3.1 Morphology of cellulose/expandable graphite aerogels
The microstructures of the cellulose/EG aerogels with different blend ratios between two components were detected using SEM (Fig. 2). The prepared samples had low density and the samples could be balanced on the surface of a flower (as shown in Fig. 2a), which indicated that the samples were light. As can be seen from Fig. 2b and 2c, EG was interspersed in the cellulose framework. The thickness and width of the EG monolayer was approximately 9.86 and 98.11 µm, respectively. Ice templates were used to prepare samples with air channels. An orientation porous structure was observed in all samples, as shown in Fig. 2d–i, wherein the red arrow is used to note the orientation direction and the broken line is employed to indicate the porous boundary. Note that ice crystals formed in the polymer solution. The crystals were removed during the freeze-drying process, thus resulting in the formation of an orientation porous structure. The distance between the two broken lines represented the porous diameter, which decreased with an increase in the EG concentration in the aerogels. The decrease in porous diameter was caused by EG, which prevents the formation of ice crystals because of its large size compared with cellulose molecules. The porous diameter could be controlled through the EG concentration in the mixture, which influences the filtration efficiency of the aerogel.
3.2 Mechanical properties of cellulose/expandable graphite aerogels
Mechanical properties are important parameters that determine the direction of the application and its service life. Compression experiments were conducted on the prepared samples along the porous direction, as shown in Fig. 3. Figure 3a shows the stress–strain curves of the cellulose composite aerogels. Evidently, the curves that the stress increased along with the increase in strain. The strains of the composite aerogels were elastic deformations below 8% and inelastic deformations above 8%. Figure 3b shows that with the addition of EG, the elastic modulus of the cellulose composite aerogels gradually increased. This is because the addition of EG reduced the pore sizes inside numerous of the microporous structures of the cellulose composite aerogels, thus rendering the structures denser and increasing the elastic modulus of the cellulose composite aerogels. When the strain of the cellulose composite aerogel was 75%, the corresponding stresses when the EG content gradually increased from 0–50% were 0.4832, 0.5498, 0.7756, 0.6418, 0.5669 and 0.578 MPa, respectively. The addition of EG considerably improved the mechanical properties of the cellulose composite aerogels, which lays a mechanical foundation for its future flame-retardant and filtration applications.
3.3 Thermodynamic properties of cellulose/expandable graphite aerogels
Figure 4a shows the TG curves of the prepared cellulose composite aerogels and EGs. Evident from the Fig. 4a, the thermal decomposition process of EG can be divided into three stages. In the first stage (Ⅰ), the mass loss accounted for approximately 1% at 0–230°, which is the direct volatilization of small molecules and water (Kaczor et al. 2021). In the second stage (Ⅱ), the mass loss was approximately 25% at 230–500 ℃, which was primarily due to the expansion of graphite and was the largest of the three stages. The third stage (Ⅲ) was at 700–800 ℃ and the thermal weight loss tended to be flat, primarily because of the loss of carbon-containing substances. The degradation temperature of cellulose /EG composite aerogel were lower than that of pure cellulose aerogel, as shown in Fig. 4a. This is because EG begins thermal decomposition at approximately 230°, which affects the degradation temperature of the composite aerogels.
The DSC curve of the cellulose/EG composite membrane is shown in Fig. 4b. Below 100°C, the endothermic phenomenon was caused by the evaporation of water and the endothermic peak of the pure cellulose aerogels was the largest. The endothermic peak of EG was sharper at 230°C, which is the initial expansion temperature of EG. Note that EG does not exhibit obvious expansion in the range of 0–230°C (Alonso et al. 2022). With an EG content of 10–50%, the water evaporation temperature of the composite aerogels decreased, which could be caused by the fact that EG enhances the thermal conductivity of the composite aerogels and accelerates the heating rate of the composite systems. The endothermic peak of the composite aerogels was very weak at approximately 175°C owing to the endothermic phenomenon caused by the degradation of small molecular cellulose. The DSC curve corresponds to the TG curve and the addition of EG had a certain influence on the thermal properties of cellulose aerogels.
3.3 Crystallization properties of cellulose/expandable graphite aerogels
Figure 5a shows the XRD patterns of cellulose and the cellulose aerogels with different EG contents. The characteristic peaks of cellulose could be observed at approximately 2θ = 12.28°, 20.50°, and 22.5° on the curve of the cellulose aerogel with an EG content of 0% (Fig. 5a), which represent the (1_10), (110), and (020) planes of cellulose (Nomura et al. 2020). Figure 5b is the XRD pattern of EG; herein, two primary peaks at approximately 2θ = 25.46° and 56.63° were observed, representing the (002) and (004) planes of EG, respectively. Additionally, in Fig. 5a, the presence of the height near 2θ = 25.46° denotes EG inclusion in the cellulose aerogel (Nwuzor et al. 2022). Evidently, with an increase in the EG content, the characteristic peaks of EG in the composite aerogels shifted to the left. This was caused by the growth of ice crystals during the preparation process, which expanded the interlayer spacing of EG, as shown in Fig. 5a (10–5 %). Interestingly, in the composite aerogels with EG contents of 4 % and 5 %, new EG peaks appeared at approximately 26.2° (4 %) and 25.9° (5 %), which were attributed to the fused-ring structure carbon atoms were Oxidation produces wrinkles. (As the expandable graphite interlayer spacing expands, more interlayer surface area is exposed, which is easily oxidized.)
3.4 Flame-retardancy of cellulose/expandable graphite aerogels
The effect of EG on the flame-retardant properties of aerogels was investigated by firing prepared samples (Fig. 6). In Fig. 6a, the effect of the EG content on the flame-retardant performance of the aerogels was judged by measuring the residual area of the composite aerogel after burning it under a butane flame for 15 s. Evidently, the burned area of the cellulose aerogel without EG was nearly zero. For composite aerogels containing EG, no combustion was observed after contact with an open flame. The color of the residues after being extinguished was black and the residual areas of the five aerogel samples with different EG contents after burning were 164.57, 149.91, 241.49, 142.97, and 195.57 mm2. The residual area of the sample with 30% EG content was the largest, which indicates that that it had the best flame-retardant effect. This result indicates that the flame-retardancy of EG-doped cellulose aerogels was significantly improved.
Figure 6b is the self-extinguishing image for composite aerogels with different EG concentrations after being removing from the flame. For the pure cellulose aerogel, the self-extinguishing time was 5.41 s. When 10% EG was added to the cellulose aerogel, the self-extinguishing time extended to 18.91 s. The increase in burning time is attributed to the slow burning speed caused by the addition of EG. EG could absorb heat produced from burning, thereby breaking its continuous process and reducing the burning rate. The composite aerogel has a very small flux, thus causing the flame to burn slowly. However, when the EG content was 20%, the self-extinguishing time of the composite aerogel was only 0.363 s, which is because of the large amount of EG swelling that prevented air from entering the composite aerogel. When the EG content was 30%, the self-extinguishing time of the composite aerogel was approximately 0.155 s, which was close to the best effect. Thus, the addition of EG significantly improved the flame-retardant properties of the cellulose aerogels.
The flame-retardance of EG is attributed to the solidified phase flame-retardant mechanism, which realizes flame-retardance by delaying or interrupting solid substances to generate flammable substances (Xia et al. 2022; Zhang et al. 2022). Evidently, the EG expanded after being heated to form interwoven worms, as shown in Fig. 7a and b, and the pores of EG in this shape were denser and more uniform in the aerogel structures. The porous carbonized layer had sufficient thermal stability to isolate the flame-retardant body from the heat source, thereby delaying and terminating the decomposition of the polymer (Zhang et al. 2021b).
Moreover, EG was filled in the cellulose skeleton in a sheet shape (Fig. 7a), and the thickness of the sheet was approximately 10 µm, as observed from the Fig. 2g. After exposure to an open flame, the volume of EG increased rapidly and the generated swelling material covered the surface of the porous membrane, thus changing from a sheet to a worm shape (Wang et al. 2018). The diameter reached 98.11 µm and the volume expanded instantly; hence, the structure was loose, porous, and curved, thereby forming a very good thermal insulation layer, which can quickly suffocate the flame and block the entry of air and heat radiation, as shown in Fig. 7c and 7d. The heat release rate was low and the generated flue gas was small. In addition, the exposed acid radicals during the expansion process in EG promoted the carbonization of cellulose, thus achieving the effect of flame-retardancy.
3.5 Filtration performance of cellulose/expandable graphite aerogels
Evident from Fig. 8a, the filtration mechanism of the cellulose/EG composite aerogel was micropore interception. Cellulose/EG composite aerogels with a large number of directional pores were prepared using an ice template method. When air flows through the micropores because of the numerous interconnected networks in the cut holes in the scaffold, the particles are intercepted in the interconnected networks by virtue of the larger specific surface area of the directional pores. Thus, the addition of EG can further increase the number of micropores and their specific surface area, thereby further improving the filtration effect.
The abovementioned test results on mechanical properties and flame-retardancy indicated that the best properties and flame-retardancy was achieved with a composite cellulose aerogel with 30% EG. Therefore, the prepared aerogel with 30% EG was chosen to study the filtration performance. Using a NaCl gas-phase filter medium, the filtration efficiency of pure cellulose aerogels with particle sizes of 0.3–0.5 µm was 84%, those with particle sizes of 0.5–1.0 µm was 91%, and those with particle sizes exceeding 1.0 µm was 97% (Fig. 8b). The composite aerogels with EG considerably improved the barrier efficiency of small particle substances with sizes of 0.3 and 0.5 µm in an NaCl gas phase to 98% and 99%, respectively. Cellulose aerogels are essentially flat (96%). In general, the cellulose/EG composite aerogels have excellent filtration properties, particularly when filtering impurities containing small particles.