Abstract:

Objective Photochromic dye is a type of intelligent material with reversible photochromic function. Most organic photochromic dyes have single color-changing color spectra, which are difficult to meet the needs of diverse color changes. In order to solve the problem that the color-changing range of photochromic dyes is narrow and the color is limited to reversible changes from colorless to colored, a method for widening the color range of photochromic microcapsules is proposed.

Method Multi-colorant photochromic microcapsules were prepared by in-situ polymerization with the composite of photochromic dye spiroxazine and phase change solvent butyl stearate as the initial core material, acid dye as the extended dye, and melamine resin as the wall material. The surface morphology, characteristic functional groups and thermal properties of microcapsules were characterized using scanning electron microscope, infrared spectrometer, differential scanning calorimeter and thermogravimetric analyzer. The photochromic fabrics were prepared by screen printing using multi-colorant photochromic microcapsules as main components and fabrics as substrates, and the related color-changing properties were studied.

Results The photochromic microcapsules prepared by in-situ polymerization were spherical and had good encapsulation integrity. A small number of irregular particles on the surface was self-polymerized melamine resin, and no obvious agglomeration occered between the microcapsules. The microcapsule particle size was relatively uniform, with an average particle size of about 3 μm. The core material with the mass ratio of spioxazine to butyl stearate to acid dye of 0.05:50:0.1 was selected, and the mass ratio of the core to wall material was adjusted. When the core to wall ratio is 1:1, the encapsulation rate of photochromic microcapsules reached 73.4%. Through the infrared spectrum analysis of the photochromic microcapsule and its components, it could be inferred that there is no chemical reaction between the core material, the extended dye and the wall material. The photochromic microcapsule contained only the initial core material, C.I. Acid Red 337 and the wall material of melamine resin. The thermogravimetric analysis showed that the thermal degradation temperature of the core material was about 190 ℃, while the thermal degradation temperature of the photochromic microcapsule under the coating of melamine resin arrived at about 370 ℃, indicating that the thermal stability of the core material was improved by the microencapsulation. The photochromic microcapsules achieved reversible color change between pink and blue when irradiated by UV and D65 light source at 0 ℃. At the same ambient temperature, dark and D65 light sources showed little effect on the color recovery time of the photochromic microcapsules. Even without visible light irradiation, the photochromic microcapsules still demonstrated good color recovery. The printed patterns on fabrics achieved reversible color change under the irradiation UV and D65 light sources. No significant difference existed in the re-color state of cotton fabrics under different temperatures and color-changing cycles.

Conclusion This paper introduced a method to broaden the photochromic microcapsule chromatography. Multi-colorant photochromic microcapsules were prepared by in-situ polymerization with the compound of photochromic dye spiroxazine, phase change solvent butyl stearate and extended dye C.I. Acid Red 337 as the initial core material, and melamine resin as the wall material. The controlled mass ratio of spiroxazine to butyl stearate to extended dye C.I. Acid Red 337 is 0.05:50:0.1. When the core to wall ratio is 1:1, the prepared microcapsules have a spherical appearance with an average particle size of about 3 μm, and then capsulation rate can reach 73.4%. The photochromic microcapsules have excellent thermal stability and color-changing sensitivity. By adding acid dyes as extended colorants with different color shades and mass ratios, the photochromic microcapsules can achieve color changes between different colors, which significantly expands the color-change range of photochromic microcapsules. The microcapsules can be applied to cotton, polyester and other fabrics through screen printing process, which can give fabrics good color sensitivity and stability.

Key words: photochromic dye, extended dye, color-changing microcapsule, screen printing, photochromic fabric