Figure 2a shows the incineration ash as received, and Fig. 2b and c show the images of the fabricated SA granulate and SA powder after ball milling, respectively. The SA powder is liquid-like and has a slight change in color from white to translucent after ball milling. The color change is attributed to the low density and high porosity of the SA powder. Its density is measured to be around 0.08 g/ml and porosity is calculated to be 96.36%. Its thermal conductivity is measured to be around 0.025 W/mK.
The XRD pattern of SA powder is shown in Fig. 3. The XRD result shows a broad peak between 20˚ to 30˚ without any sharp peaks, which indicates that the SA powder is fabricated in amorphous structure . Thus, the safety of the SA powder can be assured with no or minimal chronic effects for application in building insulation .
Fig. 4 shows typical nitrogen adsorption and desorption isotherms of fabricated SA powder. The pore size distribution of the silica aerogel calculated from the adsorption and desorption data, which leads to values of the core pore radius, is shown in Fig. 5 (a and b). The Barrett-Joyner-Halenda (BJH) adsorption average pore radius (2V/A) is 82.3 Å, while the BJH desorption average pore radius (2V/A) is 64.1 Å. The BET surface area is 786 m2/g. The average pore width (4V/A by BET) is 20 nm with a narrow distribution and the BJH cumulative pore volume of pores between 0.85 nm and 150 nm radius is about 3.9 cm3/g. The surface area of the fabricated SA powder is 786 m2/g.
Figure 6 shows the result of the TGA of the fabricated SA powder. TGA shows the thermal stability of the fabricated SA powder. A slight weight loss is observed on the curve at 75–125 ˚C, which may be attributed to the evaporation of water trapped by the SA. The water content is less than 2%. A 9% weight loss is found when increasing the temperature to 375 ˚C to 775 ˚C. The onset temperature is around 418 ˚C at which the SA exposed to heat undergo the degradation. This is due to the oxidation of silicon-carbon bond (Si-CH3) to silanol groups (Si-OH) and evaporation of organic components at the SA surface . And after the heat treatment, the hydrophobic SA is converted into hydrophilic SA.
The workability of the paint was measured through viscosity. Different percentage of silica aerogel from 5–20% in volume was added into the paint, and the viscosity result is shown in Fig. 7. It shows that there is a sharp increase in viscosity after adding aerogel and the viscosity further increases with the increase of the aerogel content from 5–20%. The adding of hydrophobic SA powder can reduce the agglomeration problem of the nano-sized filler, and at the same time, enhance the interfacial interactions . This enhancement is mainly attributed to the improved mechanical anchoring between the porous SA powder and the paint matrix by mechanical interlocking generated by the pores.
The adding of SA powder not only affects the viscosity of the paint, but also contributes in the shift of surface energy . The contact angle of the paint with aerogel additives were also measured and the results are shown in Fig. 8. Figure 8 shows the influence of surface energy on wettability. As the SA content increases from 5–20%, the contact angle increases steadily, from 52.47 degrees to 64.38 degrees, indicating a better water repellence of the paint, improved self-cleaning property and mildew growth prevention. The aging and service life of the paint can thus be prolonged. This improvement is due to the addition of hydrophobic aerogel powder. The increase of the hydrophobic SA content corresponding to the increase of contact angles indicates that chemical is mainly responsible for the increase in hydrophobicity rather than surface roughness effect.
Figure 9 shows the thermal conductivity of paints with different SA content. As shown in Fig. 9, the thermal conductivity of the paints decreases with the increase of SA content owing to its superior thermal conductivity (0.025 W/mK). Thus, the enhancement of this property is beneficial for reduction of the conduction and dissipation of heat . As a result, the thermal insulation properties would be improved. These findings can be used to demonstrate the effects of SA addition on the thermal insulation performance of paints. However, due to low density of SA powder, the volume of added SA is limited by 20 %. When the content of the SA is further increased, the defects such as cracks and small lumps will be generated from the dried coating.
The thermal insulation performance of the SATIPs were measured by putting the samples onto a hotplate, as shown in Fig. 10a, the temperature of the hotplate was set to around 60 ˚C. A paint sample without SA additive was tested at left side of the hotplate as a reference for comparison, while SATIP samples with different SA content was tested at the right side of the hotplate. The IR images clearly shows that the surface temperature of the sample with SA additive is lower than that of the sample without additive, as shown in Fig. 10b. The surface temperature of the both sides of the samples was then measured concurrently by using two thermocouples. The temperature reduction was calculated by subtracting the bottom temperature from the top temperature of the samples. Figure 10c shows the temperature reduction of SATIPs with different SA content. As can be seen from the curve, the temperature reduction increases with the increase of the SA content, demonstrating the improved insulation performance by adding SA additive. Up to 12 ˚C improvement in temperature reduction is achieved compare to the one without SA. The trend is in line with the trend of the thermal conductivity of paints with different SA content.
Figure 11 shows the spectral reflectance of paints with different SA content. It can be seen that there is no obvious difference in the spectrum from visible to near infrared among all paints with 0–20% SA additive. The photons appear not to be interacting with the particles as the size of the SA particles are much larger than the wavelength of the incident light . The reflectance is contributed by the original pigment in the white paint. This spectral reflectance results indicates that the improvement of the thermal insulation performance of the SATIP is attributed to the thermal conductivity reduction from SA additive instead of the heat reflectance of paints with additive.
Figure 12 shows the thermal insulation performance of paint without (left) and with (right) 20% SA additive. The external surface temperature of the samples were monitored by using an IR thermometer. As shown in Fig. 12, the SATIP with 20% SA has better thermal insulation with the temperature difference of 1 ˚C under the external temperature of 44.6 ˚C compared with the paint without SA. This insulation performance improvement is mainly due to the thermal conductivity reduction contributed by SA additive. By adding SA into paint, the thermal conduction and convection of paint is reduced.