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Title: Silica–cellulose hybrid aerogels for thermal and acoustic

insulation applications

Author: Jingduo Feng Duyen Le Son T Nguyen Tan Chin

Nien Victor Daniel Jewell Hai M Duong

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Silica–cellulose hybrid aerogels for thermal and acoustic insulation

applications

Jingduo Feng1, #, Duyen Le2, #, Son T Nguyen2

, Tan Chin Nien, Victor1, Daniel Jewell3, Hai

Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU-HCM,

268 Ly Thuong Kiet Street, District 10, Ho Chi Minh City, Vietnam

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Department of Materials Science and Metallurgy, University of Cambridge, UK

* Corresponding author: mpedhm@nus.edu.sg

151o Their thermal conductivity was approximately 0.04 W/mK Moreover, the thermal degradation temperature for the cellulose component of the silica–cellulose aerogels showed

a 25 oC improvement over those for cellulose aerogels The sound absorption coefficients of

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the silica–cellulose aerogels with a 10 mm thickness were 0.39–0.50, better than those of cellulose aerogels (0.30–0.40) and commercial polystyrene foams When the cellulose fibre concentration increases from 1.0 to 4.0 wt %, the compressive Young’s modulus of the silica–cellulose aerogels can be enhanced 160%, up to 139 KPa This work provides a facile approach to fabricate cost-effective and promising silica–cellulose aerogels with industrial dimensions for thermal and acoustic insulation applications

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1 Introduction

Global warming is one of the essential reasons for irrevocable sea level rise and climate disasters [1] Most scientists believe that greenhouse emission is a major cause of global warming [1, 2] As a matter of fact, in the developed countries, more than 30% of the total greenhouse gas emission is caused by buildings [3] Moreover, the recent rapid development

of developing countries has led to a growth of greenhouse gas emission from buildings, as more buildings have been equipped with air-conditions One possible solution to this problem

is to integrate the building with high performance thermal insulation materials Conventional insulation materials (mineral fibres and polyurethane foam) with high sensitivity to moisture still have the largest market shares, because of the best performance per unit cost [4] Hence, fabrication of inexpensive thermal insulation materials with reasonable thermal conductivity and high moisture resistance is in need

In addition, noise is one of the major environmental challenges of the current world, considered as the most widespread and least controlled environmental pollutant [5, 6] Noise can cause negative health effects, such as loss of hearing, high blood pressure, and increased physiologic stress [7, 8] Some works have been conducted to investigate the acoustic insulation properties and applications of silica aerogels [9] However, their brittleness and high cost hinder their industrial development To our best knowledge, there have been few reports on the acoustic insulation properties of cellulose-based aerogels Therefore, in this work, the acoustic insulation properties of the silica–cellulose aerogels and their cellulose matrices were explored by an industrial approach for the first time

Moreover, 360 million tonnes of paper-related waste were generated in 2004, and paper and paperboard consumption is likely to keep increasing by approximately 2.1% each year until 2020 [10] The above facts imply huge amount of paper waste is generated every

Aerogels may be a solution to the above environmental problems Aerogels, considered as a new state of matter, are materials with numerous unique properties, such as low thermal conductivity (0.012–0.045 W/mK), low density (0.001–2.06 g/cm3), high porosity (77–99.8%), and large surface area (81–1600 m2/g) [4, 12-15] Silica aerogels normally are fragile, but their thermal conductivities are low; while cellulose aerogels can be compressed to a strain of 80%, but their thermal conductivities are relatively higher than those of silica aerogels [12, 16] On the other hand, both cellulose materials and silica aerogels are outstanding acoustic insulation materials [9, 17] Researchers want to combine the outstanding properties of silica aerogels and cellulose aerogels Hence, the silica–cellulose aerogels become promising, and have attracted a lot of research efforts [12, 18-20]

The silica–cellulose aerogels have thermal conductivities between those of their pure silica counterparts and pure cellulose counterparts, while the compressive moduli of the silica–cellulose aerogels surpass those of the pure silica or cellulose aerogels [12, 18, 19] However, no studies were available to fabricate silica–cellulose aerogels from recycled cellulose fibres that generated from paper waste Because cellulose aerogels synthesized from recycled cellulose fibres have much lower mechanical strength than that made from normal cellulose fibres, improvements on the mechanical strength of recycled cellulose aerogels are

resulted in different nano-structures of the silica phases Shi et al changed silica loading

without alternating the silica nano-structures, however, the maximum silica loading in the silica–cellulose aerogels was only 12% [24] It is interesting to investigate silica cellulose aerogels of higher silica loadings High silica loadings will improve the thermal stability of the composite aerogels due to the excellent thermal stability of silica and thus, enhance the fire retardant property of the materials [25] In addition, hydrophobicity helps them to withstand moisture, and contributes to the stability of their thermal insulation performance However, the available hydrophobic silica–cellulose aerogels in literature were all obtained via post-modifications of silica–cellulose aerogels [23, 24] In 2013, a CCl4 cold-plasma post-modification approach using gas ionization was applied on silica–cellulose aerogels, and the final aerogels have a water contact angle of approximately 132o [24] In the same year, a solvent-immersion method involving a 24 h aging followed by freeze drying was introduced for the hydrophobic modification of the silica–cellulose aerogels [23] Their water contact angle was approximately 133o, which was similar to that of the previous approach [23, 24] Both the methods might be considered as uneconomical because of either the expensive equipment or the large amount of chemicals and the long duration involved Moreover, the water contact angles of the modified composite aerogels were not super-hydrophobic

In our study, cost-effective and superhydrophobic silica-cellulose aerogels were successfully prepared for the first time from recycled cellulose fibres and methoxytrimethylsilane (MTMS) Their morphology, hydrophobicity, thermal, sound

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insulation and mechanical properties were carefully investigated The materials showed promising thermal and sound insulation properties for practical applications

2 Experiments

2.1 Materials and chemicals

Kymene 557H was supported by Ashland (Taiwan) Recycled cellulose fibres were purchased from Insul-Dek Engineering Pte Ltd (Singapore) The recycled cellulose fibres and Kymene were used without further purification Methoxytrimethylsilane (MTMS), absolute ethanol, ammonium hydroxide, ammonium fluoride were of analytical grade, purchased from Sigma-Aldrich and used as received All the solutions were made with deionized (DI) water

2.2 Fabrication of silica–cellulose aerogels

2.2.1 Preparation of silica aerogels

The catalyst solution was prepared by mixing 10.25 g of ammonium hydroxide, 0.927 g of ammonium fluoride in 50ml of DI water 6.5 ml of MTMS solution was mixed with 11 ml of ethanol and stirred for 5 minutes to prepare the MTMS–ethanol mixture Then

7 ml of DI water, 11 ml of ethanol, and 0.5 ml of the catalyst solution were mixed in another beaker for the DI water–ethanol–catalyst solution While the MTMS–ethanol mixture was still being stirred, the obtained solution of DI water–ethanol–catalyst was poured slowly into the MTMS–ethanol solution and stirred for another 15 minutes to form the silica sol The silica sol was poured into a mould, and then the gelation and the aging processes were conducted at room temperature (25 oC) for three days After solvent exchange of the gel with

DI water for three days, the obtained hydrogel was frozen and dried by using a freeze dryer (ScanVac CoolSafe freeze dryer, Labogene, Denmark) for 24 h

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2.2.1 Preparation of cellulose aerogels

The hydrophobic recycled cellulose aerogels were fabricated using a cost-effective method developed in our lab [26] Recycled cellulose aerogels of different porosities were obtained by changing the amounts of recycled cellulose fibres (0.3, 0.6, and 1.2 g), and the ratio of the volume of Kymene to the weight of recycled cellulose fibres (Kymene: recycled cellulose fibres = 10 μl: 0.15 g) was fixed The recycled cellulose fibre concentrations inside the initial mixtures (30 mL) for the cellulose aerogel fabrications were 1, 2, and 4 wt %, respectively

2.2.3 Preparation of silica–cellulose aerogels

The catalyst solution was prepared by mixing 10.25 g of ammonium hydroxide, 0.927 g of ammonium fluoride in 50 ml of DI water 6.5 ml of MTMS solution was mixed with 11 ml of ethanol and stirred for 5 minutes to prepare the MTMS–ethanol mixture Then

7 ml of DI water, 11 ml of ethanol, and 0.5 ml of the catalyst solution were mixed in another beaker for the DI water–ethanol–catalyst solution While the MTMS–ethanol mixture was still being stirred, the obtained solution of DI water–ethanol–catalyst was poured slowly into the MTMS–ethanol solution and stirred for another 15 minutes to form the silica sol

After the formation of the silica sol, the weighted hydrophobic cellulose aerogels (obtained from Section 2.2.2) with different cellulose concentrations (of 1, 2, and 4 wt % inside the initial reaction mixtures for cellulose aerogel fabrications) were immersed into the silica sol

Gelation and the aging processes of the mixtures were conducted at room temperature (25 oC) for three days After solvent exchange of the gel with DI water for three days, the obtained hydrogel was frozen and dried by using a freeze dryer (ScanVac CoolSafe freeze dryer, Labogene, Denmark) for 24 h

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2.3 Characterization

2.3.1 Characterization of structure and morphology

The morphology of the aerogel samples was investigated using a field-emission scanning electron microscopy (FE-SEM, Model S-4300 Hitachi, Japan) The samples were coated with a thin layer of gold by sputtering prior to FE-SEM

The structures of the aerogels were confirmed with the aid of X-ray diffraction (XRD,

6000 Shimadzu, Japan) The Cu-Kα radiation source (λ=0.1506 nm) was applied In addition, the scan step was 0.02o and the rate of the scan step was 0.6 o/min in the range of 5–45o (2θ)

The pore properties of the silica aerogels and silica cellulose composite aerogels were determined by nitrogen physisorption measurements with a Nova 2200e (Quantachrome) All the samples were degassed in vacuum at 80 oC for 24 h before measurement

2.3.2 Determination of SiO2 content in silica–cellulose aerogels

Before the immersion of the cellulose aerogel into the silica sol, the weight of the cellulose aerogel was recorded as Wc After the freeze-drying, the total weight of the silica–cellulose aerogel was measured as Wt The cellulose content (ω ) and the SiOc 2 contents (ω s

are calculated as:

c c t

Wω

2.3.3 Determination of water contact angle of silica–cellulose aerogels

Hydrophobicity of the aerogels was investigated by conducting a water contact angle test on the aerogels A VCA Optima goniometer (AST Products Inc., USA) was used

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to conduct this test The program was controlled by software built in function where water is dispensed from the syringe at 0.5 l every time The software then helped to calculate the water contact angle automatically The measurement was repeated at different positions of the sample and an average was taken

2.3.4 Determination of thermal conductivity of silica–cellulose aerogels and their cellulose matrices

The thermal conductivity of the aerogel was determined by a C-Therm TCi Thermal Conductivity Analyzer (C-Therm Technologies, Canada) using the modified transient plane source method under ambient conditions [27]

2.3.5 Determination of thermal stability of silica–cellulose aerogels and their cellulose matrices

The TGA (thermal gravimetric analysis) tests were performed by a DTG60H thermogravimetric analyzer (Shimadzu, Japan) to study the thermal stability of the aerogels The specimen was heated from room temperature to 800 oC at a rate of 10 oC/min in air During the heating process, the specimen was maintained at 150 oC for 1 h to remove the absorbed water

2.3.6 Determination of sound absorption coefficient of silica–cellulose aerogels and their cellulose matrices

A demonstration for the sound insulation ability of the silica–cellulose aerogels and their cellulose matrices were conducted A sound signal generator producing known sound signals with a known sound intensity (Blesi Guardian angel anti-rape alarm, 90 dB), and a sound meter (Amprobe SM-10, USA) were used to investigate the sound absorption coefficient of the silica–cellulose aerogels and their cellulose matrices With this approach,

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the sound generator was placed both inside and outside an insulation box The insulation box was fabricated by fixing a type of aerogels at all sides of the container via double-sided tap to ensure a closed system The incident sound signal was measured in both cases, at the same distance away from the sound generator The absorbed sound intensity was defined as the difference between the known sound intensity and the incident sound intensity The sound absorption coefficient was defined as the ratio of the absorbed sound intensity to the known sound intensity

2.3.7 Determination of compressive mechanical strength of silica–cellulose aerogels and their cellulose matrices

The compressive test was carried out on an Instron 5500 micrometer (USA) to investigate the compressive moduli of the aerogels During the test, the specimen was under a loading at a rate of 1 mm/min

3 Results and discussion

3.1 Morphologies and structures of the silica–cellulose aerogels

The morphologies of the silica–cellulose aerogels were characterized by FE-SEM As shown in Figures 1a–c, the cellulose matrices, fabricated with different cellulose fibre concentrations (1, 2, and 4 wt % respectively) inside the initial reaction mixture for cellulose matrices synthesis, were the three dimensional supporting frames for the silica–cellulose aerogels The resultant silica–cellulose aerogels had a silica loading of approximately 80–60

wt %, and the silica loading of the silica–cellulose aerogels decreased (from 80 to 71, and to

60 wt %) with an increase in initial cellulose fibre concentration (from 1 to 2, and to 4 wt %) inside the initial reaction mixtures for their cellulose matrix syntheses

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The cellulose matrices are strengthened by the interaction points of the cellulose fibres via hydrogen bonding [28, 29] The strengthened cellulose matrices restrict the movement of the silica particles and confine the silica particles inside their porosity [23] In addition, the interconnected silica particles stiffen the silica–cellulose aerogels by supporting the cellulose fibre matrices [23] The effects of the different cellulose contents on the microstructures of the silica–cellulose aerogels are presented in Figures 1a–c Increases in the cellulose content of the silica–cellulose aerogels leads to a more uniformly distribution of the silica particles with smaller particle sizes The changes in particle size and distribution of the silica particles could be observed in Figures 1a-c

The meso-porous structures of the silica–cellulose aerogels produced with different cellulose matrices were nearly identical A typical image of the magnified silica region of the silica–cellulose aerogels is displayed in Figure 1d Cellulose fibres cannot be observed in Figure 1d, due to their large sizes and inter-fibre distances The silica meso-porous structure developed in this work is similar to those of silica-cellulose aerogels developed by other groups with respect to morphology, resembling tentacle patterns of a coral [23, 24]

As shown in Figure 2, the X-ray diffraction patterns of the silica-cellulose aerogels can be a systematic superposition of those of the pure cellulose aerogels and the pure silica

aerogels, as reported by Cai et al [12] This result suggests that no new compound could be

detected by XRD and the chemical reaction between the cellulose fibres and the silica particles is very limited The round peaks appeared at approximately 2θ = 17o and 22o are corresponding to natural cellulose, and the round peak centring at approximately 2θ = 23o is typical for amorphous silica [30-32] For samples fabricated from recycled cellulose fibres, the sharp crystalline peaks appeared at 2θ = 12.5o, 29.4o, 36.0o, 39.4o, and 43.2o, corresponding to (0, 0, 1) crystal plane of Kaolinite (Al2Si2O5(OH)4) , and (1, 0, 4), (1, 1, 0), (1, 1, -3), and (2, 0, 2) crystalline planes of Calcite (CaCO3) Both Kaolinite and Calcite are

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widely used fillers in paper and board industry for improving brightness and smoothness of end-products [33]

3.2 The nitrogen adsorption–desorption isotherms of the silica–cellulose aerogels

The adsorption–desorption isotherms of silica–cellulose aerogels and silica aerogel are displayed in Figure 3 These isotherms are similar to a type IV isotherm, which suggests the aerogels have a well-developed meso-porosity [34, 35] As observed from Figure 4, a moderate linear relationship can be found between the silica mass concentration and the BET surface area of the aerogel The silica mass concentration for the silica–cellulose aerogels was calculated as the ratio of the mass of the composite aerogel excluding the mass of the cellulose matrix (before embedding) to the mass of the composite aerogel This relationship suggested that the meso-porous structure of the composites was not significantly affected by the different cellulose matrices, which is consistent with previous SEM findings The BET surface areas of the silica–cellulose aerogels (approximately from 198 to 296 m2/g) are higher

than or similar to those found in similar studies conducted by Demilecamps et al (90–170

m2/g) [19] and Litschauer et al (220–290 m2/g) [20]

3.3 Hydrophobicity of the silica–cellulose aerogels

Figure 5 shows a typical image of the water contact angle on the surface of the silica–cellulose aerogels The silica–cellulose aerogels exhibited an inherent super-hydrophobicity The average water contact angle was approximately 151o for all three types of the silica–cellulose aerogels, and the different cellulose matrices do not cause any significant difference among the water contact angles of the silica–cellulose aerogels This excellent water-repellent property was inherited from both the hydrophobic cellulose matrix and the silica precursor (MTMS) The hydrophobic property of the cellulose matrix (water contact angle:

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approximately 150o) was reported in our previous work [26] Meanwhile, the stable methyl group of MTMS was responsible for the excellent hydrophobicity of the silica components [36]

Water contact angle measurements were also performed on the cross-sectional area of the silica-cellulose aerogel samples It was found that there was no significant difference between the values of external water contact angle (on the external surface) and internal water contact angle (on the cross-sectional area) This confirmed that the superhydrophobicity was distributed uniformly in the composite aerogel structure

3.4 Thermal conductivity of the silica–cellulose aerogels and their cellulose aerogel matrices

To investigate the thermal insulation ability of the silica–cellulose aerogels and their cellulose matrices, thermal conductivity measurements were carried out with a C-Therm TCi Thermal Conductivity Analyzer System The cellulose aerogel matrices show low thermal conductivities (0.034–0.037 W/mK), as shown in Table 1 These values are slightly higher than those of cellulose aerogels fabricated from sodium hydroxide/urea (thiourea) dissolution systems (0.029–0.032 W/mK) [16, 37] However, the cellulose aerogel matrices in this current work are more stable than those prepared from sodium hydroxide/urea (thiourea) solutions At 300 oC, the aerogels fabricated from sodium hydroxide/urea (thiourea) dissolution systems only had an approximately 40 wt % remaining [16], as opposed to an approximately 85 wt % remaining of the cellulose aerogel matrices fabricated in this paper (shown in Figure 6) The weak thermal stability of the cellulose aerogels fabricated from sodium hydroxide/urea (thiourea) dissolution systems impedes the industrial applications of the aerogels due to safety concerns

The thermal conductivities (0.039–0.041 W/mK) of the silica–cellulose aerogels in our work were lower than those of silica–cellulose composites fabricated by other group

3.5 Thermal stability of the silica–cellulose aerogels

As shown in Figure 6, it is clear that the thermal stability of the silica–cellulose aerogels was better than that of the cellulose matrices At 150 oC, the silica–cellulose aerogels showed an approximately 2 wt % weight loss, which is less than those (an approximately 7

wt % weight loss) of their cellulose matrices This weight loss corresponds to the evaporation of adsorbed water [41] Compared to the cellulose matrices, the silica–cellulose aerogels had smaller weight loss due to their water-repellent and superhydrophobic property

A 25 oC delay in the thermal degradation of the cellulose component of the silica–cellulose aerogels was observed at 325 oC, compared with 300 oC of their cellulose matrices This increase in the degradation temperature might have been due to an interaction between the silica component and the cellulose component of the silica–cellulose aerogels at higher temperatures, and the interaction showed a positive effect on the thermal resistance of the aerogels [38, 42]

At the plateau, approximately 67, 60 and 51 wt % of the silica–cellulose aerogels (with an initial cellulose concentration of 1, 2 and 4.0 wt %, respectively) were preserved, containing the silica The differences in the weight loss were because of the variance in the

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cellulose content of the silica–cellulose aerogels The silica component significantly contributes to this increase, since silica is stable at higher temperatures [43]

3.6 Acoustic insulation properties of the silica–cellulose aerogels

Figure 7 describes the set-up of the acoustic insulation test As shown in Table 2, the sound absorption coefficient of the cellulose matrix decreases (0.399–0.303) with an increase

in the density (0.039–0.059 g/cm3) According to Motahari et al [44], the sound absorption

coefficient of aerogel decreases with an increase in the density because of an increase in sound velocity When the sound wave propagates to a depth of cellulose aerogel, the air in the pores begins to vibrate, possibly leading to the vibration of the cellulose fibres [45] During this process, because the acoustic energy is partially absorbed, the amplitude and velocity of the acoustic wave are reduced [45]

As suggested by Table 2, the sound absorption coefficients of the silica–cellulose are between 0.39 and 0.50, which are comparable to other insulation materials [46] With adding silica particles, the sound absorption coefficients were generally increased This increase can

be explained by acoustic energy partially absorbed by the interface between silica particles and cellulose fibres [47] The sound wave energy might be absorbed by interface between silica particles and cellulose fibres On the other hand, the sound absorption coefficient of the silica–cellulose aerogel (0.390) fabricated from the 1 wt % cellulose aerogel are not higher than that of the 1 wt % cellulose aerogel (0.399) This observation could be explained by the counter effects of the increase in density and the presence of the silica–cellulose interface

The sound absorption coefficient of the silica–cellulose aerogels increases with a decrease in the cellulose fibre concentration from 2 to 1 wt % This phenomenon could be attributed to a more uniform distribution and smaller sizes of silica particles [47] When the cellulose fibre concentration was increased from 2 to 4 wt % in the initial suspension for cellulose matrix fabrication, the sound absorption coefficient of the silica–cellulose aerogel

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was not significantly changed This observation may be due to the particle size and the size distribution of silica particles of the silica–cellulose aerogel fabricated from the 2 wt % cellulose aerogel were adequately small and uniform for a good sound insulation performance

3.7 Mechanical properties of the silica–cellulose aerogels

The mechanical properties of the aerogels are very important for thermal and sound insulation applications Therefore, compressive tests were carried out for the aerogel samples Compressive strain-stress curves of the silica–cellulose aerogels and the cellulose matrices are shown in Figure 8 It can be seen that silica embedment improved the mechanical strength

of the aerogels dramatically The compressive Young’s moduli of the silica–cellulose aerogels (86–169 KPa) were three to five times greater than those of their cellulose aerogel matrices (13–39 KPa) in Table 3 It is possible that the silica particles restrain the bending and mobility of cellulose fibres during compression As a result, the Young’s moduli of the silica–cellulose aerogels were higher than those of their cellulose matrices

As shown in Table 3, the Young’s modulus of the silica–cellulose composite and cellulose aerogels increased when the cellulose fibre content increased from 1 to 2 and 4% This phenomenon might be explained that the higher cellulose content leads to a more rigid supporting matrix in both of the silica-cellulose and cellulose aerogel structures

4 Conclusion

In conclusion, cost-effective silica–cellulose aerogels were successfully developed from recycled cellulose fibres and MTMS The silica–cellulose aerogels were investigated with different cellulose fibre concentrations showed excellent thermal and acoustic insulation properties The silica–cellulose aerogels were more flexible than silica aerogels and showed