Transparent CelluloseâSilica Composite Aerogels with Excellent

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Transparent Cellulose-silica composite aerogels with excellent flame retardancy via in situ sol-gel process Bin Yuan, JinMing Zhang, Qinyong Mi, Jian Yu, Rui Song, and Jun Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03211 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Sustainable Chemistry & Engineering

Transparent Cellulose-silica composite aerogels with excellent flame retardancy via in situ sol-gel process Bin Yuanabc, Jinming Zhanga, Qinyong Mia, Jian Yua*, Rui Songb*, Jun Zhanga,b* a

Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of

Sciences, Zhongguancun North First Street, Beijing, 100190, China b

College of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Yuquan Road, Beijing, 100049, China c

Jiangxi Guangyuan Chemical Co. Ltd., Guanyuan Road, Yongfeng, Ji'an, 331500,

Jiangxi, China

* Corresponding authors: [email protected] (Jian Yu); [email protected] (Rui Song); [email protected] (Jun Zhang). KEYWORDS: Aerogel, regenerated cellulose, silica, ionic liquid, supercritical CO2

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ABSTRACT: Cellulose-silica composite aerogels were prepared by in situ formation of silica nanoparticles via a two-step sol-gel process in cellulose gel, which was prepared by dissolving cotton pulp in 1-allyl-methylimidazole chloride ionic liquid (AmimCl) and then regenerating from AmimCl/water bath. Related properties were investigated with ultraviolet-visible spectrometer, scanning electron microscopy, compression tests, thermogravimetric analysis, ignition tests as well as micro-scale combustion calorimeter. The results showed that the composite aerogels displayed increasing transparency, compressive properties, and thermal and thermal-oxidative stability with increasing of silica content. The incorporation of silica nanoparticles also improved the mesoporous characteristics of aerogels including specific surface area and mesopore volume, and significantly delayed the decomposition of cellulose, suppressed the heat release during combustion. The composite aerogels with high silica content (33.6 % or more) exhibited good transparency with light transmittance as high as 78.4 % at 800 nm, even higher than the neat cellulose aerogels, and presented excellent flame retardant performance, achieving self-extinguishment after ignition. The transparent cellulose-silica composite aerogels with enhanced mechanical performance and improved flame retardancy might show great potential in a wide variety of applications.

Introduction


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Since the first report in 2001,1 cellulose aerogels have attracted extensive interest as novel kind of green biodegradable nanoporous materials. Cellulose aerogels combine unique features of native cellulose, e.g. environmental friendliness, biocompatibility, and biodegradability,2 with excellent properties of traditional aerogels, such as extraordinarily low density, high porosity, and high specific surface area.3,4 Hence, they have great potential in applications varying from cell culture and drug delivery,5-7 to thermal insulation,8-12 selective filtration or absorption,13-15 catalyst supporting,16-19 and template synthesis.20-22

Recently, concerns regarding the intrinsic inflammability of cellulose have motivated the researchers to fabricate flame retardant cellulose aerogels, which are promising for applications including lightweight building materials and domestic devices with reducing fire hazards.9,23-26 In all these works, cellulose composite aerogels were fabricated by addition or in situ formation of flame retardant agents. The addition of graphene oxides and sepiolite nanorods endowed nanocellulose composite foams with high limiting oxygen index up to 34, and good flame propagation resistance in vertical burning tests.9 Boric acid and borate were also used as effective flame retardant agents.23 Montmorillonite displayed a synergistic effect with ammonium polyphosphate, a intumescent flame retardant agent, to significantly reduce the flammability of cellulose-based aerogels in terms of heat release rate and fire growth rate.25 On the other hand, in situ synthesis of inorganic nanoparticles such as magnesium hydroxide nanoparticles in cellulose gels provided an alternative to improve flame retardancy of cellulose aerogels.24 Despite exhibiting excellent flame 3 ACS Paragon Plus Environment


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retardancy, these cellulose composite aerogels lack transparency, which limits their potential in practical applications where high light transmission is needed. In our previous work, based on transparent regenerated cellulose gels prepared by a facile method,27 aluminum hydroxide nanoparticles were in situ formed to obtain flame retardant cellulose composite aerogels with high optical transparency.26

As one of the most widely studied cellulose-based composite aerogels, cellulose-silica composite aerogels28 have been very attractive for developing porous materials with different functionalities, such as enhanced mechanical properties and thermal stability,10,11,29-39 excellent thermal insulation performance,10,11,29,31,33,35-39 increased hydrophobicity,10,31-33,38-40 and acceptable visible light transmittance.29,33,41 However, no report is available until now about the flame retardant performance of cellulose-based aerogels after incorporation of non-flammable silica.

In this study, silica was in situ prepared via a two-step sol-gel process by using tetraethoxysilane (TEOS) as precursor in a matrix of transparent cellulose gel, which was fabricated from cellulose solution in 1-allyl-methylimidazole chloride (AmimCl) by using a high concentration aqueous solution of AmimCl as the regeneration bath.26,27 The cellulose-silica composite aerogels (CSAs) obtained were characterized in terms of micro-structure, mechanical and thermal stability, and combustion behaviors. The flame retardant mechanism was suggested and compared with that of cellulose-aluminum hydroxide composite aerogels reported in our previous work.26

Experimental section 4 ACS Paragon Plus Environment

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Materials. Cotton linter pulp (DP = 650) and AmimCl (purity > 99 %) were kindly supplied by Henglian New Materials Co., Ltd. (Shandong, China). TEOS were purchased from Beijing Beihua Fine Chemical Co., Ltd. (Beijing, China). Carbon dioxide (purity 99.95 %) was provided by Beijing AP BAIF Gas Factory (Beijing, China). Cotton pulp was pre-dried in vacuum oven at 60 oC for 24 h before using. Deionized water was used for the preparation of solutions. All other chemicals were analytical grade reagents and used as received.

Preparation of cellulose gels and cellulose aerogels. The transparent cellulose gels were obtained according to our previous work.27 Briefly, 2 wt % cotton pulp was dissolved in AmimCl solvent at 80 oC for 2 h with stirring to obtain a homogeneous clear solution. After air bubbles were completely removed in a vacuum oven, the obtained solution was poured into a 70 mm × 40 mm mold, and then immersed in a regeneration bath of 60 wt% aqueous AmimCl solution to form transparent cellulose gel. It was washed with absolute ethanol until no chloride ions were detected using AgNO3 to obtain cellulose alcogel, which was further dried by supercritical CO2 at 40 o

C and 12 MPa to fabricate neat cellulose aerogel (NCA). As a typical ionic liquid,

AmimCl possesses the characteristics of excellent thermal and chemical stability and non-detectable vapor pressure, and can be recovered and reused after a simple vacuum distillation to separate water and organic solvents.

Preparation of cellulose-silica composite aerogels. TEOS was mixed with H2O and absolute ethanol under constant stirring for 10 min. The molar ratios of TEOS, 5 ACS Paragon Plus Environment


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H2O, and ethanol in the solutions were 1: 5: x with various x values of 10, 20, 40 and 80, corresponding to 27 wt%, 17 wt%, 10 wt%, and 5 wt% of TEOS, respectively. The cellulose alcogels were immersed in fresh TEOS solutions, followed by dropwise adding hydrochloric acid (0.01 M) to adjust the pH of the solution to 2~3 and stirring for 30 min. Subsequently, base catalyst (ammonia solution, 0.1 M) was added into the pre-hydrolysis TEOS solution until the pH > 7 and stirring for 15 min. Afterwards, the wet gels were then immersed in ethanol, and placed at 50 oC for 12 h for further gelation and aging. The wet gels were solvent exchanged with ethanol and then dried by supercritical CO2 to obtain cellulose-silica composite aerogels. The composite aerogels were labeled as CSA27, CSA17, CSA10, and CSA5 according to the concentration of TEOS solution. All the obtained aerogels were about 3 mm in thickness. Characterization. The density of the aerogel (ρ) was calculated from the sample weight divided by its volume. The porosity was calculated according to Equation (1),29,31

 ρ  Porosity (%) = 1 − *  × 100 (1)  ρ 

ρ * = 1.6 × (1 − ω ) + 2.1× ω (2) where ρ* is the skeleton density of aerogels, and ω is the weight percent of silica in the aerogels. Here, the densities of cellulose and silica used in the calculation were 1.6 and 2.1 g⋅cm-3, respectively. 6 ACS Paragon Plus Environment

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Light transmittance of aerogels was measured on an UV-Vis spectrometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd, China) in the visible light range from 400 to 800 nm. Nitrogen adsorption/desorption isotherms were measured on a surface area and pore-size analyzer (NOVA 2000e, Quantachrome, USA) at 77 K. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method. Pore size distributions were derived from desorption branch of the isotherms by the Barrett–Joyner–Halenda (BJH) method. The cross-section morphology of aerogels was investigated by a scanning electron microscope (SEM, S-4800, Hitachi, Japan) at 5 kV acceleration voltages, and the elemental analysis of composite aerogel was conducted using energy dispersive X-ray spectrometer (EDS) attached to the SEM. The samples for SEM observation were cryo-fractured in liquid nitrogen and sputter-coated with gold. Fourier transform infrared (FTIR) spectra of aerogel samples were recorded on a FTIR 6700 spectrometer (Nicolet, USA) in the range between 4000 and 400 cm-1. Compression tests were conducted on a universal testing machine (model 3365, Instron, USA). The samples were compressed with a crosshead speed of 1 mm⋅min-1 until a maximum load of 4 kN was reached. Thermogravimetric analysis (TGA) was performed by a TGA-7 (Perkin-Elemer, USA) in air or N2 atmosphere from 50 to 750 oC at a heating rate of 20 oC⋅min-1. The flame retardancy of aerogels was evaluated by ignition test with an alcohol burner. A micro-scale combustion calorimeter (MCC) was used for the quantitative analysis according to ASTM D7309-2007. Approximately 5 mg samples were heated in MCC-2 (Govmark, USA) from 90 oC to 750 oC at a heating rate of 1 oC⋅s-1 in a stream of N2 flowing at 80 7 ACS Paragon Plus Environment


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mL⋅min-1. Then the volatiles were mixed with a stream of O2 (20 mL⋅min-1), followed by entering a combustion furnace at 900 oC. The samples were dried at 50 oC for 12 h before analysis.

Results and Discussion

Chemical structure and morphology. In this study, cellulose hydrogels were dipped into TEOS solution to fabricate CSAs via a two-step sol-gel process followed by supercritical drying. FTIR spectra were recorded to investigate the structure information of aerogel samples, as shown in Figure 1. The spectrum of NCA is clearly different from those of CSAs. The former exhibits a broad peak centered at 3410 cm-1, assigned to O-H stretching vibration of hydroxyl groups in cellulose, and a band at 1650 cm-1, suggested the presence of absorbed water. Both bands become less pronounced in the spectra of CSAs. It may be because partial surface hydroxyl groups of cellulose and most silanols formed via hydrolysis of TEOS were consumed during sol-gel process, leading to lower hydrophilicity for CSAs. Some characteristic peaks of cellulose at 2896, 1350, 1153, and 897 cm-1 are also presented in NCA sample, which are attributed to C-H stretching vibration, deformation of C-H, C-O-C asymmetric stretching, and β-glucosidic linkage between the glucose units, respectively.42 Compared to NCA, CSAs display three new characteristic peaks in the low wavenumber range. These bands at 966, 799, and 444 cm-1 are assigned to the Si-OH stretching vibration, and Si-O-Si bending and symmetric stretching vibrations, respectively.42,43 Furthermore, the intensities of these characteristic peaks of silica 8 ACS Paragon Plus Environment

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increase with the increasing of TEOS concentration in the precursor solutions, indicating the increase of silica content in CSAs. Actually, the silica content is 15.0 wt% for CSA5, 26.9 wt% for CSA10, 33.6 wt% for CSA17, and 59.1 wt% for CSA27, respectively, calculated from the residual weight of TGA in air. Therefore, the presence of silica in the composite aerogels has been confirmed by FTIR spectra.

Figure 1. FTIR spectra of neat cellulose and cellulose-silica composite aerogels.

The cross-section morphology of NCA and CSA samples was investigated by SEM, and the representative images are shown in Figure 2. Under a low magnification (× 2,000), it is observed that all samples show a dense and smooth morphology, indicating that both porous microstructure of cellulose and solid silica particles are homogeneously distributed in the matrix with small size. It is more clearly under a high magnification (× 50,000) that the porous 3D networks with pore sizes ranging from 10 to 100 nm are formed from interconnected cellulose nanofibers in NCA sample. Although the composite aerogels retain porous microstructure, their morphology is denser than that of NCA. The SEM images of CSAs indicate that silica 9 ACS Paragon Plus Environment


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interacts with the cellulose and deposit tightly on the network to form uniformly distributed spherical silica nanoparticles. The number of nanoparticles increases with increasing silica content, while the size of nanoparticles does not change obviously. As shown in Table 1, NCA sample has a low density of 0.067 g⋅cm-3 and exhibits a high porosity of 95.8 %. With the increase in silica content, the density increases from 0.089 g⋅cm-3 of CSA5 to 0.122 g⋅cm-3 of CSA27, while the porosity decreases slightly from 94.7% to 93.6%. It is clear that the silica nanoparticles were formed in situ after the effective penetration of silica sol into the cellulose gel. They filled the voids in the interconnected network of cellulose homogeneously without aggregation, which is beneficial to promote the physico-mechanical performances of CSAs.


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Figure 2. SEM images of neat cellulose and cellulose-silica composite aerogels.

Table 1. Mesoporous characteristics of neat cellulose and cellulose-silica composite aerogels. Sample

Density

Porosity

SBET

Pore volume

Pore size

(g⋅cm-3)

(%)

(m2⋅g-1)

(cm3⋅g-1)

(nm)

NCA

0.067

95.8

206

0.59

11.4

CSA5

0.089

94.7

223

0.74

13.2

CSA10

0.095

94.5

324

1.01

12.4

CSA17

0.102

94.2

350

1.03

11.7

CSA27

0.122

93.6

438

1.31

12.0

EDS analysis was performed to further confirm the distribution of silica inside the network of CSAs. The spectrum was collected from the central cross-section of sample on an area of about 1 mm2 (Figure S1). In comparison with NCA (Inset of Figure S1a), CSA17 as representative of the composite aerogels displays a new signal at 1.74 keV assigned to silicon element besides the signals of carbon and oxygen elements presented in both samples. The visual mapping of carbon, oxygen, and 11 ACS Paragon Plus Environment


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silicon elements (see Figures S1b-d) suggest that the silica is uniformly distributed in the matrix of CSAs. Despite the fact that the accurate content can not be quantitatively calculated from EDS spectrum, it gives valuable information that high content of silicon exists inside CSAs. It is mostly due to the fact that the cellulose nanofibers are well covered by silica, as shown by SEM results.

Transparency. The digital photo of CSA17 is showed in Figure 3a as an example of the composite aerogels along with NCA as reference. By visual inspection, both samples with a thickness of 3 mm have smooth appearance and are transparent. The transparency was further measured by a UV-Vis spectrometer in the visible wavelength range of 400~800 nm. As shown in Figure 3b, all aerogels exhibit increasing transmittance with the wavelength. NCA has a light transmittance of 73.4% at 800 nm. Compared to other cellulose-based aerogels, it has relatively high transparency in consideration of sample thickness.8,26,27,29,49 With the introduction of silica, the transmittance at 800 nm decreases obviously to 66.2% for CSA5. A further increase in the silica content leads to an increase in the transmittance, i.e. from 70.0% for CSA10 at 800 nm to 75.0% for CSA17 and 78.4% for CSA27, respectively. It is worth to note that CSA17 and CSA27, which contain high silica content of 33.6 wt% and 59.1 wt%, respectively, are even more optically transparent than NCA. Cai et al.29 observed a similar result in their cellulose-silica composite aerogel with 39% silica. Therefore, CSA samples obtained in this study have pretty high transparency.26,29


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Figure 3. Digital photos of CSA17 (left) and NCA (right) (a), and transmittance curves of visible light for various aerogel samples (b) with a thickness of 3 mm.

The SEM images indicate that the homogeneous porous microstructure formed in cellulose gels was retained during in situ formation of silica and supercritical CO2 drying. Almost all pores having uniform pore size well below the wavelength of visible light account for the high transparency of aerogels. More importantly, the decreasing nanoscale inhomogeneity with increasing silica content further reduces the light scattering,3,27,29 resulting in high transmittance of visible light for CSA17 and CSA27.

Mechanical performance. The mechanical properties of aerogel samples were investigated by compressive tests, and their stress-strain curves are shown in Figure S2. All the aerogel samples display the classical deformation behavior of porous materials. At low strain, the aerogels exhibit a linear elastic regime followed by a plastic deformation plateau at intermediate strain due to the irreversible collapse of porous structure. At strains of more than approximately 40%, a sharp increase in stress indicates the presence of a densification regime after complete collapse of the porous structure. The incorporation of silica significantly improves the compressive 13 ACS Paragon Plus Environment


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strength. Meanwhile, the composite aerogels containing high content of silica exhibit excellent toughness because they can be compressed without disintegration till a strain higher than 50%.

Nitrogen adsorption-desorption isotherms. Nitrogen adsorption and desorption isotherms for various aerogel samples are presented in Figure S3. These isotherms are categorized to type IV according to the IUPAC classification with a small hysteresis loop, suggesting the presence of mesoporous structures. As shown in Table 1, the obtained BET surface area (SBET) and pore volume increase gradually from 206 m2⋅g-1 and 0.59 cm3⋅g-1 of NCA to 438 m2⋅g-1 and 1.31 cm3⋅g-1 of CSA27, respectively, with the increase of silica content. Similar results have been reported that the silica29,35 and other inorganic nanoparticles44,45 improved the mesoporous characteristics of cellulose-based composite aerogels. These results are probably attributed to the presence of high amount of silica nanoparticles filling the voids of cellulose aerogels. Therefore, more mesopores are formed in the porous network at the expense of the macropores. To the SBET, the contribution of meospores is much larger than that of macropores. Another possible reason is the reduction of the collapse and shrinkage of porous structure during supercritical drying because of enhanced mechanical strength of solid skeleton in CSAs. On the other hand, the BJH analysis displays that NCA and CSAs have similar pore size of approximately 12 nm and pore size distribution ranged from 2 to about 100 nm, as shown in Figure S3b. Therefore, despite the incorporation of silica obviously decreases the total pore volume, it improves the mesoporous characteristics in the cellulose-silica composite aerogels. 14 ACS Paragon Plus Environment

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Thermo-oxidative and thermal stability. TGA in air and N2 was applied to investigate the influence of silica nanoparticles on the thermo-oxidative and thermal stability of CSAs, respectively. Figure S4 shows the primary and derivative thermograms (DTG) of different samples, and the detailed data are summarized in Table 2.

Table 2. TGA data of neat cellulose and cellulose-silica composite aerogels in air and N2 atmosphere.

Air

N2

Air

N2

Air

N2

Char yield (wt%)

NCA

334

334

353/554

377

0.3

13.3

13.0

CSA5

344

356

357

379

15.0

28.5

15.9

CSA10

348

358

364

393

26.9

37.3

14.2

CSA17

347

357

360

383

33.6

46.9

20.0

CSA27

347

354

374

403

59.1

65.4

15.4

Sample

Tonset (ᵒC)

Tmax (ᵒC)

Residue (wt%)

NCA exhibits a onset degradation temperature (Tonset) of about 334 oC in both air and N2 atmosphere, and a sharp step of thermal degradation in the range between 300 and 400 oC with 70~80% weight loss. Comparison between results shows that the presence of oxygen accelerates the decomposition of cellulose, resulting in lower peak temperature corresponding to DTG (Tmax) in air (Table 2). Moreover, NCA has an obvious second degradation step between 500 and 600 oC in air to reach complete degradation (Figure S4a). On the other hand, a residual amount of 13.3 wt% is present at 750 oC in N2 because there are some carbonaceous residues transformed from



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cellulose in an inert atmosphere (Figure S4c). The degradation behavior of NCA is in good agreement with those of cellulose46-48 and cellulose aerogels26,29,49 reported elsewhere.

By comparison, CSAs have higher thermal and thermal-oxidative stability, on account of higher Tonset, Tmax, and residue weight irrespective of TGA atmosphere. In air, the Tonset and Tmax shifts to temperatures 10~14 oC and 13~27 oC above those of NCA, respectively. In N2, the Tonset and Tmax improve more significantly, which are higher than those of NCA by 20~24 oC and 35~56 oC, respectively. It is indicated that the deposition of silica nanoparticles on the porous network forms a protective layer on the cellulose network and stabilizes the cellulose against decomposition. On the other hand, the white residue of CSAs in air is considered to be pure silica, whose content increases with the TEOS concentration as expected. In N2, CSAs produced more grey solid residues. From the difference between these two residue weights, char yielded from cellulose in N2 can be calculated, as listed in Table 2. About 14~20 wt% char is yielded in CSAs, while 13 wt% char remains in NCA. These results suggest that the silica nanoparticles serve as inert barriers to suppress the thermal decomposition of cellulose and increase the char yield in CSAs as compared to NCA, which would contribute to improve the flame retardancy of cellulose-based aerogels.

Flame retardancy

Ignition test. Ignition and combustion tests were conducted to elucidate the flame retardant performance of different samples. Figure 4 shows the images of NCA and 16 ACS Paragon Plus Environment

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CSA17 samples ignited by an alcohol burner, which was captured from a recorded video (Video S1). NCA was easy to ignite and continued to burn intensely after removal of the fire due to the high flammability of cellulose. In contrast, the ignited CSA5 and CSA10 had obviously small flame and burned slowly. When the silica content reached 33.6 wt%, the ignited CSA17 self-extinguished within about 10 s. For CSA27 with 59.1 wt% silica content, the flame extinguished quickly once the fire source was removed. Consequently, the presence of silica nanoparticles significantly improves the flame retardancy of cellulose-based aerogels.

Figure 4. Images of CSA17 (a) and NCA (b) samples after ignition by an alcohol burner.

Figure S5 shows the morphology of residual CSA17 after combustion and its corresponding EDS results. The porous microstructure is maintained in the residue, which consists of dense aggregates of nanoparticles typical for the silica aerogels obtained by regular sol-gel process.3,29,35 The EDS results indicate that most of cellulose was burned up due to obvious reduction of the signal assigned to carbon element in comparison with the sample before combustion. Meanwhile, the presence



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of silica nanoparticles protects the cellulose char from complete combustion, resulting in obviously retained carbon element in the residue.

MCC analysis. Moreover, a micro-scale combustion calorimeter was used to measure the heat release rate of NCA and CSA samples against temperature, as shown in Figure 5. From the curves, quantitative information related to the combustion properties of materials can be obtained including peak heat release rate (PHRR) and its corresponding temperature (TPHRR), total heat release (THR), heat release capacity (HRC), initial decomposition temperature (Tinitial), and residual content, as listed in Table 3.

Figure 5. HRR curves of neat cellulose and cellulose-silica composite aerogels obtained from MCC.

Table 3. Results of MCC tests for neat cellulose and cellulose-silica composite aerogels. Sample

PHRR TPHRR THR HRC

Tinitial

Residue

**

THR

**

HRC


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(W/g)

(oC)

(kJ/g) (J/g K) (oC)

(wt%)

(kJ/g)

(J/g K)

NCA

361

356

11.2

327

270

10.8

11.2

327

CSA5

323

360

8.3

246

304

26.1

11.1

289

CSA10

256

375

8.1

199

305

29.8

10.9

272

CSA17

154

376

6.5

118

307

42.6

9.8

177

CSA27

56

386

3.4

43

326

57.5

8.3

105

Note: * The final HRC and THR values were normalized by the original weight of cellulose. It is shown that NCA exhibits a sharp HRR peak around 356 oC with PHRR value as high as 361 W⋅g-1, indicating that the combustion of NCA releases large amount of heat in a short time. With the incorporation of silica, the HRR peak shifts to a temperature of about 4~30 oC higher than that of NCA. Meanwhile, the PHRR decreases remarkably to a lower value from 323 W⋅g-1 of CSA5 to only 56 W⋅g-1 of CSA27. The THR and HRC gradually decreases as the silica content increases, from 11.2 kJ⋅g-1 and 246 J⋅g-1 K of NCA to 3.4 kJ⋅g-1 and 43 J⋅g-1 K of CSA27, respectively. Considering the cellulose proportion in the composite aerogels, the normalized THR and HRC is also obviously decreased with the increase of silica content. Therefore, the presence of silica nanoparticles not only slows the combustion of cellulose, but also suppresses the heat release during combustion due to the barrier effect of silica. CSA17 and CSA27 with high loading of silica have PHRR/THR just about 43%/58% and 16%/30% of NCA, respectively. Because the heat release does not sufficiently support the flame propagation, CSA17 and CSA27 display 19 ACS Paragon Plus Environment


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self-extinguishing behavior in the ignition test. These results further confirm the excellent flame retardancy of CSAs from the viewpoint of heat release behavior. The mild release of heat during combustion would endow the composite aerogels with less fire hazards.

The proposed mechanism of flame retardancy. As an inert filler, silica nanoparticles neither catalyze the thermal decomposition of cellulose or residual char like some metal and metal oxide nanoparticles, e.g. Au and Pt,49 TiO2,44 and γ-Fe2O3,45 nor promote significantly the charring of cellulose. From the above-mentioned results, it is speculated that the silica nanoparticles formed in situ cover the surface of cellulose network in aerogels as a solid barrier to protect the cellulose against decomposition. Meanwhile, the presence of high loading of silica reduces the proportion of flammable component, hence diminishing total heat release during combustion. By contrast, cellulose-aluminum hydroxide (AH) composite aerogels reported previously have a different approach for flame retardancy.26 The decomposition of AH produced a large amount of steam, which would dilute the flammable species in gas phase and reduce the temperature therein. Therefore, the TPHRR of cellulose-AH composite aerogels shifted to lower temperature with a much lower THR. The composite aerogels with an AH content of 55.8 wt% had a THR about 18 % of that of neat cellulose aerogel, which was far below that of CSA27 with a silica content of 59.1 wt% in this study. It indicates that the combination of effects from solid phase and gas phase can reduce the heat release of composite aerogels more effectively. However, the cellulose-AH composite aerogels possess the 20 ACS Paragon Plus Environment

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shortcoming of lower thermal stability at temperatures lower than 300 oC due to the decomposition of AH at relative low temperatures.

Conclusions

Cellulose-silica composite aerogels were fabricated by in situ synthesis of silica nanoparticles in the cellulose gels via a two-step sol-gel process and subsequent supercritical CO2 drying. The silica nanoparticles up to 59 wt% were incorporated and filled the voids in the nanoporous network structure of cellulose aerogels homogeneously without aggregation. Consequently, the composite aerogels displayed increasing transparency and compressive properties with the increase of silica content. On the other hand, the incorporation of silica nanoparticles improved the mesoporous characteristics of aerogels including specific surface area and mesopore volume. Moreover, the solid barrier formed by the silica nanoparticles significantly delayed the decomposition of cellulose, and suppressed the heat release during combustion, endowing the composite aerogels with excellent thermo-oxidative and thermal stability, flame retardancy, and even self-extinguishing behavior. Therefore, the cellulose-silica composite aerogels, which integrate many excellent physical and mechanical performance, show great potential in wide range of applications from green package to building insulation.

ASSOCIATED CONTENT



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Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: The results of EDS, Compression, Nitrogen adsorption-desorption isotherms, TGA, and SEM image after ignition (pdf) Video of the ignition test for NCA and CSA17 samples (avi)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.Y.).

*E-mail: [email protected] (R.S.).

*E-mail: [email protected] (J.Z.).

ORCID Jinming Zhang: 0000-0003-3404-4506

Jian Yu: 0000-0003-0591-0524

Jun Zhang: 0000-0003-4824-092X

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51273206, 51425307). 22 ACS Paragon Plus Environment

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High transparent and mechanical robust cellulose-silica composite aerogels are obtained and show high flame retardancy, even self-extinguishment after ignition.


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