Ammonium Polyphosphate with Crystalline Form V by Ammonium

May 21, 2010 - Ammonium polyphosphate (APP) with crystalline form V (APP-V) was prepared through the reaction of ammonium dihydrogen phosphate and ure...
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Ind. Eng. Chem. Res. 2010, 49, 5523–5529

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Ammonium Polyphosphate with Crystalline Form V by Ammonium Dihydrogen Phosphate Process Gousheng Liu,* Xinchun Liu, and Jianguo Yu State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, China

Ammonium polyphosphate (APP) with crystalline form V (APP-V) was prepared through the reaction of ammonium dihydrogen phosphate and urea at high temperature and under dry ammonia atmosphere. Effects such as raw material ratios, ammoniation, condensation temperature, and condensation time were examined. Its water-solubility was tested and compared with those of industrial APP-I and APP-II. FTIR, XRD, and TGA techniques were used to characterize this product. The thermal degradation of APP-V was examined and compared with that of APP-I and APP-II; the molecular structure of this APP-V was speculated and discussed. 1. Introduction Ammonium polyphosphates (APP) are drawing attention in recent years as a flame retardant ingredient to be incorporated into synthetic resins. Various crystalline forms of ammonium polyphosphate are known; the two most common forms, form I (APP-I) and form II (APP-II), are prepared by conducting a thermal condensation reaction between a condensation agent and either phosphoric acid or phosphate.1,2 The physical properties of APP-I and APP-II are distinct from each other, they differ in their size of polyphosphate chain and crystalline structure; APP-I contains chains with average sequences of PO4 tetrahedron much shorter than APP-II, resulting in their distinct applications in the manufacture of intumescent flame retardant coatings and fertilizers. APP-I has a high water solubility, which is a drawback when it is incorporated into synthetic resin and moldings produced from the synthetic resin, APP-I will hydrolyze and bleed out the molding surface to considerably reduce the electrical surface resistance of the moldings. APP-II is water insoluble and is well-known as a flame retardant ingredient for use in synthetic resins.3-5 APP-V was once prepared by Shen6 by heating APP-I and APP-II in a sealed tube at 410 °C for about a day and then quenching in liquid nitrogen. Shen reported APP-V underwent rapid conversion to APP-II at temperatures in the neighborhood of 250-300 °C, and unless the cooling of APP-V was done at a high rate the final product was a mixture containing large amounts of APP-II. Dryoff7 obtained APP-V by heating the mixture of APP-I and APP-II to 342-420 °C under a high content of ammonia atmosphere, then the material was cooled rapidly to room temperature by being quenched in liquid nitrogen. This process of crystalline transformation was so slow that almost 20 h was required. Recently, Watanabe8 synthesized APP-V by heating a mixture of ammonium orthophosphate and urea to a temperature of 320-350 °C under wet ammonia which was prepared by passing purchased ammonia gas through 29% of aqueous ammonia. For the application of APP-V, Dyroff7 reported that APP-V can be employed as a major ingredient in metal polishes (particularly soft metals, for example silver), whereas it was not as good as APP-I and APP-II in fire-retardant coating compositions because the viscosity of the paint was undesirably * To whom correspondence should be addressed. Tel: +86-2164250981. Fax: +86-21-64250981. E-mail: [email protected].

high. On the contrary, the flame retardation effect of APP-II and APP-V was examined by measuring an oxygen index of polymer materials PP and PE by Watanabe.8 The results showed that APP-II and APP-V had a flame retardation effect for PE and PP and especially that APP-V has a high flame retardation effect for PP and that a content of 10% APP-V gave enough of an effect. The effect of APP-II was not as high as that of APPV. Though a conflict for application of APP-II and APP-V in flame retardant exists, APP-V is worthy of preparation by different processes so that further application studies can be examined. By heating the mixture of ammonium dihydrogen phosphate (ADP) and urea under dry ammonia atmosphere, we find a certain proportion of APP-V in the product; thus controllable synthesis with an aim to obtaining the largest proportion of APP-V is examined thoroughly, and the molecular structure of APP-V is speculated and discussed. 2. Materials and Methods 2.1. Materials. Ammonium dihydrogen phosphate (ADP) was supplied by Sinopharm Chemical Reagent Co. Ltd., China, urea was supplied by Shanghai Lingfeng Chemical Reagent Co. Ltd., industrial samples (APP-I and APP-II) were supplied by Shanghai Xushen Nonhalogen Smoke Suppressing Fire Retardants Co. Ltd., with a measured polymerization degree of APP-I equal to 34 and APP-II equal to 994. The polymerization degree of APP-I and APP-II was measured by the Chinese chemical industrial standard “Ammonium polyphosphate for industrial use HG/T 2770-2008”. 2.2. Synthesis of APP-V. A certain amount of ADP and urea was heated to 350-390 °C, then, dry ammonia was blown into the flask, the condensation process was controlled within a certain time and temperature. The solidification and crystalline transformation time was controlled at a given time and temperature, the reaction mixture was cooled down to room temperature, crushed and pulverized to less than 50 µm for subsequent use. 2.3. Characterization. The Fourier transform infrared (FTIR) spectra were obtained using a Nicolet 6700 FTIR 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

10.1021/ie100453a  2010 American Chemical Society Published on Web 05/21/2010

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Figure 1. XRD spectra of products in different mole ratios.

(Rigaku, Tokyo, Japan) at the scanning rate of 0.02° per second in the 2θ range of 10-50°. The thermogravimetric analyses (TGA) (approximately 10 mg) were carried out at a heating rate of 10 °C/min under N2 flow of 50 mL/min, over the range of temperature (70-100 °C) by SDT Q600 (TA Co., USA). 3. Results and Discussion 3.1. Preparation of APP-V. It was experimentally found that ammonium dihydrogen phosphate (ADP) was suitable for controllable synthesis of APP-V, but diammonium phosphate (DAP) and triammonium phosphate (TAP) were not suitable as raw materials, so ADP was chosen as the raw material in this study. APP with controllable crystalline form V was prepared by the reaction of ADP with urea in the presence of dry ammonia with certain raw material ratios, ammonia aeration, condensation temperature, and condensation time, which are discussed below. To indentify the assignment of XRD peaks of the prepared products, the XRD peaks are compared with standard XRD data of APP I-VI in ref 8, especially APP-I, APP-II, and APP-V. 3.1.1. Effects of Raw Material Ratios. The weights of ADP/ urea were (g/g) 4.60/3.00, 5.75/3.00, 6.9/3.00 and 8.63/3.00; their mole ratios of ADP to urea were 0.8:1, 1:1, 1.2:1, and 1.5:1, respectively. The mixture was heated to 380-390 °C and maintained at this temperature for 20 min. Dry ammonia was blown into the mixture at a rate of 0.5 L/min and maintained for solidification time of 60 min. The XRD spectra are shown in Figure 1. Figure 1 showed that the characteristic peaks (2θ ) 12.97°, 15.82°, 24.22°, 26.07°, and 31.83°) of APP-V were present in all XRD spectra of different mole ratios, but peaks (peaks 0-2 and 7-8) not belonging to APP-V were also present with an increase of mole ratios. According to characteristic peak analysis in the literature,6,9 peaks 0-8 present in Figure 1 can be attributed to the crystalline forms as listed in Table 1. From Figure 1 and Table 1 detailed peak analysis of d, 2θ, and intensities were obtained and are listed in Table 2. Table 2 showed that peaks 7 and 8, which were trivial at small mole ratios, were enlarged with increasing mole ratios, meaning large mole ratios would lead to formation of APP-I/ II. The mole ratio of 0.8-1.0 seems to be favorable for the preparation of a large proportion of APP-V, in which peaks not belonging to APP-V were at low intensities. 3.1.2. Effects of Ammoniation. Different flow rates of ammonia were studied. Experiment no. 13: the mixture was heated to 340-350 °C and maintained at this temperature for 20 min. Dry ammonia

was blown into the mixture at rate of 4 L/min, and then the mixture was maintained at temperature of 260-270 °C for solidification time of 60 min to yield product 13. Experiment no. 14: the flow rate of dry ammonia was 3 L/min, and the mixture was maintained at temperature of 300-310 °C for solidification time of 60 min to yield product 14. Experiment no. 15: the flow rate of dry ammonia was 2 L/min, and the mixture was maintained at temperature of 300-310 °C for solidification time of 60 min to yield product 15. Experiment no. 16: the flow rate of dry ammonia was 1.5 L/min, and the mixture was maintained at temperature of 310-320 °C for solidification time of 60 min to yield product 16. Experiment no. 17: the flow rate of dry ammonia was 1.0 L/min, and the mixture was maintained at temperature of 340-350 °C for solidification time of 60 min to yield product 17. Experiment no. 18: the flow rate of dry ammonia was 0.8 L/min, and the mixture was maintained at temperature of 340-350 °C for solidification time of 60 min to yield product 18. It was found that XRD spectra of products 13-15 were similar to each other, and XRD spectra of products 16-18 were also similar to each other. For simplification, the XRD spectra of products 14 and 18 were shown in Figure 2 and compared with that of APP-V in the literature.10 Figure 2 showed that the XRD spectrum of product 18 was similar to that of APP-V in the literature;10 its peak positions and intensities were similar to that of the XRD spectrum in literature,10 but peaks not belonging to APP-V were also present, even with large intensities as in product 14. Detailed peak analysis of d, 2θ, and intensities are listed in Table 3. Table 3 showed that peaks 1-8 were enlarged with increasing flow rate of ammonia, and much higher than corresponding values of APP-V in literature.6,9 The presence of these peaks implied the product was blended with some proportions of AppI, APP-II, and APP-VI, meaning it was a transitional state. It was considered that the XRD spectra deviation between product 14 and product 18 may be caused not only by ammoniation, but also by temperature fluctuations with deviations of 50-60 °C. The solidification temperature was 300-310 °C in experiment 14 with high flow rate of ammonia, while it was 340-350 °C in experiment 18 with low flow rate of ammonia. With an increasing of flow rate of ammonia, temperature stability was actually difficult to control. Two aspects may be thus inferred: first, the blowing of ammonia not only supplied ammonia atmosphere for the reaction, but also removed heat of formation, H2O, CO2, NH3, or other dehydrated substances of small molecular weight; second, the crystalline form of APP-V was sensitive to temperature. 3.1.3. Effects of Condensation Temperature. Different condensation temperatures were studied. The weights of ADP/ urea were 5.75 g/3.00 g, the mole ratio of ADP to urea was 1:1, the flow rate of ammonia was 1.2-1.4 L/min. Experiment no. 19: the mixture was maintained at condensation temperature of 330 °C for 60 min and for solidification time of 60 min to yield product 19. Experiment no. 20: the mixture was maintained at condensation temperature of 350 °C for 60 min and for solidification time of 60 min to yield product 20.

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Table 1. Peak Analysis for Peaks 0-8 in Figure 1 XRD data in literature6,9 peaks peak peak peak peak peak peak peak peak

d (Å)

0 1 2 3 4 5 7 8

2θ (deg)

7.27-7.31 6.03-6.06 3.05 6.62 3.76 5.36 3.23 2.92

29.14

APP-V in literature6,9 I (%)

crystalline forms

71 100/55 45 100 67 89 19-32 25

pyrophosphate APP-I/II APP-II APP-VI APP-VI APP-VI APP-I/II APP-II

d (Å)

3.07

2θ (deg)

I (%)

14.72 29.23 13.43 23.73

4 11 7 3

27.2 30.28

3 3

Table 2. Peak Analysis of d, 2θ, and Intensity for Products at Different Mole Ratios 0.8:1

peak peak peak peak peak peak peak

1 2 3 4 5 7 8

1:1

1.2:1

1.5:1

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

5.96 3.04 6.53 3.73 5.28 3.22 2.90

14.84 29.34 13.56 23.84 16.78 27.66 30.76

9 13 7 11 18 7 6

6.01 3.06 6.59 3.75 5.37 3.24 2.91

14.72 29.20 13.42 23.7 16.5 27.50 30.60

6 12 6 8 20 5 2

5.98 3.06 6.54 3.74 5.34 3.23 2.91

14.80 29.20 13.52 23.78 16.58 27.58 30.68

16 23 5 19 7 7 9

5.99 3.05 6.55 3.79 5.37 3.22 2.91

14.78 29.24 13.50 23.44 16.48 27.60 30.68

27 32 5 20 14 17 12

Table 3. Peak Analysis of d, 2θ, and Intensity for Products 14, 18, Compared with That of APP-V in the Literature6,9 product 14

APP-V characteristic peaks

peak peak peak peak peak peak peak

1 2 3 4 5 7 8

product 18

APP-V in literature10

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

6.83 5.58 3.67 3.41 2.81 6.00 3.05 6.57 3.75 5.30 3.23 2.91

12.96 15.86 24.26 26.08 31.86 14.76 29.22 13.46 23.72 16.72 27.58 30.66

44 100 47 71 36 22 18 19 21 33 13 9

6.80 5.58 3.66 3.41 2.80 5.98 3.05 6.56

13.00 15.88 24.28 26.12 31.90 14.8 29.24 13.48

36 100 44 65 28 14 18 6

6.86 5.61 3.67 3.43 2.81

12.97 15.82 24.22 26.07 31.83 14.72 29.23 13.42 23.73

54 100 60 75

5.37 3.23 2.91

16.48 27.62 30.71

2 10 7

27.20 30.28

3 3

Experiment no. 21: the mixture was maintained at condensation temperature of 370 °C for 60 min and for solidification time of 60 min to yield product 21. Experiment no. 22: the mixture was maintained at condensation temperature of 390 °C for 60 min and for solidification time of 60 min to yield product 22. Experiment no. 23: the mixture was maintained at condensation temperature of 430 °C for 60 min and for solidification time of 60 min to yield product 23.

Figure 2. XRD spectra of products 14 and 18 compared with that of APP-V found in the literature.10

3.06

4 11 7 3

XRD spectra of products 19-23 are shown in Figure 3. Figure 3 showed that peaks 1 and 2, which were attributed to APP-I/II according to table 1, were at low intensities when temperature was 350-390 °C. Detailed peak analysis of d, 2θ and intensities are listed in Table 4. Table 4 showed that peaks 3-5 varied drastically, these peaks were all at low intensities when condensation temperature were below 390 °C. It was considered that crystalline transformation must occur when condensation temperatures were above 400 °C, because the characteristic peaks of APP-V diminished sharply (2θ ) 12.97°, 26.04°) or even disappeared (2θ ) 12.97°). At the same time, the characteristic peaks of APP-VI (peaks 3-5) were enlarged and even became strong main peaks, which means that a high condensation temperature will cause a transition from APP-V to APP-VI. If it were not for the heating equipment restriction, stable APP-VI might be obtained by further increasing the condensation temperature. However, this experiment did verify that the higher condensation temperature of 350-390 °C was favorable for the preparation of APP-V. Of course, some proportions of APP-I/II and VI were still present in this APP-V. 3.1.4. Effects of Condensation Time. Different condensation times were studied. The weights of ADP/urea were 5.75 g/3.00 g, the mole ratio of ADP to urea was 1:1, and the flow rate of ammonia was 1.2-1.4 L/min. Experiment no. 24: the mixture was maintained at condensation temperature of 370 °C-390 °C, dry ammonia was blown

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Table 4. Peak Analysis of d, 2θ, and Intensity for Products 19-23 and Compared with That of APP-V in the Literature6,9 product 19

Peak Peak Peak Peak Peak

1 2 3 4 5

product 20

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

6.03 3.06 6.61 3.75 5.41

14.68 29.14 13.38 23.66 16.38

20 15 2 5 12

5.96 3.04 6.53 3.73 5.34

14.86 29.34 13.54 23.80 16.60

14 10 8 14 15

5.91 3.03 6.48 3.71 5.29

14.98 29.46 13.68 23.98 16.76

11 9 6 9 16

product 22

peak peak peak peak peak

1 2 3 4 5

product 21

d (Å)

product 23

APP-V in literature

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

5.93 3.04 6.50 3.72 5.25

14.92 29.36 13.62 23.88 16.86

7 9 19 11 9

5.93

14.92

14

d (Å) 3.06

6.50 3.72 5.27

13.62 23.90 16.78

40 24 42

2θ (deg)

I (%)

14.72 29.23 13.42 23.73

4 11 7 3

Table 5. Peak Analysis of d, 2θ, and Intensity for Products 24-28 and Compared with That of APP-V in Literature6,9 product 24

peak peak peak peak peak

1 2 3 4 5

product 25

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

5.93 3.04 6.48 3.71 5.31

14.92 29.40 13.66 23.94 16.67

13 19 4 10 11

6.01 3.06 6.59 3.75 5.37

14.72 29.20 13.42 23.70 16.50

6 12 6 8 24

5.96 3.04 6.53 3.73 5.27

14.86 29.32 13.54 23.80 16.82

7 7 14 13 18

d (Å)

product 27

peak peak peak peak peak peak

1 2 3 4 5 6

product 26

d (Å)

product 28

APP-V in literature

d (Å)

2θ (deg)

I (%)

d (Å)

2θ (deg)

I (%)

5.93 3.03 6.50 3.71 5.25 3.44

14.92 29.42 13.62 23.94 16.88 25.88

9 7 20 24 29 40

6.00 3.05 6.55 3.74 5.34 3.45

14.76 29.24 13.50 23.76 16.60 25.78

4 2 44 32 52 52

3.07

2θ (deg)

I (%)

14.72 29.23 13.43 23.73

4 11 7 3

into the mixture at once, and maintained at this temperature for solidification time of 60 min to yield product 24. Experiment no. 25: the mixture was maintained at condensation temperature of 370 °C-390 °C, dry ammonia was blown into the mixture after condensation time of 10 min, and maintained at this temperature for solidification time of 60 min to yield product 25. Experiment no. 26: dry ammonia was blown into the mixture after condensation time of 20 min, other conditions were the same as no. 24 to yield product 26. Experiment no. 27: dry ammonia was blown into the mixture after condensation time of 30 min, other conditions were the same as no. 24 to yield product 27.

Experiment no. 28: dry ammonia was blown into the mixture after condensation time of 50 min, other conditions were the same as no. 24 to yield product 28. XRD spectra of products 24-28 were shown in Figure 4. Figure 4 showed that characteristic peaks of APP-V (2θ ) 12.97°, 15.82°, 24.22°, 26.07°, and 31.83°) were present in a product when the condensation time was shorter than 30 min. Detailed peak analysis of d, 2θ, and intensities are listed in Table 5. Table 5 show that characteristic peaks of APP-I/II (peaks 1 and 2) decrease with prolongation of condensation time, meaning that longer condensation time was favorable for control of APP-I/II at low intensities. Unfortunately, characteristic peaks

Figure 3. XRD spectra of products 19-23.

Figure 4. XRD spectra of products 24-28.

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Table 6. Characteristic Peak Analysis of d, 2θ, and Intensity for Optimal Prepared APP-V and Compared with That of APP-V in the Literature6,9 APP-V in literature9

APP-V in literature6

APP-V in this study

2θ (deg)

I (%)

d (Å)

I (%)

d (Å)

2θ (deg)

I (%)

15.85 26.07 12.97 24.22 31.83 33.35 38.22

100 75 66 50 25 22 14

5.61 3.43 3.67 6.86 2.69 2.81 2.36

100 75 60 54 19 15 13

5.54 3.40 6.77 3.65 2.80 2.68 2.35

15.98 26.18 13.06 24.36 31.94 33.42 38.30

100 57 36 39 26 16 11

Table 7. Water Solubility for APP-I, APP-II, and APP-V products

Figure 5. XRD spectra of optimal prepared APP-V and compared with that in the literature.

of APP-VI (peaks 3-6) increased with prolongation of condensation time. It was found that peaks 3-6 increased rapidly with prolongation of condensation time, especially when the time was longer than 30 min. When condensation time was longer than 50 min, the intensities of peaks 3-6 were even stronger than those of the APP-V peaks, which means that a longer condensation time was not favorable for APP-V. 3.1.5. Optimal Preparation of APP-V. Further studies confirmed that it was difficult to diminish all peaks not belonging to APP-V; some APP-VI was more or less blended in the products. However, the five strongest characteristic peaks were strictly identical to that of APP-V in the literature.6,9 There were two advantages of this controllable synthesis process in which APP-V was the largest proportion: first, a rapid cooling process such as quenching in liquid nitrogen was no longer required; second, the whole reaction time was shortened to 2 h, far less than 20 h reported in the literature, and therefore large quantities of industrial production with affordable efficiency and production cost were thus possible. As a new compound, no available JCPDS card for APP-V was found, and it was difficult to find any standard XRD spectrum of APP-V in literature. Figure 5 shows the optimal prepared APP-V XRD spectra compared to one found in the literature.10 The figure shows that characteristic peaks and intensities of the optimal prepared product were exactly identical to those of APP-V in the literature,10 which means that the optimal prepared product was exactly in the crystalline form of APP-V. As stated above, it was difficult to obtain pure APP-V by this process because APP-VI was blended more or less in optimal prepared APP-V. Unfortunately, those peaks not belonging to APP-V were also found in APP-V in the literature10 as shown in Figure 5, which means that APP-V in the literature was actually not pure, either. Characteristic peak analysis of d, 2θ, and intensities for optimal prepared APP-V were listed in Table 6. Table 6 showed that optimal prepared APP-V had the same interplanar spacing (d value) and peak positions (2θ) with a deviation of only (0.2° and (0.05, respectively, which means that optimal prepared APP-V was certainly synthesized by this process. It was considered that the deviation of interplanar spacing (d value) and peak positions (2θ) may be caused by heterogeneous heating or stirring in the crystalline formation process, leading to interplanar spacing compression or tension. Actually, some peaks not belonging to APP-V were also present in the literature6,9 as listed in Table 1. These peaks are

APP-I APP-II APP-V

solubility (%)

range (%) average (%)

7.60 8.80 9.20 8.20 8.57 7.90 7.5-9.5 0.22 0.31 0.28 0.37 0.43 0.27 0.2-0.4 0.74 0.78 0.70 0.77 0.73 0.79 0.7-0.8

8.38 0.31 0.75

obtained from preparation methods that are as long as 20 h or that require rapid cooling such as quenching in liquid nitrogen. 3.2. Water Solubility. The water solubility of APP-V was not known from any literature, but whether APP-V is used as a major component in metal polishes7 or as a major component in intumescent flame retardant,8 its water solubility is an essential property for further applications. The water solubility of optimal prepared APP-V was tested by method HG/T 27702008 and compared with that of APP-I and APP-II, as shown in Table 7. The process of solubility measurement is as follows: 10 ( 0.0002 g of APP is weighed and mixed with 100 mL of water in a beaker. The mixture is stirred on water bath at 25 ( 2 °C for 20 min, and then it is separated in a centrifuge at a speed of 2000 rev/min for 20 min. A 20 mL portion of the upper clear liquid is taken out into a beaker and is dried at (160 ( 5) °C until constant weight. The solubility (g/100 mL) is calculated by the following equation: F)

m1 - m2 100 20

where m1 is total weight (g) of water solution and beaker and m2 is weight (g) of the beaker. The deviation must be less than 0.1 g/100 mL for two parallel measurements. Table 7 shows that water solubility sequences were APP-I > APP-V > APP-II. As is well-known, water solubility is the macroscopical reflection of polymer structure. Usually, APP-I is ammonium polyphosphate with a low polymerization degree of 30-50 and can be easily dissolved in water.11 APP-II is ammonium polyphosphate with polymerization degree of higher than 1000 and is difficult to be dissolved in water. In the molecular structure of ammonium polyphosphate, cross-linked condensed polyphosphates involve many branching points, there are ion bonds (-O--NH4+) at the end side of each polyphosphate chain and bonds -P-O-P- at branching point. The branching point (-P-O-P-) is not sensitive to water attacking. On the contrary, ion bonds (-O--NH4+) at the end side of each polyphosphate chain are easily broken and dissolved by water attacking, leading to the solubility of the polyphosphate. Because APP-I is a structure of linear polyphosphate chains, we speculate that most of its ion bonds (-O--NH4+) in the linear polyphosphate chain (APP-I) are exposed outside the crystal, leading to its water solubility. And because APP-II is a structure of cross-linked condensed polyphosphates, we speculate that most of its ion bonds (-O--NH4+) are wrapped inside the crystal spheroid. These ion bonds wrapped inside the crystal spheroid are not easily

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Figure 6. FTIR spectra for APP-I, APP-II, and APP-V.

attacked by water, leading to water insolubility. Of course, other physicochemical evidence for the speculation is required, which will be investigated in our future study. The solubility of optimal prepared APP-V was a little larger than APP-II, and was far less than APP-I, which means that the molecular structure of APP-V was close to APP-II. 3.3. FTIR Analysis. Similarly, the FTIR of APP-V was not known from any literature. The FTIR spectra of optimal prepared APP-V and that of APP-I and APP-II are shown in Figure 6. The region of 3400-3030 cm-1 corresponded to the asymmetric stretching absorption of NH4+, and the region of 1430-1390 cm-1 corresponded to the bending absorption of NH+. These two regions did not change with the crystalline form of APP. Both APP-I and APP-II had vibration absorption peaks at 800 cm-1 and 1350-1100 cm-1. The peaks at 1100-850 cm-1 were assigned to the stretching vibration of P-O-P, and the peaks at 1350-1100 cm-1 were assigned to the stretching vibration of PdO, all these peaks were typical featured bands of polyphosphate chains5 and were not related to crystalline forms. Figure 6 also showed that the FTIR spectrum of APP-V was more similar to that of APP-II than to the spectrum of APP-I, the only difference being that APP-V had a small vibration peak at 682 cm-1, while APP-II had not. As pointed out in the literature,6 the FTIR difference between APP-I and APP-II was that APP-I had peaks at 602, 682, and 760 cm-1, while APP-II had not. However, Yi12 pointed out that the absorption peak at 682 cm-1 was caused by a lattice defect during the crystalline formation process, and it should not be a characteristic peak of APP-I. Some APP-II spectra may also exhibit a peak at 682 cm-1 if there was any lattice defect in APP-II. If the speculation of Yi12 is correct, the absorption peak at 682 cm-1 for APP-V should be also considered as a lattice defect within its crystalline form. As was shown in Figure 5, APP-VI was blended more or less in optimal prepared APP-V, leading to its lattice defect. 3.4. TG Analysis. TG analysis of optimal prepared APP-V was shown in Figure 7, TG curves of APP-I and APP-II were also shown in Figure 7. The weight loss at different thermal degradation stages were listed in Table 8. Table 8 showed that APP-II had the least weight loss of 2.42% before 300 °C, APP-I had a weight loss of 4.93%, and APP-V had a weight loss of 4.30%. This weight loss may be attributed to the thermally unstable structural groups that exist in APP as stated by Camino.11 Over the range of 300-450 °C, a key weight loss range for evaluating the performance of flame retardant, the weight loss reached 17.70% for APP-I, 16.45% for APP-II, and only 10.28% for APP-V. This weight loss may

Figure 7. TG curves for APP-I, APP-II, and APP-V. Table 8. Weight Loss at Different Stages for APP-I, APP-II, and APP-V samples APP-I APP-II APP-V in this study

∼300 °C (%) 300-450 °C (%) 450-700 °C (%) 4.93 2.42 4.30

17.70 16.45 10.28

37.46 45.26 53.43

be attributed to the branched or cross-linked phosphate chain with elimination of NH3 and H2O in the process of APP thermal degradation, implying that APP-V may be a thermally more stable than APP-I and APP-II. These results seem to explain the results of Watanabe8 where it was found that APP-V has a high flame retardation effect for PP, a content of 10% APP-V was enough for this effet, and the effect of APP-II was not so high as that of APP-V. Over the range of 450-700 °C, the weight loss reached 37.46% for APP-I, 45.26% for APP-II, and 53.43% for APPV. This weight loss may be attributed to the release of phosphoric acid, polyphosphoric acid, and metaphosphoric acid during APP decomposition.13 Figure 7 also shows that 10% weight loss for APP-I, APPII, and APP-V occurred at 324, 356, and 332 °C, respectively, which means that APP-II had the best stability for initial thermal degradation, the second was APP-V, and the worst was APP-I. Theoretically, TG performances provide an external explanation of the internal structures of the AAPs. APP-I is a linear polyphosphate chain structure with a low polymerization degree of 30-50; APP-II is a branched polyphosphate chain structure with a polymerization degree of higher than 1000. Results from the water solubility tests implied that APP-V should have a similar structure to APP-II. Camino11 pointed out that the elimination of NH3 and H2O in the process of APP thermal degradation can be divided into two steps: the first step occurred in the range of 165-280 °C, where 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 at an excess of 290 °C, where a process occurred 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. 3.5. Molecular Structure Speculation. The principle in the above explanation about APP thermal degradation along with the water solubility data gave us a hint that the structure of

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be a more thermodynamically stable structure than APP-I. The thermodynamics analysis is in agreement with TG experiment above that APP-II had better stability for initial thermal degradation than did APP-I. 4. Conclusions Controllable synthesis with the most proportion of APP-V was examined by heating a mixture of ammonium dihydrogen phosphate and urea at optimal reaction conditions. There were two advantages to this process: First, a rapid cooling process such as quenching in liquid nitrogen was no longer required; second, the whole reaction time was shortened to 2 h, far less than 20 h reported in literature and therefore large quantities of industrial production with affordable efficiency and production costs were thus possible. The XRD, FTIR, TG and water solubility of APP-V were characterized and tested, and speculation that the structure of APP-V was a cross-linked polyphosphate chain spheroid was pointed out. Acknowledgment

Figure 8. The molecular structures of APP-II and APP-V.

APP-II would be far more than the branched structure given in the literature.6,11 We speculated (this will be discussed in the following study by GPC and 31P NMR) that APP-II should be a cross-linked polyphosphate chain spheroid with most of its ion bonds (-O--NH4+) wrapped inside the crystal (because only the very few ion bonds (-O--NH4+) exposed outside the crystal spheroid can be dissolved) leading to its insolubility. The speculated molecular structure of APP-II is shown in Figure 8. Furthermore APP-V should also be a cross-linked polyphosphate chain spheroid with most of its ion bonds (-O--NH4+) wrapped inside the crystal, somewhat like the structure of APPII. Probably, APP-V is not as crystalline a structure of regular polymer as APP-II, or other crystalline forms such as APP-VI are blended in optimal prepared APP-V as shown in Figure 5, leading to AAP-V being a bit more soluble in water than APPII. If this speculation is correct, it is easy to explain the fact that APP-II can be obtained by heating APP-I at a higher temperature for a certain time.6 APP-I is a linear polyphosphate chain structure with a low polymerization degree of 30-50; two or more polyphosphate units will connect with each other to form a cross-linked structure at a high temperature by selfcondensation. The whole effect will be a cross-linked polyphosphate chain spheroid, with most of its ion bond (-O--NH4+) wrapped inside the crystal, leading to a greater degree of polymerization, insolubility, and stability in thermal degradation.11 On the basis of the speculated structure in Figure 8, it should be further stated that the said polymerization degree of 1000 for APP-II should be an aggregative concept, that is, it is a value of the summary of all the cross-linked polyphosphate chains, not a value for a single polyphosphate chain. Watanabe8 pointed out that APP-I was prepared by heating an equimolar mixture of ammonium dihydrogenorthophosphate and urea at 250-300 °C, APP-II was prepared at a temperature of 280-305 °C with wet ammonia atmosphere, and APP-V was prepared at a higher temperature of 340-350 °C. From a thermodynamics point of view, for an exothermic reaction, a higher temperature would lead to a more thermodynamically stable structure, which means that APP-II and APP-V would

This work was supported by National Science & Technology Supporting Program of China (2007BAE58B03) and West Region Project of Shanghai Science and Technology (10195801600). Literature Cited (1) Watanabe M. Process for producing ammonium polyphosphate of crystalline form II. US Patent 5,718,875, 1998. (2) Sheridan, R. C.; McCullough, J. F.; Allen, S. E. Preparation and agronomic evalution of long-chain ammonium and potassium polyphosphate. J. Agric. Food Chem. 1979, 27, 612–615. (3) 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. (4) Schacker, O.; Wanzke, W. Compounding with ammonium polyphosphate-based flame retardants. Plast., Addit. Compound. 2002, 4, 28– 30. (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. AdVan. 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) Dyroff, D. R. Crystalline form V ammonium polyphosphate and process for producing same. US Patent 3,687,695, 1972. (8) Watanabe, M; Sakurai, M; Maeda, M. Preparation of ammonium polyphosphate and its application to flame retardant. Phosphor. Res. Bull. 2009, 23, 35–44. (9) Waerstad, K. R.; McClellan, G. H. Preparation and characterization of some long-chain ammonium polyphosphate. J. Agric. Food Chem. 1976, 24, 412–415. (10) Watanabe M. Production of V type of ammonium polyphosphate. Japan Patent 9,235,110, 1997. (11) Camino, G.; Costa, L.; Trossarelli, L. Study on the mechanism of intumescence in fire retardant polymers: Part VsMechanism of formation of gaseous products in the thermal degradation of ammonium polyphosphate. Polym. Degrad. Stab. 1985, 12, 203–211. (12) Yi, D. Q.; Yang, R. J. Study of crystal defects and spectroscopy characteristics of ammonium polyphosphate. J. Beijing Inst. Technol. 2009, 18, 238–242. (13) Gu, J. W.; Zhang, G. C.; Dong, S. L.; Zhang, Q. Y.; Kong, J. Study on preparation and fire-retardant mechanism analysis of intumescent flameretardant coatings. Surf. Coat. Technol. 2007, 201, 7835–7841.

ReceiVed for reView March 1, 2010 ReVised manuscript receiVed April 20, 2010 Accepted May 11, 2010 IE100453A