Spray freeze-dried nanofibrillated cellulose aerogels with thermal superinsulating properties
Introduction
Aerogels are highly porous materials with very low density and a large specific surface. They are prepared by replacing the solvent of a gel with gas with very moderate shrinkage of gel solid network. They form a class of materials showing potential for a wide range of applications (Hüsing & Schubert, 1998a) due to their unique optical (Platzer, Wittwer, & Mielke, 1986), mechanical (Gronauer, Kadur, & Fricke, 1986), and thermal properties (Fricke, 1988). Though the first aerogel was a silica aerogel made by (Kistler, 1931), intense research for new aerogels only began in the 1970’s. Since then, inorganic aerogels have been extensively studied and used in several applications, including thermal insulating materials (Fricke, 1988), chemical adsorbents (Nguyen et al., 2014), drug carriers (Alnaief et al., 2012, Pierre and Pajonk, 2002), gas or liquid purifiers (Korhonen, Kettunen, Ras, & Ikkala, 2011) or in the aerospace industry (Jones, 2006).
In the context of a sustainable society, research is oriented towards the creation of aerogels from renewable resources. Thus, hybrid aerogels and bioaerogels are beginning to be studied (Alnaief, Alzaitoun, García-González, & Smirnova, 2011; Bendahou, Bendahou, Seantier, Grohens, & Kaddami, 2015; Borges et al., 2011, Cai et al., 2012; García-González, Alnaief, & Smirnova, 2011; Kistler, 1932, Koga et al., 2013). Bioaerogels are a new generation of aerogels prepared from biomass and most of them are prepared from polysaccharides. Cellulose is the most abundant polysaccharide on earth and it has been used for a large range of applications due to its renewability, biocompatibility and biodegradability (Huang, Yuan, & Chen, 2006). These properties are the reasons why, during the past decade, bioaerogels from cellulose have been developed (Cai, Kimura, Wada, Kuga,& Zhang, 2008; Gavillon & Budtova, 2008; Innerlohinger, Weber, & Kraft, 2006), cellulose derivatives (Fischer, Rigacci, Pirard, Berthon-Fabry, & Achard, 2006; Tan, Fung, Newman, & Vu, 2001) or nanofibrillated cellulose (NFC) (Pääkkö et al., 2008; Sehaqui, Zhou, & Berglund et al., 2011).
Some aerogels have a very low thermal conductivity at atmospheric pressure (Kistler, 1934). As a result, they form a new class of solids showing potentialities for insulating applications. Increasing research activity in bioaerogels has tried to improve their thermal properties in order to develop superinsulatingbioaerogels.
Thermal transfer in insulation materials can be divided into three contributions: conduction in a solid network, conduction in a gaseous phase and radiation through pores. The total thermal conductivity, λ, can be described by a parallel flux model (Eq. (1)) (Ebert, 2011) and calculated as the sum of each contribution (Fricke, 1986):λ = λsolid + λgas + λrad
In aerogels, solid conduction is reduced compared to non-porous materials, because of the large quantity of pores which restricts the propagation of phonons in the aerogel skeleton (Fricke & Emmerling, 1992). The gas contribution is due to the elastic collision between gas molecules. To reduce it, a 3D mesoporous structure having specific size can be used. In porous materials, this specific size is characterized by the Knudsen number which is the ratio of the free mean path of gas molecules, lg and the diameter of the pore, d (Eq. (2)) (Kaganer, 1969).Kn = lg/d
Biobased aerogels are usually obtained from hydrogels since most biopolymers are soluble in water. Drying techniques such as freeze-drying, produce materials initially known as cryogels, now also termed aerogels (Pierre & Pajonk, 2002). During the freeze-drying technique, the gel is first frozen and then dried under low pressure by sublimation of the frozen liquid. The problem is that the gel network may be destroyed by nucleation and growth of solvent crystals (Tamon, Ishizaka, Yamamoto, & Suzuki, 2000). This effect tends to create structures with very large pores. Supercritical drying can avoid the collapse of aerogel structures during liquid removal by removing the liquid/vapor surface tension and thus achieving structures with nanoporosity (Kistler, 1931). Supercritical drying is, therefore, the most commonly used method to make aerogels (Baetens, Jelle, & Gustavsen, 2011). Only a few thermal superinsulating bioaerogels have been reported to date and they were made by supercritical drying. One was prepared from pectin by Rudaz et al. (2014), it has a pore size around 40 nm and a thermal conductivity of 0.016–0.020 W/(m K). Another example is a NFC bioaerogel with a thermal conductivity of 0.018 W/(m K) and pore sizes of 2–50 nm prepared by Kobayashi, Saito, & Isogai (2014). The size of the pores is the reason why there is no aerogel made by freeze-drying with superinsulatingproperties in the literature. The best values reported for aerogels prepared by freeze-drying are around 0.029–0.030 W/(m K) (Nguyen et al., 2014; Shi, Lu, Guo, Sun, & Cao, 2013). The major drawback of this technique is that resulting macroporous structures yield relatively high thermal conductivities because the contribution of gas conduction remains rather high.
In this study, we aim at overcoming this drawback by implementing spray freeze-drying technique (SFD) to reduce the aerogel pore size down to nanometric scale. SFD is an emerging technology utilized in many industries for the production of powders (Mujumdar, 2014). This process has two main steps: first, the solution is sprayed in a cryogenic solvent and the droplets are instantly frozen into micro granules. In the second step, frozen droplets are dried by sublimation. Originally, this technique was used to study the surface area of protein particles (Benson & Ellis, 1948) and afterwards it was used for the production of sub-micron powders (Werly & Baumann, 1964). Later, SFD has been used for a wide range of applications such as food elaboration (Malecki, Shinde, Morgan, & Farkas, 1970), pharmaceutical industry (Amorij et al., 2007; Cheow, Ng, Kho, & Hadinoto, 2011) or ceramic fabrication (Capadona, Shanmuganathan, Tyler, Rowan, & Weder, 2008; Shanmugam, 2015). More recently, it has also been used to prepare aerogel microspheres from NFC (Cai et al., 2014).
The advantage of this technique is that, thanks to the fast freezing, the gel structure is protected against deformation due to ice crystal growth. It is precisely this aspect that inspired us to adapt this method to the preparation of monolithic bioaerogels. Their structural, mechanical and thermal insulation properties were compared with those of bioaerogels made by CFD. The morphologies of these aerogels were examined using scanning electron microscopy (SEM) together with microstructural characteristics that were determined by nitrogen adsorption. Finally, mechanical and thermal insulation properties were determined. This work demonstrates that the preparation of bioaerogels with nanoporous structures by the SFD technique significantly improved thermal insulation properties.
Section snippets
Materials
A 1 wt% aqueous suspension of NFC produced by TEMPO-mediated oxidation (Saito, Nishiyama, Putaux, Vignon, & Isogai, 2006) of spruce wood pulp was received from Swiss Federal Laboratories for Materials Science and Technology (EMPA, Dübendorf, Switzerland). The suspension was further characterized in order to confirm the supplier specifications. The 1.12 mmol/g carboxylate content was confirmed by conductimetric titration (SCAN-CM65:02, 2002). The dimensions were determined by AFM. The diameter and
Texture and density of the bioaerogels
Bioaerogels were prepared from NFC suspensions by CFD and SFD methods. Monolithic and homogeneous aerogels were successfully obtained, as shown in Fig. 1 (see also Fig. S4 in Supporting information).
The bulk density of bioaerogels as a function of the NFC concentration is illustrated in Fig. 3. The theoretical density (red solid line in Fig. 3) was calculated assuming there was no volume shrinkage between the gel (precursor of the aerogel) and the aerogel (dried sample). The bioaerogels
Conclusions
In this article, bioaerogels were prepared from NFC suspensions by osmotic concentration followed by drying step done by CFD and SFD methods. Monolithic and homogeneous aerogels were successfully obtained. All bioaerogels have a very low density (0.012–0.033 g/cm3) and a high porosity (98–99%). SEM characterization highlighted microstructure differences between both aerogels. When bioaerogels prepared by CFD exhibit a 2D-sheet-like morphology with macropores, bioaerogels made by SFD yield a
Acknowledgments
This work was supported by the French Environment and Energy Management Agency (ADEME) and Bretagne region. The authors thank Anthony Magueresse (IRDL, UBS, Rue de Saint Maudé, Lorient, France) for the SEM images.
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