Hyperbranched Polyurea as Charring Agent for Simultaneously

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A novel hyperbranched polyurea as charring agent for simultaneously improving flame retardancy and mechanical properties of ammonium polyphosphate/polypropylene composites Hong Yan, Zhilei Zhao, Weijuan Ge, Naien Zhang, and Qing Jin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01896 • Publication Date (Web): 09 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

A novel hyperbranched polyurea as charring agent for simultaneously improving flame retardancy and mechanical properties of ammonium polyphosphate/polypropylene composites Hong Yanab*, Zhilei Zhaoab, Weijuan Geab, Naien Zhangab, and Qing Jinab a

Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, P.R. China b

College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, P.R. China

HIGHLIGHTS ·

The hyperbranched polyurea (HBPU) is hydrophobic and thermally stable.

·

HBPU can be used as charring agent and carbonization agents simultaneously.

·

HBPU is synergistic with ammonium polyphosphate (APP).

·

HBPU and APP can remarkably enhance flame retardancy and smoke suppression of PP.

·

HBPU can eliminate negative effect of APP on the tensile strength of PP.

ABSTRACT A novel hyperbranched charring agent containing s-triazine, diphenylmethane and urea groups (HBPU) was synthesized from melamine and 4,4’-diphenylmethane diisocyanate, which is hydrophobic and thermally stable at high temperature (residual weight being 40 % at 800 °C). HBPU and ammonium polyphosphate (APP) are synergistic for the improvement of flame retardancy and smoke suppression of polypropylene (PP). With the addition of 30 % of APP and HBPU (2:1 or 3:1), PP composites could reach V-0 rating at UL-94 test, and their peak heat release rates (PHRR) were reduced by 78.6 % and 81.3 %, respectively. Besides, HBPU also improved the compatibility of APP with the PP matrix with a result of enhanced mechanical properties. In this intumescent flame retardant system, HBPU served as carbonization and blowing agents simultaneously. In addition, the interaction of HBPU and APP as well as their mechanism of flame retardation were deeply discussed. Key words: hyperbranched polyurea; hydrophobicity; charring agent 1. INTRODUCTION

1

ACS Paragon Plus Environment


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PP is widely used in all aspects of our lives owing to its unique structure, easy processing and other excellent performance. However, one of non-ignorable shortcomings of PP is flammable together with dripping 1, becoming a potential threat to the safety of people's life and property and limiting its application in some fields. Therefore, it is significant to develop flame-retarded PP to reduce the fire hazard 2-4. Intumescent flame retardants (IFRs), due to their environmental-friendliness, low toxicity, low smoke production in fire accidents and high yield of expandable char, have a very wide application to polymers, especially to PP

5-9

.

One of conventional IFRs is composed of ammonium polyphosphate (APP) as an acid source, pentaerythritol (PER) as a carbonization or charring agent and melamine (MEL) as a blowing agent. But these small molecular materials have some drawbacks, such as poor compatibility and heterogeneous dispersion in the matrix as well as migration to the surface of the matrix. In order to overcome such issues, researchers increasingly focus on the synthesis of macromolecular charring agents including polyamides

10

, hyperbranched polymers

11-14

, s-triazine-based polymers

13, 15, 16

and so on, which

could greatly improve the flame-retarding efficiency of APP for PP due to their good performance of char formation. Moreover, such macromolecular charring agents present good compatibility with the matrix. Usually, those reported s-triazine-based charring agents are almost synthesized from cyanuric chloride. Nevertheless, melamine (MEL) has many advantages over it in terms of low price, low toxicity, easy storage, and no need to use acid-binding agents during the synthesis. In this paper, melamine was used to react with 4,4’-diphenylmethane diisocyanate (MDI) to synthesize a novel hyperbranched polyurea (HBPU) as a charring agent to improve the flame-retarding efficiency of APP. Composing a lot of s-triazine and benzene rings as well as -NH- and -NH2 groups, HBPU could release incombustible gases and form char residue during the decomposition, acting as charring and blowing agents simultaneously. The combination 2


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of APP and HBPU could endow PP with good flame retardancy and mechanical properties. 2. EXPERIMENTAL 2.1. Materials MEL was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. MDI was bought from Aladdin Reagent Chemical Co., Ltd. N,N-dimethylformamide (DMF) as a solvent and triethylamine (TEA) as a catalyst were supplied by Tianjin Tianli Chemical Reagent Co., Ltd. All these chemicals were used without further purification. The commercial APP was provided by Hangzhou JLS Flame Retardants Chemical Corporation. Powdered PP (045, Ningbo Yongxing Chemical Co., Ltd., China) was used for the preparation of PP composites. 2.2. Synthesis of HBPU MDI (7.44 g, 0.03 mol), completely dissolved in DMF (100 mL), was poured into a 250 mL three-neck flask equipped with a stirrer. TEA (2-3 mL) was added dropwise into flask, followed by MEL (2.5g, 0.02 mol). The reaction was maintained at 80 °C for 8 h. Finally, the reactive system was cooled to room temperature, filtrated and rinsed with DMF and deionized water repeatedly, then dried at 60 °C in a vacuum oven overnight to obtain light yellow powder. The synthesis of HBPU is illustrated in Scheme 1.

NH2 N H 2N

N N

HN

NH

R2

NH

NH

R2

R2

NH

NH

R1

R1

NH

NH R2

NH

NH

NH

R2

R2 NH

R1

NH2

NH TEA

+

DMF

R2

80 ¡ãC 8 h NH O

N

N

D

NH

NH 2

O

NH

R1 NH

R2

NH R2

R1 NH 2

NH

NH

R2

R1

T

NH2

L

O N R 1:

N N

R2 :

O

C

C NH

Scheme 1. The synthesis of HBPU. 3


NH


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2.3. Preparation of flame retarded PP composites All the materials for the composites, including the commercial APP, the prepared HBPU and PP were dried in an oven at 80 °C for 24 h and then mechanically blended by high-speed mixer. PP composites were extruded by a double conical micro-twin-screw extruder (SJZS-10B-2, Wuhan, China). The formulations of different PP composites are presented in Table 1. The operating temperature of the extruder was maintained at 190-200 °C from hopper to die. The rotation speed of the screw was 20 rpm. Subsequently, the various specimens were made by micro-injection-moulding machine (SZS-20, Wuhan, China). 2.4. Characterization and test. Fourier transform infrared (FTIR) spectra were recorded on Nicolet-710 (Nicolet, USA) spectrometer. Nuclear magnetic resonance spectrometry (1H NMR) measurements were acquired on a Bruker AV-III 400 MHz spectrometer using DMSO-d6 as a solvent. Thermogravimetric analysis (TGA) was obtained at a linear heating rate of 10 °C/min by using a STA-409 (Netzsch, Germany) thermo-analyzer under nitrogen atmosphere. The limiting oxygen index (LOI) values were measured on a digital oxygen index meter (ZR-01, Qingdao, China) with sheet dimensions of 80×10×4 mm3 according to ASTMD 2863-10. The UL-94 vertical burning tests were conducted on a JR-SSC-A instrument (Shenyang, China) according to ASTM D3801-10, and the dimension of specimens was 130×13×1.6 mm3. All specimens (100×100×3 mm3) for cone calorimeter test (CCT) were exposed to a Fire Testing Technology apparatus with a truncated cone-shaped radiator under a heat flux of 35 kW/m2 according to ASTM E1354-16a. A field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL, Tokyo, Japan) was used to observe the morphologies of chars of different specimens after CCT as well as the fracture surfaces of PP composites. 4


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The tensile test was performed on universal material test system (RH-10, Yangzhou, China) with Type-V specimens according to ASTM D638-14 at a testing speed of 1 mm·min-1. The Izod pendulum impact resistance was tested in accordance with ASTM D256-10. 3. RESULTS AND DISCUSSION 3.1. Characterization of HBPU FTIR was used to give evidence on the structure of HBPU. Figure 1 shows FTIR spectra of MDI, MEL and HBPU. The characteristic peaks observed in the FTIR spectrum of HBPU at about 1512 cm-1 and 1638 cm-1 are for -NH-CO-NH- and C=O of -NH-CO-NH-, respectively 17. Furthermore, the disappearance of the absorption peak of -NCO at 2278 cm-1 in Fig 1 (a) means that all -NCO groups had reacted with -NH2 attached on the s-triazine ring. Moreover, the peak at 3120 cm-1 is attributed to -NH2 in MEL 18, indicating the existence of some unreacted -NH2 in HBPU.

Figure 1. FTIR spectra of MDI (a), MEL (b) and HBPU (c). The 1H NMR spectrum can further verify the chemical structure of HBPU, as shown in Figure 2. The chemical shifts at 9.38 ppm, 8.54 ppm, 8.42 ppm could be assigned to three types of N-H protons which attach with dendritic

5



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units (s-triazine with no -NH2), linear units (s-triazine with one unsubstituted -NH2) and terminal units (s-triazine with two -NH2), respectively. The multiplets of benzene rings could be assigned at 6.5-7.5 ppm 19. Moreover, the triplets centered at 3.84 ppm are attributed to -CH2 connecting two aromatic ring structures

19

, split peak deriving

from the complicated hyperbranched structures. A signal at 7.60 ppm is due to the protons of -NH2, and a peak at 7.95 ppm results from N-H in the urea groups. The chemical shift at 3.37 ppm arises from the protons of water molecules, due to the absorption of water in the solvent DMSO-d6. The structure of hyperbranched polymers is usually characterized by the degree of branching (DB) according to the Frechet's equation DB=(D+T)/(D+L+T) 20, where D, T and L refer to the number of dendritic, terminal and linear units in the polymer. Generally, the value of DB can be confirmed from the 1H NMR spectrum by comparing the integration of the peaks of different units. The value of DB for HBPU is found to be 0.51, which indicates that this polymer is hyperbranched structure (DB close to zero for linear, 0.5 for hyperbranched and 1.0 for dendrimer). In addition, the inset picture shows that the water contact angle of HBPU is 97 °, indicating the hydrophobicity of HBPU.

Figure 2. 1H NMR spectrum of HBPU (inset: water contact angle).

Figure 3. TG and DTG curves of HBPU.

3.2. Thermal stability of HBPU. Being filler, the thermal stability of HBPU should first meet the processing requirement of polymer. So the

6


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thermogravimetric analysis was done to investigate the pyrolysis of HBPU. Figure 3 illustrates the TG and DTG curves of HBPU. The initial decomposition temperature of HBPU was 250 °C based on 5 % mass loss, which can meet the processing requirement of PP. HBPU decomposed in three steps. The first step started at 250 °C and ended at 400 °C, with the weight loss of 25 wt% due to the evolution of some volatiles just like NH3 in the compound. Secondly, about 10 % mass loss from 400 °C to 500 °C was assumed to further decomposition and volatilization of the product. The third step was between 520 °C and 650 °C possibly because of degradation of aromatic moieties. The char residue at 800 °C was 40 wt%, indicating high thermal stability and high char yield of HBPU. 3.3. Flame retardancy of PP/IFRs composites The effect of HBPU on the flame-retarding efficiency of APP was evaluated by LOI, vertical burning tests (UL-94) and cone calorimeter test (CCT). The results of LOI and UL-94 tests of various specimens are listed in Table 1. PP had the lowest LOI value of 18.0 vol% and failed at UL-94 test. The only addition of APP increased the LOI value of PP1 to 23.0 vol%, but still failed at UL-94 test. Among PP/APP/HBPU systems, PP4 and PP5 both reached V-0 rating and P4 had the highest LOI value of 31.5 vol%. Table 1. Results of LOI and UL-94 tests of different specimens. PP

APP

HBPU

LOI

(wt %)

(wt %)

(wt %)

(vol %)

PP

100

0

0

PP1

70

30

PP2

70

PP3

Samples

UL-94

Dripping

18.0

NR

Yes

0

23.0

NR

Yes

0

30

23.4

NR

Yes

70

15

15

23.2

NR

Yes

PP4

70

20

10

31.5

V-0

No

PP5

70

22.5

7.5

29.5

V-0

No

PP6

70

18

12

26.5

V-2

Yes

PP7

70

12

18

22.5

V-2

Yes

PP8

70

7.5

22.5

23.4

NR

Yes

PP9

70

10

20

23.3

NR

Yes

7



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In order to further investigate the heat release of composites, PP, PP1, PP2, PP4 and PP5 were selected to be quantitatively evaluated by cone calorimeter. Cone calorimeter is an effective medium experimental device and the experiment environment is very similar with the real fire. The heat release rate (HRR) and the total heat release (THR) and the mass loss (ML) curves of selected specimens are displayed in Figure 4. The peak of heat release rate (PHRR) of virgin PP was 1193 kW/m2, while that of PP1 and PP2 were reduced to 321 kW/m2 and 419 kW/m2. PP4 and PP5 showed much lower PHRR values of 255 kW/m2 and 223 kW/m2, with reductions of 78.6 % and 81.3 %, respectively. Furthermore, total heat release (THR) is usually used to evaluate the safety of the material in a real fire. It can be seen that the addition of APP or HBPU could reduce THR of PP from 126 MJ/m2 to 91 MJ/m2 or 119 MJ/m2. PP5 had the lowest THR value of 66 MJ/m2, 47.6 % lower than that of virgin PP. The lower the PHRR and THR, the slower the thermal decomposition and flame propagation. In addition, it can be seen from ML curves that respective residues of virgin PP, PP1, PP2, PP4 and PP5 were 2.1, 22.3, 9.2, 26.5 and 42.4 %, indicating the highest char yield when APP/HBPU at 3:1. Although PP4 had the highest LOI value, its THR was much greater than that of PP5. Thus the optimum ratio of APP/HBPU should be 3:1, showing the best flame retardant performance at this ratio.

Figure 4. HRR (a), THR (b) and ML (c) curves of different specimens. Moreover, other detailed parameters, such as time to ignition (TTI), time to PHRR (tPHRR), fire propagation index (FPI), and fire growth rate index (FIGRA) are listed in Table 2. These parameters can simulate the real fire 8


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conditions. The TTI of PP composites were shorter than that of virgin PP, showing the earlier decomposition of additives to form char layer and to protect PP from further burning. The FPI is a ratio of TTI/PHRR, which is related to the effective time to escape from a large range of fire. FPI is considered to be the most clear individual indicator of fire hazards in a large scale. As we all know, the greater the FPI value, the smaller the harm 21. PP5 has the greatest FPI (0.152), suggesting the strongest resistance to fire growth. In addition, FIGRA is defined as the fire growth rate index, which is calculated as the maximum value of the function (heat release rate)/(elapsed test time)

22, 23

. FIGRA

can suggest the spread speed and scope of the flame. It is obvious that PP5 had the slowest fire spread speed. Table 2. Some detailed cone calorimetric data and results of mechanical test. Samples

TTI (s)

PHRR (kW/m2)

tPHRR (s)

FIGRA (kW/m2·s)

FPI (s·m2/kW)

Tensile strength Impact strength (MPa) (kJ/m2)

PP

58

1192

145

6.16

0.049

35.81±1.27

2.92±0.15

PP1

37

320

165

2.35

0.116

26.83±2.35

3.26±0.23

PP2

38

419

309

4.04

0.091

35.50±1.80

3.48±0.17

PP4

35

255

248

2.23

0.142

35.66±2.15

3.13±0.21

PP5

35

223

179

1.59

0.152

34.19±0.92

3.25±0.11

Figure 5. SPR (a) and TSR (b) curves of different specimens. 3.4. Smoke suppression For the flame retardant materials, the release amount of smoke is also a very important parameter, because most of the people who died in the fire are due to the smoke-caused coma. So it is significant to reduce the amount of smoke

9



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released from the combustion of the polymer. Smoke production rate (SPR) and total smoke release (TSR) curves are illustrated in Figure 5. It can be seen that they are similar to HRR and THR curves, and the values of SPR peak and TSP at the end of burning for PP5 were decreased remarkably in comparison with PP. 3.5. Effect of char structure on flame retardancy It is well known that intumescent flame retardant system usually experience a strong expansion process to form a carbonaceous protective layer. PP5 showed better flame retardancy than PP4, which must be related with the structure of char formed. Therefore, the study of intumescent char layer is helpful to understand the differences of flame retardancy of different specimens. Figure 6 shows digital photos of PP, PP1, PP2, PP4 and PP5 residues after the cone calorimeter tests. Virgin PP almost had no residue after burning. PP1 had unbroken residue without obvious expansion, while the residue of PP2 was broken to a few parts with a result that the underlying material continued burning to generate more heat and to lose more weight, which agrees well with the results of HRR, THR and ML. But for PP4 (APP/HBPU=2:1) and PP5 (APP/HBPU=3:1), much more well-expanded residues were left, especially for PP5. The well expanded char layer would be an excellent barrier for heat and mass transfer, achieving high efficient flame retardation. From the above discussion, it is clear that HBPU acted as blowing and carbonization agents simultaneously. But the high amount of HBPU in the composites is not good for the flame retardancy. Consequently, the structure of char layer must be the key to the flame retardancy.

10


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Figure 6. Top and side views of residues of PP (a, a1), PP1 (b, b1), PP2 (c, c1), PP4 (d, d1), and PP5 (e, e1) after CCT. FESEM was adopted to observe the external and internal micro-morphology of the char layers of PP1, PP2, PP4 and PP5 after cone calorimeter tests. As shown in Figure 7, the external char surface of PP1 was continuous and dense, while its internal char surface was porous and rough. But for PP2, both the external and internal char layers were composed of spherical particles in different sizes, seemly small spheres gradually growing up to big ones during burning. More tiny spheres can be seen from Figure 7b, indicating that external surface of PP2 degraded faster than internal part to form more carbonaceous nuclei due to the exposure to flame. Compared with the external residue, the internal nuclei likely had comparatively long time to merge and grow up. Actually, it is impossible for this kind of loose char layer to shield the underlying material from burning. This is why PP2 had higher PHRR, THR and ML than PP1 though HBPU had high thermal stability. With the combination of APP and HBPU, for PP4 and PP5, the gas products from HBPU decomposition contributed to the formation of intumesent char, but more HBPU (APP:HBPU=2:1, PP4) would destroy the continuity and the strength of char layer due to spherical particles (Figure 7c) so that gas products partly escaped. For PP5 (APP:HBPU=3:1), the external and internal char surfaces were smooth, continuous and compact, effectively cutting off oxygen transfer and restricting gas products beneath it to form well expanded char. In addition, from Figure 7a1, 7c1 and 7d1, it is obvious that the char of PP4 and PP5 had superior strength and expansibility than that of PP1. It follows that HBPU together with APP contributed to the strong and highly intumescent char.

11



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Figure 7. FESEM images of external and internal surface of char layers of PP1 (a, a1), PP2 (b, b1), PP4 (c, c1) and PP5 (d, d1). 3.6 . Flame retardation and char formation mechanism From the above discussion, it can be concluded that APP and HBPU are strongly synergistic. The FTIR spectrum of char residue of PP5 (Figure 8) could further prove the interaction of APP and HBPU. The peaks in range of 3000 cm-1 to 3500cm-1 are attributed to stretching vibration of -NH- and -OH 24. The absorption at 2357 cm-1 is assigned to the stretching mode of P-H

25

. The peak at 1720 cm-1 is the stretching vibration of C=O. The peak at 1642 cm-1 is

assigned to the stretching mode of aromatic compound

25, 26

. Bands appear around 1002 cm-1 and 877 cm-1 are

attributed to P-O bond in P-O-C structure 12. Moreover, the band at 1401 cm-1 is attributed to P-N structure 26. The existence of P-O-C and P-N suggest the cross-linking reaction occurred between APP and HBPU. That could be the reason that the spherical char turned to laminar char layer. Scheme 2 could help us to better understand the flame-retarding mechanism. APP decomposed to form poly-phosphoric acid (PPA) as a surface coating to cut off air while releasing NH3 after being heated to a certain temperature 27. The dehydration promoted the cross link of PPA. The tiny spherical particles decomposed from HBPU interacted with cross-linked PPA to form cross-linked char, resulting in laminar char layers. With the increase of APP (APP:HBPU=3:1), the laminar char layers turned to be 12


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continuous, smooth and compact. Covered by these laminar layers, the decomposed incombustible gases from HBPU were difficult to escape so that the intumescent char residue formed. The formation of the phosphorus and carbon-rich cross-linked char contributed to the improvement of flame retardancy and smoke suppression and to the resistance of dripping 27.

Figure 8. FTIR spectrum of the char residue of PP5.

Scheme 2. Synergistic char-forming mechanism of HBPU and APP. 3.7. Mechanical properties of PP composites To improve the flame retardancy of PP should not be the only goal for us, it will be better to keep high mechanical properties of PP composite after the addition of flame retardant fillers. The tensile and impact strength of different specimens are also listed in Table 2. Compared with PP, the loading of APP decreased the tensile strength of PP1 to a great extent, but the extra addition of HBPU could eliminate negative effect of APP on the tensile strength of PP 13



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composites. As for the impact strength, all composites have higher values than PP. It is well known that the mechanical properties of a composite strongly depend on the compatibility of filler with the matrix. In order to investigate the compatibility of fillers with the PP matrix, the fracture surfaces of different specimens were observed via FESEM, as shown in Figure 9. The specimens were frozen in liquid nitrogen for 2-4 h and then broken off. Figure 9a clearly shows that APP was incompatible to PP due to its hydrophilicity, causing the lowest tensile strength of PP1. But HBPU is of high compatibility with PP (Figure 9b) because of its hydrophobicity. For PP5, no gaps between fillers and PP were observed from Figure 9c, indicating that HBPU improved the compatibility of APP with PP. HBPU particles are much smaller than APP ones (Insets in Figure 9a and 9b), which were easily to surround APP particles owing to the formation of hydrogen bonds, as shown in Scheme 3. Consequently, the negative effect of APP on the tensile strength was eliminated. PP4 had higher tensile strength than PP5 owing to higher loading of HBPU.

Figure 9. FESEM images of fracture surfaces of PP1(a), PP2 (b) and PP5 (c) Inset: APP particles (a) and HBPU particles (b). Furthermore, all PP composites have enhanced impact strength. It is known that when an external force is applied to a material, stress will be produced. For the inorganic filler/polymer (APP/PP) composites, these inorganic fillers causes the stress concentration, leading to the occurrence of many micro-cracks in the surrounding resins, and thus absorbing deformation work

28

. As for hyperbranched polymer/polymer (HBPU/PP) composites, hyperbranched

chains forming three-dimensional network and unoccupied spaces between molecules because of the steric effects 14


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associated with the large volume of the hyperbranched structure prevent the fracture of PP composite and absorb more energy 29-31. Thus all the composites have high toughness.

Scheme 3. The interaction between HBPU and APP. 4. CONCLUSION In summary, a novel hyperbranched polyurea (HBPU) containing s-triazine and benzene rings was synthesized， which can be used as charring and blowing agents simultaneously. HBPU exhibits a good performance of char formation and a high thermostability. HBPU and APP are strongly synergistic and the optimum ratio is 1:3. The IFR system containing HBPU and APP performed a good expansibility and high yield of char under a heat. Their interaction achieved a strong and compact char layer, endowing PP with good flame retardancy and smoke suppression. More importantly, HBPU could improve the compatibility of APP with PP matrix, thus the flame-retarded PP composites could remain almost the same tensile strength as virgin PP and have enhanced impact strength. Overall, the studied flame retarded PP composites imply a promising application to furniture, electric casings, cars, electronics, interior decorations and so on. AUTHOR INFORMATION Corresponding Author ∗

Tel.: +86 351 6018942; fax: +86 351 6010311. E-mail address: [email protected].

Notes 15



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The authors declare no competing financial interest. ACKNOWLEDGMENT The work was financially supported by the National Natural Science Foundation of China (No. 21306124), Natural Science Foundation of Shanxi Province (No 2015011035) and State Foundation for Studying Abroad from China Scholarship Council (201506935022). REFERENCES

(1) Song, P.; Fang, Z.; Tong, L.; Jin, Y.; Lu, F., Effects of metal chelates on a novel oligomeric intumescent flame retardant system for polypropylene. J. Anal. Appl. Pyrol. 2008, 82, (2), 286-291. (2) Feng, C.; Zhang, Y.; Liang, D.; Liu, S.; Chi, Z.; Xu, J., Influence of zinc borate on the flame retardancy and thermal stability of intumescent flame retardant polypropylene composites. J. Anal. Appl. Pyrol. 2015, 115, 224-232. (3) Liu, J.C.; Xu, M.J.; Lai, T.; Li, B., Effect of surface-modified ammonium polyphosphate with kh550 and silicon resin on the flame retardancy, water resistance, mechanical and thermal properties of intumescent flame retardant polypropylene. Ind. Eng. Chem. Res. 2015, 54, (40), 9733-9741. (4) Qin, Z.; Li, D.; Zhang, W.; Yang, R., Surface modification of ammonium polyphosphate with vinyltrimethoxysilane: Preparation, characterization, and its flame retardancy in polypropylene. Polym. Degrad. Stab. 2015, 119, 139-150. (5) Shao, Z. B.; Deng, C.; Tan, Y.; Chen, M. J.; Chen, L.; Wang, Y. Z., An efficient mono-component polymeric intumescent flame retardant for polypropylene: preparation and application. ACS Appl. Mater. Inter. 2014, 6, (10), 7363-70. (6) Shao, Z.B.; Deng, C.; Tan, Y.; Chen, M.J.; Chen, L.; Wang, Y.Z., Flame retardation of polypropylene via a novel intumescent flame retardant: Ethylenediamine-modified ammonium polyphosphate. Polym. Degrad. Stab. 2014, 106, 88-96. (7) Qian, X. D.; Song, L.; Bihe, Y.; Yu, B.; Shi, Y. Q.; Hu, Y.; Yuen, R. K. K., Organic/inorganic flame retardants containing phosphorus, nitrogen and silicon: Preparation and their performance on the flame retardancy of epoxy resins as a novel intumescent flame retardant system. Mater. Chem. Phys. 2014, 143, (3), 1243-1252. (8) Shao, Z. B.; Deng, C.; Tan, Y.; Chen, M. J.; Chen, L.; Wang, Y. Z., An Efficient Mono-Component Polymeric Intumescent Flame Retardant for Polypropylene: Preparation and Application. ACS Appl. Mater. Inter. 2014, 6, (10), 7363-7370. (9) Yang, J. C.; Cao, Z. J.; Wang, Y. Z.; Schiraldi, D. A., Ammonium polyphosphate-based nanocoating for melamine foam towards high flame retardancy and anti-shrinkage in fire. Polym. 2015, 66, 86-93. (10) Yang, B.; Liu, H.; He, B.; Leng, J.; Chen, X., Effect of novel copolyamide charring agents on flame-retarded polypropylene. Polym. Compos. 2013, 34, (5), 634-640. (11) Ke, C. H.; Li, J.; Fang, K. Y.; Zhu, Q. L.; Zhu, J.; Yan, Q.; Wang, Y. Z., Synergistic effect between a novel hyperbranched charring agent and ammonium polyphosphate on the flame retardant and anti-dripping properties of polylactide. Polym. Degrad. Stab. 2010, 95, 763-770. (12) Li, J.; Ke, C. H.; Xu, L.; Wang, Y. Z., Synergistic effect between a hyperbranched charring agent and ammonium polyphosphate on the intumescent flame retardance of acrylonitrile-butadiene-styrene polymer. Polym. Degrad. Stab. 2012, 97, (7), 1107-1113. 16


Page 17 of 19

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(13) Wen, P. Y.; Wang, X. F.; Wang, B. B.; Yuan, B. H.; Zhou, K. Q.; Song, L.; Hu, Y.; Yuen, R. K. K., One-pot synthesis of a novel s-s-triazine-based hyperbranched charring foaming agent and its enhancement on flame retardancy and water resistance of polypropylene. Polym. Degrad. Stab. 2014, 110, 165-174. (14) Huang, Z. G.; Shi, W. F., Thermal degradation behavior of hyperbranched polyphosphate acrylate/tri(acryloyloxyethyl) phosphate as an intumescent flame retardant system. Polym. Degrad. Stab. 2007, 92, (7), 1193-1198. (15) Su, X.; Yi, Y.; Tao, J.; Qi, H.; Li, D., Synergistic effect between a novel s-triazine charring agent and ammonium polyphosphate on flame retardancy and thermal behavior of polypropylene. Polym. Degrad. Stab. 2014, 105, 12-20. (16) Feng, C.; Zhang, Y.; Liu, S.; Chi, Z.; Xu, J., Synthesis of novel s-triazine charring agent and its effect in intumescent flame-retardant polypropylene. J. Appl. Polym. Sci. 2012, 123, (6), 3208-3216. (17) Mariappan, T.; Zhou, Y.; Hao, J.; Wilkie, C. A., Influence of oxidation state of phosphorus on the thermal and flammability of polyurea and epoxy resin. Eur. Polym. J. 2013, 49, (10), 3171-3180. (18) Zheng, Z.; Sun, H.; Li, W.; Zhong, S.; Yan, J.; Cui, X.; Wang, H., Co-microencapsulation of ammonium polyphosphate and aluminum hydroxide in halogen-free and intumescent flame retarding polypropylene. Polym. Compos. 2014, 35, (4), 715-729. (19) Kannan, A. G.; Choudhury, N. R.; Dutta, N. K., Novel dendritic-poly(urethane-urea) hybrid thin films from hydrogen bond rich dendrons. Polym. 2014, 55, (20), 5132-5139. (20) Hawker C. J., R. Lee, J. M. J. Frechet, One-step synthesis of hyperbranched dendritic polyesters. J. Am. Chem. Soc. 1991, 113, (12), 4583-4588. (21) Feng, J.; Hao, J.; Du, J.; Yang, R., Using TGA/FTIR TGA/MS and cone calorimetry to understand thermal degradation and flame retardancy mechanism of polycarbonate filled with solid bisphenol A bis(diphenyl phosphate) and montmorillonite. Polym. Degrad. Stab. 2012, 97, (4), 605-614. (22) Ye, X.; Wang, Y.; Zhao, Z.; Yan, H., A novel hyperbranched poly(phosphorodiamidate) with high expansion degree and carbonization efficiency used for improving flame retardancy of APP/PP composites. Polym. Degrad. Stab. 2017, 142, 29-41. (23) B. Sundström, The development of a european fire classification system for building products: Test methods and mathematical modelling, Department of Fire Safety Engineering, Lund University, Sweden, 2007. (24) Nie, S.; Hu, Y.; Song, L.; He, Q.; Yang, D.; Chen, H., Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym. Advan. Technol. 2008, 19, (8), 1077-1083. (25) Huang, J.; Zhang, Y.; Yang, Q.; Liao, X.; Li, G., Synthesis and characterization of a novel charring agent and its application in intumescent flame retardant polypropylene system. J. Appl. Polym. Sci. 2012, 123, (3), 1636-1644. (26) Mahapatra, S. S.; Karak, N., s-S-triazine containing flame retardant hyperbranched polyamines: Synthesis, characterization and properties evaluation. Polym. Degrad. Stab. 2007, 92, (6), 947-955. (27) Deng, C. L.; Deng, C.; Zhao, J.; Fang, W. H.; Lin, L.; Wang, Y. Z., Water resistance, thermal stability, and flame retardation of polypropylene composites containing a novel ammonium polyphosphate microencapsulated by UV-curable epoxy acrylate resin. Polym. Adv. Technol. 2014, 25, 861-871. (28) Zhang, Z. Y.; Yuan, L.; Liang, G. Z.; Gu, A. J.; Qiang, Z. X.; Yang, C. W.; Chen, X. X., Unique hybridized carbon nanotubes and their high performance flame retarding composites with high smoke suppression, good toughness and low curing temperature. J. Mater. Chem. A 2014, 2, (14), 4975-4988.

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(29) Wu, L.; Lu, S.; Pan, L.; Luo, Q.; Yang, J.; Hou, L.; Li, Y.; Yu, J., Enhanced thermal and mechanical properties of polypropylene composites with hyperbranched polyester grafted sisal microcrystalline. Fiber. Polym. 2017, 17, (12), 2153-2161. (30) Santiago, D.; Fabregat-Sanjuan, A.; Ferrando, F.; De la Flor, S., Hyperbranched-modified epoxy thermosets: Enhancement of thermomechanical and shape-memory performances. J. Appl. Polym. Sci. 2017, 134, (12). (31) Yang, J.P.; Chen, Z.K.; Yang, G.; Fu, S.Y.; Ye, L., Simultaneous improvements in the cryogenic tensile strength, ductility and impact strength of epoxy resins by a hyperbranched polymer. Polym. 2008, 49, (13-14), 3168-3175.

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A novel hyperbranched polyurea as charring agent for simultaneously improving flame retardancy and mechanical properties of ammonium polyphosphate/polypropylene composites

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Hyperbranched Polyurea as Charring Agent for Simultaneously

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