Synergistic Effect between Aluminum Hypophosphite and Alkyl

Article pubs.acs.org/IECR

Synergistic Effect between Aluminum Hypophosphite and AlkylSubstituted Phosphinate in Flame-Retarded Polyamide 6 Bin Zhao, Li Chen,* Jia-Wei Long, Rong-Kun Jian, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Wangjiang Road 29, Chengdu, Sichuan 610064, China S Supporting Information *

ABSTRACT: A novel binary flame-retardant system was formed by introducing aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu) together for PA6. The optimum flame retardant formulation was 1:1 (AP:APBu, 15 wt % in total), and the resulting flame-retardant PA6 could achieve a LOI value of 28.3 vol % and UL-94 V-0 rating. Cone calorimeter testing showed the samples containing binary flame retardants became less flammable with lower peak heat release rate (PHRR, 259 kW/m2). Dynamic oscillatory rheology and simultaneous thermogravimetry−differential scanning calorimetry results proved that cross-linking reactions existed in the samples containing AP. Morphology of the char layers was analyzed via SEM and laser Raman spectroscopy, and the results demonstrated the samples containing both AP and APBu formed more effective char. These results revealed that flame-retardant synergism existed between AP and APBu when they were combined to flame retard PA6. Consequently, a brief synergistic mechanism in this system was proposed.

1. INTRODUCTION Polyamide 6 (PA6) is an important engineering plastic produced in the largest volume among polyamides family and applied in electrical and electronic fields, such as terminal blocks, electrical connectors, switch components, etc.1 However, in these fields, adequate flame retardance is imminently needed due to the flammability of PA6. In recent years, a large number of systems containing halogen2 or halogen-free flame retardants3−5 are available to flame retard PA6. Halogen-free solutions are current challenges due to environmental and health concerns; among them, phosphorus-based flame retardants such as red phosphorus and inorganic and organic phosphorus-containing chemicals are well-known to be the appropriate halogen-free alternatives for PA6.2 The applications of red phosphorus, particularly unencapsulated ones, are widely restricted for the dark color problem, potential flammability, and poisonous gas release during processing. Also, it has been reported that the oxidation of phosphorus could be strongly influenced by other additives or moisture in the polymer during both processing and application.6 Exolit OP series phosphorus-based flame retardants (mostly aluminum diethylphosphinate, OP1312, Clariant Co., Germany) have been developed and commercialized, and have proved to be effective flame retardants for polyesters, polyamides, and the corresponding glass-fiber reinforced composites.7−10 However, the primary limiting factor for using these flame retardants is due to their complex and expensive manufacturing technologies.11 In our former work, aluminum hypophosphite (AP) was used to flame-retard glass-fiber-reinforced PA6 and PBT, and results suggested that AP could be considered as an effective and low cost flame retardant for these polymers.12,13 Li and his coworkers studied the difference of flame-retardant effects between AP and magnesium hypophosphite (MP) in PA6, and found that the AP showed much better flame retardance than MP did.14 In order to enhance the flame retardance, nitrogen-containing © 2013 American Chemical Society

substances, such as melamine polyphosphate (MPP) as well as melamine cyanurate (MC), or mineral particles like zinc borate (ZnB)7−9,11,15 and different metal oxides10 were applied as synergists to combine with different phosphinates. From our previous report,16 it was found that AP showed condensedphase-dominant activity to enhance the flame retardance thanks to the formation of hypophosphorus acid-terminated and phosphinic acid-coupled PA6 fragments during combustion, which could further salify with Al3+ to form ionic cross-linking structures, while aluminum isobutylphosphinate (APBu) exhibited more gaseous-phase activity owing to the phosphoruscontaining free radicals generated from the decomposition of APBu, which could act as the scavengers and inhibitors in fire. According to the different acting mechanism of AP and APBu, one can imagine that the potential flame-retardant synergism might exist between two aluminum phosphinates. In this study, AP and APBu were combined to flame retard PA6. The flame retardance, thermal stability, combustion behavior, and flameretardant synergism were investigated comprehensively.

2. EXPERIMENTAL SECTION 2.1. Materials. PA6 (Mn ≈ 30k; product code YH-800) was manufactured by Hunan Yueyang Baling Petrochemical Co., Ltd., China. Sodium hypophosphite (NaH2PO2·H2O, A.R.), glacial acetic acid (CH 3 COOH, A.R.), tert-butanol ((CH3)3COH, A.R.), concentrated sulfuric acid (H2SO4, A.R.), aluminum chloride hexahydrate (AlCl3·6H2O, A.R.), and sodium hydroxide (NaOH, A.R.) were purchased from Chengdu Kelong Chemical Reagent Factory, China. Aluminum hypophosphite Received: Revised: Accepted: Published: 17162

March 21, 2013 October 27, 2013 November 5, 2013 November 5, 2013 dx.doi.org/10.1021/ie4009056 | Ind. Eng. Chem. Res. 2013, 52, 17162−17170

Industrial & Engineering Chemistry Research

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of the testing samples. In this part, flame-retardant effects of AP, APBu, and their mixture in PA6 were investigated, and the detailed results were summarized in Table 1. The total content of

(AP) (P% = 40.0%) was prepared according to previous reports.13,17 The detailed synthesis process of aluminum isobutylphosphinate (APBu, mixture of aluminum monoisobutylphosphinate and aluminum diisobutylphosphinate, Scheme 1) (P% = 21.7%) can be found in the Supporting Information.

Table 1. Flame Retardance of Virgin PA6 and Its Composites UL-94 (3.2 mm)

Scheme 1. Chemical Structure of APBu

sample

t1a (s)

t2a (s)

rating

LOI (vol %)

PA6 PA6-15%AP PA6-15%AP-APBu (3:1) PA6-15%AP-APBu (2:1) PA6-15%AP-APBu (1:1) PA6-15%AP-APBu (1:2) PA6-15%AP-APBu (1:3) PA6-15%APBu

15 ± 2 1±1 41 ± 2 0 0 0 0 6

35 ± 10 10 ± 1 burn to clamp 11 ± 3 4±1 5±2 6±2 15

fail V-2 fail V-1 V-0 V-0 V-0 V-2

20.5 25.7 25.9 27.8 28.3 28.3 28.2 28.1

Average combustion duration after the first (t1) and the second ignition (t2). a

2.2. Sample Preparation. PA6, AP, and APBu were all dried at 100 °C in vacuum for 5 h before using. The PA6 and flameretarded samples were prepared by melt compounding in a twinscrew extruder (L/D = 35, CTE-35). The temperature for the extruder was set as 200, 235, 245, 235, and 220 °C from feed inlet to die, respectively. After that, the snipped extrudates were compression molded at 245 °C and cut into the standard testing bars. 2.3. Measurements. Flame Retardance and Combustion Behavior. The LOI values were measured according to ASTM D 2863-97, and the dimension of the samples was 130 × 6.5 × 3.0 mm3. The UL-94 vertical burning rates were assessed according to UL-94, and the 3-dimensional size of the specimens was 130 × 13 × 3.0 mm3. The combustion behavior of the samples was analyzed with a cone calorimeter (Fire Testing Technology, East Grinstead, U.K.) according to ISO 5660-1. The foursquare samples with a size of 100 × 100 × 3 mm3 were placed in the sample holder with a retainer frame, resulting in about 88 cm2 of the sample surface being exposed to the radiation from the cone at an external heat flux of 50 kW/m2. Burning Residue Analysis. Scanning electronic microscopy (SEM, JEOL JSM-5900LV, Japan) was utilized to observe the morphology of the burning residue. The samples were previously coated with a conductive gold layer. Graphitic structures of the burning residue were analyzed with Laser Raman spectroscopy (LRS, SPEX model 1403, SPEX Industries Inc.) at room temperature. Excitation wavelengths were 532 nm using a He− Ne laser. The chars for the Raman tests were collected after cone calorimetry, and were grounded with a mortar and pestle, and then the powder was stuck onto a glass slide for testing. Thermal Properties. Thermogravimetric analysis (TGA) was performed using a Netzsch TG 209-F1 apparatus (Germany) under a 60 mL/min nitrogen flow at heating rates of 20 °C/min. Dynamic oscillatory rheological measurements of the testing samples were preformed via an Advanced Dynamic Rheometric Expansion System (ARES, Bohlin Gemini 200, U.K.) with a parallel-plate fixture (1 mm thickness and 25 mm diameter) in an oscillatory shear mode. Temperature scanning tests at a constant frequency of 1 Hz were recorded in the range 230−300 °C. Thermal behavior was performed on a Mettler Toledo simultaneous thermogravimetry−differential scanning calorimetry (TGA-DSC, STARe System, Switzerland) at a heating rate of 10 °C/min in nitrogen.

the flame retardants was maintained at 15 wt % in all flameretardant samples, and the weight ratios of AP to APBu were regulated from 3:1 to 1:3. PA6 was a flammable polymer with a very low LOI value (20.5 vol %), and could not pass the UL-94 vertical burning test. When it was flame retarded by 15 wt % AP or APBu separately, only V-2 rating could be achieved in UL-94 tests due to flame dripping; LOI values increased to 25.7 vol % and 28.1 vol %, respectively. LOI of APBu system was quite higher than that of the AP system, owing to the effective gaseous flame-retardant activity of APBu.16 In addition, considerable flame-retardant synergism could be observed when AP and APBu were combined together to flame retard PA6. When the weight ratio of AP to APBu reached 1:1 (PA6-15%AP/APBu (1:1)), the material showed the most effective flame retardance with highest LOI value of 28.3 vol %, and UL-94 V-0 rate could be obtained. Furthermore, when the ratio of AP to APBu decreased to 1:2 and 1:3, the samples also could achieve V-0 rate, and the LOI values were all above 28.0 vol %, which was much close to that of the flame-retardant PA6 with APBu alone. However, burning durations after the second ignition of PA6-15%AP/APBu (1:2) and PA6-15%AP/APBu (1:3) were longer than that of PA6-15% AP/APBu (1:1) in UL-94 test. Thus, PA6-15%AP/APBu (1:1) was chosen as the best formulation. However, when the ratio of AP to APBu increased, the LOI values began to decrease to 27.8 vol % (2:1) and 25.9 vol % (3:1), respectively, gradually getting nearer the value of PA6-15%AP. Consequently, it remained the case that the flame-retardant synergism existed between AP and APBu when they were combined to flame retardant PA6. 3.2. Thermal Analysis. Thermogravimetric Analysis. The results of thermogravimetric in N2 were summarized in Table 2 and Figure 1. During heating, the virgin PA6 could release H2O, CO, CO2, hydronitrogens, as well as hydrocarbon fragments Table 2. TG Data of the Composites under N2 Atmosphere

sample PA6 PA6-15%AP PA6-15% APBu PA6-15%AP/ APBu(1:1)

3. RESULTS AND DISCUSSION 3.1. Flame Retardance. LOI results and UL-94 vertical burning rate have been applied to evaluate the flame retardance 17163

Tmax2 (°C)

mass loss rate at Tmax (wt %/min)

residues at 700 °C (wt %)

445 395 378

459

42 30 9, 31

0 17.4 4.3

406

450

30, 6

8.3

T5% (°C)

Tmax1 (°C)

397 344 373 355

dx.doi.org/10.1021/ie4009056 | Ind. Eng. Chem. Res. 2013, 52, 17162−17170


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Figure 1. TG (a) and DTG (b) curves of the testing samples under N2.

between 400 and 500 °C.18 In our former work,16 the thermal behaviors of PA6-AP and PA6-APBu were investigated. Because of relatively low thermal stability and potential reactivity with the matrix, the introduction of 15 wt % AP lowered the initial decomposition temperature (T5%) and maximum-rate temperature (Tmax) of the decomposition by approximately 50 °C; the percentage value of the final residues was increased to 17.4 wt %. Unlike the case of AP, the introduction of 15 wt % APBu decreased T5% (by 24 °C), and slightly increased the final residue (4.3 wt %), which could be explained by the inherent thermal stability of APBu: higher initial thermal decomposition temperature but lower thermal stability in high temperature zone than those of AP.16 When AP and APBu were combined to add into PA6, T5% was lowered to 355 °C, which was in the middle of T5% values of PA6-AP and PA6-APBu; however, the main decomposition stage and the corresponding maximum mass loss rate were much closer to those of PA6-AP. Also, an additional subsequent small decomposition process occurred after the main decomposition, which was nearby the main decomposition stage of PA6-APBu sample. The residue was increased to 8.3 wt %, which was quite lower than that of PA615%AP but a little higher than that of PA6-15%APBu sample. To further study the interference of PA6 and the flame retardants, experimental and calculated TG curves of the experimental samples were exhibited in Figure 2. The calculated TG curves were derived from the following equations: M(cal)PA6 − AP = 85% × M(exp)PA6 + 15% × M(exp)AP

(1)

M(cal)PA6 ‐ APBu = 85% × M(exp)PA6 + 15% × M(exp)APBu

Figure 2. Calculated and experimental TG curves of PA6-15%AP/APBu (a), PA6-15%AP (b), and PA6-15%APBu (c) under N2.

(2)

M(cal)PA6 ‐ (AP/APBu) = 85% × M(exp)PA6 + 7.5% × M(exp)AP + 7.5% × M(exp)APBu

above suggested that PA6 reacted with AP during heating (crosslinking reaction), but not with APBu, which was consistent with the results of high additive amount of AP or APBu to flameretardant PA6.16 However, with the addition of AP and APBu together into PA6, the experimental TG curve was significantly different from the calculated one. It revealed similar trend of PA6-AP sample. However, the real final residue was much closer to the calculated one due to the smaller amount of AP in the sample. The results demonstrated that AP and APBu remain their inherent influence to the material when they were combined together to flame-retardant PA6. Cross-Linking Investigation. To further investigate the viscoelastic behaviors of virgin PA6 and the corresponding

(3)

M(cal) is the calculated residual mass of corresponding samples, while M(exp) is the experimental residual mass of PA6, AP, or APBu, respectively. As illustrated in Figure 2, the noticeable differences were shown between the experimental and calculated TG curves of PA6-15%AP. The initial decomposition temperature found from the experimental TG curve decreased by approximately 50 °C in comparison with the calculated one, while the final residue increased considerably. Contrarily, both experimental and calculated curves of PA6-15%APBu were much closer to each other. The different phenomenon mentioned 17164



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flame-retardant samples during heating, dynamic oscillatory rheological tests were conducted. Figure 3 presented functional

shear-thinning behavior during heating, but the complex viscosity values for PA6-20%APBu were much higher than that of virgin PA6, which accounted for the filling effects of APBu acting as rigid particles in PA6, while |η*| plots of PA6-20%AP first presented a decrease before 270 °C, and then subsequently increased dramatically due to the cross-linking salification between AP and PA6. When AP and APBu were combined to flame retard PA6, the initial |η*| values were exactly in the middle of the samples containing AP or APBu alone. During heating, the viscosity first decreased slightly, and then increased with increasing temperature. After 280 °C, |η*| of PA6-20%AP/ APBu (1:1) exhibited the highest value among all the samples studied. These phenomena inferred that cross-linking reactions occurred and the reactions prevailed over the trivial decomposition of the samples, which give rise to a considerable increase of complex viscosity values, just as the PA6-20%AP sample did. In some research,19−23 in the process of degradation during combustion, increased |η*| could effectively restrain the flammability of the flame-retardant samples by depressing the vigorous bubbling process of PA6 during combustion in the course of degradation, and inhibited the melt-dripping at the same time. In order to confirm the occurrence of the crosslinking reactions between PA6 and flame retardant, the thermal behaviors of the specimen were characterized by TG-DSC in N2 (Figure 4). For the DSC curves, an abnormal exothermal peak (ca. 340 °C) occurred between two endothermal peaks in both PA6-20%AP and PA6-20%AP/APBu curves; the lower peak (ca. 220 °C) was due to the melting process, and the higher one (ca.

Figure 3. Complex viscosity plotted against temperature of the testing samples in air atmosphere at a heating rate of 10 °C/min.

relationship between temperature and the complex viscosity |η*| of virgin PA6 and the flame-retardant specimens. In this case, flame-retardant specimens with 20 wt % of flame retardants were investigated. In the whole temperature range, as we discussed previously,16 both PA6 and PA6-20%APBu exhibited typical

Figure 4. TG-DSC thermograms of the samples at a heating rate of 10 °C/min in nitrogen: (a) virgin PA, (b) PA6-20%AP, (c) PA6-20%APBu, (d) PA620%AP/APBu (1:1). 17165



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380 °C) was attributed to the decomposition, suggesting the cross-linking reactions occurred.23 However, it could not find exothermal peaks in both PA6 and PA6-20%APBu curves, which were in accord with the rheological behavior. 3.3. Fire Behavior: Forced-Flaming Combustion. For better understanding the flame-retardant synergism between AP and APBu in PA6, cone calorimetric analyses were carried out to assess the combustion behaviors of the testing samples.Detailed cone calorimetric results were summarized in Table 3. Figure 5 Table 3. Cone Calorimetric Results of the Testing Samples at an External Heat Flux of 50 kW/m2 materials PA6 PA6-15% AP PA6-15% APBu PA6-15% AP/APBu (1:1)

PHRR (kW/ m2)

THR (MJ/ m2)

Residue (wt%)

FIGRAa

MAHREb (kW/m2)

TSR (m2/m2)

789 340

98 77

1 14

5.1 5.7

400 237

490 2039

300

79

10

6.0

196

1854

259

79

18

5.1

152

935

Figure 6. Mass loss curves of the testing samples at an external heat flux of 50 kW/m2.

clearly decreased for the residue char formed a physical barrier to avoid or at least inhibit heat and mass transmission. The lowest PHRR value of PA6-15%AP/APBu (1:1) indicated that flameretardant synergism occurred between two flame retardants when they were combined to flame retard PA6. Moreover, the samples containing two flame retardants exhibited higher final residue (18 wt %), lower fire growth rate value (FIGRA, 5.1), total smoke release (TSR, 935 m2/m2), and maximum average rate of heat emission value (MAHRE, 152 kW/m2) than those of the flame-retardant samples with single additive, implying better fire safety. Meanwhile, it could be observed that PA6-15%AP/ APBu (1:1) showed the lowest mass loss rate during the whole cone calorimeter tests (Figure 7). In fact, the lowest mass loss rate value was caused by the better flame retardance of the sample containing AP and APBu which could keep a small flame during the whole test. Then, more char residue and less smoke were formed for this incomplete burning. The results of LOI, vertical burning, and cone calorimeter tests reflected that when AP and APBu were combined together to flame retard PA6 in a certain weight ratio, the samples could obtain the best flame retardance. According to the flame retardant mechanisms of AP and APBu which were proposed in our prior work,16 when these two flame retardants were utilized in PA6, the condensed phase flame-retardant mechanism of AP and gas phase activity (free radical scavenge) of APBu endowed the material better flame retardance.

FIGRA is defined as fire growth rate, which is obtained by dividing the peak HRR value by the time to HRR. bAHRE is defined as the average rate of heat emission which is calculated by dividing the cumulative heat emission by time; MAHRE denotes the maximum value of AHRE. a

illustrated heat release rate (HRR) and total heat release (THR) curves of all testing samples, while mass loss and mass loss rate (MLR) information was shown in Figure 6 and 7, respectively. HRR is a measurement of the heat release per unit surface area of a burning specimen, which is considered to have significant influence on fire hazard,24 while the peak value of HRR is regarded as an essential parameter with respect to the evaluation of the fire hazard.25 The HRR of PA6 showed a significant increase after ignition up to the PHRR value and decreased sharply toward the end of burning. This kind of heat release characteristic was long-term considered as a typical case for “thin noncharring” material.25 In contrast to virgin PA6, flameretardant samples showed much sharper decrease in HRR curves which was then followed by plateau stages which were caused by char forming. The presence of flame retardants in samples reduced the PHRR compared to PA6, by about 57% in PA6-15% AP, 62% in PA6-15%APBu, and 67% in PA6-15%AP/APBu (1:1) (Figure 5a). PHRR values of flame-retardant samples

Figure 5. HRR (a) and THR (b) curves of the testing samples at an external heat flux of 50 kW/m2. 17166



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Figure 7. MLR curves of the testing samples at an external heat flux of 50 kW/m2.

Figure 8. Outer (a, b, c) and inner (a′, b′, c′) view of the testing samples after cone calorimetric analyses: (a, a′) PA6-15%AP, (b, b′) PA6-15%APBu, (c, c′) PA6-15%AP/APBu (1:1).

corresponding flame-retardant processes were illustrated. As exhibited in Figure 9, the burning residues of the samples displayed different inner surface morphology. The appearance of PA6-15%AP char residue showed an inner char layer which was piled up by small, thick, and insular char, which should be attributed to the PA6-AP sample being a typical char-formed flame retardant system, and few gaseous products could be released during combustion. However, a sample containing APBu formed continuous and compact char at a micrometer level, which was much more similar to the typical intumescent flame-retardant system but much thinner and softer. APBu mainly took effect in gaseous phase by releasing volatile

3.4. Char Residue Analysis. Figure 8 illustrated the digital photographs of the flame-retardant samples after cone calorimetric analyses. It could be easily found out that all the outer view of chars of flame retardant samples were thick and heavy and continue after burning. However, the inner chars of PA6-15%AP and PA6-15%APBu were thin, porous, and crisp. On the contrary, the inner char of PA6-15%AP/APBu (1:1) was intumescent and compact and continues, which was an effective barrier for heat and mass transmission. To better understand the char formation process during burning, the morphology of the char residue collected from the samples after LOI tests were observed by SEM, and the 17167



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Figure 9. Flame-retardant processes and scanning electron micrographs of the inner surface of the char obtained from the testing samples after LOI tests: (a) PA6-15%AP, (b) PA6-15%APBu, (c) PA6-15%AP/APBu (1:1).

Figure 10. Raman spectra of chars produced after cone calorimetric analyses: (a) PA6-15%AP, (b) PA6-15%APBu, (c) PA6-15%AP/APBu (1:1).

From Figure 10, the R values for PA6-15%AP/APBu (1:1) (R = 3.96) and PA6-15%APBu (R = 3.95) were higher than that of PA6-15%AP (R = 3.62), suggesting that the size of carbonaceous microstructures from the PA6-15%AP/APBu (1:1) sample could be quite smaller than that from the other samples. Bourbigot et al.31 reported that the smaller size of carbonaceous microstructures was related to a higher protective shield retardance. Consequently, it could be proposed that the AP and APBu mixture repressed the decrease of the carbonaceous microdomain in size during combustion, which led to the formation of higher protective shield from the charred layers.

phosphorus-containing fragments as free radical scavengers. When AP and APBu were combined together to flame retard PA6, an expected thick and continuous char layer was formed, meaning that the binary flame retardants containing AP and APBu took effect in both condensed and gaseous phase together. On one hand, the AP component mostly acted in condensed phase by salification cross-linking toward aromatization and charring; on the other hand, phosphorus-containing volatiles derived from APBu mainly acted in gaseous phase to quench the free radicals and hence to inhibit burning. Besides, volatiles in such case could play a role as blowing agents in a typical intumescent flame-retardant system, leading to an expansion of the burning zone and the formation of the swollen charring layers, which could exhibit positive influence on flame retardance.26−28 Laser Raman spectroscopy (LRS) is proved to be a suitable measurement to study the carbonaceous char of the testing materials generated after burning.29,30 Figure 10 showed the Raman spectra of the char layers after cone calorimeter testing. All of the chars exhibited two broad absorption peaks at about 1580 and 1350 cm−1 (1590 and 1360 cm−1 in this case), which should be assigned to the polyaromatic species or so-called graphitic structures.31 The peak found at about 1590 cm−1 (G peak) was the E2g vibrational mode (C−C vibrations), and the bond at 1360 cm−1 (D peak) was attributed to the defect band,32 rooted in the A1g vibration mode of the “disordered” or amorphous carbon in the char. Tuinstra and Koening found that the relative intensity ratio of D/G (noted as R, which defined as the value dividing the intensity of D peak by G peak) is conversely proportional to the in-plane microcrystalline size.33

4. CONCLUSIONS In this paper, AP and APBu were utilized together as a novel binary flame-retardant system to improve the flame retardance of PA6. Experimental results indicated that AP and APBu could not interact with each other in PA6 by heating, whereas the flameretardant synergism was found between AP and APBu in PA6. The most favorable properties for flame-retardant PA6 could be obtained when the weight ratio of AP to APBu was 1:1. In detail, the LOI value achieved 28.3 vol % with UL-94 V-0 rating for the sample with 15 wt % of the binary flame retardants. The sample containing the binary flame retardants was endowed with lower PHRR value, mass loss rate, and total smoke rate value, as well as higher residue char in comparison with those of the samples containing AP or APBu alone. More effective char layers for flame retardance were formed. Notably, cross-linking reactions occurred in samples containing AP during heating. AP component of the binary flame retardants dominated in condensed phase by salification cross-linking toward aromatiza17168



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tion and charring, while phosphorus-containing volatiles derived from APBu exerted an effect in gaseous phase to quench the free radicals and hence to inhibit burning. Additionally, the volatiles in such case could act as blowing agents in the intumescent flameretardant system, leading to expansion of the burning zone and the formation of swollen charring layers, which acted as a protective shield against the transition of heat, oxygen, and combustible volatiles, as well as further combustion. A thick and continuous char was formed finally. In the light of above issues, an expected flame-retardant synergism between AP and APBu in both condensed and gaseous phase together was proposed.

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ASSOCIATED CONTENT

S Supporting Information *

Preparation and characterization of APBu. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*L. Chen. Tel. and fax: +86-28-8541755. E-mail: l.chen.scu@ gmail.com. *Y.-Z. Wang. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports by the National Natural Science Foundation of China (grant nos. 50933005 and 51121001) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026) would be sincerely acknowledged. The authors would also like to thank the Analysis and Testing Center of Sichuan University for the laser Raman measurements.

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REFERENCES

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Synergistic Effect between Aluminum Hypophosphite and Alkyl

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