early decomposition stage. However, the difference decreases with the temperature increasing. The T50wt % of S6 and S7 are even 6℃ and 9℃ lower than that of S3,



100 80 60 40

S1 S3 S4 S5 S6 S7

100

S1 S3 S4 S5 S6 S7

Weight(%)

Weight(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20

20

a

0

0

b 100 200 300 400 500 600 700 800 Temperature(oC)

100 200 300 400 500 600 700 800

Temperature(oC)

Figure 6. TGA curves of different PP composites (a) air and (b) N2 atmosphere. Table 4. TGA data of different PP composites under air and N2 atmosphere.

Air Samples

T1wt %

T5wt %

T10wt %

T50wt %

（℃）

（℃）

（℃）

（℃）

S1

261

278

288

326

0.7

S3

145

263

272

325

2.5

S4

170

258

271

321

2.2

S5

149

252

263

322

1.2

S6

189

265

273

319

2.8

S7

189

265

273

316

2.8

Residues at 800℃

N2 Samples

T1wt %

T5wt %

T10wt %

T50wt %

（℃）

（℃）

（℃）

（℃）

S1

363

409

425

455

0

S3

142

341

417

469

5.0

S4

235

363

406

458

3.7

S5

178

346

406

458

3.7

S6

222

359

412

457

4.0

S7

230

363

415

457

4.6

Residues at 800℃

respectively. In addition, all PP composites degrade earlier than neat PP at lower temperature, while show better thermal stability at higher temperature (>350℃) due


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to the formation of char. What’s more, the char residues at 800℃ for S6 and S7 are more than that for S3 and S4. The results suggest that C or U promote the degradation of PP composites at early period and strengthen the thermal stability of char system at later stage which helps to protect the matrix from burning. The degradation of PP under N2 atmosphere is one-step degradation. T1wt % of PP is higher than that in air evidently and no char residues are kept at high temperature. The IFR makes T1wt %, T5wt % and T10wt % of PP shift to a lower temperature, but T50wt % increases. Moreover S3 has 5.0 % residues at 800 ℃. Adding nitrogenous bases into PP/IFR, the degradation temperatures from T1wt % to T10wt % for PP composites are improved evidently compared to PP/IFR. For example, the T1wt % for S6 and S7 are 80 and 88℃ higher than that of S3. It is noteworthy that S3 has the most char residues among all samples. This is opposite to that under air indicating that char formation promoted by C or U may depend on the participation of oxygen. Furthermore, the good flame retardancy of S6 and S7 may not only relate to the char amount, 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 of S3 seems compact and the expansion ratio is low which is supported by the photos after LOI tests. Several big 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. Firstly, the char residues of S7 and S11 become rough. Secondly, the char is not dense but cellular as described in the magnified photos. Intumescent and cellular char is a good barrier that can prevent the exchange of heat and gas between outside and inside effectively, and help samples to achieve good flame retardancy. While



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several big holes are observed on the surface of the char layer of S5, this is in accordance with its poor flame retardant behavior in the UL-94 test. Moreover, few large holes are observed in the char for both S7 and S11. It helps to explain why they achieve the UL-94 V0 but S5 fails.

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


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Changing the content of U, the micromorphology of char is also different. S7 containing 1 wt% U does not show 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. Due to the compatibility between bases and PP is not very good, excess U may accumulate in local area resulting in the formation of dendritic structure of char. Besides, excess gas flows may cause more and bigger pores and the char layer will become imperfect. The imperfect char leads to the poor flame retardancy for S13. Therefore, with good intumescent char layers, S7 and S11 obtain better flame retardant performances in LOI and UL-94 tests than S3 and S13. All the above results indicate that right bases and appropriate content induce the formation of intumescent char with proper porosity for PP/IFR composites. It is the special micromorphology of char that contributes to good flame retardant properties. The FT-IR spectra of char residues are presented in Figure 8. The strong absorption at 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 stretching vibration of C=C 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 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 etc26-27. All these peaks can be found in the char of S5 and S7 which indicates that introducing bases in IFR system does not change the char structure. The bases may just regulate the cooperation among acid source,



carbon source and gas source, and promote the formation of micro porous and intumescent char layers which is the main factor for enhancing the flame retardant properties of PP composites.

Transmittance(%)

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c b

16231241 981 1083

a

1623 1242 981 1083

1626 1174

997

4000 3500 3000 2500 2000 1500 1000 500 -1

Wavenumber(cm ) Figure 8. FT-IR spectra of char layers for (a) S3; (b) S5 and (c) S7

Cone calorimetry tests The combustion performance of PP composites was investigated by means of cone calorimeter, which is always used to evaluate the actual fire burning behavior of materials. It can provide the following principle fire parameters: It can provide following principle fire parameters: time to ignition (TTI), the heat release rate (HRR), the peak HRR (PHRR), time to PHRR (tPHRR), the 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. Adding nitrogenous bases into PP/IFR, the PHRR of S5 and S7 decreases further, which are 324 kW/m2 and 293 kW/m2, respectively. In addition, the HRR curves of all PP composites show two peaks, in which the first one is assigned to the development of the intumescent protective char, and the second peak is assigned to


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the degradation of charring layer.

(a)

120

S1 S3 S5 S7

800 600

(b)

100

THR(MJ/m2)

1000

HRR(kW/m2)

400 200

80 60

S1 S3 S5 S7

40 20 0

0 0

100 200 300 400 500 600 700

0

100

200

300

400

500

600

Time(s)

Time(s) 100

S1 S3 S5 S7

80

ML(wt%)

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60 40 20

(c) 0 0

100 200 300 400 500 600 700 800

Time(s) 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 cone calorimetric test TTI

PHRR

THR

tPHRR

FPI

FGR

(s)

(kW/m2)

(MJ/m2)

(s)

(m s2/kW)

(kW/m2 s)

S1

51

888

125

195

0.057

4.55

S3

45

439

111

320

0.103

1.37

S5

46

324

105

368

0.142

0.88

S7

46

293

105

376

0.157

0.78

Samples

PP has a THR 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 105 MJ/m2. The tPHRR for all PP composites is longer than that of PP, and the longest tPHRR is obtained by S7. The results mean that IFR delays the combustion



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effectively, and bases enhance the effect on the tPHRR. Moreover, the FPI which equals TTI/PHRR and FGR which 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 lower burning risk. The 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 9 (c). It is evident that mass of neat PP decreases quickly and nothing is left after combustion. While introducing IFR or IFR/bases into PP matrix, the ML of PP composites decreases slowly during the combustion and char residue increases meanwhile. Though the ML curve of S7 looks like S5, its char residues are more than S5 evidently. These results indicate that U is better than G to promote the formation of intumescent char layer and decrease the heat release, which protects material from burning.

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

Photographs of PP composites after cone calorimetric test are presented in Figure 10. There is few char residues for PP. With addition of 18 % IFR, the char layer of S3 covers the whole substrate. However, some big holes or cracks are also observed at the center of the char layer indicating that it is not good enough to protect the


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underlying resin from burning. After introducing G into PP/IFR, a continuous and compact char layer is formed for S5. But some crevasses and holes are also seen in the surface of the char. When adding U into 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 to achieve good flame retardant performances.

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 improve the LOI values of PP composites evidently. However, only U or C play 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 no less than 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 degrade at a lower temperature and improve the thermal stability and char residues at higher temperature. Most important of all, U and C as gas source promote the formation of intumescent and cellular char which is good barrier for preventing the exchange of heat and gas between 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 into PP/IFR system decreases the PHRR and THR and prolongs the tPHRR suggesting lower burning risk for PP/IFR/U. In conclusion, base can be used as a novel and green gas source of IFR when its thermal and chemical performance is suitable.



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ASSOCIATED CONTENT Supporting information The SEM morphology of the char residues for more samples after the UL-94 test was show in Figure S1.

ACKNOWLEDGEMENTS This work is ﬁnancially supported by the National Natural Science Foundation of China (No. 51473178) and the Program for Ningbo Science and Technology Innovative Team (No.2015B11005).

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For Table of Contents Use Only

Regulating effects of nitrogenous bases on char structure and flame retardancy of polypropylene/intumescent flame retardant composites Zhijing Wang1,2, Yinfeng Liu1, Juan Li2* 1. School of Materials and Science Engineering, Shanghai University, Shanghai 200444, PR China. 2.Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, PR China. *Corresponding author: phone: +86-574-86685256; fax: +86-574-86685186; Email: [email protected]

Nitrogenous base shows good regulating effects on the char morphology and flame retardant efficiency of PP composites.


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Regulating Effects of Nitrogenous Bases on the Char Structure and

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