Article pubs.acs.org/IECR
Ammonium Polyphosphate and Silicon-Containing Cyclotriphosphazene: Synergistic Effect in Flame-Retarded Polypropylene Zhaolu Qin,† Dinghua Li,*,† Yanhua Lan,† Qian Li,†,‡ and Rongjie Yang† †
School of Material Science and Engineering, National Engineering Research Center of Flame Retardant Materials, Beijing Institute of Technology, Beijing 100081, People’s Republic of China ‡ Department of Air Force Equipment, Beijing 100843, People’s Republic of China S Supporting Information *
ABSTRACT: A new synthetic non-halogen, flame-retardant cyclotriphosphazene containing the silicon functional group (APESP) was incorporated with ammonium polyphosphate (APP) to prepare flame-retarded polypropylene (PP). APESP/APP/ PP composites were prepared by melt-blending and extrusion in a twin-screw extruder with different loading levels of APESP. The combustion and thermal behaviors were investigated based on the limiting oxygen index, UL-94, and cone calorimeter test as well as thermogravimetric analysis (TGA). Then, TGA coupled with Fourier transform infrared spectroscopy was used to probe the degradation mechanism of APESP/APP/PP composites. The results indicated that a good flame-retardant performance could be achieved by incorporating APESP with APP in PP. The synergistic effect between APESP and APP was observed and is discussed here. The mechanical properties and rheological behaviors of APESP/APP/PP composites indicated that APESP significantly contributed to the dispersion and distribution of APP in the PP matrix and remarkably improved the elongation at break. The morphology of APESP/APP/PP composites was investigated using scanning electron microscopy, and the profiles indicated that APESP enhanced the compatibility of the APP/PP system, which was verified by the blending energy and Flory− Huggins parameter (χ) values of APP, APESP, and PP in a multiple-scale simulation.
1. INTRODUCTION
Therefore, the combination of APESP in the PP/APP system is studied with the aim of exploring the effect of APESP on PP/ APP composites. The interfacial compatibility of APESP/APP/ PP composites was studied using scanning electron microscopy (SEM) as well as mechanical property and rheological behavior tests. The flame retardancy of APESP/APP/PP composites was investigated using the limiting oxygen index (LOI), UL-94, and cone calorimetry tests as well as thermogravimetric analysis (TGA) and TGA coupled with Fourier transform infrared spectroscopy (TGA-FTIR).
Ammonium polyphosphate (APP) was first developed by the Monsanto Company in 1965, and it is one of the most important inorganic flame retardants1,2 and a major component of an intumescent flame retardant.3−6 APP has high phosphorus and nitrogen contents and can yield phosphoric acid below the decomposition temperature. APP is usually used as the acid source of an intumescent flame retardant for polypropylene (PP).7−9 However, the utilization of APP is limited because of its hygroscopicity and poor compatibility with the PP matrix. The high loading of APP can greatly worsen the mechanical properties of PP. In other words, flame retardancy is usually improved at the expense of the mechanical properties of flameretarded PP. Therefore, numerous modification methods have been employed to improve the interfacial compatibility of APP and the PP matrix.10−15 Silane coupling agents are the most widely used and studied among the various modifiers.16,17 In our previous work, a novel non-halogen, flame-retardant cyclotriphosphazene containing six (aminopropyl)triethoxysilicone groups, APESP, was designed as a novel flame retardant, which included phosphorus, nitrogen, and silicon, and synthesized through material purification and nucleophilic substitution.18 The compatibility of APESP with PP/APP composites was also investigated using computer simulations.19 Cyclophosphazene is a good and environmentally friendly flame retardant. Hence, we suggest that APESP has the potential to simultaneously improve the flame retardancy and interfacial compatibility of PP/APP composites. © 2015 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. APESP was synthesized in our laboratory, and the specific synthetic method was previously published.18 PP (S1003) with a melt-flow index (MFI) of 3.2 g/10 min (230 °C/2.16 kg) was provided by Yanshan Petroleum Chemical Company. APP with an average degree of polymerization n > 1000 was provided by Zibo SaiDa Flame-Retardant New Materials Co., Ltd. Antioxidant 1010 (pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]) and antioxidant 168 [tris(2,4-di-tert-butyl)phosphite] were supplied by Ciba Specialty Chemicals Company (Switzerland). Anhydrous ethanol was purchased from the Beijing Chemical Reagents Company. All of these chemicals were used without further Received: Revised: Accepted: Published: 10707
June 26, 2015 September 29, 2015 October 8, 2015 October 8, 2015 DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
Article
Industrial & Engineering Chemistry Research
Figure 1. Chemical structures of APESP and APP.
The mechanical properties were measured according to GB/ T 1040-2006 (equivalent to ISO 527-2012) with an electronic tensile testing machine (DXLL-5000) at a tensile rate of 20 mm/min. The dimensions of the dumbbell specimens were the same as those specified in GB/T 1040.2-2006. The samples were preconditioned at 23 °C and 50% RH for 24 h, and the results are the average of five measurements. Morphology characterization of the brittle fracture section of APESP/APP/PP composites was performed using an S4800 scanning electron microscope. The PP composite specimens were frozen in liquid nitrogen for 20 min and then made to fracture in a studden. Rheological behavior studies were performed on a controlled strain rheometer (RS300, Thermo HAAKE Scientific). Measurements were performed in the plate−plate configuration with a gap of 1.0 mm. The sensor type was a PP20 Ti. Oscillation frequency sweep tests were performed on APESP/ APP/PP composites at 200 °C. The range of the shearing rate was 1−5 s−1. The results are the average of three measurements. The blending compatibility of APP, APESP, and PP was simulated using Material Studio 5.5 (MS 5.5). The molecular models were established using the Visualizer module based on the molecular structure of APP, APESP, and PP, whereas the structures of the corresponding molecular model were optimized using the DMol3 module. During the simulation process, the temperature and pressure were maintained using the Andersen and Berendsen methods, respectively. The COMPASS (condensed-phase optimized molecular potentials for atomistic simulation studies) force field19,20 was used for computation of the interatomic interactions. In addition, the Flory−Huggins parameters of PP, APP, and APESP were calculated using Blends Tools. The Flory−Huggins interaction parameter (x) was calculated using eq 1, and the interaction energy (Emix) was calculated using eq 2.
purification. The chemical structures of APESP and APP are shown in Figure 1. 2.2. Preparation of APESP/APP/PP Composites. APESP is a viscous liquid at room temperature. First, a certain amount of APESP was well dispersed in 50 mL of ethanol. Simultaneously, PP was added into a high-speed mixer and elevated to 80 °C. After that, an ethanol solution of APESP was sprayed into the high-speed mixer by an atomization device and stirred for 10 min or until ethanol was completely volatilized. Then APP and antioxidants 1010 and 168 were added to the above mixture. Finally, all of the raw materials were placed in a blender and mixed on medium-low speed until well blended. APESP/APP/PP composites were prepared by melt-blending and extrusion in a twin-screw extruder (SHJ-20) with an L/D ratio of the screws of 20. The temperatures of the sections were 170, 175, 180, 185, 180, and 175 °C. The feed rate was 12 rpm, and the screw speed was 25 rpm. The test specimens were prepared using an injection-molding machine. The temperatures of the sections for the injection-molding machine were 200, 200, 190, and 170 °C. The formulations of APESP/APP/ PP composites are given in the Supporting lnformation. 2.3. Measurements. The LOI was measured according to ASTM D 2863 with an FTA II oxygen index meter (Rheometric Scientific Ltd., U.K.). The specimens used for the LOI test had dimensions of 118 mm × 6.5 mm × 3 mm. The samples were preconditioned at 23 °C and 50% relative humidity (RH) for 24 h. Vertical burning tests were performed on a CZF-5A-type instrument (Jiangning Analysis Instrument Company, China) according to the UL-94 test standard. The specimens had dimensions of 125 mm × 13 mm × 3.2 mm. The samples were preconditioned at 23 °C and 50% RH for 24 h. Cone calorimetry measurements were performed at an incident radiant flux of 50 kW/m2 according to the ISO 5660 protocol using a Fire Testing Technology apparatus (FTT 0007) with a truncated cone-shaped radiator. The specimens (100 mm × 100 mm × 3 mm) were measured horizontally without any grids. Typical results from the cone calorimeter were reproducible within ±10%, and the reported results are the average of three measurements. The samples were preconditioned at 23 °C and 50% RH for 24 h. TGA was performed with a Netzsch 209 F1 thermal analyzer. To detect the gas species, TGA-FTIR (Nicolet 6700) and the measurements were performed under a N2 atmosphere at a heating rate of 20 °C/min from 40 to 800 °C. The sample weight was 10 mg for each measurement. TGA was performed at a gas flow rate of 40 mL/min. The typical results from TGA were reproducible within ±1%, and the reported data are the average of three measurements.
x=
Emix RT
Emix =
(1)
1 Z(Eij + Eji − Eii − Ejj) 2
(2)
where Emix is the interaction energy of the blending system, R is the gas constant, T is 298 K, Z is the coordination number, and Eij is the binding energy of i and j.
3. RESULTS AND DISCUSSION 3.1. Flame Retardancy of APESP/APP/PP Composites. LOI and UL-94 tests have been widely employed to evaluate the flame retardancy of materials. The LOI and UL-94 test results are summarized in Table 1. The LOI value of the APP/ 10708
DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
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because the addition of APESP accelerates decomposition of PP. The heat release rate (HRR) curves of APESP/APP/PP composites are presented in Figure 2. The narrow and sharp
Table 1. LOI and UL-94 Test Results of APESP/APP/PP Composites sample no. (0% APESP−30% APP)/PP (2% APESP−28% APP)/PP (4% APESP−26% APP)/PP (6% APESP−24% APP)/PP (8% APESP−22% APP)/PP
UL-94 (3.2 mm)
mean T1 (s)
mean T2 (s)
mass loss (%)
LOI (%)
no rating
100
21.7
no rating
100
22.2
no rating
100
22.4
V-2
2
10
6.77
23.5
V-2
1
10
3.23
26.5
PP composite containing 30 wt % APP is 21.7%. Because of the scarcity of the carbonization agent and blowing agent, APP is not very effective in flame-retarded PP when used alone. However, the LOI values of APESP/APP/PP composites reflect obvious improvement with increasing loading of APESP in the APP/PP system. Specifically, for (2% APESP−28% APP/ PP, the LOI value is 22.2%; for (8% APESP−22% APP)/PP, the LOI value is 26.5%, and a V-2 rating is obtained in UL-94 tests. The increase of flame retardancy may be due to the synergistic effect between APESP and APP in PP. 3.2. Cone Calorimetry. Cone calorimetry is an effective approach to evaluating the combustion behavior of flameretarded materials.21 The experimental results obtained by the cone calorimetry test are summarized in Table 2, including the time to ignition (TTI), peak heat release rate (PHRR), time to PHRR (T-PHRR), average CO release (avCO), average CO2 release (avCO2), total heat release (THR), mass loss (ML), and total smoke release (TSR). For the APP/PP system, APP exhibits a good flame-retardant effect at a loading of 30 wt %, and the PHRR is reduced to 767 kW/m2 with that of the PP control (1284 kW/m2). Surprisingly, the data in Table 2, where the total amount of APP and APESP are maintained constant at 30 wt %, indicate that APESP exhibits a strong synergistic effect with APP in PP. In addition, increasing the APESP/APP ratio from 2/28 to 8/ 22 results in a clear further increasing effect in flame-retarded PP. For example, for (2% APESP−28% APP)/PP, a 34% decrease of PHRR occurs, whereas for (8% APESP−22% APP)/PP, PHRR is reduced by 68%. Also, importantly, it is clear that the THR values of APESP/APP/PP composites are decreased compared with the APP/PP system. The value of THR can be assumed to be representative of the fire spread, and a lower THR value indicates a safer material. Considering the experimental error, the TTI also shows obvious changes. The average TTI of the PP control specimens is 37 s and that of APP/PP is 22 s, whereas for APESP/APP/ PP composites, the average TTI is 17−18 s. This result occurs
Figure 2. HRR curves of APESP/APP/PP composites.
HRR curve of the PP control indicates that the PP control burns vigorously during combustion. When combined with APP only, the HRR decreases, but the sharp peak becomes wider with a little char formation capacity of (0% APESP−30% APP)/PP. For (2% APESP−28% APP)/PP, the peak of the HRR curve exhibits a further decrease. With an increase of APESP, the HRR curves become smooth and flattened. This behavior is characteristic of a polymer showing solid-state pyrolysis with increased char formation capacity. Therefore, it appears that the incorporation of APESP strongly contributes to the char formation of PP, which is consistent with the residual mass value in Table 2. The protective charred layers can prevent rapid heat release during combustion and reduce the possibility of a terrible fire. 3.3. TGA and TGA-FTIR. The combustion behavior of the materials depends on the thermal decomposition process, which feeds the flame with combustible volatiles. Therefore, we have studied thermal degradation of APESP, APP, and flameretarded PP as well as their mixtures. Figure 3 shows the TGA curves of APESP, APP, and an APESP/APP mixture (mass ratio 8:22). The calculated TGA curve of the APESP/APP mixture was the sum of the experimental TGA curves of APESP and APP. The thermal stability of APP (Figure 3a) is better than that of APESP (Figure 3d) before 600 °C. In contrast, with increasing temperatures, APESP exhibits better thermal stability than APP does. The experimental (Figure 3b) and calculated (Figure 3c) TGA curves of a mixture of APESP/APP (mass ratio 8:22) exhibit the same behavior before 500 °C. From 500 to 600 °C, TGA shows a weight loss somewhat larger than that of the
Table 2. Cone Calorimetry Data of APESP/APP/PP Composites sample PP control (0% APESP−30% (2% APESP−28% (4% APESP−26% (6% APESP−24% (8% APESP−22%
APP)/PP APP)/PP APP)/PP APP)/PP APP)/PP
TTI (s)
PHRR (kW/m2)
avCO (kg/kg)
avCO2 (kg/kg)
THR (MJ/m2)
TSR (m2/s)
residue mass (%)
37 22 18 17 18 17
1284 767 596 420 382 282
0.054 0.052 0.056 0.058 0.063 0.071
3.569 3.197 3.228 3.155 3.018 3.007
121 111 114 109 95 95
897 1074 1058 1100 914 980
0.10 21.9 39.7 51.8 56.7 60.6
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DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
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Table 3. T5%, Tmax, and Residues of APESP/APP/PP Composites sample PP control (0% APESP−30% (2% APESP−28% (4% APESP−26% (6% APESP−24% (8% APESP−22%
APP)/PP APP)/PP APP)/PP APP)/PP APP)/PP
T5% (°C)
Tmax (°C)
residue (%)
379 373 367 360 354 352
450 460 470 471 469 468
0.0 6.4 9.1 10.2 12.1 14.8
tests. The addition of APESP promotes decomposition of APP/ PP composites at lower temperature. Moreover, adding APESP somewhat improves the high-temperature thermal stability, and the residues of APESP/APP/PP composites also reflect a visible increase compared with the APP/PP system. To further explore the mechanism of APESP on the APP/PP system, especially the effect at high temperature, TGA-FTIR was performed to detect pyrolysis of the gases of the composites. The FTIR spectra show no difference for pyrolysis of the gases of APP/PP and APESP/APP/PP when the temperature is below 600 °C. However, as observed in Figure 5, for APESP/APP/PP, the relative absorption peak at approximately 1273 cm−1 appears at 700 °C, and the intensity continues to increase with the temperature, where the peak corresponds to the P−O compounds evolved,22 as well as the spectral evolution of alkanes or alkenes (around 2900 cm−1) and other decomposition products (around 1446 and 1365 cm−1). The absorbance intensities of the P−O compounds, alkanes, alkenes, and other decomposition products of APESP/ APP/PP are higher than that for APP/PP. The change in the evolved gases at high temperature corresponds to the better thermal stability for APESP/APP/PP. This also illustrates that the incorporation of APESP affects thermal decomposition of PP. 3.4. Mechanical Properties. The addition of the flame retardant generally yields a significant decrease in the mechanical properties of the polymers. APESP is designed as a novel organic−inorganic hybrid flame retardant. Therefore, APESP can be used not only as a flame retardant but also as a compatilizer to improve the interfacial compatibility of APESP/ APP/PP composites. The effects of APESP on the mechanical properties of APESP/APP/PP composites are illustrated in Figure 6a. The representative stress−strain curves of the PP control and APESP/APP/PP composites are illustrated in Figure 6b. The mechanical properties of the APP/PP composite are dramatically worsened because of the poor compatibility of APP with the PP matrix. When APESP was added, better mechanical properties were attained. The tensile strength of APESP/APP/ PP composites increased considerably compared with that of APP/PP. The elongation at break also improved greatly with increasing loading of APESP in APP/PP. The improvement in the mechanical properties indicates that APESP acts as an effective compatibilizer and dispersant, which improves the poor compatibility of APP with the PP matrix. 3.5. Morphology of APESP/APP/PP Composites. In order to investigate the effect of APESP on the compatibility of APP in the PP matrix, the brittle fracture surface of APESP/ APP/PP composites was analyzed. The composites were frozen in liquid nitrogen for 20 min and then made to frature in a studden. As shown in Figure 7, the micrograph was taken at 1000× magnification to represent the general surface of the
Figure 3. TGA curves of APESP, APP, and the APESP/APP mixture (N2 atmosphere).
calculated curve, whereas a lower weight loss than that of the calculated curve is observed above 600 °C. In addition, the residue at 800 °C is 31.8%, which is higher than the calculated value (20.1%). We can confirm that APESP affects the degradation process of APP. Therefore, the interaction between APESP and APP is detected: when the temperature is lower than 500 °C, APESP has no effect on the thermal degradation of APP. In the temperature range of 500−600 °C, thermal decomposition of APP is affected by APESP. In the hightemperature range (600−800 °C), the reaction between APESP and APP enhances the thermal stability and reduces the degradation rate of APP. Figure 4 presents the TGA curves of APESP/APP/PP composites in a N2 atmosphere. Both PP and APESP/APP/PP
Figure 4. TGA curves of APESP/APP/PP composites (N 2 atmosphere).
composites exhibit a one-step degradation process. Table 3 summarizes the specific thermal stability data of APESP/APP/ PP composites. For the APP/PP system, as observed in Table 3, the temperature at 5% mass loss (T5%) is 373 °C, and the temperature of the maximum mass loss rate (Tmax) is 460 °C. Compared with the APP/PP system, T5% of APESP/APP/PP composites is lower than that of the APP/PP system. In particular, at 8 wt % APESP, T5% is reduced by 27 °C. This result is similar to the TTI derived from the cone calorimetry 10710
DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
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Figure 5. FTIR spectra of the gas release of APESP/APP/PP composites at various temperatures.
Figure 6. Mechanical properties (a) and stress−strain curves (b) of APESP/APP/PP composites.
Figure 7. SEM images of the fracture surface of APESP/APP/PP composites.
fracture section. For the APP/PP system, APP particles did not adhere to the PP matrix, and the interface between the APP particles and PP matrix is clearly visible on the surface in Figure 7a. In this case, the mechanical properties of the APP/PP composite must be reduced because of the poor interfacial compatibility between the APP particles and PP matrix. For APESP/APP/PP composites, the APP particles were uniformly
embedded in the PP matrix. It is easy to see that APESP improves the interfacial compatibility between the APP particles and PP matrix because these defects almost disappear with the addition of APESP. Figure 8 presents micrographs of the tensile fracture surface of APESP/APP/PP composites after the mechanical property tests. The micrographs were captured at 200× magnification to 10711
DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
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Industrial & Engineering Chemistry Research
Figure 8. SEM images of the tensile fracture surface of APESP/APP/PP composites.
Newtonian regime during the test, and the shearing stress has little effect on its viscosity, while APESP/APP/PP composites exhibit higher viscosity than that of the APP/PP composite. Also, the viscosity increases with increasing concentration of APESP at lower shearing rate. This indicates that APESP/APP/ PP composites behave similarly to solidlike rheological behavior. The high melt viscosity could restrict the release of the gaseous products during the thermal degradation or combustion process and help the condensed phase to crosslink during char formation.25 Thus, the (8% APESP−22% APP)/PP composite can form a more stable char layer, which corresponds to the flat HRR curve in Figure 2. 3.7. Blending Energy and Flory−Huggins Parameters. Table 4 lists the blending energy (Emix) and Flory−Huggins parameter (χ) of the blending system at 298 K. For larger values of Emix and χ, the components are more immiscible, and phase separation may exist. Emix and χ of APP/PP are relatively high (19.79 and 33.42 kcal/mol, respectively), which make the PP/APP system incompatible because of the poor affinity of APP and PP.26,27 For the PP/APESP and APP/APESP systems, the values of Emix and χ are lower, which indicates that the compatibilities of PP/APESP and APP/APESP are better than that of the APP/PP system. The mechanical property, rheological behavior, and fracture surface results also reflect the improvement of the interfacial compatibility. Therefore, it can be concluded that APESP can be used as a compatilizer to improve the compatibility between APP and PP.
represent the general surface of the tensile fracture surface of the composite. The tensile fracture surface of APP/PP (Figure 8a) reveals brittle fracture after being stretched, and elongation in APP/PP is not observed. With the addition of APESP in APP/PP, the tensile fracture surface reveals ductile fracture, as understood from the fibrillated structure of the matrix phase due to plastic deformation. The tensile fracture surface corresponds to an improvement of the elongation at break compared with that in Figure 6. 3.6. Rheological Analysis. Some studies have reported a close relationship between the viscoelastic characteristics and flammability properties of thermoplastic polymers.23,24 The viscosity of APESP/APP/PP as a function of the shearing rate is plotted in Figure 9. The APP/PP composite exhibits a quasi-
4. CONCLUSION In this work, the effects of different levels of APESP in the APP/PP system were studied. The synergistic effect between APESP and APP significantly reduced the HRR, PHRR, and THR of APESP/APP/PP composites. In addition, the flame
Figure 9. Viscosity of APESP/APP/PP versus shearing rate.
Table 4. Blending Energy and Flory−Huggins Parameter of Blending Systems (298 K) sample
Eii(kcal/mol)
Ejj(kcal/mol)
Eij(kcal/mol)
Eji(kcal/mol)
Emix(kcal/mol)
χ
PP/APP PP/APESP APP/APESP
−3.22 −5.46 −10.78
−8.10 −13.20 −15.98
−5.53 −8.91 −9.99
−5.53 −8.91 −9.99
19.79 0.13 2.06
33.42 0.22 3.48
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retardancy of PP tended to increase with increasing loading of APESP in the APP/PP system. The LOI value increased from 22.2% [2% APESP−28% APP)/PP] to 26.5% [(8% APESP− 22% APP)/PP], and a V-2 rating is obtained during UL-94 tests. The PHRR value is reduced by 53% from 596 kW/m2 [(2% APESP−28% APP)/PP] to 282 kW/m2 [(8% APESP− 22% APP)/PP]. The improvement in the mechanical properties was verified by the rheological behavior and morphology of the brittle fracture surface of APESP/APP/PP composites. Finally, the analog computation on the blending energy and Flory−Huggins parameter of different components demonstrated that the addition of APESP enhanced the compatibility between APP and the PP matrix. All of the results indicated that a good synergistic effect was simultaneously detected in the flame retardancy and interface compatibility by incorporating APESP in the PP/APP system.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02343. Formulations of the APESP/APP/PP composites (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 10 68913066. Fax: +86 10 68913066. E-mail: dli@ bit.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are very grateful for the support received from the Joint Funds of the National Natural Science Foundation of China (Grant U 1433128).
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REFERENCES
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DOI: 10.1021/acs.iecr.5b02343 Ind. Eng. Chem. Res. 2015, 54, 10707−10713
