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Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings
Polymer Degradation and Stability 75 (2002) 543–547 www.elsevier.com/locate/polydegstab
Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings Sheng-Wu Zhu, Wen-Fang Shi* State Key Laboratory of Fire Science and Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, PR China Received 17 July 2001; accepted 27 July 2001
Abstract A hyperbranched polyurethane acrylate containing phosphorus was found to display flame retardance with a limiting oxygen index of 27.0 after being UV cured. The retardant mechanism was investigated by observing the site where retardance take places and the synergistic effect of phosphorus and nitrogen. It is shown that the best synergistic effect happens when the phosphorus content is around 0.7 wt.%. The thermo-degradation process was monitored by in-situ FTIR. The P–O–C bonds break more easily than C–O–C bonds, forming P–O–P bonds. The morphological characteristics of the charred crust of the product were studied by scanning electron microscopy based on comparison with that of the urethane acrylate without phosphorus. The presence of phosphorus promotes the formation of compact char, which protects the underlying polymeric materials from further attack from flame or heating. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Halogen-free flame retardant; Mechanism; Polyurethane acrylate; UV curing
1. Introduction Halogen free flame retardants (HFFR) have received much attention to because of the absence of toxic gases and smokes during combustion compared with halogentype flame retardants. Intumescent systems containing phosphorus and nitrogen have become more and more attractive in various industrial applications in recent years. During combustion, the HFFR forms an expanding charred crust to protect the underlying polymeric material from further attack from flame or heating. The action of flame retardants can occur across both or either of the vapour phase and the condensed phase. Halogen-type flame retardants usually follow the vapour phase mechanism whilst intumescent systems containing phosphorus and nitrogen generally follow the condensed phase mechanism. However, combustion is a complex process; there may be different mechanisms with different flame retardants. There are reports that phosphorus-oxygen species (PO) in the vapour phase,
* Corresponding author. Tel.: +86-551-360-6084; fax: +86-551360-1616. E-mail address: [email protected] (W.F. Shi).
which will react with H radicals, can act as an active flame poison [1]. Hyperbranched Polyurethane acrylates containing phosphorus (HPUA-P) showing flame retardance have been synthesized and were reported elsewhere [2]. In this article, the site of the retarding activity and the synergistic effect of the phosphorus and nitrogen which contribute to the flame retardance were investigated by comparing the values of limiting oxygen index (LOI). The degradation of the resin, especially the segments containing phosphorus was monitored by in-situ FTIR. The contribution of phosphorus to the formation of the final charred crusts and their morphological structures were compared with that of the resin without phosphorus by scanning electron microscope (SEM) measurements.
2. Experimental 2.1. Materials Toluene-2,4-diisocyanate (TDI) was used as received. Hydroxylethyl acrylate (HEA) was dried over 4 A˚ molecular sieve before use. Methoxyphenol was used as an inhibitor. All the chemicals mentioned above were
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00257-9
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S.-W. Zhu, W.-F. Shi / Polymer Degradation and Stability 75 (2002) 543–547
purchased from the First Reagent Company of Shanghai, China. Dibutyltin dilaurate (DBTDL) used as a catalyst was purchased from the Third Reagent Company of Beijing, China. The aliphatic urethane acrylate with a molecular weight of around 1500, Ebecryl 270, was kindly supplied by UCB Chemicals, Radcure Specialties, Belgium. a-hydroxy-ketone (Iragcure 184), kindly supplied as a gratis sample by Ciba Specialty Chemicals, was used as a photo-initiator. 2.2. Sample preparation HPUA-P was prepared as reported elsewhere [2]. The schematic outline of the synthesis route is shown in Fig. 1. Aromatic urethane diacrylate, denoted ATA, was prepared by reacting TDI with 2 equivalents of HEA in the presence of 0.1 wt.% DBTDL and 1000 ppm methoxyphenol at 70 C until no peak was observed at 2270 cm1 in the FTIR spectrum. 2.3. UV curing The sample containing Irgacure 184 (2 wt.%) was heated in an oven at 70 C and stirred to get a homogeneous mixture, then exposed to a UV lamp (1 KW, made by Lantian Company, Beijing) at variable conveyor speeds in air. 2.4. Measurements The FTIR spectra of the samples were recorded with a Nicolet MAGNA-IR 750 spectrometer. The LOI values of the UV cured samples were measured using a ZRY-type instrument (made in China) with the sheets of 12063 mm3 according to the standard ‘oxygen index’ test ASTM D2863-77. The SEM micrographs of the charred layers were analysed by an X650 scanning electron microscopy (Hitachi X560 scanning electron micro-
analyser). The specimens were previously coated with conductive gold layer.
3. Results and discussion 3.1. Site of the retarding activity Ravey and co-workers used the technique called ‘‘composite bar test’’ to study the activity site of flame retardants [1]. Two sample bars, one plain, the other mixed with flame retardant, were split in half lengthwise. Two of these halves, one of each type, were joined along their cut faces and held together by a few turns of lightly wound cotton thread. The composite bar was then candle-like top ignited. If only the vapour phase mechanism is involved, the two halves of the composite bar should burn down at more or less the same rate, because the gaseous retardant enters rapidly uniformly throughout the flame as a consequence of the turbulence inside the flame. If the plain half burns much faster than the retarded one, the condensed phase mechanism is also involved. However, in this case, it is very difficult to know whether it acts solely or together with vapour phase mechanism. To overcome this defect, a technique that can distinctly indicate the site of the retarding activity was devised in this work. Five sample bars were prepared according to the following methods. The first two bars were prepared with HPUA-P and Ebecryl 270 which is flammable in air, named P and E, respectively. The later three bars named PE, P/E, and E/P, respectively, were all composed of the same weight of HPUA-P and Ebecryl 270. The PE sample bar was homogenous mixture of them, while the P/E and E/P sample bars were slim Ebecryl 270 bar wrapped with HPUA-P and the inverse. All these sample bars were made according to the requirements of the LOI test. Fig. 2 shows their cross-section.
Fig. 1. A schematic outline of the synthesis of HPUA-P.
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Fig. 2. The cross-sections of the sample bars.
values of LOI are listed in Table 1. The values of LOI as functions of the nitrogen and phosphorus contents are plotted in Fig. 3. The presence of nitrogen heightens the values of LOI, as easily seen by comparing those points with the same phosphorus content. It also can be seen from the trace of DA that, although the nitrogen content keeps decreasing, the value of LOI increases along with the phosphorus content. It means that both of these two compositions can separately increase the values of LOI. Fig. 4 shows the increments of LOI values (LOI) as a function of the increments of the total content of phosphorus and nitrogen (P+N), which are derived from traces of BA, CA and DA in Fig. 3. To facilitate comparison, both axes are normalized. The LOI1 means the LOI of B, C or D and LOI2 means that of A in Fig. 3. The LOI increases nearly linearly along with (P+N). The fastest increments of LOI values on three curves appear at those points with abscissa in the range of 0.2–0.3, which all correspond to the samples
If only the vapour phase mechanism is involved, the retardant should distribute throughout the flame even if it emits from a located position. As a result, the PE, P/E and E/P bars should have nearly the same values of LOI. If only the condensed phase mechanism is involved, the HPUA-P on the surface of the sample bars should form an expanding charred layer, which protects the material inside from the flame. The values of LOI should follow the order of P=P/E > PE > E/P=E, namely, the higher the HPUA-P percent on the surface, the larger the LOI value. If the two mechanisms act simultaneously, the vapour mechanism offers the same contribution to PE, P/E and E/P because the same weights percent of HPUA-P. The retardation of PE, P/E and E/P should be decided by the contribution of condensed mechanism. Comparing P with P/E, the inner material of P bar emits retardant gaseous while Ebecryl 270 slim bar inside P/E has no contribution. Therefore, the later should be less retardant. Similarly, E should be less retardant than E/P. The values of LOI should follow the order of P> P/E> PE> E/P> E. By comparing with the two ideal situations above we can judge which mechanism is more important. The values of LOI of P, P/E, PE, E/P, E tested were 27.0, 26.5, 22.8, 19.7, 19.5, respectively. There is only little difference between P and P/E as well as E/P and E. Therefore, we concluded that HPUA-P follows the condensed phase mechanism. 3.2. Synergistic effect of phosphorus and nitrogen Phosphorus- and nitrogen-containing flame retardants are generally intumescent systems. To investigate the synergistic effect, three series of samples containing different ratios of HPUA-P to ATA or/and Ebecryl 270 were prepared. The formations of the samples and their
Fig. 3. The relationship of the values of LOI with the amount of phosphorus and nitrogen.
Table 1 The composition and the values of LOI of the samples Sample (wt.%)
1
2
3
4
5
6
7
8
9
10
11
12
13
HPUA-P Ebecryl 270 ATA Pa Na LOI
100 0 0 2.8 5.8 27.0
75 25 0 2.1 5.3 24.8
50 50 0 1.4 4.8 22.8
25 75 0 0.7 4.3 21.5
0 100 0 0 3.7 19.5
75 0 25 2.1 6.1 26.5
50 0 50 1.4 6.4 26.0
25 0 75 0.7 6.6 25.5
0 0 100 0 6.9 24.5
75 12.5 12.5 2.1 5.7 25.5
50 25 25 1.4 5.6 24.0
25 37.5 37.5 0.7 5.4 23.0
0 50 50 0 5.3 21.0
a
Theoretical values.
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Fig. 4. The normalized curves of the relationship of LOI with total amount of P and N.
containing 0.7 wt.% of phosphorus but with different nitrogen contents. It demonstrates that the synergistic effect of phosphorus and nitrogen in this system is mainly affected by the former, especially with a concentration around 0.7 wt.%. 3.3. Thermal degradation To further investigate the retardant activity of HPUA-P, the thermal degradation was monitored by in-situ FTIR spectra shown in Fig. 5. The sample was heated from 180 to 300 C, kept a heating rate of 10 K/ min and held for 10 min at each testing temperature. The urethane groups kept degrading shown from the band of 3300–3500 cm1 corresponding to the urethane N–H bond, and from the band of 1726 cm1 corre-
Fig. 5. The in-situ FT–IR of the degradation process of HPUA-P.
sponding to amide-I and the carbonyl group of acrylate. Both of them nearly disappeared at 280 C. The stretching vibration band associated with the N–H bond kept shifting to higher frequency from 3315 to 3405 cm1 with the increment of the temperature. This implies the gradual transformation of H-bonded N–H groups to free N–H bonds with the increment of the temperature. This can also be demonstrated by the shift of the carbonyl group. The band of amide-I has been reported to change with the degree of the carbonyl Hbonding [3–5]. Bands near 1710 cm1 associate with the H-bonded carbonyl groups while those near 1730 cm1 associated with the free carbonyl groups [3–5]. From Fig. 5, the FTIR spectrum of 25 C has a broad band in the region of 1660–1800 cm1. When heated, the peak sharpened and the shoulder at 1710 cm1 disappeared. The peaks of 1008 and 1074 cm1 can be assigned to the stretching vibration of the P–O–C and the C–O–C bonds, respectively [6,7]. From Fig. 5, the P–O–C bond has already begun to decompose at 180 C and degraded completely before 250 C. The C–O–C bond decomposed much slower and nearly disappeared at 280 C. A new peak located at 1080 cm1 began to emerge from 240 C and became one of the major peaks in the spectrum of 300 C, which can be assigned to the symmetric stretching of P–O–P bond [8,9]. Another peak at 880 cm1 began to emerge from 250 C. It also presented as one of the main peaks at 300 C, which can be assigned to the asymmetric stretching of P–O–P bond [8,9]. 3.4. Morphologic structures of the charred crusts Compared with TAT, HPUA-P has a lower urethane segment content, still it has a higher value of LOI. This can be attributed to the soft segments of HPUA-P containing phosphorus. Moreover, HPUA-P formed an expanding charred layer while TAT did not expand during combustion. According to the discussion on the retarding action site above has demonstrated that no vapour mechanism works during the combustion. Therefore, the presence of phosphorus in polyurethane acrylates is necessary to form the intumescent system, which should be incorporated in the formed charred layer. Fig. 6 presents the SEM photographs of the charred crusts of HPUA-P and ATA. Fig. 6(a) is the crust of HPUA-P magnified 500 times. There are many leaf-like structures (indicated as A area) ‘‘come up’’ from the crust, from which it may be imaged that these structures expanded and ‘‘stood up’’ during the combustion. These leaf-like structures and those granulated ones (indicated as B area) were then magnified 6000 times shown in Fig. 6(b) and (c), respectively. The A area has a very compact structure like a wall and there are some small ‘‘carbon balls’’ stuck to the surface. Fig. 6(d) is the crust of ATA magnified 500 times. There are some relatively smooth structures presented (indicated as A0 area), and
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Fig. 6. SEM micrographs of the charred crusts of HPUA-P and ATA: crust of (a) HPUA-P500; (b) A area 6000; (c) B area 6000; crust of (d) ATA 500; (e) A0 area 6000; (f) B area 6000.
their total area is smaller than that of the A area in Fig. 6(a). The relatively smooth and granulated structures in Fig. 6(d) (indicated as B0 area) were magnified 6000 times and are presented in Fig. 6(e) and (f), respectively. Both are rougher than the corresponding smooth and granulated structures of HPUA-P. It demonstrates that the flame retardant mechanism of HPUA-P is the compact charred layer acting as a barrier, which is formed by the synergistic effect of phosphorus and nitrogen.
Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China, No. 20074034. The authors express gratitude to UCB Chemicals, Radcure Specialties, Belgium, for the sample of Ebecryl 270, and also express gratitude to Ciba Specialty Chemicals for the sample of Iragcure 184.
References 4. Conclusions The site of retarding action of HPUA-P is just located in the condensed phase. The phosphorus and nitrogen in this system have a synergistic effect to form an intumescent system, especially when 0.7 wt.% of phosphorus is present in the molecule. During combustion, the P–O–C bonds break fast, and form P–O–P structure in the residual char, which has a positive effect to form an expanding and compact charred crust to protect the underlying polymeric material from further attack from flame or heating.
[1] Ravey M, Keidar I, Weil ED, Pearce EM. J Appl Polym Sci 1998; 68:217–29. [2] Zhu SW, Kang L, Shi WF. Polym Int, in press. [3] Goddard RJ, Cooper SL. Macromolecules 1995;28:1390–400. [4] Yu-Min Tsai, Tzyy-Lung Yuu, Yu-Hsien Tseng. Polym Int 1998; 47:445–50. [5] Yu TL, Lin TL, Tsai YM, Liu WJ. J Polym Sci Part B Polym Phys 1999;37:2673–81. [6] Liu YN, Shu WG, Zeng DM. Chinese Nonferrous Metals 1998; 8:145–8. [7] Nakayama K, Ino T, Matsubara I. J Macromol Sci-Chem 1969; A3(5):1005–20. [8] Bugajny M, Bourbigot S. Polym Int 1999;48:264–70. [9] Le Bras M, Bourbigot S, Revel B. J Mater Sci 1999;34:5777–82.
Flame retardant mechanism of hyperbranched polyurethane acrylates used for UV curable flame retardant coatings Sheng-Wu Zhu, Wen-Fang Shi* State Key Laboratory of Fire Science and Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, PR China Received 17 July 2001; accepted 27 July 2001
Abstract A hyperbranched polyurethane acrylate containing phosphorus was found to display flame retardance with a limiting oxygen index of 27.0 after being UV cured. The retardant mechanism was investigated by observing the site where retardance take places and the synergistic effect of phosphorus and nitrogen. It is shown that the best synergistic effect happens when the phosphorus content is around 0.7 wt.%. The thermo-degradation process was monitored by in-situ FTIR. The P–O–C bonds break more easily than C–O–C bonds, forming P–O–P bonds. The morphological characteristics of the charred crust of the product were studied by scanning electron microscopy based on comparison with that of the urethane acrylate without phosphorus. The presence of phosphorus promotes the formation of compact char, which protects the underlying polymeric materials from further attack from flame or heating. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Halogen-free flame retardant; Mechanism; Polyurethane acrylate; UV curing
1. Introduction Halogen free flame retardants (HFFR) have received much attention to because of the absence of toxic gases and smokes during combustion compared with halogentype flame retardants. Intumescent systems containing phosphorus and nitrogen have become more and more attractive in various industrial applications in recent years. During combustion, the HFFR forms an expanding charred crust to protect the underlying polymeric material from further attack from flame or heating. The action of flame retardants can occur across both or either of the vapour phase and the condensed phase. Halogen-type flame retardants usually follow the vapour phase mechanism whilst intumescent systems containing phosphorus and nitrogen generally follow the condensed phase mechanism. However, combustion is a complex process; there may be different mechanisms with different flame retardants. There are reports that phosphorus-oxygen species (PO) in the vapour phase,
* Corresponding author. Tel.: +86-551-360-6084; fax: +86-551360-1616. E-mail address: [email protected] (W.F. Shi).
which will react with H radicals, can act as an active flame poison [1]. Hyperbranched Polyurethane acrylates containing phosphorus (HPUA-P) showing flame retardance have been synthesized and were reported elsewhere [2]. In this article, the site of the retarding activity and the synergistic effect of the phosphorus and nitrogen which contribute to the flame retardance were investigated by comparing the values of limiting oxygen index (LOI). The degradation of the resin, especially the segments containing phosphorus was monitored by in-situ FTIR. The contribution of phosphorus to the formation of the final charred crusts and their morphological structures were compared with that of the resin without phosphorus by scanning electron microscope (SEM) measurements.
2. Experimental 2.1. Materials Toluene-2,4-diisocyanate (TDI) was used as received. Hydroxylethyl acrylate (HEA) was dried over 4 A˚ molecular sieve before use. Methoxyphenol was used as an inhibitor. All the chemicals mentioned above were
0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(01)00257-9
544
S.-W. Zhu, W.-F. Shi / Polymer Degradation and Stability 75 (2002) 543–547
purchased from the First Reagent Company of Shanghai, China. Dibutyltin dilaurate (DBTDL) used as a catalyst was purchased from the Third Reagent Company of Beijing, China. The aliphatic urethane acrylate with a molecular weight of around 1500, Ebecryl 270, was kindly supplied by UCB Chemicals, Radcure Specialties, Belgium. a-hydroxy-ketone (Iragcure 184), kindly supplied as a gratis sample by Ciba Specialty Chemicals, was used as a photo-initiator. 2.2. Sample preparation HPUA-P was prepared as reported elsewhere [2]. The schematic outline of the synthesis route is shown in Fig. 1. Aromatic urethane diacrylate, denoted ATA, was prepared by reacting TDI with 2 equivalents of HEA in the presence of 0.1 wt.% DBTDL and 1000 ppm methoxyphenol at 70 C until no peak was observed at 2270 cm1 in the FTIR spectrum. 2.3. UV curing The sample containing Irgacure 184 (2 wt.%) was heated in an oven at 70 C and stirred to get a homogeneous mixture, then exposed to a UV lamp (1 KW, made by Lantian Company, Beijing) at variable conveyor speeds in air. 2.4. Measurements The FTIR spectra of the samples were recorded with a Nicolet MAGNA-IR 750 spectrometer. The LOI values of the UV cured samples were measured using a ZRY-type instrument (made in China) with the sheets of 12063 mm3 according to the standard ‘oxygen index’ test ASTM D2863-77. The SEM micrographs of the charred layers were analysed by an X650 scanning electron microscopy (Hitachi X560 scanning electron micro-
analyser). The specimens were previously coated with conductive gold layer.
3. Results and discussion 3.1. Site of the retarding activity Ravey and co-workers used the technique called ‘‘composite bar test’’ to study the activity site of flame retardants [1]. Two sample bars, one plain, the other mixed with flame retardant, were split in half lengthwise. Two of these halves, one of each type, were joined along their cut faces and held together by a few turns of lightly wound cotton thread. The composite bar was then candle-like top ignited. If only the vapour phase mechanism is involved, the two halves of the composite bar should burn down at more or less the same rate, because the gaseous retardant enters rapidly uniformly throughout the flame as a consequence of the turbulence inside the flame. If the plain half burns much faster than the retarded one, the condensed phase mechanism is also involved. However, in this case, it is very difficult to know whether it acts solely or together with vapour phase mechanism. To overcome this defect, a technique that can distinctly indicate the site of the retarding activity was devised in this work. Five sample bars were prepared according to the following methods. The first two bars were prepared with HPUA-P and Ebecryl 270 which is flammable in air, named P and E, respectively. The later three bars named PE, P/E, and E/P, respectively, were all composed of the same weight of HPUA-P and Ebecryl 270. The PE sample bar was homogenous mixture of them, while the P/E and E/P sample bars were slim Ebecryl 270 bar wrapped with HPUA-P and the inverse. All these sample bars were made according to the requirements of the LOI test. Fig. 2 shows their cross-section.
Fig. 1. A schematic outline of the synthesis of HPUA-P.
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Fig. 2. The cross-sections of the sample bars.
values of LOI are listed in Table 1. The values of LOI as functions of the nitrogen and phosphorus contents are plotted in Fig. 3. The presence of nitrogen heightens the values of LOI, as easily seen by comparing those points with the same phosphorus content. It also can be seen from the trace of DA that, although the nitrogen content keeps decreasing, the value of LOI increases along with the phosphorus content. It means that both of these two compositions can separately increase the values of LOI. Fig. 4 shows the increments of LOI values (LOI) as a function of the increments of the total content of phosphorus and nitrogen (P+N), which are derived from traces of BA, CA and DA in Fig. 3. To facilitate comparison, both axes are normalized. The LOI1 means the LOI of B, C or D and LOI2 means that of A in Fig. 3. The LOI increases nearly linearly along with (P+N). The fastest increments of LOI values on three curves appear at those points with abscissa in the range of 0.2–0.3, which all correspond to the samples
If only the vapour phase mechanism is involved, the retardant should distribute throughout the flame even if it emits from a located position. As a result, the PE, P/E and E/P bars should have nearly the same values of LOI. If only the condensed phase mechanism is involved, the HPUA-P on the surface of the sample bars should form an expanding charred layer, which protects the material inside from the flame. The values of LOI should follow the order of P=P/E > PE > E/P=E, namely, the higher the HPUA-P percent on the surface, the larger the LOI value. If the two mechanisms act simultaneously, the vapour mechanism offers the same contribution to PE, P/E and E/P because the same weights percent of HPUA-P. The retardation of PE, P/E and E/P should be decided by the contribution of condensed mechanism. Comparing P with P/E, the inner material of P bar emits retardant gaseous while Ebecryl 270 slim bar inside P/E has no contribution. Therefore, the later should be less retardant. Similarly, E should be less retardant than E/P. The values of LOI should follow the order of P> P/E> PE> E/P> E. By comparing with the two ideal situations above we can judge which mechanism is more important. The values of LOI of P, P/E, PE, E/P, E tested were 27.0, 26.5, 22.8, 19.7, 19.5, respectively. There is only little difference between P and P/E as well as E/P and E. Therefore, we concluded that HPUA-P follows the condensed phase mechanism. 3.2. Synergistic effect of phosphorus and nitrogen Phosphorus- and nitrogen-containing flame retardants are generally intumescent systems. To investigate the synergistic effect, three series of samples containing different ratios of HPUA-P to ATA or/and Ebecryl 270 were prepared. The formations of the samples and their
Fig. 3. The relationship of the values of LOI with the amount of phosphorus and nitrogen.
Table 1 The composition and the values of LOI of the samples Sample (wt.%)
1
2
3
4
5
6
7
8
9
10
11
12
13
HPUA-P Ebecryl 270 ATA Pa Na LOI
100 0 0 2.8 5.8 27.0
75 25 0 2.1 5.3 24.8
50 50 0 1.4 4.8 22.8
25 75 0 0.7 4.3 21.5
0 100 0 0 3.7 19.5
75 0 25 2.1 6.1 26.5
50 0 50 1.4 6.4 26.0
25 0 75 0.7 6.6 25.5
0 0 100 0 6.9 24.5
75 12.5 12.5 2.1 5.7 25.5
50 25 25 1.4 5.6 24.0
25 37.5 37.5 0.7 5.4 23.0
0 50 50 0 5.3 21.0
a
Theoretical values.
546
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Fig. 4. The normalized curves of the relationship of LOI with total amount of P and N.
containing 0.7 wt.% of phosphorus but with different nitrogen contents. It demonstrates that the synergistic effect of phosphorus and nitrogen in this system is mainly affected by the former, especially with a concentration around 0.7 wt.%. 3.3. Thermal degradation To further investigate the retardant activity of HPUA-P, the thermal degradation was monitored by in-situ FTIR spectra shown in Fig. 5. The sample was heated from 180 to 300 C, kept a heating rate of 10 K/ min and held for 10 min at each testing temperature. The urethane groups kept degrading shown from the band of 3300–3500 cm1 corresponding to the urethane N–H bond, and from the band of 1726 cm1 corre-
Fig. 5. The in-situ FT–IR of the degradation process of HPUA-P.
sponding to amide-I and the carbonyl group of acrylate. Both of them nearly disappeared at 280 C. The stretching vibration band associated with the N–H bond kept shifting to higher frequency from 3315 to 3405 cm1 with the increment of the temperature. This implies the gradual transformation of H-bonded N–H groups to free N–H bonds with the increment of the temperature. This can also be demonstrated by the shift of the carbonyl group. The band of amide-I has been reported to change with the degree of the carbonyl Hbonding [3–5]. Bands near 1710 cm1 associate with the H-bonded carbonyl groups while those near 1730 cm1 associated with the free carbonyl groups [3–5]. From Fig. 5, the FTIR spectrum of 25 C has a broad band in the region of 1660–1800 cm1. When heated, the peak sharpened and the shoulder at 1710 cm1 disappeared. The peaks of 1008 and 1074 cm1 can be assigned to the stretching vibration of the P–O–C and the C–O–C bonds, respectively [6,7]. From Fig. 5, the P–O–C bond has already begun to decompose at 180 C and degraded completely before 250 C. The C–O–C bond decomposed much slower and nearly disappeared at 280 C. A new peak located at 1080 cm1 began to emerge from 240 C and became one of the major peaks in the spectrum of 300 C, which can be assigned to the symmetric stretching of P–O–P bond [8,9]. Another peak at 880 cm1 began to emerge from 250 C. It also presented as one of the main peaks at 300 C, which can be assigned to the asymmetric stretching of P–O–P bond [8,9]. 3.4. Morphologic structures of the charred crusts Compared with TAT, HPUA-P has a lower urethane segment content, still it has a higher value of LOI. This can be attributed to the soft segments of HPUA-P containing phosphorus. Moreover, HPUA-P formed an expanding charred layer while TAT did not expand during combustion. According to the discussion on the retarding action site above has demonstrated that no vapour mechanism works during the combustion. Therefore, the presence of phosphorus in polyurethane acrylates is necessary to form the intumescent system, which should be incorporated in the formed charred layer. Fig. 6 presents the SEM photographs of the charred crusts of HPUA-P and ATA. Fig. 6(a) is the crust of HPUA-P magnified 500 times. There are many leaf-like structures (indicated as A area) ‘‘come up’’ from the crust, from which it may be imaged that these structures expanded and ‘‘stood up’’ during the combustion. These leaf-like structures and those granulated ones (indicated as B area) were then magnified 6000 times shown in Fig. 6(b) and (c), respectively. The A area has a very compact structure like a wall and there are some small ‘‘carbon balls’’ stuck to the surface. Fig. 6(d) is the crust of ATA magnified 500 times. There are some relatively smooth structures presented (indicated as A0 area), and
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Fig. 6. SEM micrographs of the charred crusts of HPUA-P and ATA: crust of (a) HPUA-P500; (b) A area 6000; (c) B area 6000; crust of (d) ATA 500; (e) A0 area 6000; (f) B area 6000.
their total area is smaller than that of the A area in Fig. 6(a). The relatively smooth and granulated structures in Fig. 6(d) (indicated as B0 area) were magnified 6000 times and are presented in Fig. 6(e) and (f), respectively. Both are rougher than the corresponding smooth and granulated structures of HPUA-P. It demonstrates that the flame retardant mechanism of HPUA-P is the compact charred layer acting as a barrier, which is formed by the synergistic effect of phosphorus and nitrogen.
Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China, No. 20074034. The authors express gratitude to UCB Chemicals, Radcure Specialties, Belgium, for the sample of Ebecryl 270, and also express gratitude to Ciba Specialty Chemicals for the sample of Iragcure 184.
References 4. Conclusions The site of retarding action of HPUA-P is just located in the condensed phase. The phosphorus and nitrogen in this system have a synergistic effect to form an intumescent system, especially when 0.7 wt.% of phosphorus is present in the molecule. During combustion, the P–O–C bonds break fast, and form P–O–P structure in the residual char, which has a positive effect to form an expanding and compact charred crust to protect the underlying polymeric material from further attack from flame or heating.
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