Acrylonitrile–Butadiene–Styrene Terpolymer with Metal Hypophosphites

Jan 20, 2014 - retardant ABS were investigated by Underwriters Laboratories 94 vertical burning test (UL-94), limit oxygen index (LOI), and cone calor...
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Acrylonitrile−Butadiene−Styrene Terpolymer with Metal Hypophosphites: Flame Retardance and Mechanism Research Rong-Kun Jian, Li Chen,* Bin Zhao, Yuan-Wei Yan, Xiao-Fan Li, 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 ABSTRACT: Three metal hypophosphites, including aluminum hypophosphite (AP), magnesium hypophosphite (MP), and calcium hypophosphite (CP), were applied to flame retard acrylonitrile−butadiene−styrene (ABS). Thermal stability of three flame-retardant ABS were evaluated, and the enhancement of thermal stability were found. Flammable properties of flameretardant ABS were investigated by Underwriters Laboratories 94 vertical burning test (UL-94), limit oxygen index (LOI), and cone calorimetry. Results suggested that AP could endow the best flame retardance for ABS with a UL-94 V-0 rating and LOI value of 25.1%. The peak heat release rate of ABS-AP reduced to 174.8 kW/m2, and the total heat released was decreased to 40.9 MJ/m2. Thermogravimetric Fourier transform infrared (TG-FTIR), FTIR, and scanning electron microcsopy−energy-dispersive X-ray spectrometry (SEM-EDX) were used to characterize the gaseous products and condensed residue respectively. Results showed that the flame-retardant mechanism was attributed to the formation of a two-layer protective barrier consisting of an organic P−O−C char layer and an inorganic layer to insulate material from fire and oxygen in the condensed phase, and the generation of P• and PO• to capture the reactive radicals in the vapor phase.

1. INTRODUCTION Acrylonitrile−butadiene−styrene (ABS), with two-phase polymer systems in which polybutadiene is a discrete phase and styrene−acrylonitrile copolymer is a continuous phase, is a widely used thermoplastic resin due to its good mechanical performance, chemical resistance, and ease of processing.1−5 However, ABS has a low limit oxygen index (LOI) value and is easily combustible, mostly accompanied by the massive production of toxic gases and smoke, which restricts its wider application. With regard to that in many applications, including molded housings, automotive appliances, and electrical and electronic markets, etc., it is necessary for ABS to have a flameretardant grade.6 Nowadays, flame retardants in commercial use for ABS are mostly halogen-containing ones. Some halogen-containing flame retardants have been phased out for their proven or suspected adverse effects on the environment, and halogen-free flame retardants now are developed to meet the constantly changing demand of new regulations, standards, and test methods.7 Red phosphorus (RP) is an effective flame retardant; for example, the introduction of RP to ABS could provide a V-0 rating accompanied with a charring agent such as nylon8 and polyester.9 Unfortunately, RP easily absorbs moisture and is oxidized, which restrict its application. Metal hydroxides, such as aluminum hydroxide (ATH) and magnesium hydroxide (MH), are the most widely commercially used flame retardants.7,10−15 However, there is a fatal drawback of these two addictives in that high loading is needed, as a result of great deterioration in the physical/mechanical properties of the matrices. Dai et al.16−18 use an elastomeric polyacrylate latex (EPL) and a phosphate surfactant TX-10 (TX) to modify MH, giving a Underwriters Laboratories 94 vertical burning test (UL-94 V) V-0 rating for ABS with a high loading (60 wt %) of modified MH. Recently, metal hypophosphites, exemplified by © 2014 American Chemical Society

aluminum hypophosphite (AP), have been widely applied as flame retardant for different polymers. Zhao et al.19 report that the UL-94 rating of glass-fiber-reinforced PA6 could achieve V0 (1.6 mm) at the loading of 25 wt % AP; meanwhile, Luo et al.20 also obtain an excellent flame retardance of AP in poly(butylene terephthalate) (PBT) that a V-0 rating (1.6 mm) is achieved. To the best of our knowledge, metal hypophosphites are usually used for increasing the flame retardance of polycondensates, mostly polyamides and polyesters, but few works are focusing on the flame retardance of polyolefins with metal hypophosphites. Cai et al.21 use AP as a synergistic agent into an intumescent flame-retardant ABS, with 2 wt % addition of AP to the flame-retardant system, and it could pass the V-0 rating of the UL-94 test. In this work, aluminum hypophosphite (AP) prepared in our laboratory according to the previous literature is applied to flame-retard ABS19,22 compared with magnesium hypophosphite (MP) and calcium hypophosphite (CP). Flame-retardant properties of flame-retardant ABS (FR-ABS) composites are investigated by LOI, UL-94 V, and cone calorimetric analysis. Thermal stability behavior is researched by thermogravimetric analysis (TG). The residual morphology of FR-ABS after LOI test is investigated by scanning electronic microscopy (SEM). Moreover, TG-FTIR and scanning electron microcsopy− energy-dispersive X-ray spectrometry (SEM-EDX) analyses are used to study the flame-retardant mechanism of ABS-AP composites. Received: Revised: Accepted: Published: 2299

November 4, 2013 January 12, 2014 January 20, 2014 January 20, 2014 dx.doi.org/10.1021/ie403726m | Ind. Eng. Chem. Res. 2014, 53, 2299−2307

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2. EXPERIMENTAL SECTION 2.1. Materials. All of the materials and solvents were commercially available and were used without further purification. The ABS resin (MFI: 19 g/10 min, CHIMEI, PA-757K) was obtained from Zhenjiang, China. AP, MP, and CP were prepared according to the previous literature.19,22 2.2. Preparation of Flame-Retardant Samples. ABS resin and three different flame retardants AP, MP, and CP were dried in a vacuum oven at 80 °C for 4 and 8 h prior to being processed, respectively. In order to ensure the three flame retardants with the same phosphorus content in the composites, flame-retardant ABS samples were prepared with different contents of AP at the loadings of 20, 25, and 30 wt %, MP at the loadings of 20.8, 26.0, and 31.2 wt %, and CP at the loadings of 23.0, 28.7, and 34.4 wt %. The mixtures of ABS and flame retardants were fed into a twin-screw extruder operating at about 210 °C with a rotation speed of 100 rpm, and the extrudates were cut into pellets. 2.3. Characterization. Flame retardance of the target samples were measured by LOI, UL-94 vertical burning test, and cone calorimetric analysis. LOI values and UL-94 vertical burning ratings of virgin and added ABS with different contents of flame retardants were tested on an HC-2C oxygen index meter (Jiangning, China) according to ASTM D2863-97 and on a CZF-2 instrument (Jiangning, China) according to ASTM D3801-97, respectively. Testing bars of the samples with the three-dimensional size of 130 mm × 6.5 mm × 3.2 mm (LOI) and 130 mm × 13 mm × 3.2 mm (UL-94) were compressionmolded in 10 MPa and at 220 °C. The cone calorimeter tests were conducted to research the fire performance on a FTT cone calorimeter at different external heat fluxes of 35, 50, and 75 kW/m2 according to ISO 5660-1. Testing samples were injected into a room-temperature mold with a three-dimensional size of 100 mm × 100 mm × 3 mm by a K-TEC 40-155 injection-molding machine (Ferromatic Milacron Co., Europe), operating at 210, 220, 220, 220, and 210 °C from the inlet to the injection port. Thermogravimetric analyses (TGA, NETZSCH 209F1) were conducted at a heating rate of 10 °C/min from 40 to 700 °C under both nitrogen and air atmospheres with a purge flow rate of 60 mL/min. The structures of char residues after TGA testing were determined by Fourier transform infrared (FTIR), which was performed on a Nicolet FTIR 170SX infrared spectrometer with KBr pellets. TGA coupled with FTIR (TG-FTIR) through a heated transfer line set to 250 °C was used to detect the gaseous species during heating, and the measurements were carried out at a heating rate of 10 °C/min under nitrogen at a purge flow rate of 60 mL/min. Scanning electron microscopy (SEM) observation on a JEOL JSM-5900LV equipped with an energy-dispersive X-ray spectrometer (EDX, INCA, PENTAFETX3, OXFORD) was used to investigate the morphology and formation of burning residue after the LOI test.

Table 1. Results from LOI and UL-94 Tests of FlameRetardant ABS content of phosphorus in FR-ABS (%) 0 8.4

10.5

12.6

a

samples virgin ABS ABS−20.0% AP ABS−20.8% MP ABS−23.0% CP ABS−25.0% AP ABS−26.0% MP ABS−28.7% CP ABS−30.0% AP ABS−31.2% MP ABS−34.4% CP

LOI (%)

UL-94 (3.2 mm)a

18.5 23.5

NR NR

22.1

NR

21.5

NR

24.1

V-0

23.5

NR

22.1

NR

25.1

V-0

24.0

NR

23.5

NR

NR denotes no rating.

contents of phosphorus, ABS-AP exhibited the highest LOI values compared to those of ABS-MP or ABS-CP, and that LOI value reached 25.1% at the 30 wt % loading of AP; meanwhile, a UL-94 V-0 rating was obtained. It was interesting that the UL94 rankings of ABS added with MP or CP were all no ratings, and the LOI values were all considerably lower than that of ABS-AP. Generally, the phosphorus content was a crucial factor for the improvement in the efficiency of flame retardant;23 but, in this work, MP and CP both exhibited less effective flame retardance, though they had an equal content of phosphorus compared with that of ABS-AP. It was conjectured that the initial temperature of flame retardant was responsible for the different flame retardance; thus the thermal properties of flame retardant and FR-ABS were studied next. 3.2. Thermogravimetric Analysis of FR-ABS. To further understand the difference in the flame retardance of AP, MP, and CP, TG analysis of virgin ABS and different FR-ABS composites were detected to study the thermal stability and decomposition behavior, which were shown in Figure 1, and the corresponding data, including T5%, measured as the temperature at which 5 wt % of the sample was lost; Tmax1 and Tmax2, the temperatures obtained from DTG curves at which the maximum mass loss rate occurred during the first and second steps; and the char yield remaining at 700 °C were presented in Table 2. The thermal decomposition of ABS was revealed by only one degradation step started from 378.9 °C (T5%) and then degraded sharply with increasing temperature accompanied with a maximum mass loss rate of 20.7%/min at 424.7 °C (Tmax) in the nitrogen atmosphere, as shown in Figure 1a. And there was almost no residue remaining at 700 °C. No matter whether AP, MP, or CP was introduced to ABS, the maximum mass loss rate of ABS decreased; and with the increasing loading of flame retardant, it would decrease further, meanwhile the char residue increased gradually, as shown in Table 2. It could be seen that the initial decomposition temperature was different among ABS-AP, ABS-MP, and ABS-CP, which was attributed to the different T5% among AP, MP, and CP, and the thermal curves of three flame retardants were shown in Figure 2. The thermal decomposition of ABS under air shown in

3. RESULTS AND DISCUSSION 3.1. Flame Retardance of FR-ABS. To investigate the flame retardance of FR-ABS, the LOI values and UL-94 rating were tested, and the results were listed in Table 1. It was found that the LOI values increased with the increasing addition content of AP, MP, or CP, respectively. While at the same 2300

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Figure 1. TG and DTG curves of FR-ABS under nitrogen (a, c, e) and air (b, d, f) atmosphere.

Table 2. Thermal Decomposition Parameters of Virgin ABS and FR-ABS from TG in Nitrogen and Air Atmosphere at a Heating Rate of 10 °C/min nitrogen atmosphere

samples

T5% (°C)

Tmax (°C)

mass loss rate at Tmax (%/min)

virgin ABS ABS−20% AP ABS−25% AP ABS−30% AP ABS−20.8%MP ABS−26.0%MP ABS−31.2%MP ABS−23.0%CP ABS−28.7%CP ABS−34.4%CP

378.9 348.3 349.7 349.1 367.5 367.0 365.9 382.0 380.1 381.1

424.7 425.7 425.2 420.5 427.2 427.4 425.8 426.9 427.4 428.4

20.7 15.9 15.7 13.2 18.2 17.0 15.4 17.4 16.7 14.8

air atmosphere residue at 700 °C (wt %)

T5% (°C)

0.4 17.8 18.4 25.3 15.9 22.2 26.1 21.4 22.3 29.0

355.9 345.5 343.3 346.3 351.9 355.2 356.0 356.7 361.9 366.0

Figure 1c,d was slightly different from that under nitrogen. Two degradation steps emerged; the first one was observed in the temperature range of 300−500 °C, and then the residue formed at the first stage continued to degrade at the second stage from 500 to 600 °C; as a result, there was almost no residue left. When flame retardant was added to ABS, there was little change in the initial decomposition temperature and Tmax1.

Tmax1 (°C)

mass loss rate at Tmax1 (%/min)

414.9 402.8 400.3 407.2 405.5 401.3 397.4 417.0 418.4 418.5

19.4 12.6 11.2 11.4 21.7 23.6 23.2 16.7 17.7 16.4

Tmax2 (°C)

mass loss rate at Tmax2 (%/min)

residue at 700 °C (wt %)

551.2 546.5 554.9 554.5 523.0 521.3 522.4 528.4 523.9 530.7

2.1 0.9 0.9 0.7 1.8 1.6 1.5 1.5 1.3 1.2

0.1 18.1 24.3 28.0 19.3 25.4 30.5 23.7 26.6 31.9

Generally, the maximum mass loss rate of FR-ABS would decrease with the addition of flame retardant, which was suitable for ABS-AP and ABS-CP, while it was not applicable to ABS-MP; the maximum mass loss rates of ABS-MP were larger than virgin ABS and increased with the introduction of MP. With regard to the second degradation step, when AP, MP, or CP was added, the maximum mass loss rates (Tmax2) were all 2301

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earlier to improve the forming of a thermostable residue to protect the material, thus enhancing the thermal stability of ABS at high temperature, while MP or CP could not work as effectively as AP did. 3.3. Cone Calorimetric Results. The cone calorimeter is widely used as a tool to evaluate the flammability and burning behaviors of materials, and hence to assess the fire performance of flame-retardant ABS in this work. Various parameters are obtained from the cone calorimetric tests, including time to ignition (TTI), heat release rate (HRR), peak heat release rate (PHRR), time to peak heat release rate (TTPHRR), total heat released (THR), average effective heat combustion (av-EHC), and residual mass. Among these parameters, HRR and PHRR are important for assessing the fire safety:24 HRR determines the speed of fire development, while PHRR presents the intensity of such fire. To better understand the effect of different metal hypophosphites on the fire performance, ABS, ABS-30 wt % AP, ABS-31.2 wt % MP, and ABS-34.4 wt % CP are selected to study. Cone calorimetric results of virgin ABS and FR-ABS were shown in Figure 3 and listed in Table 3. At a heat flux of 35 kW/m2, ABS burned very fast after ignition and the HRR value sharply increased to 704.1 kW/m2 within 160.0 s, indicating an easy ignition and fire propagation behavior. When flame retardant was added, a lower TTI value was found in the ABSAP, which was caused by the lower initial decomposition temperature of AP, while the TTIs of ABS-MP and ABS-CP were almost close to that of ABS, and it was the reason that MP and CP had a higher initial decomposition temperature. Meanwhile, both PHRR and THR values decreased significantly; for example, the PHRR value of ABS-AP reduced from 704.1 to 174.8 kW/m2, and the THR value reduced from 85.5 to 40.9 MJ/m2 compared to virgin ABS. Moreover, it was worth noting that the residual mass of FR-ABS was much larger than that of virgin ABS. It was concluded that when any of the metal hypophosphites was added to ABS, it would play a positive role on restraining the flammability of ABS. EHC was a parameter

Figure 2. TG curves of flame retardants AP, MP, and CP in nitrogen.

lower than that of ABS, as seen in Figure 1d−f and Table 1. While ABS modified with AP had the lowest maximum mass loss rate, it also provided the highest maximum decomposition temperature compared to ABS-MP and ABS-CP. It could be suspected that AP decomposed earlier than ABS did, forming a protective layer beneath the burning zone to delay further decomposition of ABS at high temperature. As for MP or CP, ABS would decompose nearly simultaneously with MP, or earlier than CP, as a result; the decomposition of MP would promote ABS to further decompose, and the phenomenon was confirmed by the first maximum mass loss rate being larger than that of virgin ABS, or the decomposition of CP was too late to form an effective barrier to protect ABS. Thus it indicate that the initial decomposition temperature was responsible for the poor flame retardance of MP and CP in ABS. Compared to virgin ABS, the residues of FR-ABS after thermooxidative degradation increased with the increase of flame-retardant content, which was similar to that under nitrogen. Through the analysis above, it could be concluded that AP decomposed

Figure 3. Heat release rate (a−c), total heat release (d−f) plots of FR-ABS from cone calorimetric test at the heat flux of (a, d) 35, (b, e) 50, and (c, f) 75 kW/m2, respectively. 2302

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Table 3. Cone Calorimeter Data for Virgin ABS and FR- ABS with Different External Heat Fluxes flux (kW/m2)

samples

TTI (s)

PHRR (kW/m2)

TTPHRR (s)

THR (MJ/m2)

residual mass (wt %)

av-EHC (MJ/kg)

35

virgin ABS ABS-AP ABS-MP ABS-CP ABS-AP ABS-MP ABS-CP ABS-AP ABS-MP ABS-CP

45.0 35.0 44.0 41.0 19.0 19.0 21.0 8.0 9.0 6.0

704.1 174.8 141.3 118.4 222.6 241.9 182.1 284.3 315.1 234.9

160.0 80.0 75.0 70.0 60.0 145.0 60.0 60.0 105.0 65.0

85.5 40.9 30.6 32.3 52.9 50.4 50.7 51.7 53.3 53.4

0.0 42.5 39.2 45.4 30.2 34.7 37.7 31.7 23.4 17.5

27.2 21.9 16.9 17.8

50

75

Figure 4. TG curves (a) and FTIR spectra of the gas-phase product after degradation of AP (b), ABS (c), and ABS-AP (d) at different temperatures under nitrogen atmosphere.

temperature to the ignition would be small at a high heat flux level, and the results were shown in Figure 3b,c,e,f and listed in Table 3. It could be found that the TTI values of three flameretardant ABS were almost the same; thus the effect of onset decomposition temperature could be ignored temporarily at such a testing condition. PHRR and THR values increased with heat flux level, and the increases in PHRR and THR values were considerably smaller for ABS-AP when the heat flux was increased from 35 to 50 kW/m2 or 75 kW/m2 than for ABSMP or ABS-CP. And it indicated that ABS-AP was better in restraining the flammability of ABS than MP and CP at higher heat flux level. In other words, at low heat flux, the lower initial decomposition temperature of AP lead to higher PHRR and THR values of ABS-AP than those of ABS-MP and ABS-CP. Through the above analysis, all three flame retardants played an effective role on restraining the flammability of ABS, and with the increase of heat flux, the increase of PHRR and THR values for ABS-AP was smaller than those for ABS-MP or ABS-CP; it was confirmed that the initial decomposition temperature was the key point from which it endowed ABS-AP with the best flame retardance among the three flame-retardant ABS, and the results were in accordance with the TG analysis.

to evaluate the heat released from combustion of the volatile portion of the testing materials. Compared with the av-EHC value of virgin ABS, the av-EHC value of ABS-AP decreased, indicating that there was actually a vapor phase effect of AP. However, ABS-MP and ABS-CP had comparatively lower avEHC than ABS-AP, indicating MP and CP formed more noncombustible and less active gases than AP did, a diluting effect in other words. However, ABS-MP and ABS-CP could not pass UL-94 V-0 and exhibited lower LOI values; thus it could be conjectured that the vapor phase effect was not an essential factor in the flame retardance. It was weird that the flame retardance of ABS-AP was better than the others shown in the LOI and UL-94 test rather than the cone calorimetric test; the PHRR and THR values of ABS-MP and ABS-CP were all lower than that of ABS-AP. One reason was that the loading amount of AP was lowest, in other words that the combustible component of the ABS-AP system was highest among the three FR-ABS composites. On the other hand, according to the TG analysis, it was supposed that the initial decomposition temperature of flame retardant was related to the phenomenon. Thus cone calorimetric tests at a heat flux of 50 and 75 kW/m2 were also carried out, as the effect of initial decomposition 2303

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3.4. Flame-Retardant Mechanism of ABS-AP. 3.4.1. TGFTIR and FTIR Analysis of ABS with AP. Through the above analysis, we found AP was the best effective flame retardant for ABS compared to MP and CP. Thus, to find the flameretardant mechanism of AP, infrared spectra of the volatile products at 335, 360, 430, and 500 °C were selected from TGFTIR spectra of AP, ABS, and ABS-AP for further analysis, and the results were presented in Figure 4. The main evolved gases from AP decomposition were PH3 and H2O. The decomposition process of AP had been reported as follows: Al(H 2PO2 )3 → Al 2(HPO4 )3 + PH3 Al 2(HPO4 )3 → Al4(P2O7 )3 + H 2O

The TG-FTIR spectrum of AP was shown in Figure 4b. And the decomposition of AP was further analyzed in our previous literature. Another decomposition product was also found: that the peaks at 1235, 1075, and 875 cm−1 were referred to PO2− in H3PO2, which was also confirmed via Py-GC/MS experiment in our previous work.25 Decomposition of ABS started from the formation of butadiene monomer at 340 °C. Aromatics were coming into sight at 350 °C, at which the evolution of butadiene was still evident. As the temperature increased, styrene became more important. Aliphatic and aromatic C−H vibrations were clearly visible. The evolution of acrylonitrile began at about 400 °C and ceased by 450 °C, which was in accord with the results reported elsewhere.26 The production of ammonia was found at 430 °C in our work, which could be seen in Figure 4c. When AP was added to ABS, the evolution of PH3 still existed at 360 °C, where it ceased to evolve for AP, and it was due to the higher initial decomposition temperature of ABS-AP than AP. Compared with the spectra of three samples, it was easy to find that the absorption peak at 1235 cm−1 belonged to the PO bond of H3PO2 shifted to 1270 cm−1, it could be explained by that all that the degradation of polystyrene began at 360 °C and was complete by 450 °C, and the degradation of polybutadiene was throughout the test;24 thus it indicated that the P−H bond of H3PO2 could react with the -CC- bond of butadiene or styrene in the gas phase, leading the reaction equilibrium to the positive direction, and as a result, PO band was changed. But except for the change of PO2−, there was no other difference in the spectrum of ABS-AP compared to those of AP and ABS. While it was reported that the phosphorus-containing substance PH3, H3PO2 could generate free radicals such as P•, PO•, and so on, which captured the HO• and H• to interrupt the radical action as halogen did.7 To further investigate whether the interaction between AP and ABS existed in the condensed phase or not, residues of ABS, AP, and ABS-AP at 360, 430, and 500 °C from TG were characterized via FTIR, and the spectra were shown in Figure 5. It was noticed that ABS could fuse in the crucible during the heating process for the TG test, and mostly degrade at 430 and 500 °C; therefore almost no residues were left. Thus ABS only had the FTIR spectrum at 360 °C. By comparing the spectra, it could be found that there was also no obvious reaction between AP and ABS; meanwhile, it affirmed that the decomposition process of AP would first produce -HPO4, and then, with the temperature increasing, -HPO4 was dehydrated to form -P2O7. To further ensure the analysis result, the theoretical TG curve was calculated to compare with the experimental TG curve, as shown in Figure 6. It was found that the two curves almost

Figure 5. FTIR spectra of residues after degradation of AP, ABS, and ABS-AP at different temperatures under nitrogen atmosphere: (a) 360, (b) 430, and (c) 500 °C.

coincided with each other, except that the experimental residue value was higher than the calculated residue value, which indicated that the decomposition product of AP could indeed protect the matrix from further degradation. From the preceding analyses, it was concluded that the presence of AP would not change the decomposition tendency of ABS, except that the carbon−carbon double bond would react with the P− H bond generated from AP in the gas phase, while the gaseous products such as PH3 and H3PO2 could act in the vapor phase to generate P•, PO•, and so on to capture the highly reactive radicals such as HO• and H• formed during combustion. In the condensed phase, the decomposition product of AP could act as a protective layer covering the matrix. 3.4.2. Burning Residue Analysis of ABS-AP. A scanning electron microscope with an attached energy-dispersive X-ray spectrometer (SEM-EDX) is highlighted as a powerful tool for the advanced characterization of such complex fire residues, 2304

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since it offers high resolution in combination with both good depth of field and analysis of chemical composition.27 To further investigate the flame-retardant mechanism of ABS-AP and the char forming during burning, the morphology and chemical formation of char residue after LOI testing were characterized by SEM-EDX. It was easy to find that the inner surface of the burning residue was compact, thick, and continuous, which could form an insulating barrier to protect the matrix from heat and oxygen. With regard to the outer surface, the carbon residue was smooth and uniform, as shown in Figure 7a,b. Comparable micromorphology of residues obtained from ABS-MP and ABS-CP were also investigated, as shown in Figure 7c−f. It was found that there was not much difference in the outer surface of residues of ABS-MP and ABSCP compared with that of ABS-AP; however, the inner surfaces of the burning residues were comparatively different, that it was no longer continuous and compact, and it was with many cavities, especially for ABS-CP, indicating that the poor flame retardance of ABS-MP and ABS-CP was also related to the defects of the char residues.

Figure 6. Experimental and calculated TG curves of ABS-30% AP under N2.

Figure 7. SEM photographs of burning residues after the LOI test: with AP, (a) inner surface and (b) outer surface; with MP, (c) inner surface and (d) outer surface; with CP, (e) inner surface and (f) outer surface. 2305

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carbonize as the charring residue. It might be the reason why the concentration of C could reach 80.9 wt % in the outer surface. As for the inner surface of the burning residue, there was also carbon element but much less than that of the outer surface; instead, concentrations of phosphorus, oxygen, and alumina were much higher than those in the outer surface, suggesting that the formation of a carbon-rich charring residue was minimal, but the formation of an Al4(P2O7)3-rich inorganic barrier was dominant,23 which could also act as an effective insulating layer against further combustion. Thus, the absence of the oxygen in the ABS molecular chain led to aromatization and carbonization being hard to occur; alternatively, the formation of a continuous Al4(P2O7)3 layer played an impactful substitute role during burning, and hence extinguished the fire effectively. On the other hand, the phosphorus-containing products PH3, P4, and H3PO2 also exhibited activity in the gas phase. Through the preceding analyses, a formation model of the burning residue of ABS-AP was presented in Figure 8. The outer surface of the char residue was constituted by the P−O− C structure formed through the interaction between phosphoric acid and thermal-oxidative degradation product of ABS by oxygen in the outer surface, and the inner surface consisted of a continuous Al4(P2O7)3 layer as an inorganic barrier, Generally, the two different structures were combined to flame retard ABS. Moreover, in the vapor phase, the phosphorus-containing product could form low-reactive radicals to interrupt the burning. Therefore, AP worked both in the condensed and vapor phases to flame-retard ABS.

The element distribution data of the burning residue were listed in Table 4. First, the ratio of P to Al (P:Al) in AP was calculated by the formula: P:Al =

3 × 31 (P) = 3.4 27 (Al)

Table 4. Element Distributions Obtained from SEM-EDX of ABS-AP after LOI Tests concentration (wt %) element C O P Al a

inner surface

outer surface

± ± ± ±

80.9 ± 1.4 12.9 ± 1.8 6.2 ± 1.1 NDa

13.0 49.7 25.2 12.1

1.3 1.6 1.2 1.1

ND stands for not detected.

P:Al in the inner surface of the char residue was obtained by (ideally, the mass of Al would not change) P:Al =

25.2 (P) = 2.1 12.1 (Al)

From the above calculation results, P:Al was decreased considerably, which should be attributed to the evolution of the decomposition product such as P4, PH3, and H3PO2 from AP. In the meanwhile, it was the key point for explaining the large difference of the elemental contribution between the inner and outer surfaces. The concentration of the carbon element in the outer surface was much higher than that in the inner surface. It was known to all that ABS was not easy to carbonize despite the presence of benzene groups in the styrene moiety. However, H3PO2 would decompose to PH3 and phosphoric acid, and P4 or PH3 could react with oxygen to form phosphoric acid; then the phosphoric acid would further generate poly-, pyro-, and ultraphosphoric acid covering the surface of the burning sample and promoting the outer surface material oxidized by oxygen at the very beginning of the combustion to

4. CONCLUSION In this work, three metal hypophosphites, including AP, MP, and CP, were applied to flame-retard acrylonitrile−butadiene− styrene terpolymer (ABS) for comparison, and it was found that the addition of the three flame retardants all could enhance the thermal stability of ABS, while only AP was fit to flameretard ABS, due to the lower initial decomposition temperature than that of ABS; as a result, the residue formed early to protect

Figure 8. The formation model of the burning residues. (a) At the very beginning of combustion, AP released phosphine, hypophosphorus acid, and P4 to generate ultra/pyro/poly phosphorus acids and Al2(HPO4)3, while ABS was oxidized to form some oxygen-containing fragments; (b) ultra-/ pyro-/polyphosphorus acids dehydrated oxygen-containing ABS fragments to form P−O−C chars; (c) continuous generation of P−O−C chars occurred, and Al2(HPO4)3 was further dehydrated to form more thermo-stable aluminum pyrophosphate; (d) the outer surface of the burning residues was mainly composed of P−O−C chars, and Al2(HPO4)3 mostly comprised the inner surface. 2306

dx.doi.org/10.1021/ie403726m | Ind. Eng. Chem. Res. 2014, 53, 2299−2307

Industrial & Engineering Chemistry Research

Article

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the material. And it was also the reason that ABS-AP performed badly in PHRR and THR values compared to those of ABS-MP and ABS-CP via the cone calorimetric tests at low different heat flux; however AP could greater restrain the fire intensity of ABS than MP and CP at the higher heat flux. Moreover, through the TGA-FTIR, FTIR and SEM-EDX tests, it was concluded that, though there was no obvious reaction between AP and ABS, the products P4, PH3, and H3PO2 generated from AP could further react to form phosphoric acid and promote the oxidized material to carbonize as the outer charring residue; meanwhile, the solid-phase product of the continuous Al4(P2O7)3 layer as the inner residue played an impactful substitute role during burning. Moreover, phosphorus-containing volatiles could act in the vapor phase to generate P•, PO•, and so on to capture the highly reactive radicals such as HO• and H• formed during combustion, and hence extinguish the fire effectively.

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AUTHOR INFORMATION

Corresponding Authors

*Tel. and fax: +86-28-8541755. E-mail: [email protected]. *Tel. and fax: +86-28-8541755. E-mail: [email protected]. 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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REFERENCES

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dx.doi.org/10.1021/ie403726m | Ind. Eng. Chem. Res. 2014, 53, 2299−2307