Abstract:

Objective Low-temperature reversible thermochromic textiles are not only fashionable and attractive, but also applicable for temperature indication and anti-counterfeiting, showing the potential economic and social benefits of thermochromic textiles. However, the majority of thermochromic textiles are currently available for single-color change, e.g. from colored to colorless or colored to colored. In order to broaden the color change range of thermochromic textiles and achieve multichromatic thermochromic color change, SiO₂ shell thermochromic microcapsules with various colors were prepared and finished on cotton fabrics to obtain multichromatic thermochromic cotton fabrics.

Method In this research, tetraethyl orthosilicate was used as the silica source and fluorane-diphenyl sulfone-aliphatic alcohol as the core material to prepare multi-color SiO₂ shell thermochromic microcapsules by sol-gel method. After blending and dyeing cotton fabrics, cotton fabrics with continuous chromatographic color change with temperature change were obtained. Scanning electron microscopy was adopted to observe the morphological features of microcapsules. The chemical structure of the microcapsules was characterized employing infrared spectroscopy. The thermal stability of the microcapsules was tested with a thermogravimetric analyzer. The color change of thermochromic cotton fabrics was characterized by colorimeter.

Results The SiO₂ shell thermochromic microcapsules prepared by sol-gel method had uniform size dispersion and smooth surface (Fig. 2). The average particle size was about 0.9 μm (Fig. 3). The microcapsules demonstrated excellent thermal stability, where thermal degradation temperature was shown up to 200 ℃ (Fig. 5), much higher than daily service temperature. The microcapsules are finished onto the surface of the cotton fabric using the dip-roll method. After 50 heating-cooling cycles, the change of K/S value is only 0.01, showing the thermochromic microcapsules possess excellent thermal cycling durability (Fig. 7). The color change of cotton fabric in response to heat was still obvious. The excellent thermal stability and cycling durability could be attribute to fluorene, the developing agent, with sensitive color development, good stability and abundant color. The single-color change of thermochromic cotton fabric was obvious (Fig. 9), and the reflectance demonstrated an overall upward shift with temperature increasing, with the cotton fabric changing from colored to colorless (Fig. 8). It was possible to obtain thermochromic textiles with different color change temperatures by changing the solvent phase change point (Tab. 1). After different color-change thermochromic microcapsules of red, yellow and blue compound were coated onto the cotton fabric, the obtained multichromatic temperature sensitive color change cotton fabric was able to achieve step color change (Fig. 10). The color change performance of polychromatic color-change cotton was reliable and color change effect was obviously.

Conclusion Thermochromic microcapsules with SiO₂ shell have been successfully prepared by sol-gel method on fluorane-diphenyl sulfone-aliphatic alcohol core material. The prepared thermochromic microcapsules indicate superb microscopic morphology and uniform size dispersion. The prepared thermochromic fabric has good thermal stability and thermal cycling durability and can meet the requirements of daily use, which broadens the temperature variation range of thermochromic cotton textiles. The prepared multichromatic thermochromic textiles can respond to temperature from 20 to 55 ℃. Since the prepared thermochromic cotton fabric can achieve gradient color change, it can be utilized to indicate the temperature. It is expected that the multichromatic thermochromic cotton fabrics would be employed in the fields of food safety for temperature indication to indicate suitable storage temperature. It also can be used on anti-counterfeiting and building for environmental temperature indication. However, further increase of the temperature sensing accuracy of thermochromic materials still need further attention. Compared to inorganic thermochromic materials, organic thermochromic materials is non-toxic, and excellent in temperature sensitivity and color density. However, the color change performance of organic reversible thermochromic materials in the high temperature region is still a major problem, which is a hot spot in future research.

Key words: microcapsule, multichromatism, thermochromic, cotton fabric, sol-gel method, silica