Synthesis of a Novel Triazine-Based Hyperbranched Char Foaming

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Synthesis of a Novel Triazine-Based Hyperbranched Char Foaming Agent and the Study of Its Enhancement on Flame Retardancy and Thermal Stability of Polypropylene Panyue Wen,†,‡ Xiaofeng Wang,† Weiyi Xing,*,† Xiaming Feng,†,‡ Bin Yu,†,‡ Yongqian Shi,†,‡ Gang Tang,† Lei Song,† Yuan Hu,*,†,‡ and Richard K. K. Yuen‡,§ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, China § Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue Kowloon, Hong Kong ‡

ABSTRACT: A novel hyperbranched char foaming agent (HCFA), successfully synthesized via the polycondensation of 2chloro-4,6-di-(2-hydroxyethylamino)-s-triazine (CHT), was combined with ammonium polyphosphate (APP) to endow polypropylene (PP) with flame retardance. The study of thermal stability of various PP composites showed that HCFA/APP system could effectively improve the thermal degradation and thermal-oxidative stability of the char residues, and PP3 containing 30 wt % APP/HCFA with a 3/1 weight ratio left the highest amount of char residue at 700 °C. The results of flammability revealed that PP3 had the most effective flame retardancy and lowest fire hazards. The investigation of structure and morphology of char residues indicated that the compact and foaming char layer, as a good barrier against the transmission of heat and volatiles, was formed for PP3 during combustion.

1. INTRODUCTION PP has an extensive application in textiles, cables, and housing due to its outstanding superior processability and mechanical properties. However, the high flammability and melt dripping of PP during combustion severely limits its applications.1 Therefore, it is of significance to develop the PP systems to reduce its fire hazards. Among various means to improve the flame retardancy of PP, the intumescent flame retardants (IFRs) are of great interest and importance due to their environmental-friendliness and high char.2−6 In the last decades, the ammonium polyphosphate/pentaerythritol/melamine (APP/PER/MA) system, a typical IFR, has been widely used. However, small-molecule MA and PER, having poor compatibility with the polymer matrix, easily migrate to the surface of the samples,7,8 thus resulting in a worsening of the flame retardancy. To surmount these deficiencies, many works have been turned to the seeking out the new char foaming agents. Among them, triazine-based char foaming agents receive increasing attention owing to their effective charring capability.9−20 Owing to the outstanding properties such as a lower melt viscosity and large number of end groups21,22 compared with their linear counterparts, hyperbranched polymers have received considerable attention to apply for IFRs.23−29 The previous works demonstrated that triazines and their derivatives are good char foaming agents due to the integration of carbon sources and foaming agents and their overcoming of the weaknesses of conventional small molecules such as easy physical loss through migration to the surrounding area.23,27,28 In general, hyperbranched polymers are prepared by polycondensation of an ABx type monomer that has one “A” functional group and x “B” functional groups.21,22 The key © 2013 American Chemical Society

point of such approach is no gelation even at high concentrations and conversions, which effectively gets rid of the shortcomings of the polymerization of A2 + B(x+1) systems. AB2, the simplest ABx monomer, is usually employed to prepare the hyperbranched polymers. In the current work, a novel HCFA was synthesized using the polycondensation of AB2 type monomer, and its structure was characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance (1H NMR), and elemental analysis. Then, it was combined with APP to prepare intumescent flame retardant PP compounds. The thermal and combustion behaviors of various PP compounds were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), UL-94 test, and cone calorimetry. The structure and morphology of char residues were studied by FTIR and scanning electronmicroscopy (SEM). In addition, the effect of HCFA on the flow properties of PP compounds were also presented.

2. EXPERIMENTAL SECTION 2.1. Materials. PP resin (F401) was purchased by Yangzi Petrochemical Co. (China). The commercial product APP (phase II, the degree of polymerization >1000) was obtained from Shandong Shi’an chemical Engineering Corp. Cyanuric chloride and sodium hydride (NaH) were brought from Aladdin Chemical Reagent Corp. All the other chemicals were obtained from China National Pharmaceutical Group (ShangReceived: Revised: Accepted: Published: 17015

June 21, 2013 November 4, 2013 November 6, 2013 November 6, 2013 dx.doi.org/10.1021/ie401955n | Ind. Eng. Chem. Res. 2013, 52, 17015−17022

Industrial & Engineering Chemistry Research

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(Jiangning Analysis Instrument Co., China). The samples were of dimensions 100 × 6.5 × 3 mm3. UL-94 vertical burning tests were conducted on a CFZ-2 type instrument (Jiangning Analysis Instrument Company, China) according to ASTM D3801-1996 standard. The dimensions is 130 × 13 × 3 mm3. All specimens (100 × 100 × 3 mm3) for cone calorimeter test were exposed to a Stanton Redcroft cone calorimeter under a heat flux of 35 kW/m2 according to ISO-5660 standard procedures. Melt flow index (MFI) measurements was evaluated by a SRZ-400C melt flow instrument (Tianjin, China) using a load of 2.160 kg at 230 °C according to GB3682-2000. ESI-MS experiment was performed on Agilent 6460 Triple Quadruple mass spectrometer equipped with an ESI source, and exact masses were measured using a Thermo Scientific LTQ Orbitrap Mass Spectrometer equipped with an electrospray interface. Elemental analyses were investigated by using the Vario EL III elemental analyzer.

hai, China). 1, 4-Dioxane was dried with sodium and then distilled. 2.2. Synthesis of 2-Chloro-4, 6-di-(6-hydroxyethylamino)-s-triazine (CHT). To a solution of cyanuric chloride (92.25 g, 0.5 mol) in 300 mL acetone was added dropwise ethanolamine (64.05 g, 1.05 mol) in 200 mL water at 0 °C. After stirring for 3 h, the temperature was gradually elevated to 45 °C and an aqueous solution of NaOH (42.00 g, 1.05 mol) was added dropwise to the above reaction mixture. Thereafter, the reaction was kept at 0 °C for 10 h. The mixture was then filtered, washed with water and acetone, and dried at room temperature in a vacuum oven overnight to afford a white solid powder (98.65 g; yield, 84.5%). 1 H NMR (d6-DMSO): δ = 7.78−7.37 (N-H); 4.75−4.54 (OH); 3.67−3.38 (NHCH2); 3.38−3.12 (CH2OH). 2.3. Synthesis of HCFA. To a solution of CHT (93.4 g, 0.40 mol) in dry 1,4-dioxane (300 mL) was added dropwise NaH (20.4 g, 0.85 mol). After stirring at 100 °C for 12 h, deionized water was added to react the excess NaH, and the reaction mixture cooled to room temperature was filtered and washed with ice water and acetone and dried at room temperature in a vacuum oven overnight to obtain a white solid powder (62.49 g; yield, 79.3%). 1 H NMR (d6-DMSO): δ = 7.53−6.84 (N-H); 4.75−4.54 (OH); 4.40−4.07 (CH2O-triazine); 3.71−3.38 (NHCH2); 3.38− 3.06 (CH2OH) 2.4. Preparation of Flame Retarded PP Compounds. The samples were prepared on a two-roll mixing mill (Rheomixer XSS-300, Shanghai Ke Chuang China) with the roll speed of 100 rpm at 180 °C. APP/HCFA (30 wt %) with different ratios were added into PP matrix and the formulations of prepared samples are presented in Table 1. HCFA and APP

3. RESULTS AND DISCUSSION 3.1. Characterization of Intermediate CHT. The intermediate CHT is prepared according to the previous literature.16,30 The structure of CHT is clarified by 1H NMR (Figure 1a), FTIR (Figure 2b), elemental analysis (Table 2),

Table 1. Formulations of Various PP Compoundsa sample

PP (wt %)

APP (wt %)

PP0 PP1 PP2 PP3 PP4 PP5

100 70 70 70 70 70

30 20 22.5 24

a

HCFA (wt %)

LOI (%)

UL-94 rating

MFI (g/10 min)

10 7.5 6 30

17.0 22.0 32.5 33.0 31.5 23.5

NR* NR V-0 V-0 V-0 NR

3.3 6.6 19.1 17.3 15.8 24.6

Figure 1. 1H NMR spectra of CHT (a) and HCFA (b) in d6-DMSO.

No rating.

with the desired amount were added into melting PP for about 10 min. After mixing, the resulting samples were hot-pressed under 10 MPa at 180 °C for 10 min into sheets with a thickness of 3.0 ± 0.1 mm. 2.5. Measurements. Fourier transform infrared (FTIR) spectra were obtained by Nicolet 6700 spectrometer (Nicolet Instrument Company, U.S.A.). 1 H NMR measurements were recorded by an Avance 300 Bruker spectrometer using DMSO-d6 as a solvent. Thermogravimetric analysis (TGA) was obtained from room temperature to 700 °C at a linear heating rate of 20 °C/min by using a Q5000IR (TA Instruments) thermo-analyzer instrument under air and nitrogen atmospheres. Scanning electron microscopy (SEM) images were obtained with a scanning electron microscope AMRAY1000B at an accelerating voltage of 20 kV. Limiting oxygen index (LOI) was measured according to ASTM D2863 by utilizing the HC-2 oxygen index meter

Figure 2. FTIR spectra of cyanuric chloride (a) and CHT (b). 17016

dx.doi.org/10.1021/ie401955n | Ind. Eng. Chem. Res. 2013, 52, 17015−17022


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Table 2. Elementary Analysis Data of CHT and HCFA sample CHT HCFA

calculated found calculated found

C (%)

H (%)

N (%)

35.36 36.08 42.60 42.09

5.28 5.14 5.58 6.00

29.40 29.96 35.53 35.96

Scheme 1. Route for the Synthesis of HCFA via Polycondensation of AB2 Monomer CHT under the Catalysis of NaH

and ESI-MS (Figure 3). As shown in Figure 1a, there are clear assignments for all characteristic peaks and it is worth noting that the presence of the peak a and e at around 7.78−7.37 ppm (N-H) is possibly caused by the unequivalence of their respective chemical circumstance. In other words, the two nitrogen atoms are not planar. FTIR (Figure 2b) was used to provide further evidence to confirm its structure. It is obvious that the presence of several functional groups such as C−O (1060 cm−1), C−N (1160 cm−1), C−H (2850−2980 cm−1), and N−H (3050−3450 cm−1) confirm the successful linkage between cyanuric chloride and ethanolamine. However, the absorption peak of C−Cl at 850 cm−1 disappears completely, which contradicts with the feed ratio and reaction condition. The reasonable explanation may be that electron-donating nitrogen atoms in CHT make the absorption peak of the last C−Cl deviate from 850 cm−1. Table 2 exhibits the results of the elemental analysis of CHT. It is easy to display that the calculated data for CHT are fairly close to the experimental ones, further indicating its successful preparation. Besides, ESI-MS results of CHT (m/z: calcd. for [M + Na]+, 256.649; found, 256.057) in Figure 3 are well consistent with its chemical structure. All these results indicate that CHT is successful synthesized. 3.2. Characterization of HCFA. HCFA is synthesized via polycondensation of AB2 monomer CHT (A stands for chlorine atom and B represents hydroxyl group) under the catalysis of NaH, as shown in Scheme 1. Figure 1b presents 1H NMR spectrum of HCFA with the peak assignments. A new

peak around 4.40−4.07 ppm, which is ascribed to the proton of methylene (CH2O-triazine) provides the direct and strong evidence on the occurrence of self-condensation of AB2 monomer CHT via the reaction between triazine-Cl and hydroxyl. The integral ratio of peak f to peak b is calculated to be about 1/2, close to the theoretical result, indicating that the transformation degree of chlorine atom is nearly 100%. The elemental analysis results of HCFA are shown in Table 2. The theoretical content of C, H, and N is 42.60%, 5.58%, and 35.53%, respectively, while the found content of C, H, and N is 42.09%, 6.00%, and 35.96%, indirectly confirming the nearly full substitute of chlorine atom by hydroxyl group, which is in agreement with the results of its NMR spectrum. The successful reaction of chlorine atom with hydroxyl moieties proves the successful synthesis of HCFA to some extent. However, there are some structural defects for hyperbranched polymer, so it is difficult to clearly characterize the structure of the HCFA. The TGA curve of HCFA in nitrogen atmosphere is shown in Figure 4. HCFA presents a three-step decomposition in the

Figure 3. Mass spectrum of CHT. 17017



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thermal stability of the samples is evaluated by Td and the char yield at 700 °C, which are listed in Table 3. Table 3. TGA Data of Various PP Samples in Nitrogen and Air Atmosphere nitrogen sample T5%(°C) PP0 PP1 PP2 PP3 PP4 PP5

Figure 4. TG curve of HCFA under nitrogen atmosphere.

temperature ranges of 80−150 °C, 220−360 °C, and 360−700 °C, corresponding to the loss of water and small molecules such as NH3, the further carbonization and the thermal degradation of char residue, respectively. Besides, the char residue of HCFA at 500, 600, and 700 °C is 30.1%, 25.1%, and 22.1%, respectively, implying the high charring ability. 3.3. Thermal Stability of Various PP Compounds in Nitrogen. Thermal degradation behavior of different PP samples in nitrogen atmosphere is investigated by TGA, as shown in Figure 5. The onset decomposition temperature (Td) is defined as the temperature of 5% weight loss. The relative

385 394 356 369 363 296

air

char residue at 700 °C (%)

T5%(°C)

char residue at 700 °C (%)

1.8 20.2 16.8 21.0 15.4 8.8

286 310 287 289 298 259

0.6 3.7 12.5 13.6 11.5 7.8

Pure PP exhibits one step decomposition in the range of 350−480 °C, mainly ascribed to the degradation of polypropylene backbone, and leaves negligible char above 500 °C. The Td for the virgin PP is about 385 °C. The addition of 30 wt % HCFA into PP improves the thermal stability above 450 °C and increases the char residue to 8.8% at 700 °C but worsens its tolerance of the heat shock below 450 °C due to the early degradation of HCFA. In contrast, the 30 wt % APP addition can not only postpone the decomposition of PP backbone but also form 20.2% remains at 700 °C (Table 3), which will certainly be attributed to the protective effect of the degraded products of APP like polyphosphoric acid and polymetaphosphate. When it comes to the combination of APP with HCFA, the thermal stabilities of PP composites fall somewhere below 375 °C, howbeit they are extremely different at the higher temperature. PP3 shows the improved thermal stability and the most char residue of 21.0% among all the samples at 700 °C, which is closely associated with the positive synergistic effect between APP and HCFA when the weight ratio of APP to HCFA is 3/1 in the matrix. These results indicate that the optimum weight ratio of acid source, carbonization agent and blowing agent is crucial for the formation of the best protective char layer. Besides, it is noteworthy to study the thermal stability using the maximum weight loss rate at various degradation stages in Figure 5b. PP3 displayed the lowest weight loss rate value, indicating its lowest amount of released combustible gases. 3.4. Thermal Stability of Various PP Samples in Air. Figure 6 shows the thermal oxidative degradation behavior of various PP samples and the corresponding data were listed in Table 3. Compared with the TGA results in nitrogen atmosphere (Figure 5), PP samples in air atmosphere start to decompose at lower temperature. Moreover, another degradation stage in the high temperature range 550−700 °C is observed, which is ascribed to the oxidative decomposition of the char residues in the hot air. It is unambiguously found that the char residue of PP3 displays the highest resistance to the thermal oxidation in the high temperature range, confirmed by its highest char residue of 13.6%. 3.5. Flammability of Various PP Composites. The LOI values and the UL-94 results of various samples are given in Table 1. Pure PP exhibits a LOI value of 17.0% with no rating in the UL-94 test. The LOI values of PP/APP and PP/HCFA with 30 wt % loadings are 22.0 and 23.5, respectively, and neither can reach UL-94 V-0 rating. For PP/HCFA/APP systems, the PP samples exhibit a remarkable increase in LOI and reach V0 rating in UL-94. Moreover, PP3 has the highest

Figure 5. TG (a) and DTG (b) curves of PP samples in nitrogen atmosphere. 17018



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Figure 7. HRR (a) and THR (b) curves of various PP samples. Figure 6. TG (a) and DTG (b) curves of PP samples in air atmosphere.

Table 4. Related Cone Calorimeter Date of Various PP Samples

LOI value of 33.5% among all the samples. The flame retardance of PP/APP/HCFA is closely related to the formation of intumescent char layer, which impedes the flame propagation and protects the matrices from combustion. As a small-scale test, LOI is not a reliable indicator of how materials perform in a real fire. Cone calorimeter has been employed to evaluate the developing, spreading, and intensity of fires, from which several key parameters such as total heat release (THR) and peak HRR (PHRR) can be obtained. HRR and THR curves of various PP composites are displayed in Figure 7 and the interrelated data are listed in Table 4. Pure PP burns very fast once ignited and PHRR appeares at 1350 kW/m2. When 30 wt % APP is added, the PHRR of PP1 decreases by 36.9% and there displays a slight broad HRR. The incorporation of APP and HCFA to PP leads to remarkable reduction in PHRRs and significant extension in time to PHRR. For instance, the PHRR of PP4 is 414 kW/m2, decreased by 69.3% compared with that of neat PP, and the time to PHRR is 295 s, a lag of 155s relative to that of virgin PP. Furthermore, it is apparent that PP3 exhibits the best flame retardance among the PP/HCFA/APP systems, which can be seen from its lowest PHRR and longest time to PHRR. This further demonstrates the presence of the optimum APP/HCFA weight ratio in PP/ IFR systems, which is consistent with TGA results. Moreover, the THR of the PP composites presents similar change trends to assess their flammability. In order to judge the fire hazard of PP composites clearly, the fire growth rate index (FIGRA) is proposed. It is calculated by

sample TTI(s) ± 2 PP0 PP1 PP2 PP3 PP4 PP5

50 58 56 48 52 38

TTPH (s) ± 2

PHRR (kW/m2) ± 35

THR (MJ/m2) ± 0.5

FIGRA (kW/m2s)

140 180 350 395 295 164

1350 851 422 316 414 518

91.2 74.4 70.7 68.8 71.1 86.7

9.64 4.73 1.21 0.80 1.40 3.16

dividing PHRR by time to peak heat release (TTPH), which can estimate both the predicted fire spread rate and the size of a fire. The higher the FIGRA is, the faster the flame spreads and grows.31−33 The comparison in FIGRA has been made and recorded in Table 4. The FIGRA of PP3 is 0.8 kW/m2s, decreased by 91.7% compared with that of neat PP. Apparently, the fire risk of PP3 is the smallest while that of pure PP is the highest. The mass loss curves of various PP samples are illustrated in Figure 8. The addition of flame retardants can postpone the mass loss of PP with varying degrees. The slight lag appears for PP only with APP or HCFA, while there is a huge delay for PP containing APP and HCFA together. Among PP/HCFA/APP systems, PP3 with a 3/1 weight ratio of APP/HCFA presents the slowest mass loss rate, and the highest residual of 30.2%. These expatiate that the IFRs are very effective to delay the mass loss of the materials. This is in accordance with TGA results in air atmosphere. 17019



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PP4 with 4/1 weight ratio of APP to HCFA exhibits a slight expansion during the combustion, possibly caused by insufficient blowing agent. These results indicate that PP3 has the best intumescent char layer as a physical barrier against the transfer of the heat and combustible gases, thus resulting in the lowest PHRR and THR and highest char residue at 700 °C. SEM is further applied to investigate the morphologies of the char residues after cone calorimeter measurements, as shown in Figure 10. Clearly, PP1 has a poor char layer with many cracks and holes (Figure 10a). However, an obvious improvement on the quality of char layers for PP/APP/HCFA systems is achieved. Varying the weight ratio of APP/HCFA results in the significantly different morphologies. Some collapsed bubbles emerge on the exterior of the char of PP4 (Figure 10d), which is probably caused by the insufficient blowing agent. In contrast, there are very rich microstructure for the char of PP2 and PP3: many large folds on the surface of the former char residue and several tiny folds can be seen on the surface of its each large fold (Figure 10b, e), yet the char of latter has lots of large bubbles, which contain many smaller bubbles on its each surface (Figure 10c, f). The explanation maybe lie in the fact that the blowing agent of PP2 is so much that the forming bubbles during the combustion broke to release the inner pressure of the char, thus leading to the appearance of various folds in its cooling process, whereas PP3 containing the appropriate amount of HCFA produces the most optimum char layer; that is, there are many tiny bubbles embedded on the surface of each large bubble. These together endow its char layer with good barrier against the transmission of heat and volatiles.

Figure 8. Mass loss curves of various PP samples.

3.6. Characterization of Char Residue of Various PP Composites. The intumescent flame retardant systems always endure an expansion and the formation of the char layer; thus, the investigation of the differences of char residues after combustion is necessary. Figure 9 shows the digital photos of the residues for various PP composites collected after cone calorimeter measurements. Pure PP leaves nothing, and just a tiny residue of PP1 with 30 wt % APP is observed. The residue is intensely discontinuous and absent of any intumescence. In contrast, PP3 with a weight ratio of APP to HCFA being 3/1 engender the highly intumescent char residues. For PP2 having 30 wt % APP/HCFA with a weight ratio of 2/1 and PP5 with 30 wt % HCFA, the expansion leads to the breaking due to rapid release of large amount of volatiles from HCFA, whereas

Figure 9. Front and side views of the residues of various PP samples after cone calorimeter test. 17020



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temperature due to the less chain entanglements by the contribution of its hyperbranched structure. Therefore, HCFA can be as some lubricant to improve the flow performance of PP composites.

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CONCLUSION A novel hyperbranched char foaming agent (HCFA) was successfully prepared via the polycondensation of an AB2 monomer CHT and then combined with APP to fabricate flame retardant PP compounds. The thermal stability was studied by TGA, and the results showed that HCFA/APP system could be effective to improve the thermal degradation and thermal-oxidative stability of the char residues and PP3 with 3/1 weight ratio of APP to HCFA presented the highest thermal stability. Their flammability was investigated via LOI test, UL-94, and cone calorimeter, and the results revealed that PP3 had the highest flame retardancy. Moreover, the study of structure and morphology of char residue by SEM and FTIR indicated that PP3 formed the best char layer during the combustion, which effectively protected the inner char layer from the flame. Besides, HCFA contributed greatly to the enhancement in the flow properties of PP composites due to its hyperbranched structure. In summary, the combination of HCFA and APP has demonstrated a promising flame retardant system for PP.

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Figure 10. SEM images of the char residues of PP1 (a), PP2 [(b), its magnified image (e)], PP3 [(c), its magnified image (f)], and PP4 (d).

AUTHOR INFORMATION

Corresponding Author

*Fax/Tel: +86-551-63601664. E-mail: [email protected].

In order to prove the protective effect of intumescent char layer on the materials beneath, FTIR technology is chosen to measure the inner and outer layer structure of the char residue of PP3 and the results are observed in Figure 11. The strong

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by National Basic Research Program of China (973 Program) (2012CB719701), China Postdoctoral Science Foundation (2012M511418), Specialized Research Fund for the Doctoral Program of Higher Education (20103402110006) and National Natural Science Foundation of China (51203146).

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REFERENCES

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Figure 11. FTIR spectra of the outer (a) and inner layer (b) of the char residue of PP3 from the cone calorimeter test.

absorption peaks of CH at around 2980−2850 cm−1 and CO at 1670 cm−1 of the inner char clearly confirm that PP beneath the outer char layer does not decompose completely, indicating that the outer char layer effectively hinders the heat and oxygen from invading the polymer beneath. 3.7. Flow Properties of Various PP Composites. Flow properties of various PP samples are listed in Table 1. Virgin PP shows a MFI of 3.3 g/10 min and the addition of 30 wt % APP increases MFI to 6.6 g/10 min. A further increase of MFI is observed when HCFA is replaced by part of APP, and the more the HCFA is used, the higher the MFI of PP composites is. This is associated with lower viscosity for HCFA at high 17021



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Synthesis of a Novel Triazine-Based Hyperbranched Char Foaming

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