Preparation and Characterization of Flame-Retardant Aluminum

Oct 5, 2012 - Hybridization of α-zirconium phosphate with hexachlorocyclotriphosphazene and its application in the flame retardant poly(vinyl alcohol...
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Preparation and Characterization of Flame-Retardant Aluminum Hypophosphite/Poly(Vinyl Alcohol) Composite Bihe Yuan,†,‡ Chenlu Bao,† Yuqiang Guo,† Lei Song,† Kim Meow Liew,‡,§ and Yuan Hu†,‡,* †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui, 230026, People's Republic of China ‡ USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu, 215123, People’s Republic of China § Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue Kowloon, Hong Kong ABSTRACT: Composites based on poly(vinyl alcohol) (PVA) and aluminum hypophosphite (AHP) are prepared by solvent blending. The thermal, mechanical, and crystallinity properties of the composites are studied. AHP remarkably improves the char residue yield during the thermal degradation and combustion of AHP/PVA composites. The flame retardancy of the composites is obviously improved. The increased char which protects underlying PVA from heat and oxygen is one of the key reasons for the improved flame retardancy. The crystallinity of the composites is decreased. The glass transition temperature (Tg) and the storage modulus of the composites are increased. In the composite with 15 wt % AHP, limiting oxygen index (LOI) reaches 28.0%, UL-94 V-0 rating is obtained, Tg is increased by 7 °C, and the storage modulus is increased by 17.2% in comparison with neat PVA. Moreover, AHP/PVA composites keep high transparence and maintain most of the mechanical properties of PVA, which is advantageous for the practical application of the composites.

1. INTRODUCTION Poly(vinyl alcohol) (PVA), a well-known water-soluble polymer with biocompatibility, biodegradability, and good mechanical properties, has been widely used in protonexchange membranes,1 fibers,2 hydrogels,3 packing materials,4 and segregation apparatus,5 etc. However, PVA is highly flammable. PVA drips easily once ignited, and the limiting oxygen index (LOI) of PVA is only about 20%.6 So it is important to improve the flame retardancy of PVA in many applications such as the textile industry, furnishings, adhesive, and packing materials.7,8 Halogenated flame retardants (HFRs) are widely used to improve the flame retardancy of polymers, but HFRs have been banned in some application areas because they release toxic and corrosive gases during combustion.9,10 Nowadays, halogen-free flame retardants have attracted more and more attention. Compounds based on phosphorus, silicon, nitrogen, and other elements are replacing the use of HFRs.9 Metal hydroxide is a kind of environmentally friendly and cost-effective flame retardant. The most widely used metal hydroxide flame retardants include aluminum trihydroxide (Al(OH)3) and magnesium dihydroxide (Mg(OH)2). High additional levels of metal hydroxides (>50%) are usually required, which usually caused deterioration of mechanical properties.11 Phosphorus-based compound is another kind of effective and environmentally friendly flame retardant.9 As is well-known, the combustion of polymers are usually based on strong and fast reactions between radicals.11 Phosphorus-based flame retardants can volatilize into gaseous phase and form phosphoric radicals (PO2•, PO•, etc.) to capture the radicals for combustion, and hence decrease or even stop the combustion reactions.12 Moreover, phosphorus-based flame retardants may catalyze the char formation of polymers.11,13 © 2012 American Chemical Society

The char on the composite surface protects the underlying polymer from heat and oxygen and causes improvements in thermal stability and flame retardancy.14,15 Intumescent flame retardants (IFRs), which are composed of an acid source, carbonizing agent, and blowing agent, form expanded char layers during combustion and reduce heat and fuel transfer between the flame zone and substrate. 16 Ammonium polyphosphate (APP), a combination of acid source and blowing agent, is one of the most widely used IFRs. However, the use of APP usually results in weak interfacial adhesion, poor mechanical properties, low transparency, and so forth.17 To the best of our knowledge, only a few reports about halogen-free flame retardant PVA have been published. Wang et al. used APP and two-dimensional layered nanomaterial (montmorillonite) to improve the flame retardancy and mechanical properties of PVA,6 and they also reported a work using APP, PNOH (the ammonium salt of 2-hydroxyl5,5-dimethyl-2,2-oxo-1,3,2-dioxaphosphorinane), and metal chelates to improve the flame retardancy of PVA.18 The flame retardancy of composites was significantly improved with addition of only 0.5 wt % metal chelates. When 0.5 wt % metal chelate of Ni was added into the intumescent flame retardant system, the LOI value was increased from 30.4% to 34.2%. Liu et al. oxidized PVA by potassium permanganate (KMnO4) and then reacted with 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) by nucleophilic substitution reaction. The thermal stability, organosolubility, and flame retardancy of the DOPO-containing PVA were improved. An improvement of Received: Revised: Accepted: Published: 14065

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90 °C in a 1000 mL single-neck round-bottom flask with 4 h of mechanical stirring. The translucent mixtures were cast onto glass plates and dried in an oven at 40 °C for 2 days. The obtained humid sheets were peeled off and further heated at 80 °C for 1 day to remove residual water. 2.3. Characterization. LOI was measured using an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, P.R. China), according to GB/T 2406−93 standard. Dimensions of the samples were 140 × 52 × 0.5 mm3. Five samples were tested to obtain average values. Vertical burning test was conducted using CFZ-2 vertical burning tester (Jiangning Analysis Instrument Company, P.R. China) according to ASTM D3801 standard. Dimensions of samples were 130 × 13 × 0.5 mm3. Five samples were tested to obtain average values. TGA was carried out on a TA Q5000IR (TA Instruments, U.S.) thermo-analyzer instrument from room temperature to 800 °C at a linear heating rate of 10 °C/min under N2 atmosphere. The tests were run in triplicate, and the reproducibility was ±1 °C for temperature and ±0.2% for mass. Char residues after the LOI tests were studied by Philips Model XL30E scanning electron microscopy microscope. In order to study the catalytic carbonization behavior of AHP in PVA matrix, composites were burned in a muffle furnace at 550 °C for 30 min. The graphitization degrees of the chars were studied by SPEX-1403 Laser Raman spectrometer (SPEX Company, U.S.) with excitation by a 514.5 nm argon laser line in the backscattering geometry. The obtained chars were purified by hydrofluoric acid (HF) and nitric acid (HNO3) to remove inorganic salts and amorphous carbon, and then washed with deionized water until the PH value of supernate reached 7. The purified products were dried in an oven at 80 °C for 1 day. XRD patterns of the purified chars were obtained with a Japan Rigaku D = Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Ka tube and Ni filter (λ = 0.1542 nm). The scanning rate was 4°/min and the range was 10−60°. RTFTIR was recorded using a Nicolet MAGNA-IR 750 spectrophotometer equipped with a heating device and a temperature controller. Powders of samples were mixed with KBr, pressed into a tablet, and then placed in a ventilated oven. The temperature of the oven was raised from room temperature to 500 °C at 10 °C/min. Glass transition, crystallization, and melting behaviors were investigated by differential scanning calorimetry (DSC) using a DSC Q2000 instrument (TA Instruments Inc., U.S.). The tests were carried out in N2 atmosphere. The samples were heated from 20 to 220 °C, kept at 220 °C for 5 min, cooled to 20 °C, and then heated again to 220 °C. The heating and cooling rates were 10 °C/min in all cases. Two parallel runs were done in the case of each sample. Tensile strength and elongation at break were measured with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co. Ltd., Changchun City, P.R. China) according to the Chinese standard of GB 13022−91. The crosshead speed was 50 mm/min. The sheets were cut into dumbbell shapes and 10 parallel tests were carried out for each sample to obtain average values. Dynamic mechanical analysis (DMA) was measured using DMA Q800 apparatus (TA Instruments, Inc.), at a fixed frequency of 10 Hz from 30 to 110 °C. The linear heating rate was 5 °C/min.

23% for LOI value was obtained for DOPO-containing PVA when comparing with neat PVA.19 Metal salts of alkylphosphinic acid and hypophosphorous acid have attracted a lot of interest recently because they are effective and environmentally friendly for flame retardancy applications.20 Braun et al. found that metal phosphinate (aluminum phosphinate and zinc phosphinate) released phosphinate compounds, which inhibited the spread of fire in the gas phase, could obviously improve the flame retardancy of poly(1,4-butylene terephthalate) (PBT).21 Gallo et al. investigated the effect of aluminum phosphinate (AlPi) and metal oxides on the flame retardancy of PBT.22 When AlPi was decomposed, it released diethylphosphic acid to inhibit the flame development. The combination of AlPi, melamine polyphosphate (MPP) and zinc borate was very effective for glass-fiber reinforced polyamide 6,6.20 Hu et al. prepared flame retardant glass-fiber reinforced poly(1,4-butylene terephthalate) (GRPBT) using aluminum hypophosphite (AHP), polycarbonate and melamine, and the composites achieved UL-94 V-0 rating.23 Li et al. reported that AHP effectively promoted the char formation of polyamide 6, and the degradation product of AHP, phosphine (PH3) played an important role to improve the flame retardancy of the composites. 24 Therefore, phosphinate and hypophosphite have been promising for the flame retardancy applications in polymers. However, there is still no work on PVA-based composites. This work investigated the effect of AHP on the flame retardancy, thermal stability, crystallinity, mechanical properties, and transparence of PVA. A series of AHP/PVA flame retardant composites were prepared by solvent blending. The flame retardant properties and thermal stability of the composites were evaluated by LOI, UL-94, and thermogravimetric analysis (TGA). The morphologies and structures of the char residues were explored by scanning electron microscopy (SEM), Laser Raman spectroscopy, and X-ray diffraction (XRD). The thermal degradation processes were investigated by real time Fourier transform infrared spectrometry (RTFTIR). The mechanical properties were investigated by tensile test and dynamic mechanical analysis (DMA). Mechanisms for the improvements were discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. AHP (purity ≥96 wt %) was purchased from Wuhan Ruiji Chemical Co., Ltd., China. PVA (polymerization degree 1650−1850, hydrolysis 86.0−90.0%) was supplied by Anhui Wanwei Group Co., Ltd., P. R. China. 2.2. Preparation of AHP/PVA Composites. PVA and AHP were dried in an oven at 80 °C for 12 h before use. Table 1 lists the formulation of the PVA composites. Appropriate weight of PVA and AHP were dissolved in deionized water at Table 1. Formulation of Flame Retardant PVA Composites and LOI and UL-94 Test Results fire tests

component (wt %)

a

sample

PVA

AHP

PVA PVA-A-11 PVA-A-13 PVA-A-15 PVA-A-17

100 89 87 85 83

11 13 15 17

LOI (%)

UL-94

± ± ± ± ±

no rating V-2 (da) V-2 (da) V-0 V-0

20.5 26.5 27.5 28.0 29.5

0.5 0.5 0.5 0.5 0.5

d, dripping. 14066

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Transparency of PVA and the composites were recorded using a Shimadzu Solidspec-3700 UV−vis-NIR spectrophotometer in the scanning wavelength range of 300−900 nm.

3. RESULTS AND DISCUSSION 3.1. Flame Retardancy. LOI and UL-94 are two widely used laboratory test methods to characterize the flammability of polymeric materials. The LOI values and UL-94 results are listed in Table 1. The LOI value of PVA is only 20.5%. It cannot pass the UL-94 test due to serious dripping. When adding AHP, the LOI values of the composites are remarkably increased and higher UL-94 ratings are obtained. The LOI values of PVA-A-11 and PVA-A-15 are increased to 26.5% and 27.5%, respectively, while the two composites still cannot reach the UL-94 V-0 rating because of dripping. When more flame retardant is incorporated, the flame retardancy is further improved. The UL-94 V-0 rating can be obtained for PVA-A-15 and PVA-A-17. Therefore, AHP is an efficient flame retardant for PVA, which is more effective in comparison with APP for PVA.6 The PVA-based composite with 15 wt % APP only reached UL-94 V-1 rating. 3.2. Thermal Stability of PVA and AHP/PVA Composites. TGA was carried out to study the thermal properties of PVA, AHP, and AHP/PVA composites. The TGA and differential thermogravimetric (DTG) curves of PVA, AHP, and AHP/PVA composites under N2 atmosphere are shown in Figure 1. The initial decomposition temperature (Tinitial) is defined as the temperature at which the weight loss is 5 wt %. The thermal decomposition data of Tinitial, the temperature at maximum rate of weight loss (Tmax) and the char yield at 800 °C are presented in Table 2. The decomposition of AHP under N2 atmosphere contains two steps. The characteristic maximal mass loss peaks are at 331 and 439 °C. The decomposition process of AHP can be illustrated by the two equations as detailed below:25 Figure 1. (a) TGA and (b) DTG curves of PVA and AHP/PVA composites under N2 atmosphere.

Δ

2AI(H 2PO2 )3 → AI 2 (HPO4 )3 + 3PH3

(1)

Δ

2AI 2 (HPO4 )3 → AI4 (P2O7 )3 + 3H 2O

Table 2. TGA Data for PVA, AHP, and AHP/PVA Composites under N2 Atmosphere

(2)

Comparing the TGA curves of composites with that of neat PVA, the change is very little between 100 and 150 °C. The decomposition of PVA occurs mainly in two steps. The first mass loss between 225 and 408 °C is attributed to the elimination of water and residual acetate groups, because PVA is a kind of polyhydroxy polymer with a big amount of acetate group remained in its molecular chains.26 The second decomposition step at 408−520 °C can be assumed to be cyclization reactions accompanying with the scission of the main chains.27 The initial decomposition temperatures of AHP/PVA composites are decreased by 13−17 °C, which may be due to catalytic effect on removal of pendant groups of the main chain by AHP and its products. However, the thermal stability of the second step is enhanced. Tmax of the second degradation step of composites are increased by 30−35 °C. The char yield of composites (14.9−21.4 wt %) are remarkably improved in comparison with neat PVA (4.1 wt %). In the DTG curves, the peak height represents the rate of mass loss. The mass loss rates of composites in the first step are smaller than that of neat PVA except PVA-A-17. The second peaks in the DTG curves of composites are higher than that of neat PVA, probably due to the decomposition derivatives of AHP catalyze the formation of the carbonaceous char.

Tmax (°C) sample

Tinitial (°C)

first step

second step

residue at 800 °C (wt %)

PVA AHP PVA-A-11 PVA-A-13 PVA-A-15 PVA-A-17

290 328 277 275 275 273

338 331 356 340 340 357

430 439 465 463 461 460

4.1 75.9 14.9 17.1 19.5 21.4

Figure 2 shows the TGA curves of PVA, AHP, PVA-A-15 and calculated PVA-A-15. The calculated TGA curve of PVA-A-15 is computed by linear combination of the TGA curves of neat PVA and AHP. The formula is as follows: Wcal(T )composite = x × Wexp(T )PVA + y × Wexp(T )AHP , x+y=1

Wexp(T)PVA: Value of weight given by the experimental TGA curve of neat PVA; Wexp(T)AHP: Value of weight given by the experimental TGA curve of AHP; 14067

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Figure 2. TGA curves of PVA, AHP, PVA-A-15, and calculated PVAA-15.

Wcal(T)composite: Theoretical TGA curve of composite computed by liner combination of the TGA curves of PVA and AHP; x, y: the weight percent of the PVA and AHP in the composite, respectively. From Figure 2, it is observed that AHP changes the degradation process of PVA, and AHP promotes the char formation, which is beneficial to improve the thermal stability and flame retardancy performance of polymeric materials.28,29 3.3. Char Residue Analyses. In order to study the flame retardant mechanism, the morphologies of the chars collected after the LOI tests were investigated by SEM. Neat PVA was burned out, so no char was obtained. As shown in Figure 3a,b, some large pores are observed in the char of PVA-A-11, and some solid bubbles or spheres appear on the surface of the char. The SEM images of PVA-A-13 are similar to PVA-A-11. But the spheres become more regular. The chars of the former two cannot protect the underneath matrix effectively. A thick, compact, continuous char layer without pores is formed for PVA-A-15. This kind of char layer is a good barrier to inhibit the transfer of heat and mass.14,15 The bubbles emerge again on the surface of PVA-A-17, but the char layer is continuous, compact as good as PVA-A-15. In order to study the combustion processes, neat PVA and AHP/PVA composite sheets were cut into rectangles with almost the same thickness, and then burned in a muffle furnace at 550 °C for 5 and 30 min, respectively. Figure 4a shows the samples before being put into the furnace. All composites exhibit relatively good transparency. The pictures of samples after being burned in a muffle furnace for 5 min are given in Figure 4b. From Figure 4b, it is observed that the sample of neat PVA burns very fast and the sheet transforms into powder. The sheets of PVA-A-11 and 13 curl and cannot keep their original shapes. However, PVA-A-15 is intact. While the sample of PVA-A-17 fragments into several small pieces, which may be because the char becomes brittle with further addition of AHP. Figure 4c shows the photos of samples after being burned for 30 min, neat PVA is almost burned out. The chars of PVA-A-11 and 13 are collapsed, but PVA-A-15 keeps its original shape. Raman spectroscopy is widely used to characterize carbonaceous materials.30 Figure 5 shows Raman spectra of the char residues collected after PVA-A-11 and 15 being burned in a

Figure 3. SEM images of the char residues after LOI tests: (a,b) PVAA-11, (c,d) PVA-A-13, (e,f) PVA-A-15, and (g,h) PVA-A-17.

muffle furnace at 550 °C for 30 min. The band at 1580 cm−1 (called the G band) corresponds to the first-order scattering of the E2g vibration mode.31 A band adsorption at 1360 cm−1 (called the D band) represents disordered graphite or glassy carbons.32 The graphitization degree of the char can be calculated by ratio of integrated intensity of the D and G bands (ID/IG). The lower of ratio of ID/IG, the higher of graphitization degree.33 According to Figure 5, ID/IG ratio of PVA-A-11 (4.94) is slightly greater than the PVA-A-15 (4.84), indicating that the disordered carbons take up a larger proportion in the char residues and relatively higher graphitization degree char is obtained when increasing the flame retardant. XRD pattern of the PVA-A-15 char residue after being burned in a muffle furnace at 550 °C for 30 min is shown in Figure 6. The char was washed by HF and HNO3 to remove the inorganic salts and amorphous carbon.34 The pattern exhibits a broad diffraction peak centered at about 23° and a small peak at about 44°, which can be ascribed to (002), (100) diffraction of graphite.35 The d002 interlayer spacing obtained from the (002) peak is 0.386 nm, which is larger than that of graphite (d002 = 0.335 nm). It indicates that the graphitic levels of the carbonaceous chars are not very high and oxygencontaining functional groups bond at the basal plane of graphitized carbon,33,36 with good agreements with the Raman results. The graphitic materials are very stable at high temperature.37 3.4. Thermal Degradation of PVA and AHP/PVA Composites. RTFTIR was employed to investigate the pyrolysis processes of neat PVA and PVA-A-15. The RTFTIR spectra of neat PVA and PVA-A-15 at different pyrolysis temperatures are shown in Figure 7. Neat PVA presents characteristic absorption peaks at 3406, 2931, 1732, 1432, 1375, 14068

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Figure 5. Raman spectra of the char residues after being burned in a muffle furnace at 550 °C for 30 min of PVA-A-11 (a) and PVA-A-15 (b).

Figure 4. Digital photographs of PVA and AHP/PVA composites before (a) and after being burned in a muffle furnace at 550 °C for 5 min (b) and 30 min (c).

1256, and 1098 cm−1.38−41 The absorption at 3406 cm−1 is ascribed to O−H stretching vibration. The peaks at 2921, 1432, and 1375 cm−1 are assigned to the stretching and deformation vibration of C−H, respectively. The band at 1098 cm−1 corresponds to C−OH stretching vibration. The bands at 1732 and 1256 cm−1 are attributed to the stretching vibration of CO and C−O−C of ester groups, which is due to the incomplete alcoholysis of PVA.42 With the increase of the pyrolysis temperature, the intensities of all characteristic peaks decrease. A new peak at 1577 cm−1 attributed to CC stretching vibration of the aromatic structure appears at 250 °C,43 but it vanishes at 430 °C. When the temperature goes up to 450 °C, peaks are no longer visible, implying that PVA has been decomposed completely.

Figure 6. XRD pattern of the char residue (with acid treatment) after being burned in a muffle furnace at 550 °C for 30 min for PVA-A-15.

When compared with spectra of the neat PVA, two new peaks at 2393 and 1186 cm−1 which are attributed to AHP are found in PVA-A-15.44 The two peaks are attributed to the stretching and deformation vibration of P−H in AHP. The 14069

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Figure 7. The RTFTIR spectra of PVA (a) and PVA-A-15 (b) at different pyrolysis temperatures.

characteristic peak of O−H stretching vibration nearly disappears at 330 °C, which is lower than the neat PVA. It agrees well with TGA results that AHP catalyzes the degradation of PVA. The characteristic peak of CC stretching vibration of the aromatic structure at 1584 cm−1 keeps its relative intensity all the time. Some new peaks appear at higher temperatures. The peak around 1275 cm−1 may correspond to the stretching vibration of PO.45 The wide peak between 1000 and 1100 cm−1 should be assigned to P− O−C stretching vibration.46 The band of aluminum phosphate at about 1150 cm−1 is overlapped with the broad absorption band of P−O.23 The small peak at 975 cm−1 can be ascribed to the stretching vibration of P−O−P.45 3.5. Glass Transition, Melting and Crystallization Behaviors. The DSC curves of PVA and AHP/PVA composites are presented in Figure 8. Table 3 lists the glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (ΔHm), and crystallization temperature (Tc). It can be found that there is no obvious change in the melting temperatures. The crystallization temperatures of AHP/PVA composites are decreased by 10−13 °C. The degree of

Figure 8. The DSC heating (a) and cooling (b) scans of PVA and PVA/AHP composites at 10 °C/min. (c) Glass transition temperature measurement of PVA and AHP/PVA composites.

crystallinity (Xc) is calculated from the ratio ΔHm/ΔH0, which is the apparent and 100% crystalline melting enthalpy, 14070

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Table 3. DSC Crystallization and Melting Parameters of PVA and PVA/AHP Composites sample

Tg (°C)

Tm (°C)

Tc (°C)

ΔHm (J·g−1)

Xc (%)

PVA PVA-A-11 PVA-A-13 PVA-A-15 PVA-A-17

70 77 76 77 77

188 189 189 188 189

127 114 115 115 117

27.27 27.16 25.31 22.35 22.13

19.67 19.60 18.26 16.12 15.97

respectively. The melting enthalpy of 100% crystalline, ΔH0, is taken as 138.6 J·g−1.47 The decrease of the melting enthalpy of composites corresponds to decrease the degree of crystallinity. It indicates the existence of some interactions between the polymer and the filler to the detriment of interactions among polymer chains.48 The possible interactions between the two materials are hydrogen bond and complexation of Al3+ with O atom in PVA.24 The glass transition temperatures of composites shift to higher temperature, because the segmental motion of polymer chains is inhibited by the AHP particles.49 3.6. Mechanical Properties of PVA and AHP/PVA Composites. The elongation at break and tensile strength are shown in Table 4. The values of elongation at break are small,

Figure 9. DMA curves of PVA and AHP/PVA composites.

required in these applications. Figure 10 shows the UV−vis transmission spectra of neat PVA and AHP/PVA composites

Table 4. Mechanical Properties of PVA and AHP/PVA Composites sample PVA PVA-A-11 PVA-A-13 PVA-A-15 PVA-A-17

elongation at break (%) 2.2 2.0 2.4 2.0 1.8

± ± ± ± ±

0.3 0.2 0.4 0.3 0.4

tensile strength (MPa) 100 96 95 102 98

± ± ± ± ±

5 4 5 5 8

storage modulus at 40 °C (MPa) 3232 3496 3655 3789 5920

probably due to the low water contents. The tensile strength values of composites are not visibly changed. Thus, the addition of AHP does not reduce the mechanical properties of PVA, which is a visible advantage as compared to other flame retardants such as APP.6 Due to the incompatibility of APP with PVA, the incorporations of APP often lead to deterioration of mechanical properties of materials.6 The retain of good mechanical property of PVA is probably attributed to the relatively strong interactions and efficient load transfer between the two phases.50,51 The storage modulus obtained from DMA tests are plotted in Figure 9, and the data are shown in Table 4. The composites show higher storage moduli than neat PVA. At 40 °C, the storage moduli are increased by 8.2, 13.1, and 17.2% for PVAA-11, PVA-A-13, and PVA-A-15, respectively, in comparison with neat PVA. It is interesting that the storage modulus of PVA-A-17 is increased by 83.2%. We speculate that the network structure is formed with addition of 17 wt % AHP.32 When appropriate weight of AHP is added in PVA matrix, the network of the rigid particles is formed and the storage modulus is enhanced. As discussed above, there are some interactions existing between AHP and PVA, which is beneficial to improve the mechanical properties of the composites. Considering that AHP has an opposite effect on the crystallinity of PVA, the enhancement of mechanical properties should be attributed to the strong interfacial interaction between the two components.49,51 3.7. Transparence. As mentioned above, PVA is widely used in packing and coating materials, and high transparence is

Figure 10. UV−vis transmission spectra of neat PVA and AHP/PVA composites in the wavelength range of 300−900 nm.

membranes. The transparence of PVA remains at about 90% at 400−900 nm. It can be seen that the transparence of AHP/ PVA composites are decreased with increasing AHP content. When 15 wt % AHP is incorporated, the transmittance at 400− 900 nm is 58−71%. The good transparency of PVA is kept when using AHP as flame retardant, which may be useful for coating and packing applications. 3.8. Mechanisms of Flame Retardancy. AHP is efficient to improve the flame retardancy of PVA. It is mainly attributed to the combination of gas phase and condensed phase flame retardant mechanisms. 3.8.1. Gas Phase Mechanism. As demonstrated in section 3.2, the decomposition process of AHP can be divided into two stages. The former stage is the thermal decomposition of AHP, which produces phosphine and aluminum hydrogen phosphate. The second stage is the condensation of the acidic phosphate.24,25 Phosphine and its derivatives are radical scavengers to trap the radicals for combustion reactions. Phosphine is easily generated to phosphorus-based radicals (PO2•, PO•, etc.) in the flame zone.24 The phosphorus14071

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Scheme 1. Possible Formation Mechanism of Graphitic Char during Combustion of AHP/PVA Composites

According to the above experimental results, the possible formation mechanism of graphitic char is proposed in Scheme 1. The degradation mechanism of PVA has been widely reported; here we only list the main reactions and the intermediate products, which contribute to the formation of carbonaceous char.39,41,42,57,58 The catalytic effect of AHP is studied in this section. Elimination of water and acetate groups is the main process of the first stage of degradation of PVA.42,59 The elimination reaction generates CC and forms a polyene structure at the end of the first stage of degradation. The Lewis acid site, Al3+, and proton acid, namely, aluminum hydrogen phosphate, accelerate the elimination reaction of PVA,20,22,26,60−63 as evidenced by the decrease of the initial decomposition temperature in TGA results. In the second degradation stage, the polyenes are converted into an intermediate product of aromatic structure by Diels− Alder and intramolecular cyclization reactions.40,42,59 A wealth of documents have been reported that solid acid can catalyze the dehydrogenation and cross-linking of polymers.64,65 It is assumed that the acidic phosphate bearing characteristic of solid acid can catalyze the carbonization of PVA and accelerate the above reactions. The phosphate can also catalyze the cyclodehydrogenation reaction to transform substituted

containing species in the vapor phase trap the H• and HO•, so combustible compounds are reduced.11,12,52 AHP can act in the vapor phase by a radical mechanism to interrupt the combustion process of PVA. 3.8.2. Condensed Phase Mechanism. The char formation of polymer during combustion plays an important role in improving the flame retardancy of the material. The carbonaceous char layer acts as a physical barrier, which slows down mass and heat transfer between the gas and condensed phases and prevents the underlying material from further combustion.14,15 According to the carbon structure, the carbonaceous char consists of amorphous carbon and crystalline carbon char or graphitic carbon char.53 The in situ formation of graphitic structure char during combustion of polymer can further improve the flame retardancy of composites by reducing the releasing of flammable gaseous molecules.54,55 The graphitic carbon endows the char with good thermal stability and mechanical properties.13,37,56 As discussed above, the incorporation of AHP can increase the degree of graphitization of char residue of PVA and improve the char quality. The catalytic role played by AHP has an important influence on flame retarded PVA. The possible catalytic graphitization mechanism of AHP during combustion of AHP/PVA composite will be presented as below. 14072

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aromatic into polycyclic aromatic hydrocarbons.66,67 The polycyclic aromatic compound can be regarded as the sheet of graphite. The graphitic carbon is constituted by stacking the polycyclic aromatic species.68 The aluminum phosphates act as a template for the formation of graphitic carbon35,36 The salts of aluminum phosphate facilitate the transformation of the poorly graphitizable carbon to a relatively more crystalline char. Furthermore, the thermally stable aluminum phosphate acts as adhesive to cross-link the polyaromatic structure to make char intact and robust.13,14 And aluminum phosphate showing a ceramic-like structure can thermally stabilize and mechanically reinforce the char.69−72

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4. CONCLUSIONS In this work, AHP was incorporated into a PVA matrix and the desired flame retardant properties were obtained. The results showed that AHP was superior to APP with the addition of 15 wt %. From the Raman spectroscopy and XRD pattern of the char, it is indicated that AHP catalyzed the formation of the graphitic char. AHP decreased the initial decomposition temperature of PVA and significantly increased the char residue. Because of the existence of some interactions between the polymer and the filler, the degrees of crystallinity of the composites were decreased, while the glass transition temperatures shifted to higher temperatures. It was interesting that AHP could maintain the mechanical properties of PVA, unlike other conventional flame retardants. It was due to strong interfacial interaction between the two phases. The combination of the gas-phase and condensed-phase mechanism is the key reason for the improved flame retardancy.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-551-3601664; E-mail: [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No.51036007), the National Basic Research Program of China (973 Program) (2012CB719701) and the Opening Project of the State Key Laboratory of Fire Science of USTC (No. HZ2011-KF05).

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REFERENCES

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