Research on reversible effects and mechanism between the energy-absorbing and energy-reflecting states of chameleon-type building coatings

• Published in: Solar energy Vol. 72; no. 6; pp. 511 - 520
• Main Authors: Ma, Yiping, Zhang, Xiong, Zhu, Beirong, Wu, Keru
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.01.2002; Elsevier; Pergamon Press Inc
• Subjects: Applied sciences; Buildings; Buildings. Public works; Electron transitions; Energy absorption; Exact sciences and technology; Hydrogen; Infrared imaging; Infrared spectroscopy; Materials; Nuclear magnetic resonance spectroscopy; Paints and renderings. Protection products; Protective coatings; Raman spectroscopy; Solar energy; Solar radiation; Temperature measurement; Coating material; Buildings; Mechanism analysis; Temperature effect; Reflection; NMR spectrometry; Experimental study; Thermochromism; Infrared spectrometry; Experimental result; Thermal image; Pigments; Reversibility; Raman spectrometry; Energy absorption
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/S0038-092X(02)00029-4

In the present paper, the reversible effects between energy-absorbing and energy-reflecting states of chameleon-type building coatings were studied through demonstration of the layers’ properties using infrared thermal imaging of the layers when exposed to a sunlamp or temperature measurements of the layers during exposure to sunlight at different ambient temperatures. The reversible transforming mechanism between the energy-absorbing and energy-reflecting states of the chameleon-type building coatings was investigated with IR, Raman and 1H NMR spectroscopy. The infrared thermal image results showed that when reversibly thermochromic pigments were added to normal white building coatings, the chameleon-type building coatings could absorb energy from the sunlamp below a switching temperature of about 18°C. Absorption of energy from the sunlamp stopped automatically above the switching temperature. The results from exposure to solar radiation showed that when the temperature was below the switching temperature, the chameleon-type building coating could absorb almost the same amount of solar energy as an ordinary coloured coating, and when the temperature was above the switching temperature, the chameleon-type building coating could reflect more solar energy than the ordinary coloured coating. The above results showed that chameleon-type building coatings could contribute to a thermally comfortable building environment. The IR spectroscopy results showed that when the environmental temperature was below the switching temperature of 18°C, the lactone ring of the thermochromic pigment molecule would open and the band of CO would almost disappear. Raman spectra indicated that the band of C–O in ▪ would move to the high wave number range. From 1H NMR spectra, it could be found that there was some action between the hydrogen of the hydroxyl and the structure of ▪. During the lactone ring opening, the electron in the non-bond orbital would transit to the higher orbital and it could elongate the conjugated bridge, which would produce visible absorption and hence produce an energy-absorbing effect. However, when the environmental temperature was above the switching temperature of 18°C, the lactone ring in the molecule of the thermochromic pigment would close, the transited electron would transit back to the original orbital, hence the visible absorption would disappear, which would make the coating have an energy-reflecting effect.