, as well as the synergistic mechanism was discussed in detail. For the composites, the content of SiO 2

Transcription

1 Synergistic Effect of on Intumescent Flame-retardant Polypropylene Synergistic Effect of on Intumescent Flame-retardant Polypropylene Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng* Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai , China Received: 21 December 2012, Accepted: 11 February 2013 SUMMARY The flammability of polypropylene and its intumescent flame retardant composites consisting of ammonium polyphosphate (APP), pentaerythritol (PER) and were studied and the synergistic effect of, as well as the synergistic mechanism was discussed in detail. For the composites, the content of varied from 0 to 12 wt.% where the total addition of APP/PER/ was constant at 30 wt.%. The flammability properties were characterized by Limiting Oxygen Index (LOI) and UL-94 tests, while the synergistic mechanism of was investigated by a combination of thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy (SEM). The results showed that had a synergistic effect on the PP/APP/PER system, with an optimum addition at 3.5 wt.%. The introduction of improved the flame retardant properties of the PP/APP/PER system and the strength of the char layer. Keywords: Polypropylene, Intumescent flame retardant, Flame retardancy, 1. INTRODUCTION Polypropylene (PP) has being extensively utilized not only in industry but also daily life, such as for housing, transportation, and textile materials. However, due to the flammability properties, its application is severely restricted in many fields, such as electrical materials 1. Great efforts have been made to improve the flame-retardant properties of PP over the past decades. It is well known that flame retardants (FR) which are bromine-containing can efficiently improve the flame retardancy of PP, but their use has been limited due to safety and environmental concerns. Metal hydroxides are alternative flame retardant additives that can be used in PP. Because of the high loading Smithers Rapra Technology, 2013 of metal hydroxides, the mechanical properties of polymeric materials are accordingly affected 2-3. Therefore, halogen-free retardant additives such as intumescent flame retardant (IFR) are attracting more and more attention 4-7. It is known that the combination of ammonium polyphosphate (APP) and pentaerythritol (PER) is an effective flame retardant in PP system. APP works as both an acid source and a blowing agent during combustion, while PER which can be easily dehydrated by the acid works as the carbonizing agent IFRs are advantageous for very low smoke density, non-toxic gases and anti-dripping melt during burning; however, their low flame retardant efficiency and poor thermal *Corresponding author: Tel: ; Fax: ; zan@ecust.edu.cn stability are still two drawbacks in comparison with brominecontaining flame retardants. In order to produce effective flame retardancy, synergistic agents such as zeolites 13,14, organically-modified montmorillonite 15, calcium carbonate (CaCO 3 ), talc 16 and organoboron siloxane 17,18 have been introduced into IFRs. Many researchers have also discussed the effect of in the PP/IFR system But they have different viewpoints: some think that does not react with IFR 19, while other researchers claim that can catalyze the esterification reaction 21. In this work TGA and strength testing of the char layer are introduced to the PP/IFR system to investigate the synergistic mechanism of in detail. The morphological structure of the char layers is characterized by Limiting Oxygen Index (LOI), UL-94, Thermogravimetric Analysis (TGA), Fourier Transform Infrared (FTIR) spectrometry and Scanning Electron Microscopy (SEM). Polymers & Polymer Composites, Vol. 21, No. 7,

2 Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng 2. EXPERIMENTAL PART 2.1 Reagents PP resin (homopolymer, melt flow rate: 2.8 g/10 min) was produced by Zhenhai Petroleum China. APP (n>1000, Crystalline form II) with an average particle size of 10 μm was acquired from Shanghai Xusen China. PER (CP) was produced by Shanghai Lingfeng Chemistry China and (average particle size 10 μm) was provided by Changzhou Petroleum China. 2.2 Preparation of Blends The intumescent flame retardants (IFRs) were composed of APP, PER and. The molar ratio of APP to PER was fixed at 1.5:1, and dosage ranged from 0 to 12 wt.% in the PP/IFR system. Intumescent flame retardant PP matrix composites (IFR-PP) were prepared by mixing 70 wt.% PP powder and antioxidant B215, 30 wt.% IFR in an internal mixer (Su70-1, Suyan Technology Company, China) at 180 C with a rotation rate of 80 rpm. Afterwards, the samples were compressed into sheets using a vulcanizing machine (XQLB , Shanghai First Rubber Machinery Factory), and cut into standard samples for further mechanical and flame retardancy tests. 2.3 Flame Retardancy Tests The flame retardancy of all samples was measured by LOI and UL-94 methods. LOI data of all samples (130 mm 6.5 mm 3 mm) were obtained by an Oxygen Index Instrument (JF-3) which was produced by Jiangning Analysis Instrument Factory, China, according to the GB/T standard. The UL-94 test of samples (130 mm 6.5 mm 3 mm) was performed according to GB/T in a vertical burning tester (CZF-2) produced by Jiangning Analysis Instrument Factory, China. 2.4 Thermogravimetric Analysis Thermal analysis (TGA) was conducted under a nitrogen atmosphere from room temperature to 600 C at a scanning rate of 10 C/min by a Netzsch STA 409 Thermal Analyzer. 2.5 Scanning Electron Microscopy The morphology of the char layer, which was prepared by treating the sample at 400 C for 30 min in a muffle furnace, was observed by SEM (JSM- 6360). The surface char layer was sputter-coated with gold layer before examination. 2.6 FT-IR Test The char layers of the PP/IFR composites were characterized by FT-IR spectrometry (Nicolet 5700). The composites were put into the tin boxes, which were heated at 300 C or 400 C in the muffle furnace. 2.7 Mechanical Properties The bending strength of the samples (80 mm 10 mm 4 mm) was measured at 20 C using a CMT-4204 machine, according to GB/T Five replicate measurements were conducted for each sample and an average was chosen. The tensile strength tests were carried out according to GB/T Five replicate measurements were conducted for each sample and an average was chosen. The impact strength was measured by JJ-20 impact machine on sheets (80 mm 10 mm 4 mm) according to GB/T All the tests were run five times and an average was obtained. 2.8 Strength of the Residual Charred Layer The strength of the char layer obtained from LOI tests was examined by HY machine. The char layer was placed on a circular flat and subjected to pressure, and then the load was recorded when the char layer was broken. Five replicate measurements were conducted for each sample and an average was obtained. 2.9 Cone Calorimeter Analysis Cone Calorimeter tests were performed by using a Stanton Redcroft Cone Calorimeter following the procedure defined in ASTM E The samples were put in horizontal orientation at an incident flux of 35 kw/m 2. Sample size was 100 mm 100 mm 4 mm. The results were the average values from three tests. 3. RESULTS AND DISCUSSION 3.1. Effect of on LOI and UL-94 Rating of the PP/IFR Systems Table 1 lists the effect of on LOI and UL-94 values of the PP/IFR blend samples with constant loading of 30 wt.% total additives. The LOI value increased initially from 31.2% to 38.5% as the content of was increased to 3.5 wt.%, and beyond that content, it decreased to 24.5 %. Table 1 indicates that 3.5 wt.% was the optimal content of, which could significantly improve the LOI value. This effect might be the enhancement of char layer strength. UL-94 data in Table 1 indicate that when the loading of increased up to 4.5 wt.%, the UL-94 tests kept constant at V-0 rating; beyond that loading, the UL-94 data sharply decreased from V-1 to no rating because of the lack of main flame-retardant (APP and PER) additives. The heat release rate (HRR) is regarded as the most important parameter for measuring the development and spreading of fire. Figure 1 shows the HRR curves of PP, PP/IFR, and PP/ IFR/ wt.%. It indicates that 440 Polymers & Polymer Composites, Vol. 21, No. 7, 2013

3 Synergistic Effect of on Intumescent Flame-retardant Polypropylene Table 1. LOI and UL-94 results of PP/IFR/ composites with constant loading of 30 wt.% of the total additives PP/g APP/g PER/g /g LOI (%) UL-94 test Flaming dripping UL-94 rating ±0.3 Yes No rating ±0.1 No V ±0.2 No V ±0.2 No V ±0.3 No V ±0.1 No V ±0.2 No V ±0.3 No V ±0.2 No V ±0.1 No V ±0.2 No V ±0.3 Yes V ±0.1 Yes V ±0.2 Yes No rating ±0.3 Yes No rating ±0.2 Yes No rating the PHRR (peak of HRR) of PP/IFR/ -3.5 wt.% (231 kw/m 2 ) was the smallest compared with that of PP/ IFR (279 kw/m 2 ) and PP (589 kw/m 2 ). Figure 1. HRR curves of PP, PP/IFR, and PP/IFR/ Rate of smoke production (SPR) curves of PP, PP/IFR, and PP/IFR/ during the whole combustion process are presented in Figure 2. The order of the SPR peak values was PP/IFR/ (0.038 m 2 /s) < PP/IFR (0.043 m 2 /s) < PP (0.062 m 2 /s). These results suggest that the SPR decreased significantly when was introduced into the PP/IFR system. The results of the cone calorimeter indicate that could improve the strength of the char layer and thus play a vital role in protecting the inner matrix from combusting, and holding back the transfer of heat and oxygen. Figure 2. SPR curves of PP, PP/IFR, and PP/IFR/ 3.2 Thermogravimetric Behaviour of PP and PP/IFR Composites Thermal analysis is an effective method to investigate the thermal degradation behaviour of flameretardant materials. The TG and Polymers & Polymer Composites, Vol. 21, No. 7,

4 Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng Table 2. Effect of content on thermal stability of PP/IFR systems Samples T initial ( C) T max ( C) Residual char /600 C(%) PP PP/IFR PP/IFR/ -3.5% PP/IFR/ -9% Figure 3. (a) TGA and (b) DTG curves of PP/IFR samples with different contents under nitrogen atmosphere According to Figure 3, the TG curves of PP/IFR/ had two weight-loss stages. There were a few differences between the TG curves of PP/IFR and PP/IFR/. However, PP/IFR/ exhibited more stability than the PP/ IFR above 450 C, because could improve the strength of the char layer and form the stable char layer in the second step. Table 2 shows that the char residue increased from 14.2 wt.% to 20.8 wt.% with the increase of the loading from 0 to 9 wt.%. It indicates that the bulk of did not react with the IFR, and only improved the strength of the char layer. DTG curves of PP, PP/IFR, PP/IFR/ -3.5 wt.% and PP/IFR/ - 9 wt.% are shown in Figure 3. PP decomposed initially at 347 C, and the T max value occurred at 394 C; it had decomposed completely above 450 C. Figure 4 shows the decomposition mechanism of the PP/IFR, of which the thermal decomposition include three steps 22. The initial weight loss at about 233 C was caused by the decomposition of APP (I), experiencing the alcoholysis reaction with the action of PER (II) to form products with phosphoester-bonds (III) (Figure 4a and Figure 4b). The second step (Figure 4c) occurred at 392 C which was the main decomposition process of the composite, releasing water and ammonia to form the stable cyclic ester. Its T max was 436 C and the weight loss was 77.0 % during this step. The third step occurred at 537 C, which was the stage of pyrolysis of the ester, by a process such as a Diels-Alder reaction 22 (Figure 4d). 3.3 FTIR Analysis The FTIR spectra of the char layer of PP/IFR and PP/IFR/ samples heated at 300 C and 400 C for 30 min are shown in Figure 5. The composition of PP began at 347 C, then the peak at cm -1 corresponding to the CH 2 or CH 3 deformation vibration of aliphatic groups was set as the base peak. The peaks at cm -1 correspond to the CH 2 or CH 3 asymmetric and symmetric vibrations. Compared with the intensity of the base peak, the intensity of the absorption peaks at cm -1 in PP/IFR/ was stronger than that in PP/ IFR, which indicates that could prevent the decomposition of PP. The peak at cm -1 was assigned to the stretching mode of P-O-C in the phosphocarbonaceous complex. The peak at cm -1 was attributed to the P-O vibration of the P-O-P group 19,21,23,24. The peak at cm -1 was assigned to the symmetric vibration of PO 3 group. Compared with the intensity of the base peak, the intensity of the absorption peak at cm -1 which corresponded to Si-O vibration of Si-O-Si increased with the increase of content. There was no absorption peak at 800 cm -1 which corresponded to Si-C vibration 25, which indicates that did not take part in the reaction of the char formation, but only improved the strength of the char layer. 442 Polymers & Polymer Composites, Vol. 21, No. 7, 2013

5 Synergistic Effect of on Intumescent Flame-retardant Polypropylene Figure 4. The decomposition mechanism of the PP/IFR system 22 (a) (b) (c) (d) Polymers & Polymer Composites, Vol. 21, No. 7,

6 Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng Figure 5. FTIR spectra of char layer of samples heated at 300 C for 30 min (a) and heated at 400 C for 30 min (b) (a) (b) Figure 6. The strength of char layer after LOI testing of flame retarded composites 3.4 The Strength of Char Layers Figure 6 shows the strength of the char layer tested by compressing the char layer until its breakage using a universal test machine. With the loading of increased from 0 to 9 wt.%, the strength of the char layer increased from 0.69 MN/m 2 to 4.15 MN/m 2, which confirmed that could reinforce the char layer. However, the increased char layer did not always lead to a higher LOI value. The LOI of 9 wt.% sample was lower than that of 3.5 wt.% ; this demonstrated that the char layer could inhibit heat and mass transfer between the gas and condensed phases and protect the underlying material from heat flux. The flame retardant properties were not only related to the strength of the charred layer, but also contributed to the compactness of the char layer. 3.5 Morphology of Char Layer As a nitrogen-phosphorus composite flame retardant, APP involved an acid catalyst (phosphoric acid) and a blowing agent, and PER was considered as the carbon source. The mechanism of the intumescent flame retardant was based on the formation of the protective intumescent char layer on the surface of the burning matrix. The char layer could inhibit the interior material from the fire and block the heat and mass transfer. The morphologies of the char layer formed in the muffle furnace at 400 C were further observed by SEM (Figure 8) and digitial camera (Figure 7). Figure 7 shows photographs of char layers after the flame extinguished during the LOI test. As shown in Figure 7, PP/IFR/ -3.5 wt.% formed the biggest and the most compact char layer left at the end of the sample bar. The changes in the morphology of the char layers were helpful to explain the differences of the flame-resistance between PP/IFR and PP/IFR/. 444 Polymers & Polymer Composites, Vol. 21, No. 7, 2013

7 Synergistic Effect of on Intumescent Flame-retardant Polypropylene Figure 7. Pictures of the char layer formed after extinguishing in LOI test: (a) PP/IFR, (b) PP/IFR/ -3.5 wt.%, (c) PP/IFR/ -9 wt.% (a) Figure 8. SEM images of char layers of PP/IFR composites (a 1 ) PP/IFR 1000, (a 2 ) PP/IFR 5000, (b 1 ) PP/IFR/ -3.5 wt.% 1000, (b 2 ) PP/IFR/ -3.5 wt.% 5000, (c 1 ) PP/IFR/ -9 wt.% 1000, (c 2 ) PP/IFR/ -9 wt.% 5000 (a1) (a2) (b) (b1) (b2) (c) (c1) (c2) Figure 8 shows SEM micrographs of the char layer after burning. The surface of PP/IFR char layer was loose and had a lot of cracks. Undoubtedly, it was difficult for the char layer to totally obstruct the heat and mass flow, resulting in the relatively low flame retardant efficiency of this system. Compared with the char layer of PP/ IFR, the surface of the char layer of PP/ IFR/ -3.5 wt.% was more compact and smoother. It could be seen that efficiently improved the strength of char layer and had good binding between the filler and the char layer. However, when the loading of increased to 9 wt.%, there were lots of cracks on the surface of the char layer. Because the content of increased, its compatibility with the char layer decreases. From Figure 8(c 2 ), the agglomeration of could be seen clearly, due to the weak compatibility of with the char layer. 3.6 Synergistic Mechanism of Δ(wt) 14 =am APP (wt.%)+bm PER (wt.%)+cm SiO2 (1-1) Note:a- the content of APP, b- the content of PER, c-the content of Figure 9 and Figure 10 show TGA thermograms of APP/ and PER/ to study their reaction. The theoretical APP/ was calculated by (1-1), with the same ratio of APP/ as that of the experimental curve. Comparing the theoretical weight loss of APP/ with the experimental APP/ showed that there was no difference between two curves, which demonstrated that could not react with APP. As Figure 10 suggests, the theoretical curve of /PER was not different from the experimental curve of /PER, which demonstrated that could not react with PER. Therefore, the synergistic mechanism of was only the physical effect to improve the strength of the char layer. When burning, could move to the surface of the matrix. The char layer Polymers & Polymer Composites, Vol. 21, No. 7,

8 Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng Figure 9. TGA curves of APP, and APP/ Figure 10. TGA curves of PER, and PER/ Figure 11. The mechanical properties of PP/IFR composites (a) tensile strength, (b) elongation at break, (c) bending strength, (d) impact strength (a) (b) (c) (d) was formed, and then moved into the char layer to improve its strength. The char layer would be smoother and more compact than that of the PP/IFR system, and hence it could effectively block the transfer of heat and oxygen and protect the underlying matrix from burning. 3.7 Mechanical Properties of the PP/IFR and PP/IFR/ Blends From the mechanical performance listed in Figure 11, the tensile strength, the bending strength and the impact strength of PP/IFR/ -9 wt.% reached 20.7 MPa, 36.7 MPa, and 1.3 kj/m 2 respectively, higher than those of the PP/IFR system. However, had a small influence on the mechanical properties. Meanwhile IFR whose content was much higher than that of played a main influence on the mechanical properties. In order to 446 Polymers & Polymer Composites, Vol. 21, No. 7, 2013

9 Synergistic Effect of on Intumescent Flame-retardant Polypropylene improve the mechanical properties of PP/IFR, it was necessary to improve the compatibility of IFR and the polymer matrix. 4. CONCLUSIONs The synergistic effect of with APP/PER in the flame retardation of PP was investigated in this work. The optimal loading of in this system was 3.5 wt.% with the maximum LOI value of 38.5 %. When the loading of increased to 3.5 wt.%, the UL- 94 tests could reach V-0 rating. The results of TGA and FTIR confirmed that stayed in the condensed phases, which did not take part in the formation of the char layer and did not have a reaction with APP or PER. The SEM images and strength of the char layer indicated that the final fire-resistance of the flame retardant polymer materials was not only dependent on the strength of the char layer, but also on its surface morphology. The results suggested that a smooth and compact char layer could be formed when the loading of is 3.5 wt.%, which would effectively inhibit the transfer of heat and mass. ACKNOWLEDGEMENTS The study was financially supported by Shanghai Leading Academic Discipline Project (B502) and Shanghai Key Laboratory Project (08DZ ). REFERENCES 1. Green J., Flame retardant polymeric materials. In: Lewin M, Atlas SM, Pearce EM, editors. The flame retardation of polyolefins, Vol New York. 2. Wu N., Ding C., and Yang R.J., Polymer Degradation and Stability, 95 (2010) Li Y.T., Li B., Dai J.F., Jia H., and Gao S.L., Polymer Degradation and Stability, 93 (2008) Chen Y.H. and Wang Q., Polymers for Advanced Technologies, 181 (2007) Le Bras M., Bugajny M., Lefebvre J.M., and Bourbigot S., Polymer International, 49 (2000) Xie F., Wang Y.Z., Yang B., and Liu Y., Macromolecular Materials and Engineering, 291 (2006) Siat C., Le Bras M., and Bourbbigot S., Fire and Materials, 22 (1998) Li Q., Pingkai J., and Wei P., Macromolecular Materials and Engineering, 290 (2005) Bourbigot S., Le Bras M., Duquesne S., and Rochery M., Macromolecular Materials and Engineering, 289 (2004) Le Bras M., Bourbigot S., Delporte C., Siat C., and Le Tallec Y., Fire and Materials, 20 (1996) Camino G., Costa L., and Trossarelli L., Polymer Degradation and Stability, 6 (1984) Camino G., Costa L., and Trossarelli L., Polymer Degradation and Stability, 7 (1984) Demir H., Arkìs E., Balköse D., and Ülkü S., Polymer Degradation and Stability, 89 (2005) Bourbigot S., Le Bras M., Delobel R., Brěant P., and Tremillon J.-M., Polymer Degradation and Stability, 54 (1996) Almeras X., Le Bras M., Hornsby P., Bourbigot S., Marosi G., Keszei S., and Poutch F., Polymer Degradation and Stability, 82 (2003) Ravadits I., Toth A., Marosi G., Márton A., and Szép A., Polymer Degradation and Stability, 74 (2001) Marosi G., Márton A., Anna P., Bertalan G., Marosfoi B., and Szép A., Polymer Degradation and Stability, 77 (2002) Estevāo L.R.M., Le Bras M., Delobel R., and Nascimento R.S.V., Polymer Degradation and Stability, 88 (2005) Lie Y. and Qiang H.W., Journal of Applied Polymer Science, 115 (2010) Gao S. and Li B., Polymer for Advanced and Technologies, 22 (2011) Wei P., Hao J.W., Du J.X., Han Z.D., and Wang J.Q., Journal of Fire Sciences, 21 (2003) Wu K., Wang Z.Z., and Hu Y., Polymer for Advanced Technologies, 19 (2008) Bourbigot S., Le Bras M., and Delobel R., Carbon, 33, (1995), Gallo E., Schartel B., Acierno D., and Russo P., European Polymer Journal, 47 (2011) Thorne K., Liimatta E., and Mackenzie J.D., Journal of Materials Research, 6 (1991) Polymers & Polymer Composites, Vol. 21, No. 7,

10 Na Li, Yin Xia, Zongwen Mao, Liang Wang, Yong Guan, and Anna Zheng 448 Polymers & Polymer Composites, Vol. 21, No. 7, 2013