Investigation on Thermal Degradation of Poly(1,4-butylene

Apr 30, 2014 - Roberta Risoluti , Maria Aurora Fabiano , Giuseppina Gullifa , Stefano Vecchio Ciprioti , Stefano Materazzi. Applied Spectroscopy Revie...
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Investigation on Thermal Degradation of Poly(1,4-butylene terephthalate) Filled with Aluminum Hypophosphite and Trimer by Thermogravimetric Analysis−Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis−Mass Spectrometry Hongqiang Qu, Xin Liu, Jianzhong Xu,* Haiyun Ma, Yunhong Jiao, and Jixing Xie College of Chemistry and Environmental Science, Hebei University, No. 180 Wusi East Road, Baoding 071002, Hebei, PR China S Supporting Information *

ABSTRACT: Flame-retarded poly(1,4-butylene terephthalate) (PBT) has been prepared using aluminum hypophosphite (AHP) and tri(1-oxo-2,6,7-trioxa-1-phosphabicyclo[2,2,2]octane-methyl)phosphate (Trimer). The combustion properties of flame-retarded PBT were evaluated using limiting oxygen index, UL-94, and cone calorimetry. The cone calorimeter data indicated that the presence of PBT/AHP/Trimer imparted a significant smoke-suppressing effect. The thermal degradation properties of flame-retarded PBT were investigated using thermogravimetric analysis−Fourier transform infrared and thermogravimetric analysis−mass spectrometry. For the samples of PBT, PBT/25% AHP, and PBT/AHP/Trimer, the main gas pyrolysis products were CO2 and butadiene; the sample of PBT/AHP/Trimer generated less CO2 and butadiene and consequently retained more carbon in the matrix. The amount of phosphorus−oxygen radicals of PBT/25% AHP was 1.3 times that of PBT/AHP/Trimer, which indicated that AHP alone showed slightly stronger gaseous phase effect than the combination of AHP and Trimer. This result is further confirmed by the analysis of the char residues by X-ray photoelectron spectroscopy.

1. INTRODUCTION Poly(1,4-butylene terephthalate) (PBT) is one of the most widely used engineering plastics. It is mainly used in electronic industries and automotive production thanks to the excellent thermal resistance, chemical resistance, electrical insulation, and dimensional stability.1 Nevertheless, its flammability and serious dripping during combustion limit its potential applications. It is necessary to modify PBT in order to meet the requirements of electronic products for the flame-retardant properties of polymeric material. The traditional flame retardants for PBT consist of halogen-containing compound and antimony trioxide as a synergist.2 Although such flame retardants show remarkable efficiency, they cause environmental problems by releasing quantities of toxic and corrosive gases during combustion. Therefore, the development of halogen-free flame retardants is strongly demanded. The halogen-free flame retardants currently used for PBT mainly include phosphorus-based,3,4 nitrogen-based,5,6and siliconbased flame retardants.7 In recent years, alkyl phosphinates have been found to be particularly effective in PBT.8−10 Gallo and co-workers9 investigated the flame retardancy mechanisms of aluminum diethylphosphinate and its combination with metal oxides in PBT. The flame retardant effect of aluminum diethlyphosphinate in PBT was studied by the limiting oxygen index (LOI), cone calorimeter test, and the UL-94 test. When 10 wt % of aluminum diethlyphosphinate was added, the LOI value was increased from 21.7% to 31.3% and the UL-94 V-1 rating was achieved because of the predominant gas-phase effect and condensed-phase action. These organic phosphinate salts have been proven to be greatly efficient for PBT, but the manufacture of these salts on an industrial scale is relatively © 2014 American Chemical Society

complex and expensive, thereby limiting their extensive use as flame retardants. Therefore, hypophosphites, which have a structure similar to that of phosphinates, attracted the attention of more researchers. Hu and co-workers11−13 from the University of Science and Technology of China have studied the flame retardant efficiency of hypophosphites in glass-fiber reinforced PBT composites. The UL-94 V-0 rating was achieved when 20 wt % lanthanum hypophosphite was used. These results led to an assumption that hypophosphites could be used as an effective flame retardant. In this study, aluminum hypophosphite (AHP) and tri(1oxo-2,6,7-trioxa-1-phosphabicyclo[2,2,2]octane-methyl)phosphate (Trimer) were chosen to develop environmentally friendly flame-retarded PBT composites with enhanced flame retardancy. Trimer has high phosphorus content (21%), excellent thermal stability, and good charring effect.14 The aim of this research is to combine the flame-retarding potential of AHP and Trimer to significantly improve the flame retardancy of PBT composites. PBT composites were tested by LOI, UL-94, and cone calorimeter, thermogravimetric analysis (TGA) coupled with Fourier transform infrared spectrometry (TGA-FTIR), and TGA coupled with mass spectrometry (TGA-MS) to investigate the combustion behavior of PBT composites. These investigations were carried out to further understand the mechanism of the flame retardant. Received: Revised: Accepted: Published: 8476

December 18, 2013 March 27, 2014 April 30, 2014 April 30, 2014 dx.doi.org/10.1021/ie404297r | Ind. Eng. Chem. Res. 2014, 53, 8476−8483

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2. EXPERIMENTAL SECTION 2.1. Materials. PBT (B4500) was purchased from BASF Chemical Company, Germany. AHP was provided by Wuhan Hanye Chemical New Material Co., Ltd., China. Tri(1-oxo2,6,7-trioxa-1-phosphabicyclo[2,2,2]octane-methyl)phosphate (Trimer) was supplied by Jiangsu Yoke Technology Co., Ltd., China. The structure of Trimer is illustrated in Scheme 1.

wrapped in aluminum foil and exposed horizontally to a 35 kW/m2 external heat flux. Thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) were performed using a STA449C thermal analyzer (Netzsch, Germany) in Ar atmosphere at a heating rate of 10 °C/min. The TGA-FTIR measurements were carried out using a STA449C thermal analyzer (Netzsch, Germany) coupled with a TENSOR27 FTIR spectrometer (Bruker, Germany). About 10 mg of each sample was heated from 35 to 800 °C at a heating rate of 10 °C/min under Ar. The FTIR spectra were recorded within the 400−4000 cm−1 range. The TGA-MS characterization was performed using a STA449C-QMS403C instrument (Netzsch, Germany). In the experiment, a sample weighing approximately 10 mg was heated at 10 °C/min from ambient temperature to 800 °C in an Ar atmosphere. Mass scanning was carried out over the range m/z 2−150. X-ray photoelectron spectroscopy (XPS) data were obtained using a PerkinElmer PHI 5300 ESCA system (PerkinElmer, U.S.) at 250 W (12.5 kV at 20 mA) under a vacuum better than 10−6 Pa (10−8 Torr).

Scheme 1. Structure of Trimer

2.2. Preparation of Samples. PBT and all additives were dried at 80 °C overnight before use. The formulations (Table 1) were melt blended using a twin-screw extruder (SHJ-20,

3. RESULTS AND DISCUSSION 3.1. Flame-Retardant Properties. LOI and UL-94 tests were performed to investigate the flame retardancy of each formulation, which are summarized in Table 1. PBT is a flammable polymeric material with a LOI of 19.8%. The LOI value of the flame-retardant PBT gradually increased with an increasing amount of AHP. The LOI value was increased from 19.8% to 27.8% when 25 wt % AHP was present. In the UL-94 test, burning PBT was not extinguishable and there was serious dripping in the first stage of the combustion. As the level of AHP was increased to 25 wt %, the combustion behavior of the PBT composite was further influenced and a V-1 rating was achieved. The increase of LOI value and the improvement of flame classification in UL-94 testing (V-1 rating) both showed that AHP was an effective flame retardant for PBT. The presence of Trimer alone can slightly increase the LOI value for the combustion of PBT, but PBT/25% Trimer (sample 7) could not pass the UL-94 vertical burning test, which demonstrated that Trimer did not exert an obvious influence on the flame retardancy of PBT. When AHP and Trimer were combined in PBT, the observed LOI values for combustion were slightly lower than those for PBT containing AHP alone. A maximum LOI value of 25.9% was obtained for combustion PBT containing AHP/Trimer in a mass ratio of 7:1. Furthermore, all samples displayed a UL-94 V-0 rating when the mass ratio of AHP and Trimer was between 5:1 and 9:1. This suggests that this novel combination of flame retandants can enhance the drip-resistant performance of PBT. 3.2. Cone Calorimetry. The cone calorimeter is an effective bench scale apparatus to simulate fires.15,16 The flame retardancy properties of AHP and AHP/Trimer (mass ratio, 7:1) at a total loading of 25 wt % in PBT were quantified by cone calorimetry under radiation of 30 kW/m2 and 50 kW/ m2, respectively. The results are summarized in Table 2. As shown in Figure 1a, the heat release rate (HRR) curve for neat PBT exhibited a sharp peak, which can be explained by the combustion behavior of the sample. Under a radiation of a 30 kW/m2 heat flux, the surface of neat PBT melts and then forms a thin layer of char. The charred surface remains intact for a short period of time leading to a small peak in the HRR curve.

Table 1. Combustibility of Flame-Retarded PBT flame retardancy formulation (%)

a

UL-94 (3.2 mm)

sample

PBT

AHP

Trimer

LOI (%)

dripping

rating

1 2 3 4 5 6 7 8 9 10 11 12

100 95 90 85 80 75 75 75 75 75 75 75

− 5 10 15 20 25 − 12.50 18.75 20.83 21.88 22.50

− − − − − − 25 12.50 6.25 4.17 3.12 2.50

19.8 20.3 24.5 26.3 27.6 27.8 23.8 22.3 24.2 25.2 25.9 25.7

yes yes yes yes yes no yes yes yes no no no

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

NR: not rated.

Nanjing Giant Machinery Co., Ltd., China). The temperatures of the five zones were 220, 220, 225, 225, and 225 °C. Extruded materials were pelletized and dried. The chips were injected molded into standard specimens by using a microinjection molding machine (SZ-15, Wuhan Ruiming Plastic Machinery Co., Ltd., China). The temperatures of the melt and mold were 230 and 40 °C, respectively, and the pressure of the injection was 0.4 MPa. Also, the chips were molded by using a hot press at 225 °C to obtain 3 mm thick plaques. 2.3. Characterization. The LOI values were determined in accordance with ASTM Standard D2863-2000 using a JF-3 oxygen index meter (Jiangning Analytical Instrument Factory, China). The UL-94 test was carried out using a CZF-3 vertical flammability tester (Jiangning Analytical Instrument Factory, China) according to ASTM Standard D3801-2006. Cone calorimetry was performed using a cone calorimeter (Fire Testing Technology Ltd., U.K.) according to ISO Standard 5660. Each specimen (100 × 100 × 3 mm3) was 8477

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formed. As for PBT/25% AHP, the HRR curve for PBT/AHP/ Trimer exhibits two peaks but the first of these is less intense, suggesting a slightly different behavior. It could be that Trimer acts to maintain the integrity of the layered char formed during the early stages of combustion. As a consequence, the matrix is better protected and the first HRR peak is not as intense as that during the combustion of PBT/25% AHP. The first HRR peak for PBT/AHP/Trimer combustion appears earlier than during the combustion of neat PBT. However, the time to PHRR (tPHRR) is 10 s longer than that for PBT/25% AHP. This delayed tPHRR is favorable, as it would leave more time to escape a real fire. The PHRR and average HRR (Av-HRR) for PBT/ AHP/Trimer are reduced by 5% and 6%, respectively, compared with that of PBT/25% AHP, whereas the total heat release (THR) is increased by 15%. This could explain why the UL-94 rating of PBT/AHP/Trimer is higher than that of PBT/25% AHP. Previous studies17 at very low heat flux (30 kW/m2) have reported correlations between the UL-94 rating and some cone calorimeter measurements (the PHRR and AvHRR) but not others (the time to ignition and THR). In general, lowering the PHRR and delaying the time to peak HRR after ignition seem important phenomena for gaining higher UL-94 ratings. These results are in good agreement with the previous conclusions. Furthermore, note the average effective heat of combustion (Av-EHC) in Table 2. The EHC is defined as the heat released per unit mass of material being volatilized during combustion. It can be written as EHC = HRR/MLR, where HRR is the heat release rate and MLR is the mass loss rate. A flame retardant exhibiting flame inhibition in the gaseous phase would lead to incomplete combustion of the volatilized materials, thus reducing the HRR and thereby the EHC. Conversely, an increased EHC indicates a condensed phase effect in the flame retardant.18 Table 2 shows that the Av-EHC of PBT/25% AHP decreases, suggesting that AHP exhibites a gaseous phase effect. At the same time, the Av-EHC of PBT/AHP/Trimer is elevated compared with that of PBT/25% AHP. This suggests

Table 2. Detailed Cone Calorimeter Data under Radiation of 30 kW/m2 parameter

PBT

PBT/25% AHP

PBT/AHP/Trimer

PHRR (kW/m2) tPHRR (s) Av-HRR (kW/m2) THR (MJ/kg) Av-MLR (g/s) Av-EHC (MJ/kg) Av-SEA (m2/kg) PSPR (m2/s) TSP (m2) Av-COY (kg/kg) Av-CO2Y (kg/kg)

716.35 235 213.07 125.89 0.06 31.18 557.75 0.17 20.29 0.06 2.57

178.22 110 88.18 87.79 0.03 30.75 670.41 0.11 16.91 0.16 2.47

169.07 120 82.86 101.21 0.02 32.21 476.55 0.11 13.27 0.12 2.49

Then, the charred surface is destroyed by vigorous gas expulsion from the underlying sample. As more flammable gases are released into the environment, the specimen burns more intensely, reaching a peak HRR (PHRR) of 716.35 kW/ m2. Char is formed throughout, but once most of the sample has been consumed, after about 450 s as evidenced by the HRR curve, the residual char undergoes slow glowing combustion so that the HRR remains above zero until the end of the test. AHP can promote the formation of char on PBT during combustion. The PBT/25% AHP sample has a PHRR of 158.28 kW/m2 very early in the combustion process. The sample forms char quickly and firmly, thus protecting the underlying material so that the HRR decreases early (at approximately 90−95 s). After 95 s, the protecting shield is broken allowing further combustion as reported by a more intense peak in the HRR curve. Compared with that of neat PBT, the first HRR peak occurs earlier for PBT/25% AHP, suggesting that AHP facilitates the early stage decomposition of PBT. When a combination of AHP and Trimer is present during combustion, greater quantities of more compact char are

Figure 1. Cone calorimeter parameters of samples under radiation of 30 kW/m2: (a) heat release rate (HRR), (b) total heat release (THR), (c) smoke production rate (SPR), and (d) total smoke produced (TSP). 8478

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Figure 2. Cone calorimeter parameters of samples under radiation of 50 kW/m2: (a) HRR, (b) THR, (c) SPR, and (d) TSP.

that used alone, AHP provides flame retardancy through a slightly enhanced gaseous phase effect, whereas in combination with Trimer, the condensed phase effect is enhanced. A better understanding could be gained by analyzing the pyrolysis products from thermal decomposition. The smoke production rate (SPR), total smoke produced (TSP), and average specific extinction area (Av-SEA) are accurate reporters of the smoke generated during combustion. The peak SPR (PSPR), TSP, and Av-SEA of flame-retarded PBT are all decreased compared with those of neat PBT. The TSP and Av-SEA of PBT/AHP/Trimer are reduced by 22% and 29%, respectively, compared with those of PBT/25% AHP. These results prove that the combination of AHP and Trimer provides superior smoke suppression. However, the average CO yield (Av-COY) of flame-retarded PBT apparently increases compared with that of neat PBT, while the average CO2 yield (Av-COY) decreases. In particular, the amount of CO released is drastically enhanced when AHP is added alone because of the flame inhibition effect. CO is considered to be a fatal toxic gas in fires. In this context, the decrease of the AvCOY for PBT/AHP/Trimer compared with that of PBT/25% AHP is noteworthy. All these data imply that compared with AHP alone, the combination of AHP and Trimer leads to considerable smoke suppression, but more importantly, to a reduction in smoke toxicity. A comparison of the cone calorimetry results at different radiation powers, Figures 1 and 2, shows that at higher heat fluxes the samples decompose more rapidly. This is most clearly shown by the TSP. As shown in Figure 2d, PBT/25% AHP has a TSP larger than that of neat PBT because the char residue of PBT/25% AHP disappears more rapidly under higher heat fluxes, thereby generating more smoke in the gas phase. In contrast, the TSP for PBT/AHP/Trimer is significantly lower than that for PBT/25% AHP, highlighting the presence of stable char residue in greater quantities for PBT/AHP/Trimer than for PBT/25% AHP samples.

3.3. Thermogravimetric Analysis. To understand the interactions between AHP and Trimer, the thermal degradation behaviors of AHP, Trimer, and AHP/Trimer (mass ratio, 7:1) were tested by TGA at a heating rate of 10 °C/min in an Ar atmosphere. The TGA and DTG curves are shown in Figure 3

Figure 3. TGA and DTG curves of AHP, Trimer, and AHP/Trimer (mass ratio, 7:1) in Ar (10 °C/min).

and reveal clear differences between the thermal degradation behaviors of AHP, Trimer, and AHP/Trimer. The thermal decomposition of AHP is characterized by two steps with two maximal mass loss rates at 338.9 and 438.2 °C. The decomposition process19 of AHP can be described by the two following equations: Δ

2Al(H 2PO2 )3 → Al 2(HPO4 )3 + 3PH3 Δ

2Al 2(HPO4 )3 → Al4(P2O7 )3 + 3H 2O

PH3 and H2O are the main gas-phase decomposition products of AHP at the first and second step, respectively, leaving 67.73 wt % char residue at 800 °C. Trimer also degraded in two steps; the two main weight loss peaks occurred at approximately 363.6 and 582.3 °C. The highest peak at 363.6 °C, corresponding to a weight loss rate of 39.01%/min, is mainly attributed to the breaking of phosphate ester bonds (P−O−C), whereas the second peak, at 582.3 °C with a much lower weight loss rate 8479

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(2.62%/min), originates from the pyrolysis of char residue.20 The rate of weight loss for Trimer slows markedly after 600 °C, and the char residue of Trimer is 36.08 wt % at 800 °C. In contrast to AHP and Trimer, the thermal degradation of AHP/Trimer occurs in three steps. The initial decomposition temperature, T−5% (defined as the temperature at which 5% of the initial weight has been lost) decreases to 317.7 °C, and the first weight loss is mainly associated with the decomposition of AHP. The second weight loss event at 353.9 °C is decreased significantly because of the inhibition of AHP, whereas the third peak is caused by further decomposition of Al2(HPO4)3. The disappearance of the weight loss peak around 582.3 °C emphasizes the good thermal stability of the residues formed at high temperature, and 73.9 wt % char residue remains at 800 °C. Comparing theoretical and experimental TGA or DTG curves can reveal interactions between AHP and Trimer. The TGA curve (Wcal) is predicted to be the weighted average of the TGA curves of all the mixture components.14 That is,

formation. With the addition of Trimer, the initial decomposition is shifted about 16 °C lower, and at the end of the test, the residue increases to 32.00 wt % from 25.78 wt % for PBT/ 25% AHP. The addition of Trimer further promotes the formation of char residue. When AHP and Trimer are combined in PBT, the degradation occurs in three steps that are attributed to the pyrolysis of additives. In summary, the addition of flame-retardants lowers the decomposition temperature and the thermal stability of PBT. In fact, most flameretardants containing phosphorus or nitrogen should decrease the thermal stability of neat polymers because either condensed-phase or gas-phase dilution action needs to be effective before the drastic degradation of polymer. However, interactions between the flame-retardants and the polymer lead to the formation of large amounts of char residue which may form a barrier to heat and oxygen on the surface of the polymer. 3.4. TGA-FTIR Analysis. To identify the degradation products, the gases in the TGA furnace were analyzed by FTIR during thermal degradation (Figures 5 and 6). Figure 5

n

Wcal(T ) =

∑ xiWi (T ) i=1

where xi is the weight fraction of compound i and Wi is the TGA curve of compound i. Experimental and theoretical TGA and DTG curves of AHP/Trimer are shown in Figure 3. The main difference for the TGA curves is that the weight loss of AHP/Trimer after 338 °C is significantly lower than the predicted values, whereas the char residue at 800 °C is higher than the theoretical value of 8.39%, indicating a strong interaction between AHP and Trimer. It appears that the Al2(HPO4)3 produced during the initial decomposition of AHP leads AHP/Trimer to generate large quantities of stable char residue in the condensed phase, making it resistant to dripping and an effective flame retardant. The TGA and DTG results for neat PBT and the PBT composites are summarized in Figure 4, respectively. The

Figure 5. FTIR spectra of neat PBT under different pyrolysis temperatures.

Figure 4. TGA and DTG curves of the PBT samples in Ar.

Figure 6. FTIR spectra of pyrolysis products of the PBT samples at their maximum weight loss rates.

thermal degradation curve of neat PBT in Ar atmosphere is characterized by a single decomposition step beginning at 374.2 °C (T−5%), and the final residue accounts for only about 5.05 wt %. The decomposition of PBT is influenced by the addition of AHP. The initial decomposition temperature of PBT/25% AHP is approximately 29 °C lower than that for PBT as a consequence of the pyrolysis of AHP. On the other hand, the stable residue at 800 °C is considerably higher: 25.78 wt % instead of 5.05 wt %. The total amount lost by AHP from thermal decomposition between room temperature and 800 °C is known to be approximately 32.00 wt %. Therefore, the fact that after degradation more residue remains for PBT/25% AHP than for neat PBT indicates that the presence of AHP promotes char

shows the changes occurring during the pyrolysis of neat PBT in an inert atmosphere and at different pyrolysis temperatures. The spectra were obtained with the DTG pattern as mentioned above at two different temperatures: (i) the initial temperature and (ii) the temperature at which the decomposition rate is maximal. From Figure 5, it can be seen that the relative intensities of the peaks in the PBT spectrum changes negligibly below 374.2 °C. The main gaseous products of the decomposition of PBT are carbon dioxide, butadiene, tetrahydrofuran (THF), benzene, and ester derivatives.21 As shown in Figure 5, at the temperature of fastest decomposition, 402.0 °C, butadiene (910 cm−1), CO2 (2372 cm−1), THF (2968 cm−1), benzene (3085 and 1585 cm−1), and esters 8480

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Figure 7. Curves for the ion current of different charge-to -mass ratio variation with temperature.

(1743, 1267, and 1103 cm−1) are all released. The peak at 3582 cm−1 is attributed to H2O. These results are in good agreement with the literature. Several studies21−23 have reported initial polymer scission occurring via the six-membered cyclic transition state. According to the geometry of the chains, the major degradation route in PBT should lead to the formation of butadiene, but THF is also obtained in the degradation products. When the well-known acyl-oxygen cleavage of the ester linkages proceeds in PBT chains, intramolecular or intermolecular H+ shifts can occur, leading to hydroxylterminated units which can in turn undergo further degradation to yield THF and a carboxylic aicd-terminated chain.21 FTIR spectra of the pyrolysis products of neat PBT and the PBT composites at their maximum weight loss rates are presented in Figure 6. The number of absorbance bands for the spectra from the composites is the same as that for neat PBT, except for the very weak absorbance at 930 and 933 cm−1 which can be assigned to the P−O−C stretching vibration.24 The absorbance is so small as to be almost undetectable. This is because AHP reacts with PBT mainly in the condensed phase as well as slightly in the gaseous phase during thermal degradation; therefore, most phosphorus-containing components are retained in the char rather than being released as gases. Furthermore, the presence of the same number of peaks in the spectra indicates that the flame-retardants do not

fundamentally change the nature of the products released in the gas phase. However, differences in intensity suggest modified release rates. For instance, the absorbance intensity of esters in the spectrum of flame-retarded PBT is clearly lower than that in the spectrum of neat PBT. This indicates a probable reaction between AHP or its products and the PBT matrix or its products resulting in a reduction of volatilized esters. Peaks for the other pyrolysis products are also lower for the composites than for PBT. It has been shown above that the incorporation of AHP and Trimer into PBT promotes the formation of char residue, thereby trapping some of the volatile products that participate in the charring reaction. The char layer becomes a barrier to gaseous products, leading to a reduction in emissions. To understand this mechanism further, a detailed analysis of the yield of the different gases generated during pyrolysis was conducted using TGA-MS. 3.5. TGA-MS Analysis. Because FTIR measurements reveal only qualitative information about the functional group of the different pyrolysis products, MS was performed to understand the pyrolysis mechanism in flame-retarded PBT and identify the exact composition of the pyrolysis products. The TGA-MS system was heated from 35 to 800 °C under an Ar atmosphere at a heating rate of 10 °C/min scanning through m/z in the range of 2−150. Intense ion signals at m/z = 18, 44, 54, 72, and 78 were detected. According to the structure of PBT, m/z = 18 is attributed to H2O. CO2 is 8481

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associated with the MS signal at m/z = 44, while the peak at m/ z = 54 indicates the presence of butadiene. Fragments with m/z = 72 and m/z = 78 belong to tetrahydrofuran and benzene, respectively, with additional evidence of fragments with m/z = 41, 42 and m/z = 51, 52. These data firmly support the conclusions drawn from the TGA-FTIR results. Figure 7 shows MS signals from the main decomposition products. For flameretarded PBT, the integrated area under the water peak at m/z = 18 (IA(18)) is larger than that for neat PBT, meaning that more water is generated during the degradation process. Because water vapor is not flammable, it lowers the concentration of flammable radicals. The CO2 output of flame-retarded PBT is clearly reduced, by up to 87% in the case of PBT/AHP/Trimer. This is attributed to the larger amounts of char formed when PBT is associated with AHP. In addition, liquid aluminum pyrophosphate generated during the degradation of AHP forms a protective film on the surface of the carbonaceous char layer. Simultaneously, the presence of Trimer could promote foaming and expansion of the char layer. The carbonaceous char layer acts as a physical barrier, slowing mass and heat transfer between the gas and condensed phases and protecting the underlying material from further combustion. Furthermore, note the decreased outputs of butadiene and benzene but the slightly increased THF yield for flame-retarded PBT, the latter being explained by the acidic environment brought on by the decomposition of AHP. Indeed, in a high-temperature acidic environment, tetrahydrofuran is generated by dehydration cyclization of butanediol.21 The MS spectra for PBT/25% AHP and PBT/AHP/Trimer are similar to those for neat PBT; signals for carbon dioxide, butadiene, tetrahydrofuran, and benzene are all present. Nevertheless, some new fragments are detected that come from the degradation of PBT/25% AHP and PBT/AHP/ Trimer. The signals at m/z = 47 and m/z = 63 can be assigned to PO· and PO2·, and both are more intense for PBT/25% AHP than for PBT/AHP/Trimer. Because PO· radicals are the main species responsible for flame inhibition,25 this indicates that considering the gaseous phase effect, AHP is a stronger flame-retardant when used alone than when it is combined with Trimer. This is in good agreement with the cone calorimetry results discussed above. Hence, the gaseous phase effect proceeds as follows. First, the thermal decomposition of AHP produces phosphine and aluminum hydrogen phosphate before acidic phosphate condenses. Phosphine and its derivatives are radical scavengers that trap the radicals needed for combustion. Phosphine produces phosphorus-based radicals (PO2·, PO·, etc.) in the flame zone,26 and the phosphorus-containing species in the vapor phase trap H· and HO·,27,28 leaving fewer flammable compounds for combustion. Thus, AHP acts in the gas phase through a radical mechanism to inhibit the combustion of PBT. 3.6. XPS Analysis of Char Residue. XPS is a reliable method for the chemical analysis of surfaces. To further investigate the role of AHP in the gas phase, both the exterior and the interior chars obtained from LOI test were analyzed by XPS. The atomic concentrations given here are percentages of all detected carbon, oxygen, phosphorus, and aluminum atoms. As shown in Table 3, both the phosphorus and oxygen atomic concentrations in the exterior char are lower than those in the interior char for PBT/25% AHP. However, the atomic concentrations of phosphorus and oxygen in the exterior char are very close to those in the interior char for PBT/AHP/

Table 3. Atomic Concentrations of C, O, Al, and P in the Char Residues atomic concentrations sample PBT/25% AHP PBT/AHP/Trimer

char

C (%)

O (%)

Al (%)

P (%)

exterior interior exterior interior

71.88 54.59 60.59 58.58

19.96 33.47 29.88 31.78

3.57 3.77 2.76 2.58

4.59 8.17 6.77 7.06

Trimer. This implies that more phosphorus and oxygen atoms evaporate from PBT/25% AHP during combustion. Note that the atomic concentrations of phosphorus in both the exterior and in the interior char for PBT/AHP/Trimer are lower than those in the interior char for PBT/25% AHP, mainly because the phosphorus content in AHP (41.89%) is higher than that in Trimer (21.23%, calculated from the molecular formula).

4. CONCLUSIONS The combustion properties of flame-retarded PBT were evaluated by LOI, UL-94 tests, and cone calorimeter. For a 7:1 mass ratio of AHP and Trimer and a total loading of 25 wt %, the LOI and UL-94 rating are 25.9% and V-0, respectively. Under 30 kW/m2 radiation, the PHRR and Av-HRR of PBT/ AHP/Trimer are reduced by 5% and 6%, respectively, compared with those of PBT/25% AHP, which can explain why the UL-94 rating of PBT/AHP/Trimer is higher than that for PBT/25% AHP. Additionally, the PSPR, TSP, and Av-SEA of flame-retarded PBT are all decreased compared to those of neat PBT; the TSP and Av-SEA of PBT/AHP/Trimer are reduced by 22% and 29%, respectively, compared with those of PBT/25% AHP. The Av-COY of PBT/AHP/Trimer is also decreased compared with that of PBT/25% AHP, and under a higher heat flux (50 kW/m2), the TSP of PBT/AHP/Trimer remains lower than that of PBT/25% AHP. These results all imply that considerable reductions in smoke emission and toxicity can be achieved by combining AHP and Trimer. The mechanisms of thermal degradation and flame retardancy of PBT filled with AHP and Trimer were comprehensively cross-examined by TGA-FTIR and TGAMS. Under an inert atmosphere, the main pyrolysis products released upon thermal degradation of PBT are carbon dioxide, butadiene, tetrahydrofuran, benzene, and ester derivatives. The presence of AHP and Trimer does not fundamentally change the nature of the products released in the gas phase, but changes in the emission rates were revealed by TGA-FTIR. Filling PBT with a combination of AHP and Trimer promotes the formation of char residue. The flame retardancy effects of AHP and Trimer are mainly present in the condensed phase, but AHP also exhibits a slight gaseous phase effect as revealed by the detection of phosphorus−oxygen radicals by TGA-MS and decreased phosphorus concentrations in the exterior char of PBT/25% AHP. When AHP is added alone to PBT, the relative amount of phosphorus−oxygen radicals is higher than it is when AHP is combined with Trimer. In other words, a greater gaseous phase effect is obtained when AHP is used alone rather than in combination with Trimer. Finally, the greater condensed phase effect observed when AHP and Trimer are combined is attributed to the increase in stable char residue content brought on by enhanced production of Al2(HPO4)3 during the initial decomposition of AHP. 8482

dx.doi.org/10.1021/ie404297r | Ind. Eng. Chem. Res. 2014, 53, 8476−8483

Industrial & Engineering Chemistry Research

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Article

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ASSOCIATED CONTENT

S Supporting Information *

Abbreviations of parameters derived from cone calorimetry (Table S1); detailed cone calorimeter data at heat flux 50 kW/ m2 (Table S2); data of TGA and DTG for AHP, Trimer and AHP/Trimer (mass ratio=7:1) (Table S3); data of TGA and DTG for neat PBT and PBT composites (Table S4); TG-MS parameters for the PBT samples (Table S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Natural Science Foundation of China (Grant 21306035 and Grant 21276059).

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REFERENCES

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dx.doi.org/10.1021/ie404297r | Ind. Eng. Chem. Res. 2014, 53, 8476−8483