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A novel polymeric intumescent flame retardant: Synthesis, thermal degradation mechanism and application in ABS copolymer
Polymer Degradation and Stability 97 (2012) 1772e1778
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
A novel polymeric intumescent flame retardant: Synthesis, thermal degradation mechanism and application in ABS copolymer Xiaoping Hu a, *, Yuyang Guo a, Li Chen b, Xiuli Wang b, Liangjun Li a, Yuzhong Wang b a b
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Chem Building 29, Wangjiang Road, Sichuan University, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2011 Received in revised form 20 May 2012 Accepted 11 June 2012 Available online 17 June 2012
A novel polymeric intumescent flame retardant containing phosphorousenitrogen (PSPTR) was synthesized and characterized by FTIR, 1H NMR and 31P NMR. Moreover, a new intumescent flame retardant (IFR) system, which was composed of PSPTR and Phenol Formaldehyde Resin (PF), was used to impart flame retardancy for ABS. Flammability properties of ABS/IFR composites were investigated by Limiting Oxygen Index (LOI) and vertical burning test (UL-94), respectively. The results showed that when the total addition content was 30 wt%, the weight ratio of PSPTR to PF is 1:1, the LOI value of ABS/ IFR reached 28.2, and UL-94 reached V-1 rating. A distinct synergistic flame retardant effect exists between PSPTR and PF. The TGA data showed that the IFR (PSPTR/PF ¼ 1:1, wt%) had three weight-loss stages and had a high residue of 50.21 wt% at 700 C. The thermal degradation process of PSPTR and charforming mechanism of IFR was studied by thermogravimetric analysis/infrared spectrometry (TG-IR) detailedly. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Synthesis Characterization Synergistic flame retardancy Charring flame retardant mechanism TG-IR
1. Introduction ABS resin, a copolymer formed with acrylonitrile, butadiene and styrene, has the combined properties of insulation, easy processing, shining surface, thermal stability, good mechanical strength, resistance to oil, resistance to weather and improved impact strength [1e3]. ABS resin is widely used in automobile parts, toys, office supplies, and electric appliances, etc. However, it has a low LOI value and is flammable. The fateful drawback of ABS restricts its wider application. In order to assure public safety, the products of ABS must pass regulatory flame retardant tests. Therefore, how to improve its flame retardancy becomes a big challenge [1,4e9]. The common flame retardants for ABS are halogen-containing compounds such as decabromodiphenyl oxide (DBDPO), tetrabro mobis-phenol (TBBPA), 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPOE), etc. [10]. Because of the environmental problems [11e14], some halogen-containing flame retardants have been forbidden gradually. Consequently, it is essential that new and high-effective flame retardant systems should be developed to meet the constantly changing demand of new regulations and standards. Recently, triazine derivatives as charring agent in intumescent flame retardant system have been paid great attention
* Corresponding author. E-mail address: [email protected] (X. Hu). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.06.009
owing to the stable triazine rings and char formation easily during burning [15e17]. Meanwhile, a special kind of spirocyclic pentaerythritol bispirophosphate flame retardants are highly focused by researchers because of the environmentally-friendly consideration and excellent flame retardancy [4,5]. In order to combine the excellent flame retardant properties of the triazine derivatives and spirocyclic pentaerythritol bispirophosphate, some products mainly based on these two structures have been prepared [18e20]. For example, melamine salt of pentaerythritol phosphate (MPP) is a very effective IFR when used to polypropylene [21]. It usually experiences an intense expansion and forms protective charred layer which can protect the remaining polymer from the heat of combustion and limit the access of oxygen to the polymer [22,23]. However, these non-polymeric flame retardants have the following deficiencies: First, the flame retardant is salt, and has poor water resistance; second, the molecular weight of flame retardant is not large and easily migrate from the polymer matrix, which will damage the flame retardancy, appearance and the long-term usage of the materials. In this paper, a novel intumescent flame retardant containing spiral and triazine structure was synthesized and characterized by FTIR and 1H NMR. A novel intumescent flame retardant (IFR) system composed of PSPTR and Phenol Formaldehyde Resin (PF) was used for ABS. The flammability properties of ABS/IFR composites were investigated by LOI and UL-94 tests. Thermal degradation behavior of PSPTR, PF, IFR, and the ABS/IFR composite
X. Hu et al. / Polymer Degradation and Stability 97 (2012) 1772e1778
were analyzed by TGA and the charring mechanism of IFR was investigated by TG-IR. 2. Experimental 2.1. Materials All the starting materials and solvents were commercially available and were used without further purification. Methylene dichloride, toluene, ammonia and xylene were supplied by Chengdu Kelong Chemical Reagent Corp. Cyanuric chloride was purchased from Yingkou Sanzheng Organic Chemistry Co., LDT. The ABS resin (PA-747) was obtained commercially from Qimei Industry Stock Limited Company. Phenol formaldehyde resin (2123) was supplied by Zhengzhou Weiye Superhard Materials Limited Company. Spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC) was synthesized according to the reference [24]. FTIR (KBr): 2924 cm1 (eCH2e); 1310 cm1 (P]O); 1191, 1028 cm1 (PeOeC); 855 cm1 (spiral structure); 780, 550 cm1 (PeCl). 2-methoxyl-4,6-dichloro-1,3,5-triazine (MDCT) was prepared according to the reference [25]. FTIR (KBr): 1551, 1514, 805 cm1 (triazine structure); 2925, 1391, 1490 cm1 (eCH3); 1058 cm1 (CeCl); 1260 cm1 (]CeOeC). 2.2. Synthesis of 2,4-diamino-6-methoxy-1,3,5-triazine (DAMT) 0.3 mol MDCT was added in a glass flask and dispersed by 300 ml toluene at room temperature. Then 1.5 mol ammonia was charged for about 1 h at 40 C. Thereafter the mixture was heated to 80 C and kept under reflux for about 6 h. Successively, the reaction mixture was cooled slowly to room temperature and filtered. The white solid was washed with deionized water. The product was dried at 70 C under vacuum to a constant weight. The synthesis route is illustrated in Scheme 1. 2.3. Synthesis of PSPTR 0.11 mol DAMT, 0.1 mol SPDPC and 0.2 mol triethylamine were added in a glass flask and dispersed by 200 ml xylene. The reaction would be completed after 12 h at 140 C. The product obtained was filtered and purified with methylene dichloride and deionized water, then dried to a constant weight at 70 C in vacuum oven. The pale yellow solid powder was obtained, m.p. 208 C. The synthesis route is illustrated in Scheme 1. 2.4. Preparation of ABS/IFR composites ABS pellets were dried under vacuum at 50 C for 12 h before used. All the composites were prepared on a two-roll mixing mill
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(TR-502B, Dongguan, China) at 195 C. ABS was added to the mill at the beginning of the blending procedure. After ABS was molten, PSPTR and PF was then added to the matrix and processed about 10 min until a visually good dispersion was achieved. The resulting composites were compressed and molded into standard samples for tests. 2.5. Measurements IR spectroscopy was applied with a Nicolet-5700 FTIR spectrometer using KBr pellets. 1 H NMR and 31P NMR spectra were performed on an FT-80A NMR (400 MHz) using d6-DMSO as a solvent and TMS as an internal standard. The LOI values were surveyed on an HCe2C oxygen index meter (Jiangning, China) with sheet dimensions of 130 mm 6.5 mm 3.2 mm according to ASTM D2863e97. Vertical burning tests were conducted on a vertical burning test instrument (CZF-2-type) (Jiangning, China) with sheet dimensions of 130 mm 13 mm 3.2 mm according to ASTM D3801. TG-IR of the samples was performed using the TG 209 F1 thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from room temperature to 700 C at a heating rate of 10 C/min (N2, 50 ml/min). 3. Results and discussions 3.1. Characterization of DAMT and PSPTR Fig. 1 shows FTIR spectra of DAMT and PSPTR. The characteristic absorption peaks at 1582, 1528, 802 cm1 (triazine structure); 2953, 1464, 1366 cm1 (eCH3); 3347, 3217, 1681 cm1 (eNH2) are found. Moreover, the absorption peak of CeCl structure (1061 cm1) in MDCT is disappeared. These results indicate that the DAMT is successfully synthesized. In the FTIR spectra of PSPTR, the characteristic absorption peaks at 3423, 3217, 1747 cm1 (eNHe, eNH2), 1083 cm1 (PeN), 850 cm1 (spiral structure) are found. Compared with the peak 1310 cm1 of P]O in SPDPC, the stretching vibration of P]O bond falls to 1237 cm1 which due to the resonant effect between P]O and N [18]. The 1H NMR spectra of PSPTR are shown in Fig. 2. The peaks of H: 3.80 ppm (eCH2 in spiral structure); 4.10 ppm (eOCH3); 4.50 ppm (eNH2) and 7.13e7.26 ppm (PeNHe) were found. The structure of PSPTR is also confirmed by 31P NMR spectra shown in Fig. 3. Two obvious sharp signals are observed. One for phosphorus linked with imine (PeNH) is observed at 5.61 ppm, the other for phosphorus combined with chlorine (PeCl) is appeared at 20.75 ppm. All that information indicates that the PSPTR is
Scheme 1. The synthesis route of DAMT and PSPTR.
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Fig. 3.
Fig. 1. FTIR spectra of DAMT and PSPTR.
synthesized successfully. Moreover, the degree of polymerization of PSPTR can be calculated by the peak area values of PeNH and PeCl. The ratio of peak area of PeNH to PeCl is 12.5 which indicate the degree of polymerization is about 7 and the molecular weight is about 2591.5 . 3.2. Flammability properties LOI measurements and UL-94 tests have been widely used to evaluate the flame retardant properties of materials. Table 1 shows the LOI values and UL-94 ratings of all the specimens. It can be seen that the synergistic effect exists between PSPTR and PF. The single use of PSPTR or PF with a 30 wt% addition level only shows a little flame retardancy on ABS, and the LOI value are only 25.2 and 22.5. The flame retardant effect is still unsatisfying for the onecomponent system. However, when the content of IFR is 30 wt% and the weight ratio of PSPTR to PF is 1:1, the LOI 28.2, and UL-94 V1 can be achieved.
31
P NMR spectra of PSPTR.
3.3. Thermal stability The TGA and DTG curves of PSPTR, PF, ABS, IFR (PSPTR:PF ¼ 1:1, wt%) and ABS-4 are shown in Fig. 4. The related data were given in Table 2. The initial decomposition temperature (T5wt%) presents the temperature on which the weight-loss was 5 wt% and the Tmax means the maximum weight-loss temperature. It is easily found that the PSPTR have four weight-loss stages. The first stage may be assigned to the scission of the methoxy group around 222 C, whereas the second at around 290 C is due to the pyrolysis of the PeN bond. The third may be attributed to the decomposition of PeOeC and triazine structure at around 337 C [26]. Meanwhile, a new substance of 2,4-diamino-1,3,5-triazine polyphosphate may be formed in this step. N2-(4-amino-1,3,5-triazin-2-yl)-1,3,5triazine-2,4-diamine polyphosphonate and ammonium polyphosphate groups should be formed from 2,4-diamino-1,3,5triazine polyphosphate at high temperature. The last stage may be attributed to the cracking of N2-(4-amino-1,3,5-triazin-2-yl)1,3,5-triazine-2,4-diamine polyphosphonate and to form a complex PeN mixture at around 506 C. Two simple thermal degradation processes were found in PF. The first weight-loss stage may be due to the condensation reaction at around 360 C and the formation of diphenyl ether linkages [27]. The diphenyl ether linkages will generate the polyaromatic system at about 450 C. The second stage can be assigned to the thermal degradation of polyaromatic system at around 548 C. What is more, the IFR has a high residue char of 50.21 wt% at 700 C and three weight-loss stages. The first characteristic degradation stage appears at 200e420 C, which is the most important stage for intumescent char formation. The formation of polyphosphoric acid could catalyze PF to form diphenyl ether linkages. The second weight-loss stage around 420e510 C can be Table 1 Effect of IFR on flame retardancy of ABS.
Fig. 2. 1H NMR spectra of PSPTR.
Formulation
ABS/IFR ABS
PSPTR
PF
ABS-1 ABS-2 ABS-3 ABS-4 ABS-5 ABS-6 ABS-7
70 70 70 70 70 70 70
30 25 20 15 10 5 0
0 5 10 15 20 25 30
LOI (%)
UL-94
25.2 26.4 27.3 28.2 27.0 24.6 22.5
V-2 V-2 V-2 V-1 V-2 NR NR
X. Hu et al. / Polymer Degradation and Stability 97 (2012) 1772e1778
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Fig. 4. TGA (left) and DTG (right) curves of PSPTR, PF, ABS, IFR and ABS-4.
attributed to the thermal decomposition of diphenyl ether linkages which leads to the formation of the polyaromatic system at high temperature. The third step around 546 C is attributed to the thermal degradation of polyaromatic system. In addition, a new DTG peak is obviously observed at 420e510 C, which don’t exist in each DTG curve of PSPTR and PF. The reason may be that polyphosphoric acid, formed from PSPTR, can catalyze PF to form much more diphenyl ether linkages during the heating process. The thermal decomposition of a large amount of diphenyl ether linkages at 420e510 C will be helpful for formation of char layer which can improve the flame retardancy of ABS. The catalyzing char formation is also the reason for the observed synergistic flame retardant existed between PSPTR and PF. Furthermore, it can be seen that the pure ABS start decompose at 383 C, and has a negligible char (1.62 wt%) at 700 C. The T5wt% of the PSPTR and PF are about 249 C and 216 C, which are lower than pure ABS resin, but have higher residue of 40.25 wt% and 42.63 wt% at 700 C, respectively. For ABS-4 sample, the addition of IFR decreased the T5wt% but enhanced the thermal stability at high temperature. The char yield of the ABS-4 composite can reach 16.4 wt% at 700 C. The reason should be the results of the thermal degradation of PSPTR and PF at the lower temperature and formation of intact char layer which can protect the remaining polymer from the heat of combustion, limit the access of oxygen to the polymer. A possible synergistic chemical interaction between PSPTR and PF during the thermal process may exist. In order to further investigate the synergistic effects between PSPTR and PF, the calculated and experimental TG curves of IFR system (PSPTR/ PF ¼ 1:1, wt%) are compared in Fig. 5. It is unambiguous found that the char residue yield of the experiment is higher than the calculated value after 346 C and has an improvement of 8.78 wt% at 700 C, indicating PF plays a synergistic role in the thermal degradation and act as carbon source to participate in the formation of residue char. This result is favorable to protection of inner polymeric material from fire and oxygen.
3.4. The volatilized products of PSPTR and IFR TG-IR technique is usually used to identify volatile thermal degradation products, which can help to explain the thermal degradation mechanism of polymers [26]. According to TG-DTG curves of the PSPTR, several representative temperatures (such as 222 C, 290 C, 337 C and 506 C) are selected to study the volatile components released during the thermal degradation process. Fig. 6 shows typical FTIR spectra of the gaseous products in the different degradation stages for PSPTR. As shown in Fig. 6, there are a few of small molecular gaseous products released, such as CO2 (2371, 2317, 669 cm1) [28] and CO (2250 cm1) [26], attributed to the loss of eOCH3 group at 222 C. The intensity of the absorption peaks at 2371, 2317 and 2250 cm1 become greater with increasing temperature at 290 C, 337 C and 506 C, indicating the scissions of unstable PeOeC structure and triazine structure. The peak at 3500e3600 cm1 can be identified as H2O which may be formed with the self-condensation reaction of phosphoric acid. In addition, once the temperature reaches around 337 C, low amount gaseous product such as NH3 (3440, 964, 929, 714 cm1) [28,29] and H2C] NH (1629 cm1) [29] appear due to the dissociation of triazine structure. Moreover, the intensity of the peaks at 3440, 964, 929, 714, 1629 cm1 become more and more strong, attributed to the pyrolysis of N2-(4-amino-1,3,5-triazin-2-yl)-1,3,5-triazine-2,4diamine polyphosphonate and the formation of NH3 and CH2]NH and PeN complex mixture at around 506 C [30]. As expected, the
Table 2 Data of TGA and DTG thermograms of PSPTR, PF, ABS and ABS-4 in N2 at a heating rate of 10 C/min. Sample T5wt% ( C) char residue at 700 C (wt%)
Tmax ( C) Stage 1
Stage 2
Stage 3
Stage 4
PSPTR PF IFR ABS ABS-4
222 360 290 420 310
290 e 490 e 430
337 e 546 e e
506 e e e e
249 216 218 382 311
40.25 42.63 50.21 1.62 16.4
Fig. 5. TG curves of calculated and experiment for the IFR.
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Fig. 6. FTIR spectra of volatile products at representative temperature during thermal degradation of the PSPTR.
peak at 3335 cm1 is observed, which can be identified as C5H4 [29,31]. Fig. 7 presents the FTIR spectra of volatile products evolved during the thermal degradation of IFR at a heating rate of 10 C/min in N2. The selected temperature ranges from T5wt% to 580 C. As expected, some small molecular gaseous species, such as CO2, CO, H2O, NH3, H2C]NH, C5H4, are easily identified by their characteristic absorbance: CO2 at 2364 and 660 cm1; CO at 2285 and 2181 cm1; H2O at 3500e3600 cm1; NH3 at 3440, 964, 929 and 714 cm1; H2C]NH at 1629 cm1; C5H4 at 3333 cm1; aromatic compounds at 3085 cm1 and olefinic compounds at 3016 cm1. Those non-flammable gases can dilute the concentration of oxygen and fuel, then improving the flame retardancy of ABS. It is interesting to see that the intensity of absorption peak at 2364 cm1 increase with the temperature increasing and reaches a peak at 337 C which is agreement with the break of unstable PeOeC structure, then get a maximum at 490 C which is due to the thermal degradation of large number of diphenyl ether. Finally, the absorption peak of CO2 can’t be observed at above 550 C which means the thermal decomposition of IFR is completed. The peak around 3530 cm1 appears at 280 C and disappears at 520 C, which could be attached to the condensation reactions of PF, the self-condensation reaction of phosphoric acid and catalytic dehydration of polyphosphoric acid. In addition, the small intensity of
Fig. 7. FTIR spectra of volatilized products at various temperature during the thermal degradation of the IFR.
X. Hu et al. / Polymer Degradation and Stability 97 (2012) 1772e1778
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Scheme 2. Char-forming mechanism of IFR.
the peaks of NH3 and CH2]NH at 3440, 964, 929, and 1629 cm1 appeared at around 337 C. It would be due to the dissociation of triazine structure as above mentioned in TGA analysis. What is more, the peak at 3016 cm1 appeared at 470 C, and has the maximum peak at 546 C, which is mainly attributed to the thermal degradation of the polyaromatic systems. The peak at 3085 cm1 is presented at 520 C and has the maximum value at 546 C, which are mainly attributed to the aromatic volatilized products formed after pyrolysis of polyaromatic. Summarizing, according to the above analysis of TGA curves and TG-IR spectra, a possible char-forming mechanism of IFR is shown in Scheme 2.
played a synergistic effect in the flame retarding ABS/IFR composites. The data of TGA showed that the IFR had three weight-loss stages and a high residue char of 50.21 wt% at 700 C. The TG-IR results show that the volatilized products formed in thermal degradation of IFR are CO2, CO, NH3, H2O, C5H4, H2C]NH, CH4, olefinic and aromatic compounds which can dilute the concentration of oxygen and fuel in gas phase and favor the flame retardancy of ABS. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (contract No. 50973091).
4. Conclusions A novel intumescent flame retardant named PSPTR has been synthesized successfully. The flame retardancy of ABS copolymer was improved by the addition of IFR containing PSPTR and PF. When the content of IFR (PSPTR:PF ¼ 1:1) was 30 wt%, the LOI value of ABS/IFR composites reached 28.2, and the vertical burning test reached UL-94 V-1 rating. In addition, a suitable amount of PF
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
A novel polymeric intumescent flame retardant: Synthesis, thermal degradation mechanism and application in ABS copolymer Xiaoping Hu a, *, Yuyang Guo a, Li Chen b, Xiuli Wang b, Liangjun Li a, Yuzhong Wang b a b
State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, Chem Building 29, Wangjiang Road, Sichuan University, Chengdu 610064, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 September 2011 Received in revised form 20 May 2012 Accepted 11 June 2012 Available online 17 June 2012
A novel polymeric intumescent flame retardant containing phosphorousenitrogen (PSPTR) was synthesized and characterized by FTIR, 1H NMR and 31P NMR. Moreover, a new intumescent flame retardant (IFR) system, which was composed of PSPTR and Phenol Formaldehyde Resin (PF), was used to impart flame retardancy for ABS. Flammability properties of ABS/IFR composites were investigated by Limiting Oxygen Index (LOI) and vertical burning test (UL-94), respectively. The results showed that when the total addition content was 30 wt%, the weight ratio of PSPTR to PF is 1:1, the LOI value of ABS/ IFR reached 28.2, and UL-94 reached V-1 rating. A distinct synergistic flame retardant effect exists between PSPTR and PF. The TGA data showed that the IFR (PSPTR/PF ¼ 1:1, wt%) had three weight-loss stages and had a high residue of 50.21 wt% at 700 C. The thermal degradation process of PSPTR and charforming mechanism of IFR was studied by thermogravimetric analysis/infrared spectrometry (TG-IR) detailedly. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Synthesis Characterization Synergistic flame retardancy Charring flame retardant mechanism TG-IR
1. Introduction ABS resin, a copolymer formed with acrylonitrile, butadiene and styrene, has the combined properties of insulation, easy processing, shining surface, thermal stability, good mechanical strength, resistance to oil, resistance to weather and improved impact strength [1e3]. ABS resin is widely used in automobile parts, toys, office supplies, and electric appliances, etc. However, it has a low LOI value and is flammable. The fateful drawback of ABS restricts its wider application. In order to assure public safety, the products of ABS must pass regulatory flame retardant tests. Therefore, how to improve its flame retardancy becomes a big challenge [1,4e9]. The common flame retardants for ABS are halogen-containing compounds such as decabromodiphenyl oxide (DBDPO), tetrabro mobis-phenol (TBBPA), 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPOE), etc. [10]. Because of the environmental problems [11e14], some halogen-containing flame retardants have been forbidden gradually. Consequently, it is essential that new and high-effective flame retardant systems should be developed to meet the constantly changing demand of new regulations and standards. Recently, triazine derivatives as charring agent in intumescent flame retardant system have been paid great attention
* Corresponding author. E-mail address: [email protected] (X. Hu). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymdegradstab.2012.06.009
owing to the stable triazine rings and char formation easily during burning [15e17]. Meanwhile, a special kind of spirocyclic pentaerythritol bispirophosphate flame retardants are highly focused by researchers because of the environmentally-friendly consideration and excellent flame retardancy [4,5]. In order to combine the excellent flame retardant properties of the triazine derivatives and spirocyclic pentaerythritol bispirophosphate, some products mainly based on these two structures have been prepared [18e20]. For example, melamine salt of pentaerythritol phosphate (MPP) is a very effective IFR when used to polypropylene [21]. It usually experiences an intense expansion and forms protective charred layer which can protect the remaining polymer from the heat of combustion and limit the access of oxygen to the polymer [22,23]. However, these non-polymeric flame retardants have the following deficiencies: First, the flame retardant is salt, and has poor water resistance; second, the molecular weight of flame retardant is not large and easily migrate from the polymer matrix, which will damage the flame retardancy, appearance and the long-term usage of the materials. In this paper, a novel intumescent flame retardant containing spiral and triazine structure was synthesized and characterized by FTIR and 1H NMR. A novel intumescent flame retardant (IFR) system composed of PSPTR and Phenol Formaldehyde Resin (PF) was used for ABS. The flammability properties of ABS/IFR composites were investigated by LOI and UL-94 tests. Thermal degradation behavior of PSPTR, PF, IFR, and the ABS/IFR composite
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were analyzed by TGA and the charring mechanism of IFR was investigated by TG-IR. 2. Experimental 2.1. Materials All the starting materials and solvents were commercially available and were used without further purification. Methylene dichloride, toluene, ammonia and xylene were supplied by Chengdu Kelong Chemical Reagent Corp. Cyanuric chloride was purchased from Yingkou Sanzheng Organic Chemistry Co., LDT. The ABS resin (PA-747) was obtained commercially from Qimei Industry Stock Limited Company. Phenol formaldehyde resin (2123) was supplied by Zhengzhou Weiye Superhard Materials Limited Company. Spirocyclic pentaerythritol bisphosphorate disphosphoryl chloride (SPDPC) was synthesized according to the reference [24]. FTIR (KBr): 2924 cm1 (eCH2e); 1310 cm1 (P]O); 1191, 1028 cm1 (PeOeC); 855 cm1 (spiral structure); 780, 550 cm1 (PeCl). 2-methoxyl-4,6-dichloro-1,3,5-triazine (MDCT) was prepared according to the reference [25]. FTIR (KBr): 1551, 1514, 805 cm1 (triazine structure); 2925, 1391, 1490 cm1 (eCH3); 1058 cm1 (CeCl); 1260 cm1 (]CeOeC). 2.2. Synthesis of 2,4-diamino-6-methoxy-1,3,5-triazine (DAMT) 0.3 mol MDCT was added in a glass flask and dispersed by 300 ml toluene at room temperature. Then 1.5 mol ammonia was charged for about 1 h at 40 C. Thereafter the mixture was heated to 80 C and kept under reflux for about 6 h. Successively, the reaction mixture was cooled slowly to room temperature and filtered. The white solid was washed with deionized water. The product was dried at 70 C under vacuum to a constant weight. The synthesis route is illustrated in Scheme 1. 2.3. Synthesis of PSPTR 0.11 mol DAMT, 0.1 mol SPDPC and 0.2 mol triethylamine were added in a glass flask and dispersed by 200 ml xylene. The reaction would be completed after 12 h at 140 C. The product obtained was filtered and purified with methylene dichloride and deionized water, then dried to a constant weight at 70 C in vacuum oven. The pale yellow solid powder was obtained, m.p. 208 C. The synthesis route is illustrated in Scheme 1. 2.4. Preparation of ABS/IFR composites ABS pellets were dried under vacuum at 50 C for 12 h before used. All the composites were prepared on a two-roll mixing mill
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(TR-502B, Dongguan, China) at 195 C. ABS was added to the mill at the beginning of the blending procedure. After ABS was molten, PSPTR and PF was then added to the matrix and processed about 10 min until a visually good dispersion was achieved. The resulting composites were compressed and molded into standard samples for tests. 2.5. Measurements IR spectroscopy was applied with a Nicolet-5700 FTIR spectrometer using KBr pellets. 1 H NMR and 31P NMR spectra were performed on an FT-80A NMR (400 MHz) using d6-DMSO as a solvent and TMS as an internal standard. The LOI values were surveyed on an HCe2C oxygen index meter (Jiangning, China) with sheet dimensions of 130 mm 6.5 mm 3.2 mm according to ASTM D2863e97. Vertical burning tests were conducted on a vertical burning test instrument (CZF-2-type) (Jiangning, China) with sheet dimensions of 130 mm 13 mm 3.2 mm according to ASTM D3801. TG-IR of the samples was performed using the TG 209 F1 thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from room temperature to 700 C at a heating rate of 10 C/min (N2, 50 ml/min). 3. Results and discussions 3.1. Characterization of DAMT and PSPTR Fig. 1 shows FTIR spectra of DAMT and PSPTR. The characteristic absorption peaks at 1582, 1528, 802 cm1 (triazine structure); 2953, 1464, 1366 cm1 (eCH3); 3347, 3217, 1681 cm1 (eNH2) are found. Moreover, the absorption peak of CeCl structure (1061 cm1) in MDCT is disappeared. These results indicate that the DAMT is successfully synthesized. In the FTIR spectra of PSPTR, the characteristic absorption peaks at 3423, 3217, 1747 cm1 (eNHe, eNH2), 1083 cm1 (PeN), 850 cm1 (spiral structure) are found. Compared with the peak 1310 cm1 of P]O in SPDPC, the stretching vibration of P]O bond falls to 1237 cm1 which due to the resonant effect between P]O and N [18]. The 1H NMR spectra of PSPTR are shown in Fig. 2. The peaks of H: 3.80 ppm (eCH2 in spiral structure); 4.10 ppm (eOCH3); 4.50 ppm (eNH2) and 7.13e7.26 ppm (PeNHe) were found. The structure of PSPTR is also confirmed by 31P NMR spectra shown in Fig. 3. Two obvious sharp signals are observed. One for phosphorus linked with imine (PeNH) is observed at 5.61 ppm, the other for phosphorus combined with chlorine (PeCl) is appeared at 20.75 ppm. All that information indicates that the PSPTR is
Scheme 1. The synthesis route of DAMT and PSPTR.
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Fig. 3.
Fig. 1. FTIR spectra of DAMT and PSPTR.
synthesized successfully. Moreover, the degree of polymerization of PSPTR can be calculated by the peak area values of PeNH and PeCl. The ratio of peak area of PeNH to PeCl is 12.5 which indicate the degree of polymerization is about 7 and the molecular weight is about 2591.5 . 3.2. Flammability properties LOI measurements and UL-94 tests have been widely used to evaluate the flame retardant properties of materials. Table 1 shows the LOI values and UL-94 ratings of all the specimens. It can be seen that the synergistic effect exists between PSPTR and PF. The single use of PSPTR or PF with a 30 wt% addition level only shows a little flame retardancy on ABS, and the LOI value are only 25.2 and 22.5. The flame retardant effect is still unsatisfying for the onecomponent system. However, when the content of IFR is 30 wt% and the weight ratio of PSPTR to PF is 1:1, the LOI 28.2, and UL-94 V1 can be achieved.
31
P NMR spectra of PSPTR.
3.3. Thermal stability The TGA and DTG curves of PSPTR, PF, ABS, IFR (PSPTR:PF ¼ 1:1, wt%) and ABS-4 are shown in Fig. 4. The related data were given in Table 2. The initial decomposition temperature (T5wt%) presents the temperature on which the weight-loss was 5 wt% and the Tmax means the maximum weight-loss temperature. It is easily found that the PSPTR have four weight-loss stages. The first stage may be assigned to the scission of the methoxy group around 222 C, whereas the second at around 290 C is due to the pyrolysis of the PeN bond. The third may be attributed to the decomposition of PeOeC and triazine structure at around 337 C [26]. Meanwhile, a new substance of 2,4-diamino-1,3,5-triazine polyphosphate may be formed in this step. N2-(4-amino-1,3,5-triazin-2-yl)-1,3,5triazine-2,4-diamine polyphosphonate and ammonium polyphosphate groups should be formed from 2,4-diamino-1,3,5triazine polyphosphate at high temperature. The last stage may be attributed to the cracking of N2-(4-amino-1,3,5-triazin-2-yl)1,3,5-triazine-2,4-diamine polyphosphonate and to form a complex PeN mixture at around 506 C. Two simple thermal degradation processes were found in PF. The first weight-loss stage may be due to the condensation reaction at around 360 C and the formation of diphenyl ether linkages [27]. The diphenyl ether linkages will generate the polyaromatic system at about 450 C. The second stage can be assigned to the thermal degradation of polyaromatic system at around 548 C. What is more, the IFR has a high residue char of 50.21 wt% at 700 C and three weight-loss stages. The first characteristic degradation stage appears at 200e420 C, which is the most important stage for intumescent char formation. The formation of polyphosphoric acid could catalyze PF to form diphenyl ether linkages. The second weight-loss stage around 420e510 C can be Table 1 Effect of IFR on flame retardancy of ABS.
Fig. 2. 1H NMR spectra of PSPTR.
Formulation
ABS/IFR ABS
PSPTR
PF
ABS-1 ABS-2 ABS-3 ABS-4 ABS-5 ABS-6 ABS-7
70 70 70 70 70 70 70
30 25 20 15 10 5 0
0 5 10 15 20 25 30
LOI (%)
UL-94
25.2 26.4 27.3 28.2 27.0 24.6 22.5
V-2 V-2 V-2 V-1 V-2 NR NR
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Fig. 4. TGA (left) and DTG (right) curves of PSPTR, PF, ABS, IFR and ABS-4.
attributed to the thermal decomposition of diphenyl ether linkages which leads to the formation of the polyaromatic system at high temperature. The third step around 546 C is attributed to the thermal degradation of polyaromatic system. In addition, a new DTG peak is obviously observed at 420e510 C, which don’t exist in each DTG curve of PSPTR and PF. The reason may be that polyphosphoric acid, formed from PSPTR, can catalyze PF to form much more diphenyl ether linkages during the heating process. The thermal decomposition of a large amount of diphenyl ether linkages at 420e510 C will be helpful for formation of char layer which can improve the flame retardancy of ABS. The catalyzing char formation is also the reason for the observed synergistic flame retardant existed between PSPTR and PF. Furthermore, it can be seen that the pure ABS start decompose at 383 C, and has a negligible char (1.62 wt%) at 700 C. The T5wt% of the PSPTR and PF are about 249 C and 216 C, which are lower than pure ABS resin, but have higher residue of 40.25 wt% and 42.63 wt% at 700 C, respectively. For ABS-4 sample, the addition of IFR decreased the T5wt% but enhanced the thermal stability at high temperature. The char yield of the ABS-4 composite can reach 16.4 wt% at 700 C. The reason should be the results of the thermal degradation of PSPTR and PF at the lower temperature and formation of intact char layer which can protect the remaining polymer from the heat of combustion, limit the access of oxygen to the polymer. A possible synergistic chemical interaction between PSPTR and PF during the thermal process may exist. In order to further investigate the synergistic effects between PSPTR and PF, the calculated and experimental TG curves of IFR system (PSPTR/ PF ¼ 1:1, wt%) are compared in Fig. 5. It is unambiguous found that the char residue yield of the experiment is higher than the calculated value after 346 C and has an improvement of 8.78 wt% at 700 C, indicating PF plays a synergistic role in the thermal degradation and act as carbon source to participate in the formation of residue char. This result is favorable to protection of inner polymeric material from fire and oxygen.
3.4. The volatilized products of PSPTR and IFR TG-IR technique is usually used to identify volatile thermal degradation products, which can help to explain the thermal degradation mechanism of polymers [26]. According to TG-DTG curves of the PSPTR, several representative temperatures (such as 222 C, 290 C, 337 C and 506 C) are selected to study the volatile components released during the thermal degradation process. Fig. 6 shows typical FTIR spectra of the gaseous products in the different degradation stages for PSPTR. As shown in Fig. 6, there are a few of small molecular gaseous products released, such as CO2 (2371, 2317, 669 cm1) [28] and CO (2250 cm1) [26], attributed to the loss of eOCH3 group at 222 C. The intensity of the absorption peaks at 2371, 2317 and 2250 cm1 become greater with increasing temperature at 290 C, 337 C and 506 C, indicating the scissions of unstable PeOeC structure and triazine structure. The peak at 3500e3600 cm1 can be identified as H2O which may be formed with the self-condensation reaction of phosphoric acid. In addition, once the temperature reaches around 337 C, low amount gaseous product such as NH3 (3440, 964, 929, 714 cm1) [28,29] and H2C] NH (1629 cm1) [29] appear due to the dissociation of triazine structure. Moreover, the intensity of the peaks at 3440, 964, 929, 714, 1629 cm1 become more and more strong, attributed to the pyrolysis of N2-(4-amino-1,3,5-triazin-2-yl)-1,3,5-triazine-2,4diamine polyphosphonate and the formation of NH3 and CH2]NH and PeN complex mixture at around 506 C [30]. As expected, the
Table 2 Data of TGA and DTG thermograms of PSPTR, PF, ABS and ABS-4 in N2 at a heating rate of 10 C/min. Sample T5wt% ( C) char residue at 700 C (wt%)
Tmax ( C) Stage 1
Stage 2
Stage 3
Stage 4
PSPTR PF IFR ABS ABS-4
222 360 290 420 310
290 e 490 e 430
337 e 546 e e
506 e e e e
249 216 218 382 311
40.25 42.63 50.21 1.62 16.4
Fig. 5. TG curves of calculated and experiment for the IFR.
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Fig. 6. FTIR spectra of volatile products at representative temperature during thermal degradation of the PSPTR.
peak at 3335 cm1 is observed, which can be identified as C5H4 [29,31]. Fig. 7 presents the FTIR spectra of volatile products evolved during the thermal degradation of IFR at a heating rate of 10 C/min in N2. The selected temperature ranges from T5wt% to 580 C. As expected, some small molecular gaseous species, such as CO2, CO, H2O, NH3, H2C]NH, C5H4, are easily identified by their characteristic absorbance: CO2 at 2364 and 660 cm1; CO at 2285 and 2181 cm1; H2O at 3500e3600 cm1; NH3 at 3440, 964, 929 and 714 cm1; H2C]NH at 1629 cm1; C5H4 at 3333 cm1; aromatic compounds at 3085 cm1 and olefinic compounds at 3016 cm1. Those non-flammable gases can dilute the concentration of oxygen and fuel, then improving the flame retardancy of ABS. It is interesting to see that the intensity of absorption peak at 2364 cm1 increase with the temperature increasing and reaches a peak at 337 C which is agreement with the break of unstable PeOeC structure, then get a maximum at 490 C which is due to the thermal degradation of large number of diphenyl ether. Finally, the absorption peak of CO2 can’t be observed at above 550 C which means the thermal decomposition of IFR is completed. The peak around 3530 cm1 appears at 280 C and disappears at 520 C, which could be attached to the condensation reactions of PF, the self-condensation reaction of phosphoric acid and catalytic dehydration of polyphosphoric acid. In addition, the small intensity of
Fig. 7. FTIR spectra of volatilized products at various temperature during the thermal degradation of the IFR.
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Scheme 2. Char-forming mechanism of IFR.
the peaks of NH3 and CH2]NH at 3440, 964, 929, and 1629 cm1 appeared at around 337 C. It would be due to the dissociation of triazine structure as above mentioned in TGA analysis. What is more, the peak at 3016 cm1 appeared at 470 C, and has the maximum peak at 546 C, which is mainly attributed to the thermal degradation of the polyaromatic systems. The peak at 3085 cm1 is presented at 520 C and has the maximum value at 546 C, which are mainly attributed to the aromatic volatilized products formed after pyrolysis of polyaromatic. Summarizing, according to the above analysis of TGA curves and TG-IR spectra, a possible char-forming mechanism of IFR is shown in Scheme 2.
played a synergistic effect in the flame retarding ABS/IFR composites. The data of TGA showed that the IFR had three weight-loss stages and a high residue char of 50.21 wt% at 700 C. The TG-IR results show that the volatilized products formed in thermal degradation of IFR are CO2, CO, NH3, H2O, C5H4, H2C]NH, CH4, olefinic and aromatic compounds which can dilute the concentration of oxygen and fuel in gas phase and favor the flame retardancy of ABS. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (contract No. 50973091).
4. Conclusions A novel intumescent flame retardant named PSPTR has been synthesized successfully. The flame retardancy of ABS copolymer was improved by the addition of IFR containing PSPTR and PF. When the content of IFR (PSPTR:PF ¼ 1:1) was 30 wt%, the LOI value of ABS/IFR composites reached 28.2, and the vertical burning test reached UL-94 V-1 rating. In addition, a suitable amount of PF
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