Aluminum Hypophosphite versus Alkyl-Substituted Phosphinate in

Article pubs.acs.org/IECR

Aluminum Hypophosphite versus Alkyl-Substituted Phosphinate in Polyamide 6: Flame Retardance, Thermal Degradation, and Pyrolysis Behavior Bin Zhao, Li Chen,* Jia-Wei Long, Hong-Bing Chen, 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, Chengdu 610064, China S Supporting Information *

ABSTRACT: Aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu) were used to flame retard polyamide 6 (PA6). Addition of either AP or APBu resulted in an increased LOI value, UL-94 V-0 rating, and decreased heat release in cone calorimetric tests. However, different chemical structures of two flame retardants caused different flame-retardant effects: APBu endowed PA6 a higher LOI value and better UL-94 result than did AP. Decomposition pathways of AP, APBu, and the corresponding composites were investigated using TGA, TG-IR, Py-GC/MS, and FTIR characterization of the residues. The introduction of AP changed the thermal stability and decomposition behavior of the composites due to the cross-linking reactions occurred, which were proved by rheological analysis and TG-DSC. APBu could not essentially affect the composition of pyrolysis products and decomposition behaviors, but mainly produced phosphorus-containing free radical scavengers in the gaseous phase, which were positive to flame retardation. Finally, the proposed flame-retardant mechanisms of such systems were summarized. the former presented better flame retardancy in PA6 than the latter. Moreover, aluminum salts of phosphinic acid mixture of diisobutylphosphinic acid and monoisobutylphosphinic acid were found to be effective for glass fiber reinforced PA6 in our previous work.17 Despite their increasing development and study, there was still lack of scientific understanding as to their flame-retardant mechanism, especially thermal degradation and pyrolysis behavior. As is known, the effectiveness and mechanism of one flame retardant were not only related to the applied polymer, but also its chemical structure. In the present study, two different aluminum−phosphorus-containing flame retardants, aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu), were used to flame retard PA6. The flame retardancy, fire performance, thermal decomposition property, crosslinking behavior of AP system, pyrolysis behavior, as well as the decomposition products of flame retardant and flameretardant PA6 were discussed. The different potential flameretardant mechanism models were also revealed.

1. INTRODUCTION In recent years, effective halogen-free flame-retarded systems became one of the most popular topics of relevant materials research due to the environment problems of halogencontaining flame retardants. Phosphorus-based flame retardants like red phosphorus, inorganic, and organic phosphoruscontaining compounds were considered more effective to the most polymers, particularly to PA6.1−4 The color problem, flammability, and release of phosphine during processing limit the application of red phosphorus although it was one of the most effective flame retardants in polyamide and other polymers.5,6 In addition, metal phosphinates were proven to be effectively flame retardant active in both condensed and gaseous phases. The recently developed and commercialized aluminum diethylphosphinate (AlPi) was found to be effective in polyamides, polyesters, PMMA, and their glass fiber reinforced composites.7−10 Braun et al. found that AlPi acted mainly by flame inhibition when it was in combination with melamine polyphosphate and zinc borate in glass-fiberreinforced PA66.9 In our previous work, aluminum phenylphosphinate (BPA-Al) was proven to have low flame-retardant efficiency in glass fiber reinforced PA6;11 aluminum hypophosphite (AP) was synthesized and applied as an inorganic phosphinate flame retardant to glass fiber reinforced PA6 and PBT,12,13 and proved to be an effective flame retardant in these polymers. Yang et al.14,15 investigated the flame retardancy of PBT by aluminum hypophosphite/trivalent rare earth hypophosphite (REHP) in combination with melamine derivatives, and found that melamine derivatives reduced the condensedphase effect of REHP, but improved the flame inhibition in gaseous phase. Li et al.16 studied the effects of aluminum and magnesium hypophosphites on flame retardancy and found that © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials and Processing. PA6 pellets (Mn ≈ 30k; product code: YH-800) were purchased from Hunan Yueyang Baling Petrochemical Co., Ltd., China. PA6 was dried in an oven at 100 °C for 4 h prior to compounding. Aluminum hypophosphite (AP) and aluminum isobutylphosphinate Received: Revised: Accepted: Published: 2875

December 14, 2012 January 29, 2013 February 5, 2013 February 5, 2013 dx.doi.org/10.1021/ie303446s | Ind. Eng. Chem. Res. 2013, 52, 2875−2886

Industrial & Engineering Chemistry Research

Article

(APBu) were prepared in our laboratory. PA6 and the flame retardant were mixed in a tumbler by tumbling over all of the ingredients. The mixtures then were fed into a twin-screw extruder (L/D = 35) operating at about 220−240 °C, and then extrudates were cut into pellets. Finally, the pellets were compression molded and cut into standard testing bars. 2.2. Fire Testing. Vertical burning test was performed according to UL-94, and the dimensions of all samples with different thickness were 130 mm × 13 mm × 3.0 mm and 130 mm × 13 mm × 1.6 mm. The limiting oxygen index (LOI) values were performed according to GB/T2406.2-2009, and the dimension of all samples was 130 mm × 6.5 mm × 3.0 mm. The fire behavior of the samples was measured with a cone calorimeter device (Fire Testing Technology, East Grinstead, UK) according to ISO 5660-1. Samples with a size of 100 mm × 100 mm × 3 mm were exposed to a radiant cone (50 kW/ m2). The plaques were placed in the sample holder with a retainer frame, resulting in 88 cm2 of the sample surface being exposed to the radiation from the cone heater. 2.3. Thermal Analysis. Thermogravimetric (TG) experiments were performed using NETZSCH TG 209 F1 apparatus with a nitrogen flow of 50 mL/min. Samples (about 5 mg) were heated in Al2O3 pans, from 40 to 700 °C at heating rates of 20 °C/min. The onset decomposition temperature, T5%, at which 5 wt % of original weight was lost, and T max, at which products possessed the maximum weight loss rate, were recorded together with the residue weight. TG was coupled with a FTIR spectrometer (Nicolet 6700) via a transfer line (TG-IR) to determine the vapor products during decomposition. The temperature of the transfer line and the FTIR cell was maintained at 250 °C to avoid solidification of the volatile products. About 12.5 ± 2.5 mg of sample was put in an alumina oxide crucible and heated from 40 to 700 °C. The heating rate was set as 10 °C/min in a nitrogen atmosphere with a flow rate of 50 mL/min. Each spectrum of the series spectra was acquired from 8 scans with a resolution of 4 cm−1 every 8 s. The height of the product characteristic peak was used to evaluate the product release rate as a function of time. Solid residues collected during TG at heating rates of 10 °C/min at different thermal degradation zone were investigated via FTIR spectroscopy (Nicolet 6700). Dynamic oscillatory rheological measurements of pure PA6 and the flame-retardant composites were performed with a parallel-plate fixture (25 mm diameter and 1 mm thickness) using an Advanced Dynamic Rheometric Expansion System (ARES, Bohlin Gemini 200) in an oscillatory shear mode. Temperature scanning tests at a fixed frequency of 1 Hz were recorded in the range from 230 to 300 °C. Cross-linking behavior was performed on a NETZSCH simultaneous TGA-DSC (449C) at a heating rate of 10 °C/min in N2. 2.4. Pyrolysis GC/MS. Py-GC/MS tests were performed in a Pyroprobe (CDS5000). The pyrolysis chamber was full of He, and the relevant samples (500 μg) were heated from ambient to 600 °C at a rate of 1000 °C/min and kept for 10 s. The pyrolzer was coupled with a GC/MS operation (6890N-5975). For the operation, the temperature program of the capillary column of GC was as follows: 2 min at 40 °C, temperature increased to 300 °C at a rate of 10 °C/min and kept at 300 °C for 10 min. The injector temperature was 250 °C. MS indicator was operated in the electron impact mode at electron energy of 70 eV with the electron source being kept at about 180 °C. The detection of mass spectra was carried out using a NIST library.

3. RESULTS AND DISCUSSION 3.1. Flame Retardancy. The vertical burning tests (UL-94) and LOI were used to investigate the flame retardancy of the Table 1. Flame Retardancy of the Samples UL-94 (3.2 mm) sample PA6 PA6-15%AP PA6-20%AP PA6-25%AP PA6-15%APBu PA6-20%APBu PA6-25%APBu

t1/t2a(s) 15/35 2/11 1/5 1/1 6/15 1/3 1/2

UL-94 (1.6 mm)

rating

t1/t2 (s)

b

c

NR V-2 V-0 V-0 V-2 V-0 V-0

50/BC 30/BC 13/10 1/1 12/5 2/15 1/8

rating

LOI (%)

NR NR V-2 V-2 V-2 V-1 V-0

20.5 25.7 26.3 27.8 28.1 31.9 34.6

Average combustion times after the first and the second applications of the flame. bNo rating. cBurn to clamp. a

Figure 1. Char layer of the samples after LOI tests: (a) PA6; (b) PA620%AP; and (c) PA6-20%APBu.

Figure 2. Char layer of the samples after UL-94 tests (3.2 mm if not specifically annotated).

samples, and the results were summarized in Table 1. For APcontaining series, the LOI values increased from 20.5% to 27.8% when the content of AP reached 25 wt % in the flameretardant samples. For APBu samples, LOI values increased with the increase of APBu contents and reached 28.1% and 2876

dx.doi.org/10.1021/ie303446s | Ind. Eng. Chem. Res. 2013, 52, 2875−2886


Article

Table 2. Cone Calorimetric Data Obtained with a Heat Flux of 50 kW/m2 specimen

TTI (s)

PHRR (kW/m2)

tpa (s)

FIGRAb

MAHREc (kW/m2)

THR (MJ/m2)

TSR (m2/m2)

Av-EHC (MJ/kg)

PA6 PA6-25%AP PA6-25%APBu

48 32 37

789 180 218

155 70 100

5.09 2.57 2.18

400 104 133

98 63 73

490 1082 1208

30.8 24.6 24.9

a tp means time to PHRR. bFIGRA is calculated by dividing the peak HRR by the time to HRR. cAHRE means average rate of heat emission, which is defined as the cumulative heat emission divided by time; MAHRE denotes the maximum average rate of heat emission.

Figure 4. Vertical view of the samples after cone calorimeter tests: (a) PA6; (b) PA6-25%AP; and (c) PA6-25%APBu.

Figure 5. CO production rate curves of the samples at 50 kW/m2.

and qualitatively compact. As shown in Table 1, APBu endowed the materials higher LOI values than AP. One reason might be that APBu promoted the material forming bulk char layer (similar to the char layer of intumescent flame retardant), which could isolate heat and flammable gases from the unburned material during LOI test, while another reason was that beyond the condensed activity, APBu might exhibit gaseous-phase activity, which should be the main difference between AP and APBu-containing flame-retardant PA6. The vertical burning test (UL-94) measured flammability and flame spread of plastic materials exposed to a small flame.18 During tests, V-0 rating could be achieved when the material extinguished in less than 10 s after both first and second flame applications, which required the flame retardant to work in a short time.17 In UL-94 test, the pure PA6 could not extinguish itself, and there was serious melt dripping during combustion. However, flame-retardant samples passed V-0 rating (3.2 mm) and V-2 rating (1.6 mm) when 20 wt % or 25 wt % AP was added; sample containing 20 wt % APBu could achieve V-0 rating at thickness of 3.0 mm and V-1 rating at 1.6 mm; when the content of APBu increased to 25 wt %, a V-0 rating at 1.6 mm was obtained. Burning residues of the samples after UL-94 tests were shown in Figure 2. Pure PA6 could not form any char layer during UL-94 test. Some researchers reported that

Figure 3. Cone calorimetric results of the experimental samples at an external heat flux of 50 kW/m2 as a function of burning duration: (a) heat release rate; (b) total heat release; and (c) mass loss curves.

34.6% when the flame-retardant contents were 15 and 25 wt %, respectively. Figure 1 gave the digital photos of burning residue after LOI tests. For pure PA6, there was no char residue formed during combustion, which led to the lowest LOI value. Contrarily, samples containing AP or APBu formed obvious char layer after LOI tests: PA6 with APBu formed expandable char layer, while the char of AP system was quantitatively small 2877



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Figure 6. The results of TG-IR test (under N2) and characterization of pyrolysis products of AP: (a) TG curve; (b) Gram−Schmidt curve; (c) FTIR spectra of the evolved gases produced; and (d) FT-IR spectra of the condensed residue at different temperatures.

release (THR), total smoke rate (TSR), and average effective heat combustion (Av-EHC). Two important parameters to evaluate the fire safety, fire growth rate (FIGRA), and maximum average rate of heat emission (MARHE) were defined as the value dividing the peak HRR by the time to PHRR and maximum average rate of heat emission, respectively.20,21 The PHRR of a specimen in the cone calorimeter is not an “intrinsic” material property but may be most important with regard to the assessment of real fire hazard.20 As shown in Table 2, as compared to that of pure PA6, the PHRR values of PA6-25%AP and PA6-25%APBu samples decreased from 789 to 180 (22.8%) and 218 kW/m2 (27.6%), respectively. Only the whole HRR curve could adequately represent the fire behavior controlled by the material specific properties (char yield, effective heat of combustion, etc.), the influences from the specimen (thickness, deformation, etc.), as well as the physical and chemical mechanisms during burning (increasing and cracking char layer, endothermic reactions, release of different pyrolysis products, afterglow, etc.).20 The whole HRR curves of the samples were illustrated in Figure 3a. PA6 burned quickly after ignition, and a sharp peak turned up at 155 s, which was followed by a sudden decrease as a function of burning duration. When AP and APBu were added, the HRR curves showed strong dissimilarity to that of pure PA6, which were gradually sloping down after the maximum values reached. HRR curves of flame-retardant samples were much lower and flatter than that of PA6, which were typical for char-forming materials.9 From Table 2, it could be observed that the TTI were decreased when AP or APBu was added into PA6, which might be due to the initial decomposition of flame retardants.

unfilled PA6 could achieve a V-2 UL-94 classification due to the dripping mechanism;17,19 however, in our test it was no rating because the combustion time after the second applications of the flame was more than 30 s with flaming drops, which might be due to the different product code of PA6. As shown in Figure 2, the specimens PA6-20%AP and PA6-25%AP were burnt slowly and formed char layer, which led to the samples to extinguish and pass V-0 rating (3.2 mm). PA6-25%AP only achieved V-2 rating at 1.6 mm because during the UL-94 test there was a flaming drop igniting the cotton below after the second application of the flame, although the extinguishing time was only 1 s. Char layers of PA6-20%APBu and PA6-25%APBu were poorer than that of AP systems due to the slow burning with relatively weak flame applied during UL-94 tests, which further confirmed that condensed activity was not the only factor to flame retardancy. Combining the analyses of LOI and UL-94 tests, it was believed that both AP and APBu were effective flame retardants for PA6. However, APBu endowed the materials higher LOI values and better UL-94 rating than AP. The further mechanisms will be discussed later. 3.2. Fire Behavior. The cone calorimeter was a useful apparatus for fire safety engineers and researchers interested in quantitative analysis of materials’ flammability. It was one of the most useful bench-scale tests, which allow prediction of fire performance in real scale fires.18,20 To investigate fire behaviors of the flame-retardant samples, cone calorimeter tests with a heat flux of 50 kW/m2 were conducted. The data obtained from cone calorimeter were summarized in Table 2, including time to ignition (TTI), peak heat release rate (PHRR), time to peak heat release rate (tp), total heat 2878



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Figure 7. The results of TG-IR test (under N2) and characterization of pyrolysis products of APBu: (a) TG curve; (b) Gram−Schmidt curve; (c) FT-IR spectra of the evolved gases produced; and (d) FT-IR spectra of the condensed residue at different temperatures.

To evaluate the fire hazard of the samples, FIGRA and MAHRE had been introduced. As shown in Table 2, when either AP or APBu was added into PA6, both FIGRA and MAGRE of the flame-retardant samples decreased, suggesting a better fire safety could be obtained. The FIGRA value of PA625%APBu (2.18) was a little lower than that of PA6-25%AP (2.57), while the PA6-25%APBu possessed a bigger MAHRE value (133 kW/m2). The lower FIGRA could be attributed to the longer TTI of PA6-25%APBu than that of PA6-25%AP; the bigger MAHRE was due to the greater PHRR and THR values. Figure 3b and c illustrated the THR and mass loss curves of the materials. The curves revealed that the descending order of THR was PA6 > PA6-25%APBu > PA6-25%AP, opposite to the order of burning residues. At the same radiant flux, THR depended on the total mass loss, the effective heat of combustion (EHC) of the volatiles, and the combustion efficiency in the flame zone.20 From the above analyses, it could be concluded that AP was a flame retardant that promoted the char formation of the polymer during combustion, while the charring ability of PA6-APBu was comparatively weaker. The vertical view of the samples after cone calorimeter tests (Figure 4) could directly prove that. For PA6-25%AP, it could be observed that a thick, compact, and continuous char had been formed after cone test (Figure 4b); however, APBu only promoted a bulgy but thin and weak char after combustion (Figure 4c). Certainly, pure PA6 left nothing after burning (Figure 4a). The analysis of cone data further provided some evidence of gaseous flame-retardant mechanism in FRPA6. Apparently, the TSR value of FRPA6 increased from 490 to 1082 (PA6-AP)

Figure 8. Intensity of different pyrolysis products for APBu during TG-IR test.

Moreover, there was the same descending order for TTI and PHRR: PA6 > PA6-25%APBu > PA6-25%AP. The relationship between TTI and HRR could be explained on the basis of an early ignition allowing a less accumulation of combustible volatiles to give rise to the reduction of HRR after ignition.22 TTI of PA6-25%APBu was a little higher than that of PA6-25% AP sample, indicating that there were less flammable gases released from APBu system than from AP before ignition during cone tests, which were further confirmed by the thermogravimetric analyses. 2879



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Figure 9. TG and DTG curves of the samples under N2: (a) TG curves of PA6-AP; (b) DTG curves of PA6-AP; (c) TG curves of PA6-APBu; and (d) DTG curves of PA6-APBu.

Table 3. TG Data of the Samples under N2 Atmosphere

samples PA6 PA6-15% AP PA6-20% AP PA6-25% AP PA6-15% APBu PA6-20% APBu PA6-25% APBu

Tmax2 (°C)

mass loss rate at Tmax (wt %/min)

residues at 700 °C (wt %)

T5% (°C)

Tmax1 (°C)

397 344

445 395

42 30

0 17.4

343

398

28

18.3

339

390

26

23.8

373

378

459

9

31

4.3

373

380

460

11

29

4.5

371

382

463

14

25

6.7

Figure 10. Calculated and experimental TG curves of the composites under N2.

and 1208 m2/m2 (PA6-APBu), respectively. As shown in Figure 5, FRPA6 produced more CO than pure PA6 during combustion. Among them, PA6-AP generated greatest CO production, which was attributed to the incomplete combustion. EHC was another important parameter to evaluate the heat released from combustion of the volatile portion of the testing materials. The higher smoke production and lower EHC value indicated that noncombustible gases existed in the gaseous phase. The average EHC of PA6-AP and PA6-APBu were similar to each other, while the TSR of PA6-APBu was much higher. Consequently, PA6-APBu produced more noncombustible gases during the cone test than did PA6-AP.

Both condensed and gaseous-phase flame-retardant mechanisms worked when AP or APBu was added to PA6. PA6-AP showed more condensed-phase mechanism, while PA6-APBu exhibited more gaseous-phase mechanism. Further detailed flame-retardant mechanisms would be discussed thereinafter. 3.3. Thermal Analysis. AP and APBu. To understand the thermal stability and decomposition behaviors of the flame retardants, TG-IR tests were carried out. Meanwhile, solid residues collected during TG at a heating rate of 10 °C/min at different thermal degradation zones were investigated via FTIR spectroscopy. Figures 6 and 7 showed the related spectra of AP 2880



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Scheme 1. The Pyrolysis Mechanism of AP

Figure 11. Complex melt viscosity plotted against temperature at a heating rate of 10 °C/min in an air atmosphere.

Figure 14. The pyrogram of the pyrolysis products of APBu, PA6, and PA6-25%APBu.

Figure 6c gave FT-IR spectra of the evolved gases produced by AP at certain typical temperatures attributing to T5%, Tmax1, Tmax2, and Tfinal. It could be observed that phosphine (PH3, ∼2400 cm−1) occurred from 328 to 347 °C, which was the main gaseous-phase product at the first decomposition stage of AP. From 347 to 540 °C, PO2− anion could be found at 1235, 1075, 875 cm−1.23,24 FT-IR spectra of the AP residues at different temperatures were shown in Figure 6d: Al−O (480 cm−1) persisted in residue from room temperature to 500 °C; while characteristic vibrations for P−H (2409, 2378 cm−1) and PO2− anion (1190, 1075, 829 cm−1) gradually disappeared with the increase of sampling temperature, meaning that PO2−containing substances existed into the gaseous phase rather than persisting in the condensed residues. However, PO32−/ PO43− anion (1180 cm−1)23 could be observed in the residue from 330 to 500 °C. Figure 7a gave the TG curve of APBu under N2; some typical temperature points like T5%, Tmax, and Tfinal were marked. The initial decomposition temperature (T5%) of APBu was 356 °C, which was higher than that of AP (336 °C). The flame retardant decomposes quickly from 350 to 500 °C, and the final residue was 27 wt %, much lower than that of AP (70 wt %). The above-mentioned differences indicated that APBu could produce more volatile products than did AP. According to the TG and Gram−Schmidt results (Figure 7a and b), Figure 7c gave FT-IR spectra of the evolved gases produced by APBu at some typical temperature zones. At the initial decomposition zone (zone I and II; 319−359 °C), weak peaks of CH3− and −CH2− groups (about 2900 cm−1) were detected. When the temperature increased to 395 °C (zone III), different kinds of evolved substances were released: CH3−, −CH2− (3082, 2962 cm−1), −CH(CH3)2 (1462, 1381 cm−1), PO2− (1084, 885 cm−1), and PH3 (∼2400 cm−1), where the maximum weight

Figure 12. TG-DSC thermograms of PA6-25%AP at a heating rate of 10 °C/min in N2.

Figure 13. The pyrograms of AP, PA6, and PA6-25%AP.

and APBu, respectively. It was found that the TG curve of AP under N2 (Figure 6a) showed two stages: initial decomposition temperature (defined as the temperature of 5 wt % weight loss, T5%) was 336 °C, while the DTG peak of the first stage appeared at 343 °C (Tmax1, also the overall maximum decomposition); the second decomposition stage was relatively smooth, and the DTG peak appeared at about 430 °C (Tmax2). 2881



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Scheme 2. The Pyrolysis Mechanism of APBu

Figure 15. Schematic diagram of proposed flame-retardant mechanism model of PA6-AP composite.

to PO32−/PO43− anion (1115 cm−1) turned up. Much the same as AP, Al−O (480 cm−1) persisted in residue all the while. The intensity of different pyrolysis products for APBu during TG-IR test was illustrated in Figure 8. It was clear that different alkyl segments were released between 35 min (about 390 °C) and 42 min (about 450 °C). The signal of P−O/PO was much weaker than that of alkyl and CO2 during the test, which was in accord with the relatively low phosphorus content of the alkyl-substituted phosphinate, particularly as compared to AP.

loss occurred. When the temperature further increased, the signals of alkyl groups became weak, while the absorption of PO2− anion moved to 1175 cm−1. FT-IR spectra of the APBu residue sampled at different temperatures were shown in Figure 6d. During 360−400 °C, the signal of P−H (∼2400 cm−1) disappeared; also, the absorption of alkyl groups (2955, 2871, ∼1400 cm−1) and P−C groups (1250, 848 cm−1)25 became weak. After the sample was further heated from 450 to 600 °C, alkyl peaks disappeared at last; meanwhile, new peaks attributed 2882



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Figure 16. Schematic diagram of proposed flame-retardant mechanism model of PA6-APBu composite.

M(cal)PA6 ‐ X = 75% × M(exp)PA6 + 25% × M(exp)X

Flame-Retardant Samples. To further investigate the thermal behavior of the composites, thermogravimetric data under N2 were determined and analyzed. Figure 9 showed TG and DTG curves of the samples; and relevant data were given in Table 3. The main weight loss of pure PA6 in the range of 400−500 °C was the result of the release of water, carbon monoxide, carbon dioxide, hydronitrogens, and hydrocarbon fragments.26 Certainly, there was no residue left after heating. When AP was added, the thermal stability and decomposition behaviors of the composites changed obviously. In Table 3, results indicated that the addition of AP decreased both the initial decomposition temperature (T5%) (approximately 50−60 °C lower than that of the pure PA6) and the maximum-rate decomposition temperature (Tmax) of the samples. Low thermal stability of AP and catalytic activity for accelerating decomposition of PA6 led to the changes of weight loss behavior.13,27 With respect to the DTG curves and the relevant data of PA6-APs (Figure 9b), the introduction of AP decreased the maximal mass loss rate remarkably by interfering with the main decomposition of the matrix at about 390 °C, and the rate of mass loss was further decreased with the increase of AP. Obviously, the introduction of AP could increase the final residue of the samples largely (0−23.8 wt %). As compared to pure PA6, the initial decomposition temperature (T5%) of PA6APBu decreased to 370 °C, which was higher than that of PA6AP. The reason could be attributed to its higher initial decomposition temperature. As shown in Figure 9d, there were two mass-loss stages caused by the inherent property of APBu, which was different from the PA6-AP system. The same as AP, the addition of APBu decreased the maximal mass-loss rate in both two stages. However, unlike the AP system, APBu exhibited a slight effect on promoting the final residue of the samples after TG (0−6.7 wt %). To further study the difference between the thermogravimetric behaviors of PA6-AP and PA6APBu composites, calculated and experimental TG curves of the PA6-25%AP and PA6-25%APBu under N2 were illustrated in Figure 10. The calculated TG curves were calculated by eq 1:

(1)

where X is AP or APBu, PA6-X denotes the samples of PA625%AP or PA6-25%APBu, M(exp)PA6 and M(exp)X are the experimental mass of PA6, AP, or APBu, respectively, and M(cal)PA6‑X represents the calculated mass of PA6-X. For PA625%AP, the experimental decomposition temperature was much lower than that of calculated data (decreased by about 50 °C). As compared to the calculated final residue, the real one increased by 6 wt %. The difference between the calculated and experimental curves indicated that some reactions occurred between AP and PA6; thus the composite decomposed in the low temperature zone and formed more thermostable residue at high temperature. Contrarily, two curves of PA6-25%APBu were close to each other. The introduction of APBu slightly decreased the initial decomposition temperature and the final residue, which indicated that main decomposition products of APBu were released in gaseous phase during heating, rather than affecting the decomposition process of the matrix. For better understanding the thermal behavior and flameretardant mechanism of the composites, dynamic oscillatory rheological measurements were introduced to investigate the viscoelastic behaviors of the testing samples during the heating process. Figure 11 illustrated the temperature dependence of complex viscosity (|η*|) of the flame-retardant samples and pure PA6 for comparison. |η*| of both PA6 and PA6-25%APBu showed a linear decrease during the heating process, but the complex viscosities of the latter were much higher than those of pure PA6. It could be observed that, however, the plots of PA625%AP showed a completely different trend as a function of temperature. |η*| of PA6-25%AP first showed a decrease, and subsequently increased sharply from Tc (where |η*| starts to increase or cross-linking reaction occurs28−30), then a “Ushape” change could be observed obviously in the whole temperature range. This behavior could be explained as follows: at the beginning of the test, a gradual increase of the mobility of polymer chains resulted in a decrease in |η*| of PA6-25%AP 2883



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the analyses above, the pyrolysis mechanism of APBu could be presented as in Scheme 2. There were more products in the gaseous phase and little substance in the solid phase, which was in accord with TG and cone tests. The evaporated free radicals derived from alkylphosphinic anhydrides could scavenge H· and OH· in the flame, cut off the decomposition radical reactions to decrease the supply of combustible volatiles, and consequently exhibit flame inhibition in the gaseous phase. It could be concluded that APBu did not essentially affect the composition of pyrolysis products: the products detected in PA6-25%APBu could have been found both in flame-retardant PA6 and the pure one except for peak 13 whose structure cannot been assigned via mass spectrum. Consequently, combined with the above analyses, APBu was a flame retardant, which could supply phosphorus-containing free radical scavenger and noncombustible gases in the gaseous phase during combustion, but does not affect the decomposed products of composites in gaseous and solid phases essentially. 3.5. Proposed Flame-Retardant Mechanism Models. According to the analyses above, the proposed flame-retardant mechanism models for PA6-AP and PA6-APBu composites were illustrated in Figures 15 and 16, respectively. When the PA6-AP composite was burnt or heated, heterolytic cleavage of PA6 could occur at the main chain. Phosphine (PH3), hypophosphorous acid (H3PO2), and red phosphorus (P4) were evolved from the AP dispersed particles. In the condensed phase, the elimination of a proton from the hypophosphorous acid would lead to the formation of a radical (c). The disproportionation termination of radical (c) with PA6 segmental radicals (a) and (b) generated from the heterolytic cleavage of PA6 resulted in the formation of the hypophosphorus acid-terminated products (d) and (e). The residual proton of (d) and (e) in P−H bond could lose again to form new radicals, which would further recombine (a) and (b) to generate phosphinic acid-coupled product (f) at last. The salt formation reaction would occur between (d), (e), or (f) and the Al3+ to produce cross-linked materials. The cross-linked materials along with Al4(P2O7)3 and AlPO4 directly from AP composed the char-like residue during burning, which could act as a physical barrier to prevent the mass/heat transfer and protect the inner materials away from fire. In the gaseous phase, ε-caprolactam, alkenes, and nitriles substances were produced, which would further generate some noncombustion volatiles like NH3 and CO2 at last. For the PA6-APBu system, pyrolysis results indicated that APBu did not essentially affect the composition of condensed products, and the flame retardant mainly acted in the gaseous phase. As shown in Figure 16, APBu could produce many phosphorus-containing free radicals, which acted as the radical scavengers to combine with the H· and OH· in flame and extinguish the fire. According to the fire and TGA tests, the amount of experimental residue of PA6-APBu was close to that of the calculated value; therefore, the condensed-phase mechanism was less as illustrated in the schematic diagram. Like the AP system, the char residues were compounded with inorganic salts, little cross-linked materials, resulting in the incomplete combustion. Some noncombustion volatiles like NH3, CO2 were also evolved.

with an increase of the temperature, or the occurrence of the degradation of the polymer somewhat; and from 270 to 300 °C, a cross-linking reaction or the reaction overwhelming the trivial decomposition of the composite occurred, which resulted in an abnormal increase of |η*|. In some research,28−32 high |η*| caused by cross-linking networks could reduce the flammability of the polymer materials effectively via suppressing the vigorous bubbling process in the course of degradation during combustion, and simultaneously inhibit the dripping effectively. The cross-linking networks also promoted forming a solid and char-like heat transfer barrier, which could play an important role in extinguishing the samples. In fact, the samples with AP exhibited high residue and stable char layer after TGA, UL-94, LOI, and cone tests. To prove the existence of the cross-linking reactions, the thermal behavior of PA6-25%AP was characterized by simultaneous thermogravimetry-differential scanning calorimetry (TG-DSC) in N2 (Figure 12). For the DSC curve, a notable exothermal peak (ca. 325 °C) just between the melting (ca. 220 °C) and the decomposition peak (ca. 380 °C) belonged to the cross-linking reaction.30 3.4. Pyrolysis GC/MS. To further investigate the composition of pyrolysis products and speculate the decomposition mechanisms of two flame retardants and the corresponding composites, pyrolysis GC/MS tests were performed. Figure 13 illustrated the pyrograms of the pyrolysis products of AP, PA6, and PA6-25%AP composites, respectively. Among them, AP decomposed and released three volatile products, including phosphine (PH3), hypophosphorous acid (H3PO2), and red phosphorus (P4). PH3 and H3PO2 were detected in the gaseous phase during TG-FTIR analysis, except for P4. The differences were due to the different testing conditions, particularly for the heating programs.33 Moreover, hypophosphorous acid could react with PA6 to form cross-linking structure during heating. According to the pyrolysis analysis, the decomposition reactions of AP could be represented as in Scheme 1, which were much different from the decomposition process according to TG-IR analysis reported before.14 Pyrogram of PA6 showed the release of ε-caprolactam (peak 2)34,35 as well as different kinds of dimers (peaks 3−4),35 alkenes, and nitriles substance (peak 1). When AP was added, ε-caprolactam, alkenes, and nitriles substance from the pyrolysis of PA6 were still detected; also, red phosphorus released from AP could be found in the gaseous phase. On the one hand, main pyrolysis products of PA6 were not changed obviously as the introduction of AP. On the other hand, novel nitriles and acyclic amides were detected, indicating that the introduction of AP affected the pyrolysis process of PA6. According to the analyses above, AP played a more important role in the solid phase than in the gaseous phase, for instance, cross-linking reactions and formation of solid char layers, which could play a positive role in enhancing the thermal stability at high temperature and flame retardance. The pyrolysis products of APBu, PA6, and PA6-25%APBu composites were shown in Figure 14. Seven main pyrolysis products were detected in the gaseous phase when APBu was pyrolyzed. Alkylphosphine, red phosphorus, and other phosphorus-containing substances were produced (peaks 6− 12). Indeed, the characteristic vibrations of these volatile products were detected in TG-IR analysis except for red phosphorus. More importantly, the phosphorus-containing free radicals that could play an important role in flame-retardant composites were detected in the gaseous phase. According to

4. CONCLUSIONS In this work, aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu) were used to flame retard PA6. 2884



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The flame retardancy and fire behavior were investigated using UL-94, LOI, and cone calorimeter tests. The V-0 ratings (3.2 mm) are all achieved when the addition of the flame retardants amounts to 20 wt %. However, APBu endowed the material better UL-94 result and higher LOI values than did AP. The introduction of AP or APBu could change the shape of HRR curves of PA6, and also PHRR and THR values of the composites were decreased obviously. The thermal decomposition behaviors and decomposition products of flame retardants and the corresponding samples were investigated via TGA and TG-IR tests. Dynamic oscillatory rheological measurements were introduced to investigate the viscoelastic behaviors of PA6-AP composite of which the complex viscosity (|η*|) largely increased during heating. The presence of crosslinking reaction was proved via simultaneous TG-DSC test. From Py-GC/MS tests, the detailed pyrolysis products of the flame retardants and the flame-retardant composites were identified: without the alkyl substituents, AP mainly exhibited condensed-phase flame-retardant mechanism by the formation of phosphinic acid-coupled PA6 and hypophosphorus acidterminated PA6, which could saltify with aluminum ion to form cross-linking structures; contrarily, alkyl-substituted phosphinic salt APBu showed more gaseous-phase mechanism due to the phosphorus-containing free radicals acting as scavengers and fire inhibitors.

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ASSOCIATED CONTENT

S Supporting Information *

Pyrogram peaks and the corresponding structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86-28-8541755. E-mail: [email protected] (L.C.); [email protected] (Y.-Z.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (grant nos. 50933005 and 51121001) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT 1026) is sincerely acknowledged.

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Aluminum Hypophosphite versus Alkyl-Substituted Phosphinate in

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