Abstract
We report the fabrication of anisotropic lightweight composite foams based on commercial colloidal silica particles and TEMPO-oxidized cellulose nanofibrils (TOCNF). The unidirectional ice-templating of silica-TOCNF dispersions resulted in anisotropic foams with columnar porous structures in which the inorganic and organic components were homogeneously distributed. The facile addition of silica particles yielded a significant enhancement in mechanical strength, compared to TOCNF-only foams, and a 3.5-fold increase in toughness at a density of 20 kg m−3. The shape of the silica particles had a large effect on the mechanical properties; anisotropic silica particles were found to strengthen the foams more efficiently than spherical particles. The water uptake of the foams and the axial thermal conductivity in humid air were reduced by the addition of silica. The composite foams were super-insulating at dry conditions at room temperature, with a radial thermal conductivity value as low as 24 mW m−1 K−1, and remained lower than 35 mW m−1 K−1 up to 80% relative humidity. The combination of high strength, low thermal conductivity and manageable moisture sensitivity suggests that silica-TOCNF composite foams could be an attractive alternative to the oil-based thermal insulating materials.
Highlights
Biopolymers have recently emerged as a potentially viable alternative to oil-based thermal insulators (Zhao et al 2018), e.g. expanded polystyrene (EPS) (Horvath 1994; Jelle 2011), a widely used material for domestic insulation
We have produced silica-TEMPO-oxidized cellulose nanofibrils (TOCNF) anisotropic foams by directional ice-templating of aqueous dispersions of cellulose nanofibrils carboxylated via TEMPOmediated oxidation (TOCNF) (Fig. 1a), and two different types of industrially produced colloidal silica particles
The amount of TOCNF in the initial dispersions and in the foams was kept constant at 0.4 wt%, whereas the amount of colloidal silica was varied
Summary
Biopolymers have recently emerged as a potentially viable alternative to oil-based thermal insulators (Zhao et al 2018), e.g. expanded polystyrene (EPS) (Horvath 1994; Jelle 2011), a widely used material for domestic insulation. Insulating and strong CNM-based materials with direction-dependent heat transport properties can be produced using processing routes that take advantage of the intrinsically anisotropic properties of nanocellulose (Dri et al 2014). An example of such routes is directional ice-templating, called freezecasting (Deville 2018). Ice-templated anisotropic nanocellulose-based foams were shown to display very low thermal conductivity (down to 18 mW m-1 K-1) perpendicularly to the fibrils’ and pores’ axes (Wicklein et al 2014). Nanocellulose-based foams are moisture sensitive (Saito et al 2007; Apostolopoulou-Kalkavoura et al 2018; Guo et al 2018a; Illera et al 2018), resulting in low strength and increased thermal conductivity at high relative humidity (RH) values
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