A Novel Organophosphorus Hybrid with Excellent Thermal Stability

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A Novel Organophosphorus Hybrid with Excellent Thermal Stability: Core−Shell Structure, Hybridization Mechanism, and Application in Flame Retarding Semi-Aromatic Polyamide Xue-Bao Lin, Shuang-Lan Du, Jia-Wei Long, Li Chen,* and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: An organophosphorous hybrid (BM@Al-PPi) with unique core−shell structure was prepared through hybridization reaction between boehmite (BM) as the inorganic substrate and phenylphosphinic acid (PPiA) as the organic modifier. Fourier transform infrared spectra (FTIR), solid state 31P and 27Al magic angle spinning nuclear magnetic resonance, X−ray diffraction, and element analysis were used to investigate the chemical structure of the hybrids, where the microrod−like core was confirmed as Al-PPi aggregates generated from the reaction between BM and PPiA, and those irregular nanoparticles in the shell belonged to residual BM. Compared with the traditional dissolution−precipitation process, a novel analogous suspension reaction mode was proposed to explain the hybridization process and the resulting product. Scanning electronic microscopy further proved the core−shell structure of the hybrids. BM exhibited much higher initial decomposition temperature than that of Al-PPi; therefore, the hybrid showed better thermal stability than Al-PPi, and it met the processing temperature of semi−aromatic polyamide (HTN, for instance) as an additive-type flame retardant. Limiting oxygen index and cone calorimetric analysis suggested the excellent flame-retardant performance and smoke suppressing activity by adding the resulting hybrid into HTN. KEYWORDS: hybrid, thermal stability, flame retardance, semiaromatic polyamide, phenylphosphinic acid, boehmite

1. INTRODUCTION Hybridization technique has brought a new perspective to the development of organic−inorganic materials. These hybrids contained special chemical construction, fine frame structures and corresponding additional functions. Generally, sol−gel technology is a classical way of the hybridization involving the hydrolysis of organo-silicate, -titanate, or -aluminate precursors to multi-hydroxyl compounds which then condense into threedimensional structures.1,2 This strategy is simple and low cost, and can yield amorphous hybrid materials that can be easily shaped as films or bulks. In this way, silane coupling agents such as RSiX3 (X = Cl, alkoxy) are the best-known coupling agents.3 Alternatively, organophosphorus modifiers are drawing more attention now. Different metal oxide/phosphonate hybrids4 or the cyclic and cage compounds5 could be formed, while the solids obtained were then annealed under hydrothermal conditions to improve their degree of crystallinity. Organic monomers and modifiers combining with the inorganic part through some strong chemical bonds is a major way to prepare hybrids. It means two-phase reaction and the inorganic substrates always play a critical role on the functional products. Layered clays, montmorillonite (MMT) © 2015 American Chemical Society

and layered double hydroxide (LDH) for instance, are one kind of interesting material that keep booming. Organic compounds or polymers were used to prepare the layered clay composites can markedly improve the mechanical, thermal, and physical− chemical properties.6−9 Recently, several studies focused on synthesis of novel hybrids depended on the inorganic twodimensional nanosheets, those graphene-like structure such as hexagonal boron nitride (h-BN), carbon nitrides (C3N4), metal chalcogenides (e.g., TiS2, VS2, and MoS2) can be modified at surface directly due to their high specific surface areas.10−13 These hybrids are favored in the field of functional materials because of their special electronic properties and the excellent synthetical properties. Beside the inorganic substrates, diverse organic modifiers are considered as another critical factor for the functional hybrids. Especially those containing organophosphorus by using acids of phosphorus (phosphoric, phosphonic, and phosphinic) and their derivatives (salts, esters) received wide attention, which Received: October 27, 2015 Accepted: December 16, 2015 Published: December 28, 2015 881

DOI: 10.1021/acsami.5b10287 ACS Appl. Mater. Interfaces 2016, 8, 881−890

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ACS Applied Materials & Interfaces offer a general alternative to silicon−based coupling in the field of organic/inorganic hybrid materials because P−O−Metal and P−C linkages are quite stable, covering a wide range of application: ion exchange, proton conductors, catalysts, sensors, flame retardants, and so on.14−16 In this case, surface modification of an inorganic support is an classical way to prepare the hybrids containing organophosphorus, and the modifier originally stands for surface modifiers bearing reactive groups.17,18 Mutin et al. considered there is a competition between surface modification and dissolution−precipitation processes in the reaction between a metal oxide surface and a phosphonic acid, and a surface modified metal oxide should be the mainly product.19 Zbigniew Florjańczyk compared the production of diphenylphosphoric acid modified aluminum alkyl and boehmite. It was found that a fabric structure of modified boehmite was obtained attributed to the formation of short polymeric chain similar to the former reaction.20 This one−dimensional material was reported again that hybrid nanorods were obtained by the π−π stacking interactions between polymeric chains, and the hybrid shows better thermal stability than the supporting aluminum hydroxide particle.21 However, further study was required to study the exact formation mechanism of such a rod-like morphology. Depending on the property of polymeric metal phosphinates22 and our previous work about the aluminum phosphinate (AlPi), we considered there may be other forming mechanism and cluster for the condensation of phosphinic groups in the separate domains situation. Semiaromatic polyamide is a class of engineering plastics with higher temperature corrosion resistance and lower moisture uptake than aliphatic polyamide such as PA6,6 and PA6. PA6,T/D,T is a members of the group commercialized by Dupont company. It consists of a 6,T and a D,T segment that contained a substituted methyl group at the aliphatic chain.23 Phosphinates have been wildly used for preparing flame retarded engineering plastics on account of high−efficiency and hypotoxicity.24−27 However, in our previous studies, it could be mentioned the thermal stability of phosphates depended on the different substituent groups and the different cation, meaning that few kinds of phosphinates were proper to flame retard some special polyamides with high processing temperature.28,29 To prepare a high-temperature hybrid as flame retardant for semi-aromatic polyamide, we chose boehmite as the inorganic part. Properties of boehmite (BM, AlOOH) are similar to aluminum hydroxide Al(OH)3, both are popular aluminum substrate for hybrid. BM is a rod−like system with an orthorhombic single crystal structure, which can also be used as flame retardant for common materials.30,31 During burning, it releases water vapor, absorbs a vast amount of heat, and consequently, cools the material. Moreover, like Al (OH)3, BM produces alumina (Al2O3) residue acting as a protective layer to isolate heat and oxygen. The main difference between BM and aluminum hydroxide is the major endothermic peak of water loss, which is higher for BM (∼350 °C). Besides particle size has significant influence on the decomposition temperature.32,33 In this work, we prepared an organic−inorganic hybrid (BM@Al-PPi) by reacting BM nanoparticle with phenylphosphinic acid (PPiA) in aqueous solution. In addition, corresponding aluminum phenylphosphinate (Al-PPi) was produced by ion−exchange reaction, and served as the comparison sample to study the component and the forming mechanism of BM@Al-PPi hybrid. Chemical structure,

morphology and thermal properties of the BM@Al-PPi hybrid were investigated. BM@Al-PPi was then blended with semiaromatic polyamide (HTN) to produce flame-retardant composites. Thermal stability, flame retardance, and smoke suppression performance of these composites were studied in detail.

2. EXPERIMENTAL SECTION 2.1. Materials. Boehmite (BM, ca. 80 wt % Al2O3) was purchased from Boyuan high technology Co., China. Phenylphosphinic acid (PPiA, A.R.) was supplied by Shanghai Xiangrong Chemical Industry Co., China. Acetic acid (CH3COOH, A.R.), aluminum chloride hexahydrate (AlCl3·6H2O, A.R.), sodium dodecyl sulfate (A.R.), and sodium hydroxide (NaOH, A.R.) were purchased from Chengdu Kelong Chemical Reagent Factory. PA 6,T/D,T (HTN, 501) was kindly supplied by DuPont China. 2.2. Synthesis of Phenylphosphinic Acid Modified Boehmite (BM@Al-PPi) Hybrid. BM was added into deionize water in a mass ratio of 1:50 to prepare a homogeneous BM sol. PPiA was prediluted by acetic acid in a volume ratio of 1:5, then added to the BM sol dropwise and stirred at room temperature. The reactions were carried out in a glass vessel reactor under high shear conditions at 100 °C for 24 h. The solids were filtrated at room temperature, washed thoroughly with distilled water several times, dried overnight at 100 °C in a regular oven, and crushed into fine powders with a grinder. Depending on the chemical stability of the oxide and the reaction conditions (temperature, concentration, pH, nature of the solvent) a dissolution−precipitation process may compete with surface modification, and the metal phosphonate obtained would stayed on the surface of inorganic supporting.19 Here, we carried out this modification reaction in aqueous at high temperature up to 100 °C for a long time to make sure the dissolution−precipitation process is dominant. To separate aluminum phosphinate from BM, we chose aqueous colloidal sol dispersion of BM nanorods in this system. In addition, different hybrids were prepared through different reactions between PPiA and BM in different mass ratio. Those samples were listed in Table 1.

Table 1. Synthesis of Hybrid Samples molar ratio of PPiA and BM BM@Al-PPi-1 BM@Al-PPi-2 BM@Al-PPi-3 BM@Al-PPi-4

1:1 3:1 5:1 1:1

reaction medium water water water water

+ + + +

acetic acid acetic acid acetic acid SDS

2.3. Sample Preparation. Semiaromatic polyamide (PA6,T/D,T) was kindly supplied by DuPont China Co. Ltd. (Zytel HTN 501, HTN for short), China. The sample was dried in an oven for at least 12 h at 90−100 °C to remove moisture. HTN and the flame retardants were mixed in a tumbler by tumbling over all of the ingredients. The mixtures then were fed into a twin-screw extruder (L/D = 33) operating at about 280−315 °C, then the extrudates were cut into pellets and further dried prior to molding. Finally, the pellets were compression molded and cut into standard testing bars. 2.4. Characterization. 2.4.1. FTIR. The Fourier transform infrared (FTIR) spectra were recorded by a Nicolet FTIR 170SX spectrometer (Nicolet, America) using the KBr disk, and the wavenumber range was set from 500−4000 cm−1. All the samples were dry out in a vacuum oven before test and mixed with KBr in the agate mortar. 2.4.2. Solid-State NMR. Solid state 31P and 27Al MAS NMR spectra were collected by a Bruker AVANCEIII−500 MHz spectrometer (Bruker, Switzerland) using cross-polarization in conjunction with magic angle spinning (CPMAS). 2.4.3. ICP−AES. Specimens for elemental analysis were sampled after dissolution in the vessel. Atomic emission spectrometer (OPTIMA 2100) was used to determine the specimen’s aluminum and phosphorus content. The Al-PPi was dissolved into the dilute 882


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1178 cm−1, 1143 cm−1 (PO), 1083 cm−1 (P−O), and 1270 cm−1 (P−C).28,34 In particular, the combination of Al atom and PPiA in hybrids should be characterized in the fingerprint region such as the strong peak 585 cm−1 and the weak signal 1016 cm−1. It meant that Al-PPi was the other component of BM@Al-PPi-2. In addition, the peaks of BM turned weaker as the ratio of PPiA and BM raised from 1 to 5 (Figure S1), suggested that more BM had been consumed and more Al-PPi obtained. However, only the FTIR results werwe far from satisfaction in confirming the chemical structure of the hybrid. This would be further confirmed by solid state 27Al−NMR and 31P−NMR spectra. The 27Al MAS NMR spectrum (Figure 2) of BM@Al-

hydrochloric acid solution completely, while the hybrid solution had to centrifuge and collected the supernatant liquid for the elemental analysis. 2.4.4. XRD. X-ray diffraction (XRD) patterns were performed with power DX-1000 diffractometer (Dandong Fangyuan, China) using Cu Kα radiation (λ = 1.542 Å) at a scanning rate of 0.02° per second in the 2θ range of 5−40°. 2.4.5. SEM. The surface morphologies of samples were observed by using a JEOL JSM 5900LV scanning electronic microscope (SEM; JEOL, Japan) at the accelerating voltage of 20 kV. The surface of samples was sputter-coated with gold before examination. The Al-PPi and hybrids were smashed into powder before test. The BM was dispersed in deionized water, then a drop of diluted BM hydrosol was spread out on a glass side and dry out in oven before test. 2.4.6. XPS. X-ray photoelectron spectroscopy (XPS) of BM@Al-PPi and Al-PPi were recorded to determine the elements in the surface on a XSAM80 (Kratos Co., U.K.), using Al Kα excitation radiation (hν = 1486.6 eV) 2.4.7. Thermogravimetric Analysis (TGA). Experiments were performed using TG 209 F1 apparatus (NETZSCH, Germany) with a nitrogen flow of 60 mL/min. Samples (about 5 mg) were heated in Al2O3 pans, from 40 to 700 °C at a heating rate of 20 °C/min. The onset decomposition temperature, T5%, at which 5 wt % of original weight was lost, and Tmax, at which products possessed the maximum weight loss rate, were recorded together with the residue weight. 2.4.8. Limiting Oxygen Index. LOI values were performed according to GB/T2406.2−2009, and the dimension of all samples was 130 × 6.5 × 3.2 mm. 2.4.9. Cone Calorimetry. Combustion behavior of the flameretardant HTN samples was measured by a cone calorimeter device (Fire Testing Technology, U.K.) according to ISO 5660−1. The samples with the dimension of 100 × 100 × 3 mm were exposed to a radiant cone at a heat flow of 50 kW/m2.

3. RESULTS AND DISCUSSION 3.1. Chemical Structure of BM@Al-PPi. Chemical structure of BM@Al-PPi was first investigated via FTIR. For comparison, infrared absorption of BM and Al-PPi were recorded. Figure 1 showed the FTIR spectra of BM, Al-PPi and BM@Al-PPi-2. First, vibrations of −OH of BM located about 3292 and 3084 cm−1, also appeared at the spectra of BM@AlPPi-2. This suggested the hybrid particle still contained BM. The rest peaks of the hybrid showed high consistence with AlPPi, that signals of the phenylphosphinate anions presented at

Figure 2. 27Al MAS NMR spectra of BM, Al-PPi and BM@Al-PPi-2 hybrid.

PPi-2 showed two resonance (accompanied by several spinning sidebands): the broader one located at ca. 8.6 ppm could be attributed to the AlO6 octahedral of the residue BM;35 while the second signal was a sharp peak at ca. −9.6 ppm, which was accordant with the typical signal of aluminum with an octahedral oxygen environment and the P−O−Al−O−P polymeric polar covalent bond in Al-PPi. This result clearly indicated that bulk Al-PPi phase was formed, otherwise another broad peak should be observed due to the uniform distribution of P−O−Al and Al−O−Al bonds.36 The 31P MAS NMR of Al-PPi (Figure 3) showed two signals (accompanied by several spinning sidebands) at δ = 12.7 and 5.2 ppm, revealing the existence of the P sites in two nonequivalent environments related to the triple bridging phosphinate groups and single bridging phosphinate groups respectively of the linear chain of polymeric phosphinate.37 Similar to the 27Al−NMR, 31P MAS NMR of BM@Al-PPi was nearly coincide with PPiA, suggesting that overwhelming majority of Al-PPi was linked to a single Al atom to form a salt by destroying the crystalline structure and the Al−O bonds of BM. However, it is interesting to notice that a weak shoulder peak appeared at δ = 19.0 ppm could be split out from the main resonance signal of Al-PPi at δ = 12.7 ppm, which related to the phenylphosphinic groups linked to the surface of BM particles formed at the end of the reaction. In this regard, the residual Al−O−Al bond of BM would not be cleaved.

Figure 1. FTIR spectra of BM, Al-PPi and BM@Al-PPi-2 hybrid. 883


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Figure 3. 31P MAS NMR spectra of Al-PPi and BM@Al-PPi-2 hybrid.

Figure 4. XRD patterns of BM, Al-PPi and BM@Al-PPi-2.

In this case, the solubility of BM and Al-PPi was different in the strong acid or alkaline solution. The P−O−Al bond of AlPPi was broken after dissolving, but the octahedral aluminum crystal structure of BM would stay. Based on the results and suggestion presented above, hydrochloric acid was used for dissolving the Al-PPi components of BM@Al-PPi-2, and there still remained some undissolved white powder related to the rest of BM after stirring 1 day. Counting the absolute content of aluminum cation and phenylphosphinic anion is a direct way to measure the components of BM@Al-PPi hybrids. The elemental analysis of Al-PPi and the soluble components of BM@Al-PPi-2 were tested by ICP−AES. As shown in Table 2,

(2sinθ). Here good matching was obtained for the hexagonal system with the unit cell parameters α = 14.893(4) Å and c = 12.912(3) Å (V = 2476 Å3). First, the large diffraction peak at 2θ = 8.9° (d = 9.9 Å) in the XRD pattern was due to the polymeric chains arranged in the form of a close-packed hexagonal columnar structure, and some similar structures had been report.20,38 The distance between adjacent Al atoms linked by three phenylphosphinate bridges could be estimated as c/3 = 4.30 Å, which was close to the value of the Al···Al separation determined for simple inorganic Aluminum dihydrogen phosphate Al (H2PO4)3 and [(tBu)2AlO2P− (OC6H5)2]2,34,39,40 suggesting the similar polymeric chains were formed. In addition, assuming that the mean volume of each non−hydrogen atom in the structure should be close to 20 Å3, the number of molecules of Al-PPi in the crystal−lographic unit cell (Z) was 4. In agreement with Solid NMR test, Al-PPi was generated and dominant in BM@Al-PPi-2, while those reflections of BM became much weaker and the 2θ shifted to lower angle due to destruction of crystal structure. 3.3. Morphology of the Hybrid. A drop of BM hydrosol was spread out on a cover glass and dried out in oven for SEM test, and large amount of nanoparticles could be observed at Figure 5a. Similarly, irregular bulk was the dominating aggregation of Al-PPi nanoparticles as shown in Figure 5b. While both hybrids BM@Al-PPi-1 (Figure 5c) and BM@AlPPi-2 (Figure 5d) existed as microrod-like agglomerates of relatively larger size. In addition, it was interesting to find that both hybrids microrods covered by large amount of nanoparticles. Lots of irregular bulks could be seem in BM@Al-PPi1 systems while BM@Al-PPi-2 hybrid seemed more uniform and fewer nanoparticles appeared due to the increase of PPiA content. Such a rod−like morphology and interesting performance of hybrids with smooth surface had been reported in several works, and the destruction of crystal structure of the inorganic substrate had been mentioned several times.21,34 Depending on such measurements of hybrids above that the AlPPi and BM were two components of hybrid, we suggested that the microrod core related to the Al-PPi aggregates which became wider in BM@Al-PPi-2 than in BM@Al-PPi-1 with the PPiA content tripled. Meanwhile, the irregular nanoparticles could be related to the residue BM especially those located at

Table 2. Dissolved Al and P Elemental Test for Al-PPi and BM@Al-PPi-2 Al-PPi BM@Al-PPi-2

Al (ppm)

P (ppm)

P/Al ratio

8.63 8.73

30.59 30.15

3.54 3.45

the dissolved components of BM@Al-PPi-2 had a phosphorus to aluminum mole ratio of nearly 3.5:1, almost the same with Al-PPi that had been dissolved completely. This result also supported that the Al−O−Al bridge ligand could not be preserved after combining with phenylphosphinate groups. All these measurement results suggested that the product of the hybridization reaction between PPiA and BM consisting of Al-PPi and BM residue. 3.2. WAXD Analysis. Hybridization of inorganic compounds always leads to some changes in crystal structure, and X−ray diffraction is a useful way to prove that. Figure 4 shows the XRD patterns of BM, Al-PPi and BM@Al-PPi-2. For BM, AlO(OH) showed characteristic basal reflections at 2θ = 14.72, 28.52, and 38.66°, which were associated with the lattice planes 020, 021, and 130, respectively.37 However, after reacting with PPiA, great changes of the crystal structure could be observed in BM@Al-PPi-2. Apart from the broad and weak reflections attributed to the residual crystalline BM structure, others new strong reflections in the range of 8−35° appeared, in agreement with the experimental result of Al-PPi. Based on the strong reflections of Al-PPi, an attempt of indexing the XRD pattern was undertaken by the Bragg equation, d = nλ/ 884


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the stability of BM hydrosol would be destroyed. With continuous adding of the PPiA/acetic acid solution, many mixture droplets would form and achieve dynamic equilibrium that can be served as a special “micro reactor” (Figure 6a). We considered that excessive amounts of BM would be contained when the ratio of PPiA and BM was 1:1, and those who located at inner and interface of mixture droplets might play the role of stabilizing agent like emulsifier. Then the reaction was carried out at a high temperature, and the BM nanoparticle would be corroded gradually by PPiA. As Figure 6b shows, the crystal of BM and Al−O−Al bridge bonds would be destroyed, then free aluminum atoms released and combined by large amount of organophosphinic acid around. As a result, Al-PPi formed and agglomerated to be microrods inside the “micro reactor” as the core of the hybrid. The residual BM would be adhered to the microrod due to its hydrophilic nature to decrease the surface energy of the Al-PPi rods in aqueous solution. On the basis of the reaction model and the forming mechanism of hybrid mentioned above, the morphology of hybrids would change in different reaction condition. Here we could explain the volume of BM@Al-PPi-2 microrod was bigger and the chaotic BM residue reduced as increasing concentration of PPiA in the “micro reactor”. When the molar mass of PPiA is up to 5 times that of BM (Figure 7b), only Al-PPi rods formed and BM had been consumed. In addition, a surface−active agent SDS was used to disperse the PPiA into BM hydrosol and the BM@Al-PPi-4 was obtained (Figure S2). As a result, the stability of “micro reactor” would be destroyed, and the decline of local PPiA concentration led to a morphology change of the final products that smaller irregular bulks replaced the microrods of BM@Al-PPi-2. All the aforementioned results suggested that the microrod did not come from the crystal growth of aluminum phosphinate but aggregates, and it may be adjusted by controlling the reactive mass ratio between PPiA and BM. 3.5. Thermogravimetric Analysis of BM, Al-PPi and BM@Al-PPi-1. Thermal stability of BM, Al-PPi and BM@AlPPi-1 were evaluated by thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG) under a nitrogen atmosphere (Figure 8). The decomposition of neat BM was a single−step reaction, which attributed to the loss of external surface water, gallery water, and the partial dehydration reaction of BM. In contrast, the degradation behavior of BM@Al-PPi-1 was more akin to the pure Al-PPi, required two steps but superior in thermal stability. Initial decomposition temperature (defined as the temperature of 5 wt % weight loss, T5%) was 410 °C, nearly 50 °C higher than that of Al-PPi. For Al-PPi, DTG peak of the first stage appeared at 343 °C (Tmax1, also the overall maximum decomposition), which attributed to the condensation of phosphinic group and some fragments released such as PH3, P4, and phenylphosphinic acid.25,28 The second decomposition stage was relatively smooth, and the DTG peak appeared at about 430 °C (Tmax2), mainly involved the

Figure 5. Microscopic morphology of (a) BM nanoparticle, (b) AlPPi, (c) BM@Al-PPi-1, (d) BM@Al-PPi-2.

the surface as shell of hybrid. Some measurements focused on the surface property of material were used to prove this hypothesis. Table 3 showed the XPS data containing Al and P element for BM, Al-PPi and BM@Al-PPi hybrids, respectively. Comparing with BM, the binding energy of aluminum in AlPPi was 74.95, a little higher than that of Al−O−Al, which should be attributed to the formed P−O−Al bond. It should be noticed that detected binding energy of aluminum of both BM@Al-PPi-1 and BM@Al-PPi-2 were closer to BM instead of Al-PPi. Opposite to Al-PPi that phosphorus was 1.6 times of aluminum nearly half of the theoretical value, the aluminum content 4 times higher than phosphorus was tested in BM@AlPPi-2. With the decrease of PPiA content, aluminum content reached 9 times of phosphorus in BM@Al-PPi-1 system. Water contact angle tests of Al-PPi and hybrids were operated to study the effect of different surface on wettability. Opposite to the highly hydrophilic BM powder, Al-PPi showed hydrophobic and the value of water contact angle up to 127.1°. However, the angles of the BM@Al-PPi-1 and BM@Al-PPi-2 were 104.2 and 108.7°, respectively, meaning that the hydrophobicity of hybrids was weaker than Al-PPi due to their special structure, that the hydrophobic Al-PPi core was covered by hydrophilic BM particles. 3.4. Hybridization Mechanism. Here, we propose an analogous suspension reaction model depending on the totally opposite of hydrophilicity between the BM nanoparticle and the oganophosphinic acid. First, because of hydrophobicity, many floating droplets of PPiA would be formed when PPiA acetic acid solution added into plenty of aqueous medium. Then, the BM nanoparticles could be absorbed to the PPiA droplets to form the monodentate or bidentate phosphinic groups linked to the Al atom at BM surface. In consequence,

Table 3. XPS Result for the Surface of Hybrids and Contrast Samples BM

Al-PPi

BM@Al-PPi-2

BM@Al-PPi-1

elements

Al

P

Al

P

Al

P

Al

P

content (%) binding energy (eV) peak attribution

21.8 74.30 Al−O−Al

0

3.9 74.95 P−O−Al

6.3 133.00 P−O

15.7 74.26 Al−O−Al

3.6 133.04 P−O

27.0 73.84 Al−O−Al

3.3 132.4 P−O

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Figure 6. Analogous suspension “micro reactor” of mixture (a) and mechanism of reaction between PPiA and BM (b).

Figure 7. Formation process of BM@Al-PPi hybrid (a), microscopic morphology of Al-PPi aggregates (BM@PPi-3 without residual BM) (b), and BM@PPi-2 with BM particle coated outside (c).

extruding up to 320 °C. And the photos of composites strips of HTN/Al-PPi and HTN/BM@Al-PPi-1 were shown in appended Figure S4. Here the thermal degradation of HTN/BM@Al-PPi composite was discussed by TGA, and the comparison of calculated and experimental curve was shown as well. As shown in Figure 9 and Table 4, pure HTN has a single decomposition step started at 423 °C, and almost immediately reached its maximum 478 °C. At the end of the test, only 3.1 wt % residue stayed. With addition of BM@Al-PPi-1, T5% turned lower to 383 °C, related to the potential reaction between the initial

degradation of the rest of organophosphorus molecules. However, for BM@Al-PPi-1 hybrid, both two DTG peaks were greatly delayed by covering the thermal stable BM particles outside the hybrid, which played a role as a barrier of heat and degradation products. In addition, the thermal stability of different hybrids was compared in appended Figure S3. It was interesting to find that BM@Al-PPi-1 has the best thermal stability, and the decomposition happened in lower temperature with the decrease of BM particles shell. 3.6. Thermogravimetric Analysis of Flame-Retardant Composites. In this work, all the composites were prepared by 886


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phosphonate or phosphate structures, which played a positive role on charring to increase the decomposition residue.41 3.7. Flame Retardance and Combustion Behavior. The limiting oxygen index is a useful way for charactering the flame retardancy of material, and higher value of LOI meant better ignition resistance of material in normal environment. Pure HTN produced melt flow taking fire and heat away in LOI test, then achieved an LOI value at 23.5%. The LOI value of HTN/ BM composite was almost the same with 20 wt % BM adding (24.0%). For BM@Al-PPi-1 containing series, the LOI value increased from 23.5 to 28% when BM@Al-PPi-1 reached 15 wt % in the flame-retardant sample. This result mainly attributed to the cross−linking networks formed during combustion as a barrier for heat and fuels transfer, which could suppress the vigorous bubbling process containing large amount of fuels in the course of degradation and simultaneously inhibit the dripping effectively.25,28 The combustion performance of HTN/BM@Al-PPi-1 composites was studied by cone calorimeter test at a heat flow 50 kW/m2, and the detailed parameters were listed in Table 5. Heat release rate (HRR) value is an important parameter in characterizing fire intensity. As shown in Figure 10(a), the peak value of HRR (PHRR) was reduced significantly by 65.2 with 15% loading of BM@Al-PPi-1 compared with pure HTN, and total heat release decreased dramatically as well (Figure 10b). The reason was that the advanced decomposition of HTN released large amount of fuel to support the fire, while some fragments could be caged by the cross−linking reaction as BM@Al-PPi-1 addition. So, there was a remarkable increase of char residue (Figure 10c). The hybrid did not only show positive effect on limiting the fire grown of HTN material, but the peak of smoke produce rate and the total smoke release were reduced as well (Figure 11), which has more positive meanings for staff in the fire escape. As the sample with 30 wt % BM@Al-PPi-1 loading shows, the value of total smoke release nearly half that attributed to the combustion progress was limited earlier and more effective.

Figure 8. (a) TG and (b) DTG curves of flame retardants powder samples under N2/BM, BM@Al-PPi-1, and Al-PPi.

4. CONCLUSIONS Compared to the traditional surficial modification to inorganic substrates, a novel hybridization mode to prepare the organic− inorganic hybrid (BM@Al-PPi) with unique core−shell structure was proposed in this article, where the microrodlike core was confirmed as the Al-PPi aggregates generated from the reaction between BM and phenylphosphinic acid (PPiA), and those irregular nanoparticles in the shell belonged to residual BM. The hybridization process could be described as follows. First, PPiA reacted with the −OH groups located at BM surface and break the Al−O−Al linkages gradually. This led to destruction of a hydrosol dispersion of BM nanoparticles and ionic species of aluminum cation produced as gradual dissolution of the mineral structure. Then, PPiA anions combined with it and solid condensation happened that kept the polymeric chain growing. As time elapsed, the aggregation of Al-PPi in water solution occurred due to its hydrophobicity,

Figure 9. Calculated and experimental TG curves of HTN and flame retardant composites under N2.

degradation products of phosphinate and the polyamide matrix.24 Furthermore, loading with BM@Al-PPi-1 showed effect on the degradable behavior of HTN in the mass loss rate at high temperature and the residue was significantly higher than the calculate value. It suggested that a cross-linking reaction happened between HTN and the Al-PPi component of BM@Al-PPi, and then some fragments of degradation product of polyamide remained with the decomposition product of BM@Al-PPi such as aluminum oxide and aluminum Table 4. TGA Data of Samples under N2 Atmosphere

HTN HTN/15% BM@Al-PPi-1 exp. HTN/15% BM@Al-PPi-1 cal.

T5% (°C)

Tmax (°C)

mass loss rate at Tmax (wt %/min)

residues at 700 °C (wt %)

423 383 422

478 471

34.6 25.6

3.1 15.7 11.8

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ACS Applied Materials & Interfaces Table 5. Cone Calorimetric Data of HTN/BM@Al-PPi-1 Composites samples

PHRR (kW/m2)

THR (MJ/m2)

TTI (s)

residues (%)

TSR (m2/m2)

HTN HTN/15% BM@Al-PPi-1 HTN/30% BM@Al-PPi-1

661.2 230.4 175.2

113.9 49.3 39.6

43 29 30

0.04 25.6 47.0

2696.1 1973.8 1339.3

Figure 10. Cone calorimetric results of pure HTN and HTN composites: (a) heat release rate, (b) total heat release, and (c) mass as a function of burning time.

Figure 11. Smoke productions of pure HTN and HTN composites: (a) smoke production rate and (b) total smoke release as a function of burning time.

which gave rise to the formation of micro scale aggregates built of hexagonally packed catena−Al[OP(O)(C6H5)2]3 chains and arranged by van der Waals forces between adjacent chains.34 It should be noticed that the rod-like structure did not only rely on the one-dimensional molecular chain, but the way of crystalline grains aggregation of Al-PPi played an important role on the structure of hybrid. After the formation of hydrophobic Al-PPi, residual hydrophilic BM particles could decrease the surface energy of Al-PPi rod-like aggregates in water, therefore a unique core−shell structure was obtained. In addition, the morphology of hybrid might be controlled by the concentration of phosphinic acid in the suspending “micro reactor”. TGA

results indicated that, thanks to the high thermal stability of BM, both were much better than the traditional phenylphosphinate, thus the hybrids could be applied for flame retarding high-temperature thermoplastics, such as semiaromatic polyamide (HTN for instance), by melt extrusion. This feature of BM@Al-PPi induced special properties and performance in corresponding HTN/BM@Al-PPi flameretardant composites. TGA measurements showed that the presence of BM@Al-PPi facilitated degradation of the polyamide, and improved the char formation at 700 °C. LOI measurement showed that 15% BM@Al-PPi loading led to an 888


Research Article


(11) Huang, X.; Zeng, Z.-Y.; Zhang, H. Metal Dichalcogenide Nanosheets: Preparation, Properties and Applications. Chem. Soc. Rev. 2013, 42, 1934−1946. (12) Ramanathan, T.; Abdala, A.; Stankovich, S.; Dikin, D.; HerreraAlonso, M.; Piner, R.; Adamson, D.; Schniepp, H.; Chen, X.; Ruoff, R.; et al. Functionalized Graphene Sheets for Polymer Nanocomposites. Nat. Nanotechnol. 2008, 3, 327−331. (13) Zhou, K.-Q.; Yang, W.; Tang, G.; Wang, B.-B.; Jiang, S.-H.; Hu, Y.; Gui, Z. Comparative Study on the Thermal Stability, Flame Retardancy and Smoke Suppression Properties of Polystyrene Composites Containing Molybdenum Disulfide and Graphene. RSC Adv. 2013, 3, 25030−25040. (14) Bluemel, J. Reactions of Phosphines with Silicas: A Solid-state NMR Study. Inorg. Chem. 1994, 33, 5050−5056. (15) Guerrero, G.; Mutin, P. H.; Vioux, A. Anchoring of Phosphonate and Phosphinate Coupling Molecules on Titania Particles. Chem. Mater. 2001, 13, 4367−4373. (16) Zhang, L.-Q.; Shi, X.; Liu, S.-M.; Pareek, V. K.; Liu, J. Organic− Inorganic Hybrid Hierarchical Aluminum Phenylphosphonate Microspheres. J. Colloid Interface Sci. 2014, 427, 35−41. (17) Florjańczyk, Z.; Dębowski, M.; Wolak, A.; Malesa, M.; Płecha, J. Dispersions of Organically Modified Boehmite Particles and a Carboxylated Styrene−butadiene Latix: A Simple Way to Nanocomposites. J. Appl. Polym. Sci. 2007, 105, 80−88. (18) Bravet, D.; Guiselin, O.; Swei, G. Effect of Surface Treatment on the Properties of Polypropylene/Nanoboehmite Composites. J. Appl. Polym. Sci. 2009, 116, 373−381. (19) Mutin, P. H.; Guerrero, G.; Vioux, A. Organic−inorganic Hybrid Materials Based on Organophosphorus Coupling Molecules: From Metal Phosphonates to Surface Modification of Oxides. C. R. Chim. 2003, 6, 1153−1164. (20) Florjanczyk, Z.; Lasota, A.; Wolak, A.; Zachara, J. Organically Modified Aluminum Phosphates: Synthesis and Characterization of Model Compounds Containing Diphenyl Phosphate Ligands. Chem. Mater. 2006, 18, 1995−2003. (21) Jiajun, M.; Junxiao, Y.; Yawen, H.; Ke, C. Aluminum− Organophosphorus Hybrid Nanorods for Simultaneously Enhancing the Flame Retardancy and Mechanical Properties of Epoxy Resin. J. Mater. Chem. 2012, 22, 2007−2017. (22) Giancotti, V.; Giordano, F.; Randaccio, L.; Ripamonti, A. X-Ray Study of Zinc (II) Di-n-alkylphosphinate Copolymers. J. Chem. Soc. A 1968, 757−763. (23) Seefeldt, H.; Duemichen, E.; Braun, U. Flame Retardancy of Glass Fiber Reinforced High Temperature Polyamide by use of Aluminum Diethylphosphinate: Thermal and Thermo-oxidative Effects. Polym. Int. 2013, 62, 1608−1616. (24) Zhao, B.; Chen, L.; Long, J.-W.; Chen, H.-B.; Wang, Y.-Z. Aluminum Hypophosphite versus Alkyl-Substituted Phosphinate in Polyamide 6: Flame Retardance, Thermal Degradation, and Pyrolysis Behavior. Ind. Eng. Chem. Res. 2013, 52, 2875−2886. (25) Braun, U.; Schartel, B.; Fichera, M. A.; Jäger, C. Flame Retardancy Mechanisms of Aluminium Phosphinate in Combination with Melamine Polyphosphate and Zinc Borate in Glass-fibre Reinforced Polyamide 6, 6. Polym. Degrad. Stab. 2007, 92, 1528−1545. (26) Jian, R.-K.; Chen, L.; Chen, S.-Y.; Long, J.-W.; Wang, Y.-Z. A Novel Flame-retardant Acrylonitrile-butadiene-styrene System Based on Aluminum Isobutylphosphinate and Red Phosphorus: Flame Retardance, Thermal Degradation and Pyrolysis Behavior. Polym. Degrad. Stab. 2014, 109, 184−193. (27) Lin, G.-P.; Chen, L.; Wang, X.-L.; Jian, R.-K.; Zhao, B.; Wang, Y.-Z. Aluminum Hydroxymethylphosphinate and Melamine Pyrophosphate: Synergistic Flame Retardance and Smoke Suppression for Glass Fiber Reinforced Polyamide 6. Ind. Eng. Chem. Res. 2013, 52, 15613−15620. (28) Zhao, B.; Chen, L.; Long, J.-W.; Jian, R.-K.; Wang, Y.-Z. Synergistic Effect between Aluminum Hypophosphite and AlkylSubstituted Phosphinate in Flame-Retarded Polyamide 6. Ind. Eng. Chem. Res. 2013, 52, 17162−17170.

increase to 28.0%. By cone calorimetric results, both the heat release rate and smoke production were greatly suppressed.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b10287. FTIR spectra of hybrids with different PPiA content, morphology of BM@Al-PPi-4, comparison of thermal stability of hybrids and processing stability analysis of AlPPi and BM@Al-PPi-1. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

* Tel./Fax: +86-28-85410755. E-mail: [email protected]. * E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (grant nos. 51273115 and 51421061) and Program for Changjiang Scholars and Innovative Research Team in University (IRT. 1026) are sincerely acknowledged. The authors would also like to thank the Analysis and Testing Center of Sichuan University for the NMR measurements.

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REFERENCES

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A Novel Organophosphorus Hybrid with Excellent Thermal Stability

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