Aluminum Hydroxymethylphosphinate and Melamine Pyrophosphate

Article pubs.acs.org/IECR

Aluminum Hydroxymethylphosphinate and Melamine Pyrophosphate: Synergistic Flame Retardance and Smoke Suppression for Glass Fiber Reinforced Polyamide 6 Gong-Peng Lin, Li Chen,* Xiu-Li Wang,* Rong-Kun Jian, Bin Zhao, and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, Sichuan, China ABSTRACT: In this manuscript, synergistic effect was found when aluminum hydroxymethylphosphinate (AHMP) and melamine pyrophosphate (MPyP) with a suitable mass ratio were used together for glass-fiber-reinforced polyamide 6 composites (GFPA6). Both vertical burning test (UL-94) and limited oxygen index (LOI) results showed that GFPA6 containing 5 wt % AHMP and 25 wt % MPyP had excellent flame retardance, i.e. its LOI value was 31.0 and UL-94 reached V-0 rating even for the sample with 1.6 mm thickness. Besides, the flammable properties and smoke suppression of GFPA6 were also improved demonstrated by cone calorimetry results. The thermal stability and the decomposition activation energies (Eα) were studied by TGA. Moreover, the morphology of char after the LOI test was investigated by SEM. It was found that the interaction existing between PA6, AHMP, and MPyP resulted in the higher char content and stable char layer on the GFPA6 surface, which is helpful in improving the flame retardancy of GFPA6.

1. INTRODUCTION Polyamide 6 (PA6), as an engineering resin, is playing an important role in modern industry due to its good mechanical properties, attrition resistance, oil resistance, organic solvent resistance, and easy processing properties.1,2 However, the application of PA6 is limited because of its high moisture absorptivity, poor low-temperature impact strength, and dimensional stability as well as flammability.3 Therefore, glass-fiber-reinforced polyamide 6 (GFPA6) is more commonly used instead of PA6 at present for its improved dimensional stability, impact resistance, and moisture absorption resistance;4,5 but GFPA6 is more combustible than PA6 owing to the “candlewick” effect caused by the filled glass fiber (GF). Although traditional halogen-containing flame retardants can endow whether GFPA6 or PA6 exhibit fire resistance properties,1,6 they have been banned due to some laws and regulations, i.e. RoSH, because of toxic and corrosive gas released during their decomposition as well as bioaccumulation. It has been well demonstrated that phosphorus-containing or nitrogen-containing flame retardants are suitable for improving PA6’s flame retardancy.1,7 For example, melamine cyanurate, a nitrogen-containing flame retardant, can give PA6 with excellent flame retardance. It was reported when 8−15% melamine cyanurate was added to PA6, a UL-94 test V-0 rating can be obtained. Recently, phosphorus-containing flame retardants have attracted much attention, especially the metal salt of alkylphosphinic acid, such as Exolit OP 1312 and 1310 from Clariant Co., which are effective halogen-free flame retardants for GFPA6.8,9 However, the cost of the Exolit OP series is high because of the complicated synthesis route of the organic phosphinate. In our previous work, inorganic hypophosphite (aluminum hypophosphite) and arylphosphinate (aluminum phenylphosphinate) were synthesized and used as flame retardants for GFPA6.10,11 Although GFPA6 containing aluminum hypophosphite can achieve good flame © 2013 American Chemical Society

retardancy, it releases toxic gas phosphine during the processing and combustion because of the existence of the phosphorus− hydrogen group. This obstacle can be reduced when aluminum phenylphosphinate was used; however, its flame retardant efficiency for GFPA6 was low.11 It had been well demonstrated that the phosphorus-based flame retardants combining with the nitrogen-based products can achieve better flame retardancy for a polymer due to the synergistic effect.12−15 Liu et al. reported incorporating red phosphorus and melamine cyanurate simultaneously into PA6 can make PA6 a better flame retardant due to their complementary and synergistic effects.12 The synergism was also found between melamine and triphenylphosphate which was used as a complex flame retardant for poly(butylene terephthalate).13 In this study, another aluminum salt of hydroxymethylphosphinic acid, named aluminum hydroxymethylphosphinate (AHMP), and melamine pyrophosphate (MPyP) were used together to endow GFPA6 with flame retardance. The flame retardance and the thermal decomposition behaviors of flameretardant GFPA6 were investigated by LOI, vertical burning test (UL-94), and TGA, respectively. The flammable properties and char morphologies after the LOI test were studied by cone calorimetry and SEM.

2. EXPERIMENTAL SECTION 2.1. Materials. Polyamide 6 (PA6) and the glass fiber (GF) were supplied by DSM Engineering Plastic Company. Aluminum hydroxymethylphosphinate (AHMP, Scheme 1) was supplied by Weili Flame Retardant Chemicals Industry Co. Received: Revised: Accepted: Published: 15613

July 25, 2013 October 1, 2013 October 7, 2013 October 7, 2013 dx.doi.org/10.1021/ie402396x | Ind. Eng. Chem. Res. 2013, 52, 15613−15620

Industrial & Engineering Chemistry Research

Article

Ltd. (Chengdu, China). Melamine pyrophosphate (MPyP) was gained from Shandong Shian Chemical Co., Ltd.

by elemental analysis (EA) on a CARLO ERBA1106 instrument.

Scheme 1. Chemical Structure of AHMP (R = H or CH2OH)

3. RESULTS AND DISCUSSION 3.1. Flame Retardancy. LOI is a small-scale test that uses a variable percentage oxygen atmosphere to maintain a candlelike burn, and vertical burning rating test (UL-94) applies a small calibrated flame twice under the vertical sample for 10 s followed by a time measurement to extinguishment after each flame application. Both LOI and UL-94 rating tests were used to investigate the flame retardance of the GFPA6 composites, and the results are summarized in Table 1. As shown in Table 1, the UL-94 level of the virgin GFPA6 was no rating (NR), and its LOI value was only 22.0, indicating that the pure GFPA6 had poor flame retardance. It had been reported that the pure PA6 can get a V-2 rating due to the heavy dripping during the combustion.7 When some nitrogencontaining flame retardants such as melamine cyanurate were used, which promoted PA6 matrix dripping, PA6 can easily achieve a V-0 rating.16 However, when glass fiber was added to PA6, it made many flame retardants lose their effectiveness because of its “prevent-dripping” and “candlewick” effect. When 30% AHMP or 30% MPyP was individually added to GFPA6, the composite could not pass UL-94 tests, and its LOI value was only increased from 22.0 (pure GFPA6) to 25.0 or 27.5, respectively. However, when AHMP and MPyP were used together especially for those with a suitable mass ratio, the flame retardance of GFPA6 was improved obviously. For example, when the mass ratio of AHMP and MPyP was 1:1, the composite (GFPA6/15%AHMP/15%MPyP) reached a UL-94 V-2 rating. Increased the content of MPyP, i.e. the mass ratio of AHMP and MPyP was increased to 1:2, the composite (GFPA6/10%AHMP/20%MPyP) with 3.2/1.6 mm thickness could reach a V-0/V-1 rating. For the composite with the mass ratio of AHMP and MPyP was 1:5, both samples with 3.2 mm or 1.6 mm thickness could get a V-0 rating. Moreover, its LOI value was enhanced to 31.0. The above results demonstrated that there was a synergistic effect which existed between AHMP and MPyP especially for those with a suitable mass ratio, which could enhance the flame retardancy of GFPA6. 3.2. Char Morphology. The morphology of char residues after combustion was studied by the macroscopic and microcosmic views, respectively. The macromorphologies of GFPA6 and flame retardant GFPA6 after LOI tests are shown in Figure 1. An expansible carbon layer was formed after the flameout of the GFPA6/5%AHMP/25%MPyP composite, while no char existed for pure PA6; and a little char was observed for the composite containing only AHMP. When

2.2. Sample Preparation. The formulations of compositions are shown in Table 1. PA6 and all additives were dried at Table 1. Flame Retardance of GFPA6 Containing AHMP and MPyP UL-94 sample

thickness/mm

rating

LOI/%

GFPA6 GFPA6/30%AHMP GFPA6/5%AHMP/25%MPyP

3.2 3.2 3.2 1.6 3.2 1.6 3.2 3.2 3.2

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

22.0 25.0 31.0

GFPA6/10%AHMP/20%MPyP GFPA6/15%AHMP/15%MPyP GFPA6/20%AHMP/10%MPyP GFPA6/30%MPyP

29.0 25.5 25.0 27.5

100 °C in vacuum for 5 h before using and then tumbled the ingredients in a tumbler. After that the mixtures were extruded by a corotating a twin screw extruder (CTE35, Coperion Keya Machinery Co. Ltd., Nanjing, China) with a L/D ratio of 44, operated at 170−240 °C with the screw speed of 250 rpm. Finally the extrudates were cut into pellets. The pellets were compression molded at 240 °C and cut into standard testing bars. 2.3. Characterization. LOI and UL-94 vertical burning tests were carried on an HC-2C oxygen index meter (Jiangning, China) according to ASTM D2863-97 and a CZF-2 instrument (Jiangning, China) according to ASTMD3801-96, respectively. The dimension of samples for LOI and UL-94 vertical burning test were 130 mm × 6.5 mm × 3.2 mm and 130 mm × 13 mm × 3.2/1.6 mm, respectively. Samples with the size of 100 × 100 × 6 mm3 were used for the cone calorimeter test (FTT Cone Calorimeter) according to ISO 5660-1 at a heat flux of 50 kW/ m2. Thermogravimetric analysis (TGA) was performed on NETZSCH TG 209 F1 under nitrogen atmosphere at a flow rate of 60 mL/min. The heating rate was 20 °C/min, and the scanning scope was ranged from 40 to 700 °C. The decomposition activation energies were calculated from TG curves under different heating rates, including 5 °C/min, 10 °C/min, 20 °C/min, and 40 °C/min. The morphology of the char residues obtained after LOI tests were observed by scanning electron microscopy (SEM, JEOL JSM-5900LV) at an accelerating voltage of 20 kV. The surface of samples was sputter-coated with gold before examination. The actual phosphorus and aluminum contents of AHMP and MPyP were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA solution). The actual nitrogen content in MPyP was measured

Figure 1. Digital photos of flame retardant GFPA6 after LOI tests (a: GFPA6, b: GFPA6/30%AHMP, c: GFPA6/5%AHMP/25%MPyP, d: GFPA6/30%MPyP). 15614

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3.3. Thermal Degradation Behavior. Thermal degradation behaviors of AHMP, MPyP, and their blends with different mass ratios were investigated by TGA under N2 atmosphere. Figure 3 presents the TG and DTG curves of these flame retardants, and the relevant data are listed in Table 2. As shown in Figure 3 and Table 2, 5 wt % weight loss temperature (T5%) for AHMP, MPyP, and their blends were about 330 °C, while the maximum weight loss rate temperature (Tmax) and the char yield at 700 °C were different. The Tmax of MPyP was 392.1 °C, and its char residue was 36.2%, which was lower than that of AHMP. This was ascribed to the release of melamine at the temperature above 350 °C.17 When AHMP was mixed with a different weight ratio of MPyP, the T5% showed almost no change, while the mass loss rates decreased. Besides, the actual char residues of the complex flame retardant at 700 °C were all higher than the theoretical values, illustrating that there might be some interaction between AHMP and MPyP.10,18 The increased char residue was a benefit for the flame retardance of GFPA6, which had been demonstrated by UL-94 and LOI tests. The thermal degradation behaviors of GFPA6/AHMP/ MPyP composites under nitrogen atmosphere are shown in Figure 4, and the relative data are listed in Table 3. It was found that the T5% and Tmax of virgin GFPA6 were 410.9 and 471.4 °C, respectively, which were higher than all the flame retardant samples. It appeared a thermal decomposition region at the temperature of 500−700 °C, and the char residue at 700 °C was 30.4%, which was the same as the glass fiber content. When AHMP, MPyP, or their blend were added, the T5% and Tmax values of composites were brought forward, and the T5% of GFPA6/30%AHMP and GFPA6/30%MPyP were in accord with the T5% of AHMP and MPyP, indicating at this the weight loss of GFPA6/30%AHMP and GFPA6/30%MPyP was due to AHMP or MPyP decomposition. The Tmax of GFPA6/30% AHMP and GFPA6/30%MPyP were 406.5 and 372.5 °C, respectively, which were lower than the Tmax2 of AHMP (421.1 °C) and the Tmax1 of MPyP (392.1 °C). In addition, the rate of Tmax decreased obviously, and the residues at 700 °C increased greatly compared to GFPA6. Moreover, the char residues of GFPA6/30%MPyP were higher than theoretical residues 1, which indicated that an interaction existed between PA6 and MPyP resulting in the char formation. Jahromi et al. also found that there were strong interactions between PA6 and MPyP at 350 °C.17 When AHMP and MPyP were used together as a complex flame retardant for GFPA6, the T2% and T5% declined further.

MPyP was added alone, the char of GFPA6/30%MPyP was multihole and imperfect. The micromorphologies of char obtained after LOI tests are shown in Figure 2. Obviously, only glass fiber was observed on

Figure 2. SEM image of flame retardant GFPA6 after LOI tests (×2000) (a: GFPA6, b: GFPA6/30%AHMP, c: GFPA6/5%AHMP/ 25%MPyP, d: GFPA6/30%MPyP).

the char residue of virgin GFPA6. When AHMP was added (GFPA6/30%AHMP), besides a few glass fibers, numerous and large holes existed between the char layer, which greatly reduced the efficiency of isolating the unburned polymer matrix from the flammable gases and heat. When MPyP was used as a flame retardant, a more compact and continuous char layer was formed compared to GFPA6/30%AHMP. However, a few cavities still can be seen (Figure 2(d)) indicating the strength of char was low. As far as GFPA6/5%AHMP/25%MPyP is concerned, the char morphology is different. A smooth, continuous, and compact char layer was formed which can isolate the oxygen and prevent the flammable gas effectively. The above results confirmed that the formation of a highquality char layer is helpful to improve the flame retardance of GFPA6.

Figure 3. TG (a) and DTG (b) curves of AHMP, MPyP, and their mixtures in N2. 15615



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Table 2. TG Data of AHMP, MPyP, and Their Mixture in N2 sample

T5%a (°C)

Tmax1b (°C)

rate1 of Tmax1 (%/min)

Tmax2 (°C)

rate2 of Tmax2 (%/min)

char residue at 700 °C (%)

theoretical char residuec (%)

AHMP AHMP:MPyP=1:1 AHMP:MPyP=1:2 AHMP:MPyP=1:5 MPyP

330.7 330.1 330.8 331.4 331.5

363.6 365.7 391.2 391.2 392.1

8.1 5.2 5.6 7.3 9.3

421.1 425.6 561.0 560.5 568.9

4.2 3.8 3.4 4.4 5.5

54.9 49.3 45.7 40.6 36.2

-45.5 42.4 39.3 --

a

T5% defined as the temperature at which 5 wt % weight loss occurs. bTmax defined as the temperature at which the maximum weight loss rate occurs. Theoretical char residue is assumed that there is no reaction between AHMP and MPyP. Theoretical char residue = char residue of AHMP × WAHMP + char residue of MPyP × WMPyP. c

Figure 4. TG (a) and DTG (b) curves of GFPA6/AHMP/MPyP in N2.

Table 3. TG Data of GFPA6/AHMP/MPyP Composites in N2 sample

T2%a (°C)

T5%b (°C)

Tmax (°C)

rate of Tmax (%/min)

char residue at 700 °C (%)

theoretical char residue 1c (%)

theoretical char residue 2d (%)

GFPA6 GFPA6/30%AHMP GFPA6/5%AHMP/25%MPyP GFPA6/10%AHMP/20%MPyP GFPA6/15%AHMP/15%MPyP GFPA6/30%MPyP

381.5 307.7 270.3 254.4 259.0 308.2

410.9 331.6 314.6 304.8 309.3 335.1

471.4 406.5 354.7 365.0 381.0 372.5

27.1 15.0 15.2 14.7 17.8 22.5

30.4 48.9 43.3 45.9 44.7 44.1

-46.5 41.8 42.7 43.6 40.9

--42.2 43.7 44.8 --

a

T2% defined as the temperature at which 2 wt % weight loss occurs. bT5% defined as the temperature at which 5 wt % weight loss occurs. Theoretical residues 1 is assumed that there is no reaction between PA6, AHMP, and MPyP. dTheoretical residues 2 is assumed that there is a reaction between AHMP and MPyP (the actual char residues at 700 °C of the complex flame retardants are given in Table 2) and no reaction between PA6 and complex flame retardants.

c

Table 4. Obtained Activation Energy under Different Conversion from the Ozawa Method Eα (kJ/mol) conversion (%)

GFPA6

GFPA6/30%AHMP

GFPA6/5%AHMP/25%MPyP

GFPA6/30%MPyP

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

173.7 179.7 187.6 188.0 189.3 191.7 193.4 196.0 198.2 200.4

125.7 119.9 113.9 113.5 116.5 124.3 136.5 147.5 139.6 141.7

193.7 181.5 175.7 173.6 170.3 170.7 174.1 183.9 215.7 227.1

126.4 146.1 150.5 157.7 161.7 165.4 157.0 127.1 162.5 176.5

addition, the T2% of GFPA6/10%AHMP/20% MPyP declined to 254.4 °C, which were much lower than that of GFPA6, AHMP, and MPyP. This is probably because of the reactions between AHMP and MPyP occurring in the process of twin-

As shown in Table 3, the T5% of GFPA6/30%AHMP and GFPA6/30%MPyP had no obvious change comparing with that of AHMP and MPyP; however, the T5% of GFPA6 containing both AHMP and MPyP were decreased a great deal. In 15616



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the Eα of the flame-retardant composite increase in the later stage. Based on the above results, we can draw a conclusion that the earlier char formation and the higher stability of the char residue are important for improving the flame retardancy of GFPA6. 3.4. Cone Calorimeter. Figures 5−7 show the fire performance of GFPA6/AHMP/MPyP composites obtained

screw extrusion, which resulted in the formation of melamine hydroxymethylphosphinate whose function was almost the same as the compound “melamine phenylphosphinate”, an effective flame retardant for thermal plastics such as polyesters, polyamides, and polycarbonate.11,18 The thermal stability of melamine hydroxymethylphosphinate was much lower than that of AHMP or MPy, which made the T2% and T5% of complex flame-retardant GFPA6 composites decrease. Moreover, as shown in Table 3, the actual char residues were higher than the theoretical residues, except for the GFPA6/15% AHMP/15%MPyP, indicating that the interaction took place between PA6 and complex flame retardant in the other two composites, which played a positive role in char-forming; and this is in accord with the results of vertical burning tests, in which both of them can achieve a V-0 rating. In order to further investigate the char-formation mechanism, the decomposition activation energies (Eα) of the composites are calculated by the Ozawa method,19,20 and the detailed results are presented in Table 4. Generally speaking, the activation energy of virgin polymer or glass fiber reinforced polymer increased with conversion according to this method. It was reported that virgin PA6 decomposed mainly in one step and the Eα increased progressively with the increasing conversion.21 Hu et al.22 and Chen et al.23 found that the Eα of GFPA6 and glass fiber reinforced poly(butylene terephthalate) increased along with the conversion, respectively. Only when PA6 and poly(butylene terephthalate) reacted with the addition or the addition decomposed, the activation energy might decrease. As shown in Table 4, the Eα values of GFPA6 were similar to those of virgin PA6 because the glass fibers were treated as inert fillers. However, the thermal decomposition behaviors of the flame-retardant composites were different from that of virgin GFPA6. Adding 30% AHMP into GFPA6, its Eα value first decreased and then increased. Moreover, the Eα values were lower than those of virgin GFPA6. The phenomenon was ascribed to the decomposition of the flame retardant and the interaction between AHMP and PA6, which was in favor of the earlier char formation. However, there were numerous large holes in the char layer (Figure 2(b)) which did not isolate oxygen and heat effectively. Therefore, the flame retardancy of the composite was slightly enhanced. When 30% MPyP was added to GFPA6, the Eα of the composite increased as the conversion increased, except at a conversion of 0.35− 0.40. At this stage, the Eα value declined, which was due to the mass loss rate increase of MPyP and reaction between MPyP and PA6. Comparing the Eα values of composite containing 30% AHMP or 30% MPyP with those of GFPA6, the Eα at any conversion were low illustrating the chemical interaction taking place in an early decomposition stage and leading to the char formation. However, the char layers were not stable enough and compact, so the flame-retardant composites did not achieve a UL-94 V-0 rating. When 5% AHMP and 25% MPyP were added simultaneously, the Eα values of GFPA6 decreased at the early conversion, and then they increased at the later stage. Moreover, the Eα values were higher than those of virgin GFPA6 at a conversion of 0.05−0.10. The phenomenon was caused by the decomposition of flame retardant and the interaction between AHMP, MPyP, and PA6, which was in favor of the intumescent char formation (shown in Figure 2(c)). In addition, the char residue at higher conversion was still high (the char residue at 700 °C is 43.3% in N2), and the residue had the higher stability and compactness, which made

Figure 5. Total smoke release of GFPA6/AHMP/MPyP composites.

from the cone calorimeter tests at a heat flux of 50 kW/m2, and the detailed parameters are listed in Table 5. As smoke generated from an incomplete combustion is a major cause for death in fire, it is essential for flame retardants to keep smoke production to a minimum in order to reduce the overall fire hazard. Total smoke release (TSR) is an important parameter of smoke production, and it can play a crucial role in human survival during fire.24,25 Figure 5 shows the total smoke release of GFPA6/AHMP/MPyP composites. From it we can see clearly that the introduction of 30% AHMP into GFPA6 made composites having the highest total smoke release; however, the addition of MPyP can depress the total smoke release. The detailed data shown in Table 5 further demonstrate this. From Table 5, it was found when 30% AHMP was added to GFPA6, TSR was increased to 2255 m2/m2, which was about 2.8 times of virgin GFPA6. This illustrated that the thermal decomposition of AHMP resulted in the incomplete combustion of material. In the combustion process, although AHMP produced volatile gas that could dilute the fuel, however, due to the poor stability and compactness of char residue, the volatile gas could run out of the substrate easily. Therefore, AHMP had a poor flame retardant performance for GFPA6. In our previous work, a mixture of aluminum salts of diisobutylphosphinic acid and monoisobutylphosphinic acid (HPA-2TBA-Al)22 and aluminum phenylphosphinate10 were used as a flame-retardant for GFPA6. It was found that when 25% HPA-2TBA-Al or 30% aluminum phenylphosphinate was used, the TSR of GFPA6 were 3932 m2/m2 and 4602 m2/m2, respectively, both of which were higher than that of GFPA6/30% AHMP (2255 m2/m2). This result illustrates that although the flame retardance effectiveness of AHMP for GFPA6 was not high, the smoke production was lower than that of the other two substituent phosphinates. When 30% MPyP was added to GFPA6, the TSR decreased to 473 m2/m2, which was reduced by 40.5% compared to that of virgin GFPA6. When AHMP and MPyP were used together 15617



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Table 5. Cone Data of GFPA6/AHMP/MPyP Composites sample

PHRR (kW/m2)

time to PHRR (s)

TTI (s)

TSR (m2/m2)

FIGRAa

residues (%)

Pb (%)

Alc (%)

Nd (%)

GFPA6 GFPA6/30%AHMP GFPA6/5%AHMP/25%MPyP GFPA6/10%AHMP/20%MPyP GFPA6/15%AHMP/15%MPyP GFPA6/30%MPyP

467.7 270.0 209.8 222.9 249.9 234.9

145 405 185 80 60 95

52 30 35 28 32 43

793 2255 388 660 1916 473

3.2 0.7 1.1 2.8 4.2 2.5

30.4 48.6 49.6 47.2 46.4 44.5

0 7.8 4.9 5.5 6.0 4.3

0 2.2 0.4 0.7 1.1 0

0 0 9.7 7.8 5.8 11.7

a FIGRA is calculated by dividing the PHRR by the time to PHRR. bP is the phosphorus content, which is calculated according to the following equation:

weight of AHMP × 25.9% + weight of MPyP × 14.3% total weight of the composite

P(%) =

× 100% c

Al is the aluminum content, which is calculated according to the following equation:

Al(%) =

weight of AHMP × 7.2% × 100% total weight of the composite

d

N is nitrogen content, which is calculated according to the following equation: N(%) =

weight of MPyP × 38.9% × 100% total weight of the composite

as a complex flame retardant for GFPA6, it was found that with the increase of the MPyP amount, the TSR decreased gradually, especially for the composite containing 5% AHMP and 25% MPyP; its TSR value was reduced by 51.1%, which was almost half of the TSR value of virgin GFPA6. This demonstrated combining AHMP and MPyP with the mass ratio of 1:5 made GFPA6 having not only excellent flame retardance but also low smoke generation. Heat release rate (HRR) is the most important parameter in characterizing fire intensity which is based on the oxygen consumption principle.26,27 Figure 6 presents the heat release

protection and burn further. Different from the GFPA6/30% AHMP, this second heat release peak did not appear in the HRR curve of GFPA6/30% MPyP, indicating that the stability of the carbon layer was better than that of GFPA6/30%AHMP. Although the peak heat release rate (PHRR) of a specimen is not an ‘intrinsic’ material property, it may be the most important with regard to the assessment of real fire hazards.28 As shown in Table 5, the PHRR of GFPA6/30%AHMP was reduced by 42.3% compared to virgin GFPA6. When 30% MPyP was added to the GFPA6, the PHRR value was reduced by 49.8%. When 15% AHMP and 15%MPyP were added together into GFPA6, its PHRR value was 249.9 kW/m2, which was higher than that of the GFPA6/30%MPyP composite. While the PHRR values of GFPA6/10%AHMP/20%MPyP and GFPA6/5%AHMP/25%MPyP were lower than that of GFPA6/30%MPyP, in which the PHRR of GFPA6/5% AHMP/25%MPyP decreased to a lowest value (209.8 kW/ m2), its PHRR value was reduced by 55.1% compared to virgin GFPA6. This means the fire hazard of GFPA6/5%AHMP/25% MPyP was decreased greatly. Table 6 gives the phosphorus, aluminum, and nitrogen content of AHMP and MPyP determined by ICP-AES and Table 6. Elemental Contents of AHMP and MPyP

a

Figure 6. Heat release rate of GFPA6/AHMP/MPyP composites.

sample

P contenta (%)

Al contenta (%)

N contentb (%)

AHMP MPyP

25.9 14.3

7.2 0

0 38.9

Determined by ICP-AES. bDetermined by elemental analysis.

elemental analysis, and the phosphorus, aluminum, and nitrogen content in the flame-retardant GFPA6 composites are also listed in Table 5. Generally speaking, the efficiency of the phosphorus-containing flame retardants has a great relationship with the content of the phosphorus, and it increases along with the phosphorus content.29 However, when AHMP and MPyP is used together, the flame retardance of GFPA6 is not consistent with the phosphorus content. Only

rate of GFPA6/AHMP/MPyP composites. Compared with pure GFPA6, the heat release rates of GFPA6/AHMP/MPyP were lowered. In the HRR curve of GFPA6/30%AHMP a second heat release peak appeared at 405 s, which was probably attributed to the carbon layer not being stable and being fractured in the combustion, which made the composite lack 15618



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those with suitable P, Al, and N weight ratios had good flame retardance, for example, when the mass ratio of P, Al, and N in the composite was 1.0:0.08:2.0, the composite (GFPA6/5% AHMP/25%MPyP) had the lowest PHRR, TSR values, and highest LOI. When mass ratio of P, Al, and N was decreased to 1.00:0.18:0.97 (GFPA6/15%AHMP/15%MPyP), the composite had the poorest flame retardance. Figure 7 shows the residual mass curves of the GFPA6/ AHMP/MPyP composites. All the composites containing flame

25%MPyP were decreased greatly indicating it has better safety during a fire. TGA analysis and decomposition activation energies (Eα) data of flame-retardant GFPA6 demonstrated that there are some interactions existing between PA6, AHMP, and MPyP, which resulted in the higher char content and stable char layer on the GFPA6 surface. SEM images of char after the LOI test proved that the comparative compact and stable char layer formed on the surface of GFPA6, which isolated the air and heat from GFPA6 and prevented flammable gas overflowing from the internal polymer. Therefore, the flameretardant GFPA6 with more compact and stable carbon layer formation during the combustion will have excellent flame retardancy and fire safety.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-28-85410755. E-mail: l.chen.scu@gmail. com (L. Chen), [email protected] (X.-L. Wang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001), the Excellent Youth Foundation of Sichuan (2011JQ0007), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1026).

Figure 7. Residual mass curves of GFPA6/AHMP/MPyP composites.

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retardant had higher residues than that of GFPA6. As shown in Table 5, the residue of virgin GFPA6 at the end of the cone test was 30.4%, which was the same as the content of glass fiber; however, the residual mass of flame-retardant GFPA6 increased. Especially for the composite of GFPA6/5%AHMP/ 25% MPyP, the residual mass increased to the maximum value (49.6%), which further demonstrated the synergistic effect existence. In addition, FIGRA (equals to the value of PHRR/time to PHRR) is also an important parameter to assess the fire hazards of a material.30 As shown in Table 5, comparing with that of GFPA6, FIGRA of the GFPA6/AHMP/MPyP composite decreased, except for GFPA6/15%AHMP/15%MPyP. It was worth noting that because GFPA6/30%AHMP had second PHRR (270.0 kW/m2) at 405 s, its FIGRA value was calculated as 0.67. However, when the first PHRR (207 kW/m2) at 105 s was used, its corresponding FIGRA was 2.0. This value was higher than that of GFPA6/5%AHMP/25%MPyP (1.1). Moreover, the FIGRA of GFPA6/5%AHMP/25%MPyP was much lower than that of the other flame-retardant GFPA6, including GFPA6/10%AHMP/20%MPyP (2.8) and GFPA6/ 15%AHMP/15%MPyP (4.2). The results reconfirmed that the flame-retardant GFPA6 composites especially the GFPA6/5% AHMP/25%MPyP composite had better fire safety.

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4. CONCLUSIONS In this work, flame-retardant GFPA6 was prepared successfully by adding AHMP and MPyP as complex flame retardants. It was found only the complex flame retardant with an AHMP and MPyP mass ratio of 1:5 can endow GFPA6 with excellent flame retardance. The LOI value and UL-94 test of GFPA6/5% AHMP/25%MPyP is 31 and V-0 rating, respectively. Besides, the TSR, PHRR, and time to PHRR of GFPA6/5%AHMP/ 15619



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Aluminum Hydroxymethylphosphinate and Melamine Pyrophosphate

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