Commercially available antimony doped nano-SnO₂ particle ( n(Sb): n(Sn)=1:9) and nominal size of 20 nm was used as the thermal insulating material. Waterborne polyurethane (WPU) with 35% solid content was used as binder material (PU-218). Sodium polycarboxylic acid was used as dispersant (HT-5040). And mineral oil, polyoxy-propylene ether/polymethyl hydrosiloxane surfactants were used as defoamer (SF-51). Non-asscociated Acrysoltmase-60 was used as thickener. Non-ionic urethane rheology modifier was used as leveling agent (RM2020).

Firstly, serous fluid containing antimony doped nano-SnO₂ with solid content of 20% was prepared as following. Antimony doped nano-SnO₂ powder and certain amount of dispersant were mixed in methanol and DI water, and ammonia was added until serous fluid of pH=9 was obtained. The mixed serous fluid was then ultrasonically dispersed for 0.5 h, followed by ball milling for 24 h at 250 r/min. 0.1wt% of defoamer, 4wt% of leveling agent, 0.2wt% of thickener were added into WPU binder in sequence, and the antimony doped nano-SnO₂ serous fluid was added while the mixture was magnetically stirred. The mixture was then ball milled for 4 h, and finally light blue colored antimony doped nano-SnO₂/ WPU composite transparent thermal insulating emulsion was obtained. The as-prepared emulsion was then coated on 5 mm thick glass by a blade coater (20-100 μm), and dried at room temperature for one week.

The crystalline structure of the composite film was characterized by X-ray diffractometer (TD-3500, Tongda), and the optical transmittance was measured by UV-Vis-IR spectrometer equipped with an integrating sphere (UV-3600, Shimadzu). Thermal insulation measurement was carried out as described by Zhen Dai, et al^{[ 12]}.

The XRD pattern of the antimony doped nano-SnO₂/ WPU composite coatings is shown in Fig.1, together with the XRD pattern of the antimony doped nano-SnO₂ powders. Both patterns show typical rutile structure SnO₂ with widened peaks. The FWHM of the peaks of the antimony doped nano-SnO₂/WPU and the FWHM of the peaks of the antimony doped nano-SnO₂ powder are almost identical, i.e., Δ(2 θ)=1.31º for the coatings and Δ(2 θ)=1.35º for the powder. This means that all the process mentioned above doesn’t change the size of the antimony doped nano-SnO₂ particles. The size estimated by Scherrer formula from the FWHM of the XRD peak is about 16.5 nm, which is close to the nominal sized of the source material.

Fig. 1

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	Fig. 1 XRD patterns of the antimony doped nano-SnO2/WPU coating and the source powder

The thermal insulating functional component in the composite is antimony doped nano-SnO₂, therefore, the content of antimony doped nano-SnO₂ must has the dominate effects on the transmittance and thermal insulating properties. A set of composite coatings (20 μm) with three different contents of antimony doped nano-SnO₂ is prepared according to the method mentioned above. The volume ratio of antimony doped nano-SnO₂/WPU of this set is 1:1, 1;2, 1:3, 1:4 and 1:6, respectively, which corresponds to the contents of antimony doped nano-SnO₂ in the composite emulsion of 0.5, 0.33, 0.25, 0.2 and 0.147, respectively.

Figure 2(a) shows the transmittance spectra of this set of samples. Figure 2(b) shows the dependence of the transmittance on the content of antimony doped nano-SnO₂ at wavelength of 1400 nm. Data fitting of the data in Fig. 2(b) shows good fitting by an exponential function.

Fig. 2

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Fig. 2 Transmittance spectra of the composite coatings with different contents of antimony doped nano-SnO2particles (a) and transmittance dependence at 1400 nm on the contents of antimony doped nano-SnO2(b)(a) Volume ratios of antimony doped nano-SnO2/WPU are 1:1, 1;2, 1:3, 1:4 and 1:6, respectively; (b) Contents of antimony doped nano-SnO2Thickess of are 20 μm 0.5, 0.33, 0.25, 0.2 and 0.147, respectively

Another set of the samples is prepared for the fixed content of antimony doped nano-SnO₂ (content=0.147), but with different coating thicknesses, i.e., 20 μm, 40 μm, 60 μm, 80 μm, and 100 μm, respectively. Figure 3(a) shows the transmittance spectra of this set of samples. Figure 3(b) shows the thickness dependence of the transmittance at wavelength of 1400 nm. Data fitting of the data in Fig. 3(b) also shows good fitting by an exponential function.

Fig. 3

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	Fig. 3 Transmittance spectra of the composite coatings with different thickness (a) and transmittance dependence at 1400 nm on the contents of antimony doped nano-SnO2 (b)Content of antimony doped nano-SnO2 is 0.147

The exponential behaviors of the transmittance dependence on the antimony doped nano-SnO₂ content and the thickness of the composite coatings strongly suggest that the reduction of the transmittance in the infrared re- gion is caused by absorption of the infrared light due to electron transitions in antimony doped nano-SnO₂, but not by the reflection of infrared light. Since antimony doped nano-SnO₂ is conductive, electrons in the conduction band absorb infrared light.

To further approve this point, reflectance spectra of this set of samples are collected and are shown in Fig. 4. It’s obvious that the reflection of the composite coated glasses only show about 1.5% increase in the reflectance spectra compared to the uncoated blank glass. That means that reflection is not the main factor of the thermal insulating, but the infrared absorption of by antimony doped nano-SnO₂ particles in the composite coatings.

Fig. 4

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	Fig. 4 Reflectance spectra of the composite coatings with different thickness

According to the above results, the reduction of the infrared transmittance by the composite coatings can be formulated as T=e^{- μ cd}, where T is the transmittance, μ is the absorbance coefficient of the antimony doped nano-SnO₂ particles, c is the content of the antimony doped nano-SnO₂ particles in the composite coatings, and d is the thickness of the coatings.

The thermal insulating properties of the later set of coatings are shown in Fig. 5(a) and Fig. 5(b). Figure 5(a) shows the temperature of the composite coating surface, and Fig. 5(b) shows the temperature of inside the measurement box. It’s clear that the surface temperature of all coated samples are higher than that of blank glass, which indicating the absorption of the infrared light by the coatings. This is in accordance with our conclusion above, i.e., the blocking of infrared by the coatings is caused by the absorbance of the infrared light, but not the reflectance. The surface temperature of the 100 μm thick coatings is raised by 13℃. On the other hands, Fig. 5(b) shows that the temperature inside the measurement box is reduced for all coated sample. The temperature inside the box covered by 100 μm thick coatings is lowered 17.5℃, indicating good thermal insulation of the composite coating.

Fig. 5

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	Fig. 5 Surface temperature of the composite coated glass (a) and temperature inside the measurement box (b) of antimony doped nano-SnO2/WPU coatings with different thicknesses

The mechanism of the thermal insulating of the antimony doped nano-SnO₂/WPU is mainly due to the absorbance of infrared light by antimony doped nano-SnO₂ particles inside the composite coatings. Under illumination of sunlight, the temperature of the coated glass surface increases, while the temperature inside the measurement box decrease as much as 17.5℃. Therefore, antimony doped nano-SnO₂ particles composite coating is an effective thermal insulating coating with many advantages, such as low cost, chemically stable, easy in process, and can DIYI in home, etc.