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Oct 25, 2010 - P4O10 is highly deliquescent and reactive; techniques using non-P4O10 process are not only required for environmental protection, but a...
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A Novel Process to Prepare Ammonium Polyphosphate with Crystalline Form II and its Comparison with Melamine Polyphosphate Gousheng Liu,* Wenyan Chen, and Jianguo Yu State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, China

P4O10 is highly deliquescent and reactive; techniques using non-P4O10 process are not only required for environmental protection, but also for consideration of safety and cost in industrial-scale production. A nonP4O10 process to prepare ammonium polyphosphate with crystalline form II (APP-II) was proposed by heating the mixture of diammonium hydrogen phosphate (DAP) and melamine under wet ammonia. Its water solubility, FT-IR, XRD, and 31P NMR spectra were compared with those of commercial APP-II and melamine polyphosphate (MPoly), and interesting results were obtained. Thermogravometric analysis (TGA) showed the synthesized APP-II had better thermal stability than commercial APP-II. The reaction mechanism and molecular structure for the subject compound were investigated and discussed in detail. 1. Introduction Halogen-containing flame retardants are reported to be effective for various polymer materials. However, some of them are restricted due to their generating dense toxic smoke and corrosive products during combustion.1 Therefore, preparation of halogen-free flame retardants is the subject of extensive investigations, and has aroused a great deal of attention in recent years.2-5 Ammonium polyphosphate (APP) is a well-known component of halogen-free flame retardants. It has six crystalline forms;6,7 the commercially available forms are APP-I and APPII. APP-I has a high water solubility and is mainly used in coatings and fertilizers; APP-II is water insoluble and is an effective intumescent flame retardant for several kinds of polymer-based materials.8-11 Conventionally, APP-II is made by heating a mixture of ammonium orthophosphate, phosphorus pentoxide, and urea6,7,12 or by crystalline transformation from APP-I, orthophosphate, or pyrophosphate at 250-400 °C in a well covered container for more than 60 h.6,13 For the former process, phosphorus pentoxide must be used as condensing agent. However, phosphorus pentoxide is highly deliquescent and highly reactive and hence it is harmful to the human body, requiring more attention to operation safety in industrial-scale production. In addition, phosphorus pentoxide, which is produced from yellow phosphor ore by a dry process, has an industrial disadvantage that this high-temperature reduction consumes a large quantity of energy and yields large quantities of industrial wastes as byproduct. Nowadays, only fine chemicals such as electronic and pharmaceutical chemicals use phosphorus pentoxide as core raw materials; chemicals with phosphorus or polyphosphates are gradually being replaced by wet process phosphoric acid, named non-P4O10 process. For the later process, the long 60-h crystalline transformation time is tedious and not suitable for practical use at industrial scale. Few publications are found in the literature for preparation of APP-II by non-P4O10 process except those by Watanabe.14-17 He and his co-workers reported that APP-II was made by heating a mixture of ammonium orthophosphate and urea at 250-300 °C under wet ammonia which was made by passing air with a flow rate of 40 dm3/h through ammonia-water. The ammonia* To whom correspondence should be addressed. Tel/Fax: +86-2164250981. E-mail: [email protected].

water was exchanged to a new batch every 20 min to prevent lowering of the concentration of ammonia-water. Urea was used as condensation agent in Shen6 and Watanabe.14-17 Urea is decomposed at temperature of 160 °C; the expanding foam swelled and filled in the reactor at temperature of 280-320 °C. The linking together of two PO4 groups so as to obtain a P-O-P bond in the chain needs one molecule of urea, which decomposes into CO2 + NH3. Thus, the condensation of 1 mol orthophosphate to long chain material is accompanied by the evolution of 2 mols gas, which corresponds to a gas evolution rate of nearly 1 m3 per kg APP.18 As a result, the operation is hard to handle and the effective reaction space is decreased, a huge reactor would be required for industrial-scale production. For compensation of this disadvantage, a well-covered or pressure vessel is required.6,7 As can be predicted, the investment in pressure reactor would soar. It was found19,20 that melamine was an effective condensation agent in preparation of APP-V, and because the sublimation temperature of melamine was 300 °C, higher than the decomposition temperature 160 °C of urea, its expanding foam was not as fierce as urea under polymerization temperature of 280-320 °C. APP-II was successfully prepared by heating the mixture of diammonium hydrogen phosphate (DAP) and melamine with finely controlled conditions; no similar process was found in the literature. Comparing to the deliquescent and highly reactive P4O10, DAP is easy to handle in transportation and storage, and causes no harm to human body; the benefit and convenience are indeed obvious in industrial-scale production. Anyway, curiosity is aroused. The process to prepare APPII in this study is similar to that for melamine polyphosphate (MPoly)21 in which melamine and phosphoric acid are mixed and heated at high temperature for a certain time. MPoly is also an attractive halogen-free flame retardant used in recent years.22-24 The investigation into the reaction mechanism and molecular structure of APP-II and MPoly will help to understand the key technique for controllable synthesis of APP-II, which is the aim of this paper. 2. Materials and Methods 2.1. Materials. DAP was supplied by Sinopharm Chemical Reagent Co. Ltd., China; melamine was supplied Shanghai Lingfeng Chemical Reagent Co., Ltd.; Commercial APP-I and

10.1021/ie1014102  2010 American Chemical Society Published on Web 10/25/2010

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APP-II were supplied by Shanghai Xushen Nonhalogen Smoke Suppressing Fire Retardants Co., Ltd., with measured polymerization degrees of 34 and 994, respectively; Commercial MPoly was supplied by Zhengjiang Xingxin Chemical Factory, Jiangsu province. The polymerization degree of APP-II was measured by the Chinese chemical industrial standard “Ammonium polyphosphate for industrial use HG/T 2770-2008”. 2.2. Synthesis of APP-II. DAP (61.7 g) and melamine (4.9 g) were mixed and heated to 260-280 °C in a four-neck flask. After the mixture was melt, it was quickly transferred to a kneader. In this kneader, the temperature was kept at 280-320 °C, and wet ammonia was blown into the kneader. The wet ammonia was made by passing air with a flow rate of 2-4 L/min through ammonia-water, the concentration of ammonia-water was 3-25%, and was updated to a fresh batch for each batch experiment to prevent lowering of the concentration of ammonia. 2.3. Characterization. The Fourier transform infrared (FTIR) spectra were obtained using a Nicolet 6700 FT-IR spectrometer (Nicolet Instrument Company, USA) in the range of 400-4000 cm-1 by using the KBr disk method. The X-ray diffraction (XRD) patterns using Cu KR radiation (λ ) 1.542 Å) were performed with a powder D/MAX 2550 VB/PC diffractometer (Rigaku, Tokyo, Japan) at the scanning rate of 0.02° per second in the 2θ range of 10-50°. Thermogravimetric analysis (TGA) (approximately 10 mg) was carried out at a heating rate of 10 °C/min under air flow of 50 mL/min, over the range of 70-1000 °C by SDT Q600 (TA Co. USA). The 31 P MAS NMR spectra were collected at 202.46 MHz on a Bruker Avance 500 spectrometer using a 5-mm QNP probe. For all samples, a repetition time of 100 s and the spinning speed of 5 kHz were used. The reference used was 85% H3PO4 in aqueous solution. 3. Results and Discussion 3.1. Synthesis of APP-II and Comparison with MPoly. It was observed that when the mixture of DAP and melamine was heated to 160 °C, it began to melt. At 200 °C it turned to milklike stiff liquid. At 240 °C, slight foam was observed, and it turned to larger foam at 260 °C; the largest foam was observed at 280 °C. It was also observed that DAP began to decompose and release NH3 at 160 °C. One hour later, the melt was solidified, wet ammonia was blown into the kneader, the temperature was kept at 280-320 °C for another 1-2 h so that the crystalline transformation process was fully accomplished. Chen25 pointed out that melting was corresponding to the formation of a series of complicated complex salts, named melamine phosphate, melamine pyrophosphate, and melamine polyphosphate (n > 2), respectively. Under high temperature, DAP will partly condense by releasing NH3. The partly condensed phosphoric acid or phosphate can react with melamine to form melamine polyphosphate. Thus the formation of dimer, trimer, or melamine polyphosphoate (n > 2) is possible. Jahromi21 pointed out the composition of melamine phosphate, melamine pyrophosphate, and melamine polyphosphate can be quantitatively analyzed by means of solid state 31P NMR. It should be emphasized that the molar ratio of phosphoric acid to melamine was 1:1 in Jahromi21 and Chen,25 and the formed complex salt was melamine phosphate. But in this study, the molar ratio of DAP to melamine was as large as 8-15:1. As can be predicted, the excessive phosphate will react with all three NH2 groups in melamine, furthermore, the excessive phosphate will condense with each other to form a long chain or cross-linked polyphosphate chain. Thus, the structure of the

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Scheme 1. Structure of Complex Salt for Different Molar Ratios of Phosphoric Acid to Melamine: (a) Molar Ratio 1:1 in Literature, (b) Molar Ratio .1 in This Study

complex salt will be different depending on the ratio of DAP to melamine, as shown in Scheme 1. After the complex salt in Scheme 1 is formed, it turns to solid state. It was observed that the process of crystalline transformation and wet ammonia played a key role in preparation of APP-II. Without sufficient time for crystalline transformation and adequate wet ammonia, it was difficult to obtain good crystalline form of APP-II. The investigation into reaction mechanism of MPoly in stage of crystalline transformation will help to understand the reaction mechanism of APP-II. It was reported26 that there were two possible structures of MPoly when the formed complex salt a in Scheme 1 was subjected to heat treatment at high temperature of 280-320 °C, as shown in scheme 2. Brodski27 reported that the crystal structure of MPoly resembled those of melamine phosphate (MP) and melamine pyrophosphate (MPy), as all three compounds had a layered structure in which layers of melamine ribbons and phosphate chains alternated. The condensation from MPy to MPoly was a significant rearrangement of phosphate groups and melamine packing. It can be similarly inferred that there are two possible structures of APP-II when the formed complex salt b in Scheme 1 is heat treated, as shown in Scheme 3. Because DAP is the raw material in preparation of APP-II, the molecule in complex salt b in Scheme 1 is ammoniated, as shown in b1 and b2 in Scheme 3. As stated above, wet ammonia played a key role in the technique to prepare APP-II. It was observed that all the obtained XRD spectra were not APP-II if dry ammoniathe was used in stage of crystalline transformation. Wet ammonia seems to be necessary for the hydrolysis of ammonium ion into hydroxyl group and for the regulation to a thermodynamic stable structure. Without adequate moisture, ammonium ion cannot hydrolyze into hydroxyl group at high temperature of 280-320 °C, and condensation to long chain of polyphosphate seems to be difficult. Of course, the content of moisture and ammonia should be finely controlled, otherwise excessive ammonium ions are hydrolyzed into the hydroxyl group and the obtained product will not be APP-II. Which ammonium ion is hydrolyzed into hydroxyl group is of significance because it determines whether the linear or cross-linked polyphosphate chain is formed. When it is the end ammonium ion, the linear polyphosphate chain is expected, and when it is middle ammonium ion, the cross-linked polyphosphate chain is expected. The hydrolysis position of ammonium ion depends on the thermodynamic stability and sterically hindered structure of complex salt, which will be demonstrated in our next work by theoretical calculation. The possible reaction mechanisms of linear and cross-linked polyphosphate chains in crystalline transformation are shown in Scheme 4 and Scheme 5, respectively. Because the molar ratio of DAP to melamine is as great as 8-15:1, each branch in the melamine ring will be a long chain of ammonium polyphosphate. It is expected the ammonium

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Scheme 2. Possible Structure of MPoly: (a) Complex Salt, (a1) Dehydrated Form, (a2) Deammoniated Form

Scheme 3. Possible Structure of APP-II: (b) Complex Salt, (b1) Dehydration Form, (b2) Deammoniated Form

Scheme 4. Mechanism of Linear-Chain Propagation in Crystalline Transformation

polyphosphate chains may also react with each other to form a complicated and thermodynamic stable cross-linked structure at the stage of crystalline transformation under wet ammonia, as b6

in Scheme 5. Therefore, the molecular structure of APP-II will be a cross-linked polyphosphate19,20 chain spheroid, which is plausible speculation of an aggregated ring, as shown in Figure 1.

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Scheme 5. Mechanism of Cross-Linked Chain Propagation in Crystalline Transformation

Suggestions from crystal structure analysis27 also showed that sets of two (pyro)phosphate chains in melamine phosphate (and melamine pyrophosphate) can interact with each other; linear or cross-linked polyphosphate was thus formed. 3.2. Water Solubility. Water solubility is an essential property for its application in synthesized resins. High water solubility is a drawback when it is incorporated into synthetic resins and moldings produced from the synthetic resins, because it will hydrolyze and bleed out of the molding surface to considerably reduce the electrical surface resistance of the moldings. Water solubility of synthesized APP-II was tested by method HG/T 2770-2008, and compared with that of commercial APPII and MPoly, as shown in Table 1. The process of solubility measurement is referred to Liu et al.20 Table 1 shows that MPoly had the largest water solubility, and the synthesized APP-II is better than commercial APP-II. As is well-known, water solubility is the macroscopical reflection of polymer structure. The water solubility of APP-I is as

Figure 1. Possible molecular structure of synthesized APP-II. Table 1. Water Solubility of Synthesized APP-II, Commercial APP-II, and MPoly (g/100 mL Water) sample

solubility (g/100 mL water)

range (%) average

synthesized APP-II 0.38 0.26 0.28 0.07 0.12 0.05 0.05-0.38 commercial APP-II 0.22 0.31 0.28 0.37 0.43 0.27 0.22-0.43 commercial MPoly 0.72 0.65 0.75 0.82 0.77 0.79 0.65-0.82

0.19 0.31 0.75

large as 8.38 g/100 mL. Its speculated structure20 shows that APP-I exposed most of its ion bonds (-O--NH4+) including end side and its polyphosphate chain to outside, the ion bonds (-O--NH4+) are easily ionized by water, leading to its water solubility. For MPoly, its ion bond (-O-NH4+-) in a1 (Scheme 2) is interlined inside its molecule to form a laminated structure; water does not so easily enter the laminated structure as APPI, leading to its smaller water solubility than APP-I. Brodski27 once calculated the melamine-stacking distance in MPoly was 3.31 Å. The phosphates formed layers of polyphosphate anion chains, melamine cations formed hydrogen bonded ribbons with all molecules being parallel within one layer, and the melamine ribbons were linked to the polyphosphates through O-HN hydrogen bonds. a2 was another form of Mpoly, as pointed out by Wang,26 and this form of MPoly was water insolube due to the covalent bond P-O-C. The synthesized APP-II is a cross-linked polyphosphate. As shown in Figure 1, most of its ion bonds (-O--NH4+) are wrapped inside the crystal spheroid, these ion bonds wrapped inside the crystal spheroid are difficult to be ionized by water, leading to its water insolubility.19,20 3.3. FT-IR Spectra. FT-IR spectra of synthesized APP-II, commercial APP-II, and MPoly are shown in Figure 2. It can be seen that FT-IR spectrum of synthesized APP-II is the same as that of commercial APP-II, and different from that of MPoly. The FT-IR spectrum analysis for MPoly can be found in Wang,26 thus it is not narrated here. For the FT-IR spectra of synthesized APP-II and commercial APP-II, the region of 3400-3030 cm-1 corresponds to the asymmetric stretching absorption of NH4+, the region of 1430-1390 cm-1 corresponds to the bending absorption of NH+, and these two regions do not change with the crystalline form of APP. Both have vibration absorption peaks at 800 cm-1 and 1350-1100 cm-1. The peaks at 1100-850 cm-1 are assigned to stretching vibration of P-O-P, and peaks at 1350-1100 cm-1 are assigned to stretching vibration of PdO, all these peaks are typical featured bands of polyphosphate chains.5 The only difference between synthesized APP-II and commercial APP-II is that synthesized APP-II has a small vibration peak at 682 cm-1, while commercial APP-II does not. Generally, APP-I has characteristic peaks at 602, 682, and 762 cm-1; APPII has no peaks at these positions. As a result of the synthesis process, some APP-II may also have a small vibration peak at 682 cm-1. Yi28 pointed out that this peak was possibly caused by lattice defect during crystalline formation process, but his

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Figure 2. FT-IR spectra of synthesized APP-II, commercial APP-II, and MPoly.

Figure 3. XRD spectra of synthesized APP-II, commercial APP-II, and MPoly.

speculation was not confirmed by experiment. Based on this viewpoint, there is a possible tiny lattice defect in synthesized APP-II. Urea was used as condensation agent in commercial APP-II. It was decomposed into NH3 and CO2 at high temperature and released from APP-II, but molar ratio of 1:8-1:15 melamine used as condensation agent in this study was incorporated into APP-II with three of its branch connecting to ammonium polyphosphate chains, as shown in b4 in Scheme 4, b6 in Scheme 5, and in Figure 1. However, the synthesized APP-II presented lower intensities than the commercial APPII. This fact may be associated with the structure faults of synthesized APP-II or above-mentioned tiny lattice defect in synthesized APP-II, because melamine was used as condensation agent in this study. We are not sure how many triazine ring existed in the synthesized APP-II. Further study will be done to detect the triazine content in synthesized APP-II. There is a similar peak in FT-IR spectrum of MPoly near 682 cm-1. We are not sure whether this is the fingerprint peak

of triazine ring because seldom attention is paid to this position, although there are papers about the FT-IR spectrum analysis of triazine ring.29,30 3.4. XRD Spectra. The XRD spectra of synthesized APPII, commercial APP-II, and MPoly are shown in Figure 3. It can be seen that the XRD spectrum of synthesized APP-II is the same as that of commercial APP-II, and different from MPoly. The five strongest XRD diffraction lines in order of decreasing intensities [d (Å) ) 5.71, 6.03, 3.06, 2.92, 3.41 and 2θ ) 15.74°, 14.88°, 29.12°, 30.52°, 26.28°] are exactly the same as synthesized APP-II and commercial APP-II, meaning these two have the same crystalline structure. 3.5. 31P NMR Spectra. The 31P NMR spectra of synthesized APP-II, commercial APP-II, and commercial APP-I are shown in Figure 4. For the ordinary measurement of 31P NMR, 1.0 g of APP was weighed and mixed with 100 mL of water in a beaker, the

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Figure 4. (a) 31P NMR of synthesized APP-II. (b) 31P NMR of commercial APP-II. (c) 31P NMR of commercial APP-I.

mixture was heated in oil bath at temperature of 100 °C, and stirred for 10 min to a clear and transparent solution. Then, 0.4 mL of this test solution was taken out to a 5-mm NMR pipe together with 0.2 mL of D2O, deaerating and mixing well by ultrasound. Because APP can be hydrolyzed in hot water, though higher temperature is good for dissolution of APP which is the prerequisite for 31P NMR measurement, it is not good for polymerization degree measurement of APP. Therefore, a modified 31P NMR method31 was adopted in this study, 100 mL of water was replaced by 100 mL of 1 mol/L NaOH, APP was dissolved in D2O-NaOH solution because polyphosphate chain is stable in alkali solution.32 Comparison with the chemical shifts of ordinary liquid and solid 31P NMR, shift to a lower field was found for all species of phosphorus, the chemical shift of terminal phosphorus was at -2 to -4 ppm, the chemical shift of middle phosphorus was at -17 to -20 ppm, and the chemical shift of orthophosphate was at 4-6 ppm. As can be seen in Figure 4, signals at 5.315 ppm (Figure 4a), 5.998 ppm (Figure 4b), and 5.642 ppm (Figure 4c) are considered as the peaks of orthophosphate (PO4)3- anion for synthesized APP-II, commercial APP-II, and APP-I, respectively. Signals at -3.031 and -4.369 ppm (Figure 4a), -3.000 ppm (Figure 4b), -2.733 and -3.657 ppm (Figure 4c) are assigned

to the terminal phosphorus for synthesized APP-II, commercial APP-II, and APP-I, respectively. Signals at -18.489 and -19.522 ppm (Figure 4a), -18.615 ppm (Figure 4b), -18.248 and -19.341 ppm (Figure 4c) are assigned to the middle phosphorus for synthesized APP-II, commercial APP-II, and APP-I, respectively. It can be clearly seen that three species of phosphorus exist in synthesized APP-II, commercial APP-II and APP-I, but the integral for middle phosphorus in commercial APP-II is especially larger than in synthesized APP-II and APP-I, meaning the polyphosphate chain in commercial APP-II is more affluent than in synthesized APP-II and APP-I. This is because APP-I is a short polyphosphate chain, APP-II is a long polyphosphate chain,6,7 and synthesized APP-II is cross-linked polyphosphates as shown in Figure 1. There is less terminal phosphorus due to the cross-linkage by triazine ring in synthesized APP-II; the smallest integral of terminal phosphorus for synthesized APPII (0.1861, sum of -3.031 ppm and -4.369 ppm in Figure 4a) is proof. 3.6. TG Curves. TG analysis of synthesized APP-II, commercial APP-II, and MPoly are shown in Figure 5.

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Possibly due to the formed triazine ring in synthesized APP-II as shown in Figure 1, over the range of 300-600 °C, with the elimination of NH3 and H2O, transition to melam polyphosphate35 at 350 °C, melem polyphosphate at 450 °C, and a highly thermal stable and cross-linked (PNO)x structure,36 together with cross-linked -P-O-P- structure, may be the reason that synthesized APP-II is more thermal-degradation stable than commercial APP-II and MPoly. Of course, the flame retardant properties should be evaluated by LOI, UL-94, and flammability tests, which will be done in our next work. 4. Conclusions

Figure 5. TG curves of synthesized APP-II, commercial APP-II, and MPoly. Table 2. Weight Residuals at Different Stages of Synthesized APP-II, Commercial APP-II, and MPoly sample synthesizedAPP-II commercialAPP-II MPoly

∼300 °C (%) 300-600 °C (%) 600-700 °C (%) 97.85 97.39 87.09

64.99 50.51 43.85

24.86 37.05 32.72

The weight residuals at different thermal degradation steps are listed in Table 2. Before 300 °C, synthesized APP-II has weight residual of 97.85%, and commercial APP-II has weight residual of 97.39%, MPoly has weight residual of 98.25%. This weight loss was contributed to the thermally unstable structural groups existing in APP as stated by Camino.33 Figure 5 also shows that 10% weight loss for synthesized APP-II, commercial APP-II, and MPoly are obtained at 356.4, 351.2, and 384.5 °C, respectively, meaning MPoly has good initial thermal degradation stability, and synthesized APP-II and commercial APP-II have the same initial thermal degradation stability. Over the range of 300-600 °C, a key weight loss range to evaluate the performance of flame retardant, synthesized APPII has weight residual of 64.99%, commercial APP-II has weight residual of 50.51%, and MPoly has weight residual of 38.98%. Figure 5 also shows that 50% weight loss for synthesized APPII, commercial APP-II, and MPoly are acheived at 622.6, 600.9, and 550.0 °C, respectively. This weight loss was contributed to the branched or cross-linked polyphosphate chain with elimination of NH3 and H2O in the process of polyphosphate thermal degradation, implying synthesized APP-II is more thermally stable than commercial APP-II and MPoly over this range. Over the range of 600-700 °C, synthesized APP-II has weight residual of 24.86%, commercial APP-II has weight residual of 37.05%, and MPoly has weight residual of 30.99%. This weight loss was attributed to the release of phosphoric acid, polyphosphoric acid, and metaphosphoric acid with APP decomposition.34 Camino33 pointed out that elimination of NH3 and H2O in the process of APP thermal degradation can be divided into two steps: the first step was in the range of 165-280 °C, in which the limited amounts of NH3 and H2O eliminated from APP may be due to thermally unstable structural groups, NH3 eliminated in the first step was from chain ends. The second step was in excess of 290 °C, a process in which cross-linked polyphosphate was formed, with the elimination of NH3 and H2O and formation of a cross-linked -P-O-P- structure, transition to a more stable crystalline form may be involved.

APP-II was obtained by a non-P4O10 process, in which DAP and melamine were the main raw materials. There were two advantages of this process: First, P4O10 is not used in this process, so environmental protection, operation safety, convenience in transportation and storage, and cost control in industrialscale production are expected. Second, melamine is used as condensation agent instead of urea, less foam occurrs in period of condensation, large quantities of industrial production with affordable efficiency and production cost are thus possible. Most of its properties are similar to or better than commercial APPII, implying better flame retardant properties will be expected for the synthesized APP-II. Acknowledgment This work was supported by National Science & Technology Supporting Program of China No. 2007BAE58B03 and West Region Project of Shanghai Science and Technology No. 10195801600. Literature Cited (1) Lu, S. Y.; Hamerton, I. Recent developments in the chemistry of halogen-free flame retardant polymers. Prog. Polym. Sci. 2002, 27, 1661– 1712. (2) Wang, J. S.; Liu, Y.; Zhao, H. B.; Liu, J.; Wang, D. Y.; Song, Y. P.; Wang, Y. Z. Metal compound-enhanced flame retardancy of intumescent epoxy resins containing ammonium polyphosphate. Polym. Degrad. Stab. 2009, 94, 625–631. (3) Awad, W. H.; Wilkie, C. A. Investigation of the thermal degradation of polyurea: The effect of ammonium polyphosphate and expandable graphite. Polymer 2010, 51, 2277–2285. (4) Zhao, C. X.; Liu, Y.; Wang, D. Y.; Wang, D. L.; Wang, Y. Z. Synergistic effect of ammonium polyphosphate and layered double hydroxide on flame retardant properties of poly(vinyl alcohol). Polym. Degrad. Stab. 2008, 93, 1323–1331. (5) Nie, S. B.; Hu, Y.; Song, L.; He, Q. L.; Yang, D. D.; Chen, H. Synergistic effect between a char forming agent (CFA) and microencapsulated ammonium polyphosphate on the thermal and flame retardant properties of polypropylene. Polym. AdV. Technol. 2008, 19, 1077–1083. (6) Shen, C. Y.; Stahlheber, N. E.; Dyroff, D. R. Preparation and characterization of crystalline long-chain ammonium polyphosphates. J. Am. Chem. Soc. 1969, 91, 62–67. (7) Waerstad, K. R.; McClellan, G. H. Preparation and characterization of some long-chain ammonium polyphosphate. J. Agric. Food Chem. 1976, 24, 412–415. (8) Castrovinci, A.; Camino, G.; Drevelle, C.; Duquesne, S.; Magniez, C.; Vouters, M. Ammonium polyphosphate-aluminum trihydroxide antagonism in fire retarded butadiene-styrene block copolymer. Eur. Polym. J. 2005, 41, 2023–2033. (9) Schacker, O.; Wanzke, W. Compounding with ammonium polyphosphate-based flame retardants. Plast. Addit. Compd. 2002, 4, 28–30. (10) Duquesne, S.; Le Bras, M.; Bourbigot, S.; Delobel, R.; Camino, G.; Eling, B.; Lindsay, C.; Roels, T.; Vezin, H. Mechanism of fire retardancy of polyurethanes using ammonium polyphosphate. J. Polym. Sci. 2001, 82, 3262–3274. (11) Zhou, S.; Song, L.; Wang, Z.; Hu, Y.; Xing, W. Flame retardation and char formation mechanism of intumescent flame retarded polypropylene

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ReceiVed for reView July 7, 2010 ReVised manuscript receiVed September 26, 2010 Accepted October 10, 2010 IE1014102