Lightweight, Flexible, Thermally-Stable, and Thermally-Insulating

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Lightweight, Flexible, Thermally-Stable and ThermallyInsulating Aerogels Derived from Cotton Nanofibrillated Cellulose Jiale Qi, Yanjun Xie, Hai-Wei Liang, Yushu Wang, Tingting Ge, Yongming Song, Mengzhu Wang, Qing Li, Haipeng Yu, Zhuangjun Fan, ShouXin Liu, Qingwen Wang, Yixing Liu, Jian Li, Ping Lu, and Wenshuai Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06851 • Publication Date (Web): 29 Apr 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Sustainable Chemistry & Engineering

Lightweight, Flexible, Thermally-Stable and Thermally-Insulating Aerogels Derived from Cotton Nanofibrillated Cellulose Jiale Qi,† Yanjun Xie,† Haiwei Liang,‡ Yushu Wang,† Tingting Ge,† Yongming Song,† Mengzhu Wang,† Qing Li,*,† Haipeng Yu,† Zhuangjun Fan,§ Shouxin Liu,† Qingwen Wang,† Yixing Liu,† Jian Li,† Ping Lu,*,⊥ and Wenshuai Chen*,†

†Key

laboratory of Bio-based Material Science and Technology, Ministry of Education,

Northeast Forestry University, Harbin 150040, P. R. China ‡Department

of Chemistry, University of Science and Technology of China, Hefei 230026, P.

R. China. §Key

Laboratory of Superlight Materials and Surface Technology, Ministry of Education,

College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China ⊥ Department

of Chemistry and Biochemistry, Long Island University, Brooklyn, NY 11201,

USA

*To whom correspondence should be addressed. E-mail: [email protected] (W. Chen.); [email protected] (P. Lu ); [email protected] (Q. Li)

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ABSTRACT: Aerogels with multiple advantages have been developed for various requirements, but aerogels tend to be fragile and easy to break when bent or compressed. Herein, high-aspect-ratio cotton-derived nanofibrillated cellulose (NFC) was utilized as building blocks to construct aerogels. The cotton NFC formed strong web-like entangled structures that acted as the skeletal support of aerogels, exhibiting a density lower than that of wood-, bamboo- and rice straw-derived NFC aerogels. The cotton NFC aerogels (CoNAs) were soft, flexible and illustrated good resilience performance after compression release. The CoNAs had a high thermal stability arising from the component purity (~100% cellulose) and high relative crystallinity of cotton NFC, demonstrating their application suitability in high-temperature conditions. Further, the CoNAs exhibited an excellent thermal insulating performance and insulation stability at various temperatures owing to their porous structures and high thermal stability. The CoNAs fabricated herein are thus expected to be a novel member of the nanocellulose aerogel family owing to their intrinsic characteristics attained by integrating multiple structural and performance advantages into the one. KEYWORDS: cotton, nanofibrillated cellulose, aerogels, flexibility, thermal insulation


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INTRODUCTION Aerogels represent a kind of materials with the unique characteristics of lightweight, high porosity, and high specific surface area,1,2 which have been developed for multiple applications such as thermal insulation,3-5 adsorption,6-8 sensing,9,10 catalysis,11,12 and energy storage.13,14 The precursors and building blocks of aerogels play a critical role in producing new aerogel structures and functions. Generally, aerogels are made from inorganic materials such as silica,15 but silica aerogels are weak and brittle and easily broken even under a low tensile or compressive stress, which greatly limits their practical applications. Several strategies have been used to reinforce silica aerogels such as the integration of organic polymers or nanofillers, but a great requirement still exists for a further optimization of the structures and properties of these aerogels. Recently, various nanosized building blocks ranging from nanocarbon to polymer nanofibers have been developed to fabricate aerogels with diverse micro/nano structures and functions. As a novel nanomaterial that is mainly derived from biomass resources such as wood, nanocellulose has attracted increasing interest as a sustainable nanoscale building block for aerogels because of its one-dimensional nanostructure, high specific surface area, high Young’s modulus and specific strength, and large amount of active hydroxyl groups.16-21 Nanocellulose with various structures and surface chemistry has been developed for fabricating

various

types

of

aerogels.

Some

nanocellulose

aerogels

such

as

strong-acid-hydrolyzed cellulose nanocrystal aerogels,5 however, are still weak and fragile owing to the short and low-aspect-ratio nature of the building blocks. Several aerogels also suffer from low thermal stability because the nanocellulose was chemically modified and possessed carboxyl groups, aldehyde groups and sulfate ester groups on the surface.5,22 Thus, it is still indispensable for further optimizing the nanocellulose building blocks to assemble aerogels with multiple advantages. Nanofibrillated cellulose (NFC) is a type of nanocellulose produced from nanofibrillation of cellulose pulps through various shearing/impacting apparatuses such as a high-pressure homogenizer,23,24 grinder,25 and high-intensity ultrasonicator.26 Lightweight, flexible and 3 ACS Paragon Plus Environment


thermally insulating aerogels were obtained after freeze-/supercritical- drying of the NFC aqueous suspensions.5,27,28 For most biomass resources such as wood, bamboo and rice straw, more or less amorphous hemicellulose is still present in the as-extracted NFC. Nevertheless, the overall thermal stability of the NFC aerogels is affected by the low thermal degradation temperature of hemicellulose.29 Among the various biomass resources, cotton is recognized as a naturally pure cellulose material. Besides, the cellulose crystal size and the width of elementary fibrils within the cell walls of cotton are also different in compared with the other biomass resources. Thus, cotton NFC could be a novel building block for the nanocellulose aerogel family. We have recently developed a method for individualization of high-aspect-ratio NFC from cotton through chemical purification and pretreatment by a high-speed blender combined with a high-pressure homogenization technique.30 Cotton NFC can also be isolated through ultrasonicator31 and grinder32,33 nanofibrillaltion. The cotton NFC displays a significantly higher crystallinity and a higher thermal stability compared to that of other types of nanocellulose. Herein, we utilized cotton-derived NFC as building blocks to construct aerogels. The high-aspect-ratio cotton NFC can support a self-standing aerogel body with a low density. Our cotton NFC aerogels (CoNAs) demonstrate good mechanical resilience and better thermal stability when in compared with that of wood NFC aerogels (WoNAs), bamboo NFC aerogels (BaNAs) and rice straw NFC aerogels (RsNAs). CoNAs also display advantages similar with the other biomass resources derived nanocellulose aerogels in multiple aspects such as light weight, high flexibility, and good thermal-insulating capability. EXPERIMENTAL SECTION Materials. Raw cotton collected from the local farms in Shandong Province was utilized as the raw material for NFC production. Poplar wood-, bamboo- and rice straw- powders were first sieved through a 60 mesh and then were utilized as the raw materials to prepare NFC. Sodium chlorite, acetic acid, potassium hydroxide, sodium hydroxide, hydrochloric acid and other chemicals were directly used for experiments.


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NFC preparation. Poplar wood, bamboo and rice straw powders were purified via several chemical pretreatments.26,34 At first, the lignin of the biomass resources was removed with acidified sodium chlorite at 75 °C for 1 h; the purification process was repeated five times. Then, the as-prepared holocellulose was treated with 5 wt% potassium hydroxide at 90 °C for 2 h, and large amount of hemicellulose was removed. The as-generated pulps were further treated by acidified sodium chlorite at 75 °C for 1 h twice and 5 wt% potassium hydroxide at 90 °C for 2 h. Next, the cellulose pulp suspensions were fibrillated by a high-intensity ultrasonicator (JY99-IIDN, Ningbo Scientz Biotechnology Co., Ltd, China), at a frequency of 19.5–20.5 kHz with a cylindrical titanium alloy probe tip (2.0 cm in diameter). The ultrasonication treatment was conducted in an ice/water bath for 30 min using an output power of 800 W, resulting in the NFC suspensions. For the preparation of cotton NFC, raw cotton fibers were treated by an acidified sodium chlorite solution at 75 °C for 1 h for the removal of the impurities, where the process was repeated three times. Then, the samples were dispersed in 8 wt% sodium hydroxide at room temperature overnight, which were further heated at 80°C for 2 h. To further remove the alkali-insoluble materials, the samples were suspended in 1 wt% hydrochloric acid at 80 °C for 2 h, and purified cotton cellulose pulps were recieived. Next, the pulps were mechanically fibrillated by a high-speed blender for 4 min, and the as-generated samples were filtered through a metal mesh with 300 μm-diameter pores. The wet cake-like components that remained on the top of the mesh were repeatedly treated by the blender until all of the samples passed through the metal mesh. The collected suspension was set up overnight and the supernatant was removed. Subsequently, the suspension was centrifuged to increase the NFC concentration. Finally, the cotton NFC suspensions were magnetically stirred for 8 h and further treated in an ultrasonic bath for 30 min. Aerogel Fabrication. The NFC aqueous suspensions (15 mL) were placed in a 25-mL beaker, and subsequently frozen in a refrigerator at −18 °C for 24 h. Next, the samples were lyophilized in a freeze-dryer and NFC aerogels were obtained. For convenient, the cotton-, wood-, bamboo-, and rice straw- NFC derived aerogels are referred to as CoNA-x, WoNA-x, BaNA-x, and RsNA-x, where x is derived from the NFC contents of the NFC suspension 5 ACS Paragon Plus Environment


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before freeze drying process. For example, the as-generated cotton NFC aerogel that fabricated by freeze drying the 0.5 wt% cotton NFC suspension is denoted as CoNA-05. Characterization. The structures of the NFC and aerogels were investigated using a scanning electron microscopy (SEM; Quanta200, FEI, USA) and a field emission scanning electron microscopy (FE-SEM; Sigma300, Zeiss, Germany). XRD diffractogram was tested by an X-ray diffractometer (D/max 2600, Rigaku, Japan) with CuKα radiation (λ = 1.54178 Å) at 40 kV and 150 mA with a scan rate of 2° min−1. The dynamic rheological properties of the NFC aqueous suspension were measured using a rheometer (AR2000ex, TA instruments, America) with 40 mm-diameter flat parallel plates. The gap was 1.0 mm and the frequency sweeps were conducted over the range of 0.01–100 rad s−1 at a controlled strain amplitude of 0.05% at 25 °C. The compressive tests of the NFC aerogels were conducted by a mechanical testing machine (AI-7000S TC160701511, Gotech, Taiwan) equipped with two flat-surface compression stages. The strain ramping rate was maintained at 1 mm min−1. The aerogels were directly used for the compression test without any other treatments. The thermal stability of the aerogels was tested by a thermogravimetric analyzer (Pyris 6, Perkin-Elmer, USA) in nitrogen atmosphere and air atmosphere with a heating rate of 10 °C min−1. The thermal conductivity of the aerogels was tested by the transient hot-wire method using a thermal conductivity tester (TC3020, Xi’an Xiatech Electronic Technology Co., Ltd, China). The thermographic images were obtained by an infrared thermal camera (Ti200, Fluke, USA). The α-cellulose content of the cellulose pulps was determined by extraction with 17.5 wt% NaOH based on a previous reported method.34 Bulk Density and Porosity. The bulk density of the NFC aerogels was calculated based on the weight and volume of the aerogels. The porosity of NFC aerogels was obtained by using Equation (1): Porosity = (1−ρb/ρs) × 100%

(1)

where ρb is the bulk density and ρs is the skeletal density of the NFC aerogels. The skeletal density of the aerogels were obtained by helium pycnometry using a densimeter (3H-2000TD, Beishide Instrument-S&T. (Beijing) Co., Ltd) at room temperature. 6 ACS Paragon Plus Environment

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Weight loss and volume shrinkage ratio. The changes in weight and volume of the NFC aerogels when exposed to high temperatures were measured. The weight and volume of the NFC aerogels before thermal treatment were termed as Wa and Va, respectively. The weight and volume of the NFC aerogels after thermal treatment for 3 h were recorded as Wb and Vb.. The weight loss ratio and volume shrinkage ratio were calculated by using Equation (2) and Equation (3), respectively. Weight loss ratio = Wb / Wa× 100%

(2)

Volume shrinkage ratio = Vb./ Va× 100%

(3)

RESULTS AND DISCUSSION Cotton NFC was fabricated using a chemical pretreatment combined with nanofibrillation by a high-speed blender. After chemical purification, the α-cellulose content of cotton cellulose pulp (~100%) was significantly higher than that of wood- (83.6%), bamboo- (84.3%), and rice straw- (82.1%) cellulose pulps. Because the cotton cellulose pulp was nearly pure and possessed no additional impurities such as hemicellulose, strong hydrogen bonds existed between the cotton cellulose nanofibers within the pulps. Thus, the as-generated samples after high-speed blender nanofibrillation also contained a large number of microscale nanofiber bundles (Figure 1a). For convenience, we also denote these cotton micro-/nanofibrillated cellulose fibers as cotton NFC herein. The cotton NFC had a high aspect ratio and a web-like entangled structure (Figure 1a). The as-prepared 0.5 wt% cotton NFC aqueous suspension was stable, and no precipitation was observed even after storing the sample for several months (Figure 1a inset). The dynamic rheological performance of the NFC aqueous suspensions is given in Figure 1b, c. The storage modulus (G′) of the NFC suspensions showed a frequency-independent characteristic in the low angular frequency region. As the angular frequency increases, however, the suspensions became frequency sensitive and the G′ increased. This result is primarily due to the increased formation rate of entangled and cross-linked NFC networks at high frequencies.



Figure 1. Fabrication of CoNAs. (a) FE-SEM image of cotton NFC. Inset is a photograph of 0.5 wt% cotton NFC suspension. (b) G′ and (c) G′′ as a function of frequency for the 0.5 wt% cotton NFC suspension. (d) XRD pattern of CoNA-05, WoNA-05, BoNA-05, and RsNA-05. (e) Relative crystallinity of CoNA-05, WoNA-05, BoNA-05, and RsNA-05. (f) Density and porosity of the NFC aerogels. (g–l) SEM images of the CoNA-05: (g, j) surface structures, (h, k) cross-sectional structures, (i, l) vertical-sectional structures. Inset of (g) is a photograph of CoNA-05.

Owing to the length of the fibers and their ability to form strongly entangled networks, the G′ of the cotton NFC suspension in the high angular frequency region was greater than 8 ACS Paragon Plus Environment

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that of the wood, bamboo and rice straw NFC suspensions. A similar tendency was observed for the loss modulus (G″) versus frequency, as illustrated in Figure 1c. For most of the suspensions, the G′ was slightly higher than the G″, indicating that the suspensions would display hydrogel characteristics upon further increase of the NFC content. The suspension with 0.5 wt% NFC exhibited a good fluidity and contained a large amount of free water. Thus, a large number of ice crystals could form during the subsequent frozen process, and lightweight aerogels were obtained after freeze-drying. Because of the high purity of cotton NFC, the CoNAs displayed significantly higher crystallinity (86.9%) than that of WoNAs (73.3%), BaNAs (64.9%), and RsNAs (70.1%) (Figure 1d, e). The cotton NFC was sufficiently robust to support aerogels with little volume shrinkage during the freeze-drying process, resulting in CoNAs with low density and high porosity (Figure 1f). As shown in Figure 1g inset, a cube-like CoNA-05 with a sample volume of 18 cm3 rested stably on a leave of a plant, demonstrating the lightweight characteristic of the aerogels. The CoNA-05 had a density of 4.97 mg cm−3, which is lower than those of WoNA-05 (7.51 mg·cm−3), BaNA-05 (5.33 mg·cm−3) and RsNA-05 (5.92 mg·cm−3). The porosity of the CoNA-05 was 98.5%, which is close to those of the other aerogels. For a better understanding of the microstructure of th-e aerogels, the surface and cross-sectional structures of the CoNA-05 were examined (Figure 1 g–l). Owing to the formation of a large number of ice crystals during the frozen process, the cotton NFC was confined in the interstitial regions between the ice crystals. After freeze-drying the NFC self-aggregated, resulting in the CoNA-05 exhibiting a porous cellular structure comprising interconnected cell walls with a high degree of pore interconnectivity. The cell walls of the CoNA-05 were organized by the long and entangled cotton NFC, wherein micro- and nano-scaled pores existed among the entangled NFC throughout the entire cell wall structure.



a

b

c

d

Figure 2. Mechanical properties of various NFC aerogels. (a) Compressive stress–strain curves of NFC aerogels. (b) Compression strength and specific compression strength (SCS) of the NFC aerogels. (c) Compression modulus and specific compression modulus (SCM) of the NFC aerogels. (d) Modulus (log (E)) as a function of density (log (ρ)) of CoNAs.

The mechanical properties of the NFC aerogels are shown in Figure 2. The low-density CoNA-05 was quite soft and displayed a relatively low mechanical strength. Specifically, CoNA-05 had a compressive strength and a compressive modulus of 50.9 and 0.84 kPa, respectively, which were lower than those of WoNA-05, BaNA-05 and RsNA-05 (Figure 2a-c). The density, compressive strength and modulus of the CoNAs were improved by increasing the cotton NFC concentration in suspension prior to freeze-drying (Figure S1). Generally, the compressive modulus (log (E)) increased with increasing the density (log (ρ)) of the CoNAs (Figure 2d). Owing to the web-like entangling of the long and high-aspect-ratio NFC, the CoNAs showed a high flexibility and could be bent (Figure 3a, b) and compressed (Figure 3c) without structural damage or destruction. Compared with WoNA-05, the CoNA-05 had a high elasticity that allowed for large deformations without fracture and 10 ACS Paragon Plus Environment

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exhibited satisfactory deformation recovery ability. As shown in Figure 3c, the CoNA-05 bore a compression strain as high as 80% with a rapid recovery to approximately 60% of its original height after compression release. The ratio of the elastic resilience was increased to 80.4% after the sample was left standing for 24 h. Further, the CoNA-05 was compressed and released

repeatedly

and

an

equivalent

recovery

profile

was

observed

for

10

compression-release cycles, demonstrating its superior elasticity and robustness. In comparison, the ratio of elastic resilience of the WoNA-05, BaNA-05, and RsNA-05 was apparently lower during the 10 compression-release cycles (Figure 3d-f). a

b

c

d

e

f

Figure 3. (a, b) Digital photograph of the CoNA-05 illustrates the mechanical flexibility of the CoNAs. (c–f) The ratio of elastic resilience of the NFC aerogels after repeated compression and release. The (c) CoNA-05, (d) WoNA-05, (e) BaNA-05 and (f) RsNA-05 were compressed to 20% of their original height and then the compression was released, and the aerogels were allowed to stand for 24 h. The compression-release cycle was repeated 10 times.



The thermal stability of nanocellulose aerogels is critical, and especially for aerogel applications in high-temperature conditions. The thermogravimetric analysis (TGA) curves of the aerogels are shown in Figure 4. The CoNA-05 demonstrated a significantly higher thermal stability than the other NFC aerogels in both air and nitrogen. The thermal degradation temperatures of CoNA-05 were 318.8 °C in air and 333.2 °C in nitrogen. The high thermal stability of the CoNAs was mainly owing to the high purity (~100% cellulose) and high crystallinity of the cotton NFC. The thermal stability of WoNA-05, BaNA-05 and RsNA-05 were lower than that of CoNA-05 because thermally sensitive hemicellulose was present and covered the surface of the NFC. Generally, the thermal degradation of hemicellulose primarily occurs at 220–315 °C, while the weight loss of cellulose occurs at 315–400 °C.29 Thus, the CoNAs of pure cellulose displayed a higher thermal stability than those aerogels constructed by hemicellulose-containing NFC. a

b

c

d

Figure 4. TG curves (a, c) and DTG (b, d) curves of various NFC aerogels in (a, b) air and (c, d) nitrogen gas conditions.


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a

CoNA-05

WoNA-05 BaNA-05 RsNA-05

b

RT

150°C

200°C

250°C

300°C

c

d

Figure 5. Thermal stability of NFC aerogels. (a) Digital photographs, (b) △E, (c) weight loss ratio and (d) volume shrinkage ratio of various NFC aerogels when heated at different temperatures.

The thermal stability of the CoNAs were also compared to that of other NFC aerogels by heating the samples at various temperatures in an argon atmosphere (Figure 5). Little change was observed in any of the aerogels when heated to 150 °C, where most of the aerogel weight loss was owing to water vapor evaporation. When the temperature was increased to 200 °C, the WoNA-05, BoNA-05, and RsNA-05 exhibited thermal degradation, becoming yellow in appearance and showing a high chromatic aberration (△E), a large weight loss and visible shrinkage; primarily the result of thermal degradation of the hemicellulose in the NFC. In contrast, the CoNA-05 demonstrated a satisfied thermal stability with little color change, weight loss and volume shrinkage. Even when the temperature was increased to 250 °C, the CoNA-05 still displayed a high thermal stability with a △E of 20.5, a weight loss ratio of 5.9 % and a volume shrinkage ratio of 7.8 %. When the temperature was further increased to 300 °C, the cellulose of the cotton NFC was thermally degraded, and all the aerogels exhibited visible shrinkage and were partially converted to carbon aerogels. 13 ACS Paragon Plus Environment


a

Without sample Top view

With sample Top view

With sample Side view 74.8

Hot plate 20.4 29.3

Cold plate

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-0.8

b

c

Figure 6. Thermal insulation performance of CoNAs. (a) Thermographic images of the CoNA-05 when in contact with heating/cooling plates at different temperatures. (b) Thermal conductivity of the CoNA-05 versus temperature. (c) Thermal conductivity of the CoNAs versus porosity.

Owing to its superior thermal stability and high porosity, CoNA is a promising candidate for thermal insulators used in severe conditions. The thermographic images in Figure 6a show the thermal insulation performance of the CoNAs. A cube-like CoNA-05 (3 cm × 3 cm × 2 cm) was placed on the surface of a hot plate at ~70 °C and a cold plate at ~0 °C, where it 14 ACS Paragon Plus Environment

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showed excellent thermal stability without any observable size and structural change. The temperature of the top surface of the CoNA-05 was near the environmental temperature no matter if the bottom surface was in contact with a hot plate or a cold plate, signifying that the heat was resisted and diffused within the CoNA network. As shown in Figure S2, the thermal conductivity of the CoNA-05 was 0.0442 W m−1 K−1 and similar with that of WoNA-05 (0.0396 W m−1 K−1), BaNA-05 (0.0425 W m−1 K−1) and RsNA-05 (0.0455 W m−1 K−1) at 25 °C. With increasing temperature, the thermal conductivity of the CoNA-05 increased slowly (Figure 6b). Even at 200 °C, the CoNA-05 only had a thermal conductivity of 0.0545 W m−1 K−1, exhibiting its excellent thermal insulating capability. This proved that CoNA insulators could be used in high temperature conditions that were challenging for low thermal-stability

materials

such

as

2,2,6,6-tetramethylpiperidine-1-oxyl

radical

(TEMPO)-oxidized NFC aerogels and sulfuric acid-hydrolyzed cellulose nanocrystal aerogels.5 As illustrated in Figure 6c, the porosity played a critical role on the thermal conductivity of the CoNAs. The higher the porosity of the CoNAs was, the more air was filled and the better the thermal insulating performance. Thus, the thermal conductivity of the CoNAs were decreased when increasing the CoNA porosity.

CONCLUSIONS In summary, we have fabricated a new type of nanocellulose aerogel using cotton NFC as the building blocks. Owing to the high purity and high crystallinity of cotton NFC, the CoNAs demonstrated superior structural and functional characteristics to those of nanocellulose aerogels such as WoNAs, BaNAs and RsNAs. The CoNAs had a low density of 4.97 mg cm−3 after freeze-drying the 0.5 wt% NFC suspension. Specifically, the CoNAs were soft and flexible and exhibited satisfactory deformation recovery ability. Moreover, CoNAs demonstrated a high thermal stability with a thermal degradation temperature above 310 °C in both air and nitrogen. Owing to its high thermal stability and high porosity, the CoNAs showed excellent thermal insulation attributes even under extreme temperatures. Thus, the integration of multiple advantageous characteristics into a single material was achieved for nanocellulose aerogels by using cotton NFC as the building blocks. 15 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information. Densities and mechanical properties of various CoNAs.

ACKNOWLEDGMENTS. This work was supported in part by the National Natural Science Foundation of China (No. 31770594), Young Elite Scientists Sponsorship Program by CAST (No. 2017QNRC001), Funds supported by the Fok Ying-Tong Education Foundation, China (No. 161025), Natural Science Foundation of Heilongjiang Province, China (No. C2017006), Fundamental Research Funds for the Central Universities (No. 2572018CG01) and the Startup and Development Funds from Long Island University.


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TOC Graphic

Synopsis: Sustainable aerogels were constructed by cotton nanofibrillated cellulose, which were lightweight, flexible, thermally-stable and thermally-insulating.


Lightweight, Flexible, Thermally-Stable, and Thermally-Insulating

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