Yanlong Sui,
Lijie Qu,
Xueyan Dai,
Peihong Li,
Jinrui Zhang,
Shuai Luo and
Chunling Zhang
*
School of Materials Science and Engineering, Jilin University, Changchun 130025, PR China. E-mail: clzhang@jlu.edu.cn
First published on 27th March 2020
Toxicity and environmental issues have elicited research attention regarding the need to prepare a green flame retardant with high flame retardancy. Here, a supermolecular self-assembly technology was used to prepare nickel phytate as shell materials aggregated on aminated silica nanotemplates through electrostatic interactions as a green novel flame retardant (Ni@SiO2-PA). After incorporating the obtained core–shell structured Ni@SiO2-PA into epoxy resin (EP), the supermolecular shell effectively enhanced the adhesive property between the nanoparticles and the EP matrix. The thermal stability was improved, and the peak heat release rate decreased significantly after introducing the well-characterized Ni@SiO2-PA. The absorbance intensity of the toxic aromatic compounds also decreased. Moreover, the char yield of the EP composites was improved because of the synergetic coupled effects between the nickel phytate supermolecules and SiO2 nanotemplates. The possible fire-retardancy mechanism was hypothesized as follows. The crosslinking structure of the silica initially enabled the formation of a polymer network to prevent further decomposition. The N–P synergistic flame-retardancy system then generated a gas barrier and P-rich intumescent char. Besides, char-residue generation was catalyzed by introducing Ni2+, which isolated the heat and the exchange between oxygen and the matrix. Overall, this study proposes a novel green flame retardant that may enable significant improvements in preparing environmentally friendly organic–inorganic materials with applications in the fields of flame-retardant composites.
Currently, several halogen-free flame retardants, such as graphene,13–15 layered double hydroxide (LDH),16,17 boron nitride (BN),18,19 MoS2,20 and carbon nanotubes,12,21 have been rapidly developed. A small amount of nanofiller added into the polymer matrix can result in excellent flame retardancy while simultaneously maintaining or enhancing the other properties of the composites. Zhang et al.22 built a crosslinked network on the basis of cyanate ester (CE) and graphene oxide with P and Si (FGO). Compared with pure CE, FGO/CE resins could obtain outstanding flame retardancy when the P content was as low as 0.18 wt%, which was much lower than those of available P flame retardants. Zhou et al.23 prepared by self-assembly exfoliated MoS2 and LDH through electrostatic force and then introduced these into EP to reduce their fire hazards. Yu et al.4 performed the thermal oxidation of BN to obtain hydroxylated functionalized (BNO) and then covalently incorporated it into modified EP. The addition of BNO into the modified EP enhanced the thermal stability and oxidative stability efficiently. The dispersion quality of the nanofillers mentioned above in the polymer matrix determines their effectiveness in improving the performance of a material. Considerable effort has been dedicated to nanofillers functionalization thus far. The surface chemical modification of nanofillers by materials containing flame-retardant elements, such as N and P, can efficiently improve the flame retardancy and its compatibility with most polymers.24 Flame-retardant compounds are commonly used to modify the nanofillers. However, most of the flame-retardant compounds, such as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide and its derivatives, are obtained from nonrenewable resources, such as oil.25 Bio-based materials existing in nature as novel flame retardants are the inevitable requirement in developing a green strategy. Therefore, related research has become a topic of considerable concern.
Phytic acid (PA), which is an abundant natural product found in plant seeds and stems, has six phosphate groups that provide a variety of viable crosslinking sites.26,27 As an environmentally friendly and biocompatible organic acid, PA has a potential value in flame-retardant polymers because of its high P content. Wang et al.28 wrapped a PA-doped polypyrrole shell on bulky BN nanosheets as a flame retardant for thermoplastic polyurethane. The resulting system showed a significantly decreased peak heat release and dramatic suppression of CO and HCN releases. Shang et al.29 used melamine and bio-based PA to prepare 2D nanomaterials as flame retardants through self-assembly technology. The catalytic charring performance of the phytate structure generated P-rich intumescent char residue and volatile PO˙, which could rapture OH˙ and H˙ to terminate combustion reactions, respectively.30 Thus, PA, which can enhance the fire safety of polymers, can be used as an eco-friendly flame retardant. Cheng et al. prepared silica coating for improving the flame retardancy of silk fabric using naturally occurring phytic acid as a phosphorus precursor. The results showed that compared with the untreated silk, there was an obvious reduction in PHRR and THR as well as a significant increment in char residue.31 However, to the best of our knowledge, the combination of the aminated SiO2 and PA supermolecular technology and its application in polymer composites has never been reported.
In this work, we developed a facile procedure to prepare a new 3D organic–inorganic nanoparticle to form a core–shell flame retardant that could protect the EP matrix. Nickel phytate acted as the shell wrapped on aminated silica nanotemplates via electrostatic interaction as a green novel flame retardant (Ni@SiO2-PA). Supermolecular self-assembly technology was used and is easily available. The dispersion of Ni@SiO2-PA was observed. The thermal stability and flame retardancy were investigated. This work will trigger additional scientific interest in the development and application of bio-based materials to enhance thermal stability and fire safety during EP combustion.
Next, SiO2-PA was prepared via the strong interactions between the positive and negative charges. First, 1 g SiO2–NH2 was mixed with 100 mL of absolute ethanol under ultrasonication. Next, 5 g 70% PA solution was dissolved in 100 mL of absolute ethanol and 20 mL of deionized water and was then added into the solution above, followed by stirring for 12 h at 70 °C. The suspension was centrifuged and washed with absolute ethanol and deionized water. Then, the products were dried in an oven at 100 °C for 12 h.
The final step was the addition of Ni2+ to prepare the Ni@SiO2-PA. A total of 1 g SiO2-PA powder was dispersed in 100 mL of absolute ethanol, followed by ultrasonication for 10 min. Then, 2 g NiCl2·6H2O was dissolved in 40 mL of deionized water and added to the suspension above, and the resulting mixture was stirred for 3 h at 100 °C. Finally, the product was washed with absolute ethanol and deionized water and dried in an oven at 100 °C for 12 h.
Cone calorimetry tests were performed using a calorimeter (iCone, FTT0242) in accordance with the ISO 5660-1 standard under a heat flux of 35 kW m2 and a sample size of 100 mm × 100 mm × 5 mm. TGA-infrared spectrometry (TG-IR) was performed using a TGA (iCone, 209F3) thermogravimetric analyzer linked to an FTIR (Brook, TENSOR27) spectrophotometer from 30 °C to 800 °C at 15 °C min−1 (N2 atmosphere, flow rate of 30 mL min−1). The graphitization degree of the samples was obtained using an inVia-Plus532 confocal Raman microprobe (Renishaw, UK). The detected wavenumber was between 800 and 2000 cm−1. The mechanical properties of the EP composites were investigated using an electronic universal testing machine (WSM-5KN, China) in accordance with GBT 2567.
The XRD pattern for SiO2 and its derivatives are shown in Fig. 3. The diffraction pattern of SiO2 showed the reflection characteristics of amorphous silica.5 Compared with the pattern of SiO2, the peak positions of SiO2-PA and Ni@SiO2-PA were the same. However, the peak intensity significantly decreased in the 2θ range from 20° to 30°. This result may be due to the formation of a less ordered stacking structure. The results suggest that the presence of the flame-retardant particles wrapped on SiO2 surface changed its regularity and uniformity but still kept their amorphous features.
The TGA curves of SiO2, SiO2-PA, and Ni@SiO2-PA under the N2 atmosphere are shown in Fig. 4 and its DTG curves are shown in Fig. S1.† SiO2-PA and Ni@SiO2-PA exhibited an evident weight loss between 50 °C and 800 °C, and the solid residue was decreased at 800 °C. This result was due to the presence of unstable N–P-containing components, which decomposed the product early compared with SiO2. Ni@SiO2-PA showed a higher solid residual yield than SiO2-PA at 800 °C, thereby suggesting an improved thermal stability after the decoration of Ni2+.
To confirm the structures of SiO2-PA further, we used XPS to clarify the elemental components on the SiO2 surface. As shown in Fig. 5, the heteroatom (N, P, and Ni) peaks at 398 (N 1s), 133 (P 2p), 190 (P 2s), 856 (Ni 2p3/2), and 645 eV (Ni LMM) in the XPS spectrum of Ni@SiO2-PA appeared compared with that of pure SiO233,34 suggesting that N, P, and Ni atoms were doped into SiO2 successfully. Fig. 5b shows that the C 1s XPS spectrum of Ni@SiO2-PA was divided into four peaks. The characteristic peaks at 287.3, 286.5, 284.8, and 283.9 eV could be attributed to C–O, C–N, C–C, and C–Si, respectively.35,36 Three peaks located at 531.3, 532.4, and 533 eV were observed in the O 1s spectrum, which were attributed to P–O (Si–O), P–O–Ni, and absorbed water, respectively, as shown in Fig. 5c.2 Three peaks were detected in the N 1s XPS (Fig. 5d) at 401.1, 399.8, and 401.6 eV, corresponding to the positively charged N atoms in SiO2–NH2, N–C, and N–H bonds, respectively.37 As shown in Fig. 5e, the P 2p survey spectrum showed two main fitting peaks at 134.2 and 133.5 eV, thereby corresponding to a phosphate group and P–O bonds, respectively.28 Fig. 5f also shows peaks at 856 eV, corresponding to Ni 2P3/2, thereby indicating that Ni ions existed in Ni@SiO2-PA.34 These results demonstrated successful Ni@SiO2-PA synthesis.
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Fig. 5 XPS scans of (a) SiO2, SiO2-PA, and Ni@SiO2-PA. (b) C 1s, (c) O 1s, (d) N 1s, (e) P 2p, and (f) Ni 2p spectra of Ni@SiO2-PA. |
In general, the TG and DTG analyses of the EP composites were performed and the corresponding curves are shown in Fig. 7. In the inert atmosphere, the obtained thermal data are displayed in Table 1, and all the tested materials showed a similar one-stage degradation behavior from 300 °C to 450 °C, which could be attributed to the thermal decomposition of the epoxy chain network.39 As shown in Table 1, the T5 wt% values of the composites with a small amount of SiO2 and Ni@SiO2-PA were higher than those of pure EP, which could be attributed to the considerable thermal stability of SiO2 and its derivatives. When the incorporation of SiO2 and Ni@SiO2-PA reached 5 wt%, the initial thermal decomposition temperature of below 370 °C decreased significantly because of the early degradation of the nickel phytate shell. PA was thermally decomposed into pyrophosphate and polyphosphate, which can catalyze and cause composite decomposition.40 However, the composites incorporating SiO2 and the Ni@SiO2-PA nanospheres showed a relatively better thermal stability than pure EP due to the significant increase in residual char and the reduced mass loss rate (Fig. 7a and b). The obtained thermal data in air are displayed in Table 2. Two degradation stages were observed in the TGA curves for pure EP and its composites (Fig. 7c). The first degradation behavior for EP composites showed no difference compared with in the N2 atmosphere. The second degradation stage occurred from 500 °C to 600 °C, thereby corresponding to char degradation.33 The degradation temperature of char and its decomposition rate decreased as the amount of incorporated Ni@SiO2-PA increased (Fig. 7d). The char residues at 800 °C were also improved after adding SiO2 and Ni@SiO2-PA. When the amount of added Ni@SiO2-PA was 5 wt%, the highest char residue of approximately 10.52% was obtained, and the best thermal stability was exhibited. The evidence of the improving char yield and reducing mass loss rate can be attributed to two factors. First, the EP molecular chains connected by the amino-rich SiO2 as the crosslinking network acted as a physical barrier to protect the EP matrix from thermal degradation. Second, the PA was decomposed into polyphosphates, which were further degraded to form P-containing oxides. This process was beneficial for the formation of char residues, which can inhibit the release of the volatile products and heat exchange during combustion. Overall, adding Ni@SiO2-PA promoted catalytic char residue generation and inhibited EP matrix decomposition efficiently.
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Fig. 7 TGA (a) and DTG (b) curves of EP and its composites under N2 atmosphere. TGA (c) and DTG (d) curves of EP and its composites in air. |
Samples | T5 wt% (°C) | T50 wt% (°C) | Tmax (°C) | Residue at Tmax °C (wt%) | Residue at 800 °C (wt%) |
---|---|---|---|---|---|
EP | 367.3 | 402.1 | 384.7 | 75.78 | 13.06 |
EP/SiO21.0 | 367.8 | 405.5 | 386.5 | 75.52 | 13.26 |
EP/Ni@SiO2-PA1.0 | 365.8 | 404.5 | 387.0 | 73.07 | 15.83 |
EP/Ni@SiO2-PA3.0 | 368.5 | 405.7 | 387.1 | 74.02 | 18.44 |
EP/Ni@SiO2-PA5.0 | 359.6 | 403.8 | 385.6 | 71.28 | 22.60 |
Samples | T5 wt% (°C) | T50 wt% (°C) | Tmax1 (°C) | Tmax2 (°C) | Residue at Tmax1 °C (wt%) | Residue at 800 °C (wt%) |
---|---|---|---|---|---|---|
EP | 358.1 | 430.4 | 380.8 | 544.6 | 82.16 | 0.00 |
EP/SiO2 | 359.1 | 454.8 | 379.3 | 539.9 | 80.86 | 0.12 |
EP/Ni@SiO2-PA1.0 | 361.4 | 453.6 | 380.3 | 548.3 | 83.18 | 1.40 |
EP/Ni@SiO2-PA3.0 | 342.0 | 436.4 | 378.9 | 553.9 | 82.21 | 4.73 |
EP/Ni@SiO2-PA5.0 | 324.8 | 450.6 | 377.5 | 553.9 | 80.81 | 10.52 |
Samples | LOI (%) | Dripping | UL-94 |
---|---|---|---|
EP | 25.7 | Yes | No rating |
EP/Ni@SiO2-PA1.0 | 26.2 | No | No rating |
EP/Ni@SiO2-PA3.0 | 26.9 | No | V-1 |
EP/Ni@SiO2-PA5.0 | 28.3 | No | V-1 |
The influence of Ni@SiO2-PA with different contents on the flame retardancy of EP was investigated by cone calorimetry, which is an effective tool for evaluating the heat release properties of polymers, including the heat release rate (HRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP). The heat and smoke release curves from the cone calorimetry tests are shown in Fig. 8, and the related data are summarized in Table 4. The time to ignition values of the composites were lower than those of pure EP, which may be due to the competition between the effect of thermal conductivity and the shielding performance of the external heat radiant flux.41 High peak values of HRR (1152 kW m−2, Fig. 8a) and THR (99.91 MJ m−2, Fig. 8b) were obtained for pure EP during combustion, which revealed the flammability of EP. After the incorporation of Ni@SiO2-PA, the peak value of HRR was effectively reduced, and the THR values of EP composites were significantly lower than those of pure EP. With as low as 1.0 wt% Ni@SiO2-PA, the peak values of HRR and THR that were achieved in EP/Ni@SiO2-PA here decreased by 27.2% and 24.0%, respectively. This downward trend was in agreement with the increase in additive amount. The maximum decreases in HRR and THR that were achieved with the addition of 5.0 wt% Ni@SiO2-PA accounted for 51.6% and 49.2%, respectively. This phenomenon suggested that a part of the EP matrix in the nanocomposites was shielded during combustion. The remarkable improvements in the flame retardancy of EP/Ni@SiO2-PA could be ascribed to the cooperative effect between the SiO2 and nickel phytate. Here, the supermolecular shell can catalyze the generation of P-rich char residues, and Ni2+ can catalyze the decomposition products of EP to form a stable char, thereby decreasing the combustion fuel supply and heat release.42 SiO2 can form a Si-containing polymer crosslinked network that acts as a physical barrier. The majority of deaths in fire disasters are due to the production of highly toxic smoke particles.13 The SPR and TSP curves of pure EP and its composites are shown in Fig. 8c and d, respectively. Given its specific multiaromatic structure, pure EP exhibited a high toxic smoke yield with high SPR and TSP values. The TSP value of EP/Ni@SiO2-PA5.0 was significantly decreased from 23.26 m2 for pure EP to 11.69 m2, thereby indicating that smoke production was effectively suppressed. This result exhibited the best flame retardancy of all the samples. Combined with TGA, the significant reduction in smoke release was attributed to the presence of PA, which can be thermally decomposed into pyrophosphate and polyphosphate to catalyze the formation of a cohesive and compact char layer during combustion, which delays the permeation of O and the escape of volatile degradation products. These results confirmed the suppression effect of Ni@SiO2-PA nanoparticles on the fire hazard of the EP matrix.
Samples | TTI (s) | TPHRR (S) | PHRR (kW m−2) | THR (MJ m−2) | SP (m2) | COP (g s−1) | CO2P (g s−1) |
---|---|---|---|---|---|---|---|
EP | 83 | 140 | 1152 | 99.91 | 23.26 | 0.55 | 13.39 |
EP/Ni@SiO2-PA1.0 | 71 | 117 | 839 | 75.92 | 18.23 | 0.45 | 8.10 |
EP/Ni@SiO2-PA3.0 | 74 | 107 | 816 | 70.72 | 15.95 | 0.37 | 9.50 |
EP/Ni@SiO2-PA5.0 | 74 | 111 | 558 | 50.76 | 11.69 | 0.34 | 9.38 |
Fig. 10 presents digital photos for the residues for the pure EP and its composites after the cone calorimetry tests. Apparently, pure EP left a small amount of residue (Fig. 10a). For EP/Ni@SiO2-PA1.0, a small amount of flame retardant was insufficient to protect the matrix and did not increase the yield of the residue (Fig. 10b). With the added amount of Ni@SiO2-PA increasing, the flame-retardant EP nanocomposites produced additional char residue. For EP/Ni@SiO2-PA3.0 (Fig. 10c), an intumescence of the char layers could be observed. However, agglomerated char layers existed in the middle of the sample, which could be attributed to a relatively poor dispersion. For EP/Ni@SiO2-PA5.0 (Fig. 10d), a compact structure could be observed after the composites were burned. Then, the SEM technique coupled with EDX was used to investigate the structure and morphology of the residual char, as shown in Fig. 11. After combustion, a broken and fragile char residue was obtained for pure EP, and huge holes existed in the residual char, thereby suggesting poor thermal stability, according to its SEM image (Fig. 11a). When 1 wt% Ni@SiO2-PA was incorporated into the EP matrix, the cracks and holes significantly decreased, but they were still insufficient to protect the polymer layer inside the composites. The structural defect acted as channels for the exchange of heat, O, and gas products (Fig. 11b). For the composites incorporated with 3 wt% Ni@SiO2-PA (Fig. 11c), a bubble-like structure was observed on the resulting char. Meanwhile, the crack on the matrix disappeared. Intumescent-like bubbles can slow down the heat and mass transfer between gas and condensed phases.43 However, the presence of several ruptured bubbles revealed that the char's strength was not strong enough to withstand the expansion of the gas product.33 When the Ni@SiO2-PA content was increased to 5 wt%, a compact and rigid char residue was formed, and no evident cracks and holes were observed in its SEM image (Fig. 11d). The SiO2 nanoparticles were uniformly embedded in the matrix, as seen from high magnification. Thus, the char can act as an efficient shield for the underlying matrix. At the surface of the its residual char, N, Si, P, and Ni were detected and homogeneously distributed (Fig. 11e and f). The P-rich char residues were involved in char layer formation, which hindered the transfer of heat flow and provided good flame retardancy. Ni was deposited, thereby indicating that it participated in the charring process. Ni catalyzed the carbonization of the burning polymers and led to the formation of several types of C materials. Generally, Ni catalyzes the dehydrogenation and aromatization of the intermediate compounds, thereby forming additional protective char layers.34,36
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Fig. 10 Digital photographs of char residues after the cone calorimetry tests: (a) pure EP, (b) EP/Ni@SiO2-PA1.0, (c) EP/Ni@SiO2-PA3.0, and (d) EP/Ni@SiO2-PA5.0 composites. |
Raman spectroscopy was carried out to determine the graphitization degree of the char residues obtained from the cone calorimetry tests, as shown in Fig. 12. Raman spectra analysis revealed the presence of the D (1590 cm−1) and G bands (1361 cm−1), which were attributed to the A1g breathing vibration of the sp3-hybridized C and E2g in-plane stretching of the six-ring sp2 C, respectively.44 Here, the lower the ID/IG ratio is, the higher the graphitization degree of the residual char will be. The ID/IG ratio of pure EP was 1.47 (Fig. 12a). Nevertheless, the EP/Ni@SiO2-PA5.0 composites had a low ID/IG ratio of 1.35 (Fig. 12b), thereby indicating the formation of a char with a high degree of graphitization and a thermally stable char structure. The XPS survey scans of the residual char of pure EP and EP/Ni@SiO2-PA5.0 are shown in Fig. 12c. Si, N, P, and Ni were detected in the residual char. The intensities of the peaks responding to C 1s and O 1s in the residual char of EP/Ni@SiO2-PA5.0 were also significantly higher than those of pure EP. Fig. 13 shows the FTIR spectra of the residual char, which provide additional information on the chemical structure of the residual char. For pure EP, the peak at 3450 cm−1 was attributed to water, and the peak at approximately 1600–1500 cm−1 revealed the multiaromatic structure of the residual char. EP/Ni@SiO2-PA5.0 showed a similar char structure caused by the similar spectrum of the pure EP. However, the peaks at approximately 1102 and 1030 cm−1 were attributed to Si–O–C and Si–O–Si bonds, which were obtained from the silica crosslinked network, respectively.45 The intense peaks at approximately 1139 and 1160 cm−1 were assigned to the PO and P–O–C vibrations PA groups, respectively.2,38 The phenomenon above can verify the formation of the Si crosslink network and the participation in the charing process of the composites. PA could also catalyze the generation of char and acted as a catalyst for the dehydration process, which promoted the oxidative dehydrogenation crosslinking–charring process and increased the char yield, thereby improving the flame retardancy of the EP composites.24 Meanwhile, several kinds of C materials were formed on the Ni catalyst surface. Thus, the char layer provided an effective protection, and reduced the heat and mass transfers to protect the EP matrix, which is in accordance with the TGA results.
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Fig. 12 Raman spectra of residual chars of (a) pure EP and (b) EP/Ni@SiO2-PA5.0 composites; (c) XPS spectra of EP and EP/Ni@SiO2-PA5.0 composite residues. |
PA molecule also decomposed into substantial amounts of P-free radicals, such as and PO˙, which react with flammable radicals (H˙ and OH˙), reduce the opportunity of combustion, and produce a quenching effect.46 Second, in the condensed phase, the crosslinking structure of silica to form a polymer network prevented further matrix decomposition. Meanwhile, the decomposition products of the supermolecular-produced phosphorous oxide, which generated P-rich intumescent char, formed stable residual chars, and restricted the formation of holes and cracks. The Ni compounds also catalyzed the carbonization of the burning polymers top and formed a compact and continuous bubble-like char layer. Thus, the physical barrier formed by Ni@SiO2-PA could protect the matrix by isolating O- and heat-transfer efficiently.
In investigating Ni@SiO2-PA dispersion in the EP matrix, the SEM images of the fracture section after tensile testing for pure EP and its composites are shown in Fig. 16. As displayed in Fig. 16a, a smooth structure and a river-like pattern with consistent directions was observed, which revealed the poor mechanical properties of the pure EP. A relatively rough fracture surface could be observed with the addition of 1 wt% SiO2 particles (Fig. 16b). However, the high-magnification SEM images (Fig. 16c) showed several holes and considerable aggregation in the EP matrix, thereby suggesting that the SiO2 nanoparticles had poor interfacial interaction. When 1 wt% Ni@SiO2-PA and 5 wt% Ni@SiO2-PA were incorporated (Fig. 16d–f), a rough and uneven fracture surface was observed due to the high compatibility between the supermolecular shell and EP matrix. These results indicated that introducing Ni@SiO2-PA could have further strong interfacial interactions with the polymer matrix.
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Fig. 16 SEM images of (a) pure EP, (b and c) EP/SiO21.0, (d) EP/Ni@SiO2-PA1.0, and (e and f) EP/Ni@SiO2-PA5.0 composites. |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra00072h |
This journal is © The Royal Society of Chemistry 2020 |