Objective Conventional camouflage is achieved by the use of camouflage net or camouflage clothing with static color patterns. Although it can provide a certain camouflage effect for specific environment, it is difficult for it to adapt to complex and changeable environment. In order to overcome the disadvantage that static camouflage is easy to be identified in a changeable background, a study on adaptive discoloration camouflage fabrics was proposed, aiming to respond quickly to external changes and intelligently adjust its own characteristics to achieve a high degree of integration with the background.

Method In order to solve this problem, a new type of fluorane dye was designed and synthesized, and a highly sensitive two-component thermochromic system was prepared by physical blending of the fluorane dye and a phase change material. Microcapsule packaging technology was adopted to maintain the thermoal stability at high temperature, so as to provide the basis for realization of fast response discoloration camouflage. The thermochromic paste which can be used in natural environment camouflage was prepared by compounding discoloration microcapsules with ordinary disperse dyes in different proportions, and was screen printed on fabrics to produce thermochromic intelligent textiles.

Results The molecular structure of the dye was characterized and analyzed by ¹H NMR and high resolution mass spectrometry, and the apparent morphology and particle size of thermochromic microcapsules were observed by polarizing microscope and scanning electron microscope. The results showed that the sample was regular spherical, the surface was relatively smooth, and the average particle size was about 12 μm. The thermal stability and melting-crystallization properties of thermochromic microcapsules were also investigated. The thermogravimetric characteristic peaks of thermochromic microcapsules appeared at 220-340 ℃ and 360-500 ℃, corresponding to the thermal decomposition of core material composite and shell material polymethyl methacrylate (PMMA), respectively. The results showed that PMMA successfully encapsulated the thermochromic compound in the microcapsule and had a certain protective effect.

The micromorphology of ordinary polyester/cotton fabric and thermochromic cotton fabric was characterized by scanning electron microscope. The surface of the original polyester/cotton fabric was smooth, while the surface of the polyester/cotton fabric after layer printing was rough and the surface morphology changed obviously. A large number of microcapsules was attached to the surface of the fabric. In addition, the color performance of the fabric was explored. With the increase of the amount of thermochromic paste in the compound color paste, the fabric gradually turned yellow, the K/S value gradually decreased. The fabric color demonstrated an obvious lighter trend, which was kept stable when it reached a certain proportion. Secondly, owing to the increase of thermochromic paste, the response of the fabric to temperature change became more sensitive and more obviously. The discoloration cycle of the fabric was also tested, and the fabric showed excellent reversible discoloration performance in 100 cycles of rising and falling temperature. The camouflage fabric designed by screen printing was compared with the vegetation in nature, showing a good camouflage effect.

Conclusion A highly sensitive fluorane dye was designed and synthesized. The compound prepared by mixing with phase change material (tetradecyl alcohol) achieved color transformation at 2.5 ℃, and thermochromic microcapsules were prepared by solvent volatilization method. Using disperse dyes and thermochromic microcapsules as colorants, different proportions of green-yellow thermochromic pastes were prepared and finished on polyester/cotton fabrics by screen printing. With the increase of the ratio of thermochromic microcapsule pastes to disperse dye pastes, the color difference of the fabric before and after discoloration gradually increased. The discoloration range of the fabric is 35-37 ℃, still maintaining a narrow discoloration temperature range. After 100 discoloration cycles, the performance of the fabric kept consustancy, and the natural leaf color was successfully simulated. The material mainly realizes the change of its own color by adjusting the temperature, and has a color similar to that of different vegetation (such as green, yellow), which breaks the limitation of fixed color in traditional anti-reconnaissance methods and improves its applicability in different environments.

Keywords： fluorane dye; microcapsule; thermochromic fabric; imitation green leaf color; biomimetic fabric; smart textile