Fabrication of Fully Bio-Based Aerogels via Microcrystalline Cellulose

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Fabrication of Fully Bio-based Aerogels via Microcrystalline Cellulose and Hydroxyapatite Nanorods with Highly Effective Flame Retardant Properties Wei Yang, Peng Ping, Li-Li Wang, Timothy Bo-Yuan Chen, Anthony ChunYin Yuen, San-E Zhu, Ning-Ning Wang, Ye-Lian Hu, Pan-Pan Yang, Chen Sun, Cheng-Yang Zhang, Hong-Dian Lu, Qing Nian Chan, and Guan-Heng Yeoh ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00312 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Nano Materials

Fabrication of Fully Bio-based Aerogels via Microcrystalline Cellulose and Hydroxyapatite Nanorods with Highly Effective Flame Retardant Properties

Wei Yang a, b, 1, Peng Ping a, 1, Li-Li Wang a, 1, Timothy Bo-Yuan Chen b, Anthony Chun-Yin Yuen b, San-E Zhu a, Ning-Ning Wang a, Ye-Lian Hu a, Pan-Pan Yang a, Chen Sun a, Cheng-Yang Zhang a, Hong-Dian Lu a, *, Qing Nian Chan b, Guan-Heng Yeoh b

a

Department of Chemical and Materials Engineering, Hefei University, Hefei, Anhui

230601, People’s Republic of China b

School of Mechanical and Manufacturing Engineering, University of New South

Wales, Sydney, NSW 2052, Australia

*

Correspondence

to:

Hongdian

Lu

(E-mail:

[email protected],

86-551-62158393) 1

These authors contributed equally to this work (co-first author).

1

ACS Paragon Plus Environment

Tel:

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: In this study, hydroxyapatite (HAP) nanorods were synthesized via a facile hydrothermal method, which were used as nano-additives to prepare the flame retardant microcrystalline cellulose (MCC) composite aerogels. Flame retardant and thermal properties of MCC/HAP composite aerogels were evaluated. When tests were performed at room temperature, the composite aerogels exhibited enhanced thermal stability and low thermal conductivity but more rapid thermal dynamic transfer rate during heating and thermal dissipation rate during cooling, compared to pure MCC aerogel. The MCC aerogel containing 50 wt% HAP yielded a reduction of 93.7% in peak heat release rate (PHRR), and the smoldering occurred when exposed to a flame or the cone heater. The remarkable improvement in the flame retardant properties of MCC/HAP should be attributed to these possible mechanisms: (i) the increased thermal dynamic transfer performance during heating has an adverse effect on the increment in time to ignition and time to PHRR; (ii) the non-flammable HAP-backbone aerogel-like residual char with lower thermal conductivity coefficient, which is in-situ formed along the temperature gradient during the thermal degradation and combustion processes, exhibit a positive effect on slowing the diffusion of heat and mass as well as the adsorption of smoke. These mechanisms interact as well as compete with others during the thermal degradation or combustion processes. KEYWORDS: microcrystalline cellulose, hydroxyapatite nanorods, fire safety performance, flame retardant mechanism

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INTRODUCTION The development of bio-inspired materials being of light-weight and high strength as well as possessing strong flame retardancy is of great significance to the modern society, driven by the increased awareness for sustainable development and ecological, environmental protection. Highly porous ultra-light cellulose aerogels have attracted much interest of late. Regarded as being sustainable as well as biodegradable, they represent a new class of bio-inspired materials that can be applied in the building and construction landscape as well as in domestic appliances.1-9 Nevertheless, cellulose is highly combustible and large amounts of unwanted asphyxiated gases are usually emitted during the burning process. To circumvent the inherent flammability of cellulose, it is thus necessary that flame-retardant cellulose aerogels are fabricated with the aim of reducing the fire and toxicity hazards for general applications. Improving flame retardancy of cellulose aerogels can be realized by either adding flame retardant additives or in-situ formation of flame retardant agents.10-14 Incorporation

of

N-methylol

dimethylphosphonopropionamide

and

1,2,3,4-butanetetracarboxylic acid imparted into cellulose nanofibril aerogel has shown to have outstanding flame retardant and self-extinguishing characteristics.11 In-situ formation of inorganic nanoparticles such as silica (SiO2) and magnesium hydroxide [Mg(OH)2] nanoparticles in cellulose hydrogels or alcogels served as scaffold represent another effective approach to improve the flame retardancy of aerogels.12-14 Deposition of flame-retarded coating on the porous cellulose aerogels 3



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via a layer-by-layer technique has the propensity of tailoring the surface properties of aerogels to improve the flame-retardant performance.15 It has been reported that thin film consisting of cationic chitosan, anionic poly(vinylphosphonic acid) and clay built on cellulose nanofibril aerogels can self-extinguish and the coated aerogels have shown to be nonflammable under combustion tests. One-dimensional

(1D)

nanofillers

such

as

carbon

nanotubes,

and

two-dimensional (2D) layered compounds such as clay and graphene have exhibited superior flame retardant efficiency in polymeric aerogels.16-21 Consideration of nanocomposite aerogels such as poly (vinyl alcohol)/clay, cellulose/graphene oxide and cellulose/sepiolite has shown to yield good flame retardancy when evaluated using cone calorimetry, limiting oxygen index and vertical burning tests.22,23 It is noted that hydroxyapatite (HAP) nanoparticles are a major phosphate mineral with excellent bioactivity and biocompatibility which have drawn much interest especially

from

the

fabrication

of

artificial

bone-like

ceramic/polymer

composites.24-28 HAP also exhibits excellent flame retardant efficiency in flexible electrically conductive paper and poly (vinyl alcohol), and flame retardant synergy in intumescent flame retardant systems in poly (ethylene-co-vinyl acetate) composites.29-32 Although much of previous works have provided important insights into the improvement of flame retardancy of polymers with HAP, aspects on insulating capability, thermal transfer performance and thermal stability induced from HAP on the fire safety performance of light-weight cellulose aerogels have not been very 4


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well understood. This present study reports a strategy for manufacturing microcrystalline cellulose (MCC) aerogels with homogeneous HAP nanoparticles with the aim of improving the fire safety performance of the bio-inspired materials. HAP nanopowders were initially synthesized via a hydrothermal treatment, and MCC solution was prepared using a typical sodium hydroxide/urea method.33 The MCC/HAP composite aerogels were then fabricated by solution blending followed by freeze-drying. Thermal properties were measured and the flammability aspects of MCC aerogels were assessed to determine the extent of the introduction of HAP within the bio-inspired materials. Discussions on the flame retardant mechanism of HAP and the effect on MCC aerogels were subsequently provided.

EXPERIMENTAL SECTION Materials. Microcrystalline cellulose [MCC, (C6H10O5)n, molecular weight: ~35000], calcium nitrate tetrahydrate [Ca(NO3)2·4H2O], ammonium dihydrogen phosphate (NH4H2PO4), aqueous ammonia solution (NH4OH, 25 wt%), sodium hydroxide (NaOH), urea and epichlorohydrin (ECH) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used without further purification. Synthesis of HAP powders. HAP powders were synthesized by dissolving 0.025 mol Ca(NO3)2·4H2O and 0.015 mol NH4H2PO4 in 50 ml deionized water with a Ca/P molar ratio of 1.67:1 at 40 °C in a water bath. An aqueous ammonia solution was introduced whilst under stirring to adjust the pH value of 10. The mixture was then 5



hydrothermally treated at 180 oC for 16 h. The resultant precipitate was washed using deionized water, collected by centrifugation and dried at 70 oC for 24 h. The dried HAP was grounded to a fine powder. Preparation of MCC/HAP composite aerogels. To produce MCC/HAP aerogels, a HAP aqueous suspension (10 wt%) was ultrasonically stirred for 2 h. MCC solution was prepared via mixing 7 g NaOH, 12 g urea and 81 g deionized water together by stirring the mixture at -12 oC. 6 g MCC was gradually added by stirring for 30 min to attain a transparent solution. The solution was transferred to a low constant temperature water bath at 0 oC and the ECH of 4 ml was subsequently added by stirring for 1 h. The desired amounts of HAP suspension were then added into MCC dispersion by further stirring for 1 h. The resulting mixtures of 110 g and 25 g were poured into plastic plates of 105 mm × 105 mm × 20 mm and 30 mm× 30 mm× 30 mm, respectively, and then kept sealed at 50 oC for 12 h to achieve MCC/HAP hydrogels. After washing with water to attain pH of 7, the hydrogels were transferred into a lyophilizer and freeze-dried for 144 h. The route of the preparation for MCC/HAP composite aerogels is exhibited in Scheme 1. The formulations, apparent density (ρb) and porosity of these aerogels are listed in Table 1. Photographs of the selected MCC/HAP (1/1) composite aerogels are shown in Figure S1. It can be seen that there were no apparent aggregates of HAP found in the hydrogel and no apparent shrinkage existed when hydrogel transformed to aerogel. Characterizations. X-ray diffraction (XRD) data were obtained at room temperature using a D/max-TTR III X-ray diffractometer equipped with a Cu-Kα 6


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radiation (λ=1.5418 Å). Microscopic images were taken using a SU8010 field-emission scanning electron microscopy (FESEM, Japan). The aerogel samples were fractured in liquid nitrogen and the fracture surfaces were later coated with gold before SEM observations. The energy dispersive spectroscopy (EDS) elemental mapping (HORIBA, Ltd., Japan) was employed to determine the dispersion state of HAP nanorods in aerogel. Nitrogen adsorption-desorption measurements were performed at liquid nitrogen temperature (-196 °C) on an Autosorb IQ surface area and porosity analyzer (USA). Prior to measurements, the samples were degassed in a vacuum at 150 °C for 3 h. Thermogravimetric analysis (TGA) was conducted with a Netzsch TG209 F1 thermoanalyzer instrument (Germany). In each case, 4~10 mg specimens were heated from room temperature to 800 oC at a heating rate of 10 oC min-1 under nitrogen condition. Thermal conductivity measurements were performed using a C-Therm TCiTM thermal conductivity analyzer (Canada) on the discs at room temperature. To study the thermal dynamic transfer and thermal dissipation properties of aerogels, the variations of temperature with time for MCC and MCC/HAP aerogels were recorded by an infrared thermograph (Testo 865, Germany) and a FLIR thermaCAM (PM695, USA). Cone calorimeter experiments were performed using a FTT standard cone calorimeter (British) according to ISO 5660 standard procedures, on 100 mm × 100 mm × 10 mm aerogel, when exposed to an external heat flux of 30 kW/m2. Compression tests were carried out on a Universal Testing Machine (China) with a speed of 5 mm/min.

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RESULTS AND DISCUSSION Characterization of HAP powder. The structure of pristine HAP powder is characterized by XRD, SEM and N2 sorption analysis. The XRD patterns shown in Figure 1A have been found to be consistent with the characteristic peaks of HAP [Ca5(PO4)3(OH)] crystalline phase corresponding to JCPDS card No. 9-432. SEM image shown in Figure 1B demonstrates that HAP possesses a regular rod-like morphology with diameter of 20-60 nm and length of 80-200 nm. More importantly, HAP nanorods can be controlled to be homogeneously dispersed in deionized water at 10 wt% without precipitation after ultrasonic agitation treatment (see Figure S2 in Supporting Information). N2 adsorption-desorption isotherms of HAP powder is shown in Figure 1C. The corresponding Brunauer-Emmett-Teller (BET) data are listed in Table 1. The isotherms exhibit a typical IV behavior with H3-type hysteresis loops, according to the classification made by the International Union of Pure and Applied Chemistry (IUPAC), suggesting the formation of slit-like pore structure caused by the loose aggregates of HAP nanorods.34 The narrow peak at 1.7 nm appears in pore size distribution curve indicates the prevalence of microporous structure characteristic. The corresponding specific surface area (SBET) calculated using the Barrett-Joyner-Halenda (BJH) desorption method yields a value of 38 m2/g, and the porous volume is evaluated to be of 0.14 cm3/g. Structure and mechanical properties of MCC/HAP aerogels. From Table 1, it can be seen that the values of apparent density (ρb) increase gradually with the increasing composition of HAP. SBET and porous volume also exhibit the same 8


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tendency as ρb. The presence of HAP has little influence on the porosity of all composite aerogels but they are higher than that of pure MCC, which agrees with the observations in the SEM images. As shown in Figure 2, this demonstrates that both pure MCC and selected MCC/HAP (1/1) aerogels have a similar honeycomb-like structure constituted by curled MCC and MCC/HAP flakes, respectively. However, the cellular of MCC/HAP (1/1) are smaller resulting in a more dense structure compared to pure MCC. There are no HAP aggregations found in the MCC matrix even at higher magnifications. In order to further evaluate the dispersion of HAP nanorods in MCC/HAP (1/1) composite aerogel, the energy dispersive spectroscopy (EDS) elemental mapping was employed (Figure S3). As shown in Figure S3B and -C, the Ca and P elements in the region (Figure S3A) are homogeneously dispersed in the aerogel, indicating that the HAP nanorods are uniformly distributed in the aerogel. Pure MCC aerogel has a low thermal conductivity (κ) value of 0.038 W/m·K owing to its porous structure. For MCC/HAP, the composite aerogels still exhibit excellent thermal insulating properties with κ values as low as 0.037-0.036 W/m·K, indicating that the distribution of HAP in MCC matrix has minimal effect on the thermal conductivity properties of MCC aerogels when tested at room temperature. However, the addition of HAP reduces the resistance to compression when compared to pure MCC aerogels. Compressive strength at 10 % relative deformation (σ10) of those composite aerogels decreases monotonously with increasing HAP content. It indicates that the addition of HAP increases the brittleness of the MCC, resulting in a gradual decrease in the compressive strength of 9



the composite aerogel as the MCC content increases. Thermal decomposition of MCC/HAP aerogels. Thermogravimetry (TGA) was performed to investigate the influence of HAP on the thermal decomposition of MCC aerogels (Figure 3). The temperature of 10 wt% mass loss (T-10%), temperature at the first and second maximum mass loss rates (Tmax1 and Tmax2) obtained from the DTG (derivative TGA) curves, and the fraction of the residue remaining at 700 oC are listed in Table 1. HAP has high thermal stability upon 700 oC under a nitrogen atmosphere. The decomposition of MCC aerogel occurs in a two-stage degradation behavior in nitrogen, starting at 272 oC with the maximum mass loss rate at 345 oC and 566 oC, respectively, leaving a small amount of residue (0.18 wt%) at 700 oC. The presence of HAP shields the MCC aerogels from earlier degradation with 15-30 o

C higher in T-10%. With elevating temperature upon 300 oC, all MCC/HAP aerogels

exhibit higher thermal stability and leave large amounts of residual chars. In addition, the degradation feature of MCC changes to a one-stage degradation process due to the introduction of HAP, which is evidenced by the disappearance of second peak of mass loss rate in the DTG curves. It can thus be hypothesized that the in-situ formation of inorganic HAP-backbone efficiently inhibits the volatilization of degradation products from the bulk of the polymeric matrix to the gas phase, resulting in the formation of char residues. For example, the fraction of char residues for MCC/HAP (3/1) is 29.6 wt% at 800 oC (see Table 1), more than that for neat MCC (0.18 wt%). The content of HAP in MCC/HAP (3/1) is 25 wt%, indicating the effective carbonization caused by the HAP-backbone barrier effect, which will be 10


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further discussed in later sections. Thermal dynamic transfer and thermal dissipation properties. The thermal dynamic transfer properties of pure MCC aerogel and MCC/HAP composite aerogels were investigated by monitoring the variations of temperature with time. Aerogels with dimension of 28 mm × 28 mm × 25 mm were placed on the centre of one of the edges of a thermostatic heater (100 mm × 100 mm) with the temperature set at 170 o

C, as illustrated in Figure 4. The lateral-surface temperature of pure MCC and

selected MCC/HAP (1/1) aerogels were recorded by an infrared thermal imager, and their optical photographs were subsequently compared. Although almost the same κ values are attained, MCC/HAP (1/1) exhibits a faster increase in temperature at the first 300 s than pure MCC. This indicates that HAP accelerates the heat transfer during the heating process, which is due to the homogenous dispersion of HAP nanorods in the continuous cellular structure enhances the heat transfer. In addition, the reduction of the aperture size may also help reduce heat transfer. After 480 s, the two samples show similar optical photographs, indicating that they reach a thermal equilibrium state and form a stable temperature gradient within the aerogel. Therefore, it can be concluded that although MCC/HAP aerogels exhibit a low thermal conductivity and excellent heat insulation performance at room temperature, they show high thermal dynamic transfer performance during the heating process. Figure 5 shows the thermal dissipation properties of pure MCC aerogel and MCC/HAP composite aerogels. Before the optical photographs were taken, all samples were placed on the heater for 15 min then transferred immediately on a 11



polyurethane foam at room temperature. It can be observed that pure MCC and MCC/HAP (1/1) aerogels have the same thermal dissipation tendency. Both exhibit fast thermal dissipation rate within the initial 30 s and the rate decreases to a thermal equilibrium at 120 s. However, MCC/HAP (1/1) composite aerogel shows a decrease in temperature with time at a quicker rate when compared to pure MCC aerogel. The central temperature (M3 point) decreases from 73.1oC to 48.9 oC (∆T = 24.2 oC) in the initial 30 s for MCC/HAP (1/1), while 67.9 to 51.9 oC (∆T = 16.0 oC) for MCC. Nonetheless, they display the same thermal dissipation rates after 30 s. Reaction to fire and cone calorimetry analysis. The flammability of pure MCC aerogel and MCC/HAP composite aerogels was studied by placing samples of the materials on a wire netting which were exposed to the flame of an alcohol burner. The combustion videos can be found in Supporting Information (Video S1 and S2), and the digital photographs of chars are shown in Figure S4. Pure MCC aerogel burns rather quickly after ignition, leaving few residual char with a random shape. For MCC/HAP (1/1), HAP shows high efficiency to prevent MCC from being ignited and mechanically reinforce the porous char. The char from MCC/HAP (1/1) composite aerogel still maintain its original shape. Cone calorimetry represents an effective bench-scale method to determine the flammability properties of materials.35-38 The main parameters include the heat release rate (HRR) and especially peak of HRR (PHRR), the time to ignition (tign), the time to PHRR (tp), the rate of smoke release (RSR), and the average mass loss rate (AMLR). The burning behaviors of pure MCC and MCC/HAP aerogels are 12


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shown in Figure 6, and the corresponding parameters are summarized in Table 2. Pure MCC aerogel burns very quickly after ignition with a PHRR of 365 kW/m2, which occurs at 47 s. HAP shows a significant flame retardant effect on MCC. Comparing to pure MCC, the PHRR for MCC/HAP (3/1), (2/1) and (1/1) are reduced by 51.5, 51.8 and 93.7%, respectively. MCC/HAP (3/1) and (2/1) aerogels show much shorter tign and tp in comparison with pure MCC. tign decreases from 47 s for pure MCC to 18-26 s for MCC/HAP aerogels, while tp decreases from 55 s to 30-34 s. On the other hand, tign and tp increase with the increasing composition of HAP for the composite aerogels. When the HAP concentration increases to 50 wt%, the MCC/HAP (1/1) sample can’t be ignited, and the smoldering occurs during the whole cone calorimeter test process. The PHRR and PHRR/mass value of MCC/HAP (1/1) is 23 kW/m2 and 3.7 kW/m2·g respectively (see Table 2), showing the extremely low value compared to the other samples. The sample also results in the greatest reduction in AMLR with the greatest increase in tp. It indicates that MCC/HAP (1/1) aerogel has significant flame retardant properties. Fire performance index (FPI, defined as tign divided by PHRR), and fire growth rate (FIGRA, defined as PHRR divided by tp) are two important parameters that can be obtained from the cone calorimeter data to evaluate the materials for fire safety considerations.39,40 Higher values of FPI and lower values of FIGRA indicate better flame retardancy. As listed in Table 2, although PHRR is reduced by about 52 %, FPI of MCC/HAP (3/1) and (2/1) aerogels are almost unchanged and FIGRA decreases by only 11 and 21 %, respectively, compared to pure MCC. This is apparently caused 13



by the adverse effect of HAP on the increment in tign and tp for the two composites. Because MCC/HAP (1/1) aerogel did not ignite, FPI is unable to be evaluated directly. But it exhibits the lowest FIGRA value of 0.3, which indicates superior flame retardancy. Apart from reduction of the heat release rate, it is also essential that smoke production is suppressed during burning to minimize the fire hazards. All of the MCC/HAP aerogels show reduction in total smoke release (TSR) and rate of smoke release (RSR) with respect to pure MCC. The TSR values are reduced by 33.6, 38.6 and 74.2%, respectively, for MCC/HAP (3/1), (2/1) and (1/1); and the RSR peak are reduced by 6.2, 20 and 54.8% correspondingly. It can be concluded that the presence of HAP improves fire safety performance of MCC aerogel by improving the flame retardancy and the suppression of smoke release. However, due to the smoldering of MCC/HAP (1/1) composite aerogel, it yields the highest amount of carbon monoxide (CO) while the least amounts of carbon dioxide (CO2) in comparison with other samples, leading to the highest ratio of CO/CO2. Flame retardant mechanism. It has been shown that the fire safety performance of MCC aerogel can be significantly improved by the incorporation of HAP nanorods. The flame retardant mechanism may be postulated in the following based on the knowledge of the thermal dynamic transfer properties of aerogel itself, and the microstructure and physical properties of residual char. As discussed in previous sections, HAP possesses high thermal stability and increases the thermal dynamic transfer of MCC aerogels. This results in a negative 14


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effect on the increment in tign and tp and goes against the improvement in fire safety performance. However, this effect is reversed when HAP loading increases to 50 wt%. Another consideration is the characteristic of residual char. Digital-photographs of the char for pure MCC and MCC/HAP aerogels after cone calorimeter tests are shown in Figure 7. MCC alone is not a char-forming material, leaving only small amount of residues, whereas large quantity of residues can be found for MCC/HAP aerogels; the char yield is very close to the theoretical content of HAP in MCC/HAP which is composed of inorganic component of HAP. The XRD pattern shown in Figure 1A confirms that the char from MCC/HAP (1/1) aerogel has the same patterns as that of pristine HAP powder. The morphologies of char for pure MCC and MCC/HAP (1/1) are further elucidated by the SEM images (Figure 7E and F). Char of pure MCC is seen to consist of the accumulation of flakes. However, char from MCC/HAP (1/1) aerogel has a porous structure composed of interconnected networks of HAP particles. It demonstrates higher SBET and pore volume than that of pure MCC (listed in Table 1), which is geared towards the absorption of volatiles and smokes, resulting in the decrease in TSR and RSR. In general, regardless when the sample is exposed to a flame or the cone heater, non-flammable HAP-backbone aerogel-like residue will be in-situ formed along the temperature gradient. Since the thermal conductivity of this kind of residue is low (0.032 W/m·K), it has a better thermal-insulating effect. As a result, a thin layer of HAP residue formed on the front surface exposed to the flame or heat source acts as 15



an effective thermal insulator and mass-transport barrier to slow down the heat and mass transfer between gas and condensed phases, thus preventing further thermal degradation and/or combustion events to occur for the underlying material. As the burn duration increases, the thickness of the front residue layer will also gradually increase, creating stronger heat insulation for the unburnt material. As expected, with increasing HAP content, this thermal barrier effect will be reinforced and also weakens the negative effect of the thermal dynamic transfer on flame retardancy. Thus, smoldering occurs in MCC/HAP (1/1) aerogel. Furthermore, in order to better understand the competing effect between thermal dynamic transfer effect and thermal barrier effect, the real-time change of temperatures with time during cone tests have been recorded by a thermal infrared imager via selecting two temperature points. One is set up between the upper surface of the aerogel and the lower surface of cone heater (TSp1), and the other is above the cone heater (TSp2), as shown in Figure 8. TSp2 of MCC/HAP (3/1) and (2/1) are higher than that of pure MCC during the initial combustion stage. It indicates that the higher thermal dynamic transfer accelerates the spread of thermal energy, resulting in the decrease in tign and tp. But result from TSp1 shows that the maximum of TSp1 of MCC/HAP (3/1) and (2/1) are lower than that of pure MCC, suggesting that the segregation from HAP aerogel-like char can prevent flame penetration from the barrier to a higher place. The HAP char layer improves the thermal insulation properties and the heat diffusion rate is also faster, making it more difficult for heat to build up. This represents favorable flame suppression and reducing the probability 16


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of ignition. On the other hand, MCC/HAP (1/1) maintains a low, stable temperature for TSp1 and TSp2, because of the constant smoldering during the cone test.

CONCLUSIONS This present study establishes a strategy for the preparation of MCC aerogels containing homogeneous dispersed HAP nanorods to the highest concentration of 50 wt%. The presence of HAP improves the thermal stability of MCC aerogel and alters its degradation process from two-stage to one-stage mechanism. Although all aerogels exhibit similar thermal conductivity coefficient, the MCC/HAP composite areogels result in more rapid thermal dynamic transfer rate. The addition of HAP leads to the remarkable improvement in flame retardancy and smoke suppression properties of MCC arogels. Particularly, MCC aerogel with 50 wt% HAP nanorods smolders when exposed to either a thermal radiation source or a flame resulting from the formation of the HAP-based aerogel-like structure char. It should be noted that the improvement strongly depends on the competing factors between the dynamic heat transfer of aerogel itself and thermophysical properties of the HAP-based aerogel-like char layer, which ultimately determines the fire safety performance of the MCC/HAP composite aerogels.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications 17



website. Figure S1. Digital photographs of MCC/HAP (1/1) composites: (A) hydrogel, and (B) aerogel; Figure S2. Digital photographs of HAP aqueous suspension (10 wt%) after ultrasonication for different time: (a) 20 min; (b) 60 min; Figure S3. EDS elemental mappings of (B) calcium and (C) phosphorus in the region (A) of MCC/HAP (1/1); Figure S4. Digital photographs of chars from pure MCC and MCC/HAP (1/1) after burning on an alcohol burner; Video S1. Burning of pure MCC aerogel; Video S2. Burning of MCC-HAP composite aerogel.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Wei Yang: 0000-0003-4759-4996 Hong-Dian Lu: 0000-0002-8630-9740 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The work was financially supported by National Natural Science Foundation of China (Grant Nos. 51276054, 51403048, and 21702042), Program of Anhui Province for Outstanding Talents in University (Grant No. gxbjZD39), National 18


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undergraduate Innovation and Entrepreneurship Training Programs (Grant No. 201711059022), Natural Science Foundation in University of Anhui Province (Grant No. KJ2016A606), Talent Scientific Research Foundation of Hefei University (Grant Nos. 16-17RC07 and 16-17RC15), Natural Science Projects in Research Development Foundation of Hefei University (Grant No. 16ZR09ZDA), Program for Excellent Young Talents in University of Anhui Province (Grant No. gxfx2017098).

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25


Based

on

Ammonium


Table and Figure Captions Table 1. The formulations, porous properties, thermal properties and compression properties of pure MCC and MCC/HAP composite aerogels. Table 2. Cone calorimeter data of pure MCC and MCC/HAP composite aerogels under a heat flux of 30 kW/m2. Figure 1. (A) XRD patterns of pristine HAP powder and the char of MCC/HAP (1/1) composite aerogel; (B) SEM image of HAP powder; (C) N2 adsorption-desorption isotherm linear plots of HAP powder, MCC/HAP (1/1) composite aerogel and its corresponding char. Figure 2. SEM images of aerogels: (A) pure MCC; (B) MCC/HAP (1/1). Figure 3. (A) TGA and (B) DTG curves of pure MCC and MCC/HAP composite aerogels. Figure 4. Real-time variations of lateral-surface temperature with time for pure MCC and selected MCC/HAP (1/1) composite aerogels. Figure 5. Thermal dissipation properties of pure MCC and MCC/HAP (1/1) composite aerogels. The selected point was point M3. Figure 6. The curves of (A) HRR, (B) RSR, (C) CO and (D) CO2 of pure MCC and MCC/HAP composite aerogels under a heat flux of 30 kW/m2. Figure 7. Photographs of chars from cone calorimetry for aerogels (A) MCC, (B) MCC/HAP (3/1), (C) MCC/HAP (2/1), and (D) MCC/HAP (1/1), char% are 1.9, 24.9, 27.8, and 48, respectively; and SEM images of chars for the selected aerogels (E) MCC and (F) MCC/HAP (1/1). 26


Page 26 of 39

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Figure 8. (A) A selected thermal infrared image of pure MCC; (B) Schematic representation of the distribution of temperature points; Temperature vs time curves of pure MCC and MCC/HAP composite aerogels at the SP1 point (C) and SP2 point (D).

27



Page 28 of 39

Table 1. The formulations, porous properties, thermal properties and compression properties of pure MCC and MCC/HAP composite aerogels. ρb

Porosity

Sample (g/cm3)

(%)*

Pore

Pore

volume

radius

SBET (m2/g) 3

(cm /g)

(nm)

T-10%

Tmax1

Tmax1

Char

(κ)

σ10

(oC)

(oC)

(oC)

(%,)

(W/m•K)

(kPa)

MCC aerogel

0.060

95.8

2.0

0.003

1.8

272

345

566

0.18

0.038

379

MCC/HAP (3/1)

0.063

96.6

4.8

0.006

1.5

290

350

NA

29.6

0.037

311

0.073

96.3

4.9

0.006

1.5

300

347

NA

38.0

0.036

256

0.079

96.5

7.5

0.027

1.7

289

338

NA

54.2

0.036

218

NA

NA

38.0

0.14

1.7

NA

NA

NA

NA

NA

NA

NA

NA

38.3

0.16

1.9

NA

NA

NA

NA

0.032

NA

aerogel MCC/HAP (2/1) aerogel MCC/HAP (1/1) aerogel** Pristine HAP powder Char of MCC/HAP (1/1) aerogel *

The porosity was calculated as {1-ρb/[ρMCC*massMCC% +ρHAP*(1-massMCC%)]}%;

in which the density of MCC (ρMCC)is 1.43 g/cm3, and the density of HAP (ρHAP)is 3.16 g/cm3. All samples were tested in triplicate and the average values are reported. **

8.8 g HAP suspension was mixed with 16.2 g MCC solution; the mass ratio of

MCC/HAP was calculated as about 1/1.

28


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Table 2. Cone calorimeter data of pure MCC and MCC/HAP composite aerogels under a heat flux of 30 kW/m2. Average

Average

CO

CO2

PHRR tign

PHRR

tp

TSR

AMLR

Ratio of

Sample

Mass FPI

(sec)

(kW/m2)

(sec)

(m2/m2)

(g/m2·s)

yield

yield

FIGRA

CO/CO2

Reduct-Cone /Mass

(g)

(%) (kW/m2·g)

(kg/kg)

(kg/kg)

pure MCC

47

365

55

34.5

5.5

0.064

1.7

0.04

0.13

6.6

5.2

70.2

NA

MCC/HAP

18

177

30

22.9

4.7

0.089

2.2

0.04

0.10

5.9

5.4

32.8

51.5

26

176

34

21.2

4.2

0.084

2.1

0.04

0.15

5.2

5.9

29.8

51.8

NA

23

72

8.9

3.8

0.26

1.4

0.19

NA

0.3

7.5

3.1

93.7

(3/1) MCC/HAP (2/1) MCC/HAP (1/1)

29



Graphical Abstract

30


Page 30 of 39

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Scheme 1. The route of preparation for MCC/HAP composite aerogels.

31



Figure 1. (A) XRD patterns of pristine HAP powder and the char of MCC/HAP (1/1) composite aerogel; (B) SEM image of HAP powder; (C) N2 adsorption-desorption isotherm linear plots of HAP powder, MCC/HAP (1/1) composite aerogel and its corresponding char. 32


Page 32 of 39

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Figure 2. SEM images of aerogels: (A) pure MCC; (B) MCC/HAP (1/1).

33



Figure 3. (A) TGA and (B) DTG curves of pure MCC and MCC/HAP composite aerogels.

34


Page 34 of 39

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Figure 4. Real-time variations of lateral-surface temperature with time for pure MCC and selected MCC/HAP (1/1) composite aerogels.

35



Figure 5. Thermal dissipation properties of pure MCC and MCC/HAP (1/1) composite aerogels. The selected point was point M3.

36


Page 36 of 39

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Figure 6. The curves of (A) HRR, (B) RSR, (C) CO and (D) CO2 of pure MCC and MCC/HAP composite aerogels under a heat flux of 30 kW/m2.

37



Figure 7. Photographs of chars from cone calorimetry for aerogels (A) MCC, (B) MCC/HAP (3/1), (C) MCC/HAP (2/1), and (D) MCC/HAP (1/1), char% are 1.9, 24.9, 27.8, and 48, respectively; and SEM images of chars for the selected aerogels (E) MCC and (F) MCC/HAP (1/1). 38


Page 38 of 39

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Figure 8. (A) A selected thermal infrared image of pure MCC; (B) Schematic representation of the distribution of temperature points; Temperature vs time curves of pure MCC and MCC/HAP composite aerogels at the SP1 point (C) and SP2 point (D).

39


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